Metallic bipolar plates can significantly increase the power density of polymer electrolyte membrane fuel cells; however, they require corrosion-protection coatings with desirable physical and chemical properties. In this study, diamond-like carbon (DLC) film coatings are investigated as such coatings potentially for metallic-based bipolar plates, with the focus on the relation between the processes and properties of the coatings under different coating deposition conditions of Plasma Enhanced Chemical Vapor Depsoition (PECVD) method. Various characterization techniques are applied to study the adhesion, structure, morphology, wettability, corrosion, and electrical resistivity of the film coatings. XPS, EDAX, and SEM analyses are used to identify the ratio of sp3 (diamond-like) and sp2 (graphite-like) bonds in the coatings, surface elements, and surface morphology, respectively. Potentiodynamic polarization test is utilized to investigate the corrosion behaviors of substrates with and without DLC coatings. Further, the electrical resistivity of the DLC films is measured by the four-point probe method. The results indicate that higher deposition power along with the absence of argon gas results in more sp3 than sp2 bonds in the coating, and the electrical resistivity is increased accordingly. The coating films deposited from methane (CH4) exhibit superior adhesion to the stainless steel (SS316) substrates over those generated from acetylene (C2H2) gas. Coating films deposited on the metallic substrates change the surface wettability appreciably. Further, polarization tests show that coatings generated with a low power of 250 W and higher argon gas percentage of 30% provide better anti-corrosion protection for metallic-based bipolar plates. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Electrode structure determines the rate of transport and electrochemical reactions and is significantly affected by the catalyst deposition method. In this study, the effect of catalyst deposition is investigated on the pore structure, mass transport, and operating performance of the catalyzed electrodes prepared by the methods of catalyst coated on membrane (CCM) and catalyst coated on substrate (CCS). The result indicates that the CCS electrode is thinner, yielding larger porosity, smaller geometric pore surface area, smaller diffusion and permeation resistivity, and lower cell performance. The maximum power density of the CCS electrodes is only about 4% smaller than that of the CCM electrodes at high Pt loadings (0.4 mg.cm(-2)), while it is as much as 60% less than that of the CCM counterparts at low Pt loadings (0.1 mg.cm(-2)). The significant performance drop for the low-Pt-loading CCS electrodes is due to the relatively low surface area in the catalyst layers resulted from catalyst penetration into the pores of the gas diffusion layer, even though the mass transfer resistivity is smaller than their CCM counterparts. The CCS method is therefore unsuitable for low-Pt-loading electrodes (< 0.1 mg.cm(-2)) unless the material penetration and the resulting performance deterioration can be inhibited.

Geometric pore surface area is a significant parameter for the description of the irregular, somewhat random, porous structure of the catalyzed electrodes in polymer electrolyte membrane (PEM) fuel cells; however, its value is sensitive to the experimental methods employed, which necessitates the measurements via different methods. In this study, the geometric surface area of the porous electrode is determined by two different methods: the method of standard porosimetry (MSP) and Brunauer-Emmett-Teller (BET). The theory of fractal dimension is employed to analyze the data obtained from the MSP, and the fractal surface area calculated using nitrogen molecules as the scale is compared with the BET surface area. The results indicate that the geometric pore surface area is a property of the porous electrode that depends greatly on the "scale" size (ie, molecule size of the working fluid)-a smaller scale yields a larger value of the surface area. The surface area determined by the BET is found to be about one order of magnitude larger than that obtained by the MSP. Thus, the fractal dimension theory based on MSP demonstrates a useful tool to determine the accessible pore surface area at different length scales.

Catalyst layer (CL) has a significant impact on the overall pore structure of the entire electrodes, thereby impacting the transport processes and the performance of polymer electrolyte membrane (PEM) fuel cells. In this study, the contribution of the CL to the entire electrode structure is experimentally investigated. The electrodes are prepared by using two types of catalysts with different platinum/carbon (Pt/C) ratios and Pt loadings and characterized by the method of standard porosimetry (MSP). The results show that for the same type of catalysts, as the Pt loading is increased, both the porosity and mean pore size of the electrode decrease, whereas the pore surface area increases. For a constant Pt loading, a lower Pt/C ratio results in a thicker electrode with a smaller porosity, smaller pore size, and larger pore surface area. The fractal dimension is found to be a good representative of the complexity of the pore structure of the electrode; a larger fractal dimension is detected for a higher Pt loading and a smaller Pt/C ratio.

