Co-based Electrocatalysts Research Articles

Hard-templated preparation of mesoporous cobalt phosphide as an oxygen evolution electrocatalyst

Herein, we examine the application of mesoporous cobalt phosphide (meso-CoP) prepared by a hard-templating method as a novel non-precious electrocatalyst for the oxygen evolution reaction (OER). The obtained meso-CoP exhibits a relatively low overpotential (0.30 V at 10 mA cm−2), a decent Tafel slope (81 mV dec−1), and good long-term stability, performing better than its bulk counterpart and many state-of-the-art Co-based OER electrocatalysts. The high electrocatalytic performance of the meso-CoP toward OER is attributed to two key factors: (1) a large accessible active surface which endows the meso-CoP with numerous active sites for water oxidation; (2) a well-organized mesoporous structure which expedites the electron/ion transfer and mass transport, thereby significantly enhancing the OER kinetics. This work will provide guidance for designing inexpensive high-performance electrocatalysts with exquisite mesostructures for potential applications in commercial water-splitting technology.

ZIF-Derived Carbon Nanoarchitecture as a Bifunctional pH-Universal Electrocatalyst for Energy-Efficient Hydrogen Evolution

Development of nonprecious metal-based electrocatalysts supporting hydrogen evolution reaction (HER) in the entire pH range has gained significant importance for harvesting green and renewable energy. Herein, we developed a novel electrocatalyst based on 3D carbon nanoarchitecture hybrid, which consists of CoP nanoparticles (CoP NPs) embedded into N-doped carbon nanotubes (NCNT), grafted on carbon polyhedron (CoP/NCNT-CP) that was prepared by carbonization and low-temperature phosphatization treatment of cobalt-based zeolite imidazole framework (ZIF). Benefiting from the strong synergistic effect and unique 3D structure, the CoP/NCNT-CP hybrid loaded on Ni foam exhibited excellent electrocatalytic HER performance in base with a low overpotential of 165 mV at a current density of 10 mA cm-2, which is competitive with the previously reported Co-based hybrid electrocatalysts. Furthermore, the CoP/NCNT-CP also demonstrated high HER electrocatalytic activities in both neutral and acidic conditions with the overpotentials of 203 and 305 mV at the current density of 10 mA cm-2. Additionally, the bifunctional CoP/NCNT-CP electrode simultaneously acted as an anode for hydrazine oxidation reaction (HzOR) and a cathode for HER. Excellent catalytic performance was demonstrated in base conditions with a low cell potential of 0.89 V at 10 mA cm-2, which was much lower than the voltage of overall water splitting (1.91 V) at the same current density.

Ni- and Co-Based Heterogeneous Nanosheet Array As Efficient Electrocatalysts for Oxygen Evolution Reaction from Water

Efficient oxygen evolution reaction (OER) catalysts composed of earth-abundant elements are crucial to the development of practical and environmental-friendly water splitting system. Herein, we present the synthesis of Ni- and Co-based heterogeneous composites and their application in electrocatalytic OER. Using a straightforward two step reaction, the NiCoO2/CoO/Ni3N nanosheet arrays (NiCoON NSAs/NF) are fabricated in situ on the 3D nickel foam. We have controlled the ratio of two metal sources, reaction temperature, and the annealing time to optimize the active surface species, as well as the morphology. In the alkaline solution, the NiCoON nanocomposite has excellent catalytic OER properties such as a small overpotential of 247 mV (at 10 mA cm−2) and Tafel slope of 35 mV dec−1, which are better than those of NixCo3-xO4 and most Ni- and Co-based electrocatalysts. There are more active sites with oxygen vacancies on NiCoON NSAs after the treatment with NH3 and the face-centered cubic NiCoO2 facilitates the formation of active Ni-Co layered hydroxide/oxyhydroxide species (NiOOH) in comparison with spinal NixCo3-xO4 during the electrochemical process. In addition, the Ni3+-rich surface together with Co incorporation has been shown to favor the formation of β-NiOOH over γ-NiOOH. Our study reveals that the precise control of active species on NiCoON NSAs/NF leads to high activity and stability in OER catalysis.

Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes with Significantly Improved Efficiency

Cobalt complexes have been extensively explored in catalyzing the oxygen reduction reaction (ORR), which is the cathode reaction in fuel cells. Although they show high activities, mononuclear Co complexes typically mediate the 2e reduction of O2 to H2O2, and two Co sites are generally required to catalyze the 4e reduction of O2 to H2O. Herein we report the significantly improved efficiency of covalently grafted Co corroles on carbon nanotubes (CNTs) for the 4e ORR. Azide-containing Co corroles can be attached to alkyne-modified CNTs via azide–alkyne cycloaddition. This attachment can avoid the formation of dimeric face-to-face Co corroles. The resulted hybrid catalyzes the 4e ORR in 0.5 M H2SO4 aqueous solutions with a half-wave potential at 0.78 V versus reversible hydrogen electrode (RHE). This performance makes this hybrid one of the most efficient Co-based molecular ORR electrocatalysts. Control studies using Co corrole analogues loaded on CNTs via noncovalent interactions give ORR half-wave potential...

Aluminum-Tailored Energy Level and Morphology of Co3- x Alx O4 Porous Nanosheets toward Highly Efficient Electrocatalysts for Water Oxidation.

Tuning energy levels plays a crucial role in developing cost-effective, earth-abundant, and highly active oxygen evolution catalysts. However, to date, little attention has been paid to the effect of using heteroatom-occupied lattice sites on the energy level to engineer electrocatalytic activity. In order to explore heteroatom-engineered energy levels of spinel Co3 O4 for highly-effective oxygen electrocatalysts, herein Al atoms are directly introduced into the crystal lattice by occupying the Co2+ ions in the tetrahedral sites and Co3+ ions in the octahedral sites (denoted as Co2+ Td and Co3+ Oh , respectively). Experimental and theoretical simulations demonstrate that Al3+ ions substituting Co2+ Td and Co3+ Oh active sites, especially Al3+ ions occupying the Co2+ Td sites, optimizes the adsorption, activation, and desorption features of intermediate species during oxygen evolution reaction (OER) processes. As a result, the optimized Co1.75 Al1.25 O4 nanosheet exhibit unprecedented OER activity with an ultralow overpotential of 248 mV to deliver a current of 10 mA cm-2 , among the best Co-based OER electrocatalysts. This work should not only provide fundamental understanding of the effect of Al-occupied different Co sites in Co3-x Alx O4 composites on OER performance, but also inspire the design of low-cost, earth-abundant, and high-active electrocatalysts toward water oxidation.

Nanostructured Co-based bifunctional electrocatalysts for energy conversion and storage: current status and perspectives

Current popular nanostructured Co-based bifunctional electrocatalysts developed for HER/OER and ORR/OER in water splitting and oxygen electrodes are critically reviewed and discussed.

Thermally induced top-down nanostructuring for the synthesis of a core/shell-structured CoO/CoSx electrocatalyst

A top-down nanostructuring approach based on sequential reduction and sulfurization reactions is devised to prepare a highly active Co-based electrocatalyst from bulk Co3O4.

Interface-strengthened CoP nanosheet array with Co2P nanoparticles as efficient electrocatalysts for overall water splitting

Abstract Highly active and durable bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) play a pivotal role in overall water splitting. Herein, we demonstrate the construction of interface-strengthened CoP nanosheet array with Co2P nanoparticles as such an electrocatalyst through a facile hydrothermal reaction and the subsequent phosphorization process. The two-dimensional (2D) nanosheets with thickness of ∼55 nm expose a great number of active sites. The surface chemical state indicates that the strongly coupled CoP/Co2P electrocatalysts can adsorb or generate more targeted intermediates (e.g. OH− or OOH*) for both HER/OER. As a result, the CoP/Co2P electrocatalysts exhibit small overpotentials of 68 and 256 mV to drive 10 mA cm−2 for HER and OER, respectively, outperforming most of the recently reported Co-based electrocatalysts. Furthermore, an alkaline electrolyzer assembled by using CoP/Co2P as both cathode and anode can achieve a current density of 10 mA cm−2 at a low voltage of 1.57 V and work continuously for over 58 h. This work provides a feasible structural design for transition metal phosphides electrocatalysts with efficient and stable overall water splitting.

Efficient Electrocatalytic Proton Reduction with Carbon Nanotube-Supported Metal-Organic Frameworks.

