Process for the preparation of nitrogen doped carbon nanohorns for oxygen reduction electrocatalysis

Abstract

Nitrogen doped carbon nanohorns function as efficient metal-free oxygen reduction electrocatalysts for anion exchange membrane fuel cells. The disclosure relates to a process for the preparation of nitrogen doped carbon nanohorns with enhanced conductivity and improved surface area.

Claims

1. A process for preparation of nitrogen doped carbon nanohorns for use in oxygen reduction reactions (ORR) with enhanced conductivity and improved surface area consisting of: (a) pre-treating the carbon nanohorns; (b) annealing the carbon nanohorns of step (a) in the presence of a nitrogen source at 500-1200 C. for 1-3 hours; wherein the pre-treating of the carbon nanohorns results in functionalization of the carbon nanohorns, wherein the functionalization is carried out by mixing hydrogen peroxide with carbon nanohorns; and wherein the nitrogen doped carbon nanohorns are co-doped with a metal selected from the group consisting of Fe and Co.

2. The process according to claim 1, wherein the carbon nanohorns are single walled carbon nanohorns.

3. The process according to claim 1, wherein the nitrogen source is selected from the group consisting of urea and melamine.

4. The process according to claim 1, wherein a surface area of nitrogen doped carbon nanohorns ranges from 300 to 1500 m.sup.2 g.sup.1.

5. The process according to claim 1, wherein a conductivity of the nitrogen doped carbon nanohorns is in the range of 5-9 S cm.sup.1.

6. The process according to claim 1, wherein nitrogen doped carbon nanohorns having a surface area in the range of 300 to 1500 m.sup.2 g.sup.1 and a conductivity in the range of 5-9 S cm.sup.1 are prepared by the method.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts Scheme for synthesis of nitrogen doped single walled carbon nanohorns (NCNH).

(2) FIG. 2 shows HR-TEM images of (a) SWCNH, (b) FCNH and (c) & (d) N-800 under different magnifications.

(3) FIG. 3 shows plots corresponding to the changes in the electrical conductivity and surface area of the prepared carbon nanohorns samples. Surface area of nitrogen doped samples is increasing with annealing temperature.

(4) FIG. 4 (a) Cyclic voltammograms of N-800 recorded in nitrogen and oxygen saturated 0.1 M KOH solution at a scan rate of 50 mV s.sup.1 at an electrode rotation rate of 900 rpm. Glassy carbon electrode and Hg/HgO were used as the counter and reference electrodes respectively (b) Linear sweep voltammograms of the CNH samples and Pt/C with an platinum loading on 102 g cm.sup.2 in 0.1 M oxygen saturated KOH at a rotation speed of 1600 rpm and a scan rate of 10 mV s.sup.1. (c) K-L plots of N-600, N-800 and N-1000 at a potential of 0.22 V vs. Hg/HgO. The plots are generated from the LSVs of all the three samples conducted in oxygen saturated 0.1 M KOH solution at different rotation speeds. Theoretical K-L plots for n=4 and n=2 are also given in the plot (d) The number of electrons transferred versus the potential as calculated from the K-L plots.

(5) FIG. 5 (a) Methanol crossover study of N-800 and Pt/C at a rotation speed of 1000 rpm at 0.05 V. At 300 s; 3 M methanol was added into 0.1 M KOH electrolyte to evaluate the crossover effect. (b) Linear sweep voltammograms of N-800 before and after ADT in 0.1 M oxygen saturated KOH at a rotation speed of 1600 rpm and a scan rate of 10 mV s.sup.1. (c) Steady state polarization plot of anion exchange membrane fuel cell (AEMFC) with N-800 as cathode catalyst taken at 50 C.

(6) FIG. 6 depicts X-ray diffraction of SWCNH, FCNH, C-800 and N-800.

(7) FIG. 7 depicts Raman spectra of SWCNH, FCNH, C-800 and N-800.

(8) FIG. 8 (a) Nitrogen adsorption-desorption isotherm of FCNH, C-800, N-600, N-800 and N-1000. All nitrogen doped samples show Type II isotherms whereas CNH without doping shows Type IV isotherm. (b) Pore size distribution of FCNH, C-800, N-600, N-800 and N-1000.

