Modeling Electrolytic Conversion of Metabolic CO2 and Optimizing a Microfluidic Electrochemical Reactor for Advanced Closed Loop Life Support Systems

Abstract

Water electrolysis is required in any life support system intended to be operated in a closed loop system, as metabolic CO2 contains 87% of the O2 consumed by the crew members requiring the generation of the 13% O2 balance (assuming 100% efficiency of O2 recovery from CO2) via water electrolysis. Metabolism of food produces 0.34 kg/day/crewmember more water than consumed leading to having sufficient water to generate the 13% O2 balance via electrolysis. The current state of the art for oxygen recovery onboard the International Space Station (ISS) is the Environmental Control and Life Support System (ECLSS) Atmosphere Revitalization (AR) System. The AR includes four integrated units, which provide critical functions for life support including trace contaminant control (Trace Contaminant Control (TCC) system), CO2 removal (CO2 Removal Assembly (CDRA)), oxygen generation (Oxygen Generation Assembly (OGA)), and oxygen recovery (Carbon Dioxide Reduction System (CDRS)). CDRS is a Sabatier reactor that reduces CO2 to CH4 and water using H2 from OGA (an electrolysis unit that generates O2 as main product and H2 as byproduct) as reactant. The CDRS limits the feasibility of having a closed loop system, as it requires more water than the water metabolically generated ultimately resulting in a recovery rate of approximately 50%. NASA Marshall Space and Flight Center (MSFC) and University of Texas, Arlington (UTA) are currently developing an Engineering Development Unit (EDU) of a Microfluidic Electro Chemical Reactor (MFECR) to convert a continuous stream of CO2 and water into O2 and C2H4 at standard conditions. The novel design combines CO2 conversion and water electrolysis (currently conducted in OGA and CDRS units respectively) into one compact unit that runs at standard conditions and is theoretically capable of generating metabolic O2 with a maximum metabolic CO2 conversion of 77% while consuming less than metabolic water. This paper presents a multi-physic 3D model developed at MSFC on CO2 conversion to O2 and C2H4 at standard conditions via MFECR. In the model the electrochemical physics is coupled with all the other physics phenomena involved in the process, such as micro fluid flow, mass and heat transfer, and generation as well as conduction of DC electrical current. This work aims to use this 3D model to build a comprehensive, rigorous, and experimentally validated simulator that will be used as a valuable tool to not only assist the authors on the EDU design but also to optimize its operation.