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Article

Highly Efficient Cobalt Sulfide Heterostructures Fabricated on Nickel Foam Electrodes for Oxygen Evolution Reaction in Alkaline Water Electrolysis Cells

by
Ioannis Poimenidis
1,*,
Nikandra Papakosta
2,3,
Panagiotis A. Loukakos
2,
George E. Marnellos
4,5 and
Michalis Konsolakis
1,*
1
Lab of Matter Structure and Laser Physics, School of Production Engineering & Management, Technical University of Crete, 73100 Chania, Crete, Greece
2
Foundation for Research and Technology–Hellas, Institute of Electronic Structure and Laser, 70013 Heraklion, Greece
3
Department of Materials Science and Technology, University of Crete, Vassilika Vouton, 70013 Heraklion, Greece
4
Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
5
Chemical Process and Energy Resources Institute, Centre for Research & Technology Hellas, 57001 Thessaloniki, Greece
*
Authors to whom correspondence should be addressed.
Surfaces 2023, 6(4), 493-508; https://doi.org/10.3390/surfaces6040033
Submission received: 8 October 2023 / Revised: 7 November 2023 / Accepted: 16 November 2023 / Published: 23 November 2023
(This article belongs to the Special Issue Recent Advances in Catalytic Surfaces and Interfaces)
Browse Figures
Versions Notes

Abstract

:
Non-noble metal electrocatalysts for the oxygen evolution reaction (OER) have recently gained particular attention. In the present work, a facile one-step electrodeposition method is applied in situ to synthesize cobalt sulfide nanostructures on nickel foam (NF) electrodes. For the first time, a systematic study is carried out on the impact of the Co/S molar ratio on the structural, morphological, and electrochemical characteristics of Ni-based OER electrodes by employing Co(NO3)2·6 H2O and CH4N2S as Co and S precursors, respectively. The optimum performance was obtained for an equimolar Co:S ratio (1:1), whereas sulfur-rich or Co-rich electrodes resulted in an inferior behavior. In particular, the CoxSy@NF electrode with Co/S (1:1) exhibited the lowest overpotential value at 10 mA cm−2 (0.28 V) and a Tafel slope of 95 mV dec−1, offering, in addition, a high double-layer capacitance (CDL) of 10.7 mF cm−2. Electrochemical impedance spectroscopy (EIS) measurements confirmed the crucial effect of the Co/S ratio on the charge-transfer reaction rate, which is maximized for a Co:S molar ratio of 1:1. Moreover, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) were conducted to gain insights into the impact of the Co/S ratio on the structural and morphological characteristics of the electrodes. Notably, the CoxSy@NF electrocatalyst with an equimolar Co:S ratio presented a 3D flower-like nanosheet morphology, offering an increased electrochemically active surface area (ESCA) and improved OER kinetics.

