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Article

Bimetallic Ni–Mn Electrocatalysts for Stable Oxygen Evolution Reaction in Simulated/Alkaline Seawater and Overall Performance in the Splitting of Alkaline Seawater

by
Sukomol Barua
,
Aldona Balčiūnaitė
*,
Daina Upskuvienė
,
Jūrate Vaičiūnienė
,
Loreta Tamašauskaitė-Tamašiūnaitė
and
Eugenijus Norkus
Department of Catalysis, Center for Physical Sciences and Technology (FTMC), LT-10257 Vilnius, Lithuania
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(8), 1074; https://doi.org/10.3390/coatings14081074
Submission received: 12 July 2024 / Revised: 7 August 2024 / Accepted: 20 August 2024 / Published: 22 August 2024
(This article belongs to the Special Issue New Advance in Nanoparticles, Fiber, and Coatings)
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Versions Notes

Abstract

:
The perfect strategy for the generation of green and renewable hydrogen (H2) fuels is the direct electrocatalytic splitting of plentiful seawater rather than scarce freshwater. One of the half-reactions taking place during the electrocatalytic splitting of seawater is oxygen evolution (OER). However, the OER is affected by slow four-electron transfer kinetics as well as competitive chlorine evolution reactions (CERs) in seawater. To overcome the kinematic and competitive barriers of seawater splitting and achieve an excellent overall performance of seawater splitting, we herein report a facile, low-cost, one-step fabrication procedure of 3D structured nickel–manganese (NiMn) coatings using a dynamic hydrogen bubble template (DHBT) technique. The electrocatalytic activities of the thus synthesized catalytic materials for OER in simulated seawater (0.5 M NaCl + 1 M KOH, denoted as SSW) and alkaline natural seawater (natural seawater + 1 M KOH, denoted as ASW) were investigated using linear sweep voltammetry (LSV) at varying temperatures from 25 to 75 °C. Scanning electron microscopy (SEM) and inductively coupled plasma–optical emission spectroscopy (ICP–OES) were used to examine the surface morphology and composition of the prepared catalysts. It was found that the prepared NiMn/Ti-1 catalyst in a plating bath containing a molar ratio of 1:1 Ni2+:Mn2+ and having the lowest Mn loading of 13.43 µg cm−2 exhibited quite reasonable activity for OER in Cl− ion rich SSW and ASW. To achieve the benchmark current density of 10 mA cm−2 in SSW and ASW, the NiMn/Ti-1 electrocatalyst requires overpotentials of 386 and 388 mV, respectively. In addition, this optimal bimetallic electrocatalyst also demonstrated superior long-run stability at 1.81 V (vs. RHE) and 10 mA cm−2 for 24 h in both working electrolytes. Impressively, the two-electrode electrolyzer—NiMn/Ti-5(−)||NiMn/Ti-1(+)—needs only 1.619 V to deliver 10 mA cm−2 current density for overall alkaline seawater electrolysis, which is even 0.075 V lower than the noble metal-based electrolyzer (Pt(−)||NiMn/Ti-1(+)).

