C catalyst promoted by CoP as an efficient and robust anode catalyst in direct methanol fuel cells

C catalyst promoted by CoP as an efficient and robust anode catalyst in direct methanol fuel cells

Author's Accepted Manuscript Nanostructured PtRu/C catalyst promoted by CoP as an efficient and robust anode catalyst in direct methanol fuel cells L...

3MB Sizes 0 Downloads 24 Views

Author's Accepted Manuscript

Nanostructured PtRu/C catalyst promoted by CoP as an efficient and robust anode catalyst in direct methanol fuel cells Ligang Feng, Kui Li, Jinfa Chang, Changpeng Liu, Wei Xing

www.elsevier.com/nanoenergy

PII: DOI: Reference:

S2211-2855(15)00222-0 http://dx.doi.org/10.1016/j.nanoen.2015.05.007 NANOEN838

To appear in:

Nano Energy

Received date: 28 January 2015 Revised date: 24 April 2015 Accepted date: 10 May 2015 Cite this article as: Ligang Feng, Kui Li, Jinfa Chang, Changpeng Liu, Wei Xing, Nanostructured PtRu/C catalyst promoted by CoP as an efficient and robust anode catalyst in direct methanol fuel cells, Nano Energy, http://dx.doi.org/ 10.1016/j.nanoen.2015.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nanostructured PtRu/C catalyst promoted by CoP as an efficient and robust anode catalyst in direct methanol fuel cells Ligang Feng‡*a, Kui Li‡, Jinfa Chang, Changpeng Liu and Wei Xing*

State Key Laboratory of Electroanalytical Chemistry, Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China; Fax: 86-431-85685653, E-mail: [email protected] (W. Xing).

a

Present address: Department of Applied Physics, Chalmers University of Technology, SE-41 296,

Göteborg, Sweden. E-mail: [email protected]; [email protected] (L. Feng). Phone: +46 (0)31 772 3376 ‡

These authors contributed equally to the work.

1

Abstract: Nanostructured PtRu material is considered as the best catalyst for direct methanol fuel cells (DMFCs), but the performance decay resulting from Ru loss seriously hinders the commercial application. Here we demonstrated that the performance of nanostructured PtRu catalyst for methanol oxidation could be significantly improved by CoP material; the presence of CoP could largely slow down the loss of Ru and Pt in PtRu catalyst system, thus promising a highly active and durable performance in DMFCs. Cyclic Voltammetry results showed the peak current is 2.89 times higher than that of state-of-the-art commercial PtRu/C-JM (231.9 mA mg-1PtRu) and 3.86 times higher than that of the home-made reference (PtRu/C-H) catalyst (173.6 mA mg-1PtRu); kinetics study probed by electrochemical impedance spectroscopy showed a large reduced charge transfer resistance in the rate determining step. The highest maximum power density was achieved on this novel PtRu-CoP/C catalyst among all the evaluated catalysts at different temperatures. Specifically, a maximum power density of 85.7 mW cm-2 achieved at 30oC is much higher than that of state-of-the-art commercial PtRu/C catalyst at 70 oC (63.1 mw cm-2). Outstanding catalytic activity and stability observed on this novel PtRu-CoP/C catalyst should be attributed to a synergistic effect between the nanostructured PtRu and CoP, in which the presence of CoP increases PtRu physical stability and anti-CO poisoning ability. The present work is a significant step that opens an avenue in the development of highly active and durable catalysts for fuel cells technology, and makes PtRu catalyst system much closer for commercial application in DMFCs. KEYWORDS: PtRu system; methanol oxidation; fuel cells; synergistic effect; phosphide material

2

Introduction Direct methanol fuel cells (DMFCs) are emerged as promising alternative power sources for portable electronics and vehicles over the past few decades[1-4]. Recent progress demonstrated the hopeful commercial application as a kind of alternative green and sustainable power source [5-8].However, the sluggish kinetics of methanol oxidation severely hinders the mass commercialization of DMFCs. Though lots of inspiring catalyst material has been reported [9-13], a serious problem that performance decay due to the loss of catalyst materials is still there and severely impedes the practical long-term operation [14-18]. Thus, it is highly desired but still a challenge to develop extremely high performance catalyst for DMFC. Nanostructured PtRu material is thought to be best catalyst with both high activity and stability in DMFCs [19-22]. The outstanding catalytic ability is attributed to the so-called ‘functional mechanism’, in which Ru could provide more oxygen-containing species at lower potential and then accelerate the oxidation process of poisoning intermediates at the adjacent Pt active sites. Based on that principle, some catalytic promoters such as transition metal or metal oxide in combination with PtRu material largely improved the catalytic performances. For example, PtRu-SnO2 [23] hybrid catalysts exhibit superior catalytic properties toward methanol oxidation in terms of catalytic activity and desirable stability; TiO2 [24] combined with PtRu as a multifunctional catalyst showed significantly enhanced catalytic activity and CO tolerance ability compared with a commercial Pt/C catalyst. Unfortunately, a severe problem for this kind of catalytic promoter is unstable; as a result the long-term operation based on that kind of hybrid catalyst is still questionable. Hence development of stable catalytic promoter is a smart design strategy to increase the catalyst stability. Phosphide materials, recently, have shown excellent catalytic active and stability in the electrocatalysis for water splitting [25-27]. The facile adsorption of hydrogen on the phosphide materials might be beneficial to the dehydrogenation during methanol electrooxidation. We also have evidenced the excellent co-catalytic ability of Ni2P material in fuel cells [28-30]. The encouraging results promoted us to develop more practical catalytic promoter, and thus to promise the potential applications for fuel cell technology. By screening the potential materials, we found that CoP material 3

