Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion

Sustainable bacterial cellulose derived composites for high-efficiency hydrogen evolution reaction

Incorporating heteroatoms into carbon structure has been demonstrated to be efficient for hydrogen evolution reaction (HER). However, the preparation complexity and poor durability are insufficient for the future hydrogen economy. In this work, the preparation of ZIF-67/BC precursor with BC as the template was done for the in-situ growth of MOFs (ZIF-67) crystals, followed by the carbonization and phosphating of ZIF-67/BC to prepare the CoP-NC/CBC N-doped composite carbon material with CoP as the primary active material. The results show that as an HER catalyst, CoP-NC/CBC can provide a current density of 10 mA cm^-2 at an overpotential of 182 mV in the acidic electrolyte of 0.5 M H₂SO₄ or the same current density at an overpotential of 151 mV in the alkaline electrolyte of 1.0 M KOH. The work validates a design idea for advanced non-precious metal-based HER catalysts with high activity and stability.

Synthesis of Ketjenblack Decorated Pillared Ni(Fe) Metal-Organic Frameworks as Precursor Electrocatalysts for Enhancing the Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) have been investigated with regard to the oxygen evolution reaction (OER) due to their structure diversity, high specific surface area, adjustable pore size, and abundant active sites. However, the poor conductivity of most MOFs restricts this application. Herein, through a facile one-step solvothermal method, the Ni-based pillared metal-organic framework [Ni2(BDC)2DABCO] (BDC = 1,4-benzenedicarboxylate, DABCO = 1,4-diazabicyclo[2.2.2]octane), its bimetallic nickel-iron form [Ni(Fe)(BDC)2DABCO], and their modified Ketjenblack (mKB) composites were synthesized and tested toward OER in an alkaline medium (KOH 1 mol L-1). A synergistic effect of the bimetallic nickel-iron MOF and the conductive mKB additive enhanced the catalytic activity of the MOF/mKB composites. All MOF/mKB composite samples (7, 14, 22, and 34 wt.% mKB) indicated much higher OER performances than the MOFs and mKB alone. The Ni-MOF/mKB14 composite (14 wt.% of mKB) demonstrated an overpotential of 294 mV at a current density of 10 mA cm-2 and a Tafel slope of 32 mV dec-1, which is comparable with commercial RuO2, commonly used as a benchmark material for OER. The catalytic performance of Ni(Fe)MOF/mKB14 (0.57 wt.% Fe) was further improved to an overpotential of 279 mV at a current density of 10 mA cm-2. The low Tafel slope of 25 mV dec-1 as well as a low reaction resistance due to the electrochemical impedance spectroscopy (EIS) measurement confirmed the excellent OER performance of the Ni(Fe)MOF/mKB14 composite. For practical applications, the Ni(Fe)MOF/mKB14 electrocatalyst was impregnated into commercial nickel foam (NF), where overpotentials of 247 and 291 mV at current densities of 10 and 50 mA cm-2, respectively, were realized. The activity was maintained for 30 h at the applied current density of 50 mA cm-2. More importantly, this work adds to the fundamental understanding of the in situ transformation of Ni(Fe)DMOF into OER-active α/β-Ni(OH)2, β/γ-NiOOH, and FeOOH with residual porosity inherited from the MOF structure, as seen by powder X-ray diffractometry and N2 sorption analysis. Benefitting from the porosity structure of the MOF precursor, the nickel-iron catalysts outperformed the solely Ni-based catalysts due to their synergistic effects and exhibited superior catalytic activity and long-term stability in OER. In addition, by introducing mKB as a conductive carbon additive in the MOF structure, a homogeneous conductive network was constructed to improve the electronic conductivity of the MOF/mKB composites. The electrocatalytic system consisting of earth-abundant Ni and Fe metals only is attractive for the development of efficient, practical, and economical energy conversion materials for efficient OER activity.

Electronically Conductive, Multifunctional Polymer Binder for Highly Active, Stable, and Abundant Composite Electrodes for Oxygen Evolution

The electrolysis of water to form hydrogen and oxygen is a promising method to store renewable energy. This method requires electrodes that convert water into protons, electrons, and oxygen. We report a multifunctional polymer that conducts electrons and ions and may coreact with the electrocatalyst in the oxygen evolution reaction (OER). The electrodes were prepared in two steps from off-the-shelf reagents. They operate with low loadings of abundant catalysts and are among the most active (100 mA cm^-2 at 1.43 V vs RHE (1.41 V, iR-corrected)) and stable electrodes, reported to date under harsh conditions (85 °C, 6 M KOH, 120 h (0.69% loss over the first 14.5 h and then 0.61% loss over 105.5 h)). Control experiments on glassy carbon electrodes showed that the polycarbazole system significantly outperformed a Nafion system of the same catalyst loading. This simple strategy can be applied to other types of electrodes.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Simple Approach for Reconciling Cyclic Voltammetry with Hydrogen Adsorption Energy for Hydrogen Evolution Exchange Current

Cyclic voltammetry (CV) is a standard technique to analyze the current-potential characteristics of the hydrogen evolution reaction (HER). Herein, we develop a computational quantum-scaled CV model for the HER building on the Butler-Volmer relation for a one-step, one-charge transfer process. Owing to a universal and absolute rate constant verified by fitting to experimental cyclic voltammograms of elemental metals, we show that the model quantifies the exchange current─the main analytical descriptor for HER activity─solely using the hydrogen adsorption free energy obtained from density functional theory calculations. Furthermore, the model resolves controversies over analytical studies for HER kinetics.

Rhenium-Based Electrocatalysts for Water Splitting

Due to the contamination and global warming problems, it is necessary to search for alternative environmentally friendly energy sources. In this area, hydrogen is a promising alternative. Hydrogen is even more promising, when it is obtained through water electrolysis operated with renewable energy sources. Among the possible devices to perform electrolysis, proton exchange membrane (PEM) electrolyzers appear as the most promising commercial systems for hydrogen production in the coming years. However, their massification is affected by the noble metals used as electrocatalysts in their electrodes, with high commercial value: Pt at the cathode where the hydrogen evolution reaction occurs (HER) and Ru/Ir at the anode where the oxygen evolution reaction (OER) happens. Therefore, to take full advantage of the PEM technology for green H2 production and build up a mature PEM market, it is imperative to search for more abundant, cheaper, and stable catalysts, reaching the highest possible activities at the lowest overpotential with the longest stability under the harsh acidic conditions of a PEM. In the search for new electrocatalysts and considering the predictions of a Trasatti volcano plot, rhenium appears to be a promising candidate for HER in acidic media. At the same time, recent studies provide evidence of its potential as an OER catalyst. However, some of these reports have focused on chemical and photochemical water splitting and have not always considered acidic media. This review summarizes rhenium-based electrocatalysts for water splitting under acidic conditions: i.e., potential candidates as cathode materials. In the various sections, we review the mechanism concepts of electrocatalysis, evaluation methods, and the different rhenium-based materials applied for the HER in acidic media. As rhenium is less common for the OER, we included a section about its use in chemical and photochemical water oxidation and as an electrocatalyst under basic conditions. Finally, concluding remarks and perspectives are given about rhenium for water splitting.

