Anion-Containing Noble-Metal-Free Bifunctional Electrocatalysts for Overall Water Splitting

With ever-increasing severe environment pollutions and energy demand, it is becoming greatly important to develop sustainable energy resources as alternatives to traditional fossil fuels. As one of the ultimate clean energy sources, hydrogen (H2) energy produced by water splitting using electricity from solar radiation, wind, tide, and nuclear fusion, among other sources, has attracted more and more public attention since the late 18th century. Although noble-metal-based electrocatalysts perform well, they suffer severely from the high cost and rarity from the viewpoint of practical applications. Recently, various kinds of non-noble-metal electrocatalysts based on low cost and earth-abundant transition metals have been developed for both hydrogen evolution reaction (HER) and oxygen (O2) evolution reaction (OER). Among them, non-noble-metal bifunctional electrocatalysts (NHOBEs), showing high performances in both HER and OER, are therefore of great significance and importance in future applications. With NHOBEs, one will be able to significantly simplify the fabrication procedures of electrolyzers, and more importantly, elevate the efficiency of "all in one" electrocatalytic water-splitting systems, which will greatly favor their industrial applications by demanding substantially lowered the production costs. While the laws of NHOBEs from the same main group anionic elements such as oxygen, nitrogen, carbon, and boron families have not been well-reported so far. Herein, the recent significant progresses of NHOBEs classified by different main group anionic elements are summarized, and emphases are placed to the designs, syntheses, electrocatalytic performances in water-splitting, and future possible applications of NHOBEs. Moreover, the prospects of a number of vigorously investigated NHOBE catalysts and the possible trends of future developments are also evaluated. In addition, current challenges facing research with respect to electrocatalysts for water-splitting are discussed.