Open Circuit Voltage Decay Research Articles

A Diphosphonic Acid-Based Interlayer for Highly Efficient and Stable Inverted Perovskite Solar Cells.

We investigate an interlayer of 6,6'-bis(4-(bis(4-methoxyphenyl)amino)phenyl)-[1,1'-binaphthalene]-(2,2'-diyl)bis(oxy)bis(propane-3,1-diyl)bis(phosphonic acid) (BINOL-PA) with undoped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) coverage. The incorporation of the 1,10-bi-2-naphthol central core enhances π-π stacking and reduces charge recombination at the interface. Compared to PTAA alone (0.95 eV), BINOL-PA/PTAA exhibits a shorter distance from the Fermi energy (EF) to the valence-band maximum (VBM) (0.36 eV). Two phosphoric acid units in BINOL-PA fine-tune the molecular dipoles. Theoretical calculations reveal electrostatic surface potential differences between BINOL-PA and PTAA in their backbone structure. Open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) results suggest suppressed interface recombination. The photovoltaic conversion efficiency (PCE), short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) for the BINOL-PA/PTAA device are measured as 21.02%, 22.67 mA cm-2, 1.12 V, and 82.8%, respectively, all higher than those achieved by the PTAA device with a PCE of 18%. BINOL-PA/PTAA significantly elevates VOC and FF values compared with dopant-free PTAA alone. The champion device retains over 89% of its initial PCE after being exposed to an ambient environment without encapsulation for more than 30 days. The thermal aging test conducted under a nitrogen atmosphere demonstrates that the efficiency retention rate for BINOL-PA/PTAA displays 60% of its initial efficiency after 1500 h.

Effects of Storage Voltage upon Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are gaining attention as a safer, more cost-effective alternative to lithium-ion batteries (LIBs) due to their use of abundant and non-critical materials. A notable feature of SIBs is their ability to utilize aluminum current collectors, which are resistant to oxidation, allowing for safer storage at 0 V. However, the long-term impacts of such storage on their electrochemical performance remain poorly understood. This study systematically investigates how storage conditions at various states of charge (SOCs) affect open circuit voltage (OCV) decay, internal resistance, and post-storage cycling stability in two different Na-ion chemistries: Prussian white//hard carbon and layered oxide//hard carbon. Electrochemical Impedance Spectroscopy before and after storage shows a pronounced increase in internal resistance and a corresponding decline in cycling performance when SIBs are stored in a fully discharged state (0 V), particularly for layered oxide-based cells, illustrating the sensitivity of different SIB chemistries to storage conditions. Additionally, a novel reformation protocol is proposed that reactivates cell capacity by rebuilding the solid electrolyte interphase (SEI) layer, offering a recovery path after prolonged storage. These insights into the long-term storage effects on SIBs provide new guidelines for optimizing storage and transport conditions to minimize performance degradation, making them more viable for commercial applications.

Enhanced solid-electrolyte interface efficiency for practically viable hydrogen-air fuel cell systems

Proton exchange membrane fuel cells (PEMFCs) provide an appealing sustainable energy system, with the solid-electrolyte membrane playing a crucial role in its overall performance. Currently, sulfonated poly(1,4-phenylene ether-ether sulfone) (SPEES), an aromatic hydrocarbon polymer, has garnered considerable attention as an alternative to Nafion polymers. However, the long-term durability and stability of SPEES present a significant challenge. In this context, we introduce a potential solution in the form of an additive, specifically a core–shell-based amine-functionalized iron titanate (A–Fe2TiO5), which holds promise for improving the lifetime, proton conductivity, and power density of SPEES in PEMFCs. The modified SPEES/A–Fe2TiO5 composite membranes exhibited notable characteristics, including high water uptake, enhanced thermomechanical stability, and oxidative stability. Notably, the SPEES membrane loaded with 1.2 wt% of A–Fe2TiO5 demonstrates a maximum proton conductivity of 155 mS cm−1, a twofold increase compared to the SPEES membrane, at 80 °C under 100% relative humidity (RH). Furthermore, the 1.2 wt% of A–Fe2TiO5/SPEES composite membranes exhibited a maximum power density of 397.37 mW cm−2 and a current density of 1148 mA cm−2 at 60 °C under 100% RH, with an open-circuit voltage decay of 0.05 mV/h during 103 h of continuous operation. This study offers significant insights into the development and understanding of innovative SPEES nanocomposite membranes for PEMFC applications.

