In Situ Electrochemical Conversion of Biomass-Derived 5-Hydroxymethylfurfural into 2,5-Furandicarboxylic Acid by Time-Controlled Aerosol-Assisted Chemical Vapor Deposited FeNi Catalyst

The conversion of 5-hydroxymethylfurfural (HMF) into valuable chemicals, such as 2,5-furandicarboxylic acid (FDCA), is pivotal for sustainable chemical production, offering a renewable pathway to biodegradable plastics and high-value organic compounds. This pioneering study explores the synthesis of FeNi nanostructures via aerosol-assisted chemical vapor deposition (AACVD) for the electrochemical oxidation of HMF to FDCA. By adjusting the deposition time, we developed two distinct nanostructures: FeNi-40, which features nanowires with spherical terminations, and FeNi-80, which features aggregated spherical structures. X-ray diffraction (XRD) confirmed that both nanostructures possess a phase-pure face-centered cubic (FCC) crystal structure. Electrochemical tests conducted using FeNi nanocatalysts on Ni foam revealed that FeNi-40 requires a significantly lower onset potential for HMF oxidation (1.32 V vs RHE) compared to FeNi-80 (1.40 V vs RHE). This difference is attributed to the unique nanowire morphology of FeNi-40, which provides a higher density of active sites and a larger electrochemically active surface area, thereby enhancing the efficiency of the electrochemical process. When tested in an H-type electrolyzer with a Nafion membrane, FeNi-40 demonstrated a remarkable Faradaic efficiency of 96.42% and a high product yield, underscoring the potential of morphology-controlled FeNi nanostructures to enhance the efficiency of sustainable electrochemical processes significantly.

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.

Unraveling the Electrocatalytic Activity in HMF Oxidation to FDCA by Fine-Tuning the Degree of NiOOH Phase Over Ni Nanoparticles Supported on Graphene Oxide

The development of an efficient electrocatalyst for HMF oxidation to FDCA has been in the early stages. Herein, the NiNPs/GO-Ni-foam is fabricated as an electrocatalyst for FDCA production. However, the electrocatalytic performance of the untreated NiNPs/GO-Ni-foam is observed with moderate Faradaic efficiency (FE) (73.0%) and FDCA yield (80.2%). By electrochemically treating the NiNPs/GO-Ni-foam in an alkaline solution with positive potential at different treatment durations, the degree of NiOOH on metal surfaces is changed. The distinctive electrocatalytic activity obtained when using the different NiOOH degrees allows to understand the crucial impact of NiOOH species in HMF electrooxidation. Enhancing the portion of the NiOOH phase on the electrocatalyst surface improves electrocatalytic activity in terms of FE and FDCA yield up to 94.8±4.8% and 86.9±4.1%, respectively. Interestingly, as long as the NiOOH portion on the electrocatalyst surface is preserved or regenerated, the electrocatalyst performance can be intact even after several catalytic cycles. The theoretical study via density functional theory (DFT) also agrees with the experimental observations and confirms that the NiOOH phase facilitates the electrochemical transformation of HMF to FDCA through the HMFCA pathway, and the potential limiting step of the overall reaction is the oxidation of FFCA to FDCA.

Facile synthesis of nanostructured Ni/NiO/N-doped graphene electrocatalysts for enhanced oxygen evolution reaction

Electrocatalysts containing a Ni/NiO/N-doped graphene interface have been synthesised using the ligand-assisted chemical vapor deposition technique. NiO nanoparticles were used as the substrate to grow N-doped graphene by decomposing vapours of benzene and N-containing ligands. The method was demonstrated with two nitrogen-containing ligands, namely dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (L) and melamine (M). The structure and composition of the as-synthesized composites were characterized by XRD, Raman spectroscopy, SEM, TEM and XPS. The composite prepared using the ligand L had NiO sandwiched between Ni and N-doped graphene and showed an overpotential of 292 mV at 10 mA cm-2 and a Tafel slope of 45.41 mV dec-1 for the OER, which is comparable to the existing noble metal catalysts. The composite prepared using the ligand M had Ni encapsulated by N-doped graphene without NiO. It showed an overpotential of 390 mV at 10 mA cm-2 and a Tafel slope of 78.9 mV dec-1. The ligand-assisted CVD route demonstrates a facile route to control the microstructure of the electrocatalysts.

