Co,Fe-ions intercalated Ni(OH)2 network-like nanosheet arrays as highly efficient non-noble catalyst for electro-oxidation of urea

Developing a highly active, low-cost, and durable nanostructured catalytic material is of significant interest in fuel cell applications owing to its efficient energy conversion, ease of preparation and operation, and reduced emission of pollutants. In this work, various ratio (0.01, 0.03, and 0.05 mmol) of cobalt (Co), and iron (Fe) doped nickel hydroxide (Ni(OH)₂) nanosheet arrays were grown on Ni foam surface (Co–Ni(OH)₂ and Fe–Ni(OH)₂) via a simple one-step process. The Co–Ni(OH)₂ and Fe–Ni(OH)₂ nanosheet array on Ni foam electrodes were explored as potential candidate towards electro-oxidation of urea. Notably, 0.03 mmol Co doped Ni(OH)₂/Ni foam electrode displayed lowest-onset oxidation potential (0.21 V) and enhanced electro-oxidation of urea (59.7 mA) owing to its large amount of electrocatalytic active sites, densely assembled nanosheet array structures with porous surfaces, and electronic diffusion channels, which might promote interface electrochemical reaction. In addition, synergistic effect between Co metal with Ni(OH)₂ has also promotes enhanced electro-oxidation of urea in contrast to Fe–Ni(OH)₂ nanosheet array, Ni(OH)₂, and Ni foam electrodes. Chronoamperometric i-t curve of Co–Ni(OH)₂/Ni foam electrode obviously exhibited higher catalytic current, highly stable and durable properties in contrast to Fe–Ni(OH)₂ nanosheet arrays. As-fabricated Co–Ni(OH)₂/Ni foam can be explored as a new type of potential low-cost catalyst for electro-oxidation of urea, which reveals promising use in future fuel cell energy applications.     

Improved electrocatalytic kinetics of nickel hydroxide nanoparticles on Vulcan XC-72R carbon black towards alkaline urea oxidation reaction

Nickel hydroxide nanoparticles were fabricated on Vulcan XC-72R carbon black using various reducing agents through assisted microwave polyol process. The formed electrocatalysts using sodium borohydride [Ni(OH)₂/C–NB], ethylene glycol [Ni(OH)₂/C–EG] and a mixture of them [Ni(OH)₂/C–EGNB] displayed an electrocatalytic activity towards urea oxidation in NaOH solution. The oxidation peak potential and current density values were greatly influenced by the employed reducing agent. Lower onset and peak potential values were measured at Ni(OH)₂/C–EGNB, while Ni(OH)₂/C–EG exhibited the highest oxidation current density during urea oxidation reaction. Electroactive surface area measurements revealed that the number of available active sites for the oxidation reaction was arranged in an ascending order as Ni(OH)₂/C–NB < Ni(OH)₂/C–EGNB < Ni(OH)₂/C–EG. The diffusion coefficient of urea molecules at Ni(OH)₂/C–EG and Ni(OH)₂/C–EGNB was 14.69 and 5.90 times higher than that at Ni(OH)₂/C–NB. Stable performance was measured at all studied electrocatalysts over prolonged operation suggesting their valuable application as efficient anode materials in direct urea oxidation fuel cells.     

Porous Ni2P nanoflower supported on nickel foam as an efficient three-dimensional electrode for urea electro-oxidation in alkaline medium

Porous Ni₂P nanoflower supported on nickel foam (Ni₂P@Ni foam) electrodes are synthesized via a simple hydrothermal growth strategy accompanied with further phosphating treatment. The prepared electrodes are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). Electro-catalytic performances towards urea electro-oxidation are tested by cyclic voltammetry (CV), chronoamperometry (CA) coupled with electrochemical impedance spectroscopy (EIS). By phosphating Ni(OH)₂ precursor, the final obtained Ni₂P@Ni foam electrode presents a porous Ni₂P nanoflower structure within abundant porosity, and so exposes a large amount of electro-catalytic active sites and electronic transmission channels to accelerate the interfacial reaction. Compared with Ni(OH)₂@Ni foam precursor, the Ni₂P@Ni foam catalyst exhibits more excellent electro-catalytic activity as well as lower onset oxidation potential. Remarkably, the Ni₂P@Ni foam catalyst reaches a peak current density of 750 mA cm^?2 with an onset oxidation potential of 0.24 V (vs. Ag/AgCl) accompanied by an excellent stability in 0.60 M urea with 5.00 M KOH solutions. Benefiting from the unique porous nanosheet structure, the as-synthesized Ni₂P@Ni foam catalyst performs a highly enhanced catalytic behavior for alkaline urea electro-oxidation, indicating that the material can be hopefully applied in direct urea fuel cells.     

