Ion conductivity through TEMPO-mediated oxidated and periodate oxidated cellulose membranes

Journal Pre-proof Ion conductivity through TEMPO-mediated oxidated and periodate oxidated cellulose membranes ´ ´ S.T. Keene, A. Salleo, M. Norgren, L. C. Dahlstr¨om, V. Lopez Duran, ˚ Wagberg

PII:

S0144-8617(20)30003-5

DOI:

https://doi.org/10.1016/j.carbpol.2020.115829

Reference:

CARP 115829

To appear in:

Carbohydrate Polymers

Received Date:

4 October 2019

Revised Date:

18 December 2019

Accepted Date:

2 January 2020

´ ´ V, Keene ST, Salleo A, Norgren M, Please cite this article as: Dahlstr¨om C, Lopez Duran ˚ Wagberg L, Ion conductivity through TEMPO-mediated oxidated and periodate oxidated cellulose membranes, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115829

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Ion conductivity through TEMPO-mediated oxidated and periodate oxidated cellulose membranes C. Dahlströma, V. López Duránb, S. T. Keenec, A. Salleoc, M. Norgrena, L. Wågbergb. Department of Chemical Engineering, Fibre Science and Communication Network, Mid Sweden University, Holmgatan 10, SE-85170 Sundsvall, Sweden a

b

KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56, SE-100 44, Stockholm, Sweden c

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Department of Materials Science and Engineering, Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA

Highlights

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Ion conductivity through membranes depends on cellulose properties and electrolyte Specific ion effects could be observed for the TEMPO-oxidised cellulose Periodate oxidated cellulose showed low ion conductivity for all electrolytes Swelling was inhibited for the periodate oxidated cellulose

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Graphical abstract

Abstract

Cellulose in different forms is increasingly used due to sustainability aspects. Even though cellulose itself is an isolating material, it might affect ion transport in electronic applications. This effect is important to understand for instance in the design of cellulose-based supercapacitors. To test the ion conductivity through membranes made from cellulose

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nanofibril (CNF) materials, different electrolytes chosen with respect to the Hofmeister series were studied. The CNF samples were oxidised to three different surface charge levels via 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), and a second batch was further cross-linked by periodate oxidation to increase wet strength and stability. The outcome showed that the CNF pre-treatment and choice of electrolyte are both crucial to the ion conductivity through the membranes. Significant specific ion effects were observed for the TEMPO-oxidised CNF. Periodate oxidated CNF showed low ion conductivity for all electrolytes tested due to an inhibited swelling caused by the crosslinking reaction. Keywords: Cellulose nanofibrils; Ion conductivity; TEMPO oxidation; Periodate oxidation; Structure-property relationship; Specific ion effects

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1. Introduction

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Global growth requires an ever-increasing need for energy, which results in environmental impact due to excessive consumption of fossil fuels, which is not acceptable from a worldwide perspective. Considering the recent developments in global warming and our awareness of the limited resources, this issue is even more in focus and there is no doubt a huge need to develop new sustainable energy technologies using renewable raw materials. Energy storage devices, such as electric double layer capacitors (EDLCs), have generated great interest due to their higher power density, longer cycle lifetime and faster chargedischarge rate than batteries (Béguin & Frackowiak, 2013; Kötz & Carlen, 2000). EDLCs have a potential to be useful in different electrical applications such as electrical vehicles, balancing the power grid fluctuations and in energy harvesting from wind and sunlight. Commercial EDLCs usually contain expensive and toxic components, but recent research shows the potential in using alternative inexpensive and environmentally friendly materials (Andres, Dahlström, Blomquist, Norgren & Olin, 2018; Blomquist et al., 2017; Nyström et al., 2015). Furthermore, the construction of EDLCs is simple; two conductive electrodes are placed on each side of an isolating layer and the entire structure is soaked in an aqueous electrolyte. The electrodes consist e.g. of graphene/nano-graphite or carbon nanotubes as active material and cellulose nanofibrils (CNF) as binder (Andres et al., 2014; Hamedi et al. 2013). It needs to be stressed that a binding agent is crucial in order to form mechanically stable and durable electrodes. The capacitance stored in EDLCs is proportional to the accessible surface area of the electrodes. The hydrated ion size and the electrode pore size are key parameters (Chmiola et al. 2006). Since the electrodes consist of both the active material (i.e. graphene) and an inactive material (i.e. CNF), it is crucial to understand how the cellulose affects the ion transport within the electrode and how different electrolytes interact with cellulose. We used aqueous electrolytes due to several benefits compared to organic electrolytes, e.g. ionic conductivity, low toxicity, non-flammability and low cost. Moreover, we see a growing interest to use cellulose in different forms in energy storage applications, and besides being the most abundant natural biopolymer on earth, it shows superior properties in energy storage applications (Chun et al. 2012; Hu et al. 2009; Kang et al. 2012; Ko et al. 2017; Wang et al 2015). To the best of our knowledge, the effect of cellulose on the ion transport in an electrode is not known and needs to be established. To understand the influence of cellulose on properties such as ion conductivity, membranes made from two different types of cellulose materials 2

