Research Papers

J. Fuel Cell Sci. Technol. 2012;9(3):031001-031001-7. doi:10.1115/1.4005612.

This study fabricates a micro proton exchange membrane fuel cell (PEMFC) using micro electro mechanical systems (MEMS) technology. The active area of the membrane is 2 cm × 2 cm (4 cm2 ). The study is divided into two categories: [(1) the parametric experimental investigation, and (2) the durability test. This work is an attempt to find out how several parameters, including reheat temperature, the material of the current collector plates, the open ratio, and different cathode gases affect micro PEFMC performance. According to the experimental results obtained, both the conducting area and the material of the current collector plates exert great influences on the performance of the micro PEMFC, especially in the conducting area. The cell’s performance is finite when the gas reheat temperature is increased. The results show that the cell performance is better for an open ratio of 75% as compared to ratios of 50% and 67%. The concentration polarization is improved by increasing the air flow rate at high current densities, and if the GDL diffusive capability in the latter cell could be promoted, the differences between these two cells’ performances would be reduced. Furthermore, the performance at an operating voltage of 0.6 V was the most stable one among the four cases tested, and the performance deviation at a fixed operating voltage of 0.4 V was less than ±2.2%.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031002-031002-6. doi:10.1115/1.4006048.

Fuel cells consist of single cells that are connected in series to form a stack. This increases output voltage and therefore decreases current-dependent power losses, but the electric current of the stack has to flow through each single cell. In case of an increase of resistance or a failure of just one single cell the whole stack is affected. The failure tolerance of a parallel connection is higher. The serial and parallel connection of single solid oxide fuel cells (SOFC) is compared under the aspects of failure probability, power drop and stress on the single cells. With both a highly linearized and a complex SOFC model simulations have been accomplished of the connection of two single cells in parallel and in serial configuration. Additionally different connection concepts of 16 single cells were examined. Finally, an outlook on different other source or storage technologies for electric energy like batteries and photovoltaic cells is given.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031003-031003-7. doi:10.1115/1.4006050.

The contact resistance between gas diffusion layer and bipolar plate in a fuel cell stack is calculated through multiscale contact analysis, which deals with rough surfaces dependent on scales. The rough surface according to scale shows that the surface parameters vary with scale, leading to inaccurate contact resistance. A numerical model is established to reflect the contact interaction of carbon graphite fiber in the contact interface. Two separate analyses are performed, static analysis to determine the contact area and electrical conduction analysis to calculate the electrical contact resistance. Results show that the contact area decreases and the corresponding contact resistance increases as the scale decreases. To accurately estimate the contact resistance, an asymptotic contact resistance according to scale variation is predicted using error analysis. The computed contact resistance is validated via comparison with previously reported values.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031004-031004-8. doi:10.1115/1.4005608.

(La0.6 Sr0.4 )(Co0.2 Fe0.8 )O3–δ (LSCF) has been promised as a cathode material of solid oxide fuel cells at intermediate temperatures. Despite the many previous studies of LSCF that have been reported, the role of Co and Fe atoms in the oxygen ion conduction is still unclear. In this work, we aimed at presenting each valence, oxygen chemical diffusion coefficient (Dchem ) and activation energy (Ea ) related to Co and Fe in LSCF by in situ X-ray absorption spectroscopy (XAS) at high temperatures and during reduction. For quantitative analysis of X-ray absorption near edge structure (XANES) spectroscopy, these results indicated that the Co valence decreased more easily than the Fe valence. On the other hand, from relaxation plots of the Co and Fe valence during reduction, the values of Dchem and Ea related to Co and Fe were nearly equal. Considering equations showing the oxygen ion conductivity, these results would indicate that oxygen ion conductivity was contributed by Co with more oxygen vacancies rather than Fe. According to these results, a structural model with and without oxygen vacancies and the oxygen ion conduction mechanism of LSCF was speculated, that is, we found that oxygen ion conductivity was more closely related to Co than Fe in LSCF by direct observations of in situ XAS.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031005-031005-8. doi:10.1115/1.4005811.

