Quantifying Losses in Photoelectrode Performance Due to Single Hydrogen Bubbles

Abstract

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.

A Review of Inorganic Photoelectrode Developments and Reactor Scale‐Up Challenges for Solar Hydrogen Production

“…Depending on the mode of photoelectrolysis operation, light may propagate through a flowing two‐phase bubbly mixture and/or through bubbles adhering to an electrode surface, as shown in Figure 28c,d, respectively. [ 331,332 ] At every gas | liquid interface through which the light passes, it may undergo reflection, scatter, or refraction, leading to losses in transmitted light.…”

“…(a) and (d) Reproduced with permission. [ 332 ] Copyright 2017, American Chemical Society; (b) and (c) Reproduced with permission. [ 331 ] Copyright 2019, American Chemical Society.…”

“…It was determined from single bubble studies in a quiescent system where any kinetic and Ohmic effects were minimized, that the growth of a bubble from a 150 to a 1000 µm diameter can directly result in a photocurrent density decrease from 2% to 23%, respectively. [ 332 ] Using videographic imaging, it is possible to correlate optical losses on transparent substrates with current density, as well as to determine the bubble number density, velocity distribution of bubbles, and bubble radius distribution. [ 331 ] However, optical losses in real systems will be impacted by bubble dynamics, to which many different conditions can contribute.…”

“…Tilt is expected to impact on the buoyant force that causes bubble uplift as well the two‐phase flow field and so, its impact on any PEC reactor performance will require study. [ 332 ]…”

Bubble-induced convection stabilizes the local pH during solar water splitting in neutral pH electrolytes

“…28 Bubble formation within the electrolyte domain is ignored since bubble nucleation was observed predominantly at the electrode surface in previous reports. [28][29][30] We have recently shown that the buoyancy force and the resultant bubble distribution are mainly determined by the bubble production rate and not affected by the density of gas bubbles. 31 In this study, we therefore simplify our model by considering the gas property (i.e., density, molecular mass) of O 2 on both the anode and the cathode while adjusting the bubble production rate according to the number of electrons involved.…”