High-Throughput Template-Free Continuous Flow Synthesis of

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Applied Chemistry

High-Throughput Template-Free Continuous Flow Synthesis of Polyaniline Nanofibers Rekha Singh, Karuna Veeramani, Rishab Bajpai, and Anil Kumar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04507 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Industrial & Engineering Chemistry Research

High-Throughput Template-Free Continuous Flow Synthesis of Polyaniline Nanofibers Rekha Singh,†# Karuna Veeramani, †,‡,# Rishab Bajpai,†and Anil Kumar*,†,‡,§ †Department

of Chemistry, ‡IITB-Monash Research Academy, §National Centre for

Excellence in Technologies for Internal Security (NCETIS), Indian Institute of Technology Bombay, Powai, Mumbai-400076, India # These two authors contributed equally to this work. E-mail: [email protected]; phone +91 22-25767135; Fax +91 22-2576 7152 ABSTRACT: The large-scale and high-throughput synthesis of conjugated polymer based nanofibers always remain a challenge for the chemists due to the issues related to secondary nucleation in traditional batch processes. Typically polyaniline (PANI) nanofibers are synthesized under highly dilute conditions resulting in a very low throughput of few hundred mg per hour and a low space-time yield (STY) of 1-2 g·L-1·h-1. In this manuscript, we report the continuous flow synthesis of PANI nanofibers which results in high throughput (17-30 g·h-1) and high space-time yield (140-450 g·L-1·h-1). These polyaniline nanofibers show high surface area (42 m2·g-1), high specific capacitance (577 F·g-1) and high crystallinity. Finally, the present method is generic in nature and, in principle, can be extended for the synthesis of nanofibers of other conjugated polymers via oxidative polymerization. KEYWORDS: Continuous flow synthesis, Polyaniline nanofiber,

One-dimensional

nanomaterials, Throughput, Space-time yield, Electrochemical capacitor, Chemical oxidative polymerization.

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1. INTRODUCTION Polyaniline (PANI) is an ancient polymer which was first synthesized in the mid-nineteenth century and rediscovered again as a conductive polymer towards the end of the twentieth century. This polymer has been studied extensively not only due to its facile synthesis, environmental stability, and simple acid/base doping/dedoping chemistry but also due to its applications in diverse areas such as sensors,1-2 metallic corrosion protection, electromagnetic interference shielding,3 catalysis,4 and supercapacitors application.5-7 Another interesting feature is the tendency to form nanofibers during synthesis which opens up another dimension in their application as high-surface-area materials. One-dimensional (1D) nanomaterials have gained enormous attention during the past few years because of their unique properties especially associated with their high surface area.8 Nanomaterials based on conjugated polymers are also following this trend due to their added advantage of inexpensiveness, organic nature and ease of processability.9-10 The on-going research in the field of 1D conjugated polymern anomaterials involves their efficient method of synthesis, improved quality, and novel applications.11-12 The method of synthesis starts becoming more crucial than ever when avoiding the polydispersity of material, enhancing reproducibility and facilitating the scale up of the process is concerned. For the synthesis of PANInanofibers, the methods of synthesis can be categorized broadly into electrochemical13-14

and

chemical15-16

oxidative

polymerization.

Electrochemical

polymerization yields a good quality material deposited directly on the surface of the electrode.14 However, the need fora conducting surface and issues in scalability of the process make electrochemical methods less desirable. Within chemical methods, various methodologies were adopted for improving the yield and quality of materials. These methodologies can be broadlycategorized into templated17 and non-templated method of synthesis. In the templated synthesis, various hard (anodic aluminum oxide (AAO), silica


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nanotubes, flyash, polycarbonate track-etched filtration membranes etc.) and soft (crown ether, naphthalene sulfonic acid, aminobenzenesulfonic acid, camphorsulfonic acid etc., cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS) templates and seeds have been used.18-20 These methods suffer from the drawback of impurity and additional steps involved in removing the templates. Non-templated chemical methods have various advantages like ease of synthesis, scalability, and purity of the polymer. Solid state polymerization is another method of synthesis which gives good quality nanofiber.21 However, the yields are very low, and this method cannot be scaled up efficiently. Therefore solution phase, non-templated chemical synthesis remains a method of choice when material purity and scalability are the main targets. In the conventional solution phase bulk synthesis of PANI, nanofibersare formed due to homogeneous nucleation in the initial stages of polymerization. However, these initially formed nanofibers act as a seed for the subsequent polymerization(heterogeneous nucleation or secondary nucleation) leading to the agglomerate formation.22 Therefore, the drawback associated with the conventional solution phase bulk polymerization is the inability to prevent secondary nucleation. Many methods currently exist for improving the synthesis and yield of these nanofibers such as interfacial polymerization,23-25 polymerization under rapid mixing,26 sonochemical synthesis,27 and in static conditions (without stirring or shaking).28 The approach to inhibit secondary growth/heterogeneous nucleation iseither make the reaction faster (rapid mixing/elevated temperature etc.) or use of reagents in lower concentrations29 or a lower oxidant to monomer ratio. In many of the aforementioned works, a qualitative amount of polymer nanofibers could be produced, however, the overall yield and production rate are very low (Table 1). The key challenge, till date, for the synthesis of polyaniline nanofibers has been the development of a large-scale and high-throughput process which can also produce nanofibers of a good quality. Another barrier in scaling-up the synthesis of PANI nanofiber is the exothermic nature of the reaction.



