New insight into effect of potential on degradation of Fe-N-C catalyst for ORR

Abstract

In recent years, Fe-N-C catalyst is particularly attractive due to its high oxygen reduction reaction (ORR) activity and low cost for proton exchange membrane fuel cells (PEMFCs). However, the durability problems still pose challenge to the application of Fe-N-C catalyst. Although considerable work has been done to investigate the degradation mechanisms of Fe-N-C catalyst, most of them are simply focused on the active-site decay, the carbon oxidation, and the demetalation problems. In fact, the 2e⁻ pathway in the ORR process of Fe-N-C catalyst would result in the formation of H₂O₂, which is proved to be a key degradation source. In this paper, a new insight into the effect of potential on degradation of Fe-N-C catalyst was provided by quantifying the H₂O₂ intermediate. In this case, stability tests were conducted by the potential-static method in O₂ saturated 0.1 mol/L HClO₄. During the tests, H₂O₂ was quantified by rotating ring disk electrode (RRDE). The results show that compared with the loading voltage of 0.4 V, 0.8 V, and 1.0 V, the catalysts being kept at 0.6 V exhibit a highest H₂O₂ yield. It is found that it is the combined effect of electrochemical oxidation and chemical oxidation (by aggressive radicals like H₂O₂/radicals) that triggered the highest H₂O₂ release rate, with the latter as the major cause.

Graphical abstract

Keywords

proton exchange membrane fuel cells (PEMFCs) / oxygen reduction reaction (ORR) / Fe-N-C catalyst / potential / H₂O₂ / degradation

1 Introduction

Owing to the zero carbon emission and the high-energy conversion efficiency, proton exchange membrane fuel cells (PEMFCs) have received increasing attention. However, the commercialization of PEMFCs still face two challenges: cost and durability [1,2]. As is known, catalysts are key materials of PEMFCs [3]. Besides, to further lower the cost of PEMFCs, a variety of catalysts, for instance, Pt/C [4,5], Pt alloys [6,7], the nonprecious metal catalyst (NPMC) [8–11] and the metal-free carbon-based catalyst [3,12] have been developed, of which, the Fe-N-C catalyst has shown promising prospects due to its low cost and enhanced ORR activity [1,13–15].

In recent decades, significant progress has been made in improving the activity of Fe-N-C catalyst [2,16,17]. In 1964, Jasinski first reported the ORR activity of the cobalt phthalocyanine [18]. Later, Jahnke showed that the macrocyclic compounds exhibited a significantly enhanced ORR activity after the heat treatment process [19]. Afterward, Dodelet et al. reported a Fe-N-C catalyst with a high volumetric current density of 99 A/cm³ at 0.8 V. However, the current density decreased from 0.58 A/cm² to 0.36 A/cm² after being kept at 0.4 V for 100 h (H₂/air) [20]. Recently, the first commercial PEMFC with NPMC at the cathode was developed by Ballard Power Systems [14]. Nevertheless, the lifetime of the stacks with NPMC at the cathode are only hundreds of hours, which is far lower than the DOE target of 5000 h [21]. In this regard, the durability of Fe-N-C catalyst still remains a challenge [22].

To date, most studies on the degradation mechanisms of Fe-N-C catalyst have simply been focused on the active-site decay, the carbon oxidation, and the demetalation problems [23,24]. According to the equations in Fig. 1, H₂O₂, which is evidenced to be a key degradation source of PEMFCs, is generated through a 2e⁻ pathway [25]. Moreover, recent research indicates that both the direct 4e⁻ pathway and the 2e⁻ pathway coexist in the ORR reactions catalyzed by Fe-N-C catalyst. Besides, the 2e⁻ pathway is proved to be the dominated process [26]. Studies also suggest that H₂O₂ is prone to be decomposed, especially at the presence of Fe ions, releasing radicals like HO• or HOO• [27]. Specifically, many researches have evidenced that strong oxidants, H₂O₂ or radicals, are key degradation sources in PEMFC [28,29]. Previously, Lefevre and Dodelet [24] pointed out that the carbon matrix is vulnerable to the aggressive H₂O₂/radicals, forming the oxygen-containing functional groups (like –COOH, –COH) on the surface. Even worse, volatile CO or CO₂ would be formed eventually. In addition, by exposing Fe-N-C catalyst to acidic H₂O₂ solution, Chang and coworkers observed the severely oxidized N-doped carbon surface [28]. Besides, Kumar et al. reported that the presence of O₂ would trigger the carbon corrosion in Fe-N-C catalyst during load cycling tests, due to the attack of H₂O₂ or radicals [30]. Besides, the H₂O₂ or radicals would also attack the Nafion ionomers in catalyst layer or membrane in PEMFCs. The subsequent chemical decomposition of Nafion ionomers would result in the paralysis of proton transport networks [31].