Two-Dimensional Covalent Framework Derived Nonprecious Transition Metal Single-Atomic-Site Electrocatalyst toward High-Efficiency Oxygen Reduction

Designing an active, stable, and nonprecious metal catalyst substitute for Pt in the oxygen reduction reaction (ORR) is highly demanded for energy-efficient and cost-effective prototype devices. Single-atomic-site catalysts (SASCs) have been widely concerning because of their maximum atomic utilization and precise structural regulation. Despite being challenging, the controllable synthesis of SASCs is crucial for optimizing ORR activity. Here, we demonstrate an ultrathin organometallic framework template-assisted pyrolysis strategy to synthesize SASCs with a unique two-dimensional (2D) architecture. Electrochemical measurements revealed that Fe-SASCs displayed an excellent ORR activity in an alkaline media, having a half-wave potential and a diffusion-limited current density comparable to those of commercial Pt/C. Remarkably, the durability and methanol tolerance of Fe-SASCs were even superior to those of Pt/C. Furthermore, Fe-SASCs displayed a maximum power density of 142 mW cm^-2 with a current density of 235 mA cm^-2 as a cathode catalyst in a zinc-air battery, showing its great potential for practical applications.

Carbonized Polydopamine-Based Nanocomposites: The Effect of Transition Metals on the Oxygen Electrocatalytic Activity

The electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the most critical processes in renewable energy-related technologies, such as fuel cells, water electrolyzers, and unitized regenerative fuel cells. N-doped carbon composites have been demonstrated to be promising ORR/OER catalyst candidates because of their excellent electrical properties, tunable pore structure, and environmental compatibility. In this study, we prepared porous N-doped carbon nanocomposites (NC) by combining mussel-inspired polydopamine (PDA) chemistry and transition metals using a solvothermal carbonization strategy. The complexation between dopamine catechol groups and transition metal ions (Fe, Ni, Co, Zn, Mn, Cu, and Ti) results in hybrid structures with embedded metal nanoparticles converted to metal-NC composites after the carbonization process. The influence of the transition metals on the structural, morphological, and electrochemical properties was analyzed in detail. Among them, Cu, Co, Mn, and Fe N-doped carbon nanocomposites exhibit efficient catalytic activity and excellent stability toward ORR. This method improves the homogeneous distribution of the catalytically active sites. The metal nanoparticles in reduced (MnO, Fe₃C) or metallic (Cu, Co) oxidation states are protected by the N-doped carbon layers, thus further enhancing the ORR performance of the composites. Still, only Co nanocomposite is also effective toward OER with a potential bifunctional gap (ΔE) of 0.867 V. The formation of Co-N active sites during the carbonization process, and the strong coupling between Co nanoparticles and the N-doped carbon layer could promote the formation of defects and the interfacial electron transfer between the catalyst surface, and the reaction intermediates, increasing the bifunctional ORR/OER performance.

A Low-Temperature Synthetic Route Toward a High-Entropy 2D Hexernary Transition Metal Dichalcogenide for Hydrogen Evolution Electrocatalysis

High-entropy (HE) metal chalcogenides are a class of materials that have great potential in applications such as thermoelectrics and electrocatalysis. Layered 2D transition-metal dichalcogenides (TMDCs) are a sub-class of high entropy metal chalcogenides that have received little attention to date as their preparation currently involves complicated, energy-intensive, or hazardous synthetic steps. To address this, a low-temperature (500 °C) and rapid (1 h) single source precursor approach is successfully adopted to synthesize the hexernary high-entropy metal disulfide (MoWReMnCr)S₂ . (MoWReMnCr)S₂ powders are characterized by powder X-ray diffraction (pXRD) and Raman spectroscopy, which confirmed that the material is comprised predominantly of a hexagonal phase. The surface oxidation states and elemental compositions are studied by X-ray photoelectron spectroscopy (XPS) whilst the bulk morphology and elemental stoichiometry with spatial distribution is determined by scanning electron microscopy (SEM) with elemental mapping information acquired from energy-dispersive X-ray (EDX) spectroscopy. The bulk, layered material is subsequently exfoliated to ultra-thin, several-layer 2D nanosheets by liquid-phase exfoliation (LPE). The resulting few-layer HE (MoWReMnCr)S₂ nanosheets are found to contain a homogeneous elemental distribution of metals at the nanoscale by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) with EDX mapping. Finally, (MoWReMnCr)S₂ is demonstrated as a hydrogen evolution electrocatalyst and compared to 2H-MoS₂ synthesized using the molecular precursor approach. (MoWReMnCr)S₂ with 20% w/w of high-conductivity carbon black displays a low overpotential of 229 mV in 0.5 M  H₂ SO₄ to reach a current density of 10 mA cm^-2 , which is much lower than the overpotential of 362 mV for MoS₂ . From density functional theory calculations, it is hypothesised that the enhanced catalytic activity is due to activation of the basal plane upon incorporation of other elements into the 2H-MoS₂ structure, in particular, the first row TMs Cr and Mn.

Coherent hexagonal platinum skin on nickel nanocrystals for enhanced hydrogen evolution activity

Metastable noble metal nanocrystals may exhibit distinctive catalytic properties to address the sluggish kinetics of many important processes, including the hydrogen evolution reaction under alkaline conditions for water-electrolysis hydrogen production. However, the exploration of metastable noble metal nanocrystals is still in its infancy and suffers from a lack of sufficient synthesis and electronic engineering strategies to fully stimulate their potential in catalysis. In this paper, we report a synthesis of metastable hexagonal Pt nanostructures by coherent growth on 3d transition metal nanocrystals such as Ni without involving galvanic replacement reaction, which expands the frontier of the phase-replication synthesis. Unlike noble metal substrates, the 3d transition metal substrate owns more crystal phases and lower cost and endows the hexagonal Pt skin with substantial compressive strains and programmable charge density, making the electronic properties particularly preferred for the alkaline hydrogen evolution reaction. The energy barriers are greatly reduced, pushing the activity to 133 mA cmgeo-2 and 17.4 mA μgPt-1 at -70 mV with 1.5 µg of Pt in 1 M KOH. Our strategy paves the way for metastable noble metal catalysts with tailored electronic properties for highly efficient and cost-effective energy conversion.

pH Effects in a Model Electrocatalytic Reaction Disentangled

Varying the solution pH not only changes the reactant concentrations in bulk solution but also the local reaction environment (LRE) that is shaped furthermore by macroscopic mass transport and microscopic electric double layer (EDL) effects. Understanding ubiquitous pH effects in electrocatalysis requires disentangling these interwoven factors, which is a difficult, if not impossible, task without physical modeling. Herein, we demonstrate how a hierarchical model that integrates microkinetics, double-layer charging, and macroscopic mass transport can help understand pH effects of the formic acid oxidation reaction (FAOR). In terms of the relation between the peak activity and the solution pH, intrinsic pH effects without consideration of changes in the LRE would lead to a bell-shaped curve with a peak at pH = 6. Adding only macroscopic mass transport, we can already reproduce qualitatively the experimentally observed trapezoidal shape with a plateau between pH 5 and 10 in perchlorate and sulfate solutions. A quantitative agreement with experimental data requires consideration of EDL effects beyond Frumkin correlations. Specifically, the peculiar nonmonotonic surface charging relation affects the free energies of adsorbed intermediates. We further discuss pH effects of FAOR in phosphate and chloride-containing solutions, for which anion adsorption becomes important. This study underpins the importance of a full consideration of multiple interrelated factors for the interpretation of pH effects in electrocatalysis.

