Polymer Electrolyte Fuel Cell Stack Research Articles

Design and experimental validation of an HT-PEFC stack with metallic BPP

This paper provides detailed insight into the design, construction, production and verification of a metallic bipolar plate for High Temperature Polymer Electrolyte Fuel Cell stacks. With a focusing on applications with power demands of 5–10 kW, the active cell area is set to a maximum of 100 cm2. The double-plate-concept allows for liquid cooling in the inner bipolar plate compartment. Due to the bipolar plate production by hydroforming of thin stainless steel foils the structure of the coolant compartment is dependent on the gas flow field design. To ensure proper cooling functionality a co-design is necessary. The flow field design, in conjunction with the flow configuration of the reactants (reformate, air) and coolant, considers the effects of hydrogen and oxygen depletion on current density distribution, as well as the temperature profile on carbon monoxide poisoning. These specifications are based on previously published results. For validation, a 5-cell stack with commercial Membrane Electrode Assemblies was operated at 160 °C and 0.2 A/cm2 and regularly interrupted for the polarization curve measurement. After 4700 h of continuous operation, the test was terminated due to a rapid voltage drop in one of the cells. In this paper, it is shown that novel metallic bipolar plates from thin metal sheets can be used for the long-term operation of High Temperature Polymer Electrolyte Fuel Cell stacks.

Simple and Complex Polymer Electrolyte Fuel Cell Stack Models: A Comparison

In this paper, two distinct polymer electrolyte fuel cell stack models are constructed: a detailed numerical model (DNM) employing a fine-scale computational mesh and a coarse-mesh approach based on a distributed resistance analogy (DRA) where diffusion terms in the transport equations are replaced by rate terms. Both methods are applied to a 5-cell, high-temperature polymer electrolyte fuel cell stack with an active area of 200 cm2 per cell. The polarization curve and local current density distributions from both the DRA and DNM are compared with experimental data, finding good agreement. Temperature, pressure, Nernst potential, and species distributions are also exhibited. The DNM displays details of fine-scale local extrema not captured by the DRA; however, the latter requires orders of magnitude less computer processor power and memory for execution. Both methods provide much finer-scale results than present experimental techniques.

New design of a PEFC cathode separator of for water management

Generally, polymer electrolyte fuel cells (PEFCs) need humidifiers to prevent the drying of the membrane, but this use of humidifiers creates water management issues, such as the flooding/plugging phenomena and decreased system efficiency because of an increase in the electric energy needed for auxiliary equipment. Although most researchers have developed high-temperature membranes that do not need humidifiers, a lot of time is necessary for the development of these membranes, and these membranes drive up costs. Therefore, we propose a new cathode separator design that can recycle water generated by power generation in the same cell and a stack structure that can redistribute water collected in the cathode outlet manifold to drying cells. Because the new cathode separator has a bypass channel from the gas outlet to the gas inlet to transport excess water, a dry part in the gas inlet is supplied with excess water in the gas outlet through the bypass channel even if the PEFC is operated under dry conditions. Excess water in the PEFC stack can be transported from the cell with excess water to the drying cell through the cathode outlet manifold with a porous wall. Therefore, we confirm the influence of the plugging phenomenon in the cathode gas outlet manifold on the cell performance of each cell in the stack. As a result, the cell performance of the new cathode separator design is better than that of the standard separator under the low humidity conditions. We confirm that the plugging phenomenon in the cathode outlet manifold affects the cell performance of each cell in the stack.

Heat and fuel coupled operation of a high temperature polymer electrolyte fuel cell with a heat exchanger methanol steam reformer

In this work a methanol steam reforming (MSR) reactor has been operated thermally coupled to a high temperature polymer electrolyte fuel cell stack (HT-PEMFC) utilizing its waste heat. The operating temperature of the coupled system was 180 °C which is significantly lower than the conventional operating temperature of the MSR process which is around 250 °C. A newly designed heat exchanger reformer has been developed by VTT (Technical Research Center of Finland LTD) and was equipped with commercially available CuO/ZnO/Al2O3 (BASF RP-60) catalyst. The liquid cooled, 165 cm2, 12-cell stack used for the measurements was supplied by Serenergy A/S. The off-heat from the electrochemical fuel cell reaction was transferred to the reforming reactor using triethylene glycol (TEG) as heat transfer fluid. The system was operated up to 0.4 A cm−2 generating an electrical power output of 427 Wel. A total stack waste heat utilization of 86.4% was achieved. It has been shown that it is possible to transfer sufficient heat from the fuel cell stack to the liquid circuit in order to provide the needed amount for vaporizing and reforming of the methanol-water-mixture. Furthermore a set of recommendations is given for future system design considerations.

