Metal-doped tin oxide for electrocatalysis applications

Abstract

The present invention relates to a metal-doped tin oxide which has a BET surface area of at least 30 m 2/g, and comprises at least one metal dopant which is Sb, Nb, Ta, Bi, W, or In, or any mixture thereof, wherein the metal dopant is present in an amount of from 2.5 at % to 25 at %, based on the total amount of tin and metal dopant atoms, and is in a mixed valence state containing atoms of oxidation state OS1 and atoms of oxidation state OS2, wherein the oxidation state OS1 is >0 and the oxidation state OS2 is >OS1 and the atomic ratio of the atoms of OS2 to the atoms of OS1 is from 1.5 to 12.0.

Claims

1. A metal-doped tin oxide which has a BET surface area of at least 30 m.sup.2/g, and comprises a dopant which is Sb, wherein the metal dopant is present in an amount of from 2.5 at % to 25 at %, based on the total amount of tin and metal dopant atoms, and is in a mixed valence state containing Sb.sup.3+ atoms and Sb.sup.5+ atoms, wherein the atomic ratio of Sb.sup.5+ to Sb.sup.3+, measured by X-ray photoelectron spectroscopy, is from 5.0 to 9.0.

2. The metal-doped tin oxide according to claim 1, wherein the amount of the metal dopant is from 2.5 at % to 10.0 at %.

3. The metal-doped tin oxide according to claim 1, wherein the BET surface area of the metal-doped tin oxide is from 30 m.sup.2/g to 150 m.sup.2/g and/or the electrical conductivity of the metal doped tin oxide is at least 0.02 S/cm.

4. The metal-doped tin oxide according to claim 1, wherein the amount of the metal dopant is from 5.0 at % to 7.5 at %.

5. The metal-doped tin oxide according to claim 1, wherein the BET surface area of the metal-doped tin oxide is from 35 m.sup.2/g to 110 m.sup.2/g; and/or the electrical conductivity of the metal doped tin oxide is at least 0.03 S/cm.

6. The metal-doped tin oxide according to claim 1, wherein the atomic ratio of the Sb.sup.5+ atoms to the Sb.sup.3+ atoms is from 5.0 to 8.0.

7. A process for preparing the metal-doped tin oxide according to claim 1, comprising preparing a metal-doped precursor solid by a wet chemical synthesis from a reaction mixture comprising a tin-containing molecular precursor compound and a metal-dopant-containing molecular precursor compound, subjecting the metal-doped precursor solid to a thermal treatment.

8. The process according to claim 7, wherein the wet chemical synthesis is a sol-gel process, a chemical precipitation process, a hydrothermal synthesis process, a spray drying process, or any combination thereof.

9. The process according to claim 7, wherein the tin-containing molecular precursor compound and the metal-dopant-containing molecular precursor compound are mixed at acidic pH, and the pH is subsequently raised by adding a base until the metal-doped precursor solid precipitates; and/or wherein the wet chemical synthesis is carried out in an alcoholic solvent.

10. The process according to claim 7, wherein the tin-containing molecular precursor compound is a tin salt such as a tin halide or a tin nitrate, or a tin alkoxide, or a mixture thereof; and/or the metal-dopant-containing molecular precursor compound is a metal halide, a metal carboxylate or a metal alkoxide or any mixture thereof.

11. The process according to claim 7, wherein the wet chemical synthesis is carried out in the presence of a solid additive having a BET surface area of at least 40 m.sup.2/g.

12. The process according to claim 11, wherein the solid additive is carbon black or activated carbon, which has a BET surface area of at least 200 m.sup.2/g.

13. The process according to claim 7, wherein the thermal treatment includes heating to a temperature of from 400 to 800° C.

14. The process according to claim 11, wherein the solid additive is carbon black or activated carbon, which has a BET surface area of at least 500 m.sup.2/g.

15. A composite material, comprising the metal-doped tin oxide according to claim 1, and an electrocatalyst which is supported on the metal-doped tin oxide.

16. An electrochemical device, comprising the composite material according to claim 15.

17. The electrochemical device according to claim 16, wherein the electrochemical device is a polymer electrolyte membrane PEM water electrolyzer or a PEM fuel cell.

18. A catalyst support in an electrochemical device, comprising the metal-doped tin oxide according claim 1, wherein the electrochemical device is a polymer electrolyte membrane PEM water electrolyzer or a PEM fuel cell.

