To limit the increase in global temperature to 1.5°C by 2050, scientists worldwide are rushing to find effective decarbonization solutions. China could potentially lead the race through carbon dioxide electrolysis, according to Jinlong Gong, a professor of chemical engineering at Tianjin University.
“Our goal is to enable industry-scale electrochemical reduction of CO2, which involves capturing CO2 and, with renewable power, combining it with water to produce useful carbon-based products such as syngas, ethylene and ethanol,” says Gong.
However, some of the biggest unknowns about carbon dioxide electrolysis lie in its fundamental mechanisms. Scrutinizing “every freeze frame” in detail, Gong’s team is examining how sequences of elementary reactions could trigger changes to the speed and yield in complex catalytic reactions.
“Over the last five years, we’ve focused on reaction pathways and catalytic sites, and through theoretical modelling and experimental analyses, improved catalyst designs towards commercialization,” he says.
One major barrier is the rate-determining step (RDS), the slowest step in a chemical reaction that limits the overall speed of the reduction process. By analysing the RDS of different catalysts, the team has discovered that the adsorption of CO2 on to transition metal catalyst surfaces governs the overall reaction rate. Their study was published in Nature Communications1 in early 2022.
This sheds light on more targeted catalyst improvement, such as its surface modification, as well as controlling CO2 pressure on the surface and interfacial electric field.
They’ve also studied the atomic structures of active sites in copper-based catalytic systems to understand the nature of the sites. By analysing more than 2,000 surface sites through high throughput calculations, they’ve identified two types of active sites ― balanced Cu−Zn sites and Zn-heavy Cu−Zn sites ― for catalyst design2.
Both computational and hands-on experiments are important to gather and validate data. As Gong explains, these copper catalysts are the only type of metal-based catalyst that enable the production of multi-carbon products, known as C2+ compounds, but copper is easily oxidized.
Attempts to understand this phenomenon via thermodynamic experiments are difficult, as physical changes to the catalyst are “too microscopic and difficult to identify experimentally”, and they occur too quickly to capture with traditional electron microscopy.
To combat this, Gong and his team use an artificial neural network — a set of algorithms modelled on the human brain — to help them simulate and analyse a much larger catalyst surface, and at a significantly higher resolution, to understand the physical changes3.
“We have also collected computer data of the RDS for diverse reaction types and catalysts beyond copper, including gold, silver, tin and indium, which have then been verified experimentally,” he adds.
Gong explains that a greater understanding of catalytic sites and RDS has led to discoveries, for example, about where ethylene and ethanol are generated, or to improve catalyst design with increased surface area of their active sites.
His team now aims to increase the electrode size to more than 100 cm2, achieve a current density above 200 mA/cm2 without the loss of product selectivity, and maintain a stable operation for more than 100 hours.
“We are also working to prepare for CO2 electrolysis on an industry scale with business partners, while integrating time-resolved techniques to identify more reaction intermediates in our fundamental research,” says Gong.
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