ELECTROCATALYST COMPOSITION COMPRISING NOBLE METAL OXIDE SUPPORTED ON TIN OXIDE

Abstract

The present invention relates to a catalyst composition, comprising tin oxide particles which are at least partially coated by a noble metal oxide layer, wherein the composition contains iridium and ruthenium in a total amount of from 10 wt % to 38 wt %, and all iridium and ruthenium is oxidized, has a BET surface area of from 5 to 95 m.sup.2/g, and has an electrical conductivity at 25 C. of at least 7 S/cm.

Claims

1.-15. (canceled)

16. A catalyst composition, comprising tin oxide particles, wherein the tin oxide is optionally doped with at least one metal dopant, the tin oxide particles being at least partially coated by a noble metal oxide layer, wherein the noble metal oxide is iridium oxide or iridium-ruthenium oxide, wherein the composition contains iridium and ruthenium in a total amount of from 10 wt % to 38 wt %, and all iridium and ruthenium is oxidized, has a BET surface area of from 5 to 95 m.sup.2/g, and has an electrical conductivity at 25 C. of at least 7 S/cm.

17. The catalyst composition according to claim 16, wherein the tin oxide is a non-doped tin oxide; or wherein the tin oxide is doped with at least one metal dopant selected from Sb, Nb, Ta, Bi, W, or In, or any combination of at least two of these dopants, the one or more metal dopants being present in the tin oxide in an amount of from 2.5 at % to 20 at based on the total amount of tin and metal dopant atoms.

18. The catalyst composition according to claim 16, wherein the tin oxide is a non-doped tin oxide; or wherein the tin oxide is doped with at least one metal dopant selected from Sb, Nb, Ta, Bi, W, or In, or any combination of at least two of these dopants, the one or more metal dopants being present in the tin oxide in an amount of from 2.5 at % to 10.0 at %, based on the total amount of tin and metal dopant atoms.

19. The catalyst composition according to claim 16, wherein the total amount of iridium and ruthenium in the catalyst composition is from 15 to 35 wt %.

20. The catalyst composition according to claim 16, wherein the total amount of iridium and ruthenium in the catalyst composition is from 20 to 28 wt %.

21. The catalyst composition according to claim 16, wherein all iridium and ruthenium being present in the catalyst composition is in oxidation state +III and/or +IV.

22. The catalyst composition according to claim 16, having a BET surface area of from 5 m.sup.2/g to 90 m.sup.2/g.

23. The catalyst composition according to claim 16, having an electrical conductivity of at least 10 S/cm.

24. The catalyst composition according to claim 16, having a BET surface area of from 10 m.sup.2/g to 80 m.sup.2/g and having an electrical conductivity of at least 12 S/cm.

25. The catalyst composition according to claim 16, wherein the tin oxide is a non-doped tin oxide; the amount of iridium in the composition is within the range of from 15 to 35 wt %, the remainder being the tin oxide particles and the oxygen of the iridium oxide layer; the BET surface area of the composition is from 5 m.sup.2/g to 35 m.sup.2/g; and the electrical conductivity of the composition is from 10 to 50 S/cm.

26. The catalyst composition according to claim 16, wherein the tin oxide is a non-doped tin oxide; the amount of iridium in the composition is within the range of from 20 to 28 wt %, the remainder being the tin oxide particles and the oxygen of the iridium oxide layer; the BET surface area of the composition is from 5 m.sup.2/g to 35 m.sup.2/g; and the electrical conductivity of the composition is from 12 to 40 S/cm.

27. The catalyst composition according to claim 16, wherein the tin oxide is doped with antimony in an amount of from 2.5 at % to 20 at %, the amount of iridium in the composition is within the range of from 15 to 35 wt %, the remainder being the tin oxide particles and the oxygen of the iridium oxide layer; the BET surface area of the composition is from 15 m.sup.2/g to 90 m.sup.2/g; and the electrical conductivity of the composition is from 10 to 50 S/cm.

28. The catalyst composition according to claim 16, wherein the tin oxide is doped with antimony in an amount of from 2.5 at % to 10.0 at %; the amount of iridium in the composition is within the range of from 20 to 28 wt %, the remainder being the tin oxide particles and the oxygen of the iridium oxide layer; the BET surface area of the composition is from 30 m.sup.2/g to 80 m.sup.2/g; and the electrical conductivity of the composition is from 12 to 40 S/cm

29. A process for preparing the catalyst composition according to claim 16, which comprises dispersing tin oxide particles and dissolving a noble-metal-containing precursor compound in an aqueous medium, wherein the noble metal is iridium or ruthenium or a mixture thereof, adjusting pH of the aqueous medium to 5-10 and optionally heating the aqueous medium to a temperature of from 50 C. to 95 C., thereby depositing noble metal species on the tin oxide particles, separating the tin oxide particles from the aqueous medium and subjecting the tin oxide particles to a thermal treatment at a temperature of from 300 C. to 800 C., thereby forming a noble metal oxide layer on the tin oxide particles.

