Electrolyte Design and Engineering for Electrochemical Energy System

Abstract

Electrochemical energy conversion and storage technology is considered as a promising replacement of fossil fuels to directly convert the chemical energy to electrical energy through electrochemical reactions, which has environmental-benign emissions and excellent operational efficiencies. As key components of an electrochemical device, both electrode and electrolyte will have substantial effects on the performance of an electrochemical energy conversion and storage system. While there have been many research and development concerning electrode materials, the investigations focusing on electrolyte are rather limited. It is worth noticing that the design and preparation of an ideal electrolyte is very necessary, as it plays a critical role in establishing important properties of an electrochemical energy conversion and storage system including internal resistance, thermal stability, power density, energy density, cycle life, and so on. In this thesis, electrolytes are divided into two types by physical properties, which are liquid electrolyte and solid-state electrolyte. Liquid electrolyte can be further grouped into aqueous and non-aqueous ones based on different solvent utilization, while solid electrolyte can be further separated into all-solid-state and quasi-solid-state electrolytes. Overall, the development of electrolytes is moving from liquid towards solid electrolytes with the rapid growing demand of flexible, foldable, portable, micro and wearable electrochemical devices. In this work, a novel strategy towards hybrid aqueous electrolyte was firstly put forward for an all-aqueous redox flow battery with unprecedented high energy density.Theoretically, the electrolyte acidic/basic properties have a great influence on redox pair potential. By tuning the pH of electrolyte, the battery voltage can be effectively enhanced, finally leading to an increase in energy density. Inspired by this concept, an all-aqueous hybrid alkaline zinc/iodine flow battery is designed and demonstrated with a 0.47 V battery potential enhancement compared to the conventional counterpart. Also, a high-energy-density of 330.5 Wh L-1 was achieved for this all-aqueous hybrid alkaline zinc/iodine flow battery. It is an unprecedented record for an all-aqueous redox flow battery obtained to date, which is even 1.6 times of the highest reported energy density value. Overall, this hybrid alkaline zinc/iodine system demonstrates a new design with promising performance for an all-aqueous redox flow battery, and more importantly, opens a feasible and effective approach for achieving high-voltage high-energy-density all-aqueous electrochemical energy device. After that, I present a functionalized nanocellulose-based membrane with a laminated structure to be used as a hydroxide-conducting solid-state electrolyte. The introduced functional groups in the nanocellulose significantly boost the hydroxide conductivity (e.g., 58.8 mS cm-1 at 70oC) due to the enhanced ion-exchange capacity and the increased amorphousness of the membrane. Meanwhile, a cross-linking bonding network is formed between the functionalized graphene oxide and nanocellulose, providing the membrane with a superior mechanical property and excellent water retention. The battery using the novel membrane exhibited superior rechargeability and performance stability compared to the commercial A201 membrane. An excellent output power density was achieved when the flexible zinc-air battery was under stress at different bending angles. This novel membrane will pave the way for future research in the field of flexible energy storage devices, particularly for emerging portable and flexible electronic applications. In the last study, a functionalized graphene oxide-based membrane with three-dimensional interpenetrating structure was fabricated through a green, efficient and scalable approach. This membrane is used as a proton-conducting solid-state electrolyte in an electrochemical fuel cell gas sensor for the detection of alcohol. The graphene oxide nanosheets are inserted into the whole membrane fibrous skeleton, creating impermeable barrier layers to prevent ethanol gas penetration. The introduced functional groups in the graphene oxide significantly boost the proton conductivity due to the enhanced ion-exchange capacity. Importantly, the modification of graphene oxide facilitates the protons transportation in both in-plane and through-plane channels of the membrane. An alcohol fuel cell sensor equipped with the novel electrolyte membrane was fabricated on the basis of direct ethanol fuel cell principle, exhibiting excellent linearity, sensitivity as well as low ethanol detection limits approaching 25 ppm. This work will pave the way for future research in the field of electrochemical gas sensors as well as the graphene oxide utilization in gas detection application. In summary, this thesis focuses on the development of electrolytes, including aqueous-based hybrid electrolyte as well as functionalized nanocellulose and graphene oxide based solid electrolytes. Several applications are demonstrated with the presented electrolytes materials, paving the way for future electrolyte research in high-energy-density or flexible wearable electrochemical energy and storage systems.