Dendritic zinc surface

Fig. 4. Example.s of rough surfaces pretreated for adhesive bonding (a) microtibrous oxide on copper (cf. 29J) (b) a dendritic zinc surface (cf. [30J).

Some rough surfaces resulting from pretreatment prior to adhesive bonding (a) porous anodic oxide on aluminum (schematic) (b) dendrites of zinc electrodeposited onto a zinc surface (c) black CuO layer produced on copper (d) PTFE irradiated by argon ions (After Koh et al. 1997) (e) adhesion of copper to silica using a mechanical key (van der Putten 1993) (i) TiW islands deposited (ii) Pd activator adsorbed and HF etching (iii) electroless Cu deposited (iv) Cu electrodeposited... 

Another complication had to be matched when the zinc electrode was made reversible in a battery with unstirred electrolyte or an electrolyte gel, dendritic growth of the electrolytically deposited metal takes place. The formation of dendrites cannot be fully suppressed by the use of current collectors with large surface areas (grids, wire fabrics). However, by using improved separators combined in multi layer arrangements, the danger of short-circuiting is reduced. 

Limiting currents measured for a deposition reaction may be excessively high due to surface roughness formation near the limiting current. Rough deposits in the case of copper deposition have been mentioned several times in previous sections, since this reaction is one commonly used in limiting-current measurements. However, many other metals form dendritic or powdery deposits under limiting-current conditions, for example, zinc (N lb) and silver. Processes of electrolytic metal powder formation have been reviewed by Ibl (12). 

Fluorescence lifetime measurements on the aggregate have shown that the rate constant of the intermolecular energy transfer from the zinc porphyrin unit to the free-base porphyrin unit has been evaluated to be 3.0 x 109 s-1. This value is reasonable from a model in which dendritic donor 6b and acceptor 5a contact each other directly at their exterior surfaces (Scheme 2). Therefore, electrostatic assembly of positively and negatively charged dendrimers provides a promising supramolecular approach to construct photofunctional materials with nanometric precision. 

To address the zinc dendrite problem in nickel-zinc cells, eVionyx claims to have developed a proprietary membrane system that is nonporous, has very high ionic conductivity, is of low cost, and can block zinc dendrite penetration even in high concentrations of KOH. The polymeric membrane has an ionic species contained in a solution phase thereof. The ionic species behaves like a liquid electrolyte, while at the same time the polymer-based solid gel membrane provides a smooth impenetrable surface that allows the exchange of ions for both discharging and charging of the cell. 

Another problem that must be taken into account with plate electrodes is the formation of metal dendrites which grow from the cathode surface toward the counter electrode, causing eventually a short circuit. If a diaphragm is placed between the electrodes, the dendrites can pass through it and they finally may destroy it. The region of Zn or Cd dendrite (or sponge) formation lies on the polarization curve below the limiting current density for deposition of zinc from zincate solutions and cadmium from CdSO solutions. It seems that the formation... 

The resin side of a broken epoxy-zinc joint is shown in Fig. 2. The electron image only reveals the resin, but the zinc X-ray image also shows local concentrations of zinc in the resin. It is likely that some dendrites on the metal surface have broken... 

Although the zinc electrode is a difficult system and can deliver excellent high rate performance, it suffers from problems with the surface morphology, dendrite formation on charge, shape change, agglomeration/redistribution and passivation, etc., that limit cycle life. The problems directly result from the characteristics of the reactions of the zinc ions in solution and the reactivity of zinc metal with the electrolyte. 

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