Facile Synthesis of Cu/NiCu Electrocatalysts Integrating Alloy, Core-Shell, and One-Dimensional Structures for Efficient Methanol Oxidation Reaction

High loading of atomically exposed edge nickel sites embedded in hollow porous carbon nanofibers for enhanced methanol electrooxidation in direct methanol fuel cells

The enhancement of catalytic activity and durability for atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts in methanol oxidation reaction (MOR) anodes within direct methanol fuel cells presents a significant challenge. Here, we developed hollow porous nanofiber catalysts featuring edge Ni-N4 atomic sites through coaxial electrostatic spinning with domain-restricted Ni atoms embedded within a zeolitic imidazolium ester backbone, thereby increasing the exposure of accessible active sites (Ni: 4.96 %). The distinctive hollow porous fiber morphology and hierarchical structure facilitate convenient electronic conductivity and mass transport of reactants. Theoretical findings indicate that the surface adsorption of methanol at the edge Ni-N4 atomic sites exhibits negative free energy, promoting the adsorption and activation of reactants. Furthermore, the initial dehydrogenation step demonstrates a low free energy change, favoring reaction kinetics. The membrane electrode assembly achieved a power density of 42.2 mW cm-2 in single-cell application tests while displaying improved durability. This research provides valuable insights for future advancements in single-atom catalyst development for fuel cells or other energy applications.

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.

Synthesis of Copper Nanowires Using Monoethanolamine and the Application in Transparent Conductive Films

Copper nanowires (Cu NWs) are considered a promising alternative to indium tin oxide (ITO) and silver nanowires (Ag NWs) due to their excellent electrical conductivity, mechanical properties, abundant reserves, and low cost. They have been widely applied in various optoelectronic devices. In this study, Cu NWs were synthesized using copper chloride (CuCl2) as the precursor, monoethanolamine (MEA) as the complexing agent, and hydrated hydrazine (N2H4) as the reducing agent under strongly alkaline conditions at 60 °C. Notably, this is the first time that MEA has been employed as a complexing agent in this synthesis method for Cu NWs. Through a series of experiments, the optimal conditions for the CuCl2-MEA-N2H4 system in Cu NWs synthesis were determined. This study revealed that the presence of amines plays a crucial role in nanowire formation, as the co-ordination of MEA with copper in this system provides selectivity for the nanowire growth direction. MEA prevents the excessive conversion of Cu(I) complexes into Cu2O octahedral precipitates and exhibits an adsorption effect during Cu NWs formation. The different adsorption tendencies of MEA at the nanowire ends and lateral surfaces, depending on its concentration, influence the growth of the Cu NWs, as directly reflected by changes in their diameter and length. At an MEA concentration of 210 mM, the synthesized Cu NWs have an average diameter of approximately 101 nm and a length of about 28 μm. To fabricate transparent conductive films, the Cu NW network was transferred onto a polyethylene terephthalate (PET) substrate by applying a pressure of 20 MPa using a tablet press to ensure strong adhesion between the Cu NW-coated mixed cellulose ester (MCE) filter membrane and the PET substrate. Subsequently, the MCE membrane was dissolved by acetone and isopropanol immersion. The resulting Cu NW transparent conductive film exhibited a sheet resistance of 52 Ω sq-1 with an optical transmittance of 86.7%.

Efficient synergism of concentric ring structures and carbon dots for enhanced methanol electro-oxidation

Developing affordable and reliable electrocatalysts with high activity and stability is crucial for enhancing the practicality of direct methanol fuel cells (DMFCs). An effective and simple strategy of combining the carbon point of N-CDs (0.4 mg mL-1) with NiO/Ni for the fabrication of NiO/Ni-N-CDsV nanocomposites with a three-dimensional concentric core-shell structure was proposed to successfully prepare the electro-oxidation catalyst of methanol. The low cost of Ni-based materials and the conductive N-CDs that improve methanol catalytic performance make the composites an excellent choice as electrode materials for direct methanol fuel cells (DMFCs). The electrocatalytic behavior of methanol oxidation was studied using cyclic voltammetry and chronoamperometry. The results indicated that the catalytic activity of NiO/Ni-N-CDsV increased by 3.02 times, and the current density was stable during the operation for 83 hours, implying strong electrocatalytic stability. Furthermore, the electrocatalytic performance for ethanol, ethylene glycol, and glycerol electro-oxidation reactions was impressive. This study provides a novel foundation for the development of high-performance, cost-effective, non-noble metal catalysts for DMFC applications, contributing to the formation of commercially competitive electro-oxidation catalysts with enhanced efficiency and stability.

Nickel foam supported Mn-doped NiFe-LDH nanosheet arrays as efficient bifunctional electrocatalysts for methanol oxidation and hydrogen evolution

Electrochemical upgrading methanol into value-added formate at the anode in alkaline media enables the boosting production of hydrogen fuel at the cathode with saved energy. To achieve such a cost-effective and efficient electrocatalytic process, herein this work presents a Mn-doped nickel iron layered double hydroxides supported on nickel foam, derived from a simple hydrothermal synthesis. This developed electrocatalyst could act as an efficient bifunctional electrocatalyst for methanol-to-formate with a high faradaic efficiency of nearly 100 %, and for hydrogen evolution reaction, at an external potential of 1.5 V versus reversible hydrogen electrode. Additionally, a current density of 131.1 mA cm-2 with a decay of merely 12.2 % over 120 h continuous long-term testing was generated in co-electrocatalysis of water/methanol solution. Further density functional theoretical calculations were used to unravel the methanol-to-formate reaction mechanism arising from the doping of Fe and/or Mn. This work offers a good example of co-electrocatalysis to produce formate and green hydrogen fuel using a bifunctional electrocatalyst.

