Structural Evolution of Phosphorus Species on Graphene with a

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Structural Evolution of Phosphorus Species on Graphene with a Stabilized Electrochemical Interface Zhihong Bi, Li Huo, Qingqiang Kong, Feng Li, Jingpeng Chen, Aziz Ahmad, Xian-Xian Wei, Lijing Xie, and Cheng-Meng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21903 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Applied Materials & Interfaces

Structural Evolution of Phosphorus Species on Graphene with a Stabilized Electrochemical Interface Zhihong Bi

a, b,

Li Huoc, Qingqiang Kong

a, b,

Feng Li

a, b,

Jingpeng Chen

a, b,

Aziz

Ahmad a, Xianxian Wei d, Lijing Xie *a, Chengmeng Chen*a a

Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy

of Sciences, Taiyuan, 030001, China b

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

c

College of Materials Science and Engineering, Taiyuan University of Technology,

No.79, Yingze Street, Wanbolin District, Taiyuan 030024, China d

School of Environment and Safety, Taiyuan University of Science and Technology,

Taiyuan 030024, China * Correspondence should be addressed to Lijing Xie ([email protected]) and Chengmeng Chen ([email protected])

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Phosphorus doping is an effective approach to tailor the surface chemistry of carbon materials. In this work, two-dimensional graphene, as a simplified model for all sp2 hybrid carbon allotropes, is employed to explore the surface chemistry of P-doped carbon materials. Thermally reduced graphene oxide, with abundant residual oxygen functionalities, is doped by phosphorus heteroatoms through H3PO4 activation, followed by passivation in an inert atmosphere. The structural evolution of the phosphorus species in the carbon lattice during the thermal treatment is systematically studied by Fourier transform infrared spectrum (FT-IR), X-ray photoelectron spectroscopy spectrum (XPS), X-ray diffractometry (XRD) and Raman spectroscopy with the assistance of first principals calculations. The C3−P=O configuration is identified as the most stable structure in the graphene lattice and plays a key role in stabilizing the electrochemical interface between the electrode and electrolyte. These features enable an electrode based on P-doped graphene to exhibit an enlarged potential window of 1.5 V in an aqueous electrolyte, a remarkable improved cycling stability and an ultralow leak current. Therefore, this contribution provides insights for

designing

phosphorus-doped

carbon

materials

towards

electrocatalysis,

energy-related applications, etc. Keywords: Graphene, phosphorus species, evolution, interface, electrochemistry


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1. INTRODUCTION Carbon materials, such as graphite, porous carbon, carbon fiber, carbon nanotubes and graphene, have been widely used in the fields of medicine, separation, catalysis, energy storage and conversions.1-6 Among the many properties of carbon materials, the surface chemistry of carbon materials is considered to be an important factor in the tuning of material properties to meet the requirements of different application fields.7 To the best of our knowledge, doping the carbon matrix with heteroatoms, such as oxygen, nitrogen, boron, sulfur and phosphorus, is the most effective way to control the surface chemistry properties of carbon materials, particularly in terms of the properties of electron donors or acceptors.8 Many studies have focused on grafting different oxygen-containing functional groups to carbon materials to achieve the desired properties.9 However, It has been generally accepted that the oxygen species such as quinones or chemisorbed oxygen (carboxyl) on the surface of carbon materials are electrochemically active but unstable, which easily leads to a decrease in electrochemical performance.10 In addition, as the most common doping atom, the nitrogen atom (N) can greatly adjust the electron donor-acceptor properties of carbon materials due to the existence of its outermost lone pair of electrons, and has been extensively and deeply studied.11,12 The conjugation of the lone-pair electrons of N and the graphitic π-bonds of carbon materials can further distort the carbon structure to create defects and available active sites.1, 13 Phosphorus (P) is located in the same main group as nitrogen, possessing the same number of valence electrons. However,



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In terms of atomic radius and electron donating ability, P is larger and stronger than N. Thus, P is a highly interesting candidate for altering the surface properties of carbon materials to meet the requirements of a given application.1, 14-22 Recently, Yu et al. reported that the P-doping of ordered mesoporous carbon imparts an great electrocatalytic activity for oxygen reduction reactions (ORR).23 Liang and Su et al. reported that phosphorus-doped graphitic mesoporous carbon and phosphorus oxide clusters stabilized by carbon nanotubes can tune the catalytic selectivity

for

combustion.17,

24

oxidative

conversion,

isomerization,

dehydrogenation

and

In 2009, Hulicova-Jurcakova et al. first successfully used

phosphorus-functionalized carbon materials as supercapacitor electrodes and broadened the operating voltage to 1.5 V in aqueous electrolytes.2 Recently, the phosphorus-doped coal-based mesoporous carbon prepared by our team has been successfully applied with organic electrolytes as supercapacitor electrodes, which can work stably up to 3.0 V in Et4NBF4, resulting in an enhanced energy density up to 38.65 Wh kg-1 at a current density of 1 A g-1.25 However, the functional mechanism of the phosphorus configuration in the skeleton of P-doped carbon materials remains unclear. Mainly due to the well-developed porous structure and abundant oxygen functional groups in these phosphorus-doped carbon materials, a large number of phosphorus-doped species can be spontaneously buried into the complex carbon matrix. Thus, the insufficient exposure and availability of phosphorus species have led to ambiguity regarding the forms, structural evolution and functional mechanisms of phosphorus species in carbon crystal lattices.


