Nanoporous Heteroatom-Doped Carbons Derived from Cotton Waste

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Nanoporous Heteroatom-doped Carbons Derived from Cotton Waste: Efficient Hydrazine Oxidation Electrocatalysts Tais L. Silva, André Luiz Cazetta, Tao Zhang, Katherine Koh, Rafael Silva, Tewodros Asefa, and Vitor Almeida ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00145 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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ACS Applied Energy Materials

Nanoporous Heteroatom-doped Carbons Derived from Cotton Waste: Efficient Hydrazine Oxidation Electrocatalysts Taís L. Silvaa,b,c, André L. Cazettab, Tao Zhangd, Katherine Kohc, Rafael Silvae, Tewodros Asefac,d,*, and Vitor C. Almeidab,e,* a Federal

University of Technology – Paraná, 635 Marcílio Dias Street, Apucarana 86812-460, Paraná, Brazil Laboratory of Environmental and Agrochemistry, Department of Chemistry, State University of Maringá, 5790 Colombo Avenue, Maringá 87020-900, Paraná, Brazil c Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, USA d Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, USA e Department of Chemistry, State University of Maringá, 5790 Colombo Avenue, Maringá 87020-900, Paraná, Brazil b

KEYWORDS: Cotton denim; P-N-doped carbon; metal-free electrocatalyst; electrocatalysis; hydrazine oxidation ABSTRACT: In this work, cotton-based denim waste is successfully used as a precursor to synthesize nanoporous P- and N-codoped carbon materials that can serve as efficient electrocatalysts for the hydrazine oxidation reaction (HzOR). In the synthesis, the cotton denim waste is mixed with H3PO4 in two different denim:H3PO4 (wt.:vol.) ratios, namely 1:1 and 1:3, wherein H3PO4 serves as both an activating agent and a source of P dopant atoms while the indigo carmine dye present in denim serves as a source of N dopant atoms to the carbon materials. The resulting P- and N-co-doped carbon materials, named PNC1 and PNC3, respectively, are characterized by various analytical techniques. The XPS spectra show that PNC1 has ca. 2.17 atomic % P and 1.95 atomic % N whereas PNC3 has 2.54 atomic % P and 0.71 atomic % N. Pore analyses by N2 porosimetry indicate that PNC1 has a higher surface area (1582 m² g-¹) than PNC3 (486 m² g-¹), although the former has a lower mesopore volume (0.39 cm³ g-¹) than the latter (0.58 cm³ g-¹). The SEM images of the two materials also show some notable structural differences. The results overall indicate that the structures and compositions of the materials can be easily tailored by varying the ratio of denim:H3PO4 in the precursor. The electrocatalytic activities of PNC1 and PNC3 toward HzOR were then evaluated, and PNC3 is found to be a better electrocatalyst than PNC1. In a 100 mmol L-1 hydrazine solution in phosphate buffer saline (PBS) at pH 7.4, PNC3 electrocatalyzes the reaction at a lower peak potential (ca. 0.70 V vs. RHE) than PNC1 (ca. 0.77 V vs. RHE). Additionally, the current density obtained during HzOR over PNC3 is higher (by 1.29-times) than the one obtained over PNC1. Furthermore, the onset potential by which PNC3 electrocatalyzes HzOR (0.42 V vs. RHE) is comparable to or better than the values reported for some of the best HzOR electrocatalysts in the literature. Beside their high electrocatalytic activity, the materials remain stable during electrocatalysis of HzOR.

1. INTRODUCTION The rapid rise in global energy demand has been contributing to the overconsumption of traditional energy resources, such as petroleum, natural gas and mineral charcoal. This has, in turn, been leading to a rapid rise in the level of CO2 in the atmosphere and environmental pollutions. The negative impacts associated with the increased level of this greenhouse gas has necessitated a great effort toward the development of new alternative energy systems that can produce energy from clean, renewable and sustainable feedstocks, rather than from fossil fuels.1,2 Fuel cells, which can convert the chemical energy (the energy stored in chemical bonds) in some compounds into electrical energy, have been widely considered one of the most attractive energy systems to mitigate the overconsumption of fossil fuels. This is because fuel cells have the ability to convert the energy in

chemical compounds into electrical energy at low operational temperature, with high power density, with high efficiency (especially compared with internal combustion engines), and without causing much (direct) environmental pollution.3 Among many types of fuel cells, the direct hydrazine fuel cells (DHFCs) are gaining attention. These fuel cells operate based on the oxidation of hydrazine, a fuel that has the following four notable advantages compared with many hydrocarbon-based fuels: 1) it does not produce the greenhouse gas CO2 when it is oxidized; 2) it does not also form CO, a compound that can poison many types of catalysts; 3) it has a high theoretical electromotive force (1.56 V);4 and 4) it is exclusively composed of abundant elements (i.e., N and H), and its products during the reaction in typical DHFCs are merely N2 and H2O, which are environmentally-friendly.5,6 Although some of these properties are not unique to the hydrazine oxidation reaction (HzOR) and can also be associated with reactions used

