(A) Top to bottom (Left: control and Right: Co-V): Starting solutions, after the 1st drying step and finally the calcined foam obtained after pyrolysis. (B) Pictures of the final electrocatalytic material (top: control and bottom: Co-V). (C) SEM micrographs of the control and (D) Co-V catalysts. Elemental mapping of the Co-V materials with (E, yellow) Co and (F, blue) V elements. The elemental map signals were obtained by scanning a square of the Co-V image.
Photo of the Co-V electrocatalyst.An inexpensive new catalyst for water splitting could support the development of a future hydrogen economy by enabling hydrogen to be produced readily and inexpensively, according to researchers from McGill University in Canada and Queen Mary University of London in the UK [Merle et al., Materials Today Energy 9 (2018) 247].
Since the discovery of cobalt-phosphate electrocatalysts a decade ago, there has been resurgence of interest in water splitting – or electrolysis – to produce hydrogen and oxygen. But the oxygen evolution reaction (OER), which creates a double O-O bond from two water molecules and releases four protons and four electrons, requires extreme pH conditions. Current large-scale electrolysers rely on a strong base, potassium hydroxide (KOH), at high temperatures (80-90°C) with nickel (Ni) electrodes. Smaller, more efficient alternatives operating at lower temperatures use expensive cationic membrane technology and rare earth metal-oxide electrodes such as iridium oxide (IrO2). Currently, suitable rare earth metals such as Ir and ruthenium (Ru) cost in the region of $44 000 and $8000 per kg, respectively.
“Being able to make cheap hydrogen efficiently from water splitting requires improved efficiency in the oxygen-evolving side of the reaction,” explains Jake Barralet. “It is this reaction that limits the ease with which we can access hydrogen as a fuel source. The less power required, the more likely it is that clean power sources such as solar can be used.”
Coauthor and inventor of the catalyst, Geraldine Merle, believes she and her colleagues have come up with a low cost alternative to scarce and expensive metals like Ir or Ru in the form of a new material based on the oxides of cobalt (Co) and vanadium (V).
“We have discovered a new amorphous material made from two inexpensive materials, Co and V, which shows the lowest overpotential and appears extremely stable over time in real conditions,” says Barralet.
The material is produced using a combustion reaction between metal nitrates and citric acid, which acts as both a combustion fuel and complexing agent. The resulting amorphous compound is made up of plate-like particles with an irregular, glassy appearance.
“What is particularly fascinating is the low surface area of the material,” says Merle. “We imagined the surface area would be huge but instead the reverse is true, [with the material] essentially like a glass.”
The material has very low specific surface area and zero porosity, but also a very low overpotential in the 14-7 pH range and performs remarkably as an OER electrode under alkaline conditions, outperforming IrO2 electrocatalysts in both catalytic activity and long-term stability.
The new Co-V material – and other similar materials – could enable new cheap, sustainable, and efficient OER electrocatalysts for hydrogen production by water splitting. The advance could form the basis of a new generation of small, cheap, and efficient water-fueled hydrogen and oxygen generators.