Functionalized Graphene–Polyoxometalate Nanodots Assembly as “Organic–Inorganic” Hybrid Supercapacitors and Insights into Electrode/Electrolyte Interfacial Processes

Abstract

Abstract:The stable high-performance electrochemical electrodes consisting of supercapacitive reduced graphene oxide (rGO) nanosheets decorated with pseudocapacitive polyoxometalates (phosphomolybdate acid-H 3 PMo 12 O 40 (POM) and phosphotungstic acid-H 3 PW 12 O 40 (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 (k ET )) 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.

Self-Assembled Polyoxometalate Nanodots as Bidirectional Cluster Catalysts for Polysulfide/Sulfide Redox Conversion in Lithium–Sulfur Batteries

“…To further investigate the formation mechanism of the PW 12 nanodots, we applied digital photography to monitor dynamic changes in a reaction mixture of bare PW 12 with S (Figure S5 drastically from 53 wt % to 13 wt % after sulfur-melt infiltration into the rGO−CNT/PW 12 hybrid host, which can promote effective dispersion of POM aggregates to form uniform POM nanoparticles. 58 In comparison with rGO−CNT@S composite (Figure S3e−h), TEM images of rGO−CNT/PW 12 @S show the homogeneous distribution of sulfur onto the rGO−CNT/ PW 12 host, consistent with EDX elemental mapping results (Figure 1d−l and Figure S2).…”

“…Fourier transform infrared (FTIR) and Raman spectra both indicate that PW 12 clusters were successfully grafted onto the reduced graphene oxide−carbon nanotube (rGO−CNT) sheets due to electron transfer binding and electrostatic binding between rGO and the POM (Figure S6a,b). 58 Moreover, the real POM content of ca. 53 wt % in the rGO−CNT/PW 12 composite was verified by thermogravimetric analysis (TGA) (Figure S6e).…”

Polyoxometalates (POMs): from electroactive clusters to energy materials

“…This work has continued in recent years, suggesting that there is much that remains to be explored and to be better understood. 29 In 2012, Gomez-Romero's group conducted one of the earlier reported studies where a PMo 12 -type POM was anchored to a simple activated carbon (AC) to create a hybrid material. The hybrid exhibited significantly enhanced performance over bare AC as large current under the CV curves can be seen in Figure 3 (a).…”

Electrochemical Properties of Polyoxometalate (H3PMo12O40)-Functionalized Graphitic Carbon Nitride (g-C3N4)

“…4 a positively charged linker (usually applied to provide good interaction), the direct synergy between PMo 12 /C 3 N 4 improved the accessibility of the POM active sites, improved the redox performance, and demonstrated stable amplitude upon repeated scanning. These results, in addition to structural stability, are important factors contributing to the efficiency of cation insertion electrodes [27][28][29][30].…”