Use of urchin-like Ni(x)Co(3-x)O4 hierarchical nanostructures based on non-precious metals as bifunctional electrocatalysts for anion-exchange membrane alkaline alcohol fuel cells

Layer interfacing strategy to derive free standing CoFe@PANI bifunctional electrocatalyst towards oxygen evolution reaction and methanol oxidation reaction

Developing inexpensive, highly electrochemically active, and stable catalysts towards electrochemical studies remains challenge for researchers. In this regard, binder-free CoFe@PANI composite electrocatalyst is deposited on nickel foam (NF) substrate via successive electrodeposition of polyaniline (PANI) and CoFe-LDH at Room temperature (RT). As deposited binder-free CoFe@PANI electrocatalyst displays high electrocatalytic activity towards oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) in alkaline media. In CoFe@PANI structure, interfacing of high-electron conducting PANI establishes strong interconnection with CoFe-LDH by tuning electronic structures, which accelerates the electrochemical performance towards OER and MOR. For OER, CoFe@PANI requires low overpotential (η10) of 237 mV to reach current density (Id) of 10 mA cm-2 and displays low Tafel slope value of 46 mV dec-1 in 1 M KOH solution. Also, it displayed specific Id of 120 mA cm-2, when it was tested for MOR in 1 M KOH with 0.5 M methanol solution. The superior electrocatalytic activity of CoFe@PANI is mainly ascribed to high electrochemical active surface area (ECSA), abundant active sites and fast electron transfer between electrocatalyst and electrode surface. Of note, the current work may open new era for design and development of non-precious highly active and stable hybrid electrocatalysts at RT for various applications.

Fabrication of Flower-like Rhodium-Doped β-Ni(OH)2 as an Efficient Electrocatalyst for Methanol Oxidation Reaction in Alkaline Media

In this study, β-Ni(OH)₂ with a unique flower-like morphology was synthesized through a hydrothermal method. The doping of Rh in β-Ni(OH)₂ was achieved by a reduction method. The as-synthesized catalysts were characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy for the crystal structure, morphology, composition, and chemical state analysis. The electrochemical tests revealed that the doping of Rh can significantly increase the electrocatalytic performance of β-Ni(OH)₂ in 1.0 M KOH solution. The methanol oxidation peak current density of Rh-doped β-Ni(OH)₂ reached 95 mA cm^-2 with a Tafel slope of 40 mV dec^-1. The reason for these improvements is that Rh can suppress the phase transformation of NiOOH from β to α. Meanwhile, the electronic structure change of nickel in β-Ni(OH)₂ and the defect ratio increase caused by Rh doping are beneficial to methanol oxidation reaction (MOR) catalytic activity. Furthermore, the synergistic effect between Rh and Ni(OH)₂ improved the surface activity of β-NiOOH. The doping of Rh in β-Ni(OH)₂ is initiative in this work, which provides a new strategy to design highly efficient and cost-effective MOR electrocatalysts for alkaline DMFCs.

Impact of Erbium (Er) and Yttrium (Y) doping on BiVO4 crystal structure towards the enhancement of photoelectrochemical water splitting and photocatalytic performance

An efficient BiVO₄nanocatalyst with Erbium (Er) and Yttrium (Y) doping was synthesized via a facile microwave irradiation route and the obtained materials were further characterized through various techniques such as p-XRD, FT-IR, FE-SEM, HR-TEM, UV-Vis DRS, PL, LSV, and EISanalysis. The obtained results revealed that the rare metals induce the stabilization of the monoclinic-tetragonal crystalline structure with a distinct morphology. The yttrium doped BiVO₄ (Y-BiVO₄) monoclinic-tetragonal exhibited anefficient photoelectrochemical water splitting and photocatalytic performanceare compared to bare BiVO₄. TheY-BiVO₄ indicated increased results of photocurrent of 0.43 mA/cm²and bare BiVO₄0.24 mA/cm². Also, the Y-doped BiVO₄ nanocatalyst showed the maximum photocatalytic activity for the degradation of MB, MO, and RhB. A maximum degradation of 93%, 85%, and 91% was achieved for MB, MO, and RhB respectively, within 180 min under the visible light illumination. The photocatalytic decomposition of acetaldehyde also was performed. The improved photoelectrochemical water splitting and photocatalytic activity are due to the narrowing the bandgap, leading to extending the photoabsorption capability and reducing the recombination rate of photoexcited electron-hole pairs through the formation inner energy state of the rare earth metals. The current study disclosed that the synthesis of nanomaterials with crystal modification could be a prospectivecontender forhydrogen energy production as well as to the photocatalytic degradation of organic pollutants.To the best of our knowledge, both photocatalytic and photoelectrochemical studies were never been reported before for this type of material.

