Self-assembled graphene film to enable highly conductive and corrosion resistant aluminum bipolar plates in fuel cells

We report in this paper a novel method to form protective graphene film on aluminum substrate, which is particularly applicable to bipolar plates in proton exchange membrane (PEM) fuel cells. By simply immersing an aluminum sheet in an aqueous solution of graphene oxide (GO), a layer of cross-linked GO gel forms on the aluminum sheet, taking advantage of dissociated aluminum ions as a cross-linker. Then the cross-linked GO is converted to graphene at 400 °C in hydrogen atmosphere. The chemistry of the self-assembled GO layer and its conversion to graphene film is revealed by FTIR and XPS. Under simulated fuel cell environment the graphene coated aluminum sheet shows a corrosion current density of <1 × 10^?6 A/cm², which is around four orders of magnitude lower than a bare aluminum sheet. Meanwhile, the graphene film on aluminum results in a much lower and more stable interfacial contact resistance (ICR) of <5 mΩ cm². These enable the graphene coated aluminum sheet to meet the U.S. DOE targets of 2020 for bipolar plates in terms of both the corrosion and electrical resistance. Thus the proposed method is very promising for protecting aluminum bipolar plates in PEM fuel cells.     

Nanocomposite-carbon coated at low-temperature: A new coating material for metallic bipolar plates of polymer electrolyte membrane fuel cells

A nanocomposite-carbon layer is coated onto the surface of 316L stainless steel (SS316) using a beam of accelerated C₆₀ ions at low temperature. The coating is composed of textured graphite nanocrystals ranging in size from 1 to 2 nm, with the graphene plane normal to the coating plane; the nanocrystals are separated by amorphous carbon. This orientation of the graphene layer provides low film resistivity in the direction of the substrate normal. Corrosion resistance tests performed in aggressive anodic and cathodic environments of a polymer electrolyte membrane fuel cell (PEMFC) show that the nanocomposite-carbon coated SS316L exhibits better anticorrosion properties than does bare SS316L. The interfacial contact resistance (ICR) of the nanocomposite-carbon coated SS316L is 12 mΩ cm², which is similar to that of graphite at a compaction force of 150 N cm⁻² and lower than a target of ∼20 mΩ cm². A low value of ICR is maintained even after corrosion tests in aggressive anodic and cathodic environments. The fabricated nanocomposite-carbon coated SS316L exhibits excellent corrosion resistance and low interfacial contact resistance under simulated PEMFC bipolar plate conditions.     

Investigation of sintered stainless steel fiber felt as gas diffusion layer in proton exchange membrane fuel cells

The feasibility of using sintered stainless steel fiber felt (SSSFF) as gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs) is evaluated in this study. The SSSFF is coated with an amorphous carbon (a-C) film by closed field unbalanced magnetron sputter ion plating (CFUBMSIP) to enhance the corrosion resistance and reduce the contact resistance. The characteristics of treated SSSFF, including microscopic morphology, mechanical properties, electrical conductivity, electrochemical behavior and wettablity characterization, are systematically investigated and summarized according to the requirements of GDL in PEMFC. A membrane electrode assembly (MEA) with a-C coated SSSFF-15 GDL is fabricated and assembled with a-C coated stainless steel bipolar plates in a single cell. The initial peak power density of the single cell is 877.8 mW cm⁻² at a current density of 2324.9 mA cm⁻². Lifetime test of the single cell over 200 h indicates that the a-C coating protects the SSSFF-15 GDL from corrosion and decreases the performance degradation from 30.6% to 6.3%. The results show that the SSSFF GDL, enjoying higher compressive modulus and ductility, is a promising solution to improve fluid permeability of GDL under compression and PEMFC durability.     

