Efficient Electrooxidation of 5-Hydroxymethylfurfural Using Co-Doped Ni3 S2 Catalyst: Promising for H2 Production under Industrial-Level Current Density

Anionic regulation Fe/NiOOH electrocatalysts to boost electrooxidation performance of biomass derived 5-hydroxymethylfurfural

Nickel-based catalysts show great potential as promising candidates for the electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF). However, the limited adsorption capacity of nickel-based catalysts for OH- and HMF limits their further development. In this study, amorphous Fe/NiOOH-SOx was generated from nanosheets (Fe/NiOOH-Ni3S2), which was pre-fabricated on nickel foam via pre-reconstruction and anionic regulation strategy. The optimized catalyst Fe/NiOOH-Ni3S2 demonstrated exceptional activity in the HMF oxidation reaction (HMFOR) with a current density of 10 mA cm-2 at 1.32 V vs RHE, accompanying with the 98.9 % HMF conversion, 97.8 % 2,5-Furandicarboxylic acid (FDCA) selectivity, 96.8 % Faraday efficiency, and stability for ten cycles. The incorporating amorphous FeOOH reduces the electron density around Ni, promoting the formation of high-valent Ni spices. Meanwhile, the SOx combined with amorphous hydroxy nickel oxide provides unsaturated sites, which enhances the adsorption capacity of HMF. Density functional theory (DFT) computations reveal that the designed amorphous Fe/NiOOH and surface-adsorbed SOx collectively modulate the electronic structure of the catalyst, causing an upwards shift of the NiOOH d-band center and enhancing adsorption capacities for both HMF and OH-. This study proposes the adsorption enhancement mechanism of regulating electronic structure and offers a rational strategy for HMFOR electrocatalysts.

Selective electrooxidation of 5-hydroxymethylfurfural to value-added 2,5-furanodiformic acid: mechanism, electrolyzer system, and electrocatalyst regulation

Value-added chemical products derived from biomass have attracted wide attention in addressing global warming and fossil fuel pollution. Among them, 2,5-furanodiformic acid (FDCA), the oxidized product of 5-hydroxymethylfurfural (HMF), is an effective substitute for terylene acid extracted from petroleum to synthesize biodegradable plastics. Electrochemical oxidation is an environmentally friendly, mild reaction condition, high-efficiency process for converting HMF to FDCA. However, the electrooxidation of HMF involves six-electron transfer, normally leading to the formation of many by-products. Thus, there is still a need to construct highly selective catalysts for HMF electrooxidation to FDCA. In this review, first we have investigated the mechanism of HMF electrooxidation and summarized the electrolytic cells and product analysis methods for electrooxidation of HMF to FDCA. The factors influencing HMF electrooxidation to FDCA are also discussed. Then, the electronic structure regulation methods of various electrocatalysts including heteroatom doping, heterostructure construction, interfacial engineering, and defect engineering are systematically summarized for the highly selective electrooxidation of HMF to FDCA. Finally, future challenges and prospects are proposed for further deep understanding. It is expected that this review could provide new guidance for large-scale electrooxidation of HMF to FDCA in industry.

Electrochemical Hydrogen Production Coupling with the Upgrading of Organic and Inorganic Chemicals

Electrocatalytic water splitting powered by renewable energy is a green and sustainable method for producing high-purity H2. However, in conventional water electrolysis, the anodic oxygen evolution reaction (OER) involves a four-electron transfer process with inherently sluggish kinetics, which severely limits the overall efficiency of water splitting. Recently, replacing OER with thermodynamically favorable oxidation reactions, coupled with the hydrogen evolution reaction, has garnered significant attention and achieved remarkable progress. This strategy not only offers a promising route for energy-saving H₂ production but also enables the simultaneous synthesis of high-value-added products or the removal of pollutants at the anode. Researchers successfully demonstrate the upgrading of numerous organic and inorganic alternatives through this approach. In this review, the latest advances in the coupling of electrocatalytic H2 production and the upgrading of organic and inorganic alternative chemicals are summarized. What's more, the optimization strategy of catalysts, structure-performance relationship, and catalytic mechanism of various reactions are well discussed in each part. Finally, the current challenges and future prospects in this field are outlined, aiming to inspire further innovative breakthroughs in this exciting area of research.

Advances and Prospects of Selective Electrocatalytic Upgrading of 5-Hydroxymethylfurfural to Furan-2,5-Dicarboxylic Acid

The electrocatalytic upgrading of 5-hydroxymethylfurfural (HMF, 5-(Hydroxymethyl) furan-2-carbaldehyde) has emerged as a renewable and environmentally friendly means for the production of high-value chemicals, with the oxidation product furan-2,5-dicarboxylic acid (FDCA, 2,5-furandicarboxylic acid) possessing economic viability in substituting terephthalic acid in polymer synthesis. This article reviews the recent advancements in the selective electrocatalytic upgrading of HMF to FDCA, including the reaction pathways, mechanisms, as well as activity descriptors of HMF electrocatalytic oxidation reaction (HMFOR), alongside advanced operando characterization techniques. Subsequently, the representative HMFOR catalysts, encompassing noble metal, non-noble metal, transition metal-based catalysts and metal free-based catalysts are presented. Then strategies for regulating HMFOR activity and longevity were introduced, followed by an exploration of the future prospects for the development of HMFOR catalysts.

Highly Ordered Conductive Metal-Organic Frameworks with Chemically Confined Polyoxometalate Clusters: A Dual-Functional Electrocatalyst for Efficient H2O2 Synthesis and Biomass Valorization

The design of bifunctional and high-performance electrocatalysts that can be used as both cathodes and anodes for the two-electron oxygen reduction reaction (2e- ORR) and biomass valorization is attracting increasing attention. Herein, a conserved ligand replacement strategy is developed for the synthesis of highly ordered conductive metal-organic frameworks (Ni-HITP, HITP = 2, 3, 6, 7, 10, 11-hexaiminotriphenylene) with chemically confined phosphotungstic acid (PW12) nanoclusters in the nanopores. The newly formed Ni-O-W bonds in the resultant Ni-HITP/PW12 electrocatalysts modulate the electronic structures of both Ni and W sites, which are favorable for cathodic 2e- ORR to H2O2 production and anodic 5-hydroxymethylfurfural oxidation reaction (HMFOR) to 2, 5-furandicarboxylic acid (FDCA), respectively. In combination with the deliberately retained conductive frameworks and ordered pores, the dual-functional Ni-HITP/PW12 composites enable a H2O2 production rate of 9.51 mol gcat -1 h-1 and an FDCA yield of 96.8% at a current density of 100 mA cm-2/cell voltage of 1.38 V in an integrated 2e- ORR/HMFOR system, significantly improved than the traditional 2e- ORR/oxygen evolution reaction system. This work has provided new insights into the rational design of advanced electrocatalysts and electrocatalytic systems for the green synthesis of valuable chemicals.

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

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

Morphology-Guided Phase-Controlled Synthesis of Nickel Selenide Nanocrystals through Cation Exchange Reactions

The precise control of the crystal phase during the synthesis of nickel selenide (NixSey) nanocrystals is crucial, as crystal structure and composition significantly influence their reactivity, growth kinetics, and properties. The cation exchange (CE) method provides a versatile and robust approach for synthesizing nanomaterials, enabling precise control over phase, composition, and morphology. However, the application of this method for phase-controlled synthesis of NixSey nanocrystals has received limited research attention. Here, we present a morphology-guided CE method for the synthesis of spinel Ni3Se4 nanoparticles (NPs) and rhombic Ni3Se2 nanorods (NRs), wherein berzelianite Cu2-xSe NPs and NRs are employed as sacrificial templates for CE with Ni2+. This phase-controlled behavior, which is guided by morphology and dependent on the stacking length of the close-packed facets, relies on the rearrangement of the Se2- sublattice accompanied by CE, providing a unique and precise approach to controlling phase during nanocrystal synthesis. Additionally, the obtained Ni3Se4 NPs and Ni3Se2 NRs exhibit structure-dependent catalytic activities in the oxygen evolution reaction.

