A high tensile strength, highly conductive flexible composite sheet consisting of fibrillated PTFE polymers, performance enhancing dopants, and carbon particles useful in electrochemical devices such as flow through capacitors, capacitive deionization, electronic water purification, fuel cells, capacitors, super-capacitors and a method of preparation. The method consists of mixing the materials and calendering to the desired thickness and strength all at room temperature and leaving the material in a wet state without further drying.