**3. Results**

The schematic diagram of the preparation and the photo of PO-graphene are shown in Figure 1a,b. The preparation process included two parts: oxidation and exfoliation of graphite. The oxidation process produced the P–O groups and then the graphite was exfoliated into few-layer graphene, which possessed a platelet-like morphology (Figure 1c,d), with the promotion of water decomposition. The whole preparation process was completed within 30 min and the yield reached 37.2%.

The chemical composition of PO-graphene was determined by FT-IR, XPS and Raman spectra. As presented in Figure 2a, the PO-graphene shows clear stretching peaks of P=O, O–P–O, P–OH, and P-H at 1155, 1116, 946, and 2443–2330 cm<sup>−</sup>1, respectively. Different types of oxygen functionalities are presented at 3434 cm<sup>−</sup><sup>1</sup> (O–H stretching), 1076 cm<sup>−</sup><sup>1</sup> (C–O stretching), 980 and 861 cm<sup>−</sup><sup>1</sup> (P–O–C stretching). The peaks at 1649 (C=C skeletal vibrations from graphite) and 2856 cm<sup>−</sup><sup>1</sup> (CH2 stretching) are also detected [21]. These characteristic peaks confirm the graphene is successfully modified by P–O group.

**Figure 1.** (**a**) schematic diagram of preparation; (**b**) the photo of phosphorus oxide modified graphene (PO-graphene); (**c**) scanning electron microscope (SEM) and (**d**) transmission electron microscopy (TEM) image of PO-graphene.

The high resolution XPS spectra could provide detailed bonding and chemical information for the PO-graphene. As presented in Figure 2b, the C 1s peak can be divided into three components centered at about 284.3, 285.8 and 287.5 eV, which cloud be attributed to C–C, C–P, and C–O bonding, respectively [22]. The presence of the C–P characteristic peak confirms that P atoms have been successfully bonded into the graphene. The high-resolution P 2p spectrum (Figure 2c) shows that phosphorus bonded with graphene in two types of chemical bonding: P–C and P–O bonding, located at about 129.8 and 133.7 eV, respectively [23]. The presence of both P–C and P–O stretch models confirms that the P–O group was successfully bonded with graphene.

The Raman spectra of PO-graphene and graphite are presented in Figure 2d. The spectrum of the original graphite shows a very weak D band at 1338 cm<sup>−</sup><sup>1</sup> and strong G band at 1563 cm<sup>−</sup>1. The D band is related to the defects and disorder in the structure of the graphite due to the intervalley scattering, while the G band corresponds to the E2g vibration of sp<sup>2</sup> of C atoms. In contrast to the origin graphite, the PO-graphene shows a clearly strong D band. The intensity ratio of D to G band (*I*D/*I*G) increases from 0.1 to 0.87, manifesting that the disorder degree and structural defect (decrease in the average size of the sp<sup>2</sup> domains) in PO-graphene are enhanced due to the oxidization during the preparation [24]. The decreased layers of graphene in PO-graphene were also demonstrated by the wave shift of the G band from 1563 to 1568 cm<sup>−</sup>1.

The XRD patterns of PO-graphene and graphite are presented in Figure 2e. PO-graphene exhibits two diffraction peaks centered at 23.7◦ and 26.3◦, corresponding to the larger interlayer spacing of 0.375 and 0.338 nm, respectively. Compared with the graphite (about 0.335 nm), the larger interlayer distance is due to the accommodation of oxygen functional groups and water molecules during the preparation due to the obvious hydrophilic character [25]. This result also suggests the formation of P–O groups on graphene.

**Figure 2.** (**a**) Fourier transform Infrared spectroscopy (FT-IR) analysis of PO-graphene; (**b**) high-resolution C1s spectrum of PO-graphene; (**c**) high-resolution P2p spectrum of PO-graphene; (**d**) Raman analysis patterns of PO-graphene and graphite and (**e**) X-ray diffraction analysis (XRD) patterns of PO-graphene and graphite.

The electrochemical performance of PO-graphene is investigated by CV analysis. As shown in Figure 3a, compared with the graphite, which possesses only one single reduction peak, the PO-graphene has a more symmetry and larger peak current than that of graphite, suggesting that it has a better electrochemical performance due to the catalysis of P–O groups. The pseudo capacitance performance comes from both the redox transformation of I−↔ I<sup>−</sup>3 and the interaction between P–O groups and iodide ions. Clearly, the PO-graphene possesses three pair of redox peaks. The peaks centered at 0.27 and 0.12 V are due to the transformation of 3I−–2e ↔ I<sup>−</sup>3, the peaks at 0.53 and 0.22 V could be assigned to the transformation of 5I−–4e ↔ I<sup>−</sup>5 and the peaks at 0.65 and 0.29 V may be attributed to the interaction between P–O and iodide ions.