Bifunctional MoNi4/Nickel Foam Electro-Catalyst for Ultra-Efficient Oxidation of High-Concentration 5-Hydroxymethylfurfural and HER

The electro-catalytic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) provides an attractive route to produce 2, 5-furandicarboxylic acid (FDCA) as a substitute for terephthalic acid used in the plastics industry. Herein, we prepared MoNi4 alloy on nickel foam (NF) using a simple hydrothermal method followed by hydrogen reduction. Applied MoNi4/NF as the bifunctional electrodes for the electro-catalytic HMF oxidation reaction (HMFOR) and HER, 98.7 % FDCA yield and 97.3 % Faraday efficiency (FE) can be achieved even with HMF concentration as high as 200 mM. Notably, no obvious deactivation was observed after ten consecutive cycles. In-situ Raman, XANES and EXAFS results show that the nickel species of MoNi4/NF is first oxidized to Ni3+ species under the applied voltage, and after undergoing the electro-catalytic HMFOR, then reduced to Ni2-δ state (with a valence between 0 and +2) due to the electron-donating effect from Mo. MoNi4/NF with more than one electron transfer between Ni3+ and Ni2-δ during the HMFOR enables it to have excellent electro-catalytic oxidation ability toward HMFOR.

Electrocatalytic Biomass Oxidation via Acid-Induced In Situ Surface Reconstruction of Multivalent State Coexistence in Metal Foams

Electrocatalytic biomass conversion offers a sustainable route for producing organic chemicals, with electrode design being critical to determining reaction rate and selectivity. Herein, a prediction-synthesis-validation approach is developed to obtain electrodes for precise biomass conversion, where the coexistence of multiple metal valence states leads to excellent electrocatalytic performance due to the activated redox cycle. This promising integrated foam electrode is developed via acid-induced surface reconstruction to in situ generate highly active metal (oxy)hydroxide or oxide (MOxHy or MOx) species on inert foam electrodes, facilitating the electrooxidation of 5-hydroxymethylfurfural (5-HMF) to 2,5-furandicarboxylic acid (FDCA). Taking nickel foam electrode as an example, the resulting NiOxHy/Ni catalyst, featuring the coexistence of multivalent states of Ni, exhibits remarkable activity and stability with a FDCA yields over 95% and a Faradaic efficiency of 99%. In situ Raman spectroscopy and theoretical analysis reveal an Ni(OH)2/NiOOH-mediated indirect pathway, with the chemical oxidation of 5-HMF as the rate-limiting step. Furthermore, this in situ surface reconstruction approach can be extended to various metal foams (Fe, Cu, FeNi, and NiMo), offering a mild, scalable, and cost-effective method for preparing potent foam catalysts. This approach promotes a circular economy by enabling more efficient biomass conversion processes, providing a versatile and impactful tool in the field of sustainable catalysis.

Enhancing the Kinetics of Glucose Electro-Oxidation by Modulating the Binding Energy of Hydroxyl on Cobalt-Based Catalysts

Replacing the sluggish anodic water oxidation reaction with the glucose oxidation reaction (GOR) offers an energy-saving strategy to obtain value-added products during the hydrogen production process. However, rational design of the GOR electrocatalyst with an explicit structure-property relationship remains a challenge. In this study, by using cobalt chalcogenides as model catalysts, we performed an in-depth study of the GOR catalytic mechanism of CoS and CoSe nanosheets. Experimental and theoretical results revealed that the reaction pathway on cobalt chalcogenides strongly depends on their binding energy to hydroxyl (OHBE). For CoS with a weak OHBE, the reaction proceeds through an "electrophilic oxygen" route. While for CoSe, due to the strong OHBE, a surface reconstruction occurs before the GOR and therefore follows the "electrochemical-chemical" route. Inspired by these findings, a customized strategy was proposed to regulate the OHBE of the catalysts, which involved introducing F atoms into CoS to enhance its OHBE, and weakening the OHBE of CoSe by doping with Zn atoms. The optimized F-doped CoS and Zn-doped CoSe catalysts both exhibited significantly improved performance for GOR. This study thus provides a verifiable paradigm for improving the GOR performance via a customized strategy and sheds light on the design of novel catalysts in the future.

Electronic Promoter Breaks the Linear Scaling Relationship: Ultra-Rapid High-Temperature Synthesis of Heterostructured CoS/SnO2@C as a Bifunctional Oxygen Catalyst for Li-O2 Batteries

Li-O2 batteries urgently needs high discharge capacity and stable cycling performance, requiring effective and reliable bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, Hovenia acerba Lindl-like heterostructure composed of cobalt sulfide and tin dioxide supported on carbon substrate (CoS/SnO2@C) is prepared via CO2 laser irradiation technology. The half-wave potential of CoS/SnO2@C for the ORR is 0.88 V, while the overpotential of the OER at 10 mA cm-2 is as low as 270 mV. The Li-O2 batteries employing the bifunctional CoS/SnO2@C catalyst displays a high discharge specific capacity of 3332.25 mAh g-1 and long cycling life of 226 cycles. Additionally, theory calculations demonstrate that the construction of heterostructure decreases energy barrier of the rate-determining step (RDS) for both ORR and OER. Notably, SnO2 behaves as the electronic promoter to optimize the electronic structure of heterostructure interface and triggers charge redistribution of CoS, which weakens the adsorption strength of the *O-intermediates and allows to break the linear scaling relationship, thus further enhancing the catalytic performance of CoS/SnO2@C. This research furnishes directions for the design of heterogeneous catalysts, highlighting its great potential for application in rechargeable Li-O2 batteries.

Regulating Intermediate Adsorption and Promoting Charge Transfer of CoCr-LDHs by Ce Doping for Enhancing Electrooxidation of 5-Hydroxymethylfurfural

Electrochemical oxidation of 5-hydroxymethylfurfural (HMFOR) to generate high-value chemicals under mild conditions acts as an energy-saving and sustainable strategy. However, it is still challenging to develop electrocatalysts with high efficiency and good durability. Here, nickel foam (NF) supported CoCrCe(7.5%)-LDH (layered double hydroxides) by doping Ce into CoCr-LDH show high 5-hydroxymethylfurfural (HMF) conversion (99%), 2,5-furandicarboxylic acid (FDCA) yield (99%), and Faraday efficiency (100%) at 1.4 VRHE. The CoCrCe(7.5%)-LDH also exhibits remarkable stability with 97% conversion of HMF after 10 cycles. The X-ray absorption near-edge spectroscopy (XANES) and theoretical calculation show that Ce doping into CoCr-LDH is beneficial to the formation of high-valance Co and significantly facilitates the electron transfer, regulates the adsorption behavior of intermediates, reduces the Gibbs free energy barrier and accelerates the reaction rate. This work promotes the use of rare earth elements as electrocatalysts to promote the oxidation of HMF.