30. The process according to claim 29, wherein the tin oxide particles dispersed in the aqueous medium have a BET surface area of from 10 to 100 m.sup.2/g.

31. The process according to claim 29, wherein the noble-metal-containing precursor compound is a noble metal salt or a noble-metal-containing acid.

32. The process according to claim 29, wherein the thermal treatment is carried out at a temperature of from 500 C. to 700 C.

33. An electrochemical device, comprising the catalyst composition according to claim 16.

34. The device according to claim 34, which is a water electrolyser or a fuel cell.

35. A catalyst for an oxygen evolution reaction which comprises the catalyst composition according to claim 16.

Description

EXAMPLES

[0089] If not indicated otherwise, the parameters referred to in the present invention are determined according to the following measuring methods:

[0090] BET Surface Area

[0091] 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).

[0092] Electrical Conductivity

[0093] For measuring electrical conductivity, the oxide powders were pressed into pellets and the conductivity was determined at 25 C. by a 2 point probe method. First, 1 g of the powder samples were inserted into the Teflon tube with stainless steel bottom (electrode) of a 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 to 40 MPa and the resistance is measured at said pressure via the 2 point method with an Agilent 3458A multimeter. From the measured resistance R (in Ohm), the electrical conductivity is calculated according to:

Conductivity=d/(R A) [0094] d: distance of the 2 electrodes [0095] R: measured resistance [0096] A: electrode area (0.5 cm.sup.2)

[0097] The resistance is the sum of the following contributions: electrode contact resistance, intragrain (bulk) resistance and intergrain resistance.

[0098] In the present invention, electrical conductivity is determined at a pressure of 40 MPa.

[0099] Amount of Iridium, Ruthenium, Tin and the Optional Metal Dopant

[0100] The amounts of iridium, ruthenium, metal dopant and tin are determined by elemental analysis according to the following method: 0.04 to 0.5 g of the sample is mixed with 10 g of a mixture of 84% Li.sub.2B.sub.4O.sub.7, 1% LiBr and 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.

[0101] Oxidation State of Iridium, Ruthenium and the Optional Metal Dopant; Relative Amounts of Ir(+IV) and Ru(+IV); Atomic Ratio of Sb(+V) to Sb(+III)

[0102] Oxidation states of iridium, ruthenium and the optional metal dopant (such as Sb) are determined by X-ray photoelectron spectroscopy (XPS). The relative amounts of iridium and ruthenium in oxidation state +IV, and the atomic ratio of Sb(+V) to Sb(+III) are also determined by XPS.

[0103] The XPS analysis was carried out with a Phi Versa Probe 5000 spectrometer using monochromatic Al Ka 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 1001400 m.sup.2 area was analyzed with a pass energy of 23.5 eV.

[0104] If the metal dopant is e.g. Sb, Sb 3d and O1 s 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.

[0105] Iridium oxidation states were obtained from the Ir 4f signal with a doublet of asymmetric peaks (SGL(10)T(0.9)) for metallic iridium at 61.4 eV and 64.4 eV and an iridium oxide IrO.sub.2 contribution fitted by a doublet of symmetric peaks 1.8 eV separated from the metallic peak.

[0106] The most intense ruthenium signal, Ru 3d typically overlaps with the Carbon 1 s-Signal. Aside from the carbon contributions in the range of 284.5 eV to 290.2 eV, doublets for Ru(0), RuO.sub.2, hydrated RuO.sub.2 and RuO.sub.3 were used for the peak fit. All of these peaks show a high degree of asymmetry and were therefore described by a LF(0.6,1,200,900) peak shape in case of RuO.sub.3 and RuO.sub.2 or LF(0.25,1,45,280) in case of hydrated RuO.sub.2. The relative signal positions and peak shapes are given in the attached table:

TABLE-US-00001 Oxidation Binding energy Binding energy of state of Ru 3d5/2 [eV] Ru 3d3/2 [eV] Peak shape RuO.sub.3 282.4 286.6 LF(0.6, 1, 200, 900) RuO.sub.2 280.8 285.0 LF(0.25, 1, 45, 280) hydrated RuO.sub.2 280.6 284.8 LF(0.6, 1, 200, 900) Ru 280.1 284.3 peak shape as obtained from the measurement of pure Ru metal

[0107] Particle Morphology, Presence of a Noble Metal Oxide Layer on the Tin Oxide Particles

[0108] The presence of an iridium oxide or iridium-ruthenium oxide layer which is at least partially coating the tin oxide particles was verified by scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (EDXS mapping).

