Mohammad Qureshi, Department of Chemistry, Indian Institute of Technology Guwahati, Assam, India, for the award of the degree of Doctor of Philosophy. Adit Kumar Shah, Department of Chemistry, Indian Institute of Technology Guwahati was carried out under my supervision and was not submitted elsewhere for the degree.
Thesis Overview
Chapter 1: Current chapter discusses the urgent necessity to move towards a clean and renewable way of energy production, as excessive use of non-renewable resources cause’s
In this chapter, surfactant-free cuboidal MnCo2O4 was grown directly on the low-cost FTO substrate by a green hydrothermal method, providing a good ohmic contact, which helps in faster charge transfer between cuboidal MnCo2O4 and the FTO interface. modified with metal-free h-BN nanosheet, which exhibits excellent hole extraction ability helping to enhance the OER activity in the C-MCO/h-BN heterostructure. -MCO/h-BN exhibits an overpotential of 240 mV and a Tafel slope value of 66 mV/dec, making the electrode an efficient OER active catalyst.
Thesis overview and future perspectives
Introduction
Potentiostat
GLOBAL ENERGY AND ENVIRONMENTAL CHALLENGES
Therefore, it is estimated that for the foreseeable future, a large part of the world's energy needs will still come mostly from fossil fuels. 3%.8 CO2 in the atmosphere absorbs the infrared radiation of sunlight and thus increases the average temperature of the earth.
RENEWABLE ENERGY SOURCES
An ideal alternative for proper use of solar energy is to convert solar energy into chemical energy and store it for future use. When compared to some conventional energy sources such as gasoline (46.4 MJ kg−1), kerosene (46.2 MJ kg−1), diesel (45.6 MJ kg−1) and methane (55.5 MJ kg−1 1), hydrogen is also known as the zero-emission fuel has a higher calorific value of ~141.7 MJ kg-1,13,14 Coal gasification and steam reforming methane gas are currently the main sources of H2 production in a industrial scale, which in turn emit a large amount of greenhouse gases.15 Therefore, for sustainable, cost-effective, carbon-free and high-efficiency H2 production, there is an urgent need for an unconventional technology.
1.3 (PHOTO)ELECTROCHEMICAL WATER-SPLITTING
DIFFERENT STRATEGIES TO IMPROVE (PHOTO)ELECTROCHEMICAL PERFORMANCE
- Quantum Dot Sensitization
- Elemental Doping
- Morphology Modulation
- Co-Catalyst Modifications
- Semiconductor Heterojunction
- Electron/Hole Extraction Layer
Schematics showing various strategies that have been used for improving (photo)electrochemical water splitting efficiency of metal oxide-based electrodes (Refs. 42-48). Luyu Wang et al.; and the catalyst was found to increase the active sites in electrocatalytic water splitting.101 Different morphologies of catalyst used for (photo)electrochemical water splitting, shown in Figure 1.5.
MOTIVATION AND OBJECTIVES OF THE PRESENT WORK
While MnCo2O4, with high stability in harsh alkaline conditions, their generous amount and higher intrinsic conductivity make it a promising material for electrochemical water splitting. This chapter discusses the comprehensive synthetic protocols of the metal oxides and the co-catalysts, which were used to demonstrate (photo)electrochemical water splitting.
INTRODUCTION
REAGENTS AND MATERIALS USED
CHARACTERIZATION OF AS-SYNTHESIZED MATERIALS AND (PHOTO)ELECTROCHEMICAL DEVICES
2.4 (PHOTO)ELECTROCHEMICAL MEASUREMENTS
RHE, EoAg/AgCl = 0.1976 V at 25 °C, EAg/AgCl is the potential measured against the Ag/AgCl reference electrode, pH is the pH of the electrolyte used. RHE, 𝐸𝐻𝑔/𝐻𝑔𝑂 𝑂 = 0.098 V at 25 °C, 𝐸Hg/𝐻𝑔𝑂 is the potential measured against the Hg/HgO reference electrode, pH is the pH of the electrolyte used.
2.5 (PHOTO)ELECTROCHEMICAL PERFORMANCE PARAMETERS
- Incident Photon-to-Current Conversion Efficiency (IPCE)
- Faradaic Efficiency/Yield
- Applied Bias Photon-to-Current Efficiency (ABPE)
- Tafel Slope
- Turnover Frequency (TOF)
- ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) ANALYSIS
- Nyquist Plot
- Mott-Schottky Plot
- REFERENCES
The applied bias photon-current efficiency (ABPE) is one of the most important parameters for knowing the photoelectrode performance under applied bias conditions. The flat band (EF) and carrier density (ND) of photoelectrodes can be calculated using the following formula (2.10):13.
