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>

Solid Oxide Fuel Cells are fuel cells that operate at high temperatures usually in the range of 600oC to 1000oC and employ solid ceramics as the electrolyte. In Solid Oxide Fuel Cells oxygen ions (O2- ) are the ionic charge carriers. Solid Oxide Fuel Cells are known for their higher electrical efficiency of about 50-60% [1] compared to other types of fuel cells and are considered very suitable in stationary power generation applications. It is very important to study the effects of different parameters on the performance of Solid Oxide Fuel Cells and for this purpose the experimental or numerical simulation method can be adopted as the research method of choice. Numerical simulation involves constructing a mathematical model of the Solid Oxide Fuel Cell and use of specifically designed software programs that allows the user to manipulate the model to evaluate the system performance under various configurations and in real time. A model is only usable when it is validated with experimental results. Once it is validated, numerical simulation can give accurate, consistent and efficient results. Modeling allows testing and development of new materials, fuels, geometries, operating conditions without disrupting the existing system configuration. In addition, it is possible to measure internal variables which are experimentally difficult or impossible to measure and study the effects of different operating parameters on power generated, efficiency, current density, maximum temperatures reached, stresses caused by temperature gradients and effects of thermal expansion for electrolytes, electrodes and interconnects. iv Since Solid Oxide Fuel Cell simulation involves a large number of parameters and complicated equations, mostly Partial Differential Equations, the situation calls for a sophisticated simulation technique and hence a Finite Element Method (FEM) multiphysics approach will be employed. This can provide three-dimensional localized information inside the fuel cell. For this thesis, COMSOL Multiphysics® version 4.2a will be used for simulation purposes because it has a Batteries & Fuel Cells module, the ability to incorporate custom Partial Differential Equations and the ability to integrate with and utilize the capabilities of other tools like MATLAB ® , Pro/Engineer® , SolidWorks® . Fuel Cells can be modeled at the system or stack or cell or the electrode level. This thesis will study Solid Oxide Fuel Cell modeling at the cell level. Once the model can be validated against experimental data for the cell level, then modeling at higher levels can be accomplished in the future. Here the research focus is on Solid Oxide Fuel Cells that use hydrogen as the fuel. The study focuses on solid oxide fuel cells that use 3-layered, 4-layered and 6-layered electrolytes using pure YSZ or pure SCSZ or a combination of layers of YSZ and SCSZ. A major part of this research will be to compare SOFC performance of the different configurations of these electrolytes. The cathode and anode material used are (La0.6Sr0.4)0.95-0.99Co0.2Fe0.8O3 and Ni-YSZ respectively