Enhancement mechanism of sulfur dopants on the catalytic activity of N and P co-doped three-dimensional hierarchically porous carbon as a metal-free oxygen reduction electrocatalyst

Yongxi Zanab, Zhengping Zhangab, Meiling Dou*ab and Feng Wang*ab
aState Key Laboratory of Chemical Resource Engineering, Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: wangf@mail.buct.edu.cn
bBeijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Received 14th July 2019 , Accepted 20th August 2019

First published on 21st August 2019

Doping carbon networks with multi-heteroatoms is one of the most effective ways to achieve a carbon-based metal-free electrocatalyst with a superior activity for oxygen reduction reaction (ORR). Herein, multi-heteroatom doped three-dimensional hierarchically micro/mesoporous carbon was synthesized as an efficient metal-free electrocatalyst towards ORR by pyrolyzing thiourea (THU) with nitrogen- and phosphorus-doped (N- and P-doped) hierarchically porous carbon networks derived from cattle bone. The obtained sulfur, nitrogen and phosphorus ternary-doped (S, N and P ternary-doped) carbon material has both a high porosity for effective mass transport of ORR reactants and abundant surface active sites for efficient ORR. As identified by X-ray absorption near edge structure spectroscopy, the S dopant in the reduced form (SRed) plays a major role during the ORR catalytic process. When used as an ORR electrocatalyst, the S, N and P ternary-doped carbon with the highest content of SRed outperformed the commercial Pt/C electrocatalyst in alkaline electrolytes with a superior electrocatalytic activity (∼40 mV higher half-wave potential), outstanding electrochemical durability and a good anti-poisoning capability. This work presents an effective approach for the design and synthesis of efficient metal-free ORR electrocatalysts from earth-abundant cattle bone and analogues.

1. Introduction

The development of clean and efficient hydrogen fuel cell technologies is a crucial mission to decrease the consumption of fossil fuels and reduce emissions of CO2, NOx, SOx, etc.1–3 The oxygen reduction reaction (ORR), as a pivotal reaction at the cathode in fuel cells, requires efficient electrocatalysts to reduce the high overpotential and then accelerate the reaction kinetics to significantly improve the energy conversion efficiency.4,5 Up to now, Pt and its alloys are the most active electrocatalysts for the ORR. However, their prohibitive cost and scarce reserves as well as inferior durability caused by the migration and agglomeration of Pt nanoparticles (NPs) still hinder the commercialization of fuel cells.6 Therefore, it is extremely urgent to exploit low cost non-noble metal electrocatalysts with superior catalytic activity and durability toward the ORR.7–10 Since nitrogen (N)-doped vertically aligned carbon nanotubes were found to exhibit superior ORR activity in alkaline electrolyte in 2009, many kinds of heteroatoms-doped carbon materials (such as boron (B)-doped CNTs,11 phosphorous (P)-doped graphite,12 and sulfur (S)-doped graphene13) have been widely investigated as efficient electrocatalysts for the ORR and are regarded as a new category of promising candidates to take the place of the Pt-based electrocatalysts.14

