The fuel cell as a sustainable and clear energy technology has been seriously investigated to deal with continuously increasing global energy and environmental problems. Nonetheless, the performance of fuel cells was mainly limited by the intrinsic characteristic of sluggish kinetics for oxygen reduction reaction (ORR) on the cathode. Pt-based materials were recognized as the state-of-the-art electrocatalysts for ORR [1, 2]. Unfortunately, they had disadvantages of limited resource reserve, high cost, poor durability, and huge difficulty to recycle [3-5], which significantly hinder the large-scale application of fuel cell. Therefore, developing a low-cost ORR electrocatalyst with comparable performance to Pt-based materials is critically important for large-scale applications of this new energy devices [6, 7].

In recent years, heteroatom-doped carbon materials (HCM) have been developed as greatly promising ORR electrocatalysts due to their unique advantages of low cost, excellent catalytic properties, long-term stability and excellent resistance to methanol crossover effects [8-10]. The heteroatom doping strategy has been proved effectively to improve the ORR eletrocatalytic activity of carbon materials by controllable manipulation of the electronic delocalization of the carbon framework. Some recent reports suggested that carbon materials mixed with a variety of elements (N, B, S and P etc.) had better catalytic activity than the single atom doped carbon because of synergetic effects of co-doping [11-13]. In particular, the doped carbon materials decorated with transition metal(s) such as Co and (or) Fe were identified to be the most active non-noble metal catalysts [14]. Great efforts have been devoted to pursuing efficient fabrication strategies in order to obtain advanced HCM materials with highly open structure and appropriate doping location. Many previously reported results suggested that the choice of precursor and doping method should play a much more important role in intrinsic introducing active sites of HCM materials [7, 15-17].

For traditional synthesis strategies, it is still very difficult to obtain the co-doped carbon materials with superior nanostructure simultaneously and effective heteroatom doping states. Recently, Watanabe's group [18, 19] demonstrated a neutralization strategy between amines and acids to form novel protic salt complexes with controllable element composition which can be directly used as ideal precursors for fabricating a series of heteroatoms-doped carbon materials. Unfortunately, the HCM materials generated from directly pyrolysis of these complexes usually possess low surface area and a micropore-dominated nanostructure, which extremely weaken the utilization of active sites and mass transfer during electrocatalytic process. Han's group recently developed a novel template synthesis strategy for efficient transformation of biomass to high-quality graphene [20]. Although the graphene fabricated by this method has made it more possible to facilely achieve a highly-opened carbon skeleton, but can't provide excellent electrochemical activity for oxygen reduction [16, 21-23].

Herein, we successfully combined the FeCl₃ template synthesis strategy and novel protic salt precursors of P-phenylenediamine sulphate (PPS) to achieve N, S, Fe- tridoped mesoporous carbon nanosheets (NSFC) for oxygen reduction electrocatalysis. We have achieved the simultaneous optimization of the microscopic structure and surface functionality of the catalyst by controlling the ratio of FeCl₃ template and PPS. As a non-precious metal electrocatalyst, the optimized NSFC-3 showed the nearly catalytic ability and selectivity, while superior excellent resistance to crossover effects to commercial 30 wt% Pt/C catalysts in KOH solution (0.1 mol/L).

P-phenylenediamine (2.16 g, PPDA) dissolved in ethanol (40 mL) was gradually added into dilute sulfuric acid (4 g, 98 wt%, 20 mL water) under a N₂ atmosphere and kept in an ice-water mixture. The mixture was then stirred for 2 h at room temperature. Finally the solvent was removed with heating at 80 ℃ for 24 h, giving a pink solid which named PPS.

N, S-codoped porous carbon (NSPC) was prepared by heating PPS at 900 ℃ for 2 h with ramp rate of 3 ℃/min in N₂.

