Insight into Structural Evolution, Active Sites, and Stability of Heterogeneous Electrocatalysts

Amorphous Engineering of Scalable Metal-Organic Framework-Derived Electrocatalyst for Highly Efficient Oxygen Evolution Reaction

The engineering of amorphous metal-organic frameworks (MOFs) offers potential opportunities for the construction of electrocatalysts for efficient oxygen evolution reaction (OER). Herein, highly efficient OER performance and durability in alkaline electrolyte are discovered for MOF-derived amorphous and porous electrocatalysts, which are synthesized in a brief procedure and can be facilely produced in scalable quantities. The structural inheritance of MOF amorphous catalysts is significant for the retention of catalytic sites and the diffusion of electrolytes, and the presence of Fe sites can change the electronic structure and effectively control the adsorption behavior of important intermediates, accelerating reaction kinetics. The obtained amorphous A-FeNi can be transformed from FeNi-MOF effortlessly and instantly, and it only needs low overpotentials of 152 and 232 mV at 10 and 100 mA cm-2 with a Tafel slope of 17 mV dec-1 in 1 m KOH for OER. Moreover, A-FeNi possesses high corrosion resistance and durability, therefore A-FeNi can work continually for at least 400 h at 100 mA cm-2. This work may pave a new avenue for the design of MOFs-related amorphous electrocatalyst.

In Situ Tracking of Water Oxidation Generated Nanoscale Dynamics in Layered Double Hydroxides Nanosheets

Layered double hydroxides (LDHs) are potential catalysts for water oxidation, and it is recognized that they undergo dynamic evolution during the operation. However, little is known about the interfacial behaviors at the nanoscale under working conditions nor the underlying effects on electrocatalytic performance. Herein, using electrochemical atomic force microscopy, we in situ visualize the heterogeneous evolution of LDH nanosheets during oxygen evolution reaction (OER). By further combining density functional theory calculations, we elucidate the origin of the heterogeneous dynamics and their impact on the OER efficiency. Our findings demonstrate that NiCo LDHs transform to the catalytically active NiCoOx(OH)2-x phase during OER, and the redox transition between is accompanied by compressive and tensile strain, leading to in-plane contraction and reversible expansion of the nanosheets. Nonisotropic strain and out-of-plane strain relaxation due to defects and interparticle interactions result in cracking and wrinkling in the nanostructure, which is responsible for the partial activation and long-term deterioration of LDH electrocatalysts toward the OER. With this knowledge, we suggest and validate that engineering defects can precisely tune these dynamic behaviors, improving the OER activity and stability among LDH-based electrocatalysts.

A low-cost and efficient route for large-scale synthesis of NiCoSx nanosheets with abundant sulfur vacancies towards quasi-industrial electrocatalytic oxygen evolution

Transition-metal sulfides (TMS) have piqued a great deal of interest due to their unprecious nature and high intrinsic catalytic activity for water splitting. In this work, a low-cost and efficient route was developed, which included electrodeposition to prepare Ni-Co layered double hydroxide (NiCo-LDH) followed by ion exchange to form nickel cobalt sulfide (NiCoSx). Electrochemical reduction was used to modulate sulfur vacancies in order to produce sulfur vacancies-rich NiCoSx with nanosheet arrays on -three-dimensional nickel foam (NiCoSx-0.4/NF) with a large area of more than 250 cm2. Combining data from experiments and density functional theoretical (DFT) calculations reveals that engineered sulfur vacancies change the electronic structure, electron transfer property, and surface electron density of NiCoSx, significantly improving the free energy of water adsorption and boosting electrocatalytic activity. The developed NiCoSx-0.4/NF has long-term stability of more than 300 h at 500 mA cm-2 in 1 M KOH at ambient temperature and only needs a 289 mV overpotential at 100 mA cm-2. Remarkably, the synthesized electrocatalyst rich in sulfur vacancies, exhibits exceptional performance with a high current density of up to 1.9 A cm-2 and 1 A cm-2 in 6 M KOH and leads to overpotentials of 286 mV at 80 °C and 358 mV at 60 °C, respectively. The catalyst's practicability under quasi-industrial conditions (60 °C, 6 M KOH) is further demonstrated by its long-term stability for 220 h with only a 3.9 % potential increase at 500 mA cm-2.

