Surface Characteristics of Microporous and Mesoporous Carbons

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Surface Characteristics of Microporous and Mesoporous Carbons Functionalized with Pentafluorophenyl Groups Xiaoan Li, Farisa Forouzandeh, Abraham Joseph Kakanat, Fangxia Feng, Dustin William H. Banham, Siyu Ye, Daniel Y. Kwok, and Viola Birss ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13880 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Applied Materials & Interfaces

Surface Characteristics of Microporous and Mesoporous Carbons Functionalized with Pentafluorophenyl Groups Xiaoan Li,†,‡ Farisa Forouzandeh,† Abraham Joseph Kakanat,† Fangxia Feng,† Dustin William H. Banham,†,§ Siyu Ye,§ Daniel Y. Kwok,‡ and Viola Birss*,† †

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4 ‡ Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4 § Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby, BC, Canada V5J 5J8 *Corresponding author: [email protected]

Abstract The in-situ diazonium reduction reaction is a reliable and well known approach for the surface modification of carbon materials for use in a range of applications, including in energy conversion, as chromatography supports, in sensors, etc. Here, this approach was used for the first time with mesoporous colloid-imprinted carbons (CICs), materials that contain ordered monodisperse pores (10-100 nm in diameter) and are inherently highly hydrophilic, using a common microporous carbon (Vulcan carbon (VC)), which is relatively more hydrophobic, for a comparison. The ultimate goal of this work was to modify the CIC wettability without altering its nanostructure and also to lower its susceptibility to oxidation, as required in fuel cell and battery electrodes, by the attachment of pentafluorophenyl (-PhF5) groups onto their surfaces. This was shown to be successful for the CIC, with the –PhF5 groups uniformly coating the inner pore walls at a surface coverage of ca. 90% and allowing full solution access to the mesopores, while the –PhF5 groups deposited only on the outer VC surface, likely blocking its micropores. Contact angle kinetics measurements showed enhanced hydrophobicity, as anticipated, for both the -PhF5 modified CIC and VC materials, even revealing superhydrophobicity at times for the CIC materials. In contrast, water vapor sorption and cyclic voltammetry suggested that the 1 ACS Paragon Plus Environment

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micropores remained hydrophilic, arising from the deposition of smaller N- and O-containing surface groups, caused by a side reaction during the in-situ diazonium functionalization process. Keywords: Colloid-imprinted carbons, Vulcan carbon, Microporous, Mesoporous, Surface functionalization, Pentafluorophenyl groups, in-situ diazonium reduction reaction, Wettability. 1. Introduction High surface area carbon materials are of great interest as they are used in many applications,1-6 e.g., in battery and fuel cell electrodes, as chromatography supports, in biosensors, and more. In most cases, the surface properties of the carbons play a critical role in their function. For instance, when a high surface area carbon is used as a catalyst support in polymer electrolyte membrane fuel cells (PEMFCs),3, 7-8 surface wettability is one of the most important issues affecting the achievable power output and the durability/lifetime of the cells. These issues are not yet fully understood, due to the complex interactions between the various components within PEMFC catalyst layers. The surface properties of carbon electrodes are also vital to the performance of redox flow batteries (RFBs), as the surface groups interact in a variety of ways with the redox couples, e.g., V2+/3+ or VO2+/VO2+, accelerating or decelerating their oxidation/reduction rates.9 In another application, when highly porous carbons are used to remediate oil spills, a hydrophobic surface is essential for the sorption of oil.10-13 Thus, it is critical to both understand and control the surface wettability of carbon materials, linked closely to their surface chemistry. The in-situ diazonium reduction reaction is a relatively common method for the surface functionalization of carbon materials.14-15 In this reaction, the diazonium salt is reduced and then the aryl groups are grafted onto the carbon surface. The reduction process can be driven by electrochemical methods or by thermal decomposition.16-19 The diazonium salt is typically

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formed by the reaction between an aryl amine and a nitrite, often acid-catalyzed (note that when an alkyl nitrite is used, acid catalysts are not necessary). This method avoids the tedious and dangerous synthesis of pure diazonium salts prior to functionalization. Therefore, the in-situ diazonium reduction reaction has been used to surface-functionalize microporous carbon blacks, carbon nanotubes, and mesoporous carbons with various aryl moieties, such as the nitrophenyl and alkylphenyl groups.14-18 As a sub-set of high surface area carbon materials, mesoporous carbon powders are of interest primarily as their pores (2 - 50 nm in diameter) are more accessible to both solution and gas phase species than in microporous carbons. Among them, colloid-imprinted carbons (CICs), with their fully tunable and monodisperse pore sizes, have been shown to be promising when used as catalyst supports in PEMFCs, as the stationary phase in chromatography, and in other applications.20-26 However, our recent studies have shown that the CICs have a much more hydrophilic surface than do readily available, commercial, carbon blacks, e.g., Vulcan carbon (VC), which is widely used as a catalyst support in PEMFCs.27 It is thus anticipated to be challenging to use the CICs in their as-synthesized state for this purpose because the hydrophilic CIC surface could potentially cause flooding problems in the pores of the cathode, where water is produced.28-30 When used as the stationary phase in chromatography, the CICs must possess a wide range of surface polarity (from hydrophilic to hydrophobic) to selectively adsorb a range of analytes.31-32 It is thus necessary to tune the surface wettability of the CICs for this application as well. In the present work, we demonstrate that it is possible to increase the hydrophobicity of the CICs by attaching pentafluorophenyl (-PhF5) groups to the internal pore surfaces through the in-situ diazonium reduction reaction (Scheme 1), without significantly affecting the pore size.14-



15

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This type of surface modification may also increase the resistance of the carbons to

electrochemical corrosion when used as the cathode support in PEMFCs or as electrode materials in RFBs, work that is currently underway in our group.33 This is because of the enhanced surface hydrophobicity,8 caused by both the high electronegativity and low polarizability of the fluorine atoms.34 F

F

F

F

ONO

+

H 2N

F

F CH3CN, in N2, Reflux, 24 h

F

F

F

F

Carbon surface

Scheme 1. Functionalization of carbon surfaces with pentafluorophenyl (-PhF5) groups using insitu diazonium reduction chemistry. Since it has been shown that as-synthesized mesoporous CICs with different pore sizes (10-50 nm, controlled by the size of the silica colloid used in the CIC imprinting stage) have essentially undistinguishable surface properties (polarity),27 here we focus only on one of the CICs, namely CIC-22, with its pore size (ca. 20-25 nm) being in the middle of the size range already investigated. For comparison, microporous Vulcan carbon (VC), which is a common carbon powder used in PEMFCs, but with a significantly less polar surface than the CICs,27 was also surface-modified using the same approach. In this study, elemental analysis showed that a -PhF5 coverage of ca. 90 % was achieved on both the CIC-22 and the VC surfaces. In terms of wettability, contact angle kinetics (CAK) measurements confirmed that the –PhF5 group makes both carbons significantly more hydrophobic, making them more corrosion resistant in aqueous acidic solutions. However, water vapor sorption (WVS) studies suggested that the micropores in the walls of CIC-22 and throughout the VC powder are likely not PhF5-modified, due to steric hindrances, leaving them


