Design of Supercapacitor Electrodes Using Molecular Dynamics Simulations
Corresponding Author: Zheng Bo
Nano-Micro Letters,
Vol. 10 No. 2 (2018), Article Number: 33
Abstract
Electric double-layer capacitors (EDLCs) are advanced electrochemical devices for energy storage and have attracted strong interest due to their outstanding properties. Rational optimization of electrode–electrolyte interactions is of vital importance to enhance device performance for practical applications. Molecular dynamics (MD) simulations could provide theoretical guidelines for the optimal design of electrodes and the improvement of capacitive performances, e.g., energy density and power density. Here we discuss recent MD simulation studies on energy storage performance of electrode materials containing porous to nanostructures. The energy storage properties are related to the electrode structures, including electrode geometry and electrode modifications. Altering electrode geometry, i.e., pore size and surface topography, can influence EDL capacitance. We critically examine different types of electrode modifications, such as altering the arrangement of carbon atoms, doping heteroatoms and defects, which can change the quantum capacitance. The enhancement of power density can be achieved by the intensified ion dynamics and shortened ion pathway. Rational control of the electrode morphology helps improve the ion dynamics by decreasing the ion diffusion pathway. Tuning the surface properties (e.g., the affinity between the electrode and the ions) can affect the ion-packing phenomena. Our critical analysis helps enhance the energy and power densities of EDLCs by modulating the corresponding electrode structures and surface properties.
Highlights:
1 Capacitive behaviors of electric double-layer capacitors (EDLCs) are strongly related to electrode geometry and electrode modification.
2 Molecular dynamics (MD) studies on EDLCs’ performances of electrode materials from porous to nanostructures are summarized.
3 MD could provide guidelines for the optimum design and fabrication of active materials.
Keywords
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- P. Simon, Y. Gogotsi, Materials for electrochemical capacitors. Nat. Mater. 7(11), 845–854 (2008). https://doi.org/10.1038/nmat2297
- V. Jenel, V. Mihaela, B. Oleg, B. Dmitry, A comparative study of room temperature ionic liquids and their organic solvent mixtures near charged electrodes. J. Phys-Condens. Matter 28(46), 464002 (2016). https://doi.org/10.1088/0953-8984/28/46/464002
- A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors. J. Power Sources 157(1), 11–27 (2006). https://doi.org/10.1016/j.jpowsour.2006.02.065
- M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.L. Taberna, C.P. Grey, B. Dunn, P. Simon, Efficient storage mechanisms for building better supercapacitors. Nat. Energy 1, 16070 (2016). https://doi.org/10.1038/nenergy.2016.70
- X. Zhang, X. Sun, H. Zhang, C. Li, Y. Ma, Comparative performance of birnessite-type MnO2 nanoplates and octahedral molecular sieve (OMS-5) nanobelts of manganese dioxide as electrode materials for supercapacitor application. Electrochim. Acta 132, 315–322 (2014). https://doi.org/10.1016/j.electacta.2014.03.176
- E. Faggioli, P. Rena, V. Danel, X. Andrieu, R. Mallant, H. Kahlen, Supercapacitors for the energy management of electric vehicles. J. Power Sources 84(2), 261–269 (1999). https://doi.org/10.1016/s0378-7753(99)00326-2
- J.R. Miller, P. Simon, Materials science—electrochemical capacitors for energy management. Science 321(5889), 651–652 (2008). https://doi.org/10.1126/science.1158736
- B. Dunn, H. Ka, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices. Science 334(6058), 928–935 (2011). https://doi.org/10.1126/science.1212741
- J.R. Miller, R.A. Outlaw, B.C. Holloway, Graphene double-layer capacitor with ac line-filtering performance. Science 329(5999), 1637–1639 (2010). https://doi.org/10.1126/science.1194372
- L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 38(9), 2520–2531 (2009). https://doi.org/10.1039/b813846j
- J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P.L. Taberna, Anomalous increase in carbon capacitance at pore sizes less than 1 nm. Science 313(5794), 1760–1763 (2006). https://doi.org/10.1126/science.1132195
- S.Y. Lee, C.H. Choi, M.W. Chung, J.H. Chung, S.I. Woo, Dimensional tailoring of nitrogen-doped graphene for high performance supercapacitors. RSC Adv. 6(60), 55577–55583 (2016). https://doi.org/10.1039/c6ra07825g
- G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors. Chem. Soc. Rev. 41(2), 797–828 (2012). https://doi.org/10.1039/c1cs15060j
- Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Progress of electrochemical capacitor electrode materials: a review. Int. J. Hydrog. Energy 34(11), 4889–4899 (2009). https://doi.org/10.1016/j.ijhydene.2009.04.005
- G. Feng, S. Li, J.S. Atchison, V. Presser, P.T. Cummings, Molecular insights into carbon nanotube supercapacitors: capacitance independent of voltage and temperature. J. Phys. Chem. C 117(18), 9178–9186 (2013). https://doi.org/10.1021/jp403547k
- Y. Shim, H.J. Kim, Solvation of carbon nanotubes in a room-temperature ionic liquid. ACS Nano 3(7), 1693–1702 (2009). https://doi.org/10.1021/nn900195b
- H. Yang, J. Yang, Z. Bo, S. Zhang, J. Yan, K. Cen, Edge effects in vertically-oriented graphene based electric double-layer capacitors. J. Power Sources 324, 309–316 (2016). https://doi.org/10.1016/j.jpowsour.2016.05.072
- H. Yang, X. Zhang, J. Yang, Z. Bo, M. Hu, J. Yan, K. Cen, Molecular origin of electric double-layer capacitance at multilayer graphene edges. J. Phys. Chem. Lett. 8(1), 153–160 (2016). https://doi.org/10.1021/acs.jpclett.6b02659
- H.V. Helmholtz, Studien über electrische grenzschichten. Ann. Phys. 243(7), 337–382 (1879). https://doi.org/10.1002/andp.18792430702
- M. Gouy, Sur la constitution de la charge électrique à la surface d’un électrolyte. J. Phys. Theor. Appl. 9(1), 457–468 (1910). https://doi.org/10.1051/jphystap:019100090045700
- D.L. Chapman, A contribution to the theory of electrocapillarity. Philos. Mag. 25(148), 475–481 (1913). https://doi.org/10.1080/14786440408634187
- E. Frackowiak, Q. Abbas, F. Beguin, Carbon/carbon supercapacitors. J. Energy Chem. 22(2), 226–240 (2013). https://doi.org/10.1016/S2095-4956(13)60028-5
- O. Stern, The theory of the electrolytic double shift. Z. Elektrochem. Angew. Physik. Chem. 30, 508–516 (1924)
- P. Wu, J. Huang, V. Meunier, B.G. Sumpter, R. Qiao, Complex capacitance scaling in ionic liquids-filled nanopores. ACS Nano 5(11), 9044–9051 (2011). https://doi.org/10.1021/nn203260w
- G. Feng, R. Qiao, J. Huang, B.G. Sumpter, V. Meunier, Ion distribution in electrified micropores and its role in the anomalous enhancement of capacitance. ACS Nano 4(4), 2382–2390 (2010). https://doi.org/10.1021/nn100126w
- D.-E. Jiang, Z. Jin, J. Wu, Oscillation of capacitance inside nanopores. Nano Lett. 11(12), 5373–5377 (2011). https://doi.org/10.1021/nl202952d
- C. Merlet, B. Rotenberg, P.A. Madden, P.-L. Taberna, P. Simon, Y. Gogotsi, M. Salanne, On the molecular origin of supercapacitance in nanoporous carbon electrodes. Nat. Mater. 11(4), 306–310 (2012). https://doi.org/10.1038/nmat3260
- Y. Shim, H.J. Kim, Nanoporous carbon supercapacitors in an ionic liquid: a computer simulation study. ACS Nano 4(4), 2345–2355 (2010). https://doi.org/10.1021/nn901916m
- C. Largeot, C. Portet, J. Chmiola, P.L. Taberna, Y. Gogotsi, P. Simon, Relation between the ion size and pore size for an electric double-layer capacitor. J. Am. Chem. Soc. 130(9), 2730–2731 (2008). https://doi.org/10.1021/ja7106178
