METHOD OF PREPARING CELLULOSE FIBRES COATED WITH REDOX-ACTIVE POLYMER

Abstract

 There is provided a method comprising the steps of: a) pretreating cellulose fibres so as to obtain pretreated cellulose fibres; and b) polymerizing monomers on the pretreated cellulose fibres so as to obtain cellulose fibres coated with a redox-active polymer. There is also provided a porous structure, such as a sheet, of pretreated cellulose fibres coated with a redox-active polymer, which porous structure is preferably produced according to the method.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of coated cellulose fibres to be used e.g. for energy storage applications.

BACKGROUND

[0002] Recent advances in science and technology has created an environmental consciousness shifting the societal and industrial focus towards green products and sustainable processes.

[0003] Furthermore, the demand for energy storage is constantly increasing and the development of inexpensive, flexible, lightweight and sustainable energy-storage materials is crucial.

[0004] Composite materials are multiphase materials produced from individual components that, when combined, yields a material possessing new properties compared with the individual components.

[0005] Cellulose is a material that combines widespread availability and low-cost manufacturing with high material strength, flexibility and a functionalizable surface. In addition, paper-products have well-established production methods.

[0006] In "Nyström, G. et al. (2010) A Nanocellulose Polypyrrole Composite Based on Microfibrillated Cellulose from Wood. Journal of Physical Chemistry B, 114(12):4178-4182" microfibrillated cellulose (MFC) was coated with pyrrole and formed into composites.

[0007] However, the problem is that the industrial feasibility of this method is limited.

SUMMARY

[0008] The inventors have shown that increasing the dry weight ratio of pyrrole to cellulose fibres to above 1.5 when pyrrole is polymerized on the cellulose fibres has little effect on the charge capacity (see fig. 1). This suggest that there is a maximum amount of polypyrrole in the composites for a given type of pulp. The inventors have realized that the charge capacity can instead be further increased by pretreating the pulp.

[0009] Accordingly, the following itemized listing of embodiments is provided:

1. A method comprising the steps of:

a) pretreating cellulose fibres so as to obtain pretreated cellulose fibres; and

b) polymerizing monomers on the pretreated cellulose fibres so as to obtain cellulose fibres coated with a redox-active polymer.

2. The method of item 1, which is a method of producing a porous structure of cellulose fibres coated with a redox-active polymer and which further comprises the step:
c) forming the porous structure from the coated fibres obtained in step b).

3. The method of item 2, wherein the porous structure is a sheet.

4. The method of item 3, wherein step c) comprises forming the sheet in a paper machine from an aqueous furnish comprising the coated fibres.

5. The method of item 4, wherein the pH of the furnish in step c) is at least 5, such as 5-10, such as 6-8.

6. The method of item 5, wherein the furnish comprises a buffer.

7. The method of item 2 or 3, wherein step c) comprises dry forming, pressing and/or extrusion.

8. The method of any of the preceding items, wherein the redox-active polymer is a conducting polymer.

9. The method of item 8, wherein the monomers comprise pyrrole monomers.

10. The method of item 8, wherein the monomers are pyrrole monomers and the redox-active polymer is polypyrrole.

11. The method of any one of items 1-7, wherein the monomers comprise 3,4-ethylenedioxythiophene (EDOT) monomers.

12. The method of any one of items 1-7, wherein the monomers are 3,4-ethylenedioxythiophene (EDOT) monomers and the redox-active polymer is poly(3,4-ethylenedioxythiophene) (PEDOT).

13. The method of any one of items 1-7, wherein the monomers are thiophene (T) and the redox-active polymer is polythiophene (PT).

14. The method of any one of items 1-7, wherein the monomers are aniline (ANI) and the redox-active polymer is polyaniline (PANI).

15. The method of any one of items 1-7, wherein the monomers are acetylene (Ac) derivatives and the redox-active polymer is polyacetylene (PAc).

16. The method of any one of items 1-7, wherein the monomers are phenylene (Ph) and the redox-active polymer is polyphenylene (PPh).

17. The method of any one of items 1-7, wherein the monomers are phenylene sulfide (PhS) and the redox-active polymer is polyphenylene sulfide (PPhS).

18. The method of any one of items 1-7, wherein the monomers are phenylene vinylene (PhV) and the redox-active polymer is polyphenylene vinylene (PPhV).

19. The method of any one of the preceding items, wherein the proportion of redox-active polymer in the coated fibres obtained in step b) is at least 40% by weight, such as 40%-70% by weight, such as 40%-60% by weight.

20. The method of any one of the preceding items, wherein the proportion of redox-active polymer in the coated fibres obtained in step b) is at least 50% by weight, such as 50%-70% by weight, such as 50%-60% by weight.

