Closed-Loop Hydrogen Recovery Enabled by Electrochemical Hydrogen Separation

Space exploration missions are limited in duration due to constraints in oxygen and water storage. Water can be reclaimed from waste products by the Sabatier reaction {1}, which allows the recycling of carbon dioxide (produced by crew metabolism) into methane and water. Water could then be either directly used by the crew or electrolyzed to produce oxygen (and hydrogen) while methane is either vented overboard or used as fuel. CO2 + 4 H2 >> CH4 + 2 H2O {1}

The Sabatier reaction requires hydrogen; only half of it can be recovered through the electrolysis of water, while the other half is still lost from the closed-loop system in the form of the methane waste product. This trapped hydrogen can be further recovered via the following plasma pyrolysis reaction10 {2}: 2 CH4 >> C2H2 + 3 H2 {2}

Hydrogen must be separated from acetylene before it can be reused. Electrochemical hydrogen pumps are very good candidates as they allow an efficient and fast separation of hydrogen from gas mixtures with very good product purity. Yet a major challenge with mixtures containing acetylene and hydrogen arises from their spontaneous reaction with each other. In this work, we show that the best catalyst for pumping hydrogen (Pt/C) is also very good for the heterogenous hydrogenation of acetylene. It goes without saying that the latter needs to be suppressed (or lowered) to make the overall process viable. A Pt3Sn alloy11 showed the ability to significantly reduce the hydrogenation of acetylene (in the presence of CO) while still pumping hydrogen at a reasonable rate.

This paper discusses the development and testing of catalysts for electrochemical separation of hydrogen from acetylene while minimizing acetylene hydrogenation, and the development of cell hardware that promotes efficient gas separation while meeting the operational interfaces of the adjoining systems.