Graphical Abstract

1. Introduction

The fossil fuel crisis and global climate change have led researchers to pay increased attention to alternative energy sources. Hydrogen has great potential to be employed as an energy carrier for the forthcoming energy transition due to its high gravimetric energy density and zero carbon content [1,2,3,4]. In this regard, the electrochemical splitting of water using energy derived from intermittent renewable energy sources (green Hydrogen), such as photocatalysis, solar thermochemical, photovoltaic electrolysis, and the supercritical water gasification of biomass, has recently gained particular importance [5].
The maturity of the electrochemical water splitting process will highly contribute to the need for green hydrogen production in the years to come, which, according to the International Energy Agency analysis, will be ~150 Mt and ~435 Mt of low-carbon hydrogen production in 2030 and 2045, respectively [5].
However, it is well known that the efficiency of water electrolysis is associated with the overpotentials of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [6,7]. The high overpotential of the OER (4OH− → 2H2O + 4e− + O2 in alkaline medium), due to the demanding transfer of four electrons in the anodic charge transfer reaction, represents the main bottleneck for the limited efficiencies [8,9,10,11]. The state-of-the-art electrodes used in the OER mainly comprise Ir and Ru metals; however, their scarcity and high cost notably hinder their widespread application as electrocatalysts [12,13,14].
Significant research has recently been devoted to noble-metal-free and earth-abundant transition metal electrocatalysts [15]. Various materials, such as borides, chalcogenides, etc., have been explored for the OER. However, most of these candidates suffer from a small active electrochemical area and low electrical conductivity, negatively affecting OER kinetics [15]. Recently, the in situ growth of nanostructured catalytic materials on conductive substrates, such as nickel foam (NF), has proven an efficient approach to overcoming these obstacles [15,16].
In recent years, transition-metal-based electrocatalysts have been developed for water splitting via various techniques, such as aerosol spray [17], photochemical metal–organic deposition [18], hydrothermal processes [19,20,21,22], pulsed laser deposition [23], etc. Among them, transition metal sulfides (TMSs) have been considered excellent alternatives for the OER due to their low cost, adjustable electronic properties, and adequate conductivity, which make them suitable alternatives to the Ru and IrO2 benchmark electrodes [24,25,26].
Although various studies have been conducted using TMSs, especially with CoxSy composites, there is no systematic study on the impact of the sulfur/cobalt molar ratio on the electrochemical performance of electrocatalysts. Nonetheless, the type and concentration of the precursor compounds employed during fabrication are expected to affect the structural, morphological, and electrochemical properties of the electrocatalysts. Furthermore, in most studies, the fabricated electrocatalysts are in the form of powders, rendering the use of binders for the electrocatalyst’s anchor to the substate unavoidable, thus increasing the contact resistance [16,27]. In this regard, developing a facile and binder-free preparation protocol for highly efficient OER electrodes is paramount.
Motivated by the above challenges, the present work aims to investigate the effect of the sulfur/cobalt ratio on the OER. Although several studies have been devoted to cobalt sulfide electrocatalysts, there is no systematic study on the impact of sulfur/cobalt on their physicochemical properties and, in turn, on their OER performance. In this regard, a facile one-step electrodeposition method was used to fabricate CoxSy (x:y molar ratio) binary electrodes on a nickel foam substrate (NF), employing thiourea (CH4N2S) and cobalt nitrate hexahydrate (Co(NO3)2·6 H2O) as precursors. Various Co:S molar ratios were used during the electrodeposition process, and it was clearly disclosed that the Co:S ratio significantly affects the morphological and structural properties of the electrode and its electrochemical performance. Remarkably, the CoxSy@NF electrode with an equimolar Co:S ratio (1:1) presented the lowest overpotential values at 10 mA cm−2 (0.28 V), the lowest Tafel slope (95 mV dec−1), and the highest CDL (10.7 mF cm−2) and ECSA (537 cm2) values. On the other hand, sulfur-rich or Co-rich electrodes resulted in an inferior behavior, demonstrating the key effect of sulfur content on transition metal sulfide electrocatalysts.

2. Materials and Methods

2.1. Materials and Reagents

The chemical reagents in this work were used as received. Thiourea (98% Penta Chemical, Czech Republic-Prague), KOH (technical grade, Sigma-Aldrich, USA-VT-Burlington), ethanol (99.8%, ACROS Organics, Belgium-Antwerp), HCl (98% Sigma Aldrich), Co(NO3)2·6 H2O (Sigma-Aldrich) and nickel foam (99.8%, Beike advanced materials Store, China-Taizhou) were used as the electrodeposition substrate.

2.2. Electrodeposition on NF Substrate

The electrodeposition process was carried out by using an electrochemical station (Princeton Applied Research, USA-TN-Oak Ridge, VersaSTAT 4) equipped with a standard three-electrode electrochemical cell (Palmsens, Netherlands-Utrecht). The reference electrode was an Ag/AgCl electrode (3.5M KCl) (Palmsens), the counter electrode was a Pt wire (99% Goodfellow, USA-UT-Lindon), and the working electrode was an NF substrate.
Prior to the electrodeposition, the NF substrates were ultrasonicated in 3 M HCl for 10 min to activate the substrate and remove any organic impurities. Then, it was rinsed with deionized water, ultrasonicated again for 10 min in pure ethanol, and finally rinsed with deionized water.
The electrodeposition for all the samples lasted 10 min at 298 K, and the applied potential was −0.18 V vs. RHE. For the uniform deposition of the nanomaterials, a magnetic stirrer was used. The molar ratio of the Co(NO3)2·6 H2O and thiourea (CH4N2S) precursors during the synthesis procedure was varied in a wide range (i.e., 0:1, 1:0, 1:2, 1:1, 2:1, 4:1) to obtain both sulfur-rich and Co-rich electrodes. The as-prepared electrodes were defined as Co:S (x:y), where x:y represents the Co:S molar ratio of the CoSx@NF composites, whereas the bare nickel foam substrate was designated as NF. Pure cobalt (Co:S (1:0)) and sulfur (Co:S (0:1)) electrodes on the NF substrate (1 × 1 cm) were also fabricated for comparison purposes. After electrodeposition, a calcination procedure was applied (at 250 °C in an air atmosphere) in order to increase the material’s crystallinity and remove any surface impurities.

2.3. Structural and Morphological Characterization

The morphological characterization of the fabricated electrodes was carried out with field emission scanning electron microscopy (FE-SEM, JSM7000F, JEOL) at various magnification scales. Also, XRD (BEDE D1 with CuKa radiation) and XRF (Amptek X-123) analyses were conducted to gain insights into the structural features of the CoxSy@NF electrodes.

