Pt/Co Alloy Nanoparticles Prepared by Nanocapsule Method Exhibit a High Oxygen Reduction Reaction Activity in the Alkaline Media

Oxygen reduction reaction (ORR) catalysts are one of the main topics for fuel cells and metal/air batteries. We found that the platinum–cobalt alloy nanoparticles prepared by our original nanocapsule method exhibited a high ORR catalytic activity in alkaline solution, compared with that of the existing alloy nanoparticles prepared by different methods. The effect of alloy composition on the ORR activity was investigated to find the optimum composition (approximately 40 atom %). We also found that the enhancement of the catalytic activity in alkaline solution appeared in a very narrow range of Co content compared with that in acidic solution

Sulfonated Terpolymers Containing Alkylene and Perfluoroalkylene Groups: Effect of Aliphatic Groups on Membrane Properties and Interface with the Catalyst Layers

Two series of terpolymers (SPA-A, -B, and -C) composed of perfluoroalkylene, alkylene, and sulfonated phenylene (SP) groups were prepared to investigate the effect of aliphatic groups and their compositions onto the membrane properties for proton exchange membrane fuel cells. The composition of perfluoroalkylene (<i>m</i>) and alkylene groups (<i>n</i>) in SPA terpolymers was controlled to be <i>m</i>/<i>n</i> = 0.5/0.5 for SPA-A, 0.65/0.35 for SPA-B, and 1/0 for SPA-C. SPA terpolymers provided thin and bendable membranes with ion exchange capacity (IEC) ranging from 1.61 to 3.18 mequiv g^–1. Introducing alkylene groups into the polymer main chain was effective in achieving high IEC values. SPA-B membranes with lower alkylene group content showed slightly more developed phase-separated morphology than that of SPA-A membranes with higher alkylene group content. The developed phase separation with interconnected ionic channels resulted in high proton conductivity for SPA-B membranes. The alkylene groups in the main chain also contributed to improving mechanical properties as suggested by stress versus strain curves, in which SPA membranes exhibited higher Young’s moduli and higher yield strength than those of copolymer (SPA-C) membranes with no alkylene groups. An H₂/O₂ (or air) fuel cell with SPA-B membrane exhibited high open circuit voltage (OCV, 0.99 V at 100% RH with O₂), low ohmic resistance (0.05 Ω cm² at 100% RH with O₂), and good current/voltage performance, reflecting the properties of SPA-B membrane. However, interfacial compatibility with the catalyst layers was somewhat deteriorated with SPA-B membrane to cause lower mass activity (70 A g^–1 at 100% RH) of the cathode compared to that with SPAF membrane (102 A g^–1 at 100% RH). SPA-B membrane was durable in OCV hold test for 1000 h with slight degradation in alkylene groups

Tuning the Hydrophobic Component in Reinforced Poly(arylimidazolium)-Based Anion Exchange Membranes for Alkaline Fuel Cells

A series of imidazolium-based aromatic copolymers were synthesized using fluorinated and non-fluorinated hydrophobic monomers. The quaternized copolymers were reinforced with the plasma-treated porous polyethylene substrate to provide flexible, homogeneous membranes. Cross-sectional SEM images revealed a triple-layer structure. The reinforced membranes exhibited phase-separated morphology as confirmed through TEM images. Among the membranes, QQP-MEIm-PE-containing quinquephenylene hydrophobic groups exhibited the most balanced properties (ion conductivity, mechanical strength, and alkaline stability). In particular, QQP-MEIm-PE exhibited excellent elongation properties with 24 MPa maximum stress and 205% elongation at break. A single H2/O2 fuel cell using the QQP-MEIm-PE membrane (1.26 meq g–1) and non-PGM-(Fe–N–C) cathode achieved 222 mW cm–2, which accounted for 888 mW mg–1Pt at 560 mA cm–2. Reasonable durability was confirmed with the membrane in the operating fuel cell

