Adsorbed Hydroxyl Groups Research Articles

DFT Study of NiO@Graphene Composite for Electrochemical Oxidation of Urea in Alkaline Electrolyte

The continuous increase in the energy demand needs to pursuit a clean and renewable energy source because non-renewable energy sources such as traditional fossil fuels are limited and lead to global warming. It is necessary to bridge the gap between academia and industry with extensive research and their practical applications. Although hydrogen and oxygen evolution reactions (HER and OER respectively) have been considered as revolutionary fuel cell designs that utilize water-splitting technology, they suffer from efficiency drawbacks and can be realized in a limited pristine fresh water and with the use of noble metal catalysts. Urea oxidation reaction (UOR) is a fundamental step in fulfilling the need for practical green energy because they do not need high-voltage supply and also do not release both O2 and H2 gases simultaneously, encountered during water splitting. Furthermore, urea is an abundant component of human and animal waste, which can result in the production of problematic ammonia under normal degradation or standard hydrolysis practices. More importantly, the UOR process could provide an opportunity for waste disposal and green hydrogen production.In a typical UOR, urea in an alkali electrolyte is oxidized to the production of N2 and CO2 at the anode and H2 on the cathode from water electrolysis. This process is depicted in equations 1-3, respectively.Anode: CO(NH2)2 + 6OH- → N2 + 5H2O + CO2 + 6e- (1)Cathode: 6H2O + 6e- → 3H2 + 6OH- (2)Overall: CO(NH2)2 + H2O → N2 + H2 + CO2 (3)The UOR process is slow and inefficient under normal conditions due to the 6-electron transfer process from anode to cathode. Thus, it is necessary to modify the working electrode using a catalyst. Nickel-based materials are considered as one of the most promising groups of materials for catalysis in UOR owing to their low-cost, easy synthesis route, and abundant in nature. To improve the conductivity of Ni-based materials while maintaining the compounds’ catalytic performance, several effective strategies were applied, including the introduction of conductive support, element doping, high valence Ni-based materials, and defect engineering. Many of these effective strategies were developed to realize the commercial implementation of Ni-catalyst driven UOR. Nevertheless, the in-depth theoretical-fundamental understanding on the UOR were not studied due to its complicated multi-step gas adsorption and desorption.Moreover, one major theoretical drawback found by density functional theory calculation is the rate-limiting intermediate step of CO2 desorption during the reaction on the anode. In our previous work, we investigated the use of nano-NiO supported on eggshell membrane-derived carbon for a Ni-catalyzed UOR, and the periodic heterojunction model was selected to illustrate the impact of between the porous carbon and NiO nanoparticles. NiO also possesses many merits, including easy-to-obtain, low-cost, and exchangeable valence states. Meanwhile, graphene was employed as the alternative porous carbon for simulations. It is important to add theoretical simulation to understand the UOR process using different models, especially for the existence of single-atom model due to attractive findings. The in-depth mechanism of UOR the NiO@Graphene is still not clear. Consequently, it is useful to employ the single-atom model to investigate the role of NiO@Graphene in the UOR process.In this work, a single-atom model was built to understand the relationship of NiO@Graphene composite and its urea oxidation behavior. The single-atom model, which differs from the previous theoretical model, was applied to illustrate the influence of between graphene and NiO nanoparticles. Meanwhile, this work also served as an important theoretical supplement for the previous research. Prior to the investigation of UOR, the adsorption of hydroxyl group and urea on the NiO@Graphene was compared. Then, the adsorption of urea and CO2 on NiO and NiOOH with graphene was calculated and compared. The electron density difference map was also used to study the electron transfer of NiO@Graphene composite. Figure 1

Enhanced removal of Cu-EDTA in a three-dimensional electrolysis system with highly graphitic activated biochar produced via acidic and K2FeO4 treatment

