Electrochemical Separation, Pumping, and Storage of Hydrogen or Oxygen into Nanocapillaries Via High Pressure MEA Seals
Authors: Reeves Ryan D , Schwartz Nicholas R , Chester Gregory E , Diez Douglas S , Solomon Mitchell L , Cox Philip , Hill Justin J 
Published: February 2020

High-density storage of gases remains a major technological hurdle for many fields. The U.S. Department of Energy (DOE), for example, reduced their hydrogen storage targets for automotive applications due to the inability of technologies to reach the system-level targets.1 Without significant improvements in hydrogen storage density, hydrogen as a transportation fuel remains unfeasible. In other fields, the low volumetric and gravimetric storage densities of oxygen gas cylinders can be cumbersome or prohibitively expensive for applications like terrestrial and marine breathing apparatus’ or manned space exploration. Emerging technologies such as sorbents and chemical complexes show potential for gas storage. However, the ability to achieve application-specific gas delivery rates and high storage densities, while operating reversibly at ambient temperature, remains elusive.2, 3

In contrast, it may be possible to achieve high gas storage densities and potentially high gas delivery rates using nanocapillaries when used in conjunction with a membrane electrode assembly (MEA). Hoop stress calculations show that the pressure tolerances of cylinders are inversely proportional to the radius. Indeed, glass microcapillaries have already theoretically4 and experimentally5 demonstrated the capacity to achieve DOE hydrogen storage targets at a materials-level. This technology can be further improved by reducing the capillary radius to the nanoscale; however, the capping and pressurization of the gas in micro- or nanocapillaries remains problematic.
Presented here is the fabrication of nanocapillary arrays capped by an MEA for highly reversible storage of gases with the potential for high rate pumping and high-density storage. Very high aspect ratio and densely packed nanocapillary arrays are produced through aluminum anodization. The nanocapillary arrays are capped with either a PEM or an alkaline (anion) exchange membrane (AEM) complete with catalyst nanoparticles on either side of the membrane to form an MEA. This MEA is used to provide controllable electrochemical pumping of hydrogen or oxygen gas into and out of the nanocapillaries. The MEA also serves as a high pressure seal. A theoretical discussion of the potential volumetric and gravimetric storage densities of hydrogen and oxygen in nanocapillary arrays will be presented together with experimental results of electrochemical gas compression into lab-scale devices. The evaluation of both commercial catalyst materials and fabricated nanoparticle catalysts (<10 nm) for hydrogen and oxygen pumping will be discussed. In addition, the performance of both PEM and AEM for electrochemical pumping of oxygen will be compared. A discussion of the electrochemistry within nanocapillaries compared to planar MEAs will be given including the charge transport/transfer processes. The potential failure mechanisms and the technical obstacles to the implementation of our electrochemical membrane approach, together with the current state of the technology and overall storage capacities, will be presented.
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