Physical and Chemical Considerations for Improving Catalytic Activity and Stability of Non-Precious-Metal Oxygen Reduction Reaction Catalysts

Recent non-precious-metal catalysts (NPMCs) show promise to replace in the future platinum-based catalysts currently needed for the electroreduction of oxygen (ORR) in proton-exchange membrane fuel cells (PEMFCs). Among NPMCs, the most mature subclass of materials is prepared via the pyrolysis of metal (Fe and Co), nitrogen, and carbon precursors (labeled as metal–NC). Such materials often comprise different types of nitrogen groups and metal species, from atomically dispersed metal ions coordinated to nitrogen to metallic or metal–carbide particles, partially or completely embedded in graphene shells. While disentangling the different contributions of these species to the initial ORR activity of metal–NC catalysts with multidunous active sites is complex, following the fate of these different active sites during electrochemical aging is even more difficult. To shed light onto this, herein, six metal–NC catalysts were synthesized and characterized before/after aging with two different accelerated stress tests (AST) simulating PEMFC cathode operating conditions either in steady-state or transient conditions. The samples differed from each other by the nature of the metal (Fe or Co), the metal content, and the heating mode applied during pyrolysis. Catalysts featuring either only atomically dispersed metal-ion sites (metal–NxCy) or only metal nanoparticles encapsulated in the carbon matrix (metal@N–C) were obtained after pyrolysis of catalyst precursors containing 0.5 or 5.0 wt % of metal, respectively. All six catalysts showed high beginning-of-life ORR mass activity, but the ASTs revealed marked differences in their ORR activity at end-of-life. After the load-cycling AST (10000 cycles), metal–NC catalysts with metal–NxCy sites retained most of their initial activity at 0.8 V (60–100%), while those with metal@N–C particles retained only a small fraction of initial activity (10–20%). Metal–NC catalysts with metal–NxCy sites lost only 25% of their initial ORR activity after 30000 load cycles at 80 °C, thereby reaching the 2020 stability target defined by US Department of Energy. After 10000 start-up/shut-down cycles, no catalyst showed measurable ORR activity at 0.8 V. However, after 1000 start-up/shut-down cycles, most of the metal–NC catalysts initially comprising metal–NxCy sites showed measurable ORR activity at 0.8 V, while those initially comprising metal@N–C particles did not. Energy-dispersive X-ray spectroscopy and Raman spectroscopy measurements of the cycled rotating disk electrodes revealed that demetalation of the catalytic centers and corrosion of the carbon matrix are the main causes of ORR activity decay during load-cycling and start-up/shut-down cycling, respectively. In contrast to what could have been intuitively expected, the metal–NxCy sites are more robust to both demetalation and carbon corrosion than metal@N–C sites