RU2392698C1 - Method of manufacturing of membrane-electrode unit with bifunctional electrocatalytic layers - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention refers to catalytic chemistry, particularly to methods of manufacturing of membrane-electrode units (MEU) with bifunctional electrocatalytic layers based on platinum-group metals and designed for utilisation in reversible (regeneration) fuel cells with solid polymer electrolyte (SPE). According to the invention, method of MEU manufacturing involves layerwise coating a membrane with electrocatalytic layers based on platinum-group metals and proton -exchange polymer, gas diffucion electrodes and hot compression of the layers at 120-125°C under 50-60 kg/cm² pressure for 5-10 minutes. Two-layer electrocatalytic layer is applied at the anode side, with one layer based on iridium and proton-exchange polymer and second layer based on platinum and proton-exchange polymer, and gas diffusion electrode out of porous titanium, and cathode side is coated with electrocatalytic layers consisting of Pt on carbon carrier with different fluoroplastic and proton-exchange polymer content and gas diffusion electrode based on porous carbon material, such as carbon paper, fabric or felt.

EFFECT: reduced consumption of platinum-group metals, increased voltage in fuel cell mode, reduced voltage in electrolyte mode, improved stability of reversible fuel cell.

9 cl, 8 ex

Description

The invention relates to catalytic chemistry, and in particular to methods of manufacturing membrane electrode blocks (OIE) with bifunctional electrocatalytic layers based on platinum group metals, intended for use in reversible (regenerative) fuel cells with a solid polymer electrolyte (TPE). Reversible fuel cells can operate both in the cell mode and in the fuel cell.

As an electrocatalyst for oxygen and hydrogen electrodes of a fuel cell, carbon-supported platinum is used (US Pat. No. 6,344,428. Method of forming catalyst layer for fuel cell, published 05.02.2002). However, in a reversible fuel cell, said catalyst is not suitable, because in the electrolysis mode, the carbon carrier is oxidized, thereby the contact between the catalyst particles is broken and the voltage rises significantly. In addition, platinum is not an optimal anode electrocatalyst of the electrolyzer due to the relatively high potential for oxygen formation, which leads to an additional increase in voltage in the electrolyzer mode.

Reversible fuel cells can have both chemically reversible oxygen and hydrogen electrodes (Scheme 1), and bifunctional electrodes that do not change their oxidizing or reducing function when switching operating modes (Scheme 2).

In the first case, when changing the operating mode of a reversible fuel cell, the gases produced or consumed as a result of the electrochemical reaction on this electrode remain the same. For example, the electrode on which oxygen was released in the electrolyzer mode is also used to restore oxygen in the fuel cell mode. However, a fully reversible oxygen electrode does not exist. Optimal from the point of view of a fuel cell, hydrophobic platinum-based electrocatalytic layers on a carbon support are not optimal from the point of view of electrolysis, where hydrophilic materials and an Ir and / or Ru-based catalyst on the anode are required (and vice versa). In addition, when using such a scheme, inevitably, difficulties arise associated with corrosion and flooding of the oxygen electrode. In particular, a carbon gas-diffusion electrode with a hydrophobic microporous coating that is optimal from the point of view of the water balance in the fuel cell mode cannot be applied due to its rapid corrosion by oxygen and its radicals released in the electrolysis mode.

Scheme 2 of constructing a reversible cell, i.e. when hydrogen oxidation / oxygen evolution processes are carried out at the anode, oxygen reduction / hydrogen evolution is performed at the cathode, it requires the use of bifunctional electrocatalytic layers (electrodes) and allows the use of gas diffusion electrode materials traditional for systems with solid polymer electrolyte: porous titanium on the anode and porous carbon material at the cathode. In addition, with such a scheme for the oxygen electroreduction reaction (limiting reaction in the fuel cell mode), it is possible to use a hydrophobic platinum electrocatalyst on a carbon support (having a more developed surface area and lower platinum consumption), which is unacceptable when using the first scheme. A reversible fuel cell according to Scheme 2 is more promising, but an effective membrane-electrode block has not been created for it.

