All-Solid-State Thin-Film Electrochemical Capacitors Using Inorganic Nanosheets Electrolytes

Introduction

   These days, demands of safe, inexpensive and large-capacity energy storage devices are increasing for various applications as power sources of electrical vehicles. All-solid-state thin-film electrochemical capacitors using protons as carriers can meet such demands. However, solid-state electrolytes have large electrolyte resistance and interfacial resistance between electrolytes and electrodes, which cause depression of powers and energy densities of electrochemical capacitors. In this study, we focused on extremely thin nanosheets, which are plate-like particles with thicknesses of only a few nanometers. Electrolyte resistance was reduced by applying thin-films of stacked nanosheets to electrolytes. Also, all-solid-state thin-film electrochemical capacitors were fabricated, and evaluated by electrochemical measurements. As the proton conducting nanosheet, we selected a-zirconium hydrogenphosphate monohydrate (α-Zr(HPO₄)₂·H₂O : ZrP), composed of naturally abundant elements.

Experimental

   ZrP powders were prepared from zirconium oxide hydrate (ZrO₂·nH₂O) powders through treating with phosphoric acid by hydrothermal synthesis method. A colloidal suspension of ZrP nanosheets was obtained by exfoliation of ZrP powders induced by shaking with aqueous solution of tetrabutylammonium hydroxide. ZrP films were prepared by applying concentrated colloidal suspension of ZrP nanosheets on substrates and drying them in air. Protonic conductivities of ZrP films were measured by alternating-current impedance measurement in the frequency range from 1 MHz to 100 Hz with an applied voltage of ±100 mV.

   All-solid-state thin-film electrochemical capacitors were fabricated with ruthenium oxide hydrate (RuO₂·nH₂O : RuO₂) as both electrodes and ZrP films as electrolytes. Galvanostatic charge/discharge tests were carried out in the cell voltage range of 0 – 1 V at a constant current density of 100 mA g⁻¹.

Results & Discussions

   Fig. 1 shows protonic conductivities of the ZrP film at relative humidity (R. H.) 90% and those of the ZrP tablet at saturated vapor ^[1]. The thickness of the ZrP film was 700 nm. Protonic conductivity of the ZrP film increased as with increasing temperature, and reached 3.2×10⁻⁶ Scm⁻¹ at 80^oC. The activation energy of protonic conduction in the ZrP film was 0.28 eV. This value is close to the activation energy of the ZrP tablet (0.25 eV) ^[1], and this suggests that the mechanism of protonic conduction in ZrP films is the same to that in ZrP tablets. The thickness of the ZrP film was over one thousand times smaller than that of the ZrP tablet (1 mm), so the areal electrolyte resistance could be reduced from 1.0×10⁵ Ωcm² (ZrP tablet at 27^oC, R. H. 90%) ^[2] to 1.0×10² Ωcm² (ZrP film at 30^oC, R. H. 90%).

   Fig. 2 shows the charge and discharge curves of the all-solid-state thin-film electrochemical capacitor (RuO₂|ZrP-film|RuO₂) measured at a constant current density of 100 mA g⁻¹. No recognizable plateau regions are observed in either the charge or the discharge curves. The electrochemical capacitor exhibited a reversible capacity of 22 mAh g⁻¹ at the 10th cycle. This capacity is much higher than that of many other proton conducting all-solid-state capacitors ^{[3, 4]}, and is comparable to those of electrochemical capacitors assembled with liquid electrolytes. In addition, the cycle characteristic was good and coulombic efficiency was high (h = 93%), suggesting the influences of side reactions to be small.

References

[1] E. K. Andersen et al., Solid State Ionics 7, 301 (1982).

[2] S. De et al., Solid State Communications 134, 553 (2005).

[3] M. J. Lee et al., J. Electroceram. 17, 639 (2006).

[4] Sellam et al., Appl. Mater. Interfaces 5, 3875 (2013).