Abstract The gas diffusion layer (GDL), which is basically a carbon-based and wet-proofed porous medium, is a critical component for proton exchange membrane (PEM) fuel cells. This criticality is mainly due to its critical functions, such as governing the transport of both the reactants and by-products, maintaining a delicate balance between the water removal and membrane hydration levels, offering continuous mechanical support to the thin catalyst layer (CL), and providing sufficient pathways for electron transport. Active research has mainly centered on designing the architecture of GDLs; thus, it is essential to shed light on the state-of-the-art architectures of GDLs, the recent advances in the characteristics of GDLs, and ex situ and in situ characterization techniques that have been extensively used. Therefore, this chapter reviews the main characteristics of GDLs (e.g., gas permeability, electrical and thermal conductivity, porosity, and hydrophobicity and hydrophilicity), with a particular focus on commonly used insitu and ex situ characterization techniques and their effects on the performance of PEM fuel cells.

— Lower cost, lighter weight, and higher electrical conductivity are among many advantages of using metallic bipolar plates (BPs) over the conventional graphitic materials in PEM fuel cell applications. In this study, diamond-like carbon (DLC) coatings are deposited on top surfaces of stainless steel (SS316) and Aluminum (Al5052) substrates via Plasma Enhanced Carbon Vapor Deposition (PECVD) in order to increase the durability. Further, the effect of different plasma power intensities and type of precursor gases, including methane (CH 4 ) and acetylene (C 2 H 2 ), on the growth of DLC films is studied. Various ex-situ characterization techniques have been used to examine the strength of adhesion, morphologies, structures/compositions, and wettability of the DLC coatings. It is found that coatings produced from CH 4 gas show superior adhesion in comparison to those obtained from the C 2 H 2 gas. Results indicate that higher deposition power intensity can produce more sp 3 bonds than sp 2 bonds. It is also determined that the type of the metallic substrates has predominant roles on the morphology of the DLC coatings. In addition, a moderate power intensity of 250 W results in desirable properties in DLC, including adhesion and wettability, compared to higher and lower plasma power intensities.

The gas diffusion layer (GDL), which is basically a carbon-based and wet-proofed porous medium, is a critical component for proton exchange membrane (PEM) fuel cells. This criticality is mainly due to its critical functions, such as governing the transport of both the reactants and by-products, maintaining a delicate balance between the water removal and membrane hydration levels, offering continuous mechanical support to the thin catalyst layer (CL), and providing sufficient pathways for electron transport. Active research has mainly centered on designing the architecture of GDLs; thus, it is essential to shed light on the state-of-the-art architectures of GDLs, the recent advances in the characteristics of GDLs, and ex situ and in situ characterization techniques that have been extensively used. Therefore, this chapter reviews the main characteristics of GDLs (e.g., gas permeability, electrical and thermal conductivity, porosity, and hydrophobicity and hydrophilicity), with a particular focus on commonly used insitu and ex situ characterization techniques and their effects on the performance of PEM fuel cells.

The present study reports an experimental investigation regarding the dynamic characteristics of local current density in proton exchange membrane fuel cells during dynamic operation, important for mobile applications, by using segmented current collector method. The results indicate that for fully humidified operation, the local current density fluctuates more intensely when the cell potential is reduced, corresponding to an increase in the average cell current density (or load), whereas it is reduced considerably when the air stoichiometry is increased. Fluctuations in the local current density are lower near the flow channel inlet region and higher near the channel exit region. Operation at higher pressures enhances the cell performance, as expected, but results into severer fluctuations in the local current density. Further, it is observed that both the magnitude and the frequency of the fluctuations for the local current density are strongly influenced by the humidity condition in the reactant gas streams, and partially humidified operation results in significant fluctuations in regions near the channel inlet, suggesting close relation to the presence, distribution, and transport of water in the cell structure.

