Recent studies have shown that using the same material as both cathode and anode leads to a symmetric cell, named Symmetric Solid Oxide Fuel Cell (SSOFC) that presents several advantages compared with classic fuel cells. The main advantage of the symmetric configuration is the simplicity and cheapness of the production process of the cell. In addition, this kind of cell can effectively solve the problem of sulphur poisoning and carbon deposition on anode surface by changing airflow direction. Thus, recently SSOFC has received increasing attention in order to improve the performance with new materials for electrodes. One of the most promising classes of materials is the K2NiF4-type solid oxides that present good structural stability and high electrical conductivity. This paper has been studied a series of K2NiF4-type materials as SSOFC symmetric electrodes, evaluating physical, chemical and electric properties. The K2NiF4-type series of materials, with formula LaxSr2-xFe0.9Cu0.1O4±δ (0.6≤x≤1.4), have been synthesized as electrodes through solid state reaction route, and analysed through X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) to investigate the materials structure. By comparison of the results of electrical conductivity test and cathode impedance spectrum test, we choose the best La doping amount. The measured conductivity of electrodes decreases with La doping amount and increases with temperature. The highest value of conductivity has been achieved at T=850℃ and La content equals to 0.6, but at the same time, the activation energy is very large. The conductivity of the compound with x=0.8 is slightly reduced than x=0.6, but the activation energy is much smaller. Cathode impedance decreases with the increase of the temperature and the decrease of La content. The material with x=0.6 has the smallest activation energy. As a result, we choose La0.8Sr1.2Fe0.9Cu0.1O4±δ as electrode in later research. Then we produced proton conduction type electrolyte Ba(Zr0.1Ce0.7Y0.2)O3 (BZCY), and investigated the solid state reaction by XRD. The fuel used for the test of the single cell performance was hydrogen and air as oxidant. The measured OCV is in the range of 1.04~1.091V, very close to the theoretical value, meaning that pressure tightness is good. In the temperature region of 600~700℃, the power density increases with temperature, due to the increase of the catalytic activity and the decrease of the impedance. The highest value of power density is 25.85 mW·cm-2. In the temperature range of 600~700℃, the impedance decreases with the increase of temperature, at 700℃, the value is 14.97Ω·cm2. To investigate the influence of different conduction type of electrolyte on the cell performance, oxygen ionic conduction type electrolyte, La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM), is chosen as support for the cells. The structure of the synthesized LSGM is monophasic and present good chemical compatibility with the electrode La0.8Sr1.2Fe0.9Cu0.1O4±δ. The single cell measurement shows that the OCV is 0.91~1.02V, close to the theoretical value. The power density increases with the temperature and achieves the largest value of 25.48 mW·cm-2 at 850℃. The impedance decreases with temperature increasing, and its smallest value is 12.4Ω·cm2. Comparing the cell performances with different electrolytes, we can conclude that in the temperature region of 600~700℃, the cell with BZCY presents better performance with larger power density and smaller impedance; the performance of cell with LSGM gets much better at high temperature. To investigate the applicability of alcohol fuels, ethanol has been used as fuel. The results indicate that except T=600℃, the OCV is about 0.9V, thus the pressure tightness is acceptable. The power density and impedance of the cell share the same dependency as before. The largest power density is 23.79 mW·cm-2 and the smallest impedance is 13.02Ω·cm2, which are all very close to the cell performance using hydrogen fuel. This work demonstrated that the electrode has excellent catalysis performance with both hydrogen and ethanol.
Recentemente sono apparsi studi in letteratura che mostrano che è possibile operare celle a combustibile ad ossidi solidi (SOFC) usando lo stesso materiale al catodo e all’anodo. Queste celle sono denominate SOFC Simmetriche (Symmetric SOFC, SSOFC) e presentano diversi vantaggi in confronto alle celle tradizionali. I principali sono la semplicità della configurazione ed il minor costo di materiali e di produzione. Inoltre, grazie a questa configurazione è facile superare il problema dell’avvelenamento da Zolfo all’anodo semplicemente invertendo i flussi di aria e combustibile. Tra i materiali più promettenti per questa applicazione vi sono gli ossidi a struttura tipo K2NiF4 che presentano generalmente elevata conducibilità e buona stabilità strutturale. In questa tesi sono stati studiati una serie di ossidi a struttura tipo K2NiF4 per applicazione in celle SSOFC. I materiali sono stati preparati, caratterizzati dal punto di vista chimico-fisico ed elettrico. Sono stati sintetizzati ossidi di formula LaxSr2-xFe0.9Cu0.1O4±δ (0.6≤x≤1.4) con il metodo di reazione a stato solido; I materiali ottenuti sono stati caratterizzati tramite diffrazione di raggi X (XRD) e analisi termo-gravimetrica (TGA). Le misure di caratterizzazione elettrica hanno permessi di selezionare la composizione più efficace: la conducibilità decresce all’aumentare della quantità di lantanio, ed aumenta con la temperatura. La migliore conducibilità è stata ottenuta a 850 °C per il composto con La = 0.6 anche se l’energia di attivazione misurata risulta molto grande. Nel composto con La = 0.8, la conducibilità peggiora leggermente ma risulta ridotta in maniera significativa anche l’energia di attivazione. Nelle prove in cella, l’impedenza dei catodi diminuisce aumentando la temperatura e diminuendo la quantità di lantanio. Infine è stato quindi selezionato il materiale a formula La0.8Sr1.2Fe0.9Cu0.1O4±δ per le caratterizzazioni successive. Il materiale scelto è stato testato accoppiato a due elettroliti, uno cationico ed uno anionico. Il primo è un elettrolita conduttore protonico Ba(Zr0.1Ce0.7Y0.2)O3 (BZCY). La compatibilità chimica tra elettrodo ed elettrolita