Three-Dimensionally Costabilized Metal Catalysts toward an Oxygen

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Three-dimensionally co-stabilized metal catalysts towards oxygen reduction reaction Kun Cheng, Min Jiang, Bei Ye, Ibrahim Saana Amiinu, Xiaobo Liu, Zongkui Kou, Wenqiang Li, and Shichun Mu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03625 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 12, 2016

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Three-dimensionally co-stabilized metal catalysts towards oxygen reduction reaction Kun Cheng, Min Jiang, Bei Ye, Ibrahim Saana Amiinu, Xiaobo Liu, Zongkui Kou, Wenqiang Li and Shichun Mu* State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070, China. ABSTRACT: Improving the long term stability of metal catalyst is crucial to develop polymer electrolyte fuel cells (PEFCs). In this work, we first report an inorganic (TiO2)-organic (perflurosulfonic acid, PFSA) co-stabilized Pt catalyst supported on graphene nanosheets (GNS) (Pt-PFSA-TiO2/GNS). Herein, TiO2, as a robust wall, impedes the collision between the metal nanoparticles (NPs) in plane along the horizontal x- and y- axes, while PFSA mainly anchors the metal NPs to constrain detachment along the vertical z-axis. The resulting catalyst displays a higher oxygen reduction reaction (ORR) activity in comparison with the commercial Pt/C. Significantly, the stability is particularly better than that of only PFSA or TiO2 decorated catalysts ( Pt-PFSA/GNS or Pt-TiO2/GNS), and far better than that of Pt/C. After 6000 potential cycles, the half-wave potential (E1/2) of Pt-PFSA-TiO2/GNS decreases by only 16 mV, far less than that of Pt/C (56 mV). The excellent electrochemical property of Pt-PFSA-TiO2/GNS is predominantly attributed to the synergistic effect of PFSA and TiO2 to co-stabilize the Pt NP by anchoring and blocking Pt NPs in all three spatial directions. The structural dynamics and mechanism of enhanced properties are also discussed.

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1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have potential applications as a sustainable and clean power source because of high energy density, simple structure and environmentallyfriendly features.1-4 However, their commercialization is heavily hindered by the poor lifespan mainly caused by catalyst degradation.5, 6 Currently, the most widely used catalysts are Pt or Pt alloy NPs dispersed on carbon supports.6, 7 Unfortunately, such catalysts readily suffer from loss of electrochemically active area (ECSA) mainly owing to the dissolution-growth, migrationagglomeration and detachment of Pt-based NPs from the carbon support due to the weak metalsupport interaction and serious oxidation of the support.8-10 Furthermore, the weak stability of Ptbased catalyst also means an increased usage of noble-metal catalysts in order to prolong the lifetime of fuel cells, leading to an increased cost of PEMFCs. Therefore, improving the lifespan of such catalysts has been one of the most urgent issues to be addressed.10, 11, 13 To promote the stability of catalysts, nano-graphitic carbon materials (such as CNTs, carbon fibres, carbon nano-thorns and graphene) with high electrochemical oxidation resistance have been exploited to replace the conventional carbon black supports.8, 11-14 However, this strategy also brings about new problems such as unfavourable homogeneous loading of Pt NPs on such chemical inert support surfaces,15-18 and an inevitable oxidation under harsh electrochemical condition. Nano-ceramics (e.g. TiO2, SiC, ZrO2, TiB2, and B4C) have been applied in catalyst support materials for fuel cells because of their excellent stability under chemical and electrochemical conditions.19-25 The ceramic not only protects the support materials from oxidation, but inhibits Pt NPs from collision and growth on the supports. However, they have rare contribution to avoiding the detachment of Pt NPs from the supports. Previously, researchers have adopted polymers (e.g. PFSA, PANI or functional amine) 21, 26-29 to functionalize the metal


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catalysts, effectively obstructing Pt NPs detachment from the supports. However, polymer decorated Pt NPs could also migrate on supports, leading to aggregation.21,

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Therefore the

stability of catalysts is still a major issue to be addressed to meet the commercial application requirements for fuel cells (5,000 running hours DOE target in 20176). Here, for the first time, we combined the organic polymer and inorganic (ceramic) species to synergistically stabilize Pt NPs. As illustrated in Fig. 1 a, the traditional Pt/C catalyst has to suffer the motion of Pt NPs on the carbon support along x- and y- axis directions due to absence of confinement blocks, and the detachment from the support in z-axis (vertical) direction mainly caused by oxidation of carbon supports. A strategy of introducing polymer to decorate the Pt NP can effectively hamper the Pt NP from detachment at the z-axis, however, the migration can take place along x- and y- axes (Fig. 1 b). By contrast, if only the ceramic films (or nanoflakes, NFs) are introduced, the Pt NP can be blocked along the horizontal x- and y- axis on supports, however they have to suffer from detachment along z-axis direction (Fig. 1 c). Thus, as shown in Fig. 1 d, by combining the merit of the organic polymer (PFSA) and the nano-ceramic (TiO2) materials, the Pt NP migration in all three-dimension direction (x-, y- and z- axes) can be well confined. As expected, this novel inorganic and organic co-stabilized Pt catalyst can possess a much longer lifetime. Moreover, the addition of PFSA as the proton conductor can be of great benefit to improving the catalytic activity of the catalyst.

