Reactant Utilization Research Articles

Synthesis and characterization of CuO at nanoscale

CuO structures were synthesized by chemical precipitation method at nanoscale. The effects of different reactants including KOH+NH3·H2O, NH3·H2O+KOH, KOH or NaOH in addition to Cu(NO3)2·3H2O on the properties of CuO structures were investigated. Effects of different surfactants including zwitterionic 3-(N,N-dimethyldodecylammonio) propane-sulfonate (SB12), cationic cetyl trimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) were also investigated. Synthesized samples were characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Fourier Transform Infrared (FTIR) spectra, scanning electron microscopy (SEM) and cyclic voltammetry (CV) techniques. XRD and SEM results showed that the morphologies were significantly changed with respect to the reactant used. The significant effects of different surfactants on the structures were also observed from the experiments. The specific capacitances were changed according to the utilization of different reactants.

Reactant recirculation in electrochemical co-laminar flow cells

Co-laminar electrochemical cells utilize hydrodynamic engineering rather than a physical separator to prevent mixing of reactants. Commonly known as microfluidic fuel cells and batteries, these cells are based on stratified laminar flow of reactants in a microfluidic channel. The objective of the present work is to evaluate the feasibility and effectiveness of reactant recirculation in co-laminar flow cells with flow-through porous electrodes. Two representative device architectures are fabricated with separate outlets for reactant recirculation in Y-junction and dual-pass configurations. It is found that crossover at the electrodes reduces the open circuit potential of both cells and serves as a useful diagnostic method for assessing the degree of crossover at the outlets. Outlet concentration is monitored at a range of flow rates and indicates that asymmetric splitting of the co-laminar interface can lead to a constant 2% loss of state of charge, while the loss due to diffusion ranges from 0-40%. The dual-pass cell is shown to have greater diffusive loss due to deceleration at the splitting junction but also demonstrates better performance due to its higher convective velocity through the porous electrodes. These effects are observed during a series of discharge operations with recirculating reactants, in which the dual-pass cell results in greater reactant utilization despite its higher crossover rates. Overall, the present results demonstrate reactant recirculation as a useful strategy to combine high reactant utilization and high power density in membraneless co-laminar flow cells.

Boundary Layer Analysis of Membraneless Electrochemical Cells

A mathematical theory is presented for the charging and discharging behavior of membraneless electrochemical cells that rely on slow diffusion in laminar flow to separate the half reactions. Ion transport is described by the Nernst-Planck equations for a flowing quasi-neutral electrolyte with heterogeneous Butler-Volmer kinetics. Analytical approximations for the current-voltage relation and the concentration and potential profiles are derived by boundary layer analysis (in the relevant limit of large Peclet numbers) and validated against finite-element numerical solutions. Both Poiseuille and plug flows are considered to describe channels of various geometries, with and without porous flow channels. The tradeoff between power density and reactant crossover and utilization is predicted analytically. The theory is applied to the membrane-less Hydrogen Bromine Laminar Flow Battery and found to accurately predict the experimental and simulated current-voltage data for different flow rates and reactant concentrations, during both charging and discharging. This establishes the utility of the theory to understand and optimize the performance of membrane-less electrochemical flow cells, which could also be extended to other fluidic architectures.

Durability of Solid Oxide Electrolysis Cells for Syngas Production

Performance and durability of Ni-YSZ supported solid oxide electrolysis cells (SOECs) for co-electrolysis of H2O and CO2 at high current density was investigated. The cells consist of a Ni-YSZ support, a Ni-YSZ electrode, a YSZ electrolyte, and an LSM-YSZ electrode. The durability was examined at 800°C and electrolysis current densities of −1 or −1.5 A/cm2 with 60% reactant (H2O + CO2) utilization. The cell voltage degradation showed a strong dependence on the electrolysis current density. Electrochemical characterization of the cells showed that the degradation was mainly related to the LSM-YSZ electrode when operated at −1 A/cm2, whereas at increased current density (−1.5 A/cm2), both the Ni-YSZ and the LSM-YSZ electrode degraded. The initial degradation (0 - 200 h) is mostly due to increasing of polarization resistance, while the long term degradation is mainly caused by increasing of serial resistance.

