BRPI0401474B1 - Hybrid spray-hot pressing process for making PEM-type fuel cell electrode / membrane / electrode assemblies - Google Patents

Description

“SPRAY HYBRID PROCESS - HOT PRESS FOR

MAKING ELECTRODE / MEMBRANE / PEM TYPE FUEL CELL SETS ". The present invention relates to the technology of manufacturing electrodes for proton exchange polymeric membrane, that is, PEMFC (Proton Exchange Membrane Fuel Cell) type cell electrodes. More particularly, the present invention is characterized by the combined use of the Spray technique for applying the catalytic layer on the membrane and the Hot Press technique to produce the complete MEA (catalytic layer, diffuser layer and membrane), this process being a hybrid process. able to perform optimized (adjusted) control of process variables interdependently, as well as understand different process steps, such as using a vacuum table with temperature adjustment; special preparation of electrode precursor paint; and pressing the commercial diffuser layers onto extremely thin catalytic layers using different heating (pressing temperature) for each electrode. This process aims to reduce manufacturing costs while maintaining the best possible performance of the final MEA obtained. PEMFC are low operating temperature cells that use a polymeric membrane (DuPont Nafion®) as an electrolyte (ionic conductor) [David Linden, Handbook of Batteries and Fuel Cells. McGraw-Hill Book Company, New York, 1984J. They are the most promising, as an alternative to combustion engines, because they are robust and easy to start and stop, in addition to the inherent advantages such as high efficiency with low pollutant emissions [H. Wendt, M. Gotz, Brennstoffzellentechnik, Chemie in unserer Zeit, 6 (1997) 301-309.]. PEMFC cells also apply to stationary power generation units. The determining factor for your business success is its cost.

PEMFC fuel cells have two gas diffusion electrodes (electronic conductors), separated by a solid electrolyte (proton conductive membrane). The Electrode / Membrane / Electrode assembly is called MEA (Membrane Electrode Assembly). Gaseous diffusion electrodes are a porous electron conductive structure that encompasses the electrocatalyst. It is composed of two layers: the catalytic layer, where the electrocatalyst is dispersed and a diffuser layer. The construction of the catalytic layer has the function of maximizing the three-phase gas-electrolyte-electrocatalyst / electronic conductor interface, considerably increasing the speed of electrode processes. Gas diffusion electrodes must meet at least two important requirements [H. Wendt, M. and Linardi, M. and E. Aricó, Low Power Fuel Cells for Stationary Applications, New Chemistry, 25 (3) (2002) pp 470-476 and Wendt, H .; Gotz, M. and Linardi, M .; Fuel Cell Technology, New Chemistry, 23 (4) (2000), pp 538-546] namely: they must have high catalytic activity in order to obtain high current densities, and have a porous structure for gas diffusion. reagent whose inner surface is contacted by a thin film of the electrolyte ionomer in order to increase the electrode / electrolyte interface. Gas diffusion electrodes are extremely thin to minimize ohmic resistances. The electrocatalyst particles are in a nanometer size distribution range, generally dispersed in active carbon particles of diameters between 30 and 100 nm.

A thorough review of existing patents shows the development and state of the art of the technology of producing various types of electrodes, up to the development of today's gas diffusion electrodes, optimizing the use of the electrocatalyst in PEM cells. These developments are briefly described below.

U.S. Patent No. 3,385,780 discloses the development of a porous electrode composed of a thin layer formed of an electrically conductive substance (carbon with platinum finely divided with Pt contents of the order of 0.1 to 1). , 2 mg / cm2) and hydrophobic polymer (PTFE) pressed against another thin layer composed of hydrophobic polymer (PTFE). The production process of the porous electrode initially consists in obtaining a mixture formed by Pt-doped carbon, PTFE and a removable pore precursor substance (ammonium oxalate, ammonium carbonate and polyethylene or polypropylene), which is placed in a press. A second mixture formed of PTFH and pore precursor material is placed over the first mixture and then both are pressed at a pressure of 5000 and 12000 psi. The precursor material is subsequently removed by heating or acidic or basic leaching. The performance study of the electrodes is conducted with current densities of up to 160 mA / cm2, observing the polarization in relation to the theoretical value of the O2 electrode. It is the first patent to use a dual structure with an electrically conducting hydrophobic layer in conjunction with a gas dispersing hydrophobic layer, catalytic layer precursors and gas diffusion electrode diffuser layer. US Patent No. 4,469,579 uses a membrane called Nafion® as a solid electrolyte, a porous electrode with platinum metal group (oxides and / or metals and alloys), Sb, Sn, Ti catalysts. , V or a mixture thereof, the electrolyte (membrane) being pressed between the two electrodes.