Fig. 1


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2. Experimental section 2.1 Preparation of TiO2 nanoflakes (NF) TiO2 was prepared by hydrothermal synthesis as follows: 3 g of tetrabutyl titanate (TBT) was dissolved in 40 mL of anhydrous ethanol and 10 mL acetic acid solution, followed by agitation for 10 min at room temperature. Then 20 mL deionized water was slowly added in drops while stirring. After that the solution was transferred into a Teflon autoclave (100 mL) and kept at 170 o

C for 12 h. After cooling down to room temperature, the sample was centrifuged, washed and

dried at 80 oC to obtain the pure TiO2 powder. 2.2 Preparation of catalysts The catalysts were prepared by colloidal processes. Twenty millilitres of H2PtCl6 solution was blended with 10 times amount of ethylene glycol, and the pH was adjusted to 10-12 using 2 mol L-1 NaOH solution. The mixed solution was heated at 130 ℃ under continuous agitation for ~30 min until the solution color changed to dark brown, resulting in the Pt colloidal solution. A predetermined amount of Nafion (PFSA) solution (5 wt.%) was added dropwise to the Pt colloidal solution under continuous stirring for 30 min. Then the solution was naturally cooled down to 50 ℃. Meanwhile, 2 mg mL-1 GO solution was prepared under ultrasonic condition for more than 30 min and a predetermined amount of TiO2 powder was dispersed in an ethanol/water solution to form a homogeneous TiO2 colloidal suspension. The GO solution was added into the above solution and stirred for 2 h, followed by addition of the TiO2 suspension and kept under vigorous stirring at 130 ℃ for 2 h. After cooling down, the solution was


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centrifuged, washed and dried at 80 ℃ in vacuum condition and then the Pt-PFSA-TiO2/GNS catalyst was obtained. For comparison, the addition of Nafion solution was substituted by introducing the same volume of pure solvent (isopropanol solution) to form a Pt-TiO2/GNS catalyst. Similarly, the TiO2 suspension was replaced by the same volume of pure solvent to obtain a Pt-PFSA/GNS catalyst. The theoretical loading of TiO2 was controlled at 10% and Pt at 20%, and the amount of Nafion was assigned to be 6.7 w.% (1/3 that of Pt in mass according to the reported30). Notably, Pt-PFSA-TiO2/GNS with TiO2 content of 3 and 30 wt.% were also prepared, unless expressly stated, the label of “Pt-PFSA-TiO2/GNS” is considered as the 10 wt.% TiO2-containing catalyst ). The practical content of Pt as shown in Table S1 was measured using inductively coupled plasma atomic emission spectrometry (ICP-AES), and the procedure of ICP analysis is displayed below Table S1. 2.3 Structural characterizations X-ray diffraction (XRD) spectra were conducted on an X-ray diffractometer using Cu Ka radiation source, and the scanning rate was at 2° min-1 from 5 ° to 80 °. Fourier transform infrared spectroscopy (FTIR) was carried out to detect the presence of starch in Pt-SS/C catalyst using a Nicolet MAGNA-IR 560. TEM images and EDS analysis were conducted using the programmed JEM-2100F STEM/EDS (JEOL®, Japan) to characterize structure and morphology of corresponding samples.


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2.4 Electrochemical characterizations The electrochemical properties of catalysts were investigated by a three-electrode electrochemical cell, AUTO LAB Electrochemical System (Eco Chemie Corp.). The saturated calomel electrode (SCE) was used as the reference electrode which was calibrated with RHE before test (see supporting information, SI), and platinum black as counter electrode. A certain amount of catalyst powder was ultrasonically dispersed in 5 mL isopropanol solution and 10 µL 5 w.% NafionTM solution. The catalyst ink was then coated on a mirror polished glassy carbon electrode (the working electrode; 0.196 cm2, Ø=5 mm) to maintain a Pt loading of 15.28 ug cm-2. All measurements were carried out in 0.1 mol L-1 HClO4 solution. Cyclic voltammetry (CV) was carried out in Ar saturated 0.1 mol L-1 HClO4 solution at a sweep rate of 50 mV s-1. The ECSA of Pt was calculated by measuring the charge collected in the hydrogen adsorption region (around 0.05–0.38 V at the negative scanning) after double-layer correction, according to the following equation:31

