T he urgent need for e cient energy storage devices has resulted in a widespread and concerted research e ort into electrochemical capacitors, also called supercapacitors, in the past ten years. ese devices consist of two high-conductivity electrodes separated by an electrolyte containing mobile ionic species. As in conventional capacitors, charge is stored at the surface of the electrodes through an excess of electrons on one side of the device and electron holes on the other side, on application of a voltage between the electrodes [START_REF] Conway | Electrochemical Supercapacitors: Scienti c Fundamentals and Technological Applications[END_REF] . Unlike conventional capacitors, however, the charge on the electrodes is then balanced by the adsorption of cations and anions, respectively, at their surface. e excellent performance of supercapacitors is due to the highly reversible ion adsorption mechanism. In contrast to batteries, charging is not limited by di usion of ions in the bulk of the electrodes, and hence higher power densities can be achieved. Supercapacitors can therefore complement and sometimes even replace batteries when high power delivery or uptake is needed [START_REF] Miller | Electrochemical capacitors for energy management[END_REF] .

Two distinct families of supercapacitors can be considered. e principal one, electrochemical double-layer capacitors (EDLCs), uses porous carbon electrodes with high accessible surface area.

is acts as an 'electric sponge' , which adsorbs the ions from the electrolytes on charging and releases them at discharge. A er years of incremental developments, the discovery of a di erent charging mechanism in nanoporous carbons (with pore size smaller than 1 nm) has spurred further research into EDLCs [START_REF] Chmiola | Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer[END_REF] . As a consequence, recent work has not only been directed towards new materials and architectures, but is also aimed at understanding the physical mechanisms underlying the charge storage in such materials.

e development of in situ experiments, in particular spectroscopy and di raction, as well as advanced simulation techniques for the realistic description of complex interfacial E cient storage mechanisms for building better supercapacitors M. Salanne 1,2,3 , B. Rotenberg 1,2 , K. Naoi [START_REF] Toupin | Charge storage mechanism of MnO 2 electrode used in aqueous electrochemical capacitor[END_REF] , K. Kaneko [START_REF] Simon | Materials for electrochemical capacitors[END_REF] , P.-L. Taberna 2,6 , C. P. Grey 7 , B. Dunn 8 and P. Simon 2,4,6 * Supercapacitors are electrochemical energy storage devices that operate on the simple mechanism of adsorption of ions from an electrolyte on a high-surface-area electrode. Over the past decade, the performance of supercapacitors has greatly improved, as electrode materials have been tuned at the nanoscale and electrolytes have gained an active role, enabling more e cient storage mechanisms. In porous carbon materials with subnanometre pores, the desolvation of the ions leads to surprisingly high capacitances. Oxide materials store charge by surface redox reactions, leading to the pseudocapacitive e ect. Understanding the physical mechanisms underlying charge storage in these materials is important for further development of supercapacitors. Here we review recent progress, from both in situ experiments and advanced simulation techniques, in understanding the charge storage mechanism in carbon-and oxide-based supercapacitors. We also discuss the challenges that still need to be addressed for building better supercapacitors.

processes, has radically modi ed our understanding of nanoporous carbon-based supercapacitors.

e second family of supercapacitors, known as pseudocapacitors, is based on redox reactions that occur largely (but not exclusively) on oxide-based material surfaces [START_REF] Toupin | Charge storage mechanism of MnO 2 electrode used in aqueous electrochemical capacitor[END_REF] . Fundamental studies on pseudocapacitors remain scarce, because of the di culty of characterizing the oxide surfaces and determining the details of the charge transfer mechanism. But the recent development of nanostructured oxides and of hybrid carbon/oxide electrodes is progressively changing the way we think about pseudocapacitive charge storage.

Despite the recent progress in both the performance and fundamental understanding of the mechanisms involved in super capacitors, this body of work has not been placed in the broader context of capacitive storage. Existing reviews in the literature either focus on the properties of the materials [START_REF] Simon | Materials for electrochemical capacitors[END_REF][START_REF] Béguin | Carbons and electrolytes for advanced supercapacitors[END_REF][START_REF] Raccichini | e role of graphene for electrochemical energy storage[END_REF] or on the electrolytes [START_REF] Armand | Ionic-liquid materials for the electrochemical challenges of the future[END_REF][START_REF] Brandt | Ionic liquids in supercapacitors[END_REF] . Here we review both chemical and physical aspects of the capacitive storage mechanism in carbon-and oxide-based supercapacitors. We discuss the latest advances, which have implications not only for supercapacitors but also for other applications in which voltage-driven ion adsorption in nanoporous structures plays a key role.

Electrochemical double-layer capacitors

Charge storage in supercapacitor electrodes makes use of the electrostatic attraction between the ions of an electrolyte and the charges present at the electrode surface, which allows the formation of oppositely charged layers at the electrolyte/electrode interface. Supercapacitors use mainly porous carbon as electrode materials, thus bene ting from the high surface area available for ion adsorption. e charge separation occurring upon polarization at the electrode/electrolyte interface can be described in a crude approach using the double-layer capacitance model proposed by Helmholtz in 1853 (ref. 10):

C = ε r ε 0 A d (1)
where ε r is the electrolyte relative permittivity, ε 0 is the vacuum permittivity, d is the e ective thickness of the double layer (charge separation distance) and A is the electrode surface area. Note, however, that all these quantities (except for ε 0 ) are ill-de ned for the interfaces at play in the present case of highly disordered materials. Until 2005, it was believed that the best strategy for increasing capacitance consisted in maximizing the electrochemical doublelayer charging. e challenge at that time was therefore to develop new mesoporous carbons with the highest speci c surface area. en, following original work from the group of Aurbach [START_REF] Eliad | Ion sieving e ects in the electrical double layer of porous carbon electrodes: estimating e ective ion size in electrolytic solutions[END_REF][START_REF] Eliad | On the mechanism of selective electroadsorption in the pores of carbon molecular sieves[END_REF] , the discovery of a di erent, more e cient storage mechanism in nanopores (pore size less than 1 nm: that is to say, less than the size of solvated ions) [START_REF] Chmiola | Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer[END_REF] led to a change of view: not only the surface area but also the pore size and the carbon nanostructure drive the electrochemical performance. e community thus had to reconsider the way that charge is stored at the carbon/electrolyte interface in ultra-small pores. Since then, much e ort has been directed towards the basic understanding of charge storage mechanisms at the nano-and subnanoscales, considering both sides of the interface: the carbon structure and the electrolyte organization, including the (de)solvation of ions and their interactions under extreme con nement.

In this section, we show how the development and the use of new techniques, both theoretical and experimental, have been at the origin of major advances in carbon-based supercapacitors. First, more realistic structural models for porous carbon structures have been developed by coupling modelling and characterization techniques such as small-angle X-ray scattering (SAXS) or neutron scattering (SANS), which greatly improve our understanding of their electrochemical behaviour. In addition, the combination of advanced electrochemical techniques such as electrochemical quartz crystal microbalance (EQCM), spectroscopies such as nuclear magnetic resonance (NMR) or infrared, and characterization techniques (SAXS), together with modelling, have led to the elucidation of electrolyte organization in carbon nanopores at null potential and under polarization, as well as the charge storage mechanism and the ion dynamics in the pores. ese points are detailed below.

Structural models for nanoporous carbons. Nanoporous carbons generally consist of defective nanoscale graphitic units, with pores ranging from micropores (pore size <2 nm) to mesopores (from 2 to 50 nm). Unlike slit pores or nanotubes (Fig. 1a,b), they do not display any three-dimensional (3D) long-range ordering (Fig. 1c). Because the electrochemical performance of carbons is driven by the porous carbon/electrolyte interface, accurate and reliable characterizations of the porous network are needed for designing better supercapacitors. Speci c surface area and pore size distributions of porous material are generally obtained from gas adsorption techniques, using theoretical models. But such methods su er from several limitations when applied to microporous carbons.

Choosing an appropriate molecular probe is essential, as pores inaccessible to gas molecules may be accessible to ions from the electro lyte, depending on thermodynamic conditions (Fig. 1d,e). is is speci cally true when N 2 adsorption at 77 K is used to measure the carbon speci c surface area. e quadrupole moment of the N 2 molecule limits the access of the gas to small micropores, and reduced exibility of the porous carbon network structures at low temperatures does not necessarily re ect the ambient temperature properties. As a result, N 2 sorption does not give accurate porosity values for ultramicropores (<0.7 nm). Without a quadrupole moment and with a smaller diameter (0.335 nm compared with 0.363 nm), Ar atoms can access small micropores more easily than N 2 . Combined with a higher experimental measurement temperature and a simpler theoretical analysis, this makes Ar adsorption at 87 K more suited than N 2 for the characterization of microporous carbons [START_REF] Wang | Comparative pore structural analysis of highly porous graphene monoliths treated at di erent temperatures with adsorption of N 2 at 77.4 K and of Ar at 87.3 K and 77.4[END_REF] . A complementary approach involves using the adsorption of supercritical gas or subcritical uids (CO 2 ) at ambient temperature, which improves the adsorption kinetics.

e choice of the probe and of the experimental conditions is only part of the challenge, as the microscopic features must then be extracted from the experimental isotherm by means of an appropriate model. e Brunauer-Emmett-Teller (BET) analysis [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF] is not recommended for microporous carbons, as recently con rmed by the International Union of Pure and Applied Chemistry [START_REF]Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution[END_REF] , as it overestimates the surface of large micropores (>0.7 nm) by 10 to 40%, owing to enhanced bilayer gas adsorption on carbon pore walls. In addition, the BET surface area of ultramicropores (<0.7 nm) is underestimated owing to incomplete monolayer formation. Reliable porosity values for larger micropores (>0.7 nm) can be experimentally obtained by the subtracting pore e ect method (SPE) using a high-resolution α s -plot, at low relative pressure (from P/P 0 = 10 -6 ) [START_REF] Kaneko | Origin of superhigh microcrystalline graphitic structures of activated carbons[END_REF][START_REF] Setoyama | Simulation study on the relationship between a high resolution α s -plot and the pore size distribution for activated carbon[END_REF] . Moving from measurements of surface area to measurements of pore size distribution, quenched solid density functional theory analysis can provide reasonable structural information from high-resolution adsorption isotherms [START_REF] Neimark | Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons[END_REF] .

