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Article

Electrolessly Deposited Cobalt–Phosphorus Coatings for Efficient Hydrogen and Oxygen Evolution Reactions

by
Huma Amber
*,
Aldona Balčiūnaitė
,
Zita Sukackienė
,
Loreta Tamašauskaitė-Tamašiūnaitė
and
Eugenijus Norkus
*
Department of Catalysis, Center for Physical Sciences and Technology (FTMC), LT-10257 Vilnius, Lithuania
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(1), 8; https://doi.org/10.3390/catal15010008
Submission received: 4 November 2024 / Revised: 4 December 2024 / Accepted: 18 December 2024 / Published: 24 December 2024
(This article belongs to the Special Issue Recent Advances in Energy-Related Materials in Catalysts, 2nd Edition)
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Versions Notes

Abstract

:
Hydrogen production via water splitting is one of the latest low-cost green hydrogen production technologies. The challenge is to develop inexpensive and highly active catalysts. Herein, we present the preparation of electrocatalysts based on cobalt–phosphorus (Co-P) coatings with different P contents for hydrogen and oxygen evolution reactions (HER and OER). The Co-P coatings were deposited on the copper (Cu) surface using the economical and simple method of electroless metal deposition. The morphology, structure, and composition of the Co-P coatings deposited on the Cu surface were studied via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), while their activity for HER and OER in 1 M KOH was investigated using linear sweep voltammetry (LSV) and chrono-techniques. It was found that the catalyst activity for both HER and OER depends on the P content of the catalyst and varies based on the highest efficiency for each reaction. The Co-P coating with 11 wt% P exhibited the lowest overpotential value of 98.9 mV for the HER to obtain a current density of 10 mA cm−2 compared to the Co-P coatings with 8, 5, 1.6, and 0.4 wt% P (107.6, 165.9, 218.2, and 253.9 mV, respectively). In contrast, the lowest OER overpotential (378 mV) was observed for the Co-P coating with 8 wt% P to obtain a current density of 10 mA cm−2 as compared to the Co-P coatings with 5, 11, 1.6, and 0.4 wt% P (400, 413, 434, and 434 mV, respectively). These results suggest that the obtained catalysts are suitable for HER and OER in alkaline media.

