A Molecular Approach to Manganese Nitride Acting as a High Performance Electrocatalyst in the Oxygen Evolution Reaction

Coupling Nano and Atomic Electric Field Confinement for Robust Alkaline Oxygen Evolution

The alkaline oxygen evolution reaction (OER) is a promising avenue for producing clean fuels and storing intermittent energy. However, challenges such as excessive OH- consumption and strong adsorption of oxygen-containing intermediates hinder the development of alkaline OER. In this study, we propose a cooperative strategy by leveraging both nano-scale and atomically local electric fields for alkaline OER, demonstrated through the synthesis of Mn single atom doped CoP nanoneedles (Mn SA-CoP NNs). Finite element method simulations and density functional theory calculations predict that the nano-scale local electric field enriches OH- around the catalyst surface, while the atomically local electric field improves *O desorption. Experimental validation using in situ attenuated total reflection infrared and Raman spectroscopy confirms the effectiveness of the nano-scale and atomically electric fields. Mn SA-CoP NNs exhibit an ultra-low overpotential of 189 mV at 10 mA cm-2 and stable operation over 100 hours at ~100 mA cm-2 during alkaline OER. This innovative strategy provides new insights for enhancing catalyst performance in energy conversion reactions.

Structural and Electronic Factors behind the Electrochemical Stability of 3D-Metal Tungstates under Oxygen Evolution Reaction Conditions

Transition metal tungstates (TMTs) possess a wolframite-like lattice structure and preferably form via an electrostatic interaction between a divalent transition metal cation (MII) and an oxyanion of tungsten ([WO4]2-). A unit cell of a TMT is primarily composed of two repeating units, [MO6]oh and [WO6]oh, which are held together via several M-μ2-O-W bridging links. The bond character (ionic or covalent) of this bridging unit determines the stability of the lattice and influences the electronic structure of the bulk TMT materials. Recently, TMTs have been successfully employed as an electrode material for various applications, including electrochemical water splitting. Despite the wide electrocatalytic applications of TMTs, the study of the structure-activity correlation and electronic factors responsible for in situ structural evolution to electroactive species during electrochemical reactions is still in its infancy. Herein, a series of TMTs, MIIWVIO4 (M = Mn/Fe/Co/Ni), have been prepared and employed as electrocatalysts to study the oxygen evolution reaction (OER) under alkaline conditions and to scrutinize the role of transition metals in controlling the energetics of the formation of electroactive species. Since the [WO6]oh unit is common in the TMTs considered, the variation of the central atom of the corresponding [MO6]oh unit plays an intriguing role in controlling the electronic structure and stability of the lattice under anodic potential. Under the OER conditions, a potential-dependent structural transformation of MWO4 is noticed, where MnWO4 appears to be the most labile, whereas NiWO4 is stable up to a high anodic potential of ∼1.68 V (vs RHE). Potential-dependent hydrolytic [WO4]2- dissolution to form MOx active species, traced by in situ Raman and various spectro-/microscopic analyses, can directly be related to the electronic factors of the lattice, viz., crystal field splitting energy (CFSE) of MII in [MO6]oh, formation enthalpy (ΔHf), decomposition enthalpy (ΔHd), and Madelung factor associated with the MWO4 ionic lattice. Additionally, the magnitude of the Löwdin and Bader charges on M of the M-μ2-O-W bond is directly related to the degree of ionicity or covalency in the MWO4 lattice, which indirectly influences the electronic structure and activity. The experimental results substantiated by the computational study explain the electrochemical activity of the TMTs with the help of various structural and electronic factors and bonding interactions in the lattice, which has never been realized. Therefore, the study presented here can be taken as a general guideline to correlate the reactivity to the structure of the inorganic materials.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Controllable Electronic Transfer Tailoring d-band Center via Cobalt-Oxygen-Bridged Ru/Fe Dual-sites for Boosted Oxygen Evolution

Rational tailoring of the electronic structure at the defined active center of reconstructed metal (oxy)hydroxides (MOOH) during oxygen evolution reaction (OER) remains a challenge. With the guidance of density functional theory (DFT), herein a dual-site regulatory strategy is reported to tailor the d-band center of the Co site in CoOOH via the controlled electronic transfer at the Ru─O─Co─O─Fe bonding structure. Through the bridged O2- site, electrons are vastly flowed from the t2g-orbital of the Ru site to the low-spin orbital of the Co site in the Ru-O-Co coordination and further transfer from the strong electron-electron repulsion of the Co site to the Fe site by the Co-O-Fe coordination, which balancing the electronic configuration of Co sites to weaken the over-strong adsorption energy barrier of OH* and O*, respectively. Benefiting from the highly active of the Co site, the constructed (Ru2Fe2Co6)OOH provide an extremely low overpotential of 248 mV and a Tafel slope of 32.5 mV dec-1 at 10 mA cm-2 accompanied by long durability in alkaline OER, far superior over the pristine and Co-O-Fe bridged CoOOH catalysts. This work provides guidance for the rational design and in-depth analysis of the optimized role of metal dual-sites.

