Simple, Effective Molecular Strategy for the Design of Fuel Cell Membranes: Combination of Perfluoroalkyl and Sulfonated Phenylene Groups

Proton-conducting membranes are key materials in polymer electrolyte fuel cells. In addition to high proton conductivity and durability, a membrane must also support good electrocatalytic performance of the catalyst layer at the membrane–electrode interface. We herein propose an effective molecular approach to the design of high-performance proton-conducting membranes designed for fuel cell applications. Our new copolymer (SPAF) is a simple combination of perfluoroalkylene and sulfonated phenylene groups. Because this ionomer membrane exhibits a well-controlled finely phase-separated morphology, based on the distinct hydrophilic–hydrophobic differences along with the polymer chain, it functions well in an operating fuel cell with good durability under practical conditions. The advantages of this ionomer, unlike typical perfluorosulfonic acid ionomers (e.g., Nafion), include easy synthesis and versatility in molecular structure, enabling the fine-tuning of membrane properties

Double-Layer Ionomer Membrane for Improving Fuel Cell Performance

A double-layer ionomer membrane, thin-layer Nafion (perfluorinated sulfonic acid polymer) on a sulfonated aromatic block copolymer (SPK-bl-1), was prepared for improving fuel cell performance. Each component of the double-layer membrane showed similar phase-separated morphologies to those of the original membranes. A fuel cell with the double-layer membrane exhibited lower ohmic resistance and higher cathode performance than those with the original SPK-bl-1 membrane despite their comparable water uptake and proton conductivity. Detailed electrochemical analyses of fuel cell data suggested that the thin Nafion interlayer contributed to improving the interfacial contact between the SPK-bl-1 membrane and the cathode catalyst layer and to mitigating excessive drying of the membrane. The results provide new insight on designing high-performance fuel cells with nonfluorinated ionomer membranes such as sulfonated aromatic polymers

Sulfonated Poly(arylene ether sulfone ketone) Multiblock Copolymers with Highly Sulfonated Block. Fuel Cell Performance

Poly(arylene ether sulfone ketone) (SPESK) multiblock copolymers having highly sulfonated hydrophilic blocks were synthesized and the fuel cell performance with the copolymers was investigated. A membrane electrode assembly (MEA) using an SPESK ionomer with an ion exchange capacity of 1.8 mequiv g−1 as membrane and Nafion as the electrode binder showed comparable fuel cell performance and ohmic resistance to that using a Nafion NRE 211 membrane at 80 °C and 30% relative humidity (RH). A Nafion-free, all-SPESK MEA using SPESK as both the membrane and the binder was operable at 100 °C and 50% RH. The fuel cell performance was limited not only by the proton conductivity of the SPESK membrane but also by the low water flux through the membrane and specific adsorption of the ionomer on the platinum catalyst

Sulfonated Poly(arylene ether sulfone ketone) Multiblock Copolymers with Highly Sulfonated Blocks. Long-Term Fuel Cell Operation and Post-Test Analyses

The stability of poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes having highly sulfonated hydrophilic blocks was tested in an operating fuel cell. The electrochemical properties and drain water were monitored during the test, followed by post-test analyses of the membrane. During a 2000-h fuel cell operation test at 80 °C and 53% RH (relative humidity) and with a constant current density (0.2 A cm–2), the cell voltage showed minor losses, with slight increases in the resistance. In the drain water, anions such as formate, acetate, and sulfate were observed. Post-test analyses of the chemical structure by NMR and IR spectra revealed that the sulfonated fluorenyl group with ether linkage was the most likely to have degraded during the long-term operation, producing these small molecules. The minor oxidative degradation only slightly affected the proton conductivity, water uptake, and phase-separated morphology

Preparation and Fuel Cell Performance of Catalyst Layers Using Sulfonated Polyimide Ionomers

