An efficient ultra-thin chain-structured copper cobalt oxide/sulfide composite catalyst for electrochemical hydrogen generation

Abstract

The electroreduction of water for sustainable hydrogen production is an important process for clean energy technologies, such as water splitting and fuel cells. However, development of low-cost and efficient electrocatalysts for hydrogen evolution reaction (HER) is still a great challenge. Here we reported a kind of ultrathin chain-structured CuCo₂O₄ (CCO) and sulfured CuCo₂O₄ (S-CCO) for HER with excellent activity and good durability in alkaline solution. At a current density of 10 mA cm⁻², S-CCO nanocomposite catalyst only need an overpotential of 154 mV (vs. RHE), whereas chain-structured CCO need an overpotential of 490 mV. It demonstrates that sulfured treatment is an effective approach to enhance the catalytic activity of oxide for HER.

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Introduction

Electrochemical reduction of water through hydrogen evolution reaction (HER) is a clean and sustainable approach to generate hydrogen (H₂), which is a critical component of several clean-energy technologies, such as fuel cells.^1,2 Platinum (Pt) based catalysts play a vital role in the HER because they can catalyze the conversion reaction at high rates and low overpotentials (η).^3–11 However, the prohibitive cost and scarcity of Pt make them less attractive in practical applications. Therefore, it is highly desired for designing low-cost and efficient alternative catalysts for HER.

In recently years, a variety of catalysts have been extensively studied for HER, such as alloys, carbides, sulfide, phosphides, etc.^12,13 Among them, Ni-based electrodes are most popular catalyst in water electrolysis due to its good corrosion resistance in high pH value solutions compared with other transition metals, such as Fe or Co.¹⁴ However, the sluggish reaction kinetics limits their application. As reported by Zheng et al. previously,¹⁵ cobalt-based alloys or compounds show excellent performance for hydrogen adsorption. Furthermore, copper-based oxides and sulfides have not been well investigated,⁸ because copper is usually regarded as an inert element for HER.¹⁶

Herein, we report an ultra-thin chain-structured copper cobalt oxide/sulfide composite catalyst prepared by hydrothermal method for HER. It shows excellent activity and good durability with lower overpotentials and smaller Tafel slope in alkaline solution. These results suggest a strategy for designing non-noble metal catalysts with enhanced HER performance.

Experimental

Preparation of the catalyst

All the chemicals were purchased from Sinopharm Chemical Reagents Co. Ltd. CuCo₂O₄ was prepared by a hydrothermal method. 1.5 mmol Cu(NO₃)₂·3H₂O, 3 mmol Co(NO₃)₃·6H₂O, 27 mmol CO(NH₂)₂ were dissolved in 5 mL deionized water in sequence under magnetic stirring for 15 min to form a dark-red homogeneous solution. Then, 20 mL ethanol and 40 mL glycol were mixed under magnetic stirring to ensure the two liquids mixed uniformly. Then the dark-red solution was added to the transparent solution drop by drop and kept stirring for 30 min before being transferred to a 100 mL Teflon-lined stainless autoclave and maintained at 120 °C for 9 h. Then the autoclave was allowed to cool down to room temperature naturally. The products was washed by deionized water and ethanol several times and separated from the liquid by centrifugation before dried at 60 °C overnight in a vacuum oven. The purple product was collected and calcined at 250 °C for 180 min.

To prepare the sulfured CuCo₂O₄ composite catalyst, 100 mg CCO and 400 mg S powder were weighted and put into two alumina boats respectively before they were transferred to a quartz tube with a flow controller. The tube was flushed by Ar at 30–50 sccm for 30 min before it was heated to 425 °C at a heating rate of 60 °C min⁻¹ and then maintained at 425 °C for 30 min. The furnace cooled down naturally with the stable Ar flow. The obtained product was ground lightly in an agate mortar before test.

Characterization of materials

The X-ray powder diffraction (XRD) patterns were performed on a Bruker-AXS D8 diffractometer with Cu K_α radiation (λ = 1.5406 Å) over the 2θ range of 20–60°. Scanning electron microscopy (SEM) images were taken on a HATACHI S4800 field-emission scanning electron microscope. Transmission electron microscopy (TEM) was carried out by a transmission electron microscope (FEI Tencai F20). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a spectrometer with Mg K_α radiation (ESCALAB 250, Thermofisher Co.). The binding energy was calibrated with the C 1s position of contaminant carbon in the vacuum chamber of the XPS instrument (284.8 eV). The nitrogen adsorption and desorption isotherms at 77 K were measured with a Quantachrome NOVA4000 instrument.

