Direct growth of cobalt-rich cobalt phosphide catalysts on cobalt foil: an efficient and self-supported bifunctional electrode for overall water splitting in alkaline media

Abstract

The design of high-efficiency, economical and self-supported bifunctional electrodes for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is extremely crucial to renewable energy conversion processes, yet remains a long and arduous task. Here, we report the first example of cobalt-rich cobalt phosphide catalysts directly grown on cobalt foil (denoted as Co₂P/Co-foil) as a novel non-noble metal and integrated electrode by one-step phosphorization of a pre-oxidized Co foil. Owing to the intrinsic catalytic properties of cobalt-rich cobalt phosphide and the intimate contact between Co₂P and highly conductive Co foil, the resulting Co₂P/Co-foil electrode exhibits excellent catalytic performances for both the HER and OER in basic solution, affording a current density of 10 mA cm⁻² at low overpotentials of 157 mV for the HER and 319 mV for the OER, respectively. More importantly, these electrodes can be directly employed as both the anode and cathode in an alkaline electrolyzer, showing noble metal-like water splitting performances and long-term stability. Density functional theory (DFT) calculations suggest that the sites on the top of the P atoms in Co₂P are the most active sites for the HER. This work would open an exciting new avenue to synthesize other metal-rich metal phosphide catalysts on conductive metal foil as self-supported electrodes using this facile, cost-effective and easy scale-up fabrication method for overall water splitting.