Highly efficient iron phthalocyanine based porous carbon electrocatalysts for the oxygen reduction reaction

Abstract

Porous carbon (PC) materials with a large surface area and uniform pore sizes have been used as novel electrocatalysts to catalyze the oxygen reduction reaction (ORR). In this work, iron phthalocyanine (FePc) and magnesium oxide (MgO) were used as the precursors and hard-template, respectively, to fabricate FePc-based porous carbon non-noble metal electrocatalysts (NNMEs). The prepared FePc-based porous carbon electrocatalysts possess a high surface area of 555 m² g⁻¹ and a dual mesoporous structure with mean pore sizes of 3.6 nm and 32 nm, and display comparable ORR activity but superior durability than a commercial Pt/C electrocatalyst in alkaline solution. The excellent catalytic performance of FePc-based porous carbon electrocatalysts may be attributed to the large surface area, high density of iron based nano-particles (NPs) encapsulated in carbon layers and high content of doped nitrogen species.

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Introduction

Electrocatalysts, as the critical component for oxygen reduction, may dramatically influence the performance of proton exchange membrane fuel cells (PEMFCs). Up to date, highly efficient electrocatalysts have been mainly based on platinum or its alloy.^1,2 However, the prohibitively high cost and scarcity of platinum seriously limit the large-scale commercial application of platinum based electrocatalysts. Therefore, the design and synthesis of non-noble metal electrocatalysts (NNMEs) for Pt alternatives has been investigated for decades. So far, NNMEs have been classified to several types, including heteroatom-doped carbon,^3–5 MeN₄-macrocycles,^6–8 metal oxides^9,10 or carbides^11–13 and others.^14–16 Among these NNMEs MeN₄-macrocycles are considered to be one of the most promising alternatives to catalyze the oxygen reduction reaction (ORR) in both alkaline and acidic solutions. In 1964, Jasinski firstly reported that cobalt-phthalocyanine (CoPc) could display ORR activity in alkaline media.¹⁷ Since then, tremendous investigations on MeN₄-macrocycles as NNMEs were carried out to improve the ORR activity and stability in both alkaline and acidic media.¹⁸

However, MeN₄-macrocycles are easier to decompose under high temperature, which will result in low ORR performance and poor stability. Therefore, various carbon materials have been used to support MeN₄-macrocycles. This may effectively inhibit the decomposition of MeN₄-macrocycles during the pyrolysis process at high temperature and thus greatly enhance the ORR activity and stability. For instance, iron phthalocyanine (FePc), which is considered as one of the most promising MeN₄-macrocylces as precursors for NNMEs, has been supported on commercial carbon (Vulcan XC-72) and then thermally treated under Ar atmosphere at 700 °C.¹⁹ The obtained materials were found to be a well suitable traditional cathode materials in microbial fuel cells. Morozan and co-workers have modified FePc with different carbon nanotubes, like single-wall carbon nanotubes (SWCNTs), double-wall carbon nanotubes (DWCNTs) and multi-wall carbon nanotubes (MWCNTs) to boost ORR activity in both alkaline and acidic media.²⁰ Cao and co-researchers combined FePc with pyridine-functionalized single-walled CNTs (FePc-Py-CNTs) to exhibit high durability and electrocatalytic activity for ORR in 0.1 M KOH solution.²¹ Recently, FePc was supported on graphene nanosheets (GSs) and carbon-black (CB) EC600 (Ketjenblack) by Zhang and co-workers through a simple wet ball-milling method to increase the ORR activity in alkaline solution.²²

