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

Nitrogen-doped graphene supported single-atom catalysts for efficient electrocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid: a density functional theory study

Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a promising alternative for oxygen evolution reactions. The search for efficient catalysts has been attracting increasing scientific attention. This work explores the performance of nitrogen-doped graphene-supported single-atom catalysts (M-NC SACs) for the reaction. Hydroxide was found to compete with HMF for the adsorption site on early transition metal SACs. Electronic structure analysis showed that only the electron density of the functional group directly bonded to the metal site is significantly perturbed upon adsorption. Two reaction free energies were identified as descriptors for constructing the activity volcano. Scaling relation analysis elucidated the general mechanism of the reaction including the trend for the activation of the aldehyde and alcohol groups of HMF, the potential-limiting steps, and the preferred reaction pathways. In general, the reaction is limited by an aldehyde/alcohol oxidation step in the scenarios of weak/strong adsorption regardless of the reaction pathways. Nine promising candidate catalysts were proposed, including Mn, Sc, Co, Cd, Ru, Y, Cr, Fe, and Zn SACs with limiting potentials not exceeding 0.51 V. This work provides valuable insights into the electrocatalytic oxidation mechanism of HMF to FDCA on M-NC SACs and proposes candidate catalysts to guide future research.

Air-condition process for Constructing MeAl-LDHs/rGO (Me = Co, Ni, Fe) hybrids toward efficient oxygene evolution reaction

Transition-metal-based layered double hydroxides (LDHs) are emerging as one of the most promising candidates with superior oxygen evolution reaction (OER) activity. However, their fabrication under mild conditions remains a significant challenge. Here, we propose a low-cost, high-production-rate strategy for controlled preparation of AlOOH/reduced graphene oxide (rGO) with strong adsorption capacity for transition metal cations, and then in-situ conversion to CoAl-LDH/rGO, NiAl-LDH/rGO and FeAl-LDH/rGO under ambient conditions. Based on Density Functional Theory calculations, it is witnessed that the rational electronic coordination between Co and Al enables the charge absorption by surrounding oxygen atoms, thereby playing a critical role in reducing the adsorption free energy and overpotential of OER intermediates toward efficient OER. Consequently, the resultant CoAl-LDH/rGO hybrids exhibit an overpotential of just 368 mV at the current density of 100 mA/cm2, with a low Tafel slope of 43.2 mV/dec. Moreover, under Air Mass 1.5 Global (AM 1.5G) illumination, their solar-to-hydrogen (STH) conversion efficiency of photoelectrochemical system achieves ∼ 11.61 %, with robust operation stability up to 24 h, demonstrating great potential for practical applications as advanced OER catalysts.

Leveraging Iron in the Electrolyte to Improve Oxygen Evolution Reaction Performance: Fundamentals, Strategies, and Perspectives

Electrochemical water splitting is a pivotal technology for storing intermittent electricity from renewable sources into hydrogen fuel. However, its overall energy efficiency is impeded by the sluggish oxygen evolution reaction (OER) at the anode. In the quest to design high-performance anode catalysts for driving the OER under non-acidic conditions, iron (Fe) has emerged as a crucial element. Although the profound impact of adventitious electrolyte Fen+ species on OER catalysis had been reported forty years ago, recent interest in tailoring the electrode-electrolyte interface has spurred studies on the controlled introduction of Fe ions into the electrolyte to improve OER performance. During the catalytic process, scenarios where the rate of Fen+ deposition on a specific host material outruns that of dissolution pave the way for establishing highly efficient and dynamically stable electrochemical interfaces for long-term steady operation. This review systematically summarizes recent endeavors devoted to elucidating the behaviors of in situ Fe(aq.) incorporation, the role of incorporated Fe sites in the OER, and critical factors influencing the interplay between the electrode surface and Fe ions in the electrolyte environment. Finally, unexplored issues related to comprehensively understanding and leveraging the dynamic exchange of Fen+ at the interface for improved OER catalysis are summarized.

Sulfur oxidation mediated controllable reconstruction on LiCo1.9Fe0.1O4 for boosted electrochemical water oxidation

Appropriate contact between catalysts and reactants calls for optimized exposure of active sites in the near-surface region, which can be accomplished by tuning the surface reconstruction degree. Understanding and conducting the controllable surface reconstruction of oxygen evolution reaction (OER) catalysts lays the foundation to finetune their OER activity. Herein, we explore the construction of a tunable amorphous oxyhydroxide shell on LiCo1.9Fe0.1O4via heat-sulfurization, followed by electrochemical treatment. The 8-electron sulfide oxidation reaction (SOR) transforms the sulfide shell to amorphous oxyhydroxide and generates surface-anchored SO42-, which act together to boost the OER. The electrocatalyst with optimal sulfurization exhibits 2.57 times higher than the current density at 1.6 V vs. RHE compared to the original LCFO. This work is dedicated to understanding controllable reconstruction and designing efficient OER electrocatalysts.

Effect of Helicobacter pylori-induced gastric cancer on gastrointestinal microbiota: a narrative review

Helicobacter pylori (H. pylori) infection is a typical microbial agent that interferes with the complex mechanisms of gastric homeostasis by disrupting the balance between the host gastric microbiota and mucosa-related factors, ultimately leading to inflammatory changes, dysbiosis, and gastric cancer (GC). We searched this field on the basis of PubMed, Google Scholar, Web of Science, and Scopus databases. Most studies show that H. pylori inhibits the colonization of other bacteria, resulting in a less variety of bacteria in the gastrointestinal (GI) tract. When comparing the patients with H. pylori-positive and H. pylori-negative GC, the composition of the gastric microbiome changes with increasing abundance of H. pylori (where present) in the gastritis stage, whereas, as the gastric carcinogenesis cascade progresses to GC, oral and intestinal-type pathogenic microbial strains predominate. H. pylori infection induces a premalignant milieu of atrophy and intestinal metaplasia, and the resulting change in gastric microbiota appears to play an important role in gastric carcinogenesis. The effect of H. pylori-induced GC on GI microbiota is discussed in this review.

Termination-acidity tailoring of molybdenum carbides for alkaline hydrogen evolution reaction

Transition-metal carbides have been advocated as the promising alternatives to noble-metal platinum-based catalysts in electrocatalytic hydrogen evolution reaction over half a century. However, the effectiveness of transition-metal carbides catalyzing hydrogen evolution in high-pH electrolyte is severely compromised due to the lowered proton activity and intractable alkaline-leaching issue of transition-metal centers. Herein, on the basis of validation of molybdenum-carbide model-catalyst system by taking advantage of surface science techniques, Mo2C micro-size spheres terminated by Al3+ doped MoO2 layer exhibit a notable performance of alkaline hydrogen evolution with a near-zero onset-potential, a low overpotential (40 mV) at a typical current density of 10 mA/cm2, and a small Tafel slope (45 mV/dec), as well as a long-term stability for continuous hydrogen production over 200 h. Advanced morphology and spectroscopy characterizations demonstrate that the local -Al-OH-Mo- structures within Al-MoO2 terminations serve as strong Brønsted acid sites that accelerate the deprotonation kinetics in alkaline HER process. Our work paves an interesting termination-acidity-tailoring strategy to explore cost-effective catalysts towards water electrolysis and beyond.

