Process for the synthesis of nitrogen-doped carbon electro-catalyst

Abstract

The present invention discloses a process for the synthesis of nitrogen-doped carbon electrocatalyst with good electrochemical stability and fuel tolerance for oxygen reduction reaction (ORR) by pyrolysis of protein-rich pulse flour cooked with SiO2 nanoparticles.

Claims

1. A process for the synthesis of nitrogen-doped carbon electro-catalyst, the process comprising the steps of: (a) boiling a protein rich precursor in excess of deionized (DI) water to obtain a slurry or in a colloidal silica nanoparticle dispersion in water, wherein said dispersion is boiled up to dryness to obtain a dried product; (b) filtering the slurry of step (a) through a Whatman filter paper to obtain a filtered product; (c) pyrolyzing the filtered product of step (b) or the dried product of step (a) to obtain a pyrolyzed product wherein the pyrolysis is carried out at 800-1100 C. for 2-4 hours; (d) treating the pyrolyzed product of step (c) with concentrated hydrofluoric acid [HF] to remove oxide impurities to obtain a porous carbon product; and (e) washing the porous carbon product of step (d) with deionized water by centrifugation and drying in oven followed by pyrolysis at 800-1100 C. for 2-4 hours to obtain the nitrogen-doped carbon electro-catalyst having nitrogen doping in the range of 0.8 to 1.25%, wherein pore size of said electro-catalyst is in the range of 10 to 300 , surface area of said electro-catalyst is in the range of 600-800 m.sup.2/g.

2. The process as claimed in claim 1, wherein the protein rich precursor of step (a) is selected from the group consisting of gram flour and soyabean.

3. The process as claimed in claim 1, wherein size of said colloidal silica nanoparticle is 12 nm.

4. The process as claimed in claim 1, wherein the pyrolysis is carried out in argon atmosphere.

5. The process as claimed in claim 1, wherein the pyrolysis is carried out at 1000 C. for 4 hours.

6. The process as claimed in claim 1, wherein pore size of the Whatman filter paper in step (b) is 0.2 m.

7. The process as claimed in claim 1, wherein the oxide impurity is silica dioxide.

8. A nitrogen-doped carbon electro-catalyst comprising mesoporous carbon having nitrogen doping in the range of 0.8 to 1.25%, wherein pore size of said electro-catalyst is in the range of 10 to 300 , surface area of said electro-catalyst is in the range of 600-800 m.sup.2/g.

9. A method of using the nitrogen-doped carbon electro-catalyst as claimed in 8 in an oxygen reduction reaction.

Description

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

(1) FIG. 1. Diagrammatic representation of the synthesis protocol

(2) FIG. 2. a) TEM image of pure gram flour powder boiled in water, b) TEM image of SiO.sub.2 nanoparticle uptake in the expanded gram flour matrix, c, d) Nitrogen doped mesoporous carbon (GFMC3); the best performing catalyst, e) SEM image of the porous carbon (GFMC3, f) FE-SEM image of GFMC3 at higher resolution showing the sheet-like graphitic morphology.

(3) FIG. 3. a) XRD patterns of the food waste derived carbon catalysts b) Resistance measurement of 50 mg pellet at a pressure of 1 tonne of the carbon catalysts

(4) FIG. 4. a) BET surface area of the GFMC catalysts b) Pore-size distribution of the high surface area GFMC3

(5) FIG. 5. (a,b,c) N1 s XPS spectra of GFMC1, GFMC2, GFMC3 respectively d) N atom doping percentage

(6) FIG. 6. C1s XPS spectra of a) GFMC2 b) GFMC3

(7) FIG. 7. a) Cyclic Voltammogram of GFMCs in 0.1 M KOH at 5 mV s.sup.1 scan rate b) LSV studies of the GFMC catalysts in 0.1 M oxygen saturated KOH with an electrode rotation of 1600 rpm and scan rate is 5 mV s.sup.1. For both experiments, reference electrode is Hg/HgO and graphite rode as counter electrode.

(8) FIG. 8. a) K-L plots of the GFMCs at a potential of 0.47 V vs Hg/HgO. Plots are derived from the LSV at different rotation speed. b) Number of electron transfer calculated from the slope of K-L plot.

