Fabrication of Earth-Abundant Electrocatalysts Based on Green-Chemistry Approaches to Achieve Efficient Alkaline Water Splitting—A Review

Abstract

The fabrication of earth-abundant electrocatalysts by green-chemistry approaches for electrochemical water splitting could diminish or alleviate the use or generation of hazardous substances, which could be highly desirable to achieve efficient, green alkaline water electrolysis for clean energy production (H₂). This review started by introducing the importance of the green-chemistry approaches. Later, this paper reviewed the fabrication of high-performance earth-abundant electrocatalysts using green-chemistry approaches for electrochemical water splitting (HER and OER). Moreover, this review discussed the green-chemistry approaches for the fabrication of earth-abundant electrocatalysts including phosphide/pyrophosphate-, carbon-, oxide-, OH/OOH/LDH-, alloy/B/nitride-, and sulfide/selenide (chalcogenide)-based earth-abundant electrocatalysts. Moreover, this review discussed various green-chemistry approaches, including those used to alleviate toxic PH₃ gas emission during the fabrication of transition-metal phosphide-based electrocatalysts, to design energy-efficient synthesis routes (especially room-temperature synthesis), to utilize cheap or biodegradable substrates, and to utilize biomass waste or biomass or biodegradable materials as carbon sources for the fabrication of earth-abundant electrocatalysts. Thus, the construction of earth-abundant electrocatalysts by green-chemistry approaches for electrochemical water splitting could pave an efficient, green way for H₂ production.

1. Introduction

A green and ideal energy source of hydrogen (H₂) with a high energy density of 142 MJ kg⁻¹ can be produced through electrochemical water splitting and could address essential, challenging issues such as the depletion of fossil fuels (such as petroleum, natural gas, and coal) and increasing energy demand, while the fabrication of earth-abundant electrocatalysts through green-chemistry approaches for electrochemical water splitting could reduce or alleviate the use or generation of hazardous substances in the design, manufacture, and application of electrocatalysts, which could be highly desirable to achieve efficient, green water electrolysis [1,2,3,4,5,6,7,8]. Besides, H₂ can be produced sustainably through electrochemical water splitting by utilizing electrical energy from intermittent renewable resources such as wind and solar energies. Electrochemical water splitting is comprised of a hydrogen evolution reaction (HER) at the cathode and an oxygen evolution reaction (OER) at the anode; HER processes involve two electrons in both acid (2H⁺ + 2e⁻→ H₂) and alkaline (2H₂O + 2e⁻→ H₂ + 2OH⁻) environments, while OER processes involve four electrons in both acid (2H₂O → O₂ + 4H⁺ + 4e⁻) and alkaline (4OH⁻→ O₂ + 2H₂O + 4e⁻) environments [3,9]. Presently, noble electrocatalysts [10], such as Ru/Ir-based compounds for OER and Pt for HER, have been considered as highly active electrocatalysts for water splitting. For HER in 1 M KOH, Pt/C exhibited an overpotential (η) of −33 mV at −10 mA cm⁻² [11], suggesting its significantly high activity, and Pt/C exhibited 79% retention at −10 mA cm⁻² for 24 h [12], suggesting its high stability. For OER in 1 M KOH, the IrO₂ exhibited the η of 330 mV at 10 mA cm⁻² [13], suggesting its very high activity, and the IrO₂ exhibited a reasonable stability at the η of 280 mV after 6 h [14], suggesting its very high stability. However, the high cost and scarcity of Ru/Ir-based compounds and Pt prevent their usage. Hence, several promising strategies have been applied to attain high-performance earth-abundant electrocatalysts for HER and OER, while promising strategies such as tuning the electronic structure, enhancing the gas-evolution behavior, increasing the electrocatalytically active surface area (ECSA), increasing the metallic character, and improving the conductivity of the electrocatalysts could enhance the performance of the catalysts, and the in situ formation of thin films of oxide/oxyhydroxide as highly OER-active layers on the surface of the catalyst during OER could also improve the performance of the electrocatalysts [3,9,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. Nevertheless, the preparation of earth-abundant electrocatalysts by green-chemistry approaches for electrochemical water splitting could diminish or eliminate the use or generation of hazardous substances in the design, manufacture, and application of electrocatalysts, which is highly desirable to attain efficient, green water electrolysis [1,2,4,19].

In the year 1998, P. T. Anastas and J. C. Warner proposed a definition for green chemistry consisting of 12 principles [33]. Accordingly, green chemistry is the design, manufacture, and application of chemical products using 12 principles (Figure 1) that diminish or alleviate the use or formation of hazardous substances [33], where the 12 principles are as follows: (1) Preventing waste, (2) Atom economy, (3) Chemical synthesis with little or no hazardous substances, (4) Designing harmless chemicals, (5) Harmless solvents/auxiliaries, (6) Designing an energy efficiency synthesis route, (7) Use of renewable raw materials or feedstocks, (8) Diminishing derivatives, (9) Catalysis, (10) Designing chemical products for degradation, (11) Real-time, in-process monitoring and process control for the prevention of pollution, and (12) Use of inherently harmless chemistry for prevention of chemical accidents. Since the proposed definition for green chemistry using 12 principles, significant research has been carried out on the development of green chemistry [2,34], including the development of green nano-synthesis [1], green solvents [35], biocatalysis [36], biofuels [37], green synthesis of biomaterials [38], and polymer science [39], etc. In the year 2018, Anastas’s group illustrated “Green ChemisTREE”, which depicted significant research and developments in the field of green chemistry using the 12 above mentioned principles [2].

Besides, earth-abundant electrocatalysts have been prepared using green-chemistry approaches for electrochemical water splitting [40,41,42]. The fabrication of transition-metal phosphide-based electrocatalysts can be considered as highly active for HER and OER in alkaline media [15,16,19]. P sources such as NaH₂PO₂ and NH₄H₂PO₂ can be used for phosphorization processes. However, toxic phosphine (PH₃) gas could be produced by the decomposition of NaH₂PO₂ or NH₄H₂PO₂ in the phosphorization process [43,44]. The inhalation of PH₃ gas is highly toxic to humans and can cause harmful effects on multiple organs including the heart, liver, and kidney [45]. Moreover, toxic PH₃ gas can cause deleterious effects especially in the neurological system, such as axonal, neuronal, and vascular injuries as well as oxidative damage in the brain [46]. Therefore, several approaches have been used to alleviate toxic PH₃ gas emission during the fabrication of transition-metal phosphide-based electrocatalysts. Thus, Co₂P [41] was prepared using cobalt (II) complex (trioctylphosphine ligand) through a microwave-assisted method, which could alleviate toxic PH₃ gas emission, and it exhibited very high activity for HER (η of −95 mV at −10 mA cm⁻²) and OER (η of 260 mV at 10 mA cm⁻²), while the Co₂P//Co₂P exhibited significantly very high activity for overall water splitting (1.44 V at 10 mA cm⁻²) in 1 M KOH. Thus, the microwave-assisted method is a useful synthesis process [47]. Besides, nanostructured Ni_0.65Fe_0.35P [48] was obtained through phosphorization using phytic acid as the green organophosphorus source, which could alleviate toxic PH₃ gas emission, and it exhibited enhanced activity (η of 270 mV at 10 mA cm⁻²) and stability for OER in 1 M KOH. Moreover, FeP@NPC [49] was obtained using phytic acid as one of the nontoxic and environmentally friendly precursors, which could alleviate toxic PH₃ gas emission, and it exhibited enhanced activity (η of −214 mV at −10 mA cm⁻²) for HER in 1 M KOH.

On the other hand, sulfur-doped CoP [50] was obtained by a thiourea-phosphate-assisted method, which could alleviate toxic and lethal PH₃ gas formation, and it exhibited very high activity for HER (η of −109 mV at −10 mA cm⁻²) and OER (η of 270 mV at 10 mA cm⁻²), while the S-CoP//S-CoP exhibited high activity for overall water splitting (1.617 V at 10 mA cm⁻²) in 1 M KOH. Moreover, nanostructured sulfur-doped Co₂P [51] was obtained by a thiourea-phosphate-assisted method, which could alleviate toxic and lethal PH₃ gas formation, and it exhibited very high activity for HER (η of −105 mV at −10 mA cm⁻²) and OER (η of 288 mV at 10 mA cm⁻²), while the S-Co₂P//S-Co₂P exhibited a very high activity for overall water splitting (~1.63 V at 10 mA cm⁻²) in 1 M KOH. Besides, CoP@PC [44] was obtained by pyrolysis of the cobalt–phosphonate complex under a H₂/Ar atmosphere, where the complex was prepared by refluxing at 140 °C for 5 h; poisonous PH₃ gas-releasing P sources such as NaH₂PO₂ and NH₄H₂PO₂ were not used, and it exhibited very high activity for HER (η of −76 mV at −10 mA cm⁻²) and OER (η of 280 mV at 10 mA cm⁻²), while the CoP@PC//CoP@PC exhibited very high activity (1.6 V at 10 mA cm⁻²) and stability (negligible decay at 1.65 V for 60 h) for overall water splitting in 1 M KOH. On the other hand, Co₂P–CNT (CNT: Carbon nanotube) [52] was obtained through a solid-state pyrolysis process (solvent-free and one-pot synthesis) of cobalt acetylacetonate, triphenylphosphine, and melamine under N₂ atmosphere at 900 °C for 2 h, which could alleviate toxic PH₃ gas emission, and it exhibited very high activity for HER (η of −132 mV at −10 mA cm⁻²) and OER (η of 292 mV at 10 mA cm⁻²), while the Co₂P-CNT//Co₂P-CNT exhibited very high activity for overall water splitting (1.53 V at 10 mA cm⁻²) in 1 M KOH. Thus, pyrolysis is a useful synthesis process [53].

Designing an energy-efficient synthesis route can be one of the essential green-chemistry approaches [2,33,54]. Hence, several efforts have been used to design energy-efficient synthesis routes (especially at room temperature synthesis) for the fabrication of earth-abundant electrocatalysts. The nanostructured NiFe LDH on 3D Fe foam [31] (LDH: Layered double hydroxide) was obtained by facile corrosion engineering strategy, where a cleaned Fe foam was immersed in transparent Ni²⁺-containing solution (pH: ~5.88) in a conical flask at room temperature for 12 h, while it exhibited very high activity with robust stability (it exhibited a negligible decay at the huge current density of 1000 mA cm⁻² in 1 M KOH for 5000 h followed by a negligible decay at the huge current density of 1000 mA cm⁻² in 10 M KOH for 1050 h). Moreover, earth-abundant electrocatalysts for OER such as NiFe LDH [55], partially crystalline NiFe LDH nanosheets [56], and FeMoB [57] have been fabricated using facile synthesis routes at room temperature.

Besides, several efforts have been carried out to utilize cheap or biodegradable [40,58,59,60] substrates for the fabrication of earth-abundant electrocatalysts. NiCoP–Cu [58] exhibited enhanced activity and stability for HER and OER, and the NiCoP was prepared on pretreated Cu mesh by the electrodeposition method, where the hand-crafted Cu mesh was obtained from electronic wastes. The Ni–P–B/paper [40] exhibited a substantially high stability and activity for HER and OER, and it was obtained by a facile synthesis method using cheap filter paper as the substrate. On the other hand, NiMoO₄-bacterial nanocellullose [59] exhibited enhanced activity and stability for HER, where bacterial nanocellullose was used as the electrode matrix.

Moreover, several efforts have been carried out to utilize biomass waste, biomass, or biodegradable material as the carbon source [47,61,62,63,64,65,66,67,68,69] for the fabrication of earth-abundant electrocatalysts. NiO–C [61] exhibited enhanced activity and durability for HER; it was obtained by hydrothermal treatment followed by carbonization, where a waste egg shell membrane was used as the carbon source. The NiFeO_x–NP–C [62] exhibited enhanced activity and stability for OER, where expired milk powder was used as the carbon source in the synthesis. N-doped porous carbon nanosheets [63] exhibited enhanced activity and stability for OER, where the powder of Euonymus japonicus leaves was used as the carbon source in the synthesis. Fe–Ni₂P@NPC [64] exhibited enhanced activity and stability for OER, where biomass agarose was used as the carbon source in the synthesis. The Mo₂C–C [65] exhibited enhanced activity and stability for HER and OER, where cornstalk (a plant material) was used as the carbon source in the synthesis. C@NiMn [66] exhibited enhanced activity and stability for OER, where glucose was used as the carbon source in the synthesis.

Thus, various green-chemistry approaches have been applied for the fabrication of high-performance earth-abundant electrocatalysts for electrochemical water splitting to diminish or eliminate the use or generation of hazardous substances in the design, manufacture, and application of electrocatalysts. Nevertheless, reviews on the fabrication of earth-abundant electrocatalysts using green-chemistry approaches along with their catalytic performance in alkaline environments for electrochemical water splitting (HER and OER) have rarely been reported. In this respect, the present paper reviewed the fabrication of earth-abundant electrocatalysts using green-chemistry approaches, where their catalytic performances in alkaline environments for electrochemical water splitting (HER and OER) were also discussed. Moreover, this review discussed the green-chemistry approaches for the fabrication of earth-abundant electrocatalysts including phosphide/pyrophosphate-, carbon-, oxide-, OH/OOH/LDH-, alloy/B/nitride-, and sulfide/selenide (chalcogenide)-based earth abundant electrocatalysts. In addition, this review discussed the performance of the earth-abundant electrocatalysts for HER (including activity: η of ≤−55 mV at −10 mA cm⁻²; stability: ≥100 h; durability: ≥5000 cycles), OER (including activity: η of ≤133 mV at 10 mA cm⁻²; stability: ≥250 h; durability: ≥10,000 cycles), and overall water splitting (including activity: ≤1.51 V at 10 mA cm⁻²; stability: ≥240 h) in alkaline environments. Moreover, this review discussed various green-chemistry approaches, including those used to alleviate toxic PH₃ gas emission during the fabrication of transition-metal phosphide-based electrocatalysts, efforts used to design energy-efficient synthesis routes (especially room-temperature synthesis), to utilize cheap or biodegradable substrates [40,58,59,60], and to utilize biomass-waste or biomass or biodegradable material as carbon source [47,61,62,63,64,65,66,67] for the fabrication of earth-abundant electrocatalysts. The 12 principles of green chemistry (Figure 1) are discussed in the secondparagraph of the introduction section [2,33]. The fabrication of earth-abundant electrocatalysts using one of the 12 above mentioned principles of green chemistry was considered for this review.

2. Fabrication of Earth-Abundant Electrocatalysts Based on Green-Chemistry Approaches to Achieve Efficient Alkaline Water Splitting

2.1. Principle 1: Prevent Waste [33]

Fabrication of Phosphide-/Carbon-/Oxide-Based Earth Abundant Electrocatalysts

Recovering Cu from electronic wastes and utilizing it for energy-related applications could diminish the environmental impact of its production and also ensure its availability and sustainability. Thus, fabricating bimetallic phosphide on electronic waste-derived Cu substrate could modify the electronic structure and generate OER-active in situ surface layers during OER, which could enhance the conductivity, provide optimal adsorption energy with intermediates, and enhance the performance for HER and OER. Jothi et al. [58] observed that NiCoP–Cu exhibited enhanced activity and stability for HER and OER. The NiCoP was prepared on pretreated Cu mesh by the electrodeposition method, where the hand-crafted Cu mesh was obtained from electronic wastes. The NiCoP–Cu exhibited higher OER activity and lower charge-transfer resistance than that of NiP–Cu and CoP–Cu, while it exhibited higher HER activity than that of NiP–Cu and CoP–Cu. The NiCoP–Cu was composed of amorphous NiCoP film on Cu mesh. It contained Ni, Co, and P, which were uniformly distributed. It could exhibit the in situ formation of a NiCo oxy/hydroxide layer as the active species on the surface of the NiCoP during OER. For OER in 1 M KOH, the NiCoP–Cu exhibited a η of 220 mV at 10 mA cm⁻², suggesting its very high activity; it also exhibited reasonable stability at 1.46 V for 36 h, suggesting its very high stability, and it exhibited negligible decay at 100 mA cm⁻² after 2000 cycles of CV, suggesting its very high durability. For HER in 1 M KOH, the NiCoP–Cu exhibited a η of −178 mV at −10 mA cm⁻², suggesting its very high activity, while it also exhibited reasonable stability at η of −203 mV for 36 h, suggesting its very high stability. For overall water splitting in 1 M KOH, the NiCoP–Cu//NiCoP–Cu exhibited a potential of 1.59 V at 10 mA cm⁻² (

Table 1. Potential to achieve 10 mA cm⁻² for overall water splitting of various kinds of reported earth-abundant bifunctional electrocatalysts in 1.0 M KOH alkaline electrolyte (potential to attain >10 mA cm⁻² is mentioned and described in the footnote of this table), where these electrocatalysts are prepared based on green-chemistry approaches.

Electrocatalysts for Overall Water Splitting	Potential (V) at 10 mA cm−2	Ref.
Co2P//Co2P	1.44	[41]
S–NiP//S–NiP	1.51	[70]
Co2P–CNT//Co2P–CNT	1.53	[52]
CoP//CoP	1.54	[43]
Fe2+–NiFe LDH–CO32−//Fe2+–NiFe LDH–CO32−	1.55	[71]
N–NiCoP//N–NiCoP	1.56	[72]
N–NiCoPx//N–NiCoPx	1.57	[73]
NiCoP–Cu//NiCoP–Cu	1.59	[58]
FeP//FeP	1.59	[74]
CoP@PC//CoP@PC	1.6	[44]
S–CoP//S–CoP	1.617	[50]
S–Co2P//S–Co2P	~1.63	[51]
NiSe2–CoSe2//NiSe2–CoSe2	1.63 a	[75]
Mo2C–C//Mo2C–C	1.65	[65]
Ni–P–B/paper//Ni–P–B/paper	1.661 b	[40]
BN–Mo2C@BCN//BN–Mo2C@BCN	1.84 c	[76]

Ref.: References; 1.63 ^a: 1.63 V at 50 mA cm⁻²; 1.661 ^b: 1.661 V at 50 mA cm⁻²; 1.84 ^c: 1.84 V at 100 mA cm⁻²; CNT: Carbon nanotube; LDH: Layered double hydroxide.

