Zinc cadmium sulphide-based photoreforming of biomass-based monosaccharides to lactic acid and efficient hydrogen production

Approaches that add value to biomass through the use of photoreforming reactions offer great opportunities for the efficient use of renewable resources. Here, we constructed a novel zinc cadmium sulphide/molybdenum dioxide-molybdenum carbide-carbon (ZnxCd1-xS-y/MoO2-Mo2C-C) heterojunction which was applied to photoreforming of biomass-based monosaccharides for hydrogen and lactic acid production. Bandgap engineering effectively modulated the redox capacity of ZnxCd1-xS-y and exposed more (101) crystalline surfaces, which improved the lactic acid selectivity. The MoO2-Mo2C-C (MC) co-catalysts had unique microstructures that increased the light absorption range and the number of active sites of ZnxCd1-xS-y. These features effectively promoted the separation and migration of photogenerated carriers, which in turn enhanced the photoreforming activity. The optimised Zn0.4Cd0.6S-0/MC composites exhibited superior photocatalytic activity with a hydrogen yield of 12.2 mmol/g/h. Conversion of biomass-based monosaccharides was approximately 100 %, where xylose had the greatest lactic acid selectivity (64.1 %). Active species, including h+, ⋅O2-, ⋅OH, and 1O2, all favoured lactic acid production, where ⋅O2- played a major role in the conversion. This study demonstrates that rational design of photocatalysts can achieve the selective conversion of biomass into high value-added chemicals as well as the generation of clean energy.

High-Nuclearity Polyoxometalate-Based Metal-Organic Frameworks for Photocatalytic Oxidative Cleavage of C-C Bond

High-nuclearity polyoxometalate (POM) clusters are attractive building blocks (BBs) for the synthesis of metal-organic frameworks (MOFs) due to their high connectivity and inherently multiple metal centers as functional sites. This work demonstrates a strategy of step-wise growth on ring-shaped [P8W48O184]40- precursor, which produced two new high-nuclearity polyoxotungstates, a half-closed [H16P8W58O218]32- {W58} and a fully-closed [H16P8W64O236]32- {W64}. By in situ synthesis, unique MOFs of copper triazole-benzoic acid (HL) complexes incorporating the negatively-charged {W58} and {W64} as nodes, {Cu11(HL)9W58} HNPOMOF-1 and {Cu9(HL)9W64} HNPOMOF-2, were constructed by delicately tuning the reaction conditions, mainly solution pH, which controls the formation of {W58} and {W64}, and at the same time the protonation of triazole-benzoic acid ligand thus its coordination mode to copper ion that creates the highest nuclearity POM-derived MOFs reported to date. HNPOMOF-1 features 3D framework possessing cage-like cavities filled with exposed carboxyl groups, while the inherent 2D layer-like HNPOMOF-2 allows for facile exfoliation into ultrathin nanosheets, and the resulted HNPOMOF-2NS exhibits superior activity towards photocatalytic oxidative cleavage of C-C bond for a series of lignin models. This work not only provides a strategy to build high-nuclearity POM cluster-based frameworks, but also demonstrates their great potential as functional materials for green catalysis.

FeN4S1 Single-Atom Sites Anchored on Three-Dimensional Porous Carbon for Highly Efficient and Durable Oxygen Electrocatalysis

Precisely designing asymmetric active centers and exploring their electronic regulation effects to prepare efficient bifunctional single-atom catalysts (SACs) is important for boosting the practical applications of zinc-air batteries (ZABs). Herein, an effective strategy has been developed by introducing an axial S atom to the FeN4 active center, simultaneously assisted by pyrolyzing the graphene oxide (GO) sheathed zeolitic-imidazolate framework-8 (ZIF8) composite and constructing a three-dimensional (3D) porous framework with abundant FeN4S1 moieties. This structure can accelerate the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics owing to the modulated electronic redistribution and d-band center with a reduced energy barrier. The optimal S-Fe-NC/rGO showcases a lower voltage gap (ΔE) of 0.64 V between both the ORR and OER half-wave potentials at 10 mA cm-2, highlighting the excellent bifunctional activities. The assembled S-Fe-NC/rGO rechargeable liquid ZABs deliver a power density of 154.05 mW·cm-2 and a desirable durability of >900 h. More importantly, the corresponding flexible solid-state ZABs exhibit considerable foldability.

