Study of different aluminum alloy substrates coated with Ni–Co–P as metallic bipolar plates for PEM fuel cell applications

Pure Al and some of its alloys [Al alloy 6061 (AA6061), Al alloy 3004 (AA3004) and Al alloy 1050 (AA1050)] were coated with Ni–Co–P using electroplating power supply technique. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques were applied to study the surface morphology and chemical composition of coated aluminum substrates. Their performance against corrosion was examined using potentiodynamic polarization technique in (0.5 M H₂SO₄ + 2 ppm HF) solution. Corrosion potential values were shifted in the positive direction at all aluminum substrates after their coating with Ni–Co–P. Corrosion current density values at coated pure Al and AA1050 were decreased by ∼18.6 times, compared to those at bare substrates. The stability of coated aluminum alloys was investigated during long-time operation under cathodic environment in PEMFCs using potentiostatic polarization test at +160 mV (MMS) in air-saturated solution. Ni–Co–P/AA3004 substrate showed a high corrosion rate after short time, while coated AA6061 one slowly corroded. Interfacial contact resistance (ICR) values between metallic bipolar plates and gas diffusion layer were measured. Coating AA1050 with Ni–Co–P reduces its ICR value by 13 times. Accordingly, electroplated Ni–Co–P/AA1050 substrate can be chosen as an efficient bipolar plate material in PEMFCs.     

Degradation behaviour of Al–Fe coatings in wet-seal area of molten carbonate fuel cells

The corrosion resistance of Al–Fe coatings increases as a protective LiAlO₂ layer forms. If, however, the Al–Fe coatings lack sufficient aluminium for maintaining this protective layer, the corrosion resistance of the coating is degraded by the growth of non-protective scales, such as LiFeO₂. In this study, the degradation behaviour of Al–Fe coatings is investigated in the wet-seal environment of molten carbonate fuel cells (MCFC). Al–Fe coated specimens with various amounts of aluminium in the range 8–70 at.% and bulk specimens of Fe–23.9 Al (at.%) are prepared. A corrosion test is performed in Li/K carbonate systems at 650 °C with a single-cell and an immersion test. Test results reveal that aluminium contents in the coatings should be higher than 25 at.% in order to form and maintain a protective LiAlO₂ layer. In addition to aluminium content, the influence of microstructural features on the degradation behaviour of Al–Fe coatings is discussed.     

Deposition of gold–titanium and gold–nickel coatings on electropolished 316L stainless steel bipolar plates for proton exchange membrane fuel cells

The effects of electropolishing and coating deposition on electrical resistance and chemical stability were studied for the stainless steel bipolar plates in proton exchange membrane fuel cell (PEMFC). A series of 316L stainless steel plates, selected as the substrate for a proton exchange membrane fuel cell (PEMFC) bipolar plate, were electropolished with a solution of H₂SO₄ and H₃PO₄ at temperatures ranging from 70 to 110 °C. The surface regions of the two electropolished stainless steel plates were coated with gold and either a titanium or nickel layer using electron beam evaporation. The electropolished stainless steel plates coated in 2-μm thick gold with a 0.1-μm titanium or nickel interlayer showed remarkably smooth and uniform surface morphologies in AFM and FE-SEM images compared to the surfaces of the plates that were coated after mechanical polishing only. The electrical resistance and water contact angle of the deposited stainless steel bipolar plates are strongly dependent on the surface modification treatments (i.e., mechanical polishing versus electropolishing). ICP-MS and XPS results indicate that after electropolishing, the coating layers show excellent chemical stability after exposure to an H₂SO₄ solution of pH 3. Finally, it was concluded that before coating deposition, the surface modification using electropolishing was very suitable for enhancing the electrical property and chemical stability of the stainless steel bipolar plate.     

Improved performance of molten carbonate fuel cells with (Li/Na)2CO3 electrolytes by using BYS coated cathode at low operating temperatures

In order to improve the stack life time of MCFCs, it is necessary to reduce the operating temperature of MCFCs below 600 °C, because reduced operating temperature minimizes electrolyte loss due to evaporation and corrosion. However, at the low operating temperature below 600 °C, the cell performance of MCFCs with (Li/Na)₂CO₃ electrolyte is too low to operate the fuel cell stack and system. In this study, we have performed wettability control of the liquid molten carbonate electrolyte by coating NiO cathodes with poor wetting property of the mixed ionic and electronic conductor (MIEC) such as BYS (Bi_1.5Y_0.3Sm_0.3O_3-δ). From experiments with symmetrical cells, each polarization component with various temperatures and gas conditions were studied. To investigate effects of the BYS coated cathode on the performance of MCFCs, a 100 cm² single cell of MCFCs was employed. The performance of a 100 cm² single cell with BYS coated cathode was better than that with conventional cathode by a factor of 1.84, because BYS coated cathode reduces activation polarization and mass transfer resistance greatly.     