), suggesting its very high activity, while it also exhibited reasonable stability at 1.65 V for 24 h (

Table 2. Long-term stability of various kinds of reported earth-abundant bifunctional electrocatalysts for overall water splitting in 1 M KOH alkaline electrolyte, where these electrocatalysts are prepared based on green-chemistry approaches.

Electrocatalysts for Overall Water Splitting	Chr. Amp.	Chr. Pot.	Duration (h)	Remark after Stability Test	Ref.
FeP//FeP	YES	NA	336	Negligible decay at 1.6 V	[74]
Ni–P–B/paper//Ni–P–B/paper	YES	NA	240	91.4% retention (delivered ~100 mA cm−2)	[40]
CoP@PC//CoP@PC	YES	NA	60	Negligible decay at 1.65 V	[44]
N–NiCoPx//N–NiCoPx	YES	NA	50	Negligible decay	[73]
Mo2C–C//Mo2C–C	NA	YES	30	Negligible decay at 10 mA cm−2	[65]
S–NiP//S–NiP	NA	YES	25	Negligible decay at 100 mA cm−2	[70]
CoP//CoP	NA	YES	24	Negligible decay at 10 mA cm−2	[43]
NiCoP–Cu//NiCoP–Cu	YES	NA	24	Reasonable stability at 1.65 V	[58]
S–Co2P//S–Co2P	YES	NA	20	Negligible decay	[51]
S–CoP//S–CoP	YES	NA	20	Negligible decay at ~1.8 V	[50]
BN–Mo2C@BCN//BN–Mo2C@BCN	YES	NA	20	Reasonable stability	[76]
Co2P–CNT//Co2P–CNT	NA	YES	>13	Reasonable stability	[52]
Fe2+–NiFe LDH–CO32−//Fe2+–NiFe LDH–CO32−	YES	NA	12	Negligible decay	[71]
NiSe2–CoSe2//NiSe2–CoSe2	YES	NA	10	Negligible decay	[75]

Ref.: References; Chr. Amp.: Chronoamperometry; Chr. Pot.: Chronopotentiometry; NA: Not applicable; CNT: Carbon nanotube; LDH: Layered double hydroxide.

), suggesting its very high stability.

Grape skin is a waste biomass, and it is a biodegradable and non-toxic material that contains carbon atoms [77], which can be used as the carbon source for fabricating electrocatalysts. The N-doped porous carbon nanomesh [78] with a high surface area (1029.41 m² g⁻¹) exhibited an enhanced activity (η of 440 mV at 10 mA cm⁻²) for OER in 1 M KOH, while it was obtained by fermentation of waste grape skins (biomass waste) for two months followed by a hydrothermal process at 180 °C for 12 h followed by annealing at 800 °C under a N₂ atmosphere for 5 h.

Sewage sludge is a waste material byproduct of wastewater treatment that can be used as a source for fabricating electrocatalysts. N–Fe–S–C [79] exhibited enhanced activity and durability (negligible decay at 5 mA cm⁻² after 5000 cycles of CV) for OER in 0.1 M KOH, while it is composed of N, Fe, and S multi-doped nanoporous carbon. N–Fe–S–C was obtained by pyrolysis of dewatered and lyophilized sewage sludge at 800 °C for 2 h under a NH₃ atmosphere followed by partial etching away of the ash content.

The fabrication of transition metallic oxide embedded in a carbon matrix as the electrocatalyst for HER using waste egg shell membrane as the carbon source could render the waste material valuable. NiO–C [61] exhibited an enhanced activity (η of −565 mV at −10 mA cm⁻²) and durability (negligible decay at −10 mA cm⁻² after 500 cycles of CV) for HER in 1 M KOH, being composed of crystalline NiO nanoparticles that are distributed on the amorphous carbon matrix. The NiO–C was obtained by hydrothermal treatment followed by carbonization, where waste egg shell membrane was used as the carbon source.

Milk powder is an edible dairy product, which is composed of carbon-containing substances including proteins. Moreover, several tons of milk powder are produced around the world every year, while plenty of expired milk powder is also generated as food waste. Thus, the fabrication of transition bimetallic oxide integrated with heteroatom-doped carbon as the electrocatalyst using expired milk powder as a heteroatom-doped carbon source could diminish the usage of toxic chemicals. NiFeO_x–NP–C [62] exhibited an enhanced activity (η of ~321 mV at 10 mA cm⁻²) and stability (98.4% retention at 10 mA cm⁻² for 20 h) for OER in 1 M KOH, and it contains NiFeO_x nanoparticles, which are incorporated into N- and P-codoped porous carbon. NiFeO_x–NP–C was obtained by hydrothermal treatment at 150 °C for 24 h followed by carbonization at 800 °C for 2 h under an Ar atmosphere, where expired milk powder was used as the carbon source.

Orange peel is an abundant resource and contains functional groups such as –OH, –COOH, and –NH₂ groups [80,81,82], which could be beneficial for adsorbing many metal ions; thus, orange peel can be used as template and reductive agent for the fabrication of transition metallic oxide as the electrocatalyst. Crystalline, defect-containing, coral-like Fe₃O₄ [83] exhibited enhanced activity for OER in 1 M NaOH. The Fe₃O₄ was obtained from Fe(NO₃)₃·9H₂O without using any additives, where orange peel (waste resource) was used as the template and reductive agent, while H₂O was used as the solvent.

2.2. Principle 3: Chemical Synthesis with Little or No Hazardous Substances, and Principle 4: Designing Harmless Chemicals [33]

Principle 3 is closely connected with Principle 4 [2]. Therefore, this section discusses the green-chemistry approaches based on Principle 3 and Principle 4.

2.2.1. Fabrication of OH-/LDH-/Sulfide-/Oxide-Based Earth-Abundant Electrocatalysts

The fabrication of nanostructured, amorphous transition bimetallic hydroxide on an electrode as the electrocatalyst for OER through a simple and facile electrochemical process using the immersion of a target material in only water could alleviate the usage of any chemical additives. Amorphous, nanostructured Ni_0.71Fe_0.29(OH)_x [84] exhibited an enhanced activity (η of 296 mV at 10 mA cm⁻²), stability (negligible decay at 5 mA cm⁻² for 24 h), and durability (negligible decay at 10 mA cm⁻² after 30,000 cycles of CV) for OER in 0.1 M KOH; it was obtained on a graphite electrode through a simple and facile electrochemical process at 100 V for 10 h, where the target material (NiFe alloy) was immersed in water without any chemical additives and where graphite was used as the cathode and anode. Moreover, 3D honeycomb-like amorphous Ni(OH)₂ nanosheets [85] exhibited an enhanced activity (η of 344 mV at 10 mA cm⁻²) and durability (negligible decay at 30 mA cm⁻² after 5000 cycles of CV) for OER in 0.1 M KOH, and were obtained on a graphite electrode through a simple and facile electrochemical process at 90 V for 75 min, where the target material (Ni) was immersed in water without any chemical additives and where graphite was used as the cathode and anode. In addition, amorphous Co(OH)₂ nanosheets [86] exhibited an enhanced activity (η of 380 mV at 10 mA cm⁻²) and stability (negligible decay at 1 mA cm⁻² for 24 h) for OER in 0.1 M KOH, and were obtained on a graphite electrode through a simple and facile electrochemical process at 50 V for 1 h, where the target material was immersed in water without any chemical additives and where graphite was used as the cathode and anode.

Transition bimetallic LDH can be prepared on transition bimetallic foam by a hydrothermal treatment using only deionized water, which could obviate the use of any other chemicals. Then, heteroatom-doped bimetallic phosphide could be obtained by plasma-assisted phosphorization of the above mentioned bimetallic LDH, which could modify the electronic structure and provide a nanostructure, as well as afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for HER and OER. N–NiCoP_x [73] exhibited an enhanced activity and stability for OER. The N–NiCoP_x was obtained on cleaned NiCo foam by a hydrothermal treatment (where NiCo LDH was obtained only using water without any chemicals), followed by plasma-assisted phosphorization. The N–NiCoP_x exhibited higher HER activity and lower charge-transfer resistance than that of NiCoP_x, while it exhibited higher OER activity than that of NiCoP_x. The N–NiCoP_x is crystalline, which contains Ni₂P, Co₂P, NiCoP, CoN, and Ni₃N phases. It contains Ni, Co, N, P, and O, which are uniformly distributed. Theoretical studies suggest that the Co atom bonded with N in N–NiCoP_x could be the active center for overall water splitting, which could lower the adsorption energy for intermediates; N–NiCoP_x also exhibited a high density of electronic states around the Fermi level. It possesses a nanosheet array morphology. For OER in 1 M KOH, N–NiCoP_x exhibited a η of 298 mV at 10 mA cm⁻², suggesting its considerably high activity, while it exhibited 96.93% retention (delivered ~100 mA cm⁻²) for 100 h, suggesting its significantly very high stability; it also exhibited negligible decay at 300 mA cm⁻² after 10,000 cycles of CV, suggesting its robust durability. For HER in 1 M KOH, the N–NiCoP_x exhibited a η of −23 mV at −10 mA cm⁻² (

Table 3. Overpotential (η) to achieve −10 mA cm⁻² for hydrogen evolution reaction (HER) of various kinds of reported earth-abundant electrocatalysts in 1 M KOH alkaline electrolyte (η that attain >−10 mA cm⁻² are indicated on the η and described in the footnote of this table, while the electrolytes other than 1 M KOH are indicated on the electrocatalyst), where these electrocatalysts are prepared based on green-chemistry approaches.

Electrocatalysts for HER	η at −10 mA cm−2 (mV)	Ref.
N–NiCoPx	−23	[73]
NiSe2–CoSe2	−24	[75]
S–NiP	−55	[70]
Ni–P–B/paper	−76 a	[40]
CoP@PC	−76	[44]
N–NiCoP	−78	[72]
Co3O4–Co4N–HCC	−90	[87]
Co2P	−95	[41]
BN–Mo2C@BCN	~−100	[76]
S–Co2P	−105	[51]
Fe2+–NiFe LDH–CO32−	−106	[71]
S–CoP	−109	[50]
NiMoO4–bacterial nanocellullose	−109	[59]
CoP–C	−111	[43]
Mo2C–C	−130	[65]
Co2P–CNT	−132	[52]
Mo2C–MoP–NC	−134	[88]
Mo2C–Mo2N–C	−145	[89]
CoFe2O4–NC0.1 M KOH	−164	[90]
FeP	−165	[74]
NiCoP–Cu	−178	[58]
FeP@NPC	−214	[49]
CoxFe1−x@N–graphene	−272	[91]
NiCoP	−314	[92]
Fe2O3–Co–N–graphene1 M NaOH	~−409	[93]
NiO–C	−565	[61]

Ref.: References; −76 ^a: −76 mV at −50 mA cm⁻²; CNT: Carbon nanotube; LDH: Layered double hydroxide; HCC: Hydrophilic carbon cloth.

), suggesting its substantially high activity, while it exhibited 93.98% retention (delivered ~−100 mA cm⁻²) for 100 h (

Table 4. Long-term stability of various kinds of reported earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in 1 M KOH alkaline electrolyte, while the electrolytes other than 1 M KOH are indicated on the electrocatalyst, where these electrocatalysts are prepared based on green-chemistry approaches.

Electrocatalysts for HER	Chr. Amp.	Chr. Pot.	Duration (h)	Remark after Stability Test	Ref.
Ni–P–B/paper	YES	NA	240	94.1% retention (delivered ~ −1000 mA cm−2) at η of −345 mV	[40]
Mo2C–MoP–NC	YES	NA	120	Reasonable stability	[88]
N–NiCoPx	YES	NA	100	93.98% retention (delivered ~ −100 mA cm−2)	[73]
NiSe2–CoSe2	Yes	NA	100	97.3% retention (delivered ~ −100 mA cm−2)	[75]
N–NiCoP	YES	NA	100	Negligible decay	[72]
NiMoO4–bacterialnanocellullose	YES	NA	48	Negligible decay	[59]
Co3O4–Co4N–HCC	NA	YES	40	Negligible decay	[87]
NiCoP–Cu	YES	NA	36	Reasonable stability at η of −203 mV	[58]
CoP–C	NA	YES	24	Reasonable stability at −10 mA cm−2	[43]
CoP@PC	YES	NA	20	Negligible decay	[44]
BN–Mo2C@BCN	YES	NA	20	Reasonable stability	[76]
S–Co2P	YES	NA	20	Negligible decay at η of −150 mV	[51]
S–CoP	YES	NA	20	Negligible decay at η of −130 mV	[50]
Mo2C–C	NA	YES	20	Reasonable stability at −10 mA cm−2	[65]
Mo2C–Mo2N–C	NA	YES	>16	Negligible decay	[89]
FeP	YES	NA	15	Reasonable stability	[74]
Co2P–CNT	YES	NA	>13	Reasonable stability	[52]
Fe2+–NiFe LDH–CO32−	YES	NA	12	Negligible decay	[71]
Fe2O3–Co–N–graphene1 M NaOH	YES	NA	11	Negligible decay	[93]
S–NiP	NA	YES	10	Negligible decay at −100 mA cm−2	[70]

Ref.: References; Chr. Amp.: Chronoamperometry; Chr. Pot.: Chronopotentiometry; NA: Not applicable; CNT: Carbon nanotube; LDH: Layered double hydroxide; HCC: Hydrophilic carbon cloth.

), suggesting its considerably very high stability; it also exhibited negligible decay at −300 mA cm⁻² after 10,000 cycles of CV (

Table 5. Durability of various kinds of reported earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in 1 M KOH alkaline electrolyte, while the electrolytes other than 1 M KOH are indicated on the electrocatalyst, where these electrocatalysts are prepared based on green-chemistry approaches.

Electrocatalysts for HER	Cycles of CV	Remark after Durability Test	Ref.
N–NiCoPx	10,000	Negligible decay at −300 mA cm−2	[73]
NiSe2–CoSe2	10,000	Negligible decay at −100 mA cm−2	[75]
N–NiCoP	10,000	Negligible decay at −100 mA cm−2	[72]
Ni–P–B/paper	5000	Negligible decay at −450 mA cm−2	[40]
CoFe2O4–NC0.1 M KOH	5000	Negligible decay at −10 mA cm−2	[90]
Co2P–CNT	2000	Negligible decay at −10 mA cm−2	[52]
S–NiP	1000	Negligible decay at −100 mA cm−2	[70]
CoP–C	1000	Very slight decay at −10 mA cm−2	[43]
CoxFe1−x@N–graphene	1000	Negligible decay at −50 mA cm−2	[91]
NiO–C	500	Negligible decay at −10 mA cm−2	[61]

Ref.: References; CV: Cyclic voltammetry; CNT: Carbon nanotube.

), suggesting its robust durability. For overall water splitting in 1 M KOH, the N–NiCoP_x//N–NiCoP_x exhibited a potential of 1.57 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 50 h, suggesting its very high stability.

Transition bimetallic LDH can be prepared on transition bimetallic foam by hydrothermal treatment using only deionized water, which could obviate the use of any other chemicals, and bimetallic selenides could be obtained by selenization of the above mentioned bimetallic LDH, while it could modify the electronic structure, provide nanostructure, afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and could enhance the performance for HER and OER. Chen et al. [75] observed that NiSe₂–CoSe₂ exhibited substantially very high activity and stability for HER and OER. NiSe₂–CoSe₂ was prepared by the following steps (Figure 2): At first, NiCo LDH was obtained on pre-cleaned NiCo foam by hydrothermal treatment at 200 °C for 24 h, where only 25 mL of deionized water was added to the pre-cleaned NiCo foam, while no other chemicals were used. Finally, the NiSe₂–CoSe₂ was prepared by hydrothermal selenization at 180 °C for 24 h. The NiSe₂–CoSe₂ exhibited higher HER activity and lower charge-transfer resistance than that of NiCo LDH, while it exhibited higher OER activity than that of NiCo LDH. The NiSe₂–CoSe₂ is crystalline, which is composed of NiSe₂, CoSe₂, NiCoSe₂, Co₂O₃, and Ni₂O₃ phases, where the existence of oxides could be due to the partial oxidation of the catalyst. It contains Ni, Co, Se, and O, which are uniformly distributed. The DFT calculations suggest that the Co atoms at the hetero-interface between the NiSe₂ and CoSe₂ phases could act as the active sites for the water electrolysis. It possesses nanosheet array morphology (Thickness of the nanosheet: ~300 nm). For OER in 1 M KOH, the NiSe₂–CoSe₂ exhibited a η of 250 mV at 10 mA cm⁻² (

Table 6. Overpotential (η₁₀) to achieve 10 mA cm⁻² for oxygen evolution reaction (OER) of various kinds of reported earth-abundant electrocatalysts in 1 M KOH alkaline electrolyte (η to attain other than 10 mA cm⁻² are indicated on the η and described on the footnote of this table, while the electrolytes other than 1 M KOH are indicated on the electrocatalyst), where these electrocatalysts are prepared based on green-chemistry approaches.