Inventive Example 1

[0109] In Inventive Example 1, the catalyst composition was prepared as follows:

[0110] Non-doped tin oxide powder was used as a support material to be coated by a noble metal oxide. The tin oxide powder had a BET surface area of 25 m.sup.2/g.

[0111] 2 g of the SnO.sub.2 powder was dispersed in 400 g water, followed by adding 3.83 g of IrCl.sub.4. Subsequently, the aqueous medium was heated to 80 C. and KOH was added until pH=7. From time to time, further KOH was added so as to keep the pH at about 7.

[0112] After stirring for about 1 hour, the aqueous medium was cooled to room temperature, the SnO.sub.2 powder was separated from the aqueous medium by filtration, washed with water, and calcined in air at 600 C. for about 60 minutes.

[0113] The final catalyst composition had a BET surface area of 21 m.sup.2/g, an electrical conductivity of 25 S/cm, and an iridium content of 25 wt %. All iridium was in oxidation state +IV.

[0114] FIGS. 1a and 1b show scanning transmission electron microscopy photographs with EDXS mapping. In FIG. 1a, EDXS is specifically detecting Sn, while Ir is specifically detected by EDXS in FIG. 1b. Both photographs show the same particles. As demonstrated by FIGS. 1a and 1b, tin (in the form of tin oxide) is present in the core of each particle, while iridium (in the form of iridium oxide) is present in the outer layer (shell) which is at least partially coating the tin oxide core.

Inventive Example 2

[0115] In Inventive Example 2, the catalyst composition was prepared as follows:

[0116] Antimony-doped tin oxide (ATO) powder was used as a support material to be coated by a noble metal oxide. The ATO powder had an Sb content of 5.7 wt % and a BET surface area of 56 m.sup.2/g.

[0117] 2 g of the ATO powder was dispersed in 400 g water, followed by adding 3.83 g of IrCl.sub.4.

[0118] Subsequently, the aqueous medium was heated to 80 C. and KOH was added until pH=7. From time to time, further KOH was added so as to keep the pH at about 7.

[0119] After stirring for about 1 hour, the aqueous medium was cooled to room temperature, the ATO powder was separated from the aqueous medium by filtration, washed with water, and calcined in air at 600 C. for about 60 minutes.

[0120] The final catalyst composition had a BET surface area of 38 m.sup.2/g, an electrical conductivity of >7 S/cm, and an iridium content of 33 wt %. All iridium was in oxidation state +IV.

[0121] FIGS. 2a and 2b show scanning transmission electron microscopy photographs with EDXS mapping. In FIG. 2a, EDXS is specifically detecting Sn, while Ir is specifically detected by EDXS in FIG. 2b. Both photographs show the same particles. As demonstrated by FIGS. 2a and 2b, tin (in the form of antimony-doped tin oxide ATO) is present in the core of each particle, while iridium (in the form of iridium oxide) is present in the outer layer (shell) which is at least partially coating the ATO core.

Inventive Example 3

[0122] In Inventive Example 3, the catalyst composition was prepared as follows:

[0123] Antimony-doped tin oxide (ATO) powder was used as a support material to be coated by a noble metal oxide. The ATO powder had an Sb content of 5.5 wt % and a BET surface area of 87 m.sup.2/g.

[0124] 1.4 g of the ATO powder was dispersed in 280 g water, followed by adding 2.68 g of IrCl.sub.4. Subsequently, the aqueous medium was heated to 80 C. and KOH was added until pH=7. From time to time, further KOH was added so as to keep the pH at about 7.

[0125] After stirring for about 1 hour, the aqueous medium was cooled to room temperature, the ATO powder was separated from the aqueous medium by filtration, washed with water, and calcined in air at 600 C. for about 60 minutes.

[0126] The final catalyst composition had an electrical conductivity of >7 S/cm, and an iridium content of 24 wt %. All iridium was in oxidation state +IV. The tin oxide particles (representing the core) are at least partially coated by an iridium oxide layer (representing the shell).

Inventive Example 4

[0127] In Inventive Example 4, the catalyst composition was prepared as follows:

[0128] Antimony-doped tin oxide (ATO) powder was used as a support material to be coated by a noble metal oxide. The ATO powder had an Sb content of 11.8 wt % and a BET surface area of 95 m.sup.2/g.

