Researchers cut energy costs of CO₂ conversion

Researchers at Helmholtz-Zentrum Berlin (HZB) and their international collaborators have identified conditions that improve the electrochemical conversion of carbon dioxide (CO₂) into carbon monoxide (CO) by optimizing the size and density of silver nanoparticles.

Their findings show that silver nanoparticles approximately 10 nanometers in diameter perform best when relatively sparsely distributed on a carbon support. The team also reduced the process's energy consumption by more than 30% by modifying the anode reaction, while enabling the production of valuable chemicals, including formic acid, alongside hydrogen.

Converting carbon dioxide into valuable chemicals has attracted growing interest as a strategy to reduce greenhouse gas emissions and advance a circular carbon economy. Instead of releasing CO₂ directly into the atmosphere, this approach uses electricity to transform it into other compounds that can serve as feedstocks for chemical manufacturing and fuel production.

As part of the international GreenQUEST project, a research team led by Prashanth Menezes, a chemist at HZB, systematically investigated the performance of catalytic layers made from silver nanoparticles. The main objective was to identify conditions that would enable CO₂ conversion into CO with lower energy consumption and cost.

Carbon monoxide is an important feedstock for producing chemicals and fuels. When combined with hydrogen generated through electrochemical processes, it can contribute to the production of synthesis gas, or syngas. Syngas can subsequently be used to produce dimethyl ether (DME), which can then be catalytically converted into green liquefied petroleum gas (green LPG), consisting mainly of propane and butane.

If the electricity used for electrolysis comes from renewable sources such as solar or wind power, this process could help recycle CO₂ and reduce dependence on fossil resources. Developing catalysts that enable more efficient conversion with lower energy consumption is therefore particularly important.

Optimizing silver nanoparticle size and density

The researchers focused on silver nanoparticles deposited on an electrode alongside conductive carbon powder. They systematically varied the particles' size and density to determine how these factors affected carbon monoxide production.

Previous research had shown that making silver particles smaller does not necessarily improve catalytic performance. Very small nanoparticles tend to favor the competing hydrogen-producing reaction, potentially reducing the proportion of CO produced. Conversely, excessively large silver particles generally exhibit lower catalytic activity.

The challenge was therefore to identify the optimal balance. The results showed that silver nanoparticles approximately 10 nanometers in diameter performed best when relatively sparsely distributed across the carbon support. The reported optimal silver loading was approximately 0.2 milligrams per square centimeter of electrode area.

Transmission electron microscopy (TEM) images were used to examine the nanoparticles' size and structure. In the optimized formulation, silver accounted for approximately 20% of the catalyst's mass, while conductive carbon powder made up the remaining 80%.

The findings highlight that neither increasing the amount of silver nor reducing particle size necessarily leads to better performance. The distribution and precise dimensions of nanoparticles can determine which chemical reactions dominate on the catalyst's surface. In this study, optimizing both parameters created more favorable conditions for converting CO₂ into CO.

An alternative anode reaction cuts energy consumption

The team also modified the reaction occurring at the anode, the other electrode in the electrochemical cell. In conventional systems, CO₂ is converted into CO at the cathode, while oxygen is typically produced at the anode. The oxygen-evolution reaction requires a substantial amount of electrical energy, and the oxygen produced may have limited economic value within the process.

By introducing aldehyde compounds into the electrolyte, the researchers replaced oxygen evolution with aldehyde oxidation. This change reduced the overall energy consumption of the process by more than 30%. It also enabled the production of additional valuable chemicals, including formic acid, rather than oxygen alone.

In this configuration, the cathode converts CO₂ into CO, while formaldehyde is oxidized to formic acid at the anode. Hydrogen is also produced, and a separating membrane keeps the two sides of the electrochemical cell apart.

This design allows a single electrochemical cell to produce several useful products simultaneously. From an economic perspective, this could be significant because energy consumption is only one factor determining the industrial feasibility of electrochemical processes. Producing multiple marketable compounds could increase the overall value of the system.

Catalyst maintains performance for more than 100 hours

The optimized catalyst was also tested over an extended period. Under optimal conditions, the silver nanoparticles maintained a Faradaic efficiency close to 100% for CO production for more than 100 hours. Faradaic efficiency measures how effectively the electrical current is used to produce the target product.

The researchers also used X-ray photoelectron spectroscopy to investigate the electronic and chemical structure of the silver particles. The results indicated that the nanoparticles' active structure remained largely stable throughout the process. Catalyst stability is an important consideration for the long-term operation of such systems.

Menezes said that combining carbon monoxide production with the simultaneous generation of hydrogen and other valuable chemicals, such as formic acid, could increase the overall value of the electrochemical process. The CO and hydrogen produced could subsequently serve as feedstocks for manufacturing sustainable fuels and chemicals.

GreenQUEST aims to develop sustainable cooking fuel

The research was conducted under the GreenQUEST project, involving HZB and partner institutions in South Africa. The project's broader objective is to develop an affordable, sustainable green fuel for cooking that could provide a cleaner alternative to traditional biomass fuels.

This goal is particularly relevant to rural areas of South Africa, where firewood remains a major household cooking fuel. If developed for industrial-scale use, green LPG produced through CO₂ recycling and renewable electricity could help reduce dependence on biomass and mitigate the environmental impacts associated with traditional fuels.

The findings were published in Advanced Functional Materials. The study demonstrates that seemingly small changes in the size and density of catalytic nanoparticles can substantially affect the performance of CO₂ conversion systems. Combining approximately 10-nanometer silver nanoparticles, a carefully designed anode reaction and renewable electricity offers a potential pathway toward processes that use carbon dioxide not merely as a greenhouse gas, but as a feedstock for valuable fuels and chemicals.