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available for the attachment of smaller, more polar groups during the diazonium processing steps. Thus, it is shown that the combination of both CAK and WVS analysis leads to a deeper understanding of the surface properties of micro- and mesoporous carbon nanomaterials. 2. Materials and Methods 2.1 Synthesis and functionalization of CIC-22 and Vulcan carbon The colloid-imprinted carbon (CIC-22) powder was synthesized by following the procedure published previously,35 i.e., using mesophase pitch (MP) as the carbon precursor and 22 nm silica colloids as the template, with a mass ratio of MP:silica of 1:10. The MP was imprinted by the silica template at 400 °C for 2 h and then carbonized at 900 °C for 2 hours, using a heating ramp rate of 5 °C/min. After the removal of the silica template by refluxing in 3 M NaOH (aq.), the carbon was labelled as CIC-22. Similar to the in-situ diazonium reduction process carried out in organic solvents 36-37, the surface functionalization was carried out using the following procedure. The as-prepared CIC-22 and as-received Vulcan® carbon (VC) powders were dried overnight in air at 100 °C before surface functionalization with the pentafluorophenyl groups. 1.0 g of the carbon powder, 5.6 g of 2,3,4,5,6-pentafluoroaniline (Sigma-Aldrich), and 150 mL of acetonitrile were placed into a 250 mL flask and the mixture was then sonicated for 1 h under the flow of N2. After that, 6.2 mL of amyl nitrite was added drop-wise into the mixture in the flask under sonication and N2 protection (stoichiometrically, an excess amount of nitrite was used in order to fully utilize the aniline).36-37 Although the quantity of the reagents used in the diazonium reaction also affects the fluorine surface coverage, as shown in Table S1 and Figure S1 in the Supporting Information section, the quantities described here were expected to lead to fully coverage of the carbon surfaces. This mixture of reactants was then heated to reflux for 24 h under N2 (our preliminary results showed



that lower heating temperatures or shorter heating times resulted in a smaller coverage of the functional group on the carbon surface). The surface-functionalized carbons were then filtered and rinsed with 25 mL of dimethylformamide and 50 mL of acetonitrile. Any reaction residues present in the functionalized carbon powders were further removed by Soxhlet extraction with acetonitrile for more than 4 days. Finally, the samples were dried at 100 °C in air overnight. The functionalized carbons were denoted as CIC-22-PhF5 and VC-PhF5. 2.2 Elemental analysis and gas sorption analysis The carbon, hydrogen and nitrogen content of VC and CIC-22, before and after surface modification, was determined using combustion analysis.27 The fluorine content of the surfacefunctionalized carbons was determined using a classical wet chemistry method (potentiometric titration with La(NO3)3

38

), conducted by Micro-Analysis (Wilmington, US). The content of

inorganic residue (e.g., silica) of the carbons was determined using thermogravimetric analysis by heating each carbon sample at 2 °C/min up to 800 °C under air.27 The specific surface area, pore size distribution, and pore volume of the carbons samples was also examined using nitrogen gas adsorption/desorption analysis at ca. 77 K,39 as described previously.27 The chemical composition of the carbon samples was also determined using roomtemperature X-ray photoelectron spectroscopy (XPS), using a Kratos Axis Spectrometer (nanoFAB, University of Alberta) with a monochromatic Al Kα source (1486.71 eV). The spectrometer was calibrated using the Au 4f7/2 binding energy (84.0 eV) with reference to the Fermi level, while charging effects were corrected by using the C 1s peak binding energy at 284.8 eV. CasaXPS (V2.3.16 PR 1.6) software was used for the elemental content calculation, based on the spectrum of each sample after a Shirley background subtraction.


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2.3 Wettability measurements The wettability of the carbons, with/without surface modification, was determined using both contact angle kinetics (CAK) and water vapor sorption (WVS) measurements, as presented in our previous work.27 In brief, 0.1 g of dried carbon powder sample was pressed into a pellet (25 mm dia.) using a die-presser under a pressure of 36 MPa for 4 min. A 10 µL water droplet was released at ca. 1 mm above the carbon pellet and the droplet behavior was recorded by a high speed camera (DRS Technologies) at a frame rate of 1000 fps and a shutter time of 0.25 ms, from which the CAK data for water on the carbon samples were obtained. For the WVS measurements,27 0.1 g of dried carbon power sample was placed in a porcelain crucible and the crucible was then placed onto a platform in a chamber containing distilled water. The water was placed beneath the platform but did contact the crucible directly. The weight of the crucible plus the sample was tracked as a function of time, from which the amount of water vapor absorbed by the carbon sample was acquired. The carbon samples (VC and CIC, with and without functionalization) were placed in the same chamber at the same time and their mass changes were tracked simultaneously, thus ensuring that their WVS results were directly comparable. 2.4 Determination of electrochemical properties of carbons The electrochemical properties of the carbon samples, before and after surface functionalization, were examined by cyclic voltammetry (CV) in a room-temperature N2saturated, 0.5 M H2SO4 solution in a 3-electrode cell, as described previously.40 The working electrode was prepared using the method presented elsewhere.27 In brief, 0.01 g of carbon powder and 0.1 g of a 12 wt. % H2SO4/ethanol solution were mixed together in a vial with ultra-



sonication for 5 min, and then 0.4 g of a 1 wt% Nafion/ethanol solution was added to the vial. An ink was formed after the mixture was sonicated for ≥ 1 h. A carbon/H2SO4/Nafion film was obtained by depositing 14 µL of the ink onto the polished end of a glassy carbon (GC) rod, leaving it to dry under room conditions.27 A Pt mesh counter electrode and a reversible hydrogen (RHE) reference electrode were used for the collection of the CV response of the carbon film working electrodes. After the CV testing, the carbon film electrodes were subjected to a corrosion testing protocol, consisting of 18 cycles of potential stepping between 1.4 V vs RHE (50 s) and 0.8 V vs. RHE (10 s), as described in our previous work.33, 41 Both CV and potential stepping experiments were carried out using an EG&G PARC 173 potentiostat and an EG&G PARC 175 function generator, with the data analyzed using Chart 5 (PowerLab). 2.5 Determination of carbon powder morphology To determine the carbon morphology, the powders were deposited on conductive, adhesive carbon tape and then examined with field-emission scanning electron microscopy (FESEM). A Zeiss Sigma VP FE-SEM was employed at an accelerating voltage of 8 kV, with an InLens detector used to collect the electronic signals.27 3. Results and Discussion 3.1 Porous structure of carbon powders before and after surface functionalization Figure 1 shows the general morphology, determined by FE-SEM imaging, of the mesoporous colloid-imprinted carbon (CIC-22) and microporous Vulcan carbon (VC) powders under study here, before and after surface functionalization with the pentafluorophenyl (-PhF5) group. Both CIC-22 and CIC-22-PhF5, which are indistinguishable in the FESEM images


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(Figures 1a and 1b, respectively), have ordered spherical pores ca. 20-25 nm in diameter (measured from wall to wall), as expected, and pore necks (darker circles within the spherical pores) that connect to the adjacent pores. In comparison, the VC and VC-PhF5 powders (Figures 1b and 1c, respectively) both appear as aggregates of 10-60 nm dia particles, with the micropores within the VC particles being too small to resolve in these images. Both the VC particles and aggregates appear to be randomly connected to each other, forming textural pores that are from several to hundreds of nanometers in size.

