Systems

Overview | Semiconductors | Catalysts | Protection | Structures | Devices | Systems

Goal

Ensure alignment between energy research and systemic factors which determine the feasibility of new solar energy technologies.

Strategy

Contour plot of the levelized cost of hydrogen (LCH) produced by a plant using photovoltaic cells wired to separate electrolysis units to split water. For comparison, the current LCH of hydrogen produced from fossil energy is appx $1.39 kg^-1.¹

We use modeling and simulation tools to assess constraints and requirements – both physical and technoeconomic – which govern the technical and commercial feasibility of new energy technologies and of renewable energy systems.

We collaborate with Chengxiang Xiang, a Lewis Group alumnus, to model and simulate systems that make sunlight from fuels.  This work allows us to understand the physical constraints of such systems.

We collaborate with Prof. Eric McFarland from the University of California at Santa Barbara to analyze the technoeconomic potential of new renewable energy technologies, and to understand the constraints and requirements for commercial feasible renewable energy systems.

These efforts keep our research aligned with the big-picture context of global energy systems.

Highlights

Operational Constraints and Strategies for Systems that Reduce CO₂ from the Atmosphere

Schematics for (top) the five regions for which CO₂ transport was modeled and (center) the model used for CO₂ transport near the surface of a cathode in an electrochemical CO₂-reduction cell, showing solution species included in the model. (bottom) Steady-state limiting current densities as a function of pH for a six-electron/six-proton reduction of CO₂ fed by air containing CO₂ at a concentration of 400 ppm.²

Sunlight-driven systems that reduce carbon dioxide are still in the research and development stage and face challenges with regard to the discovery of catalysts and the design of system architectures.  However, modeling and simulation allow an analysis of the upper limits of attainable efficiencies for systems that capture and convert CO₂ from the atmosphere to fuels.   Specifically, upper limits of efficiencies can be estimated based on the rates of mass transport of CO₂ through the atmosphere to the sites of chemical reduction to fuels.

In collaboration with Yikai Chen and Chengxiang Xiang, we used a multiphysics model to evaluate the mass-transport limitations for CO₂ at the current concentration in the atmosphere (400 ppm) through five different atmospheric regions: the troposphere, atmospheric boundary layer, canopy layer, membrane layer, and the aqueous electrolyte layer of an electrochemical CO₂-reduction cell.²  We additionally considered the concentration overpotential by accounting for diffusion, migration, convection, and the bulk electrochemical reactions in such a cell.

The low diffusion coefficient, combined with the low solubility of CO₂ in aqueous solutions constrains the steady-state limiting current density of CO₂-reduction systems fed with CO₂ at atmospheric concentration (400 ppm) to < 0.1 A cm^-2, regardless of the efficiency of the CO₂-reduction catalysts. This result is consistent with estimates of oceanic CO₂ uptake fluxes that have been developed in conjunction with carbon-cycle analyses for use in atmosphere-ocean circulation models.  Hence, the CO₂ capture area must be 100- to 1000-fold larger than the solar collection area to enable a > 10% efficient sunlight-driven CO₂-reduction system.

We also evaluated strategies to improve the steady-state efficiency of CO₂-reduction devices, including accelerating the transport of CO₂ in solution using as-yet-undiscovered catalysts that enhance the rate of the acid-base reactions in the bicarbonate buffer system and a cell architecture based on the use of an ultrathin polymeric membrane electrolyte.  Although these strategies could allow the CO₂ mass-transport limitations associated with flux to and within the electrolyte of laboratory-scale CO₂ reduction to be overcome, atmospheric flux limitations will constrain the operation of systems intended to reduce atmospheric CO₂ at a regional or global scale.

References

1. Shaner, M. R.; Atwater, H. A.; Lewis, N. S.; McFarland, E. W., A Comparative Technoeconomic Analysis of Renewable Hydrogen Production Using Solar Energy. Energy Environ. Sci. 2016, Advance Article.
2. Chen, Y.; Lewis, N. S.; Xiang, C., Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO₂. Energy Environ. Sci. 2015, 8, 3663-3674.

_{by Kimberly Papadantonakis, June 2016.}