Novel Electrode Designs and Configurations in Secondary Iron-Air Batteries

Abstract

Stationary energy storage solutions are urgently needed due to the challenges we face caused by the anthropogenic climate change. While Lithium-Ion batteries are still considered as state of the art within this field, the availability of lithium is limited.The iron-air battery system is mainly superior due to the low criticality of the active materials. Iron is cheap and abundant, while air is supplied by the ambient atmosphere. During discharging oxygen from ambient air is reduced (oxygen reduction reaction, ORR) and evolved during charging (oxygen evolution reaction, OER). One of the main challenges towards stable metal-air batteries is the air-electrode. Those usually consist of a catalyst, a conductive additive, and – depending on the electrode design- of a binder. Carbon-based materials offer an excellent intrinsic activity towards the oxygen reduction and a high specific surface area. However, carbon corrodes in the potential range of the oxygen evolution reaction. Our work focuses on the development of novel electrode formulations for PTFE-bonded electrodes and novel self-supported electrode structures - based on galvanic deposition of nickel - and their implementation in iron-air batteries.Within a first approach a mixed catalyst composed of MnO2 and NiO was investigated in a PTFE-bonded electrode with nickel as conductive additive. The electrode reached an initial activity of 0.77 V vs. RHE @ -25 mA cm-2 (ORR) and 1.57 V vs. RHE @ 25 mA cm-2 (OER) in 6 M KOH at room temperature. The potentials were stable for about 60 cycles (one cycle consisted of 60 min). MnO2 corrodes at OER potentials, which results in a decrease of activity. However, a partial substitution of MnO2 by NiO is beneficial for the cycling stability. In a second approach, a bifunctional nickel-cobalt-oxide based bifunctional catalyst was investigated within the same electrode system. An initial activity of 0.71 V vs. RHE and 1.63 V vs. RHE was observed for this system. Stable OER performance but a decrease ORR activity was observed within this system over time during cycling. To increase the cycling stability, dendritic nickel structures with a high intrinsic surface area were prepared with galvanic deposition. Our results indicate that the efficiency and stability of the battery can be improved by using this approach. Figure 1