Activated-Carbon Supercapacitor

Build an electric double-layer capacitor from activated carbon and sodium sulfate, then charge, discharge, and estimate its capacitance
Advanced🕐90 min▲Moderate hazardelectrochemistry

Summary

A battery stores energy in a chemical reaction; a supercapacitor stores it by piling ions onto a vast carbon surface — no bonds made or broken. In this build you’ll make an electric double-layer capacitor (EDLC) from activated carbon and a sodium sulfate electrolyte, charge and discharge it, and measure its capacitance. You’ll see first-hand why a capacitor’s voltage ramps smoothly (unlike a battery’s flat plateau) and why EDLCs win on cycle life and power but lose on energy density.

History

The double-layer effect — ions arranging against a charged surface — was described by Helmholtz in the 1850s. It became practical energy storage only when high-surface-area carbons (1000–3000 m²/g) were paired with it in the late 20th century. Today supercapacitors handle power bursts that would wear out a battery: regenerative braking, camera flashes, and memory backup.

Hazards & preparation

Note

Low-toxicity materials, but a few cautions:

  • Optional charcoal activation uses zinc chloride (corrosive) and high heat — gloves, ventilation, and rinse electrodes thoroughly before use.
  • Keep cell voltage below ~1.0 V; above that the water electrolyses and gasses.
  • Use graphite/carbon or stainless current collectors — aluminium corrodes in sodium sulfate. See the Safety page.

Part A — Prepare the Electrodes

Materials: 5–10 g activated carbon (aquarium-grade, crushed fine), graphite powder, CMC or PTFE binder, current collectors (graphite sheet, carbon cloth, or stainless mesh), separator (filter paper), a clamp, and a multimeter.

Steps:

  1. Mix activated carbon with ~10% graphite powder for conductivity.
  2. Add binder (a few drops of CMC solution) drop by drop to a thick paste.
  3. Spread a thin, even ~1–2 mm layer onto each current collector.
  4. Press firmly and dry several hours (room temperature or 60 °C).
  5. Cut two identical electrodes — 3×3 cm is a good start.

Part B — Assemble the Cell

  1. Soak the filter-paper separator in saturated sodium sulfate (~28 g/100 mL).
  2. Stack: collector / electrode / wet separator / electrode / collector.
  3. Clamp firmly — pressure ensures good contact — and connect leads to the outer collectors.

Part C — Charge, Discharge, Measure

  1. Charge at 0.8–0.9 V for 30–60 s (a USB supply through a 100 Ω series resistor works).
  2. Disconnect and immediately read the open-circuit voltage — it should hold near the charge voltage.
  3. Discharge through a 100–1000 Ω load, logging voltage over time.
  4. Plot voltage vs time: a roughly linear fall is the capacitor’s signature (a battery instead holds a flat plateau then drops off a cliff).

Estimate capacitance from the discharge:

\[C = I \cdot \frac{\Delta t}{\Delta V}\]

A good 3×3 cm cell might reach 0.5–2 F. Wire three cells in series (~2.4 V) to light a red LED for a moment.

What you should see

The cell charges in under a minute and, disconnected, holds its voltage for a while before slowly self-discharging. Under load the voltage slides down in a near-straight line rather than a battery’s plateau — the hallmark of electrostatic storage. A stack of three briefly powers an LED, discharging far faster than any of the chemical cells would.

Symptom Likely cause Fix
Won’t hold charge Electrodes touching through separator Thicker/undamaged separator; more clamp pressure
Very low capacitance Poor carbon or thick electrode Finer activated carbon; thinner, even coating
Voltage decays fast when charging past ~1 V Water electrolysing Keep below ~1.0 V per cell
High internal resistance Loose stack or dry separator Clamp firmly; fully wet the separator

The Science

Apply a voltage and ions in the electrolyte migrate to the electrode surfaces: a sheet of cations coats the negative electrode, anions the positive — the electric double layer. Because the carbon’s surface area is enormous, an enormous amount of charge parks there, and because no chemical bonds form, the process is almost perfectly reversible — hundreds of thousands of cycles.

That’s the whole trade-off with a battery:

Property Battery EDLC
Storage Chemical (bulk reaction) Electrostatic (surface)
Charge/discharge Minutes–hours Seconds
Cycle life 500–2000 100,000+
Energy density High Low (10–100× less)
Power density Low Very high

The linear discharge curve follows directly from \(Q = CV\): charge leaves in proportion to voltage, so voltage falls steadily — whereas a battery’s fixed reaction potential holds voltage nearly constant until the reactants run out.

Questions to Explore

  1. Why does the supercapacitor discharge in a straight line while a battery holds a plateau?

    Hint / answer

    A capacitor’s voltage is proportional to its stored charge (\(V = Q/C\)), so as charge drains the voltage falls steadily. A battery’s voltage is fixed by its reaction potential and stays nearly constant until the reactants are used up.

  2. Why does activated carbon give so much more capacitance than a plain graphite plate?

    Hint / answer

    Capacitance scales with surface area. Activated carbon’s porous structure offers thousands of m² per gram for ions to sit against, versus the tiny area of a flat plate.

  3. Why must you keep each cell below about 1 V? What sets that ceiling?

    Hint / answer

    Above ~1.2 V water begins to electrolyse into hydrogen and oxygen, wasting energy and gassing the cell. The aqueous electrolyte’s stability window caps the per-cell voltage, which is why you stack cells in series for more.

Going further

  • Self-discharge curve. Charge fully, disconnect, and log voltage every 5 minutes for an hour.
  • Ragone plot. Measure energy and power for this cell and a battery cell and plot them on log–log axes — how engineers compare storage technologies.
  • In the Electrochemistry track: contrast this bond-free storage with the redox chemistry of Homemade Battery Cells.