Patent Application: US-99726209-A

Abstract:
the invention disclosed relates to porous carbon of spherical morphology having tuned porosity and to a method of making same , comprising : providing a precursor solution , by combining in an aqueous solution a colloidal silica template material and a water - soluble pyrolyzable carbon source , wherein the particle size of the colloidal silica template and the colloidal silica / carbon source weight ratio are controlled , atomizing the precursor solution into small droplets by ultrasonic spray pyrolysis directing the droplets into a high temperature furnace operating at a temperature of 700 - 1200 ° c ., under an inert gas atmosphere , where the droplets are transformed into solid spherical composite carbon / silica particles , collecting the resulting composite carbon / silica particles exiting from the furnace , and removing the silica from the particles , to provide substantially pure porous carbon of spherical morphology having tuned porosity defined by surface area and pore size . the porous carbon according to the invention is used as catalyst supports in pem fuel cells , as electrodes in supercapacitors and lithium in batteries , for hydrogen storage and as earners for drug delivering .

Description:
in this invention , we adopted a combination of two strategies to controllably synthesize porous carbon spheres : ( 1 ). using colloidal silica as templates to duplicate porous carbon . the surface area and porosity of the duplicated porous carbons is tuned by controlling the particle size of the silica colloids template and the ratio of silica / carbon source . colloidal silica can be synthesized by hydrolyzing tetraethoxysilane , which is much easier than preparing ordered mesoporous silica templates . alternatively , many low priced colloidal silica products with well - defined colloid sizes are commercially available . ( 2 ). using ultrasonic spray pyrolysis ( usp ) technique to form spherical porous carbon . theoretically , spherical particles have the highest stack density in a definite volume . porous carbon spheres are ideal for the applications in electrochemical porous electrodes . usp technique has the ability to produce sub - micrometer solid spherical particles starting from liquid precursors . we use this technique to transfer the liquid mixture of colloidal silica and water - soluble carbon source material ( such as sucrose , pyrrole and aniline to spherical carbon - silica composite particles , and then etch silica by means of a strong acid or base to form porous carbon spheres . as shown in fig1 , the detailed process of the invention includes five steps : ( 1 ) preparing precursor solution . colloidal silica prepared by hydrolyzing tetraethoxysilane or commercially available colloidal silica was used as templates . sucrose or pyrrole or aniline or other pyrolyzable carbon containing compounds was used as carbon source . in a container 10 , appropriate amounts of colloidal silica and carbon source were dissolved in di water , respectively , depending on the target surface area and porosity . then , the two solutions are mixed with a constant stirring for 30 minutes . acid ( hcl , h 2 so 4 , h 3 po 4 etc .) was then added into the mixed solution quickly with rigorous stirring , to adjust the ph to 1 to 3 . oxidizing agents such as fecl 3 , h 2 o 2 etc . can be added to initialize the polymerization . the colloid particle size of colloidal silica templates and the amount of colloidal silica and carbon source were selected as per the requirement of carbon surface area and porosity . for example , 4 g ludox ® tm40 ( 40 wt %, dupont ) of template particle size of 22 nm and 4 g sucrose ( i . e . a weight ratio of 1 : 1 ) could result in porous carbon spheres with a pore size distribution of ˜ 22 nm and specific surface area of ˜ 1200 m 2 / g . if using 8 g sucrose ( i . e . a weight ratio of 1 : 2 ), the specific surface area drop down to ˜ 860 m 2 / g . the achieved specific surface area could be in a broad range of 50 to 3000 m 2 / g , depending on the weight ratio ( from 1 : 4 to 4 : 1 ) and the template colloidal particle size ( from 1 nm to 100 nm ). a colloidal particle size range of 20 - 40 nm is useful for fuel cell catalyst supports . ( 2 ) atomizing precursor solution . the precursor solution is then fed to an atomizer 12 e . g . an ultrasonic four - unit array atomizer associated with a 14 , to pulverize the solution into small droplets . the atomizer can theoretically produce uniform spherical droplets of a particle size of 0 . 