Multidimensional Nonstoichiometric Electrode Materials for Electrochemical Energy Conversion and Storage

Abstract

number of compounds deviate from definite composite. For example, Wüstite type FeO was found to occur in varied Fe/O ratios with Fe vacancies over a narrow limit in nature, [2,3] while stoichiometric form of FeO was obtained only under high pressure. [4] With the discovery of more chemical compounds with compositions deviating from their stoichiometric counterparts, a new family of materials, which is now recognized as nonstoichiometric material (NMs) or berthollides in honor of Berthollet, has drawn increasing research interest in materials chemistry.Structurally, nonstoichiometry can be induced from defect engineering, element doping, or substitution. [5,6] Although the resulting composition and elemental ratio are allowed to vary in a wide range, the phase and space group of nonstoichiometric compounds are generally identical to the parent stoichiometric counterparts. The type of nonstoichiometry is multidimensional, ranging from point defects (vacancy, substitution or interstitial), local defect (Frenkel pair or cation mixing), linear substitution (typically in layered compounds) to surface defect, bulk composition variation, and spatially dependent concentration gradient. [7] Thermodynamically, deviation from stoichiometry is always accompanied by the change in the system Gibbs free energy and/or redox of characteristic ions, which would endow the materials with multiple physicochemical features in modulating electronic structures, chemical valence, charge storage, carrier diffusion, and stability. [8][9][10][11] Since nonstoichiometry widely exists in many types of compounds including oxides, nitrides, carbides, sulfides, and alloys, it is desirable to develop deep understanding over the fundamental origin, related features, and their contributions to practical functionality.In the context of developing renewable energy conversion and storage, functional electrode materials are crucial to determine the electrochemical performance, stability, and cost. [11,12] In electrocatalysis, exploring highly active electrode materials for hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER) is a key to boost the performance of water splitting, fuel cell, and metalair battery. [13][14][15] Meanwhile, electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides a promising pathway of ammonia synthesis from N 2 and H 2 O alternative to the traditional contaminative Haber-Bosch process. [16] For batteries, the cell voltage, reversible capacity, and cycling Nonstoichiometry plays an important role in determining physicochemical properties such as conductivity, activity, and stability due to the compositioninduced change in phase, local atomic environment, and electronic structure. A variety of materials with nonstoichiometry have emerged in electrochemical energy conversion and storage, which necessitates a solid understanding of their formation mechanism and structure-function relationship. This review presents a summary of the progress made in this ...

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