Answer
For unlike charges, the electric potential energy is negative.
For like charges, the electric potential energy is positive.
Work Step by Step
Assume the potential energy is zero when the charges are infinitely far apart. The electric potential energy of two unlike charges is negative because it would take positive work to separate them and bring the system to zero potential energy.
The electric potential energy of two like charges is positive. Positive work must be done to cram the charges together, because they repel each other.
