The electrolyte in electrochemical systems plays a central role both literally and figuratively.  For low temperature systems, whether liquid or polymer electrolyte, the chemical and physical properties of this component has functions that go far beyond a linear ohmic response.  Non-linear aspects arise related to the role of water in devices such as fuel cells. The separator aspect of the membrane dictates that it play roles in mediating cross-over in multi-component systems.  In practical systems, the concentrated solutions to which a membrane might be exposed render a detailed understanding of its behavior critical.  And, of course, electrolytes must permeate through porous electrodes to complete the circuit and allow the primary transduction events to occur at an active electrode surface site.

These aspects play major roles in fuel cells, batteries and other electrochemical devices that are at the forefront of electrochemists’ contribution to the energy and environment fields in this symposium.  The scientist and engineer working on these devices often have two choices:  find a fundamental solution to the issue through developing advanced electrolytes or work around the problem with the materials at hand.  (I leave it to the reader to decide which choice would be favored by the scientist and engineer respectively!)  Nonetheless, each approach informs the other: it is important to have some sense of the underpinnings of the problem from each perspective.  As each new and potentially more complicated system is studied, it is necessary to review the available understanding of physical chemistry and transport of the electrolyte and, as needed, augment it.

In this presentation, after a brief overview of some early efforts and resolutions, examples of the state of the art of our understanding of electrolyte functions in redox flow batteries and in fuel cell electrodes will be presented.  Descriptions of processes occurring at scales from molecular to macro-scale will be presented.  Of particular interest in the flow battery context, and of general interest for electrosynthesis, is a description of the complex interplay between component interactions with each other and with membranes. For the fuel cell electrodes, unraveling of processes that connect structure and function at multiple levels is a primary unknown.  In particular, the behavior of ionomer within the layer and the relationship of that behavior to ionomer interactions with the other components will be discussed. 

Finally, in each case, these aspects will be placed in context of device-level issues that are caused and prospects for possible or actual solutions to these issues will be discussed.  To conclude reflections will be presented on issues that were ‘left behind’ incompletely resolved during development of some devices.