- Z. Bo, H. Yang, S. Zhang, J. Yang, J. Yan, K. Cen, Molecular insights into aqueous NaCl electrolytes confined within vertically-oriented graphenes. Sci. Rep. 5, 14652 (2015). https://doi.org/10.1038/srep14652
- C. Merlet, C. Péan, B. Rotenberg, P.A. Madden, B. Daffos, P.L. Taberna, P. Simon, M. Salanne, Highly confined ions store charge more efficiently in supercapacitors. Nat. Commun. 4, 2701 (2013). https://doi.org/10.1038/ncomms3701
- J. Vatamanu, L. Cao, O. Borodin, D. Bedrov, G.D. Smith, On the influence of surface topography on the electric double layer structure and differential capacitance of graphite/ionic liquid interfaces. J. Phys. Chem. Lett. 2(17), 2267–2272 (2011). https://doi.org/10.1021/jz200879a
- L. Xing, J. Vatamanu, G.D. Smith, D. Bedrov, Nanopatterning of electrode surfaces as a potential route to improve the energy density of electric double-layer capacitors: insight from molecular simulations. J. Phys. Chem. Lett. 3(9), 1124–1129 (2012). https://doi.org/10.1021/jz300253p
- J. Vatamanu, M. Vatamanu, D. Bedrov, Non-faradaic energy storage by room temperature ionic liquids in nanoporous electrodes. ACS Nano 9(6), 5999–6017 (2015). https://doi.org/10.1021/acsnano.5b00945
- E. Paek, A.J. Pak, K.E. Kweon, G.S. Hwang, On the origin of the enhanced supercapacitor performance of nitrogen-doped graphene. J. Phys. Chem. C 117(11), 5610–5616 (2013). https://doi.org/10.1021/jp312490q
- S. Kerisit, B. Schwenzer, M. Vijayakumar, Effects of oxygen-containing functional groups on supercapacitor performance. J. Phys. Chem. Lett. 5(13), 650–653 (2014). https://doi.org/10.1021/jz500900t
- S.-W. Park, A.D. DeYoung, N.R. Dhumal, Y. Shim, H.J. Kim, Y. Jung, Computer simulation study of graphene oxide supercapacitors: charge screening mechanism. J. Phys. Chem. Lett. 7(7), 1180–1186 (2016). https://doi.org/10.1021/acs.jpclett.6b00202
- J. Vatamanu, O. Borodin, D. Bedrov, G.D. Smith, Molecular dynamics simulation study of the interfacial structure and differential capacitance of alkylimidazolium bis(trifluoromethanesulfonyl)imide [CNMIM][TFSI] ionic liquids at graphite electrodes. J. Phys. Chem. C 116(14), 7940–7951 (2012). https://doi.org/10.1021/jp301399b
- H.M. Jeong, J.W. Lee, W.H. Shin, Y.J. Choi, H.J. Shin, J.K. Kang, J.W. Choi, Nitrogen-doped graphene for high-performance ultracapacitors and the importance of nitrogen-doped sites at basal planes. Nano Lett. 11(6), 2472–2477 (2011). https://doi.org/10.1021/nl2009058
- A.J. Pak, E. Paek, G.S. Hwang, Impact of graphene edges on enhancing the performance of electrochemical double layer capacitors. J. Phys. Chem. C 118(38), 21770–21777 (2014). https://doi.org/10.1021/jp504458z
- A.C. Forse, C. Merlet, J.M. Griffin, C.P. Grey, New perspectives on the charging mechanisms of supercapacitors. J. Am. Chem. Soc. 138(18), 5731–5744 (2016). https://doi.org/10.1021/jacs.6b02115
- H. Wang, T.K.J. Koester, N.M. Trease, J. Segalini, P.-L. Taberna, P. Simon, Y. Gogotsi, C.P. Grey, Real-time NMR studies of electrochemical double-layer capacitors. J. Am. Chem. Soc. 133(48), 19270–19273 (2011). https://doi.org/10.1021/ja2072115
- M.D. Levi, G. Salitra, N. Levy, D. Aurbach, J. Maier, Application of a quartz-crystal microbalance to measure ionic fluxes in microporous carbons for energy storage. Nat. Mater. 8(11), 872–875 (2009). https://doi.org/10.1038/nmat2559
- M.D. Levi, N. Levy, S. Sigalov, G. Salitra, D. Aurbach, J. Maier, Electrochemical quartz crystal microbalance (EQCM) studies of ions and solvents insertion into highly porous activated carbons. J. Am. Chem. Soc. 132(38), 13220–13222 (2010). https://doi.org/10.1021/ja104391g
- K. Li, Z. Bo, J. Yan, K. Cen, Solid-state NMR study of ion adsorption and charge storage in graphene film supercapacitor electrodes. Sci. Rep. 6, 39689 (2016). https://doi.org/10.1038/srep39689
- W.-Y. Tsai, P.-L. Taberna, P. Simon, Electrochemical quartz crystal microbalance (EQCM) study of ion dynamics in nanoporous carbons. J. Am. Chem. Soc. 136(24), 8722–8728 (2014). https://doi.org/10.1021/ja503449w
- J.M. Griffin, A.C. Forse, W.-Y. Tsai, P.-L. Taberna, P. Simon, C.P. Grey, In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors. Nat. Mater. 14(8), 812–819 (2015). https://doi.org/10.1038/nmat4318
- A.C. Forse, J.M. Griffin, H. Wang, N.M. Trease, V. Presser, Y. Gogotsi, P. Simon, C.P. Grey, Nuclear magnetic resonance study of ion adsorption on microporous carbide-derived carbon. Phys. Chem. Chem. Phys. 15(20), 7722–7730 (2013). https://doi.org/10.1039/c3cp51210j
- S. Leyva-Garcia, K. Nueangnoraj, D. Lozano-Castello, H. Nishihara, T. Kyotani, E. Marallon, D. Cazorla-Amoros, Characterization of a zeolite-templated carbon by electrochemical quartz crystal microbalance and in situ Raman spectroscopy. Carbon 89, 63–73 (2015). https://doi.org/10.1016/j.carbon.2015.03.016
- Q. Zhang, K. Scrafford, M. Li, Z. Cao, Z. Xia, P.M. Ajayan, B. Wei, Anomalous capacitive behaviors of graphene oxide based solid-state supercapacitors. Nano Lett. 14(4), 1938–1943 (2014). https://doi.org/10.1021/nl4047784
- J.N. Barisci, G.G. Wallace, R.H. Baughman, Electrochemical quartz crystal microbalance studies of single-wall carbon nanotubes in aqueous and non-aqueous solutions. Electrochim. Acta 46(4), 509–517 (2000). https://doi.org/10.1016/s0013-4686(00)00634-4
- B. Peng, H. Zhang, H. Shao, Y. Xu, X. Zhang, H. Zhu, Thermal conductivity of monolayer MOS2, MOSe2, and WS2: interplay of mass effect, interatomic bonding and anharmonicity. RSC Adv. 6(7), 5767–5773 (2016). https://doi.org/10.1039/c5ra19747c
- X. Liu, G. Zhang, Q.-X. Pei, Y.-W. Zhang, Phonon thermal conductivity of monolayer MOS2 sheet and nanoribbons. Appl. Phys. Lett. 103(13), 133113 (2013). https://doi.org/10.1063/1.4823509
- J.-W. Jiang, H.S. Park, T. Rabczuk, Molecular dynamics simulations of single-layer molybdenum disulphide (MOS2): Stillinger–Weber parametrization, mechanical properties, and thermal conductivity. J. Appl. Phys. 114(6), 064307 (2013). https://doi.org/10.1063/1.4818414
- Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage. Adv. Mater. 23(42), 4828–4850 (2011). https://doi.org/10.1002/adma.201100984
- J. Lee, J. Kim, T. Hyeon, Recent progress in the synthesis of porous carbon materials. Adv. Mater. 18(16), 2073–2094 (2006). https://doi.org/10.1002/adma.200501576
- C. Liu, F. Li, L.-P. Ma, H.-M. Cheng, Advanced materials for energy storage. Adv. Mater. 22(8), E28–E62 (2010). https://doi.org/10.1002/adma.200903328
- Z. Yu, L. Tetard, L. Zhai, J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions. Energy Environ. Sci. 8(3), 702–730 (2015). https://doi.org/10.1039/c4ee03229b
- R.L. McGreevy, L. Pusztai, Reverse Monte Carlo simulation: a new technique for the determination of disordered structures. Mol. Simul. 1(6), 359–367 (1988). https://doi.org/10.1080/08927028808080958
- C.H. Turner, J. Pikunic, K.E. Gubbins, Influence of chemical and physical surface heterogeneity on chemical reaction equilibria in carbon micropores. Mol. Phys. 99(24), 1991–2001 (2001). https://doi.org/10.1080/00268970110087254
- N.N. Rajput, J. Monk, F.R. Hung, Ionic liquids confined in a realistic activated carbon model: a molecular simulation study. J. Phys. Chem. C 118(3), 1540–1553 (2014). https://doi.org/10.1021/jp408617j
- V. Presser, M. Heon, Y. Gogotsi, Carbide-derived carbons—from porous networks to nanotubes and graphene. Adv. Funct. Mater. 21(5), 810–833 (2011). https://doi.org/10.1002/adfm.201002094
