Amorphous CoO Research Articles

Enrichment of Active Hydrogen at Amorphous CoO/Cu2O Heterojunction Interfaces Enhances Electrocatalytic Nitrate Reduction to Ammonia.

The reduction of nitrate into valuable ammonia via electrocatalysis offers a green and sustainable synthetic pathway for ammonia. The electrocatalytic nitrate reduction reaction (NO3RR) encompasses two crucial reaction steps: nitrate deoxygenation and nitrite hydrogenation. Notably, the nitrite hydrogenation reaction is regarded as the rate-determining step of the process. Herein, the amorphous CoO support introduced for the construction of the a-CoO/Cu2O tandem catalyst provides sufficient active hydrogen and synergistically catalyzes the NO3RR. The a-CoO/Cu2O catalyst showed excellent performance with a maximum NH3 Faradaic efficiency of 95.72% and a maximum yield rate of 0.96mmol h-1 mgcat -1 at -0.4V. In the flow cell, the maximum NH3 yield rate of 12.14mmol h-1 mgcat -1 is achieved at -800mA. The high NO3RR activity of a-CoO/Cu2O is attributed to the synergistic cascade effect of amorphous CoO and Cu2O at the heterojunction interface, where Cu2O serves as the adsorption site for NO3 -, while the accelerated active hydrogen generation of amorphous CoO promotes the nitrite hydrogenation reaction. This work provides a strategy for designing multi-site cascade catalysts centered on amorphous structures to achieve efficient NO3RR.

Patterns of Formation of Binary Cobalt–Magnesium Oxide Combustion Catalysts of Various Composition

In order to establish the formation patterns of the Co–Mg oxide system, samples with different Co:Mg ratios and heat treatment temperatures were synthesized and studied. A study of the samples confirmed the phase transition of MgxCo2–xO4 spinels into the corresponding solid solutions at 800–900 °C. The similarity of the formation patterns for different compositions is shown. The rocksalt oxide in low-temperature samples is an anion-modified paracrystalline phase that forms a “true” solid solution only upon spinel decomposition. The TPR profiles of the decomposed Co3O4 spinel show surface Co3O4 peaks and a wide peak corresponding to the well-crystallized CoO, while partial Co3O4 TPR up to 380 °C results in dispersed and amorphous CoO. The high-temperature non-stoichiometric samples are poorly reduced, indicating their low oxygen reactivity. Spinel reoxidation after heat treatment to 1100 °C by calcination at 750 °C showed complete regeneration for MgCo2O4–Co3O4 samples and its absence in case of an excess of MgO relative to stoichiometry.

Steering surface reconstruction of hybrid metal oxides for efficient oxygen evolution reaction in water splitting and zinc-air batteries

Surface reconstruction yields real active species in electrochemical oxygen evolution reaction (OER) conditions; however, rationally regulating reconstruction in a targeted manner for constructing highly active OER electrocatalysts remains a formidable challenge. Here, an electrochemical activation strategy with selective etching was utilized to guide the reconstruction process of a hybrid cobalt-molybdenum oxide (CoMoO4/Co3O4@CC) in a favorable direction to improve the OER performance. Both in-situ Raman and multiple ex-situ characterization tools demonstrate that controlled surface reconstruction can be easily achieved through Mo etching, with the formation of a dynamically stable amorphous-crystalline heterostructure. Theoretical calculations together with experimental results reveal that the synergistic effects between amorphous CoOOH and crystalline Co3O4 are crucial in enhancing the catalytic performance. Consequently, the reconstructed CoMoO4/Co3O4@CC exhibits a low overpotential of 250 mV to achieve a current density of 10 mA cm−2 in 1 M KOH, and more importantly it can be practiced in electrolytic water splitting and rechargeable zinc-air batteries devices, achieving ultra-long stability for over 500 and 1200 h, respectively. This work provides a promising route for the construction of high-performance electrocatalysts.

Unveiling the charge storage mechanisms of Co-based perovskite fluoride in a mild aqueous electrolyte.