is a novel effective catalyst promoter for PtRu nanocatalyst in direct methanol fuel cells. Significantly improved catalytic activity and stability were observed on CoP modified PtRu nanocatalyst towards methanol oxidation by cyclic voltammetry measurements. Upon integration into the anode of a direct methanol fuel cell, the highest maximum power density was achieved on PtRu-CoP/C catalyst among all the reference catalyst at different temperatures. Excellent discharge stability was also observed at 0.3 V on PtRu-CoP/C catalyst over 12 hours. ICP-AES-MS results demonstrated that the presence of CoP could largely slow down the dissolution of Pt and Ru elements, but no Co element was detected. The facile preparation approach of CoP from inexpensive reagents makes it ideally a new member in the catalyst promoter family. The remarkable discharge ability of nanostructured PtRu catalyst in combination with CoP promises a strong potential application, and this finding is a significant step to make PtRu catalyst system much stronger in direct methanol fuel cell technology.

4

Results and discussion The crystal structure of PtRu-CoP/C material was probed by XRD technology, and the patterns are shown in Figure 1. Broaden peaks for Pt face-centred cubic (FCC) structure are observed for all the PtRu catalyst with different CoP loadings. Some CoP diffraction peaks are observed on PtRu catalyst with CoP loading of 40% and 50%, but they are not observed on other samples. The invisible peaks of CoP in the hybrid catalyst is probably due to the coverage of PtRu on the surface or the low CoP contents, because the diffraction peaks of CoP in the CoP/C support could be clearly observed (Figure S1). Alloy between Pt and Ru was formed resulting from the incorporation of the base-metal into the Pt FCC structure, thus 2θ values for PtRu catalysts shift to a higher direction compared with the pure Pt. No distinct diffraction peaks related to the tetragonal RuO2 or hexagonal close-packed Ru phases were observed [31]. It should be pointed out that the formation of PtRuCo(P) alloy is not possible, because CoP itself is firstly prepared and then used as a support that is more stable in the acid condition and during the synthesis ethylene glycol as a weak reduce agent is not able to reduce the CoP itself. The particle size and lattice parameter were calculated according to Scherrer formula and Vegard’s law based on Pt (220) peak, and the alloy degree was calculated based on the formula proposed by Antolini and co-workers [32-34] (See the supporting information). The relevant parameters are compared in Table S1. The average crystal size is around 3 nm for all the prepared PtRu catalyst, and the lattice parameter is also very close, but it is reduced compared with the pure Pt metal (0.3915 nm) due to the formation of PtRu alloy. The alloy degree of Ru in all the home-made catalyst was around 10%, which is lower than that of the commercial PtRu catalyst. It has been pointed out that the active form of the catalyst is a mixture of phases rather than a bimetallic alloy [35]; thus the less content of the alloyed Ru in the hybrid catalyst might be promising a high activity. Typical transmission electron microscopy (TEM) images of PtRu/C-JM, PtRu/C-H and PtRuCoP/C-40% are shown in Figure 2, and those for other samples in Figure S2. It is evident that Pt nanoparticles are observed on the carbon support for all the samples. Specifically, for PtRu-CoP/C-40% catalyst, the average particle size is calculated be 2.64 nm with a narrow size distribution (Figure 2c); While it is ca. 3.31 and 3.49 nm for commercial PtRu/C-JM and home-made PtRu/C-H catalyst 5

respectively. High Resolution Transmission Electron Microscopy (HR-TEM) was further shown for PtRu-CoP/C-40% catalyst (Figure 3), the lattice fringes of Pt and CoP are visible. The distance of 0.226 nm corresponds to the spacing of Pt (111) planes, while that of 0.190 and 0.280 nm correspond to the spacing of CoP (211) and (011) planes. Selected Area Electron Diffraction (SAED) pattern was done and the diffraction rings assigned to (111), (200) and (220) reflections were well in agreement with the XRD results of Pt crystal structure. The composition was further probed by EDX and element mapping on a randomly selected area and a typical result is shown in Figure S3. The elements of Pt, Ru, Co and P are clearly visible, and the content of Pt, Ru and Co is 19.92 wt. %, 10.15 wt. %, 18.29 wt. % respectively, which agrees well with the nominal contents. X-ray photoelectron spectroscopy (XPS) was used to probe the electronic interaction for all the catalysts. Typical Pt 4f and Ru 3p spectra of all PtRu catalyst are shown in Figure S4a, and the binding energy of all peaks is referenced to a C 1s value of 284.6 eV. It is observed that the peaks of Pt 4f and Ru 3p were shifted to the lower binding energy direction compared with PtRu/C-H catalyst with the increase of CoP loading, which indicated the presence of metal and support interaction. The dependence of the CoP ratio on the chemical shifts of the Pt 4f and Ru 3p peaks was shown in Figure S4b, it is clearly observed that a maximum peak position shift of Pt 4f and Ru 3P happened on the PtRu-CoP/C-40% catalyst by increasing the CoP loading. It means that CoP loading of 40% would give rise to the largest electronic effect on the Pt and Ru in the hybrid catalyst. Unfortunately, the peaks for Co 2p and P 2p were not clearly observed even with more collection cycles (Figure S4), which may be due to the coverage of PtRu on the surface. Owing to the surface oxidation, XPS spectra of Pt 3d can be fitted by two pairs of overlapping Lorentzian curves according to the presence of the metallic and oxidized forms of Pt. For clear, the peaks of Pt 4f and Ru 3p for PtRu-CoP/C-40%, PtRu/C-JM and PtRu/C-H catalysts are further fitted and compared in Figure 4, and others see Figure S5. The fitting curves of Pt 4f and Ru 3p peaks of X-ray photoelectron spectra of samples are in accordance with the NIST [36] and literatures[37-41]. Because the preparation method and different treatments could affect the materials’ electronic state, it is more reasonable to understand the electronic effect by comparing the two home-made catalysts, and see the PtRu/C-JM as a role of 6

