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The cathode of an SOFC

The cathode (that is the electrode at the air side of the fuel cell) has to reduce the oxygen molecules (O 2) of the air to oxygen ions (O 2 -) which then migrate to (and through) the electrolyte. The cathode has a porous structure to enable gas transport to the cathode's surface. It must also supply electrons for the reduction of the oxygen and therefore has to be a good electronic conductor.
On the right side of the drawing the oxygen transport process is sketched: The oxygen of the air is adsorbed at the cathode's surface, takes up two electrons and moves - as an oxygen ion - towards the electrolyte.
The electrochemical active area (three phase boundary "of cathode, electrolyte and gas space) is limited to the direct vicinity of the electrolyte. There are two ideas to solve this drawback: A layer consisting of a mixture of cathode and electrolyte material (so-called "functional cathode layer") is inserted between the electrolyte and the conventional cathode. thus the actual area where the electrochemical reaction can take place is considerably increased. Another starting point is to look for materials which have an intrinsic conductivity for electrons and oxygen ions. In this case there is no longer a restriction of the "three phase boundary" reaction to the small area near the interface of cathode and electrolyte.

Fig.: sketch of the electrochemical reactions at the air side of the fuel cell.

Currently, there Sr x MnO 3-δ, a complex oxide with a perovskite structure, is used as a cathode. This material fulfills the requirements with respect to processing and to the chemical stability during operation as fuel cell cathodes. The coefficient of thermal expansion also matches well to that of the electrolyte. y Sr x MnO 3-δ exhibits high electronic conductivity, nevertheless, the ionic conductivity is nearly zero. In the design of Forschungszentrum Jülich's SOFC the cathode (incl. the functional layer) is deposited by Wet Powder Spraying (WPS) or alternatively by Screen printing.
There are candidates for novel cathode materials that exhibit electronic as well as ionic conductivity, eg x Sr y Fe 1-z Co z O 3-δ. However, these materials react with the electrolyte, and the coefficient of thermal expansion is too high .
The research activities of IWV-1 in the field of cathodes for SOFCs aim at the optimization of current cathode materials and the development of novel functional ceramics to enhance the power density and to decrease the operating temperature of the fuel cells.

Electrolyte for Solid Oxide Fuel Cells

The electrolyte is the central part of an SOFC (Solid Oxide Fuel Cell). Within the electrolyte the oxygen ions (O 2 -), which are reduced on the air electrode side (cathode), are transported and are reacting with eg hydrogen to form water on the fuel electrode side (anode). Vice versa electrons (e -) were formed and are moving in the opposite direction and are available for an outside current use.
Nowadays the mostly used electrolyte material is zirconia (ZrO 2). Zirconia at ambient conditions is a poor ionic conductor. If ZrO 2 is heated up to temperatures above 2000 ° C it becomes ionic conducting due to a phase transformation from tetragonal to cubic structure. By adding stabilizing agents like calcia (CSZ), yttria (YSZ) or Scandia (ScSZ) the cubic structure is stabilized even at ambient conditions. Because of this stabilization zirconia becomes a reasonable ionic conductor at SOFC operating temperatures (750-1000 ° C) and can be used as electrolyte material. Ionic conduction proceeds through oxygen vacancies due to insertion of a di-or trivalent element (Ca 2 +, Y 3 +, Sc 3 +) instead of the tetravalent Zr 4 +. The lack of positive charge is balanced by free oxygen lattice sites. Through these free sites oxygen can move through the cubic structure.

Besides gastightness good ionic conductivity of the electrolyte is the most important characteristic the material should have. If the gastightness of the electrolyte is insufficient a reaction between oxygen (cathode side) and hydrogen (anode side) may occur. Gastightness is ensured by the electrolyte sintering at temperatures of approx. 1400 ° C. Lower temperatures lead to inadquate gastightness, but pure YSZ could not be sintered to high densities below 1400 ° C. In contrast to the high sintering temperature which is necessary for the electrolyte the cathode tolerates only maximum temperatures of approx. 1200 ° C. If sintered at higher values the amount of triple phase boundaries reduces drastically due to enhanced sintering. These two facts are the one major reason for R & D on electrolyte material in Jülich which is the reduction of the sintering temperature of the electrolyte (goal ≤ 1300 ° C). Decreasing sintering temperatures can be reached by a) using nanosized starting materials or b) the use of sintering additives. Additional R & D is focused on the coating technologies for SOFC electrolytes.