Pre-oxidized and nitrided stainless steel alloy foil for proton exchange membrane fuel cell bipolar plates. Part 2: Single-cell fuel cell evaluation of stamped plates

Thermal (gas) nitridation of stainless steel alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant nitride containing surface layers (Cr₂N, CrN, TiN, V₂N, VN, etc.) of interest for fuel cells, batteries, and sensors. This paper presents results of proton exchange membrane (PEM) single-cell fuel cell studies of stamped and pre-oxidized/nitrided developmental Fe-20Cr-4V weight percent (wt.%) and commercial type 2205 stainless steel alloy foils. The single-cell fuel cell behavior of the stamped and pre-oxidized/nitrided material was compared to as-stamped (no surface treatment) 904L, 2205, and Fe-20Cr-4V stainless steel alloy foils and machined graphite of similar flow field design. The best fuel cell behavior among the alloys was exhibited by the pre-oxidized/nitrided Fe-20Cr-4V, which exhibited ∼5-20% better peak power output than untreated Fe-20Cr-4V, 2205, and 904L metal stampings. Durability was assessed for pre-oxidized/nitrided Fe-20Cr-4V, 904L metal, and graphite plates by 1000+ h of cyclic single-cell fuel cell testing. All three materials showed good durability with no significant degradation in cell power output. Post-test analysis indicated no metal ion contamination of the membrane electrode assemblies (MEAs) occurred with the pre-oxidized and nitrided Fe-20Cr-4V or graphite plates, and only a minor amount of contamination with the 904L plates.     

Corrosion behavior of TiN-coated stainless steel as bipolar plate for proton exchange membrane fuel cell

Stainless steel is a potential material to be used as the bipolar plate for proton exchange membrane fuel cell (PEFC) because of its suitable physical and mechanical properties. Several coating techniques have been applied to improve its corrosion resistance. But seldom study is focused on the microstructure evolution with corrosion. In the present study, the use of TiN-coated stainless steel as the bipolar plate is evaluated. Two surface coating techniques, pulsed bias arc ion plating (PBAIP) and magnetron sputtering (MS), are adoped to prepare the TiN-coated stainless steel. Their corrosion resistances and electrical conductivities of the coated substrates are evaluated. The performance shows strong dependance on microstructural characteristics. The corrosion of SS304/Ti₂N/TiN prepared by MS mainly occurs on the grain boundary. The corrosion of SS304/TiN prepared by PBAIP mainly takes place from the large particles on the coating. The Ti₂N/TiN multilayer coating provides superb corrosion protective layer for stainless steel. Both the TiN and Ti₂N/TiN coatings provide low contact resistance.     

质子交换膜燃料电池双极板的设计与模拟分析

质子交换膜燃料电池是直接将化学能转换为电能的装置,双极板上的流道结构对燃料电池的工作性能具有较大的影响。根据应用要求设计了具有平行流道、蛇形流道及希尔伯特分形流道的双极板结构,模拟计算了氢气在不同类型的流道和气体扩散层中的分布状态,分析了燃料电池的输出电流密度和功率密度随电极间电压的变化特点,比较了不同的流道结构对燃料电池输出电流密度的影响,以及不同的工作温度及气体压强的情况下,燃料电池输出电流密度随温度及压强的变化规律。     

Development of a one-dimensional and semi-empirical model for a high temperature proton exchange membrane fuel cell

High temperature proton exchange membrane fuel cells (HT-PEMFC), which operate between 160 °C and 200 °C, can be generally used in portable and stationary power generation applications. In this study, a one-dimensional, semi-empirical, and steady-state model of a HT-PEMFC fed with a gas mixture consisting of hydrogen and carbon monoxide is developed. Some modeling parameters are adjusted using empirical data, which are obtained conducting experiments on a HT-PEMFC for different values of Pt loading and cell temperature. For adjusting these parameters, the total summation of the square of the difference between the cell voltages found using the experimental and theoretical methods is minimized using genetic algorithm. After finding the values of the adjusted parameters, the effects of different cell temperature, Pt loading, phosphoric acid (PA) percentage, and different binders (PBI and PVDF) on the performance of the fuel cell are examined. It was found that, the performance of the fuel cell using PVDF binder exhibited better performance as compared to that using PBI binder.     