Defect-Promoted Ni-Based Layer Double Hydroxides with Enhanced Deprotonation Capability for Efficient Biomass Electrooxidation

Ni-based hydroxides are promising electrocatalysts for biomass oxidation reactions, supplanting the oxygen evolution reaction (OER) due to lower overpotentials while producing value-added chemicals. The identification and subsequent engineering of their catalytically active sites are essential to facilitate these anodic reactions. Herein, the proportional relationship between catalysts' deprotonation propensity and Faradic efficiency of 5-hydroxymethylfurfural (5-HMF)-to-2,5 furandicarboxylic acid (FDCA, FEFDCA ) is revealed by thorough density functional theory (DFT) simulations and atomic-scale characterizations, including in situ synchrotron diffraction and spectroscopy methods. The deprotonation capability of ultrathin layer-double hydroxides (UT-LDHs) is regulated by tuning the covalency of metal (M)-oxygen (O) motifs through defect site engineering and selection of M3+ co-chemistry. NiMn UT-LDHs show an ultrahigh FEFDCA of 99% at 1.37 V versus reversible hydrogen electrode (RHE) and retain a high FEFDCA of 92.7% in the OER-operating window at 1.52 V, about 2× that of NiFe UT-LDHs (49.5%) at 1.52 V. Ni-O and Mn-O motifs function as dual active sites for HMF electrooxidation, where the continuous deprotonation of Mn-OH sites plays a dominant role in achieving high selectivity while suppressing OER at high potentials. The results showcase a universal concept of modulating competing anodic reactions in aqueous biomass electrolysis by electronically engineering the deprotonation behavior of metal hydroxides, anticipated to be translatable across various biomass substrates.

Role of decomposition products in the oxidation of cyclohexene using a manganese(III) complex

Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)₂). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water.

Water Oxidation by a Copper(II) Complex with 6,6'-Dihydroxy-2,2'-Bipyridine Ligand: Challenges and an Alternative Mechanism

Recently, copper(II) complexes have been extensively investigated as oxygen-evolution reaction (OER) catalysts through a water-oxidation reaction. Herein, new findings regarding OER in the presence of a Cu(II) complex with 6,6'-dihydroxy-2,2'-bipyridine ligand are reported. Using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, Raman spectroscopy, in situ visible microscopy, in situ visible spectroelectrochemistry, X-ray absorption spectroscopy, and electrochemistry, it is hypothesized that the film formed on the electrode's surface in the presence of this complex causes an appropriated matrix to produce Cu (hydr)oxide. The resulting Cu (hydr)oxide could be a candidate for OER catalysis. The formed film could form Cu (hydr)oxide and stabilize it. Thus, OER activity increases in the presence of this complex.

Perovskite-Socketed Sub-3 nm Copper for Enhanced CO2 Electroreduction to C2

In situ socketing metal nanoparticles onto perovskite oxides has shown great potential in heterogeneous catalysis, but its employment in boosting ambient CO₂ electroreduction (CER) is unexplored. Here, a CER catalyst of perovskite-socketed sub-3 nm Cu equipped with strong metal-support interactions (SMSIs) is constructed to promote efficient and stable CO₂ -to-C₂₊ conversion. For such a catalyst, plentiful sub-3 nm ellipsoid Cu particles are homogeneously and epitaxially anchored on the perovskite backbones, with concomitant creation of significant SMSIs. These SMSIs are able to not only modulate electronic structure of active Cu and facilitate adsorption/activation of key intermediates, but also to strengthen perovskite-Cu adhesion and intensify resistance to structural degradation. Beneficial from these advantageous merits, when evaluated in CER, it performs comparably to or better than most reported Cu-based heteronanostructures. Relative to a physical-mixture counterpart, it features marked improvements (up to 6.2 folds) in activity and selectivity for C₂₊ , together with greatly boosted stability (>80 h). This work gives a new avenue to rationally design more advanced Cu-based heteronanostructures for CER.

Recent Progress of Diatomic Catalysts: General Design Fundamentals and Diversified Catalytic Applications

In recent years, some experiments and theoretical work have pointed out that diatomic catalysts not only retain the advantages of monoatomic catalysts, but also introduce a variety of interactions, which exceed the theoretical limit of catalytic performance and can be applied to many catalytic fields. Here, the interaction between adjacent metal atoms in diatomic catalysts is elaborated: synergistic effect, spacing enhancement effect (geometric effect), and electronic effect. With regard to the classification and characterization of various new diatomic catalysts, diatomic catalysts are classified into four categories: heteronuclear/homonuclear, with/without carbon carriers, and their characterization measures are introduced and explained in detail. In the aspect of preparation of diatomic catalysts, the widely used atomic layer deposition method, metal-organic framework derivative method, and simple ball milling method are introduced, with emphasis on the formation mechanism of diatomic catalysts. Finally, the effective control strategies of four diatomic catalysts and the key applications of diatomic catalysts in electrocatalysis, photocatalysis, thermal catalysis, and other catalytic fields are given.