NiCo layered double hydroxide/hydroxide nanosheet heterostructures for highly efficient electro-oxidation of urea

Urea oxidation is an important reaction for direct urea fuel cells as well as hydrogen production and/or water remediation via electrolysis using urea-rich wastewater. The key to efficient urea oxidation is to explore a well-designed high-performing catalyst. Herein, NiCo layered double hydroxide/hydroxide (NiCo LDH/NiCo(OH)₂) microspheres composed of ultrasmall nanosheets have been grown on Ni foam by a solution method at room temperature. The NiCo LDH/NiCo(OH)₂ heterostructures have been confirmed by TEM and XRD analysis. The high activity with a small onset potential of 0.29 V vs. Hg/HgO is mainly attributed to the rich NiCo LDH-NiCo(OH)₂ interfaces and the bimetallic nature of the catalysts. The NiCo LDH/NiCo(OH)₂ heterostructures can be promising catalysts for urea oxidation and offer new insights into the design of high-performance nickel-based catalysts.     

Defects-rich nickel nanoparticles grown on nickel foam as integrated electrodes for electrocatalytic oxidation of urea

Designing highly efficient, low cost and long-term stable electro-catalysts is the key step for the commercial applications of fuel cells. Electro-oxidation of urea, a hydrogen-rich fuel, is the anodic reaction of direct urea fuel cells. Herein, defects-rich nickel nanoparticles grown on nickel foam as integrated electrodes have been designed and easily fabricated by incomplete reduction of Ni(OH)₂. The Ni²⁺ defects coupled with oxygen vacancies are proposed to be mainly present in the form of amorphous NiO_x, which is the island phase in the metallic nickel nanoparticles and confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy. The synergistic effect between metallic metal with high conductivity and numerous defects with good affinity to O contributes to the high catalytic activity towards oxidation of urea with an onset potential of 0.35 V vs Hg/HgO in 2 M KOH +0.33 M urea. Additionally, the defects-rich nickel nanoparticles present good long-term stability.     

Synergistic effects of Ni1−xCox-YSZ and Ni1−xCux-YSZ alloyed cermet SOFC anodes for oxidation of hydrogen and methane fuels containing H2S

Preparation and performance of bimetallic Ni_(1−x)Co_x-YSZ and Ni_(1−x)Cu_x-YSZ anodes were tested to overcome common deficiencies of carbon and sulfur poisoning in SOFCs. Ni_1−xCo_xO-YSZ and Ni_(1−x)Cu_xO-YSZ precursors were synthesized via co-precipitation of their respective chlorides. Single cell solid oxide fuel cells of these bimetallic anodes were tested in H₂, CH₄, and H₂S/CH₄ fuel mixtures. Addition of Cu²⁺ into the NiO lattice resulted in large metal particle sizes and decreased SOFC performance. Addition of Co²⁺ into the NiO lattice to form Ni_0.92Co_0.08O-YSZ anode precursor produced a cermet with a large BET surface area and active metal surface area, thus increasing the rate of hydrogen oxidation for this sample. The performance of both bimetallics was found to quickly degrade in dry CH₄ due to carbon deposition and lifting of the anode from the electrolyte. However, Ni_0.69Co_0.31-YSZ showed superior activity in a 10% (v/v) H₂S/CH₄ fuel mixture, surpassing performance with H₂ fuel, thereby demonstrating the exciting prospect of using sulfidated Ni_(1−x)Co_x-YSZ as SOFC anodes in sulfur containing methane streams. The active anode becomes a sulfidated alloy (Ni-Co-S) under operating conditions. This anode showed enhanced performance, which surpassed those of sulfidated Ni and Co anodes, thereby suggesting a synergistic behaviour in the Ni-Co-S anode.     

