Effects of Oxygen-Containing Functional Groups on Supercapacitor

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Letter

Effects of Oxygen-Containing Functional Groups on Supercapacitor Performance Sebastien Kerisit, Birgit Schwenzer, and Vijayakumar Murugesan J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2014 Downloaded from http://pubs.acs.org on June 22, 2014

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The Journal of Physical Chemistry Letters

Effects of Oxygen-Containing Functional Groups on Supercapacitor Performance Sebastien Kerisit*, Birgit Schwenzer, and M. Vijayakumar* Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352 USA AUTHOR INFORMATION Corresponding Authors * [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Molecular dynamics (MD) simulations of the interface between graphene and the ionic liquid 1butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM OTf) were carried out to gain molecular-level insights into the performance of graphene-based supercapacitors and, in particular, determine the effects of the presence of oxygen-containing defects at the graphene surface on their integral capacitance. The MD simulations predict that increasing the surface coverage of hydroxyl groups negatively affects the integral capacitance, whereas the effect of the presence of epoxy groups is much less significant. The calculated variations in capacitance are found to be directly correlated to the interfacial structure. Indeed, hydrogen bonding between hydroxyl groups and SO3 moieties prevents BMIM+ and OTf- ions from interacting favorably in the interfacial layer and restrains the orientation and mobility of OTf- ions, thereby reducing the interfacial permittivity of the ionic liquid. The results of the simulations can facilitate the rational design of electrode materials for supercapacitors.

TOC GRAPHICS

KEYWORDS graphene; graphene oxide; ionic liquids; molecular dynamics; interface.


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Interest in graphene-based supercapacitors (electrical double-layer capacitors, EDLCs) has increased drastically over the last several years.1-6 Graphene has become a material of choice for EDLCs due to its large surface area and good electronic conductivity. Many recently reported EDLCs contain ionic liquids, either as the electrolyte or as part of a composite electrode material.7,8 Ionic liquids are safe, environmentally friendly alternatives to traditional electrolytes and offer a large electrochemical stability window and high thermal stability.9,10 The theoretical capacitance of graphene-based EDLC devices has been determined to be 550 F/g.11,12 However, reported capacitance values are in the range of 36 to 261 F/g for most devices.1,7,12-14 This unsatisfying performance underlines the lack of fundamental understanding of realistic interfaces between graphene and ionic liquids. Based on our previously published results,15 we hypothesize that the observed spread in reported capacitance values is at least partially due to variations in the quality of the graphene used.16 Most theoretical studies assume the use of pure, defect-free graphene;17-21 however, chemical synthesis routes almost always lead to the formation of oxygen-containing functional groups such as epoxy and hydroxyl groups22,23 and our recent work suggested that graphene-ionic liquid interactions depend on the amount and type of oxygen-containing defects present on graphene sheets.15 These defects may directly affect the specific capacitance of the overall device, which is the ratio of the charge stored to the supplied voltage:

C=

ε rε o d

(1)

A

where C is the specific capacitance, εr and εo are the relative permittivity and the permittivity in vacuum, respectively, A is the specific accessible surface area of the electrode, and d is the Debye length, which is a function of the thickness of the EDLC or the distance between the


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electrodes. In this work, we build upon the results we obtained previously from combined experimental and molecular modeling studies15,24 and report on the variations in capacitance of a graphene-ionic liquid system as a function of the amount (C/O ratio) and type (hydroxyl or epoxy) of oxygen-containing defects at the graphene surface. Molecular simulations are used to allow for a direct, one-to-one comparison of the influence of the two defect types on the capacitance. The results of the simulations are expected to contribute to the growing amount of molecular-level insights gained from computational modeling10 and thus facilitate the rational design of graphene-based supercapacitors. The room-temperature ionic liquid of interest here is 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM OTf), as shown in Figure 1.


