Activating lattice oxygen in NiFe-based (oxy)hydroxide for water electrolysis

Room-temperature synthesis of NiFe-hexamethylenetetramine as lattice oxygen involved electrocatalyst for efficient oxygen evolution reaction

The design and synthesis of an oxygen evolution reaction (OER) electrocatalyst following lattice oxygen mechanism (LOM) through a straightforward strategy is crucial for achieving efficient electrocatalytic hydrogen production; however, it remains a formidable challenge. Herein, a novel and highly efficient LOM-based OER electrocatalyst, NiFe-hexamethylenetetramine (NiFe-HMT) coordination compound, is fabricated through a straightforward co-precipitation strategy at room temperature within 30 min. The obtained NiFe-HMT exhibits remarkable OER activity with low overpotentials of 269 and 352 mV to achieve 100 and 1000 mA cm-2, respectively. Experimental results and theoretical calculations reveal that the incorporation of Fe can effectively activate the lattice oxygen in the reconstructed oxyhydroxides, thereby shifting the OER pathway from adsorbate evolution mechanism to LOM. Additionally, compared with NiFe-LDH, NiFe-HMT is more favorable for forming highly active oxyhydroxides and exhibits more significant lattice oxygen activity. Furthermore, NiFe-HMT can be scaled up to more than 10 g in a single batch and stored stability for over 142 days without any significant decline in activity, thereby indicating its potential for large-scale implementation. This study provides valuable insights into developing high-performance OER electrocatalysts following the LOM pathway.

Direct recovery of high-purity uranium from fluoride-containing nuclear wastewater via extraction materials with ensemble Lewis sites and a tandem electrochemical device

Electrochemical uranium extraction from real nuclear wastewater with a high concentration of fluoride ions (F-) represents a promising strategy for the efficient treatment of radioactive wastewater and the recovery of the valuable uranium resource. However, the current progress suffers from the interference of extremely high concentration of F- and undesired purity of the final uranium product. Herein, we constructed the neighboring ensemble Lewis acid-base pair sites (ensemble Lewis sites) in bismuth oxides as the extraction material, which was integrated into a designed tandem electrochemical device for efficient recovery of high-purity uranium from real nuclear wastewater. The mechanistic study revealed that the ensemble Lewis sites dramatically enhance the binding of all dominant uranyl fluoride (UO2Fx) species through the simultaneous strengthened chemical bonds with U, O, and F atoms. Besides, the tandem electrochemical device rationally controlled the reaction period to U3O8, avoiding the undesired crystalline transformation to K2U2O7. Through 3 h electrolysis in 1 L of real nuclear wastewater, the extraction efficiency of uranium reached 99.9 % with only 2.2 % impurities of alkali metals in extracted uranium product, outperforming the previous work. This study offers an effective method for the recovery of uranium resources in real nuclear wastewater.

Solid-State Proton Donors in Hydroxyl-Rich Ni3Ge2O5(OH)4 for Solar-Driven Hydrogen Evolution

The hydrogen evolution reaction (HER) through water splitting is one of the most promising solutions to global energy and environmental challenges. In this study, using hydroxyl-rich Ni3Ge2O5(OH)4 as a photocatalyst, a novel, kinetically self-activated solar-driven gas-solid system for H2 production was demonstrated, where solid-state lattice hydroxyls (Ni-OH) serve as proton donors due to their low reaction barrier. The sustainable regeneration of Ni-OH is achieved through gaseous H2O molecule dissociation at the resultant oxygen vacancies (OV), which plays a critical role in enhancing proton activity. Under 5 h of testing, the system achieved a cocatalyst-free H2 yield of 311.8 μmol g-1 at the gas-solid interface, approximately 35 times higher than that of the triphasic system, and even 3.4 and 2.1 times higher than H2O splitting systems using triethanolamine or methanol as sacrificial agents, respectively. This work provides valuable insights into the design of advanced photocatalysts and the development of sustainable H2 production systems.

Nickel-iron alloy films electrodeposited from metal salt-l-serine deep eutectic solvent for alkaline oxygen evolution reaction

Active and stable electrocatalysts for oxygen evolution reaction (OER) is crutial in the widespread application of hydrogen production from water electrolyzers. Here, deep eutectic solvents formed from metal chlorides and l-serine are utilized as the electrolyte to deposit metal films (Ni, Fe, and Ni-Fe alloys) under potentiostatic conditions onto nickel foam (NF) substrate. The deposited films are characterized, and the Ni-Fe alloy films constituting of nanometer-sized crystalline regions. Electron interaction between Ni and Fe is observed. In 1 M KOH, Ni-Fe/NF requires only 219 mV overpotential to achieve a current density of 10 mA cm-2, and can stably catalyze OER in alkaline solutions. Compared to the Ni/NF and Fe/NF, the high OER activity of Ni-Fe/NF is not originated from improved number of active sites, but from the enhanced intrinsic activity, evidenced by the improved OER kinetics caused by the electron interaction. The acidity of the Ni site increases due to the introduction of Fe, and lattice oxygen mechanism is involved in the OER process on Ni-Fe/NF. After long-term OER, metal oxides and (oxy)hydroxides are formed at the surface.

Oxide Heterostructure Engineering Drives Stable Lattice Oxygen Evolution for Highly Efficient and Robust Water Electrolysis

Achieving a highly active and stable oxygen evolution reaction (OER) is critical for the implementation of water electrolysis in green hydrogen production but remains challenging. Steering the OER pathway from an adsorbate evolution mechanism (AEM), where a metal site serves as the active site, to the lattice oxygen mechanism (LOM) has been found to enhance OER activity; however, it suffers from low stability. In this work, we propose to construct CuOx/Co3O4 heterointerface, which enables the realization of a stable LOM pathway. The lattice oxygen characteristics are modulated near the heterointerface, resulting in a shift in the reaction pathway from AEM to LOM. In situ X-ray Absorption Fine Structure results further reveal that the valence state of cobalt is stabilized during the OER process, which alleviates corrosion of cobalt and maintains LOM stability. Consequently, the obtained CuOx/Co3O4 exhibits outstanding activity and stability for overall water electrolysis in freshwater, natural seawater, and high-salt wastewater, with a low overpotential of 308 mV at 100 mA cm-2 and stable overall water electrolysis at 500 mA cm-2 for 100 h. Our work demonstrates interface engineering as an effective strategy to activate and stabilize lattice oxygen, advancing the design of high-performance electrocatalysts for energy and environmental applications.

Hierarchical Fe-based electrocatalyst for lattice oxygen mediated water oxidation with Industrial-Level activity

Rational design transition metal-based electrocatalysts for oxygen evolution reaction (OER) at large current densities is vital for industrial applications of alkaline water electrolysis. Here, we present a three-dimensional catalyst comprises of Fe2O3 nanoparticles that are highly dispersed on FeMoO4 nanorods, supported on Ni foam, featuring a hierarchical heterostructure and array morphology. The resulting Fe2O3/FeMoO4/NF electrodes exhibit remarkable OER catalytic activity, achieving overpotentials of 315 mV and 352 mV at current densities of 1000 mA cm-2 and 2000 mA cm-2, respectively, while maintaining outstanding long-term durability (>900 h) at a current density of 500 mA cm-2. In-situ Fourier-transform infrared (FTIR) spectroscopy and theoretical studies demonstrate the direct OO radical coupling for lattice-oxygen-mediated mechanism (LOM)-dominated O2 evolution, thereby breaking the scaling relationship limitation and accelerating reaction kinetics. Operando electrochemical impedance spectroscopy implies fast charge transport. The superior OER performance can be attributed to the abundant heterointerfaces between the active phases in hierarchical structure and the enhanced intrinsic activity through the LOM mechanism. This work paves an avenue for constructing advanced electrocatalysts with industrial-level activity and offering a promising approach for practical applications in alkaline water electrolysis.

