An Ir/Ni(OH)2 Heterostructured Electrocatalyst for the Oxygen Evolution Reaction: Breaking the Scaling Relation, Stabilizing Iridium(V), and Beyond

Modulating Electronic Structure of Iridium Single-Atom Anchored on 3D Fe-Doped β-Ni(OH)2 Catalyst with Nanopyramid Array Structure for Enhanced Oxygen Evolution Reaction

Developing high-performance electrocatalysts for oxygen evolution reaction (OER) is crucial in the pursuit of clean and sustainable hydrogen energy, yet still challenging. Herein, a spontaneous redox strategy is reported to achieve iridium single-atoms anchored on hierarchical nanosheet-based porous Fe doped β-Ni(OH)2 pyramid array electrodes (SAs Ir/Fe-β-Ni(OH)2), which exhibits high OER performance with a low overpotential of 175 mV at 10 mA cm-2 and a remarkable OER current density in alkaline electrolyte, surpassing Fe-β-Ni(OH)2/NF and IrO2 by 31 and 38 times at 1.43 V versus RHE, respectively. OER catalytic mechanism demonstrates that the conversion of *OH→*O and the active lattice O content can be significantly improved due to the modulation effect of the Ir single atoms on the local electronic structure and the redox behavior of FeNi (oxy) hydroxide true active species. This work provides a promising insight into understanding the OER enhancement mechanism for Ir single-atoms modified FeNi-hydroxide systems.

Direct Electroplating Ruthenium Precursor on the Surface Oxidized Nickel Foam for Efficient and Stable Bifunctional Alkaline Water Electrolysis

Water electrolysis to produce hydrogen (H2) using renewable energy is one of the most promising candidates for realizing carbon neutrality, but its reaction kinetics is hindered by sluggish anodic oxygen evolution reaction (OER). Ruthenium (Ru) in its high-valence state (oxide) provides one of the most active OER sites and is less costly, but thermodynamically unstable. The strong interaction between Ru nanoparticles (NPs) and nickel hydroxide (Ni(OH)2) is leveraged to directly form Ru-Ni(OH)2 on the surface of a porous nickel foam (NF) electrode via spontaneous galvanic replacement reaction. The formation of Ru─O─Ni bonds at the interface of the Ru NPs and Ni(OH)2 (Ru-Ni(OH)2) on the surface oxidized NF significantly enhance stability of the Ru-Ni(OH)2/NF electrode. In addition to OER, the catalyst is active enough for the hydrogen evolution reaction (HER). As a result, it is able to deliver overpotentials of 228 and 15 mV to reach 10 mA cm-2 for OER and HER, respectively. An industry-scale evaluation using Ru-Ni(OH)2/NF as both OER and HER electrodes demonstrates a high current density of 1500 mA cm-2 (OER: 410 mV; HER: 240 mV), surpassing commercial RuO2 (OER: 600 mV) and Pt/C based performance (HER: 265 mV).

Nickel Hydroxide-Based Electrocatalysts for Promising Electrochemical Oxidation Reactions: Beyond Water Oxidation

Transition metal hydroxides have attracted significant research interest for their energy storage and conversion technique applications. In particular, nickel hydroxide (Ni(OH)2), with increasing significance, is extensively used in material science and engineering. The past decades have witnessed the flourishing of Ni(OH)2-based materials as efficient electrocatalysts for water oxidation, which is a critical catalytic reaction for sustainable technologies, such as water electrolysis, fuel cells, CO2 reduction, and metal-air batteries. Coupling the electrochemical oxidation of small molecules to replace water oxidation at the anode is confirmed as an effective and promising strategy for realizing the energy-saving production. The physicochemical properties of Ni(OH)2 related to conventional water oxidation are first presented in this review. Then, recent progress based on Ni(OH)2 materials for these promising electrochemical reactions is symmetrically categorized and reviewed. Significant emphasis is placed on establishing the structure-activity relationship and disclosing the reaction mechanism. Emerging material design strategies for novel electrocatalysts are also highlighted. Finally, the existing challenges and future research directions are presented.

Synergistic Assistance of Ir Clusters and NiCo2O4 Nanosheets Interfaces in Direct O-O Coupling for High-Efficiency Alkaline Oxygen Evolution

Adopting noble metals on non-noble metals is an effective strategy to balance the cost and activity of electrocatalysts. Herein, a thorough analysis of the synergistic OER is conducted at the heterogeneous interface formed by Ir clusters and NiCo2O4 based on DFT calculations. Specifically, the electrons spontaneously bring an eg occupancy of interfacial Ir close to unity after the absorbed O, providing more transferable electrons for the conversion of the absorbed O-intermediates. Besides, the diffuse distribution of electrons in the Ir 5d orbital fills the antibonding orbital after O is absorbed, avoiding the desorption difficulties caused by the stronger Ir-O bonds. The electrons transfer from Ir to Co atoms at the heterogeneous interface and fill the Co 3d band near the Fermi level, stimulating the interfacial Co to participate in the direct O-O coupling (DOOC) pathway. Experimentally, the ultrathin-modulated NiCo2O4 nanosheets are used to support Ir clusters (Ircluster-E-NiCo2O4) by the electrodeposition method. The as-synthesized Ircluster-E-NiCo2O4 catalyst achieves a current density of 10 mA cm-2 at an ultralow overpotential of 238 mV and works steadily for 100 h under a high current of 100 mA cm-2, benefiting from the efficient DOOC pathway during the OER.

Confined Space Dual-Type Quantum Dots for High-Rate Electrochemical Energy Storage

Owing to the quantum size effect and high redox activity, quantum dots (QDs) play very essential roles toward electrochemical energy storage. However, it is very difficult to obtain different types and uniformly dispersed high-active QDs in a stable conductive microenvironment, because QDs prepared by traditional methods are mostly dissolved in solution or loaded on the surface of other semiconductors. Herein, dual-type semiconductor QDs (Co9S8 and CdS) are skillfully constructed within the interlayer of ultrathin-layered double hydroxides. In particular, the expandable interlayer provides a very suitable confined space for the growth and uniform dispersion of QDs, where Co9S8 originates from in situ transformation of cobalt atoms in laminate and CdS is generated from interlayer pre-embedding Cd2+. Meanwhile, XAFS and GGA+U calculations are employed to explore and prove the mechanism of QDs formation and energy storage characteristics as compared to surface loading QDs. Significantly, the hybrid supercapacitors achieve a high energy density of 329.2 µWh cm-2, capacitance retention of 99.1%, and coulomb efficiency of 96.9% after 22 000 cycles, which is superior to the reported QDs-based supercapacitors. These findings provide unique insights for designing and developing stable, ordered, and highly active QDs.

