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Dual Heteroatom-Doped Carbon Monoliths derived from Catalyst-free Preparation of Porous Polyisocyanurate for Oxygen Reduction Reaction Naveen Kumar Chandrasekaran, Karuppiah Selvakumar, Viji PremKumar, Saravanakumar Muthusamy, Sakkarapalayam Murugesan Senthil Kumar, and Rangasamy Thangamuthu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01440 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Sustainable Chemistry & Engineering

Dual Heteroatom-Doped Carbon Monoliths derived from Catalyst-free Preparation of Porous Polyisocyanurate for Oxygen Reduction Reaction

Naveen Chandrasekaran, a* Karuppiah Selvakumar, b Viji Premkumar,a Saravanakumar Muthusamy,a Sakkarapalayam Murugesan Senthil Kumar,b* and Rangasamy Thangamuthub

a

Electroplating and Metal Finishing and Technology Division, bMaterials Electrochemistry Division, CSIR-Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, 630 003, India. Email: [email protected], and [email protected]

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Abstract: Tris(4-isocyanatophenyl)methane (TIPM) and N,N′-Dimethyl formamide react at roomtemperature with no externally added catalyst to yield polyisocyanurate (PIR) gels. The obtained PIR gels were converted to N and S-doped porous carbon monoliths by thermal treatment at 1000 oC with elemental sulfur under inert conditions. The PIR linkage acts as precursor for carbon and nitrogen, %S doping was varied by changing the concentrations of elemental sulfur during pyrolysis. The optimized concentration of sulfur (5.6%) into the carbon matrix displayed excellent oxygen reduction activity with direct four-electron transfer relative to its pristine counterparts by, 1. Introducing micro-and mesopores in addition to the already existing macropores by etching the carbon surface (confirmed by N2 sorption isotherms and microscopic images) with the increase in the external surface area providing more active centers and efficient diffusion of electrolyte ions. 2. Providing more –C-S-C- active species than oxidized sulfur species (confirmed by XPS and FT-IR) with more oxygen adsorption sites 3. Filling the micropores of the carbon as a monolayer affording increased electronic conductivity to the amorphous carbon. This simple and facile method of incorporating N-and S- together into the porous carbon matrix can be considered as an alternate for non-precious metal catalysts for oxygen reduction reaction.

Key words: Nitrogen, sulfur, porous carbon, oxygen reduction reaction

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Introduction: Open-cell foams with interconnected organic or inorganic nanoparticles are referred to as aerogels. Due to its porous microstructure, aerogels exhibit many attractive properties such as high specific surface area, ultra-low density suitable for many technological applications.1–5 To preserve its tenuous microstucture, wet gels are dried using supercritical or freeze drying techniques. Organic aerogels are typically obtained by polymerization of organic molecules such as resorcinol-formladehyde, melamine-formaldehyde and isocyanate etc.6–8 Structural and physico-chemical properties of aerogels can be controlled by the choice of monomers, catalyst and the solvent. Organic aerogels are of importance due to its conversion to carbon aerogels at higher temperatures under inert conditions. Carbon aerogels can be prepared as monoliths and thin films, a facet that can be used in multifarious technological applications such as catalysts supports, adsorption, insulation and energy storage and conversion.9–14 Carbon allotropes doped with heteroatoms (nitrogen, sulfur, boron and phosphorous) are most sought after alternates for precious metal catalysts for energy conversion reactions such as hydrogen evolution, oxygen evolution and reduction reactions.15–20 The improved electroactivity of these materials on par with benchmark precious metal/metal oxide catalysts can be traced to the activation of pi electrons of carbon by the lone pair electrons of nitrogen which in turn enables the positively charged carbon to effectively reduce oxygen molecules.21–23Jasiniki et al. developed nitrogen doped carbons from Co-phthalocyanines as metal free catalysts for oxygen reduction reactions (ORR).15 Recently, Yang et al., demonstrated a facile approach for synthesis of sulfur-doped graphene for ORR by pyrolysis of graphene oxide and benzyl disulfide as carbon and sulphur precursor.24 S-doped carbon aerogels derived upon pyrolysis of resorcinol