Ni/NiO@NC as a highly efficient and durable HER electrocatalyst derived from nickel(II) complexes: importance of polydentate amino-acid ligands

Research on Ni/NiO electrocatalysts has advanced significantly, but the main obstacles to their use and commercialization remain their relatively ordinary activity and stability. In this paper, a chelating structure based on the coordination of multidentate ligands and Ni(II) is proposed to limit the growth of Ni and Ni oxide grains. These features reduce the particle size of Ni/NiO, increase particle dispersion, and maintain the high activity and stability of the catalyst. Aspartic acid, as a polydentate ligand, could coordinate with Ni2+ to form structurally stable chelate rings. The latter can limit grain growth, but also coat the active core with thin carbon layers after calcination to further achieve the confinement and protection of nanoparticles. The hydrogen evolution overpotential of prepared nitrogen-doped graphitized carbon shells (Ni/NiO@NC) nanoparticles was 100 mV (vs. RHE) when the current density was 10 mA cm-2 in 1 M KOH. The hydrogen evolution overpotential increased by only 4 mV after 6000 continuous cyclic-voltammetry scans. Moreover, when coated on different conductive substrates, the overpotential of this catalyst dropped to 34.6 mV (vs. RHE) at a current density of 10 mV cm-2. The lowest overpotential of the composite was only 194.9 mV at a current density of 100 mA cm-2, which is comparable with that of noble metal-based electrocatalysts. This work provides a plausible method for designing high-performance electrocatalysts of small size.

Bifunctional Al-Doped Cobalt Ferrocyanide Nanocube Array for Energy-Saving Hydrogen Production via Urea Electrolysis

The very slow anodic oxygen evolution reaction (OER) greatly limits the development of large-scale hydrogen production via water electrolysis. By replacing OER with an easier urea oxidation reaction (UOR), developing an HER/UOR coupling electrolysis system for hydrogen production could save a significant amount of energy and money. An Al-doped cobalt ferrocyanide (Al-Co2Fe(CN)6) nanocube array was in situ grown on nickel foam (Al-Co2Fe(CN)6/NF). Due to the unique nanocube array structure and regulated electronic structure of Al-Co2Fe(CN)6, the as-prepared Al-Co2Fe(CN)6/NF electrode exhibited outstanding catalytic activities and long-term stability to both UOR and HER. The Al-Co2Fe(CN)6/NF electrode needed potentials of 0.169 V and 1.118 V (vs. a reversible hydrogen electrode) to drive 10 mA cm-2 for HER and UOR, respectively, in alkaline conditions. Applying the Al-Co2Fe(CN)6/NF to a whole-urea electrolysis system, 10 mA cm-2 was achieved at a cell voltage of 1.357 V, which saved 11.2% electricity energy compared to that of traditional water splitting. Density functional theory calculations demonstrated that the boosted UOR activity comes from Co sites with Al-doped electronic environments. This promoted and balanced the adsorption/desorption of the main intermediates in the UOR process. This work indicates that Co-based materials as efficient catalysts have great prospects for application in urea electrolysis systems and are expected to achieve low-cost and energy-saving H2 production.

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.

Interface engineering of the NiO/CeO2@NF heterostructure to boost the electro-oxidation of 5-hydroxymethylfurfural

The synthesis of furan-based platform chemicals from abundant and renewable biomass-based hexoses plays an important role in the development and utilization of biomass energy. The electrochemical 5-hydroxymethylfurfural oxidation reaction (HMFOR) represents a promising route for synthesizing the 2,5-furandicarboxylic acid (FDCA) product which is a high value-added biomass-based monomer. Interface engineering is an effective strategy to adjust the electronic structure, optimize the adsorption of intermediates, and expose more active sites, thus attracting extensive attention for designing efficient HMFOR electrocatalysts. Herein, a NiO/CeO₂@NF heterostructure with an abundant interface is designed for boosting the HMFOR performance under alkaline conditions. At 1.475 V vs. RHE, the conversion of HMF is nearly 100%, the selectivity of FDCA is 99.0%, and the faradaic efficiency is as high as 98.96%. The NiO/CeO₂@NF electrocatalyst also exhibits robust stability for HMFOR for 10 cycles. When coupled with the cathode hydrogen evolution reaction (HER) in alkaline medium, the yields of FDCA and hydrogen production are 197.92 and 600 μmol cm^-2 h^-1, respectively. The NiO/CeO₂@NF catalyst is also suitable for the electrocatalytic oxidation of other biomass-derived platform compounds. The abundant interface between NiO and CeO₂, which can regulate the electronic properties of Ce and Ni atoms, improve the oxidation state of Ni species, regulate intermediate adsorption, and promote electron/charge transfer, makes the most contribution to high HMFOR performance. This work will provide a simple route for the design of heterostructured materials and reveal the application prospect of interface engineering for promoting the upgrading of biomass derivatives.

