Ti₄O₇ supported IrO_x for anode reversal tolerance in proton exchange membrane fuel cell

Abstract

Fuel starvation can occur and cause damage to the cell when proton exchange membrane fuel cells operate under complex working conditions. In this case, carbon corrosion occurs. Oxygen evolution reaction (OER) catalysts can alleviate carbon corrosion by introducing water electrolysis at a lower potential at the anode in fuel shortage. The mixture of hydrogen oxidation reaction (HOR) and unsupported OER catalyst not only reduces the electrolysis efficiency, but also influences the initial performance of the fuel cell. Herein, Ti₄O₇ supported IrO_x is synthesized by utilizing the surfactant-assistant method and serves as reversal tolerant components in the anode. When the cell reverse time is less than 100 min, the cell voltage of the MEA added with IrO_x/Ti₄O₇ has almost no attenuation. Besides, the MEA has a longer reversal time (530 min) than IrO_x (75 min), showing an excellent reversal tolerance. The results of electron microscopy spectroscopy show that IrO_x particles have a good dispersity on the surface of Ti₄O₇ and IrO_x/Ti₄O₇ particles are uniformly dispersed on the anode catalytic layer. After the stability test, the Ti₄O₇ support has little decay, demonstrating a high electrochemical stability. IrO_x/Ti₄O₇ with a high dispersity has a great potential to the application on the reversal tolerance anode of the fuel cell.

Key words： proton exchange membrane fuel cell (PEMFC); fuel starvation; cell reverse; reversal tolerance anode; oxygen evolution reaction

1 Introduction

Proton exchange membrane fuel cell (PEMFC) has attracted wide attention in recent years due to its advantage of high conversion efficiency, high power density, and environmentally friendliness [1]. However, its cost, performance, and durability are the three major challenges in the process of large-scale commercialization [2,3].

The supply of hydrogen may be delayed or insufficient when the fuel cell is operating under complicated conditions, such as start-up [4], rapid load change [5], and when flooded [6,7]. In particular, the anode potential increases rapidly to about 1.5 V or higher when fuel starvation happens in a cell of the stack. In this case, other reactions like water electrolysis and carbon corrosion will occur to provide protons and electrons for the current output [8,9]. The reaction formulas can be expressed as [6,10,11]

{\mathrm{2H}}_{\mathrm{2}}\mathrm{O}\to {\mathrm{O}}_{\mathrm{2}}+{\mathrm{4H}}^{+}+{\mathrm{4e}}^{–},

(1){\mathrm{E}}^{\mathrm{0}}=\mathrm{1}.\mathrm{229\; V}\left(\mathrm{vs}.\mathrm{SHE}\right)\mathrm{;}

\mathrm{C}+{\mathrm{2H}}_{\mathrm{2}}\mathrm{O}\to {\mathrm{CO}}_{\mathrm{2}}+{\mathrm{4H}}^{+}+{\mathrm{4e}}^{–},

(2){\mathrm{E}}^{\mathrm{0}}=\mathrm{}\mathrm{0.}\mathrm{2}\mathrm{0}\mathrm{7\; V}\left(\mathrm{vs}.\mathrm{SHE}\right)\mathrm{;}

\mathrm{C}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{CO}+{\mathrm{2H}}^{+}+{\mathrm{2e}}^{–},

(3){\mathrm{E}}^{\mathrm{0}}=\mathrm{}\mathrm{0.}\mathrm{518\; V}\left(\mathrm{vs}.\mathrm{SHE}\right).

It is well known that the reaction rate of carbon corrosion is much slower than that of water electrolysis [1,12,13]. Therefore, water electrolysis usually takes place first during cell reversal process. Unfortunately, the hydrogen oxidation reaction (HOR) catalyst like Pt/C will lose its function after a period of time due to the poisoning caused by carbon oxidation products [14–16]. Subsequently, an anode potential will rise rapidly attributing to the dominance of carbon corrosion [17]. The decay of carbon support would cause the aggregation or shedding of Pt particles, thus seriously damaging the anode catalytic layer (CL) [18]. The heat generated by carbon corrosion will lead to the formation of pinholes in proton exchange membrane [1]. In addition, the fuel and oxidant will mix in the reaction chamber, resulting in a catastrophic failure [19]. System control strategies are proposed in order to monitor the health of the stack system and find abnormalities in time, such as detecting the emitted CO₂ [8,9] and using it as an indicator to diagnose the abnormality of the system, detecting the localized anode potential [20,21], and monitoring the change in the stoichiometric ratio of the reactant gas [22], etc. However, these strategies require the support of additional systems so that the cost and complexity of the stack system will be increased. Reversal tolerance anode (RTA) is a reliable method based on materials to replace system strategies. The anode catalyst layer is usually doped with OER catalysts to extending the reaction time of water electrolysis in cell reverse, so as to alleviate the rate of carbon corrosion [23,24].

