Exploring electrolyte organization in · presence of signals with ... water on activated ... coconut shell and

Supporting Information Supercapacitors consist of two activated carbon-‐based electrodes (coatings on aluminium current collectors) soaked with an electrolyte (1M tetraethylammonium tetrafluoroborate in acetonitrile -‐ TEABF4 in ACN) and electrically isolated by a porous separator placed between the electrodes (ex situ 11B NMR shows that it contains 10-‐20% of the TEABF4 electrolyte). The electrodes prepared from the two carbon materials (carbon A and B, SI: part A) have comparable gas adsorption properties (Table S1), however, a slightly higher capacitance value is displayed by carbon B (111 F/g against 95 F/g for carbon A). Previous works using neutron diffraction1 have found that carbons produced at temperatures ranging between 800° and 1200°C are made of graphene sheets, which are more or less curved (SI: part B, Figure S 2), and adsorption of various molecules and ions can occur between these graphene sheets. The presence of signals with larger chemical shifts in the 13C MAS NMR spectra and the ratio of the D and G bands in the Raman spectra of these two carbons indicate that carbon B (from KOH activated coke) is more structurally disordered, probably with a larger amount of defects creating curvature in the graphene sheets (SI: part B, Figure S 3). In the NMR magnet, the graphene-‐like sheets create ring currents and a diamagnetic shift of -‐6 ppm is observed for molecules between graphene layers. For static samples, the anisotropy of this shift, if any, may broaden the NMR peaks of the adsorbed molecules and lead to overlapping of ACN and TEA+ resonances (see SI, Part C, Figure S 4, for a comparison between the 1H and 13C spectra of ACN in the electrolyte solution and neat ACN adsorbed between graphene layers). 2 Moreover, the dipolar couplings with neighbouring spins broaden the NMR peaks (see the static and MAS spectra from Figure S 4, part C from SI). MAS removes these effects. However, the spinning speed has been limited to 5 kHz to avoid separating the liquid and solid phases by centrifugation:3 the comparison of MAS NMR spectra at 5 kHz with static spectra shows no MAS-‐induced desorption of previously

1 Petkov, V., Difrancesco, R.G., Billinge, S.J.L., Acharya, M. & Foley, H. C. Local structure of nanoporous carbons. Phil. Mag. B79, 1519-‐1530 (1999). 2 Kibalchenko, M., Payne, M.C., Yates, J.R. Magnetic Response of Single-‐Walled Carbon Nanotubes Induced by an External Magnetic Field. ACS Nano 5, 537-‐545 (2011). 3 Dickinson, L.M., Harris, R.K., Shaw, J.A., Chinn, M. & Norman, P.R. Oxygen-‐17 and deuterium NMR investigation into the adsorption of water on activated carbon. Magn. Reson. Chem. 38, 918-‐924 (2000).

adsorbed ACN (Figure S 4, part C, SI). The resolution in 13C NMR spectra is further enhanced with 1H decoupling (for the J-‐coupling), and the resulting spectra confirm the absence of skin depth effects resulting from carbon black conductivity additive in our samples, as the 1H decoupling RF irradiation (with half the penetration depth of the 13C RF) manages to decouple the whole sample (SI: part D, Figure S 5). Fortunately, the presence of paramagnetic centres in the carbon material, as observed by EPR on most activated carbons, 4 , 5 considerably accelerates the 13C and 11B longitudinal relaxation (T1 < 8 s), thereby shortening the magnetization recovery delay to several seconds. This allowed us to record natural abundance (1%) direct detection 13C NMR spectra of the electrolyte inside the nanoporous carbon in less than two hours, and the 11B (S=3/2 and 80.42% abundant) NMR spectra in 5 minutes. Of course, such experimental times do not allow capturing the state of the electrolyte immediately after charging/discharging the supercapacitor. The self-‐discharge of our supercapacitors is less than 10% after 24 hours and is measured as follows: the system is maintained at maximum voltage (potentiostatic mode) during 30 minutes. The self-‐discharge is the subsequent voltage drop, which occurs at open circuit. In potentiostatic mode the current continuously decreases to reach a very low value after 30 minutes. Therefore, the voltage drop, which corresponds to relaxation after opening the circuit, is much smaller than the drop observed in a typical galvanostatic charge/discharge experiment. In order to avoid unpreventable ACN evaporation from the wet carbon powders inside the MAS rotor, the samples have been mildly dried under N2 flow of 100mL/min. After drying, they showed exceptional stability with no change in the NMR spectra over days. To further check the effect of drying, the 13C static and slow MAS (1 kHz) NMR spectra of a slurry (no pumping and no charge/discharge cycling) made with carbon A powder and the 1M TEABF4 solution have been recorded separately. No electrolyte enters into the carbon porosity as a result of the drying process; as no shift or no additional peak shifted by -‐6 ppm appear in the spectra at various stage of the ACN evaporation (Figure S 6. part E, SI). We also recorded the 13C static and MAS-‐NMR spectra of a cycled supercapacitor electrode, after a charge-‐discharge cycle with and without drying it for 15 4 Jackson, C. & Wynne-‐Jones, W.F.K. Electron spin resonance in carbonized organic polymers—I. Carbon 2, 227-‐252 (1964). 5 Ottaviani, M.F. & Mazzeo, R. EPR characterization of graphitized and activated micro-‐ and meso-‐porous carbons. Micropor. Mesopor. Mater. 141, 61-‐68 (2011).

