Coating of stainless steel and titanium bipolar plates for anticorrosion in PEMFC: A review

Anticorrosion coating for stainless steel (SS) and titanium bipolar plates were evaluated to improve the corrosion resistance and electrical conductivity in PEMFC. The PEMFC offers clean and environmentally friendly usage in electrical power systems. The bipolar plates contribute 60%–80% of the total components of PEMFC stack with electrical conductivity >100 S cm^?1. Therefore, high conductivity and corrosion resistance are observed for long-term operations in PEMFC. Recent works has developed the cost-effective and feasible alternative materials to replace graphite bipolar plates. Metallic materials, such as SS and titanium, possess good electrical conductivity but poor corrosion resistance. Coating of SS and titanium bipolar plates can improve the corrosion resistance of metallic bipolar plates. Excellent performance of bipolar plates was recorded by using NbC coating for stainless steel materials. The ICR value using plasma surface alloying method was 8.47 mΩ cm² with a low current density (I_corr) between 0.051 and 0.058 μA cm^?2. The criteria for both current densities (<1 μA cm^?2) and electrical conductivity (<10 mΩ cm²) met the DOE's 2020 technical targets. In addition, conventional air brush method can be used for fabricating multilayer coatings onto substrates because it is self-cleaning, low cost and offers high volume and large area production. Vapor deposition method, a highly advanced coating technology using PVD, suitable for coating bipolar plates because it is environmentally friendly and can be used in high temperatures, producing materials with good impact strength and excellent abrasion resistance. PEMFC cost is still too high for large scale commercialization, which is the cost of raw material and processing to allow fabrication of thinner plates contributes substantially to the total PEMFC cost. Some future works on fuel cell anticorrosion research with reasonable coating method is suggested to reduce the cost in order to facilitate the move toward commercialization especially for SS and titanium bipolar plates.     

The effect of annealing on the properties of diamond-like carbon protective antireflection coatings

In addition to its similarity to genuine diamond film, diamond-like carbon (DLC) film has many advantages, including its wide band gap and variable refractive index. Therefore, as one of the diverse applications, DLC film can be utilized as a protective coating for IR windows and an anti-reflective coating for solar cells. For this study, DLC films were prepared by the radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) method on silicon substrates using methane (CH₄) and hydrogen (H₂) gas. We examined the effects of the post-annealing temperature and the annealing ambient on structural, electrical and optical properties of DLC films. The films were annealed at temperatures ranging from 300 to 900 °C in steps of 200 °C using rapid thermal annealing equipment in nitrogen ambients. The thickness of the film was observed by scanning electron microscopy (SEM) and surface profile analysis. The variation of structure according to the annealing treatment was examined using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The reflectance of DLC thin film was investigated by UV–vis spectrometry and its electrical properties were investigated using a four point probe and I–V meter. The carrier lifetime of the film was also checked.     

Diamond-like carbon based low-emissive coatings

A novel low-emissive (low-E) coating based on plasma enhanced chemical vapor deposited hydrogenated diamond-like amorphous carbon (DLC) films has been developed. The coating is a three-layer structure comprised of a nano-thin Ag layer sandwiched between DLC layers. The tunable optical properties that the DLC layers afford make it extremely appealing for large-scale, low-cost manufacturing of low-E coatings. We have shown that the refractive index of DLC films can be precisely tuned between 1.55 and 2.15 at a wavelength of 550 nm, while the optical gap can be tuned between 1.7 and 4.0 eV. The tunable optical properties allow greater flexibility and provide an additional parameter for optimization of low-E coatings. The tunability also allows for simple fabrication of multi-layer or graded-layer low-E coatings. Initial results of the DLC based low-E coating have a mid-IR reflectance of approximately 95% and a transmittance of approximately 70% over the visible frequencies.     

