Low-overpotential method

One possible strategy in the development of low-overpotential methods for the electroreduction of C02 is to employ a catalyst in solution in the electrochemical cell, A few systems are known that employ homogeneous catalysts and these are based primarily on transition metal complexes. A particularly efficient catalyst is (Bipy)Re[CO]3Cl, where Bipy is 2,2 bipyridine, which was first reported as such by Hawecker et al. in 1983. In fact, this first report concerned the photochemical reduction of C02 to CO. However, they reasoned correctly that the complex should also be capable of catalysing the electrochemical reduction reaction. In 1984, the same authors reported that (Bipy)Re[C013CI catalysed the reduction of C02 to CO in DMF/water/ tetraalkylammonium chloride or perchlorate with an average current efficiency of >90% at —1.25 V vs. NHE (c. —1.5V vs. SCE). The product analysis was performed by gas chromatography and 13C nmr and showed no other products. 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

Pahnore GTR, Bertschy H, Bergens SH, Whitesides GM. 1998. A methanol/dioxygen biofuel cell that uses NAD -dependent dehydrogenases as catalysts Application of an electro-enzymatic method to regenerate nicotinamide adenine dinucleotide at low overpotentials. J Electroanal Chem 443 155-161. 

Equation (50) forms the basis upon which v can be evaluated (e.g. (1) by the radioactive tracer method to evaluate simultaneously and ), (2) by comparing i values at appropriate potentials for different reactant activities (3) coupling information from high and low overpotential regions of steady-state polarization curves " (extrapolated io and charge-transfer resistance, Rcr, respectively) (4) or by back-reaction correction analysis. 2 qqie first two methods involve determination of v at any single potential while the latter two procedures must assume that the same mechanism (and hence v) applies at different potentials (at which individual measurements are required) and that the reverse reaction occurs by the same path and has the same transition state and thus rate-determining step [for both forward (cathodic) and reverse reactions]. 

Allen and Hickling (34) suggested an alternative method allowing the use of data obtained at low overpotentials. Equation 3.4.11 can be rewritten as... 

Although Pd has superior initial performance at low overpotential, it is quickly deactivated over time. Pan et al. from Tekion Inc. showed that at 40 °C in a DFAFC enviromnent 30 % of the power was lost in the first 3 h of operation [49]. The degradation is attributed to the accumulation of COads like species on the Pd nanoparticle surface during continuous operation [49-52]. Reactivation of the surface has been demonstrated by both electrochemical [49, 53, 54] and non-electrochemical [51] methods. This activity instability drives the search for more stable and active catalysts. 

Polarization curves of protCHi exchange membrane fuel cells deviate from the simulated curve in Fig. 3a. Most significant is the low open-circuit potential of 0.9-1.0 V, as opposed to the reversible potential of approximately 1.2 V. The low open-circuit potential has a number of contributing factors, which include multiple reactions that set up a mixed potential, crossover of H2 or O2 through the membrane, and finite resistance effects of voltage measurement devices. These effects cannot be modeled with the overpotential method discussed here. Nonetheless, it is interesting to see how well the overpotential model can approximate a real curve. 

The electrochemical kinetics from Tafel slopes obtained at low overpotentials (region 1 in Figure 10.5) is related to the monomer oxidation on the metal and the polymerization initiation. At high overpotentials after the change of slope (region 2 in Figure 10.5) a reaction order equal to that obtained from microgravimetric determinations is attained. Both empirical kinetics overlap the one obtained from Tafel slopes at low overpotentials when a polypyrrole electrode is used. This means that the Tafel slope is an adequate method to study electrochemical polymerization (monomer oxidation on the polymer) when a polymeric electrode is used. 

The catalytic activity of a chalcogenide towards ORR can reach a level of 30—40% of the Pt catalyst activity. The Tafel slope varies from 100 mV/dec up to 167 mV/dec, depending on the materials used and the preparation method. Duron et al. [66] reported two Tafel slopes for ORR on a RuxSy(CO)n cluster 124 mV/dec at low overpotential and 254 mV/dec at high overpotential. Susac et al. [72] reported a Tafel slope of 167 mV/dec on Co-Se catalyst. There seems to be no reported data in the literature about the exchange current density of ORR catalyzed by chalcogenides. 

We have already mentioned that linear Tafel plots cannot be obtained by simple elimination of one of the exponents in Eq. (5.2) or (5.6), because the remaining EAC surface concentration still depends on the overvoltage. In this connection, the possibilities to carry out experiments at constant EAC concentration have attracted considerable attention. RDE voltammetry making it possible to realize this condition was suggested by Miller et al. [12, 13] and was referred to as isosurface concentration voltammetry (ICV). This method was substantiated theoretically for quasi-reversible reduction in relatively simple systems (metal- metal ions and redox systems) for low overpotentials [12-14] and for the Tafel regions [15,16]. Later, complex systems were considered in this respect [17]. 

The problem with rapid electrode reactions is that they become mass transport controlled at a rather low overpotential, masking any possibility for the determination of kinetic parameters. In such cases it is desirable to have methods which require only very small overpotential so that diffusion is still a minor factor in the kinetics. The measurement of cell impedance at the equilibrium potential gives, under certain conditions, such a possibility. 

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