Surface microstructure and performance of TiN monolayer film on titanium bipolar plate for PEMFC

The influence of bias voltage on surface microstructure of TiN films deposited on Ti substrate by multi-arc ion plating was systematically investigated. The TiN films were characterized using X-ray diffraction, scanning electron microscopy and atomic force microscopy. The corrosion resistance was tested by potentiodynamic polarization and electrochemical impedance spectroscopy at 70–80 °C in the simulated PEMFC cathode environment. The results show that the surface microstructure of TiN film depends strongly on the bias voltages. At the bias voltage of −100 V, TiN film shows the optimum surface microstructure with the lowest surface roughness R_z of 0.039 μm tested by AFM and relatively high compactness. The optimized TiN film exhibits excellent corrosion resistance with corrosion current density of 0.87 μA/cm² in a 0.5 M H₂SO₄ + 2 ppm HF solution at 80 °C with air and a low interfacial contact resistance (ICR) value of 3.0 mΩ cm² at a compaction force of 140 N/cm². These results support TiN as a promising coating material for Ti bipolar plates.     

Fabrication of electrochemically interconnected MoO3/GO/MWCNTs/graphite sheets for high performance all-solid-state symmetric supercapacitor

Binder-free MoO₃/GO/MWCNTs/graphite sheets were successfully fabricated via electrodeposition of graphene oxide and functionalized multi walled carbon nanotube onto graphite sheets followed by electrodeposition of molybdenum oxide. The capacitive behavior of the MoO₃/GO/MWCNTs/G sheet was found to be superior with respect to those of MoO₃/MWCNTs/graphite and MoO₃/GO/G sheets. The high wettability, interconnected structure and synergetic effects between MoO₃, GO and MWCNTs made the MoO₃/GO/MWCNTs/G sheet exhibited a high areal capacitance of 103 mF cm⁻² at current density of 0.7 mA cm⁻² in 1.0 M KCl. An all-solid-state symmetric supercapacitor device prepared by using the MoO₃/GO/MWCNTs/G sheet as both positive and negative electrodes showed high cell voltage of 2.5 V and remarkable cycle life of 86.8% retention after 2000 cycles, suggesting the possibility for practical applications in energy storage device.     

Formation of a protective TiN layer by liquid phase plasma electrolytic nitridation on Ti–6Al–4V bipolar plates for PEMFC

The current work mainly investigates the corrosion resistance and conductivity of TiN layer on Ti–6Al–4V with liquid phase plasma electrolytic nitridation for PEMFC bipolar plate (BP). The X-ray diffraction (XRD) results show that TiN phase presents in the nitriding samples. The surface morphology analysis indicates that higher CH₄NO₂ concentration tends to form more compact structure. The potentiodynamic polarization test indicates that sample N-600 prepared by 600 g/L CH₄NO₂ concentration possesses the lowest corrosion current density of 0.57 μA cm⁻² in the simulated PEMFC cathode potential (+0.6 V_SCE), which completely satisfies the DOE 2020 technical targets. Meanwhile, sample N-600 also exhibits the highest corrosion resistance and stability in the potentiostatic polarization, electrochemical impedance spectroscopy (EIS) and high potential (+1.6 V_SCE) polarization test. Interfacial contact resistance (ICR) results show that sample N-600 possesses the lowest ICR value of 6 ± 0.4 mΩ cm², which fully meets the DOE 2020 requirements.     

Chromium nitride/Cr coated 316L stainless steel as bipolar plate for proton exchange membrane fuel cell

Chromium nitride/Cr coating has been deposited on surface of 316L stainless steel to improve conductivity and corrosion resistance by physical vapor deposition (PVD) technology. Electrochemical behaviors of the chromium nitride/Cr coated 316L stainless steel are investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environments, and interfacial contact resistance (ICR) are measured before and after potentiostatic polarization at anodic and cathodic operation potentials for PEMFC. The chromium nitride/Cr coated 316L stainless steel exhibits improved corrosion resistance and better stability of passive film either in the simulated anodic or cathodic environment. In comparison to 316L stainless steel with air-formed oxide film, the ICR between the chromium nitride/Cr coated 316L stainless steel and carbon paper is about 30 mΩ cm² that is about one-third of bare 316L stainless steel at the compaction force of 150 N cm⁻². Even stable passive films are formed in the simulated PEMFC environments after potentiostatic polarization, the ICR of the chromium nitride/Cr coated 316L stainless steel increases slightly in the range of measured compaction force. The excellent performance of the chromium nitride/Cr coated 316L stainless steel is attributed to inherent characters. The chromium nitride/Cr coated 316L stainless steel is a promising material using as bipolar plate for PEMFC.     

