Metallic Nickel Nitride Nanosheets Realizing Enhanced Electrochemical Water Oxidation

Boron modification promoting electrochemical surface reconstruction of NiFe-LDH for efficient and stable freshwater/seawater oxidation catalysis

Transition metal-based electrocatalysts generally take place surface reconstruction in alkaline conditions, but little is known about how to improve the reconstruction to a highly active oxyhydroxide surface for an efficient and stable oxygen evolution reaction (OER). Herein, we develop a strategy to accelerate surface reconstruction by combining boron modification and cyclic voltammetry (CV) activation. Density functional theory calculations and in-situ/ex-situ characterizations indicate that both B-doping and electrochemical activation can reduce the energy barrier and contribute to the surface evolution into highly active oxyhydroxides. The formed oxyhydroxide active phase can tune the electronic configuration and boost the OER process. The reconstructed catalyst of CV-B-NiFe-LDH displays excellent alkaline OER performance in freshwater, simulated seawater, and natural seawater with low overpotentials at 100 mA cm-2 (η100: 219, 236, and 255 mV, respectively) and good durability. This catalyst also presents outstanding Cl- corrosion resistance in alkalized seawater electrolytes. The CV-B-NiFe-LDH||Pt/C electrolyzer reveals prominent performance for alkalized freshwater/seawater splitting. This study provides a guideline for developing advanced OER electrocatalysts by promoting surface reconstruction.

Redox Promoted Rapid and Deep Reconstruction of Defect-Rich Nickel Precatalysts for Efficient Water Oxidation

Understanding the reconstruction mechanism to rationally design cost-effective electrocatalysts for oxygen evolution reaction (OER) is still challenging. Herein, a defect-rich NiMoO4 precatalyst is used to explore its OER activity and reconstruction mechanism. In situ generated oxygen vacancies, distorted lattices, and edge dislocations expedite the deep reconstruction of NiMoO4 to form polycrystalline Ni (oxy)hydroxides for alkaline oxygen evolution. It only needs ≈230 and ≈285 mV to reach 10 and 100 mA cm-2, respectively. The reconstruction boosted by the redox of Ni is confirmed experimentally by sectionalized cyclic voltammetry activations at different specified potential ranges combined with ex situ characterization techniques. Subsequently, the reconstruction route is presented based on the acid-base electronic theory. Accordingly, the dominant contribution of the adsorbate evolution mechanism to reconstruction during oxygen evolution is revealed. This work develops a novel route to synthesize defect-rich materials and provides new tactics to investigate the reconstruction.

Chitosan-assisted hydrogen adsorption and reversibility of Ni-doped hierarchical carbon scaffolds

We investigated the effects of chitosan (CS) on the hydrogen adsorption and reversibility of hierarchical carbon scaffold (HCS) loaded with Ni nanoparticles. As size-controllable, stabilizing, and shape-directing agents for the green synthesis of metal nanoparticles of CS, Ni nanoparticles with uniform distribution and shape are deposited onto HCS. The latter results in the superior specific surface area of Ni nanoparticles for hydrogen chemisorption. The best hydrogen adsorption capacities at room temperature under 20-70 bar H2 of 0.5-1.70 wt% H2 were obtained from 10 wt% Ni-doped HCS-CS. Although macropores of the HCS collapsed upon cycling due to hydrogen pressure applied during adsorption, average hydrogen capacities of 1.17 ± 0.05 wt% H2 (T = 25 °C and p(H2) = 50 bar) were maintained for 14 cycles. This is because not only uniform distribution and shape of Ni nanoparticles and microporous structures of the HCS were preserved upon cycling but also the interactions between Ni and heteroatoms (N and O) of the HCS and CS prevented particle agglomeration.

Electronic redistribution through the interface of MnCo2O4-Ni3N nano-urchins prompts rapid In situ phase transformation for enhanced oxygen evolution reaction

One of the most coveted objectives in the realm of energy conversion technologies is the development of highly efficient and economically viable electrocatalysts for the oxygen evolution reaction. The commercialization of such techniques has thus far been impeded by their slow response kinetics. One of the many ways to develop highly effective electrocatalysts is to judiciously choose a coupling interface that maximizes catalyst performance. In this study, the in situ electrochemical phase transformation of MnCo2O4-Ni3N into MnCo2O4-NiOOH is described. The catalyst has an exceptional overpotential of 224 mV to drive a current density of 10 mA cm-2. Strong interfacial contact is seen in the MnCo2O4-Ni3N catalyst, leading to a considerable electronic redistribution between the MnCo2O4 and Ni3N phases. This causes an increase in the valence state of Ni, which makes it an active site for the adsorption of *OH, O*, and *OOH (intermediates). This charge transfer facilitates the rapid phase transformation to form NiOOH from Ni3N. At a higher current density of 300 mA cm-2, the catalyst remained stable for a period of 140 h. DFT studies also revealed that the in situ-formed NiOOH on the MnCo2O4 surface results in superior OER kinetics compared to that of NiOOH alone.

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.

