Asphaltene-Derived Metal-Free Carbons for Electrocatalytic

Research Article

Cite This: ACS Appl. Mater. Interfaces 2019, 11, 27697−27705 www.acsami.org

Asphaltene-Derived Metal-Free Carbons for Electrocatalytic Hydrogen Evolution † ⊥ ‡ ⊥ † Jayashree Swaminathan, , Shayan Enayat, , Ashokkumar Meiyazhagan,*, † § † † ∥ ‡ Francisco C. Robles Hernandez, , Xiang Zhang, Robert Vajtai, , Francisco M. Vargas, † and Pulickel M. Ajayan*, † ‡ Department of Materials Science & NanoEngineering and Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States § Department of Mechanical Engineering Technology, University of Houston, Houston, Texas 77204-4020, United States ∥ Interdisciplinary Excellence Centre, Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Belá ter1,́ Szeged H-6720, Hungary

*S Supporting Information

ABSTRACT: The design of new and improved catalysts is an exciting field and is being constantly improved for the development of economically, highly efficient material and for the possible replacement of platinum (Pt)-based catalysts. In this, carbon-based materials play a pivotal role due to their easy availability and environment friendliness. Herein, we report a simple technique to synthesize layered, nitrogen- doped, porous carbon and activated carbons from an abundant petroleum asphaltene. The derived nitrogen-doped carbons were found to possess a graphene-like nanosheet (N-GNS) texture with a significant percentage of nitrogen embedded into the porous carbon skeleton. On the other hand, the activated porous carbon displayed a surface area (SA) of 2824 m2/g, which is significantly higher when compared to the nitrogen-doped carbons (SA of ∼243 m2/g). However, the nonactivated N-GNS were considered as an attractive candidate due to their high electrochemical active surface area, the presence of a mixture of porous structures, uniform layers, and effective doping of nitrogen atoms within the carbon matrix. Importantly, the hydrogen evolution reaction activity of the derived N-GNS sample illustrates a significant catalytic performance when compared to that of other nonfunctionalized carbons. Our current finding demonstrates the possibility of converting the asphaltene wastes into a high- value-functionalized porous carbon for catalytic applications. KEYWORDS: porosity, water splitting, asphaltene, doping, carbon

■ INTRODUCTION pose a limitation on the scaling-up process.17,18 An alternative Downloaded via UNIV OF HOUSTON MAIN on October 31, 2019 at 15:49:03 (UTC). Carbon forms a basis for the chemistry of life and is found would be to explore processing techniques to produce graphene-mimic materials using carbon-based renewable abundantly in all living systems. The ability of carbon to form 19,20 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. different degrees of hybridizations such as sp, sp2, and sp3 resources. Few attempts have been explored earlier to giving rise to allotropes addresses them as a noble candidate in synthesize graphene-like sheets using inexpensive resources; fi 1−3 fi among them, asphaltene is considered as a promising numerous elds of science and technology. Speci cally, the 21−23 discovery of two-dimensional (2D) graphene4 and their candidate. derivatives such as graphene oxide (GO)5 and reduced Asphaltene is the heaviest fraction of crude oil consisting of graphene oxide (rGO) have broadened the scientific attention a combination of diverse functional groups such as large − due to their remarkable properties.6 9 Also, the existence of polyaromatic and polyaliphatic cyclic systems interconnected via short alkyl chains.23 It is one of the well-known low-cost outstanding thermal, mechanical, and electrical capabilities of fi this honeycomb-networked 2D graphene has advanced it as a (typically, $0.05/pound) byproducts of the oil re nery process possible candidate for studies such as catalyst, functional with an annual output of around million tons. In general, − ∼ composites, energy storage, flexible sensors, etc.10 14 Corre- asphaltenes are composed of carbon ( 77 wt %), hydrogen − − − spondingly, the presence of appropriate surface area, excellent (5 8 wt %), nitrogen (1 3 wt %), sulfur (1 4 wt %), oxygen − conductivity, and mechanical flexibility has attracted more (4 7 wt %), and traces of metals (e.g., nickel, vanadium, interest toward the utilization of these materials as a promising electrocatalyst for water-splitting applications.15,16 However, Received: March 25, 2019 the possibility of bulk production of these materials requires Accepted: July 10, 2019 sophisticated technique and expensive raw materials, which Published: July 10, 2019

