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

Enhanced electrooxidation of 5-hydroxymethylfurfural over a ZIF-67@β-Ni(OH)2/NF heterostructure catalyst: Synergistic effects and mechanistic insights

The electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a crucial step in biomass valorization. While Ni(OH)2 exhibits promise as an electrocatalyst due to its dynamic Ni2+/Ni3+ redox transitions, its limited HMF adsorption capacity and high onset potential hinder its widespread application. Herein, we report the design and synthesis of a ZIF-67@β-Ni(OH)2/NF heterostructure electrocatalyst featuring a non-homogeneous interface, resulting in an advanced onset-potential of the HMF oxidation reaction (HMFOR), a higher current density than β-Ni(OH)2 and significantly enhances HMF adsorption. Consequently, the optimized catalyst achieves 100 % HMF conversion and a remarkable 98.97 % FDCA yield with a high Faraday efficiency of 95.21 % at 1.45 VRHE. High-performance liquid chromatography (HPLC), open-circuit potential (OCP) and in-situ electrochemical impedance (in-situ EIS) tests confirm the synergistic effect of ZIF-67, demonstrating its crucial role in optimizing the β-Ni(OH)2 catalytic interface and facilitating enhanced adsorption and conversion of both HMF and OH-. This work provides a valuable strategy for engineering high-performance non-homogeneous nickel-based electrocatalysts for efficient HMF oxidation.

High voltage driven MnO2/CuO for efficient oxidation of 5-hydroxymethylfurfural

Electrocatalytic 5-hydroxymethylfurfural oxidation to 2,5-furandicarboxylic acid (FDCA) faces challenges like low efficiency at high voltage. This study fabricated MnO2/CuO, achieving 98.2% Faraday efficiency (72.12 mA cm-2) at 1.6 VRHE and 80.2% at 1.7 VRHE, which offers a novel approach for high-voltage FDCA production.

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.

Engineering Oxygen Intermediates Adsorption on Amorphous NiFe Alloys for Highly Active and Selective Electrochemical Biomass Conversion

Electrochemical 5-hydroxymethylfurfural (HMF) oxidation reaction (HMFOR) offers a promising route to transform biomass into value-added chemicals. However, the competing oxygen evolution reaction (OER) greatly limits the HMFOR selectivity. Herein, we report a facile doping strategy to engineer oxygen intermediates adsorption on amorphous NiFe alloys to boost highly selective electrochemical HMF oxidation to produce 2,5-furandicarboxylic acid (FDCA), among which, amorphous Mn-doped NiFeB alloy displays a low HMFOR onset potential of 1.35 V vs. RHE, achieving 100 % HMF conversion with 88 % FDCA selectivity at an applied potential of 1.4 V vs. RHE, outperforming amorphous NiFeB (73 % FDCA selectivity) and Mo-doped NiFeB (65 % FDCA selectivity) alloys. Experimental characterizations suggest that the introduction of Mn/Mo into amorphous NiFeB alloy can increase/decrease its electronic density and thus strengthen/weaken oxygen intermediates adsorption. Operando experiments indicate that the amorphous Mn-doped NiFeB alloy can significantly reduce the onset potential to form active Ni3+ species, which spontaneously react with HMF via nucleophile dehydrogenation to form FDCA. Furthermore, in situ infrared spectroscopy measurements verify that the HMF oxidation pathway follows the 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) route rather than the 2,5-diformyfuran (DFF) route.

Boosting Photocatalytic Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran via Atomically Bridged Indium in In-SnS2

The photocatalytic conversion of biomass-based platform molecules, such as 5-hydroxymethylfurfural (HMF), holds significant importance for the utilization of biomass resources. This study focuses on the unique ability of atomically bridged indium (In) atoms that encourages inactive SnS2 surface and steer the selective HMF oxidation process under solar light. Experimental results suggest that In confined SnS2 structure provides not only favorable sites and electronic structures for the synergistic activation of HMF/O2 but also benefit in charge carrier dynamics, thus influencing the overall activity and selectivity of the SnS2 catalyst. In addition, in-situ spectroscopy and density functional theory (DFT) analysis uncovered the multifunctional role of In sites in promoting the key steps of the catalytic process, from reactive oxygen species (ROS) generation to regulated adsorption/activation of *HMF which serves as the rate limiting step of the overall HMF oxidation process. Consequently, the optimized In-SnS2-0.75 photocatalyst demonstrated excellent photo oxidation performance, reaching a high HMF conversion efficiency, yield, and selectivity of 91.2, 73 and 80 % respectively, in just two hours of the reaction. This study highlights the strategic approach of rationally designed catalytic systems in order to tune the ROS and the product distribution of the HMF oxidation process.