(9) FIG. 9 depicts corresponding to the changes in surface area of the prepared carbon nanohorns samples without nitrogen doping (SWCNH, FCNH) annealed at different temperatures (C-800 and C-1000 to the FCNH samples annealed at 800 and 1000 C. respectively). Knotting of nanowindow of FCNH is happening at 1000 C., which leads to the reduction in the surface area.

(10) FIG. 10 depicts deconvoluted XPS spectra of N1s of (a) N-600, (b) N-800, (c) N-1000; (d) the estimated values of the different types of nitrogen in all the annealed samples.

(11) FIG. 11 depicts linear sweep voltammograms of Pt/C before and after ADT in 0.1 M oxygen saturated KOH at a rotation speed of 1600 rpm and a scan rate of 10 mV s.sup.1. Glassy carbon electrode and Hg/HgO were used as counter and reference electrode respectively.

(12) FIG. 12 depicts Steady state polarization plot of anion exchange membrane fuel cell (AEMFC) with Pt/C as cathode catalyst at 50 C.

(13) FIG. 13 depicts (a) Linear sweep voltammograms of the NCNH prepared by annealing FCNH and melamine mixture, FeNCNH and Pt/C with an platinum loading on 60 g cm.sup.2 in 0.1 M oxygen saturated KOH at a rotation speed of 1600 rpm and a scan rate of 10 mV s.sup.1. (b) Linear sweep voltammograms of FeNNCNH before and after ADT in 0.1 M oxygen saturated KOH at a rotation speed of 1600 rpm and a scan rate of 5 mV s.sup.1.

DETAILED DESCRIPTION OF THE INVENTION

(14) The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

(15) The present invention provide a process for the preparation nitrogen doped carbon nanohoms such with enhanced conductivity and improved surface area comprising following steps: (a) Pre-treating the carbon nanohorns; (b) Annealing the carbon nanohorns of step (a) in the presence of a nitrogen source and optionally in the presence of a metal at 500-1200 C. for 1-3 hours.

(16) The process for the preparation nitrogen doped carbon nanohorns as described above, wherein the carbon nanohorn is preferably single walled carbon nanohoms and source of nitrogen is selected from urea, melamine and such like. The metal is selected from Fe and Co.

(17) The process for the preparation nitrogen doped carbon nanohorns as described above, wherein surface area of nitrogen doped carbon nanohorns is ranges from 300 to 1500 m.sup.2 g.sup.1.

(18) SWCNH is an assembly of hundreds of carbon nanohorns with diameter around 60 to 80 nm. Each nanohorn has a diameter around 3-4 nm. Surface area of this material varies from 300 to 400 m.sup.2/g.

(19) Functionalization of SWCNH improved its surface area from 325 to 1384 m.sup.2/g and it further increased to 1836 m.sup.2/g after nitrogen doping. Nitrogen doping further improved the electric conductivity and it prevented the knotting of pores. The usually observed problem of pore knotting in SWCNH is overcome by the disclosed process.

(20) While the HR-TEM images indicate that annealing at higher temperature in the presence of argon atmosphere does not make substantial deformation in their morphology (Refer FIG. 2), EDAX analysis shows the presence of nitrogen in the samples. Amount of nitrogen is varying with the annealing temperature. N-600 shows the highest weight percentage of nitrogen (9.31 wt. %) and the nitrogen content reduces with increase in the temperature. N-800 has 7.42 wt. % nitrogen while N-1000 has 6.37 wt. %.

(21) In an aspect of the invention, the ORR of the synthesized compositions was studied. Nitrogen in the synthesised composition has pyridinic coordination which is responsible for the reduction in the over potential of ORR. N-800 (SWCNH annealed at 800 C. with urea) shows higher activity towards ORR compared to other NCNH (annealed at 600 and 1000 C. with urea) and undoped nanohorns. N-800 reduces oxygen molecule to hydroxide ion through a four electron pathway in alkaline medium. N-800 shows 50 mV higher over potential towards ORR compared to Pt/C.