Highly Stable Mo-NiO@NiFe-Layered Double Hydroxide Heterojunction Anode Catalyst for Alkaline Electrolyzers with Porous Membrane

Heterostructure catalysts are considered as promising candidates for promoting the oxygen evolution reaction (OER) process due to their strong electron coupling. However, the inevitable dissolution and detachment of the heterostructure catalysts are caused by the severe reconstruction, dramatically limiting their industrial application. Herein, the NiFe-layered double hydroxide (LDH) nanosheets attached on Mo-NiO microrods (Mo-NiO@NiFe LDH) by the preoxidation strategy of the core NiMoN layer are synthesized for ensuring the high catalytic performance and stability. Owing to the enhanced electron coupling and preoxidation process, the obtained Mo-NiO@NiFe LDH exhibits a superlow overpotential of 253 mV to achieve a practically relevant current density of 1000 mA cm-2 for OER with exceptional stability over 1200 h. Notably, the overall water splitting system based on Mo-NiO@NiFe LDH reveals remarkable stability, maintaining the catalytic activity at a current density of 1000 mA cm-2 for 140 h under industrial harsh conditions. Furthermore, the Mo-NiO@NiFe LDH demonstrates outstanding activity and long-term durability in a practical alkaline electrolyzer assembly with a porous membrane, even surpassing the performance of IrO2. This work provides a new sight for designing and synthesizing highly stable heterojunction electrocatalysts, further promoting and realizing the industrial electrocatalytic OER.

Surface Reconstruction on Metal Nitride during Photo-oxidation

The efficient conversion and storage of solar energy for chemical fuel production presents a challenge in sustainable energy technologies. Metal nitrides (MNs) possess unique structures that make them multi-functional catalysts for water splitting. However, the thermodynamic instability of MNs often results in the formation of surface oxide layers and ambiguous reaction mechanisms. Herein, we present on the photo-induced reconstruction of a Mo-rich@Co-rich bi-layer on ternary cobalt-molybdenum nitride (Co3 Mo3 N) surfaces, resulting in improved effectiveness for solar water splitting. During a photo-oxidation process, the uniform initial surface oxide layer is reconstructed into an amorphous Co-rich oxide surface layer and a subsurface Mo-N layer. The Co-rich outer layer provides active sites for photocatalytic oxygen evolution reaction (POER), while the Mo-rich sublayer promotes charge transfer and enhances the oxidation resistance of Co3 Mo3 N. Additionally, the surface reconstruction yields a shortened Co-Mo bond length, weakening the adsorption of hydrogen and resulting in improved performance for both photocatalytic hydrogen evolution reaction (PHER) and POER. This work provides insight into the surface structure-to-activity relationships of MNs in solar energy conversion, and is expected to have significant implications for the design of metal nitride-based catalysts in sustainable energy technologies.

Nitrogen Plasma Activates CoMn-Layered Double Hydroxides for Superior Electrochemical Oxygen Evolution

Bimetallic layered double hydroxide is considered an ideal electrocatalytic material. However, due to the poor electrical conductivity of the bimetallic layered structure, obtaining highly active and stable catalysts through facile regulation strategies remains a great challenge. Herein, we use a simple corrosion strategy and nitrogen plasma technology to convert cobalt-based metal-organic frameworks into nitrogen-doped CoMn bimetallic layered double hydroxides (CoMn-LDH). Under the condition of regulating the local coordination environment of the catalytic active site and the presence of rich oxygen vacancy defects, N@CoMn-LDH/CC generates a low overpotential of 219 mV at 10 mA cm-2, which exceeds that of the commercial RuO2 catalyst. Density functional theory calculation shows that nitrogen doping improves the adsorption energy of the Mn site for oxygen evolution intermediates and reduces the reaction energy barrier of the Co site. Meanwhile, experiments and theoretical calculations verify that the mechanism of nitrogen doping regulating the oxygen evolution reaction (OER) follows the lattice oxygen oxidation mechanism, avoiding the collapse of the structure caused by catalyst reconstruction, thus improving the stability of oxygen evolution. This work provides a new simple strategy for the preparation of catalysts for a superior electrocatalytic oxygen evolution reaction.