Nature Materials – Supplementary Information – DOI:/ ??? Exploring electrolyte organization in supercapacitor electrodes with solid-‐state NMR, M.Deschamps, E.Gilbert, P.Azais, E.Raymundo-‐Pinero, M.Ramzi Ammar, P.Simon, D.Massiot, F.Béguin

Exploring electrolyte organization insupercapacitor electrodes with solid-state NMR

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minutes under N2 flow. Drying the samples only removes some of the ACN molecules, while no significant perturbation is detected for the TEA+ NMR signals (Figure S 7, part E, SI). Hence, the drying stage is expected to remove the solvent molecules which are not adsorbed between graphene layers, as the ACN molecules detected in the carbon A powder soaked with only ACN and dried for 15 minutes, Figure S 4) or which do not belong to the solvation shell of the electrolyte ions. In figure 1, the free ACN (no shift) is outside the space between graphene layers, and we can reasonably consider that it should have been removed by the drying stage (as in Figure S 4) if it would not solvate the free TEA+ and BF4-‐ ions. A) Electrolyte and carbon electrodes TEABF4 electrolyte

Figure S 1. a) 3D representation of the tetraethylammonium (TEA+) cation –top-‐, tetrafluoroborate (BF4-‐) anion –middle-‐ and of the acetonitrile (ACN) solvent molecule. The TEA+ cation (b) and BF4-‐ anion (c) are expected to experience a diamagnetic shift in their NMR spectra while inside the space between two graphene-‐like layers, which are identified as adsorption sites. The graphene-‐like layers are made here of portions of C60 molecules in order to illustrate the relative sizes of each entity. The carbon electrodes Carbon A was prepared by carbonisation of coconut shell and subsequent steam activation at 1000 °C plus a treatment under nitrogen at 700 °C. Carbon B is a coke chemically activated by KOH between 700 °C and 800 °C and repeatedly washed with water before being further treated under nitrogen at 600 °C. The electrodes provided by Batscap (France) were made from the activated carbons A or B, a conductive additive (10 wt%) and a binder (polyvinylidene di-‐fluoride, PVdF, 10 wt%), and obtained by stretching under heating on a 30 µm thick aluminum foil. After calendering, the total thickness and loading of the electrodes were around 130 µm and 6.5 mg.cm-‐2, respectively.

