,

Indian Journal of Chemical Technology Vol. 6, November 1999, pp. 317-324

Electrodeposition mechanism of aluminium from aluminium chloride-N-(n-butyl)pyridinium chloride room temperature molten salt

M R Ali· , A Nishikata & T Tsuru

Department of Metallurgical Engineering, Tokyo Institute of Technology, 2-1 2-1 O-okayama, Meguro-ku, Tokyo, Japan.

Received 5 October 1998; accepted 27 October 1999

Electrodeposition of aluminium has been carried out by controlled-current and controlled-potential methods from acidic aluminum(lII) chloride-N-(n-butyl)pyridinium chloride (BPC) molten bath at room temperatUre. The electrodeposition of aluminium from acidic AlClrBPC melt occurs via instantaneous nucleation mechanism in the very initial stage of the crystal growth. The deposition reaction mechanisms of aluminium in the acidic AlCh-BPC molten bath are revealed by electrochemical analysis. The experimental Tafel slo6e of 20 mY dec- I and the calculated transfer coefficient (ue) of 3 suggest that the rate determining step is a chemical reaction involving the release of the complexing agents via three consecutive single electron transfer steps. The influence of various conditions on electrodeposition and the morphology of the electrodeposited layers have been investigated by X-ray di'ffractometry and scanning electron microscopy. On increasing the current density smaller particle size and better adhesiveness of the electrodeposited layers have been obtained. The cathodic current efficiency for the deposition of Aluminium is about 99.8%.

Aluminium is a very important metal in industry owing to its many excellent characteristics, e.g., good electrical and thennal conductivities, low density, special ductility etc. It is used widely as a construction material for automobi les, av iation, household appliances, containers and electronic devices l

-3

. Aluminium coatings have appeared as an extremely important way for the corrosion protection of steel in recent years because it is always covered w ith a passivating native oxide film . Although the industrial need for a high quality AI coating has'been recognized for many years, there is still no industrial electroplating bath for AI. Considerable attention has a lready been given to find a suitable electrolyte that can be used in either Al electrowinning or AI electroplating at ambient temperature so as to reduce energy consumption and facilitate the operation. Unfortunately, aluminium cannot be e lectrodeposited from protic solvents because decomposition of the solvent with hydrogen evolution occurs at the potential required to deposit the meta l; so that the current efficiency of A I is essentially zero in prot ic solvent bath. Therefore, only molten salt and water­free inorganic or organ ic e lectrolyte systems will presumably be suitable for electrolytic deposition of a luminum. The aluminum-chlori(k / alkali-metal

.. ror cOITespondence Permanent address : Department of 1\ r plied Chemistry & Chemical Techno logy, Rajshahi llntver: il '. Rll'\nat>i '-~:lng,a esh

halide system with its lower melting point received a great deal of attention, but it was found later that pyridine compound mixed with AICh fonn systems that are molten at, or near, room temperature. More attention has since been given to these mixtures.

In 1951 , Hurley and Wier 4 reported the use of the system 2: I AICh-ethylpyridinium brom ide (EPB), which was molten at room temperature, as a plating bath for AI. Unfortunately, it was fo und that the me lting point of the AIClr EPB system increased sharply as the composition was changed5

. Only the 2: I melt was molten at room temperature. There have been several reports by Osteryoung and co-workers 5- 9

on studies with AICh-N-(n-buty l)pyridin ium chloride (BPC) in recent years . They have shown that this system is molten at ambient temperatures over a w ide composition range (mo lar ratio 0.75: I to 2: I AICI 3-

BPC)6, and that it has excellent properties as a so lvent for inorganic, organic and organo-metall ic studies.

F r AICh-BPC system, Raman spectroscopic studies 'o indicate that in the neutral 1' 1 melt, aluminium is present almost entirely as AICI4- ,

whereas in the 2: I melt it is present as AI 2Ch-. In melts having a molar ratio between I : I and 2: I , both AhCI 7- and A IC I~- will be pr sent. Osteryoung and co-workers7

•9 have shown the fo llowing {'quii ibrill nl

10 eXIst in t1llS svstem~

318 KNDIAN J. CHEM. TECHNOL., NOVEMBER 1999

I'"

D ,-~

\ I-

8 ---1=

1= " 7 "

.J

~I t-b

7

'~I'---

" 6

2

3

4 5

Fig. I-Schematic diagram of the electrolytic cell : I) tungsten wire, 2) bulk compartment, 3) counter electrode (anode), 4) working electrode (cathode), 5) bulk electro lyte, 6) glass frits, 7) reference electrode compartment, 8) reference electrode.