Morphology-Dependent Electrocatalytic Behavior of Cobalt Chromite toward the Oxygen Evolution Reaction in Acidic and Alkaline Medium

Exploiting an affordable, durable, and high-performance electrocatalyst for the oxygen evolution reaction (OER) under lower pH condition (acidic) is highly challengeable and much attractive toward the hydrogen-based energy technologies. A spinel CoCr₂O₄ is observed as a potential noble-metal-free candidate for OER in alkaline medium. The presence of Cr further leads to electronic structure modulation of Co₃O₄ and thereby greatly increases the corrosive resistance toward OER in acidic environment. Herein, a typical CoCr₂O₄ with three different morphologies was synthesized for the very first time and employed as an electrocatalyst for OER in alkaline (1 M KOH) and acidic (0.5 M H₂SO₄) medium. Moreover, different morphologies display a different intrinsic exposed active site and thereby display different electrocatalytic activities. Likewise, the CoCr₂O₄ Mic (synthesized by the microwave heating method) displays a higher catalytic activity toward OER and delivers a low overpotential of 293 and 290 mV to attain 10 mA/cm² current density and smaller Tafel slope values of 40 and 151 mV/dec, respectively, in alkaline and acidic environment than the synthesized CoCr₂O₄ Wet (wet-chemically synthesized) and CoCr₂O₄ Hyd (hydrothermally synthesized). Moreover, CoCr₂O₄ Mic exhibits a long-term durability of 24 h (1 M KOH) and 10.5 h (0.5 M H₂SO₄). The optimized Co-O bond energy in OER condition makes the CoCr₂O₄ Mic superior than the CoCr₂O₄ Hyd and CoCr₂O₄ Wet. Moreover, the substitution of Cr induces the electron delocalization around the Co active species and thereby, positive shifting of the redox potential leads to providing an optimal binding energy for OER intermediates. Also, interestingly, this work represents a catalytic activity trend by a simple experimental result without any complex theoretical calculation. The morphology-dependent electrocatalytic activity obtained in this work will provide a new strategy in the field of electrochemical conversion and energy storage application.

Effective Formation of a Mn-ZIF-67 Nanofibrous Network via Electrospinning: An Active Electrocatalyst for OER in Alkaline Medium

Finding the active center in a bimetallic zeolite imidazolate framework (ZIF) is highly crucial for the electrocatalytic oxygen evolution reaction (OER). In the present study, we constructed a bimetallic ZIF system with cobalt and manganese metal ions and subjected it to an electrospinning technique for feasible fiber formation. The obtained nanofibers delivered a lower overpotential value of 302 mV at a benchmarking current density of 10 mA cm^-2 in an electrocatalytic OER study under alkaline conditions. The obtained Tafel slope and charge-transfer resistance values were 125 mV dec^-1 and 4 Ω, respectively. The kinetics of the reaction is mainly attributed from the ratio of metals (Co and Mn) present in the catalyst. Jahn-Teller distortion reveals that the electrocatalytic active center on the Mn-incorporated ZIF-67 nanofibers (Mn-ZIF-67-NFs) was found to be Mn³⁺ along with the Mn²⁺ and Co²⁺ ions on the octahedral and tetrahedral sites, respectively, where Co²⁺ ions tend to suppress the distortion, which is well supported by density functional theory analysis, molecular orbital study, and magnetic studies.

Intermolecular Energy Gap-Induced Formation of High-Valent Cobalt Species in CoOOH Surface Layer on Cobalt Sulfides for Efficient Water Oxidation

Transition metal-based electrocatalysts will undergo surface reconstruction to form active oxyhydroxide-based hybrids, which are regarded as the "true-catalysts" for the oxygen evolution reaction (OER). Much effort has been devoted to understanding the surface reconstruction, but little on identifying the origin of the enhanced performance derived from the substrate effect. Herein, we report the electrochemical synthesis of amorphous CoOOH layers on the surface of various cobalt sulfides (CoS_α ), and identify that the reduced intermolecular energy gap (Δ_inter ) between the valence band maximum (VBM) of CoOOH and the conduction band minimum (CBM) of CoS_α can accelerate the formation of OER-active high-valent Co⁴⁺ species. The combination of electrochemical and in situ spectroscopic approaches, including cyclic voltammetry (CV), operando electron paramagnetic resonance (EPR) and Raman, reveals that Co species in the CoOOH/Co₉ S₈ are more readily oxidized to CoO₂ /Co₉ S₈ than in CoOOH and other CoOOH/CoS_α . This work provides a new design principle for transition metal-based OER electrocatalysts.