1. Introduction

Growing concern over climate change and the energy crisis has led to considerable research into alternative energy storage and conversion systems [1]. Hydrogen (H₂) is a green energy source with a high energy density and can be converted into energy without releasing carbon dioxide. As a result, it has attracted significant attention from both academic researchers and industry professionals [2,3]. It is anticipated that electrolysis-based hydrogen production will become the predominant method for generating hydrogen, due to the efficiency and high controllability of this process [4]. The process of electrochemical water splitting (EWS) has potential as a sustainable energy conversion method that does not result in environmental contamination [5]. EWS has been demonstrated to be the most promising approach for sustainable hydrogen production. It enables the conversion of intermittent and recyclable electrical energy, derived from sources such as solar, wind, and marine power, into chemical energy in the form of clean hydrogen fuel [6]. The EWS process can be divided into two principal stages: the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. This offers an effective method for the production of high-purity hydrogen; however, the process’s high cost and low efficiency limit its practical applications (it is estimated that only 4% of the global hydrogen produced is derived from water electrolysis) as commercial electrolyzers are typically operational at cell voltages within the 1.8–2.0 V range, which is considerably higher than the minimum value of 1.23 V, as determined by theoretical calculation. However, electrocatalysts on both the cathode and anode electrodes can significantly reduce the overpotentials in the HER and OER, thereby facilitating charge transfer between the electrodes and the electrolytes [7,8,9].
Currently, the catalysts that exhibit the greatest efficiency for the HER and OER are platinum group metals, iridium, and rhodium-based compounds, respectively [10,11,12,13,14,15,16,17]. However, the scarcity and high cost of noble metal catalysts have resulted in their limited application on a widespread basis [18,19,20,21]. Therefore, it is essential to develop efficient, low-cost, and earth-abundant non-precious metal-based bifunctional electrocatalysts for the HER and OER [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. Over the past decade, an enormous amount of research has been conducted in this area [24,25,26,38]. Many transition metal-based compounds, including transition metal oxides [26,39,40,41,42,43,44], hydroxides [45,46,47,48], sulfides [16,21,49,50], phosphides [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79], nitrides [80,81], selenides [82,83], and transition metal nanoparticle-decorated N-doped carbon [57,58,84,85,86,87,88,89,90], have been observed to exhibit excellent electrocatalytic properties in the HER and OER, which have led to a significant surge in research activity in this area. Transition metal phosphides (TMPs) have recently been the subject of increased research interest due to their remarkable activity, stability, and conductivity [52,53,60,62,75,77,78,79]. Among transition metal phosphides (TMPs), cobalt-based catalysts have demonstrated superior activity [25,53].
The utilization of uncomplicated, cost-effective, and efficient techniques for catalyst fabrication can also serve to reduce the overall cost of hydrogen production through the electrolysis of water [91]. Cobalt-based coatings can be prepared via electroplating [92], vapor deposition [93], or magnetron sputtering methods [94], which require significant energy input and utilize complex apparatus. Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic plating method that involves several simultaneous reactions in an aqueous solution [95]. As a well-established method for preparing coatings of metals and alloys, electroless plating appears to be a convenient and energy-saving method for preparing cobalt-based coatings as it does not necessitate the use of complicated equipment or hazardous power sources [96]. In comparison with the processes of electroplating, vapor deposition, and magnetron sputtering, the method of electroless plating is also an inexpensive and convenient technique for the preparation of metallic coatings. Furthermore, the coatings produced via the electroless plating method demonstrate a notable degree of thickness uniformity on substrates with complex geometries. Since its initial description by Brenner and Riddell in 1947 [97], electroless cobalt plating has been the subject of extensive research. The majority of research in this field is focused on the development of plating solutions and the optimization of plating conditions, both of which can influence the composition, structure, and properties of cobalt-based coatings [98,99]. In the preparation of pure cobalt–phosphorus (Co-P) coatings, the corresponding electroless plating solutions are typically formulated with sodium hypophosphite [100] as the reducing agent. Additionally, the electroless plating solutions comprise various complexing agents, cobalt salts, pH regulators, buffering agents, and additives, along with the reducing agents [101]. The electroless Co-P plating process is typically conducted in alkaline plating solutions at elevated temperatures (above 60 °C). This results in a reduction in the stability of the electroless Co-P plating solutions and an increase in energy consumption. Moreover, the palladium activation step is essential for electroless Co-P plating on substrates such as copper, which are resistant to hypophosphite oxidation. Consequently, the complexity and the cost of the plating process are increased [102].
The mechanism of cathodic HER in electrocatalytic water splitting comprises three principal steps: Volmer, Heyrovsky, and Tafel reactions. This sequence is illustrated below, with the asterisk (*) denoting the active sites on the surface of the electrocatalyst in an alkaline media [103].
Volmer reaction: H2O + e− + * ↔ H* + OH− (b~120 mV dec−1)
Heyrovsky reaction: H* + H2O + e− ↔ H2 + OH− + * (b~40 mV dec−1)
Tafel reaction: H* + H* ↔ H2 + 2* (b ~ 30 mV dec−1)
In this context, b represents the Tafel slope derived from the HER polarization curves. In contrast, in alkaline electrolytes, the anodic OER mechanism involves the breaking of the O–H bond and the formation of the O–O bond, occurring through four-electron transfer steps. The mechanism of OER has been demonstrated in Equations (4)–(8) for an alkaline medium, as outlined in reference [38].
OH− + * → OH* + e−
OH* + OH− → O* + H2O + e−
2O* → 2* + O2(g)
O* + OH− → OOH* + e−
OOH* + OH− → * + O2(g) + H2O(l) + e−
Significant efforts have been dedicated to the development of Co-based electrocatalysts with diverse morphologies (including powders and nanostructures) and compositions for applications in the HER and OER [26,50,84,85]. Typically, the specific surface area of the catalysts can be efficiently enhanced by modifying their morphologies and the electronic structure of the active center [54]. Furthermore, doping with non-metallic elements enables control over the catalyst conductivity, ultimately enhancing the performance of the electrocatalyst [44,45]. Co has been the subject of a considerable amount of research due to its exceptional catalytic activity for both the HER and OER under alkaline conditions [25,40,56,86,104]. Cobalt phosphide (CoP) is regarded as one of the most active and stable HER catalysts [57,58,59]. It is crucial to enhance the HER and OER performance of transition metal phosphides in alkaline solutions as water splitting in alkaline conditions is the most promising technology. The generation of hydrogen intermediates (H*) (Volmer step) in the HER in alkaline media is a challenging process [28]. Furthermore, the metal on the surface of transition metal phosphides is readily oxidized to oxo/hydroxo species. Accordingly, the objective is to develop inexpensive and highly active catalysts for the production of hydrogen via water splitting. We deposited cobalt–phosphorous (Co-P) coatings on copper surfaces using an electroless deposition method, varying the phosphorus content in order to optimize the activity of the catalysts. It was found that the catalyst’s activity for both the hydrogen and oxygen evolution reactions depended on the phosphorus content. The Co-P coating containing 11 wt% P exhibited the most efficient hydrogen evolution, while the coating containing 8 wt% P showed the most efficient oxygen evolution. These findings suggest that the synthesized catalysts are promising candidates for use in alkaline media for water splitting, potentially leading to a more sustainable and cost-effective method for hydrogen production.

2. Results and Discussion

2.1. Coatings, Microstructure, and Morphology Studies

The principal aim of this study was to develop an effective catalyst for the HER and OER in an alkaline (1.0 M KOH) medium using a straightforward methodology. Subsequently, two-component coatings comprising Co and P were deposited on Cu sheets via a simple electroless deposition process, using sodium hypophosphite as the reducing agent at varying concentrations. Figure 1 illustrates the surface morphologies and the corresponding EDX spectra of the Cu sheet-supported Co–P catalysts.
The surface morphology of the coatings was examined using scanning electron microscopy (SEM). Figure 1 shows the SEM images of the prepared Co-P catalysts with different P contents (a–e). The surface morphology of the Co-P was observed to be compact, smooth, and free of cracks where the Co particles appeared to be uniformly distributed. A typical globular morphology consisting of smaller nodules can be seen in the top side views of the Co-P catalysts in Figure 1c. As the concentration of P increased on the catalyst, the formation of nodular structures was observed, which subsequently covered the surface of the substrate (Figure 1c,d).
The composition of the coating elements deposited onto the Cu substrate surface was determined via EDX analysis, and the results of which are presented in Table 1. It can be observed that the fabricated Co-P/Cu electrocatalysts exhibited a Co content of approximately 99.62–88.87 wt% and a P content of approximately 0.38–11.13 wt%. The P content in the prepared catalysts was gradually enhanced with an increase in the NaH2PO2 concentration from 0.25 M to 4.5 M in the plating bath.
The corresponding EDX spectra of all the samples confirm the presence of Co and P in the coatings.

