Nitrogen-Fluorine Dual Doped Porous Carbon Derived from Silk

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Nitrogen-Fluorine Dual Doped Porous Carbon Derived from Silk Cotton as Efficient Oxygen Reduction Catalyst for Polymer Electrolyte Fuel Cells Srinu Akula, Bhuvaneshwari Balasubramaniam, Prabakaran Varathan, and Akhila Kumar Sahu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00100 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Applied Energy Materials

Nitrogen-Fluorine Dual Doped Porous Carbon Derived from Silk Cotton as Efficient Oxygen Reduction Catalyst for Polymer Electrolyte Fuel Cells Srinu Akula,a,b Bhuvaneswari Balasubramaniam,c Prabakaran Varathan a and Akhila Kumar Sahua,b, * ABSTRACT: Porous carbon derived from silk cotton and heteroatom engineering is explored in this study towards developing metal-free electro catalysts for oxygen reduction (ORR). Individual and dual doping of N and F heteroatoms was conducted to regulate defects and the pore geometry to the porous carbon matrix. Microscopic analysis of N-F co-doped cotton carbon (N-F/CTC) undergoes morphological amendments in its textural properties and defects responsible in creating active sites for ORR. N-F/CTC catalyst exhibits excellent ORR catalytic activity, methanol and CO tolerance in the alkaline medium that makes it as potential metal-free ORR catalyst for the polymer electrolyte membrane fuel cell. N-F/CTC catalyst is subjected to 10,000 repeated potential cycles with no degradation in its activity. XPS analysis of N-F/CTC catalyst revealed the presence of N in the form of pyridinic-N, pyrrolic-N, graphitic-N, active species and F in the form of C-F ionic and C-F semi-ionic active forms. The maximum C-C bond polarization, charge re-distribution and high spin densities in the carbon matrices attained by all these active forms present in the catalyst and synergistically enhance the ORR activity. KEYWORDS: heteroatoms, dual doping, silk cotton, porous carbon, oxygen reduction reaction, polymer electrolyte membrane fuel cell. a

CSIR - Central Electrochemical Research Institute-Madras unit, CSIR Madras Complex,

Taramani, Chennai - 600 113, India. b

Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical

Research Institute, Karaikudi, India- 630 003. c

Indian Institute of Technology, Kanpur, Uttar Pradesh, India – 208016.

*Corresponding author. E-mail: [email protected] (A. K. Sahu).

1 Environment ACS Paragon Plus

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1. Introduction Oxygen reduction reaction (ORR), a cathodic reaction, which decides the overall efficiency of any energy generation devices, such as fuel cells and metal-air batteries needs to focus extensively.

1,2

The sluggish reaction kinetics of ORR associated with strong chemical

bond between two oxygen atoms (498 KJ/mole) significantly hinders the performance of the electrochemical devices.3 Till date, platinum (Pt) and its alloy based materials have been employed as the most efficient catalysts to facilitate ORR by catalyzing O2 into H2O via fourelectron pathway with a lower over-potential. 4,5 Nevertheless, wide commercialization of Ptbased catalysts is greatly impeded by its high cost, lake of natural abundance, poor stability in fuel cell environment and low tolerance to CO poisoning.

6,7

Since last decade, exhaustive

research efforts have been made to exploit the alternative low cost ORR electrocatalysts by employing either ultra-low Pt loading catalysts, 8-11 transition metals based, 12-15 or metal free electrocatalysts based on heteroatom engineering principles.

16-20

Among several approaches,

heteroatom doping on long range high surface area porous carbon is attractive and obviate major limitations and mechanistic hurdles faced by metal-based catalysts. 21-25 Heteroatoms doped carbon catalysts have found the best, owing to low cost, least vulnerability to methanol/CO and efficient activity towards ORR. 26,27 The doping of heteroatoms (N, F, P, B etc.,) to the carbon materials, effectively disrupt the electro neutrality of the carbon matrix, and catalyze the ORR kinetics quite similar to precious metal catalysts.

28-32

Variation in

electronic charge densities and spin densities interrupts the electro-neutrality in carbon matrix and thus creates many active catalytic sites.

33-35

The maximum charge re-distribution and

high spin densities occur by co-doping of multiple heteroatoms with higher electronegativity deference leads to create defects induced many active sites that accelerates the ORR activity synergistically. 36-38


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Many studies have been recently reported on heteroatoms co-doping to various carbon nanomaterials such as graphene, porous carbon, multiwalled carbon nanotubes, graphitic carbon nanofibers, carbon nano-ribbons etc.

39-46

Porous carbon looks promising and is

widely synthesized using hard templates such as SBA-15, MCM-41 and colloidal silica which provides highly ordered porous carbon structurally replica to the hard template. However, the tedious synthesis process limits the wide range applications of these methods 47. Alternatively, carbon derived from bio-regular sources/polymers through various carbonizing methods delivers irregular porous structured carbon with relatively wider pore sizes.

48-50

Synthesis of porous carbon from high abundance renewable sources viz. coffee waste, fruit peels, sewage wastes etc. can also be an alternative choice for transforming waste to valuable materials.

51-54

Reports are also available on synthesis of porous carbon from agricultural

sources viz. products from various plants, flowers/seeds, cotton fibres etc.55-58 Lin et al reported the ORR properties of N doped cotton fibres with appreciable ORR activity in 0.1 M KOH electrolyte. Carbon derived from silk cotton is merely used to study its behaviour in Supercapacitors, lithium ion battery and other application. 59-62 Investigation is not been done on the heteroatoms doping to the carbon derived from silk cotton and its electrochemical understanding towards fuel cell applications.

