Stacking Energy Research Articles

Enhanced chromonic stacking assisted by a hydrogel network

We report that the stacking order in lyotropic chromonic liquid crystal (LCLC) Sunset Yellow FCF (SSY) is enhanced when incorporated into a cationic hydrogel via the ionic bonds. The chromonic-incorporated gel is initially opaque but transparent and shows a birefringence at high temperatures. The stacking correlation length at room temperature (≈25 °C) is as large as 5.97 nm, corresponding to about 18 molecules in a single stack, and the stacking energy is found to be 7.96 k B T. Both values are larger than those in a normal SSY solution with the respective concentration (≈1 M).

High performance solid-state supercapacitors based on highly conductive organogel electrolyte at low temperature

Solid-state supercapacitors have advantages of leakage free and flexibility but usually have low energy density at low temperature. This is largely due to the significantly declined ionic conductivity as well as the relatively low voltage window of the gel electrolytes. Here we designed a low temperature tolerant organogel electrolyte by systematically tuning the solvents’ ionic conductivity, melting point and electrochemical stability via acetonitrile (AN), methyl formate (MF), and propylene carbonate (PC), respectively. The tuned gel electrolyte of polymer metrix of poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) with salt of ionic electrolyte 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) exhibited ionic conductivity of 2.95 mS cm−1, mechanical strain rate of 350% and voltage window of 0–4 V at low temperature of −60 °C. The stack cell of solid-state supercapacitor using activated carbon as electrode films exhibited capacitance retention of 98.5% at −60 °C compared with that under room temperature, a 3.9% capacitance attenuation after 10,000 charge/discharge cycles, and exceptional stack energy density of 30.8 Wh kg−1, at least three times higher than the state-of-the-art solid-state supercapacitors.

Conformal surface-nanocoating strategy to boost high-performance film cathodes for flexible zinc-ion batteries as an amphibious soft robot

To satisfy the increasing power demand for the rapid development of wearable and portable electronics, the design of quasi-solid-state aqueous zinc-ion batteries (QAZIBs) requires the advancement of flexible electrodes possessing excellent electrochemical performance and extraordinary mechanical strength. This purpose could be realized by utilizing electrospinning to obtain flexible film cathodes composed of layer-structured transition metal dichalcogenides@nitrogen-doped carbon nanofibers (TMD@N-CNFs), attributed to the synergetic effect between the nanostructured TMDs (for capacity) and interwoven 1D N-CNFs (for flexibility). However, their potentials are seriously impeded by the dissolution and structural instability of active materials during zinc-ion (de)intercalation process, leading to insufficient rate capability and much-shortened cycle life. Herein, to address this big challenge, we have demonstrated the approach of developing an ultrathin Al2O3 coating layer by atomic-layer-deposition (ALD) to greatly boost the electrochemical performance of vanadium diselenide nanosheets@N-CNFs (Al2O3@VSe2 NSs@N-CNFs) film cathode in flexible QAZIBs. Moreover, the ALD-Al2O3 nanocoating provides desperately necessary mechanical robustness in the cross-2D nanosheet direction of VSe2 standing on the N-CNFs surface, as confirmed by finite element simulation results. Consequently, the QAZIB delivers an outstanding stack energy density of ∼125 Wh kg−1, an ultrahigh rate capability of 65.2% capacity retention after 200-fold increase in current density and an excellent cycling performance of 86.2% retention after 2500 cycles. Finally, we integrate the flexible QAZIB into a soft inchworm robot, which achieves reversible actuation both on land and in water and keeps good electrochemical stability, showing the prospective potentials of introducing energy storage devices for soft robotics with broad multifunctional applications.

On electrically tunable stacking domains and ferroelectricity in moir\xe9 superlattices

It is well known that stacking domains form in moiré superlattices due to the competition between the interlayer van der Waals forces and intralayer elastic forces, which can be recognized as polar domains due to the local spontaneous polarization in bilayers without centrosymmetry. We propose a theoretical model which captures the effect of an applied electric field on the domain structure. The coupling between the spontaneous polarization and field leads to uneven relaxation of the domains, and a net polarization in the superlattice at nonzero fields, which is sensitive to the moiré period. We show that the dielectric response to the field reduces the stacking energy and leads to softer domains in all bilayers. We then discuss the recent observations of ferroelectricity in the context of our model.

