Catalytic electrodes for fuel cells or electrolysis cells and processes for producing said electrodes

Abstract

A method for preparing an array (13) of vertically aligned carbon nanotubes (14) for use in a catalytic electrode for a fuel cell or electrolysis cell comprises providing an array (13) of vertically aligned carbon nanotubes (14) obtained by a vapor growth process in which a precursor of a carbon nanotube growth catalyst is continuously added to a feed gas, and depositing a plurality of platinum nanodots (15) on the outer surface of the vertically aligned carbon nanotubes by using a vapor deposition process such as ALD. The ALD process advantageously uses a platinum source gas that is Pt(PF3)4. The nanodots can be protected by nanocaging.

Description

[Technical Field]

[0001] The present invention relates to the field of electrochemistry, more precisely to the subfield of electrodes for electrochemical processes in aqueous systems. The present invention relates to novel catalytic electrodes comprising platinum nanoparticles that can be used in fuel cells operating in aqueous systems, in particular fuel cells using molecular oxygen and hydrogen. They can also be used in electrolysis cells operating in aqueous systems, in particular electrolysis cells splitting water into molecular oxygen and hydrogen. The present invention also relates to a novel process for fabricating such catalytic electrodes using vertically aligned carbon nanotubes (VACNTs), and a deposition method for forming platinum nanoparticles on the VACNTs. The catalytic electrodes can be incorporated into polymer films. [Background technology]

[0002] Catalytic electrodes for fuel cells, which combine gaseous oxygen and hydrogen to form water, have been known for a long time, at least since the 1950s, when early spacecraft missions sought a self-sustaining and reliable source of electrical energy. Fuel cell electrodes comprising platinum nanoparticles dispersed on porous carbon materials were the subject of several early patents (see, for example, U.S. Pat. No. 3,231,428, which disclosed Pt-Ir nanoparticles) and were featured in the landmark 1973 textbook "Modern Electrochemistry" by J. O'M. Bockris and A.K.N. Reddy (see Vol. 2, pp. 1382–1395). Today, these electrodes function very well, but they are expensive due to their high platinum content, and their electrochemical mass activity (expressed in watts per gram of platinum) is quite low. In other words, only a small fraction of the platinum deposited on the electrode is actually active. It is clear that adding less platinum to achieve the same catalytic effect would be desirable.

[0003] Many different approaches have been taken to increase the platinum utilization efficiency in catalytic electrodes. Vertically aligned carbon nanotubes (VACNTs) are candidates for fabricating membranes for proton exchange membrane fuel cells (PEMFCs) because their special structure improves gas diffusion, water drainage, and platinum utilization. However, VACNTs cannot be grown on polymer substrates (such as those used in PEMFCs) and therefore cannot be used directly on their growth substrates, making them difficult to handle. In arrays formed with VACNTs, most nanotubes are parallel; more precisely, their main direction (i.e., their long axis) is approximately perpendicular to the substrate on which they are grown. Although some deviation from perpendicularity can occur, it is included in the term "vertically" aligned carbon nanotubes.

[0004] In their paper "Vertically Aligned Carbon Nanotube Electrodes for Proton Exchange Membrane Fuel Cells Operating at High Current Densities," S. Murata et al., Journal of Power Sources 253 (2014), pp. 104–113, propose a multi-step method for fabricating such catalytic membranes. In this method, platinum particles are generated on VACNTs (grown on a stainless steel substrate coated with an iron catalyst and an inert oxide) by infiltration of a platinum salt solution followed by reduction. Then, after fixing them on the membrane using an ionomer immersion process, the VACNTs are transferred from the steel substrate to the membrane by hot pressing, after which the original steel substrate is peeled off. This process allows the VACNTs to be coated with small platinum particles (approximately 2–2.5 nm in size), which are well dispersed across the entire surface and do not form agglomerates. However, this is a rather laborious and complicated process. Murata's paper emphasizes that while vertical alignment of carbon nanotubes is important for fabrication, it is irrelevant to the performance of the resulting platinum-filled catalytic electrode because the hot pressing process destroys the alignment of the VACNTs.

[0005] Shen et al. ("Pt-coated vertically aligned carbon nanotubes as electrodes for proton exchange membrane fuel cells," Procedia Engineering 93 (2014), pp. 34-42) reported a method for depositing compact platinum films on VACNTs by sputtering. However, the thickness of these films can reach hundreds of nanometers, blocking access to very small pores and resulting in a loss of specific surface area. This technique does not appear to solve the problem of efficient utilization of platinum in catalytic membranes.

[0006] WO 2022 / 047351, granted to Air Liquide, discloses a method for forming platinum nanodots on a substrate using a continuous gas-phase reaction process (known as atomic layer deposition (ALD) or pulsed chemical vapor deposition (pulsed CVD)) that uses a specific platinum precursor molecule, Pt(PF3)4. A variety of carbon materials can be used as substrates. These nanodots enable platinum utilization efficiencies of up to 75%. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide a catalyst electrode for a fuel cell with improved mass activity and potentially improved efficiency, thereby reducing the platinum loading of the catalyst electrode and ultimately its cost.

[0008] The present invention also aims to provide a simple method for obtaining proton exchange membranes for fuel cells using VACNTs as a porous material supporting finely dispersed platinum particles, which results in improved mass activity, improved catalyst utilization, and thereby reduced total catalyst loading. [Means for solving the problem]

[0009] The inventors have recognized that in prior art fuel cell catalyst electrodes, a significant portion of the platinum particles deposited on the substrate are not catalytically active. According to the present invention, this problem is solved by a novel catalyst electrode comprising platinum dots deposited on vertically aligned carbon nanotubes (VACNTs).

[0010] VACNT arrays are designed to cover a large external surface (typically 100 m 2 VACNTs have a high surface area (greater than 1000 nm / g), which is readily accessible to chemical species in the gas phase (such as gaseous precursors for ALD or pulsed CVD), but their mobility between the nanotubes forming the VACNT array is limited. The inventors have surprisingly discovered that the specific structural characteristics of VACNT arrays and VACNTs have a significant effect on the uniformity of platinum nanodots deposited by ALD or pulsed CVD, particularly those nanodots obtained by the above-mentioned gas phase deposition processes (ALD or pulsed CVD) using Pt(PF3)4 in the presence of an oxidizing or reducing agent.

[0011] According to one aspect of the invention, the method of the present invention uses VACNT arrays grown in a reaction chamber from the vapor phase, where a carbon source gas and a catalyst precursor are simultaneously introduced. This is a one-step growth process in which a precursor of the growth catalyst (typically ferrocene droplets dissolved in toluene) is added sequentially to the vapor phase that serves as the growth atmosphere (typically containing acetylene as the carbon source) for the carbon nanotube growth process.

[0012] Continuous addition of the growth catalyst to the feed gas can be achieved by periodically injecting an aerosol comprising the catalyst into the VACNT growth reactor; this process is known, for example, from WO 2004 / 000727 (granted by the Atomic Energy and Alternative Energies Agency). Such periodic injection is typically performed using an injection pump similar to those used in diesel engines to deliver pulses at very short time intervals; due to this pulsed injection, there is no measurable catalyst depletion in the gas phase between two successive pulses, which is also referred to as "quasi-continuous" injection.

[0013] Compared with VACNT arrays obtained by a two-step process in which a catalyst is deposited on a growth substrate and then carbon nanotubes are grown from the vapor phase, the VACNT arrays obtained by the above one-step process exhibit at least two distinctive features: the presence of catalyst particles at the center of the carbon nanotubes and the presence of numerous defects on the outer surface of the carbon nanotubes, which can facilitate the anchoring and growth of platinum nanodots during subsequent processing steps.

[0014] The inventors have discovered that when VACNT arrays obtained by this one-step process are used in a platinum nanodot deposition process, catalyst films with improved properties and higher platinum electrochemical mass activity are obtained. In contrast, two-step VACNT growth processes, in which a growth catalyst is deposited on a substrate in the first step, followed by the carbon nanotube growth process itself, do not result in VACNT arrays constructed from carbon nanotubes with graphitic planes (layers) that are inclined (i.e., form angles) with respect to the main nanotube direction.

