Phosphorous and Nitrogen Dual-Doped Carbon as a Highly Efficient Electrocatalyst for Sodium-Oxygen Batteries


 Sodium-oxygen batteries have been regarded as promising energy storage devices due to their low overpotential and high energy density. Its applications, however, still face formidable challenges due to the lack of understanding about the influence of electrocatalysts on discharge products. Here, a phosphorous and nitrogen dual-doped carbon (PNDC) based cathode is synthesized to increase the electrocatalytic activity and to stabilize the NaO2 nanoparticle discharge products, leading to enhanced cycling stability when compared with the nitrogen-doped carbon (NDC). The PNDC air cathode exhibits a quite low overpotential (0.36 V) and long cycling stability for 120 cycles. The reversible formation/decomposition and stabilize ability of NaO2 discharge products are clearly proven by in-situ synchrotron X-ray diffraction and ex-situ X-ray diffraction. Based on the density functional theory calculation, the PNDC has much stronger adsorption energy (-2.85 eV) for NaO2 than that of NDC (-1.80 eV), which could efficiently stabilize the NaO2 discharge products.


Introduction
Lithium-oxygen batteries have received great attention during the past several years because of their high energy density and power density. 1,2,3 Unfortunately, there are still many problems impede its practical application, such as high overpotential and poor cycling stability. 4,5,6,7,8,9 The main reason for these problems could be attributed to the formation of an extremely unstable superoxide intermediate (O 2 − ), which could not be stabilized by small Li + ions due to the mismatch according to the hard and soft acid base (HSAB) theory and can react with non-aqueous electrolytes or carbon-based air cathodes. 10,11,12 By contrast, as a soft Lewis acid compared with Li + , Na + could effectively stabilize the soft Lewis base O 2 − ions. Therefore, sodium-oxygen batteries (Na-O 2 batteries) have been considered as a more promising system due to the formation of more stable discharge product, sodium superoxide (NaO 2 ), leading to low overpotential and high reversibility. 13,14,15,16,17,18,19,20,21,22,23,24 However, other distinct sodium oxides, such as sodium peroxide (Na 2 O 2 ) and sodium peroxide dihydrate (Na 2 O 2 ·2H 2 O), also have been reported as main discharge products of Na-O 2 batteries, which could signi cantly affect the electrochemical performance in such aspects as overpotential, energy e ciency, and cycling stability. 25,26,27,28,29,30,31,32,33,34,35 In addition, even with NaO 2 as the primary discharge product, it also could be transformed into Na 2 O 2 ·2H 2 O with spontaneous dissolution and ionization. 36,37,38,39 Therefore, it is important to achieve an in-depth understanding of the in uence of electrocatalysts on the composition of discharge products and to prevent the transformation of NaO 2 , which is a prerequisite for developing e cient catalysts with high-performance and long-life cycling stability. 40,41,42 Owing to the tunability of their structure and surface properties, carbonaceous based materials have been extensively studied as e cient catalysts for various catalytic reactions. 43,44,45,46,47 The catalytic performance of carbon based materials could also be improved through doping with different elements, such as phosphorus, sulfur, or boron. 48,49,50,51,52,53,54 Thus, it is considered that carbonaceous materials with tunable structure and surface properties are one of the most promising types of catalytic materials for Na-O 2 chemistry.
In this work, a heteroatom-doped carbon-based cathode has been designed, and the in uence of carbon catalysts doped with different heteroatoms has been studied with respect to the electrocatalytic activities, composition of discharge products, and electrocatalytic performance of Na-O 2 batteries. Carbon materials with different doping, namely, the phosphorus and nitrogen dual-doped carbon (PNDC) and nitrogen-doped carbon (NDC), were fabricated and studied as electrocatalysts for the Na-O 2 batteries.
When the two different doped carbon materials were evaluated as air cathodes for the Na-O 2 batteries, different electrocatalytic performance were observed due to their different electrocatalytic activity and distinct stabilize abilities to the discharge products. Thanks to the strong adsorption energy, the PNDC could prevent the dissolution and ionization of NaO 2 nanoparticle discharge product to Na 2 O 2 ·2H 2 O.
Therefore, PNDC based Na-O 2 batteries demonstrate a quite low overpotential of about 0.36 V and excellent cycling stability for 120 cycles.

