Zinc-air Flow Batteries Research Articles

The Influence of Dimethyl Sulfoxide as Electrolyte Additive on Anodic Dissolution of Alkaline Zinc-Air Flow Battery

The present work describes the effects of dimethyl sulfoxide (DMSO) in KOH aqueous electrolyte on the performance of a zinc-air flow battery. Aqueous electrolytes containing 7 M KOH and (0 to 20)% v/v DMSO were studied revealing a critical role of DMSO on the dissolution and deposition of zinc. The anodic zinc dissolution process was studied via cyclic voltammetry, Tafel polarization and electrochemical impedance spectroscopy (EIS). The presence of DMSO showed improved zinc dissolution performance with the highest peak of zinc dissolution being the electrolyte containing 5% v/v DMSO. Tafel analysis demonstrated a significant decrease in polarization resistance and an increase in corrosion rate due to the introduction of DMSO to the electrolyte. This suggests that DMSO has the ability to suspend zinc oxide in the electrolyte, thus preventing passivation of the zinc surface. EIS results revealed that by adding DMSO to the electrolyte, charge transfer resistance increased. This is attributed to the formation of passive layers having arisen from DMSO adsorption, the formation of zincate ions in the vicinity of the zinc surface, and the deposition of discharged products. A difference in Nyquist plots was observed for 20% v/v DMSO/KOH and 0% v/v DMSO/KOH electrolytes implying non-Debye relaxation behavior taking place due to the surface effects. The electrolytes were implemented in a zinc-air flow battery. Maximum power densities of 130 mW/cm2 (5% v/v DMSO) and 125 mW/cm2 (20% v/v DMSO) were obtained and were observed to be about 43% and 28% higher than that of the DMSO-free electrolyte. Results indicated that when 20% v/v DMSO was added to KOH solution, there was 67% zinc utilization efficiency (550 mAh/g) which provided 20% improvement in discharge capacity. Further, the battery with 20% v/v DMSO demonstrated excellent cyclability. Overall, DMSO shows great promise for enhancement of zinc dissolution/deposition in zinc-air batteries.

Optimization of pulse current on energy storage of zinc-air flow batteries

The energy storage of a zinc-air flow battery subject to a pulse current is experimentally addressed. The energy storage occurs in the form of zinc reduction during the charging process. The controlling parameters, affecting the zinc reduction, are an electrolyte temperature, a pulse current, a pulse frequency, and a duty cycle. A pulse current provides more controlling parameters compared to a uniform DC current. The Taguchi method is used to optimize the control parameters and maximize zinc reduction and energy storage efficiency. Five levels for four control parameters are adopted in the Taguchi method, and an L25 orthogonal table is introduced for experimental tests. The optimum set of control parameters is obtained as temperature 40 °C, current 1.4 A, pulse frequency 50 Hz, and duty cycle 85%. In this case, 3.668g zinc is produced during 1 h of charging process. By using the pulse current, the weight of produced zinc particles is increased up to 1.394g by compared to the regular DC current. Therefore, using pulse current parameters does contribute to the improvement of energy storage efficiency. Finally, the SEM study of the produced zinc particles demonstrates that employing a pulse current provides finer zinc particles compared to a DC current.

In situ growing catalytic sites on 3D carbon fiber paper as self-standing bifunctional air electrodes for air-based flow batteries

Noble metal-free bifunctional catalysts are highly desirable for both oxygen reduction and evolution reactions in air-based flow batteries, but remain a big challenge. Herein, micropores dominated nitrogen and sulfur codoped carbon (NSC) film is directly growing on conductive carbon fiber paper (CFP) as self-standing air electrode through the electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole and followed pyrolysis process. NSC film provides abundant active sites, while conductive CFP with 3D macro-networks offers affluent channels for electron and mass transfer. Benefit from the strong interaction between NSC and CFP, the resultant CFP@NSC displays highly catalytic activity for both oxygen reduction (comparable half-wave potential to commercial Pt/C) and oxygen evolution (lower overpotential than commercial IrO2), along with robust stability. As a proof of practical usage, zinc air flow battery with CFP@NSC displays lower charge voltage and higher discharge voltage, compared with commercial Pt/C and IrO2 coated CFP electrode. Importantly, zinc air flow battery also exhibits excellent cycling stability including nearly 100% retention of coulombic efficiency and energy efficiency over 3400 min, representing the most efficient bifunctional oxygen electrode reported so far.

