Rapid transport between fluid and solid phases can yield high power density in electrochemical energy storage systems, and efficient chemical separations in chromatography.  Transport is often slower in the solid phase, and in long, narrow channels of the fluid phase. Careful architecture can lead to significant improvement of overall transport rates.  A known architectural theme that can achieve this is hierarchical porosity, where there are a few long, wide channels connecting to many shorter, narrower channels that provide greater access to the solid phase.

We have fabricated and tested hierarchically porous metal hydride electrodes and chromatography columns with overall dimensions in the millimeter range. The fluid channels are tens or hundreds of micrometers wide.  The solid layers are tens of micrometers thick, and penetrated by pores that are about 10 nanometers wide. We achieve this through electrodeposition of thick layers of block copolymer-templated nanoporous palladium onto a fine wire mesh or photoresist-patterned wafer.  Pieces of these materials are then stacked and fixed in place to form a defined geometry that is macroscopic in three dimensions. Palladium forms a hydride under mild chemical or electrochemical conditions.  We have measured hydrogen transport rates into or out of these structures electrochemically and by mass spectrometry. Transport rates are significantly faster than for cases where one or both scales of porosity are absent. We expect that similar improvements could be demonstrated for other multiphase chemical systems, such as electrodes that store lithium ions, or organic solutes for chemical separations.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2015‑10657A