Electrocatalytic Activity of Functionalized Carbon Paper Electrodes

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Electrocatalytic Activity of Functionalized Carbon Paper Electrodes and their Correlation to the Fermi Level Derived from Raman Spectra Ashutosh Singh, Nael Yasri, Kunal Karan, and Edward P.L. Roberts ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00180 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Applied Energy Materials

Electrocatalytic Activity of Functionalized Carbon Paper Electrodes and their Correlation to the Fermi Level Derived from Raman Spectra Ashutosh K. Singh, Nael Yasri, Kunal Karan, and Edward PL Roberts* Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4 *Corresponding author: Tel. +1 403 714-3228; email [email protected]

ABSTRACT Carbon paper electrodes are employed for different electrochemical applications such as flow batteries and fuel cells. However, redox reactions such as VO2+/VO2+ in a vanadium redox flow battery have been found to possess relatively slow kinetics, resulting in significant activation losses during operation. In this work, we demonstrate a facile and scalable method for nitrogen doping of carbon paper electrodes, leading to superior electrocatalytic activity. The effect of pyrolytic pretreatments under different conditions on the performance of carbon paper were also studied to elucidate their electrocatalytic activity from a material physics perspective, using Raman spectroscopy. The 2D Raman signature, a specific feature of the carbon structures, was employed to understand the effect of different pretreatments on the Fermi level of the carbon papers, which could help us elucidate their intrinsic electron transfer kinetics. The full wave half maximum of the 2D Raman band, and the intensity ratio I2D/IG were used to indicate changes in the Fermi level relative to the untreated carbon paper, and hence the electrocatalytic properties, which were confirmed using voltammetric techniques. Although heating of carbon paper in air at around 500°C (a widely used method for activating carbon paper electrodes) increases the surface area by about 16-times compared to untreated and nitrogendoped carbon paper, the latter exhibits superior electrocatalytic property for VO2+/VO2+, [Fe(CN)6]3-/4-, and the oxygen reduction reaction. This study provides greater physical insights into different pretreatments in terms of the energy barrier at the interface, which will aid the pursuit for better carbon-based electrode materials and provide mechanistic details about charge transfer processes at the interface. Keywords: Carbon papers, N-doped, Raman mapping, Fermi level, electrode functionalization.

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INTRODUCTION Carbon paper is a commonly used material for electrochemical applications such as fuel cells1, redox flow batteries2, electro-analysis3, bio-electrochemical growth 4,5 and electrochemical capacitors 6. Its wide applicability can be attributed to its high surface area, electrochemical activity, electronic conductivity, porosity, ease of handling and relatively good mechanical strength7. Carbon paper is usually prepared via process sintering of polyacrylonitrile (PAN)7 fibers impregnated with a carbonizable resin, to form an integrated system of carbon fibers and carbonized resin8. Carbon paper electrodes are characterized by long connected highly conductive fibers, high fluid permeability, and the carbon surface can be modified to tailor the targeted electrochemical reactions 9. These properties enable, in some cases, the use of carbon paper for several functions in the same electrochemical device. For example, in fuel cells, carbon paper acts as a conducting catalyst support and as a hydrophobic gas diffusion layer allowing reactant gas (hydrogen or oxygen) to access the catalyst 10. Many studies have been performed to modify the surface properties of carbon paper and other carbonaceous materials to enhance electrochemical performance or to increase the operating lifetime. These include, but are not limited to, surface activation, modifying the pore size distribution and functionalizing with surface functional groups by employing suitable pretreatment methods.11 A common pretreatment involves heating in air, which is known to introduce oxygen functional groups on the carbon surface, to increase hydrophilicity, and may enhance the redox catalysis of electrode.11 Liu et al. utilized CO2 to incorporate oxygen functional groups on carbon paper, increasing surface area, and hence decreasing the charge-transfer resistance of carbon paper electrodes in vanadium redox flow batteries (VRFB)

12.

They found that the charge transfer resistance decreased after CO2

functionalization from 970 to 120 mΩ cm2, leading to a 13% increase in the VRFB energy efficiency. However, a careful control of activation temperature is required during treatment, since the ohmic resistance of the carbon paper electrode increases with the activation temperature 12. Other approaches to increase the active area and porosity without heating have included laser ablation to create macro-pores of various sizes on carbon paper electrodes 13. Although this route was found to increase the surface area as well as improve mass-transport, the process is only suitable for surface treatment, with the modification is limited to the exposed surface of the treated carbon paper. The addition of carbon nanomaterials such as carbon nanotubes, graphene, nanosphere, and multifunctional nano-carbon composites to increase the active surface area of carbon paper have also been reported.11 For example, Manahan et al. 14 prepared a nanoporous layer of multiwall carbon nanotubes (MWCNT) on carbon paper to improve the performance of VRFBs.


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Metal and metal oxides anchored on the surface of carbon fibers were also found to enhance performance in batteries and fuel cells.11,15 The instability of the metal-carbon structure in acidic environments can present a challenge for the application of this type of catalyst for fuel-cells, VRFB and for the oxygen reduction reaction (ORR) in metal air batteries. 16 Recently, there has been increasing interest in metal-free electrocatalysis using carbon materials doped with heteroatoms such as nitrogen, boron, phosphorous, fluorine, and Sulphur, along with co-doping of these elements.17 This approach was found to be effective as it intrinsically modifies the chemical properties of the pristine carbon without altering the physical characteristics (such as surface area and porosity). The chemical properties are modified due to coupling between electronically active (electron-donating or electron-withdrawing) moieties on the carbon structure.18 Nitrogen-doping of carbon materials has been found to alter its electronic structure.18 The heteroatoms present in the graphitic framework modifies the electrochemical properties of the material by favoring interfacial electron transfer processes and thus enhancing some redox reactions at the electrode interface.18 Nitrogen doping introduces defects into the carbon structure, and the presence of nitrogen valence electrons contribute to the π–electrons in the C-C system, leading to an increase in the n–type conductivity.18 The most common configurations of nitrogen doped onto carbon are pyridinic, pyrrolic, quaternary and pyridinic N-oxides.18 The slight difference in electronegativity between nitrogen and carbon induces a weak permanent dipole associated with the C–N bond. The presence of weak polarity increases surface energy which enhances wettability as compared to the non-polar C=C bond18 In addition, the presence of dipoles on the carbon surface facilitates adsorption of species when N-doped carbon paper is used as an electrode, and at the same time, the n-type conductivity facilitates interfacial charge transfer, such as in oxygen reduction and vanadium redox reactions 19. For example, Shao et al.

