MEMBRANE ELECTRODE WITH ULTRA-LOW OXYGEN MASS TRANSFER RESISTANCE

Abstract

A membrane electrode with ultra-low oxygen mass transfer resistance includes an anode catalyst layer, a proton exchange membrane (PEM), and a cathode catalyst layer. A catalyst in the cathode catalyst layer is negatively charged, and the cathode catalyst layer is further doped with a negatively charged carbon carrier. A carbon carrier of the cathode catalyst layer in the membrane electrode is negatively charged, thereby optimizing the distribution of ionomers to achieve the purpose of reducing an oxygen mass transfer resistance in the cathode catalyst layer. In addition, an appropriate amount of the negatively charged carbon carrier is doped to increase a local oxygen concentration near active sites. In conclusion, the two methods of modifying with a negative charge and doping a negatively charged carbon carrier are used to optimize the local mass transfer resistance in an electrode and thus improve the cell performance.

Claims

1. A membrane electrode with an ultra-low oxygen mass transfer resistance, comprising an anode catalyst layer, a proton exchange membrane (PEM), and a cathode catalyst layer, wherein a catalyst in the cathode catalyst layer is negatively charged, and the cathode catalyst layer is further doped with a negatively charged carbon carrier.

2. The membrane electrode according to claim 1, wherein a catalyst carrier is a carbon carrier; and the carbon carrier is negatively charged and then loaded with platinum nanoparticles to obtain a negatively charged platinum-carbon catalyst.

3. The membrane electrode according to claim 2, wherein a mass ratio of the platinum nanoparticles to the negatively charged carbon carrier in the negatively charged platinum-carbon catalyst is 1:1.

4. The membrane electrode according to claim 3, wherein the negatively charged carbon carrier is prepared by subjecting the carbon carrier to a hydrothermal reaction with concentrated sulfuric acid to modify a carbon surface with a sulfate anion.

5. The membrane electrode according to claim 1, wherein the negatively charged carbon carrier is prepared by subjecting the carbon carrier to a hydrothermal reaction with concentrated sulfuric acid to modify a carbon surface with a sulfate anion.

6. The membrane electrode according to claim 4, wherein a mass ratio of the carbon carrier to the concentrated sulfuric acid is 1:3; and the hydrothermal reaction is conducted at 180° C. for 24 h.

7. The membrane electrode according to claim 4, wherein the carbon carrier comprises XC-72 and KJ-600.

8. A fabrication method of the membrane electrode according to claim 1, comprising the following steps: S1: subjecting a carbon carrier to a hydrothermal reaction with concentrated sulfuric acid to modify a carbon surface with a sulfate anion to obtain a negatively charged carbon carrier; S2: loading a first part of the negatively charged carbon carrier with platinum nanoparticles to obtain a negatively-charged platinum-carbon catalyst; S3: adding the negatively-charged platinum-carbon catalyst and a Nafion solution in a mass ratio of 2:5 to a solvent, adding a second part of the negatively charged carbon carrier to the solvent to obtain a mixture, and stirring the mixture by ball-milling for 10 h to 16 h to obtain a cathode catalyst layer slurry, wherein a mass ratio of the second part of the negatively charged carbon carrier to the negatively-charged platinum-carbon catalyst is (0.3-0.6):1; and S4: spraying the cathode catalyst layer slurry on a first side of a Nafion proton membrane by electrostatic spraying, spraying an anode catalyst layer slurry on a second side of the Nafion proton membrane, and then drying the Nafion proton membrane to obtain the membrane electrode comprising the anode catalyst layer, the PEM, and the cathode catalyst layer.

9. The fabrication method according to claim 8, wherein in S4, the anode catalyst layer has a Pt load of 0.05 to 0.1 mg.Math.cm.sup.−2.

10. The fabrication method according to claim 8, wherein in S4, the cathode catalyst layer has a Pt load of 0.05 to 0.1 mg.Math.cm.sup.−2.

11. The membrane electrode according to claim 5, wherein a mass ratio of the carbon carrier to the concentrated sulfuric acid is 1:3; and the hydrothermal reaction is conducted at 180° C. for 24 h.

12. The membrane electrode according to claim 5, wherein the carbon carrier comprises XC-72 and KJ-600.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Other features, objectives, and advantages of the present invention will become more apparent by reading the detailed description of non-limiting embodiments with reference to the drawings.

[0029] FIG. 1 is a schematic diagram illustrating the overall electrode mass transfer resistances in catalyst layers modified with different charges;

[0030] FIG. 2 is a schematic diagram illustrating the air cell performance of catalyst layers modified with different charges; and

[0031] FIG. 3 is a schematic diagram illustrating the air cell performance of the negatively charged catalyst layers in Embodiments 1 to 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] The present invention is described in detail below with reference to the embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those of ordinary skill in the art can further make several variations and improvements without departing from the idea of the present invention. These all fall within the protection scope of the present invention.

[0033] The membrane electrode required by the present invention was fabricated by electrostatic spraying.

[0034] A catalyst layer slurry required for the spraying was prepared.

[0035] 1. Preparation of materials XC-72 carbon was mixed with concentrated sulfuric acid (in a mass ratio 1:3) and reacted at 180° C. for 24 h, such that the carbon was loaded with a negative charge to obtain a negatively charged carbon carrier. 2 g of the negatively charged carbon carrier was dispersed in 200 ml of an aqueous solution, then 2.5 g of chloroplatinic acid hexahydrate was added, ultrasonic treatment is performed for 30 min; and 6 ml of a 0.5 mol L.sup.−1 sodium borohydride solution was added dropwise to reduce the chloroplatinic acid into platinum nanoparticles to obtain a negatively charged platinum-carbon catalyst.

