Chapter 1

1.1 Introduction

One of the most fundamental laws of physics is that energy can’t be created or destroyed, but only changed from one form to another, or converted. In Pakistan, electricity contributes about 43.5% of total energy demand and its share in households and businesses is increasing. The growing role of electricity presents both opportunities for greater efficiency and lower carbon emissions than traditional sources, but presents challenges in maintaining a consistent balance between generation and usage. Optimising energy management is essential in particular when taking into account the variability of generation from renewables and the ever-increasing number of electric vehicles. With fossil fuels diminishing quickly and environmental impacts becoming an issue, attention to renewable energy technologies has been seriously increased.

Solar energy is a renewable resource but is limited by availability, accessibility, and the environmental risks, but it is a promising option for global energy. Solar panels (also known as PV or photovoltaic cells) are panels that transform sunlight directly into electricity. Photovoltaic technology has undergone several generations over the years, and each generation has been more economical, and overall, more efficient, than the last.

1.2 Solar Cell Generations

1.2.1 First Generation

The first generation of solar cells have been based on crystalline silicon (c-Si) technology, which comprises monocrystalline cells and polycrystalline cells. They’re extremely reliable, have been on-task for more than 25 years and fall in the 15-26% efficiency range – that’s why they’re now the predominant part of the market. The entire industry has pretty much constructed its supply chain on them, so to speak, and there’s a proven, reliable manufacturing line for these cells. However, crystalline silicon solar cells also have their disadvantages, including expensive manufacturing, energy consuming production and mechanical stiffness, making them unsuitable for lightweight or flexible applications.

1.2.2 Second Generation

The thin-film technology-over the years, amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS)-has been used in second-generation solar cells. They are ideal for requirements where you need something that is flexible, lightweight, or covers large areas and reduces the material used and the costs of manufacturing. Some of them are rare and potentially toxic (particularly CdTe and CIGS) and their efficiencies typically range between 8% and 20% – a reduction compared to crystalline silicon. Even though thin-film panels are not the most efficient, they end up in applications where bend-ability matters, such as building-integrated PVs or handheld devices.

1.2.3 Third Generation

By utilizing novel materials and device architectures, third-generation solar cells (also known as emerging photovoltaic) will be able to transcend the traditional constraints on stability, efficiency, and cost. This group of devices includes organic solar cells, dye-sensitized solar cells (DSSCs), perovskite solar cells, and quantum-dot based devices. Although not as stable or efficient (typically only 8-13% compared to silicon), DSCs are incredibly attractive materials as they’re inexpensive to produce, semi-transparent and functional on flexible substrates.

Perovskite cells have already achieved efficiencies exceeding 25%, however, problems such as lead toxicity and environmental instability still prevent widespread implementation in the majority of applications. Organic cells based on conjugated polymers and small molecules are still only moderate in efficiency and durability, but they are light in weight and their optical properties can be tuned. Quantum-dot solar cells have the potential to reach very high efficiencies as a result of the plethora of exciton generation events, but these devices are at the proof-of-concept stage and have challenges with stability and up-scaling.

                                          Figure 1: generations of solar cell

1.3 Dye-Sensitized Solar Cells (DSSCs)

1.3.1 Working Principal

Dye-sensitized solar cells (DSSCs), first reported by O’Regan and Grätzel in 1991, are photoelectrochemical devices that emulate natural photosynthesis. In a DSSC, dye molecules absorb photons and become excited, injecting electrons into the conduction band of a semiconductorCurrent is produced as these electrons move to the external circuit. A redox electrolyte, usually the iodide/triiodide pair, regenerates the oxidized dye molecules, and the counter electrode catalyzes the reduction of triiodide back to iodide, completing the cycle.

Figure 2: DSSC cell

1.3.2 Components of DSSCs and Their Working

The photoanode, dye sensitizer, electrolyte, and counter electrode are the four primary parts of a dye-sensitized solar cell (DSSC). Every element directly affects the device’s performance and is essential to the photoelectrochemical process as a whole.

1.3.2.1 The photoanode

A transparent conducting oxide (TCO) glass substrate, such as fluorine-doped tin oxide (FTO), is often coated with a nanostructured semiconductor, most frequently titanium dioxide (TiO₂), as the photoanode. Its main functions are to efficiently transport electrons and offer a vast surface area for dye adsorption. The dye molecules introduce electrons into the TiO₂ conduction band when they are stimulated by sunlight. Electric current is subsequently produced by these electrons as they move through the mesoporous TiO₂ network and arrive at the external circuit.

                                               Figure 3: Photoanode for DSSC

 

 

1.3.2.2 Dye sensitizer

Dye sensitizer it absorbs sunlight and starts the electron injection process. While photons excite the dye molecules, the excited electrons are donated to the conduction band of the semiconductor. The oxidized dye molecules are then reduced, by electrons donated from the electrolyte. Organic dyes and natural pigments have also been investigated as potential candidates, however ruthenium-based complexes are very common due to their wide absorption range and stability.

                                                       Figure 4: Dye for DSSC

1.3.2.3 Electrolyte

The electrolyte serves as a medium for charge transport and frequently contains the iodide/triiodide (I⁻/I₃⁻) redox pair. It returns the dye to its ground state by regenerating the oxidized dye molecules through electron donation. Iodide ions undergo oxidation during this process, forming triiodide, which subsequently moves in the direction of the counter electrode. Despite having strong ionic conductivity, liquid electrolytes have drawbacks such leakage and long-term instability.

1.3.2.4 Counter Electrode

By catalyzing the reduction of triiodide ions back to iodide, the counter electrode completes the circuit. Platinum’s superior electrical conductivity and catalytic activity make it the most often utilized material. However, large-scale use is restricted by its high cost and poor long-term stability in iodide electrolytes. In order to replace platinum and increase the stability and economy of DSSCs, substitute materials such conducting polymers, carbon-based nanostructures, and transition metal sulfides are being researched.