B) The nanoporous carbon materials: models and characterization The nanotexture of the fresh electrodes (Table S1) was characterized using N2 adsorption at -‐196 °C (AUTOSORB-‐1, Quantachrome). The samples were outgassed 12 h at 200°C before the analysis. The specific surface area SBET was determined from the nitrogen adsorption isotherms using the BET (Brunauer – Emett -‐ Teller) method. The volumes of micropores and pores in the range from 1.2 nm to 50 nm were calculated by applying the NLDFT (Non-‐Local Density Functional Theory) method to the N2 adsorption data. The CO2 adsorption measurements carried out at 0 °C were used to calculate the ultramicropore volume by applying the DR equation (Dubinin-‐Radushkevich). Table S1 Properties of the electrodes prepared with carbons A and B Carbon A Carbon B SBET(N2) (m²/g) 1071 1191 Vtotal (cm3/g) 0.72 0.70 Vultramicroa (d<0.7 nm) (cm3/g)

0.51 0.40

Vmicrob (d<2 nm) (cm3/g) 0.42 0.40 V b (1.2<d<50 nm) (cm3/g)

0.37 0.45

Electrode capacitance (F/g)

95 111

Carbon capacitance (F/g) 119 138 a: obtained by applying the Dubinin-‐Radushkevich equation to the CO2 adsorption data b: obtained by applying the NLDFT method to the N2 adsorption data Several nanotexture models have been proposed for nanoporous carbon materials in the literature. Three models are shown below in Figure S 2. The structural organization of carbons A and B was characterized by 13C MAS-‐NMR and Raman micro-‐spectroscopy (Figure S 3). The 13C NMR spectra show that the signal intensity for chemical shifts higher than 120 ppm is larger for carbon B. Recent studies indicate that larger chemical shifts indicate smaller CCC angles in conjugated carbon systems, and therefore the curvature of the surface graphene-‐like in carbon B is expected to be larger, resulting from the presence of 5 or 7 members rings, whereas the signal of 13C around 120 ppm, similar to the graphite peak, corresponds to graphene layers with a more “planar” character.6

6 Deschamps, M., Cadars, S., Gilbert, E., Azaïs, P., Raymundo-‐Pinero, E., Béguin, F., Massiot, D. A solid-‐state NMR study of C70: a model molecule for amorphous carbons. Solid State NMR 42, 81-‐86 (2012).

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Figure S 2. Models of nanotexture of amorphous porous carbon materials (Reprinted from top: reference [7 ], with permission of Taylor & Francis Ltd, bottom left: reference [8], Copyright 2005, with permission from Elsevier, and bottom right: reference [9], with permission of Taylor & Francis Ltd).

Figure S 3. A: 13C solid-‐state NMR spectrum of carbon A (blue) and B (green), using a 30° excitation pulse, 14 kHz MAS and a “short” relaxation delay of 1s (to select the fast relaxing component of the amorphous carbon and remove the contribution of the PVDF binder). B: Raman spectroscopy of carbon A (blue) and carbon B (green), with excitation laser line 7 Harris, P.J.F. New Perspectives on the Structure of Graphitic Carbons. Critical Reviews in Solid State and Materials Sciences 30, 235 (2005). 8 Kumar, A., Lobo, R.F. & Wagner, N.J. Porous amorphous carbon models from periodic Gaussian chains of amorphous polymers. Carbon 43, 3099-‐3111 (2005). 9 Acharya, M., Michael S. Strano, Jonathan P. Mathews, Simon J. L. Billinge, Valeri Petkov, Shekhar Subramoney & Henry C. Foley. Simulations of nanoporous carbons: A chemically constrained structure. Phil. Mag. B79, 1499-‐1518 (1999).

