Hierarchical porous N-P-coupled carbons as metal-free bifunctional electro-catalysts for oxygen conversion

Accepted Manuscript Full Length Article Hierarchical Porous N-P-Coupled Carbons as Metal-free Bifunctional Electrocatalysts for Oxygen Conversion Z. Zhou, A. Chen, X. Fan, A. Kong, Y. Shan PII: DOI: Reference:

S0169-4332(18)32511-X https://doi.org/10.1016/j.apsusc.2018.09.095 APSUSC 40399

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

14 June 2018 14 August 2018 10 September 2018

Please cite this article as: Z. Zhou, A. Chen, X. Fan, A. Kong, Y. Shan, Hierarchical Porous N-P-Coupled Carbons as Metal-free Bifunctional Electro-catalysts for Oxygen Conversion, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.09.095

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Hierarchical

Porous

N-P-Coupled

Carbons

as

Metal-free

Bifunctional Electro-catalysts for Oxygen Conversion Z. Zhou a, A. Chen a, X. Fan a, A. Kong a* and Y. Shan a* a School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai, 200241, P.R.China

Abstract: Hierarchical porous N and P co-doped carbons were facilely synthesized by employing

phenoxycycloposphazenes (PCPZs)

as multi-source of nitrogen,

phosphorus and carbon. The pyrolysis of PCPZs that supported on monodispersed silica spheres produced high-surface graphitized carbons (N,P-GCs) with abundant surface N-P coupling sites. The N,P-GCs were demonstrated as efficient metal-free electro-catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The pore structures of these carbons for exposing more active sites could be adjusted by importing varisized SiO2 spheres (10-50 nm) as hard templates. Typically, N, P-GC-1000 using 30 nm SiO2 as templates exhibits the ORR half-wave potential of 0.85 V in 0.1 M KOH solution, with a loading density of 0.35 mg cm-2 on the electrodes, which is comparable to commercial Pt/C catalyst (0.1 mg cm-2, 20 wt%, JM). Moreover, the OER over N, P-GC-1000 electrodes (0.65 mg cm-2) could also reach 10 mA cm-2 current density at a lower potential of 1.56 V, close to typical RuO2 electrode (1.59 V, 0.2 mg cm-2, Premetek). This work not only suggested a simple and effective pathway to prepare N-P-doped carbon with N,P-coupled sites, but also provided a very efficient metal-free bifunctional carbon electro-catalyst for ORR and OER. *Corresponding author. E-mail addresses: [email protected]

Keywords Non-metal catalysts; N-P coupling; Oxygen Reduction Reaction; Oxygen Evolution Reaction 1.

Introduction

Proton exchange membrane fuel cells (PEMFC) and water-spitting system are deemed to be the most prospective energy generation and storage devices by the interconversion of water and clean renewable energy (H2 and O2).1 The hearts of these devices are oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In ORR, water is formed by combining oxygen molecules and electrons and the reverse process occurs in OER.

2,3

However, both of these processes need high

overpotential to be triggered and thus electro-catalysts are required to accelerate and optimize these reactions in the practical application. Current commercial catalysts with satisfying electro-performance are precious metal-based catalysts (Pt, RuO2, etc.). But all of them have disadvantages as expensive cost, natural store deficiency and poor long-time durability, which become barriers for the mass market. 4-8 Diverse attentions and various efforts have focused on the replacement by non-precious metal catalysts (NPMC) or pure metalloid catalysts (PMC) for oxygen conversion.9-17 In comparison, PMC are commonly more stable and methanol-tolerant than NPMC, thus more appreciated as substitutes for traditional electro-catalysts.18 A novel class of carbon catalysts doped with various metalloid heteroatoms was found with splendid electro-properties. Among all heteroatoms, nitrogen is preferred by its similar atomic radius but different electronic configurations with carbon, which 2

endows N-doped carbons with new electronic structures in the minimum lattice mismatch.19,20 On the basis of N-doping, importing another heteroatom and forming co-doped carbon can verify the polarities and electronic distribution of the catalyst to further optimize its electro-activity.21 Phosphorus is additionally appreciated due to its similar electronic structure to nitrogen but better electron-donating ability. It also has the tendency to induce charge density by its lone pair electrons in 3p orbitals as well as embrace the lone pairs in O2 via its empty 3d orbitals.