Figure 5: counter electrode for DSSC

1.4 CoNi₂S₄/rGO Nanocomposite as Photoanode

1.4.1 CoNi₂S₄Properties       

The transition metal sulfide cobalt–nickel sulfide (CoNi₂S₄) has a spinel-like structure. Because cobalt and nickel ions coexist in various oxidation states, it has a narrow band gap, strong redox activity, and great electrical conductivity. Because of these characteristics, CoNi₂S₄ is ideal for electrochemical and photoelectrochemical applications, such as solar cells and supercapacitors. CoNiS₄ offers a large number of active sites and rapid charge transfer in DSSCs, which makes it easier for the dye to efficiently inject electrons. CoNi₂S₄ nanoparticles’ propensity to aggregate, which lowers the surface area accessible for dye adsorption and obstructs electron transport, is a significant drawback.

1.4.2 The function of rGO, or reduced graphene oxide

Reduced graphene oxide (rGO), a two-dimensional carbon nanomaterial, possesses a high specific surface area, excellent chemical stability, and electrical conductivity. rGO when used as conductive scaffold is capable of enhancing electron transport and suppressing charge recombination when integrated into DSSC photoanodes. Additionally, its multilayer structure provides a stable anchoring nanoparticle support which improves the dispersion of photoactive molecules. rGO acts as a conductive network which improves the interfacial interaction between the dye molecules and the photoanode surface and accelerates the charge transport.

1.4.3 Synergistic Effect of CoNi₂S₄/rGO Nanocomposite 

When CoNi₂S₄ and rGO were coupled as a nanocomposite, the synergistic effect surpasses the shortcomings of each component. CoNi₂S₄ not only acts as the catalytically active material and visible light absorber, but also rGO acts as the electron transport material and aggregation inhibitor of the nanoparticles. This hybrid assembly excludes the charge recombination and enhances the electron injection efficiency and offers more active sites for the adsorption of the dye. Hence the CoNi₂S₄/rGO nanocomposite photoanode demonstrates enhanced light harvesting and photoconversion efficiency as compared to traditional TiO2-based photoanode. Thus, thanks to the above advantages, CoNi₂S₄/rGO nanocomposites were considered as a potential system for designing the next generation DSSCs.

1.5 Problem Statement

The synergistic effect of the formation of CoNi₂S₄ and rGO was obtained as a nanocomposite, which has a synergistic effect superior to the sparseness of each material. And the CoNi₂S₄ with considerable catalytic activity and visible light absorption, rGO, which enhances the electron transport and suppresses RSA of the nanoparticles, are crucial elements in the assembled system. This hybrid structure features a higher quantity of active sites for adsorption of the dye, the superior electron injection performance, and inhibition of charge recombination. Consequently, the power conversion efficiency and the light collection efficiency of the CoNi₂S₄/rGO nanocomposite photoanode are higher than that of the conventional TiO₂ based photoanode. Taking these advantages into account, CoNi₂S₄/rGO nanocomposites are a promising candidate for the fabrication of next generation DSSCs.

Sinergistic effect of combination in which CoNi₂S₄ and rGO are combined as nanocomposite outperforms shortcomings of materials used individually. CoNi₂S₄ fills with high catalytic activity and visible light absorption provided by rGO with good electron transfer properties for the inhibition of nanoparticle aggregation. In addition to enhancement of electron injection efficiency and suppression of charge recombination, this hybrid structure provides additional dye-adsorbing surface area. Furthermore, the light collecting efficiency and power conversion efficiency of the CoNi₂S₄/rGO nanocomposite photoanode are enhanced compared with traditional TiO2 based photoanodes. Therefore, CoNi₂S₄/rGO nanocomposites are promising as a candidate for the design of next-generation DSSCs.

1.6 Objectives of the Study

The main aims of this study are:

• To synthesize CoNi₂S₄/rGO nanocomposites for photoanode application by using a controlled hydrothermal process.
• To create DSSCs using photoanodes made of CoNi₂S₄/rGO nanocomposite and evaluate how well they work in comparison to traditional TiO₂-based DSSCs.
• To assess the stability, efficiency, and charge transfer processes of the photovoltaic system
• To illustrate how CoNi₂S₄ and rGO work together to improve light harvesting and electron transport in DSSCs.
• To use cutting-edge methods to describe the produced nanocomposites’ optical, morphological, and structural characteristic

 

 

Chapter 2

2.1 Literature Review

Transition metal sulfides (TMSs) have attracted much attention recently as efficient counter electrode (CE) materials for dye-sensitized solar cell (DSSC). Among them, cobalt-nickel sulfide (CoNi₂S₄) has been investigated in great detail because of its unique structural, electrical and catalytic properties. In order to enhance the electrochemical performance and energy-conversion efficiency, different morphologies and composites of CoNi₂S₄ have been explored by several research groups. The basic framework of relevant study papers is shown below. Zhiewi Shi et al (2016) was one of the first investigations of CoNi₂S₄ as the counter electrode. Using the synergistic effect of nickel and cobalt ions, their experiments showed that the material has better electrical catalytic activity than the well-known platinum (Pt) CE. The power conversion efficiency (PCE) was enhanced as a result to 3.43% making it an attractive alternative to the costly Pt-based electrode.

In 2017, L. Chen and colleagues considered how nanostructuring might frustratingly further extend electrochemical activity. CoNi₂S₄ (CNT-like nanorod) had an efficiency of 4.10%, due to strong redox reaction. The group then shaped the material into nanoribbons, which increased surface area and lowered resistance to the flow of charge. Therefore the electrocatalytic activity significantly improved with the much higher efficiency of 7.03%. The results obtained clearly indicate the catalyst morphology as a key parameter for the enhancement of its performance.   

Sankar et al. (2021) further improved the performance of the supercapacitor by introducing the reduced graphene oxide (rGO) to the CoNi₂S₄ matrix to fabricate CoNi₂S₄-rGO composite. Moreover, the rGO creates better electron-transfer channels that significantly lowers recombination losses and enhances charge-transfer kinetics. Compared with the pure CoNi₂S₄ nanostructure, the composite displayed remarkable electrocatalytic activities of high efficiency of 8.93%. This work reveals the importance of carbon hybridization for improved electrochemical performance of transition-metal sulfides. 

