Channel Dimension Constraints for Miniature Low Humidity PEM Fuel Cells

Numerous applications exist requiring power for small loads (<5W) with minimal mass operating in extreme ambient conditions. Making progress toward reducing stack mass, we investigate the influence of flow field channel depth and endplate compression on cell performance. The best performance was found at endplate compressions of 139 psi, cathode channel depths of 0.032 in and anode channel depths of 0.032 in. The maximum power mass-density achieved with these 4.84 cm2 cells was 16.8 mW/g in a single cell stack. If deployed in a multicell stack, this same performance would translate to a power mass-density of 45.3 mW/g, nearing the performance of off-the-shelf lithium ion batteries (approximately 70 mW/g).

The Design and Development of an Evaluation System for Redox Characteristics of Anode Supported SOFCs Using In-Situ Acoustic Emission and Electrochemical Technique

In this study, a redox evaluation system for anode supported SOFCs using in-situ acoustic emission (AE) and electrochemical technique has been developed. The system consists of a gas blending unit, moisture controlling unit, AE cell evaluation probe, gas cooling exhaust, electrochemical cell test system and AE signal measurement system. The anode supported coin cells, which have the same thickness dimension as practical SOFCs have, can be evaluated under temperature and atmosphere controlled conditions. The oxygen partial pressure in the anodic atmospheres can be gradually controlled from air to reducing atmosphere using the gas blending unit which is connected to 6 gas cylinders. Humidity in the anodic atmospheres can be controlled by moisture controlling unit which consists of 2 bubblers form 0.86% (5°C saturation) up to 80% (94°C saturation). Redox process of the anode can be simulated in this system by controlled three oxidation modes, i.e. O2 gas oxidation, steam oxidation and electrochemical oxidation, which correspond to actual troubles, i.e. gas leakage, degradation of downstream and fuel depletion, respectively. An AE transducer can monitor the cell condition via an inner tube for a guide of exhaust from the cathode. Redox cell test for the anode supported coin cell has been examined at 770°C using this system. After the reduction of the anode substrate in moist H2, current 0.5Acm−2 loaded to the cell. And then H2 gas concentration had been reduced by stages. The cell voltage was down to below −6V after H2 gas concentration was reduced to pH2 = 2%. This drastic cell voltage drop and AE signal generation occurred at the same time. It is considered that Ni re-oxidation with fracture started at this time. Local delamination between anode and electrolyte, and also cracks at the electrolyte and cathode were observed after redox test. It was confirmed that AE sensing is effective for redox evaluation.

Numerical Study on Electric Characteristics and Thermal Stresses of Solid Oxide Fuel Cells

A generalized, three-dimensional (3D) mathematical model of solid oxide fuel cells (SOFCs) for various geometries is constructed in this paper. A finite-volume method is applied to calculate the electric characteristics, which is based on the fundamental conservation law of mass, energy and electrical charge. The electrical potential distribution, the current density distribution, the concentrations distribution of the chemical species and the temperature profile are calculated by solving the governing equations of a single-unit model with double channels of co-flow and counter-flow pattern using the commercial computational fluid dynamic software Fluent. The internal steam reforming and the water shift reactions are taken into account in the mathematical model. The Knudsen diffusion is considered for computation of the gases diffusion in the porous electrodes and the concentration overpotential. The Butler-Volmer equation and the function of the reaction gases composition for the exchange density are used in the model to analyze the activation overpotential. Numerical simulations are performed for a planar geometry solid oxide fuel cell and the detailed features of the temperature, the electrical potential distribution and the gases composition are illustrated. The simulation results agree well with the Benchmark results for planar configuration. With the simulated temperature profile in the planar SOFC, the finite-element method is employed to calculate the thermal stress distribution in the planar solid oxide fuel cell. A 3D finite-element model consists of positive electrode-electrolyte-negative electrode (PEN) and interconnects assembly is constructed by using commercial finite-element code Abaqus. The effects of temperature profile, electrodes and electrolyte thickness, and coefficients of thermal expansion (CTE) mismatch between components are characterized. The calculated results indicate that the maximum stress appears on the electrode and electrolyte interface. The value and distribution of the thermal stress are the functions of the applied materials CTE, applied temperature profiles and the thicknesses of electrode and electrolyte. The calculated results can be applied as the guide for the SOFC materials selection and the SOFC structure design.

Recent Advances in High Temperature Electrolysis at Idaho National Laboratory: Single Cell Tests

An experimental investigation on the performance and durability of single solid oxide electrolysis cells (SOECs) is under way at the Idaho National Laboratory. In order to understand and mitigate the degradation issues in high temperature electrolysis, single SOECs with different configurations from several manufacturers have been evaluated for initial performance and long-term durability. A new test apparatus has been developed for single cell and small stack tests from different vendors. Single cells from Ceramatec Inc. show improved durability compared to our previous stack tests. Single cells from Materials and Systems Research Inc. (MSRI) demonstrate low degradation both in fuel cell and electrolysis modes. Single cells from Saint Gobain Advanced Materials (St. Gobain) show stable performance in fuel cell mode, but rapid degradation in the electrolysis mode. Electrolyte-electrode delamination is found to have significant impact on degradation in some cases. Enhanced bonding between electrolyte and electrode and modification of the microstructure help to mitigate degradation. Polarization scans and AC impedance measurements are performed during the tests to characterize the cell performance and degradation.