Hollow carbon nanoreactors integrating NiFe-LDH nanodots with adjacent La single atoms for efficient oxygen electrocatalytic reactions

Optimizing both mass transport and electronic structure of the active component is of interest to obtain electrocatalysts with superior oxygen evolution reaction (OER) performance. Here, we miniaturized the classical NiFe-layered double hydroxides (NiFe-LDHs) and integrated them into S/N co-doped hollow hierarchical porous carbon (SNHPC) loaded with rare earth La single atoms (La SAs) to obtain nanoreactors. The unique carbon framework induced uniform deposition of LDH nanodots and ensured adequate exposure during electrocatalysis. The advantages of the carbon carrier for the local electric field and interfacial OH- layer density in the catalytic process were confirmed by finite element simulations. The well-designed NiFe-LDH@La SNHPC exhibited satisfactory activity (overpotential of 251 mV at 10 mA cm-2) and stability in alkaline media, exceeding those of commercial RuO2. Impressively, a cathode catalyst combining NiFe-LDH@La SNHPC with Pt/C can be stabilized in rechargeable zinc-air batteries (ZABs) for more than 350 h. Theoretical calculations indicated that the introduction of La SAs modified the electronic structures of the NiFe-LDH nanodots, activated lattice oxygen activity, optimized the adsorption strength of the intermediates, and reduced rate-determining step energy barriers in OER. This study provides guidance for the preparation and design of sub-microreactors and information on the strong electron interaction effects induced by rare earth species.

Activating Surface Oxygen in Ce/Mo-Doped Ni Oxyhydroxide for Synergistically Enhancing Furfural Oxidation and Hydrogen Evolution at Ampere-Level Current Densities

The integration of biomass-platform molecule oxidation with water electrolysis is a promising strategy to reduce energy consumption in hydrogen production and obtain high-value chemicals simultaneously, yet the efficiency of organic oxidation requires further improvement. Herein, we developed a highly efficient Ce, Mo co-doped Ni-based (oxy)hydroxide catalyst, where Mo with high spin state promotes the adsorption of furfural (FA), while Ce activates surface lattice oxygen (OL), lowering the energy barrier for OL─OH coupling to form OOH, the key intermediate for high current densities. The catalyst achieves an industrial-grade current density of 1000 mA cm- 2 at a remarkably low potential of 1.46 V versus RHE in furfural oxidation, with exceptional selectivity (99.4%) and Faradaic efficiency (97.7%) for furoic acid. When deployed as anode in an anion-exchange membrane reactor, the NiMoCe/NF catalyst sustains a current density of 500 and 1000 mA cm- 2 at a cell voltage of only 1.85 and 2.15 V, respectively, surpassing most reported continuous flow electrolyzers limited to 200 mA cm- 2. Moreover, the system exhibits outstanding durability after 200 h of continuous operation. This work provides critical insights into the rational design of catalysts for energy-efficient biomass valorization coupled with industrial hydrogen production.

Rapid synthesis of metastable materials for electrocatalysis

Metastable materials are considered promising electrocatalysts for clean energy conversions by virtue of their structural flexibility and tunable electronic properties. However, the exploration and synthesis of metastable electrocatalysts via traditional equilibrium methods face challenges because of the requirements of high energy and precise structural control. In this regard, the rapid synthesis method (RSM), with high energy efficiency and ultra-fast heating/cooling rates, enables the production of metastable materials under non-equilibrium conditions. However, the relationship between RSM and the properties of metastable electrocatalysts remains largely unexplored. In this review, we systematically examine the unique benefits of various RSM techniques and the mechanisms governing the formation of metastable materials. Based on these insights, we establish a framework, linking RSM with the electrocatalytic performance of metastable materials. Finally, we outline the future directions of this emerging field and highlight the importance of high-throughput approaches for the autonomous screening and synthesis of optimal electrocatalysts. This review aims to provide an in-depth understanding of metastable electrocatalysts, opening up new avenues for both fundamental research and practical applications in electrocatalysis.

Isotope Techniques in Chemical Wastewater Treatment: Opportunities and Uncertainties

A comprehensive and in-depth analysis of reaction mechanisms is essential for advancing chemical water treatment technologies. However, due to the limitations of conventional experimental and analytical methods, the types of reactive species and their generation pathways are commonly debatable in many aqueous systems. As highly sensitive diagnostic tools, isotope techniques offer deeper insights with minimal interference from reaction conditions. Nevertheless, precise interpretations of isotope results remain a significant challenge. Herein, we first scrutinized the fundamentals of isotope chemistry and highlighted key changes induced by the isotope substitution. Next, we discussed the application of isotope techniques in kinetic isotope effects, presenting a roadmap for interpreting KIE in sophisticated systems. Furthermore, we summarized the applications of isotope techniques in elemental tracing to pinpoint reaction sites and identify dominant reactive species. Lastly, we propose future research directions, highlighting critical considerations for the rational design and interpretation of isotope experiments in environmental chemistry and related fields.

Balancing Hydrogen Evolution and Hydrogenation Reaction via Facet Engineering for Efficient Conversion of Nitrate to Ammonia in Actual Wastewater

Due to the competitive relationship between nitrate reduction reaction (NO3 -RR) and hydrogen evolution reaction (HER), the conventional approach to improve Faradaic efficiency is to select a catalyst without HER activity. Nevertheless, such a strategy not only limits the application of HER catalysts in NO3 -RR, but also causes insufficient hydrogen source, thereby sacrificing ammonia yield rate. We believe that HER catalysts should not be excluded from hydrogenation reduction. Herein, taking traditional water electrolysis material Co3O4 as model system, we reveal that the oxygen vacancies on crystal facet can greatly promote water dissociation and capture HER intermediate for NO3 -RR, successfully shifting the reaction pathway from hydrogen evolution to nitrate hydrogenation. Beyond material development, we construct a hybrid reactor and achieve an ammonia recovery rate of 1216.8 g-N m-2 d-1 in nuclear industry wastewater with ultra-high nitrate concentration. This study breaks through the limitation of HER catalyst in NO3 -RR, which provides a significant insight into the catalyst designing and hydrogenation mechanism.

Bicarbonate boosts the anaerobic generation of reactive oxygen species by hydrochar: Surface oxygenated functional groups activation and hexavalent chromium removal

Hydrochar can generate reactive oxygen species (ROS) via activating molecular oxygen under sunlight. However, whether and how hydrochar generates ROS in the dark and anaerobic environments remains unknown. Herein, we found that combining hydrochar with the co-existing bicarbonate could generate superoxide radicals (O2•-) under alkaline conditions, with the maximum concentration reaching 24.7 μmol L-1. Interestingly, the O2•-generation was not affected by eliminating the molecular oxygen but highly depended on the abundance of the oxygenated functional groups (OFGs) on hydrochar surface. The results of electrochemical analysis, density functional theory calculation, and surface characterization elucidated that the bicarbonate first inner-sphere complexed with surface OFGs, followed by the electron transfer from bicarbonate to OFGs. This enabled the activation of oxygen inside OFGs into active oxygen (O*) while bicarbonate was oxidized to carbonate radical (CO3•-). The CO3•- further reacts with O* through an oxygen transfer mechanism, resulting in the generation of O2•-. The generated O2•- was used for the Cr(VI) treatment, which could efficiently reduce over 95 % into Cr(III). The findings provide a novel pathway for developing hydrochar-based advanced oxidation processes and highlight the potential of hydrochar in pollutant transformation mediated by ROS in dark and anaerobic water environments.

Fe-Doped Ni-Phytate/Carbon Nanotube Hybrids Integrating Activated Lattice Oxygen Participation and Enhanced Photothermal Effect for Highly Efficient Oxygen Evolution Reaction Electrocatalyst

Developing highly efficient oxygen evolution reaction (OER) electrocatalysts is critical for hydrogen production through electrocatalytic water splitting, yet it remains a significant challenge. In this study, a novel OER electrocatalyst, Fe-doped Ni-phytate supported on carbon nanotubes (NiFe-phy/CNT), which simultaneously follows lattice oxygen mechanism (LOM) and exhibits a photothermal effect, is synthesized through a facile and scalable co-precipitation method. Experimental results combined with theoretical calculations indicate that introducing Fe can facilitate the structural reconstruction of NiFe-phy/CNT to form highly active NiFe oxyhydroxides, switch OER pathway to LOM from the adsorbate evolution mechanism, and reinforce the photothermal effect to counterbalance the enthalpy change during OER process while reducing its activation energy. Therefore, under near-infrared light irradiation, NiFe-phy/CNT demonstrates exceptional OER activity, featuring low overpotentials of 237, 275, and 286 mV at 100, 500, and 1000 mA cm-2, respectively. Moreover, this electrocatalyst demonstrates the capability of large-scale synthesis and can be stored for over 120 days with a negligible decrease in activity. This work presents a novel conceptual approach to integrate lattice oxygen redox chemistry with photothermal effect for designing highly efficient OER electrocatalysts.

