Helicases use the free energy from ATP hydrolysis (ATP + H2O --> ADP + PO4) in order to move directionally along DNA (double stranded and/or single stranded) as well as peel apart the two strands of DNA. How these enzymes couple this chemical reaction to movement of the helicase and strand separation is not completely understood and is an active area of research.
Here's a few recent review papers I talking more about how helicases work and ongoing experimental work trying to tease out their mechanism:
Helicases as said above use ATP to move along the DNA strand, along the way they break hydrogen bonds I think by creating an area of negative electrostatic charge around them...At least that is what my mind is saying, but I may be mistaken