As the demand for sustainable energy grows, large-scale energy storage solutions are becoming in- creasingly essential to balance power supply and demand, ensuring grid stability and security. Acid- base flow batteries (ABFBs) have emerged as a promising energy storage solution due to their cost- effective electrolytes, high energy density, and environmental advantages. However, co-ion crossover in ABFBs remains a critical challenge, leading to pH imbalances, capacity fade, and reduced efficiency over cycling. In this study, the mechanistic aspects of co-ion transport in ABFBs are investigated, and their impact on battery performance is evaluated. Experiments are conducted under varying electrolyte concentrations, state of charge, current densities, and temperatures to quantify crossover ion flux and correlate it with key performance metrics, including round-trip efficiency, power density, and capacity fade rate. The results provide insight into the co-ion crossover mechanisms and their influence on the cycling stability of the battery. These findings contribute to the optimization of ABFBs by identifying operational strategies to mitigate co-ion crossover and enhance overall performance, advancing the development of high-efficiency ABFB systems. Keywords: Acid-base flow batteries, co-ion crossover, cycling stability, ion transport phenomena, ca- pacity fade, bipolar membranes, ion exchange membranes

In recent years, carbon dioxide and other greenhouse gases have been emitted on a large scale, leading to global warming, rising of sea levels, and causing many ecological problems. The use of fossil fuels is the chief source of large-scale emissions of CO2, but the reliability and economy of fossil fuels make it difficult for other energy sources to replace it in a short time. Reducing CO2 into other useful products like CO and CH4 is a potential solution. The electrochemical methods stand out in the field of CO2 reduction due to its mild and controllable conditions and the utilization of renewable source of energy. Design of efficient catalysts are essential for the electrochemical reduction of CO2. Transition metal dichalcogenides, such as MoS2 with high selectivity, low overpotential, and tunable structure is a potential candidate for catalysts. However, its performance still need to improve. The aim of this thesis us to develop a catalyst for CO2RR. As the intercalation of ions increases the conductivity of MoS2, the intercalation process of Ca2+ ions into MoS2 was studied. Furthermore, how the newly engineered material performs in the CO2RR was investigated.

In this study, two types of MoS2, namely pristine MoS2 and MoS2 intercalated with Ca2+ ions were used. The Linear sweep voltammetry (LSV), Electrochemical Impedance Spectroscopy (EIS) and Chronoamperometry (CA) were used to examine the electrochemical performance. Their CO2 reduction reaction performance in the KHCO3 aqueous solutions was investigated by gas chromatography (GC), high pressure liquid chromatography (HPLC), and Nuclear Magnetic Resonance spectroscopy (NMR). The catalysts were characterized before and after the intercalation process using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray Diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).

Due to global warming, there is a rising demand for green energy storage and green fuels. $CO_2$ reduction can help mitigate these demands by storing energy in chemical bonds, thereby creating fuels that are relatively easy to use and store. In this way, a new carbon cycle can be created. Catalyst materials play a key role in improving the selectivity, productivity and stability of the $CO_2$ reduction reaction ($CO_2RR$). MOFs are an interesting catalyst material for this, as their porosity enables large surface areas and their structure allows for tunable selectivity. 2D-MOFs are especially beneficial due to their higher conductivity and mass permeability compared to 3D-MOFs. Carbonization can be used to improve the limited conductivity and stability of MOFs making them more suitable for $CO_2RR$. This is why this study investigates the effects of carbonization on the catalytic capabilities of 3D and 2D-MOFs for the $CO_2RR$. Therefore, Cu-BTC was carbonized using different parameters. The resulting materials were investigated via a multitude of electrochemical techniques to further understand their catalytic capabilities. The 2D-MOFs Cu-THQ and Cu-HAB were then carbonized and the $CO_2RR$ products of the resulting catalysts were investigated. The analysis of the $CO_2RR$ products created by the different MOF derived catalysts shows that the main influence on the selectivity of these catalyst materials is the size and distribution of the Cu-cluster formed during carbonization. Since SEM and EDS results showed that the Cu-cluster formation is influenced by the applied carbonization parameters, this shows that these parameters can affect the selectivity of the resulting catalyst. A reasoning for why the Cu-clusters influence selectivity is discussed, however this remains to be proven by future work.

