At pH> 4.00, the electrodeposition of cobalt occurs via Co(OH)2 forming in the interface electrode solution (chemicalstage). In the cobalt electrodeposition process the interface electrode solution becomes alkaline due to the water reduction (Eq(5)). The local alkalization that occurs in the interface electrode solution can provoke the precipitation of the Co(OH)2 as showed in Pourbaix diagram (Fig. 1). The presence of cobalt hydroxide was confirmed with an electrochemistry quartz crystalmicrobalance (EQCM) technique, as done by Matsushima et al. [11].H3BO3 was added to the cobalt electrodeposition solution to avoid pH variations in the interface electrode solution. In this case,the electrodeposition of cobalt occurs directly, owing to Eq. (1) [12]. Eqs. (5)–(8) describe the electrodeposition process through formation of a cobalt hydroxide intermediate:
The EQCM technique supplies detailed information about variations in electrodeposition and electrodissolution mass for fine
films, as caused by oxidation and reduction processes [13–20].According to Sauerbrey’s equation, frequency variation (f)ofthe quartz crystal can be correlated with the mass variation (m) and can be written according to Eq. (9):
where f0 is the resonance frequency of the quartz crystal, A is the piezoelectric active area, i is the quartz shear modulus, K is the experimental mass coefficient, and i is the density of quartz.The current work is a continuation of previous work that studied the electrochemical recycling of cobalt fromthe spent cathodes of Li-ion batteries [8]. In this work, a greater efficiency for cobalt electrodepositionwas found at a potential equal to−1.00V at all pH values tested (1.50, 2.70, and 5.40). It was observed that the charge efficiency of cobalt electrodeposition decreased with a decrease in the pH of the solution. The cobalt nucleation process was investigatedwith the help ofmathematicalmodels proposed by Scharifker and Hills [21]. In the present work, the electrodeposition mechanism of cobalt thin films has been studied. For this reason the electrochemical quartz crystal microbalance technique was used together with potentiodynamic and potentiostatic techniques to obtain information about the electrodeposition mechanism for cobalt from the cathodes of spent Li-ion batteries. This technique is very effective in the study of the mechanism of metal electrodeposition for thin films. However, deviations from the Sauerbrey’s equation occur for thick films. The study of the electrodeposition mechanism is key in electrochemical recycling because it correlates with the structure, morphology, and properties of the cobaltfilm. The current work aims to clarify the cobalt electrodeposition mechanismas a function of pH. It follows then that the study of the cobalt electrodeposition process is of paramount importance in the electrochemical recycling of cobalt.