Recent Advances and Perspectives on Coupled Water Electrolysis for Energy-Saving Hydrogen Production

Overall water splitting (OWS) to produce hydrogen has attracted large attention in recent years due to its ecological-friendliness and sustainability. However, the efficiency of OWS has been forced by the sluggish kinetics of the four-electron oxygen evolution reaction (OER). The replacement of OER by alternative electrooxidation of small molecules with more thermodynamically favorable potentials may fundamentally break the limitation and achieve hydrogen production with low energy consumption, which may also be accompanied by the production of more value-added chemicals than oxygen or by electrochemical degradation of pollutants. This review critically assesses the latest discoveries in the coupled electrooxidation of various small molecules with OWS, including alcohols, aldehydes, amides, urea, hydrazine, etc. Emphasis is placed on the corresponding electrocatalyst design and related reaction mechanisms (e.g., dual hydrogenation and N-N bond breaking of hydrazine and C═N bond regulation in urea splitting to inhibit hazardous NCO- and NO- productions, etc.), along with emerging alternative electrooxidation reactions (electrooxidation of tetrazoles, furazans, iodide, quinolines, ascorbic acid, sterol, trimethylamine, etc.). Some new decoupled electrolysis and self-powered systems are also discussed in detail. Finally, the potential challenges and prospects of coupled water electrolysis systems are highlighted to aid future research directions.

Balancing Competitive Adsorption on Co3O4@P, N-Doped Porous Carbon to Enhance the Electrocatalytic Upgrading of Biomass Derivatives

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) represents an environmentally friendly approach to generate high-value-added chemicals from biomass. The successful electrochemical transformation of HMF during the oxidation reaction (HMFOR) necessitates an ideal adsorption interaction between HMF and OH- on the electrode surface. Yet, catalysts with a singular active site offer limited flexibility in managing the competitive adsorption of HMF and OH-. To this end, different active sites are customized in this work to construct a P and N co-doped porous carbon that wrapped Co3O4 (Co3O4@PNC). Co-doping with these two heteroatoms generates C3P = O and pyrrolic N as adsorption sites to better balance the adsorption of HMF and OH-, respectively, rather than promoting competition between the HMF and OH- on a single active site. With this design strategy, Co3O4@PNC demonstrates significant HMFOR activity, the conversion rate of HMF surpassed 99% with a 2,5-furandicarboxylic acid (FDCA) yield exceeding 95% after 2 h of electrolysis. Furthermore, it shows universal applicability in the electrooxidation of other alcohol/aldehyde substrates, yielding efficiencies of 90-99%. This work not only provides guidance for advanced electrocatalysts design toward alcohol/aldehyde oxidation but also offers insights into the utilization of biomass-derived platform chemicals.

Synergistically Enhanced Co-Adsorption of Reactant and Hydroxyl on Platinum-Modified Copper Oxide for High-Performance HMF Oxidation

Electrochemical oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) provides an environmentally friendly route for producing the sustainable polymer monomer 2,5-furandicarboxylic acid (FDCA). Thus, precisely adjusting the synergistic adsorption among key reactive species, such as HMF and OHads, on the carefully designed catalyst surface is essential for achieving satisfactory catalytic performance for HMF oxidation to FDCA as it is closely related to the adsorption strength and configuration of the reaction substrates. This kind of regulation will ultimately facilitate the improvement of HMF oxidation performance. In this work, Pt nanoparticles modified CuO nanowires (denoted as Pt/CuO@CF) are constructed for the selective electrooxidation of HMF to FDCA under alkaline conditions. The well-designed Pt/CuO@CF demonstrates highly impressive catalytic performance across a range of HMF concentrations, ranging from the commonly used concentrations to higher levels typically not explored (10, 25, 50, 75, and 100 mm) with high FEFDCA (all above 95%) and outstanding long-term stability (15 cycles). In situ experimental characterizations confirm that the designed heterogeneous interface between Pt and CuO enhances the enrichment of HMF and OHads species on the catalyst surface. Theoretical calculations reveal the anchored Pt nanoparticles reduce the adsorption barrier for HMF and OHads, thereby promoting the highly selective oxidation of HMF to FDCA.

Photothermal Assisted Biomass Oxidation for Pairing Carbon Dioxide Electroreduction with Low Cell Potential

Integrating anodic biomass valorization with carbon dioxide electroreduction (CO2RR) can produce value-added chemicals on both the cathode and anode; however, anodic oxidation still suffers from high overpotential. Herein, a photothermal-assisted method was developed to reduce the potential of 5-hydroxymethyl furfural (HMF) electrooxidation. Capitalizing on the copious oxygen vacancies, defective Co3O4 (D-Co3O4) exhibited a stronger photothermal effect, delivering a local temperature of 175.47 °C under near infrared light illumination. The photothermal assistance decreased the oxidation potential of HMF from 1.7 V over pristine Co3O4 to 1.37 V over D-Co3O4 to achieve a target current density of 30 mA cm-2, with 2,5-furandicarboxylic acid as the primary product. Mechanistic analysis disclosed that the photothermal effect did not change the HMF oxidation route but greatly enhanced the adsorption capacity of HMF. Meanwhile, faster electron transfer for direct HMF oxidation and the surface conversion to cobalt (oxy)hydroxide, which contributed to indirect HMF oxidation, was observed. Thus, rapid HMF conversion was realized, as evidenced by in situ surface-enhanced infrared spectroscopy. Upon coupling cathodic CO2RR with an atomically dispersed Ni-N/C catalyst, the Faradaic efficiencies of CO (cathode) and 2,5-furandicarboxylic acid (FDCA, anode) exceeded 90.0 % under a low cell potential of 1.77 V.

Electrooxidation of 5-Hydroxymethylfurfural via Ni2P-Ni3Se4 Heterostructure Nanosheet Arrays

The electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) has been deeply investigated. However, developing a durable electrocatalyst for fast production of FDCA at low potentials remains a challenge. Herein, we report Ni2P-Ni3Se4 heterostructure nanosheet arrays as efficient electrocatalysts for HMF electrooxidation. These nanosheet arrays were synthesized via an in situ deep eutectic solvent etching approach, followed by phosphorization and a selenization process. The optimal Ni2P-Ni3Se4 electrocatalyst could achieve 99.1% FDCA selectivity, 98.9% Faradaic efficiency, and 100% HMF conversion at 1.38 V (reversible hydrogen electrode) within 1.6 h. Density functional theory calculations demonstrate that apparent charge redistribution occurs at the Ni2P/Ni3Se4 heterointerface, which greatly enhances HMF adsorption and consequently modulates the catalytic performance. In situ Raman spectroscopy technology confirms that NiOOH is the main active species during HMF electrooxidation. This work provides a significant strategy to develop robust heterogeneous electrocatalyst for HMF electrooxidation and beyond.

An Fe-doped Ni-based oxalate framework with a favorable electronic structure for electrocatalytic water and urea oxidation

An Fe-doped Ni-based oxalate framework, synthesized via a facile co-precipitation method, is applied as an excellent bi-functional electrocatalyst for water and urea oxidation reactions. The obtained framework achieved a large current density of 100 mA cm-2 at 1.497 V and 1.375 V (vs. RHE) for the OER and UOR, highlighting its potential for practical hydrogen production.

Pt-decorated spinel MnCo2O4 nanosheets enable ampere-level hydrazine assisted water electrolysis

Coupling hydrazine oxidation reaction (HzOR) with hydrogen evolution reaction (HER) has been widely concerned for high efficiency of green hydrogen preparation with low energy consumption. However, the lacking of bifunctional electrodes with ampere-level performance severely limits its industrialization. Herein, we put forward an efficient active site anchored strategy for MnCo2O4 nanosheet arrays on nickel foam (NF) by introducing Pt species (denoted as Pt-MnCo2O4/NF), which is standing for excellent bifunctional electrodes. The Pt-MnCo2O4/NF delivers ultralow potentials of -195 mV and 350 mV at 1000 mA cm-2 as well as robust stability for HzOR and HER, respectively. The study of in-situ Raman and reaction kinetics reveal that the formation of key adsorbed *NH2 and *N2H4 intermediates and the rapidly oxidization of intermediates with a fast interfacial charge transfer on Pt-MnCo2O4/NF. Remarkably, the Pt-MnCo2O4/NF assembled two-electrode hydrazine assisted water electrolyzer realizes current density of 100 mA cm-2 and 1000 mA cm-2 at 0.16 V and 0.62 V with over 80 h stability. This work provides a promising way to design efficient electrodes for energy-saving H2 generation under ampere-level current density.