Understanding Biomass Valorization through Electrocatalysis: Transformation of Glycerol and Furan Derivatives

Electrochemical conversion of underutilized biomass provides a green approach for their valorization into high-value-added products, which constitutes a mild, safe, and green procedure. Moreover, the use of an electrochemical pathway in contrast to the classical routes (thermo or biochemical) offers several advantages in terms of product selectivity, safety, catalyst stability, and reusability. The highly variable number of tunable conditions in an electrochemical reaction offers a broad space for improvement until the optimum ones are achieved, including substrates, curent and voltage, electrodes, electrolytes, and cell set-up. The present Perspective aims to provide an informative overview into the biomass and waste valorization of furan derivatives and byproducts of biofuel refineries (glycerol) by reviewing the essential aspects of this field. We cover the fundamentals of electrochemical organic transformations, emphasizing the different parameters to consider during these procedures. We highlight the potential of electrochemical methods for biomass valorization and suggest new directions for more sustainable research.

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.

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

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

Lattice Distortions Promoting the In-Depth Reconstruction of Ni-Based Electrocatalysts with Enriched Oxygen Vacancies for the Electrochemical Oxidation of 5-Hydroxymethylfurfural toward 2,5-Furandicarboxylic Acid

The electrocatalytic 5-hydroxymethylfurfural (HMF) oxidation reaction (HMFOR) toward 2,5-furandicarboxylic acid (FDCA) has been considered a promising approach for the substitution of the energy-consuming and hazardous oxygen evolution reaction and for the valorization of renewable biomass. However, it is limited by the susceptibility of HMF to the oxidative environment and requires efficient electrocatalysts. Herein, a NiMo complex (NiMo-N) is provided as the precatalyst for the HMFOR, exhibiting favorable performances with a current density of 450 mA·cm-2 achieved at an anodic potential of 1.4 V vs RHE (similarly hereinafter) with 50 mmol/L (mM) HMF and over 95% HMF conversion and FDCA FE for at least five cycles. Combined with quasi situ and in situ analysis, it is confirmed that the extensive lattice distortions in the precatalyst facilitate the in-depth reconstruction, increasing the accessible Ni sites and defective oxygen vacancies (Ov), which would promptly convert to high-valence Ni and active O species during the reaction. The improved performance is then attributed to the incorporation of the improved chemisorption and dehydrogenation ability of HMF by the as-evolved active sites.

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.

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.

Mn-Doped Ni(OH)2 Nanosheets as High-Performance Electrocatalyst for 5-Hydroxymethylfurfural Electrooxidation

Converting 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) via electrooxidation is a sustainable approach for generating high-value chemicals from biomass. This study presents Mn-doped Ni(OH)2 nanosheets as an effective electrocatalyst for HMF electrooxidation. The Mn-doped Ni(OH)2 nanosheets were synthesized through a microwave-assisted deep eutectic solvent (DES) strategy, followed by an alkaline reflux process. The as synthesized Mn-doped Ni(OH)2 nanosheets demonstrated remarkable catalytic performance, achieving 100 % HMF conversion, 99.0 % FDCA yield, and 98.8 % Faraday efficiency. Analysis using X-ray photoelectron spectroscopy (XPS), open circuit potential (OCP), and density functional theory (DFT) revealed that Mn doping induced surface charge redistribution and electron hole formation, enhancing HMF adsorption and facilitating its oxidation. This study not only elucidates the role of Mn doping in Ni(OH)2 catalyst for HMF electrooxidation but also introduces an efficient electrocatalyst for biomass conversion.

