Enhancement Factor or Faradaic Efficiency

As already noted, the enhancement factor or faradaic efficiency, A, is defined from  

In the experiments of Figs. 4.13 and 4.14, the A value at steady-state are 74,000 and 770 respectively. A reaction is said to exhibit the NEMCA effect when A 1. 

As shown on the figure, small I/2F values lead to A values up to 3105. This implies that each O2 supplied to the catalyst causes up to 3- 10s coadsorbed O atoms to react and form C02 and H20. 

An important step in the elucidation of the origin of NEMCA was the observation1,19 that the magnitude of A for any reaction can be estimated from  

As analyzed in section 4.6, equation (4.20) can be derived theoretically and is in good qualitative agreement with experiment over five orders of magnitude (Fig. 4.23). This important relationship shows that  

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Enhancement Factor or Faradaic Efficiency A. - This is defined from ... 

Figure 11 Comparison of predicted and measured enhancement factor or Faradaic efficiency A values for some of the catalytic reactions already found to exhibit the NEMCA effect (Table 1). Reprinted with permission from Elsevier Science Ltd.

Table 13.4 provides a list of the reactions which have been studied already and shown to exhibit NEMCA. In order to compare different catalytic reactions, it is useful to define two dimensionless parameters, i.e., the enhancement factor or faradaic efficiency A and the rate enhancement ratio p. The former is defined as ... 

The NEMCA effect was first demonstrated on Pt and Ag electrodes using 8 mol% YjOj-stabilized ZrOj (YSZ), an O " conductor, as the solid electrolyte. Electrochemical 0 pumping to the catalyst electrode was found to cause an up to 60-fold (6000%) steady-state reversible enhancement in the rate of oxidation. Furthermore, this steady-state rate increase was found to be up to 3 x 10 times higher than the steady-state rate of supply of O to the catalyst, i.e., the enhancement factor A or apparent faradaic efficiency for the process is 3 x 10 . More recently, the rate of C2H4 oxidation on Rh electrodes supported on YSZ was found to reversibly increase by a factor of 90, again with faradaic efficiency values of the order of 10 . ... 

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