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formaldehyde networks in the presence of H2S displayed 4e- transfer with a more positive onset potential and excellent methanol tolerance for ORR.25 The electrocatalytic activity of the heteroatom-doped carbon allotropes can be enhanced by incorporating two or more heteroatoms into the carbon backbone. For example, the S and Ndoped carbon showed remarkably better ORR activity than its mono-doped and pristine counterparts.26–29 Antonieetti et al, reported the synthesis of S and N-doped carbon aerogels by pyrolysing a mixture of glucose (as carbon source), S-(2-thienyl)-L-cysteine (TC) and 2-thienyl carboxaldehyde (TCA) (sulfur precursors) and ovalbumin (nitrogen source).30 The resultant N, S-doped carbon networks revealed 4e- transfer for ORR. The mechanism behind enhanced electrocatalytic activity of N, S-doped carbon networks are still unclear and yet to be investigated. Inspired by the outstanding properties of both aerogels and heteroatom-doped carbon allotropes, we propose a simple and efficient method for synthesis of simultaneous N and S-doped carbon aerogels by forming a three-dimensional network of polyisocyanurate aerogels (PIRs) and by pyrolyzing PIR in the presence of elemental sulfur to yield N and S-doped carbon monoliths. The presence of nitrogen moieties in the PIR linkages acts as the source for N-doping into the carbon matrices. The sulfur content can be varied by simply changing the concentration of elemental sulfur during pyrolysis. This approach eliminates the use of multiple precursors, rigorous methods and toxic chemicals as reported in the previous methods. The electrocatalytic activities of these materials were examined towards ORR in alkaline medium. These combined structural and morphological analysis show that the observed electrocatalytic activity towards ORR (4etransfer) significantly dependent on the doping amount of sulphur.


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The improved ORR activity of S, N-doped carbon monoliths on par with the benchmark Platinum catalysts can be ascribed to the 1. Synergistic effect of heteroatoms doped into the porous carbon matrix 2. Presence of micro-,meso- and macropores, where the macropores facilitate excellent ionic diffusion and the micro- and mesopores provides more active centers for efficient ORR. 3. The increased electronic conductivity by forming a monolayer in the micropores of the amorphous carbon. Experimental Section: Materials: Desmodur RE (tris(4-isocyanatophenyl)methane) (TIPM) (∼27% in ethyl acetate), N,N′Dimethyl formamide (DMF) (99.96%) and elemental sulfur (325 mesh, 99.5%) was purchased from TCI Chemicals, Bayer-India and Alfa Aesar respectively. Synthesis of S, N-doped carbon monoliths: The PIR gels were synthesized by mixing 0.275 g (0.074 M) TIPM in 10 mL of DMF with no external catalysts added. The sol was transferred to polypropylene molds and the time of gelation was noted to be 30 minutes. The wet gels were initially washed with DMF (3× 8 h) to remove the starting materials followed by acetone washes and dried in an autoclave using supercritical CO2 (scCO2). The aerogels obtained after scCO2 drying were pyrolyzed in a tube

furnace in the presence of 25 (CA-N-S-1), 45 (CA-N-S-2), 55 (CA-N-S-3), and 62 (CA-N-S-2) wt% with and without (CA-N) elemental sulfur to yield S, N-doped and N-doped carbon monoliths respectively.



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Characterization: X-ray diffraction (XRD) patterns of the samples were obtained and analyzed using PANalytical instruments (Model no. PW3040/60 X'pert PRO, Cu Kα radiation, λ = 1.5414 Å, at a scan rate and step size of 1.2° min−1 and 0.02° s−1) analyzed using Hi-Score search-match software and X'Pert Plus crystallographic analysis software with Rietveld capability respectively. X-ray photo electron spectroscopy (XPS) of the samples was carried out using SPECS, Phoibos 100 MCD Analyzer with the pass energy of 20 eV (Al Kα anode (1486.6 eV)) in ultrahigh vacuum (5×10–10 mbar). Renishaw Ramanscope 2000 spectrometer (He-Ne 632 nm laser) was employed to obtain the Raman spectra for all monoliths. Field-emission scanning electron, high resolution-transmission electron microscope images (HRTEM) and Selective aperture electro diffraction (SAED) patterns for the samples were performed using Zeiss supra 55VP and Tecnai G2 (FEI make) respectively. To preserve the volume of the gel, supercritical drying was carried out in a critical point drier, from Quorum Technologies, Ltd, using scCO2 at critical point pressure and temperature of 73 bar and 304.35 K respectively. The PIRs were pyrolyzed in a tubular furnace under flowing Argon (120 mL/min) at 1000 °C (5 °C min−1) for 5 h in the presence of elemental sulfur. Bruker Tensor 27 Fourier transform infra-red (FT-IR) spectrometer using KBr pellets was used to perform FT-IR spectroscopy. BrunauerEmmet-Teller (BET) measurements were obtained using Micromeritics (AccuPyc 1330) instrument.