Strategies for Hydraulic Recharge of Zinc-Air Flow Batteries

The increased use of fluctuating renewable energy calls for efficient, safe and environmentally friendly energy storage solutions. To meet the variable power-, capacity- and profitability requests of mobile and stationary applications, it is necessary to identify promising materials. Zinc-air batteries with specific energy density higher than lithium-ion and low cost, highly available, eco-friendly active materials are suitable to fulfill these requirements. The use of a flow battery type with zinc-particles suspended in alkaline solution (zinc-slurry) in addition with high performance oxygen-reduction electrodes enables the development of high-power zinc-air batteries. This setup also allows the independent scaling of capacity and power. The recharging of this system can be performed due to classical electrical charging or due to a hydraulic recharge by changing the used zinc-slurry with a fresh one. The cooperative project, ZnMobil, funded by the Federal Ministry of Economic Affairs and Energy in Germany combines the experience of industrial and academic partners (Covestro Deutschland AG; Grillo-Werke AG; VARTA Microbattery GmbH; Accurec Recycling GmbH; Technical University Freiberg, Gottfried Wilhelm Leibniz University Hannover; University of Duisburg-Essen; the fuel cell research center ZBT GmbH) to develop a zinc-air flow battery with high performance and economic efficiency. Here we present strategies for the hydraulic recharging of zinc-air flow batteries. The battery system consists of a 100 cm² copper plate as current collector for the zinc-suspension electrode and an oxygen reduction electrode with a gas-diffusion layer (supplied by Covestro Deutschland AG). The zinc-slurry contains zinc particles (supplied by Grillo-Werke AG) suspended in alkaline solution (30 wt.-% KOH) and stabilized with polyacrylic acid. Depth-of-discharge and battery resistance were measured for zinc-slurries in a flow system under regulated conditions. The influence of the depth-of-discharge and several additives on the rheology of the zinc-slurries was evaluated. Depending on the current density and additive selection, it is possible to achieve energies of up to 300 Wh during discharge.

The Role of DMSO As an Electrolyte Additive in Alkaline Zinc-Air Flow Battery

Zinc-air batteries are attractive electrical energy storage due to their cost effectiveness, high specific energy density, and safety. Traditionally, the batteries use alkaline solution as an electrolyte. Besides, electrolyte additives such as potassium sulfide, zinc oxide, ethanol, surfactants, bismuth oxide, and iron oxide were introduced to suppress zinc anode passivation and corrosion. Dimethyl sulfoxide (DMSO) is one of the potential additives capable of inhibiting zinc anode passivation and corrosion effectively. This work investigates the application of DMSO in an alkaline zinc-air flow battery. Adding DMSO to potassium hydroxide (KOH) aqueous solution electrolyte revealed the positive effects on zinc dissolution.The effects of DMSO were studied by cyclic voltammetry of Fe(CN)6 3-/Fe(CN)6 4- redox couple reaction using the scan rate of 0.05 mV/s. A pair of sharp cathodic and anodic peaks were observed at 0.2 and 0.7 V (vs. Hg/HgO), corresponding to the redox reaction related to Fe(CN)6 3-/Fe(CN)6 4-. The reduction peaks between 0.04 and 0.2 V (vs. Hg/HgO) were observed for all electrolytes (KOH, DMSO/KOH). With increasing DMSO concentration from 1 to 20 v/v %, the redox peaks shifted towards less positive values. The results suggested that KOH undergoes an interaction with DMSO.Besides, the effects of DMSO (0, 1, 5, 10 and 20 v/v %) in 0.1 M KOH electrolyte on the electrochemical performance of air-cathode using MnO2 oxygen reduction reaction (ORR) electrocatalyst was examined by a rotating disk electrode technique. Linear sweep voltammetry was recorded in a potential window from 0 to -1 V (vs. Hg/HgO) at a scan rate of 0.05 mV/s and rotation rates of 100 to 3600 rpm. The Koutecky-Levich equation was applied to study kinetics and mass transfer processes of ORR in the electrolyte containing DMSO and KOH. The presence of DMSO in KOH solution causes significant improvement in the limiting current density. Also, it was observed that the limiting current density for the 5 v/v % DMSO/KOH at 3600 rpm is higher than that of 20 v/v % and also higher than that attained by KOH. The calculated number of electron transfer in 5 and 20 v/v % DMSO/KOH were 3.2 and 2.75, respectively, whilst the number of electron transfer of 2.24 was obtained for KOH electrolyte without DMSO. Overall, the results indicated that using DMSO as an electrolyte additive significantly improve zinc anode performance as well as ORR electrochemical performance. Figure 1

