{Influence of the exchange current density and overpotential for hydrogen evolution reaction on the shape of electrolytically produced disperse forms}


 

http://dx.doi.org/10.5599/jese.707    111 

J. Electrochem. Sci. Eng. 10(2) (2020) 111-126; http://dx.doi.org/10.5599/jese.707 

 
Open Access: ISSN 1847-9286 

www.jESE-online.org 
Review 

Influence of the exchange current density and overpotential for 
hydrogen evolution reaction on the shape of electrolytically 
produced disperse forms 

Nebojša D. Nikolić 

ICTM-Department of Electrochemistry, University of Belgrade, Njegoševa 12, P.O.B. 473, Belgrade, 
Serbia 
nnikolic@ihtm.bg.ac.rs; Tel.: +381 11 337 03 90; Fax: +381 11 337 03 89  

Received: July 4, 2019; Revised: August 14, 2019; Accepted: August 14, 2019 
 

Abstract 
In this study, comprehensive survey of formation of disperse forms by the electrolysis from 
aqueous electrolytes and molten salt electrolysis has been presented. The shape of 
electrolitically formed disperse forms primarily depends on the nature of metals, 
determined by the exchange current density (j0) and overpotential for hydrogen evolution 
reaction as a parallel reaction to metal electrolysis. The decrease of the j0 value leads to a 
change of shape of dendrites from the needle-like and the 2D fern-like dendrites (metals 
characterized by high j0 values) to the 3D pine-like dendrites (metals characterized by 
medium j0 values). The appearing of a strong hydrogen evolution leads to formation of 
cauliflower-like and spongy-like forms (metals characterized by medium and low j0 values). 
The other disperse forms, such as regular and irregular crystals, granules, cobweb-like, 
filaments, mossy and boulders, usually feature metals characterized by the high j0 values. 
The globules and the carrot-like forms are a characteristic of metals with the medium j0 
values. The very long needles were a product of molten salt electrolysis of magnesium 
nitrate hexahydrate. Depending on the shape of the disperse forms, i.e. whether they are 
formed without and with vigorous hydrogen evolution, formation of all disperse forms can 
be explained by either application of the general theory of disperse deposits formation or 
the concept of "effective overpotential". With the decrease of j0 value, the preferred 
orientation of the disperse forms changed from the strong (111) in the needle-like and the 
fern-like dendrites to randomly oriented crystallites in the 3D pine-like dendrites and the 
cauliflower-like and the spongy-like forms. 

Keywords 
Electrolysis; metal; morphology; powder particles; SEM. 

 

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Basic facts 

The disperse (powder, irregular) forms of metal deposits can be obtained by electrolysis from 

aqueous electrolytes and molten salt electrolysis [1,2]. The typical disperse forms obtained by 

electrolysis are: dendrites of different shape, crystals of regular and irregular shape, cobweb, 

needles, wires, carrot, globules, granules, cauliflower, the honeycomb, spongy, mossy, filaments, 

etc. The shape of disperse forms depends on the regimes and parameters of electrolysis, and the 

nature of metals. The both constant (potentiostatic and galvanostatic) and periodically changing 

(pulsating overpotential (PO), pulsating current (PC) and reversing current (RC)) regimes of 

electrolysis are used for production of disperse forms. The main parameters affecting the shape of 

disperse forms are: the type and composition of electrolytes, temperature of electrolysis, the type 

of cathode, stirring of electrolyte, the addition of specific substances known as additives, etc. 

According to the exchange current density, melting point and overpotential for hydrogen 

evolution reaction, metals are classified into three classes [3]: 

a) Class I, so-called normal metals like silver, cadmium, lead, tin and zinc. This group of metals is 

characterized by the high values of both the exchange current density (j0 > 1 A dm-2; j0 is the 

exchange current density) and overpotential for hydrogen evolution reaction, and low melting 

point, 

b) Class II, so-called intermediate metals like copper, gold and silver (ammonium electrolyte). This 

group of metals is characterized by moderate melting points, the medium exchange current 

density values (10-2 < j0 < 1 A dm-2), and the lower values of overpotential for hydrogen evolution 

than the normal metals, and 

c) Class III, so-called inert metals like nickel, cobalt, iron and platinum. This group of metals is 

characterized by the low values of the both exchange current density (10-2 > j0 > 10-12 A dm-2) 

and overpotential for hydrogen evolution reaction, and high melting points. 