The use of this system in the chlorine soda industry is described. This patent has the merit of using the Membrane Electrode Assembly (MEA) concept.

U.S. Patent No. 4,536,272 discloses the production of a porous electrode composed of finely divided carbon, catalyst (platinum, palladium, iridium, rhodium, silver, gold or a mixture of two or more of these elements), binder. (PTFE) and hydrophobic polymer particles (hardened PTFE). Obtaining hardened P TFE starts from heating PTFE powder at a temperature of 600 to 650 K within one hour. The use of hardened PTFE increased the electrode potential stability from 1000 hours to 5000 hours and decreased overvoltage.

Raistrick discloses in U.S. Patent No. 4,876,115 a fuel cell with solid polymeric membrane electrolyte and gaseous diffusion porous electrode formed by support with dispersed catalyst (platinum) particles and two layers of conductive material of protons. The catalyst loading used is between 0.3 and 0.5 mgPt / cm2. The production of the electrode is done by applying a Nafion® layer over the catalyst carbon particle layer and depositing a RuCh solution layer on them which is later oxidized to R11O2. This patent has the merit of using the MEA concept for fuel cell applications. This invention reduced the required Pt concentration on the electrode (from 4.0 to 0.35 mg / cm2) relative to cells with liquid electrolyte for the same performance due to the presence of the Nafion® membrane ionomer already in the catalytic layer. electrode

U.S. Patent No. 5,211,984 discloses two methods of forming a catalytic layer between a solid electrolyte membrane and a porous electrode. The first form of thin film production (less than 10 pm thick and catalyst loading less than 0.35 mgPt / cm2) by spraying and subsequent curing. The film is transferred to the membrane and then hot pressed to form the catalytic layer. The second innovative way to produce the catalytic layer is by applying a paint with the Na4 ionomer directly onto the membrane, drying the film at high temperature and then turning the film to the protanated form. 10 to 12% PVA is used in the paint, which through its surfactant nature promotes an adequate dispersion of the catalyst particles in the carbon support structure, acting also as a binder of the carbon and Nafion® molecules. The use of PVA allowed the formation of resistant layers. The pore distribution problem and its influence on the removal of water formed in the cathode as well as the variation of the electrode structure regarding the pore size and the possibility of the electrode becoming soaked with water obstructing O2 entry, decreasing the cell kinetics. U.S. Patent No. 5,316,871. The invention proposes the control of pore size and distribution using catalyst loads of the order of 0.03 mgPt / cm2 for the anode and 0.06 mgPt / cm2 for the cathode. The manufacture of gaseous diffusion electrode for use in protonic conductive membrane type fuel cells, manufactured using a pore precursor ink and a total porosity of 40 to 75% is described in U.S. Patent No. 5,861,222. The electrocatalysts are Pt based with charges from 0.01 to 4 mgPt / cm2 and at least one other element of the cobalt, chromium, tungsten, molybdenum, iron, copper, nickel and ruthenium group. The electrode has variable thickness from 5 to 100 nm, and porosity with bimodal distribution.

Currently, there are several different methods of manufacturing gas diffusion electrodes with their own characteristics in the literature [US 629427; WO

01/71840; WO 01/78173; WO 01/80336; WO 01/65623; WO 01/48854; WO

01/22514; WO 01/22510; WO 00/72391; WO 00/79628; WO 00/79630; US 6171721; US 6183668; US 6136463; WO 00/52775; WO 00/42670; WO 00/45448;

WO 00/30201; US 6060187; WO 00/26975; WO 00/26982; US 6042959; US 6054230; WO 00/24074; WO 00/10174; US 6017650; WO 99/66575; US 5998057;

WO 99/60648; WO 99/57772; WO 99/57773; WO 99/56335; WO 99/39840; US

5,910,378; US 5879828; US 5882810; WO 99/16137; US 5,869,416; US 5871860; US 5861222; US Patent Nos. 5843519], The patents studied cite different techniques and materials for preparing gaseous diffusion electrodes, polymeric membrane and MEAs for increased performance, cost reduction, production process automation and component life. However, none of the aforementioned processes use concurrent spray and hot pressing techniques, ie a hybrid Electrode / Membrane / Electrode (MEA) assembly process for PEM-type fuel cell applications, with optimized control of variables. associated with simplified and low cost steps, bringing together the advantages of both techniques into one: the spray technique for applying the catalytic layer on the membrane, and the hot pressing technique for the complete MEA. This unique process aims to reduce manufacturing costs, obtaining a reasonable performance of the MEA, quantified by the electrical current density supplied by the operation of the PEM cell at a potential of 0.6 V on the order of 800 mA.cm'2. .

