Pinpointing active sites for water splitting

Microscopic defects on a catalyst made of carbon and iron actively drive the hydrogen evolution reaction, one of the key steps in liberating hydrogen from water, according to a study that combines electrochemical analysis with atomic resolution microscopy (Nat. Catal. 2021, DOI: 10.1038/s41929-021-00682-2). If scientists can find catalysts that efficiently split water into hydrogen and oxygen, then the oceans could serve as a nearly limitless supply of clean-burning, carbon-free hydrogen fuel for transportation and other uses. Precious metals work well as catalysts, but they’re expensive. So a team led by Stefano Agnoli and Gaetano Granozzi of the University of Padua examined inexpensive model catalysts consisting of thin films of graphene and iron. Sandwiches made from those films can be highly active electrocatalysts for hydrogen evolution, but how they work is unclear. The researchers used electrochemical scanning tunneling microscopy to examine the films while they mediated the catalytic reaction. By homing in on variations in the scanning tunneling current that track the catalytic process, the team probed individual sites on the films and identified the most catalytically active ones. Guided by quantum calculations, the researchers found that the most active sites include defects consisting of a few missing carbon atoms that exposed an underlying iron atom, as well as bent edges of graphene resembling carpeted steps. Such defects are easily generated by roughening the films.