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in other fuel cells, such as the hydrogen oxidation reaction (HOR) used in proton exchange membrane (PEM) fuel cells, the others are. However, efforts to commercialize DHFCs in large-scale have so far been plagued by the fact that these fuel cells require good electrocatalysts to run the HzOR in them. Moreover, most of the known electrocatalysts for HzOR are either precious metals7 or toxic and leachable metals whose deployment in large-scale practical applications is unfeasible.8 Furthermore, metals such as Pt, which can be used as electrocatalysts for HzOR, may also be involved in other undesirable reactions, such as the oxidation of H2, which is produced by HzOR, rather than continuing to promote the oxidation of hydrazine.9 Attempts have been made to address some of these issues by mixing or alloying noble metals with other sustainable, earthabundant elements. Of particular interest is making metal alloys and binary metal oxides whose external surfaces are occupied by the noble metals and their cores are filled with the earthabundant element(s).7 This allows the noble metals not to be underutilized and the resulting materials to show high catalytic activity per unit mass of catalyst toward the reactions. However, here too, many of the catalysts are not immune from dissolution/leaching, poisoning, sintering, agglomeration, and oxidation during the reactions.7,10 Consequently, they tend to lose their activity over time when being used in fuel cells. Thus, inexpensive, sustainable and more stable catalysts for various reactions in fuel cells, including HzOR, are still needed. Heteroatom-doped carbon nanomaterials are emerging as great alternatives to replace many metal-based electrocatalysts, both because they have electrocatalytic activities for various reactions commonly used in fuel cells and because they are inexpensive, easy to synthesize, more eco-friendly and corrosion resistant.11 These materials constitute carbon lattices along with some hetero-elements, such as N, S, P, B, I, Li, and Se, in place of some carbon atoms. The presence of heteroatoms in carbon lattices perturbs the electronic structure of the materials and generates partially positively and partially negatively charged groups on the surfaces of the materials that can interact with reactants better.7 However, the heteroatoms do not substantially compromise the extensive π-conjugated electrons in the carbon lattice or the conductivity of the materials. As a result of these, such materials can effectively electrocatalyze various reactions.8,12 Among many heteroelements, N and P have been widely used as dopants for carbon materials because they can easily be introduced into carbon structures and create catalytically active sites in the materials. As both of them have one more valence electron than C, they provide more electrons into the system when they are introduced as dopants into a carbon material; however, as they have different electronegativity (i.e., the electronegativity of P (2.19) is lower than that of C (2.55) whereas the electronegativity of N (3.04) is higher than that of C, their effects as dopants on carbon lattice can be different. In other words, while both N and P form C-heteroatom bonds, the polarity of the bonds are quite different: C-N bonds are more polarized to N whereas C-P bonds are more polarized to C. Nonetheless, in both cases, the electro-neutrality of the bonds in the carbon materials is disrupted, enabling the atoms in the materials to interact better with reactants compared with those in their undoped counterparts. Additionally, the introduction of N or P atoms into carbon materials alters the bond lengths within the carbon materials. All of these ultimately create electrocatalytically active sites in the materials.3,7,8

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As the quest for sustainable materials is growing, making electrocatalytically active, heteroatom doped/co-doped carbon materials from sustainable sources, especially agro-industrial plant residues, is quite appealing.2,13–17 We thought that denim cotton residue, a waste that is generated in large quantity during the production of billions of jeans worldwide,18 can be a good precursor for making metal-free, heteroatom-doped carbon electrocatalysts for fuel cells. In the process, the indigo carmine dye in denim can be taken advantage of to serve as a source of N dopant atoms while the cellulose in denim forms a carbon material through pyrolysis. To this end, herein we report the synthesis of nanoporous Pand N-co-doped carbon materials that can serve as metal-free electrocatalysts for HzOR using cotton-based denim waste that contains indigo carmine dye and is treated with phosphoric acid (H3PO4) as a precursor. In the synthesis of the materials, no external N-containing reagent is needed or added to serve as N doping agent. H3PO4 is used to serve both as an activating agent and as a P-doping agent for the carbon materials. Besides, it is used to stabilize the structure of cellulose and inhibit the formation of levoglucosan during pyrolysis so that the decomposition of cellulose into volatile byproducts is reduced and the yield of the carbon product is increased.19 By varying the ratio of denim:H3PO4 (wt.:vol.), the structures, compositions and electrocatalytic properties of the carbon materials are tuned. The resulting materials are found to electrocatalyze HzOR, with good activity and current density, while remaining stable during the reaction.

2. EXPERIMENTAL PROCEDURES 2.1. Reagents and Chemicals. Cotton denim fabric waste composed exclusively of cellulose was supplied by a local store that produces denim pants and jackets. The denim waste was intentionally used as is, without any washing, in order to keep all the indigo carmine dye in it. H3PO4 (85%), KBr (FT-IR grade), 2-propanol, Nafion (5%, in lower aliphatic alcohols and water, in which 15-20% is water), hydrazine monohydrate (98%), and phosphate buffer saline (PBS, 10%, pH 7.4) were purchased from Sigma-Aldrich. All chemicals and reagents were used as received without further purification.

2.2. Synthesis of P- and N-co-doped Carbon Materials. To synthesize different P- and N-co-doped carbon materials, first 3.00 g of denim fabric were cut into small sizes (ca. 1 cm²) and then mixed with H3PO4 in denim:H3PO4 (wt.:vol.) ratios of 1:1 and 1:3 in a stainless-steel reactor that has an internal volume of 42 mL and is equipped with a removable lid and an inlet hole and an outlet hole for gases. After drying the mixtures in oven at 65 ºC for 24 h, the solid products were subjected to pyrolysis in three-stages in a furnace (EDG3P 7000) under a flow of N2 gas at a rate of 100 mL min1. In the first stage, the temperature of the furnace was increased from room temperature to 300 ºC at a rate of 5 ºC min-1 and kept at 300 ºC for 2 h. In the second stage, the temperature was increased from 300 ºC to 500 ºC at a rate of 5 ºC min-1 and kept at 500 ºC for 1 h. In the third stage, the samples were let to cool down to ca. 150 ºC under a flow of N2 gas (100 mL min-1), and then to room temperature in air. The resulting materials were ground into powder using a mortar and pestle and washed with 20 mL, 0.1 M of NaOH aqueous solution under stirring. The solid materials were recovered with vacuum filtration (using 0.45 µm Millipore® membranes) and then washed several times with distilled water until the supernatant’s pH reached ca. 6.5. The materials were then dried in oven at 110 ºC for 12 h. The material obtained from the precursor with denim:H3PO4 wt.:vol.