PPS was directly mixed with FeCl₃ template according to 1:1, 1:3, 1:5 (PPS: FeCl₃ mass ratio) by grounding in an agate mortar for 5 min, then the mixture was dried at 80 ℃ for 24 h in porcelain boat and resulted to black solid. Then the mixture was heated at 900 ℃ for 2 h with the ramp rate of 3 ℃/min in N₂. The resultant composite was etched with a HCl solution (2 mol/L) at 80 ℃ for 8 h with the stirring to remove the superfluous Fe. NSFC-X (X = the mass ratio of FeCl₃ template and PPS, X = 1, 3, 5) was obtained after drying overnight at 80 ℃.

The X-ray diffraction (XRD) instrument was examined at 40 kV and 40 mA (BRUKER D8, Cu K_α) to analyze composition. X-ray photoelectron spectroscopy (XPS) measurements were carried out on Kratos AXIS Ultra spectrometer with a source gun of Al K_α and spot size of 400 μm. The morphology of as-prepared catalysts was investigated by scanning electron microscopy (SEM) (Hitachi S-4800), transmission electron microscopy (TEM) (Philips Tecnai 12). The Brunaue-Emmett-Teller (BET) was characterized with a Micrometrics ASAP2020 analyzer (USA) at –196 ℃ to get specific surface area, pore size distribution, and pore volume of materials.

The all cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ring-disk electrode (RRDE) were tested using the catalyst ink which was prepared from 2.0 mg of prepared catalyst or commercial 30 wt% Pt/C catalyst dispersed in 1.0 mL Nafion ethanol solution (0.05 wt%) by sonication for 1 h. Then, 25.0 μL of the prepared catalyst ink or 12.5 μL of commercial 30 wt% Pt/C ink was dropped on a RDE glass carbon electrode with a diameter of 5.00 mm to prepare the working RDE electrode. And 31.5 μL of the prepared catalyst ink or 16.0 μL of commercial 30 wt% Pt/C ink was dropped on an RRDE glass carbon electrode with a diameter of 5.61 mm to prepare the working RRDE electrode. Electrochemical measurements were conducted by a three-electrode cell and used saturated calomel electrode (SCE) and a platinum sheet as the reference and counter electrodes, respectively. 0.1 mol/L KOH electrolyte was used to carry out CV, RDE, and RRDE tests on a CHI760D electrochemical workstation assembled with MSR Electrode Rotator (Pine Research Instrumentation) at room temperature. The obtained electrochemical curves in O₂-saturated electrolyte have been corrected by subtracting the obtained electrochemical curves in N₂-saturated electrolyte, considering the contribution of the electrical double layer capacity for current response. The four-electron selectivity was evaluated based on the electron transfer number (n) and HO₂⁻ yield, calculated from the following equations.

(1)

(2)

Here, I_D and I_R are the disk and ring currents, respectively, and N is the ring collection efficiency (0.37).

The synthetic process for NSFC-X has been shown in Fig. 1. According to previous reports, FeCl₃ has been developed as an efficient template for fabrication of 2D carbon nanomaterials due to the formation of layered crystal structure during recrystallization process at 80 ℃ [20]. Therefore, PPS as the sources of C, N, and S can be confined in the laminated structure of FeCl₃. After high temperature pyrolysis and acid treatment, the NSFC-X with a layered structure and N, S, Fe-tridoped states was achieved.

PPS was first directly carbonized at high temperature to get the product (denoted with NSPC). The SEM image of NSPC was shown in Fig. 2(A). It can be obviously observed that PPS was transformed into bulk carbon with promiscuous agglomerate without the assistant of FeCl₃ template.

