Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study

FINAL DEGREE THESIS Bachelor’s Degree in Materials Engineering EVALUATION OF CERIUM OXIDE AND GADOLINIUM-DOPED CERIA AS ENERGY STORAGE MATERIALS: PHYSICOCHEMICAL AND ELECTROCHEMICAL STUDY Report and Annex Author: Lubna Asbai Mohamed Supervisor: Miguel Morales Comas Department DISAT Company tutor: Mara Serrapede Company co-tutor: Pietro Zaccagnini Call: 2024, September Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. i 1 Abstract Energy storage is key to the transition towards a sustainable energy system, as it allows for the integration of intermittent renewable sources, such as solar and wind, by capturing excess energy and releasing it as needed. It also reduces dependence on fossil fuels and promotes a more resilient future. This work focused on the electrochemical and physicochemical study of gadolinium-doped cerium oxide (Gd-CeO2) and pure cerium oxide (CeO2), to evaluate their potential for energy storage. These materials stand out in solid oxide fuel cells due to their high ionic conductivity and catalytic properties. Electrochemical studies, such as cyclic voltammetry (CV), and characterization techniques like Xray diffraction (XRD) and Raman spectroscopy were conducted to analyse their structure and properties. The experiments assessed the catalytic behaviour and operating voltage range of the materials, with and without the addition of carbon black (CB), which enhances electrical conductivity. The results indicated that the materials, with the addition of CB, operate in the low-voltage region and showed potential as electrodes in asymmetric supercapacitors. Memory ii 2 Resum L'emmagatzematge d'energia és clau per a la transició cap a un sistema energètic sostenible, ja que permet integrar fonts renovables intermitents, com la solar i l'eòlica, capturant l'excés d'energia i alliberant-la segons la demanda. També redueix la dependència dels combustibles fòssils i promou un futur més resilient. Aquest treball es va centrar en l'estudi electroquímic i físic/químic de l'òxid de ceria dopat amb gadolini (Gd-CeO2) i l'òxid de ceria pur (CeO2), per avaluar el seu potencial en l'emmagatzematge d'energia. Aquests materials destaquen en piles de combustible d'òxid sòlid per la seva alta conductivitat iònica i propietats catalítiques. Es van realitzar estudis electroquímics, com la voltamperometria cíclica (CV), i tècniques de caracterització com la difracció de raigs X (XRD) i l'espectroscòpia Raman, per analitzar la seva estructura i propietats. Els experiments van avaluar el comportament catalític i el rang de voltatge de treball dels materials, amb i sense l'addició de carbon black (CB), que millora la conductivitat elèctrica. Els resultats van indicar que els materials, amb l'addició de CB, operen en la zona de baixos voltatges i van mostrar el seu potencial com a elèctrodes en supercondensadors asimètrics Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. iii 3 Resumen El almacenamiento de energía es clave para la transición hacia un sistema energético sostenible, ya que permite integrar fuentes renovables intermitentes, como la solar y la eólica, capturando el exceso de energía y liberándola según la demanda. También reduce la dependencia de combustibles fósiles y promueve un futuro más resiliente. Este trabajo se enfocó en el estudio electroquímico y fisico/químico del óxido de cerio dopado con gadolinio (Gd-CeO2) y el óxido de cerio puro (CeO2), para evaluar su potencial en el almacenamiento de energía. Estos materiales destacan en celdas de combustible de óxido sólido por su alta conductividad iónica y propiedades catalíticas. Se realizaron estudios electroquímicos como la voltamperometría cíclica (CV) y técnicas de caracterización como difracción de rayos X (XRD) y espectroscopia Raman, para analizar su estructura y propiedades. Los experimentos evaluaron el comportamiento catalítico y el rango de voltaje de trabajo de los materiales,con y sin adición de carbon black (CB), que mejora la conductividad eléctrica. Los resultados indicaron que los materiales, con la adición del CB, operan en la zona de voltajes bajos y mostraron su potencial como electrodos en supercapacitores asimétricos. Memory iv 4 Appreciations I would like to express my gratitude for the opportunity to explore the world of research, to understand how laboratories operate, to engage in data analysis, and to appreciate the relevance of these processes in our modern world. Furthermore, I am thankful for the chance to immerse myself in a new culture and language, which has significantly contributed to the enhancement of my skills and abilities beyond the academic sphere. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. v Index . 1 ABSTRACT ____________________________________________________ I 2 RESUM _______________________________________________________ II 3 RESUMEN ___________________________________________________ III 4 APPRECIATIONS ______________________________________________ IV 1. PREFACE _____________________________________________________ 1 2. INTRODUCTION ________________________________________________ 3 2.1 Objetive ............................................................................................................... 3 2.2 Scope ................................................................................................................... 3 3 THEORETICAL FRAMEWORK ______________________________________ 5 3.1 Energy storage devices ........................................................................................ 5 3.1.1 Types of energy storage devices ....................................................................... 5 3.2 Batteries............................................................................................................... 6 3.3 Supercapacitors ................................................................................................... 7 3.3.1 Electrical double layer capacitor (EDLC) mechanism ........................................ 8 3.3.2 Pseudocapacitance mechanism ........................................................................ 9 3.3.3 Hybrid supercapacitors .................................................................................... 10 3.4 Electrical double layer capacitor vs conventional capacitors ........................... 11 3.5 Pseudocapacitors vs batteries........................................................................... 11 3.6 Supercapacitor components ............................................................................. 12 3.6.1 Current collector .............................................................................................. 12 3.6.2 Active material ................................................................................................. 13 3.6.3 Electrolyte ........................................................................................................ 14 3.7 Metal oxides in energy storage ......................................................................... 14 3.8 CeO₂ in energy storage and catalysis ................................................................ 16 3.9 GDC in energy storage and catalysis ................................................................. 17 4 MATERIALS AND METHODS _____________________________________ 18 4.1 Materials used ................................................................................................... 18 4.2 Electrochemical characterization techniques ................................................... 19 4.2.1 Cyclic Voltammetry (CV) .................................................................................. 20 4.3 Performance Indicators ..................................................................................... 20 Memory vi 4.3.1 Specific Capacitance ........................................................................................ 20 4.3.2 Energy Density ................................................................................................. 21 4.3.3 Power Density .................................................................................................. 21 4.3.4 Coulombic Efficiency ....................................................................................... 22 4.3.5 Equivalent Series Resistance (ESR) .................................................................. 22 4.3.6 Cycle Life .......................................................................................................... 23 4.4 Physical-chemical characterization ................................................................... 23 4.4.1 X-Ray Diffraction (XRD) .................................................................................... 23 4.4.2 Raman Spectroscopy ....................................................................................... 24 5 ELECTROCHEMICAL CHARACTERIZATION IN WATER-BASED ELECTROLYTE. ____________________________________________________________ 26 5.1 Chapter introduction ......................................................................................... 26 5.2 Preliminary calculations .................................................................................... 26 5.2.1 Slurries creation ............................................................................................... 26 5.2.2 Electrolyte creation ......................................................................................... 27 5.3 Materials and methods ..................................................................................... 30 5.3.1 Materials .......................................................................................................... 30 5.3.2 Methods/procedure ........................................................................................ 32 5.4 Experimental conditions ................................................................................... 35 5.5 Measurements methods ................................................................................... 36 5.6 Results................................................................................................................ 37 5.6.1 Mass of the electrodes used. .......................................................................... 37 5.6.2 Onset potential and maxim current results .................................................... 38 5.7 Discussion .......................................................................................................... 39 5.7.1 Cyclic voltammetry analysis ............................................................................ 39 5.7.2 Carbon Black effect .......................................................................................... 49 5.7.3 N2 and air saturated effect ............................................................................. 50 5.8 Conclusions ........................................................................................................ 51 6 ELECTROCHEMICAL CHARACTERIZATION IN ORGANIC BASED ELECTROLYTE ____________________________________________________________ 53 6.1 Chapter introduction ......................................................................................... 53 6.2 Preliminary calculations .................................................................................... 53 6.3 Materials and methods ..................................................................................... 54 6.3.1 Materials .......................................................................................................... 