The impact of imidazolium with steric hindrance on the dissociation of phosphoric acid and the performance of high-temperature proton exchange membranes

Abstract

The low conductivity and poor stability of high-temperature proton exchange membranes (HT-PEMs) are still the main reasons limiting the practical application of high-temperature proton exchange membrane fuel cells (HT-PEMFCs). Herein, a strategy of blending polyimidazolium (P-Im) with strong steric effect in HT-PEMs is proposed to accelerate proton conduction and improve operating stability. For one thing, the steric hindrance facilitates the formation of stable ion-association complexes between imidazolium and dihydrogen phosphate, promoting the dissociation of phosphoric acid (PA). For another, it helps to enhance acid–base interaction and hydrogen bonding between P-Im and PA, thus inhibiting the leaching of PA. The proton conductivity of the blend membrane reaches 0.149 S cm⁻¹ at 200 °C, which is 1.16 times higher than that of the poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) membrane with a 24.1% lower PA uptake under the same conditions. And the corresponding peak power density is 746 mW cm⁻² without backpressure. This work presents a novel approach to enhance proton conduction efficiency in HT-PEMs from the perspective of accelerating PA dissociation by introducing additional ionic interactions and regulating the steric effect of functional groups in the polymer matrix.