Applied Catalysis B: Environmental

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1 Applied Ctlysis B: Environmentl 17 (211) Contents lists ville t ScienceDirect Applied Ctlysis B: Environmentl jo ur n l homepge: Synthesis of grphitic mesoporous crons with high surfce res nd their pplictions in direct methnol fuel cells Jing Qi,, Luhu Jing, Suli Wng, Gongqun Sun, Direct Alcohol Fuel Cell Lortory, Dlin Institute of Chemicl Physics, Chinese Acdemy of Sciences, Dlin 11623, Chin Grdute University of Chinese Acdemy of Sciences, Beijing 149, Chin r t i c l e i n f o Article history: Received 13 Jnury 211 Received in revised form 3 My 211 Accepted 1 July 211 Aville online 13 July 211 Keywords: Grphitic cron Electroctlyst Stility Methnol electro-oxidtion rection Direct methnol fuel cell s t r c t A grphitic mesoporous cron (denoted s ) ws synthesized using resorcinol nd formldehyde s cron precursors nd iron nitrte s grphitiztion ctlyst. The ws chrcterized y X-ry diffrction, Rmn spectroscopy, trnsmission electron microscopy (TEM), nd nitrogen dsorption. The results show tht the hs mesoporous structure nd high surfce re of 43 m 2 g 1, nd prticulrly, well-defined grphitic frmework. Using the s the support, PtRu/ ws synthesized to ct s n electroctlyst for the methnol oxidtion rection (MOR). A counterprt with Vulcn -72 (denoted s ) s the support ws prepred for comprison. TEM imges show tht PtRu nnoprticles re distriuted uniformly on the cron supports for oth electroctlysts. The electrochemicl ctivity of the PtRu/ towrd the MOR is slightly higher thn tht of the PtRu/ in oth hlf cell nd single cell mesurements. The 15 h stility test of single cell suggests tht the PtRu/ is excellent stle. Crown Copyright 211 Pulished y Elsevier B.V. All rights reserved. 1. Introduction Direct methnol fuel cells (DMFCs) hve een received extensive interests s lterntive power sources for oth sttionry nd moile pplictions. However, the ctivity, stility nd cost of electroctlysts still remin criticl issues to e resolved for the commerciliztion of DMFCs. Pt metl lloys supported on crons re generlly used s electroctlysts for DMFCs [1 5]. As supports for electroctlysts, the chrcteristics of crons, such s high surfce re, high grphitic chrcters, nd suitle porosity, re of considerle importnce for improving the ctivity nd stility of electroctlysts [6 8]. Cron lcks re most widely used s supports for electroctlysts. However, cron lcks re usully morphous with low grphitic chrcter, which re prone to undergo electrochemicl corrosion. Moreover, they contin lrge quntities of micropores, which oth reduce the Pt utiliztion ecuse the rectnts re hrdly ccessile to Pt prticles nchored in the micropores of crons nd lso increse the trnsport resistnce of rectnts, intermedites nd products. Recently, grphitic crons hve received much more ttention for electroctlyst supports ecuse of their high conductivity, good therml nd electrochemicl stility. The electroctlysts supported on grphitic crons, such s cron nnotues [9 11], cron nnofiers [12,13], ordered hierrchicl nnostructured Corresponding uthors. Tel.: ; fx: E-mil ddresses: sunshine@dicp.c.cn (L. Jing), gqsun@dicp.c.cn (G. Sun). crons [14], cron nnocpsule [15,16], cron nnocges [17], ordered mesoporous crons [18 2], nd cron nnocoils [21], showed good ctivity nd stility whether in cidic solutions or in rel fuel cell working conditions. Vrious conventionl synthesis methods, including rc dischrge, lser evportion, nd chemicl vpor deposition, hve een used to prepre grphitic crons [22 24]. However, up to now, the lrge-scle nd economicl production of grphitic crons is limited y the hrsh synthetic conditions nd low production yields with the synthesis methods mentioned ove [25]. An lterntive method to synthesize grphitic crons is to tret thermlly mixture contining