Nitrogen-rich carbon nano-onions for oxygen reduction reaction

Accepted Manuscript Nitrogen-rich carbon nano-onions for oxygen reduction reaction Kuntal Chatterjee, Meiyazhagan Ashokkumar, Hemtej Gullapalli, Yongji Gong, Robert Vajtai, Palanisamy Thanikaivelan, Pulickel M. Ajayan PII:

S0008-6223(18)30061-7

DOI:

10.1016/j.carbon.2018.01.052

Reference:

CARBON 12793

To appear in:

Carbon

Received Date: 26 September 2017 Revised Date:

8 January 2018

Accepted Date: 14 January 2018

Please cite this article as: K. Chatterjee, M. Ashokkumar, H. Gullapalli, Y. Gong, R. Vajtai, P. Thanikaivelan, P.M. Ajayan, Nitrogen-rich carbon nano-onions for oxygen reduction reaction, Carbon (2018), doi: 10.1016/j.carbon.2018.01.052. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Nitrogen-Rich Carbon Nano-Onions for Oxygen Reduction Reaction

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Kuntal Chatterjee,‡†* Ashokkumar Meiyazhagan,‡* Hemtej Gullapalli,‡ Yongji

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Gong,‡ Robert Vajtai,‡ Thanikaivelan Palanisamy § and Pulickel M. Ajayan‡*

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Nitrogen-Rich Carbon Nano-Onions for Oxygen Reduction Reaction

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Kuntal Chatterjee,‡†* Ashokkumar Meiyazhagan,‡* Hemtej Gullapalli,‡ Yongji

‡

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Gong,‡ Robert Vajtai,‡ Thanikaivelan Palanisamy § and Pulickel M. Ajayan‡*

Department of Material Science and NanoEngineering, Rice University, Houston,

Texas 77005, United States

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Department of Physics and Technophysics, Vidyasagar University, Midnapore,

721102, India

Advanced Materials Laboratory, CSIR-Central Leather Research Institute, Adyar,

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Chennai 600020, India

*Corresponding author(s). E-mail: [email protected] (Ashokkumar Meiyazhagan); [email protected] (Kuntal Chatterjee) and [email protected] (Pulickel M. Ajayan)

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ABSTRACT

The demand for economical, sustainable and highly efficient catalysts to replace the Pt-based

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catalysts for proper industrialization of oxygen reduction reaction (ORR) has gained tremendous scientific interest. Herein, we report a facile strategy to develop doped nitrogen-rich carbon nano-onion architectures from the renewable biological resource, collagen, for use as a metal-

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free ORR catalyst. The product contains an appreciably high percentage of nitrogen (7.5%) integrated into the carbon molecular skeleton. The materials exhibit outstanding ORR

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electrocatalytic activity with low onset potential, high current density, superior methanol crossover immunity and better durability than the benchmark Pt/C catalyst in alkaline medium. The ORR followed an efficient direct reduction pathway of 4-electron transfer kinetics. The performance analysis of different samples demonstrates that the porous graphitic carbon with

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more nitrogen content provides adequate active catalytic sites for ORR activity and the pyridinic nitrogen act as an effective promoter for ORR. The findings ascertain that renewable biomasses can be easily transformed into novel carbon nanostructures with excellent catalytic activity.

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KEYWORDS: Biomass wastes; doping; Carbon; Fuel cell; Electrocatalysis

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1. INTRODUCTION Catalysts for cathodic oxygen reduction reaction (ORR) are the essential components of fuel

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cells and metal–air batteries.[1-4] Owing to the relatively low over potential and high current density, platinum (Pt) and its alloys remain as the most common ORR catalysts. However, several concerns such as prohibitive cost, scarcity of Pt, severe intermediate tolerance, anode

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crossover, etc. impose limitations on the development of fuel cell technologies.[5-7] Hence, the search for low-cost and high-performance ORR electrocatalysts is currently underway. In this

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context, low-cost carbon-based metal-free ORR electrocatalysts have gained significant attention due to their excellent electrocatalytic activity and superior durability.[8-11] Different forms of carbon-based materials such as activated carbon,[12] porous carbon,[13,

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carbon nanotubes,[15]

and graphene[16] are actively pursued to maximize the electroactive surface area of catalysts and to improve their catalytic activity and durability. Recent studies have shown an enhanced