Fabrication and characterization of NiCu/GO and NiCu/rGO nanocomposites for fuel cell application

This study investigated the electrochemical behavior of NiCu, NiCu/GO, and NiCu/rGO nanocomposites designed by combining a modified Hummers' method and hydrothermal technique. The prepared nanocomposites are tested as electrocatalysts in direct alcohol oxidation fuel cells (DAFCs) to identify the role of GO and rGO as catalyst supports for the enhancement of the NiCu composite performance. The production of the NiCu/GO and NiCu/rGO composites was demonstrated by FTIR spectroscopy, EDX, and SEM analyses. In DAFCs experiments, NiCu/rGO has better catalytic activity than pure NiCu and NiCu/GO composites, whereas the use of rGO and GO as supports enhances the performance of NiCu by 468.2% and 377.7% in methanol and 255.6% and 105.9% in ethanol, respectively. The higher performance was caused by the increased density of active dots and the combined electronic effects in the designed catalysts. The stabilities of the catalysts and charge carriers' dynamics are studied using chronoamperometry and electrochemical impedance spectroscopy.

Enhanced Hydroxyl Adsorption in Ultrathin NiO/Cr2 O3 In-Plane Heterostructures for Efficient Alkaline Methanol Oxidation Reaction

The exploration of advanced nickel-based electrocatalysts for alkaline methanol oxidation reaction (MOR) holds immense promise for value-added organic products coupled with hydrogen production, but still remain challenging. Herein, we construct ultrathin NiO/Cr2 O3 in-plane heterostructures to promote the alkaline MOR process. Experimental and theoretical studies reveal that NiO/Cr2 O3 in-plane heterostructures enable a favorable upshift of the d-band center and enhanced adsorption of hydroxyl species, leading to accelerated generation of active NiO(OH)ads species. Furthermore, ultrathin in-plane heterostructures endow the catalyst with good charge transfer ability and adsorption behavior of methanol molecules onto catalytic sites, contributing to the improvement of alkaline MOR kinetics. As a result, ultrathin NiO/Cr2 O3 in-plane heterostructures exhibit a remarkable MOR activity with a high current density of 221 mA cm-2 at 0.6 V vs Ag/AgCl, which is 7.1-fold larger than that of pure NiO nanosheets and comparable with other highly active catalysts reported so far. This work provides an effectual strategy to optimize the activity of nickel-based catalysts and highlights the dominate efficacy of ultrathin in-plane heterostructures in alkaline MOR.

Electro-oxidation Reaction of Methanol over Reducible Ce1-x-yNixSryO2-δ: A Mechanistic Probe of Participation of Lattice Oxygen

Methanol oxidation reaction crucially depends on the formation of -OOH species over the catalyst's surface. Ni-based catalysts are by far the choice of materials, where the redox couple of Ni²⁺/Ni³⁺ facilitates the formation of -OOH species by surface reconstructions. However, it is challenging to oxidize Ni²⁺ as it generates charge-transfer orbitals near the Fermi energy level. One possible solution is to substitute Ni²⁺ with a reducible oxide support, which will not only facilitate the Ni²⁺ → Ni³⁺ oxidation but also adsorb oxygenated species like -OOH at a lower potential owing to its oxophilicity. This work shows with the help of structural and surface studies that the reducible CeO₂ support in Ni and Sr co-doped Ce_1-x-yNi_xSr_yO_2-δ solid solution can easily facilitate Ni²⁺ → Ni³⁺ oxidation as well as evolution of lattice oxygen during the methanol oxidation reaction. While the Ni³⁺ species helped in formation of -OOH surface intermediates, the evolved lattice oxygen eased the CO oxidation process in order to bring out the better CO-tolerant methanol oxidation activity over Ce_1-x-yNi_xSr_yO_2-δ. The study shows the unique importance of the electronic interactions between the active site and support and involvement of lattice oxygen in the methanol oxidation reaction.

Prussian-blue-analogue derived FeNi2S4/NiS nanoframes supported by N-doped graphene for highly efficient methanol oxidation electrocatalysis

The design of effective and robust non-noble metal electrocatalysts to enhance catalytic reaction kinetic is critical to promote methanol oxidation catalysis. Herein, hierarchical Prussian blue analogue (PBA)-derived sulfide heterostructures supported by N-doped graphene (FeNi₂S₄/NiS-NG) as efficient catalysts have been developed for methanol oxidation reaction (MOR). Benefiting from the merits of hollow nanoframes structure and heterogeneous sulfide synergy, FeNi₂S₄/NiS-NG composite not only possesses abundant active sites to boost the catalytic properties but also alleviates the CO poisoning effect during the process exhibiting favorable kinetic behavior toward MOR. Specifically, the remarkable catalytic activity (97.6 mA cm^-2/1544.3 mA mg^-1) of FeNi₂S₄/NiS-NG for methanol oxidation was achieved, superior to most reported non-noble electrocatalysts. Additionally, the catalyst showed competitive electrocatalytic stability, with a current density of over 90% after 2000 consecutive CV cycles. This study offers promising insights into the rational modulation of the morphology and components of precious-metal-free catalysts for fuel cell applications.

