Speciation, Conductivities, Diffusivities, and Electrochemical

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Speciation, Conductivities, Diffusivities, and Electrochemical Reduction as a Function of Water Content in Mixtures of Hydrated Chromium Chloride/Choline Chloride Dorrell C. McCalman, Liyuan Sun, Yong Zhang, Joan F. Brennecke,* Edward J. Maginn,* and William F. Schneider* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: We report experiments and simulations to understand the factors that control chromium (Cr3+) electrodeposition from ionic liquid solutions. Speciation, conductivities and diffusivities in mixtures of trivalent chromium chloride, water and choline chloride (CrCl3/xH2O/yChCl) were computed from molecular dynamics simulations and compared to measured ultraviolet−visible spectra, conductivities from electrical impedance spectroscopy, and cyclic voltammograms. Computed changes in Cr3+ first solvation shell and conductivity with solution composition qualitatively agree with experimental observations. The Cr3+ first solvation shell contains predominantly H2O and Cl− and the proportion of the two ligands changes with the relative bulk concentrations of each. Conductivities and diffusivities are observed to be functions of these composition variables. Variations in observed reduction current are primarily determined by dynamical properties and are less influenced by speciation.

0.7, and 1 μm min−1), and conductivities (1.5, 2, and 10 mS cm−1). Past work suggests that conductivities and speciation can be changed by varying the relative amounts of ChCl and CrCl3 or by adding other chloride-containing salts, and that these changes can also affect the electrochemistry.4,5 Abbott et al. used ultraviolet−visible spectroscopy (UV−vis) and electrospray mass spectroscopy to identify [Cr(H2O)2Cl4]− and Cr(H2O)3Cl3 species in CrCl3/6H2O/0.5ChCl.4 They also observed that the addition of 15 wt % LiCl increases the amount of [Cr(H2O)2Cl4]− in solution.5 The conductivity increased as the ChCl:CrCl3 ratio decreased and was invariant to the addition of up to 15 wt % LiCl.5 They observed the reduction of trivalent Cr complexes in CrCl3/6H2O/0.5ChCl using cyclic voltammetry.4 They attributed the two peaks in the cyclic voltammogram to the stepwise reduction of Cr3+ to Cr2+ and Cr2+ to Cr metal. Increasing LiCl content decreased the Cr3+ to Cr2+ and increased the Cr2+ to Cr metal reduction currents.5 They suggested that the change in voltammetry results from a change in speciation. These results demonstrate the need for careful studies of the effect of composition on electrochemical properties. Here we report the results of experiments and computations to further our understanding of the effect of composition on the trivalent Cr salt/IL properties, including speciation, diffusivity, conductivity, and redox potentials. We explore speciation as a

1. INTRODUCTION Chromium electrodeposition from aqueous baths containing Cr(VI) salts is a widely used technology that provides wear resistance, corrosion resistance and improved aesthetics for a host of equipment parts.1 However, there are disadvantages to this technology, which include toxicity and hydrogen evolution,1−3 the latter of which decreases current efficiency and causes embrittlement of the metallic substrate. Ionic liquids (ILs) containing trivalent Cr salts are potential solutions to the existing problems in chromium electrodeposition.1,3−8 ILs are salts with melting points at or below 100 °C. ILs can be formed from a vast number of possible cation−anion combinations, and are generally observed to have low volatility, low combustibility, high thermal stability, electrochemical stability and good solvating properties.9 An IL-based Cr bath has a lower potential for H2 evolution than an aqueous bath does. Further, trivalent Cr is less toxic than hexavalent Cr10 and requires less current for reduction to chromium metal. Abbott and co-workers have shown that chromium can be electrodeposited onto many substrates from mixtures of hydrated CrCl3 and the IL N,N,N-trimethylethanolammonium chloride (commonly known as choline chloride, or ChCl).4,5,3 Choline chloride is cheap, nontoxic, and forms a eutectic with trivalent Cr salts. Further, Abbott found electrodeposited films to be between 27 and 30 μm thick, to exhibit good corrosion resistance (700 h in salt spray test) and good hardness (Vickers pyramid number between 700 and 1500). High current efficiencies (>90%) were also reported over a wide range of current densities (0.345, 150 mA cm−2), deposition rates (0.2, © XXXX American Chemical Society