In the AICh-BPC system, the deposition of ~{uminium from AICI4 - ion cannot take place since the reduction of the n-butyl-pyridinium cations (BP+) occurs at potential more positive than that of AICI4-

9.

Aluminium deposition can therefore only proceed by the reduction of AI2Ch-. Although the AICI4 - ion cannot be reduced directly at the electrode to form aluminum, it can affect the aluminium electrodeposition reaction via the dissociation reaction Eq. (I). The deposition of aluminium from AIC h­BPC molten salts has been reported by a number of electrochemists l

(}"' 12. However, the reaction mechanism of aluminium deposition in AICh-BPC molten bath is sti ll unfounded.

The aim of the present study is to investigate on the mechan ism of aluminium deposition in the AICh­BPC molten bath and to search for optimum conditions to get smooth and silver bright aluminium electrodeposit by changing the composition.

Experimental Procedure

Chemicals-Both Aluminium chloride (AICI)), 99.99%, and n-(n-butyl)pyridinium chloride (BPC), 99 .99% in sealed glass tubes was supplied by Sowekawa Chemil;als Co., Tokyo, Japan.

60

-- 2:1 melt

40 .. ~ :::. 20

.~ on

0 c -8 d

~ -20 U

-40

-60 -1.5 -0.5 0.5 1.5

Potential, EIV vs. AVAIl"

Fig. 2-Cyclic voltammogram of I : I and 2: I (mole ratio) AIClr BPC melt on Pt electrode. Sweep rate, 0.01 V s-'.

Melt preparation-The melts were prepared by simple addition of accurately weighed aliquots of AICh to BPC to get the desired mole ratio. Since both AICh and BPC are extr~mely sensitive to moisture and oxygen, all operations were carried out in a dry glove box under an argon atmosphere containing les! than I ppm H20 and O2• BPC was placed in a glass bottle and AICI) was then very slowly added with constant stirring aided by a magnetic stirrer. The reaction of AICI) with BPC is highly exothermic and so care was taken not to permit the temperature of the mixture to rise above lOODC at which thermal decomposition occurs. The resulting melt was then purified by the vacuum evaporation of any hydrolyzed HCI produced. In case of 2: I AIC I3-BPC melt, the vacuum purification was performed after immersing some small pieces of pure Aluminium (99.99%) in the melt. The purification was carried out step wise until the melt became colourless.

Electrochemical cell and electrodes-The electrolytic cell, made of Pyrex glass with a fitted Pyrex glass cap, used in the present work is shown in Fig. I . The experimental cell had two compartments the secondary or bulk and the reference electrode compartments. The reference electrode compartment was separated from the secondary compartment by a fine porosity glass frits. The cell cap was fitted with three tungsten wires to faci litate the mounting of the secondary and reference electrodes in respective compartment. About 10 cm) of AIClrBPC melt and I cm3 of 2: 1 AIClrBPC melt were introduced into the bulk and iderence electrode compartments re~pective ly in all experiments.

ALI et al.: ELECTRODEPOSITION MECHANISM or ALUMIN IUM 319

Table I-Experimental conditions and results of the electrodeposition of aluminium from

acidic AIClr BPC molten salt. ', 35°C;b, 25°C.

