Cell resistance, compensation

Further, the operator must be able to choose the drop lifetime and the scan parameters, viz., the starting potential, direction (cathodic or anodic), rate and end potential, together with the sensitivity of the current measurement and the amplification in the ohmic cell resistance compensation circuit. Convenient additional facilities are (a) display of the polarogram on an oscilloscope, (b) delivery of hard copy of the polarograms on a chart recorder and (c) repeated recording of the polarographic curve for the same sample. 

Figure 9 The ideal assembly of a three-electrode cell. Rs= (compensated) solution resistance Rnc = uncompensated solution resistance...

D. Garreau and J.-M. Saveant, Linear Sweep Voltammetry. Compensation of Cell Resistance and Stability. Determination of Residual Uncompensated Resistance, J. Electroanal. Chem. 35 309 (1972). 

An equivalent circuit of the three-electrode cell discussed in Chapters 6 and 7 is illustrated in Figure 9.1. In this simple model, Rr is the resistance of the reference electrode (including the resistance of a reference electrode probe, i.e., salt bridge), Rc is the resistance between the reference probe tip and the auxiliary electrode (which is compensated for by the potentiostat), Ru is the uncompensated resistance between the reference probe and the working-electrode interphase (Rt is the total cell resistance between the auxiliary and working electrodes and is equal to the sum of Rc and Ru), Cdl is the double-layer capacitance of the working-electrode interface, and Zf is the faradaic impedance of the electrode reaction. 

Now it is possible to assemble microelectrodes with extremely short response times. Nevertheless, an additional problem for the reduction of the ohmic drop is that for short times high currents arise from the large concentration surface gradients. This leads to the use of on-line and real-time electronic compensation of the cell resistance combined with the use of microelectrodes [53]. 

The potentiostatic control, aimed at compensating a major fraction of the cell resistance, is accomplished with a three-electrode system and a combination of operational amplifiers and feedback loops (Fig. 4.3). Here, the reference electrode is placed as close as possible to the working electrode and... 

Note that the local term depends through R uncomp = Rceii — RComp on the compensated and the cell resistance. Therefore, it is not possible to change the strength of the global coupling without also changing the local dynamics. 

In order to estimate the input of the cathode catalyst layer on the MEA cell performance, each cell before testing in methanol solution has been evaluated in PEMFC operation conditions, namely with hydrogen on the anode side and oxygen or air on the cathode. Fig. 6 shows the dependencies of cell voltage and cell voltage compensated for membrane resistance vs. current density. 

The CV curves were recorded by using a measuring system consisting of potentiostat, generator and x-y recorder. For rapid scan rates the curves were recorded on the screen of the monitor of an IBM PC-XT computer using a programme written in our laboratory. In kinetic experiments the positive feedback technique was used for cell resistance iR compensation. 

Yamitzky C, Klein N (1975) Dynamic compensation of the overall and uncompensated cell resistance in a two- or three-electrode system. Transient techniques. Anal Chem 47 880-884. doi 10.1021/ ac60356a030... 

Note that this is correct if the cell resistivity Rq is small, or the variation of the local cell current with z is not large. If, for example, membrane humidification is strongly nonuniform along z, the variation of the ohmic term is large, and this variation must be compensated by the respective variation in t]q. Writing Equation 5.60 in the form... 

Fig. 1.1.1. A simple electronic scheme equivalent to the electrochemical cell. resistance uncompensated in the regular three-electrode system Q differential capacity of the double layer R resistance to faradaic current at the electrode surface, Rq solution resistance compensated in the three-electrode system...

The main difficulty in measuring the compensation voltage of voltaic cells is the very large resistance of the system caused by the presence of a dielectric gas phase. Therefore there are two possibilities for solving the problem reduce this resistance or measure the work of the charge transfer across the dielectric. The first possibihty is accomplished by the ionizing method and the second by condenser and jet methods. 

Apart from the necessity of excluding interferences from any diffusion potential, normal potentiometry requires accurate determination of the emf, i.e., without any perceptible drawing off of current from the cell therefore, usually one uses the so-called Poggendorff method for exact compensation measurement the later application of high-resistance glass and other membrane electrodes has led to the modern commercial high-impedance pH and PI meters with high amplification in order to detect the emf null point in the balanced system. 

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