Browsing by Author "Gong, Ke"
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Item Manipulating Water in High-Performance Hydroxide Exchange Membrane Fuel Cells through Asymmetric Humidification and Wetproofing(The Electrochemical Society, 2015-02-20) Kaspar, Robert B.; Letterio, Michael P.; Wittkopf, Jarrid A.; Gong, Ke; Gu, Shuang; Yan, Yushan; Robert B. Kaspar, Michael P. Letterio, Jarrid A.Wittkopf, Ke Gong, Shuang Gu and Yushan Yan; Kaspar, Robert B.; Letterio, Michael P.; Wittkopf, Jarrid A.; Gong, Ke; Gu, Shuang; Yan, YushanHydroxide exchange membrane fuel cells (HEMFCs) are an emerging low-cost alternative to conventional proton exchange membrane fuel cells. In addition to producing water at the anode, HEMFCs consume water at the cathode, leading to distinctive water transport behavior. We report that gas diffusion layer (GDL) wetproofing strictly lowers cell performance, but that the penalty is much higher when the anode side is wetproofed compared to the cathode side. We attribute this penalty primarily to mass transport losses from anode flooding, suggesting that cathode humidification may be more beneficial than anode humidification for this device. GDLs with little or no wetproofing perform best, yielding a competitive peak power density of 737 mW cm−2.Item Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs(Royal Society of Chemistry, 2015-08-17) Gong, Ke; Fang, Qianrong; Gu, Shuang; Li, Sam Fong Yau; Yan, Yushan; Ke Gong, Qianrong Fang, Shuang Gu, Sam Fong Yau Li and Yushan Yan; Gong, Ke; Fang, Qianrong; Gu, Shuang; Yan, YushanAs members of the redox-flow battery (RFB) family, nonaqueous RFBs can offer a wide range of working temperature, high cell voltage, and potentially high energy density. These key features make nonaqueous RFBs an important complement of aqueous RFBs, broadening the spectrum of RFB applications. The development of nonaqueous RFBs is still at its early research stage and great challenges remain to be addressed before their successful use for practical applications. As such, it is essential to understand the major components in order to advance the nonaqueous RFB technology. In this perspective, three key major components of nonaqueous RFBs: organic solvents, supporting electrolytes, and redox pairs are selectively focused and discussed, with emphasis on providing an overview of those components and on highlighting the relationship between structure and properties. Urgent challenges are also discussed. To advance nonaqueous RFBs, the understanding of both components and systems is critically needed and it calls for inter-disciplinary collaborations across expertise including electrochemistry, organic chemistry, physical chemistry, cell design, and system engineering. In order to demonstrate the key features of nonaqueous RFBs, herein we also present an example of designing a 4.5 V ultrahigh-voltage nonaqueous RFB by combining a BP/BP˙− redox pair and an OFN˙+/OFN redox pair.Item Study of novel redox flow batteries based on double-membrane, single-membrane, and membrane-less cell configurations(University of Delaware, 2016) Gong, KeWide deployment of intermittent energy generation (e.g., wind and solar) calls for low-cost energy storage system for smooth and reliable power output. Redox flow batteries (RFBs) have been identified as one of the most suitable systems for largescale energy storage. Different from conventional batteries that store energy in solid electrode, RFBs take advantage of flowable electrolytes as energy-storage media and therefore bring unprecedented freedom in independent tuning of energy and power of RFB. The method to separate two chemically reactive electrolytes plays a key role in RFB. Current RFBs adopt a single ion-exchange membrane (IEM) as separator, which can physically separate two electrolytes but ionically conduct them with commuting ions. Ever since the invention in 1974, the single-membrane configuration has enabled a tremendous amount of new combinations of elements from periodic table for battery application. However, single IEM configuration remains imperfect: 1) IEM is designed to either conduct cation while excluding anion (cation-exchange membrane, CEM), or conduct anion while excluding cation (anion-exchange membrane, AEM). This property only allows the combination of redox pairs in the same type of charge, leaving a lot of promising redox pair combinations useless; 2) IEM cannot reach 100% selectivity of commuting ion, which results in an inevitable crossover of redox pairs, causing electrolyte imbalance, coulombic efficiency and capacity loss; 3) IEM contributes the biggest voltage loss due to its large internal resistance in many RFBs, and is usually one of the most expensive components in the stack, both indirectly or directly increasing the cost of RFBs. Aiming at solving the problems in single-membrane RFBs, this work explored three possible routes that provide alternative configurations to current RFBs: 1) a double-membrane RFB that could combine redox pairs with different types of charge, and of different supporting pHs; 2) a single-membrane all-iron (all-Fe) flow battery that adopts the same elements on both sides, which is immune to the crossover of metal ions; 3) a membrane-less RFB that utilizes immiscible organic and inorganic electrolytes, which thermodynamically separate two redox species and eliminate the usage of membrane in RFB. In the double-membrane RFB design, both AEM and CEM are incorporated in cell to isolate cation and anion redox pairs respectively. A middle electrolyte is used to ionically conduct two membranes. Three examples have been successfully demonstrated: Zn-Ce (Zn(OH)4 2−/Zn vs. Ce4+/Ce3+), S-Fe (S4 2−/S2 2− vs. Fe3+/Fe2+) and Zn-Fe (Zn(OH)4 2−/Zn vs. Fe3+/Fe2+) RFBs. Zn-Ce RFB provides the highest cell voltage among all aqueous RFBs as 3.08 V. S-Fe RFB combines very inexpensive anion redox pair (S4 2−/S2 2−) and cation redox pair (Fe3+/Fe2+) together (1.22 V) and brings low electrolyte cost. Zn-Fe RFB has the best balance between high voltage (2.0 V) and low electrolyte cost, thus bringing high performance and low capital cost. Middle electrolyte was found to be an important role in controlling total cell resistance. With optimally engineered middle electrolyte, Zn-Fe RFB shows high power density (676 mW/cm2) and the lowest system cost so far among several notable RFBs, under $100/kWh, which is below the cost target for energy storage system set by Department of Energy of U.S. in the 2023 term. Such a low cost puts Zn-Fe RFB in a very promising position for future development and commercialization. In the single-membrane all-Fe RFB, the same element, iron, is used in redox pairs in both positive and negative electrolytes with different coordination chemistries. The adoption of the same element fundamentally eliminates the cross-contamination in RFBs that uses two different elements. All-Fe RFB shows good durability and stability over cycle test. The slow diffusion of coordinate agent, however, was identified as a prominent concern in capacity retention in long-term. Nonetheless, all- Fe RFB remains as a good attempt in combining redox pairs of the same element with different coordination chemistries to extend the spectrum of redox pairs for RFB application. In the membrane-less RFB design, a new separation method of redox pairs is introduced by employing immiscible organic and inorganic electrolytes. Redox pairs are thus thermodynamically separated and require no membrane. A zinc-ferrocene RFB was demonstrated as an example for this membrane-less design and good durability and stability were proved in cycle test. This concept broadens the method to construct flow battery and brings more possible combinations between organic and inorganic redox pairs in RFB application. The new designs and concepts studied in this work successfully demonstrated that invention of new cell structure could greatly enrich and diversify the category of RFBs, expanding new redox chemistries and enabling new redox pair combinations for RFB. Setting those three cell designs as frame work, we are expecting and looking forward to more exciting redox chemistries being explored.