Figure 1. Field-emission scanning electron microscopy (FE-SEM) images of (a) CIC-22, (b) CIC22-PhF5, (c) VC, and (d) VC-PhF5 powders, all supported on carbon tape. The N2 adsorption/desorption isotherms of these carbon powders were also obtained, primarily in order to track any change in their porous structure due to surface modification with the –PhF5 groups (Figure 2a). After surface functionalization, the amount of N2 adsorbed on the VC surface decreased over a wide range of relative pressures (P/Po < 0.9), while CIC-22 did not 9 ACS Paragon Plus Environment


exhibit any significant decrease at P/Po < 0.8. This suggests that -PhF5 functionalization significantly decreases the surface area of VC, as also reported by others for the surface modification of VC powders,42-43 but hardly alters the surface area of CIC-22. However, the adsorption-desorption hysteresis seen at CIC-22 at a P/Po of around 0.8 did shift to lower pressures after surface functionalization, indicating a slight shrinkage in its average pore size. In order to obtain the pore size of these carbons, the Barrett-Joyner-Halenda (BJH) method was used to analyze both the N2 adsorption and desorption branches of the isotherms, with the results shown in Figures 2b and 2c. As explained in our previous work,27 for the CICs, the pore size derived from the adsorption branch isotherms (Figure 2b) reflects the diameter of the spherical pores (Figures 1a and 1b), which is directly related to the diameter of the colloidal silica particles used for imprinting, while the desorption branch (Figure 2c) gives the size of the necks (or connecting pores) between adjacent spherical pores (Figures 1a and 1b). Figures 2b and 2c show that CIC-22 surface functionalization decreased the average pore size by ca. 3 nm (Table 1) and the pore necks by 1 nm, from 10 to 9 nm. Surface functionalization also narrowed the pore size distribution of CIC-22, as seen in both figures. These changes confirm the successful attachment of the -PhF5 groups, which are ca. 0.7 nm in size (similar to hexafluorobenzene, C6F6 44), onto the inner surface of the CIC pores. Theoretically, if the pore surface of CIC-22 is completely covered with one monolayer of –PhF5, both the pore and pore neck sizes of CIC-22 should decrease by ca. 1.5 nm (0.7 nm on each pore side). Since it is unlikely that a second -PhF5 can be bound to an already attached PhF5 group (forming more than one monolayer of -PhF5) because of the high electronegativity of fluorine, the 1-3 nm decrease in the pore size (pore necks and widest part of the pores), obtained from the BJH fitting (Figure 2b, Table 1), is quite reasonable.


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1.8

0.14

(a)

1.6

(b)

CIC-22-PhF5

0.12

Pore volume (ml/g·nm)

Quantity adsorbed (ml/g)

1.4 CIC-22

1.2 1 0.8

CIC-22-PhF5

0.6 0.4

CIC-22 0.03

0.10 0.08

CIC-22

0.02

0.06

VC

0.01

VC-PhF5

0.04

VC

VC-PhF5

0 0

0.2 0.4 0.6 0.8 Relative pressure (P/Po ) 0.5 (c) 0.4

CIC-22-PhF5

0.00 0

0.02

VC

0.2

Pore volume (ml/g·nm)

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


20

40

60

80

100

VC-PhF5

0.00

1

0

20

40 60 80 Pore size (nm)

100

CIC-22 CIC-22-PhF5 0.03

0.3

0.02

0.2

0.01

0.1

0.00

CIC-22 CIC-22-PhF5

VC

0

10

VC-PhF5

20

30

40

50

VC-PhF5

VC 0.0 0

10

20 30 40 Pore size (nm)

50

Figure 2. (a) N2 adsorption (solid line) and desorption (dashed line) isotherms for VC and CIC-22, before and after surface functionalization with pentafluorophenyl (-PhF5) groups using the diazonium reduction reaction (Scheme 1), and pore size distribution of these carbons, calculated from the (b) adsorption and (c) desorption branches of the N2 sorption isotherms (a) using the Barrett-Joyner-Halenda (BJH) method, with the t-curve of carbon black used as the standard to determine the statistical thickness of the adsorbed nitrogen film.45 The insets in (b) and (c) show the pore size distribution of VC and VC-PhF5 more clearly. For better comparison, the structural properties of the carbons, with and without surface functionalization, are summarized in Table 1. The t-plot method was used to determine the



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‘external surface area’ (Sexternal), which is defined here to include the outer particle surface as well as the surface of pores that are > 2 nm in diameter (i.e., mesopores), as well as the total micropore volume (Vmicro). The total micropore surface area (Smicro) was then obtained by subtracting Sexternal from the total surface area (SBET),27 while the total volume (VNSI, Table 1) of pores that are < 100 nm in size was obtained from the nitrogen sorption isotherms (NSI) at a relative pressure of 0.98.27

Table 1 Structural properties of VC and CIC-22, before and after surface functionalization with pentafluorophenyl (-PhF5) groups Sample a

Pore size (nm)b

SBET (m2/g)c

VNSI Sexternal Smicro Vmicro S /S V /V (m2/g)d (m2/g)e micro BET (mL/g)f (mL/g)d micro NSI

VC

220

130

90

40%

0.38

0.05

12.0%

VC-PhF5

160

100

60

40%

0.41

0.03

6.7%

CIC-22

25

400

320

80

20%

1.6

0.04

2.4%

CIC-22-PhF5

22

390

310

80

20%

1.5

0.04

2.3%

a

The “22” in the sample names is the particle size (in nanometers) of the colloidal silica template used in the colloid-imprinted carbon (CIC) synthesis process. b Pore size was obtained from the maximum in the pore size distribution plots in Figure 2b, with an estimated error of ±1 nm. c SBET = total surface area was obtained using the Brunauer-Emmett-Teller (BET) plot in the relative pressure range of 0.05 < P/Po < 0.30 (Figure 2a). d Sexternal = ‘external surface area’ (outer surface of particles + surface of pores > 2 nm in diameter) and Vmicro = micropore volume were both obtained using the t-plot method in the relative pressure range of 0.2 < P/Po < 0.5 (Figure 2a), with carbon black used as the reference material.45 e Smicro = micropore surface area was obtained by subtracting the external surface area (Sexternal) from the total surface area (SBET). f VNSI = pore volume was determined from the N2 sorption isotherms (NSI) at P/Po = 0.98 (Figure 2a). For VC, while the majority of the pores are micropores (< 2 nm in diameter), Figure 2b shows that a broad distribution of pores larger than 2 nm in size is also present. VC is typically composed of 10-60 nm particles (Figure 1c) containing mainly micropores, which contribute 40% of the total surface area of VC (Smicro/SBET in Table 1),27, 43 but the VC particles are bound