Proton-Coupled Electron Transfer in Photoelectrochemical Alcohol Oxidation Enhanced by Nickel-Based Cocatalysts

Using biomass oxidation reactions instead of water oxidation reactions is optimal for accomplishing biomass conversion and effective hydrogen generation. Here, we report that α-Fe2O3 photoanodes with a NiOOH cocatalyst exhibit excellent performance for photoelectrochemical oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). The conversion efficiency for HMF reaches 98.5 %, while the selectivity for FDCA is 94.2 %. We revealed that HMF is oxidized through a spontaneous proton-coupled electron transfer (PCET) process with the high-valent phase of the Ni-based catalyst. The dangling oxygen and bridging oxygen of the high-valent phase species serve as proton-accepting sites. Furthermore, we pointed out that the deprotonated bond dissociation free energy difference between the catalysts and alcohols is the thermodynamic trigger for the PCET process. This study provides a reasonable explanation for the alcohol oxidation reaction, which is beneficial for designing biomass conversion systems.

Sustainable production of 2,5-diformylfuran via peroxymonosulfate-triggered mild catalytic oxidation of lignocellulosic biomass

The relentless depletion of fossil fuels accentuates the urgent necessity for the sustainable synthesis of chemicals from renewable biomass. 5-Hydroxymethylfurfural (HMF), extracted from lignocellulosic biomass, emerges as a beacon of hope for conversion into value-added chemicals. However, the inherent susceptibility of its unsaturated aldehyde groups to excessive oxidation often culminates in undesired reactions, compromising both the yield and specificity of the desired products. Here, we introduce a holistic methodology for the cost-effective and ecologically responsible generation of 2,5-diformylfuran (DFF), through the heterogeneously catalyzed oxidation of HMF utilizing peroxymonosulfate (PMS) under benign conditions. This strategy, characterized by the meticulous enhancement of surface ketone groups via a mixed-salt-assisted co-pyrolysis technique, achieves an unparalleled selective activation of PMS, engendering singlet oxygen to catalyze the oxidation of HMF into DFF with a selectivity surpassing 80%. Life-cycle assessments underscore a negligible impact on human health, ecosystems, and natural resources, endorsing the holistic utilization of biomass. This integration of pyrolysis for the creation of functional carbonaceous materials within biomass conversion processes significantly enhances sustainability and economic viability, while paving green pathways for selective biomass oxidation and the production of high-value chemicals.

Visible Light-Switchable Lattice Oxygen Sites for Selective C-H and C(O)-C Bond Electrooxidation

Lattice-oxygen is highly oxidizable, ideal for electrocatalytic C-H oxidation but insufficient alone for C(O)-C bond cleavage due to the non-removable nature of lattice sites. Here, we present a visible light-assisted electrochemical method of in situ formulating removable lattice-oxygen sites in a nickel-oxyhydroxide (ESE-NiOOH) electrocatalyst. This catalyst efficiently converts aromatic alcohols and carbonyls with C(O)-C fragments from lignin and plastics into benzoic acids (BAs) with high yields (83-99 %). Without light irradiation, ESE-NiOOH's intrinsic lattice-oxygen is non-removable and inert for C(O)-C bond cleavage. In situ characterizations show light-induced lattice-oxygen removal and regeneration via OH- refilling. Theoretical calculations identify the nucleophilic oxygen attack on ketone-derived carbanion as a rate-determining step, which can be remarkably facilitated by removable lattice-oxygen to activate α-C-H bonds. As a proof-of-concept, an "electrochemical funnel" strategy is developed for high-efficiency upgrading aromatic mixtures with C(O)-C moieties into BA with up to 94 % yield. This in situ removal-regeneration approach for lattice sites opens an avenue for the tailored design of interfacial electrocatalysts to selectively upcycle waste carbon sources into valuable products.

Quasi-homogeneous photoelectrochemical organic transformations for tunable products and 100% conversion ratio