Built-In Electric Field Boosted Overall Water Electrolysis at Large Current Density for the Heterogeneous Ir/CoMoO4 Nanosheet Arrays

Advanced bifunctional electrocatalysts are essential for propelling overall water splitting (OWS) progress. Herein, relying on the obvious difference in the work function of Ir (5.44 eV) and CoMoO4 (4.03 eV) and the constructed built-in electric field (BEF), an Ir/CoMoO4/NF heterogeneous catalyst, with ultrafine Ir nanoclusters (1.8 ± 0.2 nm) embedded in CoMoO4 nanosheet arrays on the surface of nickel foam skeleton, is reported. Impressively, the Ir/CoMoO4/NF shows remarkable electrocatalytic bifunctionality toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), especially at large current densities, requiring only 13 and 166 mV to deliver 10 and 1000 mA cm-2 for HER and 196 and 318 mV for OER. Furthermore, the Ir/CoMoO4/NF||Ir/CoMoO4/NF electrolyzer demands only 1.43 and 1.81 V to drive 10 and 1000 mA cm-2 for OWS. Systematical theoretical calculations and tests show that the formed BEF not only optimizes interfacial charge distribution and the Fermi level of both Ir and CoMoO4, but also reduces the Gibbs free energy (ΔGH*, from 0.25 to 0.03 eV) and activation energy (from 13.6 to 8.9 kJ mol-1) of HER, the energy barrier (from 3.47 to 1.56 eV) and activation energy (from 21.1 to 13.9 kJ mol-1) of OER, thereby contributing to the glorious electrocatalytic bifunctionality.

Redispersing Ir Nanoparticles via a Carbon-Assisted Pyrolysis Process to Break the Activity-Stability Trade-Off of H2 Sensors

Oxide semiconductor-supported metal nanoparticles often suffer from a high-temperature gas sensing process, resulting in agglomeration and coalescence, which significantly decrease their surface activity and stability. Here, we develop an in situ pyrolysis strategy to redisperse commercial Ir particles (∼15.6 nm) into monodisperse Ir species (∼5.4 nm) on ZnO supports, exhibiting excellent sintering-resistant properties and H2 sensing. We find that large-size Ir nanoparticles can undergo an unexpected splitting decomposition process and spontaneously migrate along the encapsulated carbon layer surface during high-temperature pyrolysis of ZIF-8. This resultant monodisperse status can be integrally reserved, accompanying further oxidation sintering. The final Irred/ZnO-450-based sensor exhibits outstanding stability, H2 response (10-2000 ppm), fast response/recovery capability (7/9.7 s@100 ppm), and good moisture resistance. In situ Raman and ex situ XPS further experimentally verify that highly dispersive Ir species can promote the electron transfer process during the gas sensing process. Our strategy thus provides important insights into the design of agglomeration-resistant gas sensing materials for highly effective H2 detection.

Mesoporous Cubic Nanocages Assembled by Coupled Monolayers With 100% Theoretical Capacity and Robust Cycling

High capacity and long cycling often conflict with each other in electrode materials. Despite extensive efforts in structural design, it remains challenging to simultaneously achieve dual high electrochemical properties. In this study, we prepared brand-new completely uniform mesoporous cubic-cages assembled by large d-spacing Ni(OH)2 coupled monolayers intercalated with VO4 3- (NiCMCs) using a biomimetic approach. Such unique mesoporous structural configuration results in an almost full atomic exposure with an amazing specific surface area of 505 m2/g and atomic utilization efficiency close to the theoretical limit, which is the highest value and far surpasses all of the reported Ni(OH)2. Thus, a breakthrough in simultaneously attaining high capacity approaching the 100% theoretical value and robust cycling of 10,000 cycles is achieved, setting a new precedent in achieving double-high attributes. When combined with high-performance Bi2O3 hexagonal nanotubes, the resulting aqueous battery exhibits an ultrahigh energy density of 115 Wh/kg and an outstanding power density of 9.5 kW/kg among the same kind. Characterizations and simulations reveal the important role of large interlayer spacing intercalation units and mesoporous cages for excellent electrochemical thermodynamics and kinetics. This work represents a milestone in developing "double-high" electrode materials, pointing in the direction for related research and paving the way for their practical application.

Construction and enhancement of built-in electric field for efficient oxygen evolution reaction

The construction and regulation of built-in electric field (BIEF) are considered effective strategies for enhancing the oxygen evolution reaction (OER) performance of transition metal-based electrocatalysts. Herein, we present a strategy to regulate the electronic structure of nickel-iron layered double hydroxide (NiFe-LDH) by constructing and enhancing the BIEF induced by in-situ heterojunction transformation. This concept is demonstrated through the design and synthesis of Ag2S@S/NiFe-LDH (p-n heterojunction) and Ag@S/NiFe-LDH (Mott-Schottky heterojunction). Benefiting from the larger BIEF of Mott-Schottky heterojunction, efficient electron transfer occurs at the interface between silver (Ag) and NiFe-LDH. As a result, Ag@S/NiFe-LDH exhibits excellent OER performance, requiring only a 232 mV overpotential at 1 M KOH to achieve a current density of 100 mA cm-2, with a small Tafel slope of 73 mV dec-1, as well as excellent electrocatalytic durability. Density functional theory (DFT) calculations further verified that stronger BIEF in Mott-Schottky heterojunction enhances the electron interaction at the interfaces, reduces the energy barrier for the rate-determining step (RDS), and accelerates the OER kinetics. This work provides an effective strategy for designing catalyst with larger BIEF to enhance electrocatalytic activity.