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together as agglomerates or aggregates,27 resulting in a wide range of mesopores (textural pores that are 2-100 nm is size, or even larger) between the VC particles,43 as seen in Figure 1c. Figure 2b shows that surface functionalization of VC has led to a decrease in the total volume of pores that are smaller than 15 nm in diameter. It is also seen that VC-PhF5 has more pores that are > 20 nm in size than does VC, suggesting that the functionalization chemistry has affected primarily the inter-particle VC properties. Similar phenomena have also been observed previously when VC was surface modified with other functional groups.46 This may be caused by the further aggregation of VC particles/agglomerates during the surface functionalization process,42 decreasing the number of textural pores smaller than 15 nm in size and forming more of the larger textural mesopores (> 20 nm) between particles/aggregates,43 as seen in Figure 2b. Table 1 confirms that surface functionalization results in a significant decrease in the total surface area (SBET) of VC and only a small increase in its total pore volume (VNSI). The decrease in surface area may be attributed to the blockage of the entrance of the VC micropores (< 2 nm)46 by the -PhF5 groups (~ 0.7 nm) and also to some aggregation of the VC nanoparticles/agglomerates. This would result in a net decrease in both the total micropore surface area (Smicro) and the external (textural) surface area (Sexternal), but may not necessarily impact the percentage of micropore surface area (Smicro/SBET). At the same time, the aggregation of VC particles/agglomerates, after functionalization, causes the formation of additional textural mesopores. This results in an overall only minor change in the VC pore volume (VNSI) (0.41 mL/g vs 0.38 mL/g, Table 1), but a significant decrease in Vmicro/VNSI for VC (6.7 % vs 12 %, Table 1). Compared to VC, the surface functionalization of CIC-22 with the -PhF5 groups resulted in only a minor decrease in total surface area and pore volume. This is likely due to the larger



pore size of CIC-22 as well as the stability of its nanostructure (Figure 1b vs 1a) during the functionalization process. The -PhF5 groups are too small in size to block the relatively large CIC-22 pores, leaving the mesopore surfaces fully accessible to the adsorbate (N2), with little effect expected on the mass transport of N2 through the pores. The average size of the CIC-22 particles is about 10 µm,27 so the outer surface of the particles contributes little to their total surface area when compared to the large number of ca. 25 nm pores (Table 1) inside the CIC particles. Therefore, any aggregation of the CIC-22 particles would not significantly affect the total surface area (SBET, Table 1). These are the main reasons why surface modification has had much less influence on the surface area of CIC-22 than VC. It is interesting to note that the attachment of the -PhF5 groups to the surfaces of both VC and CIC-22 did not significantly change the percentage of micropore surface area for either of the two carbons (Smicro/SBET, Table 1). This suggests that surface modification does not alter the structure of the VC or the CIC-22 pore walls, in contrast to what was seen in earlier work after heat-treatment.27 This is also reflected by the only small change in the volume percentage of micropores in CIC-22 after surface functionalization (Vmicro/VNSI, Table 1), while the significant decrease in Vmicro/VNSI of VC is due to particle aggregation, as explained above. In comparison, heat treatment at 1500 °C in a N2 atmosphere was shown to significantly decrease both the

Smicro/SBET and Vmicro/VNSI ratios of VC and the CICs.27 3.2 Elemental composition of carbons before and after surface functionalization The elemental composition of VC and CIC-22, before and after functionalization with the -PhF5 groups, was evaluated to determine the concentration of fluorine groups grafted onto the carbon surfaces, with the results shown in Table 2. The attachment of the -PhF5 groups, assumed to occur at up to monolayer coverages on the accessible carbon surfaces (external surfaces of the


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particles and internal surface of mesopores larger than 2 nm in size), resulted in a higher fluorine content for CIC-22-PhF5 vs VC-PhF5, fully consistent with the difference in their PhF5accessible surface areas (Sexternal, Table 1).

Table 2 Elemental content (wt. %) of VC and CIC-22, before and after surface functionalization with pentafluorophenyl (-PhF5) groups Solid residue Coverage of -PhF5 Sample C (%)a H (%)a N (%)a F (%)b O (%)d (%)c groups (µmol/m2)e VC 98.4 0.8 0.1 0.7 ± 0.5 VC-PhF5 92.4 1.0 0.8 3.7 2.1 ± 0.6 3.2 CIC-22 93.0 1.0 0.0 1.8 4.1 ± 2.1 CIC-22-PhF5 85.3 1.1 1.0 7.6 1.2 3.7 ± 2.1 2.9 a Carbon, hydrogen and nitrogen contents obtained by combustion analysis, with an error of ± 0.3 % for each element. b Fluorine content determined using potentiometric titration with La(NO3)3, with an error of ± 0.3 %. c Solid residue in the CICs obtained from thermogravimetric analysis (TGA) in air, with an estimated error of ± 2.0 %. d Obtained assuming that O% = 100% – (C% + H% + N% + F% + solid residue %). e Coverage of -PhF5 groups on functionalized carbon surfaces obtained by normalizing the fluorine content to the external surface area (Sexternal, Table 1) of the corresponding carbon before functionalization, assuming that the -PhF5 groups were grafted only onto the surface of pores that are > 2 nm in diameter. The surface functional group coverage of both VC and CIC-22 (Table 2) was calculated by dividing the number of moles of bonded -PhF5 by the surface area of the pre-surfacefunctionalized carbons (Sexternal), excluding the micropores (Table 1). This calculation was based on the assumption that the -PhF5 groups cannot penetrate the micropores, due to steric hindrances, but would be covalently bonded to the surface of all of the other pores (diameter > 2 nm) and also on the outer surface of the particles. Table 2 shows that both VC-PhF5 and CIC-22PhF5 have a similar-PhF5 surface coverage, 3.2 and 2.9 µmol/m2, respectively. This similarity is indirect evidence that the analysis is being done correctly, since both carbons were functionalized under the same conditions.



Page 16 of 43

If the -PhF5 group covalently bonds through a C-C single bond (Schemes 2a and 2b) onto planar graphene surfaces or on graphene edges, the groups should be oriented perpendicularly to the carbon surface. Assuming that a -PhF5 group rotates freely along the axis of a single bond, it will take up a surface area of ca. 50 Å2, similar to the cross-sectional area of a hexafluorobenzene (C6F6) molecule.44 Thus, the maximum coverage (one monolayer) of the PhF5 groups on a carbon surface would be 3.4 µmol/m2, based on this assumption. The observed coverage of ~ 3 µmol/m2 of VC-PhF5 and CIC-22-PhF5 (Table 2) therefore suggests that the -PhF5 groups cover about 90% of the accessible VC and CIC-22 surface area, which is very reasonable. F

(a)

(b)

Top view PhF5

Side view

F

F

Side view F

F

F F

F

F

F

Scheme 2. Two likely forms of -PhF5 bonded to the carbon surfaces, (a) at a planar graphene surface and (b) at a surface composed of graphene edges. In Table 2, it is also seen that surface functionalization leads to a significant increase in the nitrogen content of both VC and CIC-22, as well as an increase in the oxygen content of VC. Figure 3 shows the X-ray photoelectron spectroscopy (XPS) results for the non-modified and functionalized VC and CIC-22 powders, confirming the presence of N groups after the diazonium modification, with a binding energy of ~ 400 eV. The N atoms in the surfacemodified carbons may be present in the form of pentafluorophenyl-azo (-N=N-PhF5) groups,46 covalently bonded to the carbon surfaces, since PhF5-N=N-O-C5H9 and PhF5-N≡N+ are both important intermediates during the functionalization reaction.47-48 The N atoms may also be


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present in other chemical forms, some of which may contain oxygen (e.g., PhF5-N=N-Oor -NO),36, 49 as will be addressed in the following sections.

