Composite materials

Abstract

A mixed metal oxide material of tungsten and titanium is provided for use in a fuel cell. The material may comprise less than approximately 30 at. % tungsten. The mixed metal oxide may form the core of a core-shell composite material, used as a catalyst support, in which a catalyst such as platinum forms the shell. The catalyst may be applied as a single monolayer, or up to 20 monolayers.

Claims

1. A core-shell catalyst consisting of a core particle and a shell catalyst, wherein said core particle has a diameter of from 10-50 nm and comprises a mixed metal oxide material of tungsten and titanium, wherein the mixed metal oxide material comprises less than 30 atomic % tungsten, calculated on a metals only basis, and wherein said shell catalyst comprises 1 to 20 monolayers of a catalyst material.

2. The core-shell catalyst of claim 1, wherein the shell catalyst comprises platinum or platinum alloy.

3. The core-shell catalyst of claim 2, wherein the shell catalyst consists of 1 to 20 monolayers of platinum or platinum alloy.

4. The core-shell catalyst of claim 3, wherein the shell catalyst consists of 1 to 14 monolayers of platinum or platinum alloy.

5. The core-shell catalyst of claim 4, wherein the shell catalyst consists of 1 to 6 monolayers of platinum or platinum alloy.

6. A fuel cell comprising the core-shell catalyst of claim 5.

7. The core-shell catalyst of claim 2, wherein the mixed metal oxide material comprises less than 15 atomic % tungsten.

8. The core-shell catalyst of claim 7, wherein said core particle consists of said mixed metal oxide.

9. The core-shell catalyst of claim 8, wherein the shell catalyst consists of 1 to 14 monolayers of platinum or platinum alloy.

10. The core-shell catalyst of claim 9, wherein the shell catalyst consists of 1 to 6 monolayers of platinum or platinum alloy.

11. A fuel cell comprising the core-shell catalyst of claim 10.

12. The core-shell catalyst of claim 7, wherein the mixed metal oxide material comprises from 6 to 11 atomic % tungsten.

13. The core-shell catalyst of claim 12, wherein said core particle consists of said mixed metal oxide.

14. The core-shell catalyst of claim 13, wherein the shell catalyst consists of 1 to 14 monolayers of platinum or platinum alloy.

15. The core-shell catalyst of claim 14, wherein the shell catalyst consists of 1 to 6 monolayers of platinum or platinum alloy.

16. A fuel cell comprising the core-shell catalyst of claim 15.

17. The core-shell catalyst of claim 12, wherein the mixed metal oxide material comprises from 7 to 9 atomic % tungsten.

18. The core-shell catalyst of claim 17, wherein said core particle consists of said mixed metal oxide.

19. The core-shell catalyst of claim 18, wherein the shell catalyst consists of 1 to 14 monolayers of platinum or platinum alloy.

20. The core-shell catalyst of claim 19, wherein the shell catalyst consists of 1 to 6 monolayers of platinum or platinum alloy.

21. A fuel cell comprising the core-shell catalyst of claim 20.

22. The core-shell catalyst of claim 1, wherein the titanium oxide is in a crystalline form.

23. The core-shell catalyst of claim 22, wherein the crystalline titanium oxide is anatase titanium oxide.

24. A fuel cell comprising the core-shell catalyst of claim 1.

25. A method of producing a core-shell catalyst, the method comprising: forming a core particle having a diameter of from 10-50 nm and comprising a mixed metal oxide material of tungsten and titanium, wherein the material comprises less than 30 atomic % tungsten, calculated on a metals only basis; and forming a catalytic shell layer comprising 1 to 20 monolayers of a catalyst material on the core particle.

26. The method of claim 25, wherein the mixed metal oxide material comprises less than approximately 15 atomic % tungsten.