- W.T. Gu, G. Yushin, Review of nanostructured carbon materials for electrochemical capacitor applications: advantages and limitations of activated carbon, carbide-derived carbon, zeolite-templated carbon, carbon aerogels, carbon nanotubes, onion-like carbon, and graphene. Wiley Interdiscip. Rev. Energy 3(5), 424–473 (2014). https://doi.org/10.1002/wene.102
- D.S. Su, R. Schloegl, Nanostructured carbon and carbon nanocomposites for electrochemical energy storage applications. Chemsuschem 3(2), 136–168 (2010). https://doi.org/10.1002/cssc.200900182
- J.C. Palmer, A. Llobet, S.H. Yeon, J.E. Fischer, Y. Shi, Y. Gogotsi, K.E. Gubbins, Modeling the structural evolution of carbide-derived carbons using quenched molecular dynamics. Carbon 48(4), 1116–1123 (2010). https://doi.org/10.1016/j.carbon.2009.11.033
- C. Pean, B. Daffos, B. Rotenberg, P. Levitz, M. Haefele, P.-L. Taberna, P. Simon, M. Salanne, Confinement, desolvation, and electrosorption effects on the diffusion of ions in nanoporous carbon electrodes. J. Am. Chem. Soc. 137(39), 12627–12632 (2015). https://doi.org/10.1021/jacs.5b07416
- C. Pean, C. Merlet, B. Rotenberg, P.A. Madden, P.-L. Taberna, B. Daffos, M. Salanne, P. Simon, On the dynamics of charging in nanoporous carbon-based supercapacitors. ACS Nano 8(2), 1576–1583 (2014). https://doi.org/10.1021/nn4058243
- M.Z. Bazant, B.D. Storey, A.A. Kornyshev, Double layer in ionic liquids: overscreening versus crowding. Phys. Rev. Lett. 106(4), 046102 (2011). https://doi.org/10.1103/PhysRevLett.106.046102
- C. Lian, D.-E. Jiang, H. Liu, J. Wu, A generic model for electric double layers in porous electrodes. J. Phys. Chem. C 120(16), 8704–8710 (2016). https://doi.org/10.1021/acs.jpcc.6b00964
- A.J. Pak, G.S. Hwang, Molecular insights into the complex relationship between capacitance and pore morphology in nanoporous carbon-based supercapacitors. ACS Appl. Mater. Interfaces 8(50), 34659–34667 (2016). https://doi.org/10.1021/acsami.6b11192
- S. Schweizer, R. Meissner, M. Amkreutz, K. Thiel, P. Schiffels, J. Landwehr, B.J.M. Etzold, J.-R. Hill, Molecular modeling of microporous structures of carbide-derived carbon-based supercapacitors. J. Phys. Chem. C 121(13), 7221–7231 (2017). https://doi.org/10.1021/acs.jpcc.6b12774
- M. Chen, S. Li, G. Feng, The influence of anion shape on the electrical double layer microstructure and capacitance of ionic liquids-based supercapacitors by molecular simulations. Molecules 22(2), 241 (2017). https://doi.org/10.3390/molecules22020241
- C. Merlet, C. Péan, B. Rotenberg, P.A. Madden, P. Simon, M. Salanne, Simulating supercapacitors: can we model electrodes as constant charge surfaces? J. Phys. Chem. Lett. 4(2), 264–268 (2013). https://doi.org/10.1021/jz3019226
- J. Yang, Z. Bo, H. Yang, H. Qi, J. Kong, J. Yan, K. Cen, Reliability of constant charge method for molecular dynamics simulations on EDLCs in nanometer and sub-nanometer spaces. Chemelectrochem 4(10), 2486–2493 (2017). https://doi.org/10.1002/celc.201700447
- J. Vatamanu, D. Bedrov, O. Borodin, On the application of constant electrode potential simulation techniques in atomistic modelling of electric double layers. Mol. Simul. 43(10–11), 838–849 (2017). https://doi.org/10.1080/08927022.2017.1279287
- L.L. Zhang, R. Zhou, X.S. Zhao, Graphene-based materials as supercapacitor electrodes. J. Mater. Chem. 20(29), 5983–5992 (2010). https://doi.org/10.1039/c000417k
- J. Chen, C. Li, G. Shi, Graphene materials for electrochemical capacitors. J. Phys. Chem. Lett. 4(8), 1244–1253 (2013). https://doi.org/10.1021/jz400160k
- R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, The role of graphene for electrochemical energy storage. Nat. Mater. 14(3), 271–279 (2015). https://doi.org/10.1038/nmat4170
- K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films. Science 306(5696), 666–669 (2004). https://doi.org/10.1126/science.1102896
- M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors. Nano Lett. 8(10), 3498–3502 (2008). https://doi.org/10.1021/nl802558y
- J. Xia, F. Chen, J. Li, N. Tao, Measurement of the quantum capacitance of graphene. Nat. Nanotechnol. 4(8), 505–509 (2009). https://doi.org/10.1038/nnano.2009.177
- C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321(5887), 385–388 (2008). https://doi.org/10.1126/science.1157996
- A.A. Kornyshev, Double-layer in ionic liquids: paradigm change? J. Phys. Chem. B 111(20), 5545–5557 (2007). https://doi.org/10.1021/jp067857o
- J. Vatamanu, L. Xing, W. Li, D. Bedrov, Influence of temperature on the capacitance of ionic liquid electrolytes on charged surfaces. Phys. Chem. Chem. Phys. 16(11), 5174–5182 (2014). https://doi.org/10.1039/c3cp54705a
- Z. Hu, J. Vatamanu, O. Borodin, D. Bedrov, A comparative study of alkylimidazolium room temperature ionic liquids with FSI and TFSI anions near charged electrodes. Electrochim. Acta 145, 40–52 (2014). https://doi.org/10.1016/j.electacta.2014.08.072
- H. Zongzhi, V. Jenel, B. Oleg, B. Dmitry, A molecular dynamics simulation study of the electric double layer and capacitance of [BMIM][PF6] and [BMIM][BF4] room temperature ionic liquids near charged surfaces. Phys. Chem. Chem. Phys. 15(34), 14234–14247 (2013). https://doi.org/10.1039/c3cp51218e
- J. Hafner, Ab-initio simulations of materials using VASP: density-functional theory and beyond. J. Comput. Chem. 29(13), 2044–2078 (2008). https://doi.org/10.1002/jcc.21057
- J. Vatamanu, X. Ni, F. Liu, D. Bedrov, Tailoring graphene-based electrodes from semiconducting to metallic to increase the energy density in supercapacitors. Nanotechnology 26(46), 464001 (2015). https://doi.org/10.1088/0957-4484/26/46/464001
- L. Chen, X. Li, C. Ma, M. Wang, J. Zhou, Interaction and quantum capacitance of nitrogen/sulfur co-doped graphene: a theoretical calculation. J. Phys. Chem. C 121(34), 18344–18350 (2017). https://doi.org/10.1021/acs.jpcc.7b04551
- S. Mao, H. Pu, J. Chen, Graphene oxide and its reduction: modeling and experimental progress. RSC Adv. 2(7), 2643–2662 (2012). https://doi.org/10.1039/C2RA00663D
- A.D. DeYoung, S.-W. Park, N.R. Dhumal, Y. Shim, Y. Jung, H.J. Kim, Graphene oxide supercapacitors: a computer simulation study. J. Phys. Chem. C 118(32), 18472–18480 (2014). https://doi.org/10.1021/jp5072583
- K. Xu, X. Ji, C. Chen, H. Wan, L. Miao, J. Jiang, Electrochemical double layer near polar reduced graphene oxide electrode: insights from molecular dynamic study. Electrochim. Acta 166, 142–149 (2015). https://doi.org/10.1016/j.electacta.2015.03.101
- S. Li, G. Feng, P.T. Cummings, Interfaces of dicationic ionic liquids and graphene: a molecular dynamics simulation study. J. Phys-Condens. Matter 26(28), 284106 (2014). https://doi.org/10.1088/0953-8984/26/28/284106
- S. Jo, S.-W. Park, Y. Shim, Y. Jung, Effects of alkyl chain length on interfacial structure and differential capacitance in graphene supercapacitors: a molecular dynamics simulation study. Electrochim. Acta 247, 634–645 (2017). https://doi.org/10.1016/j.electacta.2017.06.169
- X. Liu, Y. Wang, S. Li, T. Yan, Effects of anion on the electric double layer of imidazolium-based ionic liquids on graphite electrode by molecular dynamics simulation. Electrochim. Acta 184, 164–170 (2015). https://doi.org/10.1016/j.electacta.2015.10.064
- J. Vatamanu, O. Borodin, G.D. Smith, Molecular simulations of the electric double layer structure, differential capacitance, and charging kinetics for n-methyl-n-propylpyrrolidinium bis(fluorosulfonyl)imide at graphite electrodes. J. Phys. Chem. B 115(12), 3073–3084 (2011). https://doi.org/10.1021/jp2001207
