Although the amount of oxygen evolution electrocatalyst is a factor determining its efficiency, its fundamental correlation with activity remains unclear. To address this issue, we take advantage of a urea-based chemical bath deposition method (CBD) that enables to control the amount of electrocatalyst (Fe(OH)2 and α-Co(OH)2) deposited on conducting glass. The thickness of the resulting films, whose use in electrocatalysis is unprecedented, is tuned by controlling the deposition time. The turnover frequency (TOF) for O2 generation decreases drastically as the electrocatalyst amount increases from equivalent coverages of 3.5 monolayers (ML) for Fe(OH)2 and of 0.06 ML for α-Co(OH)2 electrodes. The contrasting behavior of both hydroxides comes from the different structure of the incipient deposits, formed by small acicular nanoparticles in the case of Fe(OH)2 and larger flat microparticles in the case of α-Co(OH)2. The former structure allows a large fraction of the Fe sites to be in direct contact with solution, while such a fraction rapidly diminishes with loading for α-Co(OH)2. In addition, the resulting Co(OH)2 electrodes show TOFs similar or higher than those of electrodes prepared by more complex routes. The optimum ultrathin films are remarkably stable in alkaline media, showing that the preparation of efficient electrocatalysts for oxygen evolution with an extremely small amount of metal through a novel, facile and scalable CBD is possible.Financial support of the Spanish Ministry of Economy and Competitiveness through project MAT2015-71727-R (FONDOS FEDER) is gratefully acknowledged. J. Q. thanks to the Spanish Ministry of Education, Culture, and Sport (MECD) for the award of an FPU predoctoral grant (FPU15/02005)
