How tree nut shells can become high-powered energy stores

Turning nut shells into power

Throwing away nut shells might seem harmless, but they could actually help power future electronics. Researchers took agricultural waste shells from the Calophyllum inophyllum tree and turned them into carbon. Their goal was to build a biomass supercapacitor, a device that stores and releases electrical charge far faster than standard batteries.

To make this work, the shells first underwent carbonization, a process of heating raw organic matter without oxygen until only pure carbon remained. Solid carbon alone cannot hold much charge. Ions need open spaces to dock, much like a kitchen sponge soaking up water through thousands of tiny tunnels.

Cooking with acid and alkali

The team treated the charred shells with three different activating chemicals: sodium hydroxide, sulfuric acid, and phosphoric acid. These chemicals ate away microscopic pathways, creating hierarchical porous structures with mixed pore sizes.

The choice of chemical changed everything. In cyclic voltammetry tests, sodium hydroxide produced an interconnected network that reached a specific capacitance of 638.21 farads per gram at a scan rate of 10 millivolts per second. That measurement represents how much electrical charge a material holds per unit of weight.

However, when testing complete charge and discharge cycles, sulfuric acid came out on top. In galvanostatic charge-discharge tests, it achieved an energy density of 4.02 watt-hours per kilogram, alongside a specific capacitance of 59.06 farads per gram. The acid-etched carbon held the best overall balance of total energy storage.

What we still don't know

While the sponge analogy fits how ions enter these pores, it stops working under pressure; squashing a real sponge forces water out, but electrical charges stay locked on pore surfaces until released. We still do not know how well these electrodes survive over thousands of charge cycles. Scalability remains uncertain too, because washing carbon with strong industrial acids and bases carries real environmental costs.