Influencing Selectivity in Sunlight-Driven Carbon Dioxide Reduction

Hydrocarbon fuels have a significantly higher energy-density than hydrogen and are more easily incorporated into the existing energy infrastructure. A primary goal for solar fuels researchers is therefore to combine water-splitting with carbon dioxide reduction to produce hydrocarbon fuels. A major challenge in this area is to achieve a high product selectivity for the desired chemical species while minimizing the required electrical bias. Efforts at the Conn Center are focused on investigating and controlling the heterogeneously catalyzed carbon dioxide reduction reaction to drive the formation of specific products. This work includes the study of new catalysts, the effects of catalyst surface sites and arrangement, the effect of process temperature, the effects of ionic liquid electrolytes, and using intermediate products in a cascade system to achieve the final fuel product.

Figure 1. (a) The faradaic efficiency (FE%) of carbon dioxide reduction products at a copper electrocatalyst as measured in the Conn Center by in-line gas chromatography. (b) Schematic for a CO2 reduction electrolyzer.

Non-monolithic Tandem Solar Cells for Photoelectrolysis

Another thrust of the Conn Center’s PV/Solar Fuels theme is to design and build photovoltaics specially tailored to drive a given electrolysis reaction. The production of hydrocarbons through carbon dioxide, for instance, requires significantly more voltage input than hydrogen production through water-splitting. To provide the necessary photovoltage, multijunction solar cells are being designed that have a high voltage output chosen to operate efficiently with a given electrolyzer, while simultaneously making more efficient use of the solar spectrum than a single band gap solar cell could. Unlike the existing technology of multijunction PV grown epitaxially in one expensive wafer, this effort strives to produce a low-cost, modular form of tandem PV produced by layering thin films. The group is currently working with the emerging thin film technology of organometal halide perovskite photovoltaics to use in this modular format. Device performance is modeled with multiphysics numerical computational simulations to optimize the parameters that will lead to the highest efficiency.

Figure 2. Simulated current distribution within a diode to determine the effects of lateral series resistance at the contact layers. Inset shows the modeled current density and power density vs. applied voltage behavior.

Solar Fuels from Ambient Seawater Vapor

Terrestrial solar utilities have the drawback that they are limited by the solar flux, necessitating large land areas to harvest enough energy to be comparable to a typical coal-based power plant. Preferred utility sites thus typically favor desert-like environments, where land is cheap and the sun is strong. However, water is also a precious commodity in such regions, hurting the case for solar water-splitting to hydrogen fuel. Production of solar hydrogen from ambient water vapor over the ocean simultaneously removes the land and water limitations while overcoming the challenges of liquid seawater impurities. The Conn Center’s PV/Solar Fuels theme is characterizing seawater vapor electrolysis at realistic conditions and developing the technology for a marine solar fuels generation system.

Figure 3. (a) Current density vs. voltage for an electrolyzer operated with air humidified with seawater at different relative humidities (RH). (b) Current density vs. time at 1.6 V showing the decay in performance with liquid seawater vs. stable behavior from water vapor. (c) Cross-sectional schematic of a proposed marine solar fuel buoy, along with (d) a serpentine flow path for one unit of such a system.

Photoelectrochemical Carbon Dioxide Reduction

While an enormous body of work has examined direct solar water-splitting in a photoelectrochemical system, far less research has investigated photoelectrochemical carbon dioxide reduction. There is significant fundamental work to be done at this nexus between solar energy conversion, complex electrocatalytic fuel synthesis, and interfacial surface science. Heterogeneous dark electrocatalysis of CO2 reduction is inherently limited in its versatility by the catalyst composition and crystal structure only, whereas the photoelectrochemical strategy allows us to leverage the photoelectrode interface, spectral input, and semiconductor/electrolyte interaction as additional parameters to affect the reaction. The group is researching how the carbon dioxide reduction product selectivity is affected by catalyst deposition method, semiconductor surface functionalization, illumination effects, and ionic liquid electrolyte junction effects.