were studied; the conventional 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidised cellulose and another sample where the TEMPO-mediated oxidation was followed by periodate oxidation of the cellulose for increased wet strength and stability. Surface charge was changed from low to high for both types of cellulose materials. The electrolytes were chosen with respect to the Hofmeister series to capture ion specific effects. The main purpose of this study is therefore to identify key properties of the cellulose material in combination with cellulose-electrolyte interactions for increased ion conductivity. This knowledge is significant in order to select a suitable combination of electrolyte and type of CNF used in the electrode, to obtain high performance EDLCs. 2. Material and methods 2.1 Raw materials and chemicals

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A bleached softwood kraft pulp (K44) was supplied by SCA AB (Östrand pulp mill, Timrå, Sweden). Sodium metaperiodate (99%), was purchased from AK Scientific. Ammonium chloride (≥99%) purchased from Fluka was used. ( 4-Acetamido-2,2,6,6-tetramethylpiperidine 1-oxyl (4-AcNH-TEMPO, ≥98.0%), hydroxylamine hydrochloride (99%), 2-propanol (99.9%), sodium acetate (99%), sodium carbonate (≥99.5%), sodium chlorite (80%), sodium hypochlorite solution (10-15% available chlorine), magnesium chloride (hexahydrate >99%), and acetic acid glacial (≥99.85%) were purchased from Sigma-Aldrich. Sodium iodide ( >99.5%), sodium sulfate (≥99%), sodium hydroxide (NaOH) standard solution 1 M, and hydrochloric acid (HCl) standard solution 1 M were purchased from Merck Millipore. Deionised water was used throughout the study. The electrolytes prepared for the study were 0.5M and 1M Na2SO4, 0.5M and 1M NaI, 1 M NH4Cl and 1 M MgCl2. 2.2. Pulp washing

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Prior to use the fibres were disintegrated, washed and transferred to their sodium form as previously described (Wågberg & Hägglund, 2001). 2.3. TEMPO-mediated oxidation

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TEMPO-mediated oxidation was performed following an earlier published procedure (Tanaka, Saito, & Isogai, 2012). In brief, the fibres were dispersed in 0.1 M acetate buffer pH 4.8. Thereafter, 4-AcNH-TEMPO and sodium chlorite was added to the dispersion followed by addition of NaClO. The reaction was performed at 40 °C during three different times in order to achieve different charge densities of the fibres/fibrils: 1h, 8h and 48h. These samples will be referred to as TEMPO 1h, TEMPO 8h and TEMPO 48h throughout the text. 2.4. Periodate oxidation The three different TEMPO oxidised cellulose samples described in section 2.3. (TEMPO 1h, TEMPO 8h and TEMPO 48h) were each suspended to a concentration of 20 g/L with 6.3 wt% isopropanol and heated to 50 °C. Thereafter 1.35 g of NaIO4/g fibre were added and allowed to react between 15 to 25 min to introduce aldehydes to the cellulose and increase the wet strength of membranes prepared from fibrils from these fibres (Larsson, Gimåker, & Wågberg,