Hierarchical N-doped carbon nanofiber-loaded NiCo alloy nanocrystals with enhanced methanol electrooxidation for alkaline direct methanol fuel cells

The high catalytic activity of non-precious metals in alkaline media opens a new direction for the development of alkaline direct methanol fuel cell (ADMFC) electrocatalysts. Herein, a highly dispersed N-doped carbon nanofibers (CNFs) -loaded NiCo non-precious metal alloy electrocatalyst based on metal-organic frameworks (MOFs) was prepared, which conferred excellent methanol oxidation activity and resistance to carbon monoxide (CO) poisoning through a surface electronic structure modulation strategy. The porous electrospun polyacrylonitrile (PAN) nanofibers and the P-electron conjugated structure of polyaniline chains provide fast charge transfer channels, enabling electrocatalysts with abundant active sites and efficient electron transfer. The optimized NiCo/N-CNFs@800 was tested as an anode catalyst for ADMFC single cell and exhibited a power density of 29.15 mW cm^-2. Due to the fast charge transfer and mass transfer brought by its one-dimensional porous structure and the synergistic effect between NiCo alloy, NiCo/N-CNFs@800 is expected to be an economical, efficient and CO-resistant methanol oxidation reaction (MOR) electrocatalyst.

Trimetallic non-noble NiCoSn alloy as an efficient electrocatalyst towards methanol oxidation and oxygen reduction reactions

The development of a non-noble, highly efficient bifunctional catalyst for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) is the bottleneck in the alkaline direct methanol fuel cells (ADMFC). Ni-based bi/tri metallic alloys are the promising candidates next to the noble metals in the alkaline medium.Herein we present a facile hydrazine-assisted hydrothermal technique to synthesize a trimetallic nickel-cobalt-tin (NiCoSn) alloy as an efficient electrocatalyst for MOR and ORR reactions. The physiochemical analysis confirms the formation of trimetallic alloys with a high surface area. The as-synthesized trimetallic NiCoSn electrocatalyst exhibited superior MOR activity in terms of mass activity (509 mA mg^-1 at 1.55 V vs RHE) and stability than the bimetallic alloys in 1.0 M KOH electrolyte. Further, the trimetallic alloy delivered a lower onset and half-wave potential of 0.8 and 0.72 V vs RHE with the favorable four-electron transfer in the oxygen reduction reactions. This work highlights a facile approach for preparing Ni-based trimetallic alloys as a promising candidate for the alkaline direct methanol fuel cells/other catalytic applications.

Flower-like Layered NiCu-LDH/MXene Nanocomposites as an Anodic Material for Electrocatalytic Oxidation of Methanol

Direct methanol fuel cell (DMFC) technology has grabbed much attention from researchers worldwide in the realm of green and renewable energy-generating technologies. Practical applications of DMFCs are marked by the development of highly active, efficient, economical, and long-lasting anode catalysts. Layered double hydroxide (LDH) nanohybrids are found to be efficient electrode materials for methanol oxidation. In this study, we synthesized NiCu-LDH/MXene nanocomposites (NCMs) and investigated their electrochemical performance for methanol oxidation. The formation of NCM was verified through field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET), and X-ray photoemission spectroscopy (XPS) analyses. The cyclic voltammetry, chronoamperometry, and electron impedance spectroscopy techniques were carried out to assess the electrocatalytic ability of the methanol oxidation reaction. The incorporation of MXene enhanced the methanol oxidation 2-fold times higher than NiCu-LDH. NCM-45 exhibited high peak current density (86.9 mA cm^-2), enhanced electrochemical active surface area (7.625 cm²), and long-term stability (77.8% retention after 500 cycles). The superior performance of NCM can be attributed to the synergistic effect between Ni and Cu and, further, the electronic coupling between LDH and MXene. Based on the results, NCM nanocomposite is an efficient anodic material for the electrocatalytic oxidation of methanol. This study will open the door for the development of various LDH/MXene nanocomposite electrode materials for the application of direct methanol fuel cells.

Inhibitor and Activator: Dual Role of Subsurface Sulfide Enables Selective and Efficient Electro-Oxidation of Methanol to Formate on CuS@CuO Core-Shell Nanosheet Arrays

Selective electro-oxidation of aliphatic alcohols into value-added carboxylates at lower potentials than that of the oxygen evolution reaction (OER) is an environmentally and economically desirable anode reaction for clean energy storage and conversion technologies. However, it is challenging to achieve both high selectivity and high activity of the catalysts for the electro-oxidation of alcohols, such as the methanol oxidation reaction (MOR). Herein, a monolithic CuS@CuO/copper-foam electrode for the MOR with superior catalytic activity and almost 100% selectivity for formate is reported. In the core-shell CuS@CuO nanosheet arrays, the surface CuO directly catalyzes MOR, while the subsurface sulfide not only serves as an inhibitor to attenuate the oxidative power of the surface CuO to achieve selective oxidation of methanol to formate and prevent over-oxidation of formate to CO₂ but also serves as an activator to form more surface O defects as active sites and enhances the methanol adsorption and charge transfer to achieve superior catalytic activity. CuS@CuO/copper-foam electrodes can be prepared on a large scale by electro-oxidation of copper-foam at ambient conditions and can be readily utilized in clean energy technologies.