The superior ORR activity of heteroatom-doped carbon electrocatalysts has been shown to mainly originate from the redistribution of surface charge by breaking the electroneutrality of the sp2-hybridized carbon matrix and/or the spin density redistribution by doping heteroatoms into the carbon matrix.15 Doping multi-heteroatoms (e.g., N/P, N/S, N/B, or N/P/S) into the carbon matrix is proven to be an effective strategy to achieve high intrinsic electrocatalytic activity by expanding charge delocalization by virtue of synergistic effects of different heteroatoms.16–22 Dai et al. synthesized a B and N doped graphene material through thermal annealing of graphene oxide with boric acid, and the resulting material was evaluated to have excellent electrocatalytic activity toward the ORR that is superior to commercial Pt/C.16 Density functional theory (DFT) calculations revealed that B and N co-doping induced a strong synergistic interaction between B and N dopants with surrounding carbon atoms by changing their energy bandgap, charge density and spin density.16,17 In our previous work, we prepared a N- and P-doped three-dimensional (3-D) hierarchically porous carbon electrocatalyst (N,P-HPC) and showed that the resulting dual-doped-carbon electrocatalyst exhibited an improved ORR activity compared to the single-heteroatom (N or P)-doped HPC.23 Recently, based on both the experimental and theoretical results, the element S is proposed to be an efficient dopant to enhance the ORR activity.13,24–29 DFT calculations indicated that the S dopant primarily induced an enhanced spin density of surrounding carbon atoms that is the dominant factor for ORR activity instead of changing the surface charge distribution due to the similar electronegativities of S and C.15,30 Although S and other heteroatom multi-doped carbon electrocatalysts have been reported in some literature studies (e.g., N-, S-, and P-doped graphene (NSP-G-800),31 N-, S-, and P-doped carbon nanosheets (NPS-CNS-300-1000),32 and N-, S-, and F-doped carbon nanotubes (N/S/F-CNTs)33) and have shown relatively high electrocatalytic activity towards the ORR, the form of the S dopant and its impact on ORR performance still need to be further investigated. Besides the improvement of intrinsic electrocatalytic activity by doping multi-heteroatoms, the construction of a favorable pore structure to ensure the sufficient access of ORR reactants to catalytic active sites is also vital for the further improvement of ORR activity. A unique 3-D hierarchically porous structure with a high specific surface area (SSA) has been shown to benefit the ORR process by promoting the mass transport of ORR reactants and electroytes.34–37 Based on the above analyses, constructing a 3-D multi-heteroatom-doped (especially doping S) hierarchically porous carbon electrocatalyst with rapid mass transport properties and diverse active sites is clearly effective for further enhancing the ORR activity.38–43

In this work, we presented an effective preparation method of a high-performance S, N and P ternary-doped 3-D hierarchically porous carbon electrocatalyst (S,N,P-HPC-1) through co-pyrolysis of thiourea (THU) with N,P-HPC prepared according to the approach we previously reported.23 In particular, N,P-HPC was prepared through heat treating a mixture of dicyandiamide (DCDA), phytic acid (PA), and cattle-bone-derived hierarchically porous carbon (HPC).23 S doping effectively improves the activity of S,N,P-HPC for the ORR in both alkaline and acidic electrolytes. X-ray absorption near edge structure spectroscopy demonstrates that the S dopant is mainly in the form of reduced sulfur species (SRed) that is reported to favor the ORR activity of metal-free carbon-based electrocatalysts. Furthermore, the resulting electrocatalyst possesses a well- defined 3-D hierarchically micro/mesoporous structure with a high SSA (1533 m2 g−1) inherited from N,P-HPC. These features lead to a remarkable ORR performance for S,N,P-HPC-1 with a superior electrocatalytic activity, outstanding electrochemical durability and excellent anti-poisoning capability.

2. Experimental section

2.1 Synthesis of S,N,P-HPC electrocatalysts

Firstly, a N,P-HPC electrocatalyst with a N/P molar ratio of 1.0 was synthesized according to our previously reported approach.23 Then, the as-prepared N,P-HPC (100 mg) was mixed with THU (978 mg) as a S source in ultrapure water (20 mL), followed by ultrasonic stirring for 1 h. The obtained mixture was subsequently dried at 60 °C for 22 h. Afterwards, the product was heated to 900 °C in a tubular furnace (heating rate: 2.5 °C min−1) under an Ar atmosphere and kept for 3 h, generating the S, N and P ternary-doped HPC electrocatalyst (S,N,P-HPC-1) (P/S molar ratio of 1). S,N,P-HPC electrocatalysts with different P/S molar ratios by varying the THU content were also prepared at different pyrolysis temperatures. The corresponding electrocatalysts prepared at 900 °C were denoted as S,N,P-HPC-X (X represents the P/S molar ratios of 0.5, 1.5, 1 and 2). To determine the optimal reaction temperature, S,N,P-HPC electrocatalysts were also prepared at different pyrolysis temperatures (P/S molar ratio of 1).

