Many
photoelectrodes produce a gaseous product, such as hydrogen
or oxygen, from a liquid electrolyte and require light transmission
directly through the two-phase mixture forming at the semiconductor–electrolyte
interface. Consequently, incidence solar photons will be scattered
and reflected from the bubbly mixture leading to an additional optical
loss. In this work, these optical losses are quantified for a population
of bubbles that evolved from the vertical surface of a transparent
conductive electrode (F-SnO2) by measuring the amount of
light transmitted. The transmitted photons were collected in an integrating
sphere placed directly behind the 15 mm × 15 mm electrode to
capture the forward scattered light. The empirical results were compared
with a simple dimensionless model. Finally, mitigation strategies
are suggested and critically discussed. With progress in the development
of large scale prototype photoelectrochemical devices comes the need
to understand, quantify, and potentially resolve the issue of optical
losses from gas evolving photoelectrodes.
When scaling up photo-electrochemical processes to larger areas than conventionally studied in the laboratory, substrate performance must be taken into consideration and in this work, a methodology to assess this via an uncomplicated 2 dimensional model is outlined. It highlights that for F-doped SnO2 (FTO), which is ubiquitously used for metal oxide photoanodes, substrate performance becomes significant for moderately sized electrodes (5 cm) under no solar concentration for state of the art Fe2O3 thin films. It is demonstrated that when the process is intensified via solar concentration, current losses become quickly limiting. Methodologies to reduce the impact of substrate ohmic losses are discussed and a new strategy is proposed. Due to the nature of the photo-electrode current-potential relationship, operation at a higher potential where the photo-current saturates (before the dark current is observed) will lead to a minimum in current loss due to substrate performance. Crucially, this work outlines an additional challenge in scaling up photo-electrodes based on low conductivity substrates, and establishes that such challenges are not insurmountable.
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