Electrochemical assessment of high active area of cobalt deposited in deep eutectic solvent

Abstract

• Green electrodeposition of nanostructured Co films using deep eutectic solvent (DES). • Influence of substrate and potential conditions on the morphology and composition of Co films. • Use of outer sphere electron transfer redox pairs to assess the increase of the active area of Co films. • The validity of 1,4-benzoquinone and hexaammineruthenium(III) redox pairs probes is tested. • Lead underpotential deposition on Co films covered by gold to quantify the surface active area. Preparation of nanostructured materials with highly efficient active area is crucial for advancement in fields of electrocatalytic conversion of energy, energy storage, or biomedicine. In this regard, there are two main challenges in the electrochemical preparation of high-extended surface area of the nanostructures: 1) establishing the optimal potential conditions to enhance the active area while reducing the size of the prepared material and, 2) simultaneously, finding a practice and simple in situ methodology to qualitatively test the increase in the active surface area. Herein, we investigate the cobalt electrodeposition in choline chloride plus urea deep eutectic solvent (DES) and the effect of the DES in the increase of the active surface area of deposited Co films. In situ classical electrochemical techniques, in combination with ex situ characterization techniques, were employed to establish the best potential conditions to obtain metallic Co films with high active surface area. Then, the increase in active area was tested by using different voltammetric methodologies. The increase in intensity of the outer sphere electron transfer was tested as a tool to analyze the superficial change after cobalt deposition. Voltammetric response in organic medium using benzoquinone on Co films and in an aqueous medium using Ru(NH 3 ) 6 , after covering the Co films with gold via galvanic displacement, was assessed. Later, as alternative method, the active area of the freshly deposited Co films, after superficial modification with Au, was tested by lead underpotential deposition. Lead underpotential deposition on gold is a surface-structure process that indirectly allows the estimation of the increment in active area of prepared Co films.