Two of the most promising current trends in supercapacitor research, (i) the development of biomass based carbon electrodes, and (ii) the transition to solid thin flexible form factors via polymer electrolytes are combined and investigated. A high surface area (3312 m(2) g(-1)) mesoporous activated carbon was synthesized from chitosan biomass and showed excellent capacitive behaviour in a range of acidic, neutral, and alkaline liquid electrolytes. The performance in the neutral Li2SO4 electrolyte system was particularly promising with the chitosan AC electrodes showing a high capacitance (264 F g(-1)) similar to the values in acidic and alkaline electrolytes but with a much larger 1.8 V potential window. The chitosan AC also proved compatible with a series of solid polymer electrolytes through a detailed comparison in which solid-state chitosan supercapacitor devices were shown to closely mimic the capacitance and high rate performance of their liquid counterparts. This is an important finding as it demonstrates that high surface area, intricately porous activated carbon networks can still be readily accessible to solid electrolytes. Combining the chitosan AC with a Li2SO4-polyacrylamide (PAM) solid electrolyte enabled the fabrication of ultra-thin (<0.38 mm) supercapacitor devices which demonstrated a capacitance close to 3 times greater than analogues prepared with a commercially available microporous AC (YP-50). These chitosan devices also demonstrated high volumetric energy density (1.6 mWh cm(-3)) and power density (0.8 W cm(-1)) comparable to state-of-the-art SC devices utilizing much more expensive materials. This material system represents a simple and cost effective approach for the design of next-generation solid thin, flexible energy storage devices. (c) 2018 Elsevier Ltd. All rights reserved.