63-66

Post doping of heteroatoms brings the

defects, electronic charge redistribution and spin densities in the carbon matrix which alters the morphological and electrochemical behaviour of a catalyst. Hence choosing the type of heteroatoms and their electronegativity differences play a vital role in designing the catalysts for ORR. 67-69 Herein we explore N-F co-doped porous carbon derived from silk cotton as an efficient ORR catalyst. F doping in presence of N, drives towards more charge delocalization, spin-density variation through its attack from basal plane of the carbon matrix. N-F co-doped cotton carbon (N-F/CTC) synthesized in present study exhibits high surface area with



predominantly mesoporous in nature and the synergetic effect exerted by co-doping of heteroatoms to the carbon matrix results in enhancing the ORR activity. N-F/CTC studied here as low-cost metal-free catalyst shows excellent activity, superior electrochemical stability and CO susceptibility in alkaline conditions with better price/performance ratio applicable to PEMFC technology. 2. EXPERIMENTAL SECTION 2. 1 Materials Silk cotton collected from the tree (Bombax Ceiba), melamine (C6H3N6), ammonium fluoride (NH4F), potassium hydroxide (KOH), hydrochloric acid (HCl), nitric acid (HNO3), sulphuric acid (H2SO4) were obtained from Acros Organics. Pt/C (20 wt. % Pt on carbon) was obtained from Alfa Aesar, (Johnson Matthey Ltd). All chemicals are used as received without further purifying. De-ionized water (18.4 MΩ cm) used for the experiments was produced by a Millipore system. 2.2 Synthesis of porous carbon from silk cotton Silk cotton was washed with copious amount of DI water to remove the water-soluble impurities and sediment out the larger inorganic particles. The samples were then dried at 80 °C for 12 hrs. The dried cotton was pyrolyzed at 1000 °C for 1 h at heating ramps of 5 °C per minute under the nitrogen atmosphere. Black carbon material was collected after cooling to room temperature at the yield around 22% and further purified with 2 M aqueous HCl to remove the impurities present in the material. It was then treated with 2 M aqueous HNO3 for functionalization followed by washing with the copious amount of DI water till pH reached to neutral. The resultant cotton carbon (CTC) powder was dried at 80 °C for 12 h. under vacuum for further processing.


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2.3 Nitrogen and fluorine co-doping on porous carbon C6H3N6 and NH4F are selected as nitrogen and fluorine sources respectively for the synthesis of heteroatom-doped carbon catalyst. Individual doped (N-CTC or F-CTC) catalysts were synthesized with their respective mass ratios between CTC and precursors aiming for an optimum level of dopants into the carbon matrix. In a typical synthesis of NCTC, CTC and C6H3N6 of different wt. ratios (1:10, 1:15 and 1:20) were dispersed in 25 mL of DI water and ultrasonicated for 2 h followed by mechanical stirring overnight. The obtained mixture was dried in an air oven followed by its pyrolysis at 1000 °C temperature at the heating rate 5 °C per minute for 1 h under N2 atmosphere. For the synthesis of F-CTC, CTC and NH4F of different wt. ratios (1:5, 1:10 and 1:15) were carried out for optimization of F content in the catalyst. For the synthesis of N and F co-doped carbon (N-F/CTC), the addition of both C6H3N6 and NH4F was made with their respective optimized mass ratios to the CTC. Optimization of N and F content in the co-doped catalyst is also carried out by taking different mass ratios between carbon to the respective precursors for N (melamine), F (NH4F) viz, 1:10:15, 1:15:10, and 1:20:5 under similar operating conditions. 2.4 Physical and electrochemical characterizations Powder X-ray diffraction (XRD) technique was used to investigate the effect on carbon structure by heteroatoms doping and was obtained by BRUKER D8 Advance diffractometer using Cu-Kα as X-ray source (1.54 Å). To study the defects in the heteroatom doped catalysts (N-CTC, F-CTC and N-F/CTC), Raman spectroscopy (RFS27, Bruker) employing a Nd:YAG laser wavelength of 1064 nm was used. Textural and surface properties of heteroatom doped catalysts were characterized by N2-adsorption-desorption isotherms, measured at 77 K using Quantachrome Autosorb® iQ-MP / iQ-XR. Surface area and pore size were determined using the Brunauer–Emmett–Teller (BET) equation and pore size distribution (PSD) curves were obtained by the Barrett–Joyner–Halenda (BJH) method. Field



emission scanning electron microscopic (FE-SEM) images were recorded using a TESCAN MIRA3 LM instrument to visualize the morphology of the catalysts. An energy dispersive Xray microanalyzer (OXFORD ISI 300 EDS) attached to the electron microscope was used for elemental mapping of the catalysts. Morphological studies of CTC before and after hetero atom doping were examined by transmission electron microscope (TEM) (Tecnai 20 G2). Xray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB 250 XPS system) using monochromatic Al Kα source at 15 keV and 150 W systems was used to identify elements and their quantification. The electrochemical tests were carried out using potentiostat/galvanostat, Biologic instruments (VSP/VMP 3B-20) at 25 °C in a standard three electrode system. A glassy carbon disk (diameter: 5 mm) was used as a substrate to electrochemically characterize the heteroatom doped catalysts. Prior to drop casting of catalyst ink, GC electrode was polished using 0.3 μm alumina powder and then cleaned ultrasonically in water and ethanol to obtain a smooth electrode surface. 5 mg of each N-CTC, F-CTC and N-F/CTC Catalysts was added to 1 mL of ethanol and water mixtures (1:3 ratio) followed by addition of 10 µL of Nafion ionomer as the binder. The admixture was dispersed by ultra-sonication for 30 min to obtain the catalyst ink. 20 µL aliquot of ink was dropped on to the GC electrode surface and air dried at room temperature (30 °C). The resultant catalyst loading of ~ 500 µg cm-2 was maintained on GC electrode surface for all the samples. Graphite rod and saturated calomel electrode (SCE) were used as counter and reference electrodes respectively. All potentials in this work are referred with respect to the reversible hydrogen electrode (RHE) for convenience. Electrochemical characterization of heteroatom doped catalysts was carried out by cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) measurements in 0.1 M aqueous KOH electrolyte alkaline medium. CVs were recorded in N2 and O2 saturated electrolyte between -1.0 and 0.2 V (vs. SCE) potential windows in alkaline at a scan rate of


