A Spatially Confined gC3N4-Pt Electrocatalyst with Robust Stability

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Energy, Environmental, and Catalysis Applications

A Spatially Confined gC3N4-Pt Electrocatalyst with Robust Stability Kun Cheng, Kang Zhu, Shengli Liu, Mengxue Li, Jinhua Huang, Lihuan Yu, Zhuo Xia, Chang Zhu, Xiaobo Liu, Wenhui Li, Wangting Lu, Feng Wei, Youhua Zhou, Wanquan Zheng, and Shichun Mu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03832 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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ACS Applied Materials & Interfaces

A Spatially Confined gC3N4-Pt Electrocatalyst with Robust Stability Kun Chenga,b, Kang Zhua,c, Shengli Liua,d, Mengxue Lia,c, Jinhua Huanga,d, Lihuan Yua,d, Zhuo Xiaa,d, Chang Zhua,d, Xiaobo Liub, Wenhui Lia, Wangting Lua, Feng Weia, Youhua Zhoua,d, Wanquan Zhenga,e,*, Shichun Mub,*

a

Institution for Interdisciplinary Research, Jianghan University, 8th Triangle Lake Road, Wuhan,

Hubei, 430056, P. R. China. b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, Wuhan, Hubei, 430056, P. R. China. c

School of Chemical and Environmental Engineering, Jianghan University, 8th Triangle Lake

Road, Wuhan, Hubei, 430056, P. R. China. d

School of Physics and Information Engineering, Jianghan University, Wuhan, Hubei 430056, P.

R. China. e

Institut des Sciences Moléculaires d’Orsay, Université Paris-Sud, 91405 Orsay Cedex, France

*Corresponding author: [email protected] (Shichun Mu,Tel:+86-027-87651837); [email protected]

Keywords: Pt-based catalyst, stability, spatial confinement, graphitic carbon nitride, oxygen reduction reaction, methanol oxidation reaction.

Abstract Metal catalysts (e.g. Pt) have a variety of applications in energy conversion devices including polymer electrolyte fuel cells (PEFCs), however they commonly confront a crucial issue of poor stability. Herein, a structural model of spatially confining supported Pt nanoparticles is figured out to improve the stability of metal catalysts, wherein graphitic carbon nitride (gC3N4) supported Pt nanoparticles (gC3N4-Pt) is spatially confined by carbon nano-spheres (CNSs). The resulting CNSs-Pt/gC3N4 catalyst demonstrates a surprised retention rate of electrochemical surface area as high as 85.0%, much higher than that of the commercial Pt/C catalyst (45.2%), and the half-wave potential reduced by only 11 mV compared with 54 mV for Pt/C after 6,000 scanning cycles. Besides, CNSs also serve as conductive agent to increase electron transfer pathways on Pt surfaces, and the unique spatial confinement

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structure with an open framework ensures the mass transfer. Moreover, the methanol oxidation reaction (MOR) activity of CNSs-Pt/gC3N4 gets elevated by 2.1 times that of Pt/C in terms of the anodic peak current. The stabilized catalyst model and its derivative structures can be applied to various metal catalyst systems.


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1. Introduction Polymer electrolyte fuel cells (PEFCs) have been drawing increasing attentions in green energy and environmental fields owing to their outstanding efficiency and eco-friendly nature.1,

2

The current commercially available cathodic and anodic

catalysts towards PEFCs are Pt-based catalysts with high catalytic kinetics.3,

4

However, Pt is very precious and of extremely low-abundance in nature, therefore it is of fundamental importance to maximize its utilization efficiency through improvement of the catalytic activity with decreasing dosage, and through stability amelioration with reducing activity loss during catalytic processes.5-9 For enhancing catalytic activity of Pt-based catalysts, Pt-based nanoparticles (NPs, nanowires or films) on high surface area carbon supports are normally designed to boost their dispersion and specific surface area.4, 10, 11 However, such catalysts readily suffer from migration, aggregation and detachment of Pt-based NPs on the support under the harsh operating conditions of fuel cells, leading to poor stability.8, 12, 13 Therefore, it has been extremely urgent to improve the lifetime of Pt-based catalysts.12, 14 To enhance stability of Pt-based catalysts, efforts have been made on fabricating Pt-based alloys,15 designing unique Pt crystal structures,16 substituting commercial carbon

