CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201301079
Mesostructured Platinum-Free Anode and Carbon-Free Cathode Catalysts for Durable Proton Exchange Membrane Fuel Cells Xiangzhi Cui, Jianlin Shi,* Yongxia Wang, Yu Chen, Lingxia Zhang, and Zile Hua[a] As one of the most important clean energy sources, proton exchange membrane fuel cells (PEMFCs) have been a topic of extensive research focus for decades. Unfortunately, several critical technique obstacles, such as the high cost of platinum electrode catalysts, performance degradation due to the CO poisoning of the platinum anode, and carbon corrosion by oxygen in the cathode, have greatly impeded its commercial development. A prototype of a single PEMFC catalyzed by a mesostructured platinum-free WO3/C anode and a mesostruc-
tured carbon-free Pt/WC cathode catalysts is reported herein. The prototype cell exhibited 93 % power output of a standard PEMFC using commercial Pt/C catalysts at 50 and 70 8C, and more importantly, CO poisoning-free and carbon corrosion-resistant characters of the anode and cathode, respectively. Consequently, the prototype cell demonstrated considerably enhanced cell operation durability. The mesostructured electrode catalysts are therefore highly promising in the future development and application of PEMFCs.
Introduction Proton exchange membrane fuel cells (PEMFCs) are currently among the most important and attractive clean energy sources as an alternative to traditional energy conversion technologies. However, PEMFCs are still facing a number of vital and unavoidable challenges, including high cost due to the indispensible use of noble metals, such as platinum as the electrode catalysts; limited energy conversion efficiency; and, even more importantly, serious catalytic performance degradation due to platinum poisoning in anodes by CO impurities in the hydrogen source and carbon corrosion by oxygen in the cathodes, which leads to unsatisfactory durability and short lifetimes of the PEMFCs. Pt/C is an active electrocatalyst for hydrogen electrooxidation and is used widely in the application of PEMFCs as anode catalysts. Unfortunately, the catalyst surface of Pt is easily covered by the adsorbed CO molecules, even at very low levels of CO (e.g., 10 ppm), resulting in Pt poisoning and consequent catalytic activity loss.[1] In recent years, Pt-based and CO-tolerant composite anode catalysts, including PtM/C (M = Ru,[2] Pd,[3] Co,[4] Sn,[5] Ir[6]) and PtMOx/C (M = RuO2,[7] SnO2,[8] WO3[9]), have been reported in the literature by several groups. Among these composite catalysts, PtWO3/C has proved to be one of the most promising CO-tolerant anode materials for PEMFCs or direct methanol fuel cells (DMFCs), which results from the spe[a] Dr. X. Cui, Prof. J. Shi, Dr. Y. Wang, Dr. Y. Chen, Dr. L. Zhang, Dr. Z. Hua State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics Chinese Academy of Sciences, 1295 Dingxi Road Shanghai 200050 (P.R. China) Fax: (+ 86) 21-52413122 E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301079.
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cial electrochemical property of WO3 and the hydrogen overflow effect between Pt and WO3.[9] Nevertheless, the precious metal platinum is still necessary for these platinum-based composite catalysts mentioned above. Considering the rarity of precious metals such as platinum, ruthenium, and palladium on earth, searching for and developing noble-metal-free, costeffective anode electrocatalysts with substantially enhanced CO tolerance is one of the most urgent missions, but remains a great challenge in the large-scale application of PEMFCs. On the other hand, active carbon, which is the most widely used catalyst support material due to its high surface area and good electric conductivity (EC), is unfortunately subject to easy corrosion in the strongly acidic working conditions of PEMFCs, especially under a high oxygen content and high potential in the cathode chamber. According to thermodynamics, the carbon support would be oxidized to form surface oxides when the potential is higher than 0.207 V, which will lead to the significant loss of carbon and performance degradation of the cathodes.[10] Therefore, searching for carbon-free cathode catalysts with a strong antioxidation capability and high electron conductivity is of great importance for durable cathodes for PEMFCs. Tungsten carbide (WC), which is relatively low cost and has an especially high resistance to chemical erosion and catalyst poisoning, has attracted much attention in the electrochemical catalysis field.[11] Chhina et al. reported that WC material as an oxidation-resistant catalyst support was more thermally and electrochemically stable than currently used carbon support materials such as Vulcan XC-72R.[12] Subsequently, Yin et al. loaded PdFe nanoparticles on WC material and obtained an alcohol-tolerant PdFe/WC composite electrocatalysts for oxygen reduction reactions (ORRs).[13] Following this, Elezovic´ et al. used nanostructured WC as a support to load Pt nanoparticles, and they found that the resulting Pt/WC was a beneficial ChemSusChem 2014, 7, 135 – 145
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CHEMSUSCHEM FULL PAPERS catalyst for the ORR; this further proved the possibility of WC as a cathode catalyst support material.[14] Unfortunately, WC material prepared by conventional methods usually shows rather low surface areas (< 30 m2 g1) compared with active carbon supports; this is likely to be responsible for the unsatisfactory catalytic activity for oxygen electroreduction of the cathodes reported due to the relative low dispersion of Pt nanoparticles on these WC supports. In recent years, mesoporous materials have been widely studied in the development of catalytic systems because of their ordered pore structures and high surface areas.[15] Although mesoporous WO3 materials synthesized by soft- or hard-templating routes have been reported in the literature,[16] the hydrogen electrochemical oxidation properties of this material, especially of the nonprecious-metal-supported mesoporous WO3, have rarely been reported. In our previous work,[17] we reported that mesoporous WO3 showed significant electrochemical catalytic activity for hydrogen electro-oxidation and its catalytic activity could be further enhanced after an appropriate amount of carbon black was added to the mesoporous WO3 material by a physical mixing method. Regretfully, the electrochemical activity of mesoporous WO3/C was much lower than that of 20 wt % Pt/C catalyst because of the poor EC of the former, since no carbon black is expected to be able to enter the pore structure of mesoporous WO3 only by physical mixing and the electrons generated during the hydrogen oxidation by WO3 cannot be conducted out effectively. On the other hand, no reports on mesoporous WC or Pt-supported mesoporous WC used as cathode catalysts in PEMFCs can be found, although the synthesis of mesoporous WC has been reported by Wu et al.,[18] and non-mesoporous WC supports have been employed in cathodes of PEMFCs.[12–14] In the present work, we synthesized an electrically conductive m-WO3/C composite with carbon present in situ in the pore structure of mesoporous WO3 by route A (Scheme 1) and a high-surface-area mesoporous Pt/WC composite with Pt nanoparticles dispersed homogeneously in the pore structure of mesoporous WC by route B. These synthetic routes are different from those reported in the literature,[16, 18] and feature products with a well-defined mesoporous structure, simplicity, and high electrochemical performances, as shown in the fol-
www.chemsuschem.org lowing sections. In the m-WO3/C composite, the two interpenetrating components on the nanoscale ensure its high EC, which is essential when it is used as an anode catalyst; in the mesostructured Pt/WC composite, Pt nanoparticles are highly dispersed and supported in the WC mesoporous network and both the high catalytic activity and electron conductivity can be guaranteed when used as cathode catalysts. We report herein, for the first time, to the best of our knowledge, the uses of Pt-free m-WO3/C as an anode catalyst and carbon-free mesostructured Pt/WC composite as a cathode catalyst to fabricate a PEMFC. The single cell shows highly comparable current density and power output to that obtained by using commercial E-TEK Pt/C catalysts at a working temperature of 70 8C and, more importantly, much enhanced stability in electrochemical performance due to the almost CO-poisoning-free feature of the m-WO3/C anode and the corrosion resistance of the carbon-free mesostructured Pt/WC cathode; this marks important progress in the future wide application of PEMFCs.
Results and Discussion Structural and chemical properties of the as-prepared catalytic materials
The XRD pattern of the prepared Pt-free anode catalyst mWO3/C can be well indexed to tungsten trioxide (WO3 ; JCPDS card no. 46-1096; Figure 1 A, curve a) and the carbon characteristic peaks are almost undetectable; this indicates that the framework is composed of well-crystallized WO3 and the carbon remains amorphous in the mesostructure of WO3/C. The XRD pattern of the carbon-free cathode support m-WC can be clearly indexed to WC (JCPDS card no. 25-1047; Figure 1 A, curve b). After being loaded with Pt nanoparticles, the characteristic diffraction peaks of Pt metal can be also identified in addition to those of WC and the average crystallite sizes of Pt were found to be around 4.8, 5.1, 5.6, and 6.2 nm for the samples m-10Pt/WC, m-20Pt/WC, m-30Pt/WC, and m40Pt/WC, respectively, by using Debye–Scherer equation.[21] Moreover, according to the low-angle XRD patterns (Figure 1 B), template KIT-6 exhibits three well-resolved diffraction peaks of the (211), (220), and (321) planes of cubic symmetry (Ia3d space group). The samples m-WO3/C and m-WC all clearly show similar characteristic lowangle diffraction peaks of the (211) plane; this suggests that the m-WO3/C and m-WC products have retained the ordered mesoporous structure with the cubic Ia3d symmetry of the template. After being loaded with Pt nanoparticles, the obtained m-10Pt/WC sample still shows the characteristic lowScheme 1. The in situ carbonization replication syntheses of mesoporous WO3/C (m-WO3/C; route A) and mesoangle (211) diffraction peak, structured Pt/WC composite (route B) by using the block copolymer P123 present in the pore channels of a mesowhich can no longer be detectporous silica template as the carbon source. For convenience, the common porous structure of the silica parent ed for samples m-30Pt/WC and compound is given. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org m-40Pt/WC at increased Pt loading amounts within the mesostructure. According to the N2 sorption isotherms (Figure 2) and the corresponding pore structure parameters (Table 1), the prepared electrode materials show typical characteristics of mesoporous transition-metal oxides replicated by the wet-impregnation pathway from mesoporous silica.[22] Sample m-WO3/C shows a type IV hysteresis loop (Figure 2 A), and relative to that of the silica template (inset in Figure 2 A), the m-WO3/C composite shows a pore size distribution centered at about 4.4 nm, which matches well with the wall thickness (4.5 nm) of KIT-6. The Brunauer–Emmett–Teller (BET) specific surface area of as-prepared m-WO3/C is 108 m2 g1 and the wall thickness
Table 1. The pore structure parameters of the prepared m-WO3/C and mPt/WC composites. Sample
BET surface area [m2 g1]
BJH pore size [nm]
Thickness of wall[a] [nm]
498 6.8 4.5[b] 108 4.4 5.6[b] 143 4.4, 12.5[c] 138 4.1, 10 128 4.2, 11.4 115 3.9, 12.0 113 3.7, 12.0 p [a] Thickness of pore wall = 2d211/ 3BJH (Barrett–Joyner–Halenda) pore size. [b] Denotes the diameter of WO3 nanorods. [c] Bimodal pore size distribution maximized at 4.5 and 12.5 nm, respectively. KIT-6 raw m-WO3/C m-WC m-10Pt/WC m-20Pt/WC m-30Pt/WC m-40Pt/WC
Figure 1. A) High-angle XRD patterns of samples m-WO3/C (a), m-WC (b), m10Pt/WC (c), m-20Pt/WC (d), m-30Pt/WC (e), and m-40Pt/WC (f); the numbers before Pt indicate the amounts of Pt loaded by weight percentage. B) The corresponding low-angle XRD patterns.
