LEE- Malzeme Bilimi ve Mühendisliği-Doktora

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http://hdl.handle.net/11527/19377

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Konu "Anodic oxidation" ile LEE- Malzeme Bilimi ve Mühendisliği-Doktora'a göz atma
  • Öge
    Investigation of the catalytic performance of tin nanowires produced by aluminum anodic oxide template method for electrochemical CO2 reduction
    (Graduate School, 2023-10-17) Gönül Er, Dilan ; Ürgen, Mustafa ; 521182006 ; Materials Science and Engineering
    Climate change causes various dramatic situations, including the melting of the ice mass, flooding, and drought, and it affects even the daily life of human beings. The increased amount of anthropogenic CO2 gas in the atmosphere is one of the driving forces for global warming and climate change. Therefore, mitigation of CO2 gas has become mandatory to eliminate the harmful effects of global warming. There are mainly two mitigation methods: carbon capture-storage and carbon capture-utilization. By the utilization of carbon dioxide, not only the amount of CO2 is reduced, but also it is used as an input for value-added chemical production. Among the utilization methods, electrochemical CO2 reduction over heterogeneous metallic electrocatalyst steps forward thanks to its several advantages, such as high number of available catalysts, its ability to work at ambient conditions, scalability, and the utilization of renewable energy sources. Several products, including organic acids, hydrocarbons, and fuels, can be obtained via electrochemical CO2 reduction reactions. The techno-economic analysis performed for the feasible CO2 reduction systems have shown that formic acid is the most practical chemical to be produced via electrochemical CO2 reduction. Formic acid is used as input in various industrial areas such as agriculture, food, textile, and pharmaceutical. However, recent studies have indicated that formic acid is also a strong candidate for modern engineering applications. For instance, formic acid has an extraordinary capacity to carry H2 gas (590 l per l). This storage performance of formic acid enables the advance in the H2 driven cars. Moreover, formic acid can be used as feed for formic acid fuel cells to generate electricity. The conventional production methods of formic acid require high energy consumption and include highly complex process steps. Hence, the production of formic acid via electrochemical CO2 reduction utilizing renewable energy sources such as wind, solar, etc., is in high demand. The metallic electrocatalysts that produce formic acid or formate (depending on the pH) are Sn, Cd, Tl, In, Pb, Bi, and Hg. Among these metals, tin is one of the most studied electrocatalysts since it has low toxicity, is abundant in nature, and is cost-friendly. Tin is known to produce only formic acid as liquid product under the electrochemical CO2 reduction in liquid environment at ambient conditions. Although high Faradaic efficiency has been achieved for formic acid, there are challenging problems in scaling up the CO2 reduction system. These problems can be listed as low current density, low stability, high overpotentials, and by-product formation. Specifically, low current density due to the low solubility of CO2 in the aqueous solutions and low catalytic activity is a crucial disadvantage. To overcome the low current density, flow-cell systems, gas diffusion electrodes, and gas-phase reduction structures have been proposed since the low solubility problem of CO2 can be outframed. To increase the catalytic activity of the catalysts, meso- and nano-structured electrodes have been proposed rather than using bulk electrodes. Especially nanostructured electrodes, such as nano-rods, wires, tubes, sheets, etc., have shown promising results in terms of efficiency and activity thanks to their unique structure. In the literature, there are several studies on the production of tin-based nanostructured electrodes, tin nanowires have yet to be investigated extensively compared to the other structures. Nanowires can be a great candidate for electrocatalytic CO2 reduction since they offer high surface to volume ratio and enhanced charge and mass transfer. In recent years, a few studies on tin nanowire electrocatalysts have been published, yet the suggested production methods in these works are highly complex. To commercialize the electrochemical CO2 reduction system, the production technique is also crucial, besides the performance of the electrocatalyst. The ideal production method should be simple, low-cost, and scalable. Also, significant structural changes should be obtained by easily altering the working parameters such as potential and duration. In this study, the catalytic performance of tin nanowire electrodes produced via the AAO template method toward formate production under electrochemical CO2 reduction is investigated. By choosing AAO template method, most of the desired properties of the catalyst production technique are provided. A self-standing, interconnected, and branched-like 3D nanowire network was achieved via simple AAO template technique. Moreover, anodic oxidation in alkaline solution was applied to the as-produced tin nanowires to increase the active surface area and improve catalytic performance by increasing the oxide-related content on the electrode surface. The first of the tin nanowire production was the formation of AAO template. To obtain AAO pattern, the aluminum substrate was anodically oxidized in 0.3 M oxalic acid solution at 70 V for 30 min. Then, the zincating