Biomass Oxidation in Direct Biomass Redox Flow Fuel Cell: The Roles of Catalysts, Redox Couples, Mediators and Oxidants

This review focused on the progress catalysts, redox couples, mediators and oxidants have made in the power generation of direct biomass redox flow fuel cell (DBRFFC), identifying the challenges associated with them, and the approach toward resolving the issues to improve the power output of the direct biomass redox flow fuel cell. Ferric chloride and polyoxometalates currently used as catalysts with some redox couples, mediators, and oxidants provide less than 10% of the total harvestable electrons from the biomass degradation for power generation in DBRFFC. They are also inorganic toxic non-renewable catalysts that do not selectively break down the biomasses completely into simple reducing sugars for further oxidation into carbon dioxide and water only during electricity generation in the cell. By-products are produced which trap more electrons that would have been released for improved power generation if complete biomass oxidation were achieved. This challenge can be addressed by developing and combining organic catalyst with less quantity of the inorganic ones for more efficient biomass hydrolysis and oxidation. This review discussed biomass degradation in DBRFFC by thermo-chemical and photo-chemical means and the performances that have been achieved with the catalysts, redox couples, mediators and oxidants that were applied. Drawing from the knowledge will shape the future research direction toward reducing the usage of the non-renewable catalysts by designing and developing efficient renewable catalysts that can act in synergy with the inorganic ones for the biomass hydrolysis and oxidation for improved power generation of the DBRFFC.