One-Step Synthesis of a Binder-Free, Stable, and High-Performance Electrode; Cu-O|Cu3P Heterostructure for the Electrocatalytic Methanol Oxidation Reaction (MOR)

Although direct methanol fuel cells (DMFCs) have been spotlighted in the past decade, their commercialization has been hampered by the poor efficiency of the methanol oxidation reaction (MOR) due to the unsatisfactory performance of currently available electrocatalysts. Herein, we developed a binder-free, copper-based, self-supported electrode consisting of a heterostructure of Cu₃P and mixed copper oxides, i.e., cuprous-cupric oxide (Cu-O), as a high-performance catalyst for the electro-oxidation of methanol. We synthesized a self-supported electrode composed of Cu-O|Cu₃P using a two-furnace atmospheric pressure-chemical vapor deposition (AP-CVD) process. High-resolution transmission electron microscopy analysis revealed the formation of 3D nanocrystals with defects and pores. Cu-O|Cu₃P outperformed the MOR activity of individual Cu₃P and Cu-O owing to the synergistic interaction between them. Cu₃P|Cu-O exhibited a highest anodic current density of 232.5 mAcm^-2 at the low potential of 0.65 V vs. Hg/HgO, which is impressive and superior to the electrocatalytic activity of its individual counterparts. The formation of defects, 3D morphology, and the synergistic effect between Cu₃P and Cu-O play a crucial role in facilitating the electron transport between electrode and electrolyte to obtain the optimal MOR activity. Cu-O|Cu₃P shows outstanding MOR stability for about 3600 s with 100% retention of the current density, which proves its robustness alongside CO intermediate.

Molten Salts Approach of Poly(vinyl alcohol)-Derived Bimetallic Nickel-Iron Sheets Supported on Porous Carbon Nanosheet as an Effective and Durable Electrocatalyst for Methanol Oxidation

The preparation of metallic nanostructures supported on porous carbon materials that are facile, green, efficient, and low-cost is desirable to reduce the cost of electrocatalysts, as well as reduce environmental pollutants. In this study, a series of bimetallic nickel-iron sheets supported on porous carbon nanosheet (NiFe@PCNs) electrocatalysts were synthesized by molten salt synthesis without using any organic solvent or surfactant through controlled metal precursors. The as-prepared NiFe@PCNs were characterized by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction, and photoelectron spectroscopy (XRD and XPS). The TEM results indicated the growth of NiFe sheets on porous carbon nanosheets. The XRD analysis confirmed that the Ni_1-xFe_x alloy had a face-centered polycrystalline (fcc) structure with particle sizes ranging from 15.5 to 30.6 nm. The electrochemical tests showed that the catalytic activity and stability were highly dependent on the iron content. The electrocatalytic activity of catalysts for methanol oxidation demonstrated a nonlinear relationship with the iron ratio. The catalyst doped with 10% iron showed a higher activity compared to the pure nickel catalyst. The maximum current density of Ni_0.9Fe_0.1@PCNs (Ni/Fe ratio 9:1) was 190 mA/cm² at 1.0 M of methanol. In addition to the high electroactivity, the Ni_0.9Fe_0.1@PCNs showed great improvement in stability over 1000 s at 0.5 V with a retained activity of 97%. This method can be used to prepare various bimetallic sheets supported on porous carbon nanosheet electrocatalysts.

Recent progress on the synthesis of metal alloy nanowires as electrocatalysts

Benefiting from both one-dimensional (1D) morphology and alloy composition, metal alloy nanowires have been exploited as advanced electrocatalysts in various electrochemical processes. In this review, the synthesis approaches for metal alloy nanowires are classified into two categories: direct syntheses and syntheses based on preformed 1D nanostructures. Ligand systems that are of critical importance to the formation of alloy nanowires are summarized and reviewed, together with the strategies imposed to achieve the co-reduction of different metals. Meanwhile, different scenarios that form alloy nanowires from pre-synthesized 1D nanostructures are compared and contrasted. In addition, the characterization and electrocatalytic applications of metal alloy nanowires are briefly discussed.

A high-performance Pt-based catalyst for the methanol oxidation reaction: effect of electrodeposition mode and cocatalyst on electrocatalytic activity

The influence of supporting material, cocatalyst, and electrodeposition mode on MOR activity.

Electrochemical Conversion of Alcohols into Acidic Commodities on Nickel Sulfide Nanoparticles

The electrocatalytic oxidation of alcohols is a potentially cost-effective strategy for the synthesis of valuable chemicals at the anode while simultaneously generating hydrogen at the cathode. For this approach to become commercially viable, high-activity, low-cost, and stable catalysts need to be developed. Herein, we demonstrate an electrocatalyst based on earth-abundant nickel and sulfur elements. Experimental investigations reveal the produced NiS displays excellent electrocatalytic performance associated with a higher electrochemical surface area (ECSA) and the presence of sulfate ions on the formed NiOOH surface in basic media. The current densities reached for the oxidation of ethanol and methanol at 1.6 V vs reversible hydrogen electrode (RHE) are up to 175.5 and 145.1 mA cm^-2, respectively. At these high current densities, the Faradaic efficiency of methanol to formate conversion is 98% and that of ethanol to acetate is 81%. Density functional theory calculations demonstrate the presence of the generated sulfate groups to modify the electronic properties of the NiOOH surface, improving electroconductivity and electron transfer. Besides, calculations are used to determine the reaction energy barriers, revealing the dehydrogenation of ethoxy groups to be more favorable than that of methoxy on the catalyst surface, which explains the highest current densities obtained for ethanol oxidation.