XRD result [nm] d211 = 9.781 d211 = 9.781
Figure 2. Nitrogen sorption isotherms of A) sample m-WO3/C and the mesoporous silica template (inset), and C) mesostructured m-Pt/WC composites. B, D) The corresponding pore size distribution curves.
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CHEMSUSCHEM FULL PAPERS (diameter of WO3 nanorods or nanoparticles) is 5.6 nm; this value is a little smaller than the pore size of the silica template (6.8 nm) due to WO3 wall contraction during replication. Both m-WC and m-Pt/WC composites exhibit similar bimodal pore size distributions with maxima at 4 and 12 nm (Figure 2 D), respectively. The BET surface area of m-WC reaches 143 m2 g1, which decreases slightly after loading Pt nanoparticles in/on the mesoporous WC support. A typical SEM image (Figure 3 A) shows that the morphology of the m-WO3/C replica is disk-like with a diameter of around 500 nm and a thickness of about 100 nm. Moreover, m-WO3/C possesses a well-ordered mesostructure when viewed along the (110) direction of cubic Ia3d symmetry (Figure 3 B), although its color is darker than that of the m-WO3 sample with-
www.chemsuschem.org ure 4 D). The smaller one of d = 0.23 nm in the mesopores can be attributed to Pt metal (d = 0.226 nm, JCPDS card no. 040802) and the other of d = 0.25 nm in the framework can be ascribed to WC (according to JCPDS card no. 25-1047); these results imply the existence of Pt nanoparticles in the pores of the mesoporous WC support. More direct evidence of Pt nanoparticle dispersion in/on the WC support is the clear Pt signals easily detected by EDS (Figure S1 b in the Supporting Information) on the m-20Pt/WC composite sample. However, some of the Pt nanoparticles may exist outside of the mesoporous network of WC or aggregate together when the Pt loading reaches 40 wt %, as shown in the selected area in Figure 4 C. Figure 4 A-1–C-1 gives the corresponding platinum particle size distribution histograms. The average diameters of 4.65 (for m10Pt/WC), 5.65 (for m-30Pt/WC), and 6.3 nm (for m-40Pt/WC) and relatively narrow particle size distributions were determined; this is in good agreement with the values calculated by using the Scherer formula.
Electrochemical performance of the mesostructured catalyst materials Cyclic voltammetry tests In cyclic voltammetry (CV) tests, m-WO3/C shows a clear hydrogen oxidation peak at about 0.35 V in positive scans, and the peak current of hydrogen oxidation is even higher than that of the commercial catalyst 20Pt/C-ETK (Figure S2 A in the Supporting Information). Moreover, m-WO3/C also shows stable electrochemical catalytic activity of the anode (Figure S2 B in the Supporting Information). This is due to the formation of a hydrogen tungsten bronze compound (HxWO3) in the acidic environment of the PEMFC by hydrogen intercalation/de-intercalation into/out of the WO3 [Eqs. (1) and (2)]:[23] Figure 3. A) SEM and B) TEM images of sample m-WO3/C and its energy-dispersive X-ray spectrum (C); D) TEM image of m-WO3.
out carbon combination (Figure 3 C). No Si signal can be detected in the energy-dispersive X-ray spectroscopy (EDS) pattern (Figure 3 D) performed on different domains of templatefree m-WO3/C; there are only strong W and O signals, which indicates complete removal of the silica template. In addition, as-prepared mesostructured m-WO3/C shows much enhanced EC, which was measured to be 5.0 103 W1 m1, relative to mesoporous WO3. After the addition of a certain amount of external carbon, the EC values of mesostructured m-WO3/C-C composites were further enhanced (Table S1 in the Supporting Information). A typical TEM image of m-WC (Figure S1 a in the Supporting Information) shows an ordered mesoporous structure when viewed along the (100) direction. Pt nanoparticles, which appear as darker dots, are well dispersed in the pore structure of, or on, the mesoporous WC support for composites m-10Pt/ WC and m-30Pt/WC, according to the results shown in Figure 4 A and B, respectively. The HRTEM image of as-prepared m-20Pt/WC shows two dominant crystal lattice stripes (Fig 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
WO3 þ x Hþ þ x e ! Hx WO3
ð0 < x < 1Þ
WO3 þ 2y Hþ þ 2y e ! WO3y þ y H2 O
ð0 < y < 1Þ
ð1Þ ð2Þ
The reactions of mesoporous WO3 with H/H + in the electrolyte are reversible, so that mesoporous WO3 can act as a catalyst and show electrocatalytic activity for hydrogen oxidation. HxWO3 is a nonstoichiometric and electrically conducting compound capable of hydrogen intercalation/de-intercalation; this is comparable to hydrogen adsorption/desorption on platinum metal. The electrocatalytic activity of the mesostructured m-WO3/C composite anode catalyst for the hydrogen oxidation reaction (HOR) was measured and the results are shown in Figure 5. All catalysts showed much enhanced hydrogen oxidation current density in the H2-saturated electrolyte than that in the N2-saturated electrolyte. The m-WO3/C catalyst reveals a much higher HOR current density than that of the pure m-WO3 catalyst, and the HOR current density can be further enhanced after the addition of a certain amount of carbon black (m-WO3/C-10C), which is not only much higher than that of the m-WO3/C catalyst, but also than that of the 20 %Pt/C catalyst. The peak curChemSusChem 2014, 7, 135 – 145
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WO3/C-C composites, which enhances the electrocatalytic activity of the anode for the HOR. The exchange current density (i0) of the mesostructured mWO3/C catalyst for hydrogen oxidation was calculated according to Tafel curves (Figure S3 in the Supporting Information). A linear Tafel region can be obtained in the range of 0.1– 0.25 V and i0 can be calculated according to the Tafel equation;[24] results are shown in Table 2. The i0 value at the mWO3/C electrode is 140.6 mA cm2, which is significantly larger than that at the pure m-WO3 electrode (111.7 mA cm2); this is mainly due to the higher EC of the former. After the addition of 10 % carbon black, the i0 value of the obtained m-WO3/C-10C electrode is remarkably enhanced to 198.6 mA cm2, which is almost equal to that of commercial the 20 wt %Pt/C-ETK electrode (198.8 mA cm2). The cathode catalysts, mesostructured Pt/WC composites, show high and stable electrochemical catalytic activity in O2saturated acidic solutions (Figure 6). The m-20Pt/WC composite still retains an electroFigure 4. TEM images of samples A) m-10Pt/WC, B) m-30Pt/WC, and C) m-40Pt/WC, as well as the corresponding chemically active surface area particle size distribution histograms (A-1–C-1); D) high-resolution (HR) TEM image of m-20Pt/WC. (ECSA) of 56 m2 g1, a mass activity of 0.082 A mg1 Pt, and a specific activity of after being 146 mA cm2 Pt rent densities obtained from linear sweep voltammetry at 0.3 V cycled for 30 000 times between 0.6 and 1.0 V (Table S2 in the are 6.0, 8.8, and 11.0 mA cm2 for m-WO3, m-WO3/C, and mSupporting Information); these values are 90, 77, and 84 % of the corresponding initial values, respectively. Comparatively, WO3/C-10C catalysts, respectively, in comparison with the curthe corresponding parameters of 40Pt/C-E remain at 33, 28, rent density of 6.8 mA cm2 for 20Pt/C-E catalyst peaked at and 85 % of their initial values, respectively, under identical test around 0.2 V (Table 2). In other words, the HOR current density conditions; these values are much lower than those of the mof the m-WO3/C-10C catalyst is 62, 83, and 25 % higher than 20Pt/WC composite. Moreover, the ECSA, mass activity, and those of 20 %Pt/C-ETK, m-WO3, and m-WO3/C, respectively, on specific activity of mesostructured m-20Pt/WC remain at 81, a catalyst weight basis. The mass activity of 20 %Pt/C-ETK for 50, and 61 %, respectively, of the corresponding initial values the HOR is 97.1 mA mg1 Pt, whereas there is no Pt used in the even after being held at 1.4 V for 200 h. This indicates that the mesostructured m-WO3/C composite at all, which is very imm-20Pt/WC composite has stable electrocatalytic activity for portant in the development of nonprecious-metal anode cataoxygen reduction, which is important in the cell electrode aplysts. Together with the EC test results, the high performance plication. of the m-WO3/C-10C catalyst stems evidently from both the reversible formation of the HxWO3 structure and enhanced EC achieved by the addition of the carbon component in the
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Figure 5. Linear sweep voltammetries of m-WO3 (A), m-WO3/C (B), m-WO3/C-10C (C), and 20Pt/C-ETK (D) in N2- and H2-saturated 0.5 m H2SO4 electrolytes at ambient solution temperature and 0.1 MPa under a potential scan rate of 0.05 Vs1.
