Fe doped Ni–Co alloy by electroplating as protective coating for solid oxide fuel cell interconnect application

To obtain higher electrical conductivity and lower area specific resistance (ASR), the 10% Fe doped Ni–Co (NCF) alloy was prepared on SUS 430 steel substrate by electroplating for solid oxide fuel cells (SOFCs) interconnects application. Then, the SUS 430 steels and NCF coated steels were oxidized at 800 °C. The microstructure and oxide phase of samples were tested by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). These results proved that the NCF coated steel achieved the lower oxidation rate of 9.28 × 10⁻¹⁴ g²cm⁻⁴s⁻¹ and ASR of 14.72 mΩ cm².     

The preparation and properties of Mn–Co–O spinel coating for SOFC metallic interconnect

To meet the performance requirements of solid oxide fuel cell (SOFC) metallic interconnect, the Mn–Co–O spinel coating is prepared on the surface of AISI430 by pack cementation method to reduce the growth kinetics of oxides and inhibit the outward diffusion of Cr. The microstructural characterization shows that a dense, uniform, defect-free spinel coating is successfully fabricated on the surface of AISI430. Under the simulated SOFC cathode environment, the weight gain of coated steel (0.608 mg cm⁻²) after oxidation at 800 °C for 800 h is significantly lower than that of uncoated (1.586 mg cm⁻²). In addition, the area specific resistance (ASR) of the coated steel oxidized for 500 h is 17.69 mΩ cm², much smaller than that of the bare steel, indicating that the oxidation resistance and electrical conductivity of AISI430 are significantly improved by Mn–Co–O spinel coating. Cross-sectional observations of the Mn–Co–O spinel coating are conducted to assess the compatibility of substrate with the adjacent coating and its effectiveness in reducing the growth of the Cr₂O₃ layer.     

Oxidation behavior of ferritic stainless steel containing Nb,Nb–Si and Nb–Ti for SOFC interconnect

The effect of Nb on the oxidation kinetics, electrical conductivity and Cr evaporation behavior of FSS has been discussed depending on the Nb content and oxygen active element such as Ti and Si. Nb in ferritic stainless steel is saturated during heat treatment as NbO₂ at the outermost oxide scale and as both Nb₂O₅ and Laves phase near the oxide scale/alloy interface. Excess Nb (>4.7 wt%) suppresses precipitation of Nb₂O₅, because of rapid Laves phase growth. Nb enhances selective Ti oxidation, whereas Ti retards Nb₂O₅ precipitation near the scale/alloy interface. On the other hand, Si suppresses Nb enrichment near the scale/alloy interface and it reduces the precipitation of both Nb₂O₅ and Laves phase. Nb also suppresses Si enrichment and the formation of continuous Si oxide at the scale/alloy interface. Co-addition of Nb and Ti is effective to decrease the electrical resistance and Cr evaporation rate of oxide scale.     

Influence of precipitation on initial high-temperature oxidation of Ti–Nb stabilized ferritic stainless steel SOFC interconnect alloy

Oxidation phenomena on Laves phase forming Ti–Nb stabilized ferritic stainless steel (EN 1.4509) were studied at 650 °C by electron microscopic and electron spectroscopic methods. These investigations reveal a strong competition between Nb and Si for interfacial oxidation at the oxide–metal interface that is affected by different segregation rates of Nb and Si at elevated temperatures. In particular, formation of Si containing Laves (FeNbSi)-type intermetallic compounds in the bulk results in non-uniform distribution of Si oxide at the interface. This has direct implications to the electrical properties of this alloy in solid oxide fuel cell (SOFC) applications. Furthermore, these results provide better understanding to the controversial role of second phases (e.g. Laves, chi) on high-temperature oxidation (as recently discussed by Dae Won Yon, Hyung Suk Seo, Jae Ho Jun, Jae Myung Lee, Do Hyuong Kim, Kyoo Young Kim in Int J Hydrogen Energy 2011;36:5595–5603).     

Highly dense (Mn,Co)3O4 spinel protective coating derived from Mn–Co metal precursors for SOFC interconnect applications

The major degradation issues of solid oxide fuel cells (SOFC) are associated with the Cr₂O₃ scale growth and Cr diffusion of the Cr-based ferritic stainless steel (FSS) interconnects. Although (Mn,Co)₃O₄ has been proved as a suitable material for protecting FSS interconnects, the porous structure of the coatings prepared with the pre-synthesized spinel weakens the protective capability of the coatings. In this paper, the widely-used pre-synthesized spinel is replaced with metal precursors (Mn and Co powders). Due to the low melting point (≤1290 °C) and the volume expansion during oxidation, the metal precursors, can be effectively sintered at 900 °C in a reducing atmosphere and form dense, well-protective coatings at 850 °C in the air. The samples are characterized with X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and a 4-probe area-specific resistance (ASR) test. Compared with the coatings derived from pre-synthesized spinel, the metal-derived coatings present denser structures with better electrical conductivity (ASR = 5.76 mΩ cm²). The weight gain and ASR measurement results indicate that the metal-derived coatings significantly mitigate the increase of weight gain and ASR by inhibiting scale formation and growth, showing better protective capability for SOFC applications.     

