The specific problem is the 2D semiconductor energy levels move under working photocatalytic or photoelectrochemical conditions, due to both the applied potential (𝐸) and absorption of photons . The phenomenon called band gap renormalization (BGR) involves movement of the semiconductor band edges, resulting in a potential-or light intensity-dependent G 0 ´ that currently remains unknown or illdefined. The magnitude of BGR effects depend on the semiconductor's carrier concentration (𝑛), which changes with applied potential or illumination. BGR effects are enormous in 2D semiconductors (e.g., >360 meV in photo-excited ML-MoS2) due to strong Coulomb interactions, dielectric screening effects, and quantum confinement. In the context of photoelectrochemistry, BGR shifts the semiconductor band edges dramatically upon illuminating or applying a potential to the semiconductor, changing Δ𝐺 ∘′ (see Eqn. 1). Importantly, quantifying the extent to which applied potential, BGR, and other effects, such as illumination, tune the semiconductor band energies in these low dimensional materials remains a largely open question inspiring active research. In addition, the large band edge movement is significant because it violates a key assumption in the field of semiconductor electrochemistry: kET is essentially independent of 𝐸 because the band edges are "fixed". This assumption means ECB and EVB are independent of applied potential (𝐸) and, therefore, Δ𝐺 ∘′ is independent of 𝐸, too. Thus, these assumptions fail for 2D semiconductors. There is a critical need to quantify interfacial energetics of 2D semiconductor/electrolyte interfaces so we can rationally tune the electron transfer kinetics of these systems for solar energy conversion applications.