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Applications of magnetoelectrolysis

Citation for published version (APA): Tacken, R. A., & Janssen, L. J. J. (1995). Applications of magnetoelectrolysis. Journal of Applied , 25(1), 1-5. https://doi.org/10.1007/BF00251257

DOI: 10.1007/BF00251257

Document status and date: Published: 01/01/1995

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Download date: 03. Oct. 2021 JOURNAL OF APPLIED ELECTROCHEMISTRY 25 (1995) 1-5

REVIEWS OF APPLIED ELECTROCHEMISTRY 38 Applications of magnetoelectrolysis

R. A. TACKEN, L. J. J. JANSSEN Department of Chemical Engineering, Laboratory of Instrumental Analysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Received 31 January 1994; revised 1 June 1994

A broad overview of research on the effects of imposed magnetic fields on electrolytic processes is given. As well as modelling of mass transfer in magnetoelectrolytic cells, the effect of magnetic fields on reaction kinetics is discussed. Interactions of an imposed with cathodic crystalli- zation and anodic dissolution behaviour of are also treated. These topics are described from a practical point of view.

Nomenclature ml, m2 regression parameters (-) n charge transfer number (-) al, a2 regression parameters (-) q charge on a particle (C) B magnetic field flux density vector (T) R gas constant (Jmo1-1 K -1) c concentration (molm -3) T temperature (K) Ccx~ bulk concentration (tool m -3) t time (s) D coefficient (m2 s -1) V electrostatic potential (V) de diameter of rotating disc (m) ~3 particle velocity vector (m s -1) E strength vector (Vm -1) Ei induced electric field strength vector (V m -1) Greek symbols Es electrostatic field strength vector (Vm -1) c~ transfer coefficient (-) F force vector (N) 7 velocity gradient (s -1) F (C tool -1) AMS potential difference between phase H magnetic field strength vector (Am -1) and point just inside phase (OHP) i (Am -z) 01) iL limiting current density (A m -2) diffusion layer thickness (m) i o limiting current density without applied G0 hydrodynamic boundary layer thickness magnetic field (Am -z) I without applied magnetic field (m) I current (A) p density (kg m -3) /L limiting current (A) cr electrolyte conductivity (fU 1m -1) J current density vector (Am -2) # magnetic permeability (V s A -1 m -1) K reaction equilibrium constant u kinematic viscosity (m2 S-1) k reaction velocity constant w vorticity kb Boltzmann constant (J K -1)

1. Introduction 2. Mass transport effects

The practical relevance of the application of magnetic 2.1. Magnetohydrodynamie phenomena fields in electrochemical processes is potentially large. Improved mass transfer in cells, better deposit quality Magnetohydrodynamic phenomena arise from the and control of corrosion are just some of the effects interaction of velocity fields with electromagnetic that can be promoted. In addition, magnetic fields fields. The total force on a charged particle (elec- are powerful scientific tools in, for instance, reaction tron, ) moving in an is the kinetics and metal deposition or dissolution studies. [2]: A review paper by Fahidy [1] appeared in 1983 and F= q(E + v x B) (1) since then research has continued and progressed in the field, while its scope is broadening. This review where E is the sum of the electric and electrostatic summarizes results obtained on magnetoelectrolytic fields E1 + Es. Since the Lorentz force is capable of processes and emphasizes practically relevant aspects producing movement of charged particles such as in four main areas of interest: mass transport, reac- , a magnetic field applied during electrolysis tion kinetics, cathodic metal deposition and anodic gives rise to convection of the electrolyte. This mag- metal dissolution. netohydrodynamic effect not only influences mass

