A method for online cleaning and activation of active sites of a fe / n / c oxygen reduction catalyst

Abstract

The application discloses a method for on-line cleaning and activation of Fe / N / C oxygen reduction catalyst active sites. The method first carries out electrochemical cleaning on the Fe / N / C catalyst in hydrogen and a specific potential range, removes adsorbates or impurities adsorbed on the surface of metal active sites by using the electrochemical oxidation reaction of hydrogen, exposes and activates the Fe active sites, and then observes the Fe 3+ / Fe 2+ variable peak at about 0.85 V (vs RHE) by cyclic voltammetry scanning, integrates the electric quantity to calculate the Fe active site density. The application has the advantages of simple and fast operation, on-line performance in an electrolytic cell or a proton exchange membrane fuel cell, effective removal of active site impurities, accurate evaluation of the Fe / N / C catalyst active site density, and contribution to the differentiation and screening of catalysts with different activities, monitoring of the active site density and Fe 3+ / Fe 2+ potential change caused attenuation of the Fe / N / C catalyst during long-time operation.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy materials technology, specifically relating to a method for online cleaning and activation of active sites in Fe / N / C oxygen reduction catalysts. Background Technology

[0002] Online measurement of the number of active sites in a proton exchange membrane fuel cell (PEMFC) catalyst is an important characterization method for studying its performance and a crucial means of monitoring the catalyst's operating state. Currently, NO2 - Adsorption-desorption is the primary method for measuring the number of active sites in Fe / N / C catalysts, but it is only applicable to rotating disk electrodes (RDE), i.e., a characterization method in liquid electrolytes, and not suitable for measuring the number of active sites in Fe / N / C catalysts at the gas-liquid-solid three-phase interface in PEMFCs. In PEMFCs, Fe... 3+ / Fe 2+ Variable valence reaction (Fe(Ⅲ)-OH + H) + The number of Fe sites can be measured under operating conditions (e-→ H2O + Fe(Ⅱ)). In practical applications, the active metal sites of Fe-NC catalysts are often poisoned by adsorption of impurity molecules during preparation and storage, making it difficult for these sites to directly participate in electrochemical reactions and for their number to be assessed through electrochemical measurements. In noble metal nanocatalysts, the number of Fe sites can be measured under high potentials (>1.2 V). RHE Oxidation or metal dissolution reactions are carried out at high potentials (>1.1 V) to remove surface contaminants and expose pure metal active sites. However, for non-noble metal oxygen reduction catalysts such as Fe / N / C, their oxidation or metal dissolution reactions at high potentials (>1.1 V) are not ideal. RHE At high potentials (>1.1 V), the metal is highly unstable, and its active sites are mostly dispersed as single atoms. Once the metal dissolves or the support oxidizes and corrodes, the corresponding active sites are lost. Therefore, high potentials (>1.1 V) are unsuitable for high-energy environments. RHE The activation method under these conditions is not applicable to non-noble metal oxygen reduction catalysts such as Fe / N / C.

[0003] In summary, there is currently no definitive and effective method to determine the number of active sites in Fe / N / C catalysts for proton exchange membrane fuel cells (PEMFCs). There is an urgent need to develop a method that can clean and activate the active sites of Fe / N / C catalysts under actual fuel cell operating conditions, so as to achieve the measurement of the number and density of Fe sites under operating conditions. Summary of the Invention

[0004] The purpose of this invention is to provide a method for online cleaning and activation of active sites in Fe / N / C oxygen reduction catalysts.

[0005] To achieve the above object, the present application adopts the following technical solutions:

[0006] A method for online cleaning and activating Fe / N / C oxygen reduction catalyst, specifically, in an electrolytic cell or a proton exchange membrane fuel cell, loading Fe / N / C catalyst on an electrode, placing the electrode in a hydrogen atmosphere, applying cyclic voltammetry scanning potential, step potential or constant potential for electrochemical cleaning until stable, using electrochemical oxidation reaction of hydrogen to remove adsorbates or impurities adsorbed on the surface of metal active sites, according to Fe 3+ / Fe 2+ variable potential peak in the cyclic voltammetry curve, integrating to obtain variable potential reaction electric quantity, and calculating Fe site active site density.