In recent years, a variety of OER catalysts have been used in the RTA, such as IrO₂ [1], RuO₂ [6], IrRu alloy [25,26], etc. Of these OER catalysts, IrO₂ has the best stability, and its activity is second only to RuO₂ [27–29]. Therefore, IrO₂ can decompose water for a long time at a relatively low potential. However, it is usually added directly into the anode in the form of particulate matter in most literature. In this case, IrO₂ may agglomerate in the catalytic layer so that it cannot protect all the carbon support [30]. Based on this situation, Roh et al. [30] deposited monodisperse IrO₂ on commercial Pt/C, and effectively improved the dispersion of IrO₂ in the catalytic layer. However, the preparation steps are cumbersome. Jang et al. [31] added IrO₂/C to the anode by improving the dispersion of IrO₂ on carbon, which showed less cell performance degradation than that of adding IrO₂ after frequent cell reverse. However, the additional carbon introduction would increase the possibility of corrosion. Consequently, it is necessary to improve the dispersion of IrO₂ in the catalytic layer and develop antioxidant catalyst supports at the same time. Ti₄O₇ has been widely regarded as a corrosion-resistant and electrically conductive material [32]. There are some reports utilizing Ti₄O₇ as a support material. For example, Ioroi and Yasuda [33] used Ti₄O₇ as the support of Pt, which greatly improved the reversal tolerance of the cell under the condition of fuel starvation. Won et al. [34] used Ti₄O₇ supported PtIr alloy as a bifunctional catalyst, showing excellent oxygen reduction reaction (ORR) and OER performance compared with single metal. However, there is no report on using Ti₄O₇-supported iridium oxide as the reversal tolerant components in the presence of Pt/C.

In this study, IrO_x/Ti₄O₇ was synthesized by utilizing the surfactant-assistant method. For comparison, IrO_x without support was prepared in the same way. The electrochemical performance of OER catalysts was characterized by half-cell and full cell tests. The dispersity of the catalysts which contribute to the electrolysis performance was analyzed by transmission electron microscopy (TEM), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). In addition, the electrochemical stability of Ti₄O₇ in the three electrode system was studied. The results show that IrO_x particles have a good dispersity on the surface of Ti₄O₇ and IrO_x/Ti₄O₇ particles are uniformly dispersed on the anode catalytic layer, which make the reverse tolerance of MEA with IrO_x/Ti₄O₇ much better than MEA with IrO_x.

2 Experiment

2.1 Preparation of IrO_x and IrO_x/Ti₄O₇

The IrO_x/Ti₄O₇ was synthesized by the surfactant-assistant method [35]. Typically, 328 mg of Ti₄O₇ (Changsha Purong Chemical Engineering Inc.) was ultrasonically dispersed in 60.8 mL of distilled water (18.2 mΩ). Then, 153 mg of Pluronic® F127 (Sigma-Aldrich) was dissolved in the mixture and stirred for 30 min. After adding 626.3 mg of H₂IrCl₆·xH₂O (Tianjin Jinbolan Fine Chemical Co., Ltd.), the mixture was vigorously stirring at 60°C for 5 h. After that, the sample was added with 12.2 mL of NaBH₄ solution (1 mol/L) at the same concentration as that of F127. Ultimately, the mixture was stirring overnight at 60°C for 12 h. After the reaction, the product was washed several times with anhydrous ethanol and dried at 60°C. The mass content of IrO_x in IrO_x/Ti₄O₇ is 32% (mass fraction). For comparison, the IrO_x without support was prepared in the same way.

