Quasi-single-crystalline CoO hexagrams with abundant defects for highly efficient electrocatalytic water oxidation

Structure and oxygen vacancy engineered CuCo-layered double oxide nanotube arrays as advanced bifunctional electrocatalysts for overall water splitting

In recent years, as a green renewable energy production technology, electrochemical water splitting has demonstrated high development potential. Many materials have been reported as successful catalysts in the water-splitting field. However, it is still a huge challenge to produce bifunctional electrocatalysts for the efficient and sustainable generation of hydrogen and oxygen simultaneously. Herein, we successfully developed oxygen vacancies abundant CuCo layered double oxide (O_v-CuCo-LDO) hollow nanotube arrays (HNTAs) loaded on nickel foam as advanced electrocatalysts for total water splitting. When the current density was 10 mA cm^-2, the O_v-CuCo-LDO HNTAs exhibited outstanding onset overpotentials of 53.9 and 72.5 mV for the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline medium, respectively, because of the bimetallic synergistic effect between the cobalt and copper and the unique hollow porous structure. In addition, an as-assembled O_v-CuCo-LDO||O_v-CuCo-LDO electrolytic cell showed a small potential of 1.55 V to deliver a current density of 10 mA cm^-2. Moreover, it also showed remarkable durability after long-term overall water splitting for more than 20 h. The research results in this paper are of great interest to practical applications of the water decomposition process, providing clear and in-depth insights into preliminary robust and efficient multifunctional electrocatalysts for overall water splitting.

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.

In Situ Formation of Surface-Induced Oxygen Vacancies in Co9S8/CoO/NC as a Bifunctional Electrocatalyst for Improved Oxygen and Hydrogen Evolution Reactions

Creating oxygen vacancies and introducing heterostructures are two widely used strategies in Co-based oxides for their efficient electrocatalytic performance, yet both strategies have rarely been used together to design a bifunctional electrocatalyst for an efficient overall water splitting. Herein, we propose a facile strategy to synthesize oxygen-defect-rich Co9S8/CoO hetero-nanoparticles with a nitrogen-doped carbon shell (ODR-Co9S8/CoO/NC) through the in situ conversion of heterojunction along with surface-induced oxygen vacancies, simply via annealing the precursor Co3S4/Co(OH)2/ZIF-67. The as-prepared ODR-Co9S8/CoO/NC shows excellent bifunctional catalytic activities, featuring a low overpotential of 217 mV at 10 mA cm-2 in the oxygen evolution reaction (OER) and 160 mV at 10 mA cm-2 in the hydrogen evolution reaction (HER). This performance excellency is attributed to unique heterostructure and oxygen defects in Co9S8/CoO nanoparticles, the current work is expected to offer new insights to the design of cost-effective, noble-metal-free electrocatalysts.

Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide

The recent progress made on porphyrin-based frameworks and their applications in energy-related conversion technologies (e.g., ORR, OER and CO₂RR) and storage technologies (e.g., Zn–air batteries).

Selective Synthesis of Monodisperse CoO Nanooctahedra as Catalysts for Electrochemical Water Oxidation

Thermal decomposition is a promising route for the synthesis of metal oxide nanoparticles because size and morphology can be tuned by minute control of the reaction variables. We synthesized CoO nanooctahedra with diameters of ∼48 nm and a narrow size distribution. Full control over nanoparticle size and morphology could be obtained by controlling the reaction time, surfactant ratio, and reactant concentrations. We show that the particle size does not increase monotonically with time or surfactant concentration but passes through minima or maxima. We unravel the critical role of the surfactants in nucleation and growth and rationalize the observed experimental trends in accordance with simulation experiments. The as-synthesized CoO nanooctahedra exhibit superior electrocatalytic activity with long-term stability during oxygen evolution. The morphology of the CoO particles controls the electrocatalytic reaction through the distinct surface sites involved in the oxygen evolution reaction.

Synergistic effects in bimetallic Pd-CoO electrocatalytic thin films for oxygen evolution reaction

Bimetallic catalysts due to the synergistic effects often outperform their single-component counterparts while exhibiting structure and composition-dependent enhancement in active sites, thereby having the potential to improve the current density and over-potential parameters in the water oxidation reaction. Herein, we demonstrate a simple and rapid, yet highly efficient method to fabricate Pd–CoO films of immaculate homogeneity as characterized using different imaging and spectroscopic techniques. The SEM images revealed that the films were composed of bimetallic spherical granules wherein both metals were uniformly distributed in an atomic ratio of ~ 1:1. The time-dependent investigations of the film fabrication behavior demonstrated that the films formed in shorter deposition times (1–2 h) display more porous character, allowing better access to the reaction centers. This character was transcribed into their enhanced electrocatalytic performance toward the oxygen evolution reaction (OER). Using this specific bimetallic formulation, we could attain a low over-potential of 274 mV for a current density of 10 mA cm⁻², whereas the high current density value of > 200 mA cm⁻² was achieved while still under 600 mV of over-potential. The cycling and current generation stability was also found to be sufficiently high, which can only be attributed to the facile electron transfer processes and a higher number of active sites available in homogeneous bimetallic films.