Mechanism of H₂O₂ Decomposition by Triphenylphosphine Oxide

A decomposition mechanism of H₂O₂ by triphenylphosphine oxide (TPPO) is presented. TPPO is often incorporated in proton-exchange membrane electrolytes as a moiety to inhibit the H₂O₂-induced degradation of the membranes. However, it has not been revealed how TPPO decreases the concentration of free H₂O₂ in the membranes. Following the experimental X-ray structures, the TPPO dimer capturing two H₂O₂ molecules was used as the calculation model. The vibrational spectrum calculations for various hydration numbers show that this model correctly reproduces the spectral peaks of TPPO capturing H₂O₂. On the basis of this model, the H₂O₂ decomposition mechanism by the TPPO dimer was searched. It was consequently found that this reaction proceeds through three steps: (1) Hydrogen transfer from H₂O₂ to the PO bond of TPPO, (2) Hydrogen transfer from the −OOH group to the −OH group, and (3) O–O bond formation between O₂ groups. The calculated vibrational spectra for the reactants and intermediates indicated that the first and second steps are activated by vibrational excitations. Moreover, the third step giving low barrier heights is considered to proceed through two reaction paths: directly producing the O₂ molecule or going through an HOOOH intermediate. Interestingly, this reaction mechanism was found to use the violation of the octet rule for the PO double bond, resulting in the strong H₂O₂ binding of TPPO

Sulfonated Poly(arylene ether phosphine oxide ketone) Block Copolymers as Oxidatively Stable Proton Conductive Membranes

The introduction of triphenylphosphine oxide moiety into the hydrophilic segments of aromatic multiblock copolymers provided outstanding oxidative stability and high proton conductivity. Our designed multiblock copolymers are composed of highly sulfonated phenylene ether phosphine oxide ketone units as hydrophilic blocks and phenylene ether biphenylene sulfone units as hydrophobic blocks. High molecular weight block copolymers (<i>M</i>_w = 204–309 kDa and <i>M</i>_n = 72–94 kDa) with different copolymer compositions (number of repeat unit in the hydrophobic blocks, <i>X</i> = 30, and that of hydrophilic blocks, <i>Y</i> = 4, 6, or 8) were synthesized, resulting in self-standing, transparent, and bendable membranes by solution-casting. The block copolymer membranes exhibited well-developed hydrophilic/hydrophobic phase separation, high proton conductivity, and excellent oxidative stability due to the highly sulfonated hydrophilic blocks, which contained phenylene rings with sulfonic acid groups and electron-withdrawing phosphine oxide or ketone groups

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

Structurally Well-Defined Anion Conductive Aromatic Copolymers: Effect of the Side-Chain Length

For improving the alkaline stability and other properties of aromatic semiblock copolymer [QPE-<i>bl</i>-11a­(C1)] membranes containing benzyltrimethyl­ammonium groups, several novel hydrophilic monomers with different side-chain lengths and substitution positions were designed and synthesized for the polymerization. The pendant-type preaminated copolymers PE-<i>bl</i>-11s were quaternized using iodomethane to obtain the target QPE-<i>bl</i>-11s with well-defined chemical structure. In TEM analyses, QPE-<i>bl</i>-11a­(C3) and QPE-<i>bl</i>-11a­(C5) membranes with propyl and pentyl side-chains, respectively, showed more developed phase-separated morphology with greater hydrophilic domains (ca. 10–20 nm in width) than that of the C1 equivalent. The phase separation was more distinct and larger for the QPE-<i>bl</i>-11a membranes linked with <i>p</i>-phenylene groups in the hydrophilic part than for the QPE-<i>bl</i>-11b membranes with <i>m</i>-phenylene groups. In particular, QPE-<i>bl</i>-11b­(C5) membrane exhibited considerably smaller hydrophilic/hydrophobic domains compared to those of the other membranes. After the alkaline stability test in 1 M KOH aqueous solution at 60 °C for 1000 h, the remaining conductivity was better as increasing the side-chain length: 34% for QPE-<i>bl</i>-11a­(C1), 54% for QPE-<i>bl</i>-11a­(C3), and 72% for QPE-<i>bl</i>-11a­(C5) at 60 °C. The results suggest that the pendant alkyl chains could improve the alkaline stability and the main-chain bond position could improve morphology, water utilization, and mechanical properties of QPE-<i>bl</i>-11 membranes. An H₂/O₂ fuel cell with QPE-<i>bl</i>-11 membrane showed 139 mW cm^–2 of the maximum power density at 0.28 A cm^–2 of the current density

