Performance of stainless steel interconnects with (Mn,Co)3O4-Based coating for solid oxide electrolysis

Mixed transition-metal oxide coatings are commonly applied to stainless steel interconnects for solid oxide cell stacks. Such coatings reduce oxidation and Cr evaporation rates, leading to improved degradation rate and stack lifetime. Here, the ChromLok? MCO-based composition (Mn,Co)₃O₄ is applied to Crofer 22 APU stainless steel and evaluated specifically for application in solid oxide electrolyzer stacks operating around 800 °C and utilizing oxygen-ion-conducting solid oxide cells. The MCO coating is found to decrease the stainless steel oxidation rate by about one order of magnitude, and decrease the Cr evaporation rate by fourfold. The coating also dramatically lowers the rate of area-specific resistance increase for stainless steel coupons oxidized for 500 h with constant current applied, from 33 mΩ1cm² kh^?1 for an uncoated coupon to less than 4 mΩ1cm² kh^?1 for coated coupons. The coating is demonstrated on full-scale interconnects for single-cells, where the coating dramatically reduces degradation rate, and for a stack, which displays stable operation for 700 h.     

Electroplated Ni–Fe2O3 composite coating for solid oxide fuel cell interconnect application

Ni–Fe₂O₃ composite coating was applied onto ferritic stainless steel using the cost-effective method of electroplating for intermediate temperature solid oxide fuel cell (SOFC) interconnects application. By comparison, the coated and bare steels were evaluated at 800 °C in air corresponding to the cathode environment of SOFC. The oxidation investigations indicated that the oxidation rate of the coated steel was close to that of the bare steel after initially rapid mass gain. The mass gain of the coated steel was higher than that of the bare steel owing to the formation of double-layer oxide structure with an outer layer of (Ni,Fe)₃O₄/NiO atop an inner layer of Cr₂O₃. The area specific resistance (ASR) of the double-layer oxide scale was lower than that of the Cr₂O₃ scale thermally grown on the bare steel.     

Electrochemical hydrogen production from water electrolysis using ionic liquid as electrolytes: Towards the best device

Electrodes constructed with different electroactive materials such as platinum (Pt), nickel (Ni), 304 stainless steel (SS) and low carbon steel (LCS) have been tested in water electrolysis using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF₄). All experiments were performed at room temperature using a classical Hoffman's cell operating at atmospheric pressure and at different cathodic potentials. For the electrodes studied herein, in the presence of a 10 vol.% solution of BMI.BF₄ in water, current densities (j) in the range 10–42 mA cm⁻² were observed, with overall hydrogen production efficiencies (experimental/theoretical hydrogen production ratio) between 82 and 98%. The highest j values obtained with Pt, Ni, SS and LCS electrodes were 30, 12, 10 and 42 mA cm⁻², respectively, and all efficiencies were in the 85–99% range. These comparative results show that the LCS electrocatalyst constitutes an attractive alternative for the technological production of high purity hydrogen by water electrolysis reaction since the LCS electrode gave j and efficiencies as high as those observed with platinum electrodes.     

Experimental investigation and analysis of a new photoelectrochemical reactor for hydrogen production

In this research paper, an experimental investigation of photoactive material titanium dioxide (TiO₂) coated on 180 cm² 316 stainless steel anode is undertaken to study the photoresponse on photoelectrochemical (PEC) hydrogen production. The TiO₂ nanoparticles are first prepared via sol-gel method. A large surface 316 stainless steel anode is coated with TiO₂ nanoparticles by a dip coating apparatus at a withdraw rate of 2.5 mm/s. The nanoparticles are carried on the stainless steel substrate by two-step annealing procedure. The potentiostatic studies confirm the photoactivity of TiO₂ nanoparticles in a photoelectrochemical reactor when exposed to solar ultraviolet (UV) light. The photon to current efficiency measurements carried out on the PEC reactor with TiO₂ coated large surface stainless steel as photoanode demonstrate a significant increase of photoresponse in UV light compared to the uncoated stainless steel prepared under similar conditions. Upon illumination at a power density of 600 W/m², the hydrogen production is observed in TiO₂ coated stainless steel substrate at a measured rate of 51 ml/h while no illumination conditions show a production rate of 42 ml/h. In comparative assessments, the TiO₂ coated substrate shows an increase in photocurrent of 10 mA with an energy efficiency of 1.32% and exergy efficiency of 3.42% at an applied potential of 1.6 V. The present results show a great potential for titanium nanoparticles semiconductor metal oxide in photoelectrochemical hydrogen production application.     