Ligand-Hybridization Activates Lattice-Hydroxyl-Groups of NiCo(OH)x Nanowires for Efficient Electrosynthesis

Electrochemical dehydrogenation of hydroxides plays a crucial role in the formation of high-valence metal active sites toward 5-hydroxymethylfurfural oxidation reaction (HMFOR) to produce the value-added chemical of 2,5-furandicarboxylic (FDCA). Herein, we construct benzoic acid ligand-hybridized NiCo(OH)x nanowires (BZ-NiCo(OH)x) with ample electron-deficient Ni/Co sites for HMFOR. The robust electron-withdrawing capability of benzoic acid ligands in BZ-NiCo(OH)x speeds up the electrochemical activation and dehydrogenation of lattice-hydroxyl-groups (M2+-O-H⇌M3+-O), boosting the formation of abundant electron-deficient and high-valence Ni/Co sites. DFT calculation reveals that the deintercalation proton is prone to establishing a hydrogen bridge with the carbonyl group in benzoic acid, facilitating the proton transfer. Coupled with the synergistic oxidation of Ni/Co sites on hydroxyl and aldehyde groups, BZ-NiCo(OH)x delivers a remarkable current density of 111.20 mA cm-2 at 1.4 V for HMFOR, exceeding that of NiCo(OH)x by approximately fourfold. And the FDCA yield and Faraday efficiency are as high as 95.24 % and 95.39 %, respectively. The ligand-hybridized strategy in this work introduces a novel perspective for designing high-performance transition metal-based electrocatalysts for biomass conversion.

Pt─O Bond Accelerated Cu0/Cu+ Activity for Boosting Low-Energy Bipolar Hydrogen Production

To address the imperative challenge of producing hydrogen in a low-energy consumption electrocatalytic system, this study emphasizes the utilization of thermodynamically favorable biomass oxidation for achieving energy-efficient hydrogen generation. This research integrates ultralow PtO2-loaded flower-like nanosheets (denoted as PtO2@Cu2O/Cu FNs) with Cu0/Cu+ pairs and Pt─O bonds, thereby yielding substantial enhancement in both hydrogen evolution reaction (HER, -0.042 VRHE at 10 mA cm-2) and furfural oxidation reaction (FFOR, 0.09 VRHE at 10 mA cm-2). As validated by DFT calculations, the dual built-in electric field (BIEF) is elucidated as the driving force behind the enhanced activities, in which Pt─O bonds expedite the HER, while Cu+/Cu0 promotes low-potential FFOR. By coupling the FFOR and HER together, the resulting bipolar-hydrogen production system requires a low power input (0.5072 kWh per m3) for producing H2. The system can generate bipolar hydrogen and high value-added furoic acid, significantly enhancing hydrogen production efficiency and concurrently mitigating energy consumption.

Cobalt telluride regulated by nickel for efficient electrooxidation of 5-hydroxymethylfurfural

Replacing the anodic oxygen evolution reaction (OER) in water splitting with 5-hydroxymethylfurfural oxidation reaction (HMFOR) can not only reduce the energy required for hydrogen production but also yield the valuable chemical 2,5-furandicarboxylic acid (FDCA). Co-based catalysts are known to be efficient for HMFOR, with high-valent Co being recognized as the main active component. However, efficiently promoting the oxidation of Co2+ to produce high-valent reactive species remains a challenge. In this study, Ni-doped CoTe (CoNiTe) nanorods were prepared as efficient catalysts for HMFOR, achieving a high HMFOR current density of 65.3 mA cm-2 at 1.50 V. Even after undergoing five successive electrolysis processes, the Faradaic efficiency (FE) remained at approximately 90.7 %, showing robust electrochemical durability. Mechanistic studies indicated that Ni doping changes the electronic configuration of Co, enhancing its charge transfer rate and facilitating the oxidation of Co2+ to high-valent CoO2 species. This work reveals the effect of Ni doping on the reconfiguration of the active phase during HMFOR.

Highly Self-Healing Co3+/Ni3+ Dual-Active Species for the Electrocatalytic Oxidation of 5-Hydroxymethylfurfural

Electrocatalytic conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is significant for the sustainable production of value-added chemicals. Active sites of catalysts could enhance the activity and selectivity of the HMF oxidation reaction (HMFOR), but the self-healing ability of active sites has been commonly ignored. In this work, Co(OH)2/Ni-MOF was successfully fabricated for efficient oxidation of HMF to FDCA under mild conditions. Electrochemical and cyclic stability experiments demonstrated the high self-healing properties of the dual active sites (Co3+/Ni3+). So, the retention rate of FDCA yield can still reach 98.5%, even after 90 days. HMFOR was further coupled with 4-nitrophenol hydrogenation, which promotes the yield and Faradaic efficiency of FDCA to about 100%. Therefore, this study explores the self-healing properties of species and provides new insights for designing efficient catalysts.

Unlocking Efficient Hydrogen Production: Nucleophilic Oxidation Reactions Coupled with Water Splitting

Electrocatalytic water splitting driven by sustainable energy is a clean and promising water-chemical fuel conversion technology for the production of high-purity green hydrogen. However, the sluggish kinetics of anodic oxygen evolution reaction (OER) pose challenges for large-scale hydrogen production, limiting its efficiency and safety. Recently, the anodic OER has been replaced by a nucleophilic oxidation reaction (NOR) with biomass as the substrate and coupled with a hydrogen evolution reaction (HER), which has attracted great interest. Anode NOR offers faster kinetics, generates high-value products, and reduces energy consumption. By coupling NOR with hydrogen evolution reaction, hydrogen production efficiency can be enhanced while yielding high-value oxidation products or degrading pollutants. Therefore, NOR-coupled HER hydrogen production is another new green electrolytic hydrogen production strategy after electrolytic water hydrogen production, which is of great significance for realizing sustainable energy development and global decarbonization. This review explores the potential of nucleophilic oxidation reactions as an alternative to OER and delves into NOR mechanisms, guiding future research in NOR-coupled hydrogen production. It assesses different NOR-coupled production methods, analyzing reaction pathways and catalyst effects. Furthermore, it evaluates the role of electrolyzers in industrialized NOR-coupled hydrogen production and discusses future prospects and challenges. This comprehensive review aims to advance efficient and economical large-scale hydrogen production.

Modulating Ni-S coordination in Ni3S2 to promote electrocatalytic oxidation of 5-hydroxymethylfurfural at ampere-level current density

Electricity-driven oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a highly attractive strategy for biomass transformation. However, achieving industrial-grade current densities remains a great challenge. Herein, by modulating the water content in a solvothermal system, Ni3S2/NF with stabilized and shorter Ni-S bonds as well as a tunable coordination environment of Ni sites was fabricated. The prepared Ni3S2/NF was highly efficient for electrocatalytic oxidation of HMF to produce FDCA, and the FDCA yield and Faraday efficiency could reach 98.8% and 97.6% at the HMF complete conversion. More importantly, an industrial-grade current density of 1000 mA cm-2 could be achieved at a potential of only 1.45 V vs. RHE for HMFOR and the current density could exceed 500 mA cm-2 with other bio-based compounds as the reactants. The excellent performance of Ni3S2/NF originated from the shorter Ni-S bonds and its better electrochemical properties, which significantly promoted the dehydrogenation step of oxidizing HMF. Besides, the gram-scale FDCA production could be realized on Ni3S2/NF in a MEA reactor. This work provides a robust electrocatalyst with high potential for practical applications for the electrocatalytic oxidation of biomass-derived compounds.

