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Journal ArticleDOI

A high specific capacity membraneless aluminum-air cell operated with an inorganic/organic hybrid electrolyte

TL;DR: In this paper, a microfluidic aluminum-air cell working with KOH methanol-based anolyte was developed in order to overcome the self-discharge issue of aluminum.
About: This article is published in Journal of Power Sources.The article was published on 2016-12-30 and is currently open access. It has received 12 citations till now. The article focuses on the topics: Electrolyte.

Summary (4 min read)

1. Introduction

  • Cost-effective and high-density energy storage remains an unmet demand for applications ranging from portable electronics to large-scale grid storage.
  • Early developments of Al-air cell have achieved little commercial success, mainly due to the dependence on the use of aqueous electrolytes, which suffered severe self-discharge problems and resulted in practical energy densities inferior even to those of zinc-air systems [5].
  • Due to the low proton availability in the non-aqueous solvents, an almost complete inhibition of Al corrosion, and correspondingly, anode efficiency of nearly 100% have been demonstrated from half-cell experiments on the basis of methanol [6], ethanol [7], propanol [8], and ionic liquid electrolytes [9].
  • Non-aqueous/aqueous hybrid cell is an emerging technology to address the limitations of traditional single-electrolyte cell structures by operating electrodes in different electrolyte environments.
  • To avoid the problems associated with the use of a solid separator, laminar flow- based microfluidic electrochemical cells have been extensively researched [24-26].

2.1 Chemicals

  • Alkaline electrolytes with different concentrations of KOH were prepared by dissolving KOH pellets (≥85%, Sigma-Aldrich, Hong Kong) in methanol (≥99.9%, Merck KGaA, Germany) and in 18 MΩ deionized water (Barnstead, NANOpure DiamondTM, USA) under different test conditions.
  • Commercial A gas diffusion electrode (GDE, Hesen Company, China) with a catalyst loading of 2 mg cm-2 Pt/C (Johnson Matthey, USA) was adopted as the air cathode for oxygen reduction.

2.2 Cell fabrication and assembly

  • The cell consisted of three polyvinyl chloride (PVC) layers cut out by a carbon dioxide laser ablation system (VLS 2.30, Universal Laser System, USA).
  • The top layer contained two inlets and one outlet for electrolyte flows.
  • The active projected electrode area was 0.015 cm2, and all current and power densities were normalized by this area.
  • The three layers had thicknesses of 0.1 mm, 0.5 mm, and 0.1 mm, respectively, and they were bound together using double-side adhesive tape.
  • The electrolyte was pumped into the cell by a syringe pump (LSP02-1B, LongerPump, China), via 1.5 mm tubing bonded to the ports with epoxy.

2.3 Fluorescent dye characterization

  • Solutions of bromophenol blue (C19H10O5Br4), a methanol/water-ratio-sensitive dye, were prepared with a concentration of 1.75 mg mL-1 in solvents containing different volume ratios of methanol and deionized water.
  • The solutions were observed by Precentered Fiber Illuminator (Intensilight C-HGFI Nikon, Japan) using yellow fluorescent light with a wavelength ranging between 600 and 630 nm.
  • At this wavelength range, the methanol solution of C19H10O5Br4 is a transparent orange solution, while its water solution is opaque.
  • The color is gradually transformed when the volume ratios of methanol/water changed.
  • Then, pure C19H10O5Br4 methanol solution and water solution were pumped into the cell channel structure to investigate the flow pattern of methanol and water electrolytes at different flow rates.

2.4 Electrochemistry

  • Electrochemical measurements were conducted under room conditions.
  • The polarization curves were obtained by potentiostatic current measurement at every 0.2 V for 200s to reach a steady state, from open-circuit voltage (OCV) to 0 V, by an electrochemical workstation (CHI 660E, Shanghai Chenhua Instruments Co., Ltd., China).
  • The average value of the current data in the last 50 seconds of the sampling was used to represent the cell current at a certain voltage.
  • An external Ag/AgCl (in saturated KCl) electrode (Shanghai Lei-ci Co., Ltd., China) was used as a reference electrode to acquire the singleelectrode potentials of the cells.
  • The potential data was recorded in situ by a digital multimeter (15B, Fluke Corporation, USA).

2.5 Post-discharge characterization

  • Post-discharge products precipitated on the Al surface were first cleaned with methanol.
  • They were then dried and ground into powder for X-ray diffusion (XRD) analysis (D8 ADVANCE Diffractometer, Bruker AXS, USA) with Lynxeye detector Cu/Kα radiation operating at 40 kV/40 mA.
  • The products were identified based on the database in the JADE diffraction analysis software (Material Data Inc., USA).
  • The residual methanolbased electrolyte solution was taken out and dried by a rotary evaporator (Rotavapor R210, BUCHI, Switzerland) to remove the methanol and form white powders.
  • The powder samples from these two solutions were made into KBr pellets with a 13 mm diameter for Fourier transform infrared spectroscopy (FTIR) analysis (IRAffinity-1s, Shimadzu Co., Japan) to verify whether any other organic product was generated.

