Electrochemical characterization of porous nickel–cobalt oxide electrodes

The electrochemical characterization of mixed oxides containing cobalt and nickel prepared by cathodic electrodeposition is presented. Their catalytic properties are discussed according to the results obtained employing stationary polarization curves, voltammetry, electrochemical impedance spectroscopy, and scanning electron microscopy. Impedance results were analyzed considering a porous electrode and approximated in terms of a finite transmission line model of conical pores linked in parallel. For sake of comparison, results obtained with Co–Ni cobaltites prepared by thermal decomposition are also presented.     

Stabilization of the platinum–ruthenium electrocatalyst against the dissolution of ruthenium with the incorporation of gold

The dissolution of Ru from the PtRu electrocatalyst has been identified as one of the most critical factors in degrading the performance of polymer electrolyte membrane fuel cells (PEMFC). In this work, we prepared an Au-modified PtRu catalyst (Au/PtRu) and found that the incorporation of Au could significantly stabilize the PtRu electrocatalyst against the dissolution of Ru. The X-ray photoelectron spectroscopy (XPS) characterization of the Au/PtRu catalysts revealed that the incorporation of Au increased the oxidation potential of Ru, which is the mechanism that leads to a reduction in the dissolution of Ru from the alloyed catalyst. The XPS characterization of the cathode catalyst also showed that with the PtRu as the anode catalyst Ru appeared at the cathode, but the crossover of Ru could be reduced when the anode catalyst was changed to Au/PtRu.     

Electrochemical characterization of activated carbon–ruthenium oxide nanoparticles composites for supercapacitors

The high specific capacitance of ruthenium oxide (denoted as RuO_x) nanoparticles prepared by a modified sol–gel method with annealing in air for supercapacitors was demonstrated in this work. The specific capacitance of activated carbon (denoted as AC) measured at 5 mA/cm² is significantly increased from 26.8 to 38.7 F/g by the adsorption of RuO_x nanoparticles with ultrasonic weltering in 1 M NaOH for 30 min. This method is a promising tool in improving the performance of carbon-based double-layer capacitors. The total specific capacitance of a composite composed of 90 wt.% AC and 10 wt.% RuO_x measured at 25 mV/s is about 62.8 F/g, which is increased up to ca. 111.7 F/g when RuO_x has been previously annealed in air at 200 °C for 2 h. The specific capacitance of RuO_x nanoparticles was promoted from 470 to 980 F/g by annealing in air at 200 °C for 2 h. The nanostructure of RuO_x was examined from the transmission electron microscopic (TEM) morphology.     

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.     

Development of lanthanum strontium cobalt ferrite perovskite electrodes of solid oxide fuel cells – A review

There is an enormous driving force in solid oxide fuel cells (SOFCs) to reduce the operating temperatures from high temperatures (800–1000 °C) to intermediate and low temperatures (400–800 °C) in order to increase the durability, improve thermal compatibility and thermal cycle capability, and reduce the fabrication and materials costs. One of the grand challenges is the development of cathode materials for intermediate and low temperature SOFCs with high activity and stability for the O₂ reduction reaction (ORR), high structural stability as well as high tolerance toward contaminants like chromium, sulfur and boron. Lanthanum strontium cobalt ferrite (LSCF) perovskite is the most popular and representative mixed ionic and electronic conducting (MIEC) electrode material for SOFCs. LSCF-based materials are characterized by high MIEC properties, good structural stability and high electrochemical activity for ORR, and have played a unique role in the development of SOFCs technologies. However, there appears no comprehensive review on the development and understanding of this most important MIEC electrode material in SOFCs despite its unique position in SOFCs. The objective of this article is to provide a critical and comprehensive review in the structure and defect chemistry, the electrical and ionic conductivity, and relationship between the performance, intrinsic and extrinsic factors of LSCF-based electrode materials in SOFCs. The challenges, strategies and prospect of LSCF-based electrodes for intermediate and low temperature SOFCs are discussed. Finally, the development of LSCF-based electrodes for metal-supported SOFCs and solid oxide electrolysis cells (SOECs) is also briefly reviewed.     