ESCA =

QH mPt qH

(1)

where QH is the charge collected in the hydrogen adsorption region, mPt is the weight of Pt loading, and qH (=210 µC cm-2) is the charge required for monolayer adsorption of hydrogen on Pt surface. The polarization curves for the oxygen reduction reaction (ORR) were carried out in O2 saturated 0.1 mol L-1 HClO4 solution by linear sweep voltammetry technique from 1.1 to 0.2 V (vs. RHE) at a rotating rate of 1600 rpm and a scan rate of 10 mV s-1. The kinetic current can be


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calculated from the ORR polarization curve according to Koutecky–Levich equation,31 and the variant formula is described as follows:

ik =

id * i id - i

(2)

where ik is the kinetic current, id is the diffusion-limiting current, i is the experimentally measured current (here, at potential 0.9 V vs. RHE.). And then, the mass activity was calculated based on the following equation:

mass activity =

ik m Pt

(3)

where mPt is the amount of Pt loading. The i-t curves for catalysts were performed using chronoamperometric technique at a constant potential of 0.7 V vs. RHE. Accelerated durability test (ADT) was employed to evaluate the long-term stability of catalysts, which is widely applied to characterize durability of PEM fuel cell catalysts.32 The ADT was conducted using the same system as the CV test with a potential scan ranging from 0.6 to 1.2 V vs. RHE at a sweep rate of 100 mV s-1. CV curves were recorded after certain cycles of potential scan, and then a group of CV curves vs. potential cycling numbers were screened. 3. Results and discussion TEM images and corresponding EDX analyses of the PFSA wrapped Pt NPs are displayed in Fig. 2 a and b. The enlarged TEM image clearly demonstrates structure of the Pt NP with a lattice spacing of 0.222 nm，corresponding to (111) crystal face, in agreement with the XRD


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analysis below. The predominant particle size is 2.0 ~ 4.0 nm. The EDX spectra clearly show peaks of elemental F and S at 0.678 and 2.335 KeV, respectively, confirming the successful synthesis of PFSA stabilized NPs. Notably, due to the quite weak crystalline phase of the ultrathin layer as well as the poor stability under high energy electron beams, the PFSA morphology can hardly be identified by the TEM technique.15, 26 Fig. 2 c shows the XRD pattern of the synthesized TiO2 matches the standard XRD card of anatase TiO2 (JCPDS No. 21-1272，space group: I41/amd (141)),33,34 and consistent with the reported data.35 Wherein, 2θ = 25.2 °, 37.9 °, 47.9 °, 54.1 °, 54.9 ° and 62.6 ° corresponds to the crystal plane of (101), (004), (200), (105) , (211) and (204), respectively. The average particle size of TiO2 NFs is 12 nm, calculated from the (004) plane characteristic peak by the Scherrer equation.36 This value is close to that observed directly from the TEM image (Fig. 2 b). From the characteristic XRD peaks of Pt-TiO2/GNS and Pt-PFSA-TiO2/GNS, it can be concluded that TiO2 NFs were successfully introduced into both samples. Fig. 2 c reveals Pt NPs in catalysts belong to the face-centered cubic crystal (fcc) structure, and peaks at 2θ=39.7 ° and 46.5 ° correspond to Pt (111) and (200) crystal plane.37 An average particle size of Pt NPs is about 4 nm for these catalysts, calculated from the high 2θ angle of the (220) characteristic peak by the Scherrer equation,37 which is close to the TEM results as described below. FTIR spectra of the catalysts are shown in Fig. 2 d, and the structural formula of PFSA is shown in the inset. Two strong peaks at 1223 and 1151 cm-1 for both Pt-PFSA/GNS and Pt-PFSA-TiO2/GNS catalysts, correspond to -CF2- asymmetric and symmetric stretching vibration in main-chain of PFSA. Meanwhile, a peak at 984 cm-1 is ascribed to CF symmetric stretching vibration (-CFRCF3). Additionally, the characteristic peaks at 1056 and 970 cm-1 are attributed to the -SO3- and C-O-C symmetric stretching vibration in PFSA, respectively.38-40 These results confirm a successful


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introduction of PFSA to serve as a stabilizer and a proton conductor in both Pt-PFSA/GNS and Pt-PFSA-TiO2/GNS catalysts, which are further confirmed by EDX analyses.

