Patent Application: US-201113094594-A

Abstract:
a fe — co hybrid catalyst for oxygen reaction reduction was prepared by a two part process . the first part involves reacting an ethyleneamine with a cobalt - containing precursor to form a cobalt - containing complex , combining the cobalt - containing complex with an electroconductive carbon supporting material , heating the cobalt - containing complex and carbon supporting material under conditions suitable to convert the cobalt - containing complex and carbon supporting material into a cobalt - containing catalyst support . the second part of the process involves polymerizing an aniline in the presence of said cobalt - containing catalyst support and an iron - containing compound under conditions suitable to form a supported , cobalt - containing , iron - bound polyaniline species , and subjecting said supported , cobalt - containing , iron bound polyaniline species to conditions suitable for producing a fe — co hybrid catalyst .

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 . briefly , a hybrid fe — co catalyst was prepared in a two part process . the hybrid fe — co catalyst may be used in an electrode for the oxygen reduction reaction taking place in an electrochemical cell such as a polymer electrolyte fuel cell ( pefc ). the details of the synthesis are provided ( vide infra ). the performance of the hybrid fe — co catalyst was determined using a set - up including a rotating disk electrode ( rde ). a chi electrochemical station ( model 750b ) was used with the rde in a conventional three - electrode cell at a rotating disk speed of 900 rpm at room temperature . unless otherwise noted , the catalyst loading on the rde was held at 0 . 6 mg / cm − 2 . graphite - rod and ag / agcl ( 3 m nacl , 0 . 235 v vs . rhe , rhe is reversible hydrogen electrode ) were used as the counter and reference electrodes , respectively . orr steady - state polarization curves were measured in oxygen - saturated 0 . 5 m h 2 so 4 electrolyte with a potential step of 0 . 03 v and a period time of 30 s . pani - derived non - precious metal catalysts , such as the fe — co hybrid catalyst , were tested at the fuel cell cathode to evaluate their activities and durabilities under pefc operating conditions . catalyst inks were prepared by ultrasonically mixing catalyst powders with nafion ® solution for four hours . the inks were then applied to the gas diffusion layer ( gdl , elat lt 1400w , e - tek ) by successive brush - painting in layers until the cathode catalyst loading reached 4 mg cm − 2 . the nafion ® content in the dry catalyst was controlled around 35 wt %. a commercially - available pt - catalyzed cloth gdl ( e - tek , 0 . 25 milligrams of platinum per square centimeter ( mg pt cm − 2 ) was used as the anode . the cathode and anode were hot - pressed onto the opposite sides of a nafion ® 212 membrane . the geometric area of the mea was 5 . 0 cm 2 . fuel cell testing was carried out in a single cell with serpentine anode / cathode flow channels . pure hydrogen and oxygen humidified at 85 ° c . were supplied to the anode and cathode at a flow rate of 200 and 400 ml min − 1 , respectively . both electrodes were maintained at the same backpressure of 2 . 8 bar ( about 40 psi absolute pressure at the los alamos altitude ). fuel cell polarization plots were recorded using fuel cell test stations ( fuel cell technologies , inc .). the reference polarization fuel cell plot with a pt / c catalyst at the cathode was recorded at a cell temperature of 80 ° c . and backpressures of 2 . 8 ( h 2 )- 2 . 8 bar ( air ). elemental quantification and species analysis using x - ray photoelectron spectroscopy ( xps ) were performed using an esca 210 and microlab 310d spectrometer . mid - infrared spectra were recorded with a nicolet 670 ftir spectrometer on potassium bromide ( kbr ) pellets . the sample morphology was characterized by scanning electron microscopy ( sem ) on an fei quanta 400 esem instrument . high - resolution transmission electron microscopy ( hr - tem ) images were taken on a jeol 3000f microscope operating at 300 kv . the crystallinity of various samples was determined by x - ray diffraction ( xrd ) using a bruker axs d8 advance diffractometer with cu kα radiation . to take advantage of both co and fe catalyst properties with the aim of achieving active and durable non - precious metal catalysts , the present invention provides a non - precious metal oxygen reduction reaction catalyst and method for preparing the catalyst . the synthesis of this hybrid catalyst is a two part synthesis . the first part involves preparing a cobalt - containing catalyst support . this support has catalytic properties by itself , but is still referred to as a catalyst support because of its support function in the final co — fe hybrid catalyst . as fig2 a shows , in the first part of the synthesis , the catalyst support is synthesized by heat - treating a co species chelated with an ethylamine called ethylenediamine . the catalyst support is referred to in fig2 a as eda - co — c . although eda is part of the name shown in fig2 a , it should be understood that the catalyst support likely contains no eda . the eda is meant to convey that eda was used . this catalyst support is believed to aid catalyzing the oxygen reduction of the co — fe hybrid catalyst to more positive potentials than traditional carbon supporting materials [ 6 ]. in an embodiment of the first part , ethylenediamine ( eda ) was combined with cobalt nitrate hexahydrate ( co ( no 3 ) 2 . 6h 2 o ) to form an eda - co chelate complex in ethanol solution . this chelate complex was thoroughly impregnated into hcl - treated carbon black ( ketjenblack ec - 300j , lion akzo co . ltd ). suitable examples of carbon black are available commercially under the trade names vulcan ™ xc - 72 ( available from cabot corp ., alpharetta , ga . ), ketjen black ™ ec 300 j ( available from lion akzo co ., ltd . ), and black pearl ™ 2000 ( available from cabot corp ., alpharetta , ga .). after vacuum - drying using a rotary evaporator and subsequent high - temperature heating ( 900 ° c . in n 2 atmosphere for one hour ) and acid leaching treatments ( 0 . 5 m h 2 so 4 at 80 ° c . for eight hours ), the final eda - co — c catalyst support was obtained . the name eda - co — c does not imply that this catalyst support , after heating at 900 ° c . has any ethylenediamine left in the support , but that the support was derived from eda . this final catalyst support was used in the second part , shown in fig1 b . in an embodiment of the second part , shown in fig2 b , a pani - fe complex is anchored to the catalyst support and heated treated to produce a co — fe containing hybrid catalyst . as fig2 b shows , polyaniline ( pani ) was polymerized in - situ onto the catalyst support in the presence of iron ( iii ) chloride ( fecl 3 ) and ammonium persulfate (( nh 4 ) 2 s 2 o 8 ). ammonium persulfate is an oxidizing agent . the resulting suspension was treated by heating at 900 ° c . in n 2 gas for one hour . the result of this heating was carbonization of the pani . in an embodiment , also shown in fig2 b , subsequent chemical leaching was accomplished using sulfuric acid ( 0 . 5 m h 2 so 4 at 80 ° c . for eight hours ) followed by a second heat treatment ( 900 ° c . in n 2 gas for three hour ). this subsequent treatment further enhances the orr activity by exposing more active sites and adjusting the surface hydrophilicity of the catalyst . in an embodiment of the two part synthesis for preparing a fe — co hybrid catalyst of this invention , ketjenblack ec 300j ( akzonobel ) with bet surface area of about 950 m 2 g − 1 and good corrosion resistance was treated in hcl solution for 24 hours to remove metal impurities . 2 . 0 ml eda was reacted with 0 . 50 g co ( no 3 ) 2 . 6h 2 o ) in 500 ml ethanol solution . the resulting chelate complex was thoroughly impregnated into 0 . 40 g of the hcl - treated carbon black . after vacuum - drying using a rotary evaporator and subsequent high - temperature heating ( 900 ° c . in n 2 atmosphere for one hour ) and acid leaching treatments ( 0 . 5 m h 2 so 4 at 80 ° c . for eight hours ), the final eda - co — c catalyst support was ready for next part , which began by dispersing 0 . 4 grams of the catalyst support with an amount of about 2 - 3 milliliters polyaniline in 500 ml 0 . 5 m hcl solution . the resulting suspension was kept below 10 ° c . while 5 . 0 g of the oxidant ammonium peroxydisulfate ( aps ) and 3 . 0 g fecl 3 were added . after constant mixing for 24 hours to allow the polymerized pani to uniformly mix and cover the carbon black particles , the suspension was vacuum - dried using a rotary evaporator . the subsequent heat - treatment was performed at 900 ° c . in an inert atmosphere of a nitrogen gas for 1 hour . the heat - treated sample was then pre - leached in 0 . 