(9) FIG. 9. a) Kinetic current density of the GFMCs Calculated from the slope of K-L plot b) Methanol Tolerance studies of GFMC3 at a rotation speed of 1000 rpm in oxygen saturated 0.1 M KOH. At 500 s, 3M methanol was added in to the electrolyte in order to evaluate the cross over effect.

(10) FIG. 10. Pore size distribution of GFMC1

(11) FIG. 11. Pore size distribution of GFMC2

(12) FIG. 12. Adsorption isotherms for GFMCs

(13) FIG. 13. a) Durability of Pt/C catalyst b) Durability of GFMC3

(14) FIG. 14. Raman Spectroscopy of GFMCs

(15) FIG. 15. XPS analysis of N1s in SBMC1

(16) FIG. 16. Pore size distribution of SBMC1

(17) FIG. 17. TEM image of SBMC1

(18) FIG. 18. a) LSV at different rotations b) KL plot for SBMC1 c) Kinetic current density plots d) No. of electrons transferred

DETAILED DESCRIPTION OF THE INVENTION

(19) 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.

(20) In an embodiment, the present invention provides a process for synthesis of a high performance metal-free, nitrogen doped carbon electrocatalyst by a process involving silica templating through nanoparticle uptake followed by double pyrolysis of food waste with remarkable electrocatalytic activity towards ORR and high tolerance towards fuel like methanol with excellent electrochemical stability.

(21) In a preferred embodiment, the present invention provides a process for the synthesis of nitrogen-doped carbon electro-catalyst from food waste comprising the steps of: (a) boiling a protein rich precursor in excess of deionized (DI) water to obtain a slurry or in a colloidal silica nanoparticle dispersion in water, wherein said dispersion is boiled up to dryness to obtain a dried product; (b) filtering the slurry of step (a) through a Whatman filter paper to obtain a filtered product; (c) pyrolyzing the filtered product of step (b) or the dried product of step (a) to obtain a pyrolyzed product wherein the pyrolysis is carried out, at 800-1100 C. for 2-4 hours; (d) treating the pyrolyzed product of step (c) with concentrated hydrofluoric acid [HF] to remove oxide impurities to obtain a porous carbon product; and (e) washing the porous carbon product of step (d) with deionized water by centrifugation and drying in oven followed by pyrolysis at 800-1100 C. for 2-4 hours to obtain the nitrogen-doped carbon electro-catalyst.

(22) The present invention provide a process for the synthesis of nitrogen-doped carbon electro-catalyst from food waste wherein the food waste is protein rich precursors preferably protein enriched pulse grains.

(23) In an embodiment, the protein rich precursors may be selected from, but not limited to 1. Dry beans (Phaseolus spp. including several species now in Vigna) Kidney bean, haricot bean, pinto bean, navy bean (Phaseolus vulgaris) Lima bean, butter bean (Phaseolus lunatus) Azuki bean, adzuki bean (Vigna angularis) Mung bean, golden gram, green gram (Vigna radiata) Black gram, urad (Vigna mungo) Scarlet runner bean (Phaseolus coccineus) Ricebean (Vigna umbellata) Moth bean (Vigna aconitifolia) Tepary bean (Phaseolus acutifolius) 2. Dry broad beans (Vicia faba) Horse bean (Vicia faba equina) Broad bean (Vicia faba) Field bean (Vicia faba) 3. Dry peas (Pisum spp.) Garden pea (Pisum sativum var. sativum) Protein pea (Pisum sativum var. arvense) 4. Chickpea, garbanzo, Bengal gram (Cicer arietinum) 5. Dry cowpea, black-eyed pea, blackeye bean (Vigna unguiculata) 6. Pigeon pea, Arhar/Toor, cajan pea, Congo bean, gandules (Cajanus cajan) 7. Lentil (Lens culinaris) 8. Bambara groundnut, earth pea (Vigna subterranea) 9. Vetch, common vetch (Vicia sativa) 10. Lupins (Lupinus spp.) 11. Minor pulses, including: Lablab, hyacinth bean (Lablab purpureus) Jack bean (Canavalia ensiformis), sword bean (Canavalia gladiata) Winged bean (Psophocarpus teragonolobus) Velvet bean, cowitch (Mucuna pruriens var. utilis) Yam bean (Pachyrrizus erosus)

(24) In a preferred embodiment, the protein rich precursors are gram flour or soyabean.

