Porous carbon nanosheets (aCS) formed by carbonization and chemical activation of biomass cornstalk are presented for capacitive energy storage. The obtained carbon possesses a two-dimensional (2D) sheet-like structure which is mainly composed of abundant micropores. Depending on the amount of KOH used in the activation step, the specific surface area of the carbon sheets varies from 388 to 1736 m² g⁻¹, whereas the lateral size is in the range of 50–100 μm, and sheet thickness is around 100 nm. The capacitive performance of aCS-5 in both KOH and Na₂SO₄ aqueous electrolytes is investigated. The aCS-5 electrode in KOH electrolyte exhibits a remarkable high-rate capability due to the dramatically shortened ion pathway and more exposed external surface. While, in Na₂SO₄ it shows relatively lower capacitance retention but a stable and extended voltage window (0–1.6 V), thus delivering a specific energy of 20.2 W h kg⁻¹ at a power density of 398 W kg⁻¹. The aCS-5 with a large aspect ratio (200–500) can also serve as an electrode for assembling a solid-state capacitor (SSC) using PVA/KOH gel as a polymer electrolyte. The SSC retains the normal capacitive performance under bent and twisted states and exhibit an unusual rate capability with an areal capacitance of 136 mF cm⁻² at 0.5 mA cm⁻².