Formation mechanism and oxidation behavior of Cu–Mn spinel coating on ferritic stainless steel for solid oxide fuel cell interconnects

Buckling damage and spallation during oxidation are prime challenges in electroplated Cu–Mn oxide spinel coatings prepared for SOFC interconnects. (Cu,Mn)₃O₄ spinel coating is formed on AISI-430 stainless steel by oxidizing electroplated copper and manganese layers at 750 °C in air. Nickel strike deposition significantly reduces coating spallation. During oxidation at 750 °C, spinel coating hinders chromium migration and improves the oxidation behavior of the alloy by a decrease of 98.9% in the parabolic oxidation rate constant. Formation mechanism of the spinel is investigated, revealing that manganese initially forms MnO, Mn₂O₃, and Mn₃O₄. Over the time, copper oxidizes to CuO, and Mn₂O₃ replaces MnO and Mn₃O₄. Finally, (Cu,Mn)₃O₄ spinel forms after 6 h of oxidation by the co-existence of CuO and Mn₂O₃. It is concluded that manganese oxidation induces buckling damage due to a volume increase of up to 137% in the coating during the earliest stages of the oxidation.     

Cu doped Ni–Co spinel protective coatings for solid oxide fuel cell interconnects application

Four different amount of Cu doped Ni–Co alloy coatings were fabricated on SUS430 substrate by electroplating for solid oxide fuel cells (SOFCs) interconnects application. After oxidation at 800 °C, the microstructure and oxide phase of samples were tested by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Our experimental results indicated that the Cu addition improved the electrical behavior of Ni–Co alloy coating. Cu doping reduced the activation energy (Ea) of electrons hopping and inhibited the growth of Cr₂O₃ oxide layer. Furthermore, the oxidation kinetics and electrical properties of the alloy coatings were obtained. These results showed that the 9% Cu doped Ni–Co coated steels achieved the minimum parabolic rate constant (2.05 × 10⁻¹⁴ g²cm⁻⁴s⁻¹) and area specific resistance (14.11 mΩ cm²) after the thermostatic oxidation process.     

Sputtered Fe1·5CoNi0.5 coating: An improved protective coating for SOFC interconnect applications

In an attempt to optimize the properties of FeCoNi coating for planar solid oxide fuel cell (SOFC) interconnect application, the coating composition is modified by increasing the ratio of Fe/Ni. An Fe_1·5CoNi_0.5 (Fe:Co:Ni = 1.5:1:0.5, atomic ratio) metallic coating is fabricated on SUS 430 stainless steel by magnetron sputtering, followed by oxidation in air at 800°C. The Fe_1·5CoNi_0.5 coating is thermally converted to (Fe,Co,Ni)₃O₄ and (Fe,Co,Mn,Ni)₃O₄ without (Ni,Co)O particles. After oxidation for 1680 h, no further migration of Cr is detected in the thermally converted coating region. A low oxidation rate of 5.9 × 10^?14 g² cm^?4 s^?1 and area specific resistance of 12.64 mΩ·cm² is obtained for Fe_1·5CoNi_0.5 coated steels.     

Post-mortem evaluation of oxidized atmospheric plasma sprayed Mn–Co–Fe oxide spinel coatings on SOFC interconnectors

Interconnects employed in solid oxide fuel cells require electrically conductive protective coatings such as those based on manganese cobalt oxide spinels in order to prevent evaporation of volatile Cr(VI)-compounds and to minimize high temperature corrosion. MnCo_2−xFe_xO₄ based (where x = 0.1 and 0.3) oxide spinel protective coatings were manufactured by the atmospheric plasma spraying process on Crofer 22 APU substrates. The coated substrates were oxidized at 700 °C in air for 1000 h and post-mortem analyses were conducted to study the performance of the thermal sprayed coatings. During the high temperature oxidation, a four-point on-line measurement technique was used for area specific resistance studies. The MnCo_1.7Fe_0.3O₄ coating was tested together with the La_0.85Sr_0.15Mn_1.1O₃-spacer.     