Understanding the Dynamic Evolution of Active Sites among Single Atoms, Clusters, and Nanoparticles

Catalysis remains a cornerstone of chemical research, with the active sites of catalysts being crucial for their functionality. Identifying active sites, particularly during the reaction process, is crucial for elucidating the relationship between a catalyst's structure and its catalytic property. However, the dynamic evolution of active sites within heterogeneous metal catalysts presents a substantial challenge for accurately pinpointing the real active sites. The advent of in situ and operando characterization techniques has illuminated the path toward understanding the dynamic changes of active sites, offering robust scientific evidence to support the rational design of catalysts. There is a pressing need for a comprehensive review that systematically explores the dynamic evolution among single atoms, clusters, and nanoparticles as active sites during the reaction process, utilizing in situ and operando characterization techniques. This review aims to delineate the effects of various reaction factors on dynamic evolution of active sites among single atoms, clusters, and nanoparticles. Moreover, several in situ and operando techniques are elaborated with emphases on tracking the dynamic evolution of active sites, linking them to catalytic properties. Finally, it discusses challenges and future perspectives in identifying active sites during the reaction process and advancing in situ and operando characterization techniques.

Dual-Site Bridging Mechanism for Bimetallic Electrochemical Oxygen Evolution

Heterogeneous dual-site electrocatalysts are emerging cutting-edge materials for efficient electrochemical water splitting. However, the corresponding oxygen evolution reaction (OER) mechanism on these materials is still unclear. Herein, based on a series of in situ spectroscopy experiments and density function theory (DFT) calculations, a new heterogeneous dual-site O-O bridging mechanism (DSBM) is proposed. This mechanism is to elucidate the sequential appearance of dual active sites through in situ construction (hybrid ions undergo reconstruction initially), determine the crucial role of hybrid dual sites in this mechanism (with Ni sites preferentially adsorbing hydroxyls for catalysis followed by proton removal at Fe sites), assess the impact of O-O bond formation on the activation state of water (inducing orderliness of activated water), and investigate the universality (with Co doping in Ni(P4O11)). Under the guidance of this mechanism, with Fe-Ni(P4O11) as pre-catalyst, the in situ formed Fe-Ni(OH)2 electrocatalyst has reached a record-low overpotential of 156.4 mV at current density of 18.0 mA cm-2. Successfully constructed Fe-Ni(P4O11)/Ti uplifting the overall efficacy of the phosphate from moderate to superior, positioning it as an innovative and highly proficient electrocatalyst for OER.

Surface Reconstruction Regulation of Co3N Through Heterostructure Engineering Toward Efficient Oxygen Evolution Reaction

Oxygen evolution reaction (OER) electrocatalysts generally experience structural and electronic modifications during electrocatalysis. This phenomenon, referred to as surface reconstruction, results in the formation of catalytically active species that act as real OER sites. Controlling surface reconstruction therefore is vital for enhancing the OER performance of electrocatalysts. In this study, a new approach is introduced of heterostructure engineering to facilitate the surface reconstruction of target catalysts. Using MnCo carbonate hydroxide (MnCo─CH)@Co3N as a demonstration, it is discovered that the surface reconstruction occurs more readily and rapidly on MnCo─CH@Co3N than on Co3N. More interestingly, during the reconstruction process, Mn species migrate to the surface, enabling the in situ formation of highly active Mn-doped CoOOH. Consequently, the MnCo─CH@Co3N catalyst after reconstruction exhibits a low overpotential of 257 mV at 10 mA cm-2, compared to 379 mV of individual Co3N. This work offers fresh perspectives on understanding the enhanced OER performance of heterostructure electrocatalysts and the role of heterostructure in promoting surface reconstruction.

Deep Reconstruction of Ruthenate Perovskite Oxide Enables Efficient pH-Universal Hydrogen Evolution Reaction

Designing highly efficient, stable, and pH-universal perovskites for hydrogen evolution reaction (HER) is urgently needed yet remains a grand challenge. Herein, a titanium-containing strontium ruthenate (SrTi0.5Ru0.5O3, STRO) is developed as an excellent HER electrocatalyst in a wide pH range. The introduction of Ti into SrRuO3 significantly reduces the size of STRO, endowing with a high reactivity that facilitates a deep surface-reconstruction during HER. Furthermore, Sr2+ leaching triggered reconstruction leads to STRO breaking into tiny nanoparticles accompanied by high-valence ruthenium (Ru) species reducing to metallic Ru. The generated active species, increased accessible sites, and improved electrical conductivity greatly boost HER. The reconstructed STRO displays remarkable HER activities with exceptional low overpotentials of 18, 24, and 55 mV at 10 mA cm-2 in 1 m KOH, 0.5 m H2SO4, and 1 m PBS, respectively, surpassing most perovskites reported previously and comparable to or even outperforming that of commercial Pt/C. Moreover, the STRO exhibits excellent stabilities over 200 h in alkaline and acidic media, superior to that of Pt/C. This work not only provides insights into structure reconstruction of perovskites during HER, but also opens new perspectives for developing high-efficiency and pH-universal electrocatalysts for future energy applications.

In Situ Reconstructing NiFe Oxalate Toward Overall Water Splitting

Surface reconstruction plays an essential role in electrochemical catalysis. The structures, compositions, and functionalities of the real catalytic species and sites generated by reconstruction, however, are yet to be clearly understood, for the metastable or transit state of most reconstructed structures. Herein, a series of NiFe oxalates (NixFe1- xC2O4, x = 1, 0.9, 0.7, 0.6, 0.5, and 0) are synthesized for overall water splitting electrocatalysis. Whilst NixFe1-xC2O4 shows great hydrogen evolution reaction (HER) activity, the in situ reconstructed NixFe1-xOOH exhibits outstanding oxygen evolution reaction (OER) activity. As identified by the in situ Raman spectroscopy and quasi-in situ X-ray absorption spectroscopy (XAS) techniques, reconstructions from NixFe1-xC2O4 into defective NixFe1-xOOH and finally amorphous NixFe1-xOOH active species (R-NixFe1-xOOH) are confirmed upon cyclic voltammetry processes. Specifically, the fully reconstructed R-Ni0.6Fe0.4OOH demonstrates the best OER activity (179 mV to reach 10 mA cm-2), originating from its abundant real active sites and optimal d-band center. Benefiting from the reconstruction, an alkaline electrolyzer composed of a Ni0.6Fe0.4C2O4 cathode and an in situ reconstructed R-Ni0.6Fe0.4OOH anode achieves a superb overall water splitting performance (1.52 V@10 mA cm-2). This work provides an in-depth structure-property relationship understanding on the reconstruction of catalysts and offers a new pathway to designing novel catalyst.

