With the rapid growth and development of proton exchange membrane fuel cell (PEMFC) technology there has been an increasing demand for clean and sustainable global energy applications.While there are many device-level and infrastructure challenges still to be overcome before wide commercialization can be realized, increasing the PEMFC power density is a critical technical challenge, with ambitious goals proposed globally. For example, the short-term and long-term goals of the Japan New Energy and Industrial Technology Development Organization (NEDO) are 6 kW L -1 by 2030 and 9 kW L -1 by 2040, respectively. To this end, we propose technical development directions required for next-generation high power density PEMFCs. This perspective comprehensively embraces the latest advanced ideas for improvements in the membrane electrode assembly (MEA) and its components, bipolar plate (BP), integrated BP-MEA design, with regard to water and thermal management, and materials. The realization of these ideas is expected to be encompassed in next-generation PEMFCs with the aim of achieving a high power density.

This review summarizes the major advances since 2012 in highly permeable and CO2-selective polymer-based membrane materials.

ABSTRACT:This article reviews the recent progress made over the past years based on naphthalene-based sulfonated polyimides (SPIs) in terms of proton conductivity, membrane swelling behavior, membrane stability toward water, and fuel cell performance in polymer electrolyte fuel cells (PEFCs) or direct methanol fuel cells (DMFCs). The structure-property relationship of SPI membranes is discussed in details with respect to the chemical structure of various sulfonated diamines and morphology of SPI membranes from the viewpoints of viscosity, mechanical strength and proton conductivity. Ion exchange capacity (IEC), basicity of sulfonated diamine, configuration (para-, meta-, or ortho-orientation) and chemical structure of polymer chain (linear or net-work) show great influence on the water stability and mechanical strength of SPI membrane. The SPIs with a branched/crosslinked structure and derived from highly basic sulfonated diamines display reasonably high water stability of more than 200-300 h in water at 130 C, suggesting high potential as PEMs operating at temperatures up to 100 C. The SPI membranes have fairly high proton conductivity at higher relative humidities and low methanol permeability. The water and methanol crossover through membrane under the fuel cell operation conditions is not controlled by electro-osmosis due to proton transport but by diffusion due to activity difference. This is quite different from the case of perfluorosulfonated membranes such as Nafion and results in the advantageous effects on fuel cell performance. SPI membranes displayed high PEFC performances comparable to those of Nafion 112. In addition, SPI membranes displayed higher performances in DMFC systems with higher methanol concentration (20-50 wt %), which is superior to Nafion and have high potential for DMFC applications at mediate temperatures (40-80 C In the past decades, great interest has been focused on the development of polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) as a clean power source of energy for transportation, stationary and portable power applications.1,2 Fuel cells with high performance, high durability and potentially lower cost are greatly required. Polymer electrolyte membrane (PEM) is one of the key components in PEFC and DMFC systems. Perfluorosulfonic acid copolymer membranes, such as DuPont's Nafion membrane, are the state-of-the-art PEMs commercially available due to their high proton conductivity and excellent chemical stability.3,4 However, because of their high cost, low operational temperature below 80 C and large methanol crossover, there has been much interest in alternative PEMs. Many efforts have been done in the development of PEMs based on sulfonated aromatic hydrocarbon polymers. [5][6][7][8][9] The main problem existed in the hydrocarbon PEMs is the membrane stability under fuel cell conditions and low conducting performance at low moisture atmosphere. The balance between ion exchange capacity (IEC), proton conductivity and mechanical stability of a PEM ...

The water stability of sulfonated copolyimides (SPIs) derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), sulfonated diamines of 4,4‘-bis(4-aminophenoxy)biphenyl-3,3‘-disulfonic acid (pBAPBDS), and 2,2‘- or 3,3‘-bis(3-sulfopropoxy)benzidine (2,2‘- or 3,3‘-BSPB) and nonsulfonated diamines was investigated in detail from viewpoints of viscosity, mechanical strength, proton conductivity, weight loss, and hydrolysis products eluted into the soaking water. With the aging in water or 100% relative humidity vapor at 130 °C, the polymer chain scission took place mainly in the early stage but thereafter slightly, and as a result the SPI membranes kept the reasonably high mechanical properties even after 192 h. The branched/cross-linked SPI membranes prepared by incorporating a flexible triamine kept better mechanical properties. The weight loss and sulfur loss of <10% were observed for the pBAPBDS-based SPIs. This was due mainly to the elution of hydrolysis product, the oligomer of NTDA, and sulfonated diamine, which did not contain the nonsulfonated diamine moieties. With the aging at 130 °C, the proton conductivity did not change for the pBAPBDS-based SPIs, but for the BSPB-based SPIs it decreased 20% in water and much more at the lower relative humidities because of the cleavage of the sulfopropoxy group. The accelerated water stability tests reveal that the water stability of the present SPI membranes is not sufficiently high at 130 °C but is high enough for PEFC and DMFC applications at least at 80 °C.

