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Summer Research Fellowship Programme of India's Science Academies

Prussian blue analogs for aqueous rechargeable batteries

Kavali Basappa Latha

Department of Chemistry, Jnana Sahyadri, Kuvempu University, Shankaraghatta 577451

Aninda J. Bhattacharyya

Department of Solid State Structural Unit, IISc, Bengalore 560012

Abstract

Non-lithium energy storage devices especially sodium ion and multivalent ion batteries are drawing attention recently due to insufficient and uneven distribution of lithium resources. Prussian blue analogs [PBAs], or metal hexacyanoferrates, are well known since 18thcentury and having the general formula AXMAY[MB(CN)6]Z.n H2O where MA and MB are usually Mn, Fe, Co, Ni, Cu or Zn and A is usually Li, Na or K. In this report, we are reporting the preparation and characterization of Prussian blue material that is CuCoHCF for aqueous battery system. The material shows good or promising capacity when it is combined with the zinc foil as anode. The initial electrochemical performance of this material in aqueous zinc in battery is promising and yet further optimizations are required to get the better capacity retention out of it.

Keywords: prussian blue analogs, aqueous battery systems, metal ion batteries, transition metal hexacyanoferrate

Abbreviations

Abbreviations
 PBPrussian Blue 
 PBAPrussian Blue Analogs 
 LIBLithium Ion Batteries 
 ARBAqueous Rechargeable Batteries 
 ARLBAqueous Rechargeable Lithium Battery 
 ARSBAqueous Rechargeable Sodium Battery 
 RFB Redox Flow Batteries
 ZIB Zinc Ion Batteries
 PXRDPowder X-Ray Diffractometry
 FTIRFourier Transformation Infrared 
 TGAThermogravemetric Analysis 
 CVCyclic Voltametry 
 FESEMField Emission Scanning Electron Microscopy 

INTRODUCTION

I Kavali Basappa Latha, here I am working on aqueous rechareable batteries.Battery is a device that converts the chemical energy contained in its active materials directly into electrical energy by means of an electrochemical oxidation – reduction (redox) reaction. In the case of the rechargeable system, the battery can be recharged by a reversible process. This type of reaction involves the transfer of electrons from one material to another through an electrical circuit.

COMPONENTS OF CELL AND BATTERIES

The cell consists of three major components

1. The anode or negative electrode-the reducing or fuel electrode which gives up electrons to the external circuit and is oxidized during electrochemical reaction.

2. The cathode or positive electrode-the oxidizing electrode-which accepts electrons from the external circuit and is reduced during electrochemical reaction.

3. The electrolyte-the ionic conductor-which provides the medium for transfer of charges as ions, inside the cell between anode andcathode. The electrolyte is typically a liquid, such as water or other solvents, with dissolved salts, acids, or alkalis to impart ionic conductivity. Some batteries use solid electrolytes, which are ionic conductors at the operating temperature of the cell.

The anode and cathode electrodes are electronically/physically isolated in the cell to prevent internal short-circuiting, but these are surrounded by the electrolyte. In cell designs a separator material is used to separate the anode and cathode electrodes mechanically. The separatoris permeable to the electrolyte in order to maintain the desired ionic conductivity.

Classification of batteries

Electrochemical batteries are identified as primary (non-rechargeable) or secondary (rechargeable), based on their capability of being electrically recharged.

Primary batteries

These batteries cannot be easily or effectively recharged electrically and hence, are discharged once and discarded[1].

Secondary batteries

The secondary batteries can be recharged electrically after the battery gets discharged. These batteries can be recharged by passing current in the opposite direction to that of the discharge current[1].

Reserve batteries

In these primary types, a key component is being separated from the rest of the battery giving important to activation. In this condition, chemical deterioration or self-discharge can be eliminated, and the battery is capable of long-term storage[1].

Fuel cells

Fuel cells are like batteries and are electrochemical galvanic cells that convert chemical energy into electrical energy.

Fuel cell technology can be classified into two categories, Direct systems and Indirect systems

WORKING PRINCIPLE OF LITHIUM ION BATTERIES

A battery consists of anode, cathode, separator, electrolyte, and two current collectors (positive and negative). The anode and cathode store the lithium. The electrolyte carries positively charged lithium ions from the anode to the cathode and vice versa through the separator. The movement of the lithium ions creates free electrons in the anode which creates a charge at the positive current collector. The electrical current then flows from the current collector through a device being powered (cell phone, computer, etc.) to the negative current collector. The separator blocks the flow of electrons inside the battery.

Discharge

While the battery is discharging and providing an electric current, the anode releases lithium ions to the cathode, generating a flow of electrons from one side to the other.

Li1-xFePO4+xLi+ +xe- →LiFePO4

Li → Li+ + e

31.gif
    Discharging.

