Iron-containing Nickel Cobalt Sulfides, Selenides and a Sulfoselenide as Active and Stable Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Solution

1. Introduction

According to the US Energy Information Administration (EIA) report, the total world energy consumption will rise to 815 quadrillion Btu (British thermal units) in 2040, a 29% increase compared to 2020 [1] , demonstrating the increasing demand for energy in the near future. Using clean and renewable energy is one of the main issues for societies. Green hydrogen (H₂), produced with renewable energy, is seen as an alternative fuel and energy storage resource in the future [2]. Water splitting is one of the most studied ways to produce H₂. With electrocatalysis, this process includes the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) [3]. The anodic reaction (OER) involves a sluggish four-electron-proton coupled transfer reaction. It is the main obstacle to an economic water-splitting process since it requires a much higher potential (1.6-2 V vs. the reversible hydrogen electrode, RHE) than the theoretical equilibrium potential of E° = 1.23 V vs. RHE [4,5,6,7,8].

The well-known benchmark materials for OER are the oxides of iridium and ruthenium (IrO₂ and RuO₂), while Pt-based materials are used as benchmarks for HER [9]. However, the high cost, scarcity, and low stability of these precious noble metals limit their practical large-scale application for water electrolysis [10,11]. Sustainable water splitting requires utilizing non-precious metals as a catalyst. Using non-precious metals with high abundance, durability, and catalytic activity, especially in OER, can effectively improve the scalability of electrocatalytic hydrogen production. Therefore, in recent years, there has been an impetus to develop high-performance, stable, and low-cost (non-noble) transition metal-based electrocatalysts such as transition metal sulfides, hydroxides, oxides, oxide-hydroxides, phosphides, and nitrides for OER [12,13,14,15,16,17,18].

Transition metal sulfides possess good conductivity and excellent mechanical and thermal stability, making them promising electrocatalysts for OER and HER and the oxygen reduction reaction (ORR) [19]. Moreover, mixed-metal sulfides, MMSs, show even higher electric conductivity and richer redox reactions through the synergistic effect of multi-transition metal ions, leading to a notable enhancement in electrocatalytic performance compared to monometallic sulfides [20,21].

Bimetallic nickel cobalt sulfide is one of the most studied and promising MMSs, extensively studied for electrocatalytic energy conversion and storage devices [22]. For example, the thiospinel NiCo₂S₄ with Ni(II) on tetrahedral (T_d) sites and Co(III) ions on octahedral (O_h) sites receives much attention in many electrocatalysis applications, including water splitting, supercapacitors, and zinc-air batteries [23,24,25,26]. However, the electrocatalytic activity and stability of NiCo₂S₄ remain lower than IrO₂ and RuO₂ in OER and lower than Pt-based catalysts for HER [27]. To improve the electrocatalytic performance of nickel cobalt sulfides, researchers have applied many different strategies, including making composites with carbon materials [28,29,30], metal oxides [31,32], oxyhydroxides [24], layer double hydroxide [28], metal sulfides [33,34], incorporating other active metals such as Ru, Ir, and Pt in the structure [35,36] and also partially replacing sulfur ions with nitrogen [37], phosphorous [38] or selenium ions [39].

The improved electrocatalytic performance of nickel cobalt sulfide in the presence of anions such as Se and P can be attributed to effectively altering the surface electron density by modulating the compound's d-band [36,40,41]. Increased electrocatalytic activity in the presence of other cations is mainly related to improving exposed active sites, reducing the charge transfer resistance, enhancing structural stability, and synergistic interactions between host and guest cations [42,43].

Although replacing anions or cations is widely used to upgrade the water-splitting performance of nickel cobalt sulfides, only a few reports applied a simultaneous cation and anion replacement to evaluate a possible synergistic effect. Recently, Deng et al. synthesized the polymetallic sulfoselenide, Co_0.31Ni_0.22Ru_0.05S_0.46Se_0.41, which showed an overpotential of η₁₀ = 261 mV (1.491 V vs. RHE) at 10 mA cm⁻², while RuO₂ needed η₁₀ = 254 mV (1.484 V vs. RHE) at the same condition [44]. However, ruthenium limits its classification as a completely non-precious metal electrocatalyst.

Here we establish a facile two-step hydrothermal strategy to synthesize iron-containing nickel cobalt sulfides, selenides and sulfoselenide as non-precious polymetallic compounds. A series of mixed-metal iron nickel cobalt carbonate hydroxide hydrates, (Fe_xNi_1-x)CoCH-(y), were synthesized and used as a precursor for sulfurization, selenization, and sulfoselenization.

