Molecular Facts on the Structure and Dynamics of Electrolyte Species

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Molecular Facts on the Structure and Dynamics of Electrolyte Species in Cu-Cl Cycle for Hydrogen Generation: An Insight From Molecular Dynamic Simulations Pooja Sahu, Sk. Musharaf Ali, Kalsanka Trivikram Shenoy, and Sadhana Mohan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01650 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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The Journal of Physical Chemistry

Molecular Facts on the Structure and Dynamics of Electrolyte Species in Cu-Cl Cycle for Hydrogen Generation: An Insight from Molecular Dynamic Simulations Pooja Sahu1,2, Sk. Musharaf Ali*1,2, K.T. Shenoy1 and S. Mohan1 1

Chemical Engineering Division, Bhabha Atomic Research Center, Mumbai, Maharashtra, India 400085 2 Department of Chemical Science, Homi Bhabha National Institute, Mumbai, Maharashtra, India 400094

Abstract Cu complex, which are the key chemical species in well-known Cu-Cl hybrid thermochemical cycles and also in numerous metal hydrometallurgical and sedimentary deposit processes, displays wide variety of structural and dynamical characteristics that are further complicated by the presence of multiple oxidation states of Cu ions with different coordination chemistry, therefore are difficult to explore from experiments alone. In this article, therefore, an attempt has been made to understand the coordination behavior of Cu complex using MD simulations. The study provides compelling evidence of experimentally observed multiple stoichiometry of Cu ions i.e. 1:6:0, 1:5:1, and 1:4:2 for Cu+:H2O: Cl- and 1:6:0 for Cu++:H2O: Cl-. The presence of anionic Cu complex; [Cu+Cl2]-.2H2O, [Cu+Cl2].3H2O, [Cu++Cl3]-.H2O and [Cu++Cl3]-.2H2O was captured in presence of excess chloride ions. Furthermore, the probability distribution profiles have been estimated to determine the most possible complex in the considered systems. The results establish structural and dynamical reformation of Cu complex with change in the salt concentration or variation in solvent medium in which they are dissolved. Moreover, the structure and kinetics of Cu ions in Cu-Cl electrolyser has been explored over a large range of electric field, by extending the simulated systems for varied strength of electric fields. It has been observed that with increase in the strength of electric field, the water molecules lose their coordination strength with central Cu ions, which on other hand, results into significant change in the structure of captured complex. The diffusion dynamics of ions is altered while applying the electric field, which is furthermore modified while increasing the strength of electric field beyond a critical limit. In fact, the diffusion mechanism of ions was seen to be transformed from Brownian like to linear motion and then to hopping diffusion with the increasing strength of electric field. To the best of our knowledge, this is first time, when the multiple oxidation states of Cu ion are explored using MD simulations and the coexisting pictures of multiple coordination and the solvent effects have been clearly revealed. Also to the date, the present article is first one to report the insights of the structure and dynamics of ions in Cu-Cl electrolyser over a wide range of electric field. The present studies will be very helpful in understanding the mechanism involved in numerous metal hydrometallurgical and sedimentary deposit processes and to comprehend the analogies involved in the electrode reactions of Cu-Cl cycle for hydrogen generation.