In-situ electrochemical reconstruction and modulation of adsorbed hydrogen coverage in cobalt/ruthenium-based catalyst boost electroreduction of nitrate to ammonia

The electroreduction of nitrate offers a promising, sustainable, and decentralized route to generate valuable ammonia. However, a key challenge in the nitrate reduction reaction is the energy efficiency of the reaction, which requires both a high ammonia yield rate and a high Faradaic efficiency of ammonia at a low working potential (≥-0.2 V versus reversible hydrogen electrode). We propose a bimetallic Co-B/Ru12 electrocatalyst which utilizes complementary effects of Co-B and Ru to modulate the quantity of adsorbed hydrogen and to favor the specific hydrogenation for initiating nitrate reduction reaction at a low overpotential. This effect enables the catalyst to achieve a Faradaic efficiency for ammonia of 90.4 ± 9.2% and a remarkable half-cell energy efficiency of 40.9 ± 4% at 0 V versus reversible hydrogen electrode. The in-situ electrochemical reconstruction of the catalyst contributes to boosting the ammonia yield rate to a high level of 15.0 ± 0.7 mg h-1 cm-2 at -0.2 V versus reversible hydrogen electrode. More importantly, by employing single-entity electrochemistry coupled with identical location transmission electron microscopy, we gain systematic insights into the correlation between the increase in the catalyst's active sites and its structural transformations during the nitrate reduction reaction.

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

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

Fe-S dually modulated adsorbate evolution and lattice oxygen compatible mechanism for water oxidation

Simultaneously activating metal and lattice oxygen sites to construct a compatible multi-mechanism catalysis is expected for the oxygen evolution reaction (OER) by providing highly available active sites and mediate catalytic activity/stability, but significant challenges remain. Herein, Fe and S dually modulated NiFe oxyhydroxide (R-NiFeOOH@SO4) is conceived by complete reconstruction of NiMoO4·xH2O@Fe,S during OER, and achieves compatible adsorbate evolution mechanism and lattice oxygen oxidation mechanism with simultaneously optimized metal/oxygen sites, as substantiated by in situ spectroscopy/mass spectrometry and chemical probe. Further theoretical analyses reveal that Fe promotes the OER kinetics under adsorbate evolution mechanism, while S excites the lattice oxygen activity under lattice oxygen oxidation mechanism, featuring upshifted O 2p band centers, enlarged d-d Coulomb interaction, weakened metal-oxygen bond and optimized intermediate adsorption free energy. Benefiting from the compatible multi-mechanism, R-NiFeOOH@SO4 only requires overpotentials of 251 ± 5/291 ± 1 mV to drive current densities of 100/500 mA cm-2 in alkaline media, with robust stability for over 300 h. This work provides insights in understanding the OER mechanism to better design high-performance OER catalysts.

Unraveling the Role of Particle Size and Nanostructuring on the Oxygen Evolution Activity of Fe-Doped NiO

Nickel-based oxides and oxyhydroxide catalysts exhibit state-of-the-art activity for the sluggish oxygen evolution reaction (OER) under alkaline conditions. A widely employed strategy to increase the gravimetric activity of the catalyst is to increase the active surface area via nanostructuring or decrease the particle size. However, the fundamental understanding about how tuning these parameters influences the density of oxidized species and their reaction kinetics remains unclear. Here, we use solution combustion synthesis, a low-cost and scalable approach, to synthesize a series of Fe0.1Ni0.9O samples from different precursor salts. Based on the precursor salt, the nanoparticle size can be changed significantly from ∼2.5 to ∼37 nm. The OER activity at pH 13 trends inversely with the particle size. Using operando time-resolved optical spectroscopy, we quantify the density of oxidized species as a function of potential and demonstrate that the OER kinetics exhibits a second-order dependence on the density of these species, suggesting that the OER mechanism relies on O-O coupling between neighboring oxidized species. With the decreasing particle size, the density of species accumulated is found to increase, and their intrinsic reactivity for the OER is found to decrease, attributed to the stronger binding of *O species (i.e., a cathodic shift of species energetics). This signifies that the high apparent OER activity per geometric area of the smaller nanoparticles is driven by their ability to accumulate a larger density of oxidized species. This study not only experimentally disentangles the influence of the density of oxidized species and intrinsic kinetics on the overall rate of the OER but also highlights the importance of tuning these parameters independently to develop more active OER catalysts.

Elevated Water Oxidation by Cation Leaching Enabled Tunable Surface Reconstruction

Water electrolysis is one promising and eco-friendly technique for energy storage, yet its overall efficiency is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Therefore, developing strategies to boost OER catalyst performance is crucial. With the advances in characterization techniques, an extensive phenomenon of surface structure evolution into an active amorphous layer was uncovered. Surface reconstruction in a controlled fashion was then proposed as an emerging strategy to elevate water oxidation efficiency. In this work, Cr substitution induces the reconstruction of NiFexCr2-xO4 during cyclic voltammetry (CV) conditioning by Cr leaching, which leads to a superior OER performance. The best-performed NiFe0.25Cr1.75O4 shows a ~1500 % current density promotion at overpotential η=300 mV, which outperforms many advanced NiFe-based OER catalysts. It is also found that their OER activities are mainly determined by Ni : Fe ratio rather than considering the contribution of Cr. Meanwhile, the turnover frequency (TOF) values based on redox peak and total mass were obtained and analysed, and their possible limitations in the case of NiFexCr2-xO4 are discussed. Additionally, the high activity and durability were further verified in a membrane electrode assembly (MEA) cell, highlighting its potential for practical large-scale and sustainable hydrogen gas generation.

Catalytic Water Electrolysis by Co-Cu-W Mixed Metal Oxides: Insights from X-ray Absorption Spectroelectrochemistry

Mixed metal oxides (MMOs) are a promising class of electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Despite their importance for sustainable energy schemes, our understanding of relevant reaction pathways, catalytically active sites, and synergistic effects is rather limited. Here, we applied synchrotron-based X-ray absorption spectroscopy (XAS) to explore the evolution of the amorphous Co-Cu-W MMO electrocatalyst, shown previously to be an efficient bifunctional OER and HER catalyst for water splitting. Ex situ XAS measurements provided structural environments and the oxidation state of the metals involved, revealing Co2+ (octahedral), Cu+/2+ (tetrahedral/square-planar), and W6+ (octahedral) centers. Operando XAS investigations, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), elucidated the dynamic structural transformations of Co, Cu, and W metal centers during the OER and HER. The experimental results indicate that Co3+ and Cu0 are the active catalytic sites involved in the OER and HER, respectively, while Cu2+ and W6+ play crucial roles as structure stabilizers, suggesting strong synergistic interactions within the Co-Cu-W MMO system. These results, combined with the Tafel slope analysis, revealed that the bottleneck intermediate during the OER is Co3+ hydroperoxide, whose formation is accompanied by changes in the Cu-O bond lengths, pointing to a possible synergistic effect between Co and Cu ions. Our study reveals important structural effects taking place during MMO-driven OER/HER electrocatalysis and provides essential experimental insights into the complex catalytic mechanism of emerging noble-metal-free MMO electrocatalysts for full water splitting.