Improvements in correction were demonstrated by Patel et al. (2025) through the method of fabricating a CoNi₂S₄-rGO film using a spray-coating procedure. The resultant homogeneous coating reduces the charge-transfer resistance to an extremely low level and enhances device stability. As a result, the once-thought optimal efficiency of the DSSC was obtained as high as 9.85%, which is the highest efficiency value in the considered studies. In addition, the study finds that CoNi₂S₄/rGO-based electrodes are scalable and long-term stable, which is a good candidate for real-life applications.

In conclusion, the literature shows a distinct pattern in the advancement of counter electrodes based on CoNi₂S₄: Compared to Pt, base CoNi₂S₄ exhibits superior catalytic activity. Nanostructuring (nanorods, nanoribbons) increases efficiency, decreases resistance, and increases surface area. Higher efficiencies are achieved by further accelerating charge transfer in hybrid composites (CoNi₂S₄ with rGO). High efficiency and long-term device stability are made possible by modern fabrication techniques(spray-coating). Overall, efficiency has grown steadily from 3.43% to 9.85% as a result of the transition from bulk CoNi₂S₄ to nanostructures and finally to hybrid composites. This indicates that CoNi₂S₄/rGO-based electrodes are a very promising substitute for traditional Pt electrodes in next-generation DSSCs.

Prior research on CoNi₂S₄ and its composites has mostly concentrated on their application as counter electrodes in DSSCs, where catalytic activity and efficiency were greatly enhanced by morphology (nanorods, nanoribbons) and carbon-based hybridization (rGO). However, we have taken a different approach in our research by using CoNi₂S₄/rGO composites as photoanodes.Although the majority of earlier research has focused on CoNi₂S₄ and its composites as counter electrodes in DSSCs, our study adopts a novel strategy by using CoNi₂S₄/rGO composites as photoanodes in different ratios (1:0.5, 1:1, and 1:2). This change in use enables a better comprehension of the ways in which these hybrid nanostructures may affect the dynamics of recombination, electron transport, and light absorption in the photoanode layer. We generated three distinct ratios of CoNi₂S₄/rGO (1:0.5, 1:1, and 1:2) in order to better understand the impact of composition.

When compared to the other ratios, the 1:2 ratio performed the best since it had the longest electron lifespan (19.8 ms) and the lowest resistance values (Rs and Rct). These results clearly show that a larger percentage of rGO improves electron transport, lowers recombination, and increases device stability overall.Therefore, our study shows that a straightforward yet efficient method to improve the efficiency of DSSCs when utilized as photoanodes is to optimize the ratio of CoNi₂S₄ to rGO. This method not only supports earlier research but also creates new opportunities for using CoNi₂S₄/rGO composites in applications other than counter electrodes. The results obtained indicate that conductivity increases and charge-transport routes are improved when more rGO is added to the composite, while recombination losses are reduced. Thus, the optimum combination of conductivity, catalytic activity, and stability is obtained with 1:2 CoNi₂S₄/rGO photoanode.

Chapter 3

Synthesis and Characterization Techniques

This chapter will explain two things, first, it will explain how beneficial my research is. Specifically, we will report the hydrothermal method and material selection for the CoNi₂S₄/rGO nanocomposite synthesis with various rGO ratios (1:0.5, 1:1, 1:2). Second, it will present a comprehensive overview of all characterization techniques, including XRD and electrochemical techniques such as CV, LSV, CA and EIS.

3.2 Hydrothermal Method

The hydrothermal synthesis process consists of various important steps for the production of nanomaterials with controlled properties. At first, the precursor materials are dissolved in water or an appropriate solvent in order to be a uniform solution. If needed, the pH of this solution is adjusted to give the correct chemical environment for the reaction. This solution is then poured into a sealed autoclave, a mighty vessel which is potent enough to endure high pressures and temperatures. The autoclave is maintained at a specified temperature (typically 100 to 220^o) for a specified time period (typically five to twenty-four hours). Under such conditions, the solution is under a pressure and this will lead to a dissolution of the precursors as well as an inducing of nucleation and growth of the nanocrystals or the nanostructures. After the reaction period, the autoclave was cooled to normal temperature.

The resultant solid nanomaterials are collected from the liquid by centrifugation or filtration and extensively washed to remove impurities. Finally, these nanomaterials are dried and, depending on the final use, may be further treated (calcination etc.). This process allows the synthesis conditions, such as temperature, pressure, pH, contact time, and precursor concentration, to be controlled in order to exactly optimize the size, shape, and composition of the synthesized nanomaterials. Besides, the hydrothermal method is also of importance in fabrication of controlled nanomaterials as efficient photoanodes for dye-sensitized solar cells, thus enhancing their performance and stability.

3.1.1 Chemical Used

Only high-quality chemicals were used to ensure the results are accurate and reliable. Potassium permanganate (KMnO₄, ~99%) was taken as the strong oxidant and graphite powder (C, ~99% pure) was chosen as the carbon source. Hydrogen peroxide (H₂O₂, ~33%) with which to reduce the amount of residual manganese species, concentrated sulfuric acid (H₂SO₄, ~97%) and phosphoric acid (H₃PO₄, ~85%) as acidic reaction media to facilitate oxidation. In the synthesis step, hydrazine hydrate (N₂H₄-H₂O, ~80 %) was used as a strong reducing agent. Furthermore, as metal precursors for the creation of the composite material, cobalt sulphate heptahydrate (CoSO₄·7H₂O, ~98%) and nickel sulphate heptahydrate (NiSO₄·7H₂O, ~98%) were used. Hydrochloric acid (HCl, approximately 37%) was employed for dissolving and pH correction, while thiourea (CH₄N₂S, approximately 98%) was utilized as a sulphur source.

 3.1.2 Making Graphene Oxide (Go)

Modified Hummers method was utilized for synthesizing Go in powder form [37]. In brief, sulfuric acid and ortho-phosphoric acid were mixed in a ratio of 9:1 (27 mL/3 mL) with 0.225 g of pure graphite powder under continuous stirring. After stirring for thirty minutes, the solution was gradually supplemented with 1.32 g of KMnO₄, a strong oxidizing agent, which was added slowly over the course of one hour. The mixture was then vigorously stirred for six hours, eventually turning into a dark green paste. To halt the oxidation and completely remove the oxidizing agent, 50 mL of cold DI water was added dropwise into the dark green paste, followed by 0.657 mL of H₂O₂, which produced significant heat.