19

found that doping nitrogen on mesoporous carbon enhances the rate of the VO2+/VO2+ redox

reaction in VRFB. They found that the reversibility of the redox couple VO2+/VO2+ is greatly improved on N-doped mesoporous carbon, leading to increased energy efficiency. Conventionally, nitrogen doping of carbon is achieved by thermal treatment of a graphitic substrate in the presence of nitrogen containing precursors. Table 1 shows some of the reported methods and precursors employed for nitrogen doping, the range of nitrogen-content after treatment (in atomic %), along with the corresponding preparation method and drawbacks. In most cases, nitrogen doping was performed by heating carbon samples under ammonia (NH3) gas. For instance, Dengsheng et al. synthesized N-doped graphene by heating a graphene substrate under a constant flow of ammonia (NH3) and argon gas at different temperatures 20. They reported, that the optimum doping temperature for best performance in Proton Exchange Membrane Fuel cells (PEMFCs) is about 900 ⁰C. However,


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this process could involve significant hazards in terms of large-scale implementation, along with strict environmental regulations related to the storage and transportation of ammonia. Alternative organic and/or inorganic materials containing nitrogen functional groups such as chitosan 21, glucosamine 22, and pyridine23 have also been utilized as precursors for N-doping of carbon paper. Although the percentage of N-doping reported has been promising (see Table 1), the control of treatment conditions and the choice of precursor must be selected carefully for safe operation and also minimizing the codoping of other elements present in the precursor. For example, some of the precursors are toxic (e.g. pyridine 23 and polypyrrole24), while some contain other functional groups that can simultaneously codope and impact the performance of the carbon paper electrodes (e.g. bromide in tribromophenol 25, or oxygenated groups in glucosamine and chitosan 21,26).


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Table 1. Reported methods and nitrogen precursors employed for nitrogen doping of carbon materials

Synthesis technique

Precursor used

N doping at %

Drawbacks

Ref.

1

Thermal treatment

NH3; NNDMTa; NNDMPb; NNDMVc; melamine; urea, polyacetonitrile; and polyaniline

1.1% – 10 %

Toxic, expensive, flammable precursors

20,27, 28–30

2.

Hydrothermal treatment

CPAd, hydrazine, NH3, glucosamine, and chitosane

~ 8%

Requires post treatment and metal ion catalyst in some cases.

21,22,26,31

3.

CVD treatment

NH3, Acetonitrile, Pyridine

1.2% – 9%

Metal catalysts required, Expensive technique

23,32

4.

Template synthesis SBA 15

DABe, Polypyrrole, Amino - glucose

2–5%

Complex and tedious process

24,33,34

5.

Plasma treatment

N2 gas, NH3

1.3 – 8.5 %

High electric field required, expensive

35,36

6.

Solvothermal treatment

TBPF-NH3f

Surface functionalization by nitrogen

Longer duration required, Bromide ions produced

25

a

NNDMT is 3, N, N-dimethylethanolamine, b NNDMP is N, N-dimethyl-propanediamine, c NNDMV is N, N-dimethylformamide, d CPA is 3–chloro propylamine, e DAB is Diaminobenzene (source of C and N), and f TBPF is 2,4,6 – tribromophenol ferrocene in the presence of ammonia (250 ⁰C for 24 h).


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Consideration of the electron transfer processes occurring at the electrode-electrolyte interfacial of carbon paper electrode is important for the determination of the suitability and ultimate applications of materials in electrochemical cells. For example, peroxide formation as a product of the ORR at the electrode interface is one of the main degradation mechanisms of catalyst and membrane in fuel cells37. Depending on the type of electrode material or the type of catalyst present the ORR can either lead to peroxide (by a 2-electron transfer) or to water (by a 4-electron transfer)

38.

The type of doping on

carbon paper electrode can influence the reaction mechanism. For example, Stephen et al. reported that doping nitrogen on carbon nanofibers (CNF) altered the dissolved oxygen reduction reaction from a 2-electron transfer reaction to a 4-electron transfer process 38. On the other hand, many studies have determined that failure of electrochemical systems was caused by H2O2 generation 39,40. For example, Massa et al.

40

reported that carbon corrosion occurs in a fuel cell membrane electrode assembly

(MEA) due to the generation of peroxides at the cathode

40.

Thus, using nitrogen doping on the

backbone structure of a carbon cathode in an MEA can contribute to the overall effort of limiting the in-situ generation of peroxide and hence help in enhancing the system durability. In this study, we demonstrate a facile synthesis of nitrogen-doped carbon paper (N-doped carbon paper) and study its electrochemical characteristics for redox reactions and the oxygen reduction reaction. The challenges in conventional heat pretreatment processes for carbon paper were overcome by using a room temperature liquid ammonium hydroxide (NH4OH) precursor, followed by annealing under nitrogen (N2), avoiding the requirement for continuous supply of a hazardous precursor during thermal treatment. The modification of carbon structure in terms of defects and charge carrier concentration at various stages of the process has been studied to elucidate and visualize changes in the carbon electronic band structure. The changes to Fermi level of the carbon is inferred from the Raman spectra. The relationship between the Fermi level and electrochemical activity is investigated, providing experimental evidence of a connection between these characteristics. This study adds critical dimensions to carbon electrocatalytic studies by helping researchers gain deeper insights into how surface modifications influence the electrochemical behavior of carbon materials.