21. The method of any one of the preceding items, wherein the pretreatment of step a) comprises chemical pretreatment.

22. The method of item 21, wherein the chemical pretreatment comprises (2,2,6,6-tetramethylpiperidin-i-yl)oxyl (TEMPO) oxidation, nitroxyl radical oxidation, chlorite oxidation, periodate oxidation, peroxide oxidation, alkali metal nitrite oxidation, alkali metal nitrate oxidation, ozone oxidation, oxone oxidation, permanganate oxidation, carboxymethylation, sulfonation or phosphorylation.

23. The method of item 21 or 22, wherein the chemical pretreatment comprises alkali treatment, optionally under pressure.

24. The method of item 23, wherein the fibres are exposed to a pH of at least 10, such as at least 11, such as at least 12.

25. The method of item 23 or 24, wherein the alkali comprises NaOH.

26. The method of any one of the preceding items, wherein the pretreatment of step a) comprises mechanical pretreatment.

27. The method of any one of the preceding items, wherein the mechanical pretreatment comprises the application of a compressing or shearing force.

28. The method of any one of the preceding items, wherein the mechanical pretreatment comprises refining, preferably low consistency (LC) refining.

29. The method of any one of the preceding items, wherein the pretreatment of step a) comprises steam explosion.

30. The method of any one of the preceding items, wherein the pretreated cellulose fibres are obtained in step a) as an aqueous fibre suspension having a Schopper-Riegler (SR) number (ISO 5267-1:1999) of 10-80, such as 20-77, such as 30-77.

31. The method of any one of the preceding items, wherein the cellulose fibres are never-dried cellulose fibres.

32. The method of any one of the preceding items, wherein the cellulose fibres are wood cellulose fibres.

33. The method of any one of the preceding items, wherein the cellulose fibres are hardwood fibres or softwood fibres, preferably softwood fibres.

34. The method of any one of the preceding items, wherein the cellulose fibres are bleached fibres or unbleached fibres, preferably bleached fibres.

35. The method according to item 34, wherein the brightness (ISO 2470-1:2016) of the bleached fibres is at least 78 %, such at least 80 %, such as at least 83 %.

36. The method of any one of the preceding items, wherein the cellulose fibres are kraft fibres.

37. The method of any one of the preceding items, wherein fibres are washed between steps a) and b) and/or between steps b) and c).

38. The method of any one of the preceding items, wherein the charge capacity of the coated fibres obtained in step b) is at least 130 C/g, such as at least 150 C/g and wherein the charge capacity is the amount of oxidation charge measured in 2 M NaCl (aq.) with cyclic voltammetry between -0.9 V and 0.3 V vs Ag/AgCl with a scan rate of 5 mV/s divided by the total weight of the coated fibres.

39. The method of any one of the preceding items, wherein the charge capacity of the coated fibres obtained in step b) is 50-150 C/cm³, such as 100-120 C/cm³, and wherein the charge capacity is the amount of oxidation charge for example measured in 2 M NaCl (aq.) with cyclic voltammetry between -0.9 V and 0.3 V vs Ag/AgCl with a scan rate of 5 mV/s divided by the volume of the coated fibre sample.

40. The method of any one of the preceding items, wherein the specific surface area of the coated fibres obtained in step b) is in the range of 4.0-10.0 m²/g, such as 5.0-9.0 m²/g, when measured according to standard BET analysis of nitrogen sorption isotherms.

41. The method of any one of the preceding items, wherein the dry weight ratio of pretreated cellulose fibres to monomers in step b) is in the range of 0.8:1.0 to 1.8:1.0, such as 1.0:1.0 to 1.5:1.0.

42. The method of any one of the preceding items, wherein the charge density of the pretreated fibres obtained in step a) is 100-1500 µeq/g, such as 100-1000 µeq/g when measured according to SCAN-CM 65:02.

43. The method of any one of the preceding items, wherein step b) is carried out in water, optionally in the presence of a detergent.

44. A porous structure of pretreated cellulose fibres coated with a redox-active polymer, which porous structure is preferably produced according to any one of the preceding items.

45. The porous structure of item 44, which is a sheet.

46. The porous structure of item 44 or 45, wherein the redox-active polymer is a conducting polymer.

47. The porous structure of item 46, wherein the redox-active polymer is polypyrrole.

48. The porous structure of item 44 or 45, wherein the redox-active polymer is poly(3,4-ethylenedioxythiophene) (PEDOT).

49. The porous structure of item 44 or 45, wherein the redox-active polymer is polythiophene (PT).

50. The porous structure of item 44 or 45, wherein the redox-active polymer polyaniline (PANI).

51. The porous structure of item 44 or 45, wherein the redox-active polymer is polyacetylene (PAc).

52. The porous structure of item 44 or 45, wherein the redox-active polymer is polyphenylene (PPh).

53. The porous structure of item 44 or 45, wherein the redox-active polymer is polyphenylene sulfide (PPhS).

54. The porous structure of item 44 or 45, wherein the redox-active polymer is polyphenylene vinylene (PPhV).

55. The porous structure of any one of items 44-54, wherein the proportion of redox-active polymer is at least 40% by weight, such as 40%-70% by weight, such as 40%-60% by weight.