black

supports

with

more

stable

ones,17-19

or

conducting

decoration/modification of the support surface.14 However, it still remains a challenge to satisfy the tough requirement of stability for commercial applications.11 Recently, one strategy has displayed great success in stability amelioration by confinement, i.e,



spatially confining Pt-based NPs in certain space with one solid-phase.20-25 Yang et al.20 encapsulated PtPd NPs in hollow mesocellular SiO2 foam, and the resulting catalyst exhibited less decrease of electrochemical surface area (ECSA) than that of commercial Pt/C after long-term potential cycling. Cheng et al.21 deposited ZrO2 at outside of Pt NPs to fix Pt NPs by atomic layer deposition (ALD) method. The designed catalyst had much enhanced electrochemical stability as well as good catalytic activity compared with Pt/C catalysts. Our group combined TiO2 nanosheets and perfluorosulfonic acid (PFSA, polymer) to co-stabilize Pt NPs in the three-dimensional directions (x,y,z) on graphene surfaces, which effectively elevated the stability of Pt catalysts;22 Moreover, by constructing nanocarbon-intercalated graphene nanosheet scaffold (a sandwich structure), the subsequently deposited Pt NPs showed greatly improved electrochemical stability where the Pt NPs were spatially limited in the sandwich space.23 However, the treatment process of these confinement strategies is commonly complex (eg. hollow mesocellular foam) and highly cost (eg. ALD), homogeneously modified Pt NPs are hard to obtain (eg. TiO2 nanosheets), and the mass transfer is readily limited (eg. suffered by graphene sheets). Graphitic carbon nitride (gC3N4), despite a poor conductivity, possesses many advantages, such as the excellent electrochemical stability and good hydrophilicity, to serve as support material for the Pt-based catalysts.26-28 However, the reported gC3N4 was predominately in a kind of bulk state with a low specific surface area.29-31 To resolve the mentioned problems of gC3N4 and enhance Pt-based catalyst stability, in this study, ultrathin gC3N4 nanosheets with atomic-scale thickness are successfully


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fabricated, by virtue of which a highly stable catalyst is designed (CNSs-Pt/gC3N4). The catalyst involves that unltrathin gC3N4 nanosheet supported Pt NPs (Pt/gC3N4) are embedded into the vacancy of accumulated scaffold of carbon nano-spheres (CNSs). As illustrated in Fig. 1 a, the supported Pt NPs are spatially confined by CNSs. Noteworthily, the scaffold of CNSs owns a porous framework ensuring mass transfer. Simultaneously, CNSs can behave as a conductive agent, compensating the poor conductivity of gC3N4. The resulting CNSs-Pt/gC3N4 catalyst exhibits outstanding electrochemical stability towards oxygen reduction reaction (ORR) as well as methanol oxidation reaction (MOR).

Figure 1 (a) Schematic of arbon nano-spheres (CNSs) confining gC3N4 supported Pt catalyst, and (b) its synthesis process, including (i) preparation of gC3N4 to serve as the support; (ii) synthesis of gC3N4 supported Pt nanoparticles through colloidal method; (ii) carbon nanospheres are introduced to fabricate CNSs-Pt/gC3N4.



