From coconut shell biomass to oxygen reduction reaction catalyst: Tuning porosity and nitrogen doping

Heteroatom-doped carbon materials, despite showing excellent performance as oxygen reduction reaction (ORR) catalysts, are not commercially viable in fuel cells due to their high cost and use of hazardous materials during synthesis. The earth-abundant renewable and sustainable biomass can be a low-cost carbon precursor for the synthesis of activated porous carbons with high surface area, ordered porosity and tunable pore sizes and could provide a good alternative to platinum-based catalysts. Coconut shell is one of the best-known biomass for the production of high quality porous activated carbon, owing to its unique cellular structure. Further, the surface area and porosity of biomass-derived porous carbons can be fine-tuned and optimized by controlling synthetic parameters during the pyrolysis, carbonization, and activation. Nitrogen doping of porous carbons can further enhance their ORR activity. The review focuses specifically on the activation and tuning of coconut shell-derived porous carbons as ORR catalysts, a topic not yet thoroughly evaluated in other reviews of biomass-derived activated carbons. In this review, (i) various activation methods, including self-activation to avoid the use of harsh chemicals for developing desired pore structures, (ii) control of porosity and nitrogen doping to improve ORR catalytic performance, (iii) use of microwave heating to reduce synthesis time, energy and cost, and (iv) scalable production of porous carbons for different applications are discussed.