Method of preparing a mesoporous carbon composite material

Abstract

A method of preparing a mesoporous carbon composite material having a mesoporous carbon phase and preformed metal nanoparticles located within the mesoporous carbon phase. The present invention also relates to a mesoporous carbon composite material and to a substrate having a film of such mesoporous carbon composite material.

Claims

1. A method of preparing a mesoporous carbon composite material comprising a mesoporous carbon phase and preformed metal nanoparticles located within the mesoporous carbon phase, the method comprising the steps: a) providing a solution of carbon composite precursors, the solution of carbon composite precursors comprising a structure directing agent capable of forming micelles or lamellar structures, one or several poylmerizable carbon precursor components and a first solvent; b) inducing the solution of carbon composite precursors to polymerize to form a dispersion of polymer in the first solvent, and separating the polymer from the first solvent; c) providing preformed stabilized metal nanoparticles; d) mixing the polymer and the preformed stabilized metal nanoparticles, wherein during the mixing, either the polymer or the preformed stabilized metal nanoparticles or both are dispersed in a second solvent; e) stabilizing the mixture of step d) by subjecting it to a stabilization heat treatment in the range of from 80° C. to 120° C.; and f) subjecting the product of step e) to a carbonization heat treatment in the range of from 500° C. to 1000° C.

2. The method according to claim 1, wherein the method additionally comprises a step: drying the mixture resulting from step d) to yield a solid, which step is performed between steps d) and e).

3. The method according to claim 1, wherein the structure directing agent capable of forming micelles or lamellar structures is a surfactant selected from the group consisting of nonionic surfactants, cationic surfactants, anionic surfactants or zwitterionic surfactants ore mixtures thereof.

4. The method according to claim 3, wherein the surfactant, is a nonionic surfactant, which is a block copolymer.

5. The method according to claim 4 wherein the block copolymer is a poloxamer.

6. The method according to claim 1, wherein the polymerizable carbon precursor components comprise at least one phenolic compound and, optionally, at least one crosslinkable aldehyde compound, wherein the at least one crosslinkable aldehyde compound is added to the solution during step a) or at the beginning of step b).

7. The method according to claim 6, wherein the at least one phenolic compound is selected from the group consisting of phenol, catechol, resorcinol, dihydroquinone, phloroglucinol, cresol, halophenol, aminophenol, hydroxybenzoic acid, and dihydroxybiphenyl.

8. The method according to claim 6, wherein the at least one crosslinkable aldehyde compound is selected from the group consisting of formaldehyde, organoaldehydes, and organodialdehydes, represented by formulae HCHO, R—CHO and OHC—R—CHO, respectively, wherein R is a bond, a straight-chained, branched or cyclic hydrocarbonyl group, which can be either saturated or unsaturated, typically containing at least 1, 2, or 3 carbon atoms and up to 4, 5, 6, 7, 8, 9, or 10 carbon atoms.

9. The method according to claim 6, wherein the at least one crosslinkable aldehyde compound is formaldehyde.

10. The method according to claim 1, wherein the preformed metal nanoparticles are nanoparticles of one or more metals selected from the group consisting of Sn, Cu, Ag, Au, Zn, Cd, Hg, Cr, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt.

11. The method according to claim 10, wherein the preformed metal nanoparticles are nanoparticles of one or more metals selected from the group consisting of Pt, Pd, Ru, Rh, Ir, Os and Ru.

12. The method according to claim 10, wherein the preformed metal nanoparticles are nanoparticles of one or more metals selected from the group consisting of Pd, Ru, Rh and Ir.

13. The method according to claim 1, wherein the preformed stabilized metal nanoparticles do not include carbon nanoparticles, wherein said carbon nanoparticles are carbon blacks, carbon onions, fullerenes, carbon nanodiamonds and carbon nanobuds.

14. The method according to claim 1, wherein the preformed stabilized metal nanoparticles have a metallic core of one or several metals selected from the group consisting of Sn, Cu, Ag, Au, Zn, Cd, Hg, Cr, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt stabilized by a quaternary ammonium cation or another ionic stabilizing agent.

15. A method of preparing a mesoporous carbon composite material comprising a mesoporous carbon phase and preformed metal nanoparticles located within the mesoporous carbon phase, the method comprising the steps: a) providing a solution of carbon composite precursors, the solution of carbon composite precursors comprising a structure directing agent capable of forming micelles or lamellar structures, one or several poylmerizable carbon precursor components and a first solvent; b) inducing the solution of carbon composite precursors to polymerize to form a dispersion of polymer in the first solvent, and separating the polymer from the first solvent; c) providing preformed stabilized metal nanoparticles; d) mixing the polymer and the preformed stabilized metal nanoparticles, wherein during the mixing, either the polymer or the preformed stabilized metal nanoparticles or both are dispersed in a second solvent; e) stabilizing the mixture of step d) by subjecting it to a stabilization heat treatment in the range of from 80° C. to 120° C.; f) subjecting the product of step e) to a carbonization heat treatment in the range of from 500° C. to 1000° C.; and g) applying the mixture resulting from step d) to a substrate to form a polymer film having micelles or lamellar structures and metal nanoparticles within, which step is performed between steps d) and e).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows an embodiment of a method in accordance with the present invention. More specifically, in this example, a solution of carbon composite precursors comprises a poloxamer, e.g. Pluronic® F127, as a structure directing agent that is capable of forming micelles, resorcinol as phenolic compound and formaldehyde as crosslinkable aldehyde compound. The preformed colloidal metal nanoparticles are colloidal PdNP (i.e. palladium nanoparticles) in THF.