Melt Deposition conditions Deposit Current Evaluation of

composition, mole Controlled -potential, plating, Controlled -current plating, thickness, efficiency, deposit ratio of AICI) :BPC App!:;!d potential, £IV Applied current, i/A m-2

J,lm %

1.l6:1" - 0.30 1.22:1" -D.27 1.33: I" -D.20 1.l 4:1" -D.25

-5 -10

- 20

- 8

- 16

-32

Platinum (0 .1 x 5 x 50 mm) or mild steel (0.2 x 10 x 50 mm) and pure alUminium to.) x 10 x 50 mm) plates were used as working and counter electrodes respectively, which were immersed in the bulk electrolyte. Pure aluminium plate (0 .5 x 2 x 50 mm) was used as a reference electrode and was immersed in 2: I AlCh-BPC melt of the reference electrode compartment. The melt level in the reference compartment was kept slightly higher than the bulk melt. Throughout this work, all potentials were quoted with respect to this A1I2: I AlCI 3-BPC reference electrode which would be written as All A13

+ electrode in this paper.

Electrochemical instrumentation-All cyclic voltammetry, chronoamperometry and chronopotentiometry for different electrolytes were performed using a Hokudo Denko HAB-151 potentiostatigalvanostat, Tokyo, Japan, equipped with a potential sweeper. Cycl ic voltammogram were recorded on an X-V recorder (Type 3077, Yokogawa, Japan). Chronoamperograms and chronopotentio­grams were recorded on an X-t recorder (SP-H5 P, Riken Denshi Co., Ltd., Japan). All electrochemical measurements were carried out at room temperature. The res istance of the electrolytes (2: I and 1.5: 1 AICh-BPC melts) in the three electrode systems were measured by the impedance method with a frequency response analyzer (FRA) coupled to a GPlB and an

4 Fine particles, smooth surface

6 Average deposit, smooth surface

6 Big particles

6 A verage deposit, smooth surface

6 90 Big particles 6 98 A verage deposit, smooth

surface 6 99.9 Small particles, smooth

surface

6 99.9 A verage deposit, smooth surface

6 99.9 A verage deposit, smooth surface

6 99.9 Small particles, smooth surface

IBM computer. A more detailed description of the instruments in impedance measurement techniques was reported earlier13

. Impedance measurements were carried out between 10 kHi and 10 mHz. The resistance of the electrolytes was obtained from the measured impedance in the high frequency range. The measured solution resistance of the electrolytes were 132 and 137 ncm for 2:1 and 1.5:1 AICh-BPC melts respectively.

Bulk aluminium was electrodeposited onto platinum and steel cathodes by controlled-current and controlled-potential methods at 25°C from acidic AIC I3-BPC melts of different mole ratios.

Deposit characterization- The surface morphology of the electrodepos ited layer was noticed by scaiming e lectron microscope (SEM) (ABT-55 , TOPCO , Tokyo, Japan) equipped with an Olympus camera. The crystall in ity and structure of the electrodeposited aluminium sample was studied by X-ray di ffractometer (Shimadzu, Japan, XD-3A) using Ni­fi ltered CuKa radiation (30 kV, 20 mA) with a measuring speed of 1° min- I and time constant 2 s. The X-ray d iffraction patterns were recorded on a chart recorder. The aluminium content of the electrodeposited fi lm was quantitatively analyzed by electron probe microanalyzer (EPMA)14, (EPMA-8705, Shimadzu, Japan), using the standard sample of pure al uminium (99 .99%).

320 INDIAN 1. CHEM. TECHNOL., NOVEMBER 1999

8.0

6.0

~ 4.0

~ 2.0 .~ II)

0.0 ~

~ ~ -2.0 ~ U -4.0

~.OL-~~~~--~~~~~--~~~

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2

Potential, ElY vs. AUAJ3+

Fig. 3-Cathodic and anodic polarization curves of acidic AIClr BPC melt. Working electrode, Pt; sweep rate, 0.00 1 Y S- I.

Reduction Oxidation

20 mV/dec 20 mVidec

10°L---~~--~--~~~--~--~--~

-0.25 -0.15 -0.05 0.05 0.15

Potential, ElY vs. AU AP+

Fig. 4- Tafel plot for the reducti on ofaluminum(ll l) in 2 : I AIClr BPC melt. Sweep rate, 0.5 mY S- I; re fe rence electrode, AIIA IJ+ (in 2: I AI ClrBPC melt).

