High reversibility between ORR and OER should be guaranteed in rechargeable metal air batteries, while fuel cells and water splitting are based on either forward or backward reaction of the oxygen-to-water conversion. Platinum has been known as the best ORR catalyst while the oxide layer formed on its surface at oxidative conditions deteriorates the catalytic activity for OER seriously. Iridium or ruthenium oxides have been regarded as the best OER catalysts. However, their electrocatalytic activities of ORR are not as high as those of OER, significantly inferior to other catalysts. Iridium alloys with transition metals, as another form of iridium-containing catalysts (not the oxide form), catalyzed ORR efficiently while any forms of ruthenium did not work as the ORR catalysts. It is challengeable to develop a catalyst with high electroactivities for both ORR and OER.

—Lee et al.

Perovskite oxides—functional materials with formula ABO₃ or A₂BO₄, where A = alkaline and/or rare earth metal, B= transition metal)—have been studied as electrocatalysts for both OER and ORR. Perovskite ORR electroactivities are much inferior to other ORR catalysts such as nitrogen-doped carbons; perovskite OER activities are relatively more comparable to other OER catalysts.

The UNIST researchers found that physically mixing perovskite with polypyrrole/carbon composites (pPy/C), significantly reduced the overpotentials of ORR and OER on perovskite oxide catalysts—without any strong interaction between the two.

The team suggested a sequential role beween pPy and the oxide catalysts for ORR to explain the synergistic effects:

1. Oxygen is doped into pPy as a form of superoxide or partially charged oxygen species.

2. The doped species are transferred from pPy to active sites of perovskite oxide catalysts.

3. The oxygen species on perovskite are completely reduced.

Resources

• Dong-Gyu Lee, et al., (2016) “Polypyrrole-assisted oxygen electrocatalysis on perovskite oxides”, EES doi: 10.1039/C6EE03501A