An oxygen-rich carbon for supercapacitors has been obtained by one-step carbonization of a natural biopolymer from seaweeds, e.g., sodium alginate. Although the specific surface area of this carbon is very low, it gives a capacitance comparable to the best activated carbons available. Due to its low porosity (see figure), this material has high density and good electrical conductivity, also advantageous for supercapacitor applications.