Figure 3. X-ray photoelectron spectra (XPS) of (a) VC, (b) VC-PhF5, (c) CIC-22, and (d) CIC22-PhF5. The inset within each figure shows the high-resolution XPS spectrum of the corresponding sample at around 400 eV to better analyze the N signal, while the tables at their left top corners give the elemental content estimation based on the XPS data. The small amount of F and N present in the CIC-22 powder (c) is due to a minor contamination problem.



3.3 Wettability of carbons before and after surface functionalization It was shown above that surface functionalization of the CIC-22 and VC powders with the -PhF5 groups did not alter the CIC-22 structural properties to any major extent (Figure 1). However, in the case of VC, attachment of the –PhF5 groups lowered the BET-determined surface area (Table 1), which may be due to blocking of the entrances to the micropores by the –PhF5 groups and/or causing further aggregation of the VC particles/aggregates. At the same time, surface functionalization did alter the elemental content of both carbons, adding F, as expected, and some O and N (Table 2). As has been reported previously for heat-treated CICs,27 contact angle kinetics (CAK) and water vapor sorption (WVS) methods are good methods for determining the wettability of solid surfaces and are used here to better understand the surface chemistry of the carbon powders after surface modification.

3.3.1 Contact angle kinetics (CAK) study Figure 4a shows the sequential images of a water droplet falling on pressed VC and CIC22 pellets (before and after functionalization with -PhF5), as well as the time taken for the water droplet to be fully taken up by the pellet (last row). Figure 4b shows the CAK data for the water droplets with time in the first 200 ms of contact with the carbon pellet surfaces. As discussed previously,27 water droplets have a higher contact angle on as-prepared CIC-22 than on VC at short times (t < 200 ms, Figure 4), but water is fully absorbed in a much shorter time by the CIC pellet than the VC pellet (0.8 s vs 10.7 s, Figure 4a). The higher initial contact angles seen for the as-synthesized CIC-22 are attributed mainly to the rougher surface of the CIC-22 pellet (caused by its larger particle size), while its shorter water-uptake time reflects its very hydrophilic surface when compared to VC.27


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b)

180 160 140

Contact angle (°)

Page 19 of 43

120 100 80 60 VC CIC-22 VC-PhF5

40 20

CIC-22-PhF5

0 0

20 40 60 80 100 120 140 160 180 200 Time (ms)

Figure 4. (a) Sequential images of water droplets deposited on pressed pellets of carbon powders, before and after surface functionalization with the pentafluorophenyl groups, and (b) contact angle kinetics (CAK) of water droplets after deposition onto the pressed pellets, within the first 200 ms of contact (data from (a)). The time to 100% dryness (last row in (a)) refers to the time that it takes for the pellet to fully absorb the water droplet deposited on it. The times to dryness for the -PhF5 functionalized carbons were so long (~ 1 h) that the droplets eventually evaporated, instead of being absorbed by the pellets. A contact angle of 180 ° in (b) indicates that the water droplet has fully rebounded from the sample surface (as seen for CIC-22-PhF5 at t = 25 ms in (a)). Compared to VC, a water droplet deposited on the VC-PhF5 pellet shows much higher contact angles at all times (Figure 4) and exhibits a static contact angle of ca. 125 ° (t = 1 s, Figure 4a), suggesting a significantly enhanced hydrophobicity. A high static contact angle of water has also been reported for other fluorinated carbon powders.50-51 The exact time to 100% dryness cannot be reported here because the water droplet was not absorbed by the VC-PhF5 pellet and, instead, it finally evaporated after ca. 1 h. It should be noted that the high contact angle of water on VC-PhF5, shown in Figure 4, is due not only to the hydrophobic nature of the PhF5-modified carbon surface, but also to the roughness of the pellets.27 This results in a higher contact angle than obtained for a perfectly smooth surface with the same hydrophobicity, known as the lotus effect.52-53 However, compared to heat-treated VC,27 VC-PhF5 exhibits a much 19 ACS Paragon Plus Environment


higher contact angle, which suggests that surface modification with the -PhF5 groups results in a more hydrophobic character. In terms of CIC-22, it is also much more hydrophobic after functionalization than before, as shown by the high contact angle of water on the CIC-22-PhF5 pellet and the long time to 100% dryness (Figure 4). A water droplet deposited on the CIC-22-PhF5 pellet surface can even rebound from the surface, reflecting superhydrophobicity, a combined resultant phenomenon of intrinsic surface hydrophobicity and surface roughness.54 As compared to heat-treated CIC-22,27 CIC-22-PhF5 exhibits a higher contact angle, which suggests that surface modification with -PhF5 groups is more effective than thermal treatments in increasing the hydrophobicity. The water droplet on CIC-22-PhF5 shows higher contact angles than on VC-PhF5 (~ 160 ° vs 125 °, Figure 4b), at least partly due to their different particle sizes (~30 nm of VC-PhF5 vs ~ 10 µm of CIC-22-PhF5).27 Here, it is believed that surface modification did not change any of the carbon particle sizes, as discussed above. (Note: This is different from the size of the VC agglomerates in Figure 1, since agglomerates can be deformed during the die-pressing process when the pellet was prepared.) Based on these CAK results, it is still not clear which of the two carbons, VC-PhF5 or CIC-22-PhF5, is intrinsically more hydrophobic.

3.3.2 Water vapor sorption (WVS) study Water vapor sorption (WVS) studies of the carbons, with/without surface functionalization, were also carried out to confirm the wettability results obtained from the macroscopic contact angle kinetic measurements (Figure 4) and also to obtain some new insights regarding the wettability at the microscopic scale. Figure 5a shows the WVS results for a certain set of samples with time, where the volume of water vapor sorbed on the carbon powders was calculated from the mass increase of the samples in a water vapor environment at room


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temperature, with an error of ±0.01 mL/g.27 Other batches of samples were examined on different days, but we were not able to average the WVS results from these different batches as the experimental conditions, e.g., the ambient temperature in the lab, seemed to vary somewhat. Even so, the trends between samples were always the same, as all of the carbon samples were placed in the same water vapor chamber at the same times so that they all experienced the same humidity and lab conditions each time. CIC-22 is seen to sorb significantly more water than does VC, with the volume of water sorbed by CIC-22 being close to the pore volume obtained from the nitrogen sorption isotherms (Table 1). This confirms that CIC-22, as-synthesized, is very hydrophilic, as reported previously by us.27 As expected, the amount of water sorbed by CIC-22-PhF5 has decreased significantly (Figure 5a) vs at CIC-22 alone, confirming its much lowered wettability. In comparison, as-received VC is seen to take up nearly as much water as CIC-22-PhF5, even without surface modification. This is not surprising, as VC was heat-treated to ca. 1400 °C in its commercial preparation in order to maximize its conductivity,41 a process that will also remove most of its surface oxide groups. This link between VC powder preparation and its wettability and crystallinity has been reported previously by us,41 as well as by others.43, 55 The water uptake for the VC, VC-PhF5 and CIC-22-PhF5 samples is observed to decrease somewhat between 200 and 300 hours, likely as the water vapor concentration in the chamber was somewhat different each time the chamber was opened to measure the sample mass, with time for re-equilibration then required. Notably, a relatively hydrophobic sample cannot retain adsorbed water for very long and thus the WVS results will be very sensitive to changes in the relative humidity. We normally consider this as part of the error present in the WVS experiments



and thus we always repeated each measurement multiple times until three successive data points give the same value before considering that the experiment was complete. (Figure 5a)

b) 5.0 CIC-22

4.0 VWVS/Vmono

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

Page 22 of 43

3.0

VC-PhF5

2.0

VC

1.0 CIC-22-PhF5 0.0 0

100

200

300 400 Time (h)