Synthesis and Characterization of MOF-Derived Structures: Recent Advances and Future Perspectives

Due to their facile tunability, metal-organic frameworks (MOFs) are employed as precursors and templates to construct advanced functional materials with unique and desired chemical, physical, mechanical, and morphological properties. By tuning MOF precursor composition and manipulating conversion processes, various MOF-derived materials commonly known as MOF derivatives can be constructed. The possibility of controlled and predictable properties makes MOF derivatives a preferred choice for numerous advanced technological applications. The innovative synthetic designs besides the plethora of interdisciplinary characterization approaches applicable to MOF derivatives provide the opportunity to perform a myriad of experiments to explore the performance and offer key insight to develop the next generation of advanced materials. Though there are many published works of literature describing various synthesis and characterization techniques of MOF derivatives, it is still not clear how the synthesis mechanism works and what are the best techniques to characterize these materials to probe their properties accurately. In this review, the recent development in synthesis techniques and mechanisms for a variety of MOF derivates such as MOF-derived metal oxides, porous carbon, composites/hybrids, and sulfides is summarized. Furthermore, the details of characterization techniques and fundamental working principles are summarized to probe the structural, mechanical, physiochemical, electrochemical, and electronic properties of MOF and MOF derivatives. The future trends and some remaining challenges in the synthesis and characterization of MOF derivatives are also discussed.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Nanoengineered, Pd-doped Co@C nanoparticles as an effective electrocatalyst for OER in alkaline seawater electrolysis

Water electrolysis is considered one of the major sources of green hydrogen as the fuel of the future. However, due to limited freshwater resources, more interest has been geared toward seawater electrolysis for hydrogen production. The development of effective and selective electrocatalysts from earth-abundant elements for oxygen evolution reaction (OER) as the bottleneck for seawater electrolysis is highly desirable. This work introduces novel Pd-doped Co nanoparticles encapsulated in graphite carbon shell electrode (Pd-doped CoNPs@C shell) as a highly active OER electrocatalyst towards alkaline seawater oxidation, which outperforms the state-of-the-art catalyst, RuO2. Significantly, Pd-doped CoNPs@C shell electrode exhibiting low OER overpotential of ≈213, ≈372, and ≈ 429 mV at 10, 50, and 100 mA/cm2, respectively together with a small Tafel slope of ≈ 120 mV/dec than pure Co@C and Pd@C electrode in alkaline seawater media. The high catalytic activity at the aforementioned current density reveals decent selectivity, thus obviating the evolution of chloride reaction (CER), i.e., ∼490 mV, as competitive to the OER. Results indicated that Pd-doped Co nanoparticles encapsulated in graphite carbon shell (Pd-doped CoNPs@C electrode) could be a very promising candidate for seawater electrolysis.

Accelerate charge separation in Cu2O/MoO2 photocathode for photoelectrocatalytic hydrogen evolution

Photoelectrocatalyzing water reduction is a potential approach to building a green and sustainable society. As a benchmark photocathode, Cu2O receives much attention but faces serious charge recombination and photocorrosion. This work prepared an excellent Cu2O/MoO2 photocathode via in situ electrodeposition. A systematical study of theory and experiment demonstrates that MoO2 not only effectively passivates the surface state of Cu2O as well as accelerates reaction kinetics as a cocatalyst, but also promotes the directional migration and separation of photogenerated charge. As expected, the constructed photocathode exhibits a highly enhanced photocurrent density and an appealing energy transformation efficacy. Importantly, MoO2 can inhibit the reduction of Cu+ in Cu2O via a formed internal electric field and shows excellent photoelectrochemical stability. These findings pave the way to designing a high-activity photocathode with high stability.

Development of polyoxometalate-based Ag-H2biim inorganic-organic hybrid compounds functionalized for the acid electrocatalytic hydrogen evolution reaction

The electrocatalytic hydrogen evolution reaction (HER) is an ideal method for hydrogen production. Transition metal complex electrocatalysts exhibit poor HER activity due to excessive or weak adsorption of H during the electrochemical reduction of water to molecular hydrogen in acidic environments. Developing specific functional complex materials as desired catalysts is challenging. Here, an electrochemical surface restructuring strategy of polyoxometalate (POM)-modified Ag materials toward the HER with a dramatically decreased overpotential under acidic aqueous conditions is established. We prepared two POM [SiW12O40]4- (SiW12)/[P2W18O62]6- (P2W18)-based Ag-2,2'-biimidazole (H2biim) inorganic-organic hybrid compounds (1 and 2) via the hydrothermal method and these two compounds undergo an electrochemical restructuring process in 0.5 M H2SO4 during the HER, in which Ag nanoparticles are in situ formed with the basic structures of SiW12 and P2W18 being maintained. The activated catalysts (1-AC-RDE and 2-AC-RDE) exhibit good electrocatalytic activity for the HER with good long-term stability, and the required overpotentials at a current density of 10 mA cm-2 are 112 mV (1-AC-RDE) and 91 mV (2-AC-RDE) with Tafel slopes of 77 mV dec-1 and 65 mV dec-1, respectively. The excellent electron-proton storage and transferability of SiW12 and P2W18 may provide a solution for the insufficient capture of H by Ag, leading to an effective self-optimizing behavior and superior acidic HER activity.

