Claims:We Claim
1. A Zinc-Air battery, comprising Zinc as anode; Nickel titanate-carbon as cathode and electrocatalyst; and polymer gel electrolyte housed in a vented case.
2. The Zinc-Air battery as claimed in claim 1, wherein the electrolyte is a gel of potassium salt of polyacrylic acid in a solution of alkali metal hydroxide and zinc salt.
3. The Zinc-Air battery as claimed in claim 2, wherein the alkali metal hydroxide is selected from a group comprising potassium hydroxide and sodium hydroxide.
4. The Zinc-Air battery as claimed in claim 2, wherein the zinc salt is selected from a group comprising zinc acetate , zinc chloride and the like.
5. The Zinc-Air battery as claimed in claim 1, wherein the battery is of energy density ranging from about 800 to about 900 Wh/KgZn at about 5 mA/cm2 current density and of stability about 1000 charge-discharge cycles at about 5 mA/cm2.
6. A method of fabrication of Zinc-Air battery comprising Zinc as anode; Nickel titanate-carbon as cathode and electrocatalyst; and polymer gel electrolyte, said method comprising acts of-
a) preparing the cathode, comprising acts of –
i) preparing a slurry of nickel titanate-carbon in solvent comprising a binder;
ii) coating the slurry on to a gas diffusion layer; and drying to obtain nickel titanate-carbon cathode.
b) preparing polymer-gel electrolyte, comprising acts of- dissolving about 4-9% of potassium salt of polyacrylic acid in about 6M solution of alkali metal hydroxide and about 0.2M zinc salt; and
c) housing polymer-gel electrolyte between Nickel titanate-Carbon cathode, and Zinc anode in a vented case.
7. The method of fabrication of Zinc-Air battery as claimed in claim 6, wherein the solvent is selected from a group comprising N-methyl-2-pyrrolidone, ethanol, isopropanol and the like; and binder is selected from a group comprising tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer (nafion), Polyvinylidene fluoride (PVDF) and the like.
8. The method of fabrication of Zinc-Air battery as claimed in claim 6, wherein the drying is carried out at a temperature ranging from about 80 °C to about 90 °C for a period ranging from about 22 h to about 24 h.
9. The method of fabrication of Zinc-Air battery as claimed in claim 6, wherein the polymer-gel electrolyte is of thickness ranging from about 700µm to about 900µm.
10. A Zinc-Air battery, comprising plurality of batteries of claim 1 arranged in series or in parallel.
, Description:TECHNICAL FIELD
The present invention is in relation to electrochemistry. In particular, the invention is in relation to an electrochemical storage device, primary and rechargeable Zinc-Air battery. The Zinc-Air battery is based on bi-functional ORR/OER electrocatalyst and hydrogel-based aqueous electrolyte. The battery is of energy density 800-900 Wh/KgZn and rechargeability and stability of about 1000 charge-discharge cycles at 5 mA/cm2. The battery is economically viable due to simple method of fabrication, stable and efficient, cost effective electrodes and solid polymer gel based- electrolyte.
BACKGROUND OF INVENTION
The ubiquitous usage of electronic devices in day-to-day life is augmenting beyond leaps and bounds. The usage and demand has put in lot of impetus on the quality of battery used in electronic devices in terms of power and reusability. Among the different types of batteries manufactured and used, Metal-Air batteries have lion’s share across electronic industry and automobile applications owing to well established functions and use.
Zinc-Air battery in particular commands prominent position amongst the different Metal-Air batteries. The basics of both Zinc-Air primary and rechargeable batteries are known in the art. The recent focus on the primary Zinc-Air battery is to develop high power density devices with low cost and long duration of usage. The present technology requires lot of efforts to improve the duration of the performance of the battery with high power density. The rechargeable batteries are preferred considering the convenience of re-use, economical and environmental aspects. Mainly, Zinc-Air battery is only mechanically rechargeable. Main challenge in the electrical recharging technology is the difficulty in recharging the batteries based on catalyzing reactions involving oxygen evolution. The problem associated with the dendritic formation of zinc while charging resulting in shorting of the battery is another major issue. Also, they suffer from short cycle life.
There are many attempts in the recent past to improve the capacity/overall performance of the Zinc-Air battery. Patent literature JP2009-9983 discusses about the minimization of dendrite formation by the use of dendrite formation inhibitor; WO2010/109670 proposes the usage of electrolyte membrane of layered double hydroxides; US20140227616 discusses about the usage of inorganic solid electrolyte to reduce the dendrite and carbon dioxide incorporation.
Inspite of consistent research in the arena of Metal-Air battery, the need of a battery with high performance and high cycle life is still eluding. The present invention is aimed to provide a high performing Zinc-Air battery with over improved cycle life. Accordingly, the present invention provides a battery based on bi-functional ORR/OER electrocatalyst and hydrogel-based aqueous electrolyte.
SUMMARY OF INVENTION:
Accordingly, the present invention provides a Zinc-Air battery, comprising Zinc as anode; Nickel titanate-carbon as cathode and electrocatalyst; and polymer gel electrolyte housed in a vented case; a method of fabrication of Zinc-Air battery comprising Zinc as anode; Nickel titanate-carbon as cathode and electrocatalyst; and polymer gel electrolyte, said method comprising acts of-
a) preparing the cathode, comprising acts of –
i) preparing a slurry of nickel titanate-carbon in solvent comprising a binder;
ii) coating the slurry on to a gas diffusion layer; and drying to obtain nickel titanate-carbon cathode.
b) preparing polymer-gel electrolyte, comprising acts of- dissolving about 4-9% of potassium salt of polyacrylic acid in about 6M solution of alkali metal hydroxide and about 0.2M zinc salt; and
c) housing polymer-gel electrolyte between Nickel titanate-Carbon cathode, and Zinc anode in a vented case; and
a Zinc-Air battery, comprising plurality of batteries of present invention arranged in series or in parallel.
BRIEF DESCRIPTION OF FIGURES:
The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.
Figure 1: a) ORR activity for (i) NiTiO3(red) and Pt/C (black) in 0.5 M KOH. b) Linear sweep voltammograms for OER on NiTiO3(black), in 0.5 M KOH at 5 mV/s.

