BET Surface
3.6. Lithium oxygen cell assembly and testing
3.7.1. Structural analysis
(i) Phase analysis
The as synthesized catalyst was analyzed by powder XRD. As shown in Figure 3.3 the strong and broad peak in the range of 2 theta value of 15o to 35o can be assigned to amorphous silica. At the same time, no obvious carbon related diffraction peaks can be detected, and the carbon content of SiO2/NC materials is confirmed to be completely amorphous nature.
Figure 3.3. XRD pattern of SiO2/NC and NC.
70 (ii) Morphology analysis
The morphology of SiO2 and composite SiO2/NC nanosphere was analyzed by FE-SEM. The monodisperse SiO2 exhibits spherical morphology with an average diameter of 350 nm (Figure 3.4 a,b). After hydrothermal reaction and carbonization, the as prepared SiO2 nanospheres were decorated with a thin film coating of carbon and agglomerated together shown in Figure 3.4 (c,d).
(Continue)
(a) (b)
(c) (d)
71
Figure 3.4. SEM images of as synthesized sample: (a, b) SiO2; (c, d) SiO2/NC and (e) NC
(ii) Xray Photoelectron spectroscopy (XPS)
The element compositions of the prepared SiO2/NC are determined by XPS measurement and the content of different elements are carbon, nitrogen and oxygen as 75.5, 10.24 and 8.44 percentage, respectively. Based on the XPS results, N-doping of the prepared SiO2/NC is confirmed. The SiO2/NC has N and O content is in the range of 10.24, and 8.44 at atomic percentage basis, respectively. In case of C1s, four different groups of C=C/C-C in sp2-hybridized domains, C-O/C-N (epoxy, hydroxyl or pyrrolic N), C=O/C=N (carbonyl, pyridinic N or quaternary N), and O=C-O (carboxyl) groups, are determined by the peaks at 284.4, 285.3 and 286.7 eV, respectively. In case of N1s, four types of nitrogen-containing groups could be observed. The peaks at about 398.2, 400.4, 401.2, and 402.5–403.4 eV could be attributed to pyridinic N (N-6), pyrrolic N (N-5), quaternary N (N–Q), and pyridine-N-oxide (N–X), respectively [116, 117].
(e)
72
Figure 3.5. Deconvoluted XPS data of SiO2/NC (a) Carbon 1s spectra (b) Nitrogen 1s spectra.
(a)
(b)
73 3.7.2. Electrochemical studies
(i) ORR catalysis
The LSV measurement on a RDE set up were conducted to assess the ORR activity of SiO2/NC in O2-saturated 0.1 M KOH solution. As shown in Figure 3.6, SiO2/NC samples were tested with various rotating speeds from 400 rpm to 2025 rpm. Furthermore, in the corresponding j−1 vs.
ω−1/2 plots, which shows an excellent agreement with the linear relationship at different voltages described in Koutecky-Levich (K-L) equation as follows
j−1 = jK−1 diffusion current density, respectively; ω is the rotation speed of the working electrode, n is the transferred electron number per oxygen molecule during the ORR process, F (F = 96485 C mol−1) is the Faraday constant, CO (CO = 1.2 × 10−6 mol cm−3) is the bulk concentration of O2, DO (DO = 1.9 × 10−5 cm2 s−1) is the diffusion coefficient of O2 in the 0.1 M KOH solution, ν(ν = 0.01 cm2 s−1) is the kinematic viscosity.
In order to compare the ORR activity of the catalyst, we evaluate the ORR activity of NC sample and elucidate that the SiO2/NC has high ORR catalytic activity when compared with NC this is mainly because of thin film deposition of nitrogen doped carbon on the surface of SiO2
74
leads to increase in mass transfer region. The E1/2 and onset value of SiO2/NC is 0.81 V and 0.76 V, respectively, which is much higher than NC. Moreover from Koutecky Levich plot it exhibits the 4 electron transfer ORR reaction.
Figure 3.6. Rotating disk voltammograms of SiO2/NC catalyst in O2 saturated 0.1 M KOH with sweep rate of 5mVs-1 at the different rotation rates indicated. The insets in show corresponding Koutecky–Levich plots (J−1 versus ω−0.5) at different potential.
75
Figure 3.7. Comparative LSV polarization curves of SiO2/NC, NC with commercial 20 wt % Pt/C catalyst at a scan rate of 10mV.s-1, measured in 0.1M KOH electrolyte
76 3.7.3.Lithium-oxygen battery performance
(i) Effect of solvent
To examine the preferable solvent for testing the performance of lithium-oxygen battery, we used tetraethylglycoldimethylether(TEGDME) and dimethyl sulfonyl oxide (DMSO) by using same catalyst of SiO2/NC and NC at the discharge current of 150mA.g-1. Using DMSO solvent as an electrolyte the discharge capacity of SiO2/NC catalyst shows 18,588 mAh.g-1, this is much higher than TEGDME solvent electrolyte and it capacity lasts for 15,655 mAh.g-1. In addition to that the discharge voltage of DMSO and TEGDME electrolyte base Lithium oxygen battery shows a platue voltage of 2.76 V and 2.51 V respectively, this means the ORR is much facile in DMSO compare with TEGDME electrolyte.
Figure 3.8.Discharge capacity plot of cathode sample prepared using SiO2/NC catalyst using various electrolytes in the current density of 150mAh.g-1
77 (ii) Catalytic activity performance
In the present study, the full discharge study of lithium-oxygen battery has been evaluated using SiO2/NC and NC catalysts with a current density of 150mA.g-1, and it shows the discharge capacity of 18588 mAh.g-1 and 9544 mAh.g-1 respectively. The increase in capacity of SiO2/NC is mainly because of incorporation SiO2 in the catalytic part and which results in adsorption of hydroxyl molecule from the electrolyte which results in increase of mass transfer region of oxygen reduction reaction.