NiOx Passivation in Perovskite Solar Cells: From Surface Reactivity to Device Performance.

Nonstoichiometric nickel oxide (NiOx) is one of the very few metal oxides successfully used as hole extraction layer in p-i-n type perovskite solar cells (PSCs). Its favorable optoelectronic properties and facile large-scale preparation methods are potentially relevant for future commercialization of PSCs, though currently low operational stability of PSCs is reported when a NiOx hole extraction layer is used in direct contact with the perovskite absorber. Poorly understood degradation reactions at this interface are seen as cause for the inferior stability, and a variety of interface passivation approaches have been shown to be effective in improving the overall solar cell performance. To gain a better understanding of the processes happening at this interface, we systematically passivated specific defects on NiOx with three different categories of organic/inorganic compounds. The effects on NiOx and the perovskite (MAPbI3) deposited on top were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Here, we find that the perovskite's structural stability and film formation can be significantly affected by the passivation treatment of the NiOx surface. In combination with density functional theory (DFT) calculations, a likely origin of NiOx-perovskite degradation interactions is proposed. The surface passivated NiOx layers were incorporated into MAPbI3-based PSCs, and the influence on device performance and operational stability was investigated by current-voltage (J-V) characterization, impedance spectroscopy (IS), and open circuit voltage decay (OCVD) measurements. Interestingly, we find that a superior structural stability due to interface passivation must not relate to high operational stability. The discrepancy comes from the formation of excess ions at the interface, which negatively impacts all solar cell parameters.

Mitigating self-discharge in supercapacitors through strategic mesoporous structural modification of NiCo layered double hydroxides

Nickel - Cobalt layered double hydroxide (NiCo LDH) based supercapacitors suffer from severe self-discharge due to their uneven charge distribution, which affects the application of supercapacitors (SCs) in real life. In this paper, three-dimensional self-supported mesoporous NiCo layered double hydroxide mesoporous on nickel foam (NF) (NiCo LDHMs/NF) was prepared by H2O2 etching. The self-discharge phenomenon of supercapacitors assembled using the reaction-derived NiCo LDHMs/NF as the positive electrode and activated carbon as the negative electrode is significantly suppressed. Comparing with the original NiCo LDH devices, the successful construction of mesoporous structures in NiCo LDH resulted in slower open-circuit voltage (OCV) decay (0.93 V vs. 0.59 V in the range of open-circuit voltage of 1.8 V for the same time of 7200 S), and lower leakage current (0.09 mA vs. 0.18 mA at 1.8 V). Due to the formation of mesoporous structure, the charged ions adsorbed on the electrode have more storage sites. Which reduces the movement or loss of charged ions to inhibit the charge redistribution, so it has a high specific capacitance and the self-discharge phenomenon has been greatly reduced.

A green approach to synthesis of Ag-doped CeO2 nanorods embedded reduced graphene oxide nanocomposite for excellent photocatalytic and antimicrobial activity

In this paper, the Ag-doped cerium oxide nanorods embedded reduced graphene oxide (Ag@CeO2/rGO) nanocomposite was synthesized via Punica Granatum peel extract for for photocatalytic degradation of methylene blue (MB) and 4-nitrophenol (4-NP). The microstructural results confirmed the successful decoration of Ag-doped CeO2 nanorods on rGO matrix. The photocatalytic properties, including photocatalytic degradation, charge transfer kinetics and photocurrent generation, are systematically investigated using electrochemical impedance spectroscopy (EIS), photocurrent transient response (PCTR) and open circuit voltage decay (OCVD). The results of photocatalytic dye degradation measurements indicated that Ag@CeO2/rGO nanocomposite is more effective than pristine CeO2 to degrade the MB dye, and the degradation rate reached 100 % in 60 min. The decomposition of MB with Ag@CeO2/rGO nanostructure followed first-order reaction kinetics with a reaction rate constant (Ka) of 0.0198 min−1. The EIS and OCVD measurements revealed that the Ag doping and incorporation of rGO could suppress the recombination probability in CeO2 by the separation of photo-generated electron–hole pairs, which leads to the enhanced photocurrent generation and photocatalytic activity. The photocurrent density of Ag@CeO2/rGO, CeO2/rGO and pristine CeO2 are 215, 167.4 and 98.8 mAcm−2, respectively.