Manganese‐Catalysed Deuterium Labelling of Anilines and Electron‐Rich (Hetero)Arenes

There is a constant need for deuterium‐labelled products for multiple applications in life sciences and beyond. Here, a new class of heterogeneous catalysts is reported for practical deuterium incorporation in anilines, phenols, and heterocyclic substrates. The optimal material can be conveniently synthesised and allows for high deuterium incorporation using deuterium oxide as isotope source. This new catalyst has been fully characterised and successfully applied to the labelling of natural products as well as marketed drugs.

Surface Design Strategy of Catalysts for Water Electrolysis

Hydrogen, a new energy carrier that can replace traditional fossil fuels, is seen as one of the most promising clean energy sources. The use of renewable electricity to drive hydrogen production has very broad prospects for addressing energy and environmental problems. Therefore, many researchers favor electrolytic water due to its green and low-cost advantages. The electrolytic water reaction comprises the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Understanding the OER and HER mechanisms in acidic and alkaline processes contributes to further studying the design of surface regulation of electrolytic water catalysts. The OER and HER catalysts are mainly reviewed for defects, doping, alloying, surface reconstruction, crystal surface structure, and heterostructures. Besides, recent catalysts for overall water splitting are also reviewed. Finally, this review paves the way to the rational design and synthesis of new materials for highly efficient electrocatalysis.

Constructing nickel-iron oxyhydroxides integrated with iron oxides by microorganism corrosion for oxygen evolution

SignificanceOxygen evolution reaction plays a striking role in renewable energy storage and conversion technologies. However, efficient electrocatalysts are necessary to promote this kinetically sluggish reaction. These reported down-top preparation methods of highly efficient transition metal hydroxides usually need delicate control in complex environments, limiting their large-scale application. An iron-oxidizing bacteria corrosion approach is developed to construct iron oxides top-down from iron-oxidizing bacteria corrosion integrated with nickel-iron oxyhydroxides to boost oxygen evolution. This study demonstrates a natural bacterial corrosion strategy to prepare highly efficient electrodes top-down for efficient water oxidation, which may stimulate broad interest in multidisciplinary integration of innovative nanomaterials and emerging technologies.

The Pivotal Role of s-, p-, and f-Block Metals in Water Electrolysis: Status Quo and Perspectives

Transition metals, in particular noble metals, are the most common species in metal-mediated water electrolysis because they serve as highly active catalytic sites. In many cases, the presence of nontransition metals, that is, s-, p-, and f-block metals with high natural abundance in the earth-crust in the catalytic material is indispensable to boost efficiency and durability in water electrolysis. This is why alkali metals, alkaline-earth metals, rare-earth metals, lean metals, and metalloids receive growing interest in this research area. In spite of the pivotal role of these nontransition metals in tuning efficiency of water electrolysis, there is far more room for developments toward a knowledge-based catalyst design. In this review, five classes of nontransition metals species which are successfully utilized in water electrolysis, with special emphasis on electronic structure-catalytic activity relationships and phase stability, are discussed. Moreover, specific fundamental aspects on electrocatalysts for water electrolysis as well as a perspective on this research field are also addressed in this account. It is anticipated that this review can trigger a broader interest in using s-, p-, and f-block metals species toward the discovery of advanced polymetal-containing electrocatalysts for practical water splitting.

Nonprecious transition metal nitrides as efficient oxygen reduction electrocatalysts for alkaline fuel cells

Hydrogen fuel cells have attracted growing attention for high-performance automotive power but are hindered by the scarcity of platinum (and other precious metals) used to catalyze the sluggish oxygen reduction reaction (ORR). We report on a family of nonprecious transition metal nitrides (TMNs) as ORR electrocatalysts in alkaline medium. The air-exposed nitrides spontaneously form a several-nanometer-thick oxide shell on the conductive nitride core, serving as a highly active catalyst architecture. The most active catalyst, carbon-supported cobalt nitride (Co₃N/C), exhibited a half-wave potential of 0.862 V and achieved a record-high peak power density among reported nitride cathode catalysts of 700 mW cm^-2 in alkaline membrane electrode assemblies. Operando x-ray absorption spectroscopy studies revealed that Co₃N/C remains stable below 1.0 V but experiences irreversible oxidation at higher potentials. This work provides a comprehensive analysis of nonprecious TMNs as ORR electrocatalysts and will help inform future design of TMNs for alkaline fuel cells and other energy applications.