Recent Advances in Water-Splitting Electrocatalysts Based on Electrodeposition

Green hydrogen is being considered as a next-generation sustainable energy source. It is created electrochemically by water splitting with renewable electricity such as wind, geothermal, solar, and hydropower. The development of electrocatalysts is crucial for the practical production of green hydrogen in order to achieve highly efficient water-splitting systems. Due to its advantages of being environmentally friendly, economically advantageous, and scalable for practical application, electrodeposition is widely used to prepare electrocatalysts. There are still some restrictions on the ability to create highly effective electrocatalysts using electrodeposition owing to the extremely complicated variables required to deposit uniform and large numbers of catalytic active sites. In this review article, we focus on recent advancements in the field of electrodeposition for water splitting, as well as a number of strategies to address current issues. The highly catalytic electrodeposited catalyst systems, including nanostructured layered double hydroxides (LDHs), single-atom catalysts (SACs), high-entropy alloys (HEAs), and core-shell structures, are intensively discussed. Lastly, we offer solutions to current problems and the potential of electrodeposition in upcoming water-splitting electrocatalysts.

In Situ Surface Restructuring of Amorphous Ni-Doped CoMo Phosphate-Based Three-Dimensional Networked Nanosheets: Highly Efficient and Durable Electrocatalyst for Overall Alkaline Water Splitting

Developing cost-efficient bifunctional electrocatalysts with high efficiency and durability for the production of green hydrogen and oxygen is a demanding and challenging research area. Due to their high earth abundance, transition metal-based electrocatalysts are alternatives to noble metal-based water splitting electrocatalysts. Herein, binder-free three-dimensional (3D) networked nanosheets of Ni-doped CoMo ternary phosphate (Pi) were prepared using a facile electrochemical synthetic strategy on flexible carbon cloth without any high-temperature heat treatment or complicated electrode fabrication. The optimized CoMoNiPi electrocatalyst delivers admirable hydrogen (η₁₀ = 96 mV) and oxygen (η₁₀ = 272 mV) evolution performances in 1.0 M KOH electrolyte. For overall water splitting in a two-electrode system, the present catalyst demands only 1.59 and 1.90 V to reach current densities of 10 and 100 mA/cm², respectively, which is lower than that of the Pt/C||RuO₂ couple (1.61 V @ 10 mA/cm², 2 V > @ 100 mA/cm²) and many other catalysts reported previously. Furthermore, the present catalyst delivers excellent long-term stability in a two-electrode system continuously over 100 h at a high current density of 100 mA/cm², exhibiting nearly 100% faradic efficiency. The unique 3D amorphous structure with high porosity, a high active surface area, and lower charge transfer resistance provides excellent overall water splitting. Notably, the amorphous structure of the present catalyst favors the in situ surface reconstruction during electrolysis and generates very stable surface-active sites capable of long-term performance. The present work provides a route for the preparation of multimetallic-Pi nanostructures for various electrode applications that are easy to prepare and have superior activity, high stability, and low cost.

Doping of Cr to Regulate the Valence State of Cu and Co Contributes to Efficient Water Splitting

Water electrolysis in alkaline media is the most promising technology for hydrogen production, but efficient electrocatalysts are required to reduce the overpotential in HER and OER processes. In this work, the multicomponent transition metal catalyst Cr-Cu/CoO_x was loaded on copper foam by electrodeposition and annealing, and the catalyst exhibited excellent electrochemical activity. The HER overpotential is 21 mV and the OER overpotential is 252 mV at a current density of 10 mA cm^-2. The overall water splitting voltage is 1.51 V, even better than the Pt/C//RuO₂ two-electrode system (1.61 V). The excellent performance of this catalyst is mainly derived from the close synergistic interaction among Cu, Co, and Cr. The doping of Cr modulates the valence states of Cu and Co at the active sites and improves the adsorption of various reaction intermediates. Density functional theory (DFT) calculations show that the doping of Cr can optimize the adsorption of the reaction intermediate H*. Meanwhile, the high-valent Cr and Co promote hydrolysis through strong adsorption with OH^-. The present work provides a reasonable strategy for designing low-cost transition metals as efficient catalysts for water electrolysis.

Heterostructured Ultrathin Two-Dimensional Co-FeOOH Nanosheets@1D Ir-Co(OH)F Nanorods for Efficient Electrocatalytic Water Splitting

It is highly desirable to develop high-performance and robust electrocatalysts for overall water splitting, as the existing electrocatalysts exhibit poor catalytic performance toward hydrogen and oxygen evolution reactions (HER and OER) in the same electrolytes, resulting in high cost, low energy conversion efficiency, and complicated operating procedures. Herein, a heterostructured electrocatalyst is realized by growing Co-ZIF-67-derived 2D Co-doped FeOOH on 1D Ir-doped Co(OH)F nanorods, denoted as Co-FeOOH@Ir-Co(OH)F. The Ir-doping couples with the synergy between Co-FeOOH and Ir-Co(OH)F effectively modulate the electronic structures and induce defect-enriched interfaces. This bestows Co-FeOOH@Ir-Co(OH)F with abundant exposed active sites, accelerated reaction kinetics, improved charge transfer abilities, and optimized adsorption energies of reaction intermediates, which ultimately boost the bifunctional catalytic activity. Consequently, Co-FeOOH@Ir-Co(OH)F exhibits low overpotentials of 192/231/251 and 38/83/111 mV at current densities of 10/100/250 mA cm^-2 toward the OER and HER in a 1.0 M KOH electrolyte, respectively. When Co-FeOOH@Ir-Co(OH)F is used for overall water splitting, cell voltages of 1.48/1.60/1.67 V are required at current densities of 10/100/250 mA cm^-2. Furthermore, it possesses outstanding long-term stability for OER, HER, and overall water splitting. Our study provides a promising way to prepare advanced heterostructured bifunctional electrocatalysts for overall alkaline water splitting.

