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Thesis advisor: Udayan Mohanty / Metal-oxygen batteries recently emerge as one of the most promising post-Li-ion energy storage technologies. The key feature of this technology lies in the conversion reactions of O2 at the cathode. Such a chemistry promises the highest theoretical energy densities due to the contribution from the cathode reactions. However, the conversion between various oxygen-based species suffer severe kinetic penalties, resulting in poor energy efficiencies and low rate capabilities. To promote these reactions, catalysts with desired functionality and stability are needed. On the other hand, the O2-based chemistry incurs severe parasitic chemical reactions against various cell components, including the anode, the cathode and the electrolyte. Consequently, the reported cyclabilities of metal-oxygen batteries remain much worse than required. While stable cathode and anode candidates have been developed, further advance of this technology still hinges on developing stable electrolyte and efficient catalyst to ensure prolonged and stable cell operations. In the first part of this thesis, two distinct strategies were exploited as proof-of-concept demonstrations on the catalyst design for metal-oxygen batteries. For one, using Li-O2 batteries as a study platform, we show that the stability of catalyst can be heavily dependent on the synthesis history. A novel approach, namely carbothermal shock method, was found to enable superior chemical and structural stability of the catalyst compared to those of the catalyst prepared by conventional methods. For another, using Mg-O2 batteries as prototypical system, we demonstrate a strategy using two redox mediators that concertedly operate for discharge and recharge. As a result, a total overpotential reduction by ca. 600 mV can be achieved through manipulating the charge transfer mechanism. To meet the need of a stable electrolyte for metal-oxygen batteries, in the second part of this thesis, we analyzed the decomposition pathways of the electrolyte in the presence of reactive oxygen species. Using Li-O2 battery as a model system, we address this issue by employing a water-in-salt (WiS) electrolyte that eliminates organic solvents all together. WiS was found stable under Li-O2 battery operation conditions. When carbon was used as a cathode, much longer cycling numbers (>70) can be achieved in WiS than in organic ones. When carbon was replaced with a carbon-free cathode (TiSi2 nanonets decorated with Ru catalyst), over 300 reversible cycles was measured. The unique feature of WiS also enables other opportunities beyond O2 chemistry in metal-oxygen batteries. Toward the end of this thesis, we employ WiS for electrochemical CO2 reduction reactions. By controlling the concentration of H2O in WiS, the rate determining step on Au catalyst was found to be the first electron transfer from the electrode to CO2. Moreover, the reduced H2O activity by WiS significantly suppressed hydrogen evolution reactions, through which high selectivity toward CO can be measured. Our study provides important knowledge base on the design of electrolyte for future optimizations. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

One of the major trends of development of solid oxide fuel cells is to reduce the operating temperature from the high temperature range (>950°C) and intermediate temperature range (750-850°C) to the low temperature range (450-650°C). Development of low temperature oxygen ion conducting electrolytes is focused on single-phase materials including Bi2O3 and CeO2-based oxides. These materials have high ion conductivity at the low temperature range, but they are unstable in reducing environments and they are also electronic conductors. In the present research, three types of multiphase materials, Ce0.887Y0.113O1.9435 (CYO)-ZrO2, CYO- yttria-stabilized zirconia (YSZ), and CuO-CYO were investigated. We found that the conductivity of multiphase electrolyte CuO-CYO with a mass ratio of 1:3 is at least 4 times greater than that of CYO and 10 times greater than that of YSZ, the most commonly used material, obtained in the present experiments at 600°C. The enhancement of conductivity in multiphase materials correlates with the level of mismatch between the two phases. Large mismatches in terms of valance and structure result in high vacancy density and hence high oxygen ion conductivity at grain boundaries. This study demonstrates that synthesis of multiphase ceramic materials is a feasible new avenue for development of oxygen ion electrolyte material for low temperature SOFCs.

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In this study, the CVD diamond films were grinded using composite electroplating in-process sharpening (CEPIS). At an applied load of 4.2 kg, a rotation speed of 200 rpm and 20 rpm for the upper and lower spindles, the effects of NiCl2 (nickel chloride) content of electrolyte (in the range of 5-75 g/l) and cathode current density (in the range of 0-7.5 ASD) on the structure of the nickel-diamond composite coating and its grinding ability were investigated. Based on the experiment results, the coating structure became porous and the coating thickness increased with decreasing NiCl2 content of electrolyte during the composite electroplating process. During CEPIS process, the material removal rate of diamond film has no obvious concern with the NiCl2 content at low cathode current density (below 2.5 ASD). However, its material removal rate rapidly increased to a saturated value with increasing NiCl2 content at high cathode current density (above 5 ASD). The NiCl2 content to achieve a saturated material removal rate increased with increasing cathode current density. In this study, the maximum removal rate of diamond film is about 0.093 mg/min at the NiCl2 content of 75 g/l and the cathode current density of 5 ASD.

In the past few years, YSZ (Yttria Stabilized Zirconia) had been the dominate electrolyte material of high temperature (>1000¢J) solid oxide fuel cells (SOFCs). Nowadays, CGO (Cerium Gadolinium Oxide) material has been considered as preferred electrolytes for solid oxide fuel cells (IT-SOFCs) used in the temperature range of 600¢J~800¢J due to their excellent oxygen-ion conductivity compared to YSZ. The performance of unit cells can be improved when an anode functional layer (AFL) is employed between the anode composite substrate and the electrolyte. Therefore, AFL has been a lot of use in SOFCs In this study, the deposition system of EAVD (Electrostatic Assisted Vapor Deposition) was employed to deposite CGO electrolyte films and NiO-CGO anode functional layer. In this study, deposition parameters such as deposition temperature, flow rate and concentration of precursor solution were varied to figure out their effects for the resultant films. Finally, the OCV of unit cells was also measured in this study. In this work, the optimum concentration of precursor solution for NiO-CGO anode functional layer and CGO films were 0.2 M and 0.3 M, respectively. The optimum deposition temperature and flow rate for this two films were both 400¢J and 6 mL/hr, respectively. When the cells were test with H2 as fuel and air as oxidant, the unit cell of Ni-CGO/CGO/BSCF with CGO film thickness of 25 £gm exhibited an maximum OCV of 0.86 V at 500¢J and the other unit cell of Ni-CGO/AFL/CGO/BSCF with 25 £gm CGO film thickness and 10 £gm AFL exhibited an maximum OCV of 0.91 V at 500¢J.