Ionization energy is defined as the minimum energy required to remove an electron from its ground state ($n=1$) to a point of 'infinite separation' from the nucleus (where $n\to \infty$). At infinity, the potential energy is zero and the electron is no longer bound. For hydrogen, this energy is $E_{\infty }-E_1=0-(-13.6\text{eV})=13.6\text{eV}$.