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Boosting Fuel Cell Durability under Shut-Down/Start-Up Conditions using Hydrogen Oxidation-Selective Metal-Carbon Hybrid Core-Shell Catalyst Jeonghee Jang, Monika Sharma, Daeil Choi, Yun Sik Kang, Youngjin Kim, Jiho Min, Hukwang Sung, Namgee Jung, and Sung Jong Yoo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06309 • Publication Date (Web): 02 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Materials & Interfaces

Boosting Fuel Cell Durability under Shut-Down/Start-Up Conditions using Hydrogen Oxidation-Selective Metal-Carbon Hybrid CoreShell Catalyst a†Daeil Choi, b†Yun Sik Kang, b YoungjinaKim, a Hukwang Jeongheea† Jang, Monika Sharma, Jiho Min, a a* b* Sung, Namgee Jung, Sung Jong Yoo aGraduate

School of Energy Science and Technology (GEST), Chungnam National Univers Daejeon, 34134, Republic of Korea bFuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul KEYWORDS: Durability, Hydrogen oxidation reaction, Polymer electrolyte membrane fuel cells, Reverse current, Selectivity

ABSTRACT: Performance degradation generated by reverse current flow during fuel cell shut-do commercialization of polymer electrolyte membrane fuel cells in automobile applicati the formation boundaries H on Pt surfaces and the occurrence of undesired oxygen reduction re 2/O 2 of severe degradation of carbon supports and Pt catalysts in cathode due to an increase of the directly prevent the formation of H in the anode, we propose a unique metal-carbon hybrid 2/O 2 boundaries having Pt nanoparticles encapsulated in nanoporous carbon shells Thisfor hybrid selective catalyst H exh 2 permeation. hydrogen oxidation reaction (HOR) selectivity along with fully subdued ORR activity du stability of the carbon molecular sieves. Furthermore, the HOR selective catalyst effe single cell under shut-down/start-up conditions.

To solve the problem generated by the reverse c 1. INTRODUCTION scientists Polymer electrolyte membrane fuel cells (PEMFCs) have are proposed various strategies f system management promising energy conversion devices for the future due to high techniques to the develop 17-24 As one of the simplest operat support materials. energy conversion efficiency and environmentally friendly techniques, the electrodes can be purged with a reaction mechanism without pollutant emission. However, there as 2N to completely remove the electrochemic are still many problems to impede commercialization of 17,18 However, PEMFC, for example, low performance and gases. durability of this method may cause the incre system cost and volume because additional g 1-4 catalyst and polymer membrane. In terms of durability, it is accessories should be equipped in PEMFC syst well known that the PEMFC performance is severely degraded development highly durable support materia under harsh conditions such as long-term operation,of fuel graphitized carbon materials such as carbon nan starvation, and shut-down/start-up. Especially, under the shutand flow carbon nanofibers down/start-up condition, the reverse current generated by (CNFs) have been extensi replace amorphous carbons since they have hi transient fuel starvation in the anode causes critical degradation 19-21Alternatively, stability due to enhanced crystallinity.metal of the cathode catalyst layer. When PEMFC operation is shutoxides such as and TiO SnO have received a great deal o 2 2 down, air exists in both the anode and cathode due to O 2 as new support materials because of crossover through the polymer membrane orattention direct leakage 22-24 Consequently, stability than carbon materials. both th from outside air. However, aftersupplied H into the 2 is again graphitized carbons and metal oxides were mor anode to start the fuel cell operation, are H 2/O 2 boundaries against the corrosion compared to amorphous carb locally formed in the anode catalyst layer, which results in the shut-down/start-up increased cathode potential by ~1.5 V. As a result, in the condition. the practical point of view, cathode, carbon supports are rapidly oxidizedHowever, (corroded)from and Pt nanoparticles are dissolved from the carbon loading surface of Pt nanoparticles and on the cathode sup 5-16 necessary to secure high PEMFC performance, it i agglomerated. deposit large amount of Pt nanoparticles on t

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gas will be immediately adsorbed on the Pt surfa H2, which may result in the2/O formation onH the 2 boundaryof Pt surface and the generation of reverse current. a practical perspective, to simultaneousl selectivity for the HOR and electrochemical st 2 blocking layers, it is necessary to design a nov decorated with molecular sieve layer having n nanopores but also a rigid structure. In this work, we present, for the first time carbon molecular sieve-encapsulated Pt nanop as a hybrid anode catalyst which exhibits high and excellent stability even after a number of (Figure 1). Furthermore, it is clearly demonstrate shut-down/start-up conditions, the hydroge selective Pt nanoparticles effectively sup current flow in a single cell, thereby signific durability of the fuel cell.