1 - 10 μm . other conventional atomizers such as air - pressurized , electrostatic ones could be used for atomizing the solution . a squirm or syringe pump 16 was used to transport the solution into the vessel and keep the solution level constant in the vessel . high purity ( 99 . 999 %) nitrogen was used as carrier gas to carry the formed droplets through a 2 - inch quartz tube 18 , which was placed in a high temperature tube furnace 20 . a flow controller 22 is used to control the flow of nitrogen gas . ( 3 ) pyrolysizing droplets . the droplets were transformed into solid spherical particles in the tube furnace 20 ( maximum 1200 ° c ., e . g . a furnace produced by theimcraft inc ., usa ). in a first part of the tube furnace , carbon source chemical was polymerized and the droplets were dehydrated . at the central zone of the tube furnace , carbon was formed onto nano - sized silica particles by carbonizing the precursor in inert gas atmosphere ( such as n 2 , ar , he ) at a temperature range of 700 - 1200 ° c . ( 4 ) collecting carbon - silica composite particles . the formed carbon - silica solid spherical particles were collected in a water bubbling container 24 . nitrogen carries the products into the container to deposit the solid and dissolve the residual chemicals into water . the carrier gas was vented out through a fume hood . ( 5 ) etching silica . the collected particles were filtered and washed with water based solvent several times to eliminate the residual chemicals on the surface of carbon - silica composite . then , strong base or acid was added to the carbon - silica composite , stirring for 1 - 10 hours to etch silica . this step is repeated twice to completely etch silica from the carbon spheres . after filtering and washing several times and drying at the temperature higher than 100 c , porous carbon spheres were attained . the prepared carbon spheres were characterized by means of sem , tem , and surface area / porosity analysis . carbon spheres with different surface area and porosity were synthesized by using different particle - size colloidal silica template and different weight ratios of silica and carbon source chemical . the particle size of the carbon spheres was in the range 100 nm - 2000 nm depending on synthesis parameters such as precursor concentration , atomizer frequency and the gas flow rate . the pore size of porous carbon spheres , and hence the colloidal silica template size could be at the range of 1 ˜ 100 nm , depending upon the use / application , which covers the definitions of micropore (& lt ; 2 nm ), mesopore ( 2 ˜ 50 nm ) and macropore (& gt ; 50 nm ). and , various pores could be designed to coexist in a carbon sphere as per the needs of different applications . the specific surface area of porous carbon spheres could be attained up to 3000 m 2 / g by controlling the synthesis parameters . in this example , porous carbon spheres were synthesized by 22 - nm colloidal silica templates , according to the detailed process described above . in this case , sucrose was used as carbon source , with the silica to carbon weight ratio of 2 : 1 . fig2 a shows the sem picture of the carbon - silica composite particles synthesized by 22 - nm colloidal silica templates . the composite particles have completely spherical shape and smooth surface . fig2 b shows the sem picture of the carbon spheres after etching silica . fig2 c is a zoomed picture of a single carbon sphere . it is clear that the etching process doesn &# 39 ; t destroy the spherical shape of the primary particles . the silica content was etched from the carbon matrix , which resulted in a honeycomb - like carbon sphere with many uniform nanosized pores . the tem picture of a single carbon sphere ( fig2 d ) shows that the carbon sphere is hollow . the particle size of porous carbon sphere displays a unimodal distribution around 1000 nm , as shown in fig3 . for analysis purposes , in order to insure the complete removal of silica from the carbon sphere , thermal gravimeter ( tg ) was carried out in an air flowing between room temperature and 700 ° c . ( fig4 ). as shown , the porous carbon sphere was dramatically burnt around 525 ° c . after 560 ° c ., no residual exists any more , indicating that the porous sphere contains 100 % carbon without silica . it is noted that the tg experiment is to confirm the silica was completely removed from carbon spheres . it is a characterization , not a preparation step . fig5 shows the surface area and porosity information provided by nitrogen adsorption and desorption experiments . commercially available vulcan 72 carbon black was also measured as a reference . the specific surface area calculated by bet ( brunauer - emmett - teller ) method is 1200 m 2 / g for the prepared carbon spheres while 245 m 2 / g for vulcan 72 