500

600

Figure 5. (a) Water vapor sorption (WVS) data for VC and CIC-22, before and after surface functionalization with pentafluorophenyl (-PhF5) groups, in a water vapor atmosphere at room temperature. (b) Water vapor sorption data (VWVS) of the carbons in (a), normalized to their corresponding surface area (SBET in Table 1). Note: Vmono = the volume of one monolayer of water molecules close-packed on the carbon surfaces, obtained using the equation: Vmono = SBET M H O σ H O N A ρH O , where SBET is the specific surface area of the carbon powders (Table 2

1),

MH O 2

2

2

= molar mass of water, σH O = cross-sectional area of water molecules (0.106 nm2),56-57 2

NA = Avogadro’s constant, and ρH O = water density at 23°C. 2

The surprising result in Figure 5a is that, while VC-PhF5 takes up less water than VC alone at the beginning of the experiment (t < 300 h, Figure 5a ), as expected, it then sorbs a little more water than VC did after longer times of exposure to water vapor (t > 500 h). Since all of the samples in Figure 5a were present together in the same chamber and were thus exposed to the identical water vapor environment, the data are fully comparable with each other. Thus, the longterm WVS behavior of VC-PhF5 vs VC alone does not follow the expected trend of a lower degree of hydrophilicity with functionalization, as CIC-22 clearly showed (Figure 4). In order to validly compare the WVS mass gain data for the different carbon powders (Figure 5a), the raw WVS results were corrected for the surface area of the corresponding carbon 22 ACS Paragon Plus Environment

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samples (SBET, Table 1) and then re-plotted (Figure 5b) to better gauge the relative wettability at the microscopic level. Vmono is the calculated volume of one closely packed monolayer of water on the carbon surface, having an area of SBET, assuming that the surface is hydrophilic and thus becomes fully covered with water.27 The VWVS/Vmono values (Figure 5b) can therefore be viewed as indicative of the number of layers of water sorbed on the sample surface. Although this may not indicate the real adsorption behavior of water vapor on a porous carbon surface, it can still reflect the propensity of the sample to sorb water. This provides very useful information regarding its relative wettability.27 As seen in Figure 5b, the non-functionalized CIC-22 still exhibits much higher

VWVS/Vmono ratios than the other samples, indicative of its very hydrophilic surface.27 At the same time, surface functionalization with the fluorine groups made the CIC-22-PhF5 the most hydrophobic among these samples, as seen from its very low VWVS/Vmono ratio over the full time range. This is also fully consistent with what was seen in the CAK experiments for these samples (Figure 4). However, the VC-PhF5 sample exhibits VWVS/Vmono ratios that are almost the same as for VC in the first 300 h of exposure to water vapor, and then it is seen to sorb even more water than does VC at longer times. This again suggests that VC-PhF5 is somewhat more hydrophilic than VC, which is contrary to what was concluded for VC from the CAK results (Figure 4). Although VC continues to be much more hydrophobic than CIC-22 before functionalization (Figure 5), after surface area correction, the -PhF5-functionalized VC takes up more water vapor per unit surface area than does CIC-22-PhF5 (Figure 5b). An effort will be made to clarify this in the following section.



3.4 Further understanding of the –PhF5 functionalized carbon surfaces The contradiction between the measured relative wettability of VC and VC-PhF5, in particular, based on the CAK and WVS methods can perhaps be understood better by taking into account the different wetting processes involved in the two methods. After water droplet deposition on the VC-PhF5 and CIC-22-PhF5 surfaces in the CAK experiments, the contact angle will depend mainly on the distribution of hydrophobic sites, such as the fluorinated surface groups as well as trapped air (caused by roughness of the pellet surfaces). If these sites dominate the surface, this will prevent the water droplets from spreading on the carbon pellet surface during the CAK measurements, even if some hydrophilic domains (polar groups) are present. A similar discussion was presented in the macroscopic explanation of the wetting of Nafion films by an impacting water droplet.54 The high coverage of the hydrophobic -PhF5 groups on VC-PhF5 (~ 90 % coverage, when calculated based only on the external surface area of the microporous VC particles,) must dominate the observed surface wettability of the VC-PhF5 pellets in the macroscopic CAK experiments, hindering the water droplets from spreading and thus exhibiting high contact angles (Figure 4). In comparison, the fresh (as-received) VC particle surfaces are somewhat more hydrophilic (θ < 90 °) and thus do not prevent the water droplets from spreading. Therefore, VC shows a smaller contact angle than does VC-PhF5 in the short-term CAK measurements, as was expected (Figure 4). In contrast, when the carbon powder samples were exposed to water vapor in the WVS experiments, which give microscopic wettability information, any polar groups present on the surface (e.g., C-OH and COOH on CIC-22), even at low coverage, could help to nucleate water condensation.27 Therefore, the WVS results will depend on the area-specific density of both


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polar and non-polar groups, as a surface with a higher polar surface group density will take up more water vapor per unit area of surface (giving a higher VWVS/Vmono ratio). Also, a micropore in any of the carbon structures could be considered as a surface polar group in the WVS experiment, since it can quickly take up water vapor.58-59 Although VC and VC-PhF5 both have a similar micropore density (Smicro/SBET ~ 40%, Table 1), VC-PhF5 has a higher nitrogen and oxygen surface content than does VC (Table 2) and a lower surface area (160 m2/g vs 220 m2/g, Table 1). This means that VC-PhF5 must have a higher surface density of polar groups than VC (area-specific density of oxygen atoms is 6.5 µmol/m2 for VC-PhF5 vs 2.1 µmol/m2 for VC, based on the total surface area, SBET). Since the presence of -PhF5 groups (~ 90 % coverage on the external surface) cannot prevent water vapor from nucleating around any polar domains on the outer VC particle surface, the higher density of surface polar groups could be the reason why VC-PhF5 takes up more water vapor than does VC (Figure 5 ). It is of interest to better understand the origin and nature of the nitrogen and oxygen groups present on the VC-PhF5 surfaces (Table 2). As mentioned above, nitrogen may be present as surface azo (-N=N-PhF5 or –O-N=N-PhF5) groups, but this would not explain the significant increase also in the oxygen content of VC after surface functionalization. However, considering the instability of the amyl nitrite reagent used in the diazonium reduction reaction (Scheme 1), it may have reacted directly with the carbon surface during the functionalization process, following Scheme 3. At high temperatures (82 °C, boiling point of acetonitrile), amyl nitrite will dissociate,4748

accelerated by protic reagents (such as C-OH or COOH groups), forming ·NO radicals or

[NO]+ ions.