- M. Wu, W. Li, S. Li, G. Feng, Capacitive performance of amino acid ionic liquid electrolyte-based supercapacitors by molecular dynamics simulation. RSC Adv. 7(46), 28945–28950 (2017). https://doi.org/10.1039/c7ra00443e
- E. Sedghamiz, M. Moosavi, Tricationic ionic liquids: structural and dynamical properties via molecular dynamics simulations. J. Phys. Chem. B 121(8), 1877–1892 (2017). https://doi.org/10.1021/acs.jpcb.6b10766
- Y.N. Ahn, S.H. Lee, G.S. Lee, H. Kim, Effect of alkyl branches on the thermal stability of quaternary ammonium cations in organic electrolytes for electrochemical double layer capacitors. Phys. Chem. Chem. Phys. 19(30), 19959–19966 (2017). https://doi.org/10.1039/c7cp03209a
- J. Vatamanu, O. Borodin, Ramifications of water-in-salt interfacial structure at charged electrodes for electrolyte electrochemical stability. J. Phys. Chem. Lett. 8(18), 4362–4367 (2017). https://doi.org/10.1021/acs.jpclett.7b01879
- R.S. Kuhnel, D. Reber, A. Remhof, R. Figi, D. Bleiner, C. Battaglia, “Water-in-salt” electrolytes enable the use of cost-effective aluminum current collectors for aqueous high-voltage batteries. Chem. Commun. 52(68), 10435–10438 (2016). https://doi.org/10.1039/c6cc03969c
- S. Kondrat, P. Wu, R. Qiao, A.A. Kornyshev, Accelerating charging dynamics in subnanometre pores. Nat. Mater. 13(4), 387–393 (2014). https://doi.org/10.1038/nmat3916
- G. Feng, P.T. Cummings, Supercapacitor capacitance exhibits oscillatory behavior as a function of nanopore size. J. Phys. Chem. Lett. 2(22), 2859–2864 (2011). https://doi.org/10.1021/jz201312e
- S.R. Varanasi, S.K. Bhatia, Capacitance optimization in nanoscale electrochemical supercapacitors. J. Phys. Chem. C 119(31), 17573–17584 (2015). https://doi.org/10.1021/acs.jpcc.5b04254
- D.E. Jiang, J. Wu, Unusual effects of solvent polarity on capacitance for organic electrolytes in a nanoporous electrode. Nanoscale 6(10), 5545–5550 (2014). https://doi.org/10.1039/C4NR00046C
- D.-E. Jiang, Z. Jin, D. Henderson, J. Wu, Solvent effect on the pore-size dependence of an organic electrolyte supercapacitor. J. Phys. Chem. Lett. 3(13), 1727–1731 (2012). https://doi.org/10.1021/jz3004624
- R. Burt, K. Breitsprecher, B. Daffos, P.-L. Taberna, P. Simon, G. Birkett, X.S. Zhao, C. Holm, M. Salanne, Capacitance of nanoporous carbon-based supercapacitors is a trade-off between the concentration and the separability of the ions. J. Phys. Chem. Lett. 7(19), 4015–4021 (2016). https://doi.org/10.1021/acs.jpclett.6b01787
- B. Uralcan, I.A. Aksay, P.G. Debenedetti, D.T. Limmer, Concentration fluctuations and capacitive response in dense ionic solutions. J. Phys. Chem. Lett. 7(13), 2333–2338 (2016). https://doi.org/10.1021/acs.jpclett.6b00859
- M.H. Kowsari, L. Tohidifar, Tracing dynamics, self-diffusion, and nanoscale structural heterogeneity of pure and binary mixtures of ionic liquid 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide with acetonitrile: insights from molecular dynamics simulations. J. Phys. Chem. B 120(41), 10824–10838 (2016). https://doi.org/10.1021/acs.jpcb.6b08396
- Q. Zhang, P. Xie, X. Wang, X. Yu, Z. Shi, S. Zhao, Thermodynamic and transport properties of spiro-(1,1′)-bipyrrolidinium tetrafluoroborate and acetonitrile mixtures: a molecular dynamics study. Chin. Phys. B 25(6), 066102 (2016). https://doi.org/10.1088/1674-1056/25/6/066102
- H. Yang, J. Yang, Z. Bo, X. Chen, X. Shuai, J. Kong, J. Yan, K. Cen, Kinetic-dominated charging mechanism within representative aqueous electrolyte-based electric double-layer capacitors. J. Phys. Chem. Lett. 8(15), 3703–3710 (2017). https://doi.org/10.1021/acs.jpclett.7b01525
- C. Lian, K. Liu, H. Liu, J. Wu, Impurity effects on charging mechanism and energy storage of nanoporous supercapacitors. J. Phys. Chem. C 121(26), 14066–14072 (2017). https://doi.org/10.1021/acs.jpcc.7b04869
- P. Wu, J. Huang, V. Meunier, B.G. Sumpter, R. Qiao, Voltage dependent charge storage modes and capacity in subnanometer pores. J. Phys. Chem. Lett. 3(13), 1732–1737 (2012). https://doi.org/10.1021/jz300506j
- J. Vatamanu, O. Borodin, G.D. Smith, Molecular insights into the potential and temperature dependences of the differential capacitance of a room-temperature ionic liquid at graphite electrodes. J. Am. Chem. Soc. 132(42), 14825–14833 (2010). https://doi.org/10.1021/ja104273r
- R.K. Kalluri, D. Konatham, A. Striolo, Aqueous NaCl solutions within charged carbon-slit pores: partition coefficients and density distributions from molecular dynamics simulations. J. Phys. Chem. C 115(28), 13786–13795 (2011). https://doi.org/10.1021/jp203086x
- T. Ohba, N. Kojima, H. Kanoh, K. Kaneko, Unique hydrogen-bonded structure of water around Ca ions confined in carbon slit pores. J. Phys. Chem. C 113(29), 12622–12624 (2009). https://doi.org/10.1021/jp9030688
- N.N. Rajput, J. Monk, R. Singh, F.R. Hung, On the influence of pore size and pore loading on structural and dynamical heterogeneities of an ionic liquid confined in a slit nanopore. J. Phys. Chem. C 116(8), 5169–5181 (2012). https://doi.org/10.1021/jp212440f
- S. Salemi, H. Akbarzadeh, S. Abdollahzadeh, Nano-confined ionic liquid [emim][PF6] between graphite sheets: a molecular dynamics study. J. Mol. Liq. 215, 512–519 (2016). https://doi.org/10.1016/j.molliq.2016.01.035
- J. Kong, Z. Bo, H. Yang, J. Yang, X. Shuai, J. Yan, K. Cen, Temperature dependence of ion diffusion coefficients in nacl electrolyte confined within graphene nanochannels. Phys. Chem. Chem. Phys. 19(11), 7678–7688 (2017). https://doi.org/10.1039/c6cp08752c
- S.A. Kislenko, R.H. Amirov, I.S. Samoylov, Influence of temperature on the structure and dynamics of the [BMIM][PF6] ionic liquid/graphite interface. Phys. Chem. Chem. Phys. 12(37), 11245–11250 (2010). https://doi.org/10.1039/c0cp00220h
- R. Singh, J. Monk, F.R. Hung, Heterogeneity in the dynamics of the ionic liquid [BMIM+][PF6 −] confined in a slit nanopore. J. Phys. Chem. C 115(33), 16544–16554 (2011). https://doi.org/10.1021/jp2046118
- W. Yuan, Y. Zhou, Y. Li, C. Li, H. Peng, J. Zhang, Z. Liu, L. Dai, G. Shi, The edge- and basal-plane-specific electrochemistry of a single-layer graphene sheet. Sci. Rep. 3, 2248 (2013). https://doi.org/10.1038/srep02248
- C. Zhan, Y. Zhang, P.T. Cummings, D.-E. Jiang, Computational insight into the capacitive performance of graphene edge planes. Carbon 116, 278–285 (2017). https://doi.org/10.1016/j.carbon.2017.01.104
- V.N. Popov, Carbon nanotubes: properties and application. Mater. Sci. Eng. R-Rep. 43(3), 61–102 (2004). https://doi.org/10.1016/j.mser.2003.10.001
- H.J. Dai, Carbon nanotubes: opportunities and challenges. Surf. Sci. 500(1–3), 218–241 (2002). https://doi.org/10.1016/s0039-6028(01)01558-8
- M. Trojanowicz, Analytical applications of carbon nanotubes: a review. Trac-trends Anal. Chem. 25(5), 480–489 (2006). https://doi.org/10.1016/j.trac.2005.11.008
- L. Yang, B.H. Fishbine, A. Migliori, L.R. Pratt, Molecular simulation of electric double-layer capacitors based on carbon nanotube forests. J. Am. Chem. Soc. 131(34), 12373–12376 (2009). https://doi.org/10.1021/ja9044554
- A. Dive, S. Banerjee, Ion storage in nanoconfined interstices between vertically aligned nanotubes in electric double-layer capacitors. J. Electrochem. Energy Convers. 15(1), 011001 (2017). https://doi.org/10.1115/1.4037582
- K. Dong, G. Zhou, X. Liu, X. Yao, S. Zhang, A. Lyubartsev, Structural evidence for the ordered crystallites of ionic liquid in confined carbon nanotubes. J. Phys. Chem. C 113(23), 10013–10020 (2009). https://doi.org/10.1021/jp900533k
