FeN@N-doped graphitic biochars derived from hydrothermal-microwave pyrolysis of cellulose biomass for fuel cell catalysts

Iron-nitrogen moieties encapsulated in the N-doped graphitic and hierarchical biochars are derived from the hydrothermal pyrolysis and microwave NH₃ activation of cellulose and lignin. The physicochemical properties of these biochars are identified by using various analytical instruments and their application as electrocatalysts in oxygen reduction reaction (ORR) is also evaluated in alkaline solution. As compared to the lignin-derived biochars (named as L-y), the cellulose-derived biochars (named as C-x) have superior activities toward ORR due to the formation of highly graphitic carbon shells (as proofed by using X-ray diffraction and high-resolution transmission electron microscope) in the biochars. Moreover, the C-1% (i.e., 1% of iron in the cellulose-derived biochars) exhibits the best electrocatalytic performance (onset potential = −0.05 V vs. Ag/AgCl) and follows a near four-electron ORR route, which could be ascribed to the more active iron-nitrogen sites (as evidenced by X-ray photoelectron and X-ray absorption spectroscopy) covered with hierarchical carbon shells (as identified by N₂ adsorption-desorption isotherms). The cellulose-derived biochars with distinctive iron-nitrogen retained in the shell of N-doped graphitic and hierarchical carbons by employing a time-saving and energy-efficient microwave activation procedure may render a potential application in the ORR.