2. RESULTS AND DISCUSSION The carbon molecular sieve-encapsulated Pt deposited on carbon support (Pt@C/C) were fab Figure 1. Schematic diagrams of change in HOR selectivity of simple thermal decomposition and post-anne chemically-modified Pt catalyst and carbon molecular sieveAfter Pt(acac) 2 precursor, oleylamine as a surfacta encapsulated Pt catalyst during a long-term PEMFC operation. carbon supports were well mixed in 1-octadecene was defect heated sites up to and 300 ℃ and the reaction tempe supports due to inert surface structure (few 1 h. Through the filtration of th hydrophobic property) of the graphitizedmaintained carbons or for poor thermal decomposition, as-prepared Pt@C/C ( compatibility between Pt and the 25,26 metalthe oxide supports. sample was produced. To form the carbon shell l Furthermore, nanoparticles and to control the crystalline carbon shells, Pt@C/C ASP was annealed at tw the metal oxide supports need additional doping of Nb or(600 Sb toand 900 temperatures 27,28 conductivity. enhance their electrical Therefore, to ℃ for Pt@C/C 600 and Pt@C/C 900 samples, respect overcome the limitation of the system management techniques Ar atmosphere for 1 h. using inert gases and the replacement of As cathode shown support in Figure S1, transmission electron mi materials, it is required to develop a fundamental approach to ASP, Pt@C/C 600, and Pt@C (TEM) images of Pt@C/C directly prevent the H formation on Pt surfaces 2/O 2 boundary samples revealed that Pt nanoparticles with ver despite the co-existence of H anode. 2 and O 2 in the uniformly dispersed on the carbon supports. I At this stage, a few scientists recently an Pt@C/C proposed 600 and Pt@C/C 900 catalysts had the Pt pa interesting anode catalyst structure (~3.0 composed of Pt as Pt@C/C ASP (~2.9 nm) alth nm) similar nanoparticles coated by organic molecules asannealed molecular were atsieve much high temperature. In addi layers to directly block2 the gas access to the Pt ofsurface O resolution TEM (HR-TEM) images of Pt@C/C 900 although HO @C/C 600et inal. Figures 2 and S1 clearly shows the exis 2 and 2 are co-existed in the anode. Markovic developed Pt crystalline surfaces chemically with carbon modified shell with an interspacing of ~0.34 nm, 32,33 calix[4]arene molecules having a widethe rim graphitic structure for carbon, encapsulating the Pt nanopart 29,30 selective HOR. While the ORR activity of the Pt surface was comparison to commercial Pt/C. selectively reduced by tuning the surface of The coverage X-ray photoelectron spectroscopy (XPS) s calix[4]arene molecules, the chemically-modified Pt surfaces prepared samples (Figure S2) revealed that nitroge showed a comparable HOR performances to a bare Pt surface. detected in only Pt@C/C ASP sample while the Similarly, Kim et al. replaced calix[4]arene molecules byin less nitrogen content heat-treated Pt@C/C 600 an expensive dodecanethiol and functionalized 12H25SH), (C samples. From the XPS analysis, it was clarifie carbon-supported Pt (Pt/C) catalysts with molecules forASP the Nthe moiety in Pt@C/C may originate from oleyla 31 Through this interesting idea, it was believed same purpose. we used excess amount of oleylamine as a surfa that the reverse current problem could be solved due tobut high synthesis, it evaporated from heat-treated HOR selectivity of chemically-modified Pt catalysts. except as-synthesized Pt@C/C ASP sample becau However, in long-term operation of PEMFC, theof organic point oleylamine is ~360 °C. Therefore, molecules chemically functionalized onmolecules the Pt surface were can not be carbon source in our synth easily detached since the binding strength of the molecules formation of carbonmay shells. not be strong enough to maintain their adsorption during a Therefore, the carbon shell layer formation ca number of potential cycles (Figure 1). Once the Pt surface is by the carbon absorption under the Pt surfac exposed at the sites where the molecules are removed from, on O the surface. The carbon sourc 2 graphitization

