(771a) Featured: Energy Conversion Considerations in Solar-Driven Photoelectrochemical Water Splitting and Carbon Dioxide Reduction

A practical method to use sunlight to generate chemical fuels could dramatically change the landscape of global energy generation and storage. One specific implementation of this concept is an integrated photoelectrochemical (PEC) device that operates without added electrical bias with sunlight as the only energy input, producing either hydrogen by water splitting or carbon-based fuels via carbon dioxide reduction. Experimental demonstrations of such systems date back to the 1970s, and, since that time, there have been improvements in both the functional understanding and performance of the devices.

There are a number of design considerations which arise when considering the full solar to fuel conversion system, both for water splitting and carbon dioxide reduction. These include the type and number of semiconductor junctions used to generate the requisite photovoltage, the use of co-catalysts, the electrolyte conditions, a product separation method such as a membrane, and the geometric layout and size of components.

To date, all approaches which have a solar to hydrogen (STH) conversion efficiency of >5% employ either 2 or more semiconductor junctions arranged in a voltage-additive tandem configuration. The degree of integration varies widely, from fully integrated â??artificial leavesâ? to systems with decoupled photovoltaics and catalysts. A small subset of devices employ a mechanism, such as a proton or anion conducting membrane, to separate products and yield a pure H₂ stream. Reported STH conversion efficiencies range from <1% to over 20% [1]. However, there are very few reports of long term operational stability, which is a clear prerequisite for a positive energy return on investment [2].

Electrochemical reduction of CO₂ to fuel molecules such as methanol and ethanol could form the basis for the production of renewable and sustainable transportation fuels, replacing the fossil fuels which are used today. There has been substantial recent progress in solar driven PEC CO2 reduction, with a number of reports of energy conversion efficiencies of over 1%. While these values are larger than those estimated for biomass generation by plants and algae (up to 1%) [3], the natural system utilizes dilute CO₂ in the atmosphere as a feed stock and generates a separated fuel product. Thus, product separation and the selective production fuels remain as outstanding challenges for the PEC technology.

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

[1] Ager, J. W.; Shaner, M. R.; Walczak, K. A.; Sharp, I. D.; Ardo, S. Energy Environ. Sci. 2015, 8 (10), 2811â??2824.

[2] Sathre, R.; Scown, C. D.; Morrow, W. R.; Stevens, J. C.; Sharp, I. D.; Ager, J. W.; Walczak, K.; Houle, F. A.; Greenblatt, J. B. Energy Environ. Sci. 2014, 7, 3264â??3278.

[3] Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A.; Moore, T. A.; Moser, C. C.; Nocera, D. G.; Nozik, A. J.; Ort, D. R.; Parson, W. W.; Prince, R. C.; Sayre, R. T. Science. 2011, 332, 805â??809.