Παραγωγή υδρογόνου σε υβριδικούς ηλεκτροχημικούς αντιδραστήρες - οξείδωση προϊόντων της ενζυμικής υδρόλυσης της κυτταρίνης σε ηλεκτρόδια μη-πολύτιμων μετάλλων

Βαϊοπούλου, Τριανταφυλλιά; Vaiopoulou, Triantafyllia

dc.contributor.author	Βαϊοπούλου, Τριανταφυλλιά	el
dc.contributor.author	Vaiopoulou, Triantafyllia	en
dc.date.accessioned	2024-09-03T07:06:38Z
dc.date.available	2024-09-03T07:06:38Z
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/60109
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.27805
dc.rights	Αναφορά Δημιουργού-Μη Εμπορική Χρήση-Όχι Παράγωγα Έργα 3.0 Ελλάδα	*
dc.rights.uri	http://creativecommons.org/licenses/by-nc-nd/3.0/gr/	*
dc.subject	Glucose oxidation	en
dc.subject	Hybrid reactor	en
dc.subject	Metal oxides	en
dc.subject	Electrochemical evaluation	en
dc.subject	Catalytic activity	en
dc.subject	Hydrogen production	en
dc.title	Παραγωγή υδρογόνου σε υβριδικούς ηλεκτροχημικούς αντιδραστήρες - οξείδωση προϊόντων της ενζυμικής υδρόλυσης της κυτταρίνης σε ηλεκτρόδια μη-πολύτιμων μετάλλων	el
heal.type	bachelorThesis
heal.classification	Chemical Engineering	en
heal.classification	Electrochemistry	en
heal.language	el
heal.access	campus
heal.recordProvider	ntua	el
heal.publicationDate	2023-10-12
heal.abstract	The ability to oxidize enzymatic hydrolysis products of cellulose in nonprecious metal electrodes is studied in order to operate a hybrid electrochemical water hydrolysis-hydrogen production reactor. The said typical reactor contains two electrodes, an electrolytic cell and a current source, as the water desintegrates into hydrogen and oxygen through these two electrodes which are arrabged in water. More specifically, during the typical electrol ysis process the imposed current passes through the electrodes leading to water’s chemical bond cleavage, thus to hydrogen and oxygen production. The main goal of the hybrid reactor is to maximize both the energy and cost efficiency of the method. Therefore, the design of a two-compartment electrolytic cell is being considered. In the anodic compartment, the oxidation of an organic compound will take place, while in the cathodic compartment, hydrogen production will occur. These two compartments of the electrolytic cell will be separated by a suitable membrane. The aim of this concept is to reduce the required voltage by redesigning the anodic part of the process, allowing the oxidation of the organic compound to occur at a more negative electrode potential compared to the oxidation of water to oxygen, and at a lower overpotential. The objective is to achieve a required voltage lower than 1.3 V which is necessary for water electrolysis. The objective of this study is to investigate the implementation of the previously mentioned process, which involves oxidizing carbohydrates, specifically D-glucose, with the goal of producing hydrogen and value-added products such as gluconic and glucaric acids. More specifically this thesis targets to find non-precious metal electrodes with the following characteristics: • Enhanced catalytic ability in the oxidation of glucose. • Reduced construction cost (hence non-precious metals) to potentially enable scala bility. • The ability to oxidize glucose at low potentials to avoid by-products and improve the energy efficiency of the hydrogen production process. • The ability to form an active surface that is resistant to degradation and yields high yields in the specific reaction, thus avoiding unwanted parallel reactions and pro ducing the desired value-added products. The research focuses on several key aspects. It involves the identification of non-precious metals and metal oxides believed to be capable of oxidizing glucose. The study explores ideal conditions for the formation of oxides to achieve desired properties for glucose oxidation, if attainable. It also delves into the kinetics and mechanism of oxidation at the respective working electrodes where possible, through experiments, extrapolations, and Tafel linear adjustments. Additionally, a method for the detection and quantification of oxidation products using a gold electrode is examined to develop a comprehensive un derstanding of the mechanism and the ability to oxidize glucose from the correspond ing non-precious metal electrode. It’s worth noting that the existence of the reaction is confirmed through ion-exchange chromatography. The materials studied in the work in clude manganese dioxide MnO2, pure nickel, and oxide films of these materials, such as Ni(OH)2/NiO(OH) , NiO, Ni3O4 και Ni2O3. Initially, the reduction synthesis of manganese oxide films is studied with the following desired characteristics: • It should be inert with respect to the substrate. • It should isolate the substrate from the electrolytic solution by forming a continuous film without cracks. • It should exhibit resistance within the desired pH range. • It should allow for the oxidation of glucose with satisfactory yields at low potentials, specifically more