Differences in the effects of Co and CoO on the performance of Ni(OH)2 electrode in Ni/MH power battery

The effects of metallic cobalt (Co) and cobalt monoxide (CoO), as additives in positive electrodes, on the electrochemical performance of nickel/metal hydride (Ni/MH) power batteries are studied. Commercial Co and CoO are charged at 50 °C in 6 M KOH solution. The oxidation mechanism of cobalt materials is investigated by observing structural and morphological evolutions during charging. A pure Co₃O₄-type phase is formed when the starting material is CoO. When Co is used, a cobalt oxyhydroxide (CoOOH) phase is present, together with a tricobalt tetroxide (Co₃O₄) phase. In both cases, the cobalt concentration in the electrolyte decreases during oxidation. The final product is dependent on the solubility of cobalt and the kinetics of the reaction that consumes cobalt tetrahydroxide [Co(OH)₄]²⁻. The highly compact CoOOH phase, which works well between the nickel foam frame and nickel hydroxide [Ni(OH)₂] particles, enhances the power performance of Ni/MH power battery. The Co₃O₄ phase, which works well in connecting Ni(OH)₂ particles, improves the capacitive performance of Ni/MH power battery.     

Synthesis of γ-CoOOH and its effects on the positive electrodes of nickel batteries

High-valence (3.40) cobalt compounds were synthesized by a chemical precipitation (CP) method. Nickel hydroxide electrodes were then prepared using the synthesized compounds as conducting agents. The effects of high-valence cobalt additions on electrode properties, such as charge–discharge characteristics, electrode reaction reversibility, and cyclic voltammetric performance, were investigated by cyclic voltammetry, X-ray diffraction (XRD), and resistance measurement. The results demonstrate that the addition of high-valence cobalt helps to form a conductive network in the electrode, thus greatly increasing the cycling performance and simultaneously accomplishing an extremely high utilization of the active material in the positive electrode. For comparison, active materials of Co(OH)₂, β-CoOOH, and CoO were also included in the test. It was found that cobalt with a valence of 3.40 has a higher performance within one cycle. The utilization of the active material can reach 100.6% at a discharge rate of 0.2 C.     

Electrochromic characterization of Co(OH)2 thin film prepared by sol–gel process

Electrochemical, spectroscopic and structural measurements were used to characterize the electrochromic behavior and stability of sol–gel deposited Co(OH)₂ thin films. These films were prepared from polymeric solutions containing cobalt methoxyethoxide precursor by spin coating technique. The as-deposited films are amorphous and show crystalline structure after heat treatment at 450°C. Sol–gel-deposited cobalt hydroxide films show reversible electrochromic response in 1 M LiClO₄/ propylene carbonate solution beyond 500 cycles. The structural and chemical properties of the films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. Spectral transmittance change was T_p=29.9–60.2% for cobalt hydroxide films. It is argued that reversible lithium insertion capacity, good cyclic reversibility of Co(OH)₂ films make them suitable as counterelectrode layers in the electrochromic devices.     

Effect of precursor nature on the performance of palladium–cobalt electrocatalysts for direct methanol fuel cells

The performance of platinum-free palladium–cobalt catalysts in oxygen reduction was investigated for a direct methanol fuel cell. The dependence of catalytic activity on precursor nature was determined for two classes of precursors; namely, palladium chloride and palladium nitrate. The nitrate precursor exhibits much higher catalytic performance than the chloride precursor. X-ray absorption fine structure (XAFS) spectra indicate that the structure of palladium catalyst prepared from nitrate is much closer to Pd₃Co structure that can explain high catalytic activity. The MEA prepared from the nitrate catalyst achieved the peak power density of 125 mW cm⁻², which is much higher than 19 mW cm⁻² measured on the cell prepared from the chloride catalyst.     

Effect of cobalt electroless deposition on nickel hydroxide electrodes

The effects of cobalt additive on the positive electrode surface of nickel alkaline batteries are investigated. Electrode surface modifications by electroless cobalt deposits were made at different immersion times. The performance of nickel hydroxide electrodes was studied by optical techniques, such as scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and electrochemical methods as cyclic voltammetry, charge–discharge curves and electrochemical impedance spectroscopy (EIS). According to these results, electroless cobalt deposits obtained with 5 min of immersion time in the electroless-bath exhibit a better electrode performance.     

