Sustainable activated carbon–metal oxide cathodes for Al–air batteries

The ever-growing worldwide demand for an energy storage system that is high-density and sustainable has pushed the research work in the next generation of batteries. The Aluminium-air (Al-air) batteries stand out among them because they are safe, and the use of the Al-air battery has the potential of producing a high energy density, the aluminum is very abundant, and the battery is also very cheap. They, however, possess several barriers to their commercialization, all of which put the sluggish Oxygen Reduction Reaction (ORR) kinetics of the air cathode near the top of the list. This review is dedicated to the synthesis of the advanced air cathode catalysts, which are based on metal oxide and M-doped activated carbon, and focuses specifically on green routes of synthesis. We focus on the biomass, which is the most popular and renewable raw material as a precursor of activated carbon, with the particular consideration given to the environmentally friendly process of its activation, which gives a hierarchical porous structure to the obtained product, an essential condition for the best foundation of mass transfer. This is followed by the discussion on synergistic combinations of cheap and earth-abundant transition metal oxides (e.g., MnO2, Co3O4) with these carbon scaffolds. We revise a multiplicity of green doping strategies, e.g., in-situ and hydrothermal process, and they fine-tuned the electrocatalytic activity to the ORR by producing a greater abundance of active sites and electronic structural adjustment in the composite material. The fundamental mechanisms responsible for the performance enhancement, including the synergistic interaction between the carbon support and the metal oxide dopants, have been thoroughly analyzed. Finally, we have been giving an analytical overview of how these green-synthesized catalysts have been behaving in Al-air battery systems and present our final opinion on the already existing bottlenecks and research avenues that need to be carried out to achieve the transition between the research lab and application.