This study is an in-depth exploration of the charge storage mechanisms of KCoF3 in 1 M Na2SO4 mild aqueous electrolytes via an array of ex situ/in situ physicochemical/electrochemical methods, especially the electrochemical quartz crystal microbalance (EQCM) technique, showing a combination of conversion, insertion/extraction and adsorption mechanisms. Specifically, during the first charge phase, Co(OH)2 is formed/oxidized into amorphous CoOOH and Co3O4, and then CoOOH undergoes partial proton extraction to yield CoO2, which is simultaneously accompanied by the transformation of Co3O4 into CoOOH and (hydrated) CoO2. During the first discharge process, the partial insertion of H+ into (hydrated) CoO2 leads to the formation of CoOOH and Co3O4, with the conversion of Co3O4 into CoOOH and both Co3O4 and CoOOH undergoing further transformations into (hydrated) Co(OH)2via the insertion of H+. This work offers valuable references for the development of aqueous energy storage.

Crystalline-amorphous Co/Co3O4 derived from a low-temperature etching of ZIF-67 for oxygen-invovled catalytic reactions

Development of a simple method for synthesizing transition metal-based bifunctional catalysts is of great importance in the context of oxygen-involved electrochemical energy conversion and storage devices. In this effort, electrochemical catalysts consisting of amorphous cobalt and cobalt oxides on graphene oxide (GO) were fabricated using a solvothermal process followed by etching ZIF-67 with oxygen-rich functional groups on GO (≈ 29 at. %) in a weak reduction atmosphere at 400 ℃. During the calcination process forming the ZIF-67@GO-based materials, exposed cobalt metal centers are created by partial vaporization and oxidation of ZIF-67. Also, metal ions are reduced to form Co and partially combine with oxygen from GO to generate amorphous Co3O4. Moreover, the porous framework structure of ZIF is retained under the mild calcination temperatures used. Benefiting from their porous framework and heteroatoms-doped composition, the ZIF-67@GO-based materials have low overpotentials of 319 mV at 10 mA cm−2 and high catalytic activities toward the oxygen evolution and oxygen reduction reactions in an alkaline electrolyte. In addition, the liquid zinc-air battery assembled with the ZIF-67@GO composite produced at a calcination temperature of 400 ℃ displays a high performance. Overall, the results demonstrate that the ZIF-67@GO composites have the potential to be high-performing catalysts in energy conversion devices. The readily controllable and high-yielding calcination method in this study is expected to serve as a model for new strategies to synthesize low-cost and efficient bifunctional electrocatalysts.

Giant exchange bias by tuning Co/CoO core/shell structure

In this report, the exchange-bias effect of Co/CoO core/shell nanoparticles with the amorphous CoO shell has been investigated. At low temperatures, the developed spin-glass state with net magnetic moments in the CoO shell provides a unidirectional anisotropy in the Co core via exchange coupling, leading to a giant exchange bias field up to 3.6 kOe. By improving the crystallinity of CoO shell by performing surface oxidation at an elevated temperature, the spin-glass ordering is suppressed by the antiferromagnetic ordering, resulting in a significantly lower exchange-bias field (2.2 kOe) and a prominent training effect due to the existence of multiple easy axes in the multi-crystalline CoO shell. Theoretical studies confirm that the emerging spin-glass state in the CoO shell has a significant impact on the exchange bias of the system. These new findings provide a novel approach toward manipulating and fine-tuning the exchange bias effects in nanoscale systems for spintronics applications.

Co2P/CoP quantum dots surface heterojunction derived from amorphous Co3O4 quantum dots for efficient photocatalytic H2 production

Amorphous/crystalline heterostructures show excellent potential in the hydrogen evolution reaction (HER) as they can significantly facilitate surface adsorption and redox reactions. Herein, a unique amorphous Co2P/crystalline CoP quantum dots (Co2P/CoP QDs) Type-II surface heterojunction was derived from amorphous Co3O4 QDs via phosphorization. The intimate contact between Co2P QDs and CoP QDs was conducive to charge transfer, thereby promoting surface reaction kinetics. The unique structure and properties were beneficial to providing more active sites and controlling the electronic structures thus making amorphous/crystalline composites show superior photocatalytic hydrogen (H2) production performance. Additionally, the amorphous Co2P QDs had a plethora of unsaturated bonds and abundant defects; the disordered structure led to increased active sites that promoted surface reaction kinetics. Due to the synergistic effect of the quantum confinement of QDs and the surface heterojunction, the charge transfer efficiency of Co2P/CoP QDs was extremely high, and high H2 evolution activity and photostability were achieved. The maximum H2 generation rate over the Co2P/CoP QDs composite reached 11.88 mmol h−1 g−1 with an apparent quantum efficiency (AQE) of 3.88 % at 420 nm, which is roughly 20-times that of the pure Co3O4 QDs. In addition, high photostability was realized; even the photocatalyst that stood for a week reached initial photoactivity. This work offers a novel idea for reasonably establishing amorphous/crystalline photocatalysts to achieve efficient H2 evolution.