Impact of pressure on carbon films by PECVD toward high deposition rates and high stability as metallic bipolar plate for PEMFCs

Stainless-steel bipolar plates (BPPs) are widely used in place of graphite bipolar plates in proton exchange membrane fuel cells (PEMFCs). Amorphous hydrogenated carbon (a-C:H) coatings are widely used to improve the conductivity and corrosion resistance of metal bipolar plates. However, a-C:H coatings prepared by the sputtering method cannot be applied to quantity production on account of its low deposition rate. Our paper focuses on a-C:H coatings applied at metallic BPPs for PEMFCs produced by direct-current plasma-enhanced chemical vapor deposition (DC-PECVD) with high deposition rates. The effects of adjusting deposition pressures on the structure and properties of coatings have been investigated. The results show that the a-C:H coating deposited at 8 Pa deposition pressure have high stability, with a high deposition rate of 37.5 nm/min. As the deposition pressure increased, sp²-hybridized carbon atoms increased, the larger microcrystalline carbon clusters are found, and the structure will undergo different structural transformations in a-C:H coatings. Overall, the a-C:H coatings deposited at 8 Pa are attractive to be applied in metallic bipolar plate with have high deposition rates, dense coating structure, and proper carbon structure.     

Corrosion resistant quaternary Al–Cr–Mo–N coating on type 316L stainless steel bipolar plates for proton exchange membrane fuel cells

Insufficient corrosion resistance, electrical conductivity and wettability of bipolar plates are some of the important issues affecting the performance of hydrogen fuel cells. To address these issues, an amorphous Al–Cr–Mo–N coating is deposited on type 316L stainless steel using direct current (DC) magnetron sputtering. The electrochemical corrosion behaviour is investigated under simulated fuel cell anode (H₂-purging) and cathode (air-purging) environment consisting of 0.5 M H₂SO₄ + 2 ppm NaF at 70 ± 2 °C. The corrosion current density is reduced to 0.02 μA cm⁻² comparable to the commercially used Ta/TaN coatings. The polarization resistance increases by two orders of magnitude and the interfacial contact resistance (ICR) reduces significantly due to the application of the coating. Further, the coating shows better water management due to high hydrophobicity than the bare stainless steel.     

Surface modifications of aluminum alloy 5052 for bipolar plates using an electroless deposition process

This study tries to replace graphite bipolar plates in fuel cells with surface-modified aluminum alloy 5052. To improve the surface characteristics of Al alloy, Ni–Mo–P coatings were deposited on the substrates under various pH values and concentrations of sodium molybdate (Na₂MoO₄) by an electroless deposition process. The effects of the controlling conditions on the microstructure and the corrosion resistance of these deposits were examined. Moreover, the thermal stability and the corrosion resistance of Ni–Mo–P coatings were compared with those of Ni–P deposits in various attacking environments. The experimental results indicate that the electrical conductivity of all deposits produced in this experiment is superior to the U.S. DOE's target. The optimum Ni–Mo–P coating, which is produced in a solution containing 4.13 × 10⁻² M Na₂MoO₄ at pH 7.0 and 70 °C, possesses superior corrosion resistance in a mixed acidic environment. It is also found that Ni–Mo–P coatings exhibit better thermal stability, and superior long-term corrosion resistance than Ni–P deposits. The Ni–Mo–P deposits, therefore, are promising for applications in protecting coatings for bipolar plates.     

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.     

Investigation of single-layer and multilayer coatings for aluminum bipolar plate in polymer electrolyte membrane fuel cell

Aluminum bipolar plates offer good mechanical performance and availability for mass production while allow up to 65% lighter than stainless steel. To improve the corrosion resistance and surface electrical conductivity of aluminum bipolar plates, several coatings, including TiN, CrN, C, C/TiN and C/CrN, are deposited on aluminum alloy 5052 (AA-5052) by close field unbalanced magnetron sputter ion plating. Scanning electron microscope (SEM) results show that the coatings containing carbon layer are denser than TiN and CrN. Although the potentiodynamic test results show improved corrosion resistance by all the coatings, the potentiostatic test results reveal different stability of these coatings in PEMFC environments. Comparing the SEM images of these coatings after potentiostatic test, C/CrN multilayer coating exhibits the best stability. C/CrN multilayer coated AA-5052 has the lowest metal ion concentration after potentiostatic test, being 11.12 ppm and 1.29 ppm in PEMFC cathodic and anodic environments, respectively. Furthermore, the interfacial contact resistance (ICR) of the bare AA-5052 is decreased from 61.58 mΩ-cm² to 4.08 mΩ-cm² by C/CrN multilayer coating at the compaction force of 150 N-cm⁻². Therefore, C/CrN multilayer coating is a good choice for surface modification of aluminum bipolar plate.     

