Search

Commercial Li-ion batteries typically incorporate a small amount of high-capacity silicon (Si)-based materials in the composite graphite-based anode to increase the energy density of the battery. However, very little is known about the effects of Si on the fast-charging behavior of composite anodes. Herein, we examine the effects of the Si/graphite ratio in the composite anode on the fast-charging behavior of full cells. We show that addition of Si increases the rate capability from 1C to 8C and improves the capacity retention in early cycles at 6C due to reduced overpotential in constant current charging cycles. The impacts of Si content on fast-charging aging were identified by Post-Test characterization. Despite realizing benefits of available capacity and reduced Li plating at 6C, silicon–electrolyte interactions lead the time-dependent cell performance to fade quickly in the long term. The Post-Test analysis also revealed the thickening of the electrode and nonuniform distribution of electrolyte decomposition products on the Si-containing anodes, as well as the organic-rich solid electrolyte interphase (SEI), which are the factors behind cell degradation. Our study sheds insight on the advantages and disadvantages of Si/graphite composite anodes when they are used in fast-charging applications and guides further research in the area by designing an optimized composition of Si incorporated in a mature graphite matrix.

Commercial 18,650 lithium-ion batteries are cycled at different discharge current rates. It reveals that an accelerated capacity fade occurs for cells at a low discharge rate which is attributed to the loss of lithium inventory (LLI) from the differential voltage analysis (DVA). Cells using high discharge rates exhibit more kinetic loss at the same capacity retention from the analysis of impedance. Characterization techniques, i.e., post-mortem analysis including scanning electron microscopy (SEM) and ex-situ x-ray diffraction (XRD), galvanostatic tests, and in-situ XRD on half-cells made with cathodes and anodes retrieved from the 18,650 batteries, are used to further address the degradation factors. It indicates that the kinetic loss of the high discharge cells can be ascribed to the cathode where more particles are cracked and pulverized. Degradation on the anode is the primary reason for accelerated capacity fade occurring at the low discharge rate. The low discharge current deepens the discharge depth leading the graphite accessing into a higher potential over de-lithiation. Worse interphases and dense agglomerated structure are found from SEM images, which is deemed to result from the anode cycled at a high potential where large volumetric change happens as evidenced by the cycling of new anode half-cells.

The growth of nickel (Ni) particles in the porous anode is one of the most critical issues in solid oxide fuel cells (SOFC). It reduces the density of triple-phase boundaries (TPBs) over time and increases the polarization resistance of SOFC. Most of the three-dimensional models that are used to simulate this phenomenon in detail are numerically exhausting and as such intractable for on-line applications. This work presents a two-dimensional, microstructural model of reduced complexity as a trade-off between the numerical load and the level of detail. The model of Ni agglomeration is based on the power-law coarsening theory. The resulting model was validated by comparing the relative density of TPBs and the cell voltage to the experimentally measured values. It was shown that the calculated values closely fit the measured data. The advantage of the proposed model is that it takes lower computational load during the simulation compared to the complex phase field models and is suitable for estimation of SOFC electric performance over time.

An elementary kinetic model is established to represent the coupled behavior of (electro)-chemistry, transport and degradation processes in the porous Ni/YSZ anode of solid oxide fuel cells (SOFC). The model is applied to support the development and evaluation of solid carbon formation mechanisms at Ni/YSZ anodes. The simulations of cells operated on partially reformed hydrocarbons show that performance and degradation are influenced significantly by the operation temperature and applied potential. Specifically, at OCV and high temperature (1023 K), a surface carbon layer is formed which covers Ni surface and Ni three-phase boundary, blocking heterogeneous and charge-transfer reactions. However, at lower temperature (923 K) carbon growth mainly proceeds inside of anode porous phase leading to significant diffusion polarization. Literature experimental data is used for validation of the model.