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ratio of 1:1 was labeled as PNC1, and the one obtained from a denim:H3PO4 wt.:vol. ratio of 1:3 was labeled as PNC3. For comparative studies, a control carbon material was synthesized using denim waste not pre-treated with H3PO4 as a precursor, with otherwise the same procedure. The material was named “WPC” to represent a carbon material without P dopants. 2.3. Characterization. The compositions of PNC1 and PNC3 materials were analyzed using Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). FT-IR spectra were obtained with a Thermo Scientific Nicolet iS 10 FT-IR spectrometer, using samples prepared on KBr pellets. The spectra were obtained in a range of 400 to 4000 cm-1, with an acquisition rate of 32 scans/min and a resolution of 4 cm-1. XPS spectra were acquired using a Kratos Axis Ultra XPS spectrometer operating with an emission current of 10 mA and a monochromatic source of Al Kα X-ray beam, which has an energy of 1486.7 eV. The XPS peaks associated with C 1s, O 1s, N 1s and P 2p were further deconvoluted to obtain more information about the different species present in the materials. The compositions of the materials were further analyzed with a LECO CHN628 elemental analyzer using 50 mg of samples and high-purity O2 (99.99%) and He (99.995%) gases. The temperature in the primary furnace was maintained at 950 ºC while the one for the afterburning was kept at 850 ºC. The data were then analyzed using a CHN628 software, version 1.30. The porosity of the materials were characterized by N2 porosimetry at 77 K using a QuantachromeTM Nova 1200e instrument. The Brunauer-Emmett-Teller (BET) equation was applied on the linear fit of the N2 adsorption-desorption data or isotherms in the relative pressure range (p/pº) of 0.05 to 0.20 to determine the surface areas (SBET) of the materials. The maximum amounts of N2 adsorbed at a relative pressure (p/pº) of 0.99 were used to obtain the total pore volume (Vt) of the materials. The Barrett-Joyner-Halenda (BJH) method was applied on the data to obtain the mesopore volume (Vm) of the materials. The average pore size (Aps) of the materials was calculated with the equation 4Vt/SBET.20,21 The pore size distributions of the materials were obtained using the non-local density functional theory (NLDFT)-based method.22 The Raman spectra of the materials were acquired using a SENTERRA II dispersive Raman microscope (Brüker Optik) operating with a radiation source at a wavelength of 663 nm. The spectra were acquired in the wavenumber range of 2000 to 800 cm-1 and with an acquisition rate of 10 scans/min. The structural and morphological features and energy-dispersive Xray (EDX) spectra of the materials and the cotton denim were investigated using a QuantaTM field emission gun scanning electron microscope (FEG-SEM, model FEG 250, FEI Co.). For the analyses, the carbon samples did not need metallization as they are conductive; however, the non-conducting denim cotton needed one, which was done using Au. Transmission electron microscopy (TEM) images of the materials were acquired using a Tecnai T-12 TEM instrument (FEI Co.). 2.4. Electrocatalysis. The electrocatalytic performances of PNC1 and PNC3 materials and the control material WPC for HzOR were evaluated using a VersaSTAT 3 potentiostat (Princeton Applied Research or PAR). For the experiments, a three-electrode cell, which comprised a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and a glassy carbon electrode (GCE) containing the synthesized carbon materials as a working electrode, was used. To prepare the working electrode for each catalyst, the carbon material was dispersed in 2-propanol (with a concentration of 10 g L-1), and the homogeneous suspension (2 µL) was casted

on a GCE and dried in air. It was then coated with Nafion (2 µL, 5% in 2-propanol) and dried in air for 5 min. To determine how the electrocatalytic activity of the materials changes at different concentrations of hydrazine, cyclic voltammograms (CVs) for HzOR over the materials in different concentrations of hydrazine, in the range of 0 to 100 mmol L-1 in 15 mL of PBS (0.01 mol L-1 at pH 7.4), were obtained. The CVs were collected at a scan rate of 10 mV s-1. To evaluate the effect of scan rate on the electrocatalytic activity of the materials, the reaction was run at different scan rates, ranging from 10 to 100 mV s-1 using 50 mmol L-1 of hydrazine solution in PBS. In both cases, the CVs were acquired in a potential range of 0.10 to 1.30 V vs. RHE. Chronoamperometric analyses were carried out at a potential of 0.60 V vs. RHE for 10, 50 and 100 mmol L-1 hydrazine solutions in PBS. During each measurement, the working electrode was taken out of the solution every 1 h to remove bubbles formed on its surfaces by the HzOR and then placed back into the cell to continue the run.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterizations of PNC Materials. Two nanostructured P- and N-co-doped carbon materials are obtained by pyrolyzing cotton-based denim waste that are pre-treated with and contain two different amounts of H3PO4. The denim waste is purposely obtained from a local industry in order to explore if such materials, which would otherwise make it into landfills, can be used as starting materials to derive useful carbon-based electrocatalysts for fuel cells from. After pyrolyzing the H3PO4-treated denim wastes, and then washing the products with dilute NaOH solution, two heteroatom-doped carbon electrocatalysts are obtained. More specifically, by using denim:H3PO4 (wt.:vol.) ratios of 1:1 and 1:3, respectively, as precursors, P- and N-co-doped carbon (PNC) materials, denoted PNC1 and PNC3, are synthesized. In the synthesis, the denim waste has provided the C atoms as well as the N dopant atoms whereas the H3PO4 has provided the P dopant atoms, besides serving as an activating agent, to the carbon materials. The types of functional groups present in the materials is characterized by FT-IR spectroscopy. The FT-IR spectra of PNC1 and PNC3 (Figure 1) show similar types of bands, indicating the presence of similar functional groups in both materials. The band centered at ca. 3450 cm-¹ is characteristic of the stretching vibration of O-H bonds of carboxylic and phenolic groups, and possibly some physisorbed water molecules, which are common in such materials. The bands observed at ca. 1600, 1250 and 1090 cm-¹ can be assigned to the stretching vibrations of C=O, C-OH and C-O-C groups, respectively. The bands observed in the range of 1090 to 660 cm-¹ are attributable to the stretching vibrations of P=O, P-C (of aromatic carbon), P-O, P-OH, and P-C groups.8,23,24


ACS Applied Energy Materials 80 70 PNC3

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Figure 1. FT-IR spectra of cotton denim waste-derived P- and Nco-doped carbon materials, PNC1 and PNC3. The materials are synthesized using H3PO4 as an activating agent, with denim waste:H3PO4 wt.:vol. ratios of 1:1 and 1:3, respectively.