process was applied to make the AAO template electrodepositable. The main drawback of the AAO template method is the formation of a non-conductive barrier layer at the bottom of the pores, which prevents the direct usage of the template for electrodeposition. By fast and easy zincating step, AAO templates can be prepared for the electrolytic solutions without any significant change and/or loss of structure. Tin was electrodeposited into the AAO template in tin sulphate-containing bath under constant potential. Then, AAO template and the remnant aluminum were dissolved in 3 M NaOH solution at 60°C until the vigorous H2 gas evolution stops. The obtained tin nanowire electrodes were characterized by SEM and Raman analysis. The catalytic performance tests were conducted in a custom-made H-cell filled with 0.1 M KHCO3 at potentiostatic mode for 1 h. The catalytic behavior was expressed as Faradaic efficiency toward formate. To compare the performance, high purity tin foil was subjected to similar reduction experiments. The SEM analysis showed that tin nanowire electrodes possessed highly ordered and interconnected branched-like structure, including nanowires having 7 µm length and 150 nm diameter. Moreover, it was found via Raman analysis that the surface contained poor crystalline and non-stoichiometric oxide structures. The reduction experiments indicated that tin nanowire electrodes achieved 10 times higher current density values than tin foil at every reduction potential. However, the increase in formate production was not as high as current density values, and only 6-fold higher production amount was observed. Moreover, Faradaic efficiency values reached only 30% on the tin nanowire electrodes, while 60% efficiency was obtained over tin foil. This result indicated that a significant portion of the total charge is consumed by side reactions, i.e., CO and H2 formation rather than formate. The low efficiency values obtained over tin nanowire electrodes were attributed to the destruction of the oxide content on the nanowires during the AAO removal in the concentrated NaOH solution. Since the oxide layer has been proved as the key catalytic part for the formate production over tin-based catalysts by in-situ and in-operando studies, the removal of the oxide content results in the decay of the catalytic performance toward formate. In concentrated alkaline solutions, tin oxides can dissolve, and the electrode ends up containing mostly metallic tin. To further prove this phenomenon, tin foil was immersed in 3 M NaOH solution at 60°C for 2 hours, and then CO2 reduction experiment was run over this electrode. The result showed that the efficiency reduced from 60 to 15% after etching in alkaline solution. To reform the destructed oxide layer, anodic oxidation in 1 M NaOH at room temperature was performed. The applied potential was set to 4 V to obtain a crack-free porous oxide structure over tin nanowires. For comparison, tin foil was also anodically oxidized. The SEM analysis showed that anodically oxidized tin nanowires enlarged approximately 50 nm, and pores formation on the wires with 30-40 nm diameter was observed. Raman analysis gave peaks of the crystalline SnO2 structure along with the SnO and Sn3O4. The catalytic performance was enhanced significantly, and 87% Faradaic efficiency with ca. 15 mA.cm-2 current density at -1.6 V vs. Ag/AgCl was achieved over anodically oxidized tin nanowire electrodes. Moreover, 9- and 12-times higher formate production was achieved over anodically oxidized tin nanowire electrode compared to anodically oxidized tin foil and untreated tin foil, respectively. However, current density values for both tin nanowire and tin foil decreased after anodic oxidation due to the semi-conductive character of the tin oxide structures. The improvement in the catalytic behavior after anodic oxidation was attributed to the increased surface area and oxide layer formation on the electrode surface; the latter was more dominant. The ECSA calculations showed that anodically oxidized tin nanowire electrode possessed only 4-fold higher active area than anodically oxidized tin foil. The formate production amount was much higher than the surface area increase, therefore the improved catalytic activity was not only related to the active surface area. Furthermore, the kinetics of the electrochemical CO2 reduction reactions was faster over anodically oxidized tin nanowire electrodes since lower polarization resistance value was obtained via EIS analysis. As an important indicator for the catalytic performance, stability was tested over both anodically oxidized tin nanowire and tin foil electrodes for 12 h at -1.5 V vs. Ag/AgCl. Within the first 30 min, the oxide layer over the tin foil flaked off from the surface; therefore, the experiment was not continued. The anodically oxidized tin nanowire electrode preserved its structural integrity throughout the experiment and gave ca. 64% Faradaic efficiency. However, after 5 hours, a slight decline was observed in the current density values. The SEM images revealed the enlargement of the nanowires and fillings between the nanowires. The current density loss was related to the decline in the surface area. The Raman analysis indicated the absence of the crystalline SnO2 related peaks but the intensified of the already existing peaks for SnOx structures and new peak formation for SnO structure. These results concluded that the survival of the metastable oxide structures over tin nanowires is crucial for preserving the efficiency value toward formate even if the current density is lost.
