Novel electrocatalyst produces valuable fuels using CO2

Jeff Fitlow, Rice University

RESEARCHERS at Rice University, US, have developed an electrocatalytic reactor that could be used to produce valuable fuels using carbon dioxide (CO₂).

In the study the researchers focussed on production of formic acid (HCOOH), which is an energy carrier that can be used directly in fuel cells. Additionally, it can be used indirectly in hydrogen fuel cells. According to Haotian Wang, Associate Professor in the Department of Chemical and Biomolecular Engineering at Rice, formic acid can store nearly 1,000 times as much energy as the same volume of hydrogen gas, which is difficult to compress. This is currently a big challenge for hydrogen fuel cell cars, said Wang.

The team at Rice developed an electrocatalyst which is able to achieve an energy conversion efficiency of up to 48.5%, meaning that nearly half of electrical energy could be stored in formic acid.

Traditionally, CO₂ is reduced in a liquid electrolyte, for example, water containing a salt such as sodium chloride or potassium bicarbonate. Using this method, the end product is obtained in a mixture with the salt ions, requiring costly and energy-intensive purification steps for separation. The developed device avoids the need for purification by employing polymer-based solid electrolytes for ion transport.

In the developed electrocatalytic device, CO₂ is reduced to formate (HCOO^-) by an HCOOH^-selective bismuth catalyst at the cathode. The negatively-charged ions are then driven by an electrical field into the middle solid electrolyte channel. On the anode side, water oxidation occurs generating protons (H⁺) that can move into the solid electrolyte channel to compensate the charge.

Depending on the electrolyte used, formic acid is formed at the left (H⁺-conductor) or right (HCOO^--conductor) interface between channel and membrane. The researchers employed solid electrolytes coated with sulfonic ligands to conduct positive charge, or amino acid functional groups to conduct negative charge. The HCOOH could then diffuse away in a flow of water or gas through the channel.

Using the developed device, the researchers were able to generate formic acid continuously for 100 hours, with negligible degradation of the reactor’s components, including the nanoscale catalysts.

The researchers found that the rate of water flow through the product chamber determines the concentration of formic acid. With the current setup they can achieve nearly 30% by weight formic acid using slow throughput. The researchers believe they will be able to achieve higher concentrations using next-generation reactors that “accept gas flow to bring out pure formic acid vapours”.

According to Chuan Xia, lead author of the study and Postdoctoral Researcher at Rice, the current process is easy to scale up because generation of the bismuth catalyst is achieved using simple hydrolysis of commercial bismuth nitrate. Currently, catalyst production is achieved on a milligramme or gramme scale, but the team at Rice developed a method capable of producing their catalyst at a kilogramme scale.

Xia said that the developed electrochemical device could be extended to many other electrochemical synthesis applications in the future.

For example, the reactor developed can be “retooled” to produce higher value products. This can be achieved by replacing the bismuth catalyst with other electrocatalysts. In the study the researchers used a copper-catalyst to produce acetic acid, ethanol, and propanol. Copper catalyst is able to generate multiple C₂₊ oxygenate fuels.

Furthermore, the study demonstrated that the reactor could be upgraded to a symmetric four-chamber configuration enabling simultaneous production of high purity products such as, potassium hydroxide, oxygen, and formic acid. In the future, such a configuration could be used to simultaneously produce sodium hydroxide, chlorine, and formic acid using a brine stream and CO₂.

Currently, the group is working to further develop the solid-electrolyte and electrocatalyst design to further improve the energy efficiency.

The current research was supported by Rice, and the US Department of Energy Office of Science User Facilities.

Wang said: “The big picture is that carbon dioxide reduction is very important for its effect on global warming as well as for green chemical synthesis.”

“If the electricity comes from renewable sources like the sun or wind, we can create a loop that turns carbon dioxide into something important without emitting more of it.”

Nature Energy: http://doi.org/dbg3