polyvinyl butyral composite as a stable binder for castable supercapacitor electrodes in aqueous electrolytes

Journal of Power Sources 279 (2015) 323e333

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Polyvinylpyrrolidone/polyvinyl butyral composite as a stable binder for castable supercapacitor electrodes in aqueous electrolytes M. Aslan a, D. Weingarth a, P. Herbeck-Engel a, I. Grobelsek a, V. Presser a, b, * a b

INM e Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany Saarland University, Campus D2 2, 66123 Saarbrücken, Germany

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Polyvinylpyrrolidone/polyvinyl butyral (PVP/PVB) mixtures can be used as binder.
PVP/PVB mixtures are suitable for composite electrodes for aqueous supercapacitors.
PVP/PVB-bound electrodes can be applied directly on a current collector.
PVP/PVB-bound electrodes perform better then when using polyvinylidene difluoride.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 November 2014 Received in revised form 16 December 2014 Accepted 30 December 2014 Available online 31 December 2014

Mixtures of polyvinylpyrrolidone/polyvinyl butyral (PVP/PVB) are attractive binders for the preparation of carbon electrodes for aqueous electrolyte supercapacitors. The use of PVP/PVB offers several key advantages: They are soluble in ethanol and can be used to spray coat or drain cast activated carbon (AC) electrodes directly on a current collector. Infrared spectroscopy and contact angle measurements show that the PVP-to-PVB ratio determines the degree of binder hydrophilicity. Within our study, the most favorable performance was obtained for AC electrodes with a composition of AC þ 1.5 mass% PVP þ 6.0 mass% PVB; such electrodes were mechanically stabile and water resistant with a PVP release of less than 5% of total PVP while PVB itself is water insoluble. Compared to when using PVDF, the specific surface area (SSA) of the assembled electrodes was 10% higher, indicating a reduced pore blocking tendency. A good electrochemical performance was observed in different aqueous electrolytes for composite electrodes with the optimized binder composition: 160 F g1 at 1 A g1 for 1 M H2SO4 and 6 M KOH and 120 F g1 for 1 M NaCl. The capacitance was slightly reduced by 2.5% after cycling to 1.2 V with 1.28 A g1 in 1 M NaCl for 10,000 times. © 2014 Elsevier B.V. All rights reserved.

Keywords: Supercapacitor Polymer binder Casting Electrode manufacturing

1. Introduction Efficient and environmentally friendly storage and recovery of

* Corresponding author. INM e Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany. E-mail address: [email protected] (V. Presser). http://dx.doi.org/10.1016/j.jpowsour.2014.12.151 0378-7753/© 2014 Elsevier B.V. All rights reserved.

electrical energy has emerged as a key bottleneck to the large scale implementation of renewable energy to the power grid and for many mobile applications [1,2]. Supercapacitors have attracted particular attention as high-power, high-efficiency devices for electrochemical energy storage (EES) [3,4]. The energy storage mechanism of supercapacitors can be divided in two categories: electrical double-layer capacitance as a result of reversible ion

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electrosorption and pseudocapacitance as a result of fast and reversible redox reaction on electroactive surfaces [5e7]. Most commercial supercapacitors employ organic electrolytes with acetonitrile as the solvent. [8] Using an organic electrolyte enables utilization of lightweight current collector materials like aluminum and an electrochemical operation voltage window, U, of around 2.7 V [9] In particular, a large voltage window is preferable to improve the energy and power density which both scale with U2. Even higher cell voltages are possible when employing ionic liquids; [10e13] however, cost considerations and limitations to the power density impede the widespread utilization of this interesting group of solvent-free electrolytes until now. [14]. Albeit the predominance of organic electrolytes (including novel organic solvents [15] or ionic liquids [10]), aqueous electrolytes have seen a renaissance for supercapacitor devices. In principle, the dielectric constant (at 25  C: 78 for water vs. 36 for acetonitrile) [16,17] and ionic conductivity [18] of aqueous electrolytes are much higher than for organic electrolytes which facilitates a high device capacitance and superior power handling. Also, the harsh requirements for drying carbons for use in organic electrolytes are severely lowered in aqueous media which greatly decreases the energy needed for device fabrication. [4] Furthermore, aqueous electrolytes enable to capitalize faradaic reactions and the device energy density can be increased significantly when adapting highly reversible redox reactions. [19,20]. Yet, a major limitation to aqueous media has been the rather small stability window for stable cell performance, determined by the theoretical limit of 1.23 V. In a symmetric (two-electrode) cell, the pH-dependency of the electrochemical stability window leads to even lower practical values below 1 V for typical electrolytes such as H2SO4 or KOH. Recent work on pH-neutral electrolytes has reenergized the research on aqueous systems by showing that a stable maximum cell voltage between 1.6 and 1.9 V can be reached for thousands of charge and discharge cycles. [21] Knowledge on the facile production of high performance carbon electrodes for aqueous systems is also needed for other emerging electrochemical technologies, such as capacitive deionization (CDI) [22] or capacitive mixing energy extraction (Capmix) [23,24]. Activated carbon is the most commonly used electrode material in supercapacitors due to the combination of high specific surface area, moderate or low cost, and high material abundance. To obtain electrodes from activated carbon powders, it is necessary to blend carbon particles with polymer binders like polyvinylidene difluoride, PVDF, or polytetrafluoroethylene, PTFE. Both of these polymers contain fluorine and the electrode preparation may require the use of toxic chemicals, like N-methyl-2-pyrrolidone, NMP. Recently, we have introduced polyvinylpyrrolidone, PVP, as an effective and mechanically stable binder for castable or sprayable carbon electrodes. [25] While employing a more environmentally friendly preparation route compared to conventional binders like polyvinylidene difluoride, the applicability of PVP remains limited to organic electrolytes. The increased interest in aqueous supercapacitors and emerging electrochemical technologies has motivated us to investigate a strategy to overcome the issue of high PVP water solubility. At the same time, our proposed method will maintain an overall environmentally friendly synthesis approach together with facile film application to the current collector, high mechanical stability, and electrochemical performance stability. Our approach is based on blending PVP with an additional polymer. As known from the biomedical literature, [26] PVP can be blended with polyvinyl butyral, PVB, without leading to a phase separation up to a PVP-toPVB-ratio of 2-to-3 for PVP with 11 mass% polyvinyl alcohol content. [27] In our study, we present an evaluation of different types of PVP/PVB mixtures as binder for drain casted and spray coated

