Ionothermal carbonization of biomass to construct sp²/sp³ carbon interface in N-doped biochar as efficient oxygen reduction electrocatalysts

The oxygen reduction reaction (ORR) is critical to various energy conversion and storage systems. Developing cost-effective and high performance metal-free carbon as an alternative to Pt-based ORR electrocatalytsts is a promising strategy for the commercialization of these clean energy conversion technologies. In this work, the microalgae biomass with a high N content was used as a precursor to prepare N-doped biochar material via an ionic liquid-assisted carbonization process. Experimental results show that the sp²/sp³ carbon interface and pore structure of the as-synthesized biochar material could be tuned by the polymerization of the ionic liquids and the catalysis of Fe species in the carbonization process. The as-synthesized N-doped biochar material exhibited robust electrocatalytic ORR performance with a high catalytic activity, excellent long-term stability and great tolerance to methanol and CO. The pyridinic N species was identified as the active catalytic site for the ORR. A Zn-air battery was assembled using the N-doped biochar as the air cathode catalyst and showed favourable performance. This work provides a straightforward strategy to prepare efficient and low-cost functional carbon materials for clean energy conversion from naturally abundant biomass.