Various methods are known for manufacturing bifunctional electrocatalytic layers for a reversible fuel cell.

Optimization of bifunctional electrocatalyst for PEM unitized regenerative fuel cell, Sung-Dae Yim, Won-Yong Lee et al., Electrochimica Acta 50 (2004), 713-718, proposes as an electrocatalyst for a reversible hydrogen cell fuel cell Pt black in an amount of 4.0 mg / cm ² , on the oxygen electrode - composition Pt-Ir (chemically deposited Ir on Pt in a ratio of 50:50 wt.%) With a flow rate of 4.0 mg / cm ² . Carbon paper is used as gas diffusion electrodes.

The disadvantage of this electrocatalyst and the membrane-electrode block based on it is the low specific active surface of the Pt black and Pt-Ir composition (for both catalysts it is the same and amounts to S _BET = 27 m ² / g) and, as a consequence, a rather high consumption of platinum group metals - 4.0 mg / cm ² , in addition, the electrocatalyst does not provide efficient operation according to scheme 2, because Ir is not an optimal catalyst for the oxidation of hydrogen.

A known method of manufacturing a membrane-electrode unit with bifunctional electrocatalytic layers is a prototype (US patent No. 6838205. "Bifunctional catalytic electrode", publ. 10.04.2003). The manufacturing method consists in applying a reversible fuel cell to the proton-exchange polymer membrane as an electrocatalytic layer of one electrode of an IrO ₂ -RuO ₂ mixture with a content of RuO _{2 of} 5-85 mol%, and as an electrocatalytic layer of a second electrode of a mixture of Pt black and solid solution RuO ₂ -IrO ₂ (1: 1 mol.%), The content of Pt black is 40-70 wt.%, RuO ₂ -IrO ₂ - 60-30 wt.%, The optimal ratio of 60 wt.% Pt and 40 wt.% RuO ₂ -IrO ₂ . The deposition density of the electrocatalyst 4 mg / cm ² . The composition of the electrocatalytic layers includes a proton-exchange polymer in an amount of 30-60 vol.%. After applying the electrocatalytic layer to the membrane, the resulting membrane-electrode block is dried in an electric oven at 150 ° C with a nitrogen purge.

Gas diffusion electrodes are made of thin sheets of metal (titanium, zirconium, hafnium, niobium, aluminum, copper, nickel) or metal grids or thin metal felts. To prevent the formation of an oxide film, a layer of noble metal is applied to them or made of oxidation-resistant alloys, stainless steel. To impart hydrophobic properties, alternating hydrophobic and hydrophilic zones are created on the electrode surface on a scale from micron to about one millimeter (by treating the electrode surface with a perfluorocarbon solution).

The disadvantage of the prototype is the high consumption of platinum group metals, the lack of uniform hydrophobization of the gas diffusion electrode of the fuel cell, in addition, the low efficiency of the electrode according to scheme 2, the relatively low voltage in the fuel cell mode and insufficient stability when switching the operating mode repeatedly.

The technical result to which the invention is directed is to reduce the consumption of platinum group metals, increase the voltage in the fuel cell mode, reduce the voltage in the electrolysis mode, increase the stability of the reversible fuel cell.

To this end, a method for manufacturing a membrane-electrode block with bifunctional electrocatalytic layers for a reversible fuel cell is proposed, which consists in layer-by-layer deposition of electrocatalytic layers on the membrane based on platinum group metals and proton-exchange polymer, gas diffusion electrodes and hot pressing of said layers, while applied from the side anode a two- or three-layer electrocatalytic layer based on iridium and platinum and a proton-exchange polymer and a gas diffusion electrode made of porous titanium, and on the cathode side a two-layer electrocatalytic layer consisting of Pt on a carbon carrier with different contents of fluoroplastic and proton-exchange polymer, and a gas diffusion electrode based on porous carbon material.