Thermodynamic analysis of synthetic hydrocarbon fuel production in pressurized solid oxide electrolysis cells

A promising way to store wind and solar electricity is by electrolysis of H2O and CO2 using solid oxide electrolysis cells (SOECs) to produce synthetic hydrocarbon fuels that can be used in existing fuel infrastructure. Pressurized operation decreases the cell internal resistance and enables improved system efficiency, potentially lowering the fuel production cost significantly. In this paper, we present a thermodynamic analysis of synthetic methane and dimethyl ether (DME) production using pressurized SOECs, in order to determine feasible operating conditions for producing the desired hydrocarbon fuel and avoiding damage to the cells. The main parameters of cell operating temperature, pressure, inlet gas composition and reactant utilization are varied to examine how they influence cell thermoneutral and reversible potentials, in situ formation of methane and carbon at the Ni–YSZ electrode, and outlet gas composition. For methane production, low temperature and high pressure operation could improve the system efficiency, but might lead to a higher capital cost. For DME production, high pressure SOEC operation necessitates higher operating temperature in order to avoid carbon formation at higher reactant utilization. Optimal operating conditions are dependent on the total system design.

Performance and Durability of Solid Oxide Electrolysis Cells for Syngas Production

Performance and durability of Ni/YSZ based solid oxide electrolysis cells (SOECs) for co-electrolysis of H2O and CO2 at high current density were investigated. The cells consist of a Ni/YSZ support, a Ni/YSZ fuel electrode, a YSZ electrolyte, and a LSM-YSZ oxygen electrode. The cell durability was examined at 800°C and electrolysis current density of -1 or -1.5 A/cm2 with 60% reactant (H2O+CO2) utilization. The cell voltage degradation showed a strong dependence on the electrolysis current density, with an overall cell voltage degradation rate of 0.24 mV/h at -1 A/cm2 and of 0.82 mV/h at -1.5 A/cm2. Electrochemical characterization of the cells showed that the degradation was mainly related to the LSM/YSZ electrode when operated at -1 A/cm2, whereas at increased current density (-1.5 A/cm2), both the Ni/YSZ and LSM/YSZ electrodes showed degradation.

PEMFC Flow Channel Geometry Optimization: A Review

The proton exchange membrane fuel cell (PEMFC) is a particularly promising energy conversion device for use in stationary or vehicular applications. PEMFCs provide high efficiency and power density, with zero emissions, low operating temperatures, quick start-up, and long lifetime. While the usage of PEMFCs has been on the increase, their commercialization has been hindered by technical issues such as water flooding in their cathodes. Flow field optimization is one approach to mitigating these issues, as the geometry of the flow channels within a PEMFC influences reactant transport, water management, and reactant utilization efficiency, and thus the final performance of a PEMFC system. This paper looks at some of the recent research that has been focused on modeling PEMFCs, exploring phenomena in them, and improving their performance, especially through flow field optimization. This paper shows how such modeling can provide useful information for PEMFC optimization, and, based on the research reviewed, presents recommendations that can be implemented in optimizing the design of a PEMFC bipolar for maximum performance. Among more traditional designs, the reviewed research shows that a serpentine flow field with small channel and rib size would perform the best at low operating voltages, and could be further improved by utilizing diverging channels with varying heights. Furthermore, additions such as baffles have been shown to improve the performance of various flow channel designs.

Analysis and comparative study of pulsating current of fuel cells by inverter load with different power converter topologies

Fuel cells are being increasingly used for stand alone and grid connected systems in wide range of applications due to their high efficiency and low emissions. Because of unregulated nature of fuel cell voltage a power conditioning unit, consisting of DC–DC converter and an inverter, is invariably used as an interface between the fuel cell and the load in a typical fuel cell system for ac applications. Major issues with the use of fuel cells for ac applications are the low frequency pulsating current propagation on to the fuel cell side and dynamic response to various loads during transient conditions. Low frequency pulsating currents are reported to affect reactant utilization, degrade the performance and life of fuel cells. These current ripples can be reduced by filters with passive elements having bulky inductor and capacitor in the dc-link between the fuel cell and the inverter but, it will add to the weight and cost. DC–DC converters of different configurations are being used in the power conditioning unit of fuel cell systems. These converters are operated at high frequencies and the filtering units of these converters have minimal effect on low frequency ripple. But, it is observed that different configurations of power conditioner with same filtering components perform differently for the low frequency current ripple of the inverter load by mitigating the power mismatch between load and source at the DC link. This paper investigates and compares the low frequency current ripple mitigation by cascaded converters with conventional push–pull and also with series connected converters in the power conditioning stage of fuel cell system for ac applications. Parameters such as peak switching currents, the percentage of peak to DC level of low frequency current ripple are analyzed using these conversion topologies in power conditioning unit. The analytical and simulation results related to the study are presented. Key results are verified with experimental work.

