Ultrathin amorphous CoFeP nanosheets derived from CoFe LDHs by partial phosphating as excellent bifunctional catalysts for overall water splitting

Non-noble-metal electrocatalysts of transition metal phosphides have been considered as promising catalysts for hydrogen evolution reaction (HER); however, their bifunctional catalytic activities are limited by unsatisfactory performance in oxygen evolution reaction (OER). Here we report ultrathin amorphous CoFeP nanosheets derived from CoFe layered double hydroxides (LDHs) as bifunctional electrocatalysts with a simple phosphorization method. The partial phosphating induced by sodium hypophosphite can greatly reduce the thickness of nanosheets and promote the polycrystalline nature of the materials, leading to more active sites exposed and faster electronic transport. The optimal levels of phosphating agents are 1.8 g for OER and 2.4 g for HER, respectively, etched by which the resultant CoFeP materials can exhibit excellent electrocatalytic performances toward OER, HER and overall water splitting in alkaline media. At the current densities of 100 and 20 mA cm⁻², ultralow overpotentials of 242 and 80 mV are achieved for OER and HER, individually, which are 128 mV lower than RuO₂ and 6 mV slightly lower than Pt, respectively. In a cell voltage of 1.62 V, the CoFeP materials reacted with 1.8 and 2.4 g phosphating agents and used as the anode and the cathode, respectively, can achieve 100 mA cm⁻² current density and exhibit extraordinary durability for 30 h in overall water splitting. These superior electrocatalytic properties of ultrathin amorphous CoFeP nanosheets are comparable to those of state-of-the-art noble metal and other reported catalysts for OER, HER and overall water splitting. This work proposes a promising strategy for excellent bifunctional electrocatalysts by tailoring the morphology and crystallinity degree based on LDHs through a facile partial phosphating method.