Photoelectrochemical (PEC) organic transformation at the anode coupled with cathodic H2 generation is a potentially rewarding strategy for efficient solar energy utilization. Nevertheless, achieving the full conversion of organic substrates with exceptional product selectivity remains a formidable hurdle in the context of heterogeneous catalysis at the solid/liquid interface. Here, we put forward a quasi-homogeneous catalysis concept by using the reactive oxygen species (ROS), such as ·OH, H2O2 and SO4•-, as a charge transfer mediator instead of direct heterogeneous catalysis at the solid/liquid interface. In the context of glycerol oxidation, all ROS exhibited a preference for first-order reaction kinetics. These ROS, however, showcased distinct oxidation mechanisms, offering a range of advantages such as ∼ 100 % conversion ratios and the flexibility to tune the resulting products. Glycerol oxidative formic acid with Faradaic efficiency (FE) of 81.2 % was realized by the H2O2 and ·OH, while SO4•- was preferably for glycerol conversion to C3 products like glyceraldehyde and dihydroxyacetone with a total FE of about 80 %. Strikingly, the oxidative coupling of methane to ethanol was successfully achieved in our quasi-homogeneous system, yielding a remarkable production rate of 12.27 μmol h-1 and an impressive selectivity of 92.7 %. This study is anticipated to pave the way for novel approaches in steering solar-driven organic conversions by manipulating ROS to attain desired products and conversion ratios.

Molybdenum-doping to enhance the deprotonation ability of nickel-based hydroxide electrocatalysts for ethanol oxidation

With technological advancements, the practical application of ethanol oxidation reaction (EOR) is becoming increasingly promising, yet the need for higher ethanol concentrations highlights the growing importance of the deprotonation ability (Ni2+ to Ni3+) of the catalyst. The deprotonation ability is the key step for nickel-based catalysts in EOR, as it is essential for Ni2+ to continuously undergo deprotonation to transform into Ni3+ in order to maintain the continuous EOR. Herein, we developed Mo-doped Ni(OH)2 nanosheets by a hydrothermal method. The Mo-doped Ni(OH)2 nanosheets show excellent EOR performance due to the high valence doping of Mo, the onset potential of the oxidation peak (Ni2+ to Ni3+) appears at a position with a small overpotential,. The in-situ Raman spectroscopy technique further characterized the increase in NiOOH in the process of EOR. The Mo-doped Ni(OH)2 nanocomposite catalyst facilitates the oxidation of Ni2+ into Ni3+. Based on the above theoretical guidance, Mo-doped Fe/Ni(OH)2 nanosheets was designed and synthesized. The outstanding EOR performance of the Mo-Fe/Ni(OH)2-3 showed a potential of 1.352 V at 10 mA cm-2. The catalyst was used to design three-electrode reversible zinc-ethanol-air battery (T-RZEAB), which effectively overcomes the opposing kinetic and thermodynamic requirements for EOR and oxygen reduction reaction (ORR) catalysts in the oxygen electrode. The charging voltage of T-RZEAB with Mo-Fe/Ni(OH)2-3 is 240 mV lower than that of a traditional zinc-air battery at 25 mA cm-2.

Hydrogen-Bond-Assisted Electrocatalytic Semi-Oxidation of 5-Hydroxymethylfurfural into 2,5-Diformylfuran by Operando Dissociated N-Oxyl Mediator

The conversion of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) is a promising approach for enhancing biomass utilization. Nevertheless, traditional methods using noble metal catalysts face challenges due to high costs and poor selectivity towards DFF. Herein, we developed a novel catalytic electrode integrating N-hydroxyphthalimide (NHPI) into a metal-organic framework on a hydrophilic carbon cloth. This design significantly enhances the selective adsorption of HMF due to stronger hydrogen-bond interaction between the electrode's hydrophilic surface and the C(sp3)-OH group in HMF compared to the C(sp2)=O in DFF. Additionally, the electro-driven dissociation of the NHPI-linker generates stabilized N-Oxyl radicals that promote selective semi-oxidation of HMF under neutral conditions. As a result, this approach achieves a high yield rate of 138.2 mol molcat -1 h-1 with a selectivity of 96.7 % for the HMF-to-DFF conversion. This work introduces a novel strategy for designing catalytic electrodes with stabilized N-Oxyl radicals, and offers a promising method for electrocatalytic DFF synthesis, leveraging hydrogen-bond interaction between electrode surface and HMF.