Synthesis and electrochemical properties of Li[Ni0.8Co0.1Mn0.1]O2 and Li[Ni0.8Co0.2]O2 via co-precipitation

Spherical Li[Ni_0.8Co_0.2−xMn_x]O₂ (x = 0, 0.1) with phase-pure and well-ordered layered structure have been synthesized by heat-treatment of spherical [Ni_0.8Co_0.2−xMn_x](OH)₂ and LiOH·H₂O precursors. The structure, morphology, electrochemical properties, and thermal stability of Li[Ni_0.8Co_0.2−xMn_x]O₂ (x = 0, 0.1) were studied. The average particle size of the powders was about 10–15 μm and the size distribution was narrow due to the homogeneity of the metal hydroxide [Ni_0.8Co_0.2−xMn_x](OH)₂ (x = 0, 0.1). The Li[Ni_0.8Co_0.2−xMn_x]O₂ (x = 0, 0.1) delivered a discharge capacity of 197–202 mAh g⁻¹ and showed excellent cycling performance. Compared to Li[Ni_0.8Co_0.2]O₂, Li[Ni_0.8Co_0.1Mn_0.1]O₂ exhibited greater thermal stability resulting from improved structural stability due to Mn substitution.     

Controlled synthesis of three-dimensional branched Mo–NiCoP@NiCoP/NiXCoYH2PO2 core/shell nanorod heterostructures for high-performance water and urea electrolysis

It is necessary to design reasonably efficient bifunctional electrocatalyst, but it is still a difficult problem for the water and urea electrolysis. Therefore, we firstly constructed a novel Mo–NiCoP@NiCoP/Ni_XCo_YH₂PO₂ (MNCP@NCP/Ni_XCo_YH₂PO₂) core/shell nanorod heterostructure by hydrothermal and two-step phosphating on nickel foam (NF). It is worth noting that Mo-doping could availably regulate the electronic structure of NiCoP(NCP), resulting in the increased exposure of the active center and the increased inherent activity of each site. Furthermore, a strategy of improving catalyst activity was proposed, that is, the NiCoP nanorod core and Mo–NiCoP/Ni_XCo_YH₂PO₂ nanorod shell was constructed by the two phosphating reactions to come into being mixed transition-metal phosphides (TMPs), thus improving the synergistic catalytic effect of the material. In addition, the water and urea electrolysis apparatus was installed from two MNCP@NCP/Ni_XCo_YH₂PO₂ electrodes to actuate a current density of 10 mA cm^?2, the necessary cell voltage was merely 1.348 V in 1.0 M KOH with 0.5 M urea for urea electrolysis, while the higher 1.522 V of cell voltage was required in 1.0 M KOH for water electrolysis, which is one of the best catalytic activities reported so far. Experimental results show that the oxyhydroxide is the real active site during urea electrolysis process. Density functional theory calculation shows that the doping of Mo and Co increase the water adsorption energy and conductivity of the oxyhydroxide material, so the water splitting performance of the catalyst is improved. Therefore, this work provided a new way to design bifunctional electrocatalysts by Mo-doping and two-step phosphating process.     

Sprouts-like Fe(OH)2 hetero-nanostructures assembly on selenium layered nickel foam (NiF–Se) as an efficient and durable catalyst for electro-oxidation of urea