precursor of cron nd metl slts s grphitiztion ctlyst [21,25,26]. By this method, grphitic nnoprticles were generted round the in situ generted metl prticles, wheres morphous cron ws generted from cron precursor wy from the metl prticles [25,26]. Therefore, the distriution of the metl prticles in the cron precursors is crucil to the grphitic chrcter nd the qulifiction of the grphitic cron. Furthermore, it ws reported tht incresing the concentrtion of metl slts could increse the yield of hollow grphitic nnoprticles, ttriuted to the higher dispersion of metl prticles, ut lwys results in low surfce re of the otined cron mterils [25]. Therefore, the synthesis of grphitic mesoporous crons with high surfce res is still gret chllenge with the conventionl methods. In this pper, we report novel ut simple method to synthesize mesoporous cron with high surfce re nd grphitic chrcter () y introducing trisodium citrte, well-known complex /$ see front mtter. Crown Copyright 211 Pulished y Elsevier B.V. All rights reserved. doi:1.116/j.pct

2 96 J. Qi et l. / Applied Ctlysis B: Environmentl 17 (211) ing gent, to the synthesis procedure nd employing resorcinol nd formldehyde s precursors nd iron nitrte s grphitiztion ctlyst. The otined ws used s the support for PtRu nnoprticles synthesized y polyol method nd counterprt employing Vulcn -72 (denoted s ) s the support ws lso prepred for comprison. The ctivities of the PtRu/ nd the PtRu/ towrd the methnol oxidtion rection (MOR) were investigted in.5 M H 2 SO 4 contining 1 M CH 3 OH. DMFCs with the PtRu/ nd the PtRu/ s node electroctlysts, respectively, were fricted nd tested t 75 C. The stility of the PtRu/ electroctlyst ws lso investigted in the single cell working conditions. 2. Experimentl 2.1. Synthesis of the cron The ws synthesized using resorcinol nd formldehyde s cron precursors, iron nitrte complexed with trisodium citrte s grphitiztion ctlyst, nd silic sol (Ek chemicls Co., Ltd., prticle size of SiO 2 = 14 nm; density = 1.3 g cm 3 ) s templte. The synthesis procedure is descried riefly s follows: the required mounts of iron nitrte nd trisodium citrte were mixed in n queous solution. Next, the silic sol ws dded into the solution with ultrsoniclly stirring for 3 min, followed y the ddition of the resorcinol nd the formldehyde. The resulting mixture with molr rtio of iron nitrte/trisodium citrte/silic/resorcinol/formldehyde =.4:.4:1:2:4 ws cured t 85 C for 3 h in closed glss tue nd then cronized under nitrogen tmosphere t 9 C for 3 h. The otined composite ws refluxed in 3 M NOH solution t 1 C for 6 h to remove the silic prticles, nd then refluxed in 5 M HNO 3 solution t 1 C for 12 h to remove the residue iron prticles. After filtered, wshed, nd dried t 1 C for 8 h in n oven, the ws otined Preprtion of the electroctlysts The otined ws refluxed t 1 C in 1% H 2 O 2 for 6 h efore use s the support. The ws treted with sme procedure s the. Chloropltinic cid (H 2 PtCl 6 6H 2 O) nd ruthenium chloride (RuCl 3 ) were used s the precursors of Pt nd Ru, respectively. In rief, the required mounts of H 2 PtCl 6 6H 2 O nd RuCl 3 were dded to 2 ml of ethylene glycol under mgnetic stirring for out 3 min. The ph vlue of the solution ws djusted to 13 with sodium hydroxide. The solution ws heted to 16 C nd mintined t this temperture for 3 h. After the otined mixture ws cooled down to 8 C, the required mount of or ws dded under mgnetic stirring for out 3 min to otin homogeneous slurry. Then suitle mount of 1.5 M HCl queous solution used s sedimenttion promoter ws dded to the slurry. After cooled down to room temperture, the slurry ws filtered, wshed, nd dried t 75 C for 8 h in vcuum oven. Finlly, the electroctlysts with nominl metl loding of 45 wt.