Realizing Efficient Overall Water Splitting by Tuning the Cobalt Content in Self-Supported Ni_x−Co_y−P Microarrays

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 Ni₂P or Ni_x−Co_y−P can be obtained with tunable composition. Among the obtained phosphides with different Ni and Co contents, Ni₁−Co₁−P exhibits a low overpotential of only 90 mV at 10 mA cm⁻² for HER in 1 M KOH, whereas that of Ni₂P is as high as 137 mV. Furthermore, Ni₁-Co₁-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 Ni₁−Co₁−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 Ni₁-Co₁-P has been optimized, showing a much smaller Gibbs free energy change of absorbed intermediates during the HER and OER than Ni₂P. Considering the ease in fabrication and the superior performance, this material displays good potential for industrial applications at a large scale.