A 500 W stack operating at medium temperatures was designed, manufactured and tested. Nanocomposite Nafion membranes/electrodes (MEA1-NZr) assemblies, containing a commercial yttria stabilized zirconia (YSZ) as a filler, were developed to work over 80 °C. A 100 cm2 cell active area with a parallel serpentine flow field as reactants distributor was used for stack realization. Preliminary electrochemical tests in a single cell and in two-cells short stack were performed at 120 °C, 3 barabs and fully hydrated gases, reaching a rated power at 50 A of about 30W in single cell and 70W in short stack. Finally, a 20 cells stack was assembled, with composite MEA-NZr, and tested at 120 °C, 3 barabs and partially humidified gases. In these conditions, a power of 433 W at 50 A was reached. Comparing these results with the short stack performance at the same current (50 A), a 24% of power loss, which corresponds to 7 W/cell, was recorded. This performance reduction could be explained considering the scale up effect passing from 2-cells to 20-cells stack. The obtained results show that the developed composite membrane-electrodes assemblies and the designed stack are suitable for working at higher temperature than traditional polymer electrolyte membrane fuel cells.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031006-031006-8. doi:10.1115/1.4006052.

This research study investigates bubble and liquid circulation patterns in a vertical column photobioreactor (PBR) both experimentally as well as computationally using computational fluid dynamics (CFD). Dispersed gas-liquid flow in the rectangular bubble column PBR are modeled using Eulerian-Lagrangian approach. A low Reynolds number k-epsilon CFD model is used to describe the flow pattern near the wall. A flat surface bubble column PBR is used to achieve sufficient light penetration into the system. Bubble size distribution measurements were completed using a high-speed digital camera. Operating parameters, bubble flow patterns, and internal hydrodynamics of a bubble column reactor were studied, and the numerical simulations presented for the hydrodynamics in a bubble column PBR account for bubble phenomena that have not been sufficiently accounted for in previous research. Bubble size and shape affect the hydrodynamics as does bubble interaction with other bubbles (multiple bubbles in a flow versus single bubbles and wall effects on bubble(s) that are not symmetrical or bubbles not centered on the reactor cross-section). Understanding the bubble movement patterns will aid in predicting other design parameters like mass transfer (bubble to liquid and liquid to bubble), heat transfer (within the PBR and between the PBR and environment surrounding the PBR), and interaction forces inside the PBR. The computational results are validated with experimental data and from current literature.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031007-031007-9. doi:10.1115/1.4006053.

A one-dimensional, dynamic proton exchange membrane fuel cells stack model is developed in this paper, where the transports of reactant and water (in both liquid and vapor phase) are described by partial differential equations (PDEs) in gas diffusion layers (GDLs) of both anode and cathode, and the lumped model is applied to channels and MEA. The boundary conditions needed for PDEs in GDLs are provided by the lumped model. In addition, the convection term is considered in PDEs for GDLs to describe the convection effect on hydrogen gas purge process on the anode side. As a result, the purge effect under medium current density (corresponding to ohmic polarization dominated region) can be simulated in an efficient manner by improving the mass transfer and reducing the effect of water back diffusion from cathode to anode. The presented gas purge model is validated by the experimental data obtained from our laboratory as well as other research group. The influence factors to the gas purge schedule on the anode side, such as the purge interval and purge time, are investigated as well.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031008-031008-8. doi:10.1115/1.4006054.

This work studied internal loss factors (flow rate, pressure, partial pressure, voltage, current, temperature, etc.) in a unit cell and stack of solid oxide fuel cells (SOFC) with different separator materials, operation temperature and types of gas supply channels by employing computational fluid dynamics (CFD). A steel separator (AISI430) was superior to a ceramic plate (LaCrO3 ) in an aspect of thermal stress due to high thermal conductivity but inferior at average current density and fuel utilization rate. As initial temperature at the cell inlet was lowered from 950 °C to 650 °C per each pattern of gas flow (co-flow and counter-flow), useful data were acquired to analyze a performance drop. I-V curves at 650 °C and 900 °C, which involved various parameters as separator materials and directions of gas supply, compared performance characterization between low and high temperature SOFC and also implied the most effective combination.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031009-031009-7. doi:10.1115/1.4006474.