Model-Based Analysis of an Integrated Zinc-Air Flow Battery/Zinc Electrolyzer System

This work aims at analyzing an integrated system of a zinc-air flow battery with a zinc electrolyzer for energy storage application. For efficient utilization of inherently intermittent renewable energy sources, safe and cost-effective energy storage systems are required. A zinc-air flow battery integrated with a zinc electrolyzer shows great promise as an electricity storage system due to its high specific energy density at low cost. A mathematical model of the system was developed. The model was implemented in MATLAB and validated against experimental results. The validation of the model was verified by the agreement between the simulation and experimental polarization characteristic. The behavior and performance of the system were then examined as a function of different operating parameters: the flow rate of the electrolyte, the initial concentration of potassium hydroxide (KOH) and the initial concentration of zincate ion. These parameters significantly affected the performance of the system. The influence of the hydrogen evolution reaction (HER) on the performance of the system was investigated and discussed as it significantly affected the coulombic efficiencies of both the zinc-air flow battery and the zinc electrolyzer. Optimal KOH concentration was found to be about 6 to 7 M. Whilst increased KOH concentration enhanced the discharge energy of the battery, it also increased HER of both the battery and the electrolyzer. However, higher initial concentration of zincate ion reduced HER and improved the coulombic efficiency of the system. Besides, a higher flow rate of electrolyte enhanced the performance of the system especially at a high charge/discharge current by maintaining the concentration of active species in the cell. Nevertheless, the battery suffered from a higher rate of HER at a high flow rate. It was noted that the model-based analysis provided better insight into the behavioral characteristics of the system leading to an improved design and operation of the integrated system of zinc-air flow battery with the zinc electrolyzer.

Vector Flow Imaging of a Highly Laden Suspension in a Zinc-Air Flow Battery Model.

Flow batteries using suspension electrodes, e.g., zinc-air flow batteries (ZABs), have recently gained renewed interest as potential candidates for grid energy storage or mobile applications. The performance of ZABs depends on the local flow conditions of the suspension in the electrochemical cell, which acts as an electrode. Hence, it is crucial to measure and understand the complex flow characteristics of such solid-liquid suspensions. The investigated suspension electrode is an opaque slurry that consists of microscopic zinc particles and an aqueous potassium hydroxide electrolyte. Commonly, ultrasound Doppler velocimetry is used for flow imaging in opaque fluids. However, due to the high particle concentration in the suspension electrode, strong scattering and wavefront distortions of the ultrasound are introduced. In this paper, we show that this results in an increased measurement uncertainty for Doppler-based velocity estimation. Instead, ultrasound image velocimetry is applied to measure the 2-D and two-component flow field in the zinc-electrolyte suspension. This is possible by adapting the measurement system to the suspension with a calibration setup. The total measurement uncertainties of 4.1% and 2.5% for the axial and lateral flow components are derived from the calibration measurements. For the first time, the flow field of such a suspension could be measured in a scaled fluidic model of a ZAB. The comparison of the estimated flow rates from the velocity profiles showed good agreement to a gravimetric reference. A significant difference in the flow characteristics of a macroscopically homogeneous electrolyte and the same electrolyte loaded with 8 vol.-% zinc particles, i.e., the suspension electrode, was found. Along with the demonstration of the measurement technique for opaque, concentrated suspensions, the measurement data will be used to calibrate and validate numerical models for comparable multiphase fluids.

In Situ Charge Exfoliated Soluble Covalent Organic Framework Directly Used for Zn-Air Flow Battery.

