This study has been mainly focused on high temperature solid oxide electrolysis cells (HT-SOECs) for steam electrolysis. The compositions, microstructures and metal catalysts for SOEC cathodes based on (La₀.₇₅Sr₀.₂₅)₀.₉₅Mn₀.₅Cr₀.₅O₃ (LSCM) have been investigated. Hydrogen production amounts from SOECs with LSCM cathodes have been detected and current-to-hydrogen efficiencies have been calculated. The effect of humidity on electrochemical performances from SOECs with cathodes based on LSCM has also been studied. LSCM has been applied as the main composite in HT-SOEC cathodes in this study. Cells were measured at temperatures up to 920°C with 3%steam/Ar/4%H₂ or 3%steam/Ar supplied to the steam/hydrogen electrode. SOECs with LSCM cathodes presented better stability and electrochemical performances in both atmospheres compared to cells with traditional Ni cermet cathodes. By mixing materials with higher ionic conductivity such as YSZ(Y₂O₃-stabilized ZrO₂ ) and CGO(Ce₀.₉Gd₀.₁O₁.₉₅ ) into LSCM cathodes, the cell performances have been improved due to the enlarged triple phase boundary (TPB). Metal catalysts such as Pd, Fe, Rh, Ni have been impregnated to LSCM/CGO cathodes in order to improve cell performances. Cells were measured at 900°C using 3%steam/Ar/4%H₂ or 3%steam/Ar and AC impedance data and I-V curves were collected. The addition of metal catalysts has successfully improved electrochemical performances from cells with LSCM/CGO cathodes. Improving SOEC microstructures is an alternative to improve cell performances. Cells with thinner electrolytes and/or better electrode microstructures were fabricated using techniques such as cutting, polishing, tape casting, impregnation, co-pressing and screen printing. Thinner electrolytes gave reduced ohmic resistances, while better electrode microstructures were observed to facilitate electrode processes. Hydrogen production amounts under external potentials from SOECs with LSCM/CGO cathodes were detected by gas chromatograph and current-to-hydrogen efficiencies were calculated according to the law of conservation of charge. Current-to-hydrogen efficiencies from these cells at 900°C were up to 80% in 3%steam/Ar and were close to 100% in 3%steam/Ar/4%H₂. The effect of humidity on SOEC performances with LSCM/CGO cathodes has been studied by testing the cell in cathode atmospheres with different steam contents (3%, 10%, 20% and 50% steam). There was no large influence on cell performances when steam content was increased, indicating that steam diffusion to cathode was not the main limiting process.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19730号 / 工博第4185号 / 新制||工||1645(附属図書館) / 32766 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 安部 武志, 教授 陰山 洋 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

The research presented is based on alternative anode materials for high temperature steam electrolysis. The key to commercially viable renewable energy economy is based on energy storage of intermittent sources. Hydrogen is the preferred form of energy storage for solid oxide electrolysis cells. However, conventional anode material lanthanum strontium manganite (LSM), suffers from poor ionic conductivity, thus prohibiting much of the bulk electrode from providing an enhanced electrochemical performance. This study explores the use of a double-layered perovskite system with mixed electronic and ionic conductivity for use as anode material. Specifically, the SmBa₁₋ₓSrₓCo₂O[sub](5+δ) system (SBSCO) is analyzed for characteristics that may enhance the performance and feasibility of SBSCO as an alternative anode material to LSM. Previous in-house work showed SmBa₀.₅Sr₀.₅Co₂O[sub](5+δ) had the lowest area specific resistance of any double- layered material reported. Here the system is further explored by studying the full range of compositions. From X-ray diffraction analysis, increased Sr substitution leads to a tetragonal phase change in SBSCO. High temperature x-ray diffraction of compositions showed thermal stability of structure. Magnetization measurements are reported for selected compositions. The stability of SBSCO was examined in CO₂ containing atmospheres. Despite containing alkaline earth metals, the system offers limited CO₂ tolerance. A set of thermodynamic parameters is presented based on CO₂ partial pressure and temperature. Model indicates SBSCO is a stable electrode material for both electrolysis and fuel cell modes. Compositions were tested for steam electrolysis performance with the use of YSZ electrolyte, and Ni-YSZ and La₀.₄Sr₀.₄Ni₀.₀₆Ti₀.₉₄O₂.₉₄ cathodes. SmBa₀.₃Sr₀.₇Co₂O[sub](5+δ) had the highest performance for compositions (0≤x≤1) based on I-V curves and impedance measurements. Stability tests were conducted in potentiostatic mode and no delamination was observed for SBSCO in microstructural analysis after testing. From these studies, SBSCO is demonstrated to be a suitable for application in electrolysis and an alternative for LSM as anode material.