56. The porous structure of any one of items 44-54, wherein the proportion of redox-active polymer is at least 50% by weight, such as 50%-70% by weight, such as 50%-60% by weight.

57. The porous structure of any one of items 44-56, wherein the pretreated cellulose fibres are oxidized, carboxymethylated, sulfonated or phosphorylated.

58. The porous structure of any one of items 44-57, wherein the pretreated cellulose fibres are never-dried cellulose fibres.

59. The porous structure of any one of items 44-58, wherein the cellulose fibres are wood cellulose fibres

60. The porous structure of any one of items 44-59, wherein the pretreated cellulose fibres are hardwood fibres or softwood fibres, preferably softwood fibres.

61. The porous structure of any one of items 44-60, wherein the cellulose fibres are bleached fibres or unbleached fibres, preferably bleached fibres.

62. The porous structure of any one of items 44-61, wherein the cellulose fibres are kraft fibres.

63. The porous structure of any one of items 44-62, wherein the charge capacity of the coated fibres is at least 150 C/g and wherein the charge capacity is the amount of oxidation charge measured in 2 M NaCl (aq.) with cyclic voltammetry between -0.9 V and 0.3 V vs Ag/AgCl with a scan rate of 5 mV/s divided by the total weight of the coated fibres.

64. The porous structure of any one of items 44-63, wherein the specific surface area of the coated fibres is in the range of 4.0-10.0 m²/g, such as 5.0-9.0 m²/g, when measured according to standard BET analysis of nitrogen sorption isotherms.

65. An energy storage device comprising a porous structure according to any one of items 44-64.

66. Use of a porous structure according to any one of items 44-64:

as an electrode in an energy storage device;

for electrochemically controlled ion release;

for electrochemically controlled ion extraction;

in an oxygen sensor;

in a microbial or enzymatic fuel cell; or

as a light to heat conversion membrane, e.g. in solar distillation of salt water.

BRIEF DESCRIPTION OF THE FIGURES

[0010] 

Fig. 1 shows the charge capacity after polymerization as a function of the weight ratio of pyrrole monomer to cellulose before polymerization.

Fig. 2 shows voltammograms obtained with a scan rate of 5 mV/s in 2 M NaCl for the composites of example 2. It is seen that the composite prepared with never-dried pulp has higher electroactivity than the composite prepared with dried pulp that was resuspended before synthesis. The increased electroactivity is explained by higher concentration of polypyrrole.

Fig. 3 shows the cell potential (V) during 1 h of operation of the set-up of example 8.

Fig. 4 shows the cell potential (V) during 204 min of operation of the set-up of example 9.

DETAILED DESCRIPTION

[0011] As a first aspect of the present disclosure, there is provided a method comprising the steps of:

a) pretreating cellulose fibres so as to obtain pretreated cellulose fibres; and

b) polymerizing monomers on the pretreated cellulose fibres so as to obtain cellulose fibres coated with a redox-active polymer ("coated fibres").

[0012] The cellulose fibres are preferably never-dried cellulose fibres. They are typically wood fibres, such as hardwood fibres or softwood fibres. Softwood fibres are preferred. Cellulose fibres can also be obtained from non-wood sources such as straw (e.g. wheat straw, rice straw, corn straw or sorghum straw), bagasse, hemp, bamboo, reed, grass (e.g. mischanthus), jute, flax and sisal.

[0013] The cellulose fibres are preferably bleached, but unbleached fibres are not excluded. The brightness (ISO 2470-1:2016) of the bleached fibres is typically at least 78 %, preferably at least 80 %, such as at least 83 %, such as at least 85 %.

[0014] The pulping method used to prepare the cellulose fibres may be kraft or sulphite pulping. Accordingly, the fibres may be kraft fibres or sulphite fibres, such as dissolving fibres.

[0015] The pretreatment of step a) may for example comprise chemical pretreatment. Examples of chemical pretreatments are (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidation, nitroxyl radical oxidation, chlorite oxidation, periodate oxidation, peroxide oxidation, alkali metal nitrite oxidation, alkali metal nitrate oxidation, ozone oxidation, oxone oxidation, permanganate oxidation, carboxymethylation, sulfonation and phosphorylation.