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In the present study, we choose thermally reduced graphene oxide, with an open two-dimensional planar structure, as a simplified model to exclude the interference of a complex carbon matrix and investigate the mechanisms of phosphorus doping and structure evolution. Here, we adopt a general phosphorus doping method including the H3PO4-activation of partially thermally reduced graphene oxide at 800 °C and the further high-temperature passivation of phosphorus-doped graphene at 1000 °C. By this way, we can properly explore the interactions between carbon, phosphorus and oxygen. The chemical bonding states of phosphorus with carbon and oxygen are discussed mainly based on a comparison between X-ray photoelectron spectroscopy (XPS) spectra and density functional theory (DFT) calculations. Furthermore, we also correlate the type of P-doped configuration with the electrochemical performance of P-doped graphene. We expect this work will provide an important insight into the identification of the most-optimized P-doped configuration and facilitate the rational design of P-doped carbon materials with desirable properties for various applications. 2. RESULTS AND DISCUSSION The microstructures of G600, G800, PG800 and PG800S (G600: graphene oxide reduced at 600 °C; G800: G600 carbonized at 800 °C, PG800: G600 activated with phosphoric acid at 800 °C, and PG800S: PG800 further passivated at 1000 °C) are characterized by transmission electron microscopy (TEM). The TEM images of all samples (Figure 1 a-d) show similar two-dimensional open planar morphologies with no clear difference. Furthermore, the presence of C, O and P elements is confirmed by the energy dispersive spectrum (EDS) (Figure 1e). As evidenced by the corresponding



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EDX elemental maps, the C, O and P elements are uniformly distributed on the PG800S sheets.

Figure 1. TEM images of (a) G600, (b) G800, (c) PG800 and (d) PG800S. (e) An STEM image of PG800S and the corresponding EDX elemental mapping images. The existence of phosphorus functional groups in all the samples is investigated via FT-IR, as shown in Figure 2a. The superposition of multiple signals can be seen between 1193 –1180 cm-1. In this range, the peak at 1085 cm-1 can be attributed to the ionized linkage of P+–O–26-27 and the symmetrical vibration of P–O–P in polyphosphate chains.28-29 The peak at 1193–1180 cm-1 may also be assigned to the superposition of three signals, namely, those of P=O, O–C (P–O–C)

30-31

and

P=OOH.31 Figure 2b is the enlarged region of Figure 2a from 640 to 700 cm-1. The peak appearing at 668 cm-1 is attributed to C-P stretching. As is evident, the peak intensity of PG800S is significantly stronger than that of PG800, which proves that the passivation process can effectively promote the formation and stabilization of the C-P bond.26 Although the FT-IR shows clear evidence of phosphorus species, the superposition of multiple functional groups between 1085 -1190 cm-1 is far from


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enough to further understand the existence and evolution of phosphorus functional groups.

Figure 2. (a) (b) FT-IR spectrum. (c) XPS surveys of G800, PG800 and PG800S. (d) High-resolution P 2p spectra of PG800 and PG800S. Thus, the chemical composition and bonding configurations of C, O and P on the surface of all the samples are further semiquantitatively elaborated by XPS analysis, as shown in Figure 2c. The XPS shows three characteristic peaks at approximately 131, 284 and 532 eV corresponding to the P 2p, C 1s and O 1s peaks in the activated samples (PG800 and PG800S), while P cannot be observed in the carbonized sample (G800) (Figure 2c), further illustrating the successful doping of P atoms into the activated samples. With the help of the XPS analysis, the specific percent contents of the surface elements in G800, PG800 and PG800S samples are recorded in Table 1. With the activation and passivation, the percent content of carbon tends to decrease.



