Patent Application
20090246570
This invention relates to a novel method for measuring a crossover loss relative to a membrane electrode assembly for fuel cells. The invention also relates to a measuring apparatus based on the measuring method. The invention still relates to various type of application devices based on the measuring method.
In recent years, a direct methanol fuel cell DMFC (Direct Methanol Fuel Cell) using methanol as a fuel has been expected as an electric source for portable devices in place of lithium ion secondary cells, and extensive developments have been made in order to work forward the practical use thereof.
The electric generation unit of DMFC has a structure wherein a cathode catalyst layer and an anode catalyst layer are provided on opposite sides of a proton conductive solid polymer electrolyte membrane. This is called membrane electrode assembly (Membrane Electrode Assembly). The cathode catalyst layer and the anode catalyst layer are each made of a matrix wherein catalyst-supporting carbon and a solid polymer electrolyte are appropriately mixed together, and the electrode reaction is carried out at the three-phase interface where the catalyst on carbon, the solid polymer electrode and a reactant are in contact with one another. Carbon linkage is a path of electrons and solid polymer electrolyte linkage becomes a path of protons.
DMFC is such that reactions of the formulas (1) and (2), respectively, occur at the anode catalyst layer and the cathode catalyst layer and electricity is taken out
CH3OH+H2O→CO2+6H++6e− (1)
O2+4H++4eu−→2H2O (2)
The total reaction formula combining (1) and (2) is as follows
CH3OH+3/2O2→CO2+H2O (3)
It has been accepted that DMFC has an energy density of about 10 times greater than lithium secondary cells theoretically. At present, however, the output of MEA is lower than that of a lithium secondary cell, thus not yet arriving at practice.
For improving the output of MEA, there are approaches for improving catalysts and an electrolyte membrane and optimizing an MEA structure. Of these, an improvement in electrolyte membrane is a key for improving the output of MEA. The properties required for the electrolyte membrane are those two including (1) high proton conductivity and (2) a low pervious amount of methanol. The proton conductivity of (1) relates to a resistance of the electrolyte membrane. If the proton conductivity is low, the resistance increases, thereby inviting an output lowering. The pervious amount of methanol of (2) is concerned with so-called “crossover” wherein methanol at the anode arrives at the cathode after passage through the electrolyte membrane. The methanol arriving at the cathode reacts with oxygen on the cathode catalyst chemically, thereby generating heat. This causes an overvoltage to be increased at the cathode owing to the crossover, thereby lowering the output of MEA. A lowering of output voltage caused by the crossover is called “crossover loss”.
Methods of measuring an amount of crossover includes (i) a measurement of a transmitted current density (J. Electrochem. Soc., 147 (2) 466, (2000)) by the Gotesfeld method, (ii) a measurement of a coefficient of methanol permeability by gas chromatography, (iii) a measurement of a coefficient of methanol permeability by liquid chromatography, and the like.
According to the method (i), because the electrode reaction differs from the electrode reaction of DMFC, the crossover loss which is a lowering of output voltage of MEA cannot be estimated. More particularly, an electric current of a reaction differing from an actual DMFC reaction formula is measured.
With (ii) and (iii), judging from a membrane thickness and a time, a coefficient of methanol permeability is calculated to compare the crossover amounts of methanol with each other.
Although the methanol crossover current and the coefficient of methanol permeability become a measure for a crossover amount, interrelation with the crossover loss is not clear, so that it cannot be estimated how much is a crossover loss.
In this way, although a crossover loss is a factor that is important for determining an output in actual fuel cells, the method of direct measurement has never been known up to now and thus, other crossover amounts are used for a measure. It is unclear what meaning is involved in the measured crossover current density or a permeability coefficient. For instance, if these values are reduced to ½, a crossover loss is not reduced to ½.
In view of the above, the present invention has for its object the provision of a novel measuring method of measuring a methanol crossover loss directly.
The measuring method for evaluating a crossover amount of methanol of the membrane electrode assembly by measuring a voltage of a membrane electrode assembly which comprises a cathode catalyst layer reducing an oxidative gas and an anode catalyst layer oxidizing a methanol aqueous solution, a proton conductive solid polymer electrolyte membrane layer arranged therebetween.
