Answer
The potential energy is positive for repelling particles it's maximum value if they are right next to each other.
Work Step by Step
So, Now the force act to separate them and as they are separated the potential energy decrease and approaches 0 as the distance approaches infinity.
If the particles attract each other then the potential energy is zero or will be an infinite distance between them and as the distance decrease, the potential energy has to decrease, i.e to be negative because the force always acts in the direction of the potential energy decrease .
