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Electrochemistry and Electric Grid > Advanced Fuel Cell Systems

Metal Oxide Nanotubes and Photo-Excitation Effects: New Approaches for Low-to-Intermediate Temperature Solid Oxide Fuel Cells

Start Date: July 2007
Status: Completed
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Investigators

Paul C. McIntyre, Materials Science and Engineering, Stanford University; Shriram Ramanathan, School of Engineering and Applied Sciences, Harvard University

Objective

The goal of this project is to enhance the power density and low-temperature efficiency of solid oxide fuel cells manufactured by atomic layer deposition. These enhancements will be achieved by engineering the morphology of the electrolyte at the nanoscale, and by photo-excitation of the membrane surface.

Background

The efficiency of a fuel cell is limited by the loss mechanisms inherent to its operation. The power density of a fuel cell is limited by the area of its electrolyte membrane. These two operational parameters are related by the fact that thinner electrolytes not only limit the resistive losses within the fuel cell, but they also allow the incorporation of more active area into a given stack volume.

Ultra-thin electrolyte membranes for solid oxide fuel cells (SOFCs) must meet at least two criteria: they must be free of pinholes through which gases could leak, and they must be of the correct chemical composition and crystalline structure. Atomic layer deposition (ALD) is capable of making a pinhole-free layer of material two to three orders of magnitude thinner than state-of-the-art electrolyte membranes. Furthermore, this layer is highly conformal to the substrate, even over rough or convolved surfaces. The chemical composition of the membrane is determined by the mix of precursors admitted to the deposition chamber, and it has been shown that the crystalline structure can be controlled through a sequence of deposition steps at different compositions and thicknesses, followed by annealing at a specified temperature.

Electrolyte thickness is not the only parameter that governs efficiency. The nature of the chemical reactions that take place at the surface of the membrane (the “triple phase boundary”) is also responsible for lost work in a fuel cell. Absorption by surface chemical species of photons in the ultraviolet range may reduce the activation energy for these reactions, thereby improving cell efficiency.

Approach

This research into novel structures for SOFCs will begin with a thorough investigation of the properties of ALD-fabricated yttria stabilized zirconia (YSZ) membranes. A combination of in-situ transmission electron microscopy and electron diffraction methods will be used to characterize the evolution of the membranes during the annealing process. The ion-conductivity of the ultra-thin films will be compared to that reported for bulk crystals and ceramics.

A porous electrode layer must be applied to the surface of the electrolyte membrane for the fuel cell to be operational. The use of ALD to form a perovskite mixed electron-ion conductor layer for this purpose will be investigated. Once a perovskite deposition process has been developed, experiments to determine both the oxygen diffusivity through the YSZ and the surface oxygen exchange kinetics on bare YSZ and on perovskite-coated YSZ will be conducted.

Figure 1

Figure 1: Forest of germanium nanowires grown on a silica substrate.

While a relatively modest increase in membrane surface area can be achieved via membrane thickness reduction, the major increase in membrane area proposed by this research will be achieved by forming the electrolyte as a series of YSZ nanotubes instead of planar YSZ sheets. The nanotubes will be manufactured by growing a forest of germanium nanowires on a silica substrate (Figure 1).

This forest will act as a template, and the YSZ layer will be grown atop the germanium. The initial germanium nanowire outer diameter will thus define the inner diameter of the final metal oxide nanotubes. ALD-coated nanowires will then be released from the substrate surface by selective wet or dry etching of the underlying silica (Figure 2).

Figure 2

Figure 2: (a) Schematic of a nanotube array grown on silica with some of the
substrate etched away, and (b) detail of a single YSZ nanotube.

The current/voltage performance of YSZ nanotube membranes as fuel cells will be characterized to determine cell efficiency. Impedance spectroscopy will be used to analyze the electrode and electrolyte contributions to the total impedance of both planar and nanotube ALD-YSZ membranes. Arrays of nanotube ALD-YSZ samples will be fabricated with varying catalyst/electrode materials to find the optimum electrode formulation.

The effects of photo-excitation of the membrane-electrode assembly will be studied near the end of the project. By suitably altering the wavelength of light, the band structure effects on ion conductivity and catalytic effects on surface reaction kinetics will be investigated.