EXPERIMENTAL Carbon Paper pretreatment: Commercial PAN-based carbon papers (Sigracet GDL-39AA, Ion Power, USA8) were pretreated prior to electrochemical characterization. The mean fiber diameter of the untreated carbon paper was ~ 9 μm, and its thickness was 280 ± 30 μm, ash content < 0.25%, electrical resistivity < 5 mΩ cm2, porosity 85%, and mean pore diameter 42 – 44 μm. Prior to treatment, carbon papers were cleaned by sonicating


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in isopropyl alcohol for 10 minutes to remove organic impurities. Nitrogen doping of carbon paper (Ndoped carbon paper) was performed by soaking carbon paper in 3.5 M ammonium hydroxide solution (NH4OH, Alfa Aesar) for 45 minutes with 5 minutes of initial sonication. After soaking, the carbon paper was dried in air and then heated in a tubular furnace at 500C for 1 h under nitrogen. The Ndoped carbon paper was characterized and compared with untreated carbon paper and with carbon papers that had been pretreated by two different methods: 1) soaking in 3.5 M NH4OH solution and leaving to dry in air under ambient conditions (precursor treated carbon paper), and 2) heat treating in air at 500C for 5h (partially oxidized carbon paper). The conditions used for the partially oxidized carbon paper were chosen to ensure a high oxygen content and to contrast with the N-doped carbon paper. Where the carbon paper was heated, the rate of heating and cooling rate was maintained at 5C min1. Material Characterization Elemental analysis for carbon paper samples was performed using field-emission scanning electron microscopy (FESEM) (Philips XL 30 SEM, Philips, Tokyo, Japan) coupled with Energy Dispersive X-ray (EDX) (Bruker XFlash 6|30, Bruker, Germany) mapping technique with an accelerating voltage of 200 V at Ultra high vacuum (UHV). Raman mapping was performed on a region of 150  150 μm of the sample, using a WiTec Alpha-300 series confocal Raman microscope utilizing 532 nm diode laser. XPS spectra were obtained using Kartos Axis ULTRA photoelectron spectrometer (Kratos analytical limited, UK) with an Al-Alpha excitation source (1436 eV) under vacuum (5 × 10-10 torr). The atomic concentration of respective elements, peak fitting for high resolution data and other data processing was performed using CasaXPS software. Nitrogen adsorption/desorption isotherms for Brunauer–Emmett–Teller (BET) surface area determination was obtained using a Micromeritics Tristar II 3020. All the samples were degassed for 10 hours at 150⁰C under high vacuum to remove any adsorbed species before nitrogen adsorption analysis. Electrochemical characterization: All electrochemical measurements were performed using a three-electrode system with PGSTAT 320N (Metrohm Autolab, Netherlands) potentiostat at room temperature. Carbon paper samples with a geometric area of 0.5 cm2 (0.5 cm x 1 cm) were used as the working electrode, with a platinum wire counter electrode, and a saturated calomel reference electrode (SCE). The electrolyte was sparged with Argon gas (except where stated) to remove any dissolved oxygen. All potentials in this work have been adjusted to a standard hydrogen electrode (SHE) reference. IR compensation was carried out using the


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current interrupt method. Cyclic voltammetry (CV) was performed for the following electrolyte solutions: (i)

[Fe(CN)6]4−/3−: 2.5 mM potassium hexacyanoferrate(III) K3[Fe(CN)6] + 2.5 mM potassium hexacyanoferrate(III) K4[Fe(CN)6] in 0.5 M KCl supporting electrolyte.

(ii)

VOSO4: 1 M Vanadium oxy-sulphate in 3 M H2SO4 solution

(iii)

ORR-Alkaline: O2-saturated in 0.1 M KOH solution

In addition, for the [Fe(CN)6]4−/3− solution, electrochemical impedance spectroscopy (EIS) was performed using a potential amplitude of 10 mV and frequency range of 100 kHz to 100 mHz.

RESULTS AND DISCUSSION N-doping was performed on carbon paper via pyrolytic treatment in an inert (N2) atmosphere, using a liquid ammonium hydroxide precursor to achieve the N-doping. During this treatment, the nitrogen atoms of the ammonium hydroxide diffuse into the carbon paper and are thermally doped into the carbon structure at high temperatures (~500 °C). Energy dispersive X-ray (EDX) spectra mapping of the N-Doped carbon paper is shown in Figure 1 (a) and (b). The SEM images show the two components used in the preparation of the carbon paper: the carbon fibers (ca. 9 μm diameter) and the carbonized/graphitized resin8. The EDX analysis in Figure 1 (b) shows the presence of nitrogen as bright orange spots on carbon paper samples. (EDX mapping of N doped carbon paper sample, as well as higher resolution SEM image are provided in Figure S1 (a) and (b)) Nitrogen doping was also confirmed from the XPS survey scan (Figure S2), the atomic concentration of C, N, and O was found to be 91.57 %, 1.39 %, and 7.04 %, respectively. Many studies have doped nitrogen on carbon structures using various N-precursor, including melamine and NH3 gas (see Table 1) with the percentage of N-doping ranging from 1.1 to 10 %. The configurations of N-doped carbon paper were further characterized by examining the high-resolution N1s spectra. Figure 1 (c) shows that the N1s peak can be deconvoluted into three components centered at 401.63, 399.55, and 403.75 eV, corresponding to quaternary-N, pyridinic-N, and pyridinic N-oxides, respectively. The dominant form was the quaternary-N. Although quaternary N-doping will involve slightly higher binding energy (401.63 eV) as compared with pyridinic-N (399.55 eV), and pyridinic N-oxides (403.75 eV), quaternary N-doping may be favored on the graphite basal planes. Quaternary-N doping of carbon has been shown to offer enhanced durability and electrochemical activity 41,42.


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(a)

(b)

1000 μm

1000 μm

(c)

Figure 1. Scanning Electron Microscopy and EDX mapping of N-doped carbon paper (a) SEM

image of carbon paper sample, (b) EDX analysis where bright orange spots show broad distribution of nitrogen over carbon paper sample confirming the presence of nitrogen, and c) high-resolution XPS of N1s spectrum for N-doped carbon paper sample with deconvoluted peaks showing different Nitrogenous functional groups in sample.

