JPH0520868B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains] This invention uses pure hydrogen or methanol as a fuel,
This invention relates to a method for manufacturing a porous gas diffusion electrode for a fuel cell that generates electricity through an electrochemical reaction using hydrogen-rich reformed gas obtained by reforming methane, LNG, etc., and using air or oxygen as an oxidizing agent.

[Prior art and its problems]

This type of porous gas diffusion electrode generally has a porous electrode base material made of carbon fiber or carbon powder on one side, and a catalyst powder in which a base metal is supported on fine carbon powder, and a bond for bonding these catalyst powders. It consists of a catalyst layer in which polytetrafluoroethylene (PTFE), which is both a water repellent and a water repellent, is uniformly dispersed.

The electrochemical reaction in the catalyst layer of such an electrode is
The electrolytic solution permeates from one side of the catalyst layer, the reactive gas diffuses from the electrode base material side, and this occurs at the three-phase interface where the electrolytic solution and the reactive gas come into contact at the noble metal catalyst part. In order to maintain this electrochemical reaction stably for a long period of time, it is important to improve the stability of the noble metal catalyst under the cell operating conditions, but even more importantly, it is important to improve the stability of the noble metal catalyst under the cell operating conditions. The goal is to ensure a stable gas diffusion path for a long period of time. In order to create such gas diffusion passages in the catalyst layer, in the conventional technology, a precious metal-supported catalyst is ultrasonically dispersed in a dispersion medium such as ion-exchanged water, and then a predetermined amount of PTFE is added per weight of the catalyst. Porous gas diffusion electrodes were fabricated by applying the mixed solution onto an electrode base material treated with water-repellent treatment such as PTFE, and firing at a temperature at which PTFE melts. Electrodes produced by this method are considered to have a multilayer structure in which the amount of PTFE is increased from the electrolyte side to the base material side in order to strengthen water repellency from the electrolyte side to the electrode base material side. .

However, even in an electrode with such a configuration,
When looking at the microstructure of each layer, we can see that the catalyst powder
PTFE is a material that is uniformly dispersed within the layer.
Since the structure is not equipped with a gas diffusion channel network to ensure stable electrochemical reactions over a long period of time, when electrodes made using this method are incorporated into a battery and operated for a long period of time, the electrolyte The drawback was that it gradually penetrated into the catalyst layer, causing gas diffusion inhibition, making it impossible to obtain the desired battery output.

[Purpose of the invention]

The present invention has been made in view of the above, and an object of the present invention is to provide a gas diffusion electrode that can form a continuous gas diffusion passage network in the thickness direction of the electrode and obtain stable output for a long period of time under fuel cell operating conditions. With the goal.

[Key points of the invention]

According to the present invention, this purpose is achieved by mixing an aqueous dispersion of a noble metal-supported carbonaceous powder and a fluororesin with a fluororesin aggregate prepared in advance to form a catalyst mixture, and applying the mixture to a porous substrate. This is accomplished by firing to form a continuous gas diffusion channel network of fluororesin aggregates within the catalyst layer.

[Embodiments of the invention]

Examples of the present invention will be described below. 1~2wt% of 6g of precious metal supported carbon black catalyst, which is made by supporting 10wt% of platinum on carbon black fine powder.
After ultrasonic dispersion for about 5 minutes in 150 c.c. of ion-exchanged water containing a surfactant, the
Teflon equivalent containing 40wt% PTFE
30J (PTFE dispersion suspension manufactured by Mitsui Dupont Fluorochemicals) is gradually added to the catalyst dispersion while stirring with a stirrer to prepare a catalyst coating solution. On the other hand, in order to create a gas diffusion channel network in the catalyst layer, 2 c.c. of Teflon 30J suspension was dropped into 20 c.c. of inpropyl alcohol and stirred with a stirrer for 5 to 10 minutes to coagulate Teflon. Prepare the liquid. Next, these catalyst coating liquids and Teflon coagulation liquid were mixed to form a catalyst mixture, which was applied by suction onto an electrode base material that had been previously treated with water repellent treatment, and after drying, it was heated at 330°C for 5 minutes. A porous gas diffusion electrode was prepared by firing.

FIG. 1 shows the structure of the gas diffusion electrode formed in this way.The gas diffusion electrode 1 has a porous electrode base material 2 made of carbon fibers with a layer on one side for causing an electrochemical reaction. It is composed of a catalyst layer 3. In this configuration, the catalyst layer 3 serves as a binder for bonding the carbon catalyst powder 4 supporting a noble metal such as platinum and the carbon catalyst powder, and at the same time serves as a binder for imparting water repellency to the catalyst layer. The PTFE powder 5 is made of PTFE, and the PTFE powder is agglomerated into clumps to form a continuous gas diffusion passage network in the thickness direction of the catalyst layer.

Figure 2 shows the electrodes fabricated according to the embodiment of the present invention and the electrodes fabricated using the conventional technique assembled into a phosphoric acid fuel cell at an operating temperature of 190°C and a current density of 200mA/2.
This shows the results of a continuous discharge test when operated at ^cm3 . As is clear from the figure, the characteristics of the conventional electrode gradually deteriorate over time, and approximately
Although a rapid deterioration in characteristics was observed after 8000 hours, almost no deterioration in characteristics over time was observed in the electrode of the present invention. This is because in the gas diffusion electrode of the present invention, a gas diffusion channel network made of Teflon aggregates is formed continuously in the electrode thickness direction within the catalyst layer, so even if the battery is operated for a long period of time, the electrolyte remains This is thought to be because clogging of gas diffusion passages due to penetration is prevented.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, a pre-prepared fluororesin agglomerate is mixed with an aqueous dispersion of precious metal-supported carbonaceous powder and a fluororesin to form a catalyst mixture. By applying the coating to a solid substrate and firing it to form a continuous gas diffusion channel network made of fluororesin aggregates within the catalyst layer, even when the battery is operated for a long period of time, the gas caused by the penetration of the electrolyte can be prevented. The effect of preventing clogging of the diffusion path and lengthening the life of the electrode can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a main part showing the structure of an electrode obtained according to an example of the present invention, and FIG. 2 is a graph showing battery life characteristics when using an electrode according to an example of the present invention. 1... Gas diffusion electrode, 2... Porous electrode base material,
3...Catalyst layer, 4...Precious metal supported carbon catalyst powder, 5...PTFE powder, 6...Gas diffusion passage network.

Claims (1)

[Claims] 1. A gas characterized by mixing a pre-prepared fluororesin agglomerate with an aqueous dispersion of precious metal-supported carbonaceous powder and fluororesin to form a catalyst mixture, which is applied to a porous substrate and then fired. Method for manufacturing a diffusion electrode.