Additionally, large surface area of NNMEs plays a critical role to enhance the ORR catalytic performances through exposing more active sites and facilitating the mass transport of reactants and products.²³ However, MeN₄-macrocycles themselves generally do not possess enough porous system for ORR. Therefore, template-assisted method has to be used to obtain FePc based porous materials. As reported, mesoporous carbon vesicle (MCV) and ordered mesoporous carbon (OMC) were combined with FePc to modify the ORR activity and stability.^24–26 FePc-based ordered mesoporous carbon (Fe-N-OMC) electrocatalysts have been synthesized by introducing FePc precursors into the channels of mesoporous SBA-15, followed by the thermal pyrolysis in inert atmosphere and silica removal by hydrofluoric acid solution. Herein, we synthesize novel NNMEs fabricated with FePc and magnesium oxide (MgO) as a precursor and a hard template, respectively. After the removal of MgO by hydrochloric acid solution, FePc based novel porous carbon NNMEs (FePc-PC) were obtained. The large surface area of 555 m² g⁻¹ could be achieved through optimizing the pyrolysis temperature and the mass ratio of FePc to MgO. Compared with the commercial Pt/C electrocatalyst, our novel FePc-PC electrocatalysts exhibit comparable ORR activity and much better durability and tolerance to methanol in O₂-saturated 0.1 M KOH solution.

Experimental

Materials

The iron phthalocyanine (FePc) was purchased from Acros (Geel, Belgium). Magnesium oxide (MgO) nanoparticles was purchased from Aladdin Industrial Corporation (Shanghai, China). Dichloromethane and hydrochloric acid was purchased from Sinopharm Chemical Reagent Co., Ltd. (GR, Shanghai, China) and Jinfeng Chemical Reagents (Tianjin, China), respectively.

Preparation of FePc-PC materials

Typically, 100 mg of FePc was mixed with 400 mg of MgO nano-particles in 60 mL of dichloromethane, and then the suspension was continuously stirred for 20 min at room temperature. Finally, the suspension was evaporated to remove dichloromethane, followed by drying overnight at room temperature in the vacuum oven. The powders were collected and pyrolyzed at 900 °C for 2 h under flowing Ar atmosphere. The as-prepared samples were further treated with 2 M HCl to remove MgO, and then washed with ultrapure water till the filtrate becomes neutral. The final product was dried at 60 °C overnight, and named as FePc-PC (900), where 900 represents the pyrolysis temperature. For comparison, different mass ratio of FePc to MgO were prepared with the same method. In addition, FePc-PC was also pyrolyzed at the lower temperatures of 600, 700, or 800 °C and higher temperature like 1000 °C, respectively. Notably, the higher temperature such as 1000 °C resulted in so tiny amount of carbon materials that was very difficult to collect for further acid-leaching and electrochemical test.

Characterization

X-ray powder diffraction (XRD) were recorded on a PANalytical Empyrean-100 diffractometer with Cu K_α radiation at a scanning rate at 6° min⁻¹. N₂ adsorption–desorption was performed on MIC ASAP 2020. The specific surface area and pore size distribution was calculated by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. X-ray photoelectron spectroscopy (XPS) were carried out on a Thermo ESCALAB 250Xi photoelectron spectrometer. Transmission electron microscopy (TEM) was operated on a JEM-2100F at 200 kV. High resolution TEM (HRTEM) and element mapping were operated on FEI Tecnai G2 F30 at 300 kV.

Electrochemical analysis

The as-prepared samples were measured in a standard three-electrode electrochemical cells with a Hg/HgO as a reference electrode, the Pt net as a counter electrode and the catalyst-coated glassy carbon (GC) disk as a working electrode. The electrolyte (0.1 M KOH) was provided by ultrapure water (18.2 MΩ cm). Here, the catalyst ink was prepared to be 2 mg mL⁻¹ by mixing the catalyst with ethanol, ultrapure water and 5 wt% Nafion (V_ethanol : V_water : V_Nafion = 9 : 1 : 0.1), followed by ultrasonication for at least 10 min. Then 40 μL ink was dropped onto the GC disk. The geometric surface area of GC was 0.196 cm². The total mass loading of NNMEs was 0.4 mg cm⁻². For comparison, the Johnson Matthey 20 wt% Pt/C ink was prepared to be 1 mg mL⁻¹, and 20 μL was drawn and loaded onto the GC. As a result, the platinum loading was 20 μg_Pt cm⁻².