Direct One-Step Growth of Bimetallic Ni2Mo3N on Ni Foam as an Efficient Oxygen Evolution Electrocatalyst

A simple and economical synthetic route for direct one-step growth of bimetallic Ni2Mo3N nanoparticles on Ni foam substrate (Ni2Mo3N/NF) and its catalytic performance during an oxygen evolution reaction (OER) are reported. The Ni2Mo3N/NF catalyst was obtained by annealing a mixture of a Mo precursor, Ni foam, and urea at 600 °C under N2 flow using one-pot synthesis. Moreover, the Ni2Mo3N/NF exhibited high OER activity with low overpotential values (336.38 mV at 50 mA cm-2 and 392.49 mV at 100 mA cm-2) and good stability for 5 h in Fe-purified alkaline electrolyte. The Ni2Mo3N nanoparticle surfaces converted into amorphous surface oxide species during the OER, which might be attributed to the OER activity.

Comprehensive Understandings into Complete Reconstruction of Precatalysts: Synthesis, Applications, and Characterizations

Reconstruction induced by external environment (such as applied voltage bias and test electrolytes) changes catalyst component and catalytic behaviors. Investigations of complete reconstruction in energy conversion recently receive intensive attention, which promote the targeted design of top-performance materials with maximum component utilization and good stability. However, the advantages of complete reconstruction, its design strategies, and extensive applications have not achieved the profound understandings and summaries it deserves. Here, this review systematically summarizes several important advances in complete reconstruction for the first time, which includes 1) fundamental understandings of complete reconstruction, the characteristics and advantages of completely reconstructed catalysts, and their design principles, 2) types of reconstruction-involved precatalysts for oxygen evolution reaction catalysis in wide pH solution, and origins of limited reconstruction degree as well as design strategies/principles toward complete reconstruction, 3) complete reconstruction for novel material synthesis and other electrocatalysis fields, and 4) advanced in situ/operando or multiangle/level characterization techniques to capture the dynamic reconstruction processes and real catalytic contributors. Finally, the existing major challenges and unexplored/unsolved issues on studying the reconstruction chemistry are summarized, and an outlook for the further development of complete reconstruction is briefly proposed. This review will arouse the attention on complete reconstruction materials and their applications in diverse fields.

Perspective on intermetallics towards efficient electrocatalytic water-splitting

Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.

Autologous manganese phosphates with different Mn sites for electrocatalytic water oxidation

Herein, we report two autologous phosphates obtained from the same parent material for electrocatalytic water oxidation. These two phosphates have many similarities except the coordination structure of the Mn centers. It has been straightforwardly observed that the highly asymmetric geometry of Mn2P2O7 can stabilize the active Mn(iii) to promote water oxidation.

Two-dimensional IrN2 monolayer: An efficient bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

The development of bifunctional electrocatalysts with good stability and high efficiency for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for renewable energy conversion and storage. Herein, by means of swarm-intelligence structure search and density functional theory (DFT) computations, we proposed a novel kind of two-dimensional (2D) monolayer with hypercoordinate structure as electrocatalysts for ORR/OER, namely, transition dinitride (TMN₂, TM = V, Co, Rh, Pd, W, Re, and Ir) monolayer. Our result revealed that these TMN₂ monolayers have excellent thermal, dynamic and chemical stability, as well as inherent metallic nature for their practical applications in electrocatalysis. More interestingly, among all 2D TMN₂ materials, the IrN₂ monolayer was suggested to perform as an ideal bifunctional electrocatalyst for ORR/OER with a low overpotential of 0.47 and 0.27 V, respectively, which is comparable to Pt and Ir- or Ru-based oxides. Furthermore, by examining the d-band centers of the active sites in different TMN₂ monolayers, we well rationalized the superior catalytic activity of IrN₂ monolayer for ORR/OER. Our findings not only further enrich 2D nanomaterials with hypercoordinate structure, but also open a new door to develop bifunctional oxygen electrocatalysts with high efficiency.

