Optimised catalyst boosts artificial photosynthesis

"Carbon capture and renewable energy are two promising technologies, but there are problems," says Phil De Luna, one of the lead authors of a paper published in Nature Chemistry.

"Carbon capture technology is expensive, and solar and wind power are intermittent. You can use batteries to store energy, but a battery isn't going to power an airplane across the Atlantic or heat a home all winter: for that you need fuels."

De Luna and his co-lead authors Xueli Zheng and Bo Zhang - who conducted their work under the supervision of Professor Ted Sargent - aim to address both challenges at once, and they are looking to nature for inspiration.

They are designing an artificial system that mimics how plants and other photosynthetic organisms use sunlight to convert CO2 and water into molecules that humans can later use for fuel.

As in plants, their system consists of two linked chemical reactions: one that splits H2O into protons and oxygen gas, and another that converts CO2 into carbon monoxide, or CO. (The CO can then be converted into hydrocarbon fuels through an established industrial process called Fischer-Tropsch synthesis.)

"Over the last couple of years, our team has developed very high-performing catalysts for both the first and the second reactions," says Zhang, who contributed to the work while a post-doctoral fellow at U of T and is now a professor at Fudan University. "But while the second catalyst works under neutral conditions, the first catalyst requires high pH levels in order to be most active."

That means that when the two are combined, the overall process is not as efficient as it could be, as energy is lost when moving charged particles between the two parts of the system.

The team has now overcome this problem by developing a new catalyst for the first reaction - the one that splits water into protons and oxygen gas. Unlike the previous catalyst, this one works at neutral pH, and under those conditions it performs better than any other catalyst previously reported.

"It has a low overpotential, which means less electrical energy is needed to drive the reaction forward," says Zheng, who is now a postdoctoral scholar at Stanford University. "On top of that, having a catalyst that can work at the same neutral pH as the CO2 conversion reaction reduces the overall potential of the cell."

In the paper, the team reports the overall electrical-to-chemical power conversion efficiency of the system at 64%. According to De Luna, this is the highest value ever achieved for such a system, including their previous one, which only reached 54%.

The new catalyst is made of nickel, iron, cobalt and phosphorus, all elements that are low-cost and pose few safety hazards. It can be synthesised at room temperature using relatively inexpensive equipment, and the team showed that it remained stable as long as they tested it, a total of 100 hours.

Armed with their improved catalyst, the Sargent lab is now working to build their artificial photosynthesis system at pilot scale. The goal is to capture CO2 from flue gas - for example, from a natural gas-burning power plant - and use the catalytic system to efficiently convert it into liquid fuels.

"We have to determine the right operating conditions: flow rate, concentration of electrolyte, electrical potential," says De Luna. "From this point on, it's all engineering."

The team and their invention are semi-finalists in the NRG COSIA Carbon XPRIZE, a $20 million challenge to "develop breakthrough technologies that will convert CO? emissions from power plants and industrial facilities into valuable products."