Herein, we report a novel strategy for the fabrication of almond shell biochar-based materials containing highly dispersed graphene nanosheet structures by sequential treatment with hydrochloric acid and potassium ferrate. The resulting material serving as particle electrodes endowed the three-dimensional electrolysis system with excellent and stable Cu-ethylene diamine tetraacetic acid (Cu-EDTA) decomplexation and mineralization ability, along with Cu(Ⅱ), chemical oxygen demand, and dissolved total organic carbon removal efficiencies of 96.8%, 92.5%, and 86.2%, respectively. In contrast to excess Cu(Ⅱ) or EDTA, the complete complexation between Cu(Ⅱ) and EDTA facilitated EDTA degradation. During the reaction, the adsorption toward Cu(Ⅱ), rather than EDTA, in the complexing system, alleviated the burden of electrochemical oxidation for Cu-EDTA decomplexation. This, together with direct and indirect electrocatalytic oxidation mediated by adsorbed hydroxyl radicals on the particle electrode surface, contributed to the enhanced removal of Cu-EDTA. Density functional theory calculations further demonstrated the main types of active sites and the importance of the graphene nanosheet structure of the particle electrodes. This work provides different perspectives on the electrocatalytic removal of heavy metal complexes.

Lattice-Confined Ir Clusters on Pd Nanosheets with Charge Redistribution for the Hydrogen Oxidation Reaction under Alkaline Conditions.

Electrocatalysts with high activity and long-term stability for the hydrogen oxidation reaction (HOR) under alkaline conditions is still a major challenge for anion exchange membrane fuel cells (AEMFCs). Herein, a heterostructured Ir@Pd electrocatalyst with ultrasmall Ir nanoclusters (NCs) epitaxially confined on Pd nanosheets (NSs) for catalyzing the sluggish alkaline HOR is reported. Apparent charge redistribution occurs across the heterointerface, and both experimental and theoretical results suggest that the electrons transfer from Pd to Ir, which consequently greatly weakens the hydrogen binding on Pd. More interestingly, the interfacial epitaxy results in the formation of Ir/IrO2 Janus nanostructures, where the partially oxidized Ir species away from the interface further optimize the hydroxyl adsorption behavior. The unique Ir@Pd heterostructure eventually shows an optimal balance between hydrogen and hydroxyl adsorption, and hence exhibits impressive HOR activity with an exchange current density of up to 7.18mA cm-2 in 0.1 m KOH solution. In addition, the Ir@Pd electrocatalyst exhibits negligible activity degradation owing to the confinement effect of the unique epitaxial interface.

Exploring the activation pathway of photo-induced electrons in facets-dependent I− doped BiOCl nanosheets for PCPNa degradation

Electrons can degrade pentachlorphenate sodium (PCPNa) directly or activate molecular oxygen to produce and ·OH for its degradation. However, less work has been performed to control such two kinds of reaction pathway by modifying BiOCl. Herein, we firstly regulated the reaction pathway between electrons and PCPNa by adjusting the amount of surface oxygen vacancies (OVs) and surface adsorbed hydroxyl groups in I− doped BiOCl exposed with different facets. OVs on (001) facets-exposed I− doped BiOCl enabled large amount of PCPNa to adsorb on its surface and facilitated the direct reaction between electrons and PCPNa. In contrary, more surface adsorbed hydroxyl groups and oxygen on (010) facets-exposed I− doped BiOCl can retard the direct reaction between electrons and PCPNa via lowering the adsorption of PCPNa and increasing the activation of molecular oxygen by electrons. Although more and ·OH generated in I− doped (010)-facets exposed BiOCl, I− doped (001)-facets exposed BiOCl exhibited better photocatalytic activity. We proposed that the direct reaction between electrons and PCPNa can enhance the utilization efficiency of photogenerated electrons and improve photocatalytic degradation efficiency of PCPNa.