27. The method of claim 26, wherein the mixed metal oxide material comprises from 6 to 11 atomic % tungsten.

28. The method of claim 27, wherein the mixed metal oxide material comprises from 7 to 9 atomic % tungsten.

29. The method of claim 28, wherein the shell layer consists of 1 to 14 monolayers of platinum or platinum alloy.

30. The method of claim 29, wherein the shell layer consists of 1 to 6 monolayers of platinum or platinum alloy.

31. The method of claim 27, wherein the shell layer consists of 1 to 14 monolayers of platinum or platinum alloy.

32. The method of claim 31, wherein the shell layer consists of 1 to 6 monolayers of platinum or platinum alloy.

33. The method of claim 25, wherein the shell layer consists of 1 to 20 monolayers of platinum or platinum alloy.

34. The method of claim 33, wherein the shell layer consists of 1 to 14 monolayers of platinum or platinum alloy.

35. The method of claim 34, wherein the shell layer consists of 1 to 6 monolayers of platinum or platinum alloy.

36. The method of claim 25, wherein the titanium oxide is anatase titanium oxide.

37. The method of claim 26, wherein the shell layer consists of 1 to 14 monolayers of platinum or platinum alloy.

38. The method of claim 37, wherein the shell layer consists of 1 to 6 monolayers of platinum or platinum alloy.

Description

(1) FIG. 1 is a schematic cross-section of a core-shell structure and an equivalent layer structure used to model the core shell particle behavior;

(2) FIG. 2 is a schematic representation of particle growth and nucleation onto supports with a low number of nucleation sites and an increased number of nucleation sites;

(3) FIG. 3 is a schematic view of an electrochemical array substrate;

(4) FIG. 4 is a chart showing the potential at which 1 A raw current is reached during the oxygen reduction slow cycling experiment for Pt supported on amorphous and anatase titanium oxide. Error bars are plus and minus one standard deviation;

(5) FIG. 5 is a chart showing the potential at which 3 A raw current is reached during the oxygen reduction slow cycling experiment for amorphous and anatase titanium oxide. Error bars are plus and minus one standard deviation;

(6) FIG. 6 is a chart showing an example O.sub.2 reduction slow cycling CV for 0.7 ML of Pt supported on amorphous TiO.sub.x indicating the two different potentials where ignition potentials have been compared; and

(7) FIG. 7 is a chart showing the potential at which 1 A raw current is reached during the oxygen reduction slow cycling experiment for Pt supported on anatase tungsten doped titanium oxide. Error bars are plus and minus one standard deviation.

(8) For ease of reference, the mixed metal oxide of tungsten and titanium is represented as TiWO.sub.x. However, it will be appreciated that this is not a chemical formula indicating any specific stoichiometry, but merely a shorthand indication of the elemental composition of the material and is not to be taken as limiting on the stoichiometry of the material.

(9) Thin film models of core-shell systems were produced with different platinum loadings and using titanium oxide supports with different crystallinity and varying levels of tungsten. The thin film models were tested for their activity towards the oxygen reduction reaction.

(10) Thin film models of core-shell structures (see FIG. 1) with a tungsten-doped titanium oxide core and a platinum shell have been produced, which have the same (or similar) activity for oxygen reduction as bulk platinum. Traditionally, dopants have been added to metal oxides in order to increase conductivity by creating defects (such as oxygen vacancies). However, in addition to increased conductivity, dopants can also help to create defects on the surface of the metal oxides and therefore create more sites for nucleation of a metal overlayer. This would cause smaller particles to be formed and lead to a more complete metal film at a lower equivalent thickness (see FIG. 2). A film of platinum should approach the oxygen reduction capacity of bulk platinum, since a significant number of platinum atoms will be in contact.

(11) Metal oxides also offer a stable alternative to carbon supports (which are prone to oxidative destruction) and would therefore increase the lifetime of fuel cells. One may also expect a more effective utilisation (activity per mass of platinum).

(12) The inventors found that, as the loading of platinum was decreased on all of the support materials studied, the ignition potential (see below) for the oxygen reduction reaction initially remained constant. However, below a certain critical thickness (d.sub.crit), the ignition potential (see below) started to decrease (i.e. the overpotential for the oxygen reduction reaction increases, or the electrodes are less active for the oxygen reduction reaction). At high loadings of platinum, the ignition potential was similar to a bulk platinum electrode. This suggests a core-shell model structure, where enough Pt atoms are in contact to behave as the bulk metal. As the platinum started to break into discrete particles, the ignition potential decreased.