Nanoscale Grain Boundary-Weakened Ce-O Covalency and Surface Confinement Intrinsically Boosting Ceria Surface Oxygen Reactivity

Promoting the reactivity of surface lattice oxygen atoms of oxide nanomaterials is critical for enhancing their catalytic performances in oxidation, hydrogenation, and electrocatalytic reactions; however, the fundamental electronic mechanisms governing this surface reactivity have long remained insufficiently understood. Here, we reveal the electronic mechanism of how the nanoscale grain boundary (GB) boosts the intrinsic surface reactivity of CeO2 nanomaterials, in which GBs are introduced by pyrolyzing the precursors of cerium carbonate and formate. The results of X-ray absorption near-edge structures (XANES) at the O K- and Ce L3-edges reveal that GBs can reduce the degree of covalency of Ce-O bonds, while H2-TPR and Raman spectra show that this decreased orbital overlap can further weaken the confinement strength of surface oxygen atoms by the lattice potential. This electronic effect can fundamentally boost the leaving activity of surface lattice oxygen atoms, which further promotes the formation of oxygen vacancies and the activation of the O2 molecules to oxidize benzyl alcohol into benzaldehyde with 100% selectivity. This structure-function relationship based on reduction in lattice covalency provides a new electronic perspective to understand how GBs and size reduction enhance nanomaterial surface reactivity.

High Performance Capacitive Deionization Cathode of Nickel Hexacyanoferrate Doped with Trace Molybdenum: Breaking the Capacity-Stability Trade-Off

Nickel hexacyanoferrate (NiHCF) is considered one of the most promising electrode materials for capacitive deionization (CDI) because of its excellent cycling stability and other advantages. However, the poor desalination ability and low adsorption capacity caused by a single reaction site seriously restrict its practical application. Here, a high-valence transition metal element Mo-doped strategy is proposed to utilize traces of Mo6+ to activate the inert-sites within NiHCF. The Mo-doped NiHCF electrodes (NiMox-HCF) with multiple pairs of redox active sites and long cycling capability were designed, breaking the trade-off between high adsorption capacity and cycling stability. The results show that the preferred NiMo0.3-HCF not only retains the excellent cycling stability (91% after 200 cycles), but also possesses a high adsorption capacity of 90.92 mg g-1 at 1.2 V, which is much better than those of NiHCF under the same conditions. In addition, it has good practical application in simulated brackish water and circulating cooling water. Structural characterization and theoretical calculations indicate that the Mo doping strategy enhanced electronic conductivity while maintaining structural stability. This research proposes a new method to activate inert sites using high-valence elements, thus effectively balancing the adsorption capacity and cycling stability of the electrode.

Enhancing catalytic durability in alkaline oxygen evolution reaction through squaric acid anion intercalation

The corrosive acidic interfacial microenvironment caused by rapid multi-step deprotonation of alkaline oxygen evolution reaction in industrial high current water electrolysis is one of the key problems limiting its stability. Some functional anions derived from electrocatalysis exhibit special functionalities in modulating the interface microenvironment, but this matter has not received adequate attention in academic discussions. Here we show that the coordinate squaric acid undergoes a dissolve-re-intercalation process in alkaline oxygen evolution, leading to its stabilization within the Fe-doped NiOOH interlayer in the form of the squaric acid anions (NiFe-SQ/NF-R). These intercalated squaric acid anions stabilizes OH- through multiple hydrogen bond interactions, which is conducive to maintaining high catalytic interface alkalinity. Hence, the interfacial acidification of prepared NiFe-SQ/NF-R is inhibited, resulting in a tenfold prolong in its catalytic durability (from 65 to 700 h) when exposed to 3.0 A cm-2, as opposed to NiFe-LDH/NF-R. This derived functional anion guarantees the enduring performance of the NiFe-derived electrocatalyst under high current densities by controlling the interfacial alkalinity.

High-entropy alloy enables multi-path electron synergism and lattice oxygen activation for enhanced oxygen evolution activity

Electrocatalytic oxygen evolution reaction (OER) is key to several energy technologies but suffers from low activity. Leveraging the lattice oxygen activation mechanism (LOM) is a strategy for boosting its activity. However, this approach faces significant thermodynamic challenges, requiring high-valent oxidation of metal ions without compromising their stability. We reveal that high-entropy alloys (HEAs) can efficiently activate the LOM through synergistic multi-path electron transfer. Specifically, the oxidation of nickel is enhanced by this electron transfer, aided by the integration of weaker Co-O bonds, enabling effective LOM at the Ni-Co dual-site. These insights allow the design of a NiFeCoCrW0.2 HEA that exhibits improved activity, achieving an overpotential of 220 mV at a current density of 10 mA cm-2. It also demonstrates good stability, maintaining the potential with less than 5% variation over 90 days at 100 mA cm-2 current density. This study sheds light on the synergistic effects that confer high activity in HEAs and contribute to the advancement of high-performance OER electrocatalysts.

High-Throughput DFT-Assisted Design of Electrode for Efficient High-Temperature Electrochemical Dehydrogenation

Protonic ceramic electrolysis cell (PCEC) is a promising technique to enable efficient dehydrogenation reactions for producing valuable chemicals, but is still limited by the lack of stable electrocatalysts to achieve efficient O─H/C─H dissociation. In this work, upon high-throughput first-principles calculations, Ba(Zr,Co,Fe,M)O3-based (M represents dopants) perovskite is formulated, and oxygen vacancy formation energy (Δ E v f ${{\Delta}}E_{\mathrm{v}}^{\mathrm{f}}$ ) and hydration energy (ΔEhydr) are taken as two key performance indicators to screen potential PCEC electrode materials derived from this category. Trivalent doping elements, particularly Y, Yb, Er, and Tm, achieve a good balance betweenΔ E v f ${{\Delta}}E_{\mathrm{v}}^{\mathrm{f}}$ and ΔEhydr. Experiments further validate that the BaZr0.125Co0.375Fe0.375Tm0.125O3-δ showed impressive dehydrogenation reaction activity, with faradaic efficiency as high as 98.90% in water electrolysis, and outstanding ethane conversion rate (67.60%) and ethylene yield (62.62%) for ethane dehydrogenation reaction at 700 °C. The computational approach can be applied to the rational design of novel electrode materials for other electrochemical reactions in energy and environment devices.

Passivating Lattice Oxygen in ZnO Nanocrystals to Reduce its Interactions with the Key Intermediates for a Selective Photocatalytic Methane Oxidation to Methanol

An inevitable overoxidation process is considered as one of the most challenging problems in the direct conversion of methane (CH4) to methanol (CH3OH), which is limited by the uncontrollable cracking of key intermediates. Herein, we have successfully constructed a photocatalyst, the Fe-doped ZnO hollow polyhedron (Fe/ZnOHP), for the highly selective photoconversion of CH4 to CH3OH under mild conditions. In situ experiments and density functional theory calculations confirmed that the introduction of Fe was able to decrease the energy level of the O 2p orbital, which passivated the activity of lattice oxygen in ZnO nanocrystals. This passivation effect greatly weakened the interaction between *CH3 and lattice oxygen, thus facilitating the conversion of *CH3O to *CH3 intermediate rather than the direct desorption of *CH3O. As a result, Fe/ZnOHP exhibited excellent CH3OH generation rate (ca. 1009 μmol gcat -1 h-1) and selectivity (ca. 96 %) in the photocatalytic conversion of CH4 at room temperature and low pressure.

