Following the Hodge decomposition of regular vector fields we can decompose the second member of any Liénard system into 2 (non-unique) polynomials, the first corresponding to potential and the second to Hamiltonian dynamics. This polynomial Hodge decomposition is called potential-Hamiltonian, denoted PH-decomposition, and we give it for any polynomial differential system of dimension 2. We will give in a future Note an algorithm expliciting the PH-decomposition in the neighborhood of particular orbits, like a limit-cycle for Liénard systems, the method being applicable for any polynomial differential system of dimension 2.