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50 mV s-1 respectively. LSVs were performed to measure the onset/half-wave potential (E1/2) of the catalysts in O2-saturated 0.1 M KOH solution at the rotation speed of 1600 rpm. LSVs were also performed with different rotational speeds (viz. 800, 1200, 1600, 2000 and 2400 rpm) in O2-saturated KOH at a scan rate of 5 mV s-1 to find the number of electron transfer from K-L plots. Electrochemical stability of the catalysts was also evaluated in the alkaline environment by repeating the CVs up to 10,000 potential cycles in O2-saturated electrolytes. LSVs were then recorded after 10,000 repeated potential cycles in order to understand the electrochemical performance of the catalyst after durability. CH3OH tolerance and CO sensitivity of the catalyst were evaluated by LSVs in O2-saturated 0.1 M KOH with 1 M CH3OH and CO tolerance at a scan rate of 5 mV s-1. Electrolytes were bubbled with ~20 ppm CO contained N2 gas at 0.10 V vs. RHE in alkaline for 1h for CO susceptibility test. Prior to record LSVs, pure N2 gas was purged for 1h to remove the CO species in the electrolyte followed by bubbled with O2 till saturation. The results were compared with the commercial 20 wt. % Pt on carbon. Rotating ring-disk electrode (RRDE) experiments with GC disk of 5 mm diameter and a Pt ring (Pine Instruments) were performed to corroborate the percentage peroxide yield and number of electron transfer during the ORR process. 20 μL of as-prepared catalyst slurry was dropped on to GC disk to prepare the RRDE working electrode and allowed to air dry at room temperature. Standard Calomel Electrode (SCE) and Graphite rod were used as reference and counter electrodes respectively during the electrochemical Analysis. 2.5 Membrane electrode assembly (MEA) fabrication and fuel cell performance evaluation Commercial gas diffusion layers (GDL, SGL DC-35) were used as backing layers for both anode and cathode. N-F/CTC catalyst was dispersed separately in ethanol followed by the addition of 10 wt. % Fumion FAA-3 ionomer with continuous sonication for 30 min. The



resultant slurry was brush coated on cathode GDL to achieve ~2 mg cm-2 catalyst loading. Commercial Pt/C (20 wt. %) was coated to the anode with Pt loading of 0.5 mg cm-2. Both the electrodes were soaked in 1 M aqueous KOH solution for 12 h for the exchange of Brions from ionomer to OH-. Commercial Fumatech membranes of thickness 50 μm were used as anion exchange membrane for making MEAs. The membrane is treated with 2 M aqueous KOH at room temperature for 48 h in order to replace Cl- ions with OH- ions in the membrane. MEAs were obtained by sandwiching Fumatech membrane between the electrodes and hot pressed at 85 °C at the pressure of 25 kg cm-2. MEAs were assembled in fuel cell text fixtures (Fuel cell Tech. Inc., USA) of 5 cm2 active area with a parallel serpentine flow field on graphite plates. Gaseous H2 and O2 were fed into anode and cathode side of the cell respectively at a flow rate of ~180 mL min-1 through bubble humidifiers to maintain maximum relative humidity in the cell. Galvanostatic polarization studies were made using LCN100-36 electronic loads from Bitrode Corporation, USA. Experiments were carried out at 40 °C for anion exchange membrane fuel cell (AEMFC) under ambient pressure. 3. RESULTS AND DISCUSSION 3.1 Morphology and structure Carbon material obtained after annealing of raw silk cotton was purified by a simple acid purification process by treating in 0.2 M HCl followed by 0.5 M HNO3 aqueous solution to remove the impurities. Figure 1a shows the XRD patterns of carbon from silk cotton which shows two peaks corresponds to C (002) and C (100) at the 2 theta of 24 and 43 respectively. Defects analysis and influence of heteroatoms doping to the carbon matrices is deeply studied by Raman spectroscopy shown in Figure 1b. Raman active D-bands associated with defects and disorders in the carbon matrices are positioned at 1336 cm-1 for CTC and doped catalysts (N-CTC, F-CTC, and N-F/CTC) as well. The peaks positioned at


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1596 cm-1 are the Raman active G-bands for all the catalysts which are due to E2g vibration mode occurs for sp2 bonds in the carbon networks. No shift in the peak positions is observed for both D and G bands for all the catalysts. However, noteworthy changes in the ID/IG ratio are observed for the catalysts. The higher ID/IG ratio for N-CTC (1.02), F-CTC (1.03) than pristine CTC (1.00) is found. This is due to replacement of carbon atoms in the carbon network by N and F which induce defects due to E2g vibrations of higher electronegative atoms in polarising C-C bonds in the graphitic network. The Highest ID/IG ratio found for NF/CTC (1.07) is accredited maximum defects occurred due to widespread C-C bond polarisation in presence of both N and F heteroatoms. The relationship between ID/IG and the average size of sp2 cluster [La] or the optical band gap [Eg] of carbon materials is proposed by Ferrari and Robertson 70

Id C" 2  C '  La  2 Ig Eg

(1)

Where C ' and C" are constants;  is the Raman relaxation length of the D band scattering. La is the cluster diameter. According to the above equation, the band gap of carbon materials is determined by π states of sp2 sites.