The reliability of battery performance is crucial for the implementation of long-term large-scale energy storage. The promising Iron-Air battery is known for its suitable properties relying on its advantageous iron-based anode, characterized by its high volumetric energy density, robustness, material abundance, low cost and scalability. The anode does however present drawbacks, such as self-discharge, hydrogen evolution reaction (HER) and initial- and gradual surface passivation. In previous studies the effect of porosity and specific additives have been presented as important factors in tackling these drawbacks. This multidisciplinary research aimed to study porosity and additive effects on the electrochemical behaviour and performance of iron-based mechanically-stable anodes, manufactured at elevated temperature and pressure. Specifically, the effect of 10wt% pore former K2CO3 on the initial capacity and on the passivation behaviour was studied by galvanostatic charge-discharge, with prior investigation on pore formation. In addition, the effect of high-performance additives (Bi2S3, Bi2O3, ZnS and FeS) was investigated on HER kinetics and the passivation behaviour, by cyclic voltammetry (CV) and galvanostatic cycling, respectively. Surface characterization was performed by optical microscopy, scanning
electron microscopy (SEM), and profilometry. Post-test analysis was carried out by optical microscopy and X-ray Photoelectron Spectroscopy (XPS). An intensification of surface cracks was observed for the K2CO3-rich sample before testing and its initial discharge capacity is 27 mAh/g less than a sample without pore former, but passivated later than the pristine sample in the full cell. Amongst the additives, the highest cycle of 100% capacity retention was marked at cycle 41 for FeS and for Bi2S3 at cycle 29 in the full cell, both showing 100% retention in the half cell up to 36 cycles. FeS might benefit from its readily available soluble reservoir of S2− ions. Bi2O3 showed the lowest capacity retention which might be explained by low conductivity, low solubility and/or lack of beneficial role of S2− ions. However, the additive Bi2O3 showed great reversibility of discharge products in the CV, confirmed by the lower O1s peaks and the lower respective Fe2O3 and FeOOH peak in XPS spectra. The pristine sample showed in the CV over the cycles increased current density and slope near the HER potential, with low reversibility. Apart from Bi2O3 and ZnS, the pristine sample showed lower capacity retention than FeS and Bi2S3, confirming the effective working of these additives. This systematic study portrays a good starting point for further studying porosity and additive effects on the electrochemical behaviour in hot-pressed anodes for the upcoming Iron-Air battery. Improvements can be assigned to cell design, electrolyte control and further detailed porosity characterization. Another type of current collector
might withstand higher current densities and anode thickness reduction can lead to higher discharge capacities over its lifespan.

The overdependence on the tradition energy source have caused a serious of ecologi-cal problems such as the increasing emission of carbon dioxide. To combat this and achieve carbon neutrality, attention has shifted towards sustainable energy, particu-larly wind, tidal, and solar energy. The solar cell catches significant interest due to its ability to directly convert solar energy into electricity through the photovoltaic (PV) effect. The silicon heterojunction (SHJ) cells got a lot of attention with high efficiency and simple fabrication process. The record efficiency of SHJ solar cell is 26.81% from Longi [1]. The fabrication process for SHJ solar cells involves the use of plasma en-hanced chemical vapor deposition (PECVD). However, the use of toxic and flammable gases in the doping process poses safety risks in laboratories. The parasitic absorption introduced by a doped layer, leads to drawbacks in solar cell performance. Conse-quently, alternative materials like dopant-free carrier selective contacts (CSCs), divided into hole transport layers (HTL) and electron transport layers (ETL), have attracted con-siderable interest. Transition metal oxides (TMO) and metal fluorides stand out in do-pant-free material research.
This study investigates three metal fluorides (LiF, MgFX, and SrFX) as ETL in SHJ solar cells. Initially, four plasma treatments (PTP, PT, PTB, and noPT) are examined as inter-face treatments in combination with LiF as ETL in SHJ solar cells. Results indicate that PTP is the most compatible with LiF, yielding the highest efficiency. Additionally, stud-ying the impact of LiF thickness on cell performance reveals that a 1 nm thickness shows higher Voc and FF, resulting in higher efficiency. Further exploration into the deposition order of transparent conductive oxide (TCO) and LiF highlights that depos-iting LiF on the rear side preserves passivation properties, ensuring device perfor-mance. Investigations into metal electrode contacts on the rear side reveal that a sput-tered Ti/Ag contact with LiF provides the best performance due to its no-venting dep-osition process. On the front side, copper plating proves superior to screen printing due to better-controlled grids and a less aggressive process, benefiting cell perfor-mance.
We expand the optimal parameters obtained from previous experiments to MgFX and SrFX. We explore the thickness influence on cell performance. The optimal thickness of MgFX and SrFX are 1 nm and 4 nm, respectively. Finally, SHJ solar cells integrated with MoOX as HTL and LiF, MgFX, and SrFX as ETL show significant improvement in light response within the 300-600 nm wavelength range. The combination of MgFX/MoOX yields the highest efficiency, with the champion device exhibiting 716 mV of Voc, 38.02 mA/cm2 of Jsc, 76% of FF, and 20.69% efficiency, surpassing reported efficiencies within the same cell structure.