© 2019 American Chemical Society 27697 DOI: 10.1021/acsami.9b05309 ACS Appl. Mater. Interfaces 2019, 11, 27697−27705 ACS Applied Materials & Interfaces Research Article etc.).24,25 Still, their molecular structure is not clearly understood, but the widely accepted model is the island molecular structure in which asphaltenes are composed of a core of seven fused aromatic rings with several aliphatic chains attached to the central core.23,26 Few exciting studies have been reported previously, explaining possible methods of synthesizing carbon-based products such as fullerene,27,28 carbon microspheres,29 carbon nanotubes,30 and graphene − sheets,21 23 by utilizing petroleum asphaltene. However, the asphaltene-derived carbon materials were popularly studied for applications such as Li-ion batteries,26,30,31 supercapacitors,32,33 carbon dioxide capture and reduction,34 etc. Never- theless, to mention, there were no clear reports explaining methods of the development of an asphaltene-derived catalyst for water-splitting applications. The presence of a graphene- like backbone via π−π and hydrogen-bonding interactions will modify the local electron density of neighboring carbon atoms and facilitates enhanced hydrogen evolution. Inspired by this strategy, we designed an experiment to synthesize graphene- like nanosheets utilizing asphaltene as an ideal supramolecular carbon precursor. Similarly, the generation of green fuel “Hydrogen” via electrochemical water splitting using low-cost industrial byproducts or wastes with higher efficiency would be considered as a value-added material following wealth from the waste approach. Figure 1. Characterization of 3h-N-GNS (a) X-ray diffraction (XRD), (b) Raman spectrum, (c) X-ray photoelectron spectroscopy (XPS) ■ RESULTS AND DISCUSSION survey spectrum with a high-resolution scan for (d) core-level carbon C(1s), (e) core-level nitrogen N(1s), and (f) core-level oxygen O(1s) In this article, we explored a simple method for the preparation spectra. The peaks are deconvoluted into component curves. of activated porous carbon (A-PAC) and nitrogen-doped graphene-like nanosheets (N-GNS) with an interconnected hierarchical porous structure using asphaltene extracted from crude oil as the precursor. The detailed method of the formation of A-PAC and N-GNS and the effect of temperature and nitrogen doping at different time intervals were described in the experimental section. In brief, the nitrogen-doped carbon samples were prepared by the heat treatment of asphaltene at varying temperatures (700, 800, and 900 °C) and intervals (1, 3, and 5 h) of ammonia (NH3) gas treatment. Importantly, we demonstrated methods to functionalize the graphene nanosheets (GNS) with nitrogen atoms and employed them as a potential candidate for water-splitting applications. In comparison, the nonactivated 3 h nitrogen- doped GNS (3h-N-GNS) displayed the best performance due to the presence of higher proportion of nitrogen atoms, porosity, and conductivity and their characterization results are shown in Figure 1a−f. The thermogravimetric analysis (TGA) of the pristine asphaltene was carried out under an inert condition to understand their evolution process (Figure S1). A major weight loss was observed between 400 and 600 °C, which was primarily due to the disintegration of volatile organic molecules, aliphatic chains, and other residual matters beyond which the graphitization process occurs leading to the formation of a porous carbon network. This observation was supported by scanning electron microscopy (SEM) results Figure 2. Structural characterization of derived carbons: (a, b) SEM (Figure 2a−d) showing the formation of porous carbons. ff image of 3h-N-GNS, (c, d) SEM image of A-PAC, (e) transmission The powder X-ray di raction (PXRD) pattern of the derived electron microscopy (TEM) image of 3h-N-GNS showing graphene- 3h-N-GNS (Figure 1a) displays two peaks around 2θ = 24 and like sheets, and (f) TEM image of 3h-N-GNS displaying an increase 43°, which corresponds to the presence of (002) and (101) in the interlayer distance due to doping of nitrogen atoms. reflections.35,36 The observed plane resembles crystalline graphitic carbon, and this observation coincides with the sheets. The observed d-spacing (0.36/0.37 nm) with a high-resolution TEM (HRTEM) result (Figure 2f). Moreover, significant degree of disorder is due to the distortion in the the presence of highly crystalline (002) peak suggests the derived carbon material, which arises due to the doping of formation of a regularly ordered graphitic carbon in graphene nitrogen atoms. A similar observation was previously reported