Comparing Charge Dynamics in Organo-Inorganic Halide Perovskite: Solid-State versus Solid-Liquid Junctions

In this study, we explore the dynamics of a perovskite-electrolyte photoelectrochemical cell, pivotal for advancing electrolyte-gated field effect transistors, water-splitting photoelectrochemical and photocatalytic cells, supercapacitors, and CO2 capture and reduction technologies. The instability of hybrid perovskite materials in aqueous electrolytes presents a significant challenge, yet recent breakthroughs have been achieved in stabilizing organo-inorganic halide perovskite films. This stabilization is facilitated by employing liquid electrolytes, specifically those formed by dissolving tetrabutylammoniumperchlorate in dichloromethane. A critical aspect of this research is the comparative analysis of charge and ion kinetics at the perovskite/liquid electrolyte interface versus the perovskite/solid charge transport layer interface. Employing Intensity Modulated Photocurrent Spectroscopy (IMPS), Open-Circuit Voltage Decay (OCVD), and Capacitance-Frequency (C-F) methods, the study scrutinizes charge dynamics in both perovskite/electrolyte and perovskite/solid interfaces. Furthermore, the investigation extends to contrasting the properties of solid–liquid and solid-state junctions, focusing on mobile ions, electric field impacts, and electron-hole transport. The research also examines variations in recombination resistance and ionic double layer charging in perovskite-based devices, aiming to elucidate the operational mechanisms and kinetic complexities at the hybrid perovskite/electrolyte interface.

Suppressing the self-discharge of high-frequency supercapacitors using electrolytes containing BaTiO3 nanoparticles

High-frequency supercapacitors (HF–SCs) are promising electric energy storage devices and alternating current line filters. However, severe self-discharge of HF–SCs causes significant energy loss and limits their applications. Current self-discharge suppression methods for supercapacitors typically lead to decreased rate performance and hence cannot be applied to HF–SCs directly. In this work, barium titanate (BTO) nanoparticles are employed as an electrolyte additive for HF–SCs to reduce self-discharge. By adding BTO nanoparticles into the electrolyte, both leakage current and decay of open circuit voltage of the devices are reduced without sacrificing the specific capacitance and high-frequency response. At a charging voltage of 2 V, the leakage current is reduced by 49 % (3.91 vs. 1.98 μA), while the time for the voltage to drop from 2.0 to 1.0 V is extended by 4.5 times (2300 vs. 10240 sec). When the HF–SCs with BTO electrolyte are employed to store the energy generated by a triboelectric nanogenerator, the time for charging the cells to 2.0 V is reduced by 82 %. Mechanistic analysis indicates that the reduced self-discharge can be attributed to the fluid electrorheological effect and the enhanced localized electric field within the gaps among BTO nanoparticles that lead to sluggish diffusion and migration of reactive species, so the diffusion-controlled faradaic reaction process is suppressed.

