Co oxide nanostructures for electrocatalytic water-oxidation: effects of dimensionality and related properties

Promising Electrocatalytic Water and Methanol Oxidation Reaction Activity by Nickel Doped Hematite/Surface Oxidized Carbon Nanotubes Composite Structures

Tailoring the precise construction of non-precious metals and carbon-based heterogeneous catalysts for electrochemical oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) is crucial for energy conversion applications. Herein, this work reports the composite of Ni doped Fe₂ O₃ (Ni-Fe₂ O₃ ) with mildly oxidized multi-walled CNT (O-CNT) as an outstanding Mott-Schottky catalyst for OER and MOR. O-CNT acts as a co-catalyst which effectively regulates the charge transfer in Ni-Fe₂ O₃ and thus enhances the electrocatalytic performance. Ni-Fe₂ O₃ /O-CNT exhibits a low onset potential of 260 mV and overpotential 310 mV @ 10 mA cm^-2 for oxygen evolution. Being a Mott-Schottky catalyst, it achieves the higher flat band potential of -1.15 V with the carrier density of 0.173×10²⁴  cm^-3 . Further, in presence of 1 M CH₃ OH, it delivers the MOR current density of 10 mA cm^-2 at 1.46 V vs. RHE. The excellent electrocatalytic OER and MOR activity of Ni-Fe₂ O₃ /O-CNT could be attributed to the synergistic interaction between Ni-doped Fe₂ O₃ and O-CNT.

Tailoring the Morphology of Cost-Effective Vanadium Diboride Through Cobalt Substitution for Highly Efficient Alkaline Water Oxidation

Design and development of efficient, economical, and durable electrocatalysts for oxygen evolution reaction (OER) are of key importance for the realization of electrocatalytic water splitting. To date, VB₂ and its derivatives have not been considered as electrocatalysts for water oxidation. Herein, we developed a series of electrocatalysts with a formal composition of V_1-xCo_xB₂ (x = 0, 0.05, 0.1, and 0.2) and employed them in an oxygen-evolving reaction. The incorporation of Co into the VB₂ structure caused a dramatic transformation in the morphology, resulting in a super low overpotential of 200 mV at 10 mA cm^-2 for V_0.9Co_0.1B₂ and displaying much greater performance compared to the noble-metal catalyst RuO₂ (290 mV). The longevity of the best-performing sample was assessed through the exposure to the current density of 10 mA cm^-2, showing relative durability after 12 h under 1 M KOH conditions. The Faradaic efficiency tests corroborated the initiation of OER at 1.45 V (vs RHE) and suggested a potential region of 1.50-1.55 V (vs RHE) as the practical OER region. The facile electron transfer between metal(s)-metalloid, high specific surface area, and availability of active oxy-hydroxy species on the surface were identified as the major contributors to this superior OER performance.

A Cr-FeOOH@Ni-P/NF binder-free electrode as an excellent oxygen evolution reaction electrocatalyst

Refining the size of nanoparticles to exhibit larger specific surface areas and expose much more active sites is of great significance for enhancing the oxygen evolution reaction (OER) activity of the electrocatalyst, but still a tremendous challenge. Herein, a Cr-FeOOH@Ni₂P-Ni₅P₄/NF (Cr-FeOOH@Ni-P/NF) catalyst was constructed by electrodepositing a layer of CrFe oxyhydroxides on the self-grown Ni-P nanoparticles, which exhibits ultrafine nanoparticles and thus superexcellent electrocatalytic OER performance. The final catalyst affords ultra-low overpotentials of 144 mV and 210 mV to achieve current densities of 10 and 50 mA cm^-2, respectively. Meanwhile, it demonstrates robust stability for at least 80 hours with no activity decay. This strategy of refining nanoparticles on a three-dimensional electrode has once again been further demonstrated to be feasible and highly effective and opens a new door for the exploration of electrocatalysts with excellent comprehensive properties.

Bimetallic copper nickel sulfide electrocatalyst by one step chemical bath deposition for efficient and stable overall water splitting applications

Transition metal sulfides have been intensively investigated as an effective catalyst for overall water splitting application due to their inexorable bifunctional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity. However, the chalcogenides are oxidised during the OER process and hence limit the stability of the electrocatalyst. The synthesized materials should have a higher oxidation state corresponding to the active species in order to improve the stability. In this study, we have employed a one-step chemical bath deposition (CBD) route to synthesis bimetallic copper nickel sulfide (CuNiS) electrocatalyst. We have accomplished a superior OER electrocatalytic activity with a lower overpotential of 337 mV at 10 mA/cm² current density and a small Tafel slope of 43 mV/dec. Also, we have achieved an excellent HER activity with a very low overpotential of 99 mV at 10 mA/cm² and a Tafel slope of 63 mV/dec. The constructed electrolyzer attained a lower cell voltage of only 1.55 V to reach the current density of 10 mA/cm²_. The stability test carried at a high current density of 200 mA/cm² for 50 h showed less than 5% increase in Ni³⁺ active species at the surface ensure the stable performance nature. Thus, this work provides a promising methodology for the synthesis of bimetallic sulfides for enhanced electrocatalytic water splitting with exceptional reliability.