[0015] Therefore, a first object of the present invention is a method for producing an array of vertically aligned carbon nanotubes for use in a catalytic electrode of a fuel cell or electrolysis cell, comprising the steps of: - providing an array of vertically aligned carbon nanotubes obtained by a vapor growth process in which a precursor of a carbon nanotube growth catalyst is added continuously to a feed gas; - depositing a plurality of platinum nanodots on the outer surface of the VACNTs using a first vapor deposition process;

[0016] In an advantageous embodiment, the platinum nanodots are deposited by a chemical vapor deposition (CVD) technique, preferably atomic layer deposition (ALD) or pulsed CVD, using a specific gaseous precursor, namely Pt(PF3)4, in the presence of an oxidizing or reducing agent. The oxidizing agent can be oxygen radicals or HO, O2, O3, or mixtures thereof. The reducing agent can be molecular hydrogen. The terms ALD and pulsed CVD are used interchangeably in the present invention to refer to a two-step sequential vapor deposition process, in which two independent precursor reactants are introduced in each step and separated by a gas purge.

[0017] According to yet another aspect of the present invention, the platinum nanodot deposition process is carried out at a temperature below 300° C., preferably below 275° C., more preferably between 25° C. and 275° C., and even more preferably between 50° C. and 250° C. Heating the surface of VACNTs to temperatures above about 300° C. in the presence of platinum can lead to thermal degradation of the carbon nanotubes.

[0018] The first vapor deposition process advantageously comprises a sequence of alternating cycles, each comprising an exposure time and a purge time, during which the array is exposed to Pt(PF3)4 gas and a reactive gas, preferably selected from the group consisting of H2, HO, O2, O3, NO2, oxygen radicals and mixtures thereof, NH3, SiH4, Si2H6, Si3H8, SiH2Me2, SiH2Et2, N(SiH3)3, SiH2(NEt2)2, other Si-H containing reactants, hydrogen radicals, hydrazine, methylhydrazine, amines, NO, NO, borane, B2H6, CH4, CH6, CH3I, and mixtures thereof. The duration of the exposure time and / or such purge time in each of the alternating cycles is advantageously between 0.1 seconds and 60 minutes, preferably between 1 second and 1000 seconds, and more preferably between 10 seconds and 100 seconds. The number of said sequences is advantageously between 2 and 100, preferably between 5 and 35, even more preferably between 8 and 30, and most preferably between 10 and 25.

[0019] According to an advantageous feature of the present invention, the average diameter of the platinum nanodots is 0.7 nm to 5 nm, preferably 1 nm to 5 nm, and more preferably 1 nm to 4 nm. Because only the surface of the platinum nanodots is catalytically active, the smaller the particle, the greater the surface area-to-volume ratio, and the higher the specific activity (activity per gram of platinum). However, the smaller the particle, the more susceptible it is to degradation over time, particularly due to Ostwald ripening. This effect is particularly pronounced for particles less than 2 nm. Above 5 nm, the specific activity drops significantly. Particles less than 2 nm can also be used, but in that case, it is preferable to protect them by nanocaging, as described below.

[0020] In an advantageous embodiment of the present invention, the first vapor deposition process is carried out so that the total amount of platinum in the vertically aligned carbon nanotube array is greater than 10% by weight, preferably greater than 20% by weight, more preferably greater than 30% by weight, and most preferably greater than 40% by weight.

[0021] According to another feature of the invention, the volumetric mass of the VACNT array is 0.10 g / cm 3 More than 0.15 g / cm 3 More preferably, 0.20 g / cm 3 more preferably at least 0.25 g / cm 3 (not considering the substrate on which the VACNT arrays are deposited). 3 It is preferable that the value does not exceed 0.5 g / cm 3 It is more preferable not to exceed 100%. If the volumetric mass of the VACNT array is too large, the nanotubes may form bundles with poor access to most of the tube surface, resulting in non-uniform deposition of platinum nanodots. If the volumetric mass is too small, the precursors in the ALD or pulsed CVD process appear to be used less efficiently.

[0022] According to yet another aspect of the present invention, the stability of platinum nanodots can be enhanced by encapsulating them in nanocages made from inorganic oxides deposited by a region-selective vapor deposition process. According to this advantageous embodiment, the process of the present invention comprises the further step of first treating the VACNT arrays comprising platinum nanodots with a region-selective blocking agent, e.g., a surfactant, that is applied to the platinum nanodots but not to the nanotube surface or the VACNT substrate, and then depositing an inorganic porous oxide using a second vapor deposition process. The vapor deposition process is typically an ALD process, in which the inorganic oxide is deposited by ALD on the carbon nanotubes but not on the platinum nanodots.

[0023] Finally, in a further step, the blocking agent is removed. The selective blocking agent is selected so that it can selectively adhere to the platinum dots, deposits only on the surface of the platinum nanodots, and does not contaminate the ALD reactor during the deposition of the inorganic oxide. Surfactants such as amines, thiols, or acids can be used as blocking agents. In particular, the selective blocking agent is advantageously selected from alkylamines, allylamines such as oleylamine, alkylthiols, or carboxylic acids such as oleic acid.

[0024] The second vapor deposition process is advantageously carried out so that the platinum nanodots have an average diameter of between 0.7 nm and 5 nm, preferably between 1 nm and 4 nm, more preferably between 1 nm and 3 nm.

[0025] The second vapor deposition process advantageously comprises a sequence of alternating cycles, each comprising an exposure period and a purge period, during which the array is exposed to a metal organic precursor of a metal element and a reactive gas to form an inorganic oxide of the metal element, the metal element preferably being selected from the group consisting of zirconium, niobium, tantalum, vanadium, tungsten, molybdenum, titanium, hafnium, cobalt, nickel, yttrium, cerium, lanthanum, rare earth elements or other elements of the lanthanide series.

[0026] Another object of the present invention is an array of vertically aligned carbon nanotubes for use in a catalytic electrode of a fuel cell or electrolysis cell, characterized in that the nanotubes are provided with a plurality of platinum nanodots on the outer surface thereof, the carbon nanotubes having graphitic faces oriented perpendicular to the main direction of the nanotubes. Advantageously, the volumetric mass of the array of vertically aligned carbon nanotubes is 0.10 g / cm. 3 ~0.45g / cm 3 , preferably 0.15 g / cm 3 ~0.30g / cm 3 without taking into account the substrate on which the VACNT array is deposited. The average diameter of the platinum nanodots is preferably 0.7 nm to 5 nm, preferably 1 nm to 5 nm, more preferably 1 nm to 4 nm, and most preferably 1 nm to 3 nm. The plurality of platinum nanodots advantageously comprise face-centered cubic platinum crystals.

[0027] Another object of the invention is the use of the array according to the invention for manufacturing catalytic electrodes for fuel cells or electrolysis cells.

[0028] Yet another object of the present invention is a polymer film comprising an array of vertically aligned carbon nanotubes, characterized in that the carbon nanotubes have graphitic faces oriented perpendicular to the main direction of the nanotubes, and a plurality of platinum nanodots on the outer surface of the carbon nanotubes. [Brief explanation of the drawings]

[0029] 1-20 illustrate various aspects of the present invention, some of which relate to the prior art for comparison.