(Section 1)			(Section 2)
Electrocatalysts for OER	η10 (mV)	Ref.	Electrocatalysts for OER	η10 (mV)	Ref.
S-graphene foam0.1 M KOH	128	[94]	Ni0.71Fe0.29(OH)x0.1 M KOH	296	[84]
Fe2+–NiFe LDH–CO32−	128	[71]	N–NiCoPx	298	[73]
NiFe LDH	133	[56]	PEMAc@CNT	298	[95]
NiFe LDH	~180	[55]	Ultrathin Cu–Co(OH)2	300	[96]
NiFe LDH	200	[31]	CoSe@NCNT–NC	301	[97]
Fe–Ni pyrophosphate	210	[98]	CuO	310 b	[42]
NixFe3-xO4–Ni	218	[99]	Co2P–Co@PN–C	311	[100]
NiCoP–Cu	220	[58]	NiFeOx–NP–C	~321	[62]
exfoliated NiFe LDH–C	220	[101]	CoV2O6	324	[102]
N–NiCoP	225	[72]	CoOOH with oxygen vacancies	330	[103]
FeP	227	[74]	NiCo2S4	337	[104]
S–NiP	229	[70]	NiCoP	340	[92]
Fe3O41 M NaOH	234	[83]	rGO–C–CoB0.1 M KOH	340	[105]
NiSe2–CoSe2	250	[75]	Ni(OH)20.1 M KOH	344	[85]
FeMoB	253	[57]	BN–Mo2C@BCN	~360 c	[76]
Co2P	260	[41]	CoFe2O4	360	[106]
NiFe–MoS2	260	[107]	Co@C0.1 M KOH	370 d	[108]
Ni–P–B/paper	263 a	[40]	Co2P–NP–CNT	370	[109]
Co(OH)2 with lattice distortion	265	[110]	NPSC–Co2Fe10.1 M KOH	~370	[111]
Ni@Ni2P–N–C nanofiber–CNT	269	[112]	CoFe2O4–NC0.1 M KOH	~380	[90]
S–CoP	270	[50]	Co(OH)20.1 M KOH	380	[86]
Ni0.65Fe0.35P	270	[48]	Fe–Ni2P@NPC0.1 M KOH	390	[64]
Geobacter–rGO	270	[113]	Fe–N–graphene0.1 M KOH	393	[114]
C@NiMn	270	[66]	NiCo2S40.1 M KOH	400	[115]
Mo2C–C	274	[65]	NiO–MWCNT0.5 M KOH	409	[116]
CoP–C	277	[43]	N–C0.1 M KOH	~345 e	[63]
CoP@PC	280	[44]	CoZnOH	~430	[117]
S–Co2P	288	[51]	N–C	440	[78]
Co2P–CNT	292	[52]	Co–TiO20.5 M KOH	474	[118]
exfoliated NiFe LDH	292	[119]	NS–C0.1 M KOH	~545	[120]

Ref.: References; η₁₀: η at 10 mA cm⁻²; 263 ^a: 263 mV at 50 mA cm⁻²; 310 ^b: 310 mV at 25 mA cm⁻²; ~360 ^c: ~360 mV at 100 mA cm⁻²; 370 ^d: 370 mV at 6.43 A g⁻¹; ~345 ^e: ~345 mV at 5 mA cm⁻²; CNT: Carbon nanotube; LDH: Layered double hydroxide; rGO: Reduced graphene oxide; PEMAc: Poly(ethylene-alt-maleic acid); MWCNT: Multi-walled carbon nanotube.

), suggesting its very high activity, while it exhibited 95.2% retention (delivered ~100 mA cm⁻²) for 100 h, suggesting its significantly very high stability; it also exhibited negligible decay at 150 mA cm⁻² after 10,000 cycles of CV, suggesting its substantially very high durability. For HER in 1 M KOH, the NiSe₂–CoSe₂ exhibited a η of −24 mV at −10 mA cm⁻², suggesting its considerably very high activity, while it exhibited 97.3% retention (delivered ~ −100 mA cm⁻²) for 100 h, suggesting its significantly very high stability, and it exhibited negligible decay at −100 mA cm⁻² after 10,000 cycles of CV, suggesting its substantially very high durability. For overall water splitting in 1 M KOH, the NiSe₂–CoSe₂//NiSe₂–CoSe₂ exhibited a potential of 1.63 V at 50 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 10 h, suggesting its very high stability.

Exfoliation of bimetallic LDH as the electrocatalyst for OER through an ultrasonic process using only water could alleviate the usage of toxic solvents. The crystalline, exfoliated NiFe LDH–C single-/few-layer nanosheets [101] exhibited enhanced activity (η of 220 mV at 10 mA cm⁻²) and stability (reasonable stability at 10 mA cm⁻² for 12 h) for OER in 1 M KOH, while the exfoliation was achieved by an ultrasonic process for 15 min in water. In the XPS spectra, the binding energies of Fe 2p_1/2 and Fe 2p_2/3 peaks were shifted for exfoliated NiFe LDH-C when compared with those of bulk NiFe LDH-C, suggesting the modified electronic structure, while the exfoliated NiFe LDH-C exhibited higher OER activity and lower charge-transfer resistance than that of bulk NiFe LDH-C.

NiFe LDH can be prepared by hydrothermal treatment, while Ostwald ripening-driven in situ exfoliation can be achieved by a subsequent hydrothermal treatment, and it could modify the electronic structure and provide nanostructure, which could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and enhance the performance for OER. Chen et al. [119] observed that the exfoliated NiFe LDH exhibited enhanced activity and stability for OER. The exfoliated NiFe LDH was prepared by the following steps (Figure 3a–e): At first, Cu_xO was obtained on Cu mesh by heating the Cu mesh at 500 °C for 2 h under air atmosphere. Then, the NiFe LDH was grown on Cu_xO/Cu mesh by hydrothermal treatment at 120 °C for 15 h using a precursor solution, while Ostwald ripening driven in situ exfoliation was achieved by a subsequent hydrothermal treatment at 160 °C for 8 h (autoclave was unopened) without adding any other reagent or surfactant. The exfoliated NiFe LDH exhibited higher OER activity and electrochemically active surface areas than that of bulk NiFe LDH. The exfoliated NiFe LDH is crystalline. It contains Ni, Fe, and O, which are uniformly distributed. It contains Ni²⁺, and Fe³⁺. It possesses ultrathin nanosheet morphology (thickness: ~10 nm). For OER in 1 M KOH, the NiFe LDH exhibited a η of 292 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at 10 mA cm⁻² for >60 h, suggesting its very high stability.

The fabrication of hollow-structured, transition bimetallic sulfide as the electrocatalyst for OER through a facile synthesis route without using surfactant and template at relatively low-temperature could be desirable to achieve efficient water electrolysis. The NiCo₂S₄ [115] exhibited enhanced activity (η of 400 mV at 10 mA cm⁻²), stability (reasonable stability at η of 400 mV for ~11 h), and durability (8 mV decay at 10 mA cm⁻² after 1000 cycles of CV) for OER in 0.1 M KOH. It is polycrystalline, which is composed of cubic NiCo₂S₄ phase. It possesses hollow sphere morphology with high surface area, while it contains mesopores. The NiCo₂S₄ was obtained by a surfactant-free and template-free method using an binary solution comprising of N,N-dimethylformamide and ethylene glycol through a reflux route at low temperature (<200 °C).

Besides, the Co–TiO₂ [118] exhibited enhanced activity (η of 474 mV at 10 mA cm⁻²) for OER in 0.5 M KOH, composed of Co-doped anatase TiO₂ nanoparticles, while it was obtained by the sol–gel method, where gelatin (biodegradable material) was used in the preparation process.

2.2.2. Fabrication of Phosphide-Based Earth-Abundant Electrocatalysts

The fabrication of cobalt phosphide using cobalt (II) complex (trioctylphosphine ligand) through a microwave-assisted method could alleviate toxic PH₃ gas emission, while it could generate metallic character, modify the electronic structure, generate OER-active thin in situ surface layers during OER, and provide ultrathin nanostructure, and could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and enhance the performance for HER and OER. Jin et al. [41] observed that Co₂P exhibited enhanced activity for HER and OER. The Co₂P was prepared using cobalt (II) complex (trioctylphosphine ligand) through a microwave-assisted method, which could alleviate toxic PH₃ gas emission. The Co₂P nanowire exhibited higher OER activity than that of CoP nanowire and CoP film. The Co₂P is composed of crystalline cobalt phosphide with the obvious lattice planes of (112) and (020). In the XPS spectra (XPS: X-ray photoelectron spectroscopy), the Co₂P exhibited lower binding energy for Co 2p when compared with that of CoP, suggesting the modified electronic structure, while the Co₂P exhibited a peak at 778.2 eV, which is almost close to the metallic Co (Co⁰), suggesting the existence of metallic character in Co₂P (Valence state of Co could be ~+0.3), which could be due to the rapid phosphorization of the microwave-assisted method. Moreover, the density-of-states (DOS) near the Fermi level reveal that the Co₂P exhibited higher intensity of the electrons when compared with that of CoP, suggesting the superior electrical conductivity of Co₂P. It could exhibit the in situ formation of thin Co oxo/hydroxide layers as an active species on the surface of the Co₂P during OER. It possesses ultrathin nanowire morphology. For OER in 1 M KOH, the Co₂P exhibited a η of 260 mV at 10 mA cm⁻², suggesting its very high activity. For HER in 1 M KOH, the Co₂P exhibited a η of −95 mV at −10 mA cm⁻², suggesting its very high activity. For overall water splitting at 10 mA cm⁻² in 1 M KOH, the Co₂P//Co₂P affords cell voltage of 1.44 V, where this activity was higher than that of the activity of electrocatalysts prepared using toxic P source ((Ni_0.33Fe_0.67)₂P//(Ni_0.33Fe_0.67)₂P (1.49 V) [121] and Co₄Ni₁P//Co₄Ni₁P (1.59 V) [122]).

The fabrication of transition metal phosphide as the electrocatalyst from a molecular metal phosphide precursor using relatively low-temperature could be desirable to achieve efficient water electrolysis. The crystalline FeP nanoparticles [74] exhibited enhanced activity and stability for HER and OER. The FeP was obtained by conversion of a molecular iron phosphide precursor at a hot injection condition (low-temperature) in oleylamine (CH₃(CH₂)₇CH=CH(CH₂)₇CH₂NH₂), where the precursor was composed of Fe₂P₃ core having mixed-valence Fe^IIFe^III sites, which was bridged by an asymmetric cyclo-P₍₂₊₁₎³⁻ ligand, while the FeP was electrophoretically deposited on Ni foam, which was used as the anode and cathode. For OER in 1 M KOH, the FeP exhibited a η of 227 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability at 1.46 V for 15 h, suggesting its very high stability. For HER in 1 M KOH, the FeP exhibited a η of −165 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability for 15 h, suggesting its very high stability. For overall water splitting in 1 M KOH, the FeP//FeP exhibited a potential of 1.59 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at 1.6 V for 336 h, suggesting its substantially high stability.

The fabrication of transition bimetallic phosphide as the electrocatalyst using triphenylphosphine as the phosphorus source at relatively low temperature could alleviate toxic PH₃ gas emission. The crystalline NiCoP nanoparticles [92] exhibited enhanced activity for HER and OER. The NiCoP was obtained using triphenylphosphine (PPh₃) as the phosphorus source at relatively low temperature (250 °C), where NaBH₄ was used to facilitate the reaction. For OER in 1 M KOH, the NiCoP exhibited a η of 340 mV at 10 mA cm⁻², suggesting its high activity. For HER in 1 M KOH, the NiCoP exhibited a η of −314 mV at −10 mA cm⁻², suggesting its high activity.

The fabrication of heteroatom-doped metal phosphide as the electrocatalyst through the electrochemical deposition method could alleviate toxic PH₃ gas emission. The nanostructured S-doped NiP (S–NiP) [70] exhibited enhanced activity and stability for HER and OER, where the S doping could modify the electronic structure, while the nanostructure could facilitate the gas evolution. The S–NiP was obtained on pretreated Cu substrate by pulse electrochemical deposition method. For OER in 1 M KOH, the S–NiP exhibited a η of 229 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at 100 mA cm⁻² for 10 h, suggesting its very high stability, and it exhibited negligible decay at 100 mA cm⁻² after 1000 cycles of CV, suggesting its very high durability. For HER in 1 M KOH, the S–NiP exhibited a η of −55 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at −100 mA cm⁻² for 10 h, suggesting its very high stability, and it exhibited negligible decay at −100 mA cm⁻² after 1000 cycles of CV, suggesting its very high durability. For overall water splitting in 1 M KOH, the S–NiP//S–NiP exhibited a potential of 1.51 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at 100 mA cm⁻² for 25 h, suggesting its very high stability.

The fabrication of heteroatom-doped metal phosphide through a thiourea-phosphate-assisted method could alleviate toxic and lethal PH₃ gas formation, while it could modify the electronic structure, generate OER-active in situ surface layers during OER, and provide nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for HER and OER. The nanostructured sulfur-doped Co₂P [51] exhibited enhanced activity and stability for HER and OER. The S–Co₂P was obtained by a thiourea-phosphate-assisted method, which could alleviate toxic and lethal PH₃ gas formation. For OER in 1 M KOH, the S–Co₂P exhibited a η of 288 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at η of 340 mV for 20 h, suggesting its very high stability. For HER in 1 M KOH, the S–Co₂P exhibited a η of −105 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at η of −150 mV for 20 h, suggesting its very high stability. For overall water splitting in 1 M KOH, the S–Co₂P//S–Co₂P exhibited a potential of ~1.63 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 20 h, suggesting its very high stability.

Moreover, Anjum et al. [50] observed that S-CoP exhibited enhanced activity and stability for HER and OER. The sulfur-doped CoP was obtained by a thiourea-phosphate-assisted method, which could alleviate toxic and lethal PH₃ gas formation. It was prepared by pouring transparent aqueous solution of cobalt nitrate hexahydrate to the pre-heated (at 60 °C) transparent aqueous urea-phosphate solution (H₃PO₄ + (NH₂)₂CS → H₂S + (NH₂)CO(NH₂)·H₃PO₄), while the mixture was subjected to hydrothermal treatment at 160 °C for 24 h ((NH₂)CO(NH₂)·H₃PO₄ + xH₂S + Co²⁺→ S-doped Co·urea·phosphate) followed by thermal reduction at 600 °C for 3 h under H₂ atmosphere. The S–CoP exhibited higher HER activity and lower charge-transfer resistance than that of CoP, while it exhibited higher OER activity than that of CoP. The S–CoP is composed of S-doped CoP, and it exhibited the orthorhombic crystal structure of CoP, while the average crystal size is reduced from ~ 32 nm to ~ 20 nm due to the S doping in CoP. It possesses nanoparticle morphology. It could exhibit the in situ formation of a CoO/Co₃O₄ layer as the active species on the surface of the S–CoP during OER. The density of states (DOS) of S–CoP at the Fermi level was higher than that of bare CoP for the (011) and (111) planes, suggesting the existence of more electrons in the conduction band for S–CoP. For OER in 1 M KOH, the S–CoP exhibited a η of 270 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability at η of ~350 mV for 20 h, suggesting its very high stability. For HER in 1 M KOH, the S–CoP exhibited a η of −109 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at η of −130 mV for 20 h, suggesting its very high stability. For overall water splitting in 1 M KOH, the S–CoP//S–CoP exhibited a potential of 1.617 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at ~1.8 V for 20 h, suggesting its very high stability.

Toxic PH₃ gas could be produced by the decomposition of NaH₂PO₂ during the fabrication of electrocatalysts, while passing toxic PH₃ gas into the cupric sulfate solution could alleviate its emission in environment. The CoP–C [43] exhibited enhanced activity and stability for HER and OER, while the ultrathin CoP nanosheet is crystalline. The CoP was obtained by precipitation followed by low-temperature phosphorization and passivation under an O₂/N₂ mixture. In the phosphorization process, toxic PH₃ gas could be produced by the decomposition of NaH₂PO₂, where toxic PH₃ gas is passed into the cupric sulfate solution (CuSO₄) to prevents its emission in environment, while the PH₃ gas could be converted into H₃PO₄ and H₂SO₄ (4CuSO₄ + PH₃ + 4H₂O → 4Cu↓ + H₃PO₄ + 4H₂SO₄). For OER in 1 M KOH, the CoP–C exhibited a η of 277 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability at 10 mA cm⁻² for 24 h, suggesting its very high stability, and it exhibited negligible decay at 30 mA cm⁻² after 1000 cycles of CV, suggesting its high durability. For HER in 1 M KOH, the CoP-C exhibited a η of −111 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability at −10 mA cm⁻² for 24 h, suggesting its high stability, and it exhibited very slight decay at −10 mA cm⁻² after 1000 cycles of CV, suggesting its high durability. For overall water splitting in 1 M KOH, the CoP//CoP exhibited a potential of 1.54 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at 10 mA cm⁻² for 24 h, suggesting its very high stability.

Plasma treatment can be considered as a useful technology to improve the surface properties of the materials [123,124]. The fabrication of heteroatom-doped bimetallic phosphide as the electrocatalyst using plasma-assisted low-temperature phosphorization of transition bimetallic hydroxide could alleviate the usage of heavy metal ions. The N–NiCoP [72] exhibited enhanced activity and stability for HER and OER. The N–NiCoP was obtained by hydrothermal treatment followed by plasma-assisted low-temperature phosphorization. The N–NiCoP exhibited higher HER activity and lower charge-transfer resistance than that of NiCoP, while it exhibited higher OER activity than that of NiCoP. The N–NiCoP is crystalline, which contains CoP, Ni₂P, Co₂NiP₄, Ni₃N, and CoN phases. It contains Co, Ni, P, and N, which are uniformly distributed. It possesses polyhedron morphology. It exhibited high density of electronic states around the Fermi level. For OER in 1 M KOH, the N–NiCoP exhibited a η of 225 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 100 h, suggesting its substantially very high stability, and it exhibited negligible decay at 50 mA cm⁻² after 10,000 cycles of CV, suggesting its significantly very high durability. For HER in 1 M KOH, the N–NiCoP exhibited a η of −78 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 100 h, suggesting its considerably very high stability, and it exhibited negligible decay at −100 mA cm⁻² after 10,000 cycles of CV, suggesting its robust durability. For overall water splitting in 1 M KOH, the N–NiCoP//N–NiCoP exhibited a potential of 1.56 V at 10 mA cm⁻², suggesting its very high activity.