(25) The present invention provides a process for the synthesis of nitrogen-doped carbon electro-catalyst from food waste.

(26) The present invention provides a process for the synthesis of nitrogen-doped carbon electro-catalyst from food waste, wherein the said carbon catalyst is porous with pore size in the range of 10 to 300 .

(27) The onset potential of the electrocatalyst prepared by the process described herein was determined against Hg/HgO electrode and the results are: (GFMC10.0598, GFMC20.0059, GFMC30.0871, SBMC10.0583, Pt/C0.07 (V).

(28) In an embodiment, the heteroatom in the electrocatalyst is 1% doping, the resistance is 1-10 ohms for a 50 gm pellet at a pressure of 1 ton, the O.sub.2 functionality (10-20%) and the surface area of the catalyst is in the range of 600-800 m.sup.2/g. The durability, methanol stability and limiting current density of the catalyst are represented in the Figures.

EXAMPLES

(29) 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.

(30) Materials and Instrumentation

(31) Gram flour and Soyabean grains were obtained commercially and were grounded to a fine powder. Ludox [Colloidal silica nanoparticle (size 12 nm) dispersion in water] and conc. hydrofluoric acid (HF) was obtained from Sigma Aldrich. Split tube furnace with alumina tube and argon atmosphere was used for the pyrolysis process.

(32) Three carbon samples were prepared from gram flour powder. The samples were named as gram flour-derived mesoporous carbon (GFMC) and Soyabean-derived mesoporous carbon (SBMC).

Example 1

Preparation of Gram Flour-Derived Mesoporous Carbon (GFMC1)

(33) 5 gm gram flour was boiled in excess deionized (DI) water for 5 hours. The slurry was filtered through a Whatman filter paper of pore size 0.2 m. The filtered product was pyrolyzed at 1000 C./4 hrs in flowing argon atmosphere. The sample was then treated with conc. HF solution to remove oxide impurities. The product was washed with DI water by centrifugation several times and then dried in oven. The dried sample was again treated at 1000 C./4 hrs in argon atmosphere. This sample was then characterized.

Example 2

Preparation of Gram Flour-Derived Mesoporous Carbon (GFMC2)

(34) 5 gm gram flour was boiled for 5 hours in 50 ml Ludox solution up to dryness. The SiO.sub.2-Gram flour slurry obtained was pyrolyzed in argon atmosphere at 1000 C. in a split tube for 4 hours. The obtained product was treated with conc. HF to remove silica nanoparticles and generate porous carbon. The carbon was recovered and washed with DI water by centrifugation several times. It was dried overnight in an oven and then re-heated at 1000 C. for 4 hrs in argon atmosphere. The final material was used for characterization.

Example 3

Preparation of Gram Flour-Derived Mesoporous Carbon (GFMC3)

(35) 5 gm gram flour was boiled for 5 hours in 200 ml Ludox solution up to dryness. The SiO.sub.2-Gram flour slurry obtained was pyrolyzed in argon atmosphere at 1000 C. in a split tube for 4 hours. The obtained product was treated with conc. HF to remove silica nanoparticles and generate porous carbon. The carbon was recovered and washed with DI water by centrifugation several times. It was dried overnight in an oven and then re-heated at 1000 C. for 4 hrs in argon atmosphere. The final material was used for characterization.

Example 4

Preparation of Soyabean-Derived Mesoporous Carbon (SBMC1)

(36) 5 gm soyabean powder was boiled for 5 hours in 50 ml Ludox solution up to dryness. The SiO.sub.2-Gram flour slurry obtained was pyrolyzed in argon atmosphere at 1000 C. in a split tube for 4 hours. The obtained product was treated with conc. HF to remove silica nanoparticles and generate porous carbon. The carbon was recovered and washed with DI water by centrifugation several times. It was dried overnight in an oven and then re heated at 1000 C. for 4 hrs in argon atmosphere. The final material was used for characterization.

