1、CatalysisScience theseresults indicate that all the catalysts obey a direct dehydroge-nation pathway. The direct oxidation or dehydrogenationmechanism is further confirmed by deconvoluted positivescans for all the catalysts as shown in Fig. 5(cf). Thedeconvoluted positive scans confirm that the supe
2、riority ofthe composite supported Pd-NCs is due to the fact that it fol-lows a direct dehydrogenation pathway through an activeintermediate route.The obtained current values were normalized to the ECSAvalues to obtain the specific activity; the specific activityvalues exhibit enhanced performance, w
3、hich is consistentwith the improved electrochemical active surface areas of thecatalysts. The current values were also normalized to theloading of Pd-NCs to obtain the specific mass activity, wherethe loading was the same for all materials. To evaluate the ef-fect of supported and unsupported Pd-NCs
4、 on support mate-rials, mass activities were calculated and compared. The posi-tive peak potential for Pd-NCs/rGO/MWCNTs appeared nearlyat 0.40 V with a corresponding specific mass activity of ca.1450 A g1Pd, while for unsupported Pd-NCs, the specificmass activity is ca. 400 A g1Pdat 0.60 V. As show
5、n in the CVprofile, the electro-oxidation of formic acid for Pd-NCs/rGO/MWCNTs is also compared with Pd-NCs/rGO and Pd-NCs/MWCNTs. The Pd-NCs/rGO-MWCNTs catalyst shows 10 timeshigher current density/specific mass activity thanunsupported Pd-NCs, 5 times higher activity than Pd-NCs/MWCNTs and 4 times
6、 higher activity than Pd-NCs/rGO cata-lysts for the electro-oxidation of formic acid. The order ofobtained mass activities among all the catalysts is observedto be Pd-NCs/rGO/MWCNTs Pd-NCs/rGO Pd-NCs/MWCNTs Pd-NCs, which is clearly displayed in Fig. 6a. Thehigher activity of the Pd-NCs/rGO/MWCNTs ca
7、talyst is attrib-uted to the higher synergistic effect offered by the rGO/MWCNTs composite support.34This effect can be explainedby correlation of the above discussion of the base CVs of thecatalysts. Overall, catalysts show higher catalytic activity thanalready reported results.3034Table 1 ECSA, pe
8、ak current density of mass activity and specific activity for all catalystsNo. Catalyst ECSAa(m2g1Pd)SAb(mA cm2) MSAc(A g1Pd)1 Pd-NCs 20.9 18.7 3502 Pd-NCs/MWCNTs 42.6 26.4 4853 Pd-NCs/rGO 46.2 57.6 10384 Pd-NCs/rGO/MWCNTs 74.7 78.9 1436aElectrochemical active surface area.bSpecific activity.cMass a
9、ctivity at positive peak scan.Fig. 5 (a) CV results of formic acid oxidation reaction for all catalystsin 0.1 M HClO4+ 1 M HCOOH solution at 50 mV s1. (b) FAORcomparison with commercial Pd/C. (cf) Deconvoluted FAOR positivescans for all catalysts (green line and red line correspond to active andpois
10、on intermediate pathways, respectively.Fig. 6 (a) Column representation of peak current densities at positivescan for formic acid oxidation reaction activity. (b) Positive peakcurrent up to 0.40 V vs. RHE for formic acid oxidation to evaluate theeffect of used carbon based materials on onset potenti
11、al value.Catalysis Science & Technology PaperPublished on 15 February 2016. Downloaded by Xiamen University on 26/02/2016 02:48:26. View Article OnlineCatal. Sci. Technol. This journal is The Royal Society of Chemistry 2016The intercalation of MWCNTs as a spacer into the rGOsheet inhibits restacking
12、 of rGO. Moreover, the utilization ofopen-end pipes as provided by the MWCNTs, and the greatersurface area of rGO sheets, both provide enriched transportpaths and short axial transmission distances for the electronsand electrolyte ions in the electrode. As a result of these phe-nomenon, the composit
13、e support material displays higherspecific capacitance and transference of electrons in anodicapplication. The above stated synergistic effects induced thesignificantly enhanced activity of the overall catalyst material.The features of an excellent catalyst in the case of electro-oxidation have two
14、key parameters i.e. higher current densityand lower onset oxidation potential. In our work the catalystwith the highest oxidation current/mass activity also showedlower onset oxidation potential. The onset oxidation potentialis also improved and shifted towards negative potentials sys-tematically wi
15、th the advancement of each carbon based mate-rial according to the order presented in Fig. 6a. This shift ofonset oxidation potential towards lower potential for carbonsupported Pd-NCs is clearly seen in Fig. 6b. For a better ex-planation of FAOR activity, the CA measurements were alsocarried out at
16、 constant potentials of ca. 0.30 V and 0.40 V forup to 300 seconds as shown in Fig. 7(a and b).A constant decrease in the current values for all the catalystsis observed in current-time transition curves. This decrease isascribed to the poisoning intermediates in the formic acidelectro-oxidation pro
17、cess at lower potentials. In order to fur-ther probe the valuable information about the inherent kinet-ics of the electron transfer process and poisoning tolerance,EIS was carried out along with CA measurements for all thecatalysts, conducted in 0.1 M HClO4 + 1 M HCOOH solution.The EIS results provi
18、ded a strong resemblance with the CAand CV measurements. In Fig. 7(c and d) at potentials of 0.30V and 0.40 V, the Nyquist impedance plots show two arcedareas for all the catalysts. In order to measure the resistanceduring the FAOR process the first arc semicircle is presentedin the inset zoom view
19、for both the potentials. The interceptof the EIS first half semicircle curve with the x-axis representsthe equivalent series resistance (ESR), which mainly arisesfrom the electrolyte resistance since the catalyst effect is ex-cluded. It is clearly seen from the results that the ESR valueof Pd-NCs/rG