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2008). The reaction was stopped by washing with deionised water until the conductivity was below 5 μS/cm. These samples will be referred to as Periodate 1h, Periodate 8h and Periodate 48h throughout the text, where the numbers refer to the times used for TEMPO oxidation. 2.5. Carbonyl content and surface charge The carbonyl content and surface charge for all samples are summarized in Table 1 (detailed description is found in supporting information). From this table it is also clear that the material with the highest degree of TEMPO oxidation also contains some aldehyde groups without periodate oxidation which is expected (Saito & Isogai, 2004). Table 1. Surface charge and carbonyl content of modified fibrils

TEMPO oxidation

TEMPO and periodate oxidation

Sample name

Surface charge (μeq/g)

TEMPO 1h

270 ± 4

0

TEMPO 8h

479 ± 13

0

TEMPO 48h

802 ± 56

0.16 ± 0.03

Periodate 1h

229 ± 21

0.96 ± 0.06

Periodate 8h

1.12 ± 0.06

782 ± 9

1.06 ± 0.07

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2.6. Nanocellulose preparation

429 ± 12

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Periodate 48h

Carbonyl content (mmol/ g)

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Chemical structure

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Chemical modification

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After chemical modification the fibres were fluidized in a Microfluidizer M-110eh (Microfluidics Inc., USA) first by one pass through a 400 μm and a 200 μm chamber at a pressure of 900 bar followed by three passes through a 200 μm and a 100 μm chamber at 1500 bar. 2.7. Membrane preparation

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CNF fibrils were first diluted to a concentration of 0.2 wt% and dispersed for 15 min at 10000 rpm using an IKA Ultra Turrax followed by vacuum filtration using a 45 μm membrane (Durapore, Merck Millipore). After filtration, a membrane of the same type was placed on top of the membrane and this assembly was placed in-between two paperboards. Finally the membrane was dried for 15 min at 93 °C under a reduced pressure of 95 kPa, using the dryers of a Rapid Köthen sheet former (Paper Testing Instruments, Pattenbach, Austria). 2.8. Microscopy

2.8.1. Atomic force microscopy A CNF dispersion of 15 mg/L was first prepared and later used to adsorb the fibrils onto glow-discharged silicon wafers (WaferNet INC., USA) using polyethyleneimine (PEI) (60 kDa) as anchoring layer. The size of the fibrils was measured by using atomic force microscope (AFM) (Mutlimode IIIa, Veeco Instruments, CA), Fig. 1. The surface was scanned in

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ScansAsyst mode in air by use of an E piezoelectric scanner, the cantilevers had a nominal resonance of 70 kHz and a spring constant of 0.4 N/m. The surface roughness, defined as the root-mean square value (Rq), of the membranes was calculated by averaging three different points in a 5x5 μm2 area. 2.8.2. Scanning electron microscopy Surfaces images were taken were obtained using a field emission scanning electron microscope (FESEM; MAIA3 model 2016, TESCAN). Secondary electron images were generated using a 3 kV accelerating voltage and a 4.7 mm working distance.

𝑆𝑤𝑒𝑙𝑙𝑖𝑛𝑔 (%) =

𝑇𝑠 −𝑇0 𝑇0

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2.8.3. Light microscopy Cross-section images were taken using Leica M205 C Stereomicroscope. A razor blade was used to cut pieces of size 0.5*0.5 cm2 from each membrane TEMPO 48h and Periodate 48h. Images were taken at the same positions before and after being soaked in electrolyte for 20 min. The average thickness for each sample was calculated from 20 measurements. The degree of swelling was calculated according to equation 1. ∙ 100

(1)

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Where Ts is the thickness after swelling and T0 is the initial thickness before soaking in electrolyte/water.

