Patent Application: US-55200404-A

Abstract:
an apparatus for use of carbonized charcoal powder as an electrode is provided . charcoal is provided as a powder , carbonized , and placed in a container by which compressive pressure is applied to the carbonized - charcoal powder via one or more sides of the container . as a result of the compressive pressure the packed - bed of carbonized - charcoal powder manifests a resistivity of less than about 1 ohm - cm and is suitable for use as an electrode in a fuel cell , battery or electrolyzer . the apparatus is adapted with electrical contacts to conduct electric flow to or from the electrode and adapted for communication of an electrolyte with the electrode .

Description:
to ensure that the charcoal has a high electrical conductivity , it must be heated (“ carbonized ”) at a temperature of at least about 900 ° c . or more for at least a few minutes prior to its use as an electrode . usually the charcoal carbonization step is accomplished in an oxygen - free environment , but some oxygen ( i . e . air ) can be present . the charcoal may be ground either before or after carbonization to a fine particle size , preferably so that a substantial portion ( greater than about 80 % of the particles ) are of a size & lt ; 1 mm . if the charcoal is not ground prior to carbonization , the carbonization time must be sufficient to permit the center of the largest charcoal lumps to reach the desired carbonization temperature and remain at that temperature for a few minutes . referring to fig1 , an apparatus 1 according to the invention comprises a vertical , cylindrical tube 16 that retains the carbonized - charcoal bed 11 , a piston 10 within the tube 16 that delivers a compressive force to the carbonized - charcoal bed 11 , and a base 12 within the tube 16 that retains the carbonized - charcoal bed against the compressive force of the piston . the piston 10 , the base 12 , or the tube 16 must be fabricated from an electrically conductive material to enable electricity to flow to or from the carbonized - charcoal bed . if sufficient electrical connection is made with piston 10 and base 12 , tube 16 may be made of a porous insulator , such as alumina , to provide for a liquid or gaseous electrolyte a way to contact the bed 11 . the electrolyte may be provided in the annular space 20 and may communicate with another electrode ( not shown ) through appropriate interelectrode connection ( not shown ). an electrically conductive wire 13 leads to a source or sink for electrons ( not shown ). force may be applied to the piston by a pneumatic or hydraulic cylinder 6 that can be held conveniently by framework 2 that also supports the tube 16 and the base 12 . alternatively , instead of base 12 , another piston and cylinder arrangement similar to 4 through 10 may be used . in that case , bed 11 will be compressed between two moveable pistons . a pressure transducer 4 may be used to monitor and control the force delivered to the piston 10 . when the carbonized - charcoal electrode is used in a battery or fuel cell , the tube 16 may be made of a porous material to permit a liquid or gaseous electrolyte to contact the carbonized - charcoal bed 11 . in some cases it may be desirable to electrically isolate the carbonized - charcoal bed 11 from the support framework 2 and end walls 3 . in this case electrical insulators 14 ( e . g . teflon ) may be employed . air or hydraulic fluid may be applied through conduit 5 to drive the cylinder 6 . the reach to piston 8 can be adjusted with screw jack 7 . the piston 10 is attached to plunger rod 9 and receiving piston 8 . to provide another electrical connection to the bed 11 , the pistons 8 and 10 and rod 9 are all electrically conductive and electrically connected to wire 18 . tube 16 and rod 9 are supported and electrically insulated from framework 2 by insulating disk 15 ( such as teflon ). if desired , tube 16 may be fabricated of an electrically conductive material and electrically insulated from either piston 10 or base 12 . wire 18 or 13 will then be connected to tube 16 instead of piston 8 or base 12 . if tube 16 is electrically conductive , then contact of bed 11 with the electrolyte will be through piston 10 or base 12 , either of which may be fabricated of a porous material . the resistivity of the bed 11 may be measured by ohm - meter 17 by opening switch 19 b and closing switch 19 a . when the apparatus 1 is used as electrode in a fuel cell , battery or electrolyzer , switch 19 a is open and switch 19 b is closed . the following examples are provided for the purpose of illustration and are not intended to limit the invention in any way . samples ( 0 . 5 g ) of 20 / 40 mesh macadamia nutshell charcoal which had been carbonized for 10 min at temperatures of 650 , 750 , 850 , 950 , and 1050 ° c ., were loaded into the apparatus shown in fig1 and the electrical resistivity of the packed - bed , carbonized - charcoal electrode was measured as a function of applied pressure . referring to fig2 , the electrical resistivity of the carbonized - charcoal packed bed decreased by more than five orders of magnitude as the carbonization temperature increased from 650 to 1050 ° c . similarly , in fig2 the electrical resistivity of the packed bed of carbonized - charcoal powder decreased by about a factor of 10 as the applied pressure delivered by the piston 10 ( see fig1 ) increased from 0 to about 8 mpa . for comparison , graphite powder was loaded into the apparatus and the resistivity of the graphite powder electrode was measured as a function of increasing pressure . as shown in fig2 an electrode composed of a packed bed of macadamia nutshell charcoal carbonized at 1050 ° c . manifested an electrical resistivity ( 0 . 