One-dimensional hierarchical porous carbon nanofibers with cobalt oxide in a hollow channel for electrochemical applications

One-dimensional hierarchical porous carbon nanofibers (CNFs) embedded with Co3O4 nanoparticles in a hollow channel (PPMCo) are fabricated by coaxial electrospinning followed by thermal treatment. The degree to which the CNF surface is exposed to Co3O4 nanoparticles was controlled by the cobalt(II) acetate concentration. The well-controlled structure of PPMCo with porous structure, heteroatoms, and amorphous Co3O4 nanoparticles provided fast ion transport and large reaction surface area, resulting in effective ion migration to the active site and a high rate capacity of the electrode. Benefitting from the unique structure, the PPMCo supercapacitor electrodes displays a high specific capacitance of 188 Fg−1 at 1 mAcm−2, rate capability of 82% when the current density is increased from 1 to 20 mAcm−2, and cycling stability of 93% for 10,000 cycles. The good capacitive performance of the PPMCo electrode is attributed to the synergistic effect of the hierarchical porosity, electroactive material of Co3O4, high effective surface area, and polar effects by heteroatoms.

Superhydrophilic/Superaerophobic Hierarchical NiP2@MoO2/Co(Ni)MoO4 Core–Shell Array Electrocatalysts for Efficient Hydrogen Production at Large Current Densities

Rationally constructing low-cost, high-efficiency, and durable electrocatalysts toward the hydrogen evolution reaction at large current densities is imperative for water splitting, especially for large-scale industrial applications. Herein, a hierarchical core-shell NiP2@MoO2/Co(Ni)MoO4 cuboid array electrode with superhydrophilic/superaerophobic properties is successfully fabricated and the formation mechanism of the core-shell structure is systematically investigated. Through an in situ partially converted gas-solid reaction during the phosphating process, Ni and Co elements are leached and rearranged to form NiP2 particles and amorphous CoO as the shell layer and the inner undecomposed Co(Ni)MoO4 crystals serve as the core layer. Because of its seamless core-shell structure and superhydrophilicity/superaerophobicity of hierarchical cuboid arrays, NiP2@MoO2/Co(Ni)MoO4 exhibits superior HER activity in 1 M KOH with only an overpotential of 297 mV to deliver 1000 mA cm-2 and can work steadily for 650 h at 200 mA cm-2. Remarkably, when coupled with NiFe LDH for overall water splitting, it can drive an AA battery with an ultralow cell voltage of 1.49 V to deliver 10 mA cm-2. This work sheds new light on designing large-current-density efficient HER electrocatalysts for large-scale industrial applications.

In-situ transformation constructs CoTe/Co/CoO nanosheet arrays with rich grain boundaries to enhance electrochemical performance

Development of high conductivity and amorphous structure is at great significance in electrochemical storage applications. Two-dimensional nanosheet array has a large surface area and abundant active sites, showing great potential in the field of energy storage. Among many materials, Co-based oxides or hydroxides have high theoretical specific capacity and are widely used as electrode materials for hybrid supercapacitors, but low electrical conductivity inhibits their development. Here, Co(OH)2 nanosheet arrays are randomly transformed into CoTe, metallic Co, and CoO composite (CoTe/Co/CoO) during the hydrothermal process. The CoTe/Co/CoO nanosheets have abundant grain boundaries due to the multi-component effect. Thanks to the high elasticity of amorphous CoO, and the high conductivity of CoTe and Co, CoTe/Co/CoO composite nanosheet arrays exhibit excellent electrochemical performance, such as high capacity (501 C g−1 at 5 mA cm−2) and excellent cycle stability (95.2% capacity retention after 4000 cycles). Moreover, asymmetric supercapacitors are assembled with CoTe/Co/CoO as positive electrode and activated carbon as negative electrode, the developed device delivers a maximum energy density of 35.29 Wh Kg−1 with superb cycling stability.

