(Invited) Hierarchical “Core-Shell” Low-Loading Pt Electrocatalysts for the Oxygen Reduction Reaction Based on a Graphene “Core” and a Carbon Nitride “Shell”

Abstract

A major restructuring of the energy system is underway at the global level, with the purpose to curtail the dependence from fossil fuels and minimize the emissions of greenhouse gases [1]. Thus, a shift of paradigm is necessary towards the widespread implementation of innovative energy conversion and storage technologies. In this regard, a pivotal role is to be played by electrochemical energy conversion and storage (EECS) systems owing to their independence from geographical constrains, facile scalability and outstanding efficiency [2]. Among the different families of EECS, the technology of proton exchange membrane fuel cells (PEMFCs) is expected to play a pivotal role in such a milieu. Indeed, PEMFCs are compact and do not require moving parts. Hence, PEMFCs are ideal for application in light-duty vehicles or in small-size stationary applications such as auxiliary power units (APUs) and household systems able to store reversibly the energy obtained from renewables [3]. PEMFCs also exhibit an energy conversion efficiency that is two-three times larger with respect to that of competing traditional technologies such as internal combustion engines (ICEs) [4]. PEMFCs operate by converting into electrical energy the chemical energy associated with the oxidation of hydrogen. One of the most important bottlenecks in this process is the oxygen reduction reaction (ORR), that takes place at the PEMFC cathode. The ORR is sluggish and it must be promoted by suitable electrocatalysts (ECs) to ensure that the PEMFC achieves a performance level compatible with the intended application [5]. The most effective ORR ECs for PEMFCs include active sites based on platinum, whose scarcity in Earth’s crust triggers the risk to incur in supply bottlenecks [6]. This is a major drawback inhibiting the large-scale rollout of PEMFCs. Hence, the development of ECs that are high-performing, durable and comprise a low loading of platinum (giving so rise to “Low-Pt ECs”) is a major goal of PEMFC research. This work overviews the development of a new family of low-Pt ECs for the ORR. The active sites of the ECs are found on the surface of sub-nanometric clusters (SNCs) consisting of PtMx alloys, where M is a first-row transition metal (e.g., Ni, Cu). M operates as a “co-catalyst” and raises the intrinsic performance of each active site much above the Pt baseline [7]. With respect to the Pt nanoparticles (NPs) adopted in state-of-the-art ORR ECs, the SNCs increase the utilization of Pt atoms included therein by up to ca. one order of magnitude and make them much more available for electrocatalytic purposes. Consequently, the specific power yielded by the PEMFCs including the proposed low-Pt ECs is significantly raised. Values as high as 14 kW/gPt are achieved, that exceed the target set by the DoE for 2020 (i.e., 8 kW/gPt) [8]. The support of the low-Pt ECs described here exhibits a “core-shell” morphology; it comprises a hierarchical graphene-based (H-GR) “core” that is covered by a carbon nitride (CN) “shell”. H-GR consists of a combination of: (i) highly defected graphene nanoplatelets; and (ii) carbon black NPs [9]. The resulting system provides a broad surface area and allows for the facile transport of mass and charge through the system. The CN “shell” includes less than 5 wt% of N to prevent the introduction of ohmic drops associated with the transport of electrons in the system. The SNCs are stabilized on the EC surface by means of strong interactions with: (i) defects of the graphene nanoplatelets; and (ii) C- and N-based ligands of the CN “shell”, making up the “coordination nests”. As a result, the low-Pt described here exhibit an outstanding durability. This work takes into consideration the various families of low-Pt ECs based on H-GR supports developed so far by our group, and discusses extensively the interplay between: (i) the preparation parameters (e.g., synthesis of the support, chemical/electrochemical activation steps) [10]; (ii) the physicochemical properties (e.g., chemical composition, morphology, structure); (iii) the electrochemical behavior (e.g., intrinsic activity and ORR reaction mechanism); and (iv) the performance in a single PEMFC tested under operating conditions. Finally, the results are compared with the state of the art and new research avenues are suggested. 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