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To identify the effect of the annealing pro particle structure, X-ray diffraction (XRD) p Pt@C/C samples were obtained. In all the Pt@C/C peaks at 2θ % 39.8°, 46.3°, and 67.5° correspondi (200), and (220) facets, respectively, coinci 39 Moreover, commercial Pt/C (Figure 2c). from the full widt at half maximum (FWHM) of the 40,41 Pt (220) it was peaks, confirmed that the average crystallite size remained the same even after annealing proc complemented the results of TEM analysis ( Accordingly, the annealing process regardless hardly affected the Pt particle size and distribu On the other hand, as shown in Figure S3, c voltammograms (CVs) of Pt@C/C catalysts we different from that of the commercial Pt/C alt samples had similar particle size. Pt@C/C ASP drastic decrease upd inand the OH Hadsorption areas as ad compared to the commercial Pt/C. Depending upon (Figure S2), it was expected that a number of o molecules might be chemically adsorbed on the Pt@C/C ASP, which resulted in oleylaminenanoparticles %% chemically-modified Pt n Furthermore, it was clearly confirmed that with annealing temperature, the adsorption H areas were updand OH ad completely suppressed in Pt@C/C 600 and Pt@C/ depicts the notable reduction in the number o 29-31 active sites in line with previous reports. To investigate the structure of the carbon sh the Pt nanoparticles, the exposed Pt surface catalysts were identified through CO strippin (Figure S3). As shown in Figure 2d, the exposed Pt s areas of Pt@C/C catalysts were gradually red increasing the annealing temperature while the in particle size after the post-heat treatment concluded that the carbon density or thickness shells was more increased at higher annealing t After the identification of the structure of Figure 2. HR-TEM images of (a) commercial Pt/C and (b) Pt@C/C as shown inXRD Figure 3 and Figure S4, to simultaneously 900 (carbon shell-encapsulated Pt nanoparticle). (c) patterns the HOR selectivity of Pt@C/C catalysts and (d) the exposed Pt surface areas and average particle diameters of commercial Pt/C, Pt@C/C ASP, Pt@C/C 600, and Pt@C/C 900. electrochemical stability of the molecular si Oleylamine-coated Pt nanoparticle represents thepolarization structure of curves were measured after and HOR Pt@C/C ASP sample. Gray and yellow carbon shells for Pt@C/C cycle during 3000 potential cycles (accelerate 600 and Pt@C/C 900 samples indicate the carbon shells with ADT) between 0.05 and 1.05 V in Ar-saturated 0.1 4. “loosely formed large pores” and “firmly developed pores”, At thesmall initial stage, chemically-modified P respectively. carbon-encapsulated Pt@C/C (Pt@C/C 600 and Pt@ catalysts showed considerably reduced ORR activ acetylacetonates of Pt(acac) during the thermal 2 precursor indicated the same HOR perform decomposition 34 reaction, and can be absorbed intothe thecatalysts Pt commercial Pt/C. From the large difference betw 35 nanoparticles due to the carbon solubility in transition metals. and ORR activities Through a post-heat treatment, the carbon atoms absorbed of carbon-encapsulated Pt n was proved that the carbon shell as a molecular under the surface of Pt nanoparticles are rapidly segregated to sufficiently small pores suitable for the sele the outmost surface and the thin carbon layers are then formed. 42,43 H2 over 2suppress O . The carbon layers coated on the Pt surface might the migration of Pt nanoparticles on the supportHowever, materials until thein their ORR activities the change heat treatment was completed. In addition, it is well known that potential cycles were quite different althoug the higher the annealing temperature, the higher the carbon activities of the catalysts were consistent density and the smaller the pore size due to the the crystallization after ADTs (Figure S4). As expected, the ch 36-38 of the carbon structure. Therefore, it was expected that the modified Pt@C/C ASP showed a much improved ORR ac annealing temperature had a great effect on the after onlyphysical 1000 potential cycles. After 3000 c properties of the carbon shells grown on Ptperformance nanoparticles. was fully recovered due to the enlarg area (Figure 3f). The rapid change in the ORR activ

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chemically-modified Pt@C/C ASP can be because explained the by organic molecules might be loosel detachment of oleylamine molecules from Pt nanoparticles the Pt surfaceas mentioned above.

Figure 3. The change in the ORR polarization curves of a) Pt@C/C ASP, b) Pt@C/C 600, and c) Pt@C/C 9 between 0.05 and in Ar-saturated V 0.1 d)HClO The HOR polarization curves of Pt@C/C catalysts befo RHE1.05 4.M potential cycles. The ORR and HOR polarization curves of commercial Pt/C were compared with th in e) the HOR selectivity and f) the exposed Pt surface areas of Pt@C/C catalysts before and af

44In The ORR activity of Pt@C/C 600 was relatively slowly temperature (~600 ℃) might have an amorphous st increased since the carbon layers had somewhat higher adhesive sharp contrast, the ORR polarization curves of strength to stick to the Pt surface than the chemically-adsorbed were hardly changed during the ADT, which impli organic molecules. However, the stability of the carbon shells carbon molecular sieve fabricated at 900 °C had a was not sufficient to maintain the initial ORR activity during durability due to the enhanced carbon crystalli 3000 CV cycles because the carbon layerthe grown at2 molecules a of low access O were effectively inhibite

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ACS Applied Materials & Interfaces 3000 potential cycles since the small pore structure in the carbon shells did not collapse (Figure 3c). In other words, we

Figure 4. Schematic illustration of shut-down/start-up cycle tests using (a) conventional M selective MEA (anode: Pt@C/C 900, cathode: Pt/C). The change in single cell performance of (c MEA during shut-down/start-up cycles. TEM images of (e) fresh Pt/C catalyst and the cathode P (g) HOR selective MEA after 10 shut-down/start-up cycles.