A schematic illustration of position of the typical metals from each of these groups on the scale 

of the exchange current density is shown in Fig. 1. The values of their exchange current densities 

are summarized in Table 1. 

 

 
Figure 1. A schematic position of metals on a scale of the exchange current density values  

(Ag* - the ammonium electrolyte) 

This mini Author`s review gives a comprehensive survey of morphological characteristics of 

disperse forms of lead, silver and zinc (the normal metals), copper and silver (the intermediate 

metals) and nickel (the inert metal). 



Nebojša D. Nikolić J. Electrochem. Sci. Eng. 10(2) (2020) 111-126 

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Table 1. The values of the exchange current density for some technologically important metals 

 
Class of metals 

The kind  
of metals 

The exchange current 
density, j0 / A dm-2 

 
Reference: 

 Pb j0 →    [4] 

Normal metals Ag 100 − 700 [5] 

 Zn 1.84 − 8.8; 0.8 − 37 [6,7] 

Intermediate Ag (ammonium electrolyte) 0.025 [8] 

metals Cu 0.011 – 0.032 [1,9] 

Inert metals Ni 1.6   10-7 [1,9] 

Class I, so-called normal metals 

The common characteristic of this group of metals is formation of disperse (powder, irregular) 

forms starting from small overpotentials, and the absence of formation of compact deposits without 

use of additives [1,2]. There is no unique and precise way for determination of the exchange current 

density values of this group of metals, and auxiliary ways are proposed for their estimation [4,6,7].  

Lead 

The processes of lead electrodeposition belong to the very fast electrochemical processes, and 

the estimated values of the exchange current density for Pb tend to infinity [4]. Pb electrodeposition 

occurs in the conditions of the mixed ohmic-diffusion control [2,10]. The ohmic control is defined 

by a straight-line dependence of current on overpotential. The ratio of the ohmic control to the 

overall control of the electrodeposition increases with increasing concentration of Pb(II) ions [10], 

and decreasing concentration of the supporting electrolyte (NaNO3) [11] (Fig. 2). The inflection point 

at the polarization curve denotes the end of the plateau of the limiting diffusion current, and the 

fast growth of the current density with the increase of overpotential after the inflection point is 

observed (Fig. 2). 
 

 
Figure 2. Polarization curves for Pb electrodeposition from 0.10 M Pb(NO3)2 in 0.50 and 2.0 M NaNO3. 

Figure 3 shows a typical disperse forms obtained under the different electrodeposition conditions. 

The regular hexagonal crystals are a characteristic of the ohmic control (Fig. 3a).* The mixture of 

needle-like dendrites and crystals of irregular shape is obtained by electrodeposition at the 

 
* The exact experimental conditions for formation of this form, as well as all others forms shown in this study are given 
in the corresponding references indicated in Figure captions. 

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overpotential belonging to the plateau of the limiting diffusion current density (Fig. 3b). Finally, the 

two-dimensional (2D) fern-like dendrites are formed by electrodeposition outside the plateau of the 

limiting diffusion current density in the zone of the fast increase in the current density with increasing 

overpotential after the inflection point (Fig. 3c). Depending on a concentration either Pb(II) ions or 

NaNO3, the very long needle-like dendrites can be formed (Fig. 3d). 

The shape of regular hexagonal crystals does not depend on the type of electrolyte [12,13]. On the 

other hand, the shape of dendrites strongly depends on the type of electrolyte, and for example, 

dendrites formed from the complex electrolytes like acetate [12] and hydroxide [13] (Fig. 3e) were 

more branchy structure than those obtained from the basic (nitrate) electrolyte (Fig. 3c) [14]. 

Following Wranglen`s definition of a dendrite [15], dendrites obtained from the complex electrolytes 

usually belong to secondary (S) and tertiary (T) types, while those obtained from the nitrate electrolyte 

belong to primary (P) type [14]. According to Wranglen [15], a dendrite is a skeleton of a monocrystal 

and consists of a stalk and branches, thereby resembling a tree. The dendrite consisted only of the 

stalk and primary branches is referred as primary (P) dendrite. If the primary branches in turn develop 

secondary branches, the dendrite is called secondary (S). The two-dimensional (2D) dendrite refers to 

dendrites with branches that lie in the same plane as that of the primary stalk [16]. The branches 

developed from the secondary branches are known as tertiary (T) ones, etc. 