5 m h 2 so 4 at 80 ° c . for 8 hours to remove unstable and inactive species from the catalyst , and thoroughly washed in de - ionized water . finally , the catalyst was heat - treated again in nitrogen - gas atmosphere for 3 hours . the product prepared according to the two part synthesis described above will be referred to hereafter as the fe — co hybrid catalyst or the embodiment fe — co hybrid catalyst . the oxygen reduction reaction ( orr ) catalytic activity for the fe — co hybrid catalyst was measured , as were the activities for eda - co — c alone ( i . e . the catalyst support ), pani - fe — c alone , and a reference material , which was e - tek 20 wt % pt / c catalyst . the activities were measured with a rotating disk electrode ( rde ). fig2 a shows graphs that compare the activities . as fig2 a shows , relative to the heat - treated carbon ( e - tek ), a significant improvement in activity is observed with the eda - co — c sample as the onset orr potential was positively shifted from 0 . 4 to 0 . 80 v . in contrast , the pani - fe catalyst , which uses a different nitrogen precursor and transition metal , exhibits much higher orr activity , which is seen as even further positive shifts in onset and half - wave potentials ( e 1 / 2 ) to 0 . 91 and 0 . 81v , respectively . in the case of the hybrid catalyst , the utilization of eda - co — c as a support leads to a 20 mv positive shift in half - wave potential in rde tests as compared to the isolated pani - fe — c catalyst . a vigorous debate has ensued regarding whether metal atoms in these types of catalysts participate directly in the active sites [ 1 , 2 , 7 , 8 , 9 ] or merely catalyze the formation of active sites from carbon , nitrogen , and perhaps oxygen atoms [ 10 , 11 , 12 , 13 ]. it is believed that the nature of the active sites for the oxygen reduction reaction is different for co - based catalysts and fe - based catalysts . without wishing to be bound by any particular theory or explanation , it is believed that the co species of fe — co hybrid catalysts facilitates the creation of favorable c — n x morphologies , which contributes to the observed improvement in orr activity . it is also believed that fe species loaded onto the catalyst support contributes to the production of highly active , complexed fe species at these c — n morphologies . loading differences notwithstanding , the performance gap , expressed in terms of e 1 / 2 , as measured in rde experiments , between the embodiment fe — co hybrid catalyst ( 600 micrograms of catalyst per square centimeter ) and pt / c ( 60 micrograms of platinum per square centimeter ) has been reduced to approximately 35 mv . in accordance with the accelerated stress testing ( ast ) protocols proposed by the united states department of energy for fuel cell cathode catalysts , the durability of the embodiment hybrid fe — co catalyst was tested under voltammetric cycling conditions in the potential range from 0 . 6 to 1 . 0 v in n 2 - saturated 0 . 5 m h 2 so 4 . as fig2 b shows , only a 10 mv loss in half - wave potential was observed after 5 , 000 cycles . a kinetic parameter known as the tafel slope was determined to further evaluate the orr kinetic character for these materials . kinetic current densities , extracted from the steady - state polarization plot , were used to plot the tafel curves shown in fig2 c . these curves were plotted according to the koutecky - levich equation . using a linear fitting , the values of the tafel slopes are found to be approximately 59 and 84 mv dec − 1 for the eda - co — c catalyst support and pani - fe — c catalysts , respectively . the eda - co — c catalyst support has a lower tafel slope , similar to that of pani - co — c ( 67 mv dec − 1 ), but is significantly less orr active . the pani - fe — c catalyst is more active and exhibits a more than 100 mv positive shift in oxygen - reduction onset potential when compared to eda - co catalyst . this significant difference in the tafel slopes suggests different active sites and different mechanism . it is worth noting that the tafel slope for the embodiment fe — co hybrid catalyst ( 87 mv / dec ) is nearly the same as that of the pani - fe — c , suggesting that the fe - based active sites are important to the overall catalyst activity . in the case of the pt catalyst ( e - tek ), a tafel slope of 60 mv / dec was obtained at a high potential range (& gt ; 0 . 