(13) The anatase form of titanium oxide allowed the loading of platinum to be reduced further than amorphous titanium oxide before the oxygen reduction behaviour shifted away from the bulk behaviour. This occurred below approximately 5 ML equivalent thickness of Pt.

(14) The addition of tungsten to the anatase titanium oxide support showed evidence of improved activity at low loadings of platinum. This suggests that the tungsten aided the wetting of platinum onto the titanium oxide surface. That is to say, the results produced by the present inventors suggest that core-shell structures can be more easily formed when tungsten is present in the core.

EXAMPLE 1: SYNTHESIS OF SAMPLES

(15) Thin film samples were deposited using a high-throughput PVD (Physical Vapour Deposition) system to model core-shell structures (see FIG. 1b). The PVD system used is described in detail in reference 30. A range of oxide samples were synthesised onto different substrates including silicon wafers, quartz wafers and electrochemical arrays (1010 arrays of gold contact pads as represented in FIG. 3). Titanium and tungsten were deposited from electron gun (E-gun) sources. The TiWO.sub.x films were deposited in the presence of an atomic oxygen source (operating at 400 W power) with a pressure of approximately 510.sup.5 Torr. These oxide films were heated in a tube furnace to 450 C. in the presence of oxygen. Such conditions allowed the anatase structure of the undoped titanium oxide to form and would lead to high oxygen stoichiometry materials.

(16) The platinum was deposited onto the pre-deposited oxide thin films from an E-gun source. During deposition the oxide substrates were heated to 200 C. to dehydroxylate the surface. A shutter that was moved during deposition allowed different equivalent thicknesses of platinum to be deposited onto different fields. The amount of platinum deposited was calibrated by: depositing thicker films onto silicon substrates, measuring the thickness of the films by AFM (Atomic Force Microscopy), and producing a calibration curve against deposition time.

EXAMPLE 2: OXIDE CHARACTERISATION

(17) The composition of the oxide films was determined using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS, New Wave 213 nm laser & Perkin Elmer Elan 9000). This method gives the relative composition of the metallic elements, but is not capable of measuring the oxygen concentration. As the ICP-MS measurements are destructive, composition measurements were made on samples deposited onto silicon wafers. The same deposition conditions were then used to deposit onto equivalent electrochemical arrays.

(18) X-Ray diffraction (XRD) patterns were obtained using the Bruker D8 Discover diffractometer, a powerful XRD tool with a high-precision, two-circle goniometer with independent stepper motors and optical encoders for the Theta and 2 Theta circles. The D8 diffractometer system is equipped with a GADDS detector operating at 45 kV and 0.65 mA. A high intensity X-ray IS Incoatec source (with Cu K radiation) is incorporated allowing high intensity and collimated X-rays to be localised on thin film materials providing an efficient high throughput structural analysis. This analysis was carried out on oxide films deposited onto Si substrates.

EXAMPLE 3: ELECTROCHEMICAL SCREENING

(19) The high-throughput electrochemical screening equipment enables electrochemical experiments on 100 independently addressable electrodes arranged in a 1010 array in a parallel screening mode to be conducted. The equipment has been described in detail elsewhere [2, 31]. The geometric areas of the individual working electrodes on the electrochemical array are 1.0 mm.sup.2.

(20) The design of the cell and socket assembly provides a clean electrochemical environment with control of the temperature during experiments. In the experiments described, the temperature was maintained at 25 C. and a mercury/mercury sulphate (MMSE) reference electrode was used. The potential of the MMSE was measured vs. a hydrogen reference electrode prior to screening experiments and all potentials are quoted vs. the reversible hydrogen electrode (RHE). A Pt mesh counter electrode was used, in a glass compartment separated from the working electrode compartment by a glass frit. Various sweep rates were used for different experiments which are outlined in Table 1.