Raman Spectroscopy Raman spectroscopy was used to obtain information on the macroscale distribution of the defect and dopant before and after doping. Figure 2 shows a representative comparison of single Raman spectra obtained on fibers of untreated, precursor treated, nitrogen doped and partially oxidized carbon papers, whereas Table 2 summarizes the obtained parameters. Moreover, to further spatially and spectrally differentiate the morphologies of the treated carbon papers, Raman spectral mapping was also performed, and data analysis are provided in Figure S4, S5, and S6 respectively.


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For all of the treated materials, the D-band intensity increased compared to the untreated carbon paper. The nitrogen doped carbon paper shows the highest D band intensity (Figure 3 (b)). It is well established that bonding of nitrogen with graphitic materials is associated with defects due to the introduction of heteroatoms into the carbonaceous structure. These type of defects are known to increase the intensity of the D-band as compared with well-aligned crystals 43. The N-doping process led to an increase in the ID/IG of carbon paper on the fibers (with an average value of ID/IG = 1.01 ± 0.03) compared with untreated carbon paper (ID/IG = 0.82 ± 0.05) (Table 2). A similar trend was obtained by Zhang et al., who reported an increase in the ID/IG ratio from 0.26 to 0.80 after nitrogen doping of graphene 44. In comparison, the ID/IG intensity ratio of partially oxidized carbon paper was 0.85 ± 0.02, only slightly higher than the untreated carbon paper, which may be due to the introduction of oxygen functional groups onto the carbon structure. In this respect, the variation in the intensity ratio of the D and G bands, i.e. ID/IG, represents the disorder in carbon framework; that is the higher the ratio indicates more defect and decrease in crystallinity of carbon structure45. Spectral Mapping of the calculated intensity ratio of ID/IG for untreated and N-doped carbon paper overlaid on optical images, are shown in Figure 3 (a) and (b). The equivalent spectral mappings of the intensity ratio of ID/IG for partially oxidized carbon paper as well as precursor treated carbon paper, are presented in Supplementary Figure S4 (a – d). In Figure 3 (a) for untreated carbon paper, we can observe high ID/IG ratios on fibers compared with lower values on graphitic part suggesting more defective lattice structure (amorphous) on fiber than low defect graphitic part (more crystalline). The results also show clear increase in defects (indicated by ID/IG Figure 3 (a) and (b)) in the treated carbon paper, due to the introduction of nitrogen in the N-doped carbon paper. An interesting observation is that in the N-doped carbon paper (Figure 3 (b)), the ID/IG ratio is much more intense on the carbon fiber (red color in the overlay image) indicating that N-doping occurred more favorably on these sites as compared to the surrounding carbonized/graphitized resin. This suggests that the energy required to form bonds of heteroatom dopants on the amorphous fibers is less than that required for the more crystalline graphitized resin. This could be attributed to the high chemical reactivity of carbon atom in the amorphous regions compared to the carbon atom in the more crystalline resin region46. This explanation is further supported by the lower ID/IG ratio on graphitized resins after N doping (where the precursor is weakly attached or anchored); see Figure 3 (a) and (b), suggesting that the thermal annealing may results in lowering the structural stress in these regions during doping process. Thus, because of high variation in the characteristics of the graphitized resin, including the number of layers, crystal size, etc., the examples of Raman spectra included in this study were collected from the fiber regions, whereas, the mapping of Raman spectra includes both regions.


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Shifts of both D-band and G-band are signs of structural stress on carbon bonding due to the introduced heterogeneous atom

43,47

and the type of charge carriers on the surface45, respectively. It

has previously been reported that both nitrogen43 and oxygen47 doping of graphitic carbon lead to an increase in the Raman shift of the D-band. In the present work, an increase in the Raman shift of the D-band for the precursor treated, partially oxidized and N-doped carbon papers was observed, compared to the untreated carbon paper (Table 2), indicating a compression in the carbon bonding. A shift in the G-band position has been observed for doping which is associated with the introduction of both holes and electrons 45. An upshift in G-band is associated with incorporation of charged dopant which modifies the charge carrier concentration at the surface48. The Raman shift of the G-band (Table 2) of the partially oxidized, precursor-treated, and N-doped carbon paper were 6.6 cm-1, 6.7 cm-1 and 12 cm-1 higher than untreated carbon paper sample (1580 cm-1). A significant upshift in G-band was observed for N-doped carbon papers indicating a significant rise in charge carrier density due to the introduction of dopant species. Moreover, the 2D-band, unlike the D-band, is not affected by defects, and its intensity is influenced by electron-electron interactions 49. Its sensitivity towards perturbations in the electronic structure of carbon materials has been used as a diagnostic tool to monitor changes in charge carrier density on two-dimensional planar graphene

50.

The intensity of the 2D-band (I2D) is typically

evaluated in relation to the intensity of G-band (IG). Thus the ratio I2D/IG is an indicator of the charge carrier density in carbon materials, where the density of p- or n-type charge carriers is inversely related to the I2D/IG ratio 51. For example, Das et al. 51 showed significant changes in the I2D/IG ratio associated with changes in electron concentration at trace levels of doping. In this work, the I2D/IG ratio on the carbon fibers of the untreated carbon paper was almost two times higher than that observed on Ndoped carbon paper (Table 2), and the values of the partially oxidized and precursor treated carbon papers were higher than the untreated carbon paper. Since ID/IG ratio mapping (Figure 3 (b)) suggests that N-doping of the carbon paper was not homogenous, it is interesting to observe the distribution of the I2D/IG ratio on the carbon paper, corresponding to variation in the charge carrier density, which is correlated to doping level. Figure 3 (c – d) shows the distribution of the I2D/IG ratio obtained by confocal Raman mapping for untreated and N-doped carbon paper respectively. It can be observed in Figure 3(c) that the ratio of I2D/IG on untreated carbon paper was lower on carbon fibers than on the carbonized/graphitized resin. Following immersion in NH4OH and air drying at room temperature, significant increase (~ 10 times) in the intensity ratio of I2D/IG was observed (Table 2 for I2D/IG values and Figure S5 (c) for spectral mapping), which represents considerable charge carrier interactions with adsorbed NH4+ ions on the carbon fiber surface. For the N-doped carbon paper (i.e. after thermal treatment) a lower I2D/IG was observed


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(Figure 3 (d) for spectral mapping) on the fibers compared to the untreated carbon paper. In summary, a high ID/IG and low I2D/IG was observed on the N-doped carbon fibers, corresponding to increased defects (due to the doping), combined with a higher charge carrier density. In contrast, both the ID/IG and I2D/IG ratios were high on the partially oxidized carbon paper (see Table 2 for values) compared to untreated carbon paper, indicating increased defects (due to electron withdrawing oxygen functional groups) and decreased charge carrier density. The sensitivity of the 2D-band to charge carrier interactions (or electronic structures) as discussed previously can be used to indicate variations in charge carrier density, and hence the impact of different pretreatments on the surface Fermi energy level

52.