Nickel Hydroxide-Based Electrocatalysts for Promising Electrochemical Oxidation Reactions: Beyond Water Oxidation

Transition metal hydroxides have attracted significant research interest for their energy storage and conversion technique applications. In particular, nickel hydroxide (Ni(OH)2), with increasing significance, is extensively used in material science and engineering. The past decades have witnessed the flourishing of Ni(OH)2-based materials as efficient electrocatalysts for water oxidation, which is a critical catalytic reaction for sustainable technologies, such as water electrolysis, fuel cells, CO2 reduction, and metal-air batteries. Coupling the electrochemical oxidation of small molecules to replace water oxidation at the anode is confirmed as an effective and promising strategy for realizing the energy-saving production. The physicochemical properties of Ni(OH)2 related to conventional water oxidation are first presented in this review. Then, recent progress based on Ni(OH)2 materials for these promising electrochemical reactions is symmetrically categorized and reviewed. Significant emphasis is placed on establishing the structure-activity relationship and disclosing the reaction mechanism. Emerging material design strategies for novel electrocatalysts are also highlighted. Finally, the existing challenges and future research directions are presented.

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.

Electrocatalytic water-to-oxygenates conversion: redox-mediated versus direct oxygen transfer

Electrocatalytic oxygenation of hydrocarbons with high selectivity has attracted much attention for its advantages in the sustainable and controllable production of oxygenated compounds with reduced greenhouse gas emissions. Especially when utilizing water as an oxygen source, by constructing a water-to-oxygenates conversion system at the anode, the environment and/or energy costs of producing oxygenated compounds and hydrogen energy can be significantly reduced. There is a broad consensus that the generation and transformation of oxygen species are among the decisive factors determining the overall efficiency of oxygenation reactions. Thus, it is necessary to elucidate the oxygen transfer process to suggest more efficient strategies for electrocatalytic oxygenation. Herein, we introduce oxygen transfer routes through redox-mediated pathways or direct oxygen transfer methods. Especially for the scarcely investigated direct oxygen transfer at the anode, we aim to detail the strategies of catalyst design targeting the efficient oxygen transfer process including activation of organic substrate, generation/adsorption of oxygen species, and transformation of oxygen species for oxygenated compounds. Based on these examples, the significance of balancing the generation and transformation of oxygen species, tuning the states of organic substrates and intermediates, and accelerating electron transfer for organic activation for direct oxygen transfer has been elucidated. Moreover, greener organic synthesis routes through heteroatom transfer and molecular fragment transfer are anticipated beyond oxygen transfer.

Enhanced electrocatalytic biomass oxidation at low voltage by Ni2+-O-Pd interfaces

Challenges in direct catalytic oxidation of biomass-derived aldehyde and alcohol into acid with high activity and selectivity hinder the widespread biomass application. Herein, we demonstrate that a Pd/Ni(OH)2 catalyst with abundant Ni2+-O-Pd interfaces allows electrooxidation of 5-hydroxymethylfurfural to 2, 5-furandicarboxylic acid with a selectivity near 100 % and 2, 5-furandicarboxylic acid yield of 97.3% at 0.6 volts (versus a reversible hydrogen electrode) in 1 M KOH electrolyte under ambient conditions. The rate-determining step of the intermediate oxidation of 5-hydroxymethyl-2-furancarboxylic acid is promoted by the increased OH species and low C-H activation energy barrier at Ni2+-O-Pd interfaces. Further, the Ni2+-O-Pd interfaces prevent the agglomeration of Pd nanoparticles during the reaction, greatly improving the stability of the catalyst. In this work, Pd/Ni(OH)2 catalyst can achieve 100% 5-hydroxymethylfurfural conversion and >90% 2, 5-furandicarboxylic acid selectivity in a flow-cell and work stably over 200 h under a fixed cell voltage of 0.85 V.

Crystal structure optimization of copper oxides for the benzyl alcohol oxidation reaction

In this work, experimental and theoretical analyses reveal that different types of Cu wires significantly change the adsorption properties of reactant molecules and the benzyl alcohol oxidation reaction performance. In particular, CuO nanowires in situ grown on Cu foam exhibit the best performance with a low potential of 1.39 V at a current density of 200 mA cm-2, high selectivity to benzoic acid production, and good operational stability.

