CLOSED-CIRCUIT CONDITION

Excitation circuit Typical parameters of the CT Resistance of instruments of 7.5VA (a) Under energized and closed circuit condition. 

It is worth emphasizing that Eq. (5.22) is valid under both open-circuit and closed-circuit conditions and that it holds for any part of the surfaces of the catalyst and the reference electrodes. Thus, referring to the metal electrode surfaces in contact with the electrolyte (region E) it is ... 

It is important to note that equation (7.11), and thus (7.12) is valid as long as the effective double layer is present at the metal/gas interfaces. Therefore equation (7.11) is valid not only under open-circuit conditions (which is the case for the Nernst equation) but also under closed-circuit conditions, provided, of course, that the working electrode effective double layer is not destroyed. Consequently the importance of equation (7.11) is by no means trivial. 

Figure 8.26 shows the dependence of the steady-state rate on the partial pressure of methane (Fig. 8.26a) and oxygen (Fig. 8.26b) at 400°C both for open and closed circuit conditions.31... 

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 9.28. Effect of P02 on the rate of O consumption (u.) and corresponding catalyst potential ( ), U°ile, under open-circuit conditions and on the rate of O consumption (A) and corresponding A0 value under closed-circuit conditions at fixed catalyst potential Urhe-1.05 V total molar flowrate fm=l.7 l0 4 mol/s.35 Reproduced by permission of The Electrochemical Society.

Last but not least, one should check the inertness of the auxiliary electrodes in single-pellet arrangements, both under open and closed circuit conditions and, also, via the closure of the carbon balance, the appearance of coke deposition. This is especially important in systems with a variety of products (e.g. selective oxidations), where the exact value of selectivity towards specific products is of key interest. This in turn points out the importance of the use of a good analytical system and of its careful calibration. 

A typical electrochemical promotion experiment includes kinetic measurements under open and closed circuit conditions as well as study of the effect of catalyst potential or work function on catalytic rate and selectivity under steady state and transient conditions. In kinetic measurements one should change the partial pressure of each reactant while... 

The stiffness coefficients can be derived from the compliances using the following relations which hold for either open- or closed-circuit conditions and also with the symbols c and s interchanged ... 

Under closed-circuit conditions, the electrochemical reactions involve a number of sequential steps, including adsorption/desorption, surface diffusion of reactants or products, and the charge transfer to or from the electrode. Charge transfer is restricted to a narrow (almost one-dimensional) three-phase boundary (tpb) among the gaseous reactants, the electrolyte, and the electrode-catalyst. 

Closed Circuit Condition and Steady-State Potentials in a PEVD System... 

Under closed circuit conditions, the electric potential between the two electrodes (C) and (W) is set by an applied dc electric potential V. ... 

Fig. 5 Potential profiles inside the solid electrolyte (E) and product (D) under closed-circuit conditions.

The current relation in Eqn. 20 is achieved by adjusting the chemical potential of (A) at (II) under closed circuit conditions. Since the gradient in electrochemical potential is the driving force for the flow of charged particles in the multiphase PEVD system, the current density carried by (A+) in either the solid electrolyte (E) or deposit (D) can be written as ... 

Despite the growing interest in S EM Rs during the past 25 years, there is at present no industrial application of these reactors, except in the case of potentiometric sensors. Examples of the application of membrane reactors operating under closed-circuit conditions are briefly described in the following sections, with special emphasis on the laboratory set-ups used for materials studies and on membrane reactors presently in the pre-commercial phase, such as SOFCs or oxygen generators. 

Acrylaldehyde, carbon monoxide and carbon dioxide were observed as the main products under the closed circuit conditions. The... 

Part II Multicomponent Mixed Conductors Under Closed Circuit Conditions... 

Figure 7.14 Principle of aeration cells operating under (a) open and (b) closed circuit conditions. corresponds to the external resistance.

The photoproduction and subsequent separation of electron-hole pairs in the depletion layer cause the Fermi level in the semiconductor to return toward its original position before the semiconductor-electrolyte junction was established, i.e., under illumination the semiconductor potential is driven toward its flat-band potential. Under open circuit conditions between an illuminated semiconductor electrode and a metal counter electrode, the photovoltage produced between the electrodes is equal to the difference between the Fermi level in the semiconductor and the redox potential of the electrolyte. Under close circuit conditions, the Fermi level in the system is equalized and no photovoltage exists between the two electrodes. However, a net charge flow does exist. Photogenerated minority carriers in the semiconductor are swept to the surface where they are subsequently injected into the electrolyte to drive a redox reaction. For n-type semiconductors, minority holes are injected to produce an anodic oxidation reaction, while for p-type semiconductors, minority electrons are injected to produce a cathodic reduction reaction. The photo-generated majority carriers in both cases are swept toward the semiconductor bulk, where they subsequently leave the semiconductor via an ohmic contact, traverse an external circuit to the counter electrode, and are then injected at the counter electrode to drive a redox reaction inverse to that occurring at the semiconductor electrode. 

Pekridis G, Kalimeri K, Kaklidis N, Vakouftsi E, Iliopoulou EF, Athanasiou C, Mamellos GE (2007) Study of the reverse water gas shift (RWGS) reaction over Pt in a solid oxide fuel cell (SOFC) operating under open and closed-circuit conditions. Catal Today 127 337-346... 

Equation (13.40) is always valid provided the ealalyst and reference electrodes are made of the same material.It is also valid when other types of solid electrolytes are used, e.g., P"-Al203, aNa+ conductor. Equation (13.40) is mnch more general and fundamental than Equation (13.35), as it does not require the establishment of ai r specific electrochemical equilibrium. It shows that solid electrolyte cells are work function probes for their gas-exposed electrode surfaces. It also shows that SEP is essentially a work function measuring technique and that several aspects of the SEP literature reviewed in References 92,97-99 must be reexamined in the light of these findings. One can still use SEP to extract information about surface activities, provided the nature of the electrocataly tic reaction at the tpb is well known, but, even when this is not the case, the cell emf still provides a direct measure of the work function difference between the two gas-exposed electrode surfaces. Equation (13.40) also holds under closed-circuit conditions, as is further discussed in Section III.B. 

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