Electroactive and Conformal Coatings of Oxidative Chemical Vapor

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Electroactive and Conformal Coatings of oCVD Polymers for Oxygen Electroreduction Shayan Kaviani, Mahdi Mohammadi Ghaleni, Elham Tavakoli, and Siamak Nejati ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00240 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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ACS Applied Polymer Materials

Electroactive and Conformal Coatings of oCVD Polymers for Oxygen Electroreduction

Shayan Kaviani1, Mahdi Mohammadi Ghaleni1, Elham Tavakoli2, Siamak Nejati1,2*

1

Department of Chemical and Biomolecular Engineering University of Nebraska-Lincoln Lincoln, NE 68588-8286, USA 2

Department of Mechanical and Materials Engineering University of Nebraska-Lincoln Lincoln, NE 68588-8286, USA

* Corresponding author: Siamak Nejati, Email: [email protected]

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ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Polymer from 3,4-ethylenedioxythiophene(EDOT) was synthesized using oxidative Chemical Vapor Deposition (oCVD). To enable the synthesis of PEDOT two different oxidants, antimony pentachloride (SbCl5) and vanadium oxytrichloride (VOCl3), were utilized. The effect of deposition temperature on the polymer electroactivity and conductivity was evaluated by measuring the overpotential for the oxygen reduction reaction and film electrical conductivity, respectively. PEDOT films with conductivity values of ~2000 S/cm were deposited in a single step coating and doping process. X-ray photoelectron spectroscopy revealed that the residual metalloid within polymer films, in the case of SbCl5, is contributing to the hole conductivity. The electrocatalytic activity of deposited material in oxygen reduction reaction (ORR) was studied; the results indicate a direct relationship between the conductivity values and the electrocatalytic activities of the deposited films. The unique potential offered by oCVD to coat PEDOT conformally enabled us to apply a coat of electroactive polymers on complex structures of a gas diffusion layer fabric, carbon cloth. Using our approach, we imparted stable electrocatalytic activity to carbon cloth electrodes and fabricated all organic electrodes for ORR.

Keywords: Oxidative Chemical Vapor Deposition (oCVD), Poly(3,4-ethylenedioxythiophene) (PEDOT), Oxygen Reduction Reaction (ORR), Highly Conductive Polymer, Air Electrode, Liquid Oxidant, Conformal Coating, Thin Films

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1. Introduction Reduction of oxygen is a vital reaction with many implications 1. Oxygen reduction reaction (ORR) is a key reaction in some of the most promising energy conversion devices, such as fuel cells and metal-air batteries 2-3. However, the intrinsically sluggish ORR reaction and the lack of cheap and efficient air electrode catalysts are among the challenges facing large scale deployment of these devices 4-5. Currently, the platinum-group metals (PGMs) catalysts are the gold standard choice of materials in ORR 6. To rationalize the economy of the electrochemical devices relying on ORR, more affordable catalysts for ORR are in high demand. The main criteria for an alternative catalyst are high activity, durability, and low cost 7. One of the promising choices of materials are carbon-based materials

6, 8

; however, it is often argued that the design and

development of efficient non-PGM catalysts, based on carbon-based sources, have been hindered by the premature understanding of the underlying mechanism for the ORR on such materials 9-10. Conjugated polymers, as a carbon-based material, possess electrochemical activity in ORR and their activity depends on the synthetic pathway used for polymerization

11-14

. It is reported by

Winther-Jensen et. al. that vapor phase polymerization (VPP) of poly(3,4-ethylenedioxythiophene) (PEDOT) allows for synthesis of an electrode with exceptional catalytic activity in the ORR reaction 15. Similar to VPP, oxidative chemical vapor deposition (oCVD) is a unique liquid-free technique that relies on the transport of both monomer and oxidant in the vapor phase to the substrate 16. The oCVD of conjugated polymers, compared to other techniques, provides a pathway for synthesis of conformal coatings of conjugated polymers and allows for coating of complex and porous three-dimensional (3D) electrode materials 17. By utilizing oCVD, a wide range of different substrates can be processed 18. These attributes make oCVD an attractive pathway for integration 3