Considering carbon paper as an

electrode material, the exchangeability of charge carriers at the electrode interface is, of course, important for electrocatalysis, and as discussed above, a more ‘exchangeable’ electron density at the surface will enhance electron–electron interactions. A central point to be understood for ensuing discussion is that the Fermi level is inversely related to the intensity of the 2D-band, and directly related to 2D-band full wave half maximum (FWHM)52. Thus, a high value of the FWHM of the 2D peak, corresponding to a wider and less intense 2D-band, indicates a more exchangeable charge carrier and hence less energy barrier for charge transfer at the interface. In this work, it was found that partial oxidation of the carbon paper led to a decrease in the 2D-FWHM from 151 cm-1 to 61 cm-1 (Table 2), indicating a sharpening of the 2D-band, attributed to a decrease in electron density compared to untreated carbon paper. A probable reason for this decrease in electron density for partially oxidized carbon paper could be the introduction of electronegative oxygen functional groups which disrupt the aromatic π–electrons arrangement responsible for conductivity 53. For example, Ago et al. reported the effect of different oxidative treatments on the density of states of valence bands and observed an increase in the work function with these treatments on multi walls carbon nanotubes (MWCNT) 54. A reduction in electron density is associated with a downshift of Fermi energy level, leading to a higher work function and ultimately resulting in increased potential barriers for interfacial charge transfer processes. After immersion in NH4OH (precursor treated carbon paper), the 2D-FWHM of the carbon fibers decreased from 151 to 116 cm1 (Table 2), indicating neutralization of negatively charged fibers, and hence reducing the charge carrier density. After N-doping (i.e. after thermal treatment of the precursor treated carbon paper), a significant increase in the 2D-FWHM was observed, from 116 to 244 cm-1, indicating significantly enhanced electron-electron interactions and increasing interfacial charge carrier density. Thus, due to N-doping, the Fermi energy level increases resulting in a decrease of the charge transfer barrier at the interface 52.


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A change in the Raman shift of the 2D-band position can be used to indicate the type of charge induced by doping of either holes (blue shift) or electrons (red shift)51. Both N-doped and partially oxidized carbon paper show a red shift in the 2D peak (8.8 cm-1 and 2.2 cm-1, respectively), which indicates the introduction of n-type charge carriers 51. In contrast, the precursor treated carbon paper (without heating) induced a slight blue shift in the 2D peak (Table 2), indicating the presence of p-type charge carriers. The presence of holes in this case as charge carrier may be due to the adsorption of positively charged ammonium ions on the carbon surface.

Figure 2. Raman spectrum obtained on the carbon fiber of the carbon paper materials. (a) Untreated carbon paper CP, (b) N-doped carbon paper CP, (c) Precursor treated carbon paper CP and (d) Partially oxidized carbon paper CP.


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Table 2. Parameters for Raman shifts of D, G, 2D and related intensities of different carbon papers that are collected from the same represented spots of fibers as shown in Figure 2.

D-band

G-band

2D-band

shift

shift

shift

cm1

cm1

cm1

Untreated

1351

1580

N doped

1357

Pretreatment

Partially oxidized Precursor treated

(a)

ID/IG

I2D/IG

2D-

ratio

ratio

FWHM

2703

0.8

0.031

151

1592

2694

1.01

0.016

244

1357

1587

2701

0.85

0.045

61

1355

1587

2708

0.94

0.49

116

(b)

90 μm

(c)

90 μm

90 μm

(d)

90 μm

Figure 3. Confocal Raman mapping of the ID/IG ratio for (a) untreated carbon paper and (b) N-doped carbon paper, and I2D/IG ratio mapping for (c) untreated carbon paper, (d) N-doped carbon paper, superimposed on optical microscope images. The violet and red colours on ID/IG scale represent low and high defect regions, respectively while on the I2D/IG scale, they represent high and low charge carrier density, respectively. Each Raman mapping covers an area of 120  120 μm.


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Impact of Pretreatment on Surface Area The surface area of carbon electrode materials can be significantly affected by pretreatment methods. The BET surface area (SBET) determined using nitrogen adsorption experiments, for carbon paper samples at various stages of treatment are reported in Table 3. The results indicate that the surface area of N-doped (SBET 4.90 m2 g1) and precursor treated carbon papers (SBET 3.56 m2 g1) were slightly higher than the untreated carbon paper (SBET 3.31 m2 g1). The BET surface area of the partially oxidized carbon paper was much higher at 64.1 m2 g1, an increase of around 20-fold compared to the untreated carbon paper. This increase in surface area of air heated samples is probably due to the activation of carbon surface by oxygen molecules in the air, which react with surface-carbon creating surface roughness and leading to a large increase in surface area. This phenomenon of activation of carbon paper via air heating is a well-established pre-treatment method for electrochemical applications such as VRFB11,55,56. Thus, pretreatment by air activation modifies the morphology (increasing the surface area) and the surface chemistry (increased defects, oxygen functional groups, and reduced charge carrier density). With nitrogen doping, the main change is in the surface chemistry, while the surface morphology is only slightly changed. In order to investigate the impact of these changes on the application of these materials as electrodes, studies of the electrochemical behavior of the carbon papers are needed. Table 3. BET surface area analysis of the carbon paper materials. Pretreatment method

Surface area m2 g1

Untreated carbon paper

3.31

Partially oxidized carbon paper

64.1

N-doped carbon paper

4.90

Precursor treated carbon paper

3.56


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Electrochemical Characterization Vanadium redox couple: VO2+/VO2+ Carbon paper-based electrodes are extensively used in redox flow batteries, especially in VRFB 57.