Revealing Structural Evolution of Nickel Phosphide-Iron Oxide Core-Shell Nanocatalysts in Alkaline Medium for the Oxygen Evolution Reaction

Metal phosphide-containing materials have emerged as a potential candidate of nonprecious metal-based catalysts for alkaline oxygen evolution reaction (OER). While it is known that metal phosphide undergoes structural evolution, considerable debate persists regarding the effects of dynamics on the surface activation and morphological stability of the catalysts. In this study, we synthesize NiP x -FeO x core-shell nanocatalysts with an amorphous NiP x core designed for enhanced OER activity. Using ex situ X-ray absorption spectroscopy, we elucidate the local structural changes as a function of the cyclic voltammetry cycles. Our studies suggest that the presence of corner-sharing octahedra in the FeO x shell improves structural rigidity through interlayer cross-linking, thereby inhibiting the diffusion of OH-/H2O. Thus, the FeO x shell preserves the amorphous NiP x core from rapid oxidation to Ni3(PO4)2 and Ni(OH)2. On the other hand, the incorporation of Ni from the core into the FeO x shell facilitates absorption of hydroxide ions for OER. As a result, Ni/Fe(OH) x at the surface oxidizes to the active γ-(oxy)hydroxide phase under the applied potentials, promoting OER. This intriguing synergistic behavior holds significance as such a synthetic route involving the FeO x shell can be extended to other systems, enabling manipulation of surface adsorption and diffusion of hydroxide ions. These findings also demonstrate that nanomaterials with core-shell morphologies can be tuned to leverage the strength of each metallic component for improved electrochemical activities.

Intermetallic Cobalt Indium Nanoparticles as Oxygen Evolution Reaction Precatalyst: A Non-Leaching p-Block Element

Merely all transition-metal-based materials reconstruct into similar oxyhydroxides during the electrocatalytic oxygen evolution reaction (OER), severely limiting the options for a tailored OER catalyst design. In such reconstructions, initial constituent p-block elements take a sacrificial role and leach into the electrolyte as oxyanions, thereby losing the ability to tune the catalyst's properties systematically. From a thermodynamic point of view, indium is expected to behave differently and should remain in the solid phase under alkaline OER conditions. However, the structural behavior of transition metal indium phases during the OER remains unexplored. Herein, are synthesized intermetallic cobalt indium (CoIn3) nanoparticles and revealed by in situ X-ray absorption spectroscopy and scanning transmission microscopy that they undergo phase segregation to cobalt oxyhydroxide and indium hydroxide. The obtained cobalt oxyhydroxide outperforms a metallic-cobalt-derived one due to more accessible active sites. The observed phase segregation shows that indium behaves distinctively differently from most p-block elements and remains at the electrode surface, where it can form lasting interfaces with the active metal oxo phases.

Stabilizing atomic Ru species in conjugated sp2 carbon-linked covalent organic framework for acidic water oxidation

Suppressing the kinetically favorable lattice oxygen oxidation mechanism pathway and triggering the adsorbate evolution mechanism pathway at the expense of activity are the state-of-the-art strategies for Ru-based electrocatalysts toward acidic water oxidation. Herein, atomically dispersed Ru species are anchored into an acidic stable vinyl-linked 2D covalent organic framework with unique crossed π-conjugation, termed as COF-205-Ru. The crossed π-conjugated structure of COF-205-Ru not only suppresses the dissolution of Ru through strong Ru-N motifs, but also reduces the oxidation state of Ru by multiple π-conjugations, thereby activating the oxygen coordinated to Ru and stabilizing the oxygen vacancies during oxygen evolution process. Experimental results including X-ray absorption spectroscopy, in situ Raman spectroscopy, in situ powder X-ray diffraction patterns, and theoretical calculations unveil the activated oxygen with elevated energy level of O 2p band, decreased oxygen vacancy formation energy, promoted electrochemical stability, and significantly reduced energy barrier of potential determining step for acidic water oxidation. Consequently, the obtained COF-205-Ru displays a high mass activity with 2659.3 A g-1, which is 32-fold higher than the commercial RuO2, and retains long-term durability of over 280 h. This work provides a strategy to simultaneously promote the stability and activity of Ru-based catalysts for acidic water oxidation.

Anomalous Detachment Behavior and Directional Reconstruction Regulation of Leaching-Type Precatalysts for Industrial Water Electrolyzers

Current reconstruction chemistry studies are mainly operated at the laboratory scale, where the operating parameters are different from those used in industrial water electrolyzers. This gap leads to unclear reconstruction behaviors under industrial conditions and constrains the application of catalysts. Here, this work presents a new reconstruction mechanism and anomalous detachment phenomena observed in leaching-type oxygen-evolving precatalysts under industrial conditions, different from the reported results obtained under laboratory conditions. The identified detachment issues are closely linked to the production of a potassium salt separate phase, which proves sensitive to the local environment, and its instability easily leads to catalyst stripping from the substrate. By establishing detachment critical point and operating parameter-detachment correlation, a targeted reconstruction strategy is proposed to achieve smooth ligand leaching and effectively solve the detachment issue. Theoretical analyses validate the dual-site regulation in directionally reconstructed catalysts with optimized intermediate adsorption. Under industrial conditions, the coupled electrolyzer delivers an industrial-level current density at low cell voltage with prolonged durability, 1 A cm-2 at 2 V for over 340 h. This work bridges the gap of leaching-type precatalysts between laboratory test conditions and industrial operating conditions.

Mechanistic Insights into the Discharge Processes of Li-CO2 Batteries

Li-CO2 batteries facilitate renewable energy storage in a cost-effective, eco-friendly manner. However, an inadequate understanding of their reaction mechanism severely impedes their development. Here we outline recent mechanistic advances in the discharge processes of Li-CO2 batteries, particularly in terms of the theoretical aspect. First, the vital factors affecting the formation of discharge intermediates are highlighted, and a surface lithiation mechanism predominantly applicable to catalysts with weak CO2 adsorption is proposed. Subsequently, the modeling of the chemical potential of Li++e-, which is crucial for the evaluation of the theoretical limiting voltage, is detailed. Finally, challenges and future directions pertaining to the further development of Li-CO2 are discussed. In essence, this concept article seeks to inspire future experimental and theoretical studies in advancing the development of Li-CO2 electrochemical technology.

FeNi (oxy)hydroxides embedded with high-valence Mo atoms: A efficient and robust water oxidation electrocatalyst

The incorporation of high-valence transition metal atoms into FeNi (oxy)hydroxides may be a promising strategy to regulate the intrinsic electronic states, thereby reducing the thermodynamic barrier and accelerating oxygen evolution reaction (OER). Here, a high-valence Mo atoms doping route is proposed by an efficient self-reconstruction strategy to prepare MoFeNi (oxy)hydroxides for efficient alkaline OER. By using borides (MoNiB) as sacrificial template and Mo source, FeNi (oxy)hydroxides nanoflakes embedded with high-valence Mo atoms (MoFeNi) is successfully synthesized, which can modulate the electron coordination to improve the intrinsic catalytic activity. Remarkably, the obtained MoFeNi exhibits extremely low overpotential (η100 = 252 mV and η500 = 288 mV) and small Tafel slope (18.35 mV dec-1). The robust catalyst can run stably for hours at 500 mA cm-2. Characterization results and theoretical calculations confirmed that the addition of high-valence Mo effectively modulated the intrinsic electronic structure of metal sites and optimized the adsorption/desorption energy of the intermediates, accelerating OER reactions kinetics. By coupling MoFeNi anode with Pt/C cathode, anion exchange membrane (AEM) electrolyser can operate stably at 500 mA cm-2 with about less than 2.2 V. This research introduces a novel approach to develop ideal electrocatalysts through the incorporation of high-valence molybdenum species.