[0016] Another example of a chemical pretreatment is alkali treatment, which may be carried out under pressure. During alkali treatment, the fibres may be exposed to a pH of at least 10, such as at least 11, such as at least 12. The alkali may for example comprise NaOH.

[0017] As an alternative or complement to the chemical pretreatment, mechanical pretreatment may be carried out. The mechanical pretreatment typically comprises the application of compressing or shearing forces.

[0018] An example of mechanical pretreatment is refining, preferably low consistency (LC) refining, which is typically carried out at a consistency of 2-6 %.

[0019] Another example of a pretreatment is steam explosion.

[0020] The pretreated cellulose fibres are typically obtained in step a) as an aqueous fibre suspension. Such a suspension may have a Schopper-Riegler (SR) number of 10-80, such as 20-77, such as 30-77. The SR number is determined according to the standard ISO 5267-1:1999.

[0021] The charge density of the pretreated fibres obtained in step a) may for example be 100-1500 µeq/g, such as 100-1000 µeq/g, such as 200-1000 µeq/g. Charge density is measured according to SCAN-CM 65:02.

[0022] The fibres may be washed between steps a) and b), e.g. to remove substances that may interfere with the polymerization.

[0023] The redox-active polymer formed in step b) may for example be a conducting polymer, which in the context of the present disclosure means an electrically conducting polymer. An example of an electrically conducting polymer is polypyrrole.

[0024] Examples of monomers to be used in step b) comprise pyrrole (Py), 3,4-ethylenedioxythiophene (EDOT), thiophene (T), aniline (ANI), acetylene (Ac) derivatives, phenylene (Ph), phenylene sulfide (PhS), phenylene vinylene (PhV) and mixtures thereof.

[0025] Accordingly, examples of redox-active polymers are polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT), polythiophene (PT), polyaniline (PANI), polyacetylenes (PAcs), polyphenylene (PPh), polyphenylene sulfide (PPhS) and polyphenylene vinylene (PPhV).

[0026] The preferred monomer is pyrrole and the preferred redox-active polymer is polypyrrole.

[0027] Step b) is typically carried out in water, which means that the pretreated fibres are provided as a water suspension. A detergent, such as a Tween detergent, e.g. Tween 80, may be present in the water suspension when the polymerization is carried out.

[0028] The dry weight ratio of pretreated cellulose fibres to monomers in step b) is preferably in the range of 0.8:1.0 to 1.8:1.0, such as 1.0:1.0 to 1.5:1.0 (see also fig. 1)

[0029] The proportion of redox-active polymer in the coated fibres obtained in step b) may be at least 40% by weight, such as 40%-70% by weight, such as 40%-60% by weight. Preferably, said proportion is at least 50% by weight, such as 50%-70% by weight, such as 50%-60% by weight.

[0030] The charge capacity (by weight) of the coated fibres obtained in step b) is preferably at least 130 C/g, such as at least 150 C/g. The charge capacity is the amount of oxidation charge measured in 2 M NaCl (aq.) with cyclic voltammetry between -0.9 V and 0.3 V vs Ag/AgCl with a scan rate of 5 mV/s divided by the total weight of the coated fibres.

[0031] The charge capacity (by volume) of the coated fibres obtained in step b) may be 50-150 C/cm3, such as 100-120 C/cm3. Here the charge capacity is the amount of oxidation charge for example measured in 2 M NaCl (aq.) with cyclic voltammetry between -0.9 V and 0.3 V vs Ag/AgCl with a scan rate of 5 mV/s divided by the volume of the coated fibre sample.

[0032] The specific surface area of the coated fibres obtained in step b) may be in the range of 4.0-10.0 m²/g, such as 5.0-9.0 m²/g, when measured according to a protocol based on the BET method (Brunauer, Stephen, Paul Hugh Emmett, and Edward Teller. "Adsorption of gases in multimolecular layers." Journal of the American chemical society 60.2 (1938): 309-319.). Nitrogen (N2) gas adsorption isotherms were recorded using an ASAP 2020 (Micromeritics, USA) instrument. Measurements were performed at liquid nitrogen temperatures (i.e., 77 K), and the specific surface areas of the samples were obtained from the isotherms using the BET method.

[0033] The average thickness of the polymer coating on the coated fibres obtained in step b) may for example be in the range of 25-100 nm.

[0034] In one embodiment, the method of the first aspect is a method of producing a porous structure of cellulose fibres coated with a redox-active polymer, which (in addition to steps a) and b) discussed above) comprises the step:
c) forming the porous structure from the coated fibres obtained in step b).

[0035] The coated fibres may be washed between steps b) and c), e.g. to remove excess chemicals.