2.4. Electrochemical Evaluation of the Fabricated Electrodes

Electrochemical tests were undertaken for each developed electrode to assess its oxygen evolution reaction kinetics and electrochemical performance under water–alkaline electrolysis conditions. Specifically, cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) measurements were carried out, and the values of the Tafel slope, double layer capacitance (CDL), and ECSA were calculated. The above studies were accomplished using the VersaSTAT 4 electrochemical workstation, which was equipped with a Pt (99%) counter electrode and an Ag/AgCl (3.5 M KCl) reference electrode, and each electrodeposited-fabricated NF electrode acted as a working electrode. The alkaline electrolyte used in all tests was 1 M KOH. Before each measurement, N2 gas was purged into the electrochemical cell to remove any amount of dissolved oxygen gas. All the experiments were repeated at least three times to confirm the obtained results. Moreover, the obtained potential values were converted to a reversible hydrogen electrode (RHE), according to the following equation:
ERHE = EAg/AgCl + 0.059·pH + EoAg/AgCl
where EAg/AgCl is the potential measured with the Ag/AgCl reference electrode.

3. Results and Discussion

3.1. Electrodeposition

The electrochemical reaction rate and diffusion process are the main factors that determine the overall electrodeposition process. Mass transport limitations control the overall process when the charge transfer reaction is fast. On the other hand, if the diffusion rate is faster than the charge transfer reaction rate, the electrochemical reaction controls the process [28].
During the electrodeposition of the precursors used in this work, the following reactions are considered [29,30,31]:
2H2O + 2e− → 2OH− + H2
2Co2+ + 2e− → Co(OH)2(ads)
Co(OH)2(ads) +2e− → Co + 2OH−
CH4N2S + 2OH− → S2− + CH4N2O + H2O
Co2+ + S2− → CoSx (ads)
Moreover, recent studies have introduced a new synthetic route for metal sulfides via the formation of an (NH2)2CSs-M2+-OH− complex, which decomposes to the metal sulfide according to the following process [30,32]:
M2+ + CS(NH2)2 + 2OH− ⇄ M(OH)2CS(NH2)2 → MS + H2NCN + 2H2O

3.2. Structural and Morphological Evaluation

Figure 1 presents the XRD patterns of the NF, Co:S (1:0), Co:S (0:1), Co:S (1:1), Co:S (2:1), Co:S (1:2), Co:S (4:1) electrodes. The XRD analysis revealed three major peaks for the bare NF electrode at the 2θ angles of 45.2°, 52.5°, and 77°, which correspond to the (111), (200), and (220) planes of crystalline Ni, respectively. On the Co-free electrode, i.e., the Co:S (0:1) sample, there was a shift in the major peaks to 44.7°, 52.0°, and 76.5°, which are attributed to the (102), (110), and (202) crystalline planes, respectively, of the NiS phase (JCPDS file No. 02-1280) [33,34]. For the Co:S (x:y) electrodes, containing both Co and S, three major features were identified: at 44.8°, attributed to the (400) plane of the face-centered cubic Co3O4 structure (JCPDS No. 73-1701); 52.10°, ascribed to the (440) plane of the Co9S8 (PDF No.01-086-2273); and at 76.5°, attributed to the (202) crystalline plane of NiS [34,35,36]. No metallic phase of Ni was detected due to the formation of NiS or due to the low content (undetectable) of Ni. The XRF spectra and EDS analysis (Figures S1 and S2) also revealed the presence of sulfur, nickel, and cobalt on the as-prepared electrodes, further verifying the successful formation of CoxSy heterostructures according to the procedure described below (Section 2.2). These heterostructures could be responsible for the enhanced electrical conductivity and supercapacitor performance, as further discussed below on the basis of the electrochemical impedance spectroscopy (EIS) results and in agreement with relevant studies [27]. Also, it is worth noticing that S-treatment (Co:S (0:1) did not result in any structural deformation of the NF substrate, since both samples exhibited similar lattice parameters (Table S1). However, all Co-containing samples obeyed a much higher lattice parameter, indicating the formation of new CoxSy phases and strain in the crystal lattice.
The mean particle size for the as-prepared electrodes was calculated using the Scherrer equation, and the obtained values are summarized in Table 1. The smallest particle size of 27.5 nm was obtained for the Co:S (1:1) electrode. The rest of the fabricated electrodes presented higher crystallite sizes, varying between 28 and 35 nm. Interestingly, the optimum electrochemical behavior (see below) was obtained for the equimolar Co:S (1:1) electrode, probably implying a structure–performance relationship; a smaller particle size could result in an extended active electrochemical zone, facilitating the oxygen evolution reaction, in accordance with relevant studies [37]. This is further discussed below, based on electrochemical and impedance spectroscopy studies.
In Figure 2, the FE-SEM morphological analysis of the CoxSy@NF electrodes is presented. In the S-only electrode (Co:S (0:1)), no obvious modifications compared to the bare NF substrate were obtained (Figure 2). In the Co-rich electrodes (Co:S (2:1) and Co:S (4:1) samples), nanosheets of irregular morphology with dense agglomerations were observed (Figure 2a,b). Similarly, where the sulfur was in excess (Figure 2c), bulky structures were obtained without an apparent formation of nanosheets (Figure 2c). Efficient electro-catalysis requires an easy flow of the involved neutral and charged species toward and from the catalytic sites; thus, the above morphology, with the presence of agglomerates, corroborates well with the inferior performance of the Co-rich and S-rich electrodes for the OER (see below) [15].
Interestingly, in the case of CoxSy@NF electrodes with an equimolar Co:S ratio (Figure 2d–f), the formation of well-tuned 3D flower-like nanosheets was observed. This distinct morphology is most evident in higher magnification images (Figure 2e,f), where the formation of flower-like nanosheets with a vertical orientation and a thickness of ca. 7.0 nm can be observed. These findings imply the crucial effect of the Co:S ratio on the structural and morphological characteristics of the as-prepared CoxSy@NF electrodes, which are expected to affect the overall electrochemical performance. In other words, only the equimolar Co:S ratio results in a distinct nano-architecture of 3D flower-like nanosheets without creating agglomerates, which can notably enhance the ESCA; the as-formed in-plain pores may bring about additional active sites, allowing a better electrolyte flow and faster OER kinetics [37].