27698 DOI: 10.1021/acsami.9b05309 ACS Appl. Mater. Interfaces 2019, 11, 27697−27705 ACS Applied Materials & Interfaces Research Article for nitrogen-doped nanocarbons.35,37 XRD studies of pristine defects, which, in turn, is expected to provide strong chemical asphaltene and asphaltene-derived porous activated carbons reactivity for the redox chemistry and acid−base reactions to (A-PACs) are shown in the supporting information (see Figure occur. The insertion of heteroatoms such as nitrogen creates S2a). defect densities requiring the rearrangement of neighboring Raman analysis of the 3h-N-GNS sample indicates two carbon atoms and hence their electronic behavior changes. − intense bands at around 1380 and 1595 cm 1, as shown in Furthermore, the existence of defect chemistries on the surface − Figure 1b. The peak observed at 1380 cm 1 corresponds to the of carbon helps in anchoring functional groups as active sites, D-band (A1g mode), which is ascribed to the disordered while the graphitic arrangement holds active sites providing structure, or the edge defects associated with the breath mode excellent thermal stability. of the graphitic lattice vibrations. The G-band observed at The surface morphology and roughness of the formed ∼ −1 1595 cm corresponds to the active (E2g mode) of the porous carbons were examined using scanning electron 2 crystalline graphitic carbon originating from sp -bonded microscopy (SEM) (Figure 2a−d), and detailed studies were carbon atoms. The ID/IG intensity ratio of the D- and G- carried out using high-resolution transmission electron bands is observed to be 0.93 representing a higher number of microscopy (HRTEM) (Figures 2e,f and S6). The lower- structural and ordering defects. The disordering between the magnification SEM image (Figure 2a) of the derived 3h-N- graphene layers occurs primarily due to the presence of GNS shows a uniformly layered texture displaying an array of pyridinic-, pyrrolic-, and quaternary-type nitrogen function- 35 interconnected micron-sized layers (marked in red lines), and alities, as well as due to the existence of noncarbon species. consequently, the higher-magnification SEM (Figures 2b and The Raman analysis of asphaltene and A-PAC is discussed in S6)confirms the presence of well-aligned layers. The the supporting information Figure S2b, and the nitrogen- autonomous formation of regularly arranged interconnected doped samples are shown in Figure S10b. layers along with a combination of pores helps in the X-ray photoelectron spectroscopy (XPS) was carried out on enhancement of the catalytic activity of the material.41,42 both pristine asphaltene and 3h-N-GNS to reveal the surface Besides, the combinatorial effect of microlayers, pores, and the chemical state and coordination of carbon (C), nitrogen (N), nitrogen doping in N-GNS is found to increase the number of and oxygen (O) atoms. The atomic percentage of 3h-N-GNS active sites and thereby provides easy migration of channels (Figures 1c, S10a and Table S2) was found to consist of C between actives sites and the dissolved hydrogen molecules. (83.5%), N (6.2%), and O (5%), whereas the pristine Hence, we envisioned that this layered porous carbon can act asphaltene (Figure S3 and Table S2) exhibited C (93.2%), as an ideal candidate for hydrogen evolution reaction (HER). N (1%), O (3%), and S (2.9%). The deconvoluted C 1s On the other hand, the SEM image of the A-PAC (Figure spectra of 3h-N-GNS (Figure 1d) shows two peaks at binding 2c,d) shows a highly arranged uniform three-dimensional energies of 284.8 and 285.8 eV. The peak at 284.8 eV can be porous honeycomb network with a high density of fractures assigned to the presence of nonoxygenated carbon atoms such and structural defects, whereas the pristine asphaltene indicates as C−C/CC bonds present in the aromatic rings of sp2 37 dense, nonporous, micron-sized particles (Figure S5). Notably, graphitic carbon. Another peak at 285.8 eV corresponds to the lower- and higher-magnification SEM images of the A-PAC the presence of functional groups such as C−OH/C−O−Cor the presence of sp2-hybridized carbon atoms bonded to (Figure 2c,d) clearly depict the presence of long-range nitrogen in the aromatic ring. On the other hand, the honeycomb structure (highlighted in red color) with macro- deconvoluted N(1s) spectra show peaks at binding energies pore architecture and cracks. Nevertheless, the presence of a of ∼397, 398, and 400 eV, suggesting the presence of pyridinic, higher population of macropores and structural cracks in A- pyrrolic, and quaternary nitrogen, respectively (Figures 1e, PAC is found to lead toward discontinuities in transport and S10a and Table S4). The possible arrangement of the nitrogen chemical properties and the possible deterioration of the atoms is shown in Figure S4. It is seen that the 3h-N-GNS overall properties, thereby limiting their catalytic activity (Figure 2d).43 The HRTEM studies (Figures 2e,f and S6) exhibits a higher percentage of nitrogen atoms (Table S4); fi especially, the proportion of pyridinic nitrogen was found to be show the presence of signi cant crystallinity in the derived 3h- higher when compared to those of 1 and 5 h nitrogen- N-GNS, as well as the occurrence of ultrathin graphene-like functionalized carbon samples. Hence, we selected the 3h-N- sheets (Figure 2e). Well-resolved lattice fringes with an ∼ GNS as the ideal catalyst and compared the catalytic interplanar spacing of 0.37 nm match closely with the d- fl performance with the pristine asphaltene. The treatment of spacing for the (002) re ection planes of graphite, indicating a higher degree of graphitization. The process of nitrogen asphaltene with NH3 gas leads to the impregnation of nitrogen atoms into the carbon framework and changes their electronic functionalization transforms the morphology of amorphous fi behavior. Furthermore, the NH3 treatment modi es amor- asphaltene into crystalline, interconnected layered, porous 2D phous asphaltene into uniformly layered carbons with graphene nanosheets. Similarly, the observed crystallinity approximately 6.2% of nitrogen doping (Figure 1e and Table coincides well with the PXRD results. S2). It is well known that the pore architecture in the catalyst In general, the pyridinic nitrogen atoms present on the edges controls the transport phenomenon and governs the selectivity of graphitic planes bonded to two carbon atoms are considered in reactions.44 Therefore, properties, such as pore volume and − to be active sites for the catalytic process.38 40 However, the pore size distribution, are considered as an essential parameter exact catalytic role for each of the nitrogen forms in catalysts is for developing efficient catalysts. The presence of pores helps still uncertain. The 3h-N-GNS sample exhibits oxygen (Figure significantly in anchoring micro- and nanoparticles, thus 1f) peaks at 531 and 533 eV corresponding to the presence of providing channels for facile migration of foreign matters and CO and −O−COO− functional groups. The presence of byproducts.45 Hence, we evaluated the derived carbon samples − nitrogen and oxygen heteroatoms on the 3h-N-GNS carbon using N2 adsorption desorption isotherms to determine their framework eventually leads to more structural and ordering surface area and pore size distribution (Figure 3a−d).