    Charge

    While the battery is charging and the device is plugged in, the opposite happens: Lithium ions are released by the cathode and received by the anode.

    Li+ + e- →Li

    LiFePO4- → Li1-xFePO4 + xLi+ + xe-

    22 (2).gif
      chaging.

      Advantages of Lithium Ion Batteries

      There are many advantages using lithium ion battery. They include:

      1. High energy density of around 200Wh/Kg

      2.Very low self discharge rate

      3. Low maintenance

      4. No requirement for priming

      5. Different variety of types available

      Disadvantages of Lithium Ion Batteries

      Like the use of any technology, there are some disadvantages that need to be balanced against the benefits. The Li-ion battery disadvantages include:

      1. Protection required

      2. Ageing

      3. Transportation

      4. Very expensive

      5. Immature technology[1]

      Zinc Ion Batteries: To overcome the demerits of lithium ion batteries

      Lithium ion batteries (LIBs) can deliver high energy densities. In addition, LIBs present an unfavorable rate capability or power density, due to the low ion conductivity of organic electrolyte which are about two orders of magnitude lower than those of aqueous electrolytes.The high cost is also achallenge for their application in large-scale energy storage.Therefore, new strategies and endeavors are urgently required to explore new battery devices to tackle the energy crisis and environmental problems.

      Aqueous electrolytes in rechargeable batteries are of great potential to overcome these shortcomings, since non-flammable aqueous electrolytes are used instead of flammable organic electrolytes. The ionic conductivity of aqueous electrolytes are two orders of magnitude greater than that of organic electrolytes,allowing higher charge discharge rates and lower voltage drops from the electrolyte impedance. Moreover, the cost of electrolytes and assembly processes are highly reduced, due to the use of safe and low-cost aqueous electrolytes. All of these advantages indicate that aqueous rechargeable batteries (ARBs) are promising candidates for large-scale EES.

      A series of ARBs were investigated and demonstrated, such as

      1.      Aqueous rechargeable lithium batteries (ARLBs),

      2.      Aqueous rechargeable sodium batteries (ARSBs) and

      3.      Redox flow batteries (RFBs),

      Aqueous rechargeable batteries overcome some of the disadvantages of lithium ion batteries, such as high cost, and safety issues. They are more competitive with other aqueous secondary batteries, including lead-acid, Ni-Cd, and Ni-MH batteries, due to their environmental friendliness. In the past decade,their cycling performance, reversible capacity, and rate capacity have been markedly improved, due to the development of both positive and negative electrode materials[3].

      As ARLBs use lithium intercalation compound(s) as one or two electrodes based on redox reactions, and alithium-containing aqueous solution as the electrolyte, their electrochemical performance is highly dependent on both positive and negative electrode materials. Due to the decomposition potential of water being much lower than that of organic electrolytes, the intercalation/de-intercalation of the lithium ions should occur within the range of electrochemical stability of aqueous solutions. However, because of the limited lithium resources that cannot meet the consumer demands in the near future, the widespread use of lithium-based batteriesinelectrical devices and EES systems is unfavorable. Aqueous rechargeable sodium batteries have attracted more attention as alternative power sources,due to the abundant reserves of sodium. The mechanism of ARSBs is similar to that of ARLBs; sodium ions shuttle back and forth between the positive electrode and the negative one, with sodium salt electrolyte serving as the ion transfer medium. However, only afew ARSBs have been fabricated, compared with ARLBs, due to the lack of suitable sodium intercalating compounds for aqueous systems. The main reason is that the radius of the Na+ ion is much larger than that of the Li+ ion, making it more difficult to intercalate into, and de-intercalate from the lattices. Therefore, suitable sodium intercalation compounds should be two-dimensional layered structures, three-dimensional structures with large interstitial sites, or crystal structures with suitably large tunnels.

      ARBs has spanned Li+ intercalation materials with naturally abundant Na+ and K+ systems and, more recently, the intercalation chemistry of divalent cations that engage in multiple electron transfer. Rechargeable batteries based on multivalent metal ion insertion/extraction in aqueous solution, such as Mg2+, Ca2+, Zn2+ and Al3+ are found to be more promising ARB system due to potential 2-3 fold high energy density than monovalent ARBs. Studies have verified that water molecules can effectively shield the electrostatic repulsion of multivalent ion and lower the activation energy for charge transfer at electrode/electrolyte interface compared to the organic solution. Thus, the multivalent ARBs could deliver better electrochemical properties than organic ones. Among the multivalent metals, metallic zinc is promising anode candidate for aqueous batteries because of its higher specific energy, low equilibrium potential and abundance and low toxicity.

