高耐甲醇性和稳定性的纳米复合阴极催化剂

郑 宁 朱春梅 孙 斌 施祖进 刘 岩 王 远

(北京大学化学与分子工程学院,北京分子科学国家实验室,北京100871)

1 Introduction

Because of its high energy density,simplicity of handling and processing of liquid fuel,direct methanol fuel cells(DMFCs) are promising power sources for the applications in transportation and portable electronic devices.Cathode electrocatalysts with high selectivity and high durability for the oxygen reduction reaction(ORR)in the presence of methanol are vital materials in the development of high efficient DMFCs.Cathodes based on commercial Pt/C catalysts exhibit high catalytic activity for ORR.However,the methanol crossover in DMFCs equipped with such a cathode results in the competitive oxidation reaction of methanol over the cathode,which severely decreases the cellʹs performance and energy transformation efficiency.1,2Numerous efforts have been devoted to improving the performance of cathodic catalysts in the presence of methanol, including some Pt-based alloys,3-6mesoporous carbon materials as supports of Pt-based nanoparticles,6,7and non-noble metal catalysts.8-13A novel nanocomposite catalyst constructed by phthalocyanine nanocrystals and Pt nanoparticles with high methanol tolerance has been reported in our previous work.2,14Despite the advances made in this field,there are still great challenges in improving the durability of these cathode catalysts for DMFCs.

The catalytic activity and durability of Pt-based catalysts are inherently dependent on the structures and properties of the catalytic metals and support materials,as well as the interaction between them.Single walled carbon nanohorns(SWCNHs) have been proved to be a kind of advanced support material for Pt-based nanoparticles,due to their high surface area, high electrical conductivity,and high chemical stability.15-17There has been increasing evidence showing that the electrochemical and physical properties of carbon materials are extremely sensitive to heteroatoms(N,B,S,and P)doping into the carbon structures,resulting in the improved catalytic activity and durability of electrocatalysts.13,18-21Recently,we have proved that Pt nanoclusters dispersed in the aggregate of nitrogen-doped SWCNHs(NSWCNHs)exhibit obviously enhanced durability and catalytic activity for ORR in the absence of methanol.21Herein in this paper,we designed a novel nanocomposite catalyst assembled with Pt nanoclusters,TiOPc nanocrystals,and NSWCNHs.Methanol tolerant electrocatalysts with high activity and durability are expected on the basis of the merits of these materials.

2 Experimental

2.1 Materials

A carbon supported Pt catalyst(Pt/C-JM,Pt:9.4%(w)) and a Nafion®solution(5%(w))was purchased from Johnson Matthey Company(U.K.)and Aldrich(USA),respectively. H2PtCl6·6H2O(analytical reagent grade,AR grade)was supplied by Sinopharm Chemical Reagent Beijing Co.Ltd.Perchloric acid,sodium hydroxide,ethylene glycol,and ethanol of AR grade were purchased from Beijing Chemical Corp.Ultrapure water(18 MΩ),methanol(high-performance liquid chromatography grade,Fisher,USA)were used without further purification.

2.2 Catalyst synthesis

NSWCNHs applied in this work were prepared by a DC arc discharge method in air,and were heated at 400°C in air for 4 h to remove amorphous carbon.22TiOPc and“unprotected”Pt nanoclusters(stabilized with ethylene glycol and simple ions) with an average diameter of 1.7 nm used in this work were prepared according to the previously reported method.23-25In a typical synthesis of TiOPc-Pt/NSWCNH,45 mg of NSWCNHs powder was dispersed ultrasonically in a C2H5OH/H2O mixture (40 mL,volume ratio=1:1)for 30 min.Then a prepared colloidal solution of Pt nanoclusters(1.35 mL,Pt concentration:3.7 g·L-1)was added dropwise to the suspension under stirring, followed by ultrasonically treating the mixture for 20 min.After stirring overnight,solid in the suspension was filtered,leaving a colorless solution,washed with ultrapure water and C2H5OH,dried at 40°C,and then heated at 300°C in a tube furnace under nitrogen for 1 h to obtain Pt/NSWCNHs.50 mg of TiOPc and 50 mg of Pt/NSWCNHs were ultrasonically mixed in 30 mL of ethanol for 20 min.After stirring overnight at room temperature,the suspension was centrifuged,and the obtained precipitate was washed with C2H5OH and then dried at 50°C in a vacuum oven to obtain TiOPc-Pt/NSWCNH. TiOPc-Pt/C was prepared by the same method using Pt/C-JM and the same TiOPc as starting materials.

2.3 Characterization

The morphologies of catalysts were studied by transmission electron microscope(TEM,Hitachi H-9000NAR,Japan).X-ray diffraction(XRD)measurements were carried out on a Rigaku-2500Pc X-ray diffractometer(Japan)with Cu radiation at 40 kV and 300 mA.Inductively coupled plasma atomic emission spectrometry(ICP-AES,Leeman Co.,USA)was used to analyze the content of Pt in the prepared catalysts.The content of N in the prepared NSWCNH was analyzed by elementary analysis(Elementar vario MICRO CUBE,vario EL,Elementar Analysensysteme,Germany).X-ray photoelectron spectroscopy(XPS)measurements were carried out with an Axis Ultra photoelectron spectrometer with a monochromatic Al KαX-ray (1486.7 eV)source.The binding energies were referenced to the C 1s binding energy of contamination carbon at 284.5 eV.