To obtain a well-defined and continuous mesoporous carbon material, PPS was simply mixed with FeCl₃ template. As SEM shown in Fig. 2(B), the NSFC-3 had a separated lamellar structure and the thickness of about 12.2 nm. FeCl₃ was evidently functional as the template which could exfoliate the carbide products of PPS into ultra-thin two-dimensional nanosheets. The TEM shown in Fig. 2(C) further showed that the surface of NSFC-3 was not smooth under the field of vision. A large amount of folds and crisscross connected channels at the edge have been formed and couldn't see any obvious metal particles on the material surface which instructed that the most part of Fe introduced by FeCl₃ have been acid etched. The folds and channels can not only improve the specific surface area of NSFC-3, but also increase the exposure degree of catalytic active sites. As shown in Fig. 2(D), the high resolution TEM (HRTEM) image of NSFC-3 sample demonstrated abundant microporous and mesoporous structures, which were probably formed by the irregular 2D template structures as well as acid etching of Fe-containing compounds (Fe, Fe₃C, and FeS_x etc.). The above results indicated that the FeCl₃ did serve as an efficient 2D template in the process of recrystallization, and the standard slice structure and abundant pore channels, micropores and mesopores were formed.

Changing the ratio of FeCl₃ and PPS would influence the microstructure and ORR catalytic activity of NSFC, so a series of NSFC-X has been synthesized and measured. As shown in Fig. 3(A) and (C), NSFC-1 (the ratio of FeCl₃ and PPS was 1:1) presented a two-dimensional slice layer structure, but these patches were confined to the uneven surface, illustrating that less FeCl₃ couldn't make them form integrated two-dimensional template, only some nanoscale fragments displayed in the image. As shown in Fig. 2(B) and (C), when the ratio was increased to 3:1, NSFC-3 presented a micron grade 2D nanoscale lamellar structure with around 12 nm in thickness. The inerratic two-dimensional nanosheets were taken shape, and there were many connected channels on the surface of NSFC-3. While the connected channels vanished, and perfectly smooth lamellas were formed when the ratio of FeCl₃ and PPS was increased to 5:1 as shown in Fig. 3(B) and (D). Although NSFC-5 had the same complete laminar structure as NSFC-3, but the smooth surface severely limited the exposure area of catalytic active sites. The above comparison showed that only the appropriate ratio of FeCl₃ and PPS could optimize the structure and surface function simultaneously. It was expected that the NSFC-3 had a perfect two-dimensional structure and a relatively rough surface for larger specific surface area and better active site exposure.

To further study the effect of FeCl₃ to the carbonization product of PPS, we tested the samples using the BET method, XRD and XPS. As the BET measurements of NSPC and NSFC-3 shown in Fig. 4(A) and (B), the obtained NSPC had a specific surface area of 139 m²/g and pore volume of 0.159 cm³/g. By contrast, the NSFC-3 had a specific surface area of 702 m²/g and pore volume of 0.559 cm³/g, which were nearly 5 times and 3.5 times of NSPC, respectively (Fig. 4(A)). These results demonstrated that the specific surface area and pore volume were significantly improved with the addition of FeCl₃. This result was mainly attributed to the function of two-dimensional template of FeCl₃, and the connected channels and the abundant micropores and mesopores formed in the pyrolysis process, simultaneously. FeCl₃ was uniform mixed with the precursor then inserted the layer stripping and catalyzed the formation of cotton-like two-dimensional carbon nanofilms in heating treatment. The presence of micropores and mesopores further enhanced the specific surface area of the product. The graph of pore diameter distribution proved this point convincingly (Fig. 4(B)), the dV/dlgD numerical values of NSPC and NSFC-3 both were greater than zero ranging from 1 to 100 nm, which indicated that they had hierarchical porous structure incorporating micropores, mesopores and macropores simultaneously, and the number of aperture sizes of various sizes of NSFC-3 was larger than NSPC, especially within the range 4.5 to 6.7 nm.