Mo5N6 nanosheets for fluorescent quenching and target recognition: Highly selectively sensing of sodium hexametaphosphate

Typical fluorescent biosensors use fluorescently labeled ssDNA for target recognition and nanomaterials for signal transduction. Herein, we propose a reverse sensing strategy that Mo5N6 nanosheets are used for target recognition while fluorescein (FAM)-labeled ssDNA only serves for signal generation. We discover that Mo5N6 nanosheets show high fluorescence quenching ability (>95%) and selective recognition for sodium hexametaphosphate (SHMP). After FAM-labeled ssDNA is adsorbed on Mo5N6 nanosheets, the fluorescence is quenched due to the photoinduced electron transfer (PET) effect between FAM and Mo5N6 nanosheets. SHMP can specifically displace the adsorbed FAM-labeled ssDNA from Mo5N6 nanosheets, resulting in more than 80% fluorescence recovery on addition of 5 μmol L-1 SHMP. This biosensor can sensitively detect SHMP down to 150 nmol L-1 and selectively recognize SHMP over glucose, lactose, common amino acids, Zn2+, Mg2+, Ca2+ and other phosphates (such as Na2HPO4, sodium pyrophosphate, sodium tripolyphosphate). This biosensor also shows great potential for the detection of SHMP in bacon sample. This work not only provides a facile sensitive and selective biosensor for SHMP but also exploits the application of transition metal nitrides in the field of sensing and biosensing.

Tungsten doped FeCoP2 nanoparticles embedded into carbon for highly efficient oxygen evolution reaction

Designing active and stable electrocatalysts with economic efficiency for oxygen evolution reaction (OER) is essential for developing water splitting process at an industrial scale. Herein, we rationally designed a tungsten doped iron cobalt phosphide incorporated with carbon (Wx-FeCoP2/C), prepared by a mechanochemical approach. X-ray photoelectron spectroscopy (XPS) revealed that the doping of W led to an increasing of Co3+/Co2+ and Fe3+/Fe2+ molar ratios, which contributed to the enhanced OER performance. As a result, a current density of 10 mA cm-2 was achieved in 1 M KOH at an overpotential of 264 mV on the optimized W0.1-FeCoP2/C. Moreover, at high current density of 100 mA cm-2, the overpotential value was 310 mV, and the corresponding Tafel slope was measured to be 48.5 mV dec-1, placing it among the best phosphide-based catalysts for OER. This work is expected to enlighten the design strategy of highly efficient phosphide-based OER catalysts.

Cation Exchange in Colloidal Transition Metal Nitride Nanocrystals

Transition metal nitride (TMN)-based nanostructures have emerged as promising materials for diverse applications in electronics, photonics, energy storage, and catalysis due to their highly desirable physicochemical properties. However, synthesizing TMN-based nanostructures with designed compositions and morphologies poses challenges, especially in the solution phase. The cation exchange reaction (CER) stands out as a versatile postsynthetic strategy for preparing nanostructures that are otherwise inaccessible through direct synthesis. Nevertheless, exploration of the CER in TMNs lags behind that in metal chalcogenides and metal phosphides. Here, we demonstrate cation exchange in colloidal metal nitride nanocrystals, employing Cu3N nanocrystals as starting materials to synthesize Ni4N and CoN nanocrystals. By controlling the reaction conditions, Cu3N@Ni4N and Cu3N@CoN core@shell heterostructures with tunable compositions can also be obtained. The Ni4N and CoN nanocrystals are evaluated as catalysts for the electrochemical oxygen evolution reaction (OER). Remarkably, CoN nanocrystals demonstrate superior OER performance with a low overpotential of 286 mV at 10 mA·cm-2, a small Tafel slope of 89 mV·dec-1, and long-term stability. Our CER approach in colloidal TMNs offers a new strategy for preparing other metal nitride nanocrystals and their heterostructures, paving the way for prospective applications.

Functionality Modulation Toward Thianthrene-based Metal-Free Electrocatalysts for Water Splitting

The development of metal-free bifunctional electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) is significant but rarely demonstrated. Porous organic polymers (POPs) with well-defined electroactive functionalities show superior performance in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Precise control of the active sites' local environment requires careful modulation of linkers through the judicious selection of building units. Here, a systematic strategy is introduced for modulating functionality to design and synthesize a series of thianthrene-based bifunctional sp2 C═C bonded POPs with hollow spherical morphologies exhibiting superior electrocatalytic activity. This precise structural tuning allowed to gain insight into the effects of heteroatom incorporation, hydrophilicity, and variations in linker length on electrocatalytic activity. The most efficient bifunctional electrocatalyst THT-PyDAN achieves a current density of 10 mA cm─2 at an overpotential (η10) of ≈65 mV (in 0.5 m H2SO4) and ≈283 mV (in 1 m KOH) for HER and OER, respectively. THT-PyDAN exhibits superior activity to all previously reported metal-free bifunctional electrocatalysts in the literature. Furthermore, these investigations demonstrate that THT-PyDAN maintains its performance even after 36 h of chronoamperometry and 1000 CV cycling. Post-catalytic characterization using FT-IR, XPS, and microscopic imaging techniques underscores the long-term durability of THT-PyDAN.

In Situ Electrochemical Restructuring B-Doped Metal-Organic Frameworks as Efficient OER Electrocatalysts for Stable Anion Exchange Membrane Water Electrolysis

Metal organic frameworks (MOFs) are promising as effective electrocatalysts toward oxygen evolution reaction (OER). However, the origin of OER activity for MOF-based electrocatalysts is still unclear because of their structure reconstruction during electrocatalysis process. Here, a novel MOF (B-MOF-Zn-Co) with spherical superstructure is developed by hydrothermal treatment of zeolitic imidazolate framework-Zn, Co (ZIF-Zn-Co) using boric acid. The resultant B-MOF-Zn-Co shows high OER activity with a low overpotential of 362 mV at 100 mA cm-2. Remarkably, B-MOF-Zn-Co displays excellent stability with only 3.6% voltage delay over 300 h at 100 mA cm-2 in alkaline electrolyte. Surprisingly, B-MOF-Zn-Co thoroughly transforms into B-doped CoOOH (B-CoOOH) during electrolysis process, which is served as actual active material for high OER electrocatalytic performance. The newly-formed B-CoOOH possesses lower energy barrier of potential-determining step (PDS) for OOH* formation compared with CoOOH, benefiting for high OER activity. More importantly, B-MOF-Zn-Co based anion exchange membrane water electrolytic cell (AEMWE) demonstrates continuously durable operation with stable current density of 200 mA cm-2 over 300 h, illustrating its potential application in practice water electrolysis. This work offers an in situ electrochemical reconstruction strategy for the development of stable and effective OER electrocatalysts toward practice AEMWE.