[0030] [Figure 1] 1 shows a schematic cross-section of a prior art catalyst membrane having catalyst particles dispersed on a granular porous carbon support. [Figure 2] Figures 2a and 2b show schematic cross-sections of a catalyst membrane according to the present invention, with catalyst particles dispersed on an array of vertically aligned carbon nanotubes, and represent different embodiments of such a catalyst membrane. [Figure 3] Electron microscope images of carbon nanotubes grown by a one-step process are shown in Figure 3a and b before and after the deposition of platinum nanodots. The arrows indicate the iron-rich phase, which is the residue of catalyst particles trapped in the hollow carbon nanotubes. [Figure 4] Transmission electron microscope images of carbon nanotubes grown using a process that used ferrocene as the catalyst precursor and acetylene as the carbon source are shown. Figure 4a represents the two-step growth process and shows intact nanotubes with oriented graphitic planes. Figure 4b represents the one-step growth process and shows unoriented graphitic planes. [Figure 5] Schematic diagrams clearly illustrating what can be seen in FIG. 4: nanotubes with aligned graphitic walls (FIG. 5a) and defective nanotubes (FIG. 5b) exhibiting graphitic planes oriented approximately perpendicular to the nanotube's main axis, and more typically forming angles of about 30° to about 90° with said main axis. [Figure 6] 1 shows the 19F NMR spectrum of Pt(PF3)4 used in the first vapor deposition process example. [Figure 7] Referring to Example 1, the platinum loading (FIG. 7a) and utilization efficiency of the Pt(PF3)4 precursor on the VACNT powder (FIG. 7b) are shown. [Figure 8] In relation to Example 1, the utilization efficiency of Pt(PF3)4 precursor (Fig. 8a) and platinum loading into conventional porous carbon powder (reference KB) (Fig. 8b) are shown. [Figure 9] FIG. 1 shows TGA analysis of 18-cycle Pt ALD samples (three replicates, curve (b)) compared to untreated VACNT (curve (a)) relating to Example 1. The solid line shows the residual mass (expressed in wt %) and the dotted line shows the DTA. [Figure 10] 10 shows XPS analysis of Pt ALD samples with different cycle numbers, related to Example 1. [Figure 11] 1 shows BF-TEM images and nanodot size distribution as a function of the number of Pt(PF3)4 / H2 cycles during ALD deposition of platinum nanodots, related to Example 1. Data is shown for 2, 4, 8, and 18 cycles. [Figure 12-13] This relates to Example 4 and to VACNT arrays grown using a one-step growth process. This shows transmission electron microscope images of platinum nanodots deposited using the ALD process described in Example 2 after four ALD cycles at two different magnifications (FIG. 12a) and (FIG. 12b). The size distribution of the platinum nanodots is given in a histogram (FIG. 12c). FIG. 13 relates to Example 4 and shows the same type of information as FIG. 12 after 18 ALD cycles. [Figure 14-15] 12 after 4 ALD cycles. Figure 15 relates to Example 4 and to VACNT arrays grown with a two-step process. Figure 16 relates to Example 4 and shows the same type of information as Figure 12 after 18 ALD cycles. [Figure 16] In relation to Example 5, the surface activity (Fig. 16a) and mass activity (Fig. 16b) are shown for some catalyst samples as the current measured at specific potentials (0.90 V and 0.95 V) divided by the mass or surface of platinum. [Figure 17] Referring to Example 6, the mass activities measured at up to five different potentials (0.6 V, 0.7 V, 0.8 V, 0.9 V, 0.95 V) for different catalyst samples are shown. [Figure 18]

[0033] Figure 7 shows the results of TGA measurements obtained on a commercial multi-walled carbon nanotube powder after 4 (curve (b)), 8 (curve (c)), and 18 (d) ALD cycles using Pt(PF) / H, related to Example 7. Curve (a) relates to the untreated powder. [Figure 19] Referring to Example 7, the utilization efficiency of Pt(PF3)4 precursor (Fig. 19a) and platinum loading into conventional carbon nanotube powder (Fig. 19b) are shown. [Figure 20] 20a and 20b show TEM-BF and STEM-HAADF images of the same area of ​​the sample after four ALD cycles using Pt(PF3)4 / H2, respectively, related to Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0031] Unless otherwise specified, all percentage values ​​refer to percent by weight.

[0032] The present invention provides an improved catalytic VANCT array that can be used in the fabrication of catalytic electrodes, particularly for use in fuel cells and electrolysis cells. Such electrodes typically comprise a polymer membrane comprising a layer of porous carbon material on which a catalyst is formed. Figure 1 shows a schematic representation of a prior art catalytic membrane 1, comprising a porous carbon layer 3 comprising porous carbon particles 4 deposited on a carbon substrate 2. Platinum particles 5 and 6 are deposited on the porous carbon layer 3 and within the open pores of the carbon layer 3, and a suitable polymer sheet 7 (typically an ionomer) is deposited on the carbon layer. Platinum particles 6 embedded within the porous network of the carbon particles can be easily passivated, while platinum particles 5 on the surface of the porous carbon particles are not at risk of being embedded by reaction products or structural changes in the catalyst support.

[0033] Figure 2 shows two schematic diagrams of catalyst membranes 10 using VACNT arrays 13 as the porous carbon material. The VACNT arrays are formed by individual parallel carbon nanotubes 14 of approximately the same length deposited on a carbon substrate 12. As can be seen by comparing them with Figure 1, gas molecules (represented by small arrows in Figures 1 and 2) have much easier access to the ionomer membranes 2 and 12 through the VACNT arrays 13. This is because the diffusion paths of gas molecules are linear, whereas the diffusion paths across the porous carbon particles 4 can be more tortuous. In fuel cell catalyst membranes, the gas molecules can be molecular hydrogen and molecular oxygen, which recombine to form water. All of the platinum particles 6 are formed on the outside of the tubes, with no pores except in the space inside the tubes. An ionomer sheet 17 covers the platinum nanodots 15. It should be noted that the difference between the embodiments depicted in Figures 2a and 2b is the presence of a protective layer 18 that protects the platinum article from oxidation, which may be formed from niobium oxide particles.

[0034] The inventors have surprisingly discovered that there is a significant difference between catalytic electrodes based on VACNT arrays grown by a one-step process and those grown by a two-step process when the same amount of platinum is deposited using the same platinum deposition process according to the present invention. In particular, the inventors have discovered that VACNT arrays obtained by a two-step process, in which a growth catalyst is deposited on a substrate in the first step, followed by the VACNT growth process itself in the second step, are unable to improve the efficiency of platinum in the catalytic film or electrode.

[0035] Here, we briefly recapitulate the principles of the one-step VACNT growth method for preparing carbon nanotubes, first described in the aforementioned International Publication No. 2004 / 000727. This process proceeds by pyrolyzing a carbon source gas, i.e., acetylene, in the presence of a catalyst precursor, such as ferrocene, in a high-temperature reaction chamber. A liquid containing at least one liquid hydrocarbon precursor of carbon and at least one metal compound precursor of a catalytic metal is formed into finely divided liquid particles, such as droplets, under pressure using a periodic injection system. The finely divided particles, such as droplets, are then carried by a carrier gas flow containing a gaseous carbon source and introduced into the reaction chamber, where carbon nanotube deposition and growth occur. Ferrocene, toluene, and acetylene can be used as the catalyst precursor, the solvent for the liquid hydrocarbon precursor and catalyst precursor, and the gaseous carbon source, respectively. This process produces arrays of vertically aligned multiwalled carbon nanotubes.

[0036] We have observed two differences between VACNT arrays grown by a one-step process and those grown by a two-step process: the presence or absence of catalyst particles within the carbon nanotubes and the occurrence of defects on the carbon nanotube surface. While catalyst particles provide a relatively simple means of distinguishing the two classes of carbon nanotubes, their presence does not directly correlate with the observed differences in the deposition of platinum nanodots between the two classes of carbon nanotubes. On the other hand, the presence of defects on the carbon nanotube surface has been shown to be related to the deposition of TiO nanoparticles by atomic layer deposition (see L. Acauan et al., "Effect of Various Defects in Vertically Aligned Carbon Nanotubes on the Formation of TiO Nanoparticles by Atomic Layer Deposition," ACS Applied Materials & Interfaces, vol. 8(25), pp. 16444-50, 2016). However, these surface defects are extremely difficult to detect and identify.