Fig. 1. AFM images of different fibrils. First row from left to right: TEMPO 1h, TEMPO 8h and TEMPO 48 h; second row: periodate 1h, periodate 8h, and periodate 48h.

2.9. Specific Surface Area CNF membranes were prepared as described in section 2.7., but instead of drying the membrane the solvent was exchanged from water to ethanol. Thereafter, the membranes were dried using supercritical drying. The Brunauer-Emmett-Teller (BET) specific surface area was then determined by physisorption using an ASAPTM 2020 Micromeritics. (Detailed description is found in supporting information). 5

2.10. Electrochemical impedance spectroscopy

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2.10.1. Description of the technique Potentio-electrochemical Impedance Spectroscopy (PEIS) was performed using a BioLogic SP-300 Potentiostat using saturated silver/silver chloride (Ag/AgCl) reference electrode and a platinum wire counter electrode. The measurement setup is schematically shown in Fig. 2. The potential was set to 0 V vs. the reference electrode potential (VRE) for 5 seconds then an oscillatory voltage of 10 mV vs. VRE was applied with a frequency fAC sweeping from 3 MHz to 50 mHz with ten points per decade. Each data point was averaged over NA = 10 measurements. A high capacitance PEDOT:PSS electrode was used as the working electrode (WE) to reduce the impedance of the electrode surface. The impedance spectrum of the WE was measured prior to inserting membrane to extract CPEDOT:PSS and Relectrolyte for the electrolyte itself to fit independently of the inserted membrane. Bode representations of the measurements for each of the tested electrolytes and sample were imported into MatLab R2016b and fitted to the impedance model described below using the Curve Fitting Toolbox using a least-squares fitting algorithm.

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Fig. 2. Schematics of the electrochemical impedance spectroscopy measurement setup and the corresponding circuit model.

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2.10.2. Mathematical description of the evaluation of impedance of the membranes The impedance of the membrane coupled to the PEDOT:PSS high capacitance electrode is modelled using the equivalent circuit model from the attached schematic Fig. 2, which corresponds to Equation 2 and 3: 𝑍𝑅𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝑍𝐶𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒

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𝑍𝑡𝑜𝑡 (𝑓𝐴𝐶 ) = 𝑍𝑅𝑚𝑒𝑑𝑖𝑢𝑚 + 𝑍

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𝑍𝑡𝑜𝑡 (𝑓𝐴𝐶 ) = 𝑅𝑚𝑒𝑑𝑖𝑢𝑚 +

𝑅𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 +𝑍𝐶𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒

+ 𝑍𝐶𝑃𝐸𝐷𝑂𝑇:𝑃𝑆𝑆

𝑅𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 ⁄2𝜋𝑖𝑓𝐴𝐶 𝐶𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝑅𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 +1⁄2𝜋𝑖𝑓𝐴𝐶 𝐶𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒

+

1 2𝜋𝑖𝑓𝐴𝐶 𝐶𝑃𝐸𝐷𝑂𝑇:𝑃𝑆𝑆

(2) (3)

Where Rmedium is the electrolyte resistance, Rmembrane is the resistance to ion transport through the membrane, Cmembrane is the capacitance of the membrane, i is the imaginary number (-1)0.5, fAC is the oscillation frequency of the applied voltage, and CPEDOT:PSS is the capacitance of the PEDOT:PSS electrode. We fitted the impedance spectra to this model using the Trust-Region least squares fitting procedure in MatLab’s curve fitting toolbox. 3. Results and Discussion