8,22-24

The N-P-codoping in

the carbon frameworks often result into the enhancement of the catalytic performance for ORR and OER.25 However, the reported N, P-codoped carbon catalysts which used graphene, fullerenes or carbon nanotube (CNT) as carriers cannot often deny the contribution of trace metal impurities during the catalytic process. For some N, P-codoped carbons without such carriers, their electrocatalytic performance still need further improvements for acting as advanced ORR and OER bifunctional catalysts.26 Herein, we attempted to prepare a series of metal-free N,P-doped carbons with the efficient electro-catalytic performance for both ORR and OER. The commercial phenoxycycloposphazene (PCPZ) was specially chosen as a tri-resource of N, P and C owing to its pre-existing N-P coupling moieties and eradication of any metal trace. Silica nanospheres with different diameters were employed as hard-templates to create highly conductive porous carbon structure and avoid simple mechanic stacking and sintering, as shown in Figure 1.27 After simple grinding, pyrolysis and etching, metal-free carbon catalysts with N-P-coupling sites and hierarchical porosities were synthesized and exhibited more prominent electrocatalytic performance for both ORR 3

and OER reactions. 2. Experimental 2.1 Preparation of N, P-GC materials 1 g of PCPZ and 0.6 g of 30 nm silica sphere were mixed and grinded in an agate mortar for 20 min. The resulting white powder was calcined to 900-1100 °C at a heating rate of 2 C min-1 and preserved for 4 h in a quartz tube furnace under nitrogen atmosphere. After the etching the SiO2 template by 5 wt% HF solution for 24 h, the mixture was filtrated and washed to pH=7. In order to optimize the porous structure and graphitization degree of the carbon framework, different diameters of SiO2 sphere was added during the synthesis process in different proportion under various calcining temperature, the prepared materials were named as N, P-GC-d (d is the SiO2 diameter ) or N, P-GC-n (n is the thermal temperature). For comparison, the PCPZs without SiO2 spheres were directly calcined to give N,P-codoped carbons (N, P-GC-1000-A). 2.2 Physical characterization. A JEM-2010 transmission electron microscope was employed to record the transmission electron microscope (TEM) images at an acceleration voltage of 120 kV. Powder X-ray diffraction patterns (XRD) were performed on a D8 Advance X-ray diffractometer (Bruker AXS, Germany) with Cu Kα radiation（λ=1.5406A）. A Hitachi S-4800 scanning electron microscope (SEM) was engaged to determine the morphology. N2 adsorption/desorption measurements were conducted at 77 K on the QUADRASORBEVO Gas Sorption Surface Area and Pore Size Analyzer. X-ray 4

photoelectron spectroscopy (XPS) measurements were gained on ThermoFischer ESCALAB 250Xi. Fourier transform infrared (FT-IR) spectra were measured on infrared spectrophotometer (Bruker Tensor 27) in KBr pellets. A GX-PT-1500 (150) instrument with a 532 nm excitation laser (1 mW) was used to achieve Raman spectroscopy. 2.3 Electrochemical test. The electro-catalytic activities were measured by cyclic voltammetry (CV) and ring-rotated disk electrode (RRDE) on an CHI760C electrochemical workstation, which includes a conventional three-electrode cell system containing a working glass carbon RDE (Pine, 5 mm), a reference electrode (Ag/AgCl, 3M KCl electrode), and a counter electrode (Pt electrode) for both ORR and OER analysis. The electro-catalytic activities for ORR in alkaline electrolyte were conducted in O2-saturated 0.1 M KOH solution. The measurements in acidic electrolyte were carried out in O2-saturated 0.1 M HClO4 solution. The catalyst suspensions were prepared by dispersing and sonicating 10 mg of as-synthesized N, P-GC in 1.25 mL of anhydrous ethanol and 30 μL of a 5 wt % Nafion solution. A glassy carbon electrode surface was polished by nano-Al2O3 and dispersed by deionized water and anhydrous ethanol for 3 minutes, before pipetting catalyst ink on it, then drying under infrared light at room temperature. The loading of N, P-GC on the working electrode was 0.35 mg cm−2 in 0.1 M KOH solution and 0.6 mg cm−2 in 0.1 M HClO4 solution. A commercial 20 wt% Pt/C catalyst obtained from JM Company was used for comparison. The Pt/C loading was 0.1 mg cm-2 under both conditions. The ORR current was gained by subtracting 5