Microstructural factors of electrodes affecting the performance of anode-supported thin film yttria-stabilized zirconia electrolyte (∼1 μm) solid oxide fuel cells

The effects of the microstructural factors of electrodes, such as the porosity and pore size of anode supports and the thickness of cathodes, on the performance of an anode-supported thin film solid oxide fuel cell (TF-SOFC) are investigated. The performance of the TF-SOFC with a 1 μm-thick yttria-stabilized zirconia (YSZ) electrolyte is significantly improved by employing anode supports with increased porosity and pore size. The maximum power density of the TF-SOFCs increases from 370 mW cm⁻² to 624 mW cm⁻² and then to over 900 mW cm⁻² at 600 °C with increasing gas transport at the anode support. Thicker cathodes also improve cell performance by increasing the active reaction sites. The maximum power density of the cell increases from 624 mW cm⁻² to over 830 mW cm⁻² at 600 °C by changing the thickness of the lanthanum strontium cobaltite (LSC) cathode from 1 to 2-3 μm.     

Pulse power tests on nickel oxide electrodes for nickel—zinc electric vehicle batteries

In the development of non-sintered nickel oxide electrodes for NiZn electric vehicle (EV) batteries, maintaining adequate power performance was of particular concern. In the systems studied, the power output was limited by the nickel oxide electrodes. Simple pulse power tests were useful in characterizing the power performance in such cells. Although the cell impedance was not a simple resistance, the effective impedance at the end of a high rate discharge pulse had a resistive nature. This simplified the test procedures so that an accurate estimate of peak power could be obtained from one measurement. Measurements of the dependence on state of charge showed that the power output at 50% depth of discharge was representative of the power capability available during discharge.A method was devised to project power performance expected in a NiZn cell from NiCd cell tests. This was useful in testing the durabili     

Ideal pseudocapacitive performance of the Mn oxide anodized from the nanostructured and amorphous Mn thin film electrodeposited in BMP–NTf2 ionic liquid

A novel electrochemical route to prepare the Mn oxide with ideal pseudocapacitive performance is successfully proposed in this study. In n-butylmethylpyrrolidinium bis(trifluoromethylsulfony)imide (BMP–NTf₂) ionic liquid, a nanostructured and amorphous Mn film could be electrodeposited on a Ni substrate. The metallic Mn films were then anodized in Na₂SO₄ aqueous solution by the potentiostatic, galvanostatic, and cyclic voltammetric methods and transformed to Mn oxides. It was confirmed that the different anodization courses would cause variations in the material characteristics of the prepared Mn oxides and therefore in their pseudocapacitive performance. The Mn oxide anodized by the cyclic voltammetric method showed the most promising specific capacitance of 402 F g⁻¹; moreover, its capacitance retained ratio after 500 charge–discharge cycles was as high as 94%.     

Fabrication–microstructure–performance relationships of reversible solid oxide fuel cell electrodes–review

Abstract

A reversible solid oxide fuel cell system can act as an energy storage device by storing energy in the form of hydrogen and heat, buffering intermittent supplies of renewable electricity such as tidal and wave generation. The most widely used electrodes for the cell are lanthanum strontium manganate–yttria stabilised zirconia and Ni–yttria stabilised zirconia. Their microstructure depends on the fabrication techniques, and determines their performance. The concept and efficiency of reversible solid oxide fuel cells are explained, along with cell geometry and microstructure. Electrode fabrication techniques such as screen printing, dip coating and extrusion are compared according to their advantages and disadvantages, and fuel cell system commercialisation is discussed. Modern techniques used to evaluate microstructure such as three-dimensional computer reconstruction from dual beam focused ion beam–scanning electron microscopy or X-ray computed tomography, and computer modelling are compared. Reversible cell electrode performance is measured using alternating current impedance on symmetrical and three electrode cells, and current/voltage curves on whole cells. Fuel cells and electrolysis cells have been studied extensively, but more work needs to be done to achieve a high performance, durable reversible cell and commercialise a system.     

