journal of the electrochemical society 164 0013-4651/2017...
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Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F2170013-46512017164(4)F2177$3700 copy The Electrochemical Society
Platinum Supported on Functionalized Carbon Nanotubes forOxygen Reduction Reaction in PEMAEM Hybrid Fuel CellsWenjiao Huangab John M Ahlfieldalowast Xinsheng Zhangb and Paul A Kohlalowastlowastz
aSchool of Chemical and Biomolecular Engineering Georgia Institute of Technology Atlanta Georgia30332-0100 USAbState Key Laboratory of Chemical Engineering East China University of Science and Technology Shanghai 200237Peoplersquos Republic of China
Platinum supported on oxygen and nitrogen functionalized carbon nanotubes are evaluated and compared to the commerciallyavailable platinum catalysts (PtC) in hybrid PEMAEM H2O2 and direct methanol fuel cells (DMFCs) The functionalized carbonnanotubes are synthesized by the sonichemical method The existence of oxygen and nitrogen functional groups on the nanotubes isconfirmed by X-ray photoelectron spectroscopy (XPS) The size of the platinum structures on the functionalized carbon nanotubes ismeasured by transmission electron microscopy (TEM) and the crystallographic properties are evaluated by X-ray diffraction (XRD)The performance of the catalyst for the oxygen reduction reaction (ORR) in each hybrid fuel cell is found to be dependent on thetype of the functional groups on the carbon nanotubes The platinum supported on both nitrogen and oxygen functionalized carbonnanotubes(PtCNTON) and on oxygen functionalized carbon nanotubes (PtCNTOX) are more effective in hybrid H2O2 fuel cellscompared to the PtC However the PtCNTON is not stable in hybrid DMFCs while PtCNTOX exhibited the best performanceThe result suggests that CNTON may facilitate undesired methanol crossover reactionscopy 2017 The Electrochemical Society [DOI 10114920191704jes] All rights reserved
Manuscript submitted November 7 2016 revised manuscript received January 11 2017 Published January 24 2017 This was Paper1381 presented at the Phoenix Arizona Meeting of the Society October 11ndash15 2015
Fuel cells are very attractive technology for conversion of chem-ical energy to electrical energy1 The technology offers a clean andefficient alternative for energy in portable electronics and automo-tive transportation Fuel cells can have high efficiency and producezero emissions when using hydrogen as the fuel However fuels likemethanol are more convenient than hydrogen gas because of easyhandling and storage2 The most commonly used type of fuel cellis the proton exchange membrane (PEM) fuel cell with a Nafionmembrane3ndash7 However the PEM fuel cell faces problems includinghigh cost of noble-metal catalysts and carbon monoxide poisoning8
For PEM direct methanol fuel cells (DMFCs) there is the additionalproblem of methanol crossover910 Methanol crossover leads to achemical short circuit at the cathode which decreases the efficiencyof PEM DMFCs11 Anion exchange membrane (AEM) fuel cells area relatively new alternative to PEM cells Non-noble catalysts arepossible because AEM fuel cells operate in alkaline conditions12 andplatinum catalysts can have longer life span due to higher resistanceto carbon monoxide poisoning in AEM fuel cells13ndash16 Additionallycathode flooding17 is less of a problem because water is consumed atthe cathode (not produced as in PEM cells)18 For the AEM DMFCthe undesirable methanol crossover is potentially lower because ofthe reverse direction of ion flow However lower ionic conductivityand the reactantsrsquo dependence on relative humidity remain obstaclesto commercialization19
The hybrid fuel cell has been designed to exploit the advantages ofAEM and PEM fuel cells2021 In the hybrid configuration the cathodeis operated at high pH while the anode is operated at low pH as shownin Figure 1 The ionomer at the cathode is an anion exchange ionomer(AEI) such as poly-(arylene ether) with octafluoro-biphenyl groups22
The anode can be made using Nafion as the ionomer Water is formedon the interface of PEMAEM cathode junction due to the flow ofprotons from the anode to the cathode Since the oxygen reduction re-action (ORR) at the cathode consumes water the water produced at theinterface can provide self-humidification for the cathode21 Addition-ally the high pH environment can lower carbon monoxide poisoningThe electrode reactions in a hybrid H2O2 fuel cell are as follows
Anode H2 rarr 2H+ + 4eminus
Cathode O2 + 2H2O + 4eminus rarr 4OHminus
lowastElectrochemical Society Student MemberlowastlowastElectrochemical Society Fellow
zE-mail kohlgatechedu
The electrode reactions in hybrid DMFCs are as follows
Anode CH3OH + H2O rarr 6H+ + 6eminus + CO2
Cathode O2 + 3H2O + 6eminus rarr 6OHminus
In order to decrease the cost of the fuel cell there is interest in creat-ing high efficiency non-platinum catalysts especially for the cathodePlatinum catalysts supported on carbon have been the best electro-chemical catalysts for the oxygen reduction reaction but the stabilityremains the problem2324 Developing a superior catalyst support couldimprove the properties of platinum catalyst2526 Carbon nanotubes area stable material found to have a high electrical conductivity becauseof their three-dimensional structure27 As a result carbon nanotubesare a promising support material for electrocatalysts28 Based on thestudies of Li et al29 platinum supported on carbon nanotubes havehigher activity than platinum supported on carbon black for ORR in
Figure 1 Configuraion of a cathode hybrid fuel cell
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F218 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Table I Physical Properties of the AEI in this study
Ionic functional group Quinuclidine
Structure of the groupWater uptake () 25
Ion exchange capacity (mmolg) 13
DMFCs At the same time introducing oxygen containing groups andnitrogen containing groups on the carbon support have significantlychanged the activity of the platinum catalysts The doping of oxy-gen could activate the π electrons of carbon nanotubes because oftheir high electron density which makes O2 utilization more efficientIshizaki et al30 showed that the surface functional groups play a moreimportant role than the bulk oxygen content and defect sites of thenanocarbon in ORR The order of the most effective oxygen containinggroups for ORR activity is as follows carboxyl gt carbonyl gt hydroxylgroups Zhong et al31 synthesized two types of oxygen doped carbonnanofiber supported platinum catalysts and through rotating disk elec-trode studies found that the carboxylic acid groups were more activethan the hydroxyl groups Nitrogen containing groups on carbon nan-otubes can improve the performance of conversion devices becauseelectron-rich nitrogen can activate π electrons of carbon nanotubesthrough conjugation with their lone-pair electrons Artyushkova etal32 believe that the pyrrolic nitrogen can catalyze O2 reduction tohydrogen peroxide and pyridinic can reduce hydrogen peroxide towater in a 2eminus reaction Also Xing et al33 found that the carbon atomneighboring a pyridinic nitrogen should be the main site for ORRamong pyrrolic pyridinic and graphic nitrogen doped carbon
In this paper the surface of carbon nanotubes functionalized withoxygen-containing and nitrogen-containing groups were studied inorder to evaluate effectiveness of the catalysts for ORR in hybridH2O2 fuel cell and hybrid DMFCs
Experimental
Catalysts preparationmdashCarbon nanotubes (CNTs) (gt95 car-bon Aldrich) with diameter of 6 to 9 nm and length of 5 μm wereused as the catalyst support The CNTs were functionalized via sono-chemical treatment34 For the oxidized CNT (CNTOX) 02 g of CNTpowder was added to a solution of nitric acid (180 ml) and sulfuricacid (160 ml) followed by sonication for 2 hours at 60C To obtainCNTON 02 g CNTOX was suspended in 100 ml ammonia hydroxideand sonicated for additional 2 hours at 60C
The synthesis of 40 wt PtCNTOX and PtCNTON was carriedout by the chemical reduction method31 100 mg of CNTOXCNTONwas dispersed in 18 ml ethylene glycol Then 6 ml of 20 wt chloro-platinic acid in ethylene glycol was added dropwise to the suspension
Figure 2 Platinum ratio by TGA
Figure 3 TEM image of (a) PtC (b) PtCNTOX (c) PtCNTON and (d)XRD patterns
After stirring for 12 hours 2 ml NN-dimethylformanide was addedThe pH was set to 13 by adding sodium hydroxide in ethylene glycolsolution The mixture was held at 130C in an oil bath for 4 hours whilestirring The solid catalyst was collected and washed with deionizedwater The final solid product was dried at 120C in vacuum overnight
The electrocatalytic performance of the catalysts for ORR wastested in AEMPEM hybrid fuel cell The high pH electrode (AEMelectrode) was fabricated with PtC PtCNTOX and PtCNTON cat-alysts and an anion exchange ionomer (AEI) The metal loading ofthe electrodes was 1 mgcm2 The AEI is a poly (arylene ether) (PAE)ionomer with octafluoro-biphenyl groups22 The physical propertiesof the AEM are shown in Table I The AEI was stored as a 2 wtsolution in dimethylformamide
The catalyst ink for AEM electrode was prepared by mixing AEMelectrode catalysts and AEI with 300 mg of isopropyl alcohol (IPA)The ionomer-to-catalyst ratio was investigated at 10 15 1720 for each catalyst to determine the ratio for optimal performancein the hybrid hydrogen fuel cell and hybrid DMFC The ionomer-to-catalyst ratio that exhibited the best performance for each catalystwas compared The catalysts ink for PEM electrode was prepared bymixing Pt-RuC catalysts (50ndash25) 100 mg of deionized water5 wt Nafion and 300 mg propanol The ionomercatalysts ratiowas 15 The slurry was sonicated for 30 minutes and sprayed ontohydrophobic (Toray TGPH-090) and hydrophilic (Toray 2050L) gasdiffusion layer (GDL)20 The PEM electrode dried with a heat gunfor 30 minutes The AEM electrodes were soaked in 1 M sodiumhydroxide for 30 minutes and soaked in deionized water overnightThe hybrid membrane electrode assembly (MEA) was prepared byspraying 5 wt Nafion in propanol onto the surface of each electrodeNafion 115 membrane was pressed between the two electrodes for 20minutes at 212F and 2 MPa
Physical characterizationmdashThe transmission electron mi-croscopy (TEM) was carried out with a Hitachi HT7700 TEM todetermine the size of the platinum supported on the CNTs X-raydiffraction (XRD) was carried out on a PANalytical X-ray facilityto investigate crystallographic properties of catalysts The surfaceproperties of the catalysts were investigated by X-ray photoelectronspectroscopy (XPS) on a Thermos Scientific K-Alpha XPS Thermo-gravimetric analysis (TGA) was carried out on a TGA Q50 from TA