INTRODUCTION
Recent reports show protocols such as electrodeposition, hydrothermal synthesis, drop-casting, spray pyrolysis and flux-mediated one-pot dissolution process to improve photocurrent densities yields, but fall short of the theoretical photocurrent maxima of CuBi2O4.9-13. , in-situ growth using drop-casting synthetic procedure is most efficient and convenient for uniform and stable synthesis of CuBi2O4 directly over FTO. In this procedure, the structure of CuBi2O4 forms over FTO is by nucleation of CuBi2O4 nanoparticles, which helps fast diffusion of the charge carrier to the surface and increased surface area yielding more reaction sites.
EXPERIMENTAL SECTION
- Fabrication of CuBi 2 O 4 Photocathode
- Synthesis of Reduced Graphene Oxide (RGO)
- Fabrication of CuBi 2 O 4 /RGO Photocathode
The CuBi2O4/RGO photoelectrode was heated at 250 oC in room atmosphere for 1 hour to complete one complete cycle of RGO deposition on the FTO/CuBi2O4 surface, hereafter referred to as CuBi2O4/RGO-1. Other photocathodes with increasing cycles of RGO and named CuBi2O4/RGO-X (X=2-5) were produced by a similar procedure.
RESULTS AND DISCUSSIONS
- Powder X-Ray Diffraction (PXRD) Analysis
- Raman Spectroscopy Analysis
- UV-Visible Spectra Analysis
The peak centered at 92 cm-1 corresponds to the B2g in-plane mode of vibration of Bi rhombohedra, while 126 cm-1 corresponds to the A1g mode of translational vibration of CuO4 along the Z axis. It can be seen that the absorption throughout the visible range gradually increases with the increase in the number of cycles of RGO.
- Morphological and Structural Analysis
- O 4 O 4 /RGO-4
- O 4 /RGO-4
- X-Ray Photoelectron Spectroscopy (XPS) Analysis
- O 4 /RGO-4 Cu, Bi, C,O
- Photoelectrochemical Measurements
FESEM images of CuBi2O4-RGO-4 (Figure 3.4 (b)) show a homogeneous distribution of reduced graphene oxide on the surface of CuBi2O4. The photocurrent density of the CuBi2O4/RGO-4 photocathode remains constant within the error limits in the first two measurements.
Wavelength(nm)
Electrochemical Impedance Spectroscopy (EIS) Analysis
The Nyquist plot of CuBi2O4 and CuBi2O4/RGO-4 photocathode is fitted to an equivalent circuit shown in the inset of Figure 3.12 (a), where RS is the series resistance; Rbulk is the charge transfer resistance within the bulk of the semiconductor, Rct is charge transfer resistance between photocathode and electrolyte interface. Bode phase plot of CuBi2O4 and CuBi2O4/RGO-4 showing an enhanced lifetime of photoexcited electrons in CuBi2O4/RGO-4 then pristine CuBi2O4.
Stability and Faradaic Yield Measurements
Frequency (Hz) CuBi2O4
Mechanism for Enhanced HER Performance
Therefore, in the modified electrode CuBi2O4/RGO-4, when electrons are generated in CuBi2O4/RGO-4 after application of sunlight, RGO acting as a charge sink immediately extracts electrons from the conduction band of CuBi2O4. Hence reducing recombination, and increasing charge injection efficiency leads to improved water reduction efficiency of CuBi2O4/RGO-4.
CONCLUSIONS
RGO also stimulates charge injection from photocathode to electrolyte solution, resulting in higher photoelectrochemical reduction of water. 3 for CuBi2O4/RGO-4 from the Mott-Schottky plot confirms the proposed mechanism for the enhanced photoelectrochemical water reduction.
INTRODUCTION
Current work highlighted the dual modification of BiVO4, for improved PEC water oxidation efficiency by changing its electronic structure, deliberately creating oxygen vacancies to change the surface energy, facilitating charge transfers and improving the surface reaction kinetics for water oxidation. Herein, CNQDs facilitate a type II-type heterojunction with BiVO4:In and thereby enhance the water oxidation kinetics of (BiVO4:In)-CNQD photoanode.
EXPERIMENTAL SECTION
- Fabrication of BiVO 4 :In photoanode
- Synthesis of g-C 3 N 4 Quantum Dots (CNQD)
- Fabrication of (BiVO 4 :In)-CNQD Photoanode
In-situ grown BiVO4:In was immersed in the CNQD solution for different time intervals (7 h, 8 h and 9 h). The CNQD-filled BiVO4:In photoanode was further heated at 500 oC for 2 h to form the (BiVO4:In)-CNQD photoanode.
