Oxygen separation membranes

There has been considerable interest in Ce02 as a component of solid oxide fuel cells, especially as an anode material, and also for use in oxygen separation membranes. The material shows a wide nonstoichiometry range, with oxygen vacancies as the... 

Figure 9.3 Oxygen-separation membrane - GT integration for production of oxygen, after [15].

Figure 9.4 Simplified sketch of the mixed conducting oxygen-separation membrane reactor part in the AZEP concept, after [20], Second combustor placed before the turbine improves efficiency, but also increases C02 emission.

Mixed-conducting (A,Ln)M03 (M = Fe, Co), promising as cathodes of intermediate-temperature SOFCs, oxygen-separation membranes, and electrocatalysts for high- and low-temperature processes involving oxygen. 

As is clear solid oxide electrolytes are not useful for applications as oxygen separation membrane, unless operated with external circuitry (oxygen pump) or as a constituent phase of a dual-phase membrane. Both modes of operation, classified in this paper as electrochemical oxygen separation, are briefly discussed in Section 10.4.3. But we first start with a discussion of the models that have been developed to describe the oxygen semi-permeability of solid oxide... 

The surface kinetics is crucial for a membrane in operation. For oxygen separation membranes, it has proven to be rate limiting in many cases, and has been studied extensively. For proton-conducting materials and membranes, much less has been done, partly because surface kinetics has been less of a problem up to now. Still, we believe membrane materials will be better and thinner, until the surfaces eventually become rate limiting. We, therefore, mention a couple of techniques for studying hydrogen exchange kinetics. 

There are two main approaches for fabricating oxygen separation membranes. The first is to use a mixture of two distinct materials - an oxygen-ion conductor such as doped zirconia, and an electron conductor such as a noble metal or a doped perovskite (see Fig. 6.1). The second approach is to use a material which is capable of conducting both oxygen ions and electrons (see Fig. 6.2). 

A conventional planar geometry, which has been widely adopted in soUd oxide fuel cell technology, has very significant problems in the case of a pressure-driven oxygen separation membrane. The main reason is the large pressure differential. 

Similar processes took place in the case of porous YSZ coating (powder and polymer ratio of 5 1). The only difference is the amount of nanocrystalline YSZ derived from the polymer was not sufficient to fill all space in the initial YSZ framework resulting a porous ceramic material with well-developed connection between particles (Figure 3-3c). This type of material cannot be used as an oxygen separation membrane because it has open porosity, but it can be useful as the YSZ skeleton for the electrodes, as it has high effective surface area for exchange with the gaseous phase. 

Improvements in cell design and engineering have paralleled the Electropox process development. Advanced cell concepts have evolved that incorporate both electronic and ionic conducting functionalities into a single membrane. Oxygen transport rates have been demonstrated with these materials that may permit their use as oxygen separating membranes in chemical reactors. 

MIEC Perovskites as Oxygen Separation Membrane Materials for the Oxy-fuel Combustion Power Production... 

High-temperature ceramic oxygen separation membrane system on laboratory scale... 

Kharton, V.V., Yaremchenko, A.A., Kovalevsky, A.V. et al. (1999) Perovskite-type oxides for high-temperature oxygen separation membranes. Journal of Membrane Science, 163 (2), 307-317. 

Zeng, Q., Zuo, Y, Fan, C. and Chen, C. (2009) C02-tolerant oxygen separation membranes targeting CO2 capture application. Journal of Membrane Science, 335, 140-144. 

Oxygen separation membranes can be used in catalytic oxidation processes that require a continuous supply of oxygen. Depending on the mediator of the electronic flux, dense ceramic MRs can be divided into three types ... 

In addition to the oxygen separation membranes, the proton conducting membranes can also be applied to reduce NO emission by combining heterogeneous catalysis and solid state electrochemistry. The solid electrolytes in MRs serve to electrochemically control chemisorptive bonds and enhance catalytic activity. Figure 8.10 shows the schematic diagram of a steam electrolysis cell constructed with a proton conductor for reducing NO. Steam is electrolyzed at the anode. It shows the produced H+ is electrochemically pumped to the cathode and reacts with NO to produce Nj and HjO ... 

IshiharaT,Tsuruta Y,TodakaT, Nishiguchi H and Takita Y (2002), Fe doped LaGa03 perovskite oxide as an oxygen separating membrane for CH4 partial oxidation . Solid State Ionics, 152/153,709-714. 

Finally, dense ceramic oxide membrane-based eCMRs (Badwal Ciacchi, 2001 Fouletier Ghetta, 2009 Hibino, Ushiki, Kuwahara, 1997), which are available in both planar and tubular designs, have been investigated for O2 separation and compression at HT. Of course, the conventional (non-electrochemical) HT oxygen separation membranes are mixed oxide/electronic ceramics (Badwal Ciacchi, 2001). 

---