[0037] Figure 3 shows an electron microscope image of carbon nanotubes grown using a one-step process with ferrocene as the catalyst precursor and acetylene as the carbon source. Carbon-containing iron particles are observed at the center of the nanotubes, and their chemical nature was identified using energy-dispersive spectroscopy (EDS) performed under an electron microscope. These iron-rich particles are rarely observed in carbon nanotubes grown using a two-step process. A possible scientific explanation for this is that in the two-step process, in the first step, a catalyst is deposited on a growth substrate. "The carbon source gas adheres to the catalyst surface and decomposes into carbon atoms, which then dissolve and diffuse into the catalyst. When the carbon atoms reach supersaturation within the catalyst, they detach from the top of the catalyst to form VACNTs on top, while the catalyst particles remain attached to the substrate," also known as the "bottom growth pattern." (Quoted from the right column of page 1595 of the paper "Recent Advances in the Synthesis and Applications of Vertically Aligned Carbon Nanotube Materials" by S. Huang et al., Nanotechnology Review 2021;20:1592-1623.) On the other hand, a one-step process in which the iron catalyst is free-floating and does not interact with the substrate follows the so-called "top growth pattern," in which "carbon atoms are separated from the catalyst-substrate junction to form VACNT structures. That is, the catalyst particles are lifted up and sealed by the catalyst particles during VACNT growth. Generally, VACNTs perform the "top growth pattern" when the catalyst has weak interactions with the substrate" (quoted from Huang, see above).

[0038] Figure 3a shows a carbon nanotube grown by a one-step process before the platinum particles are deposited, and Figure 3b shows a similar carbon nanotube grown by a one-step vapor-phase process after the platinum particles are deposited using the process of the present invention. The platinum particles appear as small black dots, while two larger black dots, about 10 nanometers in diameter, represent carbon-containing iron particles, i.e., the growth catalysts.

[0039] Figure 4 shows transmission electron microscope images of carbon nanotubes grown using a process that uses ferrocene as a catalyst precursor and acetylene as a carbon source. Figure 4a represents a two-step growth process and shows a perfect nanotube with aligned graphite planes. Figure 4b represents a one-step growth process and shows unaligned graphite planes. It can be seen that certain graphite planes typically form angles of about 30° to about 90° with the nanotube's major axis, with some graphite planes oriented nearly perpendicular to the nanotube's major axis. This structural defect results in specific surface defects. Figure 5 shows a schematic representation of these two structures. Figure 5a shows a perfectly aligned graphite plane, while Figure 5b shows a defective carbon nanotube with specific surface defects due to structural defects. The inventors believe (but do not wish to be bound by this theory) that the presence of graphitic planes that typically form angles between about 30° and about 90° with the nanotube's major axis (and in some cases are oriented nearly perpendicular to the nanotube's major axis) indicates other, more subtle, surface defects distributed across the outer surface of the carbon nanotube, which explains why platinum nanodots can be deposited much more uniformly on such carbon nanotubes compared to defect-free nanotubes grown by the two-step process.

[0040] Here we describe the platinum nanodot deposition process and the differences in results obtained after platinum nanodot deposition using VACNT arrays grown using a one-step growth process and VACNT arrays grown using a two-step process.

[0041] The method of the present invention comprises depositing a plurality of platinum nanodots on the outer surface of the VACNTs using a first vapor deposition process. The first vapor deposition process uses a specific gaseous platinum precursor, preferably Pt(PF3)4. For VACNT arrays grown by a two-step process, this first vapor deposition process results in poor uniformity of platinum nanodot distribution (expressed as percent coverage of the substrate surface), particularly many aggregates and uncovered areas. For VACNT arrays grown by a one-step process, good uniformity of platinum nanodot distribution is observed, particularly the absence of aggregates and large uncovered areas. When Pt(PF3)4 is used for the first vapor deposition process, the plurality of platinum nanodots comprise face-centered cubic platinum crystals.

[0042] In this first vapor deposition process, the platinum utilization efficiency of VACNT arrays grown in a two-step process and a one-step process is similar, with a particular embodiment of the platinum deposition process of the present invention requiring approximately 18 deposition cycles to achieve 40 wt. % platinum loading.

[0043] However, the diameter distribution of platinum nanodots at high platinum loading (40 wt. % platinum) using the vapor deposition process of the present invention was found to be very non-uniform on VACNT arrays grown by the two-step process, with particularly high concentrations of platinum particles with diameters greater than 10 nm, which is undesirable for the use of such VACNT arrays in catalytic electrodes. At the same platinum loading of 40 wt. % on VACNT arrays grown by the one-step process, the average size of platinum nanodots was found to be 3 nm, with a narrow size distribution. These results are consistent with a volumetric mass of 0.08 g / cm for VACNTs grown by the two-step process. 3 , about 0.25 g / cm for VACNTs grown in a one-step process. 3 and the volumetric mass values ​​reported herein refer to the mass of the nanotubes only and do not include the growth substrate.

[0044] In terms of volumetric mass, according to an advantageous embodiment of the present invention, the volumetric mass of the VACNT array is 0.15 g / cm 3 greater than 0.20 g / cm 3 and more preferably at least 0.25 g / cm 3 The value is 0.70 g / cm 3 It is desirable that the density does not exceed 0.5 g / cm 3 The value shall not exceed 0.25 g / cm 3 ~0.3g / cm 3 Values ​​between 0.25 g / cm and 0.35 g / cm have shown excellent results. In an advantageous embodiment, for VACNTs grown in a one-step process, the volumetric mass of the VACNTs is about 0.25 g / cm. 3 is.

[0045] We have also found significant differences between catalytic electrodes using the platinum nanodot vapor-phase deposition process with Pt(PF3)4 according to the present invention and catalytic electrodes using different platinum precursors with the same platinum nanodot deposition process, even when the underlying VACNT array is the same, especially when the underlying VACNT array is grown using a one-step process. Furthermore, significant differences are observed when using a commercially available carbon support. These differences can be explained in a number of ways.

[0046] Using a commercially available carbon support (Ketjen Black "KBEC300J" as reference) with the platinum nanodot vapor-phase deposition process using Pt(PF3)4 according to the present invention, described in detail below, we found that platinum utilization efficiency during the first vapor-phase deposition process was low. After eight deposition cycles, only 13.2 wt% platinum was achieved on KBEC300J (0.35 g), compared with 24.4 wt% for the VACNT array (0.35 g) grown by the one-step process, indicating significantly poorer Pt(PF3)4 nucleation and growth behavior on KBEC300J. Similarly, for yet another commercially available carbon support reference ("Vulcan XC72," 1 g), 48 cycles were required to achieve 31 wt% loading, whereas the VACNT array (0.35 g) grown by the one-step process required only 18 cycles to achieve 38 wt% loading. The coverage was nonuniform, with many agglomerates and uncovered areas. When a different precursor gas, specifically Pt(MeCp)Me3, was used for platinum deposition, it was not possible to reach 40 wt% platinum loading in VACNT arrays grown in a one-step process. Pt(MeCp)Me3 required an O2-based ALD process at high temperatures (250 °C), which induced thermal degradation of the VACNTs during the vapor-phase deposition process of Pt nanodots.

[0047] We now describe in more detail the so-called first vapor deposition process. In an advantageous embodiment of the process according to the present invention, platinum nanodots are deposited on VACNT arrays by sequentially injecting a Pt precursor and a co-reactant in a pulsed CVD process (ALD, also known as atomic layer deposition) in a reactor vessel equipped with vacuum and heating means. The VACNT arrays are first dried under a flow of inert gas (e.g., nitrogen) at temperatures preferably above 150°C for at least 1 hour. After drying, the Pt precursor and reactant, i.e., Pt(PF3)4 and H2 gas, respectively, are alternately introduced into the vessel in sequences lasting tens to hundreds of seconds, with the two sequences separated by purging with inert gas. These sequences can be repeated multiple times, typically several tens of times, until the desired platinum loading is achieved. The entire vapor deposition process is carried out at temperatures below 300°C, preferably between 25°C and 275°C, and more preferably between 50°C and 250°C.

[0048] In one advantageous embodiment, the duration of each Pt(PF3)4 pulse is between about 40 seconds and about 800 seconds, preferably between about 100 seconds and about 400 seconds, and the duration of each H2 pulse is between about 50 seconds and about 1000 seconds, preferably between about 100 seconds and about 750 seconds. They are separated by an inert gas purge of between about 200 seconds and about 1000 seconds. By way of example, each sequence consists of a Pt(PF3)4 pulse of between about 150 seconds and about 250 seconds, a nitrogen purge of between 500 seconds and about 700 seconds, and an H2 pulse of between about 500 seconds and about 700 seconds. This sequence is typically repeated 10 to 15 times until the desired platinum loading is achieved.

