Copper sulfide is a common mineral which exhibits varying electrochemical and oxidative behaviors, depending on its crystal structure. These differences can lead to a variety of applications including hydrometallurgy, bioleaching and flotation.
The crystalline solids of copper sulfide have been found to be useful in the production of thermoelectric materials because of their mixed (e-, Cu+) conducting properties and low calcination temperatures required. These properties ease synthesis and reduce the CO2 footprint.
Unlike oxides, which require high calcination temperatures to form, copper sulfides are able to be formed from the elements at much lower temperatures, even in air at room temperature. They also exhibit a high degree of thermal stability and have a wide variety of physical properties such as ionic conductivity, chemical inertness and electrical insulation.
Chalcopyrite, the most common copper sulfide, crystallizes in a tetragonal system with each metal atom coordinated by a S atom. The tetragonal crystal system is a natural form of asymmetry which has been found in a number of other minerals including quartz and pyrite.
The tetragonal structure is associated with low lattice thermal conductivity and good thermoelectric figures of merit at a range of potentials. This results in an ideal model for transition-metal chalcogenide thermoelectric materials.
In order to further understand the effect of the tetragonal structure on diffusion, we measured the resistances Rint, Rpore and Rdep for potentials of -0.80 V/SCE on static electrodes with different tetrahedral sulfur doping (SH-) concentrations. As shown in Figure 2, the values of Rpore decreased for both the pore and deposit at higher potentials, which is consistent with the larger currents observed in the polarization curves.