Enhancement of Selective Catalytic Oxidation of Lignin β-O-4 Bond via Orbital Modulation and Surface Lattice Reconstruction

The orbital modulation and surface lattice reconstruction represent an effective strategy to regulate the interaction between catalyst interface sites and intermediates, thereby enhancing catalytic activity and selectivity. In this study, the crystal surface of Au-K/CeO2 catalyst can undergo reversible transformation by tuning the coordination environment of Ce, which enables the activation of the Cβ-H bond and the oxidative cleavage of the Cβ-O and Cα-Cβ bonds, leading to the cleavage of 2-phenoxy-1-phenylethanol. The t2g orbitals of Au 5d hybridize with the 2p orbitals of lattice oxygen in CeO2 via π-coordination, modulating the coordination environment of Ce 4 f and reconstructing the lattice oxygen in the CeO2 framework, as well as increasing the oxygen vacancies. The interface sites formed by the synergy between Au clusters in the reconstructed Ce-OL1-Au structure and doped K play dual roles. On the one hand, it activates the Cβ-H bond, facilitating the enolization of the pre-oxidized 2-phenoxy-1-phenylethanone. On the other hand, through single-electron transfer involving Ce3+ 4f1 and the adsorption by oxygen vacancies, it enhances the oxidative cleavage of the Cβ-O and Cα-Cβ bonds. This study elucidates the complex mechanistic roles of the structure and properties of Au-K/CeO2 catalyst in the selective catalytic oxidation of lignin β-O-4 bond.

Elucidating the Chirality-Induced Spin Selectivity Effect of Co-Doped NiO Deposited on Ni Foam for Highly Stable Zn-Air Batteries

The urgent need to alleviate global warming and limit the consumption of fossil fuels has prompted the development of rechargeable Zn-air batteries (ZABs) considering their superior energy density, safety, and cost-effectiveness. However, the sluggish reaction kinetics of the oxygen evolution reaction (OER) and the unfavorable properties of conventional OER catalysts (including low electrical conductivity and the use of active site-blocking binders) hinder the development of practically viable ZABs. Herein, we report a distinct approach for directly synthesizing cobalt-doped nickel oxide (Co-NiO) with a chiral structure on porous Ni foam via a one-step hydrothermal process. The chirality-induced spin selectivity (CISS) boosts the OER kinetics, while Co doping elevates the electrical conductivity and the abundance of active sites on the catalyst. The chiral Co-NiO demonstrates an OER current density of 10 mA cm-2 at 1.58 V versus the reversible hydrogen electrode, outperforming both achiral Co-NiO and undoped NiO. Furthermore, a chiral Co-NiO-based rechargeable ZAB demonstrates a high open-circuit potential (1.57 V), a low charge/discharge overpotential (0.71 V), and excellent stability for 960 h (40 days) because the CISS effect mitigates the production of the corrosive singlet oxygen. These results represent a prominent pathway for the advancement of ZABs using the low-cost oxygen evolution catalyst modulated by the CISS effect and heteroatomic doping.

Tailoring Octahedron-Tetrahedron Synergism in Spinel Catalysts for Acidic Water Electrolysis

The instability issues of oxide-based electrocatalysts during the oxygen evolution reaction (OER) under acidic conditions, caused by the oxidation and dissolution of the catalysts along with the current-capacitance effect, constrain their application in proton exchange membrane water electrolysis (PEMWE). To address these challenges, we tailored the spinel structure of Co3O4 and exploited the synergism between the tetrahedron and octahedron sites by partially substituting Co with Ni and Ru (denoted as NiRuCoOx), respectively. Such a catalyst design creates a Ru-O-Ni electronic coupling effect, facilitating a direct dioxygen radical-coupled OER pathway. Density-functional theory (DFT) calculations and in situ Raman spectroscopy results confirm that Ru is the active site in the diatomic oxygen mechanism while Ni stabilizes lattice oxygen and the Ru-O bonding. The designed NiRuCoOx catalyst exhibits an exceptionally low overpotential of 166 mV to achieve a current density of 10 mA cm-2. Moreover, when serving as the anode in PEMWE, the NiRuCoOx requires 1.72 V to reach a current density of 3A cm-2, meeting the 2026 target set by the U.S. Department of Energy (DOE: 1.8 V@3A cm-2). The PEMWE device can operate stably for more than 1500 h with a significantly reduced performance decay rate of 0.025 mV h-1 compared to commercial RuO2 (2.13 mV h-1). This work provides an efficient method for tailoring the octahedron-tetrahedron sites of spinel Co3O4, which significantly improves the activity and stability of PEMWE.

Best practices for in-situ and operando techniques within electrocatalytic systems

In-situ and operando techniques in heterogeneous electrocatalysis are a powerful tool used to elucidate reaction mechanisms. Ultimately, they are key in determining concrete links between a catalyst's physical/electronic structure and its activity en route to designing next-generation systems. To this end, the exact execution and interpretation of these lines of experiments is critical as this determines the strength of conclusions that can be drawn and what uncertainties remain. Instead of focusing on how techniques were used to understand systems, as is the case with most reviews on the topic, this work instead initiates a nuanced discussion of 1) how to best carry out each technique and 2) initiate a nuanced analysis of which level of insights can be drawn from the set of in-situ or operando experiments/controls carried out. We focus on several commonly used techniques, including vibrational (IR, Raman) spectroscopy, X-ray absorption spectroscopy and electrochemical mass spectrometry. In addition to this, we include sections of reactor design and the link with theoretical modelling that are applicable across all techniques. While we focus on heterogeneous electrocatalysis, we make links when appropriate to the areas of photo- and thermo-catalytic systems. We highlight common pitfalls in the field, how to avoid them, and what sets of complementary experiments may be used to strengthen the analysis. We end with an overview of what gaps remain in in-situ and operando techniques and what innovations must be made to overcome them.

Blocking Effect Retards Electron Release from Asymmetric Active Units for Selective Seawater Oxidation

During seawater electrolysis, chloride ion (Cl-) adsorption at the anode leads to an inevitable competitive chloride oxidation reaction (ClOR) with the oxygen evolution reaction (OER), compromising the long-term stability of the electrolysis process. Furthermore, Ni-based OER electrocatalysts are challenged by activity degradation due to the overoxidation of Ni3+. In response, we present a design of oxygen-vacancy-regulated asymmetric Nb-O-Ni bonds aimed at selective seawater oxidation. The experimental and in situ characterization results indicate that the blocking effect of oxygen vacancies effectively alleviates the electron release of Ni3+ and the electron enrichment of Nb5+ on asymmetric Nb-O-Ni bonds, achieving a stable and selective OER in alkaline seawater. Density functional theory (DFT) calculations reveal that oxygen vacancies in Nb-O-Ni bonds optimize the adsorption strength of reaction intermediates and break up the scaling relationship between *OH and *OOH intermediates. The constructed anion exchange membrane electrolysis cell achieves a cost efficiency of $1.07 per GGE (gasoline gallon equivalent) for H2 production at a current density of 1000 mA cm-2, maintaining operational stability for 100 h at 500 mA cm-2.

Operando unveiling the activity origin via preferential structural evolution in Ni-Fe (oxy)phosphides for efficient oxygen evolution

Non-noble metal-based heteroatom compounds demonstrate excellent electrocatalytic activity for the oxygen evolution reaction (OER). However, the origin of this activity, driven by structure evolution effects, remains unclear due to the lack of effective in situ/operando techniques. Herein, we employ the operando quick-scan x-ray absorption fine structure (Q-XAFS) technique coupled with in situ controlled electrochemical potential to establish a structure-activity correlation of the OER catalyst. Using Ni-Fe bimetallic phosphides as a model catalyst, operando Q-XAFS experiments reveal that the structural transformation initiates at the preferential oxidation of Fe sites over Ni sites. The in situ-generated O-Fe-P structure serves as the origin of the enhanced electrocatalytic OER activity of the catalyst, a finding supported by theoretical calculations. This work provides crucial insights into understanding the reaction mechanism of the state-of-the-art Ni-Fe-based OER electrocatalysts, thus advancing the rational design of more efficient OER electrocatalysts.