27699 DOI: 10.1021/acsami.9b05309 ACS Appl. Mater. Interfaces 2019, 11, 27697−27705 ACS Applied Materials & Interfaces Research Article − a mixture of micro meso-sized pores helps in the rapid N2 uptake at the high-pressure range, which makes 3h-N-GNS as an interesting candidate for catalytic applications. We further used t-plot to evaluate the micro- and mesopore volumes in the derived A-PAC and 3h-N-GNS, and their results are shown in Table 1. For superior catalytic performance, nanoscale porosity

a Table 1. BET Analysis Results of A-PAC and 3h-N-GNS

SBET Smic Vt Vmic Dpore samples (m2/g) (m2/g) (cm3/g) (cm3/g) (nm) A-APC 2824 497 2.01 0.27 2.84 3h-N-GNS 243 125 0.16 0.05 2.65 a Dpore is the average pore diameter. Vt is the total pore volume calculated at p/p0 = 0.99.

is highly desirable, facilitating higher mass transfer fluxes and Figure 3. Brunauer−Emmett−Teller (BET) measurement of the extraordinary active loading. Moreover, the surface of carbon is − porous carbons: (a) N2 adsorption desorption isotherms of 3h-N- uniformly layered and rich in defects, which leads to an − GNS, (b) pore size distribution of 3h-N-GNS, (c) N2 adsorption outstanding catalytic performance. desorption isotherms of A-PAC, and (d) pore size distribution of A- Apart from porosity measurement through physisorption PAC. studies, we evaluated the electrochemical porosity of the derived 3h-N-GNS samples using potentiodynamic polar- The specific surface area of the A-PAC was found to be 2824 ization technique. Electrochemical porosity measurement m2/g, while the 3h-N-GNS shows significantly less surface area evaluates the real active surface area of the electrode and ∼ 2 calculates the molecular packing at both the inner and outer of 243 m /g. Interestingly, both carbons are found to exhibit 49−51 type IV isotherms (Figure 3a,c), which are typical for a surfaces. Hence, we carried out potentiodynamic polar- mesoporous material with a multilayer adsorption capability.46 ization measurements on both the substrate (i.e., glassy carbon The key feature in both the samples is the increase in volume (GC)) and the 3h-N-GNS sample (see Figure S7). The electrochemical porosity of 3h-N-GNS is found to be 4.6%. It adsorbed at a higher p/p0, which is caused due to the fl adsorption in the mesopores, as well as the observation of the demonstrates the in uence of pyridinic, pyrrolic, and hysteresis category. Furthermore, a distinct increase in the quaternary nitrogen on the electrochemically active sites in 3h-N-GNS. Thus, from the geometrical and electrochemical adsorbate volume in the lower p/p0 region indicates the presence of more micropores coupled with mesopores. porosity measurements, it is clear that 3h-N-GNS has favorable fi electrochemically active microstructures, which facilitates a Speci cally, a careful investigation of the 3h-N-GNS samples ffi (Figure 3b) displayed a higher percentage of adsorbate at a conductive pathway for e cient HER. To determine the electrical conductivity and charge-transfer kinetics, electro- relatively lower pressure p/p0 (0.02), displaying a greater proportion of micropore distribution.47 Both the carbons chemical impedance spectroscopy was studied at an alternating displayed a significant adsorbate uptake starting at around current (AC) amplitude of 10 mV and sweeping the frequency 0.4p/p and they further increase in the adsorbed volume from 1 MHz to 1 mHz. The obtained Nyquist plot is shown in 0 Figure 4a and is interpreted as an equivalent circuit (inset of reaching 0.95p/p0. This increase in the adsorbed volume at a Figure 4a) with compiled solution resistance (Rs), charge- higher p/p0 was also due to the capillary condensation, which occurs below the condensation pressure of the adsorbates. Typically, the capillary condensation is considered as a secondary process that requires preformation of an adsorbed layer onto the walls of the pores, which are formed due to the multilayer adsorption. The A-PAC and 3h-N-GNS carbons displayed pore size maxima of 1.9 and 1.6 nm, respectively. However, the 3h-N-GNS samples exhibit a greater quantity of ultra-micropores (<7 Å) in association with super-micropores (7−20 Å).48 The pore size distribution of the A-PAC carbon (Figure 3d) shows the presence of a higher number of mesopores (2−50 nm) with an average micropore width of 1.83 nm (Horvath−Kawazoe method) and an approximate pore width of 19.2 nm (Barrett−Joyner−Halenda (BJH) method). On the other hand, the 3h-N-GNS samples displayed the presence of a higher proportion of microporous (<2 nm) and few mesoporous carbons with an average pore width of 0.4 nm. The BJH pore size distribution plot supports this observation, displaying an average pore width of <5 nm. The hierarchical pores range from less than a nanometer to a few Figure 4. (a) Nyquist plot; (b) AC conductivity; (c) Mott-Schottky nanometers with inter- and intralayers implying a dense plot of asphaltene and 3h-N-GNS sample; and (d) Summary of packing of 3h-N-GNS layers. Correspondingly, the presence of Nyquist plot, DC conductivity, and Mott Schott analysis.