(22) Among the different nitrogen doped systems, N-800 shows the highest conductivity (9.61 S cm.sup.1) compared to N-600 (7.39 S cm.sup.1) and N-1000 (7.35 S cm.sup.1). The lower conductivity of N-1000 compared to N-800 may be attributed to its high surface area, but still this value is higher than that of C-800 SWCNH annealed at 800 C. without urea), FCNH and SWCNH. The conductivity of nanohorn without doping is in the order of C-800 (7.07 S cm.sup.1)>SWCNH (6.57 S cm.sup.1)>FCNH (4.95 S cm.sup.1). This indicates that the functionalization of SWCNH (FCNH) along with the enhancements in the surface area decreases the conductivity of the material but its annealed product (C-800) attains enhanced conductivity due to the removal of the functional moieties.

(23) NCNH prepared from FCNH and melamine mixture showing better ORR activity compared to same prepared from FCNH and urea mixture. Onset potential (0.7 V vs Hg/HgO) of NCNH is almost same as that of commercial Pt/C. After Fe coordination with NCNH (FeNCNH) ORR activity further improved which is even higher than Pt/C in terms of onset potential (0.1 V vs Hg/HgO) and half wave potential (0.026V vs Hg/HgO). Moreover, ADT (accelerated durability test) analysis shows that ORR activity of FeNCNH is increasing with increasing potential cycle compared Pt/C. Excellent ORR activity of FeNCNH is mainly attributed to the FeNC coordination and high surface area of the electrocatalyst. Refer FIG. 13. The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention.

EXAMPLES

(24) Following examples are given by way of illustration therefore should not be construed to limit the scope of the invention.

Example 1: Pre-Treatment of Carbon Nanohorns

(25) 2 g of SWCNH were mixed well with 100 ml methanol solution in order to remove its puffy nature. After filtrating this mixture, black powder were dried under vacuum at a temperature 80 C. for complete removal of methanol and resulting nanohorns were treated as pure SWCNH. 2 g of resulting SWCNH were mixed with 250 ml of 30% hydrogen peroxide in a round bottom flask and refluxed for 5 hours at 60 C. After functionalization, resulting solution were filtered and washed several time with deionized water for the complete removal of hydrogen peroxide. Resulting carbon nanohoms cakes were dried under vacuum for 12 h at temperature 80 C. This material was treated as functionalized single wall carbon nanohoms (FCNH).

Example 2: Synthesis of N-600

(26) 50 mg of FCNH was mixed with 250 mg urea using mortar and pestle followed by annealing at 600 C. for one hour in argon atmosphere. Resulting material was used as NCNHs referring to as N-600 without any purification.

Example 3: Synthesis of N-800

(27) 50 mg of FCNH was mixed with 250 mg urea using mortar and pestle followed by annealing at 800 C. for one hour in argon atmosphere. Resulting material was used as NCNHs referring to as N-800 without any purification.

Example 4: Synthesis of N-1000

(28) 50 mg of FCNH was mixed with 250 mg urea using mortar and pestle followed by annealing at 1000 C. for one hour in argon atmosphere. Resulting material was used as NCNHs referring to as N-1000 without any purification

Example 5: Reference Example

(29) For comparison FCNH was annealed at 800 C. for 1 h without urea and was named as C-800.

Example 6: Synthesis of Nitrogen Doped Carbon Nanohorns Using Melamine (NCNH)

(30) 900 mg of melamine powder was first dissolved in 30 ml of distilled water by sonicating for 15 minutes followed by the addition of 300 mg functionalized single walled carbon nanohorn at temperature (at room temperature (25 C.). After complete mixing of melamine and single walled carbon nanohorn, the solvent was evaporated at 70 C. The obtained powder was annealed at high temperature (900 C.) in argon atmosphere for 3 hour in order to get nitrogen doped single walled carbon nanohoms. Morphology of nanohorns after high temperature annealing is intact and it has a spherical morphology with size in between 60-90 nm. Surface area of NCNH is 1327 m.sup.2/g which is less compared to NCNH prepared using FCNH and urea mixture. This reduction in surface area is mainly attributed the deposition of carbon derived from melamine during high temperature annealing on NCNH. Total nitrogen content in nanohorn is 2.2 wt. % which is compared to the NCNH prepared using urea. However, ORR activity of NCNH prepared using melamine as nitrogen source showing comparable onset potential with that of commercial 40% Pt/C.