Enhancing hydrogen production from steam electrolysis in molten hydroxides via selection of non-precious metal electrodes

There are still gaps in the field of reference electrode that is needed to assist electrolysis in high temperature electrolytes (e.g. molten hydroxides) for H₂ gas production. This research aims to fill the gaps by preparing Ni/Ni(OH)₂ reference electrode and more importantly testing its effectiveness against important performance factors including; ion conducting membrane (e.g. mullite tubes), internal electrolyte composition, working temperature and electrochemical control (e.g. potential scan rate). Then, this reference electrode was used to study the electrocatalytic activity various cheaper working electrode materials including; stainless steel (St.st), Ni, Mo and Ag in comparison with Pt by the means of chronoamperometry and voltammetry. The effect of introducing steam into electrolyte (eutectic mixture of NaOH and KOH) on the electrocatalytic activity of these working electrodes was also studied. It was observed that the potential of hydrogen evolution with different working electrodes followed an order as; Pt > Ni > St. st > Ag > Mo (positive to negative). The performance of each working electrode was confirmed through chronoamperometry for hydrogen evolution at a constant potential of −0.7 V. It was also found in cyclic voltammetry and confirmed by chronoamperometry that the introduction of steam was apparent as increasing the current density at cathodic limit for hydrogen evolution. This study could help to develop non-precious metal electrodes for the production of hydrogen fuel. In future, there will be a potential in the threshold concentration of steam for H₂ gas production.     

Development of Fe2O3–TiO2 mixed oxide incorporated Ni–P coating for electrocatalytic hydrogen evolution reaction

Considering the electronic parameters and chemical characteristics, a synergistic catalytic effect of Fe₂O₃ along with TiO₂ could be achieved for electrochemical reactions if both the oxides are produced in a mixed oxide form. The present study explored the mixed oxide composite viz; Fe₂O₃–TiO₂, synthesized via thermal decomposition method, to increase the catalytic efficiency of Ni–P electrodes, the well known catalytic electrodes for hydrogen evolution reaction in alkaline medium. The incorporation of the Fe₂O₃–TiO₂ mixed oxide into Ni–P matrix substantially reduced overpotential during hydrogen evolution reaction (HER) in 32% NaOH solution. A significant improvement on the electrochemical activity of the Ni–P coated electrodes was achieved as evidenced from the results of Tafel and impedance studies. The incorporation of Fe₂O₃–TiO₂ mixed oxide composite into the Ni–P matrix has improved both metallurgical and electrochemical characteristics and hence its amount of incorporation should be optimum. The electrodes exhibited high stability under dynamic experimental conditions. The role of the composite and the possible mechanism are discussed in this paper.     

A 4.2% efficient flexible dye-sensitized TiO2 solar cells using stainless steel substrate

An efficient flexible dye-sensitized solar cells (DSSCs) using stainless steel supporting substrate for fabricating nanocrystalline TiO₂ film electrodes were developed, intending to improve the photoelectrochemical properties of plastic substrate-based DSSCs. The most important advantage of a stainless steel-based TiO₂ film electrode over a plastic-based electrode lies in its high-temperature sinterability. Optimal photovoltaic properties were obtained with a cell where the TiO₂ film was coated on both ITO- and SiO_x-sputtered stainless steel (denoted as TiO₂/ITO/SiO_x/StSt). The photocurrent of the flexible cells with a TiO₂/ITO/SiO_x/StSt electrode increased significantly, leading to a much higher overall solar conversion efficiency η of 4.2% at 100 mW/cm², based on short-circuit photocurrent density, open-circuit voltage and fill factor of 11.2 mA/cm², 0.61 and 0.61 V, respectively, than those reported for cells with plastic substrates.     