Energy Cost of Quantum Circuit Optimisation: Predicting That Optimising Shor’s Algorithm Circuit Uses 1 GWh

Quantum circuits are difficult to simulate, and their automated optimisation is complex as well. Significant optimisations have been achieved manually (pen and paper) and not by software. This is the first in-depth study on the cost of compiling and optimising large-scale quantum circuits with state-of-the-art quantum software. We propose a hierarchy of cost metrics covering the quantum software stack and use energy as the long-term cost of operating hardware. We are going to quantify optimisation costs by estimating the energy consumed by a CPU doing the quantum circuit optimisation. We use QUANTIFY, a tool based on Google Cirq, to optimise bucket brigade QRAM and multiplication circuits having between 32 and 8,192 qubits. Although our classical optimisation methods have polynomial complexity, we observe that their energy cost grows extremely fast with the number of qubits. We profile the methods and software and provide evidence that there are high constant costs associated to the operations performed during optimisation. The costs are the result of dynamically typed programming languages and the generic data structures used in the background. We conclude that state-of-the-art quantum software frameworks have to massively improve their scalability to be practical for large circuits.

Piezoelectric energy harvesting from rail track vibration using frequency up-conversion mechanism

Piezoelectric energy harvesting has great powering potential in railway applications, but current piezoelectric literature for rail track vibration mainly focuses on a single piezoelectric element or piezoelectric cantilever, whose output power is usually at the level of µW. In this work, a piezo stack energy harvester using the frequency up-conversion method with attractive magnetic force is proposed and experimentally validated for harvesting energy from rail track vibration. It consists of two parts, an inertial mass system and a piezo stack system. The frequency up-conversion mechanism is realised by allowing the inertial mass to attach and detach the piezo stack transducer periodically with the help of the attractive magnetic force. The attractive magnetic force contributes to the collision between the two systems, especially when the excitation frequency is near the resonant frequency. A theoretical model of the proposed energy harvester considering the colliding motion and magnetic force is established. A prototype is fabricated and can generate a peak power of 96.69 mW and average power of 2.59 mW at an input acceleration of 1 g applied at 9 Hz with an optimum load impedance of 200 Ω.

Simulation and Experimental Study of Piezoelectric Stack Energy Harvester with Arch Beam Applied to the Railway System

Simulation and Experimental Study of Piezoelectric Stack Energy Harvester with Arch Beam Applied to the Railway System

Detecting and locating microseismic events with stacking velocity analysis for surface monitoring

Microseismic surface monitoring may suffer from the signal-to-noise ratio (SNR) in the data and lack of datasets to constrain a realistic velocity model. Therefore, an 1D depth velocity, which is derived from the well log and calibrated with perforation shot, is commonly utilized in microseismic monitoring. In this study, we simultaneously detect and locate microseismic events by applying a stacking method without using a depth velocity model. Since the 1D layered velocity model could be approximately equivalent to a RMS velocity, we borrow the velocity analysis concept in traditional exploration seismic data processing and apply it on mocroseismic surface monitoring. We stack the perforation shot waveform along a hyperbolic curve and pick the maximum stacking energy to obtain a stacking velocity above the target zone. The microseismic events are automatically detected and located by a stacking procedure with the known stacking velocity in time domain. The events are then converted from time to depth using the average velocity calculated from the perforation shot. Synthetic examples show that our approach can successfully recover the true locations of events with a large amount of noises added to the data. The real data results also suggest that this method is applicable for effectively detecting and locating events with low SNR. Although the approach with stacking velocity is limited to the layered media, this restriction is acceptable for many unconventional reservoirs.

Increasing Stack Energy Density Without Lifetime Penalty by Increasing Electrode Loading in Single Crystal Li[Ni0.5Mn0.3Co0.2]O2/Graphite Pouch Cells

Li-ion cell energy density can be improved by increasing the thickness of both the positive and negative electrode coatings, thus minimizing the fractional volume and mass from inactive cell materials. This study compares Li[Ni0.5Mn0.3Co0.2]O2 (NMC532)/graphite pouch cells with regular positive electrode loading (19.3 mg cm−2) and high positive electrode loading (29.2 mg cm−2) where both cell types have appropriate negative electrode loading. The results indicate similar capacity retention and impedance growth in long term cycling (when Li-plating is avoided) as well as similar rates of parasitic reactions at various upper cutoff potentials. Increasing the positive and negative loadings by 50%, as demonstrated here, can lead to an 8%–10% increase in volumetric energy density without a lifetime penalty.