Molten Carbonate Fuel cells are high temperature fuel cells suitable for distributed generation and combined heat and power, and are today being installed on commercial basis in sizes from 100kW to several MW. Novel applications for MCFC which have attracted interest lately are MCFC used for CO2 separation from combustion flue gas, and high temperature electrolysis with reversible fuel cells. In the first application, the intrinsic capability of the MCFC to concentrate CO2 from the cathode to the anode side through the cell reaction is utilized. In the second application, the high operating temperature and relatively simple design of the MCFC is utilized in electrolysis, with the aim to produce a syngas mix which can be further processed into hydrogen of synthetic fuels. In this thesis, the effect on fuel cell performance of operating a small lab-scale molten carbonate fuel cell in conditions which simulate those that would apply if the fuel cell was used for CO2 separation in combustion flue gas was studied. Such operating conditions are characterized especially by a low CO2 concentration at the cathode compared to normal operating conditions. Sulfur contaminants in fuel gas, especially H2S, are known poisoning agents which cause premature degradation of the MCFC. Furthermore, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. This makes poisoning by sulfur contaminants of great concern for MCFC development. In this thesis, the effect of sulfur contaminants at both anode and cathode on fuel cell degradation was evaluated in both normal and in low CO2 simulated flue gas conditions.      The results suggested that the poisoning effect of SO2 at the cathode is similar to that of H2S at the anode, and that it is possibly due to a transfer of sulfur from cathode to anode. Furthermore, in combination with low CO2 conditions at the cathode, SO2 contaminants cause fuel cell poisoning and electrolyte degradation, causing high internal resistance. By using a small lab-scale MCFC with commercial materials and standard fuel cell operating conditions, the reversible MCFC was demonstrated to be feasible. The electrochemical performance was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes. The separate electrodes were studied in fuel cell and electrolysis modes under different operating conditions. It was shown that the fuel cell exhibited lower polarization in MCEC mode than in MCFC mode, and a high CO2 concentration at the fuel cell anode reduced the polarization in electrolysis mode, which suggested that CO2 is reduced to produce CO or carbonate. / Smältkarbonatbränsleceller (MCFC) är en typ av högtemperaturbränsleceller som är anpassade för kombinerad el- och värmeproduktion i mellan-till stor skala. Idag installeras MCFC på kommersiell basis i storlekar mellan 100kW och flera MW. En ny typ av tillämpning för MCFC som har väckt intresse på senare tid är användandet av MCFC för CO2-avskiljning i kombination med konventionell elproduktion genom förbränning. En annan ny tillämpning är högtemperaturelektrolys genom användandet av reversibla bränsleceller. I det första fallet utnyttjas att CO2 kan koncentreras från katod- till anodsidan, vilket sker genom cellreaktionen för MCFC. I det andra fallet utnyttjas den höga arbetstemperaturen och den relativt enkla cell-designen för att använda reversibla MCFC till elektrolys, med syfte att producera en syngas-blandning som kan förädlas till vätgas eller till syntetiskt bränsle. I denna avhandling studeras effekten på bränslecellens prestanda genom att operera en MCFC i lab-skala med driftförhållanden som simulerar de som förväntas uppkomma om bränslecellen användes för CO2-avskiljning ur rökgaser från förbränning. Dessa driftförhållanden karaktäriseras av låg CO2-koncentration på katodsidan jämfört med normal drift. Svavelföroreningar i bränsle, speciellt H2S, är kända för att orsaka förgiftning av anoden, vilket i sin tur försämrar bränslecellens prestanda. Dessutom innehåller rökgaser ofta SO2, vilket antas orsaka korrosion och förgiftning av katoden. Detta gör effekten av svavelföroreningar till ett prioriterat ämne för utvecklingen av MCFC. I denna avhandling undersöks effekten av svavelföroreningar på både anod- och katodsidan, i normala driftförhållanden och i förhållanden med låg CO2 som simulerar användandet av rökgaser för CO2-avskiljning. Resultaten tyder på att effekten av förgiftning med SO2 på katoden liknar den med H2S på anoden, och att detta kan vara orsakat av en transport av svavel från katod till anod. Vidare, i kombination med låg CO2 koncentration på katoden så orsakar SO2-föroreningar elektrolytdegradering, vilket orsakar hög inre resistans. Genom att använda en liten MCFC i lab-skala med kommersiella material och standardförhållanden för MCFC påvisades att reversibla smältkarbonatbränsleceller kan vara ett lovande koncept. Den elektrokemiska prestandan av både cell och separata elektroder undersöktes både som bränslecell (MCFC)och vid elektrolys (MCEC). Resultaten visade att cellen uppvisade lägre polarisation vid elektrolys än som bränslecell, och att ten hög CO2-koncentration på det som är bränslecellens anodsida gav upphov till en minskad elektrodpolarisation, vilket indikerar att CO2 reduceras för att producera CO eller karbonat. / <p>QC 20141028</p>