Optimization of Purge Cycle for Dead-Ended Anode Fuel Cell Operation

This paper focuses on the optimization of the purge cycle for dead-ended anode (DEA) operation of a proton exchange membrane (PEM) fuel cell. Controling the purge interval at given operating conditions can optimize the fuel cell efficiency and hydrogen loss during the purge. For this optimization, a model capturing the liquid water and nitrogen accumulation in the anode and the purge flow behavior is presented. A target range of purge interval is then defined based on the minimal purge time that removes the plug of liquid and nitrogen in the channel end and the maximum purge interval beyond which hydrogen is wasted since hydrogen molar fraction all along the channel has been restored to one. If the purge is sufficiently long that all of the accumulated water and nitrogen are removed then the power output in the subsequent cycle (galvanostatic operation) would be highest, compared with incomplete purges which do not fully restore hydrogen concentration in the anode. Such purge schedule, however, is associated with certain amount of hydrogen loss. Therefore, there is a trade-off between hydrogen loss and power output, and a corresponding purge interval that produces the largest efficiency. The optimum purge intervals for different cycle durations are identified. The calculated DEA efficiencies are compared with flow-through (FT) operation. The analysis and model-based optimization methodology presented in this paper can be used for optimizing DEA operation of PEMFC with minimum experimentation and development time.

Modeling of Proton Exchange Membrane Fuel Cell (PEMFC) Performance by Using Genetic Algorithm-Polynomial Neural Network (GA-PNN) Hybrid System

In the present study, a genetic algorithm-polynomial neural network (GA-PNN) was used for modeling proton exchange membrane fuel cell (PEMFC) performance, based on some numerical results which were correlated with experimental data. Thus, the current density was modeled in respect of input (design) variables, i.e., the variation of pressure at the cathode side, voltage, membrane thickness, anode transfer coefficient, relative humidity of inlet fuel and relative humidity of inlet air. The numerical data set for the modeling was divided into train and test sections. The GA-PNN model was introduced with 80% of the numerically-validated data and the remaining data was used for testing the appropriateness of the GA-PNN model by means of two statistical criteria.

Characterization of the Fracture Energy of a PFCB/PVDF Polymer Electrolyte Fuel Cell Membrane Using a Knife Slit Test

Polymer electrolyte membranes (PEM) undergo hygrothermal stress cycling in an operating fuel cell which may lead to pinhole or crack formation and propagation resulting in membrane failure. The fracture energy of a material, measured by fracture tests, is the energy needed for a crack to propagate throughout the material. In this study, the fracture energy of a promising novel fuel cell membrane comprised of a blend of a sulfonated perfluorocyclobutane (PFCB) block copolymer and polyvinylidene fluoride (PVDF) is investigated in various environmental conditions using a knife slit test. Fracture energies determined using the knife slit test have been shown to be several orders of magnitude lower, and therefore closer to the intrinsic fracture energy of a material, than those found by other fracture tests of related membranes. It is believed that the intrinsic fracture energy can give insight into the fracture resistance and durability of the polymer blend membrane. A polymer blend of 70% PFCB and 30% PVDF was tested at dry and nominally 10% relative humidity conditions at 40, 70, and 90°C, as well as at 70°C and nominally 50% relative humidity, to assess the effect of environmental conditions on fracture energy. Results show that the PFCB/PVDF blend had comparable fracture energy to a baseline fuel cell material, Nafion® NRE 211. In addition, the fracture energy of the blend was found to lie between that of the PFCB and PVDF components.

A Solid Oxide Fuel Cell Radiation Model for the Mitigation of Temperature Gradients Using a Novel Geometric Design

Solid oxide fuel cell (SOFC) systems possess the capability for highly-efficient power production at a low level of emissions. However, these fragile, high-temperature cells are prone to thermal failure, which shortens their life-span and hinders their marketability. Thermal radiation has proven itself effective at mitigating temperature gradients, and preventing thermal failure, in tubular SOFC stacks. However, the conventional planar SOFC design does not allow for sufficient radiation exchange inside the gas channels to have a significant impact on temperature gradients. For the purpose of investigating unconventional planar SOFC designs that would optimize radiation exchange, a 1-D radiation model was developed. This model focuses on the radiation exchange within the flow channels of an SOFC, and resolves a radiation profile based on the current temperature profile of the cell. This radiation model was integrated into a 1-D, planar SOFC Matlab-Simulink™ model developed for the HyPer fuel cell/gas turbine facility at NETL in Morgantown, WV. This paper describes the development of this radiation model, and verifies the model outputs against those of another SOFC radiation model. The model presented is designed to characterize SOFC operation over a broad range of geometric settings and operating parameters in order to determine the optimal conditions for radiation exchange. The capability of this model to characterize SOFC operation, for conventional and unconventional geometric designs, is presented and analyzed.