Aside from dendrites of different shape and regular and irregular crystals, the granules (Fig. 3f 

and g) and the cobweb-like particles (Fig. 3h) were also formed by Pb electrodeposition. These 

particles are usually formed by Pb electrodeposition from electrolytes with the low concentrations 

of Pb(II) ions [17,18]. 

Irrespective of the type of electrolyte, the X-ray diffraction (XRD) analysis showed the predo-

minant presence of Pb crystallites oriented in the (111) plane in all types of Pb particles [13,19,20]. 

This predominant presence of the crystallites oriented in the (111) plane can be attributed to the 

lower surface energy of this plane in relation to the other planes like (110) and (100) [21,22], where 

the values of the surface energy follow trend: γ111<γ100<γ110 , and  is the surface energy. Pb 

crystallites were only oriented in the (111) plane in the regular hexagonal particles formed in the 

ohmic control, indicating that this particle type represents monocrystal of the (111) preferred 

orientation [19]. On the other hand, aside from Pb crystallites oriented in the (111) plane, the 

presence of Pb crystallites oriented in the other planes was observed in all types of dendritic 

particles, that can be considered as follows: due to different surface energy of crystal planes, 

electrodeposition rate on them are different, and for the fcc crystal lattice, follows a trend: 

(110) > (110) > (111) [23]. The (111) plane belongs to a slow-growing plane, and in the growth 

process, this plane survives [19]. Pb crystallites oriented in this plane originate from growth centers 

present in the interior of crystals (“growth centre“). The other planes, such as (110) and (100), 

belong to the fast-growing planes, and in the growth process, these planes disappear. Pb crystallites 

oriented in these planes originate from the growth centers present on the tips, corners and edges 

(“tip”, “edge” and “corner” types). In this way, the presence of Pb crystallites oriented in these 

planes is obvious. 

Silver 

The polarization curve for silver electrodeposition from the nitrate electrolyte (0.10 M AgNO3 in 

2.0 M NaNO3) shows a relatively short plateau of the limiting diffusion current density in the range 

of overpotentials between 70 and 110 mV (Fig. 4) [24,25]. 

 



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(a) (b) (c) 

   

(d) (e) (f) 

   

(g) (h)  

  

 

Figure 3. The typical disperse forms obtained by Pb electrodeposition from 0.10 M Pb(NO3)2 in 0.50 M 

NaNO3: (a) the regular hexagonal crystal ( = 20 mV) [11], (b) the irregular crystals ( = 50 mV) [11],  

(c) the fern-like dendrite ( = 80 mV) [11], (d) the needle-like dendrite (0.10 M Pb(NO3)2 in 4.0 M NaNO3;  = 

50 mV) [11], (e) the very branchy 2D dendrite (0.10 M Pb(NO3)2 in 2.0 M NaOH;  = 80 mV) [13],  

(f) and (g) granules (0.020 M Pb(NO3)2 in 2.0 M NaNO3;  = 5 mV) [17], h) cobweb-like particles (0.010 M 

Pb(NO3)2 in 2.0 M NaNO3;  = 62.5 mV) [18] 

Morphological analysis of deposits of Ag obtained at different overpotentials showed that 

granules were formed at the low overpotential (Fig. 5a), the mixture of the needle-like dendrites 

and granules was formed at the overpotential inside the plateau of the limiting diffusion current 

density (Fig. 5b), while the 2D fern-like dendrites were formed at the overpotential outside the 

plateau of the limiting diffusion current density in the zone of the fast increase in the current density 

with increasing overpotential after the inflection point (Fig. 5c). 

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Figure 4. The polarization curve for silver electrodeposition from 0.10 M AgNO3 in 2.0 M NaNO3. 