Figure 3.9. Discharge capacity plot of cathode sample using various catalyst in the current density of 150mAh.g-1
78 (iii) Rechargeable Li-O2 battery
For rechargeable Lithium-oxygen battery, we examine the catalytic activity of the SiO2/NC by reducing the cut off capacity to 1000 mAh.g-1 and evaluated the lithium-oxygen battery cyclability performance at the current density of 150mA.g-1. The SiO2/NC catalyst shows the overpotential of 1.51 V at first cycle and in case of further cycle observation of decrease in overpotential, this significantly shows the SiO2/NC catalyst get activating by each rechargeable cycles, by transformation from SiO2 get reduced to Si material leads to activation of catalytic material and further decrement in overpotential.
Figure 3.10. Cycling performance of the as prepared SiO2/NC catalyst with the capacity cutoff of 1000 mAh.g-1 in the current density of 150mAh.g-1
79 compared with the XRD pattern of the fresh electrode, new diffraction peaks were observed for the discharged electrode. Although these peaks were very strong, they can be reasonably assigned as the Li(OH).H2O (as highlighted in Figure 3.11). These peaks indicate that LiOH.H2O is a major crystalline discharge product, and also observation of shift in the XRD pattern, this is mainly because of oxygen deficiency observed in the SiO2, which tells that the transformation of SiO2 nanosphere to Si.
Lately, the diffraction peaks of LiOH.H2O disappeared and the shift became relocated as the pristine fresh electrode when the battery was recharged to 4 V, which suggests that the discharge product LiOH.H2O is decomposed in the charging process.
The Li(OH).H2O formed on the surface of SiO2/NC nanosphere in the discharging process were directly observed by FESEM image and again disappearance of those product while recharged to 4 V.
In case of NC based electrode, the formation of both Li(OH).H2O and Li4CO3 as the discharge product, and the formation of Li4CO3 is strongly formed on the electrode surface shown by XRD pattern and FESEM image, while in the recharged electrode, the Li4CO3 discharge product is not decomposed under charging process because of strong bond formation of lithium ion with the carbonate leads to crack in the electrode surface when goes to high voltage.
80
Figure 3.11. XRD pattern analysed at different state of electrode (a) SiO2/NC catalyst and (b) NC catalyst
81
Figure 3.12. FESEM image of SiO2/NC catalyst electrode (a,b) pristine electrode (c,d) discharge electrode and (e,f) after recharged electrode.
82
Figure 3.13. FESEM image of NC catalyst electrode (a,b) pristine electrode (c,d) discharge electrode and (e,f) after recharged electrode.
83 3.8. Summary
We have successfully prepared the SiO2/NC nanosphere with thin film coated nitrogen-doped carbon on the surface of the material, by using combined sol-gel and hydrothermal approach.
Secondly, we have applied this material as the electrocatalyst for lithium-oxygen battery, for that we have studied the ORR and OER catalytic activity in alkaline electrolyte, later we have applied this catalyst for lithium-oxygen battery and its full discharge capacity of 18588 mAh.g-1 at the extracted current density of 150 mAh.g-1. Then in case of rechargeable battery, it last for 10 cycles with cut off capacity of 1000 mAh.g-1 at the current density of 150 mAh.g-1.
To understand the mechanism behind the catalytic performance, we elucidate the post analysis by XRD and FESEM of discharged and recharged electrode and confirmed the formation of and disappearance of Li(OH).H2O product while discharge and recharge, respectively.
84 Conclusions
Alternative energy storage devices namely Zinc-air battery and lithium-oxygen batteries have been studied to replace the fossil fuels in automotive application. In this upcoming technology highly active and durable electrocatalysts on the cathode side are required to catalyse oxygen reduction reaction during discharge and oxygen evolution reaction during charge for rechargeable batteries.
Here we developed primary and rechargeable zinc-air batteries with novel one dimensional nanstructured perovskite material as electrocatalyst. Firstly, we report the synthesis of a series of porous perovskite nanostructure, LaCo0.97O3- δ, with systematical Ni substitution in Co octahedral sites. The bifunctional electrocatalaytic activity of material was studied in alkaline based electrolytes. We show that the progressive replacement of Co by Ni in the LaCo0.97O3- δ perovskite structure greatly altered the OER electrocatalytic activity and the La(Co0.71Ni0.25)0.96O3-δ composition exhibited the lowest overpotential of 324 at 10 mAcm-2 in 0.1M KOH. To understand the real scale application we used the as prepared La(Co0.71Ni0.25)
0.96O3 nanostructured as cathode catalyst for aqueous zinc-air battery, which delivers high capacity of 705 mAh.g-1zinc in primary zinc-air battery. The rechargeable battery discharges with low overpotential of 0.792 V and 0.696 V at low capacity mode (400 mAh.g-1 catalyst) and high capacity mode (2500 mAh.g-1catalyst), respectively, this value is much lower than LaCo0.97O3-δ nanotube catalyst and precious Pt/C catalyst.
Secondly, we developed ultra-high energy density non-aqueous lithium-oxygen battery using thin film nitrogen doped carbon coated SiO2 material as cathode catalyst to enhance the
85
performance of lithium-oxygen battery. We are the firstly reporting metalloid material as cathode catalyst for lithium-oxygen battery and it delivers a capacity of 18588 mAh.g-1 of catalyst material at high current density of 150 mAh.g-1, which is nearly twice the capacity of nitrogen doped carbon material at same current density, and for rechargeable battery it last for ten cycle at the cutoff capacity of 1000 mAh.g-1. Later to elucidate the working mechanism behind this metalloid oxide catalyst, we reported post mortem analysis of cathode material of lithium-oxygen battery.
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