Composites Nafion-titania membranes for Polymer Electrolyte Fuel Cell (PEFC) applications at low relative humidity levels: Chemical physical properties and electrochemical performance

The incorporation of inorganic compounds into the polymeric Nafion matrix represents a possible solution to improve the mechanical characteristics of the polymeric membranes when drastic operative conditions (higher temperatures and/or lower relative humidity levels) are adopted in a Polymer Electrolyte Fuel Cell (PEFC). On the basis of previous investigations, different commercial anatase titanium oxide (TiO2) loadings were used to prepare composite membranes with the aim of reducing the humidification levels for a PEFC stack. Membranes with three different TiO2 contents (5, 10, 15 wt.%) were cast and characterized in terms of chemical-physical and electrochemical behaviour. The influence of filler was highlighted resulting in a swelling reduction of the composite membranes if compared to a pristine recast Nafion membrane cast with the same method and used as a reference. A good proton conductivity was observed for all composite Nafion-titania (N-TiO2) membranes with values ranging 1.3–1.9·10−2 S cm−1 at 60 °C and 50%RH against a value of 1.5·10−2 S cm−1 related to the reference membrane. The composite membrane containing a 10 wt.% of TiO2 has shown an improved performance respect to the reference membrane, especially at low levels of humidification and a higher stability during the accelerated stress test (AST) carried out in drastic conditions (60 °C and 50%RH), probably due to the filler capability to reduce the swelling and give stiffness to the polymeric matrix.

Direct Simulations of Coupled Transport and Reaction on Nano-Scale X-Ray Computed Tomography Images of Platinum Group Metal-Free Catalyst Cathodes

Polymer electrolyte fuel cells (PEFCs) have been developed for automotive, residential, and portable power applications. Typically, PEFC stacks use platinum (Pt) or Pt alloys as the oxygen reduction reaction (ORR) catalyst in the cathode. Although Pt performs well, its raw material cost is significant and limits future cost reductions. As an alternative, platinum group metal-free (PGM-free) catalyst may have the potential to significantly reduce PEFC costs. Despite recent advances in PGM-free catalyst ORR activity,1 the volumetric activity remains well below that of Pt cathodes and results in thick electrodes with significant transport losses. To advance PGM-free catalyst electrodes, it is important to understand the coupling between electrochemical reactions, transport phenomena, and microstructure in the catalyst layer. Here, we expand this understanding using a combination of nano-scale X-ray computed tomography (nano-CT) and direct simulations on the CT images. Nano-CT imaging was done for cyanamide-polyaniline-iron PGM-free catalyst cathodes with three different Nafion® loadings (35, 50 and 60 wt%). Using Zernike phase contrast we differentiated the solids (Nafion®, carbon, and iron) and the pore phase,2 and by staining the electrode with cesium, we distinguished the Nafion® phase by absorption contrast in a second scan. The two data sets were overlaid to complete the structure of the whole cathode.3 In other words, at each voxel we have the volume fractions of pore, Nafion®, and carbon catalyst phases. To illustrate, Figure 1a shows the Nafion® fraction over the simulation domain where the Nafion® had agglomerated and was heterogeneously distributed. The spatially-resolved volume fraction data is then input to the simulation of oxygen, proton, and electron transport as well as the local ORR volumetric current. Figure 1b shows the ionic potential and current streamlines in Nafion® phase resulting from the distributed reaction. Acknowledgement The authors gratefully acknowledge the support the support of the technology development manager Nancy Garland and funding from the US Department of Energy, the Office of Energy Efficiency and Renewable Energy, Office of Fuel Cell Technologies. The nano-CT instrument was acquired with the support of a Major Research Infrastructure award from the National Science Foundation under Grant No. 1229090.