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of electroactive polymers within porous electrodes. The oCVD synthesis of conjugated polymers has been performed mostly with solid oxidants such as FeCl3 and CuCl2 19-24, and there are a few reports on the oCVD of conjugated polymers using liquid oxidants 25-29. In this study, we utilized oCVD to synthesize thin films of PEDOT. We used antimony pentachloride (SbCl5) and vanadium oxytrichloride (VOCl3) as liquid oxidants and compared the film properties, including their electrocatalytic activity in ORR. Thin films of deposited PEDOT using SbCl5 (PEDOT-SbCl5) showed conductivities exceeding 2100 S/cm. We studied the effect of reaction conditions on the properties of deposited films using Atomic Force Microscopy (AFM), and vibrational and X-ray photoelectron spectroscopies. Results revealed that the properties of the deposited materials are strongly dependent on the choice of oxidant and the oCVD operating conditions. We also evaluated the performance of the oCVD-deposited PEDOT in ORR, using oxygen saturated alkaline aqueous solutions. We observed that the deposited material could facilitate ORR with an overpotential of 0.76 V vs. RHE. Additionally, we evaluated the number of electrons transferred during ORR. It was found that the oCVD-synthesized PEDOT catalyzes ORR in a series reaction pathway. Lastly, a conformal PEDOT coating was applied to a carbon cloth electrode, allow us to fabricate air electrodes.

2. Experimental Section 2.1. oCVD synthesis of PEDOT Deposition of PEDOT was carried out in a custom-made oCVD reactor. The chamber was pumped down to the base pressure of 8 mTorr using a dry pump (Edwards iH160). The monomer, 3,44


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ethylenedioxythiophene (98%, Ark Pharm) and oxidants, antimony pentachloride (99%, Acros Organics) and vanadium oxytrichloride (99%, Alfa Aesar), were heated in a glass jar to a temperature of 90°C and metered through precision needle valves (Swagelok). The chamber pressure was controlled by a downstream throttle valve (MKS instrument, type 153) and a pressure transducer (MKS instrument, type 626) connected to a pressure controller (MKS instruments, 946 vacuum system controller). The substrate temperature was controlled using a PID temperature controller and heating elements (Omega Engineering) inserted into a polished aluminum stage. The stage temperature was varied between 25-200 ˚C. To have comparable materials fabricated at different stage temperatures, the chemistry of EDOT polymerization was adjusted by tuning the surface availability of the monomers and oxidants; in our operation range for the total pressure, the surface availability for both components were assumed to be linearly correlated with the ratio of the partial pressure of the monomer and oxidant to their saturation pressures (Psat.) at the stage temperature. The surface availability is controlled by adjusting the flow of the inert gas (Ar) and the reactants. Tables S1 and S2 in the supporting document summarize the parameter space for our experimental work. For the post-deposition processing of the sample, we used a solution of 1M hydrochloric acid (37%, Fisher Scientific) as the rinse solution; we soaked the samples for 12 hours in the solution, and then dried them at 60 °C for 2 hours in a vacuum oven. Polymer Film Characterization FTIR spectra were collected using ALPHA spectrometer from Bruker. Infrared (IR) spectra of the deposited polymers on double-side polished silicon wafers were measured by transmission mode 5



at a resolution of 4 cm-1. All shown FTIR spectra are normalized with respect to the peak at 1085 cm-1 as the reference peak. Raman spectra were collected using a DXR Raman microscope from Thermo Scientific using 532 nm laser with ~ 1µm lateral spot size and 10 mW total power. To measure the conductivity of deposited samples, thin films of PEDOT were coated on microscope glass slides as substrate. A four-point probe device connected to a Keithley 2000 was used to measure the sheet resistivity. Constant amperage of 0.3 mA was applied to the tips of the probe using an adjustable current source. The measurement was performed at room condition with RH=48±5% and T=22±1 °C. The thickness of the samples was measured by contact profilometry using AMBIOS Technology XP-2 contact profilometer. The conductivity of samples were calculated as the inverse product of sheet resistance and film thickness. Each data point was obtained by averaging the value obtained from four measurements on each sample and reported with one standard deviation. Atomic force microscopy (AFM) measurements were carried out under ambient conditions using a Bruker Dimension Icon AFM. The measurements were performed in peak force tapping mode (SCANASYST-Air) using a silicon tip with the nominal resonant frequency (f0) of 70 kHz and the nominal tip radius of 2 nm. Height and peak force error data were recorded. The measurement was performed over 25 µm2, and the average roughness was determined from the entire sample area. X-ray photoelectron spectroscopy (XPS) was performed using a K-Alpha XPS system (ThermoFisher Scientific). Survey XPS spectra were acquired at 100 W with a pass energy of 200 eV over the range of 0−1350 eV with 1 eV resolution and 100 ms dwell time; each spectra is the average of five scans. High-resolution XPS spectrum of C1s, S2p, O1s, Cl2p, V2p, and Sb3d core electrons was acquired over a spot of approximately 400 μm with 50 W beam power and averaged five times with 0.1 eV resolutions at 50 eV pass energy with 100 ms dwell time. Scanning electron microscopy (SEM) and Energy-dispersive X-ray (EDX) 6