The fabrication of transition metal phosphide confined in carbon matrix by pyrolysis of the transition metal phosphonate complex could alleviate toxic PH₃ gas emission, while it could modify the electronic structure, generate OER-active thin in situ surface layers during OER, and provide nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for HER and OER. Wu et al. [44] observed that CoP@PC exhibited enhanced activity and stability for OER and HER. The CoP@PC was obtained by pyrolysis of the cobalt–phosphonate complex under H₂/Ar atmosphere, where the complex was prepared by refluxing at 140 °C for 5 h, where poisonous PH₃ gas releasing P sources such as NaH₂PO₂ and NH₄H₂PO₂ are not used. The CoP@PC is composed of cobalt phosphides (crystalline orthorhombic CoP with small amount of Co₂P), which are confined in porous P-doped carbon materials, where the CoP/Co₂P nanoparticles are almost homogenously dispersed along the carbonized fibers, while the CoP/Co₂P nanoparticles are enwrapped by thin layer of carbon shell. It contains Co, P, and C. It exhibited irregular lattice pattern, suggesting the existence of carbon matrix with low graphitization degree, while it exhibited I_D/I_G ratio of 0.96. It inherits the 1D nanofiber morphology of cobalt–phosphonate complex. It exhibited high specific surface area of 88.4 m² g⁻¹. In the XPS spectra, the binding energy of the P 2p_3/2 peak (130.0 eV) for the CoP@PC is lower than that of P element, while the binding energy of the Co 2p_3/2 peak (779.1 eV) for the CoP@PC is higher than that of metal Co, suggesting the modified electronic structure, which could be due to the transfer of partial electrons from Co to P. It could exhibit the in situ formation of Co oxides/hydroxides layer as the active species on the surface of the CoP@PC during OER. For OER in 1 M KOH, the CoP@PC exhibited a η of 280 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited 95.8% retention at 1.51 V for 20 h, suggesting its very high stability. For HER in 1 M KOH, the CoP@PC exhibited a η of −76 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 20 h, suggesting its very high stability. For overall water splitting in 1 M KOH, the CoP@PC//CoP@PC exhibited a potential of 1.6 V at 10 mA cm⁻² suggesting its very high activity, while it exhibited negligible decay at 1.65 V for 60 h, suggesting its very high stability.

Phytic acid can be extracted from plant materials [125,126], while fabrication of transition bimetallic phosphide as the electrocatalyst for OER using phytic acid as a green organophosphorus source could alleviate toxic PH₃ gas emission. The nanostructured Ni_0.65Fe_0.35P [48] exhibited enhanced activity (η of 270 mV at 10 mA cm⁻²) and stability (Reasonable stability at 20 mA cm⁻² for >6 h) for OER in 1 M KOH, while it was obtained through phosphorization using phytic acid as the green organophosphorus source. However, melamine should be cautiously handled in the synthesis process. The melamine is toxic, while high content of melamine in milk or food can cause adverse effects such as urinary and kidney problems, or even death [127,128]. On the other hand, melamine is a nitrogen-rich compound, which can be used as a fertilizer in soil [128], whereas the plants could absorb the melamine beyond the recommended level [129]. Moreover, melamine has been used for the manufacture of tableware [130], whereas oxidative stress and the risk of early damage to kidneys in humans can be increased due to the low-dose exposure of melamine to the workers from melamine tableware factories [131]. The FeP@NPC [49] exhibited enhanced activity (η of −214 mV at −10 mA cm⁻²) for HER in 1 M KOH, while it is composed of FeP nanoparticles, which are encapsulated in N, P codoped carbon. The FeP@NPC was obtained by the following steps using phytic acid as one of the nontoxic and environmentally friendly precursor: At first, a homogeneous powder was obtained by drying a solution A at 60 °C for overnight, where the solution A was obtained by dissolving FeCl₃·6H₂O and phytic acid in deionized water under stirring followed by adding melamine. Finally, FeP@NPC was prepared by annealing the powder at 900 °C for 2 h under N₂ atmosphere.

The fabrication of transition metallic phosphide as the electrocatalyst using triphenylphosphine as the phosphorus source through solid-state pyrolysis process could alleviate toxic PH₃ gas emission and eliminate toxic solvent usage. The Co₂P-CNT [52] exhibited enhanced activity and stability for HER and OER, while it is composed of Co₂P nanoparticles, which are encapsulated in N, P codoped carbon nanotubes. The Co₂P-CNT was obtained through solid-state pyrolysis process (solvent-free and one-pot synthesis) of cobalt acetylacetonate, triphenylphosphine, and melamine under N₂ atmosphere at 900 °C for 2 h. For OER in 1 M KOH, the Co₂P-CNT exhibited a η of 292 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited reasonable stability for >13 h, suggesting its high stability, and it exhibited negligible decay at 10 mA cm⁻² after 2000 cycles of CV, suggesting its high durability. For HER in 1 M KOH, the Co₂P-CNT exhibited a η of −132 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability for >13 h, suggesting its high stability, and it exhibited negligible decay at −10 mA cm⁻² after 2000 cycles of CV, suggesting its high durability. For overall water splitting in 1 M KOH, the Co₂P-CNT//Co₂P-CNT exhibited a potential of 1.53 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability for >13 h, suggesting its high stability. Moreover, the Co₂P-NP-CNT [109] exhibited enhanced activity (η of 370 mV at 10 mA cm⁻²), stability (86.9% retention at η of 370 mV for 10 h), and durability (negligible decay at 10 mA cm⁻² after 1000 cycles of CV) for OER in 1 M KOH, while it is composed of Co₂P nanoparticles, which are encapsulated in N, P codoped carbon nanotubes. The Co₂P-NP-CNT was obtained through solid-state pyrolysis process (solvent-free and one-pot synthesis) of Co(NO₃)₂·6H₂O, triphenylphosphine and melamine at 850 °C under N₂ for 1 h without involving toxic phosphine gas (PH₃) formation.

The fabrication of transition metallic phosphide and alloy encapsulated in heteroatom-doped nanoporous carbon as the electrocatalyst for OER using triphenylphosphine sulfide as the P and S source through solid-state pyrolysis process could alleviate toxic PH₃ gas emission and eliminate toxic solvent usage. The NPSC-Co₂Fe₁ [111] exhibited enhanced activity (η of ~370 mV at 10 mA cm⁻²) for OER in 0.1 M KOH, while it is composed of CoFe and Co₂P nanoparticles, which are encapsulated in multi-doped nanoporous carbon. The NPSC-Co₂Fe₁ was obtained through solid-state pyrolysis process (solvent-free and one-pot synthesis) of Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, triphenylphosphine sulfide (Ph₃PS) and melamine under N₂ atmosphere at 850 °C for 1 h.

H₃PO₄ can be used as an etching agent for metal-organic framework (MOF), while it could also act as a phosphorus source, where the metallic phosphide embedded in P-doped carbon as the electrocatalyst for OER can be fabricated by selective-etching of MOF using H₃PO₄ followed by carbonization under inert atmosphere, and that could obviate the usage of toxic phosphine gas forming chemicals. The Co₂P–Co@PN–C [100] exhibited enhanced activity (η of 311 mV at 10 mA cm⁻²), and stability (negligible decay at η of 311 mV for 10 h) for OER in 1 M KOH, while it is composed of Co₂P and Co, which are embedded in P, N codoped carbon. It possesses rhombic dodecahedral morphology. It contains mesopores. The Co₂P–Co@PN–C was obtained by the following steps: At first, ZIF-67 was obtained by co-precipitation method. Finally, Co₂P–Co@PN–C was prepared by selective-etching of ZIF-67 using H₃PO₄ followed by carbonization at 800 °C for 2 h under N₂ atmosphere, where H₃PO₄ could be used as an etching agent for ZIF-67, while it could also act as a phosphorus source, and that could obviate the usage of toxic phosphine gas forming chemicals.

2.2.3. Fabrication of Carbon-Containing Earth-Abundant Electrocatalysts

Polydopamine is a biomaterial [132,133], while electrocatalysts can be fabricated using polydopamine as a biopolymer-mediated green synthesis [34]. The N, S codoped carbon [120] with high surface area (488.01 m² g⁻¹) exhibited enhanced activity (η of ~545 mV at 10 mA cm⁻²) for OER in 0.1 M KOH. The NS-C was prepared by the following steps using polydopamine as a biopolymer-mediated green synthesis: It was obtained by self-polymerization followed by annealing at 900 °C for about 2 h under N₂ atmosphere.

Besides, the Mo₂C–MoP–NC [88] exhibited substantially very high activity (η of −134 mV at −10 mA cm⁻²), and stability (reasonable stability for 120 h) for HER in 1 M KOH, while it is composed of molybdenum carbide-phosphide nanodots, which are encapsulated in N-doped carbon shell. The Mo₂C–MoP–NC was obtained by drop-casting the as-prepared product on carbon fiber paper followed by carbonization at 800 °C for 3 h under Ar atmosphere, where polydopamine was used as the precursor, where the polydopamine could facilitate the Mo ions chelation without the use of toxic chelating agents.

Fe is one of the highly earth-abundant element, while Prussian blue (Iron (III) ferrocyanide (Fe^III₄[Fe^II(CN)₆]₃) is an iron based dark blue pigment. It is used in some medicines. It is used for the preparation of laundry bluing. Thus, fabrication of nanoalloys encapsulated heteroatom-doped graphene as the electrocatalyst for HER through a facile synthesis route using Prussian blue could be desirable to achieve efficient water electrolysis. The Co_xFe_1−x@N–graphene [91] exhibited enhanced activity (η of −272 mV at −10 mA cm⁻²) and durability (negligible decay at −50 mA cm⁻² after 1000 cycles of CV) for HER in 1 M KOH, while it is composed of CoFe nanoalloys, which are encapsulated in N-doped graphene layers. The Co_xFe_1−x@N–graphene was obtained by pyrolysis of Prussian blue and cobalt nitrate hexahydrate at 750 °C for 2 h.

The fabrication of transition metal carbide encapsulated heteroatom-doped graphitic nanostructures as the electrocatalyst for OER by pyrolysis of Prussian blue without using any inert gas flow could be desirable to achieve efficient water electrolysis. The Fe–Fe₃C–NC [134] exhibited enhanced activity and stability (slight decay at 10 mA cm⁻² after 1000 cycles of CV) for OER in 0.1 M KOH, while it is composed of Fe-Fe₃C-NC nanoparticles, which are encapsulated by bambo-like N-doped graphitic nanotubes and N-doped graphitic layers. The Fe-Fe₃C-NC was obtained by pyrolysis of Prussian blue at 750 °C for 2 h without using any inert gas flow.

Imidazole (C₃N₂H₄) is a biomaterial [135,136], which is incorporated into various important biological compounds including amino acid histidine, while small quantity of boric acid (H₃BO₄) is used in some medicine [137,138] and agriculture, whereas large quantity of boric acid could be hazardous to environment and human health [139,140]. Thus, fabrication of B, N co-doped metallic carbide nanoparticles embedded in a B, N co-doped carbon using imidazole as the source of C and N, and small quantity of boric acid as the source of B could be desirable to achieve efficient water electrolysis. The BN–Mo₂C@BCN [76] exhibited enhanced activity and stability for HER and OER in 1 M KOH, while it is composed of B, N co-doped molybdenum carbide nanoparticles, which are embedded in a B, N co-doped carbon. The BN–Mo₂C@BCN was synthesized by annealing the as-prepared Mo–imidazole complex with boric acid (H₃BO₄) at 900 °C for 4 h under Ar atmosphere, where imidazole was used as the precursor. For OER in 1 M KOH, the BN–Mo₂C@BCN exhibited a η of ~360 mV at 100 mA cm⁻², suggesting its high activity, while it exhibited reasonable stability for 20 h, suggesting its high stability. For HER in 1 M KOH, the BN–Mo₂C@BCN exhibited a η of ~−100 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability for 20 h, suggesting its high stability. For overall water splitting in 1 M KOH, the BN–Mo₂C@BCN//BN–Mo₂C@BCN exhibited a potential of 1.84 V at 100 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability for 20 h, suggesting its high stability.

Microwave hydrogen plasma treatment could convert the hydrophobic surface of the carbon cloth into hydrophilic surface, which could alleviate the usage of toxic oxidizing agents such as nitric acid and hydrogen peroxide, while the hydrophilic surface of the carbon cloth could facilitate the fabrication of electrocatalyst, and it could modify the electronic structure, and provide porous nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for HER. Liu et al. [87] observed that the Co₃O₄–Co₄N–HCC (HCC: Hydrophilic carbon cloth) exhibited enhanced activity and stability for HER. The Co₃O₄–Co₄N–HCC was prepared by the following steps: At first, hydrophilic carbon cloth was obtained by microwave hydrogen plasma treatment for 20 min (without heating and without using any toxic chemicals). Then, α-Co(OH)₂-HCC was obtained by pulse electrodeposition method. Finally, Co₃O₄–Co₄N–HCC was prepared by microwave nitrogen plasma treatment at 300 °C for 15 min. The Co₃O₄–Co₄N–HCC exhibited higher HER activity and lower charge-transfer resistance than that of Co₃O₄–C and Co₄N–C. The Co₃O₄–Co₄N–HCC is composed of crystalline Co₃O₄–Co₄N heterostructures on hydrophilic carbon cloth, where the Co₄N could be enriched with defects. The Co₃O₄ and Co₄N crystallites exhibit an epitaxial relationship of Co₃O₄ (331)//Co₄N (111) having a small-angle grain boundary of ~5.8°, which could enhance the charge-transfer process. In the XPS spectra, the binding energy of the peak attributed to Co−O for Co₃O₄–Co₄N–HCC is higher than that of Co₃O₄, while the binding energy of the peak attributed to Co−N for Co₃O₄–Co₄N–HCC is lower than that of Co₄N, suggesting the modified electronic structure, which could be due to the transfer of partial electrons from Co₃O₄ to Co₄N, and that could lead to synergistic effect, where the desorption of H_ad and the release of OH⁻ could be occurred during the HER (Figure 4a), and that could enhance the intrinsic activity for HER. It contains Co, N and O, which are uniformly distributed. It possesses porous nanosheet morphology. For HER in 1 M KOH, the Co₃O₄–Co₄N–HCC exhibited a η of −90 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 40 h (Figure 4b), suggesting its very high stability.

Mo-based electrocatalysts are considered as highly HER active catalysts [16,141]. Thus, fabrication of mesoporous carbon spheres with uniform distribution of Mo-based heterostructures through facile metal–organic coordination precursor-assisted synthesis could be desirable to achieve efficient water electrolysis, while it could modify the electronic structure, and provide porous nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for HER. Li et al. [89] observed that the Mo₂C–Mo₂N–C exhibited enhanced activity and stability for HER. The Mo₂C–Mo₂N–C was prepared through facile metal–organic coordination precursor-assisted synthesis by the following steps: It was obtained by carbonization of the as-prepared metal–organic precipitate at 850 °C for 2 h under Ar atmosphere followed by etching of the silica templates. The Mo₂C–Mo₂N–C exhibited higher HER activity than that of Mo₂C and MoN. The Mo₂C–Mo₂N–C possesses homogenous spherical morphology with a mesoporous structure having thin carbon layers on its surface. It possesses high surface area of 496 m² g⁻¹. It possesses abundant nanoparticles, which are uniformly distributed, where the nanoparticles could be Mo-based nanocrystallites. It is composed of crystalline Mo₂C and Mo₂N nanoparticles, which are uniformly distributed over carbon matrix. It contains Mo₂C/Mo₂N heterojunctions. It contains C, N, and Mo, where the Mo and N are uniformly distributed within the carbon layers. It contains 4.89 atomic percent of N. It could contain pyrrolic N, pyridinic N, graphitic N, N–Mo, and oxidized N. For HER in 1 M KOH, the Mo₂C–Mo₂N–C exhibited a η of −145 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay at >16 h, suggesting its high stability.

The fabrication of transition metal oxide/metal nanoparticles decorated heteroatom-doped graphene as the electrocatalyst for HER through a facile synthesis route could be desirable to achieve efficient water electrolysis. The Fe₂O₃–Co–N–graphene [93] exhibited enhanced activity (η of ~−409 mV at −10 mA cm⁻²) and stability (negligible decay for 11 h) for HER in 1 M NaOH, while it is composed of Fe₂O₃ and Co nanoparticles, which are decorated on N-doped graphene. The Fe₂O₃–Co–N–graphene was obtained by the loading of Co²⁺ and iron (II) phthalocyanine on the graphene oxide followed by one-step calcination process, where dopamine was used as the precursor. In addition, the formation of Fe₂O₃ and Co nanoparticles, doping of N, and reduction of graphene oxide were occurred in the one-step calcination process.

2.3. Principle 5: Harmless Solvents/Auxiliaries [33]

Fabrication of Oxide-/Sulfide-Based Earth-Abundant Electrocatalysts

Some of the DESs (DESs: Deep eutectic solvents) are proposed as environmentally compatible alternatives for synthesis, and some of the DESs are biodegradable and non-toxic, while DESs can be obtained by complexation of a quaternary ammonium salt with a hydrogen bond donor or metal salt, and it could possess low melting points, which could be due to the existence of huge, non-symmetric ions with low lattice energy in DESs [142]. Thus, fabrication of transition bimetallic oxide as the electrocatalyst for OER using DES could alleviate the usage of toxic solvents. The crystalline cobalt vanadate (γ–CoV₂O₆) [102] exhibited enhanced activity (η of 324 mV at 10 mA cm⁻²) and stability (~81% retention at 1.674 V for 24 h) for OER in 1 M KOH, while it is obtained using deep eutectic solvent (1:1 choline chloride–malonic acid).

