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Letter

A Computer Simulation Study of Graphene Oxide Supercapacitors: Charge Screening Mechanism Sang-Won Park, Andrew D. DeYoung, Nilesh Ramchandra Dhumal, Youngseon Shim, Hyung J Kim, and YounJoon Jung J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00202 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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The Journal of Physical Chemistry Letters

A Computer Simulation Study of Graphene Oxide Supercapacitors: Charge Screening Mechanism Sang-Won Park,† Andrew D. DeYoung,‡ Nilesh R. Dhumal,‡ Youngseon Shim,†,§ Hyung J. Kim,∗,‡,¶,k and YounJoon Jung∗,† †Department of Chemistry, Seoul National University, Seoul 08826, Korea ‡Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA ¶School of Computational Sciences, Korea Institute for Advanced Study, Seoul 02455, Korea §Present address: Samsung Advanced Institute of Technology, Suwon, Gyeonggi-do 16678, Korea kPermanent address: Carnegie Mellon University E-mail: [email protected]; [email protected]

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March 11, 2016 Abstract Graphene oxide supercapacitors in the parallel plate configuration are studied via molecular dynamics (MD) simulations. The full range of electrode oxidation from 0 % to 100 % is examined by oxidizing the graphene surface with hydroxyl groups. Two different electrolytes, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+ BF4 – ) as an ionic liquid and its 1.3 M solution in acetonitrile as an organic electrolyte, are considered. While the area-specific capacitance tends to decrease with increasing electrode oxidation for both electrolytes, its details show interesting differences between the organic electrolyte and ionic liquid, including the extent of decrease. For detailed insight into these differences, the screening mechanisms of electrode charges by electrolytes and their variations with electrode oxidation are analyzed with special attention paid to the aspects shared by and the contrasts between the organic electrolyte and ionic liquid.

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Electric double layer capacitors (EDLCs), or supercapacitors, are promising energy storage devices thanks to their high power density and reasonable energy density. 1 Their high capacitance power arises from electric double (or multiple) layers formed at interfaces with electrodes, which can quickly rearrange and completely screen at sub-nanometer length scale the electric field generated by the electrode charges. 1–9 Commonly used electrolytes are aqueous or organic solutions of salts. Ionic liquids (ILs) have also received extensive attention as an alternative electrolyte because of their attractive properties, such as low volatility, non-flammability and wide electrochemical window. 1,10,11 Another important component of supercapacitors, critical for EDLC performance, is electrodes. Graphene-based materials have received extensive scrutiny due to their excellent properties, including high stability, electrical conductivity, surface area, and electrolyte wettability. 2,12,13 Though pristine graphenes were used in several experiments, 14–17 reduced graphene oxide materials are widely employed 18–33 probably due to their processing advantage. 34,35 The results on capacitance are, however, controversial; while some measurements suggest that oxidation enhances capacitance, 26,27 others indicate graphene oxide (GO) has lower capacitance than pure graphene. 19–25 Computer simulations have mainly focused on pure graphene systems. 5–8,36–39 Very recent MD studies with GO electrodes 31–33 show that capacitance decreases with increasing electrode oxidation. In this Letter, we extend our prior work on IL-based supercapacitors with GO electrodes 31 to organic electrolyte (OE) systems. Screening mechanisms of electrode charges, their variations with electrode oxidation and influence on capacitance are analyzed and compared with the IL-based supercapacitors. Model descriptions and computational details are the same as in ref 31. Briefly, the simulation system consists of an electrolyte confined between two GO electrodes in the parallelplate configuration (Figure 1). Two different electrolytes, a pure ionic liquid and an organic electrolyte, are considered. 6–8,31 The ionic liquid system (“IL supercapacitor”) comprises 256 pairs of EMI+ and BF4 – ions, while the organic electrolyte system (“OE supercapacitor”) consists of 62 EMI+ BF4 – pairs and 634 acetonitrile molecules, corresponding to a ∼ 1.3 M