All of this illustrates the di culty of measuring the surface area of nanoporous carbons. Furthermore, the very concept of 'surface' by itself could be questionable, as it depends on the approach (that is, the probe) used to measure it [START_REF] Centeno | Capacitance in carbon pores of 0.7 to 15 nm: a regular pattern[END_REF] . erefore, methods based on molecular probes should be complemented by alternative direct characterization techniques such as SAXS and NMR spectroscopy, which can assess the whole pores of nanoporous carbons that are accessible to the electrolyte ions, and thus are directly relevant to the electrochemical measurement. Despite such limitations, it remains reasonable to discuss the dependence of the capacitance per unit surface area on pore size based on Ar adsorption data, to provide a method for comparison between samples and preparation method. Nevertheless, gravimetric capacitance (farad per gram of carbon) or volumetric capacitance (farad per volume of electrode) is preferred, as these are based on experimental, measured values, free from any theoretical model or structural considerations.

Revealing the 'real' structure of nanoporous carbons is much more complex, because it is not currently possible to resolve such structures by resorting only to experimental approaches. To that end, combinations of experimental and modelling techniques have been proposed. One may, for example, use synchrotron X-ray diffraction and SAXS experiments to obtain short-and long-range structural information on the porous carbon network. But scattering techniques usually give access to 1D structural information, and the corresponding data must be transformed into 3D structures by the use of hybrid reverse Monte Carlo simulation 20,[START_REF] Bousige | Realistic molecular model of kerogen's nanostructure[END_REF] . Combinations of NMR, pair distribution function analysis of X-ray data, and Raman spectroscopy with lattice simulations have recently been used to evaluate the size of the graphene domains in the porous carbons [START_REF] Forse | New insights into the structure of nanoporous carbons from NMR, Raman, and pair distribution function analysis[END_REF] . Alternatively, quenched molecular dynamics has been used successfully to describe the structure of nanoporous carbons without resorting to experimental structural data [START_REF] Palmer | Modeling the structural evolution of carbide-derived carbons using quenched molecular dynamics[END_REF][START_REF] Palmer | Atomistic models for disordered nanoporous carbons using reactive force elds[END_REF] .

We nally note that most studies so far have focused on the structure of the nanoporous carbon only, whereas the ability to predict the key features of the structure needed to optimize the charge storage also requires a deeper understanding of the structures and dynamics of the con ned electrolyte.

Wetting of the pores at null potential. Until recently, the charging of supercapacitors was assumed to arise from a simple mechanism of the applied potential driving the ions into the porous carbon network. e rst in situ NMR studies have shown that this is not the case: even at low electrolyte loadings, NMR signals are observed that are shi ed towards lower frequencies, as shown in Fig. 2a (refs 25-29), owing to the aromatic ring-current e ects of graphitic domains and magnetic susceptibility of the carbon.

e results

indicate that ions (along with the solvent) are adsorbed within the pores. e starting situation at 0 V is therefore better characterized by the interpenetration of the two heterogeneous structures -the solid carbon material and the electrolyte. Molecular dynamics simulations further con rmed this picture, showing that an ionic liquid in contact with a nanoporous carbon electrode spontaneously lls the highly con ned nanopores, even without any applied eld [START_REF] Merlet | On the molecular origin of supercapacitance in nanoporous carbon electrodes[END_REF][START_REF] Merlet | Highly con ned ions store charge more e ciently in supercapacitors[END_REF][START_REF] Shim | Nanoporous carbon supercapacitors in an ionic liquid: a computer simulation study[END_REF] . NMR spectroscopy is quantitative and thus provides the concentrations of ions adsorbed inside the electrode. e latter follow a linear trend with respect to their counterparts inside the bulk electro lyte [START_REF] Gri N | In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors[END_REF] (Fig. 2b), thereby con rming the a nity of the ions for the carbon. As the ions di use between pores during the signal acquisition, however, they experience various environments, resulting in relatively broad NMR spectra [START_REF] Merlet | Lattice simulation method to model di usion and NMR spectra in porous materials[END_REF] . It is therefore not straightforward to determine the population of the various adsorption sites (for example mesopores versus micropores) using this technique. Another approach is SANS or X-rays. In an empty porous carbon, the signal at small wave vectors originates from the contrast between the carbon atoms and the pore voids. For example, nanopores with a characteristic length scale d of 1-2 nm display a signature around 2π/d ≈ 3-6 nm -1 . When the pores are lled with the electrolyte, this contrast vanishes and the signal decreases. An example of the application of in situ SANS to a typical activated carbon, either empty or loaded with an organic electrolyte, is shown in Fig. 2c (ref. 27).

e decrease in the scattering intensity depends on the wave vector, revealing an incomplete wetting of the smallest pores.

In contrast, Bañuelos et al. combined SANS and molecular dynamics simulations to investigate the adsorption of a roomtemperature ionic liquid (RTIL) inside a hierarchical porous carbon at various loadings [START_REF] Bañuelos | Densi cation of ionic liquid molecules within a hierarchical nanoporous carbon structure revealed by small angle scattering and molecular dynamics simulation[END_REF] and concluded that the RTIL covers the pore surfaces uniformly, rather than leaving some pore surfaces unsaturated. eir pictures of the wetting of mesopores and micropores are shown in Fig. 2d ande, respectively. Di erent results were observed in the case of aqueous electrolytes [START_REF] Boukhalfa | Small-angle neutron scattering for in situ probing of ion adsorption inside micropores[END_REF] , suggesting that the surface chemistry of the carbon and the nature of the electrolyte strongly a ect the wetting properties. Recently, Kondrat and Kornyshev have proposed to use 'ionophobic' pores for designing a new generation of supercapacitors [START_REF] Kondrat | Pressing a spring: what does it take to maximize the energy storage in nanoporous supercapacitors?[END_REF] . eir idea, which remains to be realized experimentally, is built on the fact that such pores would be lled for high voltage only; this could potentially result in larger energy densities and rapid rates of charge and discharge.

Desolvation in nanopores. e discovery that ions could access carbon pores smaller than their solvated ion size has revitalized research into ion con nement in carbon nanopores. From electrochemical measurements, it was proposed that ions lose a part of their solvation shell to access these small nanopores (<1 nm). In addition, the capacitance was strongly enhanced when the carbon pore size was in the same range as the ion size [START_REF] Chmiola | Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer[END_REF][START_REF] Largeot | Relation between the ion size and pore size for an electric double-layer capacitor[END_REF][START_REF] Galhena | Understanding capacitance variation in sub-nanometer pores by in situ tuning of interlayer constrictions[END_REF][START_REF] Raymundo-Pinero | Relationship between the nanoporous texture of activated carbons and their capacitance properties in di erent electrolytes[END_REF] . Although such observations were made using carbon with tailored pore sizes, the use of conventional electrochemical techniques failed to give quantitative information on the extent of desolvation. e development of in situ techniques such as EQCM has led to important advances in addressing this issue. Figure 3a shows a schematic of an EQCM set-up; the change of the quartz resonance frequency re ects the change of weight of the active material during the polarization. e use of EQCM to study ion adsorption in porous carbons was pioneered by Levi and Aurbach [START_REF] Levi | In situ electrochemical quartz crystal admittance methodology for tracking compositional and mechanical changes in porous carbon electrodes[END_REF][START_REF] Levi | Application of a quartz-crystal microbalance to measure ionic uxes in microporous carbons for energy storage[END_REF][START_REF] Sigalov | Electrochemical quartz crystal admittance studies of ion adsorption on nanoporous composite carbon electrodes in aprotic solutions[END_REF] . Figure 3b shows typical changes of ion population as a function of the charge accumulated in a porous carbon electrode during polarization in aqueous electrolyte with varying alkali cations [START_REF] Levi | In situ electrochemical quartz crystal admittance methodology for tracking compositional and mechanical changes in porous carbon electrodes[END_REF] . In this example, from the associated mass change and comparison with the theoretical mass change expected for the adsorption of bare ions from Faraday's law, it was shown that the adsorbed Li + , Na + , K + and Cs + ions are tightly attached to 2.0, 0.8, 0.1 and 0.0 water molecules on average. e con nement of the ions is therefore accompanied by a substantial decrease in their solvation number (number of solvent molecules around the ion), thus con rming previous results [START_REF] Ohkubo | Restricted hydration structures of Rb and Br ions con ned in slit-shaped carbon nanospace[END_REF] .

