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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, YBa2Cu3O7-x, has been found4 to corrode readily in water by the following reaction:

2YBa2Cu3O7 + 3H2O => Y2BaCuO5 + 3Ba(OH)2 + 5CuO + 1/2 O2 Eq. 2.1
Ba(OH)2 + CO2 => BaCO3 + H2O Eq. 2.2

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 YBa2Cu3O7-x, La1.85Sr0.15CuO4, 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:

YBa2Cu3O7 + 14H+ => Y+3 + 2Ba+2 + 2Cu+2 + Cu+3 + 7H2O Eq. 2.3

Other common reagents that are known to react with YBa2Cu3O7-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:

YBa2Cu3O7-x > Tl2Ba2Ca2Cu3O10+x > Bi2Sr2CaCu2O8+x >
La1.85Sr0.15CuO4 > Nd1.85Ce0.15CuO4 > Nd1.85Th0.15CuO4

The most corrosion resistant cuprates were Nd1.85Ce0.15CuO4 and Nd1.85Th0.15CuO4. 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.

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, DEp, 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 DEp values much greater than 59 mV.

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, DEp, 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 DEp values much greater than 59 mV. (Adapted from reference3)

Cyclic voltammetry at 100 mV/s for a 2.5 mM TCNQ solution recorded at: (A) YBa2Cu3O7-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) La1.85Sr0.15CuO4 electrode in dry CH3CN / 0.1 M Et4NBF4; (D) same as (C) with 4.5% water (v/v) added to the electrolytic solution; (E) Nd1.85Ce0.15CuO4 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.

Figure 2.1: Cyclic voltammetry at 100 mV/s for a 2.5 mM TCNQ solution recorded at: (A) YBa2Cu3O7-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) La1.85Sr0.15CuO4 electrode in dry CH3CN / 0.1 M Et4NBF4; (D) same as (C) with 4.5% water (v/v) added to the electrolytic solution; (E) Nd1.85Ce0.15CuO4 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 YBa2Cu3O7-x as the most reactive of the common cuprate phases. The high reactivity of YBa2Cu3O7-x makes it a challenge to exploit this material in the context of electrochemical measurements.

The lattice structures for YBa2Cu3O7-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 YBa2Cu3O7-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 Y0.6Ca0.4Ba1.6La0.4Cu3O7-x which has a transition temperature of 83 K and has been found10, 11 to be approximately 100 times more stable than its parent compound.

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Updated on: April 15, 2010 8:26 PM