Materials characterizations
Polypyrrole (PPy) hollow nanotubes were rst synthesized through the polymerization of pyrrole monomer with methyl orange as template. Afterwards, PNDC was fabricated by annealing PPy at 300 ºC in argon atmosphere with NaH 2 PO 2 as the P precursor, as illustrated in Fig. 1a. NDC was also prepared through directly annealing PPy at 300 ºC in argon atmosphere. The structure and morphology of the samples were studied by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), as shown in Fig Fig. 2). Then energy dispersive X-ray spectroscopy (EDS) mapping was conducted to study the element distributions within the tubal carbon ( Fig. 1 and Supplementary Figs. 2 and 3). As shown in Fig. 1e and f, the doping elements (P and N) are homogeneously distributed in the carbon tubal structures of the PNDC. In the NDC, nitrogen is also evenly distributed throughout the whole tubal structure ( Supplementary Fig. 2). In addition, the high resolution TEM images reveal the porous structure of PNDC, which could supply plentiful active sites and adequate space for gas diffusion and electrolyte impregnation as well as discharge products accommodation. (Supplementary Fig. 4).
Further information on the structure and surface chemical status of the samples were obtained from Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) (Fig. 1g-h and Supplementary Fig. 5-7). The surface chemistries of PNDC and NDC are different due to their differences in element doping. As shown in Supplementary Fig. 5a, the XPS survey scan of PNDC demonstrates the presence of P, N, and C without any impurities on the surface, which is consistent with the EDS results. The P and N elemental content is 2.36 atom % and 8.26 atom %, respectively. NDC also has similar N content of about 8.50 atom % ( Supplementary Fig. 6a). The high-resolution C 1 s spectra of PNDC ( Supplementary Fig. 5b) and NDC ( Supplementary Fig. 6b) can be deconvoluted into three component peaks, corresponding to C-C/C = C, C-N, and C = O. The only difference is that the C = O peak of PNDC (288.6 eV) is shifted to higher binding energy compared with that of NDC (287.9 eV), which may be caused by the doping with P atoms. On the other hand, the high-resolution N1s spectra of PNDC and NDC show three nitrogen species (pyridinic-N, pyrrolic-N, and graphitic-N), implying that part of the pyrrolic-N atoms within the polypyrrole rings are transformed to pyridinic-N and graphitic-N ( Fig. 1h and Supplementary Fig. 6c). Compared with NDC, the graphitic-N peak of PNDC is shifted to lower binding energy (from 402.4 to 401.7 eV). In addition, when compared with NDC, the content of graphitic-N for PNDC decreased from 17.0% to 10.1%, while the content of pyrrolic-N increased from 64.9% to 67.3% and the content of pyridinic-N increased from 18.1% to 22.6%. The higher content of pyrrolic-N at the edges could improve the charge mobility and electrocatalytic activity of PNDC. The P doping in PNDC was also con rmed by XPS with a typical P-C bond centered at 132.9 eV and P-O bond centered at 133.8 eV. 55 As shown in Supplementary Fig. 7, both PNDC and NDC have the typical carbon D band and G band. The D band located at 1350 cm − 1 could be attributed to disordered carbon atoms, while the G band observed at 1580 cm − 1 can be ascribed to sp 2hybridized graphitic carbon atoms. In addition, the I D /I G intensity ratio slightly decreases from NDC (1.19) to PNDC (1.14), indicating that the defects are reduced after introducing P atoms.