Dispersion of Ionomer in Catalyst Ink of Polymer Electrolyte Fuel Cell Visualized By Cryogenic Transmission Electron Microscopy

Fuel cell electric vehicles (FCEVs) are an effective solution to reduce the emission of carbon dioxide (CO2) and the consumption of petroleum fuel. The biggest issues to popularize FCEVs are cost reduction of polymer electrolyte fuel cell (PEFC) power systems, and the development of hydrogen infrastructure. On the development of PEFC stack for automotive application, the cost of catalyst and application accounts for 49% of the total stack cost at 500,000 units per year1). This estimation indicates that reducing platinum loading is important for the cost reduction of PEFC stack. However, low platinum loading increases the activity and mass transport losses and lowers the performance of PEMFC stack. Increasing oxygen reduction reaction (ORR) activity of catalyst materials such as platinum alloys is an effective way to decrease the above losses. Optimizing the structure of catalyst layer is also required to reduce the mass transport loss. This means the preparation process of catalyst layer is the important factor to reduce the performance losses on low platinum loading.In general, wet coating process is used for forming PEFC catalyst layer. Catalyst ink, dispersed platinum loaded carbon (Pt/C) and ionomer in solvent, are coated on the substrate or polymer electrolyte membrane. The catalyst ink coated substrate is dried to obtain the catalyst layer. Therefore, understanding the structure of catalyst ink is needed to control the structure formation of catalyst layer. Due to the fluidity of catalyst ink, visualization by cryogenic electron microscopy is an effective way to analyze the structure of catalyst ink. We observed the dispersion of Pt/C in catalyst ink by cryogenic scanning electron microscopy (Cryo-SEM)2). The result suggests that the dispersion of Pt/C aggregates correlates to the structure and performance of catalyst layer. However, the resolution of Cryo-SEM was insufficient to detect ionomer molecules in catalyst ink because of their molecular size.For above reason, we tried to visualize the dispersion of ionomer molecules by Cryo-TEM. Sample preparation for the observation of Cryo-TEM was based on the rapid freezing technique used in structural analysis of biological specimens3). The frozen thin film of catalyst ink were prepared by immersion into liquid Ethane. Observation samples were cooled by liquid helium in order to reduce the damage for the ionomer by electron irradiation. In this study, we focused on the effect of two factors, which were the solvent and Pt/C, on the dispersion of ionomer molecules. Cryo-TEM image of catalyst ink is shown in Fig.1. The net-like structure formed by ionomer strings were observed in case of catalyst ink. The average width of the ionomer string formed the net-like structure is less than 10nm approximately, which are the same dimension as the result analyzed by small angle X-ray scattering in previous study4). These results suggest the possibility that observed ionomer strings are bundled ionomer molecules together. The end part of the ionomer net-like structure seems to connect to the surface of the Pt/C aggregate. In this presentation, we will report the considerable mechanism that ionomer molecules form the net-like structure.

Corrosion Investigation of Ni-Free Nitrided Stainless Steel Material As Bipolar Plate for Polymer Electrolyte Fuel Cell

Bipolar plates are particularly required mechanical strength and corrosion resistance for automobile application of polymer electrolyte fuel cell (PEFC). Thin metallic bipolar plates enable to realize small-size and light-weight PEFC stacks. However, there is some problem for cost, durability, and so on. In this study, we report an improvement of the corrosion resistance of Ni-free stainless steel (SUS445) by a nitriding heat treatment. Nitriding heat treatment of Ni-free stainless steel (SUS445) was performed using a vacuum furnace under nitrogen atmosphere at 1473 K. Electrochemical measurements were carried out using a three-electrode electrochemical cell having a Pt counter electrode, an Ag/Ag2SO4 reference electrode, the SUS445 and the nitrided stainless steel (SUS445N) as a working electrodes in Ar-saturated 0.5 mol dm-3 H2SO4 solution at room temperature. Linear sweep voltammogram was obtained from a rest potential to 1.1 V vs. SHE at a sweep rate of 0.33 mV s-1. Linear sweep voltammograms of SUS445, SUS445N, and SUS316 are shown in Figure 1. The voltammogram of SUS445 has current peak corresponds to a formation of passivation layer at -0.2 V vs. SHE. The onset potential of the SUS445N voltammogram shifts toward the positive direction and there is no current peak of the passivation layer formation. The current density of SUS445N is one order of magnitude lower than that of SUS445 and SUS316. It is found that the corrosion resistance of SUS445 improved by the nitrogen heat treatment. Moreover, PEFC operation test was performed using SUS445N bipolar plates and graphite one for a comparison. The two cell voltage at 0.25 A cm-2 were almost the same up to 2500 h power generation. Therefore, the Ni-free nitrided SUS445 exhibited a high durability as the bipolar plate of PEFC. Figure 1