[0049] This first vapor deposition process is an ALD process for platinum nanodot deposition, in which the following co-reactants can be used instead of H2 gas: HO, O2, O3, NO2, oxygen radicals and mixtures thereof, NH3, SiH4, Si2H6, Si3H8, SiH2Me2, SiH2Et2, N(SiH3)3, SiH2(nEt2)2, other Si-H containing reactants, hydrogen radicals, hydrazine, methylhydrazine, borane, amines, NO, NO, and mixtures thereof.

[0050] According to an essential feature of the present invention, platinum nanodots are deposited by ALD using Pt(PF3)4 on arrays of VACNTs grown in a one-step process using a floating catalyst generated from precursors added continuously or quasi-continuously to a carbon source gas. The inventors believe (without wishing to be bound by this theory) that Pt(PF3)4 can preferentially dissociate at local defects on the surface of the carbon nanotubes and that these local defects occur more frequently in VACNT arrays made by the one-step process than in VACNT arrays made by the two-step process. Although these local defects have not been identified, this tentative explanation is plausible given the inventors' findings below.

[0051] Pt(PF3)4 is a highly symmetric molecule, with a radius of gyration of approximately 3.04 Å. The PF3 entity is significantly more stable than the Pt-P bond: the PF bond energy is 488 kJ / mol for the Pt(PF3)4 molecule, compared with 83 kJ / mol for the Pt-P bond. For the Pt(PF3)3 molecule, the corresponding bond energies are 483 kJ / mol and 95 kJ / mol. Our calculations indicate that the adsorption of Pt(PF3)3 on graphene is more favorable than that of Pt(PF3)4. While the total dissociation of PF3 from Pt(PF3)4 is energetically unfavorable by 80.6 kJ / mol, the formation of a more stable intermediate with a Pt-P bond length of 3.56 Å requires only an activation energy of 59.9 kJ / mol (equivalent to 24 kT at 298 K). Since the adsorption of Pt(PF3)3 on the graphene surface is more favorable than that of Pt(PF3)4, this may shift the equilibrium towards the dissociation of Pt(PF3)4.

[0052] This so-called "dissociative physisorption" mechanism can also be qualitatively understood by the geometric rearrangement of the molecules upon separation of one PF3 unit: in Pt(PF3)4 the Pt coordination is tetrahedral, whereas in Pt(PF3)3 it is coplanar, resulting in a shorter C-Pt bond length.

[0053] Our calculations indicate that the adsorption energy of Pt(PF3)3 on graphene is higher (72 kJ / mol) and the Pt-C bond length (3.46 Å) is shorter (52 kJ / mol and 4.78 Å, respectively, for Pt(PF3)4). Furthermore, the energy difference between Pt(PF3)3 on the carbon atom and Pt(PF3)3 on the ring center is below the kT value, which should allow for easy mobility of Pt(PF3)3 species on the surface. This provides at least a qualitative explanation for the ability of these Pt(PF3)3 species to diffuse across the graphene surface until they find a local defect that is energetically most favorable for bonding and subsequent molecular dissociation to release atomic platinum. This theory is supported by the finding that the C-Pt bond length for Pt(PF3)4 adsorption on a perfect graphene surface is 4.8 Å, compared with 4.6 Å for 5-7 ring defects, 4.1 Å for dangling CH3 groups, 3.9 Å for dangling CH2 groups, and 4.2 Å for the vertical ends of carbon nanotubes. Therefore, Pt(PF3)4 adsorbs on the substrate as Pt(PF3)3-C+PF3(g). Pt(PF3)3 molecules are relatively mobile on the carbon surface, and during the purge period of a pulsed CVD or ALD sequence, Pt(PF3)3 molecules migrate to one of the hypothetical defects. The Pt(PF3)3 bond is stronger at the defect, and as a result, once anchored, the Pt(PF3)3 molecule is immobile, allowing the nucleation of platinum nanodots.

[0054] In catalytic applications, platinum nanodots tend to increase in size, which is known to shorten their useful life in catalytic electrodes. This can be prevented by applying a thin, porous, protective inorganic coating to all portions of the substrate between the platinum nanodots; this process, known as "nanocaging" of the platinum dots, is known as such and is described below. Such thin, porous inorganic coatings can be deposited by a vapor deposition process, such as ALD. This vapor deposition process for creating the porous inorganic coating is referred to herein as the "second" vapor deposition process.

[0055] The so-called second vapor deposition process will now be described in more detail. Preferred porous inorganic coatings used to form metal oxide nanocagings are selected from the following list (but are not limited to): niobium oxide, tantalum oxide, vanadium oxide, tungsten oxide, molybdenum oxide, titanium oxide, zirconium oxide, hafnium oxide, cobalt oxide, nickel oxide, yttrium oxide, cerium oxide, lanthanum oxide, and other elements of the rare earth / lanthanide series. Niobium(V) oxide is a particularly preferred oxide for the inorganic coating.

[0056] Examples of metal oxide precursors used in this second vapor deposition process include, but are not limited to, Nb(=NtBu)(nEt2)3, Nb(=NtBu)(nMe2)3, Nb(=NtBu)Cp(nEt2)2, Nb(=NtBu)Cp(nMe2)2, Nb(=NbtBu)(MeCp)(nEt2)2, Nb(=NtBu)(MeCp)(nMe2)2, Nb(=NtBu)(OtBu)2(nEt2), Nb(=NtBu)(OtBu)2(nEt2), Ta(=NtBu)(nEt2)3, Ta(=NtBu)(nM e2)3, Ta(=NtBu)Cp(nEt2)2, Ta(=NtBu)Cp(nMe2)2, Ta(=NbtBu)(MeCp)(nEt2)2, Ta(=NtBu)(MeCp)(nMe2)2, Ta(=NtBu)(OtBu)2(nEt2), Ta(=NtBu)( Zr( nBuCp)(nMe2)3, Zr(iBuCp)(nMe2)3, Hf(nMe2)4, Hf(nEtMe)4, HfCp(nMe2)3, HfMeCp(nMe2)3, Hf(EtCp)(nMe2)3, Hf(nPrCp)(nMe2)3, Hf(tBuCp)(nM e2)3, Hf(nBuCp)(nMe2)3, Hf(iBuCp)(nMe2)3, Ti(nMe2)4, Ti(nEt2)4, TiCp(nMe2)3, Ti(Me5Cp)(nMe2)3, Ti(Me5Cp)(oMe)3, Ti(Me5Cp)(OiPr)3, Ti (OiPr)4, V(nMe2)4, V(nEtMe)4, W(CO)6, W(MeCp)(CO)3(NO), W(Me3CHD)(CO)3, W(=NtBu)2(nMe2)2, W(=NtBu)2(nHtBu)2, W(=NtBu)2(OtBu)2, WH2(i PrCp)2, Mo(CO)6, Mo(MeCp)(CO)3(NO), Mo(Me3CHD)(CO)3, Mo(=NtBu)2(nMe2)2, Mo(=NtBu)2(nHtBu)2, Mo(=NtBu)2(OtBu)2, MoH2(iPrCp)2, CoCp2,Co(MeCp)2, Co(EtCp)2, CoCp(CO)2, Co(iPr2-amd)2, Co(CO)3(NO), CoCp(CO)2, Co(iPr2-amd)2, Co(CO)3(NO), NiCp2, Ni( MeCp)2, Ni(EtCp)2, Ni(iPr2-amd)2, Ni(tBu2-amd)2, Ni(allyl)(iPr2-amd), Ni(PF3)4, Y(MeCp)3, Y(EtCp)3, Y(iPrCp)3, Y (nBuCp)3, Y(EtCp)(iPr2-amd)2, Y(MeCp)(iPr2-amd)2, Ce(iPrCp)3, Ce(nBuCp)3, Ce(EtCp)(iPr2-amd)2, Ce(iPrCp)(iP r2-amd)2, La(iPrCp)3, La(nBuCp)3, La(MeCp)(iPr2-amd)2, La(EtCp)(iPr2-amd)2, La(iPrCp)(iPr2-amd)2 or mixtures thereof. ,

[0057] It is desirable not to coat the platinum nanodots themselves, as this could reduce their catalytic activity, but to coat only the carbon nanotube substrate between adjacent platinum nanodots. This can be achieved by protecting the platinum nanodots with a blocking agent before depositing the inorganic coating. The deposition of zirconium dioxide by ALD onto blocking-agent-protected platinum nanodots is known per se (so-called "nanocaging") from the publication by N. Cheng et al., "Encapsulation of Highly Stable Platinum Nanoparticles in Zirconia Nanocages by Area-Selective Atomic Layer Deposition for the Oxygen Reduction Reaction," Advanced Materials, Vol. 27, No. 2, January 14, 2015, pp. 277-281.

