Hierarchical CuCo₂S₄ nanoarrays for high-efficient and durable water oxidation electrocatalysis

Abstract

It is highly attractive to design and develop earth-abundant electrocatalysts toward high-efficiency water oxidation electrocatalysis in alkaline media. In this communication, we report the in situ hydrothermal sulfidization preparation of a hierarchical CuCo₂S₄ nanoarray on copper foam (CuCo₂S₄/CF) from its CuCo₂-hydroxide nanowire array precursor. When used as a 3D catalyst electrode for water oxidation, the as-prepared CuCo₂S₄/CF is superior in catalytic activity, demanding overpotentials of only 259 and 295 mV to achieve 60 and 100 mA cm⁻² in 1.0 M KOH, respectively. Moreover, it also shows strong electrochemical durability with high turnover frequency values of 0.069 and 0.390 mol O₂ s⁻¹ at overpotentials of 300 and 400 mV, respectively.

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The drastic depletion of fossil fuels and growing global environmental concerns call for an urgent demand for exploiting eco-friendly and renewable energy carriers.^1,2 Clean and sustainable hydrogen is regarded as an ideal candidate to replace fossil fuels.^3,4 Water electrolysis provides us an attractive way to produce pure hydrogen on a large scale.^5,6 Involving a multi-electron transfer process, anodic water oxidation remains a bottleneck for water electrolysis, thus efficient water oxidation catalysts (WOCs) are required to accelerate the kinetics, for achieving high current densities at minimal overpotentials.^7–9 Although Ru- and Ir-based oxides show the highest catalytic activity toward water oxidation, their low abundance and high cost greatly hinder their widespread commercial uses.¹⁰

Tremendous efforts have been devoted to developing earth abundant WOCs for more energy-efficient water electrolysis.^11–22 Transition metal sulfides (TMSs) have been widely investigated for energy storage and catalysis, such as lithium ion batteries,^23–25 supercapacitors,^26–28 hydrodesulfurization,^29–31 and electrochemical water splitting.^32–40 For catalyzing water oxidation, bimetallic TMSs are more competitive than monometallic ones.^35,41–43 Among them, big-cell sulphospinels can expose a large number of edge sites, with more rapid electron transfer pathways, smaller optical band gaps, and better redox reactions, offering great benefits to enhance electrochemical performances.^38,44–46 Recent works demonstrated that nanoarray catalysts have obvious advantages of exposing more active sites and facilitating diffusion of electrolytes and evolved gas.^{6,11,13,47,48} Moreover, nanoarray catalysts with hierarchical feature would offer more active sites and have attracted increasing attention.^48–51 Therefore, hierarchical structured transition metal sulphospinels hold great promise for electrochemical water oxidation.

Herein, we describe the fabrication of a hierarchical CuCo₂S₄ nanoarray on copper foam (CuCo₂S₄/CF) by hydrothermally sulfidizing its bimetallic hydroxide nanowire array precursor (see the ESI† for preparation details). The as-prepared CuCo₂S₄/CF exhibits superior activity for water oxidation, needing overpotentials of only 259 and 295 mV to achieve 60 and 100 mA cm⁻² in 1.0 M KOH, respectively, outperforming monometallic cobalt sulphide on CF (Co–S/CF) and copper sulphide on CF (Cu–S/CF). Notably, this system also demonstrates strong long-term electrochemical durability with high turnover frequency (TOF) values of 0.069 and 0.390 mol O₂ s⁻¹ at overpotentials of 300 and 400 mV, respectively.