2. Materials and Methods

2.1. Materials

All commercial chemicals were used as received without any purification. Cobalt chloride hexahydrate CoCl₂·6H₂O (CAS: 7791-13-1 98% abcr GmbH, Karlsruhe, Germany), nickel chloride hexahydrate NiCl₂·6H₂O (CAS: 7791-20-0 98%, ACS reagent, Roth, Karlsruhe, Germany), ammonium iron(II) sulfate hexahydrate (NH₄)₂Fe(SO₄)₂·6H₂O (CAS: 7783-85-9 BioUltra, 99% Sigma Aldrich, St. Louis, MO, USA), sodium sulfide nonahydrate Na₂S·9H₂O (CAS: 1313-84-4 98% Acros Organics, New Dehli, India), selenium dioxide SeO₂ (CAS: 7446-08-4 98%, Sigma Aldrich, St. Louis, MO, USA ), urea OC(NH₂)₂ (CAS: 57-13-6 ACS reagent 99.5%, Sigma Aldrich, St. Louis, MO, USA ), potassium hydroxide KOH (CAS: 1310-58-3,1N, Roth, Karlsruhe, Germany), N-methyl-2-pyrrolidone NMP (CAS: 872-32-2 95%, abcr, Karlsruhe, Germany), hydrazine monohydrate N₂H₄·H₂O (CAS: 7803-57-8, Thermo Scientific, Kandel, Germany), polyvinylidene fluoride PVDF (CAS: 24937-79-9, Sigma Aldrich, Karlsruhe, Germany ), carbon black Vulcan XC-72R (Fuelcellstore, Bryan , TX, USA). Nickel foam NF was purchased from Recemat BV, Cell Material Engineering, The Netherlands. Ultrapure water was produced using the Sartorius Arium mini device. Before using NF, it was cut into 1×2 cm² pieces and cleaned with acetone (CAS: 67-64-1, ACS reagent, ≥99.5%, Sigma Aldrich, St. Louis, MO, USA), hydrochloric acid (CAS: 7647-01-0, 1 mol L^–1, Geel, Belgium), ultrapure water, and ethanol (CAS: 64-17-5, 98%, Th. Geyer, Renningen, Germany) for 15 min in an ultrasonic bath and then dried at 100 °C in a vacuum oven for 15 min.

2.2. Preparation of nickel cobalt carbonate hydroxide (NiCoCH) and iron nickel cobalt carbonate hydroxide (Fe_xNi_1–x)CoCH-(y) precursors

The NiCoCH sample was prepared according to the previously reported method by Chen et al. [20]. The amount of 950 mg (4.00 mmol) CoCl₂·H₂O, 475 mg (2.00 mmol) NiCl₂·6H₂O, and 1.1 g (18 mmol) urea was added to 40 mL of ultrapure water and stirred for 10 min. Then the obtained solution was transferred into a Teflon-lined stainless-steel autoclave and heated to 120 °C for 6 h. The product was washed five times with ultrapure water (50 mL each) and two times with ethanol (25 mL each), then dried in a vacuum oven at 60 °C overnight. Yield = 860 mg. (Fe_xNi_1–x)CoCH-(y) precursors were synthesized by the same method except that the amounts of 98, 196, 294, or 392 mg (0.25, 0.50, 0.75, 1.0 mmol) of (NH₄)₂Fe(SO₄)₂·6H₂O were added. Yields = 875 mg, 900 mg, 917 mg, and 930 mg, respectively.

Hereafter (Fe_xNi_1–x)CoCH-(y) was used to refer to the iron-containing nickel cobalt carbonate hydroxide with y = 0.25, 0.50, 0.75, and 1.0 mmol of (NH₄)₂Fe(SO₄)₂·6H₂O added to the reaction mixture, (Fe_xNi_1–x)CoCH-(y) refers to all samples.

2.3. Preparation of iron nickel cobalt sulfides, selenide, and sulfoselenide

Iron-containing nickel cobalt sulfide samples were prepared by hydrothermal sulfidation of the (Fe_xNi_1–x)CoCH-(y) precursors. A chosen amount of iron-containing precursor (125 mg) was dispersed in 40 mL of ultrapure water in an 80 mL Teflon-lined autoclave and stirred for 20 min. After that, 750 mg (3.125 mmol) of Na₂S·9H₂O was added. The resultant suspension was transferred to the oven, and the temperature was kept at 160 °C for 12 h. The obtained product was washed five times with ultrapure water (50 mL each) and two times with ethanol (20 mL each), then dried in a vacuum oven at 60 °C overnight. Yields about 90 mg.