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This result can be attributed to instability at the defects/edges of the graphene whereby carbon is easily replaced by oxygen or phosphorus, mainly through the production of C=O or P=O bonds32-33, or is lost in the form of pyrolytic CO or CO2.7, 34-36

The amounts of surface oxygen and phosphorus increase significantly, implying

that H3PO4 serves as an effective oxidant and can greatly promote the formation of oxygen-carbon structures (O-C). Table 1 Elemental analysis of G800, PG800, and PG800S by XPS. Elemental Content (at. %) Sample

C 1s

O 1s

P 2p

G800

95.69

4.31

0.00

PG800

93.16

4.90

1.93

PG800S

90.20

6.95

2.85

Moreover, to further understand the chemical bond configuration of phosphorus in the activated graphene, the high-resolution P 2p peaks of both PG800 and PG800S are deconvoluted, as shown in Figure 2d. Three major peaks at 132.3 eV (C3-P=O), 133.1 eV (C-P-O) and 134.0 eV (C-O-P) appear for both PG800 and PG800S, whereas the peak at 130.2 eV (C3-P) is completely absent from the PG800S spectrum. The quantitative results for the phosphorus species in both PG800 and PG800S are listed in Table 2. From activation to passivation, It clearly found the proportions of both C3-P=O and C-P-O are significantly enhanced, while the opposite is true for the proportions of C3-P and C-O-P. It is worth noting that the proportion of C3-P=O increases from 46.6% to 54.3%, while that of C3-P decreases from 13.6% to 0, indicating the stable nature of C3-P=O compared with that of C3-P.37 These results


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provide strong evidence for the main transformation of C3-P and C−O−P linkages into C3-P=O and C-P-O during the passivation process at 1000 °C under an Ar atmosphere. The presence of increased C3-P=O enhances the wettability of graphene surface, which plays a key role in the improving the electrochemical performance of supercapacitor.38 Previous studies have determined that C-P-O bonds are present in phosphoric acid-activated carbon by XPS and NMR analysis.39-40 Our work discloses that the C3-P=O bond type is the most stable, effective and optimized chemical bonding state, which is consistent with previous works.34, 41-42 These results indicate that the P atoms act as a link between O and C atoms, and coordinate the electrons of C atoms by synergy with O atoms. That is to say, the P atom, like a bridge, plays a critical role in stabilizing the bonding between C and O. Based on the above analysis, scheme 1 in SI presents a possible schematic diagram of changes in carbon, phosphorus and oxygen bonding conditions in PG800.



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Table 2 Results of the deconvolution of the P 2p peaks of G800, PG800, and PG800S. P 2p peak (eV) Relative content of phosphorus species (%)

Sample

P1 (C3-P)

P2 (C3-P=O)

P3 (C-P-O)

P4 (C-O-P)

130.2

132.3

133.1

134.0

PG800

13.6

46.6

22.5

17.4

PG800S

0.0

54.3

32.5

13.2

To support the abovementioned experimental results, we construct theoretical models of phosphorus in the various bonding configurations deduced from the XPS analysis. As shown in Figure 3a, eight types of phosphorus-doped models including C3-P, C3-P=O, C−O−P (SV), C−O−P (DV), C−P−O (SV-1), C−P−O (DV-1), C−P−O (SV-2) and C−P−O (DV-2) are constructed. Among the structures, SV and DV represent the single and double vacancy defects formed by removing a C atom and a C-C dimer from the complete graphene lattice; the numbers 1 and 2 indicate the phosphorus atom is connected to one and two carbon atoms. According to the XPS elemental content analysis, the formation and binding energies of different models under the condition of phosphorus concentrations of 3.1, 2.0 and 1.6% are calculated (Figure 3a and b). Figure 3a shows the binding energies of these eight structures decrease according to the sequence, C−O−P (SV) > C3−P > C−O−P (DV) > C−P−O (SV-2) > C−P−O (DV-1) > C−P−O (SV-1) > C−P−O (DV-2) > C3−P=O, highlighting the C3−P=O model as the most stable. In addition, the formation energies of these eight structures (in Figure 3b) decrease in the order of C−O−P (SV) > C−O−P (DV) >


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C−P−O (SV-2) > C−P−O (SV-1) > C−P−O (DV-1) > C3−P > C−P−O (DV-2) > C3−P=O. It is worth mentioning that the formation energy of C3−P=O is negative, while those of the others are positive. This result implies that C3−P=O bonding is readily achievable for experimental realization. Therefore, it is precisely because of the stable trigonal pyramidal phosphorus-containing configuration—mainly composed of C3-P=O—in the carbon lattice that a notable increase in the spacing of the graphene layers occurs during the passivation process (We will discuss it later). These computational data fully support our aforementioned experimental results. Particularly, the C3-P=O configuration is the most actively stable of the structures embedded in the graphene lattice.

Figure 3. Theoretical models of phosphorus in various bonding configurations and the corresponding values for the (a) binding energy and (b) formation energy. Note: The graphene lattice size and corresponding phosphorus content in the models are represented by blue (4 × 4 - 3.1%), pink (5 × 5 -2.0%) and green (6 × 6 -1.6%). Furthermore, we calculate the partial density of states (PDOS) of the phosphorus-containing models to further understand the role of individual P atoms in the various bonding configurations. From Figure 4, there is a distinct resonance state



