Credit: Nat. Catal.

An easy-to-prepare catalyst consisting of isolated metal atoms embedded at various points across functionalized graphene sheets can carry out a key step in water splitting, according to a study (Nat. Catal. 2018, DOI: 10.1038/s41929-018-0158-6).

If water could be split easily and inexpensively into molecular hydrogen (H 2 ) and oxygen (O 2 ), the world could draw a nearly limitless supply of clean-burning hydrogen fuel from the oceans. Inspired by nature’s use of a metal cluster (CaMn 4 O 5 ) to generate O 2 from water during photosynthesis, scientists have designed various multimetal-atom catalysts to facilitate the process, called the water oxidation reaction (WOR).

Several researchers have shown that various synthetic manganese-cluster catalysts actively mediate WOR. What remained unknown is whether a simpler, less expensive catalyst based on single manganese atoms rather than clusters can do the job actively and energy efficiently.

Yes, it can, concludes a team led by Can Li of the Dalian Institute of Chemical Physics. By reacting manganese chloride, graphene oxide, and ammonia, the researchers made a material in which isolated manganese atoms are embedded across a graphene surface, each surrounded by four nitrogen atoms. The team conducted water-oxidation tests to evaluate the material’s catalytic properties.

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The researchers found that in contrast with pure graphene and nitrogen-doped graphene, which were inactive, their MnN 4 -graphene catalyst exhibited a WOR turnover frequency of up to 215 per second. Turnover frequency is a measure of catalytic activity that describes how many times a catalyst carries out a reaction in a given period. The MnN 4 -graphene value is nearly 100 times as high as that of other synthetic Mn-based catalysts and in the ballpark of naturally occurring ones. The team also found that the new catalyst mediates WOR at relatively low overpotential values, an indication of high energy efficiency.