Nowadays the awareness of the causes and consequences of climate change is fueling new investments in the renewable energy field. The European Union is a world leader in research and development (R&D) for solutions that could allow a more sustainable society, and hydrogen will play a very important role in the achievement of this goal in the next decades. Electrolysis represents the best alternative to produce green hydrogen. Solid Oxide Electrolysis Cells (SOECs) in particular offer different advantages when compared to the others electrolyzers, like the very high electrochemical conversion efficiency, the possibility to be operated in both electrolysis and fuel cell modes, the low materials costs, and the opportunity to directly convert hydrocarbons, CO and other high energy-density gases. The focus of this project is on two different SOEC technologies: a state-of-the-art ceramic-supported cell and a recently developed Metal-Supported Cell (MSC), designed to improve the thermal and mechanical stability of the first one. Particular importance is given to the electrolysis performances and degradation assessment at low temperature. All the tested cells went through a 500-hours durability test in SOEC mode, performed in galvanostatic conditions at 650 °C feeding air as sweep gas to the positive electrode and a mixture 50% steam – 50% hydrogen to the negative one. The ceramic-supported cell showed an average degradation rate in terms of voltage increase equal to 13.28% per 1000 h. The cell voltage increased linearly throughout the whole durability test duration, which in this case was performed at 0.25 A/cm2. The loss of performances is attributed to degradation of the negative electrode. Ceramic-supported cells are designed to work at higher temperatures, anyway the degradation rate observed at 650 °C with the cell tested for this project was consistent with the one obtained at 800 °C in a past test. Two metal-supported cells were tested and both shown unexpected behaviour during durability tests, carried out at 0.25 A/cm2 with the first MSC and at 0.5 A/cm2 with the second one. Cells voltage did not increase, as would be a normal consequence of degradation, instead activation processes were observed as constant reduction of the measured voltage. Oxidation of the metallic supports occurred during the first characterization in both cells; instead, during durability test the reducing atmosphere held constantly at the negative electrode allowed to slowly restore the metallic structure and consequently increase its electric conductivity. The very promising results in terms of high stability in electrolysis operation, together with the optimistic cost projections, suggest that in the near future MSC technology will play a relevant role in energy market. Further tests are essential to achieve a better knowledge of such cell’s behaviour and R&D should be aimed to overcome the problems related to metal support oxidation.
La consapevolezza sulle cause e conseguenze del cambiamento climatico sta alimentando nuovi investimenti nel settore dell’energia rinnovabile. L’Unione Europea è leader mondiale nella ricerca e sviluppo di soluzioni che possano portare ad una società più sostenibile, e la produzione rinnovabile di idrogeno nei prossimi decenni giocherà un ruolo fondamentale per raggiungere gli obiettivi stabiliti. L’elettrolisi rappresenta la miglior alternativa per produrre idrogeno verde. In particolare gli elettrolizzatori basati su celle ad ossidi solidi (SOEC dall’inglese) offrono diversi vantaggi rispetto alle altre tecnologie, come l’alta efficienza di conversione, la possibilità di essere operati sia come celle a combustibile sia come elettrolizzatori, i bassi costi dei materiali, e la capacità di convertire idrocarburi, CO ed altri gas ad alta densità energetica. In questo progetto l’attenzione è rivolta a due diverse tipologie di SOEC: celle con supporto ceramico, rappresentanti lo stato dell’arte di tale tecnologia, e celle con supporto metallico, recentemente sviluppate per migliorare la resilienza agli stress termici e meccanici delle prime. Particolare importanza è data alla valutazione delle prestazioni e del degrado ottenuti operando a bassa temperatura. Con tutte le celle è stato eseguito un test di durabilità galvanostatico di 500 ore a 650 °C, operando in elettrolisi ed alimentando aria all’elettrodo positivo per aiutare la rimozione dei prodotti di reazione e una miscela 50% vapore – 50% idrogeno all’elettrodo negativo. La cella a supporto ceramico, testata applicando una densità di corrente di 0.25 A/cm2, ha mostrato un peggioramento delle prestazioni, in termini di