EQCM was also used in organic electrolytes in combination with carbons of 1-nm average pore diameter [START_REF] Tsai | Electrochemical quartz crystal microbalance (EQCM) study of ion dynamics in nanoporous carbons[END_REF] . A solvation number between three and four could be calculated for 1-ethyl-3-methylimidazolium cations con ned in the 1-nm pores, much smaller than the eight molecules for cations in the bulk electrolyte. e joint use of NMR spectroscopy and EQCM allowed the e ects from the cations and anions to be separated and the cation solvation number to be determined [START_REF] Gri N | In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors[END_REF] . ese results clearly indicate that the partial desolvation of the ions occurs when accessing small pores. e decrease in the ion solvation number could also be quanti ed directly in molecular dynamics simulations [START_REF] Merlet | Highly con ned ions store charge more e ciently in supercapacitors[END_REF] . Importantly, these studies point out a di erence in the energy storage mechanism depending on the electrode polarity, as described below in more detail. Charge storage mechanism. Although the basic principle underlying charge storage in EDLCs, namely the adsorption of ions at the surface of the electrode, is well established, the corresponding microscopic mechanisms have remained elusive until recently. e Gouy-Chapman-Stern theory, which has been the cornerstone of theoretical electrochemistry for over a century, predicts that near an extended surface, the electrode charge is balanced by the polarization of the electrolyte. e decay of the ionic charge distribution and of the electrostatic potential occurs over the 'Debye length' , typically in the 1-10 nm range, which depends on the electrolyte concentration and solvent permittivity. But this picture is of limited help in understanding the interfacial structure and thermo dynamics in the case of supercapacitors, owing to the strong ionic correlations at high electrolyte concentration or in pure ionic liquids [START_REF] Fedorov | Ionic liquids at electri ed interfaces[END_REF] and to the marked e ects of con nement [START_REF] Merlet | On the molecular origin of supercapacitance in nanoporous carbon electrodes[END_REF][START_REF] Merlet | Highly con ned ions store charge more e ciently in supercapacitors[END_REF][START_REF] Shim | Nanoporous carbon supercapacitors in an ionic liquid: a computer simulation study[END_REF][START_REF] Xing | On the atomistic nature of capacitance enhancement generated by ionic liquid electrolyte con ned in subnanometer pores[END_REF] .

e behaviour of such electrolytes near metallic walls, noticed as early as 1952 [START_REF] Freise | Zur theorie der di usendoppeltschicht[END_REF] , has called for a paradigm change [START_REF] Fedorov | Ionic liquids at electri ed interfaces[END_REF][START_REF] Kornyshev | Double-layer in ionic liquids: paradigm change?[END_REF] , and recent advances in simulation techniques [START_REF] Limmer | Charge uctuations in nanoscale capacitors[END_REF] have shown that it is also important to consider the electrolyte structure at the electrode surface [START_REF] Merlet | e electric double layer has a life of its own[END_REF][START_REF] Kornyshev | ree-dimensional double layers[END_REF] . Recent claims based on surface force apparatus experiments have also revived the debate on the di erences between electrochemical interfaces involving pure ionic liquids and those of solvent-based electrolytes [START_REF] Gebbie | Ionic liquids behave as dilute electrolyte solutions[END_REF][START_REF] Perkin | Is a stern and di use layer model appropriate to ionic liquids at surfaces?[END_REF][START_REF] Bozym | Anomalous capacitance maximum of the glassy carbon-ionic liquid interface through dilution with organic solvents[END_REF][START_REF] Lee | Are room-temperature ionic liquids dilute electrolyte?[END_REF] .

In the case of planar graphitic electrodes, the Gouy-Chapman-Stern theory predicts an increase in the capacitance with the applied potential. is theory does not, however, capture two important features, namely the ionic correlations and the nite surface area accessible to the ions. is explains why it fails at reproducing, even qualitatively, experimental results in highly concentrated electrolytes [START_REF] Kornyshev | Double-layer in ionic liquids: paradigm change?[END_REF] . Mean-eld theories accounting for these e ects and molecular simulations [START_REF] Fedorov | Ionic liquids at electri ed interfaces[END_REF][START_REF] Bazant | Double layer in ionic liquids: overscreening versus crowding[END_REF] show that the di erential capacitances may display various shapes (the 'camel' or 'bell' shapes) depending on the nature of the electrolyte. But these capacitance values remain too small to be exploited e ciently in supercapacitor devices.

An important consideration for extreme con nement, when pore sizes are comparable to the diameter of the ions, is that the electronic charge inside the electrode is compensated by an imbalance in the number of co-and counter-ions inside the pores. In such a 'superionic state' , the local breakdown of electro-neutrality in the inter stitial uid is compensated by the creation of a corresponding charge on the metallic surface that e ciently screens the interactions between like-charged ions [START_REF] Kondrat | Superionic state in double-layer capacitors with nanoporous electrodes[END_REF][START_REF] Kondrat | A superionic state in nano-porous double-layer capacitors: insights from Monte Carlo simulations[END_REF] . Several processes may lead to the overall excess of counter-ions inside an electrode: adsorption of counter-ions, swapping of co-ions for counter-ions, or desorption of co-ions [START_REF] Gri N | Ion counting in supercapacitor electrode using NMR spectroscopy[END_REF] . For a given combination of electrodes and electrolyte (nature of the ions, presence and choice of a solvent), one or several of these mechanisms are observed as a function of voltage. NMR, molecular dynamics, infrared spectroscopy, SAXS and EQCM suggest that ion swapping is the most common process at low voltage, but for large ion sizes and/or at high voltage, adsorption of the sole counter-ions is also observed, as illustrated in Fig. 3c. Desorption of co-ions seems to be less frequent. Recent in situ NMR measurements on a solventbased electrolyte have shown that the charge storage mechanism can di er depending on the electrode polarization; ion swapping was observed for positive polarization, whereas counter-ion adsorption dominated for negative polarization. is suggests that multiple factors may in uence how excess ionic charge is achieved, including relative sizes and mobilities of co/counter-ions, kinetic phenomena and ion rearrangement over multiple cycles. Coupling NMR with EQCM also showed that both counter-ion adsorption and ion swapping are accompanied by the entrance or departure of solvent molecules [START_REF] Gri N | In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors[END_REF] . In pure ionic liquids, molecular simulations suggest that the exchange occurs almost without changing the volume occupied by the liquid inside the electrode [START_REF] Merlet | On the molecular origin of supercapacitance in nanoporous carbon electrodes[END_REF] , but this conclusion should be strengthened by considering further combinations of cations and anions, and requires experimental con rmation.

e fundamental understanding of the microscopic charging mechanisms achieved in the past ve years provides a strong basis for the design of better supercapacitors, by suggesting new strategies for the optimization of charge storage through appropriate combinations of electrode structures, ions and solvent. e widespread use of experimental and simulation tools developed to establish these mechanisms will be the key to the success of this endeavour. Dynamics of charging and discharging. e key feature of supercapacitors, for example with respect to batteries, is their very high power density: they are able to charge or discharge within a few seconds. e increase in capacitance should therefore not be achieved at the price of a decrease in power. Conventional theories predict that liquids su er from noticeable slowdown of their dynamics when they are strongly con ned, disqualifying in principle the use of microporous carbons for supercapacitors. Fortunately, such e ects have not been observed in experiments performed on carbidederived carbons (CDCs), for which a maximal characteristic discharge time of 20 seconds was obtained for the smallest average pore size [START_REF] Chmiola | Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer[END_REF] . is trend is further con rmed by electrochemical impedance spectroscopy. is technique o ers the possibility of measuring inpore resistivities in the low-frequency region. ese resistivities are usually of the order of 50 to 200 ohm cm for electrolytes made of organic ions dissolved in acetonitrile at room temperature -only slightly higher than for the same liquids under bulk conditions [START_REF] Pean | Con nement, desolvation and electrosorption e ects on the di usion of ions in nanoporous carbon electrodes[END_REF] . e fast charging of supercapacitors impedes the application of the usual in situ techniques to monitor their dynamical behaviour. In many cases, the time needed to record a spectrum or a di ractogram is longer than their charging time. However, techniques such as infrared spectroscopy or SAXS have been used to study charge/discharge cycles of supercapacitors at a low scan rate of 5 mV s -1 . Figure 4a andb shows the relative evolution of the signal with time and potential, in two di erent experiments. In one case, a supercapacitor made of 1-ethyl-3-methylimidazolium bis(tri uoromethylsulfonyl)imide ionic liquid and a CDC electrode is studied by infrared spectroscopy [START_REF] Richey | Ion dynamics in porous carbon electrodes in supercapacitors using in situ infrared spectroelectrochemistry[END_REF] . In the second case, the adsorption of an aqueous CsCl electrolyte inside a microporous activated carbon is studied by SAXS [START_REF] Prehal | Tracking the structural arrangement of ions in carbon supercapacitor nanopores using in-situ small angle X-ray scattering[END_REF] . In both cases, the response of the liquid closely follows the applied potential, with a lag of only a few seconds. ese observations are consistent with a timescale of seconds for charging microporous supercapacitors. NMR and magnetic resonance imaging (MRI) studies have highlighted the role of the cell design in the timescale of ionic response during charging. Using a non-optimal 'shi ed' cell design to allow electrodes to be studied independently, in situ NMR experiments revealed a lag between the applied potential and charge build up on the electrode even at a low scan rate, which is due to ohmic drop [START_REF] Wang | Real-time NMR studies of electrochemical double-layer capacitors[END_REF] . is issue was subsequently addressed through the use of MRI which allows spatial resolution of the two electrodes in a conventional cell design [START_REF] Ilott | Multinuclear in situ magnetic resonance imaging of electrochemical double-layer capacitors[END_REF] .

e microscopic origin of the observed fast charging has been studied using molecular simulations. In particular, molecular dynamics provides the trajectories of the molecules and is perfectly suited for extracting transport properties such as di usion coecients. Recent work has shown that the di usion coe cients of the ions are generally one to two orders of magnitude smaller in the electrodes than in the bulk liquid [START_REF] Pean | Con nement, desolvation and electrosorption e ects on the di usion of ions in nanoporous carbon electrodes[END_REF][START_REF] Kondrat | Accelerating charging dynamics in subnanometre pores[END_REF] , but strong variations are observed with the lling of the electrode in the case of ionic liquids [START_REF] Kondrat | Accelerating charging dynamics in subnanometre pores[END_REF][START_REF] He | e importance of ion packing on the dynamics of ionic liquids during micropore charging[END_REF] . Indeed, Kondrat et al. have shown that in slit pores, the ions can even diffuse faster than in the bulk at intermediate voltages [START_REF] Kondrat | Accelerating charging dynamics in subnanometre pores[END_REF] . In CDCs, this e ect is not observed but the good connectivity between the pores was shown to be an important characteristic, allowing the larger pores to play the role of electrolyte reservoirs as shown in Fig. 4c (ref. 65). Injecting molecular dynamics results into an equivalent circuit model then leads to estimated charging times of 1 to 10 s for CDC electrodes [START_REF] Pean | On the dynamics of charging in nanoporous carbon-based supercapacitors[END_REF] , further demonstrating the good predictive power of these simulations.