% nd Pt:Ru tomic rtio of 1:1 were otined nd denoted s PtRu/ nd PtRu/ Physicl chrcteriztion X-ry diffrction (XRD) ptterns were recorded on Rigku D/mx-24 X-ry diffrctometer using Cu K rdition. The scn rte ws 5 min 1 with n ngulr resolution of.2. The tue voltge nd the tue current were mintined t 4 kv nd 1 ma, respectively. Rmn spectr were mesured t room temperture with Join-Yvon T64 triple stge spectrogrph with spectrl resolution of 2 cm 1. The lser line t 325 nm of He Cd lser ws used s n excittion source with n output of 25 mw. The power of lser t the smple ws out 3 mw. Trnsmission electron microscopy (TEM) ws crried out using JEOL JEM-211EM microscope operted t 1 kv. Nitrogen dsorption mesurements were performed t 196 C on Micromeritics ASAP 2 volumetric dsorption system. Energy dispersive X-ry spectroscopy (EDX) nlysis ws crried out on JSM-636LV scnning electron microscope equipped with Oxford INCA energy dispersive X-ry spectroscope. The ccelerting voltge ws 2 kv Electrochemicl chrcteriztions of the PtRu/ nd the PtRu/ electroctlysts The electrochemicl chrcteriztions were crried out in three-electrode electrochemicl cell controlled y CHI76B potentiostt/glvnostt t room temperture. A thin porous film on glssy cron (GC) disk with dimeter of 5 mm ws used s the working electrode. The thin film electrode ws fricted s follows. Typiclly, 5 mg of the electroctlyst ws ultrsoniclly suspended in 2 ml of ethnol nd 5 L of Nfion solution (5 wt.%, Du Pont) for 3 min to form homogeneous ink. Then 2 L of the ink ws spred onto the surfce of the GC electrode with micropipette to form uniform electroctlyst film on the GC. A Pt-foil nd sturted clomel electrode were employed s the counter nd the reference electrodes, respectively. All potentils in this work were referred to the reversile hydrogen electrode (RHE). The electrode ws scnned first severl cycles etween.8 nd.84 V in N 2 -sturted.5 M H 2 SO 4 with scn rte of 1 mv s 1 to clen the electrode surfce efore the se cyclic voltmmetry (CV) curves were recorded t scn rte of 1 mv s 1. The upper limit of the potentil ws set t.84 V to void ny possile dissolution of Ru. The ctlytic ctivities of the electroctlysts towrd the MOR were mesured in N 2 -sturted.5 M H 2 SO M CH 3 OH solution y potentiodynmic mesurements etween.8 nd.84 V with scn rte of 1 mv s 1. Cron monoxide (CO) ws used s proing molecules to evlute the electrochemicl surfce re (ECSA) of the PtRu/C electroctlysts. CO stripping experiments were descried s follows. The electrode ws scnned first severl cycles etween.8 nd.84 V in N 2 -sturted.5 M H 2 SO 4 with scn rte of 1 mv s 1 to clen the electrode surfce. Then the electrode potentil ws first held t.8 V in CO-sturted.5 M H 2 SO 4 solution for 3 min to ensure sturted CO dsorption on the electroctlyst surfce. After tht the gs ws switched to N 2 for 3 min keeping the potentil t.8 V to remove ny CO from the solution. Then the electrode ws scnned etween.8 nd 1.8 V t scn rte of 1 mv s 1 nd CO stripping voltmmogrms were recorded Performnce evlution for the PtRu/ nd the PtRu/ s the node electroctlysts of single cells The single cells with the PtRu/ nd the PtRu/ s the node electroctlysts were tested on fuel cell test pltform (Arin Instrument Corp., USA) t 75 C. The memrne electrode ssemly (MEA) ws fricted ccording to the method in the literture [27]. The 4 wt.% Pt/C (Johnson Mtthey Inc.) ws used in the cthode nd the Pt loding on the cthode ws 1 mg cm 2. The 45 wt.% PtRu/ or 45 wt.% PtRu/ ws used in the node nd the metl loding on the node is 2 mg cm 2. A Nfion -115 memrne ws used s the solid electrolyte. The MEA ws prepred y plcing the node nd the cthode on ech side of the memrne nd hot pressing t 12 C nd pressure of 3 MP for 1 min. 1 M CH 3 OH solution ws fed into the node with flow rte of 1 ml min 1 nd O 2 with.2 MP pressure ws fed to the cthode. The ctive geometric re of the single cell ws 2 cm 2 cm.

