Electrode kinetics crystal face

There are pros and cons for each method of electrode preparation. The polycrystalline electrodes are cheap and also are nearest in character to those used in practical reactors inindustiy. However, a polycrystal consists ofinumerable grains (bits) of the electrode material, each having a different crystal orientation and hence a different catalytic property. One way of manufacturing an original metal may differ from another in the distribution of crystal faces of different kinds. Thus, irreproducibility of results in electrode kinetics is not only due to inadequate purification of solution,... 

Single crystals, cut so that a certain crystal face is exposed to the solution, offer better definition and therefore reproducibility of results. By using electron diffraction measurements (which require a vacuum), one can determine the crystal face exposed to the solution before and after electrochemical measurements and hence ascertain if any change in crystal orientation has occurred as a result of contact with the solution. Such techniques, introduced by Hubbard in the 1970s, began a seminal change in electrode kinetics, the full fruits of which are still to be obtained. 

Swathirajan and Bruckenstein [113] have recently discussed the thermodynamics and kinetics of underpotential deposition phenomena on polycrystalline electrodes. Since work functions and surface structural features vary in different crystal faces, upd phenomena on single crystals have shown distinctive differences [109]. 

Figures 2-4 show a comparison of the ring-disk electrode for the oxygen reduction kinetic data along with the base voltammetry in oxygen-free solutions for each Vi hkl) surface. Clearly, the kinetics of the ORR on VtQikl) surfaces vary with crystal face in a different manner depending on the solution. In perchloric acid solution. Figure 2, the variation in activity at 0.8-0.9 V is relatively small between the three low-index faces, with the activity increasing in the order (100) < (110) (lll). A similar structural sensitivity is observed in KOH, Figure 3, with the activity...

Polarization curves over a wide range of overpotentials, both anodic and cathodic, for the hydrogen reaction at 1 atm on rotating Pt(hkl) disk electrodes at 298 K are shown in Figure 12. At low positive overpotentials, the order of activity for the HOR increased in the sequence (111) (100) <<(110). These differences in activity with crystal face can be attributed to the different state of adsorbed hydrogen and to different effects of these states on the mechanism of the hydrogen reaction [52,53]. The HOR on Pt(lll) and (100) in alkaline solution is purely kinetically controlled... 

Most electrochemical kinetic measurements have been made on polyciystals, i.e., norma] metals. However, metal surfaces, in reality, consist of many facets— patches—on each of which the crystals have what is called a specific orientation. In this section, some results of electrodic measurements are described in which the surfaces of the electrode catalysts are no longer the ill-defined mixture of many kinds of ciystal orientations found in polyciystals. The results described here will be those obtained on crystals prepared in such a way that one ciystal face only—having a specific, known orientation—is exposed to the solution. 

Different crystallographic planes of a semiconductor electrode usually exhibit different reaction kinetics. It was found in III-V compounds in indifferent electrolytes that the (lll)B face terminated with the anion plane (P, As) etched faster than the (lll)A face containing the cations (Ga, In) [47]. The planes composed entirely of metal atoms react more slowly than any other crystal plane because of the stable metal oxide layer, which can be formed on such planes. Consequently on these planes termed etch stop planes, provision of reactants (diffusion control) is not rate-limiting. In Si, the (100) planes are known to etch faster than (111) planes in alkaline solutions. This property is at the origin of various apphcations, such as texturization of silicon surface [formation of pyramids on (100) planes], which allows reduction of reflectivity of the front surface of solar cells and Si micromachining [48]. The semiconductor surface may be shaped during the anodic dissolution... 

---