CNTs give catalyst the support it needs

Researchers from Jilin University in China have developed a catalyst that could support renewable energy storage and transformation technologies [Wen et al., Materials Today Energy 10 (2018) 153-160]. While sunlight, wind, and waves offer sustainable sources of energy, supply is intermittent so storage technologies are a vital component of any system. Splitting water into hydrogen, which can be used as a fuel, and oxygen is a promising strategy, but the sluggish oxygen evolution reaction (OER) requires a catalyst.

“Although there are many reports of transition metal oxides supported on N-doped carbon materials showing good catalytic activity for oxygen evolution and oxygen reduction reactions in alkaline electrolytes, they are unstable in acid solutions,” says Jingqi Guan, who led the research.

Guan and his colleagues combined two promising strategies to create a stable, acid-resistant catalyst for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for regenerative fuel cells and rechargeable metal-air batteries.

“The most stable metal oxide in acidic electrolyte for OER should be iridium oxide (IrO₂), while carbon materials (e.g. carbon nanotubes) are one of the best candidate supports because of their excellent electrical conductivity,” explains Guan.

The researchers employed a simple approach to impregnate multiwalled carbon nanotubes (MWCNTs) with ultrafine IrO₂ nanoparticles. Initially, a solution containing the metal ions was mixed with the MWCNTs. The mixture was then reduced in hydrogen at 400°C to transform the metal ions into Ir nanoparticles, which were further oxidized by air to IrO₂ at low temperatures.

The most promising catalyst, containing 2% IrO₂ (IrO₂@CNT), shows superior electrochemical OER activity at 10 mA cm^-2, with low overpotentials (of 217 mV and 272 mV, respectively) in alkali (1.0 M KOH) and acid (0.5 M H₂SO₄) conditions.

“This is the first time that IrO₂ on CNTs has been reported as an efficient catalyst for ORR,” says Guan. “It is also the first time that IrO₂ on CNTs has shown superior electrochemical OER activity.”

The new catalyst is comparable in activity for the ORR as commercial platinum/carbon catalysts and among the best reported to date for the OER. Its stable and efficient performance for these two key reactions makes it a suitable candidate for regenerative fuel cells and rechargeable metal-air batteries, especially with acid electrolytes.

There is a drawback to the new catalyst, however, in that Ir is a precious metal and too scare and expensive for large-scale use. However, the researchers are working on catalysts with IrO₂ nanoparticles diameters less than 1 nm to reduce the Ir content.

“Since most transition metal oxides are unstable in acid solution, this IrO₂@CNT catalyst is promising for renewable energy storage and transformation applications,” says Guan.