Firstly, cyclic voltammetry (CV) cures were evaluated to clean the surface of electrocatalysts by purging with N₂ at least 10 min in KOH solution. The cycling potential range was from −0.8 to 0.2 V (vs. Hg/HgO) and the scanning rate was 100 mV s⁻¹.

Secondly, a constant flow rate of oxygen was bubbled into alkaline solution at least 10 min to get a O₂-saturated atmosphere. ORR polarization curves of electrocatalysts with different rotation rates (625 to 2025 rpm) were carried out in O₂-saturated alkaline solution at a scanning rate of 5 mV s⁻¹ at the potential from −0.8 to 0.2 V (vs. Hg/HgO).

Thirdly, rotating ring disk electrode (RRDE) purchased from PINE instrument in USA was used to obtain ORR polarization curves in O₂-saturated 0.1 M KOH media from −0.8 to 0.2 V (vs. Hg/HgO). The rotation rate was 1600 rpm and the positive scanning rate was 5 mV s⁻¹. The ORR polarization curves were kept under a certain potential of 0.2 V (vs. Hg/HgO).

As for the accelerated durability test (ADT), the freshly prepared electrode was repeatedly scanned for 2000 cycles from −0.4 to 0.2 V (vs. Hg/HgO) at the scanning rate of 100 mV s⁻¹. The ORR polarization curves of FePc-PC (900) and commercial Pt/C were obtained after certain cycle numbers (0, 200, 400, 600, 1000, 1500 and 2000 cycles) to track the degradation trend.

The current–time (i–t) curves of FePc-PC (900) and 20 wt% Pt/C were measured at the constant potential of −0.3 V (vs. Hg/HgO) for 1000 s at a rotation rate of 1600 rpm in O₂-saturated 0.1 M KOH solution. And the 10% (v/v) methanol was injected at approximately 400 s to confirm the selectivity of the electrocatalysts between ORR and methanol oxidation.

Results and discussion

X-ray diffraction (XRD) in Fig. 1a presents the structure information of FePc-PC (900). The relatively sharp diffraction peaks at approximately 25° and 43° correspond to (0 0 2) and (1 0 1) index of graphitic carbon, suggesting the formation of graphitized structure of FePc-PC during pyrolysis under high-temperature.²⁷ Clearly, others located at 25.8°, 30.1°, 35.6°, 43.3°, 57.0° and 62.9° are identified to γ-Fe₂O₃ (JCPDS #39-1346).²⁸ The formation of Fe₂O₃ NPs should be caused by the reaction of Fe species in FePc with the tiny amount of oxygen, which may be absorbed in the solution used for the synthesis of FePc-PC or contained in the impure Ar used for the pyrolysis of FePc-PC. The TEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Fig. 1c–e) show that Fe₂O₃ NPs are highly dispersed on the porous carbon materials. As shown in Fig. 1c and d, many macropores (>100 nm) could be clearly observed in FePc-PC (900). These macropores should be created by the removal of hard-template of MgO with diameter larger than 100 nm (Fig. 1b). These Fe₂O₃ NPs should be tightly trapped within the carbon layers of porous carbon materials because they can not be dissolved even after the acid-leaching (Fig. 1c–e). The HRTEM image of FePc-PC (900) in Fig. 1f reveals that the lattice distance is 2.086 Å, well consistent with the lattice spacing of 2.089 Å of the plane (4 0 0) for γ-Fe₂O₃. This further confirms that iron oxides encapsulated in FePc-PC (900) should exist as γ phase.

Fig. 1 (a) XRD pattern of FePc-PC (900), (b) TEM image of MgO NPs, (c) and (d) TEM images of FePc-PC (900), (e) HADDF-STEM image and (f) HRTEM image of FePc-PC (900).

As shown in Fig. 2a, a large amount of NPs could be clearly observed. A small area in Fig. 2a was also selected for element mapping analysis. The results show that the elements of C, N and O are uniformly distributed in the selected area (Fig. 2b–d). Notably, the uniform distribution of N element within carbon matrix in Fig. 2c suggests that high content of N element have been successfully doped into carbon materials. The encapsulated NPs contains Fe element (Fig. 2e). Based on the analysis above, these NPs should be ascribed to γ-Fe₂O₃ coated by N-doped porous carbon materials because washing with hydrochloric acid solution can not remove them.