The electrocatalytic layer of the anode contains: in the first layer deposited on the membrane, iridium and platinum in an amount of from 0 to 10% Pt with respect to Ir, in the second layer - iridium and platinum in an amount of from 95 to 100% Pt with respect to Ir.

In this case, an intermediate electrocatalytic layer of Pt-Ir is applied from the side of the anode between the first and second layer at a ratio of Pt: Ir = 50: 50 mol%.

The electrocatalytic layer of the anode contains Ir-containing oxides: IrO ₂ , or solid solutions IrO ₂ -RuO ₂ , or IrO ₂ -RuO ₂ -SnO ₂ , in an amount of 0.5-2.0 mg / cm ² and Pt in an amount 0.5-1.5 mg / cm ² .

The Pt content in the electrocatalytic layer of the cathode is 0.5-0.8 mg / cm ² , in the first layer 0.2-0.3 mg Pt / cm ² , the fluoroplastic content is 3-5% of the mass of the electrocatalytic layer, in the second layer - 0.3-0.5 mg Pt / cm ² , the content of fluoroplastic 10-12% of the mass of the electrocatalytic layer.

The content of the proton-exchange polymer from the cathode side is 15-20% by weight of the electrocatalytic layer.

The content of the proton-exchange polymer from the anode side is 5-7% by weight of the electrocatalytic layer.

At the same time, carbon paper, fabric, and felt are used as the carbon material.

While hot pressing is carried out at a temperature of 120-125 ° C, a pressure of 50-60 kg / cm ² for 5-10 minutes.

It is known that the highest rate of oxygen evolution is observed for iridium or iridium-containing electrocatalysts, and hydrogen oxidation for platinum.

The technical result is due to the fact that in the case of hydrogen oxidation, the reaction zone is localized mainly on the surface of the electrocatalytic layer facing the gas diffusion electrode. In the case of oxygen evolution, the reaction proceeds mainly in the area of the electrocatalytic layer at the membrane surface. In this regard, a method is proposed for layer-by-layer deposition of electrocatalytic layers of Pt and Ir or Ir-containing oxides, the electrocatalytic layer facing the membrane having a higher content of Ir or its oxides, which allows one to obtain characteristics not inferior to those for the oxygen evolution reaction on pure iridium during electrolysis. And the layer facing the gas diffusion electrode has a higher platinum content, which provides high performance in the oxidation of hydrogen in the fuel cell mode.

On the one side of the membrane (cathode side), a Pt electrocatalyst (40 wt.%) Is applied in layers on a carbon support with various degrees of hydrophobicity - near the membrane, the deposition density of the catalyst is 0.2-0.3 mg Pt / cm ² with a hydrophobicity of 3-5 wt. .% fluoroplastic, then 0.3-0.5 mg Pt / cm ² with a fluoroplastic content of 10-12 wt.%.

When applying the electrocatalyst, 15-20 wt.% Proton-exchange polymer is added to it. The electrocatalyst is applied to a proton-exchange polymer membrane, for example Nafion.

Pt and Ir (or Ir-containing oxides) are applied layerwise to the other side of the membrane (anode) with a flow rate of each metal of 0.5-2.0 mg / cm ² , with the Ir layer located near the membrane containing from 0 to 10% Pt with respect to Ir (or Ir-containing oxides), and the layer at the surface of the gas diffusion electrode contains from 95 to 100% Pt with respect to Ir or Ir-containing oxides. The composition of the electrocatalytic layer includes 5-7 wt.% Proton-exchange polymer.

Thus as the anode electrocatalyst can be used instead of Ir solid solution of IrO ₂ -RuO ₂ -SnO ₂ with RuO ₂ content of 1 to 30 mol.%, SnO ₂ of from 1 to 40 mol.%.

