Abstract
We report the structure and charge transport properties of a novel solid-state proton conductor obtained by acid–base chemistry via proton transfer from 12-tungstophosphoric acid to imidazole. The resulting material (henceforth named Imid3WP) is a solid salt hydrate that, at room temperature, includes four water molecules per structural unit. To our knowledge, this is the first attempt to tune the properties of a heteropolyacid-based solid-state proton conductor by means of a mixture of water and imidazole, interpolating between water-based and ionic liquid-based proton conductors of high thermal and electrochemical stability. The proton conductivity of Imid3WP·4H2O measured at truly anhydrous conditions reads 0.8 × 10–6 S cm–1 at 322 K, which is higher than the conductivity reported for any other related salt hydrate, despite the lower hydration. In the pseudoanhydrous state, that is, for Imid3WP·2H2O, the proton conductivity is still remarkable and, judging from the low activation energy (Ea = 0.26 eV), attributed to structural diffusion of protons. From complementary X-ray diffraction data, vibrational spectroscopy, and solid-state NMR experiments, the local structure of this salt hydrate was resolved, with imidazolium cations preferably orienting flat on the surface of the tungstophosphate anions, thus achieving a densely packed solid material, and water molecules of hydration that establish extremely strong hydrogen bonds. Computational results confirm these structural details and also evidence that the path of lowest energy for the proton transfer involves primarily imidazole and water molecules, while the proximate Keggin anion contributes with reducing the energy barrier for this particular pathway.
Highlights
Achieving high proton conductivities in the solid state is one of the biggest challenges in materials science.[1,2] In this context, heteropolyacids have attracted considerable attention because of their unique structure and the record proton conductivity among solids, that is, up to 0.18 S cm−1 at room temperature for the 12-tungstophosphoric acid hydrate.[3]
The strongest among solid acids is 12-tungstophosphoric acid, which adopts the so-called Keggin structure with one central P atom surrounded by four O atoms, that are connected to 12 corner shared WO3 units, resulting in the molecular formula H3[P(W3O10)4] · nH2O, see Figure 1
The material that we report is a salt consisting in Keggin anions (WP3−) and imidazolium cations (Imid+); it has never been reported before and is a material that bridges the gap between proton conducting solid acids and ion conducting salt hydrates
Summary
Achieving high proton conductivities in the solid state is one of the biggest challenges in materials science.[1,2] In this context, heteropolyacids have attracted considerable attention because of their unique structure and the record proton conductivity among solids, that is, up to 0.18 S cm−1 at room temperature for the 12-tungstophosphoric acid hydrate.[3] Since a number of solid electrolytes based on heteropolyacids have been reported, which have good potential for use as solid catalysts and in different electrochemical devices including the intermediate temperature H2/O2 fuel cell. The precise location of the acidic protons (H+) on the surface of the Keggin anion in the anhydrous state has been a subject of debate, it seems that terminal (Ot) and bridging (Ob) oxygens are the preferred sites.[4] By contrast, in the hydrated state, protons are located in Received: June 30, 2021 Published: August 18, 2021
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