- O.N. Kalugin, V.V. Chaban, V.V. Loskutov, O.V. Prezhdo, Uniform diffusion of acetonitrile inside carbon nanotubes favors supercapacitor performance. Nano Lett. 8(8), 2126–2130 (2008). https://doi.org/10.1021/nl072976g
- R.J. Mashl, S. Joseph, N.R. Aluru, E. Jakobsson, Anomalously immobilized water: a new water phase induced by confinement in nanotubes. Nano Lett. 3(5), 589–592 (2003). https://doi.org/10.1021/nl0340226
- L. Yang, S. Garde, Modeling the selective partitioning of cations into negatively charged nanopores in water. J. Chem. Phys. 126(8), 084706 (2007). https://doi.org/10.1063/1.2464083
- T. Ohba, Fast ion transportation associated with recovering hydration shells in a nanoelectrolyte between conical carbon nanopores during charging cycles. J. Phys. Chem. C 121(19), 10439–10444 (2017). https://doi.org/10.1021/acs.jpcc.7b02326
- D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna, P. Simon, Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon. Nat. Nanotechnol. 5(9), 651–654 (2010). https://doi.org/10.1038/nnano.2010.162
- G. Feng, D.-E. Jiang, P.T. Cummings, Curvature effect on the capacitance of electric double layers at ionic liquid/onion-like carbon interfaces. J. Chem. Theory Comput. 8(3), 1058–1063 (2012). https://doi.org/10.1021/ct200914j
References
P. Simon, Y. Gogotsi, Materials for electrochemical capacitors. Nat. Mater. 7(11), 845–854 (2008). https://doi.org/10.1038/nmat2297
V. Jenel, V. Mihaela, B. Oleg, B. Dmitry, A comparative study of room temperature ionic liquids and their organic solvent mixtures near charged electrodes. J. Phys-Condens. Matter 28(46), 464002 (2016). https://doi.org/10.1088/0953-8984/28/46/464002
A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors. J. Power Sources 157(1), 11–27 (2006). https://doi.org/10.1016/j.jpowsour.2006.02.065
M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.L. Taberna, C.P. Grey, B. Dunn, P. Simon, Efficient storage mechanisms for building better supercapacitors. Nat. Energy 1, 16070 (2016). https://doi.org/10.1038/nenergy.2016.70
X. Zhang, X. Sun, H. Zhang, C. Li, Y. Ma, Comparative performance of birnessite-type MnO2 nanoplates and octahedral molecular sieve (OMS-5) nanobelts of manganese dioxide as electrode materials for supercapacitor application. Electrochim. Acta 132, 315–322 (2014). https://doi.org/10.1016/j.electacta.2014.03.176
E. Faggioli, P. Rena, V. Danel, X. Andrieu, R. Mallant, H. Kahlen, Supercapacitors for the energy management of electric vehicles. J. Power Sources 84(2), 261–269 (1999). https://doi.org/10.1016/s0378-7753(99)00326-2
J.R. Miller, P. Simon, Materials science—electrochemical capacitors for energy management. Science 321(5889), 651–652 (2008). https://doi.org/10.1126/science.1158736
B. Dunn, H. Ka, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices. Science 334(6058), 928–935 (2011). https://doi.org/10.1126/science.1212741
J.R. Miller, R.A. Outlaw, B.C. Holloway, Graphene double-layer capacitor with ac line-filtering performance. Science 329(5999), 1637–1639 (2010). https://doi.org/10.1126/science.1194372
L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 38(9), 2520–2531 (2009). https://doi.org/10.1039/b813846j
J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P.L. Taberna, Anomalous increase in carbon capacitance at pore sizes less than 1 nm. Science 313(5794), 1760–1763 (2006). https://doi.org/10.1126/science.1132195
S.Y. Lee, C.H. Choi, M.W. Chung, J.H. Chung, S.I. Woo, Dimensional tailoring of nitrogen-doped graphene for high performance supercapacitors. RSC Adv. 6(60), 55577–55583 (2016). https://doi.org/10.1039/c6ra07825g
G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors. Chem. Soc. Rev. 41(2), 797–828 (2012). https://doi.org/10.1039/c1cs15060j
Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Progress of electrochemical capacitor electrode materials: a review. Int. J. Hydrog. Energy 34(11), 4889–4899 (2009). https://doi.org/10.1016/j.ijhydene.2009.04.005
G. Feng, S. Li, J.S. Atchison, V. Presser, P.T. Cummings, Molecular insights into carbon nanotube supercapacitors: capacitance independent of voltage and temperature. J. Phys. Chem. C 117(18), 9178–9186 (2013). https://doi.org/10.1021/jp403547k
Y. Shim, H.J. Kim, Solvation of carbon nanotubes in a room-temperature ionic liquid. ACS Nano 3(7), 1693–1702 (2009). https://doi.org/10.1021/nn900195b
H. Yang, J. Yang, Z. Bo, S. Zhang, J. Yan, K. Cen, Edge effects in vertically-oriented graphene based electric double-layer capacitors. J. Power Sources 324, 309–316 (2016). https://doi.org/10.1016/j.jpowsour.2016.05.072
H. Yang, X. Zhang, J. Yang, Z. Bo, M. Hu, J. Yan, K. Cen, Molecular origin of electric double-layer capacitance at multilayer graphene edges. J. Phys. Chem. Lett. 8(1), 153–160 (2016). https://doi.org/10.1021/acs.jpclett.6b02659
H.V. Helmholtz, Studien über electrische grenzschichten. Ann. Phys. 243(7), 337–382 (1879). https://doi.org/10.1002/andp.18792430702
M. Gouy, Sur la constitution de la charge électrique à la surface d’un électrolyte. J. Phys. Theor. Appl. 9(1), 457–468 (1910). https://doi.org/10.1051/jphystap:019100090045700
D.L. Chapman, A contribution to the theory of electrocapillarity. Philos. Mag. 25(148), 475–481 (1913). https://doi.org/10.1080/14786440408634187
E. Frackowiak, Q. Abbas, F. Beguin, Carbon/carbon supercapacitors. J. Energy Chem. 22(2), 226–240 (2013). https://doi.org/10.1016/S2095-4956(13)60028-5
O. Stern, The theory of the electrolytic double shift. Z. Elektrochem. Angew. Physik. Chem. 30, 508–516 (1924)
P. Wu, J. Huang, V. Meunier, B.G. Sumpter, R. Qiao, Complex capacitance scaling in ionic liquids-filled nanopores. ACS Nano 5(11), 9044–9051 (2011). https://doi.org/10.1021/nn203260w
G. Feng, R. Qiao, J. Huang, B.G. Sumpter, V. Meunier, Ion distribution in electrified micropores and its role in the anomalous enhancement of capacitance. ACS Nano 4(4), 2382–2390 (2010). https://doi.org/10.1021/nn100126w
D.-E. Jiang, Z. Jin, J. Wu, Oscillation of capacitance inside nanopores. Nano Lett. 11(12), 5373–5377 (2011). https://doi.org/10.1021/nl202952d
C. Merlet, B. Rotenberg, P.A. Madden, P.-L. Taberna, P. Simon, Y. Gogotsi, M. Salanne, On the molecular origin of supercapacitance in nanoporous carbon electrodes. Nat. Mater. 11(4), 306–310 (2012). https://doi.org/10.1038/nmat3260
Y. Shim, H.J. Kim, Nanoporous carbon supercapacitors in an ionic liquid: a computer simulation study. ACS Nano 4(4), 2345–2355 (2010). https://doi.org/10.1021/nn901916m
C. Largeot, C. Portet, J. Chmiola, P.L. Taberna, Y. Gogotsi, P. Simon, Relation between the ion size and pore size for an electric double-layer capacitor. J. Am. Chem. Soc. 130(9), 2730–2731 (2008). https://doi.org/10.1021/ja7106178
Z. Bo, H. Yang, S. Zhang, J. Yang, J. Yan, K. Cen, Molecular insights into aqueous NaCl electrolytes confined within vertically-oriented graphenes. Sci. Rep. 5, 14652 (2015). https://doi.org/10.1038/srep14652
C. Merlet, C. Péan, B. Rotenberg, P.A. Madden, B. Daffos, P.L. Taberna, P. Simon, M. Salanne, Highly confined ions store charge more efficiently in supercapacitors. Nat. Commun. 4, 2701 (2013). https://doi.org/10.1038/ncomms3701
J. Vatamanu, L. Cao, O. Borodin, D. Bedrov, G.D. Smith, On the influence of surface topography on the electric double layer structure and differential capacitance of graphite/ionic liquid interfaces. J. Phys. Chem. Lett. 2(17), 2267–2272 (2011). https://doi.org/10.1021/jz200879a
L. Xing, J. Vatamanu, G.D. Smith, D. Bedrov, Nanopatterning of electrode surfaces as a potential route to improve the energy density of electric double-layer capacitors: insight from molecular simulations. J. Phys. Chem. Lett. 3(9), 1124–1129 (2012). https://doi.org/10.1021/jz300253p