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solution of EMI+ BF4 – . Two GO electrodes, each modeled as a flat, rigid graphene sheet with area A0 (= 3.396 × 3.431 nm2 ), are separated by d(= 6.6 nm) and situated at z = ±z0 (z0 = 3.3 nm) parallel to the xy-plane. Each electrode comprises 448 carbon atoms and is decorated by hydroxyl groups. System nuclear dynamics were simulated in an orthorhombic box with dimensions 3.396 × 3.431 × 30.0 nm3 in the canonical ensemble at 350 K using the GROMACS program. 40 Three different surface charge densities, σS = 0, ±0.215 and ±0.43 e nm−2 corresponding to total electrode charges of 0, ±2.5 and ±5e, were considered by assigning partial charges to carbon atoms uniformly (e = elementary charge). The electrodes located at z = −z0 and +z0 were charged positive (“anode”) and negative (“cathode”), respectively (Figure 1). Ten different trajectories were simulated for each charged supercapacitor, while five trajectories were simulated in the discharged case. For further details, including information on force field parameters, the reader is referred to ref 31. We begin with the electric potential and specific capacitance of the supercapacitors. The half-cell potential ∆Φi (i = anode, cathode) and cell voltage ∆Φcell are

∆Φanode = Φ(z = −z0 ) − Φ(z = 0) ; ∆Φcathode = Φ(z = 0) − Φ(z = z0 ) ; ∆Φcell = Φ(z = −z0 ) − Φ(z = z0 ) = ∆Φanode + ∆Φcathode .

(1)

Here Φ(z) is the electric potential at z calculated by integrating the Possion equation 3,5

Φ(z) = −4π ρ¯α (z) = A−1 0

Z

z ′

′

dz (z − z )

−∞ Z x0 Z y0

all X

ρ¯α (z ′ ) ;

(2)

α

dx′ dy ′ ρα (x′ , y ′ , z)

(A0 = 4x0 y0 ) ,

−x0 −y0

where ρα (x′ , y ′ , z) is the local charge density of species α (α = EMI+ , BF4 – , CH3 CN, or P electrode) at position (x′ , y ′ , z), ρ¯α (z) is its average over x and y, and all α is the sum over all species. The area-specific capacitance ctot S —capacitance of the cell, normalized to the

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electrode surface area A0 —is

ctot S =

|σS | ; δ∆Φcell

ciS =

|σS | ; δ∆Φi

1 ctot S

=

1 canode S

+

1 ccathode S

,

(3)

where ciS is the area-specific electrode capacitance. Simulation results are presented in Figure 2. There δ denotes the difference of the charged and discharged supercapacitors, i.e., δ∆Φi = ∆Φi (charged) − ∆Φi (discharged). As the electrode oxidation level increases, δ∆Φcell and ctot S generally increases and decreases, respectively, for both IL and OE supercapacitors. Previously, this trend for IL supercapacitors was attributed to two factors: decreasing reorganization ability of IL ions and widening gap of double layers as electrode oxidation increases. 31 One of the most salient features of Figure 2a is the large difference in δ∆Φcell between IL and OE at 0 and ∼ 100 % oxidation. Furthermore, δ∆Φcell and ctot S for the ionic liquid system show, respectively, a rapid increase and decrease from 0 to ∼ 10 % oxidation, while those for the organic electrolyte remain nearly constant at low oxidation. We note that though it may not be practical for supercapacitors due to low conductivity, 18 highly oxidized GO was included in our analysis to gain complete insight into the influence of oxidation. 31 Comparison of half-cell potentials in Figure 2cd reveals that the origin of the pronounced difference in δ∆Φcell between the organic electrolyte and ionic liquid systems at low and high electrode oxidation is the cathode. Their anode potentials with respect to the discharged electrode exhibit nearly an identical behavior; δ∆Φanode increases almost linearly with % oxidation. By contrast, IL and OE show markedly different behaviors in δ∆Φcathode . Specifically, δ∆Φcathode for the IL supercapacitor increases rapidly from 0 to ∼ 25 % oxidation, whereas it decreases slightly for the OE supercapacitor. Above 30 %, δ∆Φcathode of OE exhibits more rapid increase than IL. The cathode-anode asymmetry in the half cell potential is usually ascribed to differing screening ability of ions, which depends on factors, such as ion size, shape and charge distri-