70

The ID/IG ratio increases as the degree of crystallinity

decreases or defects increases. If the ID/IG ratio increases, the band gap [Eg] reduces and the π band widens. The highest ID/IG ratio observed for N-F/CTC catalyst indicates the reduced band gap and widened π states of active C-C bond of this catalyst which help in improving the ORR activity by the formation of lowest energized active complex. Figure 1c shows N2 physisorption isotherms illustrating the textural properties of catalysts. The well-defined adsorption-desorption isotherm with H4 type hysteresis loop associated with capillary condensation of inert species at higher relative pressure in the mesopores is exhibited by all the catalysts. The pore geometry that includes porous inner cavities and aggregated pores of



any porous materials, usually quantified by measuring BET surface area, micropores volume and BJH pore-size distributions analysis. Table 1 contains BET surface areas for CTC, NCTC, F-CTC, and N-F/CTC and is found to be ~750, ~830, ~800 and ~950 m2 g−1 respectively. The corresponding pore volumes are found to be 0.21, 0.12, 0.18 and 0.07 cm3 g−1 respectively; pore diameters are 1.8, 1.6, 1.5 and 1.4 nm respectively (Figure 1d). Well shaped hysteresis loop indicates the porous structure of N-CTC lies primarily in the mesoporous range (Figure 1c). F doping to CTC slightly lowers the pore volume and volume of adsorption with thin distinct hysteresis loop. It is found that the rapid desorption occurs at high pressure primarily from the aggregated secondary cavities. In the case of N-F co-doped catalyst, highest surface area is observed through a little tapering the pore diameter/pore volume (Figure 1d). The gradual N2 uptake in the relative pressure range of 0.5 < P/P0 < 0.9 is observed and is allied with highest N2 adsorbed volume in the mesopores along with the fatty hysteresis loop leads to larger surface area. Combination of mesopores, micropores exist in the N-F co-doped CTC facilitate fast diffusion of O2 species at the active catalytic sites and enhance the ORR activity.


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Page 11 of 37

800

Table 1. BET surface area, pore volume and pore diameter, elemental composition measure from XPS analysis and parameters viz, kinetic current density and mass activity of the catalysts

Average pore diameter (nm)

C

O

N

F

750.10

0.21

1.8

96.11

3.6

---

---

N-CTC

830.32

0.12

1.6

95.61

2.03

2.33

---

-89.3

121.0

F-CTC

800.26

0.18

1.5

95.72

1.82

---

2.44

-64.74

98.02

N-F/CTC

950.43

0.07

1.4

94.11

1.66

2.54

1.65

-202.7

200.1

(0 0 2)

(1 0 0)

-1

CTC

(a)

Absorbed volume (cm g ) STP

CTC

3

(0 0 2)

N-CTC

(1 0 0)

(0 0 2)

(1 0 0)

F-CTC

(0 0 2)

N-F/CTC

(1 0 0)

. 40

50

60

2 (Degrees)

70

(b)

G band

CTC

80

ID/IG = 1.00

D band

N-CTC

ID/IG = 1.02

F-CTC

ID/IG = 1.04

N-F/CTC

-1

30

-1

20

3

10

1200

1400

1600

-1

1800

Raman shift (cm )

2000

250

2200

(c)

CTC

200 150 420

N-CTC

360 300 300 225 150 720 630 540 450 0.0

F-CTC

N-F/CTC 0.2

0.3 0.2 0.1 0.0

0.6

-1

0.8

1.0

(d)

CTC

0.08 0.04 0.00 0.16

N-CTC

F-CTC

0.08 0.00 0.08 0.00 0

0.4

Relative pressure (PPo )

N-F/CTC

0.04

ID/IG = 1.07 1000

Electrochemical parameters @ 0.75 V vs. RHE Mass ik (mA cm-2) activity (mA g-1) ---------

Average pore volume (cm3 g-1)

SBET (m2 g-1)

Catalyst

Elemental composition (At. % )

Pore Volume (cm g Å )

Intensity (a.u.)

Intensity (a.u.)

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


2


4 6 8 Pore Diameter (nm)

10

12


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Figure 1. (a) XRD patterns, (b) Raman spectra for CTC, N-CTC, F-CTC and N-F/CTC catalysts; (c) N2 adsorption-desorption hysteresis loops, (d) corresponding pore size distribution of all the catalysts

Morphological investigations by TEM for CTC, N-CTC, F-CTC and N-F/CTC catalysts are shown in Figure 2. Wider irregular rigid porous structures are observed in CTC, while well-defined porous structures are observed for N-CTC (Figure 2b) and F-CTC (Figure 2c) catalysts. Remarkably, the well-spread finest porous structure is observed for NF/CTC catalyst (Figure 2d). These observations are also well aligned with the results obtained from N2 adsorption isotherms. TEM image mappings with the uniform heteroatoms distribution in the carbon matrices of each catalyst are also shown in Figure S1.

(a)

(c)

CTC

F-CTC

(b)

N-CTC

(d)

N-F/CTC

Figure 2. TEM images of (a) CTC, (b) N-CTC, (c) F-CTC and (d) N-F/CTC catalysts with corresponding SAD patterns shown in inset to the figures. The scale bars in the SAD patterns are 20 nm.