[0007] Further, the position of the Fe 3+ / Fe 2+ variable potential peak is 0.8-0.9 V RHE .

[0008] In the electrolytic cell, the cleaning and activating steps are loading Fe / N / C catalyst on an electrode to obtain a working electrode, placing the working electrode in a hydrogen-saturated electrolyte, applying cyclic voltammetry scanning potential, step potential or constant potential until stable, and then according to Fe 3+ / Fe 2+ variable potential peak in the cyclic voltammetry curve, integrating to obtain variable potential reaction electric quantity, and calculating Fe site active site density.

[0009] Further, the electrolyte is H2SO4 solution or HClO4 solution.

[0010] Further, the upper limit potential of the applied cyclic voltammetry scanning is 0.85-1.05 V RHE , and the lower limit potential is 0.0-0.8 V RHE .

[0011] Further, the upper limit potential of the applied step potential is 0.85-1.05 V RHE , and the lower limit potential is 0.0-0.8 V RHE .

[0012] Further, the potential range of the applied constant potential is 0.85-1.05 V RHE .

[0013] In the proton exchange membrane fuel cell, the online cleaning and activating steps are using Fe / N / C catalyst as a cathode catalytic layer and platinum-based catalyst as an anode catalytic layer, introducing hydrogen into the cathode, and controlling the cell voltage to apply cyclic voltammetry scanning voltage, step voltage or constant voltage to clean and activate the Fe / N / C catalyst, and then according to Fe 3+ / Fe2+ The variable peak is used to obtain the variable reaction electric quantity, and the Fe site active site density is calculated.

[0014] Further, the upper limit voltage of the cyclic voltammetry scanning voltage is 0.85-1.05 V, and the lower limit voltage is 0.0-0.8 V.

[0015] Further, the upper limit voltage of the step voltage is 0.85-1.05 V, and the lower limit voltage is 0.0-0.8 V; the voltage interval of the constant voltage is 0.85-1.05 V.

[0016] The preparation method of the Fe / N / C oxygen reduction catalyst is not specifically limited.

[0017] The significant advantages of the present application are:

[0018] The present application uses hydrogen oxidation reaction to in-situ clean the metal sites of the Fe / N / C catalyst, and the hydrogen is electrochemically oxidized on the surface of the metal site, which can promote the desorption of the impurities adsorbed on the site, so as to expose and activate the clean metal active sites. Subsequently, the Fe 3+ / Fe 2+ The electrochemical signal generated by the variable potential reaction realizes the measurement of the number of Fe active sites. The method can effectively remove the adsorbents or toxic impurities of the active sites, restore the catalyst activity, and measure the active site density and Fe 3+ / Fe 2+ The variable potential is suitable for the electrolytic cell measurement system and the proton exchange membrane fuel cell. The activation method of the present application is simple and fast, and can effectively distinguish and screen the Fe / N / C catalysts with different activities, monitor the active site density and Fe 3+ / Fe 2+ The attenuation caused by the potential change improves the research and development efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 (a) is a cyclic voltammetry curve comparison of the FePc derived Fe / N / C catalyst electrode in the 0.1 M H2SO4 electrolyte saturated with argon (black solid line), hydrogen (red solid line) and re-switched to argon (blue solid line) at 0.2 V-1.05 V (vs. RHE) cyclic voltammetry scanning potential; (b) is an enlarged view of the high potential part in (a).

[0020] Figure 2 The Fe / N / C catalyst electrode after hydrogen oxidation cleaning and activation adsorbs NO2 -Comparison of cyclic voltammograms of the catalyst before (blue solid line) and after (red solid line) poisoning, electrolyte: HAc / NaAc buffer solution with pH=5.2.

[0021] Figure 3 : (a) Cyclic voltammograms of FePc-derived Fe / N / C catalyst electrode in 0.1 M H2SO4 electrolyte saturated with argon (black solid line), hydrogen (dashed line) and argon again (red solid line), respectively, where the cleaning potential applied in hydrogen atmosphere is 0.3 V-0.85 V (vs. RHE); (b) is the enlarged view of the high potential part of (a).

[0022] Figure 4 : (a) Comparison of cyclic voltammograms of FePc-derived Fe / N / C catalyst electrode before (black solid line) and after (red solid line) cleaning, electrolyte: 0.1 M H2SO4 saturated with argon, cleaning condition: 1.0 V (vs. RHE) constant potential applied in hydrogen saturated 0.1 M H2SO4 electrolyte for 5 min; (b) is the enlarged view of the high potential part of (a).

