The effect of pyrolysis temperature on the N conversion of biochar derived from the residue of Chlorella vulgaris after lipid extraction

Abstract

The application of biomass-based carbon materials in catalyzing oxygen reduction reactions has gained a renewed momentum due to their abundant heteroatoms and inherent structures. In this study, the effect of pyrolysis temperature on the nitrogen conversion of nitrogen-enriched biomass waste-microalgae residue carbon catalysts, and their application in microbial fuel cells, were studied. Temperature exhibits a significant impact on the effective nitrogen functional groups, playing a vital role in the oxygen reduction reaction process. Results revealed that an interconversion may occur among the nitrogen functionalities, where Pyrrolic-N, which is the dominant N formed at 700 ℃, was converted to Pyridinic-N as the temperature increased to 800 ℃. Graphitic-N has the best thermal stability; thus, it occupies the highest ratio at 900 ℃. In addition, the increment in temperature resulted in the loss of nitrogen. The biochar prepared at 800 ℃ exhibited the most outstanding oxygen reduction reaction activity (0.8444 V vs. RHE), due to its relatively high nitrogen content (5.75 at%), more active nitrogen forms (Pyridinic-N and Graphitic-N) and appropriate pore structure. The maximum power density of a microbial fuel cell assembled with MRC-800 as cathode catalyst was 442.5 mW m−2. This research not only presents a fundamental study on the nitrogen conversion of biochar, but also provides a promising metal-free electrode catalyst for microbial fuel cells.