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To isolate the effect on ion resistivity from the CNF we made all-cellulose membranes and studied the effect on ion transport separately. The anionic surface charge was altered from low to high to investigate if electrostatic interactions that occur between the cellulose surface and the electrolyte ions, affecting the ion conductivity. Two different types of cellulose were used; the conventional TEMPO-mediated oxidated cellulose and the periodate oxidised cellulose, where the latter was used to include the effect from wet strength and stability of membranes. The electrolytes used in this study were chosen to reveal possible ion-specific effects according to the foundation of the Hofmeister series (Hofmeister, 1888; Zhang and Cremer, 2006; Kunz, 2009). Fig. 3 shows that the ions might interact with the cellulose and induce swelling or deswelling according to the Hofmeister series. The chloride and sodium is referred to as neutral (in the middle) (Peula-García, Ortega-Vinuesa, & Bastos-González, 2010). Therefore, to study the anions we chose NaI and Na2SO4, i.e. the neutral sodium as the cation and two anions on each side of the neutral chloride to be able to capture both possible swelling and deswelling behaviour. To study the cations we chose NH 4Cl and MgCl2, i.e. the neutral chloride as the anion and two cations on each side of the neutral sodium.

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Fig. 3. Standard direct Hofmeister series for anions and cations based on precipitation studies of negatively charged proteins (Kunz, 2009). To the right, ions have the highest stabilization power and increase swelling whereas the left hand side has the opposite effect, for these specific systems.

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It should be mentioned that the universal Hofmeister series has been altered over the last years and replaced with a spectrum of direct and reversed series depending on salt concentration, pH-effects, hydrophobicity/hydrophilicity of the surface etc. (Schwierz, Horinek, Sivan, & Netz, 2016). According to Schwierz et al. (2016) the reversed Hofmeister series is valid for cations when the surface is hydrophilic, i.e. our anionic cellulose surfaces, at 1 M salt concentration. This means that e.g. NH4+ used in this study should swell the structure if the reversed Hofmeister series is valid since the cellulose surfaces are indeed hydrophilic.

3.1. Structural effects from chemical modification An increased surface charge after TEMPO-mediated oxidation will facilitate liberation of the fibrils during homogenisation (Saito, Nishiyama, Putaux, Vignon, & Isogai, 2006), resulting in finer sized structure for all samples. The surface charge of the TEMPO samples ranges from 270 µeq/g to 802 µeq/g, and the corresponding surface charges after periodate oxidation decreased only slightly (Table 1). The fibril width is significantly larger for the periodate 7

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samples (Periodate 1h: 2-9 nm, 8h: 2-14 nm and 48h: 0.5-14 nm) compared to the TEMPO samples (TEMPO 1h: 2-6 nm, 8h: 2-4.5 nm and 48h: 2-4.5 nm), which can be explained by the difficulty to fully liberate the fibres into individual fibrils after periodate oxidation (López Durán, Hellwig, Larsson, Wågberg, & Larsson, 2018). This result naturally leads to changes in the specific surface areas of formed membranes, where the TEMPO samples show a comparably larger internal surface area than the periodate samples due to a higher frequency of smaller pores (Table 2). The SEM images in Fig. 4 also supports this argument with larger fibres/fibrils present in the periodate samples. We aimed to introduce similar aldehyde content for the periodate samples to increase the wet strength of the membranes (Larsson et al., 2008).

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Fig. 4. Scanning electron microscopy images of the surface structures of prepared membranes a) TEMPO 1h, b) TEMPO 8h, c) TEMPO 48h, d) Periodate 1h, e) Periodate 8h and f) Periodate 48h. Bar is 1 µm.

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The membranes with higher surface charge of the fibrils also showed higher specific surface area. The reason is probably two-fold: first of all, the liberation of the fibrils is enhanced by the increased charge, as demonstrated by the decrease in particle size and hence an increase in the number of particles at a specified solids content. Second, the higher charge leads to an increase repulsion between the particles during membrane formation and this inevitably leads to a more open structure of the final membrane and hence an increased specific surface area, at least before the membrane is dried. After drying in air the situation is quite different. As can be seen from Table 2 the average pore diameter decreases as the charge and specific surface area of the membranes is increased for the membranes prepared from the TEMPO oxidized fibrils. The same trend can be detected for the periodate oxidised fibrils even though the average pore sizes are a bit higher for these membranes which is in accordance with the existence of larger aggregates of fibrils for this material. Considering the rather large increase in roughness of these membranes it can also be speculated that the pore size distribution of the membranes from periodate oxidised fibrils is less monodisperse than for the other membranes. The hydrated dimension of the ions of the electrolytes in this study is Mg2+ = 0.43 nm, Na+ = 0.36 nm, NH4+ = 0.33 nm, Cl- = 0.33 nm I- = 0.33 nm (Conway, 1981) and SO42- = 0.30 nm (Tanganov, 2013). From these data it is reasonable to assume that the ions have enough space to pass through the membranes without interacting with the fibrils assuming that the structure 8