Electrochemical properties investigations of PNDC and NDC
In order to investigate the electrochemical properties of the as-prepared materials, Na-O 2 batteries were tested using PNDC and NDC as air cathodes. The speci c capacities were calculated based on the mass of active materials in the cathodes. Cyclic voltammetry curves of the two electrodes were measured to demonstrate their catalytic activities. As shown in Fig. 2a, the PNDC has higher anodic and cathodic peaks current densities and lower overpotential than the NDC electrode, indicating that the PNDC exhibits superior catalytic activity. In addition, the PNDC electrode also has better conductivity compared with the NDC electrode, as demonstrated by the Nyquist plots in Supplementary Fig. 8. The full galvanostatic discharge/charge plot in the voltage range of 1.5-4.0 V at room temperature at current density of 200 mA g − 1 is shown in Supplementary Fig. 9. The Na-O 2 batteries with the PNDC cathode achieved a discharge capacity of 6216 mAh g − 1 , which is much higher than that of the battery with the NDC electrode (4975 mAh g − 1 ). Based on the total weight of air cathode, PNDC cathode could deliver a high energy density of 440.57 Wh kg − 1 . Compared with the NDC electrode, the PNDC electrode also exhibits much lower overpotential and higher coulombic e ciency. Figure 2b presents the discharge/charge curves of Na-O 2 batteries with PNDC and NDC cathodes with a cut-off capacity of 2000 mAh g − 1 at a current density of 400 mA g − 1 . The charge plateaus at 2.45 V corresponding to the decomposition of NaO 2 discharge products. 56 It is obvious that the PNDC air cathode has much higher round trip e ciency compared with NDC air cathode, indicating PNDC exhibits better OER performance than NDC. In addition, the PNDC also demonstrates outstanding rate capability. When the current densities were increased from 100 to 200 or even 400 mA g − 1 , as shown in Fig. 2c-d and Supplementary Fig. 10, the PNDC still exhibited a low overpotential (0.36 V). Meanwhile, the recharge capacities of the PNDC air cathodes also increased from 593 to 797 mAh g − 1 , which is consistent with other reports. 36,57,58,59 In contrast, the NDC air cathode only delivered a recharge capacity of 389 mAh g − 1 even at the current density of 400 mA g − 1 . The excellent electrochemical performance and rate capability of the PNDC air cathode are primarily attributable to its high catalytic activity and excellent stabilizing ability for the NaO 2 discharge products (would be discussed later). performance with low overpotential, which is one of the best reported air cathodes for the Na-O 2 batteries ( Fig. 2f). In comparison, the NDC electrode only exhibited a recharge capacity of 216 mAh g − 1 for the rst cycle, while its discharge capacity decreased to 878 mAh g − 1 for 19 cycles. The improved cycling performance of the PNDC air electrode is primarily attributable to its excellent catalytic activity and outstanding stability during a long cycling period.