negative potentials than those required for water oxidation. In all experiments involving MnO2, a graphite electrode is used as the counter electrode, and an Ag/AgCl (KCl sat.) electrode is used as the reference electrode. For cyclic voltammetry experiments, a scan rate of v=10 mV/s is employed. The electrochemical cell used in the process utilizes a 0.1 M KMnO4 aqueous solution, which is maintained alkaline (pH > 11) with saturated NaOH to create the appropriate conditions for film formation. The working electrode is defined as the respective substrate. The following substrates are studied: stainless steel, steel, nickel in an immobilized form, and electroplated materials, as well as graphite. After conducting potentiodynamic experiments (-0.1 V to 0.6 V), potentiostatic experiments (for each open circuit potential value), and galvano static experiments (at a current density of 3 mA/cm2 ), it appears that the formation of MnO2 films yields optimal properties of the oxide through the galvanostatic method. For a constant current density and a deposition time ranging from 10 to 20 minutes, a satisfactory film thickness (based on Faraday’s law) is achieved, ensuring relative stability. Once films with the desired properties are identified, they undergo further testing. These tests include exposure to a 0.5 M NaOH alkaline solution (pH=13) and a PBS solution in the presence and absence of 20 mM glucose. The purpose of these tests is to assess the ability of each film to oxidize glucose from the respective working electrode. If the characteristic oxidative peaks indicating glucose oxidation, as indicated by the literature, can be reproduced, an overnight experiment is conducted in the hybrid reactor. The resulting solution is then sent for analysis using ion-exchange chromatography to check if the desired oxidation products are produced in detectable quantities. Despite the lack of satisfactory repeatability for the MnO2 film on a stainless steel substrate, this system is the only one that shows encouraging results in a 0.5 M NaOH/20 mM glucose alkaline solution. Chromatographically, after the overnight experiment, the resulting solution is found to contain gluconic acid with a yield of 0.7%, while simultaneously reproducing the literature results in a PBS solution in the presence of glucose. The plain steel substrate does not exhibit strong repeatability despite reproducing cyclic voltammetry results in a PBS solution containing 20 mM glucose that are similar to those in the literature. Additionally, no chromatographic evidence of products is observed for these films after the overnight experiment. On the other hand, the electroplated carbon substrate, although having fewer pores and providing an ideal structure for the formed manganese oxide, seems to reproduce the literature results after depositing the MnO2 film for cyclic voltam metry tests in PBS/20 mM glucose and for the NaOH/glucose solution. However, the film deteriorates rapidly, and there is the possibility that the nickel substrate reacts in these tests rather than the film itself. The electroplated copper substrate appears to be the least promising, as it did not yield any expected results in line with the literature. It also does not seem to facilitate glucose oxidation to any significant extent. The practical application of the plain carbon substrate with the MnO2 film in cyclic voltammetry is questionable because the stability of the electrode response is lost when applying reduction currents. This is due to either the film degrading with use, revealing the capacitive properties of the carbon, or the oxides on the film’s surface being reduced, affecting the system’s performance and properties. The nickel wire appears promising, as it seems to oxidize glucose. However, the contribution of MnO2 alone cannot be confirmed, as the substrate, nickel in this case, may also play a significant role in facilitating glucose oxidation. Next, the formation of the MnO2 film is attempted through an oxidative process using an aqueous solution of 0.1 M Mn(NO3)2 in an electrolytic cell, where the working electrode is the samples of stainless steel, the reference electrode is Ag/AgCl KCl sat., and a graphite rod serves as the counter electrode. According to the literature, various methods are tried for film formation, including galvanostatic, potentiostatic, and potentiodynamic approaches, with the main difference compared to the previous chapter being the application of oxidative currents. A compact and robust film without pores or cracks is formed using a potentiostatic method (800 mV for 300 s). This film, when placed in a PBS solution containing 20 mM glucose, reproduces the literature results. However, it does not perform similarly in the NaOH solution, raising doubts about the film’s stability concerning pH changes. While the film exhibits a response in the PBS/20 mM glucose solution, glucose oxidation is not confirmed by this system, at