1Graphitic carbon nitride (g-C 3 N 4 ) has attracted much attention as a metal-free semiconductor 2 having visible light absorption and relatively highchemicalstability. g-C 3 N 4 can reduce CO 2 to organic 3 fuels such as methanol (CH 3 OH), formic acid (HCO 2 H), and methane (CH 4 ) under visible light 4 irradiation.However, oxidation potential of g-C 3 N 4 is not enough for water oxidation. Therefore, we 5 focused on hybridization of g-C 3 N 4 and tungsten(VI) oxide (WO 3 )which has high oxidation potential 6 for water oxidation. In this study, we examined CO 2 reduction by composite photocatalyst of g-C 3 N 4 7 and WO 3 , which was prepared by three methods (mixture using an agate mortar, impregnation and 8 planetary mill). As a result, composite photocatalyst prepared with planetary millshowed the highest 9 photocatalytic activity. 10 Photodeposition of silver or gold nanoparticles only on g-C 3 N 4 of the hybrid photocatalyst 11 induced an increase in CH 3 OH because the loaded metal nanoparticles play an important role in 12 multi-electron reduction of CO 2 . Photocatalytic activity of the Au-loaded hybrid photocatalyst 13 composed of g-C 3 N 4 and WO 3 was1.7-times higher than that of the hybrid photocatalyst without Au 14 loading. 15 In addition, we investigated photocatalytic reaction mechanism of composite photocatalyst 16 by double-beam photoacoustic spectroscopy. This result revealed Z-scheme reaction proceed in the 17 composite photocatalyst to maintain high oxidation ability of WO 3 and high reduction ability of 18 g-C 3 N 4 , resulting in high photocatalytic activity. 19 20 21 Dioxide; Z-scheme reaction 23 24 25 10 Therefore,visible-light-drivenmaterials with high efficiency and stability represent a central challenge 11 in the field of photocatalytic CO 2 conversion for energy-oriented use. 12 Recently, a visiblelight-responsivegraphitic carbon nitride (g-C 3 N 4 ) photocatalyst with high 13 reduction abilitywas attracted much attention since hydrogen evolution over g-C 3 N 4 had reported to 14 proceed from water under visible-light irradiation in the presence of sacrificial reagents [17].Since 15 Wang and co-workers first reported polymeric graphitic carbon nitride (g-C 3 N 4 ) as a novel 16 photocatalyst that exhibited photoactivity for H 2 productionunder visible-light irradiation, [18] many 17 efforts have been made to synthesize g-C 3 N 4 through thermal treatment of some nitrogen-rich organic 18 precursors, such as cyanamid, dicyanamide, triazine and heptazine derivatives[19-26]. Band gap and 19flat-band potential of g-C 3 N 4 were reported to be 2.67 eV and -1.42 V, respectively (versus Ag/AgCl, 20 pH 6.6) [27]. Although, oxidation ability of g-C 3 N 4 is low enough for efficient oxidation of water, it 21 has high reduction ability because of the high conduction band potential. Moreover, g-C 3 N 4 can be 22 synthesized by a simple and low-cost route, and it has relatively high stability under light irradiation in 23 water solution as well as in acid or base solutions due to the s...

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Development of stimuli-responsive, shape-transformable materials is fundamentally and practically important for smart actuators. Herein, we design and synthesize a bilayer hydrogel by assembling a polycationic (polyMETAC/HEAA) layer with polyelectrolyte effect and a polyzwitterionic (polyVBIPS) layer with antipolyelectrolyte effect together. The bilayer hydrogels adopt a pseudo-double-network structure, and both polyelectrolyte and polyzwitterionic layers have salt-responsive swelling and shrinkage properties, but in a completely opposite way. The resulting polyMETAC/HEAA-polyVBIPS bilayer hydrogels exhibit bidirectional bending in response to salt solutions, salt concentrations, and counterion types. Such bidirectional bending of this bilayer hydrogel is fully reversible and triggered between salt solution and pure water multiple times. The bending orientation and degree of the bilayer hydrogel is driven by the opposite volume changes between the volume shrinking (swelling) of polyMETAC/HEAA layer and the volume swelling (shrinking) of polyVBIPS layer. Such cooperative, not competitive, salt-responsive swelling-shrinking properties of the two layers are contributed to by the polyelectrolyte and antipolyelectrolyte effects from the respective layers. Moreover, an eight-arm gripper made of this bilayer hydrogel is fabricated and demonstrates its ability to grasp an object in salt solution and release the object in water. This work provides a new shape-regulated, stimuli-responsive asymmetric hydrogel for actuator-based applications.

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