Oxygen evolution reaction (OER) electrolysis, as an important reaction involved in water splitting and rechargeable metal-air batteries, has attracted increasing attention for clean energy generation and efficient energy storage. Nickel/iron (NiFe)-based compounds have been known as active OER catalysts since the last century, and renewed interest has been witnessed in recent years on developing advanced NiFe-based materials for better activity and stability. In this review, we present the early discovery and recent progress on NiFe-based OER electrocatalysts in terms of chemical properties, synthetic methodologies and catalytic performances. The advantages and disadvantages of each class of NiFe-based compounds are summarized, including NiFe alloys, electrodeposited films and layered double hydroxide nanoplates. Some mechanistic studies of the active phase of NiFe-based compounds are introduced and discussed to give insight into the nature of active catalytic sites, which could facilitate further improving NiFe based OER electrocatalysts. Finally, some applications of NiFe-based compounds for OER are described, including the development of an electrolyzer operating with a single AAA battery with voltage below 1.5 V and high performance rechargeable Zn-air batteries. 
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A mini review of NiFe-based materials as highly active oxygen evolution reaction electrocatalysts

Poor oxygen evolution reaction (OER) catalysis limits the efficiency of H2 production from water electrolysis and photoelectrolysis routes to large-scale energy storage. Despite nearly a century of research, the factors governing the activity of OER catalysts are not well understood. In this Perspective, we discuss recent advances in understanding the OER in alkaline media for earth-abundant, first-row, transition-metal oxides and (oxy)hydroxides. We argue that the most-relevant structures for study are thermodynamically stable (oxy)hydroxides and not crystalline oxides. We discuss thin-film electrochemical microbalance techniques to accurately quantify intrinsic activity and in situ conductivity measurements to identify materials limited by electronic transport. We highlight the dramatic effect that Fe cations—added either intentionally or unintentionally from ubiquitous electrolyte impurities—have on the activity of common OER catalysts. We find new activity trends across the first-row transition metals...

Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles

We report a practical and efficient strategy for synthesizing hierarchical (meso- and macro-)porous NiO nano/micro spherical superstructures. First, β-Ni(OH)2 microspheres were self-assembled based on the coalescence and Ostwald-ripening mechanisms during refluxing in an alkaline solution with Ni(NH3)x2+ at 97 °C for 1 h under vigorous stirring. Second, hierarchical porous NiO microsphere superstructures were obtained from the precursor by a simple calcination procedure. The resulting superstructures comprised two-dimensional mesoporous NiO petal building blocks. Electrochemical data demonstrated that the hierarchical porous NiO nano/micro superstructures were capable of delivering a specific capacitance of 710 F g−1 at 1 A g−1 and offered a good specific capacitance retention of ca. 98% after 2000 continuous charge-discharge cycles. This indicates that we successfully met key requirements in terms of large specific energy density, high-rate capability and good electrochemical stability. We would expect our superstructures to be good candidates for a low-cost replacement for the state-of-the-art supercapacitor material RuO2.

Facile synthesis and self-assembly of hierarchical porous NiO nano/micro spherical superstructures for high performance supercapacitors

Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides

Ni-(oxy)hydroxide-based materials are promising earth-abundant catalysts for electrochemical water oxidation in basic media. Recent findings demonstrate that incorporation of trace Fe impurities from commonly used KOH electrolytes significantly improves oxygen evolution reaction (OER) activity over NiOOH electrocatalysts. Because nearly all previous studies detailing structural differences between α-Ni(OH)2/γ-NiOOH and β-Ni(OH)2/β-NiOOH were completed in unpurified electrolytes, it is unclear whether these structural changes are unique to the aging phase transition in the Ni-(oxy)hydroxide matrix or if they arise fully or in part from inadvertent Fe incorporation. Here, we report an investigation of the effects of Fe incorporation on structure–activity relationships in Ni-(oxy)hydroxide. Electrochemical, in situ Raman, X-ray photoelectron spectroscopy, and electrochemical quartz crystal microbalance measurements were employed to investigate Ni(OH)2 thin films aged in Fe-free and unpurified (reagent-grade)...