[0036] The porous structure may be a sheet. Accordingly, step c) may comprise forming the sheet from an aqueous furnish comprising the coated fibres, preferably in a paper machine. The paper machine typically comprises a head box, a wire section and a drying section. It may also comprise a pressing section between the wire section and the drying section. In a traditional paper machine, the pH of the furnish is preferably at least 5. Accordingly, the pH of the furnish in step c) may be at least 5, such as 5-10, such as 6-8. To obtain such a pH, the furnish may comprise a buffer. It is thus more preferred to adjust the pH with a buffer than with alkali.

[0037] Alternatives to forming the porous structure in a paper machine are dry forming, pressing and/or extrusion.

[0038] As a second aspect of the present disclosure, there is provided a porous structure of pretreated cellulose fibres coated with a redox-active polymer ("coated fibres"), which porous structure is preferably produced according to the first aspect.

[0039] In a preferred embodiment, the porous structure is a sheet, such as a sheet produced on a paper machine.

[0040] The various examples and embodiments of the first aspect described above apply to the second aspect mutatis mutandis.

[0041] The porous structure of the second aspect may advantageously form part of an energy storage device, such as a battery or capacitator.

[0042] The porous structure of the second aspect may find many applications. As a third aspect of the present disclosure, there is provided a use of a porous structure according to the second aspect:

as an electrode in an energy storage device;

for electrochemically controlled ion release;

for electrochemically controlled ion extraction;

in an oxygen sensor;

in a microbial or enzymatic fuel cell; or

as a light to heat conversion membrane, e.g. in solar distillation of salt water.

EXAMPLES

Example 1 - Polypyrrole-coating of fibres and composite production

[0043] Composites based on never-dried bleached kraft pulp from pine (9 g dry content) that had been refined in a PFI refiner with 6000 revolutions was compared to composites based on the same kraft pulp that had not been refined. The pulp samples (9 g of dry content) were diluted to 1 wt-% in 0.5 M HCl (aq.). 22.5 mL of pyrrole and 3 mL of Tween-80 were added and the suspensions were stirred for 1 h. Polymerization was initiated by the addition of 1 L 0.71 M FeCl₃ in 0.5 M HCl (aq.), and the mixtures were stirred for 30 min. The products were collected in a juice filter and washed with 2.5 L of 0.5 M HCl (aq.) followed by 2 L of 0.1 M NaCl (aq.).

[0044] Sheets of the products were formed using a SCAN Finnish Sheet former (Lorentzen & Wettre) resulting in a 165 mm x 165 mm sheets and dried under ambient conditions to a grammage of 433 g/m² for the composite with refined pulp, compared to 381 g/m² for the composite with unrefined pulp. Furthermore, the density was higher for the refined pulp composite (187 kg/m³ compared to 115 kg/m³), possibly due to shorter length of the fibres allowing closer arrangement of the material and/or because excess fibre surfaces that are not coated with polypyrrole bind to each other.

Example 2 - Polypyrrole-coating of dried and resuspended pulp fibres

[0045] Following the procedure of example 1, two composites were produced; one batch was prepared with never-dried bleached kraft pulp of pine, and another batch was prepared with dried and resuspended bleached kraft pulp of pine.

Example 3 - Polypyrrole-coating of TEMPO-oxidised pulp fibres

[0046] 50 g never-dried bleached kraft pulp from pine (15 wt-% dry content) was dispersed in 1400 mL deionized water and 50 mg of 2,2,6,6-Tetramethyl-1-piperidinine 1-oxyl (TEMPO) and 520 mg sodium bromide was added to the solution. The mixture was mechanically stirred and 100 mL of 10 wt-% sodium hypochlorite was added to initiate the reaction. The reaction was allowed to proceed for 1 hour, and pH was monitored throughout the reaction. Subsequently, the reaction was quenched by the addition of 20 mL ethanol. The cellulose was collected in a sieve and thoroughly washed with 15 L deionized water. The wet TEMPO oxidized pulp was collected and stored in its wet state (pH 8) in a fridge at 6 °C.

[0047] Wet TEMPO-oxidized pulp (403.7 mg) was mixed in 80 mL 0.5 M hydrochloric acid. 1 drop of Tween-80 and 1.5 mL pyrrole was added to the solution. 12.9 g iron chloride hexahydrate was dissolved in 100 mL 0.5 M hydrochloric acid and the polymerization was allowed to proceed for 1 h. The mixture was collected in a sieve and washed with 3 L 0.5 M hydrochloric acid and 2 L 0.1 M sodium chloride. The resulting polypyrrole-coated TEMPO-oxidized pulp was collected on a filter paper in a Büchner funnel and dried in a heat press at 80 °C for 15 minutes and in an oven at 70 °C for 45 minutes.

