Measuring proton transport through two-dimensional materials

Abstract

Two dimensional materials are sought after as proton conductors for energy conversion devices. Herein, hexagonal boron-nitride (hBN) and graphene were incorporated into Nafion|2Dmaterial|Nafion membrane assemblies and liquid phase ion transport through the membranes was measured. Preliminary testing in a swage-style electrochemical cell indicated the need for a submersed electrochemical cell to minimize contact resistance, as well as demonstrated the importance of careful membrane handling to avoid undesired ion transport. A four-electrode, Devanathan–Stachurski (D-S) cell was developed, modeled, and validated to measure ion conduction through a membrane. Then, a parallel-circuit model was proposed to distinguish proton conduction through defects from conduction through a pristine 2D material lattice. Application of the model to previously published data suggests that in chemical vapor deposition (CVD) graphene, proton conduction is dominated by transport through nanoscale defects smaller than hydrated K+ ions. Finally, an electrochemical flow cell for device level testing of Nafion|2D-material|Nafion membranes was developed and demonstrated as a vanadium redox (flow) battery (VRB).