In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH3NH3PbI3, we report the first investigation of a time-resolved pump-probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P-E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH3NH3PbI3 are consistent with a centrosymmetric, nonpolar structure.
A novel core-shell design for nano-structured electrode materials is introduced for realizing cost-effective and high-performance supercapacitors. In the proposed core-shell design, thin shell-layers of highly pseudo-capacitive materials provide the platform for surface or near-surface-based faradaic and non-faradaic reactions together with shortened ion-diffusion path facilitating fast-ion intercalation and deintercalation processes. The highly-conducting core serves as highway for fast electron transfer toward current collectors, improving both energy and power performance characteristics of the core-shell structure in relation to pristine component materials. Furthermore, use of carbon (C)-based materials as a shell layer in either electrode not only enhances capacitive performance through double-layer formation but also provides enough mechanical strength to sustain volume changes in the core material during long-cycling of the supercapacitor improving its cycle life. In order to enhance electrochemical performance in terms of specific capacitance and rate capability via core-shell architecture and nano-structuring, an asymmetric supercapacitor (ASC) is assembled using ZnO/α-Fe 2 O 3 and ZnO/C core-shell nanorods as respective negative and positive electrodes. The ASC exhibits a specific capacitance of ∼115 F/g at a scan rate of 10 mV/s in a potential window as large as 1.8 V with a response time as short as ∼39 ms and retains more than 80% of its initial capacitance after 4000 cycles. Interestingly, the ASC can deliver an energy density of ∼41 Wh/kg and a power density of ∼7 kW/kg that are significantly higher than those reported hitherto for iron-oxide-based ASCs. Global concern over ever increasing greenhouse gas emissions owing to exorbitant anthropogenic usage of fossil fuels is forcing governments across the globe to swing toward renewable energy sources. Since renewable sources of energy, like sun and wind, are intermittent in nature, it becomes mandatory to store the energy from these sources that could be retrieved on demand. Accordingly, the missing link in successful exploitation of renewable energy is its storage. 1Electrochemical storage of energy in storage-batteries is attractive but supercapacitors are gaining attention due to their compelling power densities as well as fast charge and discharge traits. 2,3 It is noteworthy that a battery is an energy device while a supercapacitor is a power device. If we require high power from a battery, we will generally extract less total energy than if we require low power. Supercapacitors can complement battery-power by allowing rapid charge and discharge cycles. Accordingly, supercapacitors complement batteries perfectly in the emerging energy-storage landscape and therefore the usage of a battery-supercapacitor hybrid structure is not only becoming increasingly common, but also inevitable.In the light of the foregoing, we have attempted to develop a high-performance asymmetric supercapacitor (ASC) using onedimensional (1-D) core-shell nanorods (NRs) e...
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