(a) (b) (c) 

   
Figure 5. The disperse forms of Ag obtained by electrodeposition from 0.10 M AgNO3 in 2.0 M NaNO3:  

(a) granules ( = 15 mV), (b) the mixture of the needle-like dendrites and granules ( = 90 mV), and  

(c) the 2D fern-like dendrite ( = 150 mV) [24]. 

The needle-like dendrites showed the strong (111) preferred orientation [24]. The strong (111) 

preferred orientation was also characteristic of the 2D fern-like dendrites, but with the larger ratio 

of Ag crystallites oriented in (220), (200) and (311) planes than in the needle-like dendrites. 

Zinc 

Unlike lead and silver, the typical polarization curve for zinc electrodeposition from an alkaline 

electrolyte showed the well defined plateau of the limiting diffusion current density, in the range of 

overpotentials between 110 and 180 mV [6] (Fig. 6). 

The filaments (Fig. 7a), grouped into mossy or spongy-like particles (Fig. 7b), are formed at the 

low overpotential. The large grains, referred as boulders, are formed at the overpotential close to 

the plateau of the limiting diffusion current density (Fig. 7c). The 2D fern-like dendrites and irregular 

crystals are formed by electrodeposition at the overpotential inside the plateau of the limiting 

diffusion current density (Fig. 7d). Finally, the very branchy 2D fern-like dendrites are formed at the 

overpotential outside the plateau of the limiting diffusion current density in the zone of the fast 

increase in the current density with increasing overpotential after the inflection point at the 

polarization curve (Fig. 7e and f). It is necessary to note formation of some positions at the electrode 

surface that are result of appearing of hydrogen evolution reaction as a parallel reaction to Zn 

electrolysis at the high overpotentials (Fig. 7e). 



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Figure 6. The polarization curve for Zn electrodeposition from 0.40 M ZnO in 6.0 M KOH. 

(a) (b) (c) 

   

(d) (e) (f) 

   
Figure 7. The disperse forms of Zn obtained by electrodeposition from 0.40 M ZnO in 6.0 M KOH: (a) 

filaments ( = 25 mV), (b) mossy ( = 25 mV), (c) large grains or boulders ( = 65 mV), (d) the mixture of the 

2D dendrites and irregular crystals ( = 150 mV), (e) and (f) the 2D fern-like dendrites ( = 225 mV) [6]. 

The intermediate metals 

The disperse forms of metals from this group are formed in the activation-diffusion and diffusion 

controls of electrodeposition, as well as in the hydrogen co-deposition range (for copper) [2]. 

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Silver (ammonium electrolyte) 

The polarization curve for silver electrodeposition from the ammonium electrolyte shows the 

well defined plateau of the limiting diffusion current density, in the range of overpotentials between 

250 and 700 mV (Fig. 8) [26]. 
 

 
Figure 8. The polarization curve for Ag electrodeposition from 0.10 M AgNO3 in 0.50 M (NH4)2SO4 with the 

addition of NH3 in excess to dissolve silver sulfate precipitate (ammonium electrolyte). 

The three-dimensional (3D) pine-like dendrites constructed from the corncob-like forms as the 

basic element were formed by electrodeposition at the overpotential belonging to the plateau of 

the limiting diffusion current density ( = 650 mV; Fig. 9a). The corncob-like forms consisted of small 

cauliflower-like agglomerates of approximately spherical grains [24]. The similar shape of the pine-

like dendrites, but more branchy structure than those obtained by electrodeposition inside the 

plateau, was formed at the overpotential outside the plateau of the limiting diffusion current density 

( = 1000 mV; Fig. 9b). 

 

(a) (b) 

  
Figure 9. The 3D pine-like dendrites formed by electrodeposition from the ammonium 

electrolyte at overpotentials of: (a) 650 mV, and (b) 1000 mV [26]. 

The ratios of Ag crystallites oriented in (200), (220) and (311) planes increased with increasing 

overpotential of the electrodeposition, leading to almost randomly oriented Ag crystallites in the 

pine-like dendrites formed at the overpotential outside the plateau of the limiting diffusion current 

density [24]. 