2.2 Material characterization

To identify the decomposed products during heat treatment, thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) and TG-mass spectrometry (TG-MS) measurements were performed on a thermogravimeter (METTLER TOLEDO TGA/DSC1) coupled with a spectrophotometer (Nicolet 6700) and a mass spectrometer (Thermo), respectively. Field-emission scanning electron microscopy (FE-SEM, FE-JSM-6701F, JEOL, Japan) was performed to obtain SEM images. Transmission electron microscopy (TEM, JEM-2100, JEM-2010F, JEOL, Japan) was employed to characterize the morphologies of the as-prepared electrocatalysts. High-resolution scanning transmission electron microscopy (HR-STEM) and elemental mapping images were obtained on a JEM-ARM200F (JEOL, Japan) microscope. An X-ray diffractometer (XRD, D/max-2500, Rigaku, Japan) with a Cu Kα radiation X-ray source (λ = 1.54056 Å) was used to record XRD patterns. A Horiba Jobin Yvon LabRAM HR800 confocal microscope was employed to obtain Raman spectra (by using a laser of 633 nm). FT-IR spectra were recorded with a Perkin Elmer Spectrum 100 FT-IR spectrometer. A Quantachrome AUTOSORB-1 instrument was used to record nitrogen adsorption–desorption isotherms to characterize the SSAs and pore size distributions (PSDs) of the electrocatalysts. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 250 spectrometer (Thermo Fisher). For calibration, the C 1s peak at 284.98 eV was adopted as the reference. X-ray absorption fine structure (XAFS) spectra were recorded at the beamline 4B7A station of the Beijing Synchrotron Radiation Facility (BSRF), and the P and S K-edge X-ray absorption near-edge structure (XANES) spectra were measured in the total electron yield mode.

2.3 Electrochemical testing

The electrochemical performance of the as-prepared electrocatalysts was evaluated at room temperature through the rotating ring-disk electrode (RRDE) technique (RRDE-3A, ALS, Japan) controlled using an ALS/DY2323 Bi-potentiostat workstation. The electrochemical measurements were carried out on a standard three-electrode system. The working electrode was prepared by coating the electrocatalyst on a glassy carbon electrode (GCE) (surface area: 0.1256 cm2). The counter electrode is a graphite rod and the reference electrode is a saturated calomel electrode (SCE). All potentials in this work are given in reference to the reversible hydrogen electrode (RHE). To prepare the working electrode, 5 mg of electrocatalyst was added into 1 mL of ethanol containing 10 μL of Nafion solution, and then a homogeneous slurry was obtained after ultrasonically dispersing for 0.5 h. Subsequently, 21 μL of the dark slurry was dropped onto a freshly polished GCE uniformly and dried under ambient conditions (loading: 0.80 mgcat cm−2). As a comparison, 5 mg of the commercial Pt/C (20 wt% Pt) was ultrasonically dispersed into 1 mL of ethanol with 50 μL of Nafion to generate the Pt/C electrode (loading: 0.40 mgcat cm−2).