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Effects of Fe Electrolyte Impurities on Ni(OH)2/NiOOH Structure and Oxygen Evolution Activity

Reversible potentials (E
R) have been measured for nickel hydroxide/oxyhydroxide couples over a range of KOH concentrations from 0·01–10 M. It is shown that the couples derived from the parentα- andβ-Ni(OH)2 systems can be distinguished by the relative change in KOH level on oxidation and reduction. In the case of couples derived from theα-class of materials a dependence of 0·470 moles of KOH per 2e change is found compared with 0·102 moles of KOH per 2e change for theβ-class of materials. Couples derived from theα- andβ-Ni(OH)2 systems can be encountered in a series of ‘activated’ and ‘de-activated’ forms having a range of formal potentialsE

 0
 ′
 . ‘Activated’. and ‘de-activated’β-Ni(OH)2/β-NiOOH couples are found to lie in the range 0·443–0·470 V whilstα-Ni(OH)2/γ-NiOOH couples lie in the range 0·392–0·440 V w.r.t. Hg/HgO/KOH. It is demonstrated for ‘de-activated’,β-Ni(OH)2/β-NiOOH couples thatE
R is independent of the degree of oxidation of the nickel cation between states of charge of 25% and 70%. SimilarlyE
R is constant for states of charge between 12% and 60% for ‘activated’α-Ni(OH)2/γ-NiOOH couples. The constant potential regions are considered to be derived from heterogeneous equilibria between pairs of co-existing phases both containing nickel in upper and lower states of oxidation. Differences inE

 0
 ′
 between the ‘activated’ and ‘de-activated’ couples are considered to be related to the degree of order/disorder in the crystal lattice.

Studies concerning charged nickel hydroxide electrodes I. Measurement of reversible potentials

In this paper the thermodynamics of mixing are applied to account for the independence of the discharge potential of the nickel hydroxide electrode as a function of nickel oxidation state. The constant potential region is considered to arise from the formation of a pair of co-existing solid solutions having a composition predetermined by the magnitude of the interactions between the oxidized and reduced species. From considerations of the excess-energy terms, it can be shown for a symmetrical potential/ composition profile, that the constant potential region is identical with the standard potentialE
0. The influence of asymmetry on the changes inE
0 are discussed. Consideration has also been made of the influence of dissociation of oxidized and/or reduced species on the potential determining equations. The removal of n-type defects from the nickel(II)-rich phase on discharge is considered to be responsible for the observed secondary discharge plateau at potentials ∼ 300 mV more cathodic than normal. This non-equilibrium behaviour can be explained in terms of a mixed pn-semiconducting material.

Studies concerning charged nickel hydroxide electrodes. II. Thermodynamic considerations of the reversible potentials

Studies concerning charged nickel hydroxide electrodes

The variation of reversible potential Er with log amoh and\(a_{H_2 O} \) has been studied for several nickel hydroxide/oxyhydroxide couples in various alkali hydroxides. Both ‘activated’ and ‘deactivated’ β-phase couples show only a small dependence ofEr with logmoh (or\(a_{H_2 O} \) where known) in LiOH, NaOH, RbOH and CsOH electrolytes. The change in MOH content on oxidation/reduction is found to be about 0.1 mol MOH per two-electron transfer and is the same as found previously in KOH. These results confirm that the bulk oxidized β-phase lattice is devoid of alkali cation although a small quantity may be adsorbed by the surface. On the other hand both ‘activated’ and ‘deactivated’ α/γ-phase couples show a marked dependence of ∼ 0.45 mol MOH per two-electron transfer in LiOH, NaOH and RbOH (at concentrations > 0.5 m), also in good agreement with earlier data for KOH. On the basis of these results a general stoichiometry can be inferred for the γ-phase, namely M0.32NiO2 · 0.7H2O where M=Li+, Na+, K+ or Rb+. Measurements imply that the Cs+ ion or the Rb+ ion at low concentration (<0.5 m) do not enter the interlayer structure of the γ-phase. This behaviour is thought to be related to the low Rb-O and Cs-O bond strengths afforded by the γ-phase structure.

F. L. Tye

Papers

Studies concerning charged nickel hydroxide electrodes I. Measurement of reversible potentials

Studies concerning charged nickel hydroxide electrodes. II. Thermodynamic considerations of the reversible potentials

Studies concerning charged nickel hydroxide electrodes

Studies concerning charged nickel hydroxide electrodes IV. Reversible potentials in LiOH, NaOH, RbOH and CsOH

Studies concerning charged nickel hydroxide L Measurement of reversible potentials electrodes.