electrodes operated in aqueous electrolytes. The advantage of PVP/ PVB-based electrodes is that they can be coated directly onto a current collector by casting or spray coating just like PVP-based electrodes. [25] This enables scalable application in cost sensitive industrial production where currently PVDF is employed; yet, unlike in conventional PVDF application to film electrodes, no toxic solvent is required. In the present manuscript, the obtained film electrodes were tested in a symmetric two electrode cell configuration using different aqueous electrolytes, namely NaCl, KOH, and H2SO4, covering a wide range of pH values. Electrochemical performance and stability data is complemented by chemical investigation, especially regarding the solubility of the PVP/PVB polymer blend. 2. Experimental description 2.1. Reagents Activated carbon, AC, powder (YP-80F, Kuraray Chemicals) was used as the active material and high molecular weight polyvinylpyrrolidone (PVP, Mw ¼ 1,300,000 g mol1, Sigma Aldrich) and polyvinyl butyral of high acetalization degree (PVB, B30HH, Kuraray Chemicals) were used as binder. We also prepared electrodes with PVDF (as powder, Sigma Aldrich) and PTFE (as a 60 mass% aqueous solution, Sigma Aldrich) for comparison. As electrolytes, we employed 1 M NaCl (purity 99.8%, Roth), 6 M KOH (purity 85%, Sigma Aldrich) and 1 M H2SO4 (purity 95e95%, Fluka). The current collector was a graphite foil with a thickness of 250 mm (Sigraflex Z, SGL Carbon) to avoid electrochemical degradation frequently observed when using, for example, stainless steel. 2.2. Electrode preparation For the electrodes containing polymeric binder (PVP, PVB, and combination of PVP þ PVB) ethanolic slurries with a solid content of 20 mass% with respect to total mass (i.e., AC þ binder þ ethanol) were prepared by ultrasound-assisted stirring in an ice bath for 5 min. For the ultrasound treatment, a Branson Sonifier 450 with a maximum power output of 400 W was used (duty cycle: 20%, output power: 30%). The amount of binder varied between 1.5 and 11.5 mass% with respect to AC; for example, the composition “AC þ 1.5 mass% PVP þ 6.0 mass% PVB” means: 1.5 mass% PVP with respect to AC and 6.0 mass% PVB with respect to AC which was abbreviated as AC þ PVP(1.5) þ PVB(6.0). The same nomenclature is used for electrodes containing PVDF or PTFE binder (e.g., AC þ PTFE(5.0) or AC þ PVDF(10.0)). After sonication, the slurries were stirred for further 5 min with a magnetic stirrer and used directly for the preparation of coatings on graphite current collectors by either drain casting or spray coating. Drain casting resulted in coatings with a thickness of 50e70 mm. For spray coating, a spray gun with nozzle diameter of 1.5 mm at 1.5$105 Pa pressure was used. The coating thickness was adjusted by the number of coating cycles between 50 mm and 250 mm for coating cycles 1 to 6. The AC þ PVP, AC þ PVB, and AC þ PVP þ PVB electrodes were dried at 90  C for 2 h in an air recirculating oven. We prepared PVDF-bound AC electrodes by the following procedure: 0.2 g of PVDF powder was dissolved in 8 g of N-methyl-2pyrrolidone, NMP, in an oil bath at 100  C under constant stirring. Then, 1.8 g of AC powder was added. The resulting mixture was thoroughly stirred for 10 min at room temperature and then for another 15 min in an ultrasound-assisted ice bath. The final slurry consisted of 10 mass% solid fraction with respect to the total mass AC plus PVDF. The PVDF containing AC slurry was drain casted on a graphite current collector and coatings of 50e70 mm thickness were