2. Experimental section Synthetic procedures

Synthesis of gC3N4: Firstly, bulk gC3N4 was prepared using melamine as precursor which was heated at 600 °C and kept for 2 h in a semi-closed tube furnace. Ultrathin gC3N4 nanosheets were prepared in a modified liquid stripping method,32, 33 during which bulk gC3N4 powder was dispersed in water/ethanol and sonicated to extract gC3N4 nanosheets, followed by centrifugation and evaporation to obtain a yellow powder. Hereinafter, without specification the description of “gC3N4” represents the ultra-thin gC3N4 nanosheet not the bulk one. Preparation of the catalyst: The CNSs-Pt/gC3N4 catalyst synthesis process is illustrated in Fig. 1 b. A H2PtCl6 solution was blended with ethylene glycol (10 times in volume) with pH of 10-12, which was heated at 130°C for 30 min under vigorous agitation to prepare Pt colloid solution. Simultaneously, gC3N4 nanosheets were dispersed in ethanol/water solution through sonication, and then were dripped into the Pt colloid solution, which were stirred for 30 min to prepare Pt/gC3N4. Subsequently, CNSs (~250 m2 g-1, Ø≈30 nm, Cabot Corp.) were introduced to fabricate the CNSs-Pt/gC3N4. The mass ratio between gC3N4 and CNSs is 1:1, and the Pt content in CNSs-Pt/gC3N4 was controlled at 20 w%. Physicochemical characterization X-ray diffraction (XRD) spectra were performed using Cu Ka radiation source at 5° min-1. Morphology of the gC3N4 nanosheets were investigated using field emission


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scanning electron microscopy (SEM, Hitachi S-4800, Japan). Transmission electron microscope (TEM) analysis was performed on JEM-2100F STEM/EDS (JEOL®, Japan). Atomic-force microscopy (AFM) technique was carried out with an AutoProbe CP/MT Scanning Probe Micro-scope (MultiTask, Veeco Instruments). X-ray photoelectron spectroscopy (XPS) measurement was conducted with a VG Scientific ESCALAB 210 electron spectrometer (Al Ka radiation, 2×10-8 Pa at 14 kV) using under a vacuum of. The real Pt content of the sample was measured using inductively coupled plasma optical emission spectrometry (ICP-OES). Electrochemical measurements Electrochemical properties were characterized on electrochemical workstation (CHI760E) adopting a three-electrode test. The saturated calomel electrode (SCE) was served as the reference electrode, a platinum plate as the counter electrode, and a glass carbon electrode (GCE, 0.196 cm2) as the working electrode. The referential potential was presented relative to reversible hydrogen electrode (RHE) calibrated from the SCE adopting previous method in Ref 22. Electrochemical surface area (ECSA) of the Pt-based catalysts was examined by performing the cyclic voltammetry (CV) test. The activity of oxygen reduction reaction (ORR), was calculated according to the polarization curves using linear sweep voltammetry (LSV) technique. The methanol oxidation reaction (MOR) and CO-stripping tests were measured using the CV technique. i-t curves were obtained adopting chronoamperometric technique under a constant potential. Accelerated durability test (ADT) was applied to examine the catalyst lifetime of PEFC,34-36



during which the potential was scanned between 0.6 to 1.2 V vs RHE at 100 mV s-1. Electrochemical impedance spectroscopy (EIS) measurement was conducted with an excitation signal of 5 mV from 105 Hz and 0.1 Hz under open circuit potential. More details of the experiment are shown in the Supporting Information (SI).

3. Results and discussions Structural identification of gC3N4 is displayed in Fig. 2. The XRD pattern evidences the successful synthesis of bulk gC3N4 by the characteristic peaks at 13.6 and 26.9°.37 The peak at 26.9° corresponds to crystal face of (002) for gC3N4,37 which is ascribed to the interlayer d-spacing of 0.331 nm. The peak at 13.6° corresponds to an in-plane peak in the direction orthogonal to the layers (1 0 0) of gC3N4, corresponding to the size of the tri-s-triazine units.37, 38 One typical SEM image (Fig. 2 a) of bulk gC3N4 presents a blocky structure which is unfavorable for supporting Pt NPs. In contrast, the refined gC3N4 nanosheets possess only a few atomic layers as the TEM image shown in Fig. 2 c and d. The thickness of gC3N4 nanosheets was determined by cross section profile analysis through AFM technique. Tens of gC3N4 nanosheets were randomly chosen to conduct analysis. The results show that the average thickness is 1.2±0.2 nm, corresponding to about 2-3 atomic layers (Fig. 2 f).39 As to the width of gC3N4 nanosheets, it is predominantly 50-200 nm. Additionally, the gC3N4 nanosheets were readily suspended in water/alcohol solution to be a white colloid liquid which can maintain stable for more than 24 hours without precipitation, as presented in the inset. This indicates good hydrophilicity of gC3N4 nanosheets. Overall, the ultrathin


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structure and the good hydrophilicity of gC3N4 nanosheets are beneficial to support the metal catalysts.