Metal-organic frameworks (MOFs) for hybrid water electrolysis: structure-property-performance correlation

Hybrid water electrolysis (HWE) is a promising pathway for the simultaneous production of high-value chemicals and clean H2 fuel. Unlike conventional electrochemical water splitting, which relies on the oxygen evolution reaction (OER), HWE involves the anodic oxidation reaction (AOR). The AORs facilitate the conversion of organic or inorganic compounds at the anode into valuable chemicals, while the cathode carries out the hydrogen evolution reaction (HER) to produce H2. Recent literature has witnessed a surge in papers investigating various AORs with organic and inorganic substrates using a series of transition metal-based catalysts. Over the past two decades, metal-organic frameworks (MOFs) have garnered significant attention for their exceptional performance in electrochemical water splitting. These catalysts possess distinct attributes such as highly porous architectures, customizable morphologies, open facets, high electrochemical surface areas, improved electron transport, and accessible catalytic sites. While MOFs have demonstrated efficiency in electrochemical water splitting, their application in hybrid water electrolysis has only recently been explored. In recent years, a series of articles have been published; yet there is no comprehensive article summarizing MOFs for hybrid water electrolysis. This article aims to fill this gap by delving into the recent progress in MOFs specifically tailored for hybrid water electrolysis. In this article, we systematically discuss the structure-property-performance relationships of various MOFs utilized in hybrid water electrolysis, supported by pioneering examples. We explore how the structure, morphology, and electronic properties of MOFs impact their performance in hybrid water electrolysis, with particular emphasis on value-added chemical generation, H2 production, potential improvement, conversion efficiency, selectivity, faradaic efficiency, and their potential for industrial-scale applications. Furthermore, we address future advancements and challenges in this field, providing insights into the prospects and challenges associated with the continued development and deployment of MOFs for hybrid water electrolysis.

Seawater electrolysis for fuels and chemicals production: fundamentals, achievements, and perspectives

Seawater electrolysis for the production of fuels and chemicals involved in onshore and offshore plants powered by renewable energies offers a promising avenue and unique advantages for energy and environmental sustainability. Nevertheless, seawater electrolysis presents long-term challenges and issues, such as complex composition, potential side reactions, deposition of and poisoning by microorganisms and metal ions, as well as corrosion, thus hindering the rapid development of seawater electrolysis technology. This review focuses on the production of value-added fuels (hydrogen and beyond) and fine chemicals through seawater electrolysis, as a promising step towards sustainable energy development and carbon neutrality. The principle of seawater electrolysis and related challenges are first introduced, and the redox reaction mechanisms of fuels and chemicals are summarized. Strategies for operating anodes and cathodes including the development and application of chloride- and impurity-resistant electrocatalysts/membranes are reviewed. We comprehensively summarize the production of fuels and chemicals (hydrogen, carbon monoxide, sulfur, ammonia, etc.) at the cathode and anode via seawater electrolysis, and propose other potential strategies for co-producing fine chemicals, even sophisticated and electronic chemicals. Seawater electrolysis can drive the oxidation and upgrading of industrial pollutants or natural organics into value-added chemicals or degrade them into harmless substances, which would be meaningful for environmental protection. Finally, the perspective and prospects are outlined to address the challenges and expand the application of seawater electrolysis.

Ultra-Dense Supported Ruthenium Oxide Clusters via Directed Ion Exchange for Efficient Valorization of 5-Hydroxymethylfurfural

Maximizing the loadings of active centers without aggregation for a supported catalyst is a grand challenge but essential for achieving high gravimetric catalytic activity, especially toward multi-step reactions. The oxidation of 5-hydroxymethylfurfural (HMF), a key biomass-derived platform molecule, into 2,5-furandicarboxylic acid (FDCA), a promising alternative to polyester monomer, is such a multi-step reaction that involves 6 proton and electron transfers. This process often demands strong alkaline environment but also suffers from the alkali-driven polymerization side-reaction. Meanwhile, neutral media ameliorates the polymerization, but lacks efficient catalyst toward deep oxidation. Herein, we devised a strategy of creating ultra-dense supported Ru oxide clusters via directed ion exchange in a Co hydroxyanion (CoHA) support material. Pyrimidine ligands were first incorporated into the CoHA interlayers, and the subsequent evacuation of pyrimidines created porous channels for the directed ion exchange with the built-in anions in CoHA, which allowed the dense and mono-disperse functionalization of RuCl6 2- anions and their resulting Ru oxide clusters. These ultra-dense Ru oxide clusters not only enable high HMF electrooxidation currents under neutral conditions but also create microscopic channels in-between the clusters for the expedited re-adsorption and oxidation of intermediates toward highly oxidized product, such as 5-formyl-2-furoic acid (FFCA) and FDCA. A two-stage HMF oxidation process, consisting of ambient conversion of HMF into FFCA and FFCA oxidation into FDCA under 60 °C, was eventually developed to first achieve a high FDCA yield of 92.1 % under neutral media with significantly reduced polymerization.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Immobilised Ruthenium Complexes for the Electrooxidation of 5-Hydroxymethylfurfural

Abundantly available biomass-based platform chemicals, including 5-hydroxymethylfurfural (HMF), are essential stepping stones in steering the chemical industry away from fossil fuels. The efficient catalytic oxidation of HMF to its diacid derivative, 2,5-furandicarboxylic acid (FDCA), is a promising research area with potential applications in the polymer industry. Currently, the most encouraging approaches are based on solid-state catalysts and are often conducted in basic aqueous media, conditions where HMF oxidation competes with its decomposition. Efficient molecular catalysts are practically unknown for this reaction. In this study, we report on the synthesis and electrocatalysis of surface-bound molecular ruthenium complexes for the transformation of HMF to FDCA under acidic conditions. Catalyst immobilisation on mesoporous indium tin oxide electrodes is achieved through the incorporation of phosphonic acid anchoring groups. Screening experiments with HMF and further reaction intermediates revealed the catalytic route and bottlenecks in the catalytic synthesis of FDCA. Utilising these immobilised electrocatalysts, FDCA yields of up to 85 % and faradaic efficiencies of 91 % were achieved, without any indication of substrate decomposition. Surface analysis by X-ray photoelectron spectroscopy (XPS) post-electrocatalysis unveiled the desorption of the catalyst from the electrode surface as a limiting factor in terms of catalytic performance.

High-valence metal sites induced by heterostructure engineering for promoting 5-hydroxymethylfurfural electrooxidation and hydrogen generation

The electrocatalytic 5-hydroxymethylfurfural (HMF) oxidation reaction coupling with hydrogen evolution reaction (HER) serves as a promising strategy to generate both high-value-added products and clean energy, which is limited by the poor catalytic efficiency of bifunctional electrocatalysts and unclear electrocatalytic mechanism for HMF oxidation reaction. Herein, we fabricate a bifunctional NiSe2-NiMoO4 heterostructure nanowire electrocatalyst for the conversion of HMF to 2,5-furandicarboxylic acid (FDCA) and simultaneous H2 production. As expected, the NiSe2-NiMoO4 exhibits outstanding activity and selectivity toward HMF oxidation reaction. In particular, at a potential of 1.50 V, the yield of FDCA could reach 98 % with a faradaic efficiency of 96.5 %, as well as excellent stability. Density functional theory calculation results demonstrate that the NiSe2-NiMoO4 heterostructure could tune the adsorption energy of HMF, facilitate high-valence active species formation, and enhance electronic conductivity. Furthermore, a two-electrode electrolyzer assembled using NiSe2-NiMoO4 as a bifunctional catalyst requires 1.53 V to acquire a current density of 50 mA cm-2, which is 201 mV lower than that of water electrolysis. This work provides new insights for designing multifunctional catalysts for biomass upgrading coupled with hydrogen evolution.