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electrocatalytic performance in the nitrogen-doped carbon nanostructures for ORR applications in alkaline medium.[17-20] This significant improvement in performance is mainly due to the presence of abundant, free-flowing sp2 hybridized π electrons. Besides, the presence of lone

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electron pairs of nitrogen atoms helps in connecting the delocalized conjugated system[9, 21, 22]

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thereby additionally improving the electrocatalytic activity of ORR to a greater extent. Nitrogen doping of carbon materials can be achieved by two conventional methods

namely ‘in-situ doping’ which is carried out during the synthesis of carbons and ‘post doping’ through post–treatment of pre-synthesized carbon nanostructures.[22-24] However, these procedures commonly require high energy consumption, expensive hardware, and multi-step processes, which eventually limit their practical applicability. To overcome these drawbacks, utilization of N-containing biomass wastes such as proteins can be advantageous due to their

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easy availability and simplicity in synthesizing N-doped carbonaceous materials.[25-30]

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synthesis of a porous microscopic structure with abundant nitrogen contents and suitable atomic N-C-N bonding motifs from natural resources was first reported by Yang et al. utilizing

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carbonized nucleobases.17 Iwazaki et al. used silk fibroin derived steam activated carbons for ORR applications.[31] Significant efforts have been made recently to synthesize carbon nanostructures using biomass resources.[28,

32-35]

For example, collagen, the most abundant

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extracellular protein present in mammalian connective tissues can be easily extracted from the animal skin wastes, and it has the potential to be an effective precursor for the synthesis of

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carbon nanostructures. Tropocollagen, the collagen monomer, has a unique structure in which three polypeptide chains form left-handed helices and are supercoiled into a right-handed triple helix. Recently, we reported a simple method for the synthesis of N-enriched carbon nanostructure using the collagen biomass wastes for Li-ion battery applications.[35] Herein, we

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report a facile method for the synthesis of N-doped carbon nano-onion (N-CNO) from collagen waste source and its high electrocatalytic activity towards ORR in alkaline media. Electrochemical measurements showed that the electrocatalytic activity (i.e.) ORR activity of the

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derived carbon samples is substantially comparable to that of commercial 20% Pt/C catalyst. Also, we observed that the best performance was achieved from the carbon sample prepared at

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750°C for 8 h with the onset potential of -50 mV vs. Ag/AgCl when compared to 81 mV for Pt/C. The sample with the highest concentration of pyridinic nitrogen among the series shows an efficient 4e– transfer ORR mechanism with the current density almost comparable to Pt/C. Additionally, the N-CNO based catalyst exhibited excellent durability performance in alkaline medium and superior methanol tolerance, when compared to the commercial Pt/C.

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2. RESULTS & DISCUSSION The detailed method of preparation of N-CNO carbon samples is presented in the experimental

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section. Typically, proteinaceous goat skin wastes were used as a precursor for the formation of N-CNO. The carbonaceous samples were prepared at three different heat treatments such as 750°C for 8 h, 1000°C for 4 h and 1000°C for 8 h, and named as 750-8, 1000-4 and 1000-8

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respectively followed by acid treatment. Powder X-ray diffraction pattern of 750-8 N-CNO sample shows peaks around 2θ = 26° and 44° (Figure 1a), which can be assigned to (002) and

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(101) diffraction peaks of crystalline graphitic carbon (JCPDS PDF#00-041-1487). Evidently, the samples treated at higher temperatures namely 1000-4 and 1000-8 displayed a greater extent of crystallinity (Figure S1a). It is noteworthy to mention that the (002) diffraction peak is broad and centered at 2θ = 26.2o, which corresponds to an increase in the interlayer spacing (d 002) than the d-spacing of graphite (3.36 A° originated from 2θ = 26.5o).[36] The increase in (d 002)

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spacing may be due to the self-doping of the hetero atom[32] or owing to the curved surface of the carbon materials as also reported by other researchers.[37]

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Raman spectra of the as-prepared carbon samples show two broad bands at ~1326 and 1590 cm−1 as displayed in Figure 1b and Figure S1b. These broad bands correspond to the

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“disordered” carbon (D mode), and the ‘graphitic’ carbon (G mode) originating from the sp2 bonded carbon atoms.[38] The calculated ratio of intensities between these modes (ID/IG), a measuring parameter for disorder in graphitic materials,[39] is 0.84. The relative strength of the D band is a result of the structural defects and edge plane exposure caused by the incorporation of the heterogeneous N atom into the graphitic layers.[34] Moreover, the full-width half maxima (FWHM) of the D band has a considerably larger value of 87 cm-1 signifying the defect-rich

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nature of the N-CNO sample. The center of the G band (1590 cm-1) matches well with the

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previous reports of carbon nano-onion.[40, 41]

Figure 1. Structural and morphological characterization of 750-8 N-CNO sample. Powder X-ray diffraction pattern (a) and Raman spectrum (b) of N-CNO sample. Low magnification (c) and

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high magnification (d) TEM of the 750-8 N-CNO sample. HRTEM showing a top view (e) and side view (f) of carbon onion structure.

