The durability investigation of a 10‐cell metal bipolar plate proton exchange membrane fuel cell stack

The metal bipolar plates (BPs) have replaced the graphite BPs in vehicle‐used proton exchange membrane fuel cell (PEMFC) stack because of their high volume power density. To investigate the durability of metal BP stack, this paper performed a durability test of 2000 hours on a 10‐cell metal BP fuel cell stack. The degradations of the average voltage and individual cell voltage in fuel cell stack were analyzed. To investigate the degradation mechanism, the stack was disassembled and the morphologies and compositions of no. 1, no. 5, and no. 10 cells after 2000 hours were characterized by SEM, TEM, and ASS. The results indicated that at 800 mA/cm², the voltage decay rate is 42.303 μV/hour and the voltage decay percentage of the stack is 14.34% after 2000 hours according to the linearly fitting result. According to the US Department of Energy (DOE) definition of fuel cell stack life, only the voltage decay rate of OCV and the tenth cell is lower than the maximum voltage degradation rates of 10 000 hours. The decreases of homogeneity of stack were the main reason for its performance degradation. Especially for the tenth cell, its performance has almost no drop. The main failure reason of this metal BP stack is structural design rather than metal corrosion. The losses of Pt catalyst and C supporting are the main reason of performance degradation.     

Water management in a novel proton exchange membrane fuel cell stack with moisture coil cooling

Water flooding causes severe degradation of the performance and lifetime of proton exchange membrane fuel cell (PEMFC). In this study, a novel PEMFC stack with in-built moisture coil cooling was designed and the effects of moisture coil cooling on water management in the new PEMFC stack under various operating conditions were investigated. The result showed that the performance of the PEMFC stack was significantly improved due to the moisture condensation under high current density, high operating temperature, high relative humidity and high operating pressure. The output power was increases by 21.62% (525.71 W) at 1600·mA cm⁻² while the increased parasitic power was no more than 35W. Moreover, degradation of the cathode catalyst layer after 100 h operation was also reduced by using moisture coil cooling. Compared with the situation without moisture condensation, the maximum decay rate of the cathode catalyst layer thickness after 100 h operation was reduced by 13.01%. Accordingly, the novel design is valuable and can be widely used in the future design of PEMFC.     

Demonstration of a 20 W class high-temperature polymer electrolyte fuel cell stack with novel fabrication of a membrane electrode assembly

Acid-doped polybenzimidazole (PBI) membrane and polytetrafluoroethylene (PTFE)-based electrodes are used for the membrane electrode assembly (MEA) in high-temperature polymer electrolyte fuel cells (HTPEFCs). To find the optimum PTFE content for the catalyst layer, the PTFE ratio in the electrodes is varied from 25 to 50 wt%. To improve the performance of the electrodes, PBI is added to the catalyst layer. With a weight ratio of PTFE to Pt/C of 45:55 (45 wt% PTFE in the catalyst layer), the fuel cell shows good performance at 150 °C under non-humidified conditions. When 5 wt% PBI is added to the electrodes, performance is further improved (250 mA cm⁻² at 0.6 V). Our 20 W class HTPEFC stack is fabricated with a novel MEA. This MEA consists of 8 layers (1 phosphoric acid-doped PBI membrane, 2 electrodes, 1 sub-gasket, 2 gas-diffusion media, 2 gas-sealing gaskets). The sub-gasket mitigates the destruction of a highly acid-doped PBI membrane and provides long-term durability to the fuel cell stack. The stack operates for 1200 h without noticeable cell degradation.     

Enhanced low-humidity performance of proton exchange membrane fuel cell by incorporating phosphoric acid-loaded covalent organic framework in anode catalyst layer

Developing self-humidifying membrane electrode assembly (MEA) is of great significance for the practical use of proton exchange membrane fuel cell (PEMFC). In this work, a phosphoric acid (PA)-loaded Schiff base networks (SNW)-type covalent organic framework (COF) is proposed as the anode catalyst layer (CL) additive to enhance the PEMFC performance under low humidity conditions. The unique polymer structure and immobilized PA endow the proposed COF network with not only excellent water retention capacity but also proton transfer ability, thus leading to the superior low humidity performance of the PEMFC. The optimization of the additive content, the effect of relative humidity (RH) and PEMFC operating temperature are investigated by means of electrochemical characterization and single cell test. At a normal operation temperature of 60 °C and 38% RH, the MEA with optimized COF content (10 wt%) showes the maximum power density of 582 mW cm^?2, which is almost 7 times higher than that of the routine MEA (85 mW cm^?2). Furthermore, a preliminary durability test demonstrates the potential of the proposed PEMFC for practice operation under low humidity environment.     