In this study, we develop selenium (Se)-iron hydroxide (NiF–Se@Fe(OH)₂) hetero-nanostructured catalyst system for fuel cell, and environmental-relevant urea-electro-oxidation reaction. For the rational engineering, the Se layers are initially deposited on the Ni foam substrate; then we grow Fe(OH)₂ hetero-nanostructures with various morphologies by introducing different ratios of Fe precursor sample. The Fe(OH)₂ ball-like nanorods to rock-like nanosheets are synthesized on the Se layered Ni foam surface by a simple hydrothermal process. The microscopic characterizations, and spectral analysis reveal the formation of Se integrated Fe(OH)₂ hetero-nanostructures such as ball-like nanorods, sprouts-like nanowire network, nanoflowers and rock-like nanosheets through effective Ni–O bond and Fe–Se bond for its typical synergism. The Se induces an interesting morphology transformation from crystalline nanorods to rock-like nanosheets structures that lead to the potential constituents for catalyst electrode that effectively merge the qualities such as high conductivity, large specific surface area, and larger catalytic active sites for electro-oxidation reaction of urea. Among them, NiF–Se@Fe(OH)₂ (8 mmol Fe(NO₃)₂) sprouts-like hetero-nanostructured network displays higher catalytic activity toward oxidation of urea (146.7 mA) with onset potential of 0.11 V vs. Ag/AgCl in 1 M NaOH + 0.1 M urea. Furthermore, the sprouts-like NiF–Se@Fe(OH)₂ nanowired network shows superior activity than the other aspect ratio's, excellent long-term stability, and reproducibility.     

Ru-doped 3D porous Ni3N sphere as efficient Bi-functional electrocatalysts toward urea assisted water-splitting

Developing efficient, stable and ideal urea oxide (UOR) electrocatalyst is key to produce green hydrogen in an economical way. Herein, Ru doped three dimensional (3D) porous Ni₃N spheres, with tannic acid (TA) and urea as the carbon and nitrogen resources, is synthesized via hydrothermal and low-temperature treated process (Ru–Ni₃N@NC). The porous nanostructure of Ni₃N and the nickel foam provide abundant active sites and channel during catalytic process. Moreover, Ru doping and rich defects favor to boost the reaction kinetics by optimizing the adsorption/desorption or dissociation of intermediates and reactants. The above advantages enable Ru–Ni₃N@NC to have good bifunctional catalytic performance in alkaline media. Only 43 and 270 mV overpotentials are required for hydrogen evolution (HER) and oxygen evolution (OER) reactions to drive a current of 10 mA cm^?2. Moreover, it also showed good electrocatalytic performance in neutral and alkaline seawater electrolytes for HER with 134 mV to drive 10 mA cm^?2 and 83 mV to drive 100 mA cm^?2, respectively. Remarkably, the as-designed Ru–Ni₃N@NC also owns extraordinary catalytic activity and stability toward UOR. Moreover, using the synthesized Ru–Ni₃N@NC nanomaterial as the anode and cathode of urea assisted water decomposition, a small potential of 1.41 V was required to reach 10 mA cm^?2. It can also be powered by sustainable energy sources such as wind, solar and thermal energies. In order to make better use of the earth's abundant resources, this work provides a new way to develop multi-functional green electrocatalysts.     

Interlaced rosette-like MoS2/Ni3S2/NiFe-LDH grown on nickel foam: A bifunctional electrocatalyst for hydrogen production by urea-assisted electrolysis

In targeting the most important energy and environmental issues in current society, the development of low-cost, bifunctional electrocatalysts for urea-assisted electrocatalytic hydrogen (H₂) production is an urgent and challenging task. In this work, interlaced rosette-like MoS₂/Ni₃S₂/NiFe-layered double hydroxide/nickel foam (LDH/NF) is successfully synthesized by a two-step hydrothermal reaction. Due to its unique interlaced heterostructure, MoS₂/Ni₃S₂/NiFe-LDH/NF exhibits excellent bifunctional catalytic activity towards the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER) in 1.0 M KOH with 0.5 M urea. In a concurrent two-electrode electrolyser (MoS₂/Ni₃S₂/NiFe-LDH/NF(+,-)), only voltage of 1.343 V is required to reach 50 mA cm⁻², which is 216 mV lower than for pure water splitting. Furthermore, after 16 h of urea electrolysis in 1.0 M KOH with 0.5 M urea, the current density remains at 98% of the original value. Thus, the catalyst is not only favorable for H₂ production, but also has great significance for the problem of urea-rich wastewater treatment.     

