Platinum works well as a catalyst in hydrogen fuel cells, but it is expensive and degrades over time. Brown University chemist Shouheng Sun and his students have developed a new material — a graphene sheet covered by cobalt and cobalt-oxide nanoparticles — that can catalyze the oxygen reduction reaction nearly as well as platinum does and is substantially more durable.
The oxygen reduction reaction occurs on the cathode side of a hydrogen fuel cell. Oxygen functions as an electron sink, stripping electrons from hydrogen fuel at the anode and creating the electrical pull that keeps the current running through electrical devices powered by the cell.
Lab tests performed by Sun and his team showed that the new graphene-cobalt material was a bit slower than platinum in getting the oxygen reduction reaction started, but once the reaction was going, the new material actually reduced oxygen at a faster pace than platinum. The new catalyst also proved to be more stable, degrading much more slowly than platinum over time. After about 17 hours of testing, the graphene-cobalt catalyst was performing at around 70 percent of its initial capacity. The platinum catalyst the team tested performed at less than 60 percent after the same amount of time.
Sun and his team used a self-assembly method that gave them more control over the material’s properties. First, they dispersed cobalt nanoparticles and graphene in separate solutions. The two solutions were then combined and pounded with sound waves to make sure they mixed thoroughly. That caused the nanoparticles to attach evenly to the graphene in a single layer, which maximizes the potential of each particle to be involved in the reaction. The material was then pulled out of solution using a centrifuge and dried. When exposed to air, outside layers of atomic cobalt on each nanoparticle are oxidized, forming a shell of cobalt-oxide that helps protect the cobalt core.