Advances in Oxygen Evolution Reaction Electrocatalysts via Direct Oxygen-Oxygen Radical Coupling Pathway

Oxygen evolution reaction (OER) is a cornerstone of various electrochemical energy conversion and storage systems, including water splitting, CO2/N2 reduction, reversible fuel cells, and rechargeable metal-air batteries. OER typically proceeds through three primary mechanisms: adsorbate evolution mechanism (AEM), lattice oxygen oxidation mechanism (LOM), and oxide path mechanism (OPM). Unlike AEM and LOM, the OPM proceeds via direct oxygen-oxygen radical coupling that can bypass linear scaling relationships of reaction intermediates in AEM and avoid catalyst structural collapse in LOM, thereby enabling enhanced catalytic activity and stability. Despite its unique advantage, electrocatalysts that can drive OER via OPM remain nascent and are increasingly recognized as critical. This review discusses recent advances in OPM-based OER electrocatalysts. It starts by analyzing three reaction mechanisms that guide the design of electrocatalysts. Then, several types of novel materials, including atomic ensembles, metal oxides, perovskite oxides, and molecular complexes, are highlighted. Afterward, operando characterization techniques used to monitor the dynamic evolution of active sites and reaction intermediates are examined. The review concludes by discussing several research directions to advance OPM-based OER electrocatalysts toward practical applications.

High-Entropy Layered Double Hydroxides for Efficient Methanol Electrooxidation

The electrocatalytic methanol oxidation reaction (MOR) is considered as an effective method to replace oxygen evolution reaction (OER) for efficient hydrogen production. However, the sluggish kinetics and the difficulty of breaking C─H bond of the Ni-based catalysts limit further application. Herein, three high-entropy layered double hydroxides (HELHs), namely ZnNiFeCoV-HELH, ZnNiFeCoCr-HELH, and ZnNiFeCoAl-HELH (denoted as V-HELH, Cr-HELH, and Al-HELH, respectively), are successfully synthesized. Among them, the V-HELH displays the lowest potential of 1.39 V at 100 mA cm-2 compared to Cr-HELH (1.41 V) and Al-HELH (1.44 V). After five cycles, the formate yield of V-HELH maintains over 95% of the first cycle with excellent stability. Such outstanding performance surpasses that of most state-of-the-art MOR catalysts reported so far. A series of experiments reveal that the V-HELH exhibits the fastest reaction kinetics and the largest number of active Ni3+ species. Further investigations and theoretical calculations prove that the V-HELH shows the strongest methanol adsorption with the lowest energy of -3.31 eV. The introduction of vanadium (V) with relatively larger tensile strain optimizes the d─band center of V-HELH (-0.54 eV) and lowers the energy barrier (-1.62 eV) from *CH3O to *CH2O. This work provides new insights for rational design of efficient MOR electrocatalysts.

Upcycling of Multi-Metal Contaminated Wastewater into High-Entropy Layered Double Hydroxide for Oxygen Evolution Reaction

The rapid growth of the electric vehicle industry has driven up nickel demand for batteries. However, the release of various metals during the smelting of nickel-containing ore leads to complex multi-metal contaminated smelting wastewater. Herein, CaFe layered double hydroxide (denoted as CaFe) is synthesized for the treatment of multi-metal contaminated wastewater, achieving removal efficiencies of 98.0%, 98.6%, 100%, and 100% for Co2+, Ni2+, Cu2+, and Zn2+, respectively. The quasi-situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) results indicate the formation of high-entropy LDH of CaCoNiCuZnFe by the isomorphic substitution of Ca2+ in CaFe. Meanwhile, lattice distortion and the formation of metal vacancies can be observed due to the introduction of metals with different ionic radii and the dissolution of Ca2+. Given the stability and abundant active sites of high-entropy material, the CaCoNiCuZnFe shows good OER performance with an overpotential of 310.7 mV at 10 mA cm-2 and long-term stability of 250 h. Density functional theory (DFT) calculations reveal that lattice distortion optimizes intermediate adsorption energy by enhancing M─O covalency and metal vacancy activates lattice oxygen by generating non-bonding oxygen, which synergistically triggers the lattice oxygen mechanism (LOM). This strategy converts multi-metal contaminated wastewater resources into valuable products and achieves dual goals of environmental remediation and resource utilization.

A photovoltaic-electrolysis system with high solar-to-hydrogen efficiency under practical current densities

The photovoltaic-alkaline water (PV-AW) electrolysis system offers an appealing approach for large-scale green hydrogen generation. However, current PV-AW systems suffer from low solar-to-hydrogen (STH) conversion efficiencies (e.g., <20%) at practical current densities (e.g., >100 mA cm-2), rendering the produced H2 not economical. Here, we designed and developed a highly efficient PV-AW system that mainly consists of a customized, state-of-the-art AW electrolyzer and concentrator photovoltaic (CPV) receiver. The highly efficient anodic oxygen evolving catalyst, consisting of an iron oxide/nickel (oxy)hydroxide (Fe2O3-NiOxHy) composite, enables the customized AW electrolyzer with unprecedented catalytic performance (e.g., 1 A cm-2 at 1.8 V and 0.37 kgH2/m-2 hour-1 at 48 kWh/kgH2). Benefiting from the superior water electrolysis performance, the integrated CPV-AW electrolyzer system reaches a very high STH efficiency of up to 29.1% (refer to 30.3% if the lead resistance losses are excluded) at large current densities, surpassing all previously reported PV-electrolysis systems.

Pressure-Induced Engineering of Surface Oxygen Vacancies on Metal Oxides for Heterogeneous Photocatalysis

Oxygen vacancies (OVs) spatially confined on the surface of metal oxide semiconductors are advantageous for photocatalysis, in particular, for O2-involved redox reactions. However, the thermal annealing process used to generate surface OVs often results in undesired bulk OVs within the metal oxides. Herein, a high pressure-assisted thermal annealing strategy has been developed for selectively confining desirable amounts of OVs on the surface of metal oxides, such as tungsten oxide (WO3). Applying a pressure of 1.2 gigapascal (GPa) on WO3 induces significant lattice compression, which would strengthen the W-O bonds and increase the diffusion activation energy for the migration of the O migration. This pressure-induced compression effectively inhibits the formation of bulk OVs, resulting in a high density of surface-confined OVs on WO3. These well-defined surface OVs significantly enhance the photocatalytic activation of O2, facilitating H2O2 production and aerobic oxidative coupling of amines. This strategy holds promise for the defect engineering of other metal oxides, enabling abundant surface OVs for a range of emerged applications.

High-Valence Metals Accelerate the Reaction Kinetics for Boosting Water Oxidation

The transition metal with high valence state in oxyhydroxides can accelerate the reaction kinetics, enabling highly intrinsic OER activity. However, the formation of high-valence transition-metal ions is thermodynamically unfavorable in most cases. Here, a novel strategy is proposed to realize the purpose and reveal the mechanism by constructing amorphous phase and incorporating of elements with the characteristic of Lewis acid or variable charge state. A model catalyst, CeO2-NiFeOxHy, is presented to achieve the modulation of valence state of active site (Ni2+→Ni3+→Ni4+) for improved OER, leading to dominant active sites with high valence state. The CeO2-NiFeOxHy electrode exhibits superior OER performance with overpotential of 214 and 659 mV at 10 and 500 mA cm-2, respectively (without IR correction), and high stability, which are much better than those of NiOxHy, NiFeOxHy and CeO2-NiOxHy. These findings provide an effective strategy to achieve the active metals with high-valence state for highly efficient OER.