54 6.3.2 Methods/Procedure ........................................................................................ 57 6.4 Experiment conditions. ..................................................................................... 60 Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. vii 6.5 Measurement methods. ................................................................................... 60 6.6 Results................................................................................................................ 61 6.6.1 Mass of the electrodes used. .......................................................................... 61 6.7 Discussion .......................................................................................................... 62 6.7.1 Al as a current collector ................................................................................... 63 6.7.2 Cu as a current collector .................................................................................. 68 6.8 Conclusions ........................................................................................................ 72 7 PHYSICAL-CHEMICAL CHARACTERIZATION _________________________ 74 7.1 X-ray Diffraction ................................................................................................ 74 7.1.1 Results and discussions ................................................................................... 74 7.2 Raman Spectroscopy ......................................................................................... 79 7.2.1 Results and discussions ................................................................................... 79 7.3 Conclusions ........................................................................................................ 81 8 ENVIRONMENTAL STUDY _______________________________________ 83 9 SOCIAL IMPACT STUDY _________________________________________ 85 10 ECONOMIC ANALYSIS __________________________________________ 87 11 CONCLUSIONS ________________________________________________ 91 12 REFERENCE __________________________________________________ 92 13 ANNEX A ___________________________________________________ 101 Memoria 6 categories: pumped hydro energy storage (PHES), gravity energy storage (GES), compressed air energy storage (CAES), and flywheel energy storage (FES). • Chemical energy storage (CES) systems, energy is stored in the chemical bonds between atoms and molecules of materials, being released during chemical reactions. CES primarily include hydrogen, synthetic natural gas and solar fuel storage systems, although they also encompass technologies such as batteries and biofuels. • Electrochemical energy storage (EcES) is one of the most widely used types of energy storage systems due to its efficiency and ability to provide both high energy and power densities. EcES systems operate primarily through electrochemical reactions and can be categorized into several types: (a) battery energy storage (BES) systems, where energy is stored within the electrodes through electrochemical reactions ; (b) Flow battery energy storage (FBES) systems and (c) pseudocapacitive supercapacitors, which store energy through fast redox reactions on the electrode surfaces, combining elements of both capacitive and battery-like behavior. These systems are crucial for applications ranging from portable electronics to grid stabilization. In conclusion, there are different types of energy storages devices (EES) that are classified according to the form of energy the store. (Classification of Energy Storage Technologies: An Overview, 2020; Tong et al., 2022) 3.2 Batteries Batteries, as it is mentioned above, are electrochemical energy storage devices that are characterized by their high energy density, their low power density and their limited life cycle. It means that they can store more energy per unit weight or volume, that they are less efficient at delivering energy quickly, and that the degradation increase with repeated use. Every battery is made up of three components: a positive electrode (cathode), a negative electrode (anode), and an electrolyte. The electrolyte must be conductive to ions and insulative to electrons, and one of the functions that it has is physically separate the positive and negative electrode. In a battery, operation is based on the chemical reaction between two reactants, A and B, which are separated by an electrolyte. This electrolyte allows the passage of ions, in this case A+, but at the same time acts as an insulator for the electrons. The process starts at electrode A. This is Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 7 oxidised and creating A+ ions and electrons, according to reaction 𝐴 → 𝐴++ 𝑒−. The ions 𝐴+ are conducted by the electrolyte, whereas the electrodes can’t pass through it, and therefore flow through an external circuit, thus generating electric current. When the 𝐴+ ions and electrons reach electrodes B, a reduction reaction occurs where the 𝐴+ ions and electrons combine with reagent B to form product C, following the reaction 𝐴++ 𝐵 + 𝑒−→ 𝐶. Thus, the sum of these reactants gives places to overall reaction 𝐴 + 𝐵 → 𝐶, which represents the conversion of the chemical energy stored in the reactants into electrical energy. (Déborah García Bello, 2020; Ferrese, 2015; Schmidt-Rohr, 2018) Figure 3.1. Visual representation of the basic operation of batteries.(Mahmud, 2022) 3.3 Supercapacitors Supercapacitors (also known as ultracapacitors) are energy storage devices that are characterized by their ability to store a large amount of energy compared to traditional capacitors (they have higher energy density than capacitors, but lower than batteries), faster charge and discharge than chemical batteries, higher power density and long service life. They are ideal for applications requiring fast energy delivery and multiple charge and discharge cycles without significant degradation. A supercapacitor (SC) cell can be simply treated as a system of a capacitor in series arrangement with a resistor (𝑅𝐸𝑆). The resistance of this resistor is usually called the equivalent series resistance (𝑅𝐸𝑆). and is essential in reflecting the power performance and energy efficiency of SCS. Memoria 8 Figure 3.2.The series RC circuit for supercapacitors. (S. Zhang & Pan, 2015) To understand its operation, it is essencial to analyze its two main energy storage mechanisms: the electric double layer capacitor (EDLC) and the pseudocapacitance. (S. Zhang & Pan, 2015; J. Zhao & Burke, 2021) 3.3.1 Electrical double layer capacitor (EDLC) mechanism In EDLC supercapacitors there’s an electrostatic storage of energy through charge separation ate the interface between the electrode and the electrolyte, forming what is known as an ‘electrical double layer’. To form this ‘electrical double layer’ a voltage is required to be applied to the terminals of the SC causing the movement of the ions in the electrolyte towards the oppositely charges electrodes. This movement of charges causes there to be a layer of ions in the electrolyte and a layer opposite charges on the electrodes, creating on the surface of each electrode an electrical double layer, an extremely thin layer, on the order of nanometres. The energy is stored electrostatically in the separation between the charges of the double layer, without significant chemical reactions. This process is reversible and fast, allowing SCs to charge and discharge quickly. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 9 Figure 3.3. Typical configuration of an EDLC cell. (Jayalakshmi & Balasubramanian, 2008) In EDLC the common active material used to create electrodes include activated carbon, carbon nanotubes and graphene. Activated carbon is the most used due to its high surface are and low cost. EDLCs supercapacitors store and release energy quickly, although with lower energy density compared to chemical batteries. Anyway, the main advantages of EDLC are its high fast charge and discharge capability, as energy can be released or stored in fractions of a second. The long lifetime, as it can exceed a lot of charge/discharge cycles without significant loss of performance and the high energy efficiency. (Revista Española de Electrónica, 2022) 3.3.2 Pseudocapacitance mechanism Pseudocapacitance mechanism stores energy through rapid and reversible redox reactions or adsorption/desorption at the electrode surface. Unlike EDLC, this mechanism involves faradic electrochemical processes, meaning that there is charge transfer between the electrode and the electrolyte, but these reactions are fast enough that the device still behaves similarly to a capacitor. Memoria 10 Figure 3.4. Schematic diagram of a pseudocapacitor, where capacitance results from charge transfer between the electrolyte and pseudocapacitive electrode. ((Hossain et al., 2020) In this mechanism the most common material used as active material in the electrodes are metal oxides (Mos) , such as ruthenium or manganese oxide), conductive polymers (such as polyaniline or polypyrrole) and composite materials. The main advantage of this mechanism are higher energy density (the energy is lower than that of batteries, but it is significantly higher than in pure EDLCs), high power, similar to EDLC, can deliver energy quickly due to fast redox reactions, and good cycling stability, better than batteries but not as good as in the EDCLs (notice that the EDLCs doesn’t have chemical reactions which makes it less prone to degradation). (Bhojane, 2022; Eftekhari & Mohamedi, 2017; N. Pronkin et al., 2022) 3.3.3 Hybrid supercapacitors Hybrid supercapacitors are a mix between electrochemical double-layer capacitors and pseudocapacitors, and like this, it’s possible to reach both energy density and power. This kind of supercapacitors can be classified into two types, the asymmetric hybrids and the battery/supercapacitor hybrids. The asymmetric hybrids are the ones that use one electrode made of carbon-based material (like in EDLCs) and another made of pseudocapacitive material (such as metal oxides). This improves energy capacity compared to EDLCs while maintaining good power. And the battery/supercapacitor hybrids are the ones that combine one electrode that functions like a battery (storing energy by ion insertion) and another that functions like a supercapacitor. They offer higher energy density, although generally with a shorter cycle life than pure supercapacitors. The main advantages of this type of SC is the higher energy density and good power output that they have and the main disadvantage is the shorter cycle life compared to pure EDLCs. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 11 3.4 Electrical double layer capacitor vs conventional capacitors Conventional capacitors store electrostatic energy in the electric field between two conductive plates separated by a dielectric, while EDLC capacitors store energy by storing ions in an electric double layer at the interface between the electrodes and the electrolyte. Therefore, in both devices, energy storage is based on charge separation, and the main difference is the extremely large surface area of the electrodes in the capacitor and the very small gap between the EDLC charge layers, which makes it have a higher capacity. Comparing the indicators performance of each technology, can be seen that the capacitance of capacitors is relatively low compared with the EDLCs capacity, which has a much higher capacity due to the large electrode are and small effective separation distance, Additionally, talking about life cycle, capacitors have long lifetimes but are limited because of the dielectric degradation, while EDLC supercapacitors have even more long lifetime due to the absence of chemical reactions and the dielectric degradation. About the charge/discharge rates, both are very fats but EDLCs are slightly slower than capacitor due to their higher capacity. One advantage of EDLCs is that their energy density is much higher than that of conventional capacitors, but in contrast, power density in conventional capacitors is much higher than in conventional capacitors. (Jayalakshmi & Balasubramanian, 2008) 3.5 Pseudocapacitors vs batteries The main difference between pseudocapacitive devices and batteries are the way they store energy. Batteries store energy through electrochemical reactions but in the bulk of the active material, which ions are incorporated into the electrodes during charging and released during discharging. Pseudocapacitors, store energy also through electrochemical reactions but in this case the reactions are redox, ion adsorption or surface intercalation at the electron boundary, at the surface of the electrode and not in the bulk of the active material, as it is in batteries. About lifetime, it is important to remember than chemical reactions degrade the electrodes, in batteries, the degradation is accelerated since the energy store occurs in the bulk, while in Memoria 12 pseudocapacitors since they have less room for action, remember that the reactions only occur in the surface, the lifetime is higher, because active material suffers less degradation. The charge/ discharge rate is fast in pseudocapacitors but slow in batteries, also due to the position of the reactions process. About energy density and power density, the energy density is lower, and the power density is higher in pseudocapcitors than in batteries, which translates to pseudocapacitos being able to store less energy but can delivery it faster than batteries. Batteries can store more energy, but the delivery is slower. (Bakker et al., 2012; Xie et al., 2018) Note that EDLCs are faster than pseudocapacitors delivering energy but pseudocapacitor can store more energy than EDLCS. 3.6 Supercapacitor components To be able to understand the principle of operation of supercapacitors it is necessary to know and to understand the main components of the cells: the current collector, the active material and the electrolyte. 3.6.1 Current collector The main objective of the collector is to ensure the transfer electrons between the external circuit and the active material. This means that it combines the charge accumulated in the active material with the charge outside the device to which SC is connected. Current collectors are typically made of highly conductive materials such as aluminium and copper, due to their low resistance and high electrochemical stability. The choice of the material depends on several factors, such as compatibility with the active material and corrosion resistance in the electrolytic environment. There are many different materials that could have been used but in designing this piece the following materials used were Aluminium and Copper (see Chapter 6, Section 6.7 Discussion). The collector has high conductivity, which minimizes resistive losses, thus increasing the efficiency of energy storage and release. Additionally, the design of the collector, including its thickness and surface texture, can affect the adhesion of the active material, affecting long-term stability and performance.(Abdisattar et al., 2022) Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 13 3.6.2 Active material The active material is the component where energy is stored in a SC. There are two main mechanisms for this storage: electrostatic adsorption of ions on the surface (EDLC) and faradic charge transfer (pseudocapacitors). There are different types of active materials: • Activated Carbon: It is the most common active material in EDLCs, due to its high surface area, which allows higher ion adsorption. It is cheap and has good electrochemical stability, but its capacitance is relatively low compared to pseudocapacitive materials. • Metal oxide materials: Oxides such as manganese dioxide (𝑀𝑛𝑂2) and ruthenium dioxide (𝑅𝑢𝑂2) offer pseudocapacitive capacitance, which allows them to store more energy than activated carbons. These materials are typically more expensive and can present stability changes. • Composite materials: Composite materials, such as metal-doped graphene oxide, are being developed to combine the advantages of different active materials, improving both capacitance and cyclic stability. It is important to choose correctly the active material because it determines the specific capacitance, energy density and cyclic stability of the supercapacitor. Materials with high surface are and effective redox properties can store more energy and be compatible with the electrolyte and current collector to minimize losses and improve device lifetime is key parameter. (Saal et al., 2021) Figure 3.5. Schematic description of the active material layer formed on the current collectors (CC) in a lithium-ion battery. (Yamada et al., 2020) Memoria 14 3.6.3 Electrolyte The electrolyte is the ionic medium that enables charge transfer between the electrodes of the SC. It is responsible for the ionic mobility within the device, which affects both, the capacitance and the internal resistance. There are different types of electrolytes, but in this section only two of then will be explained, which will be used in the development of the thesis. Aqueous electrolytes: They are usually solutions of acids, bases or salts, such as KOH. They offer high ionic conductivity and are cheap, but their voltage window is limited (around 1.23 V), which restricts the energy density of the SC. Organic electrolyte: Based on solvents such as acetonitrile or propylene carbonate, these electrolytes have a wider voltage window (up to 2.7 V), allowing higher energy density. However, they are more expensive, less conductive and present challenges in terms of safety and toxicity. The electrolyte influences several key aspects of SC performance, such as capacitance, energy density and cyclic stability. An electrolyte with high ionic conductivity can reduce the internal resistance of the device, improving charge/discharge efficiency. In addition, the stability of the electrolyte under a wide voltage range is essential to maintain the lifetime of the SC. When designing and studying SC, one must understand how these three components interact: current collector, active material and electrolyte. Each plays a key role in the overall performance of the device, and design decisions must balance conductivity, stability, energy storage capacity and cost. (Tian et al., 2021) 3.7 Metal oxides in energy storage Metal oxides (Mos) are materials used for energy storage application due to their unique electrochemical properties, such as high charge storage capacity, good stability and suitable redox properties. These materials are used in various storage technologies, including SCs, batteries and hybrid systems, for their ability to improve energy density, charge/discharge capability and cyclic stability. The main advantages of these materials are their high capacitance, as many MOs offer high specific capacitance, which increases the energy density of storage devices, and their chemical stability, Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 15 because Mos generally exhibit good chemical stability, which contributes to extended lifetime and consistent performance during charge/discharge cycles. The main challenges of disadvantages presented by these materials are their electrical conductivity, as some metal oxides have low electrical conductivity, which can limit their performance, and improving conductivity often requires combination with conductive materials or nanomaterial design. In addition, the cost and availability of certain metal oxides can be a concern for large-scale applications. Cheaper materials such as activated carbon are often preferred in commercial applications dure to their cost and accessibility. It is worth noting that there have been several recent advances and trends in nanomaterial development (enabling improved electrical conductivity and reactivity of metal oxides), in doping and structure modification, and in hybrid applications (such as now the combination of SC and batteries). MOs are promising materials for energy storage applications, with electrochemical properties that can be optimized to improve the capacity, stability and overall performance devices. Attempts are being made to improve electrical conductivity, reduce costs and develop new applications to maximize the potential of these materials in the field of energy storage. Two examples of metal oxides studied in SC applications are as follows, the magnesium oxide and the rutile oxide. • Magnesium oxide (𝑴𝒏𝑶𝟐) is known for its high capacitance and good cycle life. It exhibits various oxidation states that allow for high charge storage capacity. And it is used in SCs and batteries because of its ability to provide high energy density and its stability during repeated cycling. • Rutile oxide (𝑹𝑶𝟐) stands out for its high electrical conductivity and chemical stability. Its rutile structure allows for high charge density and efficient redox capability, ideal for energy storage applications. And it is used in batteries and hybrid systems, in catalysis and in supercapacitors, where it offers high capacitance and low equivalent series resistances (ESR), improcing energy density and cyclic stability. The metallic oxide that is the focus of this thesis is 𝐶𝑒𝑂2, a metallic oxide that, as has been shown, has not been studied by many authors as a material used in electrodes for energy storage, but has been used as conductive separator in SOFC. In the following section, the key parameters of the material in question will be described.(Banerjee & Grover, 2022; Cai et al., 2021; A. Huang et al., 2019; Y. Wang et al., 2020) Memoria 22 𝑃 = (𝑉)2 4∙𝑅𝑒𝑠𝑟∙𝑚 (Eq. 4.4) • 𝑅𝑒𝑠𝑟 is the equivalent series resistance (ohms) • m is the mass of active material (kg) • V is the operable voltage. For power per mass: Watts per kilogram (W/kg). 4.3.4 Coulombic Efficiency Coulombic efficiency is the ratio of charge recovered during discharge to the charge applied during charging of a SC. It serves as an indicator of energy losses during the charging and discharging cycles. An ideal SC would have an efficiency nearing 100%, meaning nearly all energy stored during charging can be retrieved during discharge. 𝜂𝐶𝑜𝑢𝑙𝑜𝑚𝑏 = 𝑄 𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 𝑄 𝑐ℎ𝑎𝑟𝑔𝑒 ∙100% (Eq. 4.5) • Q discharge is the total charge drawn during charging (C) • Q charge is the total charge supplied during charging (C) It is expressed as a percentage (%) 4.3.5 Equivalent Series Resistance (ESR) Equivalent series resistance refers to the internal resistance that current encounters while flowing through the SC. This includes the resistance of the electrode materials, contacts, and electrolyte. ESR can be measured during EIS, derived from the semicircle in the Nyquist plot. In case of ideal capacitors, in which the real resistance does not change over frequencies, the ESR can be calculated using the potential drop methos during galvanostatic discharge, from the voltage drop (V) immediately after discharge begins. 𝐸𝑆𝑅 = ∆𝑉 𝐼 (Eq. 4.6) • V is the voltage drop (V) just after discharge. • I is the applied current (A). The unit for ESR are ohms (Ω). Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 23 A lower ESR is preferable, as it allows the supercapacitor to deliver energy quickly with the minimal energy loss due to heat. It also directly impacts power density. 