Lattice engineering of noble metal-based nanomaterials via metal-nonmetal interactions for catalytic applications

Noble metal-based nanomaterials possess outstanding catalytic properties in various chemical reactions. However, the increasing cost of noble metals severely hinders their large-scale applications. A cost-effective strategy is incorporating noble metals with light nonmetal elements (e.g., H, B, C, N, P and S) to form noble metal-based nanocompounds, which can not only reduce the noble metal content, but also promote their catalytic performances by tuning their crystal lattices and introducing additional active sites. In this review, we present a concise overview of the recent advancements in the preparation and application of various kinds of noble metal-light nonmetal binary nanocompounds. Besides introducing synthetic strategies, we focus on the effects of introducing light nonmetal elements on the lattice structures of noble metals and highlight notable progress in the lattice strain engineering of representative core-shell nanostructures derived from these nanocompounds. In the meantime, the catalytic applications of the light element-incorporated noble metal-based nanomaterials are discussed. Finally, we discuss current challenges and future perspectives in the development of noble metal-nonmetal based nanomaterials.

Simultaneous Interface Engineering and Phase Tuning of CeO2-Decorated Catalysts for Boosted Oxygen Evolution Reaction

Heterogeneous catalysts have attracted extensive attention among various emerging catalysts for their exceptional oxygen evolution reaction (OER) capabilities, outperforming their single-component counterparts. Nonetheless, the synthesis of heterogeneous materials with predictable, precise, and facile control remains a formidable challenge. Herein, a novel strategy involving the decoration of catalysts with CeO2 is introduced to concurrently engineer heterogeneous interfaces and adjust phase composition, thereby enhancing OER performance. Theoretical calculations suggest that the presence of ceria reduces the free energy barrier for the conversion of nitrides into metals. Supporting this, the experimental findings reveal that the incorporation of rare earth oxides enables the controlled phase transition from nitride into metal, with the proportion adjustable by varying the amount of added rare earth. Thanks to the role of CeO2 decoration in promoting the reaction kinetics and fostering the formation of the genuine active phase, the optimized Ni3FeN/Ni3Fe/CeO2-5% nanoparticles heterostructure catalyst exhibits outstanding OER activity, achieving an overpotential of just 249 mV at 10 mA cm-2. This approach offers fresh perspectives for the conception of highly efficient heterogeneous OER catalysts, contributing a strategic avenue for advanced catalytic design in the field of energy conversion.

Crystal Structure Regulation of CoSe2 Induced by Fe Dopant for Promoted Surface Reconstitution toward Energetic Oxygen Evolution Reaction

Most nonoxide catalysts based on transition metal elements will inevitably change their primitive phases under anodic oxidation conditions in alkaline media. Establishing a relationship between the bulk phase and surface evolution is imperative to reveal the intrinsic catalytic active sites. In this work, it is demonstrated that the introduction of Fe facilitates the phase transition of orthorhombic CoSe2 into its cubic counterpart and then accelerates the Co-Fe hydroxide layer generation on the surface during electrocatalytic oxygen evolution reaction (OER). As a result, the Fe-doped cubic CoSe2 catalyst exhibits a significantly enhanced activity with a considerable overpotential decrease of 79.9 and 66.9 mV to deliver 10 mA·cm-2 accompanied by a Tafel slope of 48.0 mV·dec-1 toward OER when compared to orthorhombic CoSe2 and Fe-doped orthorhombic CoSe2, respectively. Density functional theory (DFT) calculations reveal that the introduction of Fe on the surface hydroxide layers will tune electron density around Co atoms and raise the d-band center. These findings will provide deep insights into the surface reconstitution of the OER electrocatalysts based on transition metal elements.

Computational chemistry for water-splitting electrocatalysis

Electrocatalytic water splitting driven by renewable electricity has attracted great interest in recent years for producing hydrogen with high-purity. However, the practical applications of this technology are limited by the development of electrocatalysts with high activity, low cost, and long durability. In the search for new electrocatalysts, computational chemistry has made outstanding contributions by providing fundamental laws that govern the electron behavior and enabling predictions of electrocatalyst performance. This review delves into theoretical studies on electrochemical water-splitting processes. Firstly, we introduce the fundamentals of electrochemical water electrolysis and subsequently discuss the current advancements in computational methods and models for electrocatalytic water splitting. Additionally, a comprehensive overview of benchmark descriptors is provided to aid in understanding intrinsic catalytic performance for water-splitting electrocatalysts. Finally, we critically evaluate the remaining challenges within this field.