Two-dimensional hetero-nanostructured electrocatalyst of Ni/NiFe-layered double oxide for highly efficient hydrogen evolution reaction in alkaline medium

High-performance, cost-effective and stable electrocatalysts are critical for hydrogen evolution reaction (HER) via water splitting in alkaline media. Herein a unique hetero-nanostructured Ni/NiFe-layered double oxide (Ni/NiFe-LDO) on Ni foam (NF) has been successfully constructed through phase-transition and controlled in-situ reduction process. Because the two-dimensional (2D) morphology of NiFe-LDO nanosheets can stabilize uniform in-situ formed nano-Ni sites, the prepared Ni/NiFe-LDO on NF displays a high catalytic activity for HER in 1.0 M KOH solution, which requires an extremely low overpotential of 29 mV to afford a current density at 10 mA cm−2, achieves a Tafel slope of 82 mV dec-1 and maintains an excellent stability of electrolysis for at least 24 h. Detail characterizations reveal that the synergistic effect between nano-Ni and NiFe-LDO nanosheets contributes to the prominent HER activity. In NiFe-LDO, Fe2O3 facilitates the adsorption of H2O on the interface region of Ni-Fe2O3, which is beneficial for water activation, then NiO offers the active sites for hydroxyl adsorption (*OH), meanwhile nano-Ni sites are active to adsorb hydrogen intermediates (H*). All of the above accelerates the water dissociation in Volmer-step reaction.

Unveiling the Origin of Robust Ferroelectricity in Sub-2 nm Hafnium Zirconium Oxide Films.

HfO2-based ferroelectrics are highly expected to lead the new paradigm of nanoelectronic devices owing to their unexpected ability to enhance ferroelectricity in the ultimate thickness scaling limit (≤2 nm). However, an understanding of its physical origin remains uncertain because its direct microstructural and chemical characterization in such a thickness regime is extremely challenging. Herein, we solve the mystery for the continuous retention of high ferroelectricity in an ultrathin hafnium zirconium oxide (HZO) film (∼2 nm) by unveiling the evolution of microstructures and crystallographic orientations using a combination of state-of-the-art structural analysis techniques beyond analytical limits and theoretical approaches. We demonstrate that the enhancement of ferroelectricity in ultrathin HZO films originates from textured grains with a preferred orientation along an unusual out-of-plane direction of (112). In principle, (112)-oriented grains can exhibit 62% greater net polarization than the randomly oriented grains observed in thicker samples (>4 nm). Our first-principles calculations prove that the hydroxyl adsorption during the deposition process can significantly reduce the surface energy of (112)-oriented films, thereby stabilizing the high-index facet of (112). This work provides new insights into the ultimate scaling of HfO2-based ferroelectrics, which may facilitate the design of future extremely small-scale logic and memory devices.

Efficient visible light photocatalytic NO abatement over SrSn(OH)6 nanowires loaded with Ag/Ag2O cocatalyst

SrSn(OH)6 (SSOH) possesses a high oxidation potential in the valence band (VB), which is suitable for photocatalytic oxidation removal of pollutants. However, the electrons in the VB of these catalysts are difficult to transition to the conduction band (CB) under visible light, which makes it difficult to utilize sunlight effectively. In this work, Ag/Ag2O is loaded on the surface of SSOH nanowires, which stimulates the interfacial charge-transfer transition on SSOH. Compared with pure-phase SSOH, the NO abatement ratio of Ag/Ag2O-SSOH under visible light irradiation is increased to 45.10%. The e− in the VB of Ag2O are excited into the CB under visible light, and are further transferred to the Ag to react with O2 to produce superoxide radicals. The photo-excited e− in the VB of SSOH enter into the VB of Ag2O through interfacial charge-transfer transition to recombine with the photo-generated holes in the VB of Ag2O, thereby leaving photo-generated holes in the VB of SSOH. The holes in the VB of SSOH have sufficient oxidizing ability to oxidize the adsorbed hydroxyl groups into hydroxyl radicals. This work provides a new perspective for photocatalytic removal of pollutants by wide band gap photocatalyst under visible light.

