Cobalt/cobalt oxide/graphene catalyst shows comparable activity and better stability as fuel cell catalyst than platinum
17 October 2012
|Left: CoO shell is shown in green, Co core in black. Right: Nanoparticles of cobalt attach themselves to a graphene substrate in a single layer. Credit: Sun Lab/Brown University. Click to enlarge.|
Chemists at Brown University have engineered a cobalt/cobalt oxide/graphene catalyst for the oxygen reduction reaction in fuel cells that shows comparative activity and better stability than a commercial platinum nanoparticle catalyst supported on carbon (C–Pt). Their report appears in the journal Angewandte Chemie International Edition.
The team led by Shouheng Sun and his students used a solution-phase self-assembly approach to produce Co/CoO core/shell nanoparticles deposited on graphene (G–Co/CoO NPs). They found that the catalytic activity for the oxygen reduction reaction in O2-saturated KOH solution depends on the thickness of the CoO shell.
The new material “has the best reduction performance of any non-platinum catalyst,” said Shaojun Guo, postdoctoral researcher in Sun’s lab and lead author of the paper.
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.
The reaction requires a catalyst, and platinum is currently the best one, noted Sun. However, platinum is expensive and degrades over time. A number of research efforts are exploring alternatives. A few researchers, including Sun and Guo, have developed new catalysts that reduce the amount of platinum required, but an effective catalyst that uses no platinum at all remains elusive.
This new graphene-cobalt material is the most promising candidate yet, the researchers say. It is the first catalyst not made from a precious metal that comes close to matching platinum’s properties.
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% of its initial capacity. The platinum catalyst the team tested performed at less than 60% after the same amount of time.
Cobalt is an abundant metal, readily available at a fraction of what platinum costs. Graphene is a one-atom-thick sheet of carbon atoms arranged in a honeycomb structure. Developed in the last few years, graphene is renowned for its strength, electrical properties, and catalytic potential.
Often, graphene nanoparticle materials are made by growing nanoparticles directly on the graphene surface. But that process is problematic for making a catalyst, Sun said, given the difficulty of controlling the size, shape, and composition of the nanoparticles.
The self-assembly method provided 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.
The researchers could control the thickness of the cobalt-oxide shell by heating the material at 70 °C for varying amounts of time. Heating it longer increased the thickness of the shell. This way, they could fine-tune the structure in search of a combination that gives top performance. In this case, they found that a 1-nanometer shell of cobalt-oxide optimized catalytic properties.
Guo, S., Zhang, S., Wu, L. and Sun, S. (2012), Co/CoO Nanoparticles Assembled on Graphene for Electrochemical Reduction of Oxygen . Angew. Chem. Int. Ed.. doi: 10.1002/anie.201206152
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