Figure 2: a) Schematics of Zn-Air battery used for the present study, different components used for the fabrication of battery. b) Photograph of PAAK-KOH gel and the corresponding impedance spectrato decipher ionic conductivity.

Figure 3: a) Galvanostatic discharge curves of present catalyst as air electrode at different current densities of 5 (black) and 10 (red) mA/cm2. Galvanostatic discharge profiles of primary zinc - air battery using present catalyst (blue), Pt/C (black) as air electrode at a current density of 5 mA/cm2.

Figure 4: a) Charge – discharge cycles at 5mA/cm2 with 10 min. cycle period. b) Charge – discharge cycled at 10 mA/cm2 with 1h. cycle period. c) Photographic image of the zinc electrode after charge - discharge cycles in gel electrolyte

DETAILED DESCRIPTION OF INVENTION
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the Figures, description and claims. It may further be noted that as used herein and in the appended claims, the singular “a” “an” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.
The present invention is in relation to a Zinc-Air battery, comprising Zinc as anode; Nickel titanate-carbon as cathode and electrocatalyst; and polymer gel electrolyte housed in a vented case.
In an embodiment of the present invention, the electrolyte is a gel of potassium salt of polyacrylic acid in a solution of alkali metal hydroxide and zinc salt.
In another embodiment of the present invention, the alkali metal hydroxide is selected from a group comprising potassium hydroxide and sodium hydroxide.
In still another embodiment of the present invention, the zinc salt is selected from a group comprising zinc acetate , zinc chloride and the like.
In still another embodiment of the present invention, the battery is of energy density ranging from about 800 to about 900 Wh/KgZn at about 5 mA/cm2 current density and of stability about 1000 charge-discharge cycles at about 5 mA/cm2.
The present invention is also in relation to a method of fabrication of Zinc-Air battery comprising Zinc as anode; Nickel titanate-carbon as cathode and electrocatalyst; and polymer gel electrolyte, said method comprising acts of-
a) preparing the cathode, comprising acts of –
i) preparing a slurry of nickel titanate-carbon in solvent comprising a binder;
ii) coating the slurry on to a gas diffusion layer; and drying to obtain nickel titanate-carbon cathode.
b) preparing polymer-gel electrolyte, comprising acts of- dissolving about 4-9% of potassium salt of polyacrylic acid in about 6M solution of alkali metal hydroxide and about 0.2M zinc salt; and
c) housing polymer-gel electrolyte between Nickel titanate-Carbon cathode, and Zinc anode in a vented case.
In still another embodiment of the present invention, the solvent is selected from a group comprising N-methyl-2-pyrrolidone, ethanol, isopropanol and the like; and binder is selected from a group comprising tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer (nafion), Polyvinylidene fluoride (PVDF) and the like.
In still another embodiment of the present invention, the drying is carried out at a temperature ranging from about 80 °C to about 90 °C for a period ranging from about 22 h to about 24 h.
In yet another embodiment of the present invention, the polymer-gel electrolyte is of thickness ranging from about 700µm to about 900µm.
The present invention is also in relation to a Zinc-Air battery, comprising plurality of batteries of batteries arranged in series or in parallel.
The present invention is on the development of an efficient and stable bi-functional catalyst for oxygen reduction (ORR) and oxygen evolution reaction (OER) in the Zinc-Air battery. The complexity involved in reaction due to its multi electron transfer makes it sluggish on most of the catalyst surfaces. A novel catalyst based on nickel titanate for efficient bi-functional activity is proposed. It is also shown to be very active in presence of flexible gel electrolyte and hence a solid state device (without any restrictions of the shape of the container) is possible. The battery delivers high specific capacity and good round trip efficiency in the gel - based electrolyte in open configuration. The battery works with ambient air and there is no need of pure oxygen from a cylinder, the working potential range is 0V to 2.6V vs. Zn/Zn+.