Design and optimization of electrodialysis process parameters for brackish water treatment

Effect of flow velocity and cell-pair thickness in electrodialysis (ED) is studied. The production cost includes pump energy, while the size of the system is considered as an output variable. The performance of ED system depends on three categories of process parameters namely water quality data, stack configuration and flow characteristic inside the stack. The design of ED system is complex due to interrelation among the system variables so the design calculation chronology steps are developed with flow-chart for the fix feed salinity of groundwater and salt removal rate. The effect of recovery ratio on capital and energy cost is studied and found unidirectional. Sparingly soluble salt present in feed decides the upper limit and obtained 70–75% recovery rate based on the feedwater quality. The optimum value of the linear flow velocity and cell-pair thickness can be obtained by the trade-off among capital cost, stack energy cost and pumping energy cost. Simultaneous effect of both the variable on minimizing the total cost gives the narrow working range of flow velocity 15–17 cm/s and 0.4–0.8 mm thickness. The minimum production cost of 0.08 USD/m3 is obtained at 16 cm/s velocity and 0.5 mm thickness.

An experimental study for optimizing the energy efficiency of a proton exchange membrane fuel cell with an open-cathode

To develop an operating strategy for maximizing the energy efficiency of open-cathode proton exchange membrane fuel cells (OCPEMFCs), the present study investigates the effect of the fan speed on the stack performance and energy efficiency using a commercially available OCPEMFC system. The temperature, voltage, and current of the stack are monitored, and the energy efficiency is calculated at various stack power levels. The results of the system with a lab-developed controller are compared with the commercial system with a built-in controller. It is found that the fan speed should be minimum to reduce the auxiliary power consumption and that the stack should be efficiently heated to enhance the electrochemical reaction. In addition, it is noticed that the stack performance dramatically drops when the stack temperature is above 75 °C, due to the membrane dehydration. Overall, the results show that the stack temperature is an important indicator for controlling the fan speed for optimization of energy efficiency, and for stack powers of 50, 60, 70, and 80 W, the peak values of energy efficiencies are 38.0%, 38.3%, 38.5%, and 38.3% at the duty cycles of 0.2, 0.2, 0.25, and 0.3, respectively, which are 28–38% higher than the commercially available OCPEMFC system.

Arene-Perfluoroarene Interactions in Solution.

A systematic study of arene-perfluoroarene interactions in solution is presented. Using a combination of NMR titration experiments, X-ray crystallography, and computational analysis, we analyze the effects of fluorination, substituents, ring size, and solvation on the arene-perfluoroarene interaction. We find that fluorination, extension of the π systems, and enhancement of solvent polarity greatly stabilize the stacking energy up to 3 orders of magnitude (Ka = <1 to 6000 M-1), with the highest Ka achieved for the interaction of water-soluble variants of perfluoronaphthalene and anthracene in buffered D2O (pD = 12). Combining computational and experimental results, we conclude that this impressive binding energy is a result of enthalpically favorable electrostatic and dispersion interactions as well as the entropically driven hydrophobic effect. The enhanced understanding of arene-perfluoroarene interactions in aqueous solution sets the stage for the implementation of this abiotic intermolecular interaction in biology and medicine.

Strongly coupled piezoelectric energy harvesters: Optimised design with over 100 mW power, high durability and robustness for self-powered condition monitoring

Harvesting ambient vibration energy is a promising method to realise self-powered wireless sensors. However, most of the energy harvesters developed to date are not suitable for real-world applications because of low power output and/or poor durability and robustness. To overcome these challenges, this work develops a strongly coupled piezoelectric stack energy harvester (PSEH) with design considerations not just on the power output but also on the durability and robustness. The PSEH took advantages of the force amplification capability of an optimised mechanical transformer and the high coupling coefficient of a 33-mode multilayer piezoelectric stack to achieve strong coupling and therefore high-power generation. To increase the durability, the piezoelectric stack was pre-compressed to prevent the development of tensile stress, to exploit the high compressive strength of piezoelectric ceramics; the maximum dynamic stress in the mechanical transformer was kept below half of the material’s fatigue limit. Plate springs were used to guide the motion of the PSEH and prevent undesired vibration to enhance robustness. A finite element model was developed for design optimisation, which links the design parameters directly to the full performance matrix including maximum power generation. When actuated at 0.5 g, 157 Hz in the lab tests, the PSEH produced a maximum average power of 140 mW with a 1-mW-bandwidth of 72 Hz and 10-mW-bandwidth of 24 Hz. The PSEH showed no performance degradation after continuously actuated at 0.3 g, 157 Hz for 7.9 h. In addition to the lab tests, on-site tests were performed by installing the PSEH in two locations of a screw air compressor. On-site tests showed that the PSEH was able to produce average power of 15.95 ± 2.3 mW and 43.19 ± 1.52 mW when the acceleration produced by the air compressor was 0.125 ± 0.012 g and 0.259 ± 0.004 g, respectively.

