High-performance asymmetric supercapacitors based on polyoxometalate-graphene nanohybrids

Abstract

Supercapacitors (SCs) have attracted much attention as energy storage devices capable of accumulating electricity from renewable sources, as well as deliver high power to smart and portable electronic devices due to their fast dynamic response, high power density, and long cycle-life [1,2]. So far, nanostructured electrodes with pseudocapacitive materials have been widely utilized to enhance the specific capacitance and energy density of supercapcitors [3,4]. However, such materials typically have low electrical conductivity (10) [5,6], degrade structurally under rigorous reaction conditions [7], and have a narrow electrochemical window (<1 V) [8,9] which can lead to high internal resistance, large irreversible capacitance loss, and poor rate capability and, consequently, result in supercapacitors with limited power density. A promising strategy to increase power density has been to extend operating cell voltage by taking advantage of an organic electrolyte, or an asymmetric cell configuration [10]. In terms of cost and safety, aqueous asymmetric supercapacitors (ASCs) are a suitable choice for commercial SCs [11]. Aqueous ASCs usually consist of a battery-type Faradic electrode for the energy source and a capacitor-type electrode for the power source, and employ aqueous electrolytes, which can increase cell voltage (up to 2 V) and hence improve both energy and power densities [11,12].Polyoxometalates (POMs) are attractive Faradic electrode materials because of their remarkable redox activity, structural integrity, and low cost [13][14][15]. However, POM-based electrodes are electrochemically unstable in aqueous solution due to the dissociation of ionic aggregates [13], which leads to unnecessary loss of specific capacitance. In addition, POM powders have low specific surface areas (<10 m 2 g −1) [16]. Therefore, POMs need to be anchored on an insoluble solid matrix with a high specific surface area to achieve high specific capacitance and stable cycle performance. In this respects, nanocarbons (e.g., carbon nanotubes [17][18][19][20] and graphene [21][22][23]) are good candidates for anchoring POMs. Moreover, their superior electrical conductivity and electrochemical/mechanical stability enable improvement in supercapacitor performance [20,22,23]. However, there has been limited success in utilizing POMs in SCs with satisfactory high specific capacitance and cycle performance, due to limited binding sites and difficulty dispersing the nanocarbons, as well as the weak interaction between POMs and nanocarbons, which still remain a challenge.Herein, we demonstrate aqueous ASCs based on a POM-graphene nanohybrid with polymeric ionic liquid (POM/PIL/G) as a positive elctrode and activated carbon (AC) as a negative electrode in an aqueous electrolyte of H 2 SO 4 . The polymeric ionic liquid (PIL) was selected as the surface functionality because of its high binding affinity for the graphene surface, and high ionic conductivity, as well as sufficient binding sites to produce a high density of POMs on the graphene surf...

Simple and efficient polyoxomolybdate-mediated synthesis of novel graphene and metal nanohybrids for versatile applications

“…The specific capacitance for MoS/rGO/CC obtained from CV at scan rate of 10 mV s -1 is higher than those of previously reported POM-based supercapacitors [27][28][29].…”

A Supercapacitor with High Specific Volumetric Capacitance Produced by 12-Phosphomolybdate Anchored on Graphene Balls

“…Benefited from the high packing density of CGB/PMo 12 (3) (1.86 g cm –3 calculated on the basis of Supporting Information eqs S1 and S2, CGB/PMo 12 (3) present particularly high volumetric capacitance (245–632 F cm –3 in the range of 1–8 A g –1 ), which is higher than that of CGB electrode (123–216 F cm –3 in the range of 1–8 A g –1 ) (Figure d). This mainly comes down to the high molecular weight, the excellent redox and stable tertiary structure with “pseudo-liquid behavior” of PMo 12 itself, and the good physical buffer of CGB with the high stability. ,, …”

“…This mainly comes down to the high molecular weight, the excellent redox and stable tertiary structure with "pseudo-liquid behavior" of PMo 12 itself, 25 and the good physical buffer of CGB with the high stability. 24,48,49 According to the theory of Dunn, the actual total capacity is composed of two parts: the faradaic part from the redox process of PMo 12 and the non-Faradaic electrical double layer capacitance (EDLC) of CGB. Therefore, the total capacitive can be analyzed by CV of different sweep rates.…”

“…10 Å) can be easily anchored on graphene . For example, Yang and co-workers reported the 12-phosphomolybdate (PMo 12 ) adsorbed on graphene, showing a high cell voltage (up to 2 V) and hence improving both energy and power densities . Second, rich surface active sites and “pseudo-liquid” behavior endow POMs with fast and excellent reversible multielectron redox properties, exhibiting great potential application for energy storage .…”

“…23 For example, Yang and co-workers reported the 12-phosphomolybdate (PMo 12 ) adsorbed on graphene, showing a high cell voltage (up to 2 V) and hence improving both energy and power densities. 24 Second, rich surface active sites and "pseudo-liquid" behavior 25 endow POMs with fast and excellent reversible multielectron redox properties, 26 exhibiting great potential application for energy storage. 27 Kumar et al fabricated (NH 4 ) 6 V 10 O 28 nanodots/rGO nanocomposites exhibiting a high capacitance of 1186 F g −1 at 1 A g −1 as a result of faradaic activity.…”