achieved the gas selectivity by controlling active thesurface pore structure area (for ofinstance, only ~2 % of a carbon shells because H different kinetic might be enough to show HOR activity similar a 2 and O 2 have 42,43 29,30 diameters. The higher the annealing temperature, bare Pt the surface). higher the carbon density and the smaller the poreConsequently, size due to theas shown in Figure 3e, the HOR se crystallization of the carbon structure, of therefore, thecatalyst annealing Pt@C/C 900 with the most stable m temperature had a great effect on the physical properties of the layers was slightly decreased after the ADT whil carbon shells grown on Pt nanoparticles. The in and carbon Pt@C/Cshells 600 catalysts completely lost both catalysts, Pt@C/C 600 and Pt@C/C selectivity 900, acted as for the HOR. From CV and CO st molecular sieve layers, but highly stable carbon shellbefore structure measurements and after the ADTs (Figure S5 a with enhanced crystallinity could be fabricated only through the it was confirmed that the decrease in the HOR sel 45,46 heat treatment at higher temperature (900 ℃). catalysts is strongly related with the change areas Meanwhile, the HOR polarization curvessurface of all the catalysts did not change at all before and after theduring ADTs (Figure 3d) the ADTs (Figure 3f). Especially, in cas because the HOR is kinetically very fast annealed and needsPt@C/C only small catalysts, their HOR selectiv

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while chemically-modified Pt/C and Pt@C/C 60 by the deformation rate of carbon shells coated on Pt completely their nanoparticles. The superior stability of the carbonlost shells ofinitial selectivity. T in Pt@C/C 900 the were suitable to maintain the HO Pt@C/C 900 catalyst was clearly demonstrated since of the hybrid catalyst due to the rigid and crys exposed Pt surface area was hardly changed (slightly increased by 8 %) even after 3000 potential cycles.structure even after 3000 potential cycles. demonstrated that the cell performance of the Through the TEM analyses after the ADTs (Figure S7 and MEA using Pt@C/C 900 catalyst in the anode w S8), it was observed that Pt nanoparticles encapsulated with changed after repeated shut-down/start-up cycl carbon layers in Pt@C/C 600 and Pt@C/C 900 maintained their the conventional MEA was considerably reduced t particle size and distribution and the carbon shell thickness its initial performance. Since the nanoporous without any noticeable change after the ADTs, which indicates molecular sieve was effectively operated in a s that the change in the exposed Pt surface were attributed mainly the shut-down/start-up cycles, therefore, the to the deformation of pore structure of the carbon layers. On the provide new carbon insights to design promising anode other hand, the particle size of Pt@C/C ASP without improving the durability of PEMFCs for aut shell layer was increased by ~1 nm after 3000 potential cycles. applications. Further, the durability of the carbon shells in Pt@C/C 900 was also measured for 5000 CV cycles (Figure S9). Although the ORR performance of the catalyst slightly due SECTION to 4.increased EXPERIMENTAL partial deformation of carbon shells after 5000 CV cycles, it was 4.1 Synthesis of Pt@C/C catalysts. Carbon black of 0.1 g sufficient to confirm that the carbon shell formed at 900 ℃ had (Vulcan XC72, Cabot) and oleylamine of 5 ml (7 greatly enhanced crystallinity and durability. Therefore, it can Aldrich) were well dispersed in 1-octadecene of 1 be concluded that the carbon shells of Pt@C/C 900 catalyst had Sigma-Aldrich) by ultrasonication for 30 mi small and rigid nanopores for ensuring high HOR selectivity acetylacetonate (Pt(acac) 0.05 g (99.99%, Sigma-Aldric 2) of and effectively preventing the reverse