Coordination environment tuning of nickel sites by oxyanions to optimize methanol electro-oxidation activity

To achieve zero-carbon economy, advanced anode catalysts are desirable for hydrogen production and biomass upgrading powered by renewable energy. Ni-based non-precious electrocatalysts are considered as potential candidates because of intrinsic redox attributes, but in-depth understanding and rational design of Ni site coordination still remain challenging. Here, we perform anodic electrochemical oxidation of Ni-metalloids (NiP_x, NiS_x, and NiSe_x) to in-situ construct different oxyanion-coordinated amorphous nickel oxyhydroxides (NiOOH-TO_x), among which NiOOH-PO_x shows optimal local coordination environment and boosts electrocatalytic activity of Ni sites towards selective oxidation of methanol to formate. Experiments and theoretical results demonstrate that NiOOH-PO_x possesses improved adsorption of OH* and methanol, and favors the formation of CH₃O* intermediates. The coordinated phosphate oxyanions effectively tailor the d band center of Ni sites and increases Ni-O covalency, promoting the catalytic activity. This study provides additional insights into modulation of active-center coordination environment via oxyanions for organic molecules transformation.

Coordination environments around metal sites are important in electrocatalysis. Here, Ni metalloid oxidation produces oxyanion doped amorphous Ni-oxyhydroxides where substitution with phosphorus is found to boost methanol electrooxidation activity.

Enhancement Strategy of Photoelectrocatalytic Activity of Cobalt-Copper Layer Double Hydroxide toward Methanol Oxidation: Cerium Doping and Modification with Porphyrin

Designing durable, high-active, and low-cost noble-metal-free photoelectrocatalysts for methanol electrooxidation is highly demanded but remains a challenge. Herein, the photoelectrocatalytic activity of cobalt-copper layer double hydroxide (CoCu-LDH) for methanol oxidation reaction (MOR) in the alkaline media under light was remarkably enhanced by cerium (Ce) doping and further by 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) modification. TCPP/Ce-CoCu-LDH exhibits a remarkable mass activity of 1788.2 mA mg^-1 in 1 mol L^-1 KOH with 1 mol L^-1 methanol under light, which is 2.3 and 1.8 times higher than that of CoCu-LDH (782.2 mA mg^-1) and Ce-CoCu-LDH (987.4 mA mg^-1). The UV-vis diffuse reflectance spectra and photoluminescence emission spectra reveal that TCPP/Ce-CoCu-LDH can effectively utilize the visible light and inhibit the electron-hole pairs' recombination because of the introduction of porphyrin. Furthermore, more active sites and the greater electrical conductivity of TCPP/Ce-CoCu-LDH also contributed to the high photoelectrocatalytic activity. Thus, TCPP/Ce-CoCu-LDH can be used as a low-cost alternative for Pt-based catalyst toward MOR.

Enhanced Hydrogenation of Levulinic Acid over Ordered Mesoporous Alumina-Supported Catalysts: Elucidating the Effect of Fabrication Strategy

In this work, three types of alumina-supported bimetallic Ni-Cu catalysts [Ni-Cu/commercial non-ordered mesoporous alumina (CMA), Ni-Cu/ordered MA (OMA), and Ni-Cu-OMA] were prepared via different fabrication strategies and investigated in the conversion of levulinic acid (LA) into γ-valerolactone and 2-methyltetrahydrofuran (2-MTHF). This study employed characterization techniques and reactions to reveal the effects of the fabrication strategy on the activities of the catalysts. It was observed that the catalysts constructed on OM supports (Ni-Cu/OMA and Ni-Cu-OMA) displayed superior catalytic performance compared to those constructed on CM supports (Ni-Cu/CMA). Specifically, Ni-Cu-OMA, which was fabricated via the one-pot evaporation-induced self-assembly strategy, exhibited the best catalytic performance, achieving a complete conversion of LA and a high selectivity of 73.0 % toward 2-MTHF in a solvent-free reaction environment. The promising activity of Ni-Cu-OMA was ascribed to the well-dispersed active sites within the framework of the support, the enhanced metal-support interaction, and the highly efficient exploitation of the synergistic effect between Ni and Cu. Detailed post-characterization techniques were also employed to highlight the outstanding stability of Ni-Cu-OMA.

Ultrasonically Surface-Activated Nickel Foam as a Highly Efficient Monolith Electrode for the Catalytic Oxidation of Methanol to Formate

Most of the current electrocatalysts for the methanol oxidation reaction are precious group metals such as Pt, Pd, and Ru. However, their use is limited due to their high cost, scarcity, and issues with carbon monoxide poisoning. We developed a simple method to prepare a nickel foam (NF)-based monolith electrode with a NiO nanosheet array structure as an efficient electrocatalyst toward the oxidation of methanol to produce formate. By a simple ultrasonic acid treatment and air oxidation at room temperature, an inert NF was converted to NiO/NF as a catalytically active electrode due to the uniform NiO nanosheet array that was rapidly formed on the surface of NiO/NF. In alkaline electrolytes containing methanol, the as-prepared NiO/NF catalysts exhibited a lower methanol oxidation reaction (MOR) potential of +1.53 V vs RHE at 100 mA cm^-2 compared to that of inert NF samples. The difference in potentials between the E_MOR and the E_OER at that current density was found to be 280 mV, indicating that methanol oxidation occurred at lower potentials as compared to the oxygen evolution reaction (OER). We also observed that the NiO/NF could also efficiently catalyze the oxidation of CO without being poisoned by it. NiO/NF retained close to 100% of its initial activity after 20,000 s of methanol oxidation tests at high current densities above 200 mA cm^-2. Because of the simple synthesis method and the enhanced catalytic performance and stability of NiO/NF, this allows methanol to be used as an OER masking agent for the energy-efficient generation of value-added products such as formic acid and hydrogen.