Investigation of reaction mechanism
In order to gain an in depth understanding of the reaction mechanism on the PNDC and NDC electrodes, in-situ synchrotron XRD, ex-situ XRD and SEM were conducted on these electrodes. As shown in Fig. 3a-b, the crystal evaluation of PNDC electrode air cathode was investigated with in-situ synchrotron, using the Powder Diffraction Beamline with λ = 0.7749 Å (Australia Synchrotron). In-situ synchrotron XRD is a crucial tool to analyze the crystallographic information of materials and it will help to determine the electrochemical reaction mechanism of Na-O 2 batteries. During the initial discharge process, there is a new diffraction peak developed at 23.15°, which could be indexed to the (220) plane of NaO 2 with dspacing of 1.93 Å (JCPDF no. 01-089-5951). Then that peak disappeared during the initial recharge process, indicating the reversible formation and decomposition of NaO 2 discharge products, which is consistent with the ex-situ XRD results (Fig. 3c-d, and Supplementary Fig. 11). With an eye on the stabilize ability of different air cathode materials for NaO 2 discharge products, the crystal structure evaluation of discharge products was also investigated by using ex-situ XRD. For the discharged PNDC air cathode, the diffraction peaks of NaO 2 discharge products could be well preserved after 8 hours rested without any obvious difference, demonstrating the excellent stabilize ability of PNDC air cathode towards the NaO 2 discharge products. For the discharged NDC air cathodes, however, the intensities of NaO 2 discharged products diffraction peaks are signi cantly decreased associated with the emerging of diffraction peaks of Na 2 O 2 ·2H 2 O (JCPDF no. 00-015-0064) after 2 hours rested. With the increase of rest times to 4 and 8 hours, the diffraction peaks of NaO 2 nally disappeared and only Na 2 O 2 ·2H 2 O could be detected, implying all the NaO 2 discharge products were transformed into Na 2 O 2 ·2H 2 O on the NDC air cathodes. As the water content in the electrolyte was less than 5 ppm, this phenomenon could be assigned to the spontaneous dissolution and ionization of NaO 2 discharge products associated with the decomposition of the electrolyte. 36 With the formation of Na 2 O 2 ·2H 2 O side products, the electrochemical performance of Na-O 2 batteries would be signi cantly diminished with increased overpotential, decreased coulombic e ciency, and declined cycling stability. As the PNDC and NDC air cathodes have quite similar morphology and structure, their diverse stabilize abilities towards NaO 2 discharge products on different air cathodes could be attributed to the different absorption energy for NaO 2 driven by their different elements doping. As shown in Supplementary Fig. 12 and Supplementary Fig. 13, NaO 2 nanoparticles discharge products within the size range of 150-200 nm were gradually formed and deposited on the surface of PNDC and NDC electrodes during discharging process. The NaO 2 nanoparticles discharge products would be decomposed and totally disappeared after recharging to 3.0 V. Supplementary Fig. 12f illustrated the ORR pathway of electrochemical formation of the NaO 2 nanoparticles discharge products.
O 2 rst adsorbs on the surface of the electrode and then undergoes a one-electron electrochemical reduction to form NaO 2 . Finally, the NaO 2 would subsequently precipitate from the solution and deposit on the electrodes to form NaO 2 nanoparticles.
To con rm our hypothesis about the outstanding stabilize ability and excellent electrochemical activity of PNDC, DFT calculations are carried out to investigate the absorption energy of NaO 2 discharge products and OER reaction processes. As can be found in Supplementary Moreover, OER reaction processes on the PNDC and NDC are shown in Fig. 4, in which ve transition states were selected with a nudged elastic band (NEB) calculation. As shown in Fig. 4a, the rst three steps are all a result of endothermic reactions and the following three steps are exothermic reactions. In the case of PNDC, only near-zero free uphill energies are required for the rst three steps. The highest free energy required is 0.33 eV for the rst step. In the case of NDC, however, much higher uphill energies are required for the rst three steps. And the step 2 even needs to overcome a free energy barrier of 1.53 eV. Based on the above DFT calculations, the PNDC possesses high intrinsic electrocatalytic activity with less free energy compared to the NDC. As illustrated in Fig. 4b, the PNDC has excellent electrocatalytic activity to decompose the NaO 2 discharge products during the OER reaction process of Na-O 2 batteries.
Moreover, it has stronger adsorption energy for NaO 2 to stabilize the discharge products of Na-O 2 batteries.

Discussion
In conclusion, the PNDC air cathode was synthesized and employed as electrocatalyst for the Na-O 2 batteries, which could facilitate the formation and stabilization of NaO 2 nanoparticles as discharge products with a quite low overpotential (0.36 V) and long cycling stability for 120 cycles. The excellent electrochemical performance of the PNDC electrodes could be attributed to its strong adsorption energy and outstanding electrocatalytic activity during recharging process. As con rmed by in-situ synchrotron XRD, ex situ XRD, and DFT calculations, the PNDC could effectively prevent the transform of NaO 2 discharge products to Na 2 O 2 ·2H 2 O side products and facilitate the decomposition of NaO 2 discharge products with its outstanding electrocatalytic activity during recharging process. In addition, this work could help us to achieve an in-depth understanding of the in uence of electrocatalysts on the composition of discharge products and provide a new direction to design high performance electrocatalysts with favorable absorption energy and electrocatalytic activity for metal-oxygen/metal carbon dioxide batteries.

Synthesis of polypyrrole
Polypyrrole was fabricated by a previously reported method. 60 Generally, 81.8 mg methyl orange and 1350 mg FeCl 3 were dissolved in 100 mL deionized water. Then, 33.5 mg pyrrole monomer was added into the above solution. After stirring at room temperature for 24 h, the polypyrrole was washed with ethanol and deionized water several times.

Synthesis of phosphorus and nitrogen dual-doped carbon
Polypyrrole was placed on a porcelain boat in a tube furnace under argon atmosphere. Another porcelain boat with NaH 2 PO 2 was put on the upstream side. Then, the sample was annealed at 300 °C for 2 h with a heating rate of 5 °C min − 1 . Finally, the sample was washed with ethanol and deionized water several times after cooling down to room temperature. To synthesize nitrogen doped carbon, the polypyrrole alone was put into the tube furnace with the same annealing conditions.