Electronic Modulation of Metal-Organic Frameworks Caused by Atomically Dispersed Ru for Efficient Hydrogen Evolution

Designing excellent electrocatalysts for the hydrogen evolution reaction (HER) is extremely significant in producing clean and sustainable hydrogen fuel. Herein, a rational strategy is developed to fabricate a promising electrocatalyst by introducing atomically dispersed Ru into a cobalt-based metal-organic framework (MOF), Co-BPDC (Co(bpdc)(H₂ O)₂ , BPDC: 4,4'-Biphenyldicarboxylic acid). The obtained CoRu-BPDC nanosheet arrays exhibit remarkable HER performance with an overpotential of 37 mV at a current density of 10 mA cm^-2 in alkaline media, which is superior to most of the MOF-based electrocatalysts and comparable to the commercial Pt/C. Synchrotron radiation-based X-ray absorption fine structure (XAFS) spectroscopy studies verify that the isolated Ru atoms are dispersed in Co-BPDC nanosheets with the formation of five-coordinated Ru-O₅ species. XAFS spectroscopy combined with density functional theory (DFT) calculations unravels that atomically dispersed Ru can modulate the electronic structure of the as-obtained Co-BPDC, contributing to the optimization of binding strength for H* and the enhancement of HER performance. This work opens a new avenue to rationally design highly-active single-atom modified MOF-based HER electrocatalysts via modulating electronic structures of MOF.

Efficient Hydrogen Evolution via 1T-MoS2 /Chlorophyll-a Heterostructure: Way Toward Metal Free Green Catalyst

Electrocatalytic hydrogen evolution reaction (HER) is regarded as a sustainable and green way for H₂ generation, which faces a great challenge in designing highly active, stable electrocatalysts to replace the state-of-art noble metal-platinum catalysts. 1T MoS₂ is highly promising in this regard, but the synthesis and stability of this is a particularly pressing task. Here, a phase engineering strategy has been proposed to achieve a stable, high-percentage (88%) 1T MoS₂ /chlorophyll-a hetero-nanostructure, through a photo-induced donation of anti-bonding electrons from chlorophyll-a (CHL-a) highest occupied molecular orbital to 2H MoS₂ lowest unoccupied molecular orbital. The resultant catalyst has abundant binding sites provided by the coordination of magnesium atom in the CHL-a macro-cycle, featuring higher binding strength and low Gibbs-free energy. This metal-free heterostructure exhibits excellent stability via band renormalization of Mo 4d orbital which creates the pseudogap-like structure by lifting the degeneracy of projected density of state with 4S in 1T MoS₂ . It shows extremely low overpotential, toward the acidic HER (68 mV at the current density of 10 mA cm^-2 ), very close to the Pt/C catalyst (53 mV). The high electrochemical-surface-area and electrochemical turnover frequency support enhanced active sites along with near zero Gibbs free energy. Such a surface-reconstruction strategy provides a new avenue toward the production of efficient non-noble-metal-catalysts for the HER with the aim of green-hydrogen production.

Mass Production of Sulfur-Tuned Single-Atom Catalysts for Zn-Air Batteries

Single-atom catalysts (SACs) show great potential for rechargeable Zn-air batteries (ZABs); however, scalable production of SACs from sustainable resources is difficult owing to poor control of the local coordination environment. Herein, lignosulfonate, a by-product of the papermaking industry, is utilized as a multifunctional bioligand for the mass production of SACs with highly active MN₄ S sites (M represents Fe, Cu, and Co) via strong metalnitrogen/sulfur coordination. This effectively adjusts the charge distribution and promotes the catalytic performance, leading to highly durable and excellent performance in oxygen reduction and evolution reactions for ZABs. This study paves the way for the industrial production of cost-effective SACs in a sustainable manner.

Synthetic Access to a Framework-Stabilized and Fully Sulfided Analogue of an Anderson Polyoxometalate that is Catalytically Competent for Reduction Reactions

Polyoxometalates (POMs) featuring 7, 12, 18, or more redox-accessible transition metal ions are ubiquitous as selective catalysts, especially for oxidation reactions. The corresponding synthetic and catalytic chemistry of stable, discrete, capping-ligand-free polythiometalates (PTMs), which could be especially attractive for reduction reactions, is much less well developed. Among the challenges are the propensity of PTMs to agglomerate and the tendency for agglomeration to block reactant access of catalyst active sites. Nevertheless, the pervasive presence of transition metal sulfur clusters metalloenzymes or cofactors that catalyze reduction reactions and the justifiable proliferation of studies of two-dimensional (2D) metal-chalcogenides as reduction catalysts point to the promise of well-defined and controllable PTMs as reduction catalysts. Here, we report the fabrication of agglomeration-immune, reactant-accessible, capping-ligand-free Co^IIMo₆^IVS₂₄^n- clusters as periodic arrays in a water-stable, hierarchically porous Zr-metal-organic framework (MOF; NU1K) by first installing a disk-like Anderson polyoxometalate, Co^IIIMo₆^VIO₂₄^m-, in size-matched micropores where the siting is established via difference electron density (DED) X-ray diffraction (XRD) experiments. Flowing H₂S, while heating, reduces molybdenum(VI) ions to Mo(IV) and quantitatively replaces oxygen anions with sulfur anions (S^2-, HS^-, S₂^2-). DED maps show that MOF-templated POM-to-PTM conversion leaves clusters individually isolated in open-channel-connected micropores. The structure of the immobilized cluster as determined, in part, by X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) analysis, and pair distribution function (PDF) analysis of total X-ray scattering agrees well with the theoretically simulated structure. PTM@MOF displays both electrocatalytic and photocatalytic competency for hydrogen evolution. Nevertheless, the initially installed PTM appears to be a precatalyst, gaining competency only after the loss of ∼3 to 6 sulfurs and exposure to hydride-forming metal ions.

Concepts Relevant for the Kinetic Analysis of Reversible Reaction Systems

The net rate of a reversible chemical reaction is the difference between unidirectional rates of traversal along forward and reverse reaction paths. In a multistep reaction sequence, the forward and reverse trajectories, in general, are not the microscopic reverse of one another; rather, each unidirectional route is comprised of distinct rate-controlling steps, intermediates, and transition states. Consequently, traditional descriptors of rate (e.g., reaction orders) do not reflect intrinsic kinetic information but instead conflate unidirectional contributions determined by (i) the microscopic occurrence of forward/reverse reactions (i.e., unidirectional kinetics) and (ii) the reversibility of reaction (i.e., nonequilibrium thermodynamics). This review aims to provide a comprehensive resource of analytical and conceptual tools which deconvolute the contributions of reaction kinetics and thermodynamics to disambiguate unidirectional reaction trajectories and precisely identify rate- and reversibility-controlling molecular species and steps in reversible reaction systems. The extrication of mechanistic and kinetic information from bidirectional reactions is accomplished through equation-based formalisms (e.g., De Donder relations) grounded in principles of thermodynamics and interpreted in the context of theories of chemical kinetics developed in the past 25 years. The aggregate of mathematical formalisms detailed herein is general to thermochemical and electrochemical reactions and encapsulates a diverse body of scientific literature encompassing chemical physics, thermodynamics, chemical kinetics, catalysis, and kinetic modeling.