Table 2. Exchange current density (i0), specific activity (SA), and mass activity (MA) of different kinds of anode electrodes in hydrogen oxidation. Electrodes
i0 [mA cm2]
SA [mA cm2][a]
MA [mA mg1 Pt][b]
m-WO3(N2) m-WO3(H2) m-WO3/C(N2) m-WO3/C(H2) m-WO3/C-10C(N2) m-WO3/C-10C(H2) 20Pt/C-ETK (N2) 20Pt/C-ETK (H2)
– 111.7 – 140.6 – 198.6 – 198.8
4.2 6.0 6.8 9.0 9.1 11.0 5.6 6.8
– – – – – – 80 97.1
[a] Intrinsic activity of the catalyst normalized to the electrochemically active surface areas. [b] Intrinsic activity of the platinum site normalized to the mass of platinum.
Figure 6. CV curves of the mesostructured m-20Pt/WC composite in O2-saturated 0.1 m HClO4 electrolyte by using the rotating-disk electrode (RDE) under a scan rate of 0.02 Vs1 at 25 8C and 0.1 MPa.
ORR activity tests To assess the ORR catalytic activity of the catalysts, samples were first loaded onto the glassy carbon electrode for CV measurements in O2- versus N2-saturated 0.5 m H2SO4 (Figure 7 A). The m-40Pt/WC catalyst showed a much higher cathodic current than that of the m-20Pt/WC catalyst because of the high platinum loading amount of the former. The ORR onset potential and peak potential of both samples are 0.95 and 0.81 V, respectively; these values are 50 mV more positive than 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
those of the commercial Pt/C-E catalyst (onset potential of 0.9 V and peak potential of 0.7 V vs. a reversible hydrogen electrode (RHE)), respectively, suggesting that the mesoporous WC support can assist Pt in enhancing the catalytic activity of mPt/WC for the ORR. RDE measurements were then used to reveal the ORR kinetics of mesostructured Pt/WC composites in O2-saturated 0.5 m H2SO4 under a rotation rate of 1600 rpm (Figure 7 B). The m-40Pt/WC catalyst outperformed other catalysts, including m-10Pt/WC, m-20Pt/WC, and m-30Pt/WC, in ChemSusChem 2014, 7, 135 – 145
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Figure 7. Mesostructured Pt/WC composites as oxygen reduction catalysts: A) CV curves of m-20Pt/WC, m-40P/WC, and 40wt %Pt/C-E on glassy carbon electrodes in O2- (solid lines) or N2-saturated 0.5 m H2SO4 (dashed lines). B) Rotating-disk voltammograms of mesostructured m-Pt/WC composites in O2-saturated 0.5 m H2SO4 under a sweep rate of 10 mV s1 at 1600 rpm. C) Rotating-disk voltammograms of the m-40Pt/WC composite in O2-saturated 0.5 m H2SO4 under a sweep rate of 10 mV s1 at the different rotation rates indicated. D) The Koutecky–Levich plots (J1 versus w1/2) of m-40Pt/WC composite at different potentials.
terms of disk current density and half-wave potential. The ORR catalytic activity of the m-40Pt/WC composite at different rotating rates was recorded (Figure 7 C, which exhibits the increasing disk current density at elevated rotation rate). The Koutecky–Levich plots in Figure 7 D reveal the linearity between J1 and w1/2 and approximate parallelism of the linear curves; this suggests first-order reaction kinetics toward the concentration of dissolved oxygen and similar electron-transfer numbers during the ORR at different potentials.[25] The electron transfer number (n) was calculated to be approximately 3.9 at 0.6–0.75 V from the slopes of Koutecky–Levich plots;[26] this suggests that the mesostructured 40Pt/WC composite favors a four-electron oxygen reduction process, which is similar to the ORR catalyzed by a commercial 40Pt/C-E catalyst measured in the same 0.5 m H2SO4 electrolyte (n 4.0 for Pt/C-E; see Figure S4 in the Supporting Information). Single cell tests Single cell test results at 50 8C with 20Pt/C-E as the anode and mesostructured m-Pt/WC composites as the cathodes are shown in Figure 8 A. The cell power output with m-30Pt/WC as the cathode catalyst reaches 314 mW cm2, which is higher than that obtained when using commercial 40Pt/C-E as the cathode (300 mW cm2). When using m-40Pt/WC as the cath 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ode catalysts, the cell power output reaches 330 mW cm2, which is 10 % higher than that of the commercial 40Pt/C-E cathode. Figure 8 B gives the cell test results when using the mesostructured m-WO3/C composites as the anode catalysts and m-40Pt/WC as the cathode at 50 8C. The Pt-free anode demonstrated an apparent power output, although the power density was lower than that with 20Pt/C-E as the anode. Furthermore, when using the m-WO3/C-C composites as anodes, the cell current and power output could be enhanced effectively, owing to contributions from the largely enhanced EC of the m-WO3/C-C composites after the addition of carbon black. An optimal external carbon addition amount is 10 wt % in the present study. Single cells with m-WO3/C-C as the anodes and m-40Pt/WC as the cathode showed a power density of 280 mW cm2, which was 93 % of that obtained with commercial electrodes (20Pt/C-E as the anode and 40Pt/C-E as the cathode). Figure 9 gives the results for cells operating at various working temperatures with m-WO3/C as the anode and m-40Pt/WC as the cathode, and E-TEK electrodes as references. The cell power output increased remarkably when the working temperature was raised from 50 to 70 8C. Such enhancements suggest that both the electrochemical oxidation of hydrogen by the mWO3/C anode catalyst and reduction of oxygen by the Pt/WC cathode catalyst can be accelerated at an elevated cell working ChemSusChem 2014, 7, 135 – 145
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Figure 8. A) Single PEMFC tests at 50 8C with commercial 20Pt/C-E as the anode catalyst and m-Pt/WC as the cathode catalysts. B) Single PEMFC tests at 50 8C with m-WO3/C as the anode catalyst and m-Pt/WC as the cathode catalysts.
temperature, resulting in enhanced current density and power output of the single cells. The power density of the cell with commercial electrodes reaches 376 mW cm2, and that with the 20Pt/C E-TEK anode and 40Pt/WC cathode reaches 413 mW cm2. More significantly, the maximum power output of the cell with m-WO3/C-10C as the anode and m-40Pt/WC as the cathode reaches 350 mW cm2 at 70 8C, which is 93.3 % of that obtained when using commercial electrode catalysts (20Pt/C-E as the anode and 40Pt/C-E as the cathode, respectively); this is important progress for the application of nonprecious-metal anode and carbon-free cathode catalysts. The above single cells were operated continuously at 500 mA for 250 h at 50 8C to test the electrochemical catalytic stability (anticathode carbon corrosion) of the mesostructured cathode catalysts, and the results are shown in Figure 10. The cell potential with commercial 20Pt/C-E as the anode and 40Pt/C-E as the cathode is 0.37 V at the test end, which is around 71 % of the initial cell potential. Whereas for the cell with m-WO3/C as the anode and m-40Pt/WC as the cathode, the end test cell potential retains at 0.45 V, which is 90 % of the initial value and far higher than that of the cell with commercial cathode catalysts; this implies much greater cell durability of the carbon-free m-40Pt/WC cathode catalyst than that of the commercial one and indicates that the carbon support may be oxidized slowly under the oxidative atmosphere of the cathode chamber, which results in the decrease in cell voltage. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 9. A) Cell voltage versus current plots of single PEMFC tests at different working temperatures with various combinations of mesostructured or commercial E-TEK catalysts as the anodes. B) The corresponding power density output versus current plots.
Figure 10. Cell voltage versus time plots of single PEMFCs with mesostructured or standard E-TEK catalysts as electrodes under continuous operation at 50 8C for 250 h at a constant current output of 500 mA.
Moreover, the cell voltage and power output obtained when using CO-containing hydrogen (1 vol % CO + 99 vol % H2) as the anode fuel were also tested to investigate the CO-tolerance ability of the anode catalysts. Figure 11 gives the test results of the cells operating continuously at 500 mA for 95 h under 50 8C by using CO-containing hydrogen as the anode fuel. The voltage output of the cell with m-WO3/C as the anode and m-40Pt/WC as the cathode does not show a signifiChemSusChem 2014, 7, 135 – 145
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www.chemsuschem.org outputs to those of standard PEMFCs with commercial E-TEK Pt/C catalysts as the electrodes; these outputs were no lower than 93 % (power density) of those of the commercial catalysts at 50 and 70 8C. More importantly, greatly enhanced durability, relative to standard PEMFCs, was achieved due to the CO-poisoning-free and carbon-corrosion-free features of the mesostructured WO3/C (Pt-free) anode and Pt/WC (carbon-free) cathode, respectively, after being operated at 50 8C for as long as 250 h. The present PEMFC, with a noble-metal-free anode and a carbon-free cathode, is expected to possess a durable performance during employment in addition to cost effectiveness.