Received: February 28, 2015 Revised: April 23, 2015

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DOI: 10.1021/acs.jpcb.5b01986 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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the initial configurations were minimized until all forces were below 10 kJ/mol/nm. A short isothermal−isobaric (NPT) simulation of one nanosecond was then used to set the initial box size. This was followed by ten nanoseconds of simulated annealing where the temperature was held at 333.15 K for the first nanosecond, raised to 533.15 K in the next nanosecond, held at 533.15 K for the next 6 ns, cooled to 333.15 K during the penultimate nanosecond, and held at 333.15 K for the final nanosecond. Following the simulated annealing, a 10 ns canonical ensemble (NVT) simulation at 333.15 K was carried out. Next, a 10 ns NPT simulation at 1 atm and 333.15 K was carried out in order to set the density. Finally a 20 ns microcanonical ensemble (NVE) simulation was used for data collection. Several mixtures with the composition CrCl3/ xH2O/yChCl, where x was set to 6, 9, 12, 15, or 18, and y to 0.5 or 2.5, were simulated.

function of H2O and ChCl content to provide insight into the electrochemical reactions occurring at the electrode. We compute radial distribution functions using molecular dynamics (MD) simulations to determine speciation and compare the results to UV−vis experiments. We measure and calculate conductivities for ChCl: CrCl3 ratios of 0.5 and 2.5 and for H2O: CrCl3 ratios of 6 to 18. We use diffusivities calculated from MD trajectories to estimate conductivities using the Nernst−Einstein equation. We compare those results to electrochemical impedance spectroscopy (EIS) conductivity measurements. We record cyclic voltammograms at the same compositions to connect speciation and conductivity on the one hand and observed current densities on the other.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Section. Choline chloride (Acros Organics, 99% purity) and chromium chloride hexahydrate (CrCl3· 6H2O, Alfa Aesar, 99.5% purity) were used as purchased without further purification. Ultrapure water (MilliQ, >18 MΩ·cm resistivity) was used in this study. UV−vis diffuse reflectance spectra were collected on a Jasco-V-670 UV−vis-NIR spectrometer with an integrating sphere detector. The conductivities were measured with an EIS system, consisting of a Solartron SI 1287 electrochemical interface and a SI 1260 impedance/gain-phase analyzer. All samples were loaded and sealed into conductivity sample cells under nitrogen in a glovebox. The sample cells were kept in an oven during measurement, and were thermally equilibrated at each temperature for at least 1 h before measurement. Cyclic voltammograms were recorded with a VoltaLab 50 potentiostat and a sealed undivided three-electrode cell to investigate the electrochemical reduction behavior of trivalent Cr, and to determine the electrochemical window (EW) of the IL. A glassy carbon (GC, Basi, d = 3 mm) microelectrode was used as the working electrode (WE). The surface of the WE was polished with an alumina paste (Basi, d = 0.05 μm), and then rinsed with deionized water and dried with a fresh lab tissue before use. Pt wire was used as the counter electrode (CE) after polishing with a diamond paste (Basi, d = 1 μm), followed by the same treatment procedures described for the WE. A reference electrode (RE) was employed, consisting of a silver wire immersed into 0.01 M AgNO3 in a 0.1 M 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide/acetonitrile solution. The potential was calibrated against a 2.5 mM ferrocene redox couple.11,12 The temperature was kept at 52 °C using an oil bath. Experiments were performed in an Mbraun glovebox with a H2O content less than 0.1 ppm and an O2 content less than 50 ppm. 2.2. Computational Details. Molecular dynamics simulations were used to study speciation and dynamics in mixtures of ChCl, CrCl3, and H2O. A standard Class I force field was used. The SPC/E model13 was used for water. Lennard-Jones parameters for chromium and choline chloride were taken from Bowron and Diaz Moreno14 and the CHARMM 22 force field,15 respectively. The other parameters for choline chloride were taken from the work of Morrow and Maginn.16 The Cr partial charge was 2.9091 e, consistent with the scaled chloride charge of −0.9697 e determined by Morrow and Maginn.16 All simulations were carried out using the Gromacs package.17−21 Additional simulation details along with sample input files are provided in the Supporting Information. We simulated several liquid compositions. Initial configurations were generated with PACKMOL.22 The energies of

