recombination are slowed down. We assign this effect to a variation in OH -ion concentration in the inner Helmholtz plane (IHP) of the electrochemical double layer with applied potential, showing that ion adsorption and desorption onto the NiO surface play an essential role as a relay in light-induced charge transfer and recombination. Our work highlights the key role of ions at the electrode surface and in the electrolyte in the realization of efficient solar to fuel devices.