      Rechargeable aqueous zinc-ion batteries (ZIBs), an alternative battery chemistry, have found the way not only for realizing environmental friendliness and safe energy storage devices but also for reducing the manufacturing costs for the next generation batteries. The energy Storage Mechanisms in Aqueous ZIB Systems is not similar to lithium/sodium-ion-based energy storage chemistry in the case of monovalent alkali metal cations. The reaction mechanisms in aqueous ZIB systems are quite complicated and controversial. According to the literature, the redox reactions in aqueous ZIB systems mainly involve three mechanisms: Zn2+ insertion/ extraction, chemical conversion reaction, and H+/Zn2+ insertion/extraction[4]. AqueousZn-ion batteries have an energy storage advantage over alkali-based batteries as they can employ Zn metal as the negative electrode, increasing energy density. The development is noticed by a limited choice of positive electrodes, which often show poor rate capability and insufficient cycle life[5].

      Prussian Lue

      Prussian blue was the first modern synthetic pigment. It is also called as Berlin blue or, in painting, Parisian or Paris blue. Prussian blue or hexacyanoferrate is emerging as a promising material for most of the advanced fields of technology. There is a huge increase in energy storage market.These materials can be synthesized easily and can also be used in large scale. It is prepared as a very fine colloidal dispersion, because the compound is not soluble in water. It contains variable amounts of other ions and its appearance depends sensitively on the size of the colloidal particles. The compound was synthesized in Berlin for the first time. It was synthesized in the laboratory of Johann Conrad Dippel[5].

      It is a mixed valent compound, which contain iron atom in both 2+ and 3+ oxidation state. Fe3+ is located in an octahedral nitrogen environment and Fe2+ is located in the carbon environment. PB has two different structural positions for water molecule to occupy. Six molecules attached to six Fe3+ forms the corner of the octahedral cavity called co-ordinated water and the rest of the water molecules fill eight possible 8C(1/4,1/4,1/4) sites of the unit cell. PB allows multiple modifications in its chemical composition[17]. The new structures which are formed from the multiple modifications are called as prussian blue analogs (PBAs). The generic formula for PBA system can be represented by AXMAY[MB(CN)6]Z. nH2O where MA and MB are usually Mn, Fe, Co, Ni, Cu or Zn and A is usually Li, Na or K. The hexacyanoferrates open frame work allows large interstitial space to host large sized ions such as sodium and potassium. Thus, PBAs have become the promising material in energy storage system.

      Prussian blue analogs belong to the large family of transition-metal hexacyanoferrates with open framework structure, abundant redox-active sites, and strong structural stability. PBAs can host for larger alkali cations, due to their large ionic channels and interstices in the lattice. Benefiting from this structural feature, PB compounds have been intensively investigated as a new alternative and low-cost Na-insertion cathode during past five years [2].

      Properties

      Prussian blue is a microcrystalline blue powder. It is insoluble, but the crystallites tend to form a colloid. Such colloids can pass through fine filters. Despite being one of the oldest known synthetic compounds, the composition of Prussian blue remained uncertain for many years. Its precise identification was complicated by three factors: Prussian blue is extremely insoluble, but also tends to form colloids. Traditional syntheses tend to afford impure compositions.

      LITERATURE REVIEW

      Information

      Recently, there have been many reports on Prussian blue as cathode materials in aqueous rechargeable batteries. The main fields of interest refer to their preparation and characterization and also to their electrochemical properties: good conduction (including electron and ions transfer), electro-analysis and electro-catalysis. In recent years, considerable interest has been directed towards the investigations of transition metal-hexacyanoferrates, also known as Prussian blue analogous.

      Here are some of the reports based on the synthesis and characterization of prussian blue materials as cathode

      1. Zhiguohou,et.al., demonstrated an aqueous electrolyte with an expanded electrochemical stability window of 2.5V by the addition of surfactant that is sodium dodecyl sulphate[SDS] to the electrolyte. An aqueous battery consisting of Na2MnFe(CN)6 as cathode and zinc as anode with ZnSO4 and Na2SO4 electrolyte, delivered a specific capacity of 137mAh/g with the operating voltage up to 2.0V, corresponding to the high energy density of 170 Wh/kg. It also exhibited excellent cycling stability and high columbic efficiency over 97% at a 0.5C rate [7].

      2. A. Holland, et al., reported anatase (TiO2) nanopowder as anode, CuHCF as cathode and aqueous Al3+/K+ electrolyte. This battery showed specific energy very low, ca. 15 mWh/g active material, a high charge and discharge rate of 20C with high energy efficiency of the cell remained above 70% for over 1750 cycle. The capacity of CuHCF is c.a. 50 mAh/g at a current density of 333 mA/g [8].

      3. Andrea paolella et.al., reported that, the Prussian blue analogs in both soluble and insoluble forms as cathode materials with Li, Na, K, Mg, Ca and Zn intercalated ionic carriers. The prussian blue structure demonstrated a very good stability and performance with a maximum capacity of 150 mAh/g has been recorded with a Na2Mg[Fe(CN)6]system upto 500 charge – discharge cycles [9].

