Recent developments in earth-abundant and non-noble electrocatalysts for water electrolysis

Exploiting the peak excess electricity from abundant but intermittent wind and solar energy or from the overnight surplus in power grids to produce a storable chemical fuel as an alternative to conventional fossil fuels is very appealing yet challenging. Hydrogen produced by water electrolysis is an ideal energy carrier for potentially scalable storage of these energy sources because it has high energy density and does not emit any pollutant or greenhouse gas upon combustion. However, overall water splitting, including hydrogen evolution reaction and oxygen evolution reaction (HER and OER, respectively), currently requires a large excess potential to expedite the reactions (200–400 mA cm−2 at cell voltages of 1.8 V–2.4 V in base), resulting in less than 4% of the world's industrial hydrogen being produced by electrolysis. To overcome this obstacle, as well as the high cost of traditional noble-metal catalysts, considerable achievements have been made recently in the development of cheap and earth-abundant electrocatalysts, including some robust catalysts approaching commercial criteria, but reviews of these electrocatalysts and their compatibility with commercial-scale water electrolysis remain lacking. In this review, we will present an overview of recent developments in the production of high-performance earth-abundant and non-noble electrocatalysts for HER and OER, as well as for overall water splitting. With an eye toward the commercialization of water electrolysis, emphasis is placed on the most efficient electrocatalysts for either HER or OER, as well as those showing sustainable capability of withstanding accelerated degradation under large current densities (≥500 mA cm−2) over long periods of time, which is critically indispensable for actual applications of this technology. The major challenges facing the production of such electrocatalysts and possible future improvements in the fabrication of robust electrocatalysts for water electrolysis are also highlighted.