Sustainable energy storage and conversion devices like rechargeable metal-air batteries and fuel cells have recently attracted significant interest due to the high energy density and considerable efficiency. Noticeably, the oxygen reduction reaction (ORR) at the cathode is kinetically sluggish, which is one of the bottlenecks that greatly restrict the extensive application of these devices [1]. Traditionally, precious Pt and corresponding derivatives are employed to realize impressive electrochemical activity together with high current density and low overpotential [2-5], though the scarcity, susceptibility to time-dependent potential drift, and easy poisoning by CO and methanol impede their applicability and availability [6-8]. Therefore, exploring readily available and sustainable alternatives to replace Pt-based noble electrocatalysts towards efficient ORR is urgently needed for the development of high-performance energy-related devices.

Within this context, considerable efforts have been dedicated in developing earth-abundant transition-metal-based and metal-free catalysts, among which carbon-based nanomaterials, containing carbon nanotubes, porous carbons and graphene, have been emerging as promising electrocatalysts owing to the distinct advantages of low cost, long-term stability, tunable surface properties, and sufficient electron conductivity [9-14]. Furthermore, doping heteroatom (e.g., N, P, B, S) into carbon frameworks can significantly improve the catalytic activity of carbon materials, as the dopant atoms can break the electroneutrality of nearby carbons and thus create the active sites, favoring the adsorption of O₂ molecules and the subsequent oxygen reduction process [15-20]. Indeed, various N-doped carbons, such as graphene sheets, carbon nanotubes, mesoporous carbon and nanoshell carbons have been successfully fabricated and presented high electrocatalytic activities for ORR [21-24]. In contrast, P-doping has been reported that it can modify the structure and performance more effectively because of the larger covalent radius and lower electronegativity of P compared with N counterpart, such as P-doped mesoporous carbon, microporous carbon, and graphite layers exhibiting improved ORR activity [25-30]. On the other hand, the ORR electrocatalytic activity is greatly dependent on the structural properties of carbons, which determines the conductivity, stability and dispersion degree of active sites of the catalyst [31]. On this consideration, constructing porous carbons with high specific surface areas along with well-developed porosity should be favorable to the efficient utilization of active sites and the improvement of mass transfer. A variety of state-of-the-art techniques have been employed to synthesize P-doped mesoporous carbons, for instance, pyrolysis of toluene and triphenylphosphine at 1000 ℃ under Ar atmosphere [25], nanocasting method using SBA-15 as template [32]. However, these methods often involve complex and hazardous synthesis processes. Previously, we fabricated P-doped mesoporous carbons using soft-templating method with organophosphonic acid as the P source [27]. The resultant material inherits high P doping levels and large surface areas, ensuring the promising ORR performance, but the apparent activity is still not satisfactory to compete with commercial Pt/C catalyst.

In this work, we report that it is feasible to derive P-doped mesoporous carbons by simply adding small amounts of iron with the assistance of organophosphonic acids. Fe species originating from FeCl₃ act as graphitization catalysts that can be removed completely by acid washing. The resultant carbons feature high surface area, narrow pore size distributions, and good conductivity, which empower them to achieve superior electrocatalytic performance. In addition, the effect of doping amount of P on the mesoporous carbons was also investigated in detail. It found that the P-MC-4 exhibited impressive electrocatalytic performance, long-term stability together with superb methanol tolerance. The results can provide new insights into the highly efficient construction of metal-free heteroatom-doped mesoporous carbons with intriguing textural characters.