Stabilization of transition metal heterojunctions inside porous materials for high-performance catalysis

Transition metal-based heterostructural materials are a class of very promising substitutes for noble metal-based catalysts for high-performance catalysis, due to their inherent internal electric field at the interface in the heterojunctions, which could induce electron relocalization and facilitate charge carrier migration between different metal sites at heterostructural boundaries. However, redox-active metal species suffer from reduction, oxidation, migration, aggregation, leaching and poisoning in catalysis, which results in heavy deterioration of the catalytic properties of transition metal-based heterojunctions and frustrates their practical applications. To improve the stability of transition metal-based heterojunctions and sufficiently expose redox-active sites at the heterosurfaces, many kinds of porous materials have been used as porous hosts for the stabilization of non-precious metal heterojunctions. This review article will discuss recently developed strategies for encapsulation and stabilization of transition metal heterojunctions inside porous materials, and highlight their improved stability and catalytic performance through the spatial confinement effect and synergistic interaction between the heterojunctions and the host matrices.

Nickel-phytic acid hybrid for highly efficient electrocatalytic upgrading of HMF

Electrocatalytic upgrading of 5-hydroxymethylfurfural (HMF) provides a promising way to obtain both high-value-added biomass-derived chemicals and clean energy. However, development of efficient electrocatalysts for oxidizing HMF with depressed side reactions remains a challenge. Herein, we report a nickel-phytic acid hybrid (Ni-PA) using natural phytic acid as building block for highly efficient electrocatalytic oxidation of HMF to 2, 5-furandicarboxylic acid (FDCA). Due to the coordination of nickel ion and phosphate groups of phytic acid molecule, high selectivity and yield of FDCA were achieved at 1.6 V vs. RHE. Besides, Ni-PA has a higher electrochemical surface area and lower charge-transfer resistance than Cu/Fe-PA, which significantly promotes the oxidation of HMF to FDCA. This work demonstrates the potential of metal-phytic acid hybrids as effective electrocatalysts for biomass valorization.

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

Hollow Ni/NiO/C composite derived from metal-organic frameworks as a high-efficiency electrocatalyst for the hydrogen evolution reaction

Metal-organic frameworks (MOFs) constitute a class of crystalline porous materials employed in storage and energy conversion applications. MOFs possess characteristics that render them ideal in the preparation of electrocatalysts, and exhibit excellent performance for the hydrogen evolution reaction (HER). Herein, H-Ni/NiO/C catalysts were synthesized from a Ni-based MOF hollow structure via a two-step process involving carbonization and oxidation. Interestingly, the performance of the H-Ni/NiO/C catalyst was superior to those of H-Ni/C, H-NiO/C, and NH-Ni/NiO/C catalysts for the HER. Notably, H-Ni/NiO/C exhibited the best electrocatalytic activity for the HER, with a low overpotential of 87 mV for 10 mA cm^-2 and a Tafel slope of 91.7 mV dec^-1. The high performance is ascribed to the synergistic effect of the metal/metal oxide and hollow architecture, which is favorable for breaking the H-OH bond, forming hydrogen atoms, and enabling charge transport. These results indicate that the employed approach is promising for fabricating cost-effective catalysts for hydrogen production in alkaline media.

