Optimization of cathode microporous layer materials for proton exchange membrane fuel cell

The microporous layer (MPL) of diffusion medium has an important impact on the water management ability of proton exchange membrane fuel cells. In this study, six kinds of carbon black were used to prepare the cathode MPL. The thickness, conductivity, pore structure, hydrophobicity, and surface microstructure of MPL were characterized. The single cell was prepared and electrochemical tests were performed. The results showed that the single cell prepared by Acetylene black (ACET) and Vulcan XC-72R has a considerable power generation performance. In addition, polyvinylidene fluoride hexafluoropropylene copolymer P(VDF-HFP) was used to replace Polytetrafluoroethylene (PTFE) as hydrophobic binder. MPL with different P(VDF-HFP) contents were prepared, and the single cell performance was investigated. The results showed that all the single cells prepared by P(VDF-HFP) were worse than that of PTFE. This study provides an important reference for further improving the performance of fuel cells from the perspective of material optimization with MPL.     

Prediction and analysis of the cathode catalyst layer performance of proton exchange membrane fuel cells using artificial neural network and statistical methods

A mathematical model was developed to investigate the cathode catalyst layer (CL) performance of a proton exchange membrane fuel cell (PEMFC). A numerous parameters influencing the cathode CL performance are implemented into the CL agglomerate model, namely, saturation and eight structural parameters, i.e., ionomer film thickness covering the agglomerate, agglomerate radius, platinum and carbon loading, membrane content, gas diffusion layer penetration content and CL thickness. For the first time, an artificial neural network (ANN) approach along with statistical methods were employed for modeling, prediction, and analysis of the CL performance, which is denoted by activation overpotential. The ANN was constructed to build the relationship between the named parameters and activation overpotential. Statistical analysis, namely, analysis of means (ANOM) and analysis of variance (ANOVA) were done on the data obtained by the trained neural network and resulted in the sensitivity factors of structural parameters and their mutual combinations as well as the best performance.     

Influence of catalyst layer polybenzimidazole molecular weight on the polybenzimidazole-based proton exchange membrane fuel cell performance

The catalyst layer (CL) of a polybenzimidazole (PBI) membrane electrode assembly (MEA) consists of Pt–C (Pt on a carbon support), PBI, and H₃PO₄. Two series of catalyst ink solutions each containing Pt–C, N,N′-dimethyl acetamide, and PBIs comprising four different molecular weights (MWs) (i.e., M_w = 1.1 × 10⁴, 4.4 × 10⁴, 9.0 × 10⁴, and 17.4 × 10⁴ g mol⁻¹) are used to fabricate CLs. One catalyst ink solution series is mixed with LiCl, while the other solution series lacks LiCl. We demonstrate that the CL prepared using a lower MW PBI has a higher electrochemical surface area, lower charge transfer resistance, and higher fuel cell performance. The addition of LiCl enhances the dispersion of the high MW PBIs in the catalyst ink solution and acts as a foaming agent in CL, thus improving fuel cell performance. However, LiCl exerts small influence on the fuel cell performance of the MEAs fabricated using low MW PBIs.     

From microstructure to the development of water and major reaction sites inside the catalyst layer of the cathode of a proton exchange membrane fuel cell

Microstructures of various sizes and shapes are fabricated on the surface of the catalyst layer (CL) of the cathode of a PEMFC, adjacent to the micro porous layer (MPL). Three major experimental results are: (1) performance is improved by up to 60% and the percentage of the increase is the same as that of the increase in interface area of CL and MPL; (2) the cell suffers no significant performance loss when Pt loading of the cathode is reduced from 1 to 0.25 mg cm⁻² and; (3) transient responses in periodical linear sweep tests show an obvious performance “jump” for all the cathodes with microstructures when approaching steady state, but none for others. Based on observations, a proposal related to the development of water and, consequently, the major reaction sites in the CL is made: there is a general water “surface” inside the CL. Major electrochemical reactions occur above (on the MPL side) of this surface and within a limited height. The surface will “move” from the membrane toward the MPL as more water is produced. The vapor generation rate (current load) relative to the removal rate of the rest of the cell components will determine the steady state position of this water surface.     

