We have discovered the first metal catalyst that can produce appreciable amounts of ethanol from carbon monoxide at room temperature and pressure—a notoriously difficult electrochemical reaction.

—Matthew Kanan, an assistant professor of chemistry at Stanford and coauthor of the Nature study

Two years ago, Kanan and Stanford graduate student Christina Li created a novel oxide-derived copper electrode material. While conventional copper electrodes consist of individual nanoparticles that just sit on top of each other, oxide-derived copper, is made of copper nanocrystals that are all linked together in a continuous network with well-defined grain boundaries. The process of transforming copper oxide into metallic copper creates the network of nanocrystals, Kanan explained.

For the Nature study, Kanan and Li built an electrochemical cell: two electrodes placed in water saturated with carbon monoxide gas. When a voltage is applied across the electrodes of a conventional cell, a current flows and water is converted to oxygen gas at one electrode (the anode) and hydrogen gas at the other electrode (the cathode). The challenge was to find a cathode that would reduce carbon monoxide to ethanol instead of reducing water to hydrogen.

The electrochemical conversion of CO₂ and H₂O into liquid fuel is ideal for high-density renewable energy storage and could provide an incentive for CO₂ capture. However, efficient electrocatalysts for reducing CO₂ and its derivatives into a desirable fuel are not available at present. Although many catalysts can reduce CO₂ to carbon monoxide (CO), liquid fuel synthesis requires that CO is reduced further, using H₂O as a H⁺ source. Copper (Cu) is the only known material with an appreciable CO electroreduction activity, but in bulk form its efficiency and selectivity for liquid fuel are far too low for practical use. In particular, H₂O reduction to H₂ outcompetes CO reduction on Cu electrodes unless extreme overpotentials are applied, at which point gaseous hydrocarbons are the major CO reduction products.

—Li et al.

In the Nature experiment, Kanan and Li used a cathode made of oxide-derived copper, with the resulting high yield of ethanol and acetate at 57% Faradaic efficiency (i.e., 57% of the electric current went into producing these two compounds from carbon monoxide). By comparison, conventional Cu nanoparticles with an average crystallite size similar to that of oxide-derived copper produced nearly exclusive H₂ (96% Faraday efficiency) under identical conditions.

The researchers attributed the ability to change the intrinsic catalytic properties of Cu for this notoriously difficult reaction to the growth of the interconnected nanocrystallites from the constrained environment of the oxide lattice.

The Stanford team has begun looking for ways to create other fuels and improve the overall efficiency of the process.

In this experiment, ethanol was the major product. Propanol would actually be a higher energy-density fuel than ethanol, but right now there is no efficient way to produce it.

—Matthew Kanan

In the experiment, Kanan and Li found that a slightly altered oxide-derived copper catalyst produced propanol with 10% efficiency. The team is working to improve the yield for propanol by further tuning the catalyst’s structure. Ultimately, Kanan would like to see a scaled-up version of the catalytic cell powered by electricity from the sun, wind or other renewable resource.

For the process to be carbon neutral, scientists will have to find a new way to make carbon monoxide from renewable energy instead of fossil fuel, the primary source today. Kanan envisions taking carbon dioxide (CO₂) from the atmosphere to produce carbon monoxide, which, in turn, would be fed to a copper catalyst to make liquid fuel. The CO₂ that is released into the atmosphere during fuel combustion would be re-used to make more carbon monoxide and more fuel—a closed-loop, emissions-free process.

Technology already exists for converting CO₂ to carbon monoxide, but the missing piece was the efficient conversion of carbon monoxide to a useful fuel that's liquid, easy to store and nontoxic. Prior to our study, there was a sense that no catalyst could efficiently reduce carbon monoxide to a liquid. We have a solution to this problem that’s made of copper, which is cheap and abundant. We hope our results inspire other people to work on our system or develop a new catalyst that converts carbon monoxide to fuel.

—Matthew Kanan

The Nature study was coauthored by Jim Ciston, a senior staff scientist with the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory.

The research was supported by Stanford University, the National Science Foundation and the US Department of Energy.

Resources

• Christina W. Li, Jim Ciston & Matthew W. Kanan (2014) “Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper,” Nature doi: 10.1038/nature13249