      4. Ramesh K. Guduru. et.al., reported the CuHCF for insertion of many ions such as Cu2+, Co2+, Pb2+, Nd2+, La3+, Sm3+, Y3+ and Ce3+ and found good reversibilityof intercalation with much decay in the capacity after many cycles. The specific capacities of CuHCF with the insertion of Nd3+, La3+, Sm3+, Y3+ and Ce3+ were 39, 38, 36, 61 and 51 mAh/g respectively. For the divalent ions Cu2+, Co2+ and Pb2+ exhibited capacities around 35, 19 and 57 mAh/g respectively.

      5. Zhen Liu, et. al., have investigated the structural and electrochemical properties of FeFe(CN)6 showed the great stability during Zn2+ ion insertion/extraction process and delivered the reversible discharge capacity of 120 mAh/g with a columbic efficiency of 99% at a current rate of 10 mA/g (≈ 0.1C) [10].

      6. Yan Yuan, et.al., have reported the synthesis of Na2Co3[Fe(CN)6]. 6.5H2O by co-precipitation method with an open framework structure, and is used as a cathode material which delivered the excellent cycling performance with 73% retention of initial capacity after 50 cycles at 50 mA/g with lithium ion batteries [11].

      7. XianyongWu, et. al., have reported the cathode material for sodium ion batteries, NaK-MnHCF@3D N-doped ultra-thin carbon networks (3DNC) electrode shows a high reversible capacity, cycling stability and good rate performance delivering a reversible capacity of ≈ 220 mAh/g at 20 mA/g, 136.6 mAh/g at 40 mA/g and 99mAh/g at 500 mA/g for over 1000 cycles[12].

      8. The reversible aluminium intercalation /deintercalation from an organic electrolyte into CuHCF is demonstrated by L. D. Reed, et. al., they have also suggested that the aluminium-solute complex is the intercalating species. The initial discharge capacities as 60 mAh/g and reversible capacity between 5 and 14 mAh/g [13].

      9. S. Jancy Sophia, et. al., reported the electrochemical behavior of the CoHCF @ Titania Nanotube (TNT) was found using cyclic voltammetry technique and voltammogram showed the reversible electron transfer behavior of CuHCF. The CuHCF@TNT-modified GC electrode gave an excellent electroanalytical oxidation of hydrazine in 0.1Mphosphate buffer(PB) medium [14]. 

      Summary

      Many works have been done on the Prussian blue materials as mentioned above. These materials have been used in various fields. It has its own importance in the energy storage system also. There are many reports on Prussian blue analogs have been reported in various multivalent ion batteries, and have shown promising results. Here, we have synthesized one such Prussian blue material, characterized and electrochemically tested for aqueous zinc ion batteries.

      Recently, there have been many reports on Prussian blue as cathode materials in aqueous rechargeable batteries. The main fields of interest refer to their preparation and characterization and also to their electrochemical properties: good conduction (including electron and ions transfer), electro-analysis and electro-catalysis. In recent years, considerable interest has been directed towards the investigations of transition metal-hexacyanoferrates, also known as Prussian blue analogous.

      METHODS

      Experimental section

      Synthesis of cobalt copper hexacyanoferrates (CoCuHCF).

      The cobalt copper hexacyaoferrates (CoCUHCF), a prussian blue analog is synthesized by a co-precipitation method. The generic formula of synthesized material is KCo0.5Cu0.5Fe(CN)6.CoCuHCF is synthesized by taking 60 mL of 0.05M Co(NO3)2.6H2O, 0.05M Cu(NO3)2.3H2O and 0.05M K3Fe(CN)6 which is added slowly to 30mL of distilled de-ionized water at 60 oC under vigorous stirring for 6h. The bluish colored precipitate is obtained. The precipitate is centrifuged and washed multiple time with deionized water, then with ethanol, and is dried at 70 oC under vacuum [15]. 

      Structural charaecterization

      Structural characterization of the synthesized material is done using PXRD, FTIR, FESEM and TGA.

      The structural analysis of cobaltcopperhexacyanoferrate (CoCuHCF) are studied by using Powder X-Ray Diffractometry (PXRD, X’Pert PRO. MPD, PANalytical). Further structural analysis is done by using Fourier Transformation Infrared spectroscopy (FTIR, Bruker, TENSOR 27). The morphology of the CoCuHCF is studied using Field Emission Scanning Electron Microscopy (Ultra 55 FESEM, kari Zeiss EDS). The thermogravemetric measurement (TG) is conducted by using Thermogravemetric Analyzer (TGA, TGA/SDTA851E) in nitrogen atmosphere from room temperature to 900 0C.

