Systems and methods of recycling water from the cathode based on passive water transport mechanisms are described. These systems and methods allow water to be recycled in a fuel cell without moving components and without using power. In addition, these systems and methods allow a water recycling system to be implemented in a fuel cell system with a relatively small form factor, such as a portable fuel cell system. In one aspect, a fuel cell system includes a fuel cell and a mixing chamber. The fuel cell has an anode side and a cathode side. The mixing chamber has a recycled water input port that is coupled to the cathode side of the fuel cell, a fuel input port, and a hydrated fuel output port that is coupled to the anode side of the fuel cell. A selectively permeable membrane is disposed upstream of an input port of the mixing chamber and is operable to selectively permit osmotic inflow of a first mixing component into the mixing chamber and to substantially prevent outflow of a second mixing component from the mixing chamber.