Hydrogen isotope separation is vital for multiple application areas including nuclear energy sectors and nuclear weapons, and is fundamentally interesting. Two-dimensional (2-D) materials such as monolayer graphene have been demonstrated in the past to selectively transport ions including hydrogen isotopes through them which provides a pathway for membrane-based isotope separation. Recent studies have shown that monolayer graphene and other 2-D materials embedded in membrane electrode assemblies (MEAs) prepared from proton exchange membranes (PEMs) such as Nafion® can selectively transport protons at rates up to 14 times higher than deuterons. The mechanism for this unusually high isotope selectivity is still under study and is thought to involve transport at localized defects sites with one or more chemical steps having a high kinetic isotope effect. The long-term stability of electrochemical hydrogen isotope separation using monolayer graphene-based membranes is crucial for the graphene to become a good sieving layer candidate for large-scale hydrogen isotope separation. The focus of this study is on the long-term stability of separating characteristics of graphene layers embedded in MEAs that actively pump protons and deuterons through them over an extended period of time. Initial results suggest that graphene embedded within Nafion sandwiches in their current form have a high isotope selectivity initially but that isotope selectivity can diminish as current is passed through the sandwich structures for extended time periods. Findings from chronoamperometry experiments performed on graphene-embedded MEAs while interchanging H₂ and D₂ gas feeds in symmetric electrochemical H/D pump cells, and potential factors affecting separation capability over long periods of time, will be discussed during the talk.