carbon black . nitrogen adsorption - desorption curves showed hysteresis at high relative pressure , which is a characteristic of mesopores . the pore size distribution data calculated from the adsorption branch of the nitrogen isotherm by the bjh ( barrett - joyner - halenda ) method showed that pores are unimodal with an average pore size of 24 nm . that is well consistent with the silica template size . in order to improve the stability of such an open frame carbon structure , a graphitic carbon sphere structure was introduced by adding a catalytic graphitization step into the procedure described in example 1 . a transition metal ion e . g . fe , co , ni or others in the & amp ; qui of a salt ( chloride , sulfate , nitrate , acetate etc .) was added into the precursor solution with a metal / carbon source weight ratio from 1 : 20 to 1 : 5 . the metal or metal oxide nanoparticles derived from the decomposition of the salt acted as a catalyst in step ( 3 ) to graphitize the porous carbon sphere . fig6 shows the xrd patterns of porous carbon sphere before and after graphitization . obvious graphite peaks can be seen in the second sample . besides the benefit of a more stable structure , the graphitic carbon sphere also has a higher electronic conductivity ( 10 s / cm ) than the pre - graphitized carbon sphere (˜ 1 s / cm ). the electronic conductivity was measured at room temperature by ac impedance spectroscopy over a frequency range 10 - 10 6 hz with a voltage of 1v , using a homemade 4 - probe device . one of the examples of applications / uses for the porous carbon according to the invention is mesoporous carbon sphere supported pt and pt alloy catalysts prepared by a co - formation procedure , for oxygen reduction reaction , particularly in proton exchange membrane fuel cells . for other applications , other noble metal alloy catalysts can be used e . g . pt — ru for methanol oxidation in dmfcs . the step of adding the catalyst particles may be done either after the formation of the spherical porous carbon , or it can be done concurrently by co - formation . one process is co - formation procedure ; another is conventional impregnation procedure ( microwave - assisted polyol method ). a co - formation procedure , which was based on the above - described procedure , was used to synthesize porous carbon sphere supported pt and pt alloy . pt salt or mixture of pt and transition metal ( co , ni , fe , mn etc .) salts were dissolved in the reaction precursor , which includes carbon source ( sucrose , pyrrole , aniline etc .) and silica colloids . the mixture precursor solution was then atomized into droplets , and heat - treated in a tube furnace in inert atmosphere ( such as n 2 , ar , he ) at a temperature range of 700 - 1200 ° c . the catalysts were obtained after silica templates were removed by etching in strong acid or base . in this case , pt or pt alloy nanoparticles were formed concurrently with the carbon spheres , and uniformly dispersed in the whole carbon matrix . in order to control the metal nanoparticles only depositing on the surface of carbon spheres , another two - step procedure can be used . the first step is to mix metal salt ( s ) with the silica colloidal solution . the metal ions with positive charges automatically adsorb onto the negative - charge surface of silica colloids . a reducing agent ( nabh 4 , formaldehyde , h 2 gas etc .) was used to form metal nanoparticles on the silica colloids . the second step is to mix hydrocarbon precursor with the silica colloid supported metal nanoparticles solution , and then following the same ultrasonic spray pyrolysis procedure to attain the samples . fig7 ( a ) shows tem pictures of a single carbon sphere supported pt catalyst , which was synthesized by using pyrrole as carbon source and 22 nm silica colloids as template with a weight ratio of 1 : 1 . a uniform size distribution of pt nanoparticle is achieved on the mesoporous carbon sphere . the average loading of pt on carbon was determined by edax to be 38 . 5 %. the average platinum particle size is around 2 - 4 nm that can be seen in fig7 ( b ). the catalytic performance of the prepared pt / mc catalyst was evaluated by rotating disk electrode technique . the commercially available 40 % e - tek pt / c was used as a reference . the procedure of electrode preparation was as follows : 20 μl 1 . 0 mg ( catalyst )/ ml ( isopropanol ) was dipped onto a 0 . 196 cm 2 glassy carbon electrode . after solvent evaporation , 10 μl 0 . 5 wt % nafion ® solution was coated onto the glassy carbon electrode . the electrochemical measurement was carried out in a three - electrode cell with oxygen - saturated 0 . 