M. Aslan et al. / Journal of Power Sources 279 (2015) 323e333

obtained. Afterwards the graphite foils coated with AC-PVDF were dried at 80  C in an air recirculating oven at ambient pressure for 10 h followed by drying at 120  C under 20 mbar vacuum for 24 h. Freestanding AC þ PTFE electrodes were prepared by the following procedure: A mixture of 2.375 g AC powder and 0.208 g of aqueous PTFE solution was homogenized in a mortar and pestle by adding drops of ethanol to a dough-like mass resulting in AC þ PTFE mixtures with 5 mass% PTFE with respect to AC plus PTFE. These mixtures were then rolled into 100 mm thick freestanding films by a twin roller (MTI HR01, MTI Corporation) and dried prior to use at 120  C in vacuum at 20 mbar for 24 h. 2.3. Mechanical and structural characterization Mechanical testing of the coatings was done with a micro scratch tester (CSEM Centre Suisse d'Electronique et de Microtechnique SA). This procedure is described in more detail in Ref. [25]; in short, a 400 mm diameter diamond ball is sliding on the coating at a given speed (10 mm min1) with a load rate of 970 mN min1 (test length: 10 mm, start load: 30 mN, end load: 1000 mN, loading rate: 970 mN min1). The position of the ball, penetration depth, and the load were recorded automatically. Scanning electron microscopy (SEM) was carried out with a JSM-7500F (JEOL) field-emission system operating at an accelerating voltage of 3 kV. The samples were studied without the application of a conductive sputter coating layer. The specific surface area (SSA) and pore size distribution of the carbon powder and binder-containing films were determined by nitrogen gas sorption measurements at 196  C (Autosorb 6B, Quantachrome Instruments) using the quenched solid density functional theory (QSDFT) kernel implemented in the AS1WIN software package from Quantachrome Instruments. [28] The isotherms were fitted assuming a slit pore shape. The BET SSA (Ref. [29]) was determined in the partial pressure range between 0.0016 and 0.05 P$P1 0 that yielded the best linear correlation for the BET equation. Raman spectra were recorded with a Renishaw inVia system operating with a laser wavelength of 532 nm (ca. 0.5 mW) focused with a 50x objective lens (numeric aperture ¼ 0.9). The recorded lateral resolution (in the focal plane) was around 2 mm and the spectral resolution was approximately 1.2 cm1. Attenuated total reflection Fourier transform infrared, ATR FTIR, spectroscopy was performed with a Bruker Tensor 27 single bounce diamond ATR system with the spectral resolution set to 16 cm1. Scans of the individual chemicals were background subtracted using the recorded IR spectrum of air and the spectra taken with the electrodes were corrected using a blank measurement of AC as the background. No specific IR absorption data was obtained for the AC powder alone. Even composite electrodes with up to 11.5 mass% of binder gave no absorption signal of sufficient intensity for a quantitative analysis. Instead, we used mixtures containing 80 mass% binder (either PVP or PVB, or combination of PVP þ PVB) as coatings on glass for the investigation of the interaction between the binder components and AC powder. Binders without AC were measured as freestanding films which were prepared by casting of ethanolic polymer/polymer mixtures on a PTFE plate which were dried before IR investigations at 90  C for 2 h. 2.4. Water stability and wetting behavior of composite electrodes The solubility of PVP in water as part of the PVP þ PVB blend was examined by leaching of test samples (i.e., electrode coatings on graphite foils) in deionized (DI) water. The amount of water was chosen so that the PVP concentration in leach water would be 50 ppm in case of total release. For first set of experiments, AC

325

samples with 6 mass% PVB and different amounts of PVP (from 1 to 7.5 mass%) were leached for 120 h under ambient conditions. For the second set of experiments, electrodes with two different compositions, namely AC þ PVP(1.5) þ PVB(6.0) and AC þ PVP(5.0) þ PVB(6.0) were leached from 1 h to 15 d. Water samples containing PVP were analyzed with highperformance liquid chromatography, HPLC, using an Agilent 1260 system. For each measurement, we used an injection volume of 10 mL and an injection pressure of 50 bar. The mobile phase was a 1:1 mixture of MeOH þ H2O. The results were quantized by standards taken from water samples with known PVP concentration. The dynamic contact angle of water was measured on coatings applied on graphite current collectors by the sessile drop technique using a drop volume of 2 mL (OCA 35, Dataphysics).

2.5. Electrical and electrochemical characterization The electrical conductivity of the samples was determined by four point probe measurements using a custom-built system. Four spring loaded gold pins in contact with the electrode material and a highly sensitive ampere-meter (Prema 4001 Digitalmultimeter) was used to determine the film resistance. The 1.5 mm diameter tips were flat and had a tip-to-tip spacing of 3.0 mm. Electrochemical characterization was performed in a custombuilt symmetric two electrode cell as sketched in Fig. S1 (Supporting information). Electrodes of 30  30 mm2 were cut from coated graphite foils, whereat on one side on a small part of 10  5 mm2 was used to fix the current cables. The assembly consisted of two electrodes separated by a 250 mm thick glass fiber membrane and was clamped between polyvinyl chloride, PVC, blocks and put in beaker filled with the electrolyte so that the flaps for electrode connections were above the liquid level. The test cell was placed in a desiccator and flushed with argon to limit the oxygen uptake of the electrolyte from the environment. All electrochemical measurements were carried out using a VSP300 potentiostat/galvanostat (BioLogic Science Instruments). Electrochemical tests encompassed cyclic voltammetry (CV) at different scan rates (1, 10, 50, and 100 mV s1) up to 1.2 V cell voltage for performance testing and galvanostatic charge and discharge with potential limitation (GCPL) with 10 s resting periods for an accurate determination of the iR-drop. Capacitance cycling stability and constant voltage holding tests were done in an electrochemical cell with a symmetric two electrode configuration as described in Ref. [30]. Cycling tests were performed by charging/discharging to 1.2 V in 1 M NaCl with 1.28 A g1 for 10,000 cycles. Aging stability of the composite electrodes was characterized by measuring the discharge capacitance after voltage-holding periods at 1.2 V for 10 h. Five charge/discharge cycles were performed for characterization and the holding was repeated 20-times resulting in a total holding time of 200 h. Only the discharge current was used for the calculation of the specific capacitance. [9,31] The gravimetric capacitance during discharging was calculated via Eq. (1):