In recent years, short side chain (SSC) ionomers with lower equivalent weights have been demonstrated to increase cell performance via more-homogenous coating and higher proton conductivity than the conventional ionomer, Nafion, used in the catalyst layer (CL). In spite of some studies on cell performance, the impact of an ionomer's structure and ratio on cell durability has hardly been reported. In this study, a systematic experimental investigation is performed to study the impact of SSC ionomer structures and ratios on fuel cell durability using a scaled up cell (45 cm(2)). A CL fabricated with an SSC ionomer is shown to perform better and be more durable than a CL prepared by Nafion in the same ratio. Moreover, increasing the ionomer ratio in the catalyst layer leads to lower cell performance at high current densities, but higher cell durability. These results highlight the importance of the structure and ratio of the ionomers in cell performance and durability.

The importance of membrane electrode assembly (MEA) conditioning for proton exchange membrane (PEM) fuel cells under various operating conditions, such as reactant flow and cell voltage-current combination, has been well recognized, but few studies have considered the impact of the cell hardware design. In this study, the impact of flow-field layout on the conditioning of MEAs has been experimentally investigated. It is shown that the MEAs conditioned with serpentine flow-field layouts on both the anode and cathode side have better performance than the MEAs conditioned with straight-parallel flow-field layouts, and that the peak power density can be increased from 0.83 W/cm(2) to 0.93 W/cm(2) (about 12% increase) for the MEAs tested under the same operating condition of using humidified hydrogen and air at atmospheric pressure. This performance improvement is mainly due to the under-rib convection of the reactant gases in serpentine flow-field layouts that provides more uniform conditioning of the entire MEAs, compared with the MEAs conditioned with straight-parallel flow-field layouts for which the portion of the MEAs under the rib is not well conditioned, due to the lack of the reactant flow there.

For polymer electrolyte membrane (PEM) fuel cells, the pore structure and small effective diffusion coefficient (EDC) of the catalyst layers have significant impact on the cell performance. In this study, both the pore structure and EDC of the catalyst layers are investigated experimentally; the pore structure of the catalyst layer is characterized by the method of standard porosimetry, and the EDC is measured by a modified Loschmidt cell for oxygen-nitrogen mixture through the catalyzed electrodes. It is found that Pt loading has a direct impact on the pore structure and consequently the EDC of the catalyzed electrodes. As the Pt loading is increased, the porosity and mean pore size of the catalyzed electrode decrease, and the EDC decreases accordingly, however, it is increased by 15-25% by increasing the temperature from 25 degrees C to 75 degrees C. The EDC of the catalyst layer is about 4.6 x 10(-7) m(2) s(-2) at 75 degrees C, compared with 25.0 x 10(-7) m(2) s(-1) for the uncatalyzed electrode at the same temperature. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

To achieve a good performance, proton exchange membrane fuel cells (PEMFCs) require a delicate water balance through the design of cells and selection of operating conditions. In this study, the impact of ionomers with different side-chain lengths, having different water retention capability, serving as binder in the catalyst layers is investigated experimentally for both catalyst layer structure and the resulting cell performance under a wide range of reactant flows and cell operating pressures. The results show that cells with the short side-chain (SSC) ionomers, having a higher water retention capability, achieve a higher cell performance, active surface area, platinum utilization, and porosity than the cells made by the conventional long side-chain (LSC) even under fully humidified condition, in contrast with studies reported in literature. This is achieved for the high reactant flow, which has a higher water removal capability. The SSC ionomer with the low equivalent weight of 720 results in a significantly higher cell performance with the maximum power density of 1.3 W/cm(2) at the high reactant flow. SSC with the equivalent weight of 790 has a good balance between water retention and water removal in the catalyst layer at a lower airflow. The performance of the cells with the SSCs is better, and it is far less sensitive than, the cells with the LSC when the operating pressure is increased.

