Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO2

Abstract

The operational constraints for a 6-electron/6-proton CO₂ reduction system that operates at the concentration of CO₂ in the current atmosphere (p_CO2 = 400 ppm) have been evaluated on a variety of scale lengths that span from laboratory scale to global scale. Due to the low concentration of CO₂ in the atmosphere, limitations due to mass transport of CO₂ from the tropopause have been evaluated through five different regions, each with different characteristic length scales: the troposphere; the atmospheric boundary layer (ABL); the canopy layer; a membrane layer; and an aqueous electrolyte layer. The resulting CO₂ conductances, and associated physical transport limitations, will set the ultimate limit on the efficiency and areal requirements of a sustainable solar-driven CO₂ reduction system regardless of the activity or selectivity of catalysts for reduction of CO₂ at the molecular level. At the electrolyte/electrode interface, the steady-state limiting current density and the concomitant voltage loss associated with the CO₂ concentration overpotential in a one-dimensional solar-driven CO₂ reduction cell have been assessed quantitatively using a mathematical model that accounts for diffusion, migration and convective transport, as well as for bulk electrochemical reactions in the electrolyte. At p_CO2 = 400 ppm, the low diffusion coefficient combined with the low solubility of CO₂ in aqueous solutions constrains the steady-state limiting current density to <0.1 mA cm⁻² in a typical electrochemical cell with natural convection and employing electrolytes with a range of pH values. Hence, in such a system, the CO₂ capture area must be 100- to 1000-fold larger than the solar photon collection area to enable a >10% efficient solar-driven CO₂ reduction system (based on the solar collection area). This flux limitation is consistent with estimates of oceanic CO₂ uptake fluxes that have been developed in conjunction with carbon-cycle analyses for use in coupled atmosphere/ocean general circulation models. Two strategies to improve the feasibility of obtaining efficient and sustainable CO₂ transport to a cathode surface at p_CO2 = 400 ppm are described and modeled quantitatively. The first strategy employs yet unknown catalysts, analogous to carbonic anhydrases, that dramatically accelerate the chemically enhanced CO₂ transport in the aqueous electrolyte layer by enhancing the acid–base reactions in a bicarbonate buffer system. The rapid interconversion from bicarbonate to CO₂ in the presence of such catalysts near the cathode surface would in principle yield significant increases in the steady-state limiting current density and allow for >10% solar-fuel operation at the cell level. The second strategy employs a thin-layer cell architecture to improve the diffusive transport of CO₂ by use of an ultrathin polymeric membrane electrolyte. Rapid equilibration of CO₂ at the gas/electrolyte interface, and significantly enhanced diffusive fluxes of CO₂ in electrolytes, are required to increase the steady-state limiting current density of such a system. This latter approach however only is feasible for gaseous products, because liquid products would coat the electrode and therefore thicken the hydrodynamic boundary layer and accordingly reduce the diffusive CO₂ flux to the electrode surface. Regardless of whether the limitations due to mass transport to the electrode surface are overcome on the laboratory scale, at global scales the ultimate CO₂ flux limitations will be dictated by mass transport considerations related to transport of atmospheric CO₂ to the boundary plane of the solar-driven reactor system. The transport of CO₂ across the troposphere/ABL interface, the ABL/canopy layer interface, and the canopy layer/electrolyte interface have therefore been assessed in this work, to provide upper bounds on the ultimate limits for the solar-to-fuel (STF) conversion efficiency for systems that are intended to effect the reduction of atmospheric CO₂ in a sustainable fashion at global scale.