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More information about the chemical compositions of PNC1 and PNC3 and the functional groups in them is acquired from their XPS spectra (Figures 2, S2 and S3, and Table S1). Specifically, five characteristic peaks, centered at ca. 133, 190, 284 and 532 eV, which can be assigned to P 2p, P 2s, C 1s and O 1s, respectively, are evident in the survey spectra (Figure 2).17,25 A small peak at 399 eV, which can be assigned to N 1s, is also seen. The signal centered at 497 eV is due to the Auger peak of Na 1s,26 which is associated with the Na species making it into the materials during their washing with NaOH solution and remaining in the materials even after their subsequent washing with water (see Materials and Methods section). The relative amounts of C, O, P and N in the materials are estimated based on the relative intensities of their peaks in the spectra. They are found to be 63.20, 32.90, 2.17 and 1.95%, respectively, for PNC1, and 66.14, 30.61, 2.54 and 0.71%, respectively, for PNC3. Besides XPS, C,H,N elemental analysis is performed to determine the presence and amounts (in wt.%) of N in the materials. The results show that PNC1 contains 40.92% C, 2.74% H and 1.16% N, whereas PNC3 contains 47.64% C, 5.36% H and 0.64% N. This indicates, once again, that there is some N dopants in both materials, albeit in small amounts. The presence of N in the materials is actually to be expected, since there is N-containing indigo carmine dye in denim, and the carbonization of such N-containing carbon precursors is known to lead to N-doped carbon materials. Thus, the N dopant atoms present in both materials must have originated from the indigo carmine dye present in the denim waste. This dye, which has two pyrrole groups (Figure S1),27 is widely used to dye denim. Meanwhile, the P atoms in both materials must have come from H3PO4, which is included in the precursors. H3PO4 is often used as an activating agent during the synthesis of carbon materials via carbonization, and the P atoms in H3PO4 are known to make it into the structures of carbon materials during this process.12,13 For example, Miao et al. synthesized P-doped carbon material containing ca. 3.42% P via high temperature treatment of H3PO4-containing soft pitch16 and Borghei et al.17 synthesized P-doped carbon possessing ca. 2.30% P via carbonization of H3PO4-treated coconut shells.

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Figure 2. XPS survey spectra of a) PNC1 and b) PNC3. The insets show peaks corresponding to N 1s.

In the high-resolution XPS spectra of both carbon materials, the peaks associated with C 1s, O 1s, N 1s and P 2p are deconvoluted (see Figures S2a-d and S3a-d and Table S1). The high-resolution C 1s peak of PNC1 is deconvoluted into four distinct peaks centered at 284.68, 286.36, 288.08 and 289.55 eV. These peaks can be assigned to C=C groups (87.02%); CO, C=N and C-P groups (8.35%); C=O and C-N groups (2.89%); and O-C=O groups (1.74%), respectively (Figure S2a). The high-resolution C 1s peak for PNC3 (Figure S3a) is deconvoluted into three main peaks centered at 284.79, 286.66 and 288.81 eV. These peaks correspond to C=C groups (86.65%); C-O, C=N and C-P moieties (9.10%); and O-C=O groups (4.11%), respectively. Moreover, a small satellite peak (0.14%) at ca. 290.93 eV, which is due to the characteristic π → π* transition of graphitic materials,25,28,29 is observed. The high-resolution O 1s peak of PNC1 (Figure S2b) is deconvoluted into three peaks centered at 531.22, 533.05, and 536.01 eV. These peaks can be attributed to C=O groups (40.12%); C-O and C-O-P groups (52.10%); and HO-C groups (5.77%), respectively.17,28,29 The corresponding high-resolution O 1s peak of PNC3 (Figure S3b) displays peaks at 531.10, 533.07 and 536.03 eV. These peaks can be assigned to C=O groups (55.60%); C-O and C-O-P moieties (41.43%); and HOC groups (2.97%), respectively. The high-resolution N 1s and P 2p peaks of both materials are also deconvoluted (see Figures S2c, S2d, S3c and S3d). The N 1s peaks of PNC1 is deconvoluted into two peaks centered at 399.64 and 400.73 eV, which can be assigned to pyrrolic N and graphitic N groups, respectively (Figure S2c).10,15 The N 1s peak of PNC3 is deconvoluted into two peaks centered at 399.96 and 400.74 eV, which can be assigned to pyrrolic N and