Fluorine Induced In Situ Formation of High Valent Nickel Species for Ultra Low Potential Electrooxidation of 5-Hydroxymethylfurfural

The Nickel-based catalysts have a good catalytic effect on the 5-hydroxymethylfurfural electrooxidation reaction (HMFOR), but limited by the conversion potential of Ni2+ /Ni3+ , 1.35 V versus RHE, the HMF electrooxidation potential of nickel-based catalysts is generally greater than 1.35 V versus RHE. Considering fluorine has the highest Pauling electronegativity and similar atomic radius of oxygen, the introduction of fluorine into the lattice of metal oxides might promote the adsorption of intermediate species, thus improving the catalytic performance. F is successfully doped into the lattice structure of NiCo2 O4 spinel oxide by the strategy of hydrothermal reaction and low-temperature fluorination. As is confirmed by in situ electrochemical impedance spectroscopy and Raman spectroscopy, the introduction of F weakens the interaction force of metal-oxygen covalent bonds of the asymmetric MT -O-MO backbone and improves the valence of Ni in tetrahedra structure, which makes it easier to be oxidized to higher valence active Ni3+ under the action of electric field and promotes the adsorption of OH- , while the decrease of Co valence enhances the adsorption of HMF with the catalyst. Combining the above reasons, F-NiCo2 O4 shows superb electrocatalytic performance with a potential of only 1.297 V versus RHE at a current density of 20 mA cm-2 , which is lower than the most catalyst.

Amorphous-crystalline heterostructure: Efficient catalyst for biomass oxidation coupled with hydrogen evolution

The development of catalysts with high activity, selectivity, and stability is critical for biomass upgrading coupled with hydrogen evolution. In this study, we present a simple method for fabricating crystalline-amorphous phase heterostructures using the etching effect of the acidic medium generated during cobalt salt hydrolysis, resulting in the formation of NiCo(OH)x-modified Ni/NiMoO4 nanosheets electrode (NiCo(OH)x/Ni/NiMoO4/NF). The nanosheets array formed during the synthesis process enlarges the surface area of the prepared catalyst, which facilitates the exposure of electrochemically active sites and improves mass transfer. Unexpectedly, the strong coupling interactions between the amorphous-crystalline heterointerface optimize the adsorption of reaction molecules and the corresponding charge transfer process, consequently boosting the catalytic activity for the 5-hydroxymethylfurfural oxidation reaction (HMFOR) and hydrogen evolution reaction (HER). Specifically, NiCo(OH)x/Ni/NiMoO4/NF catalyst requires only 1.34 V to obtain a current density of 10 mA cm-2 for HMFOR-coupled H2 evolution, and operates stably for 13 consecutive cycles with good product selectivity. This work thus provides insights into the design of efficient and robust catalysts for HMFOR-assisted H2 evolution.

Interface Charge Distribution Engineering of Pd-CeO2 /C for Efficient Carbohydrazide Oxidation Reaction

Carbohydrazide electrooxidation reaction (COR) is a potential alternative to oxygen evolution reaction in water splitting process. However, the sluggish kinetics process impels to develop efficient catalysts with the aim of the widespread use of such catalytic system. Since COR concerns the adsorption/desorption of reactive species on catalysts, the electronic structure of electrocatalyst can affect the catalytic activity. Interface charge distribution engineering can be considered to be an efficient strategy for improving catalytic performance, which facilitates the cleavage of chemical bond. Herein, highly dispersed Pd nanoparticles on CeO2 /C catalyst are prepared and the COR catalytic performance is investigated. The self-driven charge transfer between Pd and CeO2 can form the local nucleophilic and electrophilic region, promoting to the adsorption of electron-withdrawing and electron-donating group in carbohydrazide molecule, which facilitates the cleavage of C-N bond and the carbohydrazide oxidation. Due to the local charge distribution, the Pd-CeO2 /C exhibits superior COR catalytic activity with a potential of 0.27 V to attain 10 mA cm-2 . When this catalyst is used for energy-efficient electrolytic hydrogen production, the carbohydrazide electrolysis configuration exhibits a low cell voltage (0.6 V at 10 mA cm-2 ). This interface charge distribution engineering can provide a novel strategy for improving COR catalytic activity.

Electrocatalytic Lignin Valorization into Aromatic Products via Oxidative Cleavage of Cα-Cβ Bonds

Lignin is the most promising candidate for producing aromatic compounds from biomass. However, the challenge lies in the cleavage of C-C bonds between lignin monomers under mild conditions, as these bonds have high dissociation energy. Electrochemical oxidation, which allows for mild cleavage of C-C bonds, is considered an attractive solution. To achieve low-energy consumption in the valorization of lignin, the use of highly efficient electrocatalysts is essential. In this study, a meticulously designed catalyst consisting of cobalt-doped nickel (oxy)hydroxide on molybdenum disulfide heterojunction was developed. The presence of molybdenum in a high valence state promoted the adsorption of tert-butyl hydroperoxide, leading to the formation of critical radical intermediates. In addition, the incorporation of cobalt doping regulated the electronic structure of nickel, resulting in a lower energy barrier. As a result, the heterojunction catalyst demonstrated a selectivity of 85.36% for cleaving the Cα-Cβ bond in lignin model compound, achieving a substrate conversion of 93.69% under ambient conditions. In addition, the electrocatalyst depolymerized 49.82 wt% of soluble fractions from organosolv lignin (OL), resulting in a yield of up to 13 wt% of aromatic monomers. Significantly, the effectiveness of the prepared electrocatalyst was also demonstrated using industrial Kraft lignin (KL). Therefore, this research offers a practical approach for implementing electrocatalytic oxidation in lignin refining.

Self-Reconstruction of Sulfate-Terminated Copper Oxide Nanorods for Efficient and Stable 5-Hydroxymethylfurfural Electrooxidation

The electrochemical 5-hydroxymethylfurfural oxidation reaction (HMFOR) has been regarded as a viable alternative to sustainable biomass valorization. However, the transformation of the catalysts under harsh electrooxidation conditions remains controversial. Herein, we confirm the self-construction of cuprous sulfide nanosheets (Cu2S NSs) into sulfate-terminated copper oxide nanorods (CuO-SO42- NRs) during the first-cycle of the HMFOR, which achieves a near-quantitative synthesis of 2,5-furandicarboxylic acid (FDCA) with a >99.9% yield and faradaic efficiency without deactivation in 15 successive cycles. Electrochemical impedance spectroscopies confirm that the surface SO42- effectively reduces the onset potential for HMFOR, while in situ Raman spectroscopies identify a reversible transformation from CuII-O to CuIII-OOH in HMFOR. Furthermore, density functional theory calculations reveal that the surface SO42- weakens the Cu-OH bonds in CuOOH to promote the rate-determining step of its coupling with the C atom in HMF-H* resulting from HMF hydrogenation, which synergistically enhances the catalytic activity of CuO-SO42- NRs toward HMF-to-FDCA conversion.

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.

Defect-Promoted Ni-Based Layer Double Hydroxides with Enhanced Deprotonation Capability for Efficient Biomass Electrooxidation

Ni-based hydroxides are promising electrocatalysts for biomass oxidation reactions, supplanting the oxygen evolution reaction (OER) due to lower overpotentials while producing value-added chemicals. The identification and subsequent engineering of their catalytically active sites are essential to facilitate these anodic reactions. Herein, the proportional relationship between catalysts' deprotonation propensity and Faradic efficiency of 5-hydroxymethylfurfural (5-HMF)-to-2,5 furandicarboxylic acid (FDCA, FEFDCA ) is revealed by thorough density functional theory (DFT) simulations and atomic-scale characterizations, including in situ synchrotron diffraction and spectroscopy methods. The deprotonation capability of ultrathin layer-double hydroxides (UT-LDHs) is regulated by tuning the covalency of metal (M)-oxygen (O) motifs through defect site engineering and selection of M3+ co-chemistry. NiMn UT-LDHs show an ultrahigh FEFDCA of 99% at 1.37 V versus reversible hydrogen electrode (RHE) and retain a high FEFDCA of 92.7% in the OER-operating window at 1.52 V, about 2× that of NiFe UT-LDHs (49.5%) at 1.52 V. Ni-O and Mn-O motifs function as dual active sites for HMF electrooxidation, where the continuous deprotonation of Mn-OH sites plays a dominant role in achieving high selectivity while suppressing OER at high potentials. The results showcase a universal concept of modulating competing anodic reactions in aqueous biomass electrolysis by electronically engineering the deprotonation behavior of metal hydroxides, anticipated to be translatable across various biomass substrates.