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spectroscopy was performed using FEI Helios NanoLab 660 microscope. A Sartorius Cubis MSU2.7s-000-DM microbalance with readability of 0.0001 mg was used to measure the weight of carbon cloth fibers, before and after PEDOT coating. X-ray diffraction (XRD) measurements were carried out using Rigaku SmartLab Diffractometer by radiation of Cu Kα1. 2.2. Electrocatalytic Activity Evaluation The cyclic voltammograms (CV) were collected using a computer-controlled potentiostat (1000E, Gamry) with a three standard three-electrode electrochemical cell. A mirror-polished glassy carbon (GC) (Pine Research) electrode was used as a substrate for material deposition. An Ag/AgCl (KCl sat.) electrode (BASi) and a graphite rod (Gamry) were used as the reference and the counter electrode, respectively. To evaluate the number of electrons transferred, linear sweep voltammetry (LSV) was performed on a rotating disk electrode (RDE) with an electrode rotator (AFMSRCE, Pine research). For all RDE and CV measurements, an aqueous phosphate buffer solution (PBS, pH=13) was used as the electrolyte. Before each test, the electrolyte was bubbled with either oxygen or argon (Matheson) for 30 minutes. Carbon cloth fabrics were purchased from Fuel Cell Earth. All electrochemical measurements were performed at room temperature (~25°C).

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3. Results and Discussion 3.1. Vibrational Spectroscopies

Figure 1. a) FTIR and b) Raman spectra of as-deposited PEDOT-SbCl5 films synthesized at different deposition temperatures. The normalized intensity of the Cα=Cβ stretching vibration mode is highlighted in the IR spectra. c) FTIR and d) Raman spectra of as-deposited PEDOT-VOCl3 films at different deposition temperatures. Cα=Cβ streching modes and oxyethylene ring deforamtion are highlihted in the IR spectra. e) The benzoid and quinoid forms of PEDOT.

8


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The successful deposition of PEDOT films was confirmed by using vibrational spectroscopy. As shown in Figure S1 (supporting information), the absence of Cα-H in the FTIR spectra for polymer when compared with the monomer spectra, confirms the successful polymerization of EDOT molecules

30

. Figures 1a and 1c show the normalized FTIR spectra of the as-deposited PEDOT

films, using SbCl5 and VOCl3 as the oxidants, synthesized at different deposition temperatures, respectively. The peak assignment, based on previous studies and IR spectroscopy of PEDOT, is tabulated in the supporting document, Table S3. As it is shown in Figure 1a and 1c, the relative intensity of the peak at 1048 cm-1 in the FTIR spectra of the samples deposited at lower temperatures, i.e. 100 ˚C and 60 ˚C for PEDOT-SbCl5 and PEDOT-VOCl3, respectively, compared with the spectra of the samples deposited at 145˚C is lower. By looking into the peak assignment table (Table S3), we attributed this change to the degradation of oxyethylene rings. We expect that the overoxidatoin of the monomer, and consequent breakage of the oxyethylene rings, occures due to the significantly higher relative surface availability of the oxidant to monomer, at the lower deposition temperature. Figure S2 present the possible chemcial mechanisim for this degradation. Also, the sharper FTIR peaks in the range of 600-1000 cm-1 for the samples deposited at lower temperatures compared to that of synthesized at the elevated temperatures, indicates the higher number density of oligomeric chains within the samples. Figures 1a and 1c, also present that the characteristic peak for the conjugated PEDOT, C=C asymmetic stretch mode at 1515 cm-1, is a function of synthetic condition. We observed that for PEDOT-SbCl5 films, as we increased the deposition temperature, the intensity of this peak rises, reaching a relative maximum at the deposition temperature of 145 ˚C. Further increases of the deposition temperature lead to smaller intensities for this peak. A similar trend was observed in the FTIR spectra of PEDOT synthesized

9



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using VOCl3 as oxidant; increasing the deposition temperature results in an increase of this vibration mode (Supporting Information Table S4). Figures 1b and 1d represent the Raman spectra of deposited PEDOT-SbCl5 and PEDOT-VOCl3 films in the range of 1300 cm-1 to 1550 cm-1, respectively. The full range Raman spectra of oCVDPEDOT is shown in Figure S3 and the peaks assignment is tabulated in Table S5. The Raman spectra of oCVD-PEDOT using liquid oxidants are similar to that of deposited with solid oxidant (i.e. FeCl3), reported elsewhere 20. Here, we compared the Raman spectra for the polymer films, synthesized at different deposition temperatures. The peak at 1369 cm-1 is assigned to Cβ-Cβ stretching vibration 31. The peaks centered upon 1435 and 1515 cm-1 are assigned to the symmetric stretching vibration of Cα=Cβ on the five-membered thiophene ring, and to the asymmetrical Cα=Cβ, respectively 31-32. We found that for the PEDOT-SbCl5 films, by increasing the deposition temperature the width of the Cα=Cβ peak narrows down; additionally, this peak red-shifted by 5.2 cm-1 when the substrate temperature was increased from 100 °C to 145 °C; by increasing the substrate temperature beyond 145 °C, we observed that the peak blue-shifted. The value of this shift for the deposition temperatures of 160 °C and 180 °C, was found to be 4.8 cm-1 and 6.3 cm1