(2) FIG. 2 shows an embodiment of a method in accordance with the present invention, namely the synthesis procedure for ordered mesoporous carbon films with incorporated preformed Rh nanoparticles.

(3) FIG. 3 shows a transmission electron microscopic (TEM) analysis of rhodium nanoparticles deposited onto a carbon coated Cu grid.

(4) FIG. 4 shows the scanning electron microscopic (SEM) analysis of an ordered mesoporous carbon film with incorporated rhodium nanoparticles after carbonization at 700° C. in H.sub.2/Ar.

(5) FIG. 5 shows the transmission electron microscopic (TEM) analysis of an ordered mesoporous carbon film with incorporated rhodium nanoparticles after carbonization at 700° C. in H.sub.2/Ar.

(6) FIG. 6 shows a characterization by physisorption of a PtNP/OMC powder, carbonized at 700° C.

(7) FIG. 7 shows a small angle X-ray scattering (SAXS) characterization of a RuPtNP/OMC film, carbonization at 700° C. in H.sub.2/Ar.

(8) FIG. 8 shows an electrochemical characterization of a F127-templated MeNP/OMC film.

(9) FIG. 9 shows SEM and SAXS analyses of an OMC, a Pt/OMC and a PtNP/OMC film.

(10) FIG. 10 shows SEM and SAXS analyses of an OMC, a Pt/OMC and a PtNP/OMC film.

(11) FIG. 11 shows the electrocatalytic performance of F127-templated Pt-containing catalyst films in 0.5 M sulfuric acid.

(12) FIG. 12 shows the electrocatalytic evaluation of Pt-containing catalyst films in 0.5 M sulfuric acid.

(13) More specifically, FIG. 1 shows the preparation of PdNP/OMC, using a poloxamer as structure directing agent, resorcinol as phenolic compound and formaldehyde as crosslinkable aldehyde compound.

(14) FIG. 2 shows an embodiment of the employed synthesis procedure for ordered mesoporous carbon films with incorporated preformed Rh nanoparticles. To synthesize the reducing agent tetraoctylammonium triethylhydroborate (N(octyl).sub.4BEt.sub.3H), tetraoctyle ammoniumbromide is dissolved in THF and potassium triethylhydroborate in THF is added under inert atmosphere. The mixture is cooled down to 0° C. After 20 hours KBr is separated as a white precipitate by filtration and washed. RhCl.sub.3 as metal precursor is dissolved in THF. While stirred, the freshly prepared N(octyl).sub.4BEt.sub.3H solution is added to the dissolved metal precursor. After 24 hours the stabilized colloidal nanoparticles can be employed.

(15) For the synthesis of the polymer precursor, resorcinol and the pore template Pluronic F127 were dissolved in EtOH until a clear solution was obtained. Then 3 M HCl was added and the tube was shaken for 30 minutes. Thereafter formaldehyde solution (37% in water) was added with continued shaking. Ten minutes after addition of formaldehyde a white precipitate was separated via centrifugation and the remaining solution discarded. The white precipitate was washed with water and centrifuged again two times. The obtained polymer/template phase was subsequently freeze dried for 12 h to remove all volatile components.

(16) For the RhNP/OMC film synthesis the freeze-dried polymer precursor was dissolved in THF under Ar atmosphere and shaken for 10 min. RhNP colloid in THF was added. The mixture was shaken for another 10 min. The resulting homogeneous black suspension was employed for film casting. Catalyst films were deposited via dip-coating at room temperature in Ar atmosphere inside a glove-box. The coated substrates were transferred to a drying furnace and then treated for 12 h at 100° C. in air for film stabilization. The stabilized films were transferred into a tube furnace and heated with 3 K/min in H.sub.2/Ar (4 vol % H.sub.2) flow to 700° C., holding this temperature for 3 h. The films were naturally cooled down to room temperature in H.sub.2/Ar flow.

(17) FIG. 3 shows the transmission electron microscopic (TEM) analysis of rhodium nanoparticles deposited onto a carbon coated Cu grid. a) TEM micrograph, together with an TEM image in high resolution of one particle (inset). The determined distance of 2.2 Å can be assigned to the (111) lattice plane distance of cubic rhodium (PDF: 01-087-0714). b) The diameter of 360 particles was determined and plotted in a histogram. The mean diameter amounts to 1.9±0.4 nm. Small, monodisperse and stable nanoparticles can be synthesized.

(18) FIG. 4 shows the scanning electron microscopic (SEM) analysis of an ordered mesoporous carbon film with incorporated rhodium nanoparticles after carbonization at 700° C. in H.sub.2/Ar. a) Top-view SEM image. The darker round areas correspond to templated mesopores and the small lighter spots can be attributed to Rh nanoparticles. b) Cross-section SEM image of film with a film thickness of ca. 265 nm.

(19) The films show a high degree of pore ordering. The films are crack-free and show a homogeneous film thickness.

