WO2008087487A2

WO2008087487A2 - Fuel cell electrode manufactured by sedimentation

Info

Publication number: WO2008087487A2
Application number: PCT/IB2007/004394
Authority: WO
Inventors: Eva WALLNÖFER; Viktor Hacker
Original assignee: Graz University Of Technology
Priority date: 2006-10-17
Filing date: 2007-09-25
Publication date: 2008-07-24
Also published as: WO2008087487A3

Abstract

A fuel cell electrode made of carbon nanofibers, a binder, and a catalyst can be constructed using a sedimentation method. Sedimentation has certain advantages over printing, painting, or rolling methods. The electrode can be used for both solid and liquid electrolytes.

Description

FUEL CELL ELECTRODE MANUFACTURED BY SEDIMENTATION

Field

The invention relates to fuel cells, and, more particularly, to fuel cell electrodes that are manufactured by sedimentation.

Background

Tubular carbon nanofϊbers (CNF's) can be used in fuel cell electrodes because of their good electrical conductivity and structural qualities. There are several manufacturing methods available to make an electrode containing CNF's. Unfortunately, printing and painting methods are not suitable for electrodes due to the existence of small, woven aggregates of CNF's introduced by CNF fabrication process. These woven aggregates may block the nozzle of a printing machine or lead to non-homogeneous layers for painting methods. Rolling methods are possible, but they are time consuming and require specialized skills to be successfully employed. In addition, rolling methods are not usable for the fabrication of thin layers less than 500 nm. Precious metals such as silver or platinum are often used as catalysts in fuel cells. Platinum in particular is very expensive. If the amount of catalyst in a fuel cell can be reduced, then the cost for the fuel cell would go down.

What is needed is a method of manufacturing an electrode from CNF's that results in an electrode that is porous enough for gas diffusion, minimizes the amount of catalyst required, and has a low binder requirement.

Summary

A method for making an electrode for a fuel cell by sedimentation is described. The electrode contains carbon nanoiϊbers, a binder, and a catalyst. Brief description of drawings

Figs. IA-D shows examples of different layers in an electrode. Fig. 2 A shows an example of Pt plating at room temperature. Fig. 2B shows an example of Pt plating at 3 ⁰C. Figs. 3A-B show examples of electrodes with and without a current collector. Fig. 4 shows an example of experimental results. Fig. 5 shows an example of an electrode construction method.

Description The electrode described here includes an active layer (AL) and a gas diffusion layer (GDL). The active layer contains CNF, a binder (for example, polytetrafluorethylene (PTFE)), and a catalyst (for example, Ag, Pt, or any other well known catalysts). The GDL contains CNF and a binder. PTFE makes a good binder, because it is hydrophobic, it has high stability in acid and alkaline environments, and ■ it is stable at the operating temperature of many types of fuel cells. The binder contents of the active layer and the GDL may be different. For example, if the fuel cell has a liquid electrolyte, the binder content of the GDL can be higher than that of the active layer. This higher binder content in the GDL can prevent the liquid electrolyte from passing through the GDL. One advantage of CNF' s used at catalyst support is their one dimensional structure in combination with their very good electrical conductivity. The electrons that are generated or used at the fuel cell reaction have a long free pathway without internal resistance. The existence of the CNF's in a woven felt also contributes to reduce the resistance of the electrode, because not every individual fiber has to be stuck to another one with isolating binder.