determined from the gas adsorption is representative of the structure of the wet membrane. However, even if the ions are not directly geometrically obstructed they will follow a very tortuous path through the membrane and any differences noted can also be due to interactions with the cellulose. It should be noted that for all samples analysed using BET measurement, the solvent was exchanged from water to ethanol before supercritical drying, so the measured pore size might be slightly smaller than the actual pore size. However, since the membranes are used in their wet state it is suggested that the determined pore dimensions are representative of the membranes in actual use in the electrochemical evaluations.

Table 2. Properties of the cellulose membranes

15.5 10.7 10.6 15.9 11.3 12.7

Surface roughness (nm)

238 388 392 117 157 256

28.3 ± 8.7 21.7 ± 7.9 23.9 ± 6.9 30.1 ± 4.2 37.3 ± 7.4 34.3 ± 6.2

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TEMPO 1h TEMPO 8h TEMPO 48h Periodate 1h Periodate 8h Periodate 48h

Specific surface area (m2/g)

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Pore diameter (nm)

Sample

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3.1.1. Electrolyte effects on cellulose membrane swelling To investigate if and how the membranes were affected by the different electrolytes, including water, a swelling study was performed. By measuring the thickness increase after soaking the membranes in different electrolyte solutions, the influence from different ions on swelling/deswelling was obtained. The swelling behaviour was studied for the TEMPO 48h and periodate 48h membranes. These two samples were chosen since a high surface charge will increase the swelling, and therefore the effect would be easiest to measure. The soaking time for the swelling study was set to 20 minutes to match the measurement time in the electrochemical impedance spectroscopy. The result shows large differences in the degree of swelling depending on which electrolyte that was used (Fig. 5). In general the swelling of the TEMPO 48h membranes is considerable in all electrolytes except Na2SO4. However, the periodate 48h sample was only slightly swollen by the studied electrolytes. Based on the considerable improvements in wet strength, Larsson et al. 2008 suggested that the periodate oxidation and the formation of dialdehyde cellulose induces hemiacetal formation between adjacent fibrils. This hemiacetal formation would also explain the modest swelling detected for these membranes. Water was the best swelling agent for both samples. In deionised water, with Na+ as the counter-ion to the carboxyl groups, the TEMPO 48h increased in thickness by 502% and the periodate 48h increased by 114%. This comparably large swelling in water due to the osmotic pressure induced by the dissociated carboxyl groups on the modified cellulose also implies that the membranes will be sensitive to increase in ionic strength and that a significant deswelling would occur in electrolyte solutions. It is also important to stress that for these highly charged systems the divalent ions would show much larger deswelling as demonstrated by Bratko, Jönsson, & Wennerström (1986) and Guldbrand, Jönsson, Wennerström & Linse (1984) and in a recent work by Benselfelt, Engström, & Wågberg (2018). The effect of the divalent ions is both due to ion correlation and image charge effects. One could then expect a more modest swelling for the TEMPO 48h than what was observed for 9

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Mg2+ ions. However, Saito and Isogai (2005) studied ion-exchange behaviour for many different multivalent cations and found that the degree of ion-exchange for magnesium was relatively low compared to other divalent ions like calcium. Differences between magnesium and calcium can be found in other areas as well, like in the interaction with the polysaccharide alginate (Topuz, Henke, Richtering, & Groll, 2012).

