(Invited) Interplay between Surface/Porosimetric, Chemical and Electrochemical Characterization of “Core-Shell” High-Pt ORR Electrocatalysts

Abstract

A critical bottleneck of PEMFC operation is the performance of the cathode electrode, where the oxygen reduction reaction (ORR) takes place. Two of the most important phenomena that conspire to curtail the cathode performance are: (i) the sluggish kinetics of the oxygen reduction reaction (ORR) [1]; and (ii) the mass transport issues. In particular, the latter become progressively more evident as the current density of the PEMFC is increased upon approaching operating conditions that are relevant for practical applications [2].Both of these shortcomings are addressed by developing new ORR electrocatalysts (ECs). The ECs described here exhibit a “core-shell” morphology: a hierarchical graphene-based support (H-GR) “core” is covered by a carbon nitride “shell” stabilizing the active sites in “coordination nests” [3]. The “core” comprises highly defected graphene nanoplatelets [4] and carbon black nanoparticles; the latter act as spacers and facilitate the charge and mass transport phenomena occurring during the EC operation. The ECs proposed here are characterized by a significant loading of platinum (on the order of ca. 20-40 wt%), that is the “active metal”. Ni and Cu are introduced as the “co-catalysts” to further boost the ORR kinetics of the ECs [3]. The high loading of “active metal” and “co-catalyst” in the ECs is meant to minimize the bulk of the support, facilitating mass transport phenomena.The innovative support of the proposed ECs is very different from that typically adopted in the state of the art in terms of morphology and especially porosity. Thus, this latter feature is to be studied in detail to fully exploit the potential of such support and guide the synthesis of new ECs beyond today’s state of the art. This information is achieved by a comprehensive characterization of the proposed ECs by a variety of porosimetric methods involving the adsorption of suitable probe molecules.Physisorption of N2 is carried out to evaluate the specific surface area of the ECs, together with their micro- and mesoporosity. Water adsorption is executed as well, being a crucial tool to elucidate the hydrophobicity of the ECs. Indeed, the latter feature is critical to prevent electrode flooding during PEMFC operation. Furthermore, through the study of the real ECs’ density a new parameter is proposed, namely the surface area per volume “Σ” (expressed as m2/cc). This latter is of paramount importance to design efficient membrane electrode assemblies (MEAs).As a final step, the results of morphologic/porosimetric studies are correlated with: (i) the outcome of both bulk and surface chemical characterizations carried out by CHNS microanalysis, energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS); and (ii) the “ex-situ” intrinsic ORR performance and selectivity in the 4-electron mechanism determined by cyclic voltammetry with the thin-film rotating ring-disk method (CV-TF-RRDE). Acknowledgements The research leading to the results reported in this work has received funding from: (a) the European Union’s Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; and (c) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells e HELPER” funded by the University of Padova.