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Copper 

The typical polarization curve for Cu electrodeposition shows a wide and the well defined the 

plateau of the limiting diffusion current density (Fig. 10). At the first sight, it is clear that the shape 

of this polarization curve was very similar to the one obtained for Ag from the ammonium 

electrolyte. The plateau of the limiting diffusion current density corresponds to the range of 

overpotentials between 300 and 750 mV for solution containing 0.10 M CuSO4 in 0.50 M H2SO4, and 

no any difference in the length of the plateau of the limiting diffusion current density is observed 

with various concentrations of H2SO4 as the supporting electrolyte [27]. 

 

 
Figure 10. The polarization curve for Cu electrodeposition from 0.10 M CuSO4 in 0.50 M H2SO4. 

The carrot-like forms, often with the sharp tips, (Fig. 11a) and globules (Fig. 11b) are formed in 

the mixed activation-diffusion control of the electrodeposition [2,28]. The cauliflower-like forms are 

formed at the beginning of the plateau of the limiting diffusion current density before the initiation 

of the dendritic growth (Fig. 11c). 

The very branchy 3D pine-like dendrites constructed from the corncob-like forms as the basic 

element were formed at the overpotential belonging to the plateau of the limiting diffusion current 

density after an initiation of dendritic growth (Fig. 11d). The corncob-like forms were constructed 

from small cauliflower-like agglomerates of approximately spherical grains. Aside from the 3D pine-

like dendrites, cauliflower-like agglomerates of Cu grains, and holes which origin is of detached 

hydrogen bubbles were also formed at an overpotential of 625 mV which belonged to the plateau 

of the limiting diffusion current density (Fig. 11e). 

The completely different situation was observed at the overpotential outside the plateau of the 

limiting diffusion current density. Dendritic growth was completely inhibited, and the honeycomb-

like structure was formed [2, 26] (Fig. 11f). The main characteristics of this structure type are holes 

formed by detached hydrogen bubbles and small agglomerates of approximately spherical grains 

formed around them. The absence of dendritic growth at an overpotential of 1000 mV and the 

cauliflower-like character of the particles obtained after a removing the deposit from the 

honeycomb-like structure is clearly visible from Fig. 11g. 

The honeycomb-like structures are formed in the conditions of vigorous hydrogen evolution 

reaction, where vigorous hydrogen evolution changes the hydrodynamic conditions in the near-

electrode layer leading to the decrease in the thickness of the diffusion layer of the macroelectrode, 

the increase in the limiting diffusion current density, and to the decrease of the degree of the 

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diffusion control of the electrodeposition [29]. Formation of the cauliflower-like forms (Fig. 11g) 

instead of dendrites (Fig. 11d) really indicates the lower degree of the diffusion control at an 

overpotential of 1000 mV than at 625 mV. 
 

(a) (b) (c) 

   

(d) (e) (f) 

   

 (g)  

 

 

 

Figure 11. The disperse forms of Cu obtained by electrodeposition from 0.10 M CuSO4 in 0.50 M H2SO4:  

(a) the carrot-like forms ( = 210 mV) [28], (b) globules ( = 210 mV) [28], (c) the cauliflower-like forms  

( = 550 mV) [31], (d) the 3D pine-like dendrites ( = 625 mV) [31], (e) the mixture of the 3D pine-like 

dendrites, holes and cauliflower-like agglomerates of Cu grains ( = 625 mV) [31], (f) the honeycomb-like 

structure ( = 1000 mV) [31], g) the cauliflower-like particles obtained in the conditions of vigorous 

hydrogen evolution ( = 1000 mV) [31]. 

The amounts of evolved hydrogen during Cu electrolysis at high overpotentials or current 

densities are quantified by determination of the average current efficiency of hydrogen evolution 

(I,av(H2)), and there is a critical value leading to the change of the hydrodynamic conditions in the 



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near-electrode layer. This value is estimated to be about 10.0 % [30]. The values obtained at 

overpotentials of 625 and 1000 mV were 2.0 and 46.8 %, respectively [31], indicating that the value 

obtained at the overpotential inside the plateau of the limiting diffusion current density was below, 

while the value obtained at the overpotential outside the plateau of the limiting diffusion current 

density was above the critical value for the change of hydrodynamic conditions in the near-electrode 

layer.  