Liquid and solid state

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C Nuclear Magnetic Resonance (NMR) spectra were

recorded using a Bruker Avance III 400 MHz spectrometer (carbon frequency of 100 MHz, magic angle spinning (at 5 kHz) with broadband proton suppression and the CPMAS TOSS pulse sequence for spin sideband suppression). Physical dimensions and weight of the samples were used to calculate the bulk densities of the samples using the formula (ρb = mass/(π 5 ACS Paragon Plus Environment

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× r2 × h)) (where, rand h is the radius and height of the samples). Helium pycnometry (Micromeritics (AccuPyc 1330) instruments) was used to determine the skeletal densities (ρs) of the samples. The porosity values (Π) of all monoliths were determined from the bulk and skeletal density values according to equation, Π = 100 × [(1/ρs) − (1/ρb)]/(1/ρs). ---------- (1) Electrochemical Characterization: All electrocatalytic ORR measurements were performed by employing a three-electrode cell using rotating ring disc electrode (PINE, AFMSRC 2092 instrumentation) coupled with a bipotentiostat (Autolab PGSTAT 302 N). The glassy carbon (GC) disc used with a geometric area of 0.2475 cm2 and a platinum ring area 0.1866 cm2. The ring has collection efficiency value of 37%. Prior to each experiment the GC electrode surface was subjected to cleaning with alumina slurry of varying particle sizes, likely, 1 µm, 0.3 µm and 0.05 µm, respectively, followed by, ultrasonication with ethanol and distilled water to remove the traces of contamination. The catalyst ink is prepared by dispersing the catalyst (10 mg) in 1 ml of 0.5% Nafion solution followed by ultrasonication for 30 minutes. Then 5 µl of the catalyst ink was pipetted onto the GC disk electrode. Prior to testing, the 0.1 M KOH electrolyte was saturated with oxygen by purging O2 and a flow of O2 was maintained over the electrolyte during the measurements. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) curves of the different catalysts in N2 saturated 0.1 M KOH solution were also measured. The sweep rate of LSV was 5 mV s-1 and the rotation speed was varied from 100 to 2500 rpm by employing Platinum (Pt) as counter and saturated calomel as a reference electrodes.



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Results and Discussion: The gelation of PIRs is schematized in scheme 1, the isocyanate functional groups present in TIPM reacts with DMF to yield PIR gels within 30 minutes. In order to confirm the isocyanurate formation, FT-IR spectra of TIPM and PIR were compared in Figure 1a. The disappearance of –NCO (isocyanate) stretching vibration at 2160 cm-1 and the appearance of new vibrations in the spectra of PIR gel NC(O)N- (carbonyl) stretching at 1654 cm-1

31

and C-N

stretch at 1413 cm-1 confirm the formation of isocyanurate by the reaction of isocyanate present in TIPM with DMF or by the trimerization of isocyanates present in TIPM.31,32 Additionally, the -NH stretch at 3313 cm-1, and new vibration at 1509 cm-1 ( -NH bending) can be attributed to the reaction of isocyanate functional group of TIPM with traces of moisture to form polyurethane linkages.

Scheme 1. Mechanism for the synthesis of polyisocyanurate (PIR) Figure 1b compares the liquid and solid-state The peaks in the

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13

C NMR spectrum of TIPM and PIRs.

C NMR spectrum of TIPM can be accounted to the carbons present in the

aromatic ring (55, 125.1, 130.9, 131.8 and 142.1 ppm), isocyanate functional group (119 ppm) of TIPM, and traces of ethyl acetate (60 ppm). In agreement with the FTIR spectra, the disappearance of the peak at 119 ppm with the formation of new peaks at 148.9 ppm and 157.9 7 ACS Paragon Plus Environment