Metal-Based Electrocatalysts for Methanol Electro-Oxidation: Progress, Opportunities, and Challenges

Direct methanol fuel cells (DMFCs) are among the most promising portable power supplies because of their unique advantages, including high energy density/mobility of liquid fuels, low working temperature, and low emission of pollutants. Various metal-based anode catalysts have been extensively studied and utilized for the essential methanol oxidation reaction (MOR) due to their superior electrocatalytic performance. At present, especially with the rapid advance of nanotechnology, enormous efforts have been exerted to further enhance the catalytic performance and minimize the use of precious metals. Constructing multicomponent metal-based nanocatalysts with precisely designed structures can achieve this goal by providing highly tunable compositional and structural characteristics, which is promising for the modification and optimization of their related electrochemical properties. The recent advances of metal-based electrocatalytic materials with rationally designed nanostructures and chemistries for MOR in DMFCs are highlighted and summarized herein. The effects of the well-defined nanoarchitectures on the improved electrochemical properties of the catalysts are illustrated. Finally, conclusive perspectives are provided on the opportunities and challenges for further refining the nanostructure of metal-based catalysts and improving electrocatalytic performance, as well as the commercial viability.

Nickel Iron Diselenide for Highly Efficient and Selective Electrocatalytic Conversion of Methanol to Formate

The electro-oxidation of methanol to formate is an interesting example of the potential use of renewable energies to add value to a biosourced chemical commodity. Additionally, methanol electro-oxidation can replace the sluggish oxygen evolution reaction when coupled to hydrogen evolution or to the electroreduction of other biomass-derived intermediates. But the cost-effective realization of these reaction schemes requires the development of efficient and low-cost electrocatalysts. Here, a noble metal-free catalyst, Ni_{1- x} Fe_x Se₂ nanorods, with a high potential for an efficient and selective methanol conversion to formate is demonstrated. At its optimum composition, Ni_0.75 Fe_0.25 Se₂ , this diselenide is able to produce 0.47 mmol cm^-2  h^-1 of formate at 50 mA cm^-2 with a Faradaic conversion efficiency of 99%. Additionally, this noble-metal-free catalyst is able to continuously work for over 50 000 s with a minimal loss of efficiency, delivering initial current densities above 50 mA cm^-2 and 2.2 A mg^-1 in a 1.0 m KOH electrolyte with 1.0 m methanol at 1.5 V versus reversible hydrogen electrode. This work demonstrates the highly efficient and selective methanol-to-formate conversion on Ni-based noble-metal-free catalysts, and more importantly it shows a very promising example to exploit the electrocatalytic conversion of biomass-derived chemicals.

Recent Progress of Ultrathin 2D Pd-Based Nanomaterials for Fuel Cell Electrocatalysis

Pd- and Pd-based catalysts have emerged as potential alternatives to Pt- and Pt-based catalysts for numerous electrocatalytic reactions, particularly fuel cell-related reactions, including the anodic fuel oxidation reaction (FOR) and cathodic oxygen reduction reaction (ORR). The creation of Pd- and Pd-based architectures with large surface areas, numerous low-coordinated atoms, and high density of defects and edges is the most promising strategy for improving the electrocatalytic performance of fuel cells. Recently, 2D Pd-based nanomaterials with single or few atom thickness have attracted increasing interest as potential candidates for both the ORR and FOR, owing to their remarkable advantages, including high intrinsic activity, high electron mobility, and straightforward surface functionalization. In this review, the recent advances in 2D Pd-based nanomaterials for the FOR and ORR are summarized. A fundamental understanding of the FOR and ORR is elaborated. Subsequently, the advantages and latest advances in 2D Pd-based nanomaterials for the FOR and ORR are scientifically and systematically summarized. A systematic discussion of the synthesis methods is also included which should guide researchers toward more efficient 2D Pd-based electrocatalysts. Lastly, the future outlook and trends in the development of 2D Pd-based nanomaterials toward fuel cell development are also presented.

Efficient non-noble Ni–Cu based catalysts for the valorization of palmitic acid through a decarboxylation reaction

The synergistic effect Ni–Cu in the bimetallic catalyst Ni–Cu/C improved the stability and reduction temperature as well as enhanced the catalytic activity for the decarboxylation of palmitic acid.

Nickel nanocrystal/nitrogen-doped carbon composites as efficient and carbon monoxide-resistant electrocatalysts for methanol oxidation reactions

High-performance electrocatalysts for the methanol oxidation reaction (MOR) are the key to advance the application of direct methanol fuel cells. Pt-Based electrocatalysts for the MOR are limited due to their high cost, low stability and poor resistance to carbon monoxide (CO) poisoning. The development of non-noble metal-based electrocatalysts for the MOR with high activity and good stability is desired, but it remains a challenge. Herein, we report a simple strategy to prepare nickel nanocrystals embedded in a nitrogen-doped carbon matrix (Ni/N-C composite) by pyrolysis of Ni-coordinated polyaniline-poly(vinyl alcohol) hydrogels. These in situ generated Ni nanocrystals serve as active electrocatalysts for the MOR, while the nitrogen-doped carbon matrix serves as a conductive support to facilitate electron transfer and also to protect the active Ni nanocrystals. The optimal Ni/N-C@500 electrocatalyst shows a high MOR activity of 147 mA cm-2 at 1.66 V vs. the RHE in alkaline methanol solution, which is outstanding among Ni-based MOR electrocatalysts. Ni/N-C@500 also shows better stability than the Pt/C catalyst in the long-term MOR test at high current densities. Upon CO poisoning, Ni/N-C@500 retains 85% of its MOR activity, far exceeding the performance of the Pt/C catalyst (61% retention). Owing to its facile synthesis, outstanding activity and high stability, the Ni/N-C@500 composite is promising as a low-cost, efficient and CO-resistant electrocatalyst for the MOR.