      Electrochemical charaecterization.

      The electrochemical measurement is done using two electrode CR2032 coin type cells with prussian blueas working electrode, zinc metal as counter and reference electrode. The working electrode is prepared by mixing the active material, carbon black, polyvinylidene fluoride (PVDF) and graphite in the ratio of 75:10:10:5 usingcyclopentanone solvent until homogeneous slurry is obtained. The prepared slurry is then coated on carbon cloth and kept drying for around 12h. The active material mass on carbon cloth is around 2-3 mg/cm2.

      The coin cells are assembled using 0.1M ZnSO4 as electrolyte(pH=5).

      The electrolyte is purged with N2 for 30 minutes before the cell assembly inorder to remove dissolved oxygen. The assembled cells are crimped using Crimper machine. CV tests are carried out at the different scan rates (2.5, 5.0, 7.5, and 10 mV/s) using instrument CHI 645D. Galvanostatic charge-discharge experiments are conducted using Neware battery cycler at various current densities at room temperature.

      INSTRUMENTAL METHODS

      In this section, the major focus is on the different instrumental methods we have adopted in order to determine the structural morphology and electrochemical properties of the synthesized Prussian blue material. The method include PXRD, SEM, FTIR, CV and battery testing device

      INSTRUMENTAL METHODS

      Powder X-Ray Diffraction(PXRD)

      X-ray powder diffraction (XRD) is the one of the important analytical techniques which is used for identification of phase of the crystalline material. It can also give information about the dimensions of the unit cell.

      Fundamental principles of X-ray Powder Diffraction (XRD):

      Max von Laue, in 1912, discovered that the crystalline substance acts as three- dimensional diffraction grating for x-ray wavelengths. It is also applicable to the spacing of planes in a crystal lattice. X-ray diffraction has become common technique to study the crystal structure and atomic spacing. Diffraction of x-rays is based on constructive interference of x-rays and a crystalline sample. X-rays are produced in cathode ray tube and then filtered to produce monochromatic radiations. These radiations are collimated to concentrate, and directed towards the sample. The constructive interference takes place due to the interaction between the incident rays and sample by satisfying Bragg’s law (nλ=2d sin θ). This law gives the relationship between wavelength of electromagnetic radiation, diffraction angle, and distance between the different planes in the crystal lattice.

      Fourier Transform Infrared (FTIR) Spectroscopy

      Infrared spectroscopy is the important study of interaction of electromagnetic radiations with matter in IR region. It is one of the most powerful technique which gives fingerprint information on the chemical composition of the sample. Electromagnetic radiations couples with molecular vibrations in this spectral region. The molecules will get excited to their higher energy state by absorbing IR radiations. The probability of a particular IR frequency being absorbed depends on the actual interaction between this frequency and the molecule.

      Slide13_1.JPG
        Fourier transform infrared(FTIR) spectroscopy.

        Infrared spectroscopy provides a way to have a look at structural details for proteins, amino acids, water molecule and the factors of redox sites. The continuous development of new and complementary experimental strategies and theoretical approaches opens its field of application in various research areas.

        Thermogravimetric analysis (TGA)

        The TGA technique measures the mass of a sample as it is heated, cooled or held at a constant temperature in a defined atmosphere. The TGA instrument which weighs a sample continuously as it is heated to temperatures of up to 2000 °C. Various components of the sample are decomposed and the weight percentage of each resulting mass changes as temperature increases and the weight loss can be can be measured. Thermo-gravimetric analysis (TGA) is ideal for characterizing the thermal properties of materials such as plastics, elastomers and thermosets, mineral compounds and ceramics as well as for chemical and pharmaceutical products. It is also used to determine the carbon content present in the electrode composites.

        Scanning Electron Microscopy(SEM)

        Slide8_1.JPG
          Thermogravimetric analyzer.

          A scanning electron microscope (SEM) is a tool to know and study the morphology of a compound. It helps us in seeing invisible worlds of micro-space (1 micron = 10-6m) and nano-space (1 nanometer = 10-9m). By using a focused beam of electrons, SEM reveals morphology of the compound clearly in micro or nano levels. SEM can magnify an object up to 300,000 times. A scale bar is provided on an SEM image. From this, the actual size andparticles in the image can be known.

          The basic working principle of a scanning electron microscope involves a beam of electrons generated from a suitable source, generally a tungsten filament or a field emission gun, which will collide with the surface of the specimen, then the nature of the secondary electrons or back scattered electrons emitted from the surface is examined to get an idea of the surface morphology. The chemical composition of the nanostructured electrode material characterized by SEM associated with Energy dispersive spectroscopy .

