Nanoporous biocarbons obtained from waste biomass have attracted a lot of attention due to their great potential for energy storage and conversion, as well as water purification. However, the manufacturing technology for these materials requires expensive and corrosive activating agents such as KOH or ZnCl2, which is a major hurdle to their potential commercialization. In this study, an inexpensive synthetic strategy for the production of O- and N-functionalized nanoporous biocarbons using non-corrosive and inexpensive potassium citrate as an activating agent and casein as a biomass source via a one-step solid process is presented. The presence of O-functional groups in the activating agent and a large amount of N in the biomass precursor enables not only the successful incorporation of O and N atoms into the finished nanoporous biocarbons, but also nanoporosity into the materials. A range of O- and N-functionalized nanoporous biocarbons with adjustable porosity, specific surface area and pore volume are produced by varying the amount of activating agent and casein. The optimized material CPC-3, which was synthesized with a 1: 3 ratio of casein to potassium citrate, showed a high surface area (2212 m2 g-1), a large pore volume (1.11 cm3 g-1) and a considerable amount on oxygen-containing functional groups on the surface. With the advantages of excellent surface parameters and the O and N functionalities on the porous surface, the functionalized nanoporous biocarbons achieved a high CO2 adsorption capacity of 25.4 mmol g-1 at 0 ° C and 30 bar and show an impressive specific capacity of 177 F g -1 at 0.5 A g-1 in a three-electrode system in 3 M KOH as the electrolyte. Interestingly, the functionalized nanoporous biocarbon with a high proportion of micropores shows a CO2 adsorption capacity of 5.3 mmol g-1 CO2 adsorption at 1 bar / 0 ° C, which is much higher than that of the reported porous activated carbon materials and ordered mesoporous carbons. It is assumed that the unique manufacturing approach described and the multifunctionality of these fascinating materials offer exciting possibilities for the cost-effective CO2 adsorption system and highly efficient energy storage.