Oxygen open circuit potential

The open-circuit potential of most metals in sea water is not a constant and varies with the oxygen content, water velocity, temperature and metallurgical and surface condition of the metal. 

Figure 9.28 shows the dependence of the catalytic rate of oxygen consumption, r0, on the oxygen partial pressure P02 at fixed pH2 under open-circuit conditions and for a potentiostatically fixed catalyst potential Urhe (=UWr). As also shown in Fig. 9.28, the open-circuit potential UrhE increases from 0.33 to 0.8 V as the Po Ph2 ratio increases from 0.2 to 3.6. 

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

Fuel The influence of fuel gas composition on the theoretical open circuit potential of SOFCs is illustrated in Figure 8-12, following the discussion by Sverdrup, et al. (8). The oxygen/carbon (0/C) atom ratio and hydrogen/carbon (H/C) atom ratio, which define the fuel composition, are plotted as a function of the theoretical open circuit potential at 1000°C. If hydrogen is absent from the fuel gas, H/C = 0. For pure CO, 0/C = 1 for pure CO2, 0/C = 2. The data in the figure... 

Palmore et al. first demonstrated the use of ABTS in a biofuel cell cathode, combining it with laccase from Pyricularia oryzae ABTS was dissolved at 2 mM in oxygen-saturated 0.2 M acetate buffer, pH 4, 25 °C. With a glassy carbon working electrode, an open-circuit potential of 0.53 V vs SCE was observed, reflecting the presence of HABTS in low-pH solution. Protonation of ABTS shifts the redox potential to 0.57 V vs SCE. With negligible stirring, current densities of 100 / A/cm were achieved at an electrode potential of 0.4 V vs SCE. 

Figure 1, Polarization curve for the electrochemical reduction of oxygen coupled with the electrochemical oxidation of an unspecified organic impurity O.C, designates the open circuit potentials...

Another form of this definition [equation (3.6.15)] has sparked much debate in the scientific community [121-124]. In this approach Vapp (or Vbias) is taken as the absolute value of the difference between the potential at the working electrode measured with respect to a reference electrode (Vmeas) and the open circuit potential (Voc) measured with respect to the same reference electrode under identical conditions (in the same electrolyte solution and under the same illumination). In the case of a semiconductor photoanode where oxygen evolution takes place the efficiency is calculated as ... 

The measurement of open circuit potential of the catalyst during the liquid phase oxidation of alcohols provides a unique insight into the redox processes taking place on the catalyst surface. A Pt catalyst stored in air contains surface oxides and in an aqueous Na2C03 solution it behaves as an oxygen electrode. Its potential is 250-280 mV when referred to a Ag/AgCl/KCl(3 ) electrode (Figure 3). When the catalyst is... 

The process of irreversibly adsorbed Sb on Au(lll) at the open-circuit potential (close to 0.2 V) has been investigated using in situ STM [474]. The oxygenated Sb adlayer was nucleated and grown on terraces and at step edges. The oxygenated Sb domains present on terraces were roundshaped islands of a diameter ranging between 3 and 6 nm. 

The cell reaction is the transfer of oxygen from one side to the other. (See also Lambda probe). In the case of an - electrochemical equilibrium (subentry of -> equilibrium) the measured -> open circuit potential (subentry of - potential) or -> equilibrium potential (subentry of -> potential) Ueq or E (emf) can be calculated by the -> Nernst equation ... 

Fig. 15. The pit nucleation potential ( np) of a clean surface of alloy 600 in deaerated NaCl solutions as a function of chloride concentration and temperature, and the open-circuit potential (Eop) of Alloy 600 in water as a function of dissolved oxygen concentration and temperature [35]. Reproduced with permission. Copyright 1985 by the American Nuclear Society, La Grange Park, Illinois.

Fig. 5.43. Current-potential curves for ethanol oxidation and oxygen reduction on smooth and platinized platinum. OCP=Open Circuit potential (catalyst potential at which no net current flows).

From Figure 1.1.4, one can see that the open-circuit potential is not well defined in the system under discussion. One can say only that the open-circuit potential lies somewhere between the background limits. The value found experimentally will depend upon trace impurities in the solution (e.g., oxygen) and the previous history of the Pt electrode. 

This electrode, when combined with an oxygen electrode would yield a cell with an open-circuit potential of 1.21 V. A fuel cell based on methanol and air in KOH solution has been used to power television relay stations. All the reactions in Table 17.3 would yield cells with potentials of about one volt. 

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