, respectively. These trend for the Raman spectra is better shown in Figure S4 and in the

deconvoluted spectra in Figure S5 in the Supporting Information. The trend in the Raman Cα=Cβ peak as a function of deposition temperature, corroborates the observed trend for the IR peak at 1515 cm-1, where the intensity of peak shows a maximum value as a function of deposition temperature at 145 °C. These observations, along with the observed change for the values of film conductivity, shown in Figure 2, indicate that by increasing the deposition temperature to 145 ˚C the dominant resonant 10


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in the PEDOT-SbCl5 polymer chains changes from benzoid to quinoid forms, evidenced by the red-shift of the Raman peak at 1429 cm-1 33-34. The transformation from a benzoid to quinoid form has been attributed to a change from a coil conformation to a linear or expanded coil conformation 35

. Raman spectra of PEDOT-VOCl3 show the same characteristic peaks in the range of 1300-1550

cm-1, however, there is no apparent trend in these data. The full width at half max (FWHM) of the peaks for the PEDOT-VOCl3 Raman spectra is larger than that of PEDOT-SbCl5 (𝐹𝑊𝐻𝑀𝑃𝐸𝐷𝑂𝑇−𝑆𝑏𝐶𝑙5 /𝐹𝑊𝐻𝑀𝑃𝐸𝐷𝑂𝑇−𝑉𝑂𝐶𝑙3 ≈ 0.74). Moreover, the relative ratio of the peak centered upon 1435 cm-1 to the peak at 1515 cm-1 (symmetric Cα=Cβ stretching to asymmetric Cα=Cβ) in PEDOT-VOCl3 spectra is smaller than that of PEDOT-SbCl5, suggesting that more benzoid form is resonating in the film deposited using vanadium oxytrichloride 36.

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3.2. Conductivity Measurement

Figure 2. Electrical conductivity of deposited PEDOT-SbCl5 films, synthesized at different deposition temperatures (Tdep), as a function of film thickness. The filled markers indicate asdeposited films and the open markers indicate HCl rinsed sample. Each data point is an average of four measurement on one sample. The uncertainty for the thickness and conductivity measurement is shown with one standard deviation. Figure 2 presents the conductivity values of PEDOT-SbCl5 films deposited at different temperature and as a function of film thickness. We observed that by increasing the deposition temperature, the values for the conductivity of deposited films increase significantly. It has been reported that increasing the deposition temperature in oCVD enhances the conductivity of PEDOT films

22, 37

This phonemenon is attributed to the enhanced doping level and increased conjugation length 38

.

20,

. Here, we found that by increasing the deposition temperature from 100 °C to 145 °C, the

conductivity values of PEDOT-SbCl5 samples were improved by 20 fold, reaching a maximum value of 2200 S/cm; by further increase of the deposition temperature beyond 145 °C, PEDOT 12


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films with lower conductivities were obtained, See supporting Information Table S6. The observed trend in the film conductivity as the function of PEDOT deposition temperature parallels the results of the vibrational spectroscopies. Using VOCl3 as the oxidant, and at similar surface availability for the reactants, we were not able to deposit film with comparable conductivity; the highest value for the measured conductivity of PEDOT-VOCl3 was 675 S/cm-1 suggesting that the proper choice of oxidants plays a critical role in the properties of the deposited films. The measured conductivity values of PEDOT-VOCl3 is reported in Figure S6 in the Supporting Information. The literature reports that the conductivity of deposited PEDOT films could be further improved by acid treatment 19, 39. Hence, here we measured the conductivity of the PEDOT-SbCl5 films, deposited at 145 ºC and rinsed in 1M HCl. A ~30% enhancement in the film conductivity for the washed sample compared to its as-deposited counterpart was measured.