MOF-Based Chemiresistive Gas Sensors: Toward New Functionalities

The sensing performances of gas sensors must be improved and diversified to enhance quality of life by ensuring health, safety, and convenience. Metal-organic frameworks (MOFs), which exhibit an extremely high surface area, abundant porosity, and unique surface chemistry, provide a promising framework for facilitating gas-sensor innovations. Enhanced understanding of conduction mechanisms of MOFs has facilitated their use as gas-sensing materials, and various types of MOFs have been developed by examining the compositional and morphological dependences and implementing catalyst incorporation and light activation. Owing to their inherent separation and absorption properties and catalytic activity, MOFs are applied as molecular sieves, absorptive filtering layers, and heterogeneous catalysts. In addition, oxide- or carbon-based sensing materials with complex structures or catalytic composites can be derived by the appropriate post-treatment of MOFs. This review discusses the effective techniques to design optimal MOFs, in terms of computational screening and synthesis methods. Moreover, the mechanisms through which the distinctive functionalities of MOFs as sensing materials, heterostructures, and derivatives can be incorporated in gas-sensor applications are presented.

Electrochemical activated molybdenum oxides based multiphase heterostructures with high hydrogen evolution activity in alkaline condition

Electrochemical activation is an effective method for synthesizing economically feasible heterogeneous hydrogen evolution reaction (HER) electrocatalysts. Herein, we first synthesized MoO2-Co2Mo3O8precatalyst, which was electrochemically activated to produce K2Mo3O10within the original phase to form the heterogeneous structure. The electrochemically activated samples demonstrate exceptional HER activity in alkaline medium, which exhibit a low overpotential of 31 mV at current density of 10 mA cm-2(135 mV at 100 mA cm-2), as well as a small Tafel slope of 34 mV dec-1. This is due to the creation of multiphase heterostructures that prompt interfacial interactions and accelerate charge transfer. Simultaneously, the creation of additional active sites increases their intrinsic activities. The combined effects collectively enhance the HER performance. The application of this method in the preparation of HER catalysts is still relatively unexplored, thus rendering our work a pioneering contribution to the field.

Atomic coordination structural dynamic evolution of single-atom Mo catalyst for promoting H2 activation in slurry phase hydrocracking

Development of efficient catalysts with high atomic utilization and turnover frequency (TOF) for H₂ activation in slurry phase hydrocracking (SPHC) is crucial for the conversion of vacuum residue (VR). Herein, for the first time, we reported a robust and stable single atoms (SAs) Mo catalyst through a polymerization-pyrolysis-in situ sulfurization strategy for activating H₂ in SPHC of VR. An interesting atomic coordination structural dynamic evolution of Mo active sites was discovered. During hydrocracking of VR, the O atoms that coordinated with Mo were gradually replaced by S atoms, which led to the O/S exchange process. The coordination structure of the Mo SAs changed from pre-reaction Mo-O₃S₁ to post-reaction Mo-O₁S₃ coordination configurations, promoting the efficient homolytic cleavage activation of H₂ into H radical species effectively. The evolved Mo SAs catalyst exhibited robust catalytic hydrogenation activity with the per pass conversion of VR of 65 wt%, product yield of liquid oils of 93 wt%, coke content of only 0.63 wt%, TOF calculated for total metals up to 0.35 s^-1, and good cyclic stability. Theoretical calculation reveals that the significant variation of occupied Mo 4d states before and after H₂ interaction has a direct bearing on the dynamic evolution of Mo SAs catalyst structure. The lower d-band center of Mo-O₁S₃ site indicates that atomic H diffusion is easy, which is conducive to catalytic hydrogenation. The finding of this study is of great significance to the development of high atom economy catalysts for the industrial application of heavy oil upgrading technology.

Achieving High Single-Pass Carbon Conversion Efficiencies in Durable CO2 Electroreduction in Strong Acids via Electrode Structure Engineering

Acidic CO₂ reduction (CO₂ R) holds promise for the synthesis of low-carbon-footprint chemicals using renewable electricity. However, the corrosion of catalysts in strong acids causes severe hydrogen evolution and rapid deterioration of CO₂ R performance. Here, by coating catalysts with an electrically nonconductive nanoporous SiC-Nafion^TM layer, a near-neutral pH was stabilized on catalyst surfaces, thereby protecting the catalysts against corrosion for durable CO₂ R in strong acids. Electrode microstructures played a critical role in regulating ion diffusion and stabilizing electrohydrodynamic flows near catalyst surfaces. This surface-coating strategy was applied to three catalysts, SnBi, Ag, and Cu, and they exhibited high activity over extended CO₂ R operation in strong acids. Using a stratified SiC-Nafion^TM /SnBi/polytetrafluoroethylene (PTFE) electrode, constant production of formic acid was achieved with a single-pass carbon efficiency of >75 % and Faradaic efficiency of >90 % at 100 mA cm^-2 over 125 h at pH 1.

