Vanadium redox flow batteries are one of the most promising electrochemical energy storage systems on the market today, thanks to the peculiarity of decouple energy capacity and deliver power. The low power density is the main weak point, it is due to the use of electrodes not optimized for this purpose, with high electrical conductivity but with a low surface area and an electrochemical activity not specific for vanadium redox reaction. For this purpose, a nanostructured carbonaceous material was deposited on gas diffusion layer (GDL) of carbon paper 39AA Sigracet, through a novel deposition technique, with the aim of increasing the surface area available to the reaction, without affecting the permeability and diffusion of the electrolyte and without lowering the electrical conductivity. The electrode thus formed has been heated up to 1100 ° C in high vacuum, during the heat treatment it was noted an increase in the degree of graphitization and electrical conductivity, as well as the formation of carbon allotropes such as fullerenes and carbon nano-onion. The development of a characterization procedure, which includes microscopy techniques (SEM) and spectroscopy (XPS, Raman), allowed to verify the physico-chemical properties of the material and to correlate the deposition parameters on the observed properties, also predicting how these affect the electrochemical activity, moreover through the use of XPS spectroscopy it was possible to evaluate the presence of contaminants such as nitrogen and oxygen, present with a relative concentration around 2% for both atoms, their concentration decreases as the annealing temperature increases, meanwhile an increasing on carbon allotropes relative concentration is faced. To test the performance of the electrode, electrochemical tests were carried out in order to study kinetics and mass transport phenomena. Specifically, cycle voltammetry tests, polarization in a symmetrical cell and tests on a segmented full cell using a Nafion 115 membrane and a serpentine flow field which proved to be the most suitable with this type of GDL. The analysis was focused on the anode since it is believed that it is the limiting electrode of the battery. Through the CV it was possible to analyze which heat treatment leads to the best kinetics towards the oxidation reaction of Vanadium, which was found to be at 1000 ° C, with a spread of 100mV and a ratio between oxidation peak and reduction of 0.9 . From tests in symmetrical cell which was carried out varying the electrolyte distributor (flow field) and flux at SOC 50%, it was possible to calculate an electrode over potential of 60mV for a current density of 400mA/cm2. The tests in the complete cell were conducted at different current densities, so to analyze the variation of the energy, voltaic and coulombic efficiency values, the analyzed current densities are: 100, 200, 300, 350, 400 mA / cm2 and the following voltages efficiency results were obtained respectively 96.2%, 93.9%, 91.2% 89%, 88.1%, the following values were calculated by eliminating the contribution of the membrane over potential (iR correction). The use of the nanostructured graphite electrode with the presence of carbon allotropes such as fullerenes and nano-onion has led to an improvement in the generic cell efficiency of about 20% compared to the commercial version, with an energy efficiency of 91.3% , 88.3%, 90.2%, 87.8% for current density of 100, 200, 300, 350 mA / cm2 respectively. It was therefore argued that, although the presence of carbon bonded atoms such as nitrogen and oxygen is effective in improving the performance of the electrode, the greatest contribution is given by the presence of carbon allotropes increasing drastically the active sites for the redox reaction of Vanadium.
Le batterie a flusso al vanadio costituiscono uno dei sistemi elettrochimici di stoccaggio energetico tra i più promettenti oggi sul mercato, soprattutto grazie alla peculiarità di poter disaccoppiare capacità energetica e potenza. La bassa densità di potenza è il principale punto debole, essa è dovuta all’utilizzo di elettrodi non ottimizzati per tale scopo, dotati di alta conducibilità elettrica ma con attività elettrochimica non specifica verso gli ioni vanadio e con bassa area superficiale. A questo scopo è stato depositato su gas diffusion layer (GDL) di carbon paper 39AA Sigracet, attraverso una tecnica di deposizione sperimentale, un materiale carbonioso nanostrutturato con il fine di aumentare l’area superficiale disponibile alla reazione, senza influenzare la permeabilità e la diffusione dell’elettrolita e senza abbassare la conducibilità elettrica. L’elettrodo così formato ha subito un riscaldamento fino a 1100°C in alto vuoto, durante il trattamento termico si è notata un aumento del grado di grafitizzazione e della conduttività elettrica, oltre che alla formazione di allotropi del carbonio come fullereni e carbo nano-onion. L’elaborazione di una procedura di caratterizzazione, che comprende tecniche di microscopia (SEM, TEM) e spettroscopia (XPS, Raman), ha consentito di verificare le proprietà fisico-chimiche del materiale e correlare i parametri di deposizione sulle proprietà osservate, pronosticando anche come queste influiscano sull’attività elettrochimica, inoltre attraverso l’uso della spettroscopia XPS è stato possibile valutare la presenza di contaminanti come azoto e ossigeno, presenti con una concentrazione relativa che si attesta attorno al 2% per entrambi gli atomi, la loro concentrazione diminuisce all’aumentare della temperatura di annealing, mentre si ha un aumento della concentrazione relativa di allotropi del carboni. Per testare la performance dell’elettrodo sono state effettuate delle prove elettrochimiche così da studiare fenomeni di cinetica e di trasporto di massa. Nello specifico sono state effettuate delle prove ciclo voltmetriche, delle prove in cella simmetrica variando il distributore di elettrolita (flow field) e la velocità di flusso, ed infine dei test su cella totale segmentata, utilizzando