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.     

A Ti3C2Tx-carbon black-acrylic epoxy coating for 304SS bipolar plates with enhanced corrosion resistant and conductivity

A conducting and anticorrosive coating is crucial for the application of metal bipolar plates (BP) in proton exchange membrane fuel cell (PEMFC). In this work, a Ti₃C₂T_x (T)-carbon black (C)-acrylic epoxy (AE) coating is prepared on 304 stainless steel (SS) with enhanced corrosion resistance and conductivity. The corrosion resistance of the T-C-AE coating is investigated in a 0.5 M H₂SO₄ solution as compared to the AE, T, and T-AE coatings. The T-C-AE coated 304SS exhibits the strongest corrosion resistance with the most positive corrosion potential and the lowest corrosion current density of 0.00673 μA cm^?2 in all the samples, while retaining intact and compact surface morphology with the lowest metal ion dissolution even after immersed for 720 h. The addition of Ti₃C₂T_x and carbon black into the AE matrix greatly decreases interfacial contact resistance (ICR), and the T-C-AE coating achieves a low ICR of 15.5 mΩ cm^?2 under 140 N cm^?2 compaction force. The excellent anticorrosion performance is mainly attributed to the physical barrier and the cathodic protection provided by the stacked Ti₃C₂T_x (MXene) nanosheets in the T-C-AE coating. This eco-friendly, conducting, and anticorrosive T-C-AE coating has a good application prospect on SS BP of PEMFC.     

Investigation on electrochemical behavior and surface conductivity of titanium carbide modified Ti bipolar plate of PEMFC

The titanium carbide (TiC) modified layer is prepared by plasma surface modification technology on the surface of Ti plate (TA1) to meet the performance requirements of bipolar plate of PEMFC. The microstructure characterization confirms that a compact and defectless TiC modified layer is formed on the surface of Ti bipolar plate. The corrosion current density of TiC modified plate in simulated PEMFC environment is reduced by approximately an order of magnitude and the self-corrosion potential is significantly improved compared with bare Ti plate. The interfacial contact resistance (ICR) of TiC modified plate (7.5 mΩ cm², under loading pressure of 140 N) is evidently lower than bare Ti plate (98.1 mΩ cm²). Even after potentiostatic polarization, the ICR of TiC modified plate still remains at satisfactory values. Furthermore, the contact angle of TiC modified plate reaches a higher value of 112°, which is beneficial to the water discharge of PEMFC.     

Enhanced corrosion resistance of Ni/Sn nano-electrodeposited metal foam for flow field application in simulated PEMFC cathode environment

Metal foam, with large specific surface area, suffers serious corrosion problems, as flow field in proton exchange membrane fuel cell (PEMFC). Ni/Sn nanoparticles are deposited onto the surface at galvanostatic and gradient current, respectively. The morphology of coated foams is examined by scanning electron microscopy (SEM), coupled with x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The effect of deposited current on its corrosion resistance in simulated PEMFC cathode environment is evaluated by Tafel polarization test, constant potential test and electrochemical impedance spectra. The results show that the coating effectively improved the stability of metal foam in acid environment. A uniform and dense protective film is formed by Ni/Sn electrodeposition at a gradient current density from 0 to 40 mA cm^?2. Its corrosion current at 25, 50 and 80 °C, accounts around 38.0%, 47.3% and 46.7%, respectively, of the value of uncoated metal foam. The most positive corrosion current is obtained, ?0.12 mA, which is explained to higher coating resistance (R_coat). No obvious pitting is depicted in the surface morphology after 8 h, which further proves its high corrosion resistance.     