Electrochemically active site-rich nanocomposites of two-dimensional materials as anode catalysts for direct oxidation fuel cells: new age beyond graphene

Direct oxidation fuel cell (DOFC) has been opted as a green alternative to fossil fuels and intermittent energy resources as it is economically viable, possesses good conversion efficiency, as well as exhibits high power density and superfast charging. The anode catalyst is a vital component of DOFC, which improves the oxidation of fuels; however, the development of an efficient anode catalyst is still a challenge. In this regard, 2D materials have attracted attention as DOFC anode catalysts due to their fascinating electrochemical properties such as excellent mechanical properties, large surface area, superior electron transfer, presence of active sites, and tunable electronic states. This timely review encapsulates in detail different types of fuel cells, their mechanisms, and contemporary challenges; focuses on the anode catalyst/support based on new generation 2D materials, namely, 2D transition metal carbide/nitride or carbonitride (MXene), graphitic carbon nitride, transition metal dichalcogenides, and transition metal oxides; as well as their properties and role in DOFC along with the mechanisms involved.

Modulating Hydroxyl-Rich Interfaces on Nickel-Copper Double Hydroxide Nanotyres to Pre-activate Alkaline Ammonia Oxidation Reactivity

The surface hydroxyl groups of Ni_x Cu_1-x (OH)₂ play a crucial role in governing their conversion efficiency into Ni_x Cu_1-x O_x (OH)_2-x during the electro-chemical pre-activation process, thus affecting the integral ammonia oxidation reaction (AOR) reactivity. Herein, the rational design of hierarchical porous NiCu double hydroxide nanotyres (NiCu DHTs) was reported for the first time by considering hydroxyl-rich interfaces to promote pre-activation efficiency and intrinsic structural superiority (i.e., annulus, porosity) to accelerate AOR kinetics. A systematic investigation of the structure-function relationship was conducted by manipulating a series of NiCu DHs with tunable intercalations and morphologies. Remarkably, the NiCu DHTs exhibit superior AOR activity (onset potential of 1.31 V with 7.52 mA cm^-2 at 1.5 V) and high ammonia sensitivity (detection limit of 9 μm), manifesting one of the best non-noble metal AOR electrocatalysts and electro-analytical electrodetectors. This work deepens the understanding of the crucial role of surface hydroxyl groups on determining the catalytic performance in alkaline medium.

Flower-like nanosheets directly grown on Co foil as efficient bifunctional catalysts for overall water splitting

Hydrogen generation through electrochemical water decomposition is a promising method to address the global energy crisis. Herein, we report the synthesis of a series of flower-like Mo₃S₄/Co_1-xS composites on Co foil (Mo₃S₄/Co_1-xS@CF) as high-performance electrochemical water-splitting catalysts in an alkaline environment. The flower-like array structure of Mo₃S₄/Co_1-xS@CF not only increases the electrochemically active surface area of ​​the catalyst, but also facilitates the release of bubbles generated, resulting in enhanced catalytic activity. For the hydrogen evolution reaction, the Mo₃S₄/Co_1-xS@CF electrode exhibits good stability and excellent catalytic activity in 1.0 M KOH (η₁₀ = 105 mV), 1.0 M PBS (η₁₀ = 92 mV) and 0.5 M H₂SO₄ (η₁₀ = 68 mV) solutions. For the oxygen evolution reaction, the electrode displays excellent stability and catalytic activity in 1.0 M KOH solution (η₁₀ = 215 mV). When used for overall water splitting in 1.0 M KOH solution, Mo₃S₄/Co_1-xS@CF achieves a current density of 10 mA cm^-2 at a low potential of 1.58 V and maintains it stably for 40 h. This study presents a simple method for preparing transition metal-based bimetallic composite catalysts for efficient hydrogen production.