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bution. 3,6–9 While previous computational works focused mostly on counter-ions and their influence on the cathode-anode asymmetry, our prior analysis has suggested the importance of co-ions for IL. 31 A recent NMR study of microporous carbon supercapacitors also indicates that co-ions can contribute to electrode screening by “desorbing” from the electrode. 41 Here we examine respective roles played by counter-ions and co-ions in electrode charge screening. To do so, we analyze the numbers of cations and anions, NαSZ , in screening zones, defined as the interfacial electrolyte region in which the difference of the counter-ion and co-ion numbers is the same as the magnitude of the electrode charge in units of e (see Supporting Information). 6 Thus the total charge of the charged electrode and its screening zone is 0. The screening zone extends ∼ 0.4–0.8 nm from the electrode; it tends to extend further from the electrode as electrode oxidation increases because of steric hindrance arising from OH groups. 31 Ion populations in screening zones of the IL supercapacitor are displayed in Figure 3. SZ SZ In the discharged case (Figure 3a), Nanion and Ncation do not vary with electrode oxidation. SZ SZ Because of local electroneutrality, Nanion and Ncation are nearly the same. When the electrodes

become charged, populations of the counter-ions and co-ions in the screening zones increase and decrease, respectively, as expected. However, there is a notable disparity between the cathode and anode. Ion reorganization in the anode screening zone in response to electrode charging occurs mainly via increase in counter-ions BF4 – (Figure 3c), whereas both counterions, EMI+ , and co-ions, BF4 – , contribute at the cathode (Figure 3d). 31 The situation is quite different for the organic electrolyte. First, there is no appreciable ion population in the screening zones of the discharged electrodes at 0 % oxidation because the OE ion concentration is very low, compared to IL; screening zones are populated almost exclusively by acetonitrile (Figure 4a). Second, as the electrodes become oxidized by adding SZ SZ hydroxyl groups to the graphene surface, Nanion and Ncation initially grow, while acetonitrile

shows the opposite trend. The formation of hydrogen bonds between anions and hydroxyl groups is mainly responsible for the anion population increase (Figure 4b). This is accompa-

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nied by cation population increase in the screening zones to meet the local electroneutrality condition. Beyond ∼ 20 % oxidation, however, the number of anion-OH hydrogen bonds decreases, analogous to the IL case (Figure 3b), due to increasing steric hindrance between SZ SZ BF4 – and OH groups. 31 This results in the reduction of Nanion and thus Ncation . Therefore

specific interactions between the electrode surface and ions play a central role in determining the screening zone populations of ions for the discharged OE supercapacitor. Response of the organic electrolyte to charging shows an interesting cathode-anode asymmetry different from IL. At the anode, ion density reorganization in the screening zone occurs primarily via the increase of counter-ions BF4 – (Figure 4c). The exception is ∼ 10–30 % oxidation, where co-ions EMI+ also make a significant contribution presumably because the reorganization ability of BF4 – is restricted due to formation of a maximum number of hydrogen bonds with OH groups. Nevertheless, the anion dominance in the ion reorganization near the anode is similar to the IL case in Figure 3c. By contrast, the mechanism for ion reorganization near the cathode of the OE supercapacitor varies with electrode oxidation. Near 0 % oxidation, the influx of counter-ions EMI+ into the screening zone is mainly responsible for ion reorganization. Since co-ions BF4 – are SZ nearly absent there prior to charging (Nanion ≈ 0), they simply cannot contribute to screen-

ing of the negatively-charged cathode. As the electrode oxidation increases, co-ions play a SZ progressively more important role because Nanion prior to charging grows due to hydrogen-

bonding interactions with hydroxyl groups of the cathode. As a result, between ∼ 20 and ∼ 80 % oxidation, both cations and anions make a significant contribution to charge reorganization by entering and leaving the screening zone, respectively. As the cathode becomes further oxidized, counter-ions become dominant again because of significant co-ion population decrease. The GO oxidation-dependent co-ion contribution to the cathode screening is the major difference of the OE supercapacitor from the IL. The analysis above sheds light on the cathode-anode disparity and the similarities and differences of the IL and OE supercapacitors. For example, anode screening is mainly governed

