Over potential deposition

In electrochemistry, adsorbed hydrogen is denoted as either the underpotentially deposited hydrogen, Hypd, that is the H adlayer formed under thermodynamic equilibrium conditions where the coverage is changed reversibly with the potential applied or the over-potentially deposited hydrogen. Hqpd. as defined by Conway and co-workers [102] for the... 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

The optimum process would ideally involve the use of soluble anodes, as the over-potential required to drive the deposition process will be small. This is especially important with ionic liquids because the ohmic loss across the cell can be significant. In aqueous solutions the use of soluble anodes is not often possible due to passivation of the electrode surface at the operating pH. 

Fig. 8.6 Features of the double-pulse technique Model on the influence of the transition moment between nucleation pulse and growth pulse in the course of the double-pulse deposition on the Gaussian particle distribution formed after the nucleation pulse [29] (a) Gaussian particle distribution of N nuclei with radii r > tcr (T)i) for different over potentials of the first pulse ( t ib << t iAl)- The hatched area of the Gaussian distribution corresponds to the number of stable particles with radii r > rcr (tje). whereas the white area of particles of under critical size is amputated as these particles dissolve, (b) Representation of the result of the particle cut off, small (dark) particles dissolve but larger particles (white) survive under the lower overvoltage of the growth pulse.(c) If a small particle lies in the diffusion zone of a larger particle the under saturation can favor the dissolution of the smaller ones...

Under standard conditions and in the absence of kinetic hindrance, the electrode potential (versus a hydrogen electrode) determines the potential at which the corresponding metal will be deposited out of an aqueous solution. Therefore, metals that have a more negative electrode potential than the hydrogen electrode cannot be deposited from aqueous electrolytes. Kinetic barriers often disfavor the production of hydrogen over metal deposition. Thus, technically important metals, such as tin, nickel, and zinc can be electrolytically deposited out of aqueous solutions without any problems, even though their electrode potentials are lower than that of the hydrogen electrode. 

In RF discharge, the molecular dissociation glow no longer adheres to the electrode surface, and the gas phase near the electrode surface becomes the major site for the creation of chemically reactive species. Consequently, the deposition yield on electrodes drops below 0.10, and the potential deposition yield in the interelectrode space increases to over 0.90. The deposition yield on substrate. Kp s, is significantly lower than the potential deposition yield because the volume of the glow expands beyond the interelectrode space as a function W/p and increases reactor wall contamination. 

The electrochemical form of ALE makes use of underpotential deposition (UPD). the electrochemical phenomena where an atomic layer of one element frequently deposits on a second element at a potential prior to (under) that needed to deposit the element on itself fhe driving force for UPD can be thought of as resulting from the free energy of formation of a surface compound. These surface limited reactions are then used in a deposition cycle, where atomic layers of each element are deposited in turn, in order to form a monolayer of the deposit. The number of cycles performed detennines the number of compound monolayers and the thickness of the deposit. One of the main advantages of this methodology is that the electrochemical formation of a compound is broken down into a series of individually addressable steps. Each step in the cycle becomes a point of control over the deposition process. 

Additives that increase the deposition over-potential at a given current density, for instance, by altering the Tafel constants, can be considered deposit-leveling additives. Since additives are typically present in very small concentrations, their transport to the electrode is nearly always under diffusion control and sensitive to flow variations. 

Fig. 4.1 Cyclic voltammogram showing zinc deposition and de-plating for carbon black (green line) and multiwall carbon nanotube-embedded (orange line) high-density polyethylene composite electrodes, with deposition potential (DP), cross-over potential (COP) and nucleation overpotential (NOP) indicated on diagram inset (Image adapted from [3].)...

The surface films discussed in this section reach a steady state when they are thick enough to stop electron transport. Hence, as the surface films become electrically insulating, the active electrodes reach passivation. In the case of monovalent ions such as lithium, the surface films formed in Li salt solutions (or on Li metal) can conduct Li-ions, and hence, behave in general as a solid electrolyte interphase (the SEI model ). See the basic equations 1-7 related to ion transport through surface films in section la above. The potentiodynamics of SEI electrodes such as Li or Li-C may be characterized by a Tafel-like behavior at a high electrical field and by an Ohmic behavior at the low electrical field. The non-uniform structure of the surface films leads to a non-uniform current distribution, and thereby, Li dissolution from Li electrodes may be characterized by cracks, and Li deposition may be dendritic. The morphology of these processes, directed by the surface films, is dealt with later in this chapter. When bivalent active metals are involved, their surface films cannot conduct the bivalent ions. Thereby, Mg or Ca deposition is impossible in most of the commonly used polar aprotic electrolyte solutions. Mg or Ca dissolution occurs at very high over potentials in which the surface films are broken. Hence, dissolution of multivalent active metals occurs via a breakdown and repair of the surface films. 

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