Optimization of the electrical properties of Ti–Nb stabilized ferritic stainless steel SOFC interconnect alloy upon high-temperature oxidation: The role of excess Nb on the interfacial oxidation at the oxide–metal interface

Interfacial oxidation of Nb and Si at 650 °C on Laves phase forming Ti–Nb stabilized ferritic stainless steel (Fe–19Cr–0.9Si–0.2Nb–0.1Ti (at.%), grade EN 1.4509) was studied by electrochemical impedance spectroscopy and photoelectron spectroscopy. It was found that excess Nb efficiently hinders the formation of electrically resistive SiO₂ layer at the oxide–metal interface. The beneficial role of Nb was attributed to its high segregation rate and the formation of conductive oxides at the interface. However, the oxidation was strongly influenced by age-precipitation of the Laves (FeNbSi)-type intermetallic phase, which removed free Nb from the alloy solution and thus allowed SiO₂ layer to form more easily. These results can be applied to optimize the oxide scale composition by Nb alloying of the ferritic stainless steel to maintain high performance under various operation conditions, particularly in solid oxide fuel cell applications.     

Ni/CeO2–MgO catalysts supported on stainless steel plates for ethanol steam reforming

Ni/CeO₂–MgO catalysts on powder form and supported on stainless steel plates were prepared, characterized and tested towards hydrogen generation via the steam reforming reaction of ethanol. The structured catalyst was prepared by the dip-coating technique. The coatings obtained over the stainless steel plates were homogeneous and retained their integrity after the reaction experiences. The samples were characterized by SEM, TEM, XRD, ICP-AES, TPR, OSC and N₂ adsorption–desorption measurements. Catalysts presented very good stability under reaction conditions for 16 h on-stream, without showing a significant variation in the activity or product distribution. The structured catalysts presented similar activities and selectivities respect to those of the powder, whereby the deposition method did not modify the catalytic properties of the particulate material. The presence of the AISI 430 stainless steel substrate also had not a significant influence on the performance of the deposited material.     

Ag–polytetrafluoroethylene composite coating on stainless steel as bipolar plate of proton exchange membrane fuel cell

Forming a coating on metals by surface treatment is a good way to get high performance bipolar plate of proton exchange membrane fuel cell (PEMFC). In our research, Ag–polytetrafluoroethylene (PTFE) composite film was electrodeposited with silver-gilt solution of nicotinic acid by a bi-pulse electroplating power supply on 316 L stainless steel bipolar plate of PEMFC. Surface topography, contact angle, interfacial conductivity and corrosion resistance of the bipolar plate samples were investigated. Results showed that the defects on the Ag–PTFE composite coating are greatly reduced compared with those on the pure Ag coating fabricated under the same condition; and the contact angle of the Ag–PTFE composite coating with water is 114°, which is much bigger than that of the pure Ag coating (73°). In addition, the interfacial contact resistance of the composite coating stays as low as the pure Ag coating; and the bipolar plate sample with composite coating shows a close corrosion resistance to the pure Ag coating sample in potentiodynamic and potentiostatic tests. Coated 316 L stainless steel plate with Ag–PTFE composite coating exhibits well hydrophobic characteristic, less defects, high interfacial conductivity and good corrosion resistance, which shows a great potential of the application in PEMFC.     

Efficient and stable Ni–Co–Fe–P nanosheet arrays on Ni foam for alkaline and neutral hydrogen evolution

The development of highly efficient and superior durability electrocatalysts is vital to expedite hydrogen evolution reaction (HER). Herein, a mixed amorphous and nano-crystalline Ni–Co–Fe–P alloy on Ni foam after 75 s dealloying in 3 M HCl (Ni–Co–Fe–P/NF-3-75) is synthesized by the preparation strategy of two-step method consisting of electroless deposition and dealloying process. Ni–Co–Fe–P/NF-3-75 shows an excellent HER performance and high durability in both alkaline and neutral conditions by optimizing the composition of the catalysts, acid concentration, and the time of dealloying. Benefitting from the high conductivity of Ni foam carrier, coordination between polymetallic phases, and the large exposure of defects, the as-prepared Ni–Co–Fe–P/NF-3-75 requires only a low overpotential of 56 mV and 104 mV to reach the current density of 10 mA cm⁻² in 1.0 M KOH and 1.0 M phosphate buffer (PBS), respectively. Remarkably, the Ni–Co–Fe–P/NF-3-75 electrode exhibits superior cycling stability and long-term robust durability without obvious overpotential decline. The successful preparation of the Ni–Co–Fe–P/NF-3-75 catalyst indicates that this method provides an efficient way to synthesize polymetallic phosphides for hydrogen evolution reaction.     

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.     

