High energy storage capabilities of CaCu3Ti4O12 for paper-based zinc–air battery

Zinc–air batteries proffer high energy density and cyclic stability at low costs but lack disadvantages like sluggish reactions at the cathode and the formation of by-products at the cathode. To resolve these issues, a new perovskite material, CaCu3Ti4O12 (CCTO), is proposed as an efficacious electrocatalyst for oxygen evolution/reduction reactions to develop zinc–air batteries (ZAB). Synthesis of this material adopted an effective oxalate route, which led to the purity in the electrocatalyst composition. The CCTO material is a proven potential candidate for energy applications because of its high dielectric permittivity (ε) and occupies an improved ORR-OER activity with better onset potential, current density, and stability. The Tafel value for CCTO was obtained out to be 80 mV dec−1. The CCTO perovskite was also evaluated for the zinc–air battery as an air electrode, corresponding to the high specific capacitance of 801 mAh g−1 with the greater cyclic efficiency and minimum variations in both charge/discharge processes. The highest power density (Pmax) measured was 127 mW cm−2. Also, the CCTO based paper battery shows an excellent performance achieving a specific capacity of 614 mAh g−1. The obtained results promise CCTO as a potential and cheap electrocatalyst for energy applications.


Results and discussion
Physical characterization. The CCTO powder was prepared to employ an oxalate precursor route. The material obtained was earth brown. XRD characterization was conducted to obtain the material's crystal structure and phase composition, as shown in Fig. 1. The XRD patterns were obtained on the as-prepared CCTO catalyst shown in the Fig. 1a perfectly matching the ICDD data no. 01-075-1149 displayed in Fig. 1b. The results state the single-phase nature of the CCTO sample as there are no residual peaks of CuO and TiO 2 using this synthesis route. The crystallite size for CCTO was calculated to be 26 nm employing the Scherrer formula: where D is the size of crystallite; K is 0.9 (Scherrer constant); λ is 0.1546 nm; β is FWHM and θ is the position of the peak in the formula.
(1) D = K βcosθ www.nature.com/scientificreports/ XPS was also done to recognize the chemical composition, valence ions, and the species oxygenated. Supplementary Figure S1 reveals the entire XPS spectra of CCTO with peaks of Ca 2p, Cu 2p 3/2 , Cu 2p 1/2 , Ti 2p 3/2 , and O1s at respective binding energies. Figure 2a demonstrates the Ca 2p spectra of CCTO are best fitted with Ca 2p 1/2 and Ca 2p 3/2 , two spin-orbit doublets obtaining peaks at early binding energies . The peak of Ca 2p 3/2 deconvolutes and splits into peaks at 346.76 eV and 347.42 eV, whereas Ca 2p 1/2 was found at 350.62 eV. Figure 2b shows the Cu 2p spectra of CCTO acquiring Cu 2p 3/2 de-convoluted peaks at 932.19 eV, 932.53 eV, and Cu 2p 1/2 at 952.14 eV. The Fig. 2c 29 . An EDX analysis also confirms the formation of CaCu 3 Ti 4 O 12 perovskite catalyst by focussing on various areas during an EDX measurement to obtain the respective peaks, as are depicted in Supplementary Figure S2. In an EDX spectrum, the CCTO can be seen synthesized, with the quantities of Ca, Cu, Ti, and O measured in atomic percent to be 1. 35, 10.20, 7.87, and 1.71%, respectively. Table S1 shows the specifics of EDX spectra for the CaCu 3 Ti 4 O 12 sample. Scanning electron micrographs were also obtained to investigate the structure of the Ni foam loaded with the catalyst and to analyze the distribution of the material ink on the cathode, as shown in Fig. 3a,b. The EDS (Energy Dispersive X-ray Spectroscopy) elemental map displays the CCTO compound was well dispersed on the surface of the Nickel foam at the tens-of-micrometer scale producing the best catalytic results. Figure 3c-h shows the elemental distribution of Ca, Ni, Cu, O, Ti and C. The elements are seen well dispersed on the electrode surface. The CCTO catalyst was quantified with Cu and Ti in abundance, which acts as prime sites for the electro-catalytic activity for the sluggish oxygen reactions. Also, the O 2 can be seen in a considerate quantity leading to enhanced performance.
