Tailoring α-MnO$_2$ gas diffusion electrodes for enhanced oxygen reduction in aluminum-air batteries

a-MnO$_2$ is a promising, inexpensive, and readily producible catalyst for the oxygen reduction reaction (ORR) in alkaline media, but its application is limited by low electronic conductivity. In this study, we enhance the performance of a-MnO$_2$ electrodes by systematically varying the a-MnO$_2$-to-Vulcan ratio within the catalyst layer. Electrodes are evaluated in a gas diffusion electrode (GDE) half-cell, where an optimized catalyst layer composition leads to significantly improved ORR performance. By finetuning both the a-MnO$_2$ -to-Vulcan ratio and the a-MnO$_2$ loading, the electrode outperforms a commercial MnO$_2$-based electrode and approaches the performance of the Pt/C benchmark. The improvement is attributed to the presence of a three-dimensional (3D) Vulcan network electronically connecting catalytically active a-MnO$_2$ sites with the substrate. Additionally, the optimized electrodes are employed in a prototype Al-O$_2$ flow cell. Under constant oxygen flow, power densities exceed 250 mW cm$^{-2}$, which is significantly higher than that of conventional Al-air batteries. Electrochemical impedance spectroscopy combined with distribution of relaxation times (DRT) analysis enables the separation of anode and cathode charge transfer impedances without the need for an additional reference electrode. ... mehrThe analysis reveals that the anode contributes more than twice as much impedance as the cathode, highlighting the need for further anode optimization. This work demonstrates a transferable approach for catalyst layer screening under technically relevant conditions in the GDE half-cell. Subsequent measurements in an Al-O$_ $flow cell validate the approach. The methodology is widely applicable to the development of advanced electrodes for a variety of metal-air battery technologies.