Effect of operating conditions on carbon corrosion in polymer electrolyte membrane fuel cells

The influence of humidity, cell temperature and gas-phase O₂ on the electrochemical corrosion of carbon in polymer electrolyte membrane fuel cells is investigated by measuring CO₂ emission at a constant potential of 1.4 V for 30 min using on-line mass spectrometry. Carbon corrosion shows a strong positive correlation with humidity and cell temperature. The presence of water is indispensable for electrochemical carbon corrosion. By contrast, the presence of gas-phase O₂ has little effect on electrochemical carbon corrosion. With increased carbon corrosion, changes in fuel cell electrochemical characteristics become more prominent and thereby indicate that such corrosion significantly affects fuel cell durability.     

Performance of diagonal control structures at different operating conditions for polymer electrolyte membrane fuel cells

This work is focused on the selection of operating conditions in polymer electrolyte membrane fuel cells. It analyses efficiency and controllability aspects, which change from one operating point to another. Specifically, several operating points that deliver the same amount of net power are compared, and the comparison is done at different net power levels. The study is based on a complex non-linear model, which has been linearised at the selected operating points. Different linear analysis tools are applied to the linear models and results show important controllability differences between operating points. The performance of diagonal control structures with PI controllers at different operating points is also studied. A method for the tuning of the controllers is proposed and applied. The behaviour of the controlled system is simulated with the non-linear model. Conclusions indicate a possible trade-off between controllability and optimisation of hydrogen consumption.     

Effects of various operating and initial conditions on cold start performance of polymer electrolyte membrane fuel cells

Cold start is a challenging and important issue that hinders the commercialization of polymer electrolyte membrane fuel cell (PEMFC). In this study, a three-dimensional multiphase model has been developed to simulate the cold start processes in a PEMFC. Numerical simulations have been conducted for a single PEMFC starting at various operating and initial conditions, which are cell voltages, initial water contents and distributions, anode inlet relative humidity (RH), surrounding heat transfer coefficients, and cell temperatures. It is found that the heating-up time can be significantly reduced by decreasing the cell voltage and effective purge is critical for PEMFC cold start. The largest heating source at high cell voltages is the activational heat, and it becomes the ohmic heat at low cell voltages. The water freezing in the membrane is not observed when the cell is producing current due to the heat generation and the slow water diffusion into the membrane at subzero temperatures, and it is only observed after the cold start is failed, further confirming the importance of purge. Humidification of the supplied hydrogen has negligible effect on the cold start performance since only small amounts of water vapour can be taken by the gas streams at subzero temperatures. The surrounding heat transfer coefficients have significant influence on the heating-up time, indicating the importance of cell insulation or heating. The rate of cell heating up is reduced when the startup temperature is lowered due to the more sluggish electrochemical reaction kinetics.     

A liquid-feed solid polymer electrolyte direct methanol fuel cell operating at near-ambient conditions

Results on the performance of a 25 cm² liquid-feed solid polymer electrolyte direct methanol fuel cell (SPE-DMFC), operating under near-ambient conditions, are reported. The fuel cell can sustain a load current density of 100 mA cm⁻² with an output voltage of c. 450 mV at 90°C with 2 M aqueous methanol and air-fed cathode at near-ambient pressures with a catalyst loading of 5 mg cm⁻² of Pt. Preliminary data on the performance of a liquid-feed SPE-DMFC stack comprising two 25 cm² cells are also reported. These data are sufficient to suggest that further developmental work on liquid-feed SPE-DMFCs operating under near-ambient conditions (0 barg O₂ at 90°C) is well worthwhile.     

Modeling of the impact of cycling operating conditions on durability of polymer electrolyte fuel cells and its sensitivity analysis

In the paper, the impact on durability of polymer electrolyte membrane fuel cells is investigated when varying operating conditions applied in accelerated stress tests. By this, the electric potential cycling protocol in given by a non-symmetric square profile. The electrochemical degradation of a catalyst layer is caused by platinum ion dissolution and oxide coverage. These mechanisms are described by the one-dimensional Holby–Morgan model with a modified Butler–Volmer equation for the reaction rates. For efficient numerical solution of the underlying nonlinear reaction-diffusion system, a variable time-step implicit-explicit method is suggested. Computer simulations predict durability for the catalyst by using a linear extrapolation up to the full platinum surface blockage. A parameter sensitivity analysis is presented on different time scales and measures how the platinum mass loss is impacted by the variation of specific parameters.     