In a typical procedure, 0.2 mmol F127 and 20 mmol resorcinol were dissolved in a mixed solution of water/ethanol with a volume ratio of 1:1 at room temperature. After stirring for 1 h, a colorless solution was obtained. Then a desired amount of HEDP was introduced to the above mixture with further stirring of 1 h, followed by dropwised addition of 2.5 mmol formaldehyde solution under vigorous stirring. Thereafter, 1 mmol FeCl₃ was introduced, and the reaction solution was further stirred for another 1h. The above mixture was then transferred to an autoclave and aged at 80 ℃ for 2 d. The red-brown bulk product was obtained by filtration, washed with ethanol and water for three times, and dried at 80 ℃ overnight. The dried bulk product was then carbonized under N₂ atmosphere at 350 ℃ for 2 h with a ramp of 4 ℃ min^‒1 and 900 ℃ for 2 h with a ramp rate of 5 ℃ min^‒1. The calcined bulk was washed by concentrated HNO₃ for several times to remove iron species. The obtained powder was denoted as P-MC-x, wherein x denotes as the molar ratio of P to Fe, i.e., x = 4, 6, 8. P-doped mesoporous carbon (marked as P-MC) was prepared via the similar processes in the absence of FeCl₃. The undoped mesoporous carbon (marked as MC) was also prepared without the addition of HEDP and FeCl₃.

X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Focus diffractometer with 40 kV operation voltage and current at 40 mA using Cu-K_α radiation (λ = 0.15418 nm). Raman analysis was performed on a Thermo-Fisher Scientific DXR spectrometer (514 nm radiation laser). N₂ sorption experiments were undertaken on a Quantachrome NOVA2000e automated surface area analyzer isothermally at –196 ℃. Before the measurement, the samples were all degassed at 200 ℃ overnight. The specific area was determined by the Brunanuer-Emmett-Teller (BET) method and total pore volume was determined from the amount of the N₂ adsorbed at a relative pressure of 0.98. The pore size distributions were calculated from the adsorption branch using Barrett-Joyner-Halenda (BJH) method. The scanning electron microscopy (SEM) was performed on a Jeol JSF-7500L microscopy. X-ray photoelectron spectroscopy (XPS) was performed for the surface analysis of the synthesized materials, using a Kratos Axis Ultra DLD spectrometer with Al-K_α irradiation (1486.6 eV).

The electrocatalytic ORR activity of the samples was investigated with the rotating disk electrode (RDE) technique using WaveDriver 20 bipotentiostat/galvanostat electrochemical workstation (Pine Research Instrumentation) under room temperature. A typical three-electrode electrochemical cell was used, wherein the catalyst film-coated glassy carbon disks were utilized as the working electrode. Ag/AgCl (3 mol/L KCl) and Pt foil were used as the reference and counter electrodes, respectively.

For preparing the working electrode, 5 mg of samples were dispersed in 1 mL Mill-Q water under sonication to form a homogenous ink. 10 µL of the dispersion was drop casted onto a polished 5 mm-diameter glassy carbon electrode. After drying at room temperature overnight, 5 µL of Nafion solutions (0.5 wt%) was dropped on the surface of electrode and allow to dry, and the resultant electrode served as a working electrode. For the ORR test, Before the ORR measurement, the electrolyte (0.1 M KOH) was ventilated with O₂ for 30 min to ensure O₂ saturation. Cyclic voltammetry (CV) curves were measured at a scanning rate of 20 mV s^‒1 within the potential range from −1.0 to +0.2 V (vs. Ag/AgCl). For linear sweep voltammetry (LSV) tests, the potential was varied between ‒1.0 and 0.2 V (vs. Ag/AgCl) with a scan rate of 10 mV s^‒1. The number of electron transfer can be calculated from the Koutechy-Levich (K-L) equation:

(1)

(2)

(3)

where J, J_L and J_K is disk, diffusion-limiting and kinetic current densities, respectively. k is the electron-transfer rate constant. F is Faraday constant (96485 C mol^‒1). ω is the electrode rotating rate. n is the transferred electron number per oxygen molecule. ν is the kinetic viscosity (0.01 cm² s^‒1). C_O2 is the bulk concentration of O₂ (1.2×10^‒6 mol cm^‒1). D_O2 is the diffusion coefficient of O₂ in 0.1 mol/L KOH (1.9×10^‒1 cm² s ^‒1). The constant 0.2 is adopted when the rotation speed is expressed in rpm.