From Charge to Spin: An In-Depth Exploration of Electron Transfer in Energy Electrocatalysis

Catalytic materials play crucial roles in various energy-related processes, ranging from large-scale chemical production to advancements in renewable energy technologies. Despite a century of dedicated research, major enduring challenges associated with enhancing catalyst efficiency and durability, particularly in green energy-related electrochemical reactions, remain. Focusing only on either the crystal structure or electronic structure of a catalyst is deemed insufficient to break the linear scaling relationship (LSR), which is the golden rule for the design of advanced catalysts. The discourse in this review intricately outlines the essence of heterogeneous catalysis reactions by highlighting the vital roles played by electron properties. The physical and electrochemical properties of electron charge and spin that govern catalysis efficiencies are analyzed. Emphasis is placed on the pronounced influence of external fields in perturbing the LSR, underscoring the vital role that electron spin plays in advancing high-performance catalyst design. The review culminates by proffering insights into the potential applications of spin catalysis, concluding with a discussion of extant challenges and inherent limitations.

Tailoring iridium-palladium nanoparticles with Ir-rich skin: a highly durable anode electrocatalyst for acidic water electrolysis via a facile microwave-assisted chemical reduction method

Electrochemical water splitting under acidic conditions is a clean way towards producing hydrogen fuels. The slow kinetics of the oxygen evolution reaction (OER) at the anode is currently a bottleneck for commercial acceptance of this technology. Therefore, arriving at more efficient and sustainable OER electrocatalysts is highly desirable. We here demonstrate the synthesis of iridium-palladium (IrPd) alloy nanoparticles (2-5 nm) with variable average composition (Ir : Pd = 1 : 0, 1 : 1, 1 : 3, 1 : 6, 1 : 9 and 0 : 1) using a facile one-pot microwave-assisted chemical reduction method. The IrPd nanoparticles show structure- and composition-dependent OER performance in acidic media. Utilizing different reduction strengths and precursor ratios, successful alloy catalysts were prepared with Ir-rich skin and sublayers of different Pd compositions. Their structures were revealed using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and hydrogen underpotential deposition (Hupd) studies. It turned out that (1) the alloy OER catalyst also has a high electrochemically active surface area for hydrogen adsorption/desorption, (2) the OER performance is strongly dependent on the surface Ir contribution and (3) the intact Ir skin is essential for electrocatalyst stability.

A frogspawn inspired twin Mo2C/Ni composite with a conductive fibrous network as a robust bifunctional catalyst for advanced anion exchange membrane electrolyzers

Anion exchange membrane water electrolysis (AEMWE) is considered one of the most cost-effective methods for producing green hydrogen. However, the performance of AEMWE is still restrained by the slow reaction kinetics and poor ion/electron transport of catalysts. Herein, inspired by frogspawn, Mo2C nanoparticles coupled with Ni were in situ embedded into a N-doped porous carbon nanofiber network (Mo2C/NCNTs@Ni) by chemical crosslinking electrospinning combined with carbonization. The unique bionic structure can guarantee favorable overall structural flexibility and fast ion/electron transport kinetics. As a result of the robust hydrogen binding energy of Mo2C, as well as the synergistic impact between Ni and Mo2C nanoparticles and the conductive network resembling frogspawn, the catalyst developed demonstrates excellent performance in both the HER and OER. When employed as a bifunctional catalyst in water electrolysis, Mo2C/NCNTs@Ni delivers overpotentials of 155 mV and 320 mV at 10 mA cm-2 for the HER and OER, respectively. In addition, the Mo2C/NCNTs@Ni also displays excellent long-term durability during a continuous operation test under different currents for 50 h. The assembled AEMWE electrolyzers with Mo2C/NCNTs@Ni as both the anode and cathode can achieve a current density of 82.5 mA cm-2 at 1.99 V, indicating great potential for industrial water splitting. These results give an insight for the development of advanced bifunctional electrocatalysts for the next generation of green and efficient H2 production by water electrolysis.

Oxygen vacancy-rich Fe2(MoO4)3 combined with MWCNTs for electrochemical sensors of fentanyl and its analogs

Hundreds of thousands of people dying from the abuse of fentanyl and its analogs. Hence, the development of an efficient and highly accurate detection method is extremely relevant and challenging. Therefore, we proposed the introduction of oxygen defects into Fe2(MoO4)3 nanoparticles for improving the catalyst performance and combining it with multi-walled carbon nanotubes (MWCNTs) for electrochemical detection of fentanyl and its analogs. Oxygen vacancy-rich Fe2(MoO4)3 (called r-Fe2(MoO4)3) nanoparticles were successfully synthesized and characterized in detail by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Raman spectra, BET, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) and investigated by comparison with oxygen vacancy-poor Fe2(MoO4)3 (called p-Fe2(MoO4)3). The obtained oxygen vacancy-rich Fe2(MoO4)3 was ultrasonically composited with MWCNTs for modification of glassy carbon electrodes (GCEs) used for the electrochemical detection of fentanyl and its analogs. The modified MWCNT-GCE showed ultrasensitivity to fentanyl, sufentanil, alfentanil, and acetylfentanyl with limits of detection (LOD) of 0.006 µmol·L-1, 0.008 µmol·L-1, 0.018 µmol·L-1, and 0.024 µmol·L-1, respectively, and could distinguish among the four drugs based on their peak voltages. Besides, the obtained r-Fe2(MoO4)3/MWCNT composite also exhibited high repeatability, selectivity, and stability. It showed satisfactory detection performance on real samples, with recoveries of 70.53 ~ 94.85% and 50.98 ~ 82.54% in serum and urine for the four drugs in a concentration range 0.2 ~ 1 µM, respectively. The experimental results confirm that the introduction of oxygen vacancies effectively improves the sensitivity of fentanyl electrochemical detection, and this work provides some inspiration for the development of catalytic materials for electrochemical sensors with higher sensitivity.