Fig. 1a presents the X-ray diffraction (XRD) patterns for CuCo₂S₄/CF. The peaks at 26.6°, 31.3°, 38.0°, 47.0°, 50.0°, 55.0°, and 64.4° can be indexed to the (022), (113), (004), (224), (115), (044), and (335) planes of the cubic CuCo₂S₄ phase (JCPDS No. 42–1450),^52–54 respectively, and the three peaks at 43.3°, 50.4°, and 74.1° arise from the CF substrate (JCPDS No. 04–0836). Fig. S1a and S1b show the XRD patterns for Cu–S/CF and Co–S/CF, indexed to the monoclinic (JCPDS No. 83–1462) and hexagonal Cu₂S (JCPDS No. 84–0206), and cubic Co₃S₄ phases (JCPDS No. 47–1738), respectively. The scanning electron microscopy (SEM) images of the hydroxide precursor indicate that it is a nanowire array (Fig. S2, ESI†). After a hydrothermal sulfidization process, the as-prepared CuCo₂S₄ still preserves the 1D morphology but with many ultrathin nanosheets on the surface, suggesting the formation of the hierarchical structure (Fig. 1b). The low-magnification SEM image in Fig. 1c indicates that the CuCo₂S₄ is well aligned on the CF substrate, and the corresponding energy-dispersive X-ray (EDX) elemental mapping images verify the uniform distribution of Cu, Co, and S elements. Cu–S/CF, Co–S/CF, and CuCo–S with varied Co²⁺/Cu²⁺ ratios were also prepared for comparison (Fig. S2, S3, and S4, ESI†). Fig. S5 (ESI†) shows the EDX spectrum of CuCo₂S₄/CF, further showing the existence of Cu, Co, and S elements with an atomic ratio close to 1 : 2 : 4. The high-resolution transmission electron microscopy (HRTEM) analysis (Fig. 1e) taken from CuCo₂S₄ (Fig. 1d) reveals lattice fringes with an interplanar distance of 0.182 nm indexed to the (115) plane of the cubic CuCo₂S₄ phase. The selected area electron diffraction (SAED) pattern shows discrete spots of the (113), (004), (224), and (026) planes of crystalline cubic CuCo₂S₄ (Fig. 1f).

Fig. 1 (a) XRD pattern for CuCo₂S₄/CF. (b) SEM images for CuCo₂S₄/CF. (c) SEM image and EDX elemental mapping images for Cu, Co, and S elements in CuCo₂S₄/CF. (d) TEM and (e) HRTEM images for CuCo₂S₄. (f) SAED patterns for CuCo₂S₄.

The X-ray photoelectron spectroscopy (XPS) survey spectrum for CuCo₂S₄/CF (Fig. 2a) confirms the presence of Cu, Co, and S elements. The binding energies (BEs) at 952.3 and 932.5 eV in the Cu 2p region (Fig. 2b) can be assigned to Cu 2p_1/2 and Cu 2p_3/2, respectively, indicating the presence of Cu⁺.^53,54 The other two shakeup satellite peaks (identified as Sat.) at 954.0 and 934.5 eV, together with the peak at 942.5 eV, suggest the existence of Cu²⁺ arising from air exposure.^52,55 In the Co 2p region (Fig. 2c), the BEs of Co 2p_1/2 and Co 2p_3/2 appear at 792.8 and 778.0 eV, respectively, with two shakeup satellites at 796.8 and 780.8 eV, indicating the coexistence of Co²⁺ and Co³⁺.^53,54 The BE at 162.0 eV in the S 1s region (Fig. 2c) is assigned to the Cu–S and Co–S bonds, and low coordination of S²⁻ at the surface.^52,53

Fig. 2 (a) XPS survey spectrum for CuCo₂S₄. XPS spectra for CuCo₂S₄ in the (b) Cu 2p, (c) Co 2p, and (d) S 2p regions.