Covalent organic frameworks (COFs) are generally obtained as insoluble, cross-linked powders or films, hindering their superior processable properties especially for device implementation. Here, a soluble COF is created with atomically well-organized positive charged centers constrained in the planar direction, exhibiting exceptional solubility through an in situ charge exfoliation pathway. Once dissolved, the obtained true solution retains homogeneity even after standing over a year. Moreover, the as-designed soluble COF contains ordered N-coordinated Fe single atom centers and conjugated structures, providing a small work function (4.84 eV) and superior catalytic performance for oxygen reduction (high half-wave potential of ∼900 mV). The obtained COF true solution can be directly used as a highly efficient Pt-replaced catalyst for zinc-air flow batteries, generating prominent performance and outstanding stability.

Superior oxygen electrocatalysts derived from predesigned covalent organic polymers for zinc-air flow batteries.

Covalent organic polymers (COPs) as emerging porous materials with ultrahigh hydrothermal stability and well-defined and adjustable architectures have aroused great interest in the electrochemical field. Here, we reported a rational design approach for the preparation of a bifunctional electrocatalyst with the assistance of a predesigned bimetallic covalent organic polymer. With the predesigned nitrogen position and structural features of COP materials, the obtained CCOPTDP-FeNi-SiO2 catalyst affords a remarkable bifunctional performance with a positive half-wave potential (0.89 V vs. reversible hydrogen electrode: RHE, superior to the benchmark Pt/C) for ORR activity, and a low overpotential (0.31 V better than the benchmark IrO2) at 10 mA cm-2 for OER activity in alkaline solution. The potential gap between E1/2 and Ej=10 reaches 0.650 V, in line with that observed in the current state-of-the-art bifunctional oxygen electrode materials. Moreover, a homemade rechargeable Zn-air flow battery using the CCOPTDP-FeNi-SiO2 catalyst as an air cathode exhibits an almost twofold power density (112.8 vs. 64.8 mW cm-2) and a lower charge-discharge voltage gap, compared with a commercialized noble Pt/C + IrO2/C-driven Zn-air flow battery. More importantly, the CCOPTDP-FeNi-SiO2-driven battery maintains a better cycling stability compared to a noble metal-driven battery without performance decay. Accordingly, this work will open up new ways for fabricating practical oxygen electrodes for, but not limited to, metal air based battery applications.

A Mathematical Model for Dynamic Operation of Zinc-Air Battery Cells

Abstract The paper investigates the newly designed zinc-airflow battery cells and aims to build a mathematical model for such energy storage systems. The work builds on the electro-chemical characterization performed in a previous study and aims to move the focus towards a modelling framework suitable for battery management/control/supervision. In this endeavour, the internal process is abstracted in order to obtain a behavioural mathematical model which concentrates the parameters in a representative input-state-output model. Ideally, the result will replicate the behaviour and allow to simulate the real cell functioning and thus to represent a meaningful tool for design techniques which are using model-based predictions.

Bimetal-phthalocyanine based covalent organic polymers for highly efficient oxygen electrode

The feature of tunable constituents in covalent organic polymers (COPs) via combination of predesigned building units is of great interest in exploring multi-functional catalysts. Here, we report constituent-tunable metal-phthalocyanine-based covalent organic polymers as self-sacrificial precursors for fabrication of mono- (Fe or Ni) and bi-metallic (Fe-Ni) heteroatoms/carbon catalysts toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). As a result, the as-prepared FeNi-COP-800 exhibits an efficient ORR performance with a half-potential of 0.80 V as well as an IrO2-like OER activity with a potential of 1.63 V at 10 mA cm−2, which surpass those of monometallic Fe-COP-800 and Ni-COP-800. Moreover, a zinc-air flow battery using FeNi-COP-800 catalyst not only shows a higher power density than the battery using Pt/C-IrO2/C but also displays superior stability with 2.90% potential loss after 85 cycles in 175 h of practical operation. Accordingly, this work provides a facile but efficient approach in molecular incorporation of desired constituents to obtain bi/multi-active sites in a single catalyst for various catalytic roles, including, but not limited to, oxygen reduction, hydrogen evolution, oxygen evolution.