Direct growth of three-dimensional nanoflower-like structures from flat metal surfaces.

Here we report on a facile top-down approach for the direct growth of Co3O4 hierarchical nanoflowers from a bulk Co surface via chemical etching and thermal annealing. The effect of the annealing temperature was investigated, showing that amorphous Co3O4 was formed at 250 °C, while crystalline Co3O4 with notable oxygen vacancies was created at 550 °C. The formed 3D nanostructures exhibited excellent oxygen evolution reaction (OER) activities with a low overpotential of 0.34 V at 10 mA cm-2 and high durability. The proposed novel approach was further demonstrated by the direct growth of 3D NiO and CuO nanostructures on Ni and Cu substrates.

Amorphous three-dimensional porous Co3O4 nanowire network toward superior OER catalysis by lithium-induced

Co3O4 has been proven to be a potential catalyst for oxygen evolution reaction (OER) because of its low cost and abundant resources. However, pure Co3O4 has the disadvantages of poor conductivity and low catalytic activity. Here, we successfully synthesized three-dimensional (3D) network Co3O4 composed of nanowires grown on nickel foam (NF). Using a lithium-ion rocking chair transport mechanism, Co3O4 nanowires were transformed into porous amorphous Co3O4, which greatly reduces the resistance in the OER process and enhances active site exposure. Meanwhile, the increase of Co2+ greatly improved the OER performance. However, excessive cycling of lithium-ion batteries can form a thick solid electrolyte interface (SEI) on the surface, which has a negative impact on OER. Specifically, we found that 5-cycle Co3O4 has the best OER performance, showing an initial potential of 1.42 V vs RHE, a reduced overpotential of 311 mV at 50 mA cm−2 and a low Tafel slope of 76 mV decade−1, which is better than commercial RuO2.

2D Amorphous CoO Incorporated g‐C3N4 Nanotubes for Improved Photocatalytic Performance

CoO nanomaterials have been identified as a potential visible‐light‐driven photocatalyst. However, they suffer from the rapid deactivation derived from the aggregation and thermally induced oxidation of CoO. Herein, a facile and scalable strategy is proposed to effectively inhibit the aggregation and oxidation of CoO via the construction of a 2D/2D amorphous CoO (a‐CoO)/g‐C3N4(CN) nanotubular heterojunction. The synthesized 2D/2D CN/a‐CoO heterojunction shows appreciably improved activity toward rhodamine B (RhB) degradation and no deactivation is detected even after four consecutive photodegradation cycles. Moreover, the optimized 2D/2D CN/a‐CoO heterojunction exhibits hydrogen evolution rate of 428.8 μmol h−1 g−1, which is 5.9 times than that of CN under visible‐light irradiation. The superior photocatalytic performance is attributed to the large interface between 2D components, the amorphous structure of disordered a‐CoO, the multiple scattering in the tubular heterojunction, and the efficient spatial charge‐separation through CoN bond due to the synergistic coupling effects between CN and a‐CoO. This work is helpful to promote the application of a‐CoO in photocatalysis while the unique 2D/2D heterojunction could be the fundamental basis for emerging applications in energy‐related and beyond.

Vacancies and phosphorus atoms assembled in amorphous urchin-like Co3O4 for highly efficient overall water splitting

Designing highly efficient and low-cost electrocatalysts is essential for water splitting. Herein, urchin-like Co3O4 microspheres are firstly grown on nickel foam by a hydrothermal method, then Oxygen vacancies, phosphorus doping are effectively assembled in Co3O4 electrocatalysts. The introduction of oxygen vacancies and phosphorus doping will adjust the electronic structure of Co which increase the intrinsic catalytic activity and improve the adsorption energy of intermediates, simultaneously, progressively transform the crystal into randomly arranged atoms structure with short range order resulting in more active sites participate in the catalytic reaction. Moreover, the catalyst of vacancies Co3O4-Ov and phosphorus doping Co3O4–P demonstrate excellent performance in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media, Co3O4-Ov sample served as anode while Co3O4–P as cathode to form an electrolytic cell needs only 1.58 V to reach 20 mA cm−2 for overall water splitting.

