Heteroatom-Doped Carbon Nanomaterials Derived from Black Liquor for Electrochemical Oxygen Reduction Reaction

Black liquor is hazardous and one of the main byproducts in the pulp and paper industry. Its primary constituent is lignin, a carbon-based molecule serving as a precursor for the synthesis of nanostructured carbon materials. Herein, we have used black liquor as a precursor to synthesize high surface area carbons for use as electrochemical oxygen reduction reaction (ORR) catalysts. The materials were activated by a NaOH treatment and subsequently nitrogen-doped by mixing with dicyandiamide, followed by pyrolysis. Synthesis resulted in catalyst materials that showed high specific surface area (1807 and 1228 m2 g–1, respectively), high surface nitrogen content (6.7 and 5.1 at. %, respectively), and the inclusion of chromium and sulfur impurities that originated from the black liquor. The black liquor-based catalyst exhibited high ORR activity in alkaline media with a half-wave potential (E 1/2) of 780 mV and an onset potential (E onset) of 900 mV versus RHE. The resultant Zn–air battery delivered a high peak power density of 112 mW cm–2 at 171 mA cm–2 and a specific capacity of 633 mAh g–1.