2.4 Electrochemical measurements

Electrochemical tests of the prepared electrodes were conducted using a CHI 660C workstation at 30°C.A conventional three-compartment electrochemical cell containing an aqueous solution of HClO4(0.1 mol·L-1)was employed for the electrochemical tests,in which a reversible hydrogen electrode(RHE) was used as the reference electrode,and a platinum foil was used as the counter electrode.All potentials in this work were referred to that of RHE.The working electrodes were prepared as follows:TiOPc-Pt/NSWCNH,TiOPc-Pt/C or Pt/C-JM catalysts(10 mg),a Nafion®solution(100 μL,5%(w))and C2H5OH(5 mL)were mixed ultrasonically for 20 min.20 μL of the obtained suspension was transferred onto a freshly polished glassy carbon disk electrode(0.196 cm2),the electrodes were dried at room temperature and then heated at 120°C in a vacuum oven for 1 h.The current densities were expressed in terms of the geometric surface area of the electrodes except where otherwise indicated.

3 Results and discussion

Fig.1 shows the TEM images and the size distributions of Pt nanoparticles in TiOPc-Pt/NSWCNH and TiOPc-Pt/C catalysts.As shown in Figs.1(a),1(c),and 1(e),rod-like TiOPc nanocrystals were observed.NSWCNHs knitted to form electroconductive nano-nets in TiOPc-Pt/NSWCNH and most of the Pt nanoparticles with an average diameter of 1.9 nm were well dispersed in the nano-nets(Figs.1(a)and 1(d)).The average diameter of Pt nanoparticles in the TiOPc-Pt/C catalyst was 2.2 nm(Fig.1(f)).The Pt contents of TiOPc-Pt/NSWCNH and TiOPc-Pt/C were 4.6%and 4.7%(w),respectively,as measured by ICP-AES.

Fig.2 shows the N 1s XPS of NSWCNHs.Elementary analysis indicates that NSWCNHs contain 1.6%(w)of N.The N 1s spectrum from NSWCNHs consists of two peaks,locating at 398.9 and 400.8 eV,which indicates that nitrogen atoms in NSWCNHs might mainly exist in the form of pyridinic and pyrrolic nitrogen species.26

Fig.3 shows the XRD patterns of TiOPc-Pt/NSWCNH(I),Pt/ C-JM(II),and NSWCNH(III).The diffraction peaks at 6.9°, 15.5°,and 23.3°assignable to the signals from m-TiOPc,can be observed in the XRD pattern of TiOPc-Pt/NSWCNH.27A broad peak centered at 25°in the XRD pattern of Pt/C-JM is the signal from carbon black which is the characteristic of amorphous carbon with only small regions of ordered structures.The diffraction peaks related to the(002)and(100) planes of the hexagonal graphite structure appear at positions near 26°and 45°in the XRD patterns of TiOPc-Pt/NSWCNHs and NSWCNH.Two sharp peaks at 26.0°and 26.4°could be attributed to the signals from NSWCNH and a small amount of graphite present in the sample,which indicates a highly graphitic ordered structure in NSWCNHs.Diffraction peaks at 39.8°,46.2°,67.5°,and 81.3°assignable to the signals from the(111),(200),(220),and(311)planes of fcc Pt,respectively, can be observed in the XRD patterns of TiOPc-Pt/NSWCNH and Pt/C-JM.

Fig.4(a)shows cyclic voltammograms(CVs)of TiOPc-Pt/ NSWCNH,TiOPc-Pt/C,and Pt/C-JM in N2-saturated HClO4(0.1 mol·L-1)aqueous solution at 30°C with a scan rate of 50 mV·s-1.The electrochemical surface areas(ESA)of Pt particles of the prepared cathodes were calculated from CVs by integrating the area in the hydrogen underpotential deposition (HUPD)region.

Fig.3 XRD patterns of the TiOPc-Pt/NSWCNH(I),Pt/C-JM(II), and NSWCNH(III)

where QHadsand QHdesare the charges(unit in mC)associated with the hydrogen adsorption and desorption,respectively.The value of 0.21 mC·cm-2is an average charge density associated with the formation of one monolayer hydrogen atoms on a polycrystalline platinum surface.28ESAs of TiOPc-Pt/NSWCNH,TiOPc-Pt/C,and Pt/C-JM were calculated to be 0.50, 0.54,and 2.97 cm2,respectively.The Pt active surface area of TiOPc-Pt/NSWCNH is close to that of TiOPc-Pt/C,but it is much smaller than that of Pt/C-JM.The small Pt active surface area of TiOPc-Pt/NSWCNH is due to that a part of the surfaces of Pt nanoparticles are covered by TiOPc nanocrystals.2,14