3. Results and discussion

3.1 Physical characterization

Scheme 1 shows the doping of S into N,P-HPC to synthesize a S, N and P ternary-doped HPC electrocatalyst by co-pyrolyzing N,P-HPC and THU, in which THU not only acts as a S precursor but also provides additional N for the obtained electrocatalyst. The co-pyrolysis process of N,P-HPC with THU involves two stages as indicated by the TG analysis (Fig. 1a). The first pyrolysis stage appears in the range of 200–320 °C, in which a significant weight loss of 63.13% is observed probably due to the release of organic species (Fig. 1a). The TG-FTIR analysis shows several infrared absorption bands at 1375, 1539, and 2071 cm−1, which are assigned to sulfur dioxide (SO2), carbon disulfide (CS2) and carbonyl sulfide (COS), respectively (Fig. 1b and S1),44 suggesting the escape of decomposition products derived from the co-pyrolysis of N,P-HPC with THU. The TG-MS spectra display two strong peaks at m/z of 17 and 16 corresponding to the NH3+ and NH2+ ions, respectively (Fig. 1c).45,46 The second carbonization stage appears from 300 °C with approximately 36.08% weight loss, in which the gases CO2, H2S, NH3 and HCN are generated as indicated by the TG-FTIR and TG-MS analyses (Fig. 1a–c). These emerging N- and S-containing species should not only facilitate the heteroatom-doping when they flow out along the porous channels in the carbon networks but also probably create abundant pores by means of the activation of small molecular gases (e.g., NH3, CS2 and H2S).47
image file: c9cy01387c-s1.tif
Scheme 1 Illustration of the synthetic process of S,N,P-HPC electrocatalysts.

image file: c9cy01387c-f1.tif
Fig. 1 (a) TG curve of the mixture of N,P-HPC and THU. (b) The relative contents of decomposition products generated during pyrolysis according to TG-FTIR. (c) Single ion current curves of co-pyrolysis of N,P-HPC with THU according to TG-MS. Representative (d) SEM and (e) TEM images of S,N,P-HPC-1. (f) The elemental mapping images of C, N, P, and S in S,N,P-HPC-1.

The FE-SEM and TEM images display the hierarchically porous characteristic of the S,N,P-HPC electrocatalysts inherited from N,P-HPC with interconnected macropores and mesopores (Fig. 1d and e, and S2). The microstructure of S,N,P-HPC gradually deviates from N,P-HPC with pores collapsing as the THU content increases, which suggests the destruction of the carbon matrix probably caused by the escape of generated gases during pyrolysis (Fig. S3). The TEM elemental mapping images show the relatively homogeneous distribution of heteroatoms (S, N and P) over the carbon frameworks of S,N,P-HPC-1, confirming the formation of S, N and P ternary-doped HPC by the co-pyrolysis of N,P-HPC and THU (Fig. 1f). The XRD pattern of S,N,P-HPC-1 shows a broad peak at approximately 24.6°, which represents the (002) diffraction plane of graphite (Fig. 2a), indicating the formation of partially crystalline carbon with a low graphitization degree.48 The Raman spectrum of S,N,P-HPC-1 shows the D′-line, G-line, D′′-line, D-line and I-line signals at about 1620, 1560, 1470, 1350 and 1180 cm−1, which are assigned to the disordered graphitic lattice, sp2-hybridized carbon vibration, stacking defects, lattice disorder and heteroatom doping defects, respectively (Fig. 2b).49,50 The peak intensity ratio of the D-line to G-line (ID/IG) of S,N,P-HPC-1 is determined to be 1.19, higher than that of N,P-HPC (1.06), and obviously increases with increasing THU content (Fig. 2b and d, and S4). This implies that the formation of disordered carbon can be induced by multi-heteroatom doping (Fig. 2b and d, and S4), which is consistent with reported literature studies.51 N2 adsorption–desorption measurements were conducted to characterize the pore microstructure of the as-prepared S,N,P-HPC-1. S,N,P-HPC-1 exhibits a type IV isotherm, characteristic of micro/mesoporous materials with a large SSA of 1533 m2 g−1 (Fig. 2d). Unlike the heat treatment of N,P-HPC (1516 m2 g−1) without THU resulting in a decreased SSA (after pyrolysis: 1482 m2 g−1), the co-pyrolysis of N,P-HPC with THU results in a slightly larger or almost unchanged SSA of S,N,P-HPC-1 in comparison to that of N,P-HPC (1516 m2 g−1) and the SSA is also gradually increased as the THU content increases (Fig. 2c and S5, and Table S1). The PSD based on the DFT model also shows an increased micropore size (0.8–2 nm) of S,N,P-HPC-1 with a higher micropore volume (0.49 cm3 g−1) in comparison to that of N,P-HPC (0.44 cm3 g−1) (Fig. 2c and S5, and Table S1). These results reveal that THU plays a vital role in the construction of a favourable pore structure with an improved SSA through activation of decomposition products (especially NH3) by virtue of them creating a large number of pores during their escape process (Fig. 2c and d, and S5, and Table S1).52 The large SSA and well-defined pronounced micro/mesoporous structure can be expected to contribute to an improved ORR activity by benefiting the exposure of diverse active sites and facilitating the mass transport of ORR reactants for ORR.53

