Supercapacitors have emerged as a critical energy storage technology, bridging the gap between conventional capacitors and batteries. Their high power density, rapid charge-discharge capabilities, and long cycle life make them suitable for applications ranging from electric vehicles to grid stabilization. A key component influencing their performance is the electrolyte, which governs ionic conductivity, operational voltage window, and safety. Advanced electrolytes—ionic liquids, aqueous, and organic—each present unique advantages and trade-offs in these areas.
Ionic liquids are molten salts at room temperature, composed entirely of ions. They exhibit exceptional thermal stability, non-flammability, and wide electrochemical stability windows, often exceeding 4 V. This makes them ideal for high-energy-density supercapacitors. Their conductivity, however, is generally lower than aqueous or organic electrolytes, typically ranging between 1 and 20 mS/cm, depending on viscosity and ion mobility. The high cost of ionic liquids also limits their commercial viability, though research into cheaper formulations is ongoing.
Aqueous electrolytes, such as sulfuric acid (H₂SO₄) or potassium hydroxide (KOH), offer high ionic conductivity, often exceeding 100 mS/cm due to the high dielectric constant of water. This enables low internal resistance and high power output. However, their narrow voltage window—limited to about 1.2 V due to water decomposition—restricts energy density. To mitigate this, neutral aqueous electrolytes like sodium sulfate (Na₂SO₄) have been explored, pushing the stability window slightly higher while maintaining safety and low cost. Aqueous systems are inherently non-flammable, making them attractive for applications where safety is paramount.
Organic electrolytes, typically based on solvents like acetonitrile or propylene carbonate with dissolved salts such as tetraethylammonium tetrafluoroborate (TEABF₄), provide a balance between conductivity and voltage window. Their electrochemical stability allows operation up to 2.7–3 V, significantly higher than aqueous systems, while conductivity ranges between 10 and 60 mS/cm. However, organic solvents are often volatile and flammable, posing safety risks. Efforts to improve safety include the use of flame-retardant additives or alternative solvents with higher flash points.
The choice of electrolyte also impacts supercapacitor design. Ionic liquids enable compact cells due to their wide voltage window but may require heating to reduce viscosity and enhance conductivity. Aqueous electrolytes simplify thermal management but necessitate series stacking of cells to achieve higher voltages, increasing system complexity. Organic electrolytes strike a middle ground but demand robust sealing and safety mechanisms to prevent leakage and combustion.
Recent advancements focus on hybridizing electrolytes to exploit their combined benefits. For instance, mixing ionic liquids with organic solvents can enhance conductivity while retaining a wide voltage window. Similarly, water-in-salt electrolytes—highly concentrated aqueous solutions—have demonstrated voltage windows approaching 3 V by suppressing water activity. These innovations aim to optimize the trade-offs between energy density, power, and safety.
In summary, the selection of advanced electrolytes for supercapacitors involves careful consideration of conductivity, voltage window, and safety. Ionic liquids excel in stability but face cost and conductivity challenges. Aqueous electrolytes offer high conductivity and safety but suffer from limited voltage. Organic electrolytes provide a compromise but require stringent safety measures. Ongoing research continues to refine these materials, pushing the boundaries of supercapacitor performance for next-generation energy storage systems.