below the Fermi level, and there is a strong hybrid between the p orbitals of C and P in the C3-P configuration. Interestingly, these resonance states are significantly weakened when the C−P−O or C-O-P bonds form, indicating that the p-orbitals of the O atom impair the interaction force between the p-orbitals of P and C atoms. Simultaneously, a tremendous overlap of the p-orbitals of C with the P and O atoms appears in the C3-P=O, C-P-O (SV-2) and C-P-O-(DV-2) at the Fermi level. Other configurations with only C and P or C and O possess either strong resonances or no resonance near the Fermi level. These results illustrate that the strength of the binding energy between C and P and O atoms near the Fermi level has a strong dependence on the number of C atoms connected to the P atom. Furthermore, the local DOS of C3-P=O is significantly stronger than that of the other configurations below the Fermi level, revealing that C3-P=O is the most stable configuration. These results are in accordance with our XPS and FT-IR analyses. Thus, the most reasonable arrangement of C, P and O and the most reasonable interaction can be maintained by optimizing the surface chemistry of the material. In other words, the above evidence further demonstrates that P acts as a connector between C and O, coordinate the electrons of C atoms by synergy with O atoms on the surface of the carbon lattice, and enhances lattice stability.


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Figure 4. Partial density of states (PDOS) of different phosphorus configurations. The Fermi level is set to zero at the dotted line. Beyond the above, we believe that some changes in the graphene structure may occur with changes in the phosphorus species. Figure 5a shows XRD data of all the samples. The average basal spacing of G600, G800, PG800 and PG800S calculated from the (002) reflection are 0.3590, 0.3351, 0.3590 and 0.3738 nm, respectively. By contrast, PG800S shows the largest layer spacing confirming the interlayer formation of the triangular-like C3-P=O configuration during thermal treatment, which effectively prevents the graphene sheets restacking. Moreover, the intensity of the (101) peak increases after the activation and passivation treatment, revealing a more graphite-like lattice. Further structural change information for P-doped graphene can be reflected by the Raman spectrum in Figure 5b. A typical D band representing a disordered carbon atoms and a typical G band representing a sp2-hybridized graphite carbon atoms can be observed at about 1346 cm-1 and 1587 cm-1, respectively.



Interestingly, the ID/IG ratio of PG800S is significantly reduced compared with that of G800 and PG800, indicating that the increment in the optimized C3-P=O configuration favors an improved graphite-like lattice. From the nitrogen adsorption and desorption isotherm tests at 77 K (Figure S1 in SI), we further demonstrate that the significant increase in the specific surface area of PG800S (by approximately 100 m2 g-1 compared with that of the other samples) is facilitated by additions of the most stable C3-P=O configuration. This result can be attributed to the phosphorus atoms entering the carbon lattice in a more stable configuration, mainly as C3-P=O.20, 41-42 Due to the differences in the bond lengths and bond angles of C-P and C-C in the C3-P=O functional group, C3-P=O has a diamond-like triangular pyramidal structure, and this structure can effectively increase the layer spacing and minimize the restacking of graphene sheets, further resulting in the specific surface area of PG800S.43-44

Figure 5. (a) XRD patterns and (b) Raman spectra of all samples. Based on the above experimental characterization analysis and DFT calculations, we propose the evolution mechanism diagram of phosphorus-containing functional groups in the graphene lattice shown by Scheme 1. During the passivation process under an Ar atmosphere, most C−O−P linkages are transformed into C3−P=O and a


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small number of C−P−O linkages through the formation of epoxide-like intermediates, [−C−O+−P−]≠.45-46 The generation of the epoxy intermediates is because the phosphorus atom is surrounded by various oxygen atoms having high electronegativity and electron-withdrawing properties, the carbon atom is subjected to a positive potential..7 Moreover, Similar to the oxidation mechanism in the gas phase thermal oxidation mechanism of carbon materials, the formation of epoxy rings has undergone a similar process.45-46 Both the C3−P=O and C−P−O linkages are mainly formed by three general mechanisms: (1) the reduction of the original C-O-P bond to form a C−P bond by generating epoxy-like intermediates, [−C−O+−P−]≠ (as show in Scheme 1 a, b and c); (2) the scission of P−OH in the C−P−OH bond to form a C−P bond (as show in Scheme 2 a and c; and (3) the active oxygen on the epoxy-like intermediate (-O-) is transferred and adsorbed on the C3-P group to form the C3−P=O bond (as show in Scheme 1 d). The result of this evolution is a further reduction in graphene surface defects and an increase in the graphitization degree, which can be proven by the enhancement of the (101) peak in the XRD data and the change in the ID/IG value in Raman spectroscopy (as show in Figure 5a and b). More precise reaction routes for phosphorus-containing compounds from intermediate transition states to defect C-P-O bonds are as follows: (1) the release of CO is through the combination of activated oxygen in the epoxy-like intermediates with an unsaturated reactive carbon atom in the graphene defect structure, and (2) the release of H2O is through the combination of the hydroxyl groups in the phosphate-like groups with hydrogen in the defect carbon matrix, while (3) it is also possible that processes (1)



and (2) occur simultaneously (as show in Scheme 2 c). All of these mechanisms can contribute to the formation of C3−P=O and C−P−O linkages (Scheme 1 and 2).7, 36, 47 Furthermore, after the passivation process, some of the intermediate transition reactants are likely converted into different forms of C−P−O bond types, as shown in Scheme 2. Most of these different forms of C−P−O bonds are metastable so their formation ratio is far lower than that of C3−P=O bonds. From the evolution trend of the phosphorus-containing functional groups, it becomes apparent that P plays a stabilizing role when located between C and O, which is consistent with the antioxidant and flame-retardant properties possessed by phosphorus itself.7, 48-50

Scheme 1. The evolutionary pathway of phosphorus species in graphene lattice during the passivation process in Ar.


