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Energy, Environmental, and Catalysis Applications
Use of Polarization Curves and Impedance Analyses to Optimize the ‘Triple-Phase Boundary’ in K–O2 Batteries Gerald Gourdin, Neng Xiao, William D. McCulloch, and Yiying Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16321 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019
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ACS Applied Materials & Interfaces
Use of Polarization Curves and Impedance Analyses to Optimize the ‘Triple-Phase Boundary’ in K–O2 Batteries Gerald Gourdin, Neng Xiao, William McCulloch, and Yiying Wu* Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Ohio 43210, United States
ABSTRACT: K–O2 superoxide batteries have shown great potential for energy storage applications due to the unique single-electron redox processes in the oxygen or gas diffusion electrode. Optimization of the ‘Triple-Phase Boundary’—the region of the cathode where the O2, electrolyte and electrode surface are in immediate contact—is crucial for maximizing their power performance, but one that has not been explored. Herein we demonstrate an efficient method for maximizing the power capabilities of the K–O2 battery system by optimizing that interface using polarization and impedance analyses. At the one extreme, an electrolyte volume-deficient state decreases access to the electrochemically active surface area resulting in a limiting of the maximum power output of the K–O2 battery. Whereas an excess electrolyte volume state increases the diffusion path to the active surface area for the dissolved O2 inducing mass-transfer limitations sooner, which results in a decrease in the current and power output. Finally, we show that the optimal electrolyte volume closely matches the void volume of the internal cell materials
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(separators, cathode) resulting in a maximization of the electrochemically accessible surface area while minimizing the O2 diffusion path.
KEYWORDS: metal-oxygen battery; K–O2 battery; polarization; impedance; power; limiting current
INTRODUCTION Increased demand for high energy electrochemical storage has driven researchers to pursue alternatives to Li-ion batteries, long considered the metric for secondary batteries. Due to their higher theoretical energy density, metal-oxygen batteries have long been anticipated as the battery chemistry that eventually supersedes Li-ion batteries. The foremost among these, lithium-oxygen (Li–O2) batteries, have been extensively researched,1-5 but their technical hurdles—large charging overpotential, severe electrolyte/electrode decomposition, and lower round-trip efficiency—have so far inhibited their commercialization.6,7 However, K–O2 and Na–O2 batteries typically undergo just a one-electron reduction that forms a stable superoxide product through the reversible O2/O2− redox couple. This allows for much higher round-trip efficiencies (>90%), as have been demonstrated by our group and others.8-14 However, for Na−O2 batteries the discharge reactions may depend on the nature of the electrolyte, water content, or other cell conditions since Na2O2, corresponding to the two-electron-transfer process, has also been identified as a stable discharge product.15-18 In metal-oxygen batteries, as well as in fuel cells, the gas diffusion electrode (cathode) performs a vital function in regard to the device’s operation: it facilitates oxygen dissolution into the electrolyte where it can diffuse to the active carbon surface to undergo reduction. For that reason,
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ACS Applied Materials & Interfaces
it is essential to create gas-diffusion pathways that promote the efficient formation of O2-saturated electrolyte.19 The cathode material employed in metal-oxygen batteries and fuel cells has typically been composed of porous carbon materials, such as carbon fiber papers and similar materials, or films fabricated from carbon powders, nanotubes, reduced graphene oxide (RGO), etc.16,18,20-22 The physical region where the O2 atmosphere, liquid electrolyte and solid electrode surface are brought into close contact is consequently a crucial feature of the cathode and has been referred to as the ‘Triple-Phase Boundary.’19,23-27 For fuel cells28-30 and Li–O2 batteries31-36 this active area encapsulates the portion of the gas diffusion electrode containing the electrocatalyst necessary to facilitate the oxygen reduction reaction (ORR) or oxygen evolution reaction (OER), which is crucial for the efficient operation of those devices. However, neither Na–O2 nor K–O2 batteries typically require such catalysts8,16,20,37-39 and so the active surface area of the cathode is the entire fraction of the cathode’s surface area that is