Morphological and Elemental Investigations on Co–Fe–B–O Thin Films Deposited by Pulsed Laser Deposition for Alkaline Water Oxidation: Charge Exchange Efficiency as the Prevailing Factor in Comparison with the Adsorption Process

Abstract

Mixed transition-metals oxide electrocatalysts have shown huge potential for electrochemical water oxidation due to their earth abundance, low cost and excellent electrocatalytic activity. Here we present Co–Fe–B–O coatings as oxygen evolution catalyst synthesized by Pulsed Laser Deposition (PLD) which provided flexibility to investigate the effect of morphology and structural transformation on the catalytic activity. As an unusual behaviour, nanomorphology of 3D-urchin-like particles assembled with crystallized CoFe2O4 nanowires, acquiring high surface area, displayed inferior performance as compared to core–shell particles with partially crystalline shell containing boron. The best electrochemical activity towards water oxidation in alkaline medium with an overpotential of 315 mV at 10 mA/cm2 along with a Tafel slope of 31.5 mV/dec was recorded with core–shell particle morphology. Systematic comparison with control samples highlighted the role of all the elements, with Co being the active element, boron prevents the complete oxidation of Co to form Co3+ active species (CoOOH), while Fe assists in reducing Co3+ to Co2+ so that these species are regenerated in the successive cycles. Thorough observation of results also indicates that the activity of the active sites play a dominating role in determining the performance of the electrocatalyst over the number of adsorption sites. The synthesized Co–Fe–B–O coatings displayed good stability and recyclability thereby showcasing potential for industrial applications.

Graphic Abstract

Facile synthesis of Ni/NiO nanocomposites: the effect of Ni content in NiO upon the oxygen evolution reaction within alkaline media

We present the facile synthesis of Ni/NiO nanocomposites, via a solution combustion methodology, where the composition of metallic Ni within NiO is controlled by varying the annealing time, from 4 minutes up to 8 hours. The various Ni/NiO nanocomposites are studied via electrically wiring them upon screen-printed graphite macroelectrodes by physical deposition. Subsequently their electrochemical activity, towards the oxygen evolution reaction (OER), is assessed within (ultra-pure) alkaline media (1.0 M KOH). An optimal annealing time of 2 hours is found, which gives rise to an electrochemical oxidation potential (recorded at 10 mA cm-2) of 231 mV (vs. Ag/AgCl 1.46 vs. RHE). These values show the Ni/NiO nanocomposites to be significantly more electrocatalytic than a bare/unmodified SPE (460 mV vs. Ag/AgCl). A remarkable percentage increase (134%) in achievable current density is realised by the former over that of the latter. Tafel analysis and turn over frequency is reported with a likely underlying mechanism for the Ni/NiO nanocomposites towards the OER proposed. In the former case, Tafel analysis is overviewed for general multi-step overall electrochemical reaction processes, which can be used to assist other researchers in determining mechanistic information, such as electron transfer and rate determining steps, when exploring the OER. The optimal Ni/NiO nanocomposite exhibits promising stability at the potential of +231 mV, retaining near 100% of its achievable current density after 28 hours. Due to the facile and rapid fabrication methodology of the Ni/NiO nanocomposites, such an approach is ideally suited towards the mass production of highly active and stable electrocatalysts for application within the anodic catalyst layers of commercial alkaline electrolysers.

Alkaline Water Oxidation Using a Bimetallic Phospho-Boride Electrocatalyst

New oxygen evolution reaction (OER) electrocatalysts based on low-cost elements, which set new benchmark levels of activity, are vital if water electrolysis is to be applied on a global scale. Herein, a low-cost bimetallic phospho-boride catalyst was developed that showed outstanding OER activity of approximately 195 mV to achieve 10 mA cm^-2 in alkaline water electrolysis, with a minimal catalyst loading of 0.3 mg cm^-2 . The contrasting electron transfer property of the metal borides and phosphides when combined in phospho-boride modulated the electron density of the Co atom, yielding highly active CoOOH species at lower potentials. The addition of Mo at low levels further enhanced the activity by increasing the surface area and by formation of nano-crystalline domains. The combined contributions from each of the components resulted in a new benchmark mass activity of 666 A g^-1 at 300 mV overpotential. This work presents a new avenue towards fabricating electrode materials with exceptional performances.