Commercial success for proton exchange membrane fuel cell (PEMFC) requires manufacturing of its core component, membrane electrode assembly (MEA), with consistent and reliable performance. In this study, three common methods for the MEA manufacturing are investigated systematically for their impact on the cell performance under consistent preparation and cell test conditions, including catalyst coated substrate (CCS), catalyst coated membrane (CCM), and low temperature decal method (LTDM). The variations in each of these methods studied include applying an extra Nafion layer, hot press, and Pt loading. It is found that MEA manufacturing process has a significant impact on the cell performance, and this impact is significantly affected by the Pt loading. At the high Pt loading of 0.5 mg/cm(2), CCM without hot press involving gas diffusion layers (GDLs), referred to as CCM-Wo, results in the best performance with the maximum power density of 0.95 W/cm(2), although both LTDM and CCS with an extra Nafion layer (hence called N-LTDM and N-CCS) are just slightly less with the maximum power density of 0.91 W/cm(2). Hot press with GDLs is essential for CCS method to achieve a good performance, while it is not the case for CCM and N-LTDM. Applying an extra layer of Nafion on catalyst layers as in N-LTDM and N-CCS methods has a positive impact on the cell performance; whereas it is negative when it is applied on the membrane as in the N-CCM method. When the Pt loading is reduced to 0.125 mg/cm(2) (75% reduction in the Pt loading), cell performance is reduced for all the MEAs made by the three methods, but significant reduction (about 75%) is observed for CCS method, while it is less than 30% for the other two methods. Therefore, care should be taken in the MEA manufacturing for MEAs with low Pt loadings.

For polymer electrolyte membrane fuel cells (PEMFCs), the importance of durability is widely recognized, but less attention has been paid to the role of ionomers. In this study, the importance of ionomer structure in achieving high PEMFC performance and durability are investigated experimentally for different catalyst-ionomer ratios and catalyst loadings in scaled up cell (45 cm(2)). The results are compared with a conventional long side chain ionomer (LSC) under the same preparation and testing conditions. Catalyst layers (CLs) fabricated with 25 wt% of short side chain (SSC) ionomer display higher performance than 17 wt% and 30 wt%. A similar trend is also demonstrated when using the LSC ionomer. However, it is found that SSC ionomer is more compatible with CLs than LSC. This compatibility is ascribed to the higher stability of the SSC ionomer. In addition, higher performance, Pt utilization, and active surface area are measured for membrane electrode assemblies (MEAs) prepared with SSC in comparison to LSC under the same ratio. Based on the accelerated stress tests, SSC ionomer has a positive role in improving durability, as the maximum power density after 30,000 cycles decreases by 21% and 48% for MEAs prepared by SSC and LSC, respectively. Moreover, the losses in performance are more than two times greater than when the Pt loading is decreased from 0.5 mg/cm(2) to 0.125 mg/cm(2). These results highlight the importance of ionomer structure in cell performance and durability at high and low Pt loadings.

For polymer electrolyte membrane (PEM) fuel cells, the importance of mass transport property, gas permeability, in gas diffusion layer (GDL) is widely recognized with less attention being paid to catalyzed electrode (GDL with a catalyst layer). In this study, the contribution of the catalyst layer to the overall gas permeability of the electrode is experimentally investigated for different catalysts with a range of Pt loadings at various temperatures for air, oxygen and nitrogen gases. Results indicate that the gas permeability of the GDLs can be reduced by 58-77% with the presence of a catalyst layer. For the constant Pt loadings, the electrodes with higher Pt/C ratios (e.g., 60% Pt/C) show larger gas permeability than those with lower ratios (e.g., 30% Pt/C) due to their smaller thicknesses and higher porosity. Similarly, for the electrodes with the same type of catalysts, the gas permeability is higher for lower Pt loadings. Further, the effective gas permeability of the catalyst layers alone is about two orders of magnitude smaller than that of the GDLs. Additionally, operating at higher temperatures slightly enhances the permeability. Oxygen gas has a higher permeability than air and nitrogen, but the differences are small. These results highlight the importance of catalyst layer, hence the Pt loadings and Pt/C ratios, in determining the mass transport throughout the entire electrode in PEM fuel cells.