Figure 2 (a) A typical SEM image of bulk gC3N4, and (b) the corresponding XRD pattern of bulk gC3N4. (c) One TEM image of a typical ultra-thin gC3N4 nanosheet, and (d) an enlarged image of the box region. (e) An AFM image of gC3N4 nanosheets on a clean mica plate with its colloidal solution shown in the inset. (f) Cross section analysis of the gC3N4 nanosheets, corresponding to the white dotted line in (e).



TEM images of Pt NPs supported on gC3N4 nanosheets (Pt/gC3N4) are displayed in Fig. 3 a,b, which show that Pt NPs are homogeneously dispersed on gC3N4 with a narrow size distribution (mainly of 2-4 nm). As to CNSs-Pt/gC3N4, Pt NPs are present in an arc shape arrangement surrounding CNSs (Fig. 3 c,d, denoted with dashed lines). This arrangement of Pt NPs is quite different from that of Pt/gC3N4 which presents a random distribution. This can be illustrated through the schematic in the inset, where the Pt NPs are spatially confined by CNSs, and thus under the vertical electron beam of TEM, an arc-shape distribution of Pt NPs appears around CNSs. Notably, the Pt nanoparticles supported on gC3N4 are confined in the vacancy of CNS scaffolds, and therefore many Pt nanoparticles can array parallelly with the TEM electron beam, so that the projections of Pt nanoparticles can overlap together, resulting in agglomeration in vision while no real agglomeration of Pt nanoparticles takes place. In addition, as the contrast of gC3N4 in TEM image is very similar with CNSs, it is not easy to identify the gC3N4 nanosheets against with CNSs. However, when gC3N4 nanosheets are distributed at the outline of CNSs, they can be identified clearly, as pointed out by the arrows in Fig. 3 c, and more TEM images also confirm this in Fig. S1. As a benchmark, the TEM image of Pt/C (Johnson Matthey, HISPEC3000, 20 w%) is also demonstrated (Fig. 3 e,f), where the Pt NPs randomly disperse on carbon supports. In addition, XRD patterns of the samples are shown in Fig. S2. The Pt crystallite size was calculated using Scherrer equation (details shown in the supporting information). The results show that the average Pt nanoparticle size of Pt/C,

gC3N4-Pt and CNSs-Pt/gC3N4 is 2.6nm, 2.4 nm, and 2.4 nm, respectively.

This indicates that the Pt size of CNSs-Pt/gC3N4 is slightly larger than that of Pt/C, and close to that of gC3N4-Pt, which is agreeable with the following the Pt size distribution analysis through TEM images.


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Figure 3 TEM images of Pt/gC3N4 (a, b), CNSs-Pt/gC3N4 (c, d) and Pt/C (e, f). The insets in (b) and (d) are schematics of corresponding catalysts.



The electrochemical surface area (ECSA) of the catalysts was studied through electrochemical workstation. First, the cyclic voltammetry (CV) curve of Pt/gC3N4 displays no obvious hydrogen-adsorption and -desorption peaks (corresponding to ~0.05-0.38 V, Fig. S3), indicating a negligible electrochemical activity of Pt/gC3N4. This could be ascribed to the poor conductivity of gC3N4 (as evidenced from EIS data in Fig. S4 a), impeding the electron transfer which is one inevitable process of the triple phase boundary (TPB) reaction on Pt surfaces. For CNSs-Pt/gC3N4, CNSs can effectively offset the weak conductivity of gC3N4 as confirmed by the EIS results from Fig. S4 b, allowing CNSs-Pt/gC3N4 to own well ECSA as the marked region demonstrated (61.4 m2 g-1). Therefore CNSs in CNSs-Pt/gC3N4 can serve as highly conductive agent, increasing electron transfer routes to the Pt NP surface. Simultaneously, the results indicate that the spatial confinement structure possesses a porous framework, ensuring fast mass transfer. Due to lack of electrochemical activity for Pt/gC3N4, we gave up further investigation of its electrochemical stability. To investigate stability of CNSs-Pt/gC3N4, chronoamperometry (i−t) analysis was adopted. It can be seen that Pt/C loses up to ~44% of the initial current density, however, CNSs-Pt/gC3N4 decreases only by 12% (Fig. 4 a), indicating that CNSs-Pt/gC3N4 has significantly better stability than Pt/C.40-42 Subsequently, the accelerated durability test (ADT) was conducted to further investigate the catalyst lifetime. As the potential cycle increases, the adsorption peak intensity of Pt/C decreases apparently (Fig. S5). However, for CNSs-Pt/gC3N4, the intensity reduces only slightly. The ECSA values were calculated from the CV curves and normalized