Manipulation of Electron Spins with Oxygen Vacancy on Amorphous/Crystalline Composite-Type Catalyst

By substituting the oxygen evolution reaction (OER) with the anodic urea oxidation reaction (UOR), it not only reduces energy consumption for green hydrogen generation but also allows purification of urea-rich wastewater. Spin engineering of the d orbital and oxygen-containing adsorbates has been recognized as an effective pathway for enhancing the performance of electrocatalysts. In this work, we report the fabrication of a bifunctional electrocatalyst composed of amorphous RuO2-coated NiO ultrathin nanosheets (a-RuO2/NiO) with abundant amorphous/crystalline interfaces for hydrogen evolution reaction (HER) and UOR. Impressively, only 1.372 V of voltage is required to attain a current density of 10 mA cm-2 over a urea electrolyzer. The increased oxygen vacancies in a-RuO2/NiO by incorporation of amorphous RuO2 enhance the total magnetization and entail numerous spin-polarized electrons during the reaction, which speeds up the UOR reaction kinetics. The density functional theory study reveals that the amorphous/crystalline interfaces promote charge-carrier transfer, and the tailored d-band center endows the optimized adsorption of oxygen-generated intermediates. This kind of oxygen vacancy induced spin-polarized electrons toward boosting HER and UOR kinetics and provides a reliable reference for exploration of advanced electrocatalysts.

Co-Co Dinuclear Active Sites Dispersed on Zirconium-doped Heterostructured Co9 S8 /Co3 O4 for High-current-density and Durable Acidic Oxygen Evolution

Developing cost-effective and sustainable acidic water oxidation catalysts requires significant advances in material design and in-depth mechanism understanding for proton exchange membrane water electrolysis. Herein, we developed a single atom regulatory strategy to construct Co-Co dinuclear active sites (DASs) catalysts that atomically dispersed zirconium doped Co9 S8 /Co3 O4 heterostructure. The X-ray absorption fine structure elucidated the incorporation of Zr greatly facilitated the generation of Co-Co DASs layer with stretching of cobalt oxygen bond and S-Co-O heterogeneous grain boundaries interfaces, engineering attractive activity of significantly reduced overpotential of 75 mV at 10 mA cm-2 , a breakthrough of 500 mA cm-2 high current density, and water splitting stability of 500 hours in acid, making it one of the best-performing acid-stable OER non-noble metal materials. The optimized catalyst with interatomic Co-Co distance (ca. 2.80 Å) followed oxo-oxo coupling mechanism that involved obvious oxygen bridges on dinuclear Co sites (1,090 cm-1 ), confirmed by in situ SR-FTIR, XAFS and theoretical simulations. Furthermore, a major breakthrough of 120,000 mA g-1 high mass current density using the first reported noble metal-free cobalt anode catalyst of Co-Co DASs/ZCC in PEM-WE at 2.14 V was recorded.

Exceptional green hydrogen production performance of a ruthenium-modulated nickel selenide

Developing low-cost, high-efficiency and stable electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is crucial but highly challenging. Density functional theory (DFT) calculations reveal that doping ruthenium (Ru) into catalysts can effectively optimize their electronic structure, hence leading to an optimal Gibbs free energy on the catalyst surface. Herein, an ultra-low Ru (about 2.34 wt%)-doped Ni3Se2 nanowire catalyst (i.e., Ru/Ni3Se2) supported on nickel foam has been fabricated by a hydrothermal reaction followed by a chemical etching process. The unique three-dimensional (3D) interconnected nanowires not only endow Ru and Ni3Se2 with uniform distribution and coupling, but also provide higher electrical conductivity, more active sites, an optimized electronic structure and favorable reaction kinetics. Therefore, the as-obtained Ru/Ni3Se2 catalyst exhibits excellent electrocatalytic performance, with low overpotentials of 24 and 211 mV to supply a current density value of 10 mA cm-2 towards the HER and OER in an alkaline environment, respectively. Notably, the as-fabricated Ru/Ni3Se2 catalyst only requires a low voltage of 1.476 V to derive a current density of 10 mA cm-2 in the constructed two-electrode alkaline electrolyzer and exhibits exceptionally high stability. This work will provide a novel strategy for the design and fabrication of low-cost and high-performance bifunctional electrocatalysts for hydrogen production by water electrolysis.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Highly Active Pyrochlore-Type Praseodymium Ruthenate Electrocatalyst for Efficient Acid-Water Oxidation

To produce directly combustible hydrogen from water, highly active, acid-resistant, and economical catalysts for oxygen evolution reaction (OER) are needed. An electrocatalyst based on praseodymium ruthenate (Pr2Ru2O7) is presented here that greatly outperforms RuO2 for acid-water oxidation. Specifically, at 10 mA cm-2, this electrocatalyst presents a low overpotential (η) of 213 mV and markedly superior stability. Moreover, Pr2Ru2O7 presents a significant rise in turnover frequency (TOF) and a highly intrinsic mass activity of 1618.8 A gRu-1 (η = 300 mV), exceeding the most commonly reported acid OER catalysts. Density functional theory calculations and electronic structure study demonstrate that the Ru 4d-band center related to the longer Ru-O bond with a large radius of Pr ion in this pyrochlore is lower than that in RuO2, which would optimize the binding between the adsorbed oxygen species and catalytic metal sites and enhance the catalytic intrinsic activity.

