The standard electrode potential of M + / M in aqueous solution does not depend on

The stability of ${\mathrm{M}}^{+}$ ions in aqueous medium depends on three factors: (i) Enthalpy of atomisation (ii) First ionisation enthalpies of isolated gaseous atom (iii) Hydration enthalpy. 

An element in ${\mathrm{M}}^{+}$ state in aqueous medium is stabler if the electrode potential ( ${\mathrm{M}}^{+}/\mathrm{M}$) value of more negative. Down the group, the tendency to form ${\mathrm{M}}^{+}$ ion increases. In this process, ionisation of solid metal is not involved in this process.