Self-supported iron-doped nickel sulfide as efficient catalyst for electrochemical urea and hydrazine oxidation reactions

The electrochemical oxidation of urea and hydrazine over self-supported Fe-doped Ni₃S₂/NF (Fe–Ni₃S₂/NF) nanostructured material is presented. Among the various reaction conditions Fe–Ni₃S₂/NF-2 prepared at 160 °C for 8 h using 0.03 mM Fe(NO₃)₃ shows the best results for the hydrazine and urea oxidation reactions. The potential values of 0.36, 1.39, and 1.59 V are required to achieve the current density of the 100 mA cm^?2 in 1 M hydrazine (Hz), 0.33 M urea, and 1 M KOH electrolyte, respectively. The onset potential in 1 M KOH, 0.33 M Urea +1 M KOH, and 1 M Hz + 1 M KOH values are 1.528, 1.306, and 0.176 respectively. The Fe–Ni₃S₂/NF-2 shows stable performance at 10 mA cm^?2 until 50 h and at 60 mA cm^?2 over the 25 h. A cell of PtC//Fe–Ni₃S₂/NF-2 requires the potential of 0.49, 1.46, and 1.59 V for the hydrogen production in 1 M Hz + 1 M KOH, 0.33 M Urea +1 M KOH, and 1 M KOH electrolyte, respectively, at a current density of 10 mA cm^?2, and almost 90% stable for the hydrogen production over the 80 h in all electrolytes. The improvement of the chemical kinetics of urea and hydrazine oxidation is due to the synergistic effect of the adsorption and fast electron transfer reaction on Fe–Ni₃S₂/NF-2. The doped Fe ion facilitates the fast electron transfer and the surface of Ni₃S₂ support to the urea and hydrazine molecule adsorption.     

Porous flower-like nickel nitride as highly efficient bifunctional electrocatalysts for less energy-intensive hydrogen evolution and urea oxidation

Finding a suitable replacement for the high potential of anodic water electrolysis (oxygen evolution reaction (OER)) is significant for hydrogen energy storage and conversion. In this work, a simple and scalable method synthesizes a structurally unique Ni₃N nanoarray on Ni foam, Ni₃N-350/NF, that provides efficient electrocatalysis for the urea oxidation reaction (UOR) that transports 10 mA cm⁻² at a low potential of 1.34 V. In addition, Ni₃N-350/NF exhibits electro-defense electrocatalytic performance for hydrogen evolution reaction, which provides a low overpotential of 128 mV at 10 mA cm⁻². As proof of concept, all-water-urea electrolysis measurement is carried out in 1 M KOH with 0.5 M Urea with Ni₃N-350/NF as cathode and anode respectively. Ni₃N-350/NF||Ni₃N-350/NF electrode can provide 100 mA cm⁻² at a voltage of only 1.51 V, 160 mV less than that of water electrolysis, which proves its commercial viability in energy-saving hydrogen production.     

Natural-wood-fiber-assisted synthesis of Ni2P embedded in P-doped porous carbon as highly active catalyst for urea electro-oxidation

Defect and interface engineering has been established as efficient methods for altering the electrical structure and improving the activity of electrocatalysts. Here, a rational design architecture consisting of Ni₂P nanoparticles embedded in P-doped carbonized wood fibers (Ni₂P/PCWF) is synthesized by simultaneous carbonization and phosphorization. A synergistic enhancement effect between electronic structure manipulation and interface regulation is observed in Ni₂P/PCWF during the urea oxidation reaction (UOR). First, the P doping of carbon can optimize the electronic structure of Ni₂P/PCWF. Second, the charge transport process is aided by the Ni₂P nanoparticles embedded in the PCWF. Lastly, electron transfer can be accelerated by the in-situ formed heterogeneous interface between metal phosphides and metal hydroxides (hydroxyl oxides). Due to the synergy of the structural and electrical modulation, Ni₂P/PCWF exhibits remarkable electrocatalytic properties toward the UOR under alkaline conditions. It only requires 1.34 V (vs. RHE) to achieve a current density of 50 mA cm^?2, and the increase in potential at 10 mA cm^?2 for 70 h is insignificant (≈2.9%). This work supports the development of new strategies using sustainable, renewable wood fibers to develop excellent UOR catalysts for energy-saving H₂ generation.     

