The NEI (Near Equilibrium Interfaces) theme focuses on understanding and tailoring kinetic parameters controlling H fluxes across and along interfaces in the linear force-flux regime (near equilibrium). Fluxes may take place via H conduction/diffusion and chemical/electrochemical reactions, depending on H charge state and whether it changes at the interface. In each case, we seek to develop an understanding of the interfacial H transfer mechanism and its relationship to interface structure/chemistry. We also propose novel heterointerface designs that target properties inaccessible within single-phase bulk lattices to promote rapid H transport and reaction kinetics. Three types of interfaces are targeted either for their importance in devices as kinetic bottlenecks or their potential to improve H fluxes: solid-gas interfaces, solid-solid heterointerfaces, and grain boundaries. The research aims to answer the following question: What are mechanisms and descriptors to accelerate the kinetics of interfacial transport and electrochemical reactions involving hydrogenic species, under near-equilibrium conditions? Model interfaces comprising perovskites, solid acids, intercalants, and hybrid systems are being synthesized, characterized, and simulated to answer this question.