Transition metal ions regulated oxygen evolution reaction performance of Ni-based hydroxides hierarchical nanoarrays

Nickel-based hydroxide hierarchical nanoarrays (Ni_yM(OH)_x HNAs M = Fe or Zn) are doped with non-noble transition metals to create nanostructures and regulate their activities for the oxygen evolution reaction. Catalytic performance in these materials depends on their chemical composition and the presence of nanostructures. These novel hierarchical nanostructures contain small secondary nanosheets that are grown on the primary nanowire arrays, providing a higher surface area and more efficient mass transport for electrochemical reactions. The activities of the Ni_yM(OH)_x HNAs for the oxygen evolution reaction (OER) followed the order of Ni_2.2Fe(OH)_x > Ni(OH)₂ > Ni_2.1Zn(OH)_x, and these trends are supported by density functional theory (DFT) calculations. The Fe-doped nickel hydroxide hierarchical nanoarrays (Ni_2.2Fe(OH)_x HNAs), which had an appropriate elemental composition and hierarchical nanostructures, achieve the lowest onset overpotential of 234 mV and the smallest Tafel slope of 64.3 mV dec^-1. The specific activity, which is normalized to the Brunauer-Emmett-Teller (BET) surface area of the catalyst, of the Ni_2.2Fe(OH)_x HNAs is 1.15 mA cm^-2_BET at an overpotential of 350 mV. This is ~4-times higher than that of Ni(OH)₂. These values are also superior to those of a commercial IrO_x electrocatalyst.