Super-Coordinated Nickel N4 Ni1 O2 Site Single-Atom Catalyst for Selective H2 O2 Electrosynthesis at High Current Densities

Electrochemical production of hydrogen peroxide (H₂ O₂ ) from O₂ on single-atom catalysts has attracted great attention, yet the quest for robust catalysts is driven by achieving >90 % Faradaic efficiency (FE) under industrial-relevant current densities (>100 mA cm^-2 ). Herein we synthesize a catalyst that contains single nickel site coordinated by four nitrogen and two oxygen atoms (i.e., N₄ Ni₁ O₂ ) via involving carboxyl functionalized multiwall carbon nanotubes as a substrate to provide extra O coordination to the regular NiN₄ site. It has a cathodic energy efficiency of approximately 82 % and a H₂ O₂ FE of around 96 % at 200 mA cm^-2 current density, outperforming the reported single-atom catalysts for H₂ O₂ electrosynthesis.

Finding the True Catalyst for Water Oxidation at Low Overpotential in the Presence of a Metal Complex

The design of molecular-based catalysts for oxygen-evolution reaction (OER) requires more investigations for the true catalyst to be found. First-row transition metal complexes are extensively investigated for OER, but the role of these metal complexes as a true catalyst is doubtful. Some doubts have been expressed about the role of first-row transition metal complexes for OER at high overpotentials (η > 450). Generally, the detection of the true catalyst has so far been focused on high overpotentials (η > 450) because at low overpotentials (η < 450), many methods are not sensitive enough to detect small amounts of heterogeneous catalysts on the electrode surface during the first seconds of the reaction. Ni(II) phthalocyanine-tetra sulfonate tetrasodium (1) is in moderate conditions (at 20-50 °C and pH 5-13) in the absence of electrochemical driving forces, which could make it noteworthy for OER. Herein, the results of OER in the presence of 1 at low overpotentials under alkaline conditions are presented. In addition, in the presence of Ni complexes, using an Fe ion is introduced as a new method for detecting Ni (hydr)oxide under OER. Our experiments indicate that in the presence of a homogeneous OER (pre)catalyst, a deep investigation is necessary to rule out the heterogeneous catalysts formed. Our approach is a roadmap in the field of catalysis to understand the OER mechanism in the presence of a molecular Ni-based catalyst design. Our results shown in this study are likely to open up new perspectives and discussion on many molecular catalysts in a considerable part of the chemistry community.

Sodium Cobalticarborane: A Promising Precatalyst for Oxygen Evolution Reaction

Water splitting is a helpful way of converting renewable electricity into fuel. The oxygen evolution reaction (OER) is a slow reaction that provides low-cost electrons for water reduction reactions. Thus, finding an efficient, low-cost, stable, and environmentally friendly OER catalyst is critical for water splitting. Here, sodium cobalticarborane (1) is introduced as a promising precatalyst for forming an OER cobalt-based catalyst. The cobalt-based catalyst was characterized by several methods and is suggested to be Co(III) (hydr)oxide. Using fluorine-doped tin oxide, glassy carbon, platinum, and gold electrodes, the OER activity of the cobalt-based precatalyst was investigated. The overpotential for the onset of OER in the presence of 1 is 315 mV using fluorine-doped tin oxide electrodes. The onsets of OERs in the presence of 1 using gold, platinum, and glassy carbon electrodes in KOH solutions (1.0 M) turned out to be 275, 284, and 330 mV, respectively. The nanoparticles on the gold electrodes exhibit significant OER activity with a Tafel slope of 63.8 mV/decade and an overpotential at 541 mV for 50 mA/cm². In the case of the glassy carbon electrodes, a Tafel slope of 109.9 mV/decade and an overpotential of 548 mV for 10 mA/cm² is recorded for the catalyst. This paper outlines an interesting approach to synthesize cobalt oxide for OER through a slow decomposition of a precatalyst.