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graphitic N species, respectively (Figure S3c).10,15 However, these two sets of peaks can also be due to the N species in P=N and P-N linkages, respectively, which are often present in Pand N-co-doped carbon materials and which have XPS peaks in similar regions.3,30 It is worth pointing out here that there appear to be some notable differences in terms of the types of N- and P-based species present in PNC1 and PNC3 materials. Based on the N 1s XPS peak of PNC1, the proportion of pyrrolic N and N=P species is found to be 50.93% while the proportion of graphitic N and P-N species is found to be 49.07% in this material. However, based on the N 1s XPS peak of PNC3, the proportion of pyrrolic N and P=N moieties (36.72%) is lower than that of graphitic N and P-N groups (63.28%) in this material. Highresolution, deconvoluted P 2p XPS peaks (Figures S2d and S3d) exhibit two peaks centered at 132.99 and 133.86 eV for PNC1 and at 132.87 and 133.96 eV for PNC3. These peaks are attributable to P-C and P-O/P-N species, respectively.3,25 The relative amounts of P-C and P-O/P-N moieties in PNC1 are found to be 54.11% and 45.89%, while the corresponding amounts in PNC3 are found to be 73.21 and 26.79%, respectively. These differences in terms of the proportions of different species in the two materials must also be due to the different amounts of H3PO4 used to synthesize the two materials. (Note again that the amount of H3PO4 used to synthesize PNC3 is 3-times higher than the amount used to synthesize PNC1). Additionally, energy-dispersive X-ray (EDX) spectra of cotton denim waste, PNC1 and PNC3 are obtained during their SEM imaging (Figures S4 and S5). The EDX spectrum of the cotton denim precursor used for the synthesis of the carbon materials (Figure S4) shows that the material is mostly composed of C and O. The broad signal centered at ca. 2.20 keV in the spectrum is characteristic of Au, which is due to the Au that is used to metalize this sample for the SEM analysis. No peak other those associated with C, O and Au is observed in the spectrum, suggesting the absence of any impurity in the cotton denim waste. Most notably, a signal corresponding to Si (at ca. 1.75 keV) is not seen in the EDX spectrum of this material, indicating the absence of Si in the denim waste (note that Si is sometimes found in cotton). On the other hand, the EDX spectra of PNC1 and PNC3 (Figure S5) show signals associated with C, N, O, and P. A tiny peak associated with S, which is not detected by XPS analysis earlier, is also observed in the EDX spectra of both materials. Additionally, a small peak attributable to Na, which must have come from the NaOH solution used for washing the materials during their syntheses, is seen. The textural properties (surface area, pore size, etc.) of the materials are characterized with N2 porosimetry (Figure 3). The results obtained from the adsorption/desorption isotherms are further compiled in Table 1. PNC1 shows a Type-IV isotherm with H4-hysteresis loop, which is characteristic of mesoporous materials containing a substantial amount of micropores (Figure 3a).21,29 PNC3, on the other hand, shows a Type-IV isotherm with H3-hysteresis loop, which is characteristic of mesoporous materials containing slit-like pores formed by non-rigid aggregates.21,31

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Pore width (nm)

Figure 3. (a) N2 adsorption/desorption isotherms and (b) pore size distributions of PNC1 and PNC3. The latter is obtained by using the NLDFT-based pore analysis method.

As can be seen in Table 1, the BET surface area of PNC1 (1582 m² g-1) is 3.26-fold higher than that of PNC3 (486 m² g1). However, the BJH mesopore volume of PNC1 (which is 0.4 cm³ g-1) is lower than that of PNC3 (which is 0.6 cm³ g-1). The pore size distributions of the denim-derived N- and P-co-doped carbon materials (Figure 3b) clearly show that the ratio of H3PO4-to-denim in the precursors has also a direct effect on the pore sizes and pore size distributions of the materials. While both PNC1 and PNC3 have multimodal pore distributions, the prominent pores in PNC1 are at around 2.7 and 4.6 nm but those in PNC3 are at around 3.0, 5.1 and 15.1 nm (see Figure 3b). Moreover, the overall average pore diameter of PNC1 (2.5 nm) is ca. 3-times smaller than that of PNC3 (ca. 7 nm). These results suggest that varying the denim:H3PO4 ratio in the precursors leads to carbon materials with different surface areas and pore structures, besides different compositions. According to Jagtoyen and Derbyshire,19 using a relatively higher concentration of H3PO4 makes the acid penetrate the


ACS Applied Energy Materials structures of cellulose better, and then cleave the hydrogenbonded structures and chains in cellulose faster by substituting them with groups such as polyphosphates. A higher amount of H3PO4 also leads to the insertion of more P atoms into cellulose and thereby into the carbon structures derived from it, giving Prich carbon materials. Moreover, a higher amount of H3PO4 can lead to the expansion/dilatation of the material during pyrolysis, as a result of the formation of more phosphate esters as well as the effect of the acid in the process. This can subsequently result in an expanded pore structure and mesopores in the material during carbonization.32 This might be why, compared with PNC1, PNC3 has a relatively higher amount of mesopores, and conversely a relatively less amount micropores. PNC3’s lower surface area might also have to do with the formation of more phosphate and polyphosphate species, which can make some of the pores in it inaccessible to N2 gas.33 Thus, overall, a higher amount of H3PO4 facilitates the decomposition of denim fibers while also increasing the amount of mesopores and decreasing the amount of micropores and surface area of the material.

The morphological features of the materials are analyzed by SEM (Figure 5). The SEM image of PNC1 (Figure 5a) displays microparticles and some fibrous structures whereas the SEM image of PNC3 (Figure 5b) shows only microparticles and barely any fibrous structures. The structures of the materials are further investigated by TEM (Figure 6). The TEM images show that PNC1 has more nanoporous features than PNC3. This is in line with the higher BET surface area obtained for PNC1 compared with that of PNC3. a)

Table 1. Textural properties of P- and N-co-doped denimderived nanoporous carbon materials (PNC1 and PNC3). PNC1

PNC3

BET surface area (m² g-1)

1582

486

Total pore volume (cm³ g-1)

1.0

0.7

Mesopore volume (cm³ g-1)

0.4

0.6

Average pore size (nm) a

2.7, 4.6

b

b)

3.0, 5.1, 15.1 b

a Determined using the NLDFT-based method. b The average values of pore sizes in the multimodal pore distribution.

