Impact of N-Substituent and pKa of Azole Rings on Fuel Cell Performance and Phosphoric Acid Loss

Introduction of polymeric ionic liquids containing quaternary ammonium groups to construct high-temperature proton exchange membranes with high proton conductivity and stability

A series of membrane materials suitable for high-temperature proton exchange membranes (HT-PEM) were successfully prepared by introducing polymeric ionic liquids (PILs) containing quaternary ammonium groups into ether-bonded polybenzimidazole (OPBI). The structure of the cross-linked membrane has a strong interaction with phosphoric acid (PA), which enhances proton transport and PA retention. To ensure better overall performance of the cross-linked membrane, the optimal PIL content is 30 wt% (OPBI-PIL-30 %). The PA uptake of OPBI-PIL-30 % membrane was 323.24 %, and the proton conductivity at 180 ℃ was 113.94 mS cm-1, which was much higher than that of OPBI membrane. It is noteworthy that the PA retention of OPBI-PIL-30 % membrane could reach 71.38 % after 240 h of testing under the harsh environment of 80 ℃/40 % RH. The membrane showed better acid retention capacity of 86.89 % at 160 ℃ under anhydrous environment. The OPBI-PIL-20 % membrane achieved the maximum power density of 436.19 mW cm-2, attributed to its favorable mechanical characteristics and proton conductivity. By these excellent properties, it is shown that OPBI-PIL-X membranes containing quaternary ammonium groups have the potential to be applied in high temperature proton exchange membrane fuel cells (HT-PEMFCs).

Strategies for Mitigating Phosphoric Acid Leaching in High-Temperature Proton Exchange Membrane Fuel Cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) have become one of the important development directions of PEMFCs because of their outstanding features, including fast reaction kinetics, high tolerance against impurities in fuel, and easy heat and water management. The proton exchange membrane (PEM), as the core component of HT-PEMFCs, plays the most critical role in the performance of fuel cells. Phosphoric acid (PA)-doped membranes have showed satisfied proton conductivity at high-temperature and anhydrous conditions, and significant advancements have been achieved in the design and development of HT-PEMFCs based on PA-doped membranes. However, the persistent issue of HT-PEMFCs caused by PA leaching remains a challenge that cannot be ignored. This paper provides a concise overview of the proton conduction mechanism in HT-PEMs and the underlying causes of PA leaching in HT-PEMFCs and highlights the strategies aimed at mitigating PA leaching, such as designing crosslinked structures, incorporation of hygroscopic nanoparticles, improving the alkalinity of polymers, covalently linking acidic groups, preparation of multilayer membranes, constructing microporous structures, and formation of micro-phase separation. This review will offer a guidance for further research and development of HT-PEMFCs with high performance and longevity.

Unveiling CeZnOx Bimetallic Oxide: A Promising Material to Develop Composite SPPO Membranes for Enhanced Oxidative Stability and Fuel Cell Performance

The incorporation of cerium-zinc bimetallic oxide (CeZnOx) nanostructures in sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) membranes holds promise in an enhanced and durable fuel cell performance. This investigation delves into the durability and efficiency of SPPO membranes intercalated with CeZnOx nanostructures by varying the filler loading of 1, 2, and 3% (w/w). The successful synthesis of CeZnOx nanostructures by the alkali-aided deposition method is confirmed by wide-angle X-ray diffraction spectroscopy (WAXS), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses. CeZnOx@SPPO nanocomposite membranes are fabricated using a solution casting method. The intricate interplay of interfacial adhesion and coupling configuration between three-dimensional CeZnOx and sulfonic moieties of the SPPO backbone yields an enhancement in the bound water content within the proton exchange membranes (PEMs). This constructs simultaneously an extensive hydrogen bonding network intertwined with the proton transport channels, thereby elevating the proton conductivity (Km). The orchestrated reversible redox cycling involving Ce3+/Ce4+ enhances the quenching of aggressive radicals, aided by Zn2+, promoting oxygen deficiency and Ce3+ concentration. This synergistic efficacy ultimately translates into composite PEMs characterized by a mere 4% mass loss and a nominal 6% decrease in Km after rigorous exposure to Fenton's solution. Remarkably, an improved power density of 403.2 mW/cm2 and a maximum current density of 1260.6 mA/cm2 were achieved with 2% loading of CeZnOx (SPZ-2) at 75 °C and 100% RH. The fuel cell performance of SPZ-2 is 74% higher than its corresponding pristine SPPO membrane.

2,6-Azulene-based Homopolymers: Design, Synthesis, and Application in Proton Exchange Membrane Fuel Cells

Azulene-based homopolymers are of great interest from the point view of chemistry and material science. Herein, by means of Friedel-Crafts acylation to introduce solubilizing chains on the 1-position of azulene, we designed and synthesized two examples of 2,6-azulene-based homopolymers RP(Az-AC16) and P(Az-AC16). The arrangement of 2,6-azulene units is irregular for RP(Az-AC16), while P(Az-AC16) has head-to-head/tail-to-tail arranged 2,6-azulene units. Proton-responsive studies demonstrate that RP(Az-AC16) and P(Az-AC16) show reversible proton responsiveness in both solution and thin film. To utilize the dynamically reversible proton-responsive property of these polymers in thin films, RP(Az-AC16) and P(Az-AC16) were incorporated into a Nafion matrix as proton exchange membranes, wherein the Nafion/P(Az-AC16) composite membrane exhibits significant increases in proton conductivity relative to the Nafion membrane at different temperatures of each relative humidity (RH), which further results in a 64% improvement in hydrogen fuel cell output power under 30% RH at 80 °C. Our studies have realized the first solution synthesis of 2,6-azulene-based homopolymers and the first application of azulene-based π-systems in hydrogen fuel cells.

Ionic Transport Properties of Cation-Exchange Membranes Prepared from Poly(vinyl alcohol-b-sodium Styrene Sulfonate)

Membrane resistance and permselectivity for counter-ions have important roles in determining the performance of cation-exchange membranes (CEMs). In this study, PVA-based polyanions-poly(vinyl alcohol-b-sodium styrene sulfonate)-were synthesized, changing the molar percentages CCEG of the cation-exchange groups with respect to the vinyl alcohol groups. From the block copolymer, poly(vinyl alcohol) (PVA)-based CEMs, hereafter called "B-CEMs", were prepared by crosslinking the PVA chains with glutaraldehyde (GA) solution at various GA concentrations CGA. The ionic transport properties of the B-CEMs were compared with those previously reported for the CEMs prepared using a random copolymer-poly(vinyl alcohol-co-2-acrylamido-2-methylpropane sulfonic acid)-hereafter called "R-CEMs". The B-CEMs had lower water content than the R-CEMs at equal molar percentages of the cation-exchange groups. The charge density of the B-CEMs increased as CCEG increased, and reached a maximum value, which increased with increasing CGA. A maximum charge density of 1.47 mol/dm3 was obtained for a B-CEM with CCEG = 2.9 mol% and CGA = 0.10 vol.%, indicating that the B-CEM had almost two-thirds of the permselectivity of a commercial CEM (CMX: ASTOM Corp. Japan). The dynamic transport number and membrane resistance of a B-CEM with CCEG = 8.3 mol% and CGA = 0.10 vol.% were 0.99 and 1.6 Ωcm2, respectively. The B-CEM showed higher dynamic transport numbers than those of the R-CEMs with similar membrane resistances.