Fig. 5. Swelling behaviour of TEMPO 48h and periodate 48h after soaking in different electrolytes/water for 20 min (left). All electrolytes are 1 M. Light microscopy cross-section images on TEMPO 48 h before and after soaked in water for 20 minutes (right).

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3.1.2. Ion specific effects on swelling behaviour Fig. 5 shows that the swelling of TEMPO 48h in 1 M NaI is much higher than in 1 M Na2SO4. Since both electrolyte have the same cation, the difference in swelling suggests that the effect is due to the anion. The valence of the anions is different which results in ionic strengths of 3 M (Na2SO4) versus 1 M (NaI). To investigate if the ionic strength had a significant influence on the result we changed the ionic strength of Na2SO4 and NaI, and studied the swelling. The trend was opposite for the two electrolytes; a higher ionic strength for NaI increased the swelling whereas a higher ionic strength for Na2SO4 decreased the swelling (Fig. 6). Note that it is the electrolyte concentration shown in Fig. 6. (Corresponding ion strengths are 1.5 M Na2SO4, 3 M Na2SO4, 1 M NaI and 2 M NaI.)

Fig. 6. Swelling measured for Na2SO4 and NaI at different concentrations.

This effect can partly be explained by ion specific effects that promote adsorption of iodide ions to the cellulose fibril surfaces, which increases the surface charge and results in increased membrane swelling (Schwierz, Horinek, & Netz, 2013). This is even more strongly accentuated when the concentration of iodide is increased. The results agree well with Schwierz et al. (2013) who found that the anion follows the direct Hofmeister series for a 1 M solution, while the cation follows the reversed series. Aveyard et al. (1976, 1977) have shown that for aqueous salt solutions in contact with macroscopic phases, evidence of preferential adsorption effects are 10

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3.2. Ion conductivity and swelling relationship

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given by the interfacial tension measurements. The interfacial energy is affected when there is a (positive or negative) excess of the cosolute at the interface (Piculell & Nilsson 1990). The composition of a two component solution at an interface is generally different from the composition in the bulk, even for simple liquids, and we may be confident that preferential adsorption occurs, in general, for an aqueous solution of a low-molecular solute in contact with a macromolecule, colloid or any surface. All these features have their parallel in the effects of the various salts on the association equilibria of macromolecules and colloids (Norgren et al 2002, Norgren & Edlund 2003, Norgren & Mackin 2009). This is expected due to the differences in polarizability of these two ions where iodide is a large and polarizable ion and then less polar than sulfate, which makes iodide to move closer to the particle surface rather than to be in the bulk water. This can be viewed upon as adsorption of iodide and depletion of sulfate. In the case of iodide, the effects on the system will be higher swelling. In other systems with e.g. anionic and nonionic polymers, iodide will increase solubility or increase colloidal stability compared to sulfate, but it is the same effect; salting-in (Kronberg, Holmberg & Lindman 2014). Notably, this is a general effect that should not be limited to aqueous solutions (Piculell & Nilsson 1990). The results for NH4Cl (Fig. 5) follow the reversed series and increases the swelling compared to Na2SO4. The periodate membranes generally show a modest swelling regardless of the electrolyte since their structures are locked due to the hemiacetal cross-linking (Larsson et al. 2008, Erlandsson et al 2018), and therefore we do not expect significant results from ion specificity.

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The ion conductivity of the membranes differs considerably between the different samples. The periodate samples (solid circles in Fig. 7) show very low ionic conductivity, which can be attributed to the low degree of swelling, resulting in a maintained pore structure as for the dry membranes with narrow pores and a highly tortuous diffusion path and naturally to a maintained surface charge density that also, as such, will limit ion conductivity. These results were not expected from the morphology point of view in section 3.1., where the periodate samples showed larger structures and it would be reasonable to suspect a higher ion conductivity, but these results were collected for solvent exchanged samples and a following critical point drying and not for air-dried sheets.