Designing a Phenalenyl-Based Dinuclear Ni(II) Complex: An Electrocatalyst with Two Single Ni Sites for the Oxygen Evolution Reaction (OER)

A new dinuclear Ni(II) complex 1, [Ni2II(dtbh-PLY)2], is synthesized from 9-(2-(3,6-di-tert-butyl-2-hydroxybenzylidene)hydrazineyl)-1H-phenalen-1-one, dtbh-PLYH2 ligand, and structurally characterized by various analytical tools including the single-crystal X-ray diffraction (SCXRD) technique. In the solid state, both Ni(II) metal centers in complex 1 exist in a distorted square planar geometry and display the presence of rare Ni···H-C anagostic interactions to form a one-dimensional (1-D) linear motif in the supramolecular array. Complex 1 is further stabilized in the solid state by π-π-stacking interactions between the highly delocalized phenalenyl rings. The redox features of complex 1 have been analyzed by the cyclic voltammetry (CV) technique in solution as well as in the solid state, revealing the crucial involvement of both the Ni(II) metal centers for undergoing quasi-reversible oxidation reactions on the application of an anodic sweep. A complex 1-modified glassy carbon electrode, GC-1, is employed as an electrocatalyst for oxygen evolution reaction (OER) in 1.0 M KOH, giving an OER onset at 1.45 V, and very low OER overpotential, 300 mV vs the reversible hydrogen electrode (RHE) to reach 10 mA cm-2 current density. Furthermore, GC-1 displayed fast OER kinetics with a Tafel slope of 40 mV dec-1, a significantly lower Tafel slope value than those of previously reported molecular Ni(II) catalysts. In situ electrochemical experiments and postoperational UV-vis, Fourier transform infrared (FT-IR), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) studies were performed to analyze the stability of the molecular nature of complex 1 and to gain reasonable insights into the true OER catalyst.

Tailoring atomic chemistry to refine reaction pathway for the most enhancement by magnetization in water oxidation

Water oxidation on magnetic catalysts has generated significant interest due to the spin-polarization effect. Recent studies have revealed that the disappearance of magnetic domain wall upon magnetization is responsible for the observed oxygen evolution reaction (OER) enhancement. However, an atomic picture of the reaction pathway remains unclear, i.e., which reaction pathway benefits most from spin-polarization, the adsorbent evolution mechanism, the intermolecular mechanism (I2M), the lattice oxygen-mediated one, or more? Here, using three model catalysts with distinguished atomic chemistries of active sites, we are able to reveal the atomic-level mechanism. We found that spin-polarized OER mainly occurs at interconnected active sites, which favors direct coupling of neighboring ligand oxygens (I2M). Furthermore, our study reveals the crucial role of lattice oxygen participation in spin-polarized OER, significantly facilitating the coupling kinetics of neighboring oxygen radicals at active sites.

Janus electronic state of supported iridium nanoclusters for sustainable alkaline water electrolysis

Metal-support electronic interactions play crucial roles in triggering the hydrogen spillover (HSo) to boost hydrogen evolution reaction (HER). It requires the supported metal of electron-rich state to facilitate the proton adsorption/spillover. However, this electron-rich metal state contradicts the traditional metal→support electron transfer protocol and is not compatible with the electron-donating oxygen evolution reaction (OER), especially in proton-poor alkaline conditions. Here we profile an Ir/NiPS3 support structure to study the Ir electronic states and performances in HSo/OER-integrated alkaline water electrolysis. The supported Ir is evidenced with Janus electron-rich and electron-poor states at the tip and interface regions to respectively facilitate the HSo and OER processes. Resultantly, the water electrolysis (WE) is efficiently implemented with 1.51 V at 10 mA cm-2 for 1000 h in 1 M KOH and 1.44 V in urea-KOH electrolyte. This research clarifies the Janus electronic state as fundamental in rationalizing efficient metal-support WE catalysts.

In situ reconstruction of self-supported NiFeP electrodes for overall water splitting at large current density

In this study, self-supported NiFeP was fabricated on Ni mesh (NiFeP/NM) via a two-step monopulse electrodeposition and phosphorization strategy. The NiFeP/NM exhibited excellent activity through electrochemical surface reconstruction to generate true active sites, requiring low overpotentials of 349 mV and 310 mV to reach a current density of 500 mA cm-2 for the HER and OER, respectively, and exhibiting satisfactory stability in 6 M NaOH.

Room-Temperature Chemoselective Hydrogenation of Nitroarene Over Atomic Metal-Nonmetal Catalytic Pair

Constructing atomic catalytic pair emerges as an attractive strategy to achieve better catalytic performance. Herein, an atomic Ir1─P1/NPG catalyst with asymmetric Ir─N2P1 sites that delivers superb activity and selectivity for hydrogenation of various functionalized nitrostyrene is reported. In the hydrogenation reaction of 3-nitrostyrene, Ir1─P1/NPG (NPG refers to N, P-codoped graphene) shows a turnover frequency of 1197 h-1, while the reaction cannot occur over Ir1/NG (NG refers to N-doped graphene). Compared to Ir1/NG, the charge density of the Ir site in Ir1─P1/NPG is greatly elevated, which is conducive to H2 dissociation. Moreover, as revealed by density functional theory calculations and poisoning experiments, the P site in Ir1─P1/NPG is found able to bind nitrostyrene, while the neighboring Ir site provides H to reduce the nitro group in chemoselective hydrogenation of nitrostyrene. This work offers a successful example of establishing atomic catalytic pair for driving important chemical reactions, paving the way for the development of more advanced catalysts to further improve the catalytic performance.

Monometallic Ultrasmall Nanocatalysts via Different Valence Atomic Interfaces Boost Hydrogen Evolution Catalysis

Synergistic monometallic nanocatalysts have attracted much attention due to their high intrinsic activity properties. However, current synergistic monometallic nanocatalysts tend to suffer from long reaction paths due to restricted nanoscale interfaces. In this paper, we synthesized the interstitial compound N-Pt/CNT with monometallic atomic interfaces. The catalysts are enriched with atomic interfaces between higher valence Ptδ+ and Pt0, allowing the reaction to proceed synergistically within the same component with an ideal reaction pathway. Through ratio optimization, N2.42-Pt/CNT with a suitable ratio of Ptδ+ and Pt0 is synthesized. And the calculated turnover frequency of N2.42-Pt/CNT is about 37.4 s-1 (-0.1 V vs reversible hydrogen electrode (RHE)), six times higher than that of the commercial Pt/C (6.58 s-1), which is the most intrinsically active of the Pt-based catalysts. Moreover, prepared N2.42-Pt/CNT exhibits excellent stability during the chronoamperometry tests of 200 h. With insights from comprehensive experiments and theoretical calculations, Pt with different valence states in monometallic atomic interfaces synergistically accelerates the H2O dissociation step and optimizes the Gibbs free energy of H* adsorption. And the existence of desirable hydrogen transfer paths substantially facilitates hydrogen evolution reaction kinetics.

