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Electrodeposition of palladium in ionic liquids has not been actively investigated. Herein, the electrochemistry of PdCl2 for electrodeposition of palladium in 1-Ethyl-3-Methylimidazolium Chloride ([EMIM][Cl]) ionic liquid (IL) was studied at a Pt-Ir working electrode. A rotating disc was applied to allow a better understanding of the reaction kinetics. A range of techniques, such as cyclic voltammetry (CV) and current-time transient, were used to elucidate the electrochemistry of palladium of PdCl2 in [EMIM][Cl] IL under both stationary and rotating conditions. Two cathodic waves and two anodic waves were observed in CV. We propose that both of the cathodic peaks, which are separated by the surface alteration of the electrode during the cathodic scan, are produced by the electroreduction of PdCl42- to palladium, with their characteristics of mixed kinetics and mass transfer controlled, respectively. Regarding the nucleation of palladium on Pt-Ir electrode, a 3D, instantaneous nucleation mechanism was proved under quiescent conditions. The nucleation of Pd at rotating disc electrode(RDE) was fmodeled, and it was found that RDE promote instantaneous nucleation of Pd.

The anodic dissolution of copper was investigated at a copper RDE in the Lewis acidic and basic composition regions of the room-temperature AlCl3-EtMeImCl ionic liquid (IL) to assess the utility of chloroaluminate liquids as solvents for the electrochemical machining and electropolishing of copper. In the Lewis acidic IL (60 mol % AlCl3), the dissolution of Cu0 proceeds under mixed kinetic-mass transport control with an exchange current density of 7.00 mA cm−2 at 306 K and an apparent activation free energy of 19.7 kJ mol−1. A formal potential of 0.843 V was obtained for the Cu+/Cu0 reaction from potentiometric measurements. In the basic IL (< 50 mol % AlCl3), potentiometric measurements showed that the oxidation of Cu0 resulted in the formation of [CuCl2]−. In this case, the formal potential of the [CuCl2]−/Cu0 reaction is −0.412 V. At small positive overpotentials, the reaction exhibited mixed control and was first order in the chloride concentration, indicating that only a single Cl− is involved in the RDS. However, at more positive overpotentials, the reaction transitions to mass transport control, and a well-defined limiting current is observed for the anodization process. This limiting current scales linearly with the free chloride concentration in the IL.

The anodic dissolution of aluminum metal was investigated in the Lewis acidic chloroaluminate ionic liquid, aluminum chloride-1-ethyl-3-methylimidazolium chloride. The investigation was conducted on aluminum rotating disk electrodes as a function of potential, ionic liquid composition, and temperature. Two different dissolution mechanisms were realized. At modest overpotentials, dissolution takes place under mixed kinetic-mass transport control. However, as the overpotential is increased to induce higher dissolution rates and/or the ionic liquid is made more acidic, the dissolution reaction transitions to a potential-independent passivation-like process ascribed to the formation of a porous solid layer of AlCl3(s). At a fixed temperature and composition, the limiting passivation current density displays Levich behavior and also scales linearly with the concentration of AlCl4- in the ionic liquid. The heterogeneous kinetics of the Al dissolution reaction were measured in the active dissolution potential regime. The exchange current densities were independent of the composition of the ionic liquid, and the anodic transfer coefficients were close to zero and seemed to be independent of the Al grain size.

The electrochemistry of 1-ethyl-3-methylimidazolium chloride (EMIC) is important because the reduction of the 1-ethyl-3-methylimidazolium cation (EMI + ) defines the negative potential limit of room temperature molten salt electrolytes made by mixing EMIC with AlCl 3 . Since EMIC constitutes both part of the solvent and the supporting electrolyte in the molten salt, it is difficult to examine its electrochemistry quantitatively in the melt. Thus, EMIC has been studied as an analyte at a glassy carbon electrode in acetonitrile with 0.1 M tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte. EMI + is reduced at - 2.35 V measured vs. a reference electrode consisting of a silver wire in 0.1 M TBAP/CH 3 CN. Controlled potential coulometry indicates that the reduction is a one-electron process. Subsequent voltammograms exhibit two small oxidation peaks at -0.45 and -0.65 V but no new reduction peaks. Exhaustive oxidation of these two peaks gives less than 15% as many coulombs as for the initial reduction peak, indicating that the reduction of EMI' is irreversible. Comparison of the voltammograms to those for 1-methylimidazole indicates that the reduction process does not cleave the ethyl group from the five-membered ring to give the precursor of EMIC. Mass spectra of the reduction product shows no evidence of dimer formation. The dominant product of EMI + reduction is not electroactive within the accessible voltage window of acetonitrile or the molten salt. Thus, slight degradation of the electrolyte should not interfere with the operation of EMIC/AlCl 3 molten salt cells