reference. The most intense peaks are attributed to metallic platinum, Pt (0) and the other doublets can be attributed to the Pt (II) chemical state on PtO or Pt(OH)2. The relative intensity of metallic Pt (0) (Table S2) for PtRu-CoP/C-40%, PtRu/C-JM and PtRu/C-H catalyst is 73.2%, 73.8% and 75.6% respectively. Specifically, the peak position of Pt 4f for the PtRu-CoP/C-40% is at 70.1 and 73.5 eV respectively, but the peak position is shifted to the lower binding energy direction compared with the home-made PtRu/C-H catalyst, where the peaks locate at 70.6 and 74 eV. This should be caused by the partial electron transfer from the CoP to the Pt in the hybrid system, and such an electronic interaction can modify the electronic and catalytic properties of metal nanoparticles and lead to the activation of both the dispersed metal and oxide matrix toward electrode processes. Similarly results were obtained on Ru 3p peaks (Table S3). The relative intensity of metallic Ru (0) for PtRu-CoP/C40%, PtRu/C-JM and PtRu/C-H catalyst is 49.2%, 53.2% and 50.1% respectively. PtRu-CoP/C catalyst was drop-cast on a glassy carbon electrode (0.1257 cm2) for electrochemical measurements by cyclic voltammetry (CV). Typical behaviours for polycrystalline Pt are observed in the background solution (0.5 M H2SO4) for all the samples (Figure S6). CO stripping voltammetry was employed to evaluate the electrochemically active surface area (ECSA) based on the Coulombic charge required for a monolayer of COad oxidation is 420 µC cm−2. The highest ECSA (116.11 m2 mg1

PtRu)

was achieved on PtRu-CoP/C-40% catalyst (Figure S7 and Table S4) indicating the largest

catalytic active sites exposed, though the PtRu loading was the same for all catalysts. The ECSA was used to calculate the specific activity later for methanol oxidation. PtRu-CoP/C catalyst with different CoP loadings was firstly screened by CV in 0.5 M methanol and H2SO4 solution saturated by N2, and the CVs based on the mass activity are shown in Figure 5, and the specific activity in Figure S8. As expected the loading of CoP affects the final catalytic performances of PtRu nanocatalyst, and an optimal CoP loading was found to be 40% of CoP on carbon in the hybrid catalyst. As a result, the catalytic performance was done on PtRu-CoP/C-40% catalyst compared with a benchmark commercial PtRu/C catalyst (PtRu/C-JM) in Figure 5b. A homemade PtRu/C-H catalyst was also compared to check the promotion effect as to avoid the effect of catalyst preparation method. Specifically, the peak current of PtRu-CoP/C-40% is 670.5 mA mg-1PtRu, 7

which is approximately 1.89 times higher than that of PtRu-JM (231.9 mA mg-1PtRu), and 2.86 times higher than that of the home-made PtRu/C-H catalyst(173.6 mA mg-1PtRu). Owing to the different ECSA, specific activity was used to compare the intrinsic activity and the highest activity was observed on PtRu-CoP/C-40% too (Figure S8 and Table S5). The catalytic stability was evaluated by CA (Chronoamperometry) technology at 0.4 V and the results are shown in Figure S9. It is evident that PtRu-CoP/C-40% catalyst exhibited the highest stable current. After 2 hours measurements, it retains 50.75% of the initial current, while a relative low value is observed on the PtRu-JM (39.50%) and PtRu/C-H catalyst (17.38%). The above results demonstrated that PtRu-CoP/C-40% catalyst had excellent catalytic activity and stability for methanol oxidation. The enhancement effect should be due to the increased ECSA and the electronic effect. As aforementioned that PtRu-CoP/C-40% has much smaller particle size than both the commercial and home-made PtRu/C catalyst, higher ECSA could provide more active sites during the reaction and increase the catalytic efficiency. Moreover, the largest electronic effect was found on the PtRu-CoP/C-40% that will increase the activation of both the dispersed metal and matrix toward electrode processes and further promise an enhanced performance. Electrochemical Impedance Spectroscopy (EIS) was employed to probe the kinetics of methanol oxidation in detail at different potentials. The Nyquist plots for all the studied catalyst are shown in Figure S10, and typical methanol oxidation behaviours catalysed by Pt catalyst are observed (supporting information). Figure 6a shows the Nyquist plots for the impedance data collected at 0.4 V. The data was further fitted by a typical equivalent circuit (Figure 6b) to show the hidden information including conductivity, charge transfer and diffusion at the catalyst-electrolyte interface. For the equivalent circuit, inductive components (L1 and L2) are obviously needed for a well-fitting due to the external circuit inductance and usually does not involve an electrochemical process; other components have the universal meaning where RS represents the uncompensated solution resistance, RCT corresponds to a charge transfer resistance arisen from methanol oxidation, R0 is probably related to the contact resistance between the catalyst material and the glassy carbon electrode and a constant phase element (CPE) composition for the double layer capacitance. The data calculated from the equivalent circuit are compared in Table S6. The RS for all the catalyst materials is quite similar with 8

