Enhancement of oxygen reduction performance of biomass-derived carbon through co-doping with early transition metal

We report an early transition metal co-doped carbon catalyst prepared by pyrolyzing a 3D chitosan hydrogel precursor mixed with Zr and Fe species. The catalyst exhibits high activity and pronounced long-term stability for the oxygen reduction reaction (ORR) in both acidic and alkaline media. We find that co-doping with Zr can greatly enhance the catalyst’s performance, whereas doping with only Zr results in no improvement. For our optimal sample ZFNC-3-800, its current density at 0.9 V (vs. RHE) could be reach 0.64 mA/cm², which is 1.7 times that of FNC-800. Through TEM, XRD and XPS analyses, we find that co-doping with Zr can change the microstructure of the catalyst, immobilize its iron and nitrogen components and modify the distribution of N species in the final catalyst. Importantly, co-doping with Zr leads to the creation of Zr₂FeO_x species, contributing to the possible structure of N_4-x-Fe-O_x in our ZFNC-3-800 catalyst and enhance its performance. Fe atoms are much more electron-deficient in this structure than in FeN_x species and thus favor the adsorption and activation of oxygen molecules. This work is the first to explore co-doping with early transition metals and to reveal their important role in a specific catalyst.