3 J. Qi et l. / Applied Ctlysis B: Environmentl 17 (211) (2) Tle 1 Physicl properties of nd. Smple 2 2 ( ) d 2 (nm) I D/I G S BET (m 2 g 1 ) Intensity (.u.) Intensity (.u.) (1)/(11) (4) (11) D nd 2 Thet (º) G nd Rmn shift (cm -1 ) Fig. 1. () XRD ptterns nd () Rmn spectr of nd. The stility test of the DMFC with the PtRu/ s n node electroctlyst ws crried out t 75 C y keeping constnt current density of 1 ma cm 2. The node ws fed with 1 M CH 3 OH solution with flow rte of 1 ml min 1, while the cthode ws fed with O 2 of.2 MP pressure. 3. Results nd discussion 3.1. Physicl chrcteriztion of the cron supports nd electroctlysts Fig. 1 shows the XRD ptterns of the nd the. It cn e seen tht the exhiits intense diffrction peks t round 26.2, 42.8, 53.8 nd 77.8 corresponding to the ( 2), (1 )/(1 1), ( 4) nd (1 1 ) diffrction peks of grphite [JCPDS: PDF ], respectively. The diffrction pek of the ( 2) is shrper nd more intense thn tht of the, indicting tht the former exhiits higher grphitic chrcter thn the ltter. Menwhile, s listed in Tle 1, the plne spcing (d 2) for the (.34 nm), clculted y Brgg s eqution sed on the ( 2) diffrction pek, is similr to tht of grphite (.3354 nm) [26]. In contrst, the d 2 vlue for the (.36 nm) is clerly lrger thn tht of the. This lso suggests well-defined grphitic frmework of the [26]. Fig. 1 presents the Rmn spectr of the nd the. Rmn spectr of the two cron mterils show chrcteristic G ( grphite ) nd pek t 1587 cm 1 nd D ( defect ) nd pek t 141 cm 1. The G nd corresponds to n idel grphitic lttice virtion mode with E 2g symmetry. It is the only pek oserved for lrge single grphitic crystls. The D nd, which is of A 1g symmetry, is chrcteristic of disordered grphite [28]. The reltive intensity rtio etween the D nd G nds (I D /I G ) reflects the grphitic chrcter, tht is, the lower the I D /I G vlue the higher the grphitic chrcter. As listed in Tle 1, the lower I D /I G vlue for the indictes tht the possesses higher grphitic chrcter thn the, which is consistent with the ove XRD results. Fig. 2 shows the TEM imges of the nd the. It cn e seen from Fig. 2 nd tht the is minly composed of cron nnorions. The cron nnorions re overlpped together s oserved on cron nnotues due to the vn der Wls force [29]. In contrst, the shows sphericl primry prticles with n verge dimeter of 3 nm which ggregte seriously (Fig. 2c). Typicl N 2 dsorption desorption isotherms of the nd the re shown in Fig. 3. The Brunuer Emmett Teller (BET) surfce re of the nd the is 43 nd 245 m 2 g 1, respectively. The hysteresis for the desorption rnch under higher reltive pressures oserved for oth mterils strongly indictes the presence of mesopores [3]. It cn e seen tht the exhiits comprtively nrrow nd uniform pore size distriution rnging from 3 to 5 nm. While for the, wide pore size distriution rnging from 1 to 15 nm is oserved. Considering tht the primry prticle size for the is round 3 nm (see Fig. 3c), it cn e concluded tht the pores of the come minly from the secondry pores due to the ggregtion of cron nnoprticles. Fig. 2. TEM imges of ( nd ) nd (c).

