Mastering Surface Reconstruction of Metastable Spinel Oxides for Better Water Oxidation

Abstract

The development of efficient electrocatalysts that lower the overpotential of oxygen evolution reaction (OER) is of great importance in improving the overall efficiency of hydrogen fuel production by water electrolysis. [1] Commercially, precious metal oxides catalysts such as IrO 2 are used. [2] However, their elemental scarcity and high cost have triggered a search for cost-effective OER electrocatalysts such as 3d transition metal oxides. Among them, families such as the perovskite ABO 3 and the spinel AB 2 O 4 ones have attracted great attention due to their tunable structural/elemental properties allowed by A and B site cation substitution. [3,4] Perovskite ABO 3 oxides have a simple structure with rare-earth or alkaline earth element occupying cuboctahedral A-site while the B-site transition metal (TM) sites in an octahedral environment. [3] Spinel oxides, however, can be either normal or inverse structure depending on the relative occupancy of divalent and trivalent cations in the octahedral and Developing highly active electrocatalysts for oxygen evolution reaction (OER) is critical for the effectiveness of water splitting. Low-cost spinel oxides have attracted increasing interest as alternatives to noble metalbased OER catalysts. A rational design of spinel catalysts can be guided by studying the structural/elemental properties that determine the reaction mechanism and activity. Here, using density functional theory (DFT) calculations, it is found that the relative position of O p-band and M Oh (Co and Ni in octahedron) d-band center in ZnCo 2−x Ni x O 4 (x = 0-2) correlates with its stability as well as the possibility for lattice oxygen to participate in OER. Therefore, it is testified by synthesizing ZnCo 2−x Ni x O 4 spinel oxides, investigating their OER performance and surface evolution. Stable ZnCo 2−x Ni x O 4 (x = 0-0.4) follows adsorbate evolving mechanism under OER conditions. Lattice oxygen participates in the OER of metastable ZnCo 2−x Ni x O 4 (x = 0.6, 0.8) which gives rise to continuously formed oxyhydroxide as surface-active species and consequently enhances activity. ZnCo 1.2 Ni 0.8 O 4 exhibits performance superior to the benchmarked IrO 2 . This work illuminates the design of highly active metastable spinel electrocatalysts through the prediction of the reaction mechanism and OER activity by determining the relative positions of the O p-band and the M Oh d-band center.

Recent Progress on Surface Reconstruction of Earth‐Abundant Electrocatalysts for Water Oxidation

“…No clear shift of Co with same applied potential compared with open circuit suggested the coordination and conductor role. The formation of active NiOOH and leaching of Zn and Co in metastable ZnCo 1.2 Ni 0.8 O 4 were also confirmed by HRTEM and scanning transmission electron microscopy energy‐dispersive X‐ray spectroscopy (STEM‐EDS) . Diminishing peaks at 532 eV in K‐edge electron energy loss spectroscopy (EELS) of O further indicated the increasing O vacancies and structure relaxation during OER.…”

Non‐Noble‐Metal‐Based Electrocatalysts toward the Oxygen Evolution Reaction

“…The incorporations of the heteroatom dopants and MoS 2 NPs modulate the electronic structure of the electrocatalyst, produce abundant oxygen vacancies and active sites, and increase the active surface area. The hybrid nanomaterial enriched with oxygen vacancies exhibits an overpotential of 250 mV at 10 mA cm –2 .Other spinel oxide OER electrocatalysts mainly include MMn 2 O 4 , MCr 2 O 4 , inverse spinel oxides, ternary metallic spinel oxides, and spinel oxides constructed with other metal oxides to form hybrid structures, which also exhibit extraordinary OER performance 29,71. For example, Wang et al recently designed a hybrid‐MOF‐assisted method to synthesize novel Co 3 O 4 /Co‐Fe oxide double‐shelled nanoboxes (Co 3 O 4 /Co‐Fe oxide DSNBs) 72.…”

“…For this reason, we put primary focus on these non‐noble‐metal‐based electrocatalysts in this review. To overcome the high overpotential of the OER, numerous non‐noble‐metal‐based electrocatalysts have been developed, including metals/alloys,23,24 oxides,25–31 hydroxides,32–37 chalcogenides,38–42 phosphides,5,43,44 phosphates/borates,19,45,46 borides,47 nitrites,48,49 and other compounds 50,51…”

Strategies to Break the Scaling Relation toward Enhanced Oxygen Electrocatalysis

“…157 For OER, these catalysts can be oxidized and self-reconstructed into amorphous hydrous oxide phases of unknown stoichiometries at the operating potentials. 143,[158][159][160][161][162] Understanding of the reasons for their insufficient stability and mitigation of such activity loss deserves more research attention. In addition, stability tests are often performed on electrodes with a high catalyst loading and tested at constant current over a period of time; if the measured potential change is minimal, it is considered stable.…”