The solution was allowed to cool to room temperature. After one hour of strong stirring, the mixture turned brown, confirming the formation of Go. Next, 30 mL of HCl and 70 mL of DI water were added, and the mixture was centrifuged for five minutes. The excess fluid was decanted, and the remaining product was thoroughly washed several times with an HCl/water solution until a pH of 6 was achieved. Finally, the collected Go solution was dried at 70 °C in an oven to obtain Go in powder form.

3.1.3 Preparation of reduced graphene oxide (rGo)

To create reduced graphene oxide (rGo), graphene oxide (GO) must be reduced as the last stage of the process.  This final phase required dispersing the Go powder into water. Then centrifuged and sonicated to accelerate the even distribution of the suspension in water. Then, hydrazine hydrate (with 3 mg of GO/ lμl, N₂H₄.H₂0) was poured into a beaker under stirring and putting beaker onto a hot plate for sixty minutes at 80°C. Reduction of Go was confirmed by its appearance from dark brown to black colour. Then wash it many times, proceeding to the last step where the solution was dried at 70°C in an oven to obtain rGo powder.

                            Figure 3.1: Schematic illustration of rGO Synthesis

3.1.4 Preparation of CoNi₂S₄/rGo nanocomposites

CoNi₂S₄ and CoNi₂S₄/rGo composites were synthesised with varying rGo contents, maintaining weight ratios of 1:0.5, 1:1, and 1:2. was synthesised by the hydrothermal method. Take 50ml DI water and thoroughly dissolve 0.5g of cobalt sulphate hepta-hydrate, 1g of nickel sulphate hepta-hydrate and 0.756g of thiourea. Let it stir at high rpm for two hours before the solution is placed within a Teflon-walled autoclave and kept at a constant 240°C temperature for five hours. After five hours, leave the autoclaved material to cool at room temperature. Black precipitates end product verified the formation of CoNi₂S₄ and its rGo composites. The final product was washed repeatedly through distilled water, then dehydrated completely at 70°C in an oven. To prepare the composites, rGo was added to the beaker in various weight ratios (1:0.5, 1:1, and 1:2) together with nickel–cobalt and the sulphur precursor. After an hour of sonication, the mixture was subjected to the above-described further processes.

                      Figure 3.2: Schematic illustration of CoNi₂S₄/rGO composites Synthesis

3.3 Characterization Techniques

3.3.1 X-ray Diffraction,

A strong non-destructive method that is frequently used for the structural characterisation of crystalline materials is X-ray diffraction (XRD). It offers useful details on the materials’ preferred orientation, strain, lattice characteristics, crystallite size, and phase identification. XRD works based on monochromatic X-rays dispersed by periodic atomic planes in a crystal constructively interfering with one another.

3.3.1.2 Working Principle

Bragg’s Law, which establishes the condition for constructive interference of X-rays reflected from distinct crystallographic planes, serves as the fundamental basis of XRD. The atoms in the crystal lattice scatter the X-rays when a monochromatic X-ray beam strikes it. A diffraction peak is produced by constructive interference when the path difference between the rays reflected from successive planes is an integer multiple of the wavelength. Bragg’s Law mathematically gives the relationship:

n =order of diffraction (usually taken as 1)

λ = wavelength of the incident X-ray; d = interplanar spacing between atomic planes

θ = angle of incidence (Bragg angle)

   Figure 3.3: Schematic representation of X-ray diffraction based on Bragg’s Law

In the diagram above, a monochromatic X-ray beam is incident at an angle on a crystalline sample. The beam is reflected by two atomic planes separated by a distance d. Constructive interference of the reflected beams occurs when the path difference is equal to an integer multiple of the X-ray wavelength, resulting in a diffracted beam detected by the XRD detector, as shown in Figure 3.3, in which two beams interact with each other.

3.3.1.3 Crystallite Size Calculation (Scherrer Equation)

D = crystallite size

K = shape factor (typically 0.9)

λ = wavelength of X-rays

β = full width at half maximum (FWHM) of the diffraction peak (in radians)

θ = Bragg angle

This equation is most reliable for crystallites smaller than ~100 nm and assumes peak broadening is mainly due to crystallite size rather than strain or instrumental factors.

3.3.4 Cyclic Voltammetry (CV)

Cyclic Voltammetry (CV) is basically the electrochemical technique we’re all used for testing the redox dance, battery-like performance and overall stability of new materials so pretty much a non-negotiable methodology for all of us.  In the context of dye-sensitized solar cell (DSSC), CV comes in very useful to make sure that the dye guys are really doing their oxidation and reduction job properly. In short, it simply verifies that the dye is effective.  It also provides an insight into the capability of the photoanode (CoNi₂S₄/rGo) to shuttle electrons and the ability of the redox mediator (I^–/I₃^–) to trigger the catalytic step. In CV measurements, the voltage is cyclically scanned while recording the current–potential response, which helps understand the electron transfer kinetics between the electrolyte, dye, and counter electrode. When we fabricate electrodes for dye-sensitized solar cells (DSSCs), CV is among the first techniques used to understand their electrochemical properties

3.3.4.1 Principle of Cyclic Voltammetry

Cyclic voltammetry involves sweeping the potential of the working electrode linearly with time, between two set potential limits, and then reversing the scan direction to return to the starting potential. The result is a current–voltage (I–V) curve, which provides insight into the electrode’s electrochemical behaviour [52].  Working Electrode (WE): the electrode under investigation (usually coated with the active material). Counter Electrode (CE): balances the current. Reference Electrode (RE): provides a stable reference for measuring potential (e.g., Ag/AgCl or saturated calomel electrode).

3.3.5   Electrochemical Impedance Spectroscopy (EIS)

The electron transport characteristics of DSSCs were examined using Electrical Impedance Spectroscopy (EIS). Generally, the impedances spectra are represented as two relaxation loops, which can be better fit with a classical equivalent impedance network by using Constant Phase Elements (CPE). In the high-frequency region, the first semicircle corresponds to the charge-transfer resistance at the counter electrode (Rc) which is coupled to the I3- reduction reaction whereas the series resistance (Rs) is due to the FTO substrate. The second semicircle at low frequency can be attributed to the resistance of recombination of electrons at the interface between TiO₂, dye and electrolyte.

