Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation
Atomic layer deposition (ALD) offers exciting possibilities for controlling the structure and composition of surfaces on the atomic scale in heterogeneous catalysts and solid oxide fuel cell (SOFC) electrodes. However, while ALD procedures and equipment are well developed for applications involving flat surfaces, the conditions required for ALD in porous materials with a large surface area need to be very different. The materials (e.g., rare earths and other functional oxides) that are of
... ) that are of interest for catalytic applications will also be different. For flat surfaces, rapid cycling, enabled by high carrier-gas flow rates, is necessary in order to rapidly grow thicker films. By contrast, ALD films in porous materials rarely need to be more than 1 nm thick. The elimination of diffusion gradients, efficient use of precursors, and ligand removal with less reactive precursors are the major factors that need to be controlled. In this review, criteria will be outlined for the successful use of ALD in porous materials. Examples of opportunities for using ALD to modify heterogeneous catalysts and SOFC electrodes will be given. Solid oxide fuel cells are unmatched in their efficiency when converting chemical energy to electrical energy; however, improvements in the performance and stability of the electrodes are still required for commercial viability. Interestingly, there are many similarities between SOFC (solid oxide fuel cell) electrodes and some heterogeneous catalysts. SOFC electrodes must be porous and are usually composites, with different components acting as catalysts or as ionic and electronic conductors. Since the electrochemical reactions occur at three phase boundary (TPB) sites, which are defined as the sites where the gas phase, the electronic conductor, and the electrolyte all come together, structure is critically important  . The electronically conductive phase of the electrode is often also catalytic, but this is not always true. Examples exist where the addition of a separate catalytic phase can greatly improve performance     . There can also be advantages to coating one of the phases in the electrode. For example, it has been shown that films of CeO 2 or BaO [14, 15] on the Ni in some SOFC anodes can lead to improved tolerance to coking. Similar to the case for heterogeneous catalysts, engineering the structure of the electrode on an atomic scale can be very important. While scientists have been very successful in developing effective heterogeneous catalysts and SOFC electrodes, recent developments in nanotechnology have greatly expanded our ability to control the synthesis of the active sites for these applications. Among the most interesting nanotechnology tools for this purpose is atomic layer deposition. In ALD, the surface that is being modified is first allowed to react with a molecular precursor, after which the ligands of the adsorbed molecular precursor are removed in a separate step. Since the ligands from the adsorbed precursors prevent additional reactions, film growth is limited to no more than one monolayer in this cycle. However, the cycle of precursor exposure and reaction can be repeated to grow films of any desired thickness. Precursor molecules are available for much of the periodic table  ; and the deposition of pure oxides, nitrides, sulfides, and mixed oxides has been demonstrated. Overall, the layer-by-layer nature of ALD gives unprecedented control in synthesizing materials with well-defined composition and structure  .