Discharge Performance of Zinc-Air Flow Batteries Under the Effects of Sodium Dodecyl Sulfate and Pluronic F-127

Zinc-air batteries are a promising technology for large-scale electricity storage. However, their practical deployment has been hindered by some issues related to corrosion and passivation of the zinc anode in an alkaline electrolyte. In this work, anionic surfactant sodium dodecyl sulfate (SDS) and nonionic surfactant Pluronic F-127 (P127) are examined their applicability to enhance the battery performances. Pristine zinc granules in 7 M KOH, pristine zinc granules in 0–8 mM SDS/7 M KOH, pristine zinc granules in 0–1000 ppm P127/7 M KOH, and SDS coated zinc granules in 7 M KOH were examined. Cyclic voltammograms, potentiodynamic polarization, and electrochemical impedance spectroscopy confirmed that using 0.2 mM SDS or 100 ppm P127 effectively suppressed the anode corrosion and passivation. Nevertheless, direct coating SDS on the zinc anode showed adverse effects because the thick layer of SDS coating acted as a passivating film and blocked the removal of the anode oxidation product from the zinc surface. Furthermore, the performances of the zinc-air flow batteries were studied. Galvanostatic discharge results indicated that the improvement of discharge capacity and energy density could be sought by the introduction of the surfactants to the KOH electrolyte. The enhancement of specific discharge capacity for 30% and 24% was observed in the electrolyte containing 100 ppm P127 and 0.2 mM SDS, respectively.

Modelling and Simulation of Zinc-air Flow Battery with Zinc Regeneration Process

Reliable and low-cost electrical energy storage systems are required for grid integration of various intermittent sources of renewable energy. Zinc-air flow batteries have shown high potential for electricity storage application because of their high energy density at low cost. The flow batteries can reduce significantly the problem associated with the formation of dendritic zinc during recharge and the formation of a passivation film during discharge. The aims of this work include the development of mathematical models of a zinc-air flow battery (ZAFB). The developed model was implemented in MATLAB and validated with experimental data. The models were then used to investigate the behavior of the flow battery. The concentration of potassium hydroxide in the electrolyte, as well as the feed flow rate considerably affected the performance of ZAFB.

Ethanol as an electrolyte additive for alkaline zinc-air flow batteries

Zinc-air flow batteries exhibit high energy density and offer several appealing advantages. However, their low efficiency of zinc utilization resulted from passivation and corrosion of the zinc anodes has limited their broad application. In this work, ethanol, which is considered as an environmentally friendly solvent, is examined as an electrolyte additive to potassium hydroxide (KOH) aqueous electrolyte to improve electrochemical performance of the batteries. Besides, the effects of adding different percentages of ethanol (0–50% v/v) to 8 M KOH aqueous electrolyte were investigated and discussed. Cyclic voltammograms revealed that the presence of 5–10% v/v ethanol is attributed to the enhancement of zinc dissolution and the hindrance of zinc anode passivation. Also, potentiodynamic polarization and electrochemical impedance spectroscopy confirmed that adding 5–10% v/v ethanol could effectively suppress the formation of passivating layers on the active surface of the zinc anodes. Though the addition of ethanol increased solution resistance and hence slightly decreased the discharge potential of the batteries, a significant enhancement of discharge capacity and energy density could be sought. Also, galvanostatic discharge results indicated that the battery using 10% v/v ethanol electrolyte exhibited the highest electrochemical performance with 30% increase in discharge capacity and 16% increase in specific energy over that of KOH electrolyte without ethanol.

The impact of operating conditions on component and electrode development for zinc-air flow batteries

Rechargeable zinc-air flow batteries are investigated as possible technology for fast responding large-scale electrical energy storage due to the use of inexpensive, non-toxic and abundant materials, and compact system design. The operating ranges for several parameters such as flow rate (2–8 cm s−1), concentration of electrolyte (6 or 8 M KOH), charge/discharge current densities (up to 100 mA cm−2 mean), and active or passive air supply as well as their influence on the performance and stability of the electrodes are investigated and compared. Bi-catalyzed bifunctional air electrodes are tested by means of half-cell measurements achieving minimum 200 charge/discharge cycles at 50 mA cm−2 with the longest operation time being 800 h. At this current density, charge/discharge efficiencies are in the range of 50% for all tested air electrodes. End-of-life characterization by means of scanning electron microscopy reveals mechanical degradation of the electrode material. On the negative zinc electrode, zinc deposition morphology on different current collector materials (nickel, brass, and steel) is investigated using Rota-Hull cylinders showing nickel to be the most suitable material. The pulse interrupt current method is thereby successfully applied for compact zinc deposition in a broad current density range without any electrolyte additive. Subsequent scale-up of the rechargeable zinc-air flow battery and unit cell operation is finally performed for proof-of-concept.Graphical