Example 4 - Electrochemical battery 1

[0048] From the composites produced in Example 1, an electrochemical battery cell was assembled with the 1.43 g of composite as positive electrode and 0.42 g zinc foil as negative electrode separated by a filterpaper (General purpose, 0.15 mm thick, pore size 12-15 µm, Munktell, Sweden). The electrodes were contacted by graphite foils (Sigraflex grade Z, 0.15 mm thick, SGL Carbon, Germany), and the electrodes were stacked with the separator in-between, soaked in NH₄Cl (aq.) 25 wt-% and hermetically sealed in vacuum plastic sealing (OBH Nordica).

Example 5 - Electrochemical battery 2

[0049] From the composites produced in Example 1, a circular piece of the composite with 2 cm diameter weighing 25 mg was cut out. The piece was directly used as positive electrode with lithium foil as negative electrode, with a polyethylene membrane as separator (Solupor, Lyndall USA), and 1 M lithium hexafluorophosphate in ethylene carbonate:diethyl carbonate (1:1, volume/volume) as electrolyte. The composite was contacted by aluminium foil, while the lithium was contacted by copper foil, and the stack of current collectors, positive and negative electrodes with separator in-between, soaked in the electrolyte, was hermetically sealed in polymer laminated aluminium foils with the current collector tabs extended out of the cell through the seam.

Example 6 - Electrochemical battery 3

[0050] From the composites produced in Example 1, three 165 mm x 165 mm sheets with a weight of 35 g were prepared. An electrochemical battery cell was assembled with two composite sheets employed as positive electrode and a single composite sheet employed as negative electrode, with a paper as separator (InterLayer 35, BillerudKorsnäs, Sweden) soaked in 2 M NaCl (aq.) as electrolyte, and contacted with graphite foils (SigraFlex grade Z, SGL Carbon, Germany). The cell was hermetically sealed in vacuum plastic (OBH Nordica).

Example 7 - pH dependence of composite production

[0051] A composite of polypyrrole and pulp was prepared according to the general description in Example 1, but without forming a sheet of the composite. Cost efficient approaches to form electrodes of the composite are highly desired, and they include preparation in a paper machine. Implementing the sheet formation on existing paper machine equipment generally require the feedstock solution to have pH values above 5. However, increasing the pH in conducting polymer systems is usually accompanied with adverse effects on the electroactivity. The composite solutions were divided in two parts, where the pH of the solutions was increased by addition of either 0.1 M NaOH or 0.1 M pH 7 phosphate buffer solution. For the fraction with NaOH addition the value was increased to pH 6.3, and for the fraction with phosphate buffer addition the value was increased to pH 6.7.

Example 8 - Electrochemical battery 4

[0052] From the composites produced in Example 1, four pieces 1.5 cm x 1.5 cm of the composite were cut out; the individual weights of the pieces were 72±2 mg. Two battery cells connected in series were assembled by using a single composite pieces as positive and negative electrode, respectively, with a cellulose filter paper as separator (General purpose, 12-15 µm pore size, Munktell Sweden) in-between, soaked in 2 M NaCl (aq.) as electrolyte, and contacted by graphite foils foils (SigraFlex grade Z, SGL Carbon, Germany). The cells were hermetically sealed in polymer laminated aluminium foils.

[0053] The cells were charged by applying a potential of 1.5 V and connected to power a wireless thermometer (RAR801, CAPiDi, Sweden). RAR801 includes two parts; one temperature sensor with wireless transmission of the data, and one base-station that receives the data from the transmitter and displays both its own (indoor) and the transmitters (outdoor) temperature. The cells were connected to the temperature sensor with wireless transmission of the data and the cell potential was monitored during 1 h of operation.

Example 9 - Electrochemical battery 5

[0054] The procedure of Example 8 was repeated, but each electrode had the dimensions 1 cm x 2.5 cm and weighed ∼40 mg. The cells were sealed in plastic (OBH Nordica, Vacuum sealing plastic for food) with two layers of thermal bonding film (3M, TBF 588) in the seams.

[0055] The cells were charged with a potential of 3.2 V and connected to power an electronic circuit (TIDA 00524, Texas Instruments, USA) monitoring temperature and luminous flux.

Example 10 - Experimental paper machine

[0056] A composite of polypyrrole and pulp was prepared according to the general description in Example 1 and repeated to produce in the order of 2.4 kg of composite material. The material was stored in 24 L 0.1 M NaCl solution and with pH 1.8, and transferred to a 300 L container equipped with propellers and connected to a so called experimental paper machine setup (available in MoRe researched facilities in Örnsköldsvik, Sweden). The experimental paper machine setup is a miniature of a standard paper machine, producing 22.5 cm wide sheets, and is composed of a section where the material is drained on a wire, a press section, a drying cylinder section.

