Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Optical Losses at Gas Evolving Photoelectrodes: Implications for Photoelectrochemical Water Splitting Isaac Holmes-Gentle, Franky Bedoya-Lora, Faye Alhersh, and Klaus Hellgardt* Department of Chemical Engineering, Imperial College London, London SW7 2AZ, U.K.
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S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION Solar fuels offer an attractive pathway for harnessing and storing the solar resource,1 which could provide a more immediately sustainable energy production system than currently exists. Presently, this is largely based on extracting stored fossil fuel reserves.2 Photoelectrochemical production of hydrogen is one such solar fuel, and while demonstrated on a laboratory scale,3 there still exists a number of barriers to large scale implementation.4,5 Alongside economic analyses, the technical engineering design challenges need to be addressed, and in this work we aim to establish one particular challenge: the optical losses incurred at a gas evolving electrode. The record solar-to-hydrogen (STH) efficiency of unassisted water splitting systems has been improving over the past 50 years3,6,7 with notable examples of integrated PEC devices with efficiencies of 5.2%8 and 12.4%9 which employ a front-side illuminated semiconductor−electrolyte junction (i.e., light must pass through evolving gas bubbles). It is therefore now more common to see studies that report the formation of a significant number of bubbles, and an example of this can be observed in the videographic supporting information of work by Goto et al.10 As we move from these high efficiency lab scale demonstrations toward large scale devices, the issues surrounding gas evolution at photoelectrodes will need to be addressed. These include (1) coverage of active electrode sites by gaseous bubble, (2) electrolyte ionic transport and conductivity issues (ohmic losses increase as bubble plume acts as an insulating layer), and (3) optical losses due to reflection at the bubble− electrolyte interface. The first two issues have been studied for electrodes in the dark extensively. Electrode reaction site blocking has been mentioned by a number of authors for PEC systems,11,12 and there have been studies on both the effect of © XXXX American Chemical Society
bubble evolution on the effective conductivity of the electrolyte13,14 and mass transfer.15 The main experimental difference between studies for bubble generation in electrolyzers and PEC is the range of current densities, typically between 101 and 104 A m−2 for the former16,17 and 0.1 for current densities 4 mA cm−2), the bubbles move faster than at low current densities ( 20,54,56 and the experimental range measured here (0 < al < 0.3) is far lower than this. However, as this theory was derived for specular transmission only and in this experiment scattered light is collected in the integrating sphere (see the Supporting Information for calculation of acceptance angles), scattering interactions between bubbles may be significant at a lower values of al. Evidence for this would be the deviation from Ka = 1 for al > 0.2 though further experimental analysis would be required to confirm this limit. A more accurate but significantly more complex model of these optical losses would couple the experimentally determined twophase flow regime with a Monte Carlo ray tracing model. This would require an accurate optical model of each individual bubble using Mie theory for a perfectly spherical bubble or perhaps geometrical optics for bubbles that are larger than the wavelength of the light.58−62 For the size of bubbles observed in this study (30−60 μm), the scattering could be computed using Mie theory without significant computational difficulty. Our results for the population of bubbles rising from a vertical electrode can be compared with that of Dorfi et al.32 whose experimental design isolated the external quantum efficiency drop due light scattering from that due to other bubble related effects (e.g., electrode blocking). They observed an areaaveraged external quantum efficiency drop of 2.2−22.9% for single bubbles of sizes 75−500 μm on a horizontal electrode. This indicates that the total scattering coefficient Ka will be a function of the bubbles size in the aforementioned optical loss model. The work by Hu et al.34 actually showed an increase in photocurrent at low bubble sizes, but this is due to nature of their setup (e.g., the light beam dimensions are comparable to a single bubble radius, and the measurement methodology does not isolate the effect of electrode blocking from that of light scattering due to bubbles); the results cannot be compared directly with this work. As previously mentioned, the effect of radius on optical losses cannot be determined from the collected data set as the radius does not change significantly (Figure 8a), but the model can be used to demonstrate the complex relationship between optical losses, bubbles size, and bubble hydrodynamics. For example, larger bubbles could scatter more light,32 meaning Ka may increase; but for a given volumetric production rate of gas there will be more bubbles at for smaller radius (Nb ∝ rb−3), and hence al decreases with bubble radius (al ∝ rb−1). Furthermore, the terminal rise velocity and the induced convection will be significantly impacted by the bubble radius, which in turn will affect the number of bubbles observed in a frame Nb. This clearly affirms that the effect of bubble size on the overall optical loss will be complex and should be an area for further study. This case study illustrates that a simple model can capture the “order of magnitude” optical losses and, to our knowledge, is the first evaluation of the optical losses for a population of bubbles evolving from a vertical electrode. Mitigation Strategies. The ramifications of optical losses due to scattering from bubbles are significant when a scale-up of photoelectrochemical technologies is considered. Laboratory scale experiments where the electrode size is small (