The effects of compositional and environmental parameters on the kinetics of microstructural degradation are investigated for porous Ni/CGO anodes in solid oxide fuel cells (SOFC). Improved methodologies of SEM-imaging, segmentation and object recognition are described which enable a precise quantification of nickel grain growth over time. Due to these methodological improvements the grain growth can be described precisely with a standard deviation of only 5–15 nm for each time step. In humid atmosphere (60 vol.% H2O, 40% N2/H2) the growth rates of nickel are very high (up to 140%/100 h) during the initial period (1000 h) the growth rates decrease significantly to nearly 0%/100 h. In contrast, under dry conditions (97 vol.% N2, 3 vol.% H2) the growth rates during the initial period are much lower (ca. 1%/100 h) but they do not decrease over a period of 2000 h. In addition to the humidity factor there are other environmental and compositional parameters which have a strong influence on the kinetics of the microstructural degradation. The nickel coarsening is strongly depending on the gas flow rate. Also the initial microstructures and the anode compositions have a big effect on the degradation kinetics. Thereby small average grain sizes, wide distribution of particle size and high contents of nickel lead to higher coarsening and degradation rates. Whereas the nickel coarsening appears to be the dominant degradation mechanism during the initial period (1000 h) in humidified gas. Thereby the evaporation of volatile nickel species may lead to a local increase of the Ni/CGO ratio. Due to the surface wetting of CGO a continuous layer tends to form on the surface of the nickel grains which prevents further grain growth and evaporation of nickel. These phenomena lead to a microstructural reorganization between 1000 and 2300 h of exposure. This complex pattern of degradation phenomena also leads to a change of the amount of active microstructural sites that are important for catalytic reactions at the pore-nickel interfaces and for electrochemical reactions at the triple phase boundaries (TPB).

Στο πρώτο μέρος της παρούσας διατριβής παρουσιάζεται ο σχεδιασμός, η κατασκευή και η λειτουργία μιας τριοδικής SOFC με state-of-the-art υλικά ηλεκτροδίων. Η τριοδική λειτουργία ενίσχυσε την απόδοση της κυψέλης έως και 7.7% κάτω από μη-ευνοϊκές συνθήκες λειτουργίας. Το δεύτερο μέρος αφορά τη μαθηματική μοντελοποίηση των SOFC. H μοντελοποίηση μόνιμης κατάστασης περιγράφει τη λειτουργία των κεραμομεταλλικών ανόδων μέσω ενός παράγοντα αποτελεσματικότητας, η, ο οποίος σχετίζεται με το αδιάστατο ρεύμα, J, και έναν αδιάστατο ρυθμό αντίδρασης, Φ2. Η μοντελοποίηση λαμβάνει υπ’όψη αποκλειστικά την υπερχείλιση ιόντων οξυγόνου από τον ηλεκτρολύτη και αφορά διάφορες γεωμετρίες των καταλυτικών σωματίων. Το μοντέλο επιβεβαιώνεται ικανοποιητικά από πειραματικά αποτελέσματα σε state-of-the-art κυψέλη SOFC. Για τη μοντελοποίηση της απενεργοποίησης των κεραμομεταλλικών ανόδων προτείνεται ότι η αύξηση του μεγέθους των καταλυτικών σωματίων Ni της ανόδου υπακούει σε μία συνάρτηση εκθετικής αύξησης της ακτίνας. Με τη χρήση θεμελιωδών ηλεκτροχημικών εξισώσεων σχετίζεται ο ρυθμός υποβάθμισης με τη μεταβολή του μήκους ορίου τριών φάσεων, που υπολογίζεται από την συνάρτηση εκθετικής αύξησης της ακτίνας, και με δομικές και λειτουργικές παραμέτρους της ανόδου. Το μοντέλο επιβεβαιώνεται από πειραματικά δεδομένα συστοιχιών SOFC. Τέλος, μελετώνται οι ιδιότητες των κεραμομεταλλικών ανόδων με τη μέθοδο της Θερμοπρογραμματιζόμενης εκρόφησης. Με τη μέθοδο της αέριας ρόφησης αναγνωρίστηκαν τρία κύρια είδη κορυφών τα οποία είναι σε συμφωνία με κορυφές που έχουν παρατηρηθεί σε αντίστοιχα συστήματα. Εκτιμήθηκαν οι ενέργειες ενεργοποίησης της εκρόφησης των κορυφών. Με τη μέθοδο της ηλεκτροχημικής ρόφησης επιβεβαιώθηκε η ύπαρξη των κορυφών και παρατηρήθηκε μετατόπιση της κύριας κορυφής σε υψηλότερη θερμοκρασία εκρόφησης υποδηλώνοντας ισχυρότερη σύνδεση με τον καταλύτη, σε συμφωνία με τη βιβλιογραφία. In the first part of the present dissertation the design, construction and operation of a triode SOFC with state-of-the-art electrode materials is presented. The triode operation led to an increased performance of the fuel cell up to 7.7% under unfavorable operating conditions. The second part of the dissertation regards the mathematical modeling of SOFC cermet anodes. Under steady state conditions the operation of the cermet anode is described via the use of an effectiveness factor, η, that is related to the dimensionless current of the anode, J, and a dimensionless reaction rate, Φ2. The modeling accounts explicitly for the migration of the oxygen spillover species from the electrolyte and examines various geometries of the catalytic particles. The model is in good agreement with experimental data obtained from state-of-the-art SOFC cells. For the anode degradation modeling it is proposed that the radius of the Ni particles of the follows an exponential growth function, to describe the sintering induced particle growth. Using fundamental electrochemical equations the degradation is linked to the three phase boundary length, that can be estimated from the exponentially growing radius, and operational and structural data of the anode. The model is validated using SOFC stack experimental data. The third part involves the study of SOFC cermet anodes with the use of Temperature Programmed Desorption. Using gaseous adsorption we identified three peaks that are in agreement with peaks observed in similar systems. The activation energies of the desorption peaks were calculated. Using electrochemical adsorption we verified the origin of the peaks. The shift of the main peak towards higher desorption temperature implies stronger bonding of the relevant species with the catalyst, in agreement with literature data.