Electrochemical impedance spectroscopy (EIS) is a potent electrochemical technique to study the charge transport and dynamics at a DSSC electrode. Simply put, EIS is the swiss army knife of fine tuning DSSCs due to its ability to help reveal losses that are impossible to find via standard IV/CV curves, e.g., too much series resistance, recombination, or bottlenecks in the ions being transferred. The integration into a circuit model with elements including Rb, Rct, constant phase elements (CPE), Warburg impedance (Zw) allows quite accurate modelling of EEI. These models can be used to understand how macro-mechanical properties like conductivity, porosity level, and chemistry of the surface impact performance. So, if you want a cripplingly high efficiency cell then you’re basically looking for low Rs and low Rct implying high conductivity and much faster electrochemical kinetics. To produce high-performance dye-sensitized solar cells, EIS is a crucial tool to determine ion transport, reaction kinetics, and internal losses.

3.3.5.2 Nyquist plot

Using the impedance graph at various frequencies, this tool is used to measure the current’s response to an AC voltage applied to the electrodes or DSSCs. The resulting data is typically displayed as a Nyquist plot, where the real part of the impedance (Z’) is plotted on the x-axis and the negative imaginary part (−Z”) on the y-axis. The first semicircle is in the high-frequency region, appearing by the charge transport resistance at the counter electrode/electrolyte interface, while the second semicircle shows the interfacial resistance at the semiconductor/electrolyte interface. The third semicircle is due to the impedance faced by the electrons during the diffusion process (Nernst diffusion/Warburg diffusion impedance) in the electrolyte. The equivalent Randles circuit, which was produced by fitting the actual data, is shown in the EIS graph. The double-layer capacitance, solution resistance, and the charge transfer resistance are denoted by the C1/C2, R1, and R2/Rct, respectively.

3.3.5.3 Bode plot

Electrochemical Impedance Spectroscopy (EIS) measures the current response that results from applying a modest sinusoidal AC voltage to the system. The applied voltage and current typically do not follow each other exactly in time; rather, there may be a shift or temporal delay between the two signals. We call this shift the phase angle (θ). Therefore, the phase angle indicates whether the system is inductive, capacitive, or resistive. The phase angle shows the temporal difference between the applied sinusoidal voltage and the current that results in a Bode plot. Purely resistive is shown by a phase angle of 0deg, capacitive by a phase angle of 90^o and inductive by a phase angle of +90^o.

A key point in the interpretation of EIS data is the phase change as a function of frequency that provides insight into the primary electrochemical processes (charge transfer, double-layer formation, and diffusion). The Bode plot is a common method for the presentation of data obtained by electrochemical impedance spectroscopy (EIS). The phase angle (ph) and magnitude (|Z|) are plotted on a logarithmic scale versus frequency in this figure. Plotting against frequency in this manner allows for a large frequency range to be displayed on one graph with each decade given equal importance and gives a clear indication of the change in impedance with frequency. Bode graphs plainly have the obvious advantage of directly plotting frequency dependences, as opposed to Nyquist graphs, making them especially useful for systems whose impedance is strongly frequency dependent.

The behaviour of basic circuit elements in EIS can be defined as follows. When a resistor has only one real component, it displays frequency-independent impedance. As the frequency increases, a capacitor’s impedance decreases, and the current leads the voltage by 90° (phase shift of –90°). On the other hand, when frequency rises, an inductor exhibits increasing impedance, with the current behind the voltage by 90° (phase shift of +90°).  Usually, equivalent circuits made up of passive components like resistors, capacitors, inductors, and diffusion-related elements are used to interpret EIS data. Ohmic resistance of the electrolyte or substrate, charge transfer resistance of electrode reactions, double-layer capacitance at the electrode/electrolyte interface, ion diffusion in the electrolyte, and resistance from adsorbed layers or protective films are among the electrochemical processes that can be described using these elements.

Even if each element’s physical meaning can change based on the system, a variety of electrochemical processes, including deposition, corrosion, ion reduction, and film generation, can be described by equivalent circuit designs. The average amount of time that photogenerated electrons stay in the TiO₂ conduction band in dye-sensitized solar cells (DSSCs) before recombining with the oxidized species (I₃⁻) in the electrolyte is known as the electron lifetime (τe). The efficiency of the device is often increased by slower recombination caused by a longer electron lifetime. The electron lifetime is calculated from the Bode phase graph (frequency vs. phase angle) using:

𝜏𝑒 = 1/2𝜋f_max

• f_max: The frequency in the intermediate-frequency range where the phase angle reaches its highest.
• The dominant electron recombination mechanism is indicated by the peak in the Bode phase graph, and the electron lifetime is directly correlated with its frequency.
• Higher photovoltaic efficiency, improved charge transfer, and slower recombination are all associated with higher τe.
  Efficiency is typically decreased because faster recombination results from a reduced τe.
  In certain optimized nanostructures, however, the power conversion efficiency (PCE) can still be improved even with a lower τe if:
  Improved dye adsorption (more light harvesting) is provided by the substance.
  There is more light scattering inside the structure, which improves photon use.

Chronoamperometry (CA)

Chronoamperometry (CA) is a widely used electrochemical technique that offers insights into charge-transfer dynamics and diffusion-controlled processes at the electrode–electrolyte interface. In this method, a stationary electrode is used, and the system is usually unstirred, so that the transport of electroactive species mainly occurs through diffusion. A constant potential is applied to the electrode, and the resulting current is recorded over time. At this potential, the electrode surface reaction proceeds either towards reduction or oxidation, depending on the system under investigation. The fundamental redox process can be represented as: Ox + ne- → Red. Both the oxidised (Ox) and reduced (Red) species are soluble in the electrolyte. The redox process is typically assumed to be reversible, indicating rapid electron-transfer kinetics, with the overall reaction rate primarily governed by the diffusion of electroactive species to the electrode surface. The current measured in chronoamperometry has two main components: faradaic current (IFar) and capacitive current (Icap). Faradaic current (IFar) is the result of electron transfer. When the potential is first applied the concentration of Ox at the electrode surface decreases rapidly, and only diffusion from the bulk can restore it. This current is given by Cottrell’s equation: IF(t) = (n FCA C0 D1/2)/(p1/2 t1/2). The expression indicates that the faradaic current decays as t-1/2. A straight line is obtained by plotting I versus t-1/2, and it is frequently used to determine the diffusion coefficient (D) and bulk concentration (C0) of the electroactive species using the straight line.

