Nickel/cobalt oxide as a highly efficient OER electrocatalyst in an alkaline polymer electrolyte water electrolyzer

Abstract

Water electrolysis by an alkaline solid polymer electrolyte (APE) water electrolyzer is a promising approach for hydrogen production from water. In this work, Ni/Co oxides were prepared by a hydrothermal method, and used as a high-efficiency OER electrocatalyst in an APE water electrolyzer. The APE water electrolyzer assembled with the as-prepared Ni/Co oxides and home-made APE showed a nearly constant operating potential, ∼2.03 V @ 100 mA cm⁻² in 1 wt% KHCO₃ solution for about 550 h in a durability test, indicating the catalyst with good stability can meet the long-term durability requirements of the water electrolyzer. This result shows the practical application of Ni/Co oxides in an alkaline solid polymer electrolyte electrolyzer.

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Introduction

The practical utilization of renewable energy depends on the storage of the electricity produced by intermittent sources, such as solar and wind.¹ Electrochemical water electrolysis has been generally believed to be an appropriate way to utilize the electricity generated.^2,3 As a method to generate hydrogen, water electrolysis has been around for decades.^4,5 Though, in recent years, the hydrogen produced by water electrolysis accounted for a rather small proportion, the overwhelming majority of hydrogen was produced by steam reforming natural gas or other fossil fuels.⁶ But electrochemical water electrolysis can provide a clean route to a “carbon-free” energy matrix, which makes it differ from the other methods.

Among the various water electrolysis technology, aqueous alkaline electrolyte (AAE) water electrolyzer has been used to produce hydrogen on a large scale for decades.⁷ A typical AAE electrolyzer usually based on structures with a porous asbestos diaphram and strong alkaline electrolyte. However, the problems brought by the asbestos diaphram still went unsettled, such as the toxicity of asbestos, bad gas barrier property. Besides, the strong alkaline electrolyte (6 M, 10 M NaOH…) are corrosive and can react with CO₂ to form insoluble carbonates, which can precipitate on the porous asbestos diaphram and block the transport of reactants and products.⁷ What's worse, the aqueous electrolyte also resulted in a high ohmic resistance. Based upon the interaction of all of these factors, the performance of the AAE water electrolyzers is typically low, for instance, the maximum working current density (usually below 100 mA cm⁻²) and the energy efficiency (usually around 60%).^4,7–9 To overcome the drawbacks of the AAE water electrolyzer, the first solid polymer electrolyte (SPE) water electrolyzer was first proposed in the 1960s by General Electric.¹⁰ SPE water electrolyzers were built upon the advances in proton exchange membrane fuel cell technology¹¹ However, SPE water electrolysis technology still remains expensive cost, and significant cost reduction are required to put on markets, though the performance of this technology is satisfactory.

To lower the capital and operating cost, a compact electrolyzer system based on SPE water electrolyzer structure combined with alkaline solid polymer electrolyte (APE) has been proposed. The employed APE water electrolyzer architecture is sketched in Fig. S1.† In a typical APE water electrolyzer, water splitting occurred in the catalyst layer of MEA.^12,13 APE membrane was used to conduct OH⁻ and form an alkaline environment. Then the applied catalysts no longer be restricted to the noble metal catalysts or the high-cost membranes.^14–17

Many high efficient APE water electrolyzers have been showed in recent years. Leng et al.¹³ have reported an APE water electrolyzer with IrO₂ as anode catalyst and Pt black as cathode catalyst. The polarization curve afforded a current density of 399 mA cm⁻² at 1.8 V. Seetharaman et al.¹⁸ have prepared a graphene oxide modified non-noble metal electrode for APE water electrolyzer. With a ternary alloy electrode of Ni/Zn/S as cathode electrode and oxidized Ni electrode coated with graphene oxide as anode electrode. The APE water electrolyzer with non-noble metal materials gave a current density of 90 mA cm⁻² at 2 V. Xiao et al.¹⁹ have fabricated an APE water electrolyzer with an MEA made of APE membrane and Ni-based catalysts. The electrolyzer remains stable for about only 8 h, though the working voltage of the electrolyzer is about 1.8–1.85 V at a current density of 400 mA cm⁻². But these electrolyzers still exist various kind of problems, for example, relatively low working current densities, poor stability,^{12,18,20–22} mainly due to ionomer or catalysts instability.^13,19 Therefore, the catalysts and APE membrane are the key components for the future APEWE.^23,24

Here, we report a nickel oxide/cobalt oxide (NiO/CoO) hexagon nanoplate exhibiting high OER catalytic activity in basic KOH (1 M) solution. The NiO/CoO nano-hybrid is fabricated by a hydrothermal method followed by post-annealing in air at 400 °C for 2 h. We have tried to evaluate the catalytic activity and durability of as-prepared catalysts in a more realistic condition, which would enrich the valuation methodology of OER catalysts.