image file: c9cy01387c-f2.tif
Fig. 2 (a) XRD pattern and (b) Raman spectrum of S,N,P-HPC-1. (c) N2 adsorption–desorption isotherm of S,N,P-HPC-1 (inset is the corresponding DFT PSD curve). (d) The summary of SSAs and ID/IG ratios (determined by Raman spectrum) of S,N,P-HPC with different P/S molar ratios.

The elements C (80.35 at%), O (11.31 at%), N (4.35 at%), P (2.96 at%), and S (1.29 at%) on the carbon networks are shown in the XPS survey spectrum of S,N,P-HPC-1, further confirming the successful ternary-doping of heteroatoms (S, N and P) into the carbon matrix (Fig. 3a and Table S2). The higher N content of S,N,P-HPC-1 (4.35 at%) compared to that of N,P-HPC (3.20 at%) suggests that THU as a S source also supplies extra N probably arising from the emergence of NH3 and HCN species during pyrolysis as indicated by the TG-FTIR and TG-MS analyses. The deconvoluted N 1s spectrum of S,N,P-HPC-1 can be divided into four types of N species with peaks at about 399.0, 400.3, 401.1, and 402.4 eV, corresponding to pyridinic-N, pyrrolic-N, graphitic-N and oxidized pyridinic-N, respectively (Fig. 3b).54,55 Graphitic-N and pyridinic-N have been proven to contribute to the ORR activity through modifying the chemical/electronic environments of neighboring carbon atoms.56,57 The content of pyridinic-N and graphitic-N of S,N,P-HPC-1 is calculated to be 3.60 at% (82.72% of total N), higher than that of N,P-HPC (2.11 at% (65.89% of total N)), possibly caused by the introduction of THU (Fig. 3b and Table S3). The deconvoluted P 2p spectrum of S,N,P-HPC-1 shows that P mainly exists in the form P–C (133.1 eV) and P–O species (134.1 eV).58 The P–C species are considered to be the ORR active sites according to previously reported literature studies and their content is determined to be 2.35 at% (79.36% of total P) for S,N,P-HPC-1 (Fig. 3b and Table S4). The high-resolution S 2p spectrum shows that the S dopant is in the forms –C–S–C– (164.4 eV), –C[double bond, length as m-dash]S– (165.5 eV), –C–SO2–C– (167.8 eV), –C–SO3–C– (169.7 eV), and –C–SO4–C– (171.5 eV), suggesting that S heteroatoms are doped into the carbon matrix by co-pyrolysis of N,P-HPC with THU (Fig. 3c and Table S4).59 It has been reported that the –C–S–C– and –C[double bond, length as m-dash]S– species are the major contributors to ORR activity and regarded as the catalytically active ORR sites.27 The total content of –C–S–C– and –C[double bond, length as m-dash]S– species is determined to be 1.07 at% (82.78% of total S) for S,N,P-HPC-1, which is the highest among all the samples (Fig. 3c and f, and Table S5). The presence of –C–S–C– species is further detected in the C 1s spectrum and the FT-IR spectrum with the vibration peak of a C–S (∼770 cm−1) bond (Fig. S6 and S7).60,61

image file: c9cy01387c-f3.tif
Fig. 3 (a) Full XPS spectra of N,P-HPC and S,N,P-HPC-1. XPS spectra of (b) N 1s, P 2p, and (c) S 2p of S,N,P-HPC-1. (d and e) P and S K-edge XANES spectra of S,N,P-HPC with different P/S molar ratios. (f) The relative intensity ratios SRed/SOx and the contents of –C–S–C– and –C[double bond, length as m-dash]S– species of the above samples.