27700 DOI: 10.1021/acsami.9b05309 ACS Appl. Mater. Interfaces 2019, 11, 27697−27705 ACS Applied Materials & Interfaces Research Article transfer resistance (Rct), and constant phase element (Q). The Figure 5a shows linear sweep voltammograms of asphaltene, capacitance (C) at a space charge layer is estimated using 3h-N-GNS, glassy carbon (GC), and benchmark platinum at a Brugg’s equation, and the values are displayed in Table S3. scan rate of 1 mV/s in an alkaline medium. On fitting, the charge-transfer resistance reduces noticeably by 3 orders of magnitude for 3h-N-GNS compared to that for asphaltene, while the capacitance increases significantly. This observed higher capacitance and decreased Rct further support the presence of an enormous number of electrocatalytic active sites in 3h-N-GNS and their electrical conductivity.52 This observation demonstrates the importance of pyridinic and pyrrolic nitrogen atoms embedded in the crystalline graphene nanosheets. We further calculated the electrochemical active surface area (ECSA) by impedance method.53 We observed μ higher ECSA CDL (0.88 F) for 3h-N-GNS than for A-PC (0.08 μF), which might be due to the effect of doping of nitrogen atoms, which predominantly increased the active sites assisting electron transfers. Additionally, the presence of highly delocalized electron clouds on the conjugated carbon increases conductivity, enhancing active sites and facilitating superior electrochemical reactions. Thus, based on BET and ESCA measurements, nitrogen doping was found to notably influence both the geometric and electrochemical active surface areas of Figure 5. Linear sweep voltammograms of GC, asphaltene, 3h-N- the derived 3h-N-GNS. Consequently, the ultra- and super- GNS, and Pt in 0.1 M KOH (a), the corresponding steady-state microporous structures contributed toward the mass transfer of polarization curves (b), and cyclic stability and static stability (c, d) of + H ion and H2O. 3h-N-GNS toward HER, respectively. Figure 4b shows AC conductivity vs frequency of asphaltene and 3h-N-GNS sample. The conductivity observed at a lower- From Figure 5a, it is clear that asphaltene and GC display σ frequency region is assigned as DC conductivity ( DC) since intrinsically poor HER performance and exhibit activity only at both samples follow the universal power law.54 Analysis of 3h- higher-onset potentials of ∼737 and ∼705 mV, respectively. σ N-GNS shows a 2-fold increase in DC compared to that of However, the 3h-N-GNS sample shows catalytic activity at a asphaltene. Therefore, it is evident that crystallinity, meso- lower-onset potential (∼403 mV) and at a current density of porosity, and active electrochemical sites (nitrogen atoms) of 10 mA/cm2. To further evaluate the HER mechanism of 3h-N- 3h-N-GNS assist in easy electron transfer and transport. To GNS samples, Tafel plot (Figures 5b and S9) is charted, and calculate the charge carrier density and flat band potential of the Tafel slope and exchange current density are determined the prepared samples, Mott−Schottky analysis55 is carried out using the linear region of LSV. Asphaltene shows a higher at a frequency of 1 Hz (see Figure 4c,d). The 3h-N-GNS is Tafel slope (231 mV/dec) than the substrate GC (289 mV/ found to possess high electron density (1.31 × 1025 cm−3), dec), while the 3h-N-GNS sample exhibits a low Tafel slope compared to asphaltene (2.01 × 1019 cm−3) which shifts the (115 mV/dec) and a higher exchange current density (1.35 flat band potential toward the conduction band in 3h-N-GNS mA/cm2), which suggests that the rate-determining step is the (−1.21 V), while asphaltene shows −1.52 V. Besides, Volmer−Heyrovsky mechanism (electrochemical desorp- asphaltene shows a stronger capacitance dependence, which tion).57 The above observations reveal the outstanding HER is a typical behavior of a space charge layer-controlled activity of the 3h-N-GNS samples. capacitance of a poor conductor exposed in an electrolyte.56 The extended catalytic activity of the 3h-N-GNS sample was On the other hand, 3h-N-GNS exhibits weak capacitance investigated using potential cycling technique, which is dependence, which is a weak Helmholtz layer-dominated conducted from −0.1 to −0.7 V vs RHE in 0.1 M Hg/HgO capacitance and signifies the characteristic of metals. This at a scan rate of 10 mV/s (see Figure 5c). The 3h-N-GNS metallic transition might be possibly due to the presence of retained their catalytic activity even after 1000 cycles, with a enhanced conductivity in the 3h-N-GNS. The results of the negligible decrease in current density; this discloses the Nyquist plot, DC conductivity, and Mott Schott analysis are extended cyclic stability of the prepared sample. Besides, we summarized in Figure 4d, which consolidates the fact of measured the potentiostatic stability of 3h-N-GNS using the enhanced carrier concentration and conductivity observed in chronoamperometric electrolysis technique under a static 3h-N-GNS in comparison to that in asphaltene. Comprehen- potential of 650 mV, as shown in Figure 5d. The measurement sively, the presence of layered channels and a combination of displayed a stable cathodic current density of 6.9 mA/cm2 for 4 interconnected micro- and mesopores in 3h-N-GNS samples h duration, and this observed result is comparable to other are found to enhance the electrical conductivity and electro- non-noble catalysts (Table S5). Overall, the electrochemical chemical reactions. To further evaluate the HER activity, linear stability studies such as cyclic sweeping, static polarization, sweep voltammogram (LSV) was conducted in a three- confirm the long-term sustainability of the developed electro- electrode system, and the potential sweep was carried out des. from 0 to −1 V vs reversible hydrogen electrode (RHE) in 0.1 ff ■ CONCLUSIONS M NaOH. The e ect of temperature and duration of NH3 gas treatment on asphaltene is systematically studied (Figure S8), Layered, nitrogen-doped porous carbon was prepared by a ° and essentially, functionalization with NH3 gas at 900 C for 3 simple method utilizing petroleum asphaltene as the precursor. h is found to significantly improve the HER activity. The functionalization of asphaltene was carried out at a

