Ditching noble metals improves efficiency of zinc-air batteries

Zinc sample. (Reference image by Nefronus, Wikimedia Commons.)

Researchers at Hunan University, University College London and the University of Oxford have developed a new metal-nitrogen-carbon catalyst for zinc-air batteries (ZABs) that outperforms noble metal catalysts, improving the efficiency and practicality of ZAB technology.

Zinc-air batteries function by oxidizing zinc with oxygen from the air. Recent research demonstrated that a catalyst incorporating a combination of different non-noble metal atoms could increase the rate of discharging reactions and battery performance.

With this evidence in mind, the researchers generated a non-noble metal-nitrogen-carbon catalyst from iron, cobalt and nickel to improve the charging, discharging and cost efficiency of ZABs. Importantly, the team also optimized a flexible carbon dot/polyvinyl alcohol (CD/PVA) film as a solid-state ZAB electrolyte, or battery component that transfers charged atoms, creating a flexible and stable high-performance battery that could potentially be used in wearable devices.

“Rechargeable metal-air batteries are promising power sources, especially zinc-air batteries which offer high theoretical energy densities (1084 Wh kg−1), environmental friendliness, and cost-effectiveness,” head researcher Huanxin Li said in a media statement.

“Additionally, rechargeable ZABs are not only safe and stable but also portable and wearable. Significant research is currently focused on rechargeable and flexible ZABs.”

How ZABs work

Zinc-air batteries discharge and charge through two reactions: the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), respectively. These reactions are notoriously slow and require catalysts that speed the electrochemical reaction along, or electrocatalysts. While noble metals are capable of speeding the ORR and OER, issues with cost, suboptimal performance and the requirement of two different noble metals limited the overall practicality of ZAB technology.

“Developing low-cost and efficient bifunctional non-noble electrocatalysts is crucial to the commercialization of rechargeable ZABs. Among various non-noble catalysts, metal-nitrogen-carbon (M-N-C) nanomaterials have attracted particular attention due to their low price, abundant reserves, excellent electrochemical activity and high stability,” Li said.

Creating an electrocatalyst composed of three different metal atoms isn’t a trivial matter, however, due to the different interaction forces that occur with each metal atom. To address this issue, the team used zeolitic imidazolate frameworks (ZIFs), carbon-nitrogen frameworks that surround and arrange each of three metal atoms to uniformly anchor the catalytic atoms onto porous carbon at high heat.

The scientists then confirmed the distribution of the Fe, Co and Ni atoms via energy-dispersive X-ray spectroscopy, spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy.

Base metals 1, noble metals 0

Overall, the ternary Fe-Co-Ni electrocatalyst outperformed bimetal electrocatalysts (FeNi, FeCo and CoNi) and platinum and ruthenium, two noble metal electrocatalysts, in the oxygen reduction and evolution reactions.

The team believes that all three metal atoms of the ternary electrocatalyst are active and cooperating to increase catalytic activity, with Fe contributing the most to activity as the most abundant atom. The porous structure and increased surface area of the electrocatalyst likely also contribute to the enhanced catalytic activity.

Overall, the team’s rechargeable ZAB achieved a specific capacity of 846.8 mAh·gZn−1 and a power density of 135 mW·cm–2 in liquid electrolyte. The ZAB also achieved a power density of 60 mW·cm–2 using the optimized CD/PVA solid-state electrolyte, which exceeds the reported results of solid-state ZABs with other catalysts.

Importantly, the zinc-air battery developed in the study was durable, stable and capable of powering a fan and an LED screen and charging a mobile phone. The researchers are hopeful that their ternary Fe-Co-Ni electrocatalyst and CD/PVA electrolyte will spur investigations into new catalysts and electrolytes for practical, high-performance ZAB technologies.