Results and Discussion

Vollammetry of AIClrBPC melt-The cyclic vo ltammograms of I : I and 2 : I (mole ratio) AICh­BPC melts at platinum electrode are shown in Fig. 2. In 2: I melt, there is a reduction wave at about zero vo lt « 0 V) and in the reverse scan, there is a oxidation peak at 0.2 V. The increase of cathodic current at about zero volt is obviously associated with the reduction of AI 2Ch- to metallic aluminum. Bu lk al uminium has been deposi ted from this reduction wave by controlled-potenti al and also controlled­current methods. Therefore, the oxidation peak at 0.2 V, which corresponds to the reduction wave of aluminum. is attributed to th~ dissolution of the

o

.(l.21 V

.(l.13 V

__ ----- .(l.1l V

__ - - .(l.09V

0;;:::::==-_------ .(l.07 V

o 10 30 50 10 90

Time, s

Fig. 5- Potentiostatic current-time transients from 2 : I AICIJ-BPC melt at di ffe rent applied potenti al.

deposited aluminium from the Pt electrode. In the I : I melt, there is reduction wave at about - 1.0 V and in the reverse scan, there is a very sma ll oxidation peak at about -0.4 V and a second small oxidation peak at 0.35 V. In the I : I melt, the reduction potential of AICI4 - and BP+ ions are below - 1.0 V and about ~0.8 V 9 respectively. Therefore, the deposition of pure aluminium from 1: I melt is difficu·lt because BP+ ion is also reduced and deposited on the cathode. In acidic melt, the deposition of al uminium occurs by the reduction of dialuminium heptachloride ion, whose reduction potential is about zero vo lt with respect to All A1 3

+ reference e lectrode. The ac idity of the electrolyte is increased with the increase of AIC I3

concentration in the neutral me lt (I: I melt) and the activity of A2Ch - increased with the increase of acidity of the melt6

.

Fig. 3 shows the cathod ic and anodic polari zation curves for the deposition and dissolution of the deposited aluminium in different mole ratios of AICh­BPC melt. Silver-white aluminium was deposited from each of the molten salt bath which are g iven in Table 1. The equilibrium potential (Eeqm) (potential at zero current) for the AI/A2CI7- couple has been shi fted to the cathodic direction with the decrease of AICh to BPC molar ratio in the electrolytes. This behavior occurs due to the decrease of the activity of A2Ch- in the melts.

Potent iostatic current-time tran sients were measured for 2: I AICl r BPC melt on a platinum electrode. After every run the deposited a lum inium was removed from the surface by polarizing the working elechode anodically. Fig. 4 shows the

•

ALI el aL.: ELECfRODEPOSmON MECHANIS~ . OF ALUMINIUM 321

1.8 ----0-,- 0.11 V

-O.J3V

1.6 ---O.J7V

---- 0.21 V

1.4

'Sl! 1.2 .2

0.6 '---~~-'--.....I.'--.....~.Io...-.o ___ ~'--'-~~-----.J

-0.5 0.0 0.5 1.0 1.5

logt

Fig. 6--Plots of log i versus log I for the rising part of the i-I transient in Fig. 5.

potentiostatic i-I transients measured at different applied potentials. After the initial charging of the double layer, the current drops to a minimum and then rises with time, showing a maximum before finally decaying in the usual way with time. The time corresponding to the maximum and minimum of the current transient decreases considerably with increasing applied potentials. The most interesting part of the i-I transients is the rising portion of the peak which corresponds to the growth of the monolayer before overlapping the first monolayer of the growth species and it can therefore be used to determine the kinetics of the growth of nuclei!s. -Tmax

x is thus related to the time at which full coalescence of the crystallites occurs!6.

In the case of a mechanism controlled by mass transfer before overlap, there are two types of nucleation!7 .

