Ultrathin LiCoO2 Nanosheets: An Efficient Water-Oxidation Catalyst

Enhanced Oxygen Evolution and Zinc-Air Battery Performance via Electronic Spin Modulation in Heterostructured Catalysts

Beyond optimizing electronic energy levels, the modulation of the electronic spin configuration is an effective strategy, often overlooked, to boost activity and selectivity in a range of catalytic reactions, including the oxygen evolution reaction (OER). This electronic spin modulation is frequently accomplished using external magnetic fields, which makes it impractical for real applications. Herein, spin modulation is achieved by engineering Ni/MnFe2O4 heterojunctions, whose surface is reconstructed into NiOOH/MnFeOOH during the OER. NiOOH/MnFeOOH shows a high spin state of Ni, which regulates the OH- and O2 adsorption energy and enables spin alignment of oxygen intermediates. As a result, NiOOH/MnFeOOH electrocatalysts provide excellent OER performance with an overpotential of 261 mV at 10 mA cm-2. Besides, rechargeable zinc-air batteries based on Ni/MnFe2O4 show a high open circuit potential of 1.56 V and excellent stability for more than 1000 cycles. This outstanding performance is rationalized using density functional theory calculations, which show that the optimal spin state of both Ni active sites and oxygen intermediates facilitates spin-selected charge transport, optimizes the reaction kinetics, and decreases the energy barrier to the evolution of oxygen. This study provides valuable insight into spin polarization modulation by heterojunctions enabling the design of next-generation OER catalysts with boosted performance.

Layered transition metal oxides (LTMO) for oxygen evolution reactions and aqueous Li-ion batteries

This perspective paper comprehensively explores recent electrochemical studies on layered transition metal oxides (LTMO) in aqueous media and specifically encompasses two topics: catalysis of the oxygen evolution reaction (OER) and cathodes of aqueous lithium-ion batteries (LiBs). They involve conflicting requirements; OER catalysts aim to facilitate water dissociation, while for cathodes in aqueous LiBs it is essential to suppress water dissociation. The interfacial reactions taking place at the LTMO in these two distinct systems are of particular significance. We show various strategies for designing LTMO materials for each desired aim based on an in-depth understanding of electrochemical interfacial reactions. This paper sheds light on how regulating the LTMO interface can contribute to efficient water splitting and economical energy storage, all with a single material.

A comprehensive review on the electrochemical parameters and recent material development of electrochemical water splitting electrocatalysts

Electrochemical splitting of water is an appealing solution for energy storage and conversion to overcome the reliance on depleting fossil fuel reserves and prevent severe deterioration of the global climate. Though there are several fuel cells, hydrogen (H2) and oxygen (O2) fuel cells have zero carbon emissions, and water is the only by-product. Countless researchers worldwide are working on the fundamentals, i.e. the parameters affecting the electrocatalysis of water splitting and electrocatalysts that could improve the performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and overall simplify the water electrolysis process. Noble metals like platinum for HER and ruthenium and iridium for OER were used earlier; however, being expensive, there are more feasible options than employing these metals for all commercialization. The review discusses the recent developments in metal and metalloid HER and OER electrocatalysts from the s, p and d block elements. The evaluation perspectives for electrocatalysts of electrochemical water splitting are also highlighted.

Regulating Spin States in Oxygen Electrocatalysis

Developing efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate the d orbitals occupancy, which influence the strength of metal-ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.