The global need to eliminate CO2 emissions and its consequent reduction in the use of fossil fuels drives the ongoing energy transition that highly involves the research achievements of the scientific community to reach the goals of this purpose. Renewable sources like photovoltaic and wind energy, are central to this endeavor, however, the intermittency of natural resources makes it non-dispatchable and energy storage is fundamental. According to the European Roadmap [1] just a 60% of the CO2 emissions reduction goal can be achieved with available technologies and existing energy. However, the production, use and specially storage opportunities that hydrogen offers can drive non-dispatchable renewable sources to achieve its full potential by clearing up the intermittency problem as well as covering the remained 40% gap. This master's thesis aims to investigate the techno-economic feasibility of integrating a Solid Oxide Electrolyzer Cell (SOEC) into a hybrid PV-CSP(sCO2) plant. The study focuses on assessing various indicators related to electricity, energy, and hydrogen production prices. To achieve this, three different integration strategies within the hybrid PV-CSP(sCO2) plant were selected for analysis: Soec using heat from the particles coming from the receiver, soec using heat coming from the particles available in the thermal energy storage (TES) and soec recovering heat from the sCO2 power block. A sensitivity analysis was conducted on different PV sizes (MWp), battery capacities (MWh), and SOEC installed capacities (MWh) to investigate the technology's potential in the plant and determine optimal sizing of subsystems. However, the individual optimization of economic indicators presented technical and economic challenges. Scenarios allowing individual optimization of hydrogen production prices (€/kg H2) resulted in 10.9, 11.7, and 14.6 €/kg h2 for receiver, TES, and sCO2 integration strategy, respectively. These scenarios, however, require high SOEC installed capacities, leading to elevated electricity and energy production prices. On the other hand, the individual optimization of electricity and energy production prices led to better and lower results when no hydrogen production presence within the plant. However, this analysis also showed that soec capacities below 5MWh together with no installation of batteries and a new definition for calculating hydrogen production prices (LCOH) allows feasible integration of hydrogen production within the plant. LCOH(€/kg h2) results were 10.2€/kg h2, 7.6€/kg h2, and 9.4€/kg h2 for receiver, TES, and sCO2, respectively, for a soec installed capacity of 0.5MWh (119m2 size) along with energy production values not exceeding 101€/MWh. While the results present a favorable outlook for SOEC installations based on literature review data [2] [3] [4] they still face challenges when competing with the cost-efficient PEM technology, which offers 4.5-5.5€/kg H2 [5] without storage. Nonetheless, this research contributes valuable insights into the integration of SOEC technology within hybrid renewable energy systems and provides a comprehensive analysis of the techno-economic aspects related to hydrogen production following different integration strategies. The findings may inform decision-making processes and promote further advancements in sustainable energy solutions. / Det globala behovet av att eliminera CO2utsläpp och därmed minska användningen av fossila bränslen driver pågående energiomställning, som starkt involverar forskningsresultaten från vetenskapssamhället för att nå syftet med detta mål. Förnybara källor som solceller och vindkraft är centrala i detta arbete, men intermittensen hos naturliga resurser gör dem icke disponibla och energilagring är grundläggande. Enligt den europeiska vägkartan [1] kan endast 60% av målet att minska CO2-utsläppen uppnås med tillgängliga teknologier och befintlig energi. Produktionen, användningen och särskilt lagringsmöjligheterna som väte erbjuder kan emellertid driva icke-disponibla förnybara källor att nå sin fulla potential genom att lösa intermitt ensproblemet och täcka den återstående 40% klyftan. Detta examensarbete syftar till att undersöka den tekniskekonomiska genomförbarheten av att integrera en fastoxid elektrolysör (SOEC) i en hybrid PV CSP(sCO2)-anläggning. Studien fokuserar på att utvärde ra olika indikatorer relaterade till el-, energi- och vätgasproduktionspriser. För att uppnå detta har tre olika integrationsstrategier inom hybrid PV CSP(sCO2) anläggningen valts för analys: SOEC med hjälp av värme från partiklar som kommer från mottagaren, SOEC med hjälp av värme från partiklar som finns i termisk energilagring (TES) och SOEC som