Synthesis of proton conductive inorganic–organic hybrid membranes from organoalkoxysilane and hydroxyalkylphosphonic acid

Advancement toward Polymer Electrolyte Membrane Fuel Cells at Elevated Temperatures

Elevation of operational temperatures of polymer electrolyte membrane fuel cells (PEMFCs) has been demonstrated with phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. The technical perspective of the technology is simplified construction and operation with possible integration with, e.g., methanol reformers. Toward this target, significant efforts have been made to develop acid-base polymer membranes, inorganic proton conductors, and organic-inorganic composite materials. This report is devoted to updating the recent progress of the development particularly of acid-doped PBI, phosphate-based solid inorganic proton conductors, and their composite electrolytes. Long-term stability of PBI membranes has been well documented, however, at typical temperatures of 160°C. Inorganic proton-conducting materials, e.g., alkali metal dihydrogen phosphates, heteropolyacids, tetravalent metal pyrophosphates, and phosphosilicates, exhibit significant proton conductivity at temperatures of up to 300°C but have so far found limited applications in the form of thin films. Composite membranes of PBI and phosphates, particularly in situ formed phosphosilicates in the polymer matrix, showed exceptionally stable conductivity at temperatures well above 200°C. Fuel cell tests at up to 260°C are reported operational with good tolerance of up to 16% CO in hydrogen, fast kinetics for direct methanol oxidation, and feasibility of nonprecious metal catalysts. The prospect and future exploration of new proton conductors based on phosphate immobilization and fuel cell technologies at temperatures above 200°C are discussed.

Highly flexible, proton-conductive silicate glass electrolytes for medium-temperature/low-humidity proton exchange membrane fuel cells

We demonstrate highly flexible, proton-conductive silicate glass electrolytes integrated with polyimide (PI) nonwoven fabrics (referred to as "b-SS glass electrolytes") for potential use in medium-temperature/low-humidity proton exchange membrane fuel cells (PEMFCs). The b-SS glass electrolytes are fabricated via in situ sol-gel synthesis of 3-trihydroxysilyl-1-propanesulfonic acid (THPSA)/3-glycidyloxypropyl trimethoxysilane (GPTMS) mixtures inside PI nonwoven substrates that serve as a porous reinforcing framework. Owing to this structural uniqueness, the b-SS glass electrolytes provide noticeable improvements in mechanical bendability and membrane thickness, in comparison to typical bulk silicate glass electrolytes that are thick and easily fragile. Another salient feature of the b-SS glass electrolytes is the excellent proton conductivity at harsh measurement conditions of medium temperature/low humidity, which is highly important for PEMFC-powered electric vehicle applications. This beneficial performance is attributed to the presence of a highly interconnected, proton-conductive (THPSA/GPTMS-based) silicate glass matrix in the PI reinforcing framework. Notably, the b-SS glass electrolyte synthesized from THPSA/GPTMS = 9/1 (mol/mol) exhibits a higher proton conductivity than water-swollen sulfonated polymer electrolyte membranes (here, sulfonated poly(arylene ether sulfone) and Nafion are chosen as control samples). This intriguing behavior in the proton conductivity of the b-SS glass electrolytes is discussed in great detail by considering its structural novelty and Grotthuss mechanism-driven proton migration that is strongly affected by ion exchange capacity (IEC) values and also state of water.

Substituent-dominated structure evolution during sol-gel synthesis: a comparative study of sol-gel processing of 3-glycidoxypropyltrimethoxysilane and methacryloxypropyltrimethoxysilane

The sol-gel processes of 3-glycidoxypropyltrimethoxysilane (GPTMS) and methacryloxypropyltrimethoxysilane (MAPTMS) have been followed by fluorescence spectroscopy with pyranine as a photophysical probe. The experimental results showed that this probe is sensitive to the structural evolution and microenvironment polarity. The specific comparison of the structural evolution in two substituted organotrialkoxysilanes, namely, MAPTMS and GPTMS, illustrates the ability of the substituents to interact with the microenvironment via electrostatic interactions. Interestingly, these interactions determine the kinds of intermediate supramolecular structures that form during the sol-gel process and hence control the structure of the ensuing sol-gel end product. In particular, the amphiphile-like character of the MAPTMS intermediates contrasts with the biamphiphilic character of their GPTMS counterparts, driving distinctly different transient and local molecular organizations, which in turn modulate the hydrolysis and condensation reactions during the sol-gel process.