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TEM images of the N-CNO 750-8 sample are shown in Figure 1c to 1f. Higher magnification TEM image (Figure 1d) illustrates the presence of carbon onion-like structure. It is seen that the particles are hollow in interior and wrapped by multiple layers of nested concentric

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graphite shells. The high-resolution top view (Figure 1e) and side view (Figure 1f) of the derived carbon onion like particles, provide a much clear vision of the graphitic lamella. The increase in

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interlayer distance compared to the pristine graphitic layer is in agreement with the XRD and Raman results. The microstructure of the other two carbon samples (1000-4 and 1000-8) also show (Figure S3) the nano-onion like carbon structures implying that heat treatment in the present case does not change the nature of the microstructure appreciably. Similar observations

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made in our previous study as well.[35]

XPS technique was used to investigate the stoichiometric composition and chemical states of the derived N-CNO samples. The atomic distribution of the N-CNO sample treated at

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750oC for 8 h (Figure 2a) found to consist of carbon (75.2%), nitrogen (7.5%) and oxygen (17.3%). The C1s spectra (Figure 2b) displays three peaks around 284, 285 and 288 eV. The

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peak at ~284 eV corresponds to the presence of sp2 hybridized graphitic carbon in aromatic rings. The peak at ~285 eV is due to the presence of hetero atom, probably nitrogen as also verified below, in sp2 hybridized carbon. Another peak at ~288 eV suggests the presence of C=O (carbonyl) or due to carbon bonding with nitrogen functionalities.[28, 34, 42, 43] On the other hand, the nitrogen spectra, shown in Figure 2a and Figure S4(a) and S4(b), clearly reveal the presence of three different N configurations namely pyridinic-N, amino-N and pyrrolic-N with peaks originating around 398, 399, and 400.5 eV, respectively.[42, 44] The nitrogen content of 750-8

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N-CNO sample (7.5%) is substantially high compared to the other biomass derived carbon samples.[45-48] We noticed the increase in heating temperature and withheld duration leads to a decrease in the nitrogen content from 7.5 (750-8) to 3.1% (1000-8). Figure 2d precisely

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demonstrates the qualitative and quantitative variation of the nitrogen atoms in three different

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samples.

Figure 2. XPS spectra of 750-8 N-CNO sample. XPS survey spectrum (a) with a high-resolution scan for C(1s) (b) and N(1s) (c) core level peaks. The peaks are deconvoluted into component curves. (d) The content of different N species in three different N-CNO samples

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The cyclic voltammetry (CV) of the derived N-CNO samples in oxygen and argon atmosphere is shown in Figure 3a and Figure S5. The CV scan was carried out in Ar and O2 saturated 0.1 M KOH solution from 0.2 to -1.0 V vs. Ag/AgCl at a scan rate of 50 mV s−1 to

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study the electrocatalytic performance of the N-CNO samples. From Figure 3a, it is evident that the cathodic reduction peak originates at −0.32 V (vs. Ag/AgCl electrode) in O2 saturated KOH solution, whereas only capacitive behavior was seen in the case of Ar saturation. This

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observation indicates that the N-CNO has a prominent catalytic activity for ORR. To further evaluate the kinetics of the ORR at the N-CNO electrode, linear sweep voltammetry (LSV) was

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recorded in an O2 saturated 0.1 M KOH electrolyte using a rotating disk electrode (RDE) at a scan rate of 10 mV s-1. Figure 3b shows the LSV taken at 1000 rpm for the three N-CNO samples along with the benchmarking catalyst, 20% Pt/C. Interestingly, the 750-8 N-CNO sample with highest nitrogen content exhibits superior ORR activity than the other N-CNO

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samples and also the produced current density is comparable to that of benchmark Pt/C catalyst. The onset potential 50 mV (vs. Ag/AgCl) at 10 mA/cm2 is even smaller than the Pt/C. The onset potentials, as well as the current densities, are relatively inefficient for the 1000-4 and 1000-8

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samples.