1. Introduction

Energy plays the most crucial role for the development and continuous growth of modern civilization in this era, maintaining the normal operation of the global economy. The dominance of fossil fuels, in particular natural gas, coal, and oil, in industrial energy consumption persists. However, the unending consumption of non-renewable fossil fuels has led to a global crisis of energy depletion and environmental threats. Hydrogen is a diverse energy bearer with the ability to address several critical energy challenges. In addition, hydrogen energy offers a number of advantages, including clean combustion products with zero pollution and zero CO2 emissions, a high gravimetric energy density, renewable and plentiful resource reserves, and the potential to be considered one of the best promising options to replace traditional fossil fuels [1,2,3,4,5,6]. The sources of hydrogen are diverse, and production processes can be classified into three categories: green hydrogen, blue hydrogen, and gray hydrogen, among others. Green hydrogen is obtained using electrochemical water splitting technology, which is environmentally friendly and effectively reduces the use of fossil fuels [7,8,9]. Consequently, green hydrogen has become a subject of considerable research interest.
Electrochemical water splitting comprises of two half-reactions: the cathodic hydrogen evolution reaction (HER) and the anodic OER. The four-electron transfer process (4OH− → H2O + O2 + 4e−) of OER at the anode is considered the rate-limiting step for water splitting, exhibiting sluggish kinetics and necessitating a theoretical voltage equal to 1.23 V [10,11,12,13]. In recent decades, researchers have developed electrolyzers based on alkaline freshwater as the key source for water splitting. However, the limited availability of freshwater resources represents a significant challenge for the advancement of electrolyzer technology, particularly in light of the increasing global population and the associated demands of modern living standards. In comparison to the restricted availability of freshwater, seawater, which constitutes approximately 97% of the world’s total water supply, can be regarded as an inexhaustible source. It is recommended that seawater splitting be considered as a more viable and sustainable option to freshwater electrolysis for hydrogen generation [14,15,16,17].
The industrial production of high-purity hydrogen from direct or selective electrolytic splitting of seawater remains a major challenge, despite the abundant availability of seawater on our planet. The presence of aggressive chloride anions (Cl−), as well as other unfavorable ions including Ca2+, Mg2+, Na+, Br−, and SO42−, and a diverse range of bacteria, suspended solid microparticles, and other impurities, can cause the surface of electrocatalysts to become inactive and poisoned, ultimately leading to their deactivation and poisoning [18,19,20]. The CER that occurs at the anode represents the most critical encounter in the seawater electrolysis process. This reaction competes with the OER and forms aggressive substances: chloride products (e.g., HClO, Cl2, or ClO−), which have the potential to degrade the electrolyte environment and result in low anode current efficiency. Moreover, the Pourbaix diagram indicates that OER is thermodynamically more favorable than CER across the whole pH region, with a constant potential difference of up to 480 mV remaining in the high pH region. Therefore, in an alkaline medium (pH > 7.5), electrocatalytic materials with an overpotential of less than 480 mV would be effective for splitting seawater without the occurrence of side-reactions caused by chloride ions [15,20,21,22,23]. Additionally, the disadvantage of high pH is the production of insoluble precipitates (e.g., Ca(OH)2 and Mg(OH)2), which lead to the poisoning of the catalytic sites in the natural seawater system.
Nevertheless, the high costs of green hydrogen energy remain a substantial obstacle to its widespread adoption, largely due to the inherent costs associated with the hydrogen production pathway. One of the principal factors contributing to this elevated cost is the electrocatalysts employed as electrodes in the water electrolyzers. The most advanced platinum group metal (PGM) catalysts demonstrate outstanding electrocatalytic activity for water splitting, while Pt-based catalysts are the leading electrocatalysts for HER. Ru- or Ir-based oxides exhibit the most efficient performances for OER [24,25,26,27,28]. However, the high costs and limited availability of precious PGM catalysts present significant obstacles to their large-scale commercial deployment. It is therefore anticipated that research into earth-abundant, inexpensive, highly efficient and durable electrocatalytic materials with the potential to reduce the energy barriers to hydrogen/oxygen evolution in HER/OER processes will be of significant interest.
In order to overcome the large overpotential associated with seawater splitting in OER, researchers have directed considerable attention toward cost-effective and earth-abundant transition metal-based electrocatalysts, which have emerged as promising options because of their active structural sites and near-optimal activity. Over the past decade, low-cost transition metals (TMs), including nickel-based [29,30,31,32], iron-based [33,34,35,36,37], and cobalt-based [38,39,40,41,42] sulfides [43,44,45,46], phosphides [47,48,49,50], nitrides [51,52,53,54], alloys [55,56], and bimetallic LDHs [57,58,59], have emerged as alternative potential candidates for OER in alkaline freshwater/seawater and overall simulated/alkaline seawater splitting as replacements for precious metal catalysts. In order to identify the most catalytically efficient materials, two key properties must be considered: (i) the intrinsic activity of the substance, which directly regulates the chemical nature of the electrodes, and (ii) the active surface area of the catalysts. From among the early group of VIII B TMs, scientists have turned their attention to Ni-based electrodes due to the enhanced water splitting performance achieved through the formation of heterostructure hybrids and nanostructured composites, etc. with Fe, Co, and Mo [60,61,62,63,64,65]. In this context, another TM, namely Mn, has received increasing attention in recent years due to its optimal electronic configuration and higher M–H∗ bond strength [66,67]. Furthermore, Mn is capable of forming complexes with other metals by the splitting of the d-orbital, resulting in the creation of an electron-deficient eg-orbital. This change in the electronic level of the catalyst facilitates the exposure of additional active sites and enhances catalytic interactions [68]. Furthermore, Mn is recognized for its ability to reduce the overpotential in HER and OER electrolytic reactions, which makes it a highly promising candidate for the preparation of transition metal-based catalysts [69,70,71,72,73]. A recent published study by Priamushko et al. demonstrated that bimetallic oxides tailored by Mn exhibited enhanced OER kinetics due to increased active sites, catalyst stability, and current density [74]. Zhang et al. [75] showed that the incorporation of Mn as a dopant effectively enhanced the inherent activity and stability of Co–P nanosheets. Guo et al. reported a bimetallic MnCo–P/NF electrocatalyst [76] that exhibited excellent hydrogen catalytic activity, requiring an overpotential of 112 mV to achieve a higher current density of 100 mA cm−2 together with strong mechanical stability. A series of Mn-induced bimetallic and multicomponent alloy electrocatalysts have also been investigated for their potential in water electrolysis and other electrochemical reactions. These studies have demonstrated excellent catalytic activity, stability, selectivity, and protective properties against corrosion in seawater splitting [77,78,79,80,81].
In our previous publication, we demonstrated that those facile electrodeposited 3D NiMn coatings have superior bifunctional activity and long-term durability for HER and OER towards alkaline water electrolysis [82]. However, in the search for TM-based promising electrodes for a sustainable alternative to scarce freshwater, it was resolved to further investigate these low-cost catalysts to investigate their electrocatalytic performances and long-time durability for OER in alkaline natural seawater (ASW) and simulated seawater (SSW). To the extent of our knowledge, no publication has yet been reported on the catalytic activities of bimetallic Ni–Mn alloys as OER electrocatalysts for seawater splitting. However, in recent times, only a few publications have highlighted the applications of NiMn–LDH materials for overall alkaline freshwater/seawater electrolysis [57,83,84,85].