the value around 5 Ω cm2. The charge transfer resistance, an important parameter to indicate the reaction rate, is 140.3 Ω cm2 for PtRu-CoP/C-40% catalyst, which is the smallest among all the catalysts. This is consistent with the diameter of the EIS arc which correlates with charge transfer resistance. According to the RCT and Figure 5a, the PtRu-CoP/C-40% catalyst has the smallest RCT, thus the highest catalytic activity in methanol oxidation. Tafel slope was calculated on the PtRu/C-H, PtRu/C-JM and PtRu-CoP/C-40% catalyst at a scan rate of 2 mV s-1 and shown in Figure 6c. It is 119, 123 and 140 mV dec-1 respectively for PtRuCoP/C-40%, PtRu/C-JM and PtRu/C-H catalyst; and the corresponding exchange current density value was calculated to be 0.012, 0.008 and 0.07 mA cm-2 respectively. The Tafel slope was consistent with the literature reported slope of 110-195 mV dec-1 [42]. If we assume the electron transfer coefficient is 0.5, the number of electron transferred for PtRu/C-H, PtRu/C-JM and PtRu-CoP/C-40% is 0.85, 0.97 and 1 respectively. The higher exchange current density and lower Tafel slope observed on the PtRuCoP/C-40% catalyst is a sign of greater catalytic activity. Recently, Aricò etal[22] reported IrO2 as a promoter of PtRu catalyst for methanol oxidation, and they attributed the improved performance to a multifunctional catalyst. Due to the complicated methanol oxidation process, they proposed that different functionalities are needed to speed up the reaction rate during the multistep process of methanol oxidation. Similarly to the role of IrO2, CoP recently was found to be an excellent catalyst for hydrogen evolution. It is supposed that this function should be helpful for the methanol dehydrogenation, thus the methanol oxidation rate on the PtRu active site could be accelerated. From the traditional mechanism, CoP is also a good material to active water, producing the oxygen-containing species to oxidize CO and other poisoning intermediates adsorbed at adjacent Pt sites through the so-called bifunctional mechanism to accelerate the methanol oxidation process. Furthermore, the facile preparation method and earth-abundant precursors of CoP make it more completive than IrO2 promoter. Owing to the complicated process of methanol oxidation on the catalyst surface, further spectroscopic studies and theory analysis are needed to understand the catalytic mechanism.

9

In order to demonstrate the potential application performances, PtRu-CoP/C-40%, PtRu/C-JM and PtRu/C-H catalyst were integrated into a home-made direct methanol fuel cell for practical performance evaluation. The polarization at 30, 50 and 70 oC was recorded to evaluate the catalytic ability. Figure 7 shows the corresponding steady-state polarization curves for PtRu-CoP/C-40%, PtRu/C-JM and PtRu/C-H catalyst respectively at 30, 50 and 70 oC. It can be seen that PtRu-CoP/C-40% showed the highest maximum power density. Specifically the maximum power density of PtRuCoP/C-40% at 30, 50 and 70 oC is 85.8, 105.2 and 128.0 mW cm-2 respectively, which is also higher than that of the similar catalysts recently reported (Table S7). The maximum power density of 85.8 mW cm-2 at 30 oC is even much higher than that of state-of-the-art PtRu/C-JM catalyst at 70 oC (63.1 mW cm-2), indicating a very promising application in direct methanol fuel cells. A plot of the current density at 0.3V vs. the temperature (T-1) is shown in Figure 8 to probe the activation energy. It can be seen that the activation energy of PtRu-CoP/C-40% and PtRu/C-JM catalyst is very close, but is lower than that of the PtRu/C-H catalyst. The discharge ability of the catalysts is compared in Figure 9 at 0.3 V over 12 hours at 50 oC. It is evident that there is a power density decrease in the initial 2 hours for all the catalyst, but soon a highly stable discharge performance is observed on the PtRu-CoP/C-40% catalyst. The gradual increase of the power density after 8 hours on the PtRu-CoP/C-40% catalyst could be due to the heat-accumulation or activation of the catalyst and so on[17, 43] . The change of the power density was around 5% versus the initial power density that is in the scope of the deviation for stability tests. However, a trend of the performance decay was accompanied on the PtRu/C-JM catalyst and PtRu/C-H during the whole discharge. The ratio of the final power density vs. the initial value was 97.2%, 51.3% and 58.0% respectively for PtRu-CoP/C-40%, PtRu/C-JM and PtRu/C-H catalyst. The somewhat performance decay could be due to the instability of the catalyst compositions. Further physical characterization of XRD, XPS and TEM was done on the decayed catalyst to probe the performance decay. No obvious crystal structure change was observed from the XRD results (Figure S11A) and also no obvious electronic binding energy change was seen from the XPS data (Figure S11B) compared with the fresh samples. However, an increased average particle size of PtRu nanoparticles was observed on both 10