Fig. 2 HADDF-STEM image of FePc-PC (900) (a) and element mapping analyses of C (b), N (c), O (d) and Fe (e) of the selected area in (a).

The porosities of FePc-PC pyrolyzed at different temperatures (600 to 900 °C) were evaluated by N₂ adsorption–desorption characterization. As shown in Fig. 3, a type-IV isotherm was clearly observed over FePc-PC (900), which is the characteristic of mesoporous materials. In addition, the dual mesoporous structures were obtained over FePc-PC (900), with the mean pore size of 3.6 nm and 32 nm, respectively. The formation of small mesopores of 3.6–3.9 nm might be formed through the thermal cracking of organic groups in FePc precursors and then the further condensation of resulting carbon species. The formation of 32 nm mesopores might be created by the removal of iron oxides which are not well capsulated well by carbon layers and can be easily removed by HCl. Indeed, some of mesoporous carbon layers without iron oxides NPs were observed in the TEM images of FePc-PC (900) (Fig. 1d). The results in Table 1 suggest that the pyrolysis temperature markedly influences the surface area and the mesoporous structures of FePc-PC materials. When the pyrolysis temperature is 600 °C, the surface area is very low (28 m² g⁻¹), meanwhile, only relative large mesoporous structure of 34 nm was formed. However, if the pyrolysis temperature was elevated to 700 °C and above, large surface area was successfully achieved (523 and 555 m² g⁻¹, respectively), and the dual mesoporous structures were also obtained, with the mean pore size of 3.6–3.9 nm and 29–32 nm, respectively. The creation of dual mesoporous structures in FePc-PC (900) may make the active sites easily accessible for the reactants and thus are beneficial to improve ORR activity.^29,30

Fig. 3 (a) The N₂-sorption isotherm plot and (b) the corresponding pore size distribution curve of FePc-PC (900).

Table 1 The BET surface area and mean pore size of FePc-PC pyrolyzed at different temperatures from 600 to 900 °C

Electrocatalysts	SBET (m^2 g^−1)	Dave (nm)
FePc-PC (600)	28	34
FePc-PC (700)	555	3.9	31
FePc-PC (800)	523	3.9	29
FePc-PC (900)	555	3.6	32

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In addition, the chemical composition of FePc-PC (900) was determined by XPS analysis. It has been reported that during the pyrolysis macrocyclic nitrogen will be transformed to different species including graphitic, pyrrolic, Me–N, pyridine N or pyridin-N-oxide.^6,31,32 As shown in Fig. 4a, the bonding configurations of N in FePc-PC (900) reveals that N spectra can be classified to four types, including pyridinic N (398.3 eV), pyrrolic N or Fe–N (400.3 eV), graphitic N (401.1 eV) and pyridine-N-oxide (402.7 eV).³³ The overall N content of FePc-PC (900) is calculated to be about 3.9% with respect to C. The XPS spectra of Fe 2p species in FePc-PC (900) displayed two peaks at 724.3 eV and 710.9 eV, respectively. The two peaks could be reassigned as Fe 2p_1/2 and Fe 2p_3/2 of iron oxides, respectively.³¹ This further confirms the presence of iron oxide in FePc-PC, consistent with the results obtained from XRD and HRTEM characterizations.

Fig. 4 XPS spectra of FePc-PC (900): (a) N 1s and (b) Fe 2p.