Therefore, to realize the wide application of Fe-N-C catalyst in PEMFCs, it is of great necessity to characterize the H₂O₂ release rate at different PEMFC working potentials. Only by operating the PEMFCs in a condition with minimized H₂O₂ releasement, can the durability of the cell be enhanced. In this paper, a new insight into the effect of potential on degradation of Fe-N-C catalyst is provided by quantifying H₂O₂ intermediate. Stability tests were conducted in O₂ saturated 0.1 mol/L HClO₄ at different potentials, during which the ring current was recorded by RRDE after every 48 h test. Based on the experimental results, further degradation mechanisms are analyzed in detail.

2 Experiment

2.1 Chemicals

The iron(III) chloridehexahydrate (FeCl₃.6H₂O, AR), 1,4-benzenedicarboxylic acid (H₂BDC, AR), dicyandiamide (DCD, AR), and N,N-dimethylformamide (DMF, AR) were purchased from Damao (Tianjin, China), Kefeng (Shanghai, China), Guangfu (Tianjin, China) and Kermel®(Tianjin, China), respectively. The isopropanol (C₃H₈O, GPLC) was obtained from Kermel (Tianjin, China). The Nafion solution (5 wt.%) was produced by Dupont (DE, USA). The deionized water (18.2 mW/cm) was produced by a Millipore Elix water purification system (Millipore, USA).

2.2 Sample preparation

The Fe-N-C catalyst was synthesized as reported in Ref. [32]. First, the FeCl₃.6H₂O (4 mmol) and H₂BDC (4 mmol) were dissolved in 200 mL DMF. After vigorous stirring at 150°C for 12 h, the brown product was collected by centrifugal separation. Then, the precursor, denoted as the MIL-101(Fe), was obtained after being washed with DMF for three times. Secondly, the precursor was dried in vacuum at 60°C for 12 h. Thirdly, the MIL-101(Fe) (0.156 g) was added into the 100 mL ethanol solution with 0.03125 g DCD dissolved. After stirring at 80°C for 4 h, the pink powder was obtained as the ethanol volatizing out. Finally, the as-prepared sample was heated to 800°C at Ar atmosphere for 2 h and at a heating rate of 5°C/min. Afterward, the black powder was obtained by finely grounding the material obtained.

2.3 Catalyst ink preparation

The homogeneous catalyst ink was prepared by sonicating the mixture of 5 mg of catalyst, 1 mL of isopropanol, and 50 µL of Nafion solution (5 wt.%) for 0.5 h. A thin film catalyst layer (0.4 mg/cm² catalyst) was obtained by depositing 19.77 µL of catalyst ink onto the rotating ring disk electrode (RRDE, 0.2471 cm²).

2.4 RRDE tests

The electrochemical tests were conducted on an electrochemical workstation (Pine, USA) in a three-electrode system, with the rotating ring-disk electrode (RRDE, PINE AFE7R9GCPT) as the working electrode, the carbon rod as the counter electrode, and the saturated calomel electrode. Specifically, all the potentials in this paper are referred to the reversible hydrogen electrode (RHE). Before the tests, the N₂ was bubbled for at least 30 min to ensure the saturated state of the electrolyte. Then, the cyclic voltammetry (CV, 0.05 –1.2 V, 100 mV/s, 30 cycles) was conducted to fully activate the catalysts. Afterward, rotating ring-disk electrode experiments (RRDE) were performed in O₂ saturated electrolyte at 1600 r/min. During the RRDE experiments, the disk potential was ranged from 1.1 to 0.05 V at a rate of 10 mV/s. Besides, the coaxial-ring potentials were kept at 1.150 V. During the ORR process, the H₂O₂ intermediate would diffuse from the disk electrode to the Pt ring, and be oxidized at the Pt ring electrode. The ring currents and the disk currents were all corrected by the N₂-saturated current.

