CUNY Energy Institute

The phase behavior and adsorption kinetics of hard-core particles on a honeycomb lattice are studied by means of random sequential adsorption with surface diffusion. We concentrate on reversible adsorption by introducing a desorption process into our previous model and varying the equilibrium rate constant as a control parameter. We find that an exact prediction of the temporal evolution of fractional surface coverage and the surface pressure dynamics of reversible adsorption can be achieved by use of the blocking function of a system with irreversible adsorption of highly mobile particles. For systems out of equilibrium we observe several features of glassy dynamics, such as slow relaxation dynamics, the memory effect, and aging. In particular, the analysis of our system in the limit of small desorption probability shows simple aging behavior with a power-law decay. A detailed discussion of Gibbs adsorption isotherm for nonequilibrium adsorption is given, which exhibits a hysteresis between this system and its equilibrium counterpart.

Zinc-manganese dioxide (Zn|MnO2) alkaline batteries have been widely studied as the active materials exhibit high theoretical capacities (820 mAh/g for Zn and 620 mAh/g for MnO2), low cost, and are inherent safety. Furthermore, the active materials are electrochemically active in aqueous alkaline electrolytes, such as potassium hydroxide (KOH), which is beneficial from a cost and safety perspective. It is widely known that one of the issues with this system is active MnO2 poisoning by zincate anions during the 2nd electron reaction of MnO2. As a result, an electrochemically inert material, ZnMn2O4 (hetaerolite), is formed, which reduces the capacity and cycle life of Zn|MnO2 batteries. Another issue is associated with high depth-of-discharge (DOD) of each material. For example, at high DOD, Zn will undergo shape change, dendritic growth and passivation, while MnO2 will form inactive phases like Mn3O4. Therefore, the 1st electron reaction range of MnO2 is normally accessed in rechargeable Zn|MnO2 batteries with low DOD.We have observed that the use of alkaline-based hydrogel electrolytes can significantly increase the cycle life of Zn|MnO2 batteries. Large-scale Zn|MnO2 cells were tested with either hydrogel or conventional alkaline electrolytes, whose nameplate capacities were 95 Ah based on the 1st electron capacity of MnO2. The hydrogel electrolytes were formed by mixing KOH solution, acrylic acid, and potassium persulfate solution as an initiator. Galvanostatic tests were carried out at cycling rates of C/20 (4.75 A) with 20% DOD of the capacity (19 Ah). The single full discharge performance to 0 V with hydrogels was compared to that with KOH aqueous electrolytes. The cell with the KOH electrolyte showed two sigmoidal curves, which means that two steps of electrochemical reactions were undergone, achieving 131 Ah. After recharging it, the single full discharge was conducted again, but nevertheless it attained less than 4 Ah to 1 V onwards. On the contrary, the cell with the hydrogel presented one sigmoidal curve with 40 Ah to 0 V, and it was continuously able to extract discharge capacities ≥28 Ah to 1 V during 40 cycles. Thus, the hydrogel electrolytes significantly enhanced the electrochemical performance of Zn|MnO2 cells, compared to conventional KOH electrolytes, the reasons for which will be discussed. The results suggest that hydrogels could help preserve active materials without the aforementioned issues and make rechargeable Zn|MnO2 batteries achieve better performance while accessing the 1st electron reaction of MnO2.

In order to investigate air-ingress phenomena in a gas-cooled very high temperature reactor (VHTR), natural circulation experiments have been conducted in a helium flow loop after the injection of nitrogen into the lower plenum. A pair of helium analyzers were used to measure the nitrogen and helium concentrations in the lower plenum and upper plenum. The changes in the nitrogen concentration in the upper plenum were used to calculate the time required for the transport of nitrogen from the lower plenum to upper plenum through a riser flow channel made of graphite. The effect of system temperature and pressure on the rate of nitrogen transport has been studied extensively. Furthermore, a close examination of the graphite flow channel wall temperatures at different elevations showed small but sudden drops indicating the arrival of nitrogen at each elevation. From these data, the upward transport of nitrogen injected into the lower plenum under natural circulation conditions could be quantitatively investigated. The experimental findings indicate that the driving mechanisms for air transport through the reactor core of VHTR would result from both molecular diffusion and natural circulation. At low graphite temperatures in the riser, molecular diffusion is the dominating mechanism; however, as the riser temperature increases, natural circulation becomes dominant and the rate of nitrogen transport increases. Further, the time constants for these mechanisms have been calculated using a simplified species transport equation.

Nanocomposites are of great interest to electronic packaging, and the opportunities are readily comprehended. First is the ability to integrate materials cheaply, with better performance, miniaturization, and reliability for a wide range of applications, including embedded passives (capacitors, resistors), low-k materials, adhesives (electrically conductive adhesives), interconnects, and thermal interface materials [1].