27701 DOI: 10.1021/acsami.9b05309 ACS Appl. Mater. Interfaces 2019, 11, 27697−27705 ACS Applied Materials & Interfaces Research Article ff di erent temperature and by varying the duration of NH3 nitrogen functionalization of the asphaltene. The doping time treatment. The most important feature of the derived N-GNS intervals varied between 1, 3, and 5 h, and the samples are denoted fi as 1h-N-GNS, 3h-N-GNS, and 5h-N-GNS, respectively (see Table is embedding them with a signi cant percentage of nitrogen ff fl and oxygen heteroatoms, which are arranged in different S1). After the reaction, the NH3 gas was switched o , and Ar ow was turned on, and finally, the reactor was cooled down to room positions on the carbon framework. XPS analysis of N-GNS fi temperature. The derived products were washed with 10 mL of 1 M con rms the existence of oxygen and nitrogen atoms with the HCl and an excess quantity of DI water, followed by washing with possibility of the presence of a higher amount of pyridinic ethanol and acetone until the filtrate reached a pH of 7. The collected nitrogen sites. The presence of pyridinic and graphitic nitrogen powders were dried overnight at 110 °C. atoms, with surface defects, helps in charge delocalization and Characterization. XRD analysis was carried out using a Rigaku migration of electrons. D/Max Ultima II powder XRD. Raman spectra of the samples were Furthermore, the microscopic analysis of the surface of the studied at room temperature using a JY Horiba LabRAM HR. The fi excitation radiation of 514.5 nm was used with the Lexel SHG 95 activated carbons shows a signi cant amount of fracture, μ cracks, and irregular distribution of pores, while the N-GNS argon-ion laser. A laser power of 28 W was used at an excitation wavelength of 514 nm. The average size of basal planes (La)of sample resulted in the formation of uniformly layered derived carbons is calculated using the equation L = 4.4 nm × I /I , structures. The high-resolution TEM image of N-GNS a G D fi where ID and IG are the intensities of D-band and G-band observed in displayed the presence of graphene nanosheets with signi cant its Raman spectrum, respectively. TEM images were collected on a edge planes. Importantly, the BET analysis of the N-GNS JEOL 2100 F TEM with 200 kV. XPS analysis was carried out using a sample reveals the presence of ultra- and super-micropores PHI Quantera X-ray photoelectron spectrometer with a chamber − with an average pore width of ∼0.4 nm. The presence of an pressure of 5 × 10 9 Torr and an Al cathode as the X-ray source. The even distribution of fine pores, microlayers, and heteroatoms source power was set at 100 W and pass energies were of 26.00 eV for on the surface of the functionalized carbon is found to the core-level scans. The SEM analysis was taken using an FEI Quanta significantly improve the electrical conductivity and catalytic 400 microscope. The elemental analysis of asphaltene and nitrogen- doped carbon samples was carried out using the Costech elemental activity of the material. On consideration of the sustainable combustion system to study the mass percentage of nitrogen, carbon, nature of the derived carbon materials, growth is anticipated in and hydrogen. From each experiment, three samples were analyzed, the application of this material to design and develop various and the average values were tabulated. Differential scanning other functional catalysts for a wide range of applications. calorimetric (DSC) analysis of asphaltene was carried out using TA Instruments (Q600 simultaneous TGA/DSC) under the nitrogen ■ EXPERIMENTAL SECTION atmosphere at a flow rate of 50 mL/min. The heating was carried out ° ° − − Extraction of Asphaltene from Crude Oil. About 150 mL of from 25 to 1000 C at a rate of 15 C/min. Brunauer Emmett fi crude oil A (from the Gulf of Mexico) was centrifuged at 10 000 rpm Teller (BET) was used to analyze the porosity and speci c surface for 30 min to remove water, sediments, and inorganic impurities. After area of the formed carbons. The device quantachrome autosorb-3b centrifugation, the supernatant fluid