Bi-Functional Oxygen Electrodes for a Rechargeable Zinc/Air Battery with Choline Acetate Electrolyte

Abstract

The primary alkaline zinc/air battery is known for its high energy density and is commonly used as energy supply for hearing aids devices with practical energy density values in the range of 700-850 mWh gzinc -1. However, recharging process is very challenging because of its low energy efficiency (<60%) due to high overpotential and poor cycling ability of the air electrode as well as dendrite and carbonate formation in alkaline solution. To overcome some drawbacks, alternative electrolytes such as ionic liquids (IL´s) are promising candidates.1 Room Temperature Ionic liquids (RTILs) are low temperature (<100°C) molten salts composed of organic cations and anions characterized by extremely low vapor pressure, high decomposition temperature, but also by high viscosity and poor/moderate conductivity. For electrochemical applications, and more especially for metal/air battery in which oxygen reactions occur, presence of proton species in ionic liquid is preferred since aprotic ones allow “only” 1e- oxygen reduction reaction (ORR) step.2 High surface tension is also a prerequisite for triple-phase boundary formation in the gas diffusion electrode (GDE). In this work, feasibility of choline acetate (ChAcO) with zinc salt as electrolyte mixture and AB2O4structured materials as bifunctional cathode catalyst was investigated. In this work, precious metal and spinel oxide bifunctional catalyst were prepared. 20 wt% Pt was deposited on commercial carbon (C65) by chemical reduction of hexachloro-platinic acid in formaldehyde at 80°C for 1 h. MnCo2O4 (MCO) and NiCo2O4 (NCO) Spinel were synthesized by hydrothermal (HT) and hard template (SS) methods. The structural characterization of the as-prepared catalysts was performed by means of XRD, SEM/EDX and TEM. Average size of as-prepared oxide particles was estimated to approximately 28, 100, 300 and 500 nm for HT-MCO, HT-NCO, SS-MCO and SS-NCO catalysts, respectively. Gas diffusion electrodes (GDE) with catalyst loading of 1 mgcat cm-2 were fabricated by airbrush spraying. Contact angle of electrode/electrolyte interface was measured for 100 h at room temperature and 30% RH. The electrochemical activity of commercial Pt/C and spinel-based GDE for ORR and OER were studied by cyclic voltammetry (CV) under half cell condition. The EL-Cell (ECC-Air) and coin cell (CR2032) were manufactured by assembling a Zn foil as active electrode, a glass fibre separator and a GDE. The cell performance was evaluated by U-I polarization at 1-100 µA cm-2 for 1-3 h charge/discharge cycle. A preliminary screening of catalysts in terms of activity and stability was conducted in EL-Cell with dry air at RT. At 50 µA cm-2, comparison of charge/discharge kinetics of the different catalysts over 5 h/cycle is shown in fig.1a. An effect of addition of water in the IL was studied at 100 µA cm-2 in ambient air (30% RH at 22°C), 24 h/cycle for one month (fig.1b). The best system was HT-MCO that was further investigated under coin cell (CR2032) condition (Zn || ChAcO + 30 wt% H2O + 0.01 M Zn AcO || MnCo2O4/C65). At 100 µA cm-2, an energy and columbic efficiency of 50 and 100% was calculated for the 3 h cycle, respectively. The same cell was successfully tested in ambient air (RT, 30% RH) for 700 h (140 cycles) at 50 µA cm-2. Average cell voltage was 0.75 V (discharge) and 1.9 V (charge). A discharge specific capacity of 725 mAh gZn -1 was achieved at 100 µA cm-2.