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Chemical states of N, F doped catalysts are deeply investigated by XPS analysis with their elemental composition shown in Table 1. The elemental survey spectra for all the catalysts and presence of heteroatoms are determined by their respective binding energies shown in Figure S2. The deconvoluted XPS spectra for N1s, F1s for all catalysts are shown in Figure 3. The N1s spectra are deconvoluted into three forms namely, pyridinic-N, pyrrolic-N, and graphitic-N with respective binding energies of 398.8, 399.7, and 401.7 for N-CTC catalyst (Figure 3a) and 398.4, 399.2, 401.1 eV are observed in N-F-CTC catalyst (Figure 3b) respectively. Comparatively lower binding energy values are observed for three N forms in N-F-CTC catalyst. This may be due to the wide-range charge delocalization or accelerated asymmetric spin densities in carbon matrices emboldens by the presence of N, F in the carbon matrix.56 The deconvoluted F1s spectra for F-CTC, N-F/CTC catalysts (Figure 3c,d) with three components namely ionic-F, semi-ionic-F and covalent-F at respective binding energies of 685.8 eV, 688.5, and 690.6 eV for F-CTC (Figure 3c) and 685.4, 687.0 eV for N-F/CTC catalyst (Figure 3d). Semi-ionic C–F bond is more active towards ORR than the ionic bond as the former is capable of polarizing the C-C bond to a greater extent than the later while covalent-F is ORR inactive. The most active semi-ionic C-F bond occurs at lower binding energy with higher intensity in case of N-F/CTC catalyst compared to F-CTC denotes that maximized C-C bond polarization by the asset of well exerted synergistic effect. It is noteworthy that ORR inactive covalent-F is absent in N-F/CTC catalyst. 71-74 The existence of both active semi-ionic-F and ionic-F further enhance the ORR activity of N-F/CTC catalyst.



Intensity (a.u.)

N

Pyridinic 398.7 eV H

N

Pyrolic

N

399.7 eV

Graphitic 401.7 eV

F1s in F-CTC

Intensity (a.u.)

Raw data (a) Fitted data Background Pyridinic-N Pyrolic-N Graphitic-N

N1s in N-CTC

Semi-ionic-F 688.5 eV

(b)

N1s in N-F/CTC

N

H

Pyrolic 399.2 eV

N

Pyridinic 398.4 eV

N

Graphitic 401.1 eV

Intensity (a.u.)

Raw data Fitted data Background Pyridinic-N Pyrolic-N Graphitic-N

682

Raw data Fitted data Background Ionic-F Semi-ionic-F Covalent-F

(c)

Covalent-F 690.7 eV

Ionic-F 685.8 eV

396 397 398 399 400 401 402 403 404 680 Binding energy (eV)

Intensity (a.u.)

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

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684 686 688 690 Binding energy (eV)

Raw data Fitted data Background Ionic-F Semi-ionic-F

692

F1s in N-F/CTC

694 (d)

Semi-ionic-F 687.1 eV

Ionic-F 685.3 eV

396 397 398 399 400 401 402 403 404 682 683 684 685 686 687 688 689 690 691 Binding energy (eV) Binding energy (eV)

Figure 3. Deconvoluted XPS spectra for (a, b) N1s in N-CTC, N-F/CTC; (c, d) F1s in F-CTC, N-F/CTC catalysts respectively. 3.2 Electrochemical Analysis After ascertaining N and F co-doping into the carbon matrix, catalysts are investigated for their electrochemical behaviour towards ORR in alkaline medium. N and F content in the doped catalysts are optimized by varying the ratios between CTC and heteroatom-containing precursors. The electrochemical characterization viz., CVs, LSVs and K-L plots of N and F individual doped catalysts to analyse ORR activity and kinetics along with number of electron transfer during the ORR process are shown in Figure 4. The optimized mass ratios between CTC and precursors (C6N3H6 and NH4F) are 1:15 for N-CTC catalyst and 1:10 for


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F-CTC catalyst and are evaluated in identical operating environments for ORR activity. The composition optimization of N and F in the catalysts defined by virtue of their highest Ered and half-wave potentials (E1/2) obtained from CVs and LSVs respectively. Figure 4a,d shows the CVs of N-CTC, F-CTC catalysts respectively with the composition optimization recorded in N2 and O2 environments. The prominent ORR reduction peaks are detected for the catalysts in O2 atmosphere signifying their intrinsic ORR activity. The highest O2 reduction potential (Ered) is observed for ratio N-CTC (1:15) and F-CTC (1:10) catalysts at 0.80 V and 0.73 V respectively. In contrast, there is no Ered peak obtained when the O2 is replaced with N2 shown in the optimized N-CTC and F-CTC catalysts. These results are deep-rooted by LSVs recorded in O2 saturated 0.1 M KOH shown in Figure 4b,e for N-CTC and F-CTC catalysts respectively. The half-wave potential (E1/2) of 0.82 V and 0.80 V recorded for N-CTC (1:15) and F-CTC (1:10) catalysts respectively. Figure 4c,f show hydrodynamic voltammograms of N-CTC (1:15) and F-CTC (1:10) catalysts at different rotation speed. The current densities increase rapidly with the increase in rotation speed as the minimized diffusion distance of reactants species towards electrode interface with constant diffusion kinetics. The corresponding Koutecky-Levich (K-L) plots for N-CTC (1:15) and F-CTC (1:10) catalysts are shown in insets. The linearity, as well as parallelism of K-L plots, designated the consistent electron transfer (n) and reaction kinetics at various potentials with respect to the concentration of dissolved oxygen in the electrolyte. The K-L plots shown in insets which are the plots of the inverse of current density (id-1) as a function of the inverse of the square root of the rotation rate (ω-1/2), obtained from Equation 1 and 2, and are employed to calculate the number of electron transfer during the ORR process.50

id = Bω-1 / 2


(1)