Mott Schottky CoSx-MoOx@NF heterojunctions electrode for H2 production and urea-rich wastewater purification

The sluggish kinetics of oxygen evolution reaction (OER) is the bottleneck of alkaline water electrolysis. The urea oxidation reaction (UOR) with much faster kinetics was to replace OER. To further promote UOR, a heterojunction structure assembled of CoS_x and MoO_x was established, and then its superior catalytic activity was predicted by DFT calculation. After that, an ultra-thin CoS_x-MoO_x@nickel foam (CoS_x-MoO_x@NF) electrode with a Mott-Schottky structure was prepared via a facile hydrothermal method, followed by a low-temperature vulcanization. Results highlighted CoS_x-MoO_x@NF electrode presented a superior performance toward UOR, OER, and H₂ evolution reaction (HER). Notably, it exhibited excellent electrocatalytic performance for OER (1.32 V vs. RHE, 10 mA cm^-2), UOR (1.305 V vs. RHE, 10 mA cm^-2), and urea-assisted overall water splitting with a low voltage (1.38 V, 10 mA cm^-2) when CoS_x-MoO_x@NF electrode served as both anode and cathode. It is promising to use CoS_x-MoO_x@NF in an electrochemical system integrated with H₂ generation and urea-rich wastewater purification.

Layer-by-Layer Construction of a Nanoarchitecture by Polyoxometalates and Polymers: Enhanced Electrochemical Hydrogen Evolution Reaction

In this contribution, a nanoarchitectural approach was employed to produce a nanolayer of polyoxometalate (POM) on the surface of a glassy carbon electrode (GCE) to achieve a higher surface area with higher electrocatalytic activity toward the electrochemical hydrogen evolution reaction (HER). To accomplish this, the well-known layer-by-layer (LbL) technique was employed, which involved the alternate adsorption of the POM, Na_0.3[N(C₄H₉)₄]_7.7 [(Mo₃O₈)₄(O₃PC(O)(C₃H₆NH₂CH₂C₄H₃S)PO₃)₄], abbreviated as [(TBA)Mo₁₂(AleThio)₄], and polyethyleneimine (PEI) polymer. This nanolayered electrode exhibited catalytic properties toward the HER in 0.5 M H₂SO₄ with the resulting polarization curves indicating an increase in the HER activity with the increasing number of POM layers, and the overpotential required for this reaction was lowered by 0.83 V when compared with a bare GCE. The eighth PEI/[(TBA)Mo₁₂(AleThio)₄] bilayer exhibited a significantly lower HER overpotential of -0.077 V at a current density of 10 mA cm^-2. Surface characterization of the LbL-assembled nanolayers was carried out using X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy. We believe that the synergetic effect of the positively charged PEI polymer and the catalytically active molybdate POM is the cause for the successful response to the electrochemical HER.

Atomically Interfacial Engineering on Molybdenum Nitride Quantum Dots Decorated N-doped Graphene for High-Rate and Stable Alkaline Hydrogen Production

The development of low-cost, high-efficiency, and stable electrocatalysts for hydrogen evolution reaction (HER) under alkaline conditions is a key challenge in water electrolysis. Here, an interfacial engineering strategy that is capable of simultaneously regulating nanoscale structure, electronic structure, and interfacial structure of Mo₂ N quantum dots decorated on conductive N-doped graphene via codoping single-atom Al and O (denoted as AlO@Mo₂ N-NrGO) is reported. The conversion of Anderson polyoxometalates anion cluster ([AlMo₆ O₂₄ H₆ ]^3- , denoted as AlMo6) to Mo₂ N quantum dots not only result in the generation of more exposed active sites but also in situ codoping atomically dispersed Al and O, that can fine-tune the electronic structure of Mo₂ N. It is also identified that the surface reconstruction of AlOH hydrates in AlO@Mo₂ N quantum dots plays an essential role in enhancing hydrophilicity and lowering the energy barriers for water dissociation and hydrogen desorption, resulting in a remarkable alkaline HER performance, even better than the commercial 20% Pt/C. Moreover, the strong interfacial interaction (MoN bonds) between AlO@Mo₂ N and N-doped graphene can significantly improve electron transfer efficiency and interfacial stability. As a result, outstanding stability over 300 h at a current density higher than 100 mA cm^-2 is achieved, demonstrating great potential for the practical application of this catalyst.