at 514.5 nm. Raman spectra were obtained on a Renishaw Invia Reflex spectrometer (one 1800 grooves/mm grating, Edge filter), with a Leica DM2500 microscope (x50 objective). Very low incident power (1-‐2 mW) was used to avoid local heating effects. The excitation laser line was the 514.5 nm of an Ar+ laser. Acquisition times are 100s. Figure S 3 also shows the first order Raman spectra characterized by two prominent features at about 1580 cm-‐1 and 1350 cm-‐1. The 1580 cm-‐1 mode (G band) corresponds to the Raman-‐allowed E2g mode (in-‐plane C-‐C stretching of an infinite crystal), and the 1350 cm-‐1 mode (D band) corresponds to the disorder-‐induced line.10 It is well known that the increase of the G and D bands width as well as the decrease of the intensity ratios I(D)/I(G) indicate the decrease of the organization degree of the carbon material. Thus, based on these two parameters, carbon A appears to be more structurally organized or less amorphous than carbon B, confirming what is observed with 13C NMR. C) Possible MAS induced effects The carbon A powder taken from supercapacitor electrodes (see the methods section for the preparation protocol), which have never been charged, has been soaked with ACN and submitted to gentle drying under N2 flow at ambient temperature (see the Methods section). The 1H and 13C static NMR spectra of ACN in carbon A only show one methyl peak for 1H and two resonances for 13C -‐CH3CN-‐, one singlet peak for the CN group and one quartet for the CH3 group Only ACN molecules adsorbed between two graphene layers 11 remain in the porous carbon after gentle drying. This is confirmed by our NMR spectra, in which the NMR peaks are shifted by -‐6 ppm compared to the spectrum of acetonitrile alone, due to the ring currents from the graphene layers in between which they are adsorbed. Spinning the sample up to 5 kHz results in substantial line narrowing, but it did not bring about any change in the NMR spectra, and no signal of desorbed species (i.e. not shifted by -‐6 ppm) resulting from MAS induced centrifugation were ever detected. Therefore we expect that MAS will not induce any desorption of the electrolyte molecules, and sample heating from MAS will be minimal with such spinning speeds. Moreover, one can compare the centrifugation forces per molecule (mω2R = 4.3.10-‐16 N for TEA+ on the rotor wall at 5 kHz MAS) and the much stronger electrostatic forces between TEA+ and BF4-‐ ions

10 Ferrari, A.C., Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B. 61, 14095-‐14107 (2000). 11 Lastoskie, C., Gubbins, K.E. & Quirke, N. Pore size distribution analysis of microporous carbons: a density functional theory approach. J. Phys. Chem. 97, 4786-‐4796 (1993).





1 nm apart (e2/4πε0r2 = 2.3.10-‐10 N) or commonly found adsorption forces per molecule (around 1-‐100.10-‐12 N).12,13

Figure S 4. 1H (A-‐F) and 13C (G-‐J) NMR spectra of the liquid electrolyte solution (A,G) and of carbon A soaked with acetonitrile (ACN) and gently dried for 15 minutes with N2 (B-‐F, H-‐J). The spectra with carbon A have first been recorded without Magic Angle Spinning (B,H) and then with 2.5 kHz (C,D,I) and 5 kHz (E,F,J) MAS speeds. The peaks are shifted by -‐6 ppm to the right compared to the neat ACN spectra due to the local magnetic susceptibility around the molecules adsorbed between the graphene layers. The 1H spectra do not change either after 30 minutes at 2.5 kHz or 24 hours at 5 kHz. If centrifugation of ACN occurred and ACN molecules were to leave the nanoporosity, NMR signals from free ACN molecules, i.e. without any diamagnetic shift, would appear at the positions indicated by the red lines. D) Decoupling and NMR lineshapes To further increase the resolution, low RF power decoupling on 1H has also been applied to remove the multiplets stemming from the 13C-‐1H J-‐couplings in the

12 Sheiko, S.S., Sun, F.C., Randall, A., Shirvanyants, D., Rubinstein, M., Lee, H. & Matyjaszewski, K. Adsorption-‐induced scission of carbon–carbon bonds. Nature 440, 191-‐194 (2006). 13 Horinek, D., Serr, A., Geisler, M., Pirzer, T., Slotta, U., Lud, S.Q., Garrido, J.A., Scheibel, T., Hugel, T. & Netz, R.R. Peptide adsorption on a hydrophobic surface results from an interplay of solvation, surface, and intrapeptide forces. Proc. Nat. Acad. Sci. 105, 2842–2847 (2008).

13C NMR spectra (see Figure S 5). The electrodes contain carbon black to improve the electronic conductivity, and in such cases, skin depth effects may be encountered which would preclude detecting the signals from the whole sample. The skin depth is proportional to ω0-‐1/2 where ω0 is the Larmor frequency: therefore, the skin depth for 1H is expected to be roughly half the skin depth for 13C. We expect the skin depth effect can be safely neglected here: the 1H RF decoupling manages to decouple every detected 13C spins, as no residual splitting is detected, implying that the RF irradiation is applied to the whole sample and all the signals are detected.

