Dual-Function Air Cathode for Metal-Air Batteries: Integrating Oxygen Reduction Catalysis with Pulse-Power Capability

The air cathode in a metal–air battery is the lung of the energy-storage device—with power output limited by the transport of gas-phase molecular oxygen into the structure of the cathode, transport of dissolved molecular oxygen across the electrolyte-wetted electrode surface, and the rate at which that surface catalyzes electroreduction of O₂. Too much electrolyte within the electrode structure confers pneumonia and an inability to sustain even moderate loads. Zinc–air at 400 W h kg^–1 is an established battery technology with an energy density three times that of Li-ion, but its broader applicability is hampered by low specific power and limited rechargeability. We have solved the inability to recharge zinc without dendrite formation [1,2], but low specific power lies with performance limitations at the air cathode. We have solved this problem by departing from customary air-cathode fabrication approaches. We adopt a “multifunctional nanoarchitecture” design in which the solid and void components in a macroscale electrode structure are controlled and modified on the nanoscale to provide balanced pathways for electronic and ionic conductivity, facile permeation/transport of electrolyte and gas-phase O₂ through a 3D interconnected void volume, and optimal electrocatalytic turnover for O₂ reduction. We achieve this desirable blend of functionality by fabricating carbon-fiber paper filled with a pyrolytic carbon nanofoam. The nanofoam papers are produced in x–y scalable forms with variable electrode thickness (50–300 μm), tunable pore sizes (10–1000 nm), high specific surface areas (>400 m² g^–1), and a highly conductive carbon framework (>15 S cm^–1) [3]. This electrode design not only improves performance for typical air-cathode operation via efficient O₂ reduction catalysis at 10 nm–thick birnessite manganese dioxide (MnOx) “painted” onto the walls of the carbon nanofoam but also imparts new function: air-independent pulse power as derived from the pseudocapacitance of the MnOx coating. Manganese (III) sites in the post-pulsed oxide spontaneously recharge to Mn(IV) in the presence of oxygen—making the oxide ready and available for subsequent pulse-power discharge. The application of our “dual function” air cathode to alkaline Zn–air cells imparts pulse-power capability (>1 F cm^–2for >10 s) otherwise absent in a battery that already provides the advantages of high energy density, low cost, and safety of operation.

This work was funded by the Office of Naval Research.

[1]   D.R. Rolison, J.W. Long, J.F. Parker, U.S. Patent Application #20140147757.

[2]   J.F. Parker, C.N. Chervin, E.S. Nelson, J.W. Long, D.R. Rolison, Energy Environ. Sci. 7(2014) 1117–1124.

[3]   J.C. Lytle, J.M. Wallace, M.B. Sassin, A.J. Barrow, J.W. Long, J.L. Dysart, D.R. Rolison, Energy Environ. Sci. 4 (2011) 1913.