Specifically, the invention provides fabrication of gel-based rechargeable Zinc-Air battery in air- breathing mode based on a bi-functional catalyst. Zinc foil, typically of the dimension 1.5 cm ×1.5 cm with thickness of 0.25 mm is used for demonstration purpose. However, this can be varied depending on the size and the capacity required. It is also possible to use other forms of zinc, such as compacted zinc powder, coil, rod as anode and nickel titanate-carbon is used as cathode and a gel made up of 4-9 weight % partial potassium salt of poly (acrylate) (PAAK) polymer in 6 M KOH serves as the electrolyte.
Essentially, alkaline conditions lead to corrosion and loss of material in a battery. Unless the catalyst is stable for a long period and under battery (electrochemical) cycling conditions, it will be difficult to use it for any application. Currently, Nickel Titanate, NiTiO3 used extensively as pigment in paints and in tribology as a coating to reduce friction and wear in high temperature applications, is used as Bi-functional ORR and OER catalyst for the electrochemical reduction of oxygen (ORR) and oxygen evolution reaction (OER) in alkaline media. Thus, same material acts as the catalysts for both reactions (essential for metal-air battery to behave reversibly or for charge-discharge cycles). Additionally, the present material is very cheap (0.4 $/ kg) as compared to commonly used Pt and IrOx.
Experiments show that the present catalyst is very similar to Pt, the state of the art catalyst, for ORR and to standard iridium oxide catalyst for OER. Figure 1a, Table 1 and 2 provide a comparison between Pt and the present catalyst that clearly shows the advantage of the use of present material. Figure 1a shows the linear sweep voltammograms on NiTiO3 and Pt electrodes in an oxygen saturated alkaline solution. Under similar experimental conditions, NiTiO¬3- Carbon composition shows a clear positive shift in the onset potential as compared to state of art catalyst, Pt/C. This clearly demonstrates the positive effect of NiTiO¬3 as compared to the state of the art catalyst, Pt.
Figure 1b shows the electrocatalytic activity of NiTiO3 composite for water oxidation in 0.5 M aqueous KOH solution. The present catalyst of NiTiO3- composite reveals remarkable catalytic activity for OER with potential for attaining a current density of 10 mA/cm2 being 1.6V vs. RHE. An onset potential of 1.51 V corresponds to over potential of 0.37 V.
Table 1. Comparison of kinetic parameters on (i) Pt/C and present catalyst

Catalyst Onset potential/ V vs. RHE Half wave potential / V vs. RHE Tafel slope
/ (mV/dec) Mass activity/ mA/g No. of electrons @0.6 V Kinetic current density @0.6 V
Pt/C 0.91 0.81 (i) 68 (ii) 96 0.65 4 ~ 10 mA/cm2
Present catalyst 0.93 0.81 (i) 69 (ii) 102 0.61 4 ~ 10 mA/ cm2

This table compares the kinetic parameters deduced for oxygen reduction reaction (ORR) on NiTiO3 catalyst. The kinetic parameters define the efficacy of the catalyst and are generally compared with Pt. As observed from values given in the table, the present catalyst is efficient and behaves very similar to Pt/C.
Table 2. Comparison of different electrochemical parameters for OER on IrOx and the present catalyst

Catalyst Onset potential/ (V vs. RHE) Potential at 10 mA/cm2 (V vs. RHE) Tafel slope
(mV/dec)
IrOx/C 1.51 1.60 54
Present catalyst 1.51 1.55 54

The Table 2 compares the kinetic parameters deduced for oxygen evolution reaction (OER) on NiTiO3 catalyst. The kinetic parameters define the efficacy of the catalyst and are generally compared with state of the art catalyst, IrOx/C. As observed from values given in the table, the present catalyst is more efficient than IrOx/C.

