The $E^{\circ }\left({\mathrm{M}}^{2+}\mid \mathrm{M}\right)$ value for copper is positive(+ 0.34 V). What can be a reason for this?

Currently, copper has a high atomization energy and a low hydration energy. Consequently, copper's $E^{\circ }\left({\mathrm{M}}^{2+}\mid \mathrm{M}\right)$ value is positive.

The bonds between the transition metals must thus be broken using a lot of energy. Thus, transition metals have a greater enthalpy of atomization. As a result, the high effective nuclear charge and many valence electrons in transition metals contribute to their high enthalpies of atomization.

The hydration enthalpy will be larger for smaller ions because they can fit more water molecules around them and get hydrated. Down the group, the hydration enthalpy falls, and the atom's size grows as a result of the more valence shells.