The joint effect of electrical conductivity and surface oxygen functionalities of carbon supports on the oxygen reduction reaction studied over bare supports and Mn–Co spinel/carbon catalysts in alkaline media

Abstract

Supported manganese–cobalt spinel catalysts and the corresponding mesoporous carbon supports were examined in order to reveal the effect of electrical conductivity and surface oxygen functionalities on the oxygen reduction reaction (ORR) in alkaline media. Carbon replicas of comparable porosity were obtained from sucrose by nano-replication of a silica template followed by carbonization at different temperatures (650–1050 °C). The degree of their graphitization was assessed by deconvolution of the Raman spectra (1000–1800 cm⁻¹), whereas relative electrical conductivity measurements were performed by means of the contactless microwave cavity quality factor perturbation technique. The electrocatalytic activities of the obtained materials towards the ORR (gauged by the ORR onset potential and the number of transferred electrons) were tested in alkaline media using standard hydrodynamic methods. It was found that with increasing carbonization temperature, the electrical conductivity of the examined carbon materials increased, in parallel with their growing crystallinity. For the carbon samples calcined in the temperature range 650–850 °C, the ORR onset potential initially rises as the electrical conductivity increases, passes through a maximum at around 850 °C, and then falls. Such a volcano-type shape was found also in the case of manganese–cobalt spinel catalysts deposited on the examined carbon supports. The observed non-monotonous electrocatalytic behavior was accounted for by two descriptors: the carbon support electrical conductivity and the amount of surface carbonyl and quinone groups, changing in the opposite way with the carbonization temperature. It was shown that variation of the electrical conductivity multiplied by the amount of carboxyl and quinone groups with the carbon calcination temperature exhibits the same behavior as the number of electrons exchanged in the ORR, confirming that an optimal conjunction of these properties plays a decisive role. The prime advantage of the dispersed spinel active phase lies in a significant enhancement of the 4-electron reduction pathway. To the best of our knowledge, the role of electrical conductivity and the nature of the surface oxygen-bearing functional groups in the ORR were elucidated for the first time.