Correlation between properties and ORR performance of low-loading graphene-based hierarchical nanostructured electrocatalysts in single PEMFCs

This work considers two groups of electrocatalysts (ECs) for the Oxygen Reduction Reaction (ORR) characterized by a low loading of platinum (L-PGM). The L-PGM ECs, that are meant for application at the cathode of a polymer electrolyte membrane fuel cell (PEMFC), consist of: (i) a hierarchical nanostructured “core” based on graphene supported on Cu nanoparticles, that is covered by (ii) a carbon nitride “shell” embedding the ORR active sites. The latter are based on Pt and include Ni and Cu as “co-catalysts” [1]. The L-PGM ECs are prepared by customizing a protocol described elsewhere [1]. The two groups of studied L-PGM ECs are distinguished on the basis of the pyrolysis temperature adopted during the synthesis [1]. The L-PGM ECs require a post-synthesis activation process, that is achieved by a suitable electrochemical cycling process. The activation protocol, that was developed for the analysis of ORR performance by means of the rotating ring-disk electrode method (RRDE), is successfully customized for the electrode configurations used in the real membrane electrode assemblies (MEA), which are then tested in operating conditions in single PEMFC. The MEAs including the proposed L-PGM ECs are extensively tested: (i) both in the low- and in the high-current regime; and (ii) as a function of the back pressure of the reactants, with a particular reference to that of the oxygen used in the cathode feed. The performance of the MEAs in the different test conditions is correlated with the physicochemical properties of the L-PGM ECs, as determined by modulating the parameters of the protocol used to synthesize them. Particular attention is devoted to the study of the “in-situ” electrochemical kinetics of the L-PGM ECs, and on the impact of the mass and charge transport issues on the performance of a single PEMFC. Finally, it is shown that the MEAs including the proposed L-PGM ECs, despite a very low loading of platinum at the cathode (0.1 mg cm-2) achieve an outstanding specific power, up to ca. 4.8 kW/gPt,cathode. The corresponding value for the reference MEA, characterized by a Pt loading at the cathode of 0.5 mg cm-2, is ca. 1.3 kW/gPt,cathode. References [1] V. Di Noto, E. Negro, K. Vezzù, F. Bertasi, G. Nawn, The Electrochemical Society Interface, Summer 2015, 59 (2015).