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ppm in addition to carbons present in the aromatic rings of TIPM can be assigned to the formation of isocyanurates and urethane (due to the reaction of traces of moisture and isocyanate)4. Figure 2 compares the FE-SEM image and the N2 sorption isotherm of PIRs. Microscopically, PIRs displayed three-dimensionally connected smaller particles to form larger agglomerates with particle size in the order of 20-30 nm. The PIRs were found to be monolithic with the bulk (ρb) and skeletal densities (ρs) and porosity of 0.29, 1.22-1.24 g/cc and 76% respectively.4,6 The N2 sorption isotherm of PIRs displayed no saturation with a steep increase above P/Po = 0.9 exhibiting macroporosity. However, slim hysteresis and extensive specific volumes at low P/Po values signify the occurrence of meso and microporosity.6 Analysis of the BET isotherms yielded surface area (σ) of 84 m2/g. The extensive t-plot and Harkins-Jura model analysis of BET isotherm showed 68% of micro and mesopores contribution to the total specific surface area of PIRs, the additional 32% can be attributed to the presence of macroporosity. Average pore diameters calculated from the adsorption isotherm (ultimate point) and ρb, ρs values were determined to be 22.9 nm and 121 nm respectively consistent with the interconnected particles leaving macropores. In accordance with the microscopic image, the particle diameter calculated from 6/ρsσ was found to be ~60 nm.6 The PIRs were pyrolyzed under Argon at 1000 oC to yield N-doped carbon monoliths (CA-N). S-doping into the carbon matrix was obtained by pyrolyzing PIRs in the presence of 25 (CA-N-S-1), 45 (CA-N-S-2), 55 (CA-N-S-3) and 62 (CA-N-S-4) wt% of elemental sulfur. Figure S1 displays the TGA of PIR samples under N2 in the presence and absence of elemental sulfur (different concentration as mentioned above). TGA of CA-N atmosphere displayed cumulative weight loss of 60% indicating carbon yield of ~40%, whereas, the TGA of PIRs in the presence of 0.25 and 0.45, 0.55 and 0.62 wt% of elemental sulfur showed an increase in cumulative weight loss of 38, 35, 33 and 25% respectively. The increase in weight loss for the samples in the



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presence of sulfur can be pertained to the reaction of sulfur with the oxygen functionalities present in the carbon and with carbon itself to gaseous products. The sharp increase in the weight loss at ~ 220 o

C for the samples pyrolyzed in the presence of elemental sulfur can be ascribed to the formation of

linear polymeric bi-radical molecules (·S-S6-S·) of sulfur.33 At temperatures above 220 oC, complete transformation of polysulfur bridges (C-S8-C) to monosulfur bridges (C-S-C) which can act as electrical links between the carbon layers due to the conjugation of sulfur atom to the carbon.33 Figure 3 displays the N2 sorption isotherms and FE-SEM images of CA-N and CA-N-Ss samples and the physical properties measured are summarized in Table 1. Consistent with the FE-SEM image, the N2 sorption isotherm of CA-N displayed macroporosity with a rapid increase of P/Po above 0.9 with no saturation. Additionally, the considerable definite volumes adsorbed at lower P/Po indicate the existence of micropores. The surface area and total pore volume of CA-N was found to be 132 m2/g and 0.22 g/cc respectively. The mean pore radii calculated from the adsorption isotherm and densities (bulk and skeletal) were found to be 21 and 47 nm respectively was found to lie in the macroporous regime. Particle diameter calculated from the surface area and the skeletal density was found to be 23 nm, but the particle size remained higher than those noticed in microscopic images may be due to the agglomeration of particles. The N2 sorption isotherms of CA-N-S-1 and CA-N-S-2 displayed Type IV isotherm with hysteresis loop between P/Po = 0.4 and 0.8 and rapid increase in adsorption P/Po value of 0.8 displaying meso- and macropores respectively. In addition to the presence of meso- and macropores, the existence of micropores was identified by the extensive volumes adsorbed at low P/Po values. The BET analysis yielded surface areas of 617 and 889 m2/g for CA-N-S-1 and CA-N-S-2 respectively, the increase in surface area relative to the pristine CA-N can be attributed to the introduction of 9 ACS Paragon Plus Environment