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Figure 1. (top left) BMIM+ and OTf- ions. (top right) Top and side views of a randomly hydroxylated graphene sheet (10 atomic percent oxygen). (bottom) Snapshot from the simulation of BMIM OTf in contact with pure graphene sheets separated by 9.05 nm. A first series of MD simulations was carried out to determine the effects of oxygen-containing functional groups on the structure of the ionic liquid at the interface with graphene sheets at room temperature, as illustrated in Figure 1. Hydroxyl and epoxy groups were distributed randomly on the graphene sheets (Figure 1). Details of the computational approach are provided in Supporting Information. The thickness of the ionic liquid slab was chosen to minimize interferences between the two interfaces.10,25 Figure 2 shows the number density profiles for the BMIM+ cation and the OTf- anion as a function of oxygen content for both hydroxyl and epoxy functional groups. For pure graphene, the simulations predict the formation of a dense layer within approximately 0.6 nm from the surface, in agreement with the results of previous MD simulations of the structure of ionic liquids in contact with graphene sheets.26-29 As the hydroxyl content increases, the magnitude of the first peak in the density profiles of BMIM+ decreases significantly and a shift to longer distances is also observed. However, the small second peak is found to increase with increasing hydroxyl content. For OTf-, the inverse trend is observed: the first peak increases in magnitude whereas the second peak decreases.


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Figure 2. Number density profiles for the BMIM+ cation (top – based on the position of the C atom of the imidazolium ring that is bonded to both N atoms) and OTf- anion (bottom – based on the position of the S atom) as a function of hydroxyl (left) and epoxy (right) contents. Averages of both interfaces are shown in each case; full density profiles are given in Supporting Information. The orientation of the ionic liquid molecules at the interface and the effects of the presence of functional groups were determined from an order parameter defined as the Legendre polynomial:

S=

(

)

(2)

1 3 cos 2 θ − 1 2

where θ is the angle between the normal to the graphene sheet and the N-N vector or the C-S vector for BMIM+ and OTf-, respectively. If the N-N or C-S vector is parallel to the surface,


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S = -0.5 whereas S = 1 if the N-N or C-S vector is perpendicular to the surface. If the orientation of the cation or anion is random, then S = 0. In the case of pure graphene, the BMIM+ cations in contact with the surface, i.e. within the first density peak, are orientated with their imidazolium ring parallel to the surface, whereas the cations at the distance of the second, smaller peak adopt a more random orientation (Figure 3). As the hydroxyl coverage increases, although those cations closest to the surface are still oriented with their imidazolium ring parallel to the surface, the shift of the first density peak to longer distances away from the surface causes the orientation of BMIM+ cations to be more randomized. The OTf- anions show a different behavior. The first anion density peak is composed of a main peak with a shoulder at shorter distances. Anions at the distance of the shoulder are mostly aligned with their C-S vector perpendicular to the surface whereas those that are grouped under the main peak are mostly oriented parallel to the surface. As the number of surface hydroxyl groups increases, the order parameter profile does not change significantly within 0.5 nm but the intensity of the shoulder increases to the detriment of the main peak. This observation indicates that a greater proportion of the anions are found to orientate perpendicular to the surface as the hydroxyl coverage is raised.


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Figure 3. Order parameter profiles for the BMIM+ cation (top – based on the orientation of the N-N vector) and OTf- anion (bottom – based on the orientation of the C-S vector) as a function of hydroxyl (left) and epoxy (right) contents. These changes in interfacial structure are due to strong binding between OTf- anions and hydroxyl functional groups. Surface hydroxyl groups can donate hydrogen bonds to oxygen atoms of the SO3 moiety, as illustrated in Figure 4. Since these bonds are stronger than the interaction between the oxygen atoms of the SO3 moiety and the hydrogen atoms on the imidazolium ring, the increasing surface coverage of hydroxyl groups leads to a diminishing ion pairing between BMIM+ and OTf- ions (Figure 4). As a result, BMIM+ cations are pushed away from the surface and OTf- anions are attracted closer to the surface, filling gaps between surface hydroxyl groups. However, at the highest hydroxyl coverage, there is no increase in bonding


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between hydroxyl groups and OTf- anions at the interface since almost all oxygen atoms from SO3 moieties are already accepting a hydrogen bond from surface hydroxyls (Figure 4). In addition, steric hindrance at the surface created by the large hydroxyl coverage prevents some of the OTf- anions from approaching the surface allowing for a small increase in the extent of bonding between BMIM+ and OTf- ions in the dense layer.