Performance optimization of polymer electrolyte membrane fuel cells using the Nelder-Mead algorithm

The direct-search simplex method for function optimization has been adapted to performance optimization of polymer electrolyte membrane fuel cells (PEMFCs). The established method is strongly application oriented and uses only experimentally determined data for optimization. It is not restricted to discrete parameters optimums and does not require the use of third-party software or computational resources. Hence, it is easy to implement in fuel cell testing stations. The optimization consists of finding, for a given fuel cell load, an optimum set of values of the 7 fuel cell operating parameters: the fuel cell temperature, the reactants' stoichiometric ratios, the reactants' inlet relative humidity, and the reactants' outlet pressures, resulting in the highest fuel cell performance. The performance is measured using a scalar function of the operating parameters and the load and can be defined according to needs.Two PEMFC performance functions: the fuel cell voltage and the system-related fuel cell efficiency were optimized using the procedure for practically sized PEMFC stacks of two designs. With respect to the nominal operating conditions defined as optimal for each stack design by its manufacturer, the gains from the optimization procedure were up to over 12% and up to over 7% for the stack voltage and efficiency, respectively. The validation of the procedure involved 5 stack specimens and four laboratories and consistent results were obtained.     

Bipolar polymer electrolyte interfaces for hydrogen–oxygen and direct borohydride fuel cells

Direct borohydride fuel cells (DBFCs) using liquid hydrogen peroxide as the oxidant are safe and attractive low temperature power sources for unmanned underwater vehicles (UUVs) as they have excellent energy and power density and do not feature compressed gases or a flammable fuel stream. One challenge to this system is the disparate pH environment between the anolyte fuel and catholyte oxidant streams. Herein, a bipolar interface membrane electrode assembly (BIMEA) is demonstrated for maintaining pH control of the anolyte and catholyte compartments of the fuel cell. The prepared DBFC with the BIMEA yielded a promising peak power density of 110 mW cm⁻². This study also investigated the same BIMEA for a hydrogen–oxygen fuel cell (H₂–O₂ FC). The type of gas diffusion layer used and the gas feed relative humidity were found to impact fuel cell performance. Finally, a BIMEA featuring a silver electrocatalyst at the cathode in a H₂–O₂ FC was successfully demonstrated.     

Temperature control strategy for polymer electrolyte fuel cells

A polymer electrolyte fuel cell (PEFC) is an electrochemical device that converts chemical energy directly to electrical energy, and its performance greatly depends on its operating temperature. Therefore, in this paper, a novel thermodynamic PEFC model with the airflow cooling method is firstly developed for the PEFC system. Then, a novel model predictive control (MPC) controller is designed to control the stack temperature at an optimal value by adjusting the air flow rate on the basis of the developed thermodynamic PEFC model. The thermodynamic PEFC model and the designed controlling strategies are simulated and analysed in Matlab/Simulink. Three tests are conducted to estimate the reliability of the developed controllers concerning different operating conditions: (a) typical perturbation in the current load, (b) any perturbation in the current load, and (c) variation of the ambient temperature. The simulation results demonstrate that the MPC controller can effectively control the stack temperature at the desired value. Moreover, the MPC controller shows much superior effects compared with the conventional proportional integral derivative (PID) controller. In addition, the developed coolant circuit model can be easily applied to various PEFC systems. The MPC controller shows potential also for other controlling issues of PEFC systems due to its strong robustness and fast response.     

Determination of optimal operating conditions for a polymer electrolyte membrane fuel cell stack: optimal operating condition based on multiple criteria

A methodology for optimal control of the polymer electrolyte membrane fuel cell (PEMFC) with multiple criteria is presented here. In this regard, thermoelectric objectives and thermoeconomic objective are considered, simultaneously. The proposed fuel cell is a 1200 W Ballard PEMFC namely Nexa? power module. The net power density and exergetic efficiency of the PEMFC are maximized, and the unit cost of the generated power is minimized in a multi‐objective optimization procedure using the NSGA‐II (non‐dominated sorting genetic algorithm). Operating temperature and pressure, air stoichiometric coefficient at the cathode and the current density are considered as controlling parameters in order to acquire optimal performance of the PEMFC. A set of optimal solution namely the Pareto frontier is obtained, and a final optimal solution is selected from available solutions located on the Pareto frontier using the fuzzy decision‐making process based on the Bellman–Zadeh approach. Results are compared with corresponding results obtained previously in single objective optimization scenarios. It has been shown that the optimal operating condition obtained based on the multiple criteria approach has least deviation from the ideal features of the fuel cell in comparison to the corresponding optimal solution obtained in conventional single‐objective optimization approaches. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of operating conditions on durability of polymer electrolyte membrane fuel cell Pt cathode catalyst layer