In spite of very different macrostructure of the formed disperse forms (dendrites or cauliflower-

like forms), Cu crystallites were random oriented in the both types of particles [31]. This random 

orientation can be attributed to the similar micro structure, i.e. to existence of spherical morphology 

in them. Namely, the both types of disperse forms consisted of small agglomerates of approximately 

spherical grains at the micro level.  

The inert metals 

For this group of metals, there is a parallelism between metal electrodeposition process and 

hydrogen evolution reaction in the whole range of potentials and current densities. For that reason, 

it is no possible to record the polarization curve in a classical way, but it is possible with an 

application of IR drop compensation technique [2,32,33]. 

Nickel 

The spongy-like particles are formed by Ni electrodeposition in the powder production range 

(Fig. 12). The main characteristics of this particle type are holes formed by detached hydrogen 

bubbles surrounded by cauliflower-like agglomerates of approximately spherical Ni grains. It is clear 

that this particle type has the honeycomb-like structure. 

 

 
Figure 12. The spongy-like particles of Ni obtained by electrolysis from  

0.10 M NiCl2 + 1.0 M NH4Cl + 0.70 M NH4OH under the condition of limiting diffusion current density [35]. 

There is no any difference among the spongy-like particles of Ni formed from various electrolytes 

[2,33–36], and the spongy-like particles of the other metals from this group, like Co [2,33,37], and 

Fe [2,38]. It is understandable because this particle type is only determined by vigorous hydrogen 

evolution as a parallel reaction to metal electrolysis in the powder production range. 

Crystallites of Ni were random oriented in the spongy-like particles [36]. 

Molten salt electrolysis 

The XRD analysis of products of molten salt electrolysis of magnesium nitrate hexahydrate 

showed that the mixture of MgO/Mg(OH)2 was formed [39,40]. The network of the intertwined 

needles was formed in the UPD (underpotential) region (Fig. 13a). The holes formed by detached 

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hydrogen bubbles and very long needles often grouped into the flower-like aggregates were formed 

by molten salt electrolysis in the OPD (overpotential) range (Fig. 13b). The hole size decreased, while 

their number increased with increasing the overpotential of electrodeposition (13c and d). 

 

(a) (b) 

  

(c) (d) 

  
Figure 13. Morphologies of MgO/Mg(OH)2 deposits: (a) the UPD region; the network of the intertwined 

needle,  (b) the OPD region; very long needles grouped into the flower-like aggregates, (c) the dish-like hole; 

 = 200 mV, and (d) holes constructing the honeycomb-like structure;  = 1000 mV [39]. 

General discussion of the presented results 

The polarization curves for Pb, Ag, Zn and Cu normalized to the limiting diffusion current density 

values are shown in Fig. 14. Since the plateaus of the limiting diffusion current density at the 

polarization curves for Ag from the nitrate electrolyte (Ag(NIT)) and Pb are with a slope to the 

overpotential axis, the values of the current density corresponding to the inflection points are taken 

as the limiting diffusion current densities. The shape of polarization curves exclusively depends on 

the type of electrolyte, while only changes in the length of the plateau of the limiting diffusion 

current density were observed with various concentrations of both the depositing ions and the 

supporting electrolyte [6,10,11,27,30]. As a result of the decrease of the exchange current density 

(or the rate of electrodeposition), moving of the plateaus of the limiting diffusion current density 

towards the higher overpotentials of electrodeposition is observed. Simultaneously, the change of 

shape of a dendrite from the needle-like and the 2D fern-like dendrites to the 3D pine-like dendrites 

was observed with the decreasing exchange current density values.  

The process of complex formation with depositing metal ions lowers the exchange current 

density value of metal, where the degree of decrease of the exchange current density depends on 

the strength of formed complex [12,13]. For example, in the case of Ag, Ag makes enough strong 



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complex with ammonium ions (this electrolyte is denoted with Ag(AM) in Fig. 14) that a transfer of 

Ag from the group of the normal to the group of intermediate metals is enabled.  

