**1. Introduction**

In recent years, graphene and its composite materials have attracted grea<sup>t</sup> attention due to their excellent performance in various fields. Many researchers have presented that graphene plays an important role as electrodes, catalysts, and supports in energy-storage fields, such as supercapacitors, ions batteries, fuel cells and so on [1–4]. To improve the electrochemical performance of graphene, introducing other active groups (–OH, –COOH, –NO*<sup>x</sup>*, and –PO*x*) or heteroatoms, including O, P, N and B, into graphene is regarded as an effective way to adjust the electrochemical properties of graphene. The advantages of this strategy are the heteroatoms could tailor the electronic properties and average electronic charge distribution of adjacent carbon atoms of graphene. Among various heteroatom-doped graphene materials, nitrogen-doped (N-doped) graphene has been widely studied in supercapacitors [5] and batteries [6]. However, only a few studies are reported so far regarding graphene modified with phosphorus (P) and phosphorus-oxide (–PO*x*) in electrochemical energy storage field. Recently, Karthika et al. prepared a high electrochemical activity P-doped graphene by reducing graphene oxide with phosphoric acid. The prepared sample exhibited a specific discharge capacitance value of 367 F/g, which is much larger than that of graphene [7]. This study suggests that the electrochemical properties of graphene can be effectively improved by introducing P into

graphene. Therefore, it is very necessary to study the preparation and electrochemical properties of P and P–O group modified graphene. In addition, it was revealed that P-modified graphene was much more stable in air than N-modified graphene and exhibited improved n-type semiconducting behavior [8]. However, the traditional preparation methods are relatively complex and time-consuming. Wang et al. synthesized P/C hybrid through a simple ball-milling approach under argon protection. The whole preparation time was 16 h [9]. Xu et al. prepared P doped graphene via thermally annealing approach and the preparation time was more than 48 h [10]. To overcome these drawbacks, it is necessary to use another fast and efficiency method to prepare P modified graphene material. In contrast to the tradition methods, the electrochemical exfoliation approach is believed to be an effective and efficiency strategy. The whole time of preparation usually takes only a few minutes. Accordingly, the preparation of P or P–O group modified graphene through electrochemical approach is more advantageous than traditional methods.

Currently, supercapacitors (SCs) play an important role in the field of energy storage and have been applied in many areas due to high power density, good rate capability and excellent cycling stability [11]. Different from the traditional methods, a unique strategy of introducing redox additives in the electrolytes could effective improve the capacity of SCs via redox reactions at the interface of electrode and electrolyte. Therefore, the efficient use of redox electrolytes is an indispensable way to increase the electrochemical performance of SCs. In our previous studies, we have presented that the potassium iodide (KI) is an effective active redox electrolyte to enhance the capacitance of SCs due to the redox reaction between pairs of 3I−/I<sup>−</sup>3, 2I−/I2, 2I−3/I2, I2/IO<sup>−</sup>3, etc. [12–19]. The electrode materials include conductive polymers such as polyaniline and poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrene sulfonate) (PSS). Although these polymers have good electrochemical performance, the preparation process is tedious, and, because of the stability restriction of polymer material itself, iodide ions may destroy the structure when they frequently penetrate and release from the polymer. Therefore, it is necessary to apply a new material with good electrochemical performance and that is not easily damaged to improve the performance of the supercapacitor.

In recent studies, some researchers have presented that heteroatom-doped graphene has excellent electrocatalytic activity for the reduction of iodine, due to the electrons conjugation effect, charge redistribution and defect sites [20]. However, the electrochemical catalytic activity of phosphorus oxide modified graphene (PO-graphene) has not been fully revealed. Similar to doping heteroatoms, introducing P–O groups is an effective way to improve the electrochemical performance of graphene. The advantages lie in the modification of the electronic properties and the effect on electronic charge distribution of graphene. Furthermore, the P–O groups could also perform redox reactions with some electroactive ions, such as iodide ions. Therefore, the PO-graphene is a suitable electrode material to use in supercapacitors that contains KI electrolysis.

In this paper, we present a facile method to prepare high quality PO-graphene and study the electrochemical property. The PO-graphene is prepared by the one-step electrochemical anodic exfoliation method and used as the electrode in a redox supercapacitor, whose electrolyte contains KI. We investigated in detail the catalytic effect of P–O group on redox reaction of 3I−/I<sup>−</sup>3 via various electrochemical analysis. We present that the introduced P–O group in graphene could effectively improve its electrochemical performance. The whole preparation process can be completed within 30 min and the yield is about 37.2%. Owing to the catalytic effect of P–O group, the PO-graphene has a better electrochemical performance than graphite in promoting the redox reaction of I−/I<sup>−</sup>3.

#### **2. Materials and Methods**