059 ω - cm ) that was about double that of a graphite powder electrode . raw charcoal is typically exposed to temperatures below 600 ° c . when it is produced from biomass in a kiln or retort . as shown in fig2 charcoal powder manifests a good electrical conductivity ( comparable to graphite powder ) only after it is carbonized at temperatures of 900 ° c . or more . consequently , raw charcoal or charcoal exposed to temperatures below about 900 ° c . are not suitable for use as electrode material . likewise , fig2 shows that a carbonized - charcoal , packed - bed electrode manifests a good electrical conductivity ( comparable to graphite powder ) when the applied pressure to the packed bed exceeds about 1 mpa . charcoal powder contained in a basket or charcoal powder under pressure of a typical spring will not conduct electricity sufficiently well to be used as an electrode . a sample ( 0 . 49 g ) of 20 / 40 mesh coconut husk charcoal , which was carbonized at 950 ° c ., was loaded into the apparatus and the electrical resistivity of the packed - bed , coconut husk carbonized - charcoal electrode was measured as a function of applied pressure for two pressurization / depressurization cycles . as shown in fig3 , the electrical resistivity of the coconut husk carbonized - charcoal electrode decreased to a value of 0 . 18 ω - cm as the pressure applied to the electrode by the piston increased to about 6 mpa . fig3 also displays the length of the carbonized - charcoal packed bed as a function of pressure . prior to the first compression the bed was loosely packed and manifested a low electrical conductivity , but following the first compression the compacted bed was relatively dense ( 0 . 46 g / cm 3 ) and virtually incompressible . a sample ( 0 . 5 g ) of 20 / 40 mesh kukui nutshell charcoal , which was carbonized at 950 ° c ., was loaded into the apparatus and the electrical resistivity of the packed - bed , kukui carbonized - charcoal electrode was measured as a function of applied pressure for two pressurization / depressurization cycles . as shown in fig4 , the electrical resistivity of the kukui nutshell carbonized - charcoal electrode decreased to a value of 0 . 18 ω - cm as the pressure applied to the electrode by the piston increased to above 6 mpa . fig4 also displays the length of the carbonized - charcoal packed bed as a function of pressure . prior to the first compression the bed was loosely packed , but following the first compression the compacted bed was quite dense ( 0 . 82 g / cm 3 ) and virtually incompressible . a sample ( 0 . 5 g ) of 20 / 40 mesh leucaena wood charcoal , which was carbonized at 950 ° c . was loaded into the apparatus and the electrical resistivity of the packed - bed , leucaena carbonized - charcoal electrode was measured as a function of applied pressure for two pressurization / depressurization cycles . as shown in fig5 a , the electrical resistivity of the leucaena carbonized - charcoal electrode decreased to a value of 0 . 16 ω - cm as the pressure applied to the electrode by the piston increased to about 6 mpa . for comparison a 0 . 5 g sample of very fine leucaena wood charcoal powder , also carbonized at 950 ° c . was loaded into the apparatus and the electrical resistivity of the leucaena carbonized - charcoal powder electrode was measured . fig5 a shows that the electrical resistivity of this powder electrode was even lower than the 2 / 40 mesh carbonized - charcoal with a value of about 0 . 11 ω - cm at the highest pressure . fig5 b shows the electrical resistivity of the two electrodes as a function of the carbonized - charcoal density . following an initial compression the 20 / 40 mesh carbonized - charcoal bed was virtually incompressible ; whereas the fine powder evidenced some compressibility at a much higher density . taken together the results of examples 3 and 4 show that the density ( i . e . porosity ) of the carbonized - charcoal , packed - bed electrode does not significantly influence its electrical resistivity . examples 1 - 4 show that carbonized charcoal powders derived from a wide variety of different biomass materials are well suited for use as electrode materials according to the invention . while the invention has been described with reference to particular embodiments thereof , those of skill in the art will be able to make various modifications to the described embodiments without departing from the spirit and scope on the invention . it is intended that the foregoing embodiments are presented only by way of example and that , within the scope of the appended claims and equivalents thereto , the invention may be practiced otherwise than as specifically described . 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