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1. INTRODUCTION Investigations on low temperature hybrid thermochemical cycles have explored an attractive energy storage option capable of storing both the thermal energy and electrical energy1,2,3. Among the others, a significant scientific effort has been devoted to Cu-Cl cycle as a potential candidate for inexpensive hydrogen generation with very high efficiency and moderate temperature requirements4,5. Interestingly, the needs of excess thermal energy and electrical energy in Cu-Cl cycle can be easily harnessed from solar resources. Also, the implication of Cu-Cl cycle for hydrogen gas production would save the excessive consumption of fossil fuels4, 5.Within the sequence of Cu-Cl reactions to split water into hydrogen and oxygen, a large number of intermediate stages are involved which need to be explored in order to understand the process deeply and make it more efficient1-3. For example, CuCl(aqs)/HCl(aqs) electrolyte is the one of the most important key components in the hybrid Cu-Cl cycle, where aqueous Cu+ complex are oxidized to Cu++ complex and HCl(aqs) is reduced to H2.6, 7.In this step, a large variation over structural and dynamics of species is expected, knowledge of which is essential to investigate the fundamental properties of the anode/cathode reactions. Apart from this, the understanding of structures and properties of Cu complex is crucial to comprehend the various metal hydrometallurgical and sedimentary deposit processes8. Besides, Copper complexes play crucial role in plethora of industrial processes and enzyme based reactions9,10,11. Therefore, understanding of the characteristics properties of copper complex become essential to explore the intricate molecular mechanisms. Though numerous experimental12,13,14,15 as well as theoretical studies16,17,18,19,20,21,22 have been conducted to investigate this issue, however, up to now, the most fundamental information is still on dispute: what does the first shell look like? In particular, the structural details of copper complex in the aqueous phase are less definitive and have been subjected to only few complexes. The interesting observation about the color of Cu-Cl solutions, which is known to be light blue at low Cl- concentrations, but yellow-green at high Clconcentrations, inspired to explore the knowledge about the microscopic details of coordination chemistry of Cu ions. The studies showed that the microscopic structure of CuCl2 solution changes with chloride concentration, however, the coordination relationships among the complex species being formed is still unclear. Being an important species in hydrothermal fluids, several experimental studies12,

23

have been

investigated on Cu complex by utilizing the state-of-the-art X-ray techniques13, EXAFS and XANES (extended X-ray absorption fine structure and X-ray absorption near edge structure)14, 15. However, it has been almost impossible to provide unambiguous coordination details of Cu complex from these experiments. Few studies, reported by UV-vis spectroscopy24, 25, assume the presence of [Cu]2+, [CuCl]+, [CuCl2]0, [CuCl3]- and [CuCl4]2- complex in the solution and then find the concentration distribution of these species by resolution of experimental UV-vis spectra. However, it remained uncertain how the Page | 2 ACS Paragon Plus Environment

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structures of species assigned were assumed? Also, it could not be assured that the resolved absorption spectra really correspond to the species assigned and therefore hold a doubt on the reliability of the data and information obtained from the experiments.

Another way to approach the structure and properties of aqueous Cu complex utilize the quantum mechanics (QM) such as density functional theory (DFT)18, 19 and ab-initio methods18, 20, 21, 26. The studies by Kim et al.’s26, 27suggested the presence of either five-coordinated contact ion pair (CIP) structures of [CuCl(H2O)n] (n = 8 and 9) or a five coordinated solvent separated ion pair (SSIP) structure of CuCl2(H2O)n (n > 8). The results revealed the existence of five-coordinated hydration structure in infinitedilute CuCl2 solution. The5-fold coordination of Cu complex was further supported by studies with CarParrinello molecular dynamics (CPMD) simulations and neutron diffraction studies with their geometry interchanging in between square pyramidal and trigonal-bipyramidal28. However, some of the QM/MM studies predict 6-fold coordination12of Cu complex with distorted octahedral geometry and therefore controverted the 5-fold hypothesis. Also, the electronic configuration of the Cu++ ion suggests a JahnTeller29 distorted octahedral coordination geometry with four equidistant equatorial ligands and two elongated axial ligands. Recently, studies by Bryantsev et. al. proposed that 4-fold coordination of Cu complex is also possible and predicted that open 4-fold structure might be the most favorable state30. And so, the previous computational studies lack in consensus and the issues need to be resolved in order to reach one reasonable conclusion. In addition, the existence of multiple oxidation states of copper with different coordination chemistry furthermore complicates the study of aqueous copper solutions. At room temperature, the copper ions primarily exist as Cu++, However, at higher temperatures, Cu+ form is more stable25, 31. Recently, the studies by Berry et al.31 showed the reversible conversion of Cu++ to Cu+ with increase in temperature and consecutive oxidation of the other species in the solution. The results predicted the coexistence of the Cu+ and Cu++ at intermediate temperatures. Therefore, an accurate analysis method is needed to consider both of these oxidation states with multiple coordination geometries. In other words, the decisive interpretation of the experimental observations are restricted by the complex coordination chemistry of copper ions and only computational methods are expected to supplement the experimental results. Furthermore, the major shortcoming of static techniques to account the solvent effect accurately, impose a limit on the QM/MM studies. Ab initio methods, though precisely capture the dynamic nature of these systems, however, are computationally very expensive. Therefore, classical MD simulation, a force field based methods are supposed to be the better suited for this purpose.



