The surface modification of benchmarked metal catalysts using nanostructured non‐metallic materials for improved performance and stability is an active area of research and is interesting from both a fundamental and an applied perspective. Amorphous few layered nanosheets of Cr2O3 (3–5 nm) are synthesized by rapid thermal exfoliation of CrCl3 · 6H2O precursors and are characterized. The hydrogen evolution reaction (HER) studies on alkaline medium conducted with platinum and gold electrodes modified with amorphous sheets of Cr2O3 show augmented HER activity compared to the pristine ones while Cr2O3 alone is not HER active. The role of amorphous Cr2O3 as a co‐catalyst is established and the synergistic charge transfer effects while coupling Cr2O3 with metal catalysts are studied using electrochemical impedance spectroscopy. Large‐scale processability of amorphous Cr2O3 by rapid thermal treatment along with its high electrochemical stability (>2000 cycles or >50 h) in harsh alkaline conditions, where benchmarked metals fail, open new avenues in designing novel scalable catalysts by protecting the surface of noble metal catalysts without sacrificing the electrochemical performance.
The lithium-sulfur (Li-S) redox battery system is considered to be the most promising next-generation energy storage technology due to its high theoretical specific capacity (1673 mAh g−1), high energy density (2600 Wh kg−1), low cost, and the environmentally friendly nature of sulfur. Though this system is deemed to be the next-generation energy storage device for portable electronics and electric vehicles, its poor cycle life, low coulombic efficiency and low rate capability limit it from practical applications. These performance barriers were linked to several issues like polysulfide (LiPS) shuttle, inherent low conductivity of charge/discharge end products, and poor redox kinetics. Here, we review the recent developments made to alleviate these problems through an electrocatalysis approach, which is considered to be an effective strategy not only to trap the LiPS but also to accelerate their conversion reactions kinetics. Herein, the influence of different chemical interactions between the LiPS and the catalyst surfaces and their effect on the conversion of liquid LiPS to solid end products are reviewed. Finally, we also discussed the challenges and perspectives for designing cathode architectures to enable high sulfur loading along with the capability to rapidly convert the LiPS.
Free standing electrodes have grabbed attention among the researchers around the globe due to its ease of fabrication, scale-up and flexibility for stack development. In the present communication, we have described a novel approach to improve the areal capacitance of graphite fiber paper (GP) from 4.5 3 10 À3 F cm À2 to 0.677 F cm À2 by surface modification. Surface functionalised graphite paper (Oxd-GP) and its chemically reduced counterpart (Red-GP) are explored as electrode materials for supercapacitors. The materials synthesized are structurally characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Energy dispersive analysis of X-rays (EDAX) and field emission scanning electron microscopy (FESEM). The electrochemical performance of the electrodes is assessed using cyclic voltammetry (CV), galvanostatic charge-discharge (GC) analysis and electrochemical impedance spectroscopy (EIS) where both Oxd-GP and Red-GP exhibit appreciable areal capacitance while Red-GP shows excellent cycle life over 2600 cycles. Red-GP showed an increment in capacitance over 1000 cycles while attaining stability. This work suggests that modified graphite paper can be a low-cost and free-standing electrode for high performance energy storage devices.
The zinc−air battery (ZAB) is an emerging rechargeable energy storage system having high energy density (1084 Wh/kg) with safe operation and low operation cost (∼$10 kW/h). Development of a durable and efficient bifunctional catalyst is the bottleneck of rechargeable ZAB technology, and here we demonstrate a hybrid catalyst system having cobalt (Co) nanoparticles dispersed graphitic spheres as an efficient catalyst at the air electrodeperforming better than the benchmarked ones while constructing the ZAB cells. Co nanoparticles dispersed nitrogendoped graphitic nanospheres (Co@NGC-NSs) were developed using a unique synthesis strategy, and it showed excellent bifunctional catalytic activity toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline medium. The potential difference calculated with Co@NGC-NSs, considering the potential for the current density of 10 mAcm −2 for OER and current density corresponding to the half wave potential for ORR, is found to be (0.78 V) lower than that of Pt/C and IrO 2 (0.87 V) system. The zinc−air cell constructed using Co@NGC-NSs shows an open circuit voltage of 1.36 V having a maximum power density of ∼52 mW cm −2 and an energy density of 876 Wh/kg, and these values are on par with the Pt/C system while much better in terms of long-term stable performance where Pt/C is found to be failing. A detailed comparison with the reported performance of other catalyst-based ZABs indicates that the Co@NGC-NSs-based ZABs has high potential as a practically viable system, where the catalyst development is also found to be simple and nonexpensive in nature.