Embodiments of the invention are described in detail with reference to the drawings.
This embodiment is directed to a measuring method, characterized in that a crossover loss is measured from a difference between a voltage at which a cathode catalyst layer is free of an influence of methanol crossover and a voltage at which the cathode catalyst layer receives an influence of methanol crossover relative to MEA for methanol fuel cell.
In order to measure a voltage at which no influence of methanol crossover is received, it is characterized that a methanol aqueous solution is filled in an anode and an inert gas is filled in a cathode, under which a voltage is loaded between the anode and the cathode, thereby oxidizing the methanol crossovered from the anode to the cathode electrochemically. Thereafter, the measuring method is characterized in that air or oxygen is subsequently fed to the cathode to measure a variation of an open circuit voltage OCV (Open Circuit Voltage). In this measuring method, a maximum voltage measured immediately after the feed of air or oxygen in the cathode is a voltage at which methanol is substantially zero in the cathode catalyst layer, i.e. a voltage at which no influence of methanol crossover is received. If allowing to stand as it is, the cathode catalyst layer turns to a voltage, at which an influence of methanol crossover is received, with the voltage becoming constant. The difference between these voltages corresponds to a crossover loss. This embodiment is also directed to a measuring apparatus based on the above-stated measuring principle and is able to measure a crossover loss directly.
The voltage at which the cathode catalyst layer does not receive any influence of methanol crossover means a voltage in a state where a methanol aqueous solution in the cathode catalyst layer is ideally at 0 or is close to 0. The voltage at which the cathode catalyst layer receives an influence of methanol crossover means a voltage that becomes constant at a given value as a result that methanol is crossovered from the anode to the cathode, so that an overvoltage of the cathode becomes great to lower an output voltage.
In practice, where a methanol concentration is high, the top voltage measured is not a voltage at which the methanol in the cathode catalyst layer is not exactly at 0. More particularly, it is considered that if the methanol concentration is high, the methanol aqueous solution is left on the carbon surfaces or fine pores of the catalyst carrier immediately after the Gotesfeld method. Methanol is present in the electrolyte membrane. It is considered that these methanol aqueous solutions instantaneously influence the voltage upon completion of the Gotesfeld method. In this sense, it is preferred that the methanol concentration is as low as possible when the top voltage is measured. More particularly, the concentration is preferably not larger than 1 wt %. More preferably, the concentration is not larger than 0.5 wt %.
In the novel crossover loss measuring method of this embodiment, a difference between the top voltage and the plateau voltage corresponds to a crossover loss. If the methanol concentrations differ from each other for the measurements of the top voltage and the plateau voltage, correction will be necessary. In other words, the difference in methanol concentration causes a variation in anode potential, for which correction is necessary. For the correction, the Nernst equation can be used. The equilibrium electromotive force E of the equation (3) is expressed from the Nernst equation as follows.
E=E
0+2.303×(RT/nF) log [aCH3OH×P023/2/aH2O2×PCO2] (6)
(E0: theoretical electromotive force, a: activity and P: partial pressure).
Where a methanol concentration at the time of measurement of top voltage is taken as wt (wt %) and a methanol concentration at the time of measurement of plateau voltage is as wp (wt %), a voltage difference ΔVconc. ascribed to the difference in the methanol concentration is expressed as follows when taking the Nernst equation is taken into account
ΔVconc.=Ewp−Ewt=2.303×(RT/nF) log [aCH3OH, wp/aCH3OH, wt](Ewp: voltage at wp, Ewt: voltage at wt, aCH3OH, wp: activity at wp, and aCH3OH, wt: activity at wt.)=0.01×log (wp/wt) (7)
Taking the above into consideration, the methanol crossover loss ΔVcrossover loss can be obtained according to the following equation
ΔVcrossover loss=Ewtt−Ewpp+ΔVconc. (8)
(Ewtt: top voltage at wt, and Ewpp: plateau voltage at wp).
