Broers and Ketelaar

Testing the electrolyte formulations of Davtyan was the starting point for Broers and Ketelaar (1960) in their work on molten carbonate fuel cells at the University of Amsterdam. The conclusion of their investigation (begun in 1951) was that the state of the electrolyte, which was supposed by Davtyan to have been solid, was actually two phases: a molten phase consisting of carbonates, phosphates, tungstates, and silicates (a eutectic salt mixture) and a solid phase of rare earth oxides. The solid phase was formed by the Ce-La-Tsh oxides (which have high melting points) and acted as a porous frame to hold the molten electrolyte. Fuel cells that used this electrolyte with town gas as fuel and air as oxidant were unable to sustain stable performance beyond 72 hours, with some degrading within 24 hours. In tests at 650 to 800°C, the tungstate was chemically reduced, the cathode was oxidized, the anode was reduced, and components of the cells cracked.

Broers and Ketelaar settled on using carbonates as electrolyte after having considered using other electrolytes. Salts such as phosphates, chlorides, sulfates, and nitrates were converted into carbonates (which would therefore require replenishment), and borates and silicates, though more resistant to conversion into carbonates, would be too polarized when current was drawn. By using carbonate salts, Broers and Ketelaar eliminated the problem of decomposition by CO2, and they were able to reduce the concentration polarization by adding CO2 to the cathode (as Baur and Brunner had shown in 1937).

The fuel cell design had the molten carbonate electrolyte held in a matrix. The electrolyte mixture of alkali carbonates (lithium, sodium, and/or potassium carbonate) was impregnated into a porous sintered disk of magnesium oxide (MgO). The commercially available MgO was sintered at 1200°C, and the volume porosity was 40-50%. After impregnating the electrolyte into the disk, which gave an electrolyte content of 40% by weight, the electrode materials were applied to both sides of the disk as powders with thicknesses of 1 mm. The cathode was always silver (an ideal O2 + CO2 electrode with no polarization at 150 mA/cm2 and 500°C), and the best anode electrodes for hydrogen were platinum and nickel. The most active electrodes for the oxidation of carbon monoxide were platinum, platinized iron or nickel, iron, and nickel. When methane was used as fuel, steam was added to the feed, and the nickel electrode catalyzed the internal steam reforming into synthesis gas and also oxidized the hydrogen and carbon monoxide for the fuel cell reaction.

The cells (10 cm2) were tested between 550 and 700°C on air and fuels such as town gas, hydrogen, carbon monoxide, and natural gas, lasting several months under continuous operation. One cell operated for 6 months on town gas as fuel and air and CO2 as the cathode feed, but its open-circuit voltage degraded from 1.1 V down to 0.90 V in that time because of a loss in the electrolyte, which increased the cell resistance from 0.3 to 1.5 ohm. The loss of electrolyte was caused by vaporization of CO2, Li2O, Na2O, and K2O and by chemical reactions with gasket materials; more of the reactant gases leaked through the MgO disk and lowered the efficiency and performance of the electrodes.

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