Highly Active and Stable Alkaline Hydrogen Evolution Electrocatalyst Based on Ir-Incorporated Partially Oxidized Ru Aerogel under Industrial-Level Current Density

The realization of large-scale industrial application of alkaline water electrolysis for hydrogen generation is severely hampered by the cost of electricity. Therefore, it is currently necessary to synthesize highly efficient electrocatalysts with excellent stability and low overpotential under an industrial-level current density. Herein, Ir-incorporated in partially oxidized Ru aerogel has been designed and synthesized via a simple in situ reduction strategy and subsequent oxidation process. The electrochemical measurements demonstrate that the optimized Ru98 Ir2 -350 electrocatalyst exhibits outstanding hydrogen evolution reaction (HER) performance in an alkaline environment (1 M KOH). Especially, at the large current density of 1000 mA cm-2 , the overpotential is as low as 121 mV, far exceeding the benchmark Pt/C catalyst. Moreover, the Ru98 Ir2 -350 catalyst also displays excellent stability over 1500 h at 1000 mA cm-2 , denoting its industrial applicability. This work provides an efficient route for developing highly active and ultra-stable electrocatalysts for hydrogen generation under industrial-level current density.

Oxalic Acid-Assisted Vacancy Engineering Promotes Iron-Copper Sulfide Nanosheets for High-Current Density Water Oxidation

The effective defect and interface coupling are pivotal for the promotion of the catalytic activity for the oxygen evolution reaction. Herein, we report novel hybrid nanosheets with sulfur vacancies composed of FeS2 and Cu39S28 grown on Cu foam (Vs-FeS2/Cu39S28). The optimal Vs-FeS2/Cu39S28 exhibits a high current output of 500 mA cm-2 at a low overpotential of 370 mV and robust stability for 60 h at 100 mA cm-2, surpassing the values of most previously reported Cu-based catalysts. Furthermore, a two-electrode electrolyzer made by pairing the prepared catalyst with commercial Pt/C requires a low cell voltage of 1.75 V at 100 mA cm-2 and is retained over 80 h. Key to its excellent performance is the synergism between intertwined FeS2 and Cu39S28 domains, enriched by the deliberate introduction of sulfur vacancies, thus optimizing the electronic structure and causing the proliferation of catalytic active sites. This work presents a potent Cu-based electrocatalyst and emphasizes the leveraging of non-precious metals for efficient water oxidation.

A motif for B/O-site modulation in LaFeO3 towards boosted oxygen evolution

Here, a series of transition metal (Ni) doped iron-based perovskite oxides LaFe1-xNixO3-δ (x = 0, 0.25, 0.5, 0.75, 1) were prepared, and then the perovskite oxide with the optimized nickel-iron ratio was doped with non-metallic elements (N). Experimental and theoretical investigations reveal that the co-doping breaks the traditional linear constraint relationship (GOOH - GOH = 3.2 eV) and the theoretical overvoltage is reduced from 0.64 V (LaFeO3-δ) to 0.44 V (LaFe0.5Ni0.5O3-δ/N). Specifically, Ni-doping can accelerate electron transfer and improve the conductivity. Moreover, N-doping can reduce the adsorption energy of *OH/*O and enhance the adsorption energy of *OOH. We demonstrated that the optimized cation and anion co-doped LaFe0.5Ni0.5O3-δ/N perovskite oxide exhibits an excellent OER performance, with a low overpotential of 270.6 mV at 10 mA cm-2 and a small Tafel slope of 65 mV dec-1 in 1 M KOH solution, markedly exceeding that of the parent perovskite oxide LaFeO3-δ (300.9 mV) and commercial IrO2 (289.1 mV). It also delivers decent durability with no significant degradation after a 35 h stability test. This work reveals the internal mechanism of perovskite oxide by doping cation and anion for water oxidation, which broadens the idea for the rational design of new perovskite-based sustainable energy catalysts.

Nano-Scale Engineering of Heterojunction for Alkaline Water Electrolysis

Alkaline water electrolysis is promising for low-cost and scalable hydrogen production. Renewable energy-driven alkaline water electrolysis requires highly effective electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, the most active electrocatalysts show orders of magnitude lower performance in alkaline electrolytes than that in acidic ones. To improve such catalysts, heterojunction engineering has been exploited as the most efficient strategy to overcome the activity limitations of the single component in the catalyst. In this review, the basic knowledge of alkaline water electrolysis and the catalytic mechanisms of heterojunctions are introduced. In the HER mechanisms, the ensemble effect emphasizes the multi-sites of different components to accelerate the various intermedium reactions, while the electronic effect refers to the d-band center theory associated with the adsorption and desorption energies of the intermediate products and catalyst. For the OER with multi-electron transfer, a scaling relation was established: the free energy difference between HOO* and HO* is 3.2 eV, which can be overcome by electrocatalysts with heterojunctions. The development of electrocatalysts with heterojunctions are summarized. Typically, Ni(OH)2/Pt, Ni/NiN3 and MoP/MoS2 are HER electrocatalysts, while Ir/Co(OH)2, NiFe(OH)x/FeS and Co9S8/Ni3S2 are OER ones. Last but not the least, the trend of future research is discussed, from an industry perspective, in terms of decreasing the number of noble metals, achieving more stable heterojunctions for longer service, adopting new craft technologies such as 3D printing and exploring revolutionary alternate alkaline water electrolysis.

Direct Capturing and Regulating Key Intermediates for High-Efficiency Oxygen Evolution Reactions

Developing efficient oxygen evolution reaction (OER) electrocatalysts can greatly advance the commercialization of proton exchange membrane (PEM) water electrolysis. However, the unclear and disputed reaction mechanism and structure-activity relationship of OER pose significant obstacles. Herein, the active site and intermediate for OER on AuIr nanoalloys are simultaneously identified and correlated with the activity, through the integration of in situ shell-isolated nanoparticle-enhanced Raman spectroscopy and X-ray absorption spectroscopy. The AuIr nanoalloys display excellent OER performance with an overpotential of only 246 mV to achieve 10 mA cm-2 and long-term stability under strong acidic conditions. Direct spectroscopic evidence demonstrates that * OO adsorbed on IrOx sites is the key intermediate for OER, and it is generated through the O-O coupling of adsorbed oxygen species directly from water, providing clear support for the adsorbate evolution mechanism. Moreover, the Raman information of the * OO intermediate can serve as a universal "in situ descriptor" that can be obtained both experimentally and theoretically to accelerate the catalyst design. It unveils that weakening the interactions of * OO on the catalysts and facilitating its desorption would boost the OER performance. This work deepens the mechanistic understandings on OER and provides insightful guidance for the design of more efficient OER catalysts.

