Method for quantifying number of active sites on three-phase interface of M-N-C material

Abstract

The invention discloses a method for quantifying the number of active sites on a three-phase interface of an M-N-C material, which comprises the following steps: carrying out cyclic voltammetry (CV) test in a humidified H2 / N2 atmosphere, activating, and removing surface impurities; changing the potential interval to be [-0.5 V, 0.2 V] and the scanning speed to be 10mV. S <-1 >, and collecting a background curve; cathode gas is switched into NO / N2 mixed gas, and the gas flow rate is controlled to be 0.2 L.min <-1 >; the cathode gas is switched to N2 again, and purging is conducted for 30 min at the gas flow rate; the NO is fully saturated and adsorbed on the Fe site; switching cathode gas into N2, adjusting the flow rate to 0.2 L.min <-1 >, and collecting an NO desorption curve; by calculating the reduction electric quantity, the number of active sites of the M-N-C material under actual conditions of a fuel cell can be obtained, and more reliable data support is provided for performance optimization of a fuel cell catalyst.

Description

Technical Field

[0001] The invention belongs to the technical field of fuel cells, and in particular relates to a method for quantifying the number of active sites at a three-phase interface of an MNC material. Background Art

[0002] Proton exchange membrane fuel cells (PEMFCs) are widely used due to their green and efficient characteristics, especially in the transportation industry. However, their reliance on expensive and scarce platinum group metal (PGM) electrocatalysts for the slow oxygen reduction reaction (ORR) at the cathode remains a major challenge. Therefore, efforts are being made worldwide to design and develop non-precious metal catalysts, among which iron or cobalt and nitrogen-doped carbon materials prepared by pyrolysis at high temperatures, called "MNC" catalysts, are currently the most promising candidates. Thanks to the development of advanced spectroscopic techniques and characterization, it has been agreed that the main active sites of pyrolysis-type MNC catalysts are metal-nitrogen sites (MNs) embedded in a graphene-like framework. x C y At present, there are various methods for determining the MNC active site density (SD), which can be divided into the following three types:

[0003] 1. A method for quantifying metal content using spectroscopy combined with inductively coupled plasma emission spectrometry (ICP-OES). This method is mainly applicable to Fe-NC materials. First, the Fe content in Fe-NC is determined, and then the Mössbauer spectrum is used to calculate the proportion of active sites in the Fe-NC material, thereby characterizing the active site density in the catalyst. This method obtains the number of bulk phase points.

[0004] 2. The second method is the gas phase molecular probe CO method, which uses CO molecules to specifically adsorb metal active sites at a low temperature of -80°C, and then desorb at a high temperature of 800°C to calculate the adsorption amount of CO and then obtain the number of metal sites in the material.

[0005] 3. The third method is the liquid phase molecular probe nitrite NO 2- Poisoning test calculation Fe-NC site nitrite poisoning method, using NO2 - Under acidic conditions, a disproportionation reaction occurs to generate NO molecules. Assuming that the NO molecules are adsorbed on the active sites in a 1:1 ratio, a NO reduction reaction NOR is performed through electrochemical sweeping, and a 5-electron reaction occurs to generate NH3. The number of active sites of the material is calculated by calculating the reduction charge during the NOR process.

[0006] The third method is the most commonly used method because of its simple operation. This method is based on the test in the rotating disk electrode RRDE, and the test environment is an electrolyte solution, while PEMFCs occur at a more complex gas / liquid / solid three-phase interface, so it cannot reflect the number of active sites at the three-phase interface in the real environment. Therefore, it is urgent to establish a new method that can identify the real number of active sites in the cathode catalyst layer in the PEMFCs environment in order to study the decay behavior of the non-precious metal cathode catalyst layer. Summary of the invention

[0007] Technical issues solved:

[0008] In response to the shortcomings of the prior art, the present application provides a method for quantifying the number of active sites at the three-phase interface of MNC materials. In order to more realistically detect the number of metal active sites in the actual environment, the present application utilizes the specific adsorption of NO on metal sites to identify the actual site density in the cathode catalyst layer of the fuel cell.

[0009] Technical solution:

[0010] To achieve the above objectives, this application is implemented through the following technical solutions:

[0011] A method for quantifying the number of active sites at the three-phase interface of Fe-NC materials, comprising the following steps:

[0012] Step 1: Cyclic voltammetry (CV) test was performed at 40°C in a 100% humidified H2 / N2 atmosphere with a potential range of [-0.5 V, 0.5 V] and a scan rate of 100 mV·s -1 , activate and remove surface impurities;

[0013] Step 2: Change the potential interval to [-0.5V, 0.2V], and the scan rate to 10mV·s -1 , collect the background curve;

[0014] Step 3: Switch the cathode gas to a NO / N2 mixed gas with a molar fraction of 1%-10%, adsorb for 10 min at 0.5 V, and control the gas flow rate to 0.2 L min -1 ;

[0015] Step 4: Switch the cathode gas to N2 again at 0.5 L min -1 Purge at a gas flow rate of 30 min;

[0016] Step 5: Repeat steps 3 and 4 until NO is fully adsorbed on the Fe site;

[0017] Step 6: Switch the cathode gas to N2 and adjust the flow rate to 0.2L·min -1, potential interval: [-0.5 V, 0.2 V], scan rate: 10 mV·s -1 , collect NO desorption curve;

[0018] Step 7: By calculating the reduction amount, the number of active sites of the MNC material under the actual conditions of the fuel cell can be obtained.