Generally, the aniline polymerization reaction is exothermic and leads to an increase in the temperature of the reaction mixture. Therefore, controlling its temperature is very important for the large-scale production. The polymerization reaction of aniline performed with a concentration greater than 1 M and volumes more than 0.5 L can result in the overheating of the system, followed by an explosion.30-31 Therefore safety is a serious concern in scaling up the synthesis of PANI. Continuous flow synthesis has attracted much attention as a versatile method for the largescaleand controlled synthesis of polymers34 and nano-sized materials.35-39 In a batch process, controlling the shape, size, and distribution of nano-sized materials is a tedious task even at small scale reaction volume of lesser than 100 ml. However, in the continuous flow process, these properties can be well controlled due to small volumes involved, enhanced mixing within the confined space of microchannels, and better mass transport.40-41 Another advantage is that the reaction happens betweena small amountof reactants and product never sees the fresh reactants (radial mixing) leading to suppression of secondary nucleation without early termination. Therefore the quality and polydispersity of material are well controlled in flow synthesis due to the possibility of controlling reaction at small scale.42 Due to the larger surfaceto-volume ratio of the flow reactors, continuous flow processes provide a much better thermal control over conventional batch synthesis. Therefore, continuous flow synthesis should be an ideal process for the large-scale synthesis of conducting polymer nanofibers.


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Table 1. Details of PANI nanofibers synthesized by various template-free methods via chemical oxidative polymerization using ammonium peroxydisulfate (APS). Aniline Method of Synthesis

Conc. (M)

APS/Aniline

Yield

Ratio

(%)

Reaction time (h)

Throughput (g·h-1)

Space-time yield

Reference

(g·L-1·h-1)

Interfacial

0.032-0.32

0.25

6 -10

24

-

-

[23-24]

Rapid mixing

0.16

0.25

Not given

24

-

-

[22]

Nanofiber seeding

0.084

0.19

22

1.5

0.13

1.3

[18]

Rapid mixing

0.5

0.25-1.0

13.4 - 42.3

2

0.72 - 2.28

3.6 - 11.4

[26]

Dilute conditions

0.008

0.5

Not given

24

-

-

[32]

Sc CO2 as co-solvent

0.074

1.2

80

24

0.078

0.26

[33]

Static Method (no stirring or shaking)

0.16

0.25

Not given

2

-

-

[28]

Continuous flow synthesis

0.4

1.0

40 -70

0.4

17 - 30

140 - 450

This work



In this work, we report a high-throughput template-free synthesis of PANI nanofibers via continuous flow method. We obtained a high (~70-80 %) to low (~5-10%) yield of PANI nanofibers by varying the monomer and oxidant concentrations and also observed a clear structural change for the same. We could obtain a high throughput (30 g·h-1) and high spacetime yield (450 g·L-1·h-1) of PANI nanofibers, which is significantly greater than that for any batch synthesis. PANI nanofibers were characterized for their structure and morphology using various techniques. The observed BET surface area of PANI nanofibers was found to be 25-42 m2·g-1which is comparable to the reported values for PANI nanofibers.24, 43-44 We have also demonstrated the application of these high surface area PANI nanofibers in the electrochemical capacitors.

2. EXPERIMENTAL 2.1 Materials Aniline was purchased from Merck (≥99.7)and distilled under reduced pressure before use. Ammonium peroxydisulfate (APS, Merck, ≥98.0), hydrochloric acid (HCl, Merck, 35%),sulfuric acid (H2SO4, Merck, 98%), and methanol (MeOH, Merck, ≥99.9 HPLC Grade),were used without any further purification.

