Nondestructive identification method for distribution state of ionomer in catalyst layer of fuel cell

Abstract

The invention discloses a nondestructive identification method for the distribution state of an ionomer in a fuel cell catalyst layer, and the method comprises the following steps: preparing a catalyst layer sample with a flat surface, scanning a to-be-tested region in the catalyst layer sample through an atomic force microscope, and obtaining an adhesive force characteristic and a three-dimensional morphology characteristic; performing cross analysis on the adhesive force characteristics and the three-dimensional morphology characteristics to confirm an ionomer area, and performing statistical analysis on the ionomer area to quantitatively obtain ionomer distribution characteristics. According to the identification method, the ionomer and the platinum-carbon catalyst can be accurately distinguished, the accuracy of measurement of critical dimension parameters such as the coverage degree and the thickness of the ionomer is remarkably improved, meanwhile, irradiation damage possibly caused by electron beam observation is avoided, and characterization under the conditions close to actual working conditions such as damp and hot conditions is supported. The method is easy and convenient to operate, high in reliability and good in practical value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fuel cells, and particularly relates to a nondestructive identification method for distribution state of ionomer in a catalytic layer of a fuel cell. BACKGROUND

[0002] Proton exchange membrane fuel cell (PEMFC) is a kind of energy conversion device that transports fuel and oxidant to the anode and cathode catalyst sites of the cell respectively, and then the electrochemical reaction occurs on the platinum-based catalyst to directly convert chemical energy into electrical energy. With the advantages of high energy conversion efficiency, excellent dynamic response and zero direct carbon emission when using hydrogen as fuel, PEMFC continues to grow in application and shipment in the field of transportation.

[0003] The catalytic layer, as the core component inside the cell, is the main place for electrochemical reaction, and its composition and nanoscale structure directly determine the power generation efficiency of the cell. The catalytic layer is formed by spraying or squeegee coating the slurry prepared by mixing platinum carbon catalyst, ionomer and dispersant: the platinum carbon catalyst provides electrochemically active sites and builds an electronic path, and the ionomer is responsible for proton transport. Building a catalytic layer structure with high mass transfer capacity and high catalytic activity is the key to improving the power density of PEMFC. The thin layer of ionomer forms a reaction gas-ion-electron three-phase interface on the surface of platinum particles and in the pores, and its distribution state determines the relative accessibility of oxygen and protons at the reaction site, especially under low humidity conditions. Too low coverage will reduce the available active interface, while too thick ionomer film will significantly increase the local oxygen transfer resistance, resulting in a decrease in limiting current density. Therefore, the in-plane uniformity, coverage and layer thickness of ionomer have an important influence on the performance of the catalytic layer.

[0004] In order to identify the distribution of ionomer in the catalytic layer, the industry often uses transmission electron microscopy (TEM) or scanning electron microscopy (SEM) to analyze the morphology based on the contrast difference between platinum / carbon and ionomer. However, these electron microscopy methods have several limitations: (1) Conventional TEM / SEM requires imaging under high vacuum conditions, which is significantly different from the actual working state of the catalytic layer, making it difficult to reflect the in-situ morphology; (2) The interaction between high-energy electron beams and polymers may cause chain scission, crosslinking or local carbonization, resulting in distortion of the morphology and chemical composition; (3) TEM / SEM usually gives two-dimensional projection or contrast image, and the image contrast is nonlinearly affected by staining, sample thickness and inclination, making it difficult to provide quantitative topological information such as three-dimensional connectivity and thickness distribution of ionomer.

[0005] Therefore, further research is needed on how to accurately evaluate the distribution state of ionomer in the catalytic layer of a proton exchange membrane fuel cell. SUMMARY

[0006] In view of the deficiencies in the prior art, the purpose of the present application is to provide a non-destructive identification method for the distribution state of ionomers in the catalytic layer of a fuel cell, which can accurately obtain the distribution information of ionomers in the catalytic layer without destroying the sample, and the measurement process is simple, fast and reliable. In order to achieve the above-mentioned purposes and other advantages according to the present application, a non-destructive identification method for the distribution state of ionomers in the catalytic layer of a fuel cell is provided, comprising the following steps: S1, preparing a catalytic layer sample with a flat surface; S2, using an atomic force microscope to scan the test area in the catalytic layer sample to obtain adhesion force characteristics and three-dimensional topography characteristics; S3, cross-analyzing the adhesion force characteristics and three-dimensional topography characteristics to confirm the ionomer region; S4, statistically analyzing the ionomer region to quantitatively obtain the ionomer distribution characteristics.

