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Electrocatalyst cuts cost of rechargeable zinc-air batteries

08 Apr 2015  | Paul Buckley

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A team of researchers from Case Western Reserve University and the University of North Texas has claimed to have developed the first metal-free bifunctional electrocatalyst that performs just as good or even better than most metal and metal oxide electrodes in zinc-air batteries. The carbon-based catalyst works efficiently in both the oxygen reduction reaction and oxygen evolution reaction, making the battery rechargeable. It is also less costly, easy to make and more ecological than most of the alternative materials, added the scientists.

Zinc-air batteries are regarded as being safer, lighter, cheaper, more powerful and durable than lithium-ion batteries. The research is reported in the online edition of Nature Nanotechnology.

"With batteries, cost is always an issue and metal-free catalysts can reduce cost while improving performance," said Liming Dai, professor of macromolecular science and engineering at Case Western Reserve University and senior author of the study. "These batteries could be used in computers, data stations, for lighting, anyplace batteries are used now."

Dai worked with Case Western Reserve post-doctor Jintao Zhang, who performed experimental work; and North Texas University's Zhenhai Xia, professor of materials science and engineering, and Zhenghang Zhao, a PhD student, who performed theoretical simulations.

Zinc-air batteries mix oxygen from the air with zinc in a liquid alkaline electrolyte to create a charge. The batteries can have three times the energy density of lithium-ion batteries, but have been sluggish. To counter that problem, researchers are seeking different catalyst materials.

The catalyst developed by the researchers is a stable carbon aerogel, or foam, with pores ranging from 2nm to 50nm in diameter, providing enormous surface area and room for the battery electrolyte to diffuse.

The researchers followed a foam-making procedure published by Stanford University scientists in 2012. The research team polymerised molecules of the organic compound aniline into long chains in a phytic acid solution, then freeze-dried the three-dimensional hydrogel into an aerogel.

"What we did that's new is carbonised the 3D structure, changing it into a graphitic carbon material," explained Zhang.

To do that, the researchers heated the aerogel to 1,000°C in the absence of oxygen. The process, called pyrolysis, caused a thermochemical reaction, turning the foam into a graphitic network, with many graphene edges that proved to be crucial to catalysis.

"This is a low-cost, one-step, scalable process," said Dai. "The electrocatalyst produces comparable or better results than more costly materials."

The aniline infuses, or dopes, the foam with nitrogen, which enhances the oxygen reduction reaction. Phytic acid infuses the foam with phosphorus. "The co-doping of nitrogen and phosphorus enhances both the oxygen reduction and oxygen evolution reactions, as confirmed by the first-principles calculations," Xia said.

In comparisons, the carbon foam's performance in a primary, or non-rechargeable, battery and a rechargeable battery matched or surpassed that of expensive platinum/metal oxide-based catalysts. And, it had better long-term stability.

The carbon foam also matched or outperformed most previously reported metal-free catalysts, even recently developed carbon-based catalysts with metals.

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