A Template Editing Strategy to Create Interlayer-Confined Active Species for Efficient and Durable Oxygen Evolution Reaction

Unraveling the Potential Dependence of Active Structures and Reaction Mechanism of Ni-based MOFs Electrocatalysts for Alkaline OER

Nickel-based metal-organic frameworks (MOFs) with flexible structure units provide a broad platform for designing highly efficient electrocatalysts, especially for alkaline oxygen evolution reaction (OER). However, the stability of MOFs under harsh and dynamic reaction conditions poses significant challenges, resulting in ambiguous structure-activity relationships in MOFs-based OER research. Herein, Ni-benzenedicarboxylic acid-based MOF (NiBDC) is selected as prototypical catalyst to elucidate  its real active sites for OER and reaction pathway under different reaction states. Electrochemical measurements combined with X-ray absorption spectroscopy (XAS) and Raman spectroscopy reveal that the complete reconstruction of NiBDC to β-NiOOH in the chronoamperometry activation process is responsible for significantly increased OER performance. In situ XAS and Raman results further demonstrate the electro-oxidation of β-NiOOH into γ-NiOOH at high-potential state (above 1.6 V vs RHE). Furthermore, the collective evidences from key reaction intermediates and isotope-labeled products definitely unravel the potential dependence of OER mechanism: OER process at low-potential state proceeds mainly through the lattice oxygen-mediated mechanism, while adsorbate evolution mechanism emerges as the predominant pathway at high-potential state. Interestingly, the dynamically changing OER mechanism can not only reduce the required overpotential at the low-potential state but also improve the electrochemical stability of catalysts at high-potential state.

Rational Design of Ultrahigh-Loading Ir Single Atoms on Reconstructed Mn─NiOOH for Enhanced Catalytic Performance in Urea-Water Electrolysis

Investigating advanced electrocatalysts is crucial for improving the efficacy of water splitting to generate environmentally friendly fuel. The discovery of highly effective electrocatalysts, capable of driving oxygen evolution reaction (OER) and urea oxidation reaction (UOR) in urea-alkaline environments, is pivotal for advancing large-scale hydrogen production. This study aims to introduce a new method that involves creating nanosheets of high-loading iridium single atoms embedded in a manganese-containing nickel oxyhydroxide matrix (Ir@Mn─NiOOH). These nanostructures are derived from self-supported hydrate pre-catalyst nanosheets grown on nickel foam and then activated through electrochemical etching pretreatment. The Ir@Mn─NiOOH nanoarchitecture displays outstanding electrocatalytic activity, having a low overpotential of just 258 mV and a potential of 1.319 V (at 10 mA cm-2) for OER and UOR, respectively. Such extraordinary catalytic characteristics of Ir@Mn─NiOOH is mainly owing to the strong synthetic electronic interaction between Ir single atoms and Mn─NiOOH, which can change its electronic characteristics and boost electrochemical catalytic sites. This research presents a new way to produce exceptionally efficient catalysts by adding a synergistic effect to complex multi-electron processes.

Metal-Hydroxide Organic Frameworks for Aqueous Nickel-Zinc Batteries

Hydroxides exhibit a high theoretical capacity for energy storage by ion release and are often intercalated with anions to enhance the ion migration kinetics. In this study, a series of metal-hydroxide organic frameworks (MHOFs) are synthesized by intercalating aromatic organic linkers into hydroxides using I-M/Ni(OH)2 (where M = Co2+, Cu2+, Mg2+, Fe2+). The coordination environment and layer spacing (1.09 nm) of I-M/Ni(OH)2 are explored by X-ray absorption fine structure and cryo-electron microscopy. The intercalation nanostructure improves the conductivity of the hydroxides and facilitates Zn2+ migration by increasing the interlayer spacing, while enhancing the rate capability and cycling stability. Consequently, the I-Co/Ni(OH)2 material exhibites a satisfactory specific capacity of 0.35 mAh cm-2 at 3 mA cm-2 and a high peak power density of 6.78 mW cm-2. This study offers a novel perspective on the design of intercalated hydroxide and provides new insights into high-performance nickel-zinc batteres.