[0129] 2.5 g of the ATO powder was dispersed in 125 g water, followed by adding 1.61 g of IrCl.sub.4. Subsequently, the aqueous medium was heated to 80 C. and KOH was added until pH=7. From time to time, further KOH was added so as to keep the pH at about 7.

[0130] After stirring for about 1 hour, the aqueous medium was cooled to room temperature, the ATO powder was separated from the aqueous medium by filtration, washed with water, and calcined in air at 600 C. for about 60 minutes.

[0131] The final catalyst composition had an electrical conductivity of >7 S/cm, a BET surface area of 83 m.sup.2/g, and an iridium content of 17 wt %. All iridium was in oxidation state +IV. The tin oxide particles (representing the core) are at least partially coated by an iridium oxide layer (representing the shell).

Inventive Example 5

[0132] In Inventive Example 5, the catalyst composition was prepared as follows:

[0133] Antimony-doped tin oxide (ATO) powder was used as a support material to be coated by a noble metal oxide. The ATO powder had an Sb content of 5.48 wt % and a BET surface area of 71 m.sup.2/g.

[0134] 6 g of the ATO powder was dispersed in 1200 g water, followed by adding 11.5 g IrCl.sub.4. Subsequently, the aqueous medium was heated to 80 C. and KOH was added until pH=7. From time to time, further KOH was added so as to keep the pH at about 7.

[0135] After stirring for about 1 hour, the aqueous medium was cooled to room temperature, the ATO powder was separated from the aqueous medium by filtration, washed with water, and calcined in air at 600 C. for about 60 minutes.

[0136] The final catalyst composition had an electrical conductivity of 18 S/cm, a BET surface area of 52 m.sup.2/g, and an iridium content of 38 wt %. All iridium was in oxidation state +IV. The tin oxide particles (representing the core) are at least partially coated by an iridium oxide layer (representing the shell).

[0137] Testing the Electrochemical Performance and Corrosion Stability

[0138] The catalyst compositions of Inventive Examples 1 to 5 were tested for their electrochemical performance and corrosion stability under highly corrosive conditions.

[0139] For comparative purposes, the following samples were tested as well:

Comparative Example 1

[0140] Non-supported metallic iridium black powder, BET surface area: 60 m.sup.2/g.

Comparative Example 2

[0141] Non-supported iridium(IV) oxide powder; BET surface area: 25 m2/g.

[0142] Inks were prepared with all samples (i.e. the samples of Inventive Examples 1 to 5 and Comparative Examples 1-2) by dispersing the appropriate amount of catalyst composition powder in a solution of water, isopropanol and Nafion (binder), to achieve a total catalyst concentration of 6 g/L. Inks were cast onto gold foil current collectors to get an electrode loading of 120 g.sub.cat/cm.sup.2 (geometric surface area). The catalyst compositions were tested in a 0.5 M H.sub.2SO.sub.4 electrolyte. A conditioning step was performed by cycling the potential in a non OER region for 50 cycles. Linear sweep voltammograms were subsequently recorded at 1 mV/s. After 3 consecutive LSVs, a chronoamperometry step at 2 V vs. RHE was applied for 20 hours, in order to submit the catalyst composition to a stress test. Afterwards, electrolytes were collected after the electrochemical characterization and analyzed by inductively coupled plasma mass spectroscopy (ICP-MS) to determine if any iridium traces were present due to dissolution.

[0143] For evaluating catalytic activity, mass normalized current densities j [A/g.sub.Ir] at 1.9 V vs. RHE were determined.

[0144] The results are summarized in Table 1.

TABLE-US-00002 TABLE 1 Results of electrochemical and corrosion stability tests j/A g.sub.Ir.sup.1 @ 1.9 vs. Ir content in Sample RHE electrolyte (ppm) IE-1 1544.4 <0.1 IE-2 1939.4 <0.1 IE-3 2062.9 <0.1 IE-4 1464.2 <0.1 IE-5 1499.1 <0.1 CE-1 1345.7 4 CE-2 633.8 <0.1

[0145] All Inventive Examples show high activities (mass normalized current densities), demonstrating a very efficient utilization of the iridium active centers. Furthermore, as indicated by a negligible content of dissolved iridium in the surrounding electrolyte, the Inventive Examples show very high corrosion stability.

[0146] When using a catalyst composition based on metallic iridium powder (Comparative Example 1), high activity can be achieved. However, corrosion stability is adversely affected.

[0147] When using a catalyst composition based on non-supported IrO.sub.2 powder (CE 2), activity is significantly lower.

[0148] Accordingly, the Inventive Examples show an improved balance between catalytic activity and corrosion stability.