Porous nickel MCFC cathode coated by potentiostatically deposited cobalt oxide: II. Structural and morphological behavior in molten carbonate

Cobalt oxide was deposited on porous nickel by an electrodeposition technique as precursor of a novel MCFC cathode. The behavior of this cathode in molten (Li_0.52Na_0.48)₂CO₃ eutectics at 650 °C under an atmosphere of CO₂:air (30:70) was studied before and after 50 h of exposure by different techniques. Before the exposure, the deposit of cobalt corresponded to a Co₃O₄ thin layer of. This crystalline structure was identified by XRD and Raman spectroscopy. After its exposure in the eutectic melt a loss of cobalt was observed by XRD, Raman spectroscopy, XPS, EDS and ICP-AES. The change in the Co₃O₄ structure into lithium–cobalt–nickel oxide (LiCo_1−yNi_yO₂) was observed by Raman spectroscopy. The SEM micrographs for Co₃O₄-coated porous nickel showed different angular shapes with respect to porous Ni. The nickel solubility for the coated porous nickel, measured by ICP-AES, decreased with respect to uncoated nickel. The Co₃O₄-coated porous nickel cathode showed, after its immersion in the molten carbonate melt, a similar porosity but a higher pore size. LiCo_1−yNi_yO₂-coated NiO offers interesting features which combine the properties of nickel, lithium and cobalt in molten carbonate. This could be a promising novel MCFC cathode material.     

Synthesis of cobalt and nitrogen co-doped carbon nanotubes and its ORR activity as the catalyst used in hydrogen fuel cells

In hydrogen fuel cells, the sluggish oxygen reduction reaction (ORR) requires the catalysts used. Unfortunately, the precious platinum based catalysts still exhibit the best ORR activity in the commercial hydrogen fuel cells. Therefore, developing non-precious metal catalysts ORR become an important aspect for the utilization of hydrogen energy by using hydrogen fuel cells to develop non-precious catalysts and understand their active sites of ORR, herein the cobalt and nitrogen co-doped CNTs, nitrogen-doped CNTs and cobalt doped CNTs were prepared, respectively, and their catalytic properties toward ORR were tested and compared. The surface composition, microstructure and ORR performance of the samples were examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), pore/specific surface analyzer and electrochemical methods. The results demonstrate that as the catalyst, the cobalt and nitrogen co-doped CNTs owns the highest ORR limiting current density, the most positive ORR onset potential and the largest transfer electron number close to four, and thus exhibits the better ORR catalytic performance compared to the other two samples of the nitrogen-doped CNTs and the cobalt doped CNTs. The good ORR performance of cobalt and nitrogen co-doped CNTs can be attributed to its active sites of nitrogen containing functional groups, cobalt or cobalt oxides, Co-N_x structure, and the synergistic effect of these sites on ORR.     

Hydrogen trapping: Synergetic effects of inorganic additives with cobalt sulfide absorbers and reactivity of cobalt polysulfide

The biphasic product CoS₂ + Co(OH)₂ obtained by oxidation of cobalt sulfide is known to trap hydrogen at room temperature and low pressure according to a balanced reduction equation. Adding various inorganic compounds to this original absorber induces their reduction by hydrogen in the same conditions at a significant rate: (i) excess cobalt hydroxide is reduced to metallic cobalt; (ii) nitrate ions are reduced to ammonia; (iii) sulfur and sodium thiosulfate are reduced to H₂S or NaHS and Na₂S, respectively. Without a hydrogen absorber these inorganic compounds are not reduced by H₂, suggesting synergetic effects involving H₂ and the hydrogen absorber. Amorphous cobalt polysulfide, CoS₅, is also reduced by hydrogen at room temperature and releases H₂S gas. In the presence of a base to neutralize H₂S gas, the reaction rate is initially slower than with the CoS₂ + Co(OH)₂ mixture due to the higher stability of polysulfide chains but the H₂ trapping yield is improved, making CoS₅ a good candidate for H₂ trapping.     