2. Materials and Methods

2.1. Chemicals

Titanium foil with a thickness of 0.127 mm thick and 99.7% purity (Sigma–Aldrich (Saint Louis, MO, USA)), stainless steel foil with a thickness of 0.2 mm (Type 304, Alfa Aesar (Karlsruhe, Germany) GmbH & Co.), nickel sulfate hexahydrate (NiSO4.6H2O, >98%, Chempur Company (Karlsruhe, Germany)), manganese chloride tetrahydrate (MnCl2.4H2O, >99%, Chempur Company (Karlsruhe, Germany)), boric acid (H3BO3, >99.5%, Chempur Company (Karlsruhe, Germany)), KOH (98.8%, Chempur Company (Karlsruhe, Germany)), ammonium sulfate ((NH4)2SO4, >99%, Chempur Company (Karlsruhe, Germany)), H2SO4 (96%, Chempur Company (Karlsruhe, Germany)), and HCl (35–38%, Chempur Company (Karlsruhe, Germany)) were of analytical grade and were employed directly without any additional purification. All solutions were prepared using ultrapure water (18.2 MΩ·cm). The seawater was taken from the Baltic Sea in the Klaipėda coastal region of Lithuania.

2.2. Fabrication of Catalysts

The electrodes were fabricated via a one-step procedure as reported in our previous research [82]. In brief, the Ti sheets in the size of 1 × 1 cm2 were pretreated in dilute H2SO4 (1:1 vol.) at 70 °C to activate and remove any possible oxides/hydroxides present on the surface. Following this, the sheets were rinsed with distilled water and subsequently immersed in the plating bath, which comprised 0.2 M NiSO4 and 0.2 to 0.6 M MnCl2 serving as the source of Ni2+ and Mn2+ ions, respectively. Then, 0.5 M (NH4)2SO4 was used as a coating morphology modifier, while 0.3 M H3BO3 was employed as a pH stabilizer. All reagents were dissolved in ultrapure water under acid conditions, specifically 1 M HCl and 1.5 M H2SO4. The composition and electroplating conditions of the plating baths used to prepare the electrodes are detailed in Table 1. After coating, the NiMn/Ti electrocatalysts were carefully washed with deionized water, dried in air at ambient temperature, and stored for future studies.

2.3. Characterization of Catalysts

An investigation into the composition and morphology of the fabricated NiMn/Ti catalysts was carried out using a TM4000Plus scanning electron microscope with an AZetecOne detector (Hitachi, Tokyo, Japan).
The loadings of Ni and Mn were identified using an Optima 7000DV inductively coupled plasma optical emission spectrometer (Perkin Elmer, Waltham, MA, USA). ICP–OES spectra were collected at λNi 231.604 nm and λMn 257.610 nm.
X-ray diffraction patterns were acquired using a D2 Phaser X-ray diffractometer with CuKα radiation (Bruker AXS, Karlsruhe, Germany). A step scan mode was used in the 2θ from 10° to 90°. The step length was 0.04° and the count time was 1 s per step.

2.4. Electrochemical Measurements

The performances of the prepared NiMn catalysts were assessed through linear sweep voltammetry (LSV), employing a PGSTAT302 potentiostat (Metrohm Autolab B.V., Utrecht, The Netherlands). The LSVs were recorded in two different types of simulated seawater, namely Ar-deaerated simulated seawater (0.5 M NaCl +1 M KOH) and alkaline seawater (natural seawater + 1 M KOH), at a range of temperatures from 25 to 75 °C. A thermostatted electrochemical cell was utilized to evaluate the catalytic performance. The prepared NiMn/Ti catalysts (a geometric area of 2 cm2) served as the working electrode, while a Pt sheet and a saturated calomel electrode (SCE) acted as the counter and the reference electrodes, respectively. If not indicated otherwise, all potential values were recalculated to the reversible hydrogen electrode (RHE) scale according to the following equation: ERHE = ESCE + 0.242 V + 0.059 V × pHsolution.
The OER LSVs were obtained in the two operating electrolytes by scanning the electrode potential from the open circuit potential (OCP) to 2.06 V (vs. RHE) at 10 mV s−1. Furthermore, the long-time durability of the prepared NiMn/Ti-1 catalyst was assessed through the collecting of the chronopotentiometric curves (CP) at a constant current density of 10 mA cm−2. The chronoamperometric curves (CA) were collected at a constant potential of 1.81 V (vs. RHE) for 24 h in ASW and SSW. Moreover, the two-electrode seawater electrolyzer was assembled by employing the as-prepared optimal NiMn/Ti-1 electrocatalyst as the anode and our previously reported efficient NiMn/Ti-5 HER electrocatalyst [82] as the cathode.
The electrochemically active surface area (ECSA) of the catalysts was determined using double layer capacitance (Cdl) measurements. CV curves were collected at varying scan rates within the non-faradaic region. Thereafter, the charging current, Ic, of the catalysts at each scan rate was calculated using the following Equation (1):
Ic [mA] = (Ianodic−Icathodic)OCP
Then, the difference in anodic and cathodic current against the scan rate was plotted and the slope of the curve, which corresponds to the Cdl, was calculated [86,87,88] as demonstrated by Equation (2):
Slope = Cdl [mF] = ΔIC [mA]/Δν [V s−1]
For the calculation of the ECSA values, the specific capacitance (Cs) of 0.040 mF cm−2 [86,87,88] was used in Equation (3):
ECSA [cm2] = Cdl [mF]/Cs [mF cm−2]
The roughness factor (Rf) for the catalysts was determined by the following Equation (4):
Rf = ECSA/Sgeometric

3. Results and Discussion

3.1. Morphology and Microstructure Studies

Schematic fabrication of 3D NiMn/Ti catalysts via a straightforward one step electroplating approach is demonstrated in Figure 1a.
The surface morphology and structure of the NiMn/Ti-1, NiMn/Ti-2 and NiMn/Ti-3 catalysts was examined using scanning electron microscopy (SEM) as demonstrated in Figure 1b–d. The SEM micrograph demonstrated that the synthesized NiMn coating on the Ti, which was electrodeposited from an aqueous operating bath containing a molar ratio of 1:1 Ni2+:Mn2+ (denoted as NiMn/Ti-1), exhibited a typical globular morphology comprised of smaller nodules that grew with a uniform distribution, covering the Ti substrate (Figure 1b). The concentration of Mn2+ in the plating bath was found to be a determining factor in the size of the resulting nodules. As the Ni2+:Mn2+ ratio increased from 1:1 to 1:2, the nodules exhibited a notable enlargement in size (Figure 1c). As the concentration of Ni2+ remained constant (0.2 M) in all plating bath compositions, the as-synthesized NiMn/Ti-3 electrocatalyst formed an uneven, coarse, flake-like heterostructured surface as a result of a higher Ni2+:Mn2+ ratio of 1:3 in the plating bath and relatively higher cell voltage during electrodeposition, as depicted in Figure 1d.
The metal loadings deposited on the Ti surface were determined by ICP–OES analysis, the results of which are presented in Table 2. The catalysts were fabricated from different plating baths with varying molar ratios of Ni:Mn, resulting in variable Ni-loadings of ca. 72–86 wt% in the prepared catalysts, while the content of deposited Mn varied from ca. 13–28 wt%. It was vividly observed that the overall metal loadings increased significantly with higher concentration of Mn2+ in the plating bath, with a sum varying from ca. 100 up to 375 μgmetalcm−2.
Figure 2 illustrates the XRD pattern of the NiMn catalyst deposited on the Ti surface. It can be seen that the major peaks align closely with the data provided for Ti according to COD No. 9016190. The most abundant phase, as determined by the XRD peak intensities, was hexagonal Ti, with diffraction peaks at 2θ approximately 35.0°, 38.4°, 40.1°, 53.0°, 62.9°, 76.2°, 77.3° and 82.3°, in good agreement with the phase of Ti (COD No. 9016190, a = 2.95 Å, c = 4.68 Å, space group P 63/m m c), which correspond to the lattice planes of (100), (002), (101), (102), (110), (112), (201), and (004), respectively.
Moreover, the pattern also displayed peaks corresponding to the phases of Ni, MnO, and Mn2NiO4. The Ni peak was detected at 2θ approximately 44.3°, which is indexed to the (111) crystal plane (COD. No. 2102269, a = 3.5382 Å, space group Fm-3m), while the MnO peaks were observed at 2θ approximately 35.1°, 40.8°, 59.1°, and 70.7°, indexed to the (111), (200), (220), and (311) crystal planes, respectively (COD No. 1010898, a = 4.415 Å, space group Fm-3m). The peak located at 35.2° may be assigned with the cubic phase of Mn2NiO4 (COD No. 1530384, a = 8.45 Å, space group Fd-3m), which correlates with the lattice plane of (311). It may be surmised that the NiMn catalyst exhibited a Ni phase with some MnO and Mn2NiO4. Meanwhile, the Mn2NiO4 phase, which has a typical spinel structure, may prove to be a beneficial structure in enhancing the electrocatalytic activity of the prepared NiMn catalyst.

3.2. Electrocatalytic Activity for OER in SSW

To assess the OER performance of the prepared electrocatalysts, LSVs were obtained in an Ar-deaerated SSW solution of 0.5 M NaCl + 1.0 M KOH (resulting pH of 14) at 10 mV·s−1 from OCP up to 2.06 V, at a temperature range of 25 up to 75 °C (Figure 3a). To ascertain the reproducibility, three electrodes were prepared and the LSVs were accounted for each electrode. As can be seen from the data presented in Figure 3a, ca. 1.57–1.72 times higher current density was attained on the investigated catalysts with an increase in temperature from 25 up to 75 °C. Figure 3b shows the LSVs for all NiMn catalysts collected at a temperature of 25 °C. It can be observed that the catalytic activity of the catalysts was significantly influenced by the varying molar ratio of Ni2+:Mn2+. Figure 3c shows the extracted Tafel plots for the aforementioned catalysts, while Figure 3d presents a summary of the overpotential values needed to reach the benchmark current density. Moreover, the slope of the Tafel is a pivotal parameter in the field of electrochemical kinetics and is used to reflect the rate-limiting step of a given reaction. With regard to the OER, the slope of the Tafel offers an insight into the underlying mechanism of the reaction and the kinetics of the steps involved.
Figure 3c shows that the Tafel slope values were 130, 120, and 86 mV dec–1 for the fabricated NiMn/Ti-1, NiMn/Ti-2 and NiMn/Ti-3 electrocatalysts, respectively. As depicted in Figure 3d, NiMn/Ti-1 requires overpotentials of 386, 423, and 509 mV to attain current densities of 10, 20 and 50 mA cm−2, respectively, in SSW. These values are superior to those of NiMn/Ti-2 (η10 = 413 mV, η20 = 457 mV, and η50 = 557 mV) and NiMn/Ti-3 (η10 = 454 mV, η20 = 497 mV, and η50 = 603 mV). Table 3 summarizes the OER activity of the synthesized catalysts in SSW.

