Towards improved catalytic activity and stability of non-precious metal catalyst for proton exchange membrane fuel cells ?
Résumé
Proton Exchange Membrane Fuel Cell (PEMFC) have applications in portable electronics and automotive propulsion. However, the current use of cathode catalysts based on platinum is a long-term impediment to their sustainable development [1]. Recent non-precious metal catalysts (NPMCs) showed promise to replace in the future platinum-based catalysts currently needed for the electroreduction of oxygen (ORR) in PEMFCs. Among NPMCs, the most mature sub-class of materials is prepared via the pyrolysis of metal (Fe and Co), nitrogen and carbon precursors (labelled as Metal-N-C). Such materials often comprise different types of nitrogen groups and metal species, from atomically dispersed metal-ions coordinated to nitrogen (Metal-NxCy centers), to metallic or metal-carbide particles, partially or completely embedded in graphene shells (Metal@NC) [2]. However, the fate of these catalysts and their different active sites during electrochemical ageing remains an open question, thereby putting into question their industrial relevance.
Herein, six Metal-N-C catalysts differing from each other by the nature of the metal (Fe or Co), the metal content and the heating mode applied during pyrolysis were synthesized and characterized before after ageing in conditions simulating PEMFC cathode operating conditions (80°C, load-cycling or start-up/shutdown protocol, 10 k cycles). Physical, chemical and electrochemical characterizations before and after two different accelerated stress tests (ASTs) provided important correlations between structure, ORR activity and durability of the Metal-N-C catalysts. Cycling between 0.6 and 1.0 V vs. RHE at 80°C led to a small activity loss for catalysts comprising only Metal-NxCy centers but to a large activity loss for those comprising Metal@NC sites. For all catalysts, a positive correlation between residual metal content after AST and residual ORR activity was observed. Higher ORR activity losses were noticed after cycling between 1.0 and 1.5 V vs. RHE at 80°C, due to a massive corrosion of the carbon phase. A Metal-N-C catalyst based on Fe (0.5 wt. %), featuring exclusively Metal-NxCy centers revealed the most active and also the most robust in simulated PEMFC cathode environment. It lost only 25% of its initial ORR activity after 30,000 load cycles at 80°C, thereby reaching the 2020 stability target defined by the US DoE [3].