For further exploring the influence of the template dosage on NSFC-X, the NSFC-X was analyzed by BET measurement (Fig. 4(C), (D) and Table 1). As the absorption and desorption curve of nitrogen shown in Fig. 4(C), the specific surface area of NSFC-1 reached 547 m²/g when the mass ratio of FeCl₃ and PPS was 1:1, nearly 4 times as much as NSPC. The results indicated that even small quantity of FeCl₃ could significantly improve the specific surface area and pore volume of NSPC adequately. When the mass ratio of FeCl₃ and PPS increased from 1:1 to 3:1 and then further increased to 5:1 gradually, the specific surface area of NSFC-X correspondingly increased from 547 to 702 m²/g and further to 813 m²/g. As shown in Fig. 4(D), it would form more mesoporous ranging from 2 to 20 nm because of more FeCl₃ template was used.

Detailed analysis of XPS was carried out as shown in Fig. 5, to research the impact on surface chemical composition and bonding state of NSPC by FeCl₃. The survey spectra of NSPC showed four peaks typical of C 1s, N 1s, O 1s, and S 2p in all the carbon materials indicating the presence of N, O, and S dopants, and the atomic ratio respectively were 88.89%, 6.75%, 3.97% and 0.39% as shown in Table 2. Comparably the survey spectra of NSFC-3 showed five peaks typical of C 1s, N 1s, O 1s, S 2p and Fe 2p indicating the presence of N, O, S and Fe dopants occupying 88.99%, 3.30%, 6.36%, 0.91% and 0.44% respectively. The diversified and abundant element composition was benefit from the neutralization strategy. N, O and S were effective and homogeneous doped into NSPC using PPS as precursor, and the collaborative doping of various elements was realized by the in situ doping method. The results also indicated that the FeCl₃ template changed the surface chemical composition of NSPC. The contents of O and S increased while that of N decreased simultaneously. More importantly, there were Fe elements occurred in NSFC-3 from scratch. To further testify the existing form of Fe in NSFC-3, the XRD pattern was demonstrated in Fig. 5. There were only two typical diffraction peaks of carbon (002) and (101) planes at 24° and 43.5° (Fig. 5(A)) and no peaks of pure Fe or other Fe compound species, which showed that no free Fe was introduced and accorded with the SEM and TEM of NSFC-3. This conclusion was further proved in the Fe 2p spectrum shown in Fig. 5(C), the peaks at 709.5 and 713.1 eV can be assigned to the binding energies of the 2p_3/2 orbitals of Fe²⁺ and Fe³⁺ species, respectively. The peak at 722.5 and 725 eV were attributed to the binding energy of Fe²⁺ and Fe³⁺ for the 2p_1/2 band. According to previous reports, the Fe²⁺ and Fe³⁺ species were beneficial to the catalytic activity of oxygen reduction [24]. In addition, the C 1s spectra shown in Fig. 5(D) indicated that the NSPC and NSFC-3 both contained C=C, C–C and C–O peaks at the binding energies of 284.7, 285.6 and 286.5 eV respectively.

Increased performance not only depended on presence of the Fe²⁺ and Fe³⁺ species but also depended on the use of active nitrogen-doped sites and sulfur-doped sites [21, 25]. The binding states of these sites in the resulting products were revealed by high-resolution by high-resolution N 1s XPS spectra of NSPC and NSFC-3 as shown in Fig. 5(E). The spectra of both of NSPC and NSFC-3 could be deconvoluted into four types of nitrogen: pyridinic N (≈398.1 eV), pyrrolic N (≈399.0 eV), graphitic N (≈400.9 eV) and oxidized nitrogen at (≈402.7 or 405 eV) [26]. The proportion of them are 17.69%, 8.13%, 62.11%, 12.07% and 11.37%, 8.53%, 69.80 %, 10.30% in sequence as shown in Fig. 6. When adding the FeCl₃ template, the pyridinic N was decreased (from 17.69% to 11.37%) and graphitic N was increased inversely (from 62.11% to 69.80%), the two else types were invariant almost. Graphitic N was more important for the ORR catalytic activity of N-doped porous carbon than pyridinic N due to that graphitic N could accelerate the transfer of electrons to O₂ in the crystal lattice of C [27, 28].

