Open-circuit potential change

EOCPC electrochemical open-circuit potential change... 

The open circuit potential as a function of time is a qualitative method that evaluates sacrificial coating stability. The sacrificial metal removal rate in the corroding solution is controlled by the potential difference of the underlying metal and the sacrificial coating, corroding solution, and sacrificial coating thickness. The open circuit potential changes to the protected metal when the sacrificial metal is completely removed. 

In the presence of a first-order relaxation that is not influenced by mass transfer (diffusion), an open-circuit potential change induced by an ILIT perturbation is described by ... 

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. 

Fig. 5. Change of the open-circuit potential with time for steam reforming of methanol over the 30 wt% Ni-SDC and 30 wt% Ni-YSZ electrode-catalyst. Upper Ni-SDC lower Ni-YSZ. Operating conditions 800 °C, 1 atm, H20/CH30H = 2, space time = 0.37 s [9].

This equilibrium open-circuit potential for a H2/air fuel cell is calculated from thermodynamic data of reaction enthalpy and entropy changes. 

Exposure of the B210/GBL post-thermaIly treated system to the salt fog environment for 24 hours gave no rusting of any of the panels. All panels had a rust rating of 10. When the open circuit potentials were plotted (Figure 3), minor changes in potential were observed which paralleled the B210/NVP system. 

Figure 2. Changes in open circuit potential (OPC) and rust rating (RR) with temperature for post-thermally treated Bonderite 40 coated steel (B40) panels and Barex 210/N-vinylpyrrolidone (B210/NVP) photocured coatings on B40 panels.

Based on the literature, the correlation between open circuit potential rust rating and temperature was unexpected. To rule out the fact that the open circuit potential measurements did not simply represent changes in the phosphated substrate due to temperature,... 

B40 panels were also subjected to heat treatment. As shown in Figure 2, no dependence of the open circuit potential on post-thermal treatment temperature is observed. This suggested that the open circuit potential measurements reflected changes at the coating/B40 panel interface and not in the B40 panel alone. 

The open circuit potential data for the B210/NVP system mirrors the behavior of the rust ratings over the temperature range examined. A plausible explanation of the change of the open circuit potential is as follows. As temperature is increased, the composition of the various oxides and hydroxides which make up the zinc phosphate conversion layer and the base iron oxide layer undergo changes. 

These compositional changes are reflected in the changing open circuit potentials. 

In conclusion, we have shown that a simple, fast, electrochemical measurement, the open circuit potential, can be extremely useful in assessing the effect of diluent for high nitrile photocured coatings. Further work is underway to elucidate the underlying reasons behind the temperature induced changes observed in corrosion performance. 

The shape of the potential-time response is determined by the concentration changes of O and R at the electrode surface during electrolysis. The potential is related to Cq/Cr via the Nernst equation for a reversible system. The initial potential before current application is simply the rest potential or open-circuit potential (E ) of the solution, which reflects the initial Cq/C in solution. At the instant of current application, this ratio becomes finite and the potential changes to a value consistent with the Nernst equation. Early in the... 

Fig. 5.14. (c) the photoinduced change in the mass signal (m/e = 44) for carbon dioxide issued from FT0/Ti02 interface (full line) and FT0/Ti02/RuxSey (dashed line). Arrow (1) refers to the initial UV illumination on FT0/Ti02 at open circuit potential arrow (2) refers to the initial UV illumination on FTO/Ti02/RuxSey at open circuit potential and arrow (3) refers to the application of a bias of 0.3 V/SHE under UV illumination for both systems. 

Calculate the free energy change (heat change) of the cell reaction (AH) in calories for two battery systems (a) A lead-acid cell with an open-circuit voltage of 2.01 V at 15 °C and a temperature coefficient of resistance (dE/dT) of 0.0037 V/K. (b) A Zn-Hg cell (Clark cell) with an open-circuit potential of 1.4324 V at 15 °C and a temperature coefficient of 0.00019 V/K. (Bhardwaj)... 

An important measurement is the corrosion potential, Ecor. This is the open circuit potential, whose value can change with time. ECOT is a mixed potential, since the anodic and cathodic reactions are different. The partial anodic or cathodic current that flows at this potential is called the corrosion current, 7cor, and is directly related to the rate constant of the electrode reaction. 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

The time range of the electrochemical measurements has been decreased considerably by using more powerful -> potentiostats, circuitry, -> microelectrodes, etc. by pulse techniques, fast -> cyclic voltammetry, -> scanning electrochemical microscopy the 10-6-10-1° s range has become available [iv,v]. The electrochemical techniques have been combined with spectroscopic ones (see -> spectroelectrochemistry) which have successfully been applied for relaxation studies [vi]. For the study of the rate of heterogeneous -> electron transfer processes the ILIT (Indirect Laser Induced Temperature) method has been developed [vi]. It applies a small temperature perturbation, e.g., of 5 K, and the change of the open-circuit potential is followed during the relaxation period. By this method a response function of the order of 1-10 ns has been achieved. 

An adaptation of the temperature-jump method, named indirect laser-induced temperature jump [29], was used in studies of distance dependence of electron transfer at electrodes. A pulsed Nd YAG laser was used to cause a sudden (<5 ns) change in temperature (<5 K) at an electrode/electrolyte interface. The increase in temperature causes a change in the open-circuit potential. The relaxation step is a function of the dissipation of thermal energy and the rate of electron transfer between the electrode and its redox partners. 

In the earliest treatment of open-circuit potential-decay transients (729), C was identified with the double-layer capacitance, C, but it was recognized (cf. Refs. 105, 129) that this formulation did not account for changes in the coverage fractions by any electroactive intermediates involved. Conway and co-workers (126-128) were the first to treat the problem with allowance for changes in coverage of the adsorbed intermediate. However, C was interpreted as the sum of Cj, and C, and the potential-decay behavior for several... 

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