(449f) Optical Losses from Bubble Evolution in Photo-Electrochemical Reactors

Solar fuels, such as the production of hydrogen by photo-electrolysis of water, have the potential to revolutionise the long term future of energy production as they can overcome the issue of intermittency. The potential of this technology can only be realised by scaling up current research and lab scale demonstrations and henceforth, design of photo-electrochemical (PEC) reactors will become increasingly important.

Successful PEC reactor design will minimise resistive losses in solution, ensure the separation of the photo-electrolysis products and permit maximal light absorption by the photo-electrode(s). Methodologies for simultaneously achieving these requirements have not been exhaustively studied and in particular a comprehensive quantification of optical losses in PEC is missing from the literature.

In this work we consider the optics of PEC reactor design and its ramifications, focusing on the optical losses of bubbles on the surface of the photo-electrode. Bubble evolution is a complex process of bubble nucleation, detachment and rise and not only detrimentally reduces the effective surface exposed to electrolyte but absorbs, reflects and scatters incoming photons.

We experimentally demonstrate this effect and quantify the optical losses due to bubble evolution using a transparent conductive oxide such as fluorine doped tin oxide (FTO) glass as an anode. The optical losses are correlated against current density (and hence gas evolution rate) and the ramifications of this discussed. From this relationship, a modified maximum Solar-To-Hydrogen efficiency for different semiconductors (Shockley-Queisser limit) is derived.

Opportunities for the elimination of optical losses from bubble evolution through bubble suppression are discussed where pressurisation and forced convection approaches are investigated. A multi-physics model is presented that demonstrates the operational requirements in order to a achieve bubble suppression in a novel membrane-less flow reactor design, which has been outlined in previous work. Experimental verification of bubble suppression through hydrodynamic control is given and compared to the theoretical results and the potential scalability of this technology explored.