High Temperature PEM Fuel Cells and Organic Fuels

Abstract

Modern way of life demands enormous amounts of energy, which so far has been mainly produced by combustion of various types of fossil fuel. Increased amounts of atmospheric CO2 and global warming leading to severe climate changes are the consequence. There is a need to make the energy production sustainable and break the dependency on fossil fuels. Hydrogen economy provides such a solution, where hydrogen produced by renewables, such as wind and solar power, becomes the energy carrier. The storage, handling and transportation of hydrogen are the main obstacles on the route to a sustainable future when it comes to powering small and medium sized applications, transportation sector in particular. This is mainly due to the gravimetric energy density being immensely inferior to the liquid fuels gasoline and diesel. Dimethyl ether has already been identified as an excellent renewable fuel and a diesel substitute, which possesses energy density not much less than those of conventional diesel and gasoline. With its predicted widespread, there is an interest in harvesting electricity from dimethyl ether directly, rather than using it solely for combustion. High temperature PEM fuel cells provide such an opportunity. Some knowledge about the electrooxidation of DME is available, together with its limited use in low temperature PEM fuel cells, where the low temperature poses an obstacle in the form of phase separation in the fuel supply, making the cells less effective and reducing the amount of power harvested from the cells. This is completely avoided at the elevated temperatures with the additional benefit of increased kinetics. In the presented work an experimental setup for testing direct dimethyl ether high temperature fuel cells is described, proposing a novel design of an evaporator for a burst-free supply of a fuel and steam mixture. Based on the knowledge gathered with the construction and operation of the single cell setup a more versatile and flexible setup was designed and commissioned for independent testing of up to 6 cells, enabling fuel cell experiments with up to 3 gasses and a single evaporated liquid stream supply to either of the electrodes. A large number of MEAs with different component compositions have been prepared and tested in different conditions using the constructed setups to obtain a basic understanding of the nature of direct DME HT-PEM FC, to map the processes occurring inside the cells and to determine the lifetime. Additionally, comparison was made with methanol as fuel, which is the main competitor to DME in direct oxidation of organic fuels in fuel cells. For the reference, measurements have also been done with conventional hydrogen/air operation. All the experiments have been conducted at atmospheric pressure. Experiments with varying amounts of PBI in the cathode catalyst layer has shown that there is a minimum content limit for the preparation of a well dispersed catalyst ink of 15 carbon to PBI weight ratio in the currently used ink formulation. On the other hand, for the MEA operation it has been shown that too much PBI has a negative effect with large mass transport limitations as a consequence. The amount of catalyst in the electrode has also shown an effect on the performance, with the optimum being between 3 and 4 mg/cm² of a 60 wt% Pt50Ru50 catalyst on 40 wt% carbon support. Catalysts with varying Pt, Ru and Sn content on carbon support has been synthesised and used for MEA testing, with the outcome of other operation and MEA composition parameters, such as reliable fuel supply, MEA assembly and operation and the characteristics of the electrolyte membrane having a much more pronounced effect on the final performance than the catalyst composition. The increased operating temperature showed improved performances for all 3 investigated fuels, hydrogen, DME and methanol, with the additional energy supplied in terms of heat helping to overcome the kinetic barriers of the oxidation of the fuels. The resistance of the electrolyte was also found to be decreasing until 200 °C, passing which would result in a rapid membrane dryout and supposed polymerisation of the phosphoric acid, irreversibly lowering the conductivity of the membrane. By increasing the partial pressure of oxygen on the cathode side from 0.21 bar to 1 bar an overall increase in cell voltages was observed for all 3 fuels, with peak power densities increasing by 25 % and 35 % for DME and MeOH operation respectively, thereby also indirectly confirming the larger fuel crossover effect of the latter. Gas chromatography study of the anode exhaust gas at open circuit voltage revealed a small degree of internal fuel reforming with different products when operated on dimethyl ether or methanol. While methanol seemed to reform to syngas, the DME yielded methane rather than CO as one of the products. The observation of internal reforming was indirectly confirmed by electrochemical impedance spectroscopy, where the best fits were obtained when a Gerischer element describing preceding chemical reaction and diffusion was included in the equivalent circuit of a methanol/air operated cell. In general it has been shown that EIS is a powerful tool for studying MEAs and making reference electrodes unnecessary. Finally, durability studies have shown that the average lifetimes of the cells are between 300 – 600 hours, depending on the operating temperature and water content in the anode fuel supply. However, a potential to operate past 1500 hours has been demonstrated. Post-mortem analyses of the MEAs have shown that one of the reasons for the cell death was formation of pinholes in the membrane, rather than an overall thinning.