The iron-containing nickel cobalt sulfoselenide sample, Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄, was synthesized via sulfidation and selenization of (Fe_xNi_1–x)CoCH-(1.0) in one step. (Fe_xNi_1–x)CoCH-(1.0) (125 mg) was dispersed in ultrapure water and stirred for 20 min. Then 680 mg (2.70 mmol) of Na₂S·9H₂O and 50 mg (0.425 mmol) of SeO₂ were added to the suspension. Finally, 10 mL of N₂H₄·H₂O was slowly added to the suspension. The Teflon-lined stainless-steel autoclave was kept at 160 °C for 12 h. The resulting powder was washed five times with ultrapure water (50 mL each time) and two times with ethanol (20 mL each time) and dried at 60 °C overnight. Yield 100 mg.

Iron-containing nickel cobalt selenide, Fe_0.4Ni_0.7Co_1.6(Se_0.81O_0.19)₄ was synthesized by selenization of (Fe_xNi_1–x)CoCH-(1.0). (Fe_xNi_1–x)CoCH-(1.0) (125 mg) was dispersed in 30 mL of ultrapure water and stirred for 20 min followed by adding 100 mg (0.9 mmol) SeO₂. Then 10 mL of N₂H₄ was added to above mention suspension and stirred for another 10 min. The resulting suspension was transferred to a stainless autoclave and heated at 160 °C for 12 h. The obtained black powder was washed five times with ultrapure water (50 mL each) and three times with absolute ethanol (20 mL each), then dried at 60 °C in the vacuum oven overnight. Yield 110 mg.

For comparison, nickel cobalt sulfide, Ni_1.0Co_2.1(S_0.9O_0.1)₄ and nickel cobalt selenide, Ni_0.7Co_1.4(Se_0.85O_0.15)₄, and were synthesized by sulfidation or selenization, respectively of NiCoCH respectively (Supplementary Materials, Section S1). The schematic illustration of the synthesis is shown in Scheme 1.

2.4. Material characterization

Powder X-ray diffraction (PXRD) analysis was conducted at ambient temperature on a Rigaku Miniflex 600 powder diffractometer (Rigaku, Tokyo, Japan) using Cu Kα1 radiation with λ = 1.5406 Å (40 kV, 15 mA, 600 W) and a flat silicon low background with a small indent in the range of 2θ = 5°–100°. The obtained PXRD data were evaluated with the Match 3.11 software.

Scanning electron microscopy (SEM) was performed with Jeol JSM-6510LV QSEM (Jeol, Akishima, Japan) advanced electron microscope (LaB₆ cathode at 20 kV) equipped with a Bruker Xflash 410 silicon drift detector for energy-dispersive X-ray (EDX) spectroscopy.

Transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (TEM-EDX) was carried out with a FEI Tecnai G2 F20 electron microscope (FEI Co., Hillsboro, OR, USA) operated at 200 kV accelerating voltage.

A Quantachrome Autosorb-6 automatic adsorption analyzer (Quantachrome Gmbh; Odelzhausen, Germany) was used to determine nitrogen sorption isotherms for the BET surface area determination of the samples at 77 K. The samples were degassed at 90 °C in a 10⁻² mbar vacuum for 15 h before the gas sorption measurement.

X-ray photoelectron spectroscopy (XPS) data were collected using a ULVAC-PHI VersaProbe II microfocus X-ray photoelectron spectrometer (ULVAC-PHI, Chigasaki, Japan). The spectra were recorded using a polychromatic aluminum Kα X-ray source (1486.8 eV) and referenced to the carbon 1s orbital with a binding energy of 284.8 eV.

Quantification of the metal content of the samples was performed using a Perkin-Elmer PinaAcle 900T atomic absorption spectrometer (Perkin Elmer LAS GmbH, Rodgau-Jügesheim, Germany) (sample preparation described in Supplementary Materials, Section S2).

The sulfur content was determined with a VarioMICRO CHNS microanalyzer (Elementar Analysensysteme, Langenselbold, Germany).