Properties of C-doped CrTiN films on the 316L stainless steel bipolar plate for PEMFC

C doped CrTiN films were deposited on 316L stainless steel by magnetron sputtering technology to investigate corrosion resistance and electrical conductivity. The sputtering current of the C target alter to obtain various C contents. The carbon target currents are 0 A, 3 A and 6 A, respectively. The result of SEM confirms that deposited films have a dense and uniform microstructure. CrTiN coating consist of Cr, CrN and TiN phases. With the increase of C carbon target currents, Cr crystal structure vanishes, and the amorphous carbon and carbides appear. The result of the potentiodynamic polarization test in the simulate PEMFC environment reveals that C doped CrTiN coating can improve samples’ corrosion resistance. At 1.1 V (vs. SHE) potentiostatic tests, the C-6A has the lowest current density, 6.09 × 10⁻⁷ A/cm². Interfacial contact resistance decreases with the addition of C atoms. The C-6A coated sample has the lowest interfacial contact resistance values, 5.5  mΩ cm².     

Planar current collector design and fabrication for proton exchange membrane fuel cell

This paper proposes a novel planar type lightweight current collector for proton exchange membrane fuel cells (PEMFC) designed for low power portable applications. The proposed lightweight current collector, which is composed of a substrate, electrical conduction layer and corrosion resistance layer, combines the conventional metal sheet/mesh for current collecting and substrate together to reduce the possible distortion during operation caused by the mismatch due to large different mechanical properties between components. The current collector adopts FR-4 as the substrate material. The electrical conduction layer is made via coating a copper thin film using a thermo-evaporation layer. The corrosion resistance layer is made via coating a graphene thin film using spin coating and a vacuum oven process. Fabricated current collector sheet resistance measurements are conducted. The complete current collectors are assembled into a single cell PEMFC with both forced convection air-breathing cathode and self-air-breathing cathode. The related performance and stability experiments were conducted to investigate the feasibility for further applications.     

Durability improvement at high current density by graphene networks on PEM fuel cell

This study presents a novel structure of catalyst layers of membrane electrode assemblies (MEAs) by adding graphene to platinum on carbon black (Pt/C) to improve the durability at high current density operation (3 A cm⁻²). Graphene displays outstanding low electrical resistance and has the advantage of high electron mobility. It is also used in lithium ion batteries to improve electrical performance such as high rate charge/discharge capability and cycle-life stability. In this study, three MEAs are compared, and graphene is used as an excellent conductive additive in catalyst layers for better electrons transport at high current density operation. The MEA coated Pt/C mixed with 0.1 wt% graphene shows best durability for 0.3 V h⁻¹ which is almost 3.7 times better than that of without graphene additive (1.1 V h⁻¹). The graphene additive effectively extends the durability of the MEA. Furthermore, the MEAs are analyzed by AC impedance. The impedance arc of the MEA coated with Pt/C only is getting worse, but those two coated with graphene show similar and smaller impedance arcs after high current density operation for 80 h.     

Graphene oxide decorated nickel-cobalt nanosheet structures based on carbonized wood for electrocatalytic hydrogen evolution

Nickel-based materials exhibit great potential in the field of hydrogen evolution reaction (HER), however, the low catalytic active site and poor corrosion resistance still limit further application. Herein, a novel 3D self-supporting electrode of graphene oxide/nickel-cobalt/carbonized wood (GO/Ni–Co/CW) based on porous carbon is developed. The self-supporting structure of the electrode effectively prevents the shedding of catalytic materials, while the exposed active sites of the Ni–Co nanosheets ensure excellent catalysis and the decoration of GO further enhances the HER performance. Evidently, GO/Ni–Co/CW requires an overpotential of 52 mV in 0.5 M H₂SO₄ and 70 mV in 1 M KOH to achieve a current density of 10 mA cm⁻². Furthermore, the introduction of GO greatly improves the stability performance of the electrode due to its corrosion resistance, as found by the catalytic stability performance test. As a new idea, GO decorated Ni–Co nanosheets grown on wood-based porous carbon as electrodes fully combine and exploit the advantages of CW's 3D porous structure, Ni–Co nanosheets' catalytic activity, and GO's corrosion resistance, which provide an effective strategy for novel nickel-based HER electrocatalysts.     