[0007] Preferably, the step S1 specifically comprises: applying / casting a catalyst slurry on a base material to form a catalytic layer, or carefully separating the measured catalytic layer from a formed membrane electrode assembly using a scalpel and tweezers; using adhesive tape to paste the edges of the measured catalytic layer to ensure the flatness of the sample surface, and the adhesive tape should avoid covering the test area and ensure the cleanliness of the test area before pasting. Preferably, the environmental conditions for testing in the step S2 are non-vacuum and can be achieved by a temperature and humidity control device to realize a controlled temperature and humidity environment.

[0008] Preferably, the step S2 specifically comprises the following steps: using an atomic force microscope to collect force-displacement curves of the test area on the surface of the catalytic layer sample pixel by pixel under a quantitative nanomechanical measurement mode, and using a very thin scanning probe to realize high-resolution imaging; The adhesion force characteristics and three-dimensional topography characteristics are obtained by extracting the pull-off force from the pull-back section of the force-displacement curve of each pixel and mapping it as an adhesion force map while recording the sample surface topography data.

[0009] Preferably, the adhesion force is the pull-off force measured when the probe is pulled off during the force curve pull-back; The three-dimensional topography is composed of the vertical displacement between the probe and the sample recorded by the feedback control system of the AFM.

[0010] Preferably, the cross-analysis in the step S3 is baseline correction and pixel registration of the adhesion force and three-dimensional topography data, and then joint segmentation based on the simultaneous satisfaction of the adhesion force threshold and the roughness characteristics of the topography, and the intersection region obtained is determined as the ionomer distribution region.

[0011] Preferably, the statistical analysis in step S4 quantitatively obtains the distribution characteristics of the ionomer as follows: after the ionomer region is identified, the area, size distribution and three-dimensional spatial distribution characteristics of the ionomer region are statistically obtained through image analysis and data processing means, and indexes such as ionomer coverage, distribution uniformity index and average thickness are further calculated to obtain the quantitative topological characteristics of the ionomer distribution characteristics.

[0012] Compared with the prior art, the present application has the following advantages and beneficial effects: 1. The present application provides a method capable of non-destructively identifying the distribution state of ionomer in the catalyst layer, which avoids the image artifact problem caused by potential damage of electron beam radiation to ionomer materials, thereby ensuring the authenticity and reliability of the test data. Compared with the technology requiring electron beam irradiation, the present method protects the integrity of the sample.

[0013] 2. The present application uses the adhesion force imaging technology of atomic force microscopy to realize accurate differentiation between ionomer and platinum carbon catalyst, thereby quantitatively characterizing the distribution state of ionomer and significantly improving the accuracy of measurement of key dimensional parameters such as ionomer coverage and thickness.

[0014] 3. The test method proposed by the present application supports the measurement of ionomer distribution under humid heat conditions and does not depend on a high vacuum environment, which can more truly reflect the morphology and distribution characteristics of ionomer under humid heat conditions and improve the accuracy and practicality of ionomer state identification.

[0015] 4. The method of the present application can simultaneously obtain the three-dimensional spatial distribution characteristics of ionomer at the nanoscale and quantitatively extract two-dimensional area coverage and thickness and other dimensional parameters, and the quantitative data obtained is detailed and comprehensive. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 Flowchart of the non-destructive identification method of the distribution state of ionomer in the catalyst layer of the fuel cell according to the present application; Figure 2 Adhesion force distribution image of the catalyst layer of the non-destructive identification method of the distribution state of ionomer in the catalyst layer of the fuel cell according to the present application; Figure 3 Composite image of adhesion force and three-dimensional morphology of the catalyst layer of the non-destructive identification method of the distribution state of ionomer in the catalyst layer of the fuel cell according to the present application; Figure 4 Statistical image of ionomer thickness information in the catalyst layer of the non-destructive identification method of the distribution state of ionomer in the catalyst layer of the fuel cell according to the present application. DETAILED DESCRIPTION The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort fall within the protection scope of the present application.

