Realizing Efficient Overall Water Splitting by Tuning the Cobalt Content in Self‐Supported Nix−Coy−P Microarrays

Abstract Transition‐metal phosphides have been identified as a group of promising catalysts, especially for the hydrogen evolution reaction (HER). However, their practical application is still hindered by the suboptimal activity and tedious preparation. In this work, self‐supported transition‐metal phosphide microarrays have been synthesized on Ni foam via electrodeposition followed by a low‐temperature gaseous phosphorization. We discover that Ni 2 P or Ni x −Co y −P can be obtained with tunable composition. Among the obtained phosphides with different Ni and Co contents, Ni 1 −Co 1 −P exhibits a low overpotential of only 90 mV at 10 mA cm −2 for HER in 1 M KOH, whereas that of Ni 2 P is as high as 137 mV. Furthermore, Ni 1 ‐Co 1 ‐P also demonstrates a current density of 10 mA cm −2 at an overpotential of only 292 mV for the oxygen evolution reaction (OER). An overall water splitting device has been assembled with two Ni 1 −Co 1 −P electrodes, and it displays an excellent performance at 1.65 V for a current density of 20 mA cm −2 with a good stability up to 30 h. The density functional theory calculation results suggest that by incorporating Co into the phosphide, the surface electronic structure of Ni 1 ‐Co 1 ‐P has been optimized, showing a much smaller Gibbs free energy change of absorbed intermediates during the HER and OER than Ni 2 P. Considering the ease in fabrication and the superior performance, this material displays good potential for industrial applications at a large scale.