Photoreduction of carbon dioxide and photodegradation of organic pollutants using alkali cobalt oxides MCoO2 (M = Li or Na) as catalysts

The recycling of lithium batteries should be prioritized, and the use of discarded alkali metal battery electrode materials as photocatalysts merits research attention. This study synthesized alkali metal cobalt oxide (MCoO₂, M = Li or Na) as a photocatalyst for the photoreduction of CO₂ and degradation of toxic organic substances. The optimized NaCoO₂ and LiCoO₂ photocatalysts increased the photocatalytic CO₂-CH₄ conversion rate to 21.0 and 13.4 μmol g^-1 h^-1 under ultraviolet light irradiation and to 16.2 and 5.3 μmol g^-1 h^-1 under visible light irradiation, which is 17 times higher than that achieved by TiO₂ P25. The rate constants of the optimized reactions of crystal violet (CV) with LiCoO₂ and NaCoO₂ were 2.29 × 10^-2 and 4.35 × 10^-2 h^-1, respectively. The quenching effect of the scavengers and electron paramagnetic resonance in CV degradation indicated that active O₂^•-, ¹O₂, and h⁺ play the main role, whereas ^•OH plays a minor role for LiCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO-^•CH₃ adducts was a_N = 1.508 mT, a_Hβ = 1.478 mT and a_N = 1.558 mT, a_Hβ = 2.267 mT, respectively, whereas the hyperfine splitting of DMPO^+• was a_N = 1.475 mT. The quenching effect also indicated that active O₂^•- and h⁺ play the main role and that ^•OH and ¹O₂ play a minor role for NaCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO^+• adducts was a_N = 1.517 mT, a_Hβ = 1.489 mT and a_N = 1.496 mT, respectively. Discarded alkali metal battery electrode materials can be reused as photocatalysts to address environmental pollution.

Selectively anchoring single atoms on specific sites of supports for improved oxygen evolution

The homogeneity of single-atom catalysts is only to the first-order approximation when all isolated metal centers interact identically with the support. Since the realistic support with various topologies or defects offers diverse coordination environments, realizing real homogeneity requires precise control over the anchoring sites. In this work, we selectively anchor Ir single atoms onto the three-fold hollow sites (Ir₁/T_O–CoOOH) and oxygen vacancies (Ir₁/V_O–CoOOH) on defective CoOOH surface to investigate how the anchoring sites modulate catalytic performance. The oxygen evolution activities of Ir₁/T_O–CoOOH and Ir₁/V_O–CoOOH are improved relative to CoOOH through different mechanisms. For Ir₁/T_O–CoOOH, the strong electronic interaction between single-atom Ir and the support modifies the electronic structure of the active center for stronger electronic affinity to intermediates. For Ir₁/V_O–CoOOH, a hydrogen bonding is formed between the coordinated oxygen of single-atom Ir center and the oxygenated intermediates, which stabilizes the intermediates and lowers the energy barrier of the rate-determining step.

While single-atom catalysts offer well-defined structures, the homogeneity of the active sites is determined by localized coordination environments. Here, authors anchor Ir single atoms onto different sites on CoOOH and show how their distinct coordinations activate oxygen-evolving electrocatalysis

Electrode reconstruction strategy for oxygen evolution reaction: maintaining Fe-CoOOH phase with intermediate-spin state during electrolysis

Computational calculations and experimental studies reveal that the CoOOH phase and the intermediate-spin (IS) state are the key factors for realizing efficient Co-based electrocatalysts for the oxygen evolution reaction (OER). However, according to thermodynamics, general cobalt oxide converts to the CoO₂ phase under OER condition, retarding the OER kinetics. Herein, we demonstrate a simple and scalable strategy to fabricate electrodes with maintaining Fe-CoOOH phase and an IS state under the OER. The changes of phase and spin states were uncovered by combining in-situ/operando X-ray based absorption spectroscopy and Raman spectroscopy. Electrochemical reconstruction of chalcogenide treated Co foam affords a highly enlarged active surface that conferred excellent catalytic activity and stability in a large-scale water electrolyzer. Our findings are meaningful in that the calculated results were experimentally verified through the operando analyses. It also proposes a new strategy for electrode fabrication and confirms the importance of real active phases and spin states under a particular reaction condition.

The phase and spin state affect catalytic activity of Co-based catalysts for oxygen evolution reaction. Herein, the authors demonstrate a simple reconstruction strategy to fabricate electrodes maintaining a Fe-CoOOH phase and an intermediate-spin state during catalysis.