As the performances of Solid Oxide Fuel Cells (SOFC) get attractive, long term degradation becomes the main issue for this technology. Therefore it is essential to localise the origin of degradation and to understand its processes in order to find solutions and improve SOFC durability. The electrode microstructure ageing, in particular nickel grain coarsening at the anode side, is known to be a major process to cause performance loss. The increase in nickel particle size will diminish the Triple Phase Boundary (TPB), where fuel oxidation takes place, and decrease the anode electronic conductivity. These two effects degrade the electrochemical performance of the fuel electrode. Degradation is defined as the decrease of potential at constant current density with time in %/1000h or mV/1000h. This study is based on HTceramix® anode supported cells tested in stack conditions from 100 to more than 1000 hours. The anode microstructure has been characterized by Scanning Electron Microscopy (SEM). As the back scattered electron yield coefficients of nickel and yttria stabilized zirconia (YSZ) are very close, the contrast of the different phases (Ni, YSZ and pores) is low. Various techniques are used to enhance the contrast. A new technique is presented here using impregnation and SEM observation based on secondary electron yield coefficients to separate the phases. Image treatment and analysis is done with an in-house Mathematica® code. Image treatment follows four steps: 1. inhomogeneous background correction, 2. double thresholding, 3. cleaning of the binary images and 4. reconstruction of a three-phase image. Image analysis gives information about phase proportion, particle size, particle size distribution, contiguity and finally a new procedure is developed to compute TPB density. A model to describe the coarsening of the nickel particles is also developed. The model assumes an exponential growth of the nickel particles. Using a particle population balance, it estimates the growth of the nickel particles and the concomitant drop in the TPB length. This model is in very good agreement with experimental data, especially for relatively low fuel cell operation times (up to 100-200 hours). This model can be used in the estimation of operational parameters of the anode electrode such as the degradation rate using fundamental parameters of the cermet anode like the anode overpotential and the work of adhesion of the nickel particles on the YSZ substrate. This model gives the portion of stack degradation that corresponds to anode performance decrease due to particle sintering. Finally this study gives the possibility to isolate the degradation coming from the anode sintering and compare to the full SOFC stack degradation.

Nickel/zirconia-based nanostructured electrodes for solid oxide fuel cells suffer from poor stability even at intermediate temperature. This study quantifies the electrochemical and microstructural degradation of nanostructured electrodes by combining 3D tomography, electrochemical impedance spectroscopy (EIS) and mechanistic modeling. For the first time, the electrochemical degradation of nanostructured electrodes is quantified according to the fractal nature of the three-phase boundary (TPB). Using this hypothesis an excellent match between modeling and the electrochemical response is found. The origin of the degradation in microstructure and electrochemical performance can be found in the initial fractal roughness of the TPB at a length scale not detectable with state-of-the-art tomography at 30 nm resolution. This additionally implies that the hydrogen electro-oxidation takes place within 4 nm from the geometric TPB line, revealing that the electrochemical reaction zone cannot be regarded anymore as a one-dimensional line when dealing with nanoparticles.