(37) These four cases of mesoporous carbon materials (GFMC1, GFMC2, GFMC3 and SBMC1) are characterized fully for their physical properties and also tested for electrocatalytic performance in ORR. The figurative description of the general synthesis protocol is shown in FIG. 1.

(38) Results

(39) Characterizations

(40) Field Emission Scanning Electron Microscopy (FESEM, Nova NanoSEM 450) and High Resolution-Transmission Electron Microscopy (HR-TEM, FEI Tecnai 300) were used. The surface area of carbon was obtained from BET surface area measurements (Quantachrome Quadrasorb automatic volumetric instrument). X-ray photoelectron spectroscopy (XPS) (ESCA-3000, VG Scientific Ltd. UK, with a 9 channeltron CLAM4 analyzer under vacuum better than 1*10.sup.8 Torr, A1 Ka radiation (1486.6 eV) and a constant pass energy of 50 eV) was employed to study the chemical state of carbon in the materials respectively. X-ray Diffraction (XRD, Philips X'Pert PRO) and Raman spectroscopy (a confocal micro-Raman spectrometer LabRAM ARAMIS Horiba JobinYvon, with laser excitation wavelength of 612 nm) was also used.

(41) Resistance Measurement

(42) 50 mg of carbon powder in each case is placed in a die and a pressure of 1 tonne is applied. The contact is made from both the sides of the die, and the top and bottom parts of the die are separated by a flexible insulator to avoid contact. Under pressure, the resistance is measured.

(43) Adsorption Isotherms for GFMCs

(44) All the samples were measured by nitrogen adsorption-desorption experiments at 77 K. All the adsorption isotherms are of type II with typical hysteris loop at high value of p/p.sup.0. Hysteresis loop at high value of p/p.sup.0 signifies microporosity in these materials.

(45) Atomic Percentages for GFMCs

(46) TABLE-US-00001 TABLE 1 Atomic Percentages for the GMFCs Material C N O GMFC1 73.26 1 25.73 GMFC2 84.77 0.75 14.47 GMFC3 88.81 1 10.23
Raman Spectroscopy

(47) Raman spectroscopy of the GFMCs reveal the characteristic D band of carbon at 1315 cm.sup.1 and G band due to sp.sup.2 carbon 1590 cm.sup.1. The I.sub.D:I.sub.G ratio shows an expected increase in the porous GFMC2 and GFMC3 as compared to GFMC1 due to the increase in number of defects in the systems.

(48) SBMC1 as an Electrocatalyst for ORR

(49) XPS analysis shows that SBMC1 contains Carbon, Nitrogen and Oxygen in the % ratio of 22:1.25:80. Very high oxygen content is observed in this material. A slightly greater % N atom doping (1.25%) is seen in SBMC1. This may be due to increased protein content of the precursor. The nitrogen content shows peaks at 399.7 eV (pyrrolic N), 401.3 eV (graphitic N) and 4033 eV (oxygenated N) (FIG. 5)

(50) The pore size distribution of SBMC1 shows a similar hierarchical pore distribution which is responsible for enhanced catalytic activity (FIG. S6). The BET surface area of SBMC1 however is low (28 m.sup.2/g).

(51) This could be due to insufficient penetration of SiO.sub.2 nanoparticles into the seed coat of soybean grains during the synthesis. The resistance of SBMC1 pellet also shows an increase as compared to GFMCs (10.23 ohms). However the good performance in ORR in this case can be attributed to an increased nitrogen doping percentage and oxygen functionalization as compared to GFMCs. It is quite clear that SBMC1 possesses large number of catalytic sites in spite of having lesser surface area and conductivity as compared to GFMC2.

Advantages of the Present Invention

(52) a. Process is facile and does not involve tedious or cost effective procedures. b. The precursors used are inexpensive. c. The heteroatom doped carbon synthesized from these protein enriched precursors (the nitrogen constitution of the proteins contribute to the doping process) require no in-situ/ex-situ addition of any dopant. d. The new catalyst provides remarkable performance and hence offers a practical alternative to Platinum based materials.

ABBREVIATIONS

(53) ORRoxygen reduction reaction

(54) PEMFCpolymer electrolyte membrane fuel cell

(55) GMFCgram flour-derived mesoporous carbon

(56) SBFCSoyabean-derived mesoporous carbon