Elevated temperature-driven coordinative reconstruction of an unsaturated single-Ni-atom structure with low valency on a polymer-derived matrix for the electrolytic oxygen evolution reaction

A high-temperature pyrolysis-controlled coordination reconstruction resulted in a single-Ni-atom structure with a Ni-Nx-C structural unit (x = N atom coordinated to Ni). Pyrolysis of Ni-phen@ZIF-8-RF at 700 °C resulted in NiNP-NC-700 with predominantly Ni nanoparticles. Upon elevating the pyrolysis temperature from 700 to 900 °C, a coordination reconstruction offers Ni-Nx atomic sites in NiSA-NC-900. A combined investigation with X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and soft X-ray L3-edge spectroscopy suggests the stabilization of low-valent Niδ+ (0 < δ < 2) in the Ni-N-C structural units. The oxygen evolution reaction (OER) is a key process during water splitting in fuel cells. However, OER is a thermodynamically uphill reaction with multi-step proton-coupled electron transfer and sluggish kinetics, due to which there is a need for a catalyst that can lower the OER overpotentials. The adsorption energy of a multi-step reaction on a single metal atom with coordination unsaturation tunes the adsorption of each oxygenated intermediate. The promising OER activity of the NiSA-NC-900/NF anode on nickel foam was followed by the overall water splitting (OWS) using using NiSA-NC-900/NF as anode and Pt coil as the cathodic counterpart, wherein a cell potential of 1.75 V at 10 mA cm-2 was achieved. The cell potential recorded with Pt(-)/(+)NiSA-NC-900/NF was much lower than that obtained for other cells, i.e., Pt(-)/NF and NF(-)/(+)NF, which enhances the potentials of low-valent NiSAs for insightful understanding of the OER. At a constant applied potential of 1.61 V (vs. RHE) for 12 h, an small increase in current for initial 0.6 h followed by a constant current depicts the fair stability of catalyst for 12 h. Our results offer an insightful angle into the OER with a coordinatively reconstructed single-Ni-atom structure at lower valency (<+2).

Activating the MnS0.5Se0.5 Microspheres as High-Performance Cathode Materials for Aqueous Zinc-Ion Batteries: Insight into In Situ Electrooxidation Behavior and Energy Storage Mechanisms

Manganese-based materials are regarded as the most prospective cathode materials because of their high natural abundance, low toxicity, and high specific capacity. Nevertheless, the low conductivity, poor cycling performance, and controversial energy storage mechanisms hinder their practical application. Here, the MnS0.5Se0.5 microspheres are synthesized by a two-step hydrothermal approach and employed as cathode materials for aqueous zinc-ion batteries (AZIBs) for the first time. Interestingly, in-depth ex situ tests and electrochemical kinetic analyses reveal that MnS0.5Se0.5 is first irreversibly converted into low-crystallinity ZnMnO3 and MnOx by in situ electrooxidation (MnS0.5Se0.5-EOP) during the first charging process, and then a reversible co-insertion/extraction of H+/Zn2+ occurs in the as-obtained MnS0.5Se0.5-EOP electrode during the subsequent discharging and charging processes. Benefiting from the increased surface area, shortened ion transport path, and stable lamellar microsphere structure, the MnS0.5Se0.5-EOP electrodes deliver high reversible capacity (272.8 mAh g-1 at 0.1 A g-1), excellent rate capability (91.8 mAh g-1 at 2 A g-1), and satisfactory cyclic stability (82.1% capacity retention after 500 cycles at 1 A g-1). This study not only provides a powerful impetus for developing new types of manganese-based chalcogenides, but also puts forward a novel perspective for exploring the intrinsic mechanisms of in situ electrooxidation behavior.

FeCoCuMnRuB Nanobox with Dual Driving of High-Entropy and Electron-Trap Effects as the Efficient Electrocatalyst for Water Oxidation

High-entropy borides hold potential as electrocatalysts for water oxidation. However, the synthesis of the tailored nanostructures remains a challenge due to the thermodynamic immiscibility of polymetallic components. Herein, a FeCoCuMnRuB nanobox decorated with a nanosheet array was synthesized for the first time by a "coordination-etch-reduction" method. The FeCoCuMnRuB nanobox has various structural characteristics to express the catalytic performance; meanwhile, it combines the high-entropy effect of multiple components with the electron trap effect induced by electron-deficient B, synergistically regulating its electronic structure. As a result, FeCoCuMnRuB nanobox exhibits enhanced OER activity with a low overpotential (η10 = 233 mV), high TOF value (0.0539 s-1), small Tafel slope (61 mV/dec), and a satisfactory stability for 200 h, outperforming the high-entropy alloy and low-entropy borides. This work develops a high entropy and electron-deficient B-driven strategy for motivating the catalytic performance of water oxidation, which broadens the structural diversity and category of high-entropy materials.

NiFeP nanosheets for efficient and durable hydrazine-assisted electrolytic hydrogen production

Hydrazine-assisted electrochemical water splitting is an important avenue toward low cost and sustainable hydrogen production, which can significantly reduce the voltage of electrochemical water splitting. Herein, we took a simple approach to fabricate NiFeP nanosheet arrays on nickel foam (NiFeP/NF), which exhibit superior electrocatalytic activity for the hydrogen evolution reaction (HER) and the hydrazine oxidation reaction (HzOR). Our investigations revealed that the excellent electrocatalytic activity of NiFeP/NF mainly arises from the bimetallic synergistic effect, abundant electrocatalytically active sites facilitated by the porous nanosheet morphology, high intrinsic conductivity of NiFeP/NF and strong NiFeP-NF adhesion. We assembled a hydrazine-boosted electrochemical water splitting cell using NiFeP/NF as a bifunctional catalyst for both electrodes, and the overall hydrazine splitting (OHzS) exhibits a considerably low overpotential (100 mV at 10 mA cm-2), and is stable for 40 h continuous electrolysis in a 1 M KOH + 0.5 M N2H4 electrolyte. When it is applied to hydrogen production by seawater electrolysis, its catalytic activity shows strong tolerance. This work provides a promising approach for low cost, high-efficiency and stable hydrogen production based on hydrazine-assisted electrolytic seawater splitting for future applications.

Complementary probes for the electrochemical interface

The functions of electrochemical energy conversion and storage devices rely on the dynamic junction between a solid and a fluid: the electrochemical interface (EI). Many experimental techniques have been developed to probe the EI, but they provide only a partial picture. Building a full mechanistic understanding requires combining multiple probes, either successively or simultaneously. However, such combinations lead to important technical and theoretical challenges. In this Review, we focus on complementary optoelectronic probes and modelling to address the EI across different timescales and spatial scales - including mapping surface reconstruction, reactants and reaction modulators during operation. We discuss how combining these probes can facilitate a predictive design of the EI when closely integrated with theory.