The fabrication of bimetallic sulfide using DES could alleviate the usage of toxic solvents, while it could modify the electronic structure, and provide nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Jiang et al. [104] observed that the NiCo₂S₄ exhibited enhanced activity and stability for OER. The NiCo₂S₄ was obtained by one-pot hydrothermal treatment at 433.15 K for 16 h (Figure 5), where the PEGylated DES (comprising of polyethylene glycol 200 and thiourea) was used as solvent. The NiCo₂S₄ exhibited higher OER activity and lower charge-transfer resistance than that of NiS₂ and CoS₂. The NiCo₂S₄ is composed of cubic NiCo₂S₄ phase. It contains Co²⁺, Co³⁺, Ni²⁺, Ni³⁺, and S²⁻. It contains Ni, Co, and S, which are uniformly distributed. It possesses sea urchin-like nanostructures, which is composed of uniformly interconnected nanorods. It contains mesopores and micropores. It possesses high surface area of 33.1 m² g⁻¹. For OER in 1 M KOH, the NiCo₂S₄ exhibited a η of 337 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited negligible decay at 10 mA cm⁻² for ~30 h, suggesting its high stability, and it exhibited negligible decay at 50 mA cm⁻² after 2000 cycles of CV, suggesting its high durability.

2.4. Principle 6: Designing Energy Efficiency Synthesis Route [33]

Fabrication of OH-/OOH-/LDH-/B-/Sulfide-/Pyrophosphate-/Carbon-/Oxide-Based Earth-Abundant Electrocatalysts

Several efforts have been used to design energy-efficient synthesis routes (especially at room temperature synthesis) for the fabrication of earth-abundant electrocatalysts. Degradation of the materials can be called corrosion, and if the degradation rate is lower, then it suggests that the corrosion stability is higher. Nevertheless, the corrosion stability can be varied (beneficial in some cases) with respect to time interval based on the substrate and immersion solution, which could be due to the formation of desirable corrosion product [3]. The formation of iron-rust can be alleviated using corrosion engineering strategy, where, highly active and ultra-stable bimetallic LDH as the electrocatalyst for OER can be obtained by the desirable corrosion reaction of inexpensive iron substrate at ambient temperature in the presence of oxygen in aqueous environment having divalent cations. Moreover, it could modify the electronic structure, and provide nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Liu et al. [31] observed that the NiFe LDH exhibited substantially very high stability and activity for OER. The NiFe LDH was obtained by facile corrosion engineering strategy, where a cleaned Fe foam was immersed in transparent Ni²⁺ containing solution (pH: ~5.88) in a conical flask at room temperature for 12 h. The NiFe LDH on Fe plate (prepared by corrosion engineering strategy) exhibited higher OER activity and lower charge-transfer resistance than that of NiFe LDH on Ni plate (prepared by electrodeposition method). The NiFe LDH possesses the crystal structure of LDH with R-3m symmetry. It contains Fe, Ni and O, which are uniformly distributed. The atomic ratio of Fe:Ni in NiFe LDH is about 1.1:1. It contains Ni²⁺ and Fe³⁺. It possesses nanosheet array morphology (Thickness of the nanosheet: ~8 nm; Thickness of the catalyst: ~200 nm), where the nanosheet is predominantly composed of ultra-small, crystalline, nano-domains (<5 nm) along with minor quantity of amorphous nano-domains. For OER, the NiFe LDH on Fe foam exhibited negligible decay at huge current density of 1000 mA cm⁻² in 1 M KOH for 5000 h followed by negligible decay at huge current density of 1000 mA cm⁻² in 10 M KOH for 1050 h (

Table 7. Long-term stability (Part 1: 24 h to 6000 h of duration) of various kinds of reported earth-abundant electrocatalysts for oxygen evolution reaction (OER) in 1 M KOH alkaline electrolyte, while the electrolytes other than 1 M KOH are indicated on the electrocatalyst, where these electrocatalysts are prepared based on green-chemistry approaches.

Electrocatalysts for OER	Chr. Amp.	Chr. Pot.	Duration (h)	Remark after Stability Test	Ref.
NiFe LDH	NA	YES	6050	Negligible decay at 1000 mA cm−2 in 1 M KOH for 5000 h followed by negligible decay at 1000 mA cm−2 in 10 M KOH for 1050 h.	[31]
NixFe3−xO4–Ni	NA	YES	250	Negligible decay	[99]
Ni–P–B/paper	YES	NA	240	92.4% retention (delivered ~1000 mA cm−2) at η of 488 mV	[40]
N–NiCoP	YES	NA	100	Negligible decay	[72]
NiSe2–CoSe2	Yes	NA	100	95.2% retention (delivered ~100 mA cm−2)	[75]
N–NiCoPx	YES	NA	100	96.93% retention (delivered ~100 mA cm−2)	[73]
CuO	NA	YES	~70	Negligible decay at 10 mA cm−2	[42]
exfoliated NiFe LDH	NA	YES	>60	Negligible decay at 10 mA cm−2	[119]
C@NiMn	YES	NA	40	Reasonable stability at η of 300 mV	[66]
S–graphene foam0.1 M KOH	YES	NA	36	~87% retention	[94]
NiCoP–Cu	YES	NA	36	Reasonable stability at 1.46 V	[58]
Ultrathin Cu–Co(OH)2	YES	NA	36	Reasonable stability at 1.55 V	[96]
NiCo2S4	NA	YES	~30	Negligible decay at 10 mA cm−2	[104]
NiFe–MoS2	YES	NA	24	Negligible decay at 1.5 V	[107]
CoP–C	NA	YES	24	Reasonable stability at 10 mA cm−2	[43]
Ni0.71Fe0.29(OH)x0.1 M KOH	NA	YES	24	Negligible decay at 5 mA cm−2	[84]
CoV2O6	YES	NA	24	~81% retention at 1.674 V	[102]
Co(OH)20.1 M KOH	NA	YES	24	Negligible decay at 1 mA cm−2	[86]

Ref.: References; Chr. Amp.: Chronoamperometry; Chr. Pot.: Chronopotentiometry; NA: Not applicable; LDH: Layered double hydroxide.

), suggesting its robust stability. For OER at 10 mA cm⁻², the NiFe LDH [31] (prepared at room temperature) exhibited a η of 200 mV in 1 M KOH, where this OER activity is well comparable with the activity of electrocatalysts prepared above room temperature (NiFe LDH (η of 240 mV in 1 M NaOH) [5]; NiFe LDH nanoarray (η of 210 mV in 1 M NaOH) [143]; NiFe LDH (η of 261 mV in 1 M KOH) [144]).

The fabrication of NiFe LDH through facile one-pot synthesis at room temperature could be highly desirable to achieve efficient water electrolysis, while it could modify the electronic structure, and provide nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Yang et al. [55] observed that the NiFe LDH exhibited enhanced activity and stability for OER. The NiFe LDH was prepared by facile one-pot synthesis, where the precleaned Ni foam was immersed in a solution for 24 h at room temperature. The NiFe LDH exhibited higher OER activity and lower charge-transfer resistance than that of Ni(OH)₂. The NiFe LDH possesses the crystal structure of LDH. It contains Ni, Fe, and O, which are uniformly distributed. It contains Ni²⁺ and Fe³⁺. In the XPS spectra, the binding energy of the Ni 2p_3/2 and Ni 2p_1/2 peaks of NiFe LDH are higher than that of the Ni(OH)₂, while a positive peak shift is also observed from O 1s spectrum of NiFe LDH when compared with that of Ni(OH)₂, suggesting the modified electronic structure, which could be due to the transfer of partial electrons from Ni to Fe. It possesses nanoarray morphology. For OER in 1 M KOH, the NiFe LDH exhibited a η of ~180 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability at 1.48 V for 20 h, suggesting its high stability.

The fabrication of transition bimetallic LDH as the electrocatalyst for OER at room temperature using much lesser time through a facile synthesis route could be desirable to achieve efficient water electrolysis. The partially crystalline NiFe LDH nanosheets [56] exhibited enhanced activity (η of 133 mV at 10 mA cm⁻²) and stability (negligible decay at 1.60 V for 17 h) for OER in 1 M KOH. In the XPS spectra, the binding energies of Ni 2p_3/2 and Ni 2p_1/2 peaks of NiFe LDH are higher when compared with that of Ni(OH)₂, suggesting the modified electronic structure, which could be due to the transfer of electrons from the Ni to Fe. The NiFe LDH was obtained at room temperature by immersing of Ni foam in Fe(NO₃)₃ for 3 s followed by drying in air for 10 min followed by immersing in NaOH for 3 s followed by drying in air for 5 min.

The fabrication of anion-doped bimetallic LDH through annealing-free and organic solvent-free one-step electrodeposition process could be desirable to achieve efficient water electrolysis. The Fe²⁺–NiFe LDH–CO₃²⁻ [71] exhibited enhanced activity and stability for HER and OER in 1 M KOH, and it contains abundant defects, while, in the O 1s XPS spectra, the Fe²⁺–NiFe LDH–CO₃²⁻ exhibited higher proportion of oxygen vacancy when compared with that of Fe²⁺–NiFe LDH, suggesting the modified electronic structure. It was obtained through annealing-free and organic solvent-free one-step electrodeposition process (at 30 °C). For OER in 1 M KOH, the Fe²⁺–NiFe LDH–CO₃²⁻ exhibited a η of 128 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 12 h, suggesting its very high stability, and it exhibited negligible decay at 400 mA cm⁻² after 1000 cycles of CV, suggesting its very high durability. For HER in 1 M KOH, the Fe²⁺–NiFe LDH–CO₃²⁻ exhibited a η of −106 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 12 h, suggesting its very high stability. For overall water splitting in 1 M KOH, the Fe²⁺–NiFe LDH–CO₃²⁻//Fe²⁺–NiFe LDH–CO₃²⁻ exhibited a potential of 1.55 V at 10 mA cm⁻², suggesting its very high activity, while it exhibited negligible decay for 12 h, suggesting its very high stability.

The fabrication of nanostructured, ultrathin metal-doped metal hydroxide as the electrocatalyst for OER through a facile synthesis route at room temperature could be desirable to achieve efficient water electrolysis. The ultrathin Cu–Co(OH)₂ nanosheets [96] exhibited enhanced activity (η of 300 mV at 10 mA cm⁻²), and stability (reasonable stability at 1.55 V for 36 h) for OER in 1 M KOH, while it is composed of Cu-doped Co(OH)₂, and it is prepared using facile strategy at room temperature. In the XPS spectra, the binding energy of Co 2p_3/2 and Co 2p_1/2 peaks for Cu–Co(OH)₂ are higher than that of Co(OH)₂, suggesting the modified electronic structure, while the Cu–Co(OH)₂ exhibited higher OER activity than that of Co(OH)₂. Moreover, the monocrystalline CoZnOH nanosheets [117] exhibited enhanced activity (η of ~430 mV at 10 mA cm⁻²) for OER in 1 M KOH, while it was obtained by dropping of CoCl₂·6H₂O on Zn foil followed by drying.

The fabrication of ultrathin transition metal hydroxide with lattice distortion through a facile synthesis route could be desirable to achieve efficient water electrolysis, while it could modify the electronic structure, modify the atomic arrangement, alter the coordination environment, distort the crystal structure, and provide ultrathin nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Yang et al. [110] observed that the ultrathin Co(OH)₂ with lattice distortion exhibited enhanced activity and stability for OER. The ultrathin Co(OH)₂ with lattice distortion was prepared by selective removal of Ga from the ultrathin Co₉₆Ga₄ layered metal hydroxide using KOH solution, where the ultrathin Co₉₆Ga₄ layered metal hydroxide was obtained by co-precipitation method. The ultrathin Co(OH)₂ with lattice distortion possesses almost the crystal structure of hexagonal Co(OH)₂, while the crystal planes of Co(OH)₂ are slightly shifted left, which could be due to the lattice distortion in Co(OH)₂ due to the selective removal of Ga. In the XPS spectra, the binding energy of the Co 2p_3/2 peak is higher than that of the ultrathin Co₉₆Ga₄ layered metal hydroxide, suggesting the modified electronic structure. The Co K-edge EXAFS spectrum of Co(OH)₂ with lattice distortion is varied with the Co₉₆Ga₄ layered metal hydroxide (Figure 6a; EXAFS: Extended X-Ray Absorption Fine Structure), suggesting the modified electronic structure. A reduction in the oscillation amplitude is observed for the Co K-edge oscillation curve for Co(OH)₂ with lattice distortion when compared with that of Co₉₆Ga₄ layered metal hydroxide (Figure 6b), suggesting the modified atomic arrangement and coordination environment. The diffraction peaks of Co(OH)₂ with lattice distortion are shifted towards higher R value in the FT curves than that of Co₉₆Ga₄ layered metal hydroxide (Figure 6c), suggesting the modified atomic rearrangement due to the crystal lattice distortion. Thus, the estimated local structure parameters reveal that the Co−O_OH distance is 2.233 Å for Co(OH)₂ with lattice distortion, which has been higher than the distance of 2.112 Å in the Co₉₆Ga₄ layered metal hydroxide (Figure 6d). It possesses ultrathin nanosheet morphology. For OER in 1 M KOH, the ultrathin Co(OH)₂ with lattice distortion exhibited a η of 265 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited negligible decay for 20 h, suggesting its high stability, and it exhibited negligible decay at 10 mA cm⁻² after 2000 cycles of CV, suggesting its high durability.

The fabrication of transition metallic oxy hydroxide with oxygen vacancies through laser ablation in liquid strategy by irradiating the metallic target at room temperature in KOH solution could alleviate the usage of toxic solvents and chemicals. The CoOOH with oxygen vacancies [103] exhibited enhanced activity (η of 330 mV at 10 mA cm⁻²), and stability (95 % retention at η of 330 mV for 10 h) for OER in 1 M KOH. It is composed of rhombohedral phase of CoOOH with oxygen vacancies. It possesses nanosheet morphology. In the EPR spectra (EPR: Electron paramagnetic resonance), it exhibited significantly higher intensity (g = 2.003) than that of bulk CoOOH, suggesting the existence of abundant oxygen vacancies. In the XPS spectra, the intensity of Co²⁺ peak is higher for CoOOH with oxygen vacancies than that of bulk CoOOH, suggesting the modified electronic structure, while the CoOOH with oxygen vacancies exhibited higher OER activity and lower charge-transfer resistance than that of bulk CoOOH. The CoOOH with oxygen vacancies was obtained by laser ablation in liquid strategy, where the Co target was irradiated by nanosecond laser at room temperature in 1 M KOH for 20 min followed by washing and freeze-drying.

The fabrication of metal-boride based electrocatalyst for OER through a facile synthesis route at room temperature could be desirable to achieve efficient water electrolysis. The nanochain-like FeMoB [57] exhibited enhanced activity (η of 253 mV at 10 mA cm⁻²), and stability (reasonable stability at 1.49 V for 20 h) for OER in 1 M KOH, while it is precipitated by the addition of NaBH₄ solution to the aqueous mixture of metal salts (FeCl₃·6H₂O and Na₂MoO₄·2H₂O) under constant stirring at room temperature.

The fabrication of nanocomposites containing reduced graphene oxide, carbon black and cobalt borate through a facile synthesis route at room temperature could be desirable to achieve efficient water electrolysis, while it could modify the electronic structure, increase the surface area and provide porous structure, and that could afford abundant active sites, enhance the conductivity, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Sun et al. [105] observed that the rGO–C–CoB (rGO: Reduced graphene oxide) exhibited enhanced activity and stability for OER. The sandwich-like rGO–C–CoB nanocomposite was prepared through noncovalent interactions between rGO, carbon black, and cobalt ions using NaBH₄ at room temperature (Figure 7). The rGO–C–CoB contains C, B, Co, and O. It is sandwich-like rGO–C–CoB nanocomposite, which could be due to the vanderWaals and π–π interaction between the rGO sheets and carbon black particles. It contains graphitic carbon, while it could contain amorphous CoB. It possesses significantly very high surface area of 1052.3 m² g⁻¹. It exhibited very high overall pore volume of ≈1.78 cm³ g⁻¹, while it contains micropores and mesopores. For OER in 0.1 M KOH, the rGO–C–CoB exhibited a η of 340 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited negligible decay at 10 mA cm⁻² for >6 h, suggesting its high stability.

The fabrication of heterostructured alloy/MoS₂ as the electrocatalyst for OER through a facile synthesis route could be desirable to achieve efficient water electrolysis, where exfoliation of MoS₂, in situ reduction of metal precursors into alloy, and integration of alloy on exfoliated MoS₂ could be achieved by one–step facile synthesis route at room temperature. The NiFe–MoS₂ [107] exhibited enhanced activity (η of 260 mV at 10 mA cm⁻²), stability (negligible decay at 1.5 V for 24 h), and durability (negligible decay at 10 mA cm⁻² after 3000 cycles of CV) for OER in 1 M KOH. It is composed of amorphous, thin NiFe alloy nanosheets, which are assembled on the exfoliated MoS₂, and it possesses heterostructure. In the XPS spectra, the binding energies of Fe 2p_1/2 and Fe 2p_2/3 peaks are shifted for NiFe–MoS₂ when compared with that of NiFe alloy, suggesting the modified electronic structure, while the NiFe–MoS₂ exhibited higher OER activity than that of NiFe alloy. The NiFe–MoS₂ heterostructure was obtained by a one–step in situ reduction at room temperate using MoS₂, nickel acetate tetrahydrate, Iron(II) chloride tetrahydrate, NaBH₄, and deionized water, while the MoS₂ was prepared by hydrothermal treatment.

Amorphous bimetallic pyrophosphate as the electrocatalyst for OER can be prepared by co–precipitation method without using high temperature. The amorphous Fe–Ni pyrophosphate nanoparticles [98] exhibited enhanced activity (η of 210 mV at 10 mA cm⁻²), stability (negligible decay for 20 h;

Table 8. Long–term stability (Part 2: 1 h to 20 h of duration) of various kinds of reported earth–abundant electrocatalysts for oxygen evolution reaction (OER) in 1 M KOH alkaline electrolyte, while the electrolytes other than 1 M KOH are indicated on the electrocatalyst, where these electrocatalysts are prepared based on green–chemistry approaches.

