The prototype of a catalytic reactor, an electrolyzer, uses carbon dioxide as its feedstock and produces high concentrations of formic acid, pure liquid fuel.
In tests, the new electrocatalyst reached an energy conversion efficiency of about 42%. That means nearly half of the electrical energy can be stored in formic acid as liquid fuel. Formic acid produced by traditional carbon dioxide devices needs costly and energy-intensive purification steps.
"Formic acid is an energy carrier," says Rice University biomolecular engineer Haotian Wang, Ph.D. "It's a fuel-cell fuel that can generate electricity and emit carbon dioxide -- which you can grab and recycle again. It's also fundamental in the chemical engineering industry as a feedstock for other chemicals, and a storage material for hydrogen that can hold nearly 1,000 times the energy of the same volume of hydrogen gas, which is difficult to compress. That's currently a big challenge for hydrogen fuel-cell cars."
Haotian Wang. Credit: Jeff Fitlow/Rice University
Two advances made the new device possible. The first was his development of a robust, two-dimensional bismuth catalyst and the second a solid-state electrolyte that eliminates the need for salt as part of the reaction.
"Bismuth is a very heavy atom, compared to transition metals like copper, iron or cobalt," Wang said. "Its mobility is much lower, particularly under reaction conditions. So that stabilizes the catalyst." He noted the reactor is structured to keep water from contacting the catalyst, which also helps preserve it.
The polymer-based solid electrolyte is coated with sulfonic acid ligands to conduct positive charge or amino functional groups to conduct negative ions. "Usually people reduce carbon dioxide in a traditional liquid electrolyte like salty water," Wang said. "You want the electricity to be conducted, but pure water electrolyte is too resistant. You need to add salts like sodium chloride or potassium bicarbonate so that ions can move freely in water.
"But when you generate formic acid that way, it mixes with the salts. For a majority of applications you have to remove the salts from the end product, which takes a lot of energy and cost. So we employed solid electrolytes that conduct protons and can be made of insoluble polymers or inorganic compounds, eliminating the need for salts."
The rate at which water flows through the product chamber determines the concentration of the solution. Slow throughput with the current setup produces a solution that is nearly 30% formic acid by weight, while faster flows allow the concentration to be customized. The researchers expect to achieve higher concentrations from next-generation reactors that accept gas flow to bring out pure formic acid vapors.
With its current reactor, the lab generated formic acid continuously for 100 hours with negligible degradation of the reactor's components, including the nanoscale catalysts. Wang suggested the reactor could be easily retooled to produce such higher-value products as acetic acid, ethanol or propanol fuels.