Experimental Section Preparation of the materials
Figure 11. Cell voltage versus time plots of assembled single PEMFCs (A) and standard PEMFCs (B) employing mesostructured or standard E-TEK catalysts as electrodes under continuous operation at 50 8C for 95 h at a constant current output of 500 mA with pure hydrogen and CO-containing hydrogen (1 % CO + 99 % H2) as the anode fuels.
cant decline compared with that using pure hydrogen as the anode fuels over the whole time course of the cell test (Figure 11 A), whereas the cell with commercial E-TEK electrodes showed a gradually decreasing voltage output and the end cell voltage (0.4 V) was only 76 % of that obtained when using pure hydrogen as the anode fuel (0.52 V; Figure 11 B). These results indicate excellent CO tolerance (no CO poisoning) of mWO3/C as the anode compared with the remarkable Pt catalyst poisoning of the commercial 20Pt/C-ETK anode. The high CO tolerance of the m-WO3/C anode and the high anticarbon corrosion property of the m-40Pt/WC cathode mean that the PEMFCs possess high and comparable electrochemical performance and, more importantly, much more durable performance than those with commercial Pt/C-based electrodes, and consequently, a prolonged lifetime is expected in service, in addition to significantly lower Pt consumption; these points are vitally important in the future wide application of PEMFCs.
Conclusions New types of mesostructured electrode catalysts for PEMFCs with greatly enhanced durability and lower cost have been designed and successfully fabricated. The assembled PEMFCs with mesostructured electrodes exhibited comparable power 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Mesoporous WO3/C and WC were prepared from almost the same starting materials and through a similar replicating approach by using mesoporous silica as a hard template and the copolymer EO20PO70EO20 (Pluronic P123, BASF; EO = ethylene oxide, PO = propylene oxide) present in situ in the mesoporous network of mesoporous silica, which was actually the soft template for synthesizing the mesoporous silica template, as the carbon sources. Briefly, as shown in Scheme 1, the KIT-6-type mesoporous silica template was first prepared according to the published procedure by using P123 as the soft template,[19] then the as-synthesized copolymer P123containing KIT-6 (P123@KIT-6) was directly used as a hard template without removing the copolymer; this was different from the conventional template-replicating method in which the soft template must be removed from the mesostructure. Mesoporous WO3/C was obtained by route A—in situ carbonization with carbon present in situ in the pore wall of mesoporous WO3 as the carbon source. Mesoporous WC and mesostructured Pt/WC composite materials were then obtained by route B—in situ carbidation followed by homogeneous Pt nanoparticle dispersion in the pore structure of the mesoporous WC. Typically, PW12 (2.5 g) was dissolved in ethanol (5 mL) and as-synthesized P123-containing KIT-6 powder (1.0 g) was added in a sealed vessel under vacuum, then the tungsten precursor solution was cast into the pore channels of the template powder at ambient pressure. After being held under continuous vacuum for 10 min to ensure thorough tungsten precursor impregnation, the mixture was aired at room temperature overnight to allow evaporation of ethanol. One part of the resultant PW12@P123@silica composite was calcined at 500 8C for 2 h at this temperature under a flow of N2 gas, which gave a decomposed product of tungsten trioxide inside the silica template as well as in situ carbonized P123. The silica template was removed in a 2 m solution of HF. This process yielded a mesoporous WO3/C composite with carbon formed in situ present in the mesoporous network of WO3 ; this product was named m-WO3/C. To further increase the EC of the sample, 5, 10, and 15 wt % carbon black were added to as-prepared m-WO3/C under ultrasonication, and the obtained composites were named m-WO3/ C-5C, m-WO3/C-10C, and m-WO3/C-15C, respectively. The sample prepared by the same procedure with P123 removed and KIT-6 as the template was named m-WO3. Meanwhile, the other part of the resultant PW12@P123@silica composites was heated in a tube furnace from room temperature to 650 8C at 10 8C min1 and held at 650 8C for 3 h under a mixed gas flow of Ar/H2 (H2/Ar = 5:95), then heated again from 650 to 750 8C at 3 8C min1 and held at 750 8C for 3 h under the same mixed gas flow of Ar/H2, finally the samples were cooled to room temperature ChemSusChem 2014, 7, 135 – 145
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under argon. This process led to the carbidation of PW12 by P123 and the formation of mesoporous WC. Subsequently, the mesostructured Pt/WC composite was prepared by an impregnation method. Briefly, as-prepared mesoporous WC powder (0.3 g) was dispersed ultrasonically for 15 min in distilled water then a solution of H2PtCl6 (9.4 mg mL1 Pt) was added to the slurry under stirring. An excess 0.5 m solution of sodium borohydride was added dropwise slowly to the homogeneous slurry at room temperature and the reaction was continued for 12 h under stirring. The deposit was collected by centrifugation and dried at 25 8C under vacuum overnight. The composites were named m-10Pt/WC, m-20Pt/WC, m-30Pt/WC, and m-40Pt/WC.
lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky–Levich equation [Eq. (4)]:[20] 1 1 1 1 1 ¼ þ ¼ þ J JL JK Bw1=2 JK ð4Þ
B ¼ 0:62nFC 0 D0 2=3 n1=6 JK ¼ nFkC 0
in which J is the measured current density; JK and JL are the kinetic and diffusion-limiting current densities, respectively; w is the angular velocity; n is transferred electron number; F is the Faraday constant; C0 is the bulk concentration of O2 ; n is the kinematic viscosity of the electrolyte; and k is the electron-transfer rate constant.
Structural characterization The powder XRD patterns were recorded on a Rigaku D/Max-2200 PC diffractometer at 40 kV and 40 mA (CuKa radiation). TEM was performed by using a JEOL 2010 electron microscope operated at 200 kV. EDS was collected from an attached Oxford Link ISIS energy-dispersive spectrometer fixed on a JEM-2010 electron microscope operated at 200 kV. Field-emission scanning electron microscopy (FE-SEM) was performed on a JEOL JSM-6700F field-emission scanning electron microscope. N2 sorption isotherms were obtained on a Micromeritics ASAP2020M instrument at 77 K under continuous adsorption conditions. Thermogravimetry differential thermal analysis (TG-DTA) was performed by using a Netzsch STA449C instrument. The EC values of the prepared samples were measured by using an ohmmeter. The samples were pressed into discs of f= 10 1 mm at 4 MPa and the EC, k, was calculated by using Equation (3): k¼
L RS
ð3Þ
in which L is the length of the disc, R is the measured resistance, and S is the round area of the disc.
Cyclic voltammetry Electrochemical measurements were carried out on a CHI660A electrochemical workstation with a three-electrode cell. Glassy carbon disks of 6 mm in diameter (0.283 cm2) served as the substrate for the catalyst materials. Catalyst ink composed of 5 mg of sample, 25 mL of Nafion solution (5 %), and 1 mL of solvent (ethanol/water = 1:1, v/v), was dispersed ultrasonically for 20 min and a 20 mL aliquot was transferred onto the glassy carbon substrate, yielding a catalyst loading level of 0.35 mg cm2.The catalysts were characterized by CV measurements at room temperature with platinum wire and saturated Ag/AgCl/3 m KCl as the counter and the reference electrodes, respectively. All potentials in this work were referenced to a RHE. For the cathode catalyst, the electrolyte was saturated with oxygen by bubbling O2 prior to each experiment. A flow of O2 was maintained over the electrolyte during CV measurements to ensure continued O2 saturation.
RDE measurements The working electrode was prepared by the same method as that used for CV. The working electrode was scanned cathodically at a rate of 10 mV s1 with a varying rotating speed from 400 to 2025 rpm. Koutecky–Levich plots (J1 vs. w1/2) were analyzed at various electrode potentials. The slopes of the best linear fitting 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Single cell tests The cell tests were carried on the electronic load (Kikusui PLZ164WA, Japan) and carbon paper (GDL-10BC, SGL, Germany) was used as the gas-diffusion electrode. First, the catalysts were loaded on the carbon papers by a brushing method under the control of an instrument (KTQ-II). After the desired catalyst loading (0.35 mg cm2) was achieved, a thin layer of 5 wt % Nafion solution was sprayed onto the surface of each electrode (1.0 mg cm2). The pretreated Nafion 212 membrane was sandwiched between the two gas-diffusion electrodes and hot-pressed into a membrane electrode assembly (MEA; 1.0 cm 1.0 cm) at 6 MPa and 125 8C for 2 min. H2 with a flow rate of 30 sccm and O2 with a flow rate of 60 sccm were imported into the anode and cathode chambers, respectively, of the cell. For comparison, the commercial catalysts (USA, E-TEK) 20 wt %Pt/XC-72R (named 20Pt/C-ETK) and 40 wt %Pt/ XC-72R (named 40Pt/C-E) were used as the anode (0.35 mg cm2) and cathode (0.35 mg cm2) catalysts, respectively, under identical operation conditions, and the actual Pt metal loading amounts were 0.07 and 0.14 mg cm2, respectively.
