Materials characterizations
Polypyrrole (PPy) hollow nanotubes were first 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 NaH2PO2 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. 1b-h, Supplementary Figs. 1 and 2. The as prepared polypyrrole has a tubular structure with the tube diameter around 400 nm and length up to 10 µm (Supplementary Fig. 1). After the doping process, the tubal structures are well preserved for both PNDC and NDC (Fig. 1b-d and Supplementary 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 confirmed 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 sp2-hybridized graphitic carbon atoms. In addition, the ID/IG 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-O2 batteries were tested using PNDC and NDC as air cathodes. The specific 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-O2 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 efficiency. Figure 2b presents the discharge/charge curves of Na-O2 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 NaO2 discharge products.56 It is obvious that the PNDC air cathode has much higher round trip efficiency 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 NaO2 discharge products (would be discussed later). Figure 2e-g and Supplementary Fig. 10b evaluate the cyclability of PNDC and NDC electrodes at a current density of 200 mA g− 1 with a fixed specific capacity of 1000 mA h g− 1. The NDC electrode demonstrated unsatisfactory electrochemical performance when used as an air electrode in the Na-O2 batteries. Compared with NDC, the discharge and charge capacities of the PNDC are still as high as 1000 and 845 mAh g− 1 after 120 cycles, respectively, indicating the excellent energy efficiency of the Na-O2 batteries based on PNDC air cathode. The PNDC air cathodes thus exhibit better cycling performance with low overpotential, which is one of the best reported air cathodes for the Na-O2 batteries (Fig. 2f). In comparison, the NDC electrode only exhibited a recharge capacity of 216 mAh g− 1 for the first 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-O2 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 NaO2 with d-spacing 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 NaO2 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 NaO2 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 NaO2 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 NaO2 discharge products. For the discharged NDC air cathodes, however, the intensities of NaO2 discharged products diffraction peaks are significantly decreased associated with the emerging of diffraction peaks of Na2O2·2H2O (JCPDF no. 00-015-0064) after 2 hours rested. With the increase of rest times to 4 and 8 hours, the diffraction peaks of NaO2 finally disappeared and only Na2O2·2H2O could be detected, implying all the NaO2 discharge products were transformed into Na2O2·2H2O 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 NaO2 discharge products associated with the decomposition of the electrolyte.36 With the formation of Na2O2·2H2O side products, the electrochemical performance of Na-O2 batteries would be significantly diminished with increased overpotential, decreased coulombic efficiency, and declined cycling stability. As the PNDC and NDC air cathodes have quite similar morphology and structure, their diverse stabilize abilities towards NaO2 discharge products on different air cathodes could be attributed to the different absorption energy for NaO2 driven by their different elements doping. As shown in Supplementary Fig. 12 and Supplementary Fig. 13, NaO2 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 NaO2 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 NaO2 nanoparticles discharge products. O2 first adsorbs on the surface of the electrode and then undergoes a one-electron electrochemical reduction to form NaO2. Finally, the NaO2 would subsequently precipitate from the solution and deposit on the electrodes to form NaO2 nanoparticles.
To confirm our hypothesis about the outstanding stabilize ability and excellent electrochemical activity of PNDC, DFT calculations are carried out to investigate the absorption energy of NaO2 discharge products and OER reaction processes. As can be found in Supplementary Table 1, PNDC has much stronger adsorption energy (-2.85 eV) for NaO2 than that of NDC (-1.80 eV). The strong adsorption energy could inhibit the generation of liberated O2− from the dissolution of NaO2, which is a strong reagent and could react with the electrolyte to form Na2O2·2H2O side product.36 Therefore, the strong adsorption energy of the PNDC for NaO2 could excellently stabilize the NaO2 discharge products and prevent its spontaneous dissolution and ionization to form Na2O2·2H2O. This ability would enhance the electrochemical performance of the PNDC air cathode through avoiding the formation of Na2O2·2H2O side products. Moreover, OER reaction processes on the PNDC and NDC are shown in Fig. 4, in which five transition states were selected with a nudged elastic band (NEB) calculation. As shown in Fig. 4a, the first 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 first three steps. The highest free energy required is 0.33 eV for the first step. In the case of NDC, however, much higher uphill energies are required for the first 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 NaO2 discharge products during the OER reaction process of Na-O2 batteries. Moreover, it has stronger adsorption energy for NaO2 to stabilize the discharge products of Na-O2 batteries.