An aluminum – ionic liquid interface sustaining a durable Al-air battery
Graphical abstract
Introduction
Whereas Li-air technology is currently being considered as the cutting-edge technology for future batteries, it holds many concerns and challenges. Some of them include poor cycling, low power capabilities, high cost and safety concerns [1], [2], [3], [4], [5], [6]. In general, to be commercially-viable, batteries must be of high energy density and affordable. The abundance and low cost of the electro-active materials are indeed important factors. However, high battery safety is a sine qua non condition, and therefore, poses fundamental design limitations. Aluminum (Al)-based battery technologies are attractive alternatives to lithium-based battery chemistries since Al has a high volume charge capacity, (8.0 Ah·cm−3 vs. 2.06 Ah·cm−3 for Li) and is comparable with Li in terms of gravimetric charge capacity (3.0 Ah·g−1 vs. 3.9 Ah·g−1) [7], [8], [9], [10]. In addition, Al is the most abundant metal in the earth's crust, and its cost is significantly lower than that of lithium [9], [10], [11]. Above all, Al-containing batteries are inherently safer than Li-containing cells [6], [7], [8], [9].
Al-air batteries are known for utilizing aqueous alkaline solutions as electrolytes [8], [11], [12], [13], [14]. The main advantage of this specific electrolyte are high ionic conductivity and its ability to efficiently reduce oxygen into hydroxide anions [8], [12]. Nevertheless, such battery systems suffer from major drawbacks, e.g., comparably low voltage and specific energy, and extremely high corrosion rates under open circuit condition [9], [11], [13], [15]. Thus, new approaches should be developed in order to overcome the main challenges to the Al-air system in alkaline. One of those possible approaches is altering the electrolyte from an aqueous one to non-aqueous. The electrolytes of choice in the case of Al, are Room Temperature Ionic Liquids (RTIL), since “simple” organic solvents that used frequently in Li battery chemistries [16], [17], unable to activate the Al surface [9], [13], [18]. Thus, those organic solvents are incapable of removing or altering the native passivation layer on the Al surface, and hence, are not suited for utilization in Al-based power sources [9], [13], [18]. Other metal-air batteries that applied Room Temperature Ionic Liquids (RTIL) as an electrolyte have been reported. Among those, one could find Na-air [1], [19], [20] and Si -air [21], [22], [23], [24] battery systems.
RTILs offer unique properties while serving as electrolytic media. The fascinating features include low vapor pressure (even at considerably elevated temperatures), high intrinsic ionic conductivity and non-flammability, thermal and chemical stability with considerably large electrochemical windows [1], [13], [25], [26], [27], [28]. Despite their appealing properties, most of the RTIL groups are unable to provide the sufficient surface activation required to produce efficient Al/RTIL based batteries [9], [13], [18].
In our previous report [9], a new Al-air battery was presented, applying 1-ethyl-3-methylimidazolium oligofluorohydrogenate (EMIm(HF)2.3F) RTIL. This electrolyte enables the conditions necessary to fabricate a durable battery, sustaining two different surface conditions upon a contact between the anode and electrolyte: (1) the activation of the anode surface by the electrolyte – namely removing the oxide layer and allowing an active metal dissolution (discharge); and (2) relatively low corrosion rates (orders of magnitude lower than the discharge) in order to enable an efficient battery [13] (Scheme 1).
In the current work, an in-depth study of the RTIL based Al-air power source is presented. Here, we present and discuss experimental data gathered from combined surface characterization tools along with electrochemical experiments, both in half cell and also full battery discharge, with a new approach to EIS data analysis based on a genetic programing [29], [30], [31], [32], [33]; herein, an advanced progress in the study of this unique battery is shown.
Section snippets
Materials
1-ethyl-3-methyl-imidazolium oligofluorohydrogenate, EMIm(HF)2.3F, (Boulder Ionics, Inc.) was used as the electrolyte in all the presented experiments, without any further purification. Al foil, (0.25 mm thick 99.997%, Alfa Aesar) was utilized as the anode while porous carbon-based air electrode (Electric Fuel, Inc.) was used as a cathode in all the experiments. The air electrode had a surface area of 533 m2 g−1, an average pore diameter of 5.43 nm and a carbon loading of 19 mg cm−2. The air
Al anode behavior at open circuit conditions
In order to evaluate the stability of the Al potential and the related corrosion reaction with EMIm(HF)2.3F RTIL, Open Circuit Potential (OCP) and Linear Polarization Resistance (LPR), measurements as a function of time were conducted. The LPR method is an effective electrochemical tool for measuring corrosion. Monitoring the relationship between electrochemical potential and current generated between electrically charged electrodes in a process stream allows the calculation of the corrosion
Conclusions
This work describes the studies and development of a unique non-aqueous primary aluminum-air battery. This system is comprised of a pure aluminum foil as the anode and oligofluorohydrogenate room temperature ionic liquid as the electrolyte. A low self-corrosion (self-discharge) currents of up to ∼ 40 μA cm−2 were detected; these currents stabilize overtime. The activated aluminum anode potential is set on a value of −1.15 V vs. Fc/Fc+ reference electrode (RE), as it negatively shifts by
Acknowledgment
Support for this work was provided by the Grand Technion Energy Program (GTEP), the Leona & Harry B. Helmsley Charitable Trust and the 2nd Israel National Research Center for Electrochemical Propulsion (INREP 2). The authors thank Dr. Neta Shomrat for the Graphical Abstract preparation.
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Both authors contribute equally to this work.