Revealing the Underlying Mechanisms of the Stacking Order and Interlayer Magnetism of Bilayer CrBr3

Aiming to clarify the mechanisms governing the interlayer magnetic coupling, we have investigated the stacking energy and interlayer magnetism of bilayer CrBr3 systemically. The magnetic ground states of bilayer CrBr3 with different R-type and H-type stacking orders are established, which are found to be in good agreement with recent experiments [ Science 2019, 366, 983]. Further analyses indicate that the stacking energy is mainly determined by the Coulomb interaction between the interlayer nearest-neighbor Br–Br atoms, while interlayer magnetism can be understood by a competition between super-superexchange interactions involving t2g–t2g and t2g–eg orbitals and semicovalent exchange interactions of eg–eg orbitals. Our studies give an insightful understanding of the stacking order and interlayer magnetism of bilayer CrBr3, which should be useful to understand the quantum confinement effect of other van der Waals magnetic materials.

Reconstruction of moiré lattices in twisted transition metal dichalcogenide bilayers

An important step in understanding the exotic electronic, vibrational, and optical properties of the moir\'{e} lattices is the inclusion of the effects of structural relaxation of the un-relaxed moir\'{e} lattices. Here, we propose novel structures for twisted bilayer of transition metal dichalcogenides (TMDs). For $\theta\gtrsim 58.4^{\circ}$, we show a dramatic reconstruction of the moir\'{e} lattices, leading to a trimerization of the unfavorable stackings. We show that the development of curved domain walls due to the three-fold symmetry of the stacking energy landscape is responsible for such lattice reconstruction. Furthermore, we show that the lattice reconstruction notably changes the electronic band-structure. This includes the occurrence of flat bands near the edges of the conduction as well as valence bands, with the valence band maximum, in particular, corresponding to localized states enclosed by the trimer. We also find possibilities for other complicated, entropy stabilized, lattice reconstructed structures.

Moir\xe9 metrology of energy landscapes in van der Waals heterostructures

The emerging field of twistronics, which harnesses the twist angle between two-dimensional materials, represents a promising route for the design of quantum materials, as the twist-angle-induced superlattices offer means to control topology and strong correlations. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moiré superlattice encodes elusive insights into the local interlayer interaction. Here we introduce moiré metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atom scale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moiré domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moiré metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe2/WSe2. Moiré metrology establishes sought after experimental benchmarks for interlayer interaction, thus enabling accurate modelling of twisted multilayers.

Replacing thymine with a strongly pairing fifth Base: A combined quantum mechanics and molecular dynamics study

The non-natural ethynylmethylpyridone C-nucleoside (W), a thymidine (T) analogue that can be incorporated in oligonucleotides by automated synthesis, has recently been reported to form a high fidelity base pair with adenosine (A) and to be well accommodated in B-DNA duplexes. The enhanced binding affinity for A of W, as compared to T, makes it an ideal modification for biotechnological applications, such as efficient probe hybridization for the parallel detection of multiple DNA strands. In order to complement the experimental study and rationalize the impact of the non-natural W nucleoside on the structure, stability and dynamics of DNA structures, we performed quantum mechanics (QM) calculations along with molecular dynamics (MD) simulations. Consistently with the experimental study, our QM calculations show that the A:W base pair has an increased stability as compared to the natural A:T pair, due to an additional CH-π interaction. Furthermore, we show that mispairing between W and guanine (G) causes a distortion in the planarity of the base pair, thus explaining the destabilization of DNA duplexes featuring a G:W pair. MD simulations show that incorporation of single or multiple consecutive A:W pairs in DNA duplexes causes minor changes to the intra- and inter-base geometrical parameters, while a moderate widening/shrinking of the major/minor groove of the duplexes is observed. QM calculations applied to selected stacks from the MD simulations also show an increased stacking energy for W, over T, with the neighboring bases.