The electrochemical behavior of 9, 10-anthraquinone (AQ) in a basic room-temperature molten salt composed of a mixture of AlCl 3 , and 1-ethyl-3-methylimidazolium chloride (ImCl) is described. In the absence of a proton source, AQ is reduced via a quasireversible two-electron transfer to AQ(AlCl 3 ) 2- . The homogeneous chemical steps coupled to the electron transfers are displacement of Cl - from AlCl 4 - to form AQ(AlCl 3 ) 2 2

Electrodeposition of palladium-tin alloys from 1-ethyl-3-ethylimidazolium chloride―tetrafluoroborate ionic liquid was studied at 120°C. Sn(II) chloride reacts spontaneously with Pd(II) chloride, producing Sn(IV) and Pd nanoparticles. Solutions containing Sn(IV) and Pd(II) were used for potentiostatic electrodeposition of Pd-Sn. The composition of the Pd-Sn electrodeposits varied with the solution composition and deposition potential. Different alloy phases were observed with X-ray diffraction measurements. Whereas the Pd-rich Pd-Sn solid solution deposits are composed of compact nodules, the Sn-rich intermetallic Pd―Sn deposits are composed of polyhedral crystals of various phases. Compared to Pd-coated electrodes, Pd-Sn solid-solution-coated electrodes show enhanced ethanol electro-oxidation efficiency and stability in alkaline aqueous solutions. As Sn content increased, new Pd/Sn intermetallic phases formed, resulting in reduced catalytic efficiency for ethanol oxidation.

The electrodeposition of palladium-copper alloys in 1-ethyl-3-methylimidazolium chloride-tetrafluoroborate ionic liquid containing excess chloride ions was studied on indium tin oxide (ITO) coated glass electrodes at 120°C. Cyclic voltammogrammetric results indicate that the thermodynamic deposition potential of copper is more negative than that of palladium. The presence of palladium(II) reduces the overpotential required for the deposition of copper. In addition, underpotential deposition of copper on the palladium electrode was observed. In contrast, the presence of copper(II) increases the overpotential required for the deposition of palladium. Palladium-copper coatings were prepared on the ITO electrode by constant potential electrolysis. Scanning electron microscopy results indicate that the deposits had nodular morphologies. Calculations from X-ray powder diffraction data show that the crystal size of the deposit decreased with increasing deposition overpotential. The prepared palladium-copper coatings adhered well on the ITO substrates and showed a good electrocatalytic capability for the electro-oxidation of ethanol in alkaline solution.

The electrodeposition of Al-Mo-Ti ternary alloys was examined in the Lewis acidic 66.7-33.3% mole fraction aluminum chloride-l-ethyl-3-methylimidazolium chloride (AlCl 3 -EtMelmCl) room-temperature ionic liquid containing (Mo 6 Cl 8 )Cl 4 and TiCl 2 . The Mo content in the alloys varied with the applied current density and the Mo(II)/Ti(II) concentration ratio. The Ti content was small and constant at 0.6 ± 0.2% atomic fraction (a/o) and was independent of the deposition conditions. All the electrodeposited Al-Mo-Ti alloys were dense and compact and adhered well to the copper substrate. The deposit surface morphology depended on the applied current density and the Mo content of the alloys, as reported previously for the amorphous binary Al-Mo alloys. However, no amorphous glass phase could be detected in the Al-Mo-Ti ternary alloy samples; this behavior may be related to the presence of Ti. In summary, the addition of a small amount of Ti (∼ 1 a/o) to the binary Al-Mo alloys resulted in a ternary alloy with a substantially improved chloride-induced pitting corrosion resistance compared to the related Al-Mo alloy.

