Three-dimensional numerical analysis of PEM fuel cells with straight and wave-like gas flow fields channels

Using a three-dimensional computational model, numerical simulations are performed to investigate the performance characteristics of proton exchange membrane fuel cells (PEMFCs) incorporating either a conventional straight gas flow channel or a novel wave-like channel. The simulations focus particularly on the effect of the wave-like surface on the gas flow characteristics, the temperature distribution, the electrochemical reaction efficiency and the electrical performance of the PEMFCs at operating temperatures ranging from 323 K to 343 K. The numerical results reveal that the wave-like surface enhances the transport of the reactant gases through the porous layer, improves the convective heat transfer effect, increases the gas flow velocity, and yields a more uniform temperature distribution. As a result, the efficiency of the catalytic reaction is significantly improved. Consequently, compared to a conventional PEMFC, the PEMFC with a wave-like channel yields a notably higher output voltage and power density.     

Cold-start icing characteristics of proton-exchange membrane fuel cells

Understanding the icing characteristics of proton-exchange membrane fuel cells (PEMFCs) is essential for optimizing their cold-start performance. This study examined the effects of start-up temperature, current density, and microporous layer (MPL) hydrophobicity on the cold-start performance and icing characteristics of PEMFCs. Further, the cold-start icing characteristics of PEMFCs were studied by testing the PEMFC output voltage, impedance, and temperature changes at different positions of the cathode gas diffusion layer. Observation of the MPL surface after cold-start failure allowed determination of the distribution of ice formation at the catalytic layer/MPL interface. At fuel cell temperatures below 0 °C, supercooled water in the cell was more likely to undergo concentrated instantaneous freezing at higher temperatures (−4 and −5 °C), whereas the cathode tended to freeze in sequence at lower temperatures (−8 °C). In addition, a more hydrophobic MPL resulted in two successive instantaneous icing phenomena in the fuel cell and improved the cold-start performance.     

Carbon monoxide poisoning of proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) performance degrades when carbon monoxide (CO) is present in the fuel gas; this is referred to as CO poisoning. This paper investigates CO poisoning of PEMFCs by reviewing work on the electrochemistry of CO and hydrogen, the experimental performance of PEMFCs exhibiting CO poisoning, methods to mitigate CO poisoning and theoretical models of CO poisoning. It is found that CO poisons the anode reaction through preferentially adsorbing to the platinum surface and blocking active sites, and that the CO poisoning effect is slow and reversible. There exist three methods to mitigate the effect of CO poisoning: (i) the use of a platinum alloy catalyst, (ii) higher cell operating temperature and (iii) introduction of oxygen into the fuel gas flow. Of these three methods, the third is the most practical. There are several models available in the literature for the effect of CO poisoning on a PEMFC and from the modeling efforts, it is clear that small CO oxidation rates can result in much increased performance of the anode. However, none of the existing models have considered the effect of transport phenomena in a cell, nor the effect of oxygen crossover from the cathode, which may be a significant contributor to CO tolerance in a PEMFC. In addition, there is a lack of data for CO oxidation and adsorption at low temperatures, which is needed for detailed modeling of CO poisoning in PEMFCs. Copyright © 2001 John Wiley & Sons, Ltd.     

Two-phase flow in GDL and reactant channels of a proton exchange membrane fuel cell

Understanding the effect of two-phase flow in the components of proton exchange membrane fuel cells (PEMFCs) is crucial to water management and subsequently to their performance. The local water saturation in the gas diffusion layer (GDL) and reactant channels influences the hydration of the membrane which has a direct effect on the PEMFC performance. Mass transport resistance includes contributions from both the GDL and reactant channels, as well as the interface between the aforementioned components. Droplet–channel wall interaction, water area coverage ratio on the GDL, oxygen transport resistance at the GDL–channel interface, and two-phase pressure drop in the channels are interlinked. This study explores each factor individually and presents a comprehensive perspective on our current understanding of the two-phase transport characteristics in the PEMFC reactant channels.     

