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Phosphorus-doped (P-doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X-ray photoelectron spectroscopy, while the morphology of P-doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P-doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 m H 2 SO 4 . The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P-doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g À1, providing a high energy density of 11.64 Wh kg À1 and a high power density of 831 W kg À1 .Owing to the accelerating consumption of global energy, research and development on sustainable energy storage technologies have attracted a great deal of interest. Particularly, carbon nanomaterials have shown excellent performance as electrodes, catalysts, and supports in energy-storage devices, such as supercapacitors, batteries, fuel cells and hydrogen storage. [1] In recent years, graphene, a single layer of sp 2 hybridised carbon atoms, has emerged as one of the most promising carbon allotropes for energy storage devices due to its unique structure and excellent properties, [2] for example, high theoretical surface area (2630 m 2 g À1 ), [3] intrinsic carrier mobility (2 10 5 cm 2 V À1 s À1 ), [4] optical transmittance (97.7 %) [5] and superior mechanical strength. [6] To improve the performance of graphene in energy-storage applications, various attempts have been investigated. Among them, introducing heteroatoms (O, N, S, B, and P) is one of the most effective methods to tailor the electrochemical properties of graphene-based materials; [7] and the advantages lie in the modification of the electronic properties of carbon materials and the effect on electronic charge distribution of adjacent carbon atoms. [7a, 8] Nitrogen-doped (N-doped) graphene has been widely studied in supercapacitors, [9] batteries, [10] fuel cells [11] and field-effect transistors.[12] However, only a few studies are available so far regarding graphene doped with other (non-nitrogen) heteroatoms in similar applications.Recently, Zhang prepared phosphorus-doped (P-doped) graphene by a thermal treatment using graphene oxide and triphenylphosphine, and the P doping in graphene significantly improved the electrochemical properties in oxygen reduction reaction (ORR) and lithium ion batteries (LIB).[13] In another work, P/N co-doped graphene was prepared by pyrolysis of a graphene-dicyandiamide mixture in the presence of phosphoric acid, exhibiting significant enhancement of catalytic activity for ORR.[14] P-doped graphene, obtained by anneal...

Phosphorus-rich carbons (PCs) were prepared by phosphoric acid activation of waste coffee grounds in different impregnation ratios. PCs were characterized by nitrogen and carbon dioxide adsorption and X-ray photoelectron spectroscopy. The results indicate that the activation step not only creates a porous structure, but also introduces various phosphorus and oxygen functional groups to the surface of carbons. As evidenced by cyclic voltammetry, galvanostatic charge/discharge, and wide potential window tests, a supercapacitor constructed from PC-2 (impregnation ratio of 2), with the highest phosphorus content, can operate very stably in 1 M H2 SO4 at 1.5 V with only 18 % degradation after 10 000 cycles at a current density of 5 A g(-1) . Due to the wide electrochemical window, a supercapacitor assembled with PC-2 has a high energy density of 15 Wh kg(-1) at a power density of 75 W kg(-1) . The possibility of widening the potential window above the theoretical potential for the decomposition of water is attributed to reversible electrochemical hydrogen storage in narrow micropores and the positive effect of phosphorus-rich functional groups, particularly the polyphosphates on the carbon surface.

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