Supplementary Figure S3a Electrochemical characterization. To assess the electrochemical behavior, the OER and ORR measurements were taken on the Nova instrument using a three electrodes system comprising of glassy carbon rotating disk electrode (5 mm; RDE) as working electrode, Ag/AgCl, and Pt wire as reference and counter electrodes in KOH solution (0.1 M). Before evaluation, the electrolyte was infused with N 2 gas followed by O 2 gas for almost 30 min. LSV (Linear sweep voltammetry) was used to detect the oxygen reduction behavior of the perovskite material in the voltage range of 1.5 to − 0.5 V (vs. RHE) at the scan rate of 20 mV s −1 at various rotation speeds www.nature.com/scientificreports/ from 0 to 2400 rpm. Previous research has shown that the transition metals and the oxygen species act as an active sites for oxygen reactions. They can improve the structure and thus increase conductivity 3 . The best ORR catalysts have distinct surface planes and high surface water content. The ORR trend can be validly indicated by the E 1/2 , as it is commonly used to analyze the ORR catalytic activity of the electrocatalysts. Therefore Fig. 4a) shows the ORR trend with the E onset of 1.10 V and the half-wave potential (E 1/2 ) of 0.70 V. The current density of the CCTO sample was seen to be steadily increasing with the rotation speeds, which shows enhancement in the diffusion regulated procedure. In Fig. 4b K-L (Koutecky-Levich) graph was obtained using an equation shown to demonstrate the ORR pathway The I lev is directly proportional to the square root of the rate of rotation of an electrode. In the limited diffusion region, a graph of (J −1 (mA cm −2 ) -1 versus 1/w (K-L plot) was drawn. The linear nature of the graph exhibited 1 st order kinetics during ORR, and the number of the transferred electron was calculated to be 4. According to this assessment, chasing the 4 electron pathway, CCTO can effectively reduce O 2 .
To further analyze the perovskite material for oxygen evolution (OER), Linear Sweep Voltammetry (LSV) was conducted in the voltage range of 1.02-2.5 V at a scan rate of 10 mV s −1 as shown in Fig. 5a. With the onset potential (E onset ) set to 1.48 V, the graph depicts the sample's oxygen evolution reaction capability (vs. RHE). In the OER LSV curves, the overpotential (η) yielding a current density of 1 mA cm −2 is given as E j=1 . The E j=1 value of CCTO came to be 1.61 V. The Tafel slope at the onset potential was calculated to observe the rate of reaction. The Tafel slope was obtained using this equation as shown in Fig. 5b:   www.nature.com/scientificreports/ Using this equation, the Tafel slope was determined to be 80 mV dec −1. The following equation defines the potential difference (ΔE) at the oxygen electrode: The lower ΔE results in good capability in terms of OER-ORR. As a result, ΔE = 0.9 V for CCTO at the voltage complementing to 1 mA cm −2 for OER and E 1/2 in ORR at 2400 rpm, i.e., 1.61-0.7 V, demonstrates its bi-functionality.
The metal-air battery's (MAB) behavior is accredited to the redox activity of its transition metals and the interaction of orbitals. The ZAB was conceived & built to investigate and improve the efficiency, capacity, and durability of the CaCu 3 Ti 4 O 12 catalyst in conjunction with the electrolyte. To assess the performance of the asprepared catalyst in energy storage application, Two different batteries were developed: (1) a primary aqueous CaCu 3 Ti 4 O 12 battery, (2) a rechargeable CaCu 3 Ti 4 O 12 based paper ZAB.