Electrochemical measurements

Electrochemical measurements were performed in a three-electrode system at an electrochemical work station (CHI660D). The three-electrode configuration using an Ag/AgCl (KCl saturated) electrode as the reference electrode, a graphite rod as the counter electrode, and the carbon paper coated with catalysts was used as the working electrode. The working electrode was fabricated as follow: the catalysts were dispersed in N-methyl-2-pyrrolidone solvent containing 7.5 wt% polyvinylidene fluoride under sonication, in which the weight ratio of the catalyst to PVDF is 8 : 1. Then the slurry was coated onto a piece of carbon paper (length × diameter × thickness = 6 cm × 1 cm × 0.03 cm). The loading density of the catalyst was around 3 mg cm⁻². Linear sweep voltammetry with scan rate of 5 mV s⁻¹ was conducted in 0.1 M NaOH (deaerated by N₂). For a Tafel plot, the linear portion is fit to the Tafel equation. All data have been corrected for a small ohmic drop based on impedance spectroscopy. In 0.1 M NaOH (pH 13), E_(RHE) = E_(SCE) + 0.21 + 0.059 × pH (V). All the potentials reported in our manuscript were calibrated to a reversible hydrogen electrode (RHE).

Result and discussion

The phase purity and crystal structure of the products obtained were examined by XRD pattern. Fig. 1a shows that the as-prepared product after hydrothermal reaction is a spinal structure. All the diffraction peaks of the product were matched well with standard peaks of Co₃O₄ (JCPDF file No. 01-1152). The ICP-AES analysis result suggests that the composition ratio of Cu : Co : O ratio is about 1 : 2 : 4. Therefore, it is a CuCo₂O₄ (CCO) phase. After sulfurization, the product consists of three phases of CuCo₂O₄, Cu_0.33Co_0.67S₂ and CuCo₂S₄, hereafter referred to as S-CCO, as shown in Fig. 1b. It should be noted that the Cu_0.33Co_0.67S₂ has the same crystal structure with that of CoS₂ while the CuCo₂O₄ and CuCo₂S₄ have the same crystal structure with that of Co₃S₄. The diffraction peaks of 2 theta at 27.9°, 32.3°, 36.2°, 39.8°, 46.3°, 54.9° are indexed to the (111), (200), (210), (211), (220) and (311) planes of pyrite-typed Cu_0.33Co_0.67S₂, respectively. The diffraction peaks of 2 theta at 26.7°, 31.5°, 38.2°, 50.3° are indexed to the (220), (311), (400) and (511) planes of linnaeite-typed CuCo₂O₄ and CuCo₂S₄ respectively. It should be noted that the sulfurization reaction should be controlled well in order to get rid of other impurities.

Fig. 1 XRD patterns of (a) CCO and (b) S-CCO.

X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the surface composition and valence states of Cu and Co in the as-prepared S-CCO sample. Fig. 2a shows the deconvoluted spectrum of Cu 2p, which shows two peaks at binding energy of 934 and 954 eV respectively, corresponding to Cu 2p_3/2 and Cu 2p_1/2.^17,18 Strong satellite peaks were observed at higher binding energies is due to the spin orbit characteristics of Cu²⁺. This shows that the majority of Cu ions are in +2 state. The deconvoluted spectrum of Co 2p is shown in Fig. 2b. The two main peaks at binding energy of 780 and 795 eV can be assigned to Co 2p_3/2 and Co 2p_1/2.^17–19 The XPS results indicate that the valences states of Cu and Co are still remain +2 and +3 after sulfuration, respectively.

Fig. 2 XPS spectra of the as-prepared S-CCO sample: (a) Cu 2p core level XPS spectrum, (b) Co 2p spectrum.

The size and morphology of the product were examined by field-emission scanning electron microscopy (FESEM). Fig. 3a shows most of CCO particles are nanoneedles. The average diameter and length of the nanoneedles are about 5 nm and 100 nm, respectively. After sulfurization, parts of CCO changed morphology and became linnaeite and pyrite type to form a tri-phase catalyst with CCO covered with sulfide partly. After sulfurization, the morphology of the product did not change obviously.

Fig. 3 SEM images of (a) the CCO sample, and (b) the S-CCO sample.

Fig. 4 show the nitrogen adsorption–desorption isotherms and the corresponding BJH (Barret–Joyner–Halenda) pore size distribution curves of the obtained CCO sample. It shows a typical IV adsorption–desoprtion isotherm with H3-type hysteresis (Fig. 4a), a feature of mesoporous material. The measured Brunauer–Emmett–Teller (BET) area is about 150 m² g⁻¹. As shown in Fig. 4b, the average pore diameter is 47 nm, calculated from the desorption branch of the nitrogen isotherm with the BJH method. The corresponding BJH desorption cumulative volume is 0.613 cm³ g⁻¹. The high porosity can provide more active sites for reactions.