The structures of PNC1 and PNC3 are analyzed by Raman spectroscopy. The spectra, which are displayed in Figure 4, show two bands at ca. 1340 (a D-band) and 1590 cm-1 (a Gband) that are characteristics of graphitic materials. The ratios of the areas integrated under D and G bands (ID/IG) are found to be 0.34 for PNC1 and 0.30 for PNC3. These values indicate the presence of a slightly higher defect density in the structure of PNC1 compared with that of PNC3. The higher density of defect sites in PNC1 is probably due to its higher surface area, which can lead to the formation of more heteroatom-doped species on the surfaces of the material. D-band

Figure 5. SEM images of (a) PNC1 and (b) PNC3. (b)

G-band

PNC3 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

Page 6 of 12

50 nm

200 nm

(d)

(c)

PNC1

800

1000

1200 1400 1600 Raman shift (cm-1)

1800

Figure 4. Raman spectra of PNC1 and PNC3.

2000 100 nm


100 nm

Figure 6. TEM images of (a,b) PNC1 and (c,d) PNC3. The scale bars in the images are 50, 200, 100, and 100 nm, respectively.

3.2. Electrocatalytic Performances of PNC Materials for HzOR. The electrocatalytic properties of PNC1 and PNC3 materials for HzOR are evaluated using cyclic voltammetry. The cyclic voltammograms (CVs) (Figure 7) are recorded using 100 mmol L-1 of hydrazine solution in PBS (pH 7.4). The CVs demonstrated an irreversible oxidation process indicating that both materials have the ability to electrocatalyze HzOR. However, their onset potentials for the reaction are slightly different: PNC1’s is at 0.45 V vs. RHE and PNC3’s is at 0.42 V vs. RHE. This means, PNC3 electrocatalyzes the reaction better, or with a slightly lower overpotential, than PNC1 does. The peak potential of the reaction over PNC3 (ca. 0.70 V vs. RHE) is also better than that of PNC1 (ca. 0.77 V vs. RHE) (see the data in the inset of Figure 7). Furthermore, PNC3 gives a higher maximum current density (4.37 mA cm-² at 0.68 V vs. RHE) compared with PNC1 (whose current density is 3.38 mA cm-² at the same potential). In other words, compared with PNC1, PNC3 would require 70 mV lower potential to produce 1.29fold higher current density in HzOR. 6.0 5.0

Ep(V)

j (mA cm-2)

0.77 0.70

3.48 4.37

PNC1 PNC3

(a) 0 mM 10 mM 20 mM 30 mM 40 mM 50 mM 100 mM

3 2 1 0

0.2 5

0.45 0.42

0.4

(b)

4

PNC1 PNC3

2.0

4

Onset (V)

4.0 3.0

of PNC1 and PNC3. Conversely, H3PO4 is instrumental in the formation of the denim-derived carbon materials with excellent electrocatalytic activity for HzOR.

j (mA cm-2)

j (mA cm-2)

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


j (mA cm-2)

Page 7 of 12

3 2

0 mM 10 mM 20 mM 30 mM 40 mM 50 mM 100 mM

0.6 0.8 E (V vs. RHE)

1.0

1.2

1.0

1.2

Figure 8a

1.0

1

0.0 0.2

0.4

0.6 0.8 E (V vs. RHE)

1.0

1.2

Figure 7. Cyclic voltammograms (CVs) of HzOR over PNC1 and PNC3 in 100 mmol L-1 hydrazine solution in PBS (0.01 mol L-¹, pH 7.4) at a scan rate of 10 mV s-¹ and in a potential range of 0.10 to 1.30 V vs. RHE.

As can be seen in Figure S6, the control material WPC, the carbon material synthesized from denim without H3PO4 shows the lowest electrocatalytic activity for HzOR. Its onset potential is found to be 0.88 V vs. RHE, which is 460 mV higher than that of PNC3 (0.42 V vs. RHE). Additionally, the maximum current density obtained on it is only 1.20 mA cm-² (around 3.6fold lower than that of PNC3), even if the peak potential is much larger (ca. 1.34 V vs. RHE). The lower electrocatalytic activity exhibited by WPC toward HzOR can be attributed to the absence of P dopants and/or lower porosity in it. It is known that the synthesis of carbon materials via carbonization without an activating agent (e.g., H3PO4) often results in carbon materials with less porous structures.16,17 Not surprisingly, compared with PN1 and PNC3, WPC is less porous, with surface area of 263 m² g-1, total pore volume of 0.165 cm³ g-1, and mesopore volume of 0.04 cm³ g-1. Nevertheless, given that H3PO4 acts both as a source of P dopant atoms and as an activating agent, it is not possible to exclusively attribute the poor electrocatalytic activity of WPC to its low porosity. It is very likely that both features (i.e., low porosity and absence of P dopants) in it have made the electrocatalytic activity of WPC to be lower than those

0 0.2

0.4

0.6

0.8

E (V vs. RHE) Figure 8. Cyclic voltammograms (CVs) of HzOR over (a) PNC1 and (b) PNC3. The CVs are obtained using different concentrations of hydrazine solutions, in the range of 0 to 100 mmol L-1, in PBS (0.01 mol L-¹, pH 7.4) and at a scan rate of 10 mV s-¹.