Description

EXAMPLES

(1) If not indicated otherwise, the parameters referred to in the present invention are determined according to the following measuring methods:

(2) Amount of Metal Dopant

(3) The amounts of metal dopant and tin are determined by elemental analysis performed on the synthesized samples, according to the following method: 0.04 to 0.5 g of each sample is mixed with 10 g of a mixture of 84% Li.sub.2B.sub.4O.sub.7, 1% LiBr und 15% NaNO.sub.3. Using a Claisse Fluxer M4, a mixed pellet is formed. After cooling to room temperature, the elemental composition is determined using wavelength dispersive X-ray fluorescence.

(4) BET Surface Area

(5) BET surface area was determined by gas adsorption analysis using Micromeritics ASAP 2420 Surface Area and Porosity Analyzer with N.sub.2 adsorbate at 77.35 K. Prior to the measurement, samples were dried at 200° C. in vacuum overnight. The specific surface area was determined by BET theory using the multi-point method (ISO 9277:2010).

(6) Electrical Conductivity

(7) For measuring electrical conductivity, the oxide powders were pressed into pellets and the conductivity was determined at room temperature by a 2 point probe method. First, ca. 1 g of the powder samples were inserted into the Teflon tube with stainless steel bottom (electrode) of an in-house measuring cell. After the filling is completed, a second stainless steel electrode was inserted on the top, and the filled test cell is inserted in between the pressure gauge. The pressure is increased from 100 to 500 bar. At each pressure the resistance is measured via the 2 point method with an Agilent 3458A multimeter. From the measured resistance R (in Ohm), the specific powder conductivity is calculated according to:
Conductivity=d/(R A) d: distance of the 2 electrodes R: measured resistance A: electrode area (0.5 cm.sup.2)

(8) The total resistance is the sum of the following contributions: electrode contact resistance, intragrain (bulk) resistance and intergrain resistance. The resistance values are all reported at 500 bar.

(9) Atomic ratio of atoms of oxidation state OS2 to atoms of oxidation state OS1 The ratio was determined by X-ray photoelectron spectroscopy (XPS). The XPS analyses were carried out with a Phi Versa Probe 5000 spectrometer using monochromatic Al Kα radiation (49 W) and Phi charge neutralizer system. The instrument work function was calibrated to give a binding energy (BE) of 84.00 eV for the Au 4f7/2 line of metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. An analysis spot of 100×1400 μm.sup.2 area was analyzed with a pass energy of 23.5 eV.

(10) If the metal dopant is e.g. Sb, Sb 3d and O1s spectra overlap and were analyzed using CasaXPS software version 2.3.17 using Shirley background subtraction in the energy region of 528-542.5 eV binding energy. Antimony contributions were fitted with three different components: Sb(III)-doublet at 529.7 and 539.1 eV, Sb(V)-doublet at 530.9 and 540.3 eV, Plasmons at 531.9 and 541.5 eV. Additionally, three oxygen contributions were used for fitting. Relative sensitivity factors as provided by the instrument manufacturer were used for quantification.

Inventive Example 1

(11) In Inventive Example 1, an antimony-doped tin oxide was prepared as follows:

(12) All steps of the wet chemical synthesis were carried out in nitrogen atmosphere and under stirring.

(13) 0.41 g SbCl.sub.3, 250 ml ethanol, and 1.25 g HCl (32 wt %) were mixed. Subsequently, 6.14 g SnCl.sub.4 was added over 30 minutes via a dropping funnel. After 10 minutes of further stirring, 10 g carbon black (microporous, BET surface area of about 1400 m2/g) was added. Stirring was continued for 10 minutes, followed by homogenization in an ultrasonic bath for 30 minutes. Then, a mixture of 15.7 ml aqueous ammonia solution (25 wt %) and 1.32 g alanine was dropwise added via a dropping funnel over 30 minutes. Stirring was continued for 16 hours.

(14) The solid material obtained by the wet chemical synthesis was filtered in air atmosphere and washed with water. The solid was dried at 100 mbar/80° C. The solid was subjected to a thermal treatment at 700° C. (heating rate: 1° C./min) for 1 hour in a furnace in air atmosphere.

Inventive Example 2

(15) The antimony-doped tin oxide was prepared as described in Inventive Example 1 with the following modifications:

(16) The amount of SbCl.sub.3 was 0.82 g and the amount of SnCl.sub.4 was 6.14 g.