Morphology Modulation and Phase Transformation of Manganese-Cobalt Carbonate Hydroxide Caused by Fluoride Doping and Its Effect on Boosting the Overall Water Electrolysis

Increasing demands for pollution-free energy resources have stimulated intense research on the design and fabrication of highly efficient, inexpensive, and stable non-noble earth-abundant metal catalysts with remarkable catalytic activity for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Morphology control of the catalysts is widely implemented as an effective strategy to change the surface atomic coordination and increase the catalytic behavior of the catalysts. In this study, we have designed a series of Mn-Co catalyts with different morphologies on the graphite paper substrate to enhance OER and HER activities in alkaline media. The prepared catalysts with different morphologies were successfully obtained by adjusting the amount of ammonium fluoride (NH₄F) in the hydrothermal process. The electrochemical tests display that the cubic-like Mn-Co catalyst with pyramids on the faces at a concentration of 0.21 M NH₄F exhibits the best activity toward both OER and HER. The cubic-like Mn-Co catalyst with pyramids on the faces showed overpotentials of 240 and 82 mV at a current density of 10 mA cm^-2 for OER and HER, respectively. Also, the cubic-like Mn-Co catalyst with pyramids on the faces required overpotentials of 319 and 216 mV to reach the current density of 100 mA cm^-2 for OER and HER, respectively. The current density of this catalyst at η = 0.32 V was 701.05 mA cm^-2 for OER, and for HER, the current density of the catalyst was 422.89 mA cm^-2 at η = 0.23 V. The Tafel slopes of the Mn-Co catalyst with cubic-like structures with pyramids on the faces were 78 and 121 mV dec^-1 for OER and HER, respectively. A two-electrode overall water electrolysis system using this bifunctional Mn-Co catalyst exhibited low cell voltages of 1.60 in the alkaline electrolyte at the standard current density of 10 mA cm^-2 with appropriate stability. These electrochemical merits exhibit the considerable potential of the cubic-like Mn-Co catalyst with pyramids on the faces for bifunctional OER and HER applications.

Amorphous chromium oxide confined Ni/NiO nanoparticles-assembled nanosheets for highly efficient and stable overall urea splitting

Applications of urea oxidation reaction (UOR) in various sustainable energy-conversion systems are greatly hindered by its slow kinetics. Herein, we demonstrate an in-situ confined synthesis method that produces amorphous chromium oxide confined Ni/NiO nanoparticles-assembled nanosheets (Ni/NiO@CrOx) with fast reaction kinetics towards UOR. The confinement effect of the in-situ generated CrOx overlay contributes to ultrafine Ni/NiO nanoparticles, bringing about rich Ni/NiO and NiO/CrOx interfaces. In-situ Raman and electrochemical characterization show that both CrOx and metallic Ni can promote the formation of the NiOOH species and the electron transfer, leading to high intrinsic activity and fast reaction kinetics. At 1.40 V vs. reversible hydrogen electrode, the Ni/NiO@CrOx delivers a current density of 275 mA cm-2, which is about 2.6 and 6.1 times as large as those of the NiO@CrOx and NiO, respectively. In addition, the protective effect of the CrOx overlay leads to robust working stability towards UOR. Further, the Ni/NiO@CrOx nanosheets are used as bifunctional catalysts for overall urea splitting, and a small electrolysis cell voltage of 1.44 V is needed to reach the benchmark current density of 10 mA cm-2.

Ni2 Se3 -CuSex Heterostructure as a Highly Efficient Bifunctional Electrocatalyst for Urea-Assisted Hydrogen Generation

Coupling urea oxidation reaction (UOR) with hydrogen evolution reaction (HER) is an attractive alternative anode reaction for electrochemical hydrogen generation with low energy consumption. However, the development of highly efficient bifunctional electrocatalysts is still a challenge. In this work, Ni₂ Se₃ -CuSe_x heterostructure was synthesized on copper foam (Ni₃ Se₂ @CuSe_x /CF) by electrodeposition accompanied by a selenization process. Benefiting from the abundant active sites, faster reaction kinetics, and modulated electronic structure, the self-supporting Ni₃ Se₂ @CuSe_x /CF electrode exhibited superior catalytic performance. Extremely low overpotentials of 120 and 140 mV were achieved at the current density of 100 mA cm^-2 for HER/UOR, respectively. Respectively, in HER||UOR coupled electrolyzer for H₂ generation, the Ni₃ Se₂ @CuSe_x /CF||Ni₃ Se₂ @CuSe_x /CF delivered a low cell voltage of 1.49 V to reach a high current density of 100 mA cm^-2 along with good stability, outperforming most of the other well-developed materials to date. The rational design of coupled heterostructure as bifunctional electrodes is a promising approach for energy-saving H₂ production.