una membrana Nafion 115 e un flow field a serpentina che si è dimostrata essere il più adatto con questa tipologia di GDL. Si è focalizzata l’analisi sull’anodo poiché si ritiene che esso sia l’elettrodo limitante della batteria. Attraverso la CV si è potuto analizzare quale trattamento termico porti alla migliore cinetica verso la reazione di ossiriduzione del Vanadio, che si è trovato essere a 1000°C, con uno spread di 100mV e un rapporto tra picco di ossidazione e riduzione di 0,9. Dai test in cella simmetrica, con SOC al 50%, si è riusciti a calcolare un sovra potenziale dell’elettrodo di 60mV per una densità di corrente di 400mA/cm2. I test in cella completa sono stati condotti a diverse densità di corrente, così da analizzarne la variazione dei valori di efficienza energetica, voltaica e coulombica, le densità di corrente analizzate sono: 100, 200, 300, 350, 400 mA/cm2 e si sono ottenuti rispettivamente i seguenti risultati di efficienza voltaica 96,2%, 93,9%, 91,2% 89%, 88,1%, i seguenti valori sono stati calcolati eliminando il contributo di sovra potenziale dovuto alla membrana. L’utilizzo dell’elettrodo grafitico nanostrutturato con presenza di allotropi del carbonio come fullereni e carbo nano-onion ha portato ad un miglioramento dell’efficienza generica di cella del 20% circa rispetto alla versione commerciale, con una efficienza energetica di 91,3%, 88,3%, 90,2%, 87,8% per densità di corrente rispettivamente di 100, 200, 300, 350 mA/cm2. Si è quindi affermato che, sebbene la presenza di atomi legati al carbonio come azoto e ossigeno sia efficace nel migliorare le prestazioni dell’elettrodo, il maggior contributo viene dato dalla presenza di allotropi del carbonio e di siti attivi per la reazione di ossidoriduzione del Vanadio.
A high-performance nanostructured carbon electrode for vanadium redox flow battery
FIORINI GRANIERI, SIMONE
2019/2020
Abstract
Vanadium redox flow batteries are one of the most promising electrochemical energy storage systems on the market today, thanks to the peculiarity of decouple energy capacity and deliver power. The low power density is the main weak point, it is due to the use of electrodes not optimized for this purpose, with high electrical conductivity but with a low surface area and an electrochemical activity not specific for vanadium redox reaction. For this purpose, a nanostructured carbonaceous material was deposited on gas diffusion layer (GDL) of carbon paper 39AA Sigracet, through a novel deposition technique, with the aim of increasing the surface area available to the reaction, without affecting the permeability and diffusion of the electrolyte and without lowering the electrical conductivity. The electrode thus formed has been heated up to 1100 ° C in high vacuum, during the heat treatment it was noted an increase in the degree of graphitization and electrical conductivity, as well as the formation of carbon allotropes such as fullerenes and carbon nano-onion. The development of a characterization procedure, which includes microscopy techniques (SEM) and spectroscopy (XPS, Raman), allowed to verify the physico-chemical properties of the material and to correlate the deposition parameters on the observed properties, also predicting how these affect the electrochemical activity, moreover through the use of XPS spectroscopy it was possible to evaluate the presence of contaminants such as nitrogen and oxygen, present with a relative concentration around 2% for both atoms, their concentration decreases as the annealing temperature increases, meanwhile an increasing on carbon allotropes relative concentration is faced. To test the performance of the electrode, electrochemical tests were carried out in order to study kinetics and mass transport phenomena. Specifically, cycle voltammetry tests, polarization in a symmetrical cell and tests on a segmented full cell using a Nafion 115 membrane and a serpentine flow field which proved to be the most suitable with this type of GDL. The analysis was focused on the anode since it is believed that it is the limiting electrode of the battery. Through the CV it was possible to analyze which heat treatment leads to the best kinetics towards the oxidation reaction of Vanadium, which was found to be at 1000 ° C, with a spread of 100mV and a ratio between oxidation peak and reduction of 0.9 . From tests in symmetrical cell which was carried out varying the electrolyte distributor (flow field) and flux at SOC 50%, it was possible to calculate an electrode over potential of 60mV for a current density of 400mA/cm2. The tests in the complete cell were conducted at different current densities, so to analyze the variation of the energy, voltaic and coulombic efficiency values, the analyzed current densities are: 100, 200, 300, 350, 400 mA / cm2 and the following voltages efficiency results were obtained respectively 96.2%, 93.9%, 91.2% 89%, 88.1%, the following values were calculated by eliminating the contribution of the membrane over potential (iR correction). The use of the nanostructured graphite electrode with the presence of carbon allotropes such as fullerenes and nano-onion has led to an improvement in the generic cell efficiency of about 20% compared to the commercial version, with an energy efficiency of 91.3% , 88.3%, 90.2%, 87.8% for current density of 100, 200, 300, 350 mA / cm2 respectively. It was therefore argued that, although the presence of carbon bonded atoms such as nitrogen and oxygen is effective in improving the performance of the electrode, the greatest contribution is given by the presence of carbon allotropes increasing drastically the active sites for the redox reaction of Vanadium.| File | Dimensione | Formato | |
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A high-performance Nanostructured Carbon Electrode for VRFB.pdf
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Descrizione: A high-performance Nanostructured Carbon Electrode for VRFB
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https://hdl.handle.net/10589/166805