[0023] Figure 5 : Comparison of cyclic voltammograms of Fe / N / C catalyst electrode prepared by gas phase deposition before (black curve) and after (red curve) hydrogen oxidation cleaning, electrolyte: 0.1 M H2SO4 saturated with argon, cleaning condition: cyclic voltammetry scanning between 0.2 V-1.0 V (vs. RHE) in hydrogen saturated 0.1 M H2SO4 electrolyte.

[0024] Figure 6 : Comparison of cyclic voltammograms of FePc-derived Fe / N / C catalyst electrode before (black solid line) and after (red solid line) applying low potential scanning in hydrogen, the applied potential scanning (three dashed lines) ranges are 0.1 V-0.4 V (vs. RHE), 0.1 V-0.6 V (vs. RHE) or 0.1 V-0.8 V (vs. RHE), respectively.

[0025] Figure 7 : Cyclic voltammograms of PEMFC single cell assembled with Fe / N / C catalyst as cathode and Pt / C as anode before (black curve) and after (red curve) in-situ hydrogen cleaning and activation, measured under N2 atmosphere. DETAILED DESCRIPTION

[0026] In order to make the content of the present application more convenient to understand, the technical solutions of the present application will be further described below in combination with specific embodiments, but the present application is not limited to this.

[0027] Example 1

[0028] The embodiment provides a method for preparing Fe / N / C catalyst by using FePc and metal organic framework (MOF) derivation, and provides a method for activating the catalyst by electrochemical online cleaning, and the specific steps are as follows:

[0029] Step 1: 6.3 g of 2-methylimidazole is dissolved in 160 mL of methanol to obtain solution A. 5.4 g of Zn (NO3) 2.6H2O is dissolved in 100 mL of methanol to obtain solution B.

[0030] Step 2: After mixing solution A and solution B in step 1 uniformly, the mixture is reacted at 60 ℃ oil bath for 24 h, the product obtained after the reaction is centrifuged and washed, and the product is dried at 70 ℃ for 12 h to obtain MOF precursor ZIF-8.

[0031] Step 3: The MOF precursor ZIF-8 in step 2 is carbonized at 1000 ℃ for 1 h under an argon atmosphere to obtain MOF-derived carbon material.

[0032] Step 4: 20 mg of iron phthalocyanine (FePc) and 200 mg of MOF-derived carbon material in step 3 are mixed in a ball mill at a ball-to-material ratio of 10:1 for 5 minutes. Then the uniformly mixed sample is subjected to secondary high-temperature heat treatment under argon protection: the temperature is raised to 1050 °C at a temperature raising rate of 10 ℃ / min and is kept for 5 minutes, the sample is taken out after natural cooling to room temperature and is ground, and FePc-derived Fe / N / C catalyst is obtained.

[0033] Step 5: Electrochemical cleaning and characterization: the Fe / N / C catalyst synthesized above is modified on a glassy carbon electrode (working electrode) with a loading of 0.27 mg·cm -2 -2. The working electrode is placed in a 0.1 M H2SO4 electrolyte saturated with hydrogen (H2) gas, and a cyclic voltammetry potential of 0.2 V-1.05 V (vs. RHE) is applied until the cyclic voltammogram is stable and the front and back circles overlap. During this period, hydrogen oxidation reaction occurs on the surface of the metal active site. After cyclic scanning, an obvious Fe 3+ / Fe 2+ variable peak (about 0.85 V RHE ) can be observed on the cyclic voltammogram, which indicates that the Fe active site has been cleaned and activated by electrochemistry.

