Experimental Characterization of Three-Terminal Tandem Photoelectrode Voltages for Photoelectrochemical Applications

Sustainably produced liquid fuels are necessary for a clean energy future, especially for energy intensive applications such as aviation where batteries are presently not sufficiently energy dense. Photoelectrochemical carbon dioxide reduction (PEC CO₂R) is a possible route to the production of sustainable fuels. However, there are an abundance of different products from CO₂R, making selective production of a single chemical difficult and requiring expensive, energy intensive product separation. One method to improve selectivity is to design cascading reactions, such as reducing CO₂ to CO and then subsequently reducing the CO to higher order (C₂₊) products.^[1] Each reaction within the CO₂R cascade occurs at a specific thermodynamic potential, requiring a specific voltage, and has a specific stoichiometry, requiring a specific electron flux or current density. The reactions of interest for a PEC CO₂R cascade would take place on a single photoelectrode, requiring first step products to move only on the order of millimeters to the second reaction site. This work aims to develop and characterize a III-V-based photoelectrode that can drive such a cascade CO₂R photoelectrochemically, using sunlight as the energy input.<br/><br/>A three-terminal tandem (3TT) is a type of multijunction photovoltaic device where a middle terminal is added between the two subcells that comprise the tandem, rather than having no extra terminal (as in a two-terminal tandem) or having the two subcells electrically separated (a four-terminal tandem).^[2] Connecting any two of the three contacts generates a different voltage, enabling a single device to supply the different potentials required for CO₂R cascade reactions. Current densities can be manipulated by changing the relative areas of the terminals. The GaInP/GaAs 3TT tandem used in this work has two terminals contacting the electrolyte, each of which drives one step of the CO₂R cascade; the photoelectrode is illuminated from opposite side, through a third terminal. Previous modelling work^[3] has shown that a 3TT tandem could drive a PEC CO₂R cascade and possibly improve product selectivity over simpler photoelectrodes. However, the fundamental behaviors of 3TT photoelectrodes are not yet understood in relationship to the performance of 3TT photovoltaics. This work experimentally characterizes the PEC behavior of a 3TT photoelectrode. Potential and current relationships will be characterized both dry and under electrochemical conditions to provide insight into what standard reduction potentials are possible with a GaAs/GaInP 3TT device. Dry power contour plots will be used to find the max power point, as simple current-voltage curves cannot fully define 3TT operation due to presence of the middle contact.^[4] Non-aqueous PEC measurements, using regenerative redox couples, will be performed in combination with light filtering (to select the GaInP or GaAs subcell) to learn about the operational window of each electrolyte terminal. In summary, this work aims to control a 3TT photoelectrode for PEC CO₂R as a step towards the goal of sustainable fuel production. <br/><br/>[1] Gurudayal, D. Perone, S. Malani, Y. Lum, S. Haussener, J. W. Ager, <i>ACS Appl. Energy Mater.</i> <b>2019</b>, <i>2</i>, 4551–4559.<br/>[2] E. L. Warren, W. E. McMahon, M. Rienäcker, K. T. VanSant, R. C. Whitehead, R. Peibst, A. C. Tamboli, <i>ACS Energy Lett.</i> <b>2020</b>, <i>5</i>, 1233–1242.<br/>[3] C. J. Kong, E. L. Warren, A. L. Greenaway, R. R. Prabhakar, A. C. Tamboli, J. W. Ager, <i>Sustain. Energy Fuels</i> <b>2021</b>, <i>5</i>, 6361–6371.<br/>[4] J. F. Geisz, W. E. McMahon, J. Buencuerpo, M. S. Young, M. Rienäcker, A. C. Tamboli, E. L. Warren, <i>Cell Rep. Phys. Sci.</i> <b>2021</b>, <i>2</i>, 100677.