Super Proton Conductivity Through Control of Hydrogen-Bonding Networks in Flexible Metal-Organic Frameworks

Abstract

Due to the growing concern over climate change and its consequences, it requires the development of renewable energy storage and conversion systems. Fuel cells operating mainly based on hydrogen as a fuel have been considered as an attractive option for sustainable energy conversion. In particular, proton exchange membrane fuel cells (PEMFCs) are recognized as a promising candidate for transportation applications due to their mild operating conditions, high-power density and ultralow emission of pollutants. The design of proton exchange membrane with a high proton conductivity over a wide humidity range remains a challenge to improve the performance of PEMFCs. Moreover, the amorphous nature of the commonly used polymer exchange membrane materials hinders the investigation and control of the proton conduction behavior. Metal-organic frameworks (MOFs) have emerged as a new type of solid-state proton conductor, attributed to their high surface area and porosity as well as high crystallinity and structural tunability. The introduction of functional groups onto the frameworks and guest molecules into the pore spaces of MOFs allows the formation of hydrogen-bonding networks, facilitating proton conduction over a wide humidity range. Furthermore, the crystalline nature of MOFs enables the precise investigation of the hydrogen-bonding networks and the relevant proton conduction mechanism. However, it is still unclear about how the functional groups and non-aqueous proton carriers would simultaneously affect the formation of hydrogen-bonding networks within MOFs. In this regard, flexible MOFs which present a reversible dynamic structural transformation in response to external stimuli can be a suitable host material to unravel the aforementioned issues because of their potential to control the proton transfer channels. In this study, we designed a series of imidazole molecule-loaded flexible MOFs (Im@MIL-88B) to modify the hydrogen-bonding networks and the resulting proton conduction characteristics by controlling the breathing behavior. Imidazole molecules were selected as a proton carrier which possibly leads to a full breathing on the flexible framework through the strong host-guest interaction. The breathing behaviors of MIL-88B are tuned by varying the concentration of adsorbed imidazole molecules-Im@MIL-88B-SB (small breathing) and Im@MIL-88B-LB (large breathing)-and introducing functional groups onto MOF ligands-Im@MIL-88B-NH2 and Im@MIL-88B-SO3H, also investigating the pristine MIL-88B (no breathing) without the imidazole loading as a control. Im@MIL-88B-LB not only shows a larger breathing effect, but also accommodates a larger amount of encapsulated imidazole into the pore spaces. The proton conductivity of 8.93×10-2 S cm-1 at RH 95% and 60°C for Im@MIL-88B-LB is achieved, which is the highest among N-heterocyclic molecules-loaded proton conducting MOFs reported so far to the best of our knowledge, in spite of mild operating condition. The computation results suggest that the breathing effect induces the pore opening in the frameworks and the mixed configuration of water and imidazole within the transfer channels, contributing to the formation of stable and continuous hydrogen-bonding networks. Interestingly, Im@MIL-88B-LB exhibits the higher proton conductivity than even functionalized Im@MIL-88B, which may be attributed to the high concentration of proton carriers through its large imidazole-dependent structural transformation and self-adaption, while the functional groups in the pores disturb such transformation Although the functional groups in the channels can facilitate the proton conduction, such effects are offset by the limited breathing behavior, suggesting that dense hydrogen-bonding network of proton carrier is a priority factor in proton conduction. Our investigation provides the promise about the design of hydrogen-bonding networks in proton carrier-loaded systems for facile proton conduction.