Ag nanoparticles decorated urchin-like cobalt carbonate hydroxide composites for highly efficient oxygen evolution reaction

Herein, a novel composite of small amounts of Ag nanoparticles (NPs) decorated urchin-like cobalt carbonate hydroxide hydrate (CCHH) was developed for highly-efficient alkaline oxygen evolution reaction (OER). Not only can Ag colloids, as template agents, modify the morphologies of urchin-like CCHH microspheres to expose more active sites available, but also the supported Ag NPs formed by Ag colloids can transfer the electron to CCHH surfaces, accelerating the transformation of surface Co^II to Co^III/Co^IV (proton-coupled electron transfer (PCET) process). The urchin-like Ag/CCHH (0.013 mmol) precatalyst (before cyclic voltammetry (CV) activation) exhibits a better OER performance (a low overpotential of 273 mV at 10 mA cm^-2 and small Tafel slope of 65 mV dec^-1) as compared with commercial RuO₂. Furthermore, the dynamic surface self-reconstruction (surface CO₃ ^2- and OH ^- exchange) can further enhance the activities of Ag/CCHH precatalysts. Consequently, the optimal Ag/CCHH (0.013 mmol) catalyst presents a superior activity (a lower overpotential of 267 mV at 10 mA cm^-2 and markedly reduced Tafel slope to 56 mV dec^-1) along with an excellent stability after CV cycles. The study provides a feasible strategy to fully realize the low overpotential of CCHH-based OER electrocatalysts.

Mechanistic Investigation into Efficient Water Oxidation by Co-Ni-Based Hybrid Oxide-Hydroxide Flowers

Oxides are envisioned as promising catalysts to facilitate water oxidation, and the benign presence of hydroxide moieties can further enhance the catalyst performance. However, the nature of synergy between oxides and hydroxides remains elusive. In this study, we have designed a one-pot solution growth technique for the synthesis of flower-shaped N-doped-C-enveloped NiCo₂O₄/Ni_xCo_(1-x)(OH)_y catalysts with varying oxide and hydroxide contents and investigated their water oxidation behavior. The correlation between performance-determining parameters involved in water oxidation, such as the onset potential and overpotential with oxide and/or hydroxide content, oxidation states (oxides), and elemental composition (Co/Ni content), and the possible ways to achieve their optimal values are discussed in detail. Our observations conclude that the onset potential and overpotential are minimal for the hybrid oxide-hydroxide bimetallic system compared with pristine hydroxide or oxide. The optimal hybrid catalyst shows excellent current density, low Tafel slope (82 mV/dec), and low onset potential (281 mV at 2 mA/cm²) and overpotential (348 mV at 10 mA/cm²), besides enduring operational stability in alkaline medium. The low Tafel slope suggests the preferable kinetics for water oxidation, and the poisoning study reveals the direct involvement of metal as active sites. The overall study unveils the synergy in the Co-Ni-based binary transition-metal oxide-hydroxide hybrid, which makes it a potential candidate for water oxidation catalysts, and hence, it is expected that the hybrid will find applications in energy conversion devices, such as electrolyzers.

Ultrasound-treated metal-organic framework with efficient electrocatalytic oxygen evolution activity

Metal-organic frameworks (MOFs) and their derivatives are excellent candidates for electrocatalysts profiting from their unique structures and accessible active sites. Generally, due to the weak poor conductivity and catalytic activity when used as OER electrocatalysts, MOFs are more likely to be used as precursors to obtain composite catalysts through further pyrolysis treatment, rather than directly applied as OER electrocatalysts. But heat treatment usually results in structural collapse and loss of active sites. Specially, as a kind of two-dimensional (2D) materials with rapid electron transfer, metal-organic framework nanosheets (MONs) have great application potential in various fields, especially in the field of catalysis, due to the advantages of both MOFs and 2D materials. Here, we have reported a simple top-down approach to synthesize Co-MONs which can be directly adopted as efficient OER catalysts. Ultrasonic bath (40 KHz, 100 W) was employed to control the exposing of the preponderant lattice plane, which can offer plentiful active catalytic sites and accelerate ions transport. The optimized Co-MONs attain 10 mA cm^-2 at an overpotential of 309 mV with a small Tafel slope of 75.71 mV dec^-1.

