A novel energy conversion and storage system using seawater as a cathode is proposed herein. This system is an intermediate
between a battery and a fuel cell, and is accordingly referred to as a hybrid fuel cell. The circulating seawater in this opencathode system results in a continuous supply of sodium ions, which gives this system superior cycling stability that allows the
application of various alternative anodes to sodium metal by compensating for irreversible charge losses. Indeed, hard carbon
and Sn-C nanocomposite electrodes were successfully applied as anode materials in this hybrid-seawater fuel cell, yielding
highly stable cycling performance and reversible capacities exceeding 110 mAh g− 1 and 300 mAh g − 1, respectively.