Hexagonal mesoporous silica/perovskite oxides composites as unique scattering layers in photoelectrodes of high performance dye-sensitized photovoltaics

As the photoactive TiO2 film in dye-sensitized solar cells (DSSCs) is transparent to incident light owing to its nano-sized particles, the efficiency may be affected by the light loss. Therefore, light scattering layers including CeMnO3, CeFeO3, CeSnO3, CeZnO3, 3D-hexagonal mesoporous silica (HMS-3), and HMS-3:CeMnO3 composites were deposited on TiO2 layer by doctor blade technique in order to harvest more sunlight. To characterize each material, XRD, FTIR, PL, DRS, FESEM, EDS, and BET analyses were investigated. In addition, dye absorption capacity, power conversion efficiency (PCE), open-circuit voltage decay (OCVD), and charge transfer parameters were assessed. According to photovoltaic data, TiO2/50 wt% HMS-3+50 wt% CeMnO3 based DSSC brought about the maximum PCE of 7.47% with fill factor (FF), short-circuit current density (Jsc), and open-circuit voltage (Voc) of 0.48, 21.17 mA/cm2, and 0.738 V, respectively, displaying 75.4% higher PCE compared to solar cell fabricated by TiO2 (PCE = 4.26%, Jsc = 11.72, Voc = 0.685 V, FF = 0.53). The champion 50 wt% HMS-3+50 wt% CeMnO3 composite featured large particles with high porosity, which could enhance light scattering and cause the light path to be longer, based on the Mie theory. Moreover, because of the big surface area (643.80 m2/g), large pore volume (0.4282 cm3/g), and appropriate N719 dye loading of the champion 50 wt% HMS-3+50 wt% CeMnO3 composite, more dye molecules were excited, and the number of light-generated electrons was boosted, elevating the current density, facilitating faster charge transfer from the TiO2 photoanode towards the Pt electrode, and limiting electron-hole recombination to a high extent, which led to an enormous improvement in efficiency.

Metal-organic framework-decorated PTFE reinforced membranes with immobilized caffeic acid radical scavenger towards improved performance and durability of PEMFC

During the continuous operation of fuel cell, the generated free radicals have an irreversible effect on the proton exchange membrane (PEM), and the introduction of free radical scavengers is an effective strategy for improving its durability. In this study, we proposed to prepare thin and highly durable reinforced membranes by immobilizing organic radical scavenger caffeic acid on the fibrous PTFE supporting layer decorated with amino-functionalized metal-organic framework. The electrostatic force between the amino and sulphonic acid groups can induce an orderly aggregation of hydrophilic cationic groups within the Nafion, endowing the reinforced composite PEMs with superior proton conductivity. The mechanical properties and dimensional stability of the reinforced membranes were effectively enhanced by the reinforcement of the PTFE layer decorated with the MOF network, and the hydrogen crossover was mitigated. The thickness of Nafion reinforced membranes was reduced to 25 µm with the PTFE reinforcement. After assembling in a single cell, the Nafion composite membrane (PPUC-Nafion-3) with a 3 wt% doping of caffeic acid achieved a maximum power density (PDmax) of 812.64 mW cm−2 and the open circuit voltage (OCV) decay rate of only 1.11 mV h−1 over 100 h. For comparison under the same conditions, recast Nafion possessed a much lower PDmax of only 486.30 mW cm−2 and a higher OCV decay rate of 1.91 mV h−1. This study demonstrates an effective approach to developing highly durable and thin PEMs for the realization of PEMFC with improved output performance and durability simultaneously.

New equations to calculate carrier recombination lifetime of silicon epitaxial layer, based on open circuit voltage decay method

Abstract New equations for recombination lifetime calculation based on the open circuit voltage decay (OCVD) method were proposed. The new equations can yield accurate recombination lifetime values of the i-layer of PiN diodes in which a silicon epitaxial layer is employed, by eliminating the effects of carrier diffusion into p+ and n+ layers from the i-layer of PiN diodes, carrier injection into the i-layers from the depletion layer of PiN diodes, and surface recombination on the side wall of PiN diodes. To verify the effectiveness of the new equations, OCVD measurements were performed by employing technology computer-aided design (TCAD) simulation and actual PiN diodes. Although the calculated values implied the effect of epitaxial layer thickness, they were consistent with the results obtained under assumed lifetime conditions for TCAD simulation, the theory of recombination lifetime, and results in other reports.

