Imidazole microcapsules toward enhanced phosphoric acid loading of polymer electrolyte membrane for anhydrous proton conduction

Polymer electrolyte membrane (PEM) with high loading, stable ion solvents remains challenging at present and significantly impedes its practical application in energy-relevant devices including hydrogen fuel cell. Here, a series of imidazole microcapsules (ImMCs) are synthesized and utilized as distinct reservoirs to access high phosphoric acid retention for PEM. We demonstrate that the ImMCs can significantly enhance the acid loading capability using the large lumens, bringing abundant proton-hopping sites and hence significantly enhanced proton conduction of membrane. In particular, 10 wt% ImMCs can afford a 78 wt% phosphoric acid loading and a consequent 75 times' increase of proton conductivity relative to the control membrane. Additionally, the cross-linked imidazole shells render membrane high acid retention ability. The acid release is almost stopped after immersing in water for 40 min, helping the membrane to retain as high as 62% of the initially loaded phosphoric acid. These features readily impart notably boosted hydrogen fuel cell performances to composite membrane under the desired conditions of elevated temperature and reduced humidity. As a further description, the acid retention and proton conduction properties of membrane can be efficiently tailored by adjusting microcapsule architectures (lumen size and shell thickness).