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by BF4 – through its population enhancement in the screening zone for both the ionic liquid and organic electrolyte. This probably explains the close similarity of δ∆Φanode between the two. For IL, both anions and cations were found to contribute to ion density reorganization in the cathode screening zone. Thus upon charging, anions will move away from the cathode and out of its screening zone. This involves breaking of anion-OH hydrogen-bonds for the oxidized cathode. For pure graphene, on the other hand, the absence of hydrogen-bonds makes the reorganization of anions much easier. This difference in ease of anion reorganization is the likely reason for the abrupt increase in δ∆Φcathode from 0 to 5 % oxidation for the IL supercapacitor in Figure 2d. For the organic electrolyte, hydrogen-bond formation (Figure 4b) was found to increase the BF4 – population near electrodes as electrode oxidation increases in the low oxidation region (Figure 4a). Thus while individual ions’ reorganization ability may decrease, the number of anions that can contribute to cathode screening as co-ions increases with electrode oxidation. The initially decreasing trend of δ∆Φcathode in OE in Figure 2d indicates that the latter aspect, not shared by IL, is dominant over the former. The large difference in δ∆Φcathode at 0 % oxidation from the IL system arises from lack of BF4 – contribution due to its low density near the discharged cathode of the organic electrolyte system. For & 60 % SZ oxidation, δ∆Φcathode increases rapidly because Nanion of the discharged OE supercapacitor

decreases with electrode oxidation (Figure 4a), another feature absent in IL (Figure 3a). For additional insight into electrode screening, we consider electric potential changes associated with reorganization of species α,

δΦα (z) = −4π

Z

z

dz ′ (z − z ′ ) δ ρ¯α (z ′ ) ;

δ ρ¯α (z) ≡ ρ¯α,charged (z) − ρ¯α,discharged (z) .

(4)

−∞

The results are displayed in Figure 5. With OH groups neglected, the electric potential arising from electrode charges decreases linearly with z, i.e., Φelectrode (z) = −4πσS z. Therefore, a rapid rise (drop) in δΦα (z) near the anode (cathode) as z increases (decreases) from −z0

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(z0 ) represents an efficient screening of the anode (cathode) charges by α. In Figure 5ab, δΦanion (z) exhibits a more rapid rise (drop) near the anode (cathode) than δΦcation (z) regardless of the electrode oxidation level. This clearly shows high screening efficiency of anions at both electrodes of the IL supercapacitor. This was ascribed to compact and symmetric molecular shape and thus charge distribution of BF4 – , compared to EMI+ . 31 Additionally, the drop in δΦanion (z) near the cathode changes significantly from 0 % to 10 % oxidation. This confirms that the rapid change in δ∆Φcathode from 0 to ∼ 10 % oxidation in Figure 2d arises primarily from BF4 – . Variations of δΦα (z) with electrode oxidation for the OE supercapacitor in Figure 5cd are more complicated than the IL supercapacitor. Nonetheless, the trends of δΦα (z) variations, e.g., total dominance of cations in cathode screening at 0 % and high oxidation, closely mirror SZ SZ those of δNcation and δNanion . This demonstrates that ion number changes in screening zones,

δNαSZ , provide a simple but reliable measure to characterize the screening efficiency of ions. One counter-intuitive aspect is that though small, co-ions make a negative contribution to electrode screening in some cases, viz., cathode at 0 % and anode at 60 and 100 % oxidation. A small increase in the co-ion population in the screening zone, induced by large enhancement of the counter-ion population upon electrode charging, is responsible for this behavior (cf. Figure 4cd). Finally, we make a brief contact with experiments. While several measurements 19–25 indicate that oxygen-containing functional groups in graphene oxide reduce capacitance in accord with our results, some others suggest the opposite. 26–29 One possible explanation for this disagreement is rough structure of multi-layered graphene oxide, in which ions can be intercalated. According to several MD studies, confinement of ions in microporous environments can enhance the capacitance. 37,39,42–44 Pseudo-capacitance is another possibility, especially in the case of aqueous electrolytes. 26 For electrode screening by co-ions, our results are in good agreement with a recent experimental finding that anions can desorb from the cathode upon charging. 41