Reduced self-discharge of supercapacitors based on surfactant-functionalized NiAl layered double hydroxide

Nickel aluminum layered double hydroxide (NiAl LDH) is an important pseudocapacitive electrode material for its low cost, facile preparation method, and high specific capacity. Due to the lack of chemical stability, however, NiAl LDH-based supercapacitors typically suffer from fast self-discharge. Herein, we report that NiAl LDH functionalized with sodium dodecyl sulfonate (SDS) can show much suppressed self-discharge for supercapacitors assembled from NiAl LDH positive electrodes and activated carbon negative electrodes. Compared to pristine NiAl LDH, the introduction of SDS in NiAl LDH leads to slower decay of open circuit voltage (26.7 vs. 11.6 h for a voltage decay from 1.6 to 0.42 V), reduced leakage current (1.2 vs. 0.17 mA at a charging voltage of 1.6 V), and improved capacitance retention (77 % vs. 49 % after resting for 4 h). The slower self-discharge can be attributed to the negatively charged hydrophobic layer on the surface of SDS-functionalized NiAl LDH particles so that the access of OH− ions to the electrode surface is impeded. As a result, the oxidization of OH− as a main process for the self-discharge of NiAl LDH is largely suppressed.

BaSnO3-SnO2 heterojunction mesoporous photoanode for quantum dot-sensitized solar cells

Mesoporous photoanodes, as quantum dot adsorption framework and electron transfer paths, are the key components in efficient quantum dot-sensitized solar cells (QDSSCs). Binary heterojunction mesoporous photoanodes hold promise for facilitating electron transport due to the heterojunction potential, when compared to conventional mono-component mesoporous photoanodes. In this work, we construct a binary mesoporous photoanode with enhanced mesoporous and electron transport using a BaSnO3-SnO2 heterojunction, and then fabricated CdSe/CdS co-sensitized QDSSCs. The charge extraction, open-circuit voltage decay, electrochemical impedance spectroscopy, and intensity-modulated voltage/photocurrent spectroscopy measurements demonstrated that, in contrast to the BaSnO3 mesoporous photoanode, the BaSnO3-SnO2 mesoporous photoanode could effectively reduce charge recombination and extend the electron lifetime, thus promoting charge collection in the QDSSCs. Based on these merits, the BaSnO3-SnO2 mesoporous photoanode-based solar cells yielded a power conversion efficiency of 3.14%, resulting in a ca. 11% increase in power conversion efficiency, compared to the reference BaSnO3 solar cells.

Dye-sensitized solar cells with aluminum-doped zinc oxide nanoparticles-loaded electrolyte: A breakthrough solution for recombination and hysteresis suppression

Dye-sensitized solar cells (DSCs) suffer from reduced power conversion efficiency due to electron recombination with triiodide (I3−) ions at the photoanode/electrolyte interface. Organic additives are conventionally used in electrolyte to reduce recombination and improve the efficiency of the device. These additives react covalently with iodine (I2) in the electrolyte, leading to a reduction in the concentration of I3− ions. Other inorganic oxides, such as silica (SiO2), titania (TiO2), and magnesia (MgO), have been shown to improve the efficiency of the solar cell. These oxides can adsorb heavy ions to some extent. In this study, we investigated aluminum-doped zinc oxide nanoparticles (AZO NPs) as an inorganic additive with ionic adsorption properties towards I− and I3− anions in the electrolyte. We confirmed the ionic adsorption of AZO NPs through zeta potential and electrochemical impedance measurements. We observed a significant decrease in recombination between photogenerated electrons and electrolyte ions after adding AZO NPs, which resulted in an increase in open circuit voltage (VOC) and fill factor (FF) values. Furthermore, the incorporation of AZO NPs reduced the compelled hysteresis caused by the mismatch in current density-voltage curves for the forward and reverse scan direction. Mott-Schottky analysis confirmed the increase in VOC due to the negative shift in the Fermi level of the TiO2 photoanode. Additionally, open circuit voltage decay measurements indicated a longer electron lifetime at the TiO2/electrolyte interface in AZO NPs loaded electrolyte. Our findings suggest that AZO NPs have potential as an effective inorganic additive for improving the performance of DSCs.