was transferred to a 1 L beaker, BET surface analyzer was used for the measurement. Before analysis, ° and then n-pentane was added to the mixture at a ratio of 5:1 followed all of the samples were evacuated and degassed for 24 h at 200 C by stirring. The beaker containing the mixture was sealed and allowed under vacuum to remove moisture. to age for 1 day in a cold and dark area. After aging, the mixture was Electrochemical Measurements. Electrochemical measure- centrifuged further to separate the precipitated asphaltenes. The ments like electrochemical impedance (Nyquist plot, AC conductivity − deposited asphaltenes were wrapped in a 0.2 μm nylon filter paper measurement, Mott Schottky plot), linear sweep voltammograms, and transferred into a paper thimble. Next, the thimble, containing the chronoamperometry (cyclic and static stabilities), and porosity deposits, was placed in a Soxhlet extraction apparatus and washed measurements were studied using a three-electrode system with Pt with n-pentane for 2 days to remove all impurities and non-asphaltene wire and Hg/HgO (0.1 M NaOH) as the counter and reference materials from the sample. After washing, the remaining and purified electrodes, respectively. The working electrode was prepared by solid asphaltenes were dried in a vacuum oven at 110 °C for 24 h. The coating the prepared catalyst on a polished glassy carbon electrode by properties of crude oil are shown in the supporting information (see a drop-casting method. The nonactivated 3h-N-GNS was used as the Table S6). catalyst, and the results are compared with the pristine asphaltene. fl Synthesis of A-PAC. About 0.2 g of asphaltene was dissolved in The catalyst and poly(vinylidene uoride) are mixed in a 9:1 ratio, 8.5 mL toluene, and in another container, 1 g of potassium hydroxide combined with N-methyl-2-pyrrolidone, and then ground to form a (KOH) pellets was dissolved in 1.5 mL of deionized (DI) water. homogeneous paste. The formed homogeneous paste was coated on These two mixtures were combined and sonicated in an ultrasonic the polished GC and dried overnight in a vacuum desiccator. bath for 10 min. To further enhance mixing, the samples were The reference electrode potential (EHg/HgO) is converted to a homogenized using a SCILOGEX D-160 homogenizer at approx- reversible hydrogen electrode (ERHE), using the following equation imately 20 000 rpm for 10 min. The prepared emulsion was EERHE=++ Hg/HgO 0.165V 0.059pH (1) transferred to a porcelain crucible and heated to 110 °C to evaporate the solvents. The dried samples were then transferred to a tube The 0.1 M NaOH aqueous electrolyte displayed a pH of 13. furnace, heated to 700, 800, and 900 °C, and maintained for 30 min The AC conductivity (σ) is obtained from the impedance spectra with a continuous flow of nitrogen gas. After 30 min, the heating was using the relation54 stopped, and the furnace was allowed to cool to reach the room Z′ temperature. The formed carbon products were washed with 10 mL σ = of 1 M hydrochloric acid (HCl) and an excess amount of DI water, ZZ′+22 ″ (2) fi followed by washing with ethanol and acetone until the ltrate ′ ″ reached a pH of 7. The collected powder was dried overnight at 110 where Z and Z represent the real and imaginary parts of Z, ° respectively. C in an oven. The derived carbon samples are abbreviated as A-PAC fl at 700 °C, A-PAC at 800 °C, and A-PAC at 900 °C (see Table S1). The carrier density (ND) and at band potential (EFB) are calculated by the potential-dependent capacitance measurement using Functionalization of Asphaltene. A porcelain boat containing − 56 asphaltene powders was placed in the center of a quartz tube furnace Mott Schottky analysis followed by purging with argon (Ar) gas for 30 min, and then the 2 ° 2C kTB reactor was heated to reach 900 C. After 20 min of heating, the Ar ND =−−()EEFB ff eAεε 2 e (3) gas was switched o , and NH3 gas was purged into the quartz tube for 0 Ä É Å Ñ 27702 Å Ñ DOI: 10.1021/acsami.9b05309 Å ACS Appl.Ñ Mater. Interfaces 2019, 11, 27697−27705 ÇÅ ÖÑ ACS Applied Materials & Interfaces Research Article