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Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F219
Figure 4 XPS results of C1s peak (a) PtC (b) PtCNTOX (c) PtCNTON
Instrument to determine the platinum content in catalysts The catalystwas loaded in the platinum pan and heated at a ramp rate of 20Cminto 800C The sample was held at 800C for 2 hours with ambientairflow
Results and Discussion
Characterization of the catalystsmdashTGA was used to investigatethe platinum content of the catalysts in ambient air environment Thecatalysts were ramped to high temperatures (800C) where the carboncontent completely burned away within two hours The remainingweight percentage was taken to be the platinum content The TGAresults are shown in Figure 2 The platinum content of the commercialcatalyst (PtC) was determined to be 370 wt which is similar tolabeled content of 40 wt The platinum ratio of PtCNTOX andPtCNTON was found to be 346 wt and 426 wt respectively
TEM revealed that the small particle size Pt was attached andwell dispersed on the surface of the CNTs (Figures 3a 3b and 3c)The particle size of platinum on carbon oxidized carbon nanotubes(CNTOX) and nitrogen doped oxidized carbon nanotubes (CNTON)are calculated by taking the average size from the TEM analysis
Figure 5 XPS results of O1s peak (a) PtC (b) PtCNTOX (c) PtCNTONand (d) N1s peak of PtCNTON
which was 465 nm 371 nm and 443 nm respectively The platinumsupported on CNTOX had the smallest particle size
The XRD patterns shows the crystallographic properties of thecatalysts Figure 3d shows that the Pt (111) peak is the main diffractionpeak for all the three catalysts while the diffraction pattern of Pt (220)and Pt (311) have only weak peaks for PtCNTON compared to theother two catalysts
X-ray photoelectron spectroscopy (XPS) has been used to charac-terize the oxygen and nitrogen content on the functionalized carbonnanotubes as shown in Figure 4 and Figure 5 Tables II and III showthe binding energies for carbon oxygen and nitrogen Table IV shows
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F220 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Table II Binding energy of C1s peaks from XPS
Binding Energy
Peak 1(2848 eV)
Peak 2(2860 eV)
Peak 3(2878 eV)
Peak 4(2888 eV)
Carbonates(2908 eV)
PtCNTOX 733 97 53 58 58
PtCNTON714 99 66 54 66
PtC 738 82 52 59 70
the atomic ratio of the catalysts The C1s spectrum of the catalystswas deconvoluted into five peaks based on a literature review corre-sponding to the sp2-hybridized graphitic carbon (peak 1 at 2848 eV)C-O groups (peak 2 at 2860 eV) C=O groups in carbonyl or qui-nine groups (peak 3 at 2874 eV) O-C=O groups in ester or carboxylgroups (peak 4 at 2888 eV) and CO and CO2 adsorbed on the cata-lysts surface (peak 5 at 2908 eV)3435 Four peaks have been appliedto deconvolute the O1s spectra A summary of peak assignments isgiven in Table III peak 1 corresponds to carbonyl groups at 5315eV peak 2 corresponds to hydroxyl or epoxide at 5326 eV peak 3corresponds to ether-type oxygen in ester and anhydrides at 5333 eVand peak 4 corresponds to carboxyl groups at 5342 eV303637 Thenitrogen peaks are at 3985 eV and 4007 eV ascribe to pyridinicnitrogen and pyrrolic nitrogen respectively33
Based on Table II and Table III the carbon nanotubes were susces-sully functionalized with oxygen and nitrogen containing groups Ahigher ratio of carbonyl groups was found in C1s spectra of PtCNTON(peak 3 665) than PtCNTOX (554) and PtC (521) whichis in agreement with the O1s results that PtCNTON has a largerpeak 1(5322) compared to PtCNTOX (4148) and PtC (366)However based on peak 4 in C1s spectra and peak 4 in O1s spec-tra PtCNTON has lower ratio of carboxyl groups compared toPtCNTOX Additionaly it is found from atom ratio in Table IVthat functionalizing the oxidized carbon nanotubes with nitrogen con-taining groups decreased the overall surface oxygen content Thusadding nitrogen functional groups on oxidized carbon nanotube pos-sibly caused carboxyl groups to convert to carbonyl groups
Fuel cell performancemdashHydrogen fuel cellmdashThe optimum powerdensity of the catalysts was investigated in an AEMPEM hybrid fuelcell The optimum ionomer-to-catalyst ratio was found experimentallyfor the different catalysts For PtC 10 was determined to be the bestionomer-to-catalyst ratio while 17 was optimum for PtCNTOXand PtCNTON Figure 6a shows the performance of the three cata-lysts after running the fuel cell for five hours to establish steady-stateconditions After a five hour period the PtCNTON had the highestpower density peak of 82 mWcm2 as compared to the PtCNTOXcatalyst which had the lowest power density peak of 60 mWcm2After running the catalysts in a hybrid fuel cell for 15 hours Figure6b the PtCNTOX had higher power than the PtC catalyst where thepower density peak reached 72 mWcm2 The power density peak ofPtCNTON increased to 89 mWcm2 with time and had the greatestpower density among the three catalysts
Table IV Element ratio of the catalysts from XPS
Element Ratio ()
C O N
PtCNTOX 877 102 ndashPtCNTON 876 77 20
PtC 886 42 ndash
The stability of the catalysts was evaluated in an AEMPEM hybridfuel cell by monitoring the current density over a 20 hour period at250 mV Figure 6c shows the current density of the three catalystsover time The power of the fuel cell with PtC increased slightly fromthe beginning and remained stable at 250 mAcm2 for the 20-hourperiod The current density of PtCNTOX catalyst increased from238 mAcm2 to 275 mAcm2 over the first 8 hours and then becamestablized at 275 mAcm2 Finally the current density of PtCNTONwas stable at 282 mAcm2 for the length of the experiment 20 hoursIn conclusion the PtCNTON had the best peak power and stabilityperformance with the shortest start up time
The smaller size of the platinum particles may contribute to thesuperior performance of the PtCNTOX and PtCNTON catalystsThe surface area of the catalysts is larger when the particles remainsmall thereby increasing the overall catalytic activity On the otherhand the binding energy between Pt and pristine carbon nanotubesis weaker which leads to agglomeration of Pt clusters to create largerparticles38 In addition it has been shown that the oxygen containinggroups can activate the carbon nanotubes and provide a greater numberof active sites While the nitrogen containing groups can directlycatalyze the ORR29 Moreover previous study have found that thefunctional groups also can promote activity of catalysts by improvingthe structure stability of Pt with CNTs39 Compared to -O functionalatom -N functional atom can promote better structural stability Thereare claims that the hydrogen adsorption energy of Pt atom on variousfunctionalized CNTs is as follows -N lt -O lt pristine CNTs Theadsorption energy trends indicate that the catalytic activity of platinumsupported on functionalized CNTs is controlled by the functionalgroup(s) which is consistent with the experimental results in thisstudy
Hybrid direct methanol fuel cellmdashFigure 7 shows the performanceof PtC PtCNTOX and PtCNTON in a hybrid DMFCs for ORR Inhybrid DMFCs 10 ionomercatalysts ratio for PtC and 15 forPtCNTOX and PtCNTON was used to reach the optimal ratio Atthe beginning of the fuel cell test PtCNTON had the highest powerdensity peak at 14 mWcm2 and the PtC had the lowest density peakat 97 mWcm2 After operating the methanol fuel cell for five hoursthe power density peak of PtC and PtCNTOX catalysts were about thesame at 15 mWcm2 However the power density for the PtCNTONcatalyst dropped to 131 mWcm2 After operating the methanol fuelcell for 20 hours the power density peak of the PtCNTOX slowlyincreased to 16 mWcm2 while the PtC remained at 131 mWcm2and PtCNTON decreased to 11 mWcm2
The stability tests in Figure 7c show that the PtCNTON per-formance was the highest of the tested catalysts at the beginning
Table III Binding energy of O1s peaks and N1s peaks from XPS
O1s peak N1s peak
Peak 1 (5315 eV) Peak 2 (5326 eV) Peak 3 (5333 eV) Peak 4 (5342 eV) Pyridinic (3985 eV) Pyrrolic (4007 eV)
PtCNTOX 415 378 66 141 ndash ndashPtCNTON 532 114 247 107 331 669
PtC 366 264 226 144 ndash ndash
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Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F221
Figure 6 Performance Comparison of a H2-O2 hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after running for(a) 5 hours and (b) 15 hours (c) Stability test in H2-O2 hybrid AEMPEM fuel cell for 20 hours
Figure 7 Performance Comparison of a methanol hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after runningfor (a) 0 hours and (b) 5 hours and (c) 20 hours (d) Stability test of PtC PtCNTOX and PtCNTON in methanol hybrid AEMPEM fuel cell for 20 hours
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F222 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Figure 8 Internal Short Circuit Caused by Crossover Methanol
However the current density decreased with time over the first 20hours The current density of PtCNTOX increased slightly duringthe first 20 hours Thus the PtCNTOX was the most stable catalystamong the three tested
A possible reason for the unstable performance of PtCNTON inthe direct methanol fuel cell is because nitrogen containing groups maycatalyze oxidation of the methanol which has crossed over through themembranes more efficiently than other catalysts Based on previousresearch4041 methanol which has crossed through the membrane caneither directly react with oxygen (1a) or can be oxidized at the cathodewith oxygen reduction (1b) as follows
CH3OH + 3
2O2 rarr CO2 + 2H2O [1a]
CH3OH + H2O rarr CO2 + 6H+ + 6eminus [1b]
Figure 9 Nitrogen groups attracting methanol model
The latter reaction causes an internal short circuit because it resultsin lost current as shown in Figure 8 Both reactions create undesirableintermediates for the platinum catalysts
Based on Figure 5 the PtCNTON has pyrrolic nitrogen and pyri-dinic nitrogen groups and 20 more carbonyl groups (C=O) com-pared to the PtCNTOX catalyst This result suggests that the nitrogengroups make PtCNTON more active to methanol oxidation The ni-trogen anchor for the groups may facilitate the oxidation of methanolby attacking the proton of methanol as shown in Figure 9 Xionget al42 also claimed that the platinum supported on nitrogen dopedgraphene show higher electrochemical activity toward methanoloxidation
Conclusions
