In Situ Formation of Cobalt Oxide Nanocubanes as Efficient Oxygen Evolution Catalysts

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Functionality Modulation Toward Thianthrene-based Metal-Free Electrocatalysts for Water Splitting

The development of metal-free bifunctional electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) is significant but rarely demonstrated. Porous organic polymers (POPs) with well-defined electroactive functionalities show superior performance in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Precise control of the active sites' local environment requires careful modulation of linkers through the judicious selection of building units. Here, a systematic strategy is introduced for modulating functionality to design and synthesize a series of thianthrene-based bifunctional sp2 C═C bonded POPs with hollow spherical morphologies exhibiting superior electrocatalytic activity. This precise structural tuning allowed to gain insight into the effects of heteroatom incorporation, hydrophilicity, and variations in linker length on electrocatalytic activity. The most efficient bifunctional electrocatalyst THT-PyDAN achieves a current density of 10 mA cm─2 at an overpotential (η10) of ≈65 mV (in 0.5 m H2SO4) and ≈283 mV (in 1 m KOH) for HER and OER, respectively. THT-PyDAN exhibits superior activity to all previously reported metal-free bifunctional electrocatalysts in the literature. Furthermore, these investigations demonstrate that THT-PyDAN maintains its performance even after 36 h of chronoamperometry and 1000 CV cycling. Post-catalytic characterization using FT-IR, XPS, and microscopic imaging techniques underscores the long-term durability of THT-PyDAN.

Rational design of bimetal phosphide embedded in carbon nanofibers for boosting oxygen evolution

The development of non-precious metal electrocatalysts for oxygen evolution reaction (OER) is crucial for generating large-scale hydrogen through water electrolysis. In this work, bimetal phosphides embedded in electrospun carbon nanofibers (P-FeNi/CNFs) were fabricated through a reliable electrospinning-carbonization-phosphidation strategy. The incorporation of P-FeNi nanoparticles within CNFs prevented them from forming aggregation and further improved their electron transfer property. The bimetal phosphides helped to weaken the adsorption of O intermediate, promoting the OER activity, which was confirmed by the theoretical results. The as-prepared optimized P-Fe1Ni2/CNFs catalyst exhibited very high OER electrocatalytic performance, which required very low overpotentials of just 239 and 303 mV to reach 10 and 1000 mA cm-2, respectively. It is superior to the commercial RuO2 and many other related OER electrocatalysts reported so far. In addition, the constructed alkaline electrolyzer based on the P-Fe1Ni2/CNFs catalyst and Pt/C delivered a cell voltage of 1.52 V at 10 mA cm-2, surpassing the commercial RuO2||Pt/C (1.61 V) electrolyzer. It also offered excellent alkaline OER performance in simulated seawater electrolyte. This demonstrated its potential for practical applications across a broad range of environmental conditions. Our work provides new ideas for the ration design of highly efficient non-precious metal-based OER catalysts for water electrolysis.

Recent advances of modification effect in Co3O4-based catalyst towards highly efficient photocatalysis

Tricobalt tetroxide (Co3O4) has been developed as a promising photocatalyst material for various applications. Several reports have been published on the self-modification of Co3O4 to achieve optimal photocatalytic performance. The pristine Co3O4 alone is inadequate for photocatalysis due to the rapid recombination process of photogenerated (PG) charge carriers. The modification of Co3O4 can be extended through the introduction of doping elements, incorporation of supporting materials, surface functionalization, metal loading, and combination with other photocatalysts. The addition of doping elements and support materials may enhance the photocatalysis process, although these modifications have a slight effect on decreasing the recombination process of PG charge carriers. On the other hand, combining Co3O4 with other semiconductors results in a different PG charge carrier mechanism, leading to a decrease in the recombination process and an increase in photocatalytic activity. Therefore, this work discusses recent modifications of Co3O4 and their effects on its photocatalytic performance. Additionally, the modification effects, such as enhanced surface area, generation of oxygen vacancies, tuning the band gap, and formation of heterojunctions, are reviewed to demonstrate the feasibility of separating PG charge carriers. Finally, the formation and mechanism of these modification effects are also reviewed based on theoretical and experimental approaches to validate their formation and the transfer process of charge carriers.