A fuel cell/battery hybrid power system for an unmanned aerial vehicle

A hybrid propulsion system composed of a homemade Polymer electrolyte fuel cell (PEFC) with a Lithium polymer (LiPo) battery in parallel connection was developed for Unmanned aerial vehicles (UAVs). The characteristics and performance of the system were evaluated considering its dynamic load responding capability and energy efficiency. A homemade PEFC stack composed of 36-unit cells and Balance of plant (BOP) was used to construct the fuel cell system directly connected to the propulsion system. Ten cells of a 3300-mAh 40C LiPo battery were combined with the PEFC system in parallel, and the LiPo battery was only switched on when high power was required for takeoff, acceleration and landing. The independent use of the homemade PEFC system and battery for the UAV showed a good load responding capability and a high fuel cell system efficiency of approximately 45%, which was obtained during cruising. The parasitic loss and the amount of unreacted hydrogen gas discharged outside of PEFC were nearly 3.91% and 0.89%, respectively. Results of the field test flights confirmed that the hybrid propulsion system based on the parallel connection of a PEFC system and a battery power is extremely effective in operating a UAV.

An impedance model for analysis of EIS of polymer electrolyte fuel cells under platinum oxidation and hydrogen peroxide formation in the cathode

In this study, an impedance model based on electrochemical theory of platinum oxide formation has been developed and combined with the impedance model based on hydrogen peroxide formation during the oxygen reduction reaction (ORR) and reported in a previous study to characterise inductive loops in impedance spectra of polymer electrolyte fuel cells (PEFCs). To validate the theoretical treatment, the simulated frequency response predicted by the theoretical model is compared against electrochemical impedance spectroscopy (EIS) measurements carried out in an open-cathode 16cm2 H2/air PEFC stack at three different current densities. The results show that neither model in isolation (hydrogen peroxide nor platinum oxide models) can accurately reproduce the inductive loops in the EIS measurements at low frequencies. By deriving a model considering kinetics of hydrogen peroxide and platinum oxide formation, it is possible to reproduce the inductive loops at low frequencies and to estimate the DC polarisation resistance related to the slope of the polarisation curve as frequency reaches zero during EIS. This study demonstrates that different mechanisms that cause PEFC degradation and low performance could be manifested in EIS measurements simultaneously. The resulting model could support other electrochemical techniques to quantify the rates of hydrogen peroxide and platinum oxide formation during the ORR that limit the performance of PEFCs.

Non-PGM ORR Catalysts Prepared from Polyaniline-Type Polymers with Strong Affinity to Iron