J. Vatamanu, M. Vatamanu, D. Bedrov, Non-faradaic energy storage by room temperature ionic liquids in nanoporous electrodes. ACS Nano 9(6), 5999–6017 (2015). https://doi.org/10.1021/acsnano.5b00945
E. Paek, A.J. Pak, K.E. Kweon, G.S. Hwang, On the origin of the enhanced supercapacitor performance of nitrogen-doped graphene. J. Phys. Chem. C 117(11), 5610–5616 (2013). https://doi.org/10.1021/jp312490q
S. Kerisit, B. Schwenzer, M. Vijayakumar, Effects of oxygen-containing functional groups on supercapacitor performance. J. Phys. Chem. Lett. 5(13), 650–653 (2014). https://doi.org/10.1021/jz500900t
S.-W. Park, A.D. DeYoung, N.R. Dhumal, Y. Shim, H.J. Kim, Y. Jung, Computer simulation study of graphene oxide supercapacitors: charge screening mechanism. J. Phys. Chem. Lett. 7(7), 1180–1186 (2016). https://doi.org/10.1021/acs.jpclett.6b00202
J. Vatamanu, O. Borodin, D. Bedrov, G.D. Smith, Molecular dynamics simulation study of the interfacial structure and differential capacitance of alkylimidazolium bis(trifluoromethanesulfonyl)imide [CNMIM][TFSI] ionic liquids at graphite electrodes. J. Phys. Chem. C 116(14), 7940–7951 (2012). https://doi.org/10.1021/jp301399b
H.M. Jeong, J.W. Lee, W.H. Shin, Y.J. Choi, H.J. Shin, J.K. Kang, J.W. Choi, Nitrogen-doped graphene for high-performance ultracapacitors and the importance of nitrogen-doped sites at basal planes. Nano Lett. 11(6), 2472–2477 (2011). https://doi.org/10.1021/nl2009058
A.J. Pak, E. Paek, G.S. Hwang, Impact of graphene edges on enhancing the performance of electrochemical double layer capacitors. J. Phys. Chem. C 118(38), 21770–21777 (2014). https://doi.org/10.1021/jp504458z
A.C. Forse, C. Merlet, J.M. Griffin, C.P. Grey, New perspectives on the charging mechanisms of supercapacitors. J. Am. Chem. Soc. 138(18), 5731–5744 (2016). https://doi.org/10.1021/jacs.6b02115
H. Wang, T.K.J. Koester, N.M. Trease, J. Segalini, P.-L. Taberna, P. Simon, Y. Gogotsi, C.P. Grey, Real-time NMR studies of electrochemical double-layer capacitors. J. Am. Chem. Soc. 133(48), 19270–19273 (2011). https://doi.org/10.1021/ja2072115
M.D. Levi, G. Salitra, N. Levy, D. Aurbach, J. Maier, Application of a quartz-crystal microbalance to measure ionic fluxes in microporous carbons for energy storage. Nat. Mater. 8(11), 872–875 (2009). https://doi.org/10.1038/nmat2559
M.D. Levi, N. Levy, S. Sigalov, G. Salitra, D. Aurbach, J. Maier, Electrochemical quartz crystal microbalance (EQCM) studies of ions and solvents insertion into highly porous activated carbons. J. Am. Chem. Soc. 132(38), 13220–13222 (2010). https://doi.org/10.1021/ja104391g
K. Li, Z. Bo, J. Yan, K. Cen, Solid-state NMR study of ion adsorption and charge storage in graphene film supercapacitor electrodes. Sci. Rep. 6, 39689 (2016). https://doi.org/10.1038/srep39689
W.-Y. Tsai, P.-L. Taberna, P. Simon, Electrochemical quartz crystal microbalance (EQCM) study of ion dynamics in nanoporous carbons. J. Am. Chem. Soc. 136(24), 8722–8728 (2014). https://doi.org/10.1021/ja503449w
J.M. Griffin, A.C. Forse, W.-Y. Tsai, P.-L. Taberna, P. Simon, C.P. Grey, In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors. Nat. Mater. 14(8), 812–819 (2015). https://doi.org/10.1038/nmat4318
A.C. Forse, J.M. Griffin, H. Wang, N.M. Trease, V. Presser, Y. Gogotsi, P. Simon, C.P. Grey, Nuclear magnetic resonance study of ion adsorption on microporous carbide-derived carbon. Phys. Chem. Chem. Phys. 15(20), 7722–7730 (2013). https://doi.org/10.1039/c3cp51210j
S. Leyva-Garcia, K. Nueangnoraj, D. Lozano-Castello, H. Nishihara, T. Kyotani, E. Marallon, D. Cazorla-Amoros, Characterization of a zeolite-templated carbon by electrochemical quartz crystal microbalance and in situ Raman spectroscopy. Carbon 89, 63–73 (2015). https://doi.org/10.1016/j.carbon.2015.03.016
Q. Zhang, K. Scrafford, M. Li, Z. Cao, Z. Xia, P.M. Ajayan, B. Wei, Anomalous capacitive behaviors of graphene oxide based solid-state supercapacitors. Nano Lett. 14(4), 1938–1943 (2014). https://doi.org/10.1021/nl4047784
J.N. Barisci, G.G. Wallace, R.H. Baughman, Electrochemical quartz crystal microbalance studies of single-wall carbon nanotubes in aqueous and non-aqueous solutions. Electrochim. Acta 46(4), 509–517 (2000). https://doi.org/10.1016/s0013-4686(00)00634-4
B. Peng, H. Zhang, H. Shao, Y. Xu, X. Zhang, H. Zhu, Thermal conductivity of monolayer MOS2, MOSe2, and WS2: interplay of mass effect, interatomic bonding and anharmonicity. RSC Adv. 6(7), 5767–5773 (2016). https://doi.org/10.1039/c5ra19747c
X. Liu, G. Zhang, Q.-X. Pei, Y.-W. Zhang, Phonon thermal conductivity of monolayer MOS2 sheet and nanoribbons. Appl. Phys. Lett. 103(13), 133113 (2013). https://doi.org/10.1063/1.4823509
J.-W. Jiang, H.S. Park, T. Rabczuk, Molecular dynamics simulations of single-layer molybdenum disulphide (MOS2): Stillinger–Weber parametrization, mechanical properties, and thermal conductivity. J. Appl. Phys. 114(6), 064307 (2013). https://doi.org/10.1063/1.4818414
Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage. Adv. Mater. 23(42), 4828–4850 (2011). https://doi.org/10.1002/adma.201100984
J. Lee, J. Kim, T. Hyeon, Recent progress in the synthesis of porous carbon materials. Adv. Mater. 18(16), 2073–2094 (2006). https://doi.org/10.1002/adma.200501576
C. Liu, F. Li, L.-P. Ma, H.-M. Cheng, Advanced materials for energy storage. Adv. Mater. 22(8), E28–E62 (2010). https://doi.org/10.1002/adma.200903328
Z. Yu, L. Tetard, L. Zhai, J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions. Energy Environ. Sci. 8(3), 702–730 (2015). https://doi.org/10.1039/c4ee03229b
R.L. McGreevy, L. Pusztai, Reverse Monte Carlo simulation: a new technique for the determination of disordered structures. Mol. Simul. 1(6), 359–367 (1988). https://doi.org/10.1080/08927028808080958
C.H. Turner, J. Pikunic, K.E. Gubbins, Influence of chemical and physical surface heterogeneity on chemical reaction equilibria in carbon micropores. Mol. Phys. 99(24), 1991–2001 (2001). https://doi.org/10.1080/00268970110087254
N.N. Rajput, J. Monk, F.R. Hung, Ionic liquids confined in a realistic activated carbon model: a molecular simulation study. J. Phys. Chem. C 118(3), 1540–1553 (2014). https://doi.org/10.1021/jp408617j
V. Presser, M. Heon, Y. Gogotsi, Carbide-derived carbons—from porous networks to nanotubes and graphene. Adv. Funct. Mater. 21(5), 810–833 (2011). https://doi.org/10.1002/adfm.201002094
W.T. Gu, G. Yushin, Review of nanostructured carbon materials for electrochemical capacitor applications: advantages and limitations of activated carbon, carbide-derived carbon, zeolite-templated carbon, carbon aerogels, carbon nanotubes, onion-like carbon, and graphene. Wiley Interdiscip. Rev. Energy 3(5), 424–473 (2014). https://doi.org/10.1002/wene.102
D.S. Su, R. Schloegl, Nanostructured carbon and carbon nanocomposites for electrochemical energy storage applications. Chemsuschem 3(2), 136–168 (2010). https://doi.org/10.1002/cssc.200900182
J.C. Palmer, A. Llobet, S.H. Yeon, J.E. Fischer, Y. Shi, Y. Gogotsi, K.E. Gubbins, Modeling the structural evolution of carbide-derived carbons using quenched molecular dynamics. Carbon 48(4), 1116–1123 (2010). https://doi.org/10.1016/j.carbon.2009.11.033
C. Pean, B. Daffos, B. Rotenberg, P. Levitz, M. Haefele, P.-L. Taberna, P. Simon, M. Salanne, Confinement, desolvation, and electrosorption effects on the diffusion of ions in nanoporous carbon electrodes. J. Am. Chem. Soc. 137(39), 12627–12632 (2015). https://doi.org/10.1021/jacs.5b07416