2.2. Electrocatalytic Activity Towards HER

The electrocatalytic performance of the prepared catalysts for the hydrogen evolution reaction (HER) was investigated by recording linear sweep voltammograms (LSVs) in 1.0 M KOH solution at a potential scan rate of 2 mV·s⁻1 at room temperature (Figure 2).
The summarized data for the HER obtained on the Co-P/Cu catalysts are shown in Table 2.
As can be seen in Figure 2a, the catalyst activity in the HER depends on the P content of the catalyst. The Co-P coating with the highest amount of 11 wt% P exhibited the lowest overpotential value of 98.9 mV in the HER, obtaining a current density of 10 mA cm−2, compared to the Co-P coatings with 8 wt% (107.6 mV), 5 wt% (165.9 mV), 1.6 wt% (218.2 mV), and 0.4 wt% (253.9 mV) P (Figure 2c). At 10 mA cm⁻2, Co-P11/Cu has a lower overpotential than other Co-P-based and advanced noble metal catalysts (Table 3). This suggests that higher P content improves HER activity. While the obtained overpotential value of 98.9 mV is higher than those for various Pt-based catalysts (from 13.5 to 50 mV) [13,14,15,16], Co-P/NF (65 mV) [10], Fe-CoP/Ti (78 mV), Ni-Co-P/NF (85 mV) [60], and Co-P-B/CC MPs (87 mV) [99], it is lower compared to other overpotential values obtained for various Co-based catalysts (Table 3).
The reaction kinetics and mechanism of the as-prepared catalysts can be evaluated based on Tafel slopes. The plot of overpotential (η) versus log j represents the Tafel slope (Figure 2d). The calculated Tafel slopes were in the range of ca. 29.4 to 50.3 mV dec⁻1, indicating that the HER might occur through the Volmer–Heyrovsky mechanism, in which water molecules or H2 adsorbs onto an electrode to generate MHads species.