**Figure 3.** (**a**) Cyclic voltammetry (CV) profiles of PO-graphene and graphite performed in a mixture of 0.1 M KI and 0.2 M H2SO4 aqueous at 10 mV/s; (**b**) CV profiles of PO-graphene and (**c**) graphite at different scan rate; (**d**) Nyquist plots and equivalent circuit (inset) of PO-graphene and graphite; (**e**) galvanostatic charge–discharge (GCD) analysis of PO-graphene and graphite at 3.5 mA/cm2; and (**f**) GCD analysis of PO-graphene at different current density; (**g**) cycling stability analysis at 10 mA/cm2.

To investigate the charge transfer process between PO-graphene and iodide ions, CV tests at different scan rates were performed. As exhibited in Figure 3b, with increasing scan rate, the separation of the peak potentials was enlarged slightly, indicating that the oxidation and reduction of iodide ions in graphite electrode were surface-controlled process. In contrast, the redox peak current of PO-graphene (Figure 3c) was increased apparently by improving the scan rate and the shape of the curve gradually turns into a rectangle, suggesting that the interaction between PO-graphene and iodide ions was performed through a Faradic reaction.

The charge transfer performance is also investigated by EIS analysis. The Nyquist plots and equivalent circuit are shown in Figure 3d. In the Nyquist plots, the diameter of the semicircle manifests the difficulty of the charge transfer in the electrochemical system [26]. As can be seen clearly, the PO-graphene shows a smaller semicircle than that of the graphite, suggesting that it has a lower charge transfer resistance and a faster charge transfer process. The charge-transfer resistance for PO-graphene and graphite is 2.09 and 6.67 Ω/cm2, respectively. This result could be ascribed to the strong redox interaction between P–O groups and iodide ions, and this conclusion agrees well with the CV results.

Because of the convenient charge transfer character, the PO-graphene has a better charging–discharging performance than that of graphite. As shown in Figure 3e, the discharge specific capacitance of PO-graphene and graphite is calculated as 1634.2 and 87.1 F/g, respectively. The rate capacity of PO-graphene was tested with different current density. As presented in Figure 3f, the discharge specific capacitance decreased gradually with increasing current density from 6.5 mA/cm<sup>2</sup> to 16.5 mA/cm<sup>2</sup> due to the polarization. The maximum discharge specific capacitance was calculated as 904.3 F/g when the current density was 6.5 mA/cm<sup>2</sup> and it reached 342.5 F/g at 16.5 mA/cm2. This result manifests that the PO-graphene possesses a good rate performance and this discharge performance is also better than the other electrochemical systems, as shown in Table 1. The cycling stability of PO-graphene was investigated at the current density of 10 mA/cm2. As shown in Figure 3g, the discharge specific capacitance still keeps 438 F/g after 500 charge–discharge cycles. This good cycling stability is ascribed to the excellent charge transfer ability of PO-graphene. The charge-transfer resistance only increased 3.1 Ω/cm<sup>2</sup> compared to pristine during the charge–discharge process (as presented in Figure 3d, inset).


**Table 1.** Comparison of discharge specific capacitance between the phosphorus oxide modified graphene (PO-graphene) and other reported values.

## **4. Conclusions**

In summary, we applied electrochemical anodic exfoliation method to prepare phosphorus oxide modified graphene and used it as an effective electrode in a redox supercapacitor. The PO-graphene has excellent electrochemical performance due to the significant catalysis to iodide ions and convenient charge transfer capability between phosphorus oxide and iodide ions. The maximum discharge capacitance is 1634.2 F/g when the current density is 3.5 mA/cm2. The PO-graphene possesses good rate performance and cycling stability.

**Author Contributions:** L.Z., H.L. and X.C. conceived and designed the experiments; L.Z., H.L., X.G., C.L., X.C., Y.Z., Y.H., Y.Z. and Z.G. performed the experiments and analyzed the data; Y.L., X.W. and X.C. contributed the analysis tools; and L.Z. and H.L. wrote the paper.

**Acknowledgments:** This work was supported by the Natural Science Foundation of China (NSFC, Nos. 51573021, 21271031, 51063009, and 51203012); the Beijing Natural Science Foundation of China (Nos. 2132009 and 2122015); Innovation Promotion Project of Beijing Municipal Commission of Education, China (No. TJSHG2015 11021002); Opening Foundation of the State Key Laboratory of Organic-Inorganic Composites (oic-201801009); and Talents project of Beijing Organization Department (No. 2017000020124G095).

**Conflicts of Interest:** The authors declare no conflict of interest.