Materials Characterization
Powder XRD on a GBC MMA diffractometer with Cu Kα radiation was utilized to study the crystalline structures of the samples. Raman spectra were investigated with a JOBIN YVON HR800 Confocal system. A Phoibos 100 Analyzer XPS with Al Kα X-rays was used to determine the surface chemical states of the samples. A JSM-7500FA scanning electron microscope and JEM-ARM200F transmission electron microscope were utilized to measure the structures and morphologies of the materials. In-situ synchrotron XRD measurements were conducted at the Powder diffraction beamline (Australia Synchrotron) using CR2032-type coin cells with holes in the cathode part in dry oxygen atmosphere with our designed facility. A 4 mm hole sealed with the Kapton lm was punched in the anode part, which could allow the X-ray beams penetrate the cell.
Electrochemical Measurements of Na-O 2 Batteries: The electrochemical properties were measured employing CR2032-type coin cells with holes in the cathode part. To prepare the oxygen cathodes, the active materials and poly(tetra uoroethylene) solution were mixed in the ratio of 90:10 in isopropyl alcohol as catalyst slurry. The slurry was then pasted onto carbon paper and dried in a vacuum oven at 120 °C for 12 h. The typical loading of cathode active materials was 0.3 mg cm − 2 . Tetraethylene glycol dimethyl ether (TEGDME) solvent was puri ed by rotary evaporation and stored with 4 Å activated molecular sieves for two weeks before use. Sodium tri uoromethanesulfonate (sodium tri ate, NaSO 3 CF 3 ) was dried at 120 °C for 24 h under vacuum. The electrolyte, consisting of 0.5 m sodium tri ate in TEGDME, was prepared in an argon-lled glovebox. The water content in the electrolyte was less than 5 ppm as measured by Karl-Fischer titration. The cells were assembled in an argon-lled MBRAUN glove box (H 2 O level < 0.1 ppm and O 2 level < 0.1 ppm). A piece of sodium cut from a sodium cube served as the anode, and glass micro ber was used as the separator. All the measurements were carried out on Neware battery testers at room temperature in dry oxygen atmosphere with our designed facility. Cyclic voltammetry was measured on a Biologic VMP-3 electrochemical workstation with a scan rate of 0.1 mV/s. Electrochemical impedance spectroscopy curves were collected in the frequency range of 100 kHz to 0.1 Hz.

Computational Method
In this study, the density functional theory (DFT) calculation was implemented using the Vienna Ab-Initio Simulation Package. 61 The electron-ion interaction was described by projector augmented-wave (PAW) pseudopotentials. For the exchange and correlation functionals, the Perdew-Burke-Ernzerhof (PBE) version of the generalized gradient approximation (GGA) exchance-correlation was applied. 62,63 In the DFT calculation, 450 eV as the cut off kinetic energy was used for the wave functions expansion. In order to avoid the interactions between each carbon layers, a vacuum width of 15 Å was applied. In the brillouin zone integration on the grid with 3 × 3 × 1 and 5 × 5 × 1 were respectively used during the geometry optimization. To better understand the catalytic ability of NaO 2 on the surface of different PNDC, the climbing image nudged elastic band (CI-NEB) method 64 was used to simulate the possibility of degradation of NaO 2 . In addition, ve images were inserted between the initial and nal con gurations in the transition state search began with a NEB calculation. And the corrected adsorption energy of NaO 2 (∆E ads ) was de ned as Where E total is the total energy of PNDC and NDC surface adsorbed with the NaO 2 , E host is the total energy of PNDC and NDC, and E guest is the total energy of NaO 2 .

Data Availability
Data supporting the ndings of this study are available from the authors on reasonable request. See author contributions for speci c data sets.  (f) the electrochemical performance of PNDC and reported air cathodes for Na-O2 batteries; (g) comparison of cycling performance of the PNDC and NDC electrodes at a current density of 200 mA g-1.

Figure 3
Analysis of the discharge products of Na-O2 batteries. (a-b) in-situ synchrotron X-ray diffraction (XRD) pattern of the PNDC electrode with the initial galvanostatic discharge/charge curves at a current density of 200 mA g-1; (c) XRD patterns of a pristine, a discharged, a recharged and a discharged with 8 hours rested PNDC electrodes; (d) XRD patterns of discharged NDC electrodes with rest times of 0, 2, 4, and 8 hours.