The ORR and OER are at the heart of metal-air batteries, explains team leader Liming Dai of Case Western. Here, electrons from the current collector in the device reduce oxygen molecules that then combine with metal dissolved in the electrolyte or from the metal electrode during battery discharge. The reverse process occurs during charging. While lithium and zinc are the main metals commonly targeted for this type of battery, zinc-air batteries are less expensive and are safer. The problem is that although these batteries can have three times the energy density of lithium-ion batteries, they can be slow when it comes to reducing oxygen.

To increase both ORR and OER efficiencies, researchers are busy developing better catalyst materials that rely on air as the source of oxygen. Until now, precious metals, like platinum, ruthenium and iridium, and their oxides, have mainly been employed in such catalysts, but they are expensive and this is one of the main problems holding back the commercialisation of zinc-air batteries.

Nitrogen and phosphorus-doped porous nanocarbon foams

Carbon nanomaterials, such as carbon nanotubes and graphene, doped with nitrogen or phosphorus could come into their own here, says Dai. Indeed, his team is now reporting on a new way to make 3D nitrogen and phosphorus-doped porous nanocarbon foams that appear to be efficient metal-free bifunctional electrocatalysts for both the ORR and OER.

Dai’s team began by polymerizing aniline monomers in the presence of phytic acid to produce a polyaniline hydrogel via a “hard” template-free gelation process (first reported on by Stanford University scientists in 2012). “We then freeze-dried the 3D hydrogel into an aerogel, and then carbonized it (at a temperature of 1000 °C in the absence of oxygen) into a 3D mesoporous graphitic carbon foam co-doped with nitrogen (from the polyaniline) and phosphorus (from the phytic acid),” explains Dai. “The co-doping with nitrogen and phosphorus significantly enhances electrocatalytic activities for both the ORR and OER while the 3D porous graphitic structure (made up of pores ranging from 2 to 50 nm in diameter) provides enormous surface area and room for the electrolyte in the battery to diffuse, as well as electron transport through the electrode.”

Replacing expensive platinum or other metal-based ORR and OER catalysts

As a result, our nitrogen and phosphorus-doped carbon foam as metal-free electrodes for rechargeable zinc-air batteries perform as well or even better than the more expensive, state-of-the art metal-based catalysts, he tells nanotechweb.org. The foam could replace expensive platinum or other metal-based ORR and OER catalysts in a wide range of applications, including fuels cells, solar cells, metal-air batteries of course, and even water-splitting systems (in which water is decomposed into oxygen and hydrogen using sunlight – a clean and renewable way to produce energy).

Younan Xia of Georgia Tech , who was not involved in this work, believes that the new research is "interesting". "It shows how carbon-based electrocatalysts can be rationally designed and fabricated to achieve great performance and help reduce the cost of energy conversion devices without using the expensive, low-abundance noble metals," he comments.

Dai and colleagues says that they are now busy further optimising their process for producing various 3D graphitic carbon materials co-doped with different heteroatoms for use in energy and environmental applications.

The researchers detail their present work in Nature Nanotechnology doi:10.1038/nnano.2015.48.

For more on the application of nanoechnology in batteries, visit Nanotechnology's special issue at http://iopscience.iop.org/0957-4484/24/42.