Influence of Ni and Cu “Co-Catalysts” on the Properties and ORR Performance of Hierarchical Graphene-Based Electrocatalysts with a Low Loading of Platinum

Abstract

The intrinsic sluggishness of the oxygen reduction reaction (ORR) is a major bottleneck in the operation of low-temperature fuel cells. This issue is particularly relevant for proton-exchange membrane fuel cells (PEMFCs). In these systems the electrodes operate in a strongly acid environment, where the best performance is afforded by active sites based on platinum-group elements (PGMs). Accordingly, owing to the extremely low abundance of PGMs in Earth’s crust, the development of ORR electrocatalysts (ECs) comprising a minimized loading of PGMs is a major goal of both fundamental and applied research to achieve a widespread rollout of PEMFC technology. One of the best strategies to obtain high-performing ORR ECs with a minimized loading of PGMs is to devise nanocomposite systems, comprising: (i) support materials with a high electrical conductivity and large surface area (e.g., carbon black or carbon nanotubes); and (ii) PGM-based nanostructures (e.g., nanoparticles, nanowires, and nanocages) exhibiting a large specific area. This approach allows to maximize the utilization of PGM atoms and boost the EC performance. The intrinsic performance of the active sites in the ORR can be further raised by introducing in the EC suitable “co-catalysts”, typically first-row transition metals (e.g., Fe, Co, Ni, Cu). This work reports a new family of ECs with a low loading of PGMs (L-PGM) for the ORR. The ECs exhibit a “core-shell” morphology. A hierarchical graphene-based support (H-GR) “core” is covered by a carbon nitride (CN) “shell” stabilizing Pt-based nanostructures in “coordination nests” [1]. Two main components are comprised in the “core”, namely: (i) highly defected graphene nanoplatelets, that are supported on ZnO nanoparticles (NPs) [2]; and (ii) carbon black NPs, whose introduction facilitates the charge and mass transport phenomena. The CN “shell” binds the Pt-based nanostructures bearing the ORR active sites. The latter include two “co-catalysts”, i.e., Ni and Cu. This work studies the interplay between: (i) the relative stoichiometry of the Ni and Cu “co-catalysts”; and (ii) the physicochemical properties and the electrochemical performance of the ECs, as determined both “ex-situ” and in an operating PEMFC. The ECs undergo extensive characterization studies aimed at the elucidation of the chemical composition, morphology, structure, and porosity features of the ECs. Particular efforts are dedicated to study the impact of the “post-synthesis” activation steps that are necessary to maximize the performance of the ECs [3]. The cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) technique is adopted to study: (i) the ORR kinetics and reaction mechanism, by means of the Tafel analysis; (ii) the average number of electrons exchanged during the ORR, in order to determine the selectivity in the reduction of O2 to water. Finally, the most promising ECs are studied in single PEMFC running in operating conditions, with the purpose to: (i) optimize the electrode configurations and the compatibility with the proton-conducting membrane; and (ii) study the interplay between the physicochemical properties of the ECs (e.g., the morphology) and the PEMFC performance. Acknowledgement This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 785219 of the Graphene Flagship, and from the BIRD 2016 program of UNIPD.