Comparison of the performance and degradation mechanism of PEMFC with Pt/C and Pt black catalyst

Proton exchange membrane fuel cell (PEMFC) has been used in supplying power for Unmanned Underwater Vehicle which operate in a closed environment and dead-ended anode and cathode (DEAC) mode is deemed as an effective way to enhance the fuel utilization rate. Catalyst is an important factor that influences the performance and durability of PEMFC, especially in DEAC mode. In this paper, the degradation characteristics of PEMFCs with Pt black and Pt/C catalyst after 100 h operation have been investigated by electrochemical techniques and morphological characterization methods. It's shown that the degradation of Pt black catalyst layer (CL) was more severe than that of Pt/C CL. The difference of performance degradation is due to the dominant decay mechanism of these two catalysts is different. According to SEM, TEM and XPS results, the decay of Pt black catalyst is mainly caused by Pt agglomeration and oxidation, causing a higher ohmic resistance, higher mass transfer resistance and severer degradation of performance. The degradation of Pt/C catalyst is mainly due to the reduction of electrochemical surface area and carbon corrosion because the larger carbon corrosion makes micropores and the thicker supporting structure, resulting in the performance degradation.     

Life prediction of membrane electrode assembly through load and potential cycling accelerated degradation testing in polymer electrolyte membrane fuel cells

Accelerated degradation tests (ADTs) are commonly used to assess the durability of membrane electrode assembly (MEA) components consisting of polymer electrolyte membrane fuel cells (PEMFC) under harsh stress conditions, estimating their lifetime in actual use condition and uncovering their vital degradation mechanisms. ADTs apply mechanically, chemically, or thermally combined stressors to efficiently investigate the durability of MEAs. However, combined stressors for ADTs might cause biased lifetime prediction because major deterioration mechanisms of MEA components are mixed with each other. This work proposes a method to accurately predict the lifetime of MEA through empirical modeling of its performance degradation through ADTs under potential cycle (carbon corrosion) and load cycle tests (electrocatalysts). To simulate operation modes of fuel cell electric vehicles, MEAs are tested under continuous on-off cycle testing (24 h operating and 1 h break) for 5000 h. Degradation patterns of MEAs are first modeled by the empirical model. The relationship between ADTs (potential and load cycle) and continuous on-off condition is then closely examined to accurately predict MEA lifetime under actual operation environments. The proposed idea has a potential to resolve critical durability issues of MEAs by identifying intermingling effects from other constituents.     

Hybrid electrode effects on polymer electrolyte membrane fuel cells

Research on membrane electrode assemblies (MEA) is focused on reducing cost and increasing durability in polymer electrolyte membrane fuel cells (PEMFC). Development of the electrode structure and reduction of platinum (Pt) contents are studied to improve the efficiency of Pt catalysts. We studied the combined effects of improved electrode structure and reduced Pt loading. To enhance the performance of an MEA, a commercial Pt/C catalyst with micro graphite (MG) was used. The 40 wt% Pt/C catalyst content was reduced about 5, 15, 30 and 60 wt% at the cathode. MG was added as a reduced weight percent of Pt/C. Cell performance was significantly dependent on the content of MG. The MEA with 15 wt% of MG was seen to best performance compare with other MEA. These results showed that the catalyst with mixed MG improved both performance and cost savings with reduced Pt content of PEMFC.     

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.     

Diagnosis of membrane electrode assembly degradation with drive cycle test technique

One of important factors determining the lifetime of proton exchange membrane fuel cells (PEMFCs) is degradation and failure of membrane electrode assembly (MEA). The lack of effective mitigation methods is largely due to the currently limited understanding of the degradation mechanisms for fuel cell MEAs. This study adopted the accelerating degradation technique to analyze durability of MEA using drive cycle test protocol developed by Chinese NERC Fuel Cell & Hydrogen Technology to assess the long-term durability of fuel cells for vehicular application. During 900 h durability test of the MEA, the polarization curve, cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were performed as diagnostics during and on completion the test. The experimental results show that the performance degradation rate of the cell is about 70 μV h⁻¹ at the operating current density of 500 mA cm⁻², failure of the proton exchange membrane is the decisive factor leading to the failure of the MEA. And the damage of the micro-structural of catalytic layer, crucial for electrochemical reaction, is the decisive factor for the performance degradation.     

