Tuning the wettability of advanced mesoporous Fe[sbnd]N[sbnd]C catalysts for optimizing the construction of the gas/liquid/solid three-phase interface in air-cathodes

Abstract

To explore high-efficient FeNC oxygen reduction reaction (ORR) catalysts, remarkable efforts have been made to search the active sites with superior ORR activity and increase the site density and exposure. However, the active sites are not utilized in fuel cells unless they are located at the gas/liquid/solid three-phase interface that can simultaneously get in touch with ions and oxygen. Tuning the wettability of the catalyst layer (CL) is a key to enlarging the three-phase interface. Here, we adjusted the wettability of FeNC catalysts and thus obtained a series of ORR electrodes with different CL wettability. Various electrochemical methods were linked and carried out to quantitatively investigate how the wettability influences the electrode behaviors including current response, electrochemical surface area, impedance, and oxygen and ion diffusion coefficient. The results revealed that the electrode behaviors are quite sensitive to the wettability of CL. Moreover, the construction of the three-phase interface that requires a coupling development of gas and liquid channels ultimately determines the electrode performance. In detail, the liquid channels of the water superwettable electrode and the gas channels of the gas superwettable electrode were both overdeveloped, which caused that the electrolyte-wetted surfaces of the former are not fully exposed to gas and lots of gas-exposed surfaces of the latter cannot be wetted by electrolytes. While the electrode with moderate gas and water wettability succeeded in a sufficient coupling development of gas and liquid channels and thus exhibited an optimal three-phase interface and a boosted cathode performance.