Electrooxidation-enabled electroactive high-valence ferritic species in NiFe layered double hydroxide arrays as efficient oxygen evolution catalysts

Currently, engineering non-precious NiFe layered double hydroxide (NiFe-LDH) electrocatalysts with excellent oxygen evolution performances at high current densities is highly critical to promoting electrolytic water splitting producing hydrogen for large-scale commercial applications. Herein, an intrigued oxygen vacancy-rich Fe(Ⅱ)-incorporated NiFe-LDH containing electroactive high-valence ferritic species is successfully grown on Ni foam (Fe²⁺-NiFe-LDH-EO6 h@NF) through an elaborate two-step route including hydrothermal and electrooxidation, and utilized as a high-efficiency elctrocayalyst of alkaline water oxidation possessing abundant exposed active sites, excellent intrinsic catalytic activity and superior durability. Therefore, the Fe²⁺-NiFe-LDH-EO6 h@NF electrocatalyst towards oxygen evolution reaction (OER) enables the low overpotentials of 239, 285 and 350 mV for the current densities of 10, 100 and 500 mA cm^-2, respectively, a small Tafel slope of 48.3 mV dec^-1, the low onset potential of 1.451 V, and retains the catalytic activity for 40 h at the large current density of 500 mA cm^-2 as well as owns the high turnover frequency (TOF) value of 0.93 s^-1 at the overpotential of 300 mV. This work provides a promising avenue to improve the OER performances of NiFe-LDH electrocatalyst for practical applications.

On the crystal chemistry of inorganic nitrides: crystal-chemical parameters, bonding behavior, and opportunities in the exploration of their compositional space

The scarcity of nitrogen in Earth's crust, combined with challenging synthesis, have made inorganic nitrides a relatively unexplored class of compounds compared to their naturally abundant oxide counterparts. To facilitate exploration of their compositional space via a priori modeling, and to help a posteriori structure verification not limited to inferring the oxidation state of redox-active cations, we derive a suite of bond-valence parameters and Lewis acid strength values for 76 cations observed bonding to N3-, and further outline a baseline statistical knowledge of bond lengths for these compounds. Examination of structural and electronic effects responsible for the functional properties and anomalous bonding behavior of inorganic nitrides shows that many mechanisms of bond-length variation ubiquitous to oxide and oxysalt compounds (e.g., lone-pair stereoactivity, the Jahn-Teller and pseudo Jahn-Teller effects) are similarly pervasive in inorganic nitrides, and are occasionally observed to result in greater distortion magnitude than their oxide counterparts. We identify promising functional units for exploring uncharted chemical spaces of inorganic nitrides, e.g. multiple-bond metal centers with promise regarding the development of a post-Haber-Bosch process proceeding at milder reaction conditions, and promote an atomistic understanding of chemical bonding in nitrides relevant to such pursuits as the development of a model of ion substitution in solids, a problem of great relevance to semiconductor doping whose solution would fast-track the development of compound solar cells, battery materials, electronics, and more.

Low-Temperature Nitridation of Fe3O4 by Reaction with NaNH2

Low-temperature soft chemical synthesis routes to transition-metal nitrides are of interest as an alternative to conventional high-temperature ammonolysis reactions involving large volumes of chemotoxic NH₃ gas. One such method is the reaction between metal oxides and NaNH₂ at ca. 200 °C to yield the counterpart nitrides; however, there remains uncertainty regarding the reaction mechanism and product phase assemblage (in particular, noncrystalline components). Here, we extend the chemical tool box and mechanistic understanding of such reactions, demonstrating the nitridation of Fe₃O₄ by reaction with NaNH₂ at 170-190 °C, via a pseudomorphic reaction. The more reduced Fe₃O₄ precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with Fe₂O₃. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and ⁵⁷Fe Mössbauer spectroscopy, comprised 49-59 mol % ε-Fe_2+xN, accompanied by 29-39 mol % FeO_1-xN_x and 8-14 mol % γ″-FeN. The oxynitride phase was apparently noncrystalline in the recovered product but could be crystallized by heating at 180 °C. Although synthesis of transition-metal nitrides is achieved by reaction of the counterpart oxide with NaNH₂, it is evident from this investigation that the product phase assemblage may be complex, which could prove a limitation if the objective is to produce a single-phase product with well-defined electrical, magnetic, or other physical properties for applications. However, the significant yield of the FeO_1-xN_x oxynitride phase identified in this study opens the possibility for the synthesis of metastable oxynitride phases in high yield, by reaction of a metal oxide substrate with NaNH₂, with either careful control of H₂O concentration in the system or postsynthetic hydrolysis and crystallization.