Research on two‐phase flow considering hydrogen crossover in the membrane for a polymer electrolyte membrane fuel cell

Hydrogen crossover has an important effect on the performance and durability of the polymer electrolyte membrane fuel cell (PEMFC). Severe hydrogen crossover can accelerate the degradation of membrane and thus increase the possibility of explosion. In this study, a two‐phase, two‐dimensional, and multiphysics field coupling model considering hydrogen crossover in the membrane for PEMFC is developed. The model describes the distributions of reactant gases, current density, water content in membrane, and liquid water saturation in cathode electrodes of PEMFC with intrinsic hydrogen permeability, which is usually neglected in most PEMFC models. The conversion processes of water between gas phase, liquid phase, and dissolved water in PEMFC are simulated. The effects of changes in hydrogen permeability on PEMFC output performance and distributions of reactant gases and water saturation are analyzed. Results showed that hydrogen permeability has a marked effect on PEMFC operating under low current density conditions, especially on the open circuit voltage (OCV) with the increase of hydrogen permeability. On the contrary, the effect of hydrogen permeability on PEMFC at high current density is negligible within the variation range of hydrogen permeability in this study. The nonlinear relations of OCV with hydrogen diffusion rate are regressed.     

Visualization,understanding, and mitigation of process-induced-membrane irregularities in gas diffusion electrode-based polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cells (PEMFC) show substantial promise for their application in electric vehicles. For large-scale manufacturing of PEMFCs, roll-to-roll coated gas-diffusion-electrodes (GDE) offer certain advantages over other production pathways. Procedures including hot pressing and coating an ionomer overlayer may be necessary for this manufacturing pathway to enable a suitable catalyst layer/membrane interface. The same procedures may potentially introduce membrane irregularities, especially when thin membranes are used. Limited understanding exists regarding if and to what extent such irregularities impact PEMFC performance and lifetime, and therefore be considered defects.In this study, NREL's customized fuel cell hardware that enables quasi in-situ infrared (IR) thermography studies was utilized to visualize spatial hydrogen crossover and identify membrane irregularities that originated from the GDE-based MEA fabrication process. The structure of these membrane irregularities was investigated by scanning electron microscopy (SEM) and its impact on initial H₂/air performance was determined. Accelerated stress testing (AST) revealed that these irregularities develop into failure point locations. These results were validated across many MEAs with identified process-induced membrane irregularities. By selecting specific gas diffusion media properties and by fine tuning the MEA hot pressing parameters, the formation of such membrane irregularities was mitigated.     

Water transport in polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cell (PEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary applications. To simultaneously ensure high membrane proton conductivity and sufficient reactant delivery to reaction sites, water management has become one of the most important issues for PEMFC commercialization, and proper water management requires good understanding of water transport in different components of PEMFC. In this paper, previous researches related to water transport in PEMFC are comprehensively reviewed. The state and transport mechanism of water in different components are elaborated in detail. Based on the literature review, it is found that experimental techniques have been developed to predict distributions of water, gas species, temperature and other parameters in PEMFC. However, difficulties still remain for simultaneous measurements of multiple parameters, and the cell and system design modifications required by measurements need to be minimized. Previous modeling work on water transport in PEMFC involves developing rule-based and first-principle-based models, and first-principle-based models involve multi-scale methods from atomistic to full cell levels. Different models have been adopted for different purposes and they all together can provide a comprehensive view of water transport in PEMFC. With the development of computational power, application of lower length scale methods to higher length scales for more accurate and comprehensive results is feasible in the future. Researches related to cold start (startup from subzero temperatures) and high temperature PEMFC (HT-PEMFC) (operating at the temperatures higher than 100 °C) are also reviewed. Ice formation that hinders reactant delivery and damages cell materials is the major issue for PEMFC cold start, and enhancing water absorption by membrane electrolyte and external heating have been identified as the most effective ways to reduce ice formation and accelerate temperature increment. HT-PEMFC that can operate without liquid water formation and membrane hydration greatly simplifies water management strategy, and promising performance of HT-PEMFC has been demonstrated.     

Structure failure of the sealing in the assembly process for proton exchange membrane fuel cells

Sealing stability in proton exchange membrane fuel cell (PEMFC) is critical to the performance and safety of stacks. However, sealing structure failure (SSF), which leads to the leakage of reactant gases, often occurs in the assembly process or start-up operation for PEMFCs. This study aims to investigate the effects of geometrical structure and material parameters of sealing components on the sealing structure failure. Slippage angle and slippage distance are adopted to evaluate the risk of SSF. Finite element (FE) simulations are conducted with consideration of the assembly process and start-up operation. Experiments are carried out to validate the accuracy of the FE model. Influences of parameters of gasket, membrane electrode assembly (MEA) frame, sealing groove shape of bipolar plate (BPP), and gas pressure are discussed in detail. Meanwhile, the risks of SSF for the stack by using metallic and graphite BPPs are compared. It is demonstrated that material properties and geometrical parameters of sealing components in PEMFC have great effects on SSF. The methodology developed is beneficial to the understanding of the SSF, and it can also be applied to guide the design of PEMFC stack assembly process to keep a good sealing reliability.     