0 B @ Csp ¼ 4$

Z

1 tend

t0

U

C Idt A 1 $ m

(1)

with specific capacitance Csp, time t (t0: starting time of discharge, tend: end of discharging time), iR-drop corrected cell voltage U, and total mass of the electrodes m (i.e., considering carbon and the binder).

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3. Results and discussion 3.1. Structural characterization of AC and AC film electrodes We used commercial activated carbon, AC, as the active electrode material. As seen from electron micrographs, it consisted of agglomerates of porous particles with a size ranging from around 1 mme15 mm (Fig. 1A). AC þ PVP þ PVB electrodes had a very smooth surface with the typical grain morphology expected for AC powder (Fig. 1B). The same smoothness and homogeneity was seen for AC electrodes with different PVP þ PVB mixtures and resembles what has been reported previously for AC þ PVP films. [25] Scanning electron micrographs show evidence for local particleeparticle gluing with the binder seemingly located at particle junctions (inset in Fig. 1B). Also, as expected for AC, Raman spectra indicate that the carbon powder is composed of incompletely graphitized carbon (see Supporting information, Fig. S2). This can be seen from the emergence of a disorder related D-mode (1343 cm1) besides the G-mode (1605 cm1) which is related to a sp2-hybridized carbon network with an ID-to-IG ratio of 1.54 ± 0.02. [32,33].

3.2. Interaction mechanisms between AC, PVP, and PVB It is important to characterize the mechanisms by which the polymers interact in PVP/PVB mixture electrodes. For that, we carried out FTIR-ATR measurements on freestanding films for binders and binder mixtures, and also on glass supported coatings for the AC þ binder system. In the measured spectra (Fig. 2A), the most characteristic absorption band for PVP, namely tertiary amide group, is appearing at wave numbers around 1650 cm1. [34] The tertiary amid group of PVP caused a hygroscopic character of this polymer. The adsorbed water caused also a spectral downshift from 1680 to 1652 cm1 and a peak broadening towards lower wave numbers. [35] The spectral intensity of the amid band was scaled to compare the mixtures for a 2:1 and 1:4 PVP-to-PVB ratio (i.e., using PVP(66) þ PVB(33) and PVP(20) þ PVB(80)). In the mixture PVP(20) þ PVB(80), the addition of PVB resulted in a low peak contraction without peak shift as a result of little less water absorption. Here, the tertiary amid (I) could be observed at its characteristic wavenumber at 1680 cm1 beside the OH bending mode of water at 1664 cm1 combined with a bisection of the bandwidth. Thus, PVB has much more hydrophobic character with only few hydrophilic groups (e.g., OH and acetyl). The decreased water content caused by adding PVB can also be observed in the OH stretching region between 3000 and 3600 cm1 (Fig. 2A). The spectrum of PVP adsorbed water showed two OH vibrations with maxima at 3260 and 3420 cm1. The first one is

related to highly coordinated water molecules and the second one to lower coordinated water. [36,37] When adding PVB to PVP, the intensities of the band for water molecules was negligible, the amount of CH groups around 2900 cm1 was increased, and the ratio of the OH to the CH groups was decreased (Fig. 2B). Thus, we see that adding more PVB to the PVP/PVB blend significantly increases the hydrophobic character of the resulting polymer blend. Yet, we note that besides PVP also AC adsorbs water. For the two binder mixtures in combination with AC one can see an additional broadening of the tertiary amid (I) band at 1652 cm1 for the low PVB content and a shift from 1680 to 1658 cm1 for the high PVB content indicating the water absorption by AC (Fig. 2B). The comparison of the peak areas for CH band (2900 cm1) and OH band (3200e3500 cm1) gives valuable insights in the mechanisms of the hydrophobic/hydrophilic character. Table 1 shows these peak areas and corresponding OH/CH ratios for binders and AC þ binder mixtures. A high OH/CH ratio indicates a highly hydrophilic character and for PVP, a highly hydrophilic polymer, the OH/CH ratio was measured to be 3.62. Mixing PVP with 20 mass% AC powder shifts this value to 8.31. This apparent increase of hydrophilicity is not intrinsic but due to increased amount of absorbed water in the micropores of AC. This value decreases to 2.01 for AC coatings with 80 mass% PVP/PVB mixture with a PVP-to-PVB ratio of 1:4, indicating to a strong increase of hydrophobicity. These results demonstrate that the hydrophilic/hydrophobic character can be adjusted by varying the PVP/PVB ratio. The degree of hydrophilicity (or, hydrophobicity) of the binder determines important application related properties of the AC composite electrodes as it will be shown in next chapters.