home-made and commercial PtRu/C catalysts (Figure S11C for TEM images) compared with that in Figure 2, it might be a reason that caused the performance decay over the discharge process. Similar results were found in Pt-based or PtRu-based catalysts[44-46]. Fortunately, no obvious change of the PtRu particle sizes was observed on PtRu-CoP/C-40% catalyst (Figure S11C) that might give rise to a robust performance. The ICP-AES-MS (Inductivity Coupled Plasma-Atomic Emission Spectroscopy-Mass Spectroscopy) was used to probe the stability by detecting the dissolved elements of PtRu-CoP/C-40%. In order to check the physical stability of this material, the catalyst sample was exposed into 0.5 M H2SO4 solution for 24 hours and the dissolved elements were detected at varied time. As shown in Figure S12, Co, Pt and Ru elements were obviously detected after the PtRu-CoP/C-40% was exposed into the electrolyte for 1 hour that might be due to the loss of the oxidized surface, and then, the concentration of all the elements was increased very slowly. The concentration of Pt, Ru and Co was 2.85, 2.99 and 4.32 ppb[47]after 24 hours, which further confirmed an excellent physical stability of PtRu-CoP/C-40% catalyst. The stability of PtRu-CoP/C-40% after the fuel cells test was also probed by ICP-AES-MS to detect the dissolved elements in the fuel of methanol solution. Consistent with our expectation, the presence of the CoP could slow down the loss of Pt and Ru during operation. For the commercial PtRu/C-JM catalyst, the concentration of Pt and Ru found in the solution was 0.925 ppb[48] and 1.513 ppb, while it was 1.345 ppb (Pt) and 18.430 ppb (Ru) for homemade PtRu/C-H catalyst indicating a serious material loss. Here, it should be pointed out that parts of dissolved Pt resulting from the cathode may be included [43, 49]. Wang [43] reported that Ru, which played a key role in methanol electrooxidation process, was easily degraded at anode catalyst layer, and it resulted in the decline of anode catalytic activity and affected the cell performance after stability testing. However, the loss of Pt and Ru material could be slowed when the presence of CoP material; it is only 1.031 ppb (Pt) 0.502 ppb (Ru) for the PtRu-CoP/C-40% catalyst. Further, we didn’t found the presence of Co element in the solution with the detection limit of 0.001 ppb. It may suggest that the CoP in the hybrid catalyst doesn’t involve the catalytic reaction, thus it was not found in the fuel. The results further confirmed the outstanding stability of CoP as a catalytic promoter, and showed a highly promising application in 11

DMFCs. Consistent with our report, C. Cabrera [18] reported that the Pt-Ru/BDDNP (boron doped diamond particulates) catalytic system exhibited superior power density, with no decay observed in the range shown, in contrast to a commercial PtRu/C.

Conclusion Methanol electro-oxidation catalyzed by nanostructured PtRu catalyst was greatly enhanced by CoP materials. Cyclic Voltammetry results showed the peak current is 2.89 times than that of state-of-theart commercial PtRu/C-JM (231.9 mA mg-1PtRu) and 3.86 times that of the home-made reference (PtRu/C-H) catalyst (173.6 mA mg-1PtRu). The charge transfer resistance revealed by EIS is largely reduced when the presence of CoP in the PtRu catalyst, and the corresponding higher exchange current density and lower Tafel slope observed on the PtRu-CoP/C-40% catalyst is a sign of greater catalytic activity. Upon integration into a direct methanol fuel cell, PtRu-CoP/C catalyst showed the highest maximum power density at 30, 50 and 70oC compared with the reference catalysts, and the performances compared favourably to some of the best performances in the literature. A maximum power density of 85.8 mW cm-2 at 30 oC was achieved which is also higher than that of PtRu/C-JM catalyst at 70 oC (63.1 mW cm-2), indicating a very promising application. Furthermore, highly stable discharge ability was observed over 12 hours due to the slowed Pt and Ru loss. Great improvements in the catalytic activity and stability should be attributed to the synergistic effect of Pt and the unique role of ‘CoP’ in the hybrid catalyst.

12

Acknowledgements The work is supported by the National High Technology Research and Development Program of China (863 Program, 2012AA053401), the National Natural Science Foundation of China (21373199, 21433003), the Strategic priority research program of CAS (XDA0903104) and the Recruitment Program of Foreign Experts (WQ20122200077).

13

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://

14

References [1] R. Bashyam, P. Zelenay, Nature 443 (2006) 63-66. [2] J.N. Tiwari, R.N. Tiwari, G. Singh, K.S. Kim, Nano Energy 2 (2013) 553-578. [3] N.V. Long, Y. Yang, C. Minh Thi, N.V. Minh, Y. Cao, M. Nogami, Nano Energy 2 (2013) 636676. [4] W. He, X. Lin, J.H. Dickerson, J.B. Goodenough, Nano Energy 2 (2013) 1004-1009. [5] X. Zhao, M. Yin, L. Ma, L. Liang, C.P. Liu, J.H. Liao, T.H. Lu, W. Xing, Energy & Environmental Science 4 (2011) 2736-2753. [6] L. Feng, X. Zhao, J. Yang, W. Xing, C. Liu, Catal. Commun. 14 (2011) 10-14. [7] S. Sharma, B.G. Pollet, J. Power Sources 208 (2012) 96-119. [8] N. Kimiaie, K. Wedlich, M. Hehemann, R. Lambertz, M. Muller, C. Korte, D. Stolten, Energy Environ. Sci. 7 (2014) 3013-3025. [9] L. Feng, L. Yan, Z. Cui, C. Liu, W. Xing, Journal of Power Sources 196 (2011) 2469-2474. [10] L. Feng, J. Zhang, W. Cai, Liangliang, W. Xing, C. Liu, Journal of Power Sources 196 (2011) 2750-2753. [11] V. Baglio, D. Sebastián, C. D’Urso, A. Stassi, R.S. Amin, K.M. El-Khatib, A.S. Aricò, Electrochimica Acta 128 (2014) 304-310. [12] M.U. Anu Prathap, R. Srivastava, Nano Energy 2 (2013) 1046-1053. [13] V.T.T. Ho, N.G. Nguyen, C.-J. Pan, J.-H. Cheng, J. Rick, W.-N. Su, J.-F. Lee, H.-S. Sheu, B.-J. Hwang, Nano Energy 1 (2012) 687-695. [14] Y. Ma, H. Li, H. Wang, X. Mao, V. Linkov, S. Ji, O.U. Gcilitshana, R. Wang, Journal of Power Sources 268 (2014) 498-507. [15] C.-D. Dong, C.-W. Chen, C.-F. Chen, C.-M. Hung, Scientific Reports 4 (2014). [16] R. Lv, T. Cui, M.-S. Jun, Q. Zhang, A. Cao, D.S. Su, Z. Zhang, S.-H. Yoon, J. Miyawaki, I. Mochida, F. Kang, Advanced Functional Materials 21 (2011) 999-1006. [17] Y. Ito, T. Takeuchi, T. Tsujiguchi, M.A. Abdelkareem, N. Nakagawa, J. Power Sources 242 (2013) 280-288. [18] L. La-Torre-Riveros, R. Guzman-Blas, A.E. Méndez-Torres, M. Prelas, D.A. Tryk, C.R. Cabrera, ACS Appl. Mater. Interfaces 4 (2012) 1134-1147. [19] Q. Lu, B. Yang, L. Zhuang, J. Lu, J. Phys. Chem. B 109 (2005) 1715-1722. [20] A.H.C. Sirk, J.M. Hill, S.K.Y. Kung, V.I. Birss, J. Phys. Chem. B 108 (2004) 689-695. [21] B. Wu, D. Hu, Y. Kuang, B. Liu, X. Zhang, J. Chen, Angew. Chem. Int. Ed. 48 (2009) 47514754.

