Polarized graphite electrodes

At mercury and graphite electrodes the kinetics of reactions (15.21) and (15.22) can be studied separately (in different regions of potential). It follows from the experimental data (Fig. 15.6) that in acidic solutions the slope b 0.12 V. The reaction rate is proportional to the oxygen partial pressure (its solution concentration). At a given current density the electrode potential is independent of solution pH because of the shift of equilibrium potential, the electrode s polarization decreases by 0.06 V when the pH is raised by a unit. These data indicate that the rate-determining step is addition of the first electron to the oxygen molecule ... 

FIGURE 15.7 Polarization curves for anodic chlorine (1) and oxygen (2) evolution at a graphite electrode, and the current yields of chlorine as a function of potential (3). 

Figure 1. Schematic representation of the electrochemical windows of the processes, occurring during cathodic polarization of graphite electrodes in nonaqueous solutions.

When the electrolyte solutions are not too reactive, as in the case of ethereal solutions, there is no massive formation of protective surface films at potentials above Li intercalation potential, and most of the solvent reduction processes may occur at potentials lower than 0.3 V vs. Li/Li+. Hence, the passivation of the electrodes is not sufficient to prevent cointercalation of solvent molecules. This leads to an exfoliation of the graphite particles into amorphous dust (expholiated graphene planes). This scenario is demonstrated in Figure 2a as the reduction of the 002 diffraction peak21 of the graphite electrode, polarized cathodically in an ethereal solution. 

The surface reactions of graphite electrodes in many nonaqueous solutions have been investigated intensively,29 30 and the major reaction paths in a variety of alkyl carbonate solutions seem to be quite clear. Both EC and PC decompose on graphite electrodes, polarized cathodically, to form solid surface films with R0C02Li species as major components,31 and ethylene or propylene gases, respectively, as co-products. 

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

Therefore, a hybrid cell has been designed in lmol L 1 LiPF6 in 1 1 ethylene carbonate/ diethyl carbonate electrolyte by combining graphite and activated carbon as negative and positive electrodes, respectively [113], The activated carbon electrode is stable in the potential window between 1.0 and 5.0 V vs. Li, whereas the graphite electrode can be polarized down to low potential values. The mass of the electrodes should be balanced to fully take profit of the performance... 

Similar to the behavior of nonactive metal electrodes described above, when carbon electrodes are polarized to low potentials in nonaqueous systems, all solution components may be reduced (including solvent, cation, anion, and atmospheric contaminants). When the cations are tetraalkyl ammonium ions, these reduction processes may form products of considerable stability that dissolve in the solution. In the case of alkali cations, solution reduction processes may produce insoluble salts that precipitate on the carbon and form surface films. Surface film formation on both carbons and nonactive metal electrodes in nonaqueous solutions containing metal salts other than lithium has not been investigated yet. However, for the case of lithium salts in nonaqueous solvents, the surface chemistry developed on carbonaceous electrodes was rigorously investigated because of the implications for their use as anodes in lithium ion batteries. We speculate that similar surface chemistry may be developed on carbons (as well as on nonactive metals) in nonaqueous systems at low potentials in the presence of Na+, K+, or Mg2+, as in the case of Li salt solutions. The surface chemistry developed on graphite electrodes was extensively studied in the following systems ... 

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