C. Pean, C. Merlet, B. Rotenberg, P.A. Madden, P.-L. Taberna, B. Daffos, M. Salanne, P. Simon, On the dynamics of charging in nanoporous carbon-based supercapacitors. ACS Nano 8(2), 1576–1583 (2014). https://doi.org/10.1021/nn4058243
M.Z. Bazant, B.D. Storey, A.A. Kornyshev, Double layer in ionic liquids: overscreening versus crowding. Phys. Rev. Lett. 106(4), 046102 (2011). https://doi.org/10.1103/PhysRevLett.106.046102
C. Lian, D.-E. Jiang, H. Liu, J. Wu, A generic model for electric double layers in porous electrodes. J. Phys. Chem. C 120(16), 8704–8710 (2016). https://doi.org/10.1021/acs.jpcc.6b00964
A.J. Pak, G.S. Hwang, Molecular insights into the complex relationship between capacitance and pore morphology in nanoporous carbon-based supercapacitors. ACS Appl. Mater. Interfaces 8(50), 34659–34667 (2016). https://doi.org/10.1021/acsami.6b11192
S. Schweizer, R. Meissner, M. Amkreutz, K. Thiel, P. Schiffels, J. Landwehr, B.J.M. Etzold, J.-R. Hill, Molecular modeling of microporous structures of carbide-derived carbon-based supercapacitors. J. Phys. Chem. C 121(13), 7221–7231 (2017). https://doi.org/10.1021/acs.jpcc.6b12774
M. Chen, S. Li, G. Feng, The influence of anion shape on the electrical double layer microstructure and capacitance of ionic liquids-based supercapacitors by molecular simulations. Molecules 22(2), 241 (2017). https://doi.org/10.3390/molecules22020241
C. Merlet, C. Péan, B. Rotenberg, P.A. Madden, P. Simon, M. Salanne, Simulating supercapacitors: can we model electrodes as constant charge surfaces? J. Phys. Chem. Lett. 4(2), 264–268 (2013). https://doi.org/10.1021/jz3019226
J. Yang, Z. Bo, H. Yang, H. Qi, J. Kong, J. Yan, K. Cen, Reliability of constant charge method for molecular dynamics simulations on EDLCs in nanometer and sub-nanometer spaces. Chemelectrochem 4(10), 2486–2493 (2017). https://doi.org/10.1002/celc.201700447
J. Vatamanu, D. Bedrov, O. Borodin, On the application of constant electrode potential simulation techniques in atomistic modelling of electric double layers. Mol. Simul. 43(10–11), 838–849 (2017). https://doi.org/10.1080/08927022.2017.1279287
L.L. Zhang, R. Zhou, X.S. Zhao, Graphene-based materials as supercapacitor electrodes. J. Mater. Chem. 20(29), 5983–5992 (2010). https://doi.org/10.1039/c000417k
J. Chen, C. Li, G. Shi, Graphene materials for electrochemical capacitors. J. Phys. Chem. Lett. 4(8), 1244–1253 (2013). https://doi.org/10.1021/jz400160k
R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, The role of graphene for electrochemical energy storage. Nat. Mater. 14(3), 271–279 (2015). https://doi.org/10.1038/nmat4170
K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films. Science 306(5696), 666–669 (2004). https://doi.org/10.1126/science.1102896
M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors. Nano Lett. 8(10), 3498–3502 (2008). https://doi.org/10.1021/nl802558y
J. Xia, F. Chen, J. Li, N. Tao, Measurement of the quantum capacitance of graphene. Nat. Nanotechnol. 4(8), 505–509 (2009). https://doi.org/10.1038/nnano.2009.177
C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321(5887), 385–388 (2008). https://doi.org/10.1126/science.1157996
A.A. Kornyshev, Double-layer in ionic liquids: paradigm change? J. Phys. Chem. B 111(20), 5545–5557 (2007). https://doi.org/10.1021/jp067857o
J. Vatamanu, L. Xing, W. Li, D. Bedrov, Influence of temperature on the capacitance of ionic liquid electrolytes on charged surfaces. Phys. Chem. Chem. Phys. 16(11), 5174–5182 (2014). https://doi.org/10.1039/c3cp54705a
Z. Hu, J. Vatamanu, O. Borodin, D. Bedrov, A comparative study of alkylimidazolium room temperature ionic liquids with FSI and TFSI anions near charged electrodes. Electrochim. Acta 145, 40–52 (2014). https://doi.org/10.1016/j.electacta.2014.08.072
H. Zongzhi, V. Jenel, B. Oleg, B. Dmitry, A molecular dynamics simulation study of the electric double layer and capacitance of [BMIM][PF6] and [BMIM][BF4] room temperature ionic liquids near charged surfaces. Phys. Chem. Chem. Phys. 15(34), 14234–14247 (2013). https://doi.org/10.1039/c3cp51218e
J. Hafner, Ab-initio simulations of materials using VASP: density-functional theory and beyond. J. Comput. Chem. 29(13), 2044–2078 (2008). https://doi.org/10.1002/jcc.21057
J. Vatamanu, X. Ni, F. Liu, D. Bedrov, Tailoring graphene-based electrodes from semiconducting to metallic to increase the energy density in supercapacitors. Nanotechnology 26(46), 464001 (2015). https://doi.org/10.1088/0957-4484/26/46/464001
L. Chen, X. Li, C. Ma, M. Wang, J. Zhou, Interaction and quantum capacitance of nitrogen/sulfur co-doped graphene: a theoretical calculation. J. Phys. Chem. C 121(34), 18344–18350 (2017). https://doi.org/10.1021/acs.jpcc.7b04551
S. Mao, H. Pu, J. Chen, Graphene oxide and its reduction: modeling and experimental progress. RSC Adv. 2(7), 2643–2662 (2012). https://doi.org/10.1039/C2RA00663D
A.D. DeYoung, S.-W. Park, N.R. Dhumal, Y. Shim, Y. Jung, H.J. Kim, Graphene oxide supercapacitors: a computer simulation study. J. Phys. Chem. C 118(32), 18472–18480 (2014). https://doi.org/10.1021/jp5072583
K. Xu, X. Ji, C. Chen, H. Wan, L. Miao, J. Jiang, Electrochemical double layer near polar reduced graphene oxide electrode: insights from molecular dynamic study. Electrochim. Acta 166, 142–149 (2015). https://doi.org/10.1016/j.electacta.2015.03.101
S. Li, G. Feng, P.T. Cummings, Interfaces of dicationic ionic liquids and graphene: a molecular dynamics simulation study. J. Phys-Condens. Matter 26(28), 284106 (2014). https://doi.org/10.1088/0953-8984/26/28/284106
S. Jo, S.-W. Park, Y. Shim, Y. Jung, Effects of alkyl chain length on interfacial structure and differential capacitance in graphene supercapacitors: a molecular dynamics simulation study. Electrochim. Acta 247, 634–645 (2017). https://doi.org/10.1016/j.electacta.2017.06.169
X. Liu, Y. Wang, S. Li, T. Yan, Effects of anion on the electric double layer of imidazolium-based ionic liquids on graphite electrode by molecular dynamics simulation. Electrochim. Acta 184, 164–170 (2015). https://doi.org/10.1016/j.electacta.2015.10.064
J. Vatamanu, O. Borodin, G.D. Smith, Molecular simulations of the electric double layer structure, differential capacitance, and charging kinetics for n-methyl-n-propylpyrrolidinium bis(fluorosulfonyl)imide at graphite electrodes. J. Phys. Chem. B 115(12), 3073–3084 (2011). https://doi.org/10.1021/jp2001207
M. Wu, W. Li, S. Li, G. Feng, Capacitive performance of amino acid ionic liquid electrolyte-based supercapacitors by molecular dynamics simulation. RSC Adv. 7(46), 28945–28950 (2017). https://doi.org/10.1039/c7ra00443e
E. Sedghamiz, M. Moosavi, Tricationic ionic liquids: structural and dynamical properties via molecular dynamics simulations. J. Phys. Chem. B 121(8), 1877–1892 (2017). https://doi.org/10.1021/acs.jpcb.6b10766
Y.N. Ahn, S.H. Lee, G.S. Lee, H. Kim, Effect of alkyl branches on the thermal stability of quaternary ammonium cations in organic electrolytes for electrochemical double layer capacitors. Phys. Chem. Chem. Phys. 19(30), 19959–19966 (2017). https://doi.org/10.1039/c7cp03209a
J. Vatamanu, O. Borodin, Ramifications of water-in-salt interfacial structure at charged electrodes for electrolyte electrochemical stability. J. Phys. Chem. Lett. 8(18), 4362–4367 (2017). https://doi.org/10.1021/acs.jpclett.7b01879
R.S. Kuhnel, D. Reber, A. Remhof, R. Figi, D. Bleiner, C. Battaglia, “Water-in-salt” electrolytes enable the use of cost-effective aluminum current collectors for aqueous high-voltage batteries. Chem. Commun. 52(68), 10435–10438 (2016). https://doi.org/10.1039/c6cc03969c