Platinum-based catalysts play an important role in electrocatalysis of the generally sluggish oxygen reduction reaction (ORR) at the polymer electrolyte fuel cell (PEFC) cathode. However, the expensive Pt-based catalysts contribute to more than 40% of the cost of the PEFC stack for automotive applications, constraining the commercialization of fuel cell power systems.1 To lower the fuel cell catalyst cost, non-platinum group (non-PGM) catalysts have drawn attention as a promising low-cost replacement for Pt-based ORR catalysts.2The development of non-PGM catalysts have been the focus of significant research effort for years, aimed at obtaining active and durable materials, typically via the heat treatment of N-containing organic compounds and transition metal salts in inert atmosphere. One of the keys to the ultimate success of those efforts is the selection of suitable nitrogen precursors for the synthesis. In this presentation, we will demonstrate a rational design of polyaniline-type (PANI-type) polymers, as the precursors for active and durable non-PGM catalysts for oxygen reduction. Recently, we have developed well-performing non-PGM catalysts by heat-treating PANI, Fe salts, and carbon in N2 atmosphere.3 The studies of this and similar non-PGM catalysts suggest that Fe-N defects in a carbon matrix may be vital for the ORR activity of such catalysts.4 If so, it is essential to promote the formation of Fe-N bonds during the catalyst synthesis. However, current approaches are still quite rudimentary, largely limited to screening of N-rich precursors and optimizing the Fe salt loading. The rationale behind selecting N-rich precursors is still lacking. In many cases, the N-containing groups in polymers, such as PANI, have poor affinity to Fe salts limiting Fe-N bond formation in the polymer precursor before the heat treatment. An approach to increasing the population of such bonds will be given in this presentation. The subject of this study are PANI-type polymers with N-containing side groups having high affinity to Fe. By this approach, we allow Fe-N bonds to form evenly in the precursor polymers before the subsequent heat treatment. A higher Fe-N bond content is observed compared to previously used PANI-Fe precursors. Following the precursor synthesis, the polymers are converted into a carbon-based non-PGM catalyst via a heat treatment under N2. The catalyst undergoes further characterization by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy, followed by electrochemical characterization and fuel cell testing. The efficiency of the Fe uptake is also determined. This new approach paves the way to rational synthesis of non-PGM ORR catalysts via a rational design of polymer precursors with strong Fe-N interaction, ultimately resulting in a higher ORR active sites. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. References Spendelow, J.; Marcinkoski, J., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2014, (2014).Wu, G.; Zelenay, P., Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 46, 1878-1889 (2013).Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 332, 443-447 (2011).Holby, E. F.; Wu, G.; Zelenay, P.; Taylor, C. D., Structure of Fe–Nx–C Defects in Oxygen Reduction Reaction Catalysts from First-Principles Modeling. J. Phys. Chem. C 118, 14388-14393 (2014).

A New, Scalable and Low Cost Multi-Channel Monitoring System for Polymer Electrolyte Fuel Cells.

In this work a new, scalable and low cost multi-channel monitoring system for Polymer Electrolyte Fuel Cells (PEFCs) has been designed, constructed and experimentally validated. This developed monitoring system performs non-intrusive voltage measurement of each individual cell of a PEFC stack and it is scalable, in the sense that it is capable to carry out measurements in stacks from 1 to 120 cells (from watts to kilowatts). The developed system comprises two main subsystems: hardware devoted to data acquisition (DAQ) and software devoted to real-time monitoring. The DAQ subsystem is based on the low-cost open-source platform Arduino and the real-time monitoring subsystem has been developed using the high-level graphical language NI LabVIEW. Such integration can be considered a novelty in scientific literature for PEFC monitoring systems. An original amplifying and multiplexing board has been designed to increase the Arduino input port availability. Data storage and real-time monitoring have been performed with an easy-to-use interface. Graphical and numerical visualization allows a continuous tracking of cell voltage. Scalability, flexibility, easy-to-use, versatility and low cost are the main features of the proposed approach. The system is described and experimental results are presented. These results demonstrate its suitability to monitor the voltage in a PEFC at cell level.

Grasp of Correlation between Degradation Factor and Cell Position of PEFC Stack

The cost reduction and the improvement of a life performance are necessary for the dissemination of Polymer Electrolyte Fuel Cell (PEFC). If the degradation of the fuel cell was able to be diagnosed at the early stage, the life performance can be improved by applying appropriate processing to PEFC. We have been proposing the PEFC Performance Diagnose System (PPDS) which diagnoses the state of each polarization (the resistance polarization, the activation polarization and the diffusion polarization) by analyzing transient response with Turuga Electric Corporation since 2002. Since 2014, the application of our PPDS to a PEFC short stack was tried because it was able to diagnose each degradation factor of a PEFC single cell. As a result, the PPDS not only found out a degradation cell in the four cells short stack but also was able to diagnose the degradation factor of this deteriorated cell. Therefore, this paper aims to grasp the correlation between the degradation factor and the cell position of the PEFC Short Stack using our PPDS, and the PPDS was able to find the unit with the deteriorated cell when it diagnoses each unit composed of four cells in the stack. As a result, because we elucidated the correlation between the degradation factor and the cell position of a stack, we can propose the structure of a PEFC stack where deteriorating is not caused easily. Moreover, we can apply our PPDS as a maintenance device for a fuel cell vehicle and a domestic use fuel cell.