(21) TABLE-US-00001 TABLE 1 Electrochemical screening procedure. Potential limits/ Sweep rate/ Experiment Gas V vs. RHE mV s.sup.1 Bubbling Ar 20 min 3 CVs in deoxygenated Ar above solution 0.025-1.200 100 solution O.sub.2 saturation Bubbling Ar 60 s At 1.000 Bubbling O.sub.2 At 1.000 10 min O.sub.2 reduction steps Bubbling O.sub.2 in Step from 1.00 to 0.60 solution and back to 1.00 in 50 mV increments every 90 s 3 CVs in O.sub.2 saturated O.sub.2 above solution 0.025-1.200 5 solution Bubbling Ar 20 min 200 CVs stability testing Ar above solution 0.025-1.200 100 Bubbling with O.sub.2 for At 1.000 20 min 3 CVs in O.sub.2 saturated O.sub.2 above solution 0.025-1.200 5 solution O.sub.2 reduction steps Bubbling O.sub.2 in Step from 1.00 to 0.60 solution and back to 1.00 in 50 mV increments every 90 s Bubbling CO At 0.075 15 min Bubbling Ar At 0.075 20 min CO stripping Ar above solution 0.025-1.200 100

(22) The electrolyte used for all experiments was 0.5 M HClO.sub.4 prepared from concentrated HClO.sub.4 (double distilled, GFS) and ultrapure water (ELGA, 18 M cm). The gases used (Ar, O.sub.2 and CO) were of the highest commercially available purity (Air Products). Unless stated otherwise, experiments were performed under an atmosphere of argon. Oxygen reduction experiments were performed under an atmosphere of O.sub.2. During potential step measurements, oxygen was bubbled through the electrolyte. Unless otherwise noted, the maximum potential applied to the electrodes was 1.2 V vs. RHE. The screening procedure carried out on each array is outline in Table 1.

EXAMPLE 4: ANATASE AND AMORPHOUS TITANIUM OXIDE

(23) To compare amorphous and crystalline titanium oxide as a support for Pt, separate electrochemical arrays were synthesised. The as-deposited titanium oxide was amorphous. The titanium oxide was crystallised by heating in a tube furnace at 450 C. for 6 hours in the presence of oxygen. XRD confirmed the titanium oxide had been crystallised in the anatase form.

(24) FIG. 4 shows the potential at which 1 A raw current is reached (threshold value used to measure the ignition or onset potential) during the oxygen reduction slow cycling experiment (i.e. cycling at 20 mV s.sup.1 in oxygenated 0.5 M HClO.sub.4) for various equivalent thicknesses of Pt supported on amorphous and anatase titanium oxide. An ignition or onset potential is defined as the potential at which the absolute value of the current starts to increase from the background (double layer) level in a cyclic voltammetry experiment, indicating that an oxidation or reduction reaction is taking place. Average results were taken across an array row for identical Pt equivalent thicknesses. This provides a measure of the onset potential of the oxygen reduction reactionthe higher the onset potential, the more active the catalyst for the oxygen reduction reaction. It can be seen that at high equivalent thicknesses of Pt, the ignition potential remains fairly constant with decreasing equivalent thickness of Pt. This is similar for both supports.

(25) However, between 5 and 6 ML (monolayers) equivalent thickness of Pt, the ignition potential starts to decrease on both support materials. The effect on the amorphous material is more significant. Under this equivalent thickness for both supports the ignition potential decreases further. On the amorphous titanium oxide at low equivalent thicknesses, there is a large amount of scatter in the data. FIG. 5 shows the potential at which 3 A raw current is reached in the oxygen reduction slow cycling experiment for the same set of data. At this current it is even clearer that the reduction wave is shifted more significantly negative on the amorphous support. This suggests that the anatase support provides better wetting for the platinum, and therefore higher activity, down to a lower equivalent thickness of Pt.