High Valence State Sites as Favorable Reductive Centers for High-Current-Density Water Splitting

Electrochemical water splitting is a promising approach for producing sustainable and clean hydrogen. Typically, high valence state sites are favorable for oxidation evolution reaction (OER), while low valence states can facilitate hydrogen evolution reaction (HER). However, here we proposed a high valence state of Co3+ in Ni9.5 Co0.5 -S-FeOx hybrid as the favorable center for efficient and stable HER, while structural analogues with low chemical states showed much worse performance. As a result, the Ni9.5 Co0.5 -S-FeOx catalyst could drive alkaline HER with an ultra-low overpotential of 22 mV for 10 mA cm-2 , and 175 mV for 1000 mA cm-2 at the industrial temperature of 60 °C, with an excellent stability over 300 h. Moreover, this material could work for both OER and HER, with a low cell voltage being 1.730 V to achieve 1000 mA cm-2 for overall water splitting at 60 °C. X-ray absorption spectroscopy (XAS) clearly identified the high valence Co3+ sites, while in situ XAS during HER and theoretical calculations revealed the favorable electron capture at Co3+ and suitable H adsorption/desorption energy around Co3+ , which could accelerate the HER. The understanding of high valence states to drive reductive reactions may pave the way for the rational design of energy-related catalysts.

Highly Efficient Biomass Upgrading by a Ni-Cu Electrocatalyst Featuring Passivation of Water Oxidation Activity

Electricity-driven organo-oxidations have shown an increasing potential recently. However, oxygen evolution reaction (OER) is the primary competitive reaction, especially under high current densities, which leads to low Faradaic efficiency (FE) of the product and catalyst detachment from the electrode. Here, we report a bimetallic Ni-Cu electrocatalyst supported on Ni foam (Ni-Cu/NF) to passivate the OER process while the oxidation of 5-hydroxymethylfurfural (HMF) is significantly enhanced. A current density of 1000 mA cm-2 can be achieved at 1.50 V vs. reversible hydrogen electrode, and both FE and yield keep close to 100 % over a wide range of potentials. Both experimental results and theoretical calculations reveal that Cu doping impedes the OH* deprotonation to O* and hereby OER process is greatly passivated. Those instructive results provide a new approach to realizing highly efficient biomass upgrading by regulating the OER activity.

Single Site Coordination Enabled Construction of Metal-Diketimine-Linked Covalent Organic Frameworks for Boosted Electrooxidation of Biomass Derivative

Aromatic aldehydes are widely used for the construction of covalent organic frameworks (COFs). However, due to the high flexibility, high steric hindrance, and low reactivity, it remains challenging to synthesize COFs using ketones as building units, especially the highly flexible aliphatic ones. Here, the single nickel site coordination strategy is reported to lock the configurations of the highly flexible diketimine to transform discrete oligomers or amorphous polymers into highly crystalline nickel-diketimine-linked COFs (named as Ni-DKI-COFs). The strategy has been successfully extended to the synthesis of a series of Ni-DKI-COFs by the condensation of three flexible diketones with two tridentate amines. Thanks to the ABC stacking model with high amount and easily accessible single nickel (II) sites on their 1D channels, Ni-DKI-COFs are exploited as well-defined electrocatalyst platforms for efficiently electro-upgrading biomass-derived 5-Hydroxymethylfurfural (HMF) into value-added 2,5-furandicarboxylic acid (FDCA) with a 99.9% yield and a 99.5% faradaic efficiency as well as a high turnover frequency of 0.31 s-1 .

Regulating the Electronic Structure of Ni Sites in Ni(OH)2 by Ce Doping and Cu(OH)2 Coupling to Boost 5-Hydroxymethylfurfural Oxidation Performance

Biomass is a sustainable and renewable resource that can be converted into valuable chemicals, reducing the demand for fossil energy. 5-Hydroxymethylfurfural (HMF), as an important biomass platform molecule, can be converted to high-value-added 2,5-furandicarboxylic acid (FDCA) via a green and renewable electrocatalytic oxidation route under mild reaction conditions, but efficient electrocatalysts are still lacking. Herein, we rationally fabricate a novel self-supported electrocatalyst of core-shell-structured copper hydroxide nanowires@cerium-doped nickel hydroxide nanosheets composite nanowires on a copper mesh (CuH_NWs@Ce:NiH_NSs/Cu) for electrocatalytically oxidizing HMF to FDCA. The integrated configuration of composite nanowires with rich interstitial spaces between them facilitates fast mass/electron transfer, improved conductivity, and complete exposure of active sites. The doping of Ce ions in nickel hydroxide nanosheets (NiH_NSs) and the coupling of copper hydroxide nanowires (CuH_NWs) regulate the electronic structure of the Ni active sites and optimize the adsorption strength of the active sites to the reactant, meanwhile promoting the generation of strong oxidation agents of Ni³⁺ species, thereby resulting in improved electrocatalytic activity. Consequently, the optimal CuH_NWs@Ce:NiH_NSs/Cu electrocatalyst is able to achieve a HMF conversion of 98.5% with a FDCA yield of 97.9% and a Faradaic efficiency of 98.0% at a low constant potential of 1.45 V versus reversible hydrogen electrode. Meanwhile, no activity attenuation can be found after 15 successive cycling tests. Such electrocatalytic performance suppresses most of the reported Cu-based and Ni-based electrocatalysts. This work highlights the importance of structure and doping engineering strategies for the rational fabrication of high-performance electrocatalysts for biomass upgrading.

Lignosulfonate-assisted in situ synthesis of Co9S8-Ni3S2 heterojunctions encapsulated by S/N co-doped biochar for efficient water oxidation

The development of highly active and stable earth-rich electrocatalysts remains a major challenge to release the reliance on noble metal catalysts in sustainable (electro)chemical processes. In this work, metal sulfides encapsulated with S/N co-doped carbon were synthesized with a one-step pyrolysis strategy, where S was introduced during the self-assembly process of sodium lignosulfonate. Due to the precise coordination of Ni and Co ions with lignosulfonate, an intense-interacted Co₉S₈-Ni₃S₂ heterojunction was formed inside the carbon shell, causing the redistribution of electrons. An overpotential as low as 200 mV was obtained over Co₉S₈-Ni₃S₂@SNC to reach a current density of 10 mA cm^-2. Only a slight increase of 14.4 mV was observed in a 50 h chronoamperometric stability test. Density functional theory (DFT) calculations showed that Co₉S₈-Ni₃S₂ heterojunctions encapsulated with S/N co-doped carbon can optimize the electronic structure, lower the reaction energy barrier, and improve the OER reaction activity. This work provides a novel strategy for constructing highly efficient and sustainable metal sulfide heterojunction catalysts with the assistance of lignosulfonate biomass.

