Hybrid Graphene‐Polyoxometalates Nanofluids as Liquid Electrodes for Dual Energy Storage in Novel Flow Cells

Abstract

Solid Hybrid materials abound. But flowing versions of them are new actors in the materials science landscape and in particular for energy applications. This paper presents a new way to deliver nanostructured hybrid materials for energy storage, namely, in the form of nanofluids. We present here the first example of a hybrid electroactive nanofluid (HENFs) combining capacitive and faradaic energy storage mechanisms in a single fluid material. This liquid electrode is composed of reduced graphene oxide and polyoxometalates (rGO-POMs) forming a stable nanocomposite for electrochemical energy storage in novel Nanofluid Flow Cells. Two graphene based hybrid materials (rGO-phosphomolybdate, rGO-PMo and rGO-phosphotungstate, rGO-PW ) were synthesized and dispersed with the aid of a surfactant in 1 M H SO aqueous electrolyte to yield highly stable hybrid electroactive nanofluids (HENFs) of low viscosity which were tested in a home-made flow cell under static and continuous flowing conditions. Remarkably, even low concentration rGO-POMs HENFs (0.025 wt%) exhibited high specific capacitances of 273 F/g(rGO-PW ) and 305 F/g(rGO-PMo ) with high specific energy and specific power. Moreover, rGO-POM HENFs show excellent cycling stability (∼95 %) as well as Coulombic efficiency (∼77-79 %) after 2000 cycles. Thus, rGO-POM HENFs effectively behave as real liquid electrodes with excellent properties, demonstrating the possible future application of HENFs for dual energy storage in a new generation of Nanofluid Flow Cells.

From Thermal to Electroactive Graphene Nanofluids

“…In the area of electrochemical energy storage, our group has pioneered the design and study of electroactive graphene nanofluids. In this case, rGO (prepared by Hummers method, then thermally reduced at 800 • C) was dispersed in acidic aqueous electrolytes and we were able to demonstrate a very fast charge transfer [8,10]. Thus, a dispersion of rGO in 1M H 2 SO 4 stabilized in aqueous solution with the addition of a surfactant (0.5 wt% of triton X-100 and sonicated by ultrasonic bath up to 2 h), featured low viscosity values, close to those of water (in the shear rate range of 25 to 150 per s at room temperature).…”

“…We used a homemade flow cell system (carved serpentine flow 200 mm long path, 5 mm wide, and 1mm in depth, with total cell size of 7 cm × 6 cm × 1 cm) to study these hybrid electroactive nanofluids (HENFs) both under flowing and static conditions [10]. Both nanofluids showed a viscosity very close to that of water (Figure 7a).…”

“…Graphene nanofluids are prepared by dispersing graphene (or RGO) nanosheets in an adequate base fluid. They can be stabilized in organic or aqueous solvents [6,8,10,11] in the form of pure, non-oxidized graphene [11] or rGO [8], but also in the form of hybrids [6,10]. All of these solvents present useful thermal or electrochemical properties, among others, as we summarize below.…”

Can polyoxometalates enhance the capacitance and energy density of activated carbon in organic electrolyte supercapacitors?

“…POMs-based materials have been applied in various fields, including catalysis, electrochromic, magnetic, and energy storage [7,8]. In a variety of energy storage devices, including supercapacitors [7][8][9][10], lithium ion batteries [11][12][13], flow cells [14,15], fuel cells [16] we can see the great potential of POMs stemming from their multielectron redox activities. And POMs not only can deliver superior energy but also they are structurally best suited to do it at a fast rate since all of their active moieties are at the surface of the cluster.…”

Improved cyclic retention and high‐performance supercapacitive behavior of poly(diphenylamine‐co‐aniline)/phosphotungstic acid nanohybrid electrode

“…[22][23][24][25] On the other hand, keggin-type polyoxometalate (POM), particularly phosphotungstic acid (PTA) has been demonstrated for many applications including supercapacitor, sensor, catalyst, fuel cells, proton memory devices, molecular conductors, and solid-state electronic devices because of its excellent solubility, thermal stability, high proton conductivity, high electronic density, strong acidity, and reversible multielectron redox activity of electrochemistry and photochemistry. 26,27 [28][29][30][31][32][33][34][35][36] High supercapacitance was achieved due to its faradaic involvement and homogeneous distribution of POM ions that directs to higher charge delocalization and enhanced conductivity. 37 Yet, articles on the judicious utilization of PDPA/PTA/PANI copolymer combination for supercapacitor applications are scanty.…”