2.3. Electrocatalytic Activity Towards OER

The activity of the Co-P/Cu catalysts in the OER was also evaluated. The obtained data are given in Figure 3 and Table 4.
As can be seen from the LSVs recorded for the Co-P/Cu catalysts with different P contents in a 1 M KOH solution, the lowest OER overpotential (378 mV) was observed for the Co-P coating with 8 wt% P, obtaining a current density of 10 mA cm⁻2, as compared to the Co-P coatings with 0.4 wt% P (434 mV), 1.6 wt% P (434 mV), 5 wt% P (400 mV), and 11 wt% P (413 mV). In comparison with some previously reported Co-P-based and most advanced noble metal catalysts at 10 mA cm2, the overpotential of Co–P8/Cu is lower than that of the majority of the other catalysts (Table 5). This indicates that an optimal P content exists for OER activity. The Tafel slope of the Co-P/Cu catalysts ranged from 61.7 to 73.9 mV dec−1, indicating superior OER kinetics (Table 4).
The comparison of the OER performances achieved in this study on the Co-P5/Cu and Co-P8/Cu catalysts, as well as on the recently reported high-performance catalysts (Table 5), shows that the obtained overpotential values of 378 and 400 mV for the CoP8/Cu and CoP5/Cu catalysts are higher compared to RuO2 on NF (290 mV) [13], RuO2/CF catalyst (360 mV) [15], Ir/C (254 mV) [17], IrO2 commercial (339 mV) [16], Mn-CoP (288 mV) [77], CoSi-P (309 mV), CoP (300 mV) [34], and CoP film (350 mV) [33], but lower compared to overpotential values for CoP-MNA/Ni foam (390 mV] [65], CoP hollow polyhedron (400 mV), reduced mesoporous Co3O4 nanowires (400 mV) [42], and Co2P2O7 (490 mV) [43]. These results show that Co-P/Cu catalysts with higher amounts of P (8 and 11 wt%) lead to simultaneous improvements in both HER and OER activities, suggesting that these catalysts have potential as a highly efficient overall water splitting catalysts.
The electrochemically active surface areas (ECSAs) of the Co-P/Cu catalysts with P content of 5, 8, and 11 wt% were determined from measurements of the electrochemical double-layer capacitance (Cdl).
The CV curves were recorded at different scan rates under the non-faradaic region (Figure 4a–c), which was followed by the calculation of the slope of the curve obtained by plotting the difference in the anodic and cathodic current versus the scan rate (Figure 4d).
The Cdl was found to be 3488.9, 3379.1, and 4606.6 µF for Co-P5/Cu, Co-P8/Cu, and Co-P11/Cu (Figure 4d), whereas the calculated ECSA values were 87.2, 84.5, and 115.6 cm2 for Co-P5/Cu, Co-P8/Cu, and Co-P11/Cu, respectively. The ECSA of Co-P11/Cu being higher than that of Co-P8/Cu and Co-P5/Cu is due to the higher number of active sites, which contributes to the higher HER electrocatalytic activity. Notably, the catalyst with 11 wt% P did not achieve the lowest OER overpotential, suggesting that factors beyond active site availability are influencing OER performance. Other factors, such as electronic structure and conductivity, also significantly influence catalyst performance. In this case, the catalyst with 8 wt% P performed best for the OER despite having a lower ECSA than the 11 wt% P catalyst.
Additionally, the Co-P11/Cu catalyst was investigated using an accelerated degradation test (ADT) by recording 1000 cycles at a higher scan rate of 400 mV s−1 in N2-saturated 1 M KOH solution. The inset in Figure 5 shows the initial LSV and the LSV after 1000 cycles recorded at 2 mV s−1. A slight decrease was observed in the performance after 1000 cycles at a current density of 10 mA cm−2, indicating the high stability of the catalyst in the HER (the inset of Figure 5). Moreover, the SEM images show that the surface morphology is well maintained; the catalyst does not separate from the substrate after the ADT, indicating good adhesion (shown in the insets of Figure 5). Additionally, after the ADT, the chronoamperometric curve was recorded at a constant potential of −0.261 V for 15 h on the Co-P11/Cu catalyst (Figure 5). The Co-P/Cu catalyst containing 11 wt% P possessed excellent stability, accompanied by two increases to the current density, over the 15 h of operation. This can be attributed to changes in the catalyst’s structure, composition, or morphology during the stability test. Moreover, the increase in activity after the stability test may include contributions from active sites other than pristine CoP. CoP may transform into more active phases, including cobalt oxides, hydroxides, mixed metal phosphates, or CoP nanoparticles with altered surface states, which are known active catalysts for the OER/HER. These new phases are often more catalytically active than pristine CoP.
To confirm the bifunctional activity of the Co-P/Cu catalysts for overall water splitting, both cathodic (HER) (Figure 2a) and anodic (OER) (Figure 3a) polarization curves (iR-corrected) were replotted and are shown in Figure 6a. The potential difference (Δη10) between the HER and OER had a current density of ±10 mA cm−2 (η10OER − η10HER) for the Co-P/Cu catalysts with 5, 8, and 11 wt% P, representing an expected full-cell potential window. The calculated values of the full-cell potential Δη10 delivered from the corresponding HER and OER polarization curves are 1.80 V for Co-P5/Cu, 1.72 V for Co-P8/Cu, and 1.74 V for Co-P11/Cu (Figure 6a). The obtained values suggest the potential application of the catalysts for a practical overall water splitting (OWS) device in an alkaline electrolyte, employing the same electrode materials as both the anode and the cathode.
The overall catalytic performance in a two-electrode alkaline electrolyzer cell configuration using the same catalysts as both the anode and the cathode, Co-P5/Cu‖Co-P5/Cu, Co-P8/Cu‖Co-P8/Cu, and Co-P11/Cu‖Co-P11/Cu, is presented in Figure 6b. The developed Co-P5/Cu, Co-P8/Cu, and Co-P11/Cu electrodes exhibit cell potentials of 1.836 V, 1.822, and 1.813 V, respectively, at 10 mA cm−2 (Figure 6b), which are comparable to cell potential values for CoP/rGO-400‖CoP/rGO-400 (1.70 V) [22], Co-P/NC/CC‖Co-P/NC/CC (1.77 V) [23], Co-P/NC-CC‖Co-P/NC-CC (1.95 V) [23], and Co(OH)2@NCNTs@NF‖ Co(OH)2@NCNTs@NF (1.72 V) [54], as well as for Pt/C‖IrO2 (1.71 V), Pt/C‖Pt/C (1.83 V) [22], and others reported in the literature (Table 6).
The calculated energy efficiencies of the Co-P5/Cu, Co-P8/Cu, and Co-P11/Cu cells are 66.49%, 67.03%, and 67.29%, respectively. The obtained catalysts seem to be suitable candidates for practical overall water splitting applications.
The turnover frequency (TOF) is an important indicator for determining the intrinsic activity of the catalysts, which was calculated to evaluate the intrinsic HER and OER performance of the prepared Co-P/Cu catalysts at an overpotential of 120 and 400 mV, respectively. The TOF value of Co-P8/Cu catalyst was observed to be 12.08 s−1 at an overpotential of 400 mV, which significantly outperformed that of Co-P11/Cu (2.99 s−1), Co-P5/Cu 1.91 s−1), Co-P1.6/Cu (0.83 s−1), and Co-P0.4/Cu (0.29 s−1), indicating the high intrinsic properties of this catalyst. The calculated TOF values for HER show that the highest values were obtained for the Co-P/Cu catalysts with higher amounts of P at 8 and 11 wt%. The TOF value of the Co-P8/Cu catalyst was observed as 56.61 s−1 at an overpotential of 120 mV, followed by Co-P11/Cu (19.16 s−1), Co-P5/Cu (13.38 s−1), Co-P1.6/Cu (5.83 s−1), and Co-P0.4/Cu (1.86 s−1).