For proton exchange membrane fuel cells (PEMFC), the importance of catalytic (kinetic) effect is widely recognized with less attention being paid to mass transport (diffusion) effect. In this study, the importance of kinetic and diffusion effect on PEMFC performance is investigated experimentally by using different catalysts and catalyst layer (CL) fabrication methods of spraying and brush-painting, both of which are commonly used, simple and low-cost with volume-production capability. It is found that as a catalyst, 20% Pt/C has better kinetic characteristics than 60% Pt/C in terms of smaller Pt particle size, better Pt dispersion, less Pt agglomeration, and larger active surface area. For the same Pt loading, the porosity is almost identical for the CLs made of the two catalysts by the two fabrication methods, but the diffusion resistivity for the CLs made of 60% Pt/C is smaller than its counterpart for the CLs made of 20% Pt/C, due to difference in the effective diffusion coefficient and CL thickness. As a result, the performance of the PEMFC made of 60% Pt/C is better than the corresponding PEMFC made of 20% Pt/C. Similarly, for PEMFCs made of the same catalyst, the diffusion resistivity is smaller for CLs made by spraying compared with brush-painting method, leading to better performance for the PEMFC made by spraying method. These results highlight the importance of diffusion effect, hence the structure of the CLs, in determining the PEMFC performance. (C) 2017 Elsevier Ltd. All rights reserved.

Proton exchange membrane fuel cell (PEMFC) has reached the stage of early commercialization, with attention now focused on manufacturing process. In this study decal transfer as the common method of PEMFC fabrication is investigated systematically, and an effective low temperature decal transfer method (LTDM) is developed without using any chemical reagents, breaking layers, and high hot press temperatures. It is shown that this LTDM results in complete catalyst transfer yield from the decal substrate to the membrane without the formation of Nafion outer layer and delamination between the catalyst layer and membrane. Further, catalyst transfer yield is influenced by the type of substrate material, the deposition of the extra Nafion layer, and the hot press conditions. Among the seven inert substrates studied, fluorinated ethylene propylene (FEP) is shown to be the best due to its lower friction coefficient and lower contact angle (for the hydrofluoropolymers). The performance of the resulting PEMFCs is compared with the recent developments in low temperature decal method under the same fabrication and testing conditions, facilitating a solid comparison of the various methods.

Water management is one of the critical issues affecting polymer electrolyte membrane fuel cell (PEMFC) performance, durability and cost. Modifying the surface wettability of flow field channels in PEMFC can improve the water management and fuel cell performance. In present work, an ex-situ investigation of the effects of channels with different surface wettability on the two phase-flow characteristics has been conducted. Horizontal graphite channel (slightly hydrophobic) and other four channels coated with polytetrafluoroethylene (PTFE) (hydrophobic), silica/Polydimethylsiloxane (PDMS) (super-hydrophobic), silica/PDMS at the bottom wall but with the side walls being raw graphite (combined surface wettability channel), and silica coated channel (superhydrophilic), are tested and visualized at room temperature and atmospheric pressure. Pressure drop measurements and two-phase flow visualization using high speed camera have been performed. Super-hydrophobic channel has shown desirable positive effect on the two-phase flow and on the PEMFC's performance compared with the other channels, especially at high current densities.

The dynamic performance of proton exchange membrane fuel cell (PEMFC) is critically important, especially for automotive applications. In this study, the dynamic behavior of the local current density distribution inside PEMFC has been investigated experimentally by using the segmented flow field plate and printed circuit board (PCB) method. The effect of different values of air stoichiometry ratios on the dynamic characteristics of local current density has been tested. It has been found that the fluctuation of the local current density increases as the cell voltage is reduced, corresponding to the increase in the average cell current density or cell loading. Increasing the stoichiometry ratio of the cathode supply gas significantly reduces the fluctuations of the current density. The fluctuation is also lower near the flow inlet than near the flow exit. The phenomena observed are closely related to the water and reactant distributions and transports in the cell structure.

Current distribution in a proton exchange membrane fuel cell (PEMFC) is significantly influenced by reactant flow configurations. In this study, the current distribution has been measured experimentally using a segmented flow-field plate and printed circuit board (PCB). Local current distributions for a PEMFC with serpentine flow field and three different flow arrangements including co-flow, cross-flow, and counter-flow arrangements for the anode and cathode streams are investigated along with the effect of flow channel orientation. It is shown that the counter-flow arrangement yields most uniform distribution for the current density, whereas the co-flow arrangement results in a considerable variation in the current density from the reactant gas stream inlet to exit. Flow channel orientation can also impact the cell performance and the current distribution appreciably. The limiting hydrogen concentration at the anode side due to the low stoichiometry condition can have a predominant effect on the current distribution and cell performance.