Is it possible to use a fuel cell to store electricity or can it only be used to generate electricity?

no, you can not store electricity in a fuel cell. It is just like a car engine, you put fuel in and power comes out, except in this case you put H2 and O2 in and electricity comes out.

Specific types of fuel cells can be 'recharged'.
Reversing the process is to produce gas is called electrolysis. Even then the unit doesn't actually store anything, it is just converting water to H2 and O2. You can obviously store the gases which can then be used by a fuel cell to produce electricity. But a fuel cell or electrolyzer does not store anything, they simply convert one thing to another.
MOST fuel cells don't store anything, some do.