(Invited) The Life and Death of Anode-Free Lithium Metal Cells

Anode-free lithium metal cells store 60% more energy per volume than conventional lithium ion cells. Such high energy density cells will enable electric vehicles with increased range and decreased cost. The anode-free strategy of using zero excess lithium presents a number of advantages and challenges over conventional lithium metal cells. Anode-free cells exhibit ultra-high stack energy density >1200 Wh/L for the NMC532 and 811 cells investigated in this work. Lithium metal cells, on the other hand, with lithium foils thicker than 60 µm are actually less energy dense than lithium ion cells. Challenges associated with manufacturing thin lithium foils and their incorporation in cell production are avoided in anode-free cells. Safety is a concern for any cell with metallic lithium, although this often remains unaddressed in the literature. However, with no excess lithium, anode-free cells suffer from rapid capacity loss. Lifetimes of fewer than 40 cycles to 80% capacity are common. Increasing cycle life to hundreds of cycles is necessary to produce viable anode-free cells for electric vehicles.Recently, we demonstrated state-of-the-art cycle life for anode-free cells utilizing a dual-salt carbonate-based electrolyte.1 This liquid electrolyte enabled a pristine lithium morphology of tightly packed lithium grains forming a smooth lithium mosaic, shown in Fig. 1a-b. In this present work, we probe the lithium morphology at different states of charge and conclude that this morphology is composed of lithium columns (Fig. 1c-e). Incredibly, the columnar lithium enabled by dual-salt electrolyte forms a matrix of dead lithium which facilitates reversible lithium deposition.Unfortunately, this pristine morphology is not maintained. In this work, we characterize the degradation of anode-free cells with dual-salt electrolyte. We probe the deterioration of lithium morphology during aging with SEM and correlate this to electrolyte degradation. We observe increased lithium porosity via x-ray tomography and use acoustic transmission imaging to demonstrate electrolyte depletion. For safety characterization, we measure the temperature of cells during penetration with a nail and compare to cells made with other electrolytes promoted in the literature. Finally, we use the insights gained here to build anode-free cells that deliver a greater energy density than lithium ion cells for over 200 cycles. Although lifetime must still be increased, we believe anode-free lithium metal cells with liquid electrolyte present the most straightforward and lowest cost path towards viable high energy density lithium batteries. Reference s : [1] Rochelle Weber, Matthew Genovese, A. J. Louli, Samuel Hames, Cameron Martin, Ian G. Hill and J. R. Dahn, Nat. Energy, 4 (2019) 683-689. Figure 1

Study on the thermoacoustic system using moisturized stack—energy generation ratio of air and water vapor during system operation

In order to clarify the principle of oscillation in a two-phase fluid thermoacoustic engine, energy generation in a moisturized stack is theoretically investigated. Theoretical equations to express the energy generation are derived first using Raspet’s theory. The validity of derived equations was suggested by comparing the calculated value and the measured value. Furthermore, on the basis of derived equations, the distribution of the energy generation in a moisturized stack and the ratio of the energy generations in air and water are derived with the measurement results for temperature distribution in the stack. As a result, it is confirmed that the energy generation based on the phase transition of water is significant near the hot end of the stack and occupies around 60% of the total energy generation in a two-phase fluid thermoacoustic engine. It is also suggested that the optimum length of stack may exist.

A graphene-covalent organic framework hybrid for high-performance supercapacitors

Due to strong van der Waals interactions, graphene nanosheets stack together, resulting in inefficient charge storage, which significantly compromises their performance as electrodes in supercapacitors. Herein, we design a hybrid material by interacting covalent organic frameworks (COFs) into reduced graphene oxide (rGO) films or fibers. The 2D COF nanosheets contain abundant mesopores, which prevents the stacking of rGO nanosheets, enabling the efficient electrolyte ion mass transfer. The optimal COF/rGO hybrid delivers a high gravimetric specific capacitance of 321 ​F ​g−1 and volumetric specific capacitance of 237 ​F ​cm−3 in an aqueous electrolyte measured in the three-electrode configuration, representing a breakthrough in capacitive graphene electrodes. Consequently, the fabricated 2D thin-film and 1D fiber supercapacitors deliver a high device energy density of 10.3 ​Wh kg−1 or 7.9 ​mWh cm−3 in aqueous and gel electrolytes, respectively. Moreover, an ultrahigh stack energy density of 87 ​Wh L−1 ​at the power density of 638 ​W ​L−1 is achieved using an ionic liquid electrolyte, which is superior to most of the carbon-based supercapacitors reported so far, as well as commercial lead-acid and lithium thin-film batteries. Overall, this study demonstrates 2D COFs as a critical enabler to realize high-performance graphene supercapacitors. 2D COFs also have the potential to enable unique architectures for many other 2D materials.