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Scheme 2. Possible evolution mechanisms of phosphorus-containing compounds from intermediate transition states to defect C-P-O bonds. Finally, we investigate the influence of phosphorus-containing functional groups on the electrochemical properties of G800, PG800 and PG800S samples. A series of two-electrode symmetric supercapacitor systems are assembled, and their electrochemical properties are measured. As shown in Figure S2a in SI, the CV curves of all as-prepared electrodes have a quasi-rectangular shape (5 mV s-1), demonstrating the formation of good electric double layer capacitance (EDLC) behavior. When the scan rate reaches 500 mV s-1, the PG800S sample retains its good rectangular geometry curve, compared with that of the G800 and PG800 samples (Figure S2b in SI). Figure S2c and d in SI compares the galvanostatic charge/discharge curves of these samples at current densities of 1 and 30 A g-1, respectively. The substantially long discharge time and small IR drop at 1 and 30 A g-1 of PG800S compared with those of the other electrodes indicate the smaller charge transfer resistance and larger ion diffusion capacity of the sample.20, 25, 51 Figure 6a



provides the specific capacitances of all obtained samples at different current densities (from 1 to 30 A g-1), and the relevant capacitance retention values are listed in Table S1 in SI. The gravimetric specific capacitance is obtained using equation 1 in SI. PG800S delivers the highest specific capacitance of 108 F g-1 at 1 A g-1, and its capacity retention is up to 89.2% at a current density of 30 A g-1. The PG800 and G800 electrodes possess specific capacitances of 72 and 57 F g-1 at 1 A g-1 and retain 80.4 and 75.5% of their initial capacities after testing at a high current density of 30 A g-1. In the galvanostatic charge-discharge measurements, the cycling performance of G800, PG800 and PG800S is also tested at 3 A g-1 (Figure 6b). After 20000 charge-discharge cycles, 70.4% of the initial capacity is maintained for PG800S, while the capacity retentions of G800 and PG800 electrodes are limited to only 42.5% and 62.2%, respectively. These results suggest that the relatively high performance and cycling stability of PG800S can be ascribed to its high SSA (420 m2 g-1) and the high exposure of optimized C3-P=O and C-P-O bond types. Moreover, PG800 and PG800S exhibit excellent cycling stabilities, which is mainly due to the fact that the phosphorus atoms in the C3-P=O and C-P-O bonds are in the middle of C and O and thus act to block the active oxidation sites. This not only inhibits the formation of electrophilic oxygen species but also greatly stabilizes the electrochemical interface of the graphene electrode. One of the main factors affecting the energy storage of supercapacitors is the phenomenon of self-discharge due to leakage current.25 Figure 6c shows the dependence of the charging current on time. After 2.5 hours of testing, it can be seen


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that the currents of PG800 and PG800S are stable at 0.009 and 0.002 mA, respectively, which is much lower than the leakage current of 0.1 mA of G800. Moreover, after experiencing 6.5 hours of open circuit conditions, both PG800 and PG800S exhibit high voltages of 0.5146 V and 0.5723 V, respectively, but the voltage of the G800 drops to 0.1615 V (Figure 6d). These findings further suggest that phosphorus doping has a significant role in reducing the self-discharge and reducing the leakage current.52 In the carbon scaffold, more C3-P=O and C-P-O bonds are optimized by heat treatment, so that the bridging barrier effect of the P atoms therein can effectively protect the carbon lattice defect sites and thereby inhibit the negative of the electrochemical active oxidation sites. The effect, which significantly suppress the production of unstable quinone and carboxyl groups. Based on these findings, we select only the PG800S sample to evaluate the EDLC behavior at various CV scan rates. With the scan rate increasing from 5 to 500 mV s-1, PG800S maintains a quasi-rectangular shape and shows more ideal electric double layer behavior (Figure S2e in SI), which further indicates that phosphorus atoms play an important role in the electrochemical performance. Furthermore, the high voltage stability of the PG800S sample can be attributed to the bridge between the carbon and oxygen formed by the phosphorus atom, which effectively stabilizes the surface of graphene and all other carbon materials. As shown in Figure S2f in SI, PG800S presents significantly different in energy density improvements under different wide voltage windows.2, 25, 53 The energy density of PG800S reaches up to 38.1 Wh kg-1 and corresponding power density is 1959.4 W kg-1 at a current density of 1 A g-1. The energy density (Et) and



power density (Pt) of PG800S are calculated using equations 5-6 in SI.