least not to the extent that a product can be quantified and identified chromatographically after the overnight experiment in the standard setup. In conclusion, regarding manganese oxides, specifically MnO2, and their potential for glucose oxidation, it appears that despite the lack of satisfactory repeatability in the film formation method and its limited durability, it is the only system that allows for the detection of chromatographic products of glucose oxidation. This system, although it may require further improvements to mitigate the unpredictable behavior of manganese oxides, is considered the most successful and comprehensive approach, suitable for use in the hybrid reactor. The experiments conducted with a nickel substrate and manganese oxide film, as well as information from the literature, suggest that nickel can interact with the glucose oxidation process when immersed in a solution. This interaction occurs as the metal is capable of forming nickel hydroxide (Ni(OH)2) on its surface, which, during cyclic voltammetry in a NaOH 0.5 M/20 mM glucose solution, transforms into nickel oxyhydroxide (NiO(OH)) at around 0.4 V. It is hypothesized that at this po tential, nickel oxide species are formed on the electrode surface, which can subsequently oxidize glucose, resulting in increased current density. Before analyzing the experimental response of this electrode regarding its ability to oxidize glucose through impedance and cyclic experiments, the electrode’s kinetic behavior will be assessed using the Tafel equation. This analysis will involve potential-dynamic experiments, and the Tafel slope and exchange current density will be determined from the resulting log i versus potential curves. Additionally, experiments will be conducted with varying glucose concentrations (ranging from 20 to 80 mM). Apart from the anodic Tafel coefficient and exchange current density for each concentration, a plot of log c−log i will be generated, allowing the determination of the reaction order with respect to glucose concentration. The experiment will be conducted under standard conditions where the nickel wire electrode is immersed in a solution containing NaOH 0.5 M/20 mM glucose, with a reference electrode of Ag/AgCl KCl sat. and a platinum wire electrode as the counter electrode immersed in a 0.5 M NaOH solution. The experiment will be carried out for anodic currents only and in solutions of 0.5 M NaOH with varying glucose concentrations (20, 40, 60, and 80 mM). The resulting graph will include the linear portion of the E - log(i) curve along with the corresponding linear fit. It appears that for the nickel wire, all values of the Tafel coefficient are close to 0.1, while for the electrode with the oxide film, it is 0.058. Therefore, this indicates a different mechanistic scenario in the two families of electrodes and a different stage of the mechanism that determines the reaction. The electrode with the nickel oxide film exhibits more intense catalytic activity because the smaller the exponent in the Tafel equation, the lower overpotential is required to achieve the same reaction rate. Thus, this specific system seems to be more sensitive to the applied voltage in order for the reaction to occur. Regarding the i 0 , again, there is a grouping of data points, as the electrode with the oxide film shows a significant difference in orders of magnitude, enhancing the impression of catalytic activity of this system, as it pertains to the higher value of the reaction rate constant. At the same time, it raises the suspicion of the existence of a chemical stage in the mechanism to accelerate the oxidation. Finally, it is evident that it follows first-order kinetics with respect to glucose with a satisfactory linear fit (R2=0.8536). The response of the nickel wire electrode as a working electrode for glucose oxidation is further studied and explored under changing conditions in cyclic experiments. The cyclic voltammetry experiments on nickel electrodes reveal a complex behavior with broad peaks and small peaks that can vary, remain stable, or disappear depending on the surface pretreatment of the electrode. This complexity suggests the involvement of various nickel oxides and the possibility of intermediate formation and the presence of other nickel oxides. The nickel electrode shows promise as it consistently produces oxidation peaks for glucose at high current densities in a dynamic potential range of 0.35 to 0.45 V, regardless of conditions such as aeration and glucose concentration. However, it appears that this electrode may not generate detectable amounts of the expected glucose oxidation products. In order to gather valuable information regarding the behavior and characteristics of reactions occurring in the cell, experiments are conducted by immersing the nickel electrode system in a solution of NaOH 0.5 M and NaOH 0.5 M/20 mM glucose, with a reference electrode of Ag/AgCl KCl sat. and a counter