the current measured in N2-saturated electrolytes from the current measured in O2-saturated one then adjusted to the reversible hydrogen electrode (RHE). To conduct the kinetic analysis for ORR, the Koutecky-Levich (K-L) equations were used.

The OER test was conducted in O2-saturated 1 M KOH solution at 5 mV s-1 scan rate. The loading of synthesized catalysts was 0.65 mg cm-2 and the preparation the working electrode was the same as in ORR test. Tafel plots (b) were calculated from LSV curves as overpotential (η) versus log current (log J).

3 Results and discussions 3.1 Morphology and Structure of the N, P-GC In order to investigate the morphology of the prepared samples, SEM, TEM and N2 adsorption/desorption analysis were conducted. The SEM results (Figure 2a and 2b) showed that the N, P-GC-1000 was irregular 3D bulk foam with abundant circular pores full of its uneven surface. As can be seen in Figure 2c and 2d, the multi-fold and wrinkled features in TEM analysis represented the abundant pores and boundaries, which indicated a good agreement with the previous SEM analysis. The mesopores were roughly 25 nm in diameter and randomly distributed in close proximity to each other. The decline of radius compared with hired SiO2 spheres may be caused by the incomplete encirclement when mixing two ingredients. The porous nature was also confirmed by N2 adsorption/desorption measurements (Figure 3). The isotherm curve 6

for N, P-GC-1000 can be identified as type Ⅳ by its steep upwarp at relative-high pressure, which was quite characteristic as hierarchical porous materials.28.29 The concentrated pore size distribution curves (Figure 3 inset) obtained from the isotherm of N, P-GC-30 heated at different temperatures suggested that the average pore diameters

were

centered

at

0.5

nm

and

15-25

nm.

Typically,

the

Brunauer-Emmett-Teller (BET) surface area of N, P-GC-1000 was consist of 478 m2 g-1 micropore surface and 524 m2 g-1 mesopore surface (Table 1). Thereinto, mesopores were presumably resulted from the vacancy after etching SiO 2 templates. The micropores were usually attributed to the volatilization of bubble on the surface during pyrolysis. The total surface areas of each sample were as much as 1,483 m2 g-1(N, P-GC-900), 1,002 m2 g-1 (N, P-GC-1000) and 805 m2 g-1(N, P-GC-1100). All evidences demonstrated a successful synthesis of hierarchical porous N-P coupling doped carbons. The graphitization degree was inquired by selected-area electron diffraction image (SAED), X-ray diffraction (XRD) patterns and Raman spectrum. The SAED (Figure 2d inset) convinced no trace of metal impurities but only the diffuse rings of carbon and a bright center, declaring high graphitic degree of the material. 30,31 As can be seen in Figure 4a, there are obvious peaks at 22.8 and 43.8, corresponding to the (002) and (101) reflection of the graphitic-type lattice (JCPDS card no. 41-1487).32 With the raise of temperature, the diffraction peaks became sharper and stronger, suggesting more intense graphitization degree of the material. The graphitization degree of carbon frameworks can be also evaluated from the intensity ratio of D band (Defect, 7