Branched Porous Ni3N as a Catalytic Electrode for Selective Semidehydrogenation of Tetrahydroisoquinoline

Oxygen evolution in electrochemical water splitting needs a high overpotential that significantly reduces the energy efficiency. To explore an alternative anodic reaction to promote the production of hydrogen at the other end of water splitting and at the same time to get high-value-added chemicals is highly desirable. Herein, we demonstrate a novel branched porous Ni3N catalyst that is prepared for dehydrogenation of tetrahydroisoquinoline, which acts as an anodic oxidation reaction to promote H2 formation on the other end. Interestingly, the Ni3N catalytic electrode can induce effective semidehydrogenation with the selective formation of dihydroisoquinoline, which is difficult to be obtained by the usual direct synthesis route. The catalytic electrode exhibits a low potential of 1.55 V (vs RHE) for a catalytic current density of 61 mA cm-2 with dehydrogenation of tetrahydroisoquinoline and hydrogen production. In situ Raman spectra studies suggest that NiOOH is formed on the electrode surface, which mediates the oxidation semidehydrogenation process. This work also provides a strategy to fabricate nitride materials for applications beyond selective semidehydrogenation of tetrahydroisoquinoline.

Boosting urea electrooxidation on oxyanion-engineered nickel sites via inhibited water oxidation

Renewable energy-based electrocatalytic oxidation of organic nucleophiles (e.g.methanol, urea, and amine) are more thermodynamically favourable and, economically attractive to replace conventional pure water electrooxidation in electrolyser to produce hydrogen. However, it is challenging due to the competitive oxygen evolution reaction under a high current density (e.g., >300 mA cm-2), which reduces the anode electrocatalyst's activity and stability. Herein, taking lower energy cost urea electrooxidation reaction as the model reaction, we developed oxyanion-engineered Nickel catalysts to inhibit competing oxygen evolution reaction during urea oxidation reaction, achieving an ultrahigh 323.4 mA cm-2 current density at 1.65 V with 99.3 ± 0.4% selectivity of N-products. In situ spectra studies reveal that such in situ generated oxyanions not only inhibit OH- adsorption and guarantee high coverage of urea reactant on active sites to avoid oxygen evolution reaction, but also accelerate urea's C - N bond cleavage to form CNO - intermediates for facilitating urea oxidation reaction. Accordingly, a comprehensive mechanism for competitive adsorption behaviour between OH- and urea to boost urea electrooxidation and dynamic change of Ni active sites during urea oxidation reaction was proposed. This work presents a feasible route for high-efficiency urea electrooxidation reaction and even various electrooxidation reactions in practical applications.

Recent Advances in the Preparation and Application of Two-Dimensional Nanomaterials

Two-dimensional nanomaterials (2D NMs), consisting of atoms or a near-atomic thickness with infinite transverse dimensions, possess unique structures, excellent physical properties, and tunable surface chemistry. They exhibit significant potential for development in the fields of sensing, renewable energy, and catalysis. This paper presents a comprehensive overview of the latest research findings on the preparation and application of 2D NMs. First, the article introduces the common synthesis methods of 2D NMs from both "top-down" and "bottom-up" perspectives, including mechanical exfoliation, ultrasonic-assisted liquid-phase exfoliation, ion intercalation, chemical vapor deposition, and hydrothermal techniques. In terms of the applications of 2D NMs, this study focuses on their potential in gas sensing, lithium-ion batteries, photodetection, electromagnetic wave absorption, photocatalysis, and electrocatalysis. Additionally, based on existing research, the article looks forward to the future development trends and possible challenges of 2D NMs. The significance of this work lies in its systematic summary of the recent advancements in the preparation methods and applications of 2D NMs.

Aqueous pulsed electrochemistry promotes C-N bond formation via a one-pot cascade approach

Electrocatalytic C - N bond formation from inorganic nitrogen wastes is an emerging sustainable method for synthesizing organic amines but is limited in reaction scope. Integrating heterogeneous and homogeneous catalysis for one-pot reactions to construct C - N bonds is highly desirable. Herein, we report an aqueous pulsed electrochemistry-mediated transformation of nitrite and arylboronic acids to arylamines with high yields. The overall process involves nitrite electroreduction to ammonia over a Cu nanocoral cathode and subsequent coupling of NH3 with arylboronic acids catalyzed by in situ dissolved Cu(II) under a switched anodic potential. This pulsed protocol also promotes the migration of nucleophilic ArB(OH)3- and causes the consumption of OH- near the cathode surface, accelerating C - N formation and suppressing phenol byproducts. Cu(II) can be recycled via facile electroplating. The wide substrate scope, ready synthesis of 15N-labelled arylamines, and methodological expansion to cycloaddition and Click reactions highlight the great promise.

Special NaBH4 hydrolysis achieving multiple-surface-modifications promotes the high-throughput water oxidation of CoN nanowire arrays

Designing an excellent OER catalyst in an alkaline environment is severe yet essential for industrial H₂ application under the electrochemical technique. This study has achieved multiple modifications on CoN nanowires, the classic OER catalyst, via a facile room-temperature NaBH₄ spontaneous hydrolysis. This facile process simultaneously generates oxygen vacancies and robust BN species. It wraps hydrophilic BO_x motifs on the OER response CoN nanowires, producing OER active Co-N-B species, increasing active numbers and guaranteeing structural stability. It suggests that a low NaBH₄ concentration (0.1 mol L^-1) treatment endows CoN_NWAs/CC with excellent OER performance and robust structure, which can drive a current density of 50 mA cm^-2 with only 325 mV overpotentials with more than 24 hours' durability. Even, the catalyst can drive 1000 mA cm^-2 around 480 mV overpotential. This study allows a novel strategy for designing high-performance OER catalysts.

