CN106338459B

CN106338459B - Method for measuring effective diffusion coefficient of oxygen in catalyst layer of fuel cell

Info

Publication number: CN106338459B
Application number: CN201610737646.7A
Authority: CN
Inventors: 章俊良; 王超; 程晓静; 张卿雷; 沈水云; 罗柳轩; 胡宸溢; 朱凤鹃
Original assignee: Shanghai Jiaotong University
Current assignee: Shanghai Jiaotong University
Priority date: 2016-08-26
Filing date: 2016-08-26
Publication date: 2020-01-07
Anticipated expiration: 2036-08-26
Also published as: CN106338459A

Abstract

The invention designs a fuel cell with double-layer membrane electrode measurementA method for determining the effective diffusion coefficient of oxygen in a catalytic layer, comprising the steps of: s1: assembling the double-layer membrane electrode into a fuel cell; s2: detecting a limiting current of the fuel cell; s3: substituting the limiting current into a Fick law shown in a formula I to obtain a diffusion coefficient of the electrode layer; wherein, C_O2The oxygen concentration is an experimental control quantity, CPt, suf is the oxygen concentration of the Pt surface, and δ is the thickness of the simulated catalytic layer, which can be obtained by experimental measurement. The invention has the advantages that: firstly, DCL does not contain Pt and does not have electrochemical reaction, so the mass transfer characteristic can be expressed by Fick law; secondly, the manufacturing method and the material composition of the DCL are completely consistent with those of the CL, and the mass transfer characteristic of the CL can be effectively copied. The relation between the oxygen flux and the concentration is obtained by measuring the limiting current, and the effective mass transfer coefficient is calculated by utilizing Fick's law.

Description

Method for measuring effective diffusion coefficient of oxygen in catalyst layer of fuel cell

Technical Field

The invention relates to a method for measuring an effective diffusion coefficient of oxygen in a catalyst layer of a fuel cell, belonging to the technical field of fuel cells.

Background

The membrane electrode comprises an anode catalyst layer, a proton exchange membrane and a cathode catalyst layer. During the operation of the battery, oxygen diffuses into the cathode catalyst layer, and electrochemical reaction is carried out on the surface of Pt. The effective diffusion coefficient of oxygen in the cathode catalytic layer becomes an important parameter that limits the performance of the fuel cell. Different membrane electrode manufacturing processes and conditions can obtain catalytic layers with different diffusion properties, diffusion coefficients can be measured in time, and the membrane electrode performance evaluation has higher reference value. At present, the effective diffusion coefficient of oxygen in the catalytic layer is mostly obtained by calculation by using a non-experimental means or directly quoted empirical values, and the actual influence of different experimental conditions is difficult to reflect. Therefore, in-situ methods of measuring diffusion coefficients are of interest for fuel cell testing.

Disclosure of Invention

In view of the deficiencies in the prior art, it is an object of the present invention to provide a method for measuring the effective diffusion coefficient of oxygen in a catalytic layer of a fuel cell.

The invention is realized by the following technical scheme:

in a first aspect, the invention provides a double-layer membrane electrode, which comprises a cathode catalyst layer and a simulated catalyst layer attached to the surface of the cathode catalyst layer, wherein the simulated catalyst layer consists of Nafion and a catalyst carrier without Pt nano particles.

Preferably, the preparation method of the simulated catalytic layer is the same as that of the cathode catalytic layer.

In a second aspect, the present invention further provides a method for measuring an effective diffusion coefficient of oxygen in a catalyst layer of a fuel cell based on the aforementioned double-layer membrane electrode, which comprises the following steps:

s1: assembling the double-layer membrane electrode into a fuel cell;

s2: detecting a limiting current of the fuel cell;

s3: the limiting current is substituted into Fick's law shown in formula I to obtain the diffusion coefficient of the electrode layer,

wherein, C_O2Oxygen concentration is the experimental control quantity, C_Pt,sufThe oxygen concentration at the Pt surface, δ is the thickness of the simulated catalytic layer, and can be obtained by experimental measurement.

Preferably, the fuel cell has parallel flow channels of 1cm by 2 cm.

Preferably, in step S2, the detection temperature is 80 ℃, H₂The testing gas amount is 800cc/min, O₂/N₂The test gas amount of (2) was 1500 cc/min.

Preferably, in step S3, the specific derivation process is as follows:

under the condition of limiting current, the oxygen concentration on the Pt surface is 0, the formula I can be simplified into the formula II,

therefore, under the condition of a certain diffusion coefficient, the magnitude of the limiting current density is in direct proportion to the reciprocal of the electrode thickness, thereby obtaining a diffusion coefficient expression formula III,

by preparing DCLs of different thicknesses, i can be used_limDerivation of 1/delta to obtain diffusion coefficient D of electrode layer^eff _O2。

Compared with the prior art, the invention has the following beneficial effects:

1. the invention designs an experimental method for directly measuring the effective diffusion coefficient in the cathode catalytic layer, and compared with a reference empirical value or an estimated value, the mass transfer characteristic of the catalytic layer can be more truly embodied;

2. the invention adds a layer of 'simulation catalyst layer' (DCL) formed by catalyst carbon carrier on the surface of cathode Catalyst Layer (CL), the layer has the same mass transfer characteristic with CL, therefore, the gas mass transfer characteristic of CL can be separated under the condition of excluding electrochemical reaction;

3. the invention introduces an electrochemical experimental method for the first time, and realizes the field measurement of the effective diffusion coefficient.