Ir-Sn pair-site triggers key oxygen radical intermediate for efficient acidic water oxidation

The anode corrosion induced by the harsh acidic and oxidative environment greatly restricts the lifespan of catalysts. Here, we propose an antioxidation strategy to mitigate Ir dissolution by triggering strong electronic interaction via elaborately constructing a heterostructured Ir-Sn pair-site catalyst. The formation of Ir-Sn dual-site at the heterointerface and the resulting strong electronic interactions considerably reduce d-band holes of Ir species during both the synthesis and the oxygen evolution reaction processes and suppress their overoxidation, enabling the catalyst with substantially boosted corrosion resistance. Consequently, the optimized catalyst exhibits a high mass activity of 4.4 A mgIr-1 at an overpotential of 320 mV and outstanding long-term stability. A proton-exchange-membrane water electrolyzer using this catalyst delivers a current density of 2 A cm-2 at 1.711 V and low degradation in an accelerated aging test. Theoretical calculations unravel that the oxygen radicals induced by the π* interaction between Ir 5d-O 2p might be responsible for the boosted activity and durability.

Electrochemical Reversible Reforming between Ethylamine and Acetonitrile on Heterostructured Pd-Ni(OH)2 Nanosheets

Rational design of electrocatalysts is essential to achieve desirable performance of electrochemical synthesis process. Heterostructured catalysts have thus attracted widespread attention due to their multifunctional intrinsic properties, and diverse catalytic applications with corresponding outstanding activities. Here, we report an in situ restoration strategy for the synthesis of ultrathin Pd-Ni(OH)2 nanosheets. Such Pd-Ni(OH)2 nanosheets exhibit excellent activity and selectivity towards reversible electrochemical reforming of ethylamine and acetonitrile. In the acetonitrile reduction process, Pd acts as reaction center, while Ni(OH)2 provide proton hydrogen through promoting the dissociation of water. Also ethylamine oxidation process can be achieved on the surface of the heterostructured nanosheets with abundant Ni(II) defects. More importantly, an electrolytic cell driven by solar cells was successfully constructed to realize ethylamine-acetonitrile reversible reforming. This work demonstrates the importance of heterostructure engineering in the rational synthesis of multifunctional catalysts towards electrochemical synthesis of fine chemicals.

Fabricating Ru Atom-Doped Novel FeP4/Fe2PO5 Heterogeneous Interface for Overall Water Splitting in Alkaline Environment

Developing bifunctional electrocatalysts with low-content noble metals and high activity and stability is crucial for water splitting. Herein, we reported a novel Ru doped FeP4/Fe2PO5 heterogeneous interface catalyst (Ru@FeP4/Fe2PO5) for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) by heat treatment coupling electrodeposition strategy. Experiments disclosed that Ru@FeP4/Fe2PO5 proclaimed excellent catalytic activity for the OER (249 mV@100 mA cm-2) and HER (49 mV@10 mA cm-2) in a 1 M KOH environment. More importantly, the mass activity and turnover frequency of Ru@FeP4/Fe2PO5 were 117 and 108 times higher than that of commercial RuO2 at an overpotential of 300 mV during the OER, respectively. In addition, the assembled Ru@FeP4/Fe2PO5 || Ru@FeP4/Fe2PO5 system could retain superior durability in a two-electrode system for 134 h at 300 mA cm-2. Further mechanism studies revealed that Ru atoms in Ru@FeP4/Fe2PO5 act in a key role for the excellent activity during water splitting because the electronic structure of Ru sites could be optimized by the interaction between Ru and Fe atoms at the interface to strengthen the adsorption of reaction intermediates. Besides, the introduction of Ru atoms could also enhance the charge transfer, which effectually accelerates the reaction kinetics. The strategy of anchoring Ru atom on novel heterostructure provides a promising path to boost the overall activity of electrocatalysts for water splitting.

Dynamically Stabilized Electronic Regulation and Electrochemical Reconstruction in Co and S Atomic Pair Doped Fe3 O4 for Water Oxidation

The electronic regulation and surface reconstruction of earth-abundant electrocatalysts are essential to efficient oxygen evolution reaction (OER). Here, an inverse-spinel Co,S atomic pair codoped Fe3 O4 grown on iron foam (Co,S-Fe3 O4 /IF) is fabricated as a cost-effective electrocatalyst for OER. This strategy of Co and S atomic pair directional codoping features accelerates surface reconstruction and dynamically stabilizes electronic regulation. CoS atomic pairs doped in the Fe3 O4 crystal favor controllable surface reconstruction via sulfur leaching, forming oxygen vacancies and Co doping on the surface of reconstructed FeOOH (Co-FeOOH-Ov /IF). Before and after surface reconstruction via in situ electrochemical process, the Fe sites with octahedral field dynamically maintains an appropriate electronic structure for OER intermediates, thus exhibiting consistently excellent OER performance. The electrochemically tuned Fe-based electrodes exhibit a low overpotential of 349 mV at a current density of 1000 mA cm-2 , a slight Tafel slope of 43.3 mV dec-1 , and exceptional long-term electrolysis stability of 200 h in an alkaline medium. Density functional theory calculations illustrate the electronic regulation of Fe sites, changes in Gibbs free energies, and the breaking of the restrictive scaling relation between OER intermediates. This work provides a promising directional codoping strategy for developing precatalysts for large-scale water-splitting systems.

Regulating electronic states of nitride/hydroxide to accelerate kinetics for oxygen evolution at large current density

Rational design efficient transition metal-based electrocatalysts for oxygen evolution reaction (OER) is critical for water splitting. However, industrial water-alkali electrolysis requires large current densities at low overpotentials, always limited by intrinsic activity. Herein, we report hierarchical bimetal nitride/hydroxide (NiMoN/NiFe LDH) array as model catalyst, regulating the electronic states and tracking the relationship of structure-activity. As-activated NiMoN/NiFe LDH exhibits the industrially required current density of 1000 mA cm^-2 at overpotential of 266 mV with 250 h stability for OER. Especially, in-situ electrochemical spectroscopic reveals that heterointerface facilitates dynamic structure evolution to optimize electronic structure. Operando electrochemical impedance spectroscopy implies accelerated OER kinetics and intermediate evolution due to fast charge transport. The OER mechanism is revealed by the combination of theoretical and experimental studies, indicating as-activated NiMoN/NiFe LDH follows lattice oxygen oxidation mechanism with accelerated kinetics. This work paves an avenue to develop efficient catalysts for industrial water electrolysis via tuning electronic states.