Two different functionalized carbon nanotubes supported platinumcatalysts PtCNTOX and PtCNTON were synthesized and the per-formance was evaluated in a hybrid PEMAEM H2O2 fuel cell and hy-brid DMFCs It was found that in a hybrid H2O2 fuel cell PtCNTONhad the best activity for ORR at the cathode For hybrid DMFCsPtCNTON had the highest activity initially but was not sable withtime The PtCNTOX had the best stability and activity for the en-tire 20 hour test The possible reason poor PtCNTON performancein the hybrid DMFCs may be because of the proton affinity for thepyrrolic nitrogen groups with methanol fuel making methanol easierto oxidize on the cathode
Acknowledgments
This work was supported by the China Scholarship Council Igratefully acknowledge Oluwadamilola Phillips for XPS characteri-zation of catalysts and technical discussion
References
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1 p 1 (2010)3 J E Soc et al 138 2334 (1993)4 K A Mauritz and R B Moore Chem Rev 104 4535 (2004)5 S Motupally A J Becker and J W Weidner J Electrochem Soc 147 3171
(2000)6 Z Liang et al J Memb Sci 233 39 (2004)7 G Sasikumar J W Ihm and H Ryu J Power Sources 132 11 (2004)8 M Unlu J Zhou and P A Kohl J Phys Chem C 113 11416 (2009)9 A Heinzel and V M Barragan J Power Sources 84 70 (1999)
10 K Scott W Taama P Argyropoulos and K Sundmacher J Power Sources 83 204(1999) httpwwwsciencedirectcomsciencearticlepiiS0378775399003031
11 S Wasmus and a Kuver J Electroanal Chem 461 14 (1999)12 L Ma H He A Hsu and R Chen J Power Sources 241 696 (2013)13 D Y Chung et al Sci Rep 4 7450 (2014) httpwwwpubmedcentral
nihgovarticlerenderfcgiartid=4264001amptool=pmcentrezamprendertype=abstract14 X Cheng et al J Power Sources 165 739 (2007)15 N Jung et al Int J Hydrogen Energy 36 15731 (2011)
httpdxdoiorg101016jijhydene20110905416 J S Spendelow J D Goodpaster P J A Kenis and A Wieckowski J Phys Chem
B 110 9545 (2006)17 M Najjari F Khemili and S Ben Nasrallah Renew Energy 33 1824 (2008)18 J R Varcoe and R C T Slade Fuel Cells 5 187 (2005)19 C G Arges V Ramani and P N Pintauro Elctrochemical Soc Interface 31 (2010)20 M Unlu J Zhou and P a Kohl J Electrochem Soc 157 B1391 (2010)
httpjesecsdlorgcgidoi1011491346870021 M Unlu J Zhou and P A Kohl Fuel Cells 10 54 (2010)22 J Zhou K Joseph J M Ahlfield D-Y Park and P a Kohl J Electrochem Soc
160 F573 (2013) httpjesecsdlorgcgidoi1011492077306jes23 K Lee et al Electrochim Acta 54 4704 (2009)24 Y Devrim and A Albostan J Electron Mater 45 3900 (2016)
httplinkspringercom101007s11664-016-4703-225 C Alegre D Sebastian M E Galvez R Moliner and M J Lazaro Appl Catal B
Environ 192 260 (2016) httpdxdoiorg101016japcatb20160307026 Z Peng and H Yang Nano Today 4 143 (2009)27 S Hong and S Myung Nat Nanotechnol 2 207 (2007)
httpwwwnaturecomdoifinder101038nnano20078928 P J Britto K S V Santhanam A Rubio J A Alonso and P M Ajayan Adv Mater
11 154 (1999)29 W Li et al Carbon N Y 40 791 (2002) httplinkinghubelseviercom
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Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F223
30 T Ishizaki S Chiba Y Kaneko and G Panomsuwan J Mater Chem A 2 10589(2014) httpxlinkrscorgDOI=c4ta01577k
31 R Zhong Y Qin D Niu X Zhang and X Zhou Electrochim Acta 89 157 (2013)httpdxdoiorg101016jelectacta201211007
32 K Artyushkova A Serov S Rojas-Carbonell and P Atanassov J Phys Chem C119 25917 (2015)
33 T Xing et al ACS Nano 8 6856 (2014)34 R Zhong Y Qin D Niu J Tian and X Zhang J Power Sources 225 192
(2013)35 H K Jeong et al J Am Chem Soc 130 1362 (2008)36 Y C Chiang C C Liang and C P Chung Materials (Basel) 8 6484
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37 A J Plomp D S Su K P De Jong and J H Bitter J Phys Chem C 113 9865(2009)
38 W Li et al J Phys Chem B 107 6292 (2003) httppubsacsorgdoiabs101021jp022505cnhttpdxdoiorg101021jp022505c
39 B-H Kim K-R Lee Y-C Chung and M Park Phys Chem Chem Phys 18 22687(2016) httppubsrscorgenContentArticleLanding2016CPC5CP07737K
40 S Zhou et al Phys Chem Chem Phys 3 347 (2001)httpxlinkrscorgDOI=b007283o
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42 B Xiong et al Carbon N Y 52 181 (2013) httpdxdoiorg101016jcarbon201209019
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F218 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Table I Physical Properties of the AEI in this study
Ionic functional group Quinuclidine
Structure of the groupWater uptake () 25
Ion exchange capacity (mmolg) 13
DMFCs At the same time introducing oxygen containing groups andnitrogen containing groups on the carbon support have significantlychanged the activity of the platinum catalysts The doping of oxy-gen could activate the π electrons of carbon nanotubes because oftheir high electron density which makes O2 utilization more efficientIshizaki et al30 showed that the surface functional groups play a moreimportant role than the bulk oxygen content and defect sites of thenanocarbon in ORR The order of the most effective oxygen containinggroups for ORR activity is as follows carboxyl gt carbonyl gt hydroxylgroups Zhong et al31 synthesized two types of oxygen doped carbonnanofiber supported platinum catalysts and through rotating disk elec-trode studies found that the carboxylic acid groups were more activethan the hydroxyl groups Nitrogen containing groups on carbon nan-otubes can improve the performance of conversion devices becauseelectron-rich nitrogen can activate π electrons of carbon nanotubesthrough conjugation with their lone-pair electrons Artyushkova etal32 believe that the pyrrolic nitrogen can catalyze O2 reduction tohydrogen peroxide and pyridinic can reduce hydrogen peroxide towater in a 2eminus reaction Also Xing et al33 found that the carbon atomneighboring a pyridinic nitrogen should be the main site for ORRamong pyrrolic pyridinic and graphic nitrogen doped carbon
In this paper the surface of carbon nanotubes functionalized withoxygen-containing and nitrogen-containing groups were studied inorder to evaluate effectiveness of the catalysts for ORR in hybridH2O2 fuel cell and hybrid DMFCs
Experimental
Catalysts preparationmdashCarbon nanotubes (CNTs) (gt95 car-bon Aldrich) with diameter of 6 to 9 nm and length of 5 μm wereused as the catalyst support The CNTs were functionalized via sono-chemical treatment34 For the oxidized CNT (CNTOX) 02 g of CNTpowder was added to a solution of nitric acid (180 ml) and sulfuricacid (160 ml) followed by sonication for 2 hours at 60C To obtainCNTON 02 g CNTOX was suspended in 100 ml ammonia hydroxideand sonicated for additional 2 hours at 60C
The synthesis of 40 wt PtCNTOX and PtCNTON was carriedout by the chemical reduction method31 100 mg of CNTOXCNTONwas dispersed in 18 ml ethylene glycol Then 6 ml of 20 wt chloro-platinic acid in ethylene glycol was added dropwise to the suspension
Figure 2 Platinum ratio by TGA
Figure 3 TEM image of (a) PtC (b) PtCNTOX (c) PtCNTON and (d)XRD patterns
After stirring for 12 hours 2 ml NN-dimethylformanide was addedThe pH was set to 13 by adding sodium hydroxide in ethylene glycolsolution The mixture was held at 130C in an oil bath for 4 hours whilestirring The solid catalyst was collected and washed with deionizedwater The final solid product was dried at 120C in vacuum overnight
The electrocatalytic performance of the catalysts for ORR wastested in AEMPEM hybrid fuel cell The high pH electrode (AEMelectrode) was fabricated with PtC PtCNTOX and PtCNTON cat-alysts and an anion exchange ionomer (AEI) The metal loading ofthe electrodes was 1 mgcm2 The AEI is a poly (arylene ether) (PAE)ionomer with octafluoro-biphenyl groups22 The physical propertiesof the AEM are shown in Table I The AEI was stored as a 2 wtsolution in dimethylformamide
The catalyst ink for AEM electrode was prepared by mixing AEMelectrode catalysts and AEI with 300 mg of isopropyl alcohol (IPA)The ionomer-to-catalyst ratio was investigated at 10 15 1720 for each catalyst to determine the ratio for optimal performancein the hybrid hydrogen fuel cell and hybrid DMFC The ionomer-to-catalyst ratio that exhibited the best performance for each catalystwas compared The catalysts ink for PEM electrode was prepared bymixing Pt-RuC catalysts (50ndash25) 100 mg of deionized water5 wt Nafion and 300 mg propanol The ionomercatalysts ratiowas 15 The slurry was sonicated for 30 minutes and sprayed ontohydrophobic (Toray TGPH-090) and hydrophilic (Toray 2050L) gasdiffusion layer (GDL)20 The PEM electrode dried with a heat gunfor 30 minutes The AEM electrodes were soaked in 1 M sodiumhydroxide for 30 minutes and soaked in deionized water overnightThe hybrid membrane electrode assembly (MEA) was prepared byspraying 5 wt Nafion in propanol onto the surface of each electrodeNafion 115 membrane was pressed between the two electrodes for 20minutes at 212F and 2 MPa
Physical characterizationmdashThe transmission electron mi-croscopy (TEM) was carried out with a Hitachi HT7700 TEM todetermine the size of the platinum supported on the CNTs X-raydiffraction (XRD) was carried out on a PANalytical X-ray facilityto investigate crystallographic properties of catalysts The surfaceproperties of the catalysts were investigated by X-ray photoelectronspectroscopy (XPS) on a Thermos Scientific K-Alpha XPS Thermo-gravimetric analysis (TGA) was carried out on a TGA Q50 from TA
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F219
Figure 4 XPS results of C1s peak (a) PtC (b) PtCNTOX (c) PtCNTON
Instrument to determine the platinum content in catalysts The catalystwas loaded in the platinum pan and heated at a ramp rate of 20Cminto 800C The sample was held at 800C for 2 hours with ambientairflow
Results and Discussion
Characterization of the catalystsmdashTGA was used to investigatethe platinum content of the catalysts in ambient air environment Thecatalysts were ramped to high temperatures (800C) where the carboncontent completely burned away within two hours The remainingweight percentage was taken to be the platinum content The TGAresults are shown in Figure 2 The platinum content of the commercialcatalyst (PtC) was determined to be 370 wt which is similar tolabeled content of 40 wt The platinum ratio of PtCNTOX andPtCNTON was found to be 346 wt and 426 wt respectively
TEM revealed that the small particle size Pt was attached andwell dispersed on the surface of the CNTs (Figures 3a 3b and 3c)The particle size of platinum on carbon oxidized carbon nanotubes(CNTOX) and nitrogen doped oxidized carbon nanotubes (CNTON)are calculated by taking the average size from the TEM analysis
Figure 5 XPS results of O1s peak (a) PtC (b) PtCNTOX (c) PtCNTONand (d) N1s peak of PtCNTON
which was 465 nm 371 nm and 443 nm respectively The platinumsupported on CNTOX had the smallest particle size
