9 - Work of electrochemical pressurization of a pore in an oxygen ion conducting solid electrolyte and implications concerning solid oxide electrolyzer degradation

Abstract

Solid oxide electrolyzers are known to degrade by forming cracks in the electrolyte as well as by the occurrence of electrode delamination. The origin of degradation lies in the internal precipitation of oxygen and local pressurization. Internal precipitation of oxygen may occur in a perfect lattice or also in already existing pores or pores formed during longtime operation. Precipitation of oxygen can occur at numerous places but analysis of such a problem is complicated due to the random nature of such events. Here, an analysis is presented on electrochemical internal pressurization of an isolated pore in a solid electrolyte such as yttria-stabilized zirconia (YSZ). This allows one to model the mechanics of pressurization and precipitation. This precipitation occurs by the transport of oxygen ions and their local oxidation to oxygen gas releasing electrons. Transport of oxygen ions and electrons thus is necessary for internal precipitation in an operating cell. The analysis is also presented on a hypothetical piston–cylinder arrangement to pressurize a pore. A possible experiment consists of embedding a platinum wire at the tip of which exists a pore. The surface of the sample has a porous electrode. Oxygen is electrochemically transported and stored under an applied voltage. Assuming linear elasticity, calculations are presented on strain energy generated in the YSZ sample upon the pressurization of the pore. In addition, estimated is the energy stored in the gas phase assuming it to obey the ideal gas law. The calculations are made under two different assumptions: (a) Pore radius is nearly fixed—applicable at low pressures. (b) Pore radius changes upon pressurization—applicable at high pressures. All equations are developed in a closed form. It is shown that the energy stored in the gas phase is much larger than the strain energy in the solid. The kinetics of pore pressurization is addressed in which the time required to achieve a given pressure is estimated in a parametric form. This work shows that an isolated pore can in principle be pressurized to very high values.