3.3. Electrochemical Evaluation

Over the past years, several mechanisms for the OER in alkaline water electrolysis cells have been proposed [38,39]. Generally, the OER proceeds via two different routes: (a) nucleophilic water attack (WNA) or the adsorbate evolution mechanism (AEM), where MOOH (M is the active site) is initially created, and the reaction proceeds via adsorption/desorption, (b) O–O coupling, followed by O2 evolution from the coupling of two metal-oxyl radicals with different variants [24,40,41].
The following reaction scheme Is usually considered for the OER (WNA) in an alkaline medium [24]:
M + OH− → MOH + e−
MOH + OH− → MO + H2O + e−
MO + OH− → MOOH + e− (WNA)
MOOH + OH− → M + O2 + H2O + e− (WNA)
Finally, the overall reaction can be written as follows:
4OH− → O2 + 2H2O + 4e−
Furthermore, recent studies have proposed a new mechanism for the OER based on redox chemistry, defined as a lattice oxidation mechanism (LOM). In this mechanism, the lattice oxygen could be activated at the corresponding potential and could participate in the formation of O–O active intermediates during the OER [42,43,44].
In addition, when transition metal chalcogenides (TMCs), such as S, are combined with transition metals such as Co, a kinetic structural transformation of the CoxSy occurs during the OER [45]. An underlying dissolution of sulfur atoms promotes the oxidation of cobalt ions and facilitates the transformation of CoxSy nanovesicles to Co(OH)2 and then to crystalline CoOOH, initiating the OER [46,47].
In order to explore the OER kinetics of as-prepared electrocatalysts (Figure 3a), the electrochemical polarization curves were obtained in 1 M KOH, with a scan rate of 1 mV s−1 (90% iR compensation) (Figure 3a). The obtained results, summarized in Table 2, clearly show the significant impact of the Co:S ratio on the OER kinetics. When only S is deposited on NF (Co:S (0:1) sample), a significant inhibition is observed, reflected in overpotential. On the other hand, the deposition of bare cobalt over the NF substrate (Co:S (1:0) sample) has a positive effect, resulting in a lower overpotential (Figure 3a, Table 2). Notably, the fabricated electrodes containing both Co and S demonstrate lower overpotential values, implying the synergistic effect of CoxSy@NF composites for an enhanced OER [48]. It is evident that the Co:S (1:1) electrode, with an equimolar Co and S content, exhibits the lowest overpotential value of 0.28 V at a 10 mA cm−2 current density, |η10|, being far lower compared to 0.42 V of bare NF.
At this point, it is also worth noting that the S−doped NF electrodes (Co:S (0:1)), where NiS is mainly detected, exhibit an even inferior performance compared to bare NF, plausibly implying the low OER reactivity of the NiS phase under the present conditions. On the other hand, the Co:S (1:0) electrode, where CoxSy has been detected, exhibits an adequate OER performance, most probably revealing the high reactivity of the CoxSy phase towards the OER reaction. Although these findings cannot exclude the possibility of the synergistic effects between the NixSx and CoxSy phases when both co-exist on the NF surface, they imply the pivotal effect of Co and S coexistence on the improvement in OER performance.
Afterward, the Tafel slope was determined using the linear part of the Tafel plot, according to the following equation:
η = a + b log j
where n is the overpotential value, j is the current density, a is the fitting parameter, and b is the Tafel slope [4,16,49].
The calculated Tafel slopes of the fabricated electrodes (Figure 3b and Figure S2 and Table 2) further strengthen the findings regarding the enhanced electrocatalytic activity of the Co:S (1:1) electrode, presenting the lowest Tafel slope (95 mV dec−1) for the OER. The lowest Tafel slope of the Co:S (1:1) electrode confirms its faster oxygen evolution reaction kinetics, which could be attributed to the electrode’s distinct nano-architecture (Figure 2f), offering the required in-plane pores for a better electrolyte flow and an extension of the active electrocatalytic area for the OER. Moreover, the Co:S (1:1) shows excellent stability in a 30 wt% KOH solution for more than 12 h (Figure S4), demonstrating its potential for practical applications. In addition, the lower overpotential of the Co:S (1:1) at 50 mA cm−2 compared to the other CoxSy electrodes is obvious (Figure S4); this is due to the retainment of flower-like nanosheets on its surface after a prolonged stability test (Figure S5). Also, the almost unchanged XRF spectra of the Co:S (1:1) electrode, before and after the stability test, clearly reveal the stability of the aforementioned electrode (Figure S5). These findings demonstrate the stability of CoxSy-based electrodes in terms of their morphological and compositional characteristics, leading to an enhanced OER performance.