The potential to improve the CO tolerance of a high temperature proton exchange membrane fuel cell (HT-PEMFC) was investigated by introducing a platinum-ruthenium alloy as anode catalyst. The electrolyte was a H3 PO4 doped poly-2,5-benzimidazole polymer (ABPBI). The experiments were carried out at the temperatures between 403 and 443 K with a CO concentration in the H2 feed gas between 0 and 6.5 vol%. The alloy anode catalyst lowers significantly the negative influence of CO in the feed, exceeding the known temperature dependent CO poisoning mitigation in HT-PEMFCs. It was found that the voltage loss of a HT-PEMFC with PtRu anode catalyst was lower than that of a similar cell equipped with Pt anode. The dynamic cell voltage response to a current step was analyzed under CO influence, as well. The PtRu bimetallic anode electrode was found to lower the observed voltage overshoot behavior after a current step, if compared to conventional Pt anode.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031010-031010-7. doi:10.1115/1.4006475.

A three-dimensional, multicomponent, two-fluid model developed in the commercial CFD package CFX 13 (ANSYS Inc.) is used to investigate the effect of porous media compression on water transport in a proton exchange membrane fuel cell (PEMFC). The PEMFC model only consist of the cathode channel, gas diffusion layer, microporous layer, and catalyst layer, excluding the membrane and anode. In the porous media liquid water transport is described by the capillary pressure gradient, momentum loss via the Darcy-Forchheimer equation, and mass transfer between phases by a nonequilibrium phase change model. Furthermore, the presence of irreducible liquid water is taken into account. In order to account for compression, porous media morphology variations are specified based on the gas diffusion layer (GDL) through-plane strain and intrusion which are stated as a function of compression. These morphology variations affect gas and liquid water transport, and hence liquid water distribution and the risk of blocking active sites. Hence, water transport is studied under GDL compression in order to investigate the qualitative effects. Two simulation cases are compared; one with and one without compression.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):031011-031011-10. doi:10.1115/1.4006490.

Direct methanol fuel cells (DMFCs) are attractive for various applications, above all, however, as replacements for batteries or accumulators. They may be used in different power classes. A market analysis indicated that the use of a DMFC energy system in the kW class had the best chance of commercial realization if applied in forklift trucks for material handling in large distribution centers or warehouses. An advantage of such energy systems is that there is no need for the relatively time-consuming recharging of the lead-acid batteries, nor is it necessary to have spare batteries available for multishift operation. This calls for DMFC energy systems that are capable of replacing the existing Pb accumulators in terms of space requirements and energy. However, this requires considerable improvements to be made in terms of power and stability over time of DMFC systems and, in comparison to their present status, an increase of overall efficiency. Recent cost analyses for the overall system; for example, show that for the DMFC stack, a durability of at least 5000 h must be achieved with an overall efficiency for the DMFC system of at least 30%, with the constraint that the system can be operated in a water-autonomous manner up to an ambient temperature of 35 °C. As part of a joint R&D project with industrial partners, two systems were constructed and each subjected to long-term testing for 3000 and more than 8000 h, respectively, with realistic load profiles from driving cycles. In this test, the stack from the first system, DMFC V 3.3–1, displayed an aging rate of approximately 52 μV h−1 at a current density of 100 mA cm−2 . This corresponds to a performance degradation of 25% over a period of 3,000 h. The DMFC V 3.3–2 system, a modified and optimized version of the first system, also underwent long-term testing. In this case, the aging rate of the stack was only approximately 9 μV h−1 at a current density of 100 mA cm−2 . The system has thus been operated to date for more than 8000 h under realistic load profiles.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Fuel Cell Sci. Technol. 2012;9(3):034501-034501-3. doi:10.1115/1.4006051.