Replacing Ir with Ir-containing oxides can increase the specific active surface of the electrocatalyst and reduce the consumption of platinum group metals.

The following are used as gas diffusion electrodes: on the anode side, a plate of porous titanium, and on the cathode side, a porous carbon material.

OIE tests were carried out as part of a reversible element with TPE under the following conditions:

in the fuel cell mode - at a temperature of 80 ° C, a hydrogen pressure of 2.8 atm, an oxygen pressure of 3.0 atm, a humidification temperature of hydrogen of 85 ° C, H ₂ and O ₂ flow rates of 160 ml / min,

in electrolyzer mode - at a temperature of 90 ° C and atmospheric pressure of gases.

The area of the working surface of the electrodes is 7 cm ² . Membrane Nation 115.

Example 1 (prototype)

Production of the first electrode

According to the methodology described in the prototype, an electrocatalytic layer containing 21 mg of IrO ₂ and 7 mg of RuO ₂ (based on 60 mol.% IrO ₂ and 40 mol.% RuO ₂ ) and 1.5 mg of proton-exchange polymer is applied to the Nafion membrane Nafion.

Making the second electrode

On the other side of the Nafion membrane, an electrocatalytic layer is applied containing 17 mg of Pt, 11 mg of a solid solution of RuO ₂ -IrO ₂ (1: 1 mol.%), Synthesized by the Adams method (based on 60 wt.% Pt black and 40 wt.% oxides) and 1.5 mg of the Nafion proton-exchange polymer.

The electrocatalytic layers are dried in an electric oven at a temperature of 150 ° C with a nitrogen purge.

Specific surface area of Pt S _BET = 28 m ² / g, solid solution of RuO ₂ -IrO ₂ (1: 1 mol%) S _BET = 37 m ² / g, IrO ₂ S _BET = 35 m ² / g, RuO ₂ S _BET = 41 m ² / g.

Tests of the OIE made as part of a cell of a reversible fuel cell showed the following results: at a current density of 1 A / cm ^{2 the} voltage in the electrolysis mode was 1.72 V, in the fuel cell mode at 1 A / cm ^{2 it} was 0.62 V. At 30- multiple switching of the processes of electrolysis-current generation, the decrease in characteristics was 7%.

Example 2

The process is similar to that in Example 1 and differs in that in the manufacture of the first electrode, instead of 21 mg of IrO ₂ and 1.5 mg of the proton exchange polymer, 7 mg of IrO ₂ and 0.7 mg of the proton exchange polymer are applied, while the manufacture of the second electrode instead of 17 mg of Pt, 11 mg of a solid solution of RuO ₂ -IrO ₂ and 4 mg of a proton-exchange polymer cause 2.5 mg of Pt, 2.5 mg of a solid solution of RuO ₂ -IrO ₂ and 0.3 mg of a proton-exchange polymer.

Tests of the OIE made as part of a cell of a reversible fuel cell showed the following results: at a current density of 1 A / cm ^{2, the} voltage in the electrolysis mode was 1.82 V, in the fuel cell mode at 1 A / cm ^{2 it} was 0.52 V. At 30- multiple switching of the processes of electrolysis-current generation, the decrease in characteristics was 11%.

Examples 3-8 of the claimed invention

Example 3

Anode fabrication

In a 30 ml glass bottle, 7 mg of Ir mobile is placed, 0.35 mg of the Nafion proton exchange polymer (5 wt.%) And 10 ml of isopropyl alcohol are added. The mixture is mixed with an ultrasonic dispersant for 5 minutes and applied to the membrane using an airbrush. The electrocatalytic layer is dried in air at 25 ° C. Similarly, an electrocatalytic layer of Pt is prepared and sprayed onto the electrocatalytic layer of Ir.