Enhancing hydrogen oxidation electrocatalysis of nickel-based catalyst by simultaneous chemical anchoring and electronic structure regulation

Designing electrocatalysts for hydrogen oxidation reaction (HOR) in alkaline media is crucial but challenging. Among all available electrocatalysts, Ni-based materials are recognized as the most potential precious-metal-free electrocatalysts for HOR. However, they still suffer from serious problems including low activity and poor stability. In this work, a synergistic chemical anchoring and electronic structure regulation strategy is proposed to gain both high electrocatalytic activity and stability for Ni-based HOR catalyst. N-doped graphite nanoflakes (N-GFs) support with abundant N anchoring sites can stabilize the loading of Ni-based active species, giving rise to an excellent stability of the catalyst. Meanwhile, the rationally designed heterostructure in the active Ni3N/Ni can trigger the electron transfer across the heterointerface, which optimizes the binding energy of the reaction intermediates, resulting in an accelerated the Volmer reaction. Benefited from the rational design, Ni3N/Ni/N-GFs exhibits excellent mass activity (42.7 A gNi-1 at the overpotential of 50 mV) and stability (more than 24 h continuous operation). Moreover, the Ni3N/Ni heterostructure performs better in HOR electrocatalysis than individual Ni or Ni3N. These experimental results are rationalized by the theoretical simulations, which demonstrate that the heterostructure effectively weakens the hydrogen adsorption, optimizes the hydroxyl adsorption, and decreases the water formation reaction barrier.

Flow anodic oxidation: Towards high-efficiency removal of aqueous contaminants by adsorbed hydroxyl radicals at 1.5 V vs SHE

Electrochemical advanced oxidation processes (EAOPs) have emerged as a promising water treatment alternative but major breakthroughs are still needed in order for EAOPs to be competitive with traditional treatment technologies in terms of energy cost. Most existing studies have been conducted at high potentials to generate the powerful hydroxyl radical oxidant (aqueous •OH). While adsorbed hydroxyl radicals (OH*) may form at a much lower energy cost, their possible utilization is limited due to the poor mass transfer of this highly reactive species on solid electrodes. In this report, we describe a novel flow anode system using 4–16 μm Magnéli phase titanium suboxide particles as the anode material which enables the generation of a high steady state •OH concentration (5.4 × 10−12 mol m−2) at only 1.5 V (vs SHE) in a dilute electrolyte (5 mM KH2PO4). The energy cost of removal per order of selected water contaminants (tetracycline and orange II in this study) using the flow anode is 1.5-–6.7 Wh m−3, which is 1 – 4 orders of magnitude lower than that of existing techniques. The anode material used demonstrates great stability with the configuration readily scaled up. The results of this study provide new insight into a high efficiency, low cost water treatment technology for organic contaminant degradation.

Activity origin and alkalinity effect of electrocatalytic biomass oxidation on nickel nitride

Electro-oxidation of 5-hydroxymethylfurfural (HMFOR) is a promising green approach to realize the conversion of biomass into value-added chemicals. However, considering the complexity of the molecular structure of HMF, an in-depth understanding of the electrocatalytic behavior of HMFOR has rarely been investigated. Herein, the electrocatalytic mechanism of HMFOR on nickel nitride (Ni3N) is elucidated by operando X-ray absorption spectroscopy (XAS), in situ Raman, quasi in situ X-ray photoelectron spectroscopy (XPS), and operando electrochemical impedance spectroscopy (EIS), respectively. The activity origin is proved to be Ni2+δN(OH)ads generated by the adsorbed hydroxyl group. Moreover, HMFOR on Ni3N relates to a two-step reaction: Initially, the applied potential drives Ni atoms to lose electrons and adsorb OH− after 1.35 VRHE, giving rise to Ni2+δN(OH)ads with the electrophilic oxygen; then Ni2+δN(OH)ads seizes protons and electrons from HMF and leaves as H2O spontaneously. Furthermore, the high electrolyte alkalinity favors the HMFOR process due to the increased active species (Ni2+δN(OH)ads) and the enhanced adsorption of HMF on the Ni3N surface. This work could provide an in-depth understanding of the electrocatalytic mechanism of HMFOR on Ni3N and demonstrate the alkalinity effect of the electrolyte on the electrocatalytic performance of HMFOR.