Considering all these points, the present article is dedicated to understand the structure and dynamics of Cu ions, considering the both oxidation states in aqueous medium as well as in acidic environment. To the best of our knowledge, there are no complete theoretical investigations on the structure and dynamics of copper complex in acidic medium. The multiple coordination geometries of complex have been explored using molecular dynamics simulations. To answer the raised questions, complete understanding of the structure and the dynamics of cuprous/cupric chloride complex has been explored with various concentration of salt solutions, which provide not only the influence of the water activity on the association constant of the Cu-Cl complex but also the interaction of Cu-Cl with Cl- donating salt in aqueous solution. Furthermore, electrochemical reactions in Cu-Cl cycle have been investigated by simulating the copper species at high concentration of HCl solution. The results have been tested under the varied strength of electric filed in order to mimic real experimental conditions. To the best of our knowledge, this is first time, when the multiple oxidation states of Cu ion are explored using MD simulations and the coexisting pictures of multiple coordination and the solvent effects have been clearly revealed. Also to the date, the present article is first one to report the insights of electric field strength on the electrochemical reactions in Cu-Cl cycle. The ability of high-fidelity molecular simulations capture the molecular insights and microscopic understanding that can be applied to capture the lots of quantitative molecular information. Moreover, the current studies will be of great practical use as they explore the plethora of salt structures, which exhibit complex coordination chemistry for multifaceted environment.

2. SIMULATION PROTOCOL The present simulation studies were conducted using LAMMPS32 molecular dynamics package. The systems were simulated for 0.25M, 0.5M and 0.75M concentration of Cu ions in the system, corresponding to 4, 8 and 12 number of Cu ions in simulation box of dimension 29.6Åx29.6Åx29.6 Å. All these systems were simulated for monovalent Cu+, divalent Cu++ and equivalent mixture of Cu+/Cu++. Further, the acidic systems were prepared by separately dissolving 8 number of Cu+ and Cu++ respectively in 200 water molecules and 80 acid ions in a cubic box of 23.5Åx23.5Åx23.5Å that make 10N acid having 1.0M Cu ion concentration. The systems with 0.5M concentration of Cu ions were further extended for electric field of strength 0.2V/Å, 0.5V/Å and 0.8V/Å. The direction of electric filed was fixed to x direction of these systems. For water TIP4P/2005 water model33 was used. Acid in the systems has been represented by dissociated ions of H3O+/Cl- ions34, 35. The forcefield parameters of simulated model are reported in TableS1and TableS2.36,37 The interatomic interaction between the atoms was defined by Lennard-Jone (L-J) potential32 using cut-off of 10Å. For unlike intermolecular interactions, Lorentz-Berthelot mixing rules was used as shown in Equation1-3. Page | 4 ACS Paragon Plus Environment

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   12    6  N N  q q  ij  ij i j U pot (rij )    4 ij                  r r  i 1 j i 1 i 1 j i 1  rij ij ij       N

N

 ij   i   j  2

   

(1)

(2)

 ij   i j

(3)

where Upot indicates the sum of van der Wall interaction energy and electrostatic energy. σ ij and εij are the interatomic separation and interaction strength between two atoms in sequence.

Implication of external electric field adds an extra force on each atom given by

Fi  qi .E

(4)

Where is E is the strength of electric field and q is the charge of ion. Therefore, the energy added to each charged particle can be written as E   x.qE  q( x.Ex  y.Ey  zEz) (5)

So that

 E  F

(6)

However, in the present case, external electric field has been applied only in the positive x direction, by keeping Ey=0 and Ez=0. The bond stretching and angle bending have been modelled through Harmonic potential model shown by equation.7 and equation.8 respectively32.