3.3. Influence of the binder on porosity and pore size distribution The impact of the binder on the porosity of composite electrodes is an important parameter and pore blocking must be avoided to maintain a large volume of ion and electrolyte accessible pores. Table 2 provides an overview of the porosity (SSA, average pore size, and pore volume) of the initial AC (as received) and compares the resulting pore characteristics for composite electrodes with various PVP-to-PVB ratios. As seen, AC alone exhibits a BET SSA of 2112 m2 g1 (QSDFT SSA: 1818 m2 g1), a total pore volume of 1.09 cm3 g1 dominated by micropores, and a volume-weighted average pore size (d50) of 1.31 nm. Fig. 3A illustrates that the nitrogen sorption isotherm at 196  C remains almost unchanged in shape (type I with almost no hysteresis) which tells us that the pore size distribution is almost unaffected by the presence of polymer binder (e.g., for AC þ PVP(1.5) þ PVB(6.0)). This can also be concluded from the data presented in Table 2 and in Fig. 3B for the example of a PVP-to-PVB ratio of 1:4. Yet, we see a significant

Fig. 1. Scanning electron microscope, SEM, images of activated carbon, AC, powder particles. (A) AC powder. (B) Spray coated electrode on graphite using 1.5 mass% of polyvinylpyrrolidone, PVP, and 6.0 mass% polyvinyl butyral, PVB (i.e., AC þ PVP(6.0) þ PVB(1.5)). (Inset B) Binder necks at grain junctions.

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Fig. 2. FTIR-ATR spectra of the binder formulations (A) and for the binder when AC is present (B) for a PVP-to-PVB ratio of 2:1 and 1:4.

decrease in the total pore volume and the corresponding total surface area as a function of total binder content. Fig. 4 shows the BET SSA of composite electrodes with different PVP-to-PVB ratios (including only PVP and only PVB). These data are compared to an ideal mixture when assuming a negligible SSA of the binder material itself (i.e., 4 m2 g1; see also Ref. [25]) and correcting the total SSA of the electrode by the amount of binder. Thus, more “dead mass” of binder yields a lower SSA of the composite electrode. The line for an ideal mixture more or less superimposes the data found for PVP as the only binder component. The reduced pore blocking tendency of PVP compared to other binders has already been shown in Ref. [25] which is probably due to the hindered spreading of hydrophilic PVP on highly hydrophobic AC surfaces. Data for different PVP-to-PVB ratios indicate an increase in pore blocking (Fig. 4); yet, within the scatter of the method and within the statistical reproducibility, all data points for samples employing some content of PVB (including 100% PVB) are very similar. This leads to a decreased BET SSA of 1601 m2 g1 for AC þ PVP(1.5) þ PVB(10.0) compared to 1859 m2 g1 for the case of an ideal mixture without additional pore blocking (i.e., 14%). We note that the deviation from an ideal mixture remains below a value of 15% for all samples and all binder compositions. This means, the values are all in the range known for other binders, such as PTFE or PVDF. [38] The higher degree of pore blocking in PVB containing samples may be related to the more hydrophobic nature of PVB enabling a better spreading on the hydrophobic carbon surfaces. [39e41] PVDF is a highly hydrophobic binder and its pore blocking is more

pronounced in comparison to PVP/PVB binder mixtures (Table 2, Fig. 4). More precisely, the BET SSA for AC þ PVDF(10.0) is 1545 m2 g1; when we extrapolate this value to just 7.5 mass% PVDF, this translates to 1683 m2 g1. For comparison, AC þ PVP(1.5) þ PVB(6.0) electrodes exhibit a much higher BET SSA of 1852 m2 g1. 3.4. Influence of the binder on mechanical strength For the application in scaled production, mechanical handling of carbon electrodes is very important. Thus, we evaluated the mechanical stability of the films coated on graphite current collectors. The choice of graphite current collectors was motivated by their superior chemical stability in aqueous media, especially in acidic environments, compared to materials such as stainless steel. We also have to consider that different PVP-to-PVB ratios may show a different mechanical response due to the differences in their chemical structure and interaction. PVP qualities are highly soluble in polar solvents because of their charged surface groups. In contrast, PVB with a high acetalization degree has only a minor amount of polar surface groups and the resulting solubility in polar solvents is negligible. Accordingly, interaction of AC with PVB is facilitated by abundant hydrophobic groups on carbon surfaces. [40,41]. A common optimization strategy for supercapacitor electrodes is the minimization of the total binder content to balance mechanical integrity at higher amounts of binder with electrical conductivity and reduced dead mass at lower binder additions. Our

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Table 1 Peak areas of CH and OH bands obtained from IR-ATR-spectra in Fig. 2 for different binders binder mixtures and AC þ binder composite coatings. System

Peak area (Arb. units)

PVP(100) PVP(66) þ PVB(33) PVP(20) þ PVB(80) AC þ PVP(100) AC þ PVP(66) þ PVB(33) AC þ PVP(20) þ PVB(80)