[0034] Step 6: The atmosphere of the Fe / N / C catalyst electrode is switched to argon (Ar) until it is saturated, and a potential containing Fe 3+ / Fe 2+Obtain stable cyclic voltammograms for the potential range of the valence change peak, such as 0.8 V to 0.9 V (vs. RHE). For Fe near 0.85 V (vs. RHE)... 3+ / Fe 2+ Integrating the valence peaks yields the charge of electrons in the electrochemical reaction, based on the electrochemical reaction Fe(Ⅲ)-OH + H+. + The ratio of electrons to Fe sites in Fe(II) is 1:1, which allows us to obtain the number of Fe sites and the density of active sites. The calculation formula is as follows:

[0035] ;

[0036] In the formula, MSD is the density of Fe active sites, and Q... strip n is the amount of electricity in the reaction. strip It is Fe 3+ / Fe 2+ The number of electrons transferred in a variable-valence reaction is 1 electron, F is the Faraday constant, and m Fe / N / C It is the loading of the Fe / N / C catalyst.

[0037] See results Figure 1 , Figure 1 (a) and Figure 1 In (b), the black solid line represents the cyclic voltammetry curves of the Fe / N / C catalyst electrode derived from FePc in an argon-saturated 0.1 M H2SO4 electrolyte. A comparison shows that only a redox peak generated by quinone / hydroquinone functional groups on the carbon support surface is observed at approximately 0.7 VRHE, but no obvious Fe... 3+ / Fe 2+ A price change peak appears.

[0038] Figure 1 (a) and Figure 1 In (b) of the figure, the red solid line represents the cyclic voltammetry scan performed after switching from argon to hydrogen atmosphere. The stabilized cyclic voltammetry curve is shown in the figure. It can be observed that in the hydrogen environment, the current in the red curve is slightly higher than that in the black curve under argon conditions above approximately 0.8 VRHE, indicating the current generated by hydrogen oxidation; at the same time, a clear FeO appears at approximately 0.85 VRHE. 3+ / Fe 2+ The variable valence peak indicates that the presence of hydrogen removes adsorbates or impurities from the Fe active sites, "detoxifying" and exposing the previously poisoned Fe sites, thus revealing typical Fe characteristics in subsequent scans. 3+ / Fe 2+ Electrochemical transition peak.

[0039] Switch back to Ar inert atmosphere, apply 0.2 V-1.05 V (vs. RHE) scan cyclic voltammetry, as Figure 1 (a) and Figure 1 (b) in blue solid line, hydrogen oxidation current disappears, but the previously appearing Fe 3+ / Fe 2+ peak still exists. This shows that the oxidation reaction of hydrogen on the Fe active site plays a cleaning role, removing impurities on the active site and activating the Fe site; and after activation, even if it returns to a hydrogen-free environment, its electrochemical characteristics (Fe 3+ / Fe 2+ peak) can still be seen. According to the active site density calculation method in step 6, the integral calculation obtains the active site density of the FePc-derived Fe / N / C catalyst as 1.0×10 18 site·g -1 .

[0040] Further NO2 - poisoning experiments verify that the reduction peak appearing at 0.85 V RHE is indeed the variable valence reaction of Fe 3+ / Fe 2+ . The Fe / N / C electrode cleaned by hydrogen oxidation is placed in a 0.125 mol·L -1 sodium nitrite solution for 5 minutes, and after the adsorption of nitrite is saturated, the cyclic voltammetry curve is tested in a pH=5.2 acetic acid / sodium acetate buffer solution, as shown in Figure 2 . Figure 2 The blue solid line is the cyclic voltammetry curve of the Fe / N / C electrode before NO2 - poisoning, and a reduction peak can be seen at about 0.85 V (vs. RHE). However, the 0.85 V (vs. RHE) peak of the Fe / N / C electrode after NO2 - poisoning disappears. This shows that 0.85 V (vs. RHE) is a metal-related peak, because NO2 - preferentially coordinates / adsorbs with Fe active sites, making them lose electrochemical activity, and the corresponding variable valence peak no longer appears. In summary, the peak is derived from the Fe 3+ / Fe 2+ variable valence reaction of the Fe active site.

[0041] This embodiment 1 shows that the FePc-derived Fe / N / C catalyst uses hydrogen oxidation reaction to clean the Fe active site in 0.1 M H2SO4 electrolyte, which on the one hand removes impurities and exposes the metal active site, realizing the activation of the Fe site; on the other hand, after eliminating the influence of impurity poisoning, the electrochemical signal of the Fe active site itself can be reflected, so that it can be used for quantitative determination of its quantity and density.

