RU2564095C1 - Fuel-cell anode based on molybdenum bronzes and platinum and method of its manufacturing - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: as catalytic layer the anode comprises composite material made of platinum compounds and hydrogen-containing molybdenum bronzes of H_xMoO₃ composition; where x accepts values from 0.2 up to 0.4.

EFFECT: improved current stability for methanol electro-oxidation in time, essential reduction of anode operating potential at methanol electro-oxidizing up to 0,4-0,6 V, resistivity to CO admixture in hydrogen.

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Description

FIELD OF THE INVENTION

The invention relates to the field of electrochemical materials science and electrochemical technology, and more particularly to an anode of a low-temperature fuel cell with a polymer membrane using methanol or hydrogen as fuel. The invention also provides a method of manufacturing such an anode.

State of the art

International application WO 2008/110651 (published September 18, 2008) discloses a three-component catalyst of the composition Pt-Ru-MeO _x (Me is Mo, W or V) supported on a carbon substrate in which Pt and Ru are present in the form of metal nanoparticles , a Me is present in the form of oxides in various oxidation states, characterized in that the ratio of Pt / Ru atoms is in the range of 0.1-2, the ratio of Pt / Me atoms is in the range of 0.1-10. The catalyst is capable of oxidizing CO to CO ₂ at potentials starting from 0.1 V with respect to a reversible hydrogen electrode in a solution of sulfuric acid with a concentration of 0.5 mol / L. Examples of a preferred embodiment of the invention confirm the effective oxidation of CO to CO ₂ starting at a potential of 0.3 V. The catalyst in question is not intended for the direct use of methanol as a fuel.

In the application US 20060141334 (published on June 29, 2006) a solid anode catalyst for low-temperature fuel cells is described, the active layer of which is a heteropoly acid salt comprising a noble metal and / or transition metal; this salt has a molecular weight of 800 to 10,000. The noble metal is selected from the platinum group, and the transition metal may be molybdenum. The examples show the applicability of a commercially available heteropoly acid of the composition Na ₅ H ₃ [PtMo ₆ O ₂₄ ] · xH ₂ O (Andersen type structure) as a catalyst for the oxidation of methanol in a fuel cell (1 M CH ₃ OH + 0.5 M H ₂ SO ₄ ) Methanol electrooxidation to a significant extent begins only at a potential higher than 0.5 V, and the maximum anode current is reached at a potential not lower than 0.75 V with respect to a reversible hydrogen electrode (ORE) in the same solution, which is a drawback in the practical application of the developed anode .

Patent CA 2219213 (publ. 04.25.1998) discloses an anode catalyst composition comprising platinum, component M and component Y, where platinum and component M form an alloy and the Pt-M alloy is in close contact with component Y. Component M is selected from elements groups IIIA and IVA, and Y represents molybdenum, tungsten or their oxides. The proposed catalysts are effective for the oxidation of hydrogen containing about 12 parts per million CO. Their use for the electrooxidation of methanol is not described.

In the application US 20050147867 (publ. 07.06.2005) disclosed the composition of the active layer of the electrode, comprising two components. The first catalytic component corresponds to the formula Pt-Y (Y is molybdenum, tungsten or their oxides), the second catalytic component can be described by the formula Pt-M (M is, in particular, ruthenium); the first and second catalytic components form one mixed layer and are in electrical contact with each other. It was shown that the catalyst works efficiently only up to the content of CO in the gas mixture of 100 ppm, which is a significant drawback of such catalysts and makes their use unpromising.

Patent CA 2268694 (publ. 15.10.1999) discloses an anode of a fuel cell for the oxidation of methanol, which as a primary catalytically active component contains at least one platinum group metal or its alloy, characterized in that the composition of the catalytic layer also includes complexes of the transition metal with unsubstituted and substituted phthalocyanines, due to which the catalytic effect of the platinum group metal during the anodic oxidation of methanol is enhanced. Nickel or palladium is used as the transition metal. According to the data presented in the patent, the catalytic effect in relation to the electrooxidation reaction of CH ₃ OH is small compared to the Pt / Pd electrode. The patent does not contain information about the stability of the electrode material during its long-term operation.

In the application EP 1819004 (publ. 08/15/2007) a catalyst for the direct oxidation of alcohol is disclosed, containing platinum and a substance prepared from molybdenum and at least one molybdenum compound as an active component. In the potential range of 0.4-0.6 V relative to the silver chloride reference electrode, i.e. more than 0.6 V relative to OE, binary platinum-molybdenum catalysts containing 69-82% Pt, when oxidized with methanol, are capable of producing a current with a density of 5-25 mA / cm ^{2 of the} geometric surface of the electrode when loading Pt ~ 0.37 mg / cm ² . However, for the practical use of such anodes, it is desirable to oxidize methanol at less positive electrode potentials.

