Modeling and Simulation of the Spatial and Light-Intensity Dependence of Product Distributions in an Integrated Photoelectrochemical CO₂ Reduction System

Abstract

A multiphysics model that accounts for the performance of electrocatalysts and triple-junction light absorbers, as well as for the transport properties of the electrolyte and dissolved CO 2 , was used to evaluate the spatial and light-intensity dependence of product distributions in an integrated photoelectrochemical CO 2 reduction (CO 2 R) cell. Different sets of band gap combinations of triple-junction light absorbers were required to accommodate the optimal total operating current density relative to the optimal partial current density for CO 2 R. The simulated product distribution was highly nonuniform along the width of the electrode and depended on the electrode dimensions as well as the illumination intensity. To achieve the same product selectivity as in a potentiostatic, "half-cell" configuration, the electrocatalyst must retain its selectivity over a range of cathode potentials, and this range is dependent on the transport losses and current−voltage relationship of the light absorbers, the geometric parameters of the cell, and the illumination intensity.A n efficient solar-driven CO 2 reduction system 1−5 requires effective delivery of CO 2 to the electrode surface, selective reduction of CO 2 by an active electrocatalyst at the cathode, adequate ionic transport between the anolyte and catholyte chambers, and robust and costeffective methods for separating the products. Electrocatalysts including metals, 6,7 metal alloys, 8 metal oxides, 9 and semiconductors 10−14,10,15 have been investigated for electrocatalytic CO 2 reduction (CO 2 R). For most electrocatalysts operated in contact with aqueous electrolytes, the branching ratio between water reduction (i.e., the hydrogen-evolution reaction, HER) and CO 2 R depends on the applied potential. 9 For instance, in a three-electrode potentiostatic configuration, the Faradaic efficiency for the production of formate, HCOO − , at the surface of an oxidized Cu foil changes from 5.5% to 32.9% when the potential of the working electrode is changed by ∼20 mV. 9 In contrast to experiments performed using threeelectrode potentiostatic configurations, the potential at the cathode of a full photoelectrosynthetic cell depends on the reaction kinetics at the anode surface as well as transport losses associated with solution resistance, electrodialysis, pH gradients, and CO 2 concentration gradients near the surface of the cathode. Modeling and simulation has shown that many test-bed configurations for water-splitting devices produce spatially dependent potential distributions, with variations of >100 mV across the electrode surface even under constant illumination intensity. 16−19 Moreover, variation in the illumination intensity during operation additionally affects the solarto-fuel (STF) conversion efficiency for such systems.Herein, a two-dimensional numerical model has been developed using COMSOL Multiphysics to evaluate the spatial and temporal variation of the product distribution in an integrated photoelectrochemical CO 2 reduction cell driven by triple-junction l...

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