4.3.6 Cycle Life Cycle life refers to the number of charge and discharge cycles a SC can ensure before its capacity falls below a specified percentage of its original value (typically set at 80%). This metric is essential for applications that require frequent charging and discharging. A longer cycle life indicates a durable and reliable device. All these parameters and characterization techniques are essential to evaluate the performance of supercapacitors. (Banerjee et al., 2020a, 2020b; Laheäär et al., 2015; Ratha & Samantara, 2018) 4.4 Physical-chemical characterization Physical-chemical techniques provide information about the crystal structure, chemical composition, and surface properties of materials. These factors directly influence the electrochemical properties and, therefore, the performance of the material as a SCs 4.4.1 X-Ray Diffraction (XRD) X-ray diffraction is a technique used to study the crystal structure of materials. When X-rays strike a crystal, they are diffracted in specific directions that depends on the arrangements of the atoms in the crystal lattice. By analysing the diffraction patterns different parameters can be studied. First, it is possible to determine the crystal structure including the identification of different crystal phases and their proportion in a sample. Also, the presence of secondary phases or impurities. Moreover, the crystallite size and stresses can be known from a XRD analysis.(Kiani, 2023) Memoria 24 Figure 4.1 (a) Schematic illustration of the Bragg equation. (b) Geometric construction of the scattering and (c) Schematic setup of X-ray scattering setup. (Seibt & Ryan, 2021) 4.4.2 Raman Spectroscopy Raman spectroscopy is a method used to study how matter interacts with light or electromagnetic radiation based on the inelastic scattering of light. When a material is illuminated with a laser, most of the light is scattered with a change in energy that corresponds to the molecular vibrations of the material. This energy change is manifested in a Raman spectrum, which is characteristic of molecular vibrations and, therefore, of the chemical and crystalline structure of the material. Each type of chemical bond has a characteristic vibration frequency, which depends on the mass of the atoms involved and the strength of the bond, and this is why crystalline structures can be identified from the change in energy. With this technique, it is possible to identify molecular structures, crystalline phases and specific vibrational modes, which is essential to understand the composition and internal structure of a material. Also, it is possible to detect defects and impurities, as it is extremely sensitive to defects in the crystal structure, such as vacancies, interstitials or impurity inclusions, which helps to analyse the quality and purity of a material. And it is also useful to study doping and modifications, as the technique allows investigating how doping atoms or structural modifications affect the vibrational properties and, consequently, the physical properties of the material. (Moon et al., 2023; Orlando et al., 2021) Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 25 Figure 4.2. Schematic presentation of Raman spectroscopy instrument. When excitation light irradiates (green line), Rayleigh scattering (blue line) and Raman scattering (red dotted line) are released. (Iqbal et al., 2020) The use of these techniques is very important because all the parameters studied here can affect the physical properties of a material, such as conductivity and thermal stability, properties that are important and relevant in the construction of energy storage devices. Memoria 26 5 Electrochemical characterization in water-based electrolyte. 5.1 Chapter introduction The main objective of this experiment is to perform a basic evaluation of the electrochemical behaviour of the material using cyclic voltammetry (CV). The purpose is to check whether the material exhibits redox reactions, i.e. whether it can oxidize and reduce when subjected to an electrical potential. In addition, the aim is to observe whether secondary reactions occur and to assess the prominence of these reactions. In short, the objective is to determine whether the material exhibits electrochemical activity and, if so, to obtain a general idea of this behaviour. At this initial stage, it is not intended to perform a detailed analysis of the specific characteristics of the reactions. The aim is simply to verify the presence of electrochemical activity, which is a basic indication of its catalytic behaviour and its potential use in the creation of electrodes. Through this experiment, it has been possible to confirm that the materials initially proposed, CeO2 and GDC, do indeed exhibit electrochemical behaviour. It therefore makes sense to further study these materials with the aim of developing an energy storage device. 5.2 Preliminary calculations Before carrying out the experiment, it is necessary to perform certain calculations, which will be described and developed in this section. 5.2.1 Slurries creation The slurries that will be used to make the electrodes must be created. A total of 200 mg of solid mass slurry will be created from each type of slurry. The percentages of each slurry will be: • Slurry 1: 95% 𝐶𝑒𝑂2 + 5% Hydroxypropylcellulose (HPC) • Slurry 2: 95% GDC + 5% HPC • Slurry 3: 95% Carbon Black (CB) + 5% HPC • Slurry 4: 75% 𝐶𝑒𝑂2 + 20% CB + 5% HPC • Slurry 5: 75% GDC + 20% CB + 5% HPC Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 27 In addition, each slurry has a liquid part, which will be the deionized water. To know the quantity of ETOH to be used, the solubility of the HPC will have to be considered. Knowing that the solubility of the HPC in deionized water is 0.1 ml/mg and that the total calculated quantity of HPC to be used is 10 mg and from a conversion factor it is concluded that the quantity of ETOH to be used will be 1 ml: 10 𝑚𝑔 𝐻𝑃𝐶 ∙0,1 𝑚𝑙 𝑑𝑒𝑖𝑜𝑛𝑖𝑧𝑒𝑑 𝑤𝑎𝑡𝑒𝑟 1 𝑚𝑔 𝐻𝑃𝐶 = 1 𝑚𝑙 𝑑𝑒𝑖𝑜𝑛𝑖𝑧𝑒𝑑 From this data slurries can be created. 5.2.2 Electrolyte creation It was decided to check how the materials under study behaved in reduced voltage ranges, so the initial experiment was carried out using an aqueous electrolyte (remembering that in aqueous electrolytes the voltage range is limited). (McCafferty, 2010) It is known that in aqueous solutions, the pH can influence the stability of the material and the reactions that occur on the electrode surface. Therefore, it is necessary to check that the pH of the aqueous electrolyte is adequate to avoid corrosion, dissolution, or any other adverse interaction with the material. Thus, it was decided to use Pourbaix Diagrams, which are used to analyse the thermodynamic stability of materials as a function of pH and electrical potential, and thus determine if a material is stable, susceptible to corrosion or if specific products will be formed under certain conditions. (Meléndez-González et al., 2020) Memoria 28 Figure 5.1. Pourbaix Diagram of 𝐶𝑒𝑂2 with a scale E(V) vs SHE. (Channei et al., 2017) It is important to mention that the reference in the Pourbaix Diagram is SHE (Standard Hydrogen Electrode), while the cyclic voltammetries shown throughout the experiments of this chapter are vs Ag/AgCl. Therfore, if one wishes to compare both voltage values, a scale conversión would be required. After interpreting the cerium oxide diagram, it can be observe the different reactions that will take place depending on the pH and potential conditions applied. The material to be studied is CeO₂, therefore, Ceria in this species has a valence of +4. It is observed that, in order to keep CeO₂ stable, the material has to work in the ranges of the diagrama where is written ‘Passivation’ or ‘Immunity’, those ranges ensures that CeO₂ remains in its solid form, without dissolving or changing into other species. However, for optimal stability and to minimize any risk of dissolution, a basic pH would be ideal. In this case, the pH value used to perform the calculations will be 14.(Meléndez-González et al., 2020) Thus, to ensure these conditions, the aqueous electrolyte used will be KOH dissolved in deionized water, which is the most commonly used electrolyte because of having a higher ionic conductivity compared to other aqueos electrolytes .(R. Mainar et al., 2016) The concentration of the KOH it’s important, since the increase of KOH concentration decreases the electrode potential but the conductivity and ex-change current associated with reaction kinetics increase. In this experiment, a 1M KOH solution was used to seek a balance between the benefits provided by conductivity and electrode potential. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 29 Figure 5.2 Pourbaix diagram showing the regions of immunity, passivation, and corrosion of CeO2 with a scale E(V) vs SHE. So, knowing the products to work with, it will be necessary to calculate the quantities. The calculations involved in preparing a solution include the calculation of molar concentration (molarity), which involves determining how many moles of solute are needed to achieve a desired concentration in a specific volume. Additionally, there are acid-base relationships, where the relationship between pH and pOH is used to understand the acid-base equilibrium in aqueous solutions. Finally, the calculation of moles and mass is important for determining the amount of substance (in moles) and its corresponding mass (in grams) to prepare solutions of specific concentrations. The concentration of KOH to be used is 1 M, the desired pH is 14 and the volume of the solution to be created is 200 ml. So, the calculations performed were: • Step 1: Calculate the relationship between pH and pOH. For a pH of 14, the pOH is: 𝑝𝑂𝐻 = 14 −𝑝𝐻 =14 −14 = 0 • Step 2: Since the pOH is 0, the concentration of [OH-] is determined: [𝑂𝐻−] = 10−𝑝𝑂𝐻 =100= 1𝑀 Memoria 30 This means that the concentration of 𝑂𝐻− in the solution must be 1 M. • Step 3: The amount of KOH must be determined. Since potassium hydroxide (KOH) completely dissociates in water: 𝐾𝑂𝐻 → 𝐾++ 𝑂𝐻− The concentration of [𝑂𝐻−] will be equal to the concentration of KOH in the solution. So, to obtain a concentration of 1 M of [𝑂𝐻−], it is needed to dissolve 1 mol of KOH per liter of solution. • Step 4: The mass of KOH needed to prepare 200 mL (0.2 L) of a 1 M KOH solution will be calculated: 𝐾𝑂𝐻𝑚𝑎𝑠𝑠 = 0.2 𝑚𝑜𝑙𝑒𝑠 ∙ 56,11 𝑔 𝑚𝑜𝑙 =11.22 𝑔 • Step 5: The molar mass of KOH is approximately 56.11 g/mol. So, the mass of KOH needed is: 𝐾𝑂𝐻𝑚𝑎𝑠𝑠 = 0.2 𝑚𝑜𝑙𝑒𝑠 ∙ 56.11 𝑔 𝑚𝑜𝑙 =11.22 𝑔 Thus, to prepare a KOH solution with a concentration of 1 M (which will give a pH of 14), 11.22 g of KOH will need to be dissolved in 200 ml of distilled water. 5.3 Materials and methods In this section the materials and the procedure will be explained. 