The XRD patterns shows the crystallographic properties of thecatalysts Figure 3d shows that the Pt (111) peak is the main diffractionpeak for all the three catalysts while the diffraction pattern of Pt (220)and Pt (311) have only weak peaks for PtCNTON compared to theother two catalysts
X-ray photoelectron spectroscopy (XPS) has been used to charac-terize the oxygen and nitrogen content on the functionalized carbonnanotubes as shown in Figure 4 and Figure 5 Tables II and III showthe binding energies for carbon oxygen and nitrogen Table IV shows
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F220 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Table II Binding energy of C1s peaks from XPS
Binding Energy
Peak 1(2848 eV)
Peak 2(2860 eV)
Peak 3(2878 eV)
Peak 4(2888 eV)
Carbonates(2908 eV)
PtCNTOX 733 97 53 58 58
PtCNTON714 99 66 54 66
PtC 738 82 52 59 70
the atomic ratio of the catalysts The C1s spectrum of the catalystswas deconvoluted into five peaks based on a literature review corre-sponding to the sp2-hybridized graphitic carbon (peak 1 at 2848 eV)C-O groups (peak 2 at 2860 eV) C=O groups in carbonyl or qui-nine groups (peak 3 at 2874 eV) O-C=O groups in ester or carboxylgroups (peak 4 at 2888 eV) and CO and CO2 adsorbed on the cata-lysts surface (peak 5 at 2908 eV)3435 Four peaks have been appliedto deconvolute the O1s spectra A summary of peak assignments isgiven in Table III peak 1 corresponds to carbonyl groups at 5315eV peak 2 corresponds to hydroxyl or epoxide at 5326 eV peak 3corresponds to ether-type oxygen in ester and anhydrides at 5333 eVand peak 4 corresponds to carboxyl groups at 5342 eV303637 Thenitrogen peaks are at 3985 eV and 4007 eV ascribe to pyridinicnitrogen and pyrrolic nitrogen respectively33
Based on Table II and Table III the carbon nanotubes were susces-sully functionalized with oxygen and nitrogen containing groups Ahigher ratio of carbonyl groups was found in C1s spectra of PtCNTON(peak 3 665) than PtCNTOX (554) and PtC (521) whichis in agreement with the O1s results that PtCNTON has a largerpeak 1(5322) compared to PtCNTOX (4148) and PtC (366)However based on peak 4 in C1s spectra and peak 4 in O1s spec-tra PtCNTON has lower ratio of carboxyl groups compared toPtCNTOX Additionaly it is found from atom ratio in Table IVthat functionalizing the oxidized carbon nanotubes with nitrogen con-taining groups decreased the overall surface oxygen content Thusadding nitrogen functional groups on oxidized carbon nanotube pos-sibly caused carboxyl groups to convert to carbonyl groups
Fuel cell performancemdashHydrogen fuel cellmdashThe optimum powerdensity of the catalysts was investigated in an AEMPEM hybrid fuelcell The optimum ionomer-to-catalyst ratio was found experimentallyfor the different catalysts For PtC 10 was determined to be the bestionomer-to-catalyst ratio while 17 was optimum for PtCNTOXand PtCNTON Figure 6a shows the performance of the three cata-lysts after running the fuel cell for five hours to establish steady-stateconditions After a five hour period the PtCNTON had the highestpower density peak of 82 mWcm2 as compared to the PtCNTOXcatalyst which had the lowest power density peak of 60 mWcm2After running the catalysts in a hybrid fuel cell for 15 hours Figure6b the PtCNTOX had higher power than the PtC catalyst where thepower density peak reached 72 mWcm2 The power density peak ofPtCNTON increased to 89 mWcm2 with time and had the greatestpower density among the three catalysts
Table IV Element ratio of the catalysts from XPS
Element Ratio ()
C O N
PtCNTOX 877 102 ndashPtCNTON 876 77 20
PtC 886 42 ndash
The stability of the catalysts was evaluated in an AEMPEM hybridfuel cell by monitoring the current density over a 20 hour period at250 mV Figure 6c shows the current density of the three catalystsover time The power of the fuel cell with PtC increased slightly fromthe beginning and remained stable at 250 mAcm2 for the 20-hourperiod The current density of PtCNTOX catalyst increased from238 mAcm2 to 275 mAcm2 over the first 8 hours and then becamestablized at 275 mAcm2 Finally the current density of PtCNTONwas stable at 282 mAcm2 for the length of the experiment 20 hoursIn conclusion the PtCNTON had the best peak power and stabilityperformance with the shortest start up time
The smaller size of the platinum particles may contribute to thesuperior performance of the PtCNTOX and PtCNTON catalystsThe surface area of the catalysts is larger when the particles remainsmall thereby increasing the overall catalytic activity On the otherhand the binding energy between Pt and pristine carbon nanotubesis weaker which leads to agglomeration of Pt clusters to create largerparticles38 In addition it has been shown that the oxygen containinggroups can activate the carbon nanotubes and provide a greater numberof active sites While the nitrogen containing groups can directlycatalyze the ORR29 Moreover previous study have found that thefunctional groups also can promote activity of catalysts by improvingthe structure stability of Pt with CNTs39 Compared to -O functionalatom -N functional atom can promote better structural stability Thereare claims that the hydrogen adsorption energy of Pt atom on variousfunctionalized CNTs is as follows -N lt -O lt pristine CNTs Theadsorption energy trends indicate that the catalytic activity of platinumsupported on functionalized CNTs is controlled by the functionalgroup(s) which is consistent with the experimental results in thisstudy
Hybrid direct methanol fuel cellmdashFigure 7 shows the performanceof PtC PtCNTOX and PtCNTON in a hybrid DMFCs for ORR Inhybrid DMFCs 10 ionomercatalysts ratio for PtC and 15 forPtCNTOX and PtCNTON was used to reach the optimal ratio Atthe beginning of the fuel cell test PtCNTON had the highest powerdensity peak at 14 mWcm2 and the PtC had the lowest density peakat 97 mWcm2 After operating the methanol fuel cell for five hoursthe power density peak of PtC and PtCNTOX catalysts were about thesame at 15 mWcm2 However the power density for the PtCNTONcatalyst dropped to 131 mWcm2 After operating the methanol fuelcell for 20 hours the power density peak of the PtCNTOX slowlyincreased to 16 mWcm2 while the PtC remained at 131 mWcm2and PtCNTON decreased to 11 mWcm2
The stability tests in Figure 7c show that the PtCNTON per-formance was the highest of the tested catalysts at the beginning
Table III Binding energy of O1s peaks and N1s peaks from XPS
O1s peak N1s peak
Peak 1 (5315 eV) Peak 2 (5326 eV) Peak 3 (5333 eV) Peak 4 (5342 eV) Pyridinic (3985 eV) Pyrrolic (4007 eV)
PtCNTOX 415 378 66 141 ndash ndashPtCNTON 532 114 247 107 331 669
PtC 366 264 226 144 ndash ndash
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F221
Figure 6 Performance Comparison of a H2-O2 hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after running for(a) 5 hours and (b) 15 hours (c) Stability test in H2-O2 hybrid AEMPEM fuel cell for 20 hours
Figure 7 Performance Comparison of a methanol hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after runningfor (a) 0 hours and (b) 5 hours and (c) 20 hours (d) Stability test of PtC PtCNTOX and PtCNTON in methanol hybrid AEMPEM fuel cell for 20 hours
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F222 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Figure 8 Internal Short Circuit Caused by Crossover Methanol
However the current density decreased with time over the first 20hours The current density of PtCNTOX increased slightly duringthe first 20 hours Thus the PtCNTOX was the most stable catalystamong the three tested
A possible reason for the unstable performance of PtCNTON inthe direct methanol fuel cell is because nitrogen containing groups maycatalyze oxidation of the methanol which has crossed over through themembranes more efficiently than other catalysts Based on previousresearch4041 methanol which has crossed through the membrane caneither directly react with oxygen (1a) or can be oxidized at the cathodewith oxygen reduction (1b) as follows
CH3OH + 3
2O2 rarr CO2 + 2H2O [1a]
CH3OH + H2O rarr CO2 + 6H+ + 6eminus [1b]
Figure 9 Nitrogen groups attracting methanol model
The latter reaction causes an internal short circuit because it resultsin lost current as shown in Figure 8 Both reactions create undesirableintermediates for the platinum catalysts
Based on Figure 5 the PtCNTON has pyrrolic nitrogen and pyri-dinic nitrogen groups and 20 more carbonyl groups (C=O) com-pared to the PtCNTOX catalyst This result suggests that the nitrogengroups make PtCNTON more active to methanol oxidation The ni-trogen anchor for the groups may facilitate the oxidation of methanolby attacking the proton of methanol as shown in Figure 9 Xionget al42 also claimed that the platinum supported on nitrogen dopedgraphene show higher electrochemical activity toward methanoloxidation
Conclusions
Two different functionalized carbon nanotubes supported platinumcatalysts PtCNTOX and PtCNTON were synthesized and the per-formance was evaluated in a hybrid PEMAEM H2O2 fuel cell and hy-brid DMFCs It was found that in a hybrid H2O2 fuel cell PtCNTONhad the best activity for ORR at the cathode For hybrid DMFCsPtCNTON had the highest activity initially but was not sable withtime The PtCNTOX had the best stability and activity for the en-tire 20 hour test The possible reason poor PtCNTON performancein the hybrid DMFCs may be because of the proton affinity for thepyrrolic nitrogen groups with methanol fuel making methanol easierto oxidize on the cathode
Acknowledgments
This work was supported by the China Scholarship Council Igratefully acknowledge Oluwadamilola Phillips for XPS characteri-zation of catalysts and technical discussion
References
1 B C Steele and A Heinzel Nature 414 345 (2001)2 I A S Arico V Baglio and V Antonucci (CNR-ITAE Institute Messina) Chapter
1 p 1 (2010)3 J E Soc et al 138 2334 (1993)4 K A Mauritz and R B Moore Chem Rev 104 4535 (2004)5 S Motupally A J Becker and J W Weidner J Electrochem Soc 147 3171
(2000)6 Z Liang et al J Memb Sci 233 39 (2004)7 G Sasikumar J W Ihm and H Ryu J Power Sources 132 11 (2004)8 M Unlu J Zhou and P A Kohl J Phys Chem C 113 11416 (2009)9 A Heinzel and V M Barragan J Power Sources 84 70 (1999)
10 K Scott W Taama P Argyropoulos and K Sundmacher J Power Sources 83 204(1999) httpwwwsciencedirectcomsciencearticlepiiS0378775399003031
11 S Wasmus and a Kuver J Electroanal Chem 461 14 (1999)12 L Ma H He A Hsu and R Chen J Power Sources 241 696 (2013)13 D Y Chung et al Sci Rep 4 7450 (2014) httpwwwpubmedcentral
nihgovarticlerenderfcgiartid=4264001amptool=pmcentrezamprendertype=abstract14 X Cheng et al J Power Sources 165 739 (2007)15 N Jung et al Int J Hydrogen Energy 36 15731 (2011)
