This year’s Nobel Prize in Chemistry makes clear that computational approaches and artificial intelligence are transforming many fields. One promising target area is energy storage, where researchers are tackling the challenge of developing single-atom catalysts (SACs)—a class of advanced materials that boost chemical reactions.
“SAC-based materials are a new frontier in both electrochemical technologies and heterogeneous catalysis: the use of solid catalysts for liquids or gases,” explained Yong-Wei Zhang, Distinguished Principal Scientist, and Jing Yang, Senior Research Scientist Ⅱ at the A*STAR Institute of High Performance Computing (A*STAR IHPC).
A common SAC design looks like a ‘carpet’ of carbon atoms, linked together as hexagons. If the carpet were pure carbon, it would be graphene: a widely-used catalyst in lithium-ion (Li-ion) batteries. However, like a pattern of woven flowers, individual atoms of a metal element—such as zinc, silver or copper—replace carbon atoms at spaced-out intervals in SACs.
“Graphene, decorated by single atoms, can provide more active sites to store lithium ions than graphene alone, increasing their theoretical capacity for high-energy density batteries,” said Zhang and Yang. “They also change how those ions are stored, enabling faster charging and slowing the buildup of unwanted lithium protrusions (dendrites).”
Traditional SAC production methods build them atom-by-atom in a ‘bottom-up’ approach, but their metal atoms tend to clump together in the process, drastically reducing the resulting SAC’s performance. Zhang and Yang explored an alternative route through a team-up with colleagues from A*STAR IHPC; A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2); John Wang and others from the National University of Singapore; and researchers from the US and China.
Schematics of the electrochemical breakdown of zinc (Zn) clusters into high-density single-atom (SA) material. Through a repeated alloying and dealloying process with lithium ions (Li+), individual Zn atoms escape from the clusters. These atoms can be captured and stabilised by defects in the carbon substrate to form new Zn SAs.
“We proposed a ‘top-down’ synthesis approach that uses a defect-rich carbon substrate with small zinc clusters to produce SACs,” said Zhang and Yang.
Like pilling fibres, those carbon atoms sticking out from their carbon ‘carpet’ would normally be considered defects. The team devised an electrochemical approach that—through cycles of controlled charging and discharging—broke down the zinc clusters into single atoms, which migrated into the defect rich substrate to form a high-density zinc SAC (Zn-SAC) material.
A key element of their toolbox was density functional theory (DFT), a computational method that can simulate the atomic-level behaviour of materials.
“DFT calculations allow us to observe phenomena that experimental devices might miss,” said Zhang and Yang. “With DFT, we analysed the interactions between zinc clusters and lithium ions, and how these interactions break down zinc clusters to form Zn-SAC.”
The team found that on average, their Zn-SAC showed boosted lithium-ion storage capacity by 58.31percent compared to the synthesised pure carbon sample. When used in Li-ion batteries, the material also remained stable over 500 charge cycles with an energy density over 300 watt-hours per kilogramme.
As zinc is cheaper than other rare metals, the researchers believe their method could lead to more cost-effective SAC production at scale for energy applications. They are currently expanding their approach to similar single-atom systems, with an eye toward applications in Li-ion and sodium-ion batteries, as well as advanced electrocatalysts for water splitting and carbon dioxide mitigation.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of High Performance Computing (A*STAR IHPC), A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2).
