Hydrogen equivalent circuit

H. J. ENGELL The paper of Efimov and Erusalimchik is very brief. They give no equivalent circuit nor do they discuss their method of measurement thus, it is difficult to say just what they have measured. Their measurements were made, however, at a potential of about -500 mv in acid solutions. Under these conditions the rate of hydrogen evolution is relatively fast therefore, it is possible that their minimum in the capacity could have been influenced by kinetic effects. I should also point out that we obtained higher values of the capacity when we did not etch the surface by applying an anodic current before each experiment. 

The resolution of the latter two steps in the overall interfacial process of hydrogen electrooxidation should be contrasted with the impedance spectrum for the ORR at a smooth Pt/Nafion membrane interface (see Fig. 6), that includes only a single feature associated with a single rate-limiting interfacial charge-transfer step. Figure 12 shows an equivalent circuit for the process at the smooth Pt/Nafion membrane... 

Figure 3.3B. Simplified equivalent circuit for the fuel cell reactor model. The battery voltage, Vb, is the result of the chemical potential difference for hydrogen between the anode and cathode. The internal resistance, R ib, is primarily the resistance of the membrane. Rl is the external load impedance. V is the voltage across the external load and I is the current through the load.

Fig. 7.4 Electrical equivalent circuit of faradaic impedance corresponding to HER with hydrogen diffusion,...

Fig. 7.7 Electrical equivalent circuit for faradaic impedance of direct hydrogen absorption in presence of hydrogen adsorption-evolution...

Fig. 7.8 Electrical equivalent circuits of faradaic impedance corresponding to (a) indirect, Eq. (7.81), and (b) direct, Eq. (7.83), hydrogen absorption reaction with finite-length linear diffusion of hydrogen...

Fig. 14.6 Electrical equivalent circuits describing faradaic impedance of hydrogen evolution reaction...

Furthermore, by varying other experimental conditions such as current load, temperature, gas composition, and as recently shown by Andreaus et al. [2002] hydrogen humidification and membrane thickness, measured cell impedance can be split into anode impedance, cathode impedance and electrolyte resistance, without using reference electrodes. These results were used to derive appropriate equivalent circuits for the analysis of impedance spectra measured on fuel cells operating with H2/O2, H2/air and H2 + lOOppm CO/O2. The variation of the experimental conditions is also a useful method to confirm the accuracy of the equivalent circuit. 

Figure 4.5.67. Equivalent circuit used for evaluation of interpolated (time dependent) impedance spectra during hydrogen oxidation and CO poisoning of the anode and oxygen reduction at the fuel cell cathode.

In order to explain the water vapor influence, one has to refer to the following equations. According to Geyer et al. [1997], the dependence of the anodic charge transfer resistance (Ret,a in the equivalent circuit used) on the mole fraction of water vapour (X) in the gas can be explained by assuming a Butler-Volmer kinetic where the exchange current density (to) is proportional to powers of the concentrations of the reactants (hydrogen and water vapour) ... 

FIGURE 6.21 Nyquist plots with impedance arcs (left) corresponding to the simplified equivalent circuit model (right). (Adapted from Rubio, M. A., Urquia, A., and Domido, S. 2010. International Journal of Hydrogen Energy 35 2586-2590.)... 

On the basis of the equivalent circuit adopted, the slope controlled by the iron dissolution, /Fe/Fe oxide formation, /pe/FeOx hydrogen evolution, /h /h2 is given by the expression... 

The effectiveness of EIS can be greatly enhanced with the use of a reference electrode, which has a stable potential at the time of measurement [3]. A suitable reference electrode allows discernment of the different electrode losses from the overall cell response, resulting in a more appropriate equivalent circuit. Ideally, the collective responses of the anode and cathode will add to the full cell resistance. Because the use of a stable reference electrode in many fuel cell systems is difficult, one common way to examine fuel cell behavior is the use of a dynamic hydrogen electrode (DHE). In this case, one of the electrodes is used as the DHE, with hydrogen flow at this location. It is assumed that the losses associated with the DHE are minor, and all polarizations measured can be attributed to the other electrode. This approach can be dubious and is not appropriate when there are phenomena at the DHE that can affect losses, such as anode dryout in a PEFC. Note that the DHE does not have to be the actual anode in the fuel cell but can be used at either electrode to examine the polarization of the opposing electrode. For example, a DHE can be used at the cathode of a DMFC to examine the polarization behavior of the anode in the DMFC. In this case, of course, the reaction does not galvanically proceed in the desired direction, and external power from a galvanostal/potentiostat system must be applied to drive the reaction in the desired direction. 

FIGURE 7-19 Equivalent circuit diagram (A) and representative data (B)for hydrogen evolution with effects of UPD, adsorption, absorption, and diffusion reactions... 

Since the highest possible Fermi level of the photoexcited n-type anode corresponds to the flat band potential of the semiconductor anode, the Fermi level of the metallic cathode short-circuited with the photoexcited n-lype anode can also be raised up to the level equivalent to the flat band potential of the semiconductor anode. In order for the cathodic electron transfer of hydrogen redox reaction to proceed at the metallic cathode, the Fermi level 1 of the cathode needs to be higher than the Fermi level of hydrogen redox reaction. Consequently, in... 

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