[0019] Furthermore, the gas flow rate in the first step is 0.2 L min -1 .

[0020] Furthermore, in the third step, a NaOH solution tail gas treatment device needs to be added during the use of NO gas, and the NO gas will be chemically adsorbed on the Fe site and physically adsorbed in the pores of the carbon carrier.

[0021] Furthermore, the repetition of the third step and the fourth step in the fifth step is repeated twice.

[0022] Furthermore, in the sixth step, NO undergoes a 5-electron reduction reaction to generate NH3.

[0023] Furthermore, the calculation formula in the seventh step is as follows:

[0024]

[0025] In the formula, Q strip is the reduction power of adsorbed NO; N A is Avogadro's constant, which is 6.02×10 23

[0026] sites·mol -1 ;n strip is the number of electrons reduced by adsorption of NO, i.e. 5; F is the Faraday constant, i.e. 96485

[0027] C mol -1 .

[0028] Principle explanation: The specific adsorption of NO molecules on the metal center is used to form a full monolayer saturated Fe-NO adsorption configuration. Further reduction potential is applied to allow the adsorbed NO molecules to generate NH3 through five-electron electroreduction. The reduction amount (Q strip ) can be used to obtain the number of metal active centers. Since both NO and O2 molecules are linear molecules with similar molecular sizes, the metal center adsorbed by NO is the active center adsorbed by O2 at the three-phase interface. The calculated metal active center density is the active site density at the three-phase interface.

[0029] Beneficial effects:

[0030] The present application provides a method for quantifying the number of active sites at the three-phase interface of MNC materials, which has the following beneficial effects compared with the prior art:

[0031] 1. The method established in this application can be used to quantitatively analyze the number of real reaction active sites of MNC materials at the three-phase interface of fuel cells. Compared with the prior art, the present invention shows obvious advantages;

[0032] 2. The present invention can realize direct quantification of the active sites of the three-phase interface reaction, while the existing technologies mostly rely on indirect estimation and cannot accurately reflect the actual number of active sites;

[0033] 3. Through the technical solution of the present invention, the actual number of reaction sites in the MNC material can be accurately determined, providing more reliable data support for the performance optimization of fuel cell catalysts;

[0034] 4. A quantitative analysis method for the three-phase interface reaction active sites of MNC materials in fuel cells. This method can be widely used in the performance evaluation and optimization of fuel cell catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a diagram of the quantitative analysis method of Fe-NC catalytic active sites at the cathode three-phase interface of the fuel cell of the present application;

[0036] Figure 2 The polarization curves of the Fe-NC material and the NC material of the present application obtained by testing under three conditions, wherein a is the polarization curve of the Fe-NC material before poisoning, after poisoning and in a 1% NO atmosphere; b is the polarization curve of the NC material before poisoning, after poisoning and in a 1% NO atmosphere; and c is the NO reduction amount of the Fe-NC and NC materials. DETAILED DESCRIPTION

[0037] The present invention is further described in detail below in conjunction with the accompanying drawings and specific examples, but it should not be construed as limiting the present invention. Without departing from the spirit and substance of the present invention, the modification or replacement of the method, steps or conditions of the present invention all belong to the scope of the present invention. The experimental methods and reagents without specifying the specific conditions in the examples are all in accordance with the conventional conditions in the art.

[0038] Example 1

[0039] A method for quantifying the number of active sites at the three-phase interface of MNC materials, comprising the following steps:

[0040] Step 1: Cyclic voltammetry (CV) test was performed at 40°C in a 100% humidified H2 / N2 atmosphere with a gas flow rate of 0.2 L min -1, potential interval: [-0.5 V, 0.5 V], scan rate: 100 mV·s -1 , activate and remove surface impurities;

[0041] Step 2: Change the potential interval to [-0.5V, 0.2V], and the scan rate to 10mV·s -1 , collect the background curve;

[0042] Step 3: Switch the cathode gas to a NO / N2 mixed gas with a molar fraction of 1%-10%, adsorb for 10 min at 0.5 V, and control the gas flow rate to 0.2 L min -1 , considering that NO is a toxic and explosive gas, high-concentration NO gas is not used, and a NaOH solution tail gas treatment device is required during the use of NO gas to absorb unreacted NO gas and prevent leakage into the air. In this process, NO gas will be chemically adsorbed on the Fe site and physically adsorbed in the pores of the carbon carrier;

[0043] Step 4: Switch the cathode gas to N2 again at 0.5 L min -1 The gas flow rate was purged for 30 min. The purpose of this operation was to remove most of the physically adsorbed NO gas. The chemically adsorbed NO molecules were relatively stable and could not be removed by N2 purging.

