HPW/MCM‐41 Phosphotungstic Acid/Mesoporous Silica Composites as Novel Proton‐Exchange Membranes for Elevated‐Temperature Fuel Cells

Abstract

Proton-exchange membrane and direct methanol fuel cells (PEMFCs and DMFCs) have attracted much attention as clean energy sources for various applications, such as electric vehicles, portable electronics, and domestic power generation, because of their high power density, high efficiency, and low greenhouse gas emission. [1][2][3][4][5] Especially, the operation of PEMFCs and DMFCs at temperatures above 100 8C is considered to have many advantages, such as the elimination of CO poisoning of the platinum electrocatalyst, faster electrode reaction kinetics, simplified water and heat management, higher energy efficiency, and reduced usage of precious Pt and Pt alloy catalyst. [6,7] However, the state-of-the-art proton-exchange membranes (PEMs) based on perfluorosulfonic acid (PFSA), such as Nafion, are unstable at elevated temperatures (!100 8C) and proton conductivity decreases significantly due to the loss of water from the membrane under conditions of high temperatures or low humidities. [8][9][10] Therefore, development of PEMs with high proton conductivity and stability at elevated temperatures is a major challenge.Great efforts have been dedicated to developing PEMs for operation at elevated temperatures based on mesoporous or nanoporous inorganic materials. Mesoporous inorganic materials have a pore size range of 2-50 nm and are characterized by high specific surface area, nanometer-sized channels or frameworks with an ordered or disordered interconnected internal structure, and high structural stability, which make feasible potential applications as proton-exchange membranes operating at elevated temperatures. [11][12][13][14][15][16][17][18][19][20] Lu and co-workers [21] reported sol-gel-derived mesostructured zirconium phosphates with proton conductivities of about 10 À8 -10 À6 S cm À1. Colomer et al. synthesized nanoporous anatase thin films with conductivity values from 10 À5 to 10 À3 S cm À1 in the range of 33%-81% relative humidity (RH) at room temperature.[22] Li and Nogami [11] prepared proton-conducting mesoporous silica films with conductivity ranging from 10 À6 to 10 À4 S cm À1 under 40%-90% humidity. However, the proton conductivity of the pure mesoporous materials depends significantly on their textural characteristics. For instance, Colomer et al. also reported a proton conductivity of 2.0 Â 10 À2 S cm À1 on mesoporous acid-free silica xerogels and 3.78 Â 10 À2 S cm À1 on a nanoporous anatase thin film at 80 8C and 81% RH. [13,22] Yamada et al. [14] reported a TiO 2 -P 2 O 5 mesoporous nanocomposite with a proton conductivity value of 2 Â 10 À2 S cm À1 at 160 8C. Halla et al. [20] synthesized meso-SiO 2 -C 12 EO 10 OH-CF 3 SO 3 H as a new protonconducting electrolyte and reported a conductivity of 1 Â 10 À3 S cm À1 at room temperature and 90% RH. Although the conductivity values of mesoporous acid-free silica xerogels, meso-SiO 2 -C 12 EO 10 OH-CF 3 SO 3 H or anatase thin films are adequate for fuel cell applications, their performance as an electrolyte in a PEMFC has not been evaluated yet. Ya...

Exceptional durability enhancement of PA/PBI based polymer electrolyte membrane fuel cells for high temperature operation at 200 °C

“…The small angle X-ray scattering (SAXS) of meso-SiO2 after PWA impregnation shows reduced intensity of the meso-SiO2 matrix (Fig.1F), indicating aggregation of PWA within the meso-SiO2 structure. 35 Moreover, the scattering peak position after PWA impregnation remains unchanged, suggesting structural integrity. The N2 adsorption/desorption isotherms of meso-SiO2 and PWA-meso-SiO2 show type IV adsorption curves with capillary condensation effect for the mesoporous structure and H 1 type hysteresis for cylindrical mesoporous structures ( After surface modification of PWA-meso-SiO2 with dequalinium chloride hydrate (Scheme S1, ESI † ) to improve the dispersion in the PBI solution, homogenous composite membranes were obtained in the low PWA-meso-SiO2 loading range up to 15 wt%, as evidenced from the cross-sectional scanning electron microscopy imaging (Fig.…”

Fabrication of novel phosphotungstic acid functionalized mesoporous silica composite membrane by alternative gel-casting technique

“…[50,51]. The position of the scattering peaks for HPW/MCM-41 powders remains the same as that of the pristine MCM-41 without any HPW impregnated, indicating that HPW has coalesced well into the mesoporous structure of MCM-41 [5,6]. This is explained by the occurrence of the relative intensities of scattering peaks of the as-assembled HPW/MCM-41 powders, which decreased significantly with the increase of HPW content until the peak disappeared/almost disappeared [6].…”

“…In the case of MCM-41-HPW15-m membrane, the conductivity was 2.4 Â 10 À3 S cm À1 and 7.7 Â 10 À3 S cm À1 at 25 C and 80 C, respectively. Lu et al have reported that the proton conductivity of the pristine MCM-41 host is very low, i.e., 5.1 Â 10 À6 S cm À1 [5]. This indicates that the transport of protons through the pure mesoporous silica via surface hydroxyl groups is very difficult.…”

“…Phosphotungstic acid (HPW) was ordered from SigmaeAldrich. The as-assembled HPW/MCM-41 powders were prepared by a vacuum assisted impregnation method [5]. MCM-41 was treated under vacuum (<10 À1 Pa) to remove impurities and trapped air in the mesoporous …”

“…HPW exhibits interesting acid properties due to its easily accessible and reactive protons present in the structure. Although earlier effort on this material as a potential solid electrolyte has failed to meet the requirements due to its high solubility in water and strong humidity-dependent proton conductivity [37], it is interesting to note that the proton conductivity of a well dispersed HPW impregnated into mesoporous silica system by vacuum assisted impregnation method has been quite promising at low humidity [5]. Apart from the conductivity, another technical challenge for the inorganic proton conductive electrolyte is the formation of thin and dense membrane structure with high mechanical strength.…”