Non-noble metal (NNM) catalysts for fuel cells: Tuning the activity by a rational step-by-step single variable evolution

Abstract

Low-temperature fuel cells (LTFCs) based on polymer electrolyte membranes (PEMs) fed with hydrogen are being recognized to be among the best candidates as pollution-free and energy-saving power sources for electric or hybrid vehicles or portable apparatuses because of their high-energy conversion efficiency (~58 %) and zero or nearly zero emissions. Currently, cost and durability are the main limitations of FC technology to be commercialized. A significant percentage of the cost of PEMFCs comes from precious group metal (PGM) based catalysts that are used mainly for the oxygen reduction reaction (ORR). Therefore, a breakthrough in the development of cost-effective, highly performing, and durable catalysts has been identified as the determining factor for success toward PEMFC commercialization. In particular, non-noble metal (NNM) cathodic electrocatalyst gained lots of attention in recent years to replace PGM-based catalysts for the ORR. Within various NNM electrocatalysts, the most promising ones seem to be heat-treated Fe(II) and/or Co(II) chelates and macrocycles supported on carbon particles. The formation of metal–nitrogen (M–NX/C) and metal–carbon (M/C) active ensembles after the heat treatment is necessary for ORR. In this chapter we will describe an enhancement of the electrochemical activity toward ORR through a step-by-step understanding of the variables involved during the formation of active Fe–NX NNM catalysts. We adopted different approaches in order to understand the formation of active ensembles and to increase the activity by a rational step-by-step progression.