httpdxdoiorg101016jijhydene20110905416 J S Spendelow J D Goodpaster P J A Kenis and A Wieckowski J Phys Chem
B 110 9545 (2006)17 M Najjari F Khemili and S Ben Nasrallah Renew Energy 33 1824 (2008)18 J R Varcoe and R C T Slade Fuel Cells 5 187 (2005)19 C G Arges V Ramani and P N Pintauro Elctrochemical Soc Interface 31 (2010)20 M Unlu J Zhou and P a Kohl J Electrochem Soc 157 B1391 (2010)
httpjesecsdlorgcgidoi1011491346870021 M Unlu J Zhou and P A Kohl Fuel Cells 10 54 (2010)22 J Zhou K Joseph J M Ahlfield D-Y Park and P a Kohl J Electrochem Soc
160 F573 (2013) httpjesecsdlorgcgidoi1011492077306jes23 K Lee et al Electrochim Acta 54 4704 (2009)24 Y Devrim and A Albostan J Electron Mater 45 3900 (2016)
httplinkspringercom101007s11664-016-4703-225 C Alegre D Sebastian M E Galvez R Moliner and M J Lazaro Appl Catal B
Environ 192 260 (2016) httpdxdoiorg101016japcatb20160307026 Z Peng and H Yang Nano Today 4 143 (2009)27 S Hong and S Myung Nat Nanotechnol 2 207 (2007)
httpwwwnaturecomdoifinder101038nnano20078928 P J Britto K S V Santhanam A Rubio J A Alonso and P M Ajayan Adv Mater
11 154 (1999)29 W Li et al Carbon N Y 40 791 (2002) httplinkinghubelseviercom
retrievepiiS0008622302000398
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F223
30 T Ishizaki S Chiba Y Kaneko and G Panomsuwan J Mater Chem A 2 10589(2014) httpxlinkrscorgDOI=c4ta01577k
31 R Zhong Y Qin D Niu X Zhang and X Zhou Electrochim Acta 89 157 (2013)httpdxdoiorg101016jelectacta201211007
32 K Artyushkova A Serov S Rojas-Carbonell and P Atanassov J Phys Chem C119 25917 (2015)
33 T Xing et al ACS Nano 8 6856 (2014)34 R Zhong Y Qin D Niu J Tian and X Zhang J Power Sources 225 192
(2013)35 H K Jeong et al J Am Chem Soc 130 1362 (2008)36 Y C Chiang C C Liang and C P Chung Materials (Basel) 8 6484
(2015)
37 A J Plomp D S Su K P De Jong and J H Bitter J Phys Chem C 113 9865(2009)
38 W Li et al J Phys Chem B 107 6292 (2003) httppubsacsorgdoiabs101021jp022505cnhttpdxdoiorg101021jp022505c
39 B-H Kim K-R Lee Y-C Chung and M Park Phys Chem Chem Phys 18 22687(2016) httppubsrscorgenContentArticleLanding2016CPC5CP07737K
40 S Zhou et al Phys Chem Chem Phys 3 347 (2001)httpxlinkrscorgDOI=b007283o
41 A Hacquard 107 (2005) httpwwwwpieduPubsETDAvailableetd-051205-151955unrestrictedAHacquardpdf
42 B Xiong et al Carbon N Y 52 181 (2013) httpdxdoiorg101016jcarbon201209019
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F219
Figure 4 XPS results of C1s peak (a) PtC (b) PtCNTOX (c) PtCNTON
Instrument to determine the platinum content in catalysts The catalystwas loaded in the platinum pan and heated at a ramp rate of 20Cminto 800C The sample was held at 800C for 2 hours with ambientairflow
Results and Discussion
Characterization of the catalystsmdashTGA was used to investigatethe platinum content of the catalysts in ambient air environment Thecatalysts were ramped to high temperatures (800C) where the carboncontent completely burned away within two hours The remainingweight percentage was taken to be the platinum content The TGAresults are shown in Figure 2 The platinum content of the commercialcatalyst (PtC) was determined to be 370 wt which is similar tolabeled content of 40 wt The platinum ratio of PtCNTOX andPtCNTON was found to be 346 wt and 426 wt respectively
TEM revealed that the small particle size Pt was attached andwell dispersed on the surface of the CNTs (Figures 3a 3b and 3c)The particle size of platinum on carbon oxidized carbon nanotubes(CNTOX) and nitrogen doped oxidized carbon nanotubes (CNTON)are calculated by taking the average size from the TEM analysis
Figure 5 XPS results of O1s peak (a) PtC (b) PtCNTOX (c) PtCNTONand (d) N1s peak of PtCNTON
which was 465 nm 371 nm and 443 nm respectively The platinumsupported on CNTOX had the smallest particle size
The XRD patterns shows the crystallographic properties of thecatalysts Figure 3d shows that the Pt (111) peak is the main diffractionpeak for all the three catalysts while the diffraction pattern of Pt (220)and Pt (311) have only weak peaks for PtCNTON compared to theother two catalysts
X-ray photoelectron spectroscopy (XPS) has been used to charac-terize the oxygen and nitrogen content on the functionalized carbonnanotubes as shown in Figure 4 and Figure 5 Tables II and III showthe binding energies for carbon oxygen and nitrogen Table IV shows
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F220 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Table II Binding energy of C1s peaks from XPS
Binding Energy
Peak 1(2848 eV)
Peak 2(2860 eV)
Peak 3(2878 eV)
Peak 4(2888 eV)
Carbonates(2908 eV)
PtCNTOX 733 97 53 58 58
PtCNTON714 99 66 54 66
PtC 738 82 52 59 70
the atomic ratio of the catalysts The C1s spectrum of the catalystswas deconvoluted into five peaks based on a literature review corre-sponding to the sp2-hybridized graphitic carbon (peak 1 at 2848 eV)C-O groups (peak 2 at 2860 eV) C=O groups in carbonyl or qui-nine groups (peak 3 at 2874 eV) O-C=O groups in ester or carboxylgroups (peak 4 at 2888 eV) and CO and CO2 adsorbed on the cata-lysts surface (peak 5 at 2908 eV)3435 Four peaks have been appliedto deconvolute the O1s spectra A summary of peak assignments isgiven in Table III peak 1 corresponds to carbonyl groups at 5315eV peak 2 corresponds to hydroxyl or epoxide at 5326 eV peak 3corresponds to ether-type oxygen in ester and anhydrides at 5333 eVand peak 4 corresponds to carboxyl groups at 5342 eV303637 Thenitrogen peaks are at 3985 eV and 4007 eV ascribe to pyridinicnitrogen and pyrrolic nitrogen respectively33
Based on Table II and Table III the carbon nanotubes were susces-sully functionalized with oxygen and nitrogen containing groups Ahigher ratio of carbonyl groups was found in C1s spectra of PtCNTON(peak 3 665) than PtCNTOX (554) and PtC (521) whichis in agreement with the O1s results that PtCNTON has a largerpeak 1(5322) compared to PtCNTOX (4148) and PtC (366)However based on peak 4 in C1s spectra and peak 4 in O1s spec-tra PtCNTON has lower ratio of carboxyl groups compared toPtCNTOX Additionaly it is found from atom ratio in Table IVthat functionalizing the oxidized carbon nanotubes with nitrogen con-taining groups decreased the overall surface oxygen content Thusadding nitrogen functional groups on oxidized carbon nanotube pos-sibly caused carboxyl groups to convert to carbonyl groups
Fuel cell performancemdashHydrogen fuel cellmdashThe optimum powerdensity of the catalysts was investigated in an AEMPEM hybrid fuelcell The optimum ionomer-to-catalyst ratio was found experimentallyfor the different catalysts For PtC 10 was determined to be the bestionomer-to-catalyst ratio while 17 was optimum for PtCNTOXand PtCNTON Figure 6a shows the performance of the three cata-lysts after running the fuel cell for five hours to establish steady-stateconditions After a five hour period the PtCNTON had the highestpower density peak of 82 mWcm2 as compared to the PtCNTOXcatalyst which had the lowest power density peak of 60 mWcm2After running the catalysts in a hybrid fuel cell for 15 hours Figure6b the PtCNTOX had higher power than the PtC catalyst where thepower density peak reached 72 mWcm2 The power density peak ofPtCNTON increased to 89 mWcm2 with time and had the greatestpower density among the three catalysts
Table IV Element ratio of the catalysts from XPS
Element Ratio ()
C O N
PtCNTOX 877 102 ndashPtCNTON 876 77 20
PtC 886 42 ndash
The stability of the catalysts was evaluated in an AEMPEM hybridfuel cell by monitoring the current density over a 20 hour period at250 mV Figure 6c shows the current density of the three catalystsover time The power of the fuel cell with PtC increased slightly fromthe beginning and remained stable at 250 mAcm2 for the 20-hourperiod The current density of PtCNTOX catalyst increased from238 mAcm2 to 275 mAcm2 over the first 8 hours and then becamestablized at 275 mAcm2 Finally the current density of PtCNTONwas stable at 282 mAcm2 for the length of the experiment 20 hoursIn conclusion the PtCNTON had the best peak power and stabilityperformance with the shortest start up time
The smaller size of the platinum particles may contribute to thesuperior performance of the PtCNTOX and PtCNTON catalystsThe surface area of the catalysts is larger when the particles remainsmall thereby increasing the overall catalytic activity On the otherhand the binding energy between Pt and pristine carbon nanotubesis weaker which leads to agglomeration of Pt clusters to create largerparticles38 In addition it has been shown that the oxygen containinggroups can activate the carbon nanotubes and provide a greater numberof active sites While the nitrogen containing groups can directlycatalyze the ORR29 Moreover previous study have found that thefunctional groups also can promote activity of catalysts by improvingthe structure stability of Pt with CNTs39 Compared to -O functionalatom -N functional atom can promote better structural stability Thereare claims that the hydrogen adsorption energy of Pt atom on variousfunctionalized CNTs is as follows -N lt -O lt pristine CNTs Theadsorption energy trends indicate that the catalytic activity of platinumsupported on functionalized CNTs is controlled by the functionalgroup(s) which is consistent with the experimental results in thisstudy
Hybrid direct methanol fuel cellmdashFigure 7 shows the performanceof PtC PtCNTOX and PtCNTON in a hybrid DMFCs for ORR Inhybrid DMFCs 10 ionomercatalysts ratio for PtC and 15 forPtCNTOX and PtCNTON was used to reach the optimal ratio Atthe beginning of the fuel cell test PtCNTON had the highest powerdensity peak at 14 mWcm2 and the PtC had the lowest density peakat 97 mWcm2 After operating the methanol fuel cell for five hoursthe power density peak of PtC and PtCNTOX catalysts were about thesame at 15 mWcm2 However the power density for the PtCNTONcatalyst dropped to 131 mWcm2 After operating the methanol fuelcell for 20 hours the power density peak of the PtCNTOX slowlyincreased to 16 mWcm2 while the PtC remained at 131 mWcm2and PtCNTON decreased to 11 mWcm2
The stability tests in Figure 7c show that the PtCNTON per-formance was the highest of the tested catalysts at the beginning
Table III Binding energy of O1s peaks and N1s peaks from XPS
O1s peak N1s peak
Peak 1 (5315 eV) Peak 2 (5326 eV) Peak 3 (5333 eV) Peak 4 (5342 eV) Pyridinic (3985 eV) Pyrrolic (4007 eV)
PtCNTOX 415 378 66 141 ndash ndashPtCNTON 532 114 247 107 331 669
PtC 366 264 226 144 ndash ndash
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F221
