Joshua L. Hertz

Fabrication and structural characterization of self-supporting electrolyte membranes for a micro solid-oxide fuel cell

Micromachined fuel cells are among a class of microscale devices being explored for portable power generation. In this paper, we report processing and geometric design criteria for the fabrication of free-standing electrolyte membranes for microscale solid-oxide fuel cells. Submicron, dense, nanocrystalline yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) films were deposited onto silicon nitride membranes using electron-beam evaporation and sputter deposition. Selective silicon nitride removal leads to free-standing, square, electrolyte membranes with side dimensions as large as 1025 µm for YSZ and 525 µm for GDC, with high processing yields for YSZ. Residual stresses are tensile (+85 to +235 MPa) and compressive (–865 to -155 MPa) in as-deposited evaporated and sputtered films, respectively. Tensile evaporated films fail via brittle fracture during annealing at temperatures below 773 K; thermal limitations are dependent on the film thickness to membrane size aspect ratio. Sputtered films with compressive residual stresses show superior mechanical and thermal stability than evaporated films. Sputtered 1025-µm membranes survive annealing at 773 K, which leads to the generation of tensile stresses and brittle fracture at elevated temperatures (923 K).

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.

Nanocomposite Platinum–Yttria Stabilized Zirconia Electrode and Implications for Micro-SOFC Operation

Thin-film composite electrodes with nanometric grains of platinum and yttria-stabilized zirconia (YSZ) were reactively sputtered onto single-crystal yttria-stabilized zirconia. The activation polarization resistance exhibited an activation energy of 1.3-1.5 eV and was found to have an approximate inverse dependence on microelectrode radius squared, consistent with, effectively, a mixed ionic-electronic conductive nature. Area specific polarization resistances of less than 500 Ω cm 2 were achieved at 400°C in a dense thin-film electrode. The attractiveness of these nanocomposite electrodes for use in microfabricated solid oxide fuel cells (SOFCs), of interest as portable power sources, is discussed.

Packing and Size Determination of Colloidal Nanoclusters

Here we demonstrate a rapid and quantitative means to characterize the size and packing structure of small clusters of nanoparticles in colloidal suspension. Clustering and aggregation play important roles in a wide variety of phenomena of both scientific and technical importance, yet characterizing the packing of nanoparticles within small clusters and predicting their aerodynamic size remains challenging because available techniques can lack adequate resolution and sensitivity for clusters smaller than 100 nm (optical techniques), perturb the packing arrangement (electron microscopies), or provide only an ensemble average (light scattering techniques). In this article, we use electrospray-differential mobility analysis (ES-DMA), a technique that exerts electrical and drag forces on the clusters, to determine the size and packing of small clusters. We provide an analytical model to determine the mobility size of various packing geometries based on the projected area of the clusters. Data for clusters aggregated from nominally 10 nm gold particles and nonenveloped viruses of various sizes show good agreement between measured and predicted cluster sizes for close-packed spheres.

On the variability of reported ionic conductivity in nanoscale YSZ thin films

Yttria-stabilized zirconia (YSZ) is the most common material used as a solid oxide electrolyte, which is a key component in solid oxide fuel cells, solid oxide electrolysis cells, and certain chemical sensors. High efficiency in these devices requires increased oxygen ion conductance at intermediate temperatures. Nanoscale YSZ thin films are quite promising in this regard, as the conduction path may be reduced below what is conventionally achievable. Still, as the thickness is decreased to nanoscale, structural properties like lattice parameter and grain morphology are typically altered. These may affect the electrochemical properties in non-trivial ways. Recent reports on nanoscale YSZ thin films have provided inconsistent and, at times, controversial results. In this paper, we present a review of reports on nanoscale YSZ thin films, focusing principally on single component YSZ films as opposed to heterogeneous multilayer films. Reports of significantly increased conductivity come from studies that use a variety of substrates, grain morphologies, and, to some extent, film thicknesses. Mechanical strain in the films is not typically reported but is a suspected cause of the variability in conductivity.

Nonlinear thermomechanical design of microfabricated thin plate devices in the post-buckling regime

A design approach for thermomechanically stable sub-micron plates is developed utilizing the post-buckling regime via a nonlinear plate analysis. Based on the analysis results and experimental observations, local stresses are observed to have maxima in the near-post-bifurcation regime, but then to decrease significantly in the post-buckling regime. This effect is more significant with plates of larger sidelength and smaller thickness structures, enabling microfabrication of numerous plate and membrane structures that are typically considered susceptible to failure due to buckling. Using a stress-based failure criterion, rather than the typical buckling criterion, an expanded design space for thin plates beyond the traditional pre-buckling regime is revealed. A device class that benefits in both power and efficiency from thin, large-area freestanding plates is microfabricated fuel cells, particularly high-temperature solid oxide fuel cells (µSOFCs). As a demonstration of the expanded design space, µSOFCs of submicron (450 nm) layer thickness are designed, fabricated and operated in the far postbuckled regime, verifying thermomechanical stability (up to 625 °C) and functional operation. The design approach introduced here can be applied to a range of microfabricated devices such as purification membranes, electrolysis cells and biochemical sensors.

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.

Extended Poisson-Nernst-Planck modeling of membrane blockage via insoluble reaction products

A generalized time-dependent mathematical model is developed for a diffusion–migration–reaction system incorporating a pore blockage effect due to generation of insoluble precipitates in a porous membrane. The system behavior is investigated via direct numerical solution of an extended, highly non-linear equation set based on the classical Poisson–Nernst–Planck equations for ion transport. In order to treat the buildup of solid reaction products in the membrane, this novel formulation incorporates both a reaction term and a space- and time-dependent diffusivity expression based on a simple precipitation model. The model is demonstrated for a generalized case and then extended to cover the well-known reaction of silver and chloride ions to form insoluble AgCl. Time-dependent concentration profiles of all ions in the membrane are obtained and the effects of precipitate buildup in the pore space are investigated. The role of counterions in the transient behavior of the system is also clarified.

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.