Self-humidifying membrane electrode assembly prepared by adding PVA as hygroscopic agent in anode catalyst layer

In this work, a novel self-humidifying membrane electrode assembly (MEA) with addition of polyvinyl alcohol (PVA) as the hygroscopic agent into anode catalyst layer was developed for proton exchange membrane fuel cell (PEMFC). The MEA shows good self humidification performance, for the sample with PVA addition of 5 wt.% (MEA PVA5), the maximum power density can reach up to 623.3 mW·cm⁻², with current densities of 1000 mA·cm⁻² at 0.6 V and 600 mA·cm⁻² at 0.7 V respectively, at 50 °C and 34% of relative humidity (RH). It is interesting that the performance of MEA PVA5 hardly changes even if the relative humidity of both the anode and cathode decreased from 100% to 34%. The MEA PVA5 also shows good stability at low humidity operating conditions: keeping the MEA discharged at constant voltage of 0.6 V for 60 h at 34% of RH, the attenuation of the current density is less than 10%, whilst for the MEA without addition of PVA, the attenuation is high up to 80% within 5 h.     

Study of catalyst sprayed membrane under irradiation method to prepare high performance membrane electrode assemblies for solid polymer electrolyte water electrolysis

In this work, a catalyst sprayed membrane under irradiation (CSMUI) method was investigated to develop high performance membrane electrode assembly (MEA) for solid polymer electrolyte (SPE) water electrolysis. The water electrolysis performance and properties of the prepared MEA were evaluated and analyzed by polarization curves, electrochemistry impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The characterizations revealed that the CSMUI method is very effective for preparing high performance MEA for SPE water electrolysis: the cell voltage can be as low as 1.564 V at 1 A cm⁻² and the terminal voltage is only 1.669 V at 2 A cm⁻², which are among the best results yet reported for SPE water electrolysis with IrO₂ catalyst. Also, it is found that the noble metal catalysts loadings of the MEA prepared by this method can be greatly decreased without significant performance degradation. At a current density of 1 A cm⁻², the MEA showed good stability for water electrolysis operating: the cell voltage remained at 1.60 V without obvious deterioration after 105 h operation under atmosphere pressure and 80 °C.     

Experimental analysis of a degraded open-cathode PEM fuel cell stack

The well-known challenges to overcome in PEM fuel cell research are their relatively low durability and the high costs for the platinum catalysts. This work focuses on degradation mechanisms that are present in open-cathode PEM fuel cell systems and their links to the decaying fuel cell performance. Therefore a degraded, open-cathode, 20 cell, PEM fuel cell stack was analyzed by means of in-situ and ex-situ techniques. Voltage transients during external perturbations, such as changing temperature, humidity and stoichiometry show that degradation affects individual cells quite differently towards the end of life of the stack. Cells located close to the endplates of the stack show the biggest performance decay. Electrochemical impedance spectroscopy (EIS) data present non-reversible catalyst layer degradation but negligible membrane degradation of several cells. Post-mortem, ex-situ experiments, such as cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) show a significant active area loss of the first cells within the stack due to Pt dissolution, oxidation and agglomeration. Scanning electron microscope (SEM) images of the degraded cells in comparison with the normally working cells in the stack show severe carbon corrosion of the cathode catalyst layers.     

Long-term durability test for direct methanol fuel cell made of hydrocarbon membrane

A long-term durability test has been conducted for a direct methanol fuel cell (DMFC) using the commercial hydrocarbon membrane and Nafion ionomer bonded electrodes for 500 h. Membrane electrode assembly (MEA) made by a decal method has experienced a performance degradation about 34% after 500 h operation. Cross-sectional analysis of the MEA shows that the poor interfacial contact between the catalyst layers and membrane in the MEA has further deteriorated after the durability test. Therefore, the internal resistance of a cell measured by electrochemical impedance spectroscopy (EIS) has considerably increased. The delamination at the interfaces is mainly attributed to incompatibility between polymeric materials used in the MEA. Furthermore, X-ray diffraction (XRD) analysis reveals that the catalyst particles have grown; thereby decreasing the electrochemical surface area. Electron probe micro analysis (EPMA) shows a small amount of Ru crossover from anode to cathode; and its effect on the performance degradation has been analyzed.     

