Humidity based proton conductivity of calcium-l-tartrate tetrahydrate: An environmentally benign coordination polymer as a solid electrolyte

•Modified crystallization method for calcium tartrate has been developed.•The material exhibits proton conductivity in protic environments.•Conductivity of the dried sample is poor; but restored with protic vapors.•The structural topology is maintained under various conditions. Calcium tartrate, an environmentally benign coordination polymer, possessing the structural architectures as those of the other proton conducting metal-organic frameworks (MOFs) and coordination polymers (CPs), has been explored for its solid electrolyte characteristics. A simple and scalable method of crystallization, involving only a plain mixing of the inexpensive aqueous precursors, has been developed in the present work to facilitate the electrochemical characterizations of this compound. Structural authenticity and the bulk purity of the material, prepared according to the new route, are ascertained by examining the cell parameters from the single crystal X-ray diffraction analysis and also by the elemental analysis results, respectively. The structural integrity of the system, under different atmospheric conditions, has been confirmed using the FT-IR and PXRD methods. As anticipated, the material exhibits a significant proton conductivity of 3×10−5Scm−1 under 100% RH condition, which is quite comparable to those of the other proton electrolytes reported in the literature. The Ea value determined, suggests that the Grotthuss mechanism of proton conduction is obeyed during the process. The conductivity diminishes to several orders of magnitude under dry condition. However, upon re-exposing the material to the full humid or moist methanol vapors, the conductivity characteristics are restored. Nevertheless, such reversibility could not be observed under anhydrous methanol or under the hydrocarbon vapors. These results signify the crucial role of the water vapor and the protic molecules in the creation of H+ charges and the hydrogen bonded pathways in the system. As the calcium based proton conducting materials are quite scanty in the literature, the present paper, with the observation of humidity based proton conductivity in calcium tartrate, represents one of such rare reports and highlights the usefulness of this simple and environmentally non-hazardous framework for various technical applications.