15

[22] V. Baglio, R.S. Amin, K.M. El-Khatib, S. Siracusano, C. D'Urso, A.S. Arico, Phys. Chem. Chem. Phys. 16 (2014) 10414-10418. [23] S. Yang, C. Zhao, C. Ge, X. Dong, X. Liu, Y. Liu, Y. Fang, H. Wang, Z. Li, Journal of Materials Chemistry 22 (2012) 7104-7107. [24] A.L. Ocampo, Q.-Z. Jiang, Z.-F. Ma, J. Rodriguez Varela, J. de Gyves, Electrocatalysis 5 (2014) 387-395. [25] H. Du, Q. Liu, N. Cheng, A.M. Asiri, X. Sun, C.M. Li, J. Mater. Chem. A 2 (2014) 14812-14816. [26] E.J. Popczun, C.G. Read, C.W. Roske, N.S. Lewis, R.E. Schaak, Angew. Chem. Int. Ed. 53 (2014) 5427-5430. [27] L. Feng, H. Vrubel, M. Bensimon, X. Hu, Phys. Chem. Chem. Phys. 16 (2014) 5917-5921. [28] J. Chang, L. Feng, C. Liu, W. Xing, X. Hu, Angew. Chem. Int. Ed. 53 (2014) 122-126. [29] J. Chang, L. Feng, C. Liu, W. Xing, X. Hu, Energy Environ. Sci. 7 (2014) 1628-1632. [30] G. Li, L. Feng, J. Chang, B. Wickman, H. Grönbeck, C. Liu, W. Xing, ChemSusChem 7 (2014) 3374-3381. [31] L. Ma, C. Liu, J. Liao, T. Lu, W. Xing, J. Zhang, Electrochimica Acta 54 (2009) 7274-7279. [32] F. Pires, P. Corradini, V. Paganin, E. Antolini, J. Perez, Ionics 19 (2013) 1037-1045. [33] E. Antolini, L. Giorgi, F. Cardellini, E. Passalacqua, J Solid State Electrochem 5 (2001) 131-140. [34] E. Antolini, F. Cardellini, J. Alloys Compd. 315 (2001) 118-122. [35] J.W. Long, R.M. Stroud, K.E. Swider-Lyons, D.R. Rolison, J. Phys. Chem. B 104 (2000) 97729776. [36] http://srdata.nist.gov/xps/ [37] Z.-B. Wang, P.-J. Zuo, G.-J. Wang, C.-Y. Du, G.-P. Yin, The Journal of Physical Chemistry C 112 (2008) 6582-6587. [38] Y. Okawa, T. Masuda, H. Uehara, D. Matsumura, K. Tamura, Y. Nishihata, K. Uosaki, RSC Advances 3 (2013) 15094-15101. [39] J.L.G. de la Fuente, F.J. Pérez-Alonso, M.V. Martínez-Huerta, M.A. Peña, J.L.G. Fierro, S. Rojas, Catalysis Today 143 (2009) 69-75. [40] C.-Z. Li, Z.-B. Wang, X.-L. Sui, L.-M. Zhang, D.-M. Gu, S. Gu, Journal of Materials Chemistry A (2014). [41] M.-L. Lin, M.-Y. Lo, C.-Y. Mou, The Journal of Physical Chemistry C 113 (2009) 16158-16168. [42] L. Feng, Q. Lv, X. Sun, S. Yao, C. Liu, W. Xing, J. Electroanal. Chem. 664 (2012) 14-19. [43] Y. Wang, G. Liu, M. Wang, G. Liu, J. Li, X. Wang, Int. J. Hydrogen Energy 38 (2013) 90009007. 16

[44] G.-S. Park, C. Pak, Y.-S. Chung, J.-R. Kim, W.S. Jeon, Y.-H. Lee, K. Kim, H. Chang, D. Seung, J. Power Sources 176 (2008) 484-489. [45] X. Wang, W. Li, Z. Chen, M. Waje, Y. Yan, J. Power Sources 158 (2006) 154-159. [46] M.K. Jeon, J.Y. Won, K.S. Oh, K.R. Lee, S.I. Woo, Electrochimica Acta 53 (2007) 447-452. [47] The volume of solution is 40 mL; Compared with the whole PtRu mass in terms of µg, the lost Pt, Ru and Co were in the level of 10-3. Theoretically, Pt, Ru and Co mass in all was 30, 15 and 27.5 µg, respectively. [48] Here, the volume of solution is 250 mL; For easy understanding, the concentration was not transferred to the materials loss%. Compared with the whole PtRu mass in terms of mg, the lost Pt and Ru were in the level of 10-5. Theoretically, Pt and Ru mass in anode was 12 and 6 mg, respectively. [49] B. Li, Z. Yan, Q. Xiao, J. Dai, D. Yang, C. Zhang, M. Cai, J. Ma, Journal of Power Sources 270 (2014) 201-207.