electrode of platinum wire immersed in a solution of NaOH 0.5 M. In the presence of glucose, intense half-cycles are observed, indicating the presence of redox reactions for these dynamics, with low charge transfer resistance. For each direction in the quarters of the diagram, the appearance of this curve signifies the electrochemical adsorption of a chemical species on the electrode surface. The shape of the diagrams in the absence of glucose is simple, without indications of chemical species adsorption on the surface of the nickel electrode. The identification and quantification of glucose oxidation products are crucial for the study, as it is part of the goal to produce value-added products within the hybrid reactor. Based on the literature and theoretical analysis of the mechanism, the main product of glucose oxidation in an alkaline environ ment is gluconic acid. The method involves conducting cyclic experiments initially using standard solutions: pure NaOH 0.5 M (blank), glucose, gluconic acid, glucaric acid, all at the same concentrations (10-20 mM). Subsequently, after placing the NaOH/glucose solution in the cell for oxidation and allowing the reaction to proceed overnight, the solution is examined, and it is expected to show the typical fingerprints of each acid and possibly a combination of these three typical curves, along with glucose, depending on the reaction outcomes. The cyclic and galvanostatic methods are being studied. The characteristic cyclic voltammetry profiles used to identify and qualitatively distinguish the three substances: glucose, gluconic, and glucaric acids, confirm the literature references regarding the two-stage mechanism of glucose oxidation. Initially, the aldehyde group is oxidized, followed by the hydroxyl group, resulting in the final oxidation product, glucaric acid. Indeed, characteristic peaks are observed for all three substances in the mixtures tested, making the method capable of detecting at least the presence of products. The galvanostatic method, which observes the oxidation steps over time and with different slopes for each substance, shows promise. However, it is quite time-consuming, with a typical experiment lasting at least 2 hours. Additionally, the results are only interpretable when the mixture’s concentration is known, making it impractical for these specific experiments. The investigation is completed by studying the formation of catalytic nickel oxides during glucose oxidation, specifically examining the response of Ni(OH)2, NiO(OH), NiO, Ni3O4, and Ni2O3 films. It appeared that, under extreme conditions of forming thick films (at - 1.8 V), the spongy green form and porous nature of Ni(OH)2 do not allow interaction with the typical alkaline glucose solution, as the coating dissolves. Under milder conditions of film formation (-0.6 V), the gray-brown film formed responds to the alkaline glucose solution. However, the interaction with the nickel substrate in this signal is uncertain due to the thin film. NiO is entirely inert towards glucose, as the brown film formed after thermal treatment does not exhibit a characteristic reaction peak. The pair NiO(OH)/Ni(OH)2 is obtained from a working nickel electrode (electroplated square copper mesh A=1.0395 cm2 ), a counter electrode of platinum mesh, and an Ag/AgCl reference electrode. The cell solution consists of NiSO4 0.13 M, NiOAc 0.1 M, Na2SO4 0.13 M, and an anodic current density of 0.4 mA/cm2 is applied for 30 minutes to form the film. This pair seems quite promising as it exhibits reproducibility. The cyclic experiments and the Tafel plot of the corresponding chapter confirm the existence of a reaction, and the voltammograms in the glucose solution show the highest current intensity at the characteristic oxidation peak for the oxidation reaction. Particularly significant is the ability of the film’s surface to change during cyclic experiments. During the anodic scan, and only for the period when the oxidative peak indicating glucose oxidation appears, it turns completely black, and then it returns to its brown-gray form without degrading the film or losing intensity in the graph. One hypothesis that can be derived from this observation is that at low po tentials, glucose is absorbed on the oxide surface, shares its hydroxyl groups to change the type of oxide, and simultaneously, as the potential increases, the substance oxidizes. Finally, during the return sweep, the surface is cleaned from the oxidized substance, and the oxide returns to its initial form.	en
heal.advisorName	Karantonis, Antonis	en
heal.committeeMemberName	Karantonis, Antonis	en
heal.committeeMemberName	Tsakanikas, Aggelos	en
heal.committeeMemberName	Topakas, Evangelos	en
heal.academicPublisher	Εθνικό Μετσόβιο Πολυτεχνείο. Σχολή Χημικών Μηχανικών. Τομέας Επιστήμης και Τεχνικής των Υλικών (ΙΙΙ). Εργαστήριο Φυσικοχημείας	el
heal.academicPublisherID	ntua
heal.numberOfPages	149 σ.	el
heal.fullTextAvailability	false