Title: Biomorphic Co-N-C/CoOx Composite Derived from Natural Chloroplasts as Efficient Electrocatalyst for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR), as a key process on fuel cell cathodes, exerts great influence on the energy conversion efficiency of fuel cells. The complicated reaction path and relatively slow reaction velocity for ORR make it highly demanding to search for efficient electrocatalysts.^[1] Although platinum-based materials show high catalytic activity, the scarcity, high price and easy poisoning features make them unsuitable for widespread use in industry. Hence, developing low-cost and efficient electrocatalyts for ORR has been a hot topic in the past decades.^[2]

Natural chloroplasts containing big amounts of chlorophylls (magnesium porphyrin, Mg-Chl) are employed both as template and porphyrin source to synthesize biomorphic Co-N-C/CoO_x composite as electrocatalyst for oxygen reduction reaction (ORR). Cobalt-substituted chlorophyll derivative (Co-Chl) in chloroplasts is firstly obtained by successively rinsing in hydrochloric acid and cobalt acetate solutions. After calcining in nitrogen to 800 °C, Co-Chl is transferred to Co-N-C; while other parts of chloroplasts adsorbed with Co ions are transferred to CoO_x retaining the micro-architecture of chloroplast. The abundant active Co-N-C sites are protected by circumjacent bio-carbon and CoO_x to avoid leakage and agglomeration, and at the same time can overcome the poor conductivity weakness of CoO_x by directly transporting electrons to the carbonaceous skeleton. This unique synergistic effect, together with efficient bio-architecture, leads to good electrocatalytical performance for ORR. The onset and half-wave potentials are 0.89 and 0.82 V vs. RHE, respectively, with better durability and methanol tolerance than that of commercial Pt/C. Different from the traditional concept of biomorphic materials which simply utilize bio-architectures, this work provides a new example of coupling bio-derivative components with bio-architectures into one integrated system to achieve good comprehensive performance for electrocatalysts.

The results have been published in Small (Small 2019, 15, 1804855)and selected as inside front cover paper.