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to be a function of potential cycling numbers (Fig. 4 b). It exhibits that the ECSA retention rate is only 45.2% (32.77 m2 g-1) of the initial value for Pt/C (72.5 m2 g-1) after 6,000 cycles. In contrast, CNSs-Pt/gC3N4 retains as high as 85.0% (52.19 m2 g-1) of the initial value (61.4 m2 g-1). For ORR, after 6,000 cycles the half-wave potential is reduced by 54 mV for Pt/C, but only by 11 mV for CNSs-Pt/gC3N4 (Fig. 4 c,d). Meanwhile, the ORR mass activity of CNSs-Pt/gC3N4 retains as high as 78.7% (69.81 mA mg-1Pt) of its initial value (88.7 mA mg-1Pt), but it is only 32.3% (34.43 mA mg-1Pt) for Pt/C (106.6 mA mg-1Pt, insets of Fig. 4 c,d ). As to the specific area activity of Pt/C, it reduced to 71.5% (14.7 mA cm-2) of its intial value (10.5 mA cm-2) after ADT. In contrast, the specific area activity of CNSs-Pt/gC3N4 retains as high as 92.4% % (14.5 mA cm-2) of its intial value % (13.4 mA cm-2). These results evidence that CNSs-Pt/gC3N4 has much better electrochemical stability than Pt/C. As to the MOR issue, it is interesting to find that catalytic activity and toxic-intermediate tolerance of CNSs-Pt/gC3N4 are largely improved compared with Pt/C. The specific area current density of the anodic peak for CNSs-Pt/gC3N4 obains 2.1 times raised compared with that of Pt/C (1.52 vs 0.72 mA cm-2Pt, Fig. 4 e). Simultaneously, the mass activity towards MOR was evaluated by the forward peak current, which demonstrates that, the value of Pt/C and CNSs-Pt/gC3N4 is 0.522 mA mgPt-1 and 0.933 mA mgPt-1, respectively. As to the forwards/backwards current ratio which can reflect the toxic intermediate tolerence of the corresponding catalyst, it achieves a 1.5 times increase for CNSs-Pt/gC3N4 compared with Pt/C. This evidences that the toxic-intermediate tolerance of CNSs-Pt/gC3N4 is improved, which is



consistent with CO-stripping test in Fig. S6. The enhanced MOR activity could be mainly originated from the merits of CNSs which own a high adsorption capability of methanol, promoting the methanol molecules to transfer onto the Pt nanoparticles. Specifically, as well known, activated carbon materials possess high specific area and strong adsorption capability towards methanol molecules while weak interaction with water molecular, which probably arises from a strong dispersion interaction between –C-C- with the the –CH3 group in CH3OH relative to interaction beween –C-C- and H- in H2O.43 Thus the activated carbon is reported to be a good absorbent to remove methanol in methanol-containing waste water.44 Herein, as high surface area carbon material (~250 m2 g-1) with rich micropores, the CNSs surrounding Pt NPs, can absorb methanol molecules from the bulk methanol-containing solution. According to the confinement space size evaluated through ideal models (see the supporting information), as Pt nanoparticles are confined in the vacancy of the CNS scaffold, many Pt nanoparticles could contact CNSs. Therefore, the absorbed methanol molecule by CNSs can be transferred to the Pt surface, promoting the MOR. This effect promotes the reaction of TPB on Pt surfaces for MOR. Simultaneously, CO2, the final production of methanol oxidation, derived from further oxidation of CO, can be acceleratedly diffused through the lyophobic interface of CNSs, thus promoting dissolution of CO. Therefore, the MOR activity and the anti-toxic intermediate (CO) is improved for CNSs-Pt/gC3N4. The effect of CNSs is similar to the reported nanocarbon-based wall confining Pt NPs structure which improves the MOR activity of the resulting catalyst.45 In addition, the improved performance can be partially