          A typical scanning electron microscope laboratory contains a machine with three components:

          1. The microscope column, including the electron gun at the top, the column, down which the electron beam travels, and the sample chamber at the base

          2. The computer that drives the microscope, with the additional bench controls

          3. Ancillary equipment that for example, analyses composition. This will be explored in the module on microanalysis rather than here under SEM. These three components and some of the other components they contain are described and illustrated below and in the figure.

          Slide9_1.JPG
            Scanning Electron microscope and Gold Sputtering machine.

            On magnification of the particle surface, the morphological features can be thoroughly studied as the percentage of various elements present. The combination of higher magnification, greater resolution, compositional and crystallographic information makes this characterization technique one of the most advanced equipment in this field

            Cyclic voltametry

            Cyclic voltammetry is a popular electrochemical technique. It is commonly employed to study the oxidation and reduction properties of a molecular species. It can be usually employed as an investigation tool to characterize new electrode systems. CV is performed by cycling the potential of the working electrode over a potential window and measuring the resulting current. Voltammograms are the plots of Current versus Voltage. Cyclic voltammetry instrument contains a high-speed data acquisition circuitry, a fast digital function generator, apotentiostat and a galvanostat. This technique gives high performance in stability and accuracy with advanced hardware and well-functioned software. It is an analyzable research platform for corrosion, sensor, electrochemical analysis, batteries, life science and environmental chemistry etc.

            Slide11_1.JPG
              CH instrument Electrochemical analyzer and cyclic voltammogram.

              Galvanostatic charge and discharge technique

              Battery tester is one of the powerful tool to get the capacity, energy density of the active material during charging and discharging using different current rate.Galvanostat is also known as amperostat. In this technique, the cell voltage is measured as a function of time by applying a constant current between the working and the counter electrodes. The specific charge/discharge capacity can be estimated for the material from the known time. The important feature of this instrument is that it has infinite internal resistance. The device is mainly used in electrochemistry. This device differs from common constant current sources by its ability to supply and measure wide range of currents from pico-amperes to amperes of different polarities.

              Slide12_1.JPG
                Arbin instrument and battery cycling setup.

                RESULTS AND DISCUSSION

                The synthesized material’s structural characterization is done using PXRD, FTIR, TGA and SEM. The results obtained are as shown in the figure.

                Figure 10a, shows the x-ray diffraction patterns of CoCuHCFPrussian blue material. All the XRD peaks corresponds to the face centered cubic lattice (FCC, space group Fm3m, JCPDS no. 73-0687). The peaks corresponding to the planes (111), (200), (220), (311), (222), (400), (420), (422), (440), (600) and (620) at angle 2qs 150, 170, 250, 290, 320, 350, 400, 450, 520, 550 and 580 respectively are well matching with the reference XRD pattern of CoCuHCF crystals in the literature [16]. The chemical characterization of the CoCuHCF is further characterized by FTIR spectroscopy. Figure 10b, shows the peaks corresponding to 2171cm-1 and 2101 cm-1 which can be assigned to –C=N- stretching in the material. The broad peak around 3400 cm-1 is due to the –OH stretching. The dominant peak around 600 cm-1is assigned for Fe-C stretching according to the literature [17]. Figure 10c, shows TGA pattern for CoCuHCF. The weight loss at around 200 0C is due to the interstitial water in the sample and it is found to be 21.5%.Further analyzing the TGA data we came to know that, above 200 0C the weight loss is due to the material decomposition. From this one can easily conclude that, CoCuHCF material is stable upto 200 0C above which there is a thermal decomposition of the material takes place as temperature increases[18]. Further, morphological analysis of the material is done using scanning electron microscopy. Figure 10d, shows the morphology of the CoCuHCF. We can see that the particles are random in shape and are highly agglomerated. This might be due to the higher centrifugal speed while performing centrifugation [14].

                Slide1_2.JPG
                  a) PXRD b) FTIR spectra c)TGA and d) SEM of CoCuHCF at 2µm.

                  Further, the material is used for the electrochemical characterization using cyclic voltametry and galvanostatic charge-discharge techniques. Figure11, shows the cyclic voltammetric plots of Zn-CoCuHCFhalf cell in 0.1M ZnSO4 electrolyte at various scan rates for 25 cycles. As can be seen in the plot, the first oxidation peak at all scan rate is seen towards more positive values (around +1.94V) but from second cycle onwards, the oxidation peaks are shifted towards lower potential and with the further cycling, the reversible redox peaks centered around +1.8 V and +1.62 V. This might be due to the de-insertion of Zn2+in the first cycle resulting in some side reactions of the material or might be due to the structural modifications. However, the reversibility in the redox peaks are pretty good which can be seen even after 25 cycles. Further, galvanostatic charge–discharge of Zn-CoCuHCF cell is done at different densities i’e 40 mA/g and 60mA/g of the active material the capacity obtained is very low as shown in the Figure 12. This might be due to the irreversibility of planar zinc foil which is reported in the literature or may be due to the zinc dendrite formation or due to some side reactions of the cathode material with water or its dissolution. To overcome, the irreversibility seen in Zn/Zn2+ from planar electrode, the electro deposition of zinc on carbon cloth is done. This is because, the electrode deposition onthe porous matrix results in 3D- Zn structure which will give homogeneous Zn2+ deposition and it avoids the zinc dendrite formation. This might results in the improved reversibility of the anode.