current flow in a fuel cell and oleylamine of 5 ml were mixed in 20 ml of 1-oc under shut-down/start-up conditions. sonication for 30 min. After the two solutions As shown in Figure 4a and b, to demonstrate the feasibility then stirred in Ar atmosphere at 120 °C 2for O 1 h to re of practical application of Pt@C/C anodeimpurities, catalysts with high the solution temperature was incre HOR selectivity in PEMFCs, the shut-down/start-up cycle tests and kept for 2 h for thermal decomposition of Pt were conducted using a single cell with a After membrane electrode finishing the reaction, the solution was assembly (MEA) composed of Pt@C/C 900 and commercial °C, and then filtered and washed by copious hex Pt/C catalysts at the anode and cathode,Samchun respectively (HOR Pure Chemical) and ethanol (95.0 %, Sa selective MEA). The experimental results of the MEA were Chemical). The as-prepared Pt@C/C (Pt@C/C ASP) thoroughly compared with those of a conventional MEA using was dried in an oven at 60 °C, and then annealed at the commercial Pt/C catalysts at both anode and cathode. temperatures (600 and 900 °C) for 1 h in Ar atmosph carbon shell layers on Pt nanoparticles. The Pt As shown in Figure 4c and d, both the HOR selective MEA annealed at at 600 and 900 °C was designated as Pt@C and the conventional MEA showed similar performances the Pt@C/C 900, respectively. initial stage. However, the single cell performance of the HOR selective MEA was maintained even after 10 shut-down/start4.2. Fabrication of membrane electrode assemblies up cycles while that of the conventional MEA was severelycoated membrane (CCM) type(MEAs).The catalyst degraded after the 1st cycle. It was confirmed that Pt electrode assemblies (MEAs) were fabricated nanoparticles in the cathode of the conventional were method. TheMEA catalysts ink was prepared by mixin significantly agglomerated after 10 cycles to the high (Sigma Aldrich), and iso wt.%due Nafion solution cathode potential formed by reverse current flow (Figure 4e, f (IPA) (Sigma Aldrich) with the catalyst. Pt@ and Figure S10). On the other hand, in case commercial of the HOR Pt/C (20 wt%, Johnson Matthey) we selective MEA, the morphology and distribution Pt anode and cathode, respectiv catalysts of for the nanoparticles in the cathode was hardly changed (Figure 4e, g The mixture was well disperse selective MEA. and Figure S10), which clearly demonstrated ultrasonication, that the MEA the slurry was then sprayed on didn’t suffer from the reverse current flow during the shutof the Nafion 212 membrane. The active geomet down/start-up cycles. electrode was2 and 5.0the cm Pt loading were-2on 0.2 mg cm the anode and cathode, respectively, in the ME prepared CCM was sandwitched between two gas 3. CONCLUSIONS layers (GDLs) (SGL, 39 BC). A conventional ME In summary, an innovative hybrid catalyst with thestructure commercial Pt/C catalysts on both ano composed of Pt nanoparticles encapsulated with nanoporous as a control sample was addtionally prepared in graphitic carbon shells was proposed as tomentioned prevent the above. performance degradation generated by a reverse current flow 4.3. Physical characterization. Transmission electron under shut-down/start-up conditions in PEMFC operation. 2 F30 S-Twin, mocroscope (TEM) (Tecnai G FEI) was taken t Through the rational design of the pore structure in the carbon confirm the particle size and distribution o shells, the carbon molecular sieve-encapsulated Pt catalyst andORR high resolution TEM (HR-TEM) w showing high HOR selectivity along withcatalysts, fully subdued clearly identify the carbon shell layer coated o activity was successfully developed. The HOR selectivity of Pt@C/C samples. The crystal structures of Pt@ Pt@C/C 900 catalyst was reduced by only ~35of % after the ADT