Nickel-enhanced electrochemical activities of shape-tailored TiO2{001} nanocrystals for water treatment: A combined experimental and DFT studies

Understanding the reaction mechanism at the solid/liquid interface is necessary to improve the catalytic activity of electrode materials for water treatment. In this study, the photoelectrocatalytic degradation of rhodamine B (RhB) and phenol in water was studied using undoped and Ni-doped TiO2 with {001} facets (TiO2{001}). Under visible light irradiation, Ni-doped TiO2 electrodes exhibited higher contamination removal efficiency than undoped ones. Photoluminescence detection and density functional theory calculations revealed that the Ni-doped TiO2 provides a better hydroxyl radical yield and adsorption capacity than the undoped TiO2, which could be a major reason for the improvement of the catalytic activity of Ni-doped TiO2. The diffuse reflectance spectroscopy data showed that introducing the Ni2+ ions into TiO2 can shift the absorption edge towards the visible light region, which also contributes to the excellent photoelectrocatalytic performance. This work provides a strategy for improving the electrochemical performance of TiO2 and an in-depth understanding of the doping and catalytic mechanism of Ni2+ ions.

The Combined Role of Faceting and Heteroatom Doping for Hydrogen Evolution on a WC Electrocatalyst in Aqueous Solution: A Density Functional Theory Study

Tungsten carbide (WC) is an established model electrocatalyst for the hydrogen evolution reaction (HER) in aqueous solutions. In spite of extensive interest in and work on this material, a systematic atomistic understanding of the combined role of faceting and heteroatom doping for hydrogen evolution on WC electrocatalysts in aqueous solution is not available. To fill this knowledge gap, here we explore the interplay of these parameters for the electrocatalytic performance of WC by means of density functional theory (DFT) simulations. In our simulations, we explicitly account for solvent (water) effects and related electrochemical potential-dependent oxygen or hydroxyl (co)adsorption on the WC surfaces as determined by the analysis of surface Pourbaix diagrams. Among the several low-index WC surfaces screened, the DFT simulations indicate that the W-terminated (−100) and (111) surfaces of WC show comparable exchange currents of 3.908 and 0.133 mA/cm2, respectively. The simulations also suggest that V or N doping can substantially increase the HER activity of some WC surfaces to levels comparable to commercial Pt/C electrocatalysts, with current densities as large as 10.555, 1.730, and 0.636 mA/cm2 for the V-doped W-terminated (001), V-doped (101), and N-doped (101) WC surfaces, respectively. The improved HER activities are rationalized in terms of the d-band or p-band center, which are both shown to be related to the capability of tuning the hydrogen adsorption, and thence the HER performance. The calculated strong dependence of the HER activity on the presence of the electrochemical adsorption of oxygen and hydroxyls reiterates the importance of modeling realistic electrochemical conditions for WC pristine or doped surfaces. We expect the DFT results presented to provide timely and original guidelines for further theoretical and experimental studies of HER activity on WC-related electrocatalysts.

Alloying Nickel with Molybdenum Significantly Accelerates Alkaline Hydrogen Electrocatalysis

AbstractBifunctional hydrogen electrocatalysis (hydrogen‐oxidation and hydrogen‐evolution reactions) in alkaline solution is desirable but challenging. Among all available electrocatalysts, Ni‐based materials are the only non‐precious‐metal‐based candidates for alkaline hydrogen oxidation, but they generally suffer from low activity. Here, we demonstrate that properly alloying Ni with Mo could significantly promote its electrocatalytic performance. Ni4Mo alloy nanoparticles are prepared from the reduction of molybdate‐intercalated Ni(OH)2 nanosheets. The final product exhibits an apparent hydrogen‐oxidation activity exceeding that of the Pt benchmark and a record‐high mass‐specific kinetic current of 79 A g−1 at an overpotential of 50 mV. A superior hydrogen‐evolution performance is also measured in alkaline solution. These experimental data are rationalized by our theoretical simulations, which show that alloying Ni with Mo significantly weakens its hydrogen adsorption, improves the hydroxyl adsorption and decreases the reaction barrier for water formation.