Cobalt Ferrite Surface-Modified Carbon Nanotube Fibers as an Efficient and Flexible Electrode for Overall Electrochemical Water Splitting Reactions

One of the most practical and environmentally friendly ways to deal with the energy crises and global warming is to produce hydrogen as clean fuel by splitting water. The central obstacle for electrochemical water splitting is the use of expensive metal-based catalysts. For electrocatalytic hydrogen production, it is essential to fabricate an efficient catalyst for the counterpart oxygen evolution reaction (OER), which is a four-electron-transfer sluggish process. Here in this study, we have successfully fabricated cobalt-based ferrite nanoparticles over the surface of carbon nanotube fiber (CNTF) that was utilized as flexible anode materials for the OER and overall electrochemical water splitting reactions. Scanning electron microscopy images with elemental mapping showed the growth of nanoparticles over CNTF, while electrochemical characterization exhibited excellent electrocatalytic performance. Linear sweep voltammetry revealed the reduced overpotential value (260 mV@η10mAcm-2) with a small Tafel slope of 149 mV dec-1. Boosted electrochemical double layer capacitance (0.87 mF cm-2) for the modified electrode also reflects the higher surface area as compared to pristine CNTF (Cdl = 0.022 mF cm-2). Charge transfer resistance for the surface-modified CNTF showed the lower diameter in the Nyquist plot and was consequently associated with the better Faradaic process at the electrode/electrolyte interface. Overall, the as-fabricated electrode could be a promising alternative for the efficient electrochemical water splitting reaction as compared to expensive metal-based electrocatalysts.

Recent Advances of Transition Metal Basic Salts for Electrocatalytic Oxygen Evolution Reaction and Overall Water Electrolysis

Electrocatalytic oxygen evolution reaction (OER) has been recognized as the bottleneck of overall water splitting, which is a promising approach for sustainable production of H2. Transition metal (TM) hydroxides are the most conventional and classical non-noble metal-based electrocatalysts for OER, while TM basic salts [M2+(OH)2-x(Am-)x/m, A = CO32-, NO3-, F-, Cl-] consisting of OH- and another anion have drawn extensive research interest due to its higher catalytic activity in the past decade. In this review, we summarize the recent advances of TM basic salts and their application in OER and further overall water splitting. We categorize TM basic salt-based OER pre-catalysts into four types (CO32-, NO3-, F-, Cl-) according to the anion, which is a key factor for their outstanding performance towards OER. We highlight experimental and theoretical methods for understanding the structure evolution during OER and the effect of anion on catalytic performance. To develop bifunctional TM basic salts as catalyst for the practical electrolysis application, we also review the present strategies for enhancing its hydrogen evolution reaction activity and thereby improving its overall water splitting performance. Finally, we conclude this review with a summary and perspective about the remaining challenges and future opportunities of TM basic salts as catalysts for water electrolysis.

Speciation of Oxygen Functional Groups on the Carbon Support Controls the Electrocatalytic Activity of Cobalt Oxide Nanoparticles in the Oxygen Evolution Reaction