Fig. 2

TEM images and corresponding EDX analyses of the catalysts are shown in Fig. 3 a-f. It can be seen that Pt NPs in these catalysts are all well dispersed on the GNS, with a particle size of 24 nm, which is the same as the colloidal one described above. HR-TEM images of Pt-TiO2/GNS (Fig. 3 d) and Pt-PFSA-TiO2/GNS (Fig. 3 f) show the particle size of TiO2 is in the range of 6 12 nm. The lattice spacing of 0.35 nm corresponds to the (101) crystal plane of anatase TiO2, which is in agreement with the XRD observation. TiO2 nano-flakes are well dispersed around Pt NPs on the support for both Pt-TiO2/GNS and Pt-PFSA-TiO2/GNS, which is also indicated from more HR-TEM images (Fig. S2). The presence of TiO2 NFs as robust barriers is available to confine the arbitrary migration of Pt NPs on supports. The EDX analysis clearly demonstrates the presence of elemental F and S in Pt-PFSA/GNS (Fig. 3 a), elemental Ti in Pt-TiO2/GNS (Fig. 3 c), and elemental F, S and Ti in Pt-PFSA-TiO2/GNS (Fig. 3 e). These results strongly confirm PFSA, TiO2 and PFSA-TiO2 are present within the Pt-PFSA/GNS, Pt-TiO2/GNS and PtPFSA-TiO2/GNS catalysts, respectively.

Fig. 3


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Here, the effect of TiO2 contents on the catalyst performance is explored. The chronoamperometry (i-t) curves (Fig. S3) clearly indicate the increased TiO2 content from 0 to 30 wt.% can improve the stability. This can be ascribed to the increased amount of TiO2 nanoflakes around Pt particles can consolidate the stabilization of Pt catalysts by blocking the gathering of Pt NPs (indicated from Fig. S3 d-i). However, the mass activity of the corresponding catalyst suffers from decrease with the increase of TiO2 content, especially for the 30 wt.% TiO2-containing catalyst, which can be ascribed to the coverage of TiO2 on Pt NPs, impairing the activity of Pt NPs. In all, although the increased TiO2 content leads to a better stability, its mass activity suffers from loss as well (Fig. S3 c). Interestingly, the 10 wt.% TiO2 contained catalyst obtains an outstanding stability, at the same time it loses a little activity, therefore it is recommended here (Unless expressly stated, the label of “Pt-PFSA-TiO2/GNS” is the 10 wt.% TiO2-containing catalyst ). As displayed in the inset of Fig. 4, the ECSA of Pt-PFSA/GNS (76.9 m2 g-1), Pt-TiO2/GNS (73.3 m2 g-1) and Pt-PFSA-TiO2/GNS (75.2 m2 g-1) is very close to that of Pt/C (78.6 m2 g-1). It is noteworthy a higher capacity current presents in the new catalyst, which can mainly be ascribed to the contribution of GNS. Previous literature has reported that graphene-based supported Pt catalysts with high surface area have much higher capacity current than that of Pt/C under the same Pt loading.15,41,42 In addition, TiO2 has little effect on the capacity current.43 The mass activity (inset in Fig. 4 b) of Pt-TiO2/GNS (100.6 mA mgPt-1) is close to that of Pt/C (103.4 mA mgPt-1). Notably, both Pt-PFSA/GNS (115.2 mA mgPt-1) and Pt-PFSA-TiO2/GNS (113.3 mA mgPt-1) show enhanced activities compared with Pt/C, which can be ascribed to the contribution of the hydrophilic group (-SO3H) of PFSA to facilitate transfer process of reaction species


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(especially protons),21,44,45 improving the ORR activity.46 Additionally, these catalysts own very similar dynamic onset potential values because they are all Pt-based catalysts.

Fig. 4

The introduction of PFSA or TiO2 on promoting stability of the Pt catalyst has been confirmed by the chronoamperometry data (i-t curve, Fig. S4 a) and the mass activity comparison of ORR before and after ADT (Fig. S4 b-d). The i-t curves clearly show that the stability of Pt-PFSA/GNS or Pt-TiO2/GNS is better than that of Pt/GNS. The ORR activity loss after ADT, shows the retention rate of Pt/GNS is only 38.2%, lower than that of Pt-TiO2/GNS (43.2%) and Pt-PFSA/GNS (51.0%). These confirm the promoting effect of Nafion or TiO2 on enhancing stability of Pt catalyst, which is agreeable with the reported. 25,43,47,48 Most importantly, the PFSA and TiO2 co-stabilized catalyst displays a much enhanced stability than those catalysts decorated with either PFSA or TiO2. As shown in i-t curves (Fig. S5), we can establish the stability of the catalysts as the order: Pt-PFSA-TiO2/GNS > PtPFSA/GNS > Pt-TiO2/GNS > Pt/C. Fig. 5 a shows the change of ECSA values as a function of potential cycling numbers for the catalyst (original CV curves used to calculate the ECSA are displayed in Fig. S 6). After 6000 scanning cycles, the ECSA retention rate of Pt-PSFA/GNS and Pt-TiO2/GNS is 54.3% and 50.1%, respectively, both higher than that of Pt/C catalyst (39.4%), further confirming the introduction of Nafion or TiO2 can help enhance stability of the Pt


