Dedensification of Perfluorinated Sulfonicacid (PFSA) Ionomer at the Platinum-Ionomer Interfaces via Modulating Chemical Structures

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In polymer electrolyte fuel cells (PEFCs), aggravation in the electrocatalytic activity towards oxygen reduction reaction (ORR) at the real-scale electrode in membrane electrode assembly (MEA), also known as the “RDE versus MEA” challenge, is known as main hurdle in the way of achieving the ultimate platinum (Pt) or platinum group metal (PGM) employment target (~0.05 mg/cm2) for sustainability [1]. Besides, the mass transport resistance of O2 at the ionomer film covering Pt is reported as unexpectedly larger. These properties are particularly important for the PEFC systems to be widely spread in the fuel cell electric vehicle (FCEV) applications. It is now well accepted that the origin of this phenomenon is escalated from the severe densification of tetrafluoroethylene (TFE) backbone in perfluorinate sulfonicacid (PFSA) ionomer at the Pt-ionomer interfaces, followed by specific adsorption of sulfonate group (-SO3 -) onto Pt in an assistance of ether group (-O-). To address the above issue, significant efforts have been made to avoid the direct contact between Pt and ionomer during the last decade. Several improvements in the ORR activity at MEA are demonstrated by 1) embedding Pt nanoparticles into meso-porous carbon support [2], 2) forming porous polymeric film or porous graphitic carbon layer onto Pt surfaces [3], and 3) depositing ionic liquid that has a considerable ionic conductivity (~0.01 S/cm) onto Pt surfaces [4]. Although they were quite successful performance wise, they inevitably suffer from poor ionomer accessibility onto Pt even under moderate humidity conditions. To this end, the lowered electrochemically available surface area (ECSA) acts as a major drawback in obtaining better Pt mass-specific activity towards ORR. For an enhanced ionomer accessibility, the recent studies have reported effective outcomes demonstrating that the nitrogen dopants and their neighboring carbons are beneficial in loosening the ionomer structure at Pt/ionomer interface, by attracting the sulfonate groups in ionomer against Pt [5,6]. The Vulcan carbon black supports are reported as more favorable when compared with the Ketjen black, due to less micro-porous structures with better accessibility [6]. However, there remains a big challenge regarding that adsorption strength of sulfonate group either onto Pt catalyst or carbon support can be notably weaken in the typical PEFC electrode slurry conditions under water-rich environment. Herein, we propose a practical approach to strengthen the molecular attractive interactions between TFE backbone in ionomer and Vulcan carbon support, which leads to a dramatic improvement in both the ORR electrode kinetics and local O2 transport from bulk to Pt active sites. Specifically, our hypotheses are: 1) stronger hydrophobic attraction between ionomer and carbon support in the electrode slurry generates larger quantities of ionomer physically adsorbed onto carbon support over Pt catalyst, 2) such adsorbed ionomers remain their attachment onto carbon support during the electrode fabrication process under water-rich environments, 3) dedensification of ionomer at Pt/ionomer interface dramatically reduce the number density of sulfonate groups in the electrical double layer of Pt, that is helpful in achieving an improved performance when operating at cathode potentials higher than potential of zero charge (PZC), and 4) high-performance ORR electrode with superior ionomer accessibility and less sulfonate coverage can be delivered successfully. To this end, the dependence of TFE and Pt spacing distances on the ORR electrode kinetics as well as O2 transportation is explored by means of physico-chemical structural analyses and electrochemical measurements. Consequently, the dramatic improvements are demonstrated with 3-fold Pt-mass activity towards ORR and 2-fold cell power density, which paves the new way for maximizing the utilization efficiency of Pt to accelerate the commercialization of FCEVs.