1345 cm-1) and G band (Graphite, 1588 cm-1) in Raman spectrum (Figure 4b). The G band corresponds to an ideal graphitic lattice vibration mode with E2g symmetry. D band is known to be characteristic for disordered graphite and becomes sharper with increasing degree of disorder in the graphitic structure. 33 The ID/IG value was in the reverse proportion of pyrolysis temperature for the prepared samples (1.05 for N, P-GC-900, 1.04 for N, P-GC-1000 and 1.00 for N, P-GC-1100). This phenomenon means that higher pyrolysis temperature reduces defect sites in the carbon layer and reinforces the degree of sp2 carbon .34-36 3.2 Bonding Condition and Surface Composition of N, P-GC SEM-EDS element mapping technology convinced an average and extensive distribution of N, P on the surface of N, P-GC-1000 (Figure S1). FT-IR analysis was carried out to verify the bonding information of N, P-GC. As shown in Figure 5a, the peaks at 879 cm-1 and 1176 cm-1 of PCPZ precursor can be ascribed from the stretching vibrations of P-N and P=N. These signals could be also observed in N, P-GC-1000. It indicated that a few of P-N coupling sites should be remained in the surface of carbons, though undergoing the carbonization and etching of PCPZ precursors. To further inquire elemental status and content in the carbon surface, XPS analysis was conducted. The full XPS spectra of N, P-GC-1000 (Figure 5b) showed no trace of metal but only the presence of C, O and limited P (1.01 at%) and N (2.35 at%). The higher temperature would remove nitrogen as well as phosphorus heteroatoms dopants by thermal decomposition. The contents of N and P on the surface of N, 8

P-GC-900 were 3.67 and 2.01 at% (Table 2), higher than N, P-GC-1000. Higher heat-treatment temperature of 1100 C resulted into the further decreasing of surface N and P content. Corresponding, the carbon content was elevated from 79.6 at% at 900 C to 92.32 at% at 1100 C. In the N,P co-doped carbons, N and P may exist as the isolated N-dopant, the isolated P-dopant and/or as N,P-coupled dopants. The isolated N-doped, the isolated P-doped and the N,P-coupled structures were identified as very active for ORR, while the N,P-coupled doping gives the best OER performance in alkaline media. 37 After P atoms are bound to N co-dopant in the form of N-P coupled sites, these P-doped sites stabilize the graphitic N site and activate a neighboring C site for more effective OER.38.39 The high-resolution of XPS N 1S spectrum of the prepared N, P-GC-1000 (Figure 5c) presented that surface nitrogen types can be classified into four species, namely pyridinic-N (398.4 eV, N1), pyrrolic-N(399.9 eV, N2), graphitic-N (400.9 eV, N3), and oxidized-N (403.0 eV, N4).40 These diverse isolated N-doped types provided various electronic and chemical environments for its neighboring carbons. More pyrrolic-N and oxidized-N turned into pyridinic-N and graphitic-N at 1000 C, which were regarded as the most active species among four types.41 However, it is difficult to clearly distinct N type in the form of P-N by fitting N 1s spectra. Fortunately, the P 2p spectra (Figure 5d, S2c and S2f) clearly indicated three chemical environments for phosphorus atoms (isolated P-doped sites at 131.6 eV, P-N sites at 133.6 eV, and P-O sites at 134.1 eV).42 The high ratio of P-N peak in the P 2p spectra proved the successful maintenance of abundant P-N coupling sites on the carbon surface of N, 9

P-GC-1000, in agreement with FT-IR result. The formation of abundant P-N coupled sites on the surface of deriving carbons confirmed by XPS and FT-IR results should be owed to special phenoxycycloposphazene precursors with pre-casting P-N moieties. 3.3 Electrochemistry Performance for ORR The ample N-P couplings and porous features endowed N, P-GC with effective ORR catalytic performance. The influence of SiO2 diameter on the ORR catalytic performance of the prepared N, P-GC was investigated. The LSV curves (Fig. S3a) demonstrated that SiO2 with 30 nm diameter shifted more positive half-wave potential as well as onset potential (Table S2), probably because the material achieved a perfect balance between microporosity and mesoporosity by employing 30 nm spheres. This hierarchical porosity delivered electrons and oxygen molecules under the most ideal condition than others (Table S1). The mass ratio of PCPZ and SiO2 were also studied (Figure S3c). The ratio of PCPZ to SiO2 of 1 g: 0.6 g showed approximately 70 mV positive half-wave potential than other matching ratio, indicating 1 g: 0.6 g was the most desired matching ratio (Table S3). The LSV curves for N, P-GC-n prepared at different temperatures (n is the pyrolysis temperature) exhibited that N, P-GC-1000 had the most positive onset potential (E onset=1.03 V) and half-wave potential (E1/2=0.85 V), owing to the optimized local structures and active doped sites of N, P-GC-1000 at this pyrolysis temperature. The higher heat-treatment temperature was beneficial to the graphitization of carbon framework, while overheat might lead to the collapse of porosity and the decomposition of nitrogen, phosphorus-contained active 10