The electrochemistry of Zr(IV) and Zr(II) and the electrodeposition of Al-Zr alloys were examined in the Lewis acidic 66.7-33.3 mol % aluminum chloride-1-ethyl-3-methylimidazolium chloride molten salt at 353 K. The electrochemical reduction of Zr(lV) to Zr(II) is complicated by the precipitation of ZrCl 3 ; however, solutions of Zr(II) can be prepared by reducing Zr(IV) with Al wire. Al-Zr alloys can be electrodeposited from plating baths containing either Zr(IV) or Zr(II), but for a given concentration and current density, baths containing Zr(IV) lead to Al-Zr alloys with the higher Zr content. This result was traced to the diminutive concentration-dependent diffusion coefficient for Zr(II). It was possible to prepare Al-Zr alloys containing up to ∼17% atomic fraction (atom %) Zr. The structure of these deposits depended on the Zr content. Alloys containing less than 5 atom % Zr could be indexed to a disordered face-centered cubic structure similar to pure Al, whereas alloys containing ∼17 atom % Zr were completely amorphous (metallic glass). The chloride pitting potentials of alloys with more than 8 atom % Zr were approximately +0.3 V relative to pure Al.

The electrodeposition of iron and Zn-Fe alloys on polycrystalline nickel was investigated in the 40.0-60.0 mol % zinc chloride1-ethyl-3-methylimidazolium chloride ionic liquid containing iron~II! at 90°C. Iron~II! was electrochemically reduced to iron metal before the reduction of zinc~II! to zinc metal, and anomalous codeposition of Zn-Fe did not occur in this ionic liquid. Underpotential deposition of zinc on iron was observed prior to the electrodeposition of bulk zinc. Dimensionless chronoamperometric current/time transients for the underpotential deposition of zinc on iron were in good accord with the theoretical transients for the two-dimensional nucleation and/or growth of the nuclei. Zn-Fe alloys could be prepared in the potential range of the underpotential of zinc on iron or the potential range where bulk deposition of zinc occurred. Energy-dispersive spectroscopy data indicated that the composition of the Zn-Fe alloys was dependent upon the deposition potential and the iron~II! concentration in the plating solution. The morphologies of the Zn-Fe codeposits were examined by scanning electron microscopy.

The electrochemistry of tin at polycrystalline tungsten and at glassy carbon (GC) electrodes was investigated in acidic and basic zinc chloride-1-ethyl-3-methyl-imidazolium chloride (ZnCl 2 -EMIC) ionic liquids at 90°C. The electrodissolution of Sn produces a Sn(II) solution, which can be either oxidized to Sn(IV) or reduced to Sn metal. The formal potentials of the Sn(II)/Sn(0) couple in the 40.0-60.0 and 25.0-75.0 mol % ionic liquids are 0.25 and -0.24 V, respectively, vs. Zn(II)/Zn in a 50.0-50.0 mol % ionic liquid. The formal potentials of the Sn(IV)/Sn(II) couple in the 40.0-60.0 and 25.0-75.0 mol % ionic liquids are 0.78 and 0.29 V. respectively. The electrodeposition of Sn from Sn(II) at both electrodes is complicated by nucleation. Experimental current-time transients recorded at these electrodes are in good agreement with the theoretical model based on 3D nucleation. Sampled-current voltammograms constructed from chronoamperometric experiments indicated that the reduction of Sn(II) to Sn metal in the acidic ionic liquid is hindered by the adsorption of Sn(II) at tungsten, nickel, and GC electrodes. In the acidic ionic liquid, the adsorption of Sn(II) also hindered the voltammetric oxidation of Sn(II) to Sn(IV) at the tungsten electrode but not at the GC electrode. In the basic ionic liquid, however, the adsorption of Sn(II) at these electrodes is not observed. When the deposition potential was extended to the range where Zn(II) reduction occurred, coatings of Sn-Zn codeposits could be obtained. The Sn-Zn codeposits consist of a two-phase mixture of Sn and Zn. The effects of deposition potential and Sn(II) concentration on the Sn-Zn codeposits composition were investigated.