2.5. Electrochemical measurements

All electrochemical analyses were conducted on a three-electrode cell using an Interface 1010E potentiostat from Gamry Instruments at ambient temperature. The reversible hydrogen electrode, RHE, and Pt foil were used as a reference and counter electrode. Coated nickel foam, NF, was the working electrode. A slurry containing 8:1:1 mass portions of the active materials (mixed-metal sulfide, selenide, or sulfoselenide material), carbon black, Vulcan XC-72R, and polyvinylidene fluoride, PVDF, respectively, in N-methyl-2-pyrrolidone, NMP was prepared and carefully dropped on a ~1 cm² area of the NF surface, and dried at 60 °C for 12 h in a vacuum oven to prepare the coated NF electrode. Before starting the electrochemical analysis, an N₂ gas flow was passed through the electrolyte to remove dioxygen from the 1 mol L^–1 KOH electrolyte.

Linear sweep voltammetry LSV measurement was applied to determine the catalytic performance of the coated NF electrode at a scan rate of 5 mV s^–1. Before starting the LSV measurement, 20 cycles of cyclic voltammetry at a scan rate of 100 mV s^–1 were run to reach a stable electrocatalytic performance. The potentials of the LSV polarization curves were corrected by iR compensation. The chronopotentiometry at the current density of 50 mA cm^–2 for 20 h was used to evaluate the stability of the selected electrocatalyst in long-term performance. Moreover, to understand the electrode/electrolyte interface behavior, electrochemical impedance spectroscopy, EIS, was performed in the frequency range of 0.1-100 kHz at 1.5 V vs. RHE.

3. Result and Discussion

3.1. Synthesis and analysis

Nickel cobalt carbonate hydroxide (NiCoCH) and iron-containing nickel cobalt carbonate hydroxides (Fe_xNi_1–x)CoCH-(y) were synthesized from CoCl₂·6H₂O, NiCl₂·6H₂O, (NH₄)₂Fe(SO₄)₂·6H₂O and urea as precursors in hydrothermal reactions (Step 1 in Scheme 1). Four different amounts of (NH₄)₂Fe(SO₄)₂·6H₂O were used to synthesize (Fe_xNi_1–x)CoCH-(y) while keeping the NiCl₂·6H₂O and CoCl₂·6H₂O amounts constant. The samples were named (Fe_xNi_1–x)CoCH-(0.025), (Fe_xNi_1–x)CoCH-(0.05), (Fe_xNi_1–x)CoCH-(0.075), and (Fe_xNi_1–x)CoCH-(0.1), representing the use of 0.025, 0.05, 0.075, and 1.0 mmol of the iron precursor. The sulfidation and selenization of the metal carbonate hydroxide precursors was achieved through a hydrothermal sulfidation process with Na₂S·9H₂O and a selenization process with SeO₂ (Step 2 in Scheme 1). For the sulfoselenide sample, a mixture of Na₂S·9H₂O and SeO₂ was employed in the hydrothermal reaction.

Two different methods were used to determine the chemical formula of the as-prepared samples, a combination of AAS for the metal and CHNS analysis for the sulfur content (method 1) and SEM-EDX (method 2) (Supplementary Materials, Table S2-S6). Method 1 provides more precise atomic ratios of metal and S content in the samples than EDX. In EDX the emitted X-rays give a 1-2 µm depth analysis but EDX as an X-ray spectroscopy experiences matrix effects and would need standards of similar composition as the sample for peak identification and accurate quantification. For the sulfoselenide and selenide sample, AAS for the metal content was combined with EDX for the Se content. Moreover, the chemical formulae obtained from AAS + CHNS + EDX were much closer to charge balance than the EDX-derived formulae. Based on the metal to sulfur ratios obtained from method 1 and the charge balance calculation of the samples, oxygen should also be incorporated into the structure of the samples. The presence of oxygen in the structure of samples was also proven from the EDX and XPS spectrum. Therefore, the chemical formulae are given here with their estimated oxygen content. The chemical formulas resulting from methods 1 and 2 are provided in Table S6. The measured SEM-EDX data of the samples are provided in Figures S1-S8.

