New Perspectives on the Charging Mechanisms of Supercapacitors.

Supercapacitors (or electric double-layer capacitors) are high-power energy storage devices that store charge at the interface between porous carbon electrodes and an electrolyte solution. These devices are already employed in heavy electric vehicles and electronic devices, and can complement batteries in a more sustainable future. Their widespread application could be facilitated by the development of devices that can store more energy, without compromising their fast charging and discharging times. In situ characterization methods and computational modeling techniques have recently been developed to study the molecular mechanisms of charge storage, with the hope that better devices can be rationally designed. In this Perspective, we bring together recent findings from a range of experimental and computational studies to give a detailed picture of the charging mechanisms of supercapacitors. Nuclear magnetic resonance experiments and molecular dynamics simulations have revealed that the electrode pores contain a considerable number of ions in the absence of an applied charging potential. Experiments and computer simulations have shown that different charging mechanisms can then operate when a potential is applied, going beyond the traditional view of charging by counter-ion adsorption. It is shown that charging almost always involves ion exchange (swapping of co-ions for counter-ions), and rarely occurs by counter-ion adsorption alone. We introduce a charging mechanism parameter that quantifies the mechanism and allows comparisons between different systems. The mechanism is found to depend strongly on the polarization of the electrode, and the choice of the electrolyte and electrode materials. In light of these advances we identify new directions for supercapacitor research. Further experimental and computational work is needed to explain the factors that control supercapacitor charging mechanisms, and to establish the links between mechanisms and performance. Increased understanding and control of charging mechanisms should lead to new strategies for developing next-generation supercapacitors with improved performances.The authors acknowledge the Sims Scholarship Cambridge (A.C.F.), the School of the Physical Sciences of the University of Cambridge (via an Oppenheimer Research Fellowship, C.M.), EPSRC (via the Supergen consortium, A.C.F. and J.M.G.), and the EU ERC (via an Advanced Fellowship to C.P.G.) for funding. We thank Nicole Trease, Andrew Ilott, Phoebe Allan, Elizabeth Humphreys, Paul Bayley, Hao Wang, Patrice Simon, Wan-Yu Tsai, Yury Gogotsi, Mathieu Salanne, Benjamin Rotenberg, Alexei Kornyshev, Svyatoslav Kondrat and Volker Presser for collaboration, and stimulating discussions and insights into supercapacitors over the course of our research on this subject.This is the final version of the article. It first appeared from the American Chemical Society via https://doi.org/10.1021/jacs.6b0211

Ring current effects: Factors affecting the NMR chemical shift of molecules adsorbed on porous carbons

Nuclear magnetic resonance (NMR) spectroscopy is increasingly being used to study the adsorption of molecules in porous carbons, a process which underpins applications ranging from electrochemical energy storage to water purification. Here we present density functional theory (DFT) calculations of the nucleus-independent chemical shift (NICS) near various sp2-hybridized carbon fragments to explore the structural factors that may affect the resonance frequencies observed for adsorbed species. The domain size of the delocalized electron system affects the calculated NICSs, with larger domains giving rise to larger chemical shieldings. In slit pores, overlap of the ring current effects from the pore walls is shown to increase the chemical shielding. Finally, curvature in the carbon sheets is shown to have a significant effect on the NICS. The trends observed are consistent with existing NMR results as well as new spectra presented for an electrolyte adsorbed on carbide-derived carbons prepared at different temperatures

Selective observation of charge storing ions in supercapacitor electrode materials.

Nuclear magnetic resonance (NMR) spectroscopy has emerged as a useful technique for probing the structure and dynamics of the electrode-electrolyte interface in supercapacitors, as ions inside the pores of the carbon electrodes can be studied separately from bulk electrolyte. However, in some cases spectral resolution can limit the information that can be obtained. In this study we address this issue by showing how cross polarisation (CP) NMR experiments can be used to selectively observe the in-pore ions in supercapacitor electrode materials. We do this by transferring magnetisation from 13C nuclei in porous carbons to nearby nuclei in the cations (1H) or anions (19F) of an ionic liquid. Two-dimensional NMR experiments and CP kinetics measurements confirm that in-pore ions are located within Ångströms of sp2-hybridised carbon surfaces. Multinuclear NMR experiments hold promise for future NMR studies of supercapacitor systems where spectral resolution is limited