Amorphous RuPd bimetallene for hydrogen evolution reaction in acidic and alkaline conditions: a first-principles study

Metallene materials can provide a large number of active catalytic sites for the efficient use of noble metals as catalysts for hydrogen evolution reaction (HER), whereas the intrinsic activity on the surface is insufficient in crystal phase. The amorphous phase with an inherent long-range disorder can offer a rich coordinate environment and charge polarization on the surface is proposed for promoting the intrinsic catalytic activity on the surface of noble metals. Herein, we designed an amorphous RuPd (am-RuPd) structure by the first principles molecular dynamics method. The performance of the acidic HER on am-RuPd can have a huge enhancement due to the free energy change of hydrogen adsorption close to zero. In alkaline conditions, the H2O dissociation energy barrier on am-RuPd is just 0.49 eV, and it is predicted that the alkaline HER performance of am-RuPd will largely exceed that of Pt nanocrystalline sheets. This work provides a strategy for enhancing the intrinsic catalytic activity on the surface and a way to design an efficient HER catalyst based on metallene materials used in both acidic and alkaline conditions.

In Situ Creation of Surface Defects on Pd@NiPd with Core-shell Hierarchical Structure Toward Boosting Electrocatalytic Activity

A deep insight into surface structural evolution of the catalyst is a challenging issue to reveal the structure-activity relationship. In this contribution, based on a surface alloying strategy, the dual-functional Pd@NiPd catalyst with a unique core-shell hierarchical structure is developed through selective crystal growth, surface cocrystallization, directional self-assembly, and reduction process. The surface defects are created in situ on the outer NiPd alloy layer in the electrochemical redox processes, which endow the Pd@NiPd catalyst with excellent electrocatalytic activity of hydrogen generation reaction (HER) and oxygen generation reaction (OER) in alkaline media. The optimal Pd@NiPd-2 catalyst requires an overpotential of only 18 mV that is far lower than Pt/C benchmark (43 mV) at the current density of 10 mA cm-2 for the HER, and 210 mV that is far lower than RuO2 benchmark (430 mV) at 50 mA cm-2 for the OER. Density functional theory (DFT) calculations reveal that the outstanding electrocatalytic activity is originated from the creation of surface defect structure that induces a significant reduction in the adsorption and dissociation energy barriers of H2O molecules in the HER and a decrease in the conversion energy from O* to OOH* that resulted from the synergy of two adjacent Pd sites by forming O-bridge. This work affords a typical paradigm for exploiting efficient catalysts and investigating the dependence of electrocatalytic activity on the surface structural evolution.

Questing for High-Performance Electrocatalysts for Oxygen Evolution Reaction: Importance of Chemical Complexity, Active Phase, and Surface-Adsorbed Species

Rational design of advanced electrocatalysts for oxygen evolution reaction (OER) is of vital importance for the development of sustainable energy. Entropy engineering is emerging as a promising approach for the design of efficient OER electrocatalysts. However, other multi-anion/cation electrocatalysts with compositional complexity, particularly the medium-entropy and other non-equimolar cation/anion complex electrocatalysts, have not received noteworthy attention. In this perspective, we review and highlight the importance of compositionally complex catalysts and propose a concept of chemical complexity to correlate the OER catalytic activity with the contributions from the pairwise cation-anion interactions. Then, we offer a new view on the active catalytic sites being the hydroxylated reacting interface in an alkaline solution. Further, we argue that the common discrepancies between computationally predicted OER activities and experimental results stem from lack of considerations of surface-adsorbed species in modeling the active catalytic phases or sites. This perspective would facilitate achieving a renewed and profound understanding of the OER mechanism and promote efficient design of OER electrocatalysts for renewable energy conversion and storage.

Surface-Reconstructed Ru-Doped Nickel/Iron Oxyhydroxide Arrays for Efficient Oxygen Evolution

The generation of an active phase through dynamic surface reconstruction is a promising strategy for improving the activity of electrocatalysts. However, studies investigating the reconstruction process and its impact on the intrinsic properties of the catalysts are scarce. Herein, the surface reconstruction of NiFe2 O4 interfaced with NiMoO4 (Ru-NFO/NMO) facilitated by Ru doping is reported. The electrochemical and material characterizations demonstrate that Ru doping can regulate the electronic structure of NFO/NMO and induce the high-valence state of Ni3.6+ δ , facilitating the surface reconstruction to highly active Ru-doped NiFeOOH/NiOOH (SR-Ru-NFO/NMO). The optimized SR-Ru-NFO/NMO exhibits promising performance in the oxygen evolution reaction, displaying a low overpotential of 229 mV at 10 mA cm-2 and good stability at varying current densities for 80 h. Density functional theory calculations indicate that Ru doping can increase the electron density and optimize intermediate adsorption by shifting the d-band center downward. This work provides valuable insights into the tuning of electrocatalysts by surface reconstruction and offers a rational design strategy for the development of highly active oxygen evolution reaction electrocatalysts.

Directional Electrosynthesis of Adipic Acid and Cyclohexanone by Controlling the Active Sites on NiOOH

Dicarboxylic acids and cyclic ketones, such as adipic acid (AA) and cyclohexanone (CHN), are essential compounds for the chemical industry. Although their production by electrosynthesis using electricity is considered one of the most promising strategies, the application of such processes has been hampered by a lack of efficient catalysts as well as a lack of understanding of the mechanism. Herein, a series of monolithic msig/ea-NiOOH-Ni(OH)2/NF were prepared by means of self-dissolution of metal matrix components, interface growth, and electrochemical activation (denoted as msig/ea). The as-synthesized catalysts have three-dimensional cuboid-like structures formed by interconnecting nanosheets composed of NiOOH. By theoretically guided regulation of the amounts of Ni3+ and oxygen vacancies (OV), a 96.5% yield of CHN from cyclohexanol (CHA) dehydrogenation and a 93.6% yield of AA from CHN oxidation were achieved. A combined experimental and theoretical study demonstrates that CHA dehydrogenation and CHN oxidation were promoted by the formation of Ni3+ and the peroxide species (*OOH) on OV. This work provides a promising approach for directional electrosynthesis of high-purity chemicals with in-depth mechanistic insights.

Activating Commercial Nickel Foam to a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction through a Three-Step Surface Reconstruction

It is highly desired to directly use commercial nickel foam (CNF) as an electrocatalyst for the oxygen evolution reaction (OER) via simple surface reconstruction. In our research, a simple three-step preactivation process was proposed to reconstruct CNF as an efficient OER catalyst, including calcination, high-voltage treatment, and immersing in electrolyte. The optimal CNF after three-step activation reaches an excellent OER performance of 228 and 267 mV at η10 and η100 in alkaline media and can tolerate long-term tests under a large current density of 500 mA·cm-2. The promotion of each step was explored. The calcination step leads to a reconstructive surficial morphology with an enlarged active surface, providing a prerequisite for the following construction steps. The high-voltage treatment changes the valence of surface Ni species, generating phases with higher catalytic activity, and the immersing process introduces Fe heteroatoms into the surface of CNF, boosting the catalytic performance of CNF through Ni-Fe interactions. This research provides a simple method of making high-performance catalysts with accessible nickel foam, a potential for large-scale application in practical industry, and new thinking for the manipulation of Ni-based catalysts.