Numerical simulation of three-dimensional gas/liquid two-phase flow in a proton exchange membrane fuel cell

Investigation into the formation and transport of liquid water in proton exchange membrane fuel cells (PEMFCs) is the key to fuel cell water management. A three-dimensional gas/liquid two-phase flow and heat transfer model is developed based on the multiphase mixture theory. The reactant gas flow, diffusion, and chemical reaction as well as the liquid water transport and phase change process are modeled. Numerical simulations on liquid water distribution and its effects on the performance of a PEMFC are conducted. Results show that liquid water distributes mostly in the cathode, and predicted cell performance decreases quickly at high current density due to the obstruction of liquid water to oxygen diffusion. The simulation results agree well with experimental data. Translated from J Tsinghua Univ (Sci & Tech), 2006, 46(2): 252–256 [译自: 清华大学学报]     

Platinum decorated aligned carbon nanotubes: Electrocatalyst for improved performance of proton exchange membrane fuel cells

This study aims to improve the performance of proton exchange membrane fuel cells (PEMFCs) using carbon nanotubes as scaffolds to support nanocatalyst for power generation over prolonged time periods, compared to the current designs. The carbon nanotubes are prepared using chemical vapor deposition and decorated by platinum nanoparticles (Pt-NPs) using an amphiphilic approach. The PEMFC devices are then constructed using these aligned carbon nanotubes (ACNTs) decorated with Pt-NPs as the cathode. The electrochemical analyses of the PEMFC devices indicate the maximum power density reaches to 860 mW cm⁻² and current density reaches 3200 mA cm⁻² at 0.2 V, respectively, when O₂ is introduced into cathode. Importantly, the Pt usage was decreased to less than 0.2 mg cm⁻², determined by X-ray energy dispersive spectroscopy and X-ray photoelectron spectroscopy as complimentary tools. Electron microscopic analyses are employed to understand the morphology of Pt-ACNT catalyst (with diameter of 4-15 nm and length from 8 to 20 μm), which affects PEMFC performance and durability. The Pt-ACNT arrays exhibit unique alignment, which allows for rapid gas diffusion and chemisorption on the catalyst surfaces.     

Performance decay of proton-exchange membrane fuel cells under open circuit conditions induced by membrane decomposition

The degradation in performance of proton-exchange membrane fuel cells (PEMFCs) under open circuit conditions was investigated. The oxygen reduction reaction (ORR) kinetic current density at 0.9 V was found to decrease from 36 to 4 mA cm⁻² (geometric) without significant crossover increase or loss in the electrochemically active surface area. Cyclic voltammograms for the electrodes show characteristic changes, e.g. appearance of peaks at ∼0.2 V and shift of the onset of platinum oxide formation to higher potentials. It was identified that the large ORR kinetic decay has its origins in the reduction of available Pt sites due to adsorption of anions, which are postulated to be membrane decomposition products such as sulfate ions. Procedures carried out to condense water in the fuel cell led to the expulsion of anions out of the membrane electrode assembly (MEA) resulting in the partial recovery of ORR kinetic current density to 15 mA cm⁻². In order to attain complete performance recovery of the catalyst, a more effective and practical method to flush out the anions is desirable.     

Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells

A gas crossover model is developed for a high temperature proton exchange membrane fuel cell (HT-PEMFC) with a phosphoric acid-doped polybenzimidazole membrane. The model considers dissolution of reactants into electrolyte phase in the catalyst layers and subsequent crossover of reactant gases through the membrane. Furthermore, the model accounts for a mixed potential on the cathode side resulting from hydrogen crossover and hydrogen/oxygen catalytic combustion on the anode side due to oxygen crossover, which were overlooked in the HT-PEMFC modeling works in the literature. Numerical simulations are carried out to investigate the effects of gas crossover on HT-PEMFC performance by varying three critical parameters, i.e. operating current density, operating temperature and gas crossover diffusivity to approximate the membrane degradation. The numerical results indicate that the effect of gas crossover on HT-PEMFC performance is insignificant in a fresh membrane. However, as the membrane is degraded and hence gas crossover diffusivities are raised, the model predicts non-uniform reactant and current density distributions as well as lower cell performance. In addition, the thermal analysis demonstrates that the amount of heat generated due to hydrogen/oxygen catalytic combustion is not appreciable compared to total waste heat released during HT-PEMFC operations.     