2.2 Characterizations

The morphology of the synthesized catalysts was characterized by transmission electron microscopy (TEM, JEM 2100) at 120 kV. The cross-section morphology was observed by field emission scanning electron microscopy (SEM, JSM-7800F). Energy dispersive X-ray spectroscopy (EDS) mapping was conducted by the detectors on SEM at an accelerated voltage of 20 kV to observe the distributions of Pt, Ir, and Ti on the electrode surface. The crystal structure of the catalysts was characterized by X-ray diffraction (XRD, PANalytical Empyrean) with a Cu-Kα tube. X-ray photoelectron spectroscopy (XPS, ESCALABXi) with an Mg anode was utilized to reveal the chemical state of the prepared IrO_x. The catalyst loading in terms of Ir content was measured via an inductively coupled plasma optical emission spectrometer (ICP-OES, 7300DV).

2.3 Electrochemical measurements

Electrochemical measurements were performed in the N₂-saturated 0.5 mol/L H₂SO₄ solution at 30°C with a rotating disk electrode (RDE) system in the traditional three electrode system, which consisted of a glassy carbon electrode (GCE, 0.1256 cm²), a saturated calomel electrode (SCE), and a Pt tablet as the working, counter, and reference electrode, respectively. All potentials used in this study were calibrated by reversible hydrogen electrode (RHE). The catalyst ink was prepared by mixing 5 mg of powder, 1 mL of isopropyl alcohol (Aladdin), and 50 μL of Nafion solution (5%, Dupont). Then, 10 μL of the ink was deposited onto the GCE, drying naturally in air.

The stability of Ti₄O₇ was tested by potentiostatic method at 1.5 V for 5 h in a CHI 760 E electrochemical system. The linear sweep voltammetry (LSV) was carried out from 0 V to 1.4 V at 2 mV/s with a rotation rate of 1600 r/min, using the Gamry Interface 1000 E. Before the test, the catalyst was activated by cyclic voltammetry (CV) from 0 V to 1.2 V at 100 mV/s for 30 cycles.

2.4 MEA fabrication and single cell test

First, catalyst coated membrane (CCM) was prepared [36], and then MEA was prepared by hot pressing. Typically, the catalyst ink of cathode was prepared by mixing Pt/C (70% (mass fraction), Johnson Matthey) with isopropyl alcohol, deionized water, and Nafion solution. The specific composition of the anode catalyst is listed in Table 1. The Pt loadings of the cathode and anode were 0.4 and 0.2 mg/cm², respectively. The mass ratio of the ionomer/carbon (I/C ratio) was 0.7. The catalyst inks were directly sprayed on both sides of the Nafion 212 membrane. Finally, the MEA was fabricated by hot pressing the CCM with two gas diffusion layers (GDL) at 140°C, 0.1 MPa for 2 min. The effective area was 5 cm².

Tab.1 Composition of catalysts in the anode

Catalyst/(mg·cm–2)	MEA-1	MEA-2	MEA-3
Pt/C	0.2	0.2	0.2
IrOx	0	0.1	0
IrOx/Ti4O7	0	0	0.3

The single cell test was conducted on a computer-controlled home-made test bench. The polarization curve (I-V) and the electrochemical impedance spectroscopy (EIS) were measured at 75°C, with a back pressure of 0.05 MPa (gauge pressure). The flow rates of fully humidified H₂ (anode) and air (cathode) were 120 and 800 mL/min, respectively. Before testing, all the single cells were activated for 4 h. The I-V curve was measured by KFM2030 (Kikusui, Japan). The EIS was measured by Gamry Interface 5000 E under the same condition of the I-V curve with the frequency ranging from 0.1 Hz to 10 kHz. The current density was kept at 0.2 A/cm².

The cell reversal test was conducted without back pressure at 75°C and the relative humidity (RH) of anode and cathode was both 100%. First, H₂ and air were supplied to the anode and cathode for about 30 min, respectively. Then, the H₂ was suddenly replaced by N₂. After that, a constant current of 0.2 A/cm² was supplied to the cell until the cell voltage reaches –2 V [6,11,27]. The schematic diagram of the voltage reversal test station is shown in Fig. 1. The I-V curve and EIS after the cell reversal test were performed. The reversal time is defined as the time that the voltage drops from 0 to –2 V.

Fig.1 Schematic diagram of voltage reversal test station.