Modelling of Gas Flow and Electrochemical Effects in a Single Chamber-Solid Oxide Fuel Cell (SC-SOFC)

A comparison of the gas flow distribution and reactant utilization in a single chamber solid oxide fuel cell (SCSOFC) between the parallel and perpendicular cell configuration (with respect to the gas flow direction) has been discussed in detail. An electrochemical-fluid coupled numerical model in a five layer membrane electrode assembly (MEA) is developed. The model accounts for the major transports of convective and diffusive mass transfer, electrode kinetics, and potential fields. The results show gas flow and electrochemical effects inside the cell in each configuration and suggest that the parallel design provides better gas flow than perpendicular configuration. A uniform reactant utilization is observed in case of perpendicular design as compared to the parallel configuration. In order to take advantage of both the gas flow and electrochemical performance of the cell, use of a fully porous cell (including porosity of the electrolyte) is suggested in perpendicular configuration. The model can predict the effect of gas flow/ electrochemical performance, especially in situations where several cells are placed at different distances in the duct. The model is solved using COMSOL Multiphysics 3.3a, a commercial finite element analysis (FEM) software package.

Numerical Simulation of Cell Performance in Proton Exchange Membrane Fuel Cells with Contracted Flow Field Design

The cell performance and transport phenomena in a proton exchange membrane fuel cell were analyzed numerically for cells with flow channels that are contracted at the outlet. The effects of various outlet contraction ratios and the liquid water production are examined in detail. The predicted results show that, for operating voltages higher than 0.7 V, the effect of the flow field design on the cell performance is not significant due to the weaker electrochemical reaction. However, for operating voltages lower than 0.7 V, the flow field design significantly affects the cell performance. The contracted channel design forces the reactants to flow into the gas diffusion layer and the catalyst layer, which enhances liquid water removal and increases reactant utilization, so the cell performance is improved compared with the conventional design. In addition, the cell performance increases with either decreasing the height contraction ratio or decreasing the length contraction ratio, with the decreasing length contraction ratio being preferred to the decreasing height contraction ratio due to the smaller pressure drops.

Effects of serpentine flow field with outlet channel contraction on cell performance of proton exchange membrane fuel cells

A serpentine flow field with outlet channels having modified heights or lengths was designed to improve reactant utilization and liquid water removal in proton exchange membrane (PEM) fuel cells. A three-dimensional full-cell model was developed to analyze the effects of the contraction ratios of height and length on the cell performance. Liquid water formation, that influences the transport phenomena and cell performance, was included in the model. The predictions show that the reductions of the outlet channel flow areas increase the reactant velocities in these regions, which enhance reactant transport, reactant utilization and liquid water removal; therefore, the cell performance is improved compared with the conventional serpentine flow field. The predictions also show that the cell performance is improved by increments in the length of the reduced flow area, besides greater decrements in the outlet flow area. If the power losses due to pressure drops are not considered, the cell performance with the contracted outlet channel flow areas continues to improve as the outlet flow areas are reduced and the lengths of the reduced flow areas are increased. When the pressure losses are also taken into account, the optimal performance is obtained at a height contraction ratio of 0.4 and a length contraction ratio of 0.4 in the present design.

