PdNi alloys with surface segregation of Pd@hydrophilic N, O doped carbons toward hydrogen production with high current densities

A series of catalysts, which consist of PdNi alloy cores and N, O co-doped carbon (NOC) shells, are developed by pyrolysis of metal–organic framework (PdNi) for hydrogen production with high current densities. The bulk or surface Pd compositions (Pd/(Pd + Ni) molar ratios) of PdNi alloys can be well controlled by the fed ones. Meanwhile, some structural features that are favorable to HER activity, especially to achieving high HER current densities under low overpotentials, are further adjusted by the Pd compositions, including d-band center (εd) of PdNi alloy affected by surface segregation of Pd, hydrophilicity of NOC shell, and charge transfer and mass diffusion efficiency of an entire catalyst. When the bulk Pd/(Pd + Ni) molar ratio is ∼ 0.21, Pd0.21Ni0.79@NOC has a moderate εd, the highest hydrophilicity, the highest charge transfer and mass diffusion efficiency, and finally the highest electric double-layer capacitance (or electrochemical active surface area). Accordingly, it affords the optimal apparent or specific activity for hydrogen production with high current densities under low overpotentials in 0.5 M H2SO4, including a low apparent η1000 of ∼ 240 mV or specific η2500(BET) of ∼ 240.9 mV, and a high current density of ∼ 1590 mA cm−2 at only ∼ 242 mV.