Electrodes performance

Christian-Albrechts-University, Kaiserstr. 2, D-24143 Kiel, Germany 

The thermodynamics of insertion electrodes is discussed in detail in Chapter 7. In the present chapter attention is focused mainly on the general kinetic aspects of electrode reactions and on the techniques by which the transport of species within electrodes may be determined. The electrodes are treated in a general fashion as exhibiting mixed ionic and electronic transport, and attention is concentrated on the description of the coupled transport of these species. In this context it is useful to consider that an electronically conducting lead provides the electrons at the electrodes and compensates the charges of the ions transferred by the electrolyte. 

Because of the high mobility of the mobile (electroactive) species in the electrolyte, there is a tendency for these ions to move from the electrode of higher activity to the electrode of lower activity. The displacement of the ions comes to a stop when an electrical field is built up which drives the ion i in the opposite direction 

One may also look at the effect of the electrodes from the point of view of energy balance. Measurement of voltages always requires at least a small electrical current. This corresponds in the case of galvanic cells to the transfer of electroactive species from one electrode to the other. The corresponding chemical work is the change of the Gibbs energy, AG, which 

We have seen that the cell potential is generated at the interfaces between the electrodes and the electrolyte. Therefore, the composition of the electrode at this interface is important and this does not have to be identical with the bulk composition. In fact, large deviations have been observed due to segregation of some of the components of the electrode and especially due to impurities at the surface. If the surface of the electrode is equilibrated with the bulk, both have the same chemical potential of the electroactive component if that is sufficiently mobile in 

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Eor the negative electrolyte, cadmium nitrate solution (density 1.8 g/mL) is used in the procedure described above. Because a small (3 —4 g/L) amount of free nitric acid is desirable in the impregnation solution, the addition of a corrosion inhibitor prevents excessive contamination of the solution with nickel from the sintered mass (see Corrosion and corrosion inhibitorsCorrosion and corrosion control). In most appHcations for sintered nickel electrodes the optimum positive electrode performance is achieved when one-third to one-half of the pore volume is filled with active material. The negative electrode optimum has one-half of its pore volume filled with active material. 

Although carbon electrode production has been regarded as a mature business, the steady growth in demand and the need for improved electrodes has prompted ongoing development efforts in these areas (/) cost containment through raw material substitutions and process improvements (2) higher purity electrodes for those processes such as siUcon production (J) improvements in thermal shock resistance to enhance electrode performance and (4) better joining systems for prebakes. 

The formation of PbOj is favoured in solutions containing passivating anions such as SO4 and in chloride solutions of intermediate concentrations very high and very low concentrations of chloride inhibit the formation of Pb02- The platinum/lead bi-electrode performs best in seawater, and is not recommended for use in waters of high resistivity. 

Deterioration of electrode performance due to corrosion of electrode components is a critical problem. The susceptibility of MHt electrodes to corrosion is essentially determined by two factors surface passivation due to the presence of surface oxides or hydroxides, and the molar volume of hydrogen, VH, in the hydride phase. As pointed out by Willems and Buschow [40], VH is important since it governs alloy expansion and contraction during the charge-discharge cycle. Large volume changes... 

Therefore, in most cases, pH values measured at high temperatures in dilute solution should be considered approximate values only. In cases where the investigators address this problem and are careful to select a suitable electrode (namely, one that manufacturers claim to have almost hysteresis-free pH measurement and a stable isopotential point over the temperature range), the error associated with electrode performance will be small, and differences in reported pH values will correspond to differences in actual pH. In cases where pH is measured in concentrated sucrose solutions, the reported pH value should be considered as a nominal value only, and the differences in nominal pH values might not correspond to actual differences in hydrogen ion activity. 