(Titanium,chromium) nitride coatings for bipolar plate of polymer electrolyte membrane fuel cell

(Titanium, chromium) nitride [(Ti,Cr)N] coatings are synthesized on a 316L stainless-steel substrate by inductively-coupled, plasma-assisted, reactive direct current magnetron sputtering. The chemical and electrical properties of the coating are investigated from the viewpoint of it application to bipolar plates. Nanocrystallized Cr–Ti films are formed in the absence of nitrogen gas, while a hexagonal β-(Ti,Cr)₂N phase is observed at N₂ = 1.2 sccm. Well-crystallized (Ti,Cr)N films are obtained at N₂ > 2.0 sccm. The corrosion resistance of the coating is examined by potentiodynamic and potentiostatic tests in 0.05 M H₂SO₄ + 0.2 ppm HF solution at 80 °C, which simulates the operation conditions of a polymer electrolyte membrane fuel cell. The Davies method is used to measure the interfacial contact resistance between the sample and carbon paper. The (Ti,Cr)N coating exhibits the highest corrosion potential and lowest current density. In a cathode environment, the corrosion potential and current density are 0.33 V (vs. SCE) and <5 × 10⁻⁷ A cm⁻² (at 0.6 V), respectively. In an anode environment the corresponding values are 0.16 V and <−5 × 10⁻⁸ A cm⁻² at −0.1 V. The (Ti,Cr)N coatings exhibit excellent stability during potentiostatic polarization tests in both anode and cathode environments. The interfacial contact resistance decreases with deposition of the (Ti,Cr)N film, and a minimum value of 4.5 mΩ cm² is obtained at a compaction force of 150 N cm⁻², which indicates that the formation of oxide films can be successfully prevented by the (Ti,Cr)N film. Analysis with Auger electron spectroscopy reveals that the oxygen content at the surface decreases with increase in the nitrogen content.     

Evaluation of vacuum heat-treated α-C films for surface protection of metal bipolar plates used in polymer electrolyte membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) assembled with metal bipolar plates (BPPs) is a promising power source for new energy vehicles. However, metal BPPs have serious corrosion issues and the surface electrical performance degrades in the harsh PEMFC environment. Amorphous carbon (α-C) films exhibit improved properties with both high corrosion resistance and electrical conductivity. Subsequent vacuum heat treatment of the as-coated α-C films can change their phase composition and film structure, modifying their performance. This study prepared α-C films with a titanium interlayer on a SS316L substrate by DC balance magnetron sputtering and then subjected them to vacuum heat treatment at different temperatures (400–700 °C). These treated α-C films are systemically analyzed in terms of surface and cross-sectional morphologies, sp bond hybridization, interfacial electrical conductivity, and corrosion resistance. The results indicate that the conversion of sp³ to sp² and the compact density of α-C films are greatly enhanced with an increase in temperature, greatly improving the corrosion resistance and surface conductivity of the films. These promising results lead to a potential direction for the post-coating treatment of α-C films on metal BPPs in PEMFCs.     

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.     

Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells

Superior corrosion resistance and high electrical conductivity are crucial to the metallic bipolar plates towards a wider application in proton exchange membrane fuel cells. In this work, molybdenum carbide coatings are deposited in different thicknesses onto the surface of 316 L stainless steel by magnetron sputtering, and their feasibility as bipolar plates is investigated. The microstructure characterization confirms a homogenous, compact and defectless surface for the coatings. The anti-corrosion performance improves with the increase of the coating thickness by careful analysis of the potentiodynamic and potentiostatic data. With the adoption of a thin chromium transition layer and coating of a ∼1052 nm thick molybdenum carbide, an excellent corrosion current density of 0.23 μA cm⁻² is achieved, being approximately 3 orders of magnitude lower than that of the bare stainless steel. The coated samples also show a low interfacial contact resistance down to 6.5 mΩ cm² in contrast to 60 mΩ cm² for the uncoated ones. Additionally, the hydrophobic property of the coatings’ surface is beneficial for the removal of liquid water during fuel cell operation. The results suggest that the molybdenum carbide coated stainless steel is a promising candidate for the bipolar plates.     