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related to an interaction of gC3N4 to Pt atoms, adjusting the electronic structure of Pt atoms which is concluded from the XPS peak shift in Fig. S7. It is noteworthy that, the improved MOR activity can not be ascribed to the pure gC3N4, because no MOR oxidative peaks are present in CNSs-gC3N4 (Fig. S8 a). As to Pt/gC3N4, it shows no MOR catalytic peaks possibly due to the weak conductivity of gC3N4 (Fig. S8 b). Fig. 4 f reveals the stability of CNSs-Pt/gC3N4 towards MOR. It demonstrates a much lower decay rate of current intensity for CNSs-Pt/gC3N4 (7×10-6) compared with that of Pt/C (1.22×10-4) during the i-t test, proving that CNSs-Pt/gC3N4 possesses a much higher stability towards MOR relative to Pt/C. The size distribution of Pt NPs and the microstructure of the catalyst after ADT were investigated by TEM technique (Fig. 5). As to the commercial Pt/C, it presents obvious coalescence of Pt NPs with decrease of the particle number (Fig. 5 a, and more TEM images of Pt/C after ADT are shown in Fig. S9). The mean diameter of Pt NPs raises sharply from 2.95 to 8.33 nm (Fig. 5 b, the TEM images used for counting the Pt nanoparticle size are displayed in Fig. S10 and S11). By contrast, in the case of CNSs-Pt/gC3N4, the CNSs confined gC3N4-Pt structure is well retained without apparent agglomeration of Pt NPs (Fig. 5 c). The Pt NP average size raises just from 2.82 to 4.31 nm (Fig. 5 d, the original TEM images shown in Fig. S12 and S13). Accordingly, a confinement effect of CNSs on the Pt NPs is proposed to mainly contribute to the stability improvement: As the Pt NPs supported on the gC3N4 are confined in the vacancy of the CNS scaffolds, the movement, coalescence, and breakaway processes of the Pt NPs on/from the support can be effectively repressed.



We have estimated the size of the confined space according to ideal models, which shows that there are quantity of places with a comparable size of Pt NPs (more details are shown in Supporting information

and Fig. S14). Therefore, it is reasonable that

CNSs can stabilize the supported Pt NPs. Furthermore, as reported Pt NPs can be split into small species (eg. atom clusters) and even dissolved into the electrolyte, followed by reprecipitating/recrystalizing on other Pt NPs or other positions (Ostwald ripening),46, 47 leading to increase of average size for Pt NPs. As to CNSs-Pt/gC3N4, CNSs might retard the diffusion of Pt species, thus alleviating the growth of Pt NPs. Moreover, once Pt NPs in CNSs-Pt/gC3N4 are detached from the support, free Pt NPs can be limited in the confined space built by the surrounding CNSs. Additionally, the enhanced stability might be partially assigned to the excellent electrochemical stability of gC3N4 (confirmed by chronoamperometry measurements in Fig. S15), as well as to the enhanced metal-support interaction (see the XPS section in Fig. S7). However, gC3N4 is probably not the main factor for the elevated stability of the CNSs-Pt/gC3N4, which is concluded through comparison of the stability of CNSs-Pt/gC3N4 with that of the reported gC3N4 supported Pt catalysts without spatial confinement (please see the Supporting information for details).


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Figure 4 (a) Chronoamperometry (i-t) curves of Pt/C and CNSs-Pt/gC3N4 in O2-purged 0.1 mol L-1 HClO4 at 0.7 V vs RHE. The initial current intensity of the i-t test was set to be the value, at which the charging process of the double layer reached to the end in the first several minutes. (b) The variation of normalized ECSA with the potential sweeping cycles. Polarization curves of ORR for Pt/C (c) and CNSs-Pt/gC3N4 (d) before and after 6,000 potential cycles, performed with in 10 mV s-1 at 1,600 rpm.