Table 1 shows that the physicochemical properties of the support material were modified by the pre-treatment process. The particle sizes. Dp, which are summarized in the Table 1 were calculated from the X-ray diffraction patterns of prepared catalysts and a commercial catalyst(30 wt% Pt-Ru/C E-TEK) by using Scherrer s equation. To avoid the interference from other peaks, (220) peak was used. All the prepared catalysts show the particle sizes of the range from 2.0 to 2.8nm. It can be thought that these values are in the acceptable range for the proper electrode performance[7]. For the prepared catalysts, notable differences are inter-metal distances(X[nm]) compared to commercial one. Due to their larger surface areas of support materials, active metals are apart from each other more than 2 3 times distance than commercial catalyst. Pt-Ru/SRaw has the longest inter-metal distances. 

Levi E, Lancry E, Gofer Y, Aurbach D (2006) The crystal structure of the inorganic surface films formed on Mg and Li intercalation compounds and the electrode performance. J Solid State Electrochem (2006) 10 176-184... 

Figure 13. Comparison of oxygen electrode performance in H2-02 PEMFC and DMFC ( ) potential of the H2-O2 PEMFC cathode, (o) potential of the DMFC cathode, (A) DMFC cell potential.

Of great interest and importance are studies on carbon dioxide reduction on copper electrodes, performed primarily by Japanese scientists. Under certain conditions, formation of methane and ethylene with high faradaic yields (up to 90%) was observed. The efficiency and selectivity of this reaction depends very much on the purity and the state of the surface of the copper electrode. For this reason, many of the published results are contradictory. 

On the basis of this and in connection with the electrode performance in practical applications, a more detailed classification of electrodes can be given (see page 7). 

In addition to revealing constants, Bjerrum curves are a valuable diagnostic tool that can indicate the presence of chemical impurities and electrode performance problems [165]. Difference curve analysis often provides the needed seed values for refinement of equilibrium constants by mass-balance-based nonlinear least squares [118]. 

Comer, J. E. A. Hibbert, C., pH electrode performance under automatic management conditions, J. Auto. Chem. 19, 213-224 (1997). 

Generally, the hindrances in the transport processes, which take place in the porous structure of the air gas-diffusion electrodes, influence strongly the electrode performance and are of prime importance for their practical application. 

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

In addition to the chemical interactions described above, microstructural changes in the electrode can lead to performance degradation after long operation time. For example sintering of the porous structure can degrade electrode performance. In the case of LSM cathodes, the sintering ability is found to be related to the strontium dopant level and stoichiometric composition of (La, Sr)xMn03. In addition, LSM with A-site deficient compositions (x < 1) sinters more readily than their B-site deficient counterparts (x > 1) [198, 199],... 

Although the electrode performance can be improved by the introduction of compositional and microstructural gradients into the anode or cathode, the processing required to produce such graded layers also increases in complexity as the number of discrete layers increases, particularly when separate deposition and firing steps are required for each increment in layer composition or structure. 

Recent improvements in pH probe design are directed toward alleviating some of the problems mentioned above. For example, the Ingold DPAS prepressurized gel-filled pH electrode performs better than the standard liquid-filled probes, requires minimal servicing since there is no need to refill the electrode with electrolyte, and is relatively simple to install. 

As in the case with PAFC s, voltage obtained from an AFC is affected by ohmic, activation, and concentration losses. Figure 4-7 presents data obtained in the 1960 s (18) which summarizes these effects, excluding ohmic losses, for a catalyzed reaction (0.5-2.0 mg noble metal/cm ) with carbon-based porous electrodes for H2 oxidation and O2 reduction in 9 N KOH at 55-60 C. The electrode technology was similar to that employed in the fabrication of PAFC electrodes. Performance of AFC s with carbon-based electrodes has not changed dramatically since these early results were obtained. 

H2 is the average partial pressure of H2 in the system. At 190°C (374°F), the presence of 10% CO2 in H2 should cause a voltage loss of about 2 mV. Thus, diluents in low concentrations are not expected to have a major effect on electrode performance however, relative to the total anode polarization (i.e., 3 mV/100 mA/cm ), the effects are large. It has been reported (16) that with pure H2, the cell voltage at 215 mA/cm remains nearly constant at H2 utilizations up to 90%, and then it decreases sharply at H2 utilizations above this value. 

Numerous efforts have been made to develop in situ catalyst layer fabrication methods to lower Pt loading and increase platinum utilization without sacrificing electrode performance. 

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