The superior performance of the Co:S (1:1) electrode can be further revealed through a comparison with state-of-the-art CoxSy-containing electrodes over NF substrates, as well as with the benchmark Ru and Ru@IrO2 electrodes used for the OER (Table 3). It is evident that the as-prepared CoxSy@NF electrodes exhibit lower values compared to the Ru and Ru@IrO2 catalysts, but are superior compared to most of the non-noble metal catalysts, offering one of the lowest Tafel slopes despite their simpler composition (monometallic Co electrodes) and facile fabrication procedure (one-step electrodeposition).
The enhanced OER kinetics of the Co:S (1:1) electrodes can be ascribed to the formation of flower-like nanosheets over the NF substrate (Figure 2e,f); these increase the electrochemically active area and are responsible for the high utilization ratio of electrocatalytic active sites [16]. Moreover, the absence of a binder and the direct adherence of Co and S onto the NF substrate may further account for the facile electron transportation and improved electrical conductivity, as supported by EIS experiments (see below).
Moreover, to verify our assumption that the mean particle size is closely linked to the Co/S molar ratio and, in turn, to the electrochemical performance, the Tafel slope and mean grain size are plotted in Figure 4 as a function of the Co/S ratio. Notably, the mean particle size totally coincides with the Tafel slope, implying their interrelation; the small particle size offers more electrochemical active sites, facilitating the OER kinetics. Neither Co-rich nor S-rich electrodes provide the optimum performance, maximized for the equimolar Co:S ratio of 1:1; this is in perfect agreement with the electrochemical performance results (lowest Tafel slope and overpotential values).
The double-layer capacitance (CDL) and electrochemical active surface area (ECSA) were next considered in order to gain insights into the intrinsic electrocatalytic reactivity of the as-prepared electrodes. The CDL, obtained via the cyclic voltammetry method, is proportional to the dependent capacitive current (JDL) in the following equation:
J DL = C DL   ×   v A
where v stands for the scan rate (V s−1) and A for the electrode surface (cm2) [39,59,60].
The following equation can be used to obtain the ESCA values:
ESCA = C DL   ×   1 20
The value 20 μF cm−2 is the CDL value of a perfectly smooth Ni electrode [61,62].
Cyclic voltammograms with scan rates of 5 to 100 mVs−1 (Figure S4) were obtained close to the open circuit potential (±50 mV) of each electrode in a non-Faradaic region to calculate the CDL values [63,64,65] (Figure 5, Table 4). Afterward, the CDL and the ESCA values were calculated by plotting the average JDL vs. scan rate and obtaining the slope.
Cobalt deposition generally increases the CDL and ESCA values compared to the NF reference electrode. On the other hand, sulfur deposition has a detrimental effect. Remarkably, the Co:S (1:1) electrode exhibits the best CDL (10.74 mF cm−2) and ESCA (537 cm2) values, which are about one order of magnitude higher compared to the corresponding values of the NF background electrode. These findings are in line with the optimum structural (smallest particle size) and morphological (highly dispersed 3D nanosheets) characteristics of the Co:S (1:1) electrode, further corroborating the close relationship between the structural–morphological features and electrochemical performance. It is also worth noting that the high CDL values of the CoxSy@NF electrodes imply their potential use as electrodes for electrochemical supercapacitors [66,67].
Electrochemical impedance spectroscopy (EIS) measurements were next carried out to gain insights into the charge transfer and transport processes involved in the OER reaction. EIS was applied in potentiostatic mode between frequencies ranging from 10 kHz to 0.1 Hz, applying a sinusoidal alternating current (AC) potential of 10 mV (RMS). In Figure 6, the Nyquist plots of the as-prepared electrodes are presented.
The intercept point on the Z real axis of the Nyquist plot (Figure 6), at high frequencies, depicts the electrolyte and the internal electrode’s resistance (Rs) [68,69,70]. The gradient line achieved in low frequencies corresponds to the diffusion resistance known as the Warburg element, W [71]. The constant phase element (CPE) represents the double-layer capacitance between solid and ionic solutions [72]. Also, the charge transfer resistance (Rct) reflects the difficulty the charge transfer oxygen evolution reaction has in proceeding. Table 5 summarizes the Rs and Rct values of all fabricated electrodes.
It is apparent that the Co:S (1:1) electrode provides, by far, the lower Rct, resulting in an enhancement in the OER kinetics. Moreover, at higher frequencies, the Co:S (1:1) electrode exhibits the Warburg line (Figure 6), implying a better capacity and improved conductivity [71,73]. It should be noted, however, that no significant differences can be observed between the Rs values of Co-containing samples, implying the crucial role of the active metal phase (Co) in determining the electrode’s resistance. Hence, based on the present results, the excellent electrochemical performance of the Co:S (1:1) electrode could be mainly ascribed to its low Rct, thereby accelerating the charge transfer oxygen evolution reaction.