1 U bond  ka (a  a0 ) 2 2

(7)

Where ka, a and a0 are used to represent the bond constant, bond length and equilibrium bond length respectively. U angle 

1 k (   0 ) 2 2

(8)

Where kθ, θ, and θ0, indicate the angle constant, angle bending and angle bending at equilibrium respectively.



Furthermore, for long range interactions in the systems, particle-particle-particle mesh (PPPM) method38 was incorporated. In addition, the SHAKE constraint algorithm39 was applied to fix the bond and angles of water molecules and hydronium ions. Furthermore, periodic boundary conditions have been implemented in all three directions to reduce the surface effects40. All the considered systems have been energy minimized using steepest descent and conjugate gradient minimization techniques41 followed by equilibration using an isothermal–isobaric (NPT) ensemble (P = 1 atm and T = 300 K) for 20 ns dynamics to adjust the densities. The production run were performed using an NVT ensemble for 10 ns simulation length, maintaining the temperature at 300 K by coupling the solution to a thermal bath using the Nose’–Hooverthermostat41. The MD trajectories and velocities were calculated using the velocity verlet algorithm41with a time step of 1 fs. For visualization of MD trajectories and extraction of enclosed images in the article VMD 1.9.142 (visual molecular dynamics) graphical package was used.

The structural analysis of the simulated systems was carried out using radial distribution function (RDF), coordination number (CN) defined as follows:

g ( r )   2

 (r ) (r i

CN  4

rmin

r

2

j i

i

j

 r) 

V N2

 (r  r ) i

j i

g (r ) dr

ij

(9)

(10)

0

Where g(r) is the radial distribution function and ρ, N, V and δ represent the number density, total number of atoms/molecules in the system, volume and kronecker delta respectively. CN denotes the coordination number obtained by integrating the RDF up to first minima position rmin. The dynamics of the solute and solvent species has been explored using mean square displacement profiles (MSD) and velocity autocorrelation functions (VACF) defined by equation.11 and equation.12 respectively. Furthermore, the obtained MSD profiles are used to estimate the diffusion coefficient with well-known Einstein relation (equation.13)

MSD(t )  R(t )  R(0) 2

VACF(t )  v(t ).v(0) D

1 d lim R (t )  R (0) 2 t  2d dt

(11) (12) (13)


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Where R(t) and v(t) respectively represents the position and velocity of atom/molecule at time t, indicates the dimension of system.

3. RESULTS AND DISCUSSION 3.1 Structure of Cu++/Cu+ in aqueous environment The studies were initiated with systems having 0.5M concentration of Cu+ and Cu++ in the aqueous solution. The structural results show that the interaction of Cl- is higher for Cu++ than Cu+ as expected. The average coordination number of Cl- ion was observed to be 0.6 for Cu++ and 0.07 for Cu+. Also, it was seen that RDFs for Cu++-Cl drops to zero in between of first two peaks as shown in Fig.1 (a). This indicates that there is no exchange of Cl- ions between the first coordination shell and second coordination shell. In other words, the chloride ions in first coordination shell are strongly bounded to Cu++. However, the same was not true for Cu+ as few of the Cl- ions were seen to be exchange in between the 1st and 2nd coordination shell, therefore, leading to non-zero value of Cu+-Cl- RDF in between the first two peaks. Interestingly, the considerable differences can be noticed in peak height of Cu+ and Cu++ in RDFs of CuCl and Cu-Ow. Indeed, the peak magnitude are greater for Cu++ than Cu+, which is because of higher charge of Cu++, which attract more number of Cl- ions and water molecules in the coordination sphere than the Cu+. Subsequently, the structure of water in the surrounding of central Cu ion was considered and the results showed that water molecules form more ordered structure in the vicinity of Cu++ as compared to Cu+ as there was a lesser exchange of water molecules between the first and second solvation shell for Cu++ (seen from the nearly zero RDF values in between of first two peaks in Fig. 1(b). Interestingly, the average coordination number of water (Ow) was also observed to be different for both the copper ions i.e. 5.0 for Cu+ and 6.0 for Cu++. Also, it was noticed that the coordinated chloride ions and water molecules do not reside within the same coordination shell. As shown in Fig.1(c),the water molecules form primary hydration shell around the copper ions, which is further surrounded by secondary coordination shell consisted of chloride ions and water molecules. The average distance of primary hydration shell was estimated to be 2.01Å for Cu++ and 2.11Å for Cu+, whereas, the secondary shell was at 2.45Å from both the central copper ions. Furthermore, while analyzing the microscopic structure of the formed complex, it was observed that all Cu++ complexes exhibit exact 6 coordination number with complex stoichiometry of 1:6:0, 1:5:1, and 1:4:2 for Cu++:H2O:Cl-. However, the coordination number of Cu+ complex was found to be varied from 4, 5 and 6, which resulted to the average coordination number of 5.0. The possible complex stoichiometry of Cu+:H2O: Cl- were 1:4:0 and 1:3:1 for CN=4, 1:5:0 and 1:4:1 for CN=5 and Page | 7 ACS Paragon Plus Environment