Oxide-based supercapacitors

In the 1970s, Conway and others recognized that certain materials undergo redox reactions at or near their surface and yet their electrochemical properties are similar to those of supercapacitors, which store energy at the electrical double layer formed at the electrode/ electrolyte interface [START_REF] Conway | Electrochemical Supercapacitors: Scienti c Fundamentals and Technological Applications[END_REF] . ese pseudocapacitive materials lead to much higher levels of charge storage than EDLCs, owing to the redox reactions [START_REF] Toupin | Charge storage mechanism of MnO 2 electrode used in aqueous electrochemical capacitor[END_REF] . For this reason, over the past several years there has been growing interest in pseudocapacitors because of the prospect of achieving the high energy density of traditional battery electrode materials combined with the long cycle life and power density of EDLCs 66 . e mechanism of pseudocapacitance. e most widely investigated pseudocapacitors are RuO 2 and MnO 2 . As the eld has developed, certain metrics have emerged that provide a more rigorous identi cation of pseudocapacitance. In particular, materials exhibiting high energy density at high discharge/charge rates display electro chemical signatures that are representative of pseudocapacitive behaviour. An especially good model system in which to characterize pseudocapacitance is Nb 2 O 5 , as 70% of its theoretical capacity for lithium can be stored in only 1 minute (125 mAh g -1 for 1-minute discharge) [START_REF] Kim | ect of crystallinity on the rapid pseudocapacitive response of Nb 2 O 5[END_REF] . Moreover, charge storage by means of redox reactions takes place in the bulk of the material and not only at the surface as is commonly thought [START_REF] Come | Electrochemical kinetics of nanostructured Nb 2 O 5 electrodes[END_REF] . e charge storage mechanism involved here, intercalation pseudocapacitance, is not controlled by semi-in nite di usion and instead exhibits the fast kinetic response observed in double-layer capacitors (leading to high power) despite the fact that charge storage arises from redox reactions. A comparison between intercalation pseudocapacitance and the redox pseudocapacitance [START_REF] Dmowski | Local atomic structure and conduction mechanism of nanocrystalline hydrous RuO 2 from X-ray scattering[END_REF] that characterizes RuO 2 is shown in Fig. 5. Some of the key electrochemical responses associated with pseudocapacitance include: (i) a linear dependence of the potential on the state of charge; (ii) a charge-storage capacity mostly independent of rate; and (iii) redox peaks with small voltage o sets [START_REF] Augustyn | High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance[END_REF] . Materials that do not show such features should not be considered pseudocapacitive. e recent paper by Brousse et al. points out the limitations in describing materials such as Ni(OH) 2 or cobalt oxides as being pseudocapacitive [START_REF] Brousse | To be or not to be pseudocapacitive?[END_REF] .

Pseudo-intercalation reactions. Another important consideration for pseudocapacitive materials is that the structure does not undergo a phase transformation on intercalation. In this regard, when MoO 2 is prepared as a nanoscale material, its characteristic monoclinic-to-orthorhombic phase transition, which occurs on lithiation, is suppressed and the material shows the pseudocapacitive responses mentioned above. e gravimetric capacity for nanoscale MoO 2 for 2-minute discharge rate is nearly 200 mAh g -1 , far exceeding the energy density of any other material at this rate [START_REF] Kim | e development of pseudocapacitive properties in nanosized-MoO 2[END_REF] . Additional studies show that pseudocapacitive behaviour is preserved even in 40-μm-thick Nb 2 O 5 lms. Such lms have been incorporated in a hybrid cell comprising a Nb 2 O 5 negative electrode and activated carbon as the positive electrode [START_REF] Come | Electrochemical kinetics of nanostructured Nb 2 O 5 electrodes[END_REF] . Although this device showed very good performance, it was limited by the energy density of the carbon electrode, which stores energy by means of the electrical double layer. One of the more popular approaches to developing pseudocapacitor materials involves the synthesis of nanoscale materials [START_REF] Simon | Where do batteries end and supercapacitors begin?[END_REF] . Growing evidence indicates that electrochemical characteristics tend to become more capacitor-like when the crystallite is smaller than 20 nm. For example, the potential dependence on state of charge (see (i) above) can become linear owing to a larger number of surface sites for charge storage or the suppression of a phase transition. Kinetics are also in uenced by the small crystallite size, as the decrease in di usion distance leads to higher power density. In addition, the use of sweep voltammetry to separate di usioncontrolled kinetics from capacitive processes shows nanoscale materials to be more capacitor-like [START_REF] Augustyn | Pseudocapacitive oxide materials for highrate electrochemical energy storage[END_REF] . us far, the charge storage mechanisms for only a few 2D materials prepared in the form of nanosheets have been characterized. In the case of TiO 2 , a surfacecontrolled response reported for this material is expected, as nearly the entire surface of the sample is exposed to electrolyte [START_REF] Augustyn | Lithium-ion storage properties of titanium oxide nanosheets[END_REF] .

To account for the role of nanostructuring, Simon, Dunn and colleagues recently proposed that pseudocapacitance can be intrinsic or extrinsic to a material [START_REF] Come | Electrochemical kinetics of nanostructured Nb 2 O 5 electrodes[END_REF] . An intrinsic pseudocapacitor shows the various electrochemical signatures of double-layer capacitors regardless of particle size or shape: MnO 2 is a good example of intrinsic behaviour [START_REF] Brousse | To be or not to be pseudocapacitive?[END_REF][START_REF] Athouel | Variation of the MnO 2 birnessite structure upon charge/ discharge in an electrochemical supercapacitor electrode in aqueous Na 2 SO 4 electrolyte[END_REF][START_REF] Wei | Manganese oxide-based materials as electrochemical supercapacitor electrodes[END_REF] , although the structural behaviour and charge storage of the material changes from one allotrope to another [START_REF] Ghodbane | Microstructural e ects on chargestorage properties in MnO 2 -based electrochemical supercapacitors[END_REF] , as shown by in situ synchrotron X-ray di raction analyses [START_REF] Ghodbane | In situ crystallographic investigations of charge storage mechanisms in MnO 2 -based electrochemical capacitors[END_REF] . In contrast, the extrinsic term applies to materials that show pseudocapacitor characteristics only as nanoparticles. With these materials, the di usion distance is less than (Dt) 1/2 , where D is the di usion coe cient and t the time. ese materials do not show pseudocapacitive behaviour in the bulk state. e extent to which nanostructuring produces pseudocapacitance is likely to cover a wide range of transition metal oxides, sul des and nitrides, and is currently an active area of research. As shown with MoO 2 , the extrinsic pseudocapacitor approach provides the exciting opportunity of creating materials that possess both the high energy density of battery materials and the high power density of capacitors.

Hybrid supercapacitors

e desire to increase the energy density of EDLCs coupled with the growing interest in pseudocapacitor materials has led to the design of composite hybrid materials [START_REF] Rangom | Carbon nanotube-based supercapacitors with excellent ac line ltering and rate capability via improved interfacial impedance[END_REF][START_REF] Zhu | A carbon quantum dot decorated RuO 2 network: outstanding supercapacitances under ultrafast charge and discharge[END_REF] . e development of these fast charge/discharge materials is providing the basis for creating the next generation of hybrid supercapacitors.

e most advanced hybrid supercapacitors are based on the use of high-rate Li 4 Ti 5 O 12 (LTO), another lithium-ion insertion material.

is material retains a lithium capacity in excess of 120 mAh g -1 at a charge/discharge rate of 30C (full discharge in 2 min). Moreover, in contrast to MnO 2 and RuO 2 which operate in aqueous electrolytes within a limited potential window (about 1 V), LTO functions in highvoltage non-aqueous electrolytes. LTO cannot be de ned as a pseudocapacitive material, as it shows a voltage plateau and undergoes a phase change during lithiation. Because it exhibits zero strain and supports rapid Li + ion transport during the (de)intercalation reaction, however, LTO is suitable for use in high-rate electrodes in hybrid supercapacitors, devices that combine a faradaic battery electrode with a capacitive electrode. e rst generation of commercial hybrid supercapacitors, called the Li-ion capacitor, was based on combining a negative graphite electrode of a Li-ion battery with a capacitive porous-carbon positive electrode [START_REF] Aravindan | Insertion-type electrodes for nonaqueous Li-ion capacitors[END_REF] . e use of ultrafast Li-ion intercalation materials such as LTO could greatly increase the power capability of such devices, while preserving high energy density [START_REF] Naoi | High-rate nano-crystalline Li 4 Ti 5 O 12 attached on carbon nano-bers for hybrid supercapacitors[END_REF] .

