(Invited) Recent Advances in Electrocatalysts for the Oxygen Reduction Reaction Comprising a Hierarchical Graphene-Based “Core” and a Carbon Nitride “Shell” with a Low Loading of Platinum

Abstract

The paradigm shift in today’s energy harnessing, distribution and utilization is leading to massive investments in innovative technologies, that include electrochemical energy conversion and storage systems (EECS) [1, 2]. The latter exhibit a number of very attractive features, that include negligible emissions of greenhouse gases at the point of operation, a facile scalability and the independence from geographical constrains. Furthermore, EECS are typically characterized by a very high efficiency, as they are not constrained by the limits associated to Carnot’s engines [3]. A broad range of EECS are available, each well-suited for a particular family of applications. In this panorama, the incidence and relevance of proton-exchange membrane fuel cells (PEMFCs) is progressively rising. State-of-the-art PEMFCs have exited the research laboratories and are currently implemented in early products, that include both stationary systems (e.g., power plants for “zero-emission” houses) and light-duty vehicles. Despite these successes, PEMFCs still suffer from several drawbacks. In particular: (i) the performance and the durability of the functional materials involved in PEMFC operation must be increased to match the requirements set by the applications; and (ii) at the same time, the costs must be curtailed to warrant a large-scale market penetration of PEMFCs and leverage the latter’s advantages over competing traditional technologies. One key issue affecting both development areas is the electrocatalyst (EC) that promotes the oxygen reduction reaction (ORR) at the PEMFC cathode. Indeed, to bestow the PEMFC a sufficient performance and durability, state-of-the-art ECs for the ORR must include a very high loading of platinum. Thus, to mitigate the risk to incur in supply bottlenecks [4], new and improved ECs with a low-loading of Pt (“low-Pt”) must be developed. An innovative approach to obtain advanced low-Pt ECs for the ORR exhibiting a performance and durability beyond the state of the art is to devise systems where the active sites are located on the surface of PtMx sub-nanometric clusters (SNCs). PtMx are alloys between Pt and a first-row transition metal (e.g., Ni, Cu), that acts as a “co-catalyst” and promotes the intrinsic ORR kinetics reaching levels that are well beyond the Pt baseline [5]. SNCs offer important advantages over the Pt nanoparticles (NPs) used in conventional ECs for the ORR. In particular, with respect to Pt NPs, in SNCs the utilization of Pt atoms is raised by up to ca. one order of magnitude. This allows for the maximization of the Pt availability for electrocatalytic purposes. Hence, the PEMFCs mounting the low-Pt ECs comprising the SNCs can achieve a specific power that is significantly larger than the 8 kW/gPt target set by the DoE for 2020 [6]. In the proposed ECs the SNCs are located on the surface of a support that exhibits unique features. In detail, the support is based on a “core” consisting of a combination of highly defected graphene nanoplatelets and carbon black NPs [7]. Such hierarchical graphene-based (H-GR) “core” is covered by a carbon nitride (CN) “shell”, that is able to stabilize effectively the SNCs by means of “coordination nests” consisting of C- and N- ligands. The present contribution compares the proposed low-Pt ECs with the state of the art and overviews the complex correlations that are established between: the physicochemical properties of the low-Pt ECs (e.g., chemical composition, morphology and structure of both the active sites and of the support); the electrochemical performance, the reaction mechanism and the durability in the ORR; and finally the characteristic curve of the PEMFC mounting the proposed ECs as tested in operating conditions. The information thus obtained is then used to identify the most promising research approaches to pursue in order to devise next-generation low-Pt ORR ECs able to comply with the stringent requirements of tomorrow’s PEMFCs. Acknowledgement This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 785219, and from the BIRD 2018 program of UNIPD. Figure 1