[0058] The blocking agent can be a surfactant, a thiol, an acid, or an amine such as oleylamine ((9Z)octadec-9-en-1-amine, n° CAS: 112-90-3). A detailed description of nanodot caging with niobium oxide is provided below in connection with Example 3, and this same process can be used for other oxides. Examples of blocking agents include, but are not limited to, C1-C amines such as oleylamine, dodecylamine, hexadecylamine, and octadecylamine. 30 Alkylamines, C1-C 30 C1-C such as allylamine, 1-octanethiol, dodecanethiol, hexadecanethiol, octanethiol, and octadecanethiol 30 Included are alkylthiols and acids such as linoleic acid, decanoic acid, lauric acid, oleic acid, stearic acid, tetradecylphosphonic acid, and mixtures thereof.

[0059] The process of the present invention results in a uniform distribution of platinum dots across the surface of the VACNT array, with a particularly narrow and uniform size distribution. The average size of the platinum nanodots is preferably between 2 nm and 3 nm. The process of the present invention uses a platinum deposition process to avoid heating the surface of the VACNTs to temperatures above about 300°C, which can cause unacceptable degradation of the nanotubes. The process of the present invention can produce catalyst films in which the vertical alignment of the carbon nanotubes is maintained after the VACNTs are transferred from the native metal substrate to the film.

[0060] The catalyst film comprises a polymer layer deposited on the fully prepared VACNT array, along with platinum nanodots and optionally nanocaging. The polymer layer can be an ionomer layer such as Nafion. [Example]

[0061] Example 1: Platinum Deposition on VACNT Powder and Carbon Powder

[0062] Platinum nanodots were deposited on VACNT powder (provided by Nawa Technologies) and KET Jenblack EC300J (KB, provided by Lion Specialty Chemicals Co., Ltd.) by continuous injection (pulsed CVD process) of Pt precursor and co-reactant using a homemade fluidized reactor with a vibration motor. VACNT or KB powder was packed into a glass tube reactor. Stainless steel filters were attached to the bottom and top of the reactor. The reactor was evacuated, and N2 gas was flowed from the bottom of the reactor to form a fluidized bed. To dry the powder before deposition, the reactor was heated to 200 °C in an N2 stream for more than 2 hours. After drying, the reactor was kept at 150 °C, and the gas lines were kept at 40 °C to prevent precursor condensation. The Pt precursor and reactant were Pt(PF3)4 and H2 gas, respectively, and N2 was used as the carrier gas and purge gas. Pt(PF3)4 was 19 The Pt was synthesized by Air Liquide with 99% purity as demonstrated by F-NMR. For the pulsed CVD Pt deposition process, each sequence consisted of a 200-second Pt(PF3)4 pulse, a 600-second N2 purge, a 500-second H2 pulse, and a 600-second N2 purge. These sequences were repeated 2, 4, 8, or 18 times (only 18 times for VACNTs).

[0063] Using a Metra Toledo instrument, thermogravimetric analysis was performed on a sample of powdered VACNT arrays according to the present invention after a given number of ALD pulses as follows: Approximately 5 mg of powder sample was placed in a ceramic crucible. A heating ramp was performed from 25 °C to 950 °C at 10 degrees per minute in a nitrogen-oxygen atmosphere (80% and 20%, respectively). This allowed for the determination of mass loss as the carbon burned off, and ultimately the residual platinum content of the powder. The results are shown below.

[0064] Figure 6 shows the NMR spectra of the Pt(PF3)4 used, measured on a Magritek Spinsolve 60 NMR spectrometer. 19F NMR spectra were obtained, and the neat liquid product was placed in a glass NMR tube.

[0065] Figure 7 shows the Pt loading (wt%) (Figure 7a) and utilization efficiency of the Pt(PF3)4 precursor on the VACNT powder (Figure 7b) as a function of Pt(PF3)4 / H2 cycles. The Pt loading was measured using thermogravimetric analysis (TGA). The loadings were 8.1 wt% (weight percent), 14.5 wt%, 24.4 wt%, and 37.1 wt% for 2, 4, 8, and 18 cycles, respectively. The utilization efficiency of the Pt(PF3)4 precursor was very high, especially for the 2- and 4-cycle tests. The efficiency decreased with increasing cycle number under non-optimized conditions.

[0066] Figure 8 shows the corresponding curves for the KB powder. The loadings were 3.9 wt%, 7.5 wt%, and 13.2 wt% for cycles 2, 4, and 8, respectively, which were significantly lower than those deposited on VACNT powder, even though the deposition conditions were unchanged. The utilization efficiency of Pt(PF3)4 was significantly lower on the KB powder compared to the VACNT powder.

[0067] Figure 9 shows the TGA analysis of 18-cycle Pt ALD samples (three replicates, curve (b)) compared to pristine VACNTs (curve (a)). The solid line shows the residual mass (expressed in wt%) and the dotted line shows the DTA.

[0068] Figure 10 shows the X-ray spectroscopy (abbreviated as XPS) analysis of Pt ALD samples with different cycle numbers: (a) 18 cycles, (b) 8 cycles, (c) 4 cycles, (d) 2 cycles, and (e) untreated.

[0069] The particle size distribution of the Pt nanodots was investigated by transmission electron microscopy (TEM). Figure 11 shows the BF-TEM images and nanodot size distribution as a function of Pt(PF3)4 / H2 cycles (the abbreviation BF refers to bright-field mode). As the number of cycles increases, the particle size also increases, but even with an 18-cycle process, the size can be controlled mainly between 1 nm and 3 nm. Furthermore, the Pt nanodots were well dispersed on the VACNTs, and no severe aggregation of the Pt nanodots was observed even with an increased number of cycles.

[0070] Example 2: Pt deposition on VACNT / Al sheets

[0071] Pt nanodots were deposited on VACNT / Al sheets (supplied by Nawa Technologies) using a pulsed CVD process in a homemade reactor. The VACNT / Al sheets were loaded into a glass tube reactor. To dry the VACNT powder before deposition, the reactor was heated to 200 °C for over 2 h. After drying, the reactor was maintained at 150 °C while the gas lines were kept at 40 °C to prevent precursor condensation. The Pt precursor and reactants were Pt(PF3)4 and H2 gas, respectively, with N2 used as the carrier and purge gas. 99% pure Pt(PF3)4 was synthesized by Air Liquide. The Pt deposition process consisted of a 200-second Pt(PF3)4 pulse, a 600-second N2 purge, a 500-second H2 pulse, and a 600-second N2 purge sequence repeated 18 times.

[0072] To investigate whether Pt is distributed from the surface to the Al substrate in the VACNT / Al sheet, samples were cut and exposed cross-sectional images were acquired using SEM-EDS analysis (Hitachi SU9000) (the abbreviation EDS stands for energy dispersive spectroscopy). As-deposited and post-deposition annealed samples (5% H2 atmosphere) were analyzed using EDS across the thickness of the cross-sectional images. These line scans show that Pt is well distributed from the surface to the bottom of the VACNT, suggesting that Pt(PF3)4 may be a good Pt precursor for roll-to-roll processing of VACNT / Al foils. While impurities such as F remain in the as-deposited samples, the levels of these impurities are significantly reduced by post-deposition temperature treatment or other chemical treatments.