In recent work, ultracentrifugation synthesis has been explored to further extend the performance of high-rate and pseudocapacitive materials such as RuO 2 (ref. 83). e ultracentrifugation process enables the preparation of nano-sized, dimension-controlled (1D to 2D) materials, directly bound on high-surface-area carbons such as carbon nanotubes (CNT) or disordered porous carbons [START_REF] Naoi | Second generation 'nanohybrid supercapacitor': evolution of capacitive energy storage devices[END_REF] . Dimension-controlled 2D-LTO is shown in Fig. 6a. High-resolution transmission electron microscopy images clearly demonstrate that 2D-LTO nanosheets nucleate from the CNT surface and are rmly attached to it. Fast Fourier transform analysis revealed the d-spacing of lattice fringes as 4.81 Å, corresponding to the (111) plane of the 3D spinel structure [START_REF] Naoi | New generation 'nanohybrid supercapacitor[END_REF] .

e surrounding interstitial carbon in uences the growth direction, shape and size of the 2D-LTO crystals. One fascinating nanoscale morphology is that which occurs from two interconnected LTO nanosheets (a 'nanobook'; Fig. 6a). ese dimension-controlled LTO show ultrafast charge/discharge behaviour when operated in excess of 300C (full discharge in 12 s) and as high as 1,200C (full discharge in 3 s), as shown in Fig. 6c.

Energy storage devices based on hybrid supercapacitors are being commercialized, and this underscores the importance of these new materials. A three-to vefold increase in energy density compared with conventional carbon-based EDLCs has been achieved by combining a negative electrode made of ultracentrifugation-prepared LTO nanosheets with a porous-carbon positive electrode. Although the stability of the nanostructured electrode/electrolyte interfaces is still an important issue to be solved, this technology has the potential to become an alternative to EDLCs [START_REF] Naoi | Second generation 'nanohybrid supercapacitor': evolution of capacitive energy storage devices[END_REF] .

Outlook

Supercapacitor electrodes, which used to be a black box, can now be probed at the molecular scale using a variety of experimental and simulation techniques. Our view on the structure of the interface has profoundly changed in the past ten years, as summarized in Fig. 7. e discovery of a new storage mechanism inside nanoporous carbons has enabled large improvements in the energy density of commercial EDLC devices, which now reach speci c energies beyond 6-7 Wh kg -1 . ese devices are now used in power electronics, in cars (stop/start systems), and in public transport systems such as trams and buses. e next challenge will be to push this limit further (>10 Wh kg -1 ), thus establishing supercapacitors as ideal candidates in other key applications.

If we consider only porous carbon-based supercapacitors, the possibilities are virtually in nite. On the electrode side of the interface, it is possible to tune the carbon porosity, composition (by activation or using dopants) and microstructure [START_REF] Vatamanu | Capacitive energy storage: current and future challenges[END_REF] . e impact of carbon doping remains to be understood. For example, including nitrogen in the structure enhances the performance thanks to an additional faradaic contribution, so far limited to aqueous electrolytes [START_REF] Lin | Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage[END_REF] . On the electrolyte side, numerous cations, anions and solvent molecules can be used. e development of high-throughput techniques, such as the ones already applied to battery materials and electrolytes [START_REF] Curtarolo | e high-throughput highway to computational materials design[END_REF] , will be a big step forward in that direction [START_REF] Schutter | Toward new solvents for EDLCs: from computational screening to electrochemical validation[END_REF] . But there is still a need for systematically de ning the good descriptors that can be extracted from simulations in order to select a few promising systems among thousands of candidates. Clearly such descriptors can only be designed from the knowledge gained from the combination of all the information extracted from in situ experiments and understanding at the molecular level. Once the relationships among the carbon morphology, the pore size distribution, the size of the ions and the capacitance of the systems are established, the key for enhanced energy densities will depend on surface functionalization [START_REF] Pognon | Performance and stability of electrochemical capacitor based on anthraquinone modi ed activated carbon[END_REF] and on improvement of the operating potential by formulating better electrolytes [START_REF] Abbas | Strategies to improve the performance of carbon/ carbon capacitors in salt aqueous electrolytes[END_REF][START_REF] Pohlmann | Mixtures of azepanium based ionic liquids and propylene carbonate as high voltage electrolytes for supercapacitors[END_REF] .

e potential for improvement is even larger for oxide-based pseudo capacitors. Although recent studies have shed some light on the main features of the various mechanisms involved (for example simple surface redox reactions versus pseudo-intercalation), their molecular aspects remain mostly unknown. ere is, therefore, an explicit need to adapt the in situ experiments and simulations that have been successfully applied to carbon-based supercapacitors for oxide materials. But there are challenges due to the di culty in obtaining well-de ned interfaces both structurally and chemically. Certain oxide supercapacitors operate in aqueous solvents for which acid/base e ects must be taken into account, whereas other oxide systems involve the development of nanostructured, nanocrystalline metal oxide particles operating in non-aqueous electrolyte. Two-dimensional materials such as MoS 2 and the compounds known as MXenes have recently been tested for supercapacitor applications [START_REF] Acerce | Metallic 1T phase MoS 2 nanosheets as supercapacitor electrode materials[END_REF][START_REF] Ghidiu | Conductive two-dimensional titanium carbide 'clay' with high volumetric capacitance[END_REF] . Here also, pseudocapacitive e ects are at play, but it is likely that the microscopic mechanisms derived for oxide materials will not be relevant for these materials. e use of in situ characterization techniques [START_REF] Shi | Structure of nanocrystalline Ti 3 C 2 MXene using atomic pair distribution function[END_REF][START_REF] Levi | Solving the capacitive paradox of 2D MXene using electrochemical quartz-crystal admittance and in situ electronic conductance measurements[END_REF] will be a decisive step towards improving the energy density of these materials.

Fundamental understanding of ion adsorption and charge storage in supercapacitors is essential for applications and technologies. In the area of energy storage, adsorption processes in porous carbons are of central importance for several technologies. One noteworthy example is redox ow batteries [START_REF] Soloveichik | Flow batteries: current status and trends[END_REF] , both aqueous and non-aqueous, which are rapidly progressing towards commercialization because of both centralized and distributed systems that store energy from renewable sources. Biofuel cells, both enzymatic and microbial, represent a wellestablished sustainable technology in which adsorption processes on porous carbon electrodes are critical for achieving the redox processes involved in energy conversion [START_REF] Kwon | High power biofuel cells textiles from woven biscrolled carbon nanotube yarns[END_REF] . Other emerging energy technologies include ow capacitors [START_REF] Presser | e electrochemical ow capacitor: a new concept for rapid energy storage and recovery[END_REF] and 'blue energy' . e latter is based on extracting the energy associated with the mixing of fresh river and salty seawater 100 or on osmotic power harvesting under salinity gradients 101 . In addition, applications of ion adsorption in carbon-based systems extend beyond energy storage to areas such as biosensors 102 as well as gas-phase sensors for detection of toxic gases 103 , while the use of carbon electrodes for desalination represents an increasingly attractive technology for drought-stricken areas. As these examples show, the experimental and theoretical tools developed and the knowledge gained for supercapacitors provide the basis for the development of a wide range of current and future technologies.

Figure 1 |

 1 Figure 1 | Examples of nanoporous carbon structures. a-c, Although ordered structures such as slit pores (a) and nanotubes (b) exist, most supercapacitors use disordered nanoporous carbons (c). The definition of the accessible surface depends on the choice of the probe and on the nature of the porosity. d,e, In mesopores, the whole surface is accessible (d), whereas in micropores (e) some regions of the pores are inaccessible to probe molecules or ions. In e, blue and red spheres represent wall atoms at opposite sides of a micropore, and the blue and red dashed lines represent the closest surfaces to these that are accessible by the probe sphere. Where the dashed lines cross, the pore wall is inaccessible, as indicated by the grey-shaded area in e.

Figure 2 |

 2 Figure 2 | Characterization of pore wetting at null potential. a, NMR spectra with di erent electrolyte loadings inside nanoporous carbon. ppm, parts per million. b, Number of ions adsorbed inside the pores with respect to the electrolyte concentration. c, Neutron scattering experiments on dry or electrolytesaturated activated carbon electrodes. Q, scattering vector; I, intensity; ACF, activated carbon fabric; AN, acetonitrile. d,e, Adsorption of ionic liquids inside a mesopore and a micropore from molecular dynamics simulations (yellow, pore wall atoms; red, cations; blue, anions). Figure reproduced from: a, ref. 25, American Chemical Society; b, ref. 26, NPG; c, ref. 27, American Chemical Society; d,e, ref. 28, American Chemical Society.

cFigure 3 |

 3 Figure 3 | Desolvation of the ions in electrified nanopores. a, EQCM set-up. RE, reference electrode; CE, counter electrode; WE, working electrode. b, Variation of the adsorption of ions with respect to the accumulated charge per unit area in aqueous electrolytes with various alkali cations. Dashed lines represent the theoretical mass change (Faraday's law for neat ions). c, Variation of the mass with respect to the accumulated charge in an organic electrolyte. EMI, 1-ethyl-3-methylimidazolium; TFSI, bis(trifluoromethylsulfonyl)imide anion. Figure reproduced from: b, ref. 39, American Chemical Society; c, ref. 43, American Chemical Society.

Figure 4 |

 4 Figure 4 | Characterization of the charging dynamics of supercapacitors. a, Temporal variation of the normalized infrared absorbance for various charge-discharge cycles (ionic liquid electrolyte). b, Temporal variation of the SAXS intensity (right panel) for various charge-discharge cycles (aqueous CsCl electrolyte). Q, scattering vector. The evolution of the applied potential U with time is shown in the left panel. c, Time evolution of the local charge on the carbon atoms on charging in a molecular dynamics simulation (turquoise sticks, C-C bonds; red spheres, cations; green spheres, anions). Figure reproduced from: a, ref. 60, American Chemical Society; b, ref. 61, RSC; c, ref. 65, American Chemical Society.

