Synthesis and Structure-Electrochemical Property Relationships of Hybrid Imidazole-Based Proton Conductors for Medium-Temperature Anhydrous Fuel Cells

Medium-temperature anhydrous operation (above 120 °C) of polymer electrolyte membrane fuel cells (PEMFCs) has been extensively investigated, particularly for heavy-duty fuel cell applications. In this context, inorganic–organic composites based on acid–base reactions emerge as essential candidates for medium-temperature PEMFC applications. This study aims to develop a proton-conductive salt by simultaneously coordinating multiple acid species with heterocyclic imidazole (Imi), which possesses basic sites capable of reacting with acids. The goal is to create a highly proton-conductive composite suitable for anhydrous conditions. The mechanochemical milling method was employed to incorporate SiO2 as a second material into imidazole hydrochloride (ImiHCl). As a result, the addition of SiO2 to ImiHCl led to an enhancement in proton conductivity compared with imidazole (Im), imidazole hydrochloride (ImiHCl), and imidazole–SiO2 (Imi–SiO2). Furthermore, a method based on density functional theory (DFT) was proposed to predict the high/low proton conductivity, which exhibited good correlation with the experimentally obtained conductivity values. This DFT approach provided insights into the proton conduction mechanism. Through comprehensive physical characterizations (FT-IR, NMR, XRD, and TGA) and DFT calculations, it was revealed that the high proton conductivity observed in the corresponding xImiHCl-(100 – x)SiO2 composites (composition ratio of ImiHCl: x = 40–100) can be attributed to an increased number of proton species, accelerated proton dissociation facilitated by SiO2, and promotion of proton diffusion. Particularly, the xImiHCl-(100 – x)SiO2 (x = 60) electrolyte demonstrated the highest proton conductivity of 1.4 × 10–2 S cm–1. Subsequently, a PBI-based electrolyte membrane prepared using 60ImiHCl-40SiO2 significantly enhanced the fuel cell performance to 521 mW cm–2 under anhydrous conditions at 150 °C.