Potential energy is defined as the negative of the work done by a field on some object when that object moves from one point in the field to another point in the field.
In the case of a field generated from a postively charged source, and a postively charged particle, as the particle moves away from the source, the potential energy decreases. Ignoring the electrical effects related to the moving charged particle, then the total energy of the particle would be constant and equal to potential energy + kinetic energy. The particle would accelerate away from the charged source, losing potential energy and gaining kinetic energy.