To get further insight into the chemical state of heteroatoms in the carbon network, XANES measurements were performed on the S,N,P-HPC electrocatalysts. The P K-edge XANES spectrum shows an intense peak at approximately 2153.0 eV which is assigned to the P 1s → σ* transition and a shoulder peak at a lower energy by 2.0 eV attributed to the formation of P–C bonds, consistent with the XPS analysis (Fig. 3d).62 The S K-edge XANES spectrum displays two broad peaks at 2474.0 and 2482.2 eV, representing the reduced (SRed) and oxidized (SOx) sulfur species, respectively (Fig. 3e). The covalently bonded SRed species mainly comprise exocyclic S (S out of the carbon six-membered ring, i.e., C–S), heterocyclic S (S in the ring, i.e., C[double bond, length as m-dash]S) and sulfoxide (i.e., –SO–), while the SOx peak can be divided into sulfone (–SO2–), sulfonate (–SO3–) and sulfate (–SO4–) contributions.63 The relative intensity ratios SRed/SOx follow an approximate “volcanic” relation as a function of P/S molar ratio, and the S,N,P-HPC-1 with the P/S molar ratio of 1 has the highest content of SRed species (Fig. 3f).

Previously reported literature studies indicated that S doping can change the spin density of surrounding carbon atoms, which is reported to be more important for the enhancement of ORR activity in comparison to the surface charge density based on DFT calculations, and thus resulting in an enhanced intrinsic activity towards ORR.15,30

3.2 Electrochemical activity

The electrochemical results show that the highest ORR activity of S,N,P-HPC was obtained at the P/S molar ratio of 1 and the heat-treatment temperature of 900 °C (Fig. S8 and S9). The ORR performance of the electrocatalysts was firstly investigated by cyclic voltammetry (CV) measurements. All samples exhibit well-defined cathodic ORR peaks in the CV curves between 0.50 and 0.86 V upon saturation of O2 in 0.1 M KOH electrolyte, whereas they display featureless CV curves in the electrolyte saturated with N2. Notably, the S,N,P-HPC-1 electrocatalyst shows a more positive potential of 0.82 V for the ORR peak compared to the others (Fig. S8 and S9), suggesting that optimal doping of S indeed improves the ORR activity. RRDE measurement was further performed to evaluate the activity of the samples towards ORR (scan rate: 5 mV s−1; rotation rate: 1600 rpm). S,N,P-HPC-1 exhibits a significantly improved ORR activity with a half-wave potential (E1/2) of 0.881 V, 28 mV higher than that of N,P-HPC (E1/2 = 0.853 V) and even higher than those of the commercial Pt/C (E1/2 = 0.841 V) (Fig. 4a) and most reported carbon-based metal-free electrocatalysts (Table S6). Furthermore, S,N,P-HPC-1 produces a low yield of peroxide (HO2 %: below 5.0%) and the corresponding transferred electron number (n) was calculated to be close to 4.0 (Fig. 4b), demonstrating a high selectivity to the 4e transfer pathway of ORR. This result is also confirmed by the Koutecky–Levich (K–L) plots that indicate a high transferred electron number of approximately 4.0 for S,N,P-HPC-1 (similar to Pt/C (4.0)) (Fig. S10). The kinetic current density (Jk) at 0.85 V of S,N,P-HPC-1 is determined to be 14.68 mA cm−2, 2,42- and 2.86-fold that of N,P-HPC (6.05 mA cm−2) and Pt/C (3.826 mA cm−2), respectively (Fig. 4c). Tafel plots in the low overpotential region were plotted to investigate the kinetic process of ORR. This electrocatalyst shows a similar Tafel slope of 62 mV dec−1 to Pt/C, suggesting that the rate-determining step of ORR catalyzed by S,N,P-HPC-1 is probably the first electron transfer (Fig. 4d).64 Moreover, the ORR activity was also investigated in acidic electrolyte (Fig. 4e). Compared with that of N,P-HPC (E1/2 = 0.653 V), the ORR activity of S,N,P-HPC-1 (E1/2 = 0.714 V) is also enhanced in acidic electrolyte despite being inferior to that of the commercial Pt/C, indicating that S doping also enhances the ORR activity in acidic electrolyte. The K–L plots show a high transferred electron number of approximately 4.0 for S,N,P-HPC-1 and indicate the ideal four-electron path for ORR (Fig. 4f). These results suggest that the S doping can effectively improve the ORR activity in both alkaline and acidic electrolytes.
image file: c9cy01387c-f4.tif
Fig. 4 (a) RRDE curves of N,P-HPC, S,N,P-HPC-1 and the commercial Pt/C in O2-saturated 0.1 M KOH. (b) The calculated electron transfer numbers (n) and peroxide percentages (HO2%, collected at 1.5 V) of the samples. (c) Comparison of E1/2 and Jk (calculated at 0.85 V) of the above samples. (d) Tafel plots of the samples from the corresponding LSV data. (e) LSV curves of the N,P-HPC, S,N,P-HPC-1 and Pt/C electrocatalysts in O2-saturated 0.1 M HClO4 (scan rate: 5 mV s−1) with a rotation rate of 1600 rpm. (f) LSV curves of S,N,P-HPC-1 in O2-saturated 0.1 M HClO4 (scan rate: 5 mV s−1) at various rotation rates (inset shows the corresponding K–L plots (J−1 versus ω−1/2).

