Carbon-Doped Co 3 O 4 Derived from Coordination Polymers as Catalysts for the Oxygen Electrode in Metal−Air Batteries: ORR Activation through the OER

Carbon-doped mesoporous cobalt oxides (Co 3 O 4 ) were obtained through a two-step pyrolysis strategy. Four cobalt-based precursors: two metal−organic frameworks (MOFs), one linear coordination polymer and, one molecular complex, were first pyrolyzed under a nitrogen atmosphere and subsequently calcined in air to yield the final catalysts. The products were characterized through scanning electron microscopy (SEM) and X-ray diffraction, as well as Raman, energy-dispersive, and X-ray photoelectron (XPS) spectroscopies. Electrochemical studies showed E 1/2 values up to 0.81 V vs RHE for the oxygen reduction reaction (ORR) and E ( j =10 mA cm −2 ) from 1.60 V vs RHE for the oxygen evolution reaction (OER). These catalysts exhibited high stability, even after 1000 voltammetry cycles alternating between ORR and OER conditions. Remarkably, the catalysts not only retained their activity but underwent an activation process as well, characterized by an improvement in key ORR electrochemical parameters after the OER occurred. This finding is interpreted in terms of an increase of both the electrochemical surface area and the Co 2+ /Co 3+ surface concentration ratio, as determined by XPS, upon cycling the catalysts over the OER potential region. This is a consequence of the pore expansion during O 2 bubble formation, as seen by SEM. Finally, these catalysts proved to be useful to construct oxygen electrodes, and the results are relevant for the design of metal−air batteries.