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Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO_2

Chen, Yikai and Lewis, Nathan S. and Xiang, Chengxiang (2015) Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO_2. Energy and Environmental Science, 8 (12). pp. 3663-3674. ISSN 1754-5692. doi:10.1039/c5ee02908b.

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The operational constraints for a 6-electron/6-proton CO_2 reduction system that operates at the concentration of CO_2 in the current atmosphere (p_(CO_2) = 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 CO_2 in the atmosphere, limitations due to mass transport of CO_2 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 CO_2 conductances, and associated physical transport limitations, will set the ultimate limit on the efficiency and areal requirements of a sustainable solar-driven CO_2 reduction system regardless of the activity or selectivity of catalysts for reduction of CO_2 at the molecular level. At the electrolyte/electrode interface, the steady-state limiting current density and the concomitant voltage loss associated with the CO_2 concentration overpotential in a one-dimensional solar-driven CO_2 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_(CO_2) = 400 ppm, the low diffusion coefficient combined with the low solubility of CO_2 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 CO_2 capture area must be 100- to 1000-fold larger than the solar photon collection area to enable a >10% efficient solar-driven CO_2 reduction system (based on the solar collection area). This flux limitation is consistent with estimates of oceanic CO_2 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 CO_2 transport to a cathode surface at p_(CO_2) = 400 ppm are described and modeled quantitatively. The first strategy employs yet unknown catalysts, analogous to carbonic anhydrases, that dramatically accelerate the chemically enhanced CO_2 transport in the aqueous electrolyte layer by enhancing the acid–base reactions in a bicarbonate buffer system. The rapid interconversion from bicarbonate to CO_2 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 CO_2 by use of an ultrathin polymeric membrane electrolyte. Rapid equilibration of CO_2 at the gas/electrolyte interface, and significantly enhanced diffusive fluxes of CO_2 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 CO_2 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 CO_2 flux limitations will be dictated by mass transport considerations related to transport of atmospheric CO_2 to the boundary plane of the solar-driven reactor system. The transport of CO_2 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 CO_2 in a sustainable fashion at global scale.

Item Type:Article
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URLURL TypeDescription
Lewis, Nathan S.0000-0001-5245-0538
Xiang, Chengxiang0000-0002-1698-6754
Alternate Title:Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO2
Additional Information:© 2015 The Royal Society of Chemistry. Received 21st September 2015, Accepted 6th October 2015, First published online 08 Oct 2015. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993 as well as by the Gordon and Betty Moore Foundation, grant 1225. We also thank Dr Ken Caldeira and Dr Joe Berry of the Carnegie Institution for Science, Department of Global Ecology, for numerous stimulating and valuable discussions regarding the ocean/air CO_2 flux and the ABL/troposphere CO_2 transport properties.
Funding AgencyGrant Number
Department of Energy (DOE)DE-SC0004993
Gordon and Betty Moore Foundation1225
Issue or Number:12
Record Number:CaltechAUTHORS:20151102-111406745
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:61771
Deposited By: Tony Diaz
Deposited On:02 Nov 2015 20:45
Last Modified:10 Nov 2021 22:53

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