Drawings

Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:

FIG. 1 is a schematic diagram of a membrane electrode assembly with DCL;

FIG. 2 Electron microscopy measurement of electrode thickness;

FIG. 3 is an example of a limiting current linear voltammetry test;

FIG. 4 is a graph of the relationship obtained according to equation III.

Detailed Description

The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention.

Example 1

The membrane electrode required by the experiment is prepared by an electrostatic spraying method, and the structure is shown in figure 1. Firstly, Pt/C catalyst slurry is prepared, commercial Nafion is adopted as the ionic resin, and ethanol is used as the solvent. The mass fraction of carbon in the slurry was 0.39%, and the mass ratio of Nafion to carbon was 0.8. The prepared slurry was ultrasonically vibrated for 20 minutes and then used for spray coating. In the embodiment, cathode and anode catalyst layers are respectively sprayed on two surfaces of a Nafion proton membrane by adopting an electrostatic spraying method, the spraying temperature is 90 ℃, and the Pt loading capacity of the cathode and the anode is respectively 0.1mg/cm². And secondly, spraying DCL with different thicknesses outside the sprayed cathode catalytic layer. The DCL thicknesses selected in this example were 15um, 22um, 30 um. The thickness of the DCL was measured by electron microscopy (as shown in fig. 2). The carbon material used in the DCL is the carbon carrier of the catalyst in the CL, so as to ensure that the two have the same structural characteristics.

The cell test temperature in this example was 80 ℃ and the humidity was 67%. The limiting current was measured at three oxygen concentrations selected at 1%, 2% and 4%. The flow channel selected by the battery is a 1 cm-2 cm parallel flow channel, and the testing gas flow is 800cc/min (H)₂)、1500cc/min(O₂/N₂) So as to ensure that the oxygen concentration on all positions of the surface of the membrane electrode is equal.

From equation II, the limiting current density is proportional to the oxygen concentration. The limiting current test is shown in fig. 3. The effective diffusion coefficient of the catalyst layer can be obtained by measuring a polarization curve, measuring the thickness of the carbon layer of the membrane electrode by an electron microscope, drawing a relation curve described by a formula III, and obtaining the effective diffusion coefficient of the catalyst layer according to the formula III by the slope of a fitting curve in the graph as shown in figure 4.

TABLE 1 slope of the curve and diffusion coefficient from FIG. 4

The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes and modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention.

Claims

1. A method of measuring the effective diffusion coefficient of oxygen in a catalytic layer of a fuel cell, comprising the steps of:

s1: preparing double-layer membrane electrodes of simulated catalyst layers with different thicknesses;

the double-layer membrane electrode comprises a cathode catalyst layer and a simulation catalyst layer attached to the surface of the cathode catalyst layer; the simulated catalytic layer consists of a proton membrane and a catalyst carbon carrier; the preparation method of the simulated catalyst layer is the same as that of the cathode catalyst layer; the cathode catalyst layer is a catalyst layer consisting of Pt, carbon and a proton membrane;

s2: assembling the double-layer membrane electrode into a fuel cell;

s3: detecting a limiting current of the fuel cell;

s4: substituting the limiting current into a Fick law shown in a formula I to obtain a diffusion coefficient of the electrode layer;

wherein, C_O2For experimental control of oxygen concentration, C_Pt,sufThe concentration of oxygen on the surface of Pt is shown, and delta is the thickness of the simulated catalytic layer, and is obtained through experimental measurement;

in step S4, the specific derivation procedure is as follows:

under the limiting current condition, the oxygen concentration of the Pt surface is 0, and the formula I is simplified into the formula II:

therefore, under the condition of a certain diffusion coefficient, the magnitude of the limiting current density is in direct proportion to the thickness reciprocal of the simulated catalyst layer, and accordingly, a diffusion coefficient expression formula III is obtained;

by preparing simulated catalytic layers of different thicknesses, using i_limDerivation of 1/delta to obtain diffusion coefficient D of electrode layer^eff _O2。

2. The method of measuring the effective diffusion coefficient of oxygen in a catalytic layer of a fuel cell of claim 1 wherein the flow channels of the fuel cell are 1cm by 2cm parallel flow channels.

3. The method of claim 1, wherein the temperature is 80 ℃ and H is detected in step S3₂The testing gas amount is 800cc/min, O₂And N₂The test gas amount of (2) was 1500 cc/min.