Ab initio molecular dynamics study of adsorption of hydroxyl groups on graphene surface

Reduced graphene oxide is the precursor to produce graphene in a large scale; however, to date, there has been no consensus on the electronic structure of reduced graphene oxide. In this study, we carried out an ab initio molecular dynamics simulation to investigate the adsorption process of hydroxyl groups on graphene surface. During the adsorption process, the OH group needs to firstly pass through a physical adsorption complex with the OH above the bridge site of two carbon atoms, next to surmount a transition state, then to be adsorbed at the atop site of a carbon atom. With a 5×5 graphene surface, up to 6 hydroxyl groups can be adsorbed on the graphene surface, indicating the concentration coverage of the hydroxyl groups on graphene surface is about 12%. The simulation results show that the negative adsorption energy increases linearly as the number of adsorbed hydroxyl groups increases, and the band gap also increases linearly with the number of adsorbed hydroxyl groups.

Removal Mechanism Investigation of Ultraviolet Induced Nanoparticle Colloid Jet Machining.

Ultraviolet induced nanoparticle colloid jet machining is a new ultra-precision machining technology utilizing the reaction between nanoparticles and the surface of the workpiece to achieve sub-nanometer ultra-smooth surface manufacturing without damage. First-principles calculations based on the density functional theory (DFT) were carried out to study the atomic material removal mechanism of nanoparticle colloid jet machining and a series of impacting and polishing experiments were conducted to verify the mechanism. New chemical bonds of Ti-O-Si were generated through the chemical adsorption between the surface adsorbed hydroxyl groups of the TiO2 cluster and the Si surface with the adsorption energy of at least −4.360 eV. The two Si-Si back bonds were broken preferentially and the Si atom was removed in the separation process of TiO2 cluster from the Si surface realizing the atomic material removal. A layer of adsorbed TiO2 nanoparticles was detected on the Si surface after 3 min of fixed-point injection of an ultraviolet induced nanoparticle colloid jet. X-ray photoelectron spectroscopy results indicated that Ti-O-Si bonds were formed between TiO2 nanoparticles and Si surface corresponding to the calculation result. An ultra-smooth Si workpiece with a roughness of Rq 0.791 nm was obtained by ultraviolet induced nanoparticle colloid jet machining.

Optimization of Boron-Doped Diamond Thin-Layer Cell for Chemical Oxygen Demand Sensor Based on Anodic Oxidation of Organics

The advanced electrochemical oxidation process (AEOPs) has been considered as an appealing approach to overcome problems associated with the conventional water treatment methods. The use of expensive and toxic chemical oxidizing reagents in chemical oxygen demand, for example, can lead to environmental concerns of the production of secondary pollution. Anodic oxidation, one of the most popular AEOPs, offers a rapid and straightforward COD determination by removing the need for chemical oxidants. The process mainly relies on the anode material to degrade the organic molecules; thus, choosing the appropriate material is one of the keys to a successful electrochemical oxidation.Numerous electrochemical applications utilize boron-doped diamond as the anode material due to its unique properties such as wide potential window, low background current, and resistance to fouling. The employment of BDD for anodic oxidation of organics enables complete oxidation of organics via physically adsorbed hydroxyl radicals (●OH). The species offers a high standard redox potential of E° (●OH/H2O) = 2.80 V/SHE, high reactivity, and short lifetime, makes it possible to be used for the on-site electrochemical oxidation process.The objective of this study is to develop and optimize an electrochemical system for chemical oxygen demand determination based on a photoelectrochemical degradation principle. The degradation of phenols takes place in a specially designed BDD thin-layer electrode, and the amount of electrons transferred at the electrode can be measured to define the equivalent COD value. A 60μm thick films are used as a spacer, resulting in a contact area of 3 cm2. At 2.54 V vs. SHE, a sample containing 50 mg/L analytes required less than 3 mins to degrade, and the assay time for organics decay increases for a higher concentration of phenols. Incorporating the BDD electrode with cathode materials, such as Pt-coated indium tin oxide (ITO), will be employed as an attempt to improve the degradation efficiency by minimizing the voltage drop between the electrodes. Sandwiching the BDD-cathode using Nafion® membrane will be studied to remove the need for additional supporting electrolytes. The major experimental conditions of the anodic oxidation of organic molecules by BDD, such as applied potential bias, supporting electrolyte concentration, pH, and dissolved oxygen concentration, will be varied to obtain the optimum conditions.