Electronic Structure Tuning of 2D Metal (Hydr)oxides Nanosheets for Electrocatalysis

2D metal (hydr)oxide nanosheets have captured increasing interest in electrocatalytic applications aroused by their high specific surface areas, enriched chemically active sites, tunable physiochemical properties, etc. In particular, the electrocatalytic reactivities of materials greatly rely on their surface electronic structures. Generally speaking, the electronic structures of catalysts can be well adjusted via controlling their morphologies, defects, and heterostructures. In this Review, the latest advances in 2D metal (hydr)oxide nanosheets are first reviewed, including the applications in electrocatalysis for the hydrogen evolution reaction, oxygen reduction reaction, and oxygen evolution reaction. Then, the electronic structure-property relationships of 2D metal (hydr)oxide nanosheets are discussed to draw a picture of enhancing the electrocatalysis performances through a series of electronic structure tuning strategies. Finally, perspectives on the current challenges and the trends for the future design of 2D metal (hydr)oxide electrocatalysts with prominent catalytic activity are outlined. It is expected that this Review can shed some light on the design of next generation electrocatalysts.

Construction of tetrahedral CoO4 vacancies for activating the high oxygen evolution activity of Co3-xO4-δ porous nanosheet arrays

This study presents low-crystalline and non-stoichiometric cobalt oxide (Co3-xO4-δ) porous nanosheet arrays (PNAs) grown on carbon fiber cloth (CFC) (Co3-xO4-δ PNAs/CFC) by a facile in situ anodic oxidation strategy. We firstly verified that the above prepared low crystalline cobalt oxide contained tetrahedral CoO4 vacancies, resulting in the creation of O vacancies at adjacent octahedral CoO6 sites, allowing the generation of tetragonal-pyramidal CoO5 sites which were regarded as active sites and being accessible for the oxygen evolution reaction (OER) with different reaction mechanisms compared to that of traditional CoO6 sites in high-crystalline and stoichiometric Co3O4, thus endowing Co3-xO4-δ PNAs/CFC with significantly improved OER activity and superior stability compared to their crystalline counterparts (Co3O4 PNAs/CFC), as illustrated by experiments and density functional theory (DFT) calculations. This study will open up a new approach for the synthesis of defect-rich materials and provide new insight into the structure-property relationship of OER catalysts.

Enhancing the Electrocatalysis of LiNi0.5Co0.2Mn0.3O2 by Introducing Lithium Deficiency for Oxygen Evolution Reaction

LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523), as a cathode material for rechargeable lithium-ion batteries, has attracted considerable attention and been successfully commercialized for decades. NCM is also a promising electrocatalyst for the oxygen evolution reaction (OER), and the catalytic activity is highly correlated to its structure. In this paper, we successfully obtain NCM523 with three different structures: spinel NCM synthesized at low temperature (LT-NCM), disordered NCM (DO-NCM) with lithium deficiency obtained at high temperature, and layered hexagonal NCM at high temperature (HT-NCM). By introducing lithium deficiency to tune the valence state of transition metals in NCM from Ni²⁺ to Ni³⁺, DO-NCM exhibits the best catalytic activity with the lowest onset potential (∼1.48 V) and Tafel slope (∼85.6 mV dec^-1), whereas HT-NCM exhibits the worst catalytic activity with the highest onset potential (∼1.63 V) and Tafel slope (∼241.8 mV dec^-1).

A novel core–double shell heterostructure derived from a metal–organic framework for efficient HER, OER and ORR electrocatalysis

A core–double shell heterostructure Co₉S₈@Co₉S₈@MoS₂-0.5 with multiple interfaces and a tunable electronic structure was constructed as an efficient tri-functional electrocatalyst.