Manufacturing and performance assessment of stamped,laser welded,and nitrided FeCrV stainless steel bipolar plates for proton exchange membrane fuel cells

A manufacturing and single-cell fuel cell performance study of stamped, laser welded, and gas nitrided ferritic stainless steel foils in an advanced automotive bipolar plate assembly design was performed. Two developmental foil compositions were studied: Fe–20Cr–4V and Fe–23Cr–4V wt.%. Foils 0.1 mm thick were stamped and then laser welded together to create single bipolar plate assemblies with cooling channels. The plates were then surface treated by pre-oxidation and nitridation in N₂–4H₂ based gas mixtures using either a conventional furnace or a short-cycle quartz lamp infrared heating system. Single-cell fuel cell testing was performed at 80 °C for 500 h at 0.3 A/cm² using 100% humidification and a 100%/40% humidification cycle that stresses the membrane and enhances release of the fluoride ion and promotes a more corrosive environment for the bipolar plates. Periodic high frequency resistance potential-current scans during the 500 h fuel cell test and post-test analysis of the membrane indicated no resistance increase of the plates and only trace levels of metal ion contamination.     

High nitrogen stainless steel as bipolar plates for proton exchange membrane fuel cells

To investigate the applicability of high nitrogen (HN) austenitic stainless steel as bipolar plates for proton exchange membrane fuel cells (PEFCs), the polarization tests were carried out in synthetic solutions (0.05 M SO₄²⁻ (pHs 2.3, 4.3 and 5.5) +2 ppm F⁻) at 353 K. Interfacial contact resistance between the stainless steel and gas diffusion layer was measured before and after polarization. A single cell employing the HN stainless steel as bipolar plates was operated for 1000 h at 0.5 A cm⁻² (12.5 A). The single cell exhibited voltage drop of 17 mV during the operation. Corrosion products were scarcely detected for the HN stainless steel bipolar plate, as confirmed by scanning electron microscopy. After the polarization tests and single cell operation, XPS analyses were carried out to examine the resulting surface states. In the synthetic solutions to pH 4.3, the passive films mainly consisted of oxides enriched with Cr. When the solution pH was 5.5, on the other hand, the films were mainly composed of Fe-oxides. After the single cell operation for 1000 h, it was found that the passive films of the rib surface for the gas inlet part was mainly composed by Fe-oxides. On the other hand, the passive films for the ribs from center to gas outlet part were mainly made up of Cr-oxides. By combining the simulated and real operation environments, it is believed that the corrosion resistive Cr-oxides passive layer of the HN stainless steel obtained by the presence of nitrogen incorporated into the stainless steel could contribute to the maintenance of the higher cell voltage during the extensive cell operation.     

Performance of a proton exchange membrane fuel cell stack using conductive amorphous carbon-coated 304 stainless steel bipolar plates

In this study, 304 stainless steel (SS) bipolar plates are fabricated by flexible forming process and an amorphous carbon (a-C) film is coated by closed field unbalanced magnetron sputter ion plating (CFUBMSIP). The interfacial contact resistance (ICR), in-plane conductivity and surface energy of the a-C coated 304SS samples are investigated. The initial performance of the single cell with a-C coated bipolar plates is 923.9 mW cm⁻² at a cell voltage of 0.6 V, and the peak power density is 1150.6 mW cm⁻² at a current density of 2573.2 mA cm⁻². Performance comparison experiments between a-C coated and bare 304SS bipolar plates show that the single cell performance is greatly improved by the a-C coating. Lifetime test of the single cell over 200 h and contamination analysis of the tested membrane electrode assemble (MEA) indicate that the a-C coating has excellent chemical stability. A 100 W-class proton exchange membrane fuel cell (PEMFC) short stack with a-C coated bipolar plates is assembled and shows exciting initial performance. The stack also exhibits uniform voltage distribution, good short-term lifetime performance, and high volumetric power density and specific power. Therefore, a-C coated 304SS bipolar plates may be practically applied for commercialization of PEMFC technology.     