[0017] A fuel cell catalytic layer sample is taken, and the sample is scanned and observed under a peak force quantitative nanomechanical measurement mode based on an atomic force microscope (AFM) technology, and accurate evaluation of the distribution state of ionomers is realized by statistical analysis of the sample surface height fluctuation and adhesion force information.

[0018] The AFM imaging principle is that short-range interaction forces (such as van der Waals forces, electrostatic forces, adhesion forces and the like) between a sharp probe at the end of a micro-cantilever beam and a sample surface cause bending or vibration of the cantilever beam, the vibration amplitude change of the cantilever beam is monitored in real time by using a high-sensitivity laser reflection system, and is converted into an electrical signal, so that the three-dimensional topography and related adhesion characteristic information of the sample surface can be reconstructed.

[0019] The adhesion force and the three-dimensional topography are the measurement indexes for determining the types of substances on the surface of the catalytic layer. As a binder between components of the catalytic layer, ionomers have higher intrinsic adhesion than platinum carbon catalysts, and have more uneven surface topography characteristics. In the adhesion force image of the surface of the catalytic layer obtained by AFM measurement, ionomers appear as bright areas due to their high adhesion to the probe, and platinum carbon catalysts appear as dark areas due to their low adhesion to the probe, and the distribution characteristics of the ionomer region are locked by the surface topography characteristics. Based on the intrinsic property difference between ionomers and platinum carbon catalysts, and combining the imaging response of the AFM scanning probe to different adhesion characteristics and surface topography substances, the distribution characteristics of ionomers in the catalytic layer are realized quickly, efficiently and non-destructively.

[0020] Reference Figure 1 A non-destructive identification method for the distribution state of ionomers in a fuel cell catalytic layer, comprising the following steps: S1: preparing a catalytic layer sample with a flat surface. Specifically, a square catalytic layer sample with a side length of 12 mm is cut, and 3M tape is used to paste the sample around the edges to ensure the flatness of the sample surface. After pasting, a 1cm 2 measuring area is reserved in the center of the sample.

[0021] S2: Obtain the adhesion force feature and three-dimensional topography feature of the region to be tested in the catalyst layer sample by atomic force microscopy. Specifically, collect complete force-displacement curves pixel by pixel under the peak force nanomechanical measurement mode of the AFM, use a silicon probe with a nominal spring constant of 0.35 N / m; the test environment conditions are normal temperature and pressure, and the relative humidity is controlled at 50-55%RH; randomly select 3 regions in a 1 cm2 area for scanning to reduce accidental errors; the scanning area size is 1 pm 2 and 0.04 pm 2 , the resolution is 256*256 pixels; record the vertical displacement of each pixel of the probe and the sample as three-dimensional topography data, and extract the pull-off force from the retraction section as the adhesion force data, while saving the original force curve for subsequent calibration and verification, the pull-off force includes van der Waals force, capillary force and other interactions.

[0022] S3: Cross-analyze the adhesion force feature and the three-dimensional topography feature to confirm the ionomer region. Specifically, first perform plane fitting and local smoothing on the topography, and perform overall offset correction on the adhesion force, and then perform pixel-level registration on the two. Use threshold analysis on the adhesion force matrix to obtain a high adhesion candidate area, and take the intersection with the topography region with high continuous undulation characteristics, and finally determine as the ionomer region.

[0023] S4: Statistically analyze the ionomer region to quantitatively obtain the ionomer distribution feature. Specifically, use image analysis software to calculate the ratio of the ionomer area to the scanning area, and obtain the catalyst layer surface ionomer coverage of 54.3%; use connected domain counting on the ionomer fragments located between the platinum-carbon catalyst particles and fit the height distribution with a unimodal fitting model, and the fitting result shows that the average thickness of the ionomer layer in the catalyst layer is 10.5 nm.