Figure S 5. 13C MAS-‐NMR spectra of carbon A soaked with an excess of a 1M solution of TEABF4 in ACN and gently dried with N2 for 15 minutes. In A, 1H decoupling is applied to remove the multiplets induced by the J-‐coupling with the 1H spins which appear for the CH2 and CH3 peaks in the 13C spectrum without 1H decoupling, as shown in B. E) Drying the samples Evaporating the solvent in the presence of carbon A powder To check the effect of the drying stage, we recorded 13C NMR spectra of a slurry made of carbon A electrode powder and electrolyte, without prior pumping or charge/discharge cycling. Upon drying, the [TEA+]/[ACN] ratio increases, indicating that ACN evaporation has occurred. However, no electrolyte enters the nanoporosity, as seen from the unchanged peak positions between B and C.





Figure S 6. 13C static and MAS NMR spectra of 5 mg (≈10 mL) of carbon A mixed with 50 mL (40 mg) of 1M TEABF4 electrolyte in ACN. A : Static spectrum, B : initial 1kHz MAS spectrum (using 60 s recovery delay as the longitudinal relaxation is slower in the solution) and C : 1 kHz MAS NMR spectrum after drying the mixture with N2 during 6 minutes (ACN loss of 20 mg). As seen from the position of the NMR lines, no TEA+ enters the activated carbon porosity upon drying. A few mg of solvent are lost over a couple of hours indicating that spontaneous evaporation occurs during the experiment, because of leaks through the seal of the 4 mm rotor cap (shown on top). Cycled supercapacitor electrodes with and without drying As ACN evaporation may still occur while the NMR spectra are recorded, to ensure the stability and the reproducibility of our NMR experiments, we chose to mildly dry the samples before recording the MAS-‐NMR spectra. The only detected effect of drying the electrodes under N2 is to remove some of the solvent (Figure S 6 and Figure S 7). Before the N2 drying stage, the spectra show that the TEA+ cations are in the nanoporosity of the carbon A electrode, as indicated by the chemical shifts. The same diamagnetic shifts are observed with and without N2 drying, and no modification is observed in the NMR spectra, even after several days.

Figure S 7. 13C MAS NMR spectra of carbon A negative supercapacitor electrodes, after a charge/discharge cycle with 30 minutes at 2.3V and 30 minutes at 0V. In A, the static 13C NMR spectrum is shown, while the B and C spectra are obtained with MAS at 5 kHz. The supercapacitor is opened right after cycling and spontaneous drying occurs in the glove box during the recovery of the carbon from the aluminum current collector. For A and B, no drying was applied after the electrode recovery and the sample is put straight into the 4mm NMR rotor. The spectrum in C has been obtained after 15 minutes drying with N2. In all spectra, the resonances are shifted by -‐6 ppm, indicating that the detected molecules are adsorbed between graphene layers. F) Rinsing of the soaked carbon powders and of the charged electrodes

Figure S 8. 11B MAS-‐NMR spectra for carbon A electrodes soaked with an excess of electrolyte (a) and rinsed with ACN before drying (b). The intensity of the 11B NMR spectrum of the rinsed electrode (b) has been multiplied by 100 in order to show the small amount of BF4-‐ in type II sites. For the rinsed electrode, the 13C MAS-‐NMR spectrum (not shown) only shows the solvent peak.





Figure S 9. 13C MAS-‐NMR spectra of the negative (a,b) and positive (c,d) carbon A electrodes of a supercapacitor charged at 2.7V, before (a,c) and after rinsing with ACN (b,d). The cation is strongly bound to the negative electrode (a and b), and rinsing only adds a small amount of ACN. The TEA+ peak width becomes smaller which indicates its increased mobility. In the positive electrode (c and d), a large amount of the previously adsorbed TEA+ has been removed by rinsing, as the BF4-‐ anion is strongly bound to the positive electrode. G) NMR spectra Figure S 10. 13C (left) and 11B (right) MAS-‐NMR spectra for the positive (red) and negative (black) electrodes of the supercapacitors made with carbon A. The supercapacitors have been charged using the voltages and times indicated on the left. The green lines show the positions of each resonance at 0 V.





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Exploring electrolyte organization in · presence of signals with ... water on activated ... coconut shell and