Capillary Absorption Spectroscopy of Hydrogen for Martian and Lunar ISRU Applications

We present a laser absorption spectroscopy-based detection technique for measuring varying hydrogen and methane concentration in gaseous mixtures using a Capillary Absorption Spectrometer (CAS), with the aim of providing sensitive, accurate, and calibration-free measurements of molecular hydrogen and methane in a variety of gases produced by various In-Situ Resource Utilization (ISRU) processes relevant to Martian and Lunar exploration and human habitation. The system employs a hollow core fiber as the optical gas cell, which can be fabricated in different lengths without significantly increasing the footprint of the experimental setup. This increases the optical path length and, consequently, improves the sensitivity of gas detection. Building on previous work that involved the application of this technology to assessing the production of methane from carbon dioxide and water (with applicability to Martian ISRU processes) using absorption techniques near 3.37 µm, we now extend the technique to assessing the production of molecular hydrogen from water (with applicability to both Martian and Lunar ISRU processes) using absorption techniques near 2.12 µm. To demonstrate the applicability of the CAS approach to both types of ISRU processes, LAS-based measurements of molecular hydrogen in gas mixtures with methane at different concentrations were performed using both Direct Absorption Spectroscopy (DAS) and Wavelength Modulation Spectroscopy (WMS) techniques. A compact benchtop prototype of a catalytic electrolyzer, based on an anion exchange membrane (AEM), was designed, assembled, and operated with water as feedstock material to produce molecular hydrogen and oxygen at the cathode and anode, respectively. Both outlet streams were monitored with the CAS at different times; the cathode stream was monitored to measure the overall production of molecular hydrogen, while the anode stream was monitored to measure any trace hydrogen crossover in the oxygen production. Current results demonstrate the ability to reliably measure molecular hydrogen in the cathode stream, and that this hydrogen production from the catalytic electrolyzer is robust. At present, no molecular hydrogen has been detected in the anode stream, suggesting that—within the detection limits—there are no signs of hydrogen crossover.