RESULTS AND DISCUSSIONS
- Powder X-Ray Diffraction (PXRD) Analysis
2 ( Degree )
- Raman Spectroscopy Analysis
- UV-Visible Spectra Analysis
- Photoluminescence (PL) Spectra
- Morphological and Structural Analysis
- FTIR Spectra Analysis
- X-Ray Photoelectron Spectroscopy (XPS)
Raman analysis shown in Figure 4.2 was performed to further confirm the formation and to know doping in BiVO4 photoanode. FESEM image of pristine BiVO4 and (BiVO4:In) CNQD photoanode in Figure 4.5 (b) and Figure 4.5 (c), respectively, shows that there was no significant change in BiVO4 morphology with indium doping.
Wave number (cm-1 )
Wettability Test of the Photoanodes
XPS analysis of the O1s spectra of pristine BiVO4 and BiVO4:In photoanodes were shown in Figure 4.11 (a) and Figure 4.11 (d).30 With indium doping, there is an increase in the oxygen vacancy (OV) peak in BiVO4:In -photoanode. To further confirm the improved wettability of In-doped BiVO4, contact angle measurements of BiVO4 and BiVO4:In were performed, as shown in Figure 4.11 (c) and Figure 4.11 (f), respectively.
Type –II Heterojunction Between (BiVO 4 :In)-CNQD and CNQD
The contact angle value for pristine BiVO4 and BiVO4:In photoanodes was found to be 71.5° and 22.2°, respectively, confirming the degree of interaction between BiVO4:In photoanode and water compared to pristine BiVO4 photoanode. 77 Part of this chapter has been published in J. Subsequently, the oxidation of water takes place in the valence band of CNQDs to produce oxygen. a) X-ray photoelectron spectroscopy (XPS) valence band spectra analysis BiVO4:In photoanode and CNQDs.
Photoelectrochemical Measurements
To further improve the photocurrent density, CNQDs were loaded onto BiVO4:In forming a type II heterojunction, which exhibited an impressive photocurrent density of ~2.42 mA/cm2 for the photoanode (BiVO4:In) -CNQD. Optimization of the (BiVO4:In)-CNQD photoanode was performed by loading CNQDs using chemical bath deposition on the BiVO4:In photoanode for different time intervals, as shown in Figure 4.13 (d). The increase in photocurrent density with dual modification in (BiVO4:In photoanode)-CNQD was attributed to the creation of oxygen vacancy by indium doping, which increases the availability of surface charge carrier density and improves charge separation in (BiVO4:In)-CNQD.
Wavelength (nm)
- Electrochemical Impedance Spectroscopy (EIS) Analysis
- Stability and Faradaic Yield Measurements
- CONCLUSIONS
- REFERENCES
There was a ~four-fold improvement in the IPCE value of (BiVO4:In)-CNQD photoanode compared to pristine BiVO4 photoanode, which is in good agreement with the photocurrent density of (BiVO4:In)-CNQD and pristine BiVO4 photoanodes shown in Figure 4.13 (a). 𝑃𝑙𝑖𝑔ℎ𝑡 (4.1) J is the photocurrent density of the photoanodes at specific applied potential, Vb is the applied potential (V vs RHE), Plight is the incident power density of the light (mW/cm2).
Hollow cuboidal MnCo 2 O 4 /nickel phosphate heterojunction: A promising oxygen evolution reaction electrocatalyst
INTRODUCTION
MnCo2O4, a stable spinel oxide, is gaining popularity in OER, due to its low cost, eco-friendliness, tunable morphological features and high stability in alkaline environment. Surface modification of h-MCO improves the overpotential required for OER activity in addition to improving charge transfer kinetics and turnover frequency in h-MCO/NiPi.
EXPERIMENTAL SECTION
- Synthesis and Fabrication Protocol of h-MCO
The area of FTO exposed to the solution was controlled by putting polyimide tape over the FTO, which can sustain very high temperature and pH. The NiPi layer was deposited over h-MCO through the electrodeposition technique.19 In a typical synthetic procedure, Ni(NO3)2.6H2O (3 mmol) and NaH2PO4.H2O (2 mmol) were dissolved in 300 mL of 1:1 (v/v) water - and ethanol mixture solution, and the solution was kept under stirring for 1 hour.