To develop the PEM cell MEA production process by the Hybrid Spray and Hot Press method of the present invention, the following criteria were selected: a) simple steps in the manufacturing process for cost reduction; b) producing MEAs with an area of 25 cm2 to 400 cm2; c) simply using ultrapure water (ΙΟΜΩαη) as a solvent for producing the electrode precursor paint; d) use as electrolyte any Nafion® (Du Pont) membrane previously treated in hydrogen peroxide (3%) to remove organic contaminants, sulfuric acid (1.0 mol L'1, 1 hour at 80 ° C) and water ultrapure (1 hour at 80 ° C, 3 repetitions), store in ultrapure water; e) use a solution of Nafion dissolved in water. (i) use a suitable vacuum table of simple construction with heating to produce the electrodes by the spray method; g) pressing commercial diffuser layers onto the catalytic layers applied to the membrane with a differential heating hydraulic press for each electrode; h) produce extremely thin catalytic layers to minimize ohmic losses.

Unlike the state of the art, the production of the electrode precursor paint to be used by the Nafion membrane spray method has a composition characterized by ultrapure water as a solvent (2.0 to 6.0 mL); high surface area carbon black (Carbon Black) containing the nanodispersed electrocatalyst (0.1 to 0.4 g with 10 to 60% electrocatalyst) and Nafion solution in water (0.5 to 2.0 mL Nafion solution in water at 5 or 10% by mass). For example, a typical ink composition is: 4.0 mL of ultrapure water; 0.2 g Carbon Black with 20% Pt; 1.0 mL of 10% by weight Nafion solution in water.

Also characteristic of the preparation of the ink is the treatment of this suspension (the ink) in a tip ultrasound, called the Ultrasonic Cell Disruptor, unlike ultrasonic baths, to break up the carbon black agglomerates more efficiently, since all The energy of the equipment is concentrated in a small volume (the tip). Treatment time may vary from 10 to 60 seconds, with 30 seconds being the recommended time. After the suspension has been produced, it should be kept under magnetic stirring for a minimum of 12 hours. The production of the catalytic layer, Spray method, is characterized by the following procedure: 1) mounting the previously treated and cut Nafion membrane in a metal frame using Teflon joints to ensure a smooth surface; 2) placing the membrane-containing frame on a vacuum table consisting of a perforated surface treated with a vacuum pump to ensure that the membrane remains smooth. The vacuum should not be too high to damage the membrane. This procedure is important for the fixation of the paint (after application step 5) on the membrane in a homogeneous way, avoiding cracking of the layer in the cell operation, if there are roughness. The perfectly smooth membrane increases electrode longevity considerably; 3) adjust the temperature of the vacuum table; 4) adjusting the pressure of an oil filter air compressor or a dry air compressor; 5) vertically spraying the paint with a commercial airbrush for the manufacture of the first electrode at a suitable membrane height; 6) remove the metal frame with the membrane from the vacuum table and place in the oven at 100 ° C, remaining for a specified period of drying; 7) apply the other electrode on the opposite side according to the procedure described in (5); 8) repeat the procedure described in (6). The production of the MEA by the Hot Press technique is characterized by the following procedure: 9) cutting two pieces of electrophobic and partially hydrophobic porous conductive material, called diffuser layer, (eg Toray® Paper or Tissue Carbon Tissue), in the dimensions of the electrodes for use as diffuser layers; 10) place the membrane assembly with electrodes and diffuser layer between two layers of teflon® film; 11) placing the anterior assembly between the two metal plates of the hydraulic press, preheated to the desired temperatures, which are not necessarily equal for the anode and cathode. This control of the pressing temperature parameter can result in different porosities in each electrode, with consequent reduction of the mass transfer resistance, especially in the cathode, where the pores must be larger than in the anode, since in the first one the water formation occurs. that should partially flow out of the cell; 12) applying the desired pressure to the assembly at a desired time; 13) Remove the still hot assembly from the press and quench at 135 ° C for 30 minutes and cool in the room. The Hot Press technique allows extremely thin catalytic layers to be obtained which are crucial for minimizing the loss / resistance of the heat, proportional to the electrode thickness. After cooling, the MEA is ready for use in the fuel cell, as shown in Fig. 1, where there is a schematic of the electrode (a), membrane (b), and electrode (a) assembly. The diffuser layers (c) are further pressed in steps 10 and 11. The MEA manufacturing process described by this invention abdicates the use of electrode precursor paint additives, simplifying the process. The choice of Nafion solution dissolved in water concerns the same reason and is characteristic of this invention. The present invention makes use of the combined Spray and Hot Press process using a paint with water as a solvent. The present invention enables different press temperatures for the anode and cathode. In the process, only simple and easy to handle steps are involved.