4 98 J. Qi et l. / Applied Ctlysis B: Environmentl 17 (211) Volume dsored (cm 3 g -1 ) ds- des- ds- des- Pore volume (cm 3 g -1 ) Reltive pressure (P/P o ) Pore dimeter (nm) Fig. 3. () N 2 dsorption desorption isotherms for nd nd () the corresponding pore size distriution curves clculted from the desorption rnches of the nitrogen isotherms y Brrett Joyner Hlend method. The XRD ptterns of the PtRu/ nd the PtRu/ re shown in Fig. 4. In ddition to the diffrction pek from grphite ( 2) ner 26, the pek t 39.8 is ttriuted to the diffrction from Pt (1 1 1) [JCPDS: PDF 4-82]. No other diffrction peks for Pt or Ru re oserved due to the rodening of the diffrction peks, indicting extremely smll metl prticles, which is confirmed y the following TEM nlysis. The similr XRD ptterns for PtRu/C electroctlysts were reported in Refs. [31,32]. Usully the pek position of Pt is used to determine whether PtRu lloy forms or not. When inspect crefully the XRD ptterns of PtRu/ nd PtRu/, we found the pek position of Pt (1 1 1) (39.8 ) did not shift significntly compred with tht of Pt crystlline (39.8 ), indicting Pt nd Ru did not form lloy. Since no diffrction peks for Ru or RuO is oserved in the XRD ptterns, it is deduced tht Ru exists in morphous sttes in oth smples. To determine the metl compositions for the PtRu/ nd the PtRu/, the EDX nlysis ws performed nd the results re shown in Tle 2. As listed in Tle 2, the Pt nd Ru elements cn e clerly detected in oth smples, confirming the deposition of PtRu nnoprticles on the cron supports. Moreover, the metl composition for the PtRu/ nd the PtRu/ is very close to the nominl vlue (Pt:Ru = 1:1 with nominl metl loding of 45 wt.%). This result grees well with the elementl compositions for the 4 wt.% nd 6 wt.% PtRu/C electroctlysts with the nominl Pt/Ru tomic rtio of 1/1 reported in Ref. [33], where the electroctlysts were prepred y the sme method. The TEM imges for the PtRu/ nd the PtRu/ re shown in Fig. 5 nd. It cn e seen tht for oth electroctlysts, the metl prticles re distriuted uniformly on the cron supports. The corresponding histogrms of the metl prticle size distriution for the two electroctlysts re lso plotted in Fig. 5c nd d. For ech smple, out 4 metl prticles were selected rndomly nd mesured. For the PtRu/ nd the PtRu/, the verge metl prticle size is round 2.3 nd 2.2 nm, respectively Electrochemicl property of the electroctlysts in N 2 -sturted.5 M H 2 SO 4 CV curves for the PtRu/ nd the PtRu/ performed in N 2 -sturted.5 M H 2 SO 4 electrolyte re shown in Fig. 6. As shown in Fig. 6 (ctive-surfce-re normlized), the hydrogen dsorption/desorption is strongly inhiited nd distorted for the PtRu/ nd the PtRu/ due to the PtRu imetl formtion s oserved in Ref. [33]. Tle 2 EDX results for the PtRu/ nd PtRu/ electroctlysts efore 15 h stility test nd node nd cthode ctlyst lyers fter 15 h stility test. Smple Element composition EDX (wt.%) EDX (t.%) 3.3. Activity of the electroctlysts towrd the MOR Prior to investigte the MOR ctivity, the ECSA of the PtRu/ nd the PtRu/ ws chrcterized y pre-dsored CO monolyer oxidtion ( CO stripping voltmmetry ), s reported in Refs. [33,34]. PtRu/ C K:42.22 C K:8. O K:1.81 O K:13.65 Ru L:15.37 Ru L:2.98 Pt M:31.6 Pt M:3.37 Totls 1 Grphite (2) PtRu/ PtRu/ PtRu/ C K:44.95 C K:8. O K:1.22 O K:13.65 Ru L:14.1 Ru L:2.98 Pt M:3.73 Pt M:3.37 Totls 1 Anode ctlyst lyer C K:47.23 C K:89.2 F K:2.95 F K:3.52 Ru L:15.57 Ru L:3.49 Pt M:34.26 Pt M:3.98 Totls 1 Cthode ctlyst lyer C K:33.51 C K:53.99 F K:42.86 F K:43.66 Pt M:23.63 Pt M:2.34 Totls 1 Intensity (.u.) Thet (º) Fig. 4. XRD ptterns of () PtRu/ nd () PtRu/.

