6.2 Electric double-layer capacitors: materials and design

Electric double-layer capacitors (EDLCs) are a type of supercapacitor that store energy through charge separation at the electrode-electrolyte interface. This section dives into the materials and design aspects of EDLCs, focusing on electrode materials, electrolytes, separators, and cell components.

Understanding these elements is crucial for optimizing EDLC performance. We'll explore carbon-based electrodes, electrolyte types, separator materials, and cell designs that impact energy density, power density, and overall device efficiency.

Electrode Materials

Carbon-Based Materials

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• Activated carbon widely used electrode material due to high surface area, low cost, and good electrical conductivity
  • Produced by thermal or chemical activation of carbonaceous precursors (coconut shells, wood, coal)
  • Activation process creates a network of micropores and mesopores, increasing the accessible surface area for charge storage
• Carbon nanotubes (CNTs) exhibit excellent electrical conductivity and high surface area
  • Can be single-walled (SWCNTs) or multi-walled (MWCNTs)
  • CNTs enhance the power density and cyclic stability of EDLCs
• Graphene, a two-dimensional carbon material, offers high electrical conductivity and large specific surface area
  • Can be produced by mechanical exfoliation, chemical vapor deposition (CVD), or reduction of graphene oxide
  • Graphene-based EDLCs demonstrate high capacitance and excellent rate capability
• Porous electrodes engineered to maximize surface area and minimize ion transport resistance
  • Hierarchical pore structure combining micropores, mesopores, and macropores
  • Interconnected pore network facilitates rapid electrolyte infiltration and ion diffusion

Surface Modification and Composites

• Surface functionalization of carbon materials can improve wettability, electrical conductivity, and pseudocapacitance
  • Introduction of heteroatoms (nitrogen, oxygen, sulfur) through doping or surface treatment
  • Functionalization enhances the interaction between electrode and electrolyte, leading to higher capacitance
• Composite electrodes combine carbon materials with pseudocapacitive materials (metal oxides, conducting polymers)
  • Synergistic effects of double-layer capacitance and pseudocapacitance
  • Composite electrodes exhibit improved energy density and power density compared to pure carbon electrodes

Electrolyte and Separator

Electrolyte Types

• Aqueous electrolytes (H2SO4, KOH) offer high ionic conductivity and low cost
  • Limited voltage window (~ 1 V) due to water electrolysis
  • Suitable for high-power applications with moderate energy density requirements
• Organic electrolytes (propylene carbonate, acetonitrile) enable wider voltage windows (up to 3 V)
  • Higher energy density compared to aqueous electrolytes
  • Lower ionic conductivity and higher cost than aqueous electrolytes
• Ionic liquids (ILs) are molten salts at room temperature with wide electrochemical stability windows (> 3 V)
  • Non-volatile, non-flammable, and thermally stable
  • High viscosity and low ionic conductivity compared to conventional electrolytes

Separator Materials

• Separators prevent physical contact between electrodes while allowing ion transport
  • Must be electrically insulating, chemically stable, and mechanically robust
  • Porous structure with high ionic conductivity and low thickness
• Polymeric separators (polyethylene, polypropylene) are commonly used in EDLCs
  • Manufactured by wet or dry processes to create a porous membrane
  • Porosity, pore size, and tortuosity affect the ionic resistance and capacitance of the EDLC
• Cellulose-based separators (paper, nanofibrillated cellulose) offer eco-friendly alternatives
  • High porosity, good wettability, and low cost
  • Suitable for aqueous electrolytes and flexible device applications

Cell Components and Design

Current Collectors

• Current collectors provide electrical contact between the electrodes and external circuit
  • Must be highly conductive, corrosion-resistant, and compatible with the electrolyte
  • Common materials include aluminum, stainless steel, and nickel foam
• Foil-type current collectors are thin, flexible, and suitable for stacked or wound cell configurations
  • Aluminum foil widely used due to its low cost, light weight, and good conductivity
  • Surface treatments (etching, coating) can improve adhesion and contact resistance with the electrode material
• Foam-type current collectors have a three-dimensional porous structure
  • High surface area and excellent electrical contact with the electrode material
  • Nickel foam commonly used in aqueous electrolytes due to its good corrosion resistance and high porosity

Cell Design and Packaging

• Stacked cell design consists of alternating layers of electrodes, separators, and current collectors
  • Enables high electrode packing density and low internal resistance
  • Suitable for high-energy applications and prismatic cell packaging
• Wound cell design involves spirally winding the electrode-separator assembly around a central core
  • Provides high surface area and good mechanical stability
  • Commonly used in cylindrical cell packaging for high-power applications
• Pouch cell packaging offers flexibility and lightweight design
  • Electrode-separator assembly sealed in a flexible, moisture-resistant pouch
  • Allows for customizable cell shapes and sizes, suitable for portable electronics and wearable devices
• Bipolar cell design utilizes shared current collectors between adjacent cells
  • Reduces the number of components and minimizes internal resistance
  • Enables high voltage and high power density, suitable for large-scale energy storage systems