Electrocatalysts for OER	Chr. Amp.	Chr. Pot.	Duration (h)	Remark after Stability Test	Ref.
NiFe LDH	YES	NA	20	Reasonable stability at 1.48 V	[55]
Fe–Ni pyrophosphate	YES	NA	20	Negligible decay	[98]
FeMoB	YES	NA	20	Reasonable stability at 1.49 V	[57]
Co(OH)2 with lattice distortion	YES	NA	20	Negligible decay	[110]
Ni@Ni2P–N–C nanofiber–CNT	NA	YES	20	98.3 % retention at 10 mA cm−2	[112]
S–CoP	YES	NA	20	Reasonable stability at η of ~350 mV	[50]
Mo2C–C	NA	YES	20	Negligible decay at 10 mA cm−2	[65]
CoP@PC	YES	NA	20	95.8 % retention at 1.51 V	[44]
S–Co2P	YES	NA	20	Negligible decay at η of 340 mV	[51]
CoSe@NCNT–NC	YES	NA	20	Reasonable stability	[97]
NiFeOx–NP–C	NA	YES	20	98.4% retention at 10 mA cm−2	[62]
BN–Mo2C@BCN	YES	NA	20	Reasonable stability	[76]
N–C0.1 M KOH	YES	NA	20	83.6 % retention at 1.6 V	[63]
NiFe LDH	YES	NA	17	Negligible decay at 1.60 V	[56]
FeP	YES	NA	15	Reasonable stability at 1.46 V	[74]
Co2P–CNT	YES	NA	>13	Reasonable stability	[52]
Fe–N–graphene0.1 M KOH	YES	NA	>13	Reasonable stability	[114]
Fe2+–NiFe LDH–CO32−	YES	NA	12	Negligible decay	[71]
exfoliated NiFe LDH–C	NA	YES	12	Reasonable stability at 10 mA cm−2	[101]
NiCo2S40.1 M KOH	YES	NA	~11	Reasonable stability at η of 400 mV	[115]
S–NiP	NA	YES	10	Negligible decay at 100 mA cm−2	[70]
Geobacter–rGO	YES	NA	10	Negligible decay at η of 270 mV	[113]
Co2P–Co@PN–C	YES	NA	10	Negligible decay at η of 311 mV	[100]
CoOOH with oxygen vacancies	YES	NA	10	95 % retention at η of 330 mV	[103]
Co2P–NP–CNT	YES	NA	10	86.9% retention at η of 370 mV	[109]
CoFe2O4–C nanofiber0.1 M KOH	YES	NA	>9	Negligible decay	[145]
Ni0.65Fe0.35P	NA	YES	>6	Reasonable stability at 20 mA cm−2	[48]
rGO–C–CoB0.1 M KOH	NA	YES	>6	Negligible decay at 10 mA cm−2	[105]
Fe–Ni2P@NPC0.1 M KOH	YES	NA	>3	Negligible decay at 1.6 V	[64]
CoFe2O4	NA	YES	>2	Reasonable stability	[106]
Co@C0.1 M KOH	YES	NA	1	Reasonable stability at 1.6 V	[108]

Ref.: References; Chr. Amp.: Chronoamperometry; Chr. Pot.: Chronopotentiometry; NA: Not applicable; LDH: Layered double hydroxide; CNT: Carbon nanotube; rGO: Reduced graphene oxide.

), and durability (negligible decay at 150 mA cm⁻² after 2000 cycles of CV;

Table 9. Durability of various kinds of reported earth–abundant electrocatalysts for oxygen evolution reaction (OER) in 1 M KOH alkaline electrolyte, while the electrolytes other than 1 M KOH are indicated on the electrocatalyst, where these electrocatalysts are prepared based on green–chemistry approaches.

Electrocatalysts for OER	Cycles of CV	Remark after Durability Test	Ref.
Ni0.71Fe0.29(OH)x0.1 M KOH	30,000	Negligible decay at 10 mA cm−2	[84]
N–NiCoP	10,000	Negligible decay at 50 mA cm−2	[72]
NiSe2–CoSe2	10,000	Negligible decay at 150 mA cm−2	[75]
N–NiCoPx	10,000	Negligible decay at 300 mA cm−2	[73]
Ni–P–B/paper	5000	Negligible decay at 100 mA cm−2	[40]
Ni(OH)20.1 M KOH	5000	Negligible decay at 30 mA cm−2	[85]
Fe–N–graphene0.1 M KOH	5000	Negligible decay at 10 mA cm−2	[114]
N–Fe–S–C0.1 M KOH	5000	Negligible decay at 5 mA cm−2	[79]
NiFe–MoS2	3000	Negligible decay at 10 mA cm−2	[107]
CoFe2O4–NC0.1 M KOH	2500	Negligible decay at 10 mA cm−2	[90]
Fe–Ni pyrophosphate	2000	Negligible decay at 150 mAcm−2	[98]
NiCoP–Cu	2000	Negligible decay at 100 mA cm−2	[58]
Co(OH)2 with lattice distortion	2000	Negligible decay at 10 mA cm−2	[110]
C@NiMn	2000	Negligible decay at 10 mA cm−2	[66]
Co2P–CNT	2000	Negligible decay at 10 mA cm−2	[52]
CoSe@NCNT–NC	2000	Negligible decay at 40 mA cm−2	[97]
CuO	2000	Negligible decay at 250 mA cm−2	[42]
NiCo2S4	2000	Negligible decay at 50 mA cm−2	[104]
Fe2+–NiFe LDH–CO32−	1000	Negligible decay at 400 mA cm−2	[71]
S–NiP	1000	Negligible decay at 100 mA cm−2	[70]
Ni@Ni2P–N–C nanofiber–CNT	1000	Negligible decay at 80 mA cm−2	[112]
CoP–C	1000	Negligible decay at 30 mA cm−2	[43]
Co2P–NP–CNT	1000	Negligible decay at 10 mA cm−2	[109]
NiCo2S40.1 M KOH	1000	8 mV decay at 10 mA cm−2	[115]
Fe–Fe3C–NC0.1 M KOH	1000	Slight decay at 10 mA cm−2	[134]

Ref.: References; CV: Cyclic voltammetry; LDH: Layered double hydroxide; CNT: Carbon nanotube.

) for OER in 1 M KOH, while it was obtained by co–precipitation method at ambient temperature.

Transition metal nanoparticles decorated graphite can be obtained by facile synthesis route by immersing graphite into metal salt solution, while it could modify the electronic structure, and that could enhance the conductivity and provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Pandey et al. [146] observed that the Fe–graphite exhibited enhanced activity for OER. The Fe–graphite was obtained by facile one step and one chemical synthesis route (Figure 8): It was prepared by immersing the graphite into a 0.5 M of ferrous sulfate solution followed by washing with ultrapure water followed by drying in ambient condition. The Fe–graphite is composed of Fe, which are decorated on graphite, while it exhibited higher OER activity than that of graphite in 0.5 M NaOH.

Pure carbon–nanotubes could be considered as less active catalyst for OER, whereas integration of pure carbon–nanotubes with electrochemically inert polymers could stabilize the intermediates for OER through hydrogen bonding, and that could afford optimal adsorption with OER intermediates and that could enhance the OER performance. Thus, fabrication of carbon–nanotubes wrapped with polymer as a metal–free electrocatalyst for OER through a facile synthesis route could be desirable to achieve efficient water electrolysis. The PEMAc@CNT (PEMAc: Poly(ethylene–alt–maleic acid); CNT: Carbon nanotube) [95] exhibited enhanced activity (η of 298 mV at 10 mA cm⁻²) for OER in 1 M KOH, while it is composed of carbon nanotubes, which are wrapped with Poly(ethylene–alt–maleic acid). The enhanced OER activity of PEMAc@CNT could be ascribed to a synergistic effect of topological Stone–Wales defects of carbon–nanotubes as active centers and the wrapped polymer layer as a co–catalyst to stabilize the intermediates for OER through hydrogen bonding, and that could afford optimal adsorption energies with OER intermediates, and that could enhance the performance for OER. The PEMAc@CNT membrane was prepared using facile synthesis route by pouring the mixture into a clean vessel followed by the evaporation of the solvent followed by mechanical peeling, while the mixture (homogenous dispersion) was obtained by mixing the PEMAc with purified MWCNT using solvent (acetone/water).

The fabrication of transition metallic oxide on metallic foam as the electrocatalyst for OER by ultrafast fabrication process through laser ablation without using any toxic products could be highly desirable to achieve efficient water electrolysis. The CuO [42] exhibited enhanced activity (η of 310 mV at 25 mA cm⁻²), stability (negligible decay at 10 mA cm⁻² for ~70 h), and durability (negligible decay at 250 mA cm⁻² after 2000 cycles of CV) for OER in 1 M KOH, while it is composed of crystalline Cu₂O and CuO. The CuO was prepared on Cu foam by ultrafast fabrication process through laser ablation without using any toxic products.

The fabrication of nickel ferrite/Ni hybrid through annealing–free one–step electrodeposition process could be desirable to achieve efficient water electrolysis, while it could modify the electronic structure, regulate the Fermi energy level and work function, and provide nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Li et al. [99] observed that the Ni_xFe_3–xO₄–Ni exhibited enhanced activity and stability for OER. The Ni_xFe_3–xO₄–Ni was prepared by annealing–free one–step electrodeposition process. It exhibited high electrochemically active surface area and low charge-transfer resistance. The Ni_xFe_3–xO₄–Ni possesses two distinctive layers (Figure 9a), where the bottom layer is a uniform and dense thin film (Thickness: ~100 nm), whereas the upper layer is ordered nanosheets (vertically aligned nanosheet array). It is crystalline, which contains NiFe₂O₄ and metallic Ni (Figure 9b–e). It contains Ni, Fe, and O (Figure 9f–i). For OER in 1 M KOH, the Ni_xFe_3–xO₄–Ni exhibited a η of 218 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited negligible decay for 250 h, suggesting its very high stability.

2.5. Principle 7: Use of Renewable Raw Materials or Feedstocks [33]

Fabrication of B–/Carbon–/Oxide–/Se–Based Earth–Abundant Electrocatalysts

Developing a facile synthesis route for the fabrication of highly active and conductive catalyst on cheap filter paper substrate could be highly desirable to achieve efficient water electrolysis. Thus, the fabrication of the transition metal based catalyst containing B and P on cheap filter paper substrate though facile synthesis route could modify the electronic structure, generate OER-active thin in situ surface layers during OER, and that could enhance the conductivity, provide optimal adsorption energy with intermediates, and that could enhance the performance for HER and OER. Hao et al. [40] observed that the Ni–P–B/paper exhibited substantially very high stability and activity for HER and OER. Ni–P–B/paper was obtained by pretreatment of cheap filter paper for 2 s followed by electroless plating for 1 h (Figure 10a). The size of the above electrode is 1 × 0.5 cm², while this technique is scalable and flexible, where the flexible Ni–P–B/paper electrode is prepared with large size (Diameter: 12 cm; Figure 10g). Moreover, this technique is adaptable, where the conductive Ni–P–B/paper electrode is prepared with different configuration (Figure 10h). The bare paper substrate possesses a network of interwined fibers (Diameter of a single fiber: ~17.4 µm. Figure 10b), and the activated paper is composed of numerous nanoparticles (Average particle size: ~20 nm), which are homogenously deposited on the paper and the single fiber (Figure 10c), while a layer of electrocatalyst (Ni–P–B/paper) is coated on the top of the activated paper substrate and the single fiber (Figure 10d,e). The Ni–P–B catalyst possesses a disordered lattice and amorphous phase (Figure 10f). The Ni–P–B/paper contains Ni, P and B, which are uniformly distributed. The atomic ratio of Ni:P:B in Ni–P–B/paper is 16.0:1.0:2.1. In the XPS spectra, the binding energy of the P 2p_3/2 peak for the Ni–P–B/paper is lower than that of Ni–P, while the binding energy of the B 1s peak for the Ni–P–B/paper is higher than that of Ni–B, suggesting the modified electronic structure. It exhibited the sheet resistance of about 0.42 Ω sq⁻¹, suggesting its high conductivity. It could exhibit the in situ formation of NiOOH layer as the active species on the surface of the Ni–P–B/paper during OER. The Ni–P–B/paper exhibited higher OER activity and lower charge-transfer resistance than that of Ni–P–B/Ni foil and paper, while it exhibited higher HER activity than that of Ni–P–B/Ni foil and paper. For OER in 1 M KOH, the Ni–P–B/paper exhibited a η of 263 mV at 50 mA cm⁻², suggesting its very high activity, while it exhibited 92.4% retention (delivered huge current density of ~1000 mA cm⁻²) at η of 488 mV for 240 h, suggesting its significantly very high stability, and it exhibited negligible decay at 100 mA cm⁻² after 5000 cycles of CV, suggesting its very high durability. For HER in 1 M KOH, the Ni–P–B/paper exhibited a η of −76 mV at −50 mA cm⁻², suggesting its significantly very high activity, while it exhibited 94.1% retention (delivered huge current density of ~ −1000 mA cm⁻²) at η of −345 mV for 240 h, suggesting its considerably very high stability, and it exhibited negligible decay at −450 mA cm⁻² after 5000 cycles of CV, suggesting its substantially very high durability. For overall water splitting in 1 M KOH, the Ni–P–B/paper//Ni–P–B/paper exhibited a potential of 1.661 V at 50 mA cm⁻² (Figure 10i), suggesting its very high activity, while it exhibited 91.4% retention (delivered ~100 mA cm⁻²; Figure 10j) for 240 h, and it exhibited 96.2% retention (delivered huge current density of ~1000 mA cm⁻²; Figure 10j) for 12 h, suggesting its significantly very high stability.

The fabrication of sulfur–doped graphene foam using rice flour (food material) as a precursor could modify the electronic structure, increase the surface area, increase the pore volume, increase the electrochemically active surface area, and decrease the charge-transfer resistance, and that could afford abundant active sites, enhance the conductivity, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Patra et al. [94] observed that the S–graphene foam exhibited enhanced activity and stability for OER. The S–graphene foam was obtained by annealing at 800 °C for 1 h under Ar atmosphere followed by microwave treatment followed by calcination at 300 °C for 1 h, where rice flour (food material) was used as a precursor. The S–graphene foam exhibited higher OER activity, higher specific surface area (499 m² g⁻¹), higher pore volume (0.522 cm³ g⁻¹), higher roughness factor (R_f: 0.690), higher electrochemically active surface area, and lower charge-transfer resistance than that of P–graphene foam, N–graphene foam, B–graphene foam, and graphene foam. The S–graphene foam is composed of sulfur–doped graphene foam. It possesses foam like morphology. It could contain C=C/C–C, C–O, C=O, and O–C=O. For OER in 0.1 M KOH, the S–graphene foam exhibited a η of 128 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited ~87% retention for 36 h, suggesting its high stability.

Biomass [147] is a biodegradable and non–toxic material [148], and it contains abundant carbon atoms along with some heteroatoms, while it can be used as a carbon source for fabricating electrocatalysts. The N–doped porous carbon nanosheets [63] exhibited enhanced activity and stability for OER, while the theoretical calculations suggest that the pyridinic nitrogen in N–C having moderate adsorption energy with intermediates could enhance the performance for OER. The N–C was obtained by pyrolysis of the powder of Euonymus japonicus leaves at 900 °C under Ar atmosphere for 2 h followed by nitric acid pickling followed by annealing at 500 °C under Ar atmosphere for 1 h. For OER in 0.1 M KOH, the N–C exhibited a η of ~345 mV at 5 mA cm⁻², suggesting its high activity, while it exhibited 83.6% retention at 1.6 V for 20 h, suggesting its high stability.

Reduction of graphene oxide and doping/decorating of heteroatoms can be achieved by an exo–electron–transferring nonpathogenic bacterium, while it could modify the electronic structure, and that could provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Kalathil et al. [113] observed that the Geobacter–rGO (rGO: reduced graphene oxide) exhibited enhanced activity and stability for OER. The Geobacter–rGO was obtained by the reduction of GO through an exo–electron–transferring nonpathogenic bacterium (Geobacter sulfurreducens). The Geobacter–rGO exhibited higher OER activity than that of rGO. The Geobacter–rGO is composed of Geobacter sulfurreducens cells, which are wrapped with rGO sheets. It contains S, P, Fe, Cu, and N, where these elements could be originated from bacterium (Geobacter sulfurreducens). The iron species in Geobacter–rGO could be predominantly in the form of small clusters or in an amorphous phase, while few FeO nanoparticles are in crystalline form. It contains primarily single– to few–layer morphology, where the interlayer spacing between the individual sheets is about 3.6 Å, while it possesses honeycomb–like carbon lattice. It exhibited I_D/I_G ratio of 1.18, which is higher than that of abiotic GO (0.93). The C 1s XPS spectrum of Geobacter/rGO is obviously varied from abiotic GO, while the atomic ratio of C/O in Geobacter/rGO is about 5.5, which is lesser than that of abiotic GO (2.78), suggesting the modified electronic structure. For OER in 1 M KOH, the Geobacter–rGO exhibited a η of 270 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited negligible decay at η of 270 mV for 10 h, suggesting its high stability.

Starch is a polymer, where this natural material can be extracted from several renewable plant resources, and corn starch can be extracted from corn, while it can be used as a carbon source for fabricating electrocatalysts. The porous Co@C [108] exhibited enhanced activity (η of 370 mV at 6.43 A g⁻¹), and stability (reasonable stability at 1.6 V for 1 h) for OER in 0.1 M KOH, while it is composed of Co nanoparticles, which are enwrapped in graphited carbon microspheres. The Co@C was obtained by mild heating followed by heat treatment at 900 °C for 3 h under N₂ atmosphere, where corn starch was used as a carbon source.