Effect of channel-rib width ratio and relative humidity on performance of a single serpentine PEMFC based on electrochemical impedance spectroscopy

The geometry configuration of proton exchange membrane fuel cell (PEMFC) which is considered as a promising energy conversion device has great influence on PEMFC performance. In this paper, effect of channel-to-rib width ratio and relative humidity of reactant gas on the performance are compared based on two single PEMFCs. The EIS testing results below 50 A are given and analyzed. The results obtained from polarization curves, power density curves and EIS fitting results prove that: 1. Compared with cell 3:4, the anode high humidification has a greater addition to the performance of cell 1:1; 2. PEMFCs with different geometry configurations of flow field have their own suitable working condition ranges; 3. The charge transfer resistance is the dominating factor when current loading is below 2.0 A cm^?2.     

A high conductivity Cs2.5H0.5PMo12O40/polybenzimidazole (PBI)/H3PO4 composite membrane for proton-exchange membrane fuel cells operating at high temperature

A high conductivity composite proton-exchange membrane Cs_2.5H_0.5PMo₁₂O₄₀ (CsPOM)/polybenzimidazole (PBI) for use in hydrogen proton-exchange fuel cells has been prepared. The CsPOM composite membrane is insoluble in water. The composite membrane doped with H₃PO₄ showed high-proton conductivity (>0.15 S cm⁻¹) and good thermal stability. ³¹P NMR analysis has suggested the formation of a chemical bond between the CsPOM and PBI in the composite membrane. The performance of the membrane in a high-temperature proton-exchange membrane fuel cell (PEMFC) fueled with hydrogen was better than that with a phosphoric acid-doped PBI membrane under the same conditions and at temperatures greater than 150 °C. The CsPOM/PBI composite would appear to be a promising material for high-temperature PEMFC applications.     

Performance of an ultra-low platinum loading membrane electrode assembly prepared by a novel catalyst-sprayed membrane technique

Membrane electrode assemblies (MEAs) with ultra-low platinum loadings are attracting significant attention as one method of reducing the quantity of precious metal in polymer electrolyte membrane fuel cells (PEMFCs) and thereby decreasing their cost, one of the key obstacles to the commercialization of PEMFCs. In the present work, high-performance MEAs with ultra-low platinum loadings are developed using a novel catalyst-sprayed membrane technique. The platinum loadings of the anode and cathode are lowered to 0.04 and 0.12 mg cm⁻², respectively, but still yield a high performance of 0.7 A cm⁻² at 0.7 V. The influence of Nafion content, cell temperature, and back pressures of the reactant gases are investigated. The optimal Nafion content in the catalyst layer is ca. 25 wt.%. This is significantly lower than for low platinum loading MEAs prepared by other methods, indicating ample interfacial contact between the catalyst layer and membrane in our prepared MEAs. Scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) measurements reveal that our prepared MEA has very thin anode and cathode catalyst layers that come in close contact with the membrane, resulting in a MEA with low resistance and reduced mass transport limitations.     

Longevity test results for reformate polymer electrolyte membrane fuel cell stacks

Polymer electrolyte membrane fuel cell (PEMFC) stacks offer a great potential for combined heat and power (CHP) applications because of their good performance and technical maturity of the key components. Nonetheless, some developmental issues have remained open. Among those are the long-term stability with respect to performance degradation and sudden death phenomena like membrane rupture.In a development program for domestic CHP systems, PEMFC stacks intended for long-term operation on reformate were developed. Development targets were high performance, high media utilization, good longevity and low degradation rates. In this paper, results on long-term performance tests of these stacks are reported. Operating times of more than 15,000 h with degradation rates of approx. 10 μV h⁻¹ have been achieved.     

Operating characteristics of an air-cooling PEMFC for portable applications

Optimal design and proper operation is important to get designed output power of a polymer electrolyte membrane fuel cell (PEMFC) stack. The air-cooling fuel cell stack is widely used in sub kW PEMFC systems. The purpose of this study is to analyze the operating conditions affecting the performance of an air-cooling PEMFC which is designed for portable applications. It is difficult to maintain well balanced operating conditions. These parameters are the relative humidity, the temperature of the stack, the utility ratio of the reactant gas and so on. In this study a 500 W rate air-cooling PEMFC was fabricated and tested to evaluate the design performance and to determine optimal operating conditions. Moreover, basic modeling also is carried out. These results can be used as design criteria and optimal operating conditions for portable PEMFCs.     