Fig. 4 (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding BJH pore size distribution for the CCO sample.

The morphology and structure of the products were further examined by transmission electron microscopy (TEM). As shown in Fig. 5, the CCO products show chain-structured morphology, which are composed of nanoparticles with a diameter of approximately 5 nm.

Fig. 5 TEM images of CCO chain-structured nanocomposites: (a) overall product morphology; (b) high-magnification TEM image.

Fig. 6 shows an annular dark-field (ADF) scanning transmission electron microscopy (STEM) image of the sulfured CCO sample. Fig. 6b–e are the corresponding energy dispersive X-ray spectrometry (EDX) elemental mappings of Co, Cu, O and S. It can be clearly seen that Co and Cu are distributed uniformly while S and O are not the same case. Therefore, it indicates that part of CCO particles surface was covered with sulfides.

Fig. 6 (a) ADF-STEM image of S-CCO, (b) and (c–f) the corresponding elemental mappings of Cu, Co, S and O taken from the area marked.

Electrochemical activity of the S-CCO loaded on carbon paper was measured with a three-electrode setup in 0.1 M NaOH solution. For comparison, Pt and CCO were also examined. Fig. 7a shows the polarization curves with a sweep rate of 5 mV s⁻¹. The Pt catalyst exhibits the expected HER activity with a near zero overpotential (η), consistent with previous report.²⁰ The S-CCO exhibit much better catalytic activity than CCO with regard to overpotential and Tafel slope. For driving cathodic current density of 10 mA cm⁻², the S-CCO only need an overpotential of 154 mV (vs. RHE), whereas CCO need an overpotential of 490 mV. Impressively, the activity of S-CCO is favorably comparable to that of most electrocatalysts in the same electrolyte.^8,21,22 To further study of HER activity of the S-CCO, Tafel plots were fitted to the Tafel equation (η = a + b log|j|), where b is the Tafel slope. As shown in Fig. 7b, the Pt catalyst exhibits a Tafel slope of 40 mV dec⁻¹. The Tafel slope of the S-CCO is 180 mV dec⁻¹, much lower than that of the CCO (222 mV dec⁻¹), suggesting both electrocatalysts may follow a Volmer–Heyrovsky mechanism, in which the Volmer reaction is the rate-limiting step. It demonstrates that sulfured treatment is an effective approach to enhance the catalytic activity of oxide for HER. The long-term durability of the S-CCO was examined by electrolysis at a given potential. As shown in Fig. 7c, the cathodic current densities at an overpotential of 100 mV have no obvious degradation over 12 000 s, revealing good stability of the S-CCO catalyst. For comparison, the durability of the CCO sample is also shown in Fig. 7c. It indicates that both catalysts have excellent durability for 12 000 s test. The enhanced activity of the S-CCO samples may be resulted from the abundance of disulfide-terminated surface sites in the pyrite crystal structure.²³ Jin et al. found that the disulfide anions (S₂²⁻) present in all pyrite structure are important for their high electrocatalytic activity toward the HER.²³ The TEM images of the S-CCO catalyst after durability test show the morphology evolution of the catalyst, as shown in Fig. 7. It is observed that the S-CCO particles have a little bit aggregation after durability test but they are still nano-sized particles.

Fig. 7 (a) HER polarization curves, (b) corresponding Tafel plots of CCO S-CCO and Pt, and (c) durability of S-CCO in 0.1 M NaOH, (d) TEM image of S-CCO after durability test for 12 000 s, (e) high-magnification TEM image of S-CCO after durability test.

Conclusion

In conclusion, we have developed a low-cost and efficient ultra-thin chain-structured copper cobalt oxide/sulfide nanocomposite catalyst for HER. In alkaline solution, the overpotential for S-CCO catalyst is 154 mV (vs. RHE) at a current density of 10 mA cm⁻². The Tafel slope of the S-CCO is 180 mV dec⁻¹. It also shows good durability. These results suggest a strategy for designing non-noble metal catalysts with enhanced HER performance.

Acknowledgements

This work was supported by the Thousands Talents Program for the pioneer researcher and his innovation team in China. The authors also acknowledge the financial support of the National Science Foundation of China (No. 51172275 and No. 51372271) and the National Key Basic Research Program of China (No. 2012CB215402). Y. Chen's work was supported by the National Natural Science Foundation of China (Grant no. 51272050 and 51572051).

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