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3.3. X-ray Photoelectron Spectroscopy (XPS)

Figure 3. a) XPS survey, b) O1s and Sb3d, c) C1s, and d) Cl2p XPS core-level spectra of asdeposited PEDOT-SbCl5 via oCVD at deposition temperature of 145 ˚C. e) XPS survey, f) O1s, g) C1s, and h) Cl2p XPS core-level of PEDOT-SbCl5 films treated by 1M HCl. In order to better understand the chemistry and stoichiometry of the deposited materials, we performed XPS measurements on the thin films of PEDOT-SbCl5. Figure 3 presents the data for both as-deposited and HCl-washed samples. The XPS survey spectrum of as-deposited PEDOT, shown in Figure 3a, compared to that of acid-washed sample, using 1M HCl solution, presented in Figure 3e. The data indicates that the acid washing successfully removed the antimony species from the film. This can be further confirmed by comparing the Sb 3d3/2 peak, in Figure 3b and 3f. As shown in Figure 3b for the as-deposited film, we identifed five peaks. Three of these peaks were attributed to the O1s core electrons. The O1s peaks centered at 532.7 eV, 533.4 eV, and 534.8 eV were assigned to C-O, C-OH, and adsorbed water species, respectively

40-41

. The other two 14


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peaks centered at 531.4 eV and 540.8 eV, separated by 9.4 eV and with a relative intensity of 3 to 2, were attributed to the characteristic peaks of antimony, Sb3d5/2 and Sb3d3/2, respectively. As shown in Figure 3f, these two peaks disappeared after HCl treatment. On the other hand, by comparing Figures 3d and 3h, presenting Cl2p, we see that the Cl2p3/2 and Cl2p1/2 peaks centered at 199.4 eV, and 201 eV, respectively, are present in as-deposited films; after washing the samples, these peaks are no longer present in the spectrum. Based on these observations, we suggest that the chlorine species, in the as-deposited films, were covalently bonded to antimony atoms. The ratio of Sb-Cl characteristics peaks in the chlorine core-level spectra to that of antimony core-level is ~3:1 (see Supporting Information, Table S7). We expect that the chlorinated metalloid complexes are playing the role of dopant in the deposited films, as their atomic concentration agrees with the amount of doping peak in the sulfur 2p, shown in Figure S7. It is proposed that the sulfur in the thiophene ring interacts with the dopant species; theoretically, for each three sulfur atoms on the PEDOT molecule, one could be strongly coordinating with the dopant molecule 43

42-

. Figure S7 and the tabulated value in Table S7 confirms that the ratio of the doped to undoped

sulfur is 1 to 3. Thus, we propose that the antimony complexes are the main dopants in the film. The percentage of antimony in the deposited films at different temperatures follows the same trend observed earlier between the film conductivity and the deposition temperature. As shown in Table 1, the amount of antimony increases by increasing the deposition temperature up to 145 ºC; increasing the temperature beyond 145 °C results in a reduction in the Sb concentration in the sample. Because for PEDOT, the carrier density or concentration is controlled through oxidation level 44, the observed trend in conductivity data, discussed earlier in Figure 2, can be tied to the observed change in the antimony dopant species, which are the charge carriers 22. 15



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As discussed earlier, by washing the samples with HCl, the antimony species in the film were removed completely. Concomitant with this change, we observed that two new peaks appeared in the Cl2p spectrum. The new peaks centered at 196.2 eV and 197.8 eV, separated by 1.6 eV, with the relative ratio of 2:1. The peaks are attributed to the ionic chlorine species. This observation suggests that acid treatment exchanges the dopant, and chlorine species in ionic forms are the new dopant in the washed film 20. The amount of these compounds agrees with the doping peak in the S2p signal for the washed samples (Supporting Information, Table S8). Additionally, we observed that in the high-resolution XPS spectra of O1s and after HCl-treatment, shown in Figure 3f, a new peak centered at 530.6 eV appeared. We assigned this peak to C=O contamination 45-46. The C1s core electron spectra of the as-deposited and the HCl-treated samples are also displayed in Figures 3c and 3g. For the as-deposited sample, we recognized six Gaussian peaks centered at 284.4 eV, 285.6 eV, 286 eV, 286.9 eV, 287.8 eV, and 289.6 eV that point to C-C, C-S, C-O, COH, C-Cl, and π-π* groups, respectively 40, 47. The C1s spectrum of the acid-treated sample showed that the intensity of C-Cl species, potentially formed because of overoxidation of the parent monomer ring 48 (see supporting information, Figure S2), has been decreased, which is due to the nucleophilic substitution reaction. This observation is in agreement with the change in the Cl2p spectra before and after acid treatment. Also, in contrast to the as-deposited C1s spectrum, C=O groups were observed in the spectra of the HCl-washed sample. These groups are attributed to the CO2 contaminations adsorbed from the atmosphere.

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Table 1. XPS Elemental Analysis of PEDOT-SbCl5 Films at Different Deposition Temperatures. PEDOT-SbCl5

Deposition Temperature (ºC)

Elemental Percentage

C

S

O

Sb

Cl Cl-Sb

Cl-C

100

59.9

9.1

17.1

2.2

6.9

4.8

120

63.4

8.5

15.9

2.4

7.3

2.5

145

59.0

8.4

16.1

2.6

8.2

5.7

160

63.9

9.3

17.0

2.5

6

1.5

To compare the effect of oxidant on the PEDOT polymerization process, we also characterized PEDOT-VOCl3 films using XPS. The result indicates that the stoichiometry of the parent monomer is not maintained in the sample, suggesting possible degradation of EDOT monomers (see Figure S8 and Table S9, Supporting Information).

