Electrodes desorption

Graphite finds wide range of applications in the electrodes for certain types of rechargeable batteries and supercapacitors, in electro-sorption/desorption electrodes, as anodes in a number of processes of... 

Frequently a simple contact between an electrode surface and a solution is sufficient to generate reasonable adsorptive layers without adding further reagents. This is valid in particular for carbon-containing surfaces. In some cases, however, an adsorptive bond with a carbon surface may affect the enzyme function to such a degree that it is deactivated. Sometimes even denatu-ration occurs. The majority of molecules is bound only weakly, and a part of them is later lost by progressive desorption. Electrodes with adsorptive layers preferably are used for tentative experiments. They are useful mainly for fast tests. 

Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

Sometimes, in FD, the emitter electrode is heated gently either directly by an electrode current or indirectly by a radiant heat source to aid desorption of ions from its surface. 

Reaction overpotential. Both overpotentials mentioned above are normally of higher importance than the reaction overpotential. It may happen sometimes, however, that other phenomena, which occur in the electrolyte or during electrode processes, such as adsorption and desorption, are the speed-limiting factors. Crystallization overpotential. This exists as a result of the inhibited intercalation of metal ions into their lattice. This process is of fundamental importance when secondary batteries are charged, especially during metal deposition on the negative side. 

All AB, alloys are very brittle and are pulverized to fine particles in the hydrid-ing-dehydriding process (see Sec. 7.2.1). Thus electrodes must be designed to accommodate fine powders as the active material. There are several methods of electrode fabrication Sakai et al [35] pulverize the alloy by subjecting it to several hydrogen absorption-desorption cycles, before coating the resulting particles with Ni by chemical plating. The powder is mixed with a Teflon dispersion to obtain a paste which is finally roller-pressed to a sheet and then hot-pressed to an expanded nickel mesh. The fabrication of a simple paste electrode suitable for laboratory studies is reported by Petrov et al. [37],... 

Measurements of the double-layer capacitance provide valuable insights into adsorption and desorption processes, as well as into the structure of film-modified electrodes (6). 

How would you use EQCM for elucidating the electrostatic desorption of anionic DNA molecules from gold electrodes ... 

Adsorption of (C4H9)4N+ cations on pc-Sn electrodes shows splitting of the adsorption-desorption capacitance peak into a doublet with the potential difference AE 8 - 50 to 60 mV. This supports the suggestion that the differences between Euso values for different Sn planes may be of the same order.621 These data point to a surface of electropolished pc-Sn that is geometrically and energetically inhomogeneous. 

The R of electropolished Zn single-crystal face electrodes has been obtained from the shape of the adsorption-desorption peak of cyclohex-anol at various Zn and Hg surfaces.154 The roughness factor of Zn electrodes has been found to increase in the order Zn(0001) < Zn(lOlO) < Zn(llZO) with values in the range 1.1 to 1.25. 

Adsorption of aliphatic alcohols and tetra-alkylammonium cations from Na2S04 + HjO solutions on Sb electrodes has been investi-gated.721 724 Splitting of the adsorption-desorption peak into two independent maxima has been found725,726 for cyclohexanol adsorption at an electrochemically polished pc-Sb electrode accordingly, the difference between the [Pg.120]

Unlike cations, the adsorption activity of CT, Br", and I at Pt electrodes is appreciable806 and increases in the given sequence of anions. At a 0, the <7, A curves for LiC104, NaCl,NaBr, and Nal coincide, which indicates that complete desorption of halide ions takes place at negatively charged surfaces. The values of Ea=0 for a renewed Pt electrode have been found to be -0.18, -0.24, and -0.33 V (SCE in H20) for NaCl, NaBr, and Nal in DMSO, respectively. 

Thermal desorption spectra, 171 Thermodynamic equilibrium, phase transitions at, 219 Thermodynamic phase formation, passivation potential and, 218 Time resolved measurements in the microwave frequency range, 447 photo electrodes and 493 Tin... 

Figure 4.8. (a) Experimental apparatus for measuring the catalyst-electrode metal/gas interface area AG- (b) typical yco2 peak obtained upon reacting the preadsorbed O with C2H4 or CO its area gives N0. (c) Plot of N0 vs the 02 desorption time, tHe, to obtain Nc. 

After the oxygen equilibrium period, to2, the catalyst-electrode is immediately exposed to a flowing stream of ultrapure (99.999%) He. During this time period, denoted foe, molecular 02 desorption is taking place. One must choose tHe to be at least 8 times longer than the residence time (V/Fv) of the catalytic reactor (V is the reactor volume and Fv is the volumetric flowrate) to ensure that all gaseous 02 is removed from the reactor. 

Subsequently the catalyst-electrode is immediately exposed to a flowing stream of C2H4 (or CO) in He and an infrared C02 analyzer is used to monitor the mole fraction, yco2> of C02 formed by the reaction of C2FLi (or CO) with adsorbed oxygen. By integrating the peak area one determines the amount N0 (mol O) of O adsorbed on the surface after the desorption time W... 

Subsequently one plots InNo vs tHe and extrapolates to tHe=0. This plot provides the 02 desorption kinetics at the chosen temperature T. The intersect with the N0 axis gives the desired catalyst surface area NG (Fig. 4.8) from which AG can also be computed. More precisely NG is the maximum reactive oxygen uptake of the catalyst-electrode but this value is sufficient for catalyst-electrode characterization. 

The activation overpotential Tiac,w is due to slow charge transfer reactions at the electrode-electrolyte interface and is related to current via the Butler-Volmer equation (4.7). A slow chemical reaction (e.g. adsorption, desorption, spillover) preceding or following the charge-transfer step can also contribute to the development of activation overpotential. 

Very simply these equations are valid as long as ion backspillover from the solid electrolyte onto the gas-exposed electrode surfaces is fast relative to other processes involving these ionic species (desorption, reaction) and thus spillover-backspillover is at equilibrium, so that the electrochemical potential of these ionic species is the same in the solid electrolyte and on the gas exposed electrode surface. As long as this is the case, equation (5.29) and its consequent Eqs. (5.18) and (5.19) simply reflect the fact that an overall neutral double layer is established at the metal/gas interface. 

Figure 5.21. Experimental setup (inset) showing the location of the working (WE), counter (CE) and reference (RE) electrodes and of the heating element (HE) thermal desorption spectra after gaseous oxygen dosing at 673 K and an 02 pressure of 4x1 O 6 Torr on Pt deposited on YSZ for various exposure times. Oxygen exposure is expressed in kilo-langmuirs (1 kL=l0 3 Torrs). Desorption was performed with linear heating rate, ()=1 K/s.4 S Reprinted with permission from Academic Press.

Figure 5.25. Redhead plot for oxygen desorption from a Pt film deposited on YSZ for various catalyst film potentials vs Au reference electrode. The slope of each line is equal to Ed/R.7 Reprinted with permission from Academic Press.

Figure 5.26. Effect of catalyst potential on the oxygen desorption activation energy, Ed, calculated from the modified Redhead analysis for Pt, Ag and Au electrodes deposited on YSZ.44,46 Reprinted from ref. 44 with permission from the Institute for Ionics.

The reasons are analyzed in detail in Chapter 5. The equation is valid as long as the effective double layer is present at the metal/gas interfaces of the working and reference electrodes. Deviations are basically observed when ion backspillover is not faster than ion desorption or reaction (see also section 11.3). 

---