Electrolyte voltage drop

A negative (cathodic) potential of a least 850 mV with the cathodic protection applied. This potential is measured with respect to a saturated copper/copper sulfate reference electrode contacting the electrolyte. Voltage drops other than those across the structure-to-electrolyte boundary must be considered for valid interpretation of this voltage measurement. 

The Rpw resistance value depends on the electrode-electrolyte voltage drop as more redox reactions occur at higher voltage magnitudes and thus more electrons are transferred. Therefore, the higher this voltage the lower the resistance value. 

Consider the cyclic voltammetry trace of electrically activated iridium oxide (the so called AIROF) which features reversible reactions (Fig. 3.3). The scan rate is very slow, so the dynamic behavior of the Helmholtz capacitance has a negligible effect on the measured trace. The positive peaks A and B correspond to two distinct oxidation reactions at the surface of the electrode, pertaining to different electrode potentials. The negative peaks C and D correspond to reduction reactions. C matches A and D matches B, as they have similar shape. The reduction potential peak (for example at C, Epc) does not happen at a negative electrode-electrolyte voltage drop, but at a positive one even near to the potential where oxidation potential peak (at A, Epa) is located. If the surface redox reactions are fast and the reaction rate is limited by the diffusion of the reactants in the solution, the difference between the oxidation and reduction peaks is only 59 mV/n for a reaction where n electrons are transferred in the stoichiometry of the reaction. This state is called electrochemical reversibility, which means that the thermodynamic equilibrium in the redox reaction at the surface is established fast at every applied electrode potential. Note that this concept is not the same as the chemical reversibility explained before. A system can be electrochemically irreversible but chemically reversible. As seen in Fig. 3.3, iridium oxide is already electrochemically irreversible even at the very slow potential ramp of 50 mV/s, as the , 4 — is already larger than 59 mV. 

From an electrochemical viewpoint, stable pit growtli is maintained as long as tire local environment witliin tire pit keeps tire pit under active conditions. Thus, tire effective potential at tire pit base must be less anodic tlian tire passivation potential (U ) of tire metal in tire pit electrolyte. This may require tire presence of voltage-drop (IR-drop) elements. In tliis respect the most important factor appears to be tire fonnation of a salt film at tire pit base. (The salt film fonns because tire solubility limit of e.g. FeCl2 is exceeded in tire vicinity of tire dissolving surface in tlie highly Cl -concentrated electrolyte.)... 

Voltmeters and potentiometers The instruments described here are generally referred to as corrosion voltmeters. As mentioned previously, the current flowing through any potential-measurement circuit must be small to avoid errors due to polarisation. Moreover, if the current flow is too large, errors will be introduced owing to the voltage drop caused by the contact resistance between the reference electrode and the electrolyte. It is thus clear that the prime requirement of a potential measurement circuit is high resistance. 

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

Doyle et al. [40] used a mathematical model to examine the effect of separator thickness for the PVDF.HFP gel electrolyte system and found that decreasing separator thickness below 52 pm caused only a minor decrease in ohmic drop across the cell. The voltage drops in the electrodes were much more significant. They state that their model predictions were confirmed experimentally. 

Since the voltage drop between electrodes is slightly less for the same electrolyte than if a non-conducting... 

In principle, a further inexpensive method is to work at constant cell voltage. But here the potentials of the working and of the counter electrode, and all voltage drops of the electrolytes and of the cell separator are included (see Fig. 2). Thus, in most cases, clearly defined conditions at the working electrode cannot be adjusted using this operation mode (nevertheless, because of its uncomplicated realization, it is applied in most technical electrolyses to achieve approximately the desired cell current). 

The functioning of a separator will be the more difficult, the more different both electrolytes are. With increasing concentration gradients diffusion will be enhanced. It can be reduced using a less porous and/or thicker separator, but then the voltage drop increases. A special problem, that has to be avoided, is a precipitation at the surface or within the separator if the solubility of a compound is smaller in one solution compared with the other. 

The tape casting and electrophoretic deposition processes are amenable to scaleup, and thin electrolyte structures (0.25-0.5 mm) can be produced. The ohmic resistance of an electrolyte structure and the resulting ohmic polarization have a large influence on the operating voltage of MCFCs (14). FCE has stated that the electrolyte matrix encompasses 70% of the ohmic loss (15). At a current density of 160 mA/cm, the voltage drop (AVohm) of an 0.18 cm thick electrolyte structure, with a specific conductivity of -0.3 ohm cm at 650°C, was found to obey the relationship (13). 

Other Compounds Experimental studies indicate that 1 ppm As from gaseous AsHs in fuel gas does not affect cell performance, but when the level is increased to 9 ppm As, the cell voltage drops rapidly by about 120 mV at 160 mA/cm (71). Trace metals, such as Pb, Cd, Hg, and Sn in the fuel gas, are of concern because they can deposit on the electrode surface or react with the electrolyte (15). Table 6-3 addresses limits of these trace metals. 

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