P3 and ASIMO - The Honda Humanoid Robots

Details of 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.

Type: