Efficient storage technologies are required to exploit renewable energy sources such as wind and the sun. One strategy is the conversion of these energies into fuels such as hydrogen, which can be achieved by electrolysis of water—or water splitting—into H2 and O2. A range of approaches have been investigated to achieve this goal. Devices based on proton-exchange membranes have proven promising, but may ultimately not be viable because they rely on electrocatalysts made from scarce and expensive noble metals, such as Pt. Robust catalysts made from abundant elements such as Co, Ni, and Mn have also been developed for the evolution of oxygen gas from water (also called the oxygen evolution reaction, OER: 2H2O → O2 + 4H+ + 4e−), but few catalysts have been developed for the hydrogen evolution reaction (HER: 4H+ + 4e− → 2H2).
Addressing this problem, V. Artero and co-researchers from the French commission for Atomic Energy and Alternative Energies (CEA) centers in Grenoble and Saclay, and from the Free University Berlin, Germany, have recently developed a straightforward and practical approach to prepare a stable Co-based catalytic material for H2evolution.
As reported in the September issue of Nature Materials (DOI: 10.1038/NMAT3385; p. 802), Artero and co-researchers reduced Co(NO3)2·6H2O from an aqueous phosphate buffer at a fluorine-doped tin oxide electrode. Electrolysis for 3 h at –1.0 V versus Ag/AgCl resulted in a gray coating on the electrode, comprising nanoparticles of about 100 nm in diameter. This electrode, with a film thickness of catalytic material, termed H2-CoCat, of about 2 μm, was then transferred to a Co-free electrolyte where its electrocatalytic properties for H2 evolution were measured. The minimal overpotential required for H2 evolution was 50 mV, which is significantly lower than the 500–700 mV overpotentials required by other recently reported molecular Co catalysts. A H2-evolution turnover frequency of 80 h–1 per Co center at 385 mV overpotential was also demonstrated.
Characterization of the H2-CoCat material using methods including x-ray diffraction, x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy and x-ray absorption spectroscopy, suggested that H2-CoCat is amorphous, and that it is composed of nanoparticles with a cobalt oxo/hydroxo phosphate component that is principally located at the particle surface, with metallic cobalt in the bulk.
Remarkably, anodic equilibration of the H2-CoCat resulted in its conversion to the OER catalyst O2–CoCat, which catalyzes O2evolution. The switch between these two forms was fully reversible, and corresponded to a progressive and local transformation between two morphologies on the surface of the electrode. The coating therefore demonstrates a Janus-like activity, behaving as a switchable catalyst.
The researchers said that their approach could be applied to the fabrication of “an artificial-leaf device” for light-driven water splitting “even in the absence of a proton-conducting membrane separating (photo)anode and (photo)cathode,” as well as to “photo-electrodeposition [of catalysts] on heterogeneous semiconductor nanoparticles.”