Figure 6. (a) Variations in the gravimetric capacitance for a series of current densities. (b) The cycling stability of all three obtained samples at 3 A g-1. (c) The leakage current curves of all three obtained samples. (d) The self-discharging curves showing the open-circuit voltage changes over time for all three obtained samples. It is well known that phosphorus-doped carbon materials can significantly broaden the stable operating voltage in aqueous electrolytes.53 The phosphorus-doped carbon electrode can be used at higher than normal voltages (>1 V) because the phosphorus groups contribute to the blocking of electrochemical active oxidation sites. As shown in Figure 7a, we study the polarization effects of G800, PG800 and PG800S samples at various voltages to determine the difference in their retained capacitance capabilities. In the absence of the phosphoric-acid treatment, the voltage resistance of G800 can only be stabilized at a normal level. When the voltage is extended to 1.1 V,


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the polarization phenomenon becomes immediately apparent. In contrast, the stabilized voltage of phosphoric-acid-activated PG800 is increased up to 1.3 V. After the passivation process, the stabilized voltage of PG800S reaches up to 1.4 V, and the polarization is significantly reduced compared with that of G800 and PG800 (Figure 7a). Additionally, the GCD test curves of PG800S are consistent with the CV curves (Figure 7b). PG800S maintains a good triangular structure even when its operation is at 1.4 V. Figure S3a and b in SI compares the different samples under the same electrochemical test conditions. It can be seen from Figure S3a in SI that the polarization effect increases as the voltage window changes from 1 V to 1.5 V. The polarization degree of PG800 is larger than that of PG800S at high voltage, which indicates that the surface of the first activated graphene contains more unstable and easily polarizable phosphorus-containing functional groups and this polarization can be significantly improved by further passivation. This observation can be explained by the transformation of the unstable C3-P and C-O-P functional groups in PG800 into the more stable C3-P=O and C-P-O forms after the subsequent passivation process (PG800S). These results also show good agreement with the XPS and simulation results in that both C3-P=O and C-P-O, as the stable forms of the phosphorus-containing functional groups, can be correlated with the occurrence of depolarization. Hence, this evidence further demonstrates the positive effect of the phosphorus-containing functional groups on the electrochemical performance and the important role played by the passivation treatment in optimizing the configuration of the phosphorus-containing functional groups.



The widening of the cell potential window dramatically influences the energy density of a supercapacitor because of its direct relationship with the square of the working voltage (equation 5 in SI).

Figure 7. (a) Cyclic voltammograms recorded at 5 mV s-1 and (b) galvanostatic charge/discharge curves of each sample at 1 A g-1 at different potential windows in two-electrode symmetric supercapacitors. Electrochemical impedance spectroscopy (EIS) measurement can be used to efficiently explore resistance and capacitive properties of the G800, PG800 and PG800S in terms of Nyquist plots, as illustrated in Figure 8a-d. As is well known, the equivalent series resistance (ESR) consists of three main components, including the inherent ohmic resistance (Rs), interfacial charge transfer resistance (Rct) and Warburg


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diffusion resistance (Rw), which can be obtained from the intercept of the real axis by the Nyquist diagram.51 The EIS curves for the PG800 and PG800S electrodes show nearly identical patterns regarding their semicircular shapes with different resistance values (Figure 8a). The Rct in the solid electrode material is reflected by the semicircle in the intermediate frequency region, and the Rw, ie the electrolyte resistance, is indicated by the slope of the oblique line above the semicircular shoulder in the low frequency region. The semicircle radius of the PG800 electrode is substantially larger than that of the PG800S electrode, indicating that the carbon material directly activated with phosphoric acid has a large charge transfer resistance.54 This property is mainly because of the large number of unstable phosphorus-containing functional groups (such as C3-P or C-O-P) anchoring the oxygen-containing functional groups to the surface of the carbon material, resulting in a slow ion transport during the electrochemical processes. After the passivation procedure, the semicircle radius of PG800S is greatly reduced. This result shows that after the high-temperature passivation, the type and composition ratio of the surface phosphorus configurations of carbon materials are optimized. The more stable C3-P=O obtained by the high-temperature evolution effectively repairs the surface defects of graphene and increases the interlayer spacing. Consequently, the robust phosphorus species stabilizes the electrochemical interface and promotes a significant reduction in the interfacial transfer resistance. Unlike PG800 and PG800S, the G800 electrode has no semicircle region, i.e., no Rct. The reason is that G800 is only carbonized at 800 °C without H3PO4 activation, resulting in no bridging of phosphorus-containing