0021-891X © 1995 Chapman & Hall 1 2 R.A. TACKEN AND L. J. J. JANSSEN transfer, but also reaction kinetics and deposit tive diffusion layer the magnetic field superposition quality. Orientating and attracting ions can only be may be represented as a 'MHD-perturbation' model, achieved by using large B-gradient fields, because where the conventional convective diffusion equa- these communicate a potential energy larger than tions are modified by small order contributions from the ionic thermal kinetic energy (kbT ,,~ 0.025eV at MHD phenomena [1]. For the specific case of a verti- room temperature) [3]. cal electrode in a weak or moderate (up to 1 tesla) In magnetohydrodynamics, the vector and scalar v magnetic field being horizontal and perpendicular to and B fields are the most important variables;j and E the electrode, the rate of mass transport is propor- can be deduced from these. Fundamental equations in tional (based on free convective diffusion theory) to magnetohydrodynamics are as follows [2]: 91/4 [9, 10]. j= ~r(E + v x B) (2) When the magnetic field is vertical and parallel to vertical plate , the strongly enhancing B curl - =j (3) effect of magnetic field superposition cannot be pre- # dicted by a simple 'MHD-perturbation' model [11]. Op For this case, and low and moderate fields (up to -- div (or) (4) Ot 0.685T), Fahidy [4] made an attempt to model the Ov magnetic field effect. Analysing experimental data p~- + (v- grad)v + gradp =j x B + pvV2v + pF on deposition, where limiting current density (5) increased from 20 to 25.6 A m -2 on increasing a mag- netic field from 0 to 0.685 T, he proposed the inter- j2 relationship between limiting current density, iL, and E.j = -- -j. (v x B) (6) o- magnetic field flux density, B, for electrodes consist- ing of nonmagnetic material and where B is parallel d (p)- ~. grad v = ~curl (~-~)(7) p to the surface to be B B 2 iL = i ° + al Bin' (9) j x B = (B. grad) ~ - grad~-~ (8) where iL° is the limiting current density without Given sufficient boundary conditions, a rigorous appfied magnetic field. Using this equation, for magnetoelectrolytic mass transport model based on copper deposition: iL = 20Am -2, al = 10.916 and the above magnetohydrodynamic equations may in m I = 1.6435 was determined. By comparing the theory be solved. In practice, however, simplified experimental data on limiting current densities and models [1, 4] and empirical relations have been magnetic field strength with various mass transfer deduced. models, Fahidy estimated the magnitude of the corre- sponding diffusion layer thickness (5, and proposed the following equation: 2.2. Empirical results on magnetoelectrolytic mass (5 = (50 _ azBrn2 (10) transport where (5o is the hydrodynamic boundary layer thick- The effect of applying a magnetic field during electro- ness without applied magnetic field. lysis is strongest when mass transport is the control- Chopart et al. [12] showed for copper magneto- ling mode (limiting current conditions), because of electrolysis that a magnetic field induces a magneto- interactions of the field with the convective diffusion hydrodynamic velocity gradient, 7, at a rotating disc layer at the electrodes. Using laser interferometry electrode, which corresponds to techniques, this effect can be made visible [5]. The rela- tive strength of this effect is strongly dependent on the 11 = 0.678FDZ/3 coodV3 71/3 (11) mutual positions of the electrodes and the direction of Aaboubi et al. [13] showed that: the magnetic and gravity fields. In certain configu- rations (magnetic and electric fields parallel), mass 7 = kac~ (12) transport can even be retarded [6]. Thus, the limiting current IL is proportional to B1/3 4/3 Quraishi et al. [7] described the magnetic field coo • effects on natural convective mass transport with The beneficial effect of coupled electric/magnetic regards to the position (inclination) of circular disc fields is more manifest in multiple electrode cells electrodes. The magnetic field was directed parallel where enhancement in mass transport can be much to the gravity field. It was clear that mass transport larger relative to single pair electrode cells. In experi- was enhanced more when the electrodes were slightly ments in a multiple electrode cell by Mohanta et al. inclined to the horizontal, than to the vertical plane. [14], a rigorous quantitative analysis of mass trans- Ismail et al. [8] studied inclined plate electrodes in a port phenomena was hindered by the absence of limit- solenoidal field and found an optimal inclination ing current conditions at even weak magnetic fields. angle of 14 ° to the vertical. Cathodic current densities at room temperature When the magnetic field is not strong enough to ranged 3-4 times higher than the advisable limit interact significantly with the structure of the convec- for current densities in industrial copper- APPLICATIONS OF MAGNETOELECTROLYSIS 3 practice at high temperature. The use of nonuniform Only a few papers deal with the effect of a magnetic magnetic fields, such as solenoidal fields, which can field on the kinetics of electrochemical trans- be created by winding current carrying wires around fer reactions at electrode surfaces. Kinetic effects can the electrolysis cell, can increase mass transport to be modelled by defining a magnetically induced an even greater extent [8, 11, 15]. Experimental potential difference [27-29]. Kelly [28] uses Butler- results [15] show that comparable relative mass trans- Volmer kinetics to describe this, and has analysed port rates can be achieved in nonuniform fields whose the total anodic and cathodic polarization in a cell average strength is about one tenth of the uniform consisting of two titanium electrodes in a flowing field strength otherwise required. H2SO4-electrolyte. The effect of the induced potential A remark should be made regarding the empirical difference on current density can be written as (for a relationships mentioned. Fahidy [16] demonstrated purely activation controlled reduction reaction): the statistical indeterminacy of mass transfer depen- [--anF AMs'~ dence on density B. Various statis- i=kFcooexp~.