The high resolution S 2p XPS peaks were also deconvoluted, mainly into four peaks associated with −C−S−C− and −C−SO_x−C− species as shown in Fig. 5(F). Participation of FeCl₃ templates changed the bonding method of the surface doped S atom by proton salt carbide obviously. The content of −C−S−C−decreased from 78.91% to 51.74%, and the content of −C−SO_x−C− increased from 21.09% to 48.26% on the contrary. According to previous reports, the doped S increased the oxygen reduction activity mainly ascribed to the S-related active sites and the synergistic effect of N, S-codoped [21, 29, 30].

The electrocatalytic activity of the as-prepared NSPC and NSFC-3 was evaluated first by CV, linear sweep voltammetry (LSV), RDE, and RRDE measurements in KOH solution (0.1 mol/L). The CV and LSV curves have been corrected by subtracting the curves recorded under saturated O₂ from those in saturated N₂. As shown in Fig. 7(A), a weak cathodic peak for ORR was observed for NSPC, while a strong peak was observed for NSFC-3 as well as 30 wt% Pt/C. Notably, the peak position for NSFC-3 was more positive than that of 30 wt% Pt/C, suggesting the best oxygen reduction electrocatalytic activity of NSFC-3 among these prepared catalysts. As the Id curves shown in Fig. 7(B), the onset potential (E_onset, the potential when catalytic current density reaches 0.20 mA/cm²) for NSPC was –0.15 V versus SCE, while –0.03 V for NSFC-3, which was quite comparable to that of 30 wt% Pt/C (–0.05 V). Meanwhile, NSFC-3 showed a similar current density at –0.3 V (5.05 mA/cm²) to 30 wt% Pt/C (5.59 mA/cm²), but much superior to that of NSPC (3.1 mA/cm²). Furthermore, NSFC-3 had a more positive half potential (E_1/2) of –0.142 V, which was 41 mV more than that of 30 wt% Pt/C (–0.181 V).

RRDE LSV measurements were further employed to uncover the reaction pathways of the as-prepared catalysts, which detected the formed peroxide (HO₂⁻) intermediate during the ORR process as shown in Fig. 7(C). The electron transfer number (n) calculated from the ring and disc currents was greater than 3.9 for NSFC-3 over the entire potential range, which indicated that ORR proceeded mainly through a four-electron (O₂ to OH⁻) reduction pathway. In contrast, n for NSPC was lower than 3.7, even at a high potential of up to –0.8 V. Furthermore, the calculated peroxide yield (HO₂⁻%) from the RRDE data was lower than 3% for NSFC-3 over the range of –0.8 to 0 V, which was significantly lower than that of NSPC.

Besides, the stability and methanol-tolerance ability of NSFC-3 were also measured by the cycling durability test and chronoamperometry [31]. In Fig. 7(D), NSFC-3 showed no obvious decrease in onset potential over 1, 000 cycles of constant operation, while a decrease both in onset potential and current density was observed for Pt/C catalyst, suggesting that NSFC-3 had superior durability over commercial Pt/C catalyst. On the other hand, as shown in Fig. 7(E), Pt/C catalyst exhibited an obvious current decay immediately after the injection of 3 mol/L methanol, indicating the occurrence of the methanol oxidation reaction. Whereas no noticeable change of current response was observed on NSFC-3. The results demonstrated that NSFC-3 catalyst possessed considerably better tolerance to methanol crossover than 30 wt% Pt/C.

