Abstract
The voltage of an alkaline electrolyte-based battery is often limited by the narrow electrochemical stability window of water (1.23 V). As an alternative to water, ionic liquid (IL)-based electrolyte has been shown to exhibit excellent proton conducting properties and a wide electrochemical stability window, and can be used in proton conducting batteries. In this study, we used IL/acid mixtures to replace the 30 wt % KOH aqueous electrolyte in nickel/metal hydride (Ni/MH) batteries, and verified the proton conducting character of these mixtures through electrochemical charge/discharge experiments. Dilution of ILs with acetic acid was found to effectively increase proton conductivity. By using 2 M acetic acid in 1-ethyl-3-methylimidazolium acetate, stable charge/discharge characteristics were obtained, including low charge/discharge overpotentials, a discharge voltage plateau at ~1.2 V, a specific capacity of 161.9 mAh·g−1, and a stable cycling performance for an AB5 metal hydride anode with a (Ni,Co,Zn)(OH)2 cathode.
Highlights
Aqueous-based electrolyte batteries possess apparent advantages over carbonate-based electrolyteLi-ion batteries in terms of safety and cost, despite their relatively lower energy densities
The Walden rule has been widely used in ionic liquid (IL) studies to explain the conductivity–viscosity relationship and to estimate the extent of ion association [30,31,32,33]
IL/acid mixtures were applied in nickel/metal hydride (Ni/MH) batteries to replace the 30 wt% KOH aqueous electrolyte, and their proton conducting character was verified through electrochemical charge/discharge tests
Summary
Aqueous-based electrolyte batteries possess apparent advantages over carbonate-based electrolyteLi-ion batteries in terms of safety and cost, despite their relatively lower energy densities. The open circuit voltage of an aqueous electrolyte-based battery is intrinsically limited by the narrow electrochemical stability window of water (1.23 V), which restricts the selection to electrodes with higher standard potentials (more positive or negative) and presents an obstacle in the improvement of energy density. As an alternative to water and flammable non-aqueous electrolytes (e.g., carbonate, acetonitrile), ionic liquids (ILs) exhibit unique and tunable physicochemical properties, including a wide electrochemical stability window, good ionic conductivity, a wide liquidus range, negligible vapor pressure, good thermal and chemical stability, inflammability, and non-toxicity, all of which have made ILs ideal candidates for many electrochemical applications, such as batteries [1,2,3,4,5,6], fuel cells [7,8,9,10,11,12], supercapacitors [13,14,15,16] and dye-sensitized solar cells [17,18,19,20]. Our recent study on proton-conducting batteries showed a discharge capacity of 3635 mAh·g−1 for a hydrogenated amorphous silicon thin film anode using
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