Direct Pyrolysis of a Manganese-Triazolate Metal-Organic Framework into Air-Stable Manganese Nitride Nanoparticles

Although metal-organic frameworks (MOFs) are being widely used to derive functional nanomaterials through pyrolysis, the actual mechanisms involved remain unclear. In the limited studies to date, elemental metallic species are found to be the initial products, which limits the variety of MOF-derived nanomaterials. Here, the pyrolysis of a manganese triazolate MOF is examined carefully in terms of phase transformation, reaction pathways, and morphology evolution in different conditions. Surprisingly, the formation of metal is not detected when manganese triazolate is pyrolyzed in an oxygen-free environment. Instead, a direct transformation into nanoparticles of manganese nitride, Mn2N x embedded in N-doped graphitic carbon took place. The electrically conductive Mn2N x nanoparticles show much better air stability than bulk samples and exhibit promising electrocatalytic performance for the oxygen reduction reaction. The findings on pyrolysis mechanisms expand the potential of MOF as a precursor to derive more functional nanomaterials.

Ion-biosorption induced core–shell Fe2P@carbon nanoparticles decorated on N, P co-doped carbon materials for the oxygen evolution reaction

This work employs bacteria as precursors and induces a cost-effective biosorption strategy to obtain Fe₂P@carbon nanoparticles decorated on N and P co-doped carbon (Fe₂P@CNPs/NPC) materials.

Transition metal nitrides for electrochemical energy applications

This review comprehensively summarizes the progress on the structural and electronic modulation of transition metal nitrides for electrochemical energy applications.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction

This review summarizes the recent progress on MOFs and their derivatives used for OER electrocatalysis in terms of their morphology, composition and structure–performance relationship.

Highly active hollow mesoporous NiFeCr hydroxide as an electrode material for the oxygen evolution reaction and a redox capacitor

A hollow mesoporous trimetallic NiFeCr hydroxide electrode is prepared via a four-step procedure involving the fast electrodeposition of an Ni/Cr/Fe alloy onto a nickel foam substrate, followed by dealloying, oxidation, and activation. The title electrode shows an ultralow onset overpotential of 210 mV for the oxygen evolution reaction and a high specific capacity of 1768 F g^-1 as a redox capacitor.

A soft molecular 2Fe-2As precursor approach to the synthesis of nanostructured FeAs for efficient electrocatalytic water oxidation

An unprecedented molecular 2Fe-2As precursor complex was synthesized and transformed under soft reaction conditions to produce an active and long-term stable nanocrystalline FeAs material for electrocatalytic water oxidation in alkaline media. The 2Fe2As-centred β-diketiminato complex, having an unusual planar Fe2As2 core structure, results from the salt-metathesis reaction of the corresponding β-diketiminato FeIICl complex and the AsCO- (arsaethynolate) anion as the monoanionic As- source. The as-prepared FeAs phase produced from the precursor has been electrophoretically deposited on conductive electrode substrates and shown to act as a electro(pre)catalyst for the oxygen evolution reaction (OER). The deposited FeAs undergoes corrosion under the severe anodic alkaline conditions which causes extensive dissolution of As into the electrolyte forming finally an active two-line ferrihydrite phase (Fe2O3(H2O) x ). Importantly, the dissolved As in the electrolyte can be fully recaptured (electro-deposited) at the counter electrode making the complete process eco-conscious. The results represent a new and facile entry to unexplored nanostructured transition-metal arsenides and their utilization for high-performance OER electrocatalysis, which are also known to be magnificent high-temperature superconductors.

Crystalline Copper Selenide as a Reliable Non-Noble Electro(pre)catalyst for Overall Water Splitting