sites. The ORR half-wave potential over N, P-GC-1000 is 30 mV more positive than the reported N, P codoped carbons without carriers containing metal impurities (0.82 V).26 Compared with N, P-C hiring CNT and graphene carriers reported in the literature, N, P-GC-1000 exhibits close half-wave potential but larger current density (4.9 mA cm-2 at 0.2 V versus 4.3 mA cm-2 at 0.2 V).43 The ORR CV curve over N, P-GC-1000 in 0.1M KOH solution saturated with O2 had an obvious cathodic peak at the potential of 0.89 V, in contrast to the featureless curve in N2-satureated solution. Tafel curves (Figure 6e) and its corresponding slopes (Figure 6f) were also calculated to verify the catalytic ability of N, P-GC-n. The Tafel slope of N, P-GC-1000 was smaller than other two catalysts (87 mV/dec), indicating better kinetic and intrinsic catalytic activity in KOH solution. All results proved that N, P-GC-1000 can work as a remarkable ORR catalyst in alkaline solution. The catalytic performance of N, P-GC in acidic medium was also investigated in 0.1 M HClO4 solution. Unlike many reported heteroatom-doped metalloid carbons whose catalytic effects are disappointing in acidic electrolyte, the N, P-GC showed the impressive catalytic activity for ORR. The N, P-GC-1000 exhibited the prime ORR catalytic performance in acidic solution. The half-wave potential reached 0.64 V, 110 mV more positive than pure metalloid N, P codoped carbons and 20 mV better than N, P-C employing CNT or graphene carriers with metal impurities.26,43 Moreover, the limiting current density reached 6.3 mA cm-2 (Figure 6d), larger than Pt/C electrode (0.1 mg, cm-2).

11

To inquire the kinetic and catalytic process of the catalyst, the RDE polarization technique was conducted at different rotation speeds from 400 to 2500 rpm in both electrolytes (Figure S4a and S4c). With the increase of rotation speed, it took less time for oxygen molecules to reach the surface of electrode and replenish the absorbed oxygen on the surface. As a result, the current density was directly proportional to the rotation speed. The ORR curves exhibited similar platform below 0.7 V in KOH solution, suggesting approximate reaction between 0.1 and 0.7 V. The K-L curves were achieved on the basis of the potential from 0.1 to 0.6 V in alkaline medium (Figure S4b) and 0.2 to 0.4 V in acidic medium (Figure S4d). All plots at different potential were in favorable linear distribution, conforming to the characteristic of first order reaction kinetics and indicating that the reaction velocity was linearly related to the concentration of reactors.44 The slopes of K-L curves demonstrated that the ORRs in both electrolytes were nearly four electron transfer processes, which are more beneficial to maintain the efficiency than two electron transfer process and avoiding catalytic poisoning. The durability of N, P-GC-1000 in both alkaline (Figure S4e) and acidic (Figure S4f) media were investigated by continuous CV scanning. From CV curves in 0.1 M KOH, the ORR peak slightly shifted 30 mV after running 5,000 cycles. The comparison in 0.1 M HClO4 indicated no shift of the peak but the decrease of current density. The results proved better ORR catalytic stability of N, P-GC-1000.45 3.4 Catalytic Properties for the OER N-doped carbons can work as electro-catalysts for OER as reported. The OER 12

