Fundamental understanding of atomic-level processes underlying charge separation and transport is critical to the development of next-generation electric energy storage devices to enable wide-scale adoption of electric-vehicle technology and intermittent renewable energy sources (such as wind and solar). Joint Density-Functional Theory (JDFT) allows for the ab initio description of electronic systems in thermodynamic equilibrium with liquid environments and, as such, is ideally positioned to study electrode-electrolyte systems, which are among the most promising technological avenues but which are also currently poorly understood at the fundamental level due to their complexity. This thesis, after describing JDFT and its application to electrochemical environments, focuses on two physical systems of great current research interest in the growing area of supercapacitor storage systems. First, we examine possible sources of an observed but hitherto unexplained enhanced capacitance effect at the Dirac point of graphene in electrolyte environments. We propose and explore two possible explanations for this effect: (1) overscreening by ions within the fluid producing a negative effective fluid capacitance and (2) a stray parallel capacitance effect produced by the adsorption of hydroxide ions. Second, we explore the conduction properties of phthalocyanine-based metal-organic frameworks (MOFs), which represent potential high-performance materials for use as supercapacitors due to their nanoscale structure and high specific surface areas. The challenge here is to increase the electronic conduction of these materials so that the high specific surface area can be accessed as a source of capacitance by an external circuit, and we present detailed calculations of the full electronic conductivity tensor for these materials. The thesis concludes with suggestions for future experimental and theoretical work to advance the understanding of nanoscale influences on charge-storage systems.