The crystallinity of all sulfide samples was low, as evidenced by broad peaks of low intensity in the powder X-ray diffractograms, PXRDs (Figure 1a). The crystalline phases in the iron-containing nickel cobalt samples were verified as spinels by matching to the known diffractograms of NiCo₂S₄ (ICDD no. 43-1477) and Co₃S₄ (ICDD no. 75-1561) (Figure 1). The prominent diffraction peaks located at 26.8°, 31.5°, 38.1°, 50.4°, and 55.2° can be attributed to the (220), (311), (400), (511), and (440) planes of the spinel lattice [17]. In addition, in the PXRD patterns of NiCo₂S₄ and iron-containing nickel cobalt samples, the diffraction peaks at 29.9° and 52.1° can be attributed to the (311) and (440) planes, respectively, of Co₉S₈ (ICDD no. 73-1442). It should be mentioned that the presence of an Co₉S₈ impurity in NiCo₂S₄ is reported in much previous literature, including Chen et al. work [20], who first reported the formation of sea urchin-like NiCo₂S₄ using metal carbonate hydroxide as a precursor [30,37,45,46,47,48].

The content of the crystalline Co₉S₈ impurity decreases with increasing iron content and is only barely visible in Fe_0.3Ni_1.2Co_2.5(S_0.9O_0.1)₄ and no longer visible in Fe_0.6Ni_1.2Co_2.5(S_0.83O_0.17)₄. The corresponding selenides are of higher crystallinity (Figure 1b). The prominent reflection peaks of NiCo₂Se₄ and Fe_0.4Ni_0.7Co_1.6(Se_0.81O_0.19)₄ match the simulation for NiCo₂Se₄ (ICDD no. 04-006-5241), where they correspond to the (002), (311), and (-313) crystal plane located at 33.3°, 44.9°, and 51.4°, respectively [49]. By incorporating both sulfur and selenium in the structure, the crystallinity of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ from the PXRD pattern became too low and no clear crystalline phase analysis was possible anymore. Such low crystallinity was also seen in the literature for nickel sulfoselenide, oxygen-containing cobalt sulfide, and nickel sulfide nanoparticles, which were reported with excellent OER properties [50,51,52].

The morphology of the as-prepared samples was studied using scanning electron microscopy, SEM. As shown in Figure 2a, Ni_1.0Co_2.1(S_0.9O_0.1)₄ consists of needle-like structures combining parts with a sea urchin-like morphology. By increasing the iron content, the morphology became more sea urchin-like (Figure 2b-f). In the selenides and the sulfoselenide Fe_0.6Ni_1.2Co_2.5(S_0.83O_0.17)₄ sample, Figure 2 e-f, agglomerations of needle-like primary particles can be seen. The SEM-energy dispersive X-ray (EDX) mappings (Supplementary Materials, Figure S9) support the AAS- and CHNS-based elemental analysis for the chemical formulae, and SEM-EDX was the analysis of choice to determine the selenium content.

To further investigate the sulfoselenide Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄, transition electron microscopy, TEM-EDX, was performed. The TEM images, Figure 3, confirmed the needle-like microstructure. EDX-mapping also proved the uniform presence of Fe, Ni, Co, S, and Se in the sulfoselenide sample (see Supplementary Materials, Table S1 for atom ratios).

Nitrogen adsorption-desorption isotherms, and specific surface area (BET) of the samples are given in Figure S10 and Table S7.

X-ray photoelectron spectroscopy (XPS) was conducted to determine the valence state of the elements in Ni_1.0Co_2.1(S_0.9O_0.1)₄ and Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄. As shown in Figure S11, X-ray survey spectra indicate the existence of Ni, Co, and S in Ni_1.0Co_2.1(S_0.9O_0.1)₄ and of Fe, Ni, Co, S, and Se in Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄. The high-resolution spectra of the metal atoms and of Se are shown in Figure 4, those of S 2p and O1s of Ni_1.0Co_2.1(S_0.9O_0.1)₄ in Figure S12a and Figure S13a, respectively. The positions of the XPS peaks are summarized in Table S8 and S9. It should be noted that the analysis depth of XPS is only 0.7-11 nm as the detected photoelectrons can only escape from such a thin surface layer of the sample.

The metal ratios and the ratios between different oxidation states of nickel and cobalt (Table 1) were calculated by integrating the fitted peak area for each metal valence state using the Ni 2p_3/2 and Co 2p_3/2 regions. In Ni_1.0Co_2.1(S_0.9O_0.1)₄ Ni²⁺ and Co³⁺ are the prevalent oxidation states.

Based on element ratios obtained from AAS and CHNS analysis and also the metal valence states for nickel and cobalt in the Ni_1.0Co_2.1(S_0.9O_0.1)₄ sample, the chemical formula can be given as ((Ni²⁺)_0.72(Ni³⁺)_0.28)_1.0((Co²⁺)_0.27(Co³⁺)_0.72)_2.1(S_0.9O_0.1)₄ which is anion-cation charge-balanced within rounding errors.