The electrode kinetics of the redox reactions in VRFB are relatively slow, thus changes in the

electron transfer resistance at the electrode-electrolyte interface can enhance the performance 57. It has been established that the slow kinetics of the electrode reaction at the VRFB cathode (i.e. the positive electrode) is due to a complex reaction mechanism which involves at least three elementary steps: two proton and one electron transfer process (see Eq. 1)

58.

These elementary steps involve complex

intermediates and the exact mechanism depends upon the electrolyte pH and potential 59. VO2 + + H2O ⇋ VO2+ +2H + + e ―

Eq. 1

Cyclic voltammetry of the VO2+/VO2+ couple on carbon paper electrodes (Figure 4) reveals a decrease in the potential difference between the oxidation and reduction peaks (ΔEp) in the following order; precursor treated carbon paper (ΔEp = 359.25 mV) > untreated carbon paper (ΔEP = 202.52 mV) > partially oxidized carbon paper (ΔEp = 190.55 mV) > N-doped carbon paper (ΔEp = 164 mV). On the other hand, voltammetry and impedance studies on a quasi-reversible system such as the [Fe(CN)6]3/4 redox couple demonstrate a very similar trend as observed for the VO2+/VO2+ redox couple (see Supporting Information Figure S7 (a) and (b)). This trend indicates that the reversibility of the electrode process for the vanadium redox couple increased in the same order as the electrochemical kinetics for the ferricyanide/ferrocyanide redox system, with N-doped carbon paper showing the best performance in both cases. Moreover, the ratio of oxidation to reduction peak currents for VO2+/VO2+ was found to increase in a similar manner; i.e. precursor treated carbon paper < untreated carbon paper < partially oxidized carbon paper < N-doped carbon paper. These results indicate nitrogen doping of the carbon paper electrode leads to electrocatalytic enhancement as well as reversibility of the VRFB positive electrode redox couple. These results are also consistent with the 1.6 times improvement in charge – discharge capacity obtained by Kim et al. for N – functionalized graphite felt electrodes compared to oxidized graphite felt for VRFB.60 It is also evident that N-doped carbon paper (Figure 4) suppresses the oxygen evolution reaction at a potential of 1.6 V vs SHE. This is consistent with the background CV in sulfuric acid (3 mol L−1 H2SO4) solution discussed in detail in SI (See the performance of carbon papers in acid and alkaline medium in the Supporting Information: Figure S8 and S9). In this aspect, the partially oxidized carbon


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paper shows oxygen evolution and hydrogen evolution currents in 3 mol L−1 H2SO4 medium of about sixfold higher than obtained with untreated carbon paper (see Supporting Information for detailed investigation Figure S8). In contrast, the N-doped carbon paper and precursor treated carbon paper show lower oxygen and hydrogen evolution currents compared to the untreated carbon paper. As both oxygen and hydrogen evolution reactions, as well as hydrogen peroxide generation, are often parasitic side reactions, suppression of water electrolysis is beneficial for many electrochemical applications. It is important to consider why the rates of the oxygen and hydrogen evolution reactions are suppressed on the N-doped paper, while the ORR (as well as vanadium and ferrricyanide redox reactions) is enhanced (see Figure 4 and the Supporting Information Figure S7). The suppression of the OER and HER can be explained in terms of charge redistribution on the N-doped carbon paper electrodes. The lone pair on the dominant quaternary nitrogen species of the N-doped electrode can be mobilized in the surrounding π-conjugation in the carbon structure, which will result in enhancement of the nucleophilic interactions at adjacent carbon atoms. The OER takes place at the electrode surface via the well – established 4 – stage reactions shown in equations (1) – (4)61, and the HER occurs via the Volmer – Heyrovsky mechanism which involves the adsorption of hydrogen ion (H+) on the active site to be ultimately released as H2 molecules following electron transfer62. * + H2O → *OH + H+ + e-

(1)

*OH → *O + H+ + e-

(2)

*O + H2O → *OOH + H+ + e-

(3)

*OOH → * + O2 + H+ + e-

(4)

(* represents an active site on the electrode) Considering the ORR, the enhancement of nucleophilic interactions in the N-doped carbon paper increases the dissolved O2 adsorption.63 For the OER, the electronic redistribution leads to a higher energy barrier for adsorption of the intermediates formed during the OER (i.e. OH and OOH).63 This implies a reduction in the rates of reactions (1) and (3), which have been identified as the rate-limiting steps of the OER64. Suppression of the HER on N-doped carbon has also been observed by Jiang et al. in a lead acid battery application65. The charge redistribution on the N-doped carbon, leading to the formation of stable N-H bonds during the Volmer step (i.e. H+ ions adsorption


17


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on active nitrogenous sites), was identified as the cause of the slower kinetics for the HER on N-doped carbon65.

Figure 4. Cyclic voltammograms for a carbon paper working electrodes with a scan rate of 20 mV/s in 1M VOSO4 solution. The electrodes used were (i) untreated, (ii) N-doped, (iii) precursor treated, and (iv) partially oxidized carbon paper.

Further investigation about the pathway of the ORR on various carbon papers as cathodes, in particular, the number of electrons (n) transferred in alkaline media (0.1 M KOH), are included in the Supporting Information (Figure S10 – S13). Analysis of cyclic voltammetry data performed at different scan rates reveals that the ORR on N-doped carbon paper occurs via a four-electron transfer process, corresponding to H2O formation rather than H2O2. In contrast, the ORR reaction follows a two-electron transfer pathway, leading to peroxide (H2O2) production, on the partially oxidized, untreated and precursor treated carbon papers, (see Supporting Information for the Epc vs. ln(ν) data Figure S11 – S13 and Table S1). These results underline the advantages of N-doped carbon papers as cathodes for the ORR.


