Establishing the Role of Operating Potential and Mass Transfer in Multicarbon Product Generation for Photoelectrochemical CO₂ Reduction Cells Using a Cu Catalyst

Abstract

There is increasing interest in the possibility of photoelectrochemical (PEC) reduction of CO2 to C2+ products; however, the criteria for maximizing PEC solar-to-C2+ (STC2+) rates are not well understood. We report here a continuum-scale model of PEC CO2 reduction (CO2R) on Cu in 0.1 M CsHCO3 and use it to optimize the design and operating conditions for generating C2+ products. We demonstrate that the potential-dependent product distribution of CO2R on Cu requires operating near the potential that maximizes C2+ generation rates (V id), unlike PEC water splitting, which desires operation at the maximum photocurrent density. Because of this requirement, the criterion for a high STC2+ rate includes high-photocurrent semiconductors with photovoltages near V id and low series resistance. The STC2+ rate in these systems is enhanced by optimal CO2 transport and exhibits low sensitivity to dirunal solar irradiance variations.

Photovoltaic-Powered Electrochemical CO2 Reduction: Benchmarking against the Theoretical Limit

“…Actually, if the CO 2 reduction device in ref is powered by a dual-junction solar cell, then it needs to be operated at a more positive cathode potential, and consequently, its current density, CO selectivity, and the resultant STC efficiency will drop dramatically. For theoretical studies, comparatively less analysis has been performed for PEC CO 2 reduction ,,, than for water splitting; thus, studies on the comparison between these two routes are also very few . Therefore, further investigations on the direct comparison are demanded both experimentally and theoretically.…”

“…This approach has been analyzed to have a higher efficiency limit than the one-step CO 2 reduction; see Supporting Information for more details. Another approach being explored is the individual CO 2 reduction to CO and water splitting to produce hydrogen, using a downstream Fischer–Tropsch process to make the desired hydrocarbons. , Since solar CO is the intermediate of these two approaches, photovoltaic-powered electrochemical (PEC ,, ) CO 2 reduction to CO is the central technology for both of them. Despite efforts toward the development of efficient and selective catalysts, − the most recently reported solar-to-chemical (STC) energy conversion efficiency was still below 20% .…”

“…For theoretical studies, comparatively less analysis has been performed for PEC CO 2 reduction 11,13,16,35 than for water splitting; 36 thus, studies on the comparison between these two routes are also very few. 16 Therefore, further investigations on the direct comparison are demanded both experimentally and theoretically.…”

Operando Label-Free Optical Imaging of Solution-Phase Ion Transport and Electrochemistry

“…Achieving a detailed picture of ion transport in the solution phase under (electro)­chemically active conditions remains a major challenge that limits advances in applications ranging from solar fuel conversion and bipolar membranes to the understanding of electrophysiology. − Ion transport is a multiscale problem of diffusion and electrostatics that is dictated by nanoscale solvent structuring, mesoscale impacts of local environmental structure, and microscale transport. , Typically, ion transport within electrochemical systems is probed indirectly with bulk measurements of voltage and current, leaving the species concentration profiles and transport properties to be inferred through modeling and simulations. − There exist powerful techniques for probing electrode and electrolyte structure or steady-state ion distributions, − and absorptive and fluorescent labels as well as Raman-active modes have been used to probe ion transport with high resolution in specific cases, ,− but there is an outstanding need for direct universal measurements of ion transport in native microenvironments with spatiotemporal resolution. Optical detection of electrochemical processes via voltage-induced refractive index changes at electrodes also has a long history − ,− and has more recently been used for imaging electrochemical processes at electrode–electrolyte interfaces, − electric double layer dynamics, and ion intercalation in battery electrodes .…”

“…Moreover, this strategy is highly accessible because not only is the contrast mechanism general but only modest alterations to a standard optical microscope are required to image electrolyte distributions. As such, we expect this approach will be a valuable label-free probe for ionic transport in a wide range of physical, chemical, and biological contexts, including product formation and collection in (photo)­electrochemical energy conversion, , water splitting, phototriggered ion transport, ion transport in soft matter, aqueous battery function, dissolution dynamics after proper calibration of the refractive index near saturation, microfluidic ion flow, bioelectronic device function, or even label-free electrophysiology dynamics . It could also be readily combined with established strategies to optically detect oxygen and hydrogen evolution, especially since the sensitivity required to see gaseous products is orders of magnitude lesser than what we have demonstrated for solution-phase species, and to potentially probe three-dimensional ion transport in such contexts. , Second, the MSE analysis used here could serve as a general approach to reveal nondiffusive behavior or spatially dependent transport parameters.…”