As shown in Fig.4(b),linear sweep voltammetry(LSV)was conducted to investigate the catalytic activities for ORR in the presence of methanol over TiOPc-Pt/NSWCNH,TiOPc-PtC, and Pt/C-JM at 30°C.In the presence of methanol,the oxidation current derived from the methanol oxidation over Pt/C-JM was quite large,however,only small peaks for methanol oxidation were observed in the potential region from 0.92 to 1.1 V over TiOPc-Pt/C and TiOPc-Pt/NSWCNH.This indicates that the oxidation of methanol was obviously suppressed over the TiOPc nanocrystal-modified catalysts.Therefore,the onset potential with respect to the reduction currents over TiOPc-Pt/C and TiOPc-Pt/NSWCNH was 0.91 and 0.92 V,respectively, shifted by about 250-260 mV toward positive relative to that over Pt/C-JM(0.66 V).The mass activity and specific activity at 0.85 V of TiOPc-Pt/NSWCNH were 83.5 A·g-1and 0.294 mA·cm-2,respectively,which were much higher than those of Pt/C-JM.TiOPc-Pt/C and TiOPc-Pt/NSWCNH exhibited much higher methanol tolerance in comparison with Pt/C-JM.The catalytic activities of these three catalysts were summarized in the Table 1.

Fig.4 Electrochemical properties of TiOPc-Pt/NSWCNH,TiOPC-Pt/C,and Pt/C-JM at 30°C(a)CV curves tested in N2-saturated HClO4(0.1 mol·L-1)aqueous solution with a scan rate of 50 mV·s-1;(b)ORR polarization curves tested in O2-saturated HClO4 (0.1 mol·L-1)aqueous solution containing methanol(0.5 mol·L-1)with a scan rate of 5 mV·s-1;dot lines tested after potential cycling from 0.6 to 1.0 V for 15000 cycles in an O2-saturated HClO4solution containing methanol(0.5 mol·L-1)

Table 1 Catalytic activities of TiOPc-Pt/NSWCNH,TiOPc-Pt/C, and Pt/C-JM catalysts

The electrochemical stability of TiOPc-Pt/NSWCNH and TiOPc-PtC was evaluated by accelerated aging tests(AAT),cycling the potential between 0.6 and 1.0 V for 15000 cycles in an O2-saturated HClO4(0.1 mol·L-1)aqueous solution containing methanol(0.5 mol·L-1)at 30°C.The dot lines show ORR polarization curves afterAAT in Fig.4(b).It was found that there was a little negative shift(30 mV)of Eonsetover TiOPc-Pt/NSWCNH (0.89 V)after AAT compared to that over the fresh catalyst (0.92 V).But for TiOPc-Pt/C,Eonsetshifted negatively by about 200 mV after the same potential cycling test.After AAT,the catalytic activity over TiOPc-Pt/NSWCNH was much higher than that of TiOPc-Pt/C,indicating that the durability of TiOPc-Pt/NSWCNH for ORR in the presence of methanol is much higher than that of TiOPc-Pt/C.

We believe that the higher methanol tolerance of TiOPc-Pt/ NSWCNH originates from the electron transfer from the TiOPc nanocrystals to Pt nanoparticles.The Fermi level of Pt lies at-5.65 eV,29and the HOMO energy level of TiOPc locates at-5.09 eV,30so electron transfer should occur when the TiOPc nanocrystals contact Pt nanoparticles,which may accelerate the electrocatalytic reduction processes of oxygen and enhance the methanol tolerance over TiOPc-Pt/NSWCNH.2,14The high durability for ORR of TiOPc-Pt/NSWCNH is mainly derived from trapping the Pt nanoparticles in the NSWCNH nanonetwork,suppressing the migration,aggregation,and growth in size of small Pt nanoclusters during the potential cycles.21The high graphitization degree of NSWCNHs enhances the support stability against the degradation caused by Pt-catalyzed oxidation,which should be another cause of the enhanced durability relative to that of TiOPc-Pt/C.

4 Conclusions

In summary,a novel nanocomposite catalyst TiOPc-Pt/ NSWCNH with an excellent methanol tolerance as well as a high durability for ORR in the presence of methanol was prepared by“unprotected”Pt nanoclusters,TiOPc nanocrystal, and nitrogen-doped carbon nanohorns.In the presence of methanol(0.5 mol·L-1),the onset potential for ORR over the TiOPc-Pt/NSWCNH catalyst shifted by more than 260 mV toward positive relative to that over the Pt/C-JM catalyst.The mass activity and specific activity for ORR at 0.85 V over TiOPc-Pt/NSWCNH were 83.5 A·g-1and 0.294 mA·cm-2,respectively,which were much higher than that of Pt/C-JM.After the accelerated aging tests(0.6-1.0 V for 15000 cycles)in an O2-saturated HClO4aqueous solution with methanol,the onset potential with respect to the reduction current was 0.89 V over TiOPc-Pt/NSWCNH,shifted by 180 mV toward positive relative to that over TiOPc-Pt/C(0.71 V).The durability for ORR in the presence of methanol over TiOPc-Pt/NSWCNH is much higher than that of TiOPc-Pt/C.

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