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Herein we report the synthesis of efficient Co 3 O 4 electrocatalysts toward the oxygen evolution reaction (OER) in alkaline media. Co 3 O 4 films were obtained by thermal treatment of Co electrodeposited from electrolytic solutions containing glycine at different pH values. Co 3 O 4 films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The grain size of the electrocatalysts decreases to 100 nm and the oxygen vacancies content increases, as the pH of the solution was increased during the synthesis. Linear Sweep voltammetry (LSV) and Chronopotentiometry techniques were employed to evaluate the electrocatalytic activity of the films. Co 3 O 4 electrocatalysts obtained at pH values higher than 7 exhibit a better electrocatalytic performance than the obtained at pH values less than 4, such as a potential to reach 1.5 mAcm −2 of 0.6 V and 0.68 mV, respectively. Co 3 O 4 synthesized under alkaline conditions exhibit the best charge transfer resistance with 4.3 during OER electrolysis. The improvement is associated to the presence of oxygen vacancies that promote the activity and stability of the electrocatalysts toward the OER. The electrochemical oxygen evolution reaction in alkaline media (OER:plays a key role in the efficient hydrogen production and solar refinery process. The OER is very important in several electrochemical technologies such as: water electrolyzers, solar water-splitting devices and rechargeable Li-Air Batteries.1-3 The OER involves a four electron transfer process and it is more challenging due to the slow kinetics, the high over-potentials and the low catalytic stability of the electrocatalysts. 4,5 It is well known that the efficient OER electrocatalysts such as Pt, Ir or Ru introduce high cost barriers to larger-scale applications because contain precious metals 6,7 however, electrocatalysts composed of earth-abundant and inexpensive materials are a promising alternative.Recently, the investigation and development of electrocatalysts for OER is based on the use of transition metals or their compounds, such as cobalt, nickel, iron, copper, zinc and manganese. [8][9][10][11] Among all electrocatalysts reported the cobalt-containing oxides have been investigated due to their high performance for the OER.12-23 On the other hand, cobalt oxides have been used in batteries and supercapacitive applications 24-26 as well as gas sensors. 27Different paths have been investigated in order to enhance the electrocatalytic activity of the Co 3 O 4 such as: the development of new morphologies, 19,28 selection of different substrates 18 and the development of composite materials which include the use of graphene. 3,23One of the recent approaches to increase the catalytic activity of single metal oxide catalysts is defect engineering by inducing impurities such as oxygen vacancies in metal oxides. In literature [29][30][31][32] it can be found that oxygen vacancies improve electrical conductivity, optical and reductive properties and ...

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