Figure 5 |

 5 Figure 5 | Di erent types of pseudocapacitive behaviour. a, Redox pseudocapacitance in RuO 2 occurs in the near-surface region. b, Intercalation pseudocapacitance in Nb 2 O 5 (T-phase) is a bulk e ect from Li + insertion. c, Cyclic voltammetry experiments show the RuO 2 to have the traditional capacitor 'box'. d, Cyclic voltammetry experiments show that the orthorhombic Nb 2 O 5 (T-phase) exhibits redox peaks with little voltage o set. Figure reproduced from: a,b, ref. 66, RSC; c, ref. 69, American Chemical Society; d, ref. 67, Wiley.

Figure 6 |

 6 Figure 6 | Nanohybrid supercapacitor set-up and characterization. a, High-resolution transmission electron microscope images of nanocrystalline Li 4 Ti 5 O 12 prepared as 'nanosheets' and/or 'nanobooks' that are fabricated in composites with CNTs using ultracentrifugation (~75,000g). b, Nanohybrid capacitor (NHC) using an activated carbon (AC) and dimension-controlled Li 4 Ti 5 O 12 nanocrystals composited with single-walled CNTs. c, Discharge curves at various C-rates (from 10 to 1,200) after charging at a fixed rate of 10C. Figure reproduced from: a,c, ref. 85, American Chemical Society; b, ref. 84, RSC.

Figure 7 |

 7 Figure 7 | Evolution of the picture of the electrode/electrolyte interface in the past decade. a, Ions (green spheres) surrounded by solvent molecules (blue spheres) residing in pores with size between 1 and 2 nm (left), and less than 1 nm (right), as pictured in 2006. b, Adsorption of BF 4 -anions (top) and 1-ethyl-3-methylimidazolium cations (bottom) inside carbon nanotubes that are positively or negatively charged, as seen in 2010 (green, B; violet, F; turquoise, C; blue, N; white, H; black sticks, C-C bonds of the nanotubes). c, Adsorption of partially desolvated PF 6 -ion in a disordered nanoporous carbon, as simulated in 2013 (green, PF 6 -; blue, acetonitrile molecules; black, C atoms in contact with the anion; black sticks, C-C bonds of the electrode). Figure reproduced from: a, ref. 3, AAAS; b, ref. 32, American Chemical Society; c, ref. 31, NPG.

Acknowledgements

We thank our many co-workers who have contributed to the work presented in this review. C.P.G. thanks J. Gri n for his critical reading of the manuscript. e research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ ERC grant agreement no. 102539.

Additional information

Reprints and permissions information is available online at www.nature.com/reprints. Correspondence should be addressed to P.S.

Competing interests

e authors declare no competing nancial interests.


Cédric Lopez

Mathieu Roche

Rachel Panckhurst

Non-standard texts: from theoretical positions to Natural Language Processing normalisation

› annotation is not neutral › annotation is linked to interpretative frameworks › researchers should not be trapped › researchers need to conduct own annotation › full corpus (88,000 sms) & samples available for download

  88milsms.huma-num.fr/ Non-standard texts: from theoretical positions to Natural Language Processing normalisation Cédric Lopez* Mathieu Roche** Rachel Panckhurst*** *R&D, Viseo, Grenoble cedric.lopez@viseo.com **UMR TETIS,Cirad, Irstea, AgroParisTech, Montpellier; LIRMM, UMR 5506 CNRS & Université Montpellier mathieu.roche@cirad.fr ***Praxiling UMR 5267 CNRS & Université Paul-Valéry Montpellier 3 rachel.panckhurst@univ-montp3.of detected 'mistakes' › normalisation based on most frequent errors › confrontation with: traditional automatic translation, speech recognition, spelling/grammatical checker principles › comparison between different types of instant media (SMS, forums, tweets) automatic normalisation techniques PLIN Day, Louvain-la-Neuve, 12 May 2016 ARTS, LETTRES, LANGUES, SCIENCES HUMAINES ET SOCIALES MONTPELLIER


Fahad R Golra

Antoine Beugnard

Fabien Dagnat

Sylvain Guerin

Continuous Requirements Engineering using Model Federation

Researchers in software engineering have been striving to produce new methods to improve the quality of development methodologies to consequently produce quality products. Proposition of iterative and evolutionary approaches was triggered by the realization that requirements engineering is not confined to the initial phases of software development only. With this shift of perspective, requirements engineering has become more or less a continuous process in software development lifecycles. We believe that existing requirements engineering approaches and associated tooling leave a room for improvement in putting 'continuity' into practice. A continuous requirements engineering methodology needs to take into account different concerns of all the stakeholders involved in the process. Approaches like KAOS bring the multi-view nature of requirements modeling in focus by using different views for goal, responsibility, object and operation modeling. We argue that a multi-view approach, maintaining a dynamic link between the requirement models and multiple sources of requirements, can offer a better support for continuous requirements engineering methodology. In this paper, we expand on this argument and present the early findings with the associated in progress tooling support.

Introduction

Information systems are increasingly becoming complex, consequently the requirements engineering methodologies need to be more systematic and efficient. In order to deal with these complexities, requirements engineering methodologies exploit the concept of abstraction. Early system requirements specify the objectives of stakeholders at a high level of abstraction, which are then refined to a desired level of detail. Use of goal models for the elicitation and analysis of early requirements has been studied in detail e.g. KAOS [START_REF] Van Lamsweerde | Goal-oriented requirements engineering: a guided tour[END_REF] or Tropos/i* [START_REF] Bresciani | Tropos: An Agent-Oriented Software Development Methodology[END_REF]. These approaches refine the requirements to a fine-grained level and map them to system models. Different approaches map these requirements to different artifacts of the system under development or its environment through traceability links [START_REF] Pinheiro | Requirements Traceability[END_REF]. These used or produced artifacts adhere to different paradigms. For example, models of human behavior, system models, hardware models, etc. do not follow OMG's MetaObject Facility (MOF) paradigm like most other software models. Furthermore, requirements engineering is a complex process dealing with different views for various stakeholders. For example KAOS specifications are presented using four models i.e. goal, responsibility, object and operation models [START_REF] Van Lamsweerde | Goal-oriented requirements engineering: a guided tour[END_REF]. This calls for a requirements modeling approach that can handle different paradigms and multiple views of a requirements management system.

The development of an embedded system like train door operations is a good example where requirements need to take hardware-driven software synthesis into account. Apart from hardware models like electric circuits, sensor models, etc., requirements need to deal with other models like system of systems model, human behavior models, etc. Another important concern for software development is to manage the development costs of a product. Different approaches try to relate implementation costs to requirements, quite early in the software development process. This might be done, for example to prioritize requirements according to a costvalue approach [START_REF] Karlsson | A cost-value approach for prioritizing requirements[END_REF]. For such techniques, they need to have continuous synchronization between the requirements and a cost analysis document (e.g. an excel spreadsheet). Contrary to the models usually developed by software engineers, these models do not conform to MOF and thus it is hard to map their features to the requirement models used for software development. Many other examples (e.g. planning, configuration management, etc.) exist where software development teams come across the need of a multi paradigm and multi-view requirements modeling methodology that can continuously synchronize these different models.

Requirements engineering is not an initial phase of system development anymore, it continues all along the development lifecycle. So requirement models need to be connected to different artifacts, even if they are produced late in the development lifecycle to allow their continuous improvement. In this article we propose an approach that links together models from different paradigms to produce new views of the system. These dynamic links between models are maintained, so that requirements remain connected to these models (information resources). Whenever an information resource updates, the requirements of the system can be updated (manually or automatically) accordingly. We call this continuous requirements engineering, because our methodology enables the continuous update of requirements using all the linked models throughout the system development lifecycle. The contribution of this paper is the use of model federation as a technical basis for requirements management. Model federation is a methodology that allows connecting models of different paradigms [START_REF] Golra | Addressing modularity for heterogeneous multi-model systems using model federation[END_REF]. In the context of requirements engineering, it furnishes the possibility to create and maintain dynamic links between the requirement models and other artifacts (models of varying paradigms), used all along the development process.

The rest of this paper is organized as follows. First, we present model federation in Section 2. Then, in Section 3, we explain the proposed approach for requirements engineering using model federation. Then, Section 4 discusses the literature review for different approaches that are used for linking requirements to the information resources and the system. Finally, we conclude this paper in Section 5.

Model Federation

Requirements engineering serves as an interface between a system and its environment. It focuses on gathering the information (domain models) from the environment by collecting the requirements for the system to be developed. In such a scenario, the artifacts and processes of requirements engineering are tightly related to both the system under development and its environment. In the context of requirements engineering, sharing information between these processes and artifacts is handled through different activities, ranging from manual (e.g. feasibility study, requirements elicitation techniques, etc.) to semi-automatic (e.g. requirements validation, requirements management, etc.). These artifacts and other information resources used or produced by these activities often share cross-cutting concerns. These cross-cutting concerns are captured through traceability links between them, which allow to traverse from initial phase artifacts to later ones (forward traceability) or from later phase artifacts to initial ones (backward traceability) [START_REF] Pinheiro | Requirements Traceability[END_REF]. In this article we propose to use model federation to connect these artifacts and information resources to allow continuous and uninterrupted requirements engineering process that offers the technical basis to implement constant improvement methodologies. But before going into the details of our proposal, we describe the main concepts of model federation.