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micro and mesopores due to the etching and the extensive oxidation of carbon by sulfur at higher temperatures. Consistent with the N2 sorption isotherms the corresponding microscopic images of CA-N-S-1 and CA-N-S-2 showed mesoporous structure decorated on macroporous carbon accounting for the relative increase in the surface area than the pristine CA-N. Since the XRD pattern and Raman spectra of CA-N-S-1 and CA-N-S-2 displayed no peaks of elemental sulfur, the formation of mesoporous structure can be addressed to the oxidation or etching of carbon by elemental sulfur during pyrolysis. However, a drastic 3 fold decrease in the surface areas of CAN-S-3 (219 m2/g) and CA-N-S-4 (217 m2/g) was observed with the respective N2 sorption isotherms displaying no typical hysteresis loops between P/Po = 0.4 to 0.8 and with reduced specific volumes at lower P/Po values. The decrease in surface areas of CA-S-N-3 and CA-S-N4 can be ascribed to the 1. Increase in pore and particle diameter 2. Intercalation and retention of sulfur into the pores of carbon relative to the CA-S-N-1 and CA-S-N-2 counterparts. Figure 4a displays the XRD of CA-N, CA-N-S-1, CA-N-S-2, CA-S-N-3 and CA-S-N-4 samples. X-ray diffraction patterns of all the samples with and without sulfur showed the characteristic broad amorphous peaks at 26o (002) and 42o (011).When the concentration of sulfur is increased during pyrolysis a slight decrease in d-spacing values of CA-S-N-3 and CA-S-N-4 was observed. This may be due to the electronic stacking interactions between the heteroatoms doped carbon layers. The large amount of S doping in the CA-S-N-3 and CA-S-N-4 would have caused an increase in the density of electrons in the carbon network resulting in decreased interlayer distance.34 This may be also accounted as one of the reasons for the sharp decrease in surface area of these samples with decreased access of N2 molecules between the carbon layers. The conversion of PIRs to carbon was confirmed from the presence of D and G peaks from the Raman spectra as displayed in Figure 4b. The D and G peaks of all the samples can be ascribed to the distorted first-order scattering 10 ACS Paragon Plus Environment


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of the E2g mode from the sp2 domains. 35 The gradual increase in ID/IG for the samples pyrolyzed in the presence elemental sulfur relative to CA-N implies an increase in defective sites due to sulfur doping into the carbon matrix. Due to moderate and extensive oxidation of carbon by increased concentration of sulfur during pyrolysis, CA-N-S-3 and CA-N-S-4 showed higher ID/IG ratios, additionally, a nominal red shift (63 cm-1) of G band (or decrease in band gap ) in CA-N-S-4 was observed relative to other samples, a typical characteristic for n-type doping of sulfur into the carbon matrices or due to the tensile strain developed between the carbon sheets during thermal annealing.36,37 The absence of sulfur peaks in the region from 100 to 500 cm-1, suggests that the dual heteroatom doped (N, S) carbon monoliths contained no traces of sulfur impurities after pyrolysis. Furthermore, the absence of elemental sulfur in its crystalline state in both the XRD pattern and Raman spectra demonstrates the availability of sulphur as a monolayer in a highly dispersed state in the micropores which may offer good electronic conductivity for CA-N-S-1 and CA-N-S-2 samples a vital requirement for electrocatalysis.35,38,39 Figure 5a displays the XPS analysis of CA-N and CA-N-Ss, in which there are two dominant peaks at 532.68 (O1s) and 284.36 eV (C1s). Furthermore, for CA-N-Ss samples there are peaks at 169.02 (S2p), 235.2 eV (S2s) and at 400. eV (N1s) while the CA-N showed no sulphur peaks. The intensity of the sulphur peak (S2s and S2p) was found to gradually increase for the CA-N-Ss samples when the sulfur concentration was increased during heat treatment.35,38,39 Figure 5b displays highresolution spectrum of N1s for CA-N and CA-N-Ss samples, the peaks at 402.2, 401.1, and 398.6 eV correspond to the quaternary (N–(C)3), pyrrolic (N–H), and pyridinic (C–N–C) groups, respectively. 25 The availability of nitrogen species in the isocyanurate ring can be deemed responsible for the Ndoping into the carbon matrices. Figure 5c shows the high-resolution spectrum of S2p for CA-N-Ss samples with three peaks at 168.6, 169.25, and 170.5 eV, corresponding to the oxidized sulfur 11 ACS Paragon Plus Environment

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groups, i.e., –C–S(O)x–C– bonds (x = 2, 3, 4) in addition to the –C-S-C- peaks at 163.85 and 165 eV. 35,38,39

There was gradual increase in the intensities of the oxidized sulfur groups for CA-S-N-3 and

