Patent Application: US-28612002-A

Abstract:
alkali metal - carbon compounds may be formed by mixing an alkali metal with carbon . such alkali metal - carbon compounds absorb hydrogen at lower temperatures and may be useful as hydrogen storage materials in various applications , such as in hydrogen fuel cells .

Description:
alkali metals are among the most active metals . alkali metals are easily oxidized if exposed to air under ambient conditions . the commercially supplied alkali metals are unavoidably coated with compact oxides or hydroxides . thus , the operation of alkali metals is preferably conducted under inert atmosphere . mixing and pre - treatment of alkali metals and carbon are preferably carried out under inert gas atmosphere , for example , under an inert gas such as ar or he , among others . on a bench scale , an inert atmosphere may be provided , for example , in a glove box . sample transfer from the glove box to containers in a testing machine is ideally performed as quickly as possible . in the present invention , a certain amount of carbon is preferably added into the alkali metal , for example , lithium or potassium . pre - treatment of the alkali - c mixture preferably includes : mixing the carbon and alkali metal , and then pressing the carbon and alkali metal together . mixing of alkali metal and carbon may be done in a variety of ways , for example , by pounding the carbon into the alkali metal using a mortar and pestle or by milling the carbon and alkali metal in a mill such as a ball mill . preferably , the mixture is made as homogeneous as possible . the mixing is preferably done under inert gas atmosphere . pressing the mixture is preferably done under a pressure of from about 1 atm to about 10 , 000 atms . without being held to any theory , it is believed that interactions between alkali metal and c occur during pre - treatment . interaction between alkali metal and c could result in two categories of compounds being formed : 1 ) alkali - c intercalated compounds ; and , 2 ) alkali metal - carbides . for example , lithium carbides , li 2 c 2 or lic , possess the face - centred structure , which is different from li and c . alkali metal carbides are usually prepared by decomposition of c 2 h 2 in the present of alkali metal at a temperature around 300 ° c . or by calcinations of alkali metal and c at elevated temperature . in the present invention , the alkali - c compound found in the alkali metal - c mixture is an alkali metal - c intercalation compound . for example , as shown in fig5 a , the xrd characterization of an as - prepared li — c mixture demonstrates that there exist lic 6 , lic 12 , lic 24 and lithium metal as well as minor amounts of li 2 o & amp ; lioh . pure carbon structure is very weak . the li — c intercalation compounds possess a similar layer structure as that of graphite but with broadened layer inter - space , i . e ., the interlayer distance of lic 6 and lic 12 is 0 . 370 nm and 0 . 35 nm , respectively . potassium also forms k — c intercalation compounds , for example , with formulae kc 8 , kc 24 etc . like li — c interaction compounds , the k — c intercalation compounds also possess layer structure with even broader interlayer distance (˜ 0 . 51 nm ). the mixing and pressing of carbon into alkali metal at ambient temperature and inert gas atmosphere preferably results in the formation of a series of alkali - c intercalation compounds . the method of forming alkali - c intercalation compounds of the present invention is different from traditional methods in which alkali metal - c ( e . g . li — c or k — c ) intercalated compounds are synthesized , for example , by reacting evaporated li or k with carbon at high temperature or by heating the alkali - c mixture at high temperature under high pressure . in the present invention , carbon materials of any form may be used , and are preferably at least one selected from the group consisting of graphite , carbon nanotubes , carbon fibres , carbon nanofibers , carbon powders , fullerenes and activated carbon . graphite , carbon powder , activated carbon , fullerenes and carbon fibres are commercially available . carbon nanotubes and nanofibers can be obtained accordingly the previously reported methods [ 10 ]. without being bound by any theories , the formed alkali - carbon intercalated compound seems to be a catalyst for the hydrogenation of alkali metal . as illustrated by low - content hydrogen temperature - programmed - reaction ( lc - tpr ) ( fig1 ), on which diluted h 2 ( 10 % h 2 + 90 % ar ) was used as reacting gas , the hydrogen absorption by li — c ( for instance ) occurred at temperature lower than 150 ° c . ; for pure lithium , the apparent hydrogenation began at temperature around 550 ° c . the degree and rate of hydrogenation of alkali metal in the presence of carbon seems related to hydrogen pressure . the lc - tpr was conducted under a hydrogen pressure of around 1 . 