functional groups between graphene nanosheets, and the high conductivity of graphene together makes its interfacial transfer resistance is extremely small, that is, there is almost no semi-ring area. Notably, the vertical lines in the low-frequency regions of the PG800 and PG800S electrodes indicate a nearly ideal capacitive behavior, compared with that of G800. In short, although phosphorus doping can generate a charge resistance, it can also significantly increase the capacity of the material. The rate capability of the supercapacitor electrode material can be effectively reflected by the capacitance-frequency response curve. As shown in Figure 8b, the electrolyte ions in the low frequency region can easily penetrate from the void to the intermediate layer of the carbon electrode because this time is sufficient. Therefore, as the penetration of electrolyte ions promotes the increase in internal capacitance and resistance of the graphene layer, it also leads to greater capacitance and higher impedance. Gradually as the frequency increases, the electrolyte ions do not reach timely the inside of the graphene layer, and the number of ions decreases, and the capacitance and total impedance also decrease. Especially the rapid drop in capacitance and impedance is due to the incomplete and untimely entry of electrolyte ions into the interior of the graphene layer, so the capacitance and resistance behavior only responds to changes in the vicinity of the orifice. Obviously, it can be seen from Figure 8b that the PG800S has the best capacitance retention compared to other electrodes at the same frequency. The interlayer spacing of PG800S is 0.3738 nm, which is significantly greater than that of G800 (0.3351 nm) and PG800 (0.3590 nm),


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contributing to a fast ion/charge diffusion. This observation is consistent with the rate capability results at high current densities of the GCD analysis in Figure 6a.The rate at which the capacitor is reversibly charged and discharged is generally evaluated by the relaxation time (τ0) or the reciprocal of the frequency (f0). The lower the relaxation time, the faster charging and discharging capabilities for a capacitor. As shown in Figure 8c, the maximum capacitance is indicated at the peak of the curve frequency f0, and the relaxation time τ0 (τ0=1/f0) corresponds to it, which can be roughly thought of as a turning point from resistance behavior to capacitance behavior in the circuit..55 The f0 values of G800, PG800 and PG800S are 2.73 Hz, 0.37 Hz and 0.73 Hz, respectively, and the corresponding τ0 is 0.36 s, 2.70 s and 1.37 s. Although the τ0 of G800 is the smallest, the functional groups on the surface of the graphene are removed synchronously due to the high temperature of 800 °C. The electrochemical active sites on the surface of the graphene electrode are greatly reduced, which greatly accelerates the charging and discharging speeds, resulting in a great reduction in the capacitance. In contrast, although the τ0 values of PG800 and PG800S are larger, their capacities are greatly improved over that of G800. This result shows that the introduction of phosphorus species is very significant to the improvement in the capacitance. After the passivation treatment, the τ0 of PG800S is significantly lower than that of PG800. This change is consistent with the Bode plot analysis (Figure S4a in SI). Furthermore, the total capacitance (C, F g-1), the real capacitance (C′, F g-1) and the imaginary capacitance (C", F g-1) are calculated using equations 2-4 in SI as a



function of the frequency for all the samples and C represents the effective capacitance provided by the device (Figure 8d). C' reaches saturation at approximately 0.2 Hz and then drops sharply with increasing frequency and reaches a constant value above 10 Hz. Therefore, the electrodes have capacitive behavior and resistance behavior in the low frequency region and the high frequency region, respectively. This result agrees with the plots of the total impedance versus frequency (Figure S4b in SI). Apparently, the PG800S also displays the best capacitance in all samples. In addition, the change trend of the capacitance obtained by the C' curve at low frequencies is consistent with the variation of the results of the GCD analysis. All above results show that the optimized C3-P=O and C-P-O can indeed improve the surface wettability of carbon materials (It can be seen from Figure S5 in SI that it is consistent with the contact angle test results), increase the interlayer spacing with rich electrochemical active sites and play a key role in stabilizing the electrochemical interface between the electrode and electrolyte, promoting the diffusion of electrolyte ions into the interior of the electrode.


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Figure 8. (a) The Nyquist plots of all samples between 10 mHz and 100 kHz, with inset as the enlarged part of the PG800S curve. (b) The frequency response of the total capacitance obtained using equation 2 in SI. (c) The frequency response of the real capacitance (F g-1) obtained using equation 3 in SI. (d) The imaginary parts of the capacitance calculated via equation 4 in SI. 3. CONCLUSION In summary, we successfully prepared phosphorus-doped graphene materials through a phosphoric-acid activation strategy and further proposed a high-temperature passivation method to achieve the optimization of the phosphorus-containing functional groups and make their content and distribution more reasonable. We also proposed an evolution mechanism for the corresponding phosphorus species, from which it is concluded that phosphorus mainly presents as C3-P=O in carbon materials, with a small portion as metastable C-P-O, to stabilize and adjust the structure and



surface chemistry of carbon materials. In addition, the electrochemical behavior of G800, PG800 and G800S samples were investigated in two-electrode symmetric supercapacitor systems, and the data establish a clear structure-activity relationship between the phosphorus-containing functional groups and the electrochemical behavior. The C3−P=O configuration is the most stable structure in the graphene lattice and plays a key role in stabilizing the electrochemical interface between the electrode and electrolyte. These features enable the electrode based on P-doped graphene to exhibit an enlarged potential window of 1.5 V in an aqueous electrolyte, a remarkable improved cyclic stability and an ultralow leak current. We believe that the present work provides important guidance and has implications for the rational optimization of the configuration and relative content of phosphorus species in carbon materials to satisfy a broad range of practical applications in many fields, such as energy storage, environmental protection, sensor device and electrocatalysis.