1:6:0 (at most) and 1:5:1 (rare) for CN=6 respectively. The corresponding images of Cu++ and Cu+ complex are shown in Fig.2(a) and Fig.2(b) respectively. The results show the planar (distorted square) coordination of central Cu ion for CN=4, whereas for CN=5, the structure was like distorted square pyramid, which was further rearranged to distorted octahedral for CN=6.

Figure. 1 RDF for (a) Cu-Cl and (b) Cu-Ow (Cu+ and Cu++ in water are shown by solid lines of black and red color respectively, whereas Cu+ and Cu++ in the mix oxidation state are represented by dash lines of blue and magenta color in sequence) and (c) Structure of Coordination shell for central copper ion (Atom symbols for Copper, Chloride and water molecules are shown in orange, green and blue as shown)


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Figure2. Possible coordination complex for (a) Cu+ and (b) Cu++ ions



It is well known that the study of aqueous copper complexes is complicated by the coexistence of multiple oxidation states of copper, Cu+ and Cu++, which have different coordination chemistry. Copper ions predominantly exist as Cu++ at room temperature, but the Cu+ form is more stable at higher temperatures. Therefore, at the moderate temperature of anode reaction in electrochemical Cu-Cl cycle, both the Cu+ as well as Cu++ might be present. In order to meet conditions closer to the real experiments, an equimolar mixture of Cu+ and Cu++with 0.5M concentration of each was considered in the aqueous phase, named as system AB. The system was neutralized by adding equivalent chloride ions. The results show no variation in the peak positions for the considered ion pairs of Cu-Cl and Cu-Ow w.r.t. to their single oxidation state systems. In line to the single oxidation state systems, the average coordination of Cu+ and Cu++in mix system was observed to be 5 and 6 respectively. Also for Cu+, the complex with 4, 5 and 6 coordination number were noticed, on the other hand, Cu++ always showed complex formation with 6 coordination number. The most important observation about the mix systems was the presence of anionic Cu complex. The snapshots in Fig.3, show that some of the water molecules from the first solvation shell of few Cu+ were replaced with chloride ions, leading to formation of additional anionic Cu complex i.e. [Cu+Cl2]-.2H2O and [Cu+Cl2]-.3H2O besides the originally exiting complex for Cu+ and Cu++. The opinion of anionic Cu complex in presence of excess of chloride ions is not new but has been also supported by many experimental as well as theoretical studies 18. However, to the best of our knowledge, this is the first time when presence of anionic Cu complex has been captured in forcefield based MD simulation studies.