The effective use of the active phase is the main goal of the optimization of supported catalysts. However, carbon supports do not interact strongly with metal oxides, thus, oxidative treatment is often used to enhance the number of anchoring sites for deposited particles. In this study, we set out to investigate whether the oxidation pretreatment of mesoporous carbon allows the depositing of a higher loading and a more dispersed cobalt active phase. We used graphitic ordered mesoporous carbon obtained by a hard-template method as active phase support. To obtain different surface concentrations and speciation of oxygen functional groups, we used a low-temperature oxygen plasma. The main methods used to characterize the studied materials were X-ray photoelectron spectroscopy, transmission electron microscopy, and electrocatalytic tests in the oxygen evolution reaction. We have found that the oxidative pretreatment of mesoporous carbon influences the speciation of the deposited cobalt oxide phase. Moreover, the activity of the electrocatalysts in oxygen evolution is positively correlated with the relative content of the COO-type groups and negatively correlated with the C═O-type groups on the carbon support. Furthermore, the high relative content of COO-type groups on the carbon support is correlated with the presence of well-dispersed Co₃O₄ nanoparticles. The results obtained indicate that to achieve a better dispersed and thus more catalytically active material, it is more important to control the speciation of the oxygen functional groups rather than to maximize their total concentration.

Trends and progress in application of cobalt-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water splitting

There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13 V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co^(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072-1075, 2008).

Oxygen Vacancy-Induced Construction of CoO/h-TiO2 Z-Scheme Heterostructures for Enhanced Photocatalytic Hydrogen Evolution

Environmentally friendly catalysts with excellent performance and low cost are critical for photocatalysis. Herein, using hydrogenated TiO₂ (h-TiO₂) nanosheets with enriched oxygen vacancies as the support, two-dimensional CoO/h-TiO₂ Z-scheme heterostructures are fabricated for hydrogen production through photocatalytic water splitting. It is revealed that the oxygen vacancies in h-TiO₂ can inhibit the oxidation of Co²⁺ into high-valence Co³⁺ during the hydrothermal reaction and thermal treatment processes. A CoO/h-TiO₂ Z-scheme heterostructure possesses a space charge region and a built-in electric field at the interface, and oxygen vacancies in h-TiO₂ can provide more reactive sites, which synergistically improve the separation and transportation of photogenerated carriers. As a result, the photocatalytic hydrogen evolution rate achieves 129.75 μmol·h^-1 (with 50 mg of photocatalysts) on the optimized CoO/h-TiO₂ heterostructures. This work provides a new design idea for the preparation of excellent TiO₂-based photocatalysts.

Controllable growth of Fe-doped NiS2 on NiFe-carbon nanofibers for boosting oxygen evolution reaction

The construction of high-efficiency and low-cost electrocatalysts toward oxygen evolution reaction (OER) to improve the overall water decomposition performance is a fascinating route to deal with the clean energy application. Herein, Fe-doped NiS₂ crystals grown on the surface of carbon nanofibers (CNFs) encapsulated with NiFe alloy nanoparticles ((Ni,Fe)S₂/NiFe-CNFs) are fabricated through an electrospinning-calcination-vulcanization process, which has been used as a splendid electrocatalyst for OER. Benefitting from the abundant electrochemical active sites from the incorporation of Fe element in NiS₂ and the synergistic effect between NiFe-CNFs and surface sulfides, the obtained (Ni,Fe)S₂/NiFe-CNFs catalyst exhibits highly electrochemical activities and satisfactory durability toward OER in an alkaline medium with a low overpotential of only 287 mV at a high current density of 30 mA cm^-2, and with a little decline in the current retention after 48 h, suggesting its superior OER performance even compared with some noble metal-based electrocatalysts. Additionally, a two-electrode system conducted by using the (Ni,Fe)S₂/NiFe-CNFs and commercial Pt/C as electrodes, only needs a cell voltage of 1.54 V to afford 10 mA cm^-2 for overall water splitting, which is even much better than the RuO₂||Pt/C electrolyzer. This study offers a promising approach to prepare high-efficiency OER catalysts toward overall water splitting.