... (2)

for instantaneous nucleation in which the nuclei are formed at the beginning of the pulse; and

i=(3/2)1lZFAk(2Dc)3/2Mf2Nop-1I213/2 ... (3)

for progressive nucleation in which the nuclei are continuously formed during the crystal growth. In the equation, zF represents the molar charge of the depositing species, D the diffusion coefficient, M and p the molecular weight and density of the depositing materials respectively, No the number of nucleation sites at t = 0, and k the nucleation rate constant. Therefore, by plotting log i vs log t, the value of the

j........... , )

Fig. 7- Scanning electron micrographs of the coated Aluminium layer electrodeposited from 1.5: I (mole ratio) AIClrBPC melt at different applied deposition current density. Deposition current density: (a) - 5, (b) - 10 and (c) - 20 A m-2

measured slope is a constant depending primarily on the geometry and type of nucleation, where the slope is 0 .5 for instantaneous nucleation and 1.5 for progressive nucleation respectively. Fig. 5 shows the log i vs log I plot for the rising part of the i-I transients in Fig. 4. The measured slopes at applied potential -0.17 and -0.21 V are 0.55 and 0.52, which are quite close to 0.5. This suggests that the nucleation process of aluminium f~om 2: 1 AICh-BPC melt most probably proceeds instantaneously in the very initial stage of the crystal growth. On the other hand, the

322 lNDlAN J. CHEM. TECHNOL. , NOVEMBER 1999

Fig. 8- Scanning electron micrographs of the coated Aluminium layer electrodeposited from 2: I (mole ratio) AIClr BPC melt at different applied deposition current density. Deposition current density: (a) - 8, (b) - 16 and (c) - 32 A m-2

measured slopes at potential -0. 13 and -0.11 V are 0.67 and 0.72 respectively. It is difficult to connect

these values with the exact nucleation mechanism because these values are between 0.5 and 1.5. The increase of slope with decrease of applied potential could be due to decrease of the nucleation rate in the very initial stage of crystal growth.

The charge transfer mechanism-Fig. 6 shows the IR drop-compensated Tafel plot for electrodeposition

I

of aluminium and the disso lution of the deposited aluminium on a Pt electrode in 2: 1 AICb-BPC melt at

" 8 - t:!-

=- < <

" " S N t:!-

<

---' '----'

35 45 55 65 75

29/degree CuKa

~ 8 - (:!,

=- < < + ~ ts

___ L...I

35 45 55

29/degree

65

S N (:!,

< + ~ ts

75

CuKa.

( a)

"

:::-C <

;::; N t:!-;(

" ,--,LA-

85

~ lb) N N (:!,

;( +

:::- ~ c ts ;(

V '0-

85

Fig. 9-X-ray diffraction patterns of the electrodeposited Aluminium layer. Deposi tion current density : (a), - 8 and (b) -6 II m-2

; substrate: (a), Pt and (b), mild steel.

at room temperature. The Tafel slope for the reduction of aluminum(III) in 2: I and also in 1.5: I melt is 20 m V dec-I and the calculated transfer coefficient (a

c)

is 3 . Gale l 8 reported that the main aluminum­containing species in 2: I melt is A2CI7- ; whereas in the 1.5: I melt, the ratio of the mole fraction of A2C17-

and AICI4 - species is 1.2: I. The deposited product from the 1.5: I melt was also pure and silver-white aluminum. Osteryoung9 reported that aluminium deposition from AICb-BPC melt can on ly proceed by the reduction of A2Ch - at about 0 V with respect to All A13

+ reference electrode. Therefore, the starting species for the deposition of Aluminium in ac idic AICI3-BPC melt is A2Ch-. The following cathodic reaction has been reported by a number of authors for

ALI et al.: ELECTRODEPOSITION MECHANISM OF ALUMINIUM 323

the de~osition of aluminium in 2: 1 AIClrBPC meltlO-I :

... (4)

In 1954, Lemkuhl19 reported the ,deposition and dissolution reaction mechanism of aluminium from the new organoAluminium electrolyte. If we consider tile multi-step reaction for the deposition of aiuminium in acidic AICh-BPC melt, then the reduction reaction of trivalent aluminium ion has to include at least three reaction steps, which are as follows,

Ae++e-~AI2+ .