Current Advances in the Sustainable Conversion of 5-Hydroxymethylfurfural into 2,5-Furandicarboxylic Acid

2,5-Furandicarboxylic acid (FDCA) is currently considered one of the most relevant bio-sourced building blocks, representing a fully sustainable competitor for terephthalic acid as well as the main component in green polymers such as poly(ethylene 2,5-furandicarboxylate) (PEF). The oxidation of biobased 5-hydroxymethylfurfural (HMF) represents the most straightforward approach to obtain FDCA, thus attracting the attention of both academia and industries, as testified by Avantium with the creation of a new plant expected to produce 5000 tons per year. Several approaches allow the oxidation of HMF to FDCA. Metal-mediated homogeneous and heterogeneous catalysis, metal-free catalysis, electrochemical approaches, light-mediated procedures, as well as biocatalytic processes share the target to achieve FDCA in high yield and mild conditions. This Review aims to give an up-to-date overview of the current developments in the main synthetic pathways to obtain FDCA from HMF, with a specific focus on process sustainability.

Electro- and Photocatalytic Oxidative Upgrading of Bio-based 5-Hydroxymethylfurfural

Conversion of biomass into biofuels and high value-added chemicals is a promising strategy to solve the increasingly deteriorating environmental problems caused by fossil energy consumption. The development of efficient technologies and methods is the premise and guarantee to realize the high-value conversion of biomass. 5-Hydroxymethylfurfural (HMF), as a versatile biomass platform compound, is generated via dehydration of hexoses (e. g., fructose and glucose) derived from cellulosic biomass. This Review gives an overview of the advances and challenges of electro- and photocatalytic oxidation of biomass-derived HMF to high-value chemicals such as 2,5-formylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA). These strategies and methods for the preparation of high-value chemicals by electro- and photocatalytic oxidation of HMF, coupled with, for example, hydrogen evolution reaction, organic substrate reduction, CO₂ reduction reaction, or N₂ reduction reaction, were summarized and discussed. Moreover, the catalytic efficiency and mechanism of different types of catalysts were also introduced in these conversion systems.

Recent Progress in Graphene-Based Electrocatalysts for Hydrogen Evolution Reaction

Hydrogen is regarded as a key renewable energy source to meet future energy demands. Moreover, graphene and its derivatives have many advantages, including high electronic conductivity, controllable morphology, and eco-friendliness, etc., which show great promise for electrocatalytic splitting of water to produce hydrogen. This review article highlights recent advances in the synthesis and the applications of graphene-based supported electrocatalysts in hydrogen evolution reaction (HER). Herein, powder-based and self-supporting three-dimensional (3D) electrocatalysts with doped or undoped heteroatom graphene are highlighted. Quantum dot catalysts such as carbon quantum dots, graphene quantum dots, and fullerenes are also included. Different strategies to tune and improve the structural properties and performance of HER electrocatalysts by defect engineering through synthetic approaches are discussed. The relationship between each graphene-based HER electrocatalyst is highlighted. Apart from HER electrocatalysis, the latest advances in water electrolysis by bifunctional oxygen evolution reaction (OER) and HER performed by multi-doped graphene-based electrocatalysts are also considered. This comprehensive review identifies rational strategies to direct the design and synthesis of high-performance graphene-based electrocatalysts for green and sustainable applications.

CoP nanorods anchored on Ni2P-NiCoP nanosheets with abundant heterogeneous interfaces boosting the electrocatalytic oxidation of 5-hydroxymethyl-furfural

CoP nanorods anchored on Ni2P-NiCoP nanosheets have been fabricated. The heterogeneous interfaces can regulate the d-band centers of Co and Ni to approach the Fermi level, thus boosting the electrocatalytic oxidation of HMF.

Ni–NiO heterojunctions: a versatile nanocatalyst for regioselective halogenation and oxidative esterification of aromatics

Herein, we report a facile method for the synthesis of Ni–NiO heterojunction nanoparticles, which we utilized for the nuclear halogenation reaction of phenol and substituted phenols using N-bromosuccinimide (NBS).