Hydrogen production from Earth-abundant catalysts remains an important but difficult challenge. Here we report the growth of Hf12-porphyrin metal-organic frameworks (MOFs) on carbon nanotubes (CNTs) for electrocatalytic proton reduction. Covalent attachment of MOF nanoplates to conductive CNTs improves electron transfer from the electrode to Co-porphyrin active sites, which leads to effective proton reduction via protonation of a CoI-H intermediate. The Hf12-CoDBP/CNT assembly afforded a turnover number of 32 000 in 30 min with a turnover frequency of 17.7 S-1, placing it among the most active Co-based molecular electrocatalysts.

Hybrid cobalt-based electrocatalysts with adjustable compositions for electrochemical water splitting derived from Co2+-Loaded MIL-53(Fe) particles

Composite Co-based electrocatalysts exhibit synergetic electrochemical performance and play important role in water-splitting devices. Previous MOF-template route to targeting Co-electrocatalysts relies mainly on regular Co-MOFs particles, and how to obtain hybrid Co-electrocatalysts with adjustable compositions is still challenging. Here we present a facile route to obtain hybrid Co-electrocatalysts with tunable compositions by impregnating different amount of Co2+ ions in common Fe-MOF particles followed by a calcination process. Four different Co-based electrocatalysts with regular porous structures have been developed and their OER activities are investigated. Among them, metallic Co-containing composite electrocatalysts (Co/CoO/CoFe2O4) exhibit both HER and OER activity, and they are applicable to overall water splitting. This work shares a new strategy to achieve porous Co-based composite electrocatalysts and a simple route to tailor their compositions and activities.

Ultrafine Co Nanoparticles Encapsulated in Carbon-Nanotubes-Grafted Graphene Sheets as Advanced Electrocatalysts for the Hydrogen Evolution Reaction.

The rational design of an efficient and inexpensive electrocatalyst based on earth-abundant 3d transition metals (TMs) for the hydrogen evolution reaction still remains a significant challenge in the renewable energy area. Herein, a novel and effective approach is developed for synthesizing ultrafine Co nanoparticles encapsulated in nitrogen-doped carbon nanotubes (N-CNTs) grafted onto both sides of reduced graphene oxide (rGO) (Co@N-CNTs@rGO) by direct annealing of GO-wrapped core-shell bimetallic zeolite imidazolate frameworks. Benefiting from the uniform distribution of Co nanoparticles, the in-situ-formed highly graphitic N-CNTs@rGO, the large surface area, and the abundant porosity, the as-fabricated Co@N-CNTs@rGO composites exhibit excellent electrocatalytic hydrogen evolution reaction (HER) activity. As demonstrated in electrochemical measurements, the composites can achieve 10 mA cm-2 at low overpotential with only 108 and 87 mV in 1 m KOH and 0.5 m H2 SO4 , respectively, much better than most of the reported Co-based electrocatalysts over a wide pH range. More importantly, the synthetic strategy is versatile and can be extended to prepare other binary or even ternary TMs@N-CNTs@rGO (e.g., Co-Fe@N-CNTs@rGO and Co-Ni-Cu@N-CNTs@rGO). The strategy developed here may open a new avenue toward the development of nonprecious high-performance HER catalysts.

Co/CoOx Nanoparticles Embedded on Carbon for Efficient Catalysis of Oxygen Evolution and Oxygen Reduction Reactions.

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are important electrochemical reactions to realize clean energy technologies. Herein, we prepared a hybrid electrocatalyst consisting of Co/CoOx nanoparticles embedded in amorphous carbon through the simple pyrolysis of cobalt-based zeolitic terephthalate frameworks. The pyrolysis temperature significantly influenced the structure morphology and catalytic behavior. Good contact between Co/CoOx and amorphous carbon resulted in a high catalytic efficiency. The hybrid obtained under pyrolysis temperature of 600 °C exhibited the highest performance for OER, offering a stable current density of 10 mA cm-2 at 277 mV in basic media. Besides good OER behavior, it also showed good ORR performance [onset potential: ∼0.87 V vs. the reversible hydrogen electrode (RHE), diffusion-limiting current density: ∼4.9 mA cm-2 ]. This work describes a novel and efficient catalyst, and greatly expands the scope of low-cost Co-based electrocatalysts for various electrochemical reactions without the need for N-containing ligands.

