Synthesis and applications of manganese oxide - biochar composites: A systematic review across catalysis, capacitor and sorption applications

Manganese oxide biochar composites (MnOx-BCs) are at the frontier of materials development for current environmental and engineering challenges in contaminant sorption and degradation, capacitive deionisation, as well as supercapacitor research. However, the parameter space for optimisation of such composites is vast, spanning from the choice of feedstocks and synthesis procedure to post-processing. This study uses a systematic literature review methodology to provide a comprehensive view into the synthesis methods and applications of MnOx-BCs. The focus is on the manganese phase oxidation states, which are often decisive for a material's properties but not directly comparable with the (well-explored) synthesis of pure MnOx due to the chemical variability of biochars. The relationships between synthesis method, manganese phase, crystallinity and morphology, as well as biochar type are characterised. We argue that careful selection of the desired manganese phase and oxidation state are as important as careful consideration of the bulk chemistry and microstructure of the biochar phase. Much evidence already exists for fine-tuning these combinations, but there is still a clear research gap in systematically studying the biochar impact on the final MnOx phase.