In recent decades, there has been increasing concern about the impact of climate change on the earth. Various countries are actively developing sustainable energy technologies, of which solar cell is one of the new energy sources with the most attention. This thesis is based on poly−Si(O_x) cells consisting of SiO_x/poly−Si(O_x) passivating contact. Various methods have been investigated to mitigate the TCO (IWO) deposition induced passivation degradation and to optimize the screen printing process.

First, approaches to reduce the passivation degradation due to IWO deposition were explored. The power density and working pressure in IWO deposition were optimized. With utilizing 1.23 W/cm² power density and 5×10⁻³ mbar working pressure, the 𝑖𝑉_𝑂𝐶 degradation was reduced to 4.7 mV for 𝑛⁺ sample (NAOS-SiO_x) and to 7.9 mV for 𝑝⁺ sample after deposition of 75 nm IWO. Besides, hydrogenated amorphous silicon and AZO were used as buffer layers for the 𝑝⁺ SiO_x/poly−Si(O_x) samples, finding that they were effective in reducing sputtering damage. Then, the optimal post-annealing condition was investigated, which turned out to be vacuum annealing at 400 ^◦𝐶 for 30 min, recovering the 49 mV and 60 mV 𝑖𝑉_𝑂𝐶 for 𝑛⁺ sample (thermal-SiO_x) and 𝑝⁺ sample, respectively. Based on the above experiments, the first cells were prepared with a maximum efficiency of 17.4%.

Second, the existing screen printing process in the lab was optimized. The 0.37% organic solvent was added into the silver paste, reducing the viscosity of paste. The snap-off distance was changed to 0.02 mm in order to improve the continuity of printed grids. Moreover, the squeegee speed was optimized to 30 mm/s, limiting the spreading of silver paste to 57.3 𝜇m, much narrower than the initial spreading distance of 155.3 𝜇m.

Finally, a batch of poly−Si(O_x) cells were prepared by applying the above various optimized conditions. The cell with optimized IWO and optimized screen printing process performed best with an champion efficiency of 18.9%.

IIn the past decades, the rapid development of industry and economy leads to heavy dependence on fossil fuels which causes increasing concentration of CO2 in atmosphere. Now, excessive CO2 emission has accelerated global warming and caused a series of irreversible environmental problems. To address this problem, electrochemical conversion of CO2 is considered one of the most promising strategies to due to its mild operating conditions, utilization of renewable sources of energy and controllability of the process. However, CO2 reduction faces many difficulties which requires active catalysts to achieve effective conversion. Among all the catalysts, Copper (Cu)-based Metal-Organic Frameworks (MOFs) become veryattracting candidates due to their high porosity, tunable structure and special properties to convert CO2 into various hydrocarbons. In particular, two-dimension (2D) Cu-MOFs can further improve the electrical conductivity and catalytic activity. In this study, three types of Cu-based MOFs are studied as catalysts for CO2 reduction reaction (CO2RR). Among them, Cu-BTC is a conventional 3D MOF, Cu-HAB and Cu-HHTP belong to 2D MOFs. In particular, Cu-HAB has not been used as catalyst for CO2 reduction before. To investigate their catalytic performance for CO2 reduction, the surface morphologies and bond structures were first characterized by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Fourier-transform infrared spectroscopy (FTIR). Then Linear sweep voltammetry (LSV), Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS) and Chronoamperometry (CA) were used to examine the electrochemical performance and stability of them. In the last, the product selectivity of Cu-HAB and Cu-HHTP wastested using Gas Chromography (GC) and Nuclear Magnetic Resonance (NMR). From the results, 2D MOFs show higher catalytic activity, capacitance and conductivity which suggest 2D structure is beneficial for catalytic process. Especially Cu-HAB shows the best conductivity and catalytic activity due to presence of Cu-N4 coordination in the MOF structure as well as smaller particles. But from the point of view of product selectivity, Cu-HHTP performs better than Cu-HAB which may indicate Cu-O4 sites can better engage with CO2 molecule and transfer it to other products.

