Oxygen, chemical potential ionic conductivity

Schematics of an oxygen membrane reactor for catalytic POx of methane. A blown up section on the left-hand side shows the details of the ceramic membrane wall explaining the mechanism of oxygen permeation across the membrane. /- is the chemical potential of oxygen and ai and

In SOFCs, the difference in chemical potential or activity of oxygen across the electrolyte surfaces provides the electromotive force, and thus the electrical potential. Extensive research over the past decades has resulted in the development of cost-effective processes for the fabrication of thin and dense electrolyte layers. YSZ has been considered one of the best choices for high-temperature applications (>650°C) because of its feasibility of fabrication of a thin membrane, reasonable ionic conductivity, large ionic domain, and, most importantly, chemical and mechanical stability... 

In EVD, a modified form of chemical vapor deposition (CVD), an electrochemical potential gradient is used to grow a thin, dense layer of the ionic conducting oxide (e.g., yttria-stabilized zirconia) on a porous substrate. EVD is either a single-step or a two-step process depending on the nature of the substrate. For a porous substrate, the first step involves pore closure by CVD (i.e., deposition from the vapor of an oxide layer by reaction of a chloride gas precursor compound with water vapor or oxygen) ... 

Electrolytes require a high oxygen ionic conductivity and a dense microstructure to allow for gas separation of fuel and air compartment. Electronic conductivity is to be avoided since this would potentially short-circuit the SOFC. Chemical and thermomechanical stability of electrolytes with electrode materials is also an important requirement to assure long-term stability. Cubic 8 mol% yttria-doped zirconia is, due to its sufficiently and predominantly ionic conductivity, the state-of-the-art material used... 

Due to its high ionic conductivity and good chemical stability, acceptor-doped ceria can potentially be used in dense ceramic membranes for the separation of oxygen from air. 

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