Oxygen reduction interface

An important consequence of ion migration is the formation of cells where the coated surface acts as a cathode and the exposed metal at the damage acts as an anode (see Section 4.3). The reason for this is that at the metal/coating interface, the cathodic partial reaction of oxygen reduction according to Eq. (2-17) is much less restricted than the anodic partial reaction according to Eq. (2-21). The activity of such cells can be stimulated by cathodic protection. 

Parthasarathy A, Srinivasan S, Appleby AJ, et al. 1992a. Temperature dependence of the electrode kinetics of oxygen reduction at the platinum/Nafion interface—A microelectrode investigation. J Electrochem Soc 139 2530-2537. 

Fig. 3. Oxygen transport in solids. 02 is dissociated and ionized at the reduction interface to give O2 ions, which are transferred across the solid to the oxidation interface, at which they lose the electrons to return back to 02 molecules that are released to the stream, (a) In the solid electrolyte cell based on a classical solid electrolyte, the ionic oxygen transport requires electrodes and external circuitry to transfer the electrons from the oxidation interface to the reduction interface (b) in the mixed conducting oxide membrane, the ionic oxygen transport does not require electrodes and external circuitry to transfer the electrons to the reduction interface from the oxidation interface, because the mixed conductor oxide provides high conductivities for both oxygen ions and electrons.

Basura, V. L, Beattie, P. D. and Holdcroft, S. 1998. Solid-state electrochemical oxygen reduction at Pt —> Nation 117 and Pt —> BAM3G 407 interfaces. Journal ofElectroanalytical Chemistry 458 1-5. 

Zhang, L., Ma, C. and Mukerjee, S. 2004. Oxygen reduction and transport characteristics at a platinum and alternative proton conducting membrane interface. Journal of Electroanalytical Chemistry 568 273-291. 

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.

Figure 25. Adler s ID macrohomogeneous model for the impedance response of a porous mixed conducting electrode. Oxygen reduction is viewed as a homogeneous conversion of electronic to ionic current within the porous electrode matrix, occurring primarily within a distance A from the electrode/electrolyte interface (utilization region). (Adapted with permission from ref 28. Copyright 1998 Elsevier.)...

Figure 4. Reaction steps in oxygen reduction at the electrocatalyst in a PEFC (left) and a postulated molecular scale structure of the interface (right).

Figure 10. (a) Schematic illustration of EDL at solid/liquid interface, (b) Configuration of reactants in oxygen reduction reaction (ORR) in ELD at Pt/acidic solution interface. 

Fig. 13.27. Potential vs. current density plots for state-of-the-art fuel cells, o, proton exchange membrane fuel cell , solid oxide fuel cell , pressurized phosphonic acid fuel cell (PAFC) a, direct methanol fuel cell, direct methanol PAFC , alkaline fuel cell. (Reprinted from M. A. Parthasarathy, S. Srinivasan, and A. J. Appleby, Electrode Kinetics of Oxygen Reduction at Carbon-Supported and Un-supported Platinum Microcrystal-lite/Nafion Interfaces, J. Electroanalytical Chem. 339 101-121, copyright 1992, p. 103, Fig. 1, with permission from Elsevier Science.)...

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],... 

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