The NHx Group Induced Formation of 3D α-Co(OH)2 Curly Nanosheet Aggregates as Efficient Oxygen Evolution Electrocatalysts

Recently, the curly structure attracts researchers' attention due to the strain effect, electronic effect, and improved surface area, which exhibits enhanced electrocatalytic activity. However, the synthesis of metastable curved structures is very difficult. Herein, a simple room temperature coprecipitation method is proposed to synthesize 3D cobalt (Co) hydroxide (α-Co(OH)₂ ) electrocatalysts that consist of curly 2D nanosheets. The formation process of curly nanosheets is elaborated systematically and the results demonstrate that the NH_x group has great effect on the formation of curly structure. Combining the advantage of 2D curly nanosheet and 3D aggregate structure, the as-prepared α-Co(OH)₂ curly nanosheet aggregates show the best water oxidation activity with an overpotential of 269 mV at j = 10 mA cm^-2 in 1.0 m KOH. The electrocatalytic process studies demonstrate that the formation of Co^IV O species is the rate-determining step. Theoretical calculations further confirm the beneficial effect of the bent structure on the conductivity, the adsorption of OH^- and the formation of OOH* species.

Slow magnetic relaxation and water oxidation activity of dinuclear CoIICoIII and unique triangular CoIICoIICoIII mixed-valence complexes

Construction of efficient multifunctional materials is one of the greatest challenges of our time. We herein report the magnetic and catalytic characterization of dinuclear [CoIIICoII(HL1)2(EtOH)(H2O)]Cl·2H2O (1) and trinuclear [CoIIICoII2(HL2)2(L2)Cl2]·3H2O (2) mixed valence complexes. Relevant structural features of the complexes have been mentioned to correlate with their magnetic and catalytic properties. Unique structural features, especially in terms of significant distortions around the CoII centre(s), prompted us to test both spin-orbit coupling (SOC) and zero field splitting (ZFS) methodologies for the systems. The positive sign of D values has been established from X-band EPR spectra recorded in the 5-40 K temperature range and reaffirmed by CAS/NEVPT2 calculations. ZFS tensors are also extracted for the compounds along with CoIIGaIII and CoIIZnIICoIII model species. Interestingly, 1 shows slow relaxation of magnetization below 6.5 K in the presence of a 1000 Oe external dc field with two relaxation processes (Ueff = 37.0 K with τ0 = 1.57 × 10-8 s for the SR process and Ueff = 7 K with τ0 = 1.66 × 10-6 s for the FR process). As mixed valence cobalt complexes with various nuclearities are central to the quest for water oxidation catalysts, we were prompted to explore their features and to our surprise, water oxidation ability has been realized for both 1 and 2 with significant nuclearity control.

Noble-Metal-Free Electrocatalysts for Oxygen Evolution

Oxygen evolution reaction (OER) plays a vital role in many energy conversion and storage processes including electrochemical water splitting for the production of hydrogen and carbon dioxide reduction to value-added chemicals. IrO₂ and RuO₂ , known as the state-of-the-art OER electrocatalysts, are severely limited by the high cost and low earth abundance of these noble metals. Developing noble-metal-free OER electrocatalysts with high performance has been in great demand. In this review, recent advances in the design and synthesis of noble-metal-free OER electrocatalysts including Ni, Co, Fe, Mn-based hydroxides/oxyhydroxides, oxides, chalcogenides, nitrides, phosphides, and metal-free compounds in alkaline, neutral as well as acidic electrolytes are summarized. Perspectives are also provided on the fabrication, evaluation of OER electrocatalysts and correlations between the structures of the electrocatalysts and their OER activities.

Boosting reversible oxygen electrocatalysis with enhanced interfacial pyridinic-N-Co bonding in cobalt oxide/mesoporous N-doped graphene hybrids

A hybrid electrocatalyst CoO/r-NLG with a high density of pyridinic-N-Co bonds was synthesized via pyrolysis treatments of a mixture of cobalt iron precursors and laser-induced mesoporous N-doped graphene, which achieved superb reversible ORR/OER activities, offering a low efficiency loss (ΔE) of 0.635 V in a 1.0 M KOH electrolyte.

Disordering the Atomic Structure of Co(II) Oxide via B-Doping: An Efficient Oxygen Vacancy Introduction Approach for High Oxygen Evolution Reaction Electrocatalysts

The introduction of oxygen (O) vacancies has been considered to be an important but challenging way to enhance the activity of electrocatalysts for the oxygen evolution reaction (OER). Substitution by heteroatoms with high electron-donating ability may be a feasible strategy for triggering O-vacancies to maintain thermodynamic stability. Herein, density functional theory (DFT) calculations predict that the incorporation of boron (B) is favorable to the generation of O-vacancies in transition metal oxides. Then, CoO nanowires with O-vacancies are prepared via incorporation of B using a facile pyrolysis strategy. As evidenced by the combined results of electron paramagnetic resonance spectroscopy and X-ray absorption near edge structure, O-vacancies in CoO are mainly derived from the disordering of the local structure caused by B doping. DFT calculation results further reveal that the oxidation of *OOH is the rate-limiting step for O-vacancies enriched CoO in the OER and that the presence of O-vacancies can efficiently lower the reaction barrier for breaking CoO bond, contributing to the improvement of OER kinetics. As expected, the O-vacancies enriched CoO exhibits a low overpotential of 280 mV to reach the current density of 10 mA cm^-2 under basic conditions.

HMTA-assisted formation of hierarchical Co-based materials built by low-dimensional substructures as water oxidation electrocatalysts

We report a 3D Co-based micro-flower constructed from 2D nanosheets, which exhibits excellent OER performance.