Understanding Potential Decay during OCV Hold via Dry Recovery Process

Nearly ∼100 mV of open circuit voltage (OCV) decay is observed in the first 24 h of a typical OCV hold test for assessing the chemical durability of a polymer electrolyte. Significant recovery of this OCV loss has been achieved by subjecting the cell to so-called wet recovery process by lowering the cell potential often under over-humidified conditions, thereby confounding the effect of potential reduction and catalyst/ionomer interfacial reorganization due to humidification. In this work by applying a dry recovery protocol (30% RH) comprising 30 min of lower potential hold in the H2/N2 environment followed by the H2/Air environment, OCV recovery of 82%, 73%, 62% and 58 was achieved by holding potentials at 0.13, 0.2, 0.6, and 0.8 V, respectively. Small changes in electrochemically active surface area and hydrogen cross-over rate observed 48 h cannot explain the nearly 100 mV OCV. The wet recovery process (low potential and 100% RH) resulted in 85% recovery of ∼100 mV of lost OCV implying oxide coverage increase as the dominant factor for sharp OCV decay. Further, it is shown that cross-over hydrogen is not fully oxidized at OCV-like conditions (>0.6 V), highlighting the need to quantify potential-dependent crossover current for OCV decay models.

Durability Study of Frequent Dry–Wet Cycle on Proton Exchange Membrane Fuel Cell

Durability is the key issue for the proton exchange membrane fuel cell application and its commercialization. Current research usually uses the accelerated stress test to decrease the experiment time, whereas the performance evolution—especially the internal state evolution—under real use may be different from that under the accelerated stress test. In addition, studies rarely report this kind of durability in real decay scenarios. This paper investigates the seldom-reported impact of dry–wet cycles on durability in terms of open circuit voltage (OCV), inner resistance, and hydrogen crossover current at the condition of 20,000 cycles or the equivalent 400 h, while simultaneously running the test for the same time interval in the control experiment. The mechanical and chemical test is independent. Frequent dry–wet cycles make the OCV decay over 14% compared to 6.9% under the normal decay. Meanwhile, the dry–wet cycle helps to alleviate deterioration in terms of the inner resistance decline (61% vs. 37%) and in terms of the hydrogen crossover current increase (−64% vs. 15%). The inner state evolution is irregular and against common sense. The relationship between the crack, platinum transfer, and the moisture which heals the crack is the potential reason for the above-mentioned phenomena. These findings are beneficial to navigating fuel cell storage.

Ultrathin TiO2 Blocking Layers via Atomic Layer Deposition toward High-Performance Dye-Sensitized Photo-Electrosynthesis Cells

The collection of solar energy in chemical bonds via dye-sensitized photoelectrosynthesis cells (DSPECs) is a reliable solution. Herein, atomic layer deposition (ALD) introduced ultrathin blocking layers (BLs) between a mesoporous TiO2 membrane and fluorine-doped tin oxide (FTO), and much improved photoelectrochemical water oxidation performance was well documented. Samples with different BL thicknesses deposited on FTO were obtained by ALD. In the photoanode, polypyridyl Ru(II) complexes were used as photosensitizers, and Ru(bda)-type was used as a catalyst during water oxidation. Under one sun irradiation, the BL (i) increased the photocurrent density; (ii) slowed down the open-circuit voltage decay (OCVD) by electrochemical measurement; (iii) increased the photo-generated electron lifetime roughly from 1 s to more than 100 s; and (iv) enhanced the water oxidation efficiency from 25% to 85% with 0.4 V of applied voltage bias. All this pointed out that the ALD technique-prepared layers could greatly hinder the photogenerated electron–hole pair recombination in the TiO2-based photoanode. This study offers critical backing for the building of molecular films by the ALD technique to split water effectively.