In Weishan Li, Jin Lu, Jinghua Du, Dongsheng Lu, Hongyu Chen, Hong Li, Yingmin W. "Electrocatalytic oxidation ofmethanol on polyaniline-stabilized Pt-H _x MoO ₃ in sulfuric acid solution." Electrochemistry Communications. No. 7 (2005). P. 406-410 describes an electrode with a catalytic layer of the composition polyaniline-platinum-molybdenum bronze (Pan-Pt-H _x MoO ₃ , where x takes values from 0 to 2), on which the oxidation of methanol (0.1 M CH ₃ OH + 0.5 M H ₂ SO ₄ ) is possible with a potential of 0.6 V relative to the silver-chloride electrode (more than 0.8 V relative to OEV). At the initial moment of electrolysis, the currents at the Pan-Pt-H _x MoO ₃ electrode significantly exceed the currents at the Pt electrode. However, 1800 seconds after the start of electrolysis (the potential is 0.6 V relative to the silver chloride electrode), the currents at the Pan-Pt-H _x MoO ₃ electrode decrease to almost zero. This is a significant drawback of the proposed system.

In a publication by R. Justin, G. Ranga Rao, "Methanol oxidation on MoO ₃ promoted Pt / C electrocatalyst." International Journal of Hydrogen Energy. Vol. 36 (2011). P.5875-5884 describes C / Pt-MoO ₃ catalysts in which the mass ratio of Pt: MoO ₃ is 2: 1, 2: 2 and 2: 3, and the catalytic layer is deposited on a carbon support Vulcan carbon XC-72R (manufactured by Cabot Corp.). For the C / Pt-MoO ₃ catalyst (2: 2), methanol oxidation studies (1.0 M CH ₃ OH + 0.5 M H ₂ SO ₄ ) at a potential of 0.6 V relative to the silver chloride electrode (~ 0.80 In relatively OEV) showed a decrease in current from 300 to 200 mA / mg Pt during the first 1800 seconds of electrolysis, which corresponds to a decrease in catalytic activity by a third.

As an analogue of the proposed technical solution, the authors consider the anode of the methanol fuel cell described in the article XD Xiang, QM Huang, Z. Fu, YL Lin, W. Wu, SJ Hu, WS Li "H _x MoO ₃ -assisted deposition of platinum nanoparticles on MWNTs for electrocatalytic oxidation ofmethanol. " International Journal of Hydrogen Energy. Vol. 37 (2012) P. 4710-4716. This publication describes methanol electrooxidation catalysts supported on single-walled carbon nanotubes (MWNTs). Platinum nanoparticles are deposited onto said support by a polyol method using ethylene glycol as a reducing agent. The composition of the active layer of the electrode introduced molybdenum bronzes H _x MoO ₃ , where x takes a value from 0 to 2. At a potential of 0.6 V relative to the silver chloride electrode (~ 0.80 V relative to the OE) in the system of 0.5 M H ₂ SO ₄ +0.5 M CH ₃ OH the specified electrode is capable of producing a current density of at least 0.4 A / cm ^{2 of the} geometric surface of the electrode. During the experiment (1000 s), the current density decreases by about a third.

The main disadvantage of the anodes described in these scientific publications is that the electrooxidation of methanol is carried out at highly positive potentials relative to the reversible hydrogen electrode in the same solution (not lower than 0.8 V). This makes their use in real fuel cells unpromising due to the extremely small electromotive force that can be obtained in methanol-oxygen fuel cells using such anodes. In addition, the proposed electrodes are poisoned by products of strong chemisorption, which leads to a decrease in oxidation currents over time.

Thus, there is a need to develop new anodes for low-temperature methanol and hydrogen fuel cells to overcome and eliminate these disadvantages of the known technical solutions.