The H₂O₂ oxidation test was conducted by RRDE under 0.8 V (1.5 wt. % H₂O₂, 0.1 mol/L HClO₄, 2500 r/min, 10 h). To keep the content of H₂O₂ constant, the electrolyte was replaced after 5 h test. After the H₂O₂ oxidation test, the electrolyte was replaced with deionized water. To remove residual H₂O₂ in the catalyst thin film completely, the rotating ring-disk electrode was kept at a rotate speed of 200 r/min for 10 h. Finally, RRDE was conducted to evaluate the performance of catalyst. Similar to the H₂O₂ oxidation test, the control experiment was conducted in the electrolyte without H₂O₂.

The transferred electron number (n) and the yield of H₂O₂ can be calculated based on Eqs. (1) and (2) [33].

(1)n=\mathrm{4}\times {\displaystyle \frac{I_{\mathrm{D}}}{I_{\mathrm{D}}+{\displaystyle \frac{I_{\mathrm{R}}}{N}}}},

(2)\mathrm{\%}\left({\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}\right)=\mathrm{200}\times {\displaystyle \frac{{\displaystyle \frac{I_{\mathrm{R}}}{N}}}{I_{\mathrm{D}}+I_{\mathrm{R}}/N}}\times \mathrm{100}\mathrm{\%},

where I_D and I_R are the disk current and the ring current, respectively. The current collection efficiency of the Pt ring, N, is 0.37 in this paper. n is the transferred electron number of per O₂ during the ORR process.

2.5 Potentiostatic tests

To characterize the chemical stability of the catalysts, the stability tests were conducted in O₂ saturated 0.1 mol/L HClO₄. The disk electrode was held at a constant potential of 0.4 V, 0.6 V, 0.8 V, and 1.0 V for 192 h (8 days). The performance of the catalysts was evaluated by RRDE after every 48 h.

2.6 Characterizations

The morphology of the catalysts was characterized by the field-emission scanning electron microscopy (FE-SEM, JSM-7800, JEOL, Japen) and the transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). The XRD was conducted on an X-ray diffractometer (XRD, Empyrean-100, Panaco, the Netherlands) with a 2q range of 10°– 90° at 5 (°)/min. The inductively coupled plasma-mass spectrometry (ICP-MS 7300DV, Perkinelmer, USA) was used to quantify the Fe ions in the electrolyte. The XPS data were obtained by the X-ray photoelectron spectrometer (XPS; ESCALAB 250Xi, Thermo Fisher, USA).

3 Results and discussion

As shown in Fig. 2, the Fe-N-C catalyst is successfully synthesized. In Fig. 2(a), sufficient porous microstructures can be observed, which is paralleled by the apparent ‘wrinkles’ in the TEM image (Fig. 2(b)). Besides, the well dispersed Fe particles manifest that the catalysts are uniformly synthesized. Previous studies suggest that the Fe-N_x-C coordinations (Fe-N₄-C, Fe-N₂-C), the N-C_x (pyridinic-N, graphitic-N) as well as the Fe particles encapsulated in graphene carbon shells constitute the active sites of the as-prepared Fe-N-C catalyst [32,34,35]. In addition, the polarization curves and the ring currents of pristine catalysts are displayed in Figs. 2(c)–2(d). As can be seen, the half-wave potentials (E_1/2), as one of the criterions of the catalytic activity, are observed at 0.724 V in 0.1 mol/L HClO₄ and 0.854 V in the 0.1 mol/L KOH, respectively. Moreover, the ring currents, which are commonly used to evaluate the intrinsic H₂O₂ generation, were characterized by RRDE. In this work, the stability tests were conducted by holding the electrode at several constant potentials (0.4 V, 0.6 V, 0.8 V, and 1.0 V) in O₂ saturated 0.1 mol/L HClO₄ for 192 h (8 days). During the tests, after every 48 h test, the ring current and the disk current are recorded to evaluate the H₂O₂ releasement and ORR activity, respectively. In this context, the pristine catalysts are noted as P-C. Furthermore, the catalysts being kept at a constant potential for 192 h at 0.4 V, 0.6 V, 0.8 V, and 1.0 V are labeled as C-0.4 V, C-0.6 V, C-0.8 V, and C-1.0 V, respectively.

