Structure Engineering and Electronic Modulation of Transition Metal Interstitial Compounds for Electrocatalytic Water Splitting

CONSPECTUS: Hydrogen is deemed as an ideal energy carrier because of its high energy density and clean nature. Water electrolysis is fairly competitive for hydrogen production due to the conversion of renewable electricity to high-purity H2 with no carbon emission, in comparison with traditional industrial technology. However, the large-scale application is hampered by high cost partially from the use of noble metal-based catalysts to promote the kinetics of hydrogen and oxygen evolution reactions. Developing cost-efficient transition metal-based electrocatalysts, therefore, is a hopeful prospect, because they can provide d -orbital lone-pair electrons or empty d-orbitals for adsorbing different intermediates (such as H*, OH*, O*, and OOH*). As compared to transition metals and their oxides, transition metal interstitial compounds (TMICs) formed by inserting C, N, and P atoms into the interstitial sites of parent metals hold distinct advantages in their Pt-like electronic structure, high conductivity, and superior chemical stability over a wide pH range, beneficial to overcoming the high energy consumption faced by alkaline water electrolysis and the intractable stability issue of acid water electrolysis. Nevertheless, the major drawbacks are large size, high density, and sluggish ionic kinetics, resulting in ordinary electrochemical activity and low mass efficiency. Electrocatalytic performance is dominated by the intrinsic activity, the number of accessible active sites, and the capacity of charge and mass transfer. Engineering the micronano structure (small-size particles, porous structure, and ultrathin nanosheet) can expose more catalytical active sites and facilitate mass transport and gas diffusion. Meanwhile, modulating the electronic structure can optimize the adsorption energy of the intermediates to boost the intrinsic activity. Apparently, synergistic modulation of the micronano structure and electronic structure of TMICs is expected to achieve the multiobjective optimization for targeting the highly effective catalysts. In this Account, we summarize our recent efforts in the designed synthesis and structure engineering of TMICs by utilizing polyoxometalates (POMs) as metal precursors and the associated electronic modulation strategies to advance the electrocatalytic performance toward HER and OER. We start with a brief summary of the HER and OER mechanisms, which play crucial roles in the elaborate design of the relevant electrocatalysts. The advantages and disadvantages of TMICs for water electrolysis are pointed out, apart from the opportunities offered by POMs for constructing novel TMICs from size, component, and interface structure. Several efficient strategies for performance enhancement are proposed including reducing the size to expose more accessible active sites, constructing heterojunctions to provide highly active interfaces, doping heteroatoms to regulate the binding energy of intermediates, and creating pores to accelerate mass transfer, etc. Accordingly, the TMICs with controllable size and well-defined structure are highlighted, in which the positive role of tailoring the micronano structure and electronic structure on enhancing the catalytic efficiency is confirmed. Furthermore, paired electrocatalysis by using hydrogen and oxygen active species from water is proposed to produce value-added chemicals and reduce energy consumption. Finally, the remaining challenges, opportunities, and future development directions of TMICs-based materials toward electrocatalytic energy conversion are discussed.