Three-fold optimization of Pt/ionomer interface by ionic liquid-modified MOF-808 in cathode of proton exchange membrane fuel cells

The large-scale commercialization of proton exchange membrane fuel cells (PEMFCs) has been hindered by the high demand of platinum (Pt) in the cathode due to the sluggish kinetics of the oxygen reduction reaction. Reducing the amount of Pt would worsen the problems caused by the adsorption of perfluorinated sulfonic acid (PFSA) ionomers to Pt via the side chains, namely, blocking the active sites of Pt and inducing densely packed layers of fluorocarbon backbones on Pt surface to obstruct local O 2 transport at the Pt/PFSA interfaces. This work aims at optimizing the Pt/ionomer interface to mitigate the sulfonate adsorption and in the meantime to reduce the local O 2 transport resistance ( R local ), by using a porous composite of 1-butyl-3-methylimidazolium hydrogen sulfate ionic liquid (IL) modified MOF-808 (BMImHSO 4 @MOF-808) as additive in cathodic catalyst layer (CCL). Through detailed physical, spectroscopic and electrochemical characterizations, we demonstrate a three-fold optimization mechanism of Pt/ionomer interface structure by BMImHSO₄@MOF-808: the unsaturated metal sites in MOF-808 effectively inhibit the sulfonate adsorption on Pt through coordination with the sulfonates of PFSA, thereby improving catalyst utilization; the pores in MOF-808 establish efficient transport channels for gaseous oxygen, significantly reducing R local ; the IL modification layers facilitate the formation of continuous proton transport networks, increasing proton conductivity. The incorporation of BMImHSO₄@MOF-808 in a low-Pt CCL (0.1 mg Pt cm –2 ) yields a peak power density of 1.9 W cm –2 for PEMFC under H 2 -O 2 condition, and ca. 20% increase of power density under H 2 -air condition as compared with conventional CCL, indicating the prospect of IL-MOF composites as an efficient additive to enhance the performance of PEMFCs.