återvinner värme från sCO2-kraftblocket. En känslighetsanalys har genomförts för olika PVstorlekar (MWp), batterikapaciteter (MWh) och SOEC installerade kapacit eter (MWh) för att undersöka teknologins potential i anläggningen och bestämma optimal dimensionering av delsystem. Resultaten från individuell optimering av ekonomiska indikatorer ledde dock till flera tekniska och ekonomiska utmaningar. Scenarier som tillåter individuell optimering av vätgasproduktionspriser (€/kg H2) resulterade i 10, 9, 11, 7 respektive 14,6 €/kg H2 för mottagare, TES och sCO2 integrationsstrategi. Dessa scenarier kräver dock höga SOEC installerade kapaciteter, vilket leder till höga el och energipriser. Å andra sidan ledde individuell optimering av el och energiproduktionspriser till bättre och lägre resultat när ingen vätgasproduktion fanns i anläggningen. Denna analys visade också att SOEC kapaciteter under 5MWh tillsammans med ingen installation av batterier och en ny definition för beräkning av vätgasproduktionspriser (LCOH) möjliggör genomförbar integration av vätgasproduktion i anläggningen. LCOH (€/kg H2) resultaten var 10,2 €/kg h2 , 7 ,6 €/kg h2 respektive 9,4 €/kg h2 för mottagare, TES och sCO2, för en SOEC installerad kapacitet på 0,5 MWh (storlek 119m2) tillsammans med energiproduktionsvärden som inte överstiger 101 €/MWh. Medan resultaten visar en gynnsam utsikt för SOECinstallationer baserat på data från litteraturöversikter [2] [3] [4], står de ändå inför utmaningar när de konkurrerar med den kostnadseffektiva PEM teknologin, som erbjuder 4,5-5,5 €/kg H2 [5] utan lagring. Trots detta bidrar forskningen med värdefulla insikter i integrationen av SOEC teknologi i hybrid förnybara energisystem och ger en omfattande an alys av de teknisk-ekonomiska aspekterna relaterade till vätgasproduktion enligt olika integrationsstrategier. Resultaten kan informera beslutsprocesser och främja ytterligare framsteg inom hållbara energilösningar.

La réponse thermique, électrochimique et mécanique d'un électrolyseur de la vapeur d'eau à haute température (EVHT) a été analysée dans ce travail. Pour ce faire, une approche de modélisation multi-physique et multi-échelle a été employée : • Un modèle local, à l'échelle de la microstructure des électrodes, a été utilisé pour analyser le comportement électrochimique apparent des électrodes de la cellule d'électrolyse étudiée. Le fonctionnement du système au sein d'un empilement de plusieurs cellules a ensuite été analysé grâce à un modèle thermoélectrochimique à l'échelle macroscopique de l'EVHT. Un élément de validation expérimentale du modèle accompagne les résultats. • Un modèle thermomécanique pour le calcul de l'état de contrainte de l'EVHT a été développé. Celui-ci tient compte des phénomènes physiques intrinsèques à la cellule et à son fonctionnement sous courant à hautes températures et à ceux imputables aux interactions mécaniques entre la cellule et son environnement. Les données manquantes nécessaires à l'exécution des modèles ont été obtenues par la caractérisation et par des calculs d'homogénéisation de la microstructure tridimensionnelle des électrodes. Par ailleurs le comportement viscoplastique du matériau de la cathode a été mis évidence par des essais de fluage en flexion quatre points. L'étude a permis de définir un domaine de fonctionnement optimal garantissant des performances électrochimiques élevées avec des niveaux de température acceptables. Des propositions visant à réduire l'endommagement mécanique du système ont également été produites. / The thermal, electrochemical and mechanical response of a high temperature steam electrolyzer (HTSE) has been analyzed in this work. To this end, a multi-physics and multi-scale modelling approach has been employed: • A local model, at the microstructure scale of the electrodes, has been used to analyze the apparent electrochemical behavior of the electrodes related to the studied electrolysis cell. System operation, in a stack of several cells, has been then analyzed using a thermoelectrochemical model at the macroscopic scale of the HTSE. An element of experimental validation of the model comes with the results. • A thermomechanical model for the calculation of the stress state of the HTSE has been developed. In this model, the intrinsic physical phenomena of the cell, of its operation under current at high temperatures and those ascribable to the mechanical interactions between the cell and its environment have been considered. The unknown data required for the models have been obtained by the characterization and homogenization calculations of the three-dimensional microstructure of the electrodes. Besides, the viscoplastic behavior of the cathode material has been determined by a four-point bending creep test. The study made it possible to define an optimal operating zone, ensuring both high electrochemical performances and acceptable temperature levels. Proposals aiming to reduce the mechanical damage of the system have been also produced.