Carbon Derived Electrodes and Electrolytes for Electrochemical Energy Systems

Abstract

The world is witnessing the inconsiderate consumption of fossil fuels which are responsible for catastrophic climate change and global warming. Therefore, renewable energy sources (wind, solar, hydropower, biomass, etc.) play significant roles in generating electrical power from their infinite resources. These resources have advantages over non-renewable resources in terms of environmental impacts and sustainability. However, there exists a visible mismatch between energy demand and energy supply due to the intermittent nature and temporal variations of renewable energy resources and it is well documented that electrochemical energy devices can serve as efficient tools to bridge this gap. In the context of electrochemical energy devices, carbon materials have shown their various advantages as electrodes & electrolytes because of their desirable properties like high electrical conductivity, high thermal stability, ability to conduct protons, ability to function as molecular fuel barriers etc. However, in the context of aqueous metal ion batteries, utilization of carbon materials as electrodes often lead to spontaneous de-insertion reactions, undesirable proton insertions reactions leading to parasitic hydrogen evolution reactions, unwanted swelling etc. Aqueous redox flow batteries mainly suffer from their inferior energy density derived from the underutilization of dissolved electroactive species and even the employment of high surface area carbon nanotube electrodes often fail to overcome these hurdles. With regard to energy conversion devices such as water electrolysers and fuel cells, though it is predicted that graphene oxide can conduct protons and can function as effective barrier for molecular fuels; these materials have not been explored in their pristine form to shuttle protons in a practical reversible fuel cell. Therefore, the primary aim of this thesis is to incorporate carbon derived materials into the architectural configuration of electrochemical energy devices; as electrodes in energy storage devices and as electrolytes in energy conversion devices to address some of their state-of-the-art challenges and issues. As electrodes in energy storage devices, we have investigated how the activation of coulombic forces on carbon nanotube electrodes can gate the molecular transport of electroactive species which can be utilized in a redox flow battery to enhance its energy density without compromising the power capability. We have explored how the mode of synthesis of graphene-based materials leads to superior graphene electrodes for selective sodium ion adsorption while simultaneously suppressing the parasitic hydrogen evolution reaction in a rechargeable aqueous sodium-ion battery. In the electrolyte part, we have utilized pristine graphene oxide (GO) as a proton exchange membrane to construct a practical water vapor electrolyzer and a reversible fuel cell. Further, we have investigated the role of dopants in GO interlayers in enhancing its proton conduction and the implications of thickness reduction of GO membranes on fuel cross over and fuel cell performance.