High-Performance Fuel Cell Operable at 120 °C Using Polyphenlyene Ionomer Membranes with Improved Interfacial Compatibility

Oxygen- and proton-transporting open framework ionomer for medium-temperature fuel cells

Medium-temperature proton exchange membrane fuel cells (MT PEMFCs) operating at 100° to 120°C have improved kinetics, simplified thermal and water management, and broadened fuel tolerance compared with low-temperature PEMFCs. However, high temperatures lead to Nafion ionomer dehydration and exacerbate gas transportation limitations. Inspired by osmolytes found in hyperthermophiles, we developed α-aminoketone-linked covalent organic framework (COF) ionomers, interwoven with Nafion, to act as "breathable" proton conductors. This approach leverages synergistic hydrogen bonding to retain water, enhancing hydration and proton transport while reducing oxygen transport resistance. For commercial Pt/C, the MT PEMFCs achieved peak and rated power densities of 18.1 and 9.5 Watts per milligram of Pt at the cathode at 105°C fueled with H2 and air, marking increases of 101 and 187%, respectively, compared with cells lacking the COF.

Proton-conductive aromatic membranes reinforced with poly(vinylidene fluoride) nanofibers for high-performance durable fuel cells

Durability and ion conductivity are counteracting properties of proton-conductive membranes that are challenging to achieve simultaneously and determine the lifetime and performance of proton exchange membrane fuel cells. Here, we developed aromatic ionomers reinforced with nonwoven poly(vinylidene fluoride) (PVDF) nanofibers. Because of the right combination of an isotropic nonwoven PVDF with high porosity (78%) and partially fluorinated aromatic ionomers (SPP-TFP-4.0), the resulting composite membrane (SPP-TFP-4.0-PVDF) outperformed state-of-the-art chemically stabilized and physically reinforced perfluorinated Nafion XL membrane, in terms of fuel cell operation and in situ chemical stability at a high temperature (120°C) and low relative humidity (30%). The SPP-TFP-4.0-PVDF membrane exhibited excellent chemical stability and stable rupture energy at high and low RH levels, allowing it to be an alternative proton-conductive membrane to meet the U.S. Department of Energy target to be used in automobile fuel cells in 2025.

EPR Study on the Oxidative Degradation of Phenyl Sulfonates, Constituents of Aromatic Hydrocarbon-Based Proton-Exchange Fuel Cell Membranes

Sulfonated aromatic hydrocarbon-based ionomers are potential constituents of next-generation polymer electrolyte fuel cells (PEFCs). Widespread application is currently limited due to their susceptibility to radical-initiated oxidative degradation that, among other intermediates, involves the formation of highly reactive aromatic cation radicals. The intermediates undergo chain cleavage (dealkylation/dearylation) and the loss of protogenic sulfonate groups, all leading to performance loss and eventual membrane failure. Laser flash photolysis experiments indicated that cation radicals can also be formed via direct electron ejection. We aim to establish the major degradation pathway of proton-exchange membranes (PEMs). To this end, we irradiated aqueous solutions of phenyl sulfonate-type model compounds with a Xe arc lamp, thus generating radicals. The radicals were trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and the formed adducts were observed by electron paramagnetic resonance (EPR). The formed DMPO spin adducts were assigned and relative adduct concentrations were quantified by simulation of the experimental EPR spectra. Through the formation of the DMPO/•SO3 - adduct, we established that desulfonation dominates for monoaromatic phenyl sulfonates. We observed that diaryl ether sulfonates readily undergo homolytic C-O scission that produces DMPO/•aryl adducts. Our results support the notion that polyphenylene sulfonates are the most stable against oxidative attack and effectively transfer electrons from DMPO, forming DMPO/•OH. Our findings help to identify durable moieties that can be used as building blocks in the development of next-generation PEMs.

Perovskite-Carbon Joint Substrate for Practical Application in Proton Exchange Membrane Fuel Cells under Low-Humidity/High-Temperature Conditions

Highly active catalysts with promising water retention are favorable for proton exchange membrane fuel cells (PEMFCs) operating under low-humidity/high-temperature conditions. When PEMFCs operate under low-humidity/high-temperature conditions, performance attenuation rapidly occurs owing to reduced proton conductivity of dehydrated membrane electrode assemblies. Herein, we load platinum onto a perovskite-carbon joint substrate (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-σ-XC-72R) to construct a highly active and durable catalyst with good water retention capacity. We propose that the Pt/(BZCYYb-C) catalyst layer at cathode can promote the water back diffusion of produced water from the cathode to the membrane, thus preventing the decay of fuel-cell performance under low-humidity/high-temperature conditions. The catalyst exhibited outstanding mass activity of 0.542 A mg_pt^-1 at 0.9 V vs RHE. PEMFCs with such a catalyst delivered very high peak power densities (1.70/1.14 W cm^-2 under H₂-O₂/air conditions at 70 °C) and kept 85.3%/92.1% of initial performance values under low-humidity/high-temperature conditions (relative humidity 60%@70 °C/100 °C).

Protocol for synthesis and characterization of ePTFE reinforced, sulfonated polyphenylene in the application to proton exchange membrane fuel cells

Sulfonated polyphenylenes (SPPs) are one of the most promising polymers as proton exchange membranes for fuel cells (PEMFCs) because of their high proton conductivity, gas impermeability, and chemical and thermal stability. Mechanical stability needs further improvement for practical applications. Here we describe a protocol for the preparation and characterization of tetrafluorophenylene-containing SPP (SPP-TP-f) membranes reinforced with double porous ePTFE (expanded polytetrafluoroethylene) thin layers. The protocol also includes performance and durability evaluation of fuel cells using the reinforced membranes. For complete details on the use and execution of this protocol, please refer to Long and Miyatake (2021a).

ePTFE reinforced, sulfonated aromatic polymer membranes enable durable, high-temperature operable PEMFCs

Sulfonated polyphenylene (SPP)-based ionomers have been developed for electrochemical applications in recent years due to their inherent thermal and chemical stability. However, the difficult synthesis, limited solubility, and rigid backbone obstructs their progress. Herein, a new monomer, 3,3″-dichloro-2',3',5',6'-tetrafluoro-1,1':4',1″-terphenyl (TP-f) with high polymerization reactivity was designed and polymerized with sulfonated phenylene monomer to prepare SPP-based ionomers (SPP-TP-f) with high ion exchange capacity up to 4.5 mequiv g-1. The resulting flexible membranes were more proton conductive than Nafion (state-of-the-art proton exchange membrane) even at 120°C and 20% RH. Unlike typical SPP ionomers, SPP-TP-f 5.1 was soluble in ethanol and thus, could be reinforced with double expanded polytetrafluorethylene thin layers to obtain SPP-TP-f 5.1/DPTFE membrane. SPP-TP-f 5.1/DPTFE showed superior fuel cell performance to that of Nafion, in particular, at low humidity (30% RH, > 100°C) and reasonable durability under the severe accelerated conditions combining OCV hold and humidity cycling tests.

Poly(para-phenylene) ionomer membranes: effect of methyl and trifluoromethyl substituents

Poly(para-phenylene sulfonic acid) containing CF3 groups exhibits thin membrane forming capability, high proton conductivity, mechanical strength, gas barrier properties, and chemical stability as a proton conductive membrane.