New Conceptual Catalyst on Spatial High-Entropy Alloy Heterostructures for High-Performance Li-O2 Batteries

High-entropy alloys (HEAs) are attracting increased attention as an alternative to noble metals for various catalytic reactions. However, it is of great challenge and fundamental importance to develop spatial HEA heterostructures to manipulate d-band center of interfacial metal atoms and modulate electron-distribution to enhance electrocatalytic activity of HEA catalysts. Herein, an efficient strategy is demonstrated to construct unique well-designed HEAs spatial heterostructure electrocatalyst (HEA@Pt) as bifunctional cathode to accelerate oxygen reduction and evolution reaction (ORR/OER) kinetics for Li-O₂ batteries, where uniform Pt dendrites grow on PtRuFeCoNi HEA at a low angle boundary. Such atomically connected HEA spatial interfaces engender efficient electrons from HEA to Pt due to discrepancy of work functions, modulating electron distribution for fast interfacial electron transfer, and abundant active sites. Theoretical calculations reveal that electron redistribution manipulates d-band center of interfacial metal atoms, allowing appropriate adsorption energy of oxygen species to lower ORR/OER reaction barriers. Hence, Li-O₂ battery based on HEA@Pt electrocatalyst delivers a minimal polarization potential (0.37 V) and long-term cyclability (210 cycles) under a cut-off capacity of 1000 mAh g^-1 , surpassing most previously reported noble metal-based catalysts. This work provides significant insights on electron-modulation and d-band center optimization for advanced electrocatalysts.

Improved activity for the oxygen evolution reaction using a tiara-like thiolate-protected nickel nanocluster

Practical electrochemical water splitting and carbon-dioxide reduction are desirable for a sustainable energy society. In particular, facilitating the oxygen evolution reaction (OER, the reaction at the anode) will increase the efficiency of these reactions. Nickel (Ni) compounds are excellent OER catalysts under basic conditions, and atomically precise Ni clusters have been actively studied to understand their complex reaction mechanisms. In this study, we evaluated the geometric/electronic structure of tiara-like metal nanoclusters [Ni_n(PET)_2n; n = 4, 5, 6, where PET refers to phenylethanethiolate] with the same SR ligand. The geometric structure of Ni₅(SR)₁₀ was determined for the first time using single-crystal X-ray diffraction. Additionally, combined electrochemical measurements and X-ray absorption fine structure measurements revealed that Ni₅(SR)₁₀ easily forms an OER intermediate and therefore exhibits a high specific activity.

In Situ Activation Endows Orthorhombic Fluorite-Type Samarium Iridium Oxide with Enhanced Acidic Water Oxidation

Developing electrochemical catalysts for acidic water oxidation with improved activity and stability has been the key to the further popularization of proton exchange membrane electrolyzers. In this work, an orthorhombic fluorite-type samarium iridium oxide (Sm3IrO7) catalyst is synthesized by a simple solid-state reaction. After in situ activation, the as-prepared Sm3IrO7 exhibits higher mass activity and durability than that of commercial IrO2. The in-depth analyses indicate the formation of amorphous IrOx species on the surface to evolve to a new heterostructure IrOx/Sm3IrO7, along with Sm leaching during the in situ activation process. More importantly, strong electronic interactions exist between newborn IrOx species and remaining Sm3IrO7, leading to the compressed Ir-O bonds in IrOx compared to commercial IrO2, thus reducing the energy barrier for oxygen evolution reaction (OER) intermediates to improve the OER process. Based on the above-mentioned analyses, it is speculated that the actual active species for enhanced acidic water oxidation should be IrOx/Sm3IrO7, rather than Sm3IrO7 itself. Theoretical calculations confirm that the optimal energy level path of IrOx/Sm3IrO7 follows the lattice oxygen mechanism, and the energy level of surface Ir 5d orbitals is lower than O 2p orbitals in IrOx/Sm3IrO7, enabling it a superior OER activity.

Toward Electrocatalytic Methanol Oxidation Reaction: Longstanding Debates and Emerging Catalysts

The study of direct methanol fuel cells (DMFCs) has lasted around 70 years, since the first investigation in the early 1950s. Though enormous effort has been devoted in this field, it is still far from commercialization. The methanol oxidation reaction (MOR), as a semi-reaction of DMFCs, is the bottleneck reaction that restricts the overall performance of DMFCs. To date, there has been intense debate on the complex six-electron reaction, but barely any reviews have systematically discussed this topic. To this end, the controversies and progress regarding the electrocatalytic mechanisms, performance evaluations as well as the design science toward MOR electrocatalysts are summarized. This review also provides a comprehensive introduction on the recent development of emerging MOR electrocatalysts with a focus on the innovation of the alloy, core-shell structure, heterostructure, and single-atom catalysts. Finally, perspectives on the future outlook toward study of the mechanisms and design of electrocatalysts are provided.

Ultrafast Combustion Synthesis of Robust and Efficient Electrocatalysts for High-Current-Density Water Oxidation

The scalable production of inexpensive, efficient, and robust catalysts for oxygen evolution reaction (OER) that can deliver high current densities at low potentials is critical for the industrial implementation of water splitting technology. Herein, a series of metal oxides coupled with Fe2O3 are in situ grown on iron foam massively via an ultrafast combustion approach for a few seconds. Benefiting from the three-dimensional nanosheet array framework and the heterojunction structure, the self-supporting electrodes with abundant active centers can regulate mass transport and electronic structure for prompting OER activity at high current density. The optimized Ni(OH)2/Fe2O3 with robust structure can deliver a high current density of 1000 mA cm-2 at the overpotential as low as 271 mV in 1.0 M KOH for up to 1500 h. Theoretical calculation demonstrates that the strong electronic modulation plays a crucial part in the hybrid by optimizing the adsorption energy of the intermediate, thereby enhancing the efficiency of oxygen evolution. This work proposes a method to construct cheap and robust catalysts for practical application in energy conversion and storage.

