Anodic Pulses

The net reaction for the oxidation of C HpOq is given by Eq. 13 in acid electrolytes 

The 100% efficiency of CO2 production [23] is a consequence of the relatively large potentials of oxidation and the high oxidation state of CgHpOq itself. In alkaline solutions, the net reaction is  

U — t curve without adsorbed species. The electrode is free of C HpOq or COad at U 0.15V. The transition times for the oxidation of or COad can be determined as illustrated by curve b in Fig. 15. The charge gox for oxidation of QHpOq or COad is the product of current density and transition time. Instead of oxidizing the chemisorbed species at constant current, a voltage that increases linearly with potential at 

Again the subscripts in front of a symbol denote saturation coverage for the respective species. This definition is identical with the regular definition of coverage as ratio of surface excesses 

Here iij is the number of electrons required for the oxidation of a single species of type j to COj, Fj is the adsorbed amount in M/cm (surface excess). When several types of species are present, the coverage 0 

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The formation of new nuclei and of a fine-crystalline deposit will also be promoted when a high concentration of the metal ions undergoing discharge is maintained in the solution layer next to the electrode. Therefore, concentration polarization will have effects opposite those of activation polarization. Rather highly concentrated electrolyte solutions, vigorous stirring, and other means are employed to reduce concentration polarization. Sometimes, special electrolysis modes are employed for the same purposes currents that are intermittent, reversed (i.e., with periodic inverted, anodic pulses), or asymmetric (an ac component superimposed on the dc). 

Using a triple galvanostatic pulse consisting of a nickel pulse (the less noble element), an anodic pulse (to change potentials rapidly), and a copper pulse (the more noble element)... 

Square wave voltammetry achieves increased sensitivity and a derivative peak shape by applying a square wave superimposed on a staircase voltage ramp. With each cathodic pulse, there is a rush of analyte to be reduced at the electrode surface. During the anodic pulse, reduced analyte is reoxidized. The voltammogram is the difference between the cathodic and the anodic currents. Square wave voltammetry permits fast, real-time measurements not possible with other electrochemical methods. 

No literature has been published in this area but, as a rule of thumb, metals which dissolve to give complexes that have linear or tetrahedral geometries, e.g. Cu, Ag, Zn, Sn, Pb, can be reversibly deposited and etched. Those with octahedral geometries, e.g. Fe, Ni, Co and Cr, are less reversible. The exceptions to this are the very electronegative metals, most notably A1 which is difficult to electrodeposit from some ionic liquids. The reversibility is also dependent upon the type of ionic liquid and the metal being deposited. Endres has shown that the adhesion of aluminum to mild steel is greatly enhanced by an anodic pulse prior to deposition. It has been shown that this alloy was formed between the steel substrate and the aluminum coating [1],... 

Figure 3.47 Potential-charge density transients from galvanostatic pulse experiments in the system Au(111)/10-3 M Bi(C104)3 + 1 M HCIO4 at T= 298 K [3.117], Initial underpotentials A i/mV= 110 (1) 190 (2) 220 (3) 223 (4) 225 (5), anodic pulse current densitiy i = 20 mA cm. ...

In order to achieve clean catalytic surfaces before reactant adsorption and reproducible results, it is necessary to establish a repetitive pretreatment schedule of cathodic and anodic pulses of fixed duration for removal of impurities and of oxides. Figure 14 gives such a pretreatment procedure followed by a constant potential equilibration step. It is also important to establish the effect of sweep rates on the i-t curves. At slow sweep rates readsorption of the desorbed species is possible at fast sweeps oxidative removal of the adsorbate may be incomplete or the capacitance current may not be separable from the reaction current. 

Nogami et al. and Augustynski et al. showed that the activity of copper electrode, deactivated during the CO2 reduction, is recovered by anodic polarization of the electrode. Periodic anodic pulses are effective to maintain the electrocatalytic activity in prolonged electrolysis of CO2 reduction. 

While electrodeposited copper represents considerable promise for HDI and VLSI applications, simple insertion of the additive chemistry or PRC processes developed for PTHs application are not likely to be successful. Furthermore, while new additive chemistries may initially be successful, the extreme tolerances and associated control issues, impurity incorporation, and waste associated with CMP prohibit the chemistry-only approach. By considering the fundamental differences associated with the PTH and HDI as well as VLSI applications, we have developed a modulated reverse electric field process (MREF) for copper electrodeposition. In contrast to the long cathodic duty cycle-short anodic duty cycle used in the PRC process, the MREF process consists of a short cathodic duty followed by a long anodic pulse. By tuning the frequency and the cathodic to anodic charge ratio (Qc/Qa), conformal and filling capability are demonstrated for vias and trenches in the 0.5 to 100 pm size range. 

During the anodic pulse, a layer of chemisorbed oxygen covers the electrode, and during the cathodic pulse (with pulse time of 10 sec), this layer is removed. The surface is cleaned and inhibiting impurity layers are not allowed to build up, thereby maintaining a... 

Metal is dissolved during the anodic pulse, yielding a fresh surface. 

It is necessary to dissolve the metal after its deposition on the upper part of PAA. Duration of the dissolution anodic pulse may be estimated as ... 

Both alloy electrodes were reported to be quasi-reversible with respect to the dissolution of the surface atoms of the less noble components, that is, they establish partial equilibrium between the solution and the surface atoms (rather than with the bulk atoms) of the less noble components, Cu and Ag, respectively. Thus, by application of the Nemst equation, the normalized surface activities of these components that result from galvanostatic anodic pulses could be determined from the chronopotentiograms and were shown to be lower than the initial activities prior to anodic current flow. In addition to this, it could be shown that upon an interruption of the appKed current the initial potential, and hence the initial activity of the less noble alloy component, is reestablished, in the course of time, due to relaxation processes that induce a reorganization of the atoms in the electrode surface [4,112]. 

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