Figure 6 Performance Comparison of a H2-O2 hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after running for(a) 5 hours and (b) 15 hours (c) Stability test in H2-O2 hybrid AEMPEM fuel cell for 20 hours
Figure 7 Performance Comparison of a methanol hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after runningfor (a) 0 hours and (b) 5 hours and (c) 20 hours (d) Stability test of PtC PtCNTOX and PtCNTON in methanol hybrid AEMPEM fuel cell for 20 hours
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F222 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Figure 8 Internal Short Circuit Caused by Crossover Methanol
However the current density decreased with time over the first 20hours The current density of PtCNTOX increased slightly duringthe first 20 hours Thus the PtCNTOX was the most stable catalystamong the three tested
A possible reason for the unstable performance of PtCNTON inthe direct methanol fuel cell is because nitrogen containing groups maycatalyze oxidation of the methanol which has crossed over through themembranes more efficiently than other catalysts Based on previousresearch4041 methanol which has crossed through the membrane caneither directly react with oxygen (1a) or can be oxidized at the cathodewith oxygen reduction (1b) as follows
CH3OH + 3
2O2 rarr CO2 + 2H2O [1a]
CH3OH + H2O rarr CO2 + 6H+ + 6eminus [1b]
Figure 9 Nitrogen groups attracting methanol model
The latter reaction causes an internal short circuit because it resultsin lost current as shown in Figure 8 Both reactions create undesirableintermediates for the platinum catalysts
Based on Figure 5 the PtCNTON has pyrrolic nitrogen and pyri-dinic nitrogen groups and 20 more carbonyl groups (C=O) com-pared to the PtCNTOX catalyst This result suggests that the nitrogengroups make PtCNTON more active to methanol oxidation The ni-trogen anchor for the groups may facilitate the oxidation of methanolby attacking the proton of methanol as shown in Figure 9 Xionget al42 also claimed that the platinum supported on nitrogen dopedgraphene show higher electrochemical activity toward methanoloxidation
Conclusions
Two different functionalized carbon nanotubes supported platinumcatalysts PtCNTOX and PtCNTON were synthesized and the per-formance was evaluated in a hybrid PEMAEM H2O2 fuel cell and hy-brid DMFCs It was found that in a hybrid H2O2 fuel cell PtCNTONhad the best activity for ORR at the cathode For hybrid DMFCsPtCNTON had the highest activity initially but was not sable withtime The PtCNTOX had the best stability and activity for the en-tire 20 hour test The possible reason poor PtCNTON performancein the hybrid DMFCs may be because of the proton affinity for thepyrrolic nitrogen groups with methanol fuel making methanol easierto oxidize on the cathode
Acknowledgments
This work was supported by the China Scholarship Council Igratefully acknowledge Oluwadamilola Phillips for XPS characteri-zation of catalysts and technical discussion
References
1 B C Steele and A Heinzel Nature 414 345 (2001)2 I A S Arico V Baglio and V Antonucci (CNR-ITAE Institute Messina) Chapter
1 p 1 (2010)3 J E Soc et al 138 2334 (1993)4 K A Mauritz and R B Moore Chem Rev 104 4535 (2004)5 S Motupally A J Becker and J W Weidner J Electrochem Soc 147 3171
(2000)6 Z Liang et al J Memb Sci 233 39 (2004)7 G Sasikumar J W Ihm and H Ryu J Power Sources 132 11 (2004)8 M Unlu J Zhou and P A Kohl J Phys Chem C 113 11416 (2009)9 A Heinzel and V M Barragan J Power Sources 84 70 (1999)
10 K Scott W Taama P Argyropoulos and K Sundmacher J Power Sources 83 204(1999) httpwwwsciencedirectcomsciencearticlepiiS0378775399003031
11 S Wasmus and a Kuver J Electroanal Chem 461 14 (1999)12 L Ma H He A Hsu and R Chen J Power Sources 241 696 (2013)13 D Y Chung et al Sci Rep 4 7450 (2014) httpwwwpubmedcentral
nihgovarticlerenderfcgiartid=4264001amptool=pmcentrezamprendertype=abstract14 X Cheng et al J Power Sources 165 739 (2007)15 N Jung et al Int J Hydrogen Energy 36 15731 (2011)
httpdxdoiorg101016jijhydene20110905416 J S Spendelow J D Goodpaster P J A Kenis and A Wieckowski J Phys Chem
B 110 9545 (2006)17 M Najjari F Khemili and S Ben Nasrallah Renew Energy 33 1824 (2008)18 J R Varcoe and R C T Slade Fuel Cells 5 187 (2005)19 C G Arges V Ramani and P N Pintauro Elctrochemical Soc Interface 31 (2010)20 M Unlu J Zhou and P a Kohl J Electrochem Soc 157 B1391 (2010)
httpjesecsdlorgcgidoi1011491346870021 M Unlu J Zhou and P A Kohl Fuel Cells 10 54 (2010)22 J Zhou K Joseph J M Ahlfield D-Y Park and P a Kohl J Electrochem Soc
160 F573 (2013) httpjesecsdlorgcgidoi1011492077306jes23 K Lee et al Electrochim Acta 54 4704 (2009)24 Y Devrim and A Albostan J Electron Mater 45 3900 (2016)
httplinkspringercom101007s11664-016-4703-225 C Alegre D Sebastian M E Galvez R Moliner and M J Lazaro Appl Catal B
Environ 192 260 (2016) httpdxdoiorg101016japcatb20160307026 Z Peng and H Yang Nano Today 4 143 (2009)27 S Hong and S Myung Nat Nanotechnol 2 207 (2007)
httpwwwnaturecomdoifinder101038nnano20078928 P J Britto K S V Santhanam A Rubio J A Alonso and P M Ajayan Adv Mater
11 154 (1999)29 W Li et al Carbon N Y 40 791 (2002) httplinkinghubelseviercom
retrievepiiS0008622302000398
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F223
30 T Ishizaki S Chiba Y Kaneko and G Panomsuwan J Mater Chem A 2 10589(2014) httpxlinkrscorgDOI=c4ta01577k
31 R Zhong Y Qin D Niu X Zhang and X Zhou Electrochim Acta 89 157 (2013)httpdxdoiorg101016jelectacta201211007
32 K Artyushkova A Serov S Rojas-Carbonell and P Atanassov J Phys Chem C119 25917 (2015)
33 T Xing et al ACS Nano 8 6856 (2014)34 R Zhong Y Qin D Niu J Tian and X Zhang J Power Sources 225 192
(2013)35 H K Jeong et al J Am Chem Soc 130 1362 (2008)36 Y C Chiang C C Liang and C P Chung Materials (Basel) 8 6484
(2015)
37 A J Plomp D S Su K P De Jong and J H Bitter J Phys Chem C 113 9865(2009)
38 W Li et al J Phys Chem B 107 6292 (2003) httppubsacsorgdoiabs101021jp022505cnhttpdxdoiorg101021jp022505c
39 B-H Kim K-R Lee Y-C Chung and M Park Phys Chem Chem Phys 18 22687(2016) httppubsrscorgenContentArticleLanding2016CPC5CP07737K
40 S Zhou et al Phys Chem Chem Phys 3 347 (2001)httpxlinkrscorgDOI=b007283o
41 A Hacquard 107 (2005) httpwwwwpieduPubsETDAvailableetd-051205-151955unrestrictedAHacquardpdf
42 B Xiong et al Carbon N Y 52 181 (2013) httpdxdoiorg101016jcarbon201209019
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F220 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Table II Binding energy of C1s peaks from XPS
Binding Energy
Peak 1(2848 eV)
Peak 2(2860 eV)
Peak 3(2878 eV)
Peak 4(2888 eV)
Carbonates(2908 eV)
PtCNTOX 733 97 53 58 58
PtCNTON714 99 66 54 66
PtC 738 82 52 59 70
the atomic ratio of the catalysts The C1s spectrum of the catalystswas deconvoluted into five peaks based on a literature review corre-sponding to the sp2-hybridized graphitic carbon (peak 1 at 2848 eV)C-O groups (peak 2 at 2860 eV) C=O groups in carbonyl or qui-nine groups (peak 3 at 2874 eV) O-C=O groups in ester or carboxylgroups (peak 4 at 2888 eV) and CO and CO2 adsorbed on the cata-lysts surface (peak 5 at 2908 eV)3435 Four peaks have been appliedto deconvolute the O1s spectra A summary of peak assignments isgiven in Table III peak 1 corresponds to carbonyl groups at 5315eV peak 2 corresponds to hydroxyl or epoxide at 5326 eV peak 3corresponds to ether-type oxygen in ester and anhydrides at 5333 eVand peak 4 corresponds to carboxyl groups at 5342 eV303637 Thenitrogen peaks are at 3985 eV and 4007 eV ascribe to pyridinicnitrogen and pyrrolic nitrogen respectively33
Based on Table II and Table III the carbon nanotubes were susces-sully functionalized with oxygen and nitrogen containing groups Ahigher ratio of carbonyl groups was found in C1s spectra of PtCNTON(peak 3 665) than PtCNTOX (554) and PtC (521) whichis in agreement with the O1s results that PtCNTON has a largerpeak 1(5322) compared to PtCNTOX (4148) and PtC (366)However based on peak 4 in C1s spectra and peak 4 in O1s spec-tra PtCNTON has lower ratio of carboxyl groups compared toPtCNTOX Additionaly it is found from atom ratio in Table IVthat functionalizing the oxidized carbon nanotubes with nitrogen con-taining groups decreased the overall surface oxygen content Thusadding nitrogen functional groups on oxidized carbon nanotube pos-sibly caused carboxyl groups to convert to carbonyl groups
Fuel cell performancemdashHydrogen fuel cellmdashThe optimum powerdensity of the catalysts was investigated in an AEMPEM hybrid fuelcell The optimum ionomer-to-catalyst ratio was found experimentallyfor the different catalysts For PtC 10 was determined to be the bestionomer-to-catalyst ratio while 17 was optimum for PtCNTOXand PtCNTON Figure 6a shows the performance of the three cata-lysts after running the fuel cell for five hours to establish steady-stateconditions After a five hour period the PtCNTON had the highestpower density peak of 82 mWcm2 as compared to the PtCNTOXcatalyst which had the lowest power density peak of 60 mWcm2After running the catalysts in a hybrid fuel cell for 15 hours Figure6b the PtCNTOX had higher power than the PtC catalyst where thepower density peak reached 72 mWcm2 The power density peak ofPtCNTON increased to 89 mWcm2 with time and had the greatestpower density among the three catalysts
Table IV Element ratio of the catalysts from XPS
Element Ratio ()
C O N
PtCNTOX 877 102 ndashPtCNTON 876 77 20
PtC 886 42 ndash
The stability of the catalysts was evaluated in an AEMPEM hybridfuel cell by monitoring the current density over a 20 hour period at250 mV Figure 6c shows the current density of the three catalystsover time The power of the fuel cell with PtC increased slightly fromthe beginning and remained stable at 250 mAcm2 for the 20-hourperiod The current density of PtCNTOX catalyst increased from238 mAcm2 to 275 mAcm2 over the first 8 hours and then becamestablized at 275 mAcm2 Finally the current density of PtCNTONwas stable at 282 mAcm2 for the length of the experiment 20 hoursIn conclusion the PtCNTON had the best peak power and stabilityperformance with the shortest start up time
The smaller size of the platinum particles may contribute to thesuperior performance of the PtCNTOX and PtCNTON catalystsThe surface area of the catalysts is larger when the particles remainsmall thereby increasing the overall catalytic activity On the otherhand the binding energy between Pt and pristine carbon nanotubesis weaker which leads to agglomeration of Pt clusters to create largerparticles38 In addition it has been shown that the oxygen containinggroups can activate the carbon nanotubes and provide a greater numberof active sites While the nitrogen containing groups can directlycatalyze the ORR29 Moreover previous study have found that thefunctional groups also can promote activity of catalysts by improvingthe structure stability of Pt with CNTs39 Compared to -O functionalatom -N functional atom can promote better structural stability Thereare claims that the hydrogen adsorption energy of Pt atom on variousfunctionalized CNTs is as follows -N lt -O lt pristine CNTs Theadsorption energy trends indicate that the catalytic activity of platinumsupported on functionalized CNTs is controlled by the functionalgroup(s) which is consistent with the experimental results in thisstudy