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catalysts, consistent with above. Significantly, Pt-PFSA-TiO2/GNS retains 68.2% of the initial ECSA under the same conditions, demonstrating the Pt-PFSA-TiO2/GNS catalyst obtains an extremely high stability compared with those of simply using Nafion or TiO2 as anchoring agent, and far better than the commercial Pt/C catalyst (39.4%). For the polarization curves of ORR before and after ADT (Fig. 5 b~e), after 6000 potential cycles, the half-wave potentials (E1/2) of Pt-PFSA/GNS and Pt-TiO2/GNS (initial 0.861 and 0.854 V) decrease by 39 and 43 mV, respectively, less than 56 mV of Pt/C (initial 0.857 V). Similarly, the Pt mass activities of Pt-PFSA/GNS and Pt-TiO2/GNS decrease by 49.1% and 56.8%, respectively, less than that of Pt/C (66.1%). This means the introduction of PFSA or TiO2 can improve the stability of Pt catalysts, in conformity with above. Significantly, the E1/2 loss of Pt-PFSA-TiO2/GNS is only 16 mV (initial 0.859 V), and the mass activity decreases by only 30.5%, indicating Pt-PFSA-TiO2/GNS owns a much higher durability than Pt-PFSA/GNS and Pt-TiO2/GNS, and far better than that of Pt/C, which is consistent with the analytic results of the i-t curve. The change of mass activity before and after ADT has been calculated, as presented in Fig. 5 f. It shows that the retention rate of Pt-PFSA-TiO2/GNS (69.5%) is quite higher than that of Pt-PFSA/GNS (51.0%) and Pt-TiO2/GNS 43.2%), and much higher than that of Pt/C (33.9%), demonsratng the much better stability of Pt-PFSA-TiO2/GNS than others.

Fig. 5


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As shown in Fig. 6, TEM images after ADT and Pt NP size distributions before and after ADT are investigated, and the Pt particle size was counted using more than 200 particles (TEM image of Pt/C before ADT is shown in Fig. S7). After 6000 potential cycles, Pt NPs in Pt/C exhibit a serious detachment and aggregation with an increased average size from 2.98 to 8.28 nm, and the number of Pt NPs decreases obviously. The average sizes of Pt NPs in Pt-PFSA/GNS and PtTiO2/GNS increases from 2.77 to 6.24 nm, and 2.82 to 6.68 nm, respectively, which are less than that of Pt/C. Importantly, the Pt NP average size of Pt-PFSA-TiO2/GNS increases only from 2.79 to 4.86 nm. As shown in the inset of Fig. 6 d, even after ADT, the structure of PFSA-TiO2 costabilized Pt NPs is still intact. Conversely, Pt NPs in Pt/C coalesce into larger particles, as displayed in the inset of Fig. 6 a. This demonstrates that the PFSA and TiO2 co-stabilized catalyst possesses an outstanding electrochemical stability.

Fig. 6

The excellent stability of Pt-PFSA-TiO2/GNS catalyst can be attributed to 1) TiO2 has an excellent electrochemical stability, which can effectively protect the support material from electrochemical oxidation. As demonstrated in Fig. S8, CV curves of commercial carbon black shows an obvious increase on the peaks in the redox region corresponding to the formation of hydroquinone–quinone redox couple on the carbon surface49 during a electrochemical oxidation process (24 h). In contrast, after a longer time of electrochemical oxidation (29 h), The CV curve of TiO2 displays a negligible electrochemical oxidation peak, confirming excellent electrochemical durability of TiO2.


