Quantifying Losses in Photoelectrode Performance Due to Single Hydrogen Bubbles

Oxygen (O2) and hydrogen (H2) gas bubbles are the desired products from photoelectrochemical water splitting, but they are also a common source of efficiency losses in photoelectrochemical cells (PECs) that are poorly understood and often difficult to quantify. When attached to the surface of a photoelectrode, a gas bubble can induce optical, kinetic, ohmic, and mass-transport losses. The operating conditions and dynamic behavior that underlie these bubble-induced losses are complex and convoluted, but understanding these dynamics would be invaluable for selecting operating conditions and engineering photoelectrode surfaces to minimize those losses. Toward this end, we employ in situ scanning photocurrent microscopy (SPCM) to investigate the local photocurrent losses associated with isolated H2 bubbles attached to the surface of a photoelectrode. For the first time, we are able to quantify and resolve local bubble-induced photocurrent losses on a photoelectrode at the sub-bubble level. As a basis for this work, Si-based metal–insulator–semiconductor (MIS) photocathodes containing an ultrathin, continuous metal layer are studied. SPCM measurements, combined with optical modeling based on Snell’s law, are used to elucidate relationships between local photocurrent losses and bubble size. These investigations reveal increases in optical losses at larger bubble sizes (radius >100 μm) due to the photoelectrode geometry and the total internal reflection of light from the edge regions of bubbles. Finally, we apply the knowledge gained from single-bubble SPCM measurements to model the “sawtooth” current–time profile associated with dozens of bubbles evolving from a photoelectrode surface under photolimited operation and uniform AM1.5 illumination.