performance of N, P-GC-1000 was also inquired in 1 M O2-saturated KOH (Figure 7a). The overpotential of N, P-GC-1000 is the lowest when the loading reached 0.68 mg cm-2. Presumably, the overloading made the catalysts drift from the surface of electrode when rotating because of its high surface area and porous structures. The potential at 10 mA cm-2 is another vital factor to assess catalytic performances of different catalysts in OER. It only needs a potential of 1.56 V over N, P-GC-1000 electrode. It suggested this catalyst with an increased loading was among the best metal-free electro-catalysts, comparable to RuO2 electrode (0.2 mg/cm2, 1.59 V).43,46 The stability of N, P-GC-1000 for OER was additionally evaluated by scanning CV continuously for 2,000 s. From the comparison (Figure 7b), at the 10 mA cm-2 current density, the origin OER peak at 1.56 V had a slightly shift to 1.59 V, which proves that even after a long period of work, N, P-GC still exceed most OER catalysts among the metal-free carbons. To gain more kinetic information about OER activity, Tafel plots calculated from polarization curves were constructed (Figure 7c). The slope over the electrode with the most optimized loading content was 67 mV dec-1 in polarization tests, consistent with comparison results about overpotentials at 10 mA cm-2.47,48 These results indicated that N, P-GC-1000 exhibited superior performance than other metal-free electro-catalysts not only in catalytic overpotential but also from kinetic perspective. Figure 7d is the comparison between Tafel slope and potential (J=10 mA cm-2), it can be seen that smaller overpotential corresponded smaller Tafel slope and faster kinetic process, which is expected in the real application. Conclusively, the as-synthesized 13

N,P-GC carbons at a higher loading density can exhibits comparable ORR and OER performance with commercial catalysts (Pt/C, RuO2). Considerable active N-P coupled sites on the carbon surface of the derived N, P-GC-1000, together with the optimized local structures attributed to its higher ORR and OER performance. In order to investigate the role of SiO2 in the catalytic process, the electro-catalytic performance of N, P-GC-1000-A from the direct of PCPZs was also studied. As shown in Figure S5a, the ORR LSV curves in both KOH and HClO4 electrolyte are much higher overpotentials than the prepared carbons hiring SiO2 as templates. It is even difficult to acquire the half-wave potential and limiting density because of the deficiency of balanced current platform. As to OER catalytic performance, the current density is lower than 10 mA cm-2 when the potential is double to commercial RuO2. The disappointing performance of N, P-GC-1000-A should be related to the shortage of mesoporosity in its pore structures, confirmed by the inexistence of hysteresis loop in the corresponding N2-sorption analysis (Figure S5c and Table 1). The dominant microporosity of N, P-GC-1000-A induced the problems in mass transport (O2, electrolytes etc.) and reduced the accessibility of active sites, as proven by previous studies.

49,50

Different from N, P-GC-1000-A, the SiO2 nanospheres as sacrificial

templates avoided the seriously sintering of N, P-GC-1000 and endowed them abundant large-pore mesoporosity, which finally contributed to higher ORR and OER activity of N, P-GC-1000.29, 30, 51-54 4 Conclusions

14

In summary, metal-free carbons with N-P coupling sites was synthesized by a simple pyrolysis of special phenoxycycloposphazene precursors without introducing transition metal impurities, which could remarkably work as bifunctional electro-catalysts for both ORR and OER. After the thorough adjustment of SiO 2 diameter, mass ratio and pyrolysis temperature, the prepared N, P-GC-1000 not only had the very efficient catalytic performance for ORR in alkaline media (E1/2=0.85 V), but also showed the impressive catalytic performance in acidic media. The OER overpotential over N,P-GC-1000 electrode with elevated loading content was close to RuO2 electrode at 10 mV cm-2, better than most of N, P-codoped carbons. This work not only provided a high-efficient bifunctional catalyst for ORR and OER, but also proved that metal-free carbons could produce high catalytic performance by optimizing the porosities and active site types of carbon. Acknowledgments The authors are grateful to financial support from National Natural Science Foundation (No. 21303058) and Shanghai Municipal Natural Science Foundation (No. 13ZR1412400 and 11JC1403400). References [1] X.-X. Ma, X.-Q. He, T. Asefa, Hierarchically porous Co 3C/Co-N-C/G modified graphitic carbon: A trifunctional corrosion-resistant electrode for oxygen reduction, hydrogen evolution and oxygen evolution reactions, Electrochim. Acta 257 (2017) 40-48.