5m h 2 so 4 as electrolyte , platinum wire as counter electrode and standard mercury sulfide electrode as reference electrode . fig8 shows the curves of disk current density versus potential for the two catalysts under a rotating rate of 400 rpm . it can be seen that the two catalysts have similar electrochemical behavior at the kinetic zone ( high potential zone ), while the homemade carbon sphere supported catalyst is better than the commercial one at the lower potential zone . the lower polarization of pt / mc may result from its unique mesoporous structure , which facilitates mass transport during electrochemical reaction . the larger plateau limiting current density of pt / mc can be attributed to its feature of higher surface area . a higher surface area results in a larger diffusion current density passing through a thinner nafion film on the glass carbon disk electrode . the porous carbon sphere supported pt or pt alloy catalysts can be also prepared by conventional impregnation procedure . for example , a mesoporous carbon sphere material denoted as mc0411 ( 1000 m 2 / g surface area ), which was synthesized by the same experimental procedure as described in example 2 , was used as carbon support for ptco catalysts for pem fuel cells . ptco nanoparticles were deposited onto mc0411 by a microwave - assisted polyol reduction method . in order to accelerate the chemical reduction of platinum and cobalt , chloride - free chemicals , ( nh 3 ) 4 pt ( no 3 ) 2 and coac 2 , were used as the metal precursors . tetra - ethylene glycol was used as the reducing agent because its high boiling point ( 314 ° c .) is good for the alloying of platinum and cobalt . the metal precursors and the porous carbon spheres were homogeneously dispersed in the solvent of tetra - eg . then , microwave was used as a power to reduce the metal ions into metal particles on the carbon . the microwave heat treatment was set for 4 - 10 minutes to guarantee the completion of alloying . fig9 ( a ) illustrates the tem pictures of a single porous carbon sphere supported ptco alloy catalyst . fig9 ( b ) shows the particle size distribution in a zoomed carbon sphere area . it can be seen that ptco alloy nanoparticles are uniformly dispersed on the carbon spheres , with an average particle size of around 4 nm . rde measurement shows that the porous carbon sphere supported ptco alloy catalyst has a double specific activity relative to the pure pt catalyst . besides the applications in fuel cells , this invention is also promising to prepare electrode materials for supercapacitors . for example , a porous carbon sphere material ( denoted as mc1105 , 1500 m 2 / g surface area ), which was synthesized by a similar experimental procedure as described in example 1 , was used as electrode material for supercapacitors . the difference consisted in the silica to carbon weight ratio , which was equal to 3 : 1 . the capacitance property of this carbon material was evaluated by cyclic voltametric technique . 20 μl carbon ink , which consists of 10 mg mc1105 , 5 ml di water and 40 μl 5 wt % nafion ®, was coated onto a glassy carbon electrode . the thin film was dried at ambient temperature . the electrochemical measurement was carried out in a three - electrode cell with 0 . 5m h 2 so 4 as electrolyte , platinum wire as counter electrode and standard mercury sulfide electrode as reference electrode . fig1 shows the cyclic voltammograms ( 50 my / s ) of porous carbon sphere ( mc1105 ) and commercially available vulcan xc72 . the capacitance of each electrode was calculated from the capacitive current density , scan rate and carbon loading . as shown , carbon spheres show much bigger capacitive current density than vulcan xc72 . the calculated mass specific capacitance of mc1105 is 95 f / g , which is almost 5 times to that of vulcan xc72 ( 20 f / g ). ( 1 ). hydrogen storage material . porous carbon spheres have potential as hydrogen storage material owing to its high surface area and large pore volume , although the efficiency of hydrogen storage in carbon materials is still a challenge at this current stage . ( 2 ). anode material for lithium ion batteries . porous carbon spheres have favourable and controllable porosity for mass transport in electrochemical reactions . if high graphitization is accessible , porous carbon spheres may be good for intercalation material of lithium ion batteries . ( 3 ). mini carriers of drug delivery . porous carbon spheres have unique hollow structure and sub - micrometer size , which are an ideal tool for drug delivery in human body . but , this application faces the challenge of toxicity validation . 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