Acknowledgements We gratefully acknowledge financial support from the National Key Basic Research Program of China (2013CB933200), the National Natural Science Foundation of China (51202278, 21177137), the National Natural Science Foundation of Shanghai (12ZR1435200), and the Key Program for Science and Technology Commission of Shanghai Municipality (11JC1413400). Keywords: electrochemistry · heterogeneous catalysis mesoporous materials · supported catalysts · tungsten
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Received: October 10, 2013 Published online on December 30, 2013
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Mesostructured Platinum-Free Anode and Carbon-Free Cathode Catalysts for Durable Proton Exchange Membrane Fuel Cells Xiangzhi Cui, Jianlin Shi,* Yongxia Wang, Yu Chen, Lingxia Zhang, and Zile Hua[a] As one of the most important clean energy sources, proton exchange membrane fuel cells (PEMFCs) have been a topic of extensive research focus for decades. Unfortunately, several critical technique obstacles, such as the high cost of platinum electrode catalysts, performance degradation due to the CO poisoning of the platinum anode, and carbon corrosion by oxygen in the cathode, have greatly impeded its commercial development. A prototype of a single PEMFC catalyzed by a mesostructured platinum-free WO3/C anode and a mesostruc-
tured carbon-free Pt/WC cathode catalysts is reported herein. The prototype cell exhibited 93 % power output of a standard PEMFC using commercial Pt/C catalysts at 50 and 70 8C, and more importantly, CO poisoning-free and carbon corrosion-resistant characters of the anode and cathode, respectively. Consequently, the prototype cell demonstrated considerably enhanced cell operation durability. The mesostructured electrode catalysts are therefore highly promising in the future development and application of PEMFCs.
Introduction Proton exchange membrane fuel cells (PEMFCs) are currently among the most important and attractive clean energy sources as an alternative to traditional energy conversion technologies. However, PEMFCs are still facing a number of vital and unavoidable challenges, including high cost due to the indispensible use of noble metals, such as platinum as the electrode catalysts; limited energy conversion efficiency; and, even more importantly, serious catalytic performance degradation due to platinum poisoning in anodes by CO impurities in the hydrogen source and carbon corrosion by oxygen in the cathodes, which leads to unsatisfactory durability and short lifetimes of the PEMFCs. Pt/C is an active electrocatalyst for hydrogen electrooxidation and is used widely in the application of PEMFCs as anode catalysts. Unfortunately, the catalyst surface of Pt is easily covered by the adsorbed CO molecules, even at very low levels of CO (e.g., 10 ppm), resulting in Pt poisoning and consequent catalytic activity loss.[1] In recent years, Pt-based and CO-tolerant composite anode catalysts, including PtM/C (M = Ru,[2] Pd,[3] Co,[4] Sn,[5] Ir[6]) and PtMOx/C (M = RuO2,[7] SnO2,[8] WO3[9]), have been reported in the literature by several groups. Among these composite catalysts, PtWO3/C has proved to be one of the most promising CO-tolerant anode materials for PEMFCs or direct methanol fuel cells (DMFCs), which results from the spe[a] Dr. X. Cui, Prof. J. Shi, Dr. Y. Wang, Dr. Y. Chen, Dr. L. Zhang, Dr. Z. Hua State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics Chinese Academy of Sciences, 1295 Dingxi Road Shanghai 200050 (P.R. China) Fax: (+ 86) 21-52413122 E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301079.
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cial electrochemical property of WO3 and the hydrogen overflow effect between Pt and WO3.[9] Nevertheless, the precious metal platinum is still necessary for these platinum-based composite catalysts mentioned above. Considering the rarity of precious metals such as platinum, ruthenium, and palladium on earth, searching for and developing noble-metal-free, costeffective anode electrocatalysts with substantially enhanced CO tolerance is one of the most urgent missions, but remains a great challenge in the large-scale application of PEMFCs. On the other hand, active carbon, which is the most widely used catalyst support material due to its high surface area and good electric conductivity (EC), is unfortunately subject to easy corrosion in the strongly acidic working conditions of PEMFCs, especially under a high oxygen content and high potential in the cathode chamber. According to thermodynamics, the carbon support would be oxidized to form surface oxides when the potential is higher than 0.207 V, which will lead to the significant loss of carbon and performance degradation of the cathodes.[10] Therefore, searching for carbon-free cathode catalysts with a strong antioxidation capability and high electron conductivity is of great importance for durable cathodes for PEMFCs. Tungsten carbide (WC), which is relatively low cost and has an especially high resistance to chemical erosion and catalyst poisoning, has attracted much attention in the electrochemical catalysis field.[11] Chhina et al. reported that WC material as an oxidation-resistant catalyst support was more thermally and electrochemically stable than currently used carbon support materials such as Vulcan XC-72R.[12] Subsequently, Yin et al. loaded PdFe nanoparticles on WC material and obtained an alcohol-tolerant PdFe/WC composite electrocatalysts for oxygen reduction reactions (ORRs).[13] Following this, Elezovic´ et al. used nanostructured WC as a support to load Pt nanoparticles, and they found that the resulting Pt/WC was a beneficial ChemSusChem 2014, 7, 135 – 145
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CHEMSUSCHEM FULL PAPERS catalyst for the ORR; this further proved the possibility of WC as a cathode catalyst support material.[14] Unfortunately, WC material prepared by conventional methods usually shows rather low surface areas (< 30 m2 g1) compared with active carbon supports; this is likely to be responsible for the unsatisfactory catalytic activity for oxygen electroreduction of the cathodes reported due to the relative low dispersion of Pt nanoparticles on these WC supports. In recent years, mesoporous materials have been widely studied in the development of catalytic systems because of their ordered pore structures and high surface areas.[15] Although mesoporous WO3 materials synthesized by soft- or hard-templating routes have been reported in the literature,[16] the hydrogen electrochemical oxidation properties of this material, especially of the nonprecious-metal-supported mesoporous WO3, have rarely been reported. In our previous work,[17] we reported that mesoporous WO3 showed significant electrochemical catalytic activity for hydrogen electro-oxidation and its catalytic activity could be further enhanced after an appropriate amount of carbon black was added to the mesoporous WO3 material by a physical mixing method. Regretfully, the electrochemical activity of mesoporous WO3/C was much lower than that of 20 wt % Pt/C catalyst because of the poor EC of the former, since no carbon black is expected to be able to enter the pore structure of mesoporous WO3 only by physical mixing and the electrons generated during the hydrogen oxidation by WO3 cannot be conducted out effectively. On the other hand, no reports on mesoporous WC or Pt-supported mesoporous WC used as cathode catalysts in PEMFCs can be found, although the synthesis of mesoporous WC has been reported by Wu et al.,[18] and non-mesoporous WC supports have been employed in cathodes of PEMFCs.[12–14] In the present work, we synthesized an electrically conductive m-WO3/C composite with carbon present in situ in the pore structure of mesoporous WO3 by route A (Scheme 1) and a high-surface-area mesoporous Pt/WC composite with Pt nanoparticles dispersed homogeneously in the pore structure of mesoporous WC by route B. These synthetic routes are different from those reported in the literature,[16, 18] and feature products with a well-defined mesoporous structure, simplicity, and high electrochemical performances, as shown in the fol-
www.chemsuschem.org lowing sections. In the m-WO3/C composite, the two interpenetrating components on the nanoscale ensure its high EC, which is essential when it is used as an anode catalyst; in the mesostructured Pt/WC composite, Pt nanoparticles are highly dispersed and supported in the WC mesoporous network and both the high catalytic activity and electron conductivity can be guaranteed when used as cathode catalysts. We report herein, for the first time, to the best of our knowledge, the uses of Pt-free m-WO3/C as an anode catalyst and carbon-free mesostructured Pt/WC composite as a cathode catalyst to fabricate a PEMFC. The single cell shows highly comparable current density and power output to that obtained by using commercial E-TEK Pt/C catalysts at a working temperature of 70 8C and, more importantly, much enhanced stability in electrochemical performance due to the almost CO-poisoning-free feature of the m-WO3/C anode and the corrosion resistance of the carbon-free mesostructured Pt/WC cathode; this marks important progress in the future wide application of PEMFCs.
Results and Discussion Structural and chemical properties of the as-prepared catalytic materials
The XRD pattern of the prepared Pt-free anode catalyst mWO3/C can be well indexed to tungsten trioxide (WO3 ; JCPDS card no. 46-1096; Figure 1 A, curve a) and the carbon characteristic peaks are almost undetectable; this indicates that the framework is composed of well-crystallized WO3 and the carbon remains amorphous in the mesostructure of WO3/C. The XRD pattern of the carbon-free cathode support m-WC can be clearly indexed to WC (JCPDS card no. 25-1047; Figure 1 A, curve b). After being loaded with Pt nanoparticles, the characteristic diffraction peaks of Pt metal can be also identified in addition to those of WC and the average crystallite sizes of Pt were found to be around 4.8, 5.1, 5.6, and 6.2 nm for the samples m-10Pt/WC, m-20Pt/WC, m-30Pt/WC, and m40Pt/WC, respectively, by using Debye–Scherer equation.[21] Moreover, according to the low-angle XRD patterns (Figure 1 B), template KIT-6 exhibits three well-resolved diffraction peaks of the (211), (220), and (321) planes of cubic symmetry (Ia3d space group). The samples m-WO3/C and m-WC all clearly show similar characteristic lowangle diffraction peaks of the (211) plane; this suggests that the m-WO3/C and m-WC products have retained the ordered mesoporous structure with the cubic Ia3d symmetry of the template. After being loaded with Pt nanoparticles, the obtained m-10Pt/WC sample still shows the characteristic lowScheme 1. The in situ carbonization replication syntheses of mesoporous WO3/C (m-WO3/C; route A) and mesoangle (211) diffraction peak, structured Pt/WC composite (route B) by using the block copolymer P123 present in the pore channels of a mesowhich can no longer be detectporous silica template as the carbon source. For convenience, the common porous structure of the silica parent ed for samples m-30Pt/WC and compound is given. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org m-40Pt/WC at increased Pt loading amounts within the mesostructure. According to the N2 sorption isotherms (Figure 2) and the corresponding pore structure parameters (Table 1), the prepared electrode materials show typical characteristics of mesoporous transition-metal oxides replicated by the wet-impregnation pathway from mesoporous silica.[22] Sample m-WO3/C shows a type IV hysteresis loop (Figure 2 A), and relative to that of the silica template (inset in Figure 2 A), the m-WO3/C composite shows a pore size distribution centered at about 4.4 nm, which matches well with the wall thickness (4.5 nm) of KIT-6. The Brunauer–Emmett–Teller (BET) specific surface area of as-prepared m-WO3/C is 108 m2 g1 and the wall thickness
Table 1. The pore structure parameters of the prepared m-WO3/C and mPt/WC composites. Sample
BET surface area [m2 g1]
BJH pore size [nm]
Thickness of wall[a] [nm]
498 6.8 4.5[b] 108 4.4 5.6[b] 143 4.4, 12.5[c] 138 4.1, 10 128 4.2, 11.4 115 3.9, 12.0 113 3.7, 12.0 p [a] Thickness of pore wall = 2d211/ 3BJH (Barrett–Joyner–Halenda) pore size. [b] Denotes the diameter of WO3 nanorods. [c] Bimodal pore size distribution maximized at 4.5 and 12.5 nm, respectively. KIT-6 raw m-WO3/C m-WC m-10Pt/WC m-20Pt/WC m-30Pt/WC m-40Pt/WC
Figure 1. A) High-angle XRD patterns of samples m-WO3/C (a), m-WC (b), m10Pt/WC (c), m-20Pt/WC (d), m-30Pt/WC (e), and m-40Pt/WC (f); the numbers before Pt indicate the amounts of Pt loaded by weight percentage. B) The corresponding low-angle XRD patterns.
