Researchers at the US Department of Energy’s (DOE) Brookhaven National Laboratory and the University of Arkansas have developed a highly efficient catalyst for extracting electrical energy from ethanol. The ternary Au@PtIr core–shell catalyst, described in a paper in the Journal of the American Chemical Society, steers the electro-oxidation of ethanol down an ideal chemical pathway that releases the liquid fuel’s full potential of stored energy.
For the ethanol reduction reaction (EOR) in alkaline solutions, the new catalyst exhibits an activity enhancement of 6 orders of magnitude compared to AuPtIr alloy catalysts.
This catalyst is a game changer that will enable the use of ethanol fuel cells as a promising high-energy-density source of off-the-grid electrical power.—Jia Wang, the Brookhaven Lab chemist who led the work
Much of ethanol’s potential power is locked up in the carbon-carbon bonds that form the backbone of the molecule.
Direct ethanol fuel cells using proton exchange membrane or anion exchange membrane are attractive power sources for portable devices due to the high energy density of the fuel and system. Each ethanol molecule can release 12 electrons (12e) via a complete electrooxidation: CH3CH2OH + 3H2O = 2CO2 + 12H+ + 12e-, which involves cleavage of the C-C bond and multiple dehydrogenation and oxidation steps. However, incomplete ethanol oxidation reaction (EOR), often occurs with 4e transfer producing acetic acid (CH3COOH) in acid or acetate (CH3COO-) in base.—Liang et al.
The catalyst developed by Wang’s group reveals that breaking those bonds at the right time is the key to unlocking that stored energy.
Electro-oxidation of ethanol can produce 12 electrons per molecule. But the reaction can progress by following many different pathways.—Jia Wang
Most of these pathways result in incomplete oxidation: The catalysts leave carbon-carbon bonds intact, releasing fewer electrons. They also strip off hydrogen atoms early in the process, exposing carbon atoms to the formation of carbon monoxide, which poisons the catalysts’ ability to function over time.
The 12-electron full oxidation of ethanol requires breaking the carbon-carbon bond at the beginning of the process, while hydrogen atoms are still attached, because the hydrogen protects the carbon and prevents the formation of carbon monoxide.—Jian Wang
Multiple steps of dehydrogenation and oxidation are then needed to complete the process.
The new catalyst—which combines reactive elements in a unique core-shell structure that Brookhaven scientists have been exploring for a range of catalytic reactions—speeds up all of these steps.
To make the catalyst, Jingyi Chen of the University of Arkansas, who was a visiting scientist at Brookhaven during part of this project, developed a synthesis method to co-deposit platinum and iridium on gold nanoparticles. The platinum and iridium form monoatomic islands across the surface of the gold nanoparticles. That arrangement, Chen noted, is the key that accounts for the catalyst’s outstanding performance.
A schematic showing how the monoatomic islands of platinum (green) and iridium (blue) on the gold nanoparticle surface (yellow) enable a full 12-electron oxidation of ethanol without carbon monoxide poisoning. The graph illustrates the significantly higher peak current produced by the new catalyst (Au@PtIr) compared with three other catalysts: gold core/iridium shell (Au@Ir); iridium/platinum alloy (IrPt); and gold core/platinum shell (Au@Pt).
The gold nanoparticle cores induce tensile strain in the platinum-iridium monoatomic islands, which increases those elements’ ability to cleave the carbon-carbon bonds, and then strip away its hydrogen atoms, Chen said.
Zhixiu Liang, a Stony Brook University graduate student and the first author of the paper, performed studies in Wang’s lab to understand how the catalyst achieves its record-high energy conversion efficiency. He used in-situ infrared reflection-absorption spectroscopy to identify the reaction intermediates and products, comparing those produced by the new catalyst with reactions using a gold-core/platinum-shell catalyst and also a platinum-iridium alloy catalyst.
By measuring the spectra produced when the infrared light is absorbed at different steps in the reaction, this method allows us to track, at each step, what species have been formed and how much of each product. The spectra revealed that the new catalyst steers ethanol toward the 12-electron full oxidation pathway, releasing the fuel’s full potential of stored energy.—Zhixiu Liang
The next step, Wang noted, is to engineer devices that incorporate the new catalyst.
The mechanistic details revealed by this study may also help guide the rational design of future multicomponent catalysts for other applications.
This work was funded by the US Department of Energy’s Office of Science and the National Science Foundation.
Zhixiu Liang, Liang Song, Shiqing Deng, Yimei Zhu, Eli Stavitski, Radoslav R. Adzic, Jingyi Chen, and Jia X. Wang (2019) “Direct 12-Electron Oxidation of Ethanol on a Ternary Au(core)-PtIr(Shell) Electrocatalyst” Journal of the American Chemical Society doi: 10.1021/jacs.9b03474