Noble Metal-Free Nanocatalysts with Vacancies for Electrochemical Water Splitting

The fast development of nanoscience and nanotechnology has significantly advanced the fabrication of nanocatalysts and the in-depth study of the structural-activity characteristics of materials at the atomic level. Vacancies, as typical atomic defects or imperfections that widely exist in solid materials, are demonstrated to effectively modulate the physicochemical, electronic, and catalytic properties of nanomaterials, which is a key concept and hot research topic in nanochemistry and nanocatalysis. The recent experimental and theoretical progresses achieved in the preparation and application of vacancy-rich nanocatalysts for electrochemical water splitting are explored. Engineering of vacancies has shown to open up a new avenue beyond the traditional morphology, size, and composition modifications for the development of nonprecious electrocatalysts toward efficient energy conversion. First, an introduction followed by discussions of different types of vacancies, the approaches to create vacancies, and the advanced techniques widely used to characterize these vacancies are presented. Importantly, the correlations between the vacancies and activities of the vacancy-rich electrocatalysts via tuning the electronic states, active sites, and kinetic energy barriers are reviewed. Finally, perspectives on the existing challenges along with some opportunities for the further development of vacancy-rich noble metal-free electrocatalysts with high performance are discussed.

In Situ Growth of MoS2 Nanosheet Arrays and TS2 (T = Fe, Co, and Ni) Nanocubes onto Molybdate for Efficient Oxygen Evolution Reaction and Improved Hydrogen Evolution Reaction

Rationally designing efficient and low-price bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are vitally important to bring solar/electrical-to-hydrogen energy conversion processes into reality. Herein, we report on a synthetic method that leads to an in situ growth of ultrathin MoS₂ nanosheets and transition metal disulfide nanocubes onto the surface of Fe_1/3Co_1/3Ni_1/3MoO₄ nanorods for the first time. Such hybrids are found to serve as a bifunctional electrocatalyst with high activities for OER and HER, as represented by an impressive anodic and cathodic current density of 10 mA cm^-2 at 1.53 and -0.25 V, respectively. More importantly, the performance for OER is even better than that of IrO₂, the conventional noble metal electrocatalyst. These striking observations were interpreted in terms of the combination of strongly synergistic effect of multimetal components, large amount of exposed active site, and superaerophobia. The present methodology has been confirmed universal for synthesizing other molybdate solid solutions, which would open up new possibilities for designing novel non-noble bifunctional electrocatalysts for OER and HER.

Molten Salt Assisted Self-Assembly: Synthesis of Mesoporous LiCoO2 and LiMn2 O4 Thin Films and Investigation of Electrocatalytic Water Oxidation Performance of Lithium Cobaltate

Mesoporous thin films of transition metal lithiates (TML) belong to an important group of materials for the advancement of electrochemical systems. This study demonstrates a simple one pot method to synthesize the first examples of mesoporous LiCoO₂ and LiMn₂ O₄ thin films. Molten salt assisted self-assembly can be used to establish an easy route to produce mesoporous TML thin films. The salts (LiNO₃ and [Co(H₂ O)₆ ](NO₃ )₂ or [Mn(H₂ O)₄ ](NO₃ )₂ ) and two surfactants (10-lauryl ether and cethyltrimethylammonium bromide (CTAB) or cethyltrimethylammonium nitrate (CTAN)) form stable liquid crystalline mesophases. The charged surfactant is needed for the assembly of the necessary amount of salt in the hydrophilic domains of the mesophase, which produces stable metal lithiate pore-walls upon calcination. The films have a large pore size with a high surface area that can be increased up to 82 m² g^-1 . The method described can be adopted to synthesize other metal oxides and metal lithiates. The mesoporous thin films of LiCoO₂ show promising performance as water oxidation catalysts under pH 7 and 14 conditions. The electrodes, prepared using CTAN as the cosurfactant, display the lowest overpotentials in the literature among other LiCoO₂ systems, as low as 376 mV at 10 mA cm^-2 and 282 mV at 1 mA cm^-2 .

Morphology engineering of nickel molybdate hydrate nanoarray for electrocatalytic overall water splitting: from nanorod to nanosheet

We realize the modulation of nanoarray morphology from nanorod to nanosheet and improve the catalytic activity for water splitting by simply changing the hydrothermal temperature.