Regulable in-situ autoredox for anchoring synergistic Ni/NiO nanoparticles on reduced graphene oxide with boosted alkaline electrocatalytic oxygen evolution

To develop ideal alternatives to noble metal catalysts, transition metal catalysts supported on graphene have been receiving extensive attention in the field of electrochemical energy. In this work, using graphene oxide (GO) and nickel formate as precursors, Ni/NiO synergistic nanoparticles with regulable composition are anchored on reduced graphene oxide (RGO) to prepare Ni/NiO/RGO composite electrocatalysts through in-situ autoredox. Thanks to the synergistic effect of Ni³⁺ active sites and Ni electron donors, the as-prepared Ni/NiO/RGO catalysts exhibit efficient electrocatalytic oxygen evolution performance in 1.0 M KOH electrolyte. The optimal sample has an overpotential of only 275 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 90 mV dec^-1, which are very comparable to those of commercial RuO₂ catalyst. Additionally, the catalytic capacity and structure remain stable after 2000 cyclic voltammetry cycles. For the electrolytic cell assembled with the best-performing sample as anode and commercial Pt/C as cathode, the current density can reach 10 mA cm^-2 at a low potential of 1.57 V and remains stable after 30 h of continuous work. It would be expected that the as-developed Ni/NiO/RGO catalyst with high activity should have broad application prospects.

Fe-Incorporated Nickel-Based Bimetallic Metal-Organic Frameworks for Enhanced Electrochemical Oxygen Evolution

Developing cost-effective and high-efficiency catalysts for electrocatalytic oxygen evolution reaction (OER) is crucial for energy conversions. Herein, a series of bimetallic NiFe metal-organic frameworks (NiFe-BDC) were prepared by a simple solvothermal method for alkaline OER. The synergistic effect between Ni and Fe as well as the large specific surface area lead to a high exposure of Ni active sites during the OER. The optimized NiFe-BDC-0.5 exhibits superior OER performances with a small overpotential of 256 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 45.4 mV dec^-1, which outperforms commercial RuO₂ and most of the reported MOF-based catalysts reported in the literature. This work provides a new insight into the design of bimetallic MOFs in the applications of electrolysis.

Surface plasmon-enhanced photo-driven CO2 hydrogenation by hydroxy-terminated nickel nitride nanosheets

The majority of visible light-active plasmonic catalysts are often limited to Au, Ag, Cu, Al, etc., which have considerations in terms of costs, accessibility, and instability. Here, we show hydroxy-terminated nickel nitride (Ni₃N) nanosheets as an alternative to these metals. The Ni₃N nanosheets catalyze CO₂ hydrogenation with a high CO production rate (1212 mmol g^-1 h^-1) and selectivity (99%) using visible light. Reaction rate shows super-linear power law dependence on the light intensity, while quantum efficiencies increase with an increase in light intensity and reaction temperature. The transient absorption experiments reveal that the hydroxyl groups increase the number of hot electrons available for photocatalysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy shows that the CO₂ hydrogenation proceeds via the direct dissociation pathway. The excellent photocatalytic performance of these Ni₃N nanosheets (without co-catalysts or sacrificial agents) is suggestive of the use of metal nitrides instead of conventional plasmonic metal nanoparticles.

Nitrogen substitution induced lattice contraction in nickel nanoparticles for electrochemical hydrogen evolution from simulated seawater

Herein, we demonstrate a facile method for the introduction of nitrogen in the lattices of nickel nanoparticles to form NiN_x (x = 0.13, 0.20, 0.27). X-ray absorption spectroscopy reveals the contraction of the Ni-Ni bond and modulated coordination environment after nitrogen introduction. The NiN_0.20 required 87 mV overpotential for -10 mA cm^-2 cathodic current density in simulated seawater. The density functional theory calculations revealed favorable E_H2Oads and ΔG_Hads after N-introduction.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Apparent activity and specific activity of lanthanides (La, Ce, Nd) decorated Co-MOF derivatives for electrocatalytic water splitting

Lanthanide (Ln) rare Earth (RE) elements are often used to incorporate and regulate the local coordination environment and electronic configuration of transition metal based electrocatalysts for acquiring improved electrocatalytic performance. But for a given pristine electrode, is a Ln element concentrated more on promoting the apparent activity of original electrode or on enhancing its specific activity? To address this issue, Ln (La, Ce and Nd) decorated ZIF-67 derivative electrodes (Ln/Co/NC) were fabricated following with the detailed experimental testing of apparent activity and specific activity of assembled electrodes. X-ray photoelectron spectroscopy data confirmed that Ce, Nd and La have played their own role in regulating the coordination electronic structure of the surface atoms of the derived Co/NC by forming different types of chemical bonds. Electrochemical (EC) results confirmed that Ce is concentrated more on the apparent activity of derived Co/NC electrode with the smallest overpotential at 50 mA cm^-2(η₅₀), while Nd contributes more to its reaction kinetic property with the smallest value of Tafel slope in alkaline hydrogen evolution reaction process. But for oxygen evolution reaction, all of La, Ce and Nd deteriorate the apparent activity of the pristine Co/NC electrode. Comparatively, La shows a greater ability to modulate the specific activity of Co/NC with a larger electrochemical active surface area normalized current density, while Nd exhibits the best ability to re-establish the properties of reaction centers. This work illustrates the difference influence of La, Ce and Nd on the apparent activity and specific activity of the ZIF-67 derivative Co/NC electrode. It will do some favors in engineering RE elements modified composite electrodes for EC applications.