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Ion conductivity (mS cm )

70

NH4Cl

60

NaI

50 40 30 20 10

MgCl2

Na2SO4

0 0

50

100

150

200

Swelling (%)

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Fig. 7. Ion conductivity as a function of swelling for TEMPO 48 h (open circles) and periodate 48h (filled circles).

The TEMPO samples (open circles) in Fig. 7 show an increasing ion conductivity with increased swelling, with the exception of MgCl2, which indeed has a very low conductivity. This observation is reasonable since the ions will ”see” the cellulose walls less often due to a more open structure. Ions that are strongly hydrated have several water molecules with them which hinder the conductivity compared to poorly hydrated ions (Adamson, 1979). This might explain the low ion conductivity for MgCl2 where Mg2+ has a large hydrated ion size, even though the structure was swollen. 3.3. Surface charge effects on ion conductivity

3.0

NaI

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Ion conductivity (mS cm )

NaI NaI Cl NaI NH NH44Cl NH NH4Cl 44Cl Na Na22SO SO444 Na Na SO Na2SO 22SO44 MgCl MgCl MgCl 2 22 MgCl MgCl 2 2

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300

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Ion conductivity (mS cm )

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We studied the effect of surface charge on ion conductivity by using three different surface charges for both the TEMPO and the periodate samples, Fig. 8. The left graph corresponds to the TEMPO samples and the surface charge effect is only visible for NaI and NH4Cl where the ion conductivity increases with increasing surface charge. The explanation is the effect from Hofmeister series on swelling for these ions, explained in section 3.1.2. Even though the MgCl2 did swell the structure, the ion conductivity is low regardless of the cellulose surface charge. Overall, the periodate samples had low ion conductivity independent on surface charge (right graph in Fig. 8). As described earlier, the cross-linked fibril structure, inhibiting swelling, will hinder the ions to easily pass through the membranes.

400

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Surface charge ( eq g )

2.5 2.0 1.5 1.0 0.5 0.0

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Surface charge ( eq g )

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Fig. 8. Ion conductivity as a function of surface charge for the TEMPO samples (left graph) and for the periodate samples (right graph).

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4. Conclusions

This study has shown that ion conductivity through cellulose membranes indeed depends on cellulose properties and the electrolytes used. The membranes from TEMPO-oxidised followed the direct Hofmeister series for anions and the reversed series for cations, except for magnesium chloride. The latter did swell the structure significantly which normally is not found for divalent ions. At the same time the ion conductivity was low, but since the ioncellulose interactions do not seem to be significant, the low conductivity might be explained by the large hydrated ion size of Mg2+. Increasing the surface charge also increased the ion conductivity for NaI and NH4Cl due to ion specific effects.

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Even though the periodate oxidised cellulose showed larger structures, i.e. larger pores and lower surface area in the wet state, i.e. the dimensions determined after solvent exchange and critical point drying, the ion conductivity was overall smaller for all electrolytes compared to the TEMPO oxidated cellulose. The reason is the cross-linked fibril structure that inhibit the swelling and the ions are most probably obstructed by geometrical restrictions. Since cellulose in different forms is used increasingly in different electrical devices where ion conductivity is a key property, it is important to understand that both the choice of cellulose, cellulose pre-treatment and the electrolyte is crucial for the device performance. This study was performed on all-cellulose membranes and it should be mentioned that when cellulose is used as a binder in e.g. EDLCs the amount is around 10 wt% compared to the active material. Also the membranes in this study were allowed to swell freely, while used in electrodes the expansion is restricted and no swelling in the z-direction can occur. Therefore, future work should include different amounts of cellulose in the electrodes used in EDLCs and to evaluate further electrolytes, based on the Hofmeister series, in electrical measurements to investigate if ion specific effects occurs for other ion combinations.

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Acknowledgements The authors acknowledge Björn Lindman, Onur Parlak and Gregório Couto Faria for valuable discussions. Lars Wågberg also acknowledges The Knut and Alice Wallenberg (KAW) Research Foundation for financial support. This research has been supported by Treesearch.

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Appendix A. Supplementary data Supplementary information can be found in the online version. References

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