4. Conclusions

The present work systematically explored the impact of the Co/S molar ratio on the OER kinetics and electrochemical performance of CoxSy@NF electrodes. Thiourea and cobalt nitrate precursors at different stoichiometric concentrations were used to fabricate CoxSy@NF heterostructures through the electrodeposition method.
Notably, it was found that the Co:S ratio profoundly influenced the structure and morphology of the as-prepared electrodes and, in turn, their OER kinetics. The CoxSy@NF electrode with a Co:S ratio of 1:1 exhibited the lowest overpotential value at 10 mA cm−2 (0.28 V) and a Tafel slope of 95 mV dec−1, offering, in addition, a high double-layer capacitance (CDL) of 10.74 mF cm−2. Electrochemical impedance spectroscopy confirmed the key role of the Co:S ratio on charge transfer resistance, which is substantially decreased at a Co:S molar ratio of 1:1. Structural and morphological analysis disclosed that the CoxSy@NF electrocatalyst with an equimolar Co:S ratio presented a 3D flower-like nanosheet morphology, offering the smallest particle size and the highest electrochemical active area, which are both conducive to improving the OER kinetics.
The enhanced electrocatalytic activity, stability, facile synthesis route, and free-of-binders fabrication procedure render CoxSy@NF electrodes a promising earth-abundant, noble metal-free, bifunctional type of electrocatalyst for the OER in alkaline water electrolysis cells.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/surfaces6040033/s1, Figure S1. XRF analysis of the Co:S (1:1) electrode; Figure S2. EDS analysis of the as-prepared Co:S electrodes; Figure S3. Tafel slope of the fabricated electrodes in 1 M KOH electrolyte; Figure S4. Stability test of all the fabricated electrodes; Figure S5. FE-SEM of the Co:S (2:1) (a), Co:S (4:1) (b), Co:S (1:2) (c), Co:S (1:1) after prolonged stability tests. XRF spectra of the Co:S (1:1) electrode before and after the OER stability test; Figure S6. Capacitance double layer measurements with cyclic voltammetry; Figure S7. Eis fitting data and equivalent circuits for the as-prepared electrodes; Table S1. Lattice parameters of the as-prepared electrodes.

Author Contributions

Conceptualization, I.P. and M.K.; methodology, I.P. and M.K.; validation, I.P., M.K. and N.P.; formal analysis, I.P., M.K.; investigation, I.P.; resources, M.K., P.A.L. and G.E.M.; data curation, I.P.; writing—original draft preparation, I.P.; writing—review and editing, I.P., M.K., G.E.M.; visualization, I.P. and M.K.; supervision, M.K., G.E.M. and P.A.L.; project administration, M.K.; funding acquisition, M.K., G.E.M. and P.A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research has received funding from the European Union under grant agreement No 101099717–ECOLEFINS project. Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or European Innovation Council and SMEs Executive Agency (EISMEA) granting authority. Neither the European Union nor the granting authority can be held responsible for them. PL, NP acknowledge financial support from the European Union’s Horizon 2020 research and innovation program under grant agreement no 871124 Laserlab-Europe. The authors would like to acknowledge the HELLAS-CH national infrastructure (MIS 5002735) implemented under “Action for Strengthening Research and Innovation Infrastructures,” funded by the Operational Programme “Competitiveness, Entrepreneurship, and Innovation” (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting the findings of this study are available in the form of Tables/Figures within the article and its Supplementary Materials. The raw data are available from the corresponding authors upon reasonable request.