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saturated andwas humidifie and commercial Pt/C were investigated by using anode X-ray for a few seconds, 2 gas -1 into then International supplied again with 150the cc min anode until th diffraction (XRD) (D/MAX-2200 Ultima, Rigaku cell and voltage was increased upto ~0.9 V (near Corporation). The morphologies of the anode cathode voltage,cycles OCV). To accelerate the degradation of catalysts in two MEAs after 10 shut-down/start-up were shut-down/start-up cycle was repeated 10 analyzed by TEM (Talos F200X, FEI). polarization curves of the MEAs were repeatedl 4.3. Electrochemical measurment in half-cell after 1, 3, 5, and 10 shut-down/start-up cycles. All electrochemical measurements were performed in a standard three compartment electrochemical cell using a ASSOCIATED CONTENT rotating disk electrode (RDE) with a glassy carbon electrode, Pt Supporting Information. The Supporting Information wire, and Ag/AgCl electrode as working, counter, and reference available free of charge on the ACS Publications electrodes, respectively. All the potential values were reported xxxxxxx versus the reversible hydrogen electrode (RHE). The RHE Calculation of the reaction selectivity betwe calibration was conducted in an electrolyte solution TEM, HR-TEM imagesby and particle size distribution measuring the currents between the potential ranges for and thePt@C/C 900 before and after t ASP, Pt@C/C 600, hydrogen oxidation and evolution reactions with a Pt disk Electrochemical characterization; CVs, ORR, H electrode. The catalyst ink slurry was prepared by mixing a Pt@C/C ASP, Pt@C/C 600, and Pt stripping curves of prepared catalyst of 5 mg with Nafion ionomer (5 wt%, Sigmabefore and after the ADT. (PDF) Aldrich) and 2-propanol (99.5 %, Sigma-Aldrich). A drop of the catalyst ink was applied to the glassy carbon electrode AUTHOR INFORMATION (0.196 2cm , geometric surface area) and then dried. The Pt Author 2 loading on the glassy carbon was Cyclic 44.86Corresponding %%%%% *[email protected] (Namgee Jung) voltammograms (CVs) were obtained by cycling the potential %1 20 mV *[email protected] (Sung Jong Yoo) between 0.05 and V 1.05 V at a scan rate of s RHE RHE in Ar-saturated 0.1 For HClO the ORR tests in O 4. M 2-saturated %1 0.1 M HClO 4, the potential was scanned at a rate of 5 mV s Author Contributions from 0.05 to RHE 1.05 with V a rotating speed of 1600 rpm. In %1 †J.J., the HOR tests, the potential was scanned at a rate ofand 5 mV s contributed equally to this w M.S. D.C. from -0.05 toRHE 1.05 with V a rotating speed of 1600 Notesrpm in H2-saturated 0.1 4M . For HClO CO stripping measurements, The authors declare no competing financial intere pure CO gas at an ambient pressure was bubbled into 0.1 M HClO at 0.05 V ACKNOWLEDGMENT 4 for 15 min while holding the potential RHE. After the electrolyte was purged with Ar gas 20 min Thisfor work was to supported by the Korea Institute completely remove residual CO molecules Technology in the electrolyte, Evaluation and Planning (KETEP) and th CVs were obtained in Ar-saturated electrolyte with a scan rate (MOTIE) of the Republic of Trade, Industry & Energy of 20 mV-1s at room temperature and in potential range of 0.05 This work was also supported by t 20173010032100). and 1.05 V. The exposed Pt surface area was calculated by Research Foundation of Korea (NRF) grant funded by RHE government (MIST) (2018M1A2A2061975, integrating the currents in the CO oxidation peak region, 2016M3A6A7945505) and by the KIST Institutional assuming a monolayer CO charge -2 of . The 420 %% cm accelerated durability tests (ADTs) for Pt@C/C samples were performed by cycling the potential between 1.05 REFERENCES 0.05 V RHE and %1 inof VRHE for 3000 times at a scan rate Ar-saturated 100 mV s (1) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrh Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. 0.1 M HClO stripping, 4. After 3000 ADT cycles, the CVs, CO Electrocatalysis HOR and ORR polarization curves of the catalysts were on Extended and Nanoscale Pt-bi Surfaces. Nat. Mater. 2007, 6, 241-247. measured again and compared with those of the catalysts before (2) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; M the ADT. Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood 4.4. Single cell performance and shut-down/start-up P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, cycle tests. The single cell performance of Inaba, MEAs was M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K examined at 70 ℃ by using2humidified (150 cc -1 min ) and H -1)min air (800 cc supplied to the anode andIwashita, cathode,N. Scientific Aspects of Polymer Elect Durability Degradation. respectively. After the evaluation of the initial and performance of Chem. Rev. 2007, 107, 3904-395 (3) Jung, N.; Chung, D. Y.; Ryu, J.; Yoo, S. J.; Sung the MEAs, the shut-down/start-up cycle tests were conducted Nanoarchitecture and Catalyst Design for Fuel Cell Ap (Figure S11). To simulate the shut-down condition, Today 2014,humidified 9, 433-456. -1 air (150 cc)min was introduced instead of Hanode 2 into the (4) Fu, X.; Zamani, P.; Choi, J.-Y.; Hassan, F. M.; Ji until the cell voltage was dropped to ~0 V,D. which inA.; theZhang, Y.; Chen, Z. In Si C.; resulted Hoque, M. formation2/O of H in the anode. To Graphenization prevent a Ingrained with Nanoporosity in a 2 boundaries vigorous explosion between O H 2 and 2, humidified 2 gasNwas Electrocatalyst Boosting the Performance of Polym -1for Membrane Fuel Cells. Adv. Mater. 2016, 29, 1604456. injected with 150 cc a min few seconds right before air was (5) Yu, P. Gu, W.; Makharia, R.; Wagner, F. T.; Gas introduced to the anode. After the simulation ofT.; shut-down Impactcondition, of Carbon Stability on PEM Fuel Cell S process, to go back to the fuel cell The start-up Shutdown Voltage Degradation. ECS Trans. 2006, 3, 797-809. humidified N was purged instantaneously into the air2 gas