[0073] Example 3: Platinum nanodot caging

[0074] In the first sequence (leading to sample S3-R210929-3-V50), platinum nanodot deposition was performed on a VACNT / Al sheet (supplied by Nawa Technologies) by sequentially injecting the Pt precursor and co-reactant (pulsed CVD process) using a homemade reactor. The VACNT / Al sheet was loaded into a glass tube reactor. To dry the VACNT / Al sample before deposition, the reactor was heated to 200 °C for over 2 h. After drying, the reactor was kept at 150 °C, and the gas lines were maintained at 40 °C to prevent precursor condensation. The Pt precursor and reactant were Pt(PF3)4 and H2 gas, respectively, with N2 used as the carrier and purge gas. 99% pure Pt(PF3)4 was synthesized by Air Liquide. For the Pt deposition process, a sequence consisting of a 200-second Pt(PF3)4 pulse, a 600-second N2 purge, a 500-second H2 pulse, and a 600-second N2 purge sequence was repeated four times.

[0075] In the second sequence of steps, area-selective deposition of Nb2O5 by ALD was performed on the product obtained in the first sequence of steps as follows: After Pt deposition on the VACNT / Al sheet using Pt(PF3)4 and H2, the sample was immersed in 5 mM oleylamine in ethanol for 5 h. After the oleylamine treatment, the sample was rinsed three times with ethanol and dried in a vacuum desiccator. Nb2O5 was deposited by ALD at 250 °C in a homemade ALD reactor using niobium amide imide organometallic precursor (Air Liquide) and water as the Nb source and co-reactant (hereafter referred to as the "Nb precursor"), respectively. The deposition process consisted of a 30-second Nb precursor pulse, a 300-second N2 purge, a 30-second water pulse, and a 300-second N2 purge sequence, repeated three times. After deposition, the oleylamine was removed by heating in air at 200 °C.

[0076] The localization of Pt particles and the distribution of Nb2O5 on the carbon support were characterized using scanning transmission electron microscopy with energy dispersive X-ray spectroscopy under ambient conditions. It was observed that Nb2O5 deposition occurred primarily on the carbon support, with most of the Pt nanodots remaining exposed even after three Nb2O5 ALD cycles.

[0077] To confirm the stability of the three-cycle Nb2O5 ALD sample on the Pt / C catalyst, in situ environmental TEM was used. To monitor the morphological changes of the samples, the ALD sample and the untreated Pt / C catalyst were first heated at 200 °C in vacuum, and then at 350 °C while injecting air. The sample holder pressure was maintained at 4 Pa. For the three-cycle ALD sample, STEM (scanning transmission electron microscopy) images showed little change in the morphology of the Pt nanodots. However, images of the untreated sample showed severe aggregation of the Pt nanodots, and their size increased after the in situ environmental TEM test. SEM images also revealed severe damage to the carbon surface, with holes formed on the carbon surface, suggesting that a catalytic reaction between carbon and oxygen had occurred on the untreated Pt / C catalyst.

[0078] Example 4: Comparison between VACNT arrays obtained by one-step and two-step processes

[0079] Figures 12a, 12b, 13a, and 13b display transmission electron microscope images showing platinum nanodots deposited using the ALD process described in Example 2. The platinum nanodots were deposited on a VANCT array obtained by one-step pyrolysis, periodically injecting a catalyst precursor (ferrocene dissolved in toluene) into the VACNT growth reactor. The VACNT growth was based on using acetylene as the carbon source. Corresponding histograms of platinum particle size distribution are also shown (see Figures 12c and 13c), with the horizontal axis in nanometers. Figures 12a, 12b, and 12c refer to four ALD cycles, as described in Example 2, while Figures 13a, 13b, and 13c refer to 18 such ALD cycles.

[0080] Figures 14a, 14b, 15a, and 15b show transmission electron microscope images of platinum nanodots deposited on VACNT arrays obtained in a two-step process using the ALD process described in Example 2. In this process, in the first step, a VACNT growth catalyst was deposited on a VACNT growth substrate, and in the second step, VACNT arrays were grown using acetylene as the carbon source. Corresponding histograms of platinum particle size distribution are also shown (Figures 14c and 15c), with the horizontal axis in nanometers. Figures 14a, 14b, and 14c show four ALD cycles described in Example 2, and Figures 15a, 15b, and 15c show 18 such ALD cycles.

[0081] A comparison between Figures 12 and 13 on the one hand and Figures 14 and 15 on the other hand is striking, as both are based on the same ALD process for the deposition of platinum nanodots, the only difference being the preparation process for creating the VACNT arrays. It can be immediately seen that the platinum ALD process results in much coarser platinum particles on the VACNT arrays obtained with the two-step growth process than with the one-step growth process.

[0082] Example 5: Comparison of mass and surface activity of platinum nanodot catalyst deposited on VACNT arrays obtained in a one-step process according to the present invention with that of a prior art platinum nanodot catalyst on carbon powder

[0083] This electrochemical test was performed on seven samples: three samples according to the present invention with 4, 8, and 18 platinum ALD cycles, two commercially available platinum catalysts on carbon powder (reference Pt_C_Vulcan 29% and Pt_C_Vulcan 47.2%, the percentage values ​​indicating platinum loading), and two platinum nanodot catalysts prepared by ALD using a Pt(PF3)4 precursor on commercial carbon powder (Pt_XC72_ALD150C and Pt_XC72_ALD300C, where XC72 is the reference for the powder and the four commercial catalyst samples, and 150 C and 300 C are the temperatures of ALD deposition, respectively).

[0084] Electrochemical testing was performed in a conventional rotating disk electrode configuration. Catalyst powder was used to prepare ink (for which the VACNT arrays were powdered), and the ink was coated onto the tip of a rotating electrode in an electrochemical cell using an electrolyte of 0.1 M perchloric acid (HClO). A potentiostat (Biologic VPM3) was used.

[0085] The results are summarized in Figure 16 for two different potentials, 0.90 V and 0.95 V, where 0.90 V is found to be very close to the electrochemical conditions experienced by catalysts in oxygen-hydrogen fuel cells. Figure 16a shows the surface activity (abbreviated SA), [μA / cm 2 ], which is the current measured at a specific potential divided by the active surface of the platinum nanodots. Figure 16b shows the mass activity (abbreviated MA), which is expressed in [A / mg], which is the current measured at a specific potential divided by the platinum mass. At a potential of 0.90 V, it can be seen that the activity of the catalyst according to the invention (after 18 cycles of platinum deposition by ALD) is close to that of the catalyst according to the prior art.

[0086] Example 6: Comparison of mass activity of platinum nanodot catalyst deposited on VACNT array obtained by one-step process according to the present invention with that on carbon powder according to the prior art used as gas diffusion electrode

[0087] Commercially available gas diffusion electrodes (H23C8) were cut to size and transferred to the gas diffusion electrodes using a laboratory calender (TOB JS-300) with a maximum pressure of 500 N / m. Reference samples were created by coating the gas diffusion electrodes with ink prepared using catalyst powder. The ink was then dried at 70 °C for 5 minutes. The electrodes were then placed in an electrolysis cell, and an electrolyte containing 1 M perchloric acid (HClO4) was added. Mass activity was determined at different potentials using different catalysts. The results are shown in Figure 17 for different catalysts: two catalysts of the present invention (reference Pt_VACNT_150C-Nafion and Pt_VACNT_150C-ss Nafion) and three prior art catalysts (reference Pt-20%_XC72, Pt-30%_XC72, and Pt-47%_TEC).

[0088] Pt_VACNT_150C-ss Nafion corresponds to the as-prepared VACNT electrode. The activity appears to be lower than the reference catalyst. This is expected because VACNTs are highly hydrophobic, and the oxygen reduction reaction can only occur at the gas-liquid interface, i.e., on top of the VACNT carpet. A small amount of Nafion was added to the VACNT electrode (Pt_VACNT_150C-Nafion) by impregnating a droplet of Nafion solution with subsequent drying. This expands the gas-liquid interface within the VACNT carpet, allowing for better utilization of the electrode's volume. As expected, the activity of the catalyst increases significantly.