For the past few decades, the aerospace industry has been actively looking for measures to tackle the problem of localized corrosion of aluminium alloy 2024-T3. One such measure was the use of hexavalent chromium salts as corrosion inhibitors. However, recent studies have reflected the environmental concerns and health impacts associated with these compounds. Since then, there has been a quest for alternatives that can be used as corrosion inhibitors for AA2024-T3.

This thesis is aimed at studying these compounds by testing their inhibitive properties. Selecting an inhibitor from a set of compounds and recommending the best one is a crucial process. The research focuses on the screening of these inhibitors through various electrochemical techniques such as Potentiodynamic Polarisation(PDP), Linear Polarisation Resistance(LPR), and Electrochemical Impedance Spectroscopy(EIS).

The electrochemical experiments during the first 24-hour exposure of AA2024-T3 to inhibitors exhibited that 2,5-Dimercapto-1,3,4-thiadiazole acts as a corrosion accelerator, while Na-Mercaptoacetate and Mercaptobenzimidazole showed the most inhibitive performance among the tested organic compounds. On monitoring their nature during 24 hours, it was observed that they show stable polarisation resistance values after 6 hours. Among the inorganics, Cerium compounds were seen to have better inhibitive properties than the Lithiumcompounds. Additionally, surface studies such as Fourier Transform Infrared Spectroscopy (FTIR) revealed the presence of thiol/mercaptan and carboxyl groups on samples exposed toNa-Mercaptoacetate. X-ray Photoelectron Spectroscopy (XPS) showed oxides of Cerium on samples exposed to Cerium Nitrate. Finally, the correlations between the inhibition efficiencies calculated from different electrochemical testing methods were illustrated using Pearson’s correlation method.

Excess CO2 in the atmosphere has been a troublesome problem for humanity for the last few decades now. Academics, governments and industries are working together to tackle this problem to avert a global warming disaster. Research on many technologies is in progress to tackle global warming, with carbon capture being one exciting solution. The process of converting this captured carbon dioxide into something useful has gained momentum in research and academia since the 1980s. Out of the many catalysts worked upon to convert the excess CO2, copper has gained particular limelight because of its exceptional ability to reduce CO2 into valuable products such as methane, formic acid, ethane, methanol, to name a few.

The literature study associated with this project identified a knowledge gap between the present literature and the understanding of the microstructural impact on CO2 reduction. The thesis project studied the effect of microstructure changes on the electrochemical properties of copper and its subsequent impact on CO2 reduction capabilities to bridge the knowledge gap. Sigma- Aldrich copper of purity 99.999% was selected for this thesis work, and four samples: as received, electropolished, and two annealed samples were prepared. Optical microscopy and XRD measurements were used for microstructural characterization, and it was found that annealed samples lost their rolling direction, and their grain sizes increased. Evaluation of electrochemical behaviour through CV and EIS results indicated the formation of a complex triple passivation layer on the copper samples. Through evaluation of EIS results, we found that the electropolished sample had the most robust Cu2O layer.

From the CO2 reduction experiments, it was found that as-received samples had the highest F.E. for CO2 reduction, and non-electropolished samples had a high F.E. for hydrogen production. The performance of the electropolished sample stood out with a high F.E. of 46.3% for hydrocarbon production. The reason for this behaviour was, possibly, the presence of robust Cu2O layers that prevented the sample from catalyst poisoning and helped form a strong bond with CO* (carbon monoxide free radical).

The thesis opened up avenues for further research. By further investigation of factors like grain orientation and grain boundary density combined with XPS results, the understanding of the complex nature of passive layers formed on copper during CO2 reduction can be improved, which will ultimately help develop better catalysts.

When no hydrogen is able to reach the Pt/C catalyst in the anode of an operating Proton-Exchange-Membrane Fuel Cell (PEMFC), the potentials of the cathode and anode will be reversed. During this fuel cell reversal, the potential of the anode rises and the Oxygen Evolution Reaction (OER) and Carbon Oxidation Reaction (COR) will occur. Applying an OER catalyst to the anode prevents the COR to destroy the anode. Therefore, a reversal tolerant anode (RTA) is created. In this research, the RTA was made by the introduction of an OER catalyst (IrOx supported on TiOx) to the anode. Electrochemical investigations on the RTA were done with a Rotating Disc Electrode (RDE), which allowed applying potentials on the RTA that occur during fuel cell reversal. However, these potentials on an OER catalyst with a RDE set-up is known to be troublesome. This can be devoted to the formation of oxygen bubbles, which are hard to evacuate in a RDE set-up and block reactant. Therefore, a special accelerated stress test (AST) has been developed in this research to diminish the effects of oxygen bubbles. This AST was used to investigate the effects of different processing techniques on the RTA performance and durability at fuel cell reversal potentials. Pt/C and IrOx/TiOx particles could be differentiated into bigger and smaller particles on the micrometer scale based on different ball milling times used during processing. This was confirmed by laser diffraction measurements, which supplied information on the particle size distribution (PSD). Besides, differences in the catalyst layer structure were confirmed by a laser microscope. In the AST, it was found that the activity towards the OER was higher for smaller particles, which could be explained by the increased surface area. It was found for all samples in the AST that there was loss of OER activity and Electrochemical Surface Area (ECSA) of Pt. Impedance spectroscopy, XPS and SEM/EDS showed that these losses could highly probable be devoted to the decrease of the ionomer content. Finally, to mimic real fuel cell reversal conditions in a RDE set-up, adjusted chronopotentiometry measurements were developed and applied. It was found that the higher OER active smaller particles had a worse tolerance against fuel cell reversal than the bigger particles.