Oxidation behavior and electrical property of ferritic stainless steel interconnects with a Cr-La alloying layer by high-energy micro-arc alloying process

Chromium volatility, poisoning of the cathode material and rapidly decreasing electrical conductivity are the major problems associated with the application of ferritic stainless steel interconnects of solid oxide fuel cells operated at intermediate temperatures. Recently, a novel and simple high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO₃-based coatings for the type 430 stainless steel interconnects using a LaCrO₃-Ni rod as deposition electrode. In this work, a Cr-La alloying layer is firstly obtained on the alloy surface by HEMAA using Cr and La as deposition electrode, respectively, followed by oxidation treatment at 850 °C in air to form a thermally grown LaCrO₃ coating. With the formation of a protective scale composed of a thick LaCrO₃ outer layer incorporated with small amounts of Cr-rich oxides and a thin Cr₂O₃-rich sub-layer, the oxidation rate of the coated steel is reduced remarkably. A low and stable electrical contact resistance is achieved with the application of LaCrO₃-based coatings, with a value less than 40 mΩ cm² during exposure at 850 °C in air for up to 500 h.     

LaCrO3-based coatings deposited by high-energy micro-arc alloying process on a ferritic stainless steel interconnect material

Currently used ferritic stainless steel interconnects are unsuitable for practical applications in solid oxide fuel cells operated at intermediate temperatures due to chromium volatility, poisoning of the cathode material, rapidly decreasing electrical conductivity and a low oxidation resistance. To overcome these problems, a novel, simple and cost-effective high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO₃-based coatings for the type 430 stainless steel interconnects. However, it is much difficult to deposit an oxide coating by HEMAA than a metallic coating due to the high brittleness of oxide electrodes for deposition. Therefore, a Cr-alloying layer is firstly obtained on the alloy surface by HEMAA using a Cr electrode rod, followed by a LaCrO₃-based coating using an electrode rod of LaCrO₃-20 wt.%Ni, with a metallurgical bonding between the coating and the substrate. The preliminary oxidation tests at 850 °C in air indicate that the LaCrO₃-based coatings showed a three-layered microstructure with a NiFe₂O₄ outer layer, a thick LaCrO₃ sub-layer and a thin Cr₂O₃-rich inner layer, which thereby possesses an excellent protectiveness to the substrate alloy and a low electrical contact resistance.     

Oxidation resistance and electrical properties of anodically electrodeposited Mn–Co oxide coatings for solid oxide fuel cell interconnect applications

Co-rich and crack-free Mn–Co oxide coatings were deposited on AISI 430 substrates by anodic electrodeposition from aqueous solutions. The as-deposited Mn–Co oxide coatings, with nano-scale fibrous morphology and a metastable rock salt-type structure, evolved into a (Cr,Mn,Co)₃O₄ spinel layer due to the outward diffusion of Cr from the AISI 430 substrates when pretreated in air. The Mn–Co oxide coatings were reduced into metallic Co and Mn₃O₄ phases when annealed in a reducing atmosphere of 5% H₂–95% N₂. In contrast to the degraded oxidation resistance and electrical properties observed for the air-pretreated Mn–Co oxide coated samples, the H₂-pretreated Mn–Co oxide coatings not only acted as a protective barrier to reduce the Cr outward diffusion, but also improved the electrical performance of the steel interconnects. The improvement in electronic conductivity can be ascribed to the higher electronic conductivity of the Co-rich spinel layer and better adhesion of the scale to the steel substrate, thereby eliminating scale spallation.     