Sulfur filling activates vacancy-induced C-C bond cleavage in polyol electrooxidation

Using the electrochemical polyol oxidation reaction (POR) to produce formic acid over nickel-based oxides/hydroxides (NiO x H y ) is an attractive strategy for the electrochemical upgrading of biomass-derived polyols. The key step in the POR, i.e. the cleavage of the C-C bond, depends on an oxygen-vacancy-induced mechanism. However, a high-energy oxygen vacancy is usually ineffective for Schottky-type oxygen-vacancy-rich β-Ni(OH)2 (VSO-β-Ni(OH)2). As a result, both β-Ni(OH)2 and VSO-β-Ni(OH)2 cannot continuously catalyze oxygen-vacancy-induced C-C bond cleavage during PORs. Here, we report a strategy of oxygen-vacancy-filling with sulfur to synthesize a β-Ni(OH)2 (S-VO-β-Ni(OH)2) catalyst, whose oxygen vacancies are protected by filling with sulfur atoms. During PORs over S-VO-β-Ni(OH)2, the pre-electrooxidation-induced loss of sulfur and structural self-reconstruction cause the in-situ generation of stable Frenkel-type oxygen vacancies for activating vacancy-induced C-C bond cleavage, thus leading to excellent POR performances. This work provides an intelligent approach for guaranteeing the sustaining action of the oxygen-vacancy-induced catalytic mechanism in electrooxidation reactions.

Boosting Oxygen Evolution Reaction Performance on NiFe-Based Catalysts Through d-Orbital Hybridization

Anion-exchange membrane water electrolyzers (AEMWEs) for green hydrogen production have received intensive attention due to their feasibility of using earth-abundant NiFe-based catalysts. By introducing a third metal into NiFe-based catalysts to construct asymmetrical M-NiFe units, the d-orbital and electronic structures can be adjusted, which is an important strategy to achieve sufficient oxygen evolution reaction (OER) performance in AEMWEs. Herein, the ternary NiFeM (M: La, Mo) catalysts featured with distinct M-NiFe units and varying d-orbitals are reported in this work. Experimental and theoretical calculation results reveal that the doping of La leads to optimized hybridization between d orbital in NiFeM and 2p in oxygen, resulting in enhanced adsorption strength of oxygen intermediates, and reduced rate-determining step energy barrier, which is responsible for the enhanced OER performance. More critically, the obtained NiFeLa catalyst only requires 1.58 V to reach 1 A cm-2 in an anion exchange membrane electrolyzer and demonstrates excellent long-term stability of up to 600 h.

Hybrid Overlayer of Defective Ni-MOF and NiO Nanoparticles toward Efficient TiO2/CdS Type-II Heterojunction

The severe photocorrosion of cadmium sulfide (CdS) restricts its practical application for solar hydrogen production. Although remarkable progress has been achieved with an overlayer strategy for isolating the CdS surface, the lifetime of CdS-based photoanodes is still far from the actual requirements. Herein, a hybrid overlayer of defective Ni-MOF and NiO nanoparticles has been developed through the chemical bath deposition method with postannealing. This hybrid overlayer of Ni-MOF-d is coated on the surface of the TiO2/CdS type-II heterojunction. The composite photoanode exhibits a photocurrent density of 4.41 mA cm-2 at 1.23 VRHE, which is 3.47- and 1.32-fold that of CdS and TiO2/CdS, respectively. The Ni-MOF-d overlayer gives rise to a negative shift of the onset potential by 59.51 mV. After a long-term stability test of 11 h, a photocurrent retention of 70% is observed, which is among the most robust CdS-based photoanodes. The kinetics studies reveal that the performance improvements can be attributed to the multiple functions of the Ni-MOF-d hybrid overlayer, including isolating the CdS surface from the electrolyte, cocatalyzing the electrode oxidation processes, passivating the surface defect states of CdS, and facilitating the charge injection from the photoanode to the electrolyte.

Unveiling Confinement Engineering for Achieving High-Performance Rechargeable Batteries

The confinement effect, restricting materials within nano/sub-nano spaces, has emerged as an innovative approach for fundamental research in diverse application fields, including chemical engineering, membrane separation, and catalysis. This confinement principle recently presents fresh perspectives on addressing critical challenges in rechargeable batteries. Within spatial confinement, novel microstructures and physiochemical properties have been raised to promote the battery performance. Nevertheless, few clear definitions and specific reviews are available to offer a comprehensive understanding and guide for utilizing the confinement effect in batteries. This review aims to fill this gap by primarily summarizing the categorization of confinement effects across various scales and dimensions within battery systems. Subsequently, the strategic design of confinement environments is proposed to address existing challenges in rechargeable batteries. These solutions involve the manipulation of the physicochemical properties of electrolytes, the regulation of electrochemical activity, and stability of electrodes, and insights into ion transfer mechanisms. Furthermore, specific perspectives are provided to deepen the foundational understanding of the confinement effect for achieving high-performance rechargeable batteries. Overall, this review emphasizes the transformative potential of confinement effects in tailoring the microstructure and physiochemical properties of electrode materials, highlighting their crucial role in designing novel energy storage devices.