Rapid synthesis of amorphous CoO nanosheets: Highly efficient catalyst for parachlorophenol degradation by peroxymonosulfate activation

Amorphous CoO nanosheets (CoO-A) were prepared in a short time and large scale for the first time via NaBH4 reduction, and applied as a catalyst for PMS activation to remove parachlorophenol (4-CP). Approximately 100% of 4-CP (10 mg/L) was degraded by PMS (0.25 mM) activated with CoO-A (0.03 g/L) with the kinetic constants of 0.428 min−1, which was 5.7 and 214.0 times higher than crystalline CoO/PMS and Co3O4/PMS systems, respectively. The superior catalytic performance was ascribed to the synergy of large surface area and amorphous structure of CoO nanosheets. Moreover, the effects of water matrix species and reaction parameters on 4-CP degradation were systematically investigated. The hydroxyl radicals and sulfate radicals were detected and involved in CoO-A/PMS system for 4-CP oxidation, and sulfate radicals made the primary contribution. The redox circles of Co2+/Co3+ on CoO-A surface were mainly responsible for the catalytic activation of PMS, and the possible activation mechanism and degradation pathway were proposed.

Amorphous Co3O4 quantum dots hybridizing with 3D hexagonal CdS single crystals to construct a 0D/3D p-n heterojunction for a highly efficient photocatalytic H2 evolution.

Herein, a novel amorphous monodisperse Co3O4 quantum dots/3D hexagonal CdS single crystals (0D/3D Co3O4 QDs/CdS) p-n heterojunction was constructed by a simple hydrothermal and electrostatic self-assembly method. The amorphous monodispersed Co3O4 QDs (≈4.5 nm) are uniformly and tightly attached to the surface of the hexagonal CdS single crystals. The sample, 0.5% CQDs/CdS exhibits outstanding hydrogen evolution activity of 17.5 mmol h-1 g-1 with a turnover number (TON) of 4214, up to 10.3 times higher than that of pure CdS. The enhanced photocatalytic activity can be attributed to the synergistic effect of the p-n heterostructure and the quantum confinement effect of Co3O4 QDs, which significantly promoted the separation efficiency of photo-generated electrons and holes. Additionally, the sulfur vacancy also can act as electron trappers to improve carrier separation and electron transfer. The photoelectrochemical and time-resolved fluorescence (TRPL) results further certify the effective spatial charge separation. This work gives an insight into the design of the 0D/3D Co3O4 QDs/CdS p-n heterostructure for a highly efficient photocatalysis.

Cobalt coordination controlled carbon nanospheres formation and inclusion of amorphous Co3O4 and AuNPs: strongly enhanced oxygen evolution reaction with excellent mass activity.

Carbon nanospheres integrated with AuNPs and amorphous Co3O4 were fabricated by making use of cobalt coordination with AuNP surface ligands, which exhibited an enhanced oxygen evolution reaction (OER) with excellent mass activity. Co2+ coordination with AuNP surface functional molecules significantly influenced the nanostructure formation and OER activity. Nanospheres of carbon with an optimum concentration of AuNPs and Co3O4 (2) showed strong OER activity. 2 exhibited a high current density (358 mA cm-2 at an applied potential of 1.59 V) and required a low overpotential (256 mV) to generate a geometric current density (10 mA cm-2) compared to commercial RuO2 (363 mV). Importantly, 2 showed high mass activity (1352.5 mA mg-1), 14 times higher than RuO2 (93.87 mA mg-1). The low Tafel slope (52.4 mV dec-1) and charge transfer resistance along with large double layer capacitance (Cdl = 20.1) of 2 suggest strong electronic communication between the catalyst and the electrode surface and facilitated fast charge transport. Chronoamperometric studies confirmed the excellent stability of the catalyst. The present work demonstrates that the electrocatalytic activity of earth-abundant amorphous metal oxides can be strongly enhanced by integrating metallic nanoparticles (NPs) and optimizing nanostructures.

Amorphous CoOx-Decorated Crystalline RuO2 Nanosheets as Bifunctional Catalysts for Boosting Overall Water Splitting at Large Current Density

Developing efficient bifunctional water electrolysis catalysts applied at large current density is desirable. Herein, an amorphous CoOx-decorated crystalline RuO2 nanosheet catalyst self-supported ...