Compared to CNF's, commonly used spherical carbon particles have to be stuck together with binder to build a three dimensional web of carbon. Therefore, there are many contact resistances. Furthermore, the binder can be destroyed by the aggressive electrolyte resulting in the electrode losing its stability. The sedimentation process is desirably used to create the active layer. The

GDL may optionally be made by sedimentation as well. In addition to overcoming the disadvantages of other manufacturing methods (rolling, etc. described above), sedimentation has the following advantages:

• Precise control of catalyst content

• Precise control of binder content

• Uniform distribution of binder in the CNF • Lower required amounts of catalyst for a given current density

In order to create the active layer, one first starts with raw CNF. The raw CNF is washed to remove the transition metals that are introduced in the manufacturing process of the CNF. These transition metals may adversely affect the fuel cell performance. The raw CNF (for example, HTF150FF-LHT, Electrovac GmbH, Austria, with an average diameter of 150 nm) material may be purified and functionalized with oxygen-containing groups by treatment in a mixture of concentrated nitric acid and sulphuric acid under refluxing conditions. CNF-material may be loaded with a range of platinum (for example, between 10-20 wt%) according to the following procedure. Oxidized CNF's may be dispersed in distilled water (for example, 0.25 g CNF/100 ml) by a treatment with a high speed disperser and an ultrasonic bath. NaBH₄ in excess may be added and pH- value may be adjusted to 12 with 1 ml OfNH₄OH solution. The appropriate amount of dissolved H₂PtCIo may be added within 5 seconds under vigorous stirring. Reaction may be carried out at room temperature for one hour and at 3 ⁰C for 48 hours. The product may be filtered, washed with distilled water, and dried at 100 ⁰C for 12 hours.

After the CNF has been washed, a catalyst can be deposited on the CNF. One way of doing this is to (1) disperse the CNF in water, (2) deposit catalyst on the CNF using an electroless plating method, and (3) filtering, washing, and drying the catalyst plated CNF.

The catalyst loaded CNF may be examined with scanning electron microscopy (SEM). Platinum reduction with NaBH₄ at room temperature leads to a deposition of particles of the desirable size (in the range of 10 nm) for fuel cell catalyst applications. Unfortunately, catalyst particles of desirable size may agglomerate, which leads to bigger particles on the CNF that may be unevenly distributed in large areas as seen in Fig. 2 A. By lowering of the reaction temperature to 3 ⁰C, one may see significantly smaller particles, with a diameter less than 10 nm, and a better distribution as seen in Fig. 2B. The reaction time may need to be elongated to 48 hours for a complete reduction of the metal ions at the lower temperature.

After the CNF is plated with the catalyst, the active layer can be created by sedimentation. An appropriate amount of catalyst plated CNF and binder (for example, Dyneon, TF 5035 PTFE) may be dispersed in distilled water by a short treatment with a high speed disperser and an ultrasonic bath. The active layer is desirably constructed on a porous support. The support may be an already constructed GDL, or the support may be some other type of structural element such as a piece of carbon cloth. A fitting piece of carbon paper (for example, Sigracet GD Media GDL 24 BA, 190 μm thickness) or a GDL as support and an optional fitting current collector (for example, a piece of titanium grid) may be placed on a glass frit (for example, Schott Duran^® Por. 4, diameter 44 mm) and screwed in a funnel system. Sealing rings may be used to prevent the dispersed plated CNF and binder from settling on the glass frit instead of the porous support. Other types of rigid porous support may be used in place of the glass frit.

It is possible that another type of carbon paper or carbon cloth can be used as the electrode support. The eletrode support is desirably not too thick, but thick enough to give mechanical strength. The paper/cloth desirably has pores with a relatively large diameter, but it should be able to separate the CNF 's and the binder particles from the glass frit.

The solid components of the dispersion are desirably sedimented on the frit by percolation. The process may be accelerated by applying a vacuum to the percolation apparatus. Next, the active layer may be dried at 120 ⁰C and then pressed at 12 MPa for 10 seconds. It is desirable that the percolation process be completed before the CNF and the binder start to separate in the dispersion.

If the layer being sedimented has a thickness greater than 3.3 mg CNF-cm^'2, it may be desirable to build the layer by repeating the steps (sedimentation, drying, and optionally pressing) until the desired thickness is achieved. The multi-step process may lead to a more uniform thickness. Also, if too thick a layer is attempted at once, the CNF and binder may separate before the percolation process is complete.