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In summary, we studied supercapacitors composed of graphene oxide electrodes in the parallel plate configuration containing either EMI+ BF4 – or its solution in acetonitrile as an electrolyte. It was found that while the area-specific capacitance generally decreases with electrode oxidation, the ionic liquid and organic electrolyte systems show distinct behaviors mainly due to the difference in cathode capacitance. For the organic electrolyte, BF4 – play an important role in cathode screening except at very low or high oxidation. Their population enhancement through hydrogen-bond formation with electrode hydroxyl groups, compared to pure graphene or fully oxidized GO, enables BF4 – to screen negative cathode charges by leaving the screening zone in response to charging. By contrast, at 0 % and 80– 100 % oxidation, EMI+ play a central role by moving into the cathode screening zone upon charging; anions are largely unavailable for screening because of their very low population in the screening zone prior to charging. This oxidation-dependent screening mechanism at the cathode is the main difference from the IL supercapacitor, for which BF4 – are the main contributor to cathode charge screening in the entire oxidation range. For anode screening, counter-ions BF4 – play a major role for both IL and OE supercapacitors (with the exception of the organic electrolyte system at 10 % oxidation). Though limited to EMI+ and BF4 – ions and to graphene oxidation via hydroxyl groups, our analysis suggests that specific interactions of ions with GO functional groups and resulting local ion concentration changes near the electrode surface can play an important role in electrode charge screening, especially for the organic electrolyte. This opens up the possibility of optimizing the cathode and anode charge screening separately via differing functionalizations of electrode materials and/or by using multiple ionic species. In this context, it would be worthwhile to investigate in detail how the use of mixture electrolytes and/or the replacement of hydroxyl groups with other functional groups, such as epoxy 32–34 and carboxyl groups, influence the screening efficiency of the electrolyte and thus the capacitance.

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Acknowledgments This work was supported in part by NSF Grant No. CHE-1223988 and by National Research Foundation (NRF) grants funded by the Korean Government (MEST) (Nos. 2015015987 and 2015008709). A.D. acknowledges financial support from the ARCS Foundation.

Supporting Information Available Partial charge assignments, definition and results of the screening zone, profiles of number density, charge density, orientation and screening potential for each species and results for area-specific capacitance.

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(28) Hantel, M. M.; Kaspar, T.; Nesper, R.; Wokaun, A.; Kötz, R. Partially Reduced Graphite Oxide for Supercapacitor Electrodes: Effect of Graphene Layer Spacing and Huge Specific Capacitance. Electrochem. Commun. 2011, 13, 90–92. (29) Hantel, M. M.; Kaspar, T.; Nesper, R.; Wokaun, A.; Kötz, R. Partially Reduced Graphite Oxide as an Electrode Material for Electrodchemical Double-Layer Capacitors. Chem. - Eur. J. 2012, 18, 9125–9136. (30) Hantel, M. M.; Płatek, A.; Kaspar, T.; Nesper, R.; Wokaun, A.; Kötz, R. Investigation of diluted ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate electrolytes for intercalation-like electrodes used in supercapacitors. Electrochim. Acta 2013, 110, 234– 239. (31) DeYoung, A. D.; Park, S.-W.; Dhumal, N. R.; Shim, Y.; Jung, Y.; Kim, H. J. Graphene Oxide Supercapacitors: A Computer Simulation Study. J. Phys. Chem. C 2014, 118, 18472–18480. (32) Kerisit, S.; Schwenzer, B.; Vijayakumar, M. Effects of Oxygen-Containing Functional Groups on Supercapacitor Performance. J. Phys. Chem. Lett. 2014, 5, 2330–2334. (33) Xu, K.; Ji, X.; Chen, C.; Wan, H.; Miao, L.; Jiang, J. Electrochemical double layer near polar reduced graphene oxide electrode: Insights from molecular dynamic study. Electrochim. Acta 2015, 166, 142–149. (34) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228–240. (35) Compton, O. C.; Nguyen, S. T. Graphene Oxide, Highly Reduced Graphene Oxide, and Graphene: Versatile Building Blocks for Carbon-Based Materials. Small 2010, 6, 711–723.