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Page 8 of 9

(6) Yu, Y.; Li, H.; Wang, H.; Yuan, X.-Z.; Pan, A Review (27)M. Sharma, S.;on Pollet, B. G. Support Materials for Performance Degradation of Proton Exchange Membrane Fuel Cells DMFC Electrocatalysts - A Review. J. Power Sources 2012, 208 During Startup and Shutdown Processes: Causes, Consequences, and 119. Mitigation Strategies. J. Power sources 2012, 250, 10-23. (28) Tr%%%%%%%% V.; Abild-Pedersen, F.; Studt, F.; Cerri (7) Tang, H.; Qi, Z.; Ramani, M.; Elter, J. F. PEM fuel cell cathode Bligaard, T.; Rossmeisl, J. Metal Oxide-Suppor carbon corrosion due to the formation of air/fuel boundary at the anode. Overlayers as Proton-Exchange Membrane Fuel Cell J. Power sources 2006, 158, 1306-1312. ChemCatChem 2012, 4, 228–235. (8) Qin, C.; Wang, H.; Yang, D.; Li, B.; Zhang,(29) C.Genorio, Proton Exchange B.; Strmcnik, D.; Subbaraman, R.; Tri Membrane Fuel Cell Reversal: A Review. Catalysts 2016, 6, 197. Karapetrov, G.; Stamenkovic, V. R.; Pejovnik, S.; (9) Reiser, C. A.; Bregoli, L.; Patterson, T. W.;Selective Yi, J. S.;Catalysts Yang, J. D.; for the Hydrogen Oxidation and O Perry, M. L.; Jarvi, T. D. A Reverse-Current Decay Mechanism for Reactions by Patterning of Platinum with Calix[4] Fuel Cells. Electrochem. Solid-State Lett. 2005, 8, A273-A276. Nat. Mater. 2010, 9, 998–1003. (10) Yu, X.; Ye, S. Recent Advances in Activity and Durability (30) Genorio, B.; Subbaraman, R.; Strmcnik, D.; Tri Enhancement of Pt/C Catalytic Cathode in Stamenkovic, PEMFC: Part II: V. R.; Markovic, N. M. Tailoring the Degradation mechanism and durability enhancement of of carbon Stability Chemically Modified Platinum Nanoca supported platinum catalyst. J. Power Sources 2007, 172, 145–154. Highly Durable Anodes for PEM Fuel Cells. Angew. Chemie I (11) Shen, Q.; Hou, M.; Liang, D.; Zhou, Z.; Li, X.; Shao, Z.; Yi, B. 2011, 50, 5468-5472. Study on the Processes of Start-up and Shutdown(31) in Yun, Proton S.Exchange W.; Park, S. A.; Kim, T. J.; Kim, J. H.; Pak Membrane Fuel Cells. J. Power Sources 2009, 189, 1114–1119. Y. T. Hydrogen Oxidation-Selective Electrocatalysi (12) Meyers, J. P.; Darling, R. M. Model of Carbonof Corrosion in PEM Pt Ensemble Sites to Enhance the Durability of A Fuel Cells. J. Electrochem. Soc. 2006, 153, A1432-A1442. Cells. ChemSusChem 2017, 10, 489–493. (13) Takeuchi, N.; Fuller, T. F. Modeling and Investigation Carbon (32) Liu, Y.; of Jiang, H.; Zhu, Y.; Yang, X.; Li, C. Tra Loss on the Cathode Electrode during PEMFC (Fe, Operation. Co, and J. Ni) Encapsulated in Nitrogen-Doped Carb ElectroChem. Soc. 2009, 157, B135-B140. as Bi-Functional Catalysts for Oxygen Electrode Rea (14) Mittermeier, T.; Weiß, A.; Hasche, F.; Gasteiger, A.4, PEM Fuel Chem. A 2016, 1694–1701. Cell Start-Up/Shut-Down Losses vs Relative Humidity: TheWen, Impact (33) Hou, Y.; Z.;of Cui, S.; Ci, S.; Mao, S.; Chen, Water in the Electrode Layer on Carbon Corrosion. J. Electrochem. Nitrogen-Doped Graphene/Cobalt-Embedded Porous Soc. 2018, 165, F1349-F1357. Polyhedron Hybrid for Efficient Catalysis of Oxygen (15) Schwammlein, J. N.; Rheinlander, P. J.; Chen, Y.; Freyer, K. T.; Water Splitting. Adv. Funct. Mater. 2015, 25, 872–882. Gasteiger, H. A. Anode Aging during PEMFC Start-Up and Shut(34) Hoene, J. Von; Charles, R. G.; Hickam, W. M. Down:2-Air H Fronts vs Voltage Cycles. J. Electrochem.Decomposition Soc. 2018, of Metal Acetylacetonates: Mass Spe 165, F1312-F1322. J. Phys. Chem. 1958, 62, 1098–1101. (16) Dyantyi, N.; Parsons, A.; Bujlo, P.; Pasupathi, S.R. Behavioural (35) Yang, T.; Goethel, P. J.; Schwartz, J. M.; L Solubility and Diffusivity of Carbon in Metals. J. Study of PEMFC during Start‑up/shutdown Cycling for Aeronautic Applications. Mater Renew Sustain Energy 2019, 8, 4. 