Principles of Water Electrolysis and Recent Progress in Cobalt-, Nickel-, and Iron-Based Oxides for the Oxygen Evolution Reaction

Abstract

Water electrolysis that results in green hydrogen is the key process towards a circular economy. The supply of sustainable electricity and availability of oxygen evolution reaction (OER) electrocatalysts are the main bottlenecks of the process for large-scale production of green hydrogen. A broad range of OER electrocatalysts have been explored to decrease the overpotential and boost the kinetics of this sluggish half-reaction. Co-, Ni-, and Fe-based catalysts have been considered to be potential candidates to replace noble metals due to their tunable 3d electron configuration and spin state, versatility in terms of crystal and electronic structures, as well as abundance in nature. This Review provides some basic principles of water electrolysis, key aspects of OER, and significant criteria for the development of the catalysts. It provides also some insights on recent advances of Co-, Ni-, and Fe-based oxides and a brief perspective on green hydrogen production and the challenges of water electrolysis.

Keywords: cobalt; iron; nickel; oxygen evolution reaction; water splitting.

Scheme 1

A typical water electrolysis cell under alkaline conditions.

Scheme 2

An alkaline water‐splitting cell; the magnification shows a generalized mechanism of OER in the alkaline medium over a metal electrocatalyst.

Figure 1

The OER volcano plot of overpotentials of diverse electrocatalysts at 1 mA cm⁻² vs. energy differences (ΔG _*O−ΔG _*OH). Reproduced with permission from ref.  Copyright 2017, American Association for the Advancement of Science.

Scheme 3

Summary of important key parameters to evaluate OER activity, including experimental conditions and evaluation methods.

Figure 2

a) LSV curves (1) before and (2) after Ohmic drop correction on the boron‐doped diamond electrode; inset shows the corresponding Tafel plots. Reprinted with permission from ref.  Copyright 2018, Elsevier. b) Plots of NiO loading on the electrode against the value of η ₁₀ and the Tafel slope, which were derived from the LSV result after Ohmic drop correction. Reprinted with permission from ref.  Copyright 2019, American Chemical Society.

Figure 3

Nyquist plots were obtained from EIS measurements for Co₃O₄, NiO, and mixed spinel oxides before (a) and after (b) electrochemical activation. Reprinted with permission from ref. [29a] Copyright 2017, American Chemical Society.

Figure 4

Carbon oxidation and relevant current evaluation (i _C) along with the OER (i _OER) of carbon‐supported nickel boride catalyst. Adapted with permission from ref.  Copyright 2019, Wiley‐VCH Verlag GmbH & Co.

Figure 5

A summary of the catalytic stability of noble‐metal‐based electrocatalysts tested at 1 A cm⁻² in proton‐exchange‐membrane water electrolyzer. The overpotential changes (Δη=η _final−η _initial) indicates activity delay, with the results compiled from the literature. Reproduced with permission from ref.  Copyright 2017, Wiley‐VCH Verlag GmbH & Co.

Figure 6

a) Theoretical overpotential plot of doped β‐CoOOH as a function of ΔG _OH and ΔG _O−ΔG _OH. Adapted with permission from ref.  Copyright 2013, American Chemical Society. b) Water adsorption on cobalt (111) surface of defect‐free (left) and Co_3−x O₄ with Co defect sites (right); numbers in blue denote the O−H bond length of adsorbed H₂O, numbers in yellow denote bond lengths between adsorbed O and H from water species over Co and O sites of Co₃O₄. Adapted with permission from ref.  Copyright 2018, American Chemical Society.

Figure 7

a) Electrochemical in situ surface‐enhanced Raman spectra (SERS) of isotope‐labeled Co¹⁸O₂ in 0.1 M purified ¹⁶O‐KOH after conditioning in ¹⁸O‐KOH. b) In situ SERS of Co¹⁶OOH in 0.1 M Fe‐free isotope‐labeled purified KOD. c) Proposed OER mechanism on cobalt oxyhydroxide. Adapted with permission from ref. [24b] Copyright 2020, American Chemical Society.

Figure 8

a) Morphology of mixed cobalt iron oxide (Co/Fe 32) templated on SBA‐15 silica. b) Co K_β‐detected HERFD XAS spectra of sample series. c) A_1g Raman band and d) LSV curve of SBA‐15 templated cobalt iron oxide series; inset: magnification at 10 mA cm⁻². Adapted with permission from ref. [83a] Copyright 2020, American Chemical Society. e) Initial LSV curve of mesostructured cobalt nickel sample series and f) LSV curves after 150 CV scans. Adapted with permission from ref. [29a] Copyright 2017, American Chemical Society.

Figure 9

a) CV curves of NiO nanoparticles deposited on a gold‐coated electrode. The CV curves were collected in 0.5 M KOH before and after electrochemical aging and are labeled “a” and “b”. Reproduced with permission from ref.  Copyright 2014, Wiley‐VCH Verlag GmbH & Co. b) Illustration of the Bode scheme for the phase transformation on Ni(OH)₂. c) Aging effect on the OER activity of Ni(OH)₂ thin films in Fe‐free (blue) and unpurified (red) KOH electrolyte. d) Change of iron amount in Ni(OH)₂ thin films after aging in unpurified KOH. Reproduced with permission from ref.  Copyright 2015, American Chemical Society.

Figure 10

a) LSV curves and b) chronopotentiometry measurements for (012)‐O, (012), (104), and (110) facets exposed on α‐Fe₂O₃. c) Free energy diagram of OER intermediates. Adapted with permission from ref.  Copyright 2018, Wiley‐VCH Verlag GmbH & Co.

Figure 11

Proposed OER reaction mechanisms of a) FeOOH‐NiOOH; Fe ion is located on the surface of the γ‐FeOOH cluster and the rate‐determining step is OH⁻ attack on Fe=O and H atom transfer to a Ni^III‐O site. b) NiFe LDH, Fe is doped in the lattice of Ni LDH, with the rate‐determining step OH⁻ attack on Fe=O. Reprinted with permission from ref.  Copyright 2021, Wiley‐VCH Verlag GmbH & Co.

Scheme 4

Future energy landscape including usage of sustainable electricity for water electrolysis for green hydrogen production, its storage, and utilization for power and heat generation, as fuel for transportation as well as feedstock for industrial application.