3. Results and Discussion

  • 1 Design principle and chemical patterns Figure 1(b) shows the visualization optical micrographs of methanol- and water-based electrolyte flow pattern in this cell structure as a function of flow rates, from 100 μL min-1 to 1000 μL min-1.
  • A mixing segment can be easily observed in the middle part.
  • As the flow rate increased, the influence of convective flux became greater than the opposite diffusive mixing flux, shrinking the mixing segment of the electrolyte in this counter-flow architecture.
  • (c) Photograph of a hybrid electrolyte Al-air cell.

3.2.1 Discharge characteristics

  • Figure 2 shows the cell performance and individual electrode polarization curves as a function of KOH concentration and water content in the methanol-based anolyte.
  • The current density was found to be limited at lower voltages.
  • At a discharge current density of 1.0 mA cm-2, the Al-air cell achieved the highest specific capacity of 2507 mAh g-1 with 1 M electrolyte, corresponding to a Coulombic efficiency of 84.1%, which is one of the highest values achieved to date for Al anodes [28, 30-32].
  • As shown in Figure 3, under the same discharge current of 1 mA cm-2, the voltage of the cells increased with increasing KOH concentration and anolyte water content, which is consistent with the performance curves in Figure 2.
  • This drop in specific capacity with increasing water content is due to the higher aluminum corrosion rate in the anolyte with higher water content as water was directly involved in the corrosion process of aluminum.

3.2.2 Impedance spectroscopy

  • Nyquist plots of the Al-air cell working with different anolytes are shown in Figure 4(a) and (b).
  • For the cathode side, the high–frequency and low-frequency impedance respectively reflect a capacitance in the catalyst layer and the kinetic impedance of the oxygen reduction reaction [39].
  • As can be seen in ) and (b), cells with anolyte of neat methanol-based KOH solutions feature EIS curves with a high-frequency capacitive loop, a middle-frequency inductive loop, and a low-frequency line ), while the line at low frequency changes to a capacitive loop when water exists ).
  • The fitting values of different resistances in equivalent circuit are list in Table 2.
  • The phenomenon was mainly due to the improvement of solution conductivity with increasing water content.

3.3 Reaction mechanism

  • Methanol is not corrosive to most metals.
  • This characteristic shows that Al corrosion in conventional water-based electrolyte could be significantly inhibited by using a methanol-based anolyte.
  • When KOH concentration and water content in the anolyte increase, the capacity densities of Al will decrease, indicating the aggravation of corrosion reaction.
  • This section investigates the reaction mechanisms of Al in KOH methanol-based anolyte, including discharge and corrosion reactions.

3.3.1 Surface morphology

  • The morphologies of the Al surface after discharging in KOH neat methanol-based and water-based anolyte are shown in Figure 5.
  • A rough surface configuration was formed on the discharged anode surfaces in both cases, while a significant difference between the two cases can be easily observed.
  • The pore size of the Al discharged in the water-based anolyte was much larger than that of Al discharged in the methanol-based anolyte.
  • This could also be seen in their sectional views.
  • Previous researchers have studied the influences of the passive layer and Al surface morphology on impedance [41-43].

3.3.2 Discharge mechanism

  • Referring to Al, Al(OH)3 was the most common discharged product.
  • Considering the high specific capacity that Al achieved, the main reaction between Al and KOH methanol-based solution is: (3) Therefore, the discharge reaction of Al in KOH methanol-based solution is the same as that in KOH water-based solution.
  • Additional K2CO3(H2O)1.5 might come from the impurities of KOH samples and the reaction between KOH and the CO2 from the air [44].

3.3.3 Corrosion mechanism

  • In a KOH methanolbased solution, an equilibrium reaction exists as following: (4) Due to the existence of water, the corrosion reaction of Al will happen as: (5) Increasing KOH concentration drives Equation (4) to the product side, resulting in higher water content and accelerating the corrosion reaction rate.
  • This explains the decrease of specific capacity of Al in KOH methanol-based solution with higher concentrations.
  • Therefore, the total corrosion reaction of Al in KOH methanol-based solution is: (6) To investigate whether there are more complex organic products, the powder samples are extracted from the KOH methanol-based solution with and without Al discharging and made into KBr pellets for FTIR analysis.
  • The infrared scanning of the two samples shows the same results of FTIR spectra as shown in Figure 7 and Table 3, indicating there are no other complex organic compounds generated.

4. Conclusions

  • This work studied the characteristics of an Al-air cell working with methanol-based anolyte.
  • The cell was built on a non-direct counter-flow microfluidic cell platform to avoid the use of an expensive membrane.
  • The inhibition of crossover between different electrolytes was verified first by observing the flow pattern of methanol and water in the cell channel.
  • The capacity tests showed that the self-corrosion of Al could be effectively inhibited by using KOH methanol-based solution as anolyte.
  • In the experiment, the highest specific capacity of 2507 mAh g-1 was achieved (84.1% of the theoretical value), which is among the highest values achieved to date.