Examining the Integration of Fuel Cell Systems Into Buildings Through Simulation

The widespread use of combined heat and power (CHP) distributed generation (DG) for buildings could significantly increase energy efficiency and reduce greenhouse gas and air pollution emissions. By displacing both electricity from conventional centralized power plants and heat from decentralized boilers, CHP DG could reduce primary feedstock fuel consumption in the U.S. by approximately 20%, or 6,000 terawatt hours. However, optimally integrating CHP DG within buildings is challenging. This work aims to elucidate optimal system sizing and design of micro-CHP fuel cell systems (FCSs) integrated with commercial buildings. This modeling effort compares and contrasts the performance of high temperature polymer electrolyte membrane (PEM) fuel cell systems (HTPEM FCSs) and solid oxide fuel cell (SOFC) systems for commercial buildings. A parallel research effort is independently analyzing measured data from HTPEM FCSs installed in commercial buildings. Measured data from that effort is integrated into this modeling work. In certain regions, there has been a research and development and commercialization trend moving from using low temperature PEM FCSs (e.g. with a stack temperature of around 80°C) to using HTPEM FCSs (e.g. with a stack temperature of around 160°C) and to using SOFC systems (e.g. with a stack temperature of around 700°C) for CHP building applications, given the higher temperature of the available waste heat from these systems. In this work FCS performance data is coupled with building energy system models from the U.S. Department of Energy (DOE) using EnergyPlus™ whole-building energy simulation software. Using these baseline reference commercial building model data, parameters are examined including heat demand for space heating and for domestic hot water heating over time, temperatures and water flow rates associated with this heat demand, and building electrical demand over time, to evaluate FCS integration within the building. Examining the data obtained through the simulation exercise in this work, it is found that in a large office building, with heat demand temperatures in the range of 82°C for space heating and 60°C for hot water heating, an HTPEM FCS with an exhaust temperature of 47°C can potentially access, at a maximum, 19% of the total building heating demand. By contrast, in a small office building, with heat demand temperatures in the range of 23°C (supply air temperature) for space heating and 60°C for hot water heating, it is found that this HTPEM FCS can potentially access, at a maximum, 90% of the total building heating demand. Examining the temporal characteristics of the building heat demand to determine FCS sizing, it is found that a maximum of 50% of the time, the heat demand can be served with an HTPEM FCS with a thermal capacity of 8 kilowatts (kW) (0.05 kW for small office) and an electrical capacity of approximately 4.5 kilowatts-electric (kWe) (0.45 kWe for small office). A maximum of 80% of the time, the heat demand can be served with an HTPEM FCS with a thermal capacity of 85 kW (0.16 kW for small office) and an electrical capacity of approximately 73 kWe (0.14 kWe for small office). The simulation results further indicate that an SOFC has advantages over an HTPEM FCS that originate from its higher exhaust temperature (between 25°C and 315°C), which allows it to meet a greater percentage of the building heating demand (up to 100%). This enables an SOFC to serve a larger percentage of the building stock and a wider variety of building heating systems. Furthermore, if the CHP FCSs are grid independent (i.e., it is not possible to supply electrical power back to the grid), then the heat-to-power ratio of an FCS can be an important parameter. In such a scenario, the heat-to-power ratio of an SOFC (approximately 0.33) is closer to the heat-to-power ratio of a building (approximately 0.081, averaged over an entire year). In a stand-alone configuration, when the CHP DG has a heat-to-power ratio that more closely matches that of the buildings, the utilization of the DG system is likely to be higher and its economics and environmental impacts more favorable.

Forecasting Reactant Ignition in Solid Oxide Fuel Cell Systems

Solid oxide fuel cell electrochemical stacks require high quality reformate for performance and durability. Insufficiently mixed reactants, carbon deposits, or improper chemical ratios thereof can result in reactant ignition during mixing prior to catalysis. Reactant ignition can warp and plug downstream components; therefore, it is desirable to predict and mitigate reactant ignition. Leading machine learning techniques were applied to the task of predicting ignition events in prototype (diesel-fueled) solid oxide fuel cells at a 30-second event horizon, using both current signal state and up to 30 seconds of signal history to make predictions. Based upon our analysis, first-order particle filtering using Fisher discriminant meta-reasoning provided the best cross-system performance when compared to other meta-reasoning methods (e.g., logistic regression, kernel support vector machine) as well as traditional vector quantization. In this paper, we demonstrate particle filter construction using data from eleven sensors, analyze predictive performance on real-world data, and discuss modifications to handle further system design changes.