Performance of Pd-impregnated Sr1.9FeNb0.9Mo0.1O6-δ double perovskites as symmetrical electrodes for direct hydrocarbon solid oxide fuel cells

Symmetrical solid oxide fuel cell (SSOFC) is one of efficient ways to simplify preparation process, reduce manufacturing cost, and improve redox stability and reliability. Here, we report the performance of Sr-deficient Sr_1.9FeNb_0.9Mo_0.1O_6-δ (SFNM) double perovskites as symmetrical electrodes for direct-hydrocarbon solid oxide fuel cells and significant improvement of electrochemical performance. The SFNM exhibits good structural stability, suitable thermal expansion coefficient and highly chemical compatibility with Sm_0.2Ce_0.8O_1.9 (SDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM) electrolytes in both air and 5% H₂/Ar atmospheres. The area specific resistance of SFNM electrode is decreased by 3.6 and 8.4 times at 800 °C in air and H₂, respectively, as compared to the pristine Sr₂FeNbO_6-δ electrode. The electrochemical performance is further improved by introducing a small amount of Pd to form Pd-impregnated SFNM composite electrode (Pd-SFNM). The SSOFCs with Pd-SFNM after two-time impregnation treatments as the electrodes achieve impressive electrochemical performances in different fuels. The Pd-SFNM symmetrical electrode reveals good electrochemical stability operating on CH₄–CO₂ mixed gas.     

Boosting solid oxide electrolyzer performance by fine tuning the microstructure of electrodes – Preliminary study

This paper addresses the concept of adjusting the microstructure of the supports of solid oxide electrolyzers in order to boost performance. While several earlier studies focused on maximizing performance, reducing degradation, adjusting operating conditions and introducing new materials or designs of electrolysis stack, the current study addresses potential improvement of the cell by fine tuning the microstructure without any major redesign of existing cells. In the proposed approach, the study combined numerical simulations with definition of the model, which can aid in predicting the performance of a cell with adjusted porosity of the electrode support, and preparation of modified cells with alternative pore forming agents and two alternative sintering temperature levels. Supports of solid oxide cells were sintered at 1350 and 1400 °C with pore former content of 25–35%. This resulted in the porosity of supports being in the range of 47–54%. Cells with 10Sc1CeSZ and 8YSZ electrolytes were investigated in operando.The proposed approach makes it possible to quantitatively and qualitatively assess the potential gain when the reference cell is slightly modified, according to guidelines obtained from the model. It was found that the proposed method to fine tune the microstructure can result in improved performance, with a clear indication that adjusting the sintering temperature has a stronger effect on the microstructure of the support than increasing the pore forming agent.     

In situ EC-AFM study of effect of lignin on performance of negative electrodes in lead−acid batteries

The effect of lignin, which is an important additive for the negative electrode in lead–acid batteries, is studied on lead electrodes in sulfuric acid by means of potentiostatic transient measurements and in situ electrochemical atomic force microscope (EC-AFM) observations. During oxidation of the electrodes, it is confirmed that the current transition in electrolyte with 20 ppm lignin gives a broad, hill-like curve, while that in electrolyte without lignin is a sharp peak. Nevertheless, there is little difference in electrode capacity in each electrolyte throughout the whole oxidation. In electrolyte with lignin, in situ EC-AFM examination reveals a uniform deposition of lead sulfate crystals after oxidation of the electrode. These results suggest that lignin adsorbs on the electrode surface and promotes uniform diffusion of lead ions near the surface during oxidation.     

