If the current in a source of emf is in the direction of emf, then the energy of the source

In the external circuit, If current ( + ve charge) flows from higher potential to lower potential, to maintain continuous flow, the same the charge has to flow from low potential to  higher potential. Inside the electric cell to do this, the cell must be able to do work on the charge. The energy to do this work is derived from the chemical process inside the cell. Hence the energy of cell decreases.