XRD result [nm] d211 = 9.781 d211 = 9.781
Figure 2. Nitrogen sorption isotherms of A) sample m-WO3/C and the mesoporous silica template (inset), and C) mesostructured m-Pt/WC composites. B, D) The corresponding pore size distribution curves.
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CHEMSUSCHEM FULL PAPERS (diameter of WO3 nanorods or nanoparticles) is 5.6 nm; this value is a little smaller than the pore size of the silica template (6.8 nm) due to WO3 wall contraction during replication. Both m-WC and m-Pt/WC composites exhibit similar bimodal pore size distributions with maxima at 4 and 12 nm (Figure 2 D), respectively. The BET surface area of m-WC reaches 143 m2 g1, which decreases slightly after loading Pt nanoparticles in/on the mesoporous WC support. A typical SEM image (Figure 3 A) shows that the morphology of the m-WO3/C replica is disk-like with a diameter of around 500 nm and a thickness of about 100 nm. Moreover, m-WO3/C possesses a well-ordered mesostructure when viewed along the (110) direction of cubic Ia3d symmetry (Figure 3 B), although its color is darker than that of the m-WO3 sample with-
www.chemsuschem.org ure 4 D). The smaller one of d = 0.23 nm in the mesopores can be attributed to Pt metal (d = 0.226 nm, JCPDS card no. 040802) and the other of d = 0.25 nm in the framework can be ascribed to WC (according to JCPDS card no. 25-1047); these results imply the existence of Pt nanoparticles in the pores of the mesoporous WC support. More direct evidence of Pt nanoparticle dispersion in/on the WC support is the clear Pt signals easily detected by EDS (Figure S1 b in the Supporting Information) on the m-20Pt/WC composite sample. However, some of the Pt nanoparticles may exist outside of the mesoporous network of WC or aggregate together when the Pt loading reaches 40 wt %, as shown in the selected area in Figure 4 C. Figure 4 A-1–C-1 gives the corresponding platinum particle size distribution histograms. The average diameters of 4.65 (for m10Pt/WC), 5.65 (for m-30Pt/WC), and 6.3 nm (for m-40Pt/WC) and relatively narrow particle size distributions were determined; this is in good agreement with the values calculated by using the Scherer formula.
Electrochemical performance of the mesostructured catalyst materials Cyclic voltammetry tests In cyclic voltammetry (CV) tests, m-WO3/C shows a clear hydrogen oxidation peak at about 0.35 V in positive scans, and the peak current of hydrogen oxidation is even higher than that of the commercial catalyst 20Pt/C-ETK (Figure S2 A in the Supporting Information). Moreover, m-WO3/C also shows stable electrochemical catalytic activity of the anode (Figure S2 B in the Supporting Information). This is due to the formation of a hydrogen tungsten bronze compound (HxWO3) in the acidic environment of the PEMFC by hydrogen intercalation/de-intercalation into/out of the WO3 [Eqs. (1) and (2)]:[23] Figure 3. A) SEM and B) TEM images of sample m-WO3/C and its energy-dispersive X-ray spectrum (C); D) TEM image of m-WO3.
out carbon combination (Figure 3 C). No Si signal can be detected in the energy-dispersive X-ray spectroscopy (EDS) pattern (Figure 3 D) performed on different domains of templatefree m-WO3/C; there are only strong W and O signals, which indicates complete removal of the silica template. In addition, as-prepared mesostructured m-WO3/C shows much enhanced EC, which was measured to be 5.0 103 W1 m1, relative to mesoporous WO3. After the addition of a certain amount of external carbon, the EC values of mesostructured m-WO3/C-C composites were further enhanced (Table S1 in the Supporting Information). A typical TEM image of m-WC (Figure S1 a in the Supporting Information) shows an ordered mesoporous structure when viewed along the (100) direction. Pt nanoparticles, which appear as darker dots, are well dispersed in the pore structure of, or on, the mesoporous WC support for composites m-10Pt/ WC and m-30Pt/WC, according to the results shown in Figure 4 A and B, respectively. The HRTEM image of as-prepared m-20Pt/WC shows two dominant crystal lattice stripes (Fig 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
WO3 þ x Hþ þ x e ! Hx WO3
ð0 < x < 1Þ
WO3 þ 2y Hþ þ 2y e ! WO3y þ y H2 O
ð0 < y < 1Þ
ð1Þ ð2Þ
The reactions of mesoporous WO3 with H/H + in the electrolyte are reversible, so that mesoporous WO3 can act as a catalyst and show electrocatalytic activity for hydrogen oxidation. HxWO3 is a nonstoichiometric and electrically conducting compound capable of hydrogen intercalation/de-intercalation; this is comparable to hydrogen adsorption/desorption on platinum metal. The electrocatalytic activity of the mesostructured m-WO3/C composite anode catalyst for the hydrogen oxidation reaction (HOR) was measured and the results are shown in Figure 5. All catalysts showed much enhanced hydrogen oxidation current density in the H2-saturated electrolyte than that in the N2-saturated electrolyte. The m-WO3/C catalyst reveals a much higher HOR current density than that of the pure m-WO3 catalyst, and the HOR current density can be further enhanced after the addition of a certain amount of carbon black (m-WO3/C-10C), which is not only much higher than that of the m-WO3/C catalyst, but also than that of the 20 %Pt/C catalyst. The peak curChemSusChem 2014, 7, 135 – 145
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WO3/C-C composites, which enhances the electrocatalytic activity of the anode for the HOR. The exchange current density (i0) of the mesostructured mWO3/C catalyst for hydrogen oxidation was calculated according to Tafel curves (Figure S3 in the Supporting Information). A linear Tafel region can be obtained in the range of 0.1– 0.25 V and i0 can be calculated according to the Tafel equation;[24] results are shown in Table 2. The i0 value at the mWO3/C electrode is 140.6 mA cm2, which is significantly larger than that at the pure m-WO3 electrode (111.7 mA cm2); this is mainly due to the higher EC of the former. After the addition of 10 % carbon black, the i0 value of the obtained m-WO3/C-10C electrode is remarkably enhanced to 198.6 mA cm2, which is almost equal to that of commercial the 20 wt %Pt/C-ETK electrode (198.8 mA cm2). The cathode catalysts, mesostructured Pt/WC composites, show high and stable electrochemical catalytic activity in O2saturated acidic solutions (Figure 6). The m-20Pt/WC composite still retains an electroFigure 4. TEM images of samples A) m-10Pt/WC, B) m-30Pt/WC, and C) m-40Pt/WC, as well as the corresponding chemically active surface area particle size distribution histograms (A-1–C-1); D) high-resolution (HR) TEM image of m-20Pt/WC. (ECSA) of 56 m2 g1, a mass activity of 0.082 A mg1 Pt, and a specific activity of after being 146 mA cm2 Pt rent densities obtained from linear sweep voltammetry at 0.3 V cycled for 30 000 times between 0.6 and 1.0 V (Table S2 in the are 6.0, 8.8, and 11.0 mA cm2 for m-WO3, m-WO3/C, and mSupporting Information); these values are 90, 77, and 84 % of the corresponding initial values, respectively. Comparatively, WO3/C-10C catalysts, respectively, in comparison with the curthe corresponding parameters of 40Pt/C-E remain at 33, 28, rent density of 6.8 mA cm2 for 20Pt/C-E catalyst peaked at and 85 % of their initial values, respectively, under identical test around 0.2 V (Table 2). In other words, the HOR current density conditions; these values are much lower than those of the mof the m-WO3/C-10C catalyst is 62, 83, and 25 % higher than 20Pt/WC composite. Moreover, the ECSA, mass activity, and those of 20 %Pt/C-ETK, m-WO3, and m-WO3/C, respectively, on specific activity of mesostructured m-20Pt/WC remain at 81, a catalyst weight basis. The mass activity of 20 %Pt/C-ETK for 50, and 61 %, respectively, of the corresponding initial values the HOR is 97.1 mA mg1 Pt, whereas there is no Pt used in the even after being held at 1.4 V for 200 h. This indicates that the mesostructured m-WO3/C composite at all, which is very imm-20Pt/WC composite has stable electrocatalytic activity for portant in the development of nonprecious-metal anode cataoxygen reduction, which is important in the cell electrode aplysts. Together with the EC test results, the high performance plication. of the m-WO3/C-10C catalyst stems evidently from both the reversible formation of the HxWO3 structure and enhanced EC achieved by the addition of the carbon component in the
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Figure 5. Linear sweep voltammetries of m-WO3 (A), m-WO3/C (B), m-WO3/C-10C (C), and 20Pt/C-ETK (D) in N2- and H2-saturated 0.5 m H2SO4 electrolytes at ambient solution temperature and 0.1 MPa under a potential scan rate of 0.05 Vs1.