Performance of advanced automotive fuel cell systems with heat rejection constraint

Although maintaining polymer electrolyte fuel cells (PEFC) at temperatures below 80 °C is desirable for extended durability and enhanced performance, the automotive application also requires the PEFC stacks to operate at elevated temperatures and meet the heat rejection constraint, stated as Q/ΔT < 1.45 kW/°C, where Q is the stack heat load for an 80-kWe net power PEFC system and ΔT is the difference between the stack coolant temperature and 40 °C ambient temperature. We have developed a method to determine the optimum design and operating conditions for an automotive stack subject to this Q/ΔT constraint, and illustrate it by applying it to a state-of-the-art stack with nano-structured thin film ternary catalysts in the membrane electrode assemblies. In the illustrative example, stack coolant temperatures >90 °C, stack inlet pressures >2 atm, and cathode stoichiometries <2 are needed to satisfy the Q/ΔT constraint in a cost effective manner. The reference PEFC stack with 0.1 mg/cm2 Pt loading in the cathode achieves 753 mW cm−2 power density at the optimum conditions for heat rejection, compared to 964 mW cm−2 in the laboratory cell at the same cell voltage (663 mV) and pressure (2.5 atm) but lower temperature (85 °C), higher cathode stoichiometry (2), and 100% relative humidity.

Fuel cell performance assessment for closed-loop renewable energy systems

Fuel cells and electrolysis are promising candidates for future energy production from renewable energy sources. Usually, polymer electrolyte fuel cell systems run on hydrogen and air, while the most of electrolysis systems vent out oxygen as unused by-product. Replacing air with pure oxygen, fuel cell electrochemical performance, durability and system efficiency can be significantly increased with a further overall system simplification and increased reliability. This work, which represents the initial step for pure H2/O2 polymer electrolyte fuel cell operation in closed-loop systems, focuses on performance validation of a single cell operating with pure H2/O2 under different relative humidity (RH) levels, reactants stoichiometry conditions and temperature. As a result of this study, the most convenient and appropriate operative conditions for a polymer electrolyte fuel cell stack integrated in a closed loop system were selected.

Optimization of an open-cathode polymer electrolyte fuel cells stack utilizing Taguchi method

The design of open-cathode polymer electrolyte fuel cells (PEFC) stacks with forced-convection requires a careful consideration on the geometrical and operating conditions as well as the operating characteristic of PEFC stacks and fan used. This paper evaluates the effect of key geometrical and operating parameters on the stack characteristic and their interactions to the thermal, water and gas managements as well as stack performance. A validated three dimensional model for open-cathode PEFC stack with fan and immediate ambient were solved to evaluate the effect of studied parameters on the stack performance. In tandem, an L27 orthogonal array (OA) of Taguchi matrix of six factors and three level designs to determine the optimum combination of parameters as well as their interactions for high, medium and low voltage operation. The result indicates that fuel cell length plays important role on determining the fuel cell performance in term of system characteristic, current density and net power. Optimum combination of design and operating parameters were obtained with the objective function of maximizing net power generated by stack by taking into account the parasitic loads.

Combined current and temperature mapping in an air-cooled, open-cathode polymer electrolyte fuel cell under steady-state and dynamic conditions

In situ diagnostic techniques provide a means of understanding the internal workings of fuel cells so that improved designs and operating regimes can be identified. Here, for the first time, a combined current density and temperature distributed measurement system is used to generate an electro-thermal performance map of an air-cooled, air-breathing polymer electrolyte fuel cell stack operating in an air/hydrogen cross-flow configuration. Analysis is performed in low- and high-current regimes and a complex relationship between localised current density, temperature and reactant supply is identified that describes the way in which the system enters limiting performance conditions. Spatiotemporal analysis was carried out to characterise transient operations in dead-ended anode/purge mode which revealed extensive current density and temperature gradients.

A self-operated polymer electrolyte fuel cell system operating at dead-end conditions using pure hydrogen and oxygen gases

The present study reports the system configurations of a self-operated Polymer electrolyte fuel cell (PEFC) that is specially designed for dead-end operation of both pure hydrogen and oxygen gas lines, including its operation features and durability. The PEFC stack with 16 unit cells was constructed using homemade membrane electrode assemblies with 300 cm2 of apparent active area and bipolar plates. A self-operated PEFC system was developed using a homemade PEFC stack, balance of plant, and electric control unit for automatic fuel cell system operation. Performance parameters of the self-operated PEFC system, including fuel utilization, parasitic loss, released heat, purge loss, and efficiency of the PEMFC stack and system were evaluated during the automatic operation of the PEFC system. A Daily start-and-stop (DSS) test was also performed for one month to check the durability of the self-operated PEFC system. Electric efficiencies of the PEFC stack and system are approximately 60% and 49%, respectively, at an operating condition of 1.1 kW electric output; no performance degradation of the PEFC system was observed for a month during the DSS test.

