Transient photocurrents on catalyst-modified n-Si photoelectrodes: insight from dual-working electrode photoelectrochemistry

Semiconductor photoelectrodes coated with electrocatalysts are an important component of water-splitting cells that convert and store solar energy. Surface states on light-absorbing semiconductors can function as recombination centers and lower the performance of water-splitting systems. To characterize the presence and impact of surface states on catalyst-coated semiconductors, transient photoelectrochemical behavior is often studied. These experiments typically assume that the filling/emptying of surface states at the semiconductor interface causes transients to occur whenever the incident illumination intensity is perturbed. Analyzing transients may then reveal the density of surface states and their effect on carrier recombination. However, the transient technique does not directly measure the origin of the transient behavior, and the utility of the experiment requires assuming an underlying process. Here, we use a dual-working-electrode technique applied to Ni-protected n-Si photoanodes coated with Ni(Fe) (oxy)hydroxide catalyst to examine transient behavior of catalyst-coated photoelectrodes. We find that the most pronounced transients are due to catalyst redox activity. By directly measuring the catalyst redox state, we confirm that transients are related to either catalyst oxidation to Ni(Fe) oxyhydroxide or reduction to Ni(Fe) hydroxide. We also find that the redox-active catalyst moderates how quickly the depletion region and Helmholtz electrostatic potentials relax after each illumination perturbation. The results indicate that a redox-active catalyst can serve as a parallel capacitor which influences both the decay time and shape of transients. This data shows that photocurrent transients on catalyzed photoanodes are influenced by the catalyst's redox-activity and are not solely based on surface state loading/emptying.