B  0.2nFCO2 DO2

2/3

 1/ 6

Page 16 of 37

(2)

Where id is the diffusion controlled current density, n is the number of electrons exchanged per O2 molecule, F is the Faraday constant (96 485 C mol-1), CO2 is the bulk oxygen concentration in the electrolyte (1.22 × 10-6 mol cm-3), DO2 is the diffusion coefficient of molecular oxygen (1.93 × 10-5 cm2 s-1), ν is the kinematic viscosity of the electrolyte (0.01 cm2 s-1), ω is the rotations of the disk expressed in rpm (ω = 2πN, N is the linear rotation speed) and 0.2 is a constant used when ω is expressed in rpm. Linearity in the K-L plots for the catalyst suggests first ordered reaction kinetics towards ORR. The number of electron transferred (n) was found to be 3.7, 3.8, 3.96 for N-CTC, F-CTC and N-F/CTC catalysts respectively. This proposes that catalysts exhibit nearly four-electron oxygen reduction process during the cathodic reaction. The K-L plots for N-F/CTC catalysts shown in Figure S3c (inset).


Eredox= 0.72 V 1:15 N2 Eredox= 0.80 V

Eredox= 0.78 V

0

1.0

N-CTC

-1

CTC : Melamine 1:10 1:15 1:20

-2 -3 -4

E1/2= 0.82 V

-5

0.1M KOH electrolyte

-6

-2 -3 -4

-2 -1 -1

-1

Current density (mA cm )

0

0.0 -3.0

0.2

N-CTC (1:15)

0.60 V 0.50 V 0.40 V 0.30 V

-2.8 -2.6

E1/2= 0.80 V E1/2= 0.75 V

0.4 0.6 0.8 Potential V vs. RHE

-2.2

0.06 0.07 0.08 0.09 0.10 0.11





-1/2

(rpm )

800 rpm 1200 rpm 1600 rpm 2000 rpm 2400 rpm

-5 -6 -7

(c)


0.0

0.2 0.4 0.6 0.8 Potential V vs. RHE

1.0

Eredox= 0.66 V N2

0

Eredox= 0.73 V

-2 2

1:15

0 Eredox= 0.71 V

-2 0.0

0

0.2

0.4

0.6

0.8

1.0

Potential V vs. RHE

1.2

(e)

CTC : NH4F 1:5 1 : 10 1 : 15

-1 -2

E1/2=0.65 V

-3 -4

E1/2=0.80 V

-5

E1/2=0.75 V

0.1M KOH electrolyte -6

1.0

N-CTC

-2.4

2

1.2

(b)

(d)

1:10

0 -1 -2 -3

0.0 -2 -1


CTC : NH4F

-2

-2.0

0.2

-1.8 -1.7

0.4

0.6

Potential V vs. RHE

0.60 V 0.50 V 0.40 V 0.30 V

-1.9

-1

0.2

1:5

0


1:20

Current density (mA cm-2)

1:10 CTC : Melamine



ACS Applied Energy Materials 2 (a)


1 0 -1 -2 2 0 -2 -4 2 0 -2 -4 0.0


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 17 of 37

F-CTC

0.8

1.0 (f)

F-CTC (1:10)

-1.6 -1.5 -1.4

Average electron transfer number=3.3

0.06 0.07 0.08 0.09 0.10 0.11





-1/2

(rpm )

-4 -5 -6 -7


0.0

800 rpm 1200 rpm 1600 rpm 2000 rpm 2400 rpm

0.2 0.4 0.6 0.8 Potential (V vs. RHE)

Figure 4. (a), (d) CVs of N-CTC, F-CTC catalysts with different mass ratios between CTC : melamine and CTC : NH4F at 50 mV s-1; (b), (e) LSVs for N-CTC, F-CTC catalysts with different mass ratios between CTC : melamine and CTC : NH4F at 5 mV s-1 scan rate measured at 1600 rpm in O2-saturated 0.1 M aqueous KOH. (c), (f) LSVs at different rotations rates measured in O2-saturated 0.1 M aqueous KOH solution. The corresponding KL plots are shown in insets.


1.0


The synergistic effect of N and F dopants is realized by co-doping of optimized mass ratios of respective precursors. Figure 5a shows CVs for optimized N-CTC, F-CTC, and N-F/CTC catalysts in 0.1 M KOH electrolyte. It is noteworthy that, N-F/CTC catalyst showed the highest Ered potential of 0.82 V than the individually doped catalysts, demonstrating the existence of synergistic effect among the dopants. The enhanced ORR activity is also observed by the RRDE curves for the catalysts as shown in Figure 5b. The E1/2 potential of 0.86 V is recorded for N-F/CTC catalyst, higher than N-CTC and F-CTC catalysts. Such a high ORR activity of the N-F/CTC catalyst clearly proposes the maximum synergism exerted by N and F dopants favourable towards ORR. The additional doping of F to the N-doped carbon matrix further improves the catalytic activity and is mainly due to the reduced energy gap between HOMO and LUMO and the increase of electron transfer at the basal plane of carbon matrices. N-F/CTC catalyst shows outstanding activity owing to the well-established synergistic effect among N and F atoms with many active sites highly favourable for ORR. 35,55

The N and F content optimization in the co-doped catalyst is shown in Figure S3. The

highest Ered (0.82 V) in Figure S3a, E1/2 (0.86 V) in Figure S3b are found for ratio of 1:15:10 than other ratios. This could be attributed to the well-established synergistic effect between heteroatoms at this particular ratio in the carbon matrices owing to its electroneutrality principle. The temperature optimization is confirmed by varying from 900 ºC to 1100 ºC. Figure S4 shows 1000 ºC is found to be optimized by means of its highest reduction and half-wave potentials and the textural, morphological properties are well aligned with these results. It may be due to the presence of active forms such as graphitic-N and semiionic-F forms in large to enhance the ORR activity. The ORR pathway and number of electron transfer (n) of the catalysts further verified by rotating ring-disk electrode (RRDE) measurements. Formation of peroxide species (HO2-)