In the Gotesfeld method, if a voltage loaded between the anode and cathode is too low, a current to be measured is such that the crossover of methanol from the anode side to the cathode side of MEA is not rate limited, but the reaction of (4) or (5) in the catalyst layer is rate limited. Accordingly, a current to be measured does not correspond to “crossover current”. The voltage loaded between the anode and cathode is preferably 0.7V or more. If the voltage loaded between the anode and the cathode is too high, electrolysis of an electrolyte and water takes place. To avoid this, the voltage is preferably at 0.9V or less.
The time of applying a voltage between the anode and the cathode is until a crossover current value to be measured becomes constant and is preferably 1 minute to 2 hours.
For the type of inert gas fed according to the Gotesfeld method, there can be used nitrogen gas, argon gas, helium gas and the like. The flow rate may be one which allows the gas to be passed throughout the cathode and is preferably 10-1000 ml/minute. Immediately after the Gotesfeld method, the flow rate of air or oxygen being fed is preferably 10-1000 ml/minute. In addition, the cathode may be opened immediately after the Gotesfeld method, followed by subjecting to natural expiration of air to measure an open circuit voltage OCV.
In this embodiment, after the Gotesfeld method, although air or oxygen is fed to the cathode to measure an open circuit voltage OCV, the measuring time varies depending on the MEA conditions such as, for example, an amount of catalyst, a thickness of the catalyst layer and a type of electrolyte membrane. The measuring time before arriving at a plateau voltage is preferably 2 minutes to 10 hours. Moreover, if a plateau voltage can be calculated by extrapolation of a voltage change, the measuring time may be shortened.
As a method of feeding a methanol aqueous solution to the anode, there may be used a tank system wherein a given amount of a methanol aqueous solution is filled or a flow system wherein a methanol aqueous solution is passed at a given flow rate. With the flow system, the flow rate of the methanol aqueous solution is preferably 5-500 ml/minute.
In
A measuring cell is not limited critically. For instance, there can be used a single cell as shown in
This embodiment can be used for PEFC (Polymer Electrolyte Fuel Cell) using hydrogen as a fuel. With PEFC and DMFC, H+ that is produced by electrode reaction in the anode catalyst layer moves from the anode catalyst layer toward cathode catalyst layer in the electrode membrane, and water entrained with H+ also moves in the electrolyte membrane. With DMFC, methanol used as a fuel has the same size as water and is compatible therewith, thereby permitting passage through the electrolyte membrane. On the other hand, with PEFC, hydrogen gas is, more or less, dissolved in water, so that crossover takes place as water moves. Crossover also occurs through fine pores in the electrolyte membrane. Like DMFC, a crossover loss is caused wherein crossovered hydrogen increases an overvoltage at the cathode, thereby lowering an output voltage.
This embodiment can be used to measure a hydrogen crossover loss against PEFC. For the measurement of a hydrogen crossover loss, measurement by the Getesfeld method is carried out. More particularly, hydrogen gas is fed to an anode and an inert gas is fed to a cathode, under which a given voltage is loaded between the anode and the cathode. For the reaction formula, the crossovered hydrogen gas from the anode side undergoes the following reaction at the cathode side.
H2→2H++2e− (9)
The resulting proton H+ undergoes the following reaction at the anode side.
2H++2e−→H2 (10)
Like DMFC, immediately after the Gotesfeld method, hydrogen in the cathode catalyst layer instantaneously becomes substantially 0. More particularly, according to the Gotesfeld method, hydrogen in the cathode catalyst layer is consumed by electrochemical reaction. Like the flowchart of
This embodiment relates to a measuring apparatus based on the principle of the novel hydrogen crossover measuring method. In
If the principle of the crossover loss measurement of this embodiment is applied to, there can be obtained a simpler measuring method although not so accurate. In the crossover loss measurement of the embodiment, an inert gas is filled in the cathode and a voltage is loaded between the anode and the cathode. However, the feed of an inert gas needs a complicated device and a prolonged time, and measurement becomes impossible when using a so-called passive DMFC cell wherein a cathode is not subjected to forced inspiration. To avoid this, it may occur that instead of filling the cathode with an inert gas, a constant current is loaded thereby lowering an oxygen concentration on and in the surface of a cathode catalyst layer. More particularly, the loading of a current causes the DMFC cell reaction of the formula (2) to occur at the cathode and thus, oxygen is consumed, thereby lowering an oxygen concentration on or in the surface of the cathode catalyst layer. Thereafter, a voltage is loaded between the anode and the cathode, so that methanol crossovered from the anode toward the cathode is oxidized electrochemically to cause the methanol in the cathode catalyst layer to be reduced substantially to 0. Subsequently, air or oxygen is fed to the cathode and a variation of an open circuit voltage OCV thereof is measured to measure a top voltage and a plateau voltage thereby obtaining a crossover loss.