If an optional GDL layer is to be created using sedimentation, then the GDL is first sedimented on the support (carbon cloth, for example) using the process described above for the creation of the active layer, but using washed CNF and an appropriate binder amount. After the GDL layer is constructed, the active layer may be sedimented on top of the GDL using the process described above. The finished electrode may be sintered under pressure at 250 ⁰C and 12 MPa for 30 minutes. Sintering allows the binders from the active layer and the gas diffusion layer to melt and stick together.

One example of an electrode has a GDL with a PFTE content of 21% by weight and an active layer with a PFTE content of 7% and a Pt catalyst load of 0.4 mg-cm^"2. The GDL may have a CNF content of 6.6 mg CNF-cm^"2 (built in two steps) and the active layer may have a CNF content of 4.7 mg CNF-cm^'2.

Experimentally constructed electrodes were testing for functionality. The electrodes were tested in half cell tests as cathodes with a working area of 5 cm² using an electrochemical workstation Zahner IM5d. AU measurements were carried out with a test cell build of polysulphone and the electrode to be tested was fixed on a PTFE sealing. A three electrode setup was used (Reference electrode: Hg/HgO, counter electrode: Pt wire) with a 9 N KOH solution as electrolyte and pure oxygen under ambient pressure as oxidant. The electrolyte was heated up to 50 ⁰C and pumped through the cell to realize a constant concentration of the solution. All potentials were corrected for the ohmic resistance and are related to the reversible hydrogen electrode (RHE). Before the test electrodes reached their full power density, they were activated by increasing the current at operating conditions.

The results showed that the performances of the electrodes with a catalyst load of 0.7 mg Pt crn^"2 was at large similar and reached at least 350 mA-cm^"2. The overall cell current was restricted to 1.9 Ampere by the electrochemical workstation. To evaluate the best variation of GDL-thickness, AL-thickness and PTFE- content, the six best and six worst electrodes were determined at current densities of 5O₅ 100, 150, 200, 250 and 300 mA-cm^'2. The selection of six electrodes each ensured that outliers were not overvalued. At this, some variations of the parameter loomed to be assigned to good or worse performances. For example, electrodes with very thin GDLs were placed under the worst six usually but rarely under the best six. For a better interpretation, those six best and worst electrodes and thus their parameters (thickness of GDL and AL, PTFE-content) received scores accordant to their places at the given current densities. According to this, many scores in the category "best electrode" and/or little scores in the category "worst electrode" meant that a variation of a parameter had a good influence on the performance (and for a bad influence vice versa). Electrodes were evaluated using this system in the range of low (50 and 100 mA-cm^"2), medium (150 and 200 mA-cm^'2) and higher (250 and 300 mA-crn^'2) current densities and in the whole current density area. Results were clear and without contradictions. The thickness of the GDL had the biggest influence on the performance. A thin GDL (4.6 mg CNF-cm^"2) clearly led to worst performances in all current density areas, whereas a medium thick GDL (6.6 mg CNF-cm^'2) led to best performances, especially at higher current densities. A thick GDL (9.9 mg CNF-cm^'2) also had a good influence, especially at lower current densities. A middle thick AL (4.7 mg CNF-cm^"2) led to best and a thick AL (7.0 mg CNF-cm^'2) to worst performances in all current densities. A thin AL (3.5 mg CNF-cm^"2) led to good performances with the exception that at higher current densities its influence was obviously worse. It could be assumed that the thick AL had too much catalyst particles placed in zones where no electrochemical reaction could take place. As expected, the PTFE-content also had a clear influence on the performance. Electrodes with a lower PTFE-content (21-7 wt%) often had a good performance, and those with a higher PTFE-content (30-10 wt%) rarely. Since the lower binder contents prevented the electrodes from being completely flooded by the electrolyte just as well, this supported the theory that the covering of catalyst particles with binder could be reduced without a loss of stability of the electrode by using CNFs as carbon material. These results were confirmed by the fact that the electrode with the best performance at all current densities combined the variety of parameter supposed to be the best (middle thick GDL₅ middle thick AL, lower PTFE-content). Furthermore, the two electrodes with the worst performances combined a thin GDL and a thick AL. The one with the higher PTFE-content performed only up to 150 mA-cm^"2.

