A [V6B20]-based architecture incorporating two types of organic amine ligands: innovative additives for enhancing the proton conductivity of Nafion membrane

Abstract

We have successfully prepared a three-dimensional vanadoborate (VBO) with a lantern-type [V₆B₂₀] cluster by solvothermal method with the molecular formula of [EtNH₃]₂[1,3-diapH₂]₂H₂[V₆B₂₀O₅₀H₈]·4H₂O (EtNH₂ = ethylamine; 1,3-diap = 1,3-diaminopropane) (VB-1). The title complex represents a novel example of a VBO introducing two different organic amines (EtNH₂ and 1,3-diap) into the framework, which interact with the host framework and guest water molecules via N–H⋯O, O–H⋯O bonds, constructing a complex hydrogen-bonding network system for proton transport. In this study, the obtained VB-1 exhibits structural insolubility and is employed as a proton-conductive additive to modify the Nafion matrix, enabling the preparation of a thermally and aqueously stable VB-1/Nafion composite at intermediate temperatures. The CV and AC impedance studies reveal a significant enhancement in the proton conductivity of Nafion doped with VB-1 compared to pure Nafion. Remarkably, the modified membrane displays a remarkable 17.8- and 11.9-fold enhancement in proton conductivity compared to the pristine membrane at temperatures of 293 K and 343 K, respectively, surpassing the vast majority of previously reported Nafion-based composite membranes that were modified with MOFs and POMs under identical testing conditions. This improvement can be attributed primarily to the reduction in ohmic resistance caused by the hygroscopic and insoluble VBO solid, which effectively retains water within its framework through hydrogen bonding with protons at elevated temperatures, thereby facilitating efficient proton transport across the membrane. The fuel cell performance of the MEA based on the VB-1/Nafion composite membrane exhibits a significant enhancement, resulting in a 28.3% increase in peak power density compared to pure Nafion at 80 °C under 100% RH. Our study represents pioneering utilization of VBOs as fillers for proton exchange membranes (PEMs), emphasizing the imperative need for further research endeavors in this field. These findings underscore the potential application of VBOs in technologically relevant embodiments for long-distance proton conductivity through composite membranes and demonstrate their promising prospects in PEM applications.