ASSOCIATED CONTENT Supporting Information (SI) Experimental details, Figures S1-S5, Table S1, AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID

Chengmeng Chen: https://orcid.org/0000-0003-4259-9923


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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work has been supported by the following fund projects. Authors Chengmeng Chen, Lijing Xie, Qingqiang Kong, Feng Li and Zhihong Bi received funding from Scientific and Technological Key Project of Shanxi Province Grant MC2016-08. Authors Qingqiang Kong, Zhihong Bi, Li Huo and Jingpeng Chen received funding from Scientific and Technological Key Project of Shanxi Province Grant MC2016-04. Authors Chengmeng Chen and Qingqiang Kong received funding from Shanxi Scholarship Council of China Grant 2016141, Author Lijing Xie, received funding from Scientific Research Foundation for Young Scientists of Shanxi Province Grant 201601D021061, Author Xianxian Wei received funding from Scientific Research Foundation for Young Scientists of Shanxi Province Grant 201601D021134. Author Chengmeng Chen appreciates the support from the Youth Innovation Promotion Association of CAS.



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Table of Contents Graphic

There is an important transformation from C3-P and C−O−P linkages into C3-P=O and C-P-O linkages in P-doped graphene during the passivation process at 1000 °C under Ar atmosphere. Experimental and theoretical calculations have proved that the C3−P=O configuration is the most stable structure in the graphene lattice, which can enable the material an enlarged potential window of 1.5 V in aqueous electrolyte by stabilizing the electrochemical interface between the electrode and electrolyte.

Figure 1. TEM images of (a) G600, (b) G800, (c) PG800 and (d) PG800S,


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respectively. (e) STEM image and corresponding EDS elemental mapping images of PG800S.

Figure 2. (a) (b) FT-IR spectrum. (c) XPS survey of G800, PG800 and PG800S. (d) High resolution of P2p spectrum of PG800 and PG800S.

Figure 3. Theoretical models of phosphorus in various bonding configurations and the corresponding values of (a) binding energy and (b) formation energy. Note: The graphene lattice size and corresponding phosphorus content in the model are



represented by blue (4 × 4 - 3.1%), pink (5 × 5 -2.0%) and green (6 × 6 -1.6%).

Figure 4. Partial density of states (PDOS) of different phosphorus configurations. The Fermi level is set to zero at the dotted line.

Figure 5. (a) XRD pattern and (b) Raman spectrum of all samples.


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Scheme 1. Proposed evolution pathways of phosphorus species in the graphene lattice during passivation process in Ar.

Scheme 2. Possible evolution mechanisms of phosphorus containing compounds from intermediate transition states to defect C-P-O bonds.



Figure 6. (a) Variation of gravimetric capacitances with a series of current densities. (b) Cycling stability of all samples at 3 A g-1. (c) Leakage current curves of all samples. (d) Self-discharging curves of the open circuit voltage changes over time for each sample.


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Figure 7. (a) Cyclic voltammograms recorded at 5 mV s-1 and (b) galvanostatic charge/discharge curves of each sample at 1 A g-1 at different potential windows in two-electrode symmetric supercapacitor.



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Figure 8. (a) The Nyquist plots of the all the samples in the frequency range of 10 mHz and 100 kHz with inset as the enlarged part of PG800S curve, (b) Frequency response of total capacitance using equation 2 in SI, (c) The frequency response of the real capacitance (F g-1) using equation 3 in SI and (d) the imaginary parts of the capacitance calculated with the help of equation 4 in SI.

Table 1. Elemental analysis by XPS of G800, PG800, PG800S. Elemental Content (at. %) Sample

C 1s

O 1s

P 2p

G800

95.69

4.31

0.00

PG800

93.16

4.90

1.93

PG800S

90.20

6.95

2.85


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Table 2. Results of Deconvolution of the P2p Peaks of G800, PG800, PG800S P2p peak (eV) Relative contents of phosphorus species (%)

Sample

P1 (C3-P)

P2 (C3-P=O)

P3 (C-P-O)

P4 (C-O-P)

130.2

132.3

133.1

134.0

PG800

13.6

46.6

22.5

17.4

PG800S

0.0

54.3

32.5

13.2


Structural Evolution of Phosphorus Species on Graphene with a

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