Disclosure of invention

As a result of extensive research, the authors of the present invention have found that the disadvantages of the prior art can be overcome to a large extent by creating an anode of a low-temperature methanol or hydrogen fuel cell with a polymer membrane, including an electrically conductive carbon carrier, a catalytic layer and a collector, which contains a composite material as a catalytic layer prepared from platinum compounds and hydrogen-containing molybdenum bronzes composition H _x MoO _3, where x has a value from 0.2 to 0.4, and said catalyst to obtain a composite material:

a) chemically synthesize molybdenum bronze of the composition H _x MoO ₃ , where x takes a value from 0.2 to 0.4, by reduction of molybdenum (VI) oxide with zinc powder in a medium of 2-3 M hydrochloric acid at 0-10 ° C;

b) the synthesized bronze is smeared on the surface of the electrically conductive carbon carrier to obtain a precursor electrode C / H _x MoO ₃ ;

c) the precursor electrode is dried in an argon stream at a temperature of from 150 to 200 ° C and placed in an electrochemical cell with a deaerated solution containing a soluble salt of Pt (II) in 0.5-1.0 M sulfuric acid;

d) in a solution saturated with argon, currentless deposition of platinum on the precursor electrode is carried out by recharging the molybdenum bronze to obtain an electrode having a catalytic layer of nPt ⁰ (H _x-2n -MoO ₃ ), and the resulting electrode is washed 0.5-1 , 0 M sulfuric acid from platinum compounds to produce an anode of a low-temperature methanol or hydrogen fuel cell.

The first technical result is to increase the stability of the current electrolysis of methanol over time. Another technical result is a significant decrease in the working potential of the anode during the electrooxidation of methanol to 0.4-0.6 V relative to a reversible hydrogen electrode in the same solution, which is more acceptable for real electrocatalytic applications. The third technical result of the invention is the extension of the field of application of the anode of a low-temperature fuel cell with a polymer membrane not only to the process of electrooxidation of methanol, but also to the process of oxidation of hydrogen substantially contaminated with carbon monoxide. Another result is the replacement of scarce and expensive ruthenium in the composition of the anode catalyst layer with a more affordable and cheaper molybdenum without deterioration of the catalytic activity and other important electrochemical characteristics of the resulting electrode.

Technical results are achieved due to the implementation of the proposed method for the manufacture of the anode of a low-temperature methanol or hydrogen fuel cell with a polymer membrane, characterized by the following sequence of operations:

a) chemically synthesize molybdenum bronze of the composition H _x MoO ₃ , where x takes a value from 0.2 to 0.4, by reduction of molybdenum (VI) oxide with zinc powder in a medium of 2-3 M hydrochloric acid at 0-10 ° C;

b) the synthesized bronze is smeared on the surface of the electrically conductive carbon carrier to obtain a precursor electrode C / H _x MoO ₃ ;

c) the precursor electrode is dried in an argon stream at a temperature of from 150 to 200 ° C and placed in an electrochemical cell with a deaerated solution containing a soluble salt of Pt (II) in 0.5-1.0 M sulfuric acid;

d) in a solution saturated with argon, currentless deposition of platinum on the precursor electrode is carried out by recharging the molybdenum bronze to obtain an electrode having a catalytic layer of nPt (H _x-2n MoO ₃ ), and the resulting electrode is washed 0.5-1.0 M sulfuric acid from platinum compounds to produce an anode of a low temperature methanol or hydrogen fuel cell.

Not limited to a specific theory, the inventors argue that the proposed method for the manufacture of the electrode provides the formation on its surface of a catalytic layer that effectively oxidizes methanol and carbon monoxide at potentials of 0.4-0.6 V relative to OEV in the same solution. The efficiency of electrooxidation is explained by the bifunctional mechanism of electrocatalysis: CO-like particles are adsorbed on platinum particles, and oxidized molybdenum bronze ensures the presence of oxygen-containing compounds at the Pt / H _x-2n MoO ₃ interface. As a result of a chemical reaction between them, the desorption of a chemisorbed substance, which is a poison for the electrooxidation of small organic molecules, occurs from the surface of platinum. This leads to an acceleration of the electrooxidation of methanol (carbon monoxide) through particles weakly bonded to the electrode surface.

Brief Description of the Drawings

Figure 1 presents the chronoamperogram of the oxidation of methanol (system 0.5 M H ₂ SO ₄ / 1.0 M CH ₃ OH) on the anode at a potential of 0.60 V relative to OEV in accordance with one embodiment of the invention (curve a) according to compared with the anode with a layer of platinum deposited by electrodeposition from a solution of H ₂ PtCl ₆ at E 0.25 V (curve b).

Figure 2 presents the chronoamperogram of methanol oxidation (0.5 M H ₂ SO ₄ / 0.5 M CH ₃ OH system) on the anode analogue of the invention (curve b) at a potential of 0.6 V relative to the silver-silver electrode (0, 8 V relative to OEV) as compared to an anode with a platinum layer deposited on a support (single-walled carbon nanotubes MWNTs, curve a).