The RRDE results during the stability tests are displayed in Figs. 3 and 4. In Figs. 3(a), 3(c), and 3(d), the ring currents of C-0.4 V, C-0.8 V, and C-1.0 V present a sustained increased tendency throughout the tests. This indicates that the highest ring current will not be observed until the catalysts are being kept at a constant potential for 192 h. However, as for C-0.6 V, quite different phenomena are observed. As displayed in Fig. 3(b), the ring currents keep increasing at the initial 96 h, and reach the peak current of 0.07 mA at 96 h. As the stability tests further processed, the reduced ring currents are observed. To make an intuitive comparison, the max ring currents of the catalysts tested at various potentials are summarized in Fig. 4(a). It is apparently observed that the max ring current of C-0.6 V is much higher (0.07 mA) than that of C-0.4 V, C-0.8 V, and C-1.0 V. In parallel, the disk currents of the catalysts before and after the tests in Fig. 4(b) suggest that the catalyst being kept at 0.6 V suffers the most severe activity loss, with the E_1/2 decreasing from 0.724 V to 0.479 V. However, as for C-0.4 V, C-0.8 V, and C-1.0 V, only a mild performance loss is detected. Besides, the H₂O₂ yield (%(H₂O₂)) and the transferred electron number (n) are also calculated based on the RRDE results. Unlike the tendency that mentioned above, a continuously increased %(H₂O₂) (Fig. S1) and a consistently decreased n (Fig. S2) are observed. As expected, C-0.6 V exhibited the highest %(H₂O₂) and the lowest n, indicating the severe degradation of active sites.

The results above manifest that the catalyst being kept at a potential of 0.6 V exhibits the highest H₂O₂ release rate and the fastest ORR activity loss. According to the relevant publications, it is proposed that two factors could give rise to the high ring currents of C-0.6 V. First, due to the combined effects of the electrochemical oxidation and the chemical oxidation (induced by the oxidative attack of H₂O₂ or radicals), the most severe carbon corrosion happens in C-0.6 V. Relevant reports show that the large amount of oxygen-containing functional groups formed are the active sites of the undesirable 2e⁻ ORR pathway, facilitating the formation of H₂O₂ [36]. Secondly, the apparent performance loss indicates that the active sites in C-0.6 V are severely deactivated. Then, the interaction between O₂ adsorbed and the active sites decayed might be not strong enough for the cleavage of O-O bonds, facilitating the formation of H₂O₂. Finally, due to the degradation of active sites, less H₂O⁻ intermediates would be further reduced into H₂O by another 2e⁻. Consequently, the factors above directly contribute to the highest ring current of C-0.6 V. To further verify the rationality of conjectures above, the H₂O₂ oxidation test, SEM, TEM, XRD, ICP-MS and XPS were conducted.

To further confirm the destructive effect of H₂O₂, control experiments are also conducted. The H₂O₂ oxidation test was conducted by RRDE at 0.8 V (1.5 wt. % H₂O₂, 0.1 mol/L HClO₄, 2500 r/min, 10 h). As depicted in Fig. 5, the catalyst suffered H₂O₂ oxidation test exhibits an apparent performance loss (E_1/2 = 0.53 V) and a significant increase of H₂O₂ release rate. However, as for the catalyst without being exposed to H₂O₂, an alleviated catalyst degradation (E_1/2 = 0.61 V) is observed. The H₂O₂ oxidation test uncovers that the oxidation effect of H₂O₂ is the major cause of Fe-N-C catalyst degradation.

