Exchange Current Densities of and Minority Carrier Fluxes to Catalyst Nanoparticles on Photoelectrodes from Intensity Modulated Photocurrent Spectroscopy

Abstract

The photoelectrochemical production of chemicals is a promising approach to replace greenhouse gas-emitting production processes. Here, an illuminated semiconducting photoelectrode provides the electrochemical potential difference to drive the targeted electrochemical reaction. Since the catalytic activity of the pristine surface is typically limited, co-catalysts are integrated on the electrochemical interface to reduce overpotentials and their form of nanoparticles avoids parasitic light absorption. The classical approach for the choice of the catalyst is based on the activity of the bulk metal electrode, but a quantification of the catalytic activity of the deposited nanoparticle catalysts is usually not done. This can be attributed to the fact that, for photoelectrodes, standard electrochemical methods do not allow for a direct evaluation of the catalytic activity of the co-catalysts. Using intensity modulated photocurrent spectroscopy, we develop a method to quantify the catalytic activity of nanoparticle catalysts on photoelectrodes. We obtain Tafel plots, analogously to the quantification of the catalytic activity of bulk metal electrodes. From that, we derive the exchange current density of the photoelectrode. Therefore, a comparison of the catalytic activity with the bulk material of the nanoparticles and the evaluation of catalytic activities of different nanoparticle materials become possible. Furthermore, we obtain plots correlating the light-generated minority carrier flux with the band bending in the semiconducting material. These plots give direct insight into the ability of the semiconducting material to generate and separate electron–hole pairs and helps to identify performance bottlenecks of photoelectrochemical interfaces.