Oxygen reduction reaction activation polarization

Figure 1 Correlation of specific activity for oxygen reduction reaction with particle size measured as steady-state polarization values with different electrolytes, (a) 98% H3PO4, 180 °C, (b) 0.5 M H2SO4, 25 °C, and (c) 97% H3PO4, 177 °C. Also superimposed is the surface-averaged distribution (SAD) of (100) sites (solid line). (From Ref. 4.)...

Development of supported Pt electrocatalysts came as a result of intensive research on fundamental and applied aspects of electrocatalysis [especially for kinetically difficult oxygen reduction reaction (ORR)] fueled by attempts at commercialization of medium-temperature phosphoric acid fuel cells (PAFCs) in the late 1960s and early 1970s. Dispersion of metal crystallites in a conductive carbon support resulted in significant improvements in all three polarization zones (activation, ohmic, and... 

Fuel Cells, Non-Precious Metal Catalysts for Oxygen Reduction Reaction, Fig. 4 (a) Polarization curves of several Me-N-C catalysts all examined under the same experimental conditions (1.5 bar H2/O2, 1 mg cm see Ref. [12] et al. for details), (b) today s most active NPMC catalysts (p-Fe-N-C) as prepared by the pore-filling method (PFM) described in Lefevre et al. [44] and by the method described by Proietti et al. [10]. The values... 

Note that by definition, in electrochemistry, the reversible potential of the hydrogen oxidation reaction is zero at all temperatures [5]. That is why the standard hydrogen electrode is used as a reference electrode. Therefore, for hydrogen anodes E,a = OV. Activation polarization of the hydrogen oxidation reaction is much smaller than activation polarization of the oxygen reduction reaction. 

Figure 3-8 shows how the cell polarization curve is formed, by subtracting the activation polarization losses, ohmic losses, and concentration polarization losses from the equilibrium potential. Anode and cathode activation losses are lumped together, but, as mentioned before, a majority of the losses occiu on the cathode because of sluggishness of the oxygen reduction reaction. 

These examples are based on both electrodes operating in the activation polarization regime, in which the logarithm of the current is proportional to the overpotential. However, there are situations - particularly at low concentrations - in which the electrochemical reaction is limited by mass transport to the electrode surface. This is referred to as concentration polarization, and is illustrated in Figure 13.2d. In this case, above a critical overpotential the current becomes constant, which appears as a vertical line in the plot. A new mixed potential is established at the intersection of this vertical line and the cathode polarization for the oxygen reduction. This potential depends on the gas concentration, and thus can be used for the chemical sensor signal. 

We see, as a consequence, that the anodic oxygen production on the part of w-type oxide may be coupled with the cathodic oxygen reduction and/or with the cathodic hydrogen production on the part of the metal surface. The polarization curves of these reactions are schematically shown for a passive metal in Figure 22.36 and for an active metal in Figure 22.37. 

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