aumento di tensione, pari al 13.28% per 1000 h. Tale degrado è attribuito all’elettrodo negativo. Nonostante le celle a supporto ceramico siano state progettate per operare ad alte temperature, il tasso di degrado ottenuto a 650 °C si è dimostrato coerente con quello riportato in un test passato ad 800 °C. Due celle a supporto metallico sono state testate in questo studio, ed entrambe hanno mostrato un comportamento inaspettato durante i test galvanostatici, svolti a 0.25 A/cm2 con la prima cella e a 0.5 A/cm2 con la seconda. In entrambi i casi, la diminuzione della tensione misurata ha suggerito che fenomeni di attivazione hanno avuto luogo. Durante la caratterizzazione iniziale in entrambe le celle i supporti metallici si sono ossidati; invece, operando costantemente in elettrolisi per 500 ore l’atmosfera riducente mantenuta all’elettrodo negativo ha permesso il lento ripristino della struttura metallica ed il conseguente aumento della conducibilità elettrica. I risultati molto promettenti in termini di stabilità durante l’operazione in elettrolisi, insieme alle proiezioni ottimiste sui costi, suggeriscono che le celle a supporto metallico sono una tecnologia che ricoprirà un ruolo rilevante nel mercato energetico nel prossimo futuro. Ulteriori test sono necessari per comprendere a fondo il comportamento di tali celle, e la ricerca dovrebbe concentrarsi sul trovare soluzioni per i problemi relativi all’ossidazione del supporto metallico.
Ceramic and metal supported solid oxide cells : low-temperature electrolysis performances and degradation assessment
Capotondo, Federico;Caldogno, Riccardo
2020/2021
Abstract
Nowadays the awareness of the causes and consequences of climate change is fueling new investments in the renewable energy field. The European Union is a world leader in research and development (R&D) for solutions that could allow a more sustainable society, and hydrogen will play a very important role in the achievement of this goal in the next decades. Electrolysis represents the best alternative to produce green hydrogen. Solid Oxide Electrolysis Cells (SOECs) in particular offer different advantages when compared to the others electrolyzers, like the very high electrochemical conversion efficiency, the possibility to be operated in both electrolysis and fuel cell modes, the low materials costs, and the opportunity to directly convert hydrocarbons, CO and other high energy-density gases. The focus of this project is on two different SOEC technologies: a state-of-the-art ceramic-supported cell and a recently developed Metal-Supported Cell (MSC), designed to improve the thermal and mechanical stability of the first one. Particular importance is given to the electrolysis performances and degradation assessment at low temperature. All the tested cells went through a 500-hours durability test in SOEC mode, performed in galvanostatic conditions at 650 °C feeding air as sweep gas to the positive electrode and a mixture 50% steam – 50% hydrogen to the negative one. The ceramic-supported cell showed an average degradation rate in terms of voltage increase equal to 13.28% per 1000 h. The cell voltage increased linearly throughout the whole durability test duration, which in this case was performed at 0.25 A/cm2. The loss of performances is attributed to degradation of the negative electrode. Ceramic-supported cells are designed to work at higher temperatures, anyway the degradation rate observed at 650 °C with the cell tested for this project was consistent with the one obtained at 800 °C in a past test. Two metal-supported cells were tested and both shown unexpected behaviour during durability tests, carried out at 0.25 A/cm2 with the first MSC and at 0.5 A/cm2 with the second one. Cells voltage did not increase, as would be a normal consequence of degradation, instead activation processes were observed as constant reduction of the measured voltage. Oxidation of the metallic supports occurred during the first characterization in both cells; instead, during durability test the reducing atmosphere held constantly at the negative electrode allowed to slowly restore the metallic structure and consequently increase its electric conductivity. The very promising results in terms of high stability in electrolysis operation, together with the optimistic cost projections, suggest that in the near future MSC technology will play a relevant role in energy market. Further tests are essential to achieve a better knowledge of such cell’s behaviour and R&D should be aimed to overcome the problems related to metal support oxidation.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/178131