Copper Dissolution in Chloroaluminate Ionic Liquids
Authors: Wang Chen , Hussey Charles L. 
Published: February 2020

The anodic dissolution of metal substrates under convective mass-transport conditions can often lead to considerable improvement of the surface roughness of the substrate through a process known as electropolishing. Electropolishing processes, which are a cornerstone of metal surface finishing, are most commonly carried out in aqueous electrolyte solutions, especially mixtures of concentrated sulfuric and/or phosphoric acids, or in the single acids.  However, depending on the metal and conditions used, anodic dissolution in aqueous solutions sometimes results in the formation of unwanted passive films of insoluble salts and/or metal oxides, which limit the efficacy of the electropolishing process.

In some circumstances, ionic liquids or room-temperature molten salts may be suitable replacement electrolytes for the aqueous acids commonly used for electropolishing.  Among these ionic liquids are the organic salt-based chloroaluminates, such as AlCl3-1-ethyl-3-methylimidazolium chloride (EtMeImCl).  Because of their adjustable Lewis acidity, chloroaluminate ionic liquids offer a unique solvation environment for many metals. The Lewis basic AlCl3-EtMeImCl ionic liquids, which contain unbound chloride ion, provide a solvation environment that is not unlike aqueous NaCl, except without the H2O.  On the other hand, the AlCl3-rich Lewis acidic ionic liquids are a very different story because there are no “hard” ligands available to form isolable anionic complexes.  Nevertheless, many metal ions are very soluble in these ionic liquids.

In this investigation, we have focused on the anodic dissolution of copper, not only to assess the utility of acidic and basic chloroaluminates as electrolytes for electropolishing metals, but to understand the kinetics of the electrodissolution of metals in these versatile solvents.  As is well known, copper is vitally important in the electronics industry, especially with regard to the preparation of interconnects for computer chips by electrodeposition.  As with all electrochemical deposition processes, anode dissolution must be considered in the overall cell reaction.  There is also an extensive body of information about the kinetics and mechanism of the anodic dissolution of copper and its alloys under simulated marine conditions, i.e., in aqueous NaCl solutions.  Thus, another goal of this investigation is to compare the dissolution of copper in the chloride-rich ionic liquids with dissolution in aqueous chloride solutions.

In this presentation, we will describe a kinetic analysis of the anodic dissolution of copper in the Lewis acidic and basic compositions of the AlCl3-EtMeImCl ionic liquid.  An example of our research results is given in the figures below.  Figure 1 shows current density-time transits recorded at a copper RDE in the Lewis acidic ionic liquid at different potentials and at a fixed rotation rate.  Clearly, the current density, j, is potential dependent, but independent of time.   Plots of j versus the square root of electrode rotation rate, w1/2, which were constructed from data resulting from a series of experiments similar to those used to produce Fig. 1, are shown in Fig. 2.  According to the Levich equation, for a mass transport-limited process, j should show a linear dependence on w1/2 with the line passing through the origin of the plot.  This is obviously not the case here, indicating that the dissolution process probably proceeds under mixed control.  Figure 3 shows plots of 1/j versus 1/w1/2 based on the Koutecky-Levich equation for a one-electron reaction, 1/j=1/j
k
+  kc
/k
a
[1.61(FD
2/3)-1
ν1/6
] ω-1/2.  These plots clearly show a potential-dependent intercept, indicating a kinetic step in the dissolution process.  The interpretation of this data, as well as that resulting from similar experiments conducted in Lewis basic ionic liquids, will be discussed in this presentation.  

This work was funded by the Strategic Environmental Research and Development Program (SERDP) through contract DE-AC05-00OR22725 to ORNL with subcontract to the University of Mississippi.

Figure 1