Computational Modeling of Photo-Electrochemical Reduction of CO2 Utilizing Concentrated Light
Authors: Cattry Alexandre , Haussener Sophia 
Published: September 2023

Solar energy conversion of water and CO2 into H2/CO by an integrated photo-electrochemical (IPEC) cell and utilizing concentrated solar light allows for high power density and, if carefully designed, delivers high solar-to-fuel efficiency [1], [2]. In such devices, light absorption, charge generation/separation/transport, electrochemical reactions, and mass and ionic transport are required to occur simultaneously. Managing the coupled photon, charge, mass, and heat transport is central to a functioning and stable device. If not carefully designed, challenging operating conditions (high irradiation fluxes, critical thermal management) can induce irreversible damage to the IPEC cell and impact its performance. Thus, the optimal design of a high-performance device is conditioned by the development of a multi-dimensional and multi-physics computational model capable of estimating experimentally inaccessible quantities in complex porous geometries [3]. While such models have already been developed for water splitting, computational modeling of CO2 reduction under concentrated light is still missing.
We present an IPEC cell computational model operating under highly concentrated radiation that accounts for both water splitting and CO2 reduction in a zero-gap gas diffusion electrode configuration. The continuum modeling is based on the solution of the coupled conservation and transport equations: species conservation and transport with electrochemical reactions, mass and momentum conservation, conservation and transport of electron and holes in the semiconductors, and Maxwell’s equations for the light propagation and absorption. We quantify spatially resolved non-uniform fields (local temperature, partial current densities, molar species fractions, etc.), inhomogeneities (e.g., local hotspots that could damage sealing), and CO2 conversion rates to demonstrate how such simulations can help to address engineering and operating conditions challenges. Finally, we show that the data obtained in experiments of an IPEC device operating under concentrated light are in agreement with our computational findings.
Our simulations reveal that critical thermal management is required to improve IPEC cell efficiency. We provide evidence that challenging operating conditions (high solar irradiation concentration, low mass flow rate) can affect the CO partial current density, favor the competing hydrogen evolution reaction, lead to quartz window failure and decrease the photo-absorber efficiency.
Overall, this work provides a validated multi-physics computational model of an IPEC cell operating with concentrated radiation that can be used to conduct feasibility studies and provide design guidelines for such thermally integrated IPEC devices for CO2 reduction at large solar irradiation concentrations.

Keywords
: Solar fuel, simulation, concentrated light, high power
[1] S. Tembhurne, F. Nandjou, S. Haussener, A thermally synergistic photo-electrochemical hydrogen generator operating under concentrated solar irradiation, Nat. Energy. 4 (2019) 399–407. https://doi.org/10.1038/s41560-019-0373-7.
[2] E. Boutin, M. Patel, E. Kecsenovity, S. Suter, C. Janáky, S. Haussener, Photo-Electrochemical Conversion of CO2 Under Concentrated Sunlight Enables Combination of High Reaction Rate and Efficiency, Adv. Energy Mater. 12 (2022). https://doi.org/10.1002/aenm.202200585.
[3] S. Tembhurne, S. Haussener, Integrated Photo-Electrochemical Solar Fuel Generators under Concentrated Irradiation, J. Electrochem. Soc. 163 (2016) H999–H1007. https://doi.org/10.1149/2.0321610jes.