New Insights into the Electrocatalytic Mechanism of Methanol Oxidation on Amorphous Ni-B-Co Nanoparticles in Alkaline Media

Despite an increased interest in sustainable energy conversion systems, there have been limited studies investigating the electrocatalytic reaction mechanism of methanol oxidation on Ni-based amorphous materials in alkaline media. A thorough understanding of such mechanisms would aid in the development of amorphous catalytic materials for methanol oxidation reactions. In the present work, amorphous Ni-B and Ni-B-Co nanoparticles were prepared by a simple chemical reduction, and their electrocatalytic properties were investigated by cyclic voltammetry measurements. The diffusion coefficients (D0) for Ni-B, Ni-B-Co0.02, Ni-B-Co0.05, and Ni-B-Co0.1 nanoparticles were calculated to be 1.28 × 10−9, 2.35 × 10−9, 4.48 × 10−9 and 2.67 × 10−9 cm2 s−1, respectively. The reaction order of methanol in the studied transformation was approximately 0.5 for all studied catalysts, whereas the reaction order of the hydroxide ion was nearly 1. The activation energy (Ea) values of the reaction were also calculated for the Ni-B and Ni-B-Co nanoparticle systems. Based on our kinetic studies, a mechanism for the methanol oxidation reaction was proposed which involved formation of an electrocatalytic layer on the surface of amorphous Ni–B and Ni-B-Co nanoparticles. And methanol and hydroxide ions could diffuse freely through this three-dimensional porous conductive layer.

Synthesis and Electrochemical Study of Mesoporous Nickel-Cobalt Oxides for Efficient Oxygen Reduction

Development of a cost-effective and efficient electrocatalyst for the sluggish oxygen reduction reaction (ORR) is a crucial challenge for clean energy technologies. In this study, we have synthesized various Ni and Co oxide (NCO) nanomaterials via a facile coprecipitation, followed by the calcination method. The morphology of the formed NCO nanomaterials was controlled by varying the percentage of the Ni and Co precursors, leading to the formation of a template-free mesoporous spinel phase structure of Ni _xCo_{3- x}O₄. It was found that the number of the octahedral site cations and the defect sites with lower oxygen in the spinel oxides can be tunable by taking appropriate ratios of the Ni and Co precursors. The optimized NCO nanomaterial exhibits superior electrocatalytic activity compared to the mono-metal oxides of NiO and Co₃O₄ with over 3 times higher current density and ∼0.250 V lower onset potential toward ORR in a 0.1 M KOH solution. Scanning electrochemical microscopy was utilized in mapping the activity of the catalyst and monitoring the ORR products, further confirming that a four-electron transfer pathway was facilitated by the NCO nanomaterial. Moreover, the developed mesoporous NCO nanomaterial exhibits a high methanol tolerance capability and long-term stability when compared to the commercial state-of-the-art Pt/C electrocatalyst. The improvement of the catalytic activity and stability of this advanced NCO nanomaterial toward ORR may be attributed to the facile accessible mesoporous structure, and the abundance of octahedral site cations and defective oxygen sites.

Shape-controlled synthesis of Co3O4 for enhanced electrocatalysis of the oxygen evolution reaction

One-dimensional Co₃O₄ nanorods, two-dimensional nanosheets and three-dimensional nanocubes were synthesized; the effect of the morphology on their electrocatalytic activities was studied.

Two-Dimensional Mn-Co LDH/Graphene Composite towards High-Performance Water Splitting

The oxygen evolution reaction (OER) is a complex multi-step four-electron process showing sluggish kinetics. Layered double hydroxides (LDH) were reported as promising catalysts for the OER, but their low electrical conductivity restricts their widespread applications. To overcome this problem, a composite material containing Mn-Co LDH ultrathin nanosheet and highly conductive graphene was synthesized for the first time. Benefited from the high electrocatalytic activity and the superior charge transfer ability induced by these components, the new material shows superior OER activity. Used as the OER catalyst, a high current density of 461 mA cm−2 at 2.0 V vs. RHE (reversible hydrogen electrode) was measured besides shows a low overpotential of 0.33 V at 10 mA cm−2. Moreover, the new composite also shows a superior bifunctional water splitting performance as catalyst for the OER and HER (hydrogen evolution reaction) catalysts. Our results indicate that the presented material is a promising candidate for water splitting which is cheap and efficient.

Improved Charge Transfer in a Mn2O3@Co1.2Ni1.8O4 Hybrid for Highly Stable Alkaline Direct Methanol Fuel Cells with Good Methanol Tolerance

