A well characterized and predictable aging pattern is necessary for practical energy storage applications of nanoporous particles that facilitate rapid transport of ions or redox species. Here we use STEM tomography with segmentation to show that surface diffusion and grain boundary diffusion are responsible for pore evolution at intermediate and higher temperatures, respectively. This unprecedented three dimensional understanding of pore behavior as a function of temperature suggests routes for optimizing pore stability in future energy storage materials.

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Nano-structured palladium is examined as a tritium storage material with the potential to release beta-decay-generated helium at the generation rate, thereby mitigating the aging effects produced by enlarging He bubbles. Helium retention in proposed structures is modeled by adapting the Sandia Bubble Evolution model to nano-dimensional material. The model shows that even with ligament dimensions of 6-12 nm, elevated temperatures will be required for low He retention. Two nanomaterial synthesis pathways were explored: de-alloying and surfactant templating. For de-alloying, PdAg alloys with piranha etchants appeared likely to generate the desired morphology with some additional development effort. Nano-structured 50 nm Pd particles with 2-3 mn pores were successfully produced by surfactant templating using PdCl salts and an oligo(ethylene oxide) hexadecyl ether surfactant. Tests were performed on this material to investigate processes for removing residual pore fluids and to examine the thermal stability of pores. A tritium manifold was fabricated to measure the early He release behavior of this and Pd black material and is installed in the Tritium Science Station glove box at LLNL. Pressurecomposition isotherms and particle sizes of a comercial Pd black were measured.

With the increasing interest and usage of alternative energy sources, the need for reliable and efficient energy storage methods is likewise increasing. Porous nanoparticles, and in particular Pd, are being investigated for their potential use in catalysis, hydrogen storage, and electrochemistry [1]. For all of these applications, a very high surface area is desirable, with every point in the material ideally being within a few atoms of an interface. This would facilitate attributes such as high double-layer capacitance, higher reaction rates in kinetically limited interfacial reactions, and rapid charging with hydrogen [2,3]. However, in order to ensure the reliable performance of these materials for such applications, the pore connectivity, diffusion, migration, and collapse must be understood for a variety of thermal treatments. As 2-D images only provide a projection of the structure of the nanoparticles, 3-D imaging is necessary to completely describe the complex pore structure and its age-dependent evolution.Here we investigate porous nanoparticles of Pd using a combination of STEM tomography and in-situ heating experiments. Our previous results show that the pore structure begins to change as low as 100°C, and complete pore collapse occurs at 400°C. The particles in this study are heated to various temperatures and then cooled, thus "freezing in" the pore structure. This pore structure is then studied with STEM tomography and reconstructed in 3-D. Figure 1 shows results from the untreated sample used as the control. Figure 1(a) is one of 141 STEM images illustrating that porosity is visible. Figure 1(b) is a 1nm slice through the reconstruction showing that the porosity is very complex, with some pores connecting within the slice, some pores going through the slice, and some pores going to the surface. Figures 1(c) and (d) show volume and surface renders, respectively, further illustrating that the pores go through the particles and to the surfaces. Results will be presented from the heat treated samples comparing the 3-D pore structure to these control results [4].