Hybrid direct methanol fuel cellmdashFigure 7 shows the performanceof PtC PtCNTOX and PtCNTON in a hybrid DMFCs for ORR Inhybrid DMFCs 10 ionomercatalysts ratio for PtC and 15 forPtCNTOX and PtCNTON was used to reach the optimal ratio Atthe beginning of the fuel cell test PtCNTON had the highest powerdensity peak at 14 mWcm2 and the PtC had the lowest density peakat 97 mWcm2 After operating the methanol fuel cell for five hoursthe power density peak of PtC and PtCNTOX catalysts were about thesame at 15 mWcm2 However the power density for the PtCNTONcatalyst dropped to 131 mWcm2 After operating the methanol fuelcell for 20 hours the power density peak of the PtCNTOX slowlyincreased to 16 mWcm2 while the PtC remained at 131 mWcm2and PtCNTON decreased to 11 mWcm2
The stability tests in Figure 7c show that the PtCNTON per-formance was the highest of the tested catalysts at the beginning
Table III Binding energy of O1s peaks and N1s peaks from XPS
O1s peak N1s peak
Peak 1 (5315 eV) Peak 2 (5326 eV) Peak 3 (5333 eV) Peak 4 (5342 eV) Pyridinic (3985 eV) Pyrrolic (4007 eV)
PtCNTOX 415 378 66 141 ndash ndashPtCNTON 532 114 247 107 331 669
PtC 366 264 226 144 ndash ndash
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F221
Figure 6 Performance Comparison of a H2-O2 hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after running for(a) 5 hours and (b) 15 hours (c) Stability test in H2-O2 hybrid AEMPEM fuel cell for 20 hours
Figure 7 Performance Comparison of a methanol hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after runningfor (a) 0 hours and (b) 5 hours and (c) 20 hours (d) Stability test of PtC PtCNTOX and PtCNTON in methanol hybrid AEMPEM fuel cell for 20 hours
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F222 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Figure 8 Internal Short Circuit Caused by Crossover Methanol
However the current density decreased with time over the first 20hours The current density of PtCNTOX increased slightly duringthe first 20 hours Thus the PtCNTOX was the most stable catalystamong the three tested
A possible reason for the unstable performance of PtCNTON inthe direct methanol fuel cell is because nitrogen containing groups maycatalyze oxidation of the methanol which has crossed over through themembranes more efficiently than other catalysts Based on previousresearch4041 methanol which has crossed through the membrane caneither directly react with oxygen (1a) or can be oxidized at the cathodewith oxygen reduction (1b) as follows
CH3OH + 3
2O2 rarr CO2 + 2H2O [1a]
CH3OH + H2O rarr CO2 + 6H+ + 6eminus [1b]
Figure 9 Nitrogen groups attracting methanol model
The latter reaction causes an internal short circuit because it resultsin lost current as shown in Figure 8 Both reactions create undesirableintermediates for the platinum catalysts
Based on Figure 5 the PtCNTON has pyrrolic nitrogen and pyri-dinic nitrogen groups and 20 more carbonyl groups (C=O) com-pared to the PtCNTOX catalyst This result suggests that the nitrogengroups make PtCNTON more active to methanol oxidation The ni-trogen anchor for the groups may facilitate the oxidation of methanolby attacking the proton of methanol as shown in Figure 9 Xionget al42 also claimed that the platinum supported on nitrogen dopedgraphene show higher electrochemical activity toward methanoloxidation
Conclusions
Two different functionalized carbon nanotubes supported platinumcatalysts PtCNTOX and PtCNTON were synthesized and the per-formance was evaluated in a hybrid PEMAEM H2O2 fuel cell and hy-brid DMFCs It was found that in a hybrid H2O2 fuel cell PtCNTONhad the best activity for ORR at the cathode For hybrid DMFCsPtCNTON had the highest activity initially but was not sable withtime The PtCNTOX had the best stability and activity for the en-tire 20 hour test The possible reason poor PtCNTON performancein the hybrid DMFCs may be because of the proton affinity for thepyrrolic nitrogen groups with methanol fuel making methanol easierto oxidize on the cathode
Acknowledgments
This work was supported by the China Scholarship Council Igratefully acknowledge Oluwadamilola Phillips for XPS characteri-zation of catalysts and technical discussion
References
1 B C Steele and A Heinzel Nature 414 345 (2001)2 I A S Arico V Baglio and V Antonucci (CNR-ITAE Institute Messina) Chapter
1 p 1 (2010)3 J E Soc et al 138 2334 (1993)4 K A Mauritz and R B Moore Chem Rev 104 4535 (2004)5 S Motupally A J Becker and J W Weidner J Electrochem Soc 147 3171
(2000)6 Z Liang et al J Memb Sci 233 39 (2004)7 G Sasikumar J W Ihm and H Ryu J Power Sources 132 11 (2004)8 M Unlu J Zhou and P A Kohl J Phys Chem C 113 11416 (2009)9 A Heinzel and V M Barragan J Power Sources 84 70 (1999)
10 K Scott W Taama P Argyropoulos and K Sundmacher J Power Sources 83 204(1999) httpwwwsciencedirectcomsciencearticlepiiS0378775399003031
11 S Wasmus and a Kuver J Electroanal Chem 461 14 (1999)12 L Ma H He A Hsu and R Chen J Power Sources 241 696 (2013)13 D Y Chung et al Sci Rep 4 7450 (2014) httpwwwpubmedcentral
nihgovarticlerenderfcgiartid=4264001amptool=pmcentrezamprendertype=abstract14 X Cheng et al J Power Sources 165 739 (2007)15 N Jung et al Int J Hydrogen Energy 36 15731 (2011)
httpdxdoiorg101016jijhydene20110905416 J S Spendelow J D Goodpaster P J A Kenis and A Wieckowski J Phys Chem
B 110 9545 (2006)17 M Najjari F Khemili and S Ben Nasrallah Renew Energy 33 1824 (2008)18 J R Varcoe and R C T Slade Fuel Cells 5 187 (2005)19 C G Arges V Ramani and P N Pintauro Elctrochemical Soc Interface 31 (2010)20 M Unlu J Zhou and P a Kohl J Electrochem Soc 157 B1391 (2010)
httpjesecsdlorgcgidoi1011491346870021 M Unlu J Zhou and P A Kohl Fuel Cells 10 54 (2010)22 J Zhou K Joseph J M Ahlfield D-Y Park and P a Kohl J Electrochem Soc
160 F573 (2013) httpjesecsdlorgcgidoi1011492077306jes23 K Lee et al Electrochim Acta 54 4704 (2009)24 Y Devrim and A Albostan J Electron Mater 45 3900 (2016)
httplinkspringercom101007s11664-016-4703-225 C Alegre D Sebastian M E Galvez R Moliner and M J Lazaro Appl Catal B
Environ 192 260 (2016) httpdxdoiorg101016japcatb20160307026 Z Peng and H Yang Nano Today 4 143 (2009)27 S Hong and S Myung Nat Nanotechnol 2 207 (2007)
httpwwwnaturecomdoifinder101038nnano20078928 P J Britto K S V Santhanam A Rubio J A Alonso and P M Ajayan Adv Mater
11 154 (1999)29 W Li et al Carbon N Y 40 791 (2002) httplinkinghubelseviercom
retrievepiiS0008622302000398
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F223
30 T Ishizaki S Chiba Y Kaneko and G Panomsuwan J Mater Chem A 2 10589(2014) httpxlinkrscorgDOI=c4ta01577k
31 R Zhong Y Qin D Niu X Zhang and X Zhou Electrochim Acta 89 157 (2013)httpdxdoiorg101016jelectacta201211007
32 K Artyushkova A Serov S Rojas-Carbonell and P Atanassov J Phys Chem C119 25917 (2015)
33 T Xing et al ACS Nano 8 6856 (2014)34 R Zhong Y Qin D Niu J Tian and X Zhang J Power Sources 225 192
(2013)35 H K Jeong et al J Am Chem Soc 130 1362 (2008)36 Y C Chiang C C Liang and C P Chung Materials (Basel) 8 6484
(2015)
37 A J Plomp D S Su K P De Jong and J H Bitter J Phys Chem C 113 9865(2009)
38 W Li et al J Phys Chem B 107 6292 (2003) httppubsacsorgdoiabs101021jp022505cnhttpdxdoiorg101021jp022505c
39 B-H Kim K-R Lee Y-C Chung and M Park Phys Chem Chem Phys 18 22687(2016) httppubsrscorgenContentArticleLanding2016CPC5CP07737K
40 S Zhou et al Phys Chem Chem Phys 3 347 (2001)httpxlinkrscorgDOI=b007283o
41 A Hacquard 107 (2005) httpwwwwpieduPubsETDAvailableetd-051205-151955unrestrictedAHacquardpdf
42 B Xiong et al Carbon N Y 52 181 (2013) httpdxdoiorg101016jcarbon201209019
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F221
Figure 6 Performance Comparison of a H2-O2 hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after running for(a) 5 hours and (b) 15 hours (c) Stability test in H2-O2 hybrid AEMPEM fuel cell for 20 hours
Figure 7 Performance Comparison of a methanol hybrid AEMPEM fuel cell Polarization and power density of PtC PtCNTOX and PtCNTON after runningfor (a) 0 hours and (b) 5 hours and (c) 20 hours (d) Stability test of PtC PtCNTOX and PtCNTON in methanol hybrid AEMPEM fuel cell for 20 hours
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F222 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Figure 8 Internal Short Circuit Caused by Crossover Methanol
However the current density decreased with time over the first 20hours The current density of PtCNTOX increased slightly duringthe first 20 hours Thus the PtCNTOX was the most stable catalystamong the three tested
A possible reason for the unstable performance of PtCNTON inthe direct methanol fuel cell is because nitrogen containing groups maycatalyze oxidation of the methanol which has crossed over through themembranes more efficiently than other catalysts Based on previousresearch4041 methanol which has crossed through the membrane caneither directly react with oxygen (1a) or can be oxidized at the cathodewith oxygen reduction (1b) as follows
CH3OH + 3
2O2 rarr CO2 + 2H2O [1a]
CH3OH + H2O rarr CO2 + 6H+ + 6eminus [1b]
Figure 9 Nitrogen groups attracting methanol model
The latter reaction causes an internal short circuit because it resultsin lost current as shown in Figure 8 Both reactions create undesirableintermediates for the platinum catalysts
Based on Figure 5 the PtCNTON has pyrrolic nitrogen and pyri-dinic nitrogen groups and 20 more carbonyl groups (C=O) com-pared to the PtCNTOX catalyst This result suggests that the nitrogengroups make PtCNTON more active to methanol oxidation The ni-trogen anchor for the groups may facilitate the oxidation of methanolby attacking the proton of methanol as shown in Figure 9 Xionget al42 also claimed that the platinum supported on nitrogen dopedgraphene show higher electrochemical activity toward methanoloxidation
Conclusions