Effects of Anionic Surfactant Sodium Dodecyl Sulfate on Electrochemical Performances of Zinc-Air Flow Batteries

Zinc-air batteries are promising for various energy application. Nonetheless, low efficiency of zinc utilization limited their broad applicability. This work aims at examining the effects of anionic surfactant sodium dodecyl sulfate (SDS) ranging from 0 - 1 mM in 7 M KOH aqueous solution electrolyte on the discharge performance of zinc-air flow batteries. The effects on electrochemical properties and discharge performances of the batteries were investigated. Cyclic voltammograms revealed that SDS effectively inhibited the anode passivation and promoted zinc dissolution. Besides, galvanostatic discharge results indicated that the introduction of SDS could enhance discharge capacity and energy density of the batteries. The battery with electrolyte containing 0.2 mM SDS exhibited the highest discharge capacity of 360 mAh/g (24% improvement) and energy density of 385 mWh/g (16% improvement).

Strategies for Improved Depth-of-Discharge of Zinc-Air Flow Batteries

As one of the proposed post lithium-ion technologies, zinc-air batteries have received revived interest in the recent years, which is driven by the increased demand of high-performance, efficient, safe and environmentally friendly energy storage solutions. These energy storage systems need to address variable power, capacity and profitability requests. Zinc-air batteries with specific energy density higher than lithium-ion and low-cost, highly available, eco-friendly active materials are suitable to fulfill these requirements. The use of a flow battery type with zinc-particles suspended in alkaline solution (zinc-slurry) in addition with high performance oxygen-reduction electrodes enables the development of high-power zinc-air batteries. This setup also enables the independent scaling of capacity and power. The cooperative project, ZnMobil, founded by the Federal Ministry of Economic Affairs and Energy in Germany combines the experience of industrial and academic partners (Covestro Deutschland AG; Grillo-Werke AG; VARTA Microbattery GmbH; Accurec Recycling GmbH; Technical University Freiberg, Gottfried Wilhelm Leibniz University Hannover; University of Duisburg-Essen; the fuel cell research center ZBT GmbH) to develop a high-performance and low-cost zinc-air flow battery. Here we present strategies to improve the depth-of-discharge of zinc-air flow batteries. The battery system consists of a 100 cm² copper plate as current collector for the zinc-suspension electrode and an oxygen reduction electrode with gas-diffusion layer (supplied by Covestro). The zinc-slurry contains zinc particles (supplied by Grillo) suspended in alkaline solution (30 wt-% KOH) and stabilized with polyacrylic acid. The battery performance and depth-of-discharge were measured for several additives added to the zinc-slurry. Furthermore, the influence of the discharge current density on the depth-of-discharge was evaluated for these setups. Depending on the current density and additive selection, it is possible to achieve depth-of-discharge up to 90 %.

Effects of Anionic Surfactant Sodium Dodecyl Sulfate on Electrochemical Performances of Zinc-Air Flow Batteries