The aqueous zinc-air battery (ZAB) setup was developed and evaluated in 6 M KOH + 0.2 M Zn(Ac) 2 electrolyte for the energy applications, The charge/discharge curve for rechargeable ZAB is demonstrated in Fig. 6a. The highest power density (P max ) was 127 mW cm −2 . The aqueous ZAB, when being discharged galvanostatically at 5 mA cm −2 , commits a constant discharge potential ̴ 0.12 V, as shown in Fig. 6b. The calculated specific capacity of an aqueous CCTO ZAB at the current density of 5 mA cm −2 came out to be as good as 801 mAh g −1 normalized to the consumed mass of zinc with the discharge rate to be 0.047 Ah. The chronopotentiometry charge/ discharge test was used to determine the durability and cyclic efficiency of the battery, as shown in Fig. 6c. The E Discharge E Charge Where E Discharge and E Charge are the final potentials of the charge/discharge profiles for the respective cycles. Therefore, the aqueous battery's round-trip efficiency was determined to be 63%. Furthermore, the straight potential graphs in Fig. 6d demonstrate CCTO to be an efficient bifunctional electrocatalyst for the paper-based zinc-air battery due to its greater cyclic efficiency and minimum variations in both charge/discharge processes.
The flexible and eco-friendly ZABs have attracted significant research attention due to advancements in energy storage devices. In this study, a flexible and eco-friendly solid-state rechargeable ZAB was developed by loading CaCu 3 Ti 4 O 12 on nickel foam (cathode) and zinc foil (anode) with the 6 M KOH + 0.2 M Zn(Ac) 2 electrolyte soaked in Whatman filter paper . Paper's porous morphology allows the electrolyte to diffuse efficiently, using it effectively. The galvanodynamic LSV technique (Current Density vs. Voltage) was employed to elucidate the charge/discharge polarisation curves for chargeable zinc-air batteries. As shown in Fig. 7a, galvanodynamic discharge-charge (current density vs. voltage) polarisation plots were taken at varied current values, at the current density ranging from 0 to 25 mA cm −2 . Figure 7b depicts the maximum power density from the current density vs. power density polarization curve. The power density indicates the current storing capabilities, which is 5.5 mW cm −2 at 11.45 mA cm −2 for the KOH-filter paper battery. It was recognized that the power density increases with increasing scan rates from 10 mA/s to 600 mA/s. A discharge profile was obtained to determine the discharge capabilities and storing capacity of the paper-based ZAB. Figure 7c reveals the discharge curve (Capacity (mAh g −1 ) versus Potential (V) at 5 mA cm −2 . The material was determined to remain stable for good hours in ambient air. This consequence in a firm discharge potential of ̴ 1.06 V. The specific capacity of the paperbased ZAB was calculated to be 614 mAh g −1 . For evaluating the cyclic efficiency and durability of the battery, the chrono-potentiometric charge/discharge plot for CCTO at a current density of 10 mA cm −2 is shown in Fig. 7d. Even after 3.5 h of continuous use, the filter paper battery was determined to be stable within a voltage range of 1.5-2.0 V. Gradually, as the potential is obtained at 2.0 V with a constant voltage gap recommending good stability. As demonstrated in insight, the cell voltage over-potential remains constant, i.e., 0.50 V, throughout the run of 3.5 h, indicating good stability characteristics. The corresponding round trip efficiency was determined to be 75% for the filter paper thus, proving itself an efficient zinc-air battery. A comparison plot is shown in Fig. 8a, displaying the high capacity of CCTO catalyst amongst other catalysts tested [35][36][37][38] . The OCV (open circuit voltage) of the produced ZAB's was also determined to be 1.44 V, as illustrated in the Fig. 8b.

Conclusions
In summary, a CaCu 3 Ti 4 O 12 (CCTO) material was successfully synthesized using oxalate-route. The perovskite material shows enhanced Oxygen Evolution and Reduction activities giving a good bi-functional behavior for secondary Zn-air batteries. To validate the performance of the CCTO materials for energy storage, electrochemically rechargeable secondary Zn-air batteries were developed. The proposed ZAB was tested under an alkaline system using aqueous, and filter paper soaked electrolyte (6 M KOH + 0.2 M Zn(Ac) 2 ). The zinc-air cells demonstrate remarkable performance and excellent cycling stability through discharge-charge cycles with a maximum power density of 127 mW cm -2 . Furthermore, by utilizing highly flexible electrodes and a flexible filter paper-soaked electrolyte membrane, the rechargeable Zn-air battery can be fabricated into an all-solid-state one, exhibiting both excellent specific capacity and cyclic stability. All of these battery tests have confirmed that the CCTO bi-functional catalyst developed in this work outperforms existing commercial bi-functional catalysts in practical Zn-air batteries.