17

Figure caption Figure 1 XRD patterns of PtRu/C-JM, PtRu/C-H and PtRu-CoP/C. Figure 2 TEM images and corresponding particle size distribution histograms of PtRu/C-JM, PtRu/CH and PtRu-CoP/C-40% catalysts. Scale bar: 20 nm. Figure 3 HR-TEM and SAED images of PtRu-CoP/C-40% catalyst. Figure 4 XPS patterns of PtRu/C-JM, PtRu/C-H and PtRu-CoP/C-40% for Pt 4f and Ru 3p binding energy. Figure 5 (a) The electrocatalytic performance of PtRu-CoP/C-10%, PtRu-CoP/C-20%, PtRu-CoP/C30%, PtRu-CoP/C-40% and PtRu-CoP/C-50% catalysts for methanol electrooxidation in H2SO4 (0.5 M) containing CH3OH (1.0 M) at 50 mV s-1; (b) The electrocatalytic performance of PtRu/C-JM, PtRu/C-H and PtRu-CoP/C-40% catalysts for methanol electrooxidation in H2SO4 (0.5 M) containing CH3OH (1.0 M) at 50 mV s-1. Figure 6 Nyquist plots (a) of Pt/C-JM, PtRu/C-H and PtRu-CoP/C for methanol oxidation at 0.4 V and equivalent circuit (b); The Tafel curves (c) of the PtRu/C-JM, PtRu/C-H and PtRu-CoP/C-40% in 0.5 M H2SO4 solutions containing 1.0 M CH3OH. Figure 7 Steady-state polarization, and power-density curves for fuel cells employing PtRu-CoP/C40%, commercial PtRu/C-JM and PtRu/C-H as anode catalysts. Conditions: 2 M methanol at 30, 50 and 70℃. The flowing rate of methanol was 2.0 mL min-1 and the flowing rate of O2 was 200 mL min1

. The anode catalyst loading was 2 mg cm-2 of PtRu.

Figure 8 Arrhenius plot and activation energies at 0.3 V for PtRu-CoP/C-40%, commercial PtRu/CJM and PtRu/C-H catalysts. Figure 9 Discharge curves at 0.3 V (50oC) for fuel cells employing PtRu-CoP/C-40%, commercial PtRu/C-JM and PtRu/C-H as anode catalysts

18

C 002 Pt 111

Intensity / a.u.

Pt 200

Pt 220 PtRu-CoP/C-50% PtRu-CoP/C-40% PtRu-CoP/C-30% PtRu-CoP/C-20% PtRu-CoP/C-10% PtRu/C-H PtRu/C-JM

10

20

30

40

50

2θ /

60

70

80

°

Figure 1 XRD patterns of PtRu/C-JM, PtRu/C-H and PtRu-CoP/C.

Figure 2 TEM images and corresponding particle size distribution histograms of PtRu/C-JM, PtRu/CH and PtRu-CoP/C-40% catalysts. Scale bar: 20 nm.

19

Figure 3 HR-TEM and SAED images of PtRu-CoP/C-40% catalyst.

20

Intensity / a.u.

PtRu/C-JM

Pt 4f5/2

Pt 4f7/2

PtRu/C-H

PtRu-CoP/C-40%

80

78

76

74

72

70

68

Binding energy / eV

Intensity / a.u.

PtRu/C-JM

Ru 3p3/2

PtRu/C-H

PtRu-CoP/C-40%

472

470

468

466

464

462

460

458

Binding energy / eV

Figure 4 XPS patterns of PtRu/C-JM, PtRu/C-H and PtRu-CoP/C-40% for Pt 4f and Ru 3p binding energy.

21

750

j / mA mg PtRu -1

600

450

PtRu-CoP/C-10% PtRu-CoP/C-20% PtRu-CoP/C-30% PtRu-CoP/C-40% PtRu-CoP/C-50%

a

300

150

0

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

E / V vs. SCE 750

j / mA mg PtRu -1

600

PtRu/C-JM PtRu/C-H PtRu-CoP/C-40%

b

450

300

150

0 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

E / V vs. SCE

Figure 5 (a) The electrocatalytic performance of PtRu-CoP/C-10%, PtRu-CoP/C-20%, PtRu-CoP/C30%, PtRu-CoP/C-40% and PtRu-CoP/C-50% catalysts for methanol electrooxidation in H2SO4 (0.5 M) containing CH3OH (1.0 M) at 50 mV s-1; (b) The electrocatalytic performance of PtRu/C-JM, PtRu/C-H and PtRu-CoP/C-40% catalysts for methanol electrooxidation in H2SO4 (0.5 M) containing CH3OH (1.0 M) at 50 mV s-1.