Table 2. Exchange current density (i0), specific activity (SA), and mass activity (MA) of different kinds of anode electrodes in hydrogen oxidation. Electrodes
i0 [mA cm2]
SA [mA cm2][a]
MA [mA mg1 Pt][b]
m-WO3(N2) m-WO3(H2) m-WO3/C(N2) m-WO3/C(H2) m-WO3/C-10C(N2) m-WO3/C-10C(H2) 20Pt/C-ETK (N2) 20Pt/C-ETK (H2)
– 111.7 – 140.6 – 198.6 – 198.8
4.2 6.0 6.8 9.0 9.1 11.0 5.6 6.8
– – – – – – 80 97.1
[a] Intrinsic activity of the catalyst normalized to the electrochemically active surface areas. [b] Intrinsic activity of the platinum site normalized to the mass of platinum.
Figure 6. CV curves of the mesostructured m-20Pt/WC composite in O2-saturated 0.1 m HClO4 electrolyte by using the rotating-disk electrode (RDE) under a scan rate of 0.02 Vs1 at 25 8C and 0.1 MPa.
ORR activity tests To assess the ORR catalytic activity of the catalysts, samples were first loaded onto the glassy carbon electrode for CV measurements in O2- versus N2-saturated 0.5 m H2SO4 (Figure 7 A). The m-40Pt/WC catalyst showed a much higher cathodic current than that of the m-20Pt/WC catalyst because of the high platinum loading amount of the former. The ORR onset potential and peak potential of both samples are 0.95 and 0.81 V, respectively; these values are 50 mV more positive than 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
those of the commercial Pt/C-E catalyst (onset potential of 0.9 V and peak potential of 0.7 V vs. a reversible hydrogen electrode (RHE)), respectively, suggesting that the mesoporous WC support can assist Pt in enhancing the catalytic activity of mPt/WC for the ORR. RDE measurements were then used to reveal the ORR kinetics of mesostructured Pt/WC composites in O2-saturated 0.5 m H2SO4 under a rotation rate of 1600 rpm (Figure 7 B). The m-40Pt/WC catalyst outperformed other catalysts, including m-10Pt/WC, m-20Pt/WC, and m-30Pt/WC, in ChemSusChem 2014, 7, 135 – 145
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Figure 7. Mesostructured Pt/WC composites as oxygen reduction catalysts: A) CV curves of m-20Pt/WC, m-40P/WC, and 40wt %Pt/C-E on glassy carbon electrodes in O2- (solid lines) or N2-saturated 0.5 m H2SO4 (dashed lines). B) Rotating-disk voltammograms of mesostructured m-Pt/WC composites in O2-saturated 0.5 m H2SO4 under a sweep rate of 10 mV s1 at 1600 rpm. C) Rotating-disk voltammograms of the m-40Pt/WC composite in O2-saturated 0.5 m H2SO4 under a sweep rate of 10 mV s1 at the different rotation rates indicated. D) The Koutecky–Levich plots (J1 versus w1/2) of m-40Pt/WC composite at different potentials.
terms of disk current density and half-wave potential. The ORR catalytic activity of the m-40Pt/WC composite at different rotating rates was recorded (Figure 7 C, which exhibits the increasing disk current density at elevated rotation rate). The Koutecky–Levich plots in Figure 7 D reveal the linearity between J1 and w1/2 and approximate parallelism of the linear curves; this suggests first-order reaction kinetics toward the concentration of dissolved oxygen and similar electron-transfer numbers during the ORR at different potentials.[25] The electron transfer number (n) was calculated to be approximately 3.9 at 0.6–0.75 V from the slopes of Koutecky–Levich plots;[26] this suggests that the mesostructured 40Pt/WC composite favors a four-electron oxygen reduction process, which is similar to the ORR catalyzed by a commercial 40Pt/C-E catalyst measured in the same 0.5 m H2SO4 electrolyte (n 4.0 for Pt/C-E; see Figure S4 in the Supporting Information). Single cell tests Single cell test results at 50 8C with 20Pt/C-E as the anode and mesostructured m-Pt/WC composites as the cathodes are shown in Figure 8 A. The cell power output with m-30Pt/WC as the cathode catalyst reaches 314 mW cm2, which is higher than that obtained when using commercial 40Pt/C-E as the cathode (300 mW cm2). When using m-40Pt/WC as the cath 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ode catalysts, the cell power output reaches 330 mW cm2, which is 10 % higher than that of the commercial 40Pt/C-E cathode. Figure 8 B gives the cell test results when using the mesostructured m-WO3/C composites as the anode catalysts and m-40Pt/WC as the cathode at 50 8C. The Pt-free anode demonstrated an apparent power output, although the power density was lower than that with 20Pt/C-E as the anode. Furthermore, when using the m-WO3/C-C composites as anodes, the cell current and power output could be enhanced effectively, owing to contributions from the largely enhanced EC of the m-WO3/C-C composites after the addition of carbon black. An optimal external carbon addition amount is 10 wt % in the present study. Single cells with m-WO3/C-C as the anodes and m-40Pt/WC as the cathode showed a power density of 280 mW cm2, which was 93 % of that obtained with commercial electrodes (20Pt/C-E as the anode and 40Pt/C-E as the cathode). Figure 9 gives the results for cells operating at various working temperatures with m-WO3/C as the anode and m-40Pt/WC as the cathode, and E-TEK electrodes as references. The cell power output increased remarkably when the working temperature was raised from 50 to 70 8C. Such enhancements suggest that both the electrochemical oxidation of hydrogen by the mWO3/C anode catalyst and reduction of oxygen by the Pt/WC cathode catalyst can be accelerated at an elevated cell working ChemSusChem 2014, 7, 135 – 145
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Figure 8. A) Single PEMFC tests at 50 8C with commercial 20Pt/C-E as the anode catalyst and m-Pt/WC as the cathode catalysts. B) Single PEMFC tests at 50 8C with m-WO3/C as the anode catalyst and m-Pt/WC as the cathode catalysts.
temperature, resulting in enhanced current density and power output of the single cells. The power density of the cell with commercial electrodes reaches 376 mW cm2, and that with the 20Pt/C E-TEK anode and 40Pt/WC cathode reaches 413 mW cm2. More significantly, the maximum power output of the cell with m-WO3/C-10C as the anode and m-40Pt/WC as the cathode reaches 350 mW cm2 at 70 8C, which is 93.3 % of that obtained when using commercial electrode catalysts (20Pt/C-E as the anode and 40Pt/C-E as the cathode, respectively); this is important progress for the application of nonprecious-metal anode and carbon-free cathode catalysts. The above single cells were operated continuously at 500 mA for 250 h at 50 8C to test the electrochemical catalytic stability (anticathode carbon corrosion) of the mesostructured cathode catalysts, and the results are shown in Figure 10. The cell potential with commercial 20Pt/C-E as the anode and 40Pt/C-E as the cathode is 0.37 V at the test end, which is around 71 % of the initial cell potential. Whereas for the cell with m-WO3/C as the anode and m-40Pt/WC as the cathode, the end test cell potential retains at 0.45 V, which is 90 % of the initial value and far higher than that of the cell with commercial cathode catalysts; this implies much greater cell durability of the carbon-free m-40Pt/WC cathode catalyst than that of the commercial one and indicates that the carbon support may be oxidized slowly under the oxidative atmosphere of the cathode chamber, which results in the decrease in cell voltage. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 9. A) Cell voltage versus current plots of single PEMFC tests at different working temperatures with various combinations of mesostructured or commercial E-TEK catalysts as the anodes. B) The corresponding power density output versus current plots.
Figure 10. Cell voltage versus time plots of single PEMFCs with mesostructured or standard E-TEK catalysts as electrodes under continuous operation at 50 8C for 250 h at a constant current output of 500 mA.
Moreover, the cell voltage and power output obtained when using CO-containing hydrogen (1 vol % CO + 99 vol % H2) as the anode fuel were also tested to investigate the CO-tolerance ability of the anode catalysts. Figure 11 gives the test results of the cells operating continuously at 500 mA for 95 h under 50 8C by using CO-containing hydrogen as the anode fuel. The voltage output of the cell with m-WO3/C as the anode and m-40Pt/WC as the cathode does not show a signifiChemSusChem 2014, 7, 135 – 145
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www.chemsuschem.org outputs to those of standard PEMFCs with commercial E-TEK Pt/C catalysts as the electrodes; these outputs were no lower than 93 % (power density) of those of the commercial catalysts at 50 and 70 8C. More importantly, greatly enhanced durability, relative to standard PEMFCs, was achieved due to the CO-poisoning-free and carbon-corrosion-free features of the mesostructured WO3/C (Pt-free) anode and Pt/WC (carbon-free) cathode, respectively, after being operated at 50 8C for as long as 250 h. The present PEMFC, with a noble-metal-free anode and a carbon-free cathode, is expected to possess a durable performance during employment in addition to cost effectiveness.