In contrast to the Ni_1.0Co_2.1(S_0.9O_0.1)₄ sample, Ni³⁺ and Co²⁺ are the dominant valence states in Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)_4, which might be one of the reasons for having an improved the OER performance of this catalyst. It is believed that Ni³⁺ can improve electrophilicity and oxygen adsorption, which can increase the amount of NiOOH active sites during the OER reaction [53].

The Fe 2p spectrum (Figure 4c) for Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ represents two broad peaks at 712.5 eV and 725.0 eV, which can be assigned to Fe 2p_3/2 and Fe 2p_1/2, respectively [54]. Furthermore, two satellite peaks were detected at 717.5 and 734.5 eV [55,56]. The 2p_3/2 spectrum range is 710 to 720 eV including the satellite peak, while the 2p_1/2 spectrum range is 720−735 eV with the satellite peak. For iron it should be noted that the Fe 2p spectral background has a contribution from a Co_LMM and Ni_LMM Auger peaks, making an unequivocal deconvolution and peak assignment difficult [57,58]. The Se 3d XPS spectra of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ (Figure 4d) consist of two peaks centered at 55.0 and 57.0 eV, representing Se 3d_5/2, and Se 3d_3/2, respectively. The peak at 59.5 eV was attributed to SeO_X, forming on the surface due to exposure to air, and to the overlap with the Co 3p signal [39]. The high-resolution spectrum of S 2p and O 1s of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ are shown in Figure S12b and Figure 13b, respectively. Based on the elemental ratios obtained from AAS, CHNS and EDX(Se) analysis and also metal valence states for iron, nickel and cobalt in the Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ sample the chemical formula can be given as (Fe²⁺)_0.5((Ni²⁺)_0.1(Ni³⁺)_0.9)_1.0((Co²⁺)_0.87(Co³⁺)_0.13)_2.0(S_0.57Se_0.25O_0.18)₄.

3.3. Oxygen evolution reaction performance

The OER activity of the mixed-metal sulfides, selenides, and the sulfoselenide was evaluated by a three-electrode setup in 1.0 mol L^–1 KOH solution. The working electrode was prepared according to a method used by Li et al.[29], a slurry containing 8:1:1 mass portions of the active materials (mixed-metal sulfide, selenide, or sulfoselenide material), carbon black, Vulcan XC-72R, and polyvinylidene fluoride, PVDF, respectively, in N-methyl-2-pyrrolidone, NMP (further details in experimental section). As is shown in Figure 6a, polarization curves of the as-prepared electrodes revealed that the presence of iron could reduce the overpotential of the Ni_1.0Co_2.1(S_0.9O_0.1)₄ parent compound. In the polarization curves, the peaks around 1.3–1.4 eV are attributed to the oxidation of Ni²⁺/Ni³⁺ [59].

The overpotential for Ni_1.0Co_2.1(S_0.9O_0.1)₄ of 346 mV at 50 mA cm^–2 was reduced with increasing iron content in Fe_0.1Ni_1.4Co_2.9(S_0.87O_0.13)₄, Fe_0.2Ni_1.5Co_2.8(S_0.9O_0.1)₄, Fe_0.3Ni_1.2Co_2.5(S_0.9O_0.1)₄, and Fe_0.6Ni_1.2Co_2.5(S_0.83O_0.17)₄ to 318, 310, 294, and 294 mV, respectively (Figure 5a,b). The electronic interaction between Fe, Ni, and Co in the iron-containing samples alters the electronic structure, making Ni²⁺ oxidation more difficult, resulting in a positive shift in the Ni²⁺/Ni³⁺ anodic peak at 1.3–1.4 eV [30,37]. The reduction of the OER overpotential in iron-containing samples can be attributed to reducing the charge transfer resistance through the synergistic electronic interaction between Fe, Co and Ni from a charge redistribution between active sites within the samples. Density functional theory (DFT) calculations in the literature traced the synergy to a decrease in the Gibbs free energy for the formation of a MOOH intermediate, which not only enhanced the intrinsic OER activity, but also significantly improved the intrinsic conductivity of iron-containing samples, greatly facilitating the charge transfer process [41,60].