Direct synthesis and coating of advanced nanocomposite negative electrodes for Li-ion batteries via electrospraying

A direct approach for the synthesis and coating of advanced nanocomposite negative electrodes via a single-step process at low temperature is presented. Metal-oxide/PVdF nanocomposites are obtained in one step by electrospray pyrolysis of precursor solutions containing dissolved metal salts together with polyvinylidene fluoride (PVdF) as binder. In this way, small oxide nanoparticles are generated and dispersed in situ in the binder creating nanocomposite structures, while being coated at once as thin electrode layers on stainless steel coin cell cans. The intimate contact between the nanoparticles and the binder favours enhanced adhesion of the materials in the overall electrode structure and adequate electrochemical performances are obtained without any conductive additive. Three nanocomposite oxide/PVdF materials (i.e. SnO₂, CoO and Fe₂O₃) are reported here as preliminary examples of negative electrodes. The results show that this approach is suitable, not only for the fabrication of nanocomposite electrodes for Li-ion batteries, but also for other novel applications.     

Oxidation of porous stainless steel supports for metal-supported solid oxide fuel cells

Oxidation behavior of porous P434L ferritic stainless steel, used for the fabrication of metal-supported solid oxide fuel cells (MS-SOFC), is studied under anodic and cathodic atmospheres. Temperature- and atmosphere-dependence is determined for as-sintered and pre-oxidized stainless steel. Pre-oxidation reduced the long-term oxidation rate. For pre-oxidized samples, the oxidation rate in air exceeds that in humid hydrogen for temperatures above 700 °C. The influence of PrOx, LSCF-SDC, and Ni-SDC coatings is also examined. The coatings do not dramatically impact oxide scale growth. Oxidation in C-free and C-containing anodic atmospheres is similar. Addition of CO₂, CH₄, and CO to humidified hydrogen to simulate ethanol reformate does not significantly impact oxidation behavior. Cr transpiration in humid air is greatly reduced by the PrOx coating, and a PrCrO₃ reaction product is observed throughout the porous structure. A dense and protective chromia-based scale forms on steel samples during oxidation in all conditions. A thin silica enriched oxide layer also forms at the metal-scale interface. In general, the oxidation behavior at 700 °C is found to be acceptable.     

DC electrodeposition of Mn–Co alloys on stainless steels for SOFC interconnect application

High conductivity coatings that resist oxide scale growth and reduce chromium evaporation are needed to make stainless steel interconnect materials viable for long-term stable operation of solid oxide fuel cells (SOFC). Mn_1.5Co_1.5O₄ spinel is one of the most promising coatings for interconnect application because of its high conductivity, good chromium retention capability, as well as good CTE match to ferritic stainless steels. Mn–Co electrodeposition followed by oxidization is potentially a low cost method for fabrication of (Mn,Co)₃O₄ spinel coatings. This work looks at the co-deposition of Mn–Co alloys for this application. As a guide to optimize the deposition process, characterizations of the cathodic reactions and reaction potentials are done using polarization curves. It was found that as cobalt concentration was varied that the alloy composition became richer in cobalt, indicating that the deposition is regular co-deposition process. It was also found that at 0.05 M Co concentration in excess gluconate the Mn–Co alloys composition could be tuned by varying the current density. Coatings with Mn–Co around 1:1 could be obtained at a current density of 250 mA/cm². However, the higher potential increased hydrogen production making the films more porous. Oxidation of the alloy coatings showed that much of the porosities could be eliminated during oxidation. It was found in a number of samples that fully dense coatings where obtained. The composition of the oxidized coating was found to become enriched in Mn, possibly due to the Mn fast diffusion from the substrate.     