Dodecylamine-modified carbon supports for cathode in proton exchange membrane fuel cells

In this investigation, hydrophobic dodecylamine-modified carbon supports are prepared for proton exchange membrane fuel cells by organic synthesis. Well-dispersed Pt-Ru nanoparticles, with a narrow size distribution, are then deposited on the dodecylamine-modified carbon supports by methanol reduction to serve as cathodic catalysts. These dodecylamine-modified catalysts are separately mixed with either a commercial catalyst or unmodified catalyst to provide hydrophobic channels to convey the reaction gas to the active sites in the catalyst layer. The best cathode composite catalyst, containing 20-40 wt% of modified-catalyst, gives approximate 30% increase in the maximum power density, comparing to E-TEK catalyst (125 mW cm⁻²). The increase in the maximum power density is attributed to higher activity and lower resistance. This result is discussed in the context of AC-impedance and proton conductivity analysis.     

Performance prediction of PEM fuel cell cathode catalyst layer using agglomerate model

Computation of Proton Exchange Membrane (PEM) fuel cell's cathode Catalyst Layer (CL) is performed using agglomerate models in this paper, and the results are compared with homogenous one. Following our earlier homogenous model for cathode CL (see Khajeh-Hosseini et al., 2010), the focus of the present study is on agglomerate model. In this study, the derivation of agglomerate model is performed in such a way that in the simplified case when agglomerate sizes shrink to zero, the homogeneous model condition is retrieved. Validations versus two sets of experimental data are performed. For example, in one of the validation cases, Case (II), it is observed that in I_tot = 3000 [A m⁻²] the homogeneous model overestimates the performance by 80%. But the agglomerate model agrees well with the validating test cases. A set of parametric studies are performed using the agglomerate model, in which the influences of some CL structural- and cell operating-parameters are studied. A sensitivity study on the cell performance is performed to rank the influence of the parameters, with rank 1 for the most influential parameter. It is observed the agglomerate sizes possess rank 1. These results give useful guidelines for manufactures of PEMFC catalyst layers.     

Dynamic behavior of liquid water transport in a tapered channel of a proton exchange membrane fuel cell cathode

A numerical model of a proton exchange membrane fuel cell (PEMFC) cathode with a tapered channel design has been developed in order to examine the dynamic behavior of liquid water transport. Three-dimensional, transient simulations employing the level-set method (available in COMSOL 3.5a, a commercial finite element method software) have been used to explicitly track the liquid-gas interface. A liquid water droplet suspended in the center of the channel, 2 mm from the channel entrance, is subjected to airflow in the bulk of the channel. Three different cases have been studied: 1) hydrophobic bottom wall representing the gas diffusion layer and hydrophilic channel top and side walls, 2) all walls are partially wetted i.e. having a contact angle of 90°, 3) a hydrophilic bottom wall and hydrophobic top and side walls. The results show that tapering the channel downstream helps in water exhaust due to increased airflow velocity. A bottom wall, although hydrophilic, results in fast removal of water droplet as compared to partially wetted and hydrophobic bottom surfaces.     

Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/TiO2/C catalysts

Pt/TiO₂/C catalysts are employed as the cathode catalysts for proton exchange membrane fuel cell (PEMFC). The comparative studies on the Pt/C and Pt/TiO₂/C catalysts are conducted with the physical and electrochemical techniques.After the accelerating aging test (AAT), the remaining electrochemical active surface area (EAS) of the Pt/TiO₂/C catalysts is 75.6%, which is larger than that of the Pt/C catalysts (42.6%). The apparent exchange current density () of the oxygen reduction reaction (ORR) at the Pt/C catalysts decreases from 3.02 × 10⁻⁹ to 1.32 × 10⁻¹¹ A cm⁻² after the AAT. And the value of of the ORR at the Pt/TiO₂/C catalysts is 2.88 × 10⁻⁹ A cm⁻² before the AAT and 2.51 × 10⁻⁹ A cm⁻² after the AAT. Furthermore, the output performance degradation of the PEMFC using the Pt/TiO₂/C cathode catalysts is also less than that using the Pt/C catalysts. The particle size of the Pt/C catalysts increases significantly from 5.3 to 26.5 nm after the AAT. The mean particle size of the Pt/TiO₂/C catalysts is 7.3 nm before the AAT and 9.2 nm after the AAT. It can be concluded that the long-term durability of the Pt/TiO₂/C catalysts in a PEMFC is much better than that of the Pt/C catalysts.     