Model federation is an approach that provides the means to integrate multiple models conforming to different paradigms [START_REF] Golra | Addressing modularity for heterogeneous multi-model systems using model federation[END_REF]. A lot of work has been carried out to integrate models from the same paradigm to get a different views for each stakeholder. This can be accomplished through approaches like model merging, profiling or extension mechanisms provided by UML [START_REF] Selic | A Systematic Approach to Domain-Specific Language Design Using UML[END_REF], model weaving with KerMeta [START_REF] Jézéquel | Mashup of metalanguages and its implementation in the Kermeta language workbench[END_REF], or other model composition approaches. However, it becomes difficult, when models are conforming to different paradigms [START_REF] Hardebolle | Exploring Multi-Paradigm Modeling Techniques[END_REF]. Model federation approach is developed as a possible response to a request for proposal (RFP) by OMG [START_REF] Omg | Semantic Information Modeling for Federation (SIMF) Request For Proposal[END_REF]. This RFP requests submissions for a standard addressing, "federation of information across different representations, levels of abstraction, communities, organizations, viewpoints, and authorities". Thus model federation allows the integration of heterogeneous models to develop new cross-concern viewpoints/models or to synchronize the models used for designing a system. Contrary to other techniques that transform, merge or compose the models in a common paradigm, we propose to keep models in their respective paradigms to avoid redundancies. Hence, we introduce multiple modeling spaces i.e. conceptual, technological and design space, as shown in Figure 1.

The different modeling spaces of our approach serve together for the development of a complex system. The conceptual space is where new models or views are developed by federating the concepts from already existing models. These federated models are called virtual models. As virtual models reuse the concepts from various models conforming to different paradigms, the conceptual space is surrounded by multiple technological spaces. Each of these technological spaces is a collection of models adhering to a common paradigm which gives a common ground for their interpretation. Usually, the technological models are already developed to serve some specific concerns. They might even belong to some different system. Finally, a design space is a specific kind of technological space that serves for diagrammatic representations of the virtual models, using the same interaction mechanism.

Model federation is realized through virtual models to develop new concepts. We have developed a generic modeling language to define virtual models in the conceptual space. They are developed using features that can either be specifically defined for virtual models or by reusing elements of models from any of the existing technological spaces. Each of these features, serving as a building block for a virtual model, is called a flexo concept. A virtual model is responsible for managing the lifecycle of all the flexo concepts that it contains. While virtual models follow the formalisms defined by our methodology, technological models follow their own paradigm depending on their specific technological space. A virtual model can not access the elements of a technological model, unless it can interpret the formalisms used in its technical space. This is done using a connection between the technological space and the conceptual space, realized as technological connectors. These connectors allow access for reading, writing and synchronizing the information between the virtual models and the technological models. Once developed, a virtual model can be serialized back into a new or existing technological space for further development.

A virtual model is composed of flexo concepts. Some of these flexo concepts are defined specifically for the development of a virtual model, we call them local flexo concepts. Virtual models can also use certain elements from connected technological models. One way is to create a dynamic link between these local flexo concepts and the modeling elements of technological models. Virtual model A shown in Figure 1 uses two dynamic links; one to the model in technological space A and the other to the one in technological space B. The second way of using model elements from technological spaces is to translate them into flexo concepts. A remote flexo concept of a virtual model is a translated modeling element from a linked technological space, serving as a 'local proxy'. Virtual model B shown in Figure 1 contains a remote flexo concept, which is a local 'proxy' of a model element from technological space B. In both these techniques, the link between the flexo concept and the technological model is bi-directional and is maintained to synchronize any changes. A virtual model in the conceptual space can be updated when a corresponding technological model evolves and conversely, a technological model can also be updated, once the corresponding virtual model is modified. As the information is shared between multiple models, it might be accessed and modified by multiple stakeholders at the same time. We do not enforce any specific consistency model in this case, we rather give this flexibility to the designers to implement any consistency model that suits their specific application. We provide a notification system that notifies the designer/stakeholder whenever a corresponding model is updated. One can choose for automatic synchronization of models, but then human intervention would be needed for resolving conflicts. Design space is a specific technological space provided by our methodology for diagrammatic representations of the virtual models. A technological connector between conceptual space and design space allows linking the flexo concepts to their graphic representations. These bi-directional links are maintained, so a virtual model can be edited either from design space or the conceptual space.

The technological connectors allow models in conceptual space to access the models of technological spaces, however the actual dynamic link between these models is realized using model slots. Our approach of model federation can be explained through the analogy of components based design, where models serve as components, model slots as component interfaces and technological connectors as connectors. Technological connectors allow the creation of a dynamic link between a virtual model and a technological model using model slots. A model slot defines an access point both for a virtual model and a technological model to link them together. It exposes a view on the structural and behavioral contents of the technological model to the virtual model and vice-versa. Once linked to a technological model, a virtual model can read/write to its attributes using roles and execute the actions using edition actions. Roles and editionActions are to a flexo concept, what attributes and methods are to a class, except that roles and editionActions are associated to attributes and methods of a model adhering to an entirely different paradigm.

Taking the example of the requirements specification model for an embedded system, as explained in the introduction section, we can connect it to cost analysis spreadsheet 1 to prioritize requirements according to a cost-value approach. In order to realize a cost-value requirements priority model, we need to develop it as a virtual model in the conceptual space. The conceptual space needs to be connected to two technological spaces i.e. MS Excel space for cost analysis spreadsheet and MS Word space for requirements specification document 2 . Both these technological connectors are already available through our tooling support. Using these connectors, we can develop our virtual model with two flexo concepts i.e. Requirement and Priority. Requirement can be implemented as a remote flexo concept, thus making a 'proxy' for each requirements in the virtual model that is dynamically linked to the requirements of the specification document. Priority concept can directly be linked to the cost analysis document to assign a priority value to each requirement. Model slots for these links need to be developed by the modeler to connect these concepts. Even though it is not a good idea to update cost analysis document from this model, for the purpose of explanation we assume that the designer has the possibility to update/modify the cost of each requirement in the cost analysis document from the virtual model, using the edition action setCellValue(). Furthermore, any other common piece of information can be synchronized among costvalue requirements priority model, requirements specification document and cost analysis document. The cost-value requirements priority model can be developed using a graphical editor of design space, hence editing/synchronizing the flexo concepts in the virtual model.

Requirements engineering through model federation

Each stakeholders involved in the requirements engineering process can have a different view about the system. Each 1. .xlsx files are XML based spreadsheets. We consider all artifacts in software development as models.

2. Different tools like IBM Rational RequisitePro, CaseComplete, Visure requirements etc. share requirements based on MS Word format. view caters a subset of concerns related to the complete software development process. Many of these views are created as different models like requirements specification document, goal models, system models, etc. Requirement model itself has conceptual links (at times expressed as traceability links) to different artifacts produced during the development process (like feasibility reports, minutes of the meetings, design models, code, etc.) and to other information resources like organizational norms, quality standards, government policies, etc. Different artifacts and processes used or produced during the software development lifecycle reinforce the need of a multi-view approach for requirements engineering. Popular requirements engineering approaches, like KAOS, are presented through multiple views i.e. goal, responsibility, object and operation models [START_REF] Van Lamsweerde | Goal-oriented requirements engineering: a guided tour[END_REF].

As already said, we propose to use model federation as an approach for modeling the requirements, where multiple views/models of the system might not adhere to a single paradigm. In case of KAOS models, their metamodel is defined using MOF. However, other domain models used in software development may not belong to the same paradigm e.g. legacy systems, databases, reports, spreadsheets and other specific domain models like building architecture models, electric circuit diagrams, CAD models, etc. Many of these models might be needed as information resources to update software requirements. A multi-paradigm modeling approach becomes necessary to cater these needs. Model federation approach, described in Section 2, offers the possibility to integrate models from different paradigms and to create new concepts using virtual models that can use information from them.

Apart from linking the concepts of different models, model federation allows to maintain those dynamic links. This means that all these models used in the requirements engineering process remain connected to the requirements model, hence updating of the specified requirements as soon as the information resources are modified. Model federation is a modular approach that treats each model as a 'component' and connects to them using specified interfaces, the model slots. This allows adding new models to the system without the need of re-designing the complete system. This modular approach permits connecting more information resources to the requirement specification model even during late stages of software development. So artifacts developed at later phases of the process or in future iterations in case of agile development, can easily be linked with existing requirements. This support for linking new information resources to the requirements and maintaining them for synchronization allows a continuous requirements engineering process all along the software development lifecycle, regardless of its phase and iteration.

Apart from using model federation for continuous elicitation of requirements through synchronization of information resources, we propose it for developing new views by federating different models that might not follow the same paradigm. As a proof of concept we have federated the goal-oriented requirements engineering model, KAOS in our methodology. However, we are looking forward to integrate other methodologies in different technological spaces, so that a requirements engineer can use multiple approaches for the requirements engineering process. This is implemented through an abstract model of requirements engineering, Formose virtual model in the conceptual space, as shown in Figure 2. This virtual model is made up of two concepts; element refers to the decomposition of project elements and requirement, which are collected and refined for each of those elements. Note that for us, a project element does not mean an element of the system to be developed only, but it can also be a part of the development process, context of the process or the context of the software system. In the current implementation, both Element and Requirement are refined using the KAOS methodology, whose model is available in the technological space KAOS.

