Solid oxide fuel cells cathode, electrochemical reactions

In solid oxide fuel cells (SOFCs), the cathode is the material where pure oxygen or oxygen from air is reduced through the following electrochemical reaction [1]... 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

Perovskites as SOFC cathode material electrode-electrolyte reactions and electrochemical characterisation. Solid Oxide Fuel Cells 10 (Sofc-X), Pts 1 and 2 7 1015... 

The solid oxide fuel cell (SOFC) is based on a thin layer of solid ceramic electrolyte of yttria-stabilized zirconia operating at 600 to 1000°C, which transfers the oxygen ion (O ) from the cathode to the anode. The high temperature is necessary to achieve sufficient ionic conductivity [14]. The electrochemical reaction in an SOFC can be expressed as equation (1.16) to equation (1.18). 

FIGURE 12.10. Microprobe analysis of cathode/electrolyte interfaces (reaction at 1300°C for 5 h). (From Ivers-Tiffee, E., Schiessl, M., Oel, H.J., and Wersing, W. Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, Singhal, S.C. and Iwahara, H., Eds., The Electrochemical Society, Pennington, NJ, 1993, 613-622. With permission.)... 

The electrolyte used by the fuel cell is a solid gas—impermeable zirconia known as zirconium oxide (ZrOj). This ZrOj is doped with calcium oxide (CaO) to supply enough oxide ions to carry the cell current. The oxidant air or oxygen is bubbled through the molten silver cathode, which is held inside the zirconia cup. At the fuel electrode or the carbon-based anode electrode, the oxide ions are combined with carbon monoxide (CO) and give up their electrons to an external circuit. The cell by-products CO and hydrogen, which are formed in the initial fuel decomposition, are burned outside the cell to keep the fuel cell at operating temperature. The hydrogen is not involved in the electrochemical cell reaction. 

Schematically, a fuel cell may be represented as a system of two electrodes separated by electrolyte (see Fig. 1). Liquid or solid electrolytes are used in different types of cells. Fuel is supplied to the anode and oxygen or air to the cathode. The two electrodes are connected by a resistive load. The electrochemical oxidation of the fuel at the anode produces electrons. The electrons flow through the external circuit to the cathode on which oxygen is reduced. The ionic and neutral species that participate in the electrochemical reactions are different for the...

The properties of the interface at which the formation of oxide ions occurs have been of special interest [6, 7, 28—35]. While solid electrocatalysts, Pt [28, 29, 31, 32] and C [30], were studied mainly, a molten silver cathode was employed in another type of zirconia-electrolyte fuel cell developed [34,35] at the General Electric Research and Development Center in Schenectady. Since the hindrance of the electrochemical steps of the O2 reduction at the cathode surface is small [28, 32] on platinum around 1000 °C, it is hard to elucidate the reaction mechanism beyond the net reaction 1. Analysis [33] of the potential distribution curves inside Zro 9Yo 2 02.i in contact with two platinum electrodes showed at 1380°C that the electronic hole contribution to the conductivity in the bulk of the specimen depended upon as would be expected from the equilibrium of reaction 15. The partial oxygen pressure had values between 10 and 10 atm. However, if the production of oxide ions is assumed to occur at the cathode solely by reaction 15, the rate of production is much lower than the rate of loss at the anode. A cathodic reaction of the type... 

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