Anisotropic Semicrystalline Homopolymer Dielectrics for High-Temperature Capacitive Energy Storage

High-temperature dielectric polymers are in high demand for powering applications in extreme environments. Here, we have developed high-temperature homopolymer dielectrics with anisotropy by leveraging the hierarchical structure in semicrystalline polymers. The lamellae have been aligned parallel to the surface in the dielectric films. This structural arrangement resembles the horizontal alignment of nanosheet fillers in polymer nanocomposites and nanosheet-like lamellae in block copolymers, which has been proven to provide the optimal topological structure for electrical energy storage. The unique ordering of lamellae in our dielectric films endue a significantly increased breakdown strength and a reduced leakage current compared to amorphous films. This novel approach of enhancing the capacitive energy storage properties by controlled orientation of lamellae in homopolymer offers a new perspective for the design of high-temperature polymer dielectrics.

Hydrogen Bonding of Aramid Boosts High-Temperature Capacitive Properties of Polyetherimide Blends

Polyetherimide (PEI) is the state-of-the-art commercial high-temperature polymer dielectric with excellent thermal and chemical stability and relatively high high-temperature capacitive energy storage properties. The rotation of the dipoles in the PEI chains brings the β-relaxation which seriously increases the leakage current and decreases the charge-discharge efficiency. In this work, hydrogen bonds have been introduced to limit the dipole rotation of PEI by blending aramids [1,4-poly(ether fluoromethyl naphthalene amide), PNFA] into the PEI matrix. By introducing 10 wt % PNFA, the β-relaxation of the blend has been significantly reduced which could be observed from the dielectric spectrum. To explore the mechanism of limited β-relaxation, we analyze the hydrogen bonds in the blend films by infrared spectroscopy and found that the maximum content of hydrogen-bonded carbonyl formed between PNFA and PEI chains was 14.3% when the content of PNFA was 30 wt %. The content of hydrogen bonds formed between PNFA and PEI was positively correlated with the energy storage performance of the blends. The maximum discharged energy density with an efficiency above 90% of the blend film with 30 wt % PNFA reaches 4.1 J cm^-3 at 150 °C, which is about 350% higher than that of pristine PEI. This work shows that composing hydrogen bonds by the blending method could be a viable strategy for enhancing the high-temperature energy storage performance of polymer dielectrics, which could be achieved by large-scale preparation and has feasible industrial production prospects.

A Simple and Expeditious Route to Phosphate-Functionalized, Water-Processable Graphene for Capacitive Energy Storage

Phosphate-functionalized carbon-based nanomaterials have attracted significant attention in recent years owing to their outstanding behavior in electrochemical energy-storage devices. In this work, we report a simple approach to obtain phosphate-functionalized graphene (PFG) via anodic exfoliation of graphite at room temperature with a high yield. The graphene nanosheets were obtained via anodic exfoliation of graphite foil using aqueous solutions of H3PO4 or Na3PO4 in the dual role of phosphate sources and electrolytes, and the underlying exfoliation/functionalization mechanisms are proposed. The effect of electrolyte concentration was studied, as low concentrations do not lead to a favorable graphite exfoliation and high concentrations produce fast graphite expansion but poor layer-by-layer delamination. The optimal concentrations are 0.25 M H3PO4 and 0.05 M Na3PO4, which also exhibited the highest phosphorus contents of 2.2 and 1.4 at. %, respectively. Furthermore, when PFG-acid at 0.25 M and PFG-salt at 0.05 M were tested as an electrode material for capacitive energy storage in a three-electrode cell, they achieved a competitive performance of ∼375 F/g (540 F/cm3) and 356 F/g (500 F/cm3), respectively. Finally, devices made up of symmetric electrode cells obtained using PFG-acid at 0.25 M possess energy and power densities up to 17.6 Wh·kg-1 (25.3 Wh·L-1) and 10,200 W/kg; meanwhile, PFG-salt at 0.05 M achieved values of 14.9 Wh·kg-1 (21.3 Wh·L-1) and 9400 W/kg, with 98 and 99% of capacitance retention after 10,000 cycles, respectively. The methodology proposed here also promotes a circular-synthesis process to successfully achieve a more sustainable and greener energy-storage device.

Toward Superior Capacitive Energy Storage: Recent Advances in Pore Engineering for Dense Electrodes

With the rapid development of mobile electronics and electric vehicles, future electrochemical capacitors (ECs) need to store as much energy as possible in a rather limited space. As the core component of ECs, dense electrodes that have a high volumetric energy density and superior rate capability are the key to achieving improved energy storage. Here, the significance of and recent progress in the high volumetric performance of dense electrodes are presented. Furthermore, dense yet porous electrodes, as the critical precondition for realizing superior electrochemical capacitive energy, have become a scientific challenge and an attractive research focus. From a pore-engineering perspective, insight into the guidelines of engineering the pore size, connectivity, and wettability is provided to design dense electrodes with different porous architectures toward high-performance capacitive energy storage. The current challenges and future opportunities toward dense electrodes are discussed and include the construction of an orderly porous structure with an appropriate gradient, the coupling of pore sizes with the solvated cations and anions, and the design of coupled pores with diverse electrolyte ions.