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3.4. Electrocatalytic Activity for Oxygen Reduction

Figure 4. a) The cyclic voltammograms collected at a scan rate of 20 mV/s, for the as-deposited

PEDOT-SbCl5 films, coated at different deposition temperature (Tdep ) in a phosphate buffer solution of pH=13, saturated with argon (Ar) or oxygen (O2). b) The average conductivity of PEDOT-SbCl5 thin films deposited at different Tdep as a function of ORR onset c) linear sweep voltammetry traces, at different angular velocities and a scan rate of 100 mV/s, for a PEDOTSbCl5 thin films coated on a glassy carbon electrode. The inset is the representative Levich plots of the data used to extract electron transfer number.

We evaluated the electrocatalytic properties of oCVD-synthesized PEDOT by performing LSV and CV measurements in a three-electrode electrochemical cell. PEDOT films were deposited on a mirror polished glassy carbon (GC) electrode, as the working electrode, and measurements were carried out in an aqueous phosphate buffer solution (PBS) with pH of 13 as the electrolyte. Figure 4a presents cyclic voltammograms of PEDOT-SbCl5 at different deposition (Tdep) temperatures at a scan rate of 20 mV/s, while the electrolyte was saturated either by oxygen or argon. As can be seen, the oxygen reduction peak is present in the range of 0.45-0.55 V vs. RHE for all the PEDOT 18


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samples in the O2 saturated electrolyte; this peak does not exist in the electrolyte solution deaerated by argon. This observation confirms that oCVD-synthesized PEDOT catalyzes ORR. Figure 4b presents the average conductivity values extracted from Figure 2 and Table S6 as a function of the onset potential of oxygen electroreduction; the values for the onset potentials were extracted from the cyclic voltammograms and estimated from the corresponding voltage for the point where the lines, tangent to the capacitive and faradic portions of the cathodic wave, intersect. As can be seen, the electrocatalytic activity of the films for ORR follow the conductivity trends of the samples. As the deposition temperature increases from 100 ˚C to 145 ˚C, i.e., corresponding to the highest conductivity value for the as deposited films, the required overpotential for ORR shifts by 30 mV to positive values, indicating that a lower overpotential is needed for ORR. This value shifts by 20 mV and reaches 0.76 V vs. RHE after dopant exchange by HCl treatment, corresponding to the highest conductivity (2590 S/cm). The shift in ORR onset as a function of deposition temperature is presented in Figure S9. We performed LSV measurements on PEDOT films deposited on a GC rotating disk electrode. Figure 4c represent the LSV curves for PEDOT-SbCl5 films deposited at Tdep=145 ˚C at different angular velocity ranging from 250 to 2000 rpm. The LSV curves of PEDOT- SbCl5 films deposited at different temperatures are shown in Figure S10. The limiting current density increases at higher angular velocities, indicating that the diffusion-limited ORR takes place on the electrode surface 49

. The number of electron transfers for reduction of one oxygen molecule can be estimated from

RDE data using the Koutecky-Levich (K-L) equation:

19



1 1 1 = + 2/3 1/2 −1/6 i i K 0.62nFADO   C O

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(1)

Where ik is kinetic current, n is the number of transferred electrons, F is the Faraday constant, A is the electrode area, CO is the concentration of the analyte, i.e., oxygen, DO is the diffusion coefficient of the analyte, ω is the angular velocity, and ν is the kinematic viscosity of the electrolyte. In a K-L plot (inset of Figure 4c), the inverse of measured current is plotted versus the inverse square root of the rotation rate, and ik and n were estimated from the intercept and slope, respectively. The values for the diffusion coefficient of oxygen (1.9×10-5 cm2/s), the saturation concentration of oxygen (1.1×10-6 mol/cm3) in PBS solution, and the kinematic viscosity of the electrolyte (1.1×10-2 cm2/s) were taken from literature

50-51

. The calculated numbers of electron

transfer for PEDOT film deposited at 100 ºC, 120 ºC, 145 ºC, and 160 ºC were estimated to be 2.1, 2.5, 2.6, and 2.3, respectively (see Supporting Information, Figure S11). These results suggest that the oxygen reduction is likely to happen via mixed two-electron and four-electron pathways

50

.

The calculated value for n follows the trend of conductivity as a function of deposition temperature: it increases with increasing deposition temperature up to 145 ºC, and reduces above this temperature. It can be concluded that PEDOT electroactivity has a direct relation with its conductivity.

