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

Abstract

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‐Nx 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.