3. RESULTS 3.1. Speciation and UV−Vis Experiments. It has been previously shown that UV−vis spectra of chloroaquachromium(III) complexes ([Cr(H2O)6‑zClz]3‑z) exhibit bathochromatic shifts of the absorption maxima as the number of Cl− anions and H2O molecules in the complex change.23 Table 1 Table 1. Literature Data23 on the Positions of the Absorption Bands of Chloroaquochromium(III) Complexes in Acid Media absorption maximum (nm) species

a

b

[Cr(H2O)6]3+ [Cr(H2O)5Cl]2+ [Cr(H2O)4Cl2]+ [Cr(H2O)3Cl3]

407 430 450 475

575 605 635 665

summarizes these literature data for the two absorption bands in the visible region (labeled as “a” and “b”) which correspond to the d−d transitions between different levels split from the dorbital set of Cr3+.23 Absorption maxima shift 20−30 nm with each substitution of a Cl− with a H2O. We determined the predominant species present in our experiments by comparing the peak positions in the same region to those in the literature. Absorption wavelengths between those listed in Table 1 were considered to represent mixtures of different species. This approach allows only qualitative analysis of the speciation. Note that the absorption spectra for the CrCl3/xH2O/yChCl mixtures also show a slight shoulder (labeled as “c” in Figure 1 and Table 2) that is interpreted as a contribution from [Cr(H2O)2Cl4]−. Increasing the water concentration in the CrCl3/xH2O/ yChCl mixtures causes the peaks to blue shift. This result holds regardless of the choline content. The UV−vis data for mixtures of CrCl3/xH2O/yChCl studied in this work suggest an increase in the number of H2O and a decrease in the number of Cl− ions in the chloroaquachromium(III) complex as the H2O: CrCl3 ratio (x) increases or as the ChCl:CrCl3 ratio (y) decreases, as shown in Figure 1 and Table 2. 3.2. Speciation and Radial Distribution Functions from Molecular Dynamics Simulations. The center of mass radial distribution functions (Figure 2, Figure 3) of all species around a central Cr3+ ion were calculated from MD trajectories. We observed choline to prefer to coordinate to Cr3+ via its O atom (OCh+) as shown in Figure 3, panels C and D. From B

DOI: 10.1021/acs.jpcb.5b01986 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Figure 1. UV−vis absorbance spectra of CrCl3/xH2O/yChCl at ambient temperature.

Table 2. Positions of the Absorption Bands of CrCl3/xH2O/ ChCl Mixtures Measured at Ambient Temperature absorption maximum (nm)

shoulder (nm)

a

b

c

interpretation

CrCl3/xH2O/0.5ChCl x=6

464

657

698

x=9

452

634

695

x = 12

446

626

691

x = 18

442

620

689

[Cr(H2O)4Cl2]+ [Cr(H2O)3Cl3] [Cr(H2O)2Cl4]− [Cr(H2O)4Cl2]+ [Cr(H2O)2Cl4]− [Cr(H2O)4Cl2]+ [Cr(H2O)2Cl4]− [Cr(H2O)5Cl]2+ [Cr(H2O)4Cl2]+ [Cr(H2O)2Cl4]−

CrCl3/xH2O/2.5ChCl x=6

476

668

705

x=9

462

654

697

x = 12

456

637

696

x = 18

450

632

695

[Cr(H2O)3Cl3] [Cr(H2O)2Cl4]− [Cr(H2O)4Cl2]+ [Cr(H2O)3Cl3] [Cr(H2O)2Cl4]− [Cr(H2O)4Cl2]+ [Cr(H2O)2Cl4]− [Cr(H2O)4Cl2]+ [Cr(H2O)2Cl4]−

Figure 2. Radial distribution functions for Cr3+−H2O and Cr3+−Cl− in CrCl3/xH2O/0.5ChCl (A, C) and CrCl3/xH2O/2.5ChCl (B, D) computed from MD trajectories. Note that there are different scales in the figures.

coordination numbers (Figure 4). These average coordination numbers were invariant across multiple independent simulations. The total Cr3+ coordination number is consistently about 6. H2O and Cl− dominate the first solvation shell. On average every Cr3+ ion is coordinated to 2 to 4 H2O molecules or Cl− ions (Figure 4). The average number of choline ligands in the first solvation shell is 0−0.45. H2O displaces Cl− with increasing H2O content and Cl− displaces H2O with increasing ChCl content. These general trends are consistent with experimental observation. 3.3. Conductivities and Diffusivities. The conductivities for mixtures of CrCl3/x H2O/y ChCl at 60 °C (Figure 5) were both measured by EIS and calculated from MD trajectories. MD conductivities were calculated using the Nernst−Einstein relation (eq 1).

