Electrochemical fuel cells convert fuel directly into electric current by triggering a chemical reaction between the fuel and an oxidant using an electrolyte. So long as the fuel (reactant) and oxidant are constantly replenished, fuel cells can generate current indefinitely, in contrast to a conventional battery, which is a closed system with finite amounts of chemicals and that eventually loses charge.
Fuel cells can range in size from the tiny (powering watches or small appliances) to the mid-range (fuel-cell-powered cars) to grid-level storage options (in particular, renewable electricity can be used to create hydrogen, which can power a fuel cell at a later point in time). Read more.