PH-Controlled Growth of Ni-Fe Layered Double Hydroxide Electro-Catalyst for Water Oxidation with Exceptional Operational Stability

Ni-Fe layered double hydroxide (LDH) is a potential OER electro-catalyst in water electrolysis due to its layered open structure, rapid diffusion of reactant, good anion exchange and excellent hydroxyl adsorption. Its OER activity has been well reported at constant applied voltage. However, the water electrolyser utilizes renewable energy due to which it suffers irregular power supply that consequently affects its electrode activity. Therefore, the development of a stable Ni-Fe LDH under dynamic operation conditions is required. Herein, we performed pH-controlled growth of NiFe LDH (1:1) on Fe substrate in nickel sulfate solution by oxygen sparging. The prepared LDH requires a low OER overpotential of ~270 mV vs. Hg/HgO at 200 mA/cm2 in 1M KOH solution. Considering the potentiodynamic polarization curve of Ni-Fe LDH, its OER region is above 0.5 V (vs. Hg/HgO) and shutdown region is below 0.3 V (vs. Hg/HgO). The voltage fluctuation between both regions may result in phase change and structural distortion. Therefore, three stability regimes are important to consider for an actual operational stability test; upper OER regime I (constant 0.75 V (vs. Hg/HgO)), one-minute lower to upper OER regime II (0.5 to 0.75 V (vs. Hg/HgO)) and one-minute on-off regime III (0.3 to 0.75 V (vs. Hg/HgO)). The electrochemical characterization before and after each 12 hours regime showed no degradation and steady kinetics (38 mV/dec). The morphological and surface analysis using SEM, XRD, TEM and XPS confirmed that the LDH structure remained preserve after each regime.Fig. 1: Operational stability test of Ni-Fe LDH prepared by pH-controlled growth under three regimes and embedded IR-corrected polarization curve after each regime. Figure 1

Unravelling the role of interfacial chemistry evolution in the kinetics of zinc insertion into chevrel phase

The insertion kinetics of the multivalent cations into an intercalation host is conventionally understood primarily focused on the ion migration and charge redistribution properties through the bulk solid framework. Here we show that the intercalation kinetics of zinc ions into a Mo6S8 Chevrel phase is critically dependent on the interfacial chemistry evolution associated with changes in the electrolyte pH and the formation of zinc hydroxide sulfate (ZHS) during the early cycles. The sluggish initial insertion process with a hydroxyl surface groups on Mo6S8 causes a concurrent intercalation of zinc ions into the inner and outer ring vacancy sites, leading to the temporary coexistence of three reaction phases. On the contrary, characteristic two-step, two-phase reactions associated with the sequential occupation of each vacancy site are observed after the electrolyte pH change induces ZHS deposit in the interface, which eliminates hydroxyl adsorption layer on the surface, and promotes the intercalation kinetics of zinc ions. This study provides valuable insight into insertion of multivalent cations into Chevrel phase, which often exhibits a cryptic kinetic dormancy during the initial cation intercalation stages.