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3 Results and discussion

3.1 Physical characterizations of the catalysts

Physical characterizations of the catalysts were conducted by XRD, XPS, and TEM. The crystal structure of the catalysts was performed by XRD. Figure 2 depicts the XRD pattern of the synthesized IrO_x and IrO_x/Ti₄O₇. The two peaks in the spectrum of IrO_x are attributable to the most intense peak of the hollandite or rutile phases of IrO₂ [37]. It can be found that there is no obvious peak of Ir and IrO₂ in the spectrum of IrO_x/Ti₄O₇ compared to the standard JCPDS cards (JCPDS No. 15-0780; JCPDS No. 87-0715). These results indicate that the IrO_x is amorphous [38,39].

Fig.2 XRD patterns of synthesized IrO_x, IrO_x/Ti₄O₇ and initial Ti₄O₇ with the standard card for IrO₂ and Ti₄O₇.

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Fig.3 XPS spectrum of IrO_x/Ti₄O₇.

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Fig.4 TEM images and particle sizes.

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XPS was performed to investigate the electronic state of the Ir element. The XPS spectrum of Ir 4f for IrO_x and IrO_x/Ti₄O₇, as well as the full XPS spectrum of IrO_x/Ti₄O₇ is presented in Fig. S1 in Electronic Supplementary Material (ESM). Figures 3(a) and 3(b) manifest the fitted Ir 4f and O 1s spectra, respectively. Figure 3(a) shows the high resolution Ir 4f spectrum of IrO_x/Ti₄O₇. The two peaks correspond to the Ir 4f_5/2 and Ir 4f_7/2, respectively. Then, each peak was deconvoluted into two peaks for the analysis of Ir element on the IrO_x/Ti₄O₇ in detail. The most intense doublet at 62.38 and 65.28 eV can be attributed to Ir 4f_7/2 and Ir 4f_5/2, corresponding to the Ir³⁺. The weaker doublet at 63.38 and 66.48 eV can be assigned to the Ir⁴⁺ [40–42]. According to the area integral results of each fitting curve, the content of Ir in different valence states is estimated. Ir³⁺ and Ir⁴⁺ account for 47% and 53% (atomic percentage), respectively. Therefore, the synthesized IrO_x/Ti₄O₇ is likely to provide a sufficient OER activity with a perfect durability due to the increased Ir⁴⁺ states [43]. Figure 3(b) exhibits the peaks of different kinds of oxygen in the XPS spectrum of O 1s. In the catalyst, about 91% (atomic percentage) of the oxygen exists in the form of metal oxide while the rest is surface species.

Figure 4 shows the TEM images and particle sizes of Ti₄O₇ and IrO_x/Ti₄O₇. Figure 4(a) is the image of Ti₄O₇ which has irregular shapes, such as particles and blocks. The particle size of Ti₄O₇ ranges from 80 nm to 150 nm. As shown in Fig. 4(c), IrO_x nanoparticles are well dispersed on the Ti₄O₇ (the red mark in the picture). The perfect dispersion is beneficial to improve the mass specific activity of the catalyst and the electrolysis efficiency compared to the form of aggregation. The particle size and distribution of IrO_x on the Ti₄O₇ are displayed in Fig. 4(d). The particle size of IrO_x ranges from 1 to 2 nm and the average particle size is 1.53 nm. Such a small particle size is due to the effect of Pluronic®F127, which plays a huge role as a template and stabilizer. Specifically, it can trigger seeds formation and control the size, shape and dispersion of IrO_x amorphous nanoparticles [35].

3.2 Electrochemical characterizations of the catalysts

The electrochemical stability of Ti₄O₇ was tested by utilizing the potentiostatic method at 1.5 V for 5 h. Figure S2 in ESM shows the CV curves of Ti₄O₇ before and after the stability test. As seen in Fig. S2 in ESM, there is no obvious redox peak in the initial CV curve, nor in that after the test. This shows that Ti₄O₇ has not been oxidized into TiO₂ after the potentiostatic test. Ti₄O₇ still maintains its original structure and conductivity. This means that the Ti₄O₇ has an excellent electrochemical stability at 1.5 V, which is expected to be seen in the reverse of fuel cell.