Reconstructed Hydroxyl Coordination Field Enhances Mass Transfer for Efficient Electrocatalytic Water Oxidation

Mass transfer factor plays an indispensable role in high current density to accelerate the oxygen evolution reaction (OER) process, yet research on modulating reactant mass transport remains limited. Herein, by leveraging the dual acid-base properties of aluminum sites, both the activation of the electronic activity of the layer for layered double hydroxides (LDH) and construction of the interlayer hydroxide coordination field (IHCF) have been achieved through in situ electrochemical reconstruction. It not only facilitates charge transfer and the surface catalytic transformation of reaction intermediates but, most notably, the presence of the IHCF significantly enhances the mass transport of reactants. As a result, the overpotential of LDHs with IHCF is only 164 mV, significantly better than the reported Ni-based catalysts. Deuterium kinetic isotope effect experiments and pH-dependence measurements demonstrate that the IHCF effectively enhances substrate mass transport capability and structural stability, thereby accelerating the proton-coupled electron transfer process. To further validate the high mass transport characteristics, stability tests of the alkaline flow electrolyzer show that catalysts maintain over 1000 h of stability at a high current density. This work suggests that the IHCF effect can be utilized for further design and synthesis of efficient water oxidation catalysts for practical application.

Optimizing the stability of NiFeOOH via oxyanion intercalation for water oxidation at large current densities

In alkaline water splitting, transition metals (Ni, Fe) have received extensive attention, and NiFe-oxyhydroxide (NiFeOOH) is regarded as an exceptionally active electrocatalysts for oxygen evolution reaction (OER). However, maintaining the long-term stability of NiFeOOH at high current densities is challenging due to Fe segregation and catalyst degradation. Herein, this study proposes an approach to enhancing the stability of the Ni/Fe-O covalent bond by intercalating oxyanions (NO3-, PO43-, SO42-, and SeO42-) into the NiFeOOH substrate, improving its resistance to bond breakage. And the NiFeOOH-NO3- electrocatalyst was found to be optimal, achieving an overpotential of 311 mV and stable performance at 1 A cm-2 for several hundred hours. Consequently, NiFeOOH-NO3- exhibited a significantly improved OER stability, with a mere 3.33 % stability attenuation after 100 h, compared to 13.19 % for pristine NiFeOOH. Notably, the presence of NO3- in NiFeOOH effectively mitigates Fe segregation, leading to a fourfold enhancement in long-term stability relative to that of NiFeOOH without NO3- modification. Theoretical calculations show that the introduction of NO3- effectively shifts metal 3d band centers of NiFeOOH closer to the Fermi level. It is suggested that the oxyanions lead to increased strength of the Ni/Fe-O bonds, thereby inhibiting the dissolution of Fe and enhancing the stability of NiFeOOH phase. This research represents a significant advance in controlling Fe segregation to stabilize NiFe-based electrocatalysts for high-current-density water oxidation.

Lattice Oxygen Redox Dynamics in Zeolite-Encapsulated CsPbBr3 Perovskite OER Electrocatalysts

Understanding the oxygen evolution reaction (OER) mechanism is pivotal for improving the overall efficiency of water electrolysis. Despite methylammonium lead halide perovskites (MAPbX3) have shown promising OER performance due to their soft-lattice nature that allows lattice-oxygen oxidation of active α-PbO2 layer surface, the role of A-site MA or X-site elements in the electrochemical reconstruction and OER mechanisms has yet to be explored. Here, it is demonstrated that the OER mechanism of perovskite@zeolite composites is intrinsically dominated by the A-site group of lead-halide perovskites, while the type of X-site halogen is crucial for the reconstruction kinetics of the composites. Using CsPbBrxI3- x@AlPO-5 (x = 0, 1, 2, 3) as a model OER catalyst, it is found that the CsPbBr3@AlPO-5 behaves oxygen-intercalation pseudocapacitance during surface restructuring due to absence of halogen-ion migration and phase separation in the CsPbBr3, achieving a larger diffusion rate of OH- within the core-shell structure. Moreover, distinct from the single-metal-site mechanism of MAPbBr3@AlPO-5, experimental and theoretical investigations reveal that the soft lattice nature of CsPbBr3 triggers the oxygen-vacancy-site mechanism via the CsPbBr3/α-PbO2 interface, resulting in excellent OER performance. Owing to the variety and easy tailoring of lead-halide perovskite compositions, these findings pave a way for the development of novel perovskite@zeolite type catalysts for efficient oxygen electrocatalysis.

Real-Time Detection of Dynamic Restructuring in KNixFe1- xF3 Perovskite Fluorides for Enhanced Water Oxidation

Mechanistic understanding of how electrode-electrolyte interfaces evolve dynamically is crucial for advancing water-electrolysis technology, especially the restructuring of catalyst surface during complex electrocatalytic reactions. However, for perovskite fluorides, the mechanistic exploration for the influence of the dynamic restructuring on their chemical property and catalytic mechanism is unclear due to their poor conductivity that makes the definition of electrocatalyst structure difficult. Herein, for oxygen evolution reaction (OER), various operando characterizations are employed to investigate the structure-activity relationships of the KNixFe1- xF3@NF. Adding iron to the KNixFe1- xF3 structure increases metal vacancies, enhancing electrochemical reconstruction. For reconstructed KNixFe1- xF3 structure, the results from operando Raman, operando X-ray diffraction, operando UV-vis spectroscopy, and differential electrochemical mass spectrometry reveal that the surface Ni sites act as catalytic centers within the amorphous Ni(Fe)OOH active layer, and the incorporation of Fe activates oxidized oxygen ions during water oxidation. Theoretical calculations support this by demonstrating the optimized adsorption-free energy of oxygenated intermediates. Consequently, the KNi0.5Fe0.5F3@NF achieves an overpotential of 281 mV to reach OER current of 150 mA·cm-2 and maintains stable operation for 200 h. These results highlight a promising pathway to tuning OER mechanisms in perovskite fluorides and offer a new perspective for developing high-efficiency and durable OER catalysts.

Comprehensive Chlorine Suppression: Advances in Materials and System Technologies for Direct Seawater Electrolysis

Seawater electrolysis offers a promising pathway to generate green hydrogen, which is crucial for the net-zero emission targets. Indirect seawater electrolysis is severely limited by high energy demands and system complexity, while the direct seawater electrolysis bypasses pre-treatment, offering a simpler and more cost-effective solution. However, the chlorine evolution reaction and impurities in the seawater lead to severe corrosion and hinder electrolysis's efficiency. Herein, we review recent advances in the rational design of chlorine-suppressive catalysts and integrated electrolysis systems architectures for chloride-induced corrosion, with simultaneous enhancement of Faradaic efficiency and reduction of electrolysis's cost. Furthermore, promising directions are proposed for durable and efficient seawater electrolysis systems. This review provides perspectives for seawater electrolysis toward sustainable energy conversion and environmental protection.

Sulfate salt assistant fabrication of Fe-doped Ni2P modified with SO42-/carbon as highly efficient oxygen evolution reaction electrocatalyst

The incorporation of oxyanion groups offers a greater potential for enhancing the activity of oxygen evolution reaction (OER) electrocatalysts compared to traditional metal cations doping, owing to their unique configurations and high electronegativity. However, the incorporation of oxyanion groups that differ from those derived from the oxidation of anions in transition metal monoxides poses significant challenges, thereby limiting further applications of oxyanion group modification approach. Herein, we present a novel sulfate salt assistant approach to fabricate Fe-doped Ni2P modified with SO42-/carbon (Fe-Ni2P-S/C) nanofibers as highly efficient OER electrocatalyst. The optimized Fe-Ni2P-S/C nanofibers display superb OER activity, requiring low overpotentials of 266, 323, and 357 mV at 100, 500, and 1000 mA cm-2, respectively. Theoretical calculations reveal that the co-adsorption of PO43- and SO42- on the surface of reconstructed electrocatalyst can reduce the energy barrier of rate-determining step, thereby resulting in enhanced OER activity. The present study emphasizes the crucial role played by anion groups in OER activity as well as proposes a novel approach for incorporating anion groups into electrocatalysts.

