Effects of Gold Substrates on the Intrinsic and Extrinsic Activity of High-Loading Nickel-Based Oxyhydroxide Oxygen Evolution Catalysts

We systematically investigate the effects of Au substrates on the oxygen evolution activities of cathodically electrodeposited nickel oxyhydroxide (NiOOH), nickel–iron oxyhydroxide (NiFeOOH), and nickel–cerium oxyhydroxide (NiCeOOH) at varying loadings from 0 to 2000 nmol of metal/cm². We determine that the geometric current densities, especially at higher loadings, were greatly enhanced on Au substrates: NiCeOOH/Au reached 10 mA/cm² at 259 mV overpotential, and NiFeOOH/Au achieved 140 mA/cm² at 300 mV overpotential, which were much greater than those of the analogous catalysts on graphitic carbon (GC) substrates. By performing a loading quantification using both inductively coupled plasma optical emission spectrometry and integration of the Ni^2+/3+ redox peak, we show that the enhanced activity is predominantly caused by the stronger physical adhesion of catalysts on Au. Further characterizations using impedance spectroscopy and in situ X-ray absorption spectroscopy revealed that the catalysts on Au exhibited lower film resistances and higher number of electrochemically active metal sites. We attribute this enhanced activity to a more homogeneous electrodeposition on Au, yielding catalyst films with very high geometric current densities on flat substrates. By investigating the mass and site specific activities as a function of loading, we bridge the practical geometric activity to the fundamental intrinsic activity.