Electrochemical C-N coupling of CO2 and nitrogenous small molecules for the electrosynthesis of organonitrogen compounds

Electrochemical C-N coupling reactions based on abundant small molecules (such as CO₂ and N₂) have attracted increasing attention as a new "green synthetic strategy" for the synthesis of organonitrogen compounds, which have been widely used in organic synthesis, materials chemistry, and biochemistry. The traditional technology employed for the synthesis of organonitrogen compounds containing C-N bonds often requires the addition of metal reagents or oxidants under harsh conditions with high energy consumption and environmental concerns. By contrast, electrosynthesis avoids the use of other reducing agents or oxidants by utilizing "electrons", which are the cleanest "reagent" and can reduce the generation of by-products, consistent with the atomic economy and green chemistry. In this study, we present a comprehensive review on the electrosynthesis of high value-added organonitrogens from the abundant CO₂ and nitrogenous small molecules (N₂, NO, NO₂^-, NO₃^-, NH₃, etc.) via the C-N coupling reaction. The associated fundamental concepts, theoretical models, emerging electrocatalysts, and value-added target products, together with the current challenges and future opportunities are discussed. This critical review will greatly increase the understanding of electrochemical C-N coupling reactions, and thus attract research interest in the fixation of carbon and nitrogen.

Enhanced electrocatalytic performance for oxygen evolution reaction via active interfaces of Co3O4arrays@FeOx/Carbon cloth heterostructure by plasma-enhanced atomic layer deposition

Oxygen evolution reaction (OER) is a necessary procedure in various devices including water splitting and rechargeable metal-air batteries but required a higher potential to improve oxygen evolution efficiency due to its slow reaction kinetics. In order to solve this problem, a heterostructured electrocatalyst (Co₃O₄@FeO_x/CC) is synthesized by deposition of iron oxides (FeO_x) on carbon cloth (CC) via plasma-enhanced atomic layer deposition, then growth of the cobalt oxide (Co₃O₄) nanosheet arrays. The deposition cycle of FeO_xon the CC strongly influences thein situgrowth and distribution of Co₃O₄nanosheets and electronic conductivity of the electrocatalyst. Owing to the high accessible and electroactive areas and improved electrical conductivity, the free-standing electrode of Co₃O₄@FeO_x/CC with 100 deposition cycles of FeO_xexhibits excellent electrocatalytic performance for OER with a low overpotential of 314.0 mV at 10 mA cm^-2and a small Tafel slope of 29.2 mV dec^-1in alkaline solution, which is much better than that of Co₃O₄/CC (448 mV), and even commercial RuO₂(380 mV). This design and optimization strategy shows a promising way to synthesize ideally designed catalytic architectures for application in energy storage and conversion.

Electrocatalytic Hydrogen Evolution at Full Atomic Utilization over ITO-Supported Sub-nano-Ptn Clusters: High, Size-Dependent Activity Controlled by Fluxional Pt Hydride Species

A combination of density functional theory (DFT) and experiments with atomically size-selected Pt_n clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Pt_n size on the electrocatalytic activity of Pt_n (n = 1, 4, 7, and 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Pt_n size such that Pt₇/ITO and Pt₈/ITO have roughly double the activity per Pt atom compared to atoms in the surface layer of polycrystalline Pt. Both the DFT and experiment find that hydrogen under-potential deposition (H_upd) results in Pt_n/ITO (n = 4, 7, and 8) adsorbing ∼2H atoms/Pt atom at the HER threshold potential, equal to ca. double the H_upd observed for Pt bulk or nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt₁/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. The theory combines global optimization with grand canonical approaches for the influence of potential, uncovering the fact that several metastable structures contribute to the HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible Pt_nH_x/ITO structures to correctly predict the activity vs Pt_n size and applied potential. For the small clusters, spillover of H_ads from the clusters to the ITO support is significant, resulting in a competing channel for loss of H_ads, particularly at slow potential scan rates.

Proton-donating and chemistry-dependent buffering capability of amino acids for the hydrogen evolution reaction

The hydrogen evolution reaction (HER) has been widely demonstrated to have a strong dependence on pH and on the source of protons, where a clear kinetic advantage arises in acidic conditions over near-neutral and alkaline conditions due to the switch in reactant from H₃O⁺ to H₂O. Playing on the acid/base chemistry of aqueous systems can avoid the kinetic frailties. For example, buffer systems can be used to maintain proton concentration at intermediate pH, driving H₃O⁺ reduction over H₂O. In light of this, we examine the influence of amino acids on HER kinetics at platinum surfaces using rotating disk electrodes. We demonstrate that aspartic acid (Asp) and glutamic acid (Glu) can act not only as proton donors, but also have sufficient buffering action to sustain H₃O⁺ reduction even at large current density. Comparing with histidine (His) and serine (Ser), we reveal that the buffering capacity of amino acids occurs due to the proximity of their isoelectric point (pI) and their buffering pK_a. This study further exemplifies HER's dependence on pH and pK_a and that amino acids can be used to probe this relationship.

PO6 geometric configuration unit enhanced electrocatalytic performance of Co3O4 in acidic oxygen evolution

It is challenging to develop high-efficient and stable nonprecious metal-based electrocatalyst for oxygen evolution reaction (OER) in acid for proton exchange membrane (PEM) water splitting. Herein, P atoms were introduced into the lattice of spinel Co₃O₄ (P-Co₃O₄) to replace with octahedral coordinated Co³⁺ via a hydrothermal process following a thermal treatment. The formation of PO₆ geometric configuration unit in Co₃O₄ can trigger electron rearrangement around Co ions, which resulted in the high-active Co²⁺ site on the surface, significantly decreasing the energy barrier of rate-determining step for OER. Moreover, the weaker covalency of the Co 3d-O 2p bond and higher formation energy of oxygen vacancy around Co²⁺ in P-Co₃O₄ inhibited the participation of lattice oxygen during OER process, enabling that P-Co₃O₄ can work stably in acidic media. The obtained P-Co₃O₄ afforded satisfying stability over 30 h in a PEM electrolysis device with an overpotential of 400 mV@10 mA/cm² in 0.1 M HClO₄.