High pressure/high temperature forced and mixed convection experiments have been performed with helium and nitrogen at temperatures and pressures up to 893 K and 64 bar, respectively. The test section had a 16.8 mm ID flow channel in a 108 mm OD graphite column. Flow regimes included turbulent, transitional, and laminar flows with the inlet Reynolds numbers ranging from 1500 to 15,000. Due to strong heating, the local Reynolds number decreased by up to 50% over the 2.7 m test section. In addition, heat transfer degradation and flow laminarization caused by intense heating led to Nusselt numbers 20–50% lower than the values given by the modified Dittus–Boelter and modified Gnielinski correlations. Flow laminarization criteria were considered based on a dimensionless acceleration parameter (Kv) and buoyancy parameter (Bo*). Upward turbulent flows displayed higher wall temperatures than downward flows, due to the impact of flow laminarization which is not expected to affect buoyancy-opposed flows. Laminar Reynolds number flows presented an opposite behavior due to the enhancement of heat transfer for buoyancy-aided flows. At low Reynolds numbers, downward flows displayed higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases. In the entrance region of downward flows, convection heat transfer was reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancy-induced mixing caused higher convection heat transfer and lower wall temperatures.

Nanocomposite metal-insulator-metal (MIM) capacitors are of great interest due to the possibility of reducing the number of discrete components in printed circuit boards, and alternatively using embedded or directly printed capacitors in conjunction with integrated circuits. Flexible substrates also become candidates when employing low processing temperatures. In addition to compatibility with nanomanufacturing, the prospect of designing a nanocomposite dielectric, by combining colloidal nanoparticle fillers and polymer hosts, lends itself very well to the idea of tunability of the mechanical and electrical properties. A parallel plate capacitor is a deceptively simple device concept that quickly evolves into a complex problem when considering how components can be assembled into the dielectric layer to optimize for comprehensive performance, as a function of capacitance, frequency, voltage, leakage, dissipation loss and ESR. Our work in solution processing of inorganic oxide dielectrics and multiferroics, using a modified sol-gel approach allows for the preparation of a variety of formulations that can be treated as inks for deposition as layers and/or for the design of novel nanocomposite films.

Manganese Dioxide (MnO2) is one of the cheapest and most abundant materials available on earth. It is used in a number of applications like catalysis, water purification, lithium-ion batteries and many more. However, it is commonly known to most as a AA primary battery, where it is used to power remote controls, clocks, etc. MnO2 is used as a cathode in these primary batteries because of its high theoretical capacity of 617mAh/g, where it is delivered through a two electron reaction mechanism in alkaline electrolyte. The first electron reaction has been known to deliver ~308mAh/g through a solid-state proton insertion, while the second electron reaction has been known to deliver the remaining half through a dissolution-precipitation reaction. For many years MnO2 was considered to be electrochemically irreversible because of the breakdown of the crystal structure during the first electron reaction and the deleterious side-reactions during the second electron reaction that resulted in the formation of an inactive substance called hausmannite (Mn3O4).1,2 These major problems relegated this energy dense cathode to the realm of primary batteries, which otherwise would have been used for many important applications like grid energy storage. In this talk, we will present different ways of accessing the theoretical second electron capacity (617mAh/g) of MnO2 for over 3000 cycles, where the use of dopants like bismuth oxide and copper, removal of hydrophobic binders like Teflon, use of high surface area carbon and maintaining a porous electrode architecture are extremely important.3-5 We will present the diverse and complex electrochemistry of MnO2 that take place with the addition of bismuth oxide and copper as complexation and intercalation reactions are added with the solid-state and dissolution-precipitation reactions on the account of the tendency of bismuth and copper to enhance the redox activity of MnO2.6 We will also show comprehensive combinatorial cyclic voltammetry and impedance analysis, where the roles of bismuth and copper will be further elucidated, and remark on the potential regions where the complexation and intercalation regions exist in the MnO2 electrochemistry. For the first time we will show real time microscopy images and video of the formation and deformation of MnO2 through dissolution-precipitation reactions.5,6 We will also report on the role of high surface area carbon and binder on MnO2 electrochemistry, where we find that they affect the electrode architecture and porosity, which turn out to be very important for the dissolution-precipitation reactions. Finally, we will present our perspective on the current understanding of MnO2 electrochemical reactions based on the aforementioned and other advanced characterizations that we have performed. Funding: This work was supported by the New York State Research and Development Authority (NYSERDA) under Project Number 58068 and US Department of Energy ARPA-E under award number DE AR0000150.

Lithium metal is a leading candidatefor next-generation electrochemicalenergy storage and therefore a key material for the future sustainableenergy economy. Lithium has a high specific energy, low toxicity,and relatively favorable abundance. The majority of lithium productionoriginates from salt lakes and is based on long (>12 months) periodsof evaporation to concentrate the lithium salt, followed by moltenelectrolysis. Purity requires separation from base metals (Na, K,Ca, Mg, etc.), which is a time-consuming, energy-intensive process,with little control over the microstructure. Here, we show how a membrane-mediatedelectrolytic cell can be used to produce lithium thin films (5–30μm) on copper substrates at room temperature. Purity with respectto base metals content is extremely high. The cell design allows anaqueous solution to be a continuous feedstock, advocating a quick,low-energy-consumption, one-step-to-product process. The film morphologyis controlled by varying the current densities in a narrow window(1–10 mA/cm2), to produce uniform nanorods, spheres,and cubes, with significant influence over the physical and electrochemicalproperties.