[0044] Step 5: Repeat steps 3 and 4 twice until NO is fully adsorbed on the Fe site;

[0045] Step 6: Switch the cathode gas to N2 and adjust the flow rate to 0.2L·min -1 , potential interval: [-0.5 V, 0.2 V], scan rate: 10 mV·s -1 , collect the NO desorption curve. In this stage, NO will undergo a 5-electron reduction reaction to generate NH3;

[0046] Step 7: By calculating the reduction amount, the number of active sites of Fe-NC material under the actual conditions of fuel cells can be obtained. The calculation formula is as follows:

[0047]

[0048] In the formula, Q strip is the reduction power of adsorbed NO; N A is Avogadro's constant, which is 6.02×10 23 sites·mol -1 ;n strip is the number of electrons reduced by adsorption of NO, i.e. 5; F is the Faraday constant, i.e. 96485 C·mol -1 . Figure 2The polarization curves of Fe-NC material and NC material were obtained under three conditions. First, Figure 2 As can be seen in a, in a 1% NO atmosphere (blue line), Fe-NC has good NO reduction performance. Compared with the N2 atmosphere (black line), the reduction charge is more negative, and the polarization curve (red line) obtained by the full saturation NO adsorption test is more negative than the background current. Figure 2 It can be seen from figure b that the NC material cannot electro-reduce NO within the potential range, indicating that the NO molecule can be further electro-reduced at the Fe site, while the reaction activity at the carbon site is very low. Figure 2 The curve c in the figure is the curve after background subtraction, and then the NO reduction capacity is obtained. The results show that the NO reduction capacity on Fe-NC material is 73.6 C·g -1 , which is much higher than the reduction charge of NO on NC material (7.3 C·g -1 ), and then we can get SD by substituting it into the above formula. Fe-N-C 9.2×10 19 sites·g -1 , while SD NC 0.91×10 19 sites·g -1 , the two can be subtracted to exclude the influence of NC material on the weak electrical reduction of NO and the adsorption of NO molecules in a small amount of pores. Finally, the number of sites of Fe-NC material in the real environment of fuel cell reaction is 9.1×10 19 sites·g -1 This method can also be applied to other MNC catalysts, such as Co-NC, Ni-NC, Mn-NC, etc.

[0049] The examples selected in the above materials are for ease of understanding, not to limit the process. People in the same technical field can easily modify the process or transfer it to other cases without creative changes. If these changes also belong to the same claims or similar technologies of the present invention, then the original intention of the invention also includes these changes.

Claims

1. A method for quantifying the number of active sites at the three-phase interface of MNC materials, characterized in that: The specific steps are: Step 1: Cyclic voltammetry (CV) test was performed at 40°C in a 100% humidified H2 / N2 atmosphere with a potential range of [-0.5 V, 0.5 V] and a scan rate of 100 mV·s -1 , activate and remove surface impurities; Step 2: Change the potential interval to [-0.5V, 0.2V], and the scan rate to 10mV·s -1 , collect the background curve; Step 3: Switch the cathode gas to a NO / N2 mixed gas with a molar fraction of 1%-10%, adsorb for 10 min at 0.5 V, and control the gas flow rate to 0.2 L min -1 ; Step 4: Switch the cathode gas to N2 again at 0.5 L min -1 Purge at a gas flow rate of 30 min; Step 5: Repeat steps 3 and 4 until NO is fully adsorbed on the Fe site; Step 6: Switch the cathode gas to N2 and adjust the flow rate to 0.2L·min -1 , potential interval: [-0.5 V, 0.2 V], scan rate: 10 mV·s -1 , collect NO desorption curve; Step 7: By calculating the reduction amount, the number of active sites of the MNC material under the actual conditions of the fuel cell can be obtained.

2. A method for quantifying the number of active sites at the three-phase interface of MNC materials according to claim 1, characterized in that: The gas flow rate in the first step is 0.2 L·min -1 .

3. A method for quantifying the number of active sites at the three-phase interface of MNC materials according to claim 1, characterized in that: In the third step, a NaOH solution tail gas treatment device needs to be added during the use of NO gas. NO gas will be chemically adsorbed on the Fe site and physically adsorbed in the pores of the carbon carrier.

4. A method for quantifying the number of active sites at the three-phase interface of MNC materials according to claim 1, characterized in that: The repeating of the third and fourth steps in the fifth step is repeated twice.

5. The method for quantifying the number of active sites at the three-phase interface of MNC materials according to claim 1, characterized in that: In the sixth step, NO undergoes a 5-electron reduction reaction to generate NH3.

6. The method for quantifying the number of active sites at the three-phase interface of MNC materials according to claim 1, characterized in that: The calculation formula in the seventh step is as follows: In the formula, Q strip is the reduction power of adsorbed NO; N A is Avogadro's constant, which is 6.02×10 23 sites·mol -1 ;n strip is the number of electrons reduced by adsorption of NO, i.e. 5; F is the Faraday constant, i.e. 96485 C·mol -1 .

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