Optimization of Design Parameters for an Open-cathode Polymer Electrolyte Fuel Cells Stack Utilizing Taguchi Method

Abstract The design of open-cathode polymer electrolyte fuel cells (PEFC) stacks with forced-convection requires a careful consideration on the geometrical and operating conditions as well as the characteristic of PEFC stacks and fan used. The operating point should be located at the intersection point between the fan and the stack characteristic curve. This paper evaluates the effect of key parameters and operating conditions on the system characteristic and stack performance of forced convection PEFC by utilizing computational fluid dynamic approach and Taguchi statistical method. A validated three dimensional open-cathode PEFC stack model with fan and immediate ambient were employed together with an L 27 orthogonal array (OA) of Taguchi matrix of six factors and three level designs to determine the optimum combination of parameters as well as their interactions. The result indicates that fuel cell length plays important role on determining the fuel cell performance in term of current density and net power. Optimum combination of design and operating parameters were obtained with the objective function of maximizing net power generated by stack by taking into account the parasitic loads.

Combining Nitrogen Precursors in Synthesis of Non-Precious Metal ORR Catalysts with Improved Fuel Cell Performance

Platinum and platinum-alloy oxygen reduction reaction (ORR) catalysts have been implemented in the prototype, and by now also in commercial fuel cell power systems for automotive applications, successfully meeting the demanding conditions of relatively low temperature (80°C) and high acidity operating conditions of the polymer electrolyte fuel cell (PEFC). However, due to the high price of Pt, the catalyst cost is projected to account for as much as a half of the total PEFC stack cost.1 In this context, the potential replacement of Pt-based catalysts, especially in the fuel cell cathode, with non-precious metal catalysts (NPMCs) is expected to significantly reduce PEFC stack cost. Achieving such a cost benefit is, however, contingent upon reaching sufficient activity and durability with NPMC for the ORR, a requirement that the electrocatalysis research community has been striving to meet. In this presentation, we expand upon the idea of combining nitrogen precursors during NPMC synthesis to improve the performance and performance durability of non-precious metal ORR catalysts. Previously, we introduced a new non-precious metal ORR catalyst derived from two nitrogen precursors with different decomposition temperatures: easily decomposing cyanamide (CM) and more thermally stable polyaniline (PANI).2 In this approach, CM acts as an efficient pore-forming agent that allows for the formation of a highly porous structure, which in particular benefits the high-current response of the resulting electrocatalyst. Compared to the alternative approach utilizing metal-organic frameworks (MOFs),3,4 the use of cyanamide promises a very significant reduction in the NPMC cost, without sacrificing ORR activity. In an effort to further improve the PEFC performance of the two nitrogen-precursor NPMCs, we have explored approaches involving the use of ammonia treatment, the addition of various carbon supports, and inclusion of multiple transition metals. In this presentation, we will summarize the results of these studies, which have involved the extensive use of characterization techniques, such as X-ray photoelectron spectroscopy, scanning electron microscopy, scanning transmission electron microscopy, X-ray diffraction, surface area/pore distribution measurements, and nano-scale X-ray computed tomography. In the conclusion, we will also define critical factors for enhancing the performance of NPMCs in the PEFC cathode and outline directions for future research. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. Microscopy research supported through a user project at ORNL’s Center for Nanomaterials Sciences, which is an Office of Science User Facility. References Spendelow et al., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2013, (2013).Hoon T. Chung et al., “Combined nitrogen precursor approach to develop highly active non-precious metal oxygen reduction reaction catalyst.” in preparation.S. Ma et al., “Cobalt Imidazolate Framework as Precursor for Oxygen Reduction Reaction Electrocatalysts,” Chem. Eur. J. 17, 2063 (2011).Proietti et al., “Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells,” Nat. Commun. 2, 416 (2011)