FAILURE PROBABILITY OF SOLID OXIDE FUEL CELLS

The failure probability of anode supported solid oxide fuel cells (SOFCs) was analyzed combining experimental results of residual and fracture stress measurements. Oxidized and reduced anode/electrolyte half-cells composed of standard Ni and yttria stabilized zirconia materials were tested. The residual stresses were derived from the temperature dependence of unconstrained cell curvature. The fracture stress (modulus of rupture) was determined in bending tests at room and operation relevant temperature (800 degrees C). The room temperature results revealed a high compressive residual stress in the electrolyte (similar to 430 MPa) and a low average tensile stress in the anode (similar to 9 MPa). The fracture stresses varied between similar to 115 MPa for the oxidized anode at room temperature and similar to 67 Wit for the reduced anode at 800 degrees C. Thermal cycling and stack operation time did not influence the results significantly. Based on Weibull statistics a modulus of similar to 13 was determined and the influence of the cell size was elaborated. The critical stress in stack operation is discussed for different failure probabilities