Example 7: Synthesis of Fe Co-Doped Nitrogen Doped Carbon Nanohorns (FeNCNH)

(31) 900 mg of melamine powder was first dissolved in 30 ml of distilled water by sonicating for 15 minutes followed by the addition of 300 mg functionalized single walled carbon nanohorn at room temperature (25 C.). This process was followed by addition of 18 mg FeCl.sub.3. Continuous sonication was preferred so that the reactants get well dispersed in the solution. The resultant mixture was kept for continuous stirring at 70 C. till the whole water content got evaporated. The dried mixture was annealed at 900 C. in argon atmosphere for 3 hours. Annealed mixture was subjected to acid washing by sonicating it for 30 minutes in con. HCl followed by filtration. Filtrate was kept for drying in hot oven at 60 C. surface area 1315 m.sup.2 g-.sup.1. The surface areas of FeNCNH-900 and NCNH are almost comparable but are 4 times higher than that of SWCNH. However, the surface areas of NCNH and FeNCNH are found to be lower, which is assumed to be due to the deposition of carbon on the surface of nanohorn during the decomposition of melamine.

Example 8: Characterization of FCNH

(32) Morphology of SWCNH was analysed using high resolution transmission electron microscopy (HR-TEM) which is shown in FIG. 2. From FIG. 2a, SWCNH are assembled and bundled to form a dahlia like morphology, having a size of around 60-80 nm. After functionalization, some morphology variations, even though not so prominent, occur due to the formation of functional groups as well as the generation of micro and mesopores (FIG. 2b). No coalescence is observed after functionalization and the individual bundles and their petals of FCNH remain intact like untreated SWCNH. This clearly indicates that annealing at higher temperature in the presence of argon atmosphere does not make substantial deformation in their morphology.

Example 9: Comparative Data

(33) TABLE-US-00001 TABLE 1 Onset potential ORR of some of the non metal electrocatalyst recently reported. Over Catalyst potential S. loading and Onset Pt loading compared to No. References Material scan rate potential (g cm.sup.2) Pt/C 1 Wang et N-doped 10 g, scan 0.1 V vs Bulk 0.1 V al.sup.[S5] graphene rate is not Ag/AgCl Platinum ACS Nano given. 2011, 5, 4350-4358 2. Qu et al.sup.[S6] N-quantum 285 g/cm.sup.2, 0.16 V vs 57.14 0.13 V J. Am. dot 10 mV/s Ag/AgCl Chem. Soc. supported 2012, 134, graphene 15-18 3. Niu et al.sup.[S7] Nitrogen 20 g/cm.sup.2, 0.16 V vs Not given 0.14 V J. Mater. doped 10 mV/s Ag/AgCl (20% Pt/C) Chem. 2012, graphene 22, 6575-6580. 4. Chen et Nitrogen 10 g, 0.140 V vs Not given 0.07 V al.sup.[S8] doped 20 mV/s Hg/Hg.sub.2Cl.sub.2 (20% Pt/C) Adv. Mater. carbon 2013, 25, nanotubes/graphene 3192-3196. 5. Lin et al.sup.[S9] Pyridinic Not given, 10 mV/s 0.3 V vs Pt disk 0.3 V J. Mater. nitrogen Ag/AgCl Chem. 2011, doped 21, 8038-8044. graphene 6 Dai et al.sup.[S10] Functionalized 10 g, 0.12 V vs Not given 0.09 V ACS Nano graphene 10 mV/s SCE 2011, 5, 6202-6209 7 Hu et al.sup.[S11] Boron doped 102 g cm.sup.2, 0.35 V vs Not given 0.15 V Angew. carbon 10 mV/s SCE (20% and Chem. Int. nanotube 40% PtC) Ed. 2011, 50, 7132-7135 8 Dai et al.sup.[S12] Polyelectrolyte 10 g, 0.12 V vs Not given 0.09 V J. Am. functionalized 10 mV/s SCE Chem. Soc. CNT 2011, 133, 5182-5185. 9 Osaka et N-doped 480 g, 0.1 V vs Not given 0.1 V al.sup.[S13] carbon 10 mV/s Ag/AgCl (10% Pt/C) Chem. nanocapsules Commun. 2011, 47, 4463-4465 10 Peng et Phosphorous 159 g/cm.sup.2, 0.1 V vs 159.1 0.1 V al.sup.[S14] doped 10 mV/s Ag/AgCl Angew. graphite Chem. Int. layers Ed. 2011, 50, 3257-3261 11 Cho et Fe/Fe.sub.3C 286 g/cm.sup.2, 0.02 V vs 9.5 and 28.6 0.07 V al.sup.[S15] functionalized 10 mV Hg/HgO Angew. melamine Chem. Int. Ed. 2013, 52, 1026-1030 12 Present Nitrogen 255 g/ cm.sup.2, 0.026 V vs 102 0.05 V work doped 10 mV/s Hg/HgO carbon nanohorns