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Figure 3c shows ORR polarization curves of the 750-8 N-CNO sample at different rotating rates. The current densities were normalized to the geometrical area (0.196 cm2). As expected, the catalytic current density increases with increasing electrode rotating rate due to the shortened diffusion layer.[17] The corresponding steady-state diffusion plateau currents as analyzed through Koutecky−Levich (K−L) plots at −0.5, −0.6, −0.7, −0.8 and −0.9 V are shown in Figure 3d. The linearity of the K-L plots and near parallelism of the fitting lines suggest firstorder reaction kinetics of oxygen molecule reduction and similar electron transfer numbers for

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ORR at different potentials.[46] Electron transfer number (n) per oxygen molecule at the working electrode was calculated using the K−L equation[47] and found to be about 4.1, clearly indicating one-step 4 electron transfer ORR process. The result is similar to ORR catalyzed by a

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high-quality commercial Pt/C catalyst measured in the same 0.1 M KOH electrolyte solution.[15] The current densities at the same rotating rate for 1000-4 and 1000-8 samples show lower values

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compared to the 750-8 N-CNO sample (Figure S6).

Figure 3. Electrocatalytic characterization of N-CNO sample. (a) CV of 750-8 N-CNO catalyst in O2 or Ar saturated 0.1 M KOH electrolyte. (b) RDE polarization curves derived from three different heat treated samples and commercial Pt/C catalyst. (c) Voltamperograms for oxygen

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reduction of 750-8 N-CNO in O2 saturated 0.1 M KOH at various rotation speeds with a scan rate of 10 mV/s. (d) K-L plots at different potentials derived from (c). Inset of (d) shows electron

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transfer number at various voltages calculated from K-L plot. The main driving force promoting ORR activity is the presence of pyridinic nitrogen atoms, which act as active sites besides providing net positive charge on adjacent carbon atoms,

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facilitating oxygen adsorption as well as electron attraction from the anode. Additionally, the presence of a high surface area of the hollow onion like graphitic carbon assists in the better

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electrocatalytic activity of 750-8 N-CNO sample. Rotating-ring disk electrode (RRDE) was employed to study the reaction mechanism and to measure the amount of H2O2 produced by the above process. Figure 4a shows the RRDE voltammograms of 750-8 N-CNO sample which was carried out in 0.1 M KOH solution saturated with O2 at a rotation rate of 1000 rpm. The electron

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transfer number involved in the ORR can be calculated using the following equation.[48] n = 4 Idisk / (Idisk + Iring / N)

(1)

Where Iring is the Faradaic ring current, Idisk is the Faradaic disk current, and N is the collection

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efficiency, (i.e.) 0.37. The measured H2O2 with a yield less than 22% and the corresponding

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electron transfer number calculated via the above equation are shown in Figure 4b. The result is in good agreement with those obtained from the K−L plots (Figure 3d) that are based on the RDE measurements, confirming that ORR kinetics for the present N-CNO sample is mainly through a 4e– pathway. Compared to other two N-CNO samples of the present study and earlier reported biomass derived samples, the excellent electrocatalytic performance of 750-8 N-CNO sample towards ORR can be attributed to its higher pyridinic-N content and mesoporous architecture, which provides a high density of active sites and plenty of mass diffusion channels

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for ORR. The presence of a significant surface area with interconnected porosity (Figure S7)

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facilitates in superior catalytic activity.

Figure 4. Performance assessment of 750-8 N-CNO catalyst for ORR reaction kinetics and

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methanol tolerance and durability. (a) Rotating ring disk electrode (RRDE) linear sweep voltammograms of 750-8 N-CNO sample at 1000 rpm rotation speed. (b) % H2O2 and electron transfer number of 750-8 N-CNO catalyst during ORR. (c) Current density−time responses at −0.4 V in 0.1 M KOH on 750-8 N-CNO and Pt−C electrode (900 rpm) followed by the introduction of 3 M methanol. (d) Chronoamperometry of 750-8 N-CNO and Pt/C at -0.4 V in O2 saturated 0.1 M KOH.