Using the novel crossover loss measuring method of this embodiment, the lifetime of MEA can be evaluated. The degradation of an electrolyte membrane, particularly, an increase of a crossover loss influences the lifetime of MEA greatly. Since the crossover loss can be measured directly according to the measuring method of the embodiment, the lifetime of MEA can be evaluated. This embodiment relates to a device capable of evaluating a lifetime by use of the principle of the novel crossover measuring method. The lifetime evaluation device can be used for DMFC shown in
Using the novel crossover loss measuring method of this embodiment, defective MEA can be sorted. In the mass production of MEA, a certain number of defectives are produced and one of defective factors resides in a failure of electrolyte membrane. For instance, if electrolyte membranes vary in thickness, defective MEA products are produced wherein the crossover loss becomes great. When using the novel crossover loss measuring method of the embodiment, defectives can be sorted. This embodiment relates to an evaluation device of defective MEA products using the principle of the novel crossover loss measuring method.
The application of the embodiment enables MEA to have high output power. More particularly, after methanol in the cathode catalyst layer is made at or close to 0, electric power is generated, upon which there is no methanol crossover loss, so that output power can be improved. For instance, an inert gas is fed to the cathode side and a methanol aqueous solution is filled at the anode side, under which a voltage is loaded between the anode and the cathode so that the methanol crossovered toward the cathode is oxidized electrochemically so that the methanol in the cathode catalyst layer is made substantially at 0. Thereafter, air or oxygen is fed to the cathode in place of the inert gas to generate electric power. Alternatively, instead of filling the cathode with an inert gas, a given current is loaded to lower an oxygen concentration on the surface of the cathode catalyst layer, after which a voltage is loaded between the anode and the cathode so that the methanol crossovered toward the cathode is oxidized electrochemically to make the methanol in the cathode catalyst layer at 0. Subsequently, air or oxygen is fed to the cathode to generate electric power. According to the process stated above, it is expected that output power is improved correspondingly to a suppression of the methanol crossover loss.
Many factors are involved in the degradation of MEA, one of which is degradation of an electrolyte membrane, particularly, an increase of crossover loss. That is, the crossover loss increases owing to the dissolution of the electrolyte membrane or a structural change in the electrolyte membrane. If the crossover loss can be measured directly, evaluation of lifetime becomes possible.
In the mass production of MEA, a given number of defectives occur. One of factors for the defectives results from defects of electrolyte membrane. For instance, if the electrolyte membranes vary in thickness, there occur defectives of MEA whose crossover loss becomes great. The direct measurement of crossover loss enables such defective products to be sorted.
The invention is illustrated by way of embodiments. The embodiment of the invention should not be construed as limited to the following embodiments.
S-PES (ion exchange capacity of 1.3 meq/g) was provided as a membrane. A varnish was prepared by dissolving S-PES (ion exchange capacity of 1.3 meq/g) in dimethylacetamide. The concentration of the solute was set at 30 wt %. The varnish was coated onto a glass sheet by means of an applicator and dried at 80° C. for 1 hour and then at 120° C. for 3 hours in a vacuum dryer, thereby evaporating the dimethylacetamide solvent. Thereafter, the coated film was peeled off from the glass sheet and immersed overnight in a 1M H2SO4 aqueous solution and protonated to obtain a single electrolyte membrane of S-PES (ion exchange capacity of 1.3 meq/g). The thus obtained electrolyte membrane was transparent. The thickness of the electrolyte membrane was at 50 μm.
