The race to find a substitute to platinum as a fuel-cell catalyst continues, with cobalt the latest alternative to be put forth. In a Nature Communications article from last week, a Chinese-American team reported successful results by attaching a small number of cobalt atoms to nitrogen pre-embedded in a graphene substrate.
Here, we report an inexpensive, concise and scalable method to disperse the earth-abundant metal, cobalt, onto nitrogen-doped graphene (denoted as Co-NG) by simply heat-treating graphene oxide (GO) and small amounts of cobalt salts in a gaseous NH3 atmosphere. These small amounts of cobalt atoms, coordinated to nitrogen atoms on the graphene, can work as extraordinary catalysts towards HER in both acidic and basic water.
Control tests with nitrogen enhanced graphene on its own showed very little catalytic power, but the inclusion of a dusting of cobalt atoms produced a material comparable to platinum-carbon in its onset voltage (30 mV). Stress tests showed little degradation in activity over ten hours.
In the eyes of team member James Tour, from Rice University, what makes the research groundbreaking is that it is predicated on the manipulation of single atoms, rather than particles or nanoparticles. He explained, “in our process the atoms driving catalysis have no metal atoms next to them. We’re getting away with very little cobalt to make a catalyst that nearly matches the best platinum catalysts.”
Using cobalt, the researchers claim that they were able to achieve a superior hydrogen evolution reaction to that seen in any comparable metal-free catalysts, including MoS2 (see Supplementary Table 1 here). The team especially stressed the superiority of Co-NG in alkaline media, contrasting it to MoS2 and metal phosphides, which are highly active in acid, but with poor stability and limited application in alkaline electrolysis.
Given its strong stability, Co-NG is mixed as a solution that can then be formed into a ‘paper’ that could be used as a free-standing electrode, or coated onto other conductive substrates. More generally, the research points the way towards new methods of preparing extremely efficient single-atom catalysts that may hold promise as next-generation materials.