3. Materials and Methods

3.1. Chemical Reagents

Cobalt(II) sulfate CoSO4·7H2O (99.5%, Chempur, Piekary Śląskie, Poland), sodium hypophosphate (NaH2PO2, 97%, Alfa Aesar, Kandel, Germany), glycine (NH2CH2COOH, 99%, Chempur, Piekary Śląskie, Poland), palladium(II) chloride PdCl2 (59.5%, Thermo Fisher Scientific, Horsham, UK), hydrochloric acid (HCl, 35–38%, Chempur, Piekary Śląskie, Poland), potassium hydroxide KOH (98.8%, Chempur, Piekary Śląskie, Poland), Cu sheets (99%, Goodfellow Cambridge Limited, Huntingdon, England), and calcium magnesium oxide, known as “Vienna Lime” (50–100%, Kremer Pigments GmbH & Co. KG, Aichstetten, Germany), were used. All the chemicals were analytical grade and were used without any further purification. The electrolytes were prepared using deionized water from a Millipore Milli-Q Ultra system with a resistivity of 18.2 MW-cm or higher.

3.2. Preparation of Co-P/Cu Catalysts

In this study, a Cu sheet with an exposed surface area of 1 × 1 cm was utilized as the substrate for the preparation of Co-P coatings via the electroless plating method (Figure 7a).
The electroless plating process for the CoP coatings includes steps such as treating the substrate first, decapitating it, activating it, and then plating it (Figure 8).
Briefly, the Cu sheets were pretreated with calcium magnesium oxide, followed by rinsing with deionized water. Next, the sheet was treated in a solution of HCl:H2O (1:1 vol) for 1 min at 25 °C. This process was carried out to remove any residual inorganic impurities. It was then rinsed with distilled water and dried. Subsequently, the pretreated Cu sheet was immersed in a solution of 0.5 g L−1 PdCl2 for 1 min for activation, following which it was washed with deionized water before the electroless deposition, dried, and placed in a freshly prepared electroless plating solution containing 0.1 M CoSO4, 0.6 M NH2CH2COOH, and 0.25–4.5 M NaH2PO2 (pH 11). The plating bath operated at a temperature of 80 °C for 10 min. Then, the obtained Co-Px/Cu sample (Figure 7b) was taken out, rinsed with deionized water, air-dried, and used without further treatment.

3.3. Characterization of Catalysts

A TM4000Plus scanning electron microscope with an AZetecOne detector (Hitachi, Tokyo, Japan) was used for the characterization of the sample’s morphology and the distribution of elements.

3.4. Evaluation of Catalysts Activity for HER and OER

The electrocatalytic activity of the Co-Px/Cu catalysts towards HER and OER was evaluated via linear sweep voltammetry (LSV). This was conducted using a PGSTAT100 potentiostat/galvanostat (Metrohm Autolab B. V., Utrecht, The Netherlands) with a standard three-electrode electrochemical cell. The fabricated Co-P/Cu catalysts, with a geometric area of 2 cm2, were employed as working electrodes. A Pt sheet was used as a counter electrode, and a Ag/AgCl (3 M KCl) electrode was used as a reference. All reported potential values in this work were converted to the reversible hydrogen electrode (RHE) scale under the following Equation (9):
ERHE = Emeasured + 0.059·pH + EAg/AgCl (3 M KCl)
where EAg/AgCl (3 M KCl) = 0.210 V.
All the exhibited potentials were corrected using Ecorr = ERHE − iR, where R was determined via EIS.
Linear sweep voltammograms (LSVs) were recorded in an N2-saturated 1 M KOH solution at room temperature. HER and OER polarization curves were obtained by measuring from the open circuit potential (OCP) to −0.432 V (vs. RHE) and from the OCP to 1.840 V (vs. RHE), respectively, at a potential scan rate of 2 mV s⁻1. Additionally, to evaluate the long-term stability of the fabricated catalysts, the chronoamperometric curve was recorded at a constant potential of −0.261 V in 1.0 M KOH solution for 15 h. The HER and OER current densities presented in this paper have been scaled to the geometric area of the catalysts.
Tafel slopes were determined from the following Equation (10) [81]:
η = b·log j/j0
where η is the overpotential; b is the Tafel slope; j is the experimental current density, and j0 is the exchange current density. The plot of η versus log j represents the Tafel slope.
To evaluate the ECSA of the catalysts, the double layer capacitance (Cdl) was determined by recording the CVs at various scan rates under the non-faradaic region followed by the calculation of the slope of the curve obtained by plotting the difference in the anodic and cathodic current against the scan rate [41,103,104]. From the CVs, the charging current, Ic, of the electrodes at each scan rate was determined via Equation (11):
Ic [A] = (Ianodic − Icathodic)OCP.
The Cdl values were evaluated by plotting a graph of the charging current vs. the scan rate and calculating the slope, as shown by Equation (12):
Slope = Cdl [F] = ΔIC [A]/Δν [V s−1].
Then, the ECSA values were calculated using the specific capacitance (Cs) of 40 μF cm−2 [41,103,104] and Equation (13):
ECSA [cm2] = Cdl [μF]/Cs [μF cm−2].
The accelerated degradation test (ADT) was performed for 1000 cycles at a constant scan rate of 400 mV s−1, after which the stable polarization curves were recorded at 2 mV s−1 for comparison with the initial curve.
A two-electrode water electrolysis cell was constructed using two of the same CoPx/Cu electrodes as the anode and the cathode. The energy efficiency of the cell was calculated using the following Equation (14):
ηelectrolyzer = Eth/Ve at j,
where Eth = 1.23 V; Ve at j is the input voltage required to drive the electrolysis at the current density of interest. The energy efficiency calculated in this study was obtained at j = 10 mA cm−2.
The turnover frequency (TOF) value was calculated using the following Equation (14) [45]:
TOF = j × A/m × F × n,
where j is the current density at an overpotential of 400 and 120 mV for OER and HER, respectively; A is the geometric surface area of the electrode; F is the Faraday constant (96,485 C mol−1); m is the number of electrons transferred in the OER is 4 and 2 for HER, and n is the number of moles of all metal ions calculated from the ICP-OES results.