Unlocking Catalytic Potential: Encasing CoP Nanoparticles within Mesoporous CoFeP Nanocubes for Enhanced Oxygen Evolution Reaction

Efficient and durable electrocatalysts fabricated by using nanosized nonprecious-metal-based materials have attracted considerable attention for use in the oxygen evolution reaction (OER). Understanding performance disparities and structure-property relationships of various nonprecious-metal-based nanostructures is crucial for optimizing their applications. Herein, CoP nanoparticles encompassed within a CoFeP shell (named CoP/CoFeP) are fabricated. The mesoporous CoFeP shell enables effective mass transport, affords abundant active sites, and ensures the accessibility of hybrid interfaces between CoP and CoFeP. Therefore, encased CoP/CoFeP nanocubes exhibit excellent OER catalytic activity with an overpotential of 266 mV at a current density of 10 mA cm-2 in alkaline media, superior to reference hollow CoFeP nanocubes and commercial RuO2. Experimental characterization and theoretical calculations show that the encased structure of CoP/CoFeP with a rich Fe-doped shell enables electronic interactions between CoP and CoFeP, as well as accelerates structural reconstruction that exposes more active sites, yielding an enhanced OER performance. This study aims to inspire further work on nonprecious-metal catalysts with tailored nanostructures and electronic properties for the OER.

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

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

Modulation Strategies for the Preparation of High-Performance Catalysts for Urea Oxidation Reaction and Their Applications

Compared with the traditional electrolysis of water to produce hydrogen, urea-assisted electrolysis of water to produce hydrogen has significant advantages and has received extensive attention from researchers. Unfortunately, urea oxidation reaction (UOR) involves a complex six-electron transfer process leading to high overpotential, which forces researchers to develop high-performance UOR catalysts to drive the development of urea-assisted water splitting. Based on the UOR mechanism and extensive literature research, this review summarizes the strategies for preparing highly efficient UOR catalysts. First, the UOR mechanism is introduced and the characteristics of excellent UOR catalysts are pointed out. Aiming at this, the following modulation strategies are proposed to improve the catalytic performance based on summarizing various literature: 1) Accelerating the active phase formation to reduce initial potential; 2) Creating double active sites to trigger a new UOR mechanism; 3) Accelerating urea adsorption and promoting C─N bond cleavage to ensure the effective conduct of UOR; 4) Promoting the desorption of CO2 to improve stability and prevent catalyst poisoning; 5) Promoting electron transfer to overcome the inherent slow dynamics of UOR; 6) Increasing active sites or active surface area. Then, the application of UOR in electrochemical devices is summarized. Finally, the current deficiencies and future directions are discussed.

The Role of Proton in High Power Density Vanadium Redox Flow Batteries

To design high-performance vanadium redox flow batteries (VRFBs), the influence of proton on electrocatalysts cannot be neglected considering the abundance of proton in a highly acidic electrolyte. Herein, the impact of proton on metal oxide-based electrocatalysts in VRFBs is investigated, and a proton-incorporating strategy is introduced for high power density VRFBs, in addition to unraveling the catalytic mechanism. This study discloses that the metal oxide-based electrocatalyst (WO3) undergoes in situ surface reconstruction by forming H0.5WO3 after incorporating proton. Experimental and theoretical results precisely disclose the catalytic active sites. The battery with H0.5WO3 designed by a proton-incorporating strategy achieves an attractive power density of 1.12 W cm-2 and sustains more than 900 cycles without an obvious decay, verifying the outstanding electrochemical performance of H0.5WO3. This work not only sheds light on the influence of proton on electrocatalysts for rational design of advanced VRFBs catalysts but also provides guidelines for the fundamental understanding of the reaction mechanism, which is highly important for the application of VRFBs.

Reversely Trapping Isolated Atoms in High Oxidation State for Accelerating the Oxygen Evolution Reaction Kinetics

Developing efficient electrocatalysts for the oxygen evolution reaction (OER) is paramount to the energy conversion and storage devices. However, the structural complexity of heterogeneous electrocatalysts makes it a great challenge to elucidate the dynamic structural evolution and OER mechanisms. Here, we develop a controllable atom-trapping strategy to extract isolated Mo atom from the amorphous MoOx -decorated CoSe2 (a-MoOx @CoSe2 ) pre-catalyst into Co-based oxyhydroxide (Mo-CoOOH) through an ultra-fast self-reconstruction process during the OER process. This conceptual advance has been validated by operando characterizations, which reveals that the initially rapid Mo leaching can expedite the dynamic reconstruction of pre-catalyst, and simultaneously trap Mo species in high oxidation state into the lattice of in situ generated CoOOH support. Impressively, the OER kinetics of CoOOH has been greatly accelerated after the reverse decoration of Mo species, in which the Mo-CoOOH affords a markedly decreased overpotential of 297 mV at the current density of 100 mA cm-2 . Density functional theory (DFT) calculations demonstrate that the Co species have been greatly activated via the effective electron coupling with Mo species in high oxidation state. These findings open new avenues toward directly synthesizing atomically dispersed electrocatalysts for high-efficiency water splitting.

Surface passivation for highly active, selective, stable, and scalable CO2 electroreduction

Electrochemical conversion of CO2 to formic acid using Bismuth catalysts is one the most promising pathways for industrialization. However, it is still difficult to achieve high formic acid production at wide voltage intervals and industrial current densities because the Bi catalysts are often poisoned by oxygenated species. Herein, we report a Bi3S2 nanowire-ascorbic acid hybrid catalyst that simultaneously improves formic acid selectivity, activity, and stability at high applied voltages. Specifically, a more than 95% faraday efficiency was achieved for the formate formation over a wide potential range above 1.0 V and at ampere-level current densities. The observed excellent catalytic performance was attributable to a unique reconstruction mechanism to form more defective sites while the ascorbic acid layer further stabilized the defective sites by trapping the poisoning hydroxyl groups. When used in an all-solid-state reactor system, the newly developed catalyst achieved efficient production of pure formic acid over 120 hours at 50 mA cm-2 (200 mA cell current).

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.

Iron-Locked Hydr(oxy)oxide Catalysts via Ion-Compensatory Reconstruction Boost Large-Current-Density Water Oxidation

Nickel-iron based hydr(oxy)oxides have been well recognized as one of the best oxygen-evolving catalysts in alkaline water electrolysis. A crucial problem, however, is that iron leakage during prolonged operation would lead to the oxygen evolution reaction (OER) deactivation over time, especially under large current densities. Here, the NiFe-based Prussian blue analogue (PBA) is designed as a structure-flexible precursor for navigating an electrochemical self-reconstruction (ECSR) with Fe cation compensation to fabricate a highly active hydr(oxy)oxide (NiFeOx Hy ) catalyst stabilized with NiFe synergic active sites. The generated NiFeOx Hy catalyst exhibits the low overpotentials of 302 and 313 mV required to afford large current densities of 500 and 1000 mA cm-2 , respectively. Moreover, its robust stability over 500 h at 500 mA cm-2 stands out among the NiFe-based OER catalysts reported previously. Various in/ex situ studies indicate that the Fe fixation by dynamic reconstruction process can reinforce the Fe-activated effect on the OER amenable to the industrial-level large current conditions against the Fe leakage. This work opens up a feasible strategy to design highly active and durable catalysts via thermodynamically self-adaptive reconstruction engineering.

