Graphene nanosheets and graphene nanoribbons, G combined with vanadium pentoxide (VO) nanobelts (VNBs) and VNBs forming GVNB composites with varying compositions were synthesized via a one-step low temperature facile hydrothermal decomposition method as high-performance electrochemical pseudocapacitive electrodes. VNBs from vanadium pentoxides (VO) are formed in the presence of graphene oxide (GO), a mild oxidant, which transforms into reduced GO (rGOHT), assisting in enhancing the electronic conductivity coupled with the mechanical robustness of VNBs. From electron microscopy, surface sensitive spectroscopy and other complementary structural characterization, hydrothermally-produced rGO nanosheets/nanoribbons are decorated with and inserted within the VNBs’ layered crystal structure, which further confirmed the enhanced electronic conductivity of VNBs. Following the electrochemical properties of GVNBs being investigated, the specific capacitance Csp is determined from cyclic voltammetry (CV) with a varying scan rate and galvanostatic charging-discharging (V–t) profiles with varying current density. The rGO-rich composite V1G3 (i.e., VO/GO = 1:3) showed superior specific capacitance followed by VO-rich composite V3G1 (VO/GO = 3:1), as compared to V1G1 (VO/GO = 1:1) composite, besides the constituents, i.e., rGO, rGOHT and VNBs. Composites V1G3 and V3G1 also showed excellent cyclic stability and a capacitance retention of >80% after 500 cycles at the highest specific current density. Furthermore, by performing extensive simulations and modeling of electrochemical impedance spectroscopy data, we determined various circuit parameters, including charge transfer and solution resistance, double layer and low frequency capacitance, Warburg impedance and the constant phase element. The detailed analyses provided greater insights into physical-chemical processes occurring at the electrode-electrolyte interface and highlighted the comparative performance of thin heterogeneous composite electrodes. We attribute the superior performance to the open graphene topological network being beneficial to available ion diffusion sites and the faster transport kinetics having a larger accessible geometric surface area and synergistic integration with optimal nanostructured VO loading. Computational simulations via periodic density functional theory (DFT) with and without V2O5 adatoms on graphene sheets are also performed. These calculations determine the total and partial electronic density of state (DOS) in the vicinity of the Fermi level (i.e., higher electroactive sites), in turn complementing the experimental results toward surface/interfacial charge transfer on heterogeneous electrodes.
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.
Graphene-based nanomaterials (graphene nanosheets/graphene nanoribbons) decorated with vanadium pentoxide (V2O5) nanobelts (i.e. GVNBs) were synthesized via one-step low-temperature facile hydrothermal/solvothermal method as high-performance electrochemical composite electrodes. VNBs were formed in the presence of graphene oxide (GO), a mild oxidant, which transforms into reduced GO (rGOHT) assisted in enhancing the electronic conductivity with mechanical strength for GVNBs. From surface sensitive electron microscopy and spectroscopy structural characterization techniques and analyses, rGOHT nanosheets/ nanoribbons appear to be inserted into and coated with the layered crystal structure of VNBs, which further confirmed the enhanced electrical conductivity of VNBs. The electrochemical energy storage capacity of GVNBs is investigated using electrochemistry and the specific capacitance Cs are determined from both the cyclic voltammetry (CV) with scan rate and galvanostatic charge/discharge V-t profiles with varying current density. The GVNBs having rGO-rich composite V1G3 (V2O5/GO = 1:3) showed superior performance followed by V2O5-rich V3G1 (V2O5/GO = 3:1) as compared with V1G1 (V2O5/GO = 1:1) composites besides pure component (rGOHT and V2O5) materials. Moreover, V1G3 and V3G1 composites showed excellent cyclic stability and the capacitance retention of > 80% after 200 cycles. Furthermore, by performing extensive simulations and modeling of electrochemical impedance spectroscopy data, we determined various circuit parameters (charge transfer and solution resistance, double layer and low frequency capacitance). These findings highlight the comparative performance of nanocomposite hybrid electrode materials.
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