Experimental investigation on scaling and stacking up of proton exchange membrane fuel cells

The performance of a proton exchange membrane fuel cell (PEMFC) with various flow channel design (serpentine and interdigitated) with different landing to channel ratios (L:C = 1:1; 2:2) for an active area of 25 cm² and 70 cm², for single cell and two cells stack is studied and compared. The effect of back pressure on the PEMFC performance is also investigated. This study establishes a strong relation between back pressure and power output from a PEMFC. It was concluded that the interdigitated flow channel gives better results than the serpentine flow channel configuration for various landing to channel ratios. It was also found that power outputs do not proportionally increase with active area of the membrane electrode assembly (MEA). Similarly, stacking up studies with single cell and two cell stack shows that the two cell stack has reduced power densities when compared to that of a single cell. The effect of cooling channels with natural and forced convection by using induced draught fan on the performance of a PEMFC stack is also studied. Fuel distribution and temperature management are found to be the significant factors which determine the performance of a PEMFC stack.     

Influence of anisotropic bending stiffness of gas diffusion layers on the degradation behavior of polymer electrolyte membrane fuel cells under freezing conditions

The effects of gas diffusion layer’s (GDL’s) anisotropic bending stiffness on the degradation behavior of polymer electrolyte membrane fuel cells have been investigated under freezing conditions. We have prepared GDL sheet samples such that the higher stiffness direction of GDL roll is aligned with the major flow field direction of a metallic bipolar plate at angles of 0° (parallel: ‘0° GDL’) and 90° (perpendicular: ‘90° GDL’). The I-V performances before and after 1000 temperature cycles between −10 and 1 °C of 90° GDL stack are higher than those of 0° GDL stack, and the voltages of 90° GDL stack are decreased slower than those of 0° GDL stack, indicating a higher durability of 90° GDL stack. Furthermore, the values and increasing rates of high-frequency resistance of 90° GDL stack are lower than those of 0° GDL stack. However, the H₂ and air pressure differences before and after 1000 temperature cycles of 90° GDL stack are very similar to those of 0° GDL stack. The surface of anode catalyst layer (CL) of membrane-electrode assembly (MEA) with catalyst-coated membrane type in 0° GDL stack appears to be more severely damaged than that in 90° GDL stack, especially under the channels, whereas the surfaces of cathode CLs of MEAs in both 0° and 90° GDL stacks are slightly damaged after 1000 temperature cycles.     

Membrane electrode assemblies with low noble metal loadings for hydrogen production from solid polymer electrolyte water electrolysis

High performance membrane electrode assemblies (MEAs) with low noble metal loadings (NMLs) were developed for solid polymer electrolyte (SPE) water electrolysis. The electrochemical and physical characterization of the MEAs was performed by I–V curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). Even though the total NML was lowered to 0.38 mg cm⁻², it still reached a high performance of 1.633 V at 2 A cm⁻² and 80 °C, with IrO₂ as anode catalyst. The influences of the ionomer content in the anode catalyst layer (CL) and the cell temperature were investigated with the purpose of optimizing the performance. SEM and EIS measurements revealed that the MEA with low NML has very thin porous cathode and anode CLs that get intimate contact with the electrolyte membrane, which makes a reduced mass transport limitation and lower ohmic resistance of the MEA. A short-term water electrolysis operation at 1 A cm⁻² showed that the MEA has good stability: the cell voltage maintained at ∼1.60 V without distinct degradation after 122 h operation at 80 °C and atmospheric pressure.     

Controlling the microscopic morphology and permeability of catalyst layers in proton exchange membrane fuel cells by adjusting catalyst ink agglomerates