8 v ), while at low potential range , a value of 120 mv dec − 1 was observed , indicating that the orr mechanism on a pt / pto surface ( at high potential ) is different from that of a metallic pt surface ( at low potential ) [ 14 ]. in theory , a tafel slope of 120 mv dec − 1 is due to the rate - determining step associated with the first electron transfer , while a tafel slope of 60 mv dec − 1 has been explained by the migration rate of adsorbed oxygen intermediates with a temkin isotherm [ 15 ]. hence , the tafel slope of approximately 59 mv dec − 1 that was observed with the catalyst support suggests that the orr rate is primarily determined by the migration of adsorbed oxygen intermediates . for the embodiment hybrid catalyst and the pani - fe catalyst , tafel slopes between 80 and 90 mv dec − 1 are indicative of a more complicated orr mechanism . the rate - determining step most likely simultaneously involves both intermediate migration as well as charge transfer [ 16 ]. the influence of temperature on the oxygen reduction reaction was determined for pani - fe — c and the embodiment hybrid fe — co catalyst in 0 . 5 m h 2 so 4 electrolyte to measure the relative activation energies for oxygen reduction . according to eq . ( 1 ) [ 17 ], the standard apparent electrochemical energy of activation , e a , can be estimated by measuring the slope of the arrhenius plot , i . e . log j vs . 1 / t . fig2 d provides the arrhenius plot constructed at constant potential chosen in the kinetic region 0 . 88 and 0 . 90 v for these two catalysts , respectively . the e a for the oxygen reduction reaction of the embodiment fe — co hybrid catalyst ( of 27 . 4 kj / mol ) is much lower than that for pani - fe — c ( 42 . 3 kj / mol ). fuel cell polarization data recorded on the eda - co — c catalyst support , the pani - fe — c , and the embodiment fe — co hybrid catalyst are compared in the plot shown in fig3 a . in good agreement with the rde results , the open - cell voltage ( ocv ) value measured with the eda - co — c catalyst support was approximately 0 . 78 v , which is significantly lower than the pani - fe — c ( 0 . 95 v ). the ocv of the pani - fe — c is comparable to that of the embodiment fe — co hybrid catalyst . from these data , it is apparent that the hybrid catalyst performed noticeably better when the voltage was lower than 0 . 90 v . this provides further support for the promotional role of the catalyst support in oxygen reduction reaction activity . under these experimental conditions , the embodiment fe — co hybrid catalyst generates a current density of 0 . 11 a / cm 2 at 0 . 8 v . using standard testing protocols ( h 2 — o 2 / 1 . 0 - 1 . 0 bar ), in the optimized cathode layer ( see fig4 ), the catalyst volumetric activity at 0 . 8 v is 98 a / cm 3 . table 1 below shows some performance metrics for the embodiment hybrid fe — co catalyst . previous results on pani - fe — c catalysts indicate that the overall operating voltage can significantly affect their performance . for example , it was reported that no degradation was observed at 0 . 4 v , but greater than 50 % performance was lost after 200 hours of operation at 0 . 6 v [ 4 ]. it is believed that the operating voltages affect the oxidation state of the active species and change the hydrophilicity of the catalyst layer , resulting in dramatic performance variations . to study such degradation effects , the hybrid fe — co catalyst was periodically cycled between 0 . 4 and 0 . 6 v during the fuel cell life test . the results are shown in fig5 a and fig5 b . fig5 shows plots illustrating cycled systems during rotating disk electrode ( rde ) testing . the rde included a loading of 0 . 60 mg / cm 2 , and a medium of 0 . 5m sulfuric acid ( h 2 so 4 ). the disk rotated at a rate of 90 ° rotations per minute ( rpm ). the reference electrode was a ag / agcl electrode in 3 molar sodium chloride ( 3m nacl ). the counterelectrode was a graphite rod . a steady state potential program was employed . the ocp was 300 seconds ; 30 millivolt steps ; 30 seconds per step . cycling was at 25 ° c . and 60 ° c ., and the scan rate was 50 millivolts per seconds , from zero to 1 . 0 volts . both plots are for the fe — co hybrid catalyst . the medium is 0 2 - saturated 0 . 5m h 2 so 4 . fig5 a shows data plotted for 20 cycles and for 2500 cycles . the top curve plots data for 2500 cycles , and the bottom curve plots data for 20 cycles . fig5 b shows cycling in o 2 - saturated 0 . 5m h 2 so 4 electrolyte after the 20 th cycle and after the 2500 th cycle . as these plots show , there was loss of performance in the o 2 - saturated electrolyte for the hybrid catalyst . performance was observed to decrease at 0 . 6 v , and it remained stable at 0 . 4 v . partial performance recovery was observed when the catalyst was shifted from 0 . 4 to 0 . 6 v for short time . however , however , long - term operation at 0 . 