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Cyclic voltammetry at a range of scan rates (υ, mV s-1) can provide further information about the kinetics of the electrode reaction66. The voltammogram of the VO2+/VO2+ couple shows a well resolved oxidation peak at scan rates in the range of 20 - 200 mV s-1, whereas, the reduction peaks broaden when the scan rate increases (see Supporting Information Figure S14 – S17 and Table S2). Thus, the variation of peak currents with scan rate was evaluated for the resolved oxidation peaks. The oxidation peak currents (Ip) were found to be linearly dependent on the square root of scan rate (υ1/2) for all carbon paper samples indicating that the reaction was diffusion-controlled. The relationship between Ip and υ for a diffusion controlled, irreversible electrochemical reaction is described by the modified Randles – Sevcik equation (Eq. 2)66: 1

𝐷𝐹𝜐 1/2 𝑅𝑇

( )

= 0.496 (𝛼𝑛′)2 𝑛𝐹𝐴𝐶 𝐼𝑖𝑟𝑟𝑒𝑣 𝑝

Eq. 2

where A is the active surface area, α is charge transfer coefficient, n is the number of electrons transferred per molecule in the electrochemical oxidation, and 𝑛′ is the number of electrons per mole transferred before the rate-determining step (RDS). The gradient of Ip versus υ1/2 was found to increase in the following order: precursor treated carbon paper (222 mA mV1/2 s1/2) < untreated carbon paper (298 mA mV1/2 s1/2) < partially oxidized carbon paper (329 mA mV1/2 s1/2) < N-doped carbon paper (344 mA mV1/2 s1/2) [see the Supporting Information: Table S3.]. For precursor treated carbon paper, untreated carbon paper and Ndoped carbon paper, the BET surface area was relatively constant compared to partially oxidized samples, and hence the electrochemical active surface area (A) can be considered to be the same for these three electrode materials67. Thus, significant variation in the slope of Ip versus υ1/2 must be due to changes in values of (𝛼𝑛′)1/2, which represents a characteristic enhancement of the interfacial charge transfer kinetics. In conclusion, N-doping of the carbon paper leads to a noticeable enhancement of the electrochemical kinetics for the VRFB electrode reaction. Our results suggest that the enhanced activity of the partially oxidized carbon paper, widely used in VRFB, is due to an increase in the surface area (according to BET analysis and high double layer current) rather than the intrinsic electrochemical kinetics. Indeed, the peak separations observed in the CVs (Figure 4) suggest that the intrinsic electron transfer kinetics were not significantly enhanced on the partially oxidized than the untreated carbon paper electrode. In summary, the electrochemical kinetics for all of the reactions (except OER and HER) studied followed the same trend as observed for the Raman spectroscopy 2D peak parameters; i.e. the higher the 2D-FWHM the higher electron-electron interactions and increasing interfacial electronic density. These observations are consistent with a strong relationship between the Fermi level (evaluated based on the Raman spectrum) and the electrochemical kinetics.


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Durability of carbon paper electrodes The durability of electrode materials is critical for many electrochemical technologies. The stability of the carbon paper electrodes for the ORR was monitored using a constant applied potential (chronoamperometry mode) (– 0.2 V vs SHE) in an oxygen (O2) saturated alkaline solution (0.1 M KOH). The output current was significantly higher for N-doped carbon paper than the untreated, precursor treated and partially oxidized carbon paper electrodes (see Figure 5). The initial ORR current densities based on geometric electrode area of samples were 0.92 ± 0.04 mA/cm2 for untreated, 8.6 ± 0.66 mA/cm2 for N-doped, 0.74 ± 0.04 mA/cm2 for precursor treated, and 3.26 ± 0.64 mA/cm2 for partially oxidized carbon paper electrodes respectively. After four hours of operation the ORR current densities at - 0.2 V v’s SHE for these four carbon paper materials decreased to 0.26 ± 0.04 mA/cm2, 7.6 ± 0.58 mA/cm2, 0.26 ± 0.04 mA/cm2 and 1.38 ± 0.56 mA/cm2. For the N-doped carbon paper, the ORR current had decreased by less than 12 % after four hours and appeared to be relatively stable. In contrast for untreated, precursor treated, and partially oxidized carbon paper the ORR current decreased by about 72%, 57%, and 59% respectively, and the current was continuing to decrease at the end of the experiment. A decrease in current output for the ORR operation has been reported for various electrocatalytic electrodes68. For example, the common benchmark Pt/C (20%) catalyst used widely for ORR reaction, shows a decrease in the ORR current of 40 % after 3 hours in O2 saturated 0.1 M KOH solution,68 a much higher rate of decrease than the results obtained in this work using Ndoped carbon paper. Moreover, on the basis of the observed durability results in this work, the Ndoped carbon paper was found to outperform some of the other reported nitrogen-doped carbon nanostructures

68–70.

For example, Zhang et al. reported 30 % degradation for Co3O4 anchored N –

doped reduced graphene after 1 hour in O2 saturated 0.1 M KOH69. Similarly N-doped Ketjenblack incorporated into Fe-C foam showed a 30 % degradation after 4 hours ORR operation70.


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Figure 5. Chronoamperometry at - 0.2 V vs SHE in 0.1 M KOH, for carbon paper samples: (i) untreated, (ii) N-doped carbon paper, (iii) precursor treated carbon paper, and (iv) partially oxidized carbon paper.