Hydrogen production by steam reforming of ethanol over Co/CeO2 catalysts: Effect of cobalt content

The Co/CeO₂ catalysts obtained by co-precipitation method were used in the steam reforming of ethanol (SRE). The influence of cobalt active phase content (15–29 wt%), the reaction temperature (420–600 °C) and H₂O/EtOH molar ratio (12/1 and 6/1) were examined. The physicochemical characterization revealed that the cobalt content of the catalyst influences the metal-support interaction which results in catalyst performance in SRE process. The differences between catalytic properties of the Co/CeO₂ catalysts with different metal loading in SRE process decayed at 500 °C for H₂O/EtOH = 12/1. The best performance among the tested catalysts showed the 29Co/CeO₂ catalyst with the highest cobalt content, exhibiting the highest ethanol conversion, selectivity to two most desirable products and the lowest selectivity to by-products in comparison with catalysts containing smaller amount of metal. Its catalytic properties results probably from its unique physicochemical properties, i.e this catalyst contains large amount of cobalt but the metal crystallites are relatively small. Regardless cobalt content, an increase in the water-to-ethanol molar ratio in the feed increased the concentration of hydrogen an carbon dioxide and decreased formation of carbon monoxide, acetone, aldehyde and ethylene.     

Electrochemical recycling of cobalt from cathodes of spent lithium-ion batteries

In this work, cobalt from spent cellular telephone Li-ion batteries was recovered by electrochemical techniques. According to X-ray diffraction results, the composition of the positive electrode is LiCoO₂, Co₃O₄, C, and Al. The largest charge efficiency found was 96.90% at pH 5.40, potential applied of −1.00 V and a charge density of 10.0 °C cm⁻². The charge efficiency in the electrochemical recycling of cobalt decreases with the decrease in pH. The energy dispersive X-ray analysis (EDX) measurements of the electrodeposits showed that the surface is constituted of 100% cobalt. Scanning electron microscopy (SEM) showed a three-dimensional nucleus growth.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

Insights into the unique role of cobalt phosphide for boosting hydrogen evolution activity based on MIL-125-NH2

Controllable and efficient photo-catalyst is of particular importance not only for improving photo-catalytic activity of hydrogen evolution, but also for achieving more excellent performance of semiconductors in practical applications. Here two n-type semiconductors (cobalt phosphide (CoP) and MIL-125-NH₂) as well as their complex catalysts (CoP/MIL-125-NH₂) are reported and the unique role of cobalt phosphide in CoP/MIL-125-NH₂ is systematically investigated for boosting hydrogen evolution activity based on MIL-125-NH₂ under visible light-driven. CoP/MIL-125-NH₂ can exhibit a 267-fold enhanced catalytic activity compared to the pristine MIL-125-NH₂. Moreover, the excellent catalytic property of the as-prepared catalysts has been investigated in detail by several means of characterization. The findings are that the distinctive morphology and relatively ideal S_BET of MIL-125-NH₂ play particular role in the highly efficient photo-catalytic reaction system, and the highly efficient catalytic property of CoP/MIL-125-NH₂ mainly depends on unique role of cobalt phosphide having excellent photo-catalytic performance for hydrogen evolution. Based on the above results, the possible mechanism of photo-catalytic hydrogen evolution is speculated.     

Cobalt (II) salts,performing materials for generating hydrogen from sodium borohydride

Hydrogen generation through sodium borohydride (NaBH₄) hydrolysis has attracted much attention. This reaction has to be catalyzed by metal-based materials. We studied the catalytic potential of cobalt (II) and (III) salts. Some of them have never been studied, and compared to e.g. cobalt nanoparticles or powder, and cobalt borides. CoCl₂ showed the best performance. In our opinion, CoCl₂ should not be dismissed from the large number of catalysts. One could conceive portable applications using CoCl₂; this is briefly discussed. CoCl₂ was compared to both commercial cobalt boride and in-situ formed (through our hydrolysis conditions) cobalt boride. Their hydrogen generation rates were 86.3, 1.0 and 1.6 L min⁻¹ g⁻¹(Co), respectively. The hydrogen generation rate of CoCl₂ is one of the highest ones reported so far. It is assumed that cobalt boride surface evolves during the reaction and depends on the hydrolysis medium features. Further studies are required to fully explain the complex reaction mechanisms.     

