A stainless steel mesh is at the heart of an electrically powered device that synthesizes ammonia.
N. Lazouski et al., Nature Catalysis, 4 May 2020
By Robert F. Service
As windmills and solar panels multiply, the supply of renewable electricity sometimes exceeds demand. Chemists would like to put the excess to work making commodity chemicals, such as the raw materials for fertilizer and plastics, which are now produced with heat, pressure, and copious fossil fuels. The electrochemical cells that can harness renewable electricity to make these compounds have been too slow to be practical. Now, two groups report redesigning the cells to achieve a dramatic speedup—perhaps enough to put green industrial chemistry within reach.
“In the future, electrons to molecules will be a major part of how we do chemical synthesis,” says Etosha Cave, chief scientific officer of Opus 12, a startup aiming to turn renewable energy into chemicals. “These two papers help push that vision forward.”
One research group uses carbon dioxide (CO2) as its starting material to make ethanol, a fuel, and ethylene, a starting point for plastics; the other turns nitrogen (N2) into ammonia (NH3), a key component in fertilizer. Both owe their progress to advances in the catalyst-coated electrodes that drive chemical reactions between gases and liquids.
In theory, turning CO2 into hydrocarbons such as ethanol and ethylene is simple: Just add energy to the CO2 so it can steal hydrogen atoms from water. But the reactions are tricky. They take place in electrolyzers, which consist of two electrodes separated by a liquid electrolyte. At one electrode, the anode, water splits into oxygen, electrons, and hydrogen ions, or protons. The protons then migrate through the electrolyte to the cathode, where they react with CO2, which is fed in separately, to make the hydrocarbons.
In current electrolyzers, the cathode typically consists of a 3D carbon mesh dotted with tiny copper catalyst particles. Their “gas diffusion” design allows CO2 gas that infiltrates the mesh to interact with all the catalyst particles simultaneously. One side of the mesh is also in contact with the liquid electrolyte, which helps ferry protons over from the anode. But water in the electrolyte can also infiltrate the pores, blocking CO2 gas from reaching the catalyst particles.
Coating the electrode with a water-repellent, fluorine-rich polymer can help. That and other improvements have resulted in electrolyzers that efficiently convert a modest input of electricity into hydrocarbons. But only about 40% of the product compounds have two carbon atoms, as ethylene and ethanol do. Much of the rest is methane, which has one carbon and is less valuable.
Now, researchers led by Ye Wang, a chemist at Xiamen University, report that adding fluorine to the standard copper catalyst on their gas diffusion electrode changes the pathway of the reactions, making them more likely to produce two-carbon compounds. Up to 85% of the resulting products are valuable two-carbon compounds, and the setup can handle 1600 milliamps of current per square centimeter of catalyst, twice the throughput of the previous record holder, the researchers reported on 20 April in Nature Catalysis. “[It’s] definitely in the range where someone will be interested in commercializing the technology,” says Karthish Manthiram, a chemical engineer at the Massachusetts Institute of Technology.
Today, Manthiram reported parallel improvements in an electrochemical process for making NH3 using N2 from air and hydrogen (H2). The H2, made by splitting water in a separate cell, is fed into a second electrolyzer, where it splits into protons and electrons at the anode. The protons pass through a liquid electrolyte made of an organic solvent that the team spikes with lithium to speed transfer of hydrogen to nitrogen. At the cathode, a three-step chemical process takes place: Piped-in N2 splits into individual nitrogen atoms, which react with the lithium to make lithium nitride. The lithium nitride then reacts with protons and electrons coming from the anode to make NH3, regenerating the lithium.
Manthiram’s team couldn’t use a porous carbon cathode because the liquid electrolyte would flood the electrode, blocking N2 gas from catalyst particles. So the researchers replaced the carbon cathode with a stainless steel mesh, which repels the organic electrolyte and allows N2 in. The setup churned out ammonia nearly five times as fast as the previous record, the group reported in Nature Catalysis. Its efficiency for converting the energy in electrons to chemical bonds in NH3 is 40%, the highest ever achieved with an electrolyzer.
That still doesn’t come close to that of fossil fuel–based plants, which can make ammonia with up to 80% energy efficiency, but “it’s absolutely great,” says Matteo Cargnello, a chemical engineer at Stanford University. Lauren Greenlee, a chemical engineer at the University of Arkansas, Fayetteville, agrees: “I think people will really jump on this” in search of further improvements.
With oil prices crashing because of price wars and the coronavirus pandemic, companies will likely continue to rely on fossil fuels to produce ammonia, ethanol, and other commodity chemicals in the near future. But as researchers continue to improve electricity-based production methods, even cheap fossil fuels may ultimately prove no match for surplus green energy.
Source: Science Mag