Figure 8b

Meanwhile, the structural differences between PNC1 and PNC3 must have been the reason for the observed differences in their electrocatalytic activities toward HzOR. It is widely reported that the electrocatalytic activity of heteroatom-doped carbon materials depends on three main factors: (i) porosity, (ii) electronic properties, and (iii) catalytic sites and surface chemistry.123,33,34 In other words, the electrocatalytic activity of a carbon material is related not just to its porosity, and introducing a higher surface area into an electrocatalytically active carbon material might not necessarily render the material a higher electrocatalytic activity.12,33,34 This is because, besides the diffusion, adsorption and desorption of reacting species and products around the catalytically active sites for effective electrocatalysis, the electron mobility through the materials is quite important for good electrocatalysis. Thus, the key in obtaining materials with the best possible electrocatalytic activity is synthesizing them with the most optimal conductivity (which is usually greater when their porosity is lower) and with the highest density of accessible



catalytically active/defect sites (which is greater when porosity and mesoporosity is higher).34,35 In the case of carbon materials, the first factor often relates to the degree of crystallinity and delocalization of π-electrons and the second factor depends on the degree of defect sites and chemical property (surface functional groups) in the materials.8,32 Both of these can be related to the heteroatom dopants (such as N and P atoms) present in the materials. Both PNC1 and PNC3 materials have similar percentages of overall heteroatom dopants (around 3 atomic % of N and P), but PNC3 has much lower surface area or much denser structure than PNC1; thus, the higher electrocatalytic activity exhibited by PNC3, in comparison to that by PNC1, might be due to the more efficient electron mobility occurring on its denser carbon lattice. Besides, the higher amount of mesoporosity and the higher degree of multimodal pore sizes in its structure may have contributed to PNC3’s better electrocatalytic activity. Researchers have reported that introducing mesoporosity and multimodal pore structures can improve the mass transport of liquid electrolyte and reactant to the catalytic sites within such materials.17 The CVs of HzOR over PNC1 and PNC3 materials in different concentrations of hydrazine solutions are then obtained. The results are displayed in Figures 8 and S7. The current density associated with the reaction over both materials increases linearly as the concentration of hydrazine is increased

Page 8 of 12

(see insets in Figures S7a and S7b). As PNC3 had exhibited a better electrocatalytic activity for the reaction, the remaining detailed electrocatalytic studies and calculation of the number of electrons involved in the reaction were done only for it. Interestingly, the onset potential at which HzOR takes place over PNC3 (0.42 V vs. RHE) is found to be comparable to those of the best HzOR electrocatalysts reported in the literature (Table 2). (Note that these comparisons of the electrocatalytic activity of the materials are made after converting the onset potentials reported in the literature based on other reference electrodes into “vs. ERHE” using the equation: ERHE = Ereference + Eºreference + 0.059pH.) For example, bimetallic dendritic AuPd alloyed nanocrystals reported by Chen et al.,35 which are among the best electrocatalysts for HzOR, catalyzed the reaction with an onset potential of ca. 0.15 V vs. RHE in 10 mmol L-1 hydrazine in 0.5 mol L-1 H2SO4 solution. NiOx-Pt/C nanocomposite material reported by de Oliveira et al.9 electrocatalyzed HzOR at onset potential of 0.02 V vs. RHE in 0.1 mol L-1 hydrazine in 1.0 mol L-1 KOH solution. Among the heteroatom-doped carbon-based materials recently reported in the literature for HzOR, N-doped holey graphene electrocatalyzes HzOR in 10 mmol L-1 hydrazine in PBS solution at pH 7.4 with an onset potential of ca. 0.46 V vs. RHE.36 Polypyrrole-derived N- and O-co-doped mesoporous

Table 2. Comparison of the electrocatalytic properties of PNC3 for HzOR (the material reported herein) with respect to various electrocatalysts reported for HzOR in the literature. Materials

Medium

Scan rate(a)

[N2H4](b)

Peak potential(c)

Onset potential(c)

Reference

Blistered graphite

HClO4

500

5.0

1.50

0.80

37

Mn2O3-Fe2O3/carbon fibers

PBS

50

5.0

1.31(a)

0.90

38

N-S-co-doped carbon

PBS

10

50

0.60

0.38

39

Pt53Ru39Ni8 nanosponges

H2SO4

50

10

—

0.47

40

Graphene nano-hills

KOH

—

50

—

0.59(e)

41

MBCPE/Fe3O4NPs/DPB

PBS

20

0.01

0.87(e),(f)

0.72(e),(f)

42

NiCoSe2 on Ni foam

KOH

10

100

—

0.33(d)

43

Fe2O3/ECP-15

KOH

2

100

—

0.61

44

Cu-PDMC

PBS

10

50

0.72(e)

0.37(e)

45

PNC3

PBS

10

100

0.70

0.42

Current work

a

mV s-1. b mmol L-1. c vs. RHE. d MBCPE/Fe3O4NPs/DPB means magnetic bar carbon paste electrode (MBCPE) modified with magnetic Fe3O4 nanoparticles (Fe3O4NPs) and 2-(3,4-dihydroxyphenyl) benzothiazole (DPB). e Obtained by converting the potential reported in the literature in other scales to “vs. RHE” by using the equation: ERHE = Ereference + Eºreference + 0.059pH. f Estimated from the LSV or CV curves.

carbon reported by Meng et al.,10 which is the best metal-free, heteroatom-doped carbon-based catalysts reported for HzOR so far, electrocatalyzed the oxidation of 50 mmol L-1 of hydrazine in PBS with an onset potential of 0.32 V vs. RHE. Precise comparison of the electrocatalytic activities of different materials reported in the literature is obviously not fully possible, even after converting all the onset potentials to the same scale, because of the different experimental conditions and catalyst loading employed by different research groups.

Nevertheless, based on the data compiled in Table 2 and those discussed above, it can still be said that PNC3’s electrocatalytic activity is comparable to or better than those of many other effective HzOR electrocatalysts reported before. Moreover, both the onset potential and/or peak potential of HzOR over PNC3 are closer to the thermodynamic onset potential of the reaction than those of several carbon or carbon-based composite materials reported in the literature.