Synergistic Co–P bonding effect on hydrogen evolution reaction activity and supercapacitor applications of CoMoP material

Due to the synergistic effect between transition metals and hetero-atoms, transition metal-phosphide-based composites have been used as electrode materials for electrocatalytic hydrogen evolution reactions (HER) and supercapacitors, but their ideal performance has yet to be achieved. Herein, the binary transition metal phosphides, CoMoP and NiMoP, were grown on Ni-foam using a two-step hydrothermal approach and phosphorus deposition in a tube furnace. Both the CoMoP and NiMoP materials showed promising HER activity, displaying an overpotential of 137 mV and 144 mV @10 mA/cm², respectively. The theoretical studies demonstrated that the ΔG_H1 on CoMoP (?0.28 eV) was closer to zero than on NiMoP (?0.34 eV), due to the synergistic Co–P bonding, making it more accessible for H-adsorption, thereby endowing HER. In addition, the CoMoP and NiMoP materials exhibited 3507 F/g and 930 F/g energy storage capacities, respectively. Moreover, both samples had close to 100% Coulombic efficiency and about 50% capacitance retention. Based on these findings, it looks like CoMoP could be good for both the HER and supercapacitor electrodes.     

Performance of naphthalene thermosyphons with non-condensable gases – Theoretical study and comparison with data

Naphthalene thermosyphons are efficient heat transfer devices that operate within 250 and 400 °C. There is a lack of literature about naphthalene thermosyphons, especially with the presence of non-condensable gases (NCG). Thermal circuit resistance models, considering or not NCG, are developed. NCG–vapor flat front hypothesis is adopted. Condensation and evaporation heat transfer coefficients are obtained from literature correlations. Thermal resistance data provided from naphthalene thermosyphon charged with argon, is obtained using especial experimental setup. Two combinations of correlations provided good comparison with data, for thermosyphons with and without NCG. These models are successfully applied for heat exchanger design.     

Enhancing effect of MgO modification of Cu–Al spinel oxide catalyst for methanol steam reforming

Cu–Al spinel oxide (CA) as a sustained release catalyst has been successfully used in methanol steam reforming and it is highly required to improve its catalytic performance. Here, surface modification of the CA with various MgO loadings was performed. Characterization results showed that MgO dopant had strong interaction with the CA, resulting in a substantial change of the surface microstructure. Importantly, a small portion of lattice Cu²⁺ was phased out while partial Mg²⁺ cations incorporated into the spinel structure, giving rise to a variation of the cation distribution. Consequently, the change of the Cu²⁺ surrounding environment made it become hard to be reducible, thus the doped catalysts showed a lower copper releasing rate and smaller copper particles. Then, the activity and stability were enhanced when a suitable amount of MgO was highly dispersed. Excess amount of crystalline MgO gave rise to easy coking that resulted in an inferior catalytic performance.     

Performance and durability of Ni–Co alloy cermet anodes for solid oxide fuel cells

Ni alloys are examined as redox-resistant alternatives to pure Ni for solid oxide fuel cell (SOFC) anodes. Among the various candidate alloys, Ni–Co alloys are selected due to their thermochemical stability in the SOFC anode environment. Ni–Co alloy cermet anodes are prepared by ammonia co-precipitation, and their electrochemical performance and microstructure are evaluated. Ni–Co alloy anodes exhibit high durability against redox cycling, whilst the current-voltage characteristics are comparable to those of pure Ni cermet anodes. Microstructural observation reveals that cobalt-rich oxide layers on the outer surface of the Ni–Co alloy particles protect against further oxidation within the Ni alloy. In long-term durability tests using highly humidified hydrogen gas, the use of a Ni–Co cermet with Gd-doped CeO₂ suppresses degradation of the power generation performance. It is concluded that Ni–Co alloy cermet anodes are highly attractive for the development of robust SOFCs.     

Performance of Wells Turbine with Guide Vanes for Wave Energy Conversion

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A facile preparation of sulfur doped nickel–iron nanostructures with improved HER and supercapacitor performance