We collected the linear sweep voltammetry (LSV) curves to evaluate the electrocatalytic activity of CuCo₂S₄/CF (CuCo₂S₄ loading: 3.6 mg cm⁻²) toward water oxidation on a typical three-electrode setup with a scan rate of 5 mV s⁻¹ in 1.0 M KOH. For comparison, Cu–S/CF, Co–S/CF, RuO₂ (Fig. S6, see ESI† for preparation details) on CF (RuO₂/CF, with the same mass loading), and bare CF were also investigated under the same conditions. Owing to the solution resistance, all experimental data were corrected with ohmic potential drop (iR) losses to reflect the intrinsic behavior of the catalysts, and all potentials were reported on a reversible hydrogen electrode (RHE) scale except specifically stated. Fig. 3a shows the LSV curves. As observed, RuO₂/CF exhibits high activity for water oxidation with the need of a low overpotential of 276 mV to achieve 100 mA cm⁻², whereas the bare CF shows poor catalytic activity. Cu–S/CF is also active for water oxidation, requiring overpotential of 472 mV to afford 100 mA cm⁻². Although Co–S/CF shows better catalytic activity for water oxidation, it still needs an overpotential of 340 mV to drive the same current density. Impressively, our CuCo₂S₄/CF is superior in catalytic activity, demanding an overpotential of only 295 mV to deliver 100 mA cm⁻², 177 and 45 mV less than those of monometallic Cu–S/CF and Co–S/CF, respectively. The oxidative feature at 1.35 and 1.47 V (as marked by the blue dash lines) preceding water oxidation is the oxidation of the CuCo₂S₄/CF catalyst.⁵¹ Additionally, it needs a much smaller overpotential of 259 mV to afford the geometric current density of 60 mA cm⁻², comparing favorably to the behaviors of most reported non-noble metal WOCs at alkaline pH, like Al–CoP/NF (η₅₀ mA cm⁻² = 280 mV),⁵⁶ MnCo₂S₄ NA/TM (η₅₀ mA cm⁻² = 325 mV),³⁸ NiCo₂S₄ NA/CC (η₁₀₀ mA cm⁻² = 340 mV),³⁴ NiSe₂/Ti (η₂₀ mA cm⁻² = 295 mV),⁵⁷ De-LNiFeP/rGO (η₁₀ mA cm⁻² = 258 mV),¹²etc., and a detailed comparison is listed in Table S1 (ESI†). Fig. 3b presents the Tafel plots of Cu–S/CF, Co–S/CF, CuCo₂S₄/CF, and RuO₂/CF. The Tafel slope of 110 mV dec⁻¹ for CuCo₂S₄/CF is much smaller than those for Cu–S/CF (164 mV dec⁻¹) and Co–S/CF (145 mV dec⁻¹), implying a more favorable catalytic kinetics on CuCo₂S₄/CF. We also examined the effect of the Co²⁺/Cu²⁺ ratio on the water oxidation activity, and overpotentials of 450, 416, 352, and 311 mV are needed to drive a geometric current density of 100 mA cm⁻² for Cu₃Co–S/CF, Cu₂Co–S/CF, CuCo–S/CF, and CuCo₃–S/CF, respectively (Fig. S7, ESI†).

Fig. 3 (a) LSV curves of RuO₂/CF, CuCo₂S₄/CF, Cu–S/CF, Co–S/CF, and bare CF with a scan rate of 5 mV s⁻¹ for water oxidation. (b) Tafel plots for RuO₂/CF, CuCo₂S₄/CF, Co–S/CF, and Cu–S/CF. (c) Multi-current process for CuCo₂S₄/CF. The current density started at 40 mA cm⁻² and ended at 400 mA cm⁻², with an increment of 40 mA cm⁻² per 500 s without iR correction. (d) LSV curves for CuCo₂S₄/CF before and after 1000 cyclic voltammetry cycles (inset: chronopotentiometric curve for CuCo₂S₄/CF at a constant current density of 100 mA cm⁻² without iR correction). All experiments were performed in 1.0 M KOH unless specially stated.