In-situ diagnosis on performance degradation of high temperature polymer electrolyte membrane fuel cell by examining its electrochemical properties under operation

Ex-situ electrochemical characterization techniques could significantly alter or misrepresent the materials of high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) to the point where they are not reflective of their conditions during operation, resulting in difficulties in obtaining realistic fuel cell durability. To minimize this disturbance, we proposed an in-situ low-invasive technique of electrochemical impedance spectroscopy (EIS), combining with polarization curve and Tafel slope analysis, to investigate the performance degradation of HT-PEMFC. The membrane electrode assemblies (MEAs) used in the HT-PEMFC were lab-made but with commercial catalyst and poly(2,5-benzimidazole) (ABPBI) membrane. Two common test modes, i.e. steady-state operation and dynamic-state operation, were employed to mimic practical HT-PEMFC operation. By examining the changes of electrochemical properties of the HT-PEMFC under steady- or dynamic-state operation, the main mechanism for the performance degradation can be determined. The results from the study suggests that a high cell performance decay rate cannot be directly attributed to materials degradation, especially in a short-term steady-state operation. In contrast, the change of Tafel slope can be seen as a clear indicator to determine the extent of catalyst degradation of HT-PEMFC, no matter which test protocol was applied. Post-analysis of TEM on the catalysts before and after tests further confirmed the main mechanism for the performance losses of the HT-PEMFCs underwent two test protocols, while acid loss and membrane degradation were considered to be negligible during the short-term tests.     

Influence of operating temperature on cell performance and endurance of high temperature proton exchange membrane fuel cells

The relation between high temperature proton exchange membrane fuel cell (HT-PEMFC) operation temperature and cell durability was investigated in terms of the deterioration mechanism. Long-term durability tests were conducted at operational temperatures of 150, 170, and 190 °C for a HT-PEMFC with phosphoric acid-doped polybenzimidazole electrolyte membranes. Higher cell temperatures were found to result in a higher cell voltage, but decrease cell life. The reduction in cell voltage of approximately 20 mV during the long-term tests was considered to be caused both by aggregation of the electrode catalyst particles in the early stage of power generation, in addition to the effects of crossover due to the depletion of phosphoric acid in the terminal stage, which occurs regardless of cell temperature. It is expected that enhanced long-term durability for practical applications can be achieved through effective management of phosphoric acid transfer.     

Electrochemical carbon corrosion in high temperature proton exchange membrane fuel cells

Electrochemical carbon corrosion occurring in a high temperature proton exchange membrane fuel cell (HT-PEMFC) operating under non-humidification conditions was investigated by measuring CO₂ generation using on-line mass spectrometry and comparing the results with a low-temperature proton exchange membrane fuel cell (LT-PEMFC) operated under fully humidified conditions. The experimental results showed that more CO₂ was measured for the HT-PEMFC, indicating that more electrochemical carbon corrosion occurs in HT-PEMFCs. This observation is attributed to the enhanced kinetics of electrochemical carbon corrosion due to the elevated operating temperature in HT-PEMFCs. Additionally, electrochemical carbon corrosion in HT-PEMFCs showed a strong dependence on water content. Therefore, it is critical to remove the water content in the supply gases to reduce electrochemical carbon corrosion.     

Effect of surface roughness of composite bipolar plates on the contact resistance of a proton exchange membrane fuel cell

Polymer electrolyte membrane fuel cell performance strongly depends on properties of the fuel cell stack bipolar plates. Composite bipolar plates, though low cost and convenient in manufacturing, raise a major concern due to their high interfacial contact resistance caused by the mechanical treatment used to remove the polymer-rich layer on the surface. It is observed that most of this contact resistance is governed by electrical properties of the interface layer between the contacting surfaces. Measurements of contact resistance of mechanically polished composite bipolar plate/gas diffusion layer interface reveal a substantial influence of surface topography on the contact resistance, which varies significantly depending on the substrate surface treatment and roughness of composite bipolar plates.     

Pre-oxidized and nitrided stainless steel alloy foil for proton exchange membrane fuel cell bipolar plates: Part 1. Corrosion, interfacial contact resistance, and surface structure

Thermal (gas) nitridation of stainless steel alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant nitride containing surface layers (Cr₂N, CrN, TiN, V₂N, VN, etc.) of interest for fuel cells, batteries, and sensors. This paper presents results of scale-up studies to determine the feasibility of extending the nitridation approach to thin 0.1 mm stainless steel alloy foils for proton exchange membrane fuel cell (PEMFC) bipolar plates. Developmental Fe-20Cr-4V alloy and type 2205 stainless steel foils were treated by pre-oxidation and nitridation to form low-ICR, corrosion-resistant surfaces. As-treated Fe-20Cr-4V foil exhibited target (low) ICR values, whereas 2205 foil suffered from run-to-run variation in ICR values, ranging up to 2× the target value. Pre-oxidized and nitrided surface structure examination revealed surface-through-layer-thickness V-nitride particles for the treated Fe-20Cr-4V, but near continuous chromia for treated 2205 stainless steel, which was linked to the variation in ICR values. Promising corrosion resistance was observed under simulated aggressive PEMFC anode- and cathode-side bipolar plate conditions for both materials, although ICR values were observed to increase. The implications of these findings for stamped bipolar plate foils are discussed.     