Performance of Ti-Ag-deposited titanium bipolar plates in simulated unitized regenerative fuel cell (URFC) environment

Titanium with excellent corrosion resistance, good mechanical strength and lightweight is an ideal BPP material for unitized regenerative fuel cell (URFC), but the easy-passivation property accordingly results in poor cell performance. Surface modification is needed to improve the interfacial conductivity. In this study, Ti-Ag film is prepared on TA1 titanium as bipolar plates for URFC by pulsed bias arc ion plating (PBAIP). Interfacial conductivity of Ti-Ag/Ti is improved obviously, presenting an interfacial contact resistance of 4.3 mΩ cm² under 1.4 MPa. The results tested by potentiodynamic, potentiostatic and stepwise potentiostatic measures in simulated URFC environments show that Ti-Ag/Ti has good anticorrosion performance, especially at high potential. The corrosion current density of Ti-Ag/Ti is approximately 10^−5.0 A cm⁻², similar to that of uncoated titanium, at 2.00 V (vs. NHE) in a 0.5 M H₂SO₄ + 5 ppm F⁻ solution at 70 °C with pressured air purging. Ti-Ag/Ti sample also has low surface energy. The contact angle of the sample with water is 102.7°, which is beneficial for water management in URFC. The bipolar plate with cost-effective Ti-Ag film combines the prominent interfacial conductivity with the excellent corrosion resistance at high potential, showing great potential of application in URFC.     

Long-term polarization accelerated degradation of nano-thin C/Ti coated SS316L bipolar plates used in polymer electrolyte membrane fuel cells

Dynamic potentials of polymer electrolyte membrane fuel cells (PEMFCs) lead to the corrosion of metal bipolar plates and significantly increase their interfacial contact resistance (ICR). Herein, two nano-thin C/Ti coatings with different thicknesses are prepared on SS316L (C/Ti/SS) and examined under three types of polarization potential modes. The C₁₀₀Ti₆₀ coating has improved corrosion resistance than C₂₀Ti₁₄₀ coating. The C₁₀₀Ti₆₀ coating exhibits a low ICR of 2.67 mΩ cm² and a small corrosion rate of 1.47 μA/cm², highlighting the high electrical conductivity and corrosion resistance. Moreover, C₁₀₀Ti₆₀/SS achieves a slight degradation of ICR after polarization at 0.67 V and cyclic polarization between 0.43 and 0.73 V. However, the high potential of 1.43 V induces severe delamination of C layer, resulting in a remarkable increase of ICR. Analysis suggests that the Ti layer improves the oxidation resistance and the C layer could provide effective protection when the potential is below 1.13 V.     

Effect of Ni content on the electrical and corrosion properties of CrNiN coating in simulated proton exchange membrane fuel cell

In this paper, CrNiN coatings with various Ni content are deposited on 304ss bipolar plates by closed field unbalanced magnetron sputter ion plating from CrNi alloy targets. Simulative working environment of proton exchange membrane fuel cell (PEMFC) is applied to test the electrical and corrosion properties of uncoated 304ss and CrNiN-coated samples. The influence of Ni content on microstructure, phase structure, contact angle with water and electrochemical performance is investigated. Results show that all the coated samples significantly enhanced the corrosion resistance of the 304ss, and the CrN-coated 304ss sample without Ni has the best corrosion resistance of 153.8 and ?141.9 mV in the simulated anodic and cathodic environments, respectively. Electrochemical impedance spectroscopy (EIS) studies reveal that the resistance of CrN coating is higher than that of other coated samples and 304ss in the cathodic environment. Furthermore, Interfacial contact resistance (ICR) studies revealed that CrN coating has a superior ICR of 11 mΩ cm² at a compaction force of 160 N cm^?2. In addition, the contact angle of the CrNiN coatings with water is approximately 114°, which is beneficial for water management in PEMFC. Analysis result indicates that the enhanced performance of the coated 304ss bipolar plates is related to the high film density determined by closed field unbalanced magnetron sputter ion plating, and the synergistic function of the CrNiN layered structure.     

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.     

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.