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23

Figure captions Figure 1 Illustration for synthesis process of N, P-GC electro-catalysts Figure 2 (a, b) SEM images, (c, d) TEM images with inset in (d) showing SAED patterns of N, P-GC-1000 Figure 3 N2-sorption isotherm of N, P-GC-1000 with the mesopore and micropore size distribution curves (insets) of N, P-GC-900, N, P-GC-1000, N, P-GC-1100 calculated by (a) HK method and (b) BJH method, respectively. Figure 4 (a) XRD patterns of N, P-GC-900, N, P-GC-1000 and N, P-GC-1100; (b) Raman spectrum of N, P-GC-1000 Figure 5 (a) FT-IR spectra of PCPZ and N, P-GC-1000; (b) Survey XPS spectrum; (c) high-resolution spectra of N 1s and (d) P 2p of N, P-GC-1000 Figure 6 CV curve of N, P-GC-1000 in 0.1 M KOH electrolytes (a) and 0.1 M HClO4 (b); RDE polarization curves of N, P-GC hiring 30 nm SiO2 under different calcining temperature in 0.1 M KOH (c) and in 0.1 M HClO 4 electrolytes. (d) the corresponding Tafel curves (e) and their slopes (f) in 0.1 M KOH. Figure 7 The OER LSV curves of N, P-GC-1000 at different capacity (a), stability test after 2,000 s (b), corresponding Tafel slopes (c) and their OER performance comparative columns (d) in 1 M KOH solution. Table 1-Structural property for N, P-GC hiring 30 nm SiO2 templates by N2 absorption-desorption analysis Table 2-XPS elemental analysis for atomic percentage of N, P-GC

24

Graphical Abstract

25

Figures

Fig.1

26

Fig.2

27

(a)

)

N,P-GC-900 N,P-GC-1000 N,P-GC-1100

d(V) / dW

-1 3

Quantity Absorbed (cm g

(b)

N,P-GC-900 N,P-GC-1000 N,P-GC-1100

2000

1500 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0

Pore Size (nm)

20

40

60

80

100

Pore Size (nm)

1000

500 Adsorption Desorption

0 0.0

0.2

0.4

0.6

0.8

Relative pressure (P/P0) Fig.3

28

1.0

N,P-GC-900 N,P-GC-1000 N,P-GC-1100

Intensity (a u.)

(a)

10

20

30

40

50

60

2Theta (2)

D

(b)

G

ID/IG=1.05

N,P-GC-900

ID/IG=1.04

N,P-GC-1000

ID/IG=1.00

N,P-GC-1100

500

1000

1500

2000

2500 -1

Raman Shift (/cm )

Fig.4

29

3000

70

PCPZ N,P-GC-1000

C 1s

(b) Intensity (a.u.)

% Transmittance

(a)

P-N

P=N

O 1s N 1s P 2p

P-N P=N

4000

3500

3000

2500

2000

1500

1000

500

1000

800

-1

(c)

396

398

400

402

404

200

0

406

408

P 2p P-C P-N P-O

Intensity (a.u)

Intensity (a.u.) 394

400

(d)

N 1s

Pyridinic-N Pyrrolic-N Graphitic-N N-oxide

600

Binding Energy (eV)

Wavenumber (cm )

124

126

128

130

132

134

136

Binding Energy (eV)

Binding Energy (eV)

Fig.5

30

138

140

-0.6

-1.5

(a)

(b)

-0.4

-1.0 -0.5

-2

J (mA cm )

-2

J (mA cm )