5.3.1 Materials The materials used to create the slurries are: • CeO₂ and GDC • HPC • Carbon Black • Laboratorio balance • Magnetic stirrer heater • Magnetic bar • Vial rack • Beaker • Vial Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 31 Figure 5.3 Balance, vial, pipette and magnetic stirrer heater. Figure 5.4. CB, HPC, CeO2 and GDC The materials used to create the electrolyte are: • Deionized water • Potassium hydroxide (KOH) Memoria 38 5.6.2 Onset potential and maxim current results The onset potential is the voltage at which an electrochemical reaction starts. It’s the voltage at which a significant increase in the current measured starts to be observed during cyclic voltammetry. This value helps to identify the voltage needed for a specific reaction to initiate. (Determining the Onset Potential in Cyclic Voltammetry. , 2020) The maximum current value in a voltammogram correspond to the highest peak observed during the electrochemical reaction. This maximum is due to a limitation in mass transport which makes the current not be able to increase indefinitely. (Mabbott, 1983) To determine the onset potential of the different reaction that occurs on GDC, GDC+CB and CeO₂+CB, it will be necessary to extract the data from the CV plots. In the current versus potential graph, the current is observed in the region where it begins to deviate from a low and constant value, characteristic of the capacitive current associated with the electrical double layer. This change in behaviour indicates the onset of the reaction. This descending region of the current peak, where the current starts to go negatively, must be selected. A linear regression was applied to this data. The regression line represents the behaviour of the current before the significant reaction starts, and from this line, which is generally expressed as 𝐼 = 𝑚𝑉 + 𝑏 , where 𝐼 is the current in amperes (𝐴), V is the potential in volts (𝑉), m is the slope in amperes per volt (𝐴 𝑉), and b is the intercept of the line with the current axis in amperes (𝐴). The onset potential is determined by finding the point where the regression line intersects the potential axis. In this case, the current is negative and descending, so this point is obtained by solving the regression line equation for 𝐼 = 0. This is done using the formula 𝑉𝑜𝑛𝑠𝑒𝑡 =−𝑏 𝑚, where b is the intercept of the line with the current axis in amperes and m is the slope in amperes per volt (𝐴 𝑉). Based on the data obtained during the calculations of the onset potential, the following tables were generated where the onset potential of each reaction is detailed, as well as the associated peak maximum. It is important to note that all the peaks described correspond to testes conducted in air-saturated environment. First peak of the first cycle in air saturated Material Onset potential (𝑬(𝑽)𝒗𝒔 𝑨𝒈/ 𝑨𝒈𝑪𝒍) Maxim current density (𝒎𝑨 𝒄𝒎𝟐) GDC 0,046 0,0146 Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 39 CeO₂+CB 0,036 0,027 GDC+CB 0,103 0,0086 Table 5.3 Onset potential and maxim current density first peak of the first cycle In the Table 5.3 it can be seen that the onset potential of CeO₂+CB presents the lowest onset potential with a value of 0.036 vs Ag/AgCl. Then GDC+CB, with the highest value of onset potential with a voltage of 0.103 V vs Ag/AgCl. While GDC with a value of 0.046 V vs Ag/AgCl represnts an intermediate value. About maxim current density respects CeO₂+CB exhibits the highest maximum current density, reaching 0.027 mA/cm², followed by GDC with a 0,0146 mA/cm² and finally GDC+CB with a value of a 0,0086 mA/cm. First peak of the last cycle in air saturated Material Onset potential (𝑬(𝑽)𝒗𝒔 𝑨𝒈/ 𝑨𝒈𝑪𝒍) Maxim current density (𝒎𝑨 𝒄𝒎𝟐) GDC 0,0087 0,00112 CeO₂+CB -0,159 0,018 GDC+CB 0,028 0,0032 Table 5.4 Onset potential and maxim current density first peak of the last cycle As the results obtained in Table 5.4 the onset potential of CeO₂+CB presents the lowest onset potential with a value of -0.159 vs Ag/AgCl. Then GDC+CB, with the highest value of onset potential with a voltage of 0.028 V vs Ag/AgCl. While GDC with a value of 0.0087 V vs Ag/AgCl represnts an intermediate value. About maxim current density respects CeO₂+CB exhibits the highest maximum current density, reaching 0.018 mA/cm², followed by GDC+CB with 0.0032 mA/cm², and finally GDC has the lowest current density, with just 0.00112 mA/cm². 5.7 Discussion 5.7.1 Cyclic voltammetry analysis The graphs obtained during the performance of the experiments are showering in the following section. In all the graphs, two shades of the same colour can be observed. This is because one shade refers to the first cycle tested, while the other shade identifies the last cycle (see the annotations 'first' and 'last' in the legends of each graph to recognize the colour). It is important to note that this Memoria 40 difference in cycles is due to the material stabilizing as the cycles progress. Therefore, data are collected from the first cycle, when the material starts to run and the last cycles, when the material has stabilized. Also, it can be noted that the representations of the materials tested in 𝑁2 environment are plotted in ‘Solid line’ format and those tested in Air saturated are plotted in ‘Dash dot line’. Figure 5.13. Legend used to represent the graphs where the materials tested in N2 environment are plotted in ‘Solid line’ format and those tested in Air saturated are plotted in ‘Dash dot line’ Cyclic voltammetry plots: Figure 5.14. Cyclic volatmmetry curves of CeO2 studied under saturated air atmosphere Upon performing the CV of CeO₂, it was concluded that it is a very resistive material, as a clear and analyzable signal from its CV could not be obtained, as it can be seen in Figure 5.14. Therefore, it was decided not to test this material on its own again; instead, it would always be used with CB to improve its conductivity and obtain an analyzable result. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 41 Figure 5.15. Cyclic voltammetry curves of GDC studied in different environments (a) under 𝑁2 atmosphere and (b) under saturated atmosphere. For GDC in a nitrogen environment (Figure 5.15 (a)) there’s a little reduction observable since the left side of the box is negative, this means that there is a reduction happening wither while scanning negatively and positivelly. Being the solution saturate with nitrogen, it colud be due to the reduction of this gas,because also the positive scan provides negative current. It can also appreciate that the material behaviour doesn’t change siginificantly from the first and last cycle, which mean that the material behavior didn’t change during the cycles. The GDC tested in air shows a different behavior compared to the material tested in nitrogen. It can be seen how the current follows a plateau behavior, the current flows flat, undisturbed path until reaching a certain value, approximately around -0.26 V V vs Ag/AgCl. When it reaches this voltage, implying that the applied potential is sufficient to overcome an activation energy and initiate a reaction, the current starts to drop to increasingly lower values until reaching a máximum peak in value voltage of -0.44 V vs Ag/AgCl. This maximum peak limit is constrained by the efficiency of mass transport. Once this limit is reached, the current begins to rise again. Then, it can be seen another current of -0.006 to 0.005 A/g, and after this peak, it returns to the plateau profile where it started. Like this, a ‘duck’ shape peak is observed, indicating, in this case, a reduction reaction. (IUPAC convention) This pattern is observed in both, the frist and the last cycle of the cyclic voltammetry, but with a variation in the máximum peak values. As can be observed, the máximum peak of the first cycle reached more negative values of current compared to the last cycle. As for the vertical stripes in the voltammogram, it is likely due to a disturbance in the signal rather than the intrinsic behavior of the material. Unlike CeO₂, GDC can be tested on its own, highlighting that the material is less resistive. Memoria 42 That’s why a way to enhanced significantly the ionic and electronic conductivity is by cation doping the ceria which creates an increase in oxygen vacancy concentration. (B. Wang et al., 2019) And for Gadolinium is expected to be a preferable dopant in CeO2 due to the coherence in an ionic radius of Ce4+(0.097 nm) and Gd3+(0.1053 nm) for the same co-ordination number of 8. (El-Habib et al., 2022) The oxygen vacancy defect is a loss of oxygen atom from their respective position in the crystal lattice, in this case 2 ∙ 𝐶𝑒𝑂2→ 𝐶𝑒2𝑂3+1 2𝑂2 (Hu & Metiu, 2011) .The defect reaction can be presented in Kröger–Vink notation as given below for gadolinia addition in ceria. Figure 5.16. Gadolinium defect reaction presented in Kröger–Vink notation Cerium and gadolinium can be found in the Ce⁴⁺ and Gd³⁺ valence states, respectively. Therefore, an oxygen vacancy is generated to maintain charge neutrality (El-Habib et al., 2022). And this stoichiometric formation of Ce 3+ gives , causes the removal of oxygen from the samples given rise to the formation of conduction electrons. (Eguchi et al., 1992) Thus, the conversion of Ce⁴⁺ to Ce³⁺ releases electrons, and the electrons are what carry the electric current in the reduced ceria, giving rise to electronic conductivity. While ionic conduction in doped ceria takes place via an oxygen vacancy diffusion mechanism. Diffusion of oxygen ions within the fluorite structure can be seen as the thermally activated process with characteristic activation energy. (Kashyap et al., 2014) Even if the GDC is less resistive than CeO2, CB will also be introduced to this material to make it even more conductive (read 5.7.2 Effect of Carbon Black) and to increase the chances that it can be used as an electrode. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 43 Figure 5.17. Cyclic voltammetry curves of CB studied in different environments (a) under 𝑁2 atmosphere and (b) under saturated atmosphere. In Figure 5.17 (a), it can be observed that the material maintains a fairly stable box shape throughout the entire scan, maintaining it during the whole sweep. And as it happened in GDC testes, it can’t be seen a change in the behavior during the cycles. As for the sample tested in air saturated, not much can be said, as the material appears to show a somewhat disturbed aspecto, which could indicate an issue with data acquisition. Figure 5.18. Cyclic voltammetry curves of CeO2+CB studied in different environments (a) under 𝑁2 atmosphere and (b) under saturated atmosphere. Following the behaviour shown by the materials tested in nitrogen so far, CeO2+CB also presents a fairly stable box-shaped signal. It is worth noting that, in this case, there are two disturbances in the first cycle of test that decrease in intensity in the last cycle. These disturbances are likely caused by signal acquisition rather than being characteristic behavior of the material. Memoria 44 Regardinf the first cycle of the material studied in air saturated air, a plateau behaviour of the signal can be observed until reaching a point where the current starts to decrease in a voltage of apporixmatelly -0.4 V vs Ag/AgCl until it reaches a máximum peak at a value voltage slightly above of -0.5 V vs Ag/AgCl. When it reaches this limit, it grows again until it reaches a certain point where it appears to re-establish a plateau behavior, although this does not last long, as another drop in current is observed, which arrives to a certain value.After this maximum, the curren trises again due to the fact that the sacn is reversed, re-establishing a plateau profile. In the last cycle, the first peak described for the first cycle is still observable, but again, in this case, the current value is lower than in the firs cycle. And in this case, the voltage at which the reduction peak occurs is slightly higher than in the first cycle, since in the first cycle the voltage value is -0.55 V vs Ag/AgCl, while in the second, the voltage value is -0.52 V vs Ag/AgCl. All those changes throughout the cycles highlight that the material loses catalytic efficiency over the course of the cycles, toward the reduction reaction explained. Figure 5.19. Cyclic voltammetry curves of GDC+CB studied in different environments (a) under 𝑁2 atmosphere and (b) under saturated atmosphere. In Figure 5.19 (a) the behaviour of