Proton conduction in alkali metal ion-exchanged porous ionic crystals

Abstract

Proton conduction in alkali metal ion-exchanged porous ionic crystals A[CrO(OOCH)(etpy)][α-SiWO]·nHO [I-A] (A = Li, Na, K, Cs, etpy = 4-ethylpyridine) is investigated. Single crystal and powder X-ray diffraction measurements show that I-A possesses analogous one-dimensional channels where alkali metal ions (A) and water of crystallization exist. Impedance spectroscopy and water diffusion measurements of I-A show that proton conductivities are low (10-10 S cm) under low relative humidity (RH), and protons mostly migrate as HO with HO as vehicles (vehicle mechanism). The proton conductivity of I-A increases with the increase in RH and is largely dependent on the types of alkali metal ions. I-Li shows a high proton conductivity of 1.9 × 10 S cm (323 K) and a low activation energy of 0.23 eV under RH 95%. Under high RH, alkali metal ions with high ionic potentials (e.g., Li) form a dense and extensive hydrogen-bonding network of water molecules with mobile protons at the periphery, which leads to high proton conductivities and low activation energies via rearrangement of the hydrogen-bonding network (Grotthuss mechanism).

Confinement of poly(allylamine) in Preyssler-type polyoxometalate and potassium ion framework for enhanced proton conductivity

“…On the other hand, the band extended toward the higher frequency (wavenumber) (3500-3600 cm −1 ) for I and II, and also toward the lower frequency (3200 cm −1 ) for I. Previous reports by us and other groups have shown that the bands at high (3500-3600 cm −1 ) and low (3200-3400 cm −1 ) frequencies can be attributed to unperturbed (free) water molecules at the outside of a water cluster and to perturbed hydrogen-bonded water molecules inside the cluster, respectively 41,42 . Therefore, it can be reasonably stated that I possesses an extensive hydrogen-bonding network of water molecules and PAA, which agrees with the relatively large number of the water of crystallization (19 (I) > 16 (II), 15 (III)), and free water molecules (protons) at the periphery of the hydrogen-bonding network.…”

Effect of the ammonium ion on proton conduction in porous ionic crystals based on Keggin-type silicododecatungstate

“…2(c), the NH 4 + cations and the water molecules were in hydrogen-bonding distances with respect to each other [NÁ Á ÁOW = 2.62 (5) Å and OWÁ Á ÁOW = 2.76 (4)-3.24 (5) Å , where OW is the O atom of a water of crystallization]. The distances were shorter than those in I-Cs + [CsÁ Á ÁOW = 2.91 (2)-3.45 (4) Å and OWÁ Á ÁOW = 2.92 (3)-3.14 (4) Å ; Uchida et al, 2017], suggesting that the dense hydrogen-bonding network in I-NH 4 + would contribute to efficient proton conduction (see below). Nyquist plots of the impedance spectra of a pelleted sample of I-NH 4 + were measured at 303-363 K and RH95% (Fig.…”

“…SXRD analysis of I-NH 4 + showed that one-dimensional channels with a minimum aperture of 3.5 Å , which are analogous to those of I-A + (Uchida et al, 2017), existed along the c axis (Fig. 2).…”

“…We have been working on proton conduction in ionic crystals based on POMs (Tsuboi et al, 2016;Niinomi et al, 2017;Uchida et al, 2017). In one of our previous works, a series of alkali metal ion-exchanged porous ionic crystals based on Keggin-type silicododecatungstate, viz.…”

“…In one of our previous works, a series of alkali metal ion-exchanged porous ionic crystals based on Keggin-type silicododecatungstate, viz. A 2 [Cr 3 O-(OOCH) 6 (etpy) 3 ] 2 [-SiW 12 O 40 ]ÁnH 2 O (A = Li, Na, K and Cs; etpy = 4-ethylpyridine) (I-A + ) were synthesized and it was demonstrated that I-A + with alkali metal ions of high ionic potentials (z/r, where r is the ionic radius and z is the charge) form dense and extensive hydrogen-bonding networks of water molecules, which leads to high proton conductivities and low activation energies (Uchida et al, 2017).…”