Simulation on cathode catalyst layer in proton exchange membrane fuel cell: Sensitivity of design parameters to cell performance and oxygen distribution

The microstructure tuning of the cathode catalyst layer (CCL) is crucial for proton exchange membrane fuel cell (PEMFC) performance. However, a great number of studies have been devoted to the qualitative analysis of CCL design parameters and there is a lack of quantitative studies. In this paper, a cross-dimensional PEMFC agglomerate model is developed to investigate the sensitivities of the CCL design parameters to cell performance and oxygen distribution. Although the results exhibit that the impact of Pt loading on cell performance accounts for 50.7%, the total impact of the Pt radius and I/C ratio (I/C) is as high as 44.9% under the current density of 1000 mA cm^?2. In addition, the variation in I/C directly affects the CCL porosity and thickness of the ionomer on the Pt surface, which determines the oxygen distribution. Typically, under a current density of 1000 mA cm^?2, the impacts of I/C on the average and standard deviation of oxygen concentration account for 42.3% and 51.3%, respectively. The sensitivities of the parameters evolve with the increase in the current density. Pt loading and I/C dominate the cell performance, respectively, with 1200 mA cm^?2 as the demarcation point. This study points out the optimization direction for the design of high-performance CCL.     

Preparation of microporous layer for proton exchange membrane fuel cell by using polyvinylpyrrolidone aqueous solution

A new method of preparing microporous layer (MPL) for proton exchange membrane fuel cell (PEMFC) was presented in this paper. Considering the bad dispersion of PTFE aqueous suspension in the carbon slurry based on ethanol, polyvinylpyrrolidone (PVP) aqueous solution was used to prepare carbon slurry for microporous layer. The prepared gas diffusion layers (GDLs) were characterized by scanning electron microscopy, contact angle system and pore size distribution analyzer. It was found that the GDL prepared with PVP aqueous solution had higher gas permeability, as well as more homogeneous hydrophobicity. Moreover, the prepared GDLs were used in the cathode of fuel cell and evaluated with fuel cell performance and EIS analysis, and the GDL prepared with PVP aqueous solution indicated better fuel cell performance and lower ohmic resistance and mass transfer resistance.     

Thickness effects of anode catalyst layer on reversal tolerant performance in proton exchange membrane fuel cell

Real-world driving conditions will likely cause hydrogen starvation at the anode chambers of stacks to trigger voltage reversal events, posing a tremendous challenge to the durability of proton exchange membrane fuel cells (PEMFCs). The reversal-tolerant anode (RTA), a material-based solution, that inclusion of oxygen evolution reaction (OER) catalyst into the anode is usually employed to cope with the voltage reversal issue. In this work, we investigate the impact of anode catalyst layer thickness on the voltage reversal performance of the membrane-electrode assemblies (MEAs) with conventional anodes (Pt/C catalyst) and RTAs doped with IrO₂ catalyst, a representative OER catalyst. We find that regardless of how thick the anode catalyst layer is, the conventional MEAs exhibit almost similar voltage reversal behaviors and times, only about 1 min to reach the shutdown voltage (?2.5 V). As for the RTA MEAs, a surprising thickness effect that the thinner RTA with the same IrO₂ loading shows superior voltage reversal tolerance. Notably, an ultra-thin RTA (~2 μm) exhibits the reversal tolerance time of 310 min, which is five times higher reversal tolerance time than most of the reported RTAs. We conclude that this thickness effect mainly results from the ionomer distribution on the OER catalyst. Besides, we observe that the RTA with a higher ionomer content shows the better reversal tolerance performance. Our work highlights the importance of the OER Triple-Phase-Boundary (TPB) and the need for improved electrode designs for robust RTAs.     