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On the other hand, carbon supports are often oxidized in an electrochemical system, normally in the process as:49 C + H2O → CO2 + 4H+ + 4e- , or in a heterogeneous water– gas reaction, especially in the presence of Pt:51 C + H2O → H2 + CO. Thus, Pt NPs suffer from detachment, migration and agglomeration when the carbon support contacting with Pt NPs is oxidized in the conventional Pt/C catalyst.9, 10 Here, the presence of TiO2 can alleviate the oxidation process. 2) Significantly, the TiO2 NF as a solid wall, hampers Pt NPs from colliding with each other and from aggregating at the horizontal x- and y- axes. Simultaneously, Pt NPs are firmly anchored to the support by the organic PFSA material, effectively avoiding detachment along the z-axis (vertical direction). Consequently, Pt NPs are tightly confined in all three spatial directions (x-, y- and z- axes) (Fig. 1 d), thus can be effectively inhibited from agglomeration (or coalescence) on and detachment from support materials. It is worth noting that Pt NPs in Pt-PFSA-TiO2/GNS still suffer from a little growth as evidenced by the slight increase in particle size (from 2.79 to 4.86 nm). This may be due to the fact that Pt NPs can be further split into smaller particles or dissolved into ions (Pt2+/Pt4+), and they reprecipitate onto surfaces of larger particles (so called Ostwald ripening phenomenon52), or recrystallize at other positions, causing an increase of average particle size.9, 10, 53 It could also be due to plausible imperfect inter-particle dispersion which can be affected by the quantity and quality of the blocks on supports. In addition, the TiO2 structure has a rarely change over time according to observation of TEM (more TEM images of Pt-PFSA-TiO2/GNS after ADT displayed in Fig. S9 to evidence that), which can be ascribed to its quite stability nature even under harsh electrochemical condition and its protection effect on the beneath carbon material.


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Conclusions We have successfully prepared an inorganic (TiO2) and organic (perflurosulfonic acid, PFSA) three-dimensionally co-stabilized Pt nanoparticles catalyst supported on graphene (Pt-PFSATiO2/GNS). This novel catalyst displays a higher oxygen reduction reaction (ORR) activity than the commercial Pt/C. Significantly, it displayed outstanding stability in comparison with the catalyst consisting of either single species (TiO2 or PFSA), and far better stability than the commercial Pt/C. The results revealed that the TiO2 and PFSA enabled a three-dimensionally costabilizing effect on the Pt NPs. ASSOCIATED CONTENT Supporting Information. Content of Pt in catalysts determined by ICP-AES. Calibration of the reference electrode. More TEM images of the catalyst. The Pt/GNS data. CV graphs used to calculate change of ECSA value. i-t curves. TEM image of Pt/C before ADT. CV curves of TiO2 and commercial carbon black during ADT. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION *Corresponding Author. Tel: +86 27 87651837. E-mail: [email protected](Sichun M) ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51372186), the National Basic Research Development Program of China (973Program, 2012CB215504), the Natural Science Foundation of Hubei Province of China (2013CFA082), and the Fundamental Research Funds for the Central Universities (No. 135101015). The authors wish to thank


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Xiaoqing Liu and Tingting Luo for HR-TEM measurement, Yubin Sun for FTIR tests, and Liyun Tie for ICP-AES test, Materials Analysis Center of Wuhan University of Technology. REFERENCES (1) Eberle, U.; Müller, B.; von Helmolt, R., Fuel cell electric vehicles and hydrogen infrastructure: status 2012. Energ. Environ. Sci. 2012, 5, 8780-8798. (2) Zhang, H.; Jin, M. S.; Xia, Y. N., Enhancing the catalytic and electrocatalytic properties of Pt-based catalysts by forming bimetallic nanocrystals with Pd. Chem. Soc. Rev. 2012, 41, 8035-8049. (3) Yu, W. T.; Porosoff, M. D.; Chen, J. G. G., Review of Pt-Based Bimetallic Catalysis: From Model Surfaces to Supported Catalysts. Chem. Rev. 2012, 112, 5780-5817. (4) Guo, S. J.; Wang, E. K., Noble ‘metal nanomaterials: Controllable synthesis and application in fuel cells and analytical sensors. Nano Today 2011, 6, 240-264. (5) Antolini, E., Palladium in fuel cell catalysis. Energ. Environ. Sci. 2009, 2, 915-931. (6) Debe, M. K., Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486, 43-51. (7) Cao, M.; Wu, D.; Cao, R., Recent Advances in the Stabilization of Platinum Electrocatalysts for Fuel-Cell Reactions. ChemCatChem 2014, 6, 26-45. (8) Wang, Y.-J.; Wilkinson, D. P.; Zhang, J., Noncarbon support materials for polymer electrolyte membrane fuel cell electrocatalysts. Chem. Rev. 2011, 111, 7625-7651. (9) Shao, Y.; Yin, G.; Gao, Y., Understanding and approaches for the durability issues of Ptbased catalysts for PEM fuel cell. J. Power Sources 2007, 171, 558-566. (10) Cheng, K.; Kou, Z.; Zhang, J.; Jiang, M.; Wu, H.; Hu, L.; Yang, X.; Pan, M.; Mu, S., Ultrathin carbon layer stabilized metal catalysts towards oxygen reduction. J. Mater. Chem. A 2015, 3, 14007-14014. (11) De Volder, M. F.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J., Carbon nanotubes: present and future commercial applications. Science 2013, 339, 535-539. (12) Zhao, X.; Yin, M.; Ma, L.; Liang, L.; Liu, C.; Liao, J.; Lu, T.; Xing, W., Recent advances in catalysts for direct methanol fuel cells. Energ. Environ. Sci. 2011, 4, 2736-2753. (13) Li, L.; Hu, L.; Li, J.; Wei, Z., Enhanced stability of Pt nanoparticle electrocatalysts for fuel cells. Nano Res. 2014, 8, 418-440. (14) Vinayan, B. P.; Nagar, R.; Ramaprabhu, S., Synthesis and investigation of mechanism of platinum-graphene electrocatalysts by novel co-reduction techniques for proton exchange membrane fuel cell applications. J. Mater. Chem. 2012, 22, 25325-25334. (15) He, D.; Cheng, K.; Li, H.; Peng, T.; Xu, F.; Mu, S.; Pan, M., Highly active platinum nanoparticles on graphene nanosheets with a significant improvement in stability and CO tolerance. Langmuir 2012, 28, 3979-86. (16) Wang, X. X.; Tan, Z. H.; Zeng, M.; Wang, J. N., Carbon nanocages: A new support material for Pt catalyst with remarkably high durability. Sci. Rep.-UK 2014, 4. doi:10.1038/srep04437. (17) Kim, Y.-T.; Mitani, T., Surface thiolation of carbon nanotubes as supports: a promising route for the high dispersion of Pt nanoparticles for electrocatalysts. J. catal. 2006, 238, 394-401. (18) Zhang, G.; Sun, S.; Yang, D.; Dodelet, J.-P.; Sacher, E., The surface analytical characterization of carbon fibers functionalized by H 2 SO 4/HNO 3 treatment. Carbon 2008, 46,


