Multianion Transition Metal Compounds: Synthesis, Regulation, and Electrocatalytic Applications

ConspectusEnergy electrocatalysis is an essential part for the modern energy network serving as the core technique in many electrochemical energy devices such as batteries, fuel cells, electrolyzers, etc. Developing high-performance electrocatalysts, especially the pursuit for higher intrinsic electrocatalytic activity, is the eternal theme for high-efficiency energy electrocatalysis. Transition metal compounds are highly considered as promising high-performance electrocatalysts due to their facile fabrication, well-defined structure, and encouraging intrinsic activity. However, further promotion on intrinsic electrocatalytic activity of conventional transition metal compounds encounters an unavoidable bottleneck originated from the rigid homoanion structure with limited choice of anions. Facing the above issue, heteroanion substitution on pristine transition metal compounds has been proposed in some recent research. Their unique structure and properties that distinguish them from routine transition metal compounds have attracted wide attention to reveal distinct electronic structures, synthesis methodologies, and functions in energy electrocatalysis for future application.Typically, multianion transition metal compound is a type of transition metal compounds containing two or more types of anions, where different anions co-occupy the same type of sites to construct the heteroanion structure. The heteroanion structure is the core feature that distinguishes multianion transition metal compounds from routine transition metal compounds, which is regarded as a double-edged sword. On one hand, considering the huge radius difference between various types of anions, the heteroanion structure is metastable. Therefore, the synthesis and morphology regulation on multianion transition metal compounds are quite challengeable. On the other hand, the highly regulable heteroanion structure affords considerable opportunities on fundamental investigation on the structure-performance relationship as well as electrocatalytic activity enhancement via rational electronic structure modulation toward practical applications.In this Account, we summarize our and the communities' efforts on the investigation of multianion transition metal compounds as a new branch of material science. We start with a basic background on their definition and general properties. Then we conduct an in-depth discussion on the synthesis of multianion transition metal compounds via anionic exchanging strategy. The advances on morphology regulation are subsequently highlighted. Furthermore, their electrocatalytic applications are presented where the mechanism on their high intrinsic activity is summarized from the perspective of fundamental principles of chemistry and material science. Finally, we end the Account with a brief overview of the current challenges and promising directions to join different voices and efforts on the conceptual advances and potential applications for multianion transition metal compounds.