Oxygen (O 2 ) and hydrogen (H 2 ) 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 H 2 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.

This study investigates the use of membraneless electrolyzers based on angled mesh flow-through electrodes for the simultaneous production of acid and base (lye) from aqueous brine solutions. These electrolyte-agnostic flow cells are capable of producing a wide variety of acids and bases with precisely controlled pH using a simple cell design.

Due to the complex nature of probe/substrate interactions in scanning electrochemical microscopy (SECM), SECM has been primarily limited to well‐defined probe geometries and slow scan speeds that reduce imaging throughput. Herein, we show that in situ colorimetric visualization of concentration gradients using pH indicator dyes during SECM measurements can be a powerful tool for understanding the coupled influences of probe geometry and scan speed on the dynamics of localized plumes of electroactive species that mediate probe/substrate interactions. Colorimetric images of plumes generated at a band electrode reveal that probe geometry strongly influences linescan signal distortion and hysteresis at scan speeds surpassing the conventional SECM “speed limit”. Combining mechanistic information from in situ colorimetric imaging with transport models, this article reports design principles that have the potential to enable quantitative SECM with novel probe geometries and at imaging rates that are at least an order of magnitude faster than currently employed.