Atomically dispersed Ni–Ru–P interface sites for high-efficiency pH-universal electrocatalysis of hydrogen evolution

Designing catalysts with specific active sites is a crucial yet challenging task for high-efficiency electrocatalysis of hydrogen evolution reaction (HER). In this paper, we report the construction of atomically dispersed Ni–Ru–P interface sites (ISs) on Ru single-atomic sites doped nickel phosphide nanoparticles (Ru SAs–Ni 2 P NPs) with high-efficiency HER performances in a wide pH range. Especially, the as-constructed atomically dispersed Ni–Ru–P ISs on 2.20 wt% Ru SAs–Ni 2 P showed a highly active for catalyzing HER with high turnover frequencies at low overpotential, featuring a high mass activity as well as a superior stability under alkaline conditions. Based on the operando X-ray absorption spectroscopy experiments indicated that the Ni–Ru–P ISs plays an active role in the process of HER and participates in the catalytic process. Density functional theory calculations exhibited that the strong interaction in the atomically dispersed Ni–Ru–P ISs results in an optimized the hydrogen adsorption energy and an elevated hydroxyl adsorption energy, both of which help improve the HER performances. Notably, this work provides a new perspective for the design of high-efficiency HER electrocatalysts by constructing atomically dispersed ISs. In this paper, the atomically dispersed Ni–Ru–P interface sites (ISs) on Ru single-atomic sites doped nickel phosphide (Ru SAs–Ni 2 P) are successfully constructed, which display high-efficiency HER performances in a wide pH range. Especially, the as-constructed atomically dispersed Ni–Ru–P ISs on 2.20 wt% Ru SAs–Ni 2 P show highly active for catalyzing HER under alkaline conditions. The operando X-ray absorption spectroscopy experiments and density functional theory calculations indicate that the Ni–Ru–P ISs plays an active role in the process of HER and participates in the catalytic process. Thereby, this work provides a new perspective for the design of high-efficiency HER electrocatalysts by constructing atomically dispersed ISs. • Ru single-atomic sites doped nickel phosphide (Ru SAs–Ni 2 P) catalysts are prepared. • Atomically dispersed Ni–Ru–P interfacial sites (ISs) are constructed in Ru SAs–Ni 2 P. • Ni–Ru–P ISs can be used as high-efficiency pH-universal electrocatalysts for HER. • Operando XAS experiments reveal that the Ni–Ru–P ISs plays an active role for HER. • DFT indicate that Ni–Ru–P ISs are favorable for adsorption of H + and H 2 O molecules.

Tuning Lewis acidity of iron-based metal-organic frameworks for enhanced catalytic ozonation

Lewis acid sites that originate from ligand missing defects inspire diverse important applications of metal-organic frameworks (MOFs), such as heterogeneous catalysis. However, the facile regulation of Lewis acidity that promises good properties remains a challenge. Herein, we report a simple strategy to tune the Lewis acidity of iron-based MOFs. By substituting Fe centers with Ce atoms in the node (a.k.a., Ce doping), the catalytic ozonation performance of developed iron-based MOFs was highly improved. We found that both the total Lewis acidity and the salicylic acid (SA) degradation rate of Ce-doping MIL-88A(Fe) linearly correlated to the suitable Ce substitution ratio. Notably, the degradation rate and TOC removal of MIL-88A(Fe0.80Ce0.20) was about 3 and 2 times higher than those of parent MIL-88A(Fe), respectively. Reactive oxygen species (ROS), including surface adsorbed hydroxyl radicals (OHads), superoxide radicals (O2−), and singlet oxygen (1O2) were responsible for SA destruction. MIL-88A(Fe0.80Ce0.20) with enlarged Lewis acidity substantially facilitated the generation of OHads and 1O2, leading to a more efficient SA degradation and mineralization. Moreover, Ce doping engineering also ameliorated the catalytic performances of MIL-53(Fe) and MIL-101(Fe), indicating that the tuning strategy we proposed is accessible and universal. This work may provide deep insights into highly efficient MOFs explorations for water purification.