3.3. Electrocatalytic Performance for OER in ASW

Compared to fresh water, the availability of seawater could be the most sustainable resource for the implementation of electrocatalytic water electrolysis technology. However, natural seawater includes a large number of metal ions and radicals (e.g., Mg2+, Ca2+, Br−, SO42−, etc.), microbes, and solid particles that have the potential to precipitate during electrolysis and accumulate on the catalyst and electrode surface resulting in significant catalytic inefficiency. To explore the potential use of seawater, the seawater (pH ≈ 8.2) used in this study was taken from the Baltic Sea near the Klaipėda coastal region in Lithuania. To prepare the ASW, 1.0 M of KOH pellets were dissolved in the seawater. The precipitate, a white cloudy layer of insoluble metal hydroxides, was allowed to settle overnight and was then filtered from the solution. The clear filtrate was collected by decantation and filtration and was designated as ASW with a pH of ≈13.85.
The OER of the fabricated catalysts was evaluated by recording LSVs in an Ar-deaerated ASW electrolyte from OCP up to 2.06 V at 10 mV s−1 with temperatures ranging from 25 to 75 °C. Figure 4a displays the LSVs, which exhibited a significant enhancement in current density, reaching values between 1.65 and 1.75 times higher at elevated temperatures (25 up to 75 °C).
As illustrated in Figure 4b, the activity for OER of the prepared catalysts by varying the molar ratio of Ni to Mn exhibited a gradual decline in current density with increasing Mn concentrations. The anodic peak corresponding to the surface oxidation of Ni2+ to Ni3+ was found at approximately 1.55 V, indicating the creation of active sites for OER that enhanced the electrochemical process. This observation is in accordance with the findings of Luo et al., who demonstrated that the OER performance was significantly influenced by the Ni to Mn content ratios in the fabricated Nix|Mn1−xO/CNTs electrocatalysts. It was reported that when the Mn content exceeded 17%, the overpotentials of the catalysts increased, indicating that a reduction in the Mn content resulted in a notable enhancement in OER activity [89]. However, the recorded current densities of the electrodes in ASW were also markedly lower than those observed in SSW. It is anticipated that the catalytic efficiency of the synthesized electrocatalysts in ASW will be appropriately decreased due to the existence of interfering ions and impurities in seawater. Subsequently, the OER LSV curves at 25 °C were employed to construct the Tafel plots and calculate the Tafel slopes. The values of the Tafel slope were determined to be 114, 92, and 86 mV dec−1 for the fabricated NiMn/Ti-1, NiMn/Ti-2 and NiMn/Ti-3 catalysts, respectively (Figure 4c and Table 4).
Furthermore, Figure 4d and Table 4 illustrate the overpotentials required to attain current densities of 10, 20, and 50 mA cm−2 at 25 °C. Notably, the NiMn/Ti-1 electrocatalyst exhibits relatively low overpotentials of 388, 428, and 537 mV, respectively, to drive current densities of 10, 20, and 50 mA cm−2 in the alkaline natural seawater. These values are superior to those of the NiMn/Ti-2 (η10 = 406 mV, η20 = 452 mV, and η50 = 562 mV) and NiMn/Ti-3 (η10 = 446 mV, η20 = 492 mV, and η50 = 603 mV) electrocatalysts.
The abovementioned data suggest that the as-synthesized NiMn bimetallic electrocatalyst, especially NiMn/Ti-1, exhibits promising OER performance, comparable to that of recent non-precious metal-based OER catalysts in alkaline media and simulated/alkaline seawater (Table 5).
Furthermore, the ECSA of the NiMn/Ti-1 electrocatalyst was determined from the measurements of double layer capacitance (Cdl) by recording CV curves in the 1 M KOH solution at various scan rates (5–50 mVs−1) in the non-faradaic region (Figure 5a).
The plot of the charging current against the scan rate is depicted in Figure 5b. The Cdl for NiMnTi-1 was determined to be 1.17 mF, which yielded an estimated ECSA value of 29.25 cm2. The roughness factor Rf for the NiMnTi-1 electrocatalyst is approximately 15, which indicates a high level of catalyst activity. Moreover, the Rf value obtained indicates that the actual surface area of the catalyst is 15 times higher than its geometric surface area. This increased surface area furnishes a larger number of active sites for the OER. As there are more sites where OER can occur simultaneously, an increased number of active sites can facilitate higher reaction rates. This can result in a higher current density for a given potential. Furthermore, with a larger number of active sites, the catalyst can operate with greater efficiency, potentially reducing the overpotential required for the OER to occur. This makes the process more energy efficient.

3.4. Electrocatalytic Stability Investigations for OER

An additional crucial factor to consider is the electrocatalytic stability of a catalyst, as this determines its practical utility and applicability. In the case of seawater splitting OER electrocatalysts, it is essential that the electrocatalytic activity is sustainable, given the presence of abundant corrosive Cl− ions and competitive CER. Accordingly, the stability evaluation of the NiMn/Ti-1 catalyst was assessed by long-time operation assessments in both ASW and SSW electrolytes. This was conducted using the CP method at a fixed current density of 10 mA cm−2 and the CA at a constant potential of 1.81 V at 25 °C for 24 h. The findings showed that the NiMn/Ti-1 electrocatalyst possesses superior long-lasting electrocatalytic stability in both investigating electrolytes.
Figure 6a demonstrates the electrocatalytic stability of the NiMn/Ti-1 catalyst by chronopotentiometric analysis at a constant current density of 10 mA cm−2 for 24 h. After one hour of operation in both working electrolytes, the recorded potential remained almost constant throughout the progression of the OER process, exhibiting a marginal increase of only 34 mV in SSW and 40 mV in ASW. The remarkable stability of the synthesized NiMn/Ti-1 catalyst ensures enhanced structural integrity and significant chloride ion-resistivity, thereby conferring anti-corrosion properties that are beneficial for electrolysis of seawater.
Furthermore, the catalytic stability of the as-synthesized NiMn/Ti-1 catalyst was also determined by the chronoamperometry at a constant potential of 1.81 V in SSW and ASW. As illustrated in Figure 6b, the NiMn/Ti-1 electrode exhibited an excellent current retention of ca. 100% in SSW, while displaying a relatively modest deviation in current density in ASW. Following a 24-h period of continuous OER electrolysis, a decline of approximately 15% in current density was monitored in comparison to the initial reading of the current density after one hour. The intrinsically stable electrochemical properties of the materials ensure the sustained performance of the OER after the completion of the stability tests. A higher roughness factor can contribute to the enhanced stability of the electrode by distributing stresses more evenly, enhancing the adherence of catalytic layers, and potentially aiding in the management of gas bubbles.