Solid-state NMR studies of supercapacitors

Electrochemical double-layer capacitors, or ‘supercapacitors’ are attracting increasing attention as high-power energy storage devices for a wide range of technological applications. These devices store charge through electrostatic interactions between liquid electrolyte ions and the surfaces of porous carbon electrodes. However, many aspects of the fundamental mechanism of supercapacitance are still not well understood, and there is a lack of experimental techniques which are capable of studying working devices. Recently, solid-state NMR has emerged as a powerful tool for studying the local environments and behaviour of electrolyte ions in supercapacitor electrodes. In this Trends article, we review these recent developments and applications. We first discuss the basic principles underlying the mechanism of supercapacitance, as well as the key NMR observables that are relevant to the study of supercapacitor electrodes. We then review some practical aspects of the study of working devices using ex situ and in situ methodologies and explain the key advances that these techniques have allowed on the study of supercapacitor charging mechanisms. NMR experiments have revealed that the pores of the carbon electrodes contain a significant number of electrolyte ions in the absence of any charging potential. This has important implications for the molecular mechanisms of supercapacitance, as charge can be stored by different ion adsorption/desorption processes. Crucially, we show how in situ NMR experiments can be used to quantitatively study and characterise the charging mechanism, with the experiments providing the most detailed picture of charge storage to date, offering the opportunity to design enhanced devices. Finally, an outlook for future directions for solid-state NMR in supercapacitor research is offered

In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors.

Supercapacitors store charge through the electrosorption of ions on microporous electrodes. Despite major efforts to understand this phenomenon, a molecular-level picture of the electrical double layer in working devices is still lacking as few techniques can selectively observe the ionic species at the electrode/electrolyte interface. Here, we use in situ NMR to directly quantify the populations of anionic and cationic species within a working microporous carbon supercapacitor electrode. Our results show that charge storage mechanisms are different for positively and negatively polarized electrodes for the electrolyte tetraethylphosphonium tetrafluoroborate in acetonitrile; for positive polarization charging proceeds by exchange of the cations for anions, whereas for negative polarization, cation adsorption dominates. In situ electrochemical quartz crystal microbalance measurements support the NMR results and indicate that adsorbed ions are only partially solvated. These results provide new molecular-level insight, with the methodology offering exciting possibilities for the study of pore/ion size, desolvation and other effects on charge storage in supercapacitors.A.C.F., J.M.G. and C.P.G. acknowledge the Sims Scholarship (A.C.F.), EPSRC (through the Supergen consortium for J.M.G.) and the EU ERC (through an Advanced Fellowship to C.P.G.) for financial support. P.S. and W.-Y.T. acknowledge support from the European Research Council (ERC, Advanced Grant, ERC-2011-AdG, Project 291543–IONACES). P.S. also acknowledges financial support from the Chair ‘Embedded Multi-Functional Nanomaterials’ from the Airbus Group Foundation. A.C.F. and J.M.G. thank the NanoDTC Cambridge for travel funding.This is the author accepted manuscript. The final version is available from NPG at http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4318.html#abstract.

NMR Study of Ion Dynamics and Charge Storage in Ionic Liquid Supercapacitors.

Ionic liquids are emerging as promising new electrolytes for supercapacitors. While their higher operating voltages allow the storage of more energy than organic electrolytes, they cannot currently compete in terms of power performance. More fundamental studies of the mechanism and dynamics of charge storage are required to facilitate the development and application of these materials. Here we demonstrate the application of nuclear magnetic resonance spectroscopy to study the structure and dynamics of ionic liquids confined in porous carbon electrodes. The measurements reveal that ionic liquids spontaneously wet the carbon micropores in the absence of any applied potential and that on application of a potential supercapacitor charging takes place by adsorption of counterions and desorption of co-ions from the pores. We find that adsorption and desorption of anions surprisingly plays a more dominant role than that of the cations. Having elucidated the charging mechanism, we go on to study the factors that affect the rate of ionic diffusion in the carbon micropores in an effort to understand supercapacitor charging dynamics. We show that the line shape of the resonance arising from adsorbed ions is a sensitive probe of their effective diffusion rate, which is found to depend on the ionic liquid studied, as well as the presence of any solvent additives. Taken as whole, our NMR measurements allow us to rationalize the power performances of different electrolytes in supercapacitors.A.C.F., J.M.G., C.M., P.M.B., H.W., and C.P.G. acknowledge the Sims Scholarship (A.C.F), EPSRC (via the Supergen consortium, J.M.G.), The School of Physical Sciences (University of Cambridge) for funding through an Oppen- heimer Research Fellowship (C.M.), The Marie Curie FP7 International Incoming Fellowship (P.M.B.), and the EU ERC (via an Advanced Fellowship to C.P.G.) for funding. A.C.F. and J.M.G. thank the NanoDTC Cambridge for travel funding. P.S. acknowledges support from the European Research Council (ERC, Advanced Grant, ERC-2011-AdG, Project 291543−IONACES).This is the final version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/jacs.5b03958

Lattice simulation method to model diffusion and NMR spectra in porous materials.