Figure 3 shows the anode polarization curve obtained on the proposed anode in a 0.5 M solution of H ₂ SO ₄ saturated with CO on a rotating disk electrode (VDE).

The implementation of the invention

The possibility of carrying out the invention with the achievement of its technical results will be shown in the following examples.

Example 1. The manufacture of the anode

Hydrogen-containing molybdenum bronze is obtained by chemical reduction of 3.0 g of MoO _{3 with} zinc powder (the first portion is 0.5 g, the subsequent portion is 0.1 g) in a medium of 2-3 M hydrochloric acid at 0-10 ° C.

As a result of the synthesis, a red monoclinic bronze H _x MoO ₃ with a hydrogen content of x = 0.2-0.4 is obtained.

The obtained bronze without a binder is applied to a flat surface of glassy carbon (GC) or another carbon carrier in the calculation of 3.75 ± 0.3 mg per 1 cm ^{2 of the} geometric surface of the electrode. The thickness of the deposited layer of molybdenum bronzes is approximately 9 μm. The electrode thus prepared is dried in a stream of high purity argon to obtain a precursor electrode.

Before platinum precipitation, the C / H _x MoO ₃ precursor electrode was polarized in 0.5 M sulfuric acid for 30 minutes, maintaining a potential of approximately −0.1 V rel. o.e., given that a lower potential leads to the destruction of the molybdenum bronze layer. Then, the application of an external polarizing current is stopped and the sulfuric acid solution is replaced with a deaerated solution containing 0.002 mol / L K ₂ PtCl ₄ in 0.5 M sulfuric acid, which ensures the deposition of platinum without external current flowing. The duration of deposition is from 9 to 12 hours, while achieving an almost constant value of the electrode potential of approximately 0.55 V. Further exposure of the electrode to the solution does not make sense, since the deposition rate of platinum becomes very low, and the layer of molybdenum oxide compounds can be destroyed.

The resulting material was characterized by x-ray phase analysis (XRD). Reflections are observed in the diffractogram at 2θ = 33.82 °, 39.74 ° and 67.63 °, typical for platinum and at 2θ = 12.83 °, 23.40 °, 25.77 °, 27.39 °, characteristic for oxidized molybdenum bronze (CuK _α radiation).

Examples 2-9. Electrochemical oxidation of methanol

An anode with a geometric surface of 2 cm is placed in a cell filled with a solution (20-100 ml) containing from 0.1 to 1.0 mol / L methanol and 0.5 mol / L sulfuric acid. Using a potentiostat, the potential of the anode is set in the range from 0.4 to 0.6 V and the time dependence of the current flowing through the anode is recorded over a period of 10 minutes to 5 hours. The data are shown in tables 1 and 2.

Table 1 Example ⁽¹⁾ The concentration of methanol, mol / l Measurement time, min Current, μA Current reduction *,% at the beginning in the end 2 0.1 10 one hundred 70 thirty 3 0.5 one hundred 110 75 32 four 0.75 200 115 72 37 5 1,0 300 120 75 38 ⁽¹⁾ Anode potential: 0.45 V

table 2 Example ⁽²⁾ The concentration of methanol, mol / l Measurement time, min Current mA Current reduction *,% at the beginning in the end 6 0.1 10 8.0 6.0 25 7 0.5 one hundred 10.0 7.5 25 8 0.75 200 10.0 8.0 twenty 9 1,0 300 12.0 9.5 21 ⁽²⁾ Anode potential: 0.60 V * in relation to the initial value

The results show that at a potential of 0.45 V, the relative decrease in current over time is comparable to the same value for the analog anode operating at a much higher potential of 0.8 V relative to OVE, which corresponds to a value of 0.6 V relative to the silver chloride electrode, given in the article describing the analogue. Due to the fact that the data obtained in accordance with the invention and the data given in the article XD Xiang, QM Huang, Z. Fu, YL Lin, W. Wu, SJ Hu, WS Li “H _x MoO ₃ -assisted deposition of platinum nanoparticles on MWNTs for electrocatalytic oxidation of methanol. " International Journal of Hydrogen Energy. Vol. 37 (2012). P. 4710-4716, refer to different values of the electrode potential (0.6 and 0.8 V, respectively), their comparison is impossible.