Dynamic modeling of a high-temperature proton exchange membrane fuel cell with a fuel processor

A dynamic model of a high-temperature proton exchange membrane fuel cell with a fuel processor is developed in this study. In the model, a fuel processing system, a fuel cell stack, and an exhaust gas burner are modeled and integrated. The model can predict the characteristics of the overall system and each component at the steady and transient states. Specifically, a unit fuel cell model is discretized in a simplified quasi-three-dimensional geometry; therefore, the model can rapidly predict the distribution of fuel cell characteristics. Various operating conditions such as the steam-to-carbon ratio, oxygen-to-carbon ratio, and autothermal reforming inlet temperature are varied and investigated in this study. In addition, the dynamic characteristics exhibited during the transient state are investigated, and an efficiency controller is developed and implemented in the model to maintain the electrical efficiency. The simulation results demonstrate that the steam-to-carbon ratio and the oxygen-to-carbon ratio affect the electrical and system efficiency and that controlling the fuel flow rate maintains the electrical efficiency in the transient state. The model may be a useful tool for investigating the characteristics of the overall system as well as for developing optimal control strategies for enhancing the system performance.     

Modeling of high temperature proton exchange membrane fuel cells with novel sulfonated polybenzimidazole membranes

In this study, a three-dimensional, steady-state, non-isothermal numerical model of high temperature proton exchange membrane fuel cells (HT-PEMFCs) operating with novel sulfonated polybenzimidazole (SPBI) membranes is developed. The proton conductivity of the phosphoric acid doped SPBI membranes with different degrees of sulfonation is correlated based on experimental data. The predicted conductivity of SPBI membranes and cell performance agree reasonably with published experimental data. It is shown that a better cell performance is obtained for the SPBI membrane with a higher level of phosphoric acid doping. Higher operating temperature or pressure is also beneficial for the cell performance. Electrochemical reaction rates under the ribs of the bipolar plates are larger than the values under the flow channels, indicating the importance and dominance of the charge transport over the mass transport.     

Design and manufacturing of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel bipolar plates (BPPs) are regarded as promising alternatives to traditional graphite BPPs in proton exchange membrane fuel cells (PEMFCs). This technology has experienced more than 20 years development and has been partially applied in industrial production. This review surveys recent progress of entire development process for stainless steel BPPs in terms of flow field design, microforming process, joining process and coating process. Besides, assembly process considering dimensional error, shape error and assembly error are comprehensively summarized as well. Finally, technical challenges and future trends are presented for the application of stainless steel BPPs for PEMFCs.     

Corrosion resistance characteristics of stamped and hydroformed proton exchange membrane fuel cell metallic bipolar plates

Metallic bipolar plates have several advantages over bipolar plates made from graphite and composites due to their high conductivity, low material and production costs. Moreover, thin bipolar plates are possible with metallic alloys, and hence low fuel cell stack volume and mass are. Among existing fabrication methods for metallic bipolar plates, stamping and hydroforming are seen as prominent approaches for mass production scales. In this study, the effects of important process parameters of these manufacturing processes on the corrosion resistance of metallic bipolar plates made of SS304 were investigated. Specifically, the effects of punch speed, pressure rate, stamping force and hydroforming pressure were studied as they were considered to inevitably affect the bipolar plate micro-channel dimensions, surface topography, and hence the corrosion resistance. Corrosion resistance under real fuel cell conditions was examined using both potentiodynamic and potentiostatic experiments. The majority of the results exhibited a reduction in the corrosion resistance for both stamped and hydroformed plates when compared with non-deformed blank plates of SS304. In addition, it was observed that there exist an optimal process window for punch speed in stamping and the pressure rate in hydroforming to achieve improved corrosion resistance at a faster production rate.     

