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2.1 Reactivity of High-Temperature Superconductors

The reactive nature of high-temperature superconductors limits their use in performing common electrochemical procedures.  Here, it is particularly important to utilize non-aqueous environments where exposure to trace amounts of water is avoided.  For example, the prototypical high-temperature superconductor, YBa₂Cu₃O_7-x, has been found⁴ to corrode readily in water by the following reaction: 

An important by-product of corrosion according to the equation above is BaCO₃. This thermodynamically stable product can precipitate onto the surface of the electrode when water is present forming an insulating barrier that prevents faradaic electron transfer.  Acidic solutions are also known to be detrimental to most high-temperature superconductors.  Concentrated acid solutions readily dissolve ceramic pellets of materials such as YBa₂Cu₃O_7-x, La_1.85Sr_0.15CuO₄, and Bi₂Sr₂CaCu₂O₈.  Upon dissolving YBa₂Cu₃O₇ in an aqueous acidic environment,⁵ the metal ions that constitute the lattice are liberated to the solution according to:

Other common reagents that are known to react with YBa₂Cu₃O_7-x are CO and CO₂.  The availability of such species in the atmosphere places severe restrictions on the environmental conditions in which one can perform electrochemical measurements similar to that of noble metal electrodes.  This electrochemical sensitivity to corrosion has been utilized to measure the relative reactivity of various high-temperature superconductors.^6-8  A schematic description of what happens electrochemically upon the formation of an insulating barrier is shown in Illustration 2.1.  An example of this can be seen in Figure 2.1.  Measuring the peak splitting of the oxidation and reduction of the solution redox species was used to evaluate the surface reactivity in several environments.  As a control, a dry acetonitrile solution of 2.5 mM 7,7’,8,8’-tetracyanoquino dimethane (TCNQ) in 0.1M tetraethylammonium tetrafluoroborate (Et₄NBF₄) was used.  Then, 4.5% water (by volume) was added to the same solution.  Additionally, an aqueous solution containing 2.0 mM K₃Fe(CN)₆ with 0.1 M NaClO₄ at pH=5 was used.  This information combined with x-ray powder diffraction and scanning electron micrographs has yielded the following reactivity trend:

The most corrosion resistant cuprates were Nd_1.85Ce_0.15CuO₄ and Nd_1.85Th_0.15CuO₄.  Unlike the other cuprates, which have copper valences above 2.0, these materials have copper valences of less than 2.0.  This issue is believed to be a major reason for the stability of the materials. 

Illustration 2.1:     Electron transfer and corresponding voltammagrams for a pristine high-temperature superconductor electrode surface and the same electrode surface coated by a thin film of insulator. a) Rapid electron transfer occurs between a solution dissolved redox couple and a pristine high-temperature superconductor electrode surface.  The corresponding voltammagram exhibits peak splitting values, DE_p, close to 59 mV for this one electron redox couple.⁹  b) Sluggish electron transfer occurs when thin insulating layers collect on the surface of the high-temperature superconductor electrode.  The corresponding voltammagram has DE_p values much greater than 59 mV. (Adapted from reference³)

Figure 2.1:     Cyclic voltammetry at 100 mV/s for a 2.5 mM TCNQ solution recorded at:  (A) YBa₂Cu₃O_7-x  electrode in dry CH₃CN / 0.1 M Et₄NBF₄; (B) same as (A) with 4.5% water (v/v) added to the electrolytic solution;  (C) La_1.85Sr_0.15CuO₄ electrode in dry CH₃CN / 0.1 M Et₄NBF₄;  (D) same as (C) with 4.5% water (v/v) added to the electrolytic solution; (E)  Nd_1.85Ce_0.15CuO₄ electrode in dry CH₃CN / 0.1 M Et₄NBF₄;  (F) same as (E) with 4.5% water (v/v) added to the electrolytic solution.   All markers are equal to 10 mA.  (Adapted from reference³)

All measurements place YBa₂Cu₃O_7-x as the most reactive of the common cuprate phases.  The high reactivity of YBa₂Cu₃O_7-x makes it a challenge to exploit this material in the context of electrochemical measurements.

The lattice structures for YBa₂Cu₃O_7-x and related cuprate compounds possess two dimensional attributes in which high electrical conductivity exists within the a-b plane and poor conductivity is observed along the c-axis.  There are four types of layers in YBa₂Cu₃O_7-x:  Cu(2)-O, Cu(1)-O, Ba-O, oxygen deficient Y layer.  Each of these layers is receptive to cationic substitution. Some of these compositions have been found to be resistant to corrosion while retaining a relatively high transition temperature.  One example is Y_0.6Ca_0.4Ba_1.6La_0.4Cu₃O_7-x which has a transition temperature of 83 K and has been found^{10, 11} to be approximately 100 times more stable than its parent compound.