The overall high bi-functional activity and long term stability in alkaline medium offered by the catalyst led to utilize it as an electrode for gel-based rechargeable Zinc-Air battery.

The battery design utilizing a gel-based electrolyte gives several advantages over the liquid-based electrolytes. The gel electrolyte gives flexibility and stability to the device, leakage of the electrolyte is avoided and hence hazards due to internal short circuiting,water loss during overcharging are avoided.The gel ionomer layer of favorable ionic conductance 0.824 Scm-1 as well as thickness of ranging from about 700-900 µm, typically 800 µm helps in preventing penetrating dendritic growth in the ionomer phase and thus avoiding shorting between the two electrodes. In the present invention, potassium salt polyacrylic acid (PAAK) polymer of about 4-7 M alkali metal hydroxide is used as the electrolyte. However, optimum performance is observed at 6M KOH concentration. Alkaline conditions may also be achieved using other salts such as NaOH without any change in performance.
A schematic of the battery devised according to the present invention is given in the Figures 2(a) and 2(b) along with the photographic images of different components used for assembling the battery. The casing of the battery is provided by a PTFE container with dimensions of 2.5 cm ×2.5 cm ×1.5 cm. Provision for passage of ambient air is made by drilling holes on the casing. Hence, the battery works by utilizing oxygen from ambient air. The total mass of the battery can vary approximately between 1g and 2g.
Figure 2(b) Photograph of PAAK-KOH gel and the corresponding impedance spectrum. The specific conductivity of the gel polymer electrolyte (GPE) is determined using the impedance spectrum and it is 0.824 Scm-1.
Experimental:
Figure 3(a) provides the discharge behaviour of primary gel based Zinc –Air battery with NiTiO3 as the ORR catalyst. The experiment is carried out at two different drain currents, 5 and 10 mA/cm2. The experiments show that the battery can deliver a high capacity of 720 mA h/ gzn. Figure 3 (b) provides comparison of the performance with NiTiO3 and Pt/C as catalysts, revealing discharge capacity of 720 mAh/ gzn for NiTiO3 while the capacity is 561 mAh/ gzn for Pt/C.

Table 3: Comparison chart for the performance of primary, gel-based, Zinc-Air battery with Pt/C (state of the art ORR catalyst) and present catalyst as air electrodes.

Parameters Air Electrode
Pt/C Present catalyst
Operating voltage
/ V 1.2 1.2
Capacity @ 5mA/cm2
/ mA h 875 1125
Energy density (normalized based on mass of consumed zinc) Wh/kgzn 677.4 871.2

Table 3 gives a comparative chart for the performance of primary gel-based Zinc-Air battery with Pt/C and NiTiO3 catalysts. The parameters are elucidated from figure 3b. The table 3 clearly shows NiTiO3 yields a remarkable performance as cathode in primary gel based Zinc –Air battery.

Table 4: Comparison chart for the performance of commercial Zinc - Air primary battery data with the battery fabricated using the present catalyst.

OCV/V Weight of battery/ g Drain current/ mA Working Voltage/ V Capacity/ mAh/gbattery Energy density/ Wh/gbattery
Duracell battery 1.4 1.8 1.7 1.10 333 366
Energizer 1.4 1.9 1.4 0.90 326 294
Panasonic 1.4 1.76 ------ 343 ------
NeXbattery 1.4 1.6 1.45 0.90 350 315
Present invention 1.4 2.0 10 1.21 539 652

