A gas management strategy for anode recirculation in a proton exchange membrane fuel cell

The anode configuration and gas management strategy are two of factors that affect the energy efficiency of a proton exchange membrane fuel cell. In order to improve the hydrogen utilization, unused hydrogen can be recirculated to the inlet using a pump. However, impurities diffusing from the cathode to the anode may cause the dilution of hydrogen in the anode. As a result, a gas management strategy is required for the anode recirculation configuration. In this preliminary study, a novel configuration for anode recirculation and a gas management strategy are proposed and verified by experiments. Two valves are installed in the recirculation line. The anode is operated in four modes (dead-end, recirculation, compression, and purge), and the real-time local current density (LCD) is monitored for gas management purposes. The results show that the LCD distribution is uniform during the recirculation mode and nonuniform during the dead-end and compression modes. With this configuration and gas management strategy, the cycle duration is increased by a factor of 6.5.     

Cross-linked PEEK-WC proton exchange membrane for fuel cell

The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 × 10⁻² S/cm at room temperature. The proton conductivity was comparable with that of Nafion^® 117 membrane (6.2 × 10⁻² S/cm). The methanol permeability of the cross-linked membrane was 1.6 × 10⁻⁷ cm²/s, much lower than that of Nafion^® 117 membrane.     

On-line fault diagnostic system for proton exchange membrane fuel cells

In this paper, a supervisor system, able to diagnose different types of faults during the operation of a proton exchange membrane fuel cell is introduced. The diagnosis is developed by applying Bayesian networks, which qualify and quantify the cause–effect relationship among the variables of the process. The fault diagnosis is based on the on-line monitoring of variables easy to measure in the machine such as voltage, electric current, and temperature. The equipment is a fuel cell system which can operate even when a fault occurs. The fault effects are based on experiments on the fault tolerant fuel cell, which are reproduced in a fuel cell model. A database of fault records is constructed from the fuel cell model, improving the generation time and avoiding permanent damage to the equipment.     

High performance proton exchange membrane fuel cell electrode assemblies

The conventional 5-layer membrane electrode assembly (MEA) consists of a proton exchange membrane (PEM) locating at its center, two layers of Pt-C-40 (Pt content 40 wt%) locating next on both surfaces of PEM, and two gas diffusion layers (GDL) locating next on the outer surfaces of Pt-C layers (structure-a MEA). In this paper, we report three modified MEAs consisting of Pt-C-40 (Pt content 40 wt%) and Pt-C-80 (Pt content 80 wt%) catalysts. These are: (1) 7-layer structure-b MEA with a thin Pt-C-80 layer locating between Pt-C-40 layer and PEM; (2) 7-layer structure-c MEA with a thin Pt-C-80 layer locating between Pt-C-40 layer and GDL; and (3) 5-layer structure-d MEA with Pt-C-40 and Pt-C-80 mixing homogeneously and locating between PEM and GDL. Under a fixed Pt loading, we find structure-b, -c, and -d MEAs with 20-40 wt% Pt contributed from Pt-C-80 have better fuel cell performance than structure-a MEA consisting only of Pt-C-40. The reasons for the better fuel cell performance of these modified MEAs are attributed to the better feasibility for O₂ gas to reach cathode Pt particles and lower proton transport resistance in catalyst layers of the modified MEAs than structure-a MEA.     

Surrogate model for proton exchange membrane fuel cell (PEMFC)

The main goal of this work is to realize a PEMFC model that can be used efficiently for the global modelling of the fuel cell system. The modelling method proposed in the paper is an approach from an empirical point of view that allows a PEMFC model of “black-box” class to be developed. Moving least squares (MLS) have therefore been employed to approximate the cell voltage characteristics V, using an experimental dataset measured in determinate conditions. The MLS approach appears to present a good balance of response surface accuracy, smoothness, robustness, and ease of use. This kind of numerical model offers good perspectives for the systems identification, the simulation of the systems, the design and the optimization of process control, etc. The results prove that the method is suitable for predicting and describing the fuel cell behaviour in all the points of the approximation domain. The proposed model can be included in a numerical application to optimize the operation of an existing fuel cell system.     

Control of a heat-integrated proton exchange membrane fuel cell system with methanol reforming

This work presents a novel heat-integrated fuel cell stack system with methanol reforming. Its configuration is composed of fuel processing units (FPUs), proton exchange membrane (PEM) fuel cell stack, and heat exchangers (HEXs). Well mixed methanol and oxygen flows in contact with countercurrent flowing water dominates the production of hydrogen at the exit of FPUs and influences the stack temperature. The heat exchange connections can enhance the utilization of energy of FPUs. To ensure the stable steady-state operation, the model-free fuzzy incremental control scheme within the multi-loop feedback control framework is developed. Finally, the proposed system integration and control configuration are verified by closed-loop simulations.     