S. Kondrat, P. Wu, R. Qiao, A.A. Kornyshev, Accelerating charging dynamics in subnanometre pores. Nat. Mater. 13(4), 387–393 (2014). https://doi.org/10.1038/nmat3916
G. Feng, P.T. Cummings, Supercapacitor capacitance exhibits oscillatory behavior as a function of nanopore size. J. Phys. Chem. Lett. 2(22), 2859–2864 (2011). https://doi.org/10.1021/jz201312e
S.R. Varanasi, S.K. Bhatia, Capacitance optimization in nanoscale electrochemical supercapacitors. J. Phys. Chem. C 119(31), 17573–17584 (2015). https://doi.org/10.1021/acs.jpcc.5b04254
D.E. Jiang, J. Wu, Unusual effects of solvent polarity on capacitance for organic electrolytes in a nanoporous electrode. Nanoscale 6(10), 5545–5550 (2014). https://doi.org/10.1039/C4NR00046C
D.-E. Jiang, Z. Jin, D. Henderson, J. Wu, Solvent effect on the pore-size dependence of an organic electrolyte supercapacitor. J. Phys. Chem. Lett. 3(13), 1727–1731 (2012). https://doi.org/10.1021/jz3004624
R. Burt, K. Breitsprecher, B. Daffos, P.-L. Taberna, P. Simon, G. Birkett, X.S. Zhao, C. Holm, M. Salanne, Capacitance of nanoporous carbon-based supercapacitors is a trade-off between the concentration and the separability of the ions. J. Phys. Chem. Lett. 7(19), 4015–4021 (2016). https://doi.org/10.1021/acs.jpclett.6b01787
B. Uralcan, I.A. Aksay, P.G. Debenedetti, D.T. Limmer, Concentration fluctuations and capacitive response in dense ionic solutions. J. Phys. Chem. Lett. 7(13), 2333–2338 (2016). https://doi.org/10.1021/acs.jpclett.6b00859
M.H. Kowsari, L. Tohidifar, Tracing dynamics, self-diffusion, and nanoscale structural heterogeneity of pure and binary mixtures of ionic liquid 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide with acetonitrile: insights from molecular dynamics simulations. J. Phys. Chem. B 120(41), 10824–10838 (2016). https://doi.org/10.1021/acs.jpcb.6b08396
Q. Zhang, P. Xie, X. Wang, X. Yu, Z. Shi, S. Zhao, Thermodynamic and transport properties of spiro-(1,1′)-bipyrrolidinium tetrafluoroborate and acetonitrile mixtures: a molecular dynamics study. Chin. Phys. B 25(6), 066102 (2016). https://doi.org/10.1088/1674-1056/25/6/066102
H. Yang, J. Yang, Z. Bo, X. Chen, X. Shuai, J. Kong, J. Yan, K. Cen, Kinetic-dominated charging mechanism within representative aqueous electrolyte-based electric double-layer capacitors. J. Phys. Chem. Lett. 8(15), 3703–3710 (2017). https://doi.org/10.1021/acs.jpclett.7b01525
C. Lian, K. Liu, H. Liu, J. Wu, Impurity effects on charging mechanism and energy storage of nanoporous supercapacitors. J. Phys. Chem. C 121(26), 14066–14072 (2017). https://doi.org/10.1021/acs.jpcc.7b04869
P. Wu, J. Huang, V. Meunier, B.G. Sumpter, R. Qiao, Voltage dependent charge storage modes and capacity in subnanometer pores. J. Phys. Chem. Lett. 3(13), 1732–1737 (2012). https://doi.org/10.1021/jz300506j
J. Vatamanu, O. Borodin, G.D. Smith, Molecular insights into the potential and temperature dependences of the differential capacitance of a room-temperature ionic liquid at graphite electrodes. J. Am. Chem. Soc. 132(42), 14825–14833 (2010). https://doi.org/10.1021/ja104273r
R.K. Kalluri, D. Konatham, A. Striolo, Aqueous NaCl solutions within charged carbon-slit pores: partition coefficients and density distributions from molecular dynamics simulations. J. Phys. Chem. C 115(28), 13786–13795 (2011). https://doi.org/10.1021/jp203086x
T. Ohba, N. Kojima, H. Kanoh, K. Kaneko, Unique hydrogen-bonded structure of water around Ca ions confined in carbon slit pores. J. Phys. Chem. C 113(29), 12622–12624 (2009). https://doi.org/10.1021/jp9030688
N.N. Rajput, J. Monk, R. Singh, F.R. Hung, On the influence of pore size and pore loading on structural and dynamical heterogeneities of an ionic liquid confined in a slit nanopore. J. Phys. Chem. C 116(8), 5169–5181 (2012). https://doi.org/10.1021/jp212440f
S. Salemi, H. Akbarzadeh, S. Abdollahzadeh, Nano-confined ionic liquid [emim][PF6] between graphite sheets: a molecular dynamics study. J. Mol. Liq. 215, 512–519 (2016). https://doi.org/10.1016/j.molliq.2016.01.035
J. Kong, Z. Bo, H. Yang, J. Yang, X. Shuai, J. Yan, K. Cen, Temperature dependence of ion diffusion coefficients in nacl electrolyte confined within graphene nanochannels. Phys. Chem. Chem. Phys. 19(11), 7678–7688 (2017). https://doi.org/10.1039/c6cp08752c
S.A. Kislenko, R.H. Amirov, I.S. Samoylov, Influence of temperature on the structure and dynamics of the [BMIM][PF6] ionic liquid/graphite interface. Phys. Chem. Chem. Phys. 12(37), 11245–11250 (2010). https://doi.org/10.1039/c0cp00220h
R. Singh, J. Monk, F.R. Hung, Heterogeneity in the dynamics of the ionic liquid [BMIM+][PF6 −] confined in a slit nanopore. J. Phys. Chem. C 115(33), 16544–16554 (2011). https://doi.org/10.1021/jp2046118
W. Yuan, Y. Zhou, Y. Li, C. Li, H. Peng, J. Zhang, Z. Liu, L. Dai, G. Shi, The edge- and basal-plane-specific electrochemistry of a single-layer graphene sheet. Sci. Rep. 3, 2248 (2013). https://doi.org/10.1038/srep02248
C. Zhan, Y. Zhang, P.T. Cummings, D.-E. Jiang, Computational insight into the capacitive performance of graphene edge planes. Carbon 116, 278–285 (2017). https://doi.org/10.1016/j.carbon.2017.01.104
V.N. Popov, Carbon nanotubes: properties and application. Mater. Sci. Eng. R-Rep. 43(3), 61–102 (2004). https://doi.org/10.1016/j.mser.2003.10.001
H.J. Dai, Carbon nanotubes: opportunities and challenges. Surf. Sci. 500(1–3), 218–241 (2002). https://doi.org/10.1016/s0039-6028(01)01558-8
M. Trojanowicz, Analytical applications of carbon nanotubes: a review. Trac-trends Anal. Chem. 25(5), 480–489 (2006). https://doi.org/10.1016/j.trac.2005.11.008
L. Yang, B.H. Fishbine, A. Migliori, L.R. Pratt, Molecular simulation of electric double-layer capacitors based on carbon nanotube forests. J. Am. Chem. Soc. 131(34), 12373–12376 (2009). https://doi.org/10.1021/ja9044554
A. Dive, S. Banerjee, Ion storage in nanoconfined interstices between vertically aligned nanotubes in electric double-layer capacitors. J. Electrochem. Energy Convers. 15(1), 011001 (2017). https://doi.org/10.1115/1.4037582
K. Dong, G. Zhou, X. Liu, X. Yao, S. Zhang, A. Lyubartsev, Structural evidence for the ordered crystallites of ionic liquid in confined carbon nanotubes. J. Phys. Chem. C 113(23), 10013–10020 (2009). https://doi.org/10.1021/jp900533k
O.N. Kalugin, V.V. Chaban, V.V. Loskutov, O.V. Prezhdo, Uniform diffusion of acetonitrile inside carbon nanotubes favors supercapacitor performance. Nano Lett. 8(8), 2126–2130 (2008). https://doi.org/10.1021/nl072976g
R.J. Mashl, S. Joseph, N.R. Aluru, E. Jakobsson, Anomalously immobilized water: a new water phase induced by confinement in nanotubes. Nano Lett. 3(5), 589–592 (2003). https://doi.org/10.1021/nl0340226
L. Yang, S. Garde, Modeling the selective partitioning of cations into negatively charged nanopores in water. J. Chem. Phys. 126(8), 084706 (2007). https://doi.org/10.1063/1.2464083
T. Ohba, Fast ion transportation associated with recovering hydration shells in a nanoelectrolyte between conical carbon nanopores during charging cycles. J. Phys. Chem. C 121(19), 10439–10444 (2017). https://doi.org/10.1021/acs.jpcc.7b02326
D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna, P. Simon, Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon. Nat. Nanotechnol. 5(9), 651–654 (2010). https://doi.org/10.1038/nnano.2010.162
G. Feng, D.-E. Jiang, P.T. Cummings, Curvature effect on the capacitance of electric double layers at ionic liquid/onion-like carbon interfaces. J. Chem. Theory Comput. 8(3), 1058–1063 (2012). https://doi.org/10.1021/ct200914j