It is known that Pt-Ru electrodes, such as the electrode described in Chi C.-F., Yang M.-C., Weng H.-Sh. A proper amount of carbon nanotubes for improving the performance of Pt-Ru / C catalysts for methanol electrooxidation // Journal of Power Sources. Vol. 193 (2009). P.462-469 have the best catalytic activity in the methanol electrooxidation reaction. Comparison of the catalytic activity of the anode made in accordance with the invention with the data given in this article showed that the activity of the prepared samples is not inferior to the activity of the expensive and scarce Pt-Ru catalyst. In addition, when comparing the data, it should be borne in mind that the data presented in the article refer to 30 ° C, while the characteristics of the anodes in accordance with the invention were obtained at a lower temperature (25 ° C).

The relative decrease in current in time (over 800 s) for the proposed electrode is less (23%) than for the analogue of MWNTs / Pt-H _x MoO ₃ (30%). Thus, the anode made in accordance with the invention surpasses known analogues in the stability of the catalytic effect in time.

Example 10

An aqueous solution of H ₂ SO ₄ (0.5 M) was saturated for 30 minutes by bubbling CO at a rate of about 1 bubble per minute. A catalytic layer prepared in accordance with the invention is applied to the surface of a rotating disk electrode (VDE), the VDE is placed in a solution, the rotation frequency is set at 500 min ^-1 and the carbon monoxide (II) (CO) is electrooxidized in a potential-dynamic mode with a potential sweep of E _start 0.3 V to the anode side with a speed of 5 mV / s, making polarization measurements.

Effective oxidation of CO dissolved in the electrolyte begins at potentials greater than 0.3 V, and the limiting diffusion oxidation current is reached at a potential of 0.4 V, which is significantly better than the results for analogues (0.50-0.60 V), in particular - for disclosed in the application WO 2008/110651.

The high electrocatalytic activity of the proposed catalyst in the electrooxidation reaction of CO dissolved in the electrolyte suggests that the electrode should have high resistance to CO impurities in hydrogen used in the hydrogen-oxygen fuel cell, and, therefore, its incorporating element is capable of operating on hydrogen with a high CO content .

Claims (2)

1. The anode of a low-temperature methanol fuel cell with a polymer membrane, including an electrically conductive carbon carrier, a catalytic layer and a collector, characterized in that the catalyst layer contains a composite material made from platinum compounds and hydrogen-containing molybdenum bronzes of the composition H _x MoO ₃ , where x accepts a value of from 0.2 to 0.4, and the specified catalytic composite material is obtained by a method in the implementation of which:
a) chemically synthesize molybdenum bronze of the composition H _x MoO ₃ , where x takes a value from 0.2 to 0.4, by reduction of molybdenum (VI) oxide with zinc powder in a medium of 2-3 M hydrochloric acid at 0-10 ° C;
b) the synthesized bronze is smeared on the surface of the electrically conductive carbon carrier to obtain a precursor electrode C / H _x MoO ₃ ;
c) the precursor electrode is dried in an argon stream at a temperature of from 150 to 200 ° C and placed in an electrochemical cell with a deaerated solution containing a soluble salt of Pt (II) in 0.5-1.0 M sulfuric acid;
d) in a solution saturated with argon, currentless deposition of platinum on the precursor electrode is carried out by recharging the molybdenum bronze to obtain an electrode having a catalytic layer of nPt (H _x-2n MoO ₃ ) and the resulting electrode is washed 0.5-1.0 M sulfuric acid from platinum compounds to produce an anode of a low temperature methanol or hydrogen fuel cell. 2. A method of manufacturing an anode of a low temperature methanol or hydrogen fuel cell with a polymer membrane, characterized in that when implementing the specified method:
a) chemically synthesize molybdenum bronze of the composition H _x MoO ₃ , where x takes a value from 0.2 to 0.4, by reduction of molybdenum (VI) oxide with zinc powder in a medium of 2-3 M hydrochloric acid at 0-10 ° C;
b) the synthesized bronze is smeared on the surface of the electrically conductive carbon carrier to obtain a precursor electrode C / H _x MoO ₃ ;
c) the precursor electrode is dried in an argon stream at a temperature of from 150 to 200 ° C and placed in an electrochemical cell with a deaerated solution containing a soluble salt of Pt (II) in 0.5-1.0 M sulfuric acid;
d) in a solution saturated with argon, currentless deposition of platinum on the precursor electrode is carried out by recharging the molybdenum bronze to obtain an electrode having a catalytic layer of nPt ⁰ (H _x-2n MoO ₃ ), and the resulting electrode is washed 0.5-1, 0 M sulfuric acid from platinum compounds to produce an anode of a low-temperature methanol or hydrogen fuel cell.

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