These radicals and ions can react with the carbon surface to form nitroso groups,



Page 26 of 43

which could be further oxidized by oxygen when exposed to air at high temperatures (100 °C, used to dry the sample after washing),60-61 forming nitro groups (-NO2, Scheme 3). Due to steric hindrance, access of the VC micropores to the -PhF5 groups could well be prevented. This would leave the VC micropore surfaces available for reaction with the smaller nitroso and/or nitro groups, resulting in a higher polar group coverage and thus a higher degree of water uptake for VC-PhF5, compared to VC (Figure 5b). NO OH

O 1. Reflux (24 h) in amyl nitrite/CH3CN in N2

COOH

2. Oxidized in air

NO

NO2

Scheme 3. Possible reactions of carbon surfaces (either VC or CIC-22 in this work) with amyl nitrite during the –PhF5 surface functionalization reaction (Scheme 1). Since CIC-22 was chemically functionalized using the same process as VC, the same side reactions should have occurred on both surfaces, consistent with the increased N content seen also for CIC-22 after functionalization (Table 2). Along with the diazonium reduction reaction itself (Scheme 1), these side reactions (Scheme 3 as an example) could remove highly polar groups (e.g., COOH 46) from the non-functionalized CIC-22 surface, replacing them with groups of lower polarity (e.g., -NO). Alternatively, the highly active nitrite or its intermediates (e.g.,

[NO]+ )

could oxidize the C-OH groups, forming the somewhat less polar C=O groups.62 In

addition to the attachment of the –PhF5 groups, all of these reactions would decrease the hydrophilicity of CIC-22. 26 ACS Paragon Plus Environment

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Although CIC-22-PhF5 has a > 2 times higher surface area than VC-PhF5 (Table 1), the nitrogen content of these two functionalized carbons is almost the same (Table 2). This indicates that there is a lower density of nitroso/nitro groups on the CIC-22 surface during functionalization. This may be attributed to the high density of C-OH groups on the CIC-22 surfaces, which could then be converted to C=O groups, decreasing the available surface carbon atoms for –NO/–NO2 attachment. These are the likely reasons why surface functionalization does not increase the oxygen content of CIC-22 significantly (area-specific density of oxygen atoms on CIC-22-PhF5 vs CIC-22 is 6.0 µmol/m2 vs 6.4 µmol/m2, respectively). As stated above, the carbon micropores will be more easily accessed by the ·NO radicals than by the significantly larger ·PhF5 radicals during the diazonium reaction (Scheme 1). Thus, the –NO/–NO2 groups may ultimately reside predominantly inside the micropores, while the -PhF5 groups would attach to the mesopore inner surfaces of CIC-22 and to the outer surfaces of both the VC and CIC-22 particles. Compared to VC, which contains only micropores, the CIC-22 surface area arises predominantly from mesopores (only 20% of the CIC-22 surface area arises from micropores, Table 1). Thus, only a small fraction of the total number of CIC-22 pores are micropores. Thus, there are far fewer sites available for the polar –NO/–NO2 groups to attach to the surface of CIC-22 than at VC. This should then lead to a lower water uptake of CIC-22-PhF5 than VC-PhF5 in Figure 5b, as is observed. This again supports the hypothesis that the –NO/–NO2 groups are attached primarily to the micropore surfaces, while the -PhF5 groups are most likely grafted onto the external surfaces (the outer surface of both VC and CIC-22 and the internal walls of the mesopores in CIC-22). In order to further understand the surface chemistry of these carbons, cyclic voltammetry (CV) was carried out on VC-PhF5 and CIC-22-PhF5, using the non-surface modified VC and



CIC-22 as the reference states, as shown in Figure 6. First, the higher CV currents seen for CIC22 than VC are consistent with their surface area differences (Table 1). CIC-22 also exhibits a pair of large pseudo-capacitance peaks, centered at ~ 0.55 V (Figures 6c vs 6a), reflecting its higher initial density of surface oxygen groups, relative to VC.27 For both VC and CIC-22, it is seen in Figure 6 that –PhF5 surface functionalization resulted in higher cathodic currents at < 0.4 V and lower anodic currents at > 0.9 V, relative to the non-surface modified carbons (Figures 6b and 6d, as compared to Figures 6a and 6c, respectively). The higher cathodic currents at < 0.4 V may be due to the oxygen reduction reaction (ORR), arising from air trapped in the carbon pores.63 However, in this work, a sulfuric acid/ethanol solution was added to the carbon powder samples during ink preparation in order to fully wet the hydrophobic pores and avoid the presence of any trapped air bubbles within the catalyst layer. Thus, this interpretation is unlikely. Also, the sulfuric acid solution used in this work was purged with N2 for at least 30 min before collecting the CV data (Figure 6), so there should be little O2 left in the solution. This also agrees with the very small difference in the cathodic current seen in the first negative scan vs at steady-state, for both VC and CIC-22, as any oxygen present should have been consumed through the ORR if it were indeed occurring. Overall, the contribution of the ORR to the cathodic current at < 0.4 V for both of the –PhF5 modified carbons should therefore be very minor. The cathodic current observed for the –PhF5 functionalized carbons at < 0.4 V (Figures 6b vs 6a and Figures 6d vs 6c) may then be attributed to the reduction of surface-bound -NO2 groups to –NHOH (Reaction 1) or to the reduction of –NO to –NHOH (Reaction 2), where -NHOH can be further irreversibly reduced to surface-bound –NH2 (Reaction 3). These reactions are typical for surface nitrogen groups in acidic media, according to the literature.64-68


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The CVs of the –PhF5 functionalized carbons also show a pair of reversible pseudo-capacitance peaks at around 0.55 V, perhaps due to Reaction 2, in addition to the reaction of surface hydroquinone and quinone.27 Therefore, the difference between the CV responses of VC and VC-PhF5 and those of CIC-22 and CIC-22-PhF5 in Figure 6 (a-d) indicates the likely presence of the -NO/-NO2 groups on the –PhF5 functionalized carbon surfaces. -NO2 + 4e- + 4H+

-NHOH +H2O

-NO +2e- +2H+

-NHOH

(1) (2)

-NHOH +2e- +2H+

-NH2 +H2O

(3)

-N=N- + 2e- + 2H+

-N(H)-N(H)-

(4)

-N(H)-N(H)- + 2e- + 2H+

2 -NH2 +H2O

(5)

The XPS binding energy for N, seen at ~ 400 eV (Figure 3), has often been attributed to an azo (–N=N-) group36, 69-70, as mentioned above. If these –N=N- surface groups are indeed present, they can be electrochemically reduced to –NH2 (Reactions 4 and 5) at < 0.3 V (vs RHE)71-72 and oxidized at > 1 V vs RHE72-74. However, an oxidation process is not observed in the CV results for either VC-PhF5 or CIC-22-PhF5 (Figure 6). Instead, the fluorinated carbons show a much lower anodic current at > 0.9 V than do the as-received ones. This suggests that the N is more likely in the form of –NO/-NO2 groups, with the XPS peaks at ~ 400 eV in Figure 3 having sometimes been attributed to –NO groups.49, 75 Either way, the XPS data do confirm that N has been successfully incorporated onto the VC and CIC-22 surfaces as a result of our diazonium chemistry and future work will focus on determining more precisely what the surface N groups are.