CA-S-N-4 due to the increase in the concentration of elemental sulfur during pyrolysis. The elemental composition of the CA-N and CA-N-Ss given in Table 2, shows gradual decrease in carbon content for CA-N-S-3 and CA-N-S-4 samples relative to the CA-N-S-1 and CA-N-S-2 samples this may be due to the extensive oxidation of carbon content by the large amount of sulphur during pyrolysis. The S/C ratio was found to increase with the increase in %wt of sulfur during pyrolysis implying the increase in sulfur content for the CA-S-N-3 and CA-S-N-4. Nevertheless, the augment in sulfur content can be accounted to the presence of sulfur oxidized functional groups on carbon in the CA-SN-3 and CA-S-N-4 samples. The decrease in the carbon content for CA-S-N-3 and CA-S-N-4 samples by XPS was found to corroborate with the TGA. FT-IR spectra were performed to identify the functional groups present in CA-N and CA-N-S samples and displayed in Figure S2. The peaks at 3445, 2931, 1740, 1592, 1117 cm-1 for CA-N can be attributed to the OH/NH, C-N, C=O, C=C and – C-NH-C- stretching vibrations respectively. In addition to these peaks CA-N-S-3 and CA-S-N-4 displayed new peaks at 1231, 1057, 836 and 535 cm-1corresponding to the presence of –C-S-C, -SO3, C-S, -C-C=S- vibrations respectively.39 The peak at 2361 cm-1 can be ascribed to the S-H/C-N vibrations. The C-S stretching at 1365 cm-1 can be attributed to the –N-C=S bands for the CA-N-Ss samples. It is clearly evident from the FTIR spectra shows that when the sulfur concentration is increased during pyrolysis, the OH and –SO3 stretching becomes intense, the C-S and S-H vibrations decreases implying the severe oxidation of carbon or sulphur results in higher concentration of oxygen functionalities consistent with the thermal and XPS analysis. Figure S3 displays, HR-TEM images of CA-N, CA-N-S-1, CA-N-S-2, CA-N-S-3, CA-N-S-4 and its corresponding SAED patterns. All the samples displayed particulate like morphology with agglomeration, with introduction of sulphur into



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the carbon the particle size of the CA-N-Ss found to decrease relative to the pristine CA-N sample. The SAED pattern of CA-N showed diffused ring pattern with no bright spots characteristic to the amorphous nature of the sample, whereas the CA-N-S-1 and CA-N-S-2 showed bright spots which may due to the presence of polycrystalline sulphur into the pores. In contrary, the CA-N-S-3 and CAN-S-4 samples displayed also amorphous nature which may be due to the presence of increase in the oxidized sulphur species on the surface of the carbon. The XRD pattern, XPS, Raman spectra, N2 sorption isotherms and FE-SEM images collectively conclude that sulfur, 1. Occupies the pores and the defect sites of carbon and 2. Creates meso- and microporous nanostructures by oxidation of carbon. The ORR performance of as prepared CA-N and CA-N-Ss samples in 0.1 M KOH solution was determined by cyclic voltammetry (CV) and linear sweep volatammetry (LSV) using a rotating ring-disc electrode (RRDE) asembly. The CV responses of all the samples both in nitrogen and oxygen purged 0.1 M KOH electrolyte solutions were studied (See SI Fig S4). Figure 6a represents CV of CA-N-S-2 studied in N2 saturated 0.1 M KOH solution (sweep rate of 5 mV s-1) showed typical capacitive behaviour especially in the potential region between 1.1 to 0.4 V vs. RHE. On the other hand, the CV measured in O2 saturated solution for CA-N-S-2 catalyst displayed a well defined cathodic oxygen reduction peak (Epeak) at 0.80 V vs. RHE. This observation demonstrates the superior electrocatalytic activity of CA-N-S-2 catalyst towards ORR in alkaline solution than other samples. In order to understand the fundamental insight into the ORR of CA-N-S-2, detailed RDE based experiments were extended. Figure 6b illustrates the LSV recorded with different rotation rates in O2-saturated 0.1 M KOH solution at a scan rate of 5 mV s-1. The LSV show that the limited current density increases with the increase in rotation rate from 100 to 2500 rpm where as the onset potential remains unaltered (0.85 V vs. RHE). The 13 ACS Paragon Plus Environment