Materializing efficient methanol oxidation via electron delocalization in nickel hydroxide nanoribbon

Achieving a functional and durable non-platinum group metal-based methanol oxidation catalyst is critical for a cost-effective direct methanol fuel cell. While Ni(OH)₂ has been widely studied as methanol oxidation catalyst, the initial process of oxidizing Ni(OH)₂ to NiOOH requires a high potential of 1.35 V vs. RHE. Such potential would be impractical since the theoretical potential of the cathodic oxygen reduction reaction is at 1.23 V. Here we show that a four-coordinated nickel atom is able to form charge-transfer orbitals through delocalization of electrons near the Fermi energy level. As such, our previously reported periodically arranged four-six-coordinated nickel hydroxide nanoribbon structure (NR-Ni(OH)₂) is able to show remarkable methanol oxidation activity with an onset potential of 0.55 V vs. RHE and suggests the operability in direct methanol fuel cell configuration. Thus, this strategy offers a gateway towards the development of high performance and durable non-platinum direct methanol fuel cell.

Unusual Effect of Trace Water on the Structure and Activity of Nix Co1-x Electrocatalysts for the Methanol Oxidation Reaction

Highly active Ni-based catalysts have attracted much attention but are still facing challenges owing to the immature synthetic method. Herein, polyhedral Ni_x Co_1-x alloy was prepared by a facile modified polyol method in which a trace amount of water could halve the particle size of the alloy. The Ni/Co ratios in Ni_x Co_1-x alloy strictly depended on the used amount of water owing to the different solubilities of the precursors. Among them, the Ni_0.6 Co_0.4 nanoparticles obtained with 70 μL of deionized water exhibited the best performance in the methanol oxidation reaction with a peak current density of 116 mA cm^-2 in the presence of 1 m NaOH+0.5 m CH₃ OH solution, which is higher than those of Ni_0.7 Co_0.3 (80 mA cm^-2 ) and Ni_0.5 Co_0.5 (33 mA cm^-2 ). The excellent performance of Ni_0.6 Co_0.4 is attributed to the unique structure with appropriate Ni/Co ratio, which elongates the C-O bond in methanol and lowers the reaction free energy according to DFT calculations.

Low Pt-Content Ternary PtNiCu Nanoparticles with Hollow Interiors and Accessible Surfaces as Enhanced Multifunctional Electrocatalysts

Developing highly active and durable electrocatalysts with low levels of Pt content toward some crucial reactions including oxygen reduction reaction, hydrogen evolution reaction, and methanol oxidation reaction in an acidic electrolyte environment are desirable but still an open challenge for clean and efficient energy conversion. Herein, we present a facile route to synthesize low Pt-content ternary PtNiCu nanostructures with hollow interior and accessible surfaces (H-PtNiCu-AAT NPs) as enhanced multifunctional electrocatalysts. The galvanic replacement reaction and atomic diffusion between in situ preformed CuNi nanocrystals and Pt species should be responsible for the formation of hollow PtNiCu NPs. Continuous activation by acid picking and annealing treatments were performed to leach out the excessive Cu and Ni on the surfaces and to enrich Pt-content on the surface. H-PtNiCu-AAT NPs exhibit excellent activity and durability toward HER, ORR, and MOR due to the rational integration of multiple structural advantages. Strikingly, the mass activity and specific activity of H-PtNiCu-AAT NPs (0.977 A mg_Pt^-1 and 1.458 mA cm^-2) is 7.1 and 6.9 times higher than that of commercial Pt/C (0.138 A mg_Pt^-1 and 0.212 mA cm^-2) toward ORR at 0.9 V (vs RHE), respectively. This present work provides an efficient strategy for the design of low Pt-content trimetallic electrocatalysts with excellent activity and durability.

The one-pot synthesis of CuNi nanoparticles with a Ni-rich surface for the electrocatalytic methanol oxidation reaction

The use of fuel cells is one of the most promising renewable energy strategies, but they still suffer from many limitations. The high mass enthalpy of hydrogen as a fuel comes at the cost of inconveniences and risks associated with storage, transportation and utilization, while the high performance of Pt catalysts in commercial fuel cells is limited by their high cost, low earth abundance, and poor stability as a result of CO intermediate poisoning. To circumvent these dilemmas, direct methanol fuel cells (DMFCs) were developed, using methanol as a fuel and Ni as the anode catalyst. Thanks to the condensed form of the fuel, DMFCs are considered as the most promising fuel-cell solution for portable electronic devices. Usually, other elements have to be introduced into Ni-based catalysts to modify the active sites to provide better alternatives to pristine Ni metal in terms of activity and stability. In this study, we provide a mild synthetic method for the preparation of CuNi alloy nanoparticles. The proper alloying ratio leads to the suitable modification of the electronic structure of Ni, which promotes the MOR catalytic reaction on the NiCu alloy. The NiCu alloy catalyst exhibits a mass current density of 1028 mA mg_metal^-1 for the MOR at 1.55 V (vs. RHE), which is among the best values obtained from similarly prepared Ni-based catalysts.

The preparation of NiO/Ni–N/C nanocomposites and its electrocatalytic performance for methanol oxidation reaction

The NiO/Ni–N/C nanocomposites were prepared through hydrothermal method and further carbonization. The NiO/Ni–N/C₅₀₀ displays the highest MA (1043 mA mg_Ni⁻¹) and SA (18.57 mA cm⁻²) for methanol oxidation reaction.

