Rate and Faradaic efficiency

Table I. Rate and Faradaic Efficiency of CH4 and CH3OH formation from CO at...

Figure 6. Plot of In of methane formation rate vs temperature and faradaic efficiency vs temperature for electrochemical reduction of CO2 at Ru electrodes.

Fig. 4 (a) Oxygen reduction reaction pathways in the absence Qeft) and in the presence (right) of a contaminant. The presence of a contaminant affects the surface coverage G, oxygen reduction reaction product selectivity (j), and faradaic efficiency 0. (b) Platinum dissolution rate in the absence (top) and in the presence (bottom) of sulfur contamination either from the air intake or from the carbon support. The increased platinum dissolution rate favors a decrease in ionomer ionic conductivity. 

There exists, however, a second, approximate, way of estimating A on the basis of galvanostatic rate transients as outlined in section 5.2 and shown in Figure 5.6a. This approximate method is useful for gaining additional physical insight on the meaning of the faradaic efficiency A and for checking the internal consistency of experimental data with the ion backspillover mechanism. 

Methanol oxidation on Ag polycrystalline films interfaced with YSZ at 500°C has been in investigated by Hong et al.52 The kinetic data in open and closed circuit conditions showed significant enhancement in the rate of C02 production under cathodic polarization of the silver catalyst-electrode. Similarly to CH3OH oxidation on Pt,50 the reaction exhibits electrophilic behavior for negative potentials. However, no enhancement of HCHO production rate was observed (Figure 8.48). The rate enhancement ratio of C02 production was up to 2.1, while the faradaic efficiencies for the reaction products defined from... 

Figure 8.68 shows a typical galvanostatic transient under oxidizing gaseous conditions. The reaction rate is enhanced by a factor of 20 (p=21) and the faradaic efficiency A (=Ar/(I/2F)) is 1880. The behaviour is clearly electrophobic (dr/dV Xi) and strongly reminiscent of the case of C2H4 oxidation on Pt/YSZ (Fig. 4.13) with some small but important differences ... 

At t=0 a constant anodic current I=5mA is applied between the Pt catalyst film and the counter electrode. The catalyst potential, Urhe, reaches a new steady state value Urhe=1.18 V. At the same time the rates of H2 and O consumption reach, within approximately 60s, their new steady-state values rH2-4.75T0 7 mol/s, ro=4.5T0 7 mol/s. These values are 6 and 5.5 times larger than the open-circuit catalytic rate. The increase in the rate of H2 consumption (Ar=3.95T0 7 mol H2) is 1580 % higher than the rate increase, (I/2F=2.5T0 8 mol/s), anticipated from Faraday s Law. This shows clearly that the catalytic activity of the Pt catalyst-electrode has changed substantially. The Faradaic efficiency, A, defined from ... 

Figure 9.26 shows the steady state effect of applied current I on the induced changes, ArH2(=rH2 -r 2) and Ar0(=ro-io )> in the rates of consumption of H2 and O respectively, where the superscript o always denotes open-circuit conditions. The dashed lines in Fig. 9.26 are constant Faradaic efficiency, A, lines. The maximum measured A values are near 40 at low current densities. This value is in excellent qualitative agreement with the following approximate expression which can predict the magnitude of A in NEMCA studies ... 

The addition of a spillover proton to an adsorbed alkene to yield a secondary carbonium ion followed by abstraction of a proton from the C3 carbon would yield both isomers of 2-butene. The estimated faradaic efficiencies show that each electromigrated proton causes up to 28 molecules of butene to undergo isomerization. This catalytic step is for intermediate potentials much faster than the consumption of the proton by the electrochemical reduction of butene to butane. However, the reduction of butene to butane becomes significant at lower potentials, i.e., less than 0.1V, with a concomitant inhibition of the isomerization process, as manifest in Fig. 9.31 by the appearance of the maxima of the cis- and tram-butene formation rates. 

The break in the plot log I vs coincides with the observed inflection in rH2 and r0, and corresponds to the onset of Pt oxide formation.6 As shown in Fig. 10.3 the, predominantly catalytic, rates rH2 and r0 depend exponentially on catalyst potential Uriie, as in studies with solid electrolytes with slopes comparable with the Tafel slopes seen here. This explains why the observed magnitude of the faradaic efficiency A (-2-20) is in good agreement with 2F rc° /I0 (rc° is the open-circuit catalytic rate and I0 is the exchange current) which is known to predict the expected magnitude of A in solid-electrolyte studies. 

Faradaic efficiency, A, values up to 6 and rate enhancement, p, values of at least up to 13 were obtained in this study which was carried out at 50 atm pressure and using a 24-catalyst pellet reactor.16... 

Amounts of are being monitored at a wall-jet electrode. When a sample of concentration 3.23 pg cm" is squirted over the electrode, the limiting current is 152 pA. Keeping the flow rate and all other parameters constant, what is the concentration of a sample of Co when the limiting current is 214 pA Assume complete faradaic efficiency in both cases. 

When Bandi and Kuhne studied the reduction of C02 to methanol at mixed Ru02 + Ti02 electrodes (ratio 3 1) produced by coating titanium foil [65], in a C02-saturated KHC03 solution at a current density of 5 mA cm 2, only minimal C02 reduction was observed. However, the addition of electrodeposited Cu led to faradaic efficiencies of up to 30% for methanol at potentials of approximately -0.972V (versus SCE). Trace amounts of formic acid and ethanol were also observed. In this case, the rate-limiting step was surmised to be the surface recombination of adsorbed hydrogen and C02 to yield adsorbed COOH". 

The magnitude of the NEMCA effect for a given catalytic system is commonly described by two parameters, the rate enhancement ratio, p, (= r/r0, where r and rQ are the electropromoted and unpromoted reaction rate values) and the faradaic efficiency, A, (= (r - r0)/(I/nF)), where I is the current, F is the Faraday constant, and n is the charge of the promoting ion. The magnitude of A can be predicted from the parameter 2Fro/I0, where I0 is the exchange current of the catalyst-support interface [v]. 

Here L is the catalyst film thickness or nanoparticle size, k is the rate constant for depletion (reaction or desorption) of the promoting O species, Q max is its maximum possible surface concentration on the catalyst or nanoparticle surface, Ac is the metal-gas interface area of the film or nanoparticle, r is the promoted catalytic rate, and A is the Faradaic efficiency of the catalytic reaction being promoted. 

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