Because of iron doping in the NSFC-3, there is a question that how iron improved the catalytic performance. There were mainly two different hypotheses about the active sites in the Fe-N/C catalyst in previous works. One is that the Fe-N_x may act as the major active sites [32, 33] and the other means that it is same as the nonmetal N/C catalyst [34, 35]. The NSFC-3 was most probably considered to follow the former hypothesis, and the poor catalytic activity of NSPC could confirm the viewpoint that the Fe-N_x played a key role in the high catalytic activity of NSFC-3. The NSPC had higher N content (6.75%) than NSFC-3 (3.3%), while the ORR activities of the NSPC were much poorer than those of the NSFC-3. Particularly, a quite high percentage of Fe²⁺ in the NSFC-3 according to the Fe 2p XPS spectrum could provide Fe²⁺-N₄ active sites [32]. Although the Fe-N_x active sites weren't confirmed yet, the obvious improvement of ORR activity had been brought by Fe-N_x species [36].

Based on the above results, we could conclude that the NSFC-3 possessed excellent activity as well as selectivity compared with NSPC and 30 wt% Pt/C. As has been mentioned above, the excellent performance of NSFC-3 should be ascribed to the simultaneous optimization of the microscopic structure and surface functionality. The two-dimensional nanosheets, crisscross connected channels with rich micropores and mesopores provided by the FeCl₃ template made the NSFC-3 possessed of enough specific surface area and the exposure area of catalytic active sites. And the abundant and various active sites which ascribed to the protic salt method have provided concerted catalysis to the NSFC-3 [37].

The RDE and RRDE measurements of NSFC-1, NSFC-3, and NSFC-5 were shown in Fig. 8, tested after the mass ratio of FeCl₃ and PPS had been changed. As shown in Fig. 8(A), the reduction peak potential of the cyclic voltammetry curve was moving positive before negative shift as the ratio of FeCl₃ and PPS increases, which illustrated the best oxygen reduction electrocatalytic activity of NSFC-3 among these prepared catalysts. The result was further confirmed by RDE LSV curves as shown in Fig. 8(B), the onset potential (E_onset) and positive half potential (E_1/2) for NSFC-3 were 60 and 46 mV more than for NSFC-1, 50 and 30 mV more than for NSFC-5, respectively. These results further proved the excellent electrocatalytic activity for NSFC-3. The RRDE) LSV measurements were further employed to demonstrate the distinction of reaction pathways and the formed peroxide (HO²⁻) intermediate of the as-prepared catalysts as shown in Fig. 8(C) and (D). The results of the RRDE test of NSFC-X were consistent with the RDE test results, the initial potential, half wave potential and limiting current density of NSFC-3 were superior to NSFC-1 and NSFC-5. NSFC-3 also had the optimal electron transfer number (n) and calculated peroxide yield (HO₂⁻%) over the range of –0.8 to 0 V compared with NSFC-1 and NSFC-5. The excellent performance of NSFC-3 should be ascribed mainly to the perfect two-dimensional structure and a relatively rough surface for larger specific surface area and better active site exposure.

Comprehensive comparing of electrochemical test results, it went without saying that it's vital to select the template and preparation method. The participation of FeCl₃ template in the preparation process made NSPC have improved electrocatalytic performance for oxygen reduction. And NSFC-3 had the best electrocatalytic performance when appropriate ratio of FeCl₃ and PPS was chosen.

We have successfully designed and constructed a two-dimensionally ordered mesoporous carbon nanosheet catalyst through FeCl₃ assisted template method. During this process, P-phenylenediamine sulphate was successfully explored as a single, small-molecule precursor to simply but efficiently control the element composition of obtained catalysts. In comparison with traditional polymer precursors, the protic salt precursor not only simplified the synthetic procedure by direct neutralization reaction, but also incorporated N and S heteroatoms into the carbon skeleton in situ. After optimization, NSFC-3 had preponderant microstructure, great specific surface area, excellent pore volume, and rich active sites, therefore possessing similar ORR catalytic activity in both reaction current density and onset potential as well as four-electron selectivity, while superior long-term durability and methanol crossover resistance to commercial 30 wt% Pt/C catalyst in alkaline media. Notably, the excellent catalytic performance was achieved via such a simple fabrication process using inexpensive, available resources, which could significantly reduce production costs and had huge advantages for large-scale production.