Ultrafine oxygenophilic nanoalloys induced by multifunctional interstitial boron for methanol oxidation reaction

Interface construction is one of the most feasible approaches to optimize the physical and chemical properties of noble metal-based catalysts and consequently improve their catalytic performance. Herein, the design of effective reaction interfaces by bimetallic, trimetallic or polymetallic alloying has been extensively explored. In this research, metalloid boron (B) was alloyed within palladium-iridium (Pd-Ir) nanoalloy supported on nitrogen-doped graphene (NG) to promote the methanol oxidation reaction (MOR) in alkaline media. Being benefited from this, the optimum Pd7IrBx/NG catalyst exhibited enhanced EOR activity mass activity (1141.7 mA mg-1) and long-term stability (58.2 % current density retention rate after 500 cycles of cyclic voltammetry). The mechanism was further studied by electrochemical experiments and characterization, which highlighted that the multifunctional effect of electronic effect and strain effect and kinetic optimization induced by boron doping played a very positive role on MOR.

Iridium single atoms incorporated in Co3O4 efficiently catalyze the oxygen evolution in acidic conditions

Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co3O4), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir>4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co3O4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications.

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.

Nanoscale hetero-interfaces for electrocatalytic and photocatalytic water splitting

As green and sustainable methods to produce hydrogen energy, photocatalytic and electrochemical water splitting have been widely studied. In order to find efficient photocatalysts and electrocatalysts, materials with various composition, size, and surface/interface are investigated. In recent years, constructing suitable nanoscale hetero-interfaces can not only overcome the disadvantages of the single-phase material, but also possibly provide new functionalities. In this review, we systematically introduce the fundamental understanding and experimental progress in nanoscale hetero-interface engineering to design and fabricate photocatalytic and electrocatalytic materials for water splitting. The basic principles of photo-/electro-catalytic water splitting and the fundamentals of nanoscale hetero-interfaces are briefly introduced. The intrinsic behaviors of nanoscale hetero-interfaces on electrocatalysts and photocatalysts are summarized, which are the electronic structure modulation, space charge separation, charge/electron/mass transfer, support effect, defect effect, and synergistic effect. By highlighting the main characteristics of hetero-interfaces, the main roles of hetero-interfaces for electrocatalytic and photocatalytic water splitting are discussed, including excellent electronic structure, efficient charge separation, lower reaction energy barriers, faster charge/electron/mass transfer, more active sites, higher conductivity, and higher stability on hetero-interfaces. Following above analysis, the developments of electrocatalysts and photocatalysts with hetero-structures are systematically reviewed.

Ce-induced regulation of electron density enhanced the catalytic activity of Co-Mn oxides for water oxidation

The incorporation of Ce into Co-Mn oxides induced the charge redistribution of Co and Mn via electronic coupling, which facilitated the Co³⁺/Co⁴⁺ transition. Thus, the overpotential at 10 mA cm^-2 was reduced significantly by 73 mV for the oxygen evolution reaction after Ce doping.

Hierarchical NiMn/NiMn-LDH/ppy-C induced by a novel phase-transformation activation process for long-life supercapacitor

For micron-sized nickel-based hydroxides sheets, the reaction and migration of anions/water molecules in the inner region tends to lag behind those along the edge, which can cause structure mismatch and capacity degradation during cycles. Nanosizing and structure design is a feasible solution to shorten the ion/electron path and improve the reaction homogeneity. Herein, this study reports a novel three-stage strategy (self-assembly of NiMn-LDH/ppy-C - reduction to NiMn/ppy-C - in situ phase transformation into NiMn/NiMn-LDH/ppy-C) to reduce the sheet size of NiMn-LDH to nanometer. Triggered by electrochemical activation, NiMn-LDH nanosheets can hereby easily and orderly grow on the exposed active (111) crystal plane of Ni to establish NiMn-LDH/NiMn heterostructure around ppy-C. Importantly, nanosizing and hierarchical structure play a synergistic role to maintain structural integrity and to promote the electron/mass transfer kinetics. The NiMn/NiMn-LDH/ppy-C composite delivers superior cycling stability with almost no decay of capacity retention after 40,000 cycles at 5 A g^-1. Our hierarchical morphology modulation provides an ingenious, efficient way to boost the performance of Ni-based layered hydroxide materials.

Synergizing Hydrogen Spillover and Deprotonation by the Internal Polarization Field in a MoS2 /NiPS3 Vertical Heterostructure for Boosted Water Electrolysis

Hydrogen spillover (HSo) has emerged to upgrade the hydrogen evolution reaction (HER) activity of Pt-support electrocatalysts, but it is not applicable to the deprotonated oxygen evolution reaction (OER). Non-precious catalysts that can perform well in both HSo and deprotonation (DeP) are extremely desirable for a sustainable hydrogen economy. Herein, an affordable MoS₂ /NiPS₃ vertical heterostructure catalyst is presented to synergize HSo and DeP for efficient water electrolysis. The internal polarization field (IPF) is clarified as the driving force of HSo in HER electrocatalysis. The HSo from the MoS₂ edge to NiPS₃ can activate the NiPS₃ basal plane to boost the HER activity of the MoS₂ /NiPS₃ heterostructure (112 mV vs reversible hydrogen electrode (RHE) at 10 mA cm^-2 ), while for OER, the IPF in the heterostructure can facilitate the hydroxyl diffusion and render MoS₂ -to-NiPS₃ /P-to-S dual-pathways for DeP. As a result, the stacking of OER-inactive MoS₂ on the NiPS₃ surface still brings intriguing OER enhancements. With them serving as electrode couples, the overall water splitting is attested stably with a cell voltage of 1.64 V at 10 mA cm^-2 . This research puts forward the IPF as the criterion in the rational design of HSo/DeP-unified non-precious catalysts for efficient water electrolysis.

Ru-Co Pair Sites Catalyst Boosts the Energetics for the Oxygen Evolution Reaction

Manipulating the coordination environment of the active center via anion modulation to reveal tailored activity and selectivity has been widely achieved, especially for carbon-based single-atom site catalysts (SACs). However, tuning ligand fields of the active center by single-site metal cation regulation and identifying the effects on the resulting electronic configuration is seldom explored. Herein, we propose a single-site Ru cation coordination strategy to engineer the electronic properties by constructing a Ru/LiCoO₂ SAC with atomically dispersed Ru-Co pair sites. Benefitting from the strong electronic coupling between Ru and Co sites, the catalyst possesses an enhanced electrical conductivity and achieves near-optimal oxygen adsorption energies. Therefore, the optimized catalyst delivers superior oxygen evolution reaction (OER) activity with low overpotential, the high mass activity of 1000 A g_oxide ^-1 at a small overpotential of 335 mV, and excellent long-term stability. It also exhibits rapid kinetics with superior rate capability and outstanding durability in a zinc-air battery.