Figures 2 and 3 it can be seen that all the ligands form at least two well-defined coordination shells. For H2O, Cl−, and OCh+ the first peak occurs at ca. 0.2−0.24 nm. This forms the first coordination shell around a central Cr3+ ion. Other Cr3+ ions show peaks farther away from a central Cr3+ ion, with the first occurring at ca. 0.45 nm. To determine speciation, we first constructed histograms of each species type throughout each simulation. These histograms were different across independent simulations, suggesting that exchange time scales were much longer than the simulation time scales. It was possible, however, to determine average coordination numbers. To do this, we constructed cumulative radial distribution functions from the integrated radial distribution functions and extracted first solvation shell C

DOI: 10.1021/acs.jpcb.5b01986 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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1 VkBT

∑ Nqi i2Di i

(1)

In the equation above, σ is conductivity, V is volume, kB is Boltzmann’s constant, T is absolute temperature, Ni is the number of species i, qi is the charge of species i, and Di is the diffusivity of species i. The diffusivities were calculated using the Einstein relation (eq 2). D=

1 d 2 lim ⟨( ri (⃗ t ) − ri (0)) ⟩ ⃗ t →∞ 6 dt

(2)

In the above equation, D is diffusivity and the angular brackets denote an average over the squared displacement of each atom in the system. Diffusivities increase as water content increases and decrease as ChCl content increases (Figure 6). Water has

Figure 3. Radial distribution functions for Cr3+−Cr3+ and Cr3+− choline (Ch+) in CrCl3/xH2O/0.5ChCl (A, C) and CrCl3/xH2O/ 2.5ChCl (B, D) computed from MD trajectories. Note that there are different scales in the figures.

Figure 6. Diffusivities at 60 °C of species in CrCl3/xH2O/0.5ChCl (A) and CrCl3/xH2O/2.5ChCl (B) calculated from MD.

the highest diffusivity, followed by Cl−, then Ch+, and then Cr3+. Given our simulation times, a majority of the calculated diffusivities at high choline content were below the limit of reliability (10−7 cm2 s−1). Therefore, no calculated conductivity values are reported in those cases. Conductivities as measured by EIS increase as water content increases and decrease with increasing ChCl content. The calculated Nernst−Einstein conductivities reproduce this experimental trend and show good agreement with experimental values at low water content (9:1 and 12:1 H2O:CrCl3) (Figure 5). The small differences could be due to inadequacies in the force field or approximations related to the use of the idealized Nernst−Einstein equation. 3.5. Cyclic Voltammograms. The first standard reduction potential of H2O is at −0.83 V24 and the standard reduction potentials of Cr3+/Cr2+, Cr2+/Cr and Cr3+/Cr are −0.41, −0.91, and −0.74 V respectively.24 We observe a first reduction peak near −0.5 V (Figure 7), in the window between reduction potentials of Cr3+/Cr2+ and H2O reduction, consistent with Cr reduction occurring in the IL. Current density increases as water content increases, suggesting that reduction increases with increasing water content due to decreased viscosity and enhanced mass transfer. At low (x = 6) or high (x = 18) water ratios the CrCl3/xH2O/0.5ChCl systems and the CrCl3/ xH2O/2.5ChCl systems show similar current densities. However, at intermediate water ratios (x = 9, 12), the CrCl3/ x H2O/0.5 ChCl systems show larger current densities and therefore more reduction than the CrCl3/x H2O/2.5 ChCl systems.

Figure 4. Number of ligands in the first coordination shell of Cr3+ in CrCl3/x H2O/0.5 ChCl (A) and CrCl3/x H2O/2.5 ChCl (B) computed from MD trajectories.

Figure 5. Conductivity at 60 °C as a function of water content for CrCl3/x H2O/y ChCl as calculated by the Nernst−Einstein relation from MD and as measured by experiment (Expt.).