NaCl is a natural, cheap, and abundant material, while it can be used as a template for the fabrication of electrocatalysts. The Fe–N–graphene [114] exhibited enhanced activity (η of 393 mV at 10 mA cm⁻²), stability, (reasonable stability for >13 h), and durability (negligible decay at 10 mA cm⁻² after 5000 cycles of CV) for OER in 0.1 M KOH, while it is composed of Fe, N co-doped mesoporous graphene. The Fe–N–graphene was obtained by freeze–drying the homogeneous mixture containing polyacrylamide, FeCl₃·6H₂O and NaCl at 900 °C for 2 h under argon atmosphere followed by removing of NaCl template using 0.5 M H₂SO₄.

The fabrication of transition metallic phosphide embedded in carbon matrix as the electrocatalyst for OER using biomass agarose as a carbon source could alleviate the usage of toxic chemicals. The Fe–Ni₂P@NPC [64] exhibited enhanced activity (η of 390 mV at 10 mA cm⁻²), and stability (negligible decay at 1.6 V for >3 h) for OER in 0.1 M KOH, while it is composed of Fe–doped Ni₂P nanoparticles, which are incorporated in N, P–codoped porous carbon nanosheets. The Fe–Ni₂P@NPC was obtained by freeze–drying the as–prepared hydrogel followed by calcination at 750 °C for 6 h under N₂ atmosphere, where biomass agarose was used as the carbon source.

Plant cellulose is a highly abundant and sustainable biopolymer on earth, while cellulose nanofibrils with distinctive three–dimensional networks can be prepared by disintegration of plant cellulose derived from wood and bamboo. Thus, electrocatalyst can be prepared by using cellulose nanofibrils as a precursor, where the conductive cellulose nanofibrils substrate can afford 3D porous structural skeleton, while it could modify the electronic structure, and provide porous 3D nanostructure, and that could afford abundant active sites, enhance the conductivity, facilitate the gas-evolution behavior, provide optimal adsorption energy with intermediates, and that could enhance the performance for OER. Tao et al. [112] observed that the Ni@Ni₂P–N–C nanofiber–CNT (CNT: Carbon nanotube) exhibited enhanced activity and stability for OER. The Ni@Ni₂P–N–C nanofiber–CNT was prepared by using cellulose nanofibrils as 3D porous substrate (Figure 11), where the cellulose nanofibrils were obtained from natural plant fiber (bleached eucalyptus pulp powder). The Ni@Ni₂P–N–C nanofiber–CNT exhibited higher OER activity and electrochemically active surface area than that of Ni@Ni₂P. The Ni@Ni₂P–N–C nanofiber–CNT is composed of Ni@Ni₂P nanoparticles, which are encapsulated in nitrogen–doped interconnected carbonized nanofiber and carbon nanotubes. It is crystalline, which contains graphite (carbon), metallic Ni, and Ni₂P. It possesses intertwined network structure. It contains C, N, P, and Ni. It could contain graphitic–N, pyridinic–N, pyrrolic–N, and oxidized–N. It exhibited high surface area of 230.7 m² g⁻¹. It exhibited a large pore volume of 0.216 cm³ g⁻¹. It contains micropores and mesopores. For OER in 1 M KOH, the Ni@Ni₂P–N–C nanofiber–CNT exhibited a η of 269 mV at 10 mA cm⁻², suggesting its very high activity, while it exhibited 98.3% retention at 10 mA cm⁻² for 20 h (It retains its morphology and phase after stability test), suggesting its very high stability, and it exhibited negligible decay at 80 mA cm⁻² after 1000 cycles of CV, suggesting its very high durability.

The fabrication of transition bimetallic oxide as the electrocatalyst for HER using bacterial nanocellullose as the electrode matrix could diminish the usage of toxic chemicals. The NiMoO₄–Bacterial nanocellullose [59] exhibited enhanced activity (η of −109 mV at −10 mA cm⁻²), and stability (negligible decay for 48 h) for HER in 1 M KOH. The NiMoO₄–Bacterial nanocellullose was obtained by the following steps using bacterial nanocellullose as the electrode matrix: At first, bacterial nanocellullose was derived from Komagataeibacter sucrofermentans. Then, bacterial nanocellullose was converted into conductive substrate by electroless deposition. Finally, NiMoO₄ was deposited on conductive bacterial nanocellullose through electrodeposition.

The fabrication of transition bimetallic oxide integrated with carbon support as the electrocatalyst for OER using bacterial cellulose pellicles as carbon nanofiber source could diminish the usage of toxic chemicals. The CoFe₂O₄–C nanofiber [145] exhibited enhanced activity and stability (negligible decay for >9 h) for OER in 0.1 M KOH. The CoFe₂O₄–C nanofiber was obtained by using bacterial cellulose pellicles as carbon nanofiber source.

The fabrication of transition bimetallic oxide integrated with carbon support as the electrocatalyst using egg white protein (albumin) as carbon source could diminish the usage of toxic chemicals. The CoFe₂O₄–NC [90] exhibited enhanced activity (η of ~380 mV at 10 mA cm⁻²) and durability (negligible decay at 10 mA cm⁻² after 2500 cycles of CV) for OER in 0.1 M KOH, and it exhibited enhanced activity (η of −164 mV at −10 mA cm⁻²) and durability (negligible decay at −10 mA cm⁻² after 5000 cycles of CV) for HER in 0.1 M KOH, while it is composed of cobalt ferrite, which are embedded in N–doped mesoporous carbon. The CoFe₂O₄–NC was obtained by pyrolysis at 900 °C under He atmosphere, where egg white protein was used as carbon precursor.

Corn is a cereal grain, and several million tons of corn is produced around the world for every year. Therefore, cornstalks are abundant biomass. Thus, fabrication of transition metallic carbide embedded in carbon electrocatalyst for OER using cornstalk (a plant material) as the carbon source could be highly desirable to achieve efficient water electrolysis. The Mo₂C–C [65] exhibited enhanced activity and stability for HER and OER in 1 M KOH, while it is composed of hexagonal β–phase molybdenum carbide nanoparticles, which are embedded in carbon nanosheets, and it contains mesopores. The Mo₂C–C was prepared by two–step impregnation–calcination process, where cornstalk (a plant material) was used as the carbon source. For OER in 1 M KOH, the Mo₂C–C exhibited a η of 274 mV at 10 mA cm⁻², suggesting its high activity, while it exhibited negligible decay at 10 mA cm⁻² for 20 h, suggesting its high stability. For HER in 1 M KOH, the Mo₂C–C exhibited a η of −130 mV at −10 mA cm⁻², suggesting its very high activity, while it exhibited reasonable stability at −10 mA cm⁻² for 20 h, suggesting its high stability. For overall water splitting in 1 M KOH, the Mo₂C–C//Mo₂C–C exhibited a potential of 1.65 V at 10 mA cm⁻², suggesting its high activity, while it exhibited negligible decay at 10 mA cm⁻² for 30 h, suggesting its very high stability.

Gelatin is a food ingredient [149] and biodegradable [150,151,152], and it can be obtained by partial hydrolysis of collagen [153,154], while the collagen is the highly abundant protein in mammals. Moreover, several thousand tons of gelatin is produced around the world for every year. Thus, fabrication of nanostructured transition metallic oxide using gelatin in the synthesis process could be desirable to achieve efficient water electrolysis. The NiO–MWCNT [116] exhibited enhanced activity (η of 409 mV at 10 mA cm⁻²) for OER in 0.5 M KOH, where NiO was obtained from nickel nitrate while gelatin was used in the preparation process.

Glucose (C₆H₁₂O₆: aldohexose) is a biodegradable highly abundant monosaccharide, and it is highly used by most living organisms, while glucan (polysaccharides) are composed of solely of glucose, and the glucan could enhance the cell delivery systems in the central nervous system [155]. Thus, fabrication of carbon integrated bimetallic nanostructure as the electrocatalyst for OER using glucose as carbon source could be desirable to achieve efficient water electrolysis. The C@NiMn [66] exhibited enhanced activity (η of 270 mV at 10 mA cm⁻²), stability (reasonable stability at η of 300 mV for 40 h), and durability (negligible decay at 10 mA cm⁻² after 2000 cycles of CV) for OER in 1 M KOH, while it possesses sandwich–like structure, which is composed of Ni–Mn nanoparticles, which are enwrapped by graphitized carbon sheets, and it is prepared using glucose as carbon source.

Biomass is a biodegradable and non–toxic material, while transition metallic oxide as the electrocatalyst for OER can be fabricated from the aqueous extract of the biomass. The crystalline, porous Fe₃O₄ nanoparticles [156] exhibited enhanced activity for OER in 0.1 M KOH. The Fe₃O₄ was obtained by using Pandanus odoratissimus leaves as the precursor.

Agar-agar is a biopolymer, which can be extracted from certain red seaweed (Rhodophyta or red algae), while it is biodegradable and food ingredient [157,158]. Thus, fabrication of transition bimetallic oxide as the electrocatalyst for OER using agar-agar as a polymerizing agent could be desirable to achieve efficient water electrolysis. The crystalline cobalt ferrite (CoFe₂O₄) [106] exhibited enhanced activity (η of 360 mV at 10 mA cm⁻²) and stability (reasonable stability for >2 h) for OER in 1 M KOH, while it is obtained by the proteic sol–gel route, where agar-agar (biodegradable material) from red seaweed (Rhodophyta) is used as a polymerizing agent.

Besides, TiO₂–SiO₂ [159] exhibited enhanced activity for HER and OER in 1 M KOH, while it was obtained by spurting of silica over the titanium plate, where sand was used as silica source.

The fabrication of carbon integrated metallic selenide as the electrocatalyst for OER using biodegradable glucose as one of a precursor could be desirable to achieve efficient water electrolysis. However, melamine should be cautiously handled in the synthesis process because oxidative stress and the risk of early damage to kidneys in humans can be increased due to the low–dose exposure of melamine [131]. The CoSe@NCNT–NC (CNT: Carbon nanotube) [97] exhibited enhanced activity (η of 301 mV at 10 mA cm⁻²), stability (reasonable stability for 20 h), and durability (negligible decay at 40 mA cm⁻² after 2000 cycles of CV) for OER in 1 M KOH, while it is composed of crystalline CoSe nanoparticles, which are encapsulated in N–doped carbon nanotubes grafted onto N–doped carbon nanosheets, and it contains mesopores. The CoSe@NCNT–NC was obtained by a facile one–pot carbonization–selenylation process, where glucose was used as the precursor, while melamine was also used as a precursor.

Using green chemistry Principle 1 (Prevent waste [33]), various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER (

Table 10. Using green chemistry Principle 1 (Prevent waste [33]), various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER.

Electrocatalysts	Type of Catalysts		Green Chemistry Principle 1 (Prevent Waste [33])	Ref.
	HER	OER
NiCoP–Cu	Yes	Yes	The fabrication of bimetallic phosphide on electronic waste-derived pretreated Cu substrate	[58]
N–C	No	Yes	The fabrication of N–doped porous carbon nanomesh using waste grape skins (biomass waste) as the C source	[78]
N–Fe–S–C	No	Yes	The fabrication of N–Fe–S–C using sewage sludge as thesource	[79]
NiO–C	Yes	No	The fabrication of transition metallic oxide embedded in carbon matrix using waste egg shell membrane as the carbon source	[61]
NiFeOx–NP–C	No	Yes	The fabrication of transition bimetallic oxide integrated with heteroatom–doped carbon using expired milk powder as the heteroatom–doped carbon source to diminish the usage of toxic chemicals	[62]
Fe3O4	No	Yes	The fabrication of transition metallic oxide using orange peel (waste resource) as the template and reductive agent, and using H2O as the solvent in the synthesis	[83]

Ref.: References.

). Using green chemistry Principle 3 (Chemical synthesis with little or no hazardous substances), and Principle 4 (Designing harmless chemicals) [33], various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER (

Table 11. Using green chemistry Principle 3 (Chemical synthesis with little or no hazardous substances), and Principle 4 (Designing harmless chemicals) [33] (part 1), various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER.

Electrocatalysts	Type of Catalysts		Green Chemistry Principle 3 (Chemical Synthesis with Little or No Hazardous Substances), and Principle 4 (Designing Harmless Chemicals) [33]	Ref.
	HER	OER
Ni0.71Fe0.29(OH)x	No	Yes	It was obtained through a facile electrochemical process using only water.	[84]
Ni(OH)2	No	Yes	It was obtained through a facile electrochemical process using only water.	[85]
Co(OH)2	No	Yes	It was obtained through a facile electrochemical process using only water.	[86]
N–NiCoPx	Yes	Yes	It was obtained from LDH, while the LDH was prepared using only water.	[73]
NiSe2–CoSe2	Yes	Yes	It was obtained from LDH, while the LDH was prepared using only water.	[75]
exfoliated NiFe LDH–C	No	Yes	Exfoliation of LDH through ultrasonic process using only water.	[101]
exfoliated NiFe LDH	No	Yes	Ostwald ripening driven in situ exfoliation.	[119]
NiCo2S4	No	Yes	Synthesis without using surfactant and template at low–temperature.	[115]
Co–TiO2	No	Yes	It was obtained by sol–gel method, where gelatin (biodegradable material) was used in the preparation process.	[118]
Co2P	Yes	Yes	It was prepared using cobalt (II) complex (trioctylphosphine ligand) through microwave–assisted method to alleviate toxic PH3 gas emission.	[41]
FeP nanoparticles	Yes	Yes	It was obtained from molecular metal phosphide precursor using relatively low-temperature.	[74]
NiCoP nanoparticles	Yes	Yes	It was obtained using triphenylphosphine as a phosphorus source at relatively low-temperature to alleviate toxic PH3 gas emission	[92]
S–NiP	Yes	Yes	It was obtained through electrochemical deposition method to alleviate toxic PH3 gas emission.	[70]
S–Co2P	Yes	Yes	It was obtained through a thiourea–phosphate-assisted method to alleviate toxic PH3 gas formation.	[51]
S–CoP	Yes	Yes	It was obtained through a thiourea–phosphate-assisted method to alleviate toxic PH3 gas formation.	[50]
CoP@PC	Yes	Yes	It was obtained by pyrolysis of the transition metal phosphonate complex to alleviate toxic PH3 gas emission.	[44]
Ni0.65Fe0.35P	No	Yes	It was obtained using phytic acid as a green organophosphorus source to alleviate toxic PH3 gas emission.	[48]
FeP@NPC	Yes	No	It was obtained using phytic acid as one of the nontoxic and environmentally friendly precursor.	[49]
Co2P–CNT	Yes	Yes	It was prepared using triphenylphosphine as a phosphorus source through solid-state pyrolysis process to alleviate toxic PH3 gas emission and eliminate toxic solvent usage.	[52]
NPSC–Co2Fe1	No	Yes	It was prepared using triphenylphosphine sulfide as a P and S source through solid-state pyrolysis process to alleviate toxic PH3 gas emission and eliminate toxic solvent usage	[111]
Co2P–Co@PN–C	No	Yes	It was obtained by selective etching of MOF using H3PO4 followed by carbonization under inert atmosphere, where the H3PO4 could act as an etching agent for MOF, and also act as a phosphorus source, and that could obviate the usage of toxic phosphine gas forming chemicals.	[100]

Ref.: References; LDH: Layered double hydroxide; CNT: Carbon nanotube; MOF: Metal–organic framework.

and

Table 12. Using green chemistry Principle 3 (Chemical synthesis with little or no hazardous substances), and Principle 4 (Designing harmless chemicals) [33] (part 2), various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER.

Electrocatalysts	Type of Catalysts		Green Chemistry Principle 3 (Chemical Synthesis with Little or No Hazardous Substances), and Principle 4 (Designing Harmless Chemicals) [33]	Ref.
	HER	OER
NS–C	No	Yes	It was obtained using polydopamine as a biopolymer-mediated green synthesis.	[120]
Mo2C–MoP–NC	Yes	No	It was obtained using polydopamine as a biopolymer-mediated green synthesis.	[88]
CoxFe1–x@N–graphene	Yes	No	It was prepared through a facile synthesis route using Prussian blue.	[91]
Fe–Fe3C–NC	No	Yes	It was obtained by pyrolysis of Prussian blue without using any inert gas flow.	[134]
BN–Mo2C@BCN	Yes	Yes	It was obtained using imidazole as a source of C and N.	[76]
Co3O4–Co4N–HCC	Yes	No	Hydrophilic carbon cloth (HCC) was obtained by microwave hydrogen plasma treatment for 20 min without heating and without using any toxic chemicals.	[87]
Mo2C–Mo2N–C	Yes	No	It was obtained through facile metal–organic coordination precursor–assisted synthesis.	[89]
Fe2O3–Co–N–graphene	Yes	No	It was obtained through a facile synthesis route, where dopamine was used as a precursor.	[93]

Ref.: References; HCC: Hydrophilic carbon cloth.

). Using green chemistry Principle 6 (Designing energy efficiency synthesis route [33]), various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER (

Table 13. Using green chemistry Principle 6 (Designing energy efficiency synthesis route [33]), various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER.

Electrocatalysts	Type of Catalysts		Green Chemistry Principle 6 (Designing Energy Efficiency Synthesis Route [33])	Ref.
	HER	OER
NiFe LDH	No	Yes	It was prepared on Fe foam through a facile corrosion engineering strategy at room temperature.	[31]
NiFe LDH	No	Yes	It was obtained through facile one–pot synthesis at room temperature.	[55]
partially crystalline NiFe LDH nanosheets	No	Yes	It was obtained rapidly through a facile synthesis route at room temperature.	[56]
Fe2+–NiFe LDH–CO32−	Yes	Yes	It was obtained through an annealing-free and organic solvent-free one-step electrodeposition process.	[71]
Ultrathin Cu–Co(OH)2	No	Yes	It was obtained through a facile synthesis route at room temperature.	[96]
CoZnOH	No	Yes	It was obtained through a facile synthesis route.	[117]
Co(OH)2 with lattice distortion	No	Yes	It was obtained through a facile synthesis route.	[110]
CoOOH with oxygen vacancies	No	Yes	It was obtained through laser ablation in liquid strategy.	[103]
FeMoB	No	Yes	It was obtained through a facile synthesis route at room temperature.	[57]
rGO–C–CoB	No	Yes	It was obtained through a facile synthesis route at room temperature.	[105]
NiFe–MoS2	No	Yes	It was obtained through a facile synthesis route.	[107]
Fe–Ni pyrophosphate	No	Yes	It was obtained by co–precipitation method at ambient temperature.	[98]
Fe–graphite	No	Yes	It was obtained through a facile synthesis route by immersing graphite into metal salt solution.	[146]
PEMAc@CNT	No	Yes	It was obtained through a facile synthesis route.	[95]
CuO	No	Yes	It was obtained through laser ablation.	[42]
NixFe3–xO4–Ni	No	Yes	It was obtained through an annealing-free one-step electrodeposition process.	[99]

Ref.: References; LDH: Layered double hydroxide; rGO: Reduced graphene oxide; PEMAc: Poly(ethylene–alt–maleic acid); CNT: Carbon nanotube.