The progress of research on vacancies in HMF electrooxidation

5-Hydroxymethylfurfural (HMF), serving as a versatile platform compound bridging biomass resource and the fine chemicals industry, holds significant importance in biomass conversion processes. The electrooxidation of HMF plays a crucial role in yielding the valuable product (2,5-furandicarboxylic acid), which finds important applications in antimicrobial agents, pharmaceutical intermediates, polyester synthesis, and so on. Defect engineering stands as one of the most effective strategies for precisely synthesizing electrocatalytic materials, which could tune the electronic structure and coordination environment, and further altering the adsorption energy of HMF intermediate species, consequently increasing the kinetics of HMF electrooxidation. Thereinto, the most routine and effective defect are the anionic vacancies and cationic vacancies. In this concise review, the catalytic reaction mechanism for selective HMF oxidation is first elucidated, with a focus on the synthesis strategies involving both anionic and cationic vacancies. Recent advancements in various catalytic oxidation systems for HMF are summarized and synthesized from this perspective. Finally, the future research prospects for selective HMF oxidation are discussed.

CuO-Ni(OH)2 heterostructure nanosheets: a high-performance electrocatalyst for 5-hydroxymethylfurfural oxidation

CuO-Ni(OH)2 heterostructure nanosheets were designed for efficient electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furanedioic acid (FDCA). The CuO-Ni(OH)2 nanosheets exhibited impressive performance, achieving 100% HMF conversion, 99.8% FDCA yield, and 98.4% faradaic efficiency.

Effect of K+ and Ca2+ Cations on Structural Manganese(IV) Oxide for the Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

The promising influences of K+ and Ca2+ ions in the development of effective MnO2 for the selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid (FDCA) were studied for the catalytic performance under a high-pressure reaction of aqueous O2 (0.5 MPa) in a basic system. Various oxidation states of manganese in MnO2 were able to accelerate the oxidation of 5-formyl-2-furancarboxylic acid to FDCA in the rate-determining step. The results were in good agreement that Ca2+ played a key role in the highest FDCA yield up to 85% due to the associated cations on the local coordination to enhance the high surface area and the electronic effect on the manganese ion. Both K-MnO2 and Ca-MnO2 catalysts showed excellent catalytic activities without a significant change in the efficiency in the reusability experiments.

Ni/TiO2 heterostructures derived from phase separation for enhanced electrocatalysis of hydrogen evolution and biomass oxidative upgrading in anion exchange membrane electrolyzers

The construction of heterostructures is an effective strategy to enhance electrocatalysis for hydrogen evolution reactions (HERs) and biomass oxidative upgrading. In this work, a Ni/TiO2 heterostructure prepared by a phase-separation strategy was adopted as a bifunctional electrocatalyst for HERs and biomass oxidation in alkaline media. Due to the optimized hydrogen adsorption energetics as well as the interfacial water structure and hydrogen bond connectivity in the electrical double layer, Ni/TiO2 exhibited high activity for HERs with an overpotential of 28 mV at 10 mA cm-2 and good durability at 1000 mA cm-2 for over 100 h in an anion exchange membrane (AEM) electrolyzer. In addition, Ni/TiO2 showed high catalytic performance for the oxidation of biomass-based platform compound 5-hydroxymethylfurfural (HMF) to high-value added compound 2,5-furandicarboxylic acid (FDCA). Continuous production of FDCA with a yield >95% was achieved in the AEM electrolyzer for over 50 h. The superior HMF oxidation performance on the Ni/TiO2 heterostructure compared to Ni resulted from stronger HMF adsorption, lower Ni3+-O formation potential, longer Ni3+-O bond and smaller Ni crystal size.

Design and Synthesis of Porous Organic Polymers: Promising Catalysts for Lignocellulose Conversion to 5-Hydroxymethylfurfural and Derivates

In the face of the current energy and environmental problems, the full use of biomass resources instead of fossil energy to produce a series of high-value chemicals has great application prospects. 5-hydroxymethylfurfural (HMF), which can be synthesized from lignocellulose as a raw material, is an important biological platform molecule. Its preparation and the catalytic oxidation of subsequent products have important research significance and practical value. In the actual production process, porous organic polymer (POP) catalysts are highly suitable for biomass catalytic conversion due to their high efficiency, low cost, good designability, and environmentally friendly features. Here, we briefly describe the application of various types of POPs (including COFs, PAFs, HCPs, and CMPs) in the preparation and catalytic conversion of HMF from lignocellulosic biomass and analyze the influence of the structural properties of catalysts on the catalytic performance. Finally, we summarize some challenges that POPs catalysts face in biomass catalytic conversion and prospect the important research directions in the future. This review provides valuable references for the efficient conversion of biomass resources into high-value chemicals in practical applications.