Light-Driven Water Oxidation with Ligand-Engineered Prussian Blue Analogues

The elucidation of the ideal coordination environment of a catalytic site has been at the heart of catalytic applications. Herein, we show that the water oxidation activities of catalytic cobalt sites in a Prussian blue (PB) structure could be tuned systematically by decorating its coordination sphere with a combination of cyanide and bidentate pyridyl groups.  K_0.1[Co(bpy)]_2.9[Fe(CN)₆]₂ ([Cobpy–Fe]), K_0.2[Co(phen)]_2.8[Fe(CN)₆]₂ ([Cophen–Fe]), {[Co(bpy)₂]₃[Fe(CN)₆]₂}[Fe(CN)₆]_1/3 ([Cobpy2–Fe]), and {[Co(phen)₂]₃[Fe(CN)₆]₂}[Fe(CN)₆]_1/3 Cl_0.11 ([Cophen2–Fe]) were prepared by introducing bidentate pyridyl groups (phen: 1,10-phenanthroline, bpy: 2,2′-bipyridine) to the common synthetic protocol of Co–Fe Prussian blue analogues. Characterization studies indicate that [Cobpy2–Fe] and [Cophen2–Fe] adopt a pentanuclear molecular structure, while [Cobpy–Fe] and [Cophen–Fe] could be described as cyanide-based coordination polymers with lower-dimensionality and less crystalline nature compared to the regular Co–Fe Prussian blue analogue (PBA), K_0.1Co_2.9[Fe(CN)₆]₂ ([Co–Fe]). Photocatalytic studies reveal that the activities of [Cobpy–Fe] and [Cophen–Fe] are significantly enhanced compared to those of [Co–Fe], while molecular [Cobpy2–Fe] and [Cophen2–Fe] are inactive toward water oxidation. [Cobpy–Fe] and [Cophen–Fe] exhibit upper-bound turnover frequencies (TOFs) of 1.3 and 0.7 s^–1, respectively, which are ∼50 times higher than that of [Co–Fe] (1.8 × 10^–2 s^–1). The complete inactivity of [Cobpy2–Fe] and [Cophen2–Fe] confirms the critical role of aqua coordination to the catalytic cobalt sites for oxygen evolution reaction (OER). Computational studies show that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O–O bond formation. This study opens a new route toward increasing the intrinsic water oxidation activity of the catalytic sites in PB coordination polymers.

Bidentate pyridyl groups are coordinated to the catalytic cobalt sites in a cyanide-based Co−Fe structure to afford well-tuned extended network structures, which exhibit an outstanding photocatalytic performance compared to the regular Co−Fe PBA.

X-ray studies bridge the molecular and macro length scales during the emergence of CoO assemblies

The key to fabricating complex, hierarchical materials is the control of chemical reactions at various length scales. To this end, the classical model of nucleation and growth fails to provide sufficient information. Here, we illustrate how modern X-ray spectroscopic and scattering in situ studies bridge the molecular- and macro- length scales for assemblies of polyhedrally shaped CoO nanocrystals. Utilizing high energy-resolution fluorescence-detected X-ray absorption spectroscopy, we directly access the molecular level of the nanomaterial synthesis. We reveal that initially Co(acac)3 rapidly reduces to square-planar Co(acac)2 and coordinates to two solvent molecules. Combining atomic pair distribution functions and small-angle X-ray scattering we observe that, unlike a classical nucleation and growth mechanism, nuclei as small as 2 nm assemble into superstructures of 20 nm. The individual nanoparticles and assemblies continue growing at a similar pace. The final spherical assemblies are smaller than 100 nm, while the nanoparticles reach a size of 6 nm and adopt various polyhedral, edgy shapes. Our work thus provides a comprehensive perspective on the emergence of nano-assemblies in solution.