Transition-Metal-Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review.

Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is one of the most promising approaches for a sustainable energy supply. Highly active, cost-effective, and robust photoelectrodes are undoubtedly crucial for the PEC technology. To achieve this goal, transition-metal-based electrocatalysts have been widely used as cocatalysts to improve the performance of PEC cells for water splitting. Herein, this Review summarizes the recent progresses of the design, synthesis, and application of transition-metal-based electrocatalysts as cocatalysts for PEC water splitting. Mo, Ni, Co-based electrocatalysts for the hydrogen evolution reaction (HER) and Co, Ni, Fe-based electrocatalysts for the oxygen evolution reaction (OER) are emphasized as cocatalysts for efficient PEC HER and OER, respectively. Particularly, some most efficient and robust photoelectrode systems with record photocurrent density or durability for the half reactions of HER and OER are highlighted and discussed. In addition, the self-biased PEC devices with high solar-to-hydrogen efficiency based on earth-abundant materials are also addressed. Finally, this Review is concluded with a summary and remarks on some challenges and opportunities for the further development of transition-metal-based electrocatalysts as cocatalysts for PEC water splitting.

Improved electrocatalytic oxygen evolution reaction properties using PVP modified direct growth Co-based metal oxides electrocatalysts on nickel foam

Polyvinyl pyrrolidone supported ZnCo2O4 and NiCo2O4 nanoparticles with 3D nanocactus and nanoflower-like morphology, respectively, directly grown on the surface of nickel foam through a one-step hydrothermal process followed by calcination treatment were used as improved electrocatalyst for oxygen evolution reaction. This study is continuation of our previous objective about the use of polyvinyl pyrrolidone as surface stabilizer and growth modifier during nanoparticles synthesis. The resulting products were analyzed by using X-ray diffraction (XRD), field emission scanning electron micrographs (FE-SEM) equipped with energy dispersive X-ray spectrometer (EDX), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET). The calculated overpotential for ZnCo2O4 NPs is as low as 0.282 V while for NiCo2O4 NPs is 0.298 V. Additionally, ZnCo2O4 NPs had obtained a low Tafel slope of 79.90 mv dec−1 while 92.28 mV dec−1 Tafel slope for NiCo2O4 NPs. Lastly, the current density was almost retained during 24 h electrochemical running and obtained 97.74% and 96.18% efficiency, for ZnCo2O4 and NiCo2O4 NPs respectively.

The unified ordered mesoporous carbons supported Co-based electrocatalysts for full water splitting

Co-based electrocatalysts were prepared by facilely low-temperature multi-step calcination using ordered mesoporous carbons (OMCs) and cobalt salt as the precursors. According to the chemical reaction of cobalt compound, the calcined product of each step could be used as different catalysts, which could complete the full water splitting in alkaline solution together. The introduction of ordered mesoporous carbon not only make the morphology was well inherited, but also enhanced the catalytic activity. The OMCs/CoP exhibited overpotential of 134 mV in acidic media and 143 mV in basic media with a current density of 10 mA cm−2 for hydrogen evolution reaction (HER). While, in basic solution, the OMCs/Co3O4 had an overpotential value of 350 mV at 10 mA cm−2 for oxygen evolution reaction (OER). Moreover, the different Co-based electrocatalysts were assembled into one system for the full water splitting in 1 M KOH solution. This work provided a facile and efficient candidate for water splitting.

A Facile One-step Synthesis of Co 9 S 8 Electrocatalyst for Oxygen Reduction Reaction

A Co-based chalcogenide electrocatalyst has been synthesized by a facile one-step reaction of dodecacarbonyltetracobalt [Co 4 (CO) 12 ] and elemental sulfur in 1, 6-hexanediol solvent under refluxing conditions. The characterizations of the synthesized compound were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD shows the formation of cubic structure Co 9 S 8 and SEM micrograph displays cauliflower-like surface morphologies. The catalyst has an open circuit potential (OCP) of 0.75 V (vs. NHE) and shows a promising catalytic activity for the oxygen reduction. The transfer coefficient and Tafel slope are calculated to be 0.50 and 119 mV in the potential region limited by charge transfer kinetics, respectively. The catalytic activity and the electrochemical stability of the catalyst have also been compared with a commercial Pt catalyst.