Electrocatalytic Properties of Co3O4 Prepared on Carbon Fibers by Thermal Metal-Organic Deposition for the Oxygen Evolution Reaction in Alkaline Water Electrolysis

Cobalt oxide (Co₃O₄) serves as a promising electrocatalyst for oxygen evolution reactions (OER) in water-electrolytic hydrogen production. For more practical applications, advances in dry-deposition processes for the high-throughput fabrication of such Co₃O₄ electrocatalysts are needed. In this work, a thermal metal-organic deposition (MOD) technique is developed to form Co₃O₄ deposits on microscale-diameter carbon fibers constituting a carbon fiber paper (CFP) substrate for high-efficiency OER electrocatalyst applications. The Co₃O₄ electrocatalysts are deposited while uniformly covering the surface of individual carbon fibers in the reaction temperature range from 400 to 800 °C under an ambient Ar atmosphere. It is found that the microstructure of deposits is dependent on the reaction temperature. The Co₃O₄ electrocatalysts prepared at 500 °C and over exhibit values of 355-384 mV in overpotential (η₁₀) required to reach a current density of 10 mA cm^-2 and 70-79 mV dec^-1 in Tafel slope, measured in 1 M KOH aqueous solution. As a result, it is highlighted that the improved crystallinity of the Co₃O₄ electrocatalyst with the increased reaction temperature leads to an enhancement in electrode-level OER activity with the high electrochemically active surface area (ECSA), low charge transfer resistance (R_ct), and low η₁₀, due to the enhanced electrical conductivity. On the other hand, it is found that the inherent catalytic activity of the surface sites of the Co₃O₄, represented by the turnover frequency (TOF), decreases with reaction temperature due to the high-temperature sintering effect. This work provides the groundwork for the high-throughput fabrication and rational design of high-performance electrocatalysts.

Tunable Synthesis of Metal-Rich and Phosphorus-Rich Nickel Phosphides and Their Comparative Evaluation as Hydrogen Evolution Electrocatalysts

Flexible synthetic routes to crystalline metal-rich to phosphorus-rich nickel phosphides are highly desired for comparable electrocatalytic HER studies. This report details solvent-free, direct, and tin-flux-assisted synthesis of five different nickel phosphides from NiCl₂ and phosphorus at moderate temperatures (500 °C). Direct reactions are thermodynamically driven via PCl₃ formation and tuned through reaction stoichiometry to produce crystalline Ni-P materials from metal-rich (Ni₂P, Ni₅P₄) to phosphorus-rich (cubic NiP₂) compositions. A tin flux in NiCl₂/P reactions allows access to monoclinic NiP₂ and NiP₃. Intermediates in tin flux reactions were isolated to help identify phosphorus-rich Ni-P formation mechanisms. These crystalline micrometer-sized nickel phosphide powders were affixed to carbon-wax electrodes and investigated as HER electrocatalysts in acidic electrolyte. All nickel phosphides show moderate HER activity in a potential range of -160 to -260 mV to achieve current densities of 10 mA/cm² ordered as c-NiP₂ ≥ Ni₅P₄ > NiP₃ > m-NiP₂ > Ni₂P, with NiP₃ activity showing some particle size influence. Phosphorus-rich c/m-NiP₂ appears most stable under acidic conditions during extended reactions. The HER activity of these different nickel phosphides appears influenced by a combination of factors such as particle size, phosphorus content, polyphosphide anions, and surface charge.

Understanding the effect of MXene in a TMO/MXene hybrid catalyst for the oxygen evolution reaction

Very recently, it has been reported that mixed transition metal oxide (TMO)/MXene catalysts show improved performance over TMO only catalysts for the oxygen evolution reaction (OER). However, the reasoning behind this observation is unknown. In this work mixed Co(OH)2/Ti3C2Tx were prepared and characterized for the OER using ex situ and operando spectroscopy techniques in order to initiate the understanding of why mixed TMO/MXene materials show better performances compared to TMO only catalysts. This work shows that the improved electrocatalysis for the composite material compared to the TMO only catalyst is due to the presence of higher Co oxide oxidation states at lower OER overpotentials for the mixed TMO/MXene catalysts. Furthermore, the presence of the MXene allows for a more mechanically robust film during OER, making the film more stable. Finally, our results show that small amounts of MXene are more advantageous for the OER during long-term stability measurements, which is linked to the formation of TiO2. The sensitivity of MXene oxidation ultimately limits TMO/MXene composites under alkaline OER conditions, meaning mass fractions must be carefully considered when designing such a catalyst to minimize the residual TiO2 formed during its lifetime.

New Insights into the Surfactant-Assisted Liquid-Phase Exfoliation of Bi2S3 for Electrocatalytic Applications

During water electrolysis, adding an electrocatalyst for the hydrogen evolution reaction (HER) is necessary to reduce the activation barrier and thus enhance the reaction rate. Metal chalcogenide-based 2D nanomaterials have been studied as an alternative to noble metal electrocatalysts because of their interesting electrocatalytic properties and low costs of production. However, the difficulty in improving the catalytic efficiency and industrializing the synthetic methods have become a problem in the potential application of these species in electrocatalysis. Liquid-phase exfoliation (LPE) is a low-cost and scalable technique for lab- and industrial-scale synthesis of 2D-material colloidal inks. In this work, we present, to the best of our knowledge, for the first time a systematic study on the surfactant-assisted LPE of bulk Bi2S3 crystalline powder to produce nanosheets (NSs). Different dispersing agents and LPE conditions have been tested in order to obtain colloidal low-dimensional Bi2S3 NSs in H2O at optimized concentrations. Eventually, colloidally stable layered nano-sized Bi2S3 suspensions can be produced with yields of up to ~12.5%. The thus obtained low-dimensional Bi2S3 is proven to be more active for HER than the bulk starting material, showing an overpotential of only 235 mV and an optimized Tafel slope of 125 mV/dec. Our results provide a facile top-down method to produce nano-sized Bi2S3 through a green approach and demonstrate that this material can have a good potential as electrocatalyst for HER.

Evaluating Fe-Site and Vacancy Dependent Intrinsic Activity of NiFe Layered Double Hydroxides through Cavity Microelectrodes

We report evaluating the intrinsic activity of Ni(OH)₂, NiFe layered double hydroxides (LDHs), and NiFe-LDH having vacancies for oxygen evolution reaction (OER) by the use of cavity microelectrodes (CMEs) with controllable mass loading. The number of active Ni sites (N_Ni-sites) ranging from 1 × 10¹² to 6 × 10¹² is quantitatively correlated with OER current, which reveals that the introduction of Fe-sites and vacancies increases the turnover frequency (TOF) from 0.027 to 0.118 and 0.165 s^-1, respectively. Electrochemical surface area (ECSA) is further quantitatively correlated with N_Ni-sites, which indicates that N_Ni-sites per unit ECSA (N_Ni-per-ECSA) is decreased by the introduction of Fe-sites and vacancies. Therefore, the difference of OER current per unit ECSA (J_ECSA) is reduced compared with that of TOF. The results demonstrate that CMEs provide a good platform to evaluate intrinsic activity with TOF, N_Ni-per-ECSA, and J_ECSA more reasonably.