Two different functionalized carbon nanotubes supported platinumcatalysts PtCNTOX and PtCNTON were synthesized and the per-formance was evaluated in a hybrid PEMAEM H2O2 fuel cell and hy-brid DMFCs It was found that in a hybrid H2O2 fuel cell PtCNTONhad the best activity for ORR at the cathode For hybrid DMFCsPtCNTON had the highest activity initially but was not sable withtime The PtCNTOX had the best stability and activity for the en-tire 20 hour test The possible reason poor PtCNTON performancein the hybrid DMFCs may be because of the proton affinity for thepyrrolic nitrogen groups with methanol fuel making methanol easierto oxidize on the cathode
Acknowledgments
This work was supported by the China Scholarship Council Igratefully acknowledge Oluwadamilola Phillips for XPS characteri-zation of catalysts and technical discussion
References
1 B C Steele and A Heinzel Nature 414 345 (2001)2 I A S Arico V Baglio and V Antonucci (CNR-ITAE Institute Messina) Chapter
1 p 1 (2010)3 J E Soc et al 138 2334 (1993)4 K A Mauritz and R B Moore Chem Rev 104 4535 (2004)5 S Motupally A J Becker and J W Weidner J Electrochem Soc 147 3171
(2000)6 Z Liang et al J Memb Sci 233 39 (2004)7 G Sasikumar J W Ihm and H Ryu J Power Sources 132 11 (2004)8 M Unlu J Zhou and P A Kohl J Phys Chem C 113 11416 (2009)9 A Heinzel and V M Barragan J Power Sources 84 70 (1999)
10 K Scott W Taama P Argyropoulos and K Sundmacher J Power Sources 83 204(1999) httpwwwsciencedirectcomsciencearticlepiiS0378775399003031
11 S Wasmus and a Kuver J Electroanal Chem 461 14 (1999)12 L Ma H He A Hsu and R Chen J Power Sources 241 696 (2013)13 D Y Chung et al Sci Rep 4 7450 (2014) httpwwwpubmedcentral
nihgovarticlerenderfcgiartid=4264001amptool=pmcentrezamprendertype=abstract14 X Cheng et al J Power Sources 165 739 (2007)15 N Jung et al Int J Hydrogen Energy 36 15731 (2011)
httpdxdoiorg101016jijhydene20110905416 J S Spendelow J D Goodpaster P J A Kenis and A Wieckowski J Phys Chem
B 110 9545 (2006)17 M Najjari F Khemili and S Ben Nasrallah Renew Energy 33 1824 (2008)18 J R Varcoe and R C T Slade Fuel Cells 5 187 (2005)19 C G Arges V Ramani and P N Pintauro Elctrochemical Soc Interface 31 (2010)20 M Unlu J Zhou and P a Kohl J Electrochem Soc 157 B1391 (2010)
httpjesecsdlorgcgidoi1011491346870021 M Unlu J Zhou and P A Kohl Fuel Cells 10 54 (2010)22 J Zhou K Joseph J M Ahlfield D-Y Park and P a Kohl J Electrochem Soc
160 F573 (2013) httpjesecsdlorgcgidoi1011492077306jes23 K Lee et al Electrochim Acta 54 4704 (2009)24 Y Devrim and A Albostan J Electron Mater 45 3900 (2016)
httplinkspringercom101007s11664-016-4703-225 C Alegre D Sebastian M E Galvez R Moliner and M J Lazaro Appl Catal B
Environ 192 260 (2016) httpdxdoiorg101016japcatb20160307026 Z Peng and H Yang Nano Today 4 143 (2009)27 S Hong and S Myung Nat Nanotechnol 2 207 (2007)
httpwwwnaturecomdoifinder101038nnano20078928 P J Britto K S V Santhanam A Rubio J A Alonso and P M Ajayan Adv Mater
11 154 (1999)29 W Li et al Carbon N Y 40 791 (2002) httplinkinghubelseviercom
retrievepiiS0008622302000398
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F223
30 T Ishizaki S Chiba Y Kaneko and G Panomsuwan J Mater Chem A 2 10589(2014) httpxlinkrscorgDOI=c4ta01577k
31 R Zhong Y Qin D Niu X Zhang and X Zhou Electrochim Acta 89 157 (2013)httpdxdoiorg101016jelectacta201211007
32 K Artyushkova A Serov S Rojas-Carbonell and P Atanassov J Phys Chem C119 25917 (2015)
33 T Xing et al ACS Nano 8 6856 (2014)34 R Zhong Y Qin D Niu J Tian and X Zhang J Power Sources 225 192
(2013)35 H K Jeong et al J Am Chem Soc 130 1362 (2008)36 Y C Chiang C C Liang and C P Chung Materials (Basel) 8 6484
(2015)
37 A J Plomp D S Su K P De Jong and J H Bitter J Phys Chem C 113 9865(2009)
38 W Li et al J Phys Chem B 107 6292 (2003) httppubsacsorgdoiabs101021jp022505cnhttpdxdoiorg101021jp022505c
39 B-H Kim K-R Lee Y-C Chung and M Park Phys Chem Chem Phys 18 22687(2016) httppubsrscorgenContentArticleLanding2016CPC5CP07737K
40 S Zhou et al Phys Chem Chem Phys 3 347 (2001)httpxlinkrscorgDOI=b007283o
41 A Hacquard 107 (2005) httpwwwwpieduPubsETDAvailableetd-051205-151955unrestrictedAHacquardpdf
42 B Xiong et al Carbon N Y 52 181 (2013) httpdxdoiorg101016jcarbon201209019
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
F222 Journal of The Electrochemical Society 164 (4) F217-F223 (2017)
Figure 8 Internal Short Circuit Caused by Crossover Methanol
However the current density decreased with time over the first 20hours The current density of PtCNTOX increased slightly duringthe first 20 hours Thus the PtCNTOX was the most stable catalystamong the three tested
A possible reason for the unstable performance of PtCNTON inthe direct methanol fuel cell is because nitrogen containing groups maycatalyze oxidation of the methanol which has crossed over through themembranes more efficiently than other catalysts Based on previousresearch4041 methanol which has crossed through the membrane caneither directly react with oxygen (1a) or can be oxidized at the cathodewith oxygen reduction (1b) as follows
CH3OH + 3
2O2 rarr CO2 + 2H2O [1a]
CH3OH + H2O rarr CO2 + 6H+ + 6eminus [1b]
Figure 9 Nitrogen groups attracting methanol model
The latter reaction causes an internal short circuit because it resultsin lost current as shown in Figure 8 Both reactions create undesirableintermediates for the platinum catalysts
Based on Figure 5 the PtCNTON has pyrrolic nitrogen and pyri-dinic nitrogen groups and 20 more carbonyl groups (C=O) com-pared to the PtCNTOX catalyst This result suggests that the nitrogengroups make PtCNTON more active to methanol oxidation The ni-trogen anchor for the groups may facilitate the oxidation of methanolby attacking the proton of methanol as shown in Figure 9 Xionget al42 also claimed that the platinum supported on nitrogen dopedgraphene show higher electrochemical activity toward methanoloxidation
Conclusions
Two different functionalized carbon nanotubes supported platinumcatalysts PtCNTOX and PtCNTON were synthesized and the per-formance was evaluated in a hybrid PEMAEM H2O2 fuel cell and hy-brid DMFCs It was found that in a hybrid H2O2 fuel cell PtCNTONhad the best activity for ORR at the cathode For hybrid DMFCsPtCNTON had the highest activity initially but was not sable withtime The PtCNTOX had the best stability and activity for the en-tire 20 hour test The possible reason poor PtCNTON performancein the hybrid DMFCs may be because of the proton affinity for thepyrrolic nitrogen groups with methanol fuel making methanol easierto oxidize on the cathode
Acknowledgments
This work was supported by the China Scholarship Council Igratefully acknowledge Oluwadamilola Phillips for XPS characteri-zation of catalysts and technical discussion
References
1 B C Steele and A Heinzel Nature 414 345 (2001)2 I A S Arico V Baglio and V Antonucci (CNR-ITAE Institute Messina) Chapter
1 p 1 (2010)3 J E Soc et al 138 2334 (1993)4 K A Mauritz and R B Moore Chem Rev 104 4535 (2004)5 S Motupally A J Becker and J W Weidner J Electrochem Soc 147 3171
(2000)6 Z Liang et al J Memb Sci 233 39 (2004)7 G Sasikumar J W Ihm and H Ryu J Power Sources 132 11 (2004)8 M Unlu J Zhou and P A Kohl J Phys Chem C 113 11416 (2009)9 A Heinzel and V M Barragan J Power Sources 84 70 (1999)
10 K Scott W Taama P Argyropoulos and K Sundmacher J Power Sources 83 204(1999) httpwwwsciencedirectcomsciencearticlepiiS0378775399003031
11 S Wasmus and a Kuver J Electroanal Chem 461 14 (1999)12 L Ma H He A Hsu and R Chen J Power Sources 241 696 (2013)13 D Y Chung et al Sci Rep 4 7450 (2014) httpwwwpubmedcentral
nihgovarticlerenderfcgiartid=4264001amptool=pmcentrezamprendertype=abstract14 X Cheng et al J Power Sources 165 739 (2007)15 N Jung et al Int J Hydrogen Energy 36 15731 (2011)
httpdxdoiorg101016jijhydene20110905416 J S Spendelow J D Goodpaster P J A Kenis and A Wieckowski J Phys Chem
B 110 9545 (2006)17 M Najjari F Khemili and S Ben Nasrallah Renew Energy 33 1824 (2008)18 J R Varcoe and R C T Slade Fuel Cells 5 187 (2005)19 C G Arges V Ramani and P N Pintauro Elctrochemical Soc Interface 31 (2010)20 M Unlu J Zhou and P a Kohl J Electrochem Soc 157 B1391 (2010)
httpjesecsdlorgcgidoi1011491346870021 M Unlu J Zhou and P A Kohl Fuel Cells 10 54 (2010)22 J Zhou K Joseph J M Ahlfield D-Y Park and P a Kohl J Electrochem Soc
160 F573 (2013) httpjesecsdlorgcgidoi1011492077306jes23 K Lee et al Electrochim Acta 54 4704 (2009)24 Y Devrim and A Albostan J Electron Mater 45 3900 (2016)
httplinkspringercom101007s11664-016-4703-225 C Alegre D Sebastian M E Galvez R Moliner and M J Lazaro Appl Catal B
Environ 192 260 (2016) httpdxdoiorg101016japcatb20160307026 Z Peng and H Yang Nano Today 4 143 (2009)27 S Hong and S Myung Nat Nanotechnol 2 207 (2007)
httpwwwnaturecomdoifinder101038nnano20078928 P J Britto K S V Santhanam A Rubio J A Alonso and P M Ajayan Adv Mater
11 154 (1999)29 W Li et al Carbon N Y 40 791 (2002) httplinkinghubelseviercom
retrievepiiS0008622302000398
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F223
30 T Ishizaki S Chiba Y Kaneko and G Panomsuwan J Mater Chem A 2 10589(2014) httpxlinkrscorgDOI=c4ta01577k
31 R Zhong Y Qin D Niu X Zhang and X Zhou Electrochim Acta 89 157 (2013)httpdxdoiorg101016jelectacta201211007
32 K Artyushkova A Serov S Rojas-Carbonell and P Atanassov J Phys Chem C119 25917 (2015)
33 T Xing et al ACS Nano 8 6856 (2014)34 R Zhong Y Qin D Niu J Tian and X Zhang J Power Sources 225 192
(2013)35 H K Jeong et al J Am Chem Soc 130 1362 (2008)36 Y C Chiang C C Liang and C P Chung Materials (Basel) 8 6484
(2015)
37 A J Plomp D S Su K P De Jong and J H Bitter J Phys Chem C 113 9865(2009)
38 W Li et al J Phys Chem B 107 6292 (2003) httppubsacsorgdoiabs101021jp022505cnhttpdxdoiorg101021jp022505c
39 B-H Kim K-R Lee Y-C Chung and M Park Phys Chem Chem Phys 18 22687(2016) httppubsrscorgenContentArticleLanding2016CPC5CP07737K
40 S Zhou et al Phys Chem Chem Phys 3 347 (2001)httpxlinkrscorgDOI=b007283o
41 A Hacquard 107 (2005) httpwwwwpieduPubsETDAvailableetd-051205-151955unrestrictedAHacquardpdf
42 B Xiong et al Carbon N Y 52 181 (2013) httpdxdoiorg101016jcarbon201209019
) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 13020774102Downloaded on 2017-03-14 to IP
Journal of The Electrochemical Society 164 (4) F217-F223 (2017) F223
30 T Ishizaki S Chiba Y Kaneko and G Panomsuwan J Mater Chem A 2 10589(2014) httpxlinkrscorgDOI=c4ta01577k
31 R Zhong Y Qin D Niu X Zhang and X Zhou Electrochim Acta 89 157 (2013)httpdxdoiorg101016jelectacta201211007
32 K Artyushkova A Serov S Rojas-Carbonell and P Atanassov J Phys Chem C119 25917 (2015)
33 T Xing et al ACS Nano 8 6856 (2014)34 R Zhong Y Qin D Niu J Tian and X Zhang J Power Sources 225 192
(2013)35 H K Jeong et al J Am Chem Soc 130 1362 (2008)36 Y C Chiang C C Liang and C P Chung Materials (Basel) 8 6484
(2015)
37 A J Plomp D S Su K P De Jong and J H Bitter J Phys Chem C 113 9865(2009)
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