  • Öge
    Investigation of the growth kinetics and morphology transitions during porous anodization of titanium in ethylene glycol based electrolytes
    (Lisansüstü Eğitim Enstitüsü, 2022) Seçkin, Eren ; Ürgen, Mustafa Kamil ; 725516 ; Malzeme Bilimi ve Mühendisliği
    Titanium dioxide (TiO2, titania) is a transition metal oxide that has been widely used as a pigment since the early 1900s. The discovery of the photocatalytic properties of the material has made it highly attractive for photovoltaics, photocatalysis, and sensor applications. Besides, titania-based materials have been increasingly employed in the biomaterials industry due to their chemical stability and bio-compatibility. Recently, the production and use of TiO2 nanomaterials has become popular. The high surface area and possible quantum effects of titania nanostructures have provided significant advantages over micro-size use. TiO2 nanotubes have attracted great interest among various titania nanostructures, and intensive research has been carried out on their production and use. Although the anodic formation of non-porous barrier oxide films on titanium has been known for decades, the discovery of self-organized nanoporous titania production by anodization in fluoride-containing electrolytes opened a new pathway; and numerous studies have been conducted on the subject. Preliminary studies on the nanoporous anodization of titanium concentrated on understanding the anodization reactions and controlling the process. Subsequent studies mainly focused on the fabrication of nanotube structures and their use in various applications. Recently, sophisticated studies such as advanced nanotube geometries and doped nanotubes have been prominent. However, the literature shows that the reproducible fabrication of titania nanotubes of desired length and morphology, in a well-ordered structure with a clean surface, is still a challenge. Although many different morphologies have been obtained during anodization, the influence of this morphological variance on nanotube growth has not been defined with a consistent model. One of the main reasons for this deficiency is the insufficient understanding of the anodization process, which reduces effective control over the process. The necessity of a clear understanding of the formation, growth kinetics, and morphological transformations of the nanotubes constituted the main reason for starting this thesis study. In the first part of experimental studies, investigation and optimization of the anodization parameters (surface condition, stirring, aging of the electrolyte, voltage, temperature, and time) were conducted by utilizing an ethylene glycol-based anodization electrolyte containing 0.6 wt % NH4F, 1 vol % H2O. The two-step anodization procedure was optimized to improve the pre-anodization surface. The high surface roughness of the untreated Ti foil could be reduced significantly by the optimized first anodization procedure with an anodization at 50 V and 30 °C for 180 min and subsequent removal of the anodized layer. In the anodization studies carried out under different stirring rates (0 to 1000 rpm), it was observed that the stirring rate has a strong effect on the morphology. The barrier oxide film on the sample surface disintegrated faster as the stirring rate during anodization increased. It was attributed to the increase in the chemical dissolution rate due to increased agitation. In addition, it was revealed that the increase in stirring rate facilitates effective temperature control. Experimental studies on the aging and reuse of the anodization electrolyte have shown that current density behavior and morphology can vary significantly depending on the condition of the electrolyte. As the fluoride is consumed by forming water-soluble [TiF6]2- species and be solvated during anodization, the amount of free fluorine in the electrolyte gradually decreases. It was experimentally verified that the current density values shift down, and the chemical dissolving power of the electrolyte decreases in anodization conducted after extended usage or short but repetitive usage of the anodization electrolyte. However, it has also been shown that when the electrolyte is kept unused overnight after anodization, almost equivalent current density values and morphological results can be obtained as the anodization performed the previous day. We estimate that when the electrolyte is kept unused for a convenient duration after anodization, [TiF6]2- species decompose into TiF3, releasing three fluorides (for each molecule); thus, the amount of free fluoride in the electrolyte increases again. In studies examining the effects of anodization voltage and temperature on growth and morphology, it was verified that an increase in either parameter results in an increase in current density and hence an increase in growth rate. In addition, the increase in either parameter affects the morphology by accelerating the chemical dissolution. It was demonstrated that although the voltage has a slight effect on the dissolving power of the electrolyte, the temperature has a strong effect and the chemical dissolution in the electrolyte increases strongly