Karl A. Littau

CO2 separation using bipolar membrane electrodialysis

Caustic solvents such as sodium or potassium hydroxides, converted viaCO2 capture to aqueous carbonates or bicarbonates, are a likely candidate for atmospheric CO2 separation. We have performed a comprehensive experimental investigation of CO2 gas regeneration from aqueous potassium carbonate and bicarbonate solutions using bipolar membrane electrodialysis (BPMED). This system allows the regeneration of pure CO2 gas, suitable for subsequent sequestration or reaction to synthetic hydrocarbons and their products, from aqueous carbonate/bicarbonate solutions. Our results indicate that the energy consumption required to regenerate CO2 gas from aqueous bicarbonate (carbonate) solutions using this method can be as low as 100 kJ (200 kJ) per mol of CO2 in the small-current-density limit.

Heterojunction photovoltaic cell

In accordance with one aspect of the present disclosure, a solar photovoltaic device is disclosed. The semiconductor material of the solar photovoltaic device is a heterostructure of two different binary compounds of the same metal. One or both of the two different binary compounds of the same metal are doped so that they have a conduction band edge offset of greater than about 0.4 eV. The binary compound acting as the optical absorbing material of the solar photovoltaic device has a bandgap of about 1.0 eV to about 1.8 eV.Abstract A theoretical evaluation of a heterojunction photovoltaic cell is presented. Short circuit current densities, collection efficiencies, open circuit voltages, and maximum power outputs are calculated and compared to those of equivalent homojunction cells made of the two semiconductor materials forming the heterojunction. The analysis considers the abrupt p - n heterojunction and treats only the case of uniform illumination. Open circuit voltages and short circuit currents, as derived, cannot exceed the values obtained in the equivalent homojunction cells. Cell efficiencies for heterojunctions are thus bounded by the efficiencies of the corresponding homojunctions and any advantages of the cells are limited to those arising out of the window effect (low energy photons pass through one semiconductor to generate carriers in the junction region of the other): i.e., reduced surface recombination losses, and lower sheet resistances.

CO2 extraction from seawater using bipolar membrane electrodialysis

An efficient method for extracting the dissolved CO2 in the oceans would effectively enable the separation of CO2 from the atmosphere without the need to process large volumes of air, and could provide a key step in the synthesis of renewable, carbon-neutral liquid fuels. While the extraction of CO2 from seawater has been previously demonstrated, many challenges remain, including slow extraction rates and poor CO2 selectivity, among others. Here we describe a novel solution to these challenges – efficient CO2 extraction from seawater using bipolar membrane electrodialysis (BPMED). We characterize the performance of a custom designed and built CO2-from-seawater prototype, demonstrating the ability to extract 59% of the total dissolved inorganic carbon from seawater as CO2 gas with an electrochemical energy consumption of 242 kJ mol−1(CO2).

CO2 desorption using high-pressure bipolar membrane electrodialysis

The electrodialysis of gas evolving solutions may prove to be an important technology for many gas-separation applications, including CO2 and SO2 separation from mixed-gas streams. Progress on the use of electrodialysis for gas separation has been hampered by the increased resistance caused by gas bubbles on the surface of the electrodialysis membranes. This effect reduces the effective membrane surface area, causing increased voltages and reduced membrane lifetimes due to localized “hot spots” of high current density. To overcome this problem, we designed, constructed, and tested a bipolar membrane electrodialysis (BPMED) system designed to operate up to pressures as high as 20 atm. For given process conditions, operation at a sufficiently high pressure keeps all gas dissolved in solution, eliminating the problems caused by gas bubbles on the membrane surfaces. We performed CO2 desorption from aqueous bicarbonate solutions, demonstrating that high pressures decrease the resistance, voltage, and energy of the desorption process. Our results demonstrate that at high current densities (139 mA cm−2), the CO2 desorption energy from aqueous bicarbonate solutions under high-pressure operation can be 29% lower than under ambient-pressure operation.

Characteristics of lead zirconate titanate ferroelectric thick films from a screen-printing laser transfer method

Microstructures and electrical properties of lead zirconate titanate (PZT) thick films made by using a screen-printing laser transfer method have been studied. The PZT thick-film elements were first screen printed on a sapphire substrate and sintered at 1150to1250°C. Then they were bonded to a target substrate such as silicon and released from the sapphire by means of an excimer laser exposure from the backside of the sapphire substrate. This approach makes it possible to obtain highly densified films because there is less limitation on sintering conditions, and allows integrating patterned PZT thick films onto many kinds of substrates. The thick films have dielectric constants of 1397 to 1675, remnant polarization of 32to35μC∕cm2, a piezoelectric constant d31 of about −124pm∕V, and Young’s modulus Y11E(59.4GPa) of almost the same as the corresponding bulk ceramic.