[0042] Example 2

[0043] The same Fe / N / C catalyst preparation method as in Example 1 was used in this example, and the same washing and activation method was used, except that:

[0044] In Step 5, a cyclic voltammetry scan potential of 0.3 V to 0.85 V (vs. RHE) was applied until the cyclic voltammogram was stable and the before-and-after circles overlapped, and the remaining steps were the same as in Example 1.

[0045] The cyclic voltammograms before and after washing in Example 2 are shown in Figure 3 (a) and the enlarged high-potential region in Figure 3 (b), respectively. After the cyclic scanning, an obvious Fe 3+ / Fe 2+ redox peak (about 0.85 V RHE ) can be observed on the cyclic voltammogram, which indicates that the Fe active sites have been washed and activated electrochemically.

[0046] By comparing Example 1 and Example 2, it can be seen that hydrogen oxidation can occur in the upper potential range of 0.85-1.05 V RHE , which can achieve the washing and activation of Fe / N / C active sites.

[0047] Example 3

[0048] The same Fe / N / C catalyst preparation method as in Example 1 was used in this example, and the washing and activation method was different from that in Example 1, except that:

[0049] In Step 5, a constant potential of 1.0 V (vs. RHE) was applied for 5 minutes until it was stable, and the remaining steps were the same as in Example 1.

[0050] The cyclic voltammograms before and after washing in Example 3 are shown in Figure 4 (a) and (b), respectively. After the constant potential of 1.0 V (vs. RHE), an obvious Fe 3+ / Fe 2+ redox peak (about 0.85 V RHE ) can be observed on the cyclic voltammogram, which indicates that the Fe active sites have been washed and activated electrochemically.

[0051] This example 3 illustrates the method of washing Fe / N / C active sites using hydrogen oxidation reaction, and the potential application method is not limited to cyclic voltammetry method. Other electrochemical methods that can promote hydrogen oxidation of Fe sites are also applicable, such as constant potential method (Example 3) and high-low potential step.

[0052] Example 4

[0053] The present embodiment provides a method for preparing Fe / N / C catalyst by acetylacetone iron and metal organic framework (MOF) derivation, and provides a method for electrochemically online cleaning and activating the catalyst, which is different from the synthesis method and cleaning and activating method of FePc-derived Fe / N / C catalyst in Embodiment 1 in that:

[0054] Step 4, 20 mg of acetylacetone iron [Fe(acac)3] and 200 mg of MOF-derived carbon material in step 3 were respectively placed in the lower layer and the upper layer of a vacuum reaction kettle. After 6 cycles of vacuum pumping-argon introduction, the last vacuum state was maintained, and the temperature was raised to 200℃ at a speed of 15℃ / min, and then maintained for 2 hours. Subsequently, the MOF-derived carbon material adsorbed with iron source in the upper layer was transferred to a tube furnace, and pyrolysis was carried out at 800℃ in an argon atmosphere for 1 hour, thereby obtaining a Fe / N / C catalyst prepared by gas phase deposition (CVD-Fe / N / C).

[0055] The remaining steps are the same as in Embodiment 1.

[0056] The results are shown in Figure 5 After the same hydrogen oxidation online cleaning as in step 5 of Embodiment 1, the Fe / N / C catalyst has a peak current density of about 0.85 V RHE Fe 3+ / Fe 2+ variable peak, and by using the calculation method of step 6, the integral calculation obtains that the active site density of the Fe / N / C catalyst prepared by gas phase deposition is 8.0×10 17 site·g -1 .

[0057] Comparative Example 1

[0058] The present comparative example adopts a similar cleaning and activating method as in Embodiment 1, and the difference is only that one of 0.1 V-0.4 V (vs. RHE), 0.1 V-0.6 V (vs. RHE) or 0.1 V-0.8 V (vs. RHE) is applied in the cyclic voltammetry scanning potential, until the cyclic voltammogram is stable and the front and back circles overlap, and the remaining steps are the same as in Embodiment 1.

[0059] The cyclic voltammetry diagram of Comparative Example 1 applied in a hydrogen atmosphere is shown in Figure 6 No Fe 3+ / Fe 2+ variable peak is observed after cyclic scanning, indicating that the Fe active site is not cleaned and activated when the potential is lower than 0.8 V.