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The performance of the catalyst was further evaluated for mechanistic and kinetic performance using Tafel plots (Figure S8). The Tafel slope was estimated by the LSV results (illustrated in Figure 3b) for the 750-8 N-CNO catalyst sample. In the low current density region,

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the Tafel slope value of 89 mV per decade, is closer to that of the Pt/C catalyst. This observation is in good agreement with the Temkin conditions for adsorption of intermediates.[49] The Tafel slope in the higher current density region is 288 mV/decade for the 750-8 N-CNO sample, and it

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is comparable to that of Pt/C surface. Here, the result can be attributed to a change in the mechanism of ORR from Temkin to Langmuir adsorption conditions with the increasing current

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density. Such phenomenon has been reported earlier.[10, 49] From the scientific perspective, it is implied that the ORR mechanisms on the synthesized 750-8 N-CNO sample are quite similar to that of Pt-based catalysts in an alkaline medium. The

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electrocatalyst

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tested

for

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crossover

by

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chronoamperometric responses in comparison to the commercial Pt/C catalyst (Figure 4c). After the addition of 3M methanol, the ORR current for N-CNO is slightly changed and regained almost the same current within a short time, while Pt/C catalyst suffers from sharp decrease as a

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result of the mixed potential. These results indicate that, beyond the excellent electrocatalytic

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activity, the N-CNO catalyst has outstanding immunity towards methanol crossover poisoning, thereby overcomes another main challenge faced by metal-based catalysts in fuel cells. Electrochemical stability of the electrode is one of the key parameters, and that was tested under a constant potential of −0.4 V in O2 saturated 0.1 M KOH at an electrode rotation rate of 1000 rpm. Figure 4d exhibits an excellent operation stability of the catalyst in alkaline medium with only about 10% decrease in current after 6 h. In comparison, the 20% Pt/C catalyst showed a pronounced decrease in activity when tested under similar conditions (~25%, Figure 4d). The

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high current retention for the present sample is also in good rivalry with most of the reported Ndoped carbon-based catalysts.[22, 50-54] The durability of the sample was measured from the LSV graph initially and after 3000 cycles of a potential run. The decrease in half-wave potential is

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only 15 mV (Figure 5b) signifying excellent durability and stability of the catalyst. The tolerance of the 750-8 N-CNO electrode to CH3OH crossover effect, frequently encountered in fuel cells, has also been tested and confirmed by the almost identical CV spectra measured in O2 saturated

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0.1 M KOH solution before and after adding 3 M methanol (Figure 5a). Therefore, the results convincingly demonstrate that the N-CNO catalyst derived from collagen waste through a simple

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synthetic protocol is an excellent choice for direct fuel cells operated in alkaline conditions.

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Figure 5. Tolerance and stability test for 750-8 N-CNO catalyst (a) CVs of 750-8 N-CNO catalyst in O2 saturated (blue), and in 3 M methanol plus O2 saturated (red) 0.1 M KOH with a scan rate of 1000 mV/s. (b) RDE polarization curves of 750-8 N-CNO catalyst with a scan rate of 10 mV/s before and after 3000 potential cycles in O2 saturated 0.1 M KOH. 3. CONCLUSION

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In summary, we have outlined the transformation of natural waste material namely collagen into nitrogen-rich carbon nano-onion structure by a simple synthesis technique. The synthesized nitrogen rich graphitic carbon materials demonstrate excellent electrocatalytic performance

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towards ORR in alkaline medium. The best performance is revealed from the 750-8 sample with the relatively highest number of pyridinic-N sites, and the measurement shows notable onset potential and appreciably high current density with efficient 4 electron pathway for the ORR.

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Moreover, the catalyst shows superior long-term durability and higher methanol crossover resistance compared to the standard Pt/C catalyst. The process of self-doping and tailoring of the

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nitrogen concentration and bond type in a graphitic nano carbon, using a natural resource, demonstrated in this study could be applied to the design and development of various other functional catalysts. Besides, the practical importance of the derived N-CNO electrodes for ORR

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application.

ASSOCIATED CONTENT

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Supporting Information: The Supporting Information is available free of charge on the

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Publications website at DOI:

Additional details such as Materials & Method, Characterizations, XRD, Raman, SEM, TEM, XPS, Cyclic Voltammograms, LSV, BET and Tafel Plots of the derived N-CNO materials (PDF)

ACKNOWLEDGMENT

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MA, RV & PMA thank Air Force Office of Scientific Research (Grant No. AFOSR FA9550-131-0084) and Fred L. Hartley Family Foundation for financial assistance. KC acknowledges UGC-RAMAN support from UGC, India. PT thanks, CSIR, India for financial assistance under

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ZERIS Project (CSC0103).

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