The HClO4 solution

(0.1 mol L-1) was purged with O2. The insets in c and d are the mass activity towards ORR before and after ADT for the corresponding catalysts. (e) MOR catalytic performance with a sweeping rate of 50 mV s-1, in 0.1 mol L-1 HClO4 + 1 mol L-1 CH3OH solutions. The current density in is normalized by the ECSA of Pt nanoparticles. (f) i-t curves of Pt/C and CNSs-Pt/gC3N4 in 0.1 mol L-1 HClO4 + 1 mol L-1 CH3OH solutions (at 0.9 V).



Figure 5 TEM images of Pt/C (a) and CNSs-Pt/gC3N4 (c) after ADT. The Pt size distribution change for Pt/C (b) and CNSs-Pt/gC3N4 (d) after ADT.

4. Conclusions In summary, to improve electrochemical stability of the metal catalyst, a structural model is proposed, namely, supported metal catalyst is spatially confined by carbon nanospheres (CNSs). The fabrication method is relatively simple, effective, and universal. The results demonstrate that the CNSs-Pt/gC3N4 catalyst obtains greatly elevated electrochemical stability. Simultaneously, the spatial confinement structure possessing an open framework can ensure the mass transfer, and the CNSs-Pt/gC3N4


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owns an improved methanol oxidation reaction (MOR) activity compared with the commercial Pt/C. In addition, CNSs can also serve as a conductive agent to increase conductivity of the catalyst. This novel structural model and its derivative structures are also promisingly applied to many other metal catalyst systems.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51672204) and Hubei Provincial Natural Science Foundation of China (2018CFB271). The authors wish to thank Materials Analysis Center of Wuhan University of Technology for the HR-TEM measurement and ICP-AES test supports. Author contributions K. Cheng, S.C. Mu designed the experiment. K. Cheng, K. Zhu, S.L. Liu, M.X. Li prepared the samples. K. Cheng, J.H. Huang, L.H. Yu, Z. Xia, C. Zhu, X.B. Liu tested the electrochemical performances. W.H. Li, W.T. Lu, F. Wei conducted the physicochemical characterizations. K. Cheng, S.C. Mu, X.B. Liu, W.Q. Zheng, Y.H. Zhou performed electrochemical and physicochemical data analyses. K. Cheng wrote the manuscript, and S.C. Mu, W.Q. Zheng, and Y.H. Zhou gave supervisions. All authors contributed to the discussion and provided feedback on the manuscript. References (1) Wang, S.; Jiang, S. P.. Prospects of Fuel Cell Technologies. Natl. Sci. Rev. 2017, 4, 163-166. (2) Eberle, U.; Müller, B.; von Helmolt, R. Fuel Cell Electric Vehicles and Hydrogen Infrastructure: Status 2012. Energ. Environ. Sci. 2012, 5, 8780-8798. (3) Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. (4) Mistry, H.; Varela, A. S.; Kühl, S.; Strasser, P.; Cuenya, B. R. Nanostructured Electrocatalysts With Tunable Activity and Selectivity. Nat. Rev. Mater. 2016, 1, 16009~1-14. (5) Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C. Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science 2016, 354, 1414-1419. (6) Escudero-Escribano, M.; Malacrida, P.; Hansen, M. H.; Vej-Hansen, U. G.; Velazquez-Palenzuela, A.; Tripkovic, V.; Schiotz, J.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I. Tuning the Activity of Pt Alloy Electrocatalysts by Means of the Lanthanide Contraction. Science 2016, 352, 73-76. (7) Kodama, K.; Jinnouchi, R.; Takahashi, N.; Murata, H.; Morimoto, Y. Activities and Stabilities of Au-Modified Stepped-Pt Single-Crystal Electrodes as Model Cathode Catalysts in Polymer Electrolyte Fuel Cells. J. Am. Chem. Soc. 2016, 138, 4194-4200.



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A Spatially Confined gC3N4-Pt Electrocatalyst with Robust Stability

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