Figure 4. (top) Number of bonds per oxygen atom between hydrogen atoms of hydroxyl groups and oxygen atoms of the SO3 moieties. Only OTf- anions in the dense layer are considered. Atoms are considered bonded if the H-O inter-atomic distance is less than the minimum in the corresponding radial distribution function (~2.45 Å). (bottom) Number of bonds per hydrogen atom between oxygen atoms of the SO3 moieties and hydrogen atoms of the imidazolium ring. Only BMIM+ cations in the dense layer are considered. Atoms are considered bonded if the O-H inter-atomic distance is less than the first minimum in the corresponding radial distribution function (~3.77 Å). Error bars were obtained from the standard deviation of the mean when the


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trajectories were divided into four equivalent blocks. In the presence of epoxy functional groups, a similar trend is observed, i.e. the magnitude of the first BMIM+ density peak decreases while that of the first OTf- density peak increases, but the changes are not as extensive as in the presence of hydroxyl groups. These changes are due to steric effects at the interface rather than specific bonding with functional groups, as was the case in the presence of hydroxyl groups. Indeed, the extent of ion pairing, as shown by the number of bonds between SO3 moieties and hydrogen atoms of the imidazolium ring in Figure 4, is not affected by the extent of surface coverage. Additionally, this is supported by a significantly smaller first peak in the radial distribution functions between surface oxygen atoms and SO3 oxygen atoms for epoxy groups versus hydroxyl groups (Figure S2). A second series of MD simulations was carried out to determine the effects of oxygen-containing functional groups on the capacitance of the graphene-ionic liquid system. In these simulations, the charges on the C atoms of the graphene sheets were varied to yield 5 surface charge densities (0.00, 1.36, 2.72, 4.08, and 5.44 µC cm-2). In addition, the temperature was raised to 450 K in these simulations to enhance diffusion. For each simulation, the electrostatic potential φ(z) at distance z along the normal to the graphene sheets was calculated as ∆ϕ (z ) = ϕ ( z ) − ϕ ( z 0 ) = − ∫ E ( z ')dz ' z

(3)

z0

where the electric field E(z) is

E (z ) =

1 −ε0

∫

z

z0

(4)

ρ q ( z ')dz '

where ε0 is the permittivity of vacuum and the charge density ρq(z) is


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ρ q (z ) = ∑ qi ρ i (z )

(5)

i

where ρi is the density of atom i, and qi its charge. As an example, the electrostatic potential profile obtained for the case of BMIM OTf in contact with pure graphene is shown in Figure 5. As the surface charge density increases the magnitude of the potential difference between the graphene sheets and the bulk of the ionic liquid (i.e. at the half-way point between the two sheets) increases. The change in potential difference is linearly dependent on the change in surface charge density and the gradient yields the integral capacitance of the electrical double layer (C = σ/∆V). The integral capacitance calculated for BMIM OTf in contact with pure graphene is 4.45 µF cm-2. This value is consistent with that obtained by Kislenko et al.26 from a simulation of BMIM PF6 in contact with pure graphene (C = 3.6 µF cm-2).

Figure 5. (left) Electrostatic potential profiles for a range of surface charge densities for BMIM OTf in contact with pure graphene. (right) Change in capacitance as a function of surface functional group coverage for both hydroxyl and epoxy groups. Figure 5 shows the change in capacitance as a function of surface coverage for both hydroxyl and epoxy groups. In the presence of hydroxyl groups, the capacitance decreases initially with