(20) FIG. 5 shows the transmission electron microscopic (TEM) analysis an ordered mesoporous carbon film with incorporated rhodium nanoparticles after carbonization at 700° C. in H.sub.2/Ar. a) TEM micrograph, together with an TEM image in high resolution of one particle (inset). The determined distance of 2.2 Å can be assigned to the (111) lattice plane distance of cubic rhodium (PDF: 01-087-0714). b) The diameter of 90 particles was determined and plotted in a histogram. The mean diameter amounts to 1.9±0.6 nm.

(21) After carbonization the nanoparticles retain their small size. The particles are well distributed inside the mesoporous carbon matrix ensuring a high accessibility of the metal species.

(22) FIG. 6 shows the characterization by physisorption of a PtNP/OMC powder, carbonized at 700° C.: a) isotherm of a N.sub.2 physisorption measurement at 77K. b) NLDFT evaluation of the isotherm (a) with assumption of a cylindrical pore configuration. The specific surface area from BET evaluation amounts to 613 m.sup.2/g.

(23) Physisorption measurements show a high surface area and porosity of the MeNP/OMC materials. The materials exhibit a high degree of micro- and mesoporosity. NLDFT evaluation proves the abundant presence of templated mesopores with a pore diameter of ca. 5 nm.

(24) FIG. 7 shows the small angle X-ray scattering (SAXS) characterization of a RuPtNP/OMC film, carbonized at 700° C. in H.sub.2/Ar: 2D-SAXS pattern recorded in transmission mode with an incident angles of 90° (a) and 10° (b). a) The pattern shows an isotropic ring which can be attributed to a d-spacing value of 13.4 nm for the (110) plane of a mesopore lattice. b) The SAXS measurement in a tilted angle shows diffraction spots on an ellipsoidal ring. The scattering spots can be assigned to the (101) and (110) lattice planes of a contracted cubic pore system

(25) SAXS analysis proves a high degree of pore ordering. The d-spacing of the (110) plane amounts to 7.6 nm, indicating a film strong film shrinkage in direction of the film's normal of ca. 50% during template removal and carbonization.

(26) FIG. 8 shows the electrochemical characterization of a F127-templated MeNP/OMC films. The electrocatalytic activity was studied by cyclic voltammetry in 0.5 M H.sub.2SO.sub.4. a) 2.sup.nd cycle of cyclic voltammetry of MeNP/OMC catalyst films and a commercial Pt/C/Nafion reference catalyst. f) The observed overpotential at −50 mA μg.sub.Pt.sup.−1.

(27) Electrocatalytic performance studies of bimetallic PtRuNP/OMC catalysts in the HER regime reveal a high activity at low Pt loadings. FIG. 8b shows that a RuPtNP/OMC catalyst reaches a mass-based current density of −50 mA μg.sub.Pt.sup.−1 at an overpotential of −28 mV. A monometallic PtNP/OMC catalyst needs an overpotential which is about two times higher to reach the same performance. A commercial Pt/Vulcan reference catalyst needs an overpotential which is three times higher.

(28) FIG. 9 shows SEM and SAXS analyses of an OMC, a Pt/OMC (route 1) and a PtNP/OMC (route 2) film, as described in Example 8. F127 was used as templating agent and the films were carbonized for 3 h under the given conditions.

(29) a) Cross-section SEM images with FFT insets,

(30) b) SAXS patterns with incident angles of β=90°. The isotropic rings in (b) can be attributed to regular pore lattice distances.

(31) FIG. 10 shows TEM, HR-TEM and XRD analyses of a Pt/OMC (route 1) and a PtNP/OMC (route 2) film. F127 was used as templating agent and the films were carbonized at 700° C. in the given atmosphere.

(32) a) TEM micrograph of a scraped off film segment,

(33) b) particle size distribution of Pt nanoparticles determined by TEM,

(34) c) HR-TEM of a Pt particle with corresponding FFT inset and d) XRD pattern, indicated reflections of cubic Pt (04-0802) and Rietveld refinement.

(35) FIG. 11 shows electrocatalytic performances of F127-templated Pt-containing catalyst films in 0.5 M sulfuric acid. All films were carbonized at 700° C. for 3 h. Pt/OMC was carbonized in N.sub.2 and PtNP/OMC in H.sub.2/Ar.

(36) a) 2.sup.nd cycles of Pt/OMC (route 1) catalysts with Pt loading of 1.2 μg/cm.sup.2 compared to a PtNP/OMC catalyst (route 2) with 1.1 μg/cm.sup.2.

(37) b) and c) Current density at a potential of −75 mV vs. RHE at the 2.sup.nd (b) and 50.sup.th (c) cycle plotted against the geometric Pt loading of Pt/OMC and PtNP/OMC catalysts. Each point represents one measured catalyst. The mass loading was determined by WDX and StrataGem software.