5 J. Qi et l. / Applied Ctlysis B: Environmentl 17 (211) Fig. 5. TEM imges of () PtRu/ nd () PtRu/ nd the corresponding histogrms of metl prticle size distriution for (c) PtRu/ nd (d) PtRu/. The CO stripping voltmmogrms for the PtRu/C electroctlysts re shown in Fig. 7. It cn e seen tht the CO stripping profiles for oth smples re identicl, including the onset potentils nd the pek currents, except the slightly stronger ut nrrower pek for the PtRu/ compred with the PtRu/. The slightly stronger ut nrrower pek for the PtRu/ suggests tht CO d might e more esily removed on the PtRu/, which would enefit for the MOR. Assuming the chrge of CO monolyer dsorption is 42 C cm 2 [35], the ECSA ws clculted y integrting the I (ma cm -2 ) PtRu/ PtRu/ E (V vs. RHE) I (ma) I (ma) PtRu/ PtRu/ E (V vs. RHE) Fig. 6. Potentiodynmic CVs for PtRu/, nd PtRu/ in N 2-sturted.5 M H 2SO 4 with scn rte of 1 mv s 1 t 25 C. The currents were normlized y the ctive-surfce-re. Fig. 7. CO stripping voltmmogrms for PtRu/ nd PtRu/ in.5 M H 2SO 4 with scn rte of 1 mv s 1 t 25 C. ( ) 1st cycle nd ( ) 2nd cycle.

6 1 J. Qi et l. / Applied Ctlysis B: Environmentl 17 (211) I (ma mg -1 PtRu ) I (ma cm -2 ) PtRu/ PtRu/ PtRu/ PtRu/ Cell voltge (V) PtRu/ PtRu/ Current density (ma cm -2 ) Fig. 1. Performnces of DMFC single cells with different node electroctlysts. Cell temperture: 75 C; node: 2 mg PtRu cm 2, 1 M CH 3OH with flow rte of 1 ml min 1 ; cthode: 1 mg Pt cm 2 (4 wt.% Pt/C, Johnson Mtthey Inc.),.2 MP O Power density (mw cm -2 ) E (V vs. RHE) Fig. 8. Potentiodynmic mesurements for PtRu/ nd PtRu/ in N 2-sturted.5 M H 2SO M CH 3OH with scn rte of 1 mv s 1 t 25 C. Arrows indicte the direction of potentil scn. The currents were normlized y the PtRu-mss nd the ctive-surfce-re for () nd (), respectively. Cell voltge (V) ma cm -2 chrge of CO stripping. The ECSA for the PtRu/ nd the PtRu/ is 83.1 nd 85.8 m 2 g 1, respectively. The ctivities of the PtRu/ nd the PtRu/ towrd the MOR were mesured potentiodynmiclly in N 2 -sturted.5 M H 2 SO 4 contining 1 M CH 3 OH, s shown in Fig. 8. Fig. 8 nd re the PtRu-mss nd the ctive-surfce-re normlized MOR currents, respectively. The onset potentils of the MOR for oth electroctlysts re ll round.39 V, ut the MOR current density for the PtRu/ is little higher thn tht for the PtRu/ t the sme potentil, indicting tht the PtRu/ exhiits slightly higher I (ma mg -1 PtRu ) I (ma cm -2 ) PtRu/ PtRu/ PtRu/ PtRu/ Time (min) Fig. 9. Potentiosttic mesurements for PtRu/ nd PtRu/ t potentil of.54 V in N 2-sturted.5 M H 2SO M CH 3OH t 25 C. The currents were normlized y the PtRu-mss nd the ctive-surfce-re for () nd (), respectively Test time (h) Fig. 11. Stility test of DMFC single cell with the PtRu/ s the node electroctlyst. Dischrging current density: 1 ma cm 2 ; cell temperture: 75 C; node: 2 mg PtRu cm 2, 1 M CH 3OH with flow rte of 1 ml min 1 ; cthode: 1 mg Pt cm 2 (4 wt.% Pt/C, Johnson Mtthey Inc.),.2 MP O 2. ctlytic ctivity thn the PtRu/. The single cell mesurements discussed lter lso support this conclusion. Fig. 9 shows the potentiosttic curves otined t potentil of.54 V in N 2 -sturted.5 M H 2 SO 4 contining 1 M CH 3 OH for 4 min. The MOR currents were normlized y the PtRu-mss nd the ctive-surfce-re for Fig. 9 nd, respectively. For the PtRu/, fst MOR current density drop in the first 1 min is Cell voltge (V) initil fter 5 h fter 1 h fter 15 h Current density (ma cm -2 ) Fig. 12. Performnce of DMFC single cell with the PtRu/ s the node electroctlyst efore nd fter the stility test. Cell temperture: 75 C; node: 2 mg PtRu cm 2, 1 M CH 3OH with flow rte of 1 ml min 1 ; cthode: 1 mg Pt cm 2 (4 wt.% Pt/C, Johnson Mtthey Inc.),.2 MP O Power density (mw cm -2 )

7 J. Qi et l. / Applied Ctlysis B: Environmentl 17 (211) Fig. 13. TEM imges of () node ctlyst fter 15 h stility test, nd cthode ctlyst () fter nd (c) efore 15 h stility test, nd the corresponding histogrms of metl prticle size distriutions.