Results and discussion

Fig. 1 shows the TEM images of Ni_0.7Co_0.3O prepared at 180 °C with 15 h in different solvent. Ni_0.7Co_0.3O_x presents as hexagonal nanoplate. The magnified image reveals that the sample synthesized with solvent (c) water/ethanol = 20 : 10 (v/v) was regular hexagon with a diameter of ca. 200 nm. Moreover, TEM elemental mapping performed on the Ni/Co binary oxides (Fig. 1d) indicates a uniform distribution of Ni, Co and O throughout the nanoplate.

Fig. 1 The TEM images of nickel/cobalt oxide synthesized with different solvent, (a) water/ethanol = 2 : 28 (v/v), (b) water/ethanol = 5 : 25 (v/v), (c) water/ethanol = 20 : 10 (v/v), (d) TEM image of (c) and corresponding element distribution of Co, Ni, and O.

The XRD pattern of the synthesized Ni_0.7Co_0.3(OH)_x and Ni_0.7Co_0.3O_x are shown in Fig. 3a and b. The diffraction peaks at 19.2, 33.0, 38.5, 52.1, and 59.0 degree in Fig. 2a, which correspond to the (001), (100), (101), (102) and (110) planes, the XRD pattern matches well with the standard crystallographic spectrum of hexagonal Ni(OH)₂ (JCPDS card no. 14-0117) and Co(OH)₂ (JCPDS card no. 30-0443). The diffraction peaks at 31.2, 36.8, 38.5, 44.8, 59.3 and 65.2 degree in Fig. 3b, which correspond to the (220), (311), (222), (400), (511) and (440) planes, these XRD patterns are in good agreement with the standard crystallographic spectrum of cubic NiO (JCPDS card no. 47-1049) and Co₃O₄ (JCPDS card no. 42-1467). As we can see from the power XRD patterns, neither Ni nor Co metal state appeared. Based on the Hume-Rothery theory, it's possible to form a continuous solid solution between Ni(OH)₂ and Co(OH)₂ for their similar ionic radius and crystal structure.

Fig. 2 The XRD patterns of the prepared (a) Ni_0.7Co_0.3(OH)_x; (b) Ni_0.7Co_0.3O_x samples. Thermal gravity analysis of (c) Co(OH); (d) Ni(OH)₂; (e) Ni_0.7Co_0.3(OH)_x catalysts, (f) LSV curves of the prepared Ni_0.7Co_0.3(OH)_x; Ni_0.7Co_0.3O_x samples in 1 M KOH at 10 mV s⁻² and 25 °C.

Fig. 3 (a) Steady state i–V curves of an APE water electrolyzer at 50 °C; (b) stability test of the APE water electrolyzer with the nickel/cobalt oxide OER catalyst at 100 mA cm⁻² and 50 °C.

To confirm the phase transformation of Ni_0.7Co_0.3(OH)_x to Ni_0.7Co_0.3O_x under heat treatment, thermal gravity (TG) was performed to prove the structural identifications. Fig. 2c shows thermal gravity curve of the Co(OH)₂ compound. Two weight loss regions, 160–260 °C and 300–450 °C can be seen from Fig. 2c. The first weight loss in a temperature range of 160–260 °C can be ascribed to the Co(OH)₂ decomposed to Co₃O₄;²⁵ as for the region 300–450 °C corresponding to decomposition of Co₃O₄ into CoO.^26,27 The mass changes with the increase of temperature for Ni(OH)₂ (Fig. 2d) reveal, after the adsorbed and intercalated water loss, an obvious transition due to Ni(OH)₂ dehydration and the formation of NiO between 200 °C and 400 °C, and the remaining material corresponds to NiO. Based on the TG and DTG results, the samples were heated at 400 °C to ensure the complete decomposition of Ni_0.7Co_0.3(OH)_x to Ni_0.7Co_0.3O_x (Fig. 2e).