[0089] Example 7: Platinum deposition on commercial carbon nanotube powder

[0090] A powder consisting of multi-walled carbon nanotubes, sold by Nanocyl under the trade name NC7000™, was prepared. This powder was obtained from multi-walled carbon nanotubes produced by a catalytic carbon deposition process. Transmission electron microscope (TEM) images showed that the powder particles had an average diameter of approximately 9.5 nm and an average length of approximately 1.5 μm. TGA showed that the carbon purity was approximately 90% and the metal oxide content was approximately 10% (originating from the aluminum growth support and including the iron catalyst). The specific surface area, determined by the Brunauer-Emmett-Teller (BET) method, was 250 m 2 / g~300m 2 / g. Aluminum contamination of the powder was also detectable by XPS (no such contamination was found in the VACNT samples provided by Nawa Technologies). No amorphous carbon was detected by high-resolution transmission electron microscopy (HRTEM).

[0091] Figure 18 shows TGA measurements of pristine carbon nanotube powder (curve (a)) and nanotube powders that had undergone four (curve (b)), eight (curve (c)), or eighteen (curve (d)) platinum depositions by ALD using a Pt(PF3)4 precursor, corresponding to platinum loadings of 0 wt%, 6.3 wt%, 11.9 wt%, and 18.6 wt%, respectively. TGA measurements (synthetic air, temperature ramp 10 °C / min) were performed on samples dried at 200 °C in pure nitrogen, using a temperature ramp of 10 °C / min. The residual masses at the end of the TGA tests were 10.2 wt%, 16.5 wt%, 22.1 wt%, and 28.8 wt%, respectively.

[0092] Figures 19a and 19b show the percent utilization and weight percent residual mass after 4, 8, and 18 ALD cycles, respectively. Compared to Figures 7a and 7b for Example 1 (VACNT powder), the platinum utilization and residual mass are much lower when using this commercial carbon nanotube powder.

[0093] Platinum deposition by ALD using a Pt(PF3)4 precursor is possible with this commercially available carbon nanotube powder. As shown in the TEM-BF (TEM in bright-field mode) and STEM-HAADF (scanning TEM in high-angle annular dark-field mode) images after four ALD cycles using Pt(PF3)4 / H2 (see Figure 20a for TEM-BF and Figure 20b for STEM-HAADF), small nanodots are observed distributed unevenly. In some areas, few such nanodots are observed, while in other areas, aggregates are observed. Both images in Figure 20 show the same region of the sample and can be overlaid.

[0094] Combined with the lower precursor utilization efficiency observed by TGA, lower precursor nucleation is observed for the commercial nanotube powder compared to VACNTs.

Claims

1. A method for manufacturing an array of vertically oriented carbon nanotubes for use in catalytic electrodes of fuel cells or electrolytic cells, comprising the following steps: - To provide an array of vertically oriented carbon nanotubes obtained by a vapor phase growth process in which a precursor of a carbon nanotube growth catalyst is continuously added to the supply gas. - Depositing multiple platinum nanodots on the outer surface of the vertically oriented carbon nanotubes using a first vapor-phase deposition process. A method that includes [a certain feature].

2. The method according to claim 1, wherein the first gas phase deposition process is atomic layer deposition, chemical vapor deposition, or pulsed chemical vapor deposition.

3. The method according to claim 1 or 2, wherein the first gas-phase deposition process is carried out at a temperature of less than 300°C, preferably 25°C to 275°C, and more preferably 50°C to 250°C.

4. Said first vapor deposition process comprises a sequence of alternating cycles each having an exposure time and a purge time, wherein during said exposure time said array is contacted with Pt(PF ３ ) ４ gas, and preferably H ２ , H ２ O, O ２ , O ３ , NO ２ , oxygen radicals and mixtures thereof, NH ３ , SiH ４ , Si ２ H ６ , Si ３ H ８ , SiH ２ Me ２ , SiH ２ Et ２ , N(SiH ３ ) ３ , SiH ２ (NEt ２ )2, other Si-H containing reactants, hydrogen radicals, hydrazine, methylhydrazine, amines, NO, N ２ O, borane, B ２ H ６ , CH ４ , C ２ H ６ , CH ３ I, and mixtures thereof, the method of claim 2.

5. The method according to claim 4, wherein the duration of each of the alternating cycles, the exposure time and / or the purge time, is between 0.1 seconds and 60 minutes, preferably between 1 second and 1000 seconds, and more preferably between 10 seconds and 100 seconds.

6. The method according to claim 4 or 5, wherein the number of sequences is 2 to 100, preferably 5 to 35, more preferably 8 to 30, and most preferably 10 to 25.

7. The volume mass of the array of vertically oriented carbon nanotubes is 0.10 g / cm³. ３ More preferably 0.15 g / cm³ ３ Even more preferably, 0.20 g / cm³ ３ More preferably 0.30 g / cm³ ３ The method according to claim 1 or 2, wherein the method is greater than or equal to the substrate on which the VACNT array is deposited.

8. The volume mass of the array of vertically oriented carbon nanotubes is 0.70 g / cm³. ３ It should not exceed 0.50 g / cm³, preferably 0.50 g / cm³. ３ The method according to claim 1 or 2, wherein the VACNT array does not exceed a certain value and does not take into account the substrate on which the VACNT array is deposited.

9. The volume mass of the array of vertically oriented carbon nanotubes is 0.10 g / cm³. ３ ~0.45 g / cm ３ Preferably 0.15 g / cm³ ３ ~0.30 g / cm ３ The method according to claim 1 or 2, wherein the VACNT array is between and the substrate on which it is deposited is not taken into consideration.

10. The method according to claim 1 or 2, wherein the total platinum filling of the vertically oriented carbon nanotube array is higher than 10% by weight, preferably higher than 20% by weight, more preferably higher than 30% by weight, and most preferably higher than 40% by weight.

11. The method according to claim 1 or 2, further comprising the steps of first treating the array with a surfactant capable of selectively adhering to platinum dots, and then depositing an inorganic oxide using a second gas-phase deposition process.

12. The method according to claim 11, wherein the surfactant is an alkylamine, an allylamine such as oleylamine, an alkylthiol, or a carboxylic acid such as oleic acid.

13. The method according to claim 11, wherein the second vapor deposition process comprises a sequence of alternating cycles, each having an exposure time and a purge time, during which the array is exposed to an organometallic precursor of a metallic element and a reactive gas to form an inorganic oxide of the metallic element, the metallic element being preferably selected from the group consisting of zirconium, niobium, tantalum, vanadium, tungsten, molybdenum, titanium, hafnium, cobalt, nickel, yttrium, cerium, lanthanum, rare earth elements, or other elements of the lanthanide series.

14. A vertically oriented carbon nanotube array for use as a catalytic electrode in a fuel cell or electrolytic cell, characterized in that the outer surface of the nanotubes is provided with a plurality of platinum nanodots, and the carbon nanotubes have graphite surfaces that are inclined with respect to the principal direction of the nanotubes, forming an angle, or oriented perpendicularly.

15. The vertically oriented carbon nanotube array according to claim 14, wherein the carbon nanotubes have graphite surfaces that form an angle of about 30° to about 90° with respect to the main axis of the nanotubes.

16. The vertically oriented carbon nanotube array according to claim 14, wherein the average diameter of the platinum nanodots is 0.7 nm to 5 nm, preferably 1 nm to 5 nm, more preferably 1 nm to 4 nm, and most preferably 1 nm to 3 nm.

17. The vertically oriented carbon nanotube array according to claim 14, characterized in that the plurality of platinum nanodots comprise face-centered cubic platinum crystals.

18. Use of a vertically oriented carbon nanotube array according to any one of claims 14 to 17 for manufacturing a catalytic electrode for a fuel cell or electrolytic cell.

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