Electrocatalysis plays a key role in sustainable energy conversion and storage. The grand canonical treatment of electrons, which accounts for the electrochemical potential explicitly, is critical to model at the atomic scale and understand these reactions at electrified interfaces. However, such a grand canonical treatment for electrocatalytic modeling is in practice restricted to a treatment of electronic structure with density functional theory and more accurate methods are in many cases desirable. Here, we develop an original workflow combining the grand canonical treatment of electrons with many-body perturbation theory in its random phase approximation (RPA). Using the