3.3 The origin of high ORR activity

To elucidate the enhanced ORR performance, the relevance between the ORR activity and heteroatom (i.e., S, N, and P) configurations determined from the XPS and XANES spectra of S,N,P-HPC was investigated. We found that the ORR activity is positively correlated to the SRed/SOx ratio and the best ORR activity of S,N,P-HPC is achieved at the highest SRed/SOx ratio (i.e., highest content of SRed species (i.e., –C[double bond, length as m-dash]S– and –C–S–C–)) among all the samples (Fig. 5). This reveals that the SRed species play a vital role in the enhancement of the ORR activity, which is in agreement with reported literature studies (Fig. 5).65 Several studies based on experimental and DFT calculation results have identified the thiophene-like S species (i.e., –C[double bond, length as m-dash]S– and –C–S–C–) as the major activity contributor to ORR. Zhu et al. adopted DFT calculations to reveal that S doping can lower the energy barrier of O2(ads) hydrogenation and lead to a high intrinsic activity, and the ORR performance was found to be significantly affected by the content of C–S–C and pyridinic N species.37 Guo et al. synthesized a S-doped Fe/N/C electrocatalyst and found the S incorporation gave a thiophene-like structure, which can reduce the electron localization around Fe centers and improve interactions with oxygenated species.66 Unlike the N and P heteroatom doping that can change the surface charge density of carbon atoms (N and P have a higher and lower electronegativity than C, respectively), S doping tends to mainly induce a redistribution of spin density over the carbon networks but with a weak charge transfer effect since S has a similar electronegativity to C. Compared to the heteroatom-doping induced charge transfer, the heteroatom-doping induced spin redistribution has been theoretically predicted to have a more pronounced effect on the ORR active sites and thus leads to a significant improvement of ORR activity.67 Besides, pyridinic N, graphitic N and P–C species derived from N and P doping also act as active sites and contribute to the ORR activity. Based on the above analyses, we speculate that the diverse and abundant active sites originating from S, N, and P ternary-doping could create a unique electronic structure through the synergistic spin and charge effects, leading to an expanded electron delocalization and thus improving the intrinsic activity for ORR. More importantly, the 3-D hierarchical micro/mesoporous architecture of S,N,P-HPC-1 is of great importance to enhance the exposure of the accessible active sites and thus improve mass transfer of the reactants for ORR and electrolytes. All these factors are favorable to facilitate the O2 adsorption, making the multi-heteroatom-doped 3-D carbon electrocatalyst possess superior ORR activity.
image file: c9cy01387c-f5.tif
Fig. 5 The ORR activity of S,N,P-HPC electrocatalysts and relative intensity ratio SRed/SOx versus the P/S ratio.