Page 18 of 37

Page 19 of 37 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


are monitored throughout the process. The percentage peroxide yield and also the number of electron transfers (n) are calculated by the following equations. 50 % 𝑯𝟐𝑶𝟐 = 𝟐𝟎𝟎 × 𝑰

𝒅

𝒏=𝟐×𝑰

𝒅

𝑰𝒓 𝑵 + 𝑰𝒓 𝑵

𝟐𝑰𝒅 + 𝑰𝒓 𝑵

(3)

(4)

Where Id is disk current, Ir is ring current and N is current collection efficiency of Pt ring. The electron transfer number and percentage peroxide yield is calculated throughout the potential window and is shown in Figure 5c. The measured number of electron transfer during the ORR process is found to be 3.7, 3.8 and 3.96 for N-CTC, F-CTC and N-F/CTC catalysts respectively which are in good agreement with the observation obtained from K-L plots (Figure 4c, f & S3c). The commercial Pt/C catalyst shows 3.99 electrons transfer during the ORR process. Percentage of peroxide yield for N-CTC, F-CTC, N-F/CTC, and Pt/C catalysts are found to be 2.0 %, 2.2 %, 1.5 %, and 0.03 % respectively, which is a good sign of N-F/CTC catalyst as an efficient metal-free ORR catalyst. Durability, methanol and CO sensitivity of the N-F/CTC catalyst is also evaluated and compared with the commercial Pt/C catalyst. N-F/CTC catalyst evaluated its durability by subjecting to 10,000 repeated potential cycling tests between potential 0 V and 1 V vs. RHE at 50 mV s-1 scan rate in 0.1 M O2 saturated KOH electrolyte shown in Figure 5d. The corresponding ORR activity after the durability test shown in Figure 5e shows no noticeable potential shift in entire region after even 10,000 potential cycles. In contrast, Pt/C catalyst shows the huge loss in the adsorption/desorption region after even 1000 potential cycles (inset to Figure 5d) representing the large reduction in electrochemical active surface area due to carbon corrosion, Pt agglomeration, dissolution, re-deposition and migration from the electrode surface. After 1000 potential cycling, Pt/C catalyst shows significant negative shift (~150 mV) in its E1/2 potential shown in inset to Figure 5e. Thus, N-F/CTC catalyst shows 19 Environment ACS Paragon Plus


superior ORR stability over the commercial Pt/C catalyst and is exclusively credited to the porous textural features along with well-established synergistic effect of heteroatoms in NF/CTC catalyst. Moreover, this catalyst did not show any response to CH3OH or CO species present in the electrolytic system seen in Figure 5f and its actual characteristic peaks of ORR activity are retained. By contrast, CH3OH electro-oxidation peak at the anodic currents is observed for commercial Pt/C catalyst with a negative shift of 50 mV, which shows the poor ORR activity in presence of poisonous species. Similarly, the huge decrease in ORR activity of Pt/C in CO environment is due to the formation of the CO monolayer on the electrode surface and poisoning the Pt/C catalyst. From the aforesaid, N-F/CTC catalyst is advantageous over the major limitations detected in the commercial Pt based catalysts.


Page 20 of 37

0

N2 atm. Eredox= 0. 82 V

0.1M KOH elelctrolyte

0.0

0.2

0.03

0.4

0.6

0.8

1.0

Potential V vs. RHE

1.2

(b)

F-CTC N-CTC N-F/CTC Pt-C

0.02 0.01 0.0 0.00 -1.4

F-CTC (E1/2= 0.80 V) N-CTC (E1/2= 0.82 V) N-F/CTC (E1/2= 0.86 V) Pt-C (E1/2= 0.89 V)

-2.8 -4.2 -5.6 -7.0

0.0

0.2

5


2

F-CTC N-CTC N-F/CTC Pt/C

2 1 0 0.3

0.4

0.5

0.6

0.7

Potential (V vs. RHE)

0.8

3.8 3.7 3.6 3.5

0

Initial cycle 1000th cycle

-2

-2 -3 -4

Potential V vs. RHE

N-F/CTC


-4

-1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Initial 10,000th cycle

0.0 0 -1 -2 -3 -4 -5 -6 -7

0.2

0.4

0.6

0.8

Potential V vs RHE Initial cycle 1000th cycle

1.0

Pt-C

1.2

(e)

0.1 M KOH

0.0 0.2 0.4 0.6 0.8 1.0 Potential V vs RHE

Initial 10,000th cycle

-5

N-F/CTC

-6 0.0

3.9

1

-4

0

0.2 0.4 0.6 0.8 1.0 Potential V vs RHE

1.2

0 -2

3

No. of electron transfer

4

0.1 M KOH

-3

0

(d)

Pt-C

2

-2

1.0

(c) 4.0

3

-1

4

-2

N-F/CTC

-2


Eredox= 0. 73 V

6


-2 2


F-CTC

8


Eredox= 0. 80 V

0

-4

(a)


N-CTC



2 0 -2 -4 2

Peroxide yield (%)

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 21 of 37

-1 -2 -3 -4 -5

1 0 -1 -2 -3 -4 -5 -6 -7

(f)

