Hydrogen could be an environmentally friendly alternative to conventional fossil fuels, particularly if it is electrochemically produced from water in electrocatalytic or photoelectrochemical cells. However, before this can happen, scientists need to make advanced catalysts that increase the efficiency of the HER. Today, however, the most efficient HER catalysts contain expensive platinum and platinum-group metals.

Jin and colleagues made their new catalyst from ternary pyrite-type cobalt phosphosulphide (CoPS). The researchers have been studying iron pyrite (or iron disulphide, FeS2), also known as “fool’s gold”, for a few years now and had also previously found that cobalt pyrite (or cobalt disulphide, CoS2) is an excellent catalyst for HER.

Outstanding electrocatalytic properties

“In this work, we hypothesized that by replacing one sulphur atom out of the disulphur dumbshells in CoS2 with a phosphorus atom, we could tune the electronic structures of this compound and so improve the catalytic activity,” explains Jin. “We found that we could readily synthesize the ternary compound CoPS in the form of films, nanowires and nanoplatelets, and that their electrocatalytic properties were outstanding.”

The CoPS catalyst has very low catalytic overpotential for the HER of about 50 mV – a value that is very close to that of platinum-based catalysts (about 30 mV). “Our CoPS catalyst is also quite a bit better than other catalysts that do not contain noble metals,” says Jin. “For example, those made from cobalt, such as CoS2 that we reported on last year and the various cobalt phosphide catalysts that other groups made recently. CoPS catalysts are extremely stable during catalytic reactions too.”

According to the researchers, the new catalyst could find use in electrocatalytic and solar-driven photoelectrochemical water splitting. “Indeed, we have already demonstrated such applications in this work,” Jin tells nanotechweb.org. “The new hydrogen-producing catalyst is also cheap, since it is made of phosphorus and sulphur – two very common elements on Earth. Cobalt is also a thousand times cheaper than platinum – a detail that is especially important if we are to build large, square-mile-sized installations to produce hydrogen in the future.”

The team says that it is now busy trying to understand how the CoPS catalyst works in detail. “We are also looking into analogous compounds to CoPS (for example, NiPSe related compounds) and other catalytic applications of CoPS,” adds Jin. “I think this will open up a lot of new ideas for this class of compound, which has rarely been explored until now.”

The research is detailed in Nature Materials doi:10.1038/nmat4410.