(38) FIG. 12 shows an electrocatalytic evaluation of Pt-containing catalyst films in 0.5 M sulfuric acid. A Pt/OMC (route 1) catalyst with a Pt loading of 1.2 μg/cm.sup.2, a PtNP/OMC (route 2) catalyst with 1.1 μg.sub.Pt/OMC catalyst and a Pt/Vulcan/Nafion reference catalyst with 1.0 μg.sub.Pt/cm.sup.2 are compared. Tafel evaluation (potential E vs. log(current density) of the 2.sup.nd cycle (a) of a cyclic voltammetry measurement recorded in the HER regime with 20 mV/s. b) CV measurements recorded between 50 and 1200 mV: PtNP/OMC (700° C., H.sub.2/Ar), Pt/OMC (700° C., N.sub.2) and Pt/Vulcan/Nafion are compared to OMC film without Pt (800° C., N.sub.2) with a film thickness of 390 nm.

(39) Furthermore, reference is made to the following examples which are given to illustrate, not to limit the present invention.

EXAMPLES

Example 1—Preparation of Colloidal Metal Nanoparticles (RhNP)

(40) The reducing agent Tetraoctylammoniumtriethylhydroborate (N(C.sub.8H.sub.17).sub.4B (C.sub.2H.sub.5).sub.3) H was synthesized under Ar atmosphere (O.sub.2<1 ppm; H.sub.2O<1 ppm) in a glovebox. 547 mg of TOAB was initially dissolved in 1.2 ml THF. During stirring 1.0 ml of KBEt.sub.3H solution was added and a white precipitation formed. The mixture was stirred for 1 h and kept tightly sealed in a freezer (ca. 0° C.) for 20 hours. Thereafter the mixture was brought to room temperature in the glovebox and a white precipitate of KBr was removed by filtration (syringe filter, 5.0 PTFE) and washed with 0.55 ml THF. The filtrate and washing solution was filtrated again (syringe filter, 0.2 μm, Nylon) and a colorless solution was obtained.

(41) The RhNP colloid synthesis was performed under Ar atmosphere in a glovebox. 14.8 mg RhCl.sub.3 were dispersed in 8.2 ml of THF and stirred for 24 h at room temperature. The precursor dissolved completely in THF. 0.5 ml of freshly prepared N(octyl).sub.4BEt.sub.3H/THF solution was then added, upon which the RhCl.sub.3/THF mixture turned black immediately. The mixture was stirred for another 24 hours and after filtration (syringe filter 0.2 μm, Nylon) a homogeneous black colloidal solution was obtained.

Example 2—Preparation of a Solution of Carbon Composite Precursors and Subsequently of Polymer

(42) Initially 1.1 g of resorcinol and 300 mg of the pore template Pluronic F127 were dissolved in 4.5 mL of EtOH in a centrifuge tube until a clear solution was obtained. Then 4.5 mL of 3 M HCl were added and the tube was shaken for 30 minutes. Thereafter 1.2 ml of formaldehyde solution (37% in water) were added with continued shaking. Circa four minutes later the solution became turbid. Ten minutes after addition of formaldehyde a white precipitate was separated via centrifugation (7500 rpm, 10 min) and the remaining solution discarded. The white precipitate was washed with water and centrifuged again two times. The obtained polymer/template phase was subsequently freeze dried for 12 h to remove all volatile components, resulting in 855 mg of resin, which corresponds to ca. 32% of the employed components (resorcinol, formaldehyde, F127).

Example 3—Mixing of the Products of Example 1 and 2 and Further Treatment

(43) For the RhNP/OMC film synthesis the freeze-dried polymer precursor was dissolved in 2 ml of THF under Ar atmosphere and shaken for 10 min. 8 ml of RhNP colloid in THF were added. The mixture was shaken for another 10 min. The resulting homogeneous black suspension was employed for film casting. Catalyst films were deposited via dip-coating at room temperature in Ar atmosphere inside a glove-box. The withdrawal speed was 300 mm/min. The coated substrates were transferred to a drying furnace and then treated for 12 h at 100° C. in air for film stabilization. The stabilized films were transferred into a tube furnace which was purged for 2 hours with H.sub.2/Ar (4 vol % H.sub.2). Afterwards the tube furnace was heated with 3 K/min in H.sub.2/Ar flow to 700° C., holding this temperature for 3 h, and subsequent naturally cooled down to room temperature.

Example 4—Preparation of Colloidal Metal Nanoparticles (PtNP)

(44) The reducing agent Tetraoctylammoniumtriethylhydroborate (N(C.sub.8H.sub.17).sub.4BH(C.sub.2H.sub.5).sub.3) was synthesized under Ar atmosphere (O.sub.2<1 ppm; H.sub.2O<1 ppm) in a glovebox. 549 mg of TOAB was initially dissolved in 1.2 ml THF. During stirring 1.0 ml of KBEt.sub.3H solution was added and a white precipitation formed. The mixture was stirred for 1 h and kept tightly sealed in a freezer (ca. 0° C.) for 20 hours. Thereafter the mixture was brought to room temperature in the glovebox and a white precipitate of KBr was removed by filtration (syringe filter, 5.0 μm, PTFE) and washed with 0.55 ml THF. The filtrate and washing solution was filtrated again (syringe filter, 0.2 μm, Nylon) and a colorless solution was obtained.

