M-N-C Catalysts Development for PEM Fuel Cells and CO2 Electrolysis

Abstract

Several green electrochemical technologies rely on noble metal-based catalysts. This includes high temperature (HT) and low temperature (LT) proton exchange membrane fuel cells (PEMFC) and CO2 electrolysis. In the case of PEMFC, Pt-based catalysts are used at the anode and cathode, and Ag-based electrodes are commonly used for CO2 reduction. For PEMFC, the replacement of the Pt catalyst for the oxygen reduction reaction (ORR) by metal-nitrogen-carbon (M N C) catalysts using Fe-N-Cs is promising. However, Fe N Cs have lower performance and stability compared to Pt.[1] We will present three pathways to overcome these challenges: (i) Increasing the mass activity by using bimetallic Fe-Sn-N-C catalysts (Fig. 1 a):  19.3 A g-1 for Fe-Sn-N-C vs. 4.8 A g-1 for comm. Fe-N-C in 0.5 mol L-1 H3PO4 at 0.8 V[2] (ii) Impact of solvent and drying method for electrode fabrication on HT-PEMFC performance:  iso-propanol better than tert-butanol; Freeze-drying better than oven drying. (iii) Addition of small amounts of Pt to Fe-N-C[3] to maintain stability (Fig 1 b):  Maintaining the voltage over time instead of drastic voltage decrease. Moreover, the bimetallic catalyst synthesis route used for Fe-Sn-N-C catalyst (Fig 1 a) is used for fabrication of different bimetallic catalysts for the CO2 reduction reaction (CO2RR), to target CO and C2+ products (Fig 1 c). This includes application of different metals (Fe, Co, Sn, Ni and Cu) in the synthesis as well as physical analysis, e.g., XPS, HR-TEM, Mössbauer spectroscopy and XAS for determination of the nature of active sites. All in all, an overview of optimization approaches to increase performance and stability of Pt-free catalysts for ORR in PEMFC and replace Ag in CO2RR will be given.