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(35) Yu, J.; Low, J.; Xiao, W.; Zhou, P.; Jaroniec, M., Enhanced photocatalytic CO(2)reduction activity of anatase TiO(2) by coexposed {001} and {101} facets. J. Am. Chem. Soc. 2014, 136, 8839-8842. (36) Holzwarth, U.; Gibson, N., The Scherrer equation versus the'Debye-Scherrer equation'. Nat. Nanotechnol. 2011, 6, 534-534. (37) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S., A General Approach to the Size‐ and Shape‐Controlled Synthesis of Platinum Nanoparticles and Their Catalytic Reduction of Oxygen. Angew. Chem. Int. Edit. 2008, 47, 3588-3591. (38) Mu, S.; Xu, C.; Yuan, Q.; Gao, Y.; Xu, F.; Zhao, P., Degradation behaviors of perfluorosulfonic acid polymer electrolyte membranes for polymer electrolyte membrane fuel cells under varied acceleration conditions. J. Appl. Polym. Sci. 2013, 129, 1586-1592. (39) Lowry, S.; Mauritz, K., An investigation of ionic hydration effects in perfluorosulfonate ionomers by Fourier transform infrared spectroscopy. J. Am. Chem. Soc. 1980, 102, 4665-4667. (40) Jung, J.-H.; Jeon, J.-H.; Sridhar, V.; Oh, I.-K., Electro-active graphene–Nafion actuators. Carbon 2011, 49, 1279-1289. (41) Seger, B.; Kamat, P. V., Electrocatalytically Active Graphene-Platinum Nanocomposites. Role of 2-D Carbon Support in PEM Fuel Cells. J. Phys. Chem. C 2009, 113, (19), 7990-7995. (42) He, D.; Cheng, K.; Peng, T.; Sun, X.; Pan, M.; Mu, S., Bifunctional effect of reduced graphene oxides to support active metal nanoparticles for oxygen reduction reaction and stability. J. Mater. Chem. 2012, 22, 21298. (43) Jiang, Z.-Z.; Wang, Z.-B.; Chu, Y.-Y.; Gu, D.-M.; Yin, G.-P., Ultrahigh stable carbon riveted Pt/TiO 2–C catalyst prepared by in situ carbonized glucose for proton exchange membrane fuel cell. Energ. Environ. Sci. 2011, 4, 728-735. (44) Tian, Z. Q.; Jiang, S. P.; Liu, Z.; Li, L., Polyelectrolyte-stabilized Pt nanoparticles as new electrocatalysts for low temperature fuel cells. Electrochem. Commun. 2007, 9, 1613-1618. (45) Sarma, L. S.; Dai Lin, T.; Tsai, Y.-W.; Chen, J. M.; Hwang, B. J., Carbon-supported Pt– Ru catalysts prepared by the Nafion stabilized alcohol-reduction method for application in direct methanol fuel cells. J. Power Sources 2005, 139, 44-54. (46) Cheng, K.; He, D.; Peng, T.; Lv, H.; Pan, M.; Mu, S., Porous graphene supported Pt catalysts for proton exchange membrane fuel cells. Electrochim. Acta 2014, 132, 356-363. (47) Curnick, O. J.; Mendes, P. M.; Pollet, B. G., Enhanced durability of a Pt/C electrocatalyst derived from Nafion-stabilised colloidal platinum nanoparticles. Electrochem. Commun. 2010, 12, 1017-1020. (48) Curnick, O. J.; Pollet, B. G.; Mendes, P. M., Nafion®-stabilised Pt/C electrocatalysts with efficient catalyst layer ionomer distribution for proton exchange membrane fuel cells. Rsc Adv. 2012, 2, 8368-8374. (49) Kinoshita, K.; Bett, J., Potentiodynamic analysis of surface oxides on carbon blacks. Carbon 1973, 11, 403-411. (50) Eastwood, B. J.; Christensen, P. A.; Armstrong, R. D.; Bates, N. R., Electrochemical oxidation of a carbon black loaded polymer electrode in aqueous electrolytes. J. Solid State Electr. 1999, 3, 179-186. (51) Stevens, D. A.; Hicks, M. T.; Haugen, G. M.; Dahn, J. R., Ex Situ and In Situ Stability Studies of PEMFC Catalysts: Effect of Carbon Type and Humidification on Degradation of the Carbon. J. Electrochem. Soc. 2005, 152, A2309-A2315. (52) Voorhees, P. W., The theory of Ostwald ripening. J. Stat. Phys. 1985, 38, 231-252. (53) Bett, J. A. S.; Kinoshita, K.; Stonehart, P., Crystallite growth of platinum dispersed on