(45) The PtNP colloid synthesis was performed under Ar atmosphere in a glovebox. 52.7 mg PtCl.sub.2 were dispersed in 5 ml of THF and stirred for 24 h at room temperature. The precursor dissolved only partially and a brown precipitate remained visible. 1 ml of freshly prepared N(octyl).sub.4BEt.sub.3H/THF was then added, upon which the PtCl.sub.2/THF mixture turned black immediately. The mixture was stirred for another 24 hours during which nearly all precipitate was dissolved. After precipitate removal (filtration, syringe filter 0.2 μm, Nylon) a homogeneous black colloidal solution was obtained.

Example 5—Preparation of a Solution of Carbon Composite Precursors and Subsequently of Polymer

(46) Initially 1.1 g of resorcinol and 300 mg of the pore template Pluronic F127 were dissolved in 4.5 mL of EtOH in a centrifuge tube until a clear solution was obtained. Then 4.5 mL of 3 M HCl were added and the tube was shaken for 30 minutes. Thereafter 1.2 ml of formaldehyde solution (37% in water) were added with continued shaking. Circa four minutes later the solution became turbid. Ten minutes after addition of formaldehyde a white precipitate was separated via centrifugation (7500 rpm, 10 min) and the remaining solution discarded. The white precipitate was washed with water and centrifuged again two times. The obtained polymer/template phase was subsequently freeze dried for 12 h to remove all volatile components, resulting in 836 mg of resin, which corresponds to ca. 31% of the employed components (resorcinol, formaldehyde, F127).

Example 6—Mixing of the Products of Example 4 and 5 and Further Treatment

(47) For the PtNP/OMC film synthesis the freeze-dried polymer precursor was dissolved in 3.3 ml of THF under Ar atmosphere and shaken for 10 min. 1.7 ml of PtNP colloid in THF were added. The mixture was shaken for another 10 min. The resulting homogeneous black suspension was employed for film casting. Catalyst films were deposited via dip-coating at room temperature in Ar atmosphere inside a glove-box. The withdrawal speed was varied (60, 150, 300 mm/min) to obtain films of different thicknesses. The coated substrates were transferred to a drying furnace and then treated for 12 h at 100° C. in air for film stabilization. The stabilized films were transferred into a tube furnace and heated with 3 K/min in H.sub.2/Ar (4 vol % H.sub.2) flow to 700° C., holding this temperature for 3 h, and subsequent naturally cooling down to room temperature.

Example 7—Analysis of Resultant Mesoporous Carbon Composite Material

Experimental

(48) SEM images were collected on a JEOL 7401F at 10 kV. Image J program, version 1.39u (http://rsbweb.nih.gov/ij), was employed to determine pore diameters, film thicknesses, sizes of nanoparticles and to obtain fast Fourier transformations (FFT) of images. TEM images were recorded on a FEI Tecnai G.sup.2 20 S-TWIN operated at 200 kV. Colloidal PtNP or fragments of scraped off film segments were deposited on carbon-coated copper grids.

(49) SAXS analysis of MeNP/OMC films was measured at BESSY mySpot beamline with 12.518 keV and sample-to-detector distance of 753.671 mm. A marCCD detector with 3072×3072 px was employed.

(50) The electrical conductivity of PtNP/OMC coatings on SiO.sub.2 substrates was measured with a Keithley Model 6517B Electrometer employing an 8×8 pin probe head with an alternating polarity sequence of the pins.

(51) The pore system of PtNP/OMC was analyzed via N.sub.2 physisorption isotherms recorded at 77 K on powder samples using a Quantachrome Autosorb-iQ. The samples were degassed in vacuum at 150° C. for 2 h prior to physisorption analysis. The surface area and pore size was evaluated with a NLDFT equilibrium Kernel and a model assuming cylindrical pores. The surface area of MeNP/OMC films coated on both sides of double side polished Si wafers was measured with Kr physisorption at 77 K using an Autosorb-iQ (Quantachrome). Prior to adsorption measurement the samples were degassed for 2 h at 150° C. in vacuum. The surface area was calculated via the Brunauer-Emmett-Teller (BET) method.

(52) The microscopy analysis results are shown in FIGS. 3-5, the N.sub.2 physisorption results are shown in FIG. 6, the SAXS analysis is shown in FIG. 7, and the catalytic activity is exemplarily shown in FIG. 8. Moreover, the BET surface areas and conductivity values measured for some composite materials according to the present invention are shown in the following table:

(53) TABLE-US-00001 BET surface areas and conductivity values of MeNP/OMC materials MeNP/OMC BET BET BET conductivity film m.sup.2/m.sup.2 m.sup.2/g m.sup.2/cm.sup.3 S/cm Ru.sub.0.5Pt.sub.1NP 31 135 154 7.3 Ru.sub.1Pt.sub.1NP 177 794 822 3.44 Ru.sub.3Pt.sub.1NP 190 802 1058 12.26 Ru.sub.5Pt.sub.1NP 273 1349 1818 12.05 RhNP 196 474 818 9.33 PdNP 190 386 998 1.629E−05 RuRhNP 338 1056 492 3.13 PtNP 483 925 976 4.46

(54) In this table, the mass of film per area (mass depth) was calculated using the STRATAGem film analysis software (v 4.3) based on wavelength dispersive X-ray (WDX) spectra analyzed with a JEOL JXA-8530F electron microprobe at 7 and at 10 kV. The mass depth of each element can be determined individually.

