Hydrogen Bubble Templating of Fractal Ni Catalysts for Water Oxidation in Alkaline Media

Dynamic hydrogen bubble templating of Ni (Ni_DHBT) electrodes was used to prepare highly porous films with enhanced properties toward the oxygen evolution reaction (OER). Upon varying the electrodeposition conditions, Ni films with a macroporous primary structure and highly porous cauliflowerlike secondary structure were formed. These films are able to develop an extended electrochemically active surface area, up to 270-fold increase compared to a Ni plate. They exhibit stable overpotential (η₂₅₀ = 540 mV) at j = 250 mA cm_geometric^–2 in 1 M KOH electrolyte, which is 300 mV less positive than at the Ni plate. Fe incorporation onto these Ni_DHBT structures can further lower OER overpotentials to η₂₅₀ = 310 mV. Ni_DHBT films are remarkably stable over prolonged polarization and are characterized by a low Tafel slope (29 mV/decade) that extends up to j = 100 mA cm_geometric^–2, attributed to both superaerophobic characteristics with a contact angle of ca. 160° between the surface and an air bubble and superhydrophilic characteristics with less than 25° between the surface and a water droplet.