Accelerated Dynamic Reconstruction in Metal-Organic Frameworks with Ligand Defects for Selective Electrooxidation of Amines to Azos Coupling with Hydrogen Production

Electrosynthesis coupled hydrogen production (ESHP) mostly involves catalyst reconstruction in aqueous phase, but accurately identifying and controlling the process is still a challenge. Herein, we modulated the electronic structure and exposed unsaturated sites of metal-organic frameworks (MOFs) via ligand defect to promote the reconstruction of catalyst for azo electrosynthesis (ESA) coupled with hydrogen production overall reaction. The monolayer Ni-MOFs achieved 89.8 % Faraday efficiency and 90.8 % selectivity for the electrooxidation of 1-methyl-1H-pyrazol-3-amine (Pyr-NH2) to azo, and an 18.5-fold increase in H2 production compared to overall water splitting. Operando X-ray absorption fine spectroscopy (XAFS) and various in situ spectroscopy confirm that the ligand defect promotes the potential dependent dynamic reconstruction of Ni(OH)2 and NiOOH, and the reabsorption of ligand significantly lowers the energy barrier of rate-determining step (*Pyr-NH to *Pyr-N). This work provides theoretical guidance for modulation of electrocatalyst reconstruction to achieve highly selective ESHP.

Dual-site segmentally synergistic catalysis mechanism: boosting CoFeSx nanocluster for sustainable water oxidation

Efficient oxygen evolution reaction electrocatalysts are essential for sustainable clean energy conversion. However, catalytic materials followed the conventional adsorbate evolution mechanism (AEM) with the inherent scaling relationship between key oxygen intermediates *OOH and *OH, or the lattice-oxygen-mediated mechanism (LOM) with the possible lattice oxygen migration and structural reconstruction, which are not favorable to the balance between high activity and stability. Herein, we propose an unconventional Co-Fe dual-site segmentally synergistic mechanism (DSSM) for single-domain ferromagnetic catalyst CoFeSx nanoclusters on carbon nanotubes (CNT) (CFS-ACs/CNT), which can effectively break the scaling relationship without sacrificing stability. Co3+ (L.S, t2g6eg0) supplies the strongest OH* adsorption energy, while Fe3+ (M.S, t2g4eg1) exposes strong O* adsorption. These dual-sites synergistically produce of Co-O-O-Fe intermediates, thereby accelerating the release of triplet-state oxygen ( ↑ O = O ↑ ). As predicted, the prepared CFS-ACs/CNT catalyst exhibits less overpotential than that of commercial IrO2, as well as approximately 633 h of stability without significant potential loss.

Homogeneous-like photocatalysis: covalent immobilization of an iridium(III) complex onto polystyrene brushes grafted on SiO2 nanoparticles as a mass/charge transfer-enhanced platform

Current heterogeneous photocatalysis faces the major bottlenecks of limited mass transfer, charge recombination and tedious immobilization of expensive photocatalysts. In this work, fac-Ir(ppy)3 is directly anchored at a low cost via covalent linkage to poly(4-vinyl benzyl chloride) (PVBC) brushes grafted on SiO2 nanoparticles (PVBC@SiO2 NPs) via Friedel-Crafts alkylation, affording PVBC@SiO2 NP-supported fac-Ir(ppy)3 with high luminous efficacies such as emission lifetime and quantum yield. In the reductive cross-coupling of benzaldehydes/acetophenones with 1,4-dicyanobenzene (1,4-DCB), the as-fabricated photocatalyst affords benzhydrols in the same yields as homogeneous fac-Ir(ppy)3, except for o-substituted benzaldehydes/acetophenones. In terms of the same yields as homogeneous fac-Ir(ppy)3, a new catalytic model, named homogeneous-like photocatalysis, is proposed. In this catalytic model, the open stretching of PVBC brushes in DMSO enables the anchored fac-Ir(ppy)3 to catalyse the reaction in a similar manner as homogeneous fac-Ir(ppy)3, effectively avoiding charge recombination and mass transfer limitation. Furthermore, no significant decrease in yield (<5%) is observed over eight catalytic cycles, due to the good chemical and mechanical stabilities of PVBC@SiO2 NP-supported fac-Ir(ppy)3. Overall, the immobilization of fac-Ir(ppy)3 onto the PVBC brushes grafted on SiO2 NPs provides a mass/charge transfer-enhanced platform for supported photocatalysts.