Since the use of diverse synthesis approaches can induce the variation in the density of active sites, which impacts electrocatalytic performance, the strategy utilized to fabricate the electrode materials for energy devices is just as important as the materials themselves. Herein, porous NiFe-oxide nanoflowers (NiFe-NFs) and macroparticles (NiFe-MPs) and corresponding S-doped NiFe-oxide nanoflowers (NiFeS-NFs) and macroparticles (NiFeS-MPs) were fabricated using facile co-precipitation and hydrothermal-sulfurization strategies, respectively. The prepared NiFe-NFs, NiFeS-NFs, NiFe-MPs, and NiFeS-MPs materials were investigated for their electrocatalytic HER in 1 M KOH electrolyte. The results indicated that NiFe-NFs displayed an overpotential of 177 mV @ 10 mA/cm² for HER, whereas the NiFe-MPs, having similar composition, exhibited a high HER overpotential of 187 mV @ 10 mA/cm². The enhanced HER catalytic performance of NiFe-NFs was attributed to the extensive exposure of active sites at the edges and vertices of nanocubes in the NFs-architecture. Moreover, after sulfurization, NiFeS-NFs and NiFeS-MPs demonstrated a considerable enhancement in their HER activity (54 mV and 152 mV @ 10 mA/cm², respectively) as compared to un-sulfurized materials, which can be attributed to the enhanced conductivity of materials after S-doping, as supported by theoretical studies. Further, the capacitance experiments showed a significant increment in specific capacitances of NFs and MPs after sulfurization, from 69 to 604 F/g and from 185 to 514 F/g, respectively. This work shows that morphological and compositional changes in metal oxide-based materials may considerably enhance their catalytic activity and capacitance.     

Sol–gel processed thin-layer ruthenium oxide/carbon black supercapacitors: A revelation of the energy storage issues

Hydrous ruthenium oxide/carbon black nanocomposites were prepared by impregnation of the carbon blacks by differently aged inorganic RuO₂ sols, i.e. of different particle size. Commercial Black Pearls 2000^® (BP) and Vulcan^® XC-72 R (XC) carbon blacks were used. Capacitive properties of BP/RuO₂ and XC/RuO₂ composites were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in H₂SO₄ solution. Capacitance values and capacitance distribution through the composite porous layer were found different if high- (BP) and low- (XC) surface-area carbons are used as supports. The aging time (particle size) of Ru oxide sol as well as the concentration of the oxide solid phase in the impregnating medium influenced the capacitive performance of prepared composites. While the capacitance of BP-supported oxide decreases with the aging time, the capacitive ability of XC-supported oxide is promoted with increasing oxide particle size. The increase in concentration of the oxide solid phase in the impregnating medium caused an improvement of charging/discharging characteristics due to pronounced pseudocapacitance contribution of the increasing amount of inserted oxide. The effects of these variables in the impregnation process on the energy storage capabilities of prepared nanocomposites are envisaged as a result of intrinsic way of population of the pores of carbon material by hydrous Ru oxide particle.     

Development of composite LaNi0·6Fe0.4O3-δ-based air electrodes for solid oxide fuel cells with a thin-film bilayer electrolyte

In this study the bilayer composite electrodes based on LaNi₀.₆Fe₀.₄O_3-δ (LNF) electronic conductor and Bi₂O₃-based electrolytes doped with Er (Bi₁.₆Er₀.₄O₃, EDB) and Y (Bi₁.₅Y₀.₅O₃, YDB) have been developed and their performance has been investigated in the dependence on the electrolyte content and sintering conditions. The polarization resistance of the optimized electrodes with electrolyte content of 50 wt % in the functional layer and with the LNF-EDB-CuO collector is in a range of 0.65–1.09 Ω cm² at 600 °C and 0.10–0.12 Ω cm² at 700 °C. The polarization characteristics of the Bi-based electrodes are compared with those for the composite electrodes based on LNF and Ce_0.8Sm_0.2O_1.9 (SDC). The developed electrodes have been tested in a SOFC mode in the anode-supported cells with a thin film electrolyte of YSZ/YDC (Y-doped zirconia/ceria). The single cells with such cathodes are shown to have performance characteristics that are several times higher than that for the cell with a standard platinum cathode. This is due to the optimized content and dispersity of the components; high conductivity of ionic and electronic constituents of the composite electrodes; greatly extended triple phase boundary (TPB) of the electrochemical reaction and advanced electrode design with collector providing uniform current distribution.