The, the morphology of the aged catalysts was characterized. For references, the SEM and TEM images of the fresh catalysts were shown in Fig. 2, in which the micro/meso-pores and the clear carbon edges could be observed. Accordingly, the TEM images of the catalysts aged at various potentials are shown in Fig. 6. In Fig. 6(a), some vacancies, formed by the leached Fe clusters, are observed in C-0.4 V. More remarkably, as for C-0.6 V, a large amount of vacancies and the blurring boundaries are observed (Fig. 6(b) and Fig. S3), manifesting the severe Fe loss and unexpected carbon corrosion. Besides, to confirm the atomic structure and vacancies in the aged catalyst, the high-resolution TEM, the HAADF image, and the elemental maps of C-0.6 V are also provided in Fig. S4. Furthermore, the SEM images in Fig. S5 show that the original loose and porosity texture of carbon matrix is replaced with a smooth surface, indicating the severe carbon corrosion on the top surface of catalysts. However, the morphology of C-0.8 V and C-1.0 V are well preserved, with a clear carbon margins (Figs. 6(c) and 6(d)). The TEM images prove that C-0.6 V suffered from a most severe carbon corrosion, due to the electrochemical oxidation and the severe chemical oxidation by H₂O₂/radicals. It is well known that when the catalysts are kept at a relative high potential, the electrochemical oxidation of carbon would happen. As for E>0.2 V, the oxygen-containing carbon groups, for instance, –COOH, –CHO, or –C-OH, would be formed on the surface of carbon matrix. As the potential further increased to 0.9 V, CO or CO₂ would be generated [37]. Essentially, it is also shown that the chemical oxidation of carbon phases would also occur while being exposed to H₂O₂, with the oxygen-containing functional groups or the CO, CO₂ being formed [23,38]. It is important to discriminate the electrochemical oxidation and the chemical oxidation of carbon phases here. Noted that, the well remained carbon matrixes in C-0.8 V and C-1.0 V reveal that the high potential of 0.8 V or 1.0 V has a mild effect on carbon oxidation of Fe-N-C catalyst. Therefore, compared with the electrochemical potential, the attack of the aggressive H₂O₂/radicals would induce a much severer carbon corrosion in Fe-N-C catalyst. According to the RRDE results above, the catalyst being kept at 0.6 V is exposed to the largest amount of H₂O₂/radicals during the tests. Therefore, the most severe carbon corrosion is observed in C-0.6 V.

To investigate the crystal phase change and the dementalization of the catalysts, the X-ray diffraction (XRD) and the Fe release rate were characterized. As shown in Fig. 7(a), the XRD spectrums of the P-C and C-0.6 V are recorded. Compared with the fresh catalysts, the Fe₃C (22.07°, 44.66°) and the Fe₂O₃ (26.4°) in C-0.6 V are well remained [32]. Notably, in the fresh catalyst, the peak at 60° and 35.61° which are assigned to FeO and Fe₂O₃, disappear. Besides, a new peak is observed at 36.9°, indicating the formation of Fe₃O₄. In addition, according to the Scherrer equation, the average crystalline size increases from 146 nm to 191 nm after the extensive stability test at 0.6 V. The XRD results show that, after the extended stability test under 0.6 V, Fe in the catalyst is oxidized and dissolved into the electrolyte. Although most of the dissolved Fe ions are leached out, a small fraction of them would be reprecipitated in the form of iron oxides with larger size (Fe₃O₄) [39,40].