 

 
Figure 14. The polarization curves for Pb, Ag, Zn and Cu normalized to the limiting diffusion 

current density values 

All shown shapes of dendrites follow the electrochemical definition of dendrites. From the 

electrochemical point of view, a dendrite is defined as an electrode surface protrusion which tip 

grows under activation control, while electrodeposition to the macroelectrode is predominantly 

under diffusion control [1,41–45]. This definition follows from an application of the general theory 

of disperse deposit formation, based on the concept of local diffusion fields formed around the tips 

and the top edges of surface protrusions formed in the initial stage of electrodeposition and buried 

deep in the diffusion layer of the macroelectrode. Two types of local diffusion fields are responsible 

for formation and growth of disperse forms of various shape: (a) spherical – formed around the tips, 

and (b) cylindrical – formed around the top edges of growing protrusions. According to this theory, 

the radius of the spherical diffusion layer formed around the tip of the protrusion is equal to radius 

of the protrusion. During electrodeposition inside the plateau of the limiting diffusion current 

density, the outer limit of the diffusion layer of the macroelectrode was not disrupted. After the 

inflection point, the electrodeposition system remains diffusion controlled and the rapid increase in 

the current density with increasing the overpotential is a result of the fast growth of dendrites and 

a strong increase of the electrode surface area. Then, using the electrochemical definition of a 

dendrite, it follows that this sudden and rapid increase of the current density with the increasing 

overpotential can be mainly ascribed to the activation controlled electrodeposition at the tips of the 

formed dendrites. It is very clear that the tips of both primary and secondary branches contribute 

to the overall control of electrodeposition process causing the disruption of the outer limit of the 

diffusion layer of the macroelectrode.  

It is no valid the general theory of disperse deposits formation for formation of the honeycomb-

like structures. This structure type is formed in the conditions of vigorous hydrogen evolution, and the 

concept of "effective overpotential" is proposed to explain formation of this structure type [2,29]. 

According to this concept, when hydrogen evolution is vigorous enough, then electrodeposition 

process occurs at some overpotential which is effectively lower than the specified one. From 

morphological point of view, it means that morphologies of metal deposits become similar to those 

obtained at some lower overpotentials where there is no hydrogen evolution, or it is too small to 

affect the hydrodynamic conditions in the near-electrode layer. Formation of cauliflower-like 

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agglomerates of Cu grains at the overpotential outside the plateau of the limiting diffusion current 

density (Fig. 11g) instead of the dendrites formed inside the plateau of the limiting diffusion current 

density (Fig. 11d) really indicated the lower degree of the diffusion control at the overpotential outside 

than inside the plateau. In this case, the fast increase in the current density with increasing the 

overpotential after the end of the plateau of the limiting diffusion current density is a result of vigorous 

hydrogen evolution. 

In spite of vigorous hydrogen evolution accompanied by formation of numerous holes from the 

detached hydrogen bubbles (the both dish-like holes and those constructing the honeycomb-like 

structures) [39], formation of very long needles of MgO/Mg(OH)2 clearly indicates that this molten 

salt electrolysis process belongs to the very fast electrochemical process. 

Conclusions 

The following conclusions concerning formation of disperse forms by electrolysis can be derived: 

a) The shape of disperse forms depends on parameters of electrolysis like kind of electrolyte, 

b) The shape of disperse forms depends on affiliation to the determined group of metals: 

I. normal metals: the needle-like and the 2D fern-like dendrites, crystals of regular and 

irregular shape, granules, cobweb, mossy, filaments, boulders,  

II. intermediate metals: carrot-like and cauliflower-like forms, globules, the 3D pine-like 

dendrites, and 

III. inert metals: the spongy-like particles, 

c) The decrease of the exchange current density leads to the change in the shape of dendrites 

from the needle-like and the 2D fern-like dendrites to the 3D pine-like dendrites, and 

d) Vigorous hydrogen evolution changes a mechanism of formation of disperse forms from 

application of the general theory of disperse deposit formation to application of the concept of 

“effective overpotential”, and 

e) The decrease of the exchange current density leads to a change of crystal structure of disperse 

forms from the strong (111) preferred orientation observed in the needle-like and the 2D fern-

like dendrites to almost randomly oriented metal crystallites in the 3D pine-like dendrites, the 

spongy-like and the cauliflower-like particles.  

Acknowledgements: This work was supported by the Ministry of Education, Science, and Technological 
Development of the Republic of Serbia under the research project: “Electrochemical synthesis and 
characterization of nanostructured functional materials for application in new technologies” (project no. 
172046). 

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