Solid Electrolyte Membranes

A completely separate family of conducting polymers is based on ionic conduction polymers of this kind (Section 11.3.1.2) are used to make solid electrolyte membranes for advanced batteries and some kinds of fuel cell. 

M. Stoukides, Solid-Electrolyte Membrane reactors Current experience and future outlook, Catalysis Reviews - Science and Engineering 42(1 2), 1 -70 (2000). 

The generation of heat always accompanies the operation of a fuel cell. The heat is due to inefficiencies in the basic fuel-cell electrochemical reaction, crossover (residual diffusion through the fuel-cell solid-electrolyte membrane) of fuel, and electrical heating of interconnection resistances. Spatial temperature variation can occur if any of these heat-generating processes occur preferentially in different parts of the fuel cell stack. For example, non-uniform distribution of fuel across the surfaces of electrodes, different resistances between the interconnections in a stack, and variations among... 

Solid electrolyte membrane supports the electrodes, conducts ions, and achieves the reactions on its surface Transfer of heat... 

Solid electrolyte membranes such as H+ and 02 conductors in fuel cells... 

Stoukides M, (2000). Solid electrolyte membrane reactors current experience and future outlook. Catalysis Reviews Science Engineering, 42 1-70 Sun C, Stimming U, (2007). Recent anode advances in solid oxide fuel cells. Journal of Power Sources 171 247-260... 

Stoukides, M., Solid-electrolyte membrane reactors current experience and future outlook, Catalysis Reviews Science and Engineering, 2000, 42, 1-70. 

The driving force for the diffusion of across the solid electrolyte membrane is the difference in O2 concentration between the anode and cathode side. This difference is a result of depletion of O2 by partial oxidation of CzHe on the anode side. Since the concentration of oxygen in the anode has a profound effect on the concentration gradient of 0 across the electrolyte membrane, the flow pattern of fuel/air mixture around the fuel cell has to be carefully controlled through the flow geometry. 

Historically there are two major types of dense inorganic membranes that have been studied and developed extensively. They are metal membranes and solid electrolyte membranes. 

Small-size starting particles with an appropriate size distribution that lead to a maximum packing density and final fired density are essential in the sintering step to form dense membranes. The required sintering temperatures are lower because of the active state of the Hne particulate materials used. For example, when particles of an average size of 5 nm made by the alkoxide approach are used to make stabilized zirconia, 1450X instead of the usual 2(X)0X is all that is necessary to produce fully dense material [Mazdiyasni et al., 1%7]. The amount of additives such as the stabilizers affects densification of the final solid electrolyte membranes as well. Generally there is an optimum amount of stabilizer for maximum densification. Excess addition actually can lead to lower densification. 

These applications will be briefly treated in this section. As will become evident, the solid electrolyte membrane materials are either stabilized oxides or mixed oxides. Further details of science and technology of electrocatalytic membrane reactors beyond the scope of this chapter can be found in a number of excellent reviews [Ceilings et al., 1988 Stoukides, 1988]. 

Catalytic applications of solid electrolyte membrane reactors using electrochemical oxygen pumping (EOP)... 

In addition, solid electrolyte membranes potentially can be used for electrochemical oxygen pumping and as fuel cells which can produce chemicals while generating electricity. 

The issue of mismatch of thermal expansion coefficients similar to that for a composite membrane is also very critical for fuel cells. In the fuel cells, electrodes are attached to solid electrolyte membranes. Significant temperature variations during applications, pretreatments or regeneration of the membranes (e.g., decoking) can cause serious mechanical problems associated with incompatible thermal expansions of different components. A possible partial solution to the above problem is to use partially stabilized instead of fully stabilized zirconia. The former has a significantly lower thermal expansion coefficient than the latter. 

For those dense solid electrolyte membranes using metal oxides, the degree of stabilization can make a difference in the resulting thermal shock resistance. For example, the fully stabilized zirconia has poor thermal shock resistance compared to the partially stabilized zirconia. 

Similar tubular cell designs based on stabilized zirconia and P-alumina solid electrolyte membranes have been developed for hydrogen production [Ddnitz and Erdlc, 1984] and load-leveling applications [Sudworth and Tilley, 1985]. 

Dense palladium and palladium alloy membranes have been repeatedly demonstrated to show extremely high sclcctiviiies of hydrogen and certain solid electrolyte membranes... 

According to the third catalyst-membrane coupling possibility, represented in Fig. 5c, the surface of the membrane is deposited with some catalytic material. This setup is typical of solid-electrolyte membranes, where the catalyst is also playing the role of the electrode, necessary to drive the permeation of ions throughout the membrane at a desired rate. Problems may arise here concerning the fact that the catalyst per unit membrane surface is limited to some extent, and that several catalytic materials (e.g., metal oxides) are poor electricity conductors [26]. 

Concerning proton conductors, Govind and Zaho [100] stated that metal-based membranes could be outperformed by solid electrolyte membranes based on materials... 

Clark D.J., Losey R.W. and Suitor J.W., Separation of oxygen by using zirconia solid electrolyte membranes. Gas Separation and Purification 6 201 (1992). 

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