The effect of pH and halides on the corrosion process of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H₂SO₄ solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F⁻ and 10 ppm Cl⁻) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl⁻ to 100 ppm, pitting was initiated on the stainless steel surface.     

Thin chromium nitride PVD coatings on stainless steel for conductive component as bipolar plates of PEM fuel cells: Ex-situ and in-situ performances evaluation

In this paper, two types of chromium PVD coatings (100 nm) have been elaborated on 316L stainless steel (SS) by adjusting the nitrogen flow rate. The first coating is a mixture of Cr₂N and Cr, the second one is a single phase CrN. It is shown that the performances of the material are strongly dependant of the nature of the passive film formed on the chromium nitride layers due to the galvanic coupling between the coating and the substrate. The CrN coated SS shows very good corrosion resistance in simulated PEMFC media. The surface conductivity of the SS is also greatly improved and the CrN coated SS shows an interfacial contact resistance of 10 mΩ cm² at 140 N cm⁻². Five single cells of stainless steel bipolar plates coated with the CrN film were assembled for performance test. This 5 cell stack does not show any mean voltage degradation over 200 h dynamic cycling. Moreover, the performances of the CrN coated SS bipolar plates are very close to the Au-coated SS bipolar plates.     

Interface mixing and hydrogen absorption in Pd/Mg and Pd/Al/Mg thin films

Pd/Mg bilayers and Pd/Al/Mg trilayers were prepared onto glass substrates at room temperature (RT) by UHV magnetron sputtering. Mixing effects at the Pd–Mg and Al–Mg interfaces were studied in-situ, immediately after deposition, by X-ray Photoelectron Spectroscopy (XPS). Additionally, the interfaces of the Pd/Al/Mg trilayer for the Al thickness equal to 1 nm were examined. Hydrogen absorption was monitored in-situ at RT by simultaneous resistivity and optical transmittance measurements. Formation of MgH₂ phase was confirmed by ex-situ X-ray diffraction measurements. The XPS studies revealed rather sharp interface between Al and Mg layers. On the other hand, a significant interface mixing for the Pd/Mg bilayers and Pd/1 nm – Al/Mg trilayers was observed. Further studies showed that an additional layer of Al, deposited between magnesium and palladium layers, can significantly improve the hydrogen absorption kinetics at RT. The optimal thickness of the Al layer was found to be 0.5 nm.     

Influence of CrN-coating thickness on the corrosion resistance behaviour of aluminium-based bipolar plates

The electrical and corrosion properties of CrN-coated aluminium alloy Magnal-45 (Al-5083) probes have been evaluated, in order to assess their viability to be used as bipolar plates in polymer electrolyte fuel cells. To this end, ceramic micro-layers of chromium nitride (CrN) with different thicknesses (3, 4, and 5 μm) have been deposited on the surface of the Al alloy (Al-5083) using the physical vapour deposition (PVD) technique. A decrease in 2 orders of magnitude of I_corr values for the coated Al has been observed compared to the as-received Al-alloy when the probes have been exposed to simulated anodic conditions in a micro-reactor. On the other hand, when subjected to a cathodic-simulated environment, the Al-CrN probes with 3 μm and 4 μm coatings have shown a decrease in I_corr of one order of magnitude, while a variation of two orders of magnitude has also been obtained for the 5 μm coating.     