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increasing coverage up to 20 atomic percent oxygen and subsequently increases back up at 25 atomic percent. This trend mirrors that observed for the change in the extent of bonding between BMIM+ and OTf- ions in the dense layer due to hydrogen bonding between SO3 moieties and surface hydroxyls. The bonding of OTf- anions with surface functional groups limits their mobility and constrains their orientation, which decreases the permittivity of the ionic liquid at the interface and, in turn, the capacitance value (Equation 1). This finding indicates that the presence of hydroxyl groups at graphene surfaces can be detrimental to the supercapacitor capacitance. The influence of epoxy groups, however, differs significantly from that of the hydroxyl groups. Indeed, the simulations predict little to no changes in capacitance up to coverages of 10 atomic percent oxygen and only a slight decrease at higher oxygen atomic percentages. Again, this correlates well with the extent of bonding between BMIM+ and OTfions in the dense layer, which also does not show any significant changes. Although experimental studies have explored the relative performance of graphene and graphene oxide as supercapacitor electrode materials, a consensus has not been established yet and published results can even seem contradictory at times. Xu et al.30 reported graphene oxide (i.e. graphene with a significant proportion of epoxy functional groups) as the better electrode material due to a relatively higher specific capacitance than that measured for graphene. This was attributed to the redox property of epoxy groups and subsequent addition of pseudocapacitance to the final performance due to electrode-electrolyte redox interactions. On the other hand, higher performance of graphene over graphene oxide was reported by other research groups.31,32 Indeed, a recent study by Lai et al.,31 in which graphene materials with different C/O ratios were used in combination with an aqueous H2SO4 electrolyte solution, showed a trend similar to that predicted by our MD simulations, i.e. specific capacitances of ca. 50 and 2 F/g


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were observed for supercapacitors employing graphene materials with C/O atomic ratios of 5.6 and 1.7, respectively. Similarly, Wang et al.32 reported specific capacitance values of graphene materials measured in an aqueous KOH electrolyte solution that increased with increasing extent of chemical reduction of the graphene oxide materials with hydrazine. In summary, the molecular dynamics simulations show that both the quantity and the type of oxygen-containing functional groups on the graphene surface can influence supercapacitor performance. In particular, hydroxyl groups were found to affect negatively and significantly the integral capacitance, whereas the effect of epoxy groups was much less pronounced. This finding suggests that synthesis procedures that limit the formation of hydroxyl groups on graphene should be actively sought. Therefore, the molecular-level insights into the unique role of surface functional groups gained in this study provide a way for tuning and optimizing graphene-based supercapacitors beyond their current level of performance. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]; [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research was supported by the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL), a multi-program national laboratory operated by Battelle Memorial Institute for the U.S. Department of Energy (DOE) under Contract DE-


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AC05-76RL01830. The computer simulations were performed in part using the Molecular Science Computing (MSC) facilities in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research (OBER) and located at PNNL. Supporting Information Available. Details of the computational approach, full number density profiles, and surface oxygen-SO3 oxygen radial distribution functions. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Brownson, D. A. C.; Banks, C. E. Fabricating Graphene Supercapacitors: Highlighting the Impact of Surfactants and Moieties. Chem. Comm. 2012, 48, 1425-1427. (2) Choi, H.-J.; Jung, S.-M.; Seo, J.-M.; Chang, D. W.; Dai, L.; Baek, J.-B. Graphene for Energy Conversion and Storage in Fuel Cells and Supercapacitors. Nano Energy 2012, 1, 534-551. (3) Li, J.; Östling, M. Prevention of Graphene Restacking for Performance Boost of Supercapacitors - A Review. Crystals 2013, 3, 163-190. (4) Zhang, L. L.; Zhou, R.; Zhao, X. S. Graphene-Based Materials as Supercapacitor Electrodes. J. Mater. Chem. 2010, 20, 5983-5992. (5) Zhang, J.; Zhao, F.; Zhang, Z.; Chen, N.; Qu, L. Dimension-Tailored Functional Graphene Structures for Energy Conversion and Storage. Nanoscale 2013, 5, 3112-3126. (6) Tan, Y. B.; Lee, J.-M. Graphene for Supercapacitor Applications. J. Mater. Chem. A 2013, 1, 14814-14843. (7) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M. et al. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537-1541.


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Effects of Oxygen-Containing Functional Groups on Supercapacitor

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