In parallel, the Fe release rate was quantified. During the stability tests, the electrolyte was replaced after every 48 h. The content of Fe in the replaced electrolyte was quantified by ICP-MS. Then, the Fe released rate was calculated by quantifying Fe in the electrolyte. Consistent with the results above, in Fig. 7(b), C-0.6 V manifests a much higher Fe release rate than other samples. Two main reasons would be responsible for it. For one thing evidenced by TEM images in Fig. 6 and the XPS results, the most severe carbon corrosion happened at 0.6 V. The severe carbon corrosion would give rise to the destruction of catalyst, leading to the loss of Fe. Besides, the carbon corrosion would expose more Fe atoms to the acid electrolyte, aggravating the demetallization of the catalyst. For another, although the Fe³⁺ is the dominative state of Fe in the catalysts at the electrochemical potential of 0.6 V, there is still a small part of high-soluble Fe²⁺ in the catalysts [23]. Therefore, due to the combined effect of the carbon oxidation and the dissolution of Fe²⁺, at the first 100 h, the highest Fe ion release rate is observed. As the stability tests further progressed, most Fe atoms are exposed and the high-soluble Fe²⁺ is leached out, leading to a decreased Fe release rate after 100 h tests. It should be remarked that the demetalization of the catalysts would directly induce the destruction of the active sites. Worse still, the released Fe ions would catalyze the decomposition of H₂O₂, accelerating the radicals release rate. Undoubtedly, the highest Fe ion release rate at 0.6 V is detrimental to the stability of Fe-N-C catalyst.

BET, Raman, and XPS were conducted for further investigating the chemical state of catalyst. To prepare sufficient samples for the BET test, the gas diffusion electrode (GDE) was prepared by brushing Fe-N-C catalyst on the gas diffusion layer (GDL) with a catalyst loading of 4 mg/cm². Then, the stability test was conducted in a three-electrode system with GDE as the working electrode. After being kept at a constant potential for 192 h, the GDE was carefully washed with deionized water and then dried in a vacuum oven at 60°C. Afterward, the surface area of the catalyst was calculated by deducting the GDL from the GDE. As Fig. S6 shows, after being kept at 0.6 V for 192 h, the surface area of catalyst decreased by 57.23%. The significant decrease of surface area indicates the severe carbon corrosion in C-0.6 V. In parallel, Raman spectroscopy was also recorded. Commonly, Raman spectroscopy is used to detect the electronic structure and defects on carbon matrix. The D band, at 1350 cm⁻¹, mirrors the existence of the defects and the disordered crystal structures. On the contrary, the G band, at 1580 cm⁻¹, is formed by the graphitic in plane vibrations [41]. As displayed in Fig. 8, an apparent increased D/G ratio of 1.056 (the inset of Fig. 8) is observed, ascribing to the formation of defects of the graphitic carbon. In addition, compared with the pristine catalysts, C-1.0 V exhibites a mildly increased D/G ratio, implying that, the high potential of 1.0 V does not give rise to severe carbon corrosion.

To characterize the surface electronic configuration of catalyst, X-ray photoelectron spectroscopy (XPS) was recorded. In detail, after the RRDE test, the catalyst thin film on working electrode was washed with deionized water carefully. After natural drying, the catalyst thin film was stuck down by the conducting resin. Then, XPS was conducted on ESCALAB 250Xi (Thermo Scientific, 200 W, 10 kV, 20 mA) with K rays from Al target as the excitation source. As reported, all the energy spectrum data were corrected by the binding energy of C_1s (284.8 eV). The X-ray photoelectron spectroscopy was analyzed by using the XPS peak 4.1 software. Primarily, the broad scan results are summarized in Table S1, in which the absence of Fe signal is observed in the aged catalysts. Considering the fact that the XPS was used to probe the element information within a thickness of several atomic layers, it is proposed that most Fe atoms on the top surface of catalyst are dissolved into the acidic electrolyte. As a result, the little Fe contentment on the top surface of the catalysts leads to the absence of Fe signals. Additionally, the obvious decrease in C and N as well as the apparent increased O are observed in the broad scan. To get a deeper insight into the chemical state of carbon species, the detailed deconvolution of high-resolution C_1s spectra are conducted. As shown in Fig. 9(a), the C_1s curve of the fresh catalysts can be deconvoluted into 5 bonds, corresponding to the C-C (284.5 eV), the C-O (286.1 eV), the C= O (288.7 eV), the C-Fe (283.64 eV), and the C-N (287.78 eV) [42]. Specifically, due to the existence of Nafion ionomer in the aged catalysts, the peak at 291 eV is deconvoluted into three peaks at 287.3 eV, 291.3 eV, and 292.9 eV, which should be the index to the C-F, C-F₂, and C-F₃. Accordingly, the quantitative analyses are summarized in Table S2. As can be seen, compared with the P-C, C-0.6 V exhibits the most severe carbon oxidation: the C-C (the carbon in graphite) decreases by 14.4%, the C-O increases by 23%, and C=O increases by 5.86%. Consistent with the broad scan results in Table S1, the oxygen-to-carbon ratio (O/C) of C-0.6 V increases from 0.1191 to 0.9195, indicating the aggravated carbon oxidation. In addition, compared with C-0.6 V (50.43%), a much higher content of the graphite carbon is observed in C-0.4 V (55.17%), C-0.8 V (68.12%), and C-1.0 V (69.29%), demonstrating the alleviated carbon oxidation of the catalysts with lower H₂O₂ or radicals release rate. Moreover, the O_1s signals were also analyzed. As shown in Fig. S7, the O_1s is deconvoluted into 2 peaks: the C=O (carbonyl, carboxyl) at 531.2 eV and the C-O (epoxy, hydroxyl) at 533.0 eV [42]. The results manifest that the C-O content increases from 14.72% to 57.36% in C-0.6 V. However, C-0.8 V, C-1.0 V and C-0.4 V show a lower C-O of 33.51%, 33.27% and 15.79%, respectively. The results above confirm that the carbon species in C-0.6 V suffer severe oxidation.