Ae++e-~Ar

. .. (5)

... (6)

... (7)

When the first step is tht: rate determining step, the Tafel slope should be 120 mY dec - I. On the other hand, the rate determining step of the second and third step leads the Tafel slope of 40 and 24 mY dec- I respectivello. The experimental results of 20 mY dec- I does not fit for these simple reaction mechanism. The measured Tafel slope and the calculated. transfer coefficient suggest that the rate determining step for the reduction of A2Ch- ions in acidic AICh-BPC melt is a chemical reaction step involving the release of the complexing agent, which

will occur after three consecutive single-electron transfer steps. Therefore, the following mechanisms can be proposed for the deposition of aluminum:

A2Ch-+e-~ AICh(ad)+AICI4-+Cr

AICh(ad)+e-~ AICI(ad)+Cr

AICI(ad)+e-~ AICr(ad)

AICr(ad)+e-~AI+Crcrds)

... (8)

... (9)

... (10)

... (II)

The proposed reaction mechanism leads to the

theoretical Tafel slope, BE/Blog I 7 Ie = 2.303RT/ ae F =

60/ ae mY, and the transfer coefficient, ac = 3 + 0 x

p = 3, where P is a symmetry factor. Electrodeposition of Aluminium from AIClrBPC

melt-The average quality of ~ deposit is defined by

The deposited aluminium particles from low acidic (1.16: I molar ratio) electrolytes are small in size. The size of the deposited particles is increased by increasing the molar ratio of AICh to BPC. Moreover, the size of the deposited particles depends also on the applied deposition current density.

Figs/ and 8 show the scanning electron micrographs of the aluminium layer deposited from 1.5 : I and 2: I AICh -BPC melts at different applied current density. The size of the deposited particles is decreased with the increase of applied current density at constant composition and temperature. At low current density, the discharge of AI2Ch - occurs on the cathode slowly. Consequently, the growth rate ' of nuclei is greater than the rate of formation of new nuclei, and so the deposited particles should be large . As the current density increases, the formation of new nuclei favoured and the particles become finer. The current efficiency for the deposition of ~Iuminium from 1.5: I and 2: I AICh-BPC melts are given in Table I. The current efficiency for aluminium deposition frum 2: I melt IS about 99.1Wo between the applied deposition current densities of -8.0 and -32.0 A m-2

. On the other hand, the current efficiency is about 90% at applied deposition current density -5 .0 A m-2 for 1.5 : I melt. The current efficiency is also increased with the increase of applied deposition current density. The decrease of current efficiency with the decrease of applied deposition current density in low acidic melt is most likely due to the impurities present in the melt (which was not purified as much as for the 2: I melt) . The deposited layer was analyzed by the X-ray diffractometry. The diffraction peaks at 28 = 38.5°, 44.7°, 65.1 °, 78 .2° and 82.4° (Figs 9a and 9b) correspond to AI. All diffraction peaks of the deposited AI were very sharp. The X-ray diffraction pattern indicates that the deposited aluminium was of crystalline structure. The X-ray and EPMA (Electron Probe Micro Analyzer) analysis suggest that the deposited layer is composed of on ly pure aluminum.

Conclusion

its coherence and crystallization at the surface; (i) ~on-porous, crystalline and silver-white therefore, a poor deposit is the one having sever aluminium electrodeposited layers has been irregularity and appearance of a dark spongy mass on obtained by controlled-current and controlled-the surface. In the present study aluminium deposition potential methods from AIClrBPC molten sa lts experiments were carried out only from acidic melts over a wide range of composition (1.16: I to 2: I (molar ratio 1.16: 1 to 2: I AICI3-BPC) under molar ratio). The size of the deposited particles is contro lIed-potential and contro 11 ed -c urren t m eth od""sh' ___ ~d~e~cr~eC!:!a~se~d:!...-.!..!w:..!.i t~h~t~h~e---!.i.!.!.n c~ru.e~a~se"--Ql6.f"--tw.h""e"""""aw.D",,D,,,,1 i .... e ... d

324 INDIAN J. CHEM. TECHNOL. , NOVEMBER 1999

deposition current density and this is due to the enhanced rate of formation of new nuclei.

J

(ii) The electrodeposition of aluminium from acidic AlCh-BPC melt proceeds via instantaneous nucleation mechanism in the very initial stage of the crystal growth.

(iii) The experimental Tafel slope and the calculated transfer coefficient (iie) for the reduction of

AI2Ch- ion in acidic AlCb-BPC melt was 20 mY dec-1 and 3 respectively.. Following three successive electron transfer steps, the rate determining step is a chemical reaction which involves the release of the complexing agent.

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