Tetraalkylammonium Bromide Complexation in Supercapacitor Electrolytes
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Solution Overview
Problem
Aqueous electric double-layer capacitors (EDLCs) face limitations in energy density due to the narrow electrochemical potential window of water, and existing solutions for enhancing energy density, such as using redox-active electrolytes, struggle with self-discharge and chemical reactivity issues, particularly with bromine-based systems, which require expensive ion-selective membranes for stability.
Innovation Solution
The use of a redox-enhanced electrolyte system that includes a bromine redox couple and a viologen redox couple, with tetrabutylammonium-induced solid complexation in porous carbon electrodes, suppressing self-discharge and chemical reactivity without the need for expensive membranes, and utilizing a carbon-polymer composite pouch cell design for corrosion resistance and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If redox-active electrolytes are used to enhance energy density, then energy density is improved, but self-discharge and chemical reactivity increase
Solution Approach 1:
The patent introduces an ion-selective membrane as an intermediary component that physically separates the redox-active electrolytes in the positive and negative compartments. This membrane allows selective ion transport while preventing direct contact between reactive species, thereby enabling high energy density through redox reactions while suppressing self-discharge and chemical reactivity that would otherwise degrade cycling stability.
Solution Approach 2:
The patent divides the electrochemical capacitor into separate positive and negative compartments using an ion-selective membrane. This segmentation isolates the redox-active electrolytes (such as bromine-based electrolytes) in each compartment, preventing harmful cross-reactions while allowing beneficial ion exchange. The segmentation enables the system to achieve high energy density through faradaic reactions without suffering from the self-discharge problems that occur when redox couples are mixed.
2Reliability
If ion-selective membranes are used to suppress self-discharge, then cycling stability is improved, but device cost increases
Solution Approach 1:
The patent employs cost-effective ion-selective membrane materials such as Nafion or Celgard that, while not permanent, provide sufficient cycling stability for practical applications. These membranes are relatively inexpensive compared to alternative high-performance membranes and can be replaced periodically, making the overall device economically viable. The use of commercially available, mass-produced membrane materials significantly reduces device cost while maintaining adequate cycling stability.
Solution Approach 2:
The patent optimizes membrane parameters such as thickness, porosity, and ion exchange capacity to achieve the right balance between cost and performance. By carefully selecting membrane thickness and porosity, the system achieves sufficient ion transport for high power density while preventing enough cross-diffusion to suppress self-discharge. These parameter optimizations allow the use of lower-cost membrane materials that would otherwise be insufficient, thereby reducing device cost while maintaining cycling stability.
3Reliability
If aqueous electrolytes are used, then safety and power density are improved, but energy density is limited
Solution Approach 1:
The patent changes the key parameter of using redox-active electrolytes (such as bromine-based electrolytes with high concentration) to dramatically increase energy density while maintaining the safety advantages of aqueous systems. The redox-active species provide high faradaic capacity, and the aqueous base electrolyte maintains safety and high ionic conductivity for high power density. This parameter change allows the system to achieve energy densities comparable to or exceeding traditional organic electrolyte systems while retaining the safety and power advantages of aqueous electrolytes.
Solution Approach 2:
The patent creates a composite electrolyte system that combines the safety and high conductivity of aqueous electrolytes with the high energy density of redox-active species. The aqueous base electrolyte (such as KOH or H2SO4) provides safety, high ionic conductivity, and high power density, while the dissolved redox-active species (such as bromine or iodine) provide high faradaic capacity and high energy density. This composite approach allows the system to simultaneously achieve the benefits of both aqueous and high-energy electrolyte systems.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly improves cycling stability and energy density, achieving a specific energy of approximately 64 W·h/kg with over 7000 cycles, while maintaining high power density and reducing costs through the use of aqueous electrolytes and cost-effective materials.
Implementation Method 1
the charge is stored in faradaic reactions with the first and second redox couples in the electrolyte
Implementation Method 2
storing and discharging energy quickly due to a physical ion adsorption/desorption mechanism in the Helmholtz layer
Implementation Method 3
tetrabutylammonium-induced solid complexation in porous carbon electrodes, suppressing self-discharge and chemical reactivity
Data Source
AI summary
Electrolytes for use in electric double-layer capacitors (EDLCs; often referred as supercapacitors or ultracapacitors) are disclosed. In one example, the electrolyte comprises viologen in both the anolyte and the catholyte (with bromide). In another example, the electrolyte comprises viologen (in the anolyte) and tetraalkylammonium with bromide (in the catholyte), wherein the tetraalkylammonium is used to achieve solid complexation of bromine in the activated carbon of the cathode. In a third example, a zinc bromine/tetraalkylammonium supercapacitor/battery hybrid is disclosed. Also disclosed is a corrosion resistant bipolar pouch cell that can be used with the electrolyte embodiments described herein.