Capacitive current (Icap) is a result of charging of the electrical double layer at the electrode-electrolyte interface. This current reaches a maximum immediately as the double – layer charges are built up by the application of the potential step, but it decays exponentially on a time scale much shorter than the potential step, and becomes negligible compared to the faradaic contribution. The total current in chronoamperometry can therefore be written: I(t) = IFar(t) + Icap(t). At short times, the capacitive current is dominating, while at longer times the response is controlled by the diffusion-limited faradaic current. Overall, chronoamperometry can provide important details on diffusion-controlled kinetics, redox reversibility, and transport properties of electroactive species. Its capability to differentiate between capacitive and faradaic contributions makes it a key technique in studies of electrode behaviour in systems as diverse as energy storage devices, electrocatalysis, and dye-sensitised solar cells.

Chronoamperometry has a number of significant analytical applications, such as: 

1. Concentration determination – electroactivity at a point after application of a potential step allows concentration determination of an electroactive species by measuring the current. Assuming a calibration curve of current vs. concentration is obtained, it is obvious to quantify the species since the current is proportional to the concentration (Cottrell equation).
2. Coupled Chemical Reaction Analysis – The form of the current-time curve also reveals the homogeneous chemical reaction that may be going on in solution. Any deviation from the ideal Cottrell behavior most likely corresponds to follow-on chemistry such as catalytic and chemical decomposition.
3. Comparing with Current Methods: Although new and refined techniques of electrochemistry, such as cyclic voltammetry, rotating disk electrode methods and spectro-electrochemistry that allow better evaluation of concentration and reaction mechanism, have been developed, chronoamperometry remains important because of its basic nature and simplicity.
4. A explaining functions in Electrochemistry: Chronoamperometry is among the basic chronopotentiometric techniques which may be used as a foundation for other more advanced techniques. Due to its ability to clearly resolve capacitive and faradaic contributions, cell Coupled Conductance (CCC) has been a key experiment to study kinetics of every nucleation, diffusion and electrochemical mechanisms.

Linear Sweep Voltammetry (LSV)

As a basic potentiostatic sweep technique, LSV linearly sweeps the potential of the working electrode between the final and initial linear working electrode potential, while measuring the current as a function of time. As a boost: linear sweep voltammetry (LSV) refers to an electrochemical technique in which the current is measured after linearly changing the electrode potential over time. It’s mainly used to determine the potential for onset, overpotential, current density and redox response of a system. The LSV is a well-established technique for the investigations of reaction kinetics, the activity of catalysts, the process of charge transfers, and recombination phenomena. Although it provides solar cell efficiency indirectly, it is important for providing useful information about the electrochemical performance that indirectly influences efficiency. Thus, in order to quantify the efficiency of the solar cell it is extracted to its current-voltage (J-V) characteristic curve under illumination. The efficiency expression is:

In basic form, this equation represents the input light electrical energy that gets converted to electrical output energy.                                  

 

 

 Chapter 4

Results And Discussion

Characterization Techniques

4.1 Characterization of DSSC

Once finished fabricating the DSSC, we had to go through the characterization of it to see how it worked as a material and device. XRD was carried out to confirm the crystallization structure of the synthesized material. The results were confirmed by the diffraction pattern, as for the arrangement of atoms and the phase evolution. Potentiostatic electrochemical measurements were recorded using the system and a potentiostat. The resistance and capacitance of the cell was studied and the interface characteristics and the charge transfer resistance was obtained by electrochemical impedance spectroscopy (EIS). Cyclic voltammetry (CV) gave us the redox potential which is the reduced/oxidized impact on the cell’s performance. Moreover, we surveyed the I-V behavior of the device under a range of applied potentials by a linear sweep voltammetry (LSV).

This allowed calculation of the onset potential as well as the total photocurrent, which are relevant parameters for the energy conversion efficiency of the cell. Chronoamperometry (CA) was used to monitor time-dependent transients in current at constant potential which can be used to assess the long-term stability and the photo-response during long term operation. All these techniques yielded curves and plots, which give a comprehensive view of DSSC structural, electrochemical, and functional behaviour. We’ll discuss these findings in the next section.

4.2 X-Ray Diffraction

Used structural parameters are X-ray diffraction (XRD) techniques.

This technique is modeled after Bragg’s Law:

                                                              nλ = 2d sinθ

X-ray diffraction (XRD) is a non-destructive analytical characterization used to identify (identification) and characterize (structure, stage) the crystal structure of materials. To determine the crystal structure, X-ray diffraction (XRD) measurements are carried out by X ray powder diffractometer which is coupled with Cu Kα at λ = 1.5418Å within 2Ɵ range of 5 – 80֯ was at step size 4s^ (-1) (Brucker D8, Germany).

                      Fig 4.1 shows the XRD pattern for the samples CoNi₂S₄-rGO[5].

The sharp peak shows the crystallinity of prepared composites. The main diffraction peak of CoNi₂S₄-rGO observed at 16, 26, 31,32,39,50,55 and 57 can be radially indexed to the (111), (220), (131), (222), (400), (511), (440) and (531).These results indicated that CoNi₂S₄-rGO composite material shows better XRD results.

                               
                                                 Fig 4.2 shows the XRD pattern for the rGO.

Calculation of Crystallite size using Scherrer:

Micro strain using equation:   

 

The crystallite size of the prepared samples was determined using the Scherrer equation, which relates the broadening of X-ray diffraction (XRD) peaks to the size of crystalline domains. According to this relation, a smaller crystallite size results in broader diffraction peaks. In addition, the microstrain within the crystal lattice was calculated using the strain equation, which considers peak broadening due to lattice distortions.

