(Invited) Interplay between the Physicochemical Properties and the Electrochemical Performance of Hierarchial Electrocatalysts for the ORR Comprising a Graphene-based ‘Core’ and a Carbon Nitride ‘Shell’

Abstract

The sluggishness of the oxygen reduction reaction (ORR) is a major bottleneck in the operation of low-temperature fuel cells (FCs), with a particular reference to proton-exchange membrane fuel cells (PEMFCs) [1]. Hence, it is necessary to devise efficient and affordable electrocatalysts (ECs) able to minimize the overpotentials associated with the ORR. This issue is further exacerbated considering that the single chemical element affording the lowest ORR overpotentials in the highly acidic environment found at the PEMFC cathode is Pt, whose abundance in Earth’s crust is very limited [2]. This raises serious concerns towards the incipient large-scale rollout of PEMFCs, that may face serious supply bottlenecks.On these bases, massive research efforts are currently devoted to develop new ORR ECs. A serious issue faced by the researchers is that it is often very difficult to transfer the intrinsic ORR performance of an EC into a single FC. The performance of a single FC is limited by transport phenomena, that are not easy to gauge by means of the well-assessed “ex-situ” techniques routinely adopted to determine the physicochemical properties and the “pure” ORR kinetics of an EC taken on its own. Thus, the optimization of the cathodic electrocatalytic layer of the FC (that hosts the ORR EC) is a crucial step to transfer effectively the ORR performance and ultimately to maximize the current and power outputs of the FC as a whole.In order to address this issue, this work describes a characterization approach that, by combining the information derived from single fuel cell measurements collected in different current density regimes, and upon modulating the partial pressure of oxygen at the cathode, allows for the identification and quantification a few simple figures of merit. The latter: (i) picture the extent with which the intrinsic ORR performance of the cathodic EC is transferred to the single FC; and (ii) gauge the contribution of different transport phenomena in the determination of single FC performance.The results are studied with an innovative analysis framework, yielding information that is then correlated to the physicochemical properties of the cathodic EC and to its electrochemical performance as determined by “ex-situ” measurements. Important insights are thus achieved, that play a crucial role in the optimization of the cathodic electrocatalytic layer with the final purpose to maximize the performance of the FC.The proposed characterization approach and subsequent analysis framework are straightforward, and suitable for a broad variety of FCs. The latter may also include cathodic ECs (e.g., low-Pt materials exhibiting a “core-shell” morphology [3], or completely “Pt-free” systems) that are very different from the conventional Pt/C of the state of the art. In conclusion this work describes a tool, simple and of broad applicability, able to provide important contributions to facilitate the implementation of developmental cathodic ECs in single FCs in the ongoing quest to realize efficient and low-carbon energy conversion systems. Acknowledgements 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; (c) the program “Budget Integrato per la Ricerca Interdipartimentale - BIRD 2018” of the University of Padova (protocol BIRD187913); and (d) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells – HELPER” funded by the University of Padova.