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(36) Merlet, C.; Rotenberg, B.; Madden, P. A.; Taberna, P.-L.; Simon, P.; Gogotsi, Y.; Salanne, M. On the Molecular Origin of Supercapacitance in Nanoporous Carbon Electrodes. Nat. Mater. 2012, 11, 306–310. (37) Vatamanu, J.; Cao, L.; Borodin, O.; Bedrov, D.; Smith, G. D. On the Influence of Surface Topography on the Electric Double Layer Structure and Differential Capacitance of Graphite/Ionic Liquid Interfaces. J. Phys. Chem. Lett. 2011, 2, 2267–2272. (38) Vatamanu, J.; Borodin, O.; Smith, G. D. Molecular Dynamics Simulation Studies of the Structure of a Mixed Carbonate/LiPF6 Electrolyte Near Graphite Surface as a Function of Electrode Potential. J. Phys. Chem. C 2012, 116, 1114–1121. (39) Vatamanu, J.; Borodin, O.; Bedrov, D.; Smith, G. D. Molecular Dynamics Simulation Study of the Interfacial Structure and Differential Capacitance of Alkylimidazolium Bis(trifluoromethanesulfonyl)imide [Cn mim][TFSI] Ionic Liquids at Graphite Electrodes. J. Phys. Chem. C 2012, 116, 7940–7951. (40) Pronk, S.; Páll, S.; Schulz, R.; Larsson, P.; Bjelkmar, P.; Apostolov, R.; Shirts, M. R.; Smith, J. C.; Kasson, P. M.; van der Spoel, D. et al. GROMACS 4.5: a high-throughput and highly parallel open source molecular simulation toolkit. Bioinformatics 2013, 29, 845–854. (41) Griffin, J. M.; Forse, A. C.; Wang, H.; Trease, N. M.; Taberna, P.-L.; Simon, P.; Grey, C. P. Ion counting in supercapacitor electrodes using NMR spectroscopy. Faraday Discuss. 2014, 176, 49–68. (42) Shim, Y.; Kim, H. J. Nanoporous Carbon Supercapacitors in an Ionic Liquid: A Computer Simulation Study. ACS Nano 2010, 4, 2345–2355. (43) Vatamanu, J.; Hu, Z.; Bedrov, D.; Perez, C.; Gogotsi, Y. Increasing Energy Storage in Electrochemical Capacitors with Ionic Liquid Electrolytes and Nanostructured Carbon Electrodes. J. Phys. Chem. Lett. 2013, 4, 2829–2837. 16


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Figure 1: Model EDLC with an organic electrolyte composed of EMI+ , BF4 – , and CH3 CN. Graphene oxide electrodes on the left (anode) and right (cathode) are charged with +5e and −5e, respectively. Both electrodes are 10% oxidized.

20

cell

60

80

100

IL OE

(±5 e)

0

(b)

20

40

60

ctot S

80

100 2.8

IL, ± 5 e OE, ± 5 e OE, ± 2.5 e

4.0

2.4 3.5 2.0 3.0 2.5 2.2

1.6 2.2

anode

(c) δ∆Φ

cathode

(d) δ∆Φ

(+5 e)

(−5 e)

1.8

1.8

1.4

1.4

1.0

0

20

40

60

80

100

% Oxidation

Potential (V)

Potential (V)

(a) δ∆Φ

40

Capacitance (µF/cm2)