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ORR process with an on-set potential of 0.85 V gradually proceeds via a mixed control between 0.8 to 0.65 V and reaches a complete mass transfer control at 0.55 V. According to the Koutecky-Levich equation (2), the overall current density can be expressed as, 1/j = (1/jk) + (1/jd) ---------- (2) where the jd is the diffusion limiting current density and can be obtained by the following relation jd = 0.62nFCD2/3ν -1/6ω1/2 --------- (3) in which the n denotes the number of electrons involved, F is the Faraday constant, C the of oxygen concentration and D the diffusion coefficient, ν is the kinematic viscosity (0.01 cm2 s-1) and ω is the angular velocity (rad s-1). Even though very small quantities of peroxide were detected during ORR, it is unlikely to conclude the reaction followed a four-electron pathway40. Figure 7a compares the LSV curves for all the samples compared with commercial Pt/C 20% at a fixed rotation rate of 1600 rpm electrode. The observed onset potential (Eonset) of ORR for CA-N-S-2 was close to 0.85V which was found more anodic than other electrocatalysts, similarly, half-wave potential (E1/2) and its corresponding current density (j) were found to show analogous trends on this electrode. Even though, the commercial Pt/C 20% displayed a significant anodic shift in the onset potential for ORR there was an increase in current density for the CA-N-S-2 sample relative to Pt/C 20%. Figure 7b shows the Koutecky-Levich plots depicted at 0.4 V for CA-N, CA-N-Ss and Pt/C 20%. The slope of the plots for respective samples was found to vary with different sulfur concentration employed during pyrolysis. From the resultant slopes by substituting values like, C (1.14 × 10-6 mol cm-3) and D (1.73 × 10-5 cm2 s1

) which in turn yield the “n” value, i.e, the number of electrons transferred during ORR. In 14 ACS Paragon Plus Environment


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general, the direct 4e- ORR is considered favorable and usually observed on Pt and supported Pt catalysts41,42 whereas, the two-electron path is found to dominate non-noble metal catalysts.43 Ring-disc measurements were calculated to quantify the %H2O2 generated. The fraction of current generated by peroxide formation at 0.4 V was calculated using equation (4) for all samples employed in this work. % H2O2 = 2 (IR/ N) / ID + (IR / N) ------- (4) where ID is disc current, IR is ring current and N is the collection efficiency coefficient. From RRDE and ORR polarization curves, electrochemical parameters (number of electrons, %H2O2 and Tafel slope value) were determined for all the catalysts and compared in Table 3. Figure 7c displays the Tafel plots with slope values of 99, 85, 73, 74 and 70 mV/decade-1 for CA-N, CA-N-S-1, CA-S-N-3, CA-N-S-4 samples and Pt/C 20% respectively. The CA-N-S-2 sample displayed ‘n’ value, % H2O2 and Tafel slope of 4.12 (benchmark Pt/C 20% showed a n value of 4), 24% and 124 mV confirming the direct four electron transfer kinetics. In contrast, nvalues for CA-N, CA-N-S-1, CA-S-N-3 and CA-N-S-4 were determined to be 2.47, 2.95, 3.21 and 2.79 respectively suggesting two-electron ORR via a threefold increase in the % H2O2 formation limiting their applications in real-time fuel cell performance. It can be noted from Table S1 the observed ORR activity in terms of onset potential and limiting current density for our N and S doped aerogel is higher than that of reported earlier.30 The favorable four-electron ORR mechanism of CA-N-S-2 can be ascribed to the optimized sulfur doping (5.6%) into the carbon matrix, a. By etching the carbon surface providing micro-and mesopores in addition to macropores (confirmed by N2 sorption isotherms and microscopic images) with an increase in


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external surface area facilitating more active centers, efficient diffusion of electrolyte ions effectively quenching the available active sites for ORR to take place. b. With relatively more –C-S-C- than oxidized sulfur species (confirmed by XPS and FTIR) providing more oxygen adsorption sites available for ORR c. By filling the micropores of the carbon as a monolayer providing increased electronic conductivity to the amorphous carbon. Conclusions: In summary, a facile synthetic approach for polyisocyanurate monoliths by reaction of a triisocyanate, TIPM and DMF at room temperature has been demonstrated. The PIRs were converted to N, S-doped carbon monoliths pyrolysis under Ar at 800 oC. The PIR linkages act as the precursors for carbon and nitrogen doping. The %S doping was varied by changing the concentrations of elemental sulfur during pyrolysis. The optimized concentration of sulfur (5.6%) into the carbon matrix displayed excellent oxygen reduction activity with direct fourelectron transfer relative to its pristine counterparts by, 1. Introducing micro-and mesopores into the already existing macropores by etching (confirmed by N2 sorption isotherms and microscopic images) enhances external surface area thus providing more active centers and efficient diffusion of electrolyte ions. 2. Providing more –C-S-C- active species than oxidized sulfur species (confirmed by XPS and FT-IR) with more oxygen adsorption sites 3. Forming monolayer on the micropores of carbon with enhanced electrical conductivity. The underlying correlation between the structure-property-activity of the will be S, N-doped carbon monoliths further explored to comprehend the effect of stoichiometry on the electronic structure and its catalytic behavior towards ORR.



