We are using anodized aluminum oxide (AAO) for a variety of energy-related applications. Among these is as a support for amorphous metal alloy hydrogen separation membranes. The ordered nanopore array found in AAO allows for the fabrication of a membrane support with nanometer-scale surface roughness and a tunable, yet uniform, porosity. This architecture not only enhances adhesion of the membrane to the support but also facilitates the facile deposition of ultra-thin membranes without loss of film cohesion. Other benefits are the scalability and tunability of the electrochemical self-assembly mechanism used to fabricate AAO. A square foot of the support is as easily fabricated as a square inch and the uniform diameters of the pores within the array are easily tunable simply by adjusting the applied voltage that drives the electrochemical oxidation reaction. The pores are non-tortuous, channeling directly through the support which creates a direct path for hydrogen diffusion. Additionally, the pore diameter can be adjusted from 10 to 100s nm, thus avoiding the Knudsen diffusion limit. Though mild anodization can be quite slow (2 – 10 μm/hr), we have developed a process to form high‐density pores using a hard anodization technique which was recently discovered to form low‐density pores under strict temperature/acid/voltage control at rates as high as 300 μm/h. Not only were the pores fabricated for this study of high‐density, but the AAO template supports were also up to 1.6 mm thick (Figure 1). To the best of our knowledge, this is the thickest AAO template ever fabricated (which only took 23 h of anodization), exceeding the closest example in the literature by almost 20‐fold.
The membranes were fabricated as continuous thin films of amorphous metal alloys (Figure 2) and were thin enough to be surface reaction limited, exhibiting a linear increase in hydrogen flux with ΔPhydrogen. Sub-micron Pd-alloy catalytic layers are currently being investigated to improve hydrogen dissociation and reassociation rates.