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Figure 5. a) The CV curves of as-deposited PEDOT-VOCl3 films at three different temperatures of 100 ˚C, 120 ˚C, and 145 ˚C in electrolyte saturated with oxygen. b) Comparison of ORR activity of deposited PEDOT-VOCl3 (red) and PEDOT-SbCl5 (black) at a deposition temperatures of 160 °C. Voltage scan rates are 20 mV/s. We also evaluated the electrocatalytic activity of PEDOT-VOCl3, showing an apparent oxygen reduction peak in electrolyte saturated with O2 (see Supporting Information, Figure S12). Figure 5a shows the CV of deposited PEDOT-VOCl3 at different deposition temperatures. The needed overpotential to initialize the ORR decreases and shifts to more positive values as the deposition temperature increases, and reaches to the onset of 0.730 V vs. RHE for PEDOT-VOCl3 deposited at 145 ºC. Figure 5b compares the CV of PEDOT-VOCl3 and PEDOT-SbCl5 deposited at deposition temperature of 160 ºC. Although the potential onset of each material is very similar, the produced current densities differ significantly. The current densities for PEDOT-SbCl5 and PEDOT-VOCl3 are 0.26 mA/cm2 and 0.31 mA/cm2, respectively. Also, the unlike shapes of CVs of PEDOT-SbCl5 and PEDOT-VOCl3 indicate different electrocatalytic activity for a polymer synthesized using two different oxidants

52

. Thus, we can conclude the choice of oxidant is a

determinative factor for material properties.

21



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3.5. Atomic Force Microscopy (AFM)

Figure 6. AFM height images of as-deposited PEDOT films using SbCl5 at different temperatures: a) 60 ˚C, b) 90 ˚C, and c) 120 ˚C, and VOCl3 at different temperatures: d) 60 ˚C, e) 120 ˚C, and f) 160 ˚C. The average roughness factors (Ra) for the PEDOT-SbCl5 films were a) 0.71 nm, b) 0.86 nm, and c) 1.35 nm, and Ra values for the PEDOT-VOCl3 films were d) 45.7 nm, e) 7.76 nm, and f) 0.86 nm. We evaluated the surface topography of the deposited PEDOT films using AFM. Figure 6 presents the AFM height images of PEDOT films synthesized using two different oxidants and at different deposition temperatures. For all the AFM samples, the thicknesses of the deposited films were in the range of 60-80 nm. In general, by increasing the deposition temperature, more uniform morphologies were observed on the surface. For the case of PEDOT-SbCl5, we observed that at low temperatures, i.e., 60 ˚C, the formation of pinholes on the surface is predominate, while at the 22


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higher deposition temperatures, surface morphology becomes more uniform. This observation is attributed to the excess amount of acid and a higher deposition rate at the lower temperatures. Ra values for PEDOT-SbCl5 films at deposition temperatures of 60 ˚C, 90 ˚C, and 120 ˚C are 0.71 nm, 0.86 nm, and 1.35 nm, respectively. Deposition of PEDOT films using VOCl3 as the oxidant, results in deposition of films with rougher surface morphologies compared with the roughness of PEDOT-SbCl5 films. At deposition temperature of 60 ˚C, swollen features were observed on the surface; by increasing the deposition temperature, i.e., Tdep=120 ˚C, the size of these features became smaller. We attributed the formation of these domains to the low molecular weight and oligomeric materials. At higher deposition temperature, i.e., 160 ˚C, these features disappeared, and the deposited films seem very smooth with a peak-to-valley value of 9 nm. The change in the film topography as a function of deposition temperature can be better understood in the images with an area of 100 µm2 shown in the supporting information Figure S13. Ra values for PEDOTVOCl3 films deposited at temperatures of 60 ˚C, 120 ˚C, and 160 ˚C are 45.7 nm, 7.76 nm, and 0.86 nm, respectively. Previous studies showed that oCVD synthesis of PEDOT by liquid oxidant (Br2) instead of solid oxidants (FeCl3 and CuCl2) results in the formation of smoother PEDOT surfaces 27, 29, 38. However, the conductivity of the deposited material by liquid oxidant is less than that of PEDOT deposited by solid oxidants. Here, we showed fabrication of smooth PEDOT films with high conductivity.

