Phosphorene as Cathode Material for High‐Voltage, Anti‐Self‐Discharge Zinc Ion Hybrid Capacitors

Abstract

It is generally believed that supercapacitors are mainly characterized by high power density and low energy density; therefore, significant efforts have been focused on improving the energy density of electrode materials. [2,3] The potential windows of aqueous supercapacitors, especially for those based on acidic and alkaline electrolytes, are limited by their electrolyte decomposition voltage (1.23 V). Several researchers showed potentials reaching only ≈1 V, which heavily impeded the practical application of supercapacitors. [4][5][6] Two methods that have proven effective in resolving the narrow voltage range that results from water decomposition entail replacing the water electrolyte with "water in salt" (WiS) electrolytes and organic electrolytes, respectively. [7,8] The WiS electrolytes were proposed by Wang and co-workers in 2015, and their use expanded the potential window to ≈3.0 V due to the formation of an electrode-electrolyte interphase. [9] The close interaction between water molecules and the electrolyte in ultrahigh concentration makes it difficult for water to decompose, thus effectively broadening the electrochemical operating voltage range. As for the organic electrolytes, the influence of water electrolysis is exclusive, and the electrolyte operating voltage range only depends on its electrochemical stability voltage. [10][11][12] Following these two approaches, the potential windows can be effectively expanded, which is the basic requirement for supercapacitors to deliver a high output voltage.Another neglected property of supercapacitors is their self-discharge rate, the low capacity retention in open circuit state makes the supercapacitors less effective in practical applications. A general method was required to enhance the anti-self-discharge properties. The intrinsic reason for the fastself-discharge speed is that when both anode and cathode materials store electricity through adsorption behaviors, they have a barely satisfactory ion limiting ability and the ions adsorbed on the electrode during the charging process will soon diffuse into the electrolyte due to the concentration gradient. Hybrid ion capacitors can be constructed to inhibit self-discharge by incorporating insertion-type and conversion-type batterytype electrodes that have stronger force for limiting ions than through simple adsorption behaviors. For this, one of the Output voltage and self-discharge rate are two important performance indices for supercapacitors, which have long been overlooked, though these play a very significant role in their practical application. Here, a zinc anode is used to construct a zinc ion hybrid capacitor. Expanded operating voltage of the hybrid capacitor is obtained with novel electrolytes. In addition, significantly improved anti-self-discharge ability is achieved. The phosphorene-based zinc ion capacitor exploiting a "water in salt" electrolyte with a working potential can reach 2.2 V, delivering 214.3 F g −1 after 5000 cycles. The operating voltage is further extended to 2.5 V through the...

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