[0036] 2. Preparation of membrane electrode 0.062 g of the negatively charged platinum-carbon catalyst and 0.15 g of a commercial ionomer resin Nafion solution with a concentration of 20% were added to 15 ml of a mixed solvent of isopropanol and water (in a volume ratio of 3:1), then 0.031 g of the negatively charged carbon carrier was added, and a resulting slurry was subjected to ball-milling for 24 h and then sprayed on a proton membrane as a cathode.

[0037] An anode slurry was prepared from 0.062 g of a commercial 46% platinum-carbon catalyst and 0.15 g of a 20% commercial ionomer resin; and a preparation method was the same as that for the cathode slurry except that no additional carbon carrier was added and a slurry obtained after ball-milling was sprayed on the other side of the proton membrane.

[0038] The anode and cathode both had a platinum load of 0.1 mg.Math.cm.sup.−2.

[0039] In the present invention, a mass transfer resistance was tested by the limiting current method at a test temperature of 80° C. and a humidity of 67%. A 1 cm*2 cm 10-channel parallel flow field was used for the test, with test gas volumes: hydrogen: 800 cc/min and 4% oxygen-nitrogen mixed gas: 1,500 cc/min; and a test back pressure: 150 KPa.sub.abs.

[0040] A cell performance test was conducted under the following conditions: temperature: 80° C., humidity: 100%, and test back pressure: 150 KPa.sub.abs. A flow channel for cell was a 5 cm*5 cm 5-channel snakelike flow field, with a test gas membrane electrodesurement ratio: H.sub.2:Air=2:2.

Embodiments 1 to 3

[0041] In order to optimize the consumption of the negatively charged carbon carrier, the present invention adopted three embodiments in total, Embodiments 1, 2, and 3. Mass ratios of the doped negatively charged carbon carrier to the negatively charged platinum-carbon catalyst (with a platinum load of about 50%) in Embodiments 1, 2, and 3 were 0.3:1, 0.5:1, and 0.6:1, respectively.

Comparative Examples 1 to 3

[0042] Since conventional commercial catalysts generally use carbon carriers such as XC-72 and KJ-600, all of which are positively charged. In the comparative examples of the present invention, a positively charged commercial XC-72 was used to fabricate a catalyst and a membrane electrode. That is, in the comparative examples, a surface of a catalyst carbon carrier or an additionally doped carbon carrier was positively charged. A fabrication method for Comparative Examples 1 to 3 was the same as that for Embodiment 2 except that the carbon carrier and/or doped carbon carrier of the platinum-carbon catalyst used in Comparative Examples 1 to 3 was different from that of Embodiment 2. There were three specific comparative examples:

[0043] 1. The carbon carrier of the platinum-carbon catalyst in Comparative Example 1 was negatively charged (the same as Embodiment 2), and the doped carbon carrier was positively charged commercial XC-72.

[0044] 2. The carbon carrier of the platinum-carbon catalyst in Comparative Example 2 was positively charged commercial XC-72, and the doped carbon carrier was negatively charged (the same as Embodiment 2).

[0045] 3. In Comparative Example 3, both the carbon carrier of the platinum-carbon catalyst and the doped carbon carrier were positively charged commercial XC-72.

[0046] It can be seen from FIG. 1 that after the negative charge modification, the oxygen mass transfer resistance of the catalyst layer could be halved. It can be seen from FIG. 2 (in FIG. 2, the embodiment refers to Embodiment 2) that, compared with the comparative examples, the embodiment exhibited improved cell performance; and compared with the conventional membrane electrode with all carbon carriers being positively charged (comparative Example 3), the introduction of negative charges significantly improved the cell performance. When both the catalyst carbon carrier and the doped carbon carrier were negatively charged, a performance improvement of about 100 mV could be achieved. In addition, when a current was increased to 1000 mA.Math.cm.sup.−2, Comparative Example 3 began to be affected by water logging, but the embodiment did not undergo similar conditions. When a surface of a catalyst is negatively charged, an ionomer cannot tightly cover the surface of the catalyst due to electrostatic interaction, which reduces the difficulty of oxygen passing through the ionomer on the surface of the catalyst layer. Added carbon particles increase a distance among active sites and thus increase a local oxygen concentration. The two characteristics can reduce a local mass transfer resistance of the catalyst layer. Therefore, the mass transfer resistance of the catalyst layer of the embodiment is significantly reduced, and the mass transfer polarization loss of a cell is reduced.

[0047] It can be further seen from the comparison of Embodiments 1 to 3 (FIG. 3) that, when carbon is doped at a small amount (0.3:1), the improvement of the negative charge effect on mass transfer is not maximized; and when a carbon content is high (0.6:1), an electrode thickness increases and thus a bulk-phase mass transfer resistance increases, which counteracts the positive effect of negative charges. Through experiments, it can be known that a reasonable carbon doping ratio is about 0.5:1.

[0048] The embodiments of the present invention are described above. It should be understood that the present invention is not limited to the above specific implementations, and a person skilled in the art can make various variations or modifications within the scope of the claims without affecting the essence of the present invention.