High-pressure electrochemical

C.G. Yiokari, G.E. Pitselis, D.G. Polydoros, A.D. Katsaounis, and C.G. Vayenas, High pressure electrochemical promotion of ammonia synthesis over an industrial iron catalyst, /. Phys. Chem. 104, 10600-10602 (2000). 

Bard, A. J., Flarshein, W. M. and Johnston, K. P. (1988) High-pressure electrochemical oxidation of benzene at a lead dioxide electrode in aqueous bisulfate solution at 25°C to 250°C. J. Electrochem. Soc. 135, 1939-1944. 

Fig. 6.7 A high-pressure electrochemical cell with an Ag/Ag reference electrode and an Au disc working electrode a platinum wire auxiliary electrode is shown on the left (a) bomb electrical terminals, (b) brass connectors, (c) mounting plate, (d) Kel-F backing screw, (e) Kel-F electrode housing, (f) Teflon sleeve, (g) upper cell body, (h) reference-electrode compartment, (I) internal piston with a Vycor glass frit, (j) O-rings, (k) Teflon piston, and (I) screw plug. ...

Several laboratories " have developed high-pressure electrochemical techniques (here construed as excluding conductimetric techniques), with motivations ranging from the probing of supercritical aqueous electrolyte solutions ... 

Thus, the cross-reaction approach, judiciously applied, can deliver data that are otherwise inaccessible. The converse, however, may be more important - that is, if an adequate database of AV values is available, then only the easily measurable pressure dependence of A is needed to apply Eqs (5.12)-(5.14) to predict AVj2 for numerous cross reactions (algebraic summation of partial molar volumes of all the relevant solute species is an alternative route to AV12, but such data are rarely available or relevant to practical reaction media). Comparison of predicted and experimental AV data could then provide mechanistic insights. Thus, high-pressure electrochemical measurements are central to further progress in this field. 

Success in high-pressure electrochemical measurements depends largely on having clean, reproducible working electrode surfaces, which are therefore polished with fine alumina and sonicated in clean solvent before assembly of the electrochemical cell. Before a sequence of measurements at each pressure, the electrodes are cleaned electrochemically by cycling the applied potential several hundred mV either side of the potential of the couple of interest (preferably to the point of hydrogen evolution, in aqueous media). 

For non-aqueous systems, however, AV ] for Co(bpy)3 is strongly positive and comparable to AV (= —RT 8 In D/8P)j), at least for the three solvents for which good, reproducible, high-pressure electrochemical measurements could be made (Table 5.2). From the Stokes-Einstein relation... 

Figure 6 High-pressure electrochemical cell. (Reproduced with permission Sachinidis, J. I., Shalders, R. D. and Tregloan P. A. Inorganic Chemistry, Figure 1, p. 6182, 33, 1994. The American Chemical Society.)...

Sonoyama, N., M. Kirii, and T. Sakata (1999). Electrochemical reduction of CO2 at metal-porphyrin supported gas diffusion electrodes under high pressure. Electrochem. Commun. 1(6), 213-216. 

The water-in-C02 microemulsion mentioned previously in this section may provide an effective medium for generating electrical conductivity in supercritical CO2. In 2000, Ohde et al. first reported the results of voltammetric measurements for the redox reactions of ferrocene (FC) and A,fV,iV fV tetramethyl-jc-phenylenediamine (TMPD) in supercritical CO2 in the presence of a water-in-CO2 microemulsion (14). The design of their high-pressure electrochemical cell is shown in Figure 16. The same AOT/PFPE-PO4 water-in-C02 microemulsion described in Section IV.A was used in their voltammetric experiments. Well-defined voltammetric waves were obtained for FC and for TMPD in the microemulsion system as shown in Figure 17. An obvious diffusion current for the redox reaction of FC or TMPD was observed. An electrolysis experiment was also performed with TMPD. After the electrolysis at +0.3 V, the UV-Vis absorption spectrum of the sample collected in hexane was measured. The absorption peak wavelength and the shape of the peak were identical to that for TMPD + in water. The result suggests that TMPD " produced at the electrode surface was in the water core of the water-in-C02 microemulsion, as shown in Fq. (12) ... 

Figure 16 Structures of the high-pressure electrochemical cell and the microelectrode used for the voltammetry experiments in Ref. 15.

Fig. 3.9 Lc/t High-temperature-high-pressure electrochemical wall-tube cell (A) inlet, (B) pre-cell, (C) mixing dishes, (D) platinum resistor, (E) reference electrode, (F) counter electrode, (G) zircaloy nozzle, (H) outlet, (I) working electrode, (J) cell and (K) zircaloy rings. Right Typical voltammograms in Fe /Fe 1 mM in 0.2 M Na2S04 (pH 1.5) on a platinum electrode at 85 °C. Sweep rate 50 mV s, H 0.264 cm, d 0.204 cm and = 0.05 cm. Flow rates (a) 4, (b) 8, (c) 12 and (d) 20 cm min. From [259], with permission...

Hot-wire electrochemistry with directly pulse-heated microwire electrodes can be considered to be a special branch of high-temperature electrochemistry. It is characterised by two attributes (1) it can be done with common, generally available instruments and (2) in contrast to classical high-T-electrochemistry, the increased temperature is applied only where it is required, but all other regions of the cell content remain unaffected. These characteristics are of practical, not of fundamental, nature. It means that the results of this technology would be available in most cases also with classical high pressure electrochemical cells. 

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