Self-supported molybdenum nickel oxide catalytic electrode designed via molecular cluster-mediated electroplating and electrochemical activation for an efficient and durable oxygen evolution reaction

Efficient and durable nonprecious catalysts for the oxygen evolution reaction (OER) are crucial for practical water electrolysis for hydrogen production. A self-supported OER catalytic electrode with sufficient exposure of the catalyst and tight anchoring onto the current collector is vital for the catalytic activity and stability, and is therefore deemed to be a preferable tactic to enhance water electrolysis performance. Herein, a polyoxometalate (POM) molecular cluster-mediated electroplating and activation tactics are proposed to design a self-supported molybdenum nickel oxide (MoNiO_x) catalytic electrode for the OER. The MoNiO_x active layer can anchor tightly onto the Ni foam current collector with sufficient surface exposure and high structural stability, therefore enabling high alkali OER catalytic efficiency (222 mV at 10 mA cm^-2) and robust durability (only slight decay in catalytic efficiency upon 12 days of chronopotentiometry (V-t) test). Moreover, the easily processable electroplating and active protocol can serve as a general approach to prepare other OER catalytic electrodes by altering the reactants and current collectors. The current work paves a facile and universal way to design a highly active and durable molybdenum (Mo) based hybrid catalytic electrode for OER via molecular cluster-assisted electroplating and activation treatment.

Surface Design Strategy of Catalysts for Water Electrolysis

Hydrogen, a new energy carrier that can replace traditional fossil fuels, is seen as one of the most promising clean energy sources. The use of renewable electricity to drive hydrogen production has very broad prospects for addressing energy and environmental problems. Therefore, many researchers favor electrolytic water due to its green and low-cost advantages. The electrolytic water reaction comprises the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Understanding the OER and HER mechanisms in acidic and alkaline processes contributes to further studying the design of surface regulation of electrolytic water catalysts. The OER and HER catalysts are mainly reviewed for defects, doping, alloying, surface reconstruction, crystal surface structure, and heterostructures. Besides, recent catalysts for overall water splitting are also reviewed. Finally, this review paves the way to the rational design and synthesis of new materials for highly efficient electrocatalysis.

Defect engineering tuning electron structure of biphasic tungsten-based chalcogenide heterostructure improves its catalytic activity for hydrogen evolution and triiodide reduction

The design and exploration of high-efficiency and low-cost electrode catalysts are of great significance to the development of novel energy conversion technologies. In this work, metal and nonmetal heteroatoms co-doped biphasic tungsten-based chalcogenide heterostructured catalyst (Co-WS₂/P-WO_2.9) with rich defects is successfully synthesized by a vulcanization technique. The electrocatalytic performance of WS₂/WO₃ in the hydrogen evolution reaction (HER) and triiodide reduction reaction is significantly enhanced by modifying and optimizing its electronic structure through a defect engineering strategy. As an electrocatalyst for HER, the optimized Co-WS₂/P-WO_2.9 exhibits a low overpotential at 10 mA cm^-2 of 146 and 120 mV with small Tafel slopes of 86 and 74 mV dec^-1 in alkaline and acidic electrolyte, respectively. In addition, a Co-WS₂/P-WO_2.9 assembled solar cell yields a short circuit current density of 15.85 mA cm^-2, an open-circuit voltage of 0.74 V, a fill factor of 0.66, and a competitive power conversion efficiency (7.83%), which is comparable or higher than conventional Pt-based solar cell (16.02 mA cm^-2, 0.70 V, 0.63, 7.14%). The formation of a heterostructure in Co-WS₂/P-WO_2.9 leads to the presence of a built-in electric field in the interfacial region between Co-WS₂ and P-WO_2.9, which leads to an increased open-circuit voltage from 0.70 V for Pt to 0.74 V for Co-WS₂/P-WO_2.9. This work can provide a technical support for developing high-performance heterostructured catalysts, which open up a way for improving catalytic performance of heterostructured catalysts in the field of electrocatalysis.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Subnanometric Cu clusters on atomically Fe-doped MoO2 for furfural upgrading to aviation biofuels

Single cluster catalysts (SCCs) are considered as versatile boosters in heterogeneous catalysis due to their modifiable single cluster sites and supports. In this work, we report subnanometric Cu clusters dispersed on Fe-doped MoO₂ support for biomass-derived furfural upgrading. Systematical characterizations suggest uniform Cu clusters (composing four Cu atoms in average) are homogeneously immobilized on the atomically Fe-doped ultrafine MoO₂ nanocrystals (Cu₄/Fe_0.3Mo_0.7O₂@C). The atomic doping of Fe into MoO₂ leads to significantly modified electronic structure and consequently charge redistribution inside the supported Cu clusters. The as-prepared Cu₄/Fe_0.3Mo_0.7O₂@C shows superior catalytic performance in the oxidative coupling of furfural with C₃~C₁₀ primary/secondary alcohols to produce C₈~C₁₅ aldehydes/ketones (aviation biofuel intermediates), outperforming the conventionally prepared counterparts. DFT calculations and control experiments are further carried out to interpret the structural and compositional merits of Cu₄/Fe_0.3Mo_0.7O₂@C in the oxidative coupling reaction, and elucidate the reaction pathway and related intermediates.