Nanoporous electrodes of phase-dealloyed Fe83.3-xCoxSi4B8P4Cu0.7 (x = 4, 10 and 20 at.%) precursors with superior Redox performances and high stabilities

Co substitutions for Fe83.3Si4B8P4Cu0.7 precursors are effective to stabilize the nanoporous structures during dealloying in H2SO4 solutions and improve Redox performances of nanoporous electrodes in alkaline conditions. Heterogeneous nanocrystalline structures of Co-substituted Fe83.3-xCoxSi4B8P4Cu0.7 (x = 0, 4, 10 and 20 at.%) alloys consist of two phases: α-Fe(Co) phases and continuously distributed residual amorphous phases. The nanopores on as-dealloyed Fe-Co-Si-B-P-Cu alloys are smaller in size with higher Co concentrations. This is ascribed to the finer α-Fe(Co) phases distributed in the precursor alloys after nanocrystallization. Nanoporous architectures after dealloying inherit the microstructural characteristics of Fe-Co-Si-B-P-Cu precursor alloys. The short-term and long-term CV curves of nanoporous Redox electrodes in 0.5 M KOH solution demonstrate that Redox reactions have been enhanced by partial substitution of Fe by Co and the best Redox performance can be obtained on as-dealloyed Fe79.3Co4Si4B8P4Cu0.7 alloys because of the highest Redox peak current density of 90 mA cm−2 after 150 CV cycles. However, the stabilities of nanoporous Fe79.3Co4Si4B8P4Cu0.7 electrodes have a predictable decline due to the overgrowth of Fe3O4 octahedra and destruction of the ligaments during long-term Redox cycling. On the other hand, the better Redox stabilities has achieved on as-dealloyed Fe63.3Co20Si4B8P4Cu0.7 alloys with stable oxidation peak current densities of 45–47 mA cm−2 from 950 to 3000 CV cycles, about 2.3 times the nanoporous Co-free Fe83.3Si4B8P4Cu0.7 electrodes. The stabilities are considered to result from the relatively small Fe3O4 octahedra, the stabilization of the amorphous Co3O4 outmost layers and the transformation of amorphous and crystalline Co3O4 phases during long-term Redox cycling. The Co substitution of Fe83.3Si4B8P4Cu0.7 precursor alloys can achieve the high Redox performances and good stabilities of nanoporous electrodes.

The marriage of crystalline/amorphous Co/Co3O4 heterostructures with N-doped hollow carbon spheres: efficient and durable catalysts for oxygen reduction

The design of non-precious metal-N-C (M-N-C) materials with efficient oxygen reduction activity are extremely necessary for renewable energy technologies. These M-N-C electrocatalysts are often composed of metal nanoparticles and M-Nx sites simultaneously due to their structural heterogeneity induced by the high-temperature strategy. However, these metal nanoparticles usually suffer from a partial electrochemical/chemical oxidation or dissolution in strong alkaline electrolytes, which may decrease the number of active sites for electrocatalytic oxygen reduction reaction (ORR). Herein, we demonstrated a facile strategy to get this obstacle that fabricate the core-shell crystalline/amorphous Co/Co3O4 heterostructures incorporated in N-doped hollow carbon spheres (Co/Co3O4@NHCS). Specifically, the amorphous Co3O4 shell could effectively prevent the electrochemical oxidation/dissolution of metal Co core in strong alkaline solution. It could also create abundant interface that provide sufficient electron supply and reaction places, and thus make it promising the high efficiency of electron transfer. Furthermore, the N-doped defective graphitic carbon could optimize and facilitate the charge/mass transport that could maximize the catalytic activity. Deliberately coupling the crystalline/amorphous Co/Co3O4 heterostructures with Co-Nx sites are advantageous to achieve the remarkable catalytic activity and superior durability for ORR in alkaline media. As expected, the resulted Co/Co3O4@NHCS catalysts possess the superior ORR activity with a half-wave potential of 0.82V (vs. RHE), similar to that of the commercial Pt/C. Most importantly, it shows excellent durability with almost negligible loss in catalytic performance after 3500 cycles. This work opens an avenue for rational design the efficient ORR electrocatalysts toward practical applications in fuel cells.