206–210. (17) Jia, F.; Liu, F.; Guo, L.; Liu, H. Mechanisms Reverse Current (36) of Chen, L.; Mashimo, T.; Iwamoto, C.; Okudera, H.; and Mitigation Strategies in Proton Exchange Ganapathy, Membrane Fuel H. Cells S.; Ihara, H.; Zhang, J.; Abdullaev during Startups. Int. J. Hydrogen Energy 2016, 41, 6469–6475. Yoshiasa, A. Synthesis of Nanoparticles. CoC x@C Novel (18) Chen, J.; Siegel, J. B.; Stefanopoulou, A. G.; Waldecker, J. R. Nanotechnology 2013, 24, 045602. Optimization of Purge Cycle for Dead-Ended Anode Fuel Cell (37) Sun, J.; Nam, Y.; Lindvall, N.; Cole, M. T.; Teo, Operation. Int. J. Hydrogen Energy 2013, 38, 5092–5105. Y. W.; Yurgens, A. Growth Mechanism of Graphene on Surface Catalysis and Carbon Segregation. Appl. Phys. Lett. (19) Wang, X.; Li, W.; Chen, Z.; Waje, M.; Yan, Y. Durability 152107. Investigation of Carbon Nanotube as Catalyst Support for Proton Exchange Membrane Fuel Cell. J. Power Sources 2006, 158, (38)154–159. Han, Y. F.; Kumar, D.; Sivadinarayana, C.; Cle (20) Álvarez, G.; Alcaide, F.; Miguel, O.; Cabot, P. L.; Goodman, D.MartínezW. The Formation ofPd-Based PdC Catalysts in x over Huerta, M. V.; Fierro, J. L. G. Electrochemical StabilityVinyl of Carbon Vapor-Phase Acetate Synthesis: Does a Pd-Au Nanofibers in Proton Exchange Membrane Fuel Cells. ResistElectrochim. Carbide Formation? Catal. Letters 2004, 94, 131–134. (39) Hoseini, S. J.; Bahrami, M.; Dehghani, M. Fo Acta 2011, 56, 9370–9377. Snowman-like (21) Antolini, E. Graphene as a New Carbon Support for Low- Pt/Pd Thin Film and Pt/Pd/Reduced-Graph Thin Film Temperature Fuel Cell Catalysts. Appl. Catal. B Environ. 2012, 123– at Liquid-Liquid Interface by Use of Or Complexes, Suitable for Methanol Fuel Cells. RSC 124, 52–68. (22) Parrondo, J.; Han, T.; Niangar, E.; Wang, C.;13796–13804. Dale, N.; Adjemian, (40) Borchert,Oxide H.; is Shevchenko, E. V.; Robert, A.; K.; Ramani, V. Platinum Supported on Titanium-Ruthenium Kornowski, Grübel, G.; Weller, H. Determination a Remarkably Stable Electrocatayst for Hydrogen Fuel Cell A.; Vehicles. Sizes: A Comparison of TEM, SAXS, and XRD Studies o Proc. Natl. Acad. Sci. 2014, 111, 45–50 Monodisperse CoPt Langmuir 2005, 21, 1931–1936. (23) Dou, M.; Hou, M.; Liang, D.; Lu, W.; 2Shao, Z.; Yi, B. SnO 3 Particles. Cheng, X.; Nanocluster Supported Pt Catalyst with High (41) Stability for Chen, ProtonL.; Peng, C.; Chen, Z.; Zhang Catalyst Microstructure Examination of PEMFC Membra Exchange Membrane Fuel Cells. Electrochim. Acta 2013, 92, 468–473. Assemblies vs. Time.A.; J. Electrochem. Soc. 2004, 151, A48-A52. (24) Takasaki, F.; Matsuie, S.; Takabatake, Y.; Noda, Z.; Hayashi, (42) Kim, H. W.; Yoon, H. W.; Yoon, S.-M.; Yoo, B. M.; Shiratori, Y.; Ito, K.; Sasaki, K. Carbon-Free Pt Electrocatalysts Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, Supported on SnO Polymer Electrolyte Fuel Cells: Cho, Electrocatalytic 2 for Park, H. B. Selective Gas Transport Through Few-Layer Activity and Durability. J. Electrochem. Soc. 2011, 158, B1270-B1275. (25) Oh, H. S.; Kim, K.; Ko, Y. J.; Kim, H. Effect of Chemical and Graphene Oxide Membranes. Science 2013, 342, 91–95. Oxidation of CNFs on the Electrochemical Carbon Corrosion inZ.; Zhang, X.; Huang, Y.; Li, S.; M (43) Li, H.; Song, Polymer Electrolyte Membrane Fuel Cells. Int. J. Hydrogen H. J.; Energy Bao, Y.; Yu, M. Ultrathin, Molecualr-Sievin 2010, 35, 701–708. Membranes for Selective Hydrogen Separation. Science 201 (26) Han, K. I.; Lee, J. S.; Park, S. O.; Lee, S. W.; 98. Park, Y. W.; Kim, (44) Joo, for W.-J.; Lee, J.-H.; Jang, Y.; Kang, S.-G. H. Studies on the Anode Catalysts of Carbon Nanotube DMFC. Chung, J.; Lee, S.; Kim, C.; Kim, T.-H.; Yang, C.-W. Electrochim. Acta 2004, 50, 791–794.

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ACS Applied Materials & Interfaces B. L.; Whang, D.; Hwang, S. -W. Realization of Continuous Zachariasen Carbon Monolayer. Sci. Adv. 2017, 3, e1601821 (45) Emmerich, F. G. Evolution with Heat Treatment of Crystallinity in Carbons. Carbon 1995, 33, 1709-1715. (46) Worsley, M. A.; Pham, T. T.; Yan, A.; Shin, S. J.; Lee, J. R. I.; Bagge-Hansen, M.; Mickelson, W.; Zettl, A. Synthesis and Characterization of Highly Crystalline Graphene Aerogels. ACS Nano 2014, 8, 11013–11022.

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