Adjustable TiN coatings deposited with HiPIMS on titanium bipolar plates for PEMFC

TiN coatings were deposited by HiPIMS at different N₂ flow rate to improve the corrosion resistance and conductivity of metallic bipolar plates. The results show that the surface microstructure of TiN coating depends strongly on the N₂ flow rate, all the samples (N₂ flow rate: 4 to10 sccm) meet the DOE 2025 standard and exhibit good hydrophobicity, which has great potential for industrial application. Among them, at the N₂ flow rate of 8 sccm, the TiN coating shows high compactness and the optimum surface microstructure with the lowest surface roughness of 1.083 nm and the highest hardness of 31.172 GPa. The optimized TiN coating exhibits excellent corrosion resistance with corrosion current density of 0.278 μA cm^?2 and a low interfacial contact resistance value of 3.51 mΩ cm². This work has opened a new way for the large-scale preparation of high-performance metal bipolar plate coatings.     

Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC

Low-temperature solid oxide fuel cell (LTSOFC) has shown great potentials for commercial applications in clean energy generation. Seeking for low cost and easy fabrication method is one of the most important issues for LTSOFC investigations. This paper introduces a new coating spray technology, namely Low-Pressure Plasma Spray (LPPS), for efficiently manufacturing different functional coatings of LTSOFC. By applying the LPPS technique, uniform and dense Ni_0.8Co_0.15Al_0.05LiO_2?δ (NCAL) coatings were made on both solid bipolar plates and porous nickel foams to perform as protecting coatings and electrode catalyst coatings respectively. Microstructure study showed that multi phases were formed and in-situ nano-micro crystallization occurred in the coatings during the LPPS process. Around 30 W output was achieved in a 4-cell stack indicating that the LPPS sprayed NCAL coatings on bipolar plates worked well. A fuel cell based on the NCAL-coated Ni foam reached an open circuit voltage (OCV) at 1.08 V and a maximum power density of 717 mW cm^?2 at 550 °C. This study reveals that LPPS is a promising technology for fabricating coatings of LTSOFC.     

Chromium-containing carbon film on stainless steel as bipolar plates for proton exchange membrane fuel cells

A series of chromium-containing carbon films are deposited on 316L stainless steel (SS316L) as bipolar plates for proton exchange membrane fuel cells (PEMFCs) by pulsed bias arc ion plating (PBAIP). The film characterizations are evaluated by X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). Interfacial contact resistance (ICR) between the coated SS316L samples and carbon paper is measured. Potentiodynamic and potentiostatic tests in the simulated corrosive circumstance of PEMFC are conducted to evaluate the corrosion resistance of the coated SS316L samples. The results indicate the films are primarily composed of pure carbon atoms with amorphous structure, including sp³ and sp² carbon atoms. The contents of sp³ and sp² carbon atom are remarkably influenced by the doping chromium. ICR and corrosion resistance of the coated SS316L sample are greatly improved owing to the surface film. The lowest ICR between the coated SS316L sample and carbon paper is only 2.8 mΩ cm² at the compaction force of 120 N cm⁻². The ICR has a close relationship with the contents of sp³ and sp² carbon atom, and the lowest ICR is obtained for the Cr_0.23C_0.77 film with the lowest sp³ carbon atom content and highest sp² carbon atom content. The SS316L sample with Cr_0.23C_0.77 film also exhibits the best corrosion resistance. Finally, the variations of ICR and surface morphology of the coated sample before and after corrosion testing are discussed.     

Electrical and thermal transport properties of vanadium oxide thin films on metallic bipolar plates for fuel cell applications

We have focused on the in-depth comparative evaluation of the suitability of electrically-induced thermal transport characteristics of highly disordered vanadium oxide thin films deposited onto metallic bipolar plates as an expeditious self-heating source for the successful cold-start of fuel cells in a subfreezing environment. To achieve this, sol–gel derived vanadium oxide thin films on the non-polished surface of 316L austenitic and 446M ferritic substrates have been fabricated by a dip-coating process. The effects of electrical properties on thermal energy dissipation rate of the as-synthesized thin films deposited onto 316L and 446M stainless steel plates were firstly investigated and compared with each other. Subsequently, a series of physical, chemical, and structural analyses of the thin films have been performed using several analytical techniques such as the ASTM D3359, the ASTM D5946, XPS, and FE-SEM. The most important finding of this study was that the electrical resistivity of the thin films on 446M ferritic substrate was extremely low on a level of 4.8% of the 316L sample at −20 °C, and then the surface temperature rise of the thin film on 316L austenitic substrates was approximately 21.8 times greater than that of 446M ferritic substrates under simulated cold starting conditions (i.e., at a current density of 0.1 A·cm⁻² at −20 °C). Therefore, we concluded that vanadium oxide thin films on 316L austenitic stainless steel plates appears to be more applicable than those of 446M ferritic substrates for the cold-start enhancement of fuel cells from the practical point of view.     