[0060] Embodiment 5

[0061] The hydrogen cleaning and Fe site determination method described above were applied to a practical proton exchange membrane fuel cell single cell. A Fe / N / C catalyst derived from FePc was used as the cathode catalyst layer, and a commercial Pt / C catalyst was used as the anode catalyst layer. A membrane electrode was assembled and loaded into a fuel cell test cell fixture. The synthesis method of the cathode catalyst Fe / N / C was consistent with that of Example 1, and the loading amount was 1.50 mg·cm -2 -2. The platinum loading amount of the anode Pt / C catalyst was 0.5 mgPt·cm -2 -2, and the effective electrode area was 1 cm 2 2. The test temperature was 30℃, and both the cathode and anode gases were humidified by a 30℃ water tank.

[0062] In the test, pure hydrogen gas (flow rate of 200 sccm) was supplied to both the cathode and the anode, and the cathode was subjected to a cyclic voltage of 0.65 V-1.0 V for cleaning and activation. This process is a process of cleaning and activating the Fe / N / C cathode catalyst by hydrogen oxidation under the actual working condition of the fuel cell. Subsequently, the cathode gas was switched to a pure nitrogen (N2) inert atmosphere (flow rate of 200 sccm), and the cell was kept stable at 30℃ and fully humidified for 20 minutes. Then, the single cell cyclic voltammogram under the N2 atmosphere of the cathode was tested, as shown in Figure 7 Fig. 2. The Fe 3+ / Fe 2+ redox peak at about 0.85 V was integrated to calculate the active site density, and the result was 8.9×10 17 site·g -1 -2.

[0063] The above only describes the preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.

Claims

1. A method for online cleaning and activation of Fe / N / C oxygen reduction catalyst, characterized in that: In the electrolytic cell or proton exchange membrane fuel cell, the Fe / N / C catalyst is loaded on the electrode, the electrode is placed in the hydrogen atmosphere, and the cyclic voltammetry scanning potential, step potential or constant potential is applied for electrochemical cleaning until stable, the adsorbed adsorbate or impurity on the surface of the metal active site is removed by the electrochemical oxidation reaction of hydrogen, and the Fe 3+ / Fe 2+ variable peak in the cyclic voltammetry curve is integrated to obtain the variable reaction electric quantity, so as to calculate the Fe site active site density; In the electrolytic cell, the upper limit potential for applying the cyclic voltammetry scan is 0.85 to 1.05 V RHE , and the lower limit potential is 0.0 to 0.8 V RHE ; the upper limit potential for applying the step potential is 0.85 to 1.05 V RHE , and the lower limit potential is 0.0 to 0.8 V RHE ; the potential interval for applying the constant potential is 0.85 to 1.05 V RHE ; In proton exchange membrane fuel cells, the upper limit potential for cyclic voltammetry scanning is 0.85~1.05 V, and the lower limit potential is 0.0~0.8 V; the upper limit potential for step potential is 0.85~1.05 V, and the lower limit potential is 0.0~0.8 V; the potential range for constant potential is 0.85~1.05 V.

2. The method according to claim 1, characterized in that: The Fe 3+ / Fe 2+ The valence change peak is located at 0.8~0.9V. RHE .

3. The method according to claim 1, characterized in that: The cleaning and activation steps in the electrolytic cell involve loading the Fe / N / C catalyst onto the electrode to obtain the working electrode, placing the working electrode in a hydrogen-saturated electrolyte, and applying cyclic voltammetry scanning potential, step potential, or constant potential until stability is achieved. Then, based on the Fe... 3+ / Fe 2+ The variable valence peak is obtained by integration to obtain the charge of the variable valence reaction, and then the active site density of Fe sites is calculated.

4. The method according to claim 3, characterized in that: The electrolyte is an H2SO4 solution or an HClO4 solution.

5. The method according to claim 1, characterized in that: In proton exchange membrane fuel cells, the online cleaning and activation steps involve using an Fe / N / C catalyst as the cathode catalyst layer and a platinum-based catalyst as the anode catalyst layer. Hydrogen gas is introduced into the cathode, and the battery voltage is controlled by applying a cyclic voltammetric scan voltage, a step voltage, or a constant voltage to clean and activate the Fe / N / C catalyst. Then, based on the Fe... 3+ / Fe 2+ The variable valence peak is obtained by integration to obtain the charge of the variable valence reaction, and then the active site density of Fe sites is calculated.

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