Self-template synthesis of defect-rich NiO nanotubes as efficient electrocatalysts for methanol oxidation reaction

Developing robust and inexpensive non-noble metal based anode electrocatalysts is highly desirable for alkaline direct methanol fuel cells (ADMFCs). Herein, we successfully develop a facile self-template synthetic strategy for gram-grade porous NiO nanotubes (NTs) by pyrolyzing a nanorod-like Ni-dimethylglyoxime complex. The pyrolysis temperature highly correlates with the morphology and crystallinity of NiO NTs. The optimal NiO NTs exhibit a large electrochemically active surface area, a fast catalytic kinetics, and a small charge transfer resistance, which induce an outstanding electrocatalytic activity for the methanol oxidation reaction (MOR). Compared with conventional NiO nanoparticles, NiO NTs achieve a 11.5-fold increase in mass activity at 1.5 V for the MOR due to nanotubal morphology and abundant non-vacancy defects on the NiO NT surface. Moreover, NiO NTs have a higher electrocatalytic activity for the intermediates of the MOR (such as formaldehyde and formate) than conventional NiO nanoparticles, which also contribute to MOR activity enhancement. Given the facile synthesis and enhanced electrocatalytic performance, NiO NTs may be promising anode electrocatalysts for ADMFCs.

Influence of Sn Content, Nanostructural Morphology, and Synthesis Temperature on the Electrochemical Active Area of Ni-Sn/C Nanocomposite: Verification of Methanol and Urea Electrooxidation

In contrast to precious metals (e.g., Pt), which possess their electro catalytic activities due to their surface electronic structure, the activity of the Ni-based electrocatalysts depends on formation of an electroactive surface area (ESA) from the oxyhydroxide layer (NiOOH). In this study, the influences of Sn content, nanostructural morphology, and synthesis temperature on the ESA of Sn-incorporated Ni/C nanostructures were studied. To investigate the effect of the nanostructural, Sn-incorporated Ni/C nanostructures, nanofibers were synthesized by electrospinning a tin chloride/nickel acetate/poly (vinyl alcohol) solution, followed by calcination under inert atmosphere at high temperatures (700, 850, and 1000 °C). On the other hand, the same composite was formulated in nanoparticulate form by a sol-gel procedure. The electrochemical measurements indicated that the nanofibrous morphology strongly enhanced formation of the ESA. Investigation of the tin content concluded that the optimum co-catalyst content depends on the synthesis temperature. Typically, the maximum ESA was observed at 10 and 15 wt % of the co-catalyst for the nanofibers prepared at 700 and 850 °C, respectively. Study of the effect of synthesis temperature concluded that at the same tin content, 850 °C calcination temperature reveals the best activity compared to 700 and 1000 °C. Practical verification was achieved by investigation of the electrocatalytic activity toward methanol and urea oxidation. The results confirmed that the activity is directly proportionate to the ESA, especially in the case of urea oxidation. Moreover, beside the distinct increase in the current density, at the optimum calcination temperature and co-catalyst content, a distinguished decrease in the onset potential of both urea and methanol oxidation was observed.

CuO/Co(OH)2 Nanosheets: A Novel Kind of Electrocatalyst for Highly Efficient Electrochemical Oxidation of Methanol

With the booming of non-noble-metal electrocatalysts for efficient oxygen reduction reaction under alkaline conditions, corresponding anodic catalysts for methanol oxidation are urgently needed especially for direct methanol fuel cells with alkaline membranes. Here, we report the facile synthesis of a CuO/Co(OH)₂-nanosheet composite as a novel kind of high-performance electrochemical methanol oxidation reaction (MOR) catalyst. The obtained material with an optimized Cu/Co ratio shows much enhanced mass activity and area-specific activity, as well as excellent stability. The electronic structure interaction between Cu and Co, which results in the Co ion binding-energy elevation, is considered to be the origin of high MOR performance. This work promises the great potential of cobalt hydroxide as a novel kind of MOR catalyst and may arouse much interest in exploring more hydroxides as efficient nonprecious-metal electrocatalysts for MOR.

Selective Etching Induced Synthesis of Hollow Rh Nanospheres Electrocatalyst for Alcohol Oxidation Reactions

The hollow noble metal nanostructures have attracted wide attention in catalysis/electrocatalysis. Here a two-step procedure for constructing hollow Rh nanospheres (Rh H-NSs) with clean surface is described. By selectively removing the surfactant and Au core of Au-core@Rh-shell nanostructures (Au@Rh NSs), the surface-cleaned Rh H-NSs are obtained, which contain abundant porous channels and large specific surface area. The as-prepared Rh H-NSs exhibit enhanced inherent activity for the methanol oxidation reaction (MOR) compared to state-of-the-art Pt nanoparticles in alkaline media. Further electrochemical experiments show that Rh H-NSs also have high activity for the electrooxidation of formaldehyde and formate (intermediate species in the course of the MOR) in alkaline media. Unfortunately, Rh H-NSs have low electrocatalytic activity for the ethanol and 1-propanol oxidation reactions in alkaline media. All electrochemical results indicate that the order of electrocatalytic activity of Rh H-NSs for alcohol oxidation reaction is methanol (C1 ) > ethanol (C2 ) > 1-propanol (C3 ). This work highlights the synthesis route of Rh hollow nanostructures, and indicates the promising application of Rh nanostructures in alkaline direct methanol fuel cells.