Influence of Au ions irradiation on the deuterium permeation behavior in the oxidized Fe–Cr–Al ferritic steel

Our previous study has shown that an effective tritium permeation barrier (TPB) with Al₂O₃ layer found could be obtained by oxidization of a Fe–Cr–Al ferritic steel. In this study, irradiation effects on deuterium permeation behavior through oxidized Fe–Cr–Al ferritic steel (OFFS) have been investigated by Au ions irradiation followed by deuterium gas driven permeation (GDP) experiments. The deuterium permeability of the irradiated and original OFFS samples has been obtained and compared. Oxide layer has been characterized by X-ray photoelectron spectroscopy (XPS) experiment and scanning electron microscopy (SEM) experiment. The defects in the oxide layer with and without Au ions irradiation have been characterized by Doppler broadening spectrometry of positron annihilation (DBS-PA) experiments. The deuterium permeation behavior of OFFS changed owing to the Au ions irradiation, which could be attributed to the increased density of vacancy-type defects.     

Bipotential deposition of nickel–cobalt hexacyanoferrate nanostructure on graphene coated stainless steel for supercapacitors

Graphene oxide (GO) was deposited on inexpensive and mechanically stable stainless steel (SS) electrode by electrophoretic deposition (EPD) technique. GO was reduced electrochemically in NaNO₃ to obtain electrochemically reduced graphene oxide (ERGO). Next, Hybrid nickel–cobalt hexacyanofarrate (NiCoHCF) nanoparticles were deposited from solution containing Ni⁺² and Co⁺² with ratio of 1:1 on ERGO/SS by bipotential method. Morphological investigation of prepared sample by scanning electron microscopy showed the presence of nanoparticles with diameters in the range of 15–50 nm. Crystal structure of nanocomposite was investigated by X-ray diffraction technique. Electrochemical behavior of prepared film indicates that hybrid nanocomposite has higher specific capacitance (411 F g⁻¹) than ERGO (185.2 F g⁻¹) in KNO₃ solution at current density of 0.2 A g⁻¹. In other words, pseudocapacitor that is formed based on the faradaic behavior of NiCoHCF can improve the capacitive performance of ERGO.     

Electrophoretic deposition of Co–Me/ZnO (Me = Mn,Fe) ethanol steam reforming catalysts on stainless steel plates

A procedure for coating metal plates with powder catalysts was developed based on electrophoretic deposition (EPD), and tested to deposit three different Co-based catalysts for the steam reforming of ethanol on stainless steel plates. The catalysts contained 10 wt% Co and 1 wt% of Mn or Fe supported on ZnO, and were prepared by co-precipitation (Co–Mn/ZnO–P and Co–Fe/ZnO–P) and impregnation (Co–Mn/ZnO–I). EPD was performed suspending the powder catalysts in isopropanol, using a voltage of 100 V and a distance between electrodes of 2 cm. Polyethyleneimine (PEI, 1 g/L) was used as binder. Deposition time was fixed at 5 min, which gave a thickness of the catalyst layer from around 30–45 μm, depending on the catalyst. The activity of the catalyst plates was tested at 773 K using steam to carbon ratios of 3 and 4, under incomplete conversion conditions. All catalysts favored ethanol dehydrogenation to acetaldehyde, and steam reforming. Ethanol dehydration to ethylene and acetaldehyde cracking to methane and carbon monoxide were not favored, and the selectivity towards those products was very low.     

Impact of pre-oxidation on the reactivity and conductivity in H2–H2O atmosphere of a ferritic stainless steel for high temperature water vapour electrolysis

The as-rolled K41X (AISI 441) ferritic stainless steel presents unusual behaviour when exposed in H₂–H₂O atmosphere at 800 °C, with growth of non-protective iron oxides at its surface. The impact of pre-oxidation in air is investigated in this paper. The pre-oxidation induces the formation of a thin chromia layer at the steel surface. The results obtained after subsequent oxidation for 100 h in H₂–H₂O atmosphere evidence a change in the surface reactivity behaviour of the as-rolled alloy: iron oxides formation is avoided and high temperature behaviour (reactivity and conductivity) is improved. Air pre-oxidation seems a promising solution for large scale use of K41X as-rolled steel in H₂–H₂O atmosphere at high temperature.     

Study on Ni–Fe–C cathode for hydrogen evolution from seawater electrolysis

Hydrogen evolution reaction in 3.5 wt% NaCl (simulated seawater) was investigated using Ni–Fe–C cathode, prepared by cathode electrodeposits method on the matrix of A3 steel. The as-prepared Ni–Fe–C cathode coating materials has reached nanometer grade, what is more, the limit of average grain size was about 4.3 nm. As decreasing of the average coating grain sizes, hydrogen evolution overpotential was not decreasing linearly. There was a boundary average coating grain sizes of about 4.3–6.4 nm. The optimal preparation process of Ni–Fe–C cathode was listed as electroplating current density 200 A/m², temperature 30 °C, pH 1.5 and 60 min. The hydrogen overpotential was only about 65 mV, which was tested in the 3.5 wt% NaCl of 90 °C at pH 12.