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A new catalyst for the so-called hydrogen evolution reaction has been developed by researchers at Stanford University in California.
By Admin (from 21/09/2011 @ 14:00:27, in en - Science and Society, read 1370 times)

The catalyst, which is made of molybdenum disulphide nanoparticles grown on graphene, might be a real alternative to expensive platinum in future large-scale industrial and domestic applications.

Hydrogen could be an environmentally friendly alternative to conventional fossil fuels, particularly if it is electrochemically produced from ordinary seawater – a huge and abundantly available resource. However, before this can happen, scientists need to make advanced catalysts that increase the efficiency of the electrochemical hydrogen reaction (HER). Today, the most efficient HER catalysts are those made from platinum-group metals, but these are expensive.

Now, Hongjie Dai and colleagues have shown that flexible graphene oxide sheets could provide an ideal substrate for MoS2 nanoparticles. The resulting MoS2/reduced graphene oxide hybrid has a very high electrocatalytic activity for the HER that is superior to MoS2 catalysts synthesized without graphene.

 (A) Schematic solvothermal synthesis with GO sheets to afford the MoS2/RGO hybrid. (B) SEM and (inset) TEM images of the MoS2/RGO hybrid. (C) Schematic solvothermal synthesis without any GO sheets, resulting in large, free MoS2 particles. (D) SEM and (inset) TEM images of the free particles. Courtesy: JACS

(A) Schematic solvothermal synthesis with GO sheets to afford the MoS2/RGO hybrid. (B) SEM and (inset) TEM images of the MoS2/RGO hybrid. (C) Schematic solvothermal synthesis without any GO sheets, resulting in large, free MoS2 particles. (D) SEM and (inset) TEM images of the free particles. Courtesy: JACS

Catalytic edge sites

Indeed, the researches have measured a HER "Tafel slope" (which indicates the rate of a electrochemical reaction) of 41 mV/decade – a value that far exceeds the activity of previous MoS2 catalysts. This value results from the large number of catalytic edge sites on the tiny MoS2 nanoparticles and the fact that the material couples well to the underlying graphene network.

And that's not all: the hybrid catalyst also has a small overpotential, a large current density and it remains active even after 1000 cycles. "Traditional catalysts such as platinum and palladium, although very efficient, are pricey," Dai told "Given the performance and low cost of the MoS2/RGO hybrid catalyst reported in our paper, we could foresee a possible replacement of these precious metals in future large-scale industrial and domestic applications."

The Stanford researchers made their hybrid catalysts in a solvothermal reaction of ammonium tetrathiomolybdate – (NH4)MoS4 – and hydrazine in a dimethylformaide solution of graphene oxide at 200 °C overnight. During this process, graphene oxide was reduced to RGO, and (NH4)MoS4 was reduced to MoS2on RGO by hydrazine.

"We are now working on improving our catalyst and possibly integrating it into photoelectrochemical reactions," revealed Dai.

The work was published in the Journal of the American Chemical Society.

Source: NanoTechWeb

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