Loss Of Stored Energy Research Articles

Reducing the Self-Discharge Rate of Supercapacitors by Suppressing Electron Transfer in the Electric Double Layer

For supercapacitors, high self-discharge rate is an inevitable issue that causes fast decay of cell voltage and loss of stored energy. Designing supercapacitors with suppressed self-discharge for long-term energy storage has been a challenge. In this work, we demonstrate that substantially reduced self-discharge rate can be achieved by using highly concentrated electrolytes. Specifically, when supercapacitors with 14 M LiCl electrolyte are charged to 0.80 V, the open circuit voltage (OCV) drops to 0.65 V in 24 h. In stark contrast, when the electrolyte concentration is reduced to 1 M, the OCV drops from 0.80 to 0.65 V within only 0.3 h, which was 80 times faster than that with 14 M LiCl. Decreased OCV decay rate at high electrolyte concentration is also confirmed for supercapacitors with different electrolytes (e.g., LiNO3) or at higher charging voltages (1.60 V). The slow self-discharge in highly concentrated electrolyte can be largely attributed to impeded electron transfer between the electrodes and electrolyte due to the formation of hydration clusters and reduced amount of free water molecules, thereby faradaic reactions that cause fast self-discharge are reduced. Our study not only supports the newly revised model about the formation of electric double layer with the inclusion of electron transfer, but also points a direction for substantially reducing the self-discharge rate of supercapacitors.

Ion-Exchange Separators Suppressing Self-Discharge in Polymeric Supercapacitors

Author(s): Wang, K; Yao, L; Jahon, M; Liu, J; Gonzalez, M; Liu, P; Leung, V; Zhang, X; Ng, TN | Abstract: Supercapacitors offer superior cycle life and high power densities, but as energy storage devices, they are limited by self-discharge processes manifested as large potential decay and leakage current, resulting in loss of stored energy and low charging efficiency. To minimize Faradaic side reactions, this Letter has incorporated a sulfonate ion-exchange resin in separators to trap impurities and thereby suppress self-discharge in supercapacitors with PEDOT as redox electrodes. The versatile separator design is generally applicable to organic and aqueous electrolytes and compatible with a pH range of 0-14, while maintaining the device capacitance and rate performance. Temperature-dependent characteristics were analyzed to identify that the reduction of impurity concentration and diffusion was key to improve potential retention. Compared to devices using commercially available separators, the device here exhibited a lower leakage current and better charging efficiency. It was demonstrated to work with radio frequency energy-harvesting circuits and showed the potential to serve as an energy reservoir for wireless electronic applications.

A New, Generalized Electrolyte Donicity Model for Prediction of Reaction Pathways in Li-S Batteries

Li-S batteries are plagued by parasitic reactions driven by soluble reaction intermediates resulting in stored energy loss and short cycle life. Electrolytes with low solubility limits for these lithium polysulfide (LiPS) intermediates reduce parasitic reactions, improving efficiency and life of cells. Sparingly solvating electrolytes drive sulfur reduction through a solid state pathway, potentially eliminating parasitic losses due to LiPS.1 Realizing the goal of electrolyte design to enhance cell performance requires a quantitative model of LiPS solubility. Predicting LiPS Solubility with Electrolyte Donor Number (EDN): Building on the Gutman Donor Number concept, this talk will describe a new way of thinking about donicity—from the perspective of multicomponent systems (i.e. electrolytes). First, it will be shown that 23Na NMR can be used as a technique for deriving a pseudo-Gutman DN (23Na-DN) for any given electrolyte composition. This is similar to work described by Schmeisser et al., in which DNs for ionic liquids were derived from 23Na NMR.2 Next, it will be shown that an accurate descriptor for LiPS solubility also requires knowledge of the density of total available donor sites within a given electrolyte. Raman spectroscopy was used to deduce the specific populations of both solvent and anion species (i.e. establishing the coordinating behavior of solvating species to Li+ ions in solution as a function of salt concentration). Finally, it will be shown that the complete descriptor for LiPS solubility, as determined experimentally, relies on both the 23Na-DN and the structural arrangements of Li+ coordinating ligands within a given electrolyte. The combination of these two characteristics establishes an “electrolyte donor number” (EDN). Figure 1 shows that LiPS solubility values converge to a single function versus different functions with respect to the EDN and DN, respectively. Discussion on the Prediction of Reaction Pathways: The EDN model predicts LiPS solubility well. Thus, it is not surprising that it also acts as a useful tool for predicting the electrochemical reaction pathways taking place in a Li-S cell. Figure 2 shows the discharge curves for tetraglyme-based electrolytes that span a maximum range of EDN values. It is clear from this data that a higher EDN drastically changes the observed voltage profile, and therefore the mechanistic pathway.

