3.1 How supercapacitors store charge

Between a capacitor and a battery

A battery stores a lot of energy in chemical bonds but delivers it slowly. A conventional capacitor delivers energy almost at once but stores very little. Electrochemical capacitors, or supercapacitors, sit between the two. They charge and discharge in seconds to minutes, last for many thousands of cycles, and store far more energy than a conventional capacitor, though less than a battery.

The electrical double layer

When a charged electrode touches an electrolyte, ions of the opposite charge gather at its surface within about a nanometre. This double layer acts like a capacitor with a very small plate distance, so its capacitance per area is large, of the order of 10 to 30 µF per cm² of real surface. Porous carbons with surface areas of 1000 to 2000 m² per gram turn that into capacitances of the order of 100 F per gram of carbon. No electrons cross the interface, so the process is fast and very reversible, and these electric double-layer capacitors (EDLCs) can run for hundreds of thousands of cycles.

Pore size matters as much as surface area. Ions have to reach the surface, so pores that match the ion size, and a network that lets ions move quickly, decide how much of the area is used at high rates.

Pseudocapacitance and battery-type materials

Some materials store charge by fast redox reactions at or near the surface, yet still respond like a capacitor, with a current that is nearly constant across the window. This is pseudocapacitance. Classic examples are RuO₂ and MnO₂ in aqueous electrolytes, and some layered materials such as MXenes and Nb₂O₅, where ions slip between layers without phase changes.

Many other materials, such as Ni(OH)₂ or Co₃O₄ in KOH, show sharp redox peaks and a voltage plateau. They behave like battery electrodes, even when they are fast. Brousse, Bélanger and Long (2015) argued that such battery-type materials should not be called pseudocapacitive and should be reported as capacity, in C/g or mAh/g, rather than in F/g.

Telling them apart

• Shape. A rectangular CV and a triangular charge–discharge curve, as in the figure above.
• The b-value. The peak current follows i = a·v^b. A capacitive, surface-limited response gives b ≈ 1, and a diffusion-limited, battery-like one gives b ≈ 0.5.
• The Dunn method. At each potential, the current is split into k₁v (capacitive) and k₂v^½ (diffusion), following Wang and co-workers (2007). It shows how the capacitive share grows with scan rate.

Both methods assume that resistance does not distort the CV. At high scan rates iR drop and slow ion transport lower b, so use a range of scan rates where the peak potentials hardly move.

Electrolytes, windows and pH

The electrolyte also changes the chemistry. The same oxide can behave capacitively in a neutral electrolyte and battery-like in KOH, which is why comparing a material at several pH values, as we do in our group, tells a lot about how it stores charge.

Check your understanding

References and further reading

1. P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. doi:10.1038/nmat2297
2. T. Brousse, D. Bélanger, J. W. Long, To be or not to be pseudocapacitive?, J. Electrochem. Soc. 162 (2015) A5185.
3. J. Wang, J. Polleux, J. Lim, B. Dunn, Pseudocapacitive contributions to electrochemical energy storage in TiO₂ (anatase) nanoparticles, J. Phys. Chem. C 111 (2007) 14925–14931. doi:10.1021/jp074464w
4. V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597–1614. doi:10.1039/c3ee44164d
5. Y. Gogotsi, P. Simon, True performance metrics in electrochemical energy storage, Science 334 (2011) 917–918. doi:10.1126/science.1213003