For years, scientists have wondered how a metal-free material like graphene could be such a good catalyst. Well, now there’s an answer.
While graphene, an atom-thick form of carbon, has shown itself to have enormous potential for plant electronics and ultra-efficient water filtrations, its ability to act as a catalyst in fuel cells has always puzzled materials scientists.
Increasingly, graphene has been used as a replacement for expensive platinum in fuel cells where the material catalyses the oxygen reduction reaction (ORR) essential to turn chemical energy into electrical energy.
But, because graphene isn’t naturally metallic, it really shouldn’t show catalytic activity when used as a cathode, which led a team of researchers from Rice University to investigate why this is the case.
In a paper published to the journal Carbon, the team eventually discovered that trace quantities of manganese contamination from the original graphite or reactants actually hide within the graphene lattice.
When the graphene enters the right conditions – such as in a fuel cell – those minute metal bits activate the ORR.
The reason that analysis was unable to discover this until now is because the contrast between carbon and manganese atoms is so slight that the tools used to analyse them weren’t sensitive enough.
So, whereas x-ray diffraction and x-ray photoelectron spectroscopy couldn’t show the manganese, a tool called plasma mass spectrometry was able to show it loud and clear.
The team then subjected graphene samples to an acid wash, revealing that, after five or six washes, the manganese had disappeared and with it its catalyst capabilities.
The results now mean that we have to question other so-called metal-free materials.
“Single-atom catalysts can hide among graphene, and their activity is profound,” said James Tour, who led the research team.
“So, what has sometimes been attributed to the graphene was really the single metal buried into the graphene surface. Graphene is good in its own right but, in these cases, it was being made to look even better by these single metal-atom stowaways.”
Despite this, there is an upside to the research as it could help researchers find insight into how ultra-thin catalysts such as graphene can be improved.