A new catalyst created by University of Toronto engineering researchers brings them one step closer to artificial photosynthesis — a system that, just like plants, would use renewable energy to convert carbon dioxide (CO2) into stored chemical energy. By capturing carbon emissions and storing energy from solar or wind power, the invention is a huge step forward in the fight against climate change.
"Carbon capture and renewable energy are two promising technologies, but there are problems," says Phil De Luna, an author on the research paper. "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 designed 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.
Like 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.
"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."
This 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 the 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 that has been 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."
The team reports the overall electrical-to chemical power conversion efficiency of the system at 64 percent. According to De Luna, this is the highest value ever achieved for a system like this.
The new catalyst is made of nickel, iron, cobalt and phosphorous, all elements that are low-cost and have only a few safety hazards. It can be synthesized at room temperature using relatively inexpensive equipment, and the team showed that it remained stable as long as they tested it.
Armed with the 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 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."
As for what has kept him motivated throughout the project, De Luna points to the opportunity to make an impact on some of society's biggest environmental challenges.
"Seeing the rapid advancement within the field has been extremely exciting," he says. "At every weekly or monthly conference that we have, within our lab, people are smashing records left and right. There is still a lot of room to grow, but I genuinely enjoy the research, and carbon emissions are such a big deal that any improvement feels like a real accomplishment."
The paper on this research was published in Nature Chemistry.