Protective nitride formation on stainless steel alloys for proton exchange membrane fuel cell bipolar plates

Gas nitridation has shown excellent promise to form dense, electrically conductive and corrosion-resistant Cr-nitride surface layers on Ni–Cr base alloys for use as proton exchange membrane fuel cell (PEMFC) bipolar plates. Due to the high cost of nickel, Fe-base bipolar plate alloys are needed to meet the cost targets for many PEMFC applications. Unfortunately, nitridation of Fe-base stainless steel alloys typically leads to internal Cr-nitride precipitation rather than the desired protective surface nitride layer formation, due to the high permeability of nitrogen in these alloys. This paper reports the finding that it is possible to form a continuous, protective Cr-nitride (CrN and Cr₂N) surface layer through nitridation of Fe-base stainless steel alloys. The key to form a protective Cr-nitride surface layer was found to be the initial formation of oxide during nitridation, which prevented the internal nitridation typically observed for these alloys, and resulted in external Cr-nitride layer formation. The addition of V to the alloy, which resulted in the initial formation of V₂O₃–Cr₂O₃, was found to enhance this effect, by making the initially formed oxide more amenable to subsequent nitridation. The Cr-nitride surface layer formed on model V-modified Fe–27Cr alloys exhibited excellent corrosion resistance and low interfacial contact resistance under simulated PEMFC bipolar plate conditions.     

Electrically conductive polymer composite coating on aluminum for PEM fuel cells bipolar plate

Aluminum has many advantages for commercial bipolar plate of PEM fuel cell such as light weight, low cost and easy manufacturing. However, it has a low corrosion resistance under a PEM fuel cell operation condition that is a special issue of all metal bipolar plates. In this study, polypropylene composite coated with aluminum bipolar plates were fabricated to improve the corrosion resistance. However, contact resistance of polymer composite coated aluminum bipolar plate is highly increased due to high contact resistance between aluminum substrate and composite layer. Two different types of inter layers were added to improve the contact resistance. Carbon paper attached and carbon black added samples were fabricated between aluminum substrate and composite. Polyamide-imide/carbon black composite adhesive was used for carbon paper attached on the aluminum plate. The contact resistance of carbon paper attached sample was lower than that of carbon black added sample. And, corrosion resistance was tested by potentiodynamic and potentiostatic methods. The composite coated aluminum attached to carbon paper exhibited properties suitable for PEM fuel cells.     

Mechanical properties,corrosion resistance,and rubber pad forming of cold differential speed-rolled pure titanium for bipolar plates of proton-exchange membrane fuel cells

Due to problems such as pores on surface-treated coatings, the corrosion resistance of pure titanium bipolar plates for proton-exchange membrane fuel cells can be further improved by increasing the corrosion resistance of pure titanium by using differential speed-rolling (DSR); however, these materials have not yet reached the standard requirements of bipolar plates (corrosion current density i_corr＜10³ nA·cm^?2). In this work, the corrosion resistance of pure titanium was improved by optimizing the DSR process while the strength was maintained. The best corrosion resistance of the DSR pure titanium was achieved when the roller speed ratio was 2, while i_corr was 429 nA·cm^?2 in a solution of 0.5 M H₂SO₄ and 2 mg/L HF at room temperature. The formability of the DSR pure titanium for bipolar plates was verified. The optimal holding pressure range was 6.8–7.0 kN.     

Evaluation of polymer electrolyte membrane fuel cells by electrochemical impedance spectroscopy under different operation conditions and corrosion

Electrochemical impedance spectroscopy (EIS) was employed for in situ diagnosis for polymer electrolyte membrane fuel cells during operation. First, EIS was measured as a function of operation parameters such as applied current density, gas flow rates and gas humidification temperature. The resistance that correlated with conductivity of the membrane and the contact resistance between bipolar plate and gas diffusion layer (GDL) was set as R_m in the assumed equivalent circuit. The charge transfer resistances were considered for cathode (R_ct(C)). The value of R_ct(C) was sensitive to the parameters that affected cell voltage. Additionally, the diffusion resistance (R_d) was ascribed to the effect of oxygen supply and drainage of generated water. Second, the influence of corrosion of type 430 stainless steel bipolar plates was evaluated by EIS method during operation. Corrosion of the stainless steel bipolar plates resulted in an increase in the value of R_d.     

Investigation of corrosion properties with Ni–P/TiNO coating on aluminum alloy bipolar plates in proton exchange membrane fuel cell

Aluminum alloy bipolar plates have unique application potential in proton exchange membrane fuel cell (PEMFC) due to the characteristics of lightweight and low cost. However, extreme susceptibility to corrosion in PEMFC operation condition limits the application. To promote the corrosion resistance of aluminum alloy bipolar plates, a Ni–P/TiNO coating was prepared by electroless plating and closed field unbalanced magnetron sputter ion plating (CFUMSIP) technology on the 6061 Al substrate. The research results show that Ni–P interlayer improves the deposition effect of TiNO outer layer and increase the content of TiN and TiO_xN_y phases. Compared to Ni–P and TiNO single-layer coatings, the Ni–P/TiNO coating samples exhibited the lowest current density value of (1.10 ± 0.02) × 10^?6 A·cm^?2 in simulated PEMFC cathode environment. Additionally, potential cyclic polarization measurements were carried out aiming to evaluate the durability of the aluminum alloy bipolar plate during the PEMFC start-up/shut-up process. The results illustrate that the Ni–P/TiNO coating samples exhibit excellent stability and corrosion resistance.     