accessible by the electrolyte. The amount of current that potentially can be produced will be directly proportional to this electrochemically accessible, or EA, surface area,19,40 which is one of the two physical factors that limit the maximum current that can be produced by a battery. The second factor is the rate at which O2 can diffuse to the electrode surface (i.e. mass transfer limitations).17,27,41-43 Within the cathode reaction zone (Figure 1), the concentration of dissolved O2 at the electrode surface will be determined by the rate of O2 diffusion to that surface. The rate of diffusion is controlled by the concentration gradient, which arises from the solubility of oxygen and the length of the diffusion path. Therefore, the longer the diffusion distance, the slower the rate of O2 diffusion and the longer it takes to replenish any O2 consumed during reduction. Polarization, or current-potential, measurements are typically used to determine the effect of varying load levels on the performance of an energy producing device.27,42,44 This is an important
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metric in that it provides information on the peak performance capabilities of a battery under a range of loads representing operating conditions. For fuel cells, it additionally provides insight into the how the device’s performance capabilities diminish under mass-transfer limited (i.e. high current) conditions. While those same two metrics are equally important for metal-oxygen batteries, the use of polarization tests to evaluate a metal-oxygen battery’s performance under different conditions has long been overlooked. Since K–O2 batteries operate through a one-electron reduction of O2, K + + O 2 + e – KO 2 , to form thermodynamically stable KO28,13,20,45 this less complicated reaction chemistry makes the K–O2 battery’s use in the evaluation of polarization tests less involved. Electrochemical impedance spectroscopy (EIS) has also been employed in characterizing tortuosity in battery electrodes46 and optimizing electrode thickness in both batteries47 and fuel cells.48 However, unlike either Li-ion batteries or fuel cells, metal-O2 batteries form an insoluble product during discharge that accumulates in the gas-diffusion electrode. Therefore, capacity for the cathode is related to the electrochemically-accessible surface area and the available pore volume capable of hosting the discharge product. Optimization of the triplephase boundary is directly correlated with maximized electrochemically-accessible surface area. In consideration of that significant difference, it cannot be expected that the same optimization procedures that are performed for either batteries or fuel cells can be directly applied to metal–O2 batteries and achieve the same level of success. In the porous cathode, two non-optimal states can result in reduced performance. With insufficient electrolyte, the extent of the triple-phase boundary can be diminished whereas excess electrolyte can shift the O2/electrolyte interface away from the active electrode surface decreasing access to the surface by dissolved O2. As we will show here, an electrochemical impedance spectroscopy (EIS) analysis of the battery under varying electrolyte volume conditions provides
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ACS Applied Materials & Interfaces
insight into the extent of the EA surface area. This was accomplished through an analysis of the impedance data to determine the double-layer capacitance, which is proportional to the EA surface area. For each experiment, a K–O2 battery was assembled using different volumes of an electrolyte composed of the KPF6 dissolved in dimethoxyethane (DME). It should be noted that in research involving metal-O2 batteries, it has been shown that glyme-based electrolytes (DME, diglyme, tetraglyme) exhibit good stability in the presence of the superoxide anion, with DME demonstrating superior stability with the K metal due to its weaker coordinating ability.2,4,9 Polarization tests were then used to determine the effect that changes in the volume of electrolyte added during cell assembly have on performance of a K–O2 battery. An evaluation of the polarization data, together with the insight provided by the EIS analysis, was used as a method for tuning the triple-phase boundary to maximize the performance of the K–O2 battery.
EXPERIMENTAL SECTION Materials. The cathodes were cut (average diameter of 25.19 ± 0.09 mm, average mass of 26.4 ± 0.6 mg) from commercial carbon fiber paper (AvCarb P50, 0.17 mm thickness) with a reported real specific surface area of 88 cm2 mg−1 (8.8 m2 g−1)49. The cathodes were first rinsed with DI water then isopropyl alcohol and finally dried at 120 °C overnight. They were then immediately transferred into an argon-filled glovebox (