Heavy dependence on fossil fuels has caused an astonishing growth in atmospheric CO2 levels for the past decades. The increase of anthropogenic CO2 emissions accelerates global warming and sea-level rising, which could lead to irreversible environmental damages. Electrochemically converting CO2 to value-added fuels is a promising approach to reduce carbon footprint and store over-generating renewable electricity as chemical energy. Studies have shown the excellent catalytic performance of bulk MoS2 concerning the low overpotential and high product selectivity towards CO for electrochemical CO2 reduction reaction (CO2RR) in ionic liquid-based solutions. However, its performance in different configurations and electrolytes remains unexplored. In this study, two types of MoS2, namely bulk MoS2 and MoS2 film coated on a glassy carbon electrode (GCE), were used. Their CO2RR performance in the KHCO3 aqueous and ionic liquid-based solutions was investigated using electrochemical methods, gas chromatography (GC), and attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR). Additionally, the photochemical activity of MoS2 was explored using scanning vibrating electrode technique (SVET).

The selected catalysts were characterized before and after the CO2 reduction process using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). GC measurement results showed that MoS2 film coated on GCE exclusively catalyzed H2 evolution in aqueous solutions. It was also observed that higher activity was achieved on the MoS2 film mixed with glassy carbon powder. Furthermore, the MoS2 film with the changed bandgap structure also performed better in ionic liquid-based aqueous electrolyte. These findings suggest that the catalytic performance of MoS2 film for CO2RR could be enhanced by interspersing materials with high conductivity or modifying the bandgap structure.

The detection of heavy metal ions (HMI) is an essential step in water treatment.
Due to their adverse effect on human health and the environment, various agencies have set guidelines for the concentration of HMI in drinking water. However, conventional HMI detection techniques, such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectroscopy (ICP-MS), are often too expensive and not practical for on-site real-time monitoring. Electrochemical methods, including anodic stripping voltammetry (ASV) and electrochemical impedance spectroscopy (EIS), appear as alternatives for detecting HMI. Among these techniques, EIS possesses great potential in providing more information about water composition with modelling and data analysis. However, past research on the physical interpretation of the impedance response and the differentiation of different types of HMI in a mixed aqueous solution is inadequate. In the first part of the research, the electrochemical cell for HMI detection was constructed with pure platinum electrodes and the impedance response was collected for two types of HMI solutions, ZnSO4 (aq) and Pb(NO3)2(aq), and their mixed solution. The data was collected repetitively in a wide range of settings to understand the influence of varying parameters, including concentration, types of HMI, temperature, mixing ratio, and designs of the electrochemical cells. The reproducibility of results was quantified by the calculation of standard deviation. It was found that each HMI solution has its own characteristic impedance value. In addition, the change of impedance response follows a linear trend line in the Bode and Nyquist impedance plot when varying parameters such as concentration, temperature, and mixing ratio. In addition to controlled experiments, the electrochemical cell was tested in tap water and industrial water samples provided by the project’s collaborators. In the second part of the research, various techniques of surface analysis were
conducted on the surface of the platinum working electrode, including scanning
electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and Xray photoelectron spectroscopy (XPS). Since the HMI-related adsorbates were presented in the results, a hypothesis was proposed, which suggests that the adsorption/desorption process of HMIs has taken place during the EIS measurement, and this phenomenon attributes to the inductive behavior in the impedance response. Finally, equivalent circuit models were proposed based on the result of EIS measurements, surface analysis, and literature studies. Data fitting for the impedance data was carried out with Zview software version 3.5h. Charge transfer resistance and other equivalent circuit elements’ impedance values were compared to specify the differences between the two HMI.