Revealing the synergistic mechanism of the generation, migration and nearby utilization of reactive oxygen species in FeOCl-MOF yolk-shell reactors

Fenton-like reactions is attractive for environmental pollutant control, but there is an urgent need to improve the utilisation of hydroxyl radicals (·OH) in practical applications. Here, for the first time, FeOCl is encapsulated within a Metal Organic Framework (MOF) (Materials of Institut Lavoisier-101 (MIL-101(Fe))) as a yolk-shell reactor (FeOCl-MOF) by in situ growth. The interaction between FeOCl and the MOF not only increases the electron density of FeOCl, but also shifts down the d-band centre. The increase of electron density could promote the efficient conversion of H₂O₂ to ·OH catalysed by FeOCl. And the shift of the d-band centre to the lower energy level facilitates the desorption of ·OH. Experimental and theoretical calculations showed that the high catalytic performance was attributed to the unique yolk-shell structure that concentrates the catalytic and adsorption sites in a confinement space, as well as the improved electron density and d-band centre for efficient generation, rapid desorption and utilized nearby of ·OH. Which is utilized nearby by the organic pollutants adsorbed by the surface MOF, thus greatly improving the effective conversion of H₂O₂ and the ·OH utilisation (from 25.5% (Fe²⁺/H₂O₂) to 77.1% (FeOCl-MOF/H₂O₂)). In addition, a catalytic reactor was constructed to achieve continuous efficient treatment of organic pollutants. This work provides a Fenton-like microreactor for efficient generation, rapid desorption, and nearby utilization of ·OH to improve future technologies for deep water purification in complex environments.

Insight into oxygen reduction activity and pathway on pure titanium using scanning electrochemical microscopy and theoretical calculations

HYPOTHESIS

Unlike noble metals, the oxygen reduction reaction (ORR) behavior on Ti is more complicated due to its spontaneously formed oxide film. This film results in sluggish ORR kinetics and tends to be reduced within ORR potential region, causing the weak and multi-reaction coupled current. Though Ti is being used in chemical and biological fields, its ORR research is still underexplored.

EXPERIMENTS

We innovatively employed the modified reactive tip generation-substrate collection (RTG/SC) mode of scanning electrochemical microscopy (SECM) with high efficiency of 97.2 % to quantitatively study the effects of film characteristics, solution environment (pH, anion, dissolved oxygen), and applied potential on the ORR activity and selectivity of Ti. Then, density functional theory (DFT) and molecular dynamics (MD) analyses were employed to elucidate its ORR behavior.

FINDINGS

On highly reduced Ti, film properties dominate ORR behavior with promoted 4e^- selectivity. Rapid film regeneration in alkaline/O₂-saturated conditions inhibits ORR activity. Besides, ORR is sensitive to anion species in neutral solutions while showing enhanced 4e^- reduction in alkaline media. All the improved 4e^- selectivities originate from the hydrogen bond/electrostatic stabilization effect, while the decayed ORR activity by Cl^- arises from the suppressed O₂ adsorption. This work provides theoretical support and possible guidance for ORR research on oxide-covered metals.

Expediting Oxygen Evolution by Optimizing Cation and Anion Complexity in Electrocatalysts Based on Metal Phosphorous Trichalcogenides

Purposely changing the rate-determining step (RDS) of oxygen evolution reaction (OER) remains a major challenge for enhancing the energy efficiency of electrochemical splitting of water. Here we show that the OER RDS can be regulated by simply varying the cation and anion complexity in a family of the metal phosphorous trichalcogenide electrocatalysts (MPT₃ , where M=Fe, Ni; T=S, Se), achieving an exceptionally high OER activity in (Ni,Fe)P(S,Se)₃ , as demonstrated by its ultra-low Tafel slope (34 mV dec^-1 ) and a very low overpotential compared to many relevant OER catalysts. This is strongly supported by density functional theory calculations, which showed that this catalyst has a nearly optimal OER activity descriptor value of ΔG(O*)-ΔG(OH*)=1.5 eV. We also found that the activity descriptor is proportional to a newly proposed cation/anion complexity index that consists of pairwise contributions from cation-anion bonds in a catalyst compound, revealing the pivotal role of the cation-anion interactions in determining the catalyst performance and providing a simple way for predicting catalytic activities.

Phosphorus-modified cobalt single-atom catalysts loaded on crosslinked carbon nanosheets for efficient alkaline hydrogen evolution reaction

Efficient and low-cost transition metal single-atom catalysts (TMSACs) for hydrogen evolution reaction (HER) have been recognized as research hotspots recently with advances in delivering good catalytic activity without noble metals. However, the high-cost complex preparation of TMSACs and insufficient stability limited their practical applications. Herein, a simple top-down pyrolysis approach to obtain P-modified Co SACs loaded on the crosslinked defect-rich carbon nanosheets was introduced for alkaline hydrogen evolution, where Co atoms are locally confined before pyrolysis to prevent aggregation. Thereby, the abundant defects and the unsaturated coordination formed during the pyrolysis significantly improved the stability of the monatomic structure and reduced the reaction barrier. Furthermore, the synergy between cobalt atoms and phosphorus atoms was established to optimize the decomposition process of water molecules, which delivers the key to promoting the slow reaction kinetics of alkaline HER. As the result, the cobalt SAC exhibited excellent catalytic activity and stability for alkaline HER, with overpotentials of 70 mV and 192 mV at current densities of -10 mA cm^-2 and -100 mA cm^-2, respectively.

Non-Kinetic Effects Convolute Activity and Tafel Analysis for the Alkaline Oxygen Evolution Reaction on NiFeOOH Electrocatalysts

A large variety of nickel-based catalysts has been investigated for the oxygen evolution reaction (OER) in alkaline media. However, their reported activity, as well as Tafel slope values, vary greatly. To understand this variation, we studied electrodeposited Ni₈₀ Fe₂₀ OOH catalysts with different loadings at varying rotation rates, hydroxide concentrations, with or without sonication. We show that, at low current density (<5 mA cm^-2 ), the Tafel slope value is ≈30 mV dec^-1 for Ni₈₀ Fe₂₀ OOH. At higher polarization, the Tafel slope continuously increases and is dependent on rotation rate, loading, hydroxide concentration and sonication. These Tafel slope values are convoluted by non-kinetic effects, such as bubbles, potential-dependent changes in ohmic resistance and (internal) OH^- gradients. As best practise, we suggest that Tafel slopes should be plotted vs. current or potential. In such a plot, it can be appreciated if there is a kinetic Tafel slope or if the observed Tafel slope is influenced by non-kinetic effects.

