The Brown team attributed the differences to high surface roughness, hierarchical porosity, and confinement of reactive species in the foam.

Electrochemical reduction of CO₂ has been investigated at a variety of metallic electrodes, and a number of reports and reviews have been published on this subject. Among the metals studied, copper generates significant quantities of hydrocarbons such as methane and ethylene in aqueous media. Hori et al. conducted extensive studies on the electrochemical reduction of CO₂ and CO at copper electrodes and concluded that the product distribution reflected a sensitivity of adsorbed hydrogen species to the underlying structure of the copper electrode and that “surface roughening likely introduced surface defects such as steps and vacancies that are favorable for reaction of adsorbed hydrogen atoms”.

… Recent work that describes a novel approach to the fabrication of metal foams with hierarchical porosity provides an excellent opportunity for testing the effect of three-dimensional nanostructured metal surfaces and their corresponding cavities on the products produced during the electrochemical reduction of CO₂.

—Sen et al.

Copper foam, which has been developed only in the last few years, provided the surface roughness that Tayhas Palmore, professor of engineering and senior author, and her colleagues were looking for. The foams are made by depositing copper on a surface in the presence of hydrogen and a strong electric current. Hydrogen bubbles cause the copper to be deposited in an arrangement of sponge-like pores and channels of varying sizes.

After depositing copper foams on an electrode, the researchers set up experiments to see what kinds of products would be produced in an electrochemical reaction with CO₂ in water.

The experiments showed that the copper foam converted CO₂ into formic acid—a compound often used as a feedstock for microbes that produce biofuels—at a much greater efficiency than planar copper. The reaction also produced small amounts of propylene, a useful hydrocarbon that’s never been reported before in reactions involving copper.

The product distribution was unique and very different from what had been reported with planar electrodes, which was a surprise. We’ve identified another parameter to consider in the electroreduction of CO₂. It’s not just the kind of metal that’s responsible for the direction this chemistry goes, but also the architecture of the catalyst.

—Tayhas Palmore

Now that it’s clear that architecture matters, Palmore and her colleagues are working to see what happens when that architecture is tweaked. It’s likely, she says, that pores of different depths or diameters will produce different compounds from a CO₂ feedstock. Ultimately, it might be possible to tune the copper foam toward a specific desired compound.

The work in the study is part of a larger effort by Brown’s Center for the Capture and Conversion of CO₂. The Center, funded by the National Science Foundation, is exploring a variety of catalysts that can convert CO₂ into usable forms of carbon.

The Center for Capture and Conversion of CO₂ is a Center for Chemical Innovation funded by the National Science Foundation (CHE-1240020).

Resources

• Sujat Sen, Dan Liu, and G. Tayhas R. Palmore (2014) “Electrochemical Reduction of CO₂ at Copper Nanofoams,” ACS Catal., 4, pp 3091–3095 doi: 10.1021/cs500522