[0024] Figure 2 The adhesion force distribution image of the catalyst layer obtained by AFM testing, the adhesion force range of the test area is 0-3.7nN. Due to the obvious adhesion force difference between different components of the catalyst layer, the bright white area in the figure corresponds to the ionomer with high adhesion, and the dark black area corresponds to the platinum-carbon catalyst with low adhesion. Figure 3 The adhesion force and three-dimensional topography composite image of the catalyst layer obtained by AFM testing, the test area is 1 pm x 1 pm, and the surface height fluctuation is between 0-649 nm. It can be clearly seen that the ionomer with high adhesion is distributed between the low adhesion spherical platinum-carbon particles. Figure 4 The ionomer thickness information statistical image of the catalyst layer obtained by statistical analysis, the thickness distribution range is 2.5-13.5 nm; after unimodal fitting, the peak (most common thickness) is located at 10.5 nm. Through Figure 2 , 3As can be seen in Figures 4, the method accurately and quantitatively describes the spatial distribution of ionomer and extensive distribution of ionomer between platinum-carbon catalyst particles can be observed.

[0025] The number of apparatuses and the scale of processes described herein are intended to be illustrative of the application and the application, modifications and variations of which are apparent to those skilled in the art are intended to be within the scope of the application. Although embodiments of the application have been described above with particularity, the scope of the application encompasses any and all modifications and variations which could potentially be made by a person of ordinary skill in the art upon reading the description herewith. Therefore, it is intended that the application not be limited to the particular details shown and described herein, but to the appended claims and their equivalents.

Claims

1. A non-destructive identification method of ionomer distribution state in a catalytic layer of a fuel cell, characterized by, The method comprises the following steps: S1, preparing a surface flat catalyst layer sample; S2, obtaining adhesion force features and three-dimensional topography features of a region to be tested in the catalyst layer sample by scanning the region to be tested using an atomic force microscope; S3, cross-analyzing the adhesion force features and the three-dimensional topography features to confirm the flocculation region; S4, statistically analyzing the flocculation region to quantitatively obtain flocculation distribution features.

2. The method of claim 1, wherein the method is characterized by: The step S1 specifically comprises: applying a catalyst slurry on a substrate material by scraping or spraying to form a catalyst layer or carefully separating a catalyst layer to be tested from a membrane electrode assembly that has been formed using a scalpel and tweezers; using adhesive tape to paste the edges of the catalyst layer to be tested to ensure the flatness of the sample surface, and the adhesive tape should avoid covering the test region and ensure the cleanliness of the test region before pasting.

3. The method of claim 1, wherein the method is characterized by: The environmental conditions for testing in the step S2 are non-vacuum and can be achieved by a temperature and humidity control device to achieve a controlled temperature and humidity environment.

4. The method of claim 3, wherein the method is characterized by: The step S2 specifically comprises the following steps: using an atomic force microscope to collect force-displacement curves of the region to be tested on the surface of the catalyst layer sample pixel by pixel under a quantitative nanomechanical measurement mode and performing scanning measurement, and using a very thin scanning probe to achieve high-resolution imaging; the adhesion force features and the three-dimensional topography features are obtained by extracting the pull-off force from the pull-back section of the force-displacement curve of each pixel and mapping it into an adhesion force map while recording the surface topography data of the sample.

5. The method of claim 4, wherein the method is characterized by: The adhesion force is the pull-off force measured when the probe is pulled off during the pull-back of the force curve; The three-dimensional topography is composed of the vertical displacement between the probe and the sample recorded by the feedback control system of the AFM.

6. The method of claim 1, wherein the method is a non-destructive method for identifying the state of ionomer distribution in a catalytic layer of a fuel cell. The cross-analysis in the step S3 is baseline correction and pixel registration of the adhesion force and three-dimensional topography data, and then joint segmentation based on the simultaneous satisfaction of the adhesion force threshold and the roughness features of the topography, and the intersection region obtained is determined as the flocculation distribution region.

7. The method of claim 1, wherein the method is a non-destructive method for identifying the state of ionomer distribution in a catalytic layer of a fuel cell. The statistical analysis in the step S4 to quantitatively obtain the flocculation distribution features is to, after the identification of the flocculation region, statistically analyze the area, size distribution and three-dimensional spatial distribution features of the flocculation region by image analysis and data processing means, further calculate the flocculation coverage, distribution uniformity index and average thickness, and other indicators to obtain the quantitative topological features of the flocculation distribution characteristics.

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