OH/CH ratio

OH

CH

4.06 3.81 2.55 6.36 8.63 6.48

1.12 1.51 4.01 0.76 2.02 3.22

3.62 2.52 0.64 8.31 4.27 2.01

Table 2 Pore characteristics (specific surface area, SSA; volume-weighted average pore size, d50; total pore volume) obtained from nitrogen gas sorption analysis at 196  C using the BET equation and quenched solid density functional theory (QSDFT) assuming a slit pore shape for activated carbon, AC, with or without the addition of polymer binder (polyvinylpyrrolidone, PVP, polyvinyl butyral, PVB, or polyvinylidene fluoride). PVP content (mass%)

PVB content (mass%)

PVDF content (mass%)

BET SSA DFT SSA Average (m2 g1) (m2 g1) pore size (nm)

Pore volume (cm3 g1)

e 1.0 1.5 2.0 3.0 5.0 1.5 1.5 1.5 e e e e e

e 6.0 6.0 6.0 6.0 6.0 5.0 7.0 10.0 6.0 7.0 8.0 10.0 e

e e e e e e e e e e e e e 10.0

2112 1918 1852 1877 1829 1751 1892 1812 1601 1836 1706 1692 1544 1545

1.091 0.970 0.922 0.961 0.937 0.922 0.951 0.910 0.811 0.962 0.904 0.887 0.817 0.752

1818 1629 1561 1590 1544 1482 1616 1528 1356 1603 1511 1482 1352 1143

1.32 1.32 1.30 1.32 1.33 1.35 1.29 1.31 1.31 1.31 1.31 1.30 1.32 1.42

data aligns with this general rule: at higher binder concentrations, we have a higher penetration force which represents a better adhesive and cohesive character of the film (Fig. 5). To determine an optimized amount of individual binder and binder mixtures, different sets of samples were prepared and characterized: (1) AC þ PVB (X), (2) AC þ PVP(1.5) þ PVB(X), and (3) AC þ PVB(6.0) þ PVP(X). As shown in Fig. 5, the strength of the composite electrode coatings prepared with different amounts of PVB is consistently lower than coatings containing PVP/PVB mixtures. The results for coatings with AC þ PVP(1.5) þ PVB(X) indicate that a continuous increase of strength is possible up to 6 mass% PVB (fp around 350 mN; corresponding to 7.5 mass% total binder in Fig. 5). A further improvement in mechanical strength is accomplished when using higher concentrations of PVP. In our study, we increased the PVP content up to 5 mass% as an admixture to 6 mass% PVB and reached a maximum strength of 714 ± 50 mN penetration force. Yet, we note that this corresponds to a very large amount of binder (i.e., 11 mass%). To reduce the total amount of binder and to ensure mechanical stability in water, we decided that within the parameters studied in our work, the system with 1.5 mass% PVP (i.e., a ratio PVP:PVB of 1:4) was the best compromise. 3.5. Influence of the binder on the wetting behavior The wetting behavior of carbon electrodes by water is a key issue for the performance of aqueous supercapacitors. Using AC þ PVP(X) þ PVB(6.0), we see a continuous decrease in contact

Fig. 3. (A) Nitrogen sorption isotherms at 196  C of as received activated carbon, AC, without or with the addition of polymer binder (example shown: AC þ PVP(1.5) þ PVB(6.0)). (B) Calculated pore size distribution derived from quenched solid density functional theory (QSDFT). STP: standard temperature and pressure.

Fig. 4. Dependency of the specific surface area (SSA; derived from the BET equation) on the binder content. Binder content relates to PVP þ PVB.

angle for increased PVP content (Fig. 6). The data for this comparison relates to the initial contact angle; yet, we see that all samples undergo over time a transition from non-wetting to wetting. In case of AC þ PVP(1.5) þ PVB(6.0), this transition occurs progressively after 1 min (inset in Fig. 6). Coatings of just PVB exhibit a very high

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contact angle of 135 , while adding 5 mass% of PVP lower the initial contact angle to 42 . Within the studied range, we observed an almost linear correlation between the lowered contact angle and PVP content. This may be related to binding of polar groups of PVP to PVB suppressing the water solubility of PVP in PVP/PVB mixtures. 3.6. Influence of the binder on the electrical conductivity

Fig. 5. Influence of the binder composition and amount of binder on the mechanical properties (penetration depth; i.e., measure for film cohesiveness and adhesion to the current collector). Binder content relates to PVP þ PVB.

Besides mechanical stability and optimized porosity, the electric conductivity is another important property of a composite electrode. For all studied ranges of PVP-to-PVB ratios and total amount of binder (i.e., 5.5e11.5 mass%), we did not see any coherent trend and the average values for the specific resistivity ranged from 82 to 140 U m (Fig. 7). The spread was smaller for coatings only consisting of PVB (110e130 U cm). These results support our conclusion from mechanical and wetting data that PVB has a better adherence and spreading on carbon than PVP/PVB; since PVB is an isolating polymer, this translates to an increased electrical resistivity. We also note that these values are higher than what we have recently reported for electrodes only composed of PVP (41e53 U m) or rolled PTFE-bound electrodes (14e21 U m) [25]. 3.7. Water stability of composite electrodes

Fig. 6. Water contact angle on composite coatings with AC þ PVP(X) þ PVB(6.0) using the sessile drop technique. The inset shows the change of contact angle with time for AC þ PVP(1.5) þ PVB(6.0).