ε ε × −19 where e, 0, and are the elementary electron charge (1.6 10 C), Ashokkumar Meiyazhagan: 0000-0002-9765-4347 × −12 permittivity of vacuum (8.86 10 F/m), and dielectric constant of Xiang Zhang: 0000-0003-4004-5185 sample (2.5), respectively; E and EFB are the applied bias potential fl Robert Vajtai: 0000-0002-3942-8827 and built-in voltage ( at band potential), respectively; kB, T, and A are the Boltzmann constant (1.38 × 10−23 J/K), temperature (298 K), Francisco M. Vargas: 0000-0001-5686-5140 and surface area of the electrode, respectively. Author Contributions Electrochemical porosity (P) is determined by potentiodynamic The manuscript was written through contributions of all polarization measurement. In short, polarization resistance is authors. All authors have given approval to the final version of measured around the free corrosion potential, and P is obtained using the relation58 the manuscript. Author Contributions R p ⊥ s J.S. and S.E. contributed equally to this work. P =×−Δ10Ebcorr / a R p (4) Notes The authors declare no competing financial interest. where Rps and Rp are the polarization resistance of the GC substrate and catalyst, respectively. Δ ff ■ ACKNOWLEDGMENTS Ecorr is the di erence in free corrosion potential between the catalyst and the bare GC substrate, and ba is the Tafel slope (anodic) One of the authors S.J. thanks the Department of Science and of the substrate. Technology for the DST-INSPIRE Fellowship award and Electrochemical active surface area (ECSA) of samples is calculated Bhaskara Advanced Solar Energy (BASE) Internship. from the double-layer capacitance (CDL) using the equation ■ REFERENCES ECSA= CCDL / s (5) fi (1) Loh, K. P.; Bao, Q. L.; Eda, G.; Chhowalla, M. Graphene Oxide where Cs is the speci c capacitance or the capacitance of an atomically as a Chemically Tunable Platform for Optical Applications. 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27705 DOI: 10.1021/acsami.9b05309 ACS Appl. Mater. Interfaces 2019, 11, 27697−27705