The chemical and electrochemical behavior of titanium was examined in the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl 3 -EtMeImCl, molten salt at 353.2 K. Dissolved Ti(II), as TiCl 2 , was stable in the 66.7-33.3% mole fraction ( m/o composition of this melt. but slowly disproportionated in the 60.0-40.0 m/o melt. At low current densities, the anodic oxidation of Ti(0)did not lead to dissolved Ti (II). but to an insoluble passivating film of TiCl 3 . At high current densities or very positive potentials, Ti (0) was oxidized directly to Ti(IV); however, the electrogenerated Ti (IV) vaporized from the melt as TiCl 4 (g). As found by other researchers working in Lewis acidic AlCl 3 -NaCl, Ti(II) tended to form polymers as its concentration in the AlCl 3 - EtMeImCl melt was increased. The electrodeposition of Al-Ti alloys was investigated at Cu rotating disk and wire electrodes. Al-Ti alloys containing up to ∼19% atomic fraction (a/o) titanium could be electrodeposited from saturated solutions of Ti (II) in the 66.7-33.3 m/o melt at low current densities, but the titanium content of these alloys decreased as the reduction current density was increased. The pitting potentials of these electrodeposited Al-Ti alloys exhibited a positive shift with increasing titanium content comparable to that observed for alloys prepared by sputter deposition.

The electrochemistry of Cd(II) was studied with voltammetry and chronoamperometry at polycrystalline Pt, Ni, and W electrodes in the acidic zine chloride-1-ethyl-3-methylimidazolium chloride (ZnCl 2 -EMIC) ionic liquid. Cd(II) could he reduced to cadmium metal via a single-step quasi-reversible electron-transfer process. The redox potential of the Cd(II)/Cd couple shifted negatively as the acidity of the ionic liquid decreased. In addition, underpotential deposition of Cd was observed at Pt and Ni electrodes. This was related to the work function of these metals. Analysis of the chronoamperometric transient behavior revealed a three-dimensional instantaneous nucleation/growth process for the electrodeposition of pure Cd at both Pt and W substrates. When the deposition potential was extended to the potential range where Zn(II) reduction occurred, codeposition of Zn with Cd became possible. The dependency of the Cd-Zn codeposits composition on the deposition potential, temperature, and Cd(II) concentration was investigated.

The electrodeposition of tellurium and zinc telluride was investigated on a nickel electrode in the 40-60 mol % zinc chloride-1ethyl-3-methylimidazolium chloride molten salt containing propylene carbonate as a cosolvent at 40°C. Tellurium~IV! can be electrochemically reduced to tellurium in this solution. Addition of 8-quinolinol ~oxine! to the solution shifts the reduction of Te~IV! to more negative potential. Deposits of Zn-Te can be obtained through the underpotential deposition of zinc on tellurium which occurs at a potential near 20.1 V. At potentials more negative than ca. 20.5 V, tellurium can be further reduced to tellurium~2II! species which may react with zinc~II! to form Zn-Te. Energy-dispersive spectroscopy data indicate that the composition of the Zn-Te deposits is dependent upon the deposition potential and the Te~IV! concentration in the plating solution. Characteristic X-ray diffraction patterns of cubic ZnTe are observed for the electrodeposited Zn-Te samples that have been annealed at temperatures ranging from 250 to 400°C. The flatband potential of the Zn-Te electrodeposits was determined by photocurrent and impedance ~Mott-Schottky plot! experiments. The optical bandgap of the ZnTe deposits determined by optical

The buffering of 1-ethyl-3-methylimidazolium chloride-aluminum trichloride room-temperature ionic liquids (melts) using alkali metal bromides and iodides was studied. The bromide or iodide salts buffer the melts by reaction with the ion, but the bromide or iodide ions do not replace the chloride from the in the melts. Unlike melts buffered with alkali metal chlorides, it is easy to deposit the alkali metals, and thus it may be possible to use these buffered melts in power sources. In melts buffered with a mixture of lithium chloride and iodide, it is possible to both deposit and strip lithium metal. As seen with melts buffered with alkali metal chlorides, the buffered melts appear to be more acidic than expected from the low concentration of the acidic ion. © 2001 The Electrochemical Society. All rights reserved.