(26) FIG. 6 shows the voltammetry for one electrode during the oxygen reduction reaction with 0.7 ML equivalent thickness of Pt supported on amorphous titanium oxide. The two currents at which ignition potentials have been measured are indicated by grey lines. Two reduction features are seen, a small feature with a high onset potential and a larger feature with a low onset potential. These two features are probably due to inhomogeneity within the sample, i.e. some large islands of platinum leading to a high ignition potential and some smaller particles leading to the lower ignition potential. The ignition potential measured at 1 A raw current gives an indication of the onset of the first reduction feature and the ignition potential measured at 3 A raw current gives an indication of the onset of the second reduction feature.

(27) These results suggest that the platinum shows better wetting on the anatase titanium oxide, allowing bulk platinum like oxygen reduction activity to a lower equivalent thickness.

EXAMPLE 5: ANATASE TUNGSTEN TITANIUM OXIDE

(28) FIG. 7 shows the ignition potentials (potential at which 1 A raw current is reached) for the second negative cycle of the oxygen reduction slow cycling experiment. Various equivalent thicknesses of Pt were deposited onto anatase TiWO.sub.x, with varying W content. At low Pt equivalent thicknesses, the arrays containing W had higher ignition potentials (i.e. lower overpotentials). However, by the equivalent thicknesses (4.3 ML) where the highest ignition potentials are reached (the lowest overpotentials), all of the samples have similar ignition potentials. This suggests that there is better wetting of the Pt onto the tungsten-doped titanium oxide at low loadings.

(29) As indicated above, atomic percentage values given are on the total metal content. That is to say, 30 at % W indicates 70 at % Ti. The exact stoichiometry including oxygen atoms is undefined but is at or close to stoichiometric, thereby maintains oxide properties.

(30) Use as a Core-Shell Catalyst

(31) The materials described in the present application are readily scaled up from the model thin film samples to bulk core shell powder materials using known techniques, such as those described in US2010/0197490, US2007/0031722, US2009/0117257, US2006/0263675 and CN101455970. Other techniques will be readily apparent to the skilled person. The stability of the materials described is such that they are effective for use in fuel cells.