Sulfonated polyimide (SPI-8) ionomers were used as binders in the catalyst layers, and their fuel cell performance was evaluated. SPI-8 ionomers functioned well in the anode with only minor overpotential even at low humidity (50% relative humidity (RH)). In contrast, the cathode performance was significantly dependent on the content and molecular weight of the ionomers and humidity of the supplied gases. Higher molecular weight of the ionomer caused larger potential drop at high current density at 80 and 100% RH since oxygen supply and/or water discharge became insufficient due to higher water uptake (swelling) of the ionomer. Similar results were obtained at higher ionomer content, because of the increase of thickness in the catalyst layer. The mass transport was improved with decreasing humidity, however, proton conductivity became lower. While the maximum values of <i>j</i>_@0.70 V for all membrane electrode assemblies (MEAs) were ca. 0.35 A/cm², each electrode could have the different appropriate operating conditions. The results suggest that the parameters such as oxygen supply, proton conductivity, and water uptake and discharge need to be carefully optimized in the catalyst layers for achieving reasonable cathode performance with hydrocarbon ionomers

Effect of Pt Loading Percentage on Carbon Blacks with Large Interior Nanopore Volume on the Performance and Durability of Polymer Electrolyte Fuel Cells

Achieving high performance and durability at low Pt loads is an important challenge for polymer electrolyte fuel cells (PEFCs). We investigated the effect of catalyst Pt loading percentage (wt % Pt) on the performance and durability of an ultrahigh surface area carbon black (CB) with a large nanopore volume using morphological observations, nitrogen adsorption, and electrochemical performance measurements. The ratio of the surface areas of Pt on the interior and exterior surfaces of the CB affects the penetration of the ionomer into the nanosized pores. When the exterior Pt surface area is larger than that of the interior, the oxygen diffusion resistance in the ionomer increases and the performance deteriorates due to the thick covering of the ionomer on the exterior Pt. Based on durability testing that combines startup, shutdown, and galvanostatic load cycling, the main deterioration factors are dependent upon the Pt interparticle distance and the thickness of the catalyst layer, which vary with the wt % Pt. The advanced characterization and optimization of the various wt % Pt on an ultrahigh surface area CB, combined with the extensive performance and durability testing, have provided an unprecedented understanding of the reaction sites, mass transport characteristics, and stability, which are crucial for their practical application in PEFCs

Effects of Humidity and Produced Water on Specific Adsorption of High Oxygen Permeability Ionomers Composed Entirely of Cyclic Monomers on Cathode Performance for Polymer Electrolyte Fuel Cells

To enhance polymer electrolyte fuel cell (PEFC) performance, it is necessary to improve cathode ionomer performance, with attention to the development of high oxygen permeability ionomers (HOPIs) to mitigate the oxygen reduction reaction rate-limiting process of oxygen transport. We developed a new synthetic route for a cyclic monomer with a fluorosulfonyl group and synthesized a novel ionomer composed entirely of cyclic monomers. Preliminary investigations showed that it exhibited the expected HOPI performance while maintaining basic electrolyte performance. Cell evaluation of the MEA with the HOPI as a cathode ionomer confirmed improvements, especially under low humidity and high current density conditions. Overvoltage component analysis verified activation overvoltage lowering due to improved catalytic activity and concentration overvoltage decrease due to improved oxygen permeability. This was attributed to the ability of HOPI to avoid specific adsorption, to improve oxygen solubility, to improve oxygen transport due to increased ionomer permeability, and to improve Knudsen diffusion due to pore volume preservation in the catalyst layer. Careful humidity dependence examinations revealed a characteristic step in the Tafel plot under low humidity conditions, which was sensitive to ionomer differences. While the step occurrence mechanism remains debatable, we hypothesize that the step reflects the specific adsorption of the ionomer on Pt. Specific adsorption is not a static phenomenon but rather a dynamic one, which can vary according to the amount of water present near the catalyst surface. The difference in current density around the step could reflect the loss of Pt active surface area due to specific adsorption, enabling quantitative analysis of ionomer-induced performance degradation in MEAs. Cell evaluations combined with a high-performance membrane showed that an MEA using the HOPI as the cathode ionomer exhibited improved performance and high robustness versus humidity variations

Electrochemical Oxidation of Hydrolyzed Poly Oxymethylene-dimethyl Ether by PtRu Catalysts on Nb-Doped SnO_2−δ Supports for Direct Oxidation Fuel Cells