In the next step, the effect of the coexistence of sulfur and selenium was investigated. The overpotential of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ (green line)of 277 mV at 50 mA cm^–2 presents a 17 mV and 22 mV reduction compared to Fe_0.6Ni_1.2Co_2.5(S_0.83O_0.17)₄ (294 mV) and RuO₂ (299 mV) (Figure 5c,d), indicating that the insertion of selenium improves the performance of the transition metal sulfide. Moreover, the OER performance of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ at 100 mA cm^–2 only needed 299 mV while RuO₂ required 341 mV overpotential, which demonstrates the excellent electrocatalytic activity of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ even under a high current density. The boost of the OER activity of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ can be attributed to the effect of selenide incorporation that reduces the energy barrier of the OER reaction, optimizes the electronic structure of active sites via modulating of the d-band of the compound, and also accelerates the kinetics of the reaction [61].

The Tafel slopes of the samples were obtained from LSV polarization curves at a scan rate of 5 mV s^–1 using the Tafel equation (1) [62]:

η= a + b × log(j)

(1)

η is the overpotential, b is the Tafel slope, j is the current density, and c is the intercept with the y-axis. The value of the Tafel slope is one of the most useful kinetic parameters and is inversely proportional to the kinetics of the OER reaction. Hence, as demonstrated in Figure 6a,b, Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ with the lowest value of the Tafel slope presents the most favorable OER kinetics among the investigated samples. Besides, according to Krasil'shchikov's mechanistic paths (equation 2–5, M = active site, b = Tafel slope) [62,63] with their corresponding Tafel slope values, the rate-determining step of the OER reaction for Ni_1.0Co_2.1(S_0.9O_0.1)₄ (b = 132 mV dec^–1) is metal oxidation with hydroxide formation (reaction (2)). By increasing the iron content in the samples, the Tafel slope decreased and reached 87 mV dec^–1 for Fe_0.6Ni_1.2Co_2.5(S_0.83O_0.17)₄, suggesting that metal oxidation with hydroxide formation (2) and deprotonation of a metal hydroxide (3) both present rate-determining steps.

The lowest Tafel slope among the samples belongs to the sulfoselenide Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ with 75 mV dec^–1, which is again evidence for the role of selenium in enhancing the kinetics of the OER reaction. The Tafel value of 75 mV dec^–1 of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ is close to reaction (3) (b = 60 mV dec^–1) being then rate-determining in the overall OER process.

M + OH^– ⇆ MOH + e^–, b = 120 mV dec^–1

(2)

MOH +OH^– ⇆ MO^– + H₂O, b = 60 mV dec^–1

(3)

MO^– → MO + e^–, b = 45 mV dec^–1

(4)

2MO → 2M +O₂, b = 19 mV dec^–1

(5)

Electrochemical impedance spectroscopy (EIS) was conducted for the electron-transfer kinetics during the OER reaction and to justify the obtained overpotentials [64]. Figure 6c shows Nyquist plots of the samples at the potential of 1.5 V vs. RHE. The semicircle diameter of Nyquist plots is inversely proportional to the charge transfer rate across the electrode and the electrolyte that accelerates reaction kinetics. Hence, a smaller semicircle diameter represents more favorable charge transfer kinetics [24].

The smaller semicircle diameter of the Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ Nyquist plot indicates that the coexistence of iron and selenium in Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ can reduce the charge transfer resistance (Rct). Furthermore, a Voigt circuit model was applied to the Nyquist plots to evaluate the specific value for the charge resistance in the OER process. As shown in Table 2, the value of charge resistance for Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ is the lowest (0.8 Ω) compared to the other investigated samples. The results from the Rct values are in line with the recorded OER overpotentials of the samples and the electrocatalytic performance.

To elucidate the importance of sulfur and selenium in the OER electrocatalysis performance, the (Fe_xNi_1–x)CoCH-(1.0) precursor for Fe_0.6Ni_1.2Co_2.5(S_0.83O_0.17)₄ and Fe_0.5Ni_1.0Co_2.0(S_0.57 Se_0.25O_0.18)₄ was investigated and found to have an OER LSV polarization curve with a higher overpotential of 330 mV vs. RHE to reach 50 mA cm^–2 and a Tafel slope of 98 mV dec^–1, higher than the iron-containing sulfide and sulfoselenide (Table 2, Figure S14). Furthermore, the larger Nyquist plot semicircle diameter and higher charge transfer resistance (R_ct = 2.5 Ω) of (Fe_xNi_1–x)CoCH-(1.0) imply the importance of S and Se in facilitating the charge transfer through the electrode-electrolyte interface in sulfide, selenides, and sulfoselenide samples (Figure S15).