Double cross-linked polyetheretherketone proton exchange membrane for fuel cell

The proton exchange membrane based on polyetheretherketone was prepared via two steps of cross-linking. The properties of the double cross-linked membrane (water uptake, proton conductivity, methanol permeability and thermal stability) have been investigated for fuel cell applications. The prepared membrane exhibited relatively high proton conductivity, 3.2 × 10⁻² S cm⁻¹ at room temperature and 5.8 × 10⁻² S cm⁻¹ at 80 °C. The second cross-linking significantly decreased the water uptake of the membrane. The performance of direct methanol fuel cell was slightly improved as compared to Nafion^® 117 due to its low methanol permeability. The results indicated that the double cross-linked membrane is a promising candidate for the polymer electrolyte membrane fuel cell, especially for the direct methanol fuel cell due to its low methanol permeability and high stability in a methanol solution.     

About internal currents during start-up in proton exchange membrane fuel cell

This paper reports about an experimental and numerical study of the internal currents that occur during fuel cell start-up under open-circuit conditions. The internal currents were measured in a segmented cell specifically designed for this purpose and it was found that they could reach values higher than 1 A cm⁻². They result from the potential that appears at the inlet of the anode compartment while hydrogen pushes oxygen, air, or possibly nitrogen that was introduced for purging water toward the outlet. For a short time, a fraction of the channels is filled with hydrogen while the other part is still occupied by the gas initially present. The model presented in the paper demonstrates that the occurrence of internal currents can be explained mostly by capacitive effects. Carbon oxidation occurs probably simultaneously but its contribution to the internal currents is by all appearances negligible. The model also explains the transient voltage rise (over the steady state open circuit voltage) that is sometimes observed experimentally shortly after the fuel cell start-up.     

Oxygen gain analysis for proton exchange membrane fuel cells

Oxygen gain is the difference in hydrogen fuel cell performance operating on oxygen-depleted and oxygen-rich cathode fuel streams. Oxygen gain experiments provide insight into the degree of oxygen mass-transport resistance within a fuel cell. By taking these measurements under different operating conditions, or over time, one can determine how oxygen mass transport varies with operating modes and/or aging. This paper provides techniques to differentiate between mass-transport resistance within the catalyst layer and within the gas-diffusion medium for a polymer-electrolyte membrane fuel cell. Two extreme cases are treated in which all mass transfer limitations are located only (i) within the catalyst layer or (ii) outside the catalyst layer in the gas-diffusion medium. These two limiting cases are treated using a relatively simple model of the cathode potential and common oxygen gain experimental techniques. This analysis demonstrates decisively different oxygen gain behavior for the two limiting cases. For catalyst layer mass transfer resistance alone, oxygen gain values are limited to a finite range of values. However, for gas-diffusion layer mass transfer resistance alone, the oxygen gain is not confined to a finite range of values. Therefore, this work provides a straightforward diagnostic method for locating the prominent source of mass transfer degradation in a PEMFC cathode.     

A compact integrated fuel-processing system for proton exchange membrane fuel cells

A compact integrated fuel-processing system consisting of a plate-fin reformer (PFR) and a multi-stage preferential oxidation reactor is designed in this paper. The PFR, which was based on a plate-fin heat exchanger, is very compact, and reactant vaporization, methanol steam reforming and combustion are all integrated in it. Both internal plate-fins and external catalytic combustion were used to enhance heat transfer of the reformer, which offers both high methanol conversion ratio and low CO concentration, so that the water–gas shift reactor, which provides primary CO cleanup, is not necessary in this fuel-processing system. This will result in simplification of the fuel-processing system design and capital cost reduction. The performance of the main components in the fuel-processing system has been investigated. The axial temperatures of the different chambers in PFR were uniform, and the temperatures at the inlet and outlet of the PROX reactors were controlled strictly by plate-fin exchangers so that it could minimize parasitic hydrogen oxidation. In addition, the results indicated that this fuel-processing system can provide a high concentration of hydrogen and the system efficiency is always maintained above 75%. It is further demonstrated that the fuel-processing system could be operated autothermally and exhibited good test stability.     