Cathode fabrication

12.6 mg of Pt electrocatalyst (40 wt.%) Was placed in a 30 ml glass bottle on Vulcan XC-72 soot, hydrophobized with 10 wt.% F-4 fluoroplastic, 1.9 mg of Nafion proton-exchange polymer (15 wt. %) and 10 ml of isopropyl alcohol. The mixture is mixed with an ultrasonic dispersant for 5 minutes and applied to the other side of the membrane using an airbrush.

A porous titanium plate was used as gas diffusion electrodes on the anode side, and Sigracet 10bb carbon paper was used on the cathode side.

The OIE is formed by hot pressing a solid polymer membrane with electrocatalytic compositions and gas diffusion electrodes deposited on both sides at a temperature of 120 ° C and a pressure of 50 kg / cm ² for 5 minutes.

Specific surface area Pt 40 / Vulcan XC-72 S _BET = 52 m ² / g.

Tests made by the OIE in the composition of the cell of a reversible fuel cell showed the following results: at a current density of 1 A / cm ^{2 the} voltage in the electrolysis mode was 1.70 V, in the fuel cell mode at 1 A / cm ² - 0.60 V. At 30- multiple switching of the processes of electrolysis-current generation, the decrease in characteristics was 3.0%.

Example 4

The process is similar to that in Example 3 and differs in that instead of 7 mg Ir, 7 mg IrO ₂ is applied to the anode. The specific surface of IrO ₂ S _BET = 35 m ² / g.

At a current density of 1 A / cm ^{2, the} voltage in the electrolysis mode was 1.68 V, in the fuel cell mode at 1 A / cm ^{2 it} was 0.660 V. With a 30-fold switching of the electrolysis-current generation processes, the decrease in characteristics was 2.5%.

Example 5

The process is similar to that in Example 3 and differs in that instead of 7 mg of IrO ₂ , 7 mg of a solid solution of IrO ₂ -RuO ₂ (70:30 mol%) synthesized according to the Adams method is applied to the anode. The specific surface of IrO ₂ -RuO ₂ S _BET = 62 m ² / g

At a current density of 1 A / cm ^{2, the} voltage in the electrolysis mode was 1.67 V, in the fuel cell mode at 1 A / cm ^{2 it} was 0.720 V. With a 30-fold switching of current electrolysis-generation processes, the decrease in characteristics was 2.5%.

Example 6

The process is similar to that in Example 3 and differs in that instead of 7 mg of IrO ₂ , 7 mg of a solid solution of IrO ₂ -RuO ₂ -SnO ₂ (30:30:40 mol%) synthesized by the Adams method is applied to the anode. Specific surface IrO ₂ -RuO ₂ -SnO ₂ S _BET = 62 m ² / g.

At a current density of 1 A / cm ^{2, the} voltage in the electrolysis mode was 1.66 V, in the fuel cell mode at 1 A / cm ^{2 it} was 0.750 B. With a 30-fold switching of the current electrolysis-generation processes, the decrease in characteristics was 2.0%.

Example 7

The process is similar to that in Example 3 and differs in that instead of 12.6 mg of Pt electrocatalyst (40 wt.%) On soot Vulcan XC-72 hydrophobized with 10 wt.% Fluoroplastic containing 1.9 mg of the Nafion proton-exchange polymer (15 wt.%) 6.0 mg Pt (40 wt.%) is applied to the cathode on Vulcan XC-72 soot, hydrophobized with 5 wt.% fluoroplastic with the addition of 0.9 mg proton-exchange polymer, the electrocatalyst layer is dried in air, and then apply 6.6 mg of Pt (40 wt.%) on soot Vulcan XC-72, hydrophobized 10 wt.% fluoroplastic with the addition of 1.1 mg of proton-exchange polymer.

At a current density of 1 A / cm ^{2, the} voltage in the electrolysis mode was 1.65 V, in the fuel cell mode at 1 A / cm ² - 0.780 V. With a 30-fold switching of the processes of current electrolysis-generation, the decrease in characteristics was 2.0%.