Fig. 7. Specific resistance of AC composite electrodes with different amounts of binder and different binder compositions; reference data for PVDF- and PTFE-bound electrodes is also included.

In addition to the parameters studies so far, we also need to consider if the binder formulation is actually chemically stable in aqueous electrolyte. For that, we carried out leaching experiments in deionized water for up to 5 d. From HPLC analysis we can see that the amount of leached PVP (i.e., the only water-soluble component of the PVP/PVB composite) increases significantly with increased PVP content in the binder mixture (Fig. 8). Below 2 mass% of PVP admixture, the leaching is in the range of or below 5% of the total amount of PVP. It must be noted that loosing binder, especially at a small amount, does not necessarily harm the resulting electrochemical properties as long as no parasitic (electro)chemical reactions occur (as discussed later). We also see in the inset in Fig. 8 that the data after 5 d are very close to final leach concentrations. This means that while we have leached ca. 4 mass% of PVP after 5 h, only a maximum of 8 mass% is reached after 10 d and the trend of the data points does not indicate any further significant increase of that value over time. We explain

Fig. 8. Leached amount of PVP from AC þ PVP(X) þ PVB(6.0) as a function of PVP content after 5 days in water. The inset shows data for AC þ PVP(1.5) þ PVB(6.0) and AC þ PVP(5.0) þ PVB(6.0) as a function of leach time.

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this behavior by the preferential leaching of PVP that is not well bound to PVB and, henceforth, readily mobilized and dissolved in water over time. Thus, we conclude that especially a PVP-to-PVB ratio 1 can be considered as water stable after a washing step for 30 h.

Fig. 9. Electrochemical performance of AC þ PVP(1.5) þ PVB(6.0) in 1 M NaCl aqueous electrolyte. (A) Cyclic voltammetry (CV) of the as prepared electrodes. (B) Comparison of CVs at 1 and 100 mV s1 for the as prepared electrodes and after 78 d of soaking in 1 M NaCl. (C) Galvanostatic discharge data (from 1.2 V to 0 V) for different soaking times in the aqueous electrolyte.

3.8. Electrochemical measurements The first electrolyte that we studied was 1 M NaCl (Fig. 9). The behavior is what is expected from a double-layer-capacitance dominated system: the rather rectangular shape of cyclic voltammograms (Fig. 9A) transitions towards a more resistive behavior when the scan rate is increased incrementally from 1 to 100 mV s1. Interestingly, the electrochemical performance improves with time as the electrodes are soaked in aqueous electrolyte. The example shown in Fig. 9B shows that the electrochemical performance after 78 d exhibits an improved rate handling ability with decreased resistance; even though the total capacitance increases, the improvement is mainly visible in rate handling as seen from

Fig. 10. Galvanostatic discharge capacitance in 1 M NaCl as a function of the current density. (A) Influence of PVB admixture to 1.5 mass% PVP. (B) Influence of PVP admixture to 6 mass% PVB. (C) Comparison of spray and drain casted electrodes (AC þ PVP(1.5) þ PVB(6.0)) to PTFE and PVDF-bound electrodes.

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galvanostatic charge/discharge experiments (Fig. 9C). Indeed, we actually see that the equilibrium capacitance is 118 F g1 (i.e., the specific capacitance at very low current density of 0.2 A g1) immediately after the electrode has been prepared, which is very close to the value after 78 d (124 F g1; i.e., þ5%). For comparison: after 78 d, the specific capacitance only drops to 115 F g1 (i.e., 8%) at a scan rate of 4 A g1, while the as prepared electrodes only yield a capacitance of 83 F g1 at the same current density (which corresponds to a drop of 42% compared to the equilibrium capacitance). Data for 1 d and 4 d of electrode soaking in the electrolyte yield performance data in-between the aforementioned performance values. Thus, we can see that extended exposure to 1 M NaCl results in some dissolution of less-well bound PVP, but this does not translate in a loss of performance up to 1.2 V cell voltage. These data relate to the optimized composition of AC þ PVP(1.5) þ PVB(6.0). We also investigated the influence of the amount of PVB and PVP on the electrochemical properties in more detail. As seen from Fig. 10A, the by far best performance with an average capacitance of ca. 126 ± 21 F g1 (average over 3 separately prepared cells) is observed when using AC þ PVP(1.5) þ PVB(6.0). Much lower

Fig. 11. (A) Galvanostatic charge and discharge cycling of AC þ PVP(1.5) þ PVB(6.0) electrodes in 1 M NaCl at 1.3 A g1 up to 1.2 V. (B) Floating test for the same system when charging to 1.2 V and holding for 10 h followed by 5 galvanostatic charge/ discharge cycles.