Operando Forming of Lattice Vacancy Defect in Ultrathin Crumpled NiVW-Layered Metal Hydroxides Nanosheets for Valorization of Biomass

The 2D layered metal hydroxides (LMHs) have been developed for electrooxidation of 5-hydroxymethylfurfural (HMF). In this work, an effective strategy is proposed to tailor the electronic structure of active sites at the atomic level, which is by introducing defects into the lattice structure. As an example, a series of ultrathin crumpled ternary NiVW-LMH electrocatalysts with abundant lattice vacancies (denoted as NiVW_v -LMH) are prepared in this way. The introduction of tungsten (W) endows the catalyst with a special crumpled structure, which promotes the generation of lattice vacancies and thus exposes more unsaturated Ni activity sites. The NiVW_v -LMH displays superb performance in the electrooxidation of HMF. The Tafel slope for electrodehydrogenation of Ni²⁺ OH bond to Ni(OH)O species is 12.04 mV dec^-1 . The current density at 1.43 V versus reversible hydrogen electrode (RHE) toward the oxidation reaction of HMF reaches about 193 mA cm^-2 , which is better than most of the common electrocatalysts, with an 5.37-fold improvement compared with Ni(OH)₂ electrode. The preparation strategy demonstrates in this work can be useful for developing highly efficient electrocatalysts.

CoO–Co Heterojunction Covered with Carbon Enables Highly Efficient Integration of Hydrogen Evolution and 5-Hydroxymethylfurfural Oxidation

The renewable-energy-driven integration of hydrogen production and biomass conversion into value-added products is desirable for the current global energy transition, but still a challenge. Herein, carbon-coated CoO–Co heterojunction arrays were built on copper foam (CoO–Co@C/CF) by the carbothermal reduction to catalyze the hydrogen evolution reaction (HER) coupled with a 5-hydroxymethylfurfural electrooxidation reaction (HMFEOR). The electronic modulation induced by the CoO–Co heterojunction endows CoO–Co@C/CF with a powerful catalytic ability. CoO–Co@C/CF is energetic for HER, yielding an overpotential of 69 mV at 10 mA·cm−1 and Tafel slope of 58 mV·dec−1. Meanwhile, CoO–Co@C/CF delivers an excellent electrochemical activity for the selective conversion from HMF into 2,5-furandicarboxylic acid (FDCA), achieving a conversion of 100%, FDCA yield of 99.4% and faradaic efficiency of 99.4% at the lower oxidation potential, along with an excellent cycling stability. The integrated CoO–Co@C/CF||CoO–Co@C/CF configuration actualizes the H2O–HMF-coupled electrolysis at a satisfactory cell voltage of 1.448 V at 10 mA·cm−2. This work highlights the feasibility of engineering double active sites for the coupled electrolytic system.

Killing Two Birds with One Stone: Upgrading Organic Compounds via Electrooxidation in Electricity-Input Mode and Electricity-Output Mode

The electrochemically oxidative upgrading reaction (OUR) of organic compounds has gained enormous interest over the past few years, owing to the advantages of fast reaction kinetics, high conversion efficiency and selectivity, etc., and it exhibits great potential in becoming a key element in coupling with electricity, synthesis, energy storage and transformation. On the one hand, the kinetically more favored OUR for value-added chemical generation can potentially substitute an oxygen evolution reaction (OER) and integrate with an efficient hydrogen evolution reaction (HER) or CO₂ electroreduction reaction (CO₂RR) in an electricity-input mode. On the other hand, an OUR-based cell or battery (e.g., fuel cell or Zinc-air battery) enables the cogeneration of value-added chemicals and electricity in the electricity-output mode. For both situations, multiple benefits are to be obtained. Although the OUR of organic compounds is an old and rich discipline currently enjoying a revival, unfortunately, this fascinating strategy and its integration with the HER or CO₂RR, and/or with electricity generation, are still in the laboratory stage. In this minireview, we summarize and highlight the latest progress and milestones of the OUR for the high-value-added chemical production and cogeneration of hydrogen, CO₂ conversion in an electrolyzer and/or electricity in a primary cell. We also emphasize catalyst design, mechanism identification and system configuration. Moreover, perspectives on OUR coupling with the HER or CO₂RR in an electrolyzer in the electricity-input mode, and/or the cogeneration of electricity in a primary cell in the electricity-output mode, are offered for the future development of this fascinating technology.

Bioinspired Electrode for the Production and Timely Separation of Nitrile and Hydrogen

Replacing electrocatalytic oxygen evolution reaction (OER) with amine oxidation reaction is adopted to boost clean and environment-friendly energy source hydrogen (H₂ ) in water. However, the electrocatalytic reaction is severely restricted by the strong adsorption of product on the catalyst surface. Inspired by the cooperation of flavin adenine dinucleotide and mitochondria membrane in biological system, the catalysis-separation complex electrodes are introduced to promote the desorption of product and hinder its readsorption by applying polytetrafluoroethylene (PTFE)-separation membrane on the one side of electrode, which is benefit for the cleanness of active sites on the catalyst surface for the continuous production and timely separation of nitrile and hydrogen. With the intermolecular force between PTFE and nitrile, the nitrile droplets can be quickly desorbed and separated from catalyst surface of anode, and the size of nitrile droplets on the catalyst surface is only 0.23% to that without PTFE. As a result, the current at 1.49 V_RHE from the catalyst with PTFE membrane is about 33 times to that of catalyst without PTFE after long-term operation. Moreover, the cathode with PTFE membrane also achieves the rapid desorption of H₂ bubbles and stable cathodic current because of the strong absorption of PTFE to H₂ .

S-Species-Evoked High-Valence Ni2+ δ of the Evolved β-Ni(OH)2 Electrode for Selective Oxidation of 5-Hydroxymethylfurfural

An efficient NiS_x -modified β-Ni(OH)₂ electrode is reported for the selective oxidation reaction of 5-hydroxymethylfurfural (HMFOR) with excellent electrocatalytic 5-hydroxymethylfurfural (HMF) selectivity (99.4%), conversion (97.7%), and Faradaic efficiency (98.3%). The decoration of NiS_x will evoke high-valence Ni^{2+ δ} species in the reconstructed β-Ni(OH)₂ electrode, which are the real active species for HMFOR. The generated NiS_x /Ni(OH)O modulates the proton-coupled electron-transfer (PCET) process of HMFOR, where the electrocatalytically generated Ni(OH)O can effectively trap the protons from the CHO end in HMF to realize electron transfer. The oxygen evolution reaction (OER) competes with the HMFOR when NiS_x /Ni(OH)O continues to accumulate, to generate the NiS_x /NiO_x (OH)_y intermediate. Density functional theory (DFT) calculations and experimental results verify that the adsorption energy of HMF can be optimized through the increased NiS_x composition for more efficient capture of protons and electrons in the HMFOR.

Advances in Selective Electrochemical Oxidation of 5-Hydroxymethylfurfural to Produce High-Value Chemicals

The conversion of biomass is a favorable alternative to the fossil energy route to solve the energy crisis and environmental pollution. As one of the most versatile platform compounds, 5-hydroxymethylfural (HMF) can be transformed to various value-added chemicals via electrolysis combining with renewable energy. Here, the recent advances in electrochemical oxidation of HMF, from reaction mechanism to reactor design are reviewed. First, the reaction mechanism and pathway are summarized systematically. Second, the parameters easy to be ignored are emphasized and discussed. Then, the electrocatalysts are reviewed comprehensively for different products and the reactors are introduced. Finally, future efforts on exploring reaction mechanism, electrocatalysts, and reactor are prospected. This review provides a deeper understanding of mechanism for electrochemical oxidation of HMF, the design of electrocatalyst and reactor, which is expected to promote the economical and efficient electrochemical conversion of biomass for industrial applications.

Growing Co–Ni–Se nanosheets on 3D carbon frameworks as advanced dual functional electrodes for supercapacitors and sodium ion batteries

The reasonable design of electrode materials is crucial for tuning the electrochemical performances of advanced energy storage systems.