Hydrogen purification using compact pressure swing adsorption system for fuel cell

Adsorption of CO and CO₂ in mixtures of H₂/CO/CO₂ was achieved using compact pressure swing adsorption (CPSA) system to produce purified hydrogen for use in fuel cell. A CPSA system was designed by combining four adsorption beds that simultaneously operate at different processes in the pressure swing adsorption (PSA) process cycle. The overall diameter of the cylindrical shell of the CPSA is 35 cm and its height is 40 cm. Several suitable adsorbent materials for CO and CO₂ adsorption in a hydrogen stream were identified and their adsorption properties were tested. Activated carbon from Sigma–Aldrich was the adsorbent chosen. It has a surface area of 695.07 m²/g. CO adsorption capacity (STP) of 0.55 mmol/g and CO₂ at 2.05 mmol/g were obtained. The CPSA system has a rapid process cycle that can supply hydrogen continuously without disruption by the regeneration process of the adsorbent. The process cycle in each column of the CPSA consists of pressurization, adsorption, blowdown and purging processes. CPSA is capable of reducing the CO concentration in a H₂/CO/CO₂ mixture from 4000 ppm to 1.4 ppm and the CO₂ concentration from 5% to 7.0 ppm CO₂ in 60 cycles and 3600 s. Based on the mixture used in the experimental work, the H₂ purity obtained was 99.999%, product throughput of 0.04 kg H₂/kg adsorbent with purge/feed ratio was 0.001 and vent loss/feed ratio was 0.02. It is therefore concluded that the CPSA system met the required specifications of hydrogen purity for fuel cell applications.     

Dynamic modeling and simulation of air-breathing proton exchange membrane fuel cell

Small fuel cells have shown excellent potential as alternative energy sources for portable applications. One of the most promising fuel cell technologies for portable applications is air-breathing fuel cells. In this paper, a dynamic model of an air-breathing PEM fuel cell (AB-PEMFC) system is presented. The analytical modeling and simulation of the air-breathing PEM fuel cell system are verified using Matlab, Simulink and SimPowerSystems Blockset. To show the effectiveness of the proposed AB-PEMFC model, two case studies are carried out using the Matlab software package. In the first case study, the dynamic behavior of the proposed AB-PEMFC system is compared with that of a planar air-breathing PEM fuel cell model. In the second case study, the validation of the air-breathing PEM fuel cell-based power source is carried out for the portable application. Test results show that the proposed AB-PEMFC system can be considered as a viable alternative energy sources for portable applications.     

Maximum power cold start mode of proton exchange membrane fuel cell

Successful and fast cold start is important for proton exchange membrane (PEM) fuel cell in vehicular applications in addition to the desired maximum power in any case. In this study, the maximum power cold start mode is investigated in details and compared with other cold start modes based on a multiphase stack model. It is found that for the maximum power cold start mode, the current density is generally kept at high levels, and the performance improvement caused by the membrane hydration and temperature increment may not be observable. Therefore, before the melting point, the performance drops continuously. The maximum power cold start mode could better balance the heat generation and ice formation, leading to improved cold start survivability than that in the constant voltage and constant current modes, with a fast start-up generally guaranteed. Once the survivability can be ensured, the initial water content needs to be higher for fast cold start, suggesting that over purging should be avoided. The maximum power mode is suggested to be optimal for PEM fuel cell cold start based on the modeling results.     

Simulation studies of the membrane exchange assembly of an all-liquid,proton exchange membrane fuel cell

A model has been designed and constructed for the all-liquid, sodium borohydride/hydrogen peroxide fuel cell under development at the University of Illinois at Urbana-Champaign. The electrochemical behavior, momentum balance, and mass balance effects within the fuel cell are modeled using the Butler–Volmer equations, Darcy's law, and Fick's law, respectively, within a finite element modeling platform. The simulations performed with the model indicate that an optimal physical design of the fuel cell's flow channel land area or current collector exists when considering the pressure differential between channels, and the diffusion layer permeability and conductivity. If properties of the diffusion layer are known, the model is an effective method of improving the fuel cell design in order to achieve higher power density.     

Geometry optimization and pressure analysis of a proton exchange membrane fuel cell stack

For proton exchange membrane fuel cells (PEMFCs), the distribution of reactant streams in the reactor is critical to their efficiency. This study aims to investigate the optimal design of the inlet/outlet flow channel in the fuel cell stack with different geometric dimensions of the tube and intermediate zones (IZ). The tube-to-IZ length ratio, the IZ width, and the tube diameter are adjusted to optimize the geometric dimensions for the highest pressure uniformity. Four different methods, including the Taguchi method, analysis of variance (ANOVA), neural network (NN), and multiple adaptive regression splines (MARS), are used in the analyses. The results indicate the tube diameter is the most impactive one among the three factors to improve the pressure uniformity. The analysis suggests that the optimal geometric design is the tube-to-IZ length ratio of 9, the IZ width of 14 mm, and the tube diameter of 9 mm with the pressure uniformity of 0.529. The relative errors of the predicted pressure uniformity values by NN and MARS under the optimal design are 1.62% and 3.89%, respectively. This reveals that NN and MARS can accurately predict the pressure uniformity, and are promising tools for the design of PEMFCs.     