The electrocatalytic activity was evaluated on a rotating disk electrode (RDE), in a typical three-electrode configuration on a 4 mm glassy carbon disk electrode in O₂-saturated 1 M KOH electrolyte. A Pt slice (1 × 1 cm²) and a MMO served as the counter and reference electrodes, respectively. i–V curves of all the samples were measured in 1 M KOH electrolytes with a scan rate of 10 mV s⁻² at room temperature are shown in Fig. 3f. Ni_0.7Co_0.3O_x sample exhibits much better activity than the Ni_0.7Co_0.3(OH)_x with more negative potential at 10 mA cm⁻². The prepared Ni_0.7Co_0.3O_x afforded a current density of 10 mA cm⁻² at a small overpotential of a mere 0.394 V in 1 M KOH solution (1.6241V vs. RHE), about 57 mV lower than the Ni_0.7Co_0.3(OH)_x. Nevertheless, with the current density increasing, Ni_0.7Co_0.3O_x showed much better OER electrochemical activity than Ni_0.7Co_0.3(OH)_x. The in situ formed oxy-hydroxides (e.g. NiOOH, CoOOH) are proposed as the key catalytically active metal species for the OER, which have been evidenced by the previous results.^28–30 Additionally, catalysts prepared at different conditions were characterized by SEM and electrochemical method (see ESI Fig. S2–S4†).

One main challenge for electrolyzer applications is the durability of the catalysts. A single-cell stack was assembled and tested. The fabricated Ni/Co oxides (Ni_0.7Co_0.3O_x–160 °C–12 h, ∼2 mg cm⁻²) were utilized as the anode catalysts and measured after assembled into APEWE cells.

Polarization curve of APE water electrolyzer at 50 °C in 1 wt% KHCO₃ solution was shown in Fig. 3a (Cell-1) (see ESI S2 for details†). The onset voltage of the water splitting was less than 1.5 V in tested single cell with the Ni/Co oxides OER catalyst. In this term, the cell voltage was 2.0 V at 100 mA cm⁻², 2.16 V at 200 mA cm⁻². This result exhibited an electrolyzer energy efficiency of 62.7% (based on the lower heating value of hydrogen, LHV). Besides, the i–V curve of APE water electrolyzer with the commercial non-PGM OER catalysts was shown in Fig. 3a (Cell-2) (see ESI S2 for details†). In this term, the cell voltage was 2.148 V at 100 mA cm⁻², 2.26 V at 200 mA cm⁻², the performance of the commercial catalyst was lower than the as-prepared Ni/Co oxides OER catalyst.

The stability tests for APEWE cells were carried out in constant current mode at 100 mA cm⁻² and 50 °C (Fig. 3b). The cell voltage for the tested cell was relatively constant between 1.94 V and 2.05 V in 550 h running time, indicating that the catalyst with good stability can meet the long-term durability requirements of the water electrolyzer. The electrolyte solution was refreshed after 270 h to get rid of the influence of the concentrated electrolyte. Some results of recently published researches were listed in Table S2.† As we can see from Table S2,† the results of this work has good stability.

Conclusions

Ni/Co oxide electrocatalysts with well-defined morphologies were fabricated by a facile hydrothermal method. The collected results demonstrated that Ni_0.7Co_0.3O_x was an excellent OER electrolcatalyst with good stability can meet the long-term durability requirements of the APEWE water electrolyzer. We assembled a stable and available APEWE for hydrogen generation without highly concentrated alkali solution. A compact electrolyzer system based on SPE water electrolyzer structure combined with alkaline solid polymer electrolyte (APE) has been proposed to lower the capital and operating cost of water electrolysis. Then the applied catalysts no longer be restricted to the noble metal catalysts or the high-cost membranes. Thus, this work paves a viable pathway to prepare novel cobalt-based OER electrocatalyst used for APE water splitting, which certainly could be expanded to energy storage or conversion applications.

Acknowledgements

This work is financially supported by the National Natural Science Foundations of China (No. 91434106 and 21176234), and the Natural Science Foundations of Liaoning Province (No. 2014020088).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19615b

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