(34) TABLE-US-00002 TABLE 2 Onset potential ORR of some of the Fe based non-precious electrocatalyst recently reported. Over Catalyst potential loading and Onset Pt loading compared to References Material scan rate potential (g cm.sup.2) Pt/C 1 Wu et al.sup.[51] Fe.sub.3O.sub.4/N- 51 g/cm.sup.2, 0.19 V vs 51 0.15 V (Pt J. Am. GAs 10 mV/s Ag/AgCl onset is not Chem. Soc., given. 2012, 134, Compared 9082 with their own ACS nano Paper) 2. Parvez et NG/Fe 50.91 g/cm.sup.2, 0.04 V vs 50.91 0 V al.sup.[S2] 10 mV/s Ag/AgCl ACS Nano 2012, 6, 9541 3. Yin et al.sup.[S3] Fe.sub.xN/NGA 51 g/cm.sup.2, 0 V vs 51 0 V Adv. Funct. 10 mV/s Ag/AgCl Mater. 2014, 24, 2930 4. Xiang et CCOPPFe 200 g/cm.sup.2, 0.98 V vs 200 0 V al.sup.[S4] 5 mV/s RHE Angew. Chem. Int. Ed. 2014, 53, 2433 5. Hu et al.sup.[S5] Fe.sub.3C/C-800 600 g/cm.sup.2, 1.05 V vs 50 0 V Angew. 10 mV/s RHE Chem. Int. Ed. 2014, 53, 36753 6 Dai et al.sup.[S6] Functionalized 10 g, 0.12 V vs Not given 0.09 V ACS Nano graphene 10 mV/s SCE 2011, 5, 6202 7 Liang et FeN-CNT- 80 g, 5 mV/s Not given 80 g 0.018 V al.sup.[S7] OPC DOI: 10.1002/adma. 201401848 8 Dai et al.sup.[S8] SN-OMC-4 306 g/cm.sup.2, 0.05 V vs Not given 0.01 V ACS Appl. 10 mV/s Ag/AgCl Mater. Interfaces 2013, 5, 12594-12601 9 Cho et al.sup.[S9] Fe/Fe.sub.3C 286 g/cm.sup.2, 0.02 V vs 9.5 and 28.6 0.07 V Angew. functionalized 10 mV Hg/HgO Chem. Int. melamine Ed. 2013, 52, 1026 10 Present FeNCNH- 500 g/cm.sup.2, 0.026 V vs 60 0.03 V more work 900 5 mV/s Hg/HgO positive than Pt/C

ADVANTAGES OF THE INVENTION

(35) 1. A facile method to synthesize NCNHs in bulk level 2. Potential cost effective, metal-free cathode catalyst for polymer electrolyte membrane fuel cells. 3. High durability. 4. Possesses economic advantages over prevailing Pt catalyst.