F or V-induced activation of (Co, Ni)2P during electrocatalysis for efficient hydrogen evolution reaction†
A series of (Co, Ni)2P–xF electrocatalysts with different compositions and morphologies were grown on Ni foam (NF) in the absence or presence of different amounts (x mg) of NH4F. After electrolysis in the hydrogen evolution reaction (HER) process for 10 h, the electrolyzed samples, post-(Co, Ni)2P–xF, display different HER electrocatalytic activities. Among them, post-(Co, Ni)2P–10F exhibits the best activated HER performance with an overpotential (η10) of 85 mV to reach a 10 mA cm−2 HER current density, which is much better than that of (Co, Ni)2P–10F (η10 = 158 mV) and comparable to that of the commercial Pt/C benchmark (η10 = 56 mV). Furthermore, post-(Co, Ni)2P–10F exhibits good long-term durability in KOH solution. It is associated with its nanoplate arrays constructed from numerous tiny nanoparticles, and the partial transformation of metal–P into metal–O bonds during the electrolysis activation gives more suitable H adsorption free energy (ΔGH) on the O-site (−0.022 eV) than on the P-site (0.086 eV). In contrast, the free-F sample, post-(Co, Ni)2P, shows inferior HER behavior in comparison with (Co, Ni)2P. F-Incorporation plays a role in the activation process of metal phosphides, and the mechanism is explored by density functional theory (DFT) calculation. Furthermore, the V-induced activation effect on the HER performance is also observed for the electrolyzed metal phosphide. The η10 at post-(Co, Ni)2P–10F–V is 89 mV, which is much lower than that at the original sample, (Co, Ni)2P–10F–V (155 mV).