Ordered and Ultra-High Aspect Ratio Nanocapillary Arrays As a Model System

Nanocapillary arrays are attractive structures for many applications due to the relative ease and scalability of the self-assembly process of their formation. The high surface area-to-volume ratio of these structures can benefit a wide range of energy technologies such as photovoltaics, electrochemical capacitors and batteries, as well as a range of chemical technologies such as separations, storage and catalyst scaffolding. We are currently demonstrating their utility for high density storage of gases such as hydrogen and oxygen.¹ Their high aspect ratio and ordered arrangement is advantageous for low-cost, bottom-up, templated nanostructure growth and ordered assembly at the device scale.² Assembly of nanostructures the device scale in the absence of a templating structure, while maintaining the benefits of a nanomaterial, is often the dominant technical hurdle for implementation of nanomaterials into technologies. In previous work we have used AAO templating to maintain nanostructure benefits for photoelectrochemical cells with areas exceeding several square centimeters.^{3, 4} Furthermore, the confined radial dimension of nanocapillaries can be used to synthesize molecularly confined or form quantum confined nanostructures.  We have shown these effects benefit to improve double layer capacitance, as well as improving the figure of merit in nanostructured thermoelectrics.

Specifically of interest in this discussion is the formation and deep pore growth of anodized aluminum oxide (AAO)-based nanocapillary arrays as the basis for high density, safe and high rate gas storage devices. The target is to grow these ordered nanocapillaries structures to centimeters in length while maintaining a uniform 100 nm  nanocapillary diameter and an overall structure that is 100’s cm² in area. In order to produce these materials quickly, a hard anodization approach is used. Probing the limits of the fabrication has highlighted a fascinating system of interdependent length scales, transport and thermal processes, and current-potential distributions. Potentiostatic and linear sweep potentiometry during deep nanocapillary growth will be presented. Electrochemical impedance spectroscopy (EIS) of the electrolyte and within oxide barrier layer will be discussed; particularly the constant phase element dispersion behavior during deep nanocapillary growth that exemplifies this as a model electrochemical system for porous electrodes. Particularly the EIS response of the system during nanopore growth and its implications of growth mechanism and modes of failure will be presented. A discussion of the implementation of the experimental design and other factors will be discussed elsewhere.

1.            Schwartz, N.; Chester, G.; Hill, J. J., Hierarchically Structured Nanomaterials for High Density Gas Storage and Compression. In 2013 AICHE Annual Meeting, AICHE Proceedings: San Francisco, 2013; pp 1-9.

2.            Hill, J. J.; Haller, K.; Gelfand, B.; Ziegler, K. J., Eliminating Capillary Coalescence of Nanowire Arrays with Applied Electric Fields. ACS Appl. Mat. Int. 2010, 2 (7), 1992-1998.

3.            Hill, J. J.; Banks, N.; Haller, K.; Orazem, M. E.; Ziegler, J., An Interfacial and Bulk Charge Transport Model for Dye-Sensitized Solar Cells Based on Photoanodes Consisting of Core-Shell Nanowire Arrays. J. Am. Chem. Soc. 2011, 133 (46), 18663-18672.

4.            Hill, J. J.; Haller, K.; Ge, W.; Banks, N.; Ziegler, K. J., Conductive Nanowires Coated with a Semiconductive Shell as the Photoanode in Dye-Sensitized Solar Cells. Int. J. Nano. Bio. Mat. 2012, 4 (3/4), 196-212.