Electrochemistry compartments

A schemahc diagram of the DEMS apparatus is shown in Fig. 5. The electrochemistry compartment corrsists of a circular block of passivated htanirrm (a) that rests above a stainless-steel support (1) cormected to the mass spectrometer. The space between the cell body and the snpport is a Teflon membrane (j) embedded on a steel mesh (k) the membrane is 75 pm thick, has 50% porosity and pore width of 0.02 pm. The single-crystal disk (h) is the working electrode its face is in contact with the electrolyte solution and separated from the cell body by another Teflon membrane (i) that functions as a spacer to form a ca 100-pm thick electrolyte layer (j). Stop-flow or continnons-flow electrolysis can be performed with this arrangement. For the latter, flow rates have to be minimal, ca 1 pL/s, to allow ample time (ca 2 s) for the electrogenerated products to diffuse to the upper Teflon membrane. Two capillaries positioned at opposite sides of the cell body (b, e) serve as electrolyte inlet and outlet as well as connection ports to the reference (f) and two auxiliary Pt-wire electrodes (d, f). 

The combined effects of electroneutrality and the Donnan equilibrium permits us to evaluate the distribution of simple ions across a semipermeable membrane. If electrodes reversible to either the M+ or the X ions were introduced to both sides of the membrane, there would be no potential difference between them the system is at equilibrium and the ion activity is the same in both compartments. However, if calomel reference electrodes are also introduced into each compartment in addition to the reversible electrodes, then a potential difference will be observed between the two reference electrodes. This potential, called the membrane potential, reflects the fact that the membrane must be polarized because of the macroions on one side. It might be noted that polarized membranes abound in living systems, but the polarization there is thought to be primarily due to differences in ionic mobilities for different solutes rather than the sort of mechanism that we have been discussing. We return to a more detailed discussion of the electrochemistry of colloidal systems in Chapter 11. 

The potential profile through the membrane that is placed between the sample and the internal reference solution was shown in Fig. 6.3. The composition of the internal solution can be optimized with respect to the membrane and the sample solution. In the interest of symmetry, it is advisable to use the same solvent inside the electrode as is in the sample. This solution also contains the analyte ion in the concentration, which is usually in the middle of the dynamic range of the response of the membrane. The ohmic contact with the internal reference electrode is provided by adding a salt that contains the appropriate ion that forms a fast reversible couple with the solid conductor. In recent designs, gel-forming polymers have been added into the internal compartment. They do not significantly alter the electrochemistry, but add mechanical stability and convenience of handling. 

Both the - standard hydrogen electrode (SHE), which is the primary standard in electrochemistry [iv,v] and the relative hydrogen electrode (RHE) are widely used in aqueous acidic solutions. In RHE the nature and concentration of acid is the same in the reference and the main compartments. In general, it is advantageous to use the same solution in both compartments to decrease the - junction potential. By the help of RHE the -> activity effect can also be eliminated when the -> pH dependence of a — redox reaction is to be determined, since the H+ ion activity influences both the redox reactions under study and the redox reaction occurring in the reference system (1/2H2 -> H+ + e-) in the same way. 

Methylene chloride has been used for both anodic and cathodic reactions. The major advantages of using methylene chloride seem to be that certain cation radicals are more stable in methylene chloride than in the solvents usually employed in electrochemistry [425.426]. A disadvantage is that chloride ions produced at the cathode may diffuse to the anode compartment and interfere with the anodic reactions. This can partially be avoided by the addition of small amounts of acetic acid to the catholyte whereby the cathodic reaction becomes an evolution of hydrogen rather than formation of chloride ions. 

For laboratory use, most electrolyses can be achieved with a U-shaped cell equipped with a glass frit separator. In electrochemistry, the notion of the working electrode dipping in a well-defined surrounding is essential. The electrochemist defines the reaction (oxidation at the anode in the anodic compartment or reduction at the cathode in the cathodic compartment) to be achieved, and what happens at the other electrode (called counter electrode) does not really matter (although one has to avoid efficiently any diffusion of effluents formed in the other compartment). Some examples of electrolysis cells are shown and commented on at the end of this chapter (Sect. 6.15). 

Fig. 3. Single-crystal electrode and electrolysis compartment of the LEED electrochemistry system as seen through the view port of the vacuum chamber [14].

The capacity loss of the sulfur electrode can be attributed to sulfur vaporization (the vapor pressure of sulfur at 400°C is 410 torr), migration or dispersion of insoluble sulfur-containing phases from the electrode, solubilization of sulfur-containing species in the electrolyte, and/or inactivation of sulfur within the electrode compartment. Since neither the mechanism of the cell reaction nor the mechanism of sulfur loss was understood, a study of sulfur electrode chemistry and electrochemistry was made. It was expected that information gained from these studies would lead to improved performance and lifetimes of lithium-sulfur cells. 

This was probably a consequence of the hydrogen oxidation electrochemistry occurring at a significantly more positive potential than in the fuel cell case. The supported metal sulfide packing in the oxidation compartment was necessary for rapidly shifting equilibrium (7.42) upon hydrogen depletion. Typical H2S conversion rates were 95%. 

The Nemst equation (Section 7.6.2) was presented as an equation valid for redox electrode processes in electrochemistry. The Nemst concept is also used for the calculation of die potential difference across a membrane that separates two electrolyte compartments with different ion concentrations. Exchanging the natural logarithm with the common logarithm and putting n = 1 and temperature 37 °C, Eq. 7.9 becomes ... 

Experimentally constructing an H2lH reference electrode in EL media would be similar to construction of the Agl Ag. In this particular case a platinum wire should be used and the Ag" solution replaced with an solution. The separation of the reference electrode via a salt bridge tube of compartment (depending on cell designs) is advisable in order to minimise issues with leakage of reference solution into the electroactive solution. Experimentally the set-up would be similar to that used in PELs electrochemistry (see below for further details)... 

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