Effect of water on copper electrodeposition from water-containing deep eutectic solvents

Student thesis: Doctoral Thesis

Abstract

Metal electrodeposition is mostly based on aqueous systems which sometimes require the addition of chemicals that can be hazardous for the environment. Due to growing environmental regulations, Ionic Liquids (ILs) have been investigated as an alternative electrochemical media for this process. Amongst a variety of ILs, deep eutectic solvents (DESs) formulated from choline chloride and hydrogen bond donors have exhibited the most promising characteristics to be exploitable at large scale. Some of these characteristics are the tolerance to water, low volatility and toxicity, and reasonable cost.Despite these properties, DESs are viscous and the diffusivity of electroactive species is correspondingly low. This imposes mass transport constraints reflected in low deposition rates compared to either high-temperature ILs or aqueous systems. This study therefore examined if the use of water addition alleviated these problems and investigated the effect of water content on various aspects of metal electrodeposition from hydrated DES systems.'Ethaline' was the DES focus of this study since it exhibits one of the highest conductivity and lowest viscosity amongst DESs. Cu electroplating had been particularly common as it is used extensively in the electronics and microelectronic industry. Since there is a large body of existing studies on Cu electrodeposition from both ILs and aqueous systems, it can be used to benchmark metal electrodeposition from hydrated DES systems. Thereby, the 'electrolyte' used in this work comprised of the cupric chloride salt (0.2 M CuCl2*2H2O) dissolved in ethaline.Since ethaline is a hygroscopic substance, the ability of the electrolyte to incorporate water from the atmosphere was quantified using Karl Fischer titration. It was found that the electrolyte could incorporate water from 3 to 15 wt% over a month. Using these water amounts, the speciation of Cu (II) complexes, viscosity and conductivity were investigated. The behaviour of Cu (II) reduction from ethaline and the electrochemical window of ethaline were examined as a function of water content.Thereafter, Cu electrodeposition was performed from water-containing electrolytes using steel disks. The uniformity, morphology and microstructure features of Cu electrodeposits were assessed using SEM, EBSD and XRD techniques. One of the key technical issues to be resolved before practical DES-based plating systems can be deployed is the characterisation of the anode reaction. Therefore, the anodic reaction occurring at the soluble copper anode during the electrodeposition of Cu from the water-containing electrolytes was also studied.The diffusion limiting currents for Cu (II) reduction from ethaline were found to increase three times as water content increased from 1 to 15 wt%, e.g. from 6 to 18.5 mA cm-2. This occurred with minimal influence on the speciation of Cu (II) complexes, electrochemical window and deposit characteristics. This finding could be mostly attributed to the improved diffusivity of electroactive species brought about by water.This was therefore a beneficial effect since faster plating rates were achievable from hydrated DES systems than that of low water-containing ones. Cu electrodeposits were found to exhibit poor homogeneity despite the fact that increasing amounts of water increased the conductivity of the medium. Importantly, the addition of water was not detrimental to the current efficiency of the process, microstructure and purity of the Cu coatings.The analysis of the anodic reaction using a Cu soluble anode showed that the dominant anodic species were the Cu (I) complexes whereas the solution was depleted of the higher valency species, Cu (II) complexes. In the electrolyte, the apparent dissolution valency (n') was typically less than unity, corresponding to an observed mass loss greater than that expected from Faraday's law. Moreove
Date of Award1 Oct 2017
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorSudipta Roy (Supervisor) & Todd Green (Supervisor)

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