Climate change is intensifying worldwide, yet emissions of greenhouse gases like carbon dioxide (CO2) remain difficult to curb. As atmospheric CO2 levels rise, scientists are exploring novel ways to reduce emissions and reuse captured CO2. One promising pathway uses electricity generated from renewable energy sources to convert captured CO2 into useful chemicals, a process known as electrochemical reduction. These chemicals include liquid fuels like formate, which is valued for its high energy concentration, low toxicity, and ease of storage and transport.
To achieve this lofty goal, scientists rely on special materials called electrocatalysts to guide the carbon-conversion reactions through alternate chemical pathways that require less energy. However, electrocatalysts are often made from expensive precious metals like gold that can cost hundreds of dollars per gram, making their large-scale use impractical. Furthermore, electrochemical reactions tend to involve harsh operating conditions that degrade and deactivate the electrocatalysts over time, limiting their lifespan. To address these challenges, scientists are designing specialized electrocatalysts with more stable molecular structures and modified chemical compositions to minimize costs and maximize efficiency.
Researchers at the King Fahd University of Petroleum & Minerals explored whether a special type of zinc-based electrocatalyst could efficiently convert CO2 to formate. The electrocatalyst was composed of zinc ions interconnected in a 3D molecular structure, called a Zeolitic Imidazolate Framework-8 or ZIF-8. ZIF-8 has pores that trap CO2, but it doesn’t conduct electricity well and converts limited amounts of CO2 on its own. To overcome these limitations, the team incorporated nanoparticles of the conductive metal bismuth into the structure to promote CO2 capture, conductivity, and formate production.
To synthesize the electrocatalyst, the researchers combined a solution of zinc nitrate hexahydrate and bismuth nitrate pentahydrate with a chemical linker that formed physical bridges connecting the metals into a ZIF-8 structure with bismuth ions attached. They poured a strong reducing chemical into the mixture to activate the bismuth as nanoparticles, spun the mixture in a centrifuge, and dried it to recover the bismuth nanoparticle-containing ZIF-8 powder, or Bi-ZIF-8.
The team mixed the Bi-ZIF-8 powder with a glue-like chemical and coated the mixture onto conductive carbon paper to provide a supported surface for the electrocatalyst. Then they loaded the coated carbon paper into a specialized air-tight device used for electrochemical reactions, called an electrolyzer, and submerged it in a salty solution with CO2 gas bubbling through.
The researchers sequentially applied electricity to the system for 20 minutes each at 5 different rates of electric current per unit area of the electrocatalyst, or current densities, ranging from -25 to -200 milliamperes per square centimeter (mA/cm2). For comparison, this is similar to passing the current used by a small LED bulb through a surface about the size of a fingernail. By testing different current densities, they evaluated whether the electrocatalyst can convert CO2 fast enough to sustain operation under industrially-relevant conditions, which require more negative operating currents and impose greater demands on the material. They also quantified the product gases and liquids from the electrolyzer after each test.
The researchers found that ZIF-8 alone mostly produced carbon monoxide and no formate, whereas adding bismuth nanoparticles produced more formate. They analyzed the materials’ structures and observed that the bismuth nanoparticles increased ZIF-8’s electrical conductivity by 16 times and its active surface area by 11 times. The nanoparticles also suppressed competing reactions that can reduce how much formate is produced. At the same time, the ZIF-8 structure stabilized the bismuth nanoparticles and protected them from clumping or degrading.
The team also tested different operating parameters and electrolyzer setups to maximize formate production. To quantify this, they measured efficiency by calculating the fraction of electrical charge that went toward producing the desired formate product rather than unwanted byproducts. They found that operating at higher current densities and supplying CO2 directly to the electrocatalyst increased formate production up to 91% efficiency. In addition, the system maintained this high efficiency at current densities up to -150 mA/cm2, exceeding typical laboratory-scale benchmarks by about 50%.
The researchers concluded that their Bi-ZIF-8 electrocatalyst could help mitigate climate change by supporting cleaner, more sustainable energy production. They suggested the next steps will be to optimize the electrocatalyst’s composition and refine the electrolyzer’s operating conditions for larger-scale production, which could enhance the technology’s feasibility and real-world relevance.