The electrochemical activity of synthesized catalysts was measured by LSV. The curves mainly reveal the performance of IrO_x and IrO_x/Ti₄O₇ between 1.4 V and 1.65 V (versus RHE). Figure 5(a) shows the OER activity of the catalysts. The overpotential of IrO_x is 303 mV (@ 10 mA/cm²) while that of IrO_x/Ti₄O₇ is only 293 mV. This means that IrO_x/Ti₄O₇ can perform oxygen evolution reaction at a relatively low potential. As shown in Fig. 5(b), the mass specific activity (MA) of IrO_x at 1.6 V (versus RHE) is 90 mA/mg_Ir. The MA of IrO_x/Ti₄O₇ is 273 mA/mg_Ir, which is three times that of IrO_x. This can be attributed to the dispersion effect of IrO_x on Ti₄O₇, which leads to the exposure of more activity sites. The existence of Ti₄O₇ suppresses the aggregation of IrO_x to a certain extent. Besides, Ti₄O₇ reduces the amount of precious metal iridium under the condition of the same performance.

Fig.5 OER activity of the catalysts.

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3.3 Performance of single cell

The result of the half-cell test indicates that the IrO_x/Ti₄O₇ has an excellent oxygen evolution activity and the Ti₄O₇ has a good chemical stability under an acidic condition. After this, the initial electrochemical performances of the two MEAs were studied, in which 0.1 mg/cm² IrO_x (MEA-2) and IrO_x/Ti₄O₇ (MEA-3) were added respectively. For comparison, the cell performance of the traditional Pt/C anode (MEA-1) is also given in Fig. 6, from which it can be observed that the initial performances of the anodes with the two self-made OER catalysts and that of the traditional Pt/C without OER catalyst are different from each other. The I-V curves are shown in Fig. 6(a) and the performance parameters are presented in Fig. 6(b). At a current density of 1000 mA/cm², MEA-1 obtains the highest cell voltage (0.627 V) of the three MEAs, followed by MEA-2 (0.625 V). The cell voltage of MEA-3 is 0.618 V. Besides, the maximum power density of MEA-1, MEA-2, and MEA-3 are 743.4, 725.6, and 747.5 mW/cm², respectively. There is no obvious difference between the three MEAs at a current density range of less than 1200 mA/cm². However, the difference in cell voltage is obviously above 1200 mA/cm². The mass transfer problem of MEA-2 is most serious. This can be attributed to the addition of IrO_x into the anode catalyst layer of MEA-2. The resistance of electron transmission in the catalytic layer will be increased because of the poor conductivity of oxides. However, the performance of MEA-3 in the high current density region is normally worse than that of MEA-2 because it has a thicker catalytic layer. As shown in Figs. S3(a) and S3(b) in ESM, the anode catalytic layer of MEA-3 is 2.3 μm, which is 1.6 times that of MEA-2 (1.4 μm). On the contrary, MEA-3 has a better performance than MEA-2. This can be attributed to the excellent electrical conductivity of Ti₄O₇ [44] so that the adverse effect of thicker catalytic layer will be reduced. To sum up, there is no doubt that the introduction of OER catalysts into the anode catalyst layer will cause the initial performance of the cell to slightly decrease in the high current density region whether the OER catalyst is IrO_x or IrO_x/Ti₄O₇. The decrease of IrO_x/Ti₄O₇ is smaller than that of IrO_x because of the better electrical conductivity of Ti₄O₇ and the excellent dispersion of IrO_x.

Fig.6 Performance of the three MEAs.

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3.4 Cell reversal tolerance of MEA