è stata testata tramite diffrazione di raggi X, e nei test di funzionamento in cella si è utilizzato idrogeno come combustibile ed aria come ossidante. Il potenziale a circuito aperto (OCV) misurato è nel range 1.04~1.091V, in accordo con il valore teorico. Nell’intervallo di temperatura 600~700 °C, la densità di potenza cresce con la temperatura al crescere dell’attività catalitica ed al decrescere dell’impedenza. Il valore migliore ottenuto è stato pari a 25.85 mW·cm-2. Nello stesso intervallo di temperature, l’impedenza decresce e risulta, a 700 °C, pari a 14.97Ω·cm2. Il secondo è un conduttore di ioni ossigeno, La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM); Il composto è stato sintetizzato ed ottenuto monofasico; la compatibilità chimica con l’elettrodo La0.8Sr1.2Fe0.9Cu0.1O4±δ è buona. Le misure in cella hanno rivelato un OCV in accordo con quello teorico (0.91~1.02V). La densità di potenza misurata aumenta con la temperatura, ed il massimo valore ottenuto ad 850 °C è pari a 25.48 mW·cm-2. L’impedenza misurata diminuisce con il crescere della temperature ed il valore inferiore è pari a 12.4Ω·cm2. Analizzando i risultati delle due caratterizzazioni, si può concludere che nell’intervallo di temperatura 600-700 °C la cella con BZCY presenta prestazioni migliori, con elevate densità di potenza e basse impedenze; nel caso di LSGM invece si ottengono prestazioni migliori a temperature più elevate. Inoltre, è stata studiata anche la fattibilità di utilizzare altri combustibili, etanolo in particolare. Nella cella alimentata ad etanolo, l’OCV misurato è stato di 0.9V. Le densità di Potenza e impedenze misurate hanno gli stessi andamenti già misurati con Idrogeno. La migliore densità di potenza ottenuta è stata di 23.79 mW·cm-2 e la migliore impedenza di 13.02 Ω·cm2, molto vicine a quelle ottenute in idrogeno. Si può concludere che l’elettrodo selezionato ha ottime capacità catalitiche sia per l’idrogeno che per l’etanolo.
Study of K2NiF4-type electrode for symmetrical solid oxide fuel cells and cell performance based on it
CHEN, YU
2014/2015
Abstract
Recent studies have shown that using the same material as both cathode and anode leads to a symmetric cell, named Symmetric Solid Oxide Fuel Cell (SSOFC) that presents several advantages compared with classic fuel cells. The main advantage of the symmetric configuration is the simplicity and cheapness of the production process of the cell. In addition, this kind of cell can effectively solve the problem of sulphur poisoning and carbon deposition on anode surface by changing airflow direction. Thus, recently SSOFC has received increasing attention in order to improve the performance with new materials for electrodes. One of the most promising classes of materials is the K2NiF4-type solid oxides that present good structural stability and high electrical conductivity. This paper has been studied a series of K2NiF4-type materials as SSOFC symmetric electrodes, evaluating physical, chemical and electric properties. The K2NiF4-type series of materials, with formula LaxSr2-xFe0.9Cu0.1O4±δ (0.6≤x≤1.4), have been synthesized as electrodes through solid state reaction route, and analysed through X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) to investigate the materials structure. By comparison of the results of electrical conductivity test and cathode impedance spectrum test, we choose the best La doping amount. The measured conductivity of electrodes decreases with La doping amount and increases with temperature. The highest value of conductivity has been achieved at T=850℃ and La content equals to 0.6, but at the same time, the activation energy is very large. The conductivity of the compound with x=0.8 is slightly reduced than x=0.6, but the activation energy is much smaller. Cathode impedance decreases with the increase of the temperature and the decrease of La content. The material with x=0.6 has the smallest activation energy. As a result, we choose La0.8Sr1.2Fe0.9Cu0.1O4±δ as electrode in later research. Then we produced proton conduction type electrolyte Ba(Zr0.1Ce0.7Y0.2)O3 (BZCY), and investigated the solid state reaction by XRD. The fuel used for the test of the single cell performance was hydrogen and air as oxidant. The measured OCV is in the range of 1.04~1.091V, very close to the theoretical value, meaning that pressure tightness is good. In the temperature region of 600~700℃, the power density increases with temperature, due to the increase of the catalytic activity and the decrease of the impedance. The highest value of power density is 25.85 mW·cm-2. In the temperature range of 600~700℃, the impedance decreases with the increase of temperature, at 700℃, the value is 14.97Ω·cm2. To investigate the influence of different conduction type of electrolyte on the cell performance, oxygen ionic conduction type electrolyte, La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM), is chosen as support for the cells. The structure of the synthesized LSGM is monophasic and present good chemical compatibility with the electrode La0.8Sr1.2Fe0.9Cu0.1O4±δ. The single cell measurement shows that the OCV is 0.91~1.02V, close to the theoretical value. The power density increases with the temperature and achieves the largest value of 25.48 mW·cm-2 at 850℃. The impedance decreases with temperature increasing, and its smallest value is 12.4Ω·cm2. Comparing the cell performances with different electrolytes, we can conclude that in the temperature region of 600~700℃, the cell with BZCY presents better performance with larger power density and smaller impedance; the performance of cell with LSGM gets much better at high temperature. To investigate the applicability of alcohol fuels, ethanol has been used as fuel. The results indicate that except T=600℃, the OCV is about 0.9V, thus the pressure tightness is acceptable. The power density and impedance of the cell share the same dependency as before. The largest power density is 23.79 mW·cm-2 and the smallest impedance is 13.02Ω·cm2, which are all very close to the cell performance using hydrogen fuel. This work demonstrated that the electrode has excellent catalysis performance with both hydrogen and ethanol.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/108702