0 4.5

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0 0

20

40

60

80

100

% Oxidation

Figure 2: MD results for the electrochemical properties of the IL and OE supercapacitors: (a,c,d) Changes in the electric potential of the total cell (a), of the anode (c), and of the cathode (d), induced by electrode charging. (b) The area specific capacitance of the cell. Results for IL and OE supercapacitors are shown in red open circles and green filled triangles, respectively. Error bars mark ±σ, where σ is one standard deviation. The results for the IL supercapacitor are from ref 31. Analogous to the IL case, 31 ctot S determined with electrode charges ±2.5e (open yellow triangles) are essentially the same as ctot S with ±5e for the OE supercapacitor, indicating the robustness of our analysis.

18


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20

40

60

80

100

0

α = All Cation Anion

60

(b) discharged

80

100

α = Anion

0

0 (c) anode, +5e

5 δNSZ α

5 δNSZ α

40

20

NSZ α

(a) discharged

20

NH−bond

0 60

0

−5

(d) cathode, −5e

0

−5 0

20

40

60

80

100

0

20

% Oxidation

40 60 % Oxidation

80

100

Figure 3: IL supercapacitor: (a) Number of ions of species α, NαSZ , in screening zone and (b) number of anions hydrogen bonded to cathode OH groups in the discharged cell. Changes in SZ SZ NαSZ in response to charging at (c) anode and (d) cathode (δNαSZ = Nα,charged − Nα,discharged ). The results in (a) are the average over the discharged anode and cathode. Panel (b) is reprinted with permission from ref 31. Copyright 2014 American Chemical Society. 40

80

12 NSZ (CH3CN/All)

60 α = Cation Anion CH3CN All

0 (c) anode, +5e

20

40

60

80

100

SZ

δNα

δNSZ (CH3CN/All) 0

0 15

60

0

−15

0

100

(a) discharged

NSZ α 0 15

60

20

40

(b) discharged

60

80

100

Anion CH3CN

0

(d) cathode, −5e

60 δNSZ (CH3CN/All)

20

NH−bond

0 15

δNSZ α

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

−15

0 0

20

% Oxidation

40

60

80

100

% Oxidation

Figure 4: OE supercapacitor: (a) Number of ions of species α, NαSZ , in screening zone and (b) numbers of anions and acetonitrile molecules hydrogen bonded to OH groups of a discharged electrode. Changes in NαSZ in response to charging at (c) anode and (d) cathode. The results in (a) and (b) are the average over the discharged anode and cathode. 19



(a) IL, anode (+), δΦα α = cat an

40%

0

0

0

4

4 100%

0.4

0.6

0.3

Distance (nm) (c) OE, anode (+), δΦα

−4 0

10% −4 0.7

0.7

Distance (nm) 4

0

4

0

0

Distance (nm)

0.3

0.5

−4 0.2

0

0%

4

0.6

0.4

α = cat an CH3CN

40%

−4 0

100%

0.4

0.6

0

0

0.2

0.3

Distance (nm) Distance (nm) (d) OE, cathode (−), δΦα

40%

10%

100% 0.5

Potential (V)

Potential (V)

0% α = cation anion CH3CN

0.5

40%

0% −4 0

0

0.2

α = cat an

Potential (V)

Potential (V)

10%

0

4

0

Potential (V)

Potential (V)

0%

(b) IL, cathode (−), δΦα

4

−4 0

Potential (V)

4

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 20

10% −4 0.7

0.7

Distance (nm)

100% 0.5

0.3

Distance (nm)

0.6

0.4

−4 0.2

Distance (nm)

Figure 5: Profiles of electric potential changes, δΦα (z) (eq 4), associated with reorganization of individual components of the electrolyte in response to charging near (a) anode and (b) cathode in the IL supercapacitor and near (c) anode and (d) cathode in the OE supercapacitors. For a clear exposition, δΦα (z) are shown as a function of the distance ∆z from the electrode. δΦα (z) due to electrode OH groups is negligible because their dipole reorganization in response to electrode charging is very small.

20


Computer Simulation Study of Graphene Oxide ... - ACS Publications

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