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Acknowledgements C. Naveen Kumar appreciates the financial support from DST through project No GAP 10/14. S. M. Senthil Kumar would like to thank DST-UKIERI (GAP 12/15) for providing funding. K. Selvakumar thanks CSIR-New Delhi for SRF fellowship. CNK is grateful to Prof. Nicholas Leventis for his support to obtain 13C NMR spectra for all the samples. Supporting Information: The supporting information is available free of charge on the ACS Publications Website at DOI:***** Thermogravimetric analysis of polyisocyanurate (PIR) gels with different concentrations of 25 (CA-N-S-1), 45 (CA-N-S-2), 55 (CA-N-S-3), and 62 (CA-N-S-2), wt% and without sulfur (CAN), FTIR spectra of CA-N, CA-N-S-1, CA-N-S-2, CA-N-S-3 and CA-N-S-4, TEM images (left) of (A) CA-N, (B) CA-N-S-1, (C) CA-N-S-2, (D) CA-N-S-3 (E) CA-N-S-4 and its corresponding SAED patterns (right), Cyclic voltammetry (CV) responses of all the samples in N2 and O2saturated 0.1 M KOH, Table S1. Comparison of electrochemical parameters for various N, Sdoped carbon materials


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(A)

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(B)

PIR

a

TIPM

b

Figure 1. FTIR (left) and NMR (right) spectra of TIPM and PIR gels


f d

d c

e

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(A)


(B)

Figure 2. (A) FE-SEM (scale bar: 200 nm) and (B) N2 sorption isotherm PIR gels before pyrolysis (Inset: Pore size distribution from BJH desorption)


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

(A)

200 nm

(B)

200 nm

(C)

200 nm

Quantity adsorbed@STP(cc/g)Quantity adsorbed@STP(cc/g) Quantity adsorbed@STP(cc/g) Quantity adsorbed@STP(cc/g)Quantity adsorbed@STP(cc/g)


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Adsorption Desorption

800

600

400

200 700 Adsorption Desorption

600 500 400 300 200 400 Adsorption Desorption

350

300

250

200

1400

(D)


1200 1000

200 nm

(E)

200 nm

800 600 400 200 350


300 250 200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

Figure 3. FE-SEM images (left) and corresponding N2 sorption isotherms of (right) of (A) CA-N, (B) CA-N-S-1, (C) CA-N-S-2, (D) CA-N-S-3 and (E) CA-N-S-4


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(A)

(B)

Figure 4. (A) XRD and (B) Raman spectra of CA-N, CA-N-S-1, CA-N-S-2, CA-N-S-3 and CA-N-S-4



(A)

(B)

(C)

Figure 5. (A) Survey (B) High resolution N1s spectra and (C) High resolution S2p spectra of CA-N, CA-N-S-1, CA-N-S-2, CA-N-S-3 and CA-N-S-4


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(A)

(B)

Figure 6. (A) Cyclic voltammogram in N 2 and O2 saturated solution; (B) Linear sweep voltammograms (with different rotation rates) recorded for the CA-N-S-2 catalyst at a sweep rate of 5 mV s-1 in 0.1M KOH solution.



(A)

(B)

(C)

Figure 7. (A) Rotating ring disc electrode response recorded at a 5 mV s-1 scan rate with a rotation rate of 1600 rpm in O2 saturated 0.1 M KOH solution; (B) Koutecky-Levich (K-L) and (C) Tafel plots for various catalysts.


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Graphical Abstract:

Low-cost, environmental benign, metal-free dual heteroatom doped carbon catalyst as a replacement for Pt towards oxygen reduction reaction.




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Table 3. Electrochemical parameters determined from LSV at different rotation rates for CA-A, CA-N-S-1, CA-N-S-2, CA-N-S-3 and CA-N-S-4

Sample

n-value in ORR

% of H2O2

Tafel slope / mV decade-1

CA-N

2.47

62

99

CA-N-S-1

2.95

65

85

CA-N-S-2

4.12

24

124

CA-N-S-3

3.21

36

73

CA-N-S-4

2.79

54

74

Pt/C 20%

4.00

4

70


Dual Heteroatom-Doped Carbon Monoliths ... - ACS Publications

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