(55) The BET surface area per geometric surface (m.sup.2/m.sup.2) can be simply derived as a quotient of both surface areas from physisorption measurements of MeNP/OMC films. The BET surface area per film volume (m.sup.2/cm.sup.3) can be derived from the said BET surface area per geometric surface (m.sup.2/m.sup.2) by dividing the value by the film thickness determined from cross-section SEM measurements. The specific BET surface area per mass (m.sup.2/g) can be derived from the said BET surface area per geometric surface (m.sup.2/m.sup.2) by dividing the value by the the mass depth derived from WDX/StrataGem evaluation.

Example 8 Influence of the Metal Precursor on Structure and Activity

(56) Characteristics of Pt containing OMC films prepared by two synthesis routes were compared: Route 1 (ionic metal precursors) relies on the co-deposition of a polymeric carbon precursor and a structure-directing agent together with dissolved metal ions. Films synthesized via route 1 are denoted as Me/OMC. Route 2 (which is the method according to the invention, colloidal metal precursors) employs preformed colloidal metallic nanoparticles which are deposited together with a polymeric carbon precursor and a structure-directing agent. Films synthesized via route 2 are abbreviated by MeNP/OMC.

(57) The films of both routes possess comparably high weight loadings in order to study the influence of the metal precursor species on film and pore morphology as well as the influence on nanoparticle size and crystallinity. Moreover, the performance in the electrocatalytic HER is compared.

(58) 8.1 Pore Morphology

(59) The pore morphologies of a metal-free OMC, a Pt/OMC and a PtNP/OMC film are studied in FIG. 9 by cross-section SEM analyzes (FIG. 9a) and SAXS (FIG. 9b). All films were synthesized with F127 as structure directing agent. The OMC film was carbonized at 800° C. in N.sub.2, Pt/OMC at 700° C. in N.sub.2 and PtNP/OMC at 700° C. in H.sub.2/Ar. Pt/OMC has a weight loading of 2.9 wt %.sub.Pt and PtNP/OMC of 2.3 wt %.sub.Pt, as determined by WDX/StrataGem evaluation at 10 keV.

(60) Cross-section SEM (FIG. 9a) confirms that all films are homogeneous and completely penetrated by ordered mesopores. The pores of Pt/OMC via route 1 appear less ordered and less densely packed than the pores of OMC and PtNP/OMC via route 2. The FFTs of the cross-section SEM images (insets in FIG. 9a) of OMC and PtNP/OMC show spots which can be attributed to distinct pore lattice planes indicating ordered pore systems. The FFT of the SEM image of Pt/OMC shows an anisotropic ring confirming a locally ordered pore structure without a higher degree of pore ordering.

(61) SAXS studied the pore structure of all films. Each pattern recorded in transmission with 90° (FIG. 9b) features at least two circular diffraction rings. The periodic distances of these most dominant reflections amount to 7.9 nm and 13.5 nm for OMC and 7.8 nm and 13.4 nm for PtNP/OMC, respectively. The distances attributed to these two reflections are circa 25% smaller than the distances observed for Pt/OMC (10.2 nm and 18.7 nm). The smaller periodic distances prove that OMC and PtNP/OMC possess a more densely packed pore system. In case of OMC and PtNP/OMC the ratios of the periodic distances of (112) to (110) equal 3.sup.1/2≈1.7. Accordingly, the diffraction rings can be assigned to the (110) and the (112) planes of a cubic pore lattice..sup.80 The diffraction pattern of Pt/OMC cannot be assigned to a cubic pore system which confirms the observation of SEM analyses that the pore system of Pt/OMC is less densely packed. Yet, the appearances of two distinct rings indicate a higher degree of ordering.

(62) Since the OMC film as well as the PtNP/OMC in FIG. 9 show a high degree of pore ordering, neither the atmosphere nor the temperature during carbonization have a significant influence on the mesostructure. However, the development of a densely packed pore system of Pt/OMC films via route 1 is disturbed. Apparently, metal ions in a high concentration hinder the formation of an ordered mesophase.

(63) PtNP/OMC films with comparably high metal loadings show a well-ordered structure and a mesopore packing as dense as metal-free OMC films. According to this, neither the comparatively large nanoparticles nor the ammonium-based stabilizing agent disturb the mesophase formation.

(64) 8.2 Particle Size, Particle Crystallinity and Degree of Graphitization

(65) Particle size and crystallinity of nanoparticles in Pt/OMC (2.9 wt %.sub.Pt, via route 1) and PtNP/OMC (2.3 wt %.sub.Pt, via route 2) films are studied in FIG. 10 with TEM (FIG. 10a), HR-TEM (FIG. 10c) and XRD (FIG. 10d).

(66) TEM micrographs of Pt/OMC and PtNP/OMC films confirm an abundant presence of templated mesopores (FIG. 10a). The pores of PtNP/OMC appear more ordered than the pores of Pt/OMC (compare to FIG. 9). Dark spots evidence well-distributed Pt nanoparticles. The average diameters of Pt nanoparticles in Pt/OMC amount to 3.3±1.2 nm. The average diameter is slightly larger than the particles in PtNP/OMC with 3.0±1.0 nm in diameter (FIG. 10b). The particle size distribution of Pt/OMC is broader than of PtNP/OMC. Large particles with a diameter higher than 5 nm are more numerous in the Pt/OMC sample.