Design of iron oxyhydroxide nanosheets coated on Co species embedded in nanoporous carbon for oxygen evolution reaction

There is still a tremendous challenge in designing environmentally friendly oxygen evolution reaction (OER) catalysts that are inexpensive and high-performing for practical applications. Herein, the self-sacrificing template zeolitic imidazolate framework-67 (ZIF-67) was pyrolyzed under N2 atmosphere to generate Co species embedded in nanoporous carbon (Co-NC). Then, iron oxyhydroxide (FeOOH) was wrapped onto the Co-NC surface via electrodeposition to shape the Co-NC@FeOOH composites. Benefiting from the core-shell structure, high conductivity, and distributed active sites, Co-NC@FeOOH presents distinguished OER performance with a low overpotential (336 mV) at 10 mA cm-2 and small Tafel slope (49.46 mV dec-1). This work furnishes a rosy passage for receiving cost-effective electrocatalysts with high efficiency for OER.

Decoding Natural Strategy: Oxygen-Evolution Reaction on the Surface of Nickel Oxyhydroxide at Extremely Low Overpotential

Although nickel (hydr)oxides in the absence of other metal ions are conventionally deemed inefficient catalysts for the oxygen-evolution reaction (OER) under alkaline conditions, this study reveals that nickel oxyhydroxide displays an OER activity at the associated peak for Ni(II) to Ni(III) oxidation postcharge accumulation. This occurs with only 90-120 mV overpotentials (at a low current density) and a Tafel slope of 297 mV/decade in a 0.10 M KOH solution. In the initial seconds, the Faraday efficiency lingers at a relatively low 20%, which can be attributed to charge storage. Nonetheless, as the duration extends to reach the 200 s mark, the efficiency notably escalates, exceeding 80%. Additionally, a mechanism for the OER in this low-overpotential zone is proposed, grounded in our investigation of the Ni(II) to Ni(III) peak and the OER region through in situ Raman spectroscopy. Taking into account the quantity of oxygen generated and the concentrations of redox-active Ni ions in the region of the redox peak, a turnover frequency of at a potential of 4.3 × 10-4 s-1 at 1.37 V was calculated. The documented reduction in overpotential during the OER may be ascribed to the complex interplay between the process of the OER and charge accumulation. The convergence of these reciprocally influencing factors facilitates a notably low overpotential in the OER. Our findings bear substantial implications for developing highly efficient and stable electrocatalysts for the OER in water-splitting applications.

Dynamically Restructuring Nix Cry O Electrocatalyst for Stable Oxygen Evolution Reaction in Real Seawater

In the pursuit of long-term stability for oxygen evolution reaction (OER) in seawater, retaining the intrinsic catalytic activity is essential but has remained challenging. Herein, we developed a Nix Cry O electrocatalyst that manifested exceptional OER stability in alkaline condition while improving the activity over time by dynamic self-restructuring. In 1 M KOH, Nix Cry O required overpotentials of only 270 and 320 mV to achieve current densities of 100 and 500 mA cm-2 , respectively, with excellent long-term stability exceeding 475 h at 100 mA cm-2 and 280 h at 500 mA cm-2 . The combination of electrochemical measurements and in situ studies revealed that leaching and redistribution of Cr during the prolonged electrolysis resulted in increased electrochemically active surface area. This eventually enhanced the catalyst porosity and improved OER activity. Nix Cry O was further applied in real seawater from the Red Sea (without purification, 1 M KOH added), envisaging that the dynamically evolving porosity can offset the adverse active site-blocking effect posed by the seawater impurities. Remarkably, Nix Cry O exhibited stable operation for 2000, 275 and 100 h in seawater at 10, 100 and 500 mA cm-2 , respectively. The proposed catalyst and the mechanistic insights represented a step towards realization of non-noble metal-based direct seawater splitting.

Electrochemical Production of Glycolate Fuelled By Polyethylene Terephthalate Plastics with Improved Techno-Economics

Electrochemical valorization of polyethylene terephthalate (PET) waste streams into commodity chemicals offers a potentially sustainable route for creating a circular plastic economy. However, PET wastes upcycling into valuable C2 product remains a huge challenge by the lack of an electrocatalyst that can steer the oxidation economically and selectively. Here, it is reported a catalyst comprising Pt nanoparticles hybridized with γ-NiOOH nanosheets supported on Ni foam (Pt/γ-NiOOH/NF) that favors electrochemical transformation of real-word PET hydrolysate into glycolate with high Faradaic efficiency (> 90%) and selectivity (> 90%) across wide reactant (ethylene glycol, EG) concentration ranges under a marginal applied voltage of 0.55 V, which can be paired with cathodic hydrogen production. Computational studies combined with experimental characterizations elucidate that the Pt/γ-NiOOH interface with substantial charge accumulation gives rise to an optimized adsorption energy of EG and a decreased energy barrier of potential determining step. A techno-economic analysis demonstrates that, with the nearly same amount of resource investment, the electroreforming strategy towards glycolate production can raise revenue by up to 2.2 times relative to conventional chemical process. This work may thus serve as a framework for PET wastes valorization process with net-zero carbon footprint and high economic viability.