The cell reversal tests of the MEAs with OER catalysts were conducted. The voltage-time (V-T) curves during the cell reverse, the I-V curves, and EIS are presented in Fig. 7. Figure 7(a) is the V-T curve of MEA-2 and MEA-3 during the cell reverse. After H₂ was switched to N₂, the residual H₂ in the pipeline and inside the cell was quickly exhausted and the cell voltage jumped below 0 V at the same time. As observed in Fig. 7(a), MEA-2 and MEA-3 have the same time of cell reverse (about 75 min). The voltage of MEA-2 during water electrolysis drops obviously while that of MEA-3 is relatively stable. This means that the stability of the water electrolysis reaction for MEA-3 is better than that of MEA-2. The I-V curves of MEA-2 and MEA-3 are tested after cell reverse, and the results are shown in Fig. 7(b). The voltage of MEA-3 has almost no attenuation after cell reverse. However, the voltage of MEA-2 at 1000 mA/cm² is attenuated by 62.5%. Because the stability of the OER for the IrO_x is worse than that of the IrO_x/Ti₄O₇, the efficiency of OER is gradually reduced, resulting in a more serious carbon corrosion. This can also be seen from EIS (Fig. 7(c)), the initial ohmic resistances (R_Ω, which is determined by the intercept of the real axis) at a high frequency of MEA-2 and MEA-3 are 5.8 and 3.5 mΩ·cm², respectively. After undergoing the cell reverse, the R_Ω of MEA-2 increases to 15.5 mΩ·cm², while that of MEA-3 is 4.4 mΩ·cm². Besides, the growth rate of the charge transfer resistances (R_ct) of MEA-2 and MEA-3 are 56.0% and 1.4%, respectively. Obviously, the anode catalytic layer of MEA-2 has suffered from a severe carbon corrosion, which leads to the destruction of catalytic layer structure and affects the charge transfer. To further prove the excellent reverse tolerance of MEA-3, the reverse tolerance performance of MEA-1 was tested, as shown in Figs. 7(d) and 7(e). Since the anode catalyst of MEA-1 is only Pt/C and its OER activity is very low, the reverse time is only 22 s. The I-V curves of the MEA before and after the cell reverse test are given in Fig. 7(e). The voltage attenuation at 1000 mA/cm² is 5.86%. Its reverse time is two orders of magnitude shorter than that of MEA-3, but the voltage attenuation is more serious. Therefore, compared with MEA-1, MEA-3 has an excellent reverse tolerance.

Fig.7 Single cell performance of MEA-1, MEA-2 and MEA-3.

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In addition, the total reverse time of MEA-3 is 530 min (Fig. 7(f)), which is about 7 times that of MEA-2 (75 min, Fig. 7(a)). The longer time is attributed to the excellent dispersion of IrO_x on Ti₄O₇. Based on this, the OER catalyst can expose more active sites, prolonging the reaction time of water electrolysis. SEM and EDS mapping images on the surface of the catalytic layer were obtained for the distribution of OER catalysts. Figure 8(a) is the image of MEA-2, in which the Ir element shows an obvious aggregation on the surface of the catalytic layer from the element mapping. Therefore, only a small area of carbon support can be protected. In contrast, Fig. 8(b) shows that the iridium is uniformly distributed on the surface of the catalytic layer of MEA-3, which can protect most of the carbon support. Figure 8(c) illustrates the distribution of the two catalysts in the CL. The result of EDS intuitively reveals the uniform distribution of IrO_x supported by Ti₄O₇ in the catalytic layer, which provides an evidence for the reason why MEA-3 has a longer time of water electrolysis. Ti₄O₇ supported IrO_x could provide a longer time of water electrolysis at a relatively stable voltage, which is beneficial to the reverse tolerance of the MEA.

Fig.8 Distribution of IrO_x and IrO_x/Ti₄O₇ in the catalyst layer.

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4 Conclusions

In this study, Ti₄O₇ supported IrO_x with a small particle size of 1.53 nm was prepared by the surfactant-assistant method. As a catalyst support, Ti₄O₇ has shown an excellent electrochemical stability at 1.5 V. IrO_x supported on Ti₄O₇ can effectively reduce the amount of precious metal iridium and improve the catalytic efficiency for the RTA compared to the unsupported IrO_x. The reverse time of the prepared MEA with IrO_x/Ti₄O₇ in the anode has reached 530 min, which is 7 folds to that with IrO_x, indicating a better cell reverse tolerance under actual working conditions. This can be explained by the excellent dispersion of IrO_x on Ti₄O₇ and IrO_x/Ti₄O₇ on the anode catalytic layer. The initial cell performance only decreases by 1.4% at 1000 mA/cm² with the addition of IrO_x/Ti₄O₇ to the anode of the MEA for the superior conductivity of Ti₄O₇. Compared to the recently developed RTA based on carbon-free support, it is a desirable choice to add supported OER catalysts to the anode without sacrificing too much initial cell performance. In particular, support materials with a high conductivity and good corrosion resistance should be considered.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (No. 2019YFB1504502), and the Science and Technology Project of State Grid Corporation of China (SGLNDK00KJJS1900037).

Electronic Supplementary Material

ƒSupplementary material is available in the online version of this article at https://doi.org/10.1007/s11708-021-0811-7 and is accessible for authorized users.

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