(491b) Modeling and Operation of a Photo-Electrochemical Reactor for H2 Production

Sustainable fuels are required to contribute to the overall energy carrier portfolio of the world and to substitute some of the more polluting energy vectors we depend on today. For some decades now has the conversion of solar energy to usable energy (electricity or chemical fuel) been an important scientific goal. The direct conversion into electricity has been achieved and is now been driven by industry to provide ever cheaper devices for solar to electricity conversion. The production of devices of solar based fuels has been somewhat slower in its deployment, mainly due to the perceived high device costs and the intrinsic complexities involved but also due to the lower conversion that can be currently achieved. Photo-electrochemical systems, which facilitate the production of molecular hydrogen and oxygen gas directly from water and sunlight, have been pursued for the last 40 years, ever since the pioneering work by Fujishima and Honda in 1971. However, the primary focus in this area has been on the development of photo-electrode materials, for high efficiency and low-cost manufacture. The development of practical photo-electrochemical reactors for larger-scale hydrogen production has not yet received significant attention. Photo-electrochemical water splitting is a very demanding process, requiring materials that are capable of both absorbing solar radiation efficiently and remaining stable under the conditions of hydrogen or oxygen evolution. The successful nano-structuring of, especially, lower band-gap materials such as WO3 and Fe2O3, has reinvigorated interest in photo-electrochemical approaches to solar hydrogen production. Fe2O3, produced by a chemical vapor deposition process has been found to produce photocurrents greater than 0.2 A m-2 under AM1.5 sunlight. If this performance can be extended to large area electrodes, fabricated by an inexpensive route, then large-scale photo-electrolyzers based on Fe2O3 photo-anodes become a practical possibility. We have designed and built deposition systems that allow the inexpensive production of 0.01 m2 photo-anodes. These photo-anodes can be accommodated in a newly developed photo-electrochemical reactor (Figure 1), which we aim to study in order to develop a complete understanding of the issues involved in the scale-up of photo-electrochemical reactors for hydrogen production. In this context we have built a multi-physics model to describe the fluid flow as well as the current density distribution within the reactor and thus consider the operational consequences of bubble formation and sheet/electrode conductivity, channel spacing etc. Figure 1: Schematics of a flat plate photo electrochemical reactor for operation and scale-up analysis.