Selenite-Decorated Polycrystalline NiO Nanosheets Generated from Cathodic Reconstruction for Electrocatalytic Hydrogen Production

Precatalyst reconstruction in alkaline hydrogen evolution reaction (HER) usually leads to changes in the morphology, composition, and structure, thus improving the catalytic activity, which recently receives intensive attention. However, the design strategies of cathodic reconstruction and the structural features of reconstruction products have not achieved a profound understanding. Here, from the point of thermodynamic stability, metastable nickel selenite dihydrate (NiSeO₃·2H₂O) is deliberately fabricated as a precatalyst to comprehensively study the reconstruction dynamics in alkaline HER. Multiple in/ex situ techniques capture the geometric, component, and phase evolutions, proving that NiSeO₃·2H₂O can be transformed into SeO₃^2--decorated polycrystalline NiO nanosheets with rich active sites and good conductivity under alkaline HER conditions, which act as a real catalytic active species. Density functional theory calculations demonstrate that the adsorption of SeO₃^2- can further promote the HER activity of NiO due to the optimized free energy of water activation and hydrogen adsorption. As a result, the SeO₃^2--NiO catalyst exhibits a low overpotential at -10 mA cm^-2 (90 mV) and long-term stability (>100 h). This work highlights the targeted design of precatalyst to trigger and utilize cathodic reconstruction and provides an available method for the development of adsorption-modulated efficient electrocatalysts.

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

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

High-entropy NiFeCoV disulfides for enhanced alkaline water/seawater electrolysis

Creating electrocatalysts with high activity and stability to meet the needs of highly effective seawater splitting is of great importance to achieve the goal of hydrogen production from abundant seawater source, which however is still challenging owing to sluggish oxygen evolution reaction (OER) dynamics and the existed competitive chloride evolution reaction. Herein, high-entropy (NiFeCoV)S₂ porous nanosheets are uniformly fabricated on Ni foam via a hydrothermal reaction process with a sequential sulfurization step for alkaline water/seawater electrolysis. The obtained rough and porous nanosheets provide large active surface area and exposed more active sites, which can facilitate mass transfer and are conducive to the improvement of the catalytic performance. Combined with the strong synergistic electron modulation effect of multi elements in (NiFeCoV)S₂, the as-fabricated catalyst exhibits low OER overpotentials of 220 and 299 mV at 100 mA cm^-2 in alkaline water and natural seawater, respectively. Besides, the catalyst can withstand a long-term durability test for more than 50 h without hypochlorite evolution, showing excellent corrosion resistance and OER selectivity. By employing the (NiFeCoV)S₂ as the electrocatalyst for both anode and cathode to construct an overall water/seawater splitting electrolyzer, the required cell voltages are only 1.69 and 1.77 V to reach 100 mA cm^-2 in alkaline water and natural seawater, respectively, showing a promising prospect towards the practical application for efficient water/seawater electrolysis.

Navigating surface reconstruction of spinel oxides for electrochemical water oxidation

Understanding and mastering the structural evolution of water oxidation electrocatalysts lays the foundation to finetune their catalytic activity. Herein, we demonstrate that surface reconstruction of spinel oxides originates from the metal-oxygen covalency polarity in the M_T-O-M_O backbone. A stronger M_O-O covalency relative to M_T-O covalency is found beneficial for a more thorough reconstruction towards oxyhydroxides. The structure-reconstruction relationship allows precise prediction of the reconstruction ability of spinel pre-catalysts, based on which the reconstruction degree towards the in situ generated oxyhydroxides can be controlled. The investigations of oxyhydroxides generated from spinel pre-catalysts with the same reconstruction ability provide guidelines to navigate the cation selection in spinel pre-catalysts design. This work reveals the fundamentals for manipulating the surface reconstruction of spinel pre-catalysts for water oxidation.

Alkaline Ni-Zn Microbattery Based on 3D Hierarchical Porous Ni Microcathode with High-Rate Performance

Miniaturized energy storage devices with superior performance and compatibility with facile fabrication are highly desired in smart microelectronics. Typical fabrication techniques are generally based on powder printing or active material deposition, which restrict the reaction rate due to the limited optimization of electron transport. Herein, we proposed a new strategy for the construction of high-rate Ni-Zn microbatteries based on a 3D hierarchical porous nickel (Ni) microcathode. With sufficient reaction sites from the hierarchical porous structure as well as excellent electrical conductivity from the superficial Ni-based activated layer, this Ni-based microcathode is featured with fast-reaction capability. By virtue of facile electrochemical treatment, the fabricated microcathode realized an excellent rate performance (over 90% capacity retention when the current density increased from 1 to 20 mA cm^-2). Furthermore, the assembled Ni-Zn microbattery achieved a rate current of up to 40 mA cm^-2 with a capacity retention of 76.9%. Additionally, the high reactivity of the Ni-Zn microbattery is also durable in 2000 cycles. This 3D hierarchical porous Ni microcathode, as well as the activation strategy, provides a facile route for the construction of microcathodes and enriches high-performance output units for integrated microelectronics.

2D Metal-Organic Frameworks as Competent Electrocatalysts for Water Splitting

Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal-organic frameworks (MOFs) with the physicochemical properties of 2D materials such as large surface area, tunable structure, accessible active sites, and enhanced conductivity, 2D MOFs have attracted intensive attention in the field of electrocatalysis. Different strategies, such as improving the conductivities of MOFs, reducing the thicknesses of MOF nanosheets, and integrating MOFs with conductive particles or substrates, are developed to promote the catalytic performances of pristine MOFs. This review summarizes the recent advances of pristine 2D MOF-based electrocatalysts for water electrolysis. In particular, their intrinsic electrocatalytic properties are detailly analyzed to reveal important roles of inherent MOF active centers, or other in situ generated active phases from MOFs responsible for the catalytic reactions. Finally, the challenges and development prospects of pristine 2D MOFs for the future applications in overall water splitting are discussed.

Crystalline-Amorphous Interface Coupling of Ni3S2/NiPx/NF with Enhanced Activity and Stability for Electrocatalytic Oxygen Evolution

The rational design of highly efficient and stable electrocatalysts for the oxygen evolution reaction (OER) is an urgent need but remains challenging for various sustainable energy systems. How to adjust the atomic structure and electronic structure of the active center is a key bottleneck problem. Accelerating the electron transfer process and the deep self-reconstruction of active sites could be a cost-effective strategy toward electrocatalytic OER catalyst development. Here, a crystalline-amorphous (c-a) coupled Ni₃S₂/NiP_x electrocatalyst self-supported on nickel foam with an intimate interface was developed via a feasible solvothermal-electrochemistry method. The coupling interface of the crystalline structure with high conductivity and amorphous structure with numerous potential active sites could regulate the electronic structure and optimize the adsorption/desorption of O-containing species, ultimately resulting in high OER catalytic performance. The obtained Ni₃S₂/NiP_x/NF presents a low OER overpotential of 265 mV to obtain 10 mA·cm^-2 and a small Tafel slope of 51.6 mV·dec^-1. Also, the catalyst with the coupled interface exhibited significantly enhanced long-term stability compared to the other two catalysts, with <5% decay in OER activity over 20 h of continuous operation, while that of Ni₃S₂/NF and NiP_x/NF decreased by about 30 and 50%, respectively. This study provides inspiration for other energy conversion reactions in optimizing the performance of catalysts by coupling crystalline-amorphous structures.