Heterostructure iron selenide/cobalt phosphide films grown on nickel foam for oxygen evolution

Abstract

Construction of heterostructures is an effective strategy for improving electrocatalytic activity. Here, heterostructure iron selenide/cobalt phosphide films (FeSe/Co₂P) are synthesized on nickel foam (NF) substrates. Crystalline Co₂P is grown on NF (Co₂P/NF) by a solvothermal method, and an amorphous FeSe layer is electrodeposited on Co₂P/NF. The formation of the heterointerface is confirmed by high-resolution transmission electron microscopy. For the OER in 1 M KOH, FeSe/Co₂P/NF only requires overpotentials of 235 mV and 265 mV to reach 10 mA cm⁻² and 50 mA cm⁻² current densities, respectively, with a Tafel slope of 65.6 mV dec⁻¹. Compared with Co₂P/NF, the high OER activity of FeSe/Co₂P/NF does not originate from the higher electrochemically active surface area, but from enhanced OER activity per Co site. This is caused by the charge redistribution induced by the heterostructure that alters the adsorption energy of the hydroxyl group, and that leads to more facile OER kinetics as indicated by the reduced charge transfer resistance, lower Tafel slope and lower apparent activation energy. At the FeSe/Co₂P/NF surface, the lattice oxygen evolution mechanism dominates, while the adsorbate evolution mechanism prevails at the Co₂P/NF surface. The altered mechanism is probably caused by the increased pK_a of the Co^IV–OH or Co^IV–OH₂ structure in FeSe/Co₂P/NF that favors the decoupled proton–electron transfer process. FeSe/Co₂P/NF also exhibits good long-term durability, and transformation of surface metal selenides and phosphides to oxides and (oxy)hydroxides is observed after the long-term OER.