Patent Application: US-201113105714-A

Abstract:
a process for preparing a durable non - precious metal oxygen reduction electrocatalyst involves heat treatment of a ball - milled mixture of polyaniline and multiwalled carbon nanotubes in the presence of a fe species . the catalyst is more durable than catalysts that use carbon black supports . performance degradation was minimal or absent after 500 hours of operation at constant cell voltage of 0 . 40 v .

Description:
the present invention relates to catalysts useful in polymer electrolyte fuel cells . the invention also relates to polymer electrolyte fuel cells containing the catalysts and catalyst supports . the present invention further relates to methods of making the catalysts and catalyst supports . in all embodiments of the present invention , all percentages are by weight of the total composition , unless specifically stated otherwise . all ranges are inclusive and combinable . all numerical amounts are understood to be modified by the word “ about ” unless otherwise specifically indicated . all documents cited in the detailed description of the invention are , in relevant part , incorporated herein by reference ; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention . to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference , the meaning or definition assigned to that term in this document shall govern . an embodiment pani - fe - mwnt electrocatalyst was prepared using commercially available multi - walled carbon nanotubes having a bet surface area of approximately 230 square meters per gram were used . a sample of these mwnts was treated in 1 . 0 m hcl solution to remove impurities and then oxidized in a solution containing nitric acid and sulfuric acid [ 15 ]. approximately 2 . 0 milliliters of aniline was dispersed in 0 . 5 m hydrochloric acid solution . the oxidant ammonium persulfate ( nh 4 ) 2 s 2 o 8 , aps ) and ferric chloride were added . about 0 . 5 g of the resulting oxidized mwnts was mixed with the solution of polymerized pani . the resulting suspension was vacuum - dried using a rotary evaporator to produce a mixture . this mixture was ball - milled for 24 hours . after the ball milling , the resulting mixture was heated at temperature of 900 ° c . in an inert atmosphere for one hour . after cooling , the now heat - treated sample was then pre - leached in 0 . 5 m h 2 so 4 to remove unstable and inactive species . in the final step , the mixture was heat - treated again under identical conditions to the first heat treatment . the product was labeled as pani - fe - mwnts . although pani is used in this label , it merely indicates that pani was used to prepare the electrocatalyst . there is likely no pani remaining after the heat treatment . the synthesis was repeated under identical conditions with the exception that traditional carbon blacks were used instead of mwnts . the embodiment electrocatalyst labeled as pani - fe - mwnts that was prepared with mwnts as described above was compared to the catalysts prepared with carbon blacks . oxygen reduction reaction ( orr ) activity was electrochemically evaluated using a rotating disk electrode ( rde ). selectivity for the four - electron reduction of oxygen was determined by rotating - ring - disk electrode ( rrde ). performance data was recorded at a total catalyst loading of 0 . 6 milligrams per square centimeter ( mg cm − 2 ) in 0 . 5 m h 2 so 4 at a rotating disk speed of 900 rpm and room temperature . the non - precious metal electrocatalysts were further tested at the fuel cell cathode to evaluate their activity and durability under pefc operating conditions . the cathode catalyst loading was 4 mg cm − 2 . a commercially - available pt - catalyzed cloth gdl ( e - tek , 0 . 25 mg pt cm − 2 ) and nafion 1135 were used as the anode and membrane , respectively . high - resolution transmission electron microscopy ( hr - tem ) images were taken on a jeol 3000f microscope operating at 300 kv at oak ridge national laboratory . the crystallinity of various samples was determined by x - ray diffraction ( xrd ) using a bruker axs d8 advance diffractometer with cu kα radiation . fig1 a and fig1 b compare the orr activities and four - electron selectivities , respectively , of the pani - fe - c electrocatalysts as a function of the support materials ( mwnts , xc - 72 , kj - 300j , and bp2000 ) using rde and rrde . results show similar measured onset potentials of approximately 0 . 91 v for these electrocatalysts , which suggests that the carbon support does not change the nature of active site , but just affects the site distribution reflected by different half - wave potentials ( e 1 / 2 ). the most positive e 1 / 2 was observed with the bp2000 supported catalyst ; this may be attributed to its having the highest bet surface area ( approximately 1400 m 2 g − 1 ), which enables it to accommodate the highest density of active sites . the lowest h 2 o 2 yield was obtained from the kj - 300j supported electrocatalyst , which was below 1 % across at all electrode potentials . this was followed by the embodiment mwnt supported electrocatalyst , which was 2 % h 2 o 2 at 0 . 4 v . such low peroxide yields indicate an almost complete reduction of o 2 to h 2 o in a four - electron process rather than to h 2 o 2 in a much less efficient two - electron reaction . this is a truly unique result for a npmc , matching the four - electron selectivity of pt - based catalysts ( 3 - 4 % h 2 o 2 yield at 0 . 