MEA was made in the following way. Platinum-bearing carbon TEC10V50E (amount of supported Pt: 50 wt %), made by Tanaka Kikinzoku Kogyo K.K., was provided as a cathode catalyst and platinum and ruthenium-bearing carbon TEC61V54 (amount of supported Pt: 29 wt %, amount of supported Ru: 23 wt %), made by Tanaka Kikinzoku Kogyo K.K., was provided as an anode catalyst. Water and a 5 wt % Nafion solution, made by Aldrich Corp., were added to these catalysts, followed by mixing and agitating to provide catalyst slurries. The catalyst slurries, respectively, had weight ratios of TEC10V50E: water 5 wt % Nafion solution=1:1:8.46 for the cathode and TEC61V54: water 5 wt % Nafion solution=1:1:7.9 for the anode. These catalyst slurries were each applied onto a teflon sheet by use of an applicator to prepare a cathode catalyst layer and an anode catalyst layer. Thereafter, the cathode catalyst layer and the anode catalyst layer were transferred thermally to an electrolyte membrane using a hot press to provide MEA. The amounts of the catalysts were, respectively, at 1.8 mg of Pt and Ru/cm2 for the anode catalyst and 1.2 mg of Pt mg/cm2 for the cathode catalyst.
The thus made MEA was assembled in a cell shown in
The novel crossover loss measuring method of this embodiment was carried out with respect to this MEA. First, a voltage of 0.8 V was loaded for 10 minutes, followed by feeding air at 200 ml/minute in place of nitrogen gas to measure OCV. The methanol concentrations were set at 0.1, 0.3, 1.5, 10 and 20 wt %, respectively. In
With an actual cell voltage, the anode potential is influenced by the methanol concentration. More particularly, since the anode potential varies depending on the difference in methanol concentration and thus, a correction is needed therefor. For the correction, the equation (7) can be used. The crossover loss at a methanol concentration of 10 wt % is calculated below. The voltage difference ΔVconc. depending on the difference in methanol concentration between 0.3 wt % and 10 wt % is shown below using the equation (7).
ΔVconc.=E10−E0.3=2.303×(RT/nF) log [aCH3OH, 10/aCH3OH, 0.3]
(wherein E10: a voltage at 10 wt %, E0.3: a voltage at 0.3 wt %, aCH3OH, 10: an activity at 10 wt %, and aCH3OH, 0.3: an activity at 0.3 wt %)
=0.01×log (10/0.3)
=0.015 (V)
=15 (mV)
Accordingly, the crossover loss at a methanol concentration of 10 wt % can be calculated by adding a top voltage of 993 mV at a methanol concentration of 0 in the cathode catalyst layer measured at a methanol concentration of 0.3 wt % to a voltage difference of 15 mV depending on the difference in the methanol concentration, and subtracting a plateau voltage of 648 mV at a methanol concentration of 10 wt %.
ΔVcrossover loss=E0.3t−E10p+ΔVconc.=993−648+15=360 mV
Likewise, the crossover losses at 0.3, 1, 5 and 20 wt % are calculated by using a top voltage at 0.3 wt % that is a voltage at a methanol concentration of 0 in the cathode catalyst layer and correcting it with respect to the methanol concentration, with the results shown in
MEA was made under the same conditions as in the First Embodiment. The MEA was subjected to measurement of a crossover current density by use of the Gotesfeld method that is a conventional technique of measuring an amount of crossovered methanol. The measuring conditions were such that nitrogen gas was fed to the cathode at 200 ml/minute and a voltage of 0.8 V was loaded for 10 minutes. In
Nafion 112 (with a thickness of about 50 μm), made by Du Pont Kabushiki Kaisha, was used as an electrolyte membrane. In the same conditions and procedure as in the First Embodiment, MEA's were made. These MEA's were subjected to measurement of a crossover loss according to the novel crossover measuring method of this embodiment.
The MEA of the Second Embodiment was subjected to measurement of a crossover current density according to the Gotesfeld method. The results are shown in a figure. As is particularly shown in
MEA was made under the same conditions as in the First Embodiment. This MEA was assembled in a cell shown in
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/307023 | 3/28/2006 | WO | 00 | 2/6/2009 |
