Mass Transport in Gas Diffusion Layers in Polymer Electrolyte Fuel Cell

Fuel cells constitute a promising future for energy conversion. The Polymer Electrolyte Membrane Fuel Cell (PEMFC) generates electricity using hydrogen and oxygen as fuel, reactants forming only water. It thus constitutes a zero-emission power source. It can be used in a wide variety of appliances, from consumer electronics to propulsion of vehicles. As further increase in performance is needed for PEMFC to be fully commercialised, operation at higher current densities than currently possible is needed. This development calls for improved water handling in the fuel cell as flooding of the Gas Diffusion Layer (GDL), especially at the cathode, might be a limitation for cell performance. This work is done in cooperation with Cellkraft AB, under the EU-supported PEMTOOL project. Results will be used as validation data for the next generation of modelling tools in the area of research. The purpose of this work is to study mass transport related problems, such as flooding, in GDL in PEMFC and to produce validation data to the PEMTOOL project. This is done by measuring cell performance under influence of varying cell temperature, gas pressure, oxygen partial pressure and humidification of reactant gases. This report explains the function of PEMFC, the design of the specific cell used in experiments and describes how experiments are executed. Results of this work indicate a connection between flooding of the cathode and lowered oxygen partial pressure in cathode gas, while operating the cell with oversaturated reactant gases. This highlights humidification optimisation as a key factor for further increase in PEMFC performance

Mass Transport in Gas Diffusion Layers in Polymer Electrolyte Fuel Cell

Fuel cells constitute a promising future for energy conversion. The Polymer Electrolyte Membrane Fuel Cell (PEMFC) generates electricity using hydrogen and oxygen as fuel, reactants forming only water. It thus constitutes a zero-emission power source. It can be used in a wide variety of appliances, from consumer electronics to propulsion of vehicles. As further increase in performance is needed for PEMFC to be fully commercialised, operation at higher current densities than currently possible is needed. This development calls for improved water handling in the fuel cell as flooding of the Gas Diffusion Layer (GDL), especially at the cathode, might be a limitation for cell performance. This work is done in cooperation with Cellkraft AB, under the EU-supported PEMTOOL project. Results will be used as validation data for the next generation of modelling tools in the area of research. The purpose of this work is to study mass transport related problems, such as flooding, in GDL in PEMFC and to produce validation data to the PEMTOOL project. This is done by measuring cell performance under influence of varying cell temperature, gas pressure, oxygen partial pressure and humidification of reactant gases. This report explains the function of PEMFC, the design of the specific cell used in experiments and describes how experiments are executed. Results of this work indicate a connection between flooding of the cathode and lowered oxygen partial pressure in cathode gas, while operating the cell with oversaturated reactant gases. This highlights humidification optimisation as a key factor for further increase in PEMFC performance

Applications for Molten Carbonate Fuel Cells

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.QC 20141028</p

Electrochemical performance of reversible molten carbonate fuel cells

The electrochemical performance of a state-of-the-art molten carbonate cell was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes by using polarization curves and electrochemical impedance spectroscopy (EIS). The results show that it is feasible to run a reversible molten carbonate fuel cell and that the cell actually exhibits lower polarization in the MCEC mode, at least for the short-term tests undertaken in this study. The Ni hydrogen electrode and the NiO oxygen electrode were also studied in fuel cell and electrolysis cell modes under different operating conditions, including temperatures and gas compositions. The polarization of the Ni hydrogen electrode turned out to be slightly higher in the electrolysis cell mode than in the fuel cell mode at all operating temperatures and water contents. This was probably due to the slightly larger mass-transfer polarization rather than to charge-transfer polarization according to the impedance results. The CO2 content has an important effect on the Ni electrode in electrolysis cell mode. Increasing the CO2 content the Ni electrode exhibits slightly lower polarization in the electrolysis cell mode. The NiO oxygen electrode shows lower polarization loss in the electrolysis cell mode than in the fuel cell mode in the temperature range of 600-675 degrees C. The impedance showed that both charge-transfer and mass-transfer polarization of the NiO electrode are lower in the electrolysis cell than in the fuel cell mode.QC 20150623</p