(Energy Technology Division Research Award) Interplay between Synthesis, Mechanisms and Performance of Electrocatalysts and Ionomers for Ion-Exchange Membrane Fuel Cells

Abstract

Ion-exchange membrane fuel cells (IEMFCs) are a family of electrochemical energy conversion and storage devices characterized by several attractive features, including outstanding energy conversion efficiency and a clean operation. IEMFCs are a cornerstone of the “hydrogen economy”, one of the most promising avenues to decarbonize the energy sector, with the ultimate purpose to curtail the emissions of greenhouse gases and mitigate global warming. Therefore, a goal of great practical relevance is to devise highly performing, durable and inexpensive IEMFCs .IEMFCs consist of an electrolyte membrane sandwiched between two electrodes. At the membrane-electrode interfaces are located electrocatalytic layers (EC layers) comprising suitable electrocatalysts (ECs) able to promote the redox processes exploited by the IEMFC operation. The IEMFC sub-families are distinguished by the attributes of the membrane, especially in terms of: (i) chemical features (e.g., polymeric backbone); (ii) mobile ion(s) (e.g., protons for proton-exchange membrane fuel cells, PEMFCs, OH- anions for anion-exchange membrane fuel cells, AEMFCs); and (iii) operation temperature. In the environment modulated by the ionomer, the properties of the ECs , with particular reference to the chemical composition of the active sites and support morphology, are correlated to their performance. Hence, to obtain an IEMFC exhibiting a high performance and durability is imperative to rationally design membranes and ECs that are: (i) highly compatible with one another; and (ii) able to express their best performance and durability in the same set of operating conditions.This contribution overviews our research activities aimed at the development of ECs and membranes for application in PEMFCs, high-temperature PEMFCs and AEMFCs. The topics include: (i) the innovative approaches for the synthesis of the functional components; (ii) the aspects of their physicochemical characterization, together with the unique and comprehensive frameworks contrived for the integration and interpretation of the experimental outcomes; and (iii) the steps to integrate the functional components in high-performing IEMFC prototypes.The ECs considered here are meant to promote the oxygen reduction reaction (ORR), a sluggish electrochemical process that is a major bottleneck in the operation of IEMFCs fueled with hydrogen. State-of-the-art ORR ECs are based on Pt nanoparticles supported on carbon (“Pt/C” ECs). Pt/C ECs warrant a sufficient performance, but suffer from a poor durability and require a high loading of platinum, a strategic element that is prone to trigger supply bottlenecks especially in the event of a large-scale rollout of IEMFCs. The ECs devised in our research laboratory are aimed at addressing all of these issues with materials exhibiting a number of unique features, as follows. (i) Active sites with either an intrinsic ORR kinetics much improved above the Pt baseline or completely “Pt-free”, consisting of an “active metal” (e.g., Pt, Fe) whose performance is boosted by one or more “co-catalyst” (e.g., Co, Ni, Cu, Sn). (ii) Strong, covalent interactions between the active sites and the C-/N- ligands of the “coordination nests” on the surface of the EC support, warranting a high durability. (iii) A support featuring a “core-shell” morphology, wherein a “core” consisting of suitable carbon system(s) is covered by a carbon nitride (CN) “shell”, bestowing facile charge and mass transport features (Figure 1(a)). The resulting ECs bestow the IEMFC a high performance with a minimized loading of Pt. Specifically, the PEMFC prototypes mounting the ECs here described yielded more than 20 kW∙gPt -1.The second main topic of this overview is the development and the study of the conductivity mechanism of separator membranes for IEMFCs. The latter consist mainly of ionomer matrices (e.g., perfluorinated systems such as Nafion™, SPEEK and developmental anion-exchange block copolymers), both pristine and doped with suitable nanofillers (e.g., conventional ceramic oxoclusters, “core-shell” oxocluters, among many others). The matrix-nanofiller interactions modulate the physicochemical features of the resulting hybrid inorganic-organic membrane, allowing to improve crucial features for applications in IEMFCs such as: (i) mechanical stability; and (ii) conductivity in dry conditions and high temperature. Other separator membranes are overviewed, including: (i) anion-exchange ionomers based on polyketone matrices (Figure 1(b)); (ii) hybrid inorganic-organic membranes for HT-PEMFCs based on oxocluster nanofillers and polybenzimidazole-like matrices; and (iii) membranes swollen with proton-conducting ionic liquids. These membranes underwent a thorough characterization elucidating their morphology, structure, thermal properties, thermomechanical relaxations and electric response (Figure 1(c)). The integration of all the above knowledges yielded a unique, comprehensive and general framework which proved crucial to elucidate the interplay between the physicochemical properties of the different phases within each membrane and their overall conductivity mechanism. This allowed to trigger the development of new high-performing separator membranes for application in IEMFCs. Figure 1