Electrochemical water splitting remains a frontier research topic in the quest to develop artificial photosynthetic systems by using noble metal-free and sustainable catalysts. Herein, a highly crystalline CuSe has been employed as active electrodes for overall water splitting (OWS) in alkaline media. The pure-phase klockmannite CuSe deposited on highly conducting nickel foam (NF) electrodes by electrophoretic deposition (EPD) displayed an overpotential of merely 297 mV for the reaction of oxygen evolution (OER) at a current density of 10 mA cm-2 whereas an overpotential of 162 mV was attained for the hydrogen evolution reaction (HER) at the same current density, superseding the Cu-based as well as the state-of-the-art RuO2 and IrO2 catalysts. The bifunctional behavior of the catalyst has successfully been utilized to fabricate an overall water-splitting device, which exhibits a low cell voltage (1.68 V) with long-term stability. Post-catalytic analyses of the catalyst by ex-situ microscopic, spectroscopic, and analytical methods confirm that under both OER and HER conditions, the crystalline and conductive CuSe behaves as an electro(pre)catalyst forming a highly reactive in situ crystalline Cu(OH)2 overlayer (electro(post)catalyst), which facilitates oxygen (O2 ) evolution, and an amorphous Cu(OH)2 /CuOx active surface for hydrogen (H2 ) evolution. The present study demonstrates a distinct approach to produce highly active copper-based catalysts starting from copper chalcogenides and could be used as a basis to enhance the performance in durable bifunctional overall water splitting.

A Low-Temperature Molecular Precursor Approach to Copper-Based Nano-Sized Digenite Mineral for Efficient Electrocatalytic Oxygen Evolution Reaction

In the urge of designing noble metal-free and sustainable electrocatalysts for oxygen evolution reaction (OER), herein, a mineral Digenite Cu9 S5 has been prepared from a molecular copper(I) precursor, [{(PyHS)2 CuI (PyHS)}2 ](OTf)2 (1), and utilized as an anode material in electrocatalytic OER for the first time. A hot injection of 1 yielded a pure phase and highly crystalline Cu9 S5 , which was then electrophoretically deposited (EPD) on a highly conducting nickel foam (NF) substrate. When assessed as an electrode for OER, the Cu9 S5 /NF displayed an overpotential of merely 298±3 mV at a current density of 10 mA cm-2 in alkaline media. The overpotential recorded here supersedes the value obtained for the best reported Cu-based as well as the benchmark precious-metal-based RuO2 and IrO2 electrocatalysts. In addition, the choronoamperometric OER indicated the superior stability of Cu9 S5 /NF, rendering its suitability as the sustainable anode material for practical feasibility. The excellent catalytic activity of Cu9 S5 can be attributed to the formation of a crystalline CuO overlayer on the conductive Cu9 S5 that behaves as active species to facilitate OER. This study delivers a distinct molecular precursor approach to produce highly active copper-based catalysts that could be used as an efficient and durable OER electro(pre)catalysts relying on non-precious metals.

Recent advances in transition metal based compound catalysts for water splitting from the perspective of crystal engineering

A review of the recent progress on the transition metal based catalysts for water splitting with emphasis on crystal engineering.

Heteroleptic manganese compounds as potential precursors for manganese based thin films and nanomaterials

Among the five novel synthesized manganese compounds, Mn(dmampea)(ⁱPr-MeAMD) was obtained as a highly volatile liquid compound that can be used as a precursor for manganese based thin films and nanomaterials.

A core-shell structured CoMoO4•nH2O@Co1-xFexOOH nanocatalyst for electrochemical evolution of oxygen

Nickel-iron oxyhydroxide (Ni_1-xFe_xOOH) is well recognized as the best-performing oxygen evolution reaction (OER) catalyst in alkaline electrolytes, however its analogue cobalt-iron oxyhydroxide (Co_1-xFe_xOOH) is surprisingly less explored despite their structural similarity. Inspired by our recent study on high-performance HER catalyst using the nanostructured CoMoO₄•nH₂O precursor, herein, we report a facile synthesis of Co_1-xFe_xOOH catalyst derived from the same precursor and its excellent electrocatalytic properties towards the OER in alkaline electrolytes. A core-shell structured nanocatalyst consisting of disordered Co_1-xFe_xOOH layer over the surface of crystalline CoMoO₄•nH₂O nanosheets was synthesized using a simple hydrothermal method followed by anodic electrooxidation. Thus-prepared catalyst exhibited extraordinarily high and stable activity towards the OER in alkaline electrolyte, which outperformed most Co-based OER catalysts. Combined with the HER catalyst derived from the same CoMoO₄•nH₂O precursor as the cathode, we further developed and tested a simple water-splitting cell, which significantly surpasses the benchmarking IrO₂-Pt/C couple (1.63 V) and requires a voltage of only 1.517 V to afford 10 mA cm^-2 in 1.0 M KOH solution. Density functional theory calculations were conducted to gain insight into the Fe-doping induced improvement of OER activity.