Enhancing ferroelectricity in dopant-free hafnium oxide

In this study, we control the oxidant dose to promote ferroelectricity in dopant-free ALD hafnium oxide films. By lowering the oxidant dose during growth, we show that we can achieve near total suppression of the monoclinic phase in sub-10 nm hafnium oxide films with no major impurity doping. Using metal-insulator-metal structures, we demonstrate that lowering the oxidant dose can give rise to a six-fold improvement in remanent polarization. Using this technique, we observe a remanent polarization of 13.5 μC/cm2 in a 6.9 nm-thick hafnium oxide film and show that some ferroelectricity can persist in pure hafnium oxide films as thick as 13.9 nm. Using a trap-assisted tunneling model, we show the relationship between the oxidant dose and oxygen vacancy concentration in the films, suggesting a possible mechanism for the suppression of the monoclinic phase.

Front side metallization of crystalline silicon solar cells using selectively laser drilled contact openings

Selective removal of the silicon nitride dielectric layer has been demonstrated even using nanosecond range laser pulses, through carefully controlling the energy density of laser pulses. It has been found that, a laser pulse with a peak energy density of 4.3 J/cm2 or lower can remove the nitride layer without altering the underlying silicon while a pulse with a peak energy density of 4.8 J/cm2 or higher will substantially damage the silicon. With the laser energy density maintained below the threshold for silicon ablation, multiple laser pulses can be used to more completely remove the nitride layer. Also, using high quality blanket sputtered nickel film as metal contact layer and screen printed silver gridlines as an etching protection mask, a new method for front side metallization has been developed. The specific contact resistance can be reduced by about two orders of magnitude compared to the conventional screen printed and fired through silver contact, and the firing temperature can be lowered to about 500°C.

Investigation of laser ablation of silicon nitride passivation with self-doping paste for solar cell contacts

We have fabricated solar cells using a standard POCl3 process along with a PECVD SiNx AR coating. In particular, we have also used a self-doping paste with belt furnace firing in conjunction with laser ablation to remove the silicon nitride passivating layer to minimize the metal contact area. Use of laser ablation to create holes in the nitride passivation is coupled with use of a silver self-doping paste to create a selective emitter. Cell results with an efficiency of 13% were obtained. Discussion of the fabrication steps needed and the analysis of the data are given below. The cells suffer from low open circuit voltage as well as modest fill factors. Increased shading losses and unoptimized AR coating limit Jsc. The self-doping paste cells had a very high contact resistance, the root causes of which are discussed.

Characterization of a solid state air corona charging device

Two new solid state devices which produced an atmospheric air corona discharge for generating and depositing a layer of static charge for Xerographic imaging have been fabricated and characterized. One type had a parallel plate capacitive structure and the other had an interdigitated capacitive structure. It was determined that the interdigitated capacitive structure performed better than the parallel plate capacitive structure in terms of reduced power consumption, charging current stability and device reliability. Several metal electrode material alternatives were investigated and gold electrodes performed the best. The air corona’s light emission peaks were measured to be in the 350 nm to 400 nm range. Ozone gas by-product generation to ~ 13 ppm was detected for an active surface area of 5 cm^2. Charge deposition on to an imaging drum surface with a significant charging current density of 1.6E-4 A/cm^2 has been successfully demonstrated.

Developments in MEMS scale printable alkaline and Li-ion technology

Two technologies for MEMS (Microelectromechanical Systems) scale cell formation are discussed. First, the fabrication of planar alkaline cell batteries compatible with MEMS scale power storage applications is shown. Both mm scale and sub-mm scale individual cells and batteries have been constructed. The chosen coplanar electrode geometry allows for easy fabrication of series connected cells enabling higher voltage while simplifying the cell sealing and electrode formation. The Zn/Ag alkaline system is used due to the large operating voltage, inherent charge capacity, long shelf life, and ease of fabrication. Several cells have been constructed using both plated and spun-on silver. The plated cells are shown to be limited in performance due to inadequate surface area and porosity; however, the cells made from spun-on colloidal silver show reasonable charge capacity and power performance with current densities of up to 200 uA/mm2 and charge capacities of up to 18 mA-s/mm2. Second, a new printing method for interdigitated 3-D cells is introduced. A microfluidic printhead capable of dispensing multiple materials at high resolution and aspect ratio is described and used to form fine interdigitated cell features which show >10 times improvement in energy density. Representative structures enabled by this method are modeled, and the energy and power density improvements are reported.