REFERENCES

(32) 1. Peuckert, M., et al., Oxygen Reduction on Small Supported Platinum Particles. Journal of The Electrochemical Society, 1986. 133(5): p. 944-947. 2. Guerin, S., et al., Combinatorial Electrochemical Screening of Fuel Cell Electrocatalysts. Journal of Combinatorial Chemistry, 2003. 6(1): p. 149-158. 3. Gasteiger, H. A., et al., Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Applied Catalysis B: Environmental, 2005. 56(1-2): p. 9-35. 4. Xu, Z., et al., Effect of particle size on the activity and durability of the Pt/C electrocatalyst for proton exchange membrane fuel cells. Applied Catalysis B: Environmental, 2012. 111-112(0): p. 264-270. 5. de Bruijn, F. A., V. A. T. Dam, and G. J. M. Janssen, Review: Durability and Degradation Issues of PEM Fuel Cell Components. Fuel Cells, 2008. 8(1): p. 3-22. 6. Huang, S.-Y., et al., Development of a Titanium Dioxide-Supported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel Cell Applications. Journal of the American Chemical Society, 2009. 131(39): p. 13898-13899. 7. Hayden, B. E., et al., The influence of support and particle size on the platinum catalysed oxygen reduction reaction. Physical Chemistry Chemical Physics, 2009. 11(40): p. 9141-9148. 8. Huang, S.-Y., P. Ganesan, and B. N. Popov, Electrocatalytic activity and stability of niobium-doped titanium oxide supported platinum catalyst for polymer electrolyte membrane fuel cells. Applied Catalysis B: Environmental, 2010. 96(1-2): p. 224-231. 9. Antolini, E. and E. R. Gonzalez, Ceramic materials as supports for low-temperature fuel cell catalysts. Solid State Ionics, 2009. 180(9-10): p. 746-763. 10. Chen, Y., et al., Atomic layer deposition assisted PtSnO2 hybrid catalysts on nitrogen-doped CNTs with enhanced electrocatalytic activities for low temperature fuel cells. International Journal of Hydrogen Energy, 2011. 36(17): p. 11085-11092. 11. Cui, X., et al., Graphitized mesoporous carbon supported PtSnO2 nanoparticles as a catalyst for methanol oxidation. Fuel, 2010. 89(2): p. 372-377. 12. Guo, D.-J. and J.-M. You, Highly catalytic activity of Pt electrocatalyst supported on sulphated SnO2/multi-walled carbon nanotube composites for methanol electro-oxidation. Journal of Power Sources, 2012. 198(0): p. 127-131. 13. Ye, J., et al., Preparation of Pt supported on WO3-C with enhanced catalytic activity by microwave-pyrolysis method. Journal of Power Sources, 2010. 195(9): p. 2633-2637. 14. Weidner, J. W. and B. L. Garcia, Electrocatalyst support and catalyst supported thereon US 2009/0065738 A1, 2009: United States. 15. Cai, M., et al., Electrocatalyst Supports for Fuel Cells US 2007/0037041 A1, 2007: United States. 16. Do, T. B., M. Cai, and M. S. Ruthkosky, Mesoporous electrically conductive metal oxide catalyst supports WO 2009/152003 A2, 2009. 17. Adzic, R., J. Zhang, and M. Vukmirovic, Electrocatalyst for oxygen reduction with reduced platinum oxidation and dissolution rates US 2006/0263675 A1, 2006: United States. 18. Adzic, R., M. Vukmirovic, and K. Sasaki, Synthesis of metal-metal oxide catalysts and electrocatalysts using a metal cation adsorption/reduction and adatom replacement by more noble ones U.S. Pat. No. 7,704,918 B2, 2010: United States. 19. Hartl, K., et al., AuPt core-shell nanocatalysts with bulk Pt activity. Electrochemistry Communications, 2010. 12(11): p. 1487-1489. 20. Ma, Y., et al., High active PtAu/C catalyst with core-shell structure for oxygen reduction reaction. Catalysis Communications, 2010. 11(5): p. 434-437. 21. Sasaki, K., et al., Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes. Angewandte Chemie International Edition, 2010. 49(46): p. 8602-8607. 22. Shuangyin, W., et al., Controlled synthesis of dendritic Au@Pt core-shell nanomaterials for use as an effective fuel cell electrocatalyst. Nanotechnology, 2009. 20(2): p. 025605. 23. Silva, J. C. M., et al., Ethanol oxidation reactions using SnO2@Pt/C as an electrocatalyst. Applied Catalysis B: Environmental, 2010. 99(1-2): p. 265-271. 24. Wang, R., et al., Carbon supported Pt-shell modified PdCo-core with electrocatalyst for methanol oxidation. International Journal of Hydrogen Energy, 2010. 35(19): p. 10081-10086. 25. Wang, R., et al., Preparation of carbon-supported core@shell PdCu@PtRu nanoparticles for methanol oxidation. Journal of Power Sources, 2010. 195(4): p. 1099-1102. 26. Wang, W., et al., Pt overgrowth on carbon supported PdFe seeds in the preparation of core-shell electrocatalysts for the oxygen reduction reaction. Journal of Power Sources, 2010. 195(11): p. 3498-3503. 27. Wei, Z. D., et al., Electrochemically synthesized Cu/Pt core-shell catalysts on a porous carbon electrode for polymer electrolyte membrane fuel cells. Journal of Power Sources, 2008. 180(1): p. 84-91. 28. Wu, Y.-N., et al., High-performance core-shell PdPt@Pt/C catalysts via decorating PdPt alloy cores with Pt. Journal of Power Sources, 2009. 194(2): p. 805-810. 29. Lopez, M., et al., Core/Shell-type catalyst particles comprising metal or ceramic core materials and methods for their preparation, WO2008/025751. 2008. 30. Guerin, S. and B. E. Hayden, Physical Vapor Deposition Method for the High-Throughput Synthesis of Solid-State Material Libraries. Journal of Combinatorial Chemistry, 2006. 8(1): p. 66-73. 31. Guerin, S., et al., High-Throughput Synthesis and Screening of Ternary Metal Alloys for Electrocatalysis. The Journal of Physical Chemistry B, 2006. 110(29): p. 14355-14362.