“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,” report scientists from Stanford University, Stanford, Calif.
The eco-friendly method could potentially replace conventional ethanol production from corn and other crops, they add.
Current ethanol production involves high-temperature fermentation to chemically convert corn, sugarcane and other plants into liquid fuel. In addition, thousands of acres of land and vast quantities of fertilizer and water are required to grow the crops.
The new method, described in a recent Nature article, requires no fermentation and, if scaled up, could help address environmental issues surrounding ethanol production. “Our study demonstrates the feasibility of making ethanol by electrocatalysis,” says Matthew Kanan, an assistant professor of chemistry at the university who developed the technique along with Stanford graduate student Christina Li. “But we have a lot more work to do to make a device that is practical.”
The two scientists first created a metallic electrode produced from so-called “oxide-derived copper.”
“Conventional copper electrodes consist of individual nanoparticles that just sit on top of each other,” Kanan explains. “Oxide-derived copper, on the other hand, 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.”
The scientists then built an electrochemical cell consisting of two electrodes placed in water saturated with carbon monoxide gas. When they applied a small voltage, the electrode cathode made of oxide-derived copper produced ethanol and acetate with 57% faradaic efficiency, Kanan notes.
“That means 57% of the electric current went into producing these two compounds from carbon monoxide. … This represents a more than 10-fold increase in efficiency over conventional copper catalysts. Our models suggest that the nanocrystalline network in the oxide-derived copper was critical for achieving these results,” he says.
In addition, Kanan and Li found that a slightly altered oxide-derived copper catalyst produced propanol with 10% efficiency.
“Propanol would actually be a higher energy-density fuel than ethanol, but right now there is no efficient way to produce it,” he says.
Kanan believes an optimized catalyst in an appropriately configured electrolyzer could have very high Faradaic efficiency (>90%) for ethanol. “Propanol may be more challenging,” he notes. “We have been able to improve considerably on the 10% Faradaic efficiency for propanol reported … but it is still too early to determine.”
The team has two major challenges going forward. “The first is to understand the relationship between the structure of the catalyst and its activity for CO conversion,” says Kanan.
Current work focuses on identifying structural features on the nanometer and atomic scale that are required for activity and the features that determine selectivity. “We have leads on this front and several techniques available for pursuing them. The goal is to gain insight from these fundamental studies that will enable us to make more active and selective catalysts, ideally separate catalysts that are optimized for ethanol, propanol and acetate synthesis.”
The second challenge is developing prototype electrolyzers “that incorporate our catalysts and are optimized for high rates of fuel synthesis,” says Kanan.
Recently, the team took the first steps toward prototype electrolyzer assembly; a functional prototype will require about 2–3 years. Any pilot-plant scale-up will depend on the assessment of the technology at that point and the amount of commercial interest.
Kanan doesn’t anticipate a major problem scaling up. “Making Cu2O only requires Cu, air and heat and Cu is relatively cheap for an electrode material. Nonetheless, we are working to minimize the catalyst loading,” he explains.
While the catalysts appear to be robust, longer tests are required. “They are susceptible to poisoning by metal impurities in electrolyte solutions, although that problem can be addressed by using highly deionized water and minimizing the solution volume,” notes Kanan.