Monodimensional modeling and experimental study of the dynamic behavior of proton exchange membrane fuel cell stack operating in dead-end mode

The dynamic behavior of a five cells proton exchange membrane fuel cell (PEMFC) stack operating in dead-end mode has been studied at room temperature, both experimentally and by simulation. Its performances in “fresh” and “aged” state have been compared. The cells exhibited two different response times: the first one at about 40 ms, corresponding to the time needed to charge the double-layer capacitance, and the second one at about 15–20 s. The first time response was not affected by the ageing process, despite the decrease of the performances, while the second one was. Our simulations indicated that a high amount of liquid water was present in the stack, even in “fresh” state. This liquid water is at the origin of the performances decrease with ageing, due to its effect on decreasing the actual GDL porosity that in turn cause the starving of the active layer with oxygen. As a consequence, it appears that water management issue in a fuel cell operating in dead-end mode at room temperature mainly consists in avoiding pore flooding instead of providing enough water to maintain membrane conductivity.     

Preparation and properties of high performance nanocomposite proton exchange membrane for fuel cell

Various spatially enlarged organoclays were prepared by using poly(oxyproplene)-backboned quaternary ammonium salts of various molecular weights M_w 230, 400 and 2000 as the intercalating agents for Na⁺-montmorillonite. The modified MMT was utilized to improve the compatibility with Nafion^®. Sufficient interaction of the modified MMT with Nafion^® was studied by using X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS). The performance of the Nafion^®/m-MMT composite membranes for direct methanol fuel cell (DMFCs) was evaluated in terms of water uptake, ion exchange capacity (IEC), methanol permeability, proton conductivity, and cell performance. The methanol permeability of the composite membrane decreased with the increasing of m-MMT content. The proton conductivity of the membrane was lowered slightly from that of pristine Nafion^® membrane. These results led to an essential improvement in the single-cell performance of DMFCs.     

A simple electric circuit model for proton exchange membrane fuel cells

A simple and novel dynamic circuit model for a proton exchange membrane (PEM) fuel cell suitable for the analysis and design of power systems is presented. The model takes into account phenomena like activation polarization, ohmic polarization, and mass transport effect present in a PEM fuel cell. The proposed circuit model includes three resistors to approach adequately these phenomena; however, since for the PEM dynamic performance connection or disconnection of an additional load is of crucial importance, the proposed model uses two saturable inductors accompanied by an ideal transformer to simulate the double layer charging effect during load step changes. To evaluate the effectiveness of the proposed model its dynamic performance under load step changes is simulated. Experimental results coming from a commercial PEM fuel cell module that uses hydrogen from a pressurized cylinder at the anode and atmospheric oxygen at the cathode, clearly verify the simulation results.     

Degradation mitigation effects of pressure swing in proton exchange membrane fuel cells with dead-ended anode

High cost remains to be one of the primary obstacles for the commercialization of proton exchange membrane fuel cells (PEMFCs). To simplify the fuel cell system and reduce cost, dead-ended anode (DEA) is widely used. However, water and nitrogen can accumulate in the dead-ended anode, resulting in cell performance decrease and severe cell degradation. Anode pressure swing supply is a new technology which has been shown to be effective in reducing local water and nitrogen accumulation in the anode channel. In this work, the effects of pressure swing supply on fuel cell degradation have been experimentally studied. Two sets of experiments on the same fuel cell are conducted, one under conventional constant pressure operation and the other under pressure swing operation. Polarization curves show that pressure swing supply can significantly mitigate cell degradation during DEA operations. Electrochemical characterizations are performed to study the mechanisms of mitigations in cell degradation. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results show that pressure swing supply can significantly reduce electrolyte membrane degradation, but has no significant mitigation effect on the cathode catalyst degradation during DEA operation. Further examinations of the membrane-electrode-assembly (MEA) by scanning electron microscope (SEM) confirm the significant difference in membrane degradations since there is a very large difference in average thickness of the membranes after the degradation tests.     

Platinum degradation mechanisms in proton exchange membrane fuel cell (PEMFC) system: A review

Proton Exchange Membrane Fuel Cells (PEMFCs) have the perspective to intensely decrease global emission through environmentally-friendly potential. This review paper summarizes the degradation of platinum catalyst layer that has become a significant issue in the improvement of PEMFCs. The review intends to categorise and provide a clear understanding between disintegration and agglomerate that occurs during platinum degradation. In each process, different degradation mechanisms and their migration processes are presented. The improvement in platinum degradation as a function of increasing the performance of PEMFC is established. Prospects for addressing platinum degradation through the exploration of further experimental and numerical research are recommended. Lastly, this paper through recommendation attempts to prevent platinum degradation and reduces high costs associated with the replacement of catalysts in the PEMFCs.