Alternative cost-effective electrodes for hydrogen production in saline water condition

For the sustainable clean and green energy, hydrogen is considered as one of the prominent renewable energy source which attracted increasing interests in recent years. To produce this, one of the cheapest method is water electrolysis. But several challenges in water electrolysis are, to reduce the maintenance cost, energy consumption and high cost of platinum electrode material. So, in search of an alternative low cost and efficient electrode material, researchers are modifying various metals electrodes to replace the noble metal electrodes. Stainless steel (SS 304) is one of the types of carbon steel material commonly used for various applications. The aim of the work is to explore the stainless steel (SS 304), annealed at high temperature, with and without “hydrogen and argon” environment and tested the samples for hydrogen production in sea water condition (3.5% NaCl). Cr₂O₃ and MnCr₂O₄ spinel oxide formation was observed over the surface of the electrodes after annealing process. From Raman, X-ray Photoelectron Spectroscopy (XPS) and electrochemical measurements it was observed that, the sample prepared under hydrogen and argon environment is stable when compared with the rest of the samples. Decrease in relative amount of chromium oxide was observed for the sample annealed in air environment. The rate of production of hydrogen prepared under “hydrogen and argon” environment is higher and the results are discussed.     

Manufacture of Co-containing coating on AISI430 stainless steel via pack cementation approach for SOFC interconnect

Stainless steel can be applied as interconnect materials in solid oxide fuel cells (SOFCs) at operating temperatures 600–800 °C. Chromium (Cr)-forming stainless steel as an interconnect plate possesses a low oxidation resistance at high temperature and electrical conductivity, and volatility of Cr oxide scale can poison the cathode material. One effective strategy is to use a surface coating to improve interconnect performance. This work is to form cobalt (Co)-containing coatings on the surface of AISI 430 ferritic stainless steel interconnect via pack cementation approach. The resultant coating is extremely effective at heightening the oxidation resistance and electrical conductivity of AISI 430 ferritic stainless steel. The area specific resistance of samples was measured as a function of time. The area specific resistance of coated sample with 2% of activator content and holding time of 2 h is 90.21 and 108.32 mΩ cm² after 450 h of oxidation in air, respectively. Additionally, the coated sample with 2% of activator content and holding time of 2 h has a weight change of merely 0.299 and 0.231 mg/cm² after 650 h of isothermal oxidation at 800 °C, separately. The results displayed that the formation of CoFe₂O₄ spinel coating enhanced oxidation resistance by inhibiting the outward diffusion of Cr cations and the inward diffusion of oxygen anions.     

Surface coating with a high resistance to hydrogen entry under high-pressure hydrogen-gas environment

Development of a surface coating with high resistance to hydrogen entry under a high-pressure hydrogen-gas environment is presented. Two aluminum-based coatings were developed on the basis of preliminary tests: two-layer (alumina/Fe–Al) and three-layer (alumina/aluminum/Fe–Al) coatings, deposited onto cylindrical and pipe (Type 304 austenitic stainless steel) surfaces by immersion into a specially blended molten aluminum alloy. The coated specimens were exposed to hydrogen gas at 10–100 MPa at 270 °C for 200 h. Specimen hydrogen content was measured by thermal desorption analysis; hydrogen distributions were analyzed by secondary ion mass spectroscopy. Both coatings showed high hydrogen-entry resistance at 10 MPa. However, resistance of the two-layer coating clearly decreased with an increase in pressure. In contrast, the three-layer coating showed excellent hydrogen-entry resistance at a wide pressure range (10–100 MPa), achieved by the combined effect of alumina, aluminum, and Fe–Al layers.     

Hydrogen gas production from electrohydrolysis of industrial wastewater organics by using photovoltaic cells (PVC)

Electrical current generated by a photovoltaic cell (PVC) was supplied to wastewater in a mechanically mixed and sealed reactor using stainless steel electrodes. Hydrogen gas was generated by reaction of protons released from decomposition of organic compounds and electrons provided by electrical current. Gas phase was composed of 75–99% H₂ gas.     