N-doped carbon anchored CoS2/MoS2 nanosheets as efficient electrocatalysts for overall water splitting

The oriented two-dimensional porous nitrogen-doped carbon embedded with CoS₂ and MoS₂ nanosheets is a highly efficient bifunctional electrocatalyst. The hierarchical structure ensures fast mass transfer capacity in improving the electrocatalytic activity. And the greatly increased specific surface area is beneficial to expose more electrocatalytically active atoms. For oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) tests in 1 mol/L KOH solution, only 194 and 140 mV overpotential are required to achieve a current density of 10 mA/cm², respectively. Our research provides an effective strategy for synergizing the individual components in nanostructures for a wide range of electrocatalytic reactions.

Anchoring Ni/NiO heterojunction on freestanding carbon nanofibers for efficient electrochemical water oxidation

Rational design of low-cost and efficient electrocatalyst for the anodic oxygen evolution reaction (OER) to replace noble-metal-based catalysts is greatly desired for the large-scale application of water electrocatalysis. And compared with the conventional powdery catalysts, the freestanding electrode architecture is more attractive owing to the enhanced kinetics and stability. In this work, we report an electrospinning-carbonization-post oxidation strategy to develop the freestanding N-doped carbon nanofibers anchored with Ni/NiO nanoparticles (denoted as Ni/NiO-NCNFs) as efficient OER electrocatalyst. In the synthesized Ni/NiO-NCNFs, the conductive ultrathin carbon layer could promote electron transfer and thus improve the electrocatalytic activity. Meanwhile, the ratio between Ni and NiO could be regulated by tuning the oxidation duration, so as to optimize the adsorption energy of intermediates and improve the OER activity. The Ni/NiO-NCNFs prepared with the oxidation time of 3 h exhibit a promising OER activity and long-term operation durability in 0.1 M KOH solution, requiring an overpotential as small as 153 mV to achieve a current density of 10 mA cm^-2. Its overpotential is far lower than that of the reported OER catalysts. This work offers an efficient pathway to develop low-cost and highly active freestanding transitional metal-based OER electrocatalyst for potential renewable electrochemical energy conversion.

High-content atomically distributed W(V,VI) on FeCo layered double hydroxide with high oxygen evolution reaction activity

High-content atomically distributed W(V,VI) coordinated with O atoms as WO₂ moieties anchored on FeCo layered double hydroxide (FeCo LDH) nanosheets in the structure of SAC W-FeCo LDH is obtained by a facile coprecipitation method, and it presented clearly enhanced stable OER electrocatalytic activity.

Nanoengineering of Catalysts for Enhanced Hydrogen Production

Hydrogen (H2) has emerged as a sustainable energy carrier capable of replacing/complementing the global carbon-based energy matrix. Although studies in this area have often focused on the fundamental understanding of catalytic processes and the demonstration of their activities towards different strategies, much effort is still needed to develop high-performance technologies and advanced materials to accomplish widespread utilization. The main goal of this review is to discuss the recent contributions in the H2 production field by employing nanomaterials with well-defined and controllable physicochemical features. Nanoengineering approaches at the sub-nano or atomic scale are especially interesting, as they allow us to unravel how activity varies as a function of these parameters (shape, size, composition, structure, electronic, and support interaction) and obtain insights into structure–performance relationships in the field of H2 production, allowing not only the optimization of performances but also enabling the rational design of nanocatalysts with desired activities and selectivity for H2 production. Herein, we start with a brief description of preparing such materials, emphasizing the importance of accomplishing the physicochemical control of nanostructures. The review finally culminates in the leading technologies for H2 production, identifying the promising applications of controlled nanomaterials.

Confining High-Valence Iridium Single Sites onto Nickel Oxyhydroxide for Robust Oxygen Evolution

Enhancing activity and stability of iridium- (Ir-) based oxygen evolution reaction (OER) catalysts is of great significance in practice. Here, we report a vacancy-rich nickel hydroxide stabilized Ir single-atom catalyst (Ir₁-Ni(OH)₂), which achieves long-term OER stability over 260 h and much higher mass activity than commercial IrO₂ in alkaline media. In situ X-ray absorption spectroscopy analysis certifies the obvious structure reconstruction of catalyst in OER. As a result, an active structure in which high-valence and peripheral oxygen ligands-rich Ir sites are confined onto the nickel oxyhydroxide surface is formed. In addition, the precise introduction of atomized Ir not only surmounts the large-range dissolution and agglomeration of Ir but also suppresses the dissolution of substrate in OER. Theoretical calculations further account for the activation of Ir single atoms and the promotion of oxygen generation by high-valence Ir, and they reveal that the deprotonation process of adsorbed OH is rate-determining.

Monolayer NiIr-Layered Double Hydroxide as a Long-Lived Efficient Oxygen Evolution Catalyst for Seawater Splitting

Promoting the oxygen evolution reaction (OER) with saline water is highly desired to realize seawater splitting. This requires OER catalysts to resist serious corrosion and undesirable chloride oxidation. We introduce a 5d transition metal, Ir, to develop a monolayer NiIr-layered double hydroxide (NiIr-LDH) as the catalyst with enhanced OER performance for seawater splitting. The NiIr-LDH catalyst delivers 500 mA/cm² at only 361 mV overpotential with ∼99% O₂ Faradaic efficiency in alkaline seawater, which is more active than commercial IrO₂ (763 mV, 23%) and the best known OER catalyst NiFe-LDH (530 mV, 92%). Moreover, it shows negligible activity loss at up to 650 h chronopotentiometry measurements at an industrial level (500 mA/cm²), while commercial IrO₂ and NiFe-LDH rapidly deactivated within 0.2 and 10 h, respectively. The incorporation of Ir into the Ni(OH)₂ layer greatly altered the electron density of Ir and Ni sites, which was revealed by X-ray absorption fine structure and density functional theory (DFT) calculations. Coupling the electrochemical measurements and in situ Raman spectrum with DFT calculations, we further confirm that the generation of rate-limiting intermediate *O and *OOH species was accelerated on Ni and Ir sites, respectively, which is responsible for the high seawater splitting performance. Our results also provide an opportunity to fabricate LDH materials containing 5d metals for applications beyond seawater splitting.