Fabrication and Evaluation of Quasi-All-Solid-State Electrochemical Capacitors with High Energy Densities

Introduction

All-solid-state electrochemical capacitors (EC) have attracted attention as next-generation energy storage devices because they are safe, thin and chemically-stable. However, all-solid-state EC have lower energy and power densities than EC with liquid electrolytes and they have not been brought to practical application. The low energy and power densities of all-solid-state EC are mainly due to large electrolyte resistance and interfacial resistance between electrolytes and electrodes.

Usage of a small amount of liquid electrolyte as contact agent is expected to reduce the interfacial resistance. In such “quasi-all-solid-state” EC, nanoporous-structure materials with large capacity can be used as electrodes. In this study, quasi-all-solid-state EC were fabricated and evaluated. As a solid electrolyte, sulfonated tetrafluorethylene copolymer (Nafion^®) was selected. As a cathode material, the layered ruthenium dioxide (RuO₂) was selected, which has a large capacity and high power density as an electrode of an aqueous capacitor ^[1]. As an anode material, the nanoporous composite of anthraquinone (AQ) and activated carbon was selected, which has both a large capacity and a low redox potential as an electrode of an aqueous capacitor ^[2].

Experimental

The composite of AQ and activated carbon (AQ/C), in which AQ (30 wt%) was supported in the micropores of activated carbon (70 wt%), was prepared following the method reported previously ^[2].The dispersion of the electrode materials was prepared by mixing AQ/C powders and Nafion ionomer in an ethanol solvent using a revolving/rotating mixer. The weight ratio of AQ/C to Nafion was 7:3. The AQ/C anode was prepared by casting the dispersion on a current collector and drying the solvent. RuO₂ was synthesized following the method reported previously ^[1]. The RuO₂ cathode was prepared in the same casting method. It consisted of RuO₂(55 wt%), carbon black (15 wt%) and Nafion (30 wt%).

Quasi-all-solid-state capacitors, (AQ/C | Nafion | RuO₂), were fabricated by sandwiching Nafion membrane between the electrodes and hot-pressing. The weight ratio of the AQ anode to the RuO₂ cathode was 2:1. The capacitors were immersed in 0.5 M H₂SO₄and they were washed by purified water. The galvanostatic charge/discharge test for the capacitor was carried out under air atmosphere in the cell voltage range of 0–1.2 V.

Results and Discussion

The AQ/C anode showed a discharge capacity of 72 mAh (g-AQ/C)⁻¹ by the galvanostatic charge/discharge test. A plateau was observed at about −0.2 V (vs. Ag/AgCl). Thus, the following reaction is suggested ^[2]:

                 C₁₄H₈O₂+2H⁺+2e⁻ ↔ C₁₄H₈(OH)₂                 (1)

The RuO₂ cathode showed a discharge capacity of 170 mAh (g-RuO₂)⁻¹. A plateau was observed at about 0.4 V (vs. Ag/AgCl). From these results, it was estimated that the ideal capacity of the capacitor was 43 mAh (g*)⁻¹. (The g* is the total weight of the AQ/C and RuO₂ in electrodes.)

Fig. 1 shows the galvanostatic charge/discharge curves of the fabricated capacitor at a current density of 500 mA g⁻¹. The discharge capacity was 38 mAh (g*)⁻¹ and the coulombic efficiency was 88% at the 1^st cycle. This capacity corresponds to as large as 88% of the ideal one. This large capacity suggests that most of AQ reacted with proton in the AQ/C anode. The resistance in the reaction of AQ was examined by AC impedance measurement and it was 30 W at 1 Hz, which is half of the value of the all-solid-state electrochemical capacitor using gel electrolyte and MnO₂ electrodes ^[3].When the capacitor was not immersed in H₂SO₄, AQ did not react due to the large interfacial resistance. Considering these results and the fact that AQ is supported in the micropores of activated carbon ^[2], the H₂SO₄ included in the micropores of AQ/C contributed to the small resistance. The estimated energy density of the capacitor was 23 Wh kg⁻¹. It is higher than those of other all-solid-state EC previously reported (e.g. 4.8 Wh kg^−1 [3]). In addition, the capacitor kept the advantages of all-solid-state EC such as no leakage of liquid electrolyte and thinness of the device (e.g. the total thickness of electrodes and electrolyte was about 200 mm). Although improvement of a cycle characteristic is needed, a quasi-all-solid-state electrochemical capacitor with high energy density was obtained.

References

[1] K. Fukuda et al., Inorg. Chem., 49, 4391 (2010).

[2] T. Tomai et al., Sci. Rep., 4, 3591, (2014).

[3] L. Yuan et al., ACS Nano., 6(1), 656, (2012).

Fig.1 Galvanostatic charge/discharge curves of the capacitor. The g* is the total weight of the AQ/C and RuO₂in electrodes.