A facile mechanochemical preparation of Co3O4@g-C3N4 for application in supercapacitors and degradation of pollutants in water

Our study is aimed at synthesizing cobalt oxide (Co₃O₄) with graphite carbon nitride (g-C₃N₄) to form a Co₃O₄@g-C₃N₄ hybrid through a green mechanochemical one-pot synthetic approach for manufacturing efficient supercapacitor electrodes and photocatalysts. In the present study, the Co₃O₄@g-C₃N₄ hybrid revealed a significantly higher specific capacitance (C_s) (of ~ 457.2 Fg^-1 at a current density of 1 Ag^-1) than that of the pristine Co₃O_4, which proved its pseudocapacitive behavior, with a couple of redox peaks observed in three electrode measurements (obtained by using a 3.0-M KOH aqueous electrolyte). The optimized Co₃O₄@g-C₃N₄ hybrid was further embedded for a symmetric supercapacitor performance, delivering an excellent Cs of ~ 92 Fg^-1 at a current density of 1 Ag^-1; this was supplemented with a remarkable cycling stability (~ 92% over 5000 cycles). The Co₃O₄@g-C₃N₄ hybrid was further examined for photocatalysis activity using a rhodamine B (RhB) dye, and more than 95% RhB dye was degraded through the photocatalytic reduction process (after 60 min of UV irradiation). This Co₃O₄@g-C₃N₄ hybrid catalyst exhibited excellent reusability and stability and appears to be a highly efficient, cost-effective, eco-friendly, and reusable catalyst; the g-C₃N₄ present with the Co₃O₄ acted as a conductive nano-network, leading to a higher capacitive and photocatalytic performance.

Defective two-dimensional layered heterometallic phosphonates as highly efficient oxygen evolution electrocatalysts

Two-dimensional metal phosphonates can be used as OER catalysts directly and their catalytic performances can be improved greatly by combining heterometal doping and defect engineering methods.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Retention of anions in cobalt hydroxide with Ni substitution to emphasize the role of anions and cations for high current density in oxygen evolution reactions

Herein, we report the fabrication of remarkably fine nickel-substituted α-Co(OH)2 sheets using an ingenious co-precipitation method at a lower pH value. An α-CoNiOOH sheet retains the parent α-Co(OH)2 structure consisting of both tetrahedral (Td) and octahedral (Oh) sites with the retention of interlayer chloride ions, which is in contrast to the previous reports. The as-synthesized α-CoNiOOH sheet exhibits excellent oxygen evolution reactions (OERs) and produces a current of 10 mA cm-2 at an overpotential of merely 190 mV in an alkaline environment. Moreover, the α-CoNiOOH sheet attains an exceptionally high current density of 100 mA cm-2 at a low overpotential of only 270 mV. Additionally, this electrocatalyst possesses a 33 mV dec-1 Tafel slope with higher values of TOF (11 s-1) and double-layer capacitance (7.76 mF cm-2). This enhancement is attributed partially to the substitution of Ni during the conversion of α-Co(OH)2 to α-CoNiOOH and partially to the exceptionally thin sheets allowing potential octahedral sites for improved oxygen evolution reactions.

Mesoporous anion-cation-codoped Co9S8 nanorings for enhanced electrocatalytic oxygen evolution reactions

Recently, the design and synthesis of Co₉S₈ micro/nanostructures have attracted attention as electrochemical energy storage and conversion devices due to their low cost and environmental friendliness. Herein, Co₉S₈ nanorings were synthesized via a one-step solvothermal method with the incorporation of Fe ions, subsequently, properly selenized to boost their electrocatalytic performance. The morphology and structure of the series of cation and anion regulated Co₉S₈ nanorings were characterized, the electrochemical oxygen evolution reaction (OER) properties were assessed. It is worth noting that the as-prepared catalysts, especially the innovative Fe and Se ions double doped Co₉S₈ nanorings, denoted as Se/Fe-Co₉S₈-0.14, exhibited good electrocatalytic OER performance with low overpotential (298 mV) and high durability under alkaline conditions. This work provides a new perspective to develop non-noble metal Co₉S₈-based OER electrocatalysts with a superior electrocatalytic performance.

