Selectivity and mixed reactant fuel cells

Abstract

Mixed reactant fuel cells (MR-FCs), are aimed at using a uniform mixture of fuel and oxygen applied to both the anode and the cathode. This allows redesign of fuel cells with a significantly simpler construction, having potentially a higher power density, better fuel utilization and be less expensive. The challenge in realizing MR-FCs is finding selective electrodes that can enhance oxygen reduction at the cathode, fuel oxidation at the anode while inhibiting the chemical reaction between the fuel and oxygen in the gas mixture. This task is in particular challenging in solid oxide fuel cells (SOFCs), as they operate at elevated temperatures, where many reactions are easily activated and selectivity is difficult to achieve. As a result no true MR-FC of the SOFC type were reported while some were found for low temperature fuel cells (FCs). The so-called single-chamber-SOFC are not true MR-FCs as they do not contain two selective electrodes, as required. We shall discuss potential ways to search for and develop selective anodes and cathodes for SOFC type MR-FCs. We first consider material properties which should contribute to that goal. This refers to electronic properties of the bulk, band banding under adsorbed specie, point defects in the bulk and on the surface. We then proceed to show how cell design, in particular electrode structure, can contribute to selectivity. Finally operation conditions are considered and it is shown that they also can contribute to selectivity. The operation condition considered are gas mixture composition, gas mixture residence time in the hot zone, hence gas flow rate, current density and temperature. The topics discussed hold for all FC types but are crucial for the SOFC type because of the difficulty to achieve selectivity at elevated temperatures. It is suggested that a concerted effort taking advantage of all those options should allow development of a true SOFC type MR-FC.

Selective Electrodes for Mixed Reactant Fuel Cells

“…This takes advantage of the fact that the diffusivity of hydrogen is higher than that of oxygen when both are coming from the gas phase and diffuse in a pore towards the SE. The same consideration lead to short pores in the cathode (9).…”

“…At the cathode oxygen ions have to be generated and be adsorbed hence hydrogen should not be chemically adsorbed. This can be controlled by the proper choice of the work function and affinity of the cathode material (9,13). At the anode oxygen ions originating from chemisorption of oxygen should be avoided.…”

“…At the anode oxygen ions originating from chemisorption of oxygen should be avoided. This can also be achieved by choice of the anode work function and ionization energy (9,13). This will allow the adsorbed hydrogen at the anode to only react with oxygen ions emerging from the solid electrolyte, as is desired.…”

“…Control of a narrow range of properties of an electrode can be hampered when using nano particles due to high defect concentration and dangling bonds on the relative large surface. Addition of promoters to introduce local electric fields can also be considered (9). On top of the choice of the electrode material, the gas composition whether reducing or oxidizing, is of significance.…”

“…The use of a lean mixture was shown to eliminate the homogeneous reaction in cases a fuel rich mixture resulted in a reaction (5,10). Enhanced water vapor pressure in the gas mixture may also affect the electrode properties (9).…”