Abstract
The stable high-performance electrochemical electrodes consisting of supercapacitive reduced graphene oxide (rGO) nanosheets decorated with pseudocapacitive polyoxometalates (phosphomolybdate acid-H3PMo12O40 (POM) and phosphotungstic acid-H3PW12O40 (POW)) nanodots/nanoclusters are hydrothermally synthesized. The interactions between rGO and POM (and POW) components create emergent “organic–inorganic” hybrids with desirable physicochemical properties (specific surface area, mechanical strength, diffusion, facile electron and ion transport) enabled by molecularly bridged (covalently and electrostatically) tailored interfaces for electrical energy storage. The synergistic hybridization between two electrochemical energy storage mechanisms, electrochemical double-layer from rGO and redox activity (faradaic) of nanoscale POM (and POW) nanodots, and the superior operating voltage due to high overpotential yielded converge yielding a significantly improved electrochemical performance. They include increase in specific capacitance from 70 F·g−1 for rGO to 350 F·g−1 for hybrid material with aqueous electrolyte (0.4 M sodium sulfate), higher current carrying capacity (>10 A·g−1) and excellent retention (94%) resulting higher specific energy and specific power density. We performed scanning electrochemical microscopy to gain insights into physicochemical processes and quantitatively determine associated parameters (diffusion coefficient (D) and heterogeneous electron transfer rate (kET)) at electrode/electrolyte interface besides mapping electrochemical (re)activity and electro-active site distribution. The experimental findings are attributed to: (1) mesoporous network and topologically multiplexed conductive pathways; (2) higher density of graphene edge plane sites; and (3) localized pockets of re-hybridized orbital engineered modulated band structure provided by polyoxometalates anchored chemically on functionalized graphene nanosheets, contribute toward higher interfacial charge transfer, rapid ion conduction, enhanced storage capacity and improved electroactivity.
Highlights
The increasing global demand for electric energy stimulated an intense research and development activities worldwide
We propose to design and synthesize hydrothermally functional hybrid assembly composed of POM clusters and POW nanodots building blocks integrated with supercapacitive reduced graphene oxide (rGO) as potential electrochemical electrodes [45]
The rGO exhibits porous architecture composed of ultrathin nanosheets conformed to electrically conductive framework beneficial for electron transfer and ion transport while maintaining electrical conductivity with substantial accessible specific surface area for ion sorption
Summary
The increasing global demand for electric energy stimulated an intense research and development activities worldwide. Efficient energy storage solutions are required for the transition to sustainable energy sources. Among several renewable energy sources, electrochemical energy conversion and storage systems such as rechargeable secondary batteries, electrochemical supercapacitors, known as ultra-capacitors or electrochemical capacitors, pseudocapacitors and their hybrids, known as supercapbatteries, represent the most efficient and environmentally benign technologies. Electrochemical and electrocatalytical research is rapidly developing into parallel tracks striving toward improved electrode materials and subsequent implementation from portable consumer electronics to automobile and grid scale energy devices [3,4,5,6]. While rechargeable Li-ion batteries take advantage of bulk charge storage and exhibit high specific energy density, they suffer from low specific power density and cyclability. Supercapacitors and pseudocapacitors store charge through surface ion adsorption (non-faradaic) and surface redox reaction (faradaic), respectively. Electrochemical supercapacitors based on metal oxides and conducting polymers featuring voltage-changing redox processes, where faradaic and non-faradaic mechanisms take place concomitantly, are known as hybrid supercapacitors [7,8,9]
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