A three-dimensional Mn₂O₃@Co_1.2Ni_1.8O₄ hybrid was synthesized via facile two-step processes and employed as a cathode catalyst in direct methanol fuel cells (DMFCs) for the first time. Because of the unique architecture with ultrathin and porous nanosheets of the Co_1.2Ni_1.8O₄ shell, this composite exhibits better electrochemical performance than the pristine Mn₂O₃. Remarkably, it shows excellent methanol tolerance, even in a high concentration solution. The DMFC was assembled with Mn₂O₃@Co_1.2Ni_1.8O₄, polymer fiber membranes, and PtRu/C as the cathode, membrane, and anode, respectively. The power densities of 57.5 and 70.5 mW cm^-2 were recorded at 18 and 28 °C, respectively, especially the former is the best result reported in the literature at such a low temperature. The stability of the Mn₂O₃@Co_1.2Ni_1.8O₄ catalyzed cathode was evaluated, and the results show that this compound possesses excellent stability in a high methanol concentration. The improved electrochemical activity could be attributed to the narrow band gap of the hybrid, which accelerates the electrons jumping from the valence band to the conduction band. Therefore, Mn^III could be oxidized into Mn^IV more easily, simultaneously providing an electron to the absorbed oxygen.

Interdiffusion Reaction-Assisted Hybridization of Two-Dimensional Metal-Organic Frameworks and Ti3C2Tx Nanosheets for Electrocatalytic Oxygen Evolution

Two-dimensional (2D) metal-organic framework (MOF) nanosheets have been recently regarded as the model electrocatalysts due to their porous structure, fast mass and ion transfer through the thickness, and large portion of exposed active metal centers. Combining them with electrically conductive 2D nanosheets is anticipated to achieve further improved performance in electrocatalysis. In this work, we in situ hybridized 2D cobalt 1,4-benzenedicarboxylate (CoBDC) with Ti₃C₂T_x (the MXene phase) nanosheets via an interdiffusion reaction-assisted process. The resulting hybrid material was applied in the oxygen evolution reaction and achieved a current density of 10 mA cm^-2 at a potential of 1.64 V vs reversible hydrogen electrode and a Tafel slope of 48.2 mV dec^-1 in 0.1 M KOH. These results outperform those obtained by the standard IrO₂-based catalyst and are comparable with or even better than those achieved by the previously reported state-of-the-art transition-metal-based catalysts. While the CoBDC layer provided the highly porous structure and large active surface area, the electrically conductive and hydrophilic Ti₃C₂T_x nanosheets enabled the rapid charge and ion transfer across the well-defined Ti₃C₂T_x-CoBDC interface and facilitated the access of aqueous electrolyte to the catalytically active CoBDC surfaces. The hybrid nanosheets were further fabricated into an air cathode for a rechargeable zinc-air battery, which was successfully used to power a light-emitting diode. We believe that the in situ hybridization of MXenes and 2D MOFs with interface control will provide more opportunities for their use in energy-based applications.

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

The design and development of low-cost Pt-free, high-active, and durable noble-metal-free electrocatalysts for methanol electrooxidation is highly desirable but remains a challenge. Herein, unique Cu/NiCu nanowires (NWs) integrating alloy, core-shell, and one-dimensional structures are prepared by a facile one-pot strategy. It is found that the Ni-Cu surface alloying structure can effectively change the charge distribution of the atomic configuration, the core-shell structure can be optimized with the usage of Ni and Cu, and the one-dimensional structure can effectively enhance the charge transfer between the electrode surface and the active sites, making the prepared NWs promising electrocatalysts. Detailed catalytic investigations showed that the obtained Cu/NiCu NWs exhibit an enhanced electrocatalytic performance for methanol oxidation reaction (MOR). The optimized Cu/NiCu NWs in this work show a mass current density of 867.1 mA mg_metal^-1 at 1.55 V (vs. RHE) for MOR, which is far higher than those of Ni-based previously reported electrocatalysts. This work opens up a new pathway for the design and engineering of noble-metal-free alloy electrocatalysts with enhanced activity and durability.

Facile synthesis of a mechanically robust and highly porous NiO film with excellent electrocatalytic activity towards methanol oxidation

Considerable research is being conducted in searching for effective anode catalysts in alkaline direct methanol fuel cells (DMFCs). Although significant progress has been achieved, it is still challenging to prepare non-Pt catalysts with both excellent activity and good durability. Herein, a highly porous NiO film is developed by a facile and fast anodization approach. The anodic NiO film demonstrates a high surface area, large mesopore volume and small crystallite size, leading to facilitated adsorption of reaction species, easy electrolyte penetration and fast reaction kinetics. Furthermore, as anodic NiO is grown in situ on a metallic substrate with strong adhesion strength and good electrical contact, it can be used directly as an anode catalyst for methanol oxidation without the need to add any binder or conducting agent. Such an additive-free approach greatly expedites the catalyst preparation process. The anodic NiO shows lower methanol oxidation potential, higher oxidation current and better catalytic durability than most of the state-of-the-art Ni-based catalysts reported elsewhere. As anodization is a simple, low cost and easily scaled up method, the work described here provides an exciting direction to speed up the practical application of alkaline DMFCs.