Inventive Example 3

(17) The antimony-doped tin oxide was prepared as described in Inventive Example 1 with the following modifications:

(18) The amount of SbCl.sub.3 was 1.23 g and the amount of SnCl.sub.4 was 6.14 g.

Comparative Example 1

(19) A commercially available antimony-doped tin oxide having a Sb content of 8.0 wt %, a BET surface area of 41 m.sup.2/g, and an electrical conductivity of 0.0206 S/cm was used.

Comparative Example 2

(20) A commercially available antimony-doped tin oxide having a Sb content of 11.9 wt %, a BET surface area of 70 m.sup.2/g, and an electrical conductivity of 0.00342 S/cm was used.

Comparative Example 3

(21) A commercially available non-doped SnO.sub.2 having a BET surface area of 71 m.sup.2/g, and an electrical conductivity of 3.0E-05 S/cm was used.

(22) The properties are summarized in Table 1.

(23) TABLE-US-00001 TABLE 1 Properties of the samples of Inventive Examples 1-3 and Comparative Examples 1-3 Sb Sb/(Sb + Sb.sup.5+/Sb.sup.3+ BET Conductivity content Sn) molar atomic (m2/g) (S/cm) (wt %) ratio ratio Inventive 95 0.0437 5.0 0.068 6.0 Example 1 Inventive 45 0.258 11.0 0.143 8.4 Example 2 Inventive 100 0.0227 15.0 0.2 8.9 Example 3 Comparative 41 0.0206 8.0 0.105 14.3 Example 1 Comparative 70 0.00342 11.9 0.155 14.1 Example 2 Comparative 71 3.0E−05 0 0 0 Example 3
Electrochemical Performance and Stability Test

(24) From each of the powder samples of Inventive Examples 1-3 and Comparative Examples 1-3, an ink was prepared by mixing 10 mg of the powder with 2.35 ml H.sub.2O, 0.586 ml isopropanol, and 3.81 μl 5 wt % Nafion, followed by sonicating the mixture for about 30 minutes. The inks were then drop-cast on titanium foil electrodes with a loading of 120 μg/cm.sup.2.

(25) For electrochemical characterization, all samples were then submitted to an accelerated aging test protocol wherein a high anodic potential (2 V vs. RHE) is applied, and the capacitance of the sample was measured via cyclic voltametry. The electrolyte was chosen to mimic the conditions of a PEM electrolyser.

(26) The final capacitance value of each sample at the end of the accelerated aging test is listed in the following Table 2:

(27) TABLE-US-00002 TABLE 2 Final capacitance values of the accelerated aging test Final capacitance (F/g) Inventive Example 1 6.2 Inventive Example 2 4.1 Inventive Example 3 3.5 Comparative Example 1 1.9 Comparative Example 2 0.8 Comparative Example 3 0.6

(28) The antimony-doped tin oxides of the present invention show much higher capacitance values than the comparative samples. A high capacitance value means that there is a large electrochemically accessible surface area enabling an improved electron transfer from the oxide to a catalyst supported thereon.

(29) FIG. 1 shows the capacitance (measured in the accelerated aging test) as a function of time for the samples of Inventive Example 1 and Comparative Examples 1-3.

(30) With the inventive metal-doped tin oxide, electrochemical performance can be kept on a high level throughout the entire aging test. Comparative Example 2 shows a fairly high initial capacitance which is however significantly reduced during the aging test, thereby indicating an insufficient stability under the very corrosive conditions of the aging test. The electrochemical performance levels of Comparative Examples 1 and 3 are significantly lower than the one achieved by the inventive sample.

(31) For Inventive Example 1 and Comparative Example 2, it was determined how much of the metal dopant is leached from the oxide into the surrounding electrolyte during the accelerated aging test.

(32) The results are shown in Table 3.

(33) TABLE-US-00003 TABLE 3 Amount of Sb detected in the electrolyte at the end of the aging test Amount of Sb in the electrolyte [ppm] Inventive Example 1 <0.1 Comparative Example 2 0.2

(34) As confirmed by the results of Table 3, the metal-doped tin oxides of the present invention show high stability even under very corrosive conditions as used e.g. in PEM water electrolysers.

(35) As demonstrated by the Examples, the metal-doped tin oxides according to the present invention provide an improved balance between electrochemical performance (e.g. high capacitance values), sufficiently high surface area, sufficiently high conductivity, and high stability under very corrosive conditions (as they are typically used at the anode side of a PEM water electrolyser).