Polybenzimidazole containing ether units as electrolyte for high temperature proton exchange membrane fuel cells

A rapid method to synthesize poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) through a solution polycondensation under microwave irradiation is explored. Synthesis parameters affecting the molecular weight (M_w) of OPBI, including the mass ratio of solvent to P₂O₅, the monomer concentration, and reaction time, are optimized. The main characteristics of OPBI are studied, and the corresponding membrane is prepared through a solvent casting process. A series of sulfuric acid doped OPBI (H₂SO₄/OPBI) hybrid membranes with different acid doping levels (ADLs) are developed. The effects of H₂SO₄ on microstructure, ADL and electrochemical properties of these membranes are explored. Herein, the hybrid membrane shows high proton conductivity (190 mS cm⁻¹) at elevated temperature (160 °C) and anhydrous conditions, high ADL (18.73 mol of H₂SO₄ for OPBI per repeat unit, i.e., ADL = 18.73 mol PRU⁻¹) and excellent dimensional stability (40.3%). All these properties demonstrated that H₂SO₄/OPBI hybrid membrane can be used as an alternative membrane for high temperature proton exchange membrane fuel cells (HT-PEMFCs).     

Development of preform moulding technique using expanded graphite for proton exchange membrane fuel cell bipolar plates

A preform moulding technique using expanded graphite is developed to manufacture composite bipolar plates for proton exchange membrane fuel cells (PEMFCs). The preform is composed of expanded graphite, graphite flake and phenol resin. Preforms utilizing the tangled structure of expanded graphite are easily fabricated at a low pressure of 0.07–0.28 MPa. A pre-curing temperature (100 °C) slightly above the melting point of phenol powders (90 °C) induces moderate curing, but also prevents excessive curing. After the preform is placed in a steel mould, compression moulding is carried out at high pressure (10 MPa) and temperature (150 °C). The fabrication conditions are optimized by checking the electrical conductivity, flexural strength and microstructure of the composite. The optimized electrical conductivity and flexural strength, 250 S cm⁻¹ and 50 MPa, respectively, met the requirements for PEMFC bipolar plates.     

Experimental evaluation of CO poisoning on the performance of a high temperature proton exchange membrane fuel cell

We experimentally studied a high temperature proton exchange membrane (PEM) fuel cell to investigate the effects of CO poisoning at different temperatures. The effects of temperature, for various percentages of CO mixed with anode hydrogen stream, on the current-voltage characteristics of the fuel cell are investigated. The results show that at low temperature, the fuel cell performance degraded significantly with higher CO percentage (i.e., 5% CO) in the anode hydrogen stream compared to the high temperature. A detailed electrochemical analysis regarding CO coverage on electrode surface is presented which indicates that electrochemical oxidation is favorable at high temperature. A cell diagnostic test shows that both 2% CO and 5% CO can be tolerated equally at low current density (<0.3 A cm⁻²) with high cell voltage (>0.5 V) at 180 °C without any cell performance loss. At high temperature, both 2% CO and 5% CO can be tolerated at higher current density (>0.5 A cm⁻²) with moderate cell voltage (0.2-0.5 V) when the cell voltage loss within 0.03-0.05 V would be acceptable. The surface coverage of platinum catalyst by CO at low temperature is very high compared to high temperature. Results suggest that the PEM fuel cell operating at 180 °C or above, the reformate gas with higher CO percentage (i.e., 2-5%) can be fed to the cell directly from the fuel processor.     

Electrochemical behavior of surface treated metal bipolar plates used in passive direct methanol fuel cell

Corrosion resistance performance of SS316L treated by passivation solution was investigated in a simulated environment of the passive direct methanol fuel cell (DMFC). Electrochemical impedance spectroscopic (EIS) test showed that polarization resistance of untreated and treated SS316L were 1191 Ω cm² and 9335 Ω cm², respectively. The above result agreed with the Tafel slope analysis of potentiodynamic polarization curves. Comparing the untreated and treated SS316L in the simulated environment of DMFC anode working conditions, it was observed that the corrosion current density of treated SS316L as estimated by 4000 s potentiostatic test reduced from 38.7 μA cm⁻² to 0.297 μA cm⁻², meanwhile, the current densities of untreated and treated SS316L in cathode working conditions were 3.87 μA cm⁻² and 0.223 μA cm⁻², respectively. It indicated that the treated SS316L should be suitable in both anode and cathode environment of passive DMFCs. The treated SS316L bipolar plates have been assembled in a passive single fuel cell. A peak power density of 1.18 mW cm⁻² was achieved with 1 M methanol at ambient temperature.