4 v on 14 μg pt cm − 2 pt / c ) [ 16 ]. xc - 72 and bp2000 supported catalyst show slightly higher h 2 o 2 yields , possibly due to their relatively small degree of graphitization [ 17 ]. fig1 c and fig1 d compare the initial fuel cell polarization plots and life test data , respectively , obtained with the electrocatalysts supported on various carbon materials . in good agreement with rde test , fuel cell polarization plots show nearly identical performance for all electrocatalysts at high potential range ( 0 . 8 v ), but the mwnt - supported embodiment electrocatalyst offers a noticeable performance advantage in the low voltage (& lt ; 0 . 3 v ). this performance advantage is likely caused by a more open structure provided by the mwnts relative to that of carbon black [ 18 ]. importantly , in addition to offering mass - transfer benefits at high current densities , the embodiment mwnt - supported electrocatalyst shows virtually no performance degradation for more than 500 hours at a cell voltage of 0 . 40 v constantly generating a current density of 0 . 3 amperes per square centimeter ( acm − 2 ). this represents a significant improvement over the carbon black - supported catalysts that exhibit performance loss . since carbon nanotubes do not possess micro - porosity , except in the interior of the tube , and they tend to pack into a much more open structure than approximately spherical carbon - black particles , the active catalytic sites on the nanotubes are likely to be easily accessible [ 14 ]. also , mass transfer and water removal from the electrocatalyst surface should be facilitated , an important advantage , especially in the case of non - precious catalyst layers approaching 100 μm in thickness [ 19 ]. the observed higher durability of the embodiment mwnt - supported electrocatalyst also may be related to a higher degree of graphitization of mwnts , leading to an enhanced corrosion resistance and improved stability of the orr active site ( s ) [ 20 ]. in order to understand the effect of mwnts on the durability enhancement in a fuel cell , the electrocatalyst was subjected to extensive physical characterization . xrd patterns for the pani - fe catalysts supported by mwnts and kj - 300j are shown in fig2 . the results indicate that heat treatment results in a dominant formation of fes in the pani - derived catalysts [ 4 ]. the sulfur source in the catalyst system is derived from the ( nh 4 ) 2 s 2 o 8 , which was used for polymerizing the aniline to polyaniline ( pant ). unlike for the pani - fe - kj - 300j catalyst , the subsequent acid leaching was more effective for removing fes aggregates from the pani - fe - mwnts sample . this may suggest that , under identical experimental conditions , more active sites could be exposed in pani - fe - mwnts , with a possible improvement in the catalysts activity [ 6 ]. the embodiment mwnt - supported electrocatalyst was also examined using hr - tem . subtleties in its nanostructure are revealed in the images of hr - tem , and high - angle annular dark - field scanning transmission electron microscopy ( haadf - stem ), and sem of the same field of view of for the embodiment electrocatalyst as shown in fig3 . it is very likely that the aggregates observed in the embodiment electrocatalyst are fes , based on the chemical phases identified by x - ray diffraction ( xrd ). importantly , graphene - sheet - like structures were found dominant in the embodiment mwnt - supported electrocatalyst . these graphene sheets are indicated by a label co - located with the fes regions / particles . also mwnts are still obviously present in the catalysts . on the other hand , unlike the embodiment mwnt - supported electrocatalyst , no such graphene sheets were observed in carbon black supported electrocatalysts , such as the pani - fe - kj - 300j sample . thanks to the unique properties of graphene sheets , such as high surface area , good conductivity , and a graphitized basal - plane structure [ 21 ], the presence of graphene - sheet - like structures presumably contributes to the increased catalytic performance of the embodiment mwnt - supported electrocatalyst relative to the carbon black supported pani - fe ones . there appears to be a correlation between the appearance of graphene sheets and higher durability [ 6 ]. in summary , the embodiment mwnt - supported electrocatalyst exhibited much improved performance durability for oxygen reduction when compared to traditional carbon black supported materials . this represents a significant improvement over the carbon black - supported catalysts that exhibit performance loss . the unique structure of carbon nanotube in the pani - fe - mwnt catalyst would be beneficial for mass transfer , water removal from catalyst surface , corrosion resistance and electron conductivity . the presence of graphene - sheet - like structures may contribute to the increased catalytic performance . jaouen et al ., “ recent advances in non - precious metal catalysis for oxygen reduction reaction in polymer electrolyte fuel cells ,” energy environ . sci , 2011 , vol . 4 , pp . 114 - 130 . 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