2.2 Synthesis 2.2.1 Synthesis of PANI by batch method Aniline was dissolved in 100 ml of hydrochloric acid solution (1 M), and another solution of ammonium peroxydisulfate was prepared in 100 ml of hydrochloric acid solution (1 M). The freshly prepared oxidant solution was added dropwise into the monomer solution (kept in icecold water) with rapid mixing controlled by a magnetic stirrer, and the reaction was carried out at room temperature. Polymerization was observed with the appearance of the characteristic


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green color of polyaniline emeraldine salt. The polymerizationreactionwas quenched in methanol after one hour. The resultant polymer was purified by centrifugation and washed with methanol and water for three to four times each until the brown color impurity due to presence of oxidant vanished, leaving behind a transparent pale green supernatant. PANI was dried at 80 °C for overnight before measuring the yield. 2.2.2 Synthesis of PANI by continuous flow method Aniline monomer was dissolved in 100ml of hydrochloric acid solution (2 M) in a 250 ml glass bottle. In a separate glass bottle, ammonium peroxydisulfatewas dissolved in 100 ml of deionized water. Toluene was usedas a carrier solvent for plug generation of polymerizing reaction mixture. Aniline and APS solutions were pumped using Vapourtec pumps (R3 series) at the same flow rate (See Figure S1 for experimental setup in supporting information). These two solutions mix at a four-ways mixer (T = 2°C) in which toluene (pumped through the middle inlet) divides the reaction mixture into a succession of plugs which passes through a PTFE tube reactor (T = 30°C, 60 ml, 1.6 mm inner diameter). The output was collected in a beaker containing chilled methanol. The aqueous phase was washed with methanol and water for three to four times each. PANI was dried at 80 °C for overnight before measuring the yield.

2.3 Characterization FTIR spectrum of the dried PANI nanofibers was recorded in transmittance mode using a Perkin Elmer Spectrum One FTIR instrument in the scanning range of 450-4500 cm-1. KBr pellets were prepared by mixing 1-2 mg of nanofibers with finely powdered KBr and pressing the resultant mixture into a thin pellet using a Hydraulic press. UV-Visible spectra were recorded between 300-900 nm on a Varian Cary 100 Bio UV-Visible spectrophotometer. Assynthesized doped PANI nanofibers were dispersed in distilled water to form a dilute and uniform dispersion. The morphology of PANI nanofibers was studied through Scanning Electron Microscopy using the JEOL JSM-7600F FEG-SEM instrument. Samples were



prepared by dispersing around 1 mg (2.5-3.0 mg for preparing thicker films) of synthesized PANI nanofibers in around 1-2 ml of 5 mM HCl solution,45 drop-casting on Silicon wafer used as substrate and air dried overnight. The morphology of PANI nanofibers was also studied through Transmission Electron Microscopy using the JEM 2100 Ultra HRTEM 200kV instrument. Samples for the same were prepared similarly as for SEM and drop-casted (around 10µL) on copper grids and left to dry overnight before imaging. The Brunauer-Emmett-Teller (BET) surface area measurements were performed on Quanta chrome instrument (Autosorb1). The synthesized polymer samples were dried overnight under vacuum before measurement. Nitrogen gas was used as an adsorbate gas. All the electrochemical characterizations were carried out in a three-electrode setup (in 1 M H2SO4 solution) using platinum foil coated with PANI nanofibers as a working electrode, platinum wire as a counter electrode and Ag/AgCl as a reference electrode. CHI 760D potentiostat was used for cyclic voltammetry (CV) and constant current charge-discharge (CCCD) measurements. Electrical conductivity of the PANI nanofiber was measured at room temperature on pressed pellets by a four-point probe technique using a Signatone four-point probe resistivity measurement system. 0.1 g of synthesized PANI nanofibers was compressed into a pellet (diameter 2.5 cm and thickness 1.5 mm) using a hydraulic press by subjecting to a pressure of 70 MPa for 3 minutes.X-ray diffraction (XRD) experiments were carried out on Rigaku Dmax 2500 X-ray diffractometer (Japan Science Analysis Instrument Factory) in the range of 2–40°.

3. RESULTS AND DISCUSSION A general method of synthesizing PANI by chemical route involves oxidative polymerization in a strong acidic media in the presence of oxidizing agent e.g. ammoniumperoxydisulfate.46 During polymerization, the initially formed polymer remains in the nanofiber form; however, as the reaction proceeds, their shape getsdistorted due to secondary growth. For getting nanofibers, generally two strategies are used; first to polymerize in dilute conditions using


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lesser equivalents (

High-Throughput Template-Free Continuous Flow Synthesis of

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