4. Conclusions

A simple and inexpensive electroless metal deposition method was used for the preparation of efficient Co-P/Cu catalysts for the HER and OER. Using sodium hypophosphite as the reducing agent, the Co-P coatings with P contents ranging from 0.4 to 11 wt% were deposited on the Cu substrate. The performance of the CoP coatings in the HER and OER was evaluated via linear sweep voltammetry in 1 M KOH solution.
The Co-P/Cu catalysts showed varying activity for HER and OER depending on phosphorous content. It was found that the Co-P coating with 11 wt% P exhibited the lowest overpotential value of 98.9 mV in the HER, obtaining a current density of 10 mA cm−2, compared to the Co-P coatings with 8 wt% (107.6 mV), 5 wt% (165.9 mV), 1.6 wt% (218.2 mV), and 0.4 wt% (253.9 mV) P. On the other hand, the lowest OER overpotential (378 mV) was observed for the Co-P coating with 8 wt% P, obtaining a current density of 10 mA cm−2, as compared with the Co-P coatings with 5 wt% (400 mV), 11 wt% (413 mV), 1.6 wt% (434 mV), and 0.4 wt% (434 mV) P.
This study demonstrates that the phosphorus content in Co-P coatings significantly influences their electrocatalytic activity for both HER and OER. While higher phosphorus content benefits HER performance, an optimal concentration is crucial for OER. Moreover, the lowest cell potential of 1.813 V at 10 mA cm−2 was achieved by employing the Co-P/Cu catalyst with the highest P content of 11 wt% as the anode and the cathode (Co-P11/Cu‖Co-P11/Cu). The exploration of bifunctional electrocatalysts for overall water splitting using Co-P/Cu provides insights into the future design of non-noble metal electrocatalysts for hydrogen production via electrochemical water splitting.

Author Contributions

Conceptualization, L.T.-T. and E.N.; methodology, H.A.; validation, Z.S.; formal analysis, A.B. and Z.S.; investigation, H.A.; data curation, A.B.; writing—original draft preparation, E.N. and H.A.; writing—review and editing, L.T.-T.; visualization, H.A. and A.B.; supervision, L.T.-T.; project administration, L.T.-T.; funding acquisition, L.T.-T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by a grant (No. P-MIP-23-467) from the Research Council of Lithuania.