RESULTS AND DISCUSSION
- Powder X-Ray Diffraction (PXRD) Analysis
- Raman Spectroscopy Analysis
NiPi
Morphological and Structural Analysis
The Field Emission Scanning Electron Microscopy (FESEM) images (Figure 5.3(a,b)) show h-MCO grown directly over FTO substrate. Inset at Figure 5.3(a) shows the magnified view of h-MCO. The hollow cubic structure provides the more exposed active surface with better accessibility of electrochemically active sites to the electrolyte, ensuring sufficient contact between electrolyte and h-MCO for the redox process to occur.
MnCo2O4Eg
Raman shift (cm -1 )F2g
FTIR Spectra Analysis
Fourier transform infrared spectroscopy (FTIR) measurement was performed to determine the formation of NiPi and the composite, shown in Figure 5.6. The presence of two strong bands at 560 cm-1 and 640 cm-1 was attributed to vibrational bending modes of Mn-O and Co-O in MnCo2O4 spinel, resp.24 In the FTIR spectra of NiPi, vibrational.
X-Ray Photoelectron Spectroscopy (XPS) Analysis
Wave number (cm-1)
- Atomic Absorption Spectroscopy
- Faradaic Yield and Stability Measurements
- CONCLUSIONS
- REFERENCES
- INTRODUCTION
- EXPERIMENTAL SECTION
- Synthesis and Fabrication Protocol of C-MCO
- Synthesis of C-MCO/h-BN
- Mechanism for the Formation of Cuboidal MnCo 2 O 4
- RESULTS AND DISCUSSION
- Powder X-Ray Diffraction (PXRD) Analysis
- Raman Spectroscopy Analysis
Atomic absorption spectroscopy (AAS) measurement was performed to quantify the amount of h-MCO and NiPi loading in the h-MCO/NiPi electrode. The faradic yield of ∼98% was obtained for the h-MCO/NiPi electrocatalyst, confirming that the current density obtained in Figure 5.8 (a) is solely due to the OER activity of the catalyst.
Raman shift (cm -1 )F2gEgA1g
To further confirm the formation of composite C-MCO/h-BN, Raman spectra of C-MCO, h-BN nanosheets and C-MCO/h-BN were recorded, as shown in Figure 6.2. The presence of Raman bands parallel to C-MCO and h-BN nanosheets in C-MCO/h-BN indicates the formation of the composite.
MnCo2O4h-BN
- Morphological and Structural Analysis
- FTIR Spectra Analysis
- X-Ray Photoelectron Spectroscopy (XPS) Analysis
- Zeta Potential
- Electrochemical Measurements
- Faradaic Yield and Stability Measurements
Further to show, deposition of h-BN nanosheets over C-MCO, FETEM analysis of C-MCO/h-BN heterostructure was performed. HRTEM image of the composite C-MCO/h-BN in Figure 6.4 (d) with Inverse Fast Fourier transformed (IFFT) image shows different lattice spacing of 0.30 nm and 0.21 nm, which corresponds to (220) and (101) crystal plane of C -MCO and h- BN nanosheets respectively, confirming the formation of composite.27.
Turnover Frequency (sec-1)
CONCLUSIONS
Cubic MnCo2O4 modified with metal-free h-BN nanosheets, shows excellent hole extraction ability and aids in the increase of OER activity in C-MCO/h-BN heterostructure. Surface-modified C-MCO/h-BN exhibits efficient charge transfer kinetics at the working electrode and electrolyte interface, with an Rct value of 3.9 Ω.
Furthermore, C-MCO/h-BN shows ~5.5-fold enhancement in Cdl than its pristine counterpart, indicating more accessible electrochemically surfactant sites on it, therefore ~2.8-fold enhancement in TOF was observed. Green and sustainable C-MCO/h-BN electrode, exhibits excellent OER activity and exhibits 35 hours of exceptional long-term stability under harsh alkaline conditions, so it has the potential to be used in large-scale industrial applications.
Thesis overview and future perspectives
The basic principle of (photo)electrochemical water splitting and various strategies to increase the efficiency of semiconductor water splitting were also discussed. The water splitting efficiency of metal oxides is still lower than their theoretical efficiency.
LIST OF PUBLICATIONS AND
CONFERENCES/WORKSHOPS ATTENDED
Journal Articles
Devipriya Gogoi, Adit Kumar Shah, et al., Silver-grafted graphite-carbon nitride ternary hetero-junction Ag/g-C3N4 (Urea)-g-C3N4 (Thiourea) with efficient charge transfer for enhanced visible light photocatalytic green H2 production, Appl. Devipriya Gogoi, Adit Kumar Shah, et al., Step-scheme heterojunction between CdS nanowires and facet-selective assembly of Mnox-BiVO4 for efficient visible-light-driven overall water splitting, ACS Appl.
Conferences/Workshops