Transparent conductive Ga-doped ZnO/Cu multilayers prepared on polymer substrates at room temperature

In order to improve the transparency and durability of the Cu films deposited on polycarbonate (PC) substrates, a Ga-doped ZnO (GZO) layer could be deposited directly onto the Cu film. Compared with a single-layered GZO film, the GZO/Cu multilayer coatings have much lower sheet resistance and much thinner thickness. In our work, GZO/Cu multilayers were deposited by magnetron sputtering on PC substrates at room temperature. The structural, electrical, and optical properties of multilayers were investigated at various thicknesses of the Cu and GZO layers. As the Cu layer thickness increases, the resistivity decreases. As the GZO layer thickness increases, the resistivity increases. And we obtained that the transmittance of the GZO/Cu multilayer coatings was higher than that of the single Cu layer. The lowest resistivity of 5.7×10⁻⁵ Ω cm with a carrier concentration of 3.25×10²² cm⁻³ was obtained at the optimum Cu (12 nm) and GZO (10 nm) layer thickness. The best figure of merit φ_TC is 4.66×10⁻³ Ω⁻¹ for the GZO(30 nm)/Cu(12 nm) multilayer.     

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 protection of 304 stainless steel bipolar plates using TiC films produced by high-energy micro-arc alloying process

Metallic bipolar plates look promising for the replacement of graphite due to higher mechanical strength, better durability to shocks and vibration, no gas permeability, acceptable material cost and superior applicability to mass production. However, the corrosion and passivation of metals in environments of proton exchange membrane fuel cell (PEMFC) cause considerable power degradation. Great attempts were conducted to improve the corrosion resistance of metals while keeping low contact resistance. In this paper, a simple, novel and cost-effective high-energy micro-arc alloying process was employed to prepare compact titanium carbide as coatings for the type 304 stainless steel bipolar plates with a metallurgical bonding between the coating and substrate. It was found that TiC coating increased the corrosion potential of the bare steel in 1 M H₂SO₄ solution at room temperature by more than 200 mV, and decreased significantly its corrosion current density from 8.3 μA cm⁻² for the bare steel to 0.034 μA cm⁻² for the TiC-coated steel. No obvious degradation was observed for the TiC coatings after 30-day exposure in solution.     

Fabrication and characterisation of the graphite bi-polar plates used in A 0.25 kW PAFC stack

The graphite bi-polar plates were fabricated using lamination technique with polyether sulfone (PES) films (50 μm) and graphite foils (400 μn) in between the two porous graphite plates (CBC) by keeping in a specially designed and fabricated fixture with stainless steel plates at the top and bottom. The fixture was then kept in an hydraulic hot press, at loads of 10–20 tons, and heat treated at 410 °C for 30 min. Then these graphite plates were sized to 30 cm × 20 cm × 0.64 cm, leaving 0.4 mm thick graphite foil at the centre of the plate, to avoid the intermixing of the hydrogen and oxygen/air. While, the gas permeability (cm²/sec) of the plates was determined, with N₂ gas using differential pressure method, their electrical resistivity (mΩm) was measured using milliohmmeter and passing DC current to the graphite plates, at loads from 1–5 kgs. The values of permeability and electrical resistivity of the plates are found to be lower than 0.01 cm²/sec and 4–14 mΩm respectively. A stack with 6 cells was assembled using the in house developed graphite bi-polar plates, anodes and cathodes with matrix, to generate a DC power of 0.25 kW (3.6 V × 71.0 amps). It was operated for 300 h successfully using H₂ and Air, 1 bar, at 175 °C. In this paper, the detailed fabrication method of graphite bi-polar plates and their characteristics of gas permeability, electrical resistivity and the results of the 0.25 kW PAFC stack operation are presented.