An electrode with a platinum loading of 0.4 mg Pt-cm^"2 was prepared, modelled on the best one with 0.7 mg Pt-cm^"2, and its performance was measured under the same conditions as before. In comparison to the performance of the electrodes with a platinum loading of 0.7 mg Pt-cm^'2, it is shown that the choice of the best parameters, the catalyst loading could be much reduced. At lower current densities it is obviously that the lower catalyst loading had a bigger influence on the performance because of less catalyst surface. This influence was reduced at higher current densities, where the diffusion of the reaction products tended to be dominant. Figs. IA-D show examples of different layers in an electrode. Fig. IA shows an electrode 110 which has an active layer 102, a gas diffusion layer 104, and a porous support 106. Also shown is an electrolyte 108. Fig. IB shows the same elements as are shown in Fig IA, with the addition of a current collector 112 in between the gas diffusion layer 104 and the porous support 106. Fig. 1C shows the same elements as are shown in Fig IA, with the addition of a current collector 112. In this case the porous support 106 is in between the gas diffusion layer 104 and the current collector 112. Fig. ID shows the same elements as are shown in Fig IA, with the addition of a current collector 112. In this case the current collector 1 12 is in between the active layer 102 and the electrolyte 108. Fig. 2 A shows an example of Pt plating at room temperature. Fig. 2B shows an example of Pt plating at 3 ⁰C. The Pt particle sizes are larger in the plating conducted at room temperature.

Figs. 3A-B show examples of electrodes with and without a current collector. In these examples, the porous support is carbon paper. Fig. 4 shows an example of experimental results.

Fig. 5 shows an example of an electrode construction method. In 502 carbon nanofibers are washed. In 504 the washed carbon nanofibers are dispersed with a binder. In 506 a gas diffusion layer is created by sedimentation. In 508 washed carbon nanofibers are plated with a catalyst with a electroless plating method. In 510 the plated carbon nanofibers are filtered washed and dried. In 512 the plated carbon nanofibers are dispersed with a binder. In 514 an active layer is created by sedimentation on top of the gas diffusion layer. In 516 the gas diffusion layer and active layer are sintered together.

It will be apparent to one skilled in the art that the described embodiments may be altered in many ways without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their equivalents.

Claims

ClaimsWhat is claimed is:

1. An electrode for a fuel cell, the electrode comprising: an active layer, the active layer comprising: carbon nanofibers; a catalyst, the catalyst being deposited on the carbon nanofibers; and a first binder, where the first binder content is such that the first binder does not inhibit access to the catalyst.

2. The electrode of claim 1, further comprising: a gas diffusion layer, the gas diffusion layer comprising: carbon nanofibers; and a second binder.

3. The electrode of claim 1, further comprising a current collector.

4. The electrode of claim 1, where the binder is hydrophobic, where the binder has high stability in acid and alkaline environments, and where the binder is stable at an operating temperature of the fuel cell.

5. The method of making an electrode for a fuel cell, the method comprising: depositing a catalyst on carbon nanofibers; and using sedimentation to build an active layer; where the active layer comprises carbon nanofibers deposited with the catalyst and a first binder.

6. The method of claim 5, further comprising: using sedimentation to build a gas diffusion layer; and sintering the gas diffusion layer and the active layer together, where the gas diffusion layer comprises carbon nanofibers and a second binder,

7. The method of claim 5, where depositing the catalyst on the carbon nanofibers comprises: dispersing the carbon nanofibers in a solution; using an electroless plating process to deposit catalyst on the carbon nanofibers; filtering the carbon nanofibers; washing the carbon nanofibers; and drying the carbon nanofibers.

8. The method of claim 5, where the sedimentation is accelerated with vacuum percolation of a dispersion liquid through a rigid porous support.