with increasing temperature. A series of experiments were conducted to observe the effects of the anodization duration on the surface morphology. Three morphologies were obtained in the anodization experiments conducted at 50 V and 30 °C for different durations (5 min to 40 min). For 5 min and 10 min anodized samples, barrier oxide morphology, for 20 min anodized sample open-top nanotube morphology, and 40 min anodized sample nanograss morphologywere obtained. It was deduced that the surface morphology during anodization progresses in order of barrier layer, open nanotubes, and nanograss structure, showing that these morphologies are successive processes. Anodization experiments conducted at 50 V and 25 °C for different durations confirmed that the surface morphology changes in the suggested order. It was revealed that there is a correlation between the temperature and the dissolution time of the barrier oxide, which formed the basis of the proposed kinetic model. Additionally, ultrasonication and stripping with scotch tape methods were studied for clearing the sample surface from barrier layer residues and so-formed nanograss. In the second part of experimental studies, the formation and growth kinetics of the titania nanotubes were investigated by utilizing an ethylene glycol-based anodization electrolyte containing 0.3 wt % NH4F, 1 vol % H2O. A model, combining growth with morphology, has been proposed by utilizing the findings obtained in the first part. According to this model, growth is divided into three stages (I. Growth under initial barrier layer, II. Nanopore to nanotube transition, III. Nanograss formation). Stage 1 is defined as the period which includes initial barrier oxide formation, pore initiation, and nanotube growth under gradually dissolving initial barrier oxide. Formation studies conducted at 50 V and 25 °C for different durations (5 sec to 30 sec) demonstrated that ordered nanotubular formation under initial barrier oxide occurs in the early periods of the anodization. We determined that the field-assisted growth of the nanotubes occurs at a constant rate (µm C-1 cm2) at the bottom, under the gradually dissolving and still protective initial barrier oxide layer. Titanium samples were anodized at 25 °C for different durations (20 min and 80 min) remaining in Stage 1 to determine field-assisted growth rate. In both anodized samples average growth rate of ~0.7 µm C-1 cm2 was obtained. It was experimentally observed that the total dissolution time of the initial barrier oxide layer during anodization performed at 25 °C was within the range of 100 – 150 min. We have defined Stage 2 as the short period after the initial barrier oxide layer is wholly dissolved in which open tube top ordered nanotubes are seen, and nanopore/nanotube transition occurs. As Stage 2 is the sudden transition phase between Stage 1 and Stage 3, it was ignored in the kinetic calculations. Stage 3 is defined as the period for the chemical dissolution of the tube tops and the initiation and progress of nanograss formation. In this part, the nanotube film's growth and the film's shortening are examined together. Field-assisted growth continues at the bottom with the same efficiency and rate (per coulomb) (µm C-1 cm2) as it is in Stage 1. However, the simultaneous shortening of the tubes due to chemical dissolution occurs at the top at a constant rate (per time) (nm min-1). A kinetic model has been proposed to determine the persistence time of this top barrier layer. According to model, the chemical shortening rate of the nanotubes due to chemical dissolution is determined experimentally by using two different anodization durations remaining in Stage 3, which allows us to calculate the barrier oxide dissolution time (BDT). Barrier oxide dissolution times for 25 °C and 5 °C was calculated as 130.1 min and 414.1 min, respectively, and verified experimentally by anodization conducted for the corresponding BDT values. The dissolution of the top barrier layer is a chemical process, and its activation energy can be calculated using the experimentally determined parameters that allows us to determine the temperature dependence of BDT. By utilizing 25 °C and 5 °C anodization data and taking BDT-1 as the rate constant, the activation energy equivalent to 9.53 kcal mol-1 was obtained for the process. Additional experiments conducted at 35 °C, 15 °C, and -5 °C by using the theoretically calculated BDT values confirmed the model's validity. These calculations and experiments verified that temperature dependence of the barrier oxide dissolution time (BDT) shows Arrhenius-like behavior. Besides controlling the final morphology, it is shown that the plot of nanotube thickness that corresponds to BDT at different temperatures indicates a linear relation; and open-top nanotubes ranging from 6.8 µm to 10.4 µm can be obtained by tuning anodization temperature and duration according to this model. To summarize, this model gives the opportunity to tailor titania nanotube thickness (within a specific range) and desired nanotube morphology (barrier top, open tube top, or nanograss) by tuning anodization temperature and duration according to the proposed model.