Suppressing early capacitance fade of electrochemical capacitors with water-in-salt electrolytes

Gel Biopolymer Electrolytes Based on Saline Water and Seaweed to Support the Large-Scale Production of Sustainable Supercapacitors

Climate change and the demand for clean energy have challenged scientists worldwide to produce/store more energy to reduce carbon emissions. This work proposes a conductive gel biopolymer electrolyte to support the sustainable development of high-power aqueous supercapacitors. The gel uses saline water and seaweed as sustainable resources. Herein, a biopolymer agar-agar, extracted from red algae, is modified to increase gel viscosity up to 17-fold. This occurs due to alkaline treatment and an increase in the concentration of the agar-agar biopolymer, resulting in a strengthened gel with cohesive superfibres. The thermal degradation and agar modification mechanisms are explored. The electrolyte is applied to manufacture sustainable and flexible supercapacitors with satisfactory energy density (0.764 Wh kg-1 ) and power density (230 W kg-1 ). As an electrolyte, the aqueous gel promotes a long device cycle life (3500 cycles) for 1 A g-1 , showing good transport properties and low cost of acquisition and enabling the supercapacitor to be manufactured outside a glove box. These features decrease the cost of production and favor scale-up. To this end, this work provides eco-friendly electrolytes for the next generation of flexible energy storage devices.

RETRACTED: Capacitive Behavior of Aqueous Electrical Double Layer Based on Dipole Dimer Water Model

The aim of the present paper is to investigate the possibility of using the dipole dimer as water model in describing the electrical double layer capacitor capacitance behaviors. Several points are confirmed. First, the use of the dipole dimer water model enables several experimental phenomena of aqueous electrical double layer capacitance to be achievable: suppress the differential capacitance values gravely overestimated by the hard sphere water model and continuum medium water model, respectively; reproduce the negative correlation effect between the differential capacitance and temperature, insensitivity of the differential capacitance to bulk electrolyte concentration, and camel-shaped capacitance-voltage curves; and more quantitatively describe the camel peak position of the capacitance-voltage curve and its dependence on the counter-ion size. Second, we fully illustrate that the electric dipole plays an irreplaceable role in reproducing the above experimentally confirmed capacitance behaviors and the previous hard sphere water model without considering the electric dipole is simply not competent. The novelty of the paper is that it shows the potential of the dipole dimer water model in helping reproduce experimentally verified aqueous electric double layer capacitance behaviors. One can expect to realize this potential by properly selecting parameters such as the dimer site size, neutral interaction, residual dielectric constant, etc.

Elucidating the cation hydration ratio in water-in-salt electrolytes for carbon-based supercapacitors

The solvation of cations is one of the important factors that determine the properties of electrolytes. Rational solvation structures can effectively improve the performance of various electrochemical energy storage devices. Water-in-Salt (WIS) electrolytes with a wide electrochemically stable potential window (ESW) have been proposed to realize high cell potential aqueous electrochemical energy storage devices relying on the special solvation structures of cations. The ratio of H₂O molecules participating in the primary solvation structure of a cation (a cation hydration ratio) is the key factor for the kinetics and thermodynamics of the WIS electrolytes under an electric field. Here, acetates with different cations were used to prepare WIS electrolytes. And, the effect of different cation hydration ratios on the properties of WIS electrolytes was investigated. Various WIS electrolytes exhibited different physicochemical properties, including the saturated concentration, conductivity, viscosity, pH values and ESW. The WIS electrolytes with a low cation hydration ratio (<100%, an NH₄-based WIS electrolyte) or a high cation hydration ratio (>100%, a K-based WIS electrolyte and a Cs-based WIS electrolyte) exhibit more outstanding conductivity or a wide ESW, respectively. SCs constructed from active carbon (AC) and these WIS electrolytes exhibited distinctive electrochemical properties. A SC with an NH₄-based WIS electrolyte was characterized by higher capacity and better rate capability. SCs with a K-based WIS electrolyte and a Cs-based WIS electrolyte were characterized by a wider operating cell potential, higher energy density and better ability to suppress self-discharge and gas production. These results show that a WIS electrolyte with a low cation hydration ratio or a high cation hydration ratio is suitable for the construction of power-type or energy-type aqueous SCs, respectively. This understanding provides the foundation for the development of novel WIS electrolytes for the application of SCs.

Downplaying the role of water in the rheological changes of conducting polymers by using water-in-salt electrolytes

Volumetric changes associated with solvent/electrolyte exchange in electronic conducting polymers (ECPs) play an important role in the mechanical stability of the polymers, as these changes are a critical factor in ECP-based energy storage devices. Thus, the present work explores the hindering of such volumetric deformations for polypyrrole films doped with dodecylbenzenesulphonate (PPy(DBS)) by employing highly concentrated aqueous electrolytes (or water-in-salt electrolytes, WiSEs), and their effects over the corresponding electrochemical capacitor cell energy retention. Electrochemical quartz crystal microbalance with dissipation monitoring measurements for thin PPy(DBS) films in the WiSEs revealed negligible dissipation changes (ΔDn ≈ 0), in contrast with those in dilute aqueous electrolyte (ΔDn ≠ 0), indicating inexpressive structural deformation of PPy(DBS) in the WiSE. This phenomenon is observed for thick freestanding PPy(DBS) films, which presented a maximum bending angle decay from ∼56° (diluted aqueous electrolyte) to 3.5° when working in the WiSE, thus proving the hindering of film bending. The observed trends are reflected in the PPy(DBS) cell energy retention, where the use of a WiSE decreased cell energy fading by 30% after 600 cycles, in comparison with cells based on diluted electrolytes.