The results are summarized in Table X. Pure CoNi₂S₄ exhibited the largest crystallite size of 41.24 nm, with a microstrain of 3.19 × 10⁻³. When reduced graphene oxide (rGO) was introduced, the crystallite size gradually decreased. For the composite with a ratio of 1:0.5 (CoNi₂S₄/rGO), the crystallite size reduced to 38.41 nm, while at a 1:1 ratio, it further decreased to 33.84 nm. The smallest crystallite size of 29.31 nm was obtained for the 1:2 composite.

This trend shows that increasing the amount of rGO restricts the growth of CoNi₂S₄ crystallites, leading to smaller domain sizes. At the same time, the microstrain values slightly increase from 3.19 × 10⁻³ to 3.30 × 10⁻³, which suggests that the addition of rGO introduces lattice distortions and structural defects within the composite.

Importantly, we were able to shrink the crystallite size of CoNi₂S₄ by addition of rGO with a slight increase in lattice strain, which may tune the material’s electrochemical and structural properties.
4.3 Cyclic voltammetry (CV)

Cyclic voltammetry (CV) is an electrochemically based technique that is typically used to analyze the redox behavior of electrode materials. In this work we applied CV to characterize the electrochemical activity of the CoNi₂S₄/rGO materials and the fabricated devices of the DSSCs. CV curves of the materials (Figure 4.3) reveal well-defined redox peaks, indicating a reversible transfer. The peak-to-peak voltage difference (ΔE_p) is nothing but the difference between the anodic peak potential (E_pa) and the cathodic peak potential (E_pc). A positive electrode potential difference of +0.34 V indicates suitable charge transfer at electrode surface. The composite with the highest rGO content generated a higher current response compared with other composites, indicating better electrochemical activity and higher conductivity. Similarly, the CV curves (Figure 4.4, right) of these devices show recognizable redox peaks.

Their ΔEp was reduced to 0.40 V, which is less than the ΔEp of the constituent materials. This reduction indicates improved electron-transfer kinetics and an increase in reversibility of the redox process throughout the entire DSSC device. Overall, the incorporation of rGO into CoNi₂S₄ improved the redox behavior of both materials and devices. The smaller ΔEp and higher current response in the composites demonstrate excellent electrochemical activity, which is favorable for efficient charge transport in dye-sensitized solar cells (DSSC).

Figure 4.3:  CV curves of materials.

          Figure 4.4 shows the CV curves of Devices.

 4.4 Electrochemical Impedance Spectroscopy (EIS) of DSSC

            EIS is a powerful tool which is used to analyze the working of cells and batteries. It gives us data about the resistive and capacitive components of cell. We had extracted our data of EIS for DSSC with potentiostat from which we got three different curves imaginary impedance vs real impedance, impedance vs frequency and phase vs frequency. To understand the data, we had to use Randle cell which is an equivalent circuit for DSSC.

 4.4.1 Real Impedance Vs Imaginary Impedance

Impedance is defined as the resistance to the flow of alternating current. Real impedance is just like the resistance which is opposition to the flow of current whereas imaginary impedance is a complex impedance which can be defined as the whole resistance of circuit viz resistance plus capacitance, For DSSC we got the curve for imaginary and real impedance as shown in fig 4.5 and figure 4.4 shows its equivalent rundle circuit. By comparing the DSSC to Randle circuit we can understand the behavior of cell, at the start capacitive resistance is high so the current passes through solution resistance and charge transfer resistor but none of the current flows through capacitive component which shows that at low real impedance the imaginary impedance is also low and it increases when real impedance increases; which increases due to the increase of frequency.

                                Figure 4.5 shows the EIS curves of Materials.

4.4.2 Impedance Vs Frequency Graph

Another graph produced by EIS analysis is that of frequency versus impedance. We can gain a thorough understanding of how our dye-sensitized solar cell operates to this curve. The DSSC’s frequency vs. impedance graph is shown below in figure 4.6. For all electrode compositions, the impedance steadily drops as the frequency rises. CoNi₂S₄/rGO (1:0.5) has the maximum impedance at lower frequencies, signifying a greater barrier to charge transfer. The impedance is lowest at the 1:2 ratio, indicating superior conductivity and quicker charge transmission. All samples’ impedance values converge at higher frequencies, indicating decreased resistance over brief timeframes. The distinction at low frequencies emphasizes how rGO content enhances electrical channels. As a result, raising rGO reduces system resistance and increases electron mobile.

Figure 4.6 shows the Bode Impedance Plot

4.4.3 Phase Vs Frequency Graph

Additionally, EIS data provides a phase vs. frequency graph that reveals the alternating current phase change. We can observe from the graph in the figure that there is a phase shift towards 90 degrees at low frequency and zero at high frequency, indicating that there is no current lagging behind voltage. Thus, at low frequency, the DSSC’s capacitive component, or simply the capacitor, as seen in the above figure, charges, causing the current to lag by 90 degrees, causing a phase shift. At high frequency, however, Capacitive activity across the frequency range is represented by the phase angle. The sample with the largest phase shift, CoNi₂S₄/rGO (1:2), indicates a greater capacitive contribution.

The 1:0.5 ratio, on the other hand, exhibits the least phase response, suggesting dominating resistive action. The intermediate 1:1 sample combines capacitive and resistive characteristics to maintain a balanced profile. Because resistive elements predominate at high frequencies, all samples tend to have lower phase values. Capacitive characteristics and charge storage efficiency are generally enhanced by increasing rGO content.

Figure 4.7 shows the Bode Phase Angle Plot

Electron lifetime (τₑ) from Bode phase plot

We can calculate electron lifetime from Bode phase plot by using this formula:

 

 

 

 

So it clearly seems 1:2 exhibit’s good results as it has low frequency and much electron lifetime.

