Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Kevin Huang

Abstract

Cost-effective and large-scale energy storage technologies are a key enabler of grid modernization. Among energy storage technologies currently being researched, developed and deployed, rechargeable batteries are unique and important that can offer a myriad of advantages over the conventional large scale siting- and geography- constrained pumped-hydro and compressed-air energy storage systems. However, current rechargeable batteries still need many breakthroughs in material optimization and system design to become commercially viable for stationary energy storage.

This PhD research project investigates the energy storage characteristics of a new class of rechargeable solid oxide metal-air redox batteries (SOMARBs) that combines a regenerative solid oxide fuel cell (RSOFC) and hydrogen chemical-looping component. The RSOFC serves as the "electrical functioning unit", alternating between the fuel cell and electrolysis mode to realize discharge and charge cycles, respectively, while the hydrogen chemical-looping component functions as an energy storage unit (ESU), performing electrical-chemical energy conversion in situ via a H2/H2O-mediated metal/metal oxide redox reaction. One of the distinctive features of the new battery from conventional storage batteries is the ESU that is physically separated from the electrodes of RSOFC, allowing it to freely expand and contract without impacting the mechanical integrity of the entire battery structure. This feature also allows an easy switch in the chemistry of this battery. Other features include state-of-charge independent EMF, O2--enabled high rate and high capacity storage, independent design of power and energy, scalability, sustainability and safety.

The materials selection for ESU is critical to energy capacity, round-trip efficiency and cost effectiveness of the new battery. Me-MeOx redox couples with favorable thermodynamics and kinetics are highly preferable. The preliminary theoretical analysis suggests that Fe-based redox couples can be a promising candidate for operating at both high and low temperatures. Therefore, the Fe-based redox-couple systems have been selected as the baseline for this study, the constituted battery of which is termed solid oxide iron-air redox battery (or SOFeARB).

The first objective of this PhD work is aimed at demonstrating the proof-of-concept. By combining a commercial anode-supported tubular RSOFC and Fe-based redox couple, the first generation SOFeARB operated at 800oC has been demonstrated to produce an energy capacity of 348Wh/kg-Fe and round-trip efficiency of 91.5% over twenty stable charge/discharge cycles. Further system optimization leads to an 800oC-SOFeARB comprised of a commercial electrolyte-supported planar RSOFC and Fe-based redox couple; this configuration has become a standard testing system for later studies. The 800oC planar SOFeARBs have been investigated under various current densities and cycle durations. The results show that metal utilization plays a determining role in balancing the energy capacity and round-trip efficiency. Increasing metal utilization increases the energy capacity, but at the expense of lowered round-trip efficiency. From an engineering perspective, a strategy can be laid out to operate the battery at a low metal utilization (e.g., overloading the low-cost Fe-based ESU materials) as a means of achieving the required energy/power rating while retaining a high round-trip efficiency. From a computational perspective, a multi-physics-based model has also been constructed and satisfactorily verified with the experimental results obtained under high current densities.

The second objective of this work is to lower the operating temperature of SOMARBs to intermediate temperature (IT) range (e.g. 550-650oC). Two changes were made in order to enable operation at IT range: introduction of optimized Sr- and Mg- doped LaGaO3 (LSGM) based RSOFC by tape-casting and infiltration techniques, and optimization of morphology of ESU through innovative synthesis methods. The optimized battery can reach a round-trip efficiency as high as 82.5% and specific energy 91% of the theoretical value in the IT range

The third objective of this work is to improve the cyclic durability and stability of IT-SOFeARBs. The results show that the performance, reversibility and stability of a 550oC-SOFeARB can be significantly improved by nanostructuring energy storage materials synthesized from a low-cost carbothermic reaction. The 100-cycle test explicitly shows an improvement of 12.5%, 27.8% and 214% in specific energy, round-trip efficiency and stability, respectively, over the baseline battery. A more thorough investigation shows that current density has a more pronounced effect on the round-trip efficiency than the cycle duration, implying that operating a SOMARB under a relatively lower current density for a longer cyclic duration is a favorable testing condition to achieve a required energy storage capacity.

The fourth objective of this work is to explore metal-air chemistries other than Fe-air. The two new metal-air chemistries of choice are W-air and Mo-air. The selection of W and Mo as the redox metals is based on their faster kinetic rate and higher specific densities per oxygen than the Fe-based counterparts. Each battery was electrochemically compared with the baseline SOFeARB at a specific temperature. The results show that these heavy metals based SOMARBs can indeed produce higher energy density (capacity per unit volume) than the baseline battery SOFeARB by allowing more mass loading and higher oxygen storage capacity. The better kinetic rates also lead to a higher cycle efficiency and cycle stability.

In summary, this dissertation work demonstrates a new energy storage mechanism that has great potential for stationary applications. The new storage battery has been studied in the perspectives of theoretical assessment, materials development, parametric optimization, and test methodology. According to these systematic investigations, a set of standard testing and characterization protocols has been configured for future testing of larger systems. Thermodynamics and kinetics have constantly been employed to guide materials selection and electrochemical testing. The experimental results are often found consistent with the theoretical predictions.

Rights

© 2013, Xuan Zhao

Download

Share

COinS