0.6

0.6 (a)

0.2 1 st cycle

0.0

Steady-state CV

-0.2 -0.4

0.2

-0.2 Steady-state CV

0.2

0.4 0.6 0.8 Potential vs RHE (V)

1

1.2

0

1.6

(d)

CIC-22

1.2


1

1.2

CIC-22-PhF5

1.2 2 nd cycle

0.8

Specific current (A/g)


0.2

1.6 (c)

0.4 1 st cycle

0 -0.4 -0.8

Steady-state CV -1.2

2nd cycle

0.8 0.4 1 st cycle

0 -0.4

Steady-state CV

-0.8 -1.2

-1.6

-1.6 0.2


1

1.2

0 (f) 0.8 0.7

Ratio of anodic current at 1.1 V vs at 0.8 V (vs RHE)

QCorrosion /QCapacitive

0

2.5

1 st cycle

0.0

-0.6 0

3

2 nd cycle

-0.4

-0.6

(e)

VC-PhF5

0.4

2nd cycle



(b)

VC

0.4

I(1.1 V)/I(0.8 V)

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

Page 30 of 43

2 1.5 1 0.5 0

0.2


1

1.2

Ratio of corrosion charge at 1.4 V to capacitive charge at 0.8 V (vs RHE)

0.6 0.5 0.4 0.3 0.2 0.1 0

Figure 6. Cyclic voltammograms (CVs) of (a) VC, (b) VC-PhF5, (c) CIC-22, and (d) CIC-22PhF5 in N2-saturated 0.5 M H2SO4 solution at a scan rate of 10 mV/s. (e) Anodic current at 1.1 V (vs RHE) divided by the anodic current at 0.8 V (vs RHE), and (f) Corrosion charge of the carbons at 1.4 V (vs RHE) divided by the capacitive charge at 0.8 V (vs RHE) in N2-saturated 0.5 M H2SO4 solution,33, 41 both showing that –PhF5 functionalization protects both VC and CIC22 from electrochemical oxidation. 30 ACS Paragon Plus Environment

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Figure 6e shows the anodic current at 1.1 V (double layer charging plus carbon oxidation) divided by the anodic current at 0.8 V (double layer charging only) for all of the carbons, demonstrating that the surface functional groups protect the carbons from electrochemical oxidation. Thus, -PhF5 surface functionalization may be useful in enhancing the long-term stability of these carbons when they are used in an aggressive oxidizing environment, consistent with the conditions at PEM fuel cell cathodes and in agreement with the results of our parallel study.33 In that work,33 a thorough electrochemical analysis was carried out to determine the stability of –PhF5 functionalized CIC and VC (vs the non-modified) materials, using a potential stepping method (Section 2.4), with some typical results obtained shown in Figure 6f. It is seen that the grafting of the –PhF5 groups onto the carbon surfaces significantly improves the corrosion resistance of both CIC and VC by ~ 50 %. Furthermore, VC-PhF5 is seen to be more corrosion resistant than CIC-22-PhF5, just as VC is more stable than CIC-22. This is likely because the VC powders were heavily heat-treated (to > 1400 °C) during commercial preparation 41

, while CIC-22 experienced temperatures of only 900 °C, and thus its degree of crystallinity

will be much lower. Overall, using the method shown in Scheme 1, we have successfully tethered the desired -PhF5 groups onto VC and CIC-22 powder surfaces at a coverage of ca. 90 %, excluding the micropore surfaces. Due to steric hindrance, the -PhF5 groups cannot access the very small micropores, whereas smaller, more polar groups (e.g., -NO and -NO2), produced during the diazonium reaction, can. These groups were shown to influence the microscopic wettability behavior of the carbons during the WVS analyses, especially the microporous VC-PhF5 material, unexpectedly making it a little more polar than the non-functionalized VC powder.



4. Conclusions In this study, a hydrophilic, ordered, mesoporous colloid-imprinted carbon (CIC-22, formed using 22 nm silica colloids as the template and producing mesopores in the range of 2025 nm in diameter) and a more hydrophobic, microporous Vulcan carbon (VC) powder were surface modified with -PhF5 groups using an in-situ diazonium reduction reaction. This was done in order to investigate the surface properties of the functionalized carbons for future use in applications such as fuel cell and battery electrodes. For both carbons, elemental analysis confirmed the successful surface modification with the -PhF5 groups, giving an estimated surface coverage (excluding the micropore surfaces) of 90%. The results also revealed an increase in the nitrogen content of both carbons and in the oxygen content of VC, suggested to be due to the deposition of nitro groups on the surfaces (likely in the micropores), based on cyclic voltammetry data. Surface modification of the CIC-22 was shown to not alter its highly desirable ordered pore structure and high surface area, decreasing the average pore size by only 1 - 3 nm. This small change in pore size confirmed the successful attachment of ca. one monolayer of the -PhF5 groups on the internal mesopore walls. In comparison, the surface area of VC decreased by ~ 30 %, likely due to the blocking of the entrances of the VC micropores as well as enhanced agglomeration of the VC particles/aggregates. The wettability of both the non-functionalized and functionalized carbons was examined using macroscopic contact angle kinetics (CAK) measurements and microscopic water vapor sorption (WVS) methods, with the as-received carbons used as benchmarks. The CAK results showed that the -PhF5 groups make both carbons more hydrophobic, as desired, especially CIC22-PhF5, which sometimes showed superhydrophobic behavior, while the WVS results also


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confirmed the enhanced hydrophobicity especially of CIC-22-PhF5. However, the WVS data also showed that both VC-PhF5 and CIC-PhF5 continue to sorb water, likely into their micropores. It is presumed that, due to steric hindrance effects, the -PhF5 groups are not able to penetrate into micropores and thus attach only to the outer surfaces and mesopores of the carbons. In contrast, smaller polar nitro groups, formed as a surface intermediate during the diazonium surface modification process, can fit inside the micropores and attach to these available inner surfaces. Overall, this work shows that CIC-22 can be successfully made more hydrophobic by the uniform deposition of –PhF5 groups on its internal mesopore walls. This lowered wettability makes the CICs more applicable for use in PEM fuel cell catalyst layers, for example, where water flooding and corrosion problems are both exacerbated by overly hydrophilic characteristics.

Acknowledgements We gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery and Strategic Project Grant Programs for overall financial support, as well as Ballard Power Systems for the support of some of this work. NSERC and Alberta Innovates Technology Futures are also acknowledged for their scholarship support of D. Banham and F. Forouzandeh. We would also like to thank Samuel Aquino, Dr. Chris Clarkson, and Dr. Pedro Pereira for their help with the BET measurements, Dr. Josephine Hill for accesses to the die-presser, and Anusha Abhayawardhana and Dr. Scott Paulson for helpful discussions.

Supporting Information Available: The results of a group of preliminary experiments on the functionalization of VC with -PhF5 groups at different molar ratios of carbon to other reagents are provided. 33 ACS Paragon Plus Environment


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Graphic for Table of Contents or Abstract:


Surface Characteristics of Microporous and Mesoporous Carbons

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