Data Availability Statement

All data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00008 g001aCatalysts 15 00008 g001b
Figure 1. SEM views of Co-P coatings with P content of 0.4 (a), 1.6 (b), 5 (c), 8 (d), and 11 (e) wt% deposited on Cu surface. (a’–e’) The corresponding EDX spectra.
Figure 1. SEM views of Co-P coatings with P content of 0.4 (a), 1.6 (b), 5 (c), 8 (d), and 11 (e) wt% deposited on Cu surface. (a’–e’) The corresponding EDX spectra.
Catalysts 15 00008 g001aCatalysts 15 00008 g001b
Catalysts 15 00008 g002
Figure 2. (a) HER polarization curves (iR-corrected) of the Co-P/Cu catalyst in N2-saturated 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes. Bar columns of the corresponding overpotentials at 10 mA cm−2 (c) and Tafel slopes (d).
Figure 2. (a) HER polarization curves (iR-corrected) of the Co-P/Cu catalyst in N2-saturated 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes. Bar columns of the corresponding overpotentials at 10 mA cm−2 (c) and Tafel slopes (d).
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Catalysts 15 00008 g003
Figure 3. (a) OER polarization curves (iR-corrected) of the Co-P/Cu catalyst in N2-saturated 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes. Bar columns of the corresponding overpotentials at 10 mA cm−2 (c) and Tafel slopes (d).
Figure 3. (a) OER polarization curves (iR-corrected) of the Co-P/Cu catalyst in N2-saturated 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes. Bar columns of the corresponding overpotentials at 10 mA cm−2 (c) and Tafel slopes (d).
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Catalysts 15 00008 g004
Figure 4. CVs of (a) Co-P/Cu (5 wt% P), (b) Co-P/Cu (8 wt%), and (c) Co-P/Cu (11 wt%) in N2-saturated 1 M KOH in the non-faradaic potential region at different scan rates. (d) Capacitive current as a function of scan rate.
Figure 4. CVs of (a) Co-P/Cu (5 wt% P), (b) Co-P/Cu (8 wt%), and (c) Co-P/Cu (11 wt%) in N2-saturated 1 M KOH in the non-faradaic potential region at different scan rates. (d) Capacitive current as a function of scan rate.
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Catalysts 15 00008 g005
Figure 5. Chronoamperometric curve of the Co-P11/Cu catalyst in N2-saturated 1 M KOH solution at a constant potential of −0.261 V for 15 h (red line). The insets show the initial LSV and the LSV after 1000th cycle recorded at 2 mV s−1 and the corresponding SEM images of the initial and after the 1000th cycle test of the same catalyst.
Figure 5. Chronoamperometric curve of the Co-P11/Cu catalyst in N2-saturated 1 M KOH solution at a constant potential of −0.261 V for 15 h (red line). The insets show the initial LSV and the LSV after 1000th cycle recorded at 2 mV s−1 and the corresponding SEM images of the initial and after the 1000th cycle test of the same catalyst.
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Catalysts 15 00008 g006
Figure 6. (a) The predicted bifunctional activity of Co-P/Cu catalysts: values of the full-cell potential Δη10 calculated from the difference between the overpotential values (η10) at ±10 mA cm−2, obtained from the corresponding HER and OER LSVs in Figure 2a and Figure 3a. (b) LSVs for different Co-P/Cu catalysts as both anode and cathode electrocatalytic water splitting in 1 M KOH solution and the inset (b’) presents the corresponding LSVs at lower current densities. The (c) Photograph of OWS on Co-P/Cu as cathode and anode.
Figure 6. (a) The predicted bifunctional activity of Co-P/Cu catalysts: values of the full-cell potential Δη10 calculated from the difference between the overpotential values (η10) at ±10 mA cm−2, obtained from the corresponding HER and OER LSVs in Figure 2a and Figure 3a. (b) LSVs for different Co-P/Cu catalysts as both anode and cathode electrocatalytic water splitting in 1 M KOH solution and the inset (b’) presents the corresponding LSVs at lower current densities. The (c) Photograph of OWS on Co-P/Cu as cathode and anode.
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Catalysts 15 00008 g007
Figure 7. The digital photographs of the copper sheet sample both before (a) and after it was plated in a Co-P plating solution for 10 min (b).
Figure 7. The digital photographs of the copper sheet sample both before (a) and after it was plated in a Co-P plating solution for 10 min (b).
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Catalysts 15 00008 g008
Figure 8. The electroless deposition scheme of Co-P coatings on the Cu surface.
Figure 8. The electroless deposition scheme of Co-P coatings on the Cu surface.
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Table 1. Composition of CoP coatings deposited on Cu surface via EDX analysis.
Table 1. Composition of CoP coatings deposited on Cu surface via EDX analysis.
SampleElement, wt%
CoP
CoP0.4/Cu99.620.38
CoP1.6/Cu98.451.55
CoP5/Cu95.134.87
CoP8/Cu92.027.98
CoP11/Cu88.8711.13
Table 2. Summarized parameters for the HER on Co-P/Cu catalysts in alkaline media.
Table 2. Summarized parameters for the HER on Co-P/Cu catalysts in alkaline media.
SampleEonset, V at
j = −1 mA cm−2		η10 *, mVTafel Slope,
mV dec−1
Co-P0.4/Cu−0.210253.950.3
Co-P1.6/Cu−0.186218.243.9