20、O/MWCNTs is much lower than that of theother catalysts. After the small arced semicircle area, the datapoints for Pd-NCs/rGO/MWCNTs are directed towards the leftside as compared to the other materials. These results indi-cate that COadsformed during the FAOR process is being oxi-dized at the catalys
21、t surface or the catalyst is struggling to re-move the CO poisoning during FAOR. These results can becorrelated with the CA measurements, the rapid decrease injt transition curve due to the oxidation of poisoning species.The inherent kinetics of the electron transfer process dur-ing FAOR was measure
22、d by Nyquist impedance plots at fivedifferent potentials from 0.20 V to 0.60 V vs. RHE for all thecatalysts, which are provided in Fig. 8. In the case of Pd-NCs/rGO/MWCNTs the impedance arcs from 0.2 to 0.6 V, allshowed in the first quadrant a steady decrease with increas-ing potential range. The ob
23、served steadily decreasing imped-ance arc is an indicator of the rapid and smooth oxidation ofHCOOH. Since the electro-oxidation of surface adsorbedCOadsis drastically accelerated during the FAOR, the forma-tion of COadsby chemisorbed OHadsat the catalysts surfaceoccurs through a dehydration pathway
24、. In the case of Pd-NCs/rGO there is no constant first quadrant kinetics behaviorthrough this potential range, at one point the kinetics alteredand then returned to first quadrant behavior. The other twocatalysts, Pd-NCs/MWCNTs and unsupported Pd-NCs, showabnormal kinetic behavior. The reason for th
25、e non-linear ki-netics is due to the appearance of an increasingly dominantdiffusion-controlled component and a higher poisoning ofthe catalysts by means of poisoning intermediates. Despitethis poisoning effect in all the catalysts, the Pd-NCs/rGO-MWCNTs sample shows higher current density values an
26、dlower onset oxidation potential than the other catalysts atlower potential. The poisoning effect is controlled and over-come due to the surface modifications in the chemistry ofFig. 7 (a, b) Chronoamperometric results for formic acid oxidationreaction at two constant potentials of 0.30 V and 0.40 V
27、 for up to 300s for unsupported and supported Pd-NCs in 0.1 M HClO4+1MHCOOH solution vs. RHE. (c, d) Electrochemical impedance spectro-scopy results at 0.30 V and 0.40 V vs. RHE.Fig. 8 Nyquist plots for electrochemical impedance spectroscopiccurves at five different potentials from 0.20 V to 0.60 V
28、vs. RHE for allthe catalysts.Catalysis Science & TechnologyPaperPublished on 15 February 2016. Downloaded by Xiamen University on 26/02/2016 02:48:26. View Article OnlineCatal. Sci. Technol.This journal is The Royal Society of Chemistry 2016the catalyst. This surface modification is introduced by th
29、ecarbon support material. The electron transfer characteristicof the carbon support with a synergistic effect further im-proves the activity of the catalyst. The comparison of our cat-alyst with previously published Pd based catalysts is providedin Table 2. The role of the geometric factor (nanocube
30、s) isdeeply distinguished and compared with unsupported differ-ent shaped Pd nanoparticles such as polyhedrons, nano-sheets and concave nanocubes of already reported catalysts.Moreover, the role of the support material (rGO/MWCNTs) isalso compared with Pd nanoparticles supported on differentcarbon b
31、ased materials such as carbon black, CNTs,MWCNTs, GN, rGO, GN-CNTs composites. It is shown thatPd nanocubes supported on MWCNTs, rGO and rGO/MWCNTs composite exhibit remarkably higher activities forelectro-oxidation of formic acid. This higher activity may beattributed to the geometric factor (nanoc
32、ubes) and the syner-gistic effect of the support composite material in the cata-lysts. It is noted that the FAOR activity of our catalyst (Pd-NCs/rGO/MWCNTs) is five-fold higher than the commercialPd/C catalyst. These results show a significant breakthroughin anode material construction for applicat
33、ion in directformic acid fuel cells.ConclusionsIn this work, a simple but effective strategy has been appliedto provide a meaningful pathway to enhance the catalytic per-formance of fuel cells. We have reported a facile methodologyto load palladium nanocubes onto a carbon hybrid supportfor applicati
34、on as an anode material in direct formic acid fuelcells. We have found that the intercalation of MWCNTs intorGO sheets not only enhanced the surface area for electro-catalysis, but also increased the transference of electrons inthe oxidation process. These synergistic effects highly en-hanced the an
35、odic current response of the catalysts. The EISresults provided strong evidence for the synergistic enhance-ment due to the electron transfer phenomenon in compositesupport as compared to the other catalysts. Consequently inthe case of our catalyst, the higher electrocatalytic activity forthe formic
36、 acid oxidation reaction results from the synergisticeffect of the carbon support composite material in combina-tion with the fcc 100 facets exposed Pd nanocubes.AcknowledgementsThe authors acknowledge financial support from the NationalBasic Research Program of China (2011CB933700), the Na-tional N
37、atural Science Foundation of China (51572253,21271165) and NSFC-NWO cooperation (51561135011). Thiswork is also supported by USTC and Anhui GovernmentScholarship programmes.References1 R. Parsons and T. Vandernoot, J. Electroanal. Chem.,1988, 257,945.2 A. Capon and R. Parsons, J. Electroanal. Chem.,
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