Ultrafine α-CoOOH Nanorods Activated with Iron for Exceptional Oxygen Evolution Reaction

Two-dimensional oxyhydroxide materials are proved to be a potential candidate for oxygen evolution reaction (OER). Robust, efficient, and cost-effective electrocatalysts are critical to overcome the sluggish kinetics and high overpotential of OERs. Herein, a simple co-precipitation method followed by solvothermal treatment is used to synthesize Fe-doped α-CoOOH at higher pH under optimum conditions for OER. The α-Fe_0.24Co_0.76OOH/NF illustrates superior OER electrocatalytic performance and requires an overpotential of only 280 mV to produce a current density of 50 mA cm^-2 with excellent stability. The detailed analysis reveals that the exceptional OER performance originates from thin nanorods and partially due to the replacement of Fe in α-CoOOH. This work illustrates the presence of interlayer chloride ions through energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.

Amorphous nickel–iron hydroxide films on nickel sulfide nanoparticles for the oxygen evolution reaction

The development of earth-abundant and low-cost electrocatalysts with high performance toward the oxygen evolution reaction (OER) plays a key role in water splitting.

General Water-Induced Self-Exfoliation Strategy for the Ultrafast and Large-Scale Synthesis of Metal Hydroxide Nanosheets

Metal hydroxides nanosheets of atomic thickness have attracted much interest due to their great potentials in catalysis, energy storage devices, and so on. However, the lack of efficient synthesis of 2D nanosheets has critically impeded their practical applications. Herein, we develop a general water-induced self-exfoliation (WISE) strategy to achieve the fast synthesis of metal hydroxide ultrathin nanosheets with almost single-layer atom thickness in a large scale. In a typical process of layered cobalt hydroxide (LCH) nanosheets, the presynthesized cobalt acetate hydroxide precursor is directly exfoliated to form nanosheets under the attack of H₂O in a few seconds. The water-induced self-exfoliation mechanism has also been proposed based on the analysis of the designed alcohol-mediated slow-down process. In addition, the used solutions and effluents can be recycled making WISE a green, efficient, and surfactant-free method. Furthermore, this general strategy can also be applied to synthesize other layered metal hydroxide nanosheets.

Two-Dimensional Zeolitic Imidazolate Framework-L-Derived Iron-Cobalt Oxide Nanoparticle-Composed Nanosheet Array for Water Oxidation

Rational design of various functional nanomaterials using MOFs as a template provides an effective strategy to synthesize electrocatalysts for water splitting. In this work, we reported that an iron-cobalt oxide with 2D well-aligned nanoflakes assembling on carbon cloth (Fe-Co₃O₄ NS/CC), fabricated by an anion-exchange reaction followed by an annealing process, could serve as a high-performance oxygen-evolving catalyst. Specifically, the zeolitic imidazolate framework-L-Co nanosheet array (ZIF-L-Co NS/CC) was synthesized through a facile ambient liquid-phase deposition reaction, and then reacted with [Fe(CN)₆]^3- ions as precursors during the anion-exchange reaction at room temperature. Finally, the Fe-Co₃O₄ NS/CC was obtained via annealing treatment. On account of the compositional and structural superiority, this 3D monolithic anode exhibited outstanding electrocatalytic performance with a low overpotential of 290 mV to obtain a geometrical current density of 10 mA cm^-2 and good durability for water oxidation in base.

Reaction Packaging CoSe2 Nanoparticles in N-Doped Carbon Polyhedra with Bifunctionality for Overall Water Splitting

Water electrolysis is a promising approach for green and large-scale hydrogen production; however, there are still challenges for developing efficient and stable bifunctional electrocatalysts toward the hydrogen and oxygen evolution reactions. Herein, zeolitic imidazolate framework-67 was used as the precursor for the construction of CoSe₂ nanoparticles trapped in N-doped carbon (NC) polyhedra. Among as-obtained CoSe₂-NC hybrid, highly active CoSe₂ nanoparticles in sizes of 10-20 nm are encapsulated in N-doped few-layer carbon shell, avoiding their easy aggregations of CoSe₂ nanoparticles as well as enhancing the long-term stability. The unique nanostructured CoSe₂-NC hybrid with a hierarchical porosity and 3D conductive framework thus fully exerts outstanding bifunctional catalytic activity of CoSe₂ centers. As a result, the CoSe₂-NC hybrid as bifunctional catalysts for overall water splitting delivers a high current density of 50 mA cm^-2 with an applied voltage of ∼1.73 V in an alkaline electrolyte, with a promising stability over 50 000 s.