Pore network model of the cathode catalyst layer of proton exchange membrane fuel cells: Analysis of water management and electrical performance

A pore network modeling approach is developed to study multiphase transport phenomena inside a porous structure representative of the Cathode Catalyst Layer (CCL) of Proton Exchange Membrane Fuel Cell. A full coupling between two-phase transport, charge transport and heat transport is considered. The liquid water evaporation is also taken into account. The current density profile and the liquid water distribution and production are investigated to understand the liquid production mechanism inside the CCL. The results suggest that the wettability and the pore size distribution have an important impact on the water management inside the cathode catalyst layer and thus on the performances of the proton exchange membrane fuel cell. Simulations show also that Bruggemann correlation used in classical models does not predict correctly gas diffusion.     

Computational modeling and experimental verification of cathode catalyst layer on PEM fuel cells

Fuel cell systems are environmentally friendly energy converters that directly transform the chemical energy of the fuel to electricity. The proton exchange membrane (PEM) fuel cells are one of the most common type of fuel cells since they deliver high power density and are lighter and smaller when compared to the other cells. However, commercialization of the PEM fuel cells is challenging due to the high cost of its components. In addition to high catalyst costs, the problem of poor water management is also a vital issue that needs to be overcome. While the gas diffusion layer of a fuel cell is essential for removing the by-product water, the Nafion solution contained in the catalyst layer has hydrophobic properties and is crucial for both preventing the water accumulation and increasing the effectiveness of the fuel cell. In this study, the effects of Carbon:Nafion ratio on the reduction potential was investigated. The cyclic voltammograms (CV) was produced for each ratio, and it was shown that the CVs exhibit characteristics of hydrogen adsorption/desorption peaks. All the linear sweep voltammogram (LSV) curves revealed well distinguished regions of kinetic, mixed and diffusion limited reaction rate. As a result, it was observed that the ratio of 1:5 resulted higher reduction potential compared to 1:3 and 1:7. Finally, a mathematical model was purposed, in which related the rotation rate and platinum coating with the current density, in order to gain insight about the responses of the fuel cell system. The constructed model is tested and validated experimentally for various parameters that are present in the system, and it may be utilized to determine oxygen reaction activities of the catalysts without performing any unnecessary electrochemical tests.     

Development of a proton exchange membrane fuel cell cogeneration system

A proton exchange membrane fuel cell (PEMFC) cogeneration system that provides high-quality electricity and hot water has been developed. A specially designed thermal management system together with a microcontroller embedded with appropriate control algorithm is integrated into a PEM fuel cell system. The thermal management system does not only control the fuel cell operation temperature but also recover the heat dissipated by FC stack. The dynamic behaviors of thermal and electrical characteristics are presented to verify the stability of the fuel cell cogeneration system. In addition, the reliability of the fuel cell cogeneration system is proved by one-day demonstration that deals with the daily power demand in a typical family. Finally, the effects of external loads on the efficiencies of the fuel cell cogeneration system are examined. Results reveal that the maximum system efficiency was as high as 81% when combining heat and power.     

Transient computation fluid dynamics modeling of a single proton exchange membrane fuel cell with serpentine channel

The proton exchange membrane fuel cell (PEMFC) has become a promising candidate for the power source of electrical vehicles because of its low pollution, low noise and especially fast startup and transient responses at low temperatures. A transient, three-dimensional, non-isothermal and single-phase mathematical model based on computation fluid dynamics has been developed to describe the transient process and the dynamic characteristics of a PEMFC with a serpentine fluid channel. The effects of water phase change and heat transfer, as well as electrochemical kinetics and multicomponent transport on the cell performance are taken into account simultaneously in this comprehensive model. The developed model was employed to simulate a single laboratory-scale PEMFC with an electrode area about 20 cm². The dynamic behavior of the characteristic parameters such as reactant concentration, pressure loss, temperature on the membrane surface of cathode side and current density during start-up process were computed and are discussed in detail. Furthermore, transient responses of the fuel cell characteristics during step changes and sinusoidal changes in the stoichiometric flow ratio of the cathode inlet stream, cathode inlet stream humidity and cell voltage are also studied and analyzed and interesting undershoot/overshoot behavior of some variables was found. It was also found that the startup and transient response time of a PEM fuel cell is of the order of a second, which is similar to the simulation results predicted by most models. The result is an important guide for the optimization of PEMFC designs and dynamic operation.     