Corrosion and interfacial contact resistance of nanocrystalline β-Nb2N coating on 430 FSS bipolar plates in the simulated PEMFC anode environment

The corrosion of metallic bipolar plates in the proton exchange membrane fuel cells (PEMFCs) anode environment would degrade the performance and shorten the lifespan of the fuel cell. Hence, it is essential to develop a conductive coating with good corrosion resistance. Herein we demonstrate a dense, defect-free, and well-adhered nanocrystalline β-Nb₂N coating prepared on 430 ferritic stainless steel (430 FSS) via disproportionation of Nb(Ⅳ) species in molten salts. The corrosion mechanism of bare and β-Nb₂N coated 430 FSS in the simulated PEMFC anode environment is also studied by electrochemical techniques including potentiodynamic polarization, potentiostatic polarization, and electrochemical impedance spectroscopy. Results show that β-Nb₂N coating can significantly improve the corrosion resistance of the steel alloy with acceptable contact resistance. In addition, no obvious degradation is observed for the β-Nb₂N coating after potentiostatic polarization measurement for 500 h. This work offers a promising strategy to develop the corrosion protective coating on metallic bipolar plates for PEMFCs.     

Review of the membrane and bipolar plates materials for conventional and unitized regenerative fuel cells

Fuel cell or hydrogen systems offer the potential for clean, reliable and on-site energy generation. This article review current literature with the objective of identifying the latest development in membrane and bipolar plates for the conventional fuel cell and unitized regenerative fuel cell (URFC). The result shows that the choice of both the bipolar plates and the catalysts for URFC electrodes is a delicate task, for bipolar plate the corrosion in the oxygen side will be the major problem and for the electrodes a very good candidate for fuel cell mode will not function well in the electrolyser mode and therefore it is suggested that a compromise should be considered. It is recommended that aluminum, titanium or for best results titanium with a gold-coated layer is a suitable candidate as the bipolar plate and Pt/IrO_X or Pt/Ru is suitable for an oxygen side catalyst in the URFC. For the conventional fuel cell the task is more easer because the corrosion problem is no more effective and thus the main goals for most of the studies was to concentrate on bipolar plate cost reduction, increase electrical conduction and reducing the platinum loading rate for catalyst.     

Effect of manufacturing conditions on the corrosion resistance behavior of metallic bipolar plates in proton exchange membrane fuel cells

Metallic bipolar plates are one of the promising alternatives to the graphite bipolar plates in proton exchange membrane fuel cell (PEMFC) systems. In this study, stainless steel (SS304, SS316L, and SS430), nickel (Ni 270), and titanium (Grade 2 Ti) plates with an initial thickness of 51 μm were experimented as bipolar plate substrate materials in corrosion resistance tests. In addition to unformed blanks, SS316L plates were formed with stamping and hydroforming processes to obtain bipolar plates under different process conditions (stamping force, hydroforming pressure, stamping speed, hydroforming pressure rate). These bipolar plates, then, were subjected to corrosion tests, and the results were presented and discussed in detail. Potentiodynamic polarizations were performed to observe corrosion resistance of metallic bipolar plates by simulating the anodic and cathodic environments in the PEMFC. In order to determine the statistical significance of the corrosion resistance differences between different manufacturing conditions, analysis of variance (ANOVA) technique was used on the corrosion current density (I_corr, μA cm⁻²) values obtained from experiments. ANOVA for the unformed substrate materials indicated that SS430 and Ni have less corrosion resistance than the other substrate materials tested. There was a significant difference between blank (unformed) and stamped SS316L plates only in the anodic environment. Although there was no noteworthy difference between unformed and hydroformed specimens for SS316L material, neither of these materials meet the Department of Energy‘s (DOE) target corrosion rate of ≤1 μA cm⁻² by 2015 without coating. Finally, stamping parameters (i.e. speed and force levels) and hydroforming parameters (i.e. the pressure and pressure rate) significantly affected the corrosion behavior of bipolar plates.     

