Weida Shen

Improved ionic conductivity in strained yttria-stabilized zirconia thin films

Yttria-stabilized zirconia (YSZ) thin films with thickness ranging from 6 nm to 100 nm were prepared by RF sputtering on (0001) Al2O3 substrates and exhibited epitaxial growth along (111)[110] YSZ//(0001)[101¯0] Al2O3. While the thicker films exhibited oxygen ion conductivities similar to bulk samples, the thinnest films exhibited increased ionic conductivity and a reduced activation energy of 0.79 eV between 300 °C–650 °C. Concomitant with the improved conductivity of the thinner films is an increase in the out-of-plane lattice parameter, matching theoretical expectations regarding tensile strain, and the introduction of edge dislocations, which may additionally assist in-plane ionic conduction.

Reduced ionic conductivity in biaxially compressed ceria

Interfacial lattice mismatch strain has been controversially suggested as a means to alter ionic conductivity in solid ion conductors. Here, thin film multilayers composed of yttria-doped ceria (YDC) and Ce1−xZrxO2 (CZO) allow systematic quantification of the effect of biaxial compressive strain on oxygen ion conductivity in ceria. Since the lattice parameter of CZO is highly dependent on the Ce/Zr atomic ratio, its use enables precise control of the strain magnitude in neighboring lattice planes of YDC. Three series of multilayers were fabricated using Ce0.55Zr0.45O2 (CZO45), Ce0.70Zr0.30O2 (CZO30), and CeO2, with an interfacial lattice mismatch of −2.2%, −1.5%, and near-zero, respectively. The compressive strain in the YDC layers caused fairly drastic reductions in the ionic conductivity. Each 1% increase in compressive strain in the YDC yields a 1.6-fold reduction in interfacial conductivity at 650 °C and a 3-fold reduction at 450 °C. Other interfacial effects, however, were also found to have significant impact on the ionic conduction.

Ionic conductivity of YSZ/CZO multilayers with variable lattice mismatch

Heterostructured multilayers have been controversially reported to alter the oxygen ion conductivity of solid electrolytes by inducing interfacial mechanical strain. Here, we fabricated thin film multilayers composed of 9 mol% Y2O3 doped ZrO2 (YSZ) and Ce1−xZrxO2 (CZO) to systematically quantify the effects of tensile strain on the oxygen-ion conduction behavior in YSZ. A significant advantage of using CZO is that its lattice parameter can be continuously varied by adjusting the Ce/Zr atomic ratio, simplifying the strain control over the neighboring YSZ layers. Three different sets of multilayers composed of YSZ with CeO2, or with Ce0.70Zr0.30O2 (CZO30), or with Ce0.55Zr0.45O2 (CZO45) were prepared on Al2O3 substrates with interfacial lattice mismatch of +5.2%, +3.7%, and +2.9%, respectively. When decreasing the individual layer thicknesses from 35 nm to 5 nm, all of the multilayers exhibited little change of the conductivity, with values consistently near that of bulk YSZ. X-ray diffraction results indicate that the interfacial strains were largely relaxed. Suggestions that multilayers are unable to effect ionic conductivity changes must therefore consider the difficulties in obtaining lattice mismatch-based elastic strain, even at <3% mismatch.

Non-Flooding Hybrid Polymer Fuel Cell

One of the chief difficulties associated with low temperature (<100°C) fuel cells is water management. Specifically, nucleation of liquid water droplets in the electrodes (i.e., flooding) causes a drop in power output as reactants are prevented from arriving at the electrodes. Here, we describe first attempts at a hybrid polymer fuel cell that prevents flooding. A proton exchange membrane and an alkaline exchange membrane are placed on opposite sides of a porous, water-soaked layer. Water formed in the porous layer is shunted to the exterior of the fuel cell. Though the design is promising, preliminary attempts have shown that the electrolytic resistance is too high for practical use.

Using thin films to investigate heterogeneous defect chemistry

Prof. Nowick showed that the controlling mechanism of ion conduction in traditional solid electrolytes can be effectively summarized with a model built on defect chemistry within an infinite crystalline lattice. A common assumption in these models has been that, at least at low concentrations, dopants are randomly scattered within their particular sublattice. More recently, experimental tools have established means to create mesoscale compositional heterogeneity. This capability allows significant extension of the experimental space beyond what can be captured with the traditional models. Here, we survey recent experimentation that uses a sputtering technique to create films with composition Ce1-x-zZrxDzO2-z/2 (D=Y, Gd, or La) where x or z can vary through the thickness of the film at the single nanometer level. These films are used to study 1) the effect of lattice mismatch strain by modulating the Ce/Zr ratio in multilayers and 2) the effects of vacancies being trapped within planar space charge regions by locating the dopant atoms as 2-D sheets within otherwise pure CeO2 films. The films are likely to be metastable, but maintain compositional heterogeneity over experimental time scales. Current results and future possibilities for this technique are discussed.