Porous cobalt oxide nanoplates enriched with oxygen vacancies for oxygen evolution reaction

Porous cobalt oxide nanoplates enriched with oxygen vacancies are synthesized using a ligand-assisted polyol reduction method. This method enables large-scale synthesis that offers superior uniformity, solution dispersity and controllable concentration of oxygen vacancies on surface. The large surface area of porous cobalt oxide nanoplates together with enriched oxygen vacancies provide more active sites, which promote faster exchange of intermediates and more efficient electron transfer. The as prepared cobalt oxide nanoplates manifest oxygen evolution reaction (OER) overpotential as low as 306mV at 10mA/cm2 in 1M KOH, which is superior to the values of most reported Co-based electrocatalysts.

Strong Surface Hydrophilicity in Co-Based Electrocatalysts for Water Oxidation.

Developing efficient and durable oxygen evolution electrocatalyst is of paramount importance for the large-scale supply of renewable energy sources. Herein, we report the design of significant surface hydrophilicity based on cobalt oxyhydroxide (CoOOH) nanosheets to greatly improve the surface hydroxyl species adsorption and reaction kinetics at the Helmholtz double layer for high-efficiency water oxidation activity. The as-designed CoOOH-graphene nanosheets achieve a small surface water contact angle of ∼23° and a large double-layer capacitance (Cdl) of 8.44 mF/cm2 and thus could evidently strengthen surface species adsorption and trigger electrochemical oxygen evolution reaction (OER) under a quite low onset potential of 200 mV with an excellent Tafel slope of 32 mV/dec. X-ray absorption spectroscopy and first-principles calculations demonstrate that the strong interface electron coupling between CoOOH and graphene extracts partial electrons from the active sties and increases the electron state density around the Fermi level and effectively promotes the surface intermediates formation for efficient OER.

The active site exploration of Co-based non-precious metal electrocatalysts for oxygen reduction reaction

A kind of cobalt and nitrogen co-doped carbon catalyst for oxygen reduction reaction (ORR) is synthesized by pyrolyzing cobalt-melamine complexes supported on carbon at high temperature from 600 to 1000 °С. The optimum activity is obtained on Co0.25-N0.32/C-800 catalyst heat-treated at 800 °С, with the peak potential and onset potential of 0.826 and 0.930 V (vs. RHE), respectively. From the rotating disk electrode measurement using linear sweep voltammetry (LSV) and Koutechy-Levich analysis, the overall electron number transferred for ORR catalyzed by the optimum catalyst Co0.25-N0.32/C-800 is determined to be 3.43–3.87, suggesting that the ORR catalyzed by Co0.25-N0.32/C-800 is dominated by a four-electron transfer process. The catalytic active site of the catalysts is explored by Fourier transform infrared spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. The Co-N-C unit is the most efficient active site and responsible for the ORR activity enhancement in alkaline electrolyte. Accordingly, a possible ORR mechanism on the cobalt and nitrogen co-doped carbon catalyst is proposed.

A host-guest approach to fabricate metallic cobalt nanoparticles embedded in silk-derived N-doped carbon fibers for efficient hydrogen evolution

Hydrogen evolution reaction (HER) plays a key role in generating clean and renewable energy. As the most effective HER electrocatalysts, Pt group catalysts suffer from severe problems such as high price and scarcity. It is highly desirable to design and synthesize sustainable HER electrocatalysts to replace the Pt group catalysts. Due to their low cost, high abundance and high activities, cobalt-incorporated N-doped nanocarbon hybrids are promising candidate electrocatalysts for HER. In this report, we demonstrated a robust and eco-friendly host-guest approach to fabricate metallic cobalt nanoparticles embedded in N-doped carbon fibers derived from natural silk fibers. Benefiting from the one-dimensional nanostructure, the well-dispersed metallic cobalt nanoparticles and the N-doped thin graphitized carbon layer coating, the best Co-based electrocatalyst manifests low overpotential (61 mV@10 mA/cm2) HER activity that is comparable with commercial 20% Pt/C, and good stability in acid. Our findings provide a novel and unique route to explore high-performance noble-metal-free HER electrocatalysts.