(Invited) Zinc-Deposited Aluminum Electrode Prepared Using Spontaneous Deposition Method for Use in Aqueous Hybrid Capacitors

Abstract

Aqueous electrolytes have advantages compared to nonaqueous electrolytes, such as high ionic conductivity, nonflammability, low environmental load, low cost, and so on. However, the maximum operating voltages of electric double-layer capacitors (EDLCs) with aqueous electrolytes are much lower than those with nonaqueous electrolytes, which can operate at voltages over 2.5 V, because the former are limited to 1.23 V, the theoretical decomposition voltage of water, leading to low energy and power densities. So, a key to improve energy and power densities is to increase the maximum operating voltage. Hybrid capacitors (HCs), which consists of activated carbon (AC) and redox electrodes, can greatly improve the energy and power densities of EDLCs because of widened potential windows. As for aqueous HCs, metal hydroxides/oxides as a redox positive electrode have realized the maximum operating voltages of 1.4–2.0 V because they have higher potential limits and higher oxygen overpotentials than AC,1-3 leading to higher energy and power densities. In the same manner, we reported that an HC with a Zn negative electrode whose hydrogen overpotential was larger than AC also increased the maximum operating voltage due to the extension of the negative potential limits, leading to low energy and power densities.4 A copper sheet was used as a substrate for zinc electrodeposition in the previous study.4 Aluminum is a light metal and cheap. Since the standard potential of Al is more negative than that of Zn, Zn can be spontaneously deposited on an Al sheet only by immersing it in a zincate solution. In this study, we prepared Zn-deposited Al sheet negative electrodes and evaluated their electrochemical properties and capacitor performance of a HC with the negative electrode.A polished Al sheet was immersed in a zincate solution to deposit Zn metal spontaneously. After the deposition, the mirror image of the Al sheet was cloudy due to zinc deposits. EDX and XPS analyses showed that Zn deposits covered almost the whole Al surface, indicating that spontaneous deposition of Zn occurred as follows.2Al + 3Zn(OH)4 2- → 3Zn + 4OH- + 2Al(OH)4 - Cyclic voltammograms of a Zn/Al electrode were recorded in 7 M KOH aqueous solution containing 0.6 M ZnO. A reduction peak due to Zn deposition and an oxidation peak due to Zn dissolution were observed in cyclic voltammograms, which was similar to a cyclic voltammogram of a Cu electrode in the same electrolyte.An aqueous HC cell with a Zn/Al negative electrode and AC positive electrode was assembled, and its capacitor performance will be reported. References V. Khomenko, E. Raymundo-Piñero, F. Béguin, J. Power Sources, 153, 183 (2006).T. Cottineau, M. Toupin, T. Brousse, D. Bélanger, Appl. Phys. A: Mater. Sci. Process., 82, 599 (2006).H. Inoue, Y. Namba, E. Higuchi, J. Power Sources, 195, 6239 (2010).H. Inoue, T. Morimoto, S. Nohara, Electrochem. Solid-State Lett., 10, A261 (2007).