Spongy Deposit Formation

In systems characterized with low exchange current density values, electrodeposition process enters the full diffusion control at sufficiently large overpotentials [13], On the other hand, if condition 0.1 1 is fulfilled, deposition wiU be 

At low overpotentials, a small number of nuclei are formed, and they can grow independently. The limiting diffusion current density to the growing nucleus is given by  

An increase in leads to a decrease of and, at sufficiently large Tn, the electrodeposition comes under mixed activation-diffusion control, i.e., when  

On the basis of all the above facts, it can be concluded that the formation of a spongy deposit on an inert substrate may be caused by mass-transport limitations when the nucleation rate is low. Hence, suitable conditions for the formation of spongy deposits arise at low overpotentials in systems where /l io U, 54]. 

As was mentioned before, at a fixed value of the overpotential, the growth of a spongy deposit is possible if condition given by Eq. (2.128) is fulfilled. 

---

It should be noted that some other possible mechanisms of spongy deposit formation have been considered in a qualitative way, as reviewed in Refs. [114, 115], but the mechanism presented above seems to be the most probable [115]. However, the mechanism of formation of a spongy deposit over an initial coating, which is not seen in the case of cadmium but occurs in zinc deposition, requires clarification [54, 113]. For instance, the mechanism of spongy growth initiation in this case has not been elucidated. 

Spongy deposit formation can, however, be completely prevented by the PO deposition [21], as illustrated in Fig. 4.4. 

There are two conditions under which the particles of active metal placed on the surface of a macroelectrode can represent microelectrodes. The first condition is that the substrate is conducting but inert relative to the process under consideration. The second one is that the grains are sufficiently small to permit activation control of the electrochemical process on them, making thus, mixed overall control, as in the case of the tips of growing dendrites,11,12,31 or during the induction time of the formation of spongy deposits.32-34... 

When the Cd in the electroljde is depleted to such an extent that Hs begins to evolve at the cathode, the solution must be replenished with CdS04 to avoid formation of a spongy deposit (the latter also appears at excessive current densities). 

The nucleus of spongy deposit, i.e., hedgehog-like particle, appears when amplification of surface coarseness on the nucleus in spherical diffusion control growing starts. It was shown earlier [118] that this amplification is very fast so the induction time when growing nucleus enters mixed control can be taken also as induction time of spongy formation. It follows from Eqs. (2.139) and (2.140) that and decrease with increasing overpotential. 

Popov KI, Krstajic NV, Simicic MV, Bibic NM (1992) The initial stage of spongy electro-deposit formation on inert substrate. J Serb Chem Soc 57 927—933... 

Prevention of the Formation of Spongy Deposits and the Effect on Dendritic Particles... 

EPCR is used in the charging of silver-zinc storage batteries, to prevent, or to delay, the formation of spongy and dendritic deposits of zinc [20, 21]. It is impossible to obtain smooth deposits of zinc from alkaline zincate solutions during prolonged deposition at a constant rate due to formation of spongy deposits at lower and dendritic deposits at higher overpotentials [21, 22]. 

Romanov W (1963) The Zinc spongy electrodeposits formation during deposition from zincate alkaline solution. Zh Prikl Khim 36 1057-1063 (in Russian)... 

For other systems, where the simultaneous hydrogen evolution is not evident, it seems that to produce powders by this type of deposition it is necessary that the surface of the less noble metal is not completely covered by the more noble metal. An incomplete surface coverage of the less noble metal may form porous (spongy) deposits with a poor adhesion, thus leading to the formation of various shapes of powders. 

A third type of problem, that is often mistakenly confused with dendrite formation, is due to the presence of a reaction-product layer upon the growth interface if the electrode and electrolyte are not stable in the presence of each other. This leads to filamentary or hairy growth, and the deposit often appears to have a spongy character. During a subsequent discharge step the filaments often become disconnected from the underlying metal, so that they cannot participate in the electrochemical reaction, and the rechargeable capacity of the electrode is reduced. 

An experimental study of zinc electrodeposition on copper wire from 0.1 M zincate solution in 1.0 M KOH was presented by Simicic et al. [232]. A possible mechanism of the formation of spongy zinc deposits was considered. Also, it was shown that in the case of a square-wave pulsating overpotential regime, the deposit was less... 

C. i1. Burgess1 and C. Hambuechen, in 1903, investigated the various conditions requisite for the electrolytic production of a good white lead. They found that a two-compart-ment cell is necessary to obtain a pure product. When lead anodes and sodium nitrate solution are employed a certain quantity of basic lead salt is produced, and there is not therefore a 100 per cent, formation of pure lead nitrate. The reduction of sodium nitrate at copper cathodes cannot be prevented so that a certain amount of ammonia is formed, and the solution being alkaline after a time, plumbates are formed and a layer of spongy lead is deposited on the cathode. If, therefore, the cathode compartment be not separated from the anode, the loosely-deposited cathodic lead will fall into the white lead which is collecting at the bottom of the cell. 

V. Electrolyte.—The nature of the anion often has a very important influence on the physical form of the deposited metal for example, lead from lead nitrate solution is rough, but smooth deposits are obtained from silicofluoride and borofluoride solutions. The valence state of the metal may affect the nature of the deposit thus, from plumbic solutions lead is deposited in a spongy form whereas relatively large crystals are formed in plumbous solutions. In an analogous manner, smooth deposits of tin are obtained from stannate baths, but from stannite solutions the deposits are of poor quality. The difference in the behavior of different electrolytes is sometimes due to the possibility of the formation of colloidal matter which serves to give a fine-grained deposit this may be the case in the deposition of lead from silicofluoride and borofluoride solutions where a certain amount of colloidal hydrous silica or boron trioxide may be formed by hydrolysis. 

A Ni cathode that has become deactivated by impurity deposition or supposed H sorption under conditions of cathodic polarization can be activated (177) by electrodeposition of Mo species from added molybdate in solution, as found in a similar way with Co. The in situ activation is ascribed to formation of a spongy Mo-base deposit ( MoO) on the Ni during the first day of continuous water electrolysis. 

---