22

PtRu/C-JM PtRu/C-H PtRu-CoP/C-10% PtRu-CoP/C-20% PtRu-CoP/C-30% PtRu-CoP/C-40% PtRu-CoP/C-50% Fitting

a

300

Zim / ohms

200

100

0

-100

-200 0

100

200

300

400

500

600

700

800

Zre / ohms

0.45 0.40

c

E / V vs. SCE

0.35 0.30 0.25 0.20

PtRu/C-H PtRu/C-JM PtRu-CoP/C-40% fitting line

0.15 0.10 0.05 -1.5

-1.0

-0.5

0.0

0.5

1.0

-2

log (i, i in mA cm ) Figure 6 Nyquist plots (a) of Pt/C-JM, JM, PtRu/C-H PtRu/C and PtRu-CoP/C CoP/C for methanol oxidation at 0.4 V and equivalent circuit (b); The Tafel afel curves (c) of the PtRu/C-JM, PtRu/C-H and PtRu-CoP/C CoP/C-40% in 0.5 M H2SO4 solutions containing 1.0 M CH3OH. 23

PtRu-CoP/C-40%

30 oC 140 50 oC 120 70 oC

0.7

Cell Potential / V

0.6

Power Density / mW cm-2

0.8

100

0.5

80

0.4 60 0.3 40 0.2 20

0.1

0

0.0 0

100

200

300

400

500

600

700

800

900

Current Density / mA cm-2 70

PtRu/C-JM

0.8

50

0.6

40 0.4

30

o

30 C o 50 C o 70 C

0.2

20 10

0.0 0

50

100

150

200

250

300

350

400

450

Power Density / mW cm-2

Cell Potential / V

60

0 500

Current Density / mA cm -2 0.8

40

PtRu/C-H

30

Cell Potential / V

0.6

25 20

0.4 o

30 C 15 o 50 C 10 o 70 C

0.2

5 0

Power Density / mW cm-2

35

0.0 0

50

100

150

200

250

-5 300

Current Density / mA cm-2

Figure 7 Steady-state polarization, and power-density curves for fuel cells employing for PtRuCoP/C-40%, commercial PtRu/C-JM and PtRu/C-H as anode catalysts. Conditions: 2 M methanol at 30, 50 and 70oC. The flowing rate of methanol was 2.0 mL min-1 and the flowing rate of O2 was 200 mL min-1. The anode catalyst loading was 2 mg cm-2 of PtRu.

24

6.4

E = 0.3 V -1

Ea= 9.95 KJ mol

6.0

PtRu-CoP/C-40% 5.6

j / mA cm-2

-1

Ea= 9.66 KJ mol 5.2

PtRu/C-JM 4.8 -1

Ea= 13.53 KJ mol 4.4

PtRu/C-H

4.0

2.9

3.0

3.1

3.2

3.3

-3

10 T / K Figure 8 Arrhenius plot and activation energies at 0.3 V for PtRu-CoP/C-40%, commercial PtRu/CJM and PtRu/C-H catalysts.

25

100

PtRu-CoP/C-40%

Power density / mW cm

-2

90 80 70 60 50

PtRu/C-JM

40 30

PtRu/C-H

20 10 0 0

2

4

6

8

10

12

Time / Hours Figure 9 Discharge curves at 0.3 V (50oC) for fuel cells employing for PtRu-CoP/C-40%, commercial PtRu/C-JM and PtRu/C-H as anode catalysts

26

Biography of each author:

Dr. Ligang Feng is currently a post-doctoral researcher at the Department of Applied Physics at Chalmers University of Technology. He received his Ph.D degree from Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences in the end of 2011 and worked shortly at Changchun Institute of Applied Chemistry and Ecole Polytechnique Fédérale de Lausanne (EPFL). His research interests focus on energy conversion and storage, particularly water electrolysers, fuel cells, nanoscale materials, novel and cost-effective catalyst material from the earth-abundant materials and their applications in electrocatalysis. His current research focuses on optical and electrochemical combinational measurements for fuel cells catalyst.

Kui Li received his BS from Hunan University of Technology in 2012. He is currently completing his Ph. D. under the supervision of Prof. Changpeng Liu at the Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, China. His research interests focus on the development of carbon-support materials for fuel cell applications.

27

Jinfa Chang obtained his Master degree in Chemical Engineering from Yanshan University in June, 2014. He is currently pursuing his doctoral degree in Physical Chemistry under the supervision of Prof. Wei Xing in Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, China. His research interests focus on the development of low-precious and earth-abundant catalysts and there application in fuel cell and water electrolysis.

Prof. Chang-Peng Liu received his Ph. D. in Physical Chemistry in 2002 and was appointed an associate professor at CIAC in 2006. He has been in charge of and taken part in several relevant fuel cells projects (e.g. 863 programs and 973 programs China). Liu’s research focuses on the technology and performance of catalysts, electrodes MEA and stacks.

28

Prof. Wei Xing is currently director of the Clean Energy Laboratory in CIAC. After receiving his Ph.D. in Physical Chemistry, at CIAC in 1995, he worked at the Hong Kong Productivity Council (HKPC), researching the electrochemical treatment of metal surfaces. In 2001, he joined the CIAC as a professor and devoted his work to the development of advanced chemical power sources. He has published over 150 papers in peer-reviewed journals and applied over 30 patents. His research areas currently involve proton exchange membrane fuel cell from fundamental electro-catalytic processes to relevant fuel cell assembly and testing.

29

Highlights

1 Nanostructured PtRu catalyst for methanol oxidation is greatly promoted by CoP. 2 The presence of CoP could largely slow down the loss of Ru and Pt in fuel cells. 3 The catalytic activity is increased about 3-4 times owing to CoP. 4 A maximum power density of 85.7 mW cm-2 was achieved at 30oC.

30

Graphical Abstract

The performance of nanostructured PtRu catalyst for methanol oxidation could be significantly improved by CoP material; the presence of CoP could largely slow down the loss of Ru and Pt in PtRu catalyst system, thus promising a highly active and durable performance in DMFCs

31