GDC+CB in nitrogen can be seen. Following the tren of the materials studied in nitrogen, GDC+CB exhibits quite stable behaviour throughout the cycles. The shape observed in this material is different from that seen in the other materials tested in nitrogen. It is true that the box-like shape is observable at the higher potential values, but at approximately - 0.4 V vs Ag/AgCl, the material shows a rather resistive behaviour that repeats across the cycles. It shoul be noted that no conclusión has been drawn regarding th behavior displayed by the material. As for the material studied in saturated air, it follows a behaviour quite similar to that observed for CeO₂+CB in saturated air during the first cycle. The material exhibits capacitance behavior until it reaches a point where the current decreases, peaking at -0.41 V vs Ag/AgCl. After this peak, the current value rises until it stabilizes, generating again a capacitance behaviour that is not very long- Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 45 lasting, as another drop is subsequently observed, starting at -0.74 V vs Ag/AgCl. Like the behaviour observed in all the last cycles, this last cycle shows a decrease in the maximum current compared to the first one. The second drop is observable because at this point the the scan was reversed. As has been observed thorughout the discussion, no peaks were generated during the análisis of all the materilas and their cycles, in nitrogen evironment. In contrast, all materials testes in saturated air, expect for cerium oxide, which exhibited very resistive behavior, displayed peaks, specifically reduction peaks. From this observation, two conclusions can be drawn: first, the reactions obtained are due to the action of the environemnt, specifically oxygen, since in the cyclic voltammetry conducted in nitrogen, an inert environment, no significant reactions are observed. And from this conclusión, another can be inferred: if these reactions are due to the presence of oxygen, they are not inherent reactions of the material, because otherwise they would be also be observed in nitrogen, and thus, the redox reactions characteristic of CeO₂ have not been observed. It let conclude that the reacctions oberved are oxygen reduction reactions. The oxygen reduction reaction (ORR) is the cathodic branch of the oxygen electrode reactions, These reactions are important for electrochemical energy conversion and storage. The ORR is recognized as the kinetically limitng component of these devices based on oxygen electrochemistry, due to its slow kinetics. This limitation has led to the search of materials that favor these reactions, which ‘facilitate’ this reaction, materials that act as a catalysts for the reaction. The goal is to find materials that present and overpotential as closet o zero as possible concerning the onset potential of the material, that is, to minimize the overpotential. (Gõmez-Marín & Feliu, 2013) As stated, he kinetic of ORR is very slow, as it can be seen when compared to the kinetic of hyrodgen oxidation reaction. This means that the higher amount of catalyst in the cathodic range to boost this reaction is higher than the one needed for the hydrogen reduction reaction. Platinum is the most catalyst for both reactions, but the investigantions releated to this material are more focus on develop platinum catalysts for boosting the ORR, and like this the kinetic losses in low-temperatura fuel cells can be lower. Moreover, the material used is not the only consideration that has to take when study the kinetics related to oxygen reduction redactions are being study. The use of the pH media (alkaline or acidic electrolyte) can strongly affect the pathway of the reaction, and consequentlly the kinetic of the reaction. It is important to note that the kinetic of the reaction in alkaline eletrolyte is lower than in acidic electrolyte. But before doing this discussion is important to know that the oxygen reduction reaction can have two pathways, according to the exchange of electrons generated. In the two path ways electrons there’s an exchange of two electrons, while in the four electrons pathway, as the name stress, exchange four electrons. Those reaction occur through a dissociative or an associative mechanism, respectively. Once it is clear, it can be say that in both media, acidic and alkaline, the oxygen reduction reaction can be reduced following both, the two or four electrons pathway. In the Memoria 46 four electrons pathway in alkaline media, the formation of water can be seen, while in alkaline electrolyte, the OHis formed as a product. The two electrons pathway, brings to the formation of 𝐻2𝑂2, in acidic media and 𝑂𝐻2 − in basic media. In the associative mechanism of four electron pathway in alkaline media, ORR starts with the adsorption of 𝑂2 molecule. The mechanism followed by the material is the follow one (notice that * represents the active site and 𝑂2 ∗ refers to an adsorbed specie): 𝑂2+ ∗ ⇆ 𝑂2 ∗ (Reacc. 1) 𝑂2 ∗+ 2𝐻2𝑂 + 𝑒− ⇆ 𝑂𝑂𝐻∗+𝑂𝐻− (Reacc. 2) 𝑂𝑂𝐻∗+ 𝑒− ⇆ 𝑂∗+ 𝑂𝐻− (Reacc.3) 𝑂2 ∗+ 2𝐻2𝑂 + 𝑒− ⇆ 𝑂𝐻∗+𝑂𝐻− (Reacc.4) 𝑂𝐻∗+ 𝑒− ⇆ ∗ + 𝑂𝐻− (Reacc. 5) At the end, it can be observed that the 𝑂2 accepts four electrons, and therefore, it results in 4 𝑂𝐻−ions. On the contrary, if the rupture of O-O doesn’t favor, in therms of energy, the Reacc. 4, peroxide ions can be formed, following the reaction: 𝑂𝑂𝐻∗+ 𝑒− ⇆ 𝑂∗+𝑂𝑂𝐻−(Reacc. 6) In this case, the followed pathway is the two electrons pathway, following a dissociative mechanism. Here the oxygen molecule splits into two atoms, generating an addsobrtion on two different active sites. And like this, the 4 electrons pathway procedes according to the reaction Reacc. 4. and Reacc. 5.: 1 202+ ∗ ⇆ 𝑂∗ (Reacc.7) Like this, a description of the complex mechanism of the oxygen reduction raction. But now, and the main idea of this explanation is to try to understand CeO2 behaviour, so it must be explained, how CeO2 proceeds with this reactions. (Liu et al., 2016) To understand how it behaves, it is vital to highlight the redox couple Ce4+/Ce3+of cerium oxide, which indicates that the material can easily switch between realesing and storing oxygen. So, once the mechanism were explained above, it can be conclude that cerium oxide reactions associated to the oxygen reduction reactions are: 𝐶𝑒4+ + 𝐻2𝑂2 ⇆ 𝐶𝑒3+ + 𝐻𝑂𝑂 + 𝐻+(Reacc. 8) Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 47 𝐶𝑒4+ + 𝐻𝑂𝑂 ⇆ 𝐶𝑒3+ + 𝑂2+ 𝐻+(Reacc. 9) 𝐶𝑒4+ + 𝐻++ 𝑂𝐻− ⇆ 𝐶𝑒4+ + 𝐻2𝑂(Reacc. 10) 𝐶𝑒3+ + 𝐻++ 𝐻𝑂𝑂 ⇆ 𝐶𝑒4+ + 𝐻2𝑂2(Reacc. 11) As it can be be observed, 𝐶𝑒4+ tend to induce the decomposition of hydorgen peroxide and peroxyl radicals, while 𝐶𝑒3+ facilitate the scavening of hydroxyl radicals and peroxyl radicals. So, the reactions that can be seen in the CV’s are related to this ones. Since it is an associative mechanism Figure 5.20. Cyclic voltammetry oxygen reduction reactions in the (a) first and (b) last cycles. In regard to the first reduction reaction, which has been explained above. It can be observed in the Table 5.3 and 5.4, GDC presents an onset potential of 0.046 V vs Ag/AgCl for the first cycle and 0.0087 V vs Ag/AgCl for the last one. The CeO₂+CB onset potential has a value of 0.036 V vs Ag/AgCl for the first cycle and a value of -0.0159 V vs Ag/AgCl, which follows the trend of the peaks in the CV. About the GDC+CB, the onset potential values are 0.103 V vs Ag/AgCl for the first cycle, while for the last it is 0,028 V vs Ag/AgCl. This values maches the trend observable in the CV’s, where the GDC+CB goes under the reduction reaction faster than GDC and CeO₂+CB, highlithing that the GDC+CB facilitates the oxygen reduction reaction compared to the other materials. According to the density values it is considered an-error in the data adquisition since the CeO₂+CB peak is much more predominant than the other ones, even having a higher value of onset potential. So, to avoid to use wrong values that could alter the final results, the current won’t be used to discuss the efficiency of the materials studied in terms of catalysis of the oxygen reduction reaction. To make the differences in onset potential presented by each material more visual, a bar chart representation has been used, where the onset potential of each material is depicted. Memoria 54 Knowing that the solubility of CMC in distilled water is 1 ml/10 mg and that the total calculated amount of CMC to be used is 15 mg and from a conversion factor it is concluded that the amount of distilled water to be used will be 1.5 ml: 15 𝑚𝑔 𝐶𝑀𝐶 ∙ 1 𝑚𝑙 𝑑𝑒𝑠𝑡𝑖𝑙𝑙𝑒𝑑 𝑤𝑎𝑡𝑒𝑟 10 𝑚𝑔 𝐶𝑀𝐶 = 1,5 𝑚𝑙 𝑑𝑖𝑠𝑡𝑖𝑙𝑙𝑒𝑑 𝑤𝑎𝑡𝑒𝑟 6.3 Materials and methods In this section the materials and the procedure will be explained. 6.3.1 Materials All the materials used to create the slurry in the last section will be used also in this one. Just highlight the change in the binder, which instead of HPC, it will be CMC. • CMC • Deionized water Figure 6.1. CMC binder The material need as an electrolyte: • LiFP6 1 M EC/DMC (1:1) Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 55 Figure 6.2. Electrolyte used 1M LiFP6 EC/DMC (1:!) The material needs to do the coating: • Al and Cu foil • Doctor Blade Figure 6.3. Doctor blade The material need to do the cell assembly: • Reference electrode: Li metal, ribbon shape • Counter electrode: Li metal, ribbon shape • T-cell or three-electrode Swagelok cell • Spacers • Spring • Glass fibre separator Memoria 56 Figure 6.4. T-Cell structure , electrolyte, separator and electrodes inside aliminum packaging Machinery used during the experiment: • Vacuum tube • Laboratory oven for vacuum tube • Electrode Punching Machine • Glovebox Figure 6.5 Electrode Punching Machine and Glovebox Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 57 Figure 6.6. Vacuum tube with electrodes inside and laboratory oven with the vacuum tube 6.3.2 Methods/Procedure 6.3.2.1 Slurry preparation In this experiment, five different slurries were created with materials in different proportions. • Slurry 1: 75% CeO₂ + 20% CB + 5% CMC • Slurry 2: 75% GDC + 20% CB + 5% CMC The total mass of the slurry will be 300 mg and additionally, 1.5 ml of distilled water will be used. The preparation of the slurry consists of different steps: • Step 1: Distilled water and CMC are added to the test tube. A magnetic bar is added to the test tube and the test tube is then placed on the magnetic stirrer at a temperature of 60ºC to create a homogeneous mixture. • Step 2: CB is added to the mixture created in Step 1. Once added, the test tube is placed on the magnetic stirrer again until a homogeneous mixture is created. • Step 3: CeO₂ is added to slurry 1 and GDC to slurry 2 and the mixing processes mentioned above are repeated. 