(67) Lattice fringes in HR-TEM (FIG. 10c) indicate a high crystallinity of the observed Pt nanoparticles. The spacing of the lattice planes amount to 2.3 Å for both systems. This distance fits with the (111) lattice plane of cubic Pt (04-0802, Fm3m). The crystallinity and crystallite size was additionally evaluated with XRD (FIG. 10d). Both films show reflections at 40° and 46° which can be attributed to cubic Pt. Rietveld refinements give crystallite diameters of 4.5 nm for Pt/OMC and 3.0 nm for PtNP/OMC. The crystallite size of PtNP/OMC corresponds well to the particle size from TEM measurements (FIG. 10b). The crystallite diameter of Pt/OMC is larger than the average nanoparticle diameter determined by TEM which is indicative for larger Pt particles which were also observed by TEM.

(68) The reason that larger particles (>5 nm in diameter) are more numerous in highly-loaded Pt/OMC than PtNP/OMC films can be attributed to the formation mechanism of the nanoparticles. Pt nanoparticles in Pt/OMC films via route 1 are detectable by TEM at carbonization temperatures higher than 600° C. At elevated temperatures Pt atoms and small Pt clusters are moving inside the mesoporous carbon network to form nanoparticles. When the process of nanoparticle formation is finished at temperatures around 700° C., the particles are confined inside the mesopores and do not grow larger. The average particle diameters of the confined Pt nanoparticles amount to ca. 3.5 nm for all studied Pt/OMC films which were carbonized at 700° C. independent of the loading. As known from literature, thermal reduction of nanoparticles at high temperatures usually leads to a high polydispersity (Ortega-Amaya et al., 2015). In case of OMC films the mesoporous structure suppresses the formation of even larger nanoparticles (Galeano et al., 2012). It is possible that also smaller Pt clusters form during thermal reduction of Pt/OMC films. These smaller particles (<1 nm in diameter) are not detectable by TEM analysis. A part of these small Pt clusters might be trapped inside micropores or pore walls and thus is not available for the HER.

(69) Preformed nanoparticles which are introduced into the film synthesis via route 2, according to the invention, undergo a different process during carbonization. The particle diameter of the colloidal particles amounts to 2.1 nm. During carbonization some Pt species are moving inside the carbon network. Sinter processes at elevated temperatures lead to slightly larger particles (3.0 nm) and a higher polydispersity. Nevertheless, the final particle diameter of Pt in PtNP/OMC is lower than in Pt/OMC since the mobility is lower and the degree of pore confinement effect most likely more pronounced for large preformed nanoparticles.

(70) The degree of graphitization of the carbon film can be described by the XRD reflection at 2θ=24° which can be assigned to stacking of graphene. FIG. 10d shows that the reflection at 24° is more pronounced for PtNP/OMC than for Pt/OMC. The electrical conductivity also corresponds to the graphitization of the film. The conductivity of PtNP/OMC (30.4 S/cm) is four times higher than the conductivity of Pt/OMC (8.3 S/cm). Both observations indicate that PtNP/OMC has a higher degree of graphitization.

(71) In conclusion, synthesis route 2 leads to a more defined film morphology for Pt-containing OMC films at higher Pt weight loadings than route 1. The templated mesopores of PtNP/OMC are densely packed and more ordered than the mesopores of Pt/OMC. Both synthesis routes lead to crystalline particles. However, the Pt nanoparticles in PtNP/OMC are smaller. Moreover, the degree of graphitization in the PtNP/OMC sample is higher, making the film more electrically conductive. The ionic precursor in route 1 disturbs interactions of carbon precursors and template micelles during film synthesis leading to a lower degree of graphitization. Whereas, preformed nanoparticles apparently have a less pronounced impact.

(72) 8.3 HER Performance

(73) The electrocatalytic performances of Pt/OMC catalyst films prepared via route 1 and a PtNP/OMC via route 2 were studied in a RDE setup with 0.5 M sulfuric acid using repeated potential cycles. Both catalysts exhibit similar weight loadings (Pt/OMC: 2.9 wt %.sub.Pt, PtNP/OMC: 2.3 wt %.sub.Pt). FIG. 11a depicts the 2.sup.nd recorded cycle of a Pt/OMC catalyst with a geometric loading of 1.2 μg.sub.Pt/cm.sup.2 and a PtNP/OMC catalyst with 1.1 μg.sub.Pt/cm.sup.2.

(74) The film thickness and thus geometric Pt loading was adjusted by changing the withdrawal speed during the dip-coating procedure. FIG. 11 plots the current density at −75 mV vs. RHE as a function of Pt loading during the 2.sup.nd cycle (b) and the 50.sup.th cycle (c), respectively. Each point in the diagram represents one measured catalyst film.

(75) Both types of catalysts are active in the HER, but the difference in structural properties like pore ordering, conductivity as well as nanoparticle size and accessibility influence the activity (compare FIG. 11a). At current densities higher than −10 mA/cm.sup.2 PtNP/OMC clearly outperforms Pt/OMC. FIGS. 11b and c show that the observed HER current scales (within the margin of error) linearly with the geometric loading for both types of catalyst. PtNP/OMC catalysts need 2-4 times less Pt to reach the same current density in the fresh (2nd cycle, FIG. 11b) as well as in the used state (50.sup.th cycle, FIG. 11c).

