Critical review on the development of biomass waste as precursor for carbon material as electrocatalysts for metal-air batteries

The latest and state-of-art research on biomass waste-derived carbon materials synthesized for air electrodes used in metal-air batteries were succinctly and comprehensively reviewed in this report. This report also aims to provide greater understanding as well as to identify the strength and challenges of metal-air batteries (MAB), biomass wastes as precursor and the synthesis steps of carbon materials from biomass wastes as well as to propose several research routes for the shortcomings of carbon material synthesis from biomass wastes. MAB is a promising energy storage technology, but its commercialization was hindered due to the absence of a stable and active bifunctional electrocatalyst as well as costly air cathode material. The air cathode which is the key component of MAB is desired to have bifunctional catalytic properties and hierarchical porous structure. Carbon materials were known to be able to perform as an electrode material while acting as a catalyst towards the charge-discharge reactions. Due to its abundance and renewability, biomass wastes have great potential as low-cost and sustainable precursors to synthesize carbon materials as the air cathode while achieving waste reduction. The synthesis of carbon electrocatalyst from biomass waste usually include but not limited to pretreatments, carbonization, activation and doping. The synthesis method controls the structure and size, as well as the elemental content of carbon particles. Activation and doping of heteroatoms or metals were proven to enhance the efficiency and stability of the synthesized carbon material. However, each biomass waste have unique properties which requires tailored synthesis method to obtain carbon material with desired properties. Information on the synthesis methods as well as other crucial information were reviewed in this work which can help to spur more research to promote the utilization of biomass waste materials in energy storage application and to accelerate the commercialization of MAB.