Size-dependent electrocatalytic activities of printed Co₃O ₄ films for a monolithic photovoltaic-electrolytic hydrogen generation system

As an electrocatalyst in a monolithic PV-electrolytic cell for water splitting hydrogen generation Co₃O₄ films were prepared by a paste coating method using Co₃O₄ particles. Different sized Co₃O₄ particles with average diameters of 145.9, 63.3 and 36.5 nm were prepared using a supercritical hydrothermal synthesis method. Electrochemical properties with respect to the particle size in the film were investigated by evaluating overpotential, charge transfer resistance (R_ct), and number of active sites (q). The relation between overpotential in water oxidation at 5 mA/cm² and BET surface area showed a slope of -73.8 ± 6.6, implying strong particle size dependence on electrocatalytic activity. Moreover, the R_ct and q values and the actual hydrogen evolution rate indicated that the electrocatalytic activity of Co₃O₄ is attributed to physical properties (e.g. particle size) of the film. Whereas, intrinsic single site activity of the film was not significantly changed with respect to the particle size in the film.