6 v results in irreversible degradation , which suggests that the active catalytic sites are permanently blocked by local water flooding or damaged in the high potential electrochemical environment . fig6 a and 6 b show results plotted for the effect of humidity on durability . the fuel cell life of the hybrid fe — co catalyst was examined at different humidification conditions ( a ) and low catalyst loading ( b ). the anode loading was 0 . 25 milligrams of pt per square centimeter . the membrane was nafion 212 . the cell operating temperature was 80 ° c . fuel cell performance at 0 . 60 v in a lowered humidity environment was seen to be essentially reversible . the flooding mechanism , rather than active site degradation is likely responsible for much of the catalyst performance loss . however , low catalyst loading ( i . e . a thinner layer ) is still unable to improve the durability . perhaps the flooding occurred near active sites in micropores . x - ray photoelectron spectroscopy ( xps ) was used to analyze each non - precious metal catalyst at different stages of synthesis . quantitative elemental analysis indicates a nitrogen content increase from 1 . 93 atomic present ( at %) for the eda - co — c catalyst support to 5 . 49 at % for pani - fe catalysts , while the hybrid fe — co catalyst lead to an even higher nitrogen content of 6 . 00 at %. the transition metals in the hybrid catalyst that were subjected to a thorough acid leach were found to be around 0 . 81 at % for fe and undetectable for co . in non - precious metal orr catalysts of this type , the metal free doped nitrogen atoms into the carbon structures are important to the overall catalytic mechanism . the n is spectra for these non - precious metal catalysts are shown in fig7 . the two dominant nitrogen peaks are correlated to quaternary ( 401 . 1 ev ) and pyridinic ( 398 . 5 ev ) nitrogen , and reflect the relative types of nitrogen - atom doping at the interior and edge of graphitized carbon planes , respectively [ 18 ]. when compared to the catalyst support , the pani - fe sample has a relatively higher peak intensity for quaternary nitrogen , indicating that nitrogen atoms favor doping at the interior rather than at the edges of the graphene layer in the presence of fe . during the pyrolysis , pyridinic and quaternary nitrogen are in equilibrium , which is shifted towards quaternary nitrogens with the addition of fe metals [ 19 , 20 ]. it is generally accepted that pyridinic nitrogens facilitate oxygen reduction [ 6 ]. however , these data indicate that the less pyridinic n - containing fe - based catalysts are more active than the pyridinic n - rich co - based materials . the active sites related to the fe species are likely responsible for this enhanced activity in the hybrid catalyst . the morphology of the embodiment hybrid fe — co catalyst was studied using electron microscopy images ( sem and hr - tem ) at different stages in its synthesis . the morphology of the catalyst support was dominated by carbon nanostructures resembling nanotubes , nanofibers , and onion - like carbon , which result from the carbonization of the ethylenediamine . a possible growth mechanism for the metal - encapsulated onion - like carbon in this material can be approximated based on a vapor - liquid - solid model [ 18 ]. briefly , during thermal treatment in an inert atmosphere at 900 ° c ., the eda - co complex decomposes into a mixture of gaseous carbon and nitrogen along with co metal particles . these gaseous carbon and nitrogen species are gradually captured by the metal nanoparticles , which act to catalyze the formation of small nitrogen - doped carbon fragments . structural defects in these fragments , especially the dangling bonds at their , possibly act as nucleation points for further assembly and rearrangement to form the final layered structure on the surface of metal particles . of note , these small graphitic onion layers are not perfectly concentric to each other , and usually form graphitic layers that have dislocations and irregular relative curvatures . the pani - fe catalyst exhibits significant different morphologies . no dominant graphitized carbon nanostructures were observed . this significant difference highlights the major role of the different transition metals and nitrogen precursors in influencing the overall catalyst nanostructure . it is worth noting that graphene sheet structures , not observed in the catalyst support and pani - fe systems , were abundant in the fe — co hybrid catalyst . the significant changes in the various nanostructures are perhaps are due to the simultaneous presence of co and pani because similar graphene structures are also abundant in the support catalysts . the catalytic role of co metal may be to effectively catalyze pani decomposition at the atomic level , allowing for the rearrangement of the carbon and nitrogen atom , to form highly graphitic , potentially n - doped , graphene sheets [ 21 ]. in these materials , the properties of the graphene sheets ( e . g . high surface area , good conductivity , a graphitized basal - plane structure ) may contribute to the increased catalytic performance of the hybrid fe — co catalyst relative to the eda - co — c and pani - fe — c . there appears to be a correlation between the appearance of graphene sheets and higher catalyst durability [ 4 ]. the performance of the embodiment fe — co hybrid catalyst of this invention was compared to other fe — co hybrid catalysts . fig8 shows a graph of current density versus potential for three hybrid catalysts . the graph labeled 1 refers to the fe — co hybrid catalyst reported in wu et al ., “ nitrogen - doped magnetic onion - like carbon as support for pt particles in a hybrid cathode catalyst for fuel cells ,” j . mater . chem ., february 2010 , vol . 20 , pp . 3059 - 3068 . the graph labeled 2 refers to the fe — co hybrid catalyst reported in subramanian et al ., “ nitrogen - modified carbon - based catalysts for oxygen reduction reaction in polymer electrolyte membrane fuel cells ,” j . power sources , november 2008 , vol . 188 , pp . 38 - 44 . the graph labeled 3 refers to the fe — co hybrid catalyst reported in wu et al ., “ polyaniline - derived non - precious catalyst for the polymer electrolyte fuel cell cathode ,” ecs trans ., october 2008 , vol . 16 , pp . 159 - 170 . the graph labeled 4 refers to the embodiment fe — co hybrid catalyst prepared according to the two part synthesis shown in fig1 a and fig1 b . according to fig8 , catalyst 4 , i . e . the embodiment fe — co hybrid catalyst shows a significant improvement in activity based on the positive shifts in onset potential and half - wave potential relative to catalyst 1 , catalyst 2 and catalyst 3 . some parameters for the rotating disk electrode generated data are : loading was 0 . 60 mg / cm2 ; 0 . 5 m h 2 so 4 electrolyte ; 900 rpm ; temperature was 25 ° c ., ag / agcl ( 3 m nacl ) reference electrode ); graphite rod counterelectrode ; steady - state potential program ; ocp , 300 s , 30 mv steps , 30 seconds per step . electrochemical cells that include cathodes with fe — co hybrid catalysts of this invention are useful as fuel cells . embodiments of these electrochemical fuel cells of the present invention , like other fuel cells , convert fuel and oxidant to electricity and reaction product . these cells include ionomeric membranes such as a poly ( perfluorosulphonic acid ) membrane which is commercially available as nafion ® 117 , aciplex ®, or flemion ®. other ionomeric membrane materials known in the art , such as sulfonated styrene - ethylene - butylene - styrene ; polystyrene - graft - poly ( styrene sulfonic acid ); poly ( vinylidene fluoride )- graft - poly ( styrene sulfonic acid ); poly ( arylene ether ), such as poly ( arylene ether ether ketone ) and poly ( arylene ether sulfone ); polybenzimidazole ; polyphosphazene , such as poly [( 3 - methylphenooxy ) ( phenoxy ) phosphazene ] and poly [ bis ( 3 - methylphenoxy ) phosphazene ]; derivatives thereof ; and combinations thereof may also be used . in summary , a hybrid fe — co catalyst for oxygen reaction reduction was prepared . this hybrid catalyst was stable even after 5 , 000 potential cycles in aqueous electrolyte , and durable to constant voltage testing at 0 . 40 v in a real - world fuel cell system . these performance enhancements are thought to be due to the greatly reduced activation energy and the catalyzed formation of graphene - sheet - like structures in the catalyst . catalyst morphology at different synthesis stages was characterized using scanning electron microscopy ( sem ). graphene sheet structures were abundant in the hybrid catalyst . loading differences notwithstanding , the performance gap in terms of e 1 / 2 in rde testing between the hybrid fe — co catalyst ( 600 μg / cm 2 ) and pt / c ( 60 μg pt / cm 2 ) catalysts is approximately 35 mv in acidic media . the hybrid fe — co catalyst performed better than other known hybrid fe — co catalysts . whereas particular embodiments of the present invention have been illustrated and described , it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention . it is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention . lefevre et al ., “ iron - 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