Example 8

The process is similar to that in Example 3 and differs in that instead of 7 mg Ir and 7 mg Pt, 3 layers of electrocatalyst are applied to the anode: the 1st layer consists of 4 mg Ir and 0.2 mg of the proton-exchange polymer, the 2nd layer - 6 mg of a Pt-Ir solid solution (ratio of Pt: Ir = 50: 50 mol%) and 0.3 mg of a proton-exchange polymer, the third layer is 4 mg of Pt and 0.2 mg of a proton-exchange polymer. Specific surface area of Pt-Ir S _BET = 30 m ² / g.

At a current density of 1 A / cm ^{2, the} voltage in the electrolysis mode was 1.66 V, in the fuel cell mode at 1 A / cm ² - 0.780 V. With a 30-fold switching of the current electrolysis-generation processes, the decrease in characteristics was 2.0%. From example 8 it follows that an increase in the number of layers of the electrocatalyst on the anode does not lead to an improvement in the performance of a reversible fuel cell.

Thus, this method of manufacturing the OIE with bifunctional electrocatalytic layers for reversible fuel cells will reduce the consumption of platinum group metals by more than 2 times than when using the OIE prototype, increase the voltage in the fuel cell mode by 150-160 mV, reduce the voltage in the electrolysis mode by 60-70 mV, to increase the stability of the operation of a reversible fuel cell - when switching operating modes, the decrease in performance is 5-7% less.

Claims (9)

1. A method of manufacturing a membrane-electrode block with bifunctional electrocatalytic layers for a reversible fuel cell, which consists in layer-by-layer deposition of electrocatalytic layers on the membrane based on platinum group metals and proton exchange polymer, gas diffusion electrodes and hot pressing of said layers, characterized in that they are applied from the anode side a two-layer electrocatalytic layer based on iridium and platinum and a proton exchange polymer and a porous titanium gas diffusion electrode on, and on the cathode side, a two-layer electrocatalytic layer consisting of Pt on a carbon support with different contents of fluoroplastic and proton exchange polymer, and a gas diffusion electrode based on a porous carbon material. 2. The method according to claim 1, characterized in that the intermediate electrocatalytic layer of Pt-Ir is applied from the anode side between the first and second layer at a ratio of Pt: Ir = 50: 50 mol%. 3. The method according to claim 1, characterized in that the electrocatalytic layer of the anode contains: in the first layer deposited on the membrane - iridium and platinum in an amount of from 0 to 10% Pt with respect to Ir, in the second layer - iridium and platinum in an amount from 95 to 100% Pt with respect to Ir. 4. The method according to claim 1, characterized in that the electrocatalytic layer of the anode contains Ir-containing oxides of IrO ₂ , or solid solutions of IrO ₂ -RuO ₂ , or IrO ₂ -RuO ₂ -SnO ₂ in an amount of 0.5-2.0 mg / cm ² and Pt in an amount of 0.5-1.5 mg / cm ² . 5. The method according to claim 1, characterized in that the Pt content on the cathode is 0.5-0.8 mg / cm ² in the first layer deposited on the membrane is 0.2-0.3 mg Pt / cm ² and fluoroplastic 3-5% by weight of the electrocatalytic layer, in the second layer 0.3-0.5 mg Pt / cm ² and fluoroplastic 10-12% by weight of the electrocatalytic layer. 6. The method according to claim 1, characterized in that the content of the proton-exchange polymer on the cathode side is 15-20% by weight of the electrocatalytic layer. 7. The method according to claim 1, characterized in that the content of the proton-exchange polymer from the side of the anode is 5-7% by weight of the electrocatalytic layer. 8. The method according to claim 1, characterized in that the carbon material used is carbon paper, fabric, felt. 9. The method according to claim 1, characterized in that the hot pressing is carried out at a temperature of 120-125 ° C, a pressure of 50-60 kg / cm ² for 5-10 minutes