J. Robert Selman

The degradation of SOFC electrodes

The long-term performance degradation of SOFCs is due to instability of the cell components. Because SOFCs have a composite-solid structure, thermal cycling may also cause performance degradation. In this study, the degradation of the cell performance due to thermal cycles and due to aging is examined by the AC impedance method. A specially designed electrochemical method is used to study the cell degradation due to aging. A large positive overpotential is applied at the cathode in order to accelerate the performance degradation. The AC impedance response helps to understand how the aging affects the electrode structure and may lead to performance degradation. The impedance responses generated in this study also help to understand the physical meaning of various features in the impedance diagram, and to clarify some controversial questions in terms of microstructure change in the electrode.

Cooperative research on safety fundamentals of lithium batteries

A cooperative research program on the thermal characterization and safety of lithium batteries is being carried out at IIT/Center for Electrochemical Science and Engineering and Tohoku University. This research includes experimental work for commercial lithium secondary batteries and performance prediction for scaled-up batteries. In this work, we present a set of thermal characterization experiments for lithium secondary battery cells under normal and abuse conditions. These show that the rise in cell temperature depends strongly on cell chemistry as well as discharge rate. Computer simulation of the cycling of scaled-up lithium batteries shows that the cell temperature profile also depends strongly on the surface cooling rate. An effective thermal management system is required to operate these batteries safely. This paper reviews the basic information needed for intrinsically safe design.

Detonation tube studies of aluminum particles dispersed in air

A 5.487-m vertical detonation tube of 152-mm inside diameter instrumented to monitor both shock front (piezoelectric transducers) and reaction front (fiber-optic/light-detector probes) was used to study the detonation of both flake and atomized aluminum powders dispersed in air. Effects of concentration, surface-to-mass ratio, aluminum oxide coating, and type of initiation were evaluated. Flake-aluminum powder with a surface-to-mass ratio of 3 to 4 m 2 /g was readily detonated with detonation velocities as high as 1.65 km/sec and detonation pressures of about 5 MPa (compared to the Chapman-Jouguet values of 1.85 km/sec and about 2.5 MPa). The nominal 5-μm, 0.34 m 2 /g atomized-aluminum powder was also detonated, but with greater difficulty and some loss in detonation characteristics; i.e. maximum detonation velocities of 1.35 km/sec and detonation pressures of at most 3 MPa. Furthermore, in the case of the flake aluminum the induction times between shock and reaction fronts were often in the range characteristic of homogeneous detonations, approaching the limits of the instrumentation (about 1 μsec) whereas the atomized aluminum consistently gave substantially higher values: e.g., 14 μsec being the lowest. Increasing the aluminum oxide coating of either powder caused deterioration of detonation characteristics and tended to decouple, the shock and reaction fronts. Detonability and detonation characteristics were very sensitive to surface-to-mass ratio and rather insensitive to overall concentration. Spinning detonation was identified in all instances investigated for this phenomenon. Initiation of detonation in all cases required shock wave energy as obtained from the detonation of small charges of high explosive.

Wetting Characteristics of Carbonate Melts under MCFC Operating Conditions

The wetting behavior of the Au electrode under molten carbonate fuel cell (MCFC) operating conditions was investigated using a lab-scale cell with optical instrumentation. The general trends which were found were smaller contact angles, (i) under polarization than at open circuit, (ii) under an oxidizing atmosphere than under a reducing atmosphere, (iii) at high temperature rather than at lower temperature, and (iv) in Li-K carbonate than in Li-Na. Contact angles as a function of potential exhibit a dependence which resembles electrocapillary curves in aqueous solution, with the zero-charge (minimum wetting) potential and the open-circuit potential (OCP) being different, especially under a reducing atmosphere. The effect of electrolyte composition on the contact angle is interpreted in terms of activity and surface adsorption of oxides. The effect of polarization on wetting is analyzed from two different viewpoints, near OCP the electrocapillary theory is applied, while at high polarization the effect of migration is invoked. The inferior performance of Li/Na cells, compared to Li/K cells, at temperatures below 600°C, is analyzed from the viewpoint of capillary effects, and it is shown that these effects may explain the observed anode behavior and, at least in part, cathode behavior as well.

Polarization Effects on Meniscus Characteristics in Molten Carbonate

Meniscus heights on a gold fiag electrode partially submerged in Li/K carbonate (62/38 mole percent) at 650°C have been determined under a variety of O 2 -CO 2 gas mixtures at atmospheric pressure. The meniscus height as a function of electrode potential shows a characteristic minimum for each gas composition and a relatively steep increase with increasing cathodic polarization. Analysis of the near-equilibrium data, assuming electrocapillary effects only, showq that this is consistent with specific adsorption of oxide ion on the electrode