6.3.2.2 Foils preparation Once the slurries have been created, next step is to create the electrode: • Step 1: Cu foil must be place on a glass template and wet the template with ETOH. Memoria 58 • Step 2: The ETOH needs to be place on the foil and wipe it with a piece of paper to smooth the surface of the foil. The process must be repeat but this time with distilled water to avoid the hydrophobicity of the slurries with respect to the foil. • Step 3: The slurry must be place on the prepared foil. • Step 4: Doctor Blade needs to be passed to give the appropriate thickness to the electrodes, considering the thickness of the foil, (in this case, 120 mm), coating process. • Step 5: The foil that has been prepared in the previous steps has to dry overnight. These steps will be repeated with the Al foils. Figure 6.7. Foils once been coated Figure 6.8. The mechanism of the doctor blade method (Frederichi et al., 2021) 6.3.2.3 Electrodes preparation • Step 1: The electrodes are cut with a diameter of 12 mm with an Electrode Punching Machine. The electrodes were cut in an anhydrous chamber, although this step is not as critical since the material will subsequently be exposed to the action of an oven. • Step 2: The cut electrodes need to be putted into a bag made of aluminum kitchen foil. • Step 3: This bag must be putted into a laboratory oven, vacuum tube, at a temperature of 120ºC for 10 h, with the main objective of eliminating the moisture present in the material Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 59 Figure 6.9 . The foil prepared to be cut and the elctrode once it is cut. 6.3.2.4 Electrodes transport • Step 1: The electrodes must be transported in the same oven/tube where they have been dehydrated to the Glovebox without opening this tube in any moment. The electrodes must still be under vacuum when they enter the Glovebox. • Step 2: The vacuum tube must be open in the Glovebox and the electrodes must be placed there. 6.3.2.5 Cell assembly • Step 1: The weight and the thickness of the material used as working electrode must be measured and then, used. • Step 2: Two 12mm circles of metallic Li must be cut, which will serve as the reference electrode and the counter electrode. • Step 3: All the elements that must be used are prepared. The electrolyte, the three electrodes (working, reference and counter), the T-Cell, spacers, springs, separator, and a pipette, to split the electrolyte in the cell. • Step 4: The cell must be assembled with all the elements mentioned above. Figure 6.10. Cell assembly components, electrode and reference (with a glass fiber separator) once the cell has been tested Memoria 60 Figure 6.11. T-cell main components(Friebe & Schubert, 2017) 6.4 Experiment conditions. The only conditions that should be emphasized are those applicable during the assembly of the Tcells. This process must be carried out in inert environments, such as nitrogen (N2) and argon (Ar), to prevent lithium (Li) from reacting with the surrounding. (Lithium metal gas reactions). To prevent this, the T-cells should be assembled inside the glovebox, which provides high levels of inertness. In section 5.X, the importance of using inert environments in the field of electrochemistry has been discussed. 6.5 Measurement methods. In this section, the settings of the CV (Cyclic Voltammetry) to perform the experiments are explained. This characterization technique is performed with a potentiostat. The settings correspond to Cyclic Voltammetry (CV) can be seen (Figure 6.12). The scan rate is 0,1 mV/s, it is that slow because the idea is to allow enough time for both faradaic and non-faradaic processes to occur, which helps to observe the redox reactions of CeO₂ and GDC. The vertex potential for electrodes with a copper current collector range from 0.1 to 2 V vs Li/Li+, as the aim is to study how the material behaves at low voltage. Meanwhile, the vertex potential of electrodes with aluminium current collector ranges from 5.1 to 2.4 V vs Li/Li+, since in this case, the goal is to study how the material behaves at higher voltage values. Regarding the bandwidth, which is the Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 61 maximum amount of data transmitted in a given amount of time, it had to be adjusted for copper to the minimum value possible because the signal obtained during the tests contained too much noise, limiting it compression. In contrast, the materials tested with aluminium provided a good signal, even with a bandwidth of 3. As mentioned in the section Chapter 5, Section 5.5 Measurement methods, the E range is established as a safety range. As a last setting, the repetition of the cycles (n=9) is to allow the material stabilizes over time. Figure 6.12. Cyclic volatmmetry setting of (a) Al current collector electrods and (b) Cu current collector electrodes 6.6 Results 6.6.1 Mass of the electrodes used. Data of the foils used: Cu density (𝒎𝒈 𝒄𝒎𝟐 ) 5,4 Al density (𝒎𝒈 𝒄𝒎𝟐 ) 3,7 Table 6.1 Foil's density Knowing that the electrodes have a diameter of 12mm, the radius that will be used to calculate the following data will be 6mm. Memoria 62 A(𝑐𝑚2) m (mg) Cu foil 1,1304 6,10416 Al foil 1,1304 4,18248 Table 6.2. Foil's area and mass Thus, subtracting the mass of the foil used, it can be obtained that: Measurements in Cu Mass electrode (g) Mass current collector (g) Mass dry slurry (g) Mass active material (g) Thickness (mm) GDC in Cu 0,0094 0,0061 0,0033 0,002145 0,034 CeO₂ in Cu 0,00765 0,00615 0,0015 0,00100750 0,033 Table 6.3 Electrodes with Cu as a current collector measurments Measurements in Al Mass electrode (g) Mass current collector (g) Mass dry slurry (g) Mass active material (g) Thickness (mm) GDC in Al 0,0054 0,0043 0,0013 0,000845 0,029 CeO₂ in Al 0,0056 0,0041 0,0015 0,000975 0,03 Table 6.4 Electrodes with Al as a current collector measurments 6.7 Discussion The graphs obtained during the performance of the experiments are showering in the following section. It can be noted that the representation of the materials tested in Cu current collector are plotted in ‘Solid line’ format and those tested in Al current collector are plotted in ‘Dash dot line’. A more extensive explanation can be found in the corresponding section. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 63 The objective of this experiment, as mentioned before, is to study the voltage ranges in which the electrode material operates efficiently. To achieve this, its performance was investigated in both the anodic and cathodic ranges. The anodic range, where, by definition, is the range where oxidation takes place, is in this case where the material’s behavior is studied at high voltage values, while the cathodic range, is the electrode where reduction takes place, is where the material’s behavior at low voltage values will be analyzed. Thus, the current collectors will be selected based on the voltage to which they will be exposed, since not all the materials are stable in all the voltages. In the anodic range, the chosen current collector will be Al since it can operate stably up to around 3.8V vs. Li/Li+(S. Yang et al., 2024) ,whereas for the cathodic range, the chosen current collector will be Cu, since the stable electrochemical behavior of Cu occurs at potentials lower than 3 V compared to Li/Li+. (Zhu et al., 2021) To be able to study such a wide voltage range, it will not be enough to use an inorganic electrolyte, as these work within low voltage ranges. Therefore, it will be necessary to work with organic electrolytes, which have a broader operating voltage range. (Banerjee et al., 2020b; Bhat et al., 2022; Palacin, 2021) 6.7.1 Al as a current collector In the following section, the results obtained from studying the material at high voltage ranges will be discussed. To facilitate the understanding of the discussion of the graphs, it is first necessary to know what anodic dissolution is. Anodic dissolution is an electrochemical process that occurs when a conductive material, in this case, aluminum (Al) current collector, dissolves due to a redox reaction at the interface with an electrolyte while an external voltage is applied. This phenomenon is especially observed in systems where the collector material is exposed to high potentials and an electrochemical environment that favors corrosion. To understand the anodic dissolution process in this case, one must first consider the presence of Al as a current collector, the presence of LiFP6 as the electrolyte, and that trace amounts of residual water that are inevitable in assembled cells (handled in the glovebox) due to, among other factors, material processing. The choice of Al as a current collector is justified by its high electrical conductivity and low cost. However, its use presents challenges due to its operating voltage, which shows a tendency to corrode. The LiFP6 has been choice as electrolyte since it is considered the Memoria 70 So, once that it is clear, the discussion of the plots is easier to understand. In order to facilitate the compression of the plots, a legend has been created. See that purple serves to represent GDC+CB electrodes and blue CeO₂. Also, in this case, the most relevant cycles are the first and the last one, to compare the SEI action, and for that there’s once cycle, called ‘First’ and another one called ‘Last’. Figure 6.20 Legend of the electrodes with copper as a current collector Figure 6.21. Cyclic voltammetry of materiales tested with copper as a current collector Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 71 The formation of the SEI can be seen because there is irreversible charge loss. This means that the Qduring the first cycle is much larger than in subsequent cycles. The SEI keeps forming until you don’t see the stabilization of the cycles by the fact that the cycles are superimposed one on the previous one. It can be seen from Figure 6.20 and Figure 6.21 that when the material is charged for the first time, the potential decreases sufficiently to reduce component of the electrolyte, leading to the decomposition of the solvent (In this case EC and DMC) and the electrolyte salts (In this case LiFP6). This decomposition generates a series of products that form the solid electrolyte interphase (SEI). In this, a very broad reduction peak can be seen that begins at approximately 1.3 V in the first cycle. The decomposition of the carbonate-based electrolyte carbon anode material generally occur below 0.75 V, in this case, a potential value of 0.6 V is observed. This reduction value corresponds to that reported in the article (Hou et al., 2019), which states that Li–PF6–(EC) + e– → Li+–PF6–(EC) has a reduction potential of 0.59 V. Thus, this peak is associated with the decomposition of the electrolyte. Also, as voltages decreases, the lithiation (the lithium intercalation) appears at a voltage of 0.1 V. This reaction consists in the lithium ions passing through the SEI formed earlier to enter to the cathode electrode, were active material, in this case carbon black with cerium oxide, and gadolinium doped ceria and the copper current collector are. Regarding to this reaction, as the cycles progress, until the signal stabilizes (in the later cycles), the generated current is lower; thus, the glow between the SEI and, this case, the active material is less than in the subsequent cycles. (Hua et al., 2012) Also, a slight component of deintercalation of Li+ is noted, which seems more relevant in the GDC+CB case, in the potential between 0.9 and 1,4 V. Therefore, it can be concluded that the material studied in this potential window does indeed function, as the intercalation process implies the maintenance of positive charge in the cathode, thus, presenting a capacity for charge storage, while the slight deintercalation signifies the discharge process. As a result and considering the discussion for both the low and high ranges voltages, it can be concluded that the only way to create a supercapacitor with these materials would be by designing a hybrid supercapacitor. In this case, the anode, which has proven to be functional, would be made from one of the materials discussed, CeO₂+CB or GDC+CB, while the cathode would consist of a different material. Memoria 72 6.8 Conclusions The conclusions drawn from the studies conducted on aluminum were: Both CeO₂+CB and GDC+CB exhibit similar electrochemical behavior within their respective voltage ranges, indicating that they respond similarly to these applied potential and conditions. An increase in current signals a shift in material behavior associated with the onset of