Controlled synthesis of N–CuCo2S4@Ni3S2 nanoarrays as promising urea oxidation electrocatalyst

One of the most attractive means for mitigation of environmental pollution is to produce hydrogen by electrocatalytic urea splitting. In the paper, the heterogeneous interfacial rich N–CuCo₂S₄@Ni₃S₂ material was in situ synthesized on nickel foam through a typical hydrothermal and sulfuration process. This N–CuCo₂S₄@Ni₃S₂ electrode displays excellent urea oxidation performance (potential of 1.38 V@50 mA cm⁻²), which is one of the best reactivity reported so far. Experimental results show that the superior catalytic activity is attributed to the rapid charge transfer, more reaction center exposed and superior electrical conductivity. Density functional theory shows that this Ni₃S₂ material accelerates the reaction rate in the catalytic process, the introduction of this N–CuCo₂S₄ material improves the conductivity of the material, and the synergistic catalysis of the N–CuCo₂S₄ and Ni₃S₂ makes this N–CuCo₂S₄@Ni₃S₂ material exhibit superior urea oxidation activity. Notably, long-term tests have shown a decrease in catalytic activity, which suggests that the surface of the sulfide is in situ generated to oxides or hydroxides, which are truly active species. This work provides a new idea for the development of efficient and stable urea oxidation catalysts for sulfides.     

Silica based hybrid organic-inorganic materials for PEMFC application

It is of key importance to develop membrane assembly electrodes (MEAs) offering high conductivity, thermal stability and suitable performance in the fuel cell. The mesoporous materials functionalized with acid groups are appropriate candidates to improve membrane's properties. The goal of this work was to assess the addition of functionalized porous silica, bearing different acid groups, on the MEA performance in a PEM type single fuel cell. Ni₅₉Nb₄₀Pt_0.6Fe_0.4 -based amorphous alloys were applied as anode electrocatalysts. The synthesis of functionalized mesoporous silica (UGM-fx) with different acid groups, namely [SO₃H], [COOH] and [PO(OH)₂], was carried out following a nonaqueous sol gel method. The results showed that the MEA containing silica with PO(OH)₂ groups leads to an outstanding fuel cell performance compared to that of the other organic groups-based MEAs and that it outperformed a commercial Pt-based sample. This might be due to the higher proton conductivity exhibited by the phosphonic groups.     

Phase transition in V-doped cobalt hydroxide for efficient oxygen evolution and urea oxidation reaction

In this work, many kinds of V doped Co(OH)₂ electrodes were in situ synthesized on Ni foam by a one-step typical hydrothermal process. It is worth noting that the phase transition composition of the V doped Co(OH)₂ material can be modulated by the difference of the amount of the V introduced. Different crystal phase compositions show different water oxidation activities. It is worth noting that the V2–Co(OH)₂/NF electrode shows better oxygen evolution performance (Overpotential of 320 mV@50 mA cm⁻²) compared with Co(OH)₂/NF (450 mV@50 mA cm⁻²), V1–Co(OH)₂/NF (340 mV@50 mA cm⁻²) and V3–Co(OH)₂/NF (350 mV@50 mA cm⁻²) electrodes. The experimental results show that not all doping can improve the electrochemistry performance of electrodes, such as the oxidation of urea. Density functional theory calculation further proves that the doping of the V is favorable to the adsorption of water and inhibits the adsorption of urea. This study provides a new idea for the development of efficient overall water splitting catalysts.     

Facile construction of heterostructural Ni3(NO3)2(OH)4/CeO2 bifunctional catalysts for boosted overall water splitting

Interface engineering has aroused vitally widespread concern since it could be an effective strategy for exploring high-performance and low-cost water oxidation electrocatalysts. Herein, we report a hetero-structured Ni₃(NO₃)₂(OH)₄/CeO₂/NF (NNO/CeO₂/NF) electrode, exhibiting superior performance owning to the NO₃^? anion substitution for the OH^? in nickel hydroxide to form Ni₃(NO₃)₂(OH)₄, together with its interface synergy with ceria. In alkaline solution, the NNO/CeO₂/NF electrocatalyst could catalyze the OER with an overpotential of 330 mV to approach 50 mA cm^?2. Also, it needs only an overpotential of 120 mV to reach 10 mA cm^?2 for HER. Additionally, when a standard two-electrode water electrolyzer is fabricated by employing NNO/CeO₂/NF as both the cathode and anode, it can generate 10 mA cm^?2 at 1.64 V and operate steadily without performance degradation after 25 h. This research provides a novel perspective for reasonable design of advanced catalytic materials with improvements in the field of electrocatalysis.     