). Using green chemistry Principle 7 (Use of renewable raw materials or feedstocks [33]), various efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER (

Table 14. Using green chemistry Principle 7 (Use of renewable raw materials or feedstocks [33]), various efforts are performed for the fabrication of earth-abundant electrocatalysts for HER and OER.

Electrocatalysts	Type of Catalysts		Green Chemistry Principle 7 (Use of Renewable Raw Materials or Feedstocks [33])	Ref.
	HER	OER
Ni–P–B/paper	Yes	Yes	It was obtained on cheap filter paper substrate by a facile synthesis route.	[40]
S–graphene foam	No	Yes	It was obtained using rice flour (food material) as theprecursor.	[94]
N–C	No	Yes	It was obtained using powder of Euonymus japonicus leaves as the carbon source.	[63]
Geobacter–rGO	No	Yes	It was obtained by the reduction of GO through an exo-electron-transferring nonpathogenic bacterium.	[113]
Co@C	No	Yes	It was obtained using corn starch as a carbon source.	[108]
Fe–N–graphene	No	Yes	It was obtained using NaCl (a natural, cheap, and abundant material) as the template.	[114]
Fe–Ni2P@NPC	No	Yes	It was obtained using biomass agarose as the carbon source.	[64]
Ni@Ni2P–N–C nanofiber–CNT	No	Yes	It was obtained using cellulose nanofibrils as a precursor, where the cellulose nanofibrils was derived from bleached eucalyptus pulp powder.	[112]
NiMoO4–Bacterial nanocellullose	Yes	No	It was obtained using bacterial nanocellullose as the electrode matrix.	[59]
CoFe2O4–C nanofiber	No	Yes	It was obtained using bacterial cellulose pellicles as the carbon nanofiber source.	[145]
CoFe2O4–NC	Yes	Yes	It was obtained using egg white protein (albumin) as the carbon source.	[90]
Mo2C–C	Yes	Yes	It was obtained using cornstalk (a plant material) as the carbon source.	[65]
NiO–MWCNT	No	Yes	It was obtained using gelatin in the synthesis process.	[116]
C@NiMn	No	Yes	It was obtained using glucose as the carbon source.	[66]
Fe3O4 nanoparticles	No	Yes	It was obtained using Pandanus odoratissimus leaves as theprecursor.	[156]
CoFe2O4	No	Yes	It was obtained using agar-agar (biodegradable material) from red seaweed (Rhodophyta) as the polymerizing agent.	[106]
TiO2–SiO2	Yes	Yes	It was obtained by spurting of silica over the titanium plate, where sand was used as the silica source.	[159]
CoSe@NCNT–NC	No	Yes	It was obtained using biodegradable glucose as one of the precursors.	[97]

Ref.: References; rGO: reduced graphene oxide; MWCNT: Multi–walled carbon nanotube; CNT: Carbon nanotube.

).

3. Summary and Outlook

The fabrication of earth-abundant electrocatalysts by green–chemistry approaches for electrochemical water splitting could reduce or eliminate the use or production of hazardous substances, which could be highly desirable to attain efficient, green alkaline water electrolysis for clean energy production (Hydrogen). This paper starts by introducing the importance of the green–chemistry approaches for the fabrication of high–performance earth-abundant electrocatalysts for electrochemical water splitting to diminish or eliminate the use or generation of hazardous substances in the design, manufacture, and application of electrocatalysts. Later, the fabrication of earth-abundant electrocatalysts for electrochemical water splitting (HER and OER) using green–chemistry approaches has been reviewed, where their catalytic performances in alkaline environment have also been discussed. Moreover, the green–chemistry approaches for the fabrication of earth-abundant electrocatalysts including phosphide/pyrophosphate, carbon, oxide, OH/OOH/LDH, alloy/B/nitride, and sulfide/selenide (chalcogenide) based earth–abundant electrocatalysts were discussed in this review. In addition, the performance of the earth-abundant electrocatalysts for HER (including activity: η of ≤−55 mV at −10 mA cm⁻²; stability: ≥100 h; durability: ≥5000 cycles), OER (including activity: η of ≤133 mV at 10 mA cm⁻²; stability: ≥250 h; durability: ≥10,000 cycles), and overall water splitting (including activity: ≤1.51 V at 10 mA cm⁻²; stability: ≥240 h) in alkaline environment have been discussed in this review. Moreover, the various green–chemistry approaches including approaches used to alleviate toxic PH₃ gas emission during the fabrication of transition metal phosphide based electrocatalysts, efforts used to design energy–efficient synthesis route (especially room temperature synthesis), efforts used to utilize cheap or biodegradable substrate, and efforts used to utilize biomass–waste or biomass or biodegradable material as carbon source for the fabrication of earth–abundant electrocatalysts have been discussed in this review.

The following various green–chemistry approaches have been used for the fabrication of high–performance earth–abundant electrocatalysts for electrochemical water splitting in alkaline environment.

(a) Using green chemistry Principle 1 (Prevent waste [33]), the following efforts are performed for the fabrication of earth–abundant electrocatalysts for HER and OER: fabrication of bimetallic phosphide on electronic waste-derived pretreated Cu substrate [58], fabrication of N–doped porous carbon nanomesh using waste grape skins (biomass waste) as the C source [78], fabrication of N–Fe–S–C using sewage sludge as a source [79], fabrication of transition metallic oxide embedded in carbon matrix using waste egg shell membrane as carbon source [61], fabrication of transition bimetallic oxide integrated with heteroatom–doped carbon using expired milk powder as heteroatom–doped carbon source to diminish the usage of toxic chemicals [62], and fabrication of transition metallic oxide using orange peel (waste resource) as template and reductive agent, and using H₂O as solvent in the synthesis [83].

(b) Using green chemistry Principle 3 (Chemical synthesis with little or no hazardous substances), and Principle 4 (Designing harmless chemicals) [33], the following efforts are performed for the fabrication of earth-abundant electrocatalysts for HER and OER: fabrication of amorphous metallic hydroxide on an electrode through simple and facile electrochemical process using immersion of a target material in only water could alleviate the usage of any chemical additives [84,85,86], fabrication of transition bimetallic LDH on transition bimetallic foam by hydrothermal treatment using only deionized water to obviate the use of any other chemicals followed by fabrication of heteroatom-doped bimetallic phosphide by plasma-assisted phosphorization of the above bimetallic LDH [73], fabrication of transition bimetallic LDH on transition bimetallic foam by hydrothermal treatment using only deionized water to obviate the use of any other chemicals followed by fabrication of bimetallic selenides by selenization of the above bimetallic LDH [75], exfoliation of bimetallic LDH through ultrasonic process using only water to alleviate the usage of toxic solvents [101], fabrication of NiFe LDH by hydrothermal treatment followed by fabrication of exfoliated NiFe LDH by Ostwald ripening driven in situ exfoliation using a subsequent hydrothermal treatment (autoclave was unopened) without adding any other reagent or surfactant [119], fabrication of hollow–structured, transition bimetallic sulfide through a facile synthesis route without using surfactant and template at relatively low–temperature [115], fabrication of Co-doped anatase TiO₂ nanoparticles by sol–gel method, where gelatin (biodegradable material) is used in the preparation process [118], fabrication of cobalt phosphide using cobalt (II) complex (trioctylphosphine ligand) through microwave-assisted method to alleviate toxic PH₃ gas emission [41], fabrication of transition metal phosphide as the electrocatalyst from molecular metal phosphide precursor using relatively low–temperature [74], fabrication of transition bimetallic phosphide as the electrocatalyst using triphenylphosphine as a phosphorus source at relatively low–temperature to alleviate toxic PH₃ gas emission [92], fabrication of heteroatom-doped metal phosphide as the electrocatalyst through electrochemical deposition method to alleviate toxic PH₃ gas emission [70], fabrication of heteroatom-doped metal phosphide through a thiourea-phosphate-assisted method to alleviate toxic and lethal PH₃ gas formation [50,51], fabrication of metal phosphide as the electrocatalyst using NaH₂PO₂ as P source, where toxic PH₃ gas produced during phosphorization process was passed into the cupric sulfate solution to alleviate its emission in environment [43], fabrication of heteroatom-doped bimetallic phosphide as the electrocatalyst using plasma-assisted low-temperature phosphorization of transition bimetallic hydroxide to alleviate the usage of heavy metal ions [72], fabrication of transition metal phosphide confined in carbon matrix by pyrolysis of the transition metal phosphonate complex to alleviate toxic PH₃ gas emission [44], fabrication of transition bimetallic phosphide as the electrocatalyst using phytic acid as a green organophosphorus source to alleviate toxic PH₃ gas emission [48], fabrication of FeP@NPC using phytic acid as one of the nontoxic and environmentally friendly precursor [49], fabrication of transition metallic phosphide using triphenylphosphine as a phosphorus source through solid-state pyrolysis process to alleviate toxic PH₃ gas emission and eliminate toxic solvent usage [52], fabrication of transition metallic phosphide and alloy encapsulated in heteroatom-doped nanoporous carbon using triphenylphosphine sulfide as a P and S source through solid-state pyrolysis process to alleviate toxic PH₃ gas emission and eliminate toxic solvent usage [111], fabrication of metallic phosphide embedded in P–doped carbon by selective–etching of MOF using H₃PO₄ (as an etching agent for MOF, and as a phosphorus source) followed by carbonization under inert atmosphere to obviate the usage of toxic phosphine gas forming chemicals [100], fabrication of N, S codoped carbon using polydopamine as a biopolymer-mediated green synthesis [120], fabrication of Mo₂C–MoP–NC using polydopamine as a biopolymer-mediated green synthesis [88], fabrication of nanoalloys encapsulated heteroatom–doped graphene through a facile synthesis route using Prussian blue [91], fabrication of transition metal carbide encapsulated heteroatom-doped graphitic nanostructures by pyrolysis of Prussian blue without using any inert gas flow [134], fabrication of B, N co-doped metallic carbide nanoparticles embedded in a B, N co-doped carbon using imidazole as a source of C and N [76], fabrication of Co₃O₄–Co₄N–HCC (HCC: Hydrophilic carbon cloth) by microwave hydrogen plasma treatment (for HCC) for 20 min (without heating and without using any toxic chemicals) followed by pulse electrodeposition method (for α–Co(OH)₂–HCC) followed by microwave nitrogen plasma treatment [87], fabrication of mesoporous carbon spheres with uniform distribution of Mo–based heterostructures through facile metal–organic coordination precursor–assisted synthesis [89], and fabrication of transition metal oxide/metal nanoparticles decorated heteroatom–doped graphene through a facile synthesis route, where dopamine was used as a precursor [93].

(c) Using green chemistry Principle 5 (Harmless solvents/auxiliaries [33]), the following efforts are performed for the fabrication of earth–abundant electrocatalysts for OER: fabrication of transition bimetallic oxide [102] and fabrication of bimetallic sulfide [104] using deep eutectic solvents to alleviate the usage of toxic solvents.

(d) Using green chemistry Principle 6 (Designing energy efficiency synthesis route [33]), the following efforts are performed for the fabrication of earth-abundant electrocatalysts for HER and OER: fabrication of NiFe LDH on Fe foam through a facile corrosion engineering strategy at room temperature [31], fabrication of NiFe LDH through facile one–pot synthesis at room temperature [55], rapid fabrication of transition bimetallic LDH through a facile synthesis route at room temperature [56], fabrication of anion–doped bimetallic LDH through annealing-free and organic solvent-free one-step electrodeposition process [71], fabrication of nanostructured, ultrathin metal–doped metal hydroxide through a facile synthesis route at room temperature [96], fabrication of monocrystalline CoZnOH nanosheets using facile synthesis route [117], fabrication of ultrathin transition metal hydroxide with lattice distortion through a facile synthesis route [110], fabrication of transition metallic oxy hydroxide with oxygen vacancies through laser ablation in liquid strategy by irradiating the metallic target at room temperature in KOH solution to alleviate the usage of toxic solvents and chemicals [103], fabrication of metal–boride based electrocatalyst through a facile synthesis route at room temperature [57], fabrication of nanocomposites containing reduced graphene oxide, carbon black and cobalt borate through a facile synthesis route at room temperature [105], fabrication of heterostructured alloy/MoS₂ through a facile synthesis route, where exfoliation of MoS₂, in situ reduction of metal precursors into alloy, and integration of alloy on exfoliated MoS₂ occurred by a one-step facile synthesis route at room temperature [107], fabrication of amorphous bimetallic pyrophosphate as the electrocatalyst by co-precipitation method at ambient temperature [98], fabrication of transition metal nanoparticles decorated graphite by facile synthesis route by immersing graphite into metal salt solution [146], fabrication of carbon nanotubes wrapped with polymer as a metal-free electrocatalyst through a facile synthesis route [95], fabrication of transition metallic oxide on metallic foam by ultrafast fabrication process through laser ablation without using any toxic products [42], and fabrication of nickel ferrite/Ni hybrid through annealing-free one-step electrodeposition process [99].

(e) Using green chemistry Principle 7 (Use of renewable raw materials or feedstocks [33]), the following efforts are performed for the fabrication of earth-abundant electrocatalysts for HER and OER: fabrication of the transition metal based catalyst containing B and P on cheap filter paper substrate by facile synthesis route [40], fabrication of sulfur-doped graphene foam using rice flour (food material) as the precursor [94], fabrication of N-doped porous carbon nanosheets using powder of Euonymus japonicus leaves as carbon source [63], fabrication of Geobacter–rGO by the reduction of GO through an exo-electron-transferring nonpathogenic bacterium (Geobacter sulfurreducens) [113], fabrication of Co nanoparticles enwrapped in graphited carbon microspheres using corn starch as the carbon source [108], fabrication of Co@C using starch as the carbon source, fabrication of Fe–N–graphene using NaCl (natural, cheap, and abundant material) as template [114], fabrication of transition metallic phosphide embedded in carbon matrix using biomass agarose as the carbon source to alleviate the usage of toxic chemicals [64], fabrication of Ni@Ni₂P–N–C nanofiber–CNT using cellulose nanofibrils as the precursor, where the cellulose nanofibrils was derived from bleached eucalyptus pulp powder [112], fabrication of transition bimetallic oxide using bacterial nanocellullose as the electrode matrix to diminish the usage of toxic chemicals [59], fabrication of transition bimetallic oxide integrated with carbon support using bacterial cellulose pellicles as carbon nanofiber source to diminish the usage of toxic chemicals [145], fabrication of transition bimetallic oxide integrated with carbon support using egg white protein (albumin) as carbon source to diminish the usage of toxic chemicals [90], fabrication of transition metallic carbide embedded in carbon using cornstalk (a plant material) as the carbon source [65], fabrication of nanostructured transition metallic oxide using gelatin in the synthesis process [116], fabrication of carbon integrated bimetallic nanostructure using glucose as carbon source [66], fabrication of transition metallic oxide using Pandanus odoratissimus leaves as the precursor [156], fabrication of transition bimetallic oxide using agar-agar (biodegradable material) from red seaweed (Rhodophyta) as the polymerizing agent [106], fabrication of TiO₂–SiO₂ by spurting of silica over the titanium plate, where sand was used as silica source [159], and fabrication of carbon integrated metallic selenide using biodegradable glucose as one of a precursor [97].

Eventhough, it is still developing stage for the earth-abundant electrocatalysts for HER and OER to provide superior performance in practical applications, and the important factors governing the fabrication of earth-abundant electrocatalysts by green–chemistry approaches for electrochemical water splitting (HER and OER) should be considered in future research:

(1) Recently, several green–chemistry approaches have been applied for the fabrication of earth-abundant electrocatalysts for electrochemical water splitting to diminish or eliminate the use or generation of hazardous substances in the design, manufacture, and application of electrocatalysts. Nevertheless, highly promising green–chemistry approaches for the fabrication of high-performance earth-abundant electrocatalysts for HER and OER are significantly very limited. Therefore, additional progresses are obviously needed for the fabrication of high-performance earth-abundant electrocatalysts for HER and OER using several green–chemistry approaches [33] including fabrication of transition metal phosphide-based electrocatalysts without toxic PH₃ gas emission, designing synthesis with little or no hazardous substances, designing synthesis by using harmless chemicals/harmless solvents/auxiliaries, utilizing of inherently harmless chemistry for prevention of chemical accidents, designing energy-efficient synthesis route (especially ambient temperature synthesis), using cheap or biodegradable substrate, utilizing biomass-waste or biomass or biodegradable material as carbon sources [61,62,63,64,65,66,160,161,162], utilizing waste as carbon [163] or other sources, utilizing renewable raw materials [164], and designing the synthesis with atom economy.

(2) Recently, various green-chemistry approaches have been applied for the fabrication of high-performance earth-abundant electrocatalysts for electrochemical water splitting, whereas the high-performance electrocatalysts for HER and OER are very limited. Therefore, additional progresses are obviously needed for the fabrication of high-performance earth-abundant electrocatalysts for electrochemical water splitting through several green-chemistry approaches along with using promising strategies such as facilitate the gas-evolution behavior, tuning the electronic structure, enhance the conductivity, increase the metallic character, in situ formation of highly OER-active thin layer, and increase the electrocatalytically active surface area.