Acknowledgments

The authors want to acknowledge A. Manousaki for the field emission SEM.

Conflicts of Interest

The authors declare no conflict of interest.

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Surfaces 06 00033 g001
Figure 1. XRD patterns of the bare NF and CoxSy@NF electrodes with different Co:S molar ratios.
Figure 1. XRD patterns of the bare NF and CoxSy@NF electrodes with different Co:S molar ratios.
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Figure 2. FE-SEM of the NF (a), Co:S (0:1) (b), Co:S (2:1) (c), Co:S (4:1) (d), Co:S (1:2) (e), Co:S (1:1) (f), Co:S (1:1) higher magnification scale (g,h).
Figure 2. FE-SEM of the NF (a), Co:S (0:1) (b), Co:S (2:1) (c), Co:S (4:1) (d), Co:S (1:2) (e), Co:S (1:1) (f), Co:S (1:1) higher magnification scale (g,h).
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Figure 3. Electrochemical performance of the NF, Co:S (1:0), Co:S (0:1), Co:S (1:1), Co:S (2:1), Co:S (1:2), Co:S (4:1) for the OER: (a) Linear sweep voltammograms (LSV) at a scan rate of 1 mV s−1 in 1 M KOH; (b) Tafel plots of the fabricated electrodes corresponding to the LSV curves.
Figure 3. Electrochemical performance of the NF, Co:S (1:0), Co:S (0:1), Co:S (1:1), Co:S (2:1), Co:S (1:2), Co:S (4:1) for the OER: (a) Linear sweep voltammograms (LSV) at a scan rate of 1 mV s−1 in 1 M KOH; (b) Tafel plots of the fabricated electrodes corresponding to the LSV curves.
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Figure 4. Co:S molar ratio vs. mean particle size and Tafel slope of the Co:S (1:1), Co:S (2:1), Co:S (1:2), Co:S (4:1) electrodes.
Figure 4. Co:S molar ratio vs. mean particle size and Tafel slope of the Co:S (1:1), Co:S (2:1), Co:S (1:2), Co:S (4:1) electrodes.
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Figure 5. Average absolute current density vs. scan rate for CDL estimation of the NF, Co:S (1:0), Co:S (0:1), Co:S (1:1), Co:S (2:1), Co:S (1:2), and Co:S (4:1) electrodes, at scan rates of 0.005 V s−1–0.1 V s−1.
Figure 5. Average absolute current density vs. scan rate for CDL estimation of the NF, Co:S (1:0), Co:S (0:1), Co:S (1:1), Co:S (2:1), Co:S (1:2), and Co:S (4:1) electrodes, at scan rates of 0.005 V s−1–0.1 V s−1.
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Figure 6. Zoomed in Nyquist plot of Co:S (1:0), Co:S (0:1), Co:S (1:2), Co:S (2:1), Co:S (4:1), and Co:S (1:1) electrodes in 1M KOH. Insets: zoomed out Nyquist plot.
Figure 6. Zoomed in Nyquist plot of Co:S (1:0), Co:S (0:1), Co:S (1:2), Co:S (2:1), Co:S (4:1), and Co:S (1:1) electrodes in 1M KOH. Insets: zoomed out Nyquist plot.
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Table 1. Mean particle size for the NF and CoxSy@ NF electrodes with different Co:S molar ratios.
Table 1. Mean particle size for the NF and CoxSy@ NF electrodes with different Co:S molar ratios.
ElectrodeMean Particle Size (nm)
NF35.3
Co:S (1:0)28.1
Co:S (0:1)31.5
Co:S (1:2)30.1
Co:S (2:1)29.2
Co:S (4:1)32.2
Co:S (1:1)27.5
Table 2. Overpotential values (|η10|) and Tafel slope in 1M KOH of all the fabricated electrodes.
Table 2. Overpotential values (|η10|) and Tafel slope in 1M KOH of all the fabricated electrodes.
Electrode|η10| (V)Tafel Slope (mV dec−1)
NF0.42134 ± 0.01