Material characterization.
To investigate the crystal purity and structure of the prepared CCTO material, diffraction tests were performed with an X'pert diffractometer in a wide range of 2θ (5° ≤ 2θ ≤ 85°) with the step size of 0.0170 using Cu Kα 1 radiation (λ = 0.154056 nm) to evaluate the phase constitutes of the specimens. The X-ray Photo-electron Spectroscopy (XPS) characterization was performed on the Oxford Instruments (ESCA + model) Omicron Nanotechnology X-ray Photo-electron Spectroscopy system comprising a chamber with ultra-high vacuum connected to a 124 mm hemispherical electron analyzer and 1486.7 eV energy monochromatic source Al-Ka radiation. Further to obtain the composition and microstructure of the sintered pellets, FEI-Technai SEM-Sirion (equipped with Energy-Dispersive X-ray spectroscopy (EDX)) SEM (Scanning Electron Microscope) was used. To observe the specific surface area of the perovskite material, Brunauer-Emmett-Teller (BET) study was directed to get the nitrogen sorption isotherms using quantachrome Instruments Nova Touch Lx2, USA.
Electrochemical characterization. ORR-OER measurements were performed on a Metrohm Autolab (Electrochemical Workstation) in 0.1 M KOH electrolyte with a three-electrode system. A glassy carbon rotating disk electrode (RDE) with a diameter of 5 mm was taken as a working electrode, the reference electrode was Ag/ AgCl, and the counter electrode was platinum wire in KOH electrolyte.
Preparation of slurry for the working electrode. 5 mg CCTO catalyst was mixed with 10 mg of Vulcan carbon XC-72 in mortar-pestle and further dispersed in 2.5 mL Isopropyl alcohol (IPA) and 2.5 mL Distilled water (D.I) to prepare the ink for the cathode. In addition, 300 µL of Nafion solution (5 wt% (w/w)) was added. After an hour of ultra-sonication, 20 µL with the loading mass of 102 µg cm -2 of the slurry was drop cast over the glassy carbon electrode (RDE) for the electrochemical characterization. www.nature.com/scientificreports/ Development of zinc-air cell. A homemade solid-state paper-based zinc-air cell was fabricated using the CCTO perovskite catalyst. The catalyst was loaded onto a nickel foam as cathode, a filter paper saturated in 6 M KOH + 0.2 M Zn(Ac) 2 as an electrolyte, and a zinc foil as an anode. The cathode was developed by loading a slurry of Carbon black and PVDF in the ratio of 30:70 wt% dispersed in 1 ml of ethanol. The slurry completely coats the outer side of the Ni-foam, creating gas diffusion sites. The electrode is then hot pressed for 15 min at 350 °C. The slurry for the inner side was prepared by mixing the 35 mg of active perovskite material, and 35 mg of carbon black powder in 25 µL of the binder (Nafion solution: 5 wt%) was dispersed 1 ml of IPA using ultrasonication until a homogenous solution is obtained. 1 ml (loading mass = 0.033 g cm −2 ) of the slurry was coated on nickel foam (2 cm 2 ) which was further pressed at 150 °C for ten mins 39 .
To improve the performance of the ZAB, the battery components were assembled using a requisite battery cell under proper pressure. The cell consists of two Teflon sheets with 5 mm thickness containing flexibility in size with the help of four nuts and bolts at each corner. The lower sheet includes a platform where the entire battery component rests, i.e., the nickel foam (cathode), the paper-soaked electrolyte, and the zinc foil (anode). For oxygen transport, there is a provision of an air-breathing window on the upper Teflon sheet with the size 1.5 cm 2 . As the current collector, copper tapes were pasted to both electrodes, constituting the assembly ultimately. www.nature.com/scientificreports/