[0057] 20 L of 0.1 M pH 7 and 10 L of 0.2 M pH 7 buffer solution were added to the polypyrrole-coated pulp and the mixture was further diluted with deionized water to 200 L dry content of 1.2 wt-%. The mixture was stirred for 4 h prior to being added to the experimental paper machine. Two of the other 300 L containers connected to the experimental paper machine were filled with furnish prepared by diluting bleached softwood pulp. Initially, the furnish was fed to the experimental paper machine. The furnish was pumped through a system of pipes prior to entering a Fourdrinier wire screen where the water was drained. The machine was operated at a speed of 1.4 m/minute with a grammage between 180 g/m2 and 190 g/m2. The web was transferred from the wire screen to a wet press section consisting of felts and calenders applying a pressure of 7 kg/cm2 (∼700 kPa). Following the wet press section there was a dryer consisting of three sections heating the material to 70-80°C, 90-100°C and 50-60°C, respectively. Finally, the material entered another calendaring section and was rolled.

[0058] Subsequently, the mixture containing polypyrrole-coated pulp was fed to the machine and the addition of furnish was stopped, providing a transition from a web consisting of uncoated softwood fibres to polypyrrole-coated fibres entering the experimental paper machine.

Characterization methods

Cyclic voltammetry

[0059] Cyclic voltammetric experiments were performed using the composite materials as a working electrode in a three electrode setup in 2 M NaCl (aq.) with a platinum counter electrode and an Ag/AgCl reference electrode.

Cell capacity

[0060] The battery cell was discharged with a constant current of 1 mA while the cell potential was monitored as a function of time.

ζ-potential

[0061] The ζ-potential of a solution prepared with the 10 mg TEMPO oxidized bleached kraft pulp was compared with a solution prepared with bleached kraft pulp. The electrophoretic mobilities of the solutions were measured at 25 °C and pH 7 using a universial dip cell (Malvern Instruments, UK) and a ZetaSizer Nano instrument (Malvern Instruments, UK). The ζ-potential were determined from the electrophoretic mobilities using the Smoluchowski model.

Mechanical testing

[0062] Dry tensile tests were performed using a Zwick tensile tester in a conditioned room in a similar manner to ISO1924-3 standard. While the sheets prepared from pulp without PFI refinement did not exhibit enough strength to be measured, the sheets prepared from PFI refined pulp were pulled until they broke with a speed of 10 mm/min, and repeated three times. A tensile strength of 180 N/m and a tensile index of 0.51 Nm/g were obtained for the sample defining the mechanical integrity of the material, which is of importance in order to have electronic contact throughout the structure, especially for thick and large electrodes.

Results

[0063] In Example 1, both composites exhibited a reversible capacity of 137 C/g normalized with respect to the total weight of the electrode. However, the density of the composite with refined fibres was 63 % higher, meaning that the reversible capacity per volume was 63 % higher when refined fibres were used instead of unrefined fibres.

Table 1. Reversible capacity measured of the composites.

	Reversible capacity (C/g)
Example 2 (refined never-dried pulp)	140
Example 2 (refined resuspended dried pulp)	120
Example 3 (TEMPO-oxidized pulp)	155
Example 7 (pH 6.3, adjusted with NaOH)	90
Example 7 (pH 6.7, phosphate buffer)	120
Example 10 (Sheet of polypyrrole-coated fibres from the experimental paper machine)	131

[0064] The composite material of Example 3 (pyrrole-coated TEMPO-oxidised fibres) exhibited a reversible capacity of 155 C/g normalized with respect to the total weight of the electrode, which can be compared to 137 C/g from Example 1. This manifest the possibility to increase the amount of polypyrrole in the composites.

[0065] Example 2 shows that never-dried pulp works better than resuspended dried pulp. The difference in capacity/polypyrrole concentration corresponds to the difference in electroactivity demonstrated in fig. 2.

[0066] Example 7 shows that increasing the pH by addition of solutions with a weak acid and its conjugate base has a relatively small effect on the electroactivity of the polymer, whereas an pH increase obtained by the addition of alkaline solutions significantly reduces the electroactivity.

[0067] In Table 2, the capacities of the assembled cells are presented.

Table 2. Capacity of assembled cells.

	Capacity (mAh)	Normalised capacity (mAh/g)
Example 4	64	45
Example 5	0.7	27
Example 6	630

[0068] The capacity measurements show that the polypyrrole-coated fibres are compatible with different redox reactions. In example 4 a water-based electrolyte was used, whereas in example 5 an organic electrolyte with lithium salt was used. In the latter case it is shown that even though the pulp fibres are inherently hydrophilic a good wetting with the organic electrolyte is achieved.

[0069] In example 6, composite sheets were used as both positive and negative electrode, thus having a symmetric setup, and showing that also super capacitors can be produced from the composites.