Preparation of Cr-based multilayer coating on stainless steel as bipolar plate for PEMFCs by magnetron sputtering

In the present study, a multilayer composing of Cr₃Ni₂/Cr₂N/CrN is sputtered onto stainless steel. The potential of using the coated stainless steel as the bipolar plate for polymer electrolyte membrane fuel cell (PEMFC) is evaluated. The coated stainless steel exhibits improved corrosion resistance and higher electrical conductivity. The coated surface also demonstrates a hydrophobic characteristic. By using single cell test, the multilayer-coated SS304 plate exhibits an improved performance in terms of I-V properties.     

Rapid coating preparation strategy for chromium nitride coated titanium bipolar plates of proton exchange membrane fuel cells

It is critical to develop a coating with sufficient comprehensive performances and efficient preparation strategy for the commercial application of metallic bipolar plate in proton exchange membrane full cell (PEMFC). In this work, chromium nitride coatings prepared by a rapid multi-arc ion plating (MIP) process with various nano thicknesses are investigated in the simulated PEMFC cathodic environments. Both the corrosion resistance and conductivity of the coatings increase with the growth of the coating thickness, which can be attributed to the increasing equivalent diameter, density, and area fraction of the droplets formed on the coating surfaces. The chromium nitride coating with a thickness of approximately 1.0 μm has the lowest I_0.6 V (0.594 μA cm^?2) and interfacial contact resistance (ICR, 6.54 mΩ cm²@1.4 MPa after corrosion test), achieving the 2025 technical targets proposed by the US Department of Energy for bipolar plates. This work shows that rapid preparation by MIP within 12 min is a potential strategy for chromium nitride coated titanium bipolar plates of PEMFCs at industrial scale.     

Evaluation of electroplated Co-Cu metallic coatings for intermediate-temperature solid oxide fuel cells

Co-Cu alloys have been co-deposited onto 430 ferritic stainless steels via electroplating with a citrate solution. At the initial oxidation stage, a three-layer scale composed of a thin CuO outer layer, a thick (Cu,Fe,Cr)-doped Co₃O₄ middle layer and a (Cu,Fe)-doped (Co,Cr)₃O₄ inner layer was formed on the coated steel. With extended oxidation, the (Co,Cr)₃O₄ inner layer has been transformed into a Cr-rich oxide inner layer. An obvious outward diffusion of Fe appeared, leading to the formation of an (Cu,Cr,Mn)-doped (Co,Fe)₃O₄ interaction zone between the Co₃O₄-based spinel and the chromia oxides. The Co-Cu coating effectively blocked the outward migration of Cr from the substrate. No Cr element could be found in the coupled La_0·8Sr_0·2MnO₃ (LSM) plate of the coated sample after oxidized at 800 °C in air for 500 h. The highly conductive coating with a structure of CuO/Co-based spinels significantly decreased the growth of the Cr-rich oxide scale, and thus a much lower scale area specific resistance (ASR). The electrical properties and the oxidation mechanism of the coated substrates were discussed.     

Investigation of oxidation and electrical behavior of AISI 430 steel coated with Mn–Co–CeO2 composite

Ferritic stainless steels, under the working conditions of solid oxide fuel cells, form a chromium oxide layer. This layer has a low electrical conductivity and consequently reduces the efficiency of these energy converters. An action to improve the properties of the connecting plates is to use a conductive and protective layer of coating. In this study, AISI 430 stainless steel was coated with Mn–Co–CeO₂ through electroplating technique. To evaluate the oxidation behavior, isothermal and cyclic oxidation tests were used at 800 °C. Area specific resistance (ASR) of uncoated and coated specimens was also compared as a function of time during oxidation at 800 °C. Coating microstructure and oxidized samples were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) device. In isothermal oxidation, uncoated samples had more weight gain than the Mn–Co–CeO₂ coated samples. The coating layer improved oxidation resistance by limiting the diffusion of chromium cation and oxygen anion. The cyclic oxidation results showed that the Mn–Co–CeO₂ coated samples had a very good resistance to cracking and spallation. Also, the results of ASR showed that formation of MnCo₂O₄ and MnFe₂O₄ spinels and also the presence of CeO₂ resulted in reduction of area specific resistance. ASR for samples coated with Mn–Co–CeO₂ and uncoated samples was 12.4 mΩ.cm² and 38.7 mΩ.cm², respectively after 200 h of oxidation at 800 °C.