-0.2 0.0 0.2 0.4

O2

0.6

N2

0.8 0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5

O2

1.0

N2

1.5 0.0

1.2

0.2

Potential (V vs. RHE)

0.4

0.6

0.8

1.0

1.2

1.4

Potential (V vs. RHE)

-1 0

0

(c)

(d) N,P-GC-900 N,P-GC-1000 N,P-GC-1100 Pt/C

1

N,P-GC-900 N,P-GC-1000 N,P-GC-1100 Pt/C

3

2

-2

2

J (mA cm )

-2

J (mA cm )

1

4

3 4 5

5 6 6 7 0.2

0.4

0.6

0.8

1.0

1.2

0.2

Potential (V vs. RHE)

0.4

0.6

0.8

1.0

Potential (V vs. RHE) 0.2

Overpotential (V vs. NHE )

(e) -2

Log J (mA cm )

0

-1

-2

N,P-GC-900 N,P-GC-1000 N,P-GC-1100

-3 0.0

0.2

0.4

(f)

0.3

N,P-GC-900 N,P-GC-1000 N,P-GC-1100

87 mV/dec

89 mV/dec

113 mV/dec 0.4

-0.5

0.0

0.5

1.0 -2

Log J (mA cm )

Overpotential (V vs. NHE )

Fig.6

31

-2

(b) 25

Initial After 2,000 s

20 -2

10

5

15 10 5

0

0 1.4

1.5

1.6

1.7

1.4

Potential (V vs. RHE) 1.9

c

c

ec V

0

-2

Potential (J=10 mA cm

)

1.6 1.5

1.4

1.4

0.1

150

de V/

1.7

m

c

1.8

67

/de

(d)

)

50

/d

mV

mV

1.8

/de /de

1.6

60

Tafel Slope (mV dec

m

78

mV

2 RuO2

1.5

1.7

100

74

1.7

1.6

-1

(c)

c

1.8

1.5

Potential (V vs. RHE)

-2

0.50 mg cm -2 0.68 mg cm -2 0.85 mg cm -21.20 mg cm

90

Potential (V vs. RHE)

J (mA cm

-2

)

15

30

(a)

0.50 mg cm -2 0.68 mg cm -2 0.85 mg cm -2 1.20 mg cm -2RuO2 2

J (mA cm )

20

1 -2

J (mA cm )

1.3

10

0.50

0.68

0.85

1.20 -2

Loading (mg cm )

Fig.7

32

RuO2

Table 1-Structural property for N, P-GC hiring 30 nm SiO2 templates by N2 absorption-desorption analysis

SBETa

Smicrob

Smesob

Vtotalc

Vmicrob

Vmesod

Dv(d)

(m2/g)

(m2/g)

(m2/g)

(cc/g)

(cc/g)

(cc/g)

(nm)

N, P-GC-900

1483

703

780

2.86

0.32

2.54

17.3

N, P-GC-1000

1002

478

524

2.62

0.21

2.41

12.4

N, P-GC-1100

805

273

532

3.28

0.12

3.16

17.6

N, P-GC-1000-A

1084

1067

17

0.43

0.43

0

0.46

Samples

a

SBET is calculated from multi-point BET method.

b

Data obtained from t-plot method analysis.

c

Vtotal is achieved from total pore volume summary.

d

Vmeso=Vtotal-Vmicro

Table 2-XPS elemental analysis for atomic percentage of N, P-GC Samples

C (%)a

O (%) a

N (%) a

P (%) a

N, P-GC-900

79.6

14.72

3.67

2.01

N, P-GC-1000

89.49

7.14

2.35

1.01

N, P-GC-1100

92.32

5.3

1.88

0.5

a

Element content determined by the atomic weight content in elemental analysis.

33

Highlights  Metal-free catalysts with abundant N-P-coupled active sites were derived from phenoxycycloposphazene (PCPZ). 

The hierarchical porosity with ultrahigh surface area exposed more accessible active sites and optimized the transportation of oxygen molecules.

 The obtained N, P-GC foams exhibited impressive catalytic performance for both ORR and OER.

34