Co-P5/Cu −0.114165.948.1
Co-P8/Cu −0.071107.634.1
Co-P11/Cu −0.06798.929.4
* Overpotential at 10 mA cm−2.
Table 3. Comparison of HER performance of Co-P with some previously reported Co-P-based and most advanced noble metal catalysts.
Table 3. Comparison of HER performance of Co-P with some previously reported Co-P-based and most advanced noble metal catalysts.
Catalystη10, mV at
j = −10 mA cm−2		Tafel Slope,
mV dec−1		ElectrolyteRef.
Co-P11/Cu 98.929.41 M KOHThis work
Co-P8/Cu107.634.11 M KOHThis work
Ni-Co-P/NF85461 M KOH[60]
FeP NAs/CC2181461 M KOH[61]
CoP Nanoparticle170661 M KOH[55]
Ni/iP13058.51 M KOH[29]
Fe–CoP/Ti78751 M KOH[62]
Co-P-B/CC MPs8769.21 M KOH[99]
MoP/NF11454.61 M KOH[30]
Co–B/NF10398.31 M KOH[10]
Co–P/NF6559.71 M KOH[10]
NiFeLDH@NiCoP/NF12088.21 M KOH[46]
NiCoP/NF185124.41 M KOH[46]
Co/NBC1171461 M KOH[87]
CoP NS207124.51 M KOH[63]
CoP NS/C11170.91 M KOH[63]
Fe-codoped CoP/RGO145601 M KOH[1]
CoP/RGO2721341 M KOH[1]
Al-doped CoP/RGO2061061 M KOH[1]
Fe-doped CoP/RGO2081071 M KOH[1]
S:CoP NPs175711 M KOH[64]
CoP NPs216901 M KOH[64]
CoP-MNA/Ni Foam150511 M KOH[65]
CoP@BCN215521 M KOH[66]
(CoxNi1-x)2P180631 M KOH[67]
f-CoP/CoP2/Al2O3138731 M KOH[68]
Co-P/N-doped carbon matrices154511 M KOH[88]
Co-P-2188–1 M KOH[96]
CoP nanowires147691 M KOH[69]
Co2P24786.31 M KOH[70]
CoP hollow polyhedron159591 M KOH[2]
Ni-Co-P15060.61 M KOH[71]
V-doped CoP23586.1 1 M KOH[31]
CoP/PNC165701 M KOH[89]
Pt/C (20 wt% Pt/XC-72) on NF30491 M KOH[14]
Pt/C on GCE501081 M KOH[13]
Pt/C–1081 M KOH[14]
Ni-Co-P44521 M KOH[15]
20 wt% Pt/C13.558.91 M KOH[16]
Table 4. Electrochemical performance of Co-P/Cu catalysts for OER in alkaline media.
Table 4. Electrochemical performance of Co-P/Cu catalysts for OER in alkaline media.
SampleEonset, V at j = 1 mA cm−2ηonset, mVE, V at j = 10 mA cm−2η10 *, mVTafel Slope, mV dec−1
Co-P0.4/Cu1.5903601.66443473.0
Co-P1.6/Cu1.5893591.66443474.3
Co-P5/Cu1.5693391.63040061.7
Co-P8/Cu 1.5443141.60837866.4
Co-P11/Cu 1.5743441.64341373.9
* Overpotential at 10 mA cm−2.
Table 5. Comparison of the OER performance of Co-P with some previously reported Co-P-based and most advanced noble metal catalysts.
Table 5. Comparison of the OER performance of Co-P with some previously reported Co-P-based and most advanced noble metal catalysts.
Catalystη10, mV at
j = 10 mA cm−2		Tafel Slope,
mV dec−1		ElectrolyteRef.
Co-P8/Cu37866.41 M KOHThis work
Co-P5/Cu40061.71 M KOHThis work
CoP nanoparticles340991 M KOH[72]
Co2P nanorods31061.01 M KOH[73]
CoP nanowires326801 M KOH[69]
CoP nanosheet36169.51 M KOH[63]
CoP NPs@NF3201021 M KOH[64]
CoP-MNA/Ni Foam390651 M KOH[65]
CoP hollow polyhedron400571 M KOH[2]
Co-Mn oxyphosphide370521 M KOH[74]
NiCoP Nanoboxes3701151 M KOH[75]
Reduced mesoporous Co3O4 nanowires400721 M KOH[42]
O-CoP/GCE31059.91 M KOH[76]
S:CoP/NF300821 M KOH[64]
Mo-CoP/CC305561 M KOH[32]
V-doped CoP/GCE34095.71 M KOH[31]
Co-P film350471 M KOH[33]
CoPi/PCDs350-1 M KOH[17]
CL-Co2P2O7@C nanohybrids397701 M KOH[43]
Co2P2O7 nanostructure490861 M KOH[43]
Co(PO3)2 nanosheets5741061 M KOH[43]
Hydrous cobalt phosphate thin films292981 M KOH[105]
Co–Fe–P–O267301 M KOH[47]
Co phosphide/Co phosphate thin film (PCPTF)300651 M KOH[34]
Ni-P300641 M KOH[79]
Co2B360451 M KOH[35]
Mn-CoP288 77.21 M KOH[77]
SnPi@CoP–Ni5P4/NCF364521 M KOH[78]
CoSi-P3091211 M KOH[48]
RuO2 on NF290811 M KOH[13]
RuO2/CF3601641 M KOH[15]
IrO2 commercial33994.51 M KOH[16]
Ir/C25471.91 M KOH[17]
Table 6. Comparison with various electrocatalysts for overall water splitting.
Table 6. Comparison with various electrocatalysts for overall water splitting.
CatalystCell Voltage, VElectrolyteRef.
Co-P11/Cu 1.81 1 M KOHThis work
CoP/rGO-4001.701 M KOH[22]
Co-P/NC/CC1.771 M KOH[23]
Co-P/NC-CC1.951 M KOH[23]
Co(OH)2@NCNTs@NF1.721 M KOH[54]
Pt/C‖IrO21.711 M KOH[56]
Pt/C‖Pt/C1.831 M KOH[81]
Hydrous cobalt phosphate thin films1.801 M KOH[105]
CoP@PNC-DoS1.741 M KOH[36]
S:CoP NPs1.721 M KOH[64]
Co phosphide/Co phosphate thin film (PCPTF)1.921 M KOH[34]
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Amber, H.; Balčiūnaitė, A.; Sukackienė, Z.; Tamašauskaitė-Tamašiūnaitė, L.; Norkus, E. Electrolessly Deposited Cobalt–Phosphorus Coatings for Efficient Hydrogen and Oxygen Evolution Reactions. Catalysts 2025, 15, 8. https://doi.org/10.3390/catal15010008

AMA Style

Amber H, Balčiūnaitė A, Sukackienė Z, Tamašauskaitė-Tamašiūnaitė L, Norkus E. Electrolessly Deposited Cobalt–Phosphorus Coatings for Efficient Hydrogen and Oxygen Evolution Reactions. Catalysts. 2025; 15(1):8. https://doi.org/10.3390/catal15010008

Chicago/Turabian Style

Amber, Huma, Aldona Balčiūnaitė, Zita Sukackienė, Loreta Tamašauskaitė-Tamašiūnaitė, and Eugenijus Norkus. 2025. "Electrolessly Deposited Cobalt–Phosphorus Coatings for Efficient Hydrogen and Oxygen Evolution Reactions" Catalysts 15, no. 1: 8. https://doi.org/10.3390/catal15010008

APA Style

Amber, H., Balčiūnaitė, A., Sukackienė, Z., Tamašauskaitė-Tamašiūnaitė, L., & Norkus, E. (2025). Electrolessly Deposited Cobalt–Phosphorus Coatings for Efficient Hydrogen and Oxygen Evolution Reactions. Catalysts, 15(1), 8. https://doi.org/10.3390/catal15010008

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