Fig. 3c displays the multi-step chronopotentiometric curve for CuCo₂S₄/CF. The anodic current density increases from 40 to 400 mA cm⁻² with an increment of 40 mA cm⁻² per 500 s, and the potential immediately levels off at 1.52 V for the starting current value, remaining constant for the rest 500 s. Similar results are observed for other steps tested up to 400 mA cm⁻², demonstrating the excellent mass transport properties, conductivity, and mechanical robustness of the CuCo₂S₄/CF electrode.^13,58 Given that stability is also a critical criterion for the practical applications of catalysts, we thus probed the stability of our CuCo₂S₄/CF via continuous cyclic voltammetry scanning. The LSV curve shows negligible loss in current density after 1000 cycles compared with the initial one (Fig. 3d), suggesting its superior stability. An electrolysis measurement at a fixed current density of 100 mA cm⁻² further demonstrates that this catalyst has strong long-term electrochemical durability, maintaining its catalytic activity for at least 20 h (Fig. 3d, inset). CuCo₂S₄/CF still retains its hierarchical feature after the stability test (Fig. S8, ESI†). The XRD pattern also shows characteristic peaks of cubic CuCo₂S₄ but with decreased intensities (Fig. S9, ESI†). A thin amorphous layer about 2–3 nm was formed on the CuCo₂S₄ surface (Fig. S10, ESI†). We also performed XPS analysis of the sample after oxidation electrolysis. In the Cu 2p region (Fig. S11a, ESI†), the BEs at 934.5 (2p_1/2) and 954.6 eV (2p_3/2), together with the peaks at 939.5 and 941.7 eV, suggest the existence of CuO.⁵⁹ The BE at 962.8 eV indicates the presence of Cu(OH)₂.⁶⁰ In the Co 2p region (Fig. S11b, ESI†), the strong peak at 780.4 eV (2p_3/2) can be assigned to CoO.⁶¹ The peaks at 782.3 (Sat.), 795.4 (2p_1/2), and 796.8 eV arise from Co(OH)₂.^34,62 Another wide peak appearing at 787.0 eV implies the presence of CoOOH.⁶³ The peak for S²⁻ in the S 2p region can hardly be observed (Fig. S11c). In the O 1s region (Fig. S11d, ESI†), the BE at 529.3 eV and a weak peak at 531.3 eV suggest the existence of O²⁻ and OH⁻ species, respectively.⁶⁴ These observations suggest the formation of an amorphous oxide/(oxy)hydroxide layer on the CuCo₂S₄ surface as the real active phase.³⁴

To estimate the electrochemically active surface area, we determined the electrochemical double-layer capacitance (C_DL) at the solid/liquid interface for Cu–S/CF, Co–S/CF, CuCo₂S₄/CF from cyclic voltammograms (CVs).^65,66 All CVs were collected in the region of 0.608–0.708 V (Fig. S9a–c, ESI†) to ensure that the current responses are only owing to the charging of the double layer. According to the formula: j_c = vC_DL (a plot of j_c as a function of v yields a straight line with a slope equal to C_DL), the C_DL values for Cu–S/CF, Co–S/CF, and CuCo₂S₄/CF are 1.990, 2.390, and 3.300 mF cm⁻², respectively (Fig. S9d, ESI†), implying CuCo₂S₄/CF has a much larger surface area and more exposed active sites.⁶

To calculate TOF, we applied electrochemistry to quantify the surface concentration of active sites (Fig. 4a).⁶⁷ We assumed a one-electron oxidation process for the oxidation of Cu and Co metal centers in CuCo₂S₄.³⁸ There exists a linear dependence between the oxidation peak current density for redox Cu and Co species and scan rates (Fig. 4a, inset). The TOF values for CuCo₂S₄/CF are calculated to be 0.069 and 0.390 mol O₂ s⁻¹ at overpotentials of 300 and 400 mV, respectively (Fig. 4b), much higher than those for reported WOCs like MnCo₂S₄ NA/TM (∼0.20 mol O₂ s⁻¹, η = 400 mV),³⁸ Co₃O₄/N–PC (0.0015 mol O₂ s⁻¹, η = 300 mV),⁶⁸ and NiCo₂O₄@Ni–Co–B/CC (0.019 mol O₂ s⁻¹, η = 300 mV).⁶⁹

Fig. 4 (a) CVs for CuCo₂S₄/CF in the faradic capacitance current range at scan rates from 10 to 70 mV s⁻¹ (inset: the corresponding plot of oxidation peak current density versus the scan rate from CVs) in 1.0 M KOH. (b) A plot of TOF for CuCo₂S₄/CF as a function of overpotential.

In summary, a hierarchical CuCo₂S₄ nanoarray has been developed for durable water oxidation electrolysis with the need of overpotential of 295 mV to afford a geometrical catalytic current density of 100 mA cm⁻² in 1.0 M KOH. It also achieves high TOF of 0.069 and 0.390 mol O₂ s⁻¹ at overpotentials of 300 and 400 mV, respectively. This nanoarray may hold great promise as an attractive water-oxidizing catalyst material in water-splitting devices toward large-scale production of hydrogen fuels for applications.

This work was supported by the National Natural Science Foundation of China (No. 21575137).

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c7cc07259g

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