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Figure 7. a) SEM images of carbon cloth used as substrate. The scale bar is 500 μm. b) Conformal coating of PEDOT on the fibers, the scale bars are 2μm. c) Steady-state current measurement of PEDOT in ORR in PBS solution with pH 13 as the electrolyte as a function of overpotential. The inset is a piece of carbon cloth coated with PEDOT. The exceptional coating precision offered by the oCVD enabled us to coat thin films of PEDOT on the carbon cloth used in electrodes of electrochemical devices. Figure 7a shows the structure of carbon cloth. Figure 7b compares a carbon cloth fiber before and after oCVD process of EDOT. A visible conformal and thin coating of PEDOT on the surface of fiber is present. We further confirmed the presence of PEDOT on the surface of fibers using energy-dispersive X-ray (EDX) spectroscopy (Supporting Information Figure S14). The carbon cloth coated with PEDOT was used for measuring the current in a cell that provides the three-phase interface (see Supporting Information Figure S15). The performance of PEDOT films, deposited on a gas electrode, as the catalyst for ORR was evaluated using chronoamperometry measurement. PEDOT-SbCl5 was deposited on carbon cloth fibers at 145 ºC.

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Figure 7c presents the average of measured steady-state current as a function of overpotential for three samples in PBS electrolyte with pH of 13 after 15 min continuous measurement. The average loading of PEDOT on carbon cloth is estimated to be 1.397 ± 0.026 mg.cm-2. The stable electrocatalytic activity of PEDOT to reduce oxygen at three phase interface, obtained from conformal coating of material on complex structure through oCVD will open a new avenue to the design and development of air cathodes in energy conversion devices. 4. Conclusion We demonstrated oCVD of poly(3,4-ethylenedioxythiophene) (PEDOT) using two different liquid oxidants, SbCl5 and VOCl3, at elevated deposition temperatures. The deposition temperature is shown to be a vital factor influencing the electroactive properties of PEDOT. We showed that for PEDOT-SbCl5 films, electroactive properties can be improved by increasing the deposition temperature up to 145 ºC; above this temperature, the material shows less electroactivity. We reached the conductivity of 2100 S/cm for the thin films of PEDOT (~40 nm), using a single-step coating and doping process. We evaluated the electrocatalytic activity of PEDOT for oxygen reduction reaction (ORR), and demonstrated that the ORR activity of the deposited films has a direct relation to the conductivity of the material. Through X-ray photoelectron spectroscopy (XPS) studies, we showed that choice of oxidant is one of the key parameters influencing the properties of the deposited material. Acknowledgments

25



The research was performed in part in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience, which are supported by the National Science Foundation under Award ECCS: 1542182, and the Nebraska Research Initiative.

ASSOCIATED CONTENT − Supporting Information Available: Selected reaction conditions used to synthesize PEDOT-SbCl5 and PEDOT-VOCl3 films via oCVD (Tables S1 and S2). FTIR spectra of EDOT and oCVD-PEDOT (Figure S1). Assigned IR peaks (Table S3). Proposed mechanism for overoxidation of PEDOT ring (Figure S2). Value of normalized intensity of C=C in FTIR spectra (Table S4). Full range Raman Spectra of oCVD-PEDOT (Figure S3). Assigned Raman peaks (Table S5). Raman spectra of PEDOT-SbCl5 in the range of 1375 to 1475 cm-1 (Figure S4). Deconvoluted Raman Spectra of oCVD-PEDOT in three deposition temperatures (Figure S5). Measured conductivity and thickness values for PEDOT-SbCl5 films (Table S6). Conductivity of PEDOT-VOCl3 samples (Figure S6). S2p XPS spectra (Figure S7). XPS chemical state and elemental analysis of asdeposited and HCl treated PEDOT-SbCl5 (Tables S7 and S8). XPS spectra of PEDOT-VOCl3 film (Figure S8). XPS chemical state and elemental analysis of as-deposited PEDOT-VOCl3 (Table S9). Effect of conductivity on CV curves for ORR (Figure S9). LSV curves of PEDOT-SbCl5 thin films at different deposition temperatures (Figure S10). Levich plots of PEDOT-SbCl5 (Figure S11). The CV curves of asdeposited PEDOT-VOCl3 (Figure S12). AFM images of PEDOT-VOCl3 (Figure S13). EDX spectra of carbon cloth after PEDOT deposition (Figure S14). Schematic of the cell for air-electrode testing (Figure S15). XRD spectra of oCVD-PEDOT (Figure S16).

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(51) Xiang, Z.; Cao, D.; Huang, L.; Shui, J.; Wang, M.; Dai, L. Nitrogen‐Doped Holey Graphitic Carbon from 2d Covalent Organic Polymers for Oxygen Reduction. Adv. Mater. 2014, 26, 33153320. (52) Wang, Q.; Xiaoqiang, C.; Guan, W.; Zhang, L.; Fan, X.; Shi, Z.; Zheng, W. Shape-Dependent Catalytic Activity of Oxygen Reduction Reaction (Orr) on Silver Nanodecahedra and Nanocubes. J. Power Sources 2014, 269, 152-157.

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For Table of Contents Only:

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Shell: oCVD-PEDOT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21


Core: Carbon fiber


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Electroactive and Conformal Coatings of Oxidative Chemical Vapor

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