Mar 17, 2020

World's first discovery of a superionic conductor exhibiting photoswitching effect

 

Key points of the work

  • We discovered for the first time a superionic conductor exhibiting photoswitching effect.
  • The material is a superionic conductor exhibiting second harmonic generation due to the coexistence of superionic conductivity and polar crystal structure.
  • Superionic conductors are used as solid electrolytes in solid-state batteries. By taking advantage of the photoswitchable ionic conductivity of this material, turning on and off the batteries by light irradiation may become possible in the future.

 

Overview of the work

The research group of Prof. Shin-ichi Ohkoshi et al. at the University of Tokyo discovered a superionic conducting polar crystal that exhibits photoswitching effect. This crystal has a polar crystal structure [1] consisting of a three-dimensional iron-molybdenum cyanido-bridged framework containing cesium cations. This material shows a high ionic conductivity of 4 × 10−3 S cm−1 at 318 K (45 ° C) which is classified as a superionic conductor [2] . Irradiation with 532-nm light at room temperature changes the ionic conductivity from 1×10−3 S cm−1 to 6×10−5 S cm−1, exhibiting a photo-switching effect on the ionic conductivity. Furthermore, this material possesses spontaneous electric polarization [3] and shows second harmonic generation (SHG) [4] . There had been no example of a photoresponsive and SHG active superionic conductor, and such a material could lead proposal of new functional electrolytes in fuel cells. By taking advantage of the photoswitchable ionic conductivity of this material, turning on and off the batteries by light irradiation may become possible in the future.

The findings of this research published online in Nature Chemistry on March 17th, 2020.

 

Details of the work

Ionic conductors are used in various applications such as fuel cells, lithium-ion batteries, and chemical sensors. Ionic conducting materials with conductivities above 10−4 S cm−1 are called superionic conductors. In the present work, we developed a superionic conductive polar crystal that exhibits a photoswitching effect. The composition of this blue-colored crystal is Cs1.1Fe0.95[Mo(CN)5(NO)]·4H2O. This material consists of a three-dimensional iron-molybdenum cyanido-bridged framework containing cesium cations (Fig. 1a, b). Structural analysis indicates that the compound has spontaneous electric polarization due to the difference between the center of the positively charged cesium ion and the negatively charged iron-molybdenum cyanido-bridged framework (Fig. 1c). Furthermore, a one-dimensional hydrogen bond network is formed by the oxygen atom of the nitrosyl group (–N≡O) and the water molecules in the structure (Fig. 1d).

Figure 1 : (a) Crystal structure of a three-dimensional network of the developed iron–molybdenum cyanido-bridged framework containing cesium ions. (b) Photograph of the sample. (c) The arraangement of cesium ions in the iron–molybdenum cyanido-bridged framework. Spontaneous electric polarization exists along the direction indicated by arrows. (d) Proton conduction channel through one-dimensional hydrogen bonding network.

 

From the ionic conductivity measurement, the present material exhibited a remarkably high conductivity value of 4.4 × 10−3 S cm−1 at 45 °C in 100% relative humidity (RH), for which the compound is classified as a superionic conductor. This superionic conductivity is caused by the bucket-relay-like movement of the protons, where the protons are carried through the hydrogen bond network formed by the nitrosyl ligands and the water molecules.

Furthermore, we investigated the photoswitching property because this compound has a nitrosyl group which is expected to show photoresponsivity. Upon irradiation of 532-nm light to the sample inside a humidity-controlled container, the ionic conductivity changed from 1.3×10−3 to 6.3×10−5 S cm−1 (Fig. 2).

Figure 2 : Photoswitching of ionic conductivity (σ). Upper left part shows schematic of photo irradiation experiment. The lower graphs show Cole-Cole plots[7] measured before light irradiation (grey, left graph) and immediately after 532 nm light irradiation (red, right graph). The ionic conductivity returns to the value before light irradiation with time (blue, left graph). The blue and red semicircles are the fitting curves. The upper right part shows the repeatability of photo-switching of ionic conductivity.

 

After stopping the irradiation, the ionic conductivity value recovered to the original value over time. Observation of such a photoswitching phenomenon on a superionic conductor is the first example in the world. This photoswitching effect originates from the reversible change of the Mo0–N≡O bond angle by photo-irradiation due to the photo-isomerization of the NO ligand[5]. The change in the bond angle causes a breaking of the hydrogen bond between the NO ligand and the H2O molecule, resulting in a decrease of the proton conductivity.

In addition, this material exhibits both superionic conductivity and polar crystal structure that normally do not coexist. Such a coexistence in a single material is unique from the viewpoint of electrical resistance because polar crystals such as ferroelectrics and pyroelectrics[6] are usually classified as dielectrics with electric polarization (conductivity with less than 10−8 S cm−1). Therefore, we examined second harmonic generation (SHG), one of the second-order nonlinear optical effects. When the sample was irradiated by a 1040-nm laser light, a half-wavelength 520-nm light was generated due to SHG (Fig. 3). SHG was also observed from individual particles using SHG microscopy.

Figure 3 : (a) Schematic of measurement of second harmonic generation (SHG), which is one of the second order nonlinear optical effect. (b) The incident light intensity dependence of SH light intensity. The solid line is the result of fitting with a quadratic function of the incident light intensity. The inset is a log–log plot, and the solid line is the fitting with a straight line with a slope of 2. The fitting results indicate the emission of SHG. (c) SHG emission observed by an SHG microscope.

 

By taking advantage of the photoswitchable ionic conductivity of this material, turning on and off the batteries by light irradiation may become possible in the future.

The present research was supported in part by JSPS Grant-in-Aid for Specially Promoted Research (grant 15H05697).

 

Publication journal

Journal

Nature Chemistry

Title A photoswitchable polar crystal that exhibits superionic conduction (Cover picture on the April issue)
Authors Shin-ichi Ohkoshi*, Kosuke Nakagawa, Kenta Imoto, Hiroko Tokoro, Yuya Shibata, Kohei Okamoto, Yasuto Miyamoto, Masaya Komine, Marie Yoshikiyo, Asuka Namai
DOI No 10.1038/s41557-020-0427-2

 

Glossary

[1]  Polar crystal

A crystal possessing spontaneous electric polarization without an external electric field. Such a material is also called pyroelectric.

[2]  Superionic conductor

In ionic conductors, ions transport the electricity. Among ionic conductors, those exhibiting a conductivity above 10−4 S cm−1 are called superionic conductor. Conductivities above 10−4 S cm−1 are comparable to that of an aqueous electrolyte solution.

[3]  Spontaneous polarization

In a polar crystal, the center of the positive charge and that of the negative charge are shifted, and electric polarization exists without an external electric field. This electric polarization without an external electric field is called spontaneous polarization.

[4] Second harmonic generation (SHG)

Second harmonic generation is a phenomenon in which two photons interacting with a certain material are combined to form new photons with half the wavelength.

[5]  Photoisomerization

A structural arrangement (arrangement of connections among atoms) maintaining the number of atoms is called isomerization. Photoisomerization is a form of isomerization induced by photoexcitation.

[6]  Ferroelectrics and pyroelectrics

Polar crystals are also called pyroelectrics. Among them, ferroelectrics are substances whose direction of the spontaneous polarization can be altered by reversing the polarity of the external voltage.

[7]  Cole–Cole plot

Impedance (Z) measured at various frequencies is plotted on a complex plane, with the horizontal axis plotting the real part (Z') and the vertical axis plotting the imaginary part (Z"). If the sample contains a capacitor component, the plot draws a semicircle, and the crossing point with the horizontal axis corresponds to the resistance value, which is the inverse of conductivity.

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