In this study, we investigated the effects of humidity and oxygen reduction on the degradation of the catalyst of a polymer electrolyte membrane fuel cell (PEMFC) in a voltage cycling test. To elucidate the effect of humidity on the voltage cycling corrosion of a carbon-supported Pt catalyst with 3 nm Pt particles, voltage cycling tests based on 10,000 cycles were conducted using 100% relative humidity (RH) hydrogen as anode gas and nitrogen of varying humidities as cathode gas. The degradation rate of an electrochemical surface area (ECSA) was almost 50% under 189% RH nitrogen atmosphere and the Pt average particle diameter after 10,000 cycles under these conditions was about 2.3 times that of a particle of fresh catalyst because of the agglomeration of Pt particles.The oxygen reduction reaction (ORR) that facilitated Pt catalyst agglomeration when oxygen was employed as the cathode gas also demonstrated that Pt agglomeration was prominent in higher concentrations of oxygen. The ECSA degradation figure in 100% RH oxygen was similar to that in 189% RH nitrogen. It was concluded that liquid water, which was dropped under a supersaturated condition or generated by ORR, accelerated Pt agglomeration. In this paper, we suggest that the Pt agglomeration degradation occurs in a flooding area in a cell plane.     

Spatially resolved current density measurements and real-time modelling as a tool for the determination of local operating conditions in polymer electrolyte fuel cells

In this contribution a simplified, isothermal, two-phase, one-dimensional model for the calculation of the cathodic gas flow along the flow field channels of a polymer electrolyte fuel cell (PEFC) is presented. The composition of the humidified oxidant gas, average gas velocity, pressure drop, and other quantities can be calculated for any gas distributor structures with one channel. Thereby, the model requires several input parameters which have to be determined solely by experiment and pre-defined operation conditions, e.g. the water content of the feed gas, local current densities, and gas flow rates. In contrast to other models, the cross-section reduction has been taken into account which results from the penetration of the gas diffusion layer into the flow field channels due to the mounting pressure. Beyond this, the model needs no fit-parameters for further adjustment.For close examination of the factors limiting the performance of a PEFC, the DLR has developed several techniques for measuring the current density distribution with spatial resolution. In order to investigate the origin of the corresponding effects, one of these techniques has been improved by implementing the model of the cathodic gas flow as an on-line feature.The combination of a spatially resolved measurement technique with a real-time simulation gives a better understanding of the local processes within the cell and represents a helpful tool for the development of fuel cell components as well as for the optimization of the operating conditions. Exemplarily, the presentation the results for a 25 cm² serpentine flow field at different operation modes are shown in this paper.     

Expert diagnosis of polymer electrolyte fuel cells

Diagnosing faulty conditions of engineering systems is a highly desirable process within control structures, such that control systems may operate effectively and degrading operational states may be mitigated. The goal herein is to enhance lifetime performance and extend system availability. Difficulty arises in developing a mathematical model which can describe all working and failure modes of complex systems. However the expert's knowledge of correct and faulty operation is powerful for detecting degradation, and such knowledge can be represented through fuzzy logic. This paper presents a diagnostic system based on fuzzy logic and expert knowledge, attained from experts and experimental findings. The diagnosis is applied specifically to degradation modes in a polymer electrolyte fuel cell. The defined rules produced for the fuzzy logic model connect observed operational modes and symptoms to component degradation. The diagnosis is then tested against common automotive stress conditions to assess functionality.     

Membrane resistance and current distribution measurements under various operating conditions in a polymer electrolyte fuel cell

The ability to make spatially resolved measurements in a fuel cell provides one of the most useful ways in which to monitor and optimise their performance. Localised membrane resistance and current density measurements for a single channel polymer electrolyte fuel cell are presented for a range of operating conditions. The current density distribution results are compared with an analytical model that exhibited generally good agreement across a broad range of operating conditions. However, under conditions of high air flow rate, an increase in current is observed along the channel which is not predicted by the model. Under such circumstances, localised electrochemical impedance measurements show a decrease in membrane resistance along the channel. This phenomenon is attributed to drying of the electrolyte at the start of the channel and is more pronounced with increasing operating temperature.     

Operation of polymer electrolyte membrane fuel cells with dry feeds: Design and operating strategies

The operation of polymer electrolyte membrane fuel cells (PEMFCs) with dry feeds has been examined with different fuel cell flow channel designs as functions of pressure, temperature and flow rate. Auto-humidified (or self-humidifying) PEMFC operation is improved at higher pressures and low gas velocities where axial dispersion enhances “back-mixing” of the product water with the dry feed. We demonstrate auto-humidified operation of the channel-less, self-draining fuel cell, based on a stirred tank reactor; data is presented showing auto-humidified operation from 25 to 115 °C at 1 and 3 atm. Design and operating requirements are derived for the auto-humidified operation of the channel-less, self-draining fuel cell. The auto-humidified self-draining fuel cell outperforms a fully humidified serpentine flow channel fuel cell at high current densities. The new design offers substantial benefits for simplicity of operation and control including: the ability to self-drain reducing flooding, the ability to uniformly disperse water removing current gradients and the ability to operate on dry feeds eliminating the need for humidifiers. Additionally, the design lends itself well to a modular design concept.     

Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells

In this paper, the effect of porosity-graded micro-porous layer (GMPL) on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was studied in detail. The GMPL was prepared by printing micro-porous layers (MPL) with different content of NH₄Cl pore-former and the porosity of the GMPL decreased from the inner layer of the MPLs at the membrane/MPL interface to the outer layer of the MPLs at the gas diffusion electrode/MPL interface. The morphology and porosity of the GMPLs were characterized and the performance of the cell with GMPLs was compared with those having conventional homogeneous MPLs. The result demonstrates that the fuel cells consisting of GMPL have better performance than those consisting of conventional homogeneous MPLs, especially at high current densities. Micro-porous layer with graded porosity is beneficial for the electrode process of fuel cell reaction probably by facilitating the liquid water transportation through large pores and gas diffusion via small pores in the GMPLs.     

Two-phase flow measurement system for polymer electrolyte fuel cells

In this paper, a sensory system capable of measuring two-phase flow of water at the PEFC output is introduced. It works based on collecting and evaporating the liquid water that exits the PEFC in a vessel that is heated to a temperature above that of the fuel cell temperature. By measuring the vessel dew point temperature and flow rate, the mass of water in liquid and vapor phases are calculated. To demonstrate the capabilities of this measurement system, it is placed at the output of a PEFC cathode during membrane conditioning. The effect of two-phase flow on cell voltage reveals two distinct modes of liquid water transport in the PEFC cathode during membrane conditioning.     

The effect of hydrogen crossover on open-circuit voltage in polymer electrolyte membrane fuel cells

Even though the measured open-circuit voltage in a H₂-O₂ PEM fuel cell is invariably about 200-250 mV lower than that predicted from thermodynamics (1.229 V at 25 °C), there is no unequivocal explanation of this phenomenon available in the literature, although several hypotheses exist. Based on a theoretical model of mixed potential with a priori parameters, it is shown here that this voltage loss under open-circuit conditions can be attributed exclusively to hydrogen crossover and the resulting oxygen reduction reaction overpotential at the cathode. The analytical model predictions agree well with available experimental results.     

Experimental investigation on a methane fuel processor for polymer electrolyte fuel cells

This study presents experimental study on a novel methane fuel processing system for hydrogen (H₂) production. The unit includes into a single package the autothermal reformer, the CO shift converter, the preferential oxidation reactor and the internal heat exchangers. Effects of operative conditions, related to the H₂ productivity, on the performances, were investigated experimentally, in order to evaluate the integration of the fuel processor with a Polymer Electrolyte Fuel Cell (PEFC) system for residential applications. The sensitivity analysis showed that the overall performance is strongly dependent upon the operative conditions considered.     

Simulation of nanostructured electrodes for polymer electrolyte membrane fuel cells

Aligned carbon nanotubes (CNTs) with Pt uniformly deposited on them are being considered in fabricating the catalyst layer of polymer electrolyte membrane (PEM) fuel cell electrodes. When coated with a proton conducting polymer (e.g., Nafion) on the Pt/CNTs, each Pt/CNT acts as a nanoelectrode and a collection of such nanoelectrodes constitutes the proposed nanostructured electrodes. Computer modeling was performed for the cathode side, in which both multicomponent and Knudsen diffusion were taken into account. The effect of the nanoelectrode lengths was also studied with catalyst layer thicknesses of 2, 4, 6, and 10 μm. It was observed that shorter lengths produce better electrode performance due to lower diffusion barriers and better catalyst utilization. The effect of spacing between the nanoelectrodes was studied. Simulation results showed the need to have sufficiently large gas pores, i.e., large spacing, for good oxygen transport. However, this is at the cost of obtaining large electrode currents due to reduction of the number of nanoelectrodes per unit geometrical area of the nanostructured electrode. An optimization of the nanostructured electrodes was obtained when the spacing was at about 400 nm that produced the best limiting current density.     

Ultra large-scale simulation of polymer electrolyte fuel cells

This paper reports on the computational performance and detailed results of ultra large-scale simulations of a 200 cm² polymer electrolyte fuel cell (PEFC) using a 23.5 million gridpoint mesh. The computer code is based on a comprehensive single-phase PEFC model that features a detailed membrane-electrode assembly (MEA) model, electron transport, thermal and species transport, coolant heat transfer, in addition to other standard functionalities. Two cases under dry operation are simulated and compared. One case concerns an infinitely large coolant flowrate and consequently a constant temperature of bipolar plates. The other case involves a finite flowrate and a lower inlet coolant temperature designed to avoid membrane dryout in the inlet region while alleviating electrode flooding in the outlet region.