Among the obstacles for the commercialization of the MoltenCarbonate Fuel Cell (MCFC), the dissolution of thestate-of-the-art lithiated NiO cathode is considered as aprimary lifetime limiting constraint. Development ofalternative cathode materials is considered as a main strategyfor solving the cathode dissolution problem. LiFeO2and LiCoO2had earlier been reported as the most promisingalternative materials; however, they could not satisfactorilysubstitute the lithiated NiO. On the other hand, ternarycompositions of LiFeO2, LiCoO2and NiO are expected to combine some desirableproperties of each component. The aim of this work was todevelop alternative cathode materials for MCFC in the LiFeO2-LiCoO2-NiO ternary system. It was carried out byinvestigating electronic conductivity of the materials, firstin the form of bulk pellets and then in ex-situ sinteredporous-gas-diffusion cathodes, and evaluating theirelectrochemical performance by short-time laboratory-scale celloperations. Materials in the LiFeO2-NiO binary system and five ternary sub-systems,each with a constant molar ratio of LiFeO2:NiO while varying LiCoO2content, were studied. Powders withcharacteristics appropriate for MCFC cathode fabrication couldbe obtained by the Pechini method. The particle size of LiFeO2-LiCoO2-NiO powders considerably depends on thecalcination temperature and the material composition. Theelectrical conductivity study reveals the ability of preparingLiFeO2-LiCoO2-NiO materials with adequate electricalconductivity for MCFC cathode application. A bimodal pore structure, appropriate for the MCFC cathode,could be achieved in sintered cathodes prepared usingporeformers and sub-micron size powder. Further, this studyindicates the nature of the compromise to be made between theelectrical conductivity, phase purity, pore structure andporosity in optimization of cathodes for MCFC application. Cellperformance comparable to that expected for the cathode in acommercial MCFC could be achieved with cathodes prepared from20 mole% LiFeO2- 20 mole% LiCoO2- 60 mole% NiO ternary composition. It shows aniR-corrected polarization of 62 mV and a iR-drop of 46 mV at acurrent density of 160 mAcm-2at 650 °C. Altogether, this study revealsthe possibility of preparing LiFeO2-LiCoO2-NiO cathode materials suitable for MCFCapplication. Keywords: molten carbonate fuel cell (MCFC), MCFC cathode,LiFeO2-LiCoO2-NiO ternary compositions, electrical conductivity,porous gas diffusion electrodes, polarization, electrochemicalperformance, post-cell characterization.

<p>Among the obstacles for the commercialization of the MoltenCarbonate Fuel Cell (MCFC), the dissolution of thestate-of-the-art lithiated NiO cathode is considered as aprimary lifetime limiting constraint. Development ofalternative cathode materials is considered as a main strategyfor solving the cathode dissolution problem. LiFeO₂and LiCoO₂had earlier been reported as the most promisingalternative materials; however, they could not satisfactorilysubstitute the lithiated NiO. On the other hand, ternarycompositions of LiFeO₂, LiCoO₂and NiO are expected to combine some desirableproperties of each component. The aim of this work was todevelop alternative cathode materials for MCFC in the LiFeO₂-LiCoO₂-NiO ternary system. It was carried out byinvestigating electronic conductivity of the materials, firstin the form of bulk pellets and then in ex-situ sinteredporous-gas-diffusion cathodes, and evaluating theirelectrochemical performance by short-time laboratory-scale celloperations.</p><p>Materials in the LiFeO₂-NiO binary system and five ternary sub-systems,each with a constant molar ratio of LiFeO₂:NiO while varying LiCoO₂content, were studied. Powders withcharacteristics appropriate for MCFC cathode fabrication couldbe obtained by the Pechini method. The particle size of LiFeO₂-LiCoO₂-NiO powders considerably depends on thecalcination temperature and the material composition. Theelectrical conductivity study reveals the ability of preparingLiFeO₂-LiCoO₂-NiO materials with adequate electricalconductivity for MCFC cathode application.</p><p>A bimodal pore structure, appropriate for the MCFC cathode,could be achieved in sintered cathodes prepared usingporeformers and sub-micron size powder. Further, this studyindicates the nature of the compromise to be made between theelectrical conductivity, phase purity, pore structure andporosity in optimization of cathodes for MCFC application. Cellperformance comparable to that expected for the cathode in acommercial MCFC could be achieved with cathodes prepared from20 mole% LiFeO₂- 20 mole% LiCoO₂- 60 mole% NiO ternary composition. It shows aniR-corrected polarization of 62 mV and a iR-drop of 46 mV at acurrent density of 160 mAcm^-2at 650 °C. Altogether, this study revealsthe possibility of preparing LiFeO₂-LiCoO₂-NiO cathode materials suitable for MCFCapplication.</p><p>Keywords: molten carbonate fuel cell (MCFC), MCFC cathode,LiFeO₂-LiCoO₂-NiO ternary compositions, electrical conductivity,porous gas diffusion electrodes, polarization, electrochemicalperformance, post-cell characterization.</p>

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>