Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.

Li-ion batteries, which restrict their wide applications, especially for environments that require mechanical stability and extreme conditions. [8][9][10][11] Alternatively, fl exible supercapacitors, especially quasi-solid state ones, have received considerate attention recently. [12][13][14][15] Although some of the fl exible supercapacitors so far reported can provide high power density and long-term stability, the energy density is relatively low. [16][17][18][19] Thus great challenges still remain in developing the overall high-performance solid/quasi-solid-state fl exible electrochemical energy storage devices with both high energy density and high power density. [20][21][22][23][24] As a typical type of traditional aqueous rechargeable batteries, Ni/Fe battery has been studied for a long period of time, because of its relatively high safety, lowcost, and high energy density. [ 25,26 ] In general, it can provide better safety and lower cost as compared with Li-ion battery; its energy density is higher in contrast to that of common supercapacitors. However, the low power density and poor cycling ability have limited the wide application of Ni/Fe battery. [ 27 ] On the other hand, recent development of nanomaterials and nanotechnology has enabled advanced electrode materials that can greatly enhance the performance of Ni/Fe battery. For example, by proper synergizing of nanostructured active materials (FeO x , NiO, or Ni(OH) 2 ) with carbonaceous materials (such as graphene, carbon nanofi bers, and carbon nanotubes (CNTs)), the power density of Ni/Fe battery can be greatly enhanced. [ 28,29 ] In some of these previous studies, the active materials are controlled in powder forms, such that carbon black and polymer additives have been employed in electrodes, where heavy metal foils or foams are used as the current collectors. The gravimetric/volumetric capacity of the full cell is therefore limited and the cell can be hardly fl exible. Some other works have focused on fl exible electrode materials for Ni/Fe batteries, where the device is assembled using liquid electrolytes. [ 30,31 ] In addition to the overall electrochemical performance, it would be a plus to eliminate liquid electrolytes, such that the safety issue in connection with the potential leakage problem can be solved. In order to promote the application of Ni/Fe batteries as a class of energy storage components for fl exible electronics Aqueous Ni/Fe batteries have great potential as fl exible energy storage devices, owing to their low cost, low toxicity, high safety, and high energy density. However, the poor cycling stability has limited the widely expected application of Ni/Fe batteries, while the use of heavy metal substrates cannot meet the basic requirement for fl exible devices. In this work, a fl exible type of solid-state Ni/Fe batteries with high energy and power densities is rationally developed using needle-like Fe 3 O 4 and fl ake-like NiO directly grown on carbon cloth/carbon nanofi ber (CC-CF) matrix as the anode and cathode, respecti...

No abstract

due to their mechanical flexibility in volume and shape requirement, high power density, rapid charge/discharge rate, long cycle lifetimes, and remarkable stitchability. [1][2][3][4][5][6][7][8] However, one of the key challenges of the FSCs in the light of their practical applications is to increase their volumetric energy density to the value approaching to or even exceeding those of microbatteries without sacrificing the power density, cycle life, and other performance para meters. [9][10][11][12][13][14][15] Both the energy and power density of a SC is strongly dependent on the operating voltage, that is, V 2 (E = 1/2 CV 2 and P = V 2 /4R ESR , where C is the capacitance of the device, V is the operating voltage, and R ESR is the equivalent series resistance). [16][17][18][19][20][21][22][23][24][25] Therefore, increasing the voltage window would be an effective approach to achieve highefficiency FSCs.To this end, enormous efforts have been devoted to the fabrication of asymmetric FSCs (AFSCs) which make full utilization of the operational windows of both the positive and negative electrode materials. [25][26][27][28][29][30][31][32] Nevertheless, the intrinsic characteristic voltage of water splitting (1.23 V) means that an aqueous electrolyte is limited to a potential domain of around 1 V, thus constraining the operating voltage to a maxi mum of 1.8-2.0 V, [28][29][30][31][32][33] which is indeed lower than that of Fiber supercapacitors (FSCs) represent a promising class of energy storage devices that can complement or even replace microbatteries in miniaturized portable and wearable electronics. One of their main limitations, however, is the low volumetric energy density when compared with those of rechargeable batteries. Considering the energy density of FSC is proportional to CV 2 (E = 1/2 CV 2 , where C is the capacitance and V is the operating voltage), one would explore high operating voltage as an effective strategy to promote the volumetric energy density. In the present work, an all-solid-state asymmetric FSC (AFSC) with a maximum operating voltage of 3.5 V is successfully achieved, by employing an ionic liquid (IL) incorporated gel-polymer as the electrolyte (EMIMTFSI/PVDF-HFP). The optimized AFSC is based on MnO x @TiN nanowires@carbon nanotube (NWs@CNT) fiber as the positive electrode and C@TiN NWs@CNT fiber as the negative electrode, which gives rise to an ultrahigh stack volumetric energy density of 61.2 mW h cm −3 , being even comparable to those of commercially planar lead-acid batteries (50-90 mW h cm −3 ), and an excellent flexibility of 92.7% retention after 1000 blending cycles at 90°. The demonstration of employing the ILs-based electrolyte opens up new opportunities to fabricate high-performance flexible AFSC for future portable and wearable electronic devices.

A single metal–organic framework precursor is transformed into both electrodes (Co3O4 and N-doped carbon) for a flexible asymmetric supercapacitor.

Sulphospinel materials, such as MnCo2S4, are being widely investigated as a promising class of candidates for energy storage.

Direct assembling of active materials on carbon cloth (CC) is a promising way to achieve flexible electrodes for energy storage. However, the overall surface area and electrical conductivity of such electrodes are usually limited. Herein, 2D metal-organic framework derived nanocarbon nanowall (MOFC) arrays are successfully developed on carbon cloth by a facile solution + carbonization process. Upon growth of the MOFC arrays, the sites for growth of the active materials are greatly increased, and the equivalent series resistance is decreased, which contribute to the enhancement of the bare CC substrate. After decorating ultrathin flakes of MnO and Bi O on the flexible CC/MOFC substrate, the hierarchical electrode materials show an abrupt improvement of areal capacitances by around 50% and 100%, respectively, compared to those of the active materials on pristine carbon cloth. A flexible supercapacitor can be further assembled using two hierarchical electrodes, which demonstrates an energy density of 124.8 µWh cm at the power density of 2.55 mW cm .

Of the transition metals, Mn has the greatest number of different oxides, most of which have a special tunnel structure that enables bulk redox reactions. The high theoretical capacitance and capacity results from a greater number of accessible oxidation states than other transition metals, wide potential window, and the high natural abundance make MnO species promising electrode materials for energy storage applications. Although MnO electrode materials have been intensely studied over the past decade, their electrochemical performance is still insufficient for practical applications. Currently, there is a trade-off between specific capacitance and loading mass. MnO species have intrinsically poor electrical conductivity, and current structural designs are not sophisticated enough to accommodate enough redox-active sites. Recent studies have certainly made progress in increasing capacitance through making use of electrically conductive components and controlling the morphology of the MnO species to expose more surface area. To increase the capacitance of MnO electrodes to the largest extent without limiting loading mass, further structural design at the nanoscale and manipulation of the electrically conductive component are required. An ideal nanostructure is proposed to guide future research toward closing the gap between achieved and theoretical capacitance, without limiting the loading mass.