Co:S (1:0)0.31106 ± 0.7
Co:S (0:1)0.53135 ± 0.7
Co:S (1:2)0.32113 ± 0.6
Co:S (2:1)0.34110 ± 0.4
Co:S (4:1)0.3127 ± 0.7
Co:S (1:1)0.2895 ± 0.3
Table 3. Comparison with state-of-the-art Co- and S-containing electrodes for the OER in alkaline medium.
Table 3. Comparison with state-of-the-art Co- and S-containing electrodes for the OER in alkaline medium.
ElectrodesMethodTafel Slope
mV dec−1		ElectrolyteReference
NiCo2S4/Ni3S2Hydrothermal1371 M KOH[50]
NiCo-LDHs Hydrothermal 118 1 M KOH[51]
Co-Ni3S2/NF Hydrothermal method–liquid-phase vulcanization 1761 M KOH[52]
CoNiSx/NF Sulfuration process 1071 M KOH[53]
NiCo2S4/NF Hydrothermal 95 1 M KOH[54]
Co9S8 NM/NF Hydrothermal1501 M KOH[55]
NiCo2S4-NF Hydrothermal 91 1 M NaOH[56]
Ru nanoparticles Laser-generated 70 0.5 M H2SO4[57]
Ru@IrOxCharge redistribution 69 0.05 M H2SO4[58]
Co:S (1:1)@NFElectrodeposition951 M KOHThis work
Table 4. Double-layer capacitance and ECSA values for the as-synthesized electrodes.
Table 4. Double-layer capacitance and ECSA values for the as-synthesized electrodes.
ElectrodeCDL Value (mF cm−2)ECSA (cm2)
NF0.76 ± 0.1338 ± 0.5
Co:S (1:0)1.96 ± 0.1398 ± 0.4
Co:S (0:1)0.26 ± 0.0113 ± 0.3
Co:S (1:2)1.67 ± 0.0983.5 ± 0.5
Co:S (2:1)2.16 ± 0.23108 ± 0.8
Co:S (4:1)1.78 ± 0.1989 ± 0.4
Co:S (1:1)10.74 ± 0.71537 ± 1.1
Table 5. Charge transfer resistance (Rct) and electronic resistance Rs of Co:S (1:0), Co:S (0:1), Co:S (1:2), Co:S (2:1), Co:S (4:1), and Co:S (1:1) electrodes.
Table 5. Charge transfer resistance (Rct) and electronic resistance Rs of Co:S (1:0), Co:S (0:1), Co:S (1:2), Co:S (2:1), Co:S (4:1), and Co:S (1:1) electrodes.
ElectrodeRs (Ω)Rct (Ω)
Co:S (1:0)2.1 ± 0.00339.4 ± 9.5
Co:S (0:1)2.8 ± 0.004155,566 ± 3449
Co:S (1:2)2.5 ± 0.0041.9 ± 0.2
Co:S (2:1)2 ± 0.009521 ± 7
Co:S (4:1)2.3 ± 0.010.75 ± 0.03
Co:S (1:1)2.2 ± 0.0080.000006 ± 0.008
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Poimenidis, I.; Papakosta, N.; Loukakos, P.A.; Marnellos, G.E.; Konsolakis, M. Highly Efficient Cobalt Sulfide Heterostructures Fabricated on Nickel Foam Electrodes for Oxygen Evolution Reaction in Alkaline Water Electrolysis Cells. Surfaces 2023, 6, 493-508. https://doi.org/10.3390/surfaces6040033

AMA Style

Poimenidis I, Papakosta N, Loukakos PA, Marnellos GE, Konsolakis M. Highly Efficient Cobalt Sulfide Heterostructures Fabricated on Nickel Foam Electrodes for Oxygen Evolution Reaction in Alkaline Water Electrolysis Cells. Surfaces. 2023; 6(4):493-508. https://doi.org/10.3390/surfaces6040033

Chicago/Turabian Style

Poimenidis, Ioannis, Nikandra Papakosta, Panagiotis A. Loukakos, George E. Marnellos, and Michalis Konsolakis. 2023. "Highly Efficient Cobalt Sulfide Heterostructures Fabricated on Nickel Foam Electrodes for Oxygen Evolution Reaction in Alkaline Water Electrolysis Cells" Surfaces 6, no. 4: 493-508. https://doi.org/10.3390/surfaces6040033

APA Style

Poimenidis, I., Papakosta, N., Loukakos, P. A., Marnellos, G. E., & Konsolakis, M. (2023). Highly Efficient Cobalt Sulfide Heterostructures Fabricated on Nickel Foam Electrodes for Oxygen Evolution Reaction in Alkaline Water Electrolysis Cells. Surfaces, 6(4), 493-508. https://doi.org/10.3390/surfaces6040033

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