Abstract
As one of the most efficient pathways to provide clean energy, fuel cells have attracted great attention in both academic and industrial communities. Proton exchange membranes (PEMs) or proton-conducting electrolytes are the key components in fuel cell devices, which require the characteristics of high proton conductivity as well as high mechanical, chemical and thermal stabilities. Organic–inorganic hybrid PEMs can provide a fantastic platform to combine both advantages of two components to meet these demands. Due to their extremely high proton conductivity, good thermal stability and chemical adjustability, polyoxometalates (POMs) are regarded as promising building blocks for hybrid PEMs. In this review, we summarize a number of research works on the progress of POM–polymer hybrid materials and related applications in PEMs. Firstly, a brief background of POMs and their proton-conducting properties are introduced; then, the hybridization strategies of POMs with polymer moieties are discussed from the aspects of both noncovalent and covalent concepts; and finally, we focus on the performance of these hybrid materials in PEMs, especially the advances in the last five years. This review will provide a better understanding of the challenges and perspectives of POM–polymer hybrid PEMs for future fuel cell applications.
Highlights
Burning fossil fuels is the principal energy-producing pathway of the world, which emits a large amount of greenhouse gas like carbon dioxide and environmental pollution from sulfur and nitrogen oxides
POMs containing protons as the countercations have been extensively used as multifunctional proton conductors. Except for their intrinsic high proton conductivity, POMs can serve as electrostatic crosslinkers, nanoenhancers and radical decomposition catalysts etc. These features are in high demand for Proton exchange membranes (PEMs) to achieve good conductivity as well as enhanced mechanical and chemical stabilities, enabling POMs to be suitable inorganic building blocks in the fabrication of POM–polymer hybrid PEMs
POMs have been combined with polymer moieties through non-covalent and covalent interactions to prepare various hybrid materials for PEMs, where the distributed morphology of POMs can be precisely controlled down to a several nanometers scale; the proton transport efficiency and the leakage issue of POMs are gradually improved
Summary
Burning fossil fuels is the principal energy-producing pathway of the world, which emits a large amount of greenhouse gas like carbon dioxide and environmental pollution from sulfur and nitrogen oxides. One of the main groups of POMs, heteropolyacids (HPAs), displaying strong Brönsted acidity and the highest proton conductivity in their fully hydrated state among inorganic solids near ambient temperatures, are promising candidates for fabricating composite PEMs. A large number of bonded water molecules, high thermal stability, structural flexibility and mobility of HPAs are beneficial for fuel cell applications as well [11,12,13,14]. In the range of 410–440 ◦ C, a half or one water molecule, formed by acidic protons and oxygens from the host lattice lost, which resulted in the formation of denuded Keggin anion This process was more or less common to all isostructral heteropoly compounds of Keggin-type (12-molybdophosphoric, 12-tungstosilicicacid) they investigated: the dehydration process was finished about 250 ◦ C and the hexahydrates were the more stable phase [28,30,31].
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