Effect of temperature on electrochemical cells

I have set up an electrochemical cell, where both half cells are 1M of copper sulphate, except one is at room temperature, whilst the other is at a higher temperature like 80'C. I found that the hotter half cell is positive, but I can't explain why, because the half equation favouring the positive half cell is exothermic.

By positive, I presume you mean the anodic cell reaction. This is the dissolution of the metal electrode into solution by creating ions. The reason it is enhanced by an increase in temperature is due to the kinetics and associated thermodynamics of the dissolution reaction. Have a look at the kinetic and thermodynamic equations, not the electrochemical ones.