3.5. Performance of Overall Alkaline Seawater Splitting

In addition to exhibiting reasonable OER efficiency and outstanding long-time durability in both SSW and ASW, we conclude that the NiMn/Ti-1 electrocatalyst can be used as an effective and durable OER catalyst for OWS in seawater. Therefore, the optimal NiMn/Ti-1 electrocatalyst should be further investigated as an anode material for OWS in alkaline natural seawater solution. For the overall seawater-splitting performance, a two-electrode system was used, as illustrated schematically in Figure 7a. An electrolytic cell containing natural seawater and 1.0 M KOH was assembled with a synthesized NiMn/Ti-1 electrode together with our previously reported efficient NiMn/Ti-5 HER electrocatalyst [100] as the anode and cathode, respectively (Figure 7b). For comparison purposes, a two-electrode seawater electrolyzer was constructed using a fabricated NiMn/Ti-1 electrode and a Pt sheet (~2 cm2) as the anode and cathode, respectively. It is noteworthy that the electrolyzer assembly, comprising the NiMn/Ti-5(−)||NiMn/Ti-1(+) catalysts, required a cell voltage of only 1.619 V to operate at a current density of 10 mA cm−2 at 25 °C. This voltage was 0.075 V lower than that required by the electrolyzer employing Pt as the cathode: Pt(−)||NiMn/Ti-1(+) (Figure 7c). It is evident that the cell voltage of the NiMn/Ti-5(−)||NiMn/Ti-1(+) seawater electrolyzer is competitive with other recently reported non-precious metal-based materials for OWS performance in SSW/ASW, as illustrated in Figure 7d. The long-time stability of the electrolyzer NiMn/Ti-5(−)||NiMn/Ti-1(+) was also investigated in ASW. It was observed that the recorded potential remained relatively stable for the duration of the 10-h testing period (Figure 7e).
A comparative analysis of the overall seawater splitting performance using the two-electrode seawater electrolyzer reported here shows that the composition of the synthesized bimetallic electrolyzer assembly as anode and cathode has great potential for the technology of seawater splitting (Table 6).

4. Conclusions

In conclusion, we successfully synthesized 3D NiMn/Ti electrocatalysts for stable oxygen evolution reactions by seawater splitting via a one-step and facile electrochemical deposition approach. The surface morphology of the NiMn/Ti-1 electrode exhibits a unique tiny nodule-like architecture covering the substrate, while the as-fabricated NiMn/Ti-3 electrocatalyst exhibits surface roughness with flake-like heterostructures. The optimal NiMn/Ti-1 electrocatalyst displayed excellent durability and quite reasonable OER activity in chloride ion-rich simulated seawater and alkaline seawater media. Moreover, a two-electrode seawater electrolyzer assembled with the as-prepared optimal NiMn/Ti-1 electrode and with an efficient NiMn/Ti-5 electrode, as the anode and cathode, respectively, requires only 1.619 V cell voltage to achieve a current density of 10 mA cm−2 at 25 °C. This NiMn/Ti-5(−)||NiMn/Ti-1(+) bimetallic seawater electrolyzer assembly was also subjected to a long-term durability study in ASW, and it demonstrated outstanding stability with no notable change in potential.

Author Contributions

Conceptualization, A.B. and E.N.; methodology, S.B. and J.V; formal analysis, S.B.; investigation, S.B., D.U. and. J.V.; data curation, L.T.-T.; writing—original draft preparation, S.B.; writing—review and editing, E.N. and A.B.; visualization, L.T.-T.; supervision, A.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The all authors declare no conflicts of interests.