One of the critical parameters to evaluate the performance of electrocatalysts in practical applications is their long-term stability. Hence, a chronopotentiometry test at a current density of 50 mA cm^–2 for 20 h was conducted to evaluate the long-term stability performance of the Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ [47].

Figure 6d illustrates that after 20 h of OER reaction, the overpotential for obtaining 50 mA cm^–2 increased only from 277 mV to 279 mV, which is essentially constant and supports the excellent electrocatalyst stability of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ for practical applications. The more so as for RuO₂ the overpotential increases from 300 mV to 375 mV at the same chronopotentiometry test for 20 h, at 50 mA cm^–2. A recent study by Jiang et al. [65] proved that a SeO_x film on the surface could improve the catalyst stability in the OER reaction, which can be the reason behind the high stability of Fe_0.5Ni_1.0Co_2.0(S_0.57 Se_0.25O_0.18)₄ during the OER reaction (the overpotential increased from 277 to only 279 at 50 mA cm^–1 after 20 h OER reaction).

Figure 6. (a) Tafel plots and (b) bar chart of Tafel slopes of the samples. (c) Nyquist plots of selected samples and Voigt circuit model. (d) Chronopotentiometry test of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ and RuO₂.

Figure 6. (a) Tafel plots and (b) bar chart of Tafel slopes of the samples. (c) Nyquist plots of selected samples and Voigt circuit model. (d) Chronopotentiometry test of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ and RuO₂.

The superior OER activity and stability of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ material can be attributed to several effects resulting from introducing Fe and Se^2– in the structure of the nickel cobalt sulfide base material: Introducing Fe sites can enhance OER performance a) by optimizing bond energies for OER intermediates adsorbing on the electrode surface, which facilitates the OER kinetics [66], b) by overcoming the metal oxidation step and facilitating O₂ evolution [67], c) by improving the conductivity of the electrode film [68].

For comparison, the overpotential values of several high performance electrocatalysts at a current density of 50 mA cm^–2 using nickel foam as substrate are presented in Table 3. Notably, the results demonstrate that the OER performance of the sulfoselenide Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ is on par with the best electrocatalysts reported in the literature, highlighting its comparable effectiveness.

4. Conclusions

A novel trimetallic sulfoselenide, Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄, electrocatalyst was synthesized via two steps hydrothermal method. The mixed-metal sulfoselenide possessed high OER activity than to the bimetallic nickel cobalt sulfide Ni_1.0Co_2.1(S_0.9O_0.1)₄, the bimetallic nickel cobalt selenide Ni_0.7Co_1.4(Se_0.85O_0.15)₄, the trimetallic iron-containing nickel cobalt sulfides, Fe_0.1Ni_1.4Co_2.9(S_0.87O_0.13)₄, Fe_0.2Ni_1.5Co_2.8(S_0.9O_0.1)₄, Fe_0.3Ni_1.2Co_2.5(S_0.9O_0.1)_4, and Fe_0.6Ni_1.2Co_2.5(S_0.83O_0.17)₄ and the iron-containing nickel cobalt selenide Fe_0.4Ni_0.7Co_1.6(Se_0.81O_0.19)₄. The trimetallic sulfoselenide required an overpotential of only 277 mV at 50 mV cm^–2 and had favorable OER kinetics, manifested by a Tafel slope of 75 mV dec^–1. The OER performance of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ surpassed the well-known RuO₂ benchmark material where the required overpotential (300 mV to generate 50 mA cm^–2) was 23 mV higher at the same condition (1 mol L^–1 KOH). The 20 h chronopotentiometry analysis revealed that Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ has remarkable stability during long-term operation in alkaline media (the overpotential increased only from 277 to 279 mV) while at the same condition, RuO₂ showed a considerable loss in activity (such that the overpotential increased from 300 to 375 mV during the generation of 50 mA cm^–2 current density).

Furthermore, the incorporation of selenium also significantly affected the OER activity and stability by reducing the energy barrier of the OER reaction, optimizing the electronic structure of active sites by modifying the d-band of the materials. Indeed, a SeO_x film on the surface of Fe_0.5Ni_1.0Co_2.0(S_0.57Se_0.25O_0.18)₄ can enhance the long-term stability of the catalyst. Considering the improvements in OER performance, this simple two-step hydrothermal synthesis of trimetallic sulfoselenides, from transition metal carbonate hydroxide, (Fe_xNi_1–x)CoCH-(y) as precursors can be used as a facile and practical approach to produce the next generation of non-precious polymetallic polychalcogenide materials for the oxygen evolution reaction.