Initially, ORR polarization curves of electrocatalysts prepared with different mass ratio of FePc to MgO were carried out in O₂-saturated 0.1 M KOH solution. When the mass ratio of FePc to MgO is 1 : 4, the electrocatalyst displays the best ORR performance (Fig. 5a). This may mean that at the mass ratio of FePc to MgO of 1 : 4, the number of exposed active sites for ORR activity is maximum. It has been reported that the pyrolysis temperature is very important to the synthesis of efficient electrocatalysts towards the ORR activity.²⁹ Therefore, we continued to investigate the effect of pyrolysis temperature on the ORR activity. As shown in Fig. 5b, the ORR activity increases markedly with the increase of pyrolysis temperature from 600 °C to 900 °C. Compared with FePc-PC (800), FePc-PC (900) shows the same onset potential (E_onset), but larger limited current density. It suggests that the electrocatalysts carbonized at higher temperature facilitate the better oxygen diffusion. In contrast, the half-wave potential (E_1/2) and E_onset of outer performed FePc-PC (900) is approximately 24 mV and 18 mV negative than that of the commercial Pt/C catalyst in 0.1 M KOH media, respectively (Fig. 5b).

Fig. 5 The ORR polarization plots of (a) FePc-PCs prepared with different mass ratio of FePc to MgO, and (b) FePc-PC pyrolyzed at different temperature with the mass ratio of FePc to MgO of 1 : 4. Reaction conditions: rotation rate, 1600 rpm; scanning rate, 5 mV s⁻¹; O₂-saturated 0.1 M KOH electrolyte.

To gain further understanding of FePc-PC (900) for the ORR process, the CV curves of FePc-PC (900) were firstly conducted in N₂- and O₂-saturated 0.1 M KOH at a scanning rate of 100 mV s⁻¹ (Fig. 6a). Compared with the voltammogram in N₂-saturated alkaline solution, a well-defined cathodic ORR peak at −0.18 V occurred in O₂-saturated 0.1 M KOH, showing the pronounced electrocatalytic performance of FePc-PC (900) towards the oxygen reduction. Then, the ORR polarization curves with different rotation rates were undertaken from 625 to 2025 rpm at the potential range from −0.8 to 0.2 V (vs. Hg/HgO) in O₂-saturated 0.1 M KOH media (Fig. 6b). Additionally, as shown in Fig. 6c, the Koutecky–Levich (K–L) plots were analyzed based on the data from various electrode potentials (−0.25, −0.35, −0.45 and −0.55 V). The slopes of their best linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the K–L equation as follows:

J_lim = 0.62nFD^2/3γ^−1/6C_oω^1/2 (1)

F is the Faraday constant (96 485 C mol⁻¹), D is the diffusion coefficient of O₂ (D = 1.93 × 10⁻⁵ cm² s⁻¹), γ is the kinetic viscosity of the solution (γ = 0.01 cm² s⁻¹), C is the concentration of O₂ dissolved in electrolyte (C = 1.26 × 10⁻⁶ mol cm⁻³), and ω is the electrode rotation speed.³⁴ The average transfer number (n) was calculated to be 3.97 based on eqn (1), suggesting that FePc-PC (900) favored a four-electron oxygen reduction process, very similar to that of commercial Pt/C (n = 4). In addition, we also used a RRDE to discuss the electrocatalytic property of FePc-PC (900) with a constant ring potential of 0.2 V (vs. Hg/HgO) to make sure whether all the HO₂⁻ could be oxidized during the test. Herein, the % HO₂⁻ and n were calculated by the followed equations:where I_d is the disk current, I_r is the ring current, and N is the ring collection efficiency, set by the manufacturer as 0.37. The average n number for FePc-PC (900) was determined to be 3.94 from −0.8 to 0 V (vs. Hg/HgO), well consistent with the average n number of 3.97 based on the K–L plots. Here, the number (3.94) of FePc-PC (900) was very close to 3.97 of 20 wt% Pt/C (Fig. 6d). The HO₂⁻% for FePc-PC (900) was lower than 5.3%, which was lightly higher than that of commercial Pt/C (Fig. 6d). The results above indicate that the ORR reaction catalyzed by FePc-PC (900) proceeds mainly through a four electron reaction to achieve H₂O.

Fig. 6 (a) CV curves for FePc-PC (900) in O₂- and N₂-saturated 0.1 M KOH aqueous solution at a scan rate of 100 mV s⁻¹; (b) ORR polarization curves of FePc-PC (900) with various rotation rates from 625 to 2025 rpm in O₂-saturated alkaline solution; (c) K–L plots of FePc-PC (900) based on the data from (b); and (d) RRDE and n number of FePc-PC (900) and 20 wt% Pt/C at the rotation rate of 1600 rpm in O₂-saturated 0.1 M KOH. The Pt ring electrode was held at 0.2 V (vs. Hg/HgO).