Figure 3. Presence of anionic copper complex in the systems

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3.1.1. Effect of Salt Concentration Furthermore, the MD simulations were extended for three concentration; 0.25M, 0.5M and 0.75Mof Cu+/Cu++ ions in water. Each system was simulated for (a) Cu+/Cl- in water (named as system A1, A2, A3 for concentration 0.25M, 0.5M and 0.75M respectively) (b) Cu++/Cl- in water (named as system B1, B2, B3 for concentration 0.25M, 0.5M and 0.75M respectively) and (c) equimolar composition of Cu+/Cu++/Cl- in water (system AB1, AB2 and AB3 for concentration 0.25M, 0.5M and 0.75Mof each ion respectively). The densities of these simulated systems are shown in Table.1. In support of simulated data, the experiments were also conducted to estimate the density of CuCl 2 solution using Oscillating Utube density meter (Model: DMA 5000M, Antoine Parr). The results display an excellent match of experimental densities (shown by (*)) with the simulated values as the deviation was less than 2%. And so, validate the selection of forcefield parameters for the simulated systems. Both the experiments and simulation results show that the density of system increases with increase in concentration. Also, the density of these systems was observed to follow the order ρAB>ρB>ρA, which was in sequence of total number of ions in the systems. Table 1. Effect of salt concentration on density of systems in unit g/cm3 Concentration System A 1.01 0.25M 1.03 0.5M 1.04 0.75M

System B 1.01 (*1.027) 1.04(*1.056) 1.07(*1.085)

System AB 1.04 1.08 1.12

Figure4. Effect of Cu ion concentration on the coordination of (a) chloride ions and (b) water molecules for Cu+/Cu++



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Further, the results in Fig.4(a) and Fig.4(b) illustrate that with increase in concentration, the coordination of Cl- ions to Cu ions is increased, and on the other hand water hydration to these Cu ions is reduced. Interestingly, the observed growth in chloride coordination and depletion in water coordination were more significant for Cu++ than the Cu+. This indicates that for Cu++, it is easier to add more number of chloride ions in the coordination sphere than that of Cu+. In other words, it can be stated that the Cu++ have more tendency to form direct contact ion pairs (CIPs) of Cu++/Cl- by replacing water from solvent separated ion pairs (SSIPs) than the Cu+, which might be due to higher charge deficiency of Cu++. Furthermore, both the Cu ions, indicate the rise in chloride ion coordination to be more extensive than the drop in water coordination. This can be related with the two possible reasons: first, the addition of chloride ions in the coordination sphere of Cu ions, need not to replace equivalent water molecules, which is probable because chloride ions and the water molecules do not reside within the same coordination distance from central Cu ion, as discussed in previous section 3.1.Secondly, the count of Cu ions, having enhanced coordination of chloride ions and so the reduced coordination of water molecules might be too less to affect the average coordination of water. This has been made clearer furthermore by estimating the probability distribution of obtained complex stoichiometry for all these expected species. Table.2 represents the captured Cu ion complex, and the probability distribution of these complexes is shown in Fig.5. The results show that while reducing the concentration to 0.25M, only 6:0 complex were observed for Cu++ in system B1. On the other hand, Cu+ in system A1 showed 4:0, 5:0 and 6:0 complex stoichiometry with major probability, along with 3:1 and 4:1 complex stoichiometry

with minor

probability as shown in Fig.5. For the mix oxidation states of Cu ions in system AB1, the complex of 6:0 and 5:1 for Cu++ and 6:0, 5:0, 4:1, 4:0, 3:1 and 2:2 complex of Cu+ were captured. The observed 2:2 complex is indicative of anionic Cu+ complex i.e. [Cu+Cl2]-.2H2O in the system. Furthermore, for the increased concentration of 0.75M, system A3 showed 3:1, 4:0, 5:0, 6:0 complex with major contribution and 5:1 and 4:1 complex with minor contribution to probability distribution. System B3 was observed to exhibit 4:2, 5:1 and 6:0 complex ratio for Cu++. Subsequently, the system AB3 showed complex ratio of 6:0, 5:0, 4:1, 4:0, 3:1 and 2:2 for Cu+, whereas for Cu++, 6:0, 5:1 and 4:2 complex along with few 1:3 and 2:3 complex were noticed. Considering the mix Cu oxidation state systems, the formation of anionic complex of Cu++(i.e.[Cu++Cl3]-.H2O and [Cu++Cl3]-.2H2O) was first time noticed at 2.25M concentration of chloride ions in the system, on the other hand, the formation of anionic complex for Cu+ was initiated at 0.75M concentration of chloride ions. The results demonstrate that for Cu++, only with CN

Molecular Facts on the Structure and Dynamics of Electrolyte Species

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