4.4.4 Electrochemical Impedance Spectroscopy of DSSC (Devices)

The three devices’ Nyquist graphs show distinct variations in charge transfer resistance based on the CoNi₂S₄/rGO ratio. Due to its reduced rGO content, Device 1 (1:0.5) displays a big semicircle at high frequencies, indicating slower ion diffusion and high interfacial resistance. The narrower semicircle in Device 2 (1:1) indicates a balance between conductivity and structural stability as well as resistance to charge transfer. In contrast, Device 3 (1:2) confirms the advantageous role of larger rGO loading by presenting the shortest semicircle, which corresponds to the lowest resistance and most effective ion transport channels. The chosen equivalent circuit model is validated by the excellent overlap between the fitted and measured data for every device. All things considered, the EIS results unequivocally show that adding rGO improves conductivity and reduces interfacial resistance, with the 1:1 device exhibiting the highest electrochemical performance.

 

Figure 4.8 shows the EIS curves of Devices.

4.4.5 Impedance Vs Frequency Graph of Devices

As the frequency increases, the impedance gradually decreases for all electrode compositions. At lower frequencies, CoNi₂S₄/rGO (1:0.5) exhibits the highest impedance, indicating a higher barrier to charge transfer. At the 1:2 ratio, the impedance is lowest, suggesting better conductivity and faster charge transfer.  At higher frequencies, the impedance values of all samples converge, suggesting that resistance decreases over short periods of time. The difference at low frequencies highlights how electrical pathways are improved by rGO content.  Raising rGO thus improves electron mobility and lowers system resistance.

Figure 4.9 shows the Bode Impedance Plot of Devices

4.4.6 Phase Vs Frequency Graph

CoNi₂S₄/rGO (1:2), the sample with the biggest phase change, suggests a larger capacitive contribution.  Conversely, the ratio of 1:0.5 shows the least phase response, indicating dominant resistive action.  Capacitive and resistive properties are included in the intermediate 1:1 sample to preserve a balanced profile.  All samples typically have lower phase values because resistive elements are more common at high frequencies. Increasing rGO content generally improves charge storage efficiency and capacitive properties.

Figure 4.10 shows the Bode Phase Angle Plot of Devices

4.5 Linear Sweep Voltammetry (LSV) of DSSC

LSV is an electrochemical technique that measures the current response of an electrochemical cell as a function of applied potential, which is linearly swept over time. It’s a potentiostatic method where the potential between a working electrode and a reference electrode is ramped, and the resulting current is recorded. 

Figure 4.11 shows Current Vs Voltage Graphs

The Linear Sweep Voltammetry (LSV) analysis shows that CoNi₂S₄/rGO composites made with different ratios perform electrochemically. Of all the compositions tested, the sample with the 1:2 ratio exhibits the highest current response, indicating enhanced charge transfer capacity at the electrode–electrolyte interface. In order to reduce the internal resistance of the device, efficient redox processes require improved charge transfer kinetics. Consequently, the dye-sensitized solar cell (DSSC) achieves improved photovoltaic performance thanks to this rise in current density. Consequently, the optimal 1:2 ratio of CoNi₂S₄/rGO is the ideal ratio for raising the manufactured DSSC’s overall energy conversion efficiency.
4.4 Chronoamperometry (CA) of DSSC

Chronoamperometry (CA) uses a potential step at the working electrode followed by a recording of current as a function of time. This technique yields valuable new information on the long term behaviour of electrochemical systems. It is frequently used for calculating diffusion coefficients, studying other aspects of transport and the kinetics of electrochemical reactions. Since the current time response of CA allows for the determination of mechanistic information that is critical to understanding electrochemical processes at the electrode/electrolyte interface, CA is an indispensable tool.

Figure 4.12 shows Current Vs Time Graphs

The charge transport behavior of DSSCs fabricated with CoNi₂S₄/rGO composites was characterized by chromatoamperometry (CA). With this method, a potential step is introduced and the current response is measured at time. The first large current peak represents a fast electron transfer at the electrode/ electrolyte interface. The current first drops off and approaches a constant value over time which shows that the process is limited by diffusion of the ions in the electrolyte. The stability patterns and current responses of the composites are affected by the ratio of CoNi₂S₄ to rGO (1:0.5, 1:1, and 1:2). The greater current and improved stability of the 1:2 ratio indicate that better catalytic activity and more efficient charge transfer are exhibited for application in DSSCs.

 Conclusion

In the current work, successful preparation of CoNi₂S₄ and its subsequent integration with reduced graphene oxide (rGO) with different compositions (1:0.5, 1:1 and 1:2) to prepare CoNi₂S₄/rGO composites is reported. Whereas in most previous studies CoNi₂S₄ based materials have been used as counter electrodes, the composites were therefore used as photoanodes in dye-sensitized solar cells (DSSCs). And in comparison, with the different ratios, it turned out that the 1:2 CoNi₂S₄/rGO composite had the best performance. This was confirmed by cyclic voltammetry (CV) which indicated minimum potential difference between oxidation and reduction peaks to show overall good electrochemical reversibility.

Electrochemical impedance spectroscopy (EIS) indicated that the value of series resistance (R s) and charge transfer resistance (Rct) were lowest for the 1:2 composite while the Bode phase diagram showed that the electron lifetime (19.8 ms) was the longest, suggesting the highly efficient charge transport and inhibited recombination. In addition, the enhanced stability and catalytic efficiency of the composite at a narrow ratio 1:2 was validated by chronoamperometry, whereas the achieved highest photocurrent density was determined using linear sweep voltammetry (LSV). In summary, these results demonstrated the excellent potential of the CoNi₂S₄/rGO composite in the ratio of 1:2 for future energy conversion devices and snatched out to be the most stable and efficient photoanode for DSSC applications among all the studied ratios.

Future Prospective

This study provides a good foundation for the application of the prepared CoNi₂S₄/rGO composites as energy storage devices. Because the optimum ratio (1:2) provided good electrochemical performance, it is potentially useful in the development of supercapacitors. It is proposed that this compound may be used for optimizing the capacitance, energy density and long-term stability of the actual supercapacitor devices in future. Optimization of the material itself will be the main thrust to make it possess higher charge storage capacity and long working life. Due to the expanding requirements of flexible, wearable and lightweight electronic devices, the application of this material is attractive for wearable supercapacitors. Areas of research in this area will lead to the development of robust, reliable and practical devices for today’s energy requirements.

 

 

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