Electrode kinetics of oxygen reduction on gold in molten carbonate

Abstract The kinetics of oxygen reduction on gold in pure Li 2 CO 3 and Li + K carbonate eutectic melts were studied using various techniques. The steady-state current in the potentiostatic mode was very agitation-dependent, indicating that the mass transfer effect was significant even at low polarization. The stability limits of the carbonate melt and the reaction sequence of oxygen reduction were examined by the potential scan method. The kinetic parameters were determined using the potential step method. The exchange current density was found to be two orders of magnitude higher than the value suggested by potential scan measurements. The standard exchange current density in the eutectic is 11.0 mA/cm 2 at 650°C; that in pure Li 2 CO 3 at 750°C is 26.3 mA/cm 2 . The exchange reaction orders of O 2 and CO 2 , which are similar in the two melts, are quite different from the theoretical values according to the peroxide or superoxide mechanisms. The slow neutralization reaction of oxide by CO 2 , which has a strong effect on the electrode potential, is the main reason for this discrepancy.

Direct observation of the oxidation nickel in molten carbonate

Abstract Polarization of the nickel oxide (NiO) cathode limits the performance of the state-of-the-art MCFC. It is therefore important to clarify the phenomena which occur when, as is usually the case, the NiO cathode is formed in situ in the MCFC. This occurs by chemical or electrochemical reactions between nickel, which is the base material of the cathode, and molten carbonate (usually Li2CO3:K2CO3=62:38 mol%), which is the electrolyte. To clarify these formation phenomena, a direct observation method involving a telescope and CCD (charge coupled device) camera, in combination with potential measurements, is applied to the oxidation of a nickel sheet which is partially immersed in molten carbonate. In an atmosphere of pure CO2, a partially immersed nickel sheet is relatively stable, as is a gold foil even in oxidant gas. In the case of nickel exposed to oxidant gas, however, the area exposed directly to the oxidant gas is rapidly covered by an electrolyte film, and undergoes intensive chemical or electrochemical reactions with CO2 gas generation during oxidation and lithiation. As a consequence, a progressively rougher NiO surface develops over the entire sheet. After oxidation and lithiation, the non-immersed part of the sheet remains covered with electrolyte. Although the oxygen reduction current at the in situ lithiated NiO is over one order of magnitude higher than that at a gold electrode at the same applied potential, the extended meniscus region is the dominant reaction site for oxygen reduction. The same is true for the much more limited meniscus region of the gold electrode.

Conceptual design of a novel hybrid fuel cell/desalination system

Abstract A novel concept for integrating fuel cells with desalination systems is proposed and investigated in this work. Two unique case studies are discussed — the first involving a hybrid system with a reverse osmosis (RO) unit and the second — integrating with a thermal desalination process such as multi-stage flash (MSF). The underlying motivation for this system integration is that the exhaust gas from a hybrid power plant (fuel cell/turbine system) contains considerable amount of thermal energy, which may be utilized for desalination units. This exhaust heat can be suitably used for preheating the feed in desalination processes such as reverse osmosis which not only increases the potable water production, but also decreases the relative energy consumption by approximately 8% when there is an increase of just 8°C rise in temperature. Additionally, an attractive hybrid system application which combines power generation at 70%+ system efficiency with efficient waste heat utilization is thermal desalination. In this work, it is shown that the system efficiency can be raised appreciably when a high-temperature fuel cell co-generates DC power in-situ with waste heat suitable for MSF. Results indicate that such hybrid system could show a 5.6% increase in global efficiency. Such combined hybrid systems have overall system efficiencies (second-law base) exceeding those of either fuel-cell power plants or traditional desalination plants.

A model for bipolar current leakage in cell stacks with separate electrolyte loops

A model predicting leakage current in a bipolar battery stack is presented. This model applies current balance and potential balance equations to a stack and treats the electrolyte, manifold and membrane separator as resistance elements in an electric circuit analog. This results in a set of linear difference equations with constant coefficients. Leakage currents in stacks made up of different numbers of cells are predicted and the effect of each resistance component on stack performance is investigated.

Anode-supported planar solid oxide fuel cells by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) and electrostatic spray deposition (ESD): Fabrication of dense thin layers of yttria-stabilized zirconia by PE-MOCVD

This paper reports a study of plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) as a suitable technique for depositing dense, crack-free thin layers of yttria-stabilized zirconia onto porous substrates, as a step in the fabrication of anode-supported planar solid oxide fuel cells (SOFC). Our objective is to present an alternative method by which an SOFC assembly may be fabricated at lower temperature than by conventional methods. PE-MOCVD using zirconium tert butoxide (ZrTB) -and yttrium hexafluoroacetylacetonate dihydrate (Y6FA) is capable of producing the electrolyte in thin dense layers on smooth surfaces, as demonstrated for Si(110) wafers. If a porous substrate is used, the average surface pore size should not exceed 1–2 μm to obtain a dense film. The crystalline phase of the film was related to the Y6FA concentration in the gas phase using x-ray diffraction. Depth profiling, using x-ray photoelectron spectroscopy, showed that Y is present (fairly uniform) at all depths of the film. Growth rates are dependent on the applied power but independent of substrate temperature. Film density, however, shows a significant dependence on substrate temperature.