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Figure 1. Illustration of the fabrication approach of NiMn/Ti catalysts (a) and the SEM mapping images of NiMn/Ti-1 (b), NiMn/Ti-2 (c), and NiMn/Ti-3 (d) catalysts.
Figure 1. Illustration of the fabrication approach of NiMn/Ti catalysts (a) and the SEM mapping images of NiMn/Ti-1 (b), NiMn/Ti-2 (c), and NiMn/Ti-3 (d) catalysts.
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Figure 2. XRD patterns of NiMn/Ti-1. The symbols show the positions of the XRD peaks of Ti (COD. No. 9016190), Ni (COD No. 2102269), MnO (COD No. 1010898), and Mn2NiO4 (COD No. 1530384).
Figure 2. XRD patterns of NiMn/Ti-1. The symbols show the positions of the XRD peaks of Ti (COD. No. 9016190), Ni (COD No. 2102269), MnO (COD No. 1010898), and Mn2NiO4 (COD No. 1530384).
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Figure 3. The OER polarization curves recorded on NiMn/Ti catalysts in SSW at 10 mV s−1 and a temperature range from 25 to 75 °C (a), LSVs obtained at 25 °C (b), the corresponding Tafel plots (c), and the overpotential values needed to achieve 10, 20, and 50 mA cm−2 current densities (d).
Figure 3. The OER polarization curves recorded on NiMn/Ti catalysts in SSW at 10 mV s−1 and a temperature range from 25 to 75 °C (a), LSVs obtained at 25 °C (b), the corresponding Tafel plots (c), and the overpotential values needed to achieve 10, 20, and 50 mA cm−2 current densities (d).
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Figure 4. The OER LSVs recorded on NiMn/Ti catalysts in ASW at 10 mV s−1 and a temperature range of 25 to 75 °C (a), LSVs recorded at 25 °C (b), the corresponding Tafel plots (c), and the overpotential values needed to reach 10, 20, and 50 mA cm−2 current densities (d).
Figure 4. The OER LSVs recorded on NiMn/Ti catalysts in ASW at 10 mV s−1 and a temperature range of 25 to 75 °C (a), LSVs recorded at 25 °C (b), the corresponding Tafel plots (c), and the overpotential values needed to reach 10, 20, and 50 mA cm−2 current densities (d).
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Figure 5. (a) CVs of NiMnTi-1 collected in N2-deaerated 1 M KOH solution at varied scan rates. (b) Dependence of charging current on scan rate.
Figure 5. (a) CVs of NiMnTi-1 collected in N2-deaerated 1 M KOH solution at varied scan rates. (b) Dependence of charging current on scan rate.
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Figure 6. The chronopotentiometric (a) and chronoamperometric (b) curves of NiMn/Ti-1 in simulated seawater and alkaline natural seawater at a static current density of 10 mA cm−2 and a constant potential of 1.81 V for 24 h, respectively.
Figure 6. The chronopotentiometric (a) and chronoamperometric (b) curves of NiMn/Ti-1 in simulated seawater and alkaline natural seawater at a static current density of 10 mA cm−2 and a constant potential of 1.81 V for 24 h, respectively.
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Figure 7. A schematic representation of the overall alkaline seawater splitting (a), a digital photograph of the assembled two-electrode NiMn/Ti-5(−)||NiMn/Ti-1(+) cell (left) and the bubbles on the electrodes (right) (b). LSVs for the overall alkaline seawater splitting at 25 °C (c). A comparison of activity of the assembled electrolyzer with that of recently reported catalysts (d) and a long-time durability test of the assembled NiMn/Ti-5(−)||NiMn/Ti-1(+) electrolyzer (e) for the overall alkaline natural seawater splitting.
Figure 7. A schematic representation of the overall alkaline seawater splitting (a), a digital photograph of the assembled two-electrode NiMn/Ti-5(−)||NiMn/Ti-1(+) cell (left) and the bubbles on the electrodes (right) (b). LSVs for the overall alkaline seawater splitting at 25 °C (c). A comparison of activity of the assembled electrolyzer with that of recently reported catalysts (d) and a long-time durability test of the assembled NiMn/Ti-5(−)||NiMn/Ti-1(+) electrolyzer (e) for the overall alkaline natural seawater splitting.
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Table 1. The plating bath composition, along with the associated parameters of the plating condition.
Table 1. The plating bath composition, along with the associated parameters of the plating condition.
CatalystConcentration (M) Plating Conditions
NiSO4MnCl2(NH4)2SO4H3BO3pH *T (°C)j (mA cm−2)t (min)
NiMn/Ti-1
NiMn/Ti-2
NiMn/Ti-3		0.2
0.2
0.2		0.2
0.4
0.6		0.5
0.5
0.5		0.3
0.3
0.3		~12550
500		3
3
* 1.5 M H2SO4 + 1 M HCl.
Table 2. Metal loadings of various catalysts analyzed by ICP–OES.
Table 2. Metal loadings of various catalysts analyzed by ICP–OES.
CatalystNi Loadings
(µgNicm−2)		Mn Loadings
(µgMncm−2)		Total Metal Loading (µgmetalcm−2)Wt%