Table 4 gives the parameters of commercially available primary Zinc-Air batteries along with the present invention.The present invention far exceeds the performance of the primary batteries available commercially. The present set of data clearly shows that the tailored battery shows outstanding performance as compared to the commercially available batteries, even at very high drain currents.
Figure 4 shows the rechargeability of the battery tested using pulse cycling technique. Figure 4 (a) and (b) show the charge –discharge curves observed for rechargeable gel- based zinc-air battery under ambient conditions (air). The charge-discharge profiles obtained for the battery at a drain current of 5 mA/cm2 with 10 minute cycle period shows remarkable and stable behaviour with potential retention of ~ 98 % after 1000 cycles. Figure 4(b) shows the cycling behavior of the battery at a high drain current of 10 mA/cm2 with 1 h. cycle period. Even at high drain currents and long charge-discharge cycle, the battery yields stable performance. (c) The photographic image of the Zinc electrode after charge-discharge cycle in the gel electrolyte clearly shows that Zinc can be reversibly deposited giving a clear indication that the present battery is an electrically rechargeable one. The dendrite formation is observed to be lower in the gel electrolyte as compared to the liquid-based electrolyte.

Table 5: Comparison chart for the performance of gel based rechargeable Zinc - Air battery with data reported elsewhere using other catalysts. All the batteries are in open configuration with atmospheric oxygen in ambient atmospheric pressure, as the oxygen input.
Battery configuration OCV/V Drain current Initial energy efficiency Final energy efficiency No. of cycles / total duration of time cycled.
Zn/PAAK KOH/ NiTiO3 composite
(Present data)
1.4 10 mA/cm2,
5 mA/cm2 60%
61.5% ~ 60%
60% 125 cycles /125 h.
1000 cycle/ 168 h.
Zn/ KOH-hydrogel/ Co4N-CNW-CC 1.36 5 mA/cm2 34.1% 19.2% 36 cycles /12h.

Zn/PVA/ carbon fibre 1.4 2 mA/cm2 56.1% 56.1% 34 cycles /6 h.
Zn/ QA-BatteryCellulose-GO/ Co3O4
1.4 1 mA/cm2 58.5% ~58.5% 30 cycles /10 h.
Zn/ ionic liquid PP membrane/ Pt/C+IrOx/C 1.45 10 mA/cm2 60.8% 41.8 107cycles /107 h.

Table 5 compares the output of the present Zinc-Air battery with certain reported materials in the published literature. The present battery package delivers stable performance with high round trip efficiency. The values clearly show there is a lot of scope for the present technique to be developed as a commercial product.Use of NiTiO3 improves the ORR and OER characteristics while carbon improves the electronic conductivity of the composite.
Experimental- Details of fabrication of a typical battery of the present invention
• Preparation of cathode
The nickel titanate-carbon composite of the ratio 1:1, electrode slurry is made in a solvent selected from a group comprising N-Methyl-2-pyrrolidone, ethanol, isopropanol and the like, typically for experimentation N-Methyl-2-pyrrolidone has been used with a binder selected from a group comprising tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer (Nafion), polyvinylideneflouride (PVDF) and the like, typically for experimentation , nafion has been used. The as-obtained slurry is then doctor bladed on to gas diffusion layer. The electrode is then dried in vacuum at 80-90°C for 24 h. The area of the electrode is around 1 cm2 with a catalyst loading of 0.8-1 mg/cm2.
• Preparation of anode
Zinc foil of purity 99% is used in the experiment (other forms like wire, powder, coil. can also be used).The foil is cut in different shapes and used. Typical size shown in the photograph (figure 2) possesses dimensions of 1.5 cm ×1.5 cm × 0.25 mm.
• Preparation of gel-polymer electrolyte
The gel electrolyte is prepared by dissolving 4-9 wt % of potassium salt of polyacrylic acid (PAAK) in a solution containing 6M KOH and 0.2 M zinc acetate. The as-prepared polymer gel electrolyte has ionic conductivity of ~ 0.82 S cm-1.
• Fabrication of the Zn-Air battery
The casing of the battery is provided by a PTFE container with a dimension of 2.0 cm ×2.0 cm ×1.5 cm with provision for passage of air and hence the battery could work under ambient air, utilizing atmospheric oxygen. The entire mass of the battery is close to ~ 2 g.At the bottom part of the casing, provision is made for fixing zinc foil. An extension made from the foil to outside gives electrical contact. Similarly, the NiTiO3 electrode is coated on to gas diffusion electrode having an active area of 1 cm2and used as the cathode. In between the zinc foil and the air cathode, a thin layer of gel electrolyte of thickness 800 µm is filled. Holes drilled on the top part of the casing allow passage of air in to the battery.
Thus the present invention utilizes economically advantageous materials to provide a high performing stable Zinc-Air battery in air breathing mode, rechargeable with large number of cycles, in open battery configuration.