The pure mesoporous carbon was synthesized using a procedure similar to that reported elsewhere [33]. The procedure for synthesizing P doped mesoporous carbons included an organic-organic self-assembly coexisted with the in situ doping P by using HEDP through a simple autoclaving process and high-temperature carbonization under N₂ atmosphere. Actually, HEDP acted as both the P source and the catalyst for promoting the polymerization of resorcinol with formaldehyde. A small amount of FeCl₃ was introduced during the synthesis procedure, and the synthesized samples were marked as P-MC-x (x = 4, 6, 8). While, P-doped mesoporous carbon (marked as P-MC) was synthesized via the similar procedures in the absence of FeCl₃, and the undoped mesoporous carbon (marked as MC) was also synthesized without the introduction of HEDP and FeCl₃.

The morphology of as-prepared P-MC-4 catalyst was illustrated by electron microscopy measurements, as shown in Fig. 1(a). The resulting P-doped carbon material shows the rough and irregular surface with well-defined mesopores. TEM images (Fig. 1(b) and (c)) reveals the presence of a distinct wormhole-like pore structure system with the pore size of around 6 nm, which may facilitate the mass transportation during the catalytic process. The selected area electron diffraction (SAED) shows the mesoporous carbons possess a low graphitization degree. The P distribution on the resultant P-MC-4 was further investigated by EDS elemental mapping. As shown in Figs. 1(d)‒(g), the mapping image of P unveils that P was uniformly distributed in the P-MC-4, confirming the successful P doping into the carbon framework.

Wide-angle XRD pattern (Fig. 2(a)) of all the carbonaceous materials shows the presence of amorphous carbon, wherein two diffraction peaks situated at approximately 2θ = 23° and 44°, corresponding to the (002) and (100) planes of graphite with a low graphitization degree, respectively [34]. The small-angle XRD pattern of all the prepared samples was also collected and shown in Fig. 2(b). P-doped mesoporous carbons all exhibit a diffraction peak at 0.75° (2θ), revealing the positive role of the introduction of HEDP in facilitating the formation of well-defined wormhole-like pore structures. Intriguingly, the diffraction peak of P-MC-4 is more distinctive than that of P-MC, suggesting that Fe species may play an important role in enhancing the activation for carbons [35]. Additionally, the porous structure of P-MC-4 is more well-defined than both the P-MC-6 and P-MC-8. It is because the usage of high amount of P into the framework may distort the carbon lattice, resulting in irregular pore structure [36].

The porosity of the carbon catalysts was determined by nitrogen sorption technique. Table 1 lists the corresponding porous properties. All the samples show isotherms of type Ⅳ associated with typical H1 hysteresis loops (Fig. 2(c)), characteristics of mesoporous materials, which is in good agreement with the TEM observation. According to the reports, mesopores are beneficial to catalytic process due to the less limited diffusion of electrolyte ions (proton) into inside layers [37]. Notably, the pore size distribution of P-MC-4 (Fig. 2(d)) is narrower than other samples. The surface area of P-MC (721 m² g^‒1) is much larger than that of pure carbon (666 m² g^‒1), suggesting that the incorporation of P can improve the porous structure properties of carbons and generate abundant and accessible active sites. The surface area of P-MC-4 is 737 m² g^‒1, which is the largest. The surface area of P-MC-6 and P-MC-8 decreases to 685 and 678 m² g^‒1, respectively. It is proposed that the suitable doping amount of P is indispensable for the improvement of porous properties. The high doping amount of P may lead to the inferior textural properties of the corresponding carbon materials.

Raman spectra were further performed to examine the graphitization degrees of the obtained carbons. As observed in Fig. 3(a), all the carbon materials show two prominent peaks centered at approximately 1348 and 1591 cm^‒1, which can be ascribed to D and G bands that are designated to disordered and graphitic phases in carbon, respectively [38]. The intensity ratio of D and G band (I_D/I_G), as an indication for the defects of structure in carbon, is shown for each sample (Table 1). The disordered and graphitic phases in carbon are both beneficial to the electrocatalytic ORR because they can provide active sites and high electrical conductivity, respectively. Remarkably, the I_D/I_G is increased from 1.42 for MC to 1.56 for P-MC, implying that the presence of more defects in carbon material after the doping of P. The I_D/I_G of P-MC-4 is lower than that of P-MC, indicating the higher degree of graphitization. It can be inferred the addition of iron endow this carbon material with high ORR activity for the high conductivity. The I_D/I_G increases to 1.49 and 1.53 for P-MC-6 and P-MC-8, respectively, which mainly due to the increasing defects derived from more introduction of P.