Experimental Section Preparation of the materials
Figure 11. Cell voltage versus time plots of assembled single PEMFCs (A) and standard PEMFCs (B) employing mesostructured or standard E-TEK catalysts as electrodes under continuous operation at 50 8C for 95 h at a constant current output of 500 mA with pure hydrogen and CO-containing hydrogen (1 % CO + 99 % H2) as the anode fuels.
cant decline compared with that using pure hydrogen as the anode fuels over the whole time course of the cell test (Figure 11 A), whereas the cell with commercial E-TEK electrodes showed a gradually decreasing voltage output and the end cell voltage (0.4 V) was only 76 % of that obtained when using pure hydrogen as the anode fuel (0.52 V; Figure 11 B). These results indicate excellent CO tolerance (no CO poisoning) of mWO3/C as the anode compared with the remarkable Pt catalyst poisoning of the commercial 20Pt/C-ETK anode. The high CO tolerance of the m-WO3/C anode and the high anticarbon corrosion property of the m-40Pt/WC cathode mean that the PEMFCs possess high and comparable electrochemical performance and, more importantly, much more durable performance than those with commercial Pt/C-based electrodes, and consequently, a prolonged lifetime is expected in service, in addition to significantly lower Pt consumption; these points are vitally important in the future wide application of PEMFCs.
Conclusions New types of mesostructured electrode catalysts for PEMFCs with greatly enhanced durability and lower cost have been designed and successfully fabricated. The assembled PEMFCs with mesostructured electrodes exhibited comparable power 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Mesoporous WO3/C and WC were prepared from almost the same starting materials and through a similar replicating approach by using mesoporous silica as a hard template and the copolymer EO20PO70EO20 (Pluronic P123, BASF; EO = ethylene oxide, PO = propylene oxide) present in situ in the mesoporous network of mesoporous silica, which was actually the soft template for synthesizing the mesoporous silica template, as the carbon sources. Briefly, as shown in Scheme 1, the KIT-6-type mesoporous silica template was first prepared according to the published procedure by using P123 as the soft template,[19] then the as-synthesized copolymer P123containing KIT-6 (P123@KIT-6) was directly used as a hard template without removing the copolymer; this was different from the conventional template-replicating method in which the soft template must be removed from the mesostructure. Mesoporous WO3/C was obtained by route A—in situ carbonization with carbon present in situ in the pore wall of mesoporous WO3 as the carbon source. Mesoporous WC and mesostructured Pt/WC composite materials were then obtained by route B—in situ carbidation followed by homogeneous Pt nanoparticle dispersion in the pore structure of the mesoporous WC. Typically, PW12 (2.5 g) was dissolved in ethanol (5 mL) and as-synthesized P123-containing KIT-6 powder (1.0 g) was added in a sealed vessel under vacuum, then the tungsten precursor solution was cast into the pore channels of the template powder at ambient pressure. After being held under continuous vacuum for 10 min to ensure thorough tungsten precursor impregnation, the mixture was aired at room temperature overnight to allow evaporation of ethanol. One part of the resultant PW12@P123@silica composite was calcined at 500 8C for 2 h at this temperature under a flow of N2 gas, which gave a decomposed product of tungsten trioxide inside the silica template as well as in situ carbonized P123. The silica template was removed in a 2 m solution of HF. This process yielded a mesoporous WO3/C composite with carbon formed in situ present in the mesoporous network of WO3 ; this product was named m-WO3/C. To further increase the EC of the sample, 5, 10, and 15 wt % carbon black were added to as-prepared m-WO3/C under ultrasonication, and the obtained composites were named m-WO3/ C-5C, m-WO3/C-10C, and m-WO3/C-15C, respectively. The sample prepared by the same procedure with P123 removed and KIT-6 as the template was named m-WO3. Meanwhile, the other part of the resultant PW12@P123@silica composites was heated in a tube furnace from room temperature to 650 8C at 10 8C min1 and held at 650 8C for 3 h under a mixed gas flow of Ar/H2 (H2/Ar = 5:95), then heated again from 650 to 750 8C at 3 8C min1 and held at 750 8C for 3 h under the same mixed gas flow of Ar/H2, finally the samples were cooled to room temperature ChemSusChem 2014, 7, 135 – 145
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under argon. This process led to the carbidation of PW12 by P123 and the formation of mesoporous WC. Subsequently, the mesostructured Pt/WC composite was prepared by an impregnation method. Briefly, as-prepared mesoporous WC powder (0.3 g) was dispersed ultrasonically for 15 min in distilled water then a solution of H2PtCl6 (9.4 mg mL1 Pt) was added to the slurry under stirring. An excess 0.5 m solution of sodium borohydride was added dropwise slowly to the homogeneous slurry at room temperature and the reaction was continued for 12 h under stirring. The deposit was collected by centrifugation and dried at 25 8C under vacuum overnight. The composites were named m-10Pt/WC, m-20Pt/WC, m-30Pt/WC, and m-40Pt/WC.
lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky–Levich equation [Eq. (4)]:[20] 1 1 1 1 1 ¼ þ ¼ þ J JL JK Bw1=2 JK ð4Þ
B ¼ 0:62nFC 0 D0 2=3 n1=6 JK ¼ nFkC 0
in which J is the measured current density; JK and JL are the kinetic and diffusion-limiting current densities, respectively; w is the angular velocity; n is transferred electron number; F is the Faraday constant; C0 is the bulk concentration of O2 ; n is the kinematic viscosity of the electrolyte; and k is the electron-transfer rate constant.
Structural characterization The powder XRD patterns were recorded on a Rigaku D/Max-2200 PC diffractometer at 40 kV and 40 mA (CuKa radiation). TEM was performed by using a JEOL 2010 electron microscope operated at 200 kV. EDS was collected from an attached Oxford Link ISIS energy-dispersive spectrometer fixed on a JEM-2010 electron microscope operated at 200 kV. Field-emission scanning electron microscopy (FE-SEM) was performed on a JEOL JSM-6700F field-emission scanning electron microscope. N2 sorption isotherms were obtained on a Micromeritics ASAP2020M instrument at 77 K under continuous adsorption conditions. Thermogravimetry differential thermal analysis (TG-DTA) was performed by using a Netzsch STA449C instrument. The EC values of the prepared samples were measured by using an ohmmeter. The samples were pressed into discs of f= 10 1 mm at 4 MPa and the EC, k, was calculated by using Equation (3): k¼
L RS
ð3Þ
in which L is the length of the disc, R is the measured resistance, and S is the round area of the disc.
Cyclic voltammetry Electrochemical measurements were carried out on a CHI660A electrochemical workstation with a three-electrode cell. Glassy carbon disks of 6 mm in diameter (0.283 cm2) served as the substrate for the catalyst materials. Catalyst ink composed of 5 mg of sample, 25 mL of Nafion solution (5 %), and 1 mL of solvent (ethanol/water = 1:1, v/v), was dispersed ultrasonically for 20 min and a 20 mL aliquot was transferred onto the glassy carbon substrate, yielding a catalyst loading level of 0.35 mg cm2.The catalysts were characterized by CV measurements at room temperature with platinum wire and saturated Ag/AgCl/3 m KCl as the counter and the reference electrodes, respectively. All potentials in this work were referenced to a RHE. For the cathode catalyst, the electrolyte was saturated with oxygen by bubbling O2 prior to each experiment. A flow of O2 was maintained over the electrolyte during CV measurements to ensure continued O2 saturation.
RDE measurements The working electrode was prepared by the same method as that used for CV. The working electrode was scanned cathodically at a rate of 10 mV s1 with a varying rotating speed from 400 to 2025 rpm. Koutecky–Levich plots (J1 vs. w1/2) were analyzed at various electrode potentials. The slopes of the best linear fitting 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Single cell tests The cell tests were carried on the electronic load (Kikusui PLZ164WA, Japan) and carbon paper (GDL-10BC, SGL, Germany) was used as the gas-diffusion electrode. First, the catalysts were loaded on the carbon papers by a brushing method under the control of an instrument (KTQ-II). After the desired catalyst loading (0.35 mg cm2) was achieved, a thin layer of 5 wt % Nafion solution was sprayed onto the surface of each electrode (1.0 mg cm2). The pretreated Nafion 212 membrane was sandwiched between the two gas-diffusion electrodes and hot-pressed into a membrane electrode assembly (MEA; 1.0 cm 1.0 cm) at 6 MPa and 125 8C for 2 min. H2 with a flow rate of 30 sccm and O2 with a flow rate of 60 sccm were imported into the anode and cathode chambers, respectively, of the cell. For comparison, the commercial catalysts (USA, E-TEK) 20 wt %Pt/XC-72R (named 20Pt/C-ETK) and 40 wt %Pt/ XC-72R (named 40Pt/C-E) were used as the anode (0.35 mg cm2) and cathode (0.35 mg cm2) catalysts, respectively, under identical operation conditions, and the actual Pt metal loading amounts were 0.07 and 0.14 mg cm2, respectively.
Acknowledgements We gratefully acknowledge financial support from the National Key Basic Research Program of China (2013CB933200), the National Natural Science Foundation of China (51202278, 21177137), the National Natural Science Foundation of Shanghai (12ZR1435200), and the Key Program for Science and Technology Commission of Shanghai Municipality (11JC1413400). Keywords: electrochemistry · heterogeneous catalysis mesoporous materials · supported catalysts · tungsten
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Received: October 10, 2013 Published online on December 30, 2013
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