Hierarchical Porous Fe₃C@Fe-N-C Catalysts from Tannin-Fe(III) Complexes for Efficient Oxygen Reduction

The rational design of metal-nitrogen-doped carbons (M-N-C) from available and cost-effective sources featuring high electrocatalytic performance and stability is attractive for the development of viable low-temperature fuel cells. Herein, mimosa tannin, an abundant polyphenol easily extracted from the Mimosa plant, is used as a natural carbon source to produce a tannin-Fe(III) coordination complex. This process is assisted by Pluronic F127, which acts as both a surfactant and a promoter of Fe-N_x active sites. After carbonization in the presence of urea as a nitrogen precursor, this organic tannin-Fe(III) framework produces Fe₃C nanoparticles encapsulated on a Fe-N-C single-atom catalyst with hierarchical porosity. The optimal catalyst, with a Pluronic F127/mimosa tannin mass ratio of 0.5, exhibits high ORR performance in both alkaline and acidic electrolytes, with half-wave potentials of 0.87 and 0.74 V versus RHE, respectively. In addition, good performance is achieved in practical hydrogen polymer-electrolyte membrane fuel cells using OH^-- or H⁺-conducting membranes with peak power densities of 242 and 200 mW cm^-2 at cell voltages of 0.43 and 0.3 V, respectively. The synthetic approach can be explored to design new renewable M-N-C electrodes for electrochemical energy conversion or storage devices due to tannin's exceptional ability to coordinate metals.