Chemical transformation approach for high-performance ternary NiFeCo metal compound-based water splitting electrodes

https://doi.org/10.1016/j.apcatb.2021.120246Get rights and content

Highlights

  • Ternary NiFeCo LDH and phosphide electrodes were produced by exploiting corrosion of a Ni foam and subsequent phosphidation, respectively.

  • These ternary LDH and phosphide electrodes exhibited remarkably high activities for oxygen and hydrogen evolution reactions, respectively.

  • The electrodes applied to high-efficiency alkaline and anion exchange membrane water electrolyzers.

Abstract

Developing high-efficiency and cost-efficient electrodes for hydrogen and oxygen evolution reactions (HER and OER respectively) is a major challenge in water electrolysis technology. We developed a chemical transformation route that can produce both high-performance OER and HER electrodes by corrosion of a Ni foam and subsequent phosphidation process, which generated ternary NiFeCo layered double hydroxide (LDH) and phosphide nanosheets supported on the foam, respectively. Because of their compositions and high electrical conductivities, these ternary LDH and phosphide electrodes exhibited remarkably high activities for OER and HER, respectively, outperforming most of other catalysts reported to date. An alkaline water electrolyzer based on these electrodes achieved a current density of 10 mA/cm2 at an only 1.47 Vcell. In addition, anion exchange membrane water electrolyzer based on these electrodes reached a current density of 500 mA/cm2 at an only 1.75 Vcell.

Introduction

Electrochemical water splitting provides a promising Environmental-friendly route to the hydrogen (H2) production [[1], [2], [3]]. However, oxygen evolution reaction (OER) catalyst is kinetically limited at the anode of a water electrolyzer. At actual operating current density, the high overpotential required for this reaction reduces the water splitting efficiency [[4], [5], [6]]. Currently, commercial IrO2 and RuO2 catalysts are known as OER catalysts with high activity. However, cost-effective OER catalysts that have lower overpotential than existing precious catalysts are required for the efficient and economical electrochemical water splitting [[7], [8], [9]]. Moreover, the most effective catalysts for the hydrogen evolution reaction (HER) are based on Pt, which is costly and rare [[10], [11], [12], [13]]. To address this issue, various OER and HER catalysts based on non-precious, earth-abundant elements have been explored, including transition metal oxides [14,15], (oxy)hydroxides [[16], [17], [18], [19]], nitrides [20,21], phosphides [[22], [23], [24]], and sulfides [25,26]. Alkaline and Anion exchange membrane (AEM) water electrolyzers are attracting much attention because they can use non-precious metal catalysts. However, despite recent progress, most of water electrolysis reported so far require large overpotentials of ≥ 300 mV to reach current densities of ≥10 mA/cm2 in alkaline water electrolysis, and high current density of ≥500 mA/cm2 to reach ≥1.8 Vcell in AEM water electrolysis [9], and further improvement in OER and HER activities is needed to achieve low-overpotential water splitting.

Chemical transformation often allows to produce nanostructures with an unconventional morphology or composition that cannot be achieved otherwise [[23], [24], [25], [26]]. Corrosion is a chemical transformation reaction that occurs between metals with different reduction potentials facing each other [27,28]. Many studies have been done to prevent undesirable effects of the corrosion which cause damage or failure of metals and metal alloys. Here we demonstrate that corrosion can be harnessed to chemically transform a conventional metal foam into a high-performance OER electrode. Ni foams (NF) were corroded in aqueous solutions with Fe3+ and Co2+ cations to obtain OER electrodes composed of double hydroxide (LDH) nanosheets with a ternary NiFeCo layer supported by NF, which exhibited an OER current density of 10 mA/cm2 at a potential of only 1.4 V (iR corrected). To our knowledge, the presented ternary NiFeCo LDH electrode is one of the most active OER electrodes in alkaline media reported to date [8]. This OER electrode was further converted to an HER electrode consisting of a ternary NiFeCo phosphide nanosheet array via a subsequent phosphidation process, which showed an HER activity as high as a commercial Pt/C catalyst. These two electrodes were coupled to construct an alkaline water electrolyzer that achieved a current density of 10 mA/cm2 at only 1.47 Vcell (iR corrected), which was 160 mV better than the IrO2 and Pt/C catalyst pairs. We also used these electrodes to fabricate an AEM water electrolyzer that reached a cell current density of 500 mA/cm2 at only 1.75Vcell (without iR-corrected). Our electrodes showed excellent catalytic stability without compromising performance in both alkaline and AEM electrolyzers. Our synthesis method does not require the dispersion or coating technology of additional catalysts. Furthermore, this chemical transformation method is readily scalable, providing a promising route for large-area production of high-efficiency and low-cost OER and HER electrodes for efficient electrochemical water splitting.

Section snippets

Corrosion of NF

The oxide layer on the NF surface was removed by reducing heat treatment in a H2/Ar atmosphere for 2 h at 800 °C in a tube furnace. The NF was immersed in an aqueous solution in which Co(CH3COO)2 (33.3 mM) and Fe(NO3)3·9H2O (66.7 mM) were dissolved and heated at 95 °C for 36 h with magnetic stirring. The corroded electrodes were washed several times with deionized water and ethanol and dried in an oven at 80 °C for overnight.

Synthesis of NiFe LDH electrode

0.39 mmol of Ni(NO3)2·6H2O, 0.37 mmol of Fe(NO3)3·9H2O, and 1.88 mmol

Characterization of the NiFeCo LDH electrode

A surface oxide layer of NF with a porous three-dimensional structure was removed through reducing heat treatment (Fig. S1). The corrosion reaction was prepared by immersing the NF electrode in an aqueous solution in which Fe(NO3)3 and Co(CH3COO)2 were dissolved, and then heating the solution at 95 °C with magnetic stirring (Fig. 1a). The corrosion process and OH– formation were the reduction potential difference of Ni2+/Ni and Fe3+/ Fe2+ and the formation of OH– trapped by the electrons

Conclusion

In summary, we have demonstrated a chemical transformation route to produce high-efficiency and cost-efficient OER and HER electrodes using a conventional metal foam as a starting material. We successfully obtained an OER electrode by the corrosion of a NF. The corroded electrode consisted of ternary NiFeCo LDH nanosheet arrays supported on the NF, and exhibited an unprecedentedly high OER activity, which could be attributed to high electrical conductivity from direct growth of the nanosheets

Author contributions

J.L., H.J., S.M.C., J.W.H. and B.L. designed the experiments. J.L., N.K. and S.W. fabricated electrodes and performed structural analyses and electrochemical measurements. N.C.S.S. and P.J.Y. contributed to phosphidation experiment. H.J. and J.W.H. performed the DFT calculations. G.S.H. and H.S.J. contributed to the solar cell experiment. J.L., Y.S.P. and S.M.C. performed the AEM water electrolysis test. N.K. and J.H.L. contributed to the data analysis. J.L. and B.L. wrote the paper. All

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Basic Science Research Program (No. 2015R1A2A2A01006325 and 2019R1A2C2006997) and Creative Materials Discovery Program (NRF-2019M3D1A1079303) through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning.

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