Enhancing the oxygen evolution reaction activity and stability of high-valent CoOOH by switching the catalytic pathway through doping low-valent Cu

The oxygen evolution reaction (OER) is a critical process in electrochemical energy storage and conversion systems. The adsorbate evolution mechanism (AEM) pathway possesses the characteristics of high stability but slow catalytic kinetics. We propose that combining AEM with the lattice oxidation mechanism (LOM) pathway can potentially enhance the OER catalytic activity and stability. However, the triggering of LOM is an important challenge due to the high thermodynamic activation barrier of lattice oxygen. To solve this problem, we performed theoretical calculations and experiments which suggest that the introduction of low-valent Cu in CoOOH (CuxCo1-xOOH) could directionally modulate the local coordination environment of CoO bonds. This approach can activate lattice oxygen and generate oxygen vacancies to enhance the nucleophilic attack of *OH and directly establish OO coupling, thereby facilitating the smoothly switching from AEM to LOM pathway by increasing voltage and thus activating lattice oxygen in CuxCo1-xOOH. The switching of AEM and LOM enables CuxCo1-xOOH showing an outstanding overpotential of only 252 mV (10 mA cm-2) and durability of only 2.80 % degradation after 280h. This work provides a new way for designing efficient and stable electrocatalysts with AEM and LOM pathway switching.

Roll-to-Roll Flash Joule Heating to Stabilize Electrocatalysts onto Meter-Scale Ni Foam for Advanced Water Splitting

The seamless integration of electrocatalysts onto the electrode is crucial for enhancing water electrolyzers, yet it is especially challenging when scaled up to large manufacturing. Despite thorough investigation, there are few reports that tackle this integration through roll-to-roll (R2R) methodology, a technique crucial for fulfilling industrial-scale demands. Here, we develop an R2R flash Joule heating (R2R-FJH) system to process catalytic electrodes with superior performance. The electrodes exhibited improved stability and activity, showcasing an exceptional performance within an alkaline water electrolysis (AWE) system. They achieved a low operation potential of 1.66 V at 0.5 A cm-2, coupled with outstanding durability over the operation of 800 h. We further demonstrated a prototype of a rolled-up water splitting apparatus, illustrating the efficiency of R2R-FJH electrodes in producing high-purity hydrogen through advanced water oxidation. Our study emphasized the practicality and scalability of the R2R-FJH strategy in the industrial manufacturing of high-performance electrodes for water electrolysis.

Intrinsic Activity Identification of Noble Metal Single-Sites for Electrocatalytic Chlorine Evolution

Single-atom catalysts with maximal atom-utilization have emerged as promising alternatives for chlorine evolution reaction (CER) toward valuable Cl2 production. However, understanding their intrinsic CER activity has so far been plagued due to the lack of well-defined atomic structure controlling. Herein, we prepare and identify a series of atomically dispersed noble metals (e.g., Pt, Ir, Ru) in nitrogen-doped nanocarbons (M1-N-C) with an identical M-N4 moiety, which allows objective activity evaluation. Electrochemical experiments, operando Raman spectroscopy, and quasi-in situ electron paramagnetic resonance spectroscopy analyses collectively reveal that all the three M1-N-C proceed the CER via a direct Cl-mediated Vomer-Heyrovský mechanism with reactivity following the trend of Pt1-N-C>Ir1-N-C>Ru1-N-C. Density functional theory (DFT) calculations reveal that this activity trend is governed by the binding strength of Cl*-Cl intermediate (ΔGCl*-Cl) on M-N4 sites (Pt Collapse

Key Words

• Activity descriptor
• Chlorine evolution
• Electrocatalysis
• Single atom catalyst

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Affiliation(s)

Li Quan Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China
Xin Zhao School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
Li-Ming Yang Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China
Bo You Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China
Bao Yu Xia Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China

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Frustrated Lewis Pair Mediated f-p-d Orbital Coupling: Achieving Selective Seawater Oxidation and Breaking *OH and *OOH Scaling Relationship

The development of oxygen evolution reaction (OER) electrocatalyst for seawater electrolysis plays a crucial role in producing renewable hydrogen energy. However, during the seawater electrolysis process, the anode inevitably undergoes chloride oxidation reaction (ClOR) due to Cl- adsorption, making the seawater electrolysis process difficult to sustain. Inspired by the selective permeability of cell membranes, we propose a biomimetic design of frustrated Lewis pairs (FLPs) layers for selective seawater oxidation. Combining experimental results and molecular dynamics simulations, it has been demonstrated that cerium dioxide layers with FLPs sites can decompose water molecules, capture hydroxyl anions, and repel chloride ions simultaneously. DFT theoretical analysis indicates that the FLP sites regulate the Ce 4 f-O 2p-Ni 3d gradient orbital coupling, providing additional oxygen non-bonding (ONB) to stabilize the Ni-O bond and optimize the adsorption strength of intermediates, thereby breaking the *OH and *OOH scaling relationship. The assembled anion exchange membrane electrolyzers exhibit an efficiency of 95.7 % at a current density of 0.1 A cm-2 and can stably operate for 250 hours at a current density of 0.2 A cm-2.

Phase Engineering Facilitates O-O Coupling via Lattice Oxygen Mechanism for Enhanced Oxygen Evolution on Nickel-Iron Phosphide

Nickel-iron-based catalysts are recognized for their high efficiency in the oxygen evolution reaction (OER) under alkaline conditions, yet the underlying mechanisms that drive their superior performance remain unclear. Herein, we revealed the molecular OER mechanism and the structure-intermediate-performance relationship of OER on a phosphorus-doped nickel-iron nanocatalyst (NiFeP). NiFeP exhibited exceptional activity and stability with an overpotential of only 210 mV at 10 mA cm-2 in 1 M KOH and a cell voltage of 1.68 V at 1 A cm-2 in anion exchange membrane water electrolyzers. The evolution of active sites and intermediates during OER on NiFeP was in situ probed and correlated using shell-isolated nanoparticle-enhanced Raman spectroscopy, complemented by differential electrochemical mass spectrometry and density functional theory. These results provide direct evidence that OER proceeds via the lattice oxygen-mediated mechanism. Remarkably, phosphorus doping plays a critical role in stabilizing the active β-Ni(Fe)OOH phase, which facilitates the *OH deprotonation and the subsequent O-O coupling to form *OO intermediates. Our findings offer a deeper understanding of the OER mechanism, providing a clear pathway for designing next-generation OER catalysts with improved efficiency and durability.

A Mechanistic Insight into the Acidic-stable MnSb2O6 for Electrocatalytic Water Oxidation

The abundant, active, and acidic-stable catalysts for the oxygen evolution reaction (OER) are rare to proton exchange membrane-based water electrolysis. Mn-based materials show promise as electrocatalysts for OER in acid electrolytes. However, the relationship between the stability, activity and structure of Mn-based catalysts in acidic environments remains unclear. In this study, phase-pure MnSb2O6 was successfully prepared and investigated as a catalyst for OER in a sulfuric acid solution (pH of 2.0). A comprehensive mechanistic comparison between MnSb2O6 and Mn3O4 revealed that the rate-determining step for OER on MnSb2O6 is the direct formation of MnIV=O from MnII-H2O by the 2H+/2e- process. This process avoids the rearrangement of adjacent MnIII intermediates, leading to outstanding stability and activity.

Tunable heteroassembly of 2D CoNi LDH and Ti3C2 nanosheets with enhanced electrocatalytic activity for oxygen evolution

The sluggish kinetics of the oxygen evolution reaction (OER) is the bottleneck to developing hydrogen energy based on water electrolysis, which can be significantly improved using high performance catalysts. In this context, CoNi layered double hydroxide (LDH)/Ti3C2 heterostructures are obtained using electrostatic attraction of the positively charged LDH and negatively charged Ti3C2 nanosheets as the catalyst to optimize the OER performance. Such alternate stacking exhibits good catalytic activity with a lower overpotential and a small Tafel slope, outperforming their individual components. The results of density functional theory (DFT) simulation show that the charge transfers from Ti3C2 to CoNi LDH not only adjust the electron distribution, but also increase the electron density of the interfacial active sites, thus enhancing the electron transfer efficiency inside the heterostructures. Moreover, the cobalt and nickel ions exhibit a synergistic effect in supplying more electrons to adsorb the adjacent intermediates with active hydrogen and oxygen vacancies, to improve the adsorption capability and to reduce the reaction energy barriers. These findings provide a rewarding avenue towards the design of highly efficient electrocatalysts for OER.