--~ ) (13) tically justified B-exponent dependencies can be obtained when correlating the same set of experi- where AMS = potential difference between metal mental magnetic flux density/mass transfer data. phase and a point just inside the electrolyte phase Correct distinctive modelling requires data measured (OHP). Consequently, when AMS at the electrode/ over a large B-value interval. flowing electrolyte interface is changed, the rate of Finally, adverse effects of magnetohydrodynamic- electron transfer is changed. This effect was most ally induced convection have also been reported. elegantly applied by Iwakura et al. [30], who devel- Electromagnetic fields present in large oped a cell rotating in a field created by a permanent reduction cells can cause unwanted motion of the . The induced potential difference between molten electrolyte [17]. cathode and caused electrochemical reactions to proceed. In this way, direct conversion of mechani- 2.3. Magnetoelectrolytic codeposition of metals and cal energy to chemical energy is possible, using e.g. inert particles wind energy as a power source for rotating the cell. Transfer coefficients can be determined more accu- Composite materials can be produced by codeposition rately applying a magnetic field, because of an techniques: inert particles, for instance, may be increase in the potential range where Tafel's rule is embedded in a cathodically depositing metal matrix. obeyed, due to magnetohydrodynamic effects. In the Dash [18] describes the use of a magnetic field to pro- CH 2+ --+ CH + --+ CH system, transfer coefficients a duce codeposits, e.g. Cu/AI203. The combination of are not modified when a magnetic field is applied magnetohydrodynamic forces working on both elec- [12, 31]. Olivier et al. [32] and Ismail et al. [33] also trolyte ions and A1203-particles due to their surface studied copper deposition, and both suggested a charge, make it possible to codeposit particles which kinetic effect when applying a magnetic field, but did cannot be deposited using conventional techniques. not make an attempt to quantify it. Fricoteaux et aI. This change in codeposition behaviour can however [31] showed, by using radiotracers, that a magnetic also be attributed to a magnetically induced change field induces no detectable modification of the in the structure of the adsorbed ionic layer on the par- exchange current at the Cu2+/Cu-interface. Accord- ticles [19]. ing to this study, the variation in the exchange In two Japanese patents, a new method for compo- current obtained by electrochemical methods is due site electrodeposition is claimed [20, 21]. Inert par- to modifications of the structural state of the ticles were first coated with a ferromagnetic material deposit. Chiba et al. [34] and Yamashita et al. [35] (e.g. WC or A1203 coated electrolessly with ) found that a magnetic field had no effect on the and consequently codeposited in an rate-determining step in copper deposition, but did applying a nonuniform magnetic field. Codeposition increase the charge-transfer current and the efficiency can be controlled by regulating the magnetic field of the deposition process. In contrast, the rate- strength. determining step of p-benzoquinone reduction in acetonitrile shifts from mass to electron transfer 3. Kinetic effects when a strong magnetic field is applied [36]. The shift is promoted by the low viscosity of the . The effects of a magnetic field applied during poly- Chiba et al. [37] observed an increase in depo- merization [22], photochemical [23], isotopic enrich- sition efficiency applying a magnetic field; this effect is ment [24] and heterogeneous catalytic [25] reactions believed to be due to a decrease of the hydration have been investigated to some extent. Very recently, number of Zn-ions on increasing field strength, the magnetic field effect on organic chiral reactions which decreases the deposition reaction activation has received much attention [26]. It has been con- energy thus resulting in an increasing reaction cluded that the electronic structure of reaction mol- velocity. ecules and intermediates is determining for the Danilyuk et al. [38] proposed a model for reaction interaction with the field. Changes in reaction kinetic effects while applying a magnetic field during entropy have been observed. copper, nickel and deposition, which may account 4 R.A. TACKEN AND L. J. J. JANSSEN for the contradictory literature on this phenomenon. to be only a weak dependence on the type of cathode Danilyuk et al. observed that, in the regions of material and the direction of the field. Chiba et al. [44] mixed (mass transport and charge transfer) and pure also reported inhibition of dendrite growth when charge transfer kinetics, the magnetic field at a fixed applying a weak magnetic field (0.12T) during current density generates oscillations in the cathode Pb-deposition. potential. Depending on the value of the field Copper screens can be deposited on stainless strength, the electrodeposition process is either steel while electrolysing aqueous CuSO4 inhibited or accelerated. Danilyuk et al. proposed solutions in a magnetic field [50]. Simultaneous this phenomenon as arising from quantum mechani- hydronium-ion discharge and subsequent cal type interactions of the magnetic field on the con- gas evolution on the cathode surface are primarily version of the spin states of the three particles that responsible for the particular phenomenon of a interact in chemisorption at the cathode surface: an screen-type deposit structure, whose characteristics hydrated ion, an adsorption centre and an unpaired are strongly influenced by the magnetic field. Initi- conduction electron. Similar current oscillations in ally, copper is deposited only parallel to the magnetic regions of mixed control have been observed field. Thereafter cross-deposits appear. Eventually a during potentiostatic anodic dissolution of copper closely woven deposition structure appears. During (Section 5). the of magnetic wire, a thin film of magnetic alloy with uniaxial magnetic anisotropy is 4. Cathodic deposit morphology effects obtained by passing a through the fine wire itself, which serves as a cathode during electro- Applying a magnetic field during deposition, to form a magnetic field in the direction changes the crystallization behaviour of the metal of the circumference of the strand [27]. from the electrolyte. Under carefully chosen condi- tions the following effects can be promoted: (i) a 5. Anodic effects more uniform deposit morphology (microscopic as well as macroscopic) [27, 31, 33, 39, 40-43], (ii) inhi- Dash [40] was one of the first to determine the bene- bition of dendrite growth [37, 44], (iii) change in ficial effect of a magnetic field during anodic dis- macrostress of the deposit [45], (iv) increased hard- solution of metals. For copper, it was observed that ness of the deposit [29, 41, 46], (v) a more uniform when no magnetic field was applied, preferential thin- current distribution [47, 48], (vi) increased corrosion ning of a (partially immersed) anode took place at resistance [28, 29] and (vii) composition shift in alloy the air/electrolyte interface. When a magnetic field of plating [46]. 0.9 T was applied, uniform thinning at one side of the The influence of magnetic fields on crystallization anode was observed; when the magnetic field was behaviour appears most strongly at low current den- reversed, thinning took place at the opposite side. In sities, where it can be considered that the influence this way, periodic reversal of the field led to uniform of the magnetic field is larger than that of the electric dissolution without preferential at the air/ field. Chiba et al. used X-ray analysis to prove this for electrolyte interface. nickel [41]. In contradiction, Yang [49] observed that The anodic dissolution of copper in acidic the presence of a magnetic field of 0.54T, either and in neutral ones to which specific parallel or perpendicular to the cathode, had no additives, e.g. thiocyanate ions [51], have been added effect on the types of crystal orientation in Fe, Ni exhibits current oscillations at constant potential con- and Co deposits. However, Yang did observe macro- ditions. Oscillations are observed in the transition scopic effects: when the field was perpendicular to the zone between the charge-transfer and fully mass trans- cathode, the surface of the deposit became very rough fer potential regions, and appear after a certain induc- and covered with projections protruding in the direc- tion period during which the initially deposit-free tion of the field. The morphology of Cu-Ni alloy was anode becomes covered with cuprous and cupric studied by Chiba et al. [41], using scanning electron . At the onset of oscillations, the surface is microscopy (SEM). The presence of a magnetic field fully covered with oxides. enhanced the preferred growth direction, indicating Applying a magnetic field is a powerful tool to a cored or cereal-type structure, i.e. Cu-rich and study and even stop this oscillatory behaviour. Gu Ni-rich strata in the solid solution matrix. Contra- et al. [52] showed that during the induction period dictory observations on crystal growth may be the rate of copper formation is proportional explained using Danilyuk's theory [38]: certain to B -1/4. As a result of the field, the total induction values of the magnetic field influence the deposition time until oscillations start increases [53]. The oscil- reaction markedly, others do not have any influence. lations are destabilized due to mass transport or O'Brien and Santhanam [39] reported remarkably kinetic interactions [54, 55], and a shift to more uniform zinc deposits in the cathode over anode posi- positive potentials is observed [50]. Under specific tion in the electrolysis of ZnSO4, while pulsing the conditions, the field can suppress the oscillations com- current in a magnetic field. Growth of dendrites pletely. Thus, application of a magnetic field can be when depositing zinc from alkaline zincate baths used to control oxide-based corrosion of copper, was inhibited by a magnetic field [37]. There seemed and possibly of other metals. APPLICATIONS OF MAGNETOELECTROLYSIS 5

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