We synthesized Pt and PtRu catalysts supported on Nb-doped SnO_2−δ (Pt/Sn_0.99Nb_0.01O_2−δ, PtRu/Sn_0.99Nb_0.01O_2−δ) for direct oxidation fuel cells (DOFCs) using poly oxymethylene-dimethyl ether (POMM<i>n</i>, <i>n</i> = 2, 3) as a fuel. The onset potential for the oxidation of simulated fuels of POMM<i>n</i> (methanol-formaldehyde mixtures; <i>n</i> = 2, 3) for Pt/Sn_0.99Nb_0.01O_2−δ and PtRu/Sn_0.99Nb_0.01O_2−δ was less than 0.3 V vs RHE, which was much lower than those of two commercial catalysts (PtRu black and Pt₂Ru₃/carbon black). In particular, the onset potential of the oxidation reaction of simulated fuels of POMM<i>n</i> (<i>n</i> = 2, 3) for PtRu/Sn_0.99Nb_0.01O_2−δ sintered at 800 °C in nitrogen atmosphere was less than 0.1 V vs RHE and is thus considered to be a promising anode catalyst for DOFCs. The mass activity (MA) of PtRu/Sn_0.99Nb_0.01O_2−δ sintered at 800 °C was more than five times larger than those of the commercial catalysts in the measurement temperature range from 25 to 80 °C. Even though the MA for the methanol oxidation reaction was of the same order as those of the commercial catalysts, the MA for the formaldehyde oxidation reaction was more than five times larger than those of the commercial catalysts. Sn from the Sn_0.99Nb_0.01O_2−δ support was found to have diffused into the Pt catalyst during the sintering process. The Sn on the top surface of the Pt catalyst accelerated the oxidation of carbon monoxide by a bifunctional mechanism, similar to that for Pt–Ru catalysts

Self-Wrinkling in Polyacrylamide Hydrogel Bilayers

Swelling of a gel film attached to a soft substrate can induce surface instability, which results in the formation of highly ordered patterns such as wrinkles and folds. This phenomenon has been exploited to fabricate functional devices and rationalize morphogenesis. However, obtaining centimeter-scale patterns without immersing the film in a solvent remains challenging. Here, we show that wrinkles with wavelengths of up to a few centimeters can be spontaneously created during the open-air fabrication of film–substrate bilayers of polyacrylamide (PAAm) hydrogels. When the film of an aqueous pregel solution of acrylamide prepared on the PAAm hydrogel substrate undergoes open-air gelation, hexagonally packed dimples initially emerge on the surface, which later evolve into randomly oriented wrinkles. The formation of such self-organized patterns can be attributed to the surface instability resulting from autonomous water transport in the bilayer system during open-air fabrication. The temporal evolution of the patterns can be ascribed to an increase in overstress in the hydrogel film due to continued water uptake. The wrinkle wavelength can be controlled in the centimeter-scale range by adjusting the film thickness of the aqueous pregel solution. Our self-wrinkling method provides a simple mechanism for the generation of swelling-induced centimeter-scale wrinkles without requiring an external solvent, which is unachievable with conventional approaches

Impacts of Pt/Carbon Black Catalyst Surface Hydrophilicity on Ionomer Distribution and Durability during Water-Generating Load Cycling of Polymer Electrolyte Fuel Cells

The performance of polymer electrolyte fuel cells (PEFCs) is significantly influenced by the MEA structure and ionomer dispersibility. In this study, we modulated the catalyst surface hydrophilicity through surface treatment and assessed its impact on ionomer dispersibility and durability. The surface hydrophilicity was compared using nitrogen and water vapor adsorption measurements, while functional groups were analyzed using the surface-sensitive technique TOF-SIMS. Excessively hydrophilic catalysts with numerous functional groups demonstrated poor ionomer dispersion and reduced gas diffusivity due to water accumulation. Conversely, decreased hydrophilic catalysts with moderate functional groups displayed optimal ionomer dispersibility and superior I–V performance. Durability tests using water-generated load cycles revealed that a decreased hydrophilicity catalyst exhibited enhanced durability due to its moderate wettability. Our findings suggest that appropriate control of the catalyst surface condition can facilitate an improved MEA structure and its durability