Size-Dependent Electrocatalytic Water Oxidation Activity for a Series of Atomically Precise Nickel-Thiolate Clusters

The development of renewable energy technologies is critical for reducing global carbon emissions. Water splitting offers a promising renewable energy mechanism by converting water into H₂ and O₂ gas, which can directly power fuel cells or be utilized as chemical feedstocks. To increase the efficiency of water splitting, catalysts must be developed for the water reduction and water oxidation half-reactions. To promote rational catalyst design, atomically precise metal clusters (APMCs) with earth-abundant metals provide a framework for developing both structure-activity relationships and cost-effective catalysts. Previous reports on the water oxidation activity of nickel-thiolate clusters [Ni_n(SR)_2n] have not developed a systematic description of a possible size-activity relationship. Utilizing recent advancements in preparative chromatography for isolating APMCs, we have synthesized a series of Ni_n(SR)_2n (n = 4, 5, or 6) clusters as electrocatalysts for the oxygen evolution reaction. We discovered a clear size-activity and size-stability trend, with intrinsic activity and stability increasing with cluster size. Using density functional theory, we found that intrinsic activity is inversely correlated to intermediate binding energy, and by extension the oxidation potential of each cluster. Our work demonstrates the ability of APMCs to uncover previously unknown structure-activity relationships that can guide future catalyst design.

Dual Function of Hypo-d-electronic Transition Metals in the Brewer Intermetallic Phase for the Highly Efficient Electrocatalytic Hydrogen Evolution Reaction in Alkaline Electrolytes

Reported are the synthesis, material characterization, and electrocatalytic hydrogen evolution reaction (HER) in acid and alkaline electrolytes for the Brewer intermetallic phase, Nb₆Co₇ and Mo₆Co₇. It was realized that the overpotential at a current density of 10 mA/cm² (η₁₀) for Nb₆Co₇ (η₁₀ = 62 mV) and Mo₆Co₇ (η₁₀ = 143 mV) are both much lower than that of using a single Co metal (η₁₀ = 253 mV) in alkaline electrolytes. The enhancement of electrocatalytic HER activity of Nb₆Co₇ and Mo₆Co₇ can be attributed to the hypo-hyper-d-electronic interaction between Nb/Mo and Co elements. Based on the result of density functional theory calculation, alloying between Nb/Mo and Co elements will increase the antibonding state population of the Co-Co bond near the Fermi level (E_F), which induces the synergistic effect to influence the adsorption energy of the H atom (ΔG_H) on the surface of Nb₆Co₇ and Mo₆Co₇. Moreover, the role of the Nb element is not only a simple electron donor but is also an anchor position for the OH molecule (i.e., dual function) due to the bonding character of the Nb-Co bond near E_F. It can reduce the OH position effect as well as the activation energy for water dissociation, which rationalizes the high and robust HER performance of Nb₆Co₇ to that of commercial Pt/C (η₁₀ = 67 mV) in alkaline electrolytes.

Carbon-Conjugated Co Complexes as Model Electrocatalysts for Oxygen Reduction Reaction

Single-atom catalysts are a family of heterogeneous electrocatalysts widely used in energy storage and conversion. The determination of the local structure of the active metal sites is challenging, which limits the establishment of the reliable structure-property relationship of single-atom catalysts. A carbon black-conjugated complex can be used as the model catalyst to probe the intrinsic activity of metal sites with certain local structures. In this work, we prepared carbon black-conjugated [Co(phenanthroline)Cl2], [Co(o-phenylenediamine)Cl2] and [Co(salophen)]. In these catalysts, the Co complexes with well-defined structures are anchored on the edge of carbon black by pyrazine moieties. The number of electrochemical accessible Co sites can be measured from the area of the redox peaks of pyrazine linkers in the cyclic voltammetry curve. Then, the intrinsic electrocatalytic activity of one Co site can be obtained. The catalytic performances of the three catalysts towards oxygen reduction reaction in alkaline conditions were measured. Carbon black-conjugated [Co(salophen)] showed the highest intrinsic activity with the turnover frequency of 0.72 s−1 at 0.75 V vs. the reversible hydrogen electrode. The strategy developed in this work can be used to explore and verify the possible local structure of active sites proposed for single-atom catalysts.

Transplanting Gold Active Sites into Non-Precious-Metal Nanoclusters for Efficient CO2-to-CO Electroreduction

Electrocatalytic CO₂ reduction reaction (CO₂RR) is greatly facilitated by Au surfaces. However, large fractions of underlying Au atoms are generally unused during the catalytic reaction, which limits mass activity. Herein, we report a strategy for preparing efficient electrocatalysts with high mass activities by the atomic-level transplantation of Au active sites into a Ni₄ nanocluster (NC). While the Ni₄ NC exclusively produces H₂, the Au-transplanted NC selectively produces CO over H₂. The origin of the contrasting selectivity observed for this NC is investigated by combining operando and theoretical studies, which reveal that while the Ni sites are almost completely blocked by the CO intermediate in both NCs, the Au sites act as active sites for CO₂-to-CO electroreduction. The Au-transplanted NC exhibits a remarkable turnover frequency and mass activity for CO production (206 mol_CO/mol_NC/s and 25,228 A/g_Au, respectively, at an overpotential of 0.32 V) and high durability toward the CO₂RR over 25 h.

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.

Chlorocobaloxime containing N-(4-pyridylmethyl)-1,8-naphthalamide peripheral ligands: synthesis, characterization and enhanced electrochemical hydrogen evolution in alkaline medium

Two new discrete cobaloxime based complexes with the general formula [ClCo(dioxime)₂L] (1 and 2), L1 = N-(4-pyridylmethyl)-1,8-naphthalamide, L2 = 4-bromo-N-(4-pyridylmethyl)-1,8-naphthalamide have been synthesized and characterized by various spectroscopic techniques such as FT-IR, ¹H, ¹³C{¹H} NMR and PXRD. The molecular structures of both complexes have also been determined using single crystal X-ray crystallography. The solid state molecular structures revealed distorted octahedral geometry around the Co(III) central metal ion with two dioximes in the equatorial plane and axial positions are occupied by chloro and pyridine nitrogen of N-(4-pyridylmethyl)-1,8-naphthalamide ligands. Both complexes exhibit weaker non-covalent interactions (C-H⋯O, C-H⋯Cl and C-H⋯π(Centroid) in complex 1 whereas C-H⋯O and C-H⋯Br in complex 2) resulting in the formation of dimeric and 1D supramolecular structures. Furthermore, these complexes are immobilized onto the surface of activated carbon cloth (CC) and their electrocatalytic performance for the hydrogen evolution reaction (HER) has been investigated in alkaline and acidic media as well as buffer solution. In alkaline medium, we found that complex 2 exhibited impressive electrocatalytic HER activity and produced a current density of -10 mA cm^-2 at an overpotential of 260 mV, whereas complex 1 produced the same current density at an overpotential of 334 mV. An electrochemical impedance spectroscopy (EIS) spectral study revealed the faster charge transfer kinetics of complex 2 than that of complex 1. Similarly, the low Tafel slope (100 mV dec^-1) for the HER with complex 2 indicates faster HER kinetics compared to complex 1. The chronoamperometric study showed that complex 2 is stable under electrocatalytic HER conditions for 5 h without losing the initial current density and it has also been established that the complex structure is retained after electrocatalysis.