Electrodeposition of cobalt from spent Li-ion battery cathodes by the electrochemistry quartz crystal microbalance technique

Information about the cobalt electrodeposition mechanism at different pH values was obtained using an electrochemistry quartz crystal microbalance (EQCM) technique as well as potentiodynamic and potentiostatic techniques. Potentiodynamic and potentiostatic electrodeposition of ionic cobalt at pH 5.40 occurs via a direct reduction mechanism. The mass/charge relation was found to be 33.00 g mol⁻¹. At pH 2.70, electrodeposition under potentiodynamic conditions occurs via a mechanism of cobalt reduction with the formation of adsorbed hydrogen. Potentiostatic analysis verified that cobalt reduction occurs simultaneously via direct reduction and with the formation of adsorbed hydrogen. The ratio mass/charge (M/z) is 13.00 g mol⁻¹ for potentiodynamic conditions and 26.00 g mol⁻¹ for potentiostatic conditions and potentiodynamic conditions. The cobalt electrodissolution occurs directly to Co²⁺ in pH 2.7 and through of the intermediary Co⁺ that is oxidized to Co²⁺ in pH 5.4.     

Sustainable production of hydrogen via steam reforming of furfural (SRF) with Co-catalyst supported on sepiolite

Natural sepiolite is a mineral very abundant in Spain, being the main producer worldwide. This material is presented by the first time as promised support to prepare cobalt catalysts with high activity and hydrogen selectivity in the steam reforming of furfural (SRF). Two methods for cobalt incorporation on natural sepiolite, precipitation (PP) and incipient wetness impregnation (IWI), are explored and their influence in the preparation of catalysts highly active and selective in the SRF is studied. PP method makes possible to synthesize a catalytic material with high catalytic performance (high conversion of furfural, > 95%, and high hydrogen selectivity, 70%). Furthermore the production of non-desired byproducts such as CO, CH₄ and acetone, decrease significantly in the catalyst prepared by PP (CoSep). Characterization by N₂ adsorption-desorption to determine surface area, XRD, TPR and TEM shows that PP method favors the formation of smaller metallic particles of Co with a high dispersion. These differential physicochemical properties seem to explain the better catalytic performance exhibited by the sepiolitic catalyst prepared by the PP method (CoSep). In addition, this catalyst exhibits also a higher resistance to deactivation after 24 h of reaction time. This fact seems to be related to the lower coke deposition on the catalyst prepared by precipitation and particularly, with the lower sinterization of the metallic Co particles during the steam reforming of furfural.     

Boosted methane dry reforming for hydrogen generation on cobalt catalyst with small cerium dosage

Metal-support interface influences the catalytic activity and physical properties of heterocatalysts dramatically. Herein, the effect of cerium on material properties and catalytic activity of cobalt over gamma-alumina applied in dry reforming of methane (DRM) was investigated. The dispersion of cobalt over gamma-alumina was noticeably improved with low cerium dosages ranging from 0.1 to 0.5 wt%. In addition, the presence of Ce promoter on catalyst surface led to an enhancement in reducibility of cobalt oxide to cobalt metal in the catalyst activation. Using CO₂-temperature programmed desorption technique, the catalyst basicity was found to increase proportionally with cerium loading. At an optimal dosage of 0.3–0.5 wt%, the cerium promoted cobalt supported on gamma catalyst displayed outstanding performance in DRM with noticeable conversion improvements up to 11% in both methane and carbon dioxide.     