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performance is seen when increasing or decreasing the PVB content to 7 or 5 mass%. A less pronounced dependency exists for the influence of the PVP content when using 6 mass% PVB (Fig. 10B). When varying the amount of PVP from 1.0 to 5.0 mass%, we see that all data points fall within a ±10% margin; thus, when we consider that the overall amount of binder (namely PVP þ PVB) should be low without sacrificing mechanical stability (see Fig. 5), the best combination with good electrochemical performance in our study is AC þ PVP(1.5) þ PVB(6.0). To demonstrate the versatility of PVP/PVB binders for the preparation of AC composite electrodes, we also performed spray coating. Both spray coated and drain casted AC þ PVP(1.5) þ PVB(6.0) electrodes showed a highly comparable electrochemical performance when sweeping the current density from 0.2 to 8 A g1 for a set of averaged values of 3 cells (Fig. 10C). We note that due to the non-industrial laboratory-scale coating setup, we have encountered a rather high variance for the electrodes when employing drain casting (up to ±22 F g1) compared to spray coating (up to ±14 F g1) with highly comparably equilibrium capacitance (124 and 128 F g1, respectively). These data are slightly better than what is seen for electrodes prepared from PTFE or PVDF at 0.2 A g1 (namely, 115 F g1 and 124 F g1, respectively). Galvanostatic cycling up to 1.2 V at 1.28 A g1 shows very high performance stability and a minor 2.5% drop of the specific capacitance after 10,000 cycles (Fig. 11A). We also note an initial increase for the first dozens of cycles of ca. 1% corresponding to electrode conditioning. Interestingly, a continuous improvement of the device performance of up to þ8% is seen after 200 h of voltage holding (i.e., floating at 1.2 V; Fig. 11B). This can be understood as an enhanced conditioning process to overcome the intrinsically low wettability of carbon for aqueous electrolytes. The capacitance increase during potential holding can also be linked to the effect of soaking discussed before where either the carbon needs a very long time to be completely wetted or where parts of the PVP are dissolved and more pores are getting accessible. Yet, we note that these data also indicate that the presence of some PVP dissolution does not harm the electrochemical stability. SEM images after cycling also show that there is no structural degradation of the electrode material; except for the presence of some glass fibers from the separator, no changes of the electrode are observed (see Supporting information, Fig. S3). Beyond the pH-neutral 1 M NaCl system, we also investigated an acidic medium, 1 M H2SO4, and a basic medium, 6 M KOH, for the optimized electrode composition AC þ PVP(1.5) þ PVB(6.0). Both non pH-neutral media demonstrated a lower practical electrochemical stability window, as seen from the arising faradaic currents beyond 0.8 V cell voltage in cyclic voltammograms (Fig. 12A and B). Yet, we see a much higher equilibrium capacitance when galvanostatically discharging from 1.2 V to 0 V at 0.2 A g1: 176 F g1 for 1 M H2SO4 and 174 F g1 for 6 M KOH (Fig. 12C). In all cases, we see a comparable power handling ability and a very stable system performance up to ca. 4 A g1. Yet, we did not obtain such high performance when using sodium sulfate, 1 Na2SO4 (see Supporting information; Fig. S4). Further work is needed to understand this low performance with just 50 F g1 in more detail and possible chemical interactions with the PVP/PVB polymer blend. Yet, as for the provided data, we see that the new binder formulation is highly suited for at least three aqueous electrolytes at very different pH-values, namely NaCl, H2SO4, and KOH. 4. Conclusions The investigation for the use of PVB/PVP as a binder material shows that we have successfully developed a facile and

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Fig. 12. (A,B) Cyclic voltammograms in different electrolytes using AC þ PVP(1.5) þ PVB(6.0) up to 0.8 V and 1.2 V cell voltage (scan rate 5 mV s1). (C) Galvanostatic discharge capacitance in different aqueous electrolytes (from 1.2 V to 0 V).

environmentally friendly method for the preparation of castable supercapacitor electrodes for aqueous electrolytes. The investigations of the pore structure show that the electrodes are superior to standard PVDF-bound electrodes and additional pore blocking is largely reduced as compared to the latter. We also observed that the rate handling ability increases drastically after the electrodes have been soaked for several days in the electrolyte. We explain that by very long wetting time and a partly dissolving PVB content opening the pores again. While we have studied different PVP-to-PVB ratios and total binder amounts, the best performance balance was obtained for AC þ PVP(1.5) þ PVB(6.0) combining the mechanical, pore structure, electrical, and electrochemical properties. The electrochemical long-term stability has been demonstrated for more than 200 h of potential holding time and for 10,000 galvanostatic charge/discharge cycles. These data lead to the final conclusion that the mixture of 1.5 mass% PVP and 6 mass% PVB is a promising composition for the preparation of spraycoated or casted electrodes for aqueous supercapacitors and potentially other capacitive technologies, especially capacitive deionization or Capmix. Acknowledgments The INM (www.inm-gmbh.de) is part of the Leibniz Research Alliance Energy Transition (LVE). We acknowledge funding from the German Federal Ministry for Research and Education (BMBF) in support of the nanoEES3D project (award number 03EK3013) as part of the strategic funding initiative energy storage framework. The authors thank Prof. E. Arzt and Dr. P. W. de Oliveira for their continuing support, M. Zeiger for his support with Raman spectroscopy and electron microscopy, K.-P. Schmitt for his support with micro scratch measurements, and Dr. Y. Silina for the HPLC measurements (all at the INM).

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