Improving proton exchange membrane fuel cell performance with carbon nanotubes as the material of cathode microporous layer

The cathode microporous layer (MPL) is fabricated by various multiwall carbon nanotubes (CNTs), and its influence on the performance of a proton exchange membrane fuel cell (PEMFC) is evaluated. Three types of CNT with different dimensions are employed in the experiments, and the conventional MPL made by acetylene black (AB) is also considered for the purpose of comparison. The results show that the employment of CNT as MPL composition indeed may improve fuel cell performance significantly in comparison with the case of AB. The type of CNT with the largest tube diameter and straight cylinder in shape exhibits the highest cell performance. The corresponding optimal CNT loading and polytetrafluoroethylene (PTFE) content in the MPL are also evaluated. Results show that the case of cathode MPL composed of 1.5 mg cm^?2 CNT and 20 wt% PTFE exhibits the best performance in all the experimental cases. The present data reveal that the application of CNT for MPL fabrication is beneficial to promote PEMFC performance. Copyright © 2015 John Wiley & Sons, Ltd.     

Prediction of the effective conductivity of Nafion in the catalyst layer of a proton exchange membrane fuel cell

In a previous study, a simple acid catalyzed reaction (esterification) was found to predict excellently conductivity of a membrane contaminated with NH₄⁺ or Na⁺. Since measurement of the conductivity of Nafion in a catalyst layer is problematic, being able to predict this conductivity for various formulations and fuel cell conditions would be advantageous. In this study, the same methodology as before was used to examine the proton availabilities of supported Nafion (Nafion on carbon and on Pt/C), as exists in the catalyst layer used in a PEMFC, during impurity exposure (e.g., NH₃) as a means for prediction of its conductivity. It was found that the effect of NH₃ exposure on the proton composition (yH+) of supported Nafion was similar to that of N-211 under the same conditions. Determined values of yH+ were then used to estimate the effective conductivity of an ammonium-poisoned cathode layer using the correlation developed and the agglomerate model. The predicted conductivities were matched with the results available in the literature. This technique would be useful for the optimization of catalyst design and for fuel cell simulation, since it provides many benefits over conventional performance test procedures.     

Liquid water flooding process in proton exchange membrane fuel cell cathode with straight parallel channels and porous layer

Liquid water management plays a significant role in proton exchange membrane fuel cell (PEMFC) performance, especially when the PEMFC is operating with high current density. Therefore, understanding of liquid water behavior and flooding process is a critical challenge that must be addressed. To overcome PEMFC durability problems, a liquid water flooding process is studied in the cathode side of a PEMFC with straight parallel channels and a porous layer using FLUENT^® v6.3.26 software with a volume-of-fluid (VOF) algorithm and user-defined-function (UDF). The general process of liquid water flooding within this type of PEMFC cathode is investigated by analyzing the behavior of liquid water in porous layer and gas flow channels. Two important phenomena, the “first channel phenomenon” and the “last channel phenomenon”, and their effects on the flow distribution along different parallel channels are discussed.     

Enhancement of proton exchange membrane fuel cell performance by titanium-coated anode gas diffusion layer

Titanium was coated onto an anode gas diffusion layer (GDL) by direct current sputtering to improve the performance and durability of a proton exchange membrane fuel cell (PEMFC). Scanning electron microscopy (SEM) images showed that the GDLs were thoroughly coated with titanium, which showed angular protrusion. Single-cell performance of the PEMFCs with titanium-coated GDLs as anodes was investigated at operating temperatures of 25 °C, 45 °C, and 65 °C. Cell performances of all membrane electrode assemblies (MEAs) with titanium-coated GDLs were superior to that of the MEA without titanium coating. The MEA with titanium-coated GDL, with 10 min sputtering time, demonstrated the best performance at 25 °C, 45 °C, and 65 °C with corresponding power densities 58.26%, 32.10%, and 37.45% higher than that of MEA without titanium coating.