                  Slide4_1.JPG
                    Cyclic performance of the CoCuHCFhalf-cell in 0.1M ZnSO4at a). 2.5mV/s, b). 5mV/s and c), 7.5mV/s.
                    Slide6_2.JPG
                      : Galvanostatic charge-discharge of the Zn-CoCuHCFhalf-cell at a). 60 mA/g and b).40 mA/g.

                      ELECTRODEPOSITION

                      In order to increase capacity, the process of electrodeposition is done on carbon cloth to reduce the dendrite formation. Electrodeposition is the process of deposition of material on to the conducting surface from a solution containing ionic species to be deposited. Here, the deposition of zinc on carbon cloth is done using CH-instrument (CV, CHI 645) by chronopotentiometry technique.

                      In this set up carbon cloth is used as working electrode, platinum as counter electrode and saturated calomel electrode is used as reference electrode. The deposition of zinc on carbon cloth is performed at different currents ranging from 5 mA to 40mA, keeping potential constant [6].

                      Slide2_1.JPG
                        a) BeforeElectrodeposition. b) After Electrodeposition.

                        Further, the weights of carbon cloth before and after the electrodeposition is recorded and is as shown in the table below.Table 1: Electrodeposition of zinc on carbon cloth.

                        : Electrodeposition of zinc on carbon cloth.
                        Sl. No. Current (mA) Weight of the carbon cloth before electrodeposition (mg) Weight of the carbon cloth after electrodeposition(mg) Amount of zinc deposited(mg)
                        1. 5 62.4 65.1 2.7
                        2. 10 53.0 57.0 4.0
                        3. 20 51.3 61.6 10.3
                        4. 30 53.1 68.8 15.7
                        5. 40 56.8 76.7 19.9

                        It is seen from the table, as the current given for the deposition increases, the weight of zinc deposited also seen increasing, it as shown in Figure 14.

                        Slide14.JPG
                          Electrode deposition on carbon cloth corresponding to different current rates from 10mA to 40 mA.

                          PXRD is performed in order to confirm the deposition of zinc on carbon cloth. PXRD patterns of both carbon cloth and electrodeposited zinc (ED-Zn) on the carbon cloth is as shown in the Figure15. The peaks of zinc are matching well with the reference with the peaks corresponding to carbon cloth as well in the lower currents. We see that the carbon cloth peak intensities are decreasing with increase in the current given which indicates more amount of zinc is getting deposited at higher currents.

                          Slide5_1.JPG
                            PXRD of carbon cloth and electrodeposition of zinc (ED-Zn) at different currents rates.

                            For further electrochemical analysis galvanostatic charge-discharge is yet to be performed by using the zinc deposited on carbon cloth as anode and see the difference in the electrochemical properties of the cell.

                             CONCLUSION AND RECOMMENDATIONS

                            In the present report, cobalt copper hexacyanoferrate (CoCuHCF) the prussian blue material is synthesized using simple co precipitation method. PBAs have been used in energy storage system in recent years. It has been proved to be one of the promising cathode materials for aqueous rechargeable batteries. This material would allow excellent practical value with some challenges such aslow discharge capacity, poor cycling stability and low columbic efficiency. These factors have to be addressed. The performances of these electrodes are strongly affected by presence of structural imperfections such as vacancies and water molecules.Here, we have used PBA material as cathode in zinc ion batteries. The PBA material is characterized using PXRD, FTIR, TGA, SEM. The patterns obtained are all well matching with the literature. Further, electrochemical characterization is done using cyclic voltammetry and galvanostatic cycling. In cyclic voltametry, we are getting the redox peaks at +1.8 V and +1.6 V, the reversibility is good even after 25 cycles. Further, galvanostatic charge-discharge of active material is done, capacity was very low which is yet to be optimized.

                            Further, we tried to optimize the capacity of Zn-CoCuHCF by electrodeposition of zinc on carbon cloth which might reduce dendrite formation and side reactions of the active material. Hence, further more optimization has to be done in order to improve the capacity of Zn-CoCuHCF cell.

