Oxygen reduction, SOFC

Oxygen Reduction The overall reaction for the oxygen reduction at a SOFC cathode can be written as... 

The impedance polarization performance of LSM electrode is closely related to the mechanism and kinetics of the oxygen reduction reactions. 02 reduction at SOFC cathodes is the most heavily studied subject, and this subject is sufficiently broad and complex to warrant its own review. Interested readers should consult the recent excellent articles by Adler [1] and Fleig [55], Here, only the polarization performance and its influencing factors are discussed. 

Hence, catalysis related challenges for SOFC cathode are the development of cathode specifications, i.e., material and microstructure, having high catalytic activity for oxygen reduction at 600 °C, high electron and ion conductivity, and a low sensitivity for poisoning by volatile Cr species. Again, as for the anode, cost and compatibility related requirements have to be considered. 

Figure 4. Some mechanisms thought to govern oxygen reduction in SOFC cathodes. Phases a, and y refer to the eiectronic phase, gas phase, and ionic phase, respectiveiy (a) Incorporation of oxygen into the buik of the electronic phase (if mixed conducting) (b) adsorption and/or partial reduction of oxygen on the surface of the electronic phase (c) bulk or (d) surface transport of or respectively, to the oJy interface, (e) Electrochemical charge transfer of or (f) combinations of and e , respectively, across the aJy interface, and (g) rates of one or more of these mechanisms wherein the electrolyte itself is active for generation and transport of electroactive oxygen species.

The literature reviewed in sections 2—3.6 has shown that oxygen reduction on Pt is quite complex, involving several possible rate-limiting (or co-limit-ing) steps. As we will see in sections 4 and 5, this complexity is a universal feature of all SOFC cathodes, with many of the same themes and issues reappearing for other materials. We therefore highlight below several general observations about the mechanism of Pt that frame the discussion for other solid-state gas-diffusion electrodes involving O2. These observations are as follows. 

Also for cathodic oxygen reduction in low-temperature fuel cells, platinum is indispensible as a catalyst whereas the cathodic electrocatalysts in MCFCs and SOFCs are lithiated nickel oxide and lanthanum-manganese per-ovskite, respectively. Appleby and Foulkes in the Fuel Cell Handbook (101) reviewed the fundamental work as well as the technologically important publications covering electrocatalysis in fuel cells till 1989. 

An SOFC cathode normally consists of a porous matrix cast onto an oxide ion-conducting electrolyte substrate (see Figure 8.24), where the cathode porosities are typically 25-40 vol% [66,123,137], Besides, the cathode must be an electron conductor and catalytically active for the oxygen reduction reaction. However, because it is not an oxygen conductor, it must be porous with an optimized three-phase interface at which the reduction reaction takes place [33],... 

The catalysts for oxygen reduction and oxygen oxidation are materials based on substances like Co, Ni, Fe, Mn [16, 19]. One of new applications of oxygen reduction catalysts is air-metal hydride accumulator. Electrodes based on La0.1Cao.4Co03, La0. Cao.,jMn03 [18] are used in this battery. The electrodes of similar composition could be used in SOFC. An insertion of the oxides in their composition (CuO for example) leads to increasing the conductivity of system and efficiency of catalyst [30]. 

Sr-doped LaMnOj (LSM) cathode have been extensively investigated and developed as electrode materials. For oxygen reduction in SOFCs, mixed ionic and electronic conducting (MIEC) materials such as (La, Sr)(Co, FejOj (LSCF) show much higher electrochemical activity than that of LSM. However, MIEC materials based on cobaltites react readily with YSZ electrolytes to form resistive La2Zr207 and SrZrOj phases at the... 

There exist a variety of fuel cells. For practical reasons, fuel cells are classified by the type of electrolyte employed. The following names and abbreviations are frequently used in publications alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Among different types of fuel cells under development today, the PEMFC, also called polymer electrolyte membrane fuel cells (PEFC), is considered as a potential future power source due to its unique characteristics [1-3]. The PEMFC consists of an anode where hydrogen oxidation takes place, a cathode where oxygen reduction occurs, and an electrolyte membrane that permits the transfer of protons from anode to cathode. PEMFC operates at low temperature that allows rapid start-up. Furthermore, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types is not required [4]. 

Temperature Dependency of EIS Measured in SOFC at Open Cell Voltage. In order to calculate the activation energy of the oxygen reduction reaction at the cathode, hydrogen oxidation reaction at the anode and ionic conduction... 

LSM is the most commonly used cathode material in SC-SOFCs. However, the sintering process and the cell operating temperature strongly affect its catalytic activity and selectivity [23, 39, 40]. Under single-chamber operation, the cathode is required only to promote the oxygen reduction [Eq. (2.3)]. However, when sintered at low temperature, LSM shows catalytic activity towards hydrocarbon oxidation [23, 39, 40]. When sintered at higher temperatures, an increased density leads to negligible catalytic activity for methane conversion [23]. 

Solid oxide fuel cells (SOFCs), typically based on an oxide ion conducting electrolyte as previously discussed in earlier chapters, have a fundamental requirement for the high-temperature reduction of the oxidizing species. In general this reaction is viewed as the simple oxygen reduction reaction, as summarized in Eq. 1 ... 

Despite the apparent simplicity of this reaction, the process by which the oxygen reduction occurs followed by incorporation of the ionic species into the electrolyte is the subject of some debate and is dependent on the mode of operation of the cathode material. Two typical cathode types are currently utilized in SOFCs -electronic conductors and mixed ionic-electronic conductors (MIECs). The cathode reactions, while nominally the same in both types of materials, occur at different locations, and hence, the active region varies, leading to differences in the operating regime and ultimately performance. In the case of a single phase electronic conductor. 

High-Temperature Reactions, Cathode, Fig, 2 Illustration of the complex series of processes governing the oxygen reduction and incorporation processes occurring in a SOFC cathode (With permission from Ref. [1])... 

The cathode side of all fuel cells is fed with oxygen. In PEFCs and DMFCs, the oxygen reduction reaction (ORR) completely neutralizes charged particles, while in the SOFC cathode ORR converts electrons into... 

The oxygen reduction reaction occurs at the cathode. Nearly all candidate cathode materials are based on perovskite oxides such as lanthanum strontium manganite (LSM). However, the oxygen reduction reaction is particularly affected by lower operating temperatures with kinetics and transport processes that are thermally activated. The development of materials with higher electrochemical activities for the oxygen reduction reaction is critical for the development of IT-SOFCs with higher efficiencies. 

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