Zinc-air batteries are promising for various future energy application. Nevertheless, low efficiency of zinc utilization in zinc-air batteries limited their broad applicability. Therefore, extensive efforts have been carried out to address this issue by either improving a zinc anode or an electrolyte. Different approaches were reported to enhance the performance such as utilizing gel-type electrolytes, coating zinc with various chemicals and polymers and employing organic additives of the electrolyte. The electrolyte has a significant impact on the performance and cycle life of the batteries. Potassium hydroxide (KOH) has been extensively used as the electrolyte for Zn-air batteries due to its high ionic conductivity, low viscosity, and high oxygen diffusion coefficient. However, the usage of KOH revealed several technical issues such as corrosion and passivation of the zinc anode and carbonates formation leading to the battery performance deterioration. This work aims to examine the effect of utilization an anionic surfactant (sodium dodecyl sulfate, SDS) in 7 M KOH aqueous solution electrolyte on the discharge performance of zinc-air flow battery using granular zinc anode. SDS was implemented in two methods. The first method is to directly add SDS (0.0173 - 0.268 mM) to the electrolyte (zinc-KOH/SDS). The other method is to coat zinc granular using 1 M SDS (SDS/zinc-KOH). The effects of both methods on electrochemical properties and discharge performance of the batteries were then investigated. By using zinc-KOH/SDS, an 11% increase in discharge capacity over the electrolyte without SDS was observed. However, SDS/zinc-KOH had adverse effects on the battery performance by preventing zincate formation and leading to a significant drop in discharge voltage and capacity. A comparison of polarization characteristics showed that the presence of SDS caused an increase in the current for both methods. Nevertheless, by adding SDS to KOH solution ranging from 0.0173 to 0.268 mM, an increase of discharge current from 160 to 212 mA at 1 V was observed with an increase from 14 to 51% of discharge current. Besides, a 14% increase in discharge current was revealed for SDS coated zinc granular. The optimal amount of SDS was then determined. Cyclic voltammetry of three types zinc-KOH, zinc-KOH/SDS and SDS/zinc-KOH showed the main difference between two methods. Zinc-KOH and zinc-KOH/SDS demonstrated similar anodic peaks including zinc dissolution corresponding to zincate formation and zinc oxide formation whereas for SDS/zinc-KOH the peak related to zincate formation was not observed. Moreover, the anodic peak slightly shifted to the positive potential direction, and the peak intensity of zincate formation increased when SDS was introduced to the electrolyte. Electrochemical impedance spectroscopy revealed that the presence of SDS for both methods increased the charge transfer resistance in comparison with Zinc-KOH, confirming that aggregation of surfactant molecules on the zinc surface or interface KOH/zinc raised the charge transfer resistance. Figure 1

Efficient unitary oxygen electrode for air-based flow batteries

Air-based flow batteries have triggered huge interests as promising large-scale energy storage devices due to their high energy density and extremely low cost. However, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) lower their energy conversion efficiencies. Herein, we in situ grow amorphous NiSx-FeOy particles on sulfur doped carbon fiber paper (termed as NiSx-FeOy/SCFP) as practical self-supported oxygen electrode through a simple and economic synthesis route. Benefiting from the synergetic effects of amorphous NiSx-FeOy and SCFP, NiSx-FeOy/SCFP possesses massively accessible active sites, enhanced electrons and reactants transfer, and stable integrated structure, exhibiting a low overpotential (0.37 V) at 10 mA cm−2 for OER, a positive half-wave potential for ORR and a small potential gap (0.82 V). Zinc air flow battery assembled with newly developed NiSx-FeOy/SCFP shows a coulombic efficiency of 100% and a voltage efficiency of 50% at the charge-discharge current of 10 mA cm−2, and vigorously operates 110 h without performance decay, showing highly catalytic activity and super cycling stability, which are among the best performance ever reported. We believe that this work will open up new ways for fabricating practical oxygen electrode for metal air based batteries applications.

Electrochemical energy storage for renewable energy integration: zinc-air flow batteries

A 1 kW–4 kWh zinc-air flow battery has been built at Tecnicas Reunidas facilities. The battery is divided in three different stacks connected in parallel, each of them comprising 20 cells connected in series and 0.25 m3 of electrolyte. The main challenges found on scaling up include the necessity of using three electrodes per cell, electrolyte leakage, ohmic resistance and gas diffusion electrode flooding. The three electrode solution is not optimal because it adds mechanical complexity and cost but it is a robust solution to evaluate the performance of the system as a baseline. During pilot plant operation, a phenomenon called shunt currents has revealed to reduce coulombic efficiency by 18%. In order to better understand this phenomenon, a mathematical model has been developed and it has been validated using real measurements taken on one of the 20-cell stacks. The major part of the challenges were solved and technical viability has been evaluated. However, efficiency, durability and the bifunctional air electrode are the challenges to overcome before this technology becomes ready for commercialization.