Since agglomerates in catalyst inks affect the catalyst layers (CL) and membrane electrode assemblies (MEA) of proton exchange membrane fuel cell (PEMFC), it is important to study the connection among catalyst agglomerates, CL structure, and MEA performance. This study investigates the effect of Pt/GC catalyst agglomerates on the morphology and permeability of the CL by modulating the properties of the catalyst ink in two different ways. Additionally, MEA was further electrochemically tested to understand the relationship between the catalyst agglomerates and MEA performance. The result shows that High-pressure homogenization is more effective than mechanical shear mixing in dispersing the agglomerates in catalyst inks. However, the excessive homogenization pressure produced larger agglomerated particles, probably because more effective dispersion caused by higher homogenization pressure supplies new chain carriers for polymerization and higher temperature caused by higher homogenization pressure. Moreover, the surface of the CL fabricated in inks prepared by a homogenizer is more uniform, neat, and hydrophilic. But the number of secondary pores in the catalyst layer decreases at excessive homogeneous pressure, and the water permeability becomes poor, which in turn result in lower performance and higher mass transfer resistance. The electrochemical performance test results showed that the MEA with a relatively hydrophobic CL had a performance of 0.707 V at 1000 mA cm⁻², which was 30 mV higher than that with a relatively hydrophilic CL. This study provides insights for better tuning the properties of catalyst ink, CL morphology, and permeability to obtain better performance of MEA.     

Correlation between performance of polymer electrolyte membrane fuel cell and degradation of the carbon support in the membrane electrode assembly using image processing method

Long-time operation and various conditions cause the membrane electrode assembly (MEA) of polymer electrolyte membrane fuel cells (PEMFCs) to degrade, which results in decreased performance. The degradation of the MEA appears as various symptoms, such as the loss of carbon support and agglomeration of the Pt catalyst. In this paper, damage on the surface of the MEA by long-time operation and various conditions is induced intentionally by high-temperature conditions in a thermostat chamber. The MEA surface damage is photographed by scanning electron microscopy (SEM), and the loss of the carbon support that fixes the platinum catalyst is judged. Image processing is used to analyze damage on the MEA surface, and binarization processing is applied to the image processing method. SEM imagery is taken at magnifications of 100 × and the trends in quantified surface damage on the MEA according to the degradation temperature are analyzed. The correlation between the quantitative damage on the MEA surface and the performance of the PEMFC is checked. As a result, the tendency of decreasing PEMFC performance is derived from increasing quantified damage on the MEA surface.     

Self-humidifying membrane electrode assembly prepared by adding microcrystalline cellulose in anode catalyst layer as preserve moisture

A novel self-humidifying membrane electrode assemblies (MEAs) with the addition of microcrystalline cellulose (MCC) as a hygroscopic agent into anode catalyst layer was prepared to improve the performance of proton exchange membrane fuel cell (PEMFC) under low humidity conditions. The MEAs were characterized by SEM, contact angles and water uptake measurements. The MEAs with addition of MCC exhibit excellent self-humidifying single cell performance, the cell temperature for self-humidification running is up to 60 °C. As an optimized MEA with 4 wt.％ MCC in its anode catalyst layer, its current density at 0.6 V could be up to 760 mA cm⁻² under 20% of relative humidity, and remains at 680 mA cm⁻² after 22 h long time continuous testing, the attenuation of the current density is only 10%. While the current density of the blank MEA without addition of MCC degraded sharply from 300 mA cm⁻² to 110 mA cm⁻², the attenuation of the current density is high up to 70% within 2 h.     

Studies of performance degradation of a high temperature PEMFC based on H3PO4-doped PBI

In this paper, a 600 h life test of a high temperature PEMFC based on phosphoric acid (H₃PO₄)-doped polybenzimidazole (PBI) (H₃PO₄/PBI HT-PEMFC) at a current density of 714 mA cm⁻² (the beginning 510 h continuous test) and 300 mA cm⁻² (the last 90 h intermittent test) was carried out. After the life test, degradation of the MEA occurred. The H₂ crossover rate through the PBI membrane and the open circuit voltage (OCV) of the cell were tested with time. The results showed that, at the beginning of 510 h continuous test, the PBI membrane did not show much physical degradation, but during the last 90 h test there was a remarkable physical degradation which resulted in a higher H₂ crossover. The catalysts, PBI membranes and the membrane electrode assemblies (MEAs) before and after the life test were comprehensively examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). TEM results showed that the particle size of the Pt/C catalysts in the anode and cathode increased from 3.72 to 7.40 and 8.39 nm, respectively. SEM images of MEA in cross-section revealed that the PBI membrane became thin after the life test. EDS analysis implied the leaching of H₃PO₄ from the PBI membrane had occurred. Therefore, we conclude that physical degradation of PBI membrane, agglomeration of the electrocatalysts (both anode and cathode) and the leaching of H₃PO₄ from the PBI membrane were responsible for the performance degradation of the H₃PO₄/PBI HT-PEMFC.