NiMn
NiMn/Ti-186.5513.4399.9886.5613.44
NiMn/Ti-2126.440.55166.9575.7124.29
NiMn/Ti-3269.7105.25374.9571.9328.07
Table 3. Summary of electrochemical data of the catalysts under investigation for OER in SSW.
Table 3. Summary of electrochemical data of the catalysts under investigation for OER in SSW.
Catalystsj at 2.06 V (mA cm−2)η10 at 25 °C (mV)Tafel Slope
(mV dec−1)
25 °C35 °C45 °C55 °C65 °C75 °C
NiMn/Ti-1182.57203.48231.38250.69269.53286.1386130
NiMn/Ti-2144.36158.77177.15195.43210.07232.39413120
NiMn/Ti-3127.68141.57164.31182.45197.2219.2145486
Table 4. Summary of electrochemical data of the catalysts under investigation for OER in ASW.
Table 4. Summary of electrochemical data of the catalysts under investigation for OER in ASW.
Catalystsj at 2.06 V(mA cm−2)η10 at 25 °C(mV)Tafel Slope
(mV dec−1)
25 °C35 °C45 °C55 °C65 °C75 °C
NiMn/Ti-1141.97151.05177.2202.47223.75248.18388114
NiMn/Ti-2132.92147.65165.8184.67200.39219.4540692
NiMn/Ti-3123.3136.9156.51177.4195213.2344686
Table 5. Comparison of the performances of OER electrocatalysts in neutral and simulated/alkaline seawater electrolytes reported in the literature.
Table 5. Comparison of the performances of OER electrocatalysts in neutral and simulated/alkaline seawater electrolytes reported in the literature.
Catalystsη10 (mV) Tafel Slope
(mV dec−1)		ElectrolytesRef.
NiMn/Ti-1386
388		130
114		1 M KOH + 0.5 M NaCl
1 M KOH + Seawater		This work
Co-CoO@C
(denoted as ZIF67-600Ar/GF)		374–1 M KOH + Seawater[90]
oct_Cu2O-NF354901 M KOH + 0.5 M NaCl[91]
CoSe/MoSe2/NF350–1 M KOH + Seawater[92]
FTO/NiO340-1 M KOH + 0.5 M NaCl[93]
CoFe-LDH530–Simulated seawater
(pH 8.0)		[94]
Pb2Ru2O7−x500~48Natural simulated seawater (only 0.6M NaCl)[95]
Co3O4
Co3−xPdxO4		440
370		–1 M PBS + 0.5 M NaCl[96]
CaFeOx|FePO4~710–Phosphate-buffered
(0.5 M, pH 7) seawater		[97]
Co(OH)3Cl379–1 M KOH + 0.6 M NaCl[98]
ER-RP/P-SNCF-5332
346		–1 M KOH + 0.5 M NaCl
1 M KOH + Seawater		[99]
Table 6. Comparison of the performance of the two-electrode seawater electrolyzers assembled with the non-precious metal electrodes reported in the literature for SSW/ASW and natural seawater.
Table 6. Comparison of the performance of the two-electrode seawater electrolyzers assembled with the non-precious metal electrodes reported in the literature for SSW/ASW and natural seawater.
Electrodes (−||+)Potential@Current Density
(V@mA cm−2)		ElectrolyteRef.
NiMn/Ti-5||NiMn/Ti-1
Pt||NiMnTi-1		1.619@10
1.694@10		1 M KOH + SeawaterThis work
FeNiCoMnRu@CNT (−||+)1.6@101 M KOH + Seawater[56]
Pt/C/GF||ZIF67-600Ar/GF1.63@201 M KOH + Seawater[90]
oct_Cu2O-NF||oct_Cu2O-NF1.71@101 M KOH + 0.5 M NaCl[91]
CoSe/MoSe2/NF||CoSe/MoSe2/NF1.69@10
1.77@10		1 M KOH + 0.5 M NaCl
1 M KOH + Seawater		[92]
NiMoSe@CC||NiMoSe@CC1.63@101 M KOH + 0.5 M NaCl[101]
3%Er-MoO2||3%Er-MoO21.67@101.0 M KOH + 3.5% NaCl[102]
Ir0.05-Co2P/Co2P2O7 NW/NF (−||+)1.6@10
1.67@10		1 M KOH + Seawater
Seawater		[103]
CdFe-BDC||CdFe-BDC1.68@10Seawater[104]
Co-Ni-S/NF||Co-Ni-S/NF1.67@101 M KOH + 0.5 M NaCl[105]
NiFeOOH||(Co,Fe)PO41.625@101 M KOH + Seawater[106]
Cu8S5/NSC-900||Cu8S5/NSC-9001.58@10
1.65@10		1 M KOH + 0.5 M NaCl
1 M KOH + Seawater		[107]
NiCoN|NixP|NiCoN||S-(Ni,Fe)OOH1.81@10Seawater[108]
Mn-doped Ni2P/Fe2P (−||+)1.64@101 M KOH + 0.5 M NaCl[109]
RNPOH/NF||RNPOH/NF1.75@501 M KOH + Seawater[110]
Mo-CoPx/NF||Mo-CoPx/NF1.59@10
1.61@10		1 M KOH + 0.5 M NaCl
1 M KOH + Seawater		[111]
CoF-2||CoF-31.72@10
1.76@10		1 M KOH + 0.6 M NaCl
1 M KOH + Seawater		[112]
Ni-SN@C||Ni-SN@C1.72@101 M KOH + Seawater[113]
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Barua, S.; Balčiūnaitė, A.; Upskuvienė, D.; Vaičiūnienė, J.; Tamašauskaitė-Tamašiūnaitė, L.; Norkus, E. Bimetallic Ni–Mn Electrocatalysts for Stable Oxygen Evolution Reaction in Simulated/Alkaline Seawater and Overall Performance in the Splitting of Alkaline Seawater. Coatings 2024, 14, 1074. https://doi.org/10.3390/coatings14081074

AMA Style

Barua S, Balčiūnaitė A, Upskuvienė D, Vaičiūnienė J, Tamašauskaitė-Tamašiūnaitė L, Norkus E. Bimetallic Ni–Mn Electrocatalysts for Stable Oxygen Evolution Reaction in Simulated/Alkaline Seawater and Overall Performance in the Splitting of Alkaline Seawater. Coatings. 2024; 14(8):1074. https://doi.org/10.3390/coatings14081074

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Barua, Sukomol, Aldona Balčiūnaitė, Daina Upskuvienė, Jūrate Vaičiūnienė, Loreta Tamašauskaitė-Tamašiūnaitė, and Eugenijus Norkus. 2024. "Bimetallic Ni–Mn Electrocatalysts for Stable Oxygen Evolution Reaction in Simulated/Alkaline Seawater and Overall Performance in the Splitting of Alkaline Seawater" Coatings 14, no. 8: 1074. https://doi.org/10.3390/coatings14081074

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