                             REFERENCES

                            1. Hand book of batteries, third edition, edited by David Linden and Thomas B. Reddy

                            2. JiangfengQian et al.Prussian blue Cathode Materials for Sodium-Ion Batteries and Other Ion Batteries Adv. Energy Mater. 2018, 8, 1702619  

                            3. JunLiu et al.Aqueous Rechargeable Batteries for Large-scale Energy Storage. Isr.J.Chem. 2015,55, 521–536

                            4. DipanKundu et al.  A high-capacity and long-life aqueous rechargeable zinc battery using a metal oxide intercalation cathode. Zhao et al., Sci. Adv. 2018; 4eaao1761 (2018). 

                            5. AishuakKonarov et al.Present and Future Perspective on Electrode Materials for Rechargeable Zinc-Ion Batteries.2018 American Chemical Society.(2018) 

                            6. Guozhao Fang et al.Recent Advances in Aqueous Zinc-Ion. American Chemical Society. (2018).  

                            7. ZhiguaHou et al. Surfactants widens the electrochemical window of an aqueous electrolyte for better rechargeable aqueous sodium/zinc battery. The Royal Society of Chemistry. (2017). 

                            8. A. Holland et al. An aluminium battery operating with an aqueous electrolyte. Journal of Applied Electrochemistry 48:243–25.(2018) 

                            9. C. Thammawong, et al., Prussian blue-coated magnetic nanoparticles for removal of cesium from contaminated environment, Res., 2013, 15, 1689.  

                            10.Zhen Liu et al.A Prussian Blue/Zinc Secondary Battery with a Bio-Ionic Liquid− Water Mixture as Electrolyte. American Chemical Society.(2016)  

                            11. Yan Yuan et al.Na2Co3[Fe(CN)]: A ppromising cathode material for Lithium ion and Sodium ion batteries. Journal of alloy and compounds, 685.(2016) 344-349. 

                            12. YuejiaMaoa et al.Capacitance controlled, hierarchical porous 3D ultra-thin carbon networks reinforced prussian blue for high performance Na-ion battery cathode. Nano energy 58(2019) 

                            13. JancySophia et al.Hexacyanoferrate-DecoratedTitaniaNanotube: CoHCF@TNTModifiedGCEasanElectronTransfer MediatorforTheDeterminationofHydrazineinWaterSamples. ISRN Analytical Chemistry.  

                            14. Dong Jun Kim et al.An Aqueous Sodium Ion Hybrid Battery Incorporating an Organic Compound and a Prussian Blue Derivative. Adv. Energy Mater. DOI: 10.1002/aenm.201400133. 

                            15. Dong Jun Kim et al. An Aqueous Sodium Ion Hybrid Battery Incorporating an Organic Compound and a Prussian Blue Derivative. Adv. Energy Mater. 2014, 4, 1400133. 

                            16. Andrea Paolella et al. A review n hexacyanoferrate- based materials for energy storage and smart windows: Challenges and perspectives.Royal society of chemistry.(2017). 

                            17. Xiaoxi Li et al. Alginate‑enfolded copper hexacyanoferrategraphene oxide granules for adsorption of low‑concentration cesium ions from aquatic environment.

                            Journal of Radioanalytical and Nuclear Chemistry(2019) 320:655–663.

                            18. Asheeshkumar, et al. Synthesis of cobalt hexacyanoferrate nanoparticle and its hydrogen storage properties. International Journel of hydrogen energy 43(2018) 7998-8006.

                             ACKNOWLEDGEMENTS

                            I would like to thank the Indian Academy of Sciences, for selecting me as a Summer trainee under Summer Research fellowship program 2019 and funding me to carry out this project at Indian Institute of Science, Bengaluru. I want to thank the organization for providing me the best research atmosphere and all the modern facilities, which helped me, complete this project in an effective manner.

                            With deep regards and profound respect, I avail this opportunity to express my heartfelt gratitude and indebtedness to my guide Prof. Aninda J. Bhattacharyya, Department of Solid state structural chemistry unit, Indian Institute of Science for accepting me as a student trainee and providing me the opportunity to work under his excellent supervision and guiding me throughout the project. His vital support, motivation, constant encouragement, concern and valuable suggestions have played a big role in shaping this project work. I thank him for the freedom given for me during the stay in lab. I am also grateful to my co guide Ms. AkshathaV. who has shared her precious time during the process. Her immense support and boundless efforts paved me to know the basics of solid state structural chemistry to carry out my project work. She was always there with hervaluable suggestions at every step during this tenure. Also, I would like to thank all others in lab for sharing their wisdom and supporting me during the course of this research. I would also like to thank all other summer trainees working in this laboratory.

                            I also thank Prof.Y. Arthoba Naik, Chairman, Department of Chemistry, Kuvempu University, Jnanasahyadri, Shankaraghatta, for allowing me to do my project at Indian Institute of Science, Bengaluru. Last but not least, I remember with gratitude my parents and family members who were always a source of strength, support and inspiration.

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