(76) The enhanced activity of PtNP/OMC in the HER could be explained by improved transport properties and a better utilization of Pt inside the catalyst film. Pore morphology and particle size of PtNP/OMC help to explain this behavior. The pore morphology has an influence on transport properties. The mesopores in PtNP/OMC are ordered and more densely packed than in Pt/OMC which leads to a better interconnectivity of the pore system resulting in improved transport of electrolyte and H.sub.2 molecules. PtNP/OMC films show a higher degree of graphitization which results in lower ohmic resistance and thus reduced transport limitations of electrons. The average diameter of Pt nanoparticles in PtNP/OMC is smaller than in Pt/OMC. The particles in PtNP/OMC show fewer large Pt particles after carbonization. Moreover, the synthesis route prevents the formation of small Pt particles, which could be trapped inside micropores or pore walls. These effects lead to more accessible active surface area in Pt/OMC films for the HER.

(77) In summary, pore ordering, electrical conductivity as well as nanoparticle size and accessibility contribute to the HER activity of the discussed catalysts. It is, however, not possible to distinguish which property contributes most to the enhanced performance of PtNP/OMC in comparison to Pt/OMC.

(78) 8.4 Tafel Evaluation of Pt-Containing Carbon Catalyst Films

(79) Mechanistic aspects of the HER can be studied by Tafel evaluation of current-voltage curves. The potential E is plotted versus the logarithm of the current density j. The Tafel slope b indicates the rate-determining step of the HER. Values of 40 mV/dec refer to electrochemical formation of molecular H.sub.2 (Heyrovsky reaction, MeH.sub.ads+H.sup.++e.sup.−Me+H.sub.2), whereas values of 30 mV/dec are attributed to rate limitations by chemical H.sub.2 desorption via recombination of adsorbed H atoms (Tafel reaction, 2 MeH.sub.ads2 Me+H.sub.2).

(80) FIG. 12 compares the Tafel slopes of Pt/OMC via route 1, PtNP/OMC via route 2 and a commercial Pt/Vulcan catalyst prepared with Nafion via an ink-casting procedure. All films have a geometric Pt loading of ca. 1 μg.sub.Pt/cm.sup.2. The Tafel plots of the 2.sup.nd cycle of cyclic voltammetric testing in the HER regime is shown in FIG. 12a.

(81) The Tafel slopes b during the cathodic sweep of the 2.sup.nd cycle (FIG. 12a) amount to 37 mV/dec (Pt/OMC), 32 mV/dec (PtNP/OMC) and 28 mV/dec (Pt/Vulcan/Nafion). Accordingly, the rate-determining step of Pt/OMC can be attributed to the Heyrovsky reaction mechanism. PtNP/OMC and Pt/Vulcan/Nafion show a Tafel slope consistent with a Volmer-Tafel mechanism. PtNP/OMC shows a linear Tafel behaviour in a broader current density regime than Pt/OMC and Pt/Vulcan/Nafion.

(82) The regimes in which Pt/OMC and Pt/Vulcan/Nafion follow the Tafel equation range up to current densities of 6 mA/cm.sup.2 (10.sup.0.75 mA/cm.sup.2) and 4 mA/cm.sup.2 (10.sup.0.6 mA/cm.sup.2), respectively. In contrast, PtNP/OMC shows a larger regime of linear correlation which ranges from 1 mA/cm.sup.2 (10.sup.0 mA/cm.sup.2) to ca. 30 mA/cm.sup.2 (10.sup.1.5 mA/cm.sup.2). Consequently, the HER activity of PtNP/OMC is not as restricted by transport limitations as Pt/OMC and Pt/Vulcan/Nafion in the regime of current densities up to 30 mA/cm.sup.2.

(83) FIG. 12b compares the current responses during the 100.sup.th cycle of CV measurements between 50 and 1200 mV of a PtNP/OMC, a Pt/OMC, a Pt/Vulcan/Nafion and a Pt-free OMC film.

(84) In conclusion, all Pt-based catalysts show a similar behaviour in the Tafel regime. During the 2.sup.nd HER cycle PtNP/OMC seems to be less restricted by transport limitations than Pt/OMC and Pt/Vulcan/Nafion at current densities between 10 mA/cm.sup.2 and 30 mA/cm.sup.2. Furthermore, Pt/Vulcan/Nafion deviates stronger from the linear behaviour at low current densities compared to the regime at higher currents densities than the OMC-based catalysts. As shown above, PtNP/OMC catalysts have a higher mesopore ordering, a more densely packed pore system, show a higher degree of graphitization and a higher electrical conductivity. Small nanoparticles are well-dispersed throughout the film volume. The particles are not trapped in micropores or pore walls. These properties most likely decrease transport limitations of electrolyte, H.sub.2 or electrons.

(85) The features of the present invention disclosed in the specification, the claims and/or in the accompanying drawings may, both separately and in any combination thereof be material for realizing the invention in various forms thereof. Further modifications of the preferred embodiments are possible without leaving the scope of the invention which is solely defined by the claims.

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