In-situ growth of PPy/MnOx radical quenching layer for durability enhancement of proton exchange membrane in PEMFCs

Proton exchange membrane fuel cells (PEMFCs), which exhibit low environmental impact and high power density, have attracted considerable interest as clean and efficient energy conversion technology. However, the oxidative degradation of the proton exchange membrane (PEM) by in-situ generated peroxide and hydroxyl free radicals commonly leads to performance losses in PEMFC. To alleviate the chemical attack on PEM, we report an antioxidant approach to growing a polypyrrole/manganese dioxide (PPy/MnOx) radical quenching layer on a highly proton conductive membrane surface. The resulting PEM displays improved durability in PEMFC compared with the pristine membrane, as evidenced by at least 150 h stable discharging performance in an H2/O2 PEMFC at a current density of 200 mA cm−2 at 80 °C and a much slower open circuit voltage (OCV) decay. Moreover, the ex-situ conductivity and in-situ fuel cell test indicate that the porous radical quenching layer has a tiny impact on the fuel cell performance. The resultant H2/O2 PEMFC single cell containing the chemically stable PEM exhibits a promisingly high power density of over 900 mW cm−2 at 80 °C with 100% RH.

A new strategy for the morphology-controlled synthesis of Ni/Co MOFs for high-performance asymmetric supercapacitors

Ni & Co based Metal-Organic Frameworks (MOFs) with different Ni & Co ratios 1:1, 2:1, 1:2 - NCs were synthesized using Trimesic acid as linker by solvothermal process, and the same were abbreviated as NC(1:1), NC(2:1) and NC(1:2) respectively. All the samples were analysed by analytical techniques such as Powder X-ray diffraction (P-XRD), Fourier Transform-Infrared Spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) surface area, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) techniques. Morphological variations in NCs were observed from SEM images as ultrathin nanoflakes (NC(1:1)) dandelion (NC(2:1)) and nano-spikes (NC(1:2)) due to the space coordination effect. Further, NCs were checked for their electrochemical performances in both three and two-electrode systems. Among NCs, NC(1:1) exhibited remarkable specific capacity of 172C g−1 @ 2.5 A g−1 in 1 M KOH solution. Asymmetric coin cell - ACC (CR-2032) constructed using NC(1:1) and commercially purchased AC in 1 M KOH solution, exhibited a high specific capacity of 92C g−1 @ 1 A g−1 with corresponding specific energy (SE) of 23 Wh kg−1 & specific power (SP) of 90 W kg−1. The coin cells indicated 20,000 cycles stability with 90 % specific capacity retention at 5000th cycle, however it was decreased to 47 % at the end of 20,000th cycle. The self-discharge of ACC was also investigated by estimating open circuit voltage decay and interestingly the decay was only 20.4 mV h−1 after 48 h. Two-coin cells, each charged to 0–1.8 V were connected in series and utilized for its potential flashlight application in powering 12 white LEDs for 2 min.

Polyoxometalate-modified ternary copper-tungsten-sulfide nanocrystals as high-performance counter electrode materials for quantum dots-sensitized solar cells

The Cu2WS4/[Me2NH2]11[CeIII(PW11O39)2] (CWS/CeIII(PW11)2) composite is synthesized by simple solvothermal method and first applied as a counter electrode material in CdS/CdSe/ZnS-sensitized quantum dot sensitized solar cell (QDSSCs). Due to the co-existence of copper and tungsten in different valence states, the ternary CWS has more redox active sites than the binary Cu2S. Besides, the conduction band of CeIII(PW11)2 and CWS are well matched, so the electrons transport from CB of CeIII(PW11)2 to CWS is a favorable exothermic process. Brunauer-Emmett-Teller (BET), electrochemical impedance spectroscopy (EIS), Tafel-polarization test (Tafel), open-circuit voltage decay (OCVD) and cyclic voltammetry (CV) tests all confirmed that 4% CeIII(PW11)2/CWS (CWS-4) has the highest electrocatalytic activity. Under simulated sunlight, QDSSCs with CWS-4 CE delivered a power conversion efficiency (PCE) of 6.63% (Jsc = 23.42 mA cm−2, Voc = 0.62 V, FF = 0.46), which is 12% and 44% higher than QDSSCs with CWS and Cu2S CEs, respectively. CeIII(PW11)2, as an "electron-transport mediator" to compound with CWS can generate "shallow electron traps", increase active sites, and effectively inhibit the recombination of charges, thereby enhancing cell performance.