Suppressing self-discharge of supercapacitors via electrorheological effect of liquid crystals

For electric double layer capacitors (EDLCs, or supercapacitors), self-discharge has been an inevitable issue that causes decay of cell voltage and loss of stored energy, but this problem has long been ignored in the studying of supercapacitors. Due to self-discharge, applications of supercapacitors for long-term energy storage or collecting environmental energy harvested by small power devices have been severely limited. There are three main self-discharge processes for EDLCs, including i) charge diffusion and redistribution, ii) faradaic reactions, and iii) ohmic leakage. In this work, we introduced a nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) as an additive to the electrolyte to suppress the self-discharge of EDLCs. When the EDLCs are charged, the electric field in the double layers near the electrode surface induces alignment of 5CB molecules, causing much enhanced fluid viscosity via the so-called electrorheological (ER) effect. As a result, the diffusion of ions and redox species in the electrolyte can be impeded and the self-discharge rate can be reduced. Here, we have demonstrated that by adding 2% of 5CB in TEMABF4/acetonitrile electrolyte, the decay rate of open circuit potential and leakage current can be reduced by more than 80%. Simulation results confirmed the reduced contribution of both diffusion of ions and faradaic reactions to the overall self-discharge. Furthermore, when 5CB EDLCs were employed for storing energy harvested by a triboelectric nanogenerator (TENG) that has a characteristic of pulsed output current, much enhanced charging efficiency was attained compared to EDLCs without 5CB. Our study pointing to a feasible way for truly pushing supercapacitor for practical applications.

Observation of different phases during an ELM crash with the help of nitrogen seeding

A new method was applied to indirectly obtain information about the features of the crash of the H-mode edge transport barrier in consequence of an edge localized mode (ELM). The method is based on a combination of fast measurements, without spatial resolution, and relatively slow measurements, with high spatial resolution. The comparison of two different ELM scenarios in the full metal tokamak ASDEX Upgrade—a standard scenario and one with additional nitrogen seeding—revealed a two-fold nature of the ELM crash. In the case with additional nitrogen only a part of the standard crash is observed. This suggests the standard ELM crash consists of two or more consecutive events instead of a single distinct one. Some of these events are observed to be suppressed with changes in plasma parameters. The effect of the impurity seeding on different plasma parameters is documented in detail and compared to measurements conducted in machines with a carbon wall.The radial extent of the phases observed during the ELM crash differs in the kinetic profiles, with one instability extending inside the pedestal top and the other being confined to the pedestal region. This picture can explain the differences in the loss of stored energy and the change in ELM frequency which are observed for the analysed pair of discharges. It also suggests that the ELM crash starts at the pedestal top and only then affects the steep gradient region.

Pedestal characterization and stability of small-ELM regimes in NSTX

NSTX has observed transition to a desirable small-ELM regime (called type-V), in which the stored energy loss per ELM is less than 1%, by stabilizing type-I ELMs. This regime is accessed in a lower single null configuration with increased edge collisionality (ν* > 1). Coincident with the transition to this regime, a low-frequency (<10 kHz) n = 1 mode is observed at the plasma edge in magnetic and soft x-ray diagnostics, with harmonics up to n = 6 observed in some cases. Low-level density fluctuations associated with this mode are observed using microwave reflectometry, but there is no evidence that the mode is providing sufficient transport to stabilize the type-I ELMs. This mode rotates in the electron diamagnetic direction and has shown a phase inversion on USXR channels, indicating that it is resistive in nature. Discharges with type-V and type-I ELMs are both calculated to be on the peeling unstable side of the peeling–ballooning stability curve, with the type-V case at higher normalized pressure gradient and closer to the ballooning stability boundary.

The release of stored energy in heavily irradiated NaCl explosive reactions

During irradiation of NaCl with ionizing radiation at moderate temperatures (50-150 degrees C) irregular structures of very fine Na and Cl nano-precipitates are formed. The increase of the temperature to a value between 50 and 250 degrees C might induce explosive reactions between radiolytic Na and Cl in heavily irradiated NaCl, which are accompanied by a loss of stored energy up to 30% The reactions produce a very fast local temperature rise of at least 800 degrees C and are accompanied by thermal shock waves. Due to explosive reactions the crystals desintegrated into many small fragments. Experiments and theoretical calculations have been carried out to study heavily irradiated NaCl and the dynamics of the thermal shock wave.