The result for accelerated durability test (ADT) of FePc-PC (900) and commercial Pt/C were carried out in O₂-saturated 0.1 M KOH media (Fig. 7a and b). ORR polarization curves were shown after certain cycles (0, 200, 400, 600, 1000, 1500 and 2000) in O₂-saturated 0.1 M KOH. As shown in Fig. 7c the normalized current density of FePc-PC (900) lost 39% after running for 2000 cycles at the potential of 0 V (vs. Hg/HgO). However, under the same test conditions, Pt/C shows a more serious lost of 62%. It is clear that FePc-PC (900) electrocatalyst displays much better durability than commercial Pt/C in 0.1 M KOH solution (Fig. 7c). TEM characterization of FePc-PC (900) showed that after the ADT iron oxide NPs were still encapsulated within the carbon layers. The excellent stability of metal oxides NPs should be responsible to the excellent durability of FePc-PC in the alkaline media (Fig. 8).

Fig. 7 The ORR polarization curves of FePc-PC (900) electrocatalyst (a) and 20 wt% Pt/C (b) obtained at various potential cycles (0, 200, 400, 600, 1000, 1500 and 2000) at the potential range from −0.8 to 0.2 V (vs. Hg/HgO) at a scanning rate of 5 mV s⁻¹ in O₂-saturated 0.1 M KOH. The rotation rate was 1600 rpm; (c) the current density degradation of FePc-PC (900) and commercial Pt/C obtained at 0 V (vs. Hg/HgO) based on the potential cycling process at the potential of −0.4 to 0.2 V at a scanning rate of 100 mV s⁻¹; and (d) chronoamperometric responses of FePc-PC (900) and Pt/C by adding addition of 10% (v/v) methanol into O₂-saturated 0.1 M KOH at −0.3 V (vs. Hg/HgO) at 1600 rpm.

Fig. 8 The TEM images of FePc-PC (900) after the ADT for 2000 cycles.

Good tolerance towards methanol is another major concern for cathode electrocatalysts. Here, we continued to examine the effect of methanol crossover on the cathode behaviour. As shown in Fig. 7d, FePc-PC (900) and 20 wt% Pt/C against the oxidation of methanol were conducted in 0.1 M KOH in the presence of 10% (v/v) methanol injected at certain time. For FePc-PC (900), no obvious changes were observed in the current density. While, for 20 wt% Pt/C, a dramatic decay happened after the injection of methanol into alkaline solution. This might be explained that when methanol was injected, an obvious methanol oxidation reaction occurred on commercial Pt/C, however, methanol oxidation reaction was difficult to be catalyzed by our electrocatalyst FePc-PC (900), and thus ORR reaction prevailed. Therefore, FePc-PC (900) possessed a higher ORR selectivity and greater tolerance to methanol crossover than that of Pt/C electrocatalyst in alkaline environment.³⁵

Conclusions

In summary, we have successfully synthesized FePc based ORR electrocatalysts by a facile hard-template method. The prepared FePc-PC (900) electrocatalyst possesses large surface area, and dual mesoporous structure with the mean pore size of 3.6 nm and 32 nm. In the ORR test, FePc-PC (900) displays comparable ORR activity almost via four-electron pathway in comparison to commercial Pt/C in 0.1 M KOH solution. The excellent ORR performance was attributed to the high surface area and sufficient active sites primarily from N-doped carbon matrix and iron based species. Additionally, FePc-PC (900) exhibits much better stability and stronger tolerance against methanol crossover than 20 wt% Pt/C in alkaline media. This work highlight the synthesis strategy by using MgO as the hard-template to synthesize highly porous carbon materials as novel NNMEs for the ORR.

Acknowledgements

The work was financially supported by China National Natural Science Foundation (No. 21473186, 21561024 and 21503081) and by the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA09030103).

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