A method of producing a sulfonated polymer. The method includes providing a source for a quantity of a polymer having a shape. The quantity of the polymer is heated while immersed in sulfuric acid to 100-200° C. for a period time in a closed reactor containing an atmosphere and capable of holding pressure generated by a reaction between the quantity of the polymer and the sulfuric acid resulting in a sulfonated polymer, wherein substantially all the quantity of the polymer from the source is converted into sulfonated polymer. The sulfonated polymer is then removed from the reactor and dried. An electrode suitable for use as an electrode in an electrochemical energy storage cell is disclosed. The electrode contains amorphous porous carbon sheets made from a sulfonated polymer with a morphology wherein the amorphous porous carbon sheets have the morphology of the sulfonated polymer from which they are made.