Local transport phenomena and cell performance of PEM fuel cells with various serpentine flow field designs

The flow field design in bipolar plates is very important for improving reactant utilization and liquid water removal in proton exchange membrane fuel cells (PEMFCs). A three-dimensional model was used to analyze the effect of the design parameters in the bipolar plates, including the number of flow channel bends, number of serpentine flow channels and the flow channel width ratio, on the cell performance of miniature PEMFCs with the serpentine flow field. The effect of the liquid water formation on the porosities of the porous layers was also taken into account in the model while the complex two-phase flow was neglected. The predictions show that (1) for the single serpentine flow field, the cell performance improves as the number of flow channel bends increases; (2) the single serpentine flow field has better performance than the double and triple serpentine flow fields; (3) the cell performance only improves slowly as the flow channel width increases. The effects of these design parameters on the cell performance were evaluated based on the local oxygen mass flow rates and liquid water distributions in the cells. Analysis of the pressure drops showed that for these miniature PEMFCs, the energy losses due to the pressure drops can be neglected because they are far less than the cell output power.

A Mathematical Model of Ultra-Thin Catalyst Layer in PEFC

A mathematical model for an ultra-thin catalyst layer in PEFCs is introduced. It utilizes Nernst-Planck and Poisson equations. Calculated polarization curves are shown to compare favourably with published experimental data for ultra-thin catalyst layers. Aspects of current conversion, reactant, current distribution, and catalyst utilization are explored. The effect of catalyst layers thickness on the Pt utilization is discussed. This study gives us a better understanding of transport and reaction at the mesoscopic scale and it furnishes the directions for optimization of this type of catalyst layer.

Modelling and simulation of MOVPE of GaAs-based compound semiconductors in production scale Planetary Reactors

The paper deals with the global, reactor scale modelling and simulation of metal organic vapour phase epitaxy (MOVPE) growth of GaAs-based III–V compound semiconductor thin films in large production scale Planetary Reactor ® with various wafer load configurations. Reactor hardware and growth process improvements that aim at increased reactant utilisation efficiency, reduced gas consumption and improved layer thickness and compositional uniformity control at low wafer edge exclusion are discussed. The modelling approach consists of the computation of macroscopic gas phase transport phenomena in the process zone e.g. flow, heat and mass transfer, including thin film deposition on hot surfaces. Modelling and simulation results are used to guide the reactor chamber design, gas inlet layout and the choice of process parameters leading to overall productivity enhancement and increased robustness of the process. The model accuracy for MOVPE growth of the AlGaAs and GaInP material system in large reaction chambers is demonstrated, and the advantages of using modelling to optimise hardware and process design are highlighted.

Macroscopic Modeling and Simulation for Polymer Electrolyte Membrane Fuel Cells

A numerical simulation tool with macroscopic models of mass and heat transport, electrical conductance and electrochemical kinetics in PEM fuel cells was developed to predict the effect of operating, structural and material condition on a large- scale cell performance. As a result, the parameters of these macroscopic models can be adjusted to reproduce the measured I- V curves under 12 operating conditions for the different reactant concentration, relative humidity and reactant utilization. Using a set of the adjusted parameters, the effect of operating, structural and material conditions on the performance of a 300cm2 serpentine cell was investigated.

A study on the start-up and performance of a kW-class molten carbonate fuel cell (MCFC) stack

A kW-class internal-manifolded molten carbonate fuel cell (MCFC) stack (52 cells) was assembled with inorganic adhesive under a suitable stacking pressure. The organic compounds in the matrices were burnt out under the conditions of slow and uniform elevation of temperature and big flow of oxygen gas in the stack. The stacking pressure dropped with elevating temperature. The output power of the stack at 150 mA cm −2 was 1025.5 W when the reactant gas pressure and utilization were 0.5 MPa and 20%, respectively. The thermal–electrical efficiency of the stack was enhanced by increasing the pressure of the reactant. However, it was contrarily decreased when current density was increased.

Development of a Three-Dimensional Molten Carbonate Fuel Cell Model and Application to Hybrid Cycle Simulations