0 atm , and the hydrogen absorption peak was comparatively weak . to clarify the relationship between hydrogenation degree and pressure , we performed pressure - composition - isotherm ( pci ) measurement at 180 ° c . for li — c system and 120 ° c . for k — c system . pci is the commonly used method in evaluation of hydrogen storage capacity in metals or metal alloys . it measures the pressure changes during hydrogen absorption and desorption . the pci results of li — c sample are illustrated in fig2 . it can be seen that the absorption line possesses characteristics similar to the characteristics of metals , which can form metal hydrides . in the pressure range of 0 to 100 psi , the molar ratio of h /( li + c ), referred to as x , increased linearly and reached 0 . 15 . during that pressure range , absorbed hydrogen diffused into the lattice of lithium and formed random li — h solid solution . as pressure reached 100 psi , which is called the plateau pressure , the h /( li + c ) increased to 0 . 55 with pressure almost unchanged . after that , the h /( li + c ) further increased to 0 . 7 with pressure increase to 550 psi . converted to the hydrogen storage capacity , the molar ratio of h /( li + c )= 0 . 7 is equal to 9 wt %. intelligent - gravimetric - analyzer ( iga ), which also confirmed this result ( see fig3 ), measured the weight variation of hydrogen absorbed ( in mg ) during hydrogenation under 6 atms and at temperature from 25 ° c . to 250 ° c . the pci measurement of k — c system conducted at 120 ° c ., as shown in fig4 shows that x could reach 0 . 43 , which means that 80 % of k is hydrogenated . the xrd measurements were done on the as - prepared li — c ( fig5 a ) and li — c mixture after hydrogenation ( fig5 b ). it is clear that after hydrogen absorption at 180 ° c ., almost all li metal was converted to lih , and the li — c intercalation compounds , i . e . lic 6 ( situated at ˜ 2θ = 24 °) and lic 12 ( 2θ = 25 . 2 °) etc . disappeared and a pure graphite phase ( 2θ = 26 . 2 °) was developed . this result further demonstrates that with the addition of carbon , lih can be successfully synthesized at a temperature lower than 200 ° c . the alkali / c molar ratio may be adjusted to include more or less carbon . more carbon added will accelerate the hydrogenation rate , compromising hydrogen absorption capacity if the whole alkali - c mixture is considered as sorbent . less carbon will increase the hydrogen storage capacity even up to over 12 wt % ( e . g . for li — c system , a hydrogen storage capacity of about 12 . 5 % has been achieved ) but the hydrogen absorption rate is relatively slow . the following specific examples are provided to illustrate the invention . it will be understood , however , that the specific details given in each sample have been selected for purpose of illustration and are not to be construed as a limitation on the invention . generally , the experiments were conducted under similar conditions unless noted . 60 mg graphite was mixed with 350 mg lithium metal , and then the mixture was pounded with a pestle as homogeneously as possible . after that , the pounded mixture was pressed into pellets for testing . 300 mg of the above pellets were put into a pci sample container for auto - soak measurement at 180 ° c . and 30 atms of pure hydrogen . after 3 hours of absorption , 33 mg of hydrogen was absorbed . the xrd measurements show that the product only has lih , graphite and weak li 2 o phases . 120 mg of graphite was mixed with 250 mg lithium metal then the same procedure described in example 1 was followed . about 30 mg of hydrogen was absorbed with hydrogen storage capacity of 8 . 1 wt %. 240 mg of graphite was mixed with 140 mg of lithium metal , then the same procedure as example 1 was followed . 4 wt % of hydrogen was absorbed . 60 mg of multi - walled carbon nanotubes ( with average diameter of 20 nm ) was mixed with 350 mg lithium metal , then the same procedure as describe in example 1 was followed except that the absorbing temperature was changed to 160 ° c . about 4 wt % of hydrogen was absorbed . when the absorbing time was prolonged for 12 hours , 9 wt % of hydrogen was absorbed . 60 mg of activated carbon was mixed with 350 mg of lithium , then the same procedure as described in example 1 was followed except that absorbing time was 6 hours . 9 wt % of hydrogen was absorbed . 240 mg of graphite was mixed with 780 mg of potassium metal , then the same procedure as described in example 1 was followed except that k — c was exposed to hydrogen atmosphere at 120 ° c . for 4 hours . about 1 . 5 wt % of hydrogen was absorbed . to those skilled in the art , it is to be understood that many changes , modifications and variations could be made without departing from the spirit and scope of the present invention as claimed hereinafter . 1 . g . k . pitcher and g . j . kavarnos , int . j . hydrogen energy , 22 ( 6 ), 575 ( 1997 ). 3 . m . bellini , p . de natale and m . inguscio , j . astrophys , 424 , 507 ( 1994 ). 5 . a . stolarzewicz , d . neugebauer and j . grobelny , macromole . rapid comm ., 17 , 787 ( 1996 ). 6 . e . zintl and a . harder , z . phys . chem . 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