Investigation of Ni2+-doped ceria nanorods as the anode catalysts for reduced-temperature solid oxide fuel cells

Tailoring the surface chemistry of oxides has been widely used to adjust their catalytic behavior in the energy conversion and storage devices. Herein, nanorods of Ni²⁺-doped ceria (Ce_1-xNi_xO_2-δ, x = 0, 0.05, 0.1, 0.15) are synthesized via a modified hydrothermal method, and evaluated as the anode catalysts for reduced-temperature solid oxide fuel cells (SOFCs). X-Ray diffraction patterns of as-synthesized powders in air imply successful incorporation of Ni²⁺ into the fluorite lattice of ceria for x = 0.05 and 0.1, with a secondary phase of NiO observed for x = 0.15. Transmission electron microscopy (TEM) examination confirms a rod-like morphology with a diameter of 10–13 nm and a length of 55–105 nm. Exposure of these powders in H₂ at 600°C results in exsolution of some spherical Ni particles of 11 nm in diameter. Electrochemical measurements on both symmetrical anode fuel cells and functioning cathode-supported fuel cells show an order of the catalytic activity toward hydrogen oxidation - CeO_2-δ < Ce_0·95Ni_0·05O_2-δ < Ce_0·9Ni_0·1O_2-δ. The anode polarization resistances in 97% H₂ – 3% H₂O are 0.24, 0.31 and 0.37 Ω?cm² for Ce_0·9Ni_0·1O_2-δ, Ce_0·95Ni_0·05O_2-δ and CeO_2-δ at 600°C, respectively. Thin (La_0·9Sr_0.1) (Ga_0.8Mg_0.2)O_3-δ-electrolyte fuel cells with nanostructured SmBa_0.5Sr_0·5Co₂O_5+δ cathodes and Ce_0·9Ni_0·1O_2-δ anodes yield the highest power densities among the investigated series of anodes, e.g., 820 mW?cm^?2 in 97% H₂ – 3% H₂O and 598 mW?cm^?2 in 68% CH₃OH - 32% N₂. XPS analyses of reduced nanorods indicate that the highest catalytic activities of Ce_0·9Ni_0·1O_2-δ toward fuel oxidation reactions should be correlated to the presence of highly active Ni nanoparticles and increased surface active oxygen, as confirmed by substantially facilitated extraction of the lattice oxygen on the surface by H₂ in temperature-programmed reduction (TPR) measurements.     

Bimetal metal-organic framework hollow nanoprisms for enhanced electrochemical oxygen evolution

Carboxylate-based metal-organic frameworks (MOFs) have emerged as promising electrocatalyst candidates for the water splitting and metal-air batteries. Hierarchical porous structure and redox-active metal centers with unsaturated coordination sites in MOFs facilitate the enhanced catalytic activity of oxygen evolution reaction (OER). Herein, uniform hollow structured Fe-free bi-metal (Co, Ni) MOF-74 nanoprisms are successfully synthesized using a solvothermal method and (Co₁Ni₁)₃(OH)(CH₃COO)₅ as the sacrificial templates, where Co and Ni are the metal nodes and 2,5-dihydroxyterephthalic acid (H₄DOBDC) serves as the organic ligand. At an overpotential of 300 mV, CoNi MOF-74 shows a high electrocatalytic activity towards OER in 0.1 M KOH, where the current density is 10 mA cm^?2 and the Tafel slope is 65.6 mV dec^?1. Meanwhile, CoNi MOF-74 is durable that sustains in alkaline for 100 h with 83.25% retention of current density. The improved catalytic activity can be associated with the in-situ generated amorphous Ni–Co (oxy)hydroxide, as well as the electron transfer from Ni²⁺ to Co²⁺. This work elucidates the potential application of MOF materials as efficient electrocatalysts for OER.