Biomimicry-inspired fish scale-like Ni3N/FeNi3N/NF superhydrophilic/superaerophobic nanoarrays displaying high electrocatalytic performance

The mass transfer efficiency and structural stability of the electrode are critical for industrialized water electrolysis operations. Herein, the biomimicry-inspired design of Ni₃N/FeNi₃N/NF nanoarrays with a fish scale-like structure, which endowed the Ni₃N/FeNi₃N/NF nanoarrays with rapid infiltration of aqueous solution within 60 ms and 169° bubble contact angle, is demonstrated. The optimal Ni₃N/FeNi₃N/NF sample displayed catalytic activity with hydrogen evolution reaction (HER) overpotentials of only 48 mV at 10 mA cm^-2 and 102 mV at 100 mA cm^-2. Similarly, the overpotential of the anodic-coupled urea oxidation reaction (UOR) was only 1.3 V at 10 mA cm^-2 and 1.35 V at 100 mA cm^-2. Besides, the small impact resulting from the rapid bubble extraction within the Ni₃N/FeNi₃N/NF nanoarrays ensured excellent HER cycling stability over 100 h at a current density of 50 mA cm^-2. The further scale-up experiment suggests the industrialization prospects of the prepared Ni₃N/FeNi₃N/NF electrocatalysts.

Composition and Morphology Modulation of Bimetallic Nitride Nanostructures on Nickel Foams for Efficient Oxygen Evolution Electrocatalysis

Metal-nitrides-based electrocatalysts for efficient oxygen-evolution have been extensively studied as one of the most promising candidates to fulfil the demand for future energy-conversion and storage. Herein, a series of NixCo1−xO- and NixCo1−xN-based nanostructures on nickel foams were reported to show excellent activities for oxygen-evolution reaction. The catalysts were prepared and modulated rationally via a facile-hydrothermal method, followed by high-temperature calcination under air or nitrogen atmosphere. The optimal bimetallic-nitride catalyst Ni0.3Co0.7N shows a small overpotential of 268 mV at 20 mA cm−2, and a Tafel slope of 66 mV dec−1 with good stability. The enhanced OER-performance is ascribed to the synergetic effect of the unique morphology and the intrinsic catalytic property of the nanostructure after nitridation.

Plasma-Assisted Synthesis of Metal Nitrides for an Efficient Platinum-Group-Metal-Free Anion-Exchange-Membrane Fuel Cell

In comparison to the well-developed proton-exchange-membrane fuel cells, anion-exchange-membrane fuel cells (AEMFCs) permit adoption of platinum-group-metal (PGM)-free catalysts due to the alkaline environment, giving a substantial cost reduction. However, previous AEMFCs have generally shown unsatisfactory performances due to the lack of effective PGM-free catalysts that can endure harsh fuel cell conditions. Here we report a plasma-assisted synthesis of high-quality nickel nitride (Ni₃N) and zirconium nitride (ZrN) employing dinitrogen as the nitrogen resource, exhibiting exceptional catalytic performances toward hydrogen oxidation and oxygen reduction in an alkaline enviroment, respectively. A PGM-free AEMFC assembled by using Ni₃N as the anode and ZrN as the cathode delivers power densities of 256 mW cm^-2 under an H₂-O₂ condition and 151 mW cm^-2 under an H₂-air condition. Furthermore, the fuel cell shows no evidence of degradation after 25 h of operation. This work creates opportunities for developing high-performance and durable AEMFCs based on metal nitrides.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Metal-air batteries: progress and perspective

The metal-air batteries with the largest theoretical energy densities have been paid much more attention. However, metal-air batteries including Li-air/O₂, Li-CO₂, Na-air/O_2, and Zn-air/O₂ batteries, are complex systems that have their respective scientific problems, such as metal dendrite forming/deforming, the kinetics of redox mediators for oxygen reduction/evolution reactions, high overpotentials, desolution of CO₂, H₂O, etc. from the air and related side reactions on both anode and cathode. It should be the main direction to address these shortages to improve performance. Here, we summarized recently research progress in these metal-air/O₂ batteries. Some perspectives are also provided for these research fields.

Activated Ni-O-Ir Enhanced Electron Transfer for Boosting Oxygen Evolution Reaction Activity of LaNi1-x Irx O3

Tuning the structure of the active center of catalysts to atomic level provides the most efficient utilization of the active component, which plays an especially important role for precious metals. In this study, the liquid phase ion exchange method is used to introduce atomic Ir into LaNiO₃ perovskite oxide, which shows excellent catalytic performance in the oxygen evolution reaction (OER). The catalyst, LaNi_0.96 Ir_0.04 O₃ , with the optimal concentration of Ir, displays an overpotential of just 280 mV at 10 mA cm^-2 . The introduced Ir enriches the surface electron density significantly, which not only improves site-to-site electron transfer between O and Ni sites but also allows stable adsorption of the intermediates. The results of cyclic voltammetry tests reveal the superior overpotential and remarkable efficiency of the OER process because of the strong interactions in Ni-O-Ir. Moreover, the Ir atom inhibits the participation of a lattice oxygen oxidation mechanism (LOM) in LaNiO₃ that guarantees the stability of the catalyst in alkaline conditions. It is anticipated that this work will be instrumental for the preparation and study of a broad range of atomic metal-doped perovskite oxides for water splitting.