Finally, the control of the described MEA manufacturing process, object of the present invention, is optimized by seven main selected and interdependent process variables, shown in Fig. 2, with maximum, minimum values and the center point. The end result is better MEA performance from the PEM fuel cell. The following illustrates the application of the present invention with an example for hydrogen / oxygen operation. The parameter values used in the manufacture of the MEA were: 2.0 bar spray air pressure; spray distance 8 cm; spray table temperature of 100 ° C; 60 min electrode drying time; 7 ton pressing pressure; pressing temperature 127 ° C and pressing time 2 min; EC-CC1-060T carbon fabric diffuser layer from Electrochem, USA. Catalyst loads were: 0.44 mg cm-2 for the anode and 0.61 mg cm-2 for the cathode.

The operating parameters of the unit fuel cell used to obtain the polarization curve were: 300 mE / min hydrogen flow; 150mL / min oxygen flow; EE humidification temperature of 80 ° C; cell temperature of 65 ° C. Electrochem, USA PEM unit cell block of 25 cm2 electrode area was used. A polarization curve was obtained after a period of 15 hours of continuous MEA operation under the indicated conditions. A dynamic load (TDI - Model RBL488) coupled to a PC4 Potentiostat / Galvanostat / ZRA with Gamry Instruments EIS300 software was used. The curve obtained is shown in Fig. 3.

Claims (6)

1 - Hybrid Spray-Hot Press process for making PEM-type Fuel Cell Electrode / Membrane / Electrode (MEA) assemblies, characterized by controlling the adjustment of the following process variables interdependently: spray air pressure, spray distance, spray table temperature, electrode drying time, pressing pressure, pressing temperature and pressing time; and comprising the following steps: (a) using a temperature-adjusted vacuum table; (b) preparing the electrode precursor ink and treating the tip ultrasound suspension for subsequent application of the catalytic layer, in the form of said ink, onto the membrane; (c) pressing the commercial diffuser layers onto the membrane-applied catalytic layers using different heating (pressing temperature) for each electrode. Hybrid Spray-Hot-pressing process according to claim 1, characterized in that the use of temperature-adjusted vacuum permits obtaining the smooth membrane, increasing the electrode longevity and homogeneously fixing the electrode precursor paint when its subsequent application by spray technique. Hybrid Spray-Hot Press process according to claim 1, characterized in that the preparation of the electrode precursor ink comprises ultrapure water as a solvent, from 2 mL to 6 mL, high surface area carbon black containing the nanodisperse electrocatalyst; 0.1 to 0.4 g with 10 to 60% electrocatalyst, and aqueous solution of the previously treated Nafion electrolyte membrane, with 0.5 mL to 2 mL of Nafion solution in 5 or 10% water by mass. Hybrid Spray-Hot-pressing process according to claim 1, characterized in that the suspension treatment of the tip ultrasound electrode precursor paint permits efficient rupture of the carbon black agglomerates, varying the treatment time from 10 to 60 seconds. Hybrid Spray-Hot-Pressing Process according to Claim 1, characterized in that the hot-pressing process allows each electrode to be obtained with different porosity and produces extremely thin catalytic layers to minimize ohmic losses. Hybrid Spray-Hot-pressing process according to claim 1, characterized by obtaining MEA with decreased interface resistances, increased performance and an area of 25 cm to 400 cm.