DISCUSSION Figure 6 presents a schematic illustration of the energy level diagram of the electrode/electrolyte interface, where the Fermi energy level at the electrode interface alters according to the electronic density (charge carrier density) 67,71. We interpret our findings as indicating that N-doping induces a shift in Fermi energy level at the electrode surface, resulting in a reduction in the potential barrier (ΔE) between the electrode surface and the Highest Occupied Molecular Orbital (HOMO) of species in the electrolyte, and thus the interfacial charge transfer occurs at relatively higher electrocatalytic activity (Figure 6(b)). Methods utilized to indirectly estimate the Fermi levels by measuring the work function include scanning tunneling microscopy (STM)72, ultraviolet photoelectron spectroscopy (UPS) 73 and thermionic emission 74. Theoretically, monitoring the changes in the charge carrier density of the electrode can be used as an indirect approach to monitoring Fermi level75. In this study, it was found that there is a correlation between the changes in charge carrier density, the intrinsic electrochemical kinetics, and the Raman spectra, especially to the 2D signature in Raman analysis (2D-FWHM and I2D/IG). The observed trend in FWHM of the Raman 2D-band on the fibers of four different carbon paper samples was found to be consistent with the peak separation in cyclic voltammetry, and the peak current for materials with the same specific surface


21


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area. This provides a useful correlation between the 2D peak in Raman spectra and the electrocatalytic properties of carbon materials. The expected trend deviates in the case of partially oxidized carbon paper where there is a very high current and lower 2D FWHM value was observed. This deviation in the partially oxidized carbon paper may be attributed to an increase in the active surface area during pretreatment as observed in both the BET analysis and the background CV in 3M H2SO4 (see the Supplementary information).


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(a)

(b)

(c)

Figure 6. Schematic diagrams of interfacial charge transfer process showing reduction of energy barrier (ΔE) for charge transfer where grey indicates electrode surface (carbon paper in this study) and blue sphere indicates electroactive species, HOMO – (Highest Occupied Molecular Orbital) and LUMO – (Lowest Unoccupied Molecular Orbital) represents molecular orbitals in the electroactive species in the electrolyte 76: (a) before, (b) after changes in the carbon structure such as N-doping and (c) Schematic representation of the shift in the Fermi energy level for carbon paper samples (electrode) based on cone energy band structure.77 The red shaded cones indicate unoccupied density of states (DOS) and the blue shades represents electrons occupied DOS. Dashed lines across the diagram represents benchmark with a sense of positive or negative shift in Fermi level.


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According to the Raman spectra, the shift in the Fermi energy level for N-doped and partially oxidized carbon papers as compared with the untreated sample are presented schematically in Figure 6(c) based on Dirac cone electronic band structure 77. Partially oxidized carbon paper shows a decrease in charge carrier density implying a decrease of Fermi level and an increase in the potential barrier for charge transfer compared to the untreated carbon paper. In other words, partial oxidation should reduce the intrinsic electrocatalytic activity of the carbon paper. This decrease in Fermi level can be explained in terms of a difference in electronegativity between oxygen and carbon, which leads to an increase in the charge localization of the π–electron cloud. Similarly, a decrease in Fermi level and presence of p – type charge carrier (holes) was inferred from the Raman data for precursor treated carbon paper (shown in Figure 6 (c)), consistent with electrostatic interaction between the precursor (NH4+) and the carbon surface leading to charge localization. Many previous studies have suggested that the enhanced kinetics for the vanadium redox reactions with partially oxidized carbon paper is due to the formation of oxygen functional group on the carbon surface 55,56,78,79. Our results show that the enhanced activity of partially oxidized carbon paper for VRFB is associated with an increase in surface area (and perhaps hydrophilicity

80,81)

rather than

enhancement of the intrinsic electrochemical kinetics. In fact, the Fermi level and the intrinsic electron transfer kinetics were observed to decrease due to the partial oxidation. Further work is needed to establish the role of oxygen functional groups on the overall kinetics of vanadium redox reactions. CONCLUSION The effect of pretreatment on carbon paper electrodes for applications in batteries and fuel cell was studied. The conventional approach widely applied for carbon paper, heat pretreatment in the air, was found to increase active surface area rather than catalyzing electrode reaction. We demonstrate that nitrogen doping of carbon paper can be performed via pyrolytic treatment in an inert atmosphere, following a facile ammonium hydroxide pretreatment. Raman mapping analysis provided insight into heterogeneous changes to the carbon structure due to partial oxidation in air and nitrogen doping. Based on the defect distribution on nitrogen-doped carbon paper (observed using confocal Raman spectroscopy) we found that nitrogen doping occurred predominantly on the carbon fibers rather than the carbonized/graphitized resin in the carbon paper. Interestingly, despite the introduction of shifts in Fermi levels to favor the charge transfer, the N-doping was found to suppress the parasitic oxygen and hydrogen evolution reactions, presenting the significance of kinetic barriers and energy barrier at the interface. For reactions with complex mechanisms where the rate determining is associated with adsorption processes, the Fermi level cannot be used as an indicator of the overall electrochemical kinetics.


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It was found that oxygen reduction in alkali on untreated carbon paper follows a two – electron transfer pathway, leading to hydrogen peroxide which reduces the lifetime of catalyst and membrane materials in metal-air batteries and other devices. On the other hand, nitrogen-doped carbon paper was observed to follow a four-electron pathway, leading directly to form water without hydrogen peroxide production. These results point the way to new carbon paper pretreatment strategies for batteries, by combining increase in surface area (by activation or by combining with high surface area carbon nanomaterials) with enhanced intrinsic kinetics via heteroatom doping. ASSOCIATED CONTENT Supporting Information Available: XPS survey spectra, High resolution FESEM and EDX mapping, ID/IG ratio mapping for untreated, precursor treated, air heated and NCP samples, 2D FWHM mapping for all samples, I2D/IG ratio mapping for all samples, electrocatalytic studies against [Fe(CN)6]4−/3−, voltammetric studies in acid and alkaline media, electrocatalytic studies for ORR and tables. AUTHOR INFORMATION: Corresponding Author * Email [email protected] ORCID Ashutosh Kumar Singh: 0000-0002-0464-6672 Nael Yasri: 0000-0002-8170-6675 Kunal Karan: 0000-0001-5432-8050 Edward P.L. Roberts: 0000-0003-2634-0647 Notes: The authors declare no competing financial interest. ACKNOWLEDGEMENT: The authors acknowledge financial support from the Natural Science and Engineering Research Council (NSERC) of Canada (Grant Numbers 2017-495455 and 2018-03725), Alberta Innovates (Project Number ES15021), and the Canada Foundation for Innovation (Project number 32613). The authors would also like to acknowledge the NanoFAB Centre at University of Alberta who performed the XPS analysis.


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Table of Content only (Graphical abstract)


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Electrocatalytic Activity of Functionalized Carbon Paper Electrodes

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