D

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dynamics are more important than speciation in the CrCl3/ xH2O/yChCl systems for use in chromium electrodeposition. One caveat is that using conductivity as a surrogate for solution dynamics can be misleading in some cases. The conductivities (Figure 5) are less sensitive to choline chloride content than the diffusivities (Figure 6) and the voltammetry (Figure 7). For example, there is very little difference in conductivities between CrCl3/9H2O/0.5ChCl and CrCl3/9H2O/2.5ChCl, but the reduction currents are different. A partial explanation is that conductivities are proportional to the product of concentration and diffusivity. As ChCl concentrations increase, diffusivities of Ch+ and Cl− tend to decrease. This inverse relationship between concentration and diffusivities results in the observed low sensitivity of conductivity to ChCl content. Also at a certain point something else besides dynamics becomes important. For example diffusivities differ by an order of magnitude for CrCl3/18H2O/0.5ChCl and CrCl3/18H2O/ 2.5ChCl, but the voltammetries are similar. We speculate that at this water content the rate of the reaction at the electrode is slower than the rate at which species diffuse to the electrodes, but such a conclusion requires additional study.

Figure 7. Cyclic voltammogram of CrCl3/xH2O/0.5ChCl (A) and CrCl3/xH2O/2.5ChCl (B) with different H2O content on glassy carbon, with scan rate of 100 mV/s, at 52 °C.

4. DISCUSSION Care must be taken to optimize the H2O and ChCl content in mixtures of CrCl3/ H2O/ChCl for Cr electrodeposition applications. Although the radial distribution functions show that Cr3+ coordinates extensively with Cl− and H2O but not with Ch+, Ch+ does affect the properties of the mixtures and thus the performance. Literature results from Abbott show that ChCl forms a eutectic with CrCl3.6H2O and so produces a low melting point mixture.3−5 Mixing ChCl with hydrated CrCl3 can also reduce the disadvantages of aqueous baths. These disadvantages are (1) low current efficiency as electrons are used to reduce H2O to hydrogen instead of reducing Cr species, (2) the presence of Cr3+-H2O bonds which are very stable and hard to break,25 and (3) chromium hydroxide and polymeric chromium oxide formation. However, we have shown that the presence of some water increases the reduction of Cr species. Increasing H2O:ChCl ratios could increase the diffusivities and conductivities or alter speciation and thus electrochemistry. In fact, the experiments of Abbott et al. show that voltammograms are a function of added LiCl and thus likely speciation.5 Our experiments suggest a primary contribution from increased diffusivities and conductivities to observed changes in reduction. We identify four mixtures with the same species, including CrCl3/9H2O/0.5ChCl, CrCl3/12 H2O/0.5ChCl, CrCl3/12H2O/2.5ChCl and CrCl3/18H2O/2.5ChCl. These mixtures are all dominated by [Cr(H2O)4Cl2]+ and [Cr(H2O)2Cl4]− complexes. The mixtures with ChCl:CrCl3 ratios of 0.5 have conductivities of 15 and 44 mS/cm and minimum current densities of −18 and −36 mA/cm2. An increase in conductivity occurs simultaneously with an increase in current density. The mixtures with ChCl:CrCl3 ratios of 2.5 have conductivities of 25 and 59 mS/cm and minimum current densities are −10 and −35 mA/cm2. Again an increase in conductivity occurs simultaneously with an increase in current density.

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ASSOCIATED CONTENT

S Supporting Information *

Additional computational details and sample GROMACS input files for energy minimization, simulated annealing, and NPT, NVT, and NVE simulations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b01986.

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AUTHOR INFORMATION

Corresponding Authors

*(J.F.B.) E-mail: [email protected]. Telephone: 574-631-5847. Fax: (574) 631-8366. *(E.J.M.) E-mail: [email protected]. Telephone: 574-631-5687. Fax: (574) 631-8366. *(W.F.S.) E-mail: [email protected]. Telephone: (574) 6318754. Fax: (574) 631-8366. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Patrick Benaben, Mr. Daniel Fagnant, Dr. Mauricio Quiroz Guzman, Dr. Katie Maerzke, Mr. Quintin Sheridan, Mr. Surya Tiwari, and Mr. Brian Yoo for helpful discussions, the Notre Dame Center for Research Computing for computing resources, and the National Science Foundation (IIP-1237829) for financial support.

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REFERENCES

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5. CONCLUSION Simulation and experiment can be used in tandem to explore the properties of trivalent Cr salt/IL baths. For Cr electrodeposition from mixtures of CrCl3/xH2O/yChCl, care must be taken to optimize x and y such that optimal dynamics, speciation, and subsequently Cr3+ reduction are obtained. Values of x in the range 9−18 show good reduction. We conclude that conductivities are more of an indicator of performance than the speciation. This suggests that solution E

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DOI: 10.1021/acs.jpcb.5b01986 J. Phys. Chem. B XXXX, XXX, XXX−XXX