For the elicitation of requirements from multiple models, those models can be connected through their respective technological spaces. Technological space MS Word allows to connect the Requirements virtual model (VM) to different word documents like feasibility reports, project proposals, minutes of the meetings, etc. for requirements elicitation. Requirements VM allows the identification of the fragments from the model (word file in this case) that can serve as a source for requirements. These fragments are used to identify the requirements of the system under development. These identified requirements are mapped to the Formose VM for their specification. They are further linked to Goal Virtual Model (VM) for the development of goal oriented requirements model. Finally this goal model is mapped to the design space for its diagrammatic representation. Once all these models are dynamically linked with each other for carrying out the requirements engineering process, they are synchronized between themselves. Resultantly, we are able to modify the requirements model by 1) adding new models to the technological spaces and identifying new requirements from them, 2) modifying the models in the technological spaces and updating the requirements across the system (which In order to understand how model federation can be used in the context of requirements engineering, we need to understand how it changes the requirements engineering methodology. Figure 3 explains the activities involved in the proposed methodology. It needs to be noted here that this is a work in progress, so we have not yet covered requirements validation activities in this process. However, adding a technological space for formal validation of requirements is one of the future goals of our methodology (see Technological Space B Method in Figure 2). For now, our methodology is explained through the following activities:

1) Gather information resources: This activity involves the identification of possible information resources that can be used to elicit requirements. They can vary from early information resources like feasibility reports, standards and minutes of the meetings to late informations sources like deployment models, test plans, etc. 2) Federate information resources: Once the information resources are collected they need to be linked with the requirements. This is handled through the model federation approach where requirement models are developed as virtual models in the conceptual space and other models (information resources) are placed in their respective technological spaces. 3) Identify/Elicit requirements: Once the information resources are linked with the requirements model, we can identify the requirements from those information resources. For the moment, we have used the technological space for Microsoft word documents to identify multiple fragments for possible requirements. Different documents are used for this purpose like project proposals, interview transcripts, feasibility reports, etc. 4) Specify requirements: Once the requirements are identified from multiple information resources, they are specified in the requirements model. In case the requirements are already specified, they are linked with the information resources for synchronization. 5) Map requirements to project elements: In this activity, the specified requirements are mapped to the project elements. Multiple requirements can be mapped to a single project element. 6) Decompose project elements: Multiple requirements mapped to a project element describe the expected functionality from that element. This allows the decomposition of the element into sub elements, such that each sub element takes care of a subset of the requirements mapped to its parent element. 7) Define goals for project elements from requirements: All the requirements associated with a project element are considered as goals in this activity. A goal model is developed for each of these goals. 8) Refine goals to requirements: All the goal models are refined to get requirements at the leaves of the goal models. These requirements can be specified in activity 4. These activities are performed according to the control flow described by Figure 3. This is an iterative flow of activities where we see two sub-cycles in the complete iteration; one corresponds to the decomposition of project elements and the other to the refinement of requirements. A single activity, Map requirements to project elements, serves as the entry point for both these cycles. These cycles are often alternating, which describes that the system decomposition occurs in parallel to the refinement of requirements.

Tooling support for our methodology is provided through our open source project initiative, Openflexo [START_REF] Mader | Getting back to basics: Promoting the use of a traceability information model in practice[END_REF]. This tool has been developed as a generic collaborative platform for multifaceted modeling using model federation. We have developed technological connectors for different technological spaces like EMF, OWL ontologies, XML/XSD, pdf files, MS office documents for Word, Excel, PowerPoint, etc. These technological connectors allow the use of different models as information resources and can be federated to the requirements model for our continuous requirements engineering approach.

IV. RELATED WORKS

In an empirical study conducted on traceability, Ramesh and Jarke suggest that traceability links should be strongly typed in order to avoid semantic misinterpretations [START_REF] Dag | A linguistic-engineering approach to large-scale requirements management[END_REF]. They also proposed a traceability meta-model and reference model in this context. There work is focused on the rationale and definition Figure 3. Requirements engineering process with model federation is the case of requirements evolution in later stages of the software development lifecycle), 3) modifying the goal oriented requirements model through its diagram in the design space, 4) modifying the requirements directly from the virtual models. In all these cases, the modifications are reflected in other federated models.

In order to understand how model federation can be used in the context of requirements engineering, we need to understand how it changes the requirements engineering methodology. Figure 3 explains the activities involved in the proposed methodology. It needs to be noted here that this is a work in progress, so we have not yet covered requirements validation activities in this process. However, adding a technological space for formal validation of requirements is one of the future goals of our methodology (see Technological Space B Method in Figure 2). For now, our methodology is explained through the following activities:

1) Gather information resources: This activity involves the identification of possible information resources that can be used to elicit requirements. They can vary from early information resources like feasibility reports, standards and minutes of the meetings to late informations sources like deployment models, test plans, etc. 2) Federate information resources: Once the information resources are collected they need to be linked with the requirements. This is handled through the model federation approach where requirement models are developed as virtual models in the conceptual space and other models (information resources) are placed in their respective technological spaces. 3) Identify/Elicit requirements: Once the information resources are linked with the requirements model, we can identify the requirements from those information resources. For the moment, we have used the technological space for MS Word documents to identify multiple fragments for possible requirements. Different documents are used for this purpose like project proposals, interview transcripts, feasibility reports, etc.

4) Specify requirements: Once the requirements are identified from multiple information resources, they are specified in the requirements model. In case the requirements are already specified, they are linked with the information resources for synchronization. 5) Map requirements to project elements: In this activity, the specified requirements are mapped to the project elements. Multiple requirements can be mapped to a single project element. 6) Decompose project elements: Multiple requirements mapped to a project element describe the expected functionality from that element. This allows the decomposition of the element into sub elements, such that each sub element takes care of a subset of the requirements mapped to its parent element. 7) Define goals for project elements from requirements: All the requirements associated with a project element are considered as goals in this activity. A goal model is developed for each of these goals. 8) Refine goals to requirements: All the goal models are refined to get requirements at the leaves of the goal models. These requirements can be specified in activity 4. These activities are performed according to the control flow described by Figure 3. This is an iterative flow of activities where we see two sub-cycles in the complete iteration; one corresponds to the decomposition of project elements and the other to the refinement of requirements. A single activity, Map requirements to project elements, serves as the entry point for both these cycles. These cycles are often alternating, which describes that the system decomposition occurs in parallel to the refinement of requirements.

Tooling support for our methodology is provided through our open source project initiative, Openflexo [START_REF] Openflexo | Openflexo project[END_REF]. This tool has been developed as a generic collaborative platform for multifaceted modeling using model federation. We have developed technological connectors for different technological spaces like EMF, OWL ontologies, XML/XSD, pdf files, MS office documents for Word, Excel, PowerPoint, etc. These technological connectors allow the use of different models as information resources and can be federated to the requirements model for our continuous requirements engineering approach.

Related Works

In an empirical study conducted on traceability, Ramesh and Jarke suggest that traceability links should be strongly typed in order to avoid semantic misinterpretations [START_REF] Ramesh | Toward reference models for requirements traceability[END_REF]. They also proposed a traceability meta-model and reference model in this context. There work is focused on the rationale and definition of conceptual base for the development of traceability links. Mader et al. advocate the use of traceability information model as a necessary condition to employ traceability [START_REF] Mader | Getting back to basics: Promoting the use of a traceability information model in practice[END_REF]. Dag et al. propose to link requirements to domain models using linguistic engineering approach [START_REF] Dag | A linguistic-engineering approach to large-scale requirements management[END_REF]. On a conceptual level, all these techniques work well to identify which requirements need to be linked with customer wishes (domain problems), however they don't focus on the technical aspects of how these links should be created and maintained. As a complementary approach on the technical level, we focus on how these links will be maintained and what happens when the customer wishes evolve over time. Apart from customer wishes, we focus on linking requirements to many other models from the domain and from the system itself.

Most of these techniques use bi-directional model transformations for creating and maintaining such links. However, these techniques are confined to the MOF paradigm, specified by OMG. Other techniques like KerMeta using aspect weaving for multi-paradigm modeling [START_REF] Jézéquel | Mashup of metalanguages and its implementation in the Kermeta language workbench[END_REF] pose the same problems of conformity to MOF. Most of the existing techniques that allow the development of new views from existing models in the context of multiple paradigms are either bound to MOF or are not modular enough to be used in requirements engineering where new information resources can be linked to the requirements model even late in system development [START_REF] Hardebolle | Exploring Multi-Paradigm Modeling Techniques[END_REF]. Model federation is a modular approach that is not confined to MOF for linking different kinds of information resources to the requirements models. As a perspective of this RE Next paper, we are planning to add a technological space for B method that will serve for requirements verification. We will also add technological spaces other than KAOS, so that multiple approaches can be used coherently for requirements engineering in complex system development. Lastly, some industrial use cases are under study to practically validate our claims.

Conclusion

Synchronizing the information between different information resources in requirements engineering poses a hurdle for timely update of requirements. Considering all these information resources as models, we have proposed an approach of model federation that allows information sharing by maintaining a dynamic link between different models. Model federation favors the use of different views of each stakeholder that can be developed from modeling elements of different models adhering to different paradigms. Development and maintenance of multiple models in requirements engineering through model federation impacts the requirements engineering methodology as well. This methodology focuses on continuous requirements engineering activities that are carried out all along the software development lifecycle. For the moment, we have integrated goal oriented requirements engineering approach, KAOS with our core requirements model. But this work in progress is heading towards the integration of technological spaces for other requirements modeling approaches and formal verification, in the context of safety and security.

Figure 1 .

 1 Figure 1. Mapping between concepts in modeling spaces

Figure 2 .

 2 Figure 2. Model Federation for Requirements Engineering

Acknowledgment

We are thankful to French national agency, Agence Nationale de la Recherche (ANR), for funding this research under FORMOSE project ANR-14-CE28-0009.