Availability of grid-scale electric energy storage systems with response rates on the order of seconds plays a key role in wide implementation of renewable energy sources. Here, a new concept called the electrochemical flow capacitor (EFC) is presented. This new concept shares the major advantages of both supercapacitors and flow batteries, providing rapid charging/discharging while enabling the decoupling of the power and energy ratings. Like in supercapacitors, energy is stored in the electric double layer of charged carbon particles. A flowable carbon-electrolyte mixture is employed as the active material for capacitive energy storage, and is handled in a similar fashion to flow or semi-solid batteries (i.e., for charging/discharging, it is pumped into an electrochemical cell, and for storage, it is pumped into reservoirs). This study presents the proof-of-concept of this technology and reports initial EFC performance data obtained under static and intermittent flow operations.