Remarkable Reinforcement Effect in Sulfonated Aromatic Polymers as Fuel Cell Membrane

Fluorine-free aromatic ionomers are next generation materials for proton exchange membrane fuel cells (PEMFCs). In addition to high proton conductivity and chemical durability, a membrane must also have high mechanical durability under practical fuel cell operating conditions, where frequent humidity changes are involved. We herein demonstrate that a fluorine-free reinforced aromatic PEM exhibits much improved mechanical durability compared with the parent aromatic PEM under the humidity cycling test conditions. The parent PEM and the reinforcement material are a sulfonated polybenzophenone derivative (SPK, in house) and a nonwoven fabric (NF, composite of glass and PET fibers), both of which do not contain any fluorine atoms. Because the compatibility between the SPK and the reinforcement materials is high, an almost void-free, dense, homogeneous, and tough reinforced PEM is attainable even with thin membrane thickness (18 μm), leading to a reasonably high fuel cell performance. The reinforcement material improves in-plane dimensional stability and mitigates crack propagation during frequent humidity changes, resulting in high durability (more than 18 000 cycles) in the wet–dry cycling test. The advantages of this fluorine-free reinforced PEM, unlike typical reinforced PEMs (e.g., Gore-SELECT consisting of a perfluorosulfonic acid ionomer and a microporous expanded polytetrafluoroethylene support layer), include versatility in molecular design, enabling further improvement in performance and durability of PEMFCs with lower cost

Sulfonated Polybenzophenone/Poly(arylene ether) Block Copolymer Membranes for Fuel Cell Applications

Sulfonated polybenzophenone/poly­(arylene ether) block copolymers were designed and synthesized via Ni-mediated coupling polymerization. The block copolymers were obtained as high-molecular-weight (<i>M</i>_n = 70–110 kDa, <i>M</i>_w = 150–230 kDa) with low polydispersity index (<i>M</i>_w/<i>M</i>_n = 2.0–2.3). The block copolymer membranes showed well-developed hydrophilic/hydrophobic phase separation and high proton conductivity and low gas permeability. The membrane showed better fuel cell performance and durability compared with those with Nafion, state-of-the-art proton conducting membrane

Analysis of the Gold/Polymer Electrolyte Membrane Interface by Polarization-Modulated ATR-FTIR Spectroscopy

We developed a new FTIR system with two polarizers in its optics in order to conduct polarization-modulated measurements. Polarization characteristics were examined for the Kretschmann polarization-modulated attenuated total reflectance (ATR) configuration by the use of gold-sputtered films of 10–100 nm thickness on Ge and ZnSe prisms. The marked increase of the polarization characteristics for Au film thicknesses below 30–40 nm is closely associated with a large reflectivity decrease of the p-polarized radiation. A cast film of sulfonated block poly­(arylene ether sulfone ketone) membrane was formed on the Au film, and the interfacial spectra were acquired by the use of the ATR FTIR system. The interfacial spectra resemble those of the ATR spectra of the bulk membrane but exhibited strong dependence of the intensity and line shape of the vibrational modes on the Au thickness. The dependence is closely associated with a change of the polarization characteristics of the interface. Electromagnetic as well as chemical effects were concluded to be responsible for the band anomalies and enhancement