[0070] The battery formed in example 8 powered the temperature sensor during one hour of operation. During this period, data were wirelessly transmitted to the base station about every 30 seconds. The potential monitored during operation is shown in fig. 3. An overall trend of a linear decrease in cell voltage is observed in fig. 3. This is in accordance with a capacitive behavior, where the cell voltage is a linear function of the charge state, and the battery is thus suitable as a capacitor. Concerning the measurements, there are repetitive and rhythmic potential drops occurring with ∼40 s intervals. These potential drops coincide with the temperature update of the "outdoor" temperature on the base-station and the LED flashing of the transmitter. During 60 min of operation data is transmitted 93 times. Simultaneously with data transmission, an orange LED light is flashing which contributes to the power consumption. The magnitude of the potential drop varies throughout the experiment. This is most likely a consequence of the settings on the potentiostat during the recording. In the measurement the potential was recorded once every second. During the time period when the electronics transmit data the power consumption increase. But the length of these periods is probably shorter than 1 second. As a consequence, the potentiostat records merely one data point for every data transmission period. Thus, the measured potential during the period when the electronics transmit data is therefore dependent on when the potential is recorded.

[0071] During operation of the set-up of example 9, the TIDA 00524 was read once every ∼5 minutes during the first 170 min, once every minute between 170 min and 204 min, and several times per minute between 204 and 206 min. The cell potential for this experiment is presented in fig. 4. There are some visible potential drops during operation. During the first 5 minutes, these are caused by commands from the phone to program the TIDA 00524. The rhythmic potential drops every 5 minutes are caused by reading the data with a phone via NFC Tools. The temperature and luminous flux were measured every minute although this does not give rise to visual potential drops. In total, six commands were programmed, the temperature and luminous flux were sampled 206 times, and the data was read 55 times during 3 h 26 min of operation before the TIDA 00524 stopped responding.

[0072] Notably, the battery formed in example 9 powered the electronic circuit (TIDA 00524) without the use of any metals.

Claims

1. A method comprising the steps of:

a) pretreating cellulose fibres so as to obtain pretreated cellulose fibres; and

b) polymerizing monomers on the pretreated cellulose fibres so as to obtain cellulose fibres coated with a redox-active polymer.

2. The method of claim 1, which is a method of producing a porous structure of cellulose fibres coated with a redox-active polymer and which further comprises the step:
c) forming the porous structure from the coated fibres obtained in step b).

3. The method of claim 2, wherein the porous structure is a sheet and wherein step c) comprises forming the sheet from an aqueous furnish comprising the coated fibres in a paper machine.

4. The method of claim 3, wherein the aqueous furnish comprises a buffer and has a pH of at least 5, such as 5-10, such as 6-8.

5. The method of claim 2, wherein step c) comprises dry forming, pressing and/or extrusion.

6. The method of any of the preceding claims, wherein the monomers are pyrrole monomers and the redox-active polymer is polypyrrole.

7. The method of any one of the preceding claims, wherein the pretreatment of step a) comprises a chemical pretreatment selected from the group consisting of (2,2,6,6-tetramethylpiperidin-i-yl)oxyl (TEMPO) oxidation, nitroxyl radical oxidation, chlorite oxidation, periodate oxidation, peroxide oxidation, alkali metal nitrite oxidation, alkali metal nitrate oxidation, ozone oxidation, oxone oxidation, permanganate oxidation, carboxymethylation, sulfonation and phosphorylation.

8. The method of any one of the preceding claims, wherein the pretreatment of step a) comprises mechanical pretreatment, such as refining.

9. The method of any one of the preceding claims, wherein the pretreated cellulose fibres are obtained in step a) as an aqueous fibre suspension having a Schopper-Riegler (SR) number (ISO 5267-1:1999) of 10-80, such as 20-77, such as 30-77.

10. The method of any one of the preceding claims, wherein the cellulose fibres are never-dried cellulose fibres.

11. The method of any one of the preceding claims, wherein the dry weight ratio of pretreated cellulose fibres to monomers in step b) is in the range of 0.8:1.0 to 1.8:1.0, such as 1.0:1.0 to 1.5:1.0.

12. A porous structure, such as a sheet, of pretreated cellulose fibres coated with a redox-active polymer, which porous structure is preferably produced according to any one of the preceding claims.

13. The porous structure of claim 12, which is a sheet.

14. An energy storage device comprising a porous structure according to claim 12 or 13.

15. Use of a porous structure according to claim 12 or 13:

as an electrode in an energy storage device;

for electrochemically controlled ion release;

for electrochemically controlled ion extraction;

in an oxygen sensor;

in a microbial or enzymatic fuel cell; or

as a light to heat conversion membrane, e.g. in solar distillation of salt water.

Drawing