The surface chemistry properties of P-MC-4 were studied by XPS measurements. The XPS survey spectrum reflects that P-MC-4 is consisted of element C, O, and P (Fig. 3(b)). The fine C 1s XPS spectrum (Fig. 3(c)) centered at about 288.9, 286.7, 285.6 and 284.7 eV indicates the existence of carbonxyl or ester groups, C‒O, C‒O‒P, and C‒C bonding, respectively [39]. Fitting of high-resolution P 2p XPS spectrum demonstrates the existence of two types of P species, i.e., P‒C bonding (132.7 eV) and P‒O bonding (134.3 eV) in the carbons (Fig. 3(d)) [40]. XPS analysis confirms that the heteroatom P is successfully introduced into the framework of P-MC-4 catalyst. Quantitative XPS analysis indicates that P is present with 0.47 at%, while 0.64 and 0.72 at% in P-MC-6 and P-MC-8, respectively. Notably, Fe species are absent in the carbon materials because Fe was removed by concentrated acid. Herein, Fe cannot be detected in the XPS measurements. The results indicate that HEDP can be served as a highly promising phosphorous source to afford the P-doped mesoporous carbons. The existence of organic functionalities in HEDP could be favorable for the molecular interactions among formaldehyde, surfactant molecules and phloroglucinol, thus introducing heteroatom P [27]. Additionally, the introduction amount of HEDP determines the doping amount of P, which can greatly affect the catalytic activity of carbon materials.

The above results demonstrate that the introduction of heteroatom P can optimize the textural properties of carbon materials, which is considered to be beneficial for ORR as it increases the active sites. FeCl₃, as the graphitization catalyst, can promote the formation of well-defined porous structure and conductivity of carbon materials, which can further facilitate the electron transfer and mass transportation during the ORR process. To study the catalytic performance of the P-doped mesoporous carbons for the ORR, the cyclic voltammograms (CVs) of the carbon materials with a scan rate of 20 mV s^‒1 in O₂-saturated 0.1 mol/L KOH electrolyte were first performed. As demonstrated in Fig. 4(a), all samples possess a well-resolved catholic ORR peak, indicating the remarkable electrocatalytic performance for ORR. In comparison with the pure carbon, P-MC exhibits larger area under CV, indicating a larger active surface area for P-MC, in good accordance with the result obtained from the BET specific surface area. Additionally, the more positive peak potentials and stronger peak current density of P-MC manifest that the introduction of phosphorous into the framework of carbon materials can significantly facilitate the ORR process. Compared to P-MC, P-MC-4 shows an even better ORR performance, as evidenced by its more positive peak potential and stronger peak current density. It manifests the positive role of Fe species in facilitating the ORR performance. The RDE linear sweep voltammogram (LSV) recorded at a rotation speed of 1600 rpm with a scan rate of 10 mV/s is shown in Fig. 4(b). The distinct advantage with regard to the ORR limiting current density, half wave potential and onset potential is observed when introducing P into the carbon matrix. The onset potential and limiting current density of ORR on pure carbon is 0.86 V, 2.90 mA cm^‒2, respectively. Besides, an inefficient two-step ORR process is presented. Compared to the pure carbon material, the onset potential of P-MC increases positively to 0.89 V and further increases to 0.91 V for P-MC-4. Similarly, the limiting current density of P-MC-4 is 4.25 mA cm^‒2, which is much higher than that of P-MC and MC. The half-wave potential of the P-MC-4 presents a negative shift of only ~37 mV with respect to Pt/C, further indicating the higher ORR activity of P-MC-4 than that of P-MC and MC, which is comparable to commercial Pt/C catalyst. It is reported that the P-doping induced charge redistribution is responsible for the ORR performance of carbon-based catalysts [36]. Besides, the addition of FeCl₃ in the synthesis procedure can improve the porous structure and the degree of graphitization, which are attributable to the gas transportation and high conductivity, and thus favors the ORR activity.

