A Carbon-Free, Precious-Metal-Free, High-Performance O2 Electrode for Regenerative Fuel Cells and Metal-Air Batteries

There have been extensive research efforts focusing on developing technologies for renewable electricity, particularly solar photovoltaics and wind turbines. However, the intermittent and localized nature of wind and solar energy necessitates the development of a cost-effective way to balance energy supply and demand and to deliver electricity from remote places to cities.[1] The short-term option is the electricity grid; however, it can only support intermittent renewable electricity in a stable fashion up to approximately 20 % of grid capacity which means that fossil resources would still account for the remaining 80 % of grid electricity.[2] Clearly, the development of cost-effective energy storage devices is needed to establish a path toward fossil-free energy.

The development of high-performance and cost-effective electrodes for oxygen evolution and oxygen reduction is critical for enabling the use of energy storage devices based on O₂-H₂O chemistries such as metal-air batteries and unitized regenerative fuel cells (URFCs).[3] Herein, we report a precious-metal-free and carbon-free O₂ electrode synthesized via electrodeposition of manganese oxide (MnO_x) on a stainless steel (SS) substrate followed by high-temperature calcination at 480 ^oC. The MnO_x-SS electrode displays high oxygen reduction and water oxidation activities when tested in an electrochemical cell, comparable to that of a precious-metal based electrode, Pt/C-SS. Accelerated durability testing reveals the excellent stability of the MnO_x-SS electrode compared to both the Pt/C-SS electrode and a carbon-based electrode with MnO_x and Ni catalysts. This can be rationalized by the carbon-free nature of the MnO_x-SS electrode which circumvents carbon corrosion at the high electrochemical potentials during water oxidation and O₂ reduction. Integrating the MnO_x-SS electrode as the O₂ electrode in an anion exchange membrane (AEM) URFC produces round trip efficiencies of 42-45 % at 20 mA.cm^-2 over 10 cycles, and exhibits significantly enhanced durability compared to the carbon-based analogue. This work demonstrates the MnO_x-SS electrode’s potential for use as a high performance, scalable, precious-metal-free and carbon-free O₂ electrode in AEM-URFCs and metal-air batteries.

References

[1] Z. Zhan, W. Kobsiriphat, J.R. Wilson, M. Pillai, I. Kim, S.A. Barnett, Energ. Fuel. 2009, 23, 3089.

[2] W. Kempton, J. Tomić, J. Power Sources 2005, 144, 280.

[3]  J.W.D. Ng, Y. Gorlin, T. Hatsukade, T.F. Jaramillo, Adv. Energy Mater. 2013, 3, 1545.