Perfluorosulfonic acid polymer membranes, such as Nafion®, Aciplex®, and Flemion®, are attracting great attention as electrolyte membranes for fuel cells because they have high proton conductivity. In this paper, the effect of γ rays on the electrical properties of perfluorosulfonic acid membranes by applying a variable voltage and measuring the current and dimension of the sample have been investigated. In order to understand the irradiation effects on conductivity changes, we have used optical absorption measurement in the wavelength of 190 ∼ 500 nm. The results show that the electrical conductivity has been increased by increasing the absorption dose, and intensity of the absorption bands have increased by increasing the absorption dose. The electrical conductivities of the irradiated membranes at room temperature are enhanced about one order of magnitude higher than the conductivities of an unirradiated membrane. Moreover, an optical absorption measurement shows that the structures of the membranes are modified by γ radiation. Intensity absorption bands associated to fluorocarbon and peroxy radicals and C=O groups increased with increasing the dose. The change in electrical conductivity seems to be related to the high production of peroxy radicals in the irradiated membrane.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):034502-034502-6. doi:10.1115/1.4006055.

The objective of this project is to develop a proton exchange membrane (PEM) fuel cell powered scooter with a designed digital controller to regulate the air supply to PEM fuel cell stack. A 500-Watt (W) electric power train was chosen as a platform for the scooter. Two 300 W PEM fuel cell systems, each containing 63 cells, were used to charge 48-Volt batteries that powered an electric motor. The energy carrier (hydrogen) was stored in two metal hydride tanks, each one containing 85 gs of hydrogen pressurized to 250 psig. The output hydrogen pressure from each tank was maintained at 5.8 psi by a two-stage pressure regulator, and then delivered to each fuel cell stack. To regulate the voltage of each PEM fuel cell under different load conditions, two step down DC/DC converters were used. These converters were connected in series to power the motor controller and charge the batteries. The batteries then supplied power to the 500 W brushless motor mounted to the hub of the rear wheel to save space. After all modifications were completed, most of the parts of the scooter stayed the same except for the panel under the seat—where larger space is needed for accommodating the hydrogen tanks. The weight of the scooter did not change significantly, because the weight of the hydrogen tanks (6.5 kg each) and fuel cell stacks (1.7 kg each) was partially compensated by replacing the batteries from the old ones that weighed 17.5 kg to new ones that weighed 9 kg.

Commentary by Dr. Valentin Fuster
J. Fuel Cell Sci. Technol. 2012;9(3):034503-034503-4. doi:10.1115/1.4006473.

A pumpless fuel supply using pressurized fuel with autonomous flow regulation valves is presented. Since micropumps and their control circuitry consume a portion of the electrical power generated in fuel cells, fuel supply without micropumps makes it possible to provide more efficient and inexpensive fuel cells than conventional ones. The flow regulation valves in the present system maintain the constant fuel flow rate from the pressurized fuel chamber even though the fuel pressure decreases. They autonomously adjust fluidic resistance of the channel according to fuel pressure so as to maintain constant flow rate. Compared to previous pumpless fuel supply methods, the present method offers more uniform fuel flow without any fluctuation using a simple structure. The prototypes were fabricated by a polymer micromolding process. In the experimental study using the pressurized deionized water, prototypes with pressure regulation valves showed constant flow rate of 5.38 ± 0.52 μl/s over 80 min and 5.89 ± 0.62 μl/s over 134 min, for the initial pressure in the fuel chamber of 50 and 100 kPa, respectively, while the other prototypes having the same fluidic geometry without flow regulation valves showed higher and gradually decreasing flow rate. The present pumpless fuel supply method providing constant flow rate with autonomous valve operation will be beneficial for the development of next-generation fuel cells.

Commentary by Dr. Valentin Fuster

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