Estimation of hydrogen crossover through Nafion membranes in PEMFCs

It is well known that the membrane electrode assembly (MEA) of proton exchange membrane fuel cells (PEMFCs) can undergo deterioration, during long term operation, of both the electrode materials and the membrane. Hydrogen crossover, i.e., the undesired diffusion of the gas from the anode to the cathode through the membrane, has been ascribed as one of the main causes of deterioration of perfluorinated ionomer membranes, normally employed in PEMFCs. One of the effects of the hydrogen permeation across the membrane is the decrease of the cell's open circuit voltage (OCV), due to the reaction between the fuel and the oxidant at the cathode surface. Such reaction can lead to the production of peroxide radicals, causing the degradation of both the PEM and the catalyst layer. Hydrogen crossover increases when temperature, pressure and humidity of the cell rise. The hydrogen permeation rate through a very thin PEM is typically lower than 1 mA cm⁻² for a new MEA, but it can exceed 10-20 mA cm⁻² after long term operation. Various methods have been proposed to measure the rate of hydrogen crossover, mainly based on electrochemical tests on a single FC with a flow of nitrogen at the cathode, so that the steady state current corresponds to the oxidation of crossed hydrogen. Hydrogen crossover has been also determined indirectly by assuming that the changes in the OCV values are due to the passage of fuel from the anode to the cathode.In this paper, a simplified mathematical model for the direct determination of hydrogen crossover permeation rate is presented. Such a model is based on analytical expressions of the polarization terms and it is employed to determine the hydrogen crossover rate. The main results show that the hydrogen crossover current densities increased from 0.12 to 0.32 mA cm⁻², by decreasing the thickness of the membranes and increasing the operating cell temperature. Moreover, the hydrogen crossover determined for a fresh MEA was compared with that of a degraded one, exposed to repetitive freezing/thawing cycles. It was found that the hydrogen crossover for the degraded MEA was more than twice the value obtained with the fresh one at the same temperature.     

Effect of gas composition on Ru dissolution and crossover in polymer-electrolyte membrane fuel cells

Pt-Ru-based anodes are commonly used in polymer-electrolyte membrane fuel cells (PEMFCs) to provide improved CO tolerance for reformate fuel applications. However, Ru crossover from the anode to the cathode has been identified as a critical durability problem that has severe performance implications. In the present study, an anode accelerated stress test (AST) was used to simulate potential spikes that occur during fuel cell start-ups and shutdowns to induce Ru crossover. The effects of fuel gas composition, namely hydrogen and carbon dioxide concentrations, on Ru dissolution and crossover were investigated. The cell performance losses were correlated with the degree of Ru crossover as determined by the changes in cathode cyclic voltammetry (CV) characteristics and neutron activation analysis (NAA). It was found that higher hydrogen concentration in the fuel accelerated Ru crossover and that the presence of carbon dioxide hindered Ru crossover. In particular, the injection of 20 vol.% carbon dioxide during potential cycling resulted in very minor Ru crossover, which showed essentially identical performance losses and CV characteristic changes as a fuel cell composed of a Ru-free anode. The experimental results suggest that the Ru species in our Pt-Ru metal oxide catalysts need to go through a reduction step by hydrogen before dissolution. The presence of carbon dioxide may play a role in hindering the reduction step.     

Cell performance of polymer electrolyte fuel cell with urchin-like carbon supports

Urchin-like structured carbon comprising carbon nanotubes grown on Fe catalyst-seeded mesoporous carbon have shown promising results as catalyst supports for use in direct methanol fuel cells (DMFCs) and proton exchange membrane fuel cells (PEMFCs). The Fe catalyst is prepared on the mesoporous carbon by immersion process followed by a high temperature reduction. The growth of carbon nanotubes then progress, for a predetermined time, through the thermal decomposition of acetylene at 800 °C. The resulting structure, comprising intimately connected mesoporous carbon and carbon nanotubes, is shown to offer performance advantages as a catalytic support for DMFCs and PEMFCs. When the hot-pressing pressure is fixed 20 kg cm⁻² to fabricate a membrane electrode assembly (MEA) with urchin-like carbon supports, the CNT growth time is found to be 60 min for a highest maximum power density in both DMFCs and PEMFCs. The maximum power densities are 43 and 79% higher than those with purely mesoporous carbon in DMFCs and PEMFCs, respectively. In a direct comparison with commercial E-TEK catalyst, the urchin-like catalyst shows higher maximum power densities, in DMFC and PEMFC, by approximately 17 and 31%, respectively.