Morphological and Structural Evolution of Co3O4 Nanoparticles Revealed by in Situ Electrochemical Transmission Electron Microscopy during Electrocatalytic Water Oxidation

Unveiling the mechanism of electrocatalytic processes is fundamental for the search of more efficient and stable electrode materials for clean energy conversion devices. Although several in situ techniques are now available to track structural changes during electrocatalysis, especially of water oxidation, a direct observation, in real space, of morphological changes of nanostructured electrocatalysts is missing. Herein, we implement an in situ electrochemical Transmission Electron Microscopy (in situ EC-TEM) methodology for studying electrocatalysts of the oxygen evolution reaction (OER) during operation, by using model cobalt oxide Co₃O₄ nanoparticles. The observation conditions were optimized to mimic standard electrochemistry experiments in a regular electrochemical cell, allowing cyclic voltammetry and chronopotentiometry to be performed in similar conditions in situ and ex situ. This in situ EC-TEM method enables us to observe the chemical, morphological, and structural evolutions occurring in the initial nanoparticle-based electrode exposed to different aqueous electrolytes and under OER conditions. The results show that surface amorphization occurs, yielding a nanometric cobalt (oxyhydr)oxide-like phase during OER. This process is irreversible and occurs to an extent that has not been described before. Furthermore, we show that the pH and counterions of the electrolytes impact this restructuration, shedding light on the materials properties in neutral phosphate electrolytes. In addition to the structural changes followed in situ during the electrochemical measurements, this study demonstrates that it is possible to rely on in situ electrochemical TEM to reveal processes in electrocatalysts while preserving a good correlation with ex situ regular electrochemistry.

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.

Shape-controlled synthesis of Co3O4 for enhanced electrocatalysis of the oxygen evolution reaction

One-dimensional Co₃O₄ nanorods, two-dimensional nanosheets and three-dimensional nanocubes were synthesized; the effect of the morphology on their electrocatalytic activities was studied.

Synthesis of urchin-like Co3O4 spheres for application in oxygen evolution reaction

For oxygen evolution electrocatalysis of water splitting, unique urchin-shaped Co₃O₄ spheres were successfully grown on nickel foam by hydrothermal synthesis of Co(OH)F precursor and subsequent annealing method. The formation process was investigated by the evolution of phase structure and morphology with hydrothermal reaction time. And it can be explained by a 'disks-flowers-urchins' mechanism. Moreover, the Co₃O₄ urchins/NF exhibits considerable catalytic properties. It shows a low overpotential of 308 mV at a current density of 20 mA cm^-2 in alkaline solution. In the meantime, such material has a small Tafel slope of 82.1 mV dec^-1, large electrochemical active surface area and good long-term stability. The obvious promotion of oxygen evolution reaction performance can be attributed to the special morphology and the direct attachment to the substrate, which improve the exposed active sites, lower the internal resistance and accelerate the charge transport. Thus, the Co₃O₄ urchins/NF not only has a great potential promising behavior, but also provides the basis for subsequent performance improvement.

Ultrathin cobalt oxide nanostructures with morphology-dependent electrocatalytic oxygen evolution activity

Engineering compositions, structures, and defects can endow nanomaterials with optimized catalytic properties. Here, we report that cobalt oxide (CoOx) ultrathin nanosheets (UTNS, ∼1.6 nm thick) with a large number of oxygen defects and mixed cobalt valences can be obtained through a facile one-step hydrothermal protocol. The large number of oxygen defects make the ultrathin CoOx nanosheet a superior OER catalyst with low overpotentials of 315 and 365 mV at current densities of 50 and 200 mA cm-2, respectively. The stable framework-like architectures of the UTNS further ensure their high OER activity and durability. Our method represents a facile one-step preparation of CoOx nanostructures with tunable compositions, morphologies, and defects, and thus promotes OER properties. This strategy may find its wider applicability in designing active, robust, and easy-to-obtain catalysts for OER and other electrocatalytic systems.

Assembling Ni-Co phosphides/carbon hollow nanocages and nanosheets with carbon nanotubes into a hierarchical necklace-like nanohybrid for electrocatalytic oxygen evolution reaction

Currently, designing and developing high-performance, cost-effective yet stable electrocatalysts for oxygen evolution reaction (OER) is a challenging task owing to the existing high overpotential and sluggish OER kinetics. Herein, we successfully fabricated a hierarchical necklace-like nanohybrid via assembling Ni-Co mixed metal phosphides/carbon (NiCoP/C) hollow nanocages and nanosheets with carbon nanotubes (CNTs). It was revealed that Ni-Co-layered double hydroxide/carbon hollow nanocages and nanosheets derived from Co-based MOF polyhedrons (ZIF-67) could pass through CNTs and perpendicularly anchor onto the surface of CNTs, respectively, thus finally constructing a hierarchical CNTs@NiCoP/C nanohybrid via further low-temperature phosphorization. The as-assembled CNTs@NiCoP/C electrocatalyst exhibited promising electrocatalytic performance towards OER with a low overpotential of about 0.297 V for reaching a current density of 10.0 mA cm-2 and a low Tafel slope of 57.35 mV dec-1 in 1.0 M KOH owing to the advantages of its unique superstructure and multiple composition. Furthermore, the present CNTs@NiCoP/C also possessed good long-term operation stability. This synthesis strategy involving the combination of transition metal phosphides and CNTs to form hierarchical nanostructures holds promising potential for designing highly OER-active electrocatalysts.