CoPi/Co(OH)₂ Modified Ta₃N₅ as New Photocatalyst for Photoelectrochemical Cathodic Protection of 304 Stainless Steel

In this work, CoPi and Co(OH)₂ nanoparticles were deposited on the surface of Ta₃N₅ nanorod-arrays to yield a novel broad-spectrum response photocatalytic material for 304 stainless steel photocatalytic cathodic protection. The Ta₃N₅ nanorod-arrays were prepared by vapor-phase hydrothermal (VPH) and nitriding processes and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-Vis spectroscopy, respectively, to obtain morphologies, crystal structures, surface compositions, and light response range. In order to analyze the performance improvement mechanism of CoPi/Co(OH)₂ on Ta₃N₅ nanorod-arrays, the electrochemical behavior of modified and unmodified Ta₃N₅ was obtained by measuring the open circuit potential and photocurrent in 3.5 wt% NaCl solution. The results revealed that the modified Ta₃N₅ material better protects 304 stainless steel at protection potentials reaching −0.45 V.

Cobalt iron phosphide nanoparticles embedded within a carbon matrix as highly efficient electrocatalysts for the oxygen evolution reaction

Co₃FeP_x/C nanocomposites were derived from one-step phosphorization of anthraquinone-2-sulfonate (AQS2) intercalated Co₃Fe layered double hydroxides (Co₃Fe LDHs).

Design of an inherently-stable water oxidation catalyst

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation. In response, we propose using oxidatively-inert ligands to harness the metal oxides themselves. This approach is demonstrated by covalently attaching entirely inorganic oxo-donor ligands (polyoxometalates) to 3-nm hematite cores, giving soluble anionic structures, highly resistant to aggregation, yet thermodynamically stable to oxidation and hydrolysis. Using orthoperiodate (at pH 8), and no added photosensitizers, the hematite-core complex catalyzes visible-light driven water oxidation for seven days (7600 turnovers) with no decrease in activity, far exceeding the documented lifetimes of molecular catalysts under turnover conditions in water. As such, a fundamental limitation of molecular complexes is entirely bypassed by using coordination chemistry to harness a transition-metal oxide as the reactive center of an inherently stable, homogeneous water-oxidation catalyst.

A current challenge in the development of molecular water oxidation catalysts is to overcome their inherent susceptibilities to oxidative or hydrolytic degradation under turnover conditions in water. Here, the authors design an inherently-stable water oxidation catalyst using oxidatively-inert ligands to harness a reactive metal oxide.

Stabilization of Cobalt-Polyoxometalate over Poly(ionic liquid) Composites for Efficient Electrocatalytic Water Oxidation

The key to unlock a renewable, clean, and energy-dense hydrogen fuel lies in designing an efficient oxygen evolving catalyst exhibiting high activity, stability, and cost-effectiveness. This report addresses an improved activity toward oxygen evolution by a composite of cobalt-polyoxometalate [Co₄(H₂O)₂(PW₉O₃₄)₂]^10- (CoPOM) and an ionic polymer, poly(vinyl butyl imidazolium) (PVIM), in highly alkaline media. PVIM provides a stable platform for CoPOM and acts as a conductive linker between CoPOM and the electrode surface, forming a concrete solid composite, which balances the multinegative charge of CoPOM synergistically. This improved stability and conductivity of CoPOM by PVIM in the PVIM-CoPOM composite performs remarkable electrocatalytic water oxidation with a very low overpotential of 0.20 V and a very high current density of 250 mA/cm² (at 1.75 V vs RHE) with a turnover frequency (TOF) of 52.8 s^-1 in 1 M NaOH.