To identify the chemical states of N in detail, the N_1s spectra were decomposed into different peaks according to different chemical states of N. As shown in Fig. S8, the N_1s spectra are deconvoluted into 4 peaks: pyridinic N (398.7eV), pyrrolic N (400.4 eV), graphitic N (401.3 eV), and oxidized N (403.8 eV) [43]. Note that the apparent decrease in the pyridinic N and the graphitic N is observed in C-0.4 V and C-0.6 V. However, C-0.8 V and C-1.0 V manifest a higher residual N moieties. As reported, the total content of the pyridinic N and the graphitic N direct influences the ORR activity of the catalysts [25]. As discussed above, the catalyst being kept at a content potential of 0.6 V suffers the most severe H₂O₂ or ROS attack. Therefore, C-0.6 V displays a pyridinic N and the graphitic N reservation of 49.82%, with the most significant performance loss. Additionally, C-0.4 V, C-0.8 V, and C-1.0 V manifest the total content of pyridinic N and the graphitic N of 53.26%, 77.47% and 77.64%, respectively. It is apparent that the results of N_1s are in good consistent with the ORR performance variations above.

The results above confirm that compared with other samples (C-0.4 V, C-0.8 V, and C-1.0 V), C-0.6 V experiences the most severe carbon oxidation and dementalization, due to the combined effects of electrochemical oxidation and chemical oxidation (by H₂O₂/radicals). Moreover, in this case, compared with the electrochemical potential, the highest H₂O₂ released rate would more likely induce the catalyst degradation.

4 Conclusions

In this case, the by-product H₂O₂ of Fe-N-C catalyst during the stability tests at different potentials was quantified. The results show that compared with the loading voltage of 0.4 V, 0.8 V and 1.0 V, the catalyst being kept at a loading voltage of 0.6 V exhibits the highest H₂O₂ emission. It is the combined effect of the electrochemical oxidation and the chemical oxidation (H₂O₂/radicals attack, dominated) that lead to the oxidation of the carbon matrix and the decay of the active sites. In turn, the carbon corrosion would trigger the formation of the oxygen-containing functional groups, which is proved to be the active sites of the 2e⁻ ORR pathway. Besides, the decayed active sites would more likely react with O₂ in a 2e⁻ pathway and hinder the further reduction of the H₂O₂ intermediate. Consequently, the highest H₂O₂ release rate is observed in the catalysts being kept at 0.6 V. Finally, it is proposed that, a relatively higher load voltage (for instance, 0.7 V) might be appropriate for mitigating the amount of H₂O₂ or radicals in the catalyst layer.

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