A coarse-grained simulation method to predict nuclear magnetic resonance (NMR) spectra of ions diffusing in porous carbons is proposed. The coarse-grained model uses input from molecular dynamics simulations such as the free-energy profile for ionic adsorption, and density-functional theory calculations are used to predict the NMR chemical shift of the diffusing ions. The approach is used to compute NMR spectra of ions in slit pores with pore widths ranging from 2 to 10 nm. As diffusion inside pores is fast, the NMR spectrum of an ion trapped in a single mesopore will be a sharp peak with a pore size dependent chemical shift. To account for the experimentally observed NMR line shapes, our simulations must model the relatively slow exchange between different pores. We show that the computed NMR line shapes depend on both the pore size distribution and the spatial arrangement of the pores. The technique presented in this work provides a tool to extract information about the spatial distribution of pore sizes from NMR spectra. Such information is difficult to obtain from other characterisation techniques.C.M. acknowledges the School of the Physical Sciences of the University of Cambridge for funding through an Oppenheimer Research Fellowship. C.M., A.C.F., J.M.G., and C.P.G. acknowledge the Sims Scholarship (A.C.F.), EPSRC (via the Supergen consortium, J.M.G.), and the EU ERC (via an Advanced Fellowship to C.P.G.) for funding. A.C.F. and J.M.G. thank the NanoDTC Cambridge for travel funding. D.F. acknowledges EPSRC Grant No. EP/I000844/1.This is the final published version. It first appeared at http://scitation.aip.org/content/aip/journal/jcp/142/9/10.1063/1.4913368

NMR reveals the surface functionalisation of Ti3C2 MXene.

(1)H and (19)F NMR experiments have identified and quantified the internal surface terminations of Ti3C2Tx MXene. -F and -OH terminations are shown to be intimately mixed and there are found to be significantly fewer -OH terminations than -F and -O, with the proportions highly dependent on the synthesis method.We are grateful for financial support by the Oppenheimer Foundation and EPSRC. Material synthesis and characterization at Drexel University was supported by the US National Science Foundation under grant number DMR-1310245.This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C6CP00330

Cooperative Carbon Dioxide Adsorption in Alcoholamine- and Alkoxyalkylamine-Functionalized Metal-Organic Frameworks.

A series of structurally diverse alcoholamine- and alkoxyalkylamine-functionalized variants of the metal-organic framework Mg2 (dobpdc) are shown to adsorb CO2 selectively via cooperative chain-forming mechanisms. Solid-state NMR spectra and optimized structures obtained from van der Waals-corrected density functional theory calculations indicate that the adsorption profiles can be attributed to the formation of carbamic acid or ammonium carbamate chains that are stabilized by hydrogen bonding interactions within the framework pores. These findings significantly expand the scope of chemical functionalities that can be utilized to design cooperative CO2 adsorbents, providing further means of optimizing these powerful materials for energy-efficient CO2 separations

Selective nitrogen adsorption via backbonding in a metal-organic framework with exposed vanadium sites.

Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable important chemical separations1-4. Biological systems use accessible, reducing metal centres to bind and activate weakly π-acidic species, such as N2, through backbonding interactions5-7, and incorporating analogous moieties into a porous material should give rise to a similar adsorption mechanism for these gaseous substrates8. Here, we report a metal-organic framework featuring exposed vanadium(II) centres capable of back-donating electron density to weak π acids to successfully target π acidity for separation applications. This adsorption mechanism, together with a high concentration of available adsorption sites, results in record N2 capacities and selectivities for the removal of N2 from mixtures with CH4, while further enabling olefin/paraffin separations at elevated temperatures. Ultimately, incorporating such π-basic metal centres into porous materials offers a handle for capturing and activating key molecular species within next-generation adsorbents