Valorization of alcoholic wastes from the vinery industry to produce H2

This paper focuses on the development of active and stable catalysts for the steam reforming of alcoholic wastes. Two catalysts with high activity in the steam reforming of ethanol have been studied in the steam reforming of an alcoholic waste from the vinery industry, as previous step to their industrial scale up. The catalysts are based on cobalt supported on Zn-hydrotalcite-derived material and natural sepiolite. At laboratory level, the catalytic material based on natural sepiolite showed the best catalytic performance maintaining its catalytic activity for more than 160 h in presence of 50 ppm of sulfur contained in the alcoholic waste. Thus, sepiolite based catalyst was scaled up to prepare a first generation of catalytic monoliths. The reforming activity of this first generation of monoliths was found lower than their corresponding powdered catalyst. The high calcination temperature (1573 K) used in the manufacture of the first generation of monoliths was the responsible of their low performance, which was related to the sintering of cobalt catalyst in a larger crystallite size of metallic cobalt. A second generation of monoliths was prepared using a lower calcination temperature (873 K). Now, the monoliths exhibited a high catalytic performance, similar to the powdered catalyst. The excellent results obtained with the second generation of monoliths have been protected under an invention patent (E201731077). This is the first time that catalytic monoliths based on natural sepiolite promoted with Co are successfully manufactured and tested in the steam reforming of alcoholic wastes from the distillery industry to produce hydrogen.     

Evidence for H2S gas as an intermediate species in the reaction mechanism of trapping hydrogen by cobalt disulfide

Cobalt sulfide prepared by aqueous precipitation using Na₂S and a Co(II) salt is known to trap hydrogen at room temperature and low pressure. The importance of oxidation of the primary CoS precipitate with atmospheric oxygen with respect to its efficiency as a hydrogen absorber is demonstrated. This stage of oxidation produces a mixture of two solid phases: a partially crystallized cobalt hydroxide Co(OH)₂ and an amorphous cobalt sulfide CoS₂ with a Co(OH)₂/CoS₂ molar ratio of 1 as predicted by thermodynamics. This biphasic product is probably the basic cobalt sulfide CoSOH considered in older and even more recent work. This product traps molecular hydrogen with a H₂/Co molar ratio of 0.5 whereas unoxidized CoS precipitate traps almost no hydrogen (H₂/Co = 0.025). Moderate acidic treatment of the absorber at room temperature leads to the selective dissolution of Co(OH)₂. The remaining cobalt sulfide has CoS₂ stoichiometry and reacts with hydrogen to form H₂S gas and CoS. We showed that H₂S released is reactive toward bases: CoS or Na₂S were formed when H₂S reacted with Co(OH)₂ or NaOH, respectively. This proves that the hydrogen trapping reaction mechanism implies H₂S as an intermediate species.     

Porous CoO-CeO2 heterostructures as highly active and stable electrocatalysts for water oxidation

We report a CeO₂ incorporated cobalt oxide electrode with abundant nano-sized interfaces. The as-prepared heterostructures exhibit remarkable oxygen evolution reaction (OER) activity and excellent stability, surpassing the pristine cobalt oxide and the state-of-art RuO₂. Further DFT calculations and experimental results indicate that the improved OER performance of CoO-CeO₂ heterostructures originates from the hierarchically porous structure, large surface area and high electron density around Co sites. The armouphous phase is formed during OER test due to surface Ce leaching, which can further promoted the performance of the hybrids. This work elucidates the cooperative effects and in situ transformation of CeO₂-incorporated cobalt oxide in water oxidation, providing a valuable guideline for designing electrocatalysts.     

Hydrogenation of carbon monoxide over cobalt nanoparticles supported on carbon nanotubes

This paper describes a catalytic reaction of hydrogen and carbon monoxide (Fischer-Tropsch synthesis (FTS)) over carbon nanotubes (CNTs) supported cobalt nanoparticles. We have investigated the effect of calcination of the catalysts on FTS performance using X-ray diffraction (XRD), H₂ chemisorption, temperature programmed reduction (TPR), temperature programmed oxidation (TPO), and transmission electron microscopy (TEM) techniques. With the increase of outer diameter of CNTs, specific surface area of the catalyst decreases while Co particle size increased accompanying with a decrease in CO conversion. The FTS performance is similar for samples calcined in N₂ or air at temperature below 550 °C. Over 550 °C, the results are much different in that the Co/CNTs can keep its activity due to the unchanged CNTs structure in N₂ while the Co/CNTs almost lose activity owing to the loss of CNTs structure and sintering of cobalt oxide clusters in air.