Bifunctional Manganese Ferrite/Polyaniline Hybrid as Electrode Material for Enhanced Energy Recovery in Microbial Fuel Cell

Microbial fuel cells (MFCs) are emerging as a sustainable technology for waste to energy conversion where electrode materials play a vital role on its performance. Platinum (Pt) is the most common material used as cathode catalyst in the MFCs. However, the high cost and low earth abundance associated with Pt prompt the researcher to explore inexpensive catalysts. The present study demonstrates a noble metal-free MFC using a manganese ferrite (MnFe2O4)/polyaniline (PANI)-based electrode material. The MnFe2O4 nanoparticles (NPs) and MnFe2O4 NPs/PANI hybrid composite not only exhibited superior oxygen reduction reaction (ORR) activity for the air cathode but also enhanced anode half-cell potential upon modifying carbon cloth anode in the single-chambered MFC. This is attributed to the improved extracellular electron transfer of exoelectrogens due to Fe(3+) in MnFe2O4 and its capacitive nature. The present work demonstrates for the first time the dual property of MnFe2O4 NPs/PANI, i.e., as cathode catalyst and an anode modifier, thereby promising cost-effective MFCs for practical applications.

Controlled synthesis of series NixCo3-xO4 products: Morphological evolution towards quasi-single-crystal structure for high-performance and stable lithium-ion batteries

Transition metal oxides are very promising alternative anode materials for high-performance lithium-ion batteries (LIBs). However, their conversion reactions and concomitant volume expansion cause the pulverization, leading to poor cycling stability, which limit their applications. Here, we present the quasi-single-crystal Ni_xCo_3-xO₄ hexagonal microtube (QNHM) composed of continuously twinned single crystal submicron-cubes as anode materials for LIBs with high energy density and long cycle life. At the current density of 0.8 A g⁻¹, it can deliver a high discharge capacities of 1470 mAh g⁻¹ over 100 cycles (105% of the 2nd cycle) and 590 mAh g⁻¹ even after 1000 cycles. To better understand what underlying factors lead our QNHMs to achieve excellent electrochemical performance, a series of Ni_xCo_3-xO₄ products with systematic shape evolution from spherical to polyhedral, and cubic particles as well as circular microtubes consisted of spheres and square microtubes composed of polyhedra have been synthesized. The excellent electrochemical performance of QNHMs is attributed to the unique stable quasi-single-crystal structure, which can both provide efficient electrical transport pathway and suppress the electrode pulverization. It is important to note that such quasi-single-crystal structure would be helpful to explore other high-energy lithium storage materials based on alloying or conversion reactions.

Efficient and durable oxygen reduction and evolution of a hydrothermally synthesized La(Co0.55Mn0.45)0.99O3-δ nanorod/graphene hybrid in alkaline media

The increasing global energy demand and the depletion of fossil fuels have stimulated intense research on fuel cells and batteries. Oxygen electrocatalysis plays essential roles as the electrocatalytic reduction and evolution of di-oxygen are always the performance-limiting factors of these devices relying on oxygen electrochemistry. A novel perovskite with the formula La(Co0.55Mn0.45)0.99O3-δ (LCMO) is designed from molecular orbital principles. The hydrothermally synthesized LCMO nanorods have unique structural and chemical properties and possess high intrinsic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The synergic covalent coupling between LCMO and NrGO enhances the bifunctional ORR and OER activities of the novel LCMO/NrGO hybrid catalyst. The ORR activity of LCMO/NrGO is comparable to the state-of-the-art Pt/C catalyst and its OER activity is competitive to the state-of-the-art Ir/C catalyst. LCMO/NrGO generally outperforms Pt/C and Ir/C with better bifunctional ORR and OER performance and operating durability. LCMO/NrGO represents a new class of low-cost, efficient and durable electrocatalysts for fuel cells, water electrolysers and batteries.

One-dimension-based spatially ordered architectures for solar energy conversion

The current status, future developments, and challenges of one-dimension-based spatially ordered architectures in solar energy conversion are discussed and elucidated.

Electrodeposited NixCo3−xO4 nanostructured films as bifunctional oxygen electrocatalysts

Nanostructured Ni_xCo_3−xO₄ films serve as effective electrocatalysts for both the oxygen reduction reaction and oxygen evolution reaction in alkaline electrolyte.