Engineering Lattice Oxygen Regeneration of NiFe Layered Double Hydroxide Enhances Oxygen Evolution Catalysis Durability

The lattice oxygen mechanism (LOM) endows NiFe layered double hydroxide (NiFe-LDH) with superior oxygen evolution reaction (OER) activity, yet the frequent evolution and sluggish regeneration of lattice oxygen intensify the dissolution of active species. Herein, we overcome this challenge by constructing the NiFe hydroxide/Ni4Mo alloy (NiFe-LDH/Ni4Mo) heterojunction electrocatalyst, featuring the Ni4Mo alloy as the oxygen pump to provide oxygenous intermediates and electrons for NiFe-LDH. The released lattice oxygen can be timely offset by the oxygenous species during the LOM process, balancing the regeneration of lattice oxygen and assuring the enhancement of the durability. In consequence, the durability of NiFe-LDH is significantly enhanced after the modification of Ni4Mo with an impressive durability for over 60 h, much longer than that of NiFe-LDH counterpart with only 10 h. In situ spectra and first-principle simulations reveal that the adsorption of OH- is significantly strengthened owing to the introduction of Ni4Mo, ensuring the rapid regeneration of lattice oxygen. Moreover, NiFe-LDH/Ni4Mo-based anion exchange membrane water electrolyzer (AEMWE) presents an impressive durability for over 150 h at 100 mA cm-2. The oxygen pump strategy opens opportunities to balance the evolution and regeneration of lattice oxygen, enhancing the durability of efficient OER catalysts.

Transition metals-based electrocatalysts on super-flat substrate for perovskite photovoltaic hydrogen production with 13.75% solar to hydrogen efficiency

Harnessing the inexhaustible solar energy for water splitting is regarded one of the most promising strategies for hydrogen production. However, sluggish kinetics of oxygen evolution reaction (OER) and expensive photovoltaics have hindered commercial viability. Here, an adhesive-free electrodeposition process is developed for in-situ preparation of earth-abundant electrocatalysts on super-flat indium tin oxide (ITO) substrate. NiFe hydroxide exhibited prominent OER performance, achieving an ultra-low overpotential of 236 mV at 10 mA/cm2 in alkaline solution. With the superior OER activity, we achieved an unassisted solar water splitting by series connected perovskite solar cells (PSCs) of 2 cm2 aperture area with NiFe/ITO//Pt electrodes, yielding overall solar to hydrogen (STH) efficiency of 13.75 %. Furthermore, we upscaled the monolithic facility to utilize perovskite solar module for large-scale hydrogen production and maintained an approximate operating current of 20 mA. This creative strategy contributes to the decrease of industrial manufacturing expenses for perovskite-based photovoltaic-electrochemical (PV-EC) hydrogen production, further accelerating the conversion and utilization of carbon-free energy.

CeO2-δ as Electron Donor in Co0.07Ce0.93O2-δ Solid Solution Boosts Alkaline Water Splitting

Optimizing the electronic structure with increasing intrinsic stability is a usual method to enhance the catalysts' performance. Herein, a series of cerium dioxide (CeO2-δ) based solid solution materials is synthesized via substituting Ce atoms with transition metal (Co, Cu, Ni, etc.), in which Co0.07Ce0.93O2-δ shows optimized band structure because of electron transition in the reaction, namely Co3+ (3d64s0) + Ce3+ (4f15d 06s0) → Co2+ (3d74s0) + Ce4+ (4f05d06s0), with more stable electronic configuration. The in situ Raman spectra show a stable F2g peak at ≈452 cm-1 of Co0.07Ce0.93O2-δ, while the F2g peak in CeO2-δ almost disappeared during HER progress, demonstrating the charge distribution of *H adsorbed on Co0.07Ce0.93O2-δ is more stable than *H adsorbed on CeO2-δ. Density functional theory calculations reveal that Co0.07Ce0.93O2-δ solid solution increases protonation capacity and favors for formation of *H in alkaline media. General guidelines are formulated for optimizing adsorption capacity and the volcano plot demonstrates the excellent catalytic performance of Co0.07Ce0.93O2-δ solid solution. The alkaline anion exchange membrane water electrolysis based on Co0.07Ce0.93O2-δ/NiFe LDH realizes a current density of 1000 mA cm-2 at ≈1.86 V in alkaline seawater at 80 °C and exhibits long-term stability for 450 h.

A Complex Oxide Containing Inherent Peroxide Ions for Catalyzing Oxygen Evolution Reactions in Acid

Proton exchange membrane water electrolyzers powered by sustainable energy represent a cutting-edge technology for renewable hydrogen generation, while slow anodic oxygen evolution reaction (OER) kinetics still remains a formidable obstacle that necessitates basic comprehension for facilitating electrocatalysts' design. Here, we report a low-iridium complex oxide La1.2Sr2.7IrO7.33 with a unique hexagonal structure consisting of isolated Ir(V)O6 octahedra and true peroxide O22- groups as a highly active and stable OER electrocatalyst under acidic conditions. Remarkably, La1.2Sr2.7IrO7.33, containing 59 wt % less iridium relative to the benchmark IrO2, shows about an order of magnitude higher mass activity, 6-folds higher intrinsic activity than the latter, and also surpasses the state-of-the-art Ir-based oxides ever reported. Combined electrochemical, spectroscopic, and density functional theory investigations reveal that La1.2Sr2.7IrO7.33 follows the peroxide-ion participation mechanism under the OER condition, where the inherent peroxide ions with accessible nonbonded oxygen states are responsible for the high OER activity. This discovery offers an innovative strategy for designing advanced catalysts for various catalytic applications.

Tackling activity-stability paradox of reconstructed NiIrOx electrocatalysts by bridged W-O moiety

One challenge remaining in the development of Ir-based electrocatalyst is the activity-stability paradox during acidic oxygen evolution reaction (OER), especially for the surface reconstructed IrOx catalyst with high efficiency. To address this, a phase selective Ir-based electrocatalyst is constructed by forming bridged W-O moiety in NiIrOx electrocatalyst. Through an electrochemical dealloying process, an nano-porous structure with surface-hydroxylated rutile NiWIrOx electrocatalyst is engineered via Ni as a sacrificial element. Despite low Ir content, NiWIrOx demonstrates a minimal overpotential of 180 mV for the OER at 10 mA·cm-2. It maintains a stable 300 mA·cm-2 current density during an approximately 300 h OER at 1.8 VRHE and shows a stability number of 3.9 × 105 noxygen · nIr-1. The resulting W - O-Ir bridging motif proves pivotal for enhancing the efficacy of OER catalysis by facilitating deprotonation of OER intermediates and promoting a thermodynamically favorable dual-site adsorbent evolution mechanism. Besides, the phase selective insertion of W-O in NiIrOx enabling charge balance through the W-O-Ir bridging motif, effectively counteracting lattice oxygen loss by regulating Ir-O co-valency.

High-Entropy Sulfide Catalyst Boosts Energy-Saving Electrochemical Sulfion Upgrading to Thiosulfate Coupled with Hydrogen Production

Electrochemical sulfion oxidation reaction (SOR) offers a sustainable strategy for sulfion-rich wastewater treatment, which can couple with cathodic hydrogen evolution reaction (HER) for energy-saving hydrogen production. However, the corrosion and passivation of sulfur species render the inferior catalytic SOR performance, and the oxidation product, polysulfide, requires further acidification to recover cheap elementary sulfur. Here, we reported an amorphous high-entropy sulfide catalyst of CuCoNiMnCrSx nanosheets in situ growth on the nickel foam (CuCoNiMnCrSx/NF) for SOR, which achieved an ultra-low potential of 0.25 V to afford 100 mA cm-2, and stable electrolysis at as high as 1 A cm-2 for 100 h. These were endowed by the manipulated chemical environments surrounding Cu+ sites and the constructed "soft-acid" to "hard-acid" adsorption/desorption sites, enabling synergistically boosted adsorption/desorption process of sulfur species during SOR. Moreover, we developed an electrochemical-chemical tandem process to convert sulfions to value-added thiosulfate, providing a good choice for simultaneous wastewater utilization and hydrogen production.