Bifunctional catalysts of Ni nanoparticle coupled MoO2 nanorods for overall water splitting

The development of active and cost-effective bifunctional catalysts is crucial for water dissociation through electrolysis. In this study, bifunctional catalysts with Ni nanoparticles (NPs) anchored on MoO₂ nanorods have been synthesized via in situ dissolution of NiMoO₄-ZIF under an inert atmosphere without using hydrogen gas. The Ni-MoO₂ catalyst exhibits high electrocatalytic activity by modulating the calcination temperature. Benefitingfrom the MOF transformation and accompanying Ni particles' outward diffusion, a precisely designed interface heterostructure between Ni and MoO₂ was constructed. As a result, the optimized Ni-MoO₂ catalyst achieves extremely low overpotentials of only 24 mV and 275 mV at 10 mA cm^-2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. Furthermore, the catalyst required a small cell voltage of 1.55 V to deliver a current density of 10 mA cm^-2 and remained stable over 20 h for overall water splitting. The proposed MOF-derived heterojunction protocol provides a general approach for designing and fabricating transition metal oxide catalysts for water electrolysis.

Polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts for energy conversion in aqueous systems

The research progress on polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts in sustainable and clean energy conversion applications in aqueous systems is summarized.

MOF-derived ruthenium-doped amorphous molybdenum dioxide hybrid for highly efficient hydrogen evolution reaction in alkaline media

Ruthenium-doped amorphous molybdenum dioxide coupled with a reduced graphene oxide hybrid (Ru-MoO₂@PC/rGO) is synthesized using polyoxometalate-based MOFs/GO as a precursor. Benefitting from the synergistic effect of numerous exposed active sites, Ru dopants and the introduction of GO, the designed catalyst shows exceptional electrocatalytic performance toward the HER in alkaline media.

Smart Designs of Mo Based Electrocatalysts for Hydrogen Evolution Reaction

As a sustainable and clean energy source, hydrogen can be generated by electrolytic water splitting (i.e., a hydrogen evolution reaction, HER). Compared with conventional noble metal catalysts (e.g., Pt), Mo based materials have been deemed as a promising alternative, with a relatively low cost and comparable catalytic performances. In this review, we demonstrate a comprehensive summary of various Mo based materials, such as MoO2, MoS2 and Mo2C. Moreover, state of the art designs of the catalyst structures are presented, to improve the activity and stability for hydrogen evolution, including Mo based carbon composites, heteroatom doping and heterostructure construction. The structure–performance relationships relating to the number of active sites, electron/ion conductivity, H/H2O binding and activation energy, as well as hydrophilicity, are discussed in depth. Finally, conclusive remarks and future works are proposed.

Strategies for Perfect Confinement of POM@MOF and Its Applications in Producing Defect-Rich Electrocatalyst

Polyoxometalate encapsulated by metal-organic framework (POM@MOF) hybrid is an ideal precursor for electrocatalysts because it provides a homogeneous catalyst system with a flexible synergistic effect. However, controlling its fine structures to activate the derived catalyst after pyrolysis remains a challenge. The uniformly distributed carbon-defect tungsten oxides on the 3D carbon matrix were synthesized by using phospho-tungstic acid confined in HKUST-1 (PA@HKUST-1) as the precursor. By choosing ethanol as the solvent, the framework of PA@HKUST-1 was retained integrally and each pore of HKUST-1 kept intact even with holding the PA molecule inside. The well-organized structure of PA@HKUST-1 facilitated to offer a highly distributed metal source, and further transform into defect-rich catalyst. The average size of the resulting WO_xC_y/C catalysts was 1.5 nm, which was similar to a single molecule of PA. The WO_xC_y/C catalysts exhibited superior activity in the methanol oxidation reaction after being platinized due to the enhanced resistance to CO poisoning, which is also confirmed by DFT. This finding suggests a new route to design and achieve the defect-rich catalyst with controllable size.

Catalyst overcoating engineering towards high-performance electrocatalysis

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Templating Synthesis of Metal-Organic Framework Nanofiber Aerogels and Their Derived Hollow Porous Carbon Nanofibers for Energy Storage and Conversion

A kind of metal-organic framework (MOF) aerogels are synthesized by the self-assembly of uniform and monodisperse MOF nanofibers. Such MOF nanofiber aerogels as carbon precursors can effectively avoid the aggregation of nanofibers during calcination, resulting in the formation of well-dispersed hollow porous carbon nanofibers (HPCNs). Moreover, HPCNs with well-dispersion are investigated as sulfur host materials for Li-S batteries and electrocatalysts for cathode oxygen reduction reaction (ORR). On the one hand, HPCNs act as hosts for the encapsulation of sulfur into their hierarchical micro- and mesopores as well as hollow nanostructures. The obtained sulfur cathode exhibits excellent electrochemical features, good cycling stability and high coulombic efficiency. On the other hand, HPCNs exhibit better electrocatalytic activity than aggregated counterparts for ORR. Furthermore, a highly active single atom electrocatalyst can be prepared by the carbonization of bimetallic MOF nanofiber aerogels. The results indicate that well-dispersed HPCNs show enhanced electrochemical properties in contrast to their aggregated counterparts, suggesting that the dispersion situation of nanomaterials significantly influence their final performance. The present concept of employing MOF nanofiber aerogels as precursors will provide a new strategy to the design of MOF-derived nanomaterials with well-dispersion for their applications in energy storage and conversion.