In this work, we employed the electrochemical exfoliation (EE) to synthesize and tune the degree of functionalization in fluorinated graphene (FG) and studied its (degree of functionalization) effect on bromine redox reaction. The degree of functionalization was tuned by varying the concentration of the electrolyte. Physical characterization of as‐synthesized FG reveals that fluorine and oxygen content varies from 2.32 to 3.92 atomic % and from 10.46 to 21.16 atomic % respectively. The concentration of fluoride to hydroxide ion ratio should be at an optimum level in the electrolyte to get maximum degree of fluorination, which shows a sizeable effect on bromine redox reaction. The bromine redox reaction is more reversible on highly FG and less reversible on less FG and Oxygen functionalities has no effect on the reaction. The kinetic parameters such as rate constant (k) (varying from 1.45 x 10−3 to 52.43 x 10−3 mA/cm2) and exchange current density (io) (varying from 2.61 x 10−4 to 9.92 x 10−4 cm/s) suggest that nature and degree of functionalization play a vital role to improve the kinetics of redox reaction of bromine. This investigation can pave paths to design new and sustainable electrode materials for bromine‐based redox flow batteries.
Development of mechanically deformable solid state devices is receiving tremendous attention, and high ionic conductivity solid polymer electrolytes (SPEs) are highly sought after for their development. The process history‐induced polymer alignment anisotropy can lead to anisotropic conductivity to the SPEs. Here, a Li ion SPE membrane developed using poly(ethylene oxide) (PEO) and LiClO4 is demonstrated for its microstructure variations while applying external stress and the corresponding variations in the ionic conductivity are also calculated. The microstructural evolution shows that larger strain values induce large dislocations in the crystallites of PEO leading to the formation of larger amorphous regions which soften the matrix. The anisotropic mechanical responses are observed while applying cyclic strain to thicker SPEs, where the compressive measurements show softening of the matrix while tensile measurements harden the matrix. The ionic conductivities of the softened matrix are found to be enhanced while those of toughened matrix are found to be decreased. This detailed mechanical analysis along with the in situ ionic conductivity studies of PEO‐based Li ion SPE show that along with the thermal history of the polymers, process history and the anisotropic mechanical responses of the polymers also need to be considered while developing SPEs for flexible devices.
On-site peroxide generation via electrochemical reduction is gaining tremendous attention due to its importance in many fields, including water treatment technologies. Oxidized graphitic carbon-based materials have been recently proposed as an alternative to metal-based catalysts in the electrochemical oxygen reduction reaction (ORR), and in this work we unravel the role of C=O groups in graphene towards sustainable peroxide formation. We demonstrate a versatile single-step electrochemical exfoliation of graphite to graphene with a controllable degree of oxygen functionalities and thickness, leading to the formation of large quantities of functionalized graphene with tunable rate parameters, such as the rate constant and exchange current density. Higher oxygen-containing exfoliated graphene is known to undergo a two-electron reduction path in ORR having an efficiency of about 80 ± 2% even at high overpotential. Bulk production of H 2 O 2 via electrolysis was also demonstrated at low potential (0.358 mV vs RHE), yielding ≈34 mg/L peroxide with highly functionalized (≈23 atom %) graphene and ≈16 g/L with low functionalized (≈13 atom %) graphene, which is on par with the peroxide production using state-of-the-art precious-metal-based catalysts. Hence this method opens a new scheme for the single-step large-scale production of functionalized carbon-based catalysts (yield ≈45% by weight) that have varying functionalities and can deliver peroxide via the electrochemical ORR process. 432
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