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graphitized carbon black: II. Effect of liquid environment. J. Catal. 1976, 41, 124-133.


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List of Figure captions Fig. 1

Schematics of Pt NPs co-stabilized by polymer and ceramic species on carbon supports. (a) As to the conventional Pt/C catalyst, Pt NPs can motion arbitrarily in all three spatial directions (x-, y- and z- axes). (b) A strategy of introducing polymer mainly inhibiting the Pt NP from detachment in vertical direction (z-axis). (c) A strategy of introducing solid ceramic species preventing the Pt NP from motion in horizontal x- and y- directions, but not in vertical direction (z-axis). (d) Pt NPs can be tightly anchored in all three spatial degrees of freedom (x-, y- and z- axes) by combining the merits of both polymer and ceramic species.

Fig. 2

TEM (a) and HR-TEM (b) images of Pt NPs wrapped with PFSA. The inset in (a) is the corresponding EDX pattern. (c) XRD patterns of TiO2, Pt-PFSA/GNS, PtTiO2/GNS and Pt-PFSA-TiO2/GNS. (d) FTIR spectra of PFSA, Pt-PFSA/GNS and Pt-PFSA-TiO2/GNS, and the inset is the structural formula of PFSA.

Fig. 3

TEM images and corresponding EDX patterns of Pt-PFSA/GNS (a, b). Pt-TiO2/GNS (c, d) and Pt-PFSA-TiO2/GNS (e, f). The inset of (f) is a schematic of Pt NP wrapped with PFSA.

Fig. 4

(a) CV curves for Pt-PFSA/GNS, Pt-TiO2/GNS and Pt-PFSA-TiO2/GNS and commercial Pt/C in 0.1 mol L-1 HClO4 at the scan rate of 50 mV s-1. The inset is ECSA value of the catalysts. (b) Comparison of ORR curves for the catalysts with an inset of their mass activity.

Fig. 5

(a) Normalized ECSA values as a function of potential cycling numbers. Polarization curves of ORR for Pt-PFSA/GNS (b), Pt-TiO2/GNS (c), Pt-PFSA-TiO2/GNS (d) and


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Pt/C (e) before and after 6,000 potential cycles. (f) Mass activity retention rate for the catalysts after ADT. The ORR was tested at a rotating speed of 1600 rpm in an O2saturated 0.1 mol L-1 HClO4 solution with a sweep rate of 10 mV s-1. Fig. 6

TEM images and the corresponding size distribution of Pt NPs before vs. after ADT for Pt/C (a, b), Pt-PFSA/GNS (c, d), Pt-TiO2/GNS (e, f) and Pt-PFSA-TiO2/GNS (g, h). The inset is the corresponding HR-TEM image.


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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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For Table of Contents Only


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Three-Dimensionally Costabilized Metal Catalysts toward an Oxygen

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