Realizing the Excellent HER Performance of Pt3Pb2S2 by d-Orbital Electronic Modulation

Exploring new excellent electrocatalysts for the hydrogen evolution reaction (HER) is of significance for the development of hydrogen energy. Herein, a ternary chalcogenide (Pt₃Pb₂S₂) is successfully designed and synthesized using layered PtS₂ as a matrix. The energy level of the Pt 5d orbital is upshifted to the Fermi surface after replacing S atoms by Pb atoms, which results in the high conductivity of Pt₃Pb₂S₂. In addition, the low-coordinated Pt atoms inserted in the voids of [Pt₂Pb₂S₂] layers have a lower free energy of H* adsorption than do metallic Pt atoms, which endows Pt₃Pb₂S₂ with excellent HER performance. The overpotential and Tafel slope of Pt₃Pb₂S₂ toward HER activity are measured to be 43 mV at 10 mA cm^-2 and 43 mV dec^-1, respectively. More importantly, Pt₃Pb₂S₂ shows high intrinsic HER catalytic activity and long-term stability. This work provides a promising strategy for designing novel excellent transition-metal chalcogenide electrocatalysts.

A Copper-Containing Polyoxometalate-Based Metal-Organic Framework as an Efficient Catalyst for Selective Catalytic Oxidation of Alkylbenzenes

A copper-containing polyoxometalate-based metal-organic framework (POMOF), Cu^I₁₂Cl₂(trz)₈[HPW₁₂O₄₀] (HENU-7, HENU = Henan University; trz = 1,2,4-triazole), has been successfully synthesized and well-characterized. In addition, the excellent catalytic ability of HENU-7 has been proved by the selective oxidation of diphenylmethane. Under the optimal conditions, the diphenylmethane conversion obtained over HENU-7 is 96%, while the selectivity to benzophenone is 99%, which outperforms most noble-metal-free POM-based catalysts. Moreover, HENU-7 is stable to reuse for five runs without an obvious loss in activity and also can catalyze the oxidation of different benzylic C-H with satisfactory conversions and selectivities, which implied the significant catalytic activity and recyclability.

Recent progress in metal-organic framework/graphene-derived materials for energy storage and conversion: design, preparation, and application

Graphene or chemically modified graphene, because of its high specific surface area and abundant functional groups, provides an ideal template for the controllable growth of metal-organic framework (MOF) particles. The nanocomposite assembled from graphene and MOFs can effectively overcome the limitations of low stability and poor conductivity of MOFs, greatly widening their application in the field of electrochemistry. Furthermore, it can also be utilized as a versatile precursor due to the tunable structure and composition for various derivatives with sophisticated structures, showing their unique advantages and great potential in many applications, especially energy storage and conversion. Therefore, the related studies have been becoming a hot research topic and have achieved great progress. This review summarizes comprehensively the latest methods of synthesizing MOFs/graphene and their derivatives, and their application in energy storage and conversion with a detailed analysis of the structure-property relationship. Additionally, the current challenges and opportunities in this field will be discussed with an outlook also provided.

Conversion of bimetallic MOF to Ru-doped Cu electrocatalysts for efficient hydrogen evolution in alkaline media

The rational design and construction of inexpensive and highly active electrocatalysts for hydrogen evolution reaction (HER) is of great importance for water splitting. Herein, we develop a facile approach for preparation of porous carbon-confined Ru-doped Cu nanoparticles (denoted as Ru-Cu@C) by direct pyrolysis of the Ru-exchanged Cu-BTC metal-organic framework. When served as the electrocatalyst for HER, strikingly, the obtained Ru-Cu@C catalyst exhibits an ultralow overpotential (only 20 mV at 10 mA cm^-2) with a small Tafel slope of 37 mV dec^-1 in alkaline electrolyte. The excellent performance is comparable or even superior to that of commercial Pt/C catalyst. Density functional theory (DFT) calculations confirm that introducing Ru atoms into Cu nanocrystals can significantly alter the desorption of H₂ to achieve a close-to-zero hydrogen adsorption energy and thereby boost the HER process. This strategy gives a fresh impetus to explore low-cost and high-performance catalysts for HER in alkaline media.

Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules

The energy deficiency and environmental problems have motivated researchers to develop energy conversion systems into a sustainable pathway, and the development of catalysts holds the center of the research endeavors. Natural catalysts such as metalloenzymes have maintained energy cycles on Earth, thus proving themselves the optimal catalysts. In the previous research results, the structural and functional analogs of enzymes and nano-sized electrocatalysts have shown promising activities in energy conversion reactions. Mo ion plays essential roles in natural and artificial catalysts, and the unique electrochemical properties render its versatile utilization as an electrocatalyst. In this review paper, we show the current understandings of the Mo-enzyme active sites and the recent advances in the synthesis of Mo-catalysts aiming for high-performing catalysts.

Towards thermoneutral hydrogen evolution reaction using noble metal free molybdenum ditelluride/graphene nanocomposites

The development of efficient electrocatalysts for hydrogen generation is an essential task to meet future energy demand. In recent years, molybdenum ditelluride (MoTe₂) has triggered incredible research interests due to intrinsic nontrivial band gap with promising semi-metallic behaviors. In this work, 2D MoTe₂ nanosheets have been synthesized uniformly on graphene substrate through ultra-fast microwave-initiated approach, that shows a superior hydrogen evolution in acidic medium with low overpotential (~150 mV), low activation energy (8.4362 ± 1.5413 kJ mol^-1), along with a Tafel slope of 94.5 mV/decade. Interestingly, MoTe₂/graphene exhibits the enhanced electrocatalytic stability during the long cycling test, resulting an increase in specific surface area of catalyst materials. Moreover, the results from periodic plane-wave density functional theory (DFT) indicate that, the best active sites are the corner of a Mo-atom and a critical bifunctional site comprised of adjacent Mo and Te edge atoms. Furthermore, the corresponding volcano plot reveals the near thermoneutral catalytic activity of MoTe₂/graphene for hydrogen generation.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

POM-based MOFs simultaneously possessing the virtues of POMs and MOFs exhibit excellent heterogeneous catalytic properties.

α-MoC1−x nanorods as an efficient hydrogen evolution reaction electrocatalyst

Benefiting from their more exposed active sites, α-MoC_1−x nanorods exhibited significantly enhanced HER performance under both acidic and alkaline conditions.

Theoretical Study on Improving the Catalytic Activity of a Tungsten Carbide Surface for Hydrogen Evolution by Nonmetallic Doping

Tungsten carbide (WC) has received widespread attention as a new type of nonprecious metal catalyst for hydrogen evolution reaction (HER). However, it is still a challenge to improve the surface HER catalytic activity. In this work, the effects of different nonmetallic dopants on the catalytic activity and stabilities of WC (0001) surface for HER were studied by first principles methods. The effects of different types of non-metal (NM = B; N; O; P and S) and doping concentrations (ni = 25–100%) on HER catalytic activity and stability were investigated by calculating the Gibbs free energy of hydrogen adsorption (∆GH) and substitution energy. It was found that the catalytic performance can be improved by doping O and P non-metallic elements. Especially, the ∆GH with P doped is −0.04eV better than Pt (−0.085 eV), which is a potential ideal catalyst for HER. Furthermore, the electronic structure analysis was used to explore the origin of the regulation of doping on stability and catalytic activity. The results show that nonmetallic doping is an effective strategy to control the catalytic activity, which provides theoretical support for the future research of HER catalysts.

Dual Stimuli-Responsive Copper Nanoparticles Decorated SBA-15: A Highly Efficient Catalyst for the Oxidation of Alcohols in Water

In the present work, a temperature and pH-responsive hybrid catalytic system using copolymer-capped mesoporous silica particles with metal nanoparticles is proposed. The poly(2-(dimethylamino)ethyl methacrylate)(DMAEMA)-co-N-tert-butyl acrylamide) (TBA)) shell on mesoporous silica SBA-15 was obtained through free radical polymerization. Then, copper nanoparticles (CuNPs) decorated SBA-15/copolymer hybrid materials were synthesized using the NaBH₄ reduction method. SBA-15 was functionalized with trimethoxylsilylpropyl methacrylate (TMSPM) and named TSBA. It was found that the CuNPs were uniformly dispersed in the mesoporous channels of SBA-15, and the hybrid catalyst exhibited excellent catalytic performance for the selective oxidation of different substituted benzyl alcohols in water using H₂O₂ as an oxidant at room temperature. The dual (temperature and pH-) responsive behaviors of the CuNPs/p(DMAEMA-co-TBA)/TSBA catalyst were investigated using the dynamic light scattering technique. The conversion of catalytic products and selectivity were calculated using gas chromatographic techniques, whereas the molecular structure of the products was identified using ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The catalyst showed excellent catalytic activity toward the oxidation of alcohol to aldehyde in an aqueous medium below the lower critical solution temperature (LCST) and pKa values (7–7.5) of the copolymer. The main advantages of the hybrid catalyst, as compared to the existing catalysts, are outstanding alcohol conversion (up to 99%) for a short reaction time (1 h), small amount of the catalyst (5 mg), and good recyclability equal to at least five times.