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Journal ArticleDOI
TL;DR: In this article, different types of MABs are overviewed from the perspective of the metal electrodes, and the advantages and disadvantages of each system are presented, and recent advances that address challenges such as corrosion, passivation and dendrite growth are introduced.
Abstract: Metal–air batteries (MABs), which possess exceptionally high energy density and exhibit other ideal features such as low cost, environmental benignity and safety, are regarded as promising candidates for the next generation of power sources. The performance of MABs and the challenges involved in these systems are primarily related to metal electrodes. In the present work, different types of MABs are overviewed from the perspective of the metal electrodes. Most metal electrodes that have been studied in recent years are reviewed, among which Zn, Al, Mg and Fe are highlighted. The advantages and disadvantages of each system are presented, and recent advances that address challenges such as corrosion, passivation and dendrite growth are introduced. In addition, investigations focused on revealing interactions between the metal electrodes and electrolytes or exploring electrolytes to improve the performance of metal electrodes are also discussed. Finally, a general perspective on the current situation of this field and on future research directions is provided.

119 citations

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TL;DR: It is demonstrated that the presented Al-air coin cell can be recycled by a series of eco-friendly procedures using food-grade ingredients, resulting in recycled products that are environmentally safe and ready for reuse.
Abstract: Aluminum-air batteries are a promising power supply for electronics due to their low cost and high energy density. However, portable coin-type Al-air batteries operating under ambient air condition for small electronic appliances have rarely been reported. Herein, coin cell-type Al-air batteries using cost-effective and eco-friendly chitosan hydrogel membranes modified by SiO2, SnO2, and ZnO have been prepared and assembled. The Al-air coin cell employing chitosan hydrogel membrane containing 10 wt % SiO2 as a separator exhibits better discharge performance with a higher flat voltage plateau, longer discharge duration, and higher power density than the cells using a chitosan hydrogel membrane containing SnO2 or ZnO. Moreover, we also demonstrate that the presented Al-air coin cell can be recycled by a series of eco-friendly procedures using food-grade ingredients, resulting in recycled products that are environmentally safe and ready for reuse. The Al-air coin cell adopting a recycled cathode from a fully discharged Al-air coin cell using the above-mentioned procedure has shown comparable performance to cells assembled with a new cathode. With these merits of enhanced electrochemical performance and recyclability, this new Al-air coin cell with modified chitosan hydrogel membrane can find wide applications for powering portable and small-size electronics.

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TL;DR: In this article , the authors present a review of the best practices in the field of microfluidic energy conversion for the past 20 years and present opportunities for future research directions and technology advances.
Abstract: Electrochemical energy conversion is an important supplement for storage and on-demand use of renewable energy. In this regard, microfluidics offers prospects to raise the efficiency and rate of electrochemical energy conversion through enhanced mass transport, flexible cell design, and ability to eliminate the physical ion-exchange membrane, an essential yet costly element in conventional electrochemical cells. Since the 2002 invention of the microfluidic fuel cell, the research field of microfluidics for electrochemical energy conversion has expanded into a great variety of cell designs, fabrication techniques, and device functions with a wide range of utility and applications. The present review aims to comprehensively synthesize the best practices in this field over the past 20 years. The underlying fundamentals and research methods are first summarized, followed by a complete assessment of all research contributions wherein microfluidics was proactively utilized to facilitate energy conversion in conjunction with electrochemical cells, such as fuel cells, flow batteries, electrolysis cells, hybrid cells, and photoelectrochemical cells. Moreover, emerging technologies and analytical tools enabled by microfluidics are also discussed. Lastly, opportunities for future research directions and technology advances are proposed.

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TL;DR: Li-air and Zn-air batteries have been studied extensively in the past decade as mentioned in this paper, with the aim of providing a better understanding of the new electrochemical systems, and metal-air battery with conversion chemistry is a promising candidate.
Abstract: In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal-air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li-air and Zn-air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li-air and Zn-air batteries, with the aim of providing a better understanding of the new electrochemical systems.

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TL;DR: In this paper, the authors focus on the new synthesis methods that have led to these breakthroughs and analyze the improvements required from NPMC-based catalysts to match the performance of Pt-based cathodes, even at high current density.
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TL;DR: In this paper, the authors discuss the most critical challenges to developing robust, high-energy Li/air batteries and suggest future research directions to understand and overcome these challenges and predict that Li-air batteries will primarily remain a research topic for the next several years.
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TL;DR: Li et al. as mentioned in this paper constructed stable zinc-air batteries using novel catalysts for oxygen reduction and evolution reactions, but their realization is hampered by the lack of efficient and robust air catalysts.
Abstract: Metal-air batteries are promising for energy storage because of their high theoretical energy density, but their realization is hampered by the lack of efficient and robust air catalysts. Li et al. construct stable zinc-air batteries using novel catalysts for oxygen reduction and evolution reactions.

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TL;DR: In this paper, a review of aluminum-air secondary batteries is presented, including aqueous electrolyte primary batteries, aluminum air batteries, and molten salt secondary batteries, as well as solution additive to electrolytes.

567 citations