8 12 J. Qi et l. / Applied Ctlysis B: Environmentl 17 (211) oserved, then the MOR current density decys slowly. While for the PtRu/, in the whole test time, the MOR current density is higher thn tht of the PtRu/. This higher qusi-stle MOR current density for the PtRu/ is consistent with the results otined y the potentiodynmic mesurements ove. The stility of the PtRu/ under fuel cell working conditions will e investigted in detil lter. To test the ctivity of the PtRu/ nd the PtRu/ towrd the MOR under fuel cell working conditions, DMFC single cells employing the two electroctlysts s the nodes, respectively, were fricted nd tested t 75 C. The dischrge curves re shown in Fig. 1. The mximum power density of the single cell with the PtRu/ s the node electroctlyst is out 14 mw cm 2, which is similr s tht of the single cell with the PtRu/ s the node electroctlyst; however, the cell voltge for the PtRu/ is slightly higher thn tht for the PtRu/ t low current density region. It should e mentioned here tht the reproduciility of single cell performnce evlutions in our l is gurnteed. The DMFC performnce with PtRu/ s the node electroctlyst in this work is comprle with those reported in our previous work [31,36]. In Ref. [31], the mximum power density of the single cell (16 mw cm 2 ) ws resonly higher thn tht in this work since the Pt loding used in the cthode in Ref. [31] ws 1.4 mg cm 2, which ws higher thn tht in this work (i.e., 1 mg cm 2 ). In Ref. [36], the mximum power density of the single cell of out 14 mw cm 2 ws otined y using 2 wt.% Pt/C (Johnson Mtthey Inc.) on the cthode nd 6 wt.% PtRu/ (homemde) on the node t 9 C with other conditions eing the sme s in this work. We lso compre the fuel cell performnce level fricted in our l with those reported y other reserchers. In Ref. [21], the mximum power density of the single cell with the PtRu/ s the node electroctlyst ws out 15 mw cm 2 t 6 C. The etter performnces re due to the much higher Pt loding used in the cthode (5 mg cm 2, Pt lck, Johnson Mtthey Inc.) in Ref. [21] Stility tests of the single cell with the PtRu/ s the node electro-ctlyst The stility test of the DMFC with the PtRu/ s the node electroctlyst ws tested t constnt current density of 1 ma cm 2 t 75 C for 15 h. As shown in Fig. 11, the dischrge of the single cell ws periodiclly interrupted regulrly to remove the ccumulted wter in the cthode y purging oxygen. It cn e seen tht the cell voltge grdully drops during ech continuous dischrge, ut the degrdtion is reversile nd could recover prtly fter refreshment for while; however, fter operting 12 h, the cell voltge drops with little higher rte. To monitor the DMFC performnce, the dischrge curves efore nd fter the stility test with n intervl of 5 h were tested. As shown in Fig. 12, the performnce of the single cell fter operting 5 h is improved, indicted y slightly higher mximum power density thn the initil vlue, due to the improved mss-diffusion process t the higher current density region. After the stility test for 1 h, the ctivtion polriztion is devited from the initil curve; however, the mximum power density keeps constnt compred with the initil vlue. When the stility test extended to 15 h, the cell voltge t sme current density is significntly lower thn the initil vlue t the low current density region. Moreover, the mximum power density decreses from 14 to 12 mw cm 2 with out 14% loss of the power density. Our previous study [31] on the lifetime tests of the DMFC with the PtRu/ s the node electroctlyst showed tht out 25% of the power density (from 16 to 12 mw cm 2 ) lost fter operting just 556 h t sme dischrging conditions s in this work (1 ma cm 2, 75 C), nd the loss incresed to out 37% nd 5% of the power density fter operting 193 h (from 16 to 1 mw cm 2 ) nd 163 h (from 16 to 8 mw cm 2 ), respectively. It cn e seen tht the loss in cell performnce is much more serious thn tht of the DMFC with the PtRu/ s the node electroctlyst. The degrdtion of the cell performnce in the ctivtion polriztion region is minly due to the loss in ctlytic ctivities, while in the mss trnsport region oth the loss in ctlytic ctivities due to the gglomertion of the ctlysts nd the interfcil delmintion etween the memrne nd the ctlyst lyers s oserved fter the stility test contriute to the degrdtion of the DMFC. We will discuss it in detil in the next prgrph comined with the chrcteriztion on the morphology nd the elementl composition of the ctlysts fter the lifetime tests. To understnd the resons for the improved stility of the fuel cell with the PtRu/ s the node electroctlyst, we crried out TEM nd EDX nlysis of oth the node nd cthode electroctlysts fter stility tests. The TEM imges for the node nd the cthode ctlysts fter 15 h stility test re shown in Fig. 13 nd, respectively. Compred with the fresh ctlysts, the metl prticles for oth the node nd the cthode ctlysts gglomerte distinctly fter the 15 h stility test. In verge, the metl prticle size increses from 2.3 to 5 nm for node nd from 3.5 to 8 nm for cthode, respectively. The results show tht the gglomertion of the cthode ctlyst is more serious thn tht of the node ctlyst, which is resonle due to the more positive potentil of the cthode thn the node. This result is consistent with those reported in Refs. [31,37]. The serious gglomertion of the cthode ctlyst nd the oserved interfcil delmintion etween the memrne nd the ctlyst lyers contriute minly to the degrded fuel cell performnce with the PtRu/ s the node ctlyst during the stility test. Furthermore, we nlyze the elementl composition of the PtRu/ efore nd fter the stility test. The results show tht the tomic rtio of Pt to Ru efore nd fter the 15 h stility test is very close (see Tle 2). Moreover, Ru is not detected in the cthode ctlyst lyer fter the 15 h stility test, suggesting the crossover of Ru from node to cthode is negligile. In contrst, it is reported in Ref. [31] tht Ru leched out seriously fter the 22 h lifetime test. Considering sme electrode structure used in this work s tht in Ref. [31], the improved stility of the DMFC with the PtRu/ s the node electroctlyst under DMFC operting conditions proly should e due to the improved stility of PtRu nno-prticles on the. 4. Conclusions The of mesoporous structure nd well-defined grphitic frmework ws synthesized using resorcinol nd formldehyde s precursors nd iron nitrte s grphitiztion ctlyst. The ws used s the support of PtRu imetllic nnoprticles. The otined PtRu/ electroctlyst shows slightly higher ctivity towrd the MOR thn the PtRu/ mesured in oth the hlf cell nd the single cell. Importntly, the stility of the PtRu/ is improved significntly compred with the PtRu/ nd evidenced y oth the potentiosttic mesurements in hlf cells nd the lifetime tests in DMFC single cells. The serious gglomertion of the cthode ctlyst nd the oserved interfcil delmintion etween the memrne nd the ctlyst lyers contriute minly to the degrdtion of cell performnce with the PtRu/ s the node ctlyst during the stility test. The improved stilities of the PtRu/ nd the DMFC with the PtRu/ s the node electroctlyst re proly due to the improved stility of PtRu nno-prticles on the. This pproch discovered in this pper lso provides n efficient route to synthesize grphitic mesoporous crons.

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