Pt-C

O2 CH3OH CO

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential V vs RHE

O2

N-F/CTC

CH3OH

-6

CO 0.0

0.2

0.4

0.6

0.8

Potential V vs RHE

1.0

Figure 5. (a), (b) comparison CVs and RRDE curves of N-CTC, F-CTC, N-F/CTC catalysts at 50 mV s-1, 5 mV s-1 scan rates respectively; (c) number of electron transfer, HO2- produced during ORR process in O2-saturated 0.1 M KOH; (d) CVs, (e) LSVs for N-F/CTC before and after 10,000 repeated potential cycles; electrochemical data for Pt/C catalyst before and after 1,000 repeated potential cycles are shown in the insets; (f) LSVs for N-F/CTC catalyst in



presence of O2, CO, CH3OH species recorded at 1600 rpm in aqueous 0.1 M KOH at scan rate of 5 mV s-1 respectively, (corresponding sensitivity tests for Pt-C shown in inset). The quantitative analysis of kinetic current density (ik), mass activity (MA) and specific activity (SA) of catalysts are shown in Figure 6. The ik and MA of N-F/CTC catalyst is higher than individual doped catalysts which are attributed to the well-established synergistic effect between heteroatoms exerted in co-doping than the individual doping. Only 1% loss in ik and MAs for N-F/CTC catalyst before and after durability test is noted as shown in Figure 6. However, the loss in ik and mass activity of Pt/C catalyst is about 35 % under similar operating conditions. The excellent ORR activity and stability of N-F/CTC catalyst is towing to the entrenched synergistic effect between the heteroatoms.

1000

@ 0.75 V vs. RHE

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 37

ik

-2

(mA cm ) -1

MA (mA g ) -2 SA A cm Pt)

800 600 400 200 0

N-CTC

F-CTC

N-F/CTC initial

N-F/CTC final

Pt/C initial

Pt/C final

Figure 6. Kinetic current density (ik), mass activity (MA) and specific activity (SA) of all catalysts. For N-F/CTC catalyst, the data is shown at initial and after 10,000 repeated potential cycles. For Pt/C catalyst, the durability test was after 1000 potential cycles.


Page 23 of 37

The optimized catalysts are used as cathode catalysts to fabricate the MEAs and assembled in a single fuel cell fixture for the performance evaluation in alkaline conditions. The steady-state fuel cell polarization and performance curves of PEMFC are shown in Figure 7. The PEMFC comprising N-F/CTC catalyst in alkaline conditions delivers a peak power density of ~13.3 mW cm-2 at a load current density of ~79 mA cm-2 under ambient pressure and ~30 ºC temperature while, Pt-c catalyst delivers power density of 70 mW cm-2 and current density of ~200 mA cm-2. Fuel cell performance of N-CTC, F-CTC catalysts is also compared with N-F/CTC catalyst performance. Hence, N-F/CTC catalyst stands with high electrochemical stability as a metal-free ORR electrocatalyst and opens a new era in developing a cost-effective electrocatalyst for fuel cell technologies with superior stability over a long range of operations.

15 -2

0.7

12

0.5 0.4

1.0

80

0.8

60

0.6

40

0.3 0.2

-2

0.4 0.2 0

50

100

150

0

20

40

-2

0 200


60

80

-2

9

20

Pt-C

F-CTC N-CTC N-F/CTC

0.1

Power denity (mW cm )

Cell voltage (V)

0.6

0.0

Power density (mW cm )

0.8

Cell voltage (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


6 3

0 100

Current density (mA cm ) Figure 7. Fuel cell polarization and power density data of anion exchange PEMFC with 5 cm2 active area of N-F/CTC as cathode catalyst. The PEMFC performance was conducted at 35 °C under ambient pressure. Pt-C performance of AEMFC is shown in inset.



4. CONCLUSIONS Porous carbon has been synthesized from silk cotton by a robust and simple method followed by heteroatom(s) doping as efficient metal-free ORR catalyst. N-F in the carbon matrices induced more open edge defect sites that enhance the degree of disorder in the carbon materials. The onset O2 reduction potential and current density are comparable to commercial 20% Pt/C catalyst. The electrochemical stability of N-F/CTC catalyst was ascertained up to 10,000 repeated potential cycles in alkaline media and found no degradation in its ORR activity. Co-existence of various active forms of N and F brought the maximum C-C bond polarization, charge re-distribution and high spin densities in the carbon matrices and synergistically enhances the ORR activity. Thus, a process of successfully transforming an abundant agriculture product to a commercially worthwhile alternative electrocatalyst and its application towards fuel cell is demonstrated. Future work will be explored by deposition of Pt and/or Non-Pt metals metal(s) to further improve the ORR activity and fuel cell performance.


Page 24 of 37

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ASSOCIATED CONTENT

Supporting Information For TEM elemental mappings, XPS elemental survey spectra, CVs and LSVs of N-F/CTC catalyst at different weight ratios between carbon and respective precursors for the optimization of N and F contents in the co-doped catalyst. The CVs and LSVs temperature optimization of catalyst is shown in supporting information. ACKNOWLEDGMENTS Financial support from CSIR, New Delhi, under FTT project (MLP 0202) is gratefully acknowledged. S A thanks to CSIR for awarding senior research fellowship (31/20 (0171)/2018-EMR-I) to pursue research at CSIR-CECRI. We acknowledge to Prof. Ashutosh Sharma,

IIT

Kanpur

for

his

valuable

suggestions


to

the

manuscript.


Page 26 of 37

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Table of contents entry

The heteroatoms N, F dual doped high surface area porous carbon derived from the silk cotton which is applicable towards oxygen reduction reaction in polymer electrolyte fuel cells in alkaline condition.


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Graphical abstract

ACS Paragon Plus Environment

Nitrogen-Fluorine Dual Doped Porous Carbon Derived from Silk

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