Multi-electrode continuous flow microbial electrolysis cell for biogas production from acetate

Most microbial electrolysis cells (MECs) contain only a single set of electrodes. In order to examine the scalability of a multiple-electrode design, we constructed a 2.5 L MEC containing 8 separate electrode pairs made of graphite fiber brush anodes pre-acclimated for current generation using acetate, and 304 stainless steel mesh cathodes (64 m²/m³). Under continuous flow conditions and a one day hydraulic retention time, the maximum current was 181 mA (1.18 A/m², cathode surface area; 74 A/m³) within three days of operation. The maximum hydrogen production (day 3) was 0.53 L/L-d, reaching an energy efficiency relative to electrical energy input of η_E = 144%. Current production remained relatively steady (days 3–18), but the gas composition dramatically shifted over time. By day 16, there was little H₂ gas recovered and methane production increased from 0.049 L/L-d (day 3) to 0.118 L/L-d. When considering the energy value of both hydrogen and methane, efficiency relative to electrical input remained above 100% until near the end of the experiment (day 17) when only methane gas was being produced. Our results show that MECs can be scaled up primarily based on cathode surface area, but that hydrogen can be completely consumed in a continuous flow system unless methanogens can be completely eliminated from the system.     

Assessment of four different cathode materials at different initial pHs using unbuffered catholytes in microbial electrolysis cells

Nickel foam (NF), stainless steel wool (SSW), platinum coated stainless steel mesh (Pt), and molybdenum disulfide coated stainless steel mesh (MoS₂) electrodes have been proposed as catalysts for hydrogen gas production, but previous tests have primarily examined their performance in well buffered solutions. These materials were compared using two-chamber microbial electrolysis cells (MECs), and linear sweep voltammetry (LSV) in unbuffered saline solutions at two different initial pHs (7 and 12). There was generally no appreciable effect of initial pH on production rates or total gas production. NF produced hydrogen gas at a rate of 1.1 m³ H₂/m³·d, which was only slightly less than that using Pt (1.4 m³ H₂/m³·d), but larger than that obtained with SSW (0.52 m³ H₂/m³·d) or MoS₂ (0.67 m³ H₂/m³·d). Overall hydrogen gas recoveries with SSW (29.7 ± 0.5 mL), MoS₂ (28.6 ± 1.3 mL) and NF (32.4 ± 2 mL) were only slightly less than that of Pt (37.9 ± 0.5 mL). Total energy recoveries, based on the gas produced versus electrical energy input, ranged from 0.75 ± 0.02 for Pt, to 0.55 ± 0.02 for SSW. An LSV analysis showed no effect of pH for NF and Pt, but overpotentials were reduced for MoS₂ and SSW by using an initial lower pH. At cathode potentials more negative than −0.85 V (vs Ag/AgCl), NF had lower overpotentials than the MoS₂. These results provide the first assessment of these materials under practical conditions of high pH in unbuffered saline catholytes, and position NF as the most promising inexpensive alternative to Pt.     

Effect of conducting composite polypyrrole/polyaniline coatings on the corrosion resistance of type 304 stainless steel for bipolar plates of proton-exchange membrane fuel cells

A bilayer conducting polymer coating composed of an inner layer of polypyrrole (Ppy) with large dodecylsulfate ionic groups obtained by galvanostatic deposition, and an external polyaniline (Pani) layer with small SO₄²⁻ groups obtained by cyclic voltammetric deposition was prepared to protect type 304 stainless steel used for bipolar plates of a proton-exchange membrane fuel cell. The corrosion performance of the bare and coated steel in 0.3 M HCl was examined by electrochemical impedance spectroscopy, polarization and open-circuit potential measurements. The experimental results indicated that both the composite Ppy/Pani coatings and the single Ppy coatings increased the corrosion potential of the bare steel by more than 400 mV (saturated calomel electrode), and increased the pitting corrosion potential by more than 500 mV (saturated calomel electrode). The bilayer coatings could reduce the corrosion of the alloy much more effectively than the single Ppy coatings, serving as a physical barrier and providing passivity protection, with acceptable contact resistance.