3.4 Electrochemical durability and anti-poisoning capability

Durability and anti-poisoning capability are major factors that should not be ignored for the commercial application of fuel cells. The chronoamperometric measurements were carried out in O2-saturated 0.1 M KOH electrolyte at a constant voltage of 0.85 V (rotation rate: 900 rpm) to evaluate the electrochemical durability. Unlike the obvious decrease (38.61%) in the current density of the commercial Pt/C after a 10[thin space (1/6-em)]000 s test, S,N,P-HPC-1 displays superior durability with only a 3.69% decrease (Fig. 6a). Moreover, upon the addition of 3 mol L−1 methanol, the S,N,P-HPC-1 electrocatalyst shows a tiny current change that demonstrates its outstanding methanol tolerance (Fig. 6b). In contrast, an obvious decrease of the current with Pt/C is detected and that indicates the methanol poisoning effect. Since some small molecular species (e.g., SOx, NOx, and POx existing in air) tend to poison the electrocatalyst (especially Pt-based electrocatalysts) and thus decrease the performance of fuel cells,68–72 the ORR activity of the electrocatalyst was also monitored in O2-saturated 0.1 M KOH in the presence of SO32−, NO2 and HPO42−. As shown in Fig. 6d, Pt/C was severely poisoned with significant activity loss by the addition of SO32−, NO2 and HPO42− (in the order SO32− > NO2 > HPO42−) and the activity was not able to be recovered by subsequently retesting the electrocatalyst-coated electrode in fresh 0.1 M KOH electrolyte. In sharp contrast, almost no activity degradation is observed for S,N,P-HPC-1 whether in the presence of SO32− or NO2 or HPO42− (Fig. 6c). This result reveals that the as-synthesized S, N, and P ternary-doped metal-free electrocatalyst exhibits excellent anti-poisoning capability against small molecular species like SOx, NOx, and POx, making it a promising candidate for high-performance electrocatalysts toward ORR in fuel cells.

4. Conclusions

In summary, high-performance S, N, and P ternary-doped 3-D hierarchically porous carbon was developed as an ORR electrocatalyst by additionally doping S into N,P-HPC through co-pyrolysis of N,P-HPC with THU. S doping increases the types of efficient active sites in the form of SRed species identified by XANES. SRed species tend to mainly induce the redistribution of spin density and thus significantly improve the ORR activity in both alkaline and acidic electrolytes. Moreover, the as-prepared S,N,P-HPC-1 well inherits the hierarchically porous structure of N,P-HPC with an almost unchanged large SSA (1533 m2 g−1) that benefits the exposure of active sites and facilitates the mass transport of ORR reactants. Owing to these features, the optimal S,N,P-HPC-1 electrocatalyst (N (4.50 at%), P (2.96 at%) and S (1.29 at%)) exhibits a superior ORR activity, excellent electrochemical durability and outstanding anti-poisoning capability, outperforming the commercial Pt/C. Therefore, S,N,P-HPC-1 is a promising efficient carbon-based electrocatalyst for ORR to substitute Pt/C in fuel cells.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the National Natural Science Funds of China (No. 51432003 and 51732002) and the National Key R&D Program of China (2018YFB0105500). We thank Dr. S. Liu at the BSRF for the assistance with the XANES measurements.


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cy01387c

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