Linker-free chemical preparation of palladium nanoparticles on aluminum-doped zinc oxide electrodes for electrochemical water oxidation

Despite the growing popularity and promising properties of aluminum-doped zinc oxide (AZO) among other transparent conducting oxides (TCOs), modification of AZO with a suitable nano-catalyst can help to enhance its electrocatalytic properties. In this study, the surface of AZO is decorated with palladium nanoparticles (PdNPs) by simply Pd ion capturing on bare AZO from an aqueous solution of K₂PdCl₄ and successive reduction with NaBH₄ to enhance the electrocatalytic properties toward water oxidation. The effect of K₂PdCl₄ concentration is optimized for optimum PdNPs-modified AZO (PdNPs-AZO) electrodes for electrochemical water oxidation. The surface morphology, elemental composition, and electrical properties of the prepared PdNPs-AZO were examined by field emission scanning electron microscopy, energy dispersive spectroscopy, and four-in-line probe, respectively. The PdNPs-AZO electrodes, prepared with various concentrations of Pd precursors, exhibited a significant change in terms of electrode sheet resistance and resistivity from each other. The electrochemical impedance spectroscopy and cyclic voltammetry were conducted in a 0.1 M NaOH (aq.) solution to achieve quantitative information about the electrodes and electrochemical reactions toward water oxidation. The PdNPs-AZO prepared with 5.0 mM K₂PdCl₄ exhibited optimum behavior toward water oxidation with the starting oxidation potential of 625.7 mV vs. Ag/AgCl and current density of 13.8 mA cm⁻² at 1.5 V vs. Ag/AgCl.