To better understand the effect of doping amount of P on the electrocatalytic ORR performance of the P-MC-x catalyst, we compared the electrocatalytic performance of P-MC-4 with those of P-MC-6 and P-MC-8 by CV and LSV. As shown in Fig. 4(c) and (d), with the increase of the doping amount of P, the ORR performance of the catalyst decreases. It is indicated that more dopant of P may disturb the structure of carbon materials, further hampering the enhancement of electrocatalytic performance. The results clearly show that the doping amount of P plays an important role in the electrocatalytic performance of the catalyst. Furthermore, the suitable doping amount of P in the framework of carbon materials can achieve the improved catalytic performance of mesoporous carbons. The polarization curves on P-MC-4 at various rotation rates from 600‒2000 rpm for the ORR are shown in Fig. 4(e). The corresponding Koutecky-Levich plots at the potential range of 0.4‒0.6 V exhibit high linearity (Fig. 4(f)). Additionally, the number of transferred electron determined from the slope of K-L plot is 3.85, 3.80 and 3.89, respectively, which is similar to that of commercial Pt/C, suggesting a dominant 4e^- pathway towards ORR for the case of P-MC-4.

Besides catalytic activity, the durability of catalyst is also an important parameter for metal-air batteries or fuel cells. The durabilities of commercial Pt/C and P-MC-4 catalysts were investigated using a chronoamperometric response method as observed in Fig. 5(d). Remarkably, the current of P-MC-4 shows a slow reduction, and a high relative current of 91.7% is kept over 12, 000 s under a constant cathodic voltage of 0.6 V. However, the Pt/C catalyst is observed for a rapid 21.4% loss of the current. This indicates that the P-MC-4 catalyst is more durable than the commercial Pt/C catalyst. Another crucial concern for the practical application of cathode materials in direct methanol fuel cells is methanol tolerance capacity. On this context, we tested the electrocatalytic selectivity of catalysts against the electro-oxidation of methanol (0.5 mol/L). As shown in Fig. 5(a) and (b), no activity specific to methanol on P-MC-4 is observed where the characteristic peaks of ORR are maintained, implying the high selectivity towards ORR. On the contrary, commercial Pt/C catalyst (Fig. 5(c)) shows a prominent oxidation peak corresponding to methanol oxidation coupled with a negative shift of potential for ORR, indicating the poisoning of the catalyst. These results reveal our catalyst not only has an excellent stability, but also has higher catalytic selectivity toward ORR than that of Pt/C.

We employed the organophosphonic acid as the P source and phenolic resin as the carbon source to obtain the P-doped mesoporous carbons and investigated their catalytic performance toward ORR in alkaline electrolyte. It is found that the prepared catalyst exhibit a small amount of P doping coupled with well-defined wormhole-like pore structure and high surface area. With the addition of P, the electrocatalytic ORR activity improves significantly compared with the pristine mesoporous carbon materials. The introduction of a small amount of graphitization catalyst FeCl₃ greatly improves the ORR performance. A synergistic contribution is needed from both the P doping and textural properties to attain high catalytic performance. The effect of the doping amount of P on catalytic performance was also studied, showing a decrease in activity with an increase in the doping amount of P. Notably, Fe is not present in the resultant materials due to the strong acid washing. Therefore, the synthesized material is a true metal-free catalyst with outstanding catalytic performance. In addition to enhanced electrocatalytic performance, promotion in catalytic kinetics was also found, showing that P-MC-4 follows ~4 electron pathway in alkaline electrolyte for ORR. Moreover, the superior durability and immunity for methanol crossover than Pt/C make these cost-effective mesoporous carbon catalysts a potential substitute to Pt-based catalysts in direct methanol fuel cells.

This work was supported by the National Natural Science Foundation of China (21421001, 21573115).