In this paper, a three-dimensional finite volume model of a molten carbonate fuel cell (MCFC) is presented. The model applies a detailed electrochemical and thermal analysis to a planar MCFC stack of given geometry and assigned input flows conditions, material properties and assigned external heat losses, calculating global energy balances and reactant utilization, as well as internal temperature and chemical composition profiles, pressure profiles, polarization losses, and current density and voltage distribution over the stack. The model is calibrated on the available data for an experimented MCFC stack manufactured by Ansaldo Fuel Cells. The comparison of 2D versus 3D simulations is first discussed, showing the importance of the 3D model for an accurate analysis of real stack operation under significant heat loss conditions, but also showing the mutual influence of heat transfer between adjoining cells and evidencing the variable electrical performances between different cells of the same stack. As a counterpart, application of the 2D model, which takes advantage by much shorter calculation time while keeping a reasonable accuracy in the overall energy balances, is anyway indicated for complex cycle analysis and cycle optimization, where a complete 3D stack analysis may be limited to selected cases. Then, the model is applied to the off-design analysis of hybrid cycles based on a recuperated gas turbine arrangement where variable pressure ratios and fuel supply conditions are assumed for the fuel cell. The effects of the operating pressure and of the adoption of different fuel compositions are considered, with a cycle layout that may include natural gas reforming or is directly fed by a syngas provided by an external source, simulating different fuel feedstocks (e.g., coal gasification). Complete results are presented, in terms of fuel cell and overall cycle performances.

Water management along the flow channels of PEM fuel cells

AbstractWater management is one of the most critical issues for high‐performance polymer‐electrolyte‐membrane (PEM) fuel cells. A water‐flux model for vapor and liquid phases along a gas flow channel is developed. The dependency of the operating and design parameters, such as temperature, pressure, gas and liquid flow rate, and size of the fuel cell, on the water management along the flow channel is demonstrated. Issues associated with liquid water and membrane/electrode dry out as well as system water balance are identified. An “intracell” water‐exchange method, which can remove or supply water within a cell depending on local demand, is described. This water management scheme effectively minimizes the issues described above. Good cell performance with a high limiting current at low reactant pressure and high reactant utilization is demonstrated with this advanced water management system. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2594–2603, 2004

Dynamic Simulation and Optimization of Cu CVD Unit Process for Environmentally Benign Manufacturing

Environmentally benign semiconductor manufacturing requires methodologies which enable cooptimization of multiple objectives, namely environmental metrics (e.g., precursor utilization efficiency, energy consumption) simultaneously with metrics for manufacturing productivity (e.g., process cycle time) and technology performance (e.g., material or product quality). We used dynamic simulation to investigate this challenge at the unit process level, incorporating essential characteristics of physical and chemical behavior of the equipment, the process, and their dynamics for a prototype system. This enabled the evaluation of multiple metrics as a function of process recipe specifications and equipment design parameters, namely the extraction of process cycle time, reactant utilization, and energy consumption as metrics for a Cu chemical vapor deposition (CVD) unit process as the prototype. Higher temperature and pressure resulted in reduced process cycle time and increased precursor utilization efficiency, producing a situation for the manufacturing and environmental metrics. In contrast, variation in precursor flow rate produced a tradeoff situation between these metrics, which could be quantified in different process parameter regimes by the simulation. Energy consumption is dominated by vacuum pump operation, placing a premium on short cycle times. In addition, energy requirements for wafer heating are reduced at higher wafer temperature because the deposition rate of the thermally activated CVD process increases rapidly with temperature, reducing the cycle time and, therefore, the energy used. These results illustrate that dynamic simulation provides a valuable guide in design-for-environment efforts which seek cooptimization of multiple metrics, win-win situations, and quantification of tradeoffs between metrics.

Enhanced Reactor Performance with Pressure and Vacuum Swing Reaction

Product separation and regeneration of sorbent was accomplished in a novel pressure swing reactor through pressurisation, adsorption, blowdown and purge steps. The switching from sorption to reaction to regeneration was tested in a two bed sorption/reaction apparatus. Models developed for the mass and momentum transfer in the catalyst bed and sorber, were solved using orthogonal collocation within the method of lines. The effects of operating conditions and cycle configurations on performance were assessed. The results from dynamic experiments with propene metathesis to produce ethene and 2-butene in a fixed-bed catalytic reactor were in agreement with model predictions. Both pressure and vacuum swing demonstrated that conversion and product quality can be enhanced by periodic cycling with greater separation obtained with vacuum swing. The separation of products help reduce the downstream processing costs of exit mixtures, enable reactant utilisation by recycling and improve product handling at subsequent stages. The efficacy of the periodic separating reactor in terms of conversion, product purity and recovery were investigated.