Page 9 of 12

The CVs of HzOR over PNC3 at different scan rates are also acquired (Figure 9a). The linear relationship between current density (i/A) versus square root of scan rates (ʋ1/2) and the dependence of peak potential (Ep) with respect to the logarithm of scan rate (log ʋ) (Figure S8) indicate that the oxidation of hydrazine over the material depends on diffusion processes.46 The diffusion coefficient (D) is estimated from the linear fit of current density versus inverse square root of time (Figure S9) using the Cottrell equation (see Supporting Information), and its value is found to be 4.09 × 10-4 cm² s-¹. The number of electrons involved in HzOR over PNC3 is calculated using the electrochemical equations described in Supporting Information, and the value is found to be approximately 4.0. The value is not exactly 4.0, the theoretically expected value for 100% conversion of hydrazine to N2 and H2O, most likely because some of the reaction intermediates generated by HzOR can remain adsorbed on the surfaces of the carbon material.39,40 Additionally, the peak-fitted equations used to determine the diffusion coefficient (the Cottrell

j (mA cm-2)

(a)

10 mV s-1 20 mV s-1 30 mV s-1 40 mV s-1 50 mV s-1 60 mV s-1 70 mV s-1 80 mV s-1 90 mV s-1 100 mV s-1

6.0 4.0 2.0 0.0 -2.0

0.2

0.4

0.6 0.8 E (V vs. RHE)

5

Figure 9a

4

1.0

1.2

(b)

3 2 1 0

equation) and other necessary parameters can make the value slightly off from 4.0. Figure 9b shows a chronoamperometric curve for HzOR over PNC3 in hydrazine solution (100 mmol L-1) in PBS (pH 7.4). During the measurement, the electrode is taken out from the solution every 1 h to remove the bubbles forming on its surface by the reaction and is then placed back into the solution to continue the run. In Figures S10a and S10b, the corresponding chronoamperometric curves obtained in the same way for hydrazine concentrations of 10 and 50 mmol L-1 are displayed. As can be seen in all the cases, the residual current densities remain almost constant during the reactions, except when the electrodes are removed from the solutions to remove the bubbles on their surfaces. Soon after this step, the current densities return to where they were and remain constant afterwards, indicating the stability of PNC3 as electrocatalyst in HzOR under the conditions we have applied. The formation of such spikes on chronoamperograms due to bubbles is common to electrocatalysis of HzOR and other reactions that generate gaseous products.

4. CONCLUSION

8.0

j (mA cm-2)

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


0

2000

4000 6000 8000 10000 12000 Time (s)

Figure 9. (a) CVs of HzOR over PNC3 acquired at different scan rates (ranging from 10 to 100 mV s-¹) for hydrazine solution of 50 mmol L-1 in PBS (0.01 mmol L-1, pH 7.4). (b) Chronoamperometric curve obtained at 0.60 V vs. RHE for hydrazine solution of 100 mmol L-1 in PBS (0.01 mmol L-1, pH 7.4). The big spikes indicate the times at which the electrode is taken out of the solution to remove the bubbles from its surfaces and then placed back into the solution to continue the runs. The small spikes are due to the accumulation of bubbles over the surfaces of the working electrode, and their subsequent detachment naturally; this is commonly observed in chronoamperograms of electrocatalysis of HzOR and other reactions that generate gaseous products.

Nanoporous P- and N-co-doped carbon materials that can serve as efficient electrocatalysts for HzOR have been synthesized via carbonization of a cotton-based denim residue that is pre-treated with H3PO4. The presence of N and P dopant groups in the materials was confirmed by XPS and elemental analyses. The N dopants in the materials have come from the indigo carmine dye present in the denim waste. This dye is widely used to color denim in the textile industry. The work has indirectly shown that the residual dye in the denim could be successful incorporated as N dopant atoms to make electrocatalytically active carbon materials via carbonization. This also means other N-doping agents would not be necessary. The P dopant atoms in the materials have come from H3PO4, which is used an activating agent. Additionally H3PO4 has helped with the formation of mesoporous structures in the carbon materials. The ratio of denim:H3PO4 used to synthesize the materials has been found to affect the structures, compositions and electrocatalytic properties of the materials. For example, a 1:3 wt.:vol. ratio of denim:H3PO4 led to a material with better electrocatalytic activity for HzOR than a 1:1 wt.:vol. ratio of denim:H3PO4 did. The presence of dopant groups and mesoporous structures must have made these materials, especially PNC3, exhibit good electrocatalytic activity toward HzOR, with low overpotential and high current density. PNC3’s electrocatalytic activity has been found to be comparable with, or better than, those of the best performing heteroatom-doped carbon HzOR electrocatalysts reported in the literature. This work shows that cotton denim residues can be used to synthesize useful metal-free electrocatalysts for HzOR and also possibly other reactions for fuel cells.

ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website. Experimental details, characterization results, structure of indigo carmin dye, XPS spectra, EDX spectra, additional electrochemical results, Cottrell plots, and chrononoamperometric curves and electrocatalytic stability test results of materials.

AUTHOR INFORMATION



Corresponding Authors *(T.A.) Tel.: +1-848-445-2970; Fax: +1-732-445-5312; E-mail: [email protected] *(V.C.A.) Tel.: +55 44 3011-4500; Fax: +55 44 3011 4449; E-mail: [email protected]

ACKNOWLEDGMENT TLS acknowledges the financial support provided to her by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq-Brazil) for her doctoral research through a sandwich program between Brazil and USA. VCA also thanks CNPq-Brazil for his postdoctoral fellowships enabling him to carry out part of the research in the US (Grant number: 484306/2013-8). TA gratefully acknowledges the financial support of Rutgers Global (formerly the Rutgers Center for Global Advancement and International Affairs (GAIA)) and the U.S. National Science Foundation (Grant No.: 1508611), which allowed his group to conduct the research in this report.

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H O N

Energy


Nanoporous Heteroatom-Doped Carbons Derived from Cotton Waste

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