This dissertation presents my efforts to use viologens to improve the performance of glucose fuel cells and aqueous redox flow batteries. These two electrochemical systems have the potential to efficiently exploit renewable sources of energy. The contributions and significance of this work are briefly described below. Glucose Fuel cells. For glucose fuel cells, viologens were adopted as an electron mediator to facilitate the transfer of electrons from glucose to electrodes for power generation. Use of a mediator circumvents the need for precious metal electrodes to catalyze glucose oxidation. Both the oxidation efficiency and rate of glucose oxidation are important to the viability of glucose fuel cells. Oxidation efficiency is defined as the extent to which the carbons of a carbohydrate (glucose for instance) are oxidized relative to full oxidation to carbon dioxide. The efficiency measured in this study depended on the initial molar ratio of viologen to glucose and also on the rate of the regeneration of the mediator. The maximum conversion efficiency observed was ~22%, which is about three times larger than the values observed for precious-metal-based fuel cells. Rate performance is another important aspect of a glucose fuel cell. Detailed simulations demonstrated that rate performance of viologen-mediated cells was limited principally by mass transfer. The maximum obtainable current density was ~200 mA/cm2, which is significantly higher than the rates available from biological fuel cells and comparable to the values observed for precious-metal-based fuel cells. Viologen-mediated fuel cells offer the potential for higher oxidation efficiency and high current densities at a significantly lower cost. This makes viologen-mediated cells an appealing option for future development of glucose fuel cells. Redox Flow Battery. An asymmetric viologen called MMV was assessed for potential use in aqueous flow batteries to improve performance. With an asymmetric structure, MMV demonstrated one of the most negative redox potentials reported to date for organic electroactive compounds. MMV also showed a relatively high solubility in neutral electrolytes. The electrochemical reaction of MMV involved a reversible single electron transfer with fast kinetics. These characteristics support MMV as a promising anolyte for flow battery applications to improve capacity, energy density, and cell potential. MMV, however, exhibited poor cycling performance at elevated concentrations since it underwent irreversible or partially reversible side reactions. Signs of dimerization and precipitation were observed during cycling. These undesired reactions can be potentially mitigated by synthesizing asymmetric MMV derivatives that possess a higher charge than that possessed by MMV (+1). This modification can reduce the extent of dimerization by increasing repulsive forces between the monomers, and it also has the potential to reduce precipitation by increasing the solubility limit of the compounds.