corrosion when the protective layer begins to fail. The initial corrosion resistance is attributed to the formation of a protective Al₂O₃ layer on the aluminum current collector, which is compromised when voltage thresholds are exceeded. The behavior of current in response to increasing voltage suggests a relationship with the formation of a passivating layer (AlF₃) at higher voltages, affecting corrosion dynamics. The development of passivating layers increases the internal resistance of the system, leading to an IR drop that necessitates higher voltages to maintain passivation. This results in an observable hysteresis in the potential associated with the maximum current peak. Both materials cannot be effectively used as cathode materials in aluminum current collector, alternative strategies must be explored to extend the corrosion voltage limit of aluminum. If CeO₂+CB and GDC+CB are to be considered for anodic applications, further research is needed to identify methods to mitigate the corrosion issues at higher voltages to enhance their viability in practical applications. The conclusions drawn from the studies conducted on copper were: The significant charge loss observed during the first cycle indicates the formation of a solid electrolyte interphase (SEI), which stabilizes over time as subsequent cycles occur. During the initial charge, the potential drop is sufficient to cause the decomposition of electrolyte components (such as EC and DMC) and electrolyte salts (like LiFP₆), leading to the formation of the SEI. As voltage decreases, lithium intercalation occurs at approximately, indicating the movement of lithium ions through the SEI into the cathode electrode, which contains active materials like carbon black, cerium oxide, and gadolinium-doped ceria. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 73 Over successive cycles, the generated current decreases, indicating reduced interaction between the SEI and the active material. This stabilization suggests effective charge storage capabilities. The materials under study function within the specified potential window, demonstrating charge storage capacity through lithium intercalation and slight deintercalation, concluding that this material can be used as an anode material. Memoria 74 7 Physical-chemical characterization So far in the thesis, the electrochemical properties of the studied materials have been discussed. In this section, it will analyse the physicochemical properties of the studied materials to understand their electrochemical behaviour observed thus far. To achieve this goal, Raman Spectroscopy and X-Ray Diffraction will be performed (techniques explained in Chapter 4, Method and ). 7.1 X-ray Diffraction Figure 7.1 X-ray Diffraction machine 7.1.1 Results and discussions The parameters that will be discussed in this section to understand the material’s properties are the cell length, cell angle, and crystalline size. The values for each material and the corresponding parameter can be found in Table 7.1. The values extracted from the following patterns and using the CORD code allowed the obtention of the table mentioned. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 75 Figure 7.2 CeO2 pattern difraction Memoria 76 Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 77 Figure 7.3 GDC pattern difraction Figure 7.4 CORD card used to make the Rietveld refinment Memoria 78 Analyzing the diffraction patterns obtained for each material (Figure 2 and 3) and using the COD card for this, the following data have been extracted: Table 7.1 Cell length, cell angle and crystalline size of GDC and CeO2 studied in a pure state In the studied X-ray diffraction patterns, it was observed that the position of the (1 1 1) planes show a position equivalent to that compared with the standard CeO₂ (JCPDS: 34-0394), corresponding to the F2g vibrational mode of the fluorite structure. The patterns can be justified using literature. In the article ((Chen et al., 2007) the relationship between the crystallite size and the lattice parameter of ceria have been studied. It stresses that the variation in the lattice parameter is attributed to the lattice strain induced by the introduction of Ce3+ due to the formation of oxygen vacancies. This lattice stress was also observed to decrease with an increase in the particle size. While Ce3+ ions concentration increased with the reduction in the particle size. Therefore, they concluded that the increase of crystallite size resulted in the decrease of Ce3+ content, lattice strain, and lattice parameter. (Chen et al., 2007) Regarding the differences between the tested materials, it is observed that the values of the cell length of GDC_RT are slightly lower than those of CeO2_RT as well as its crystalline size. This behaviour is since the cell parameter of gadolinium doped show to be little smaller than that of porous CeO₂. This may be due to the lattice constriction effect resulting from Gd3+ ions. This can be attributed to the replacement of Ce3+ by trivalent Gd3+ ions, since the ionic radius of Gd3+ (107.8 pm) is smaller than those of Ce3+ (115 pm). Therefore, the lattice constricts upon Gd3+ doping happens.(Gao-Ren et al., 2009) Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 79 7.2 Raman Spectroscopy Raman Spectroscopy analyses were conducted with a wavelength of 514 nm, at a magnification of 50x, with a relative signal intensity of 1%, by using a Raman spectrometer. Figure 7.5 Raman Spectroscopy machine 7.2.1 Results and discussions In the following section Raman spectroscopy plots are represented. The x-axis represents the Raman shift or wavenumber (in cm-1), which is the measure of the energy difference between the incident light and the light scattered by the material. While Y-axis represents the intensity of the Raman signal, which is related to the amount of inelastically scattered light as function of the Raman shift. (Moon et al., 2023) Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 87 10 Economic analysis The economic analysis will be divided into three parts. The money allocated to labour, meaning the time dedicated by the person to carry out the work, the materials used during its execution and finally, the associated energy resources. List of materials used with higher economic impact: Material Quantity Unitary cost (€) Total cost (€) Cerium oxide 1 g 9,34/g 9,34 Gadolinium doped oxide 1 g 5/ g 5 Carbon black 1 g 1,74/g 1,74 KOH 15 1/g 15 Electrode reference Ag/AgCl 1 126 126 Glassy carbon electrode 1 371 371 Platinum counter electrode 1 268 268 T-cells and components 3 300 900 Glovebox 1 13350 13350 Potentiostat 1 11570 11570 Vacuum oven 1 1400 1400 Doctor Blade 1 44,5 44,5 Balance 1 1100 1110 Memoria 88 Stirrer 1 689 689 Electrode machine puncher 1 4000 4000 Total 33849,58 Table A. Economic impact associated to the materials used with the highest impact on the budget. Notice that X-ray diffraction and Raman Spectroscopy machines have not been added, since it haven’t been use frequently. List of hours invested by the researcher: Activity Dedicated hours (h) Cost per hour (€/h) Total cost (€) Experiment development 290 9 2610 Data analysis 100 9 900 Report writing 50 9 450 Meetings 10 9 90 Total 450 4050 Table B. Economic impact associated to hours dedicated at the project. For the hourly cost value, the minimum wage as per the Universitat Politècnica de Catalunya agreement during the internship period has been taken as a reference. List of equipment associated to the electrical consumption: Equipment Power (W) Hours use (h) Total consumption (kWh) Cost per kWh (€/kWh) Total cost (€) Potentiostat 1500 1440 2160 0,3347 723 Glovebox 2220 30 66,6 0,3347 22,3 Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 89 Oven laboratory vacuum 450 48 21,6 0,3347 7,2 Computer 500 25 12,5 0,3347 4,2 Total 6870 1543 2260,7 756,7 Table C. Economic impact associated to energy resources For the cost of kWh the price of December 2023 in Italy was taken as a reference, since the experiment were carried out in the laboratories of Politecnico di Torino. The total economic value associated to the implementation of the project. Cost (€) Impact on the final budget (%) Material cost 33.849,58 85,57 Hours dedicated cost 4.050 12,51 Energy consumption cost 756,7 1,91 TOTAL COST 38.656,28 100 Table D. Total economic impact and the percentatge of impact of each of the ítems. The total price associated to the project is 38.656,28€. The most notable impact on the final budget is the price associated with the materials used, accounting for 85.57% of the budget. During the research of the materials used, a significant cost was observed related to an electrochemistry laboratory. The second most notable contribution is the cost associated with the workforce, which represents 12.51% of the final budget, while the lowest associated cost is energy consumption, with a percentage of 1.91% of the total impact. Evaluation of cerium oxide and gadolinium-doped ceria as energy storage materials: Physicochemical and electrochemical study. 91 11 Conclusions In the first experiment, it was observed how pH affects the stability of ceria, being stable at basic pH levels. The resistivity of CeO₂ was examined, and it was found that doping with Gd decreases this resistivity due to the formation of oxygen vacancies. Furthermore, the effectiveness of carbon black in relation to the increase in electrical conductivity was noted, as both materials increased their conductivity when mixed with this material. This increase in conductivity generated by carbon black was investigated and is attributed to the formation of an interconnected carbon network, which provides pathways that facilitate electron flow. Additionally, by comparing the behavior of the material in both oxygen and nitrogen environments, the importance of the environment on the performance of the materials was highlighted, with N₂ being a more inert atmosphere. The presence of oxygen, in this case, helped us study the catalytic behavior of the materials concerning oxygen reduction reactions. It was concluded that the onset potential presented by GDC+CB for this reaction was the highest, and therefore, considering that the current value was discarded, GDC+CB was the material that exhibited the greatest catalytic efficiency regarding the ORR, which is consistent with what was expected, as the doping of cerium oxide translates to greater conductivity and catalytic activity The second experiment allowed us to study the working voltage range of the material. For this, aluminum and copper current collectors were used. It was observed that the materials studied with aluminum as the current collector presented an irreversible behavior, which was due to the anodic dissolution, making it impossible to use this material as an active material at low voltages. In contrast, the materials studied with copper showed the formation of the SEI layer and consistent repeated cycles, highlighting the reversibility of the reaction and therefore the potential application of the materials in low voltage ranges. This reveales that just asymmetric supercapacitors can be perform. The data obtained from Raman spectroscopy and X-ray diffraction support the existence of the fluorite crystalline structure, evidenced by the identification of the F2g vibrational mode in both analyses. The sample’s purity and crystalline nature of the materials were also observed, and the addition of Gd in the material can be observed. The project could help to a improve on environmental and social impact, but it must be studied deeply. 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