Binary Transition-Metal Oxide Hollow Nanoparticles for Oxygen Evolution Reaction

Synergistic Fe-Mn-Cu ternary alloys enhance bifunctional activity and stability for alkaline water splitting

Developing cost-effective, high-performance electrocatalysts for water splitting remains a critical challenge for advancing renewable energy technologies. Herein, we present a novel ternary alloy catalyst, 20Fe-80Mn-20Cu, designed and optimized for hydrogen evolution (HER) and oxygen evolution reactions (OER). The catalyst, synthesized via electrodeposition, demonstrates exceptional bifunctional activity and stability, outperforming binary (20Fe-80Mn) and benchmark electrodes, such as Pt and DSA. Linear sweep voltammetry (LSV) revealed that 20Fe-80Mn-20Cu requires a remarkably low overpotential (without iR drop correction) of 172 mV for HER and 147 mV for OER to achieve a current density of 10 mA cm- 2, significantly surpassing the performance of binary alloys and bare substrates. Tafel slope analysis further confirmed the catalytic efficiency, with values of 53 mV dec- 1 for HER and 56 mV dec- 1 for OER. Electrochemical impedance spectroscopy (EIS) revealed low charge transfer resistance, highlighting the alloy's excellent electron transport properties. Raman and XRD investigations revealed the catalyst's unique structural and compositional features, including extra crystallographic reflections indicating increased surface activity. Stability tests conducted at ± 250 mA cm- 2 over 4 days demonstrated excellent durability, with only 7% (HER) and 5% (OER) performance drops. Post-stability characterizations, including XRD and EDX, revealed Mn and Fe redistribution and Cu enrichment on the surface, as well as the formation of stable copper oxides under OER conditions. These findings establish 20Fe-80Mn-20Cu as a promising candidate for scalable water splitting, offering an energy-saving potential of up to 5.5 V per cm2 of the electrode surface. This study increases our understanding of alloy-based catalysts and demonstrates a feasible approach for efficient and sustainable hydrogen production.

3D self-assembled polar vs. non-polar NiO nanoparticles nanoengineered from turbostratic Ni3(OH)4(NO3)2 and ordered β-Ni(OH)2 intermediates

A surfactant-free ammonia and carbamide precursor-modulated engineering of self-assembled flower-like 3D NiO nanostructures based on ordered β-Ni(OH)2 and turbostratic Ni3(OH)4(NO3)2 nanoplate-structured intermediates is reported. By employing complementary structural and spectroscopic techniques, fundamental insights into structural and chemical transformations from intermediates to NiO nanoparticles (NPs) are provided. FTIR, Raman and DSC analyses show that the transformation of intermediates to NiO NPs involves subsequent loss of NO3- and OH- species through a double-step phase transformation at 306 and 326 °C corresponding to the loss of free interlayer ions and H2O species, respectively, followed by the loss of chemically bonded OH- and NO3- ions. Transformation to NiO NPs via the ammonia route proceeds as single-phase transition, accompanied with a loss of OH- species at 298 °C. The full transformation to NiO NPs of both intermediates is achieved at 350 °C through annealing in the air atmosphere. Ammonia-derived NPs maintain nanoflower morphology by self-assembling into nanoplates, which is enabled by H2O-mediated adhesion on the NiO NPs' {100} neutral surfaces. Structural transformations of turbostratic Ni3(OH)4(NO3)2 nanoplates result in the formation of NiO NPs dominantly shaped by inert polar OH-terminated (111) atomic planes, leading to the loss of the initial self-assembled 3D structure. DFT calculations support these observations, confirming that H2O adsorbs dissociatively on polar {111} surfaces, while only physisorption is energetically feasible on {100} surfaces. NiO NPs obtained via two different routes have overall different properties: carbamide-derived NPs are 3 times larger (15.5 vs. 5.4 nm), possess a larger band gap (3.6 vs. 3.2 eV) and are more Ni deficient. The intensity ratio of surface optical (SO) modes to transversal and longitudinal optical modes is ∼40 times higher in the NiO NPs obtained from β-Ni(OH)2 compared to Ni3(OH)4(NO3)2-derived NPs. The SO phonon lifetime is an order of magnitude shorter in NiO obtained from β-Ni(OH)2, reflecting a much smaller NP size. The choice of a precursor defines the size, morphology, crystallographic surface orientations and band gap of the NiO NPs, with Ni deficiency providing pathways for utilizing them as p-type materials, allowing for the precise nanoengineering of polar and neutral surface-dominated NiO NPs, which is of exceptional importance for use in catalysis.

Transition metal chalcogenides carbon-based as bifunctional cathode electrocatalysts for rechargeable zinc-air battery: An updated review

The rechargeable alkaline aqueous zinc-air batteries (ZABs) are prospective candidates to supply the energy demand for their high theoretical energy density, inherent safety, and environmental friendliness. However, their practical application is mainly restricted by the unsatisfactory efficiency of the air electrode, leading to an intense search for high-efficient oxygen electrocatalysts. In recent years, the composites of carbon materials and transition metal chalcogenides (TMC/C) have emerged as promising alternatives because of the unique properties of these single compounds and the synergistic effect between them. In this sense, this review presented the electrochemical properties of these composites and their effects on the ZAB performance. The operational fundamentals of the ZABs were described. After elucidating the role of the carbon matrix in the hybrid material, the latest developments in the ZAB performance of the monometallic structure and spinel of TMC/C were detailed. In addition, we report topics on doping and heterostructure due to the large number of studies involving these specific defects. Finally, a critical conclusion and a brief overview sought to contribute to the advancement of TMC/C in the ZABs.

Hollow Heterostructured Nanocatalysts for Boosting Electrocatalytic Water Splitting

The implementation of electrochemical water splitting demands the development and application of electrocatalysts to overcome sluggish reaction kinetics of hydrogen/oxygen evolution reaction (HER/OER). Hollow nanostructures, particularly for hollow heterostructured nanomaterials can provide multiple solutions to accelerate the HER/OER kinetics owing to their advantageous merit. Herein, the recent advances of hollow heterostructured nanocatalysts and their excellent performance for water splitting are systematically summarized. Starting by illustrating the intrinsically advantageous features of hollow heterostructures, achievements in engineering hollow heterostructured electrocatalysts are also highlighted with the focus on structural design, interfacial engineering, composition regulation, and catalytic evaluation. Finally, some perspective insights and future challenges of hollow heterostructured nanocatalysts for electrocatalytic water splitting are also discussed.

Effective Formation of a Mn-ZIF-67 Nanofibrous Network via Electrospinning: An Active Electrocatalyst for OER in Alkaline Medium

Finding the active center in a bimetallic zeolite imidazolate framework (ZIF) is highly crucial for the electrocatalytic oxygen evolution reaction (OER). In the present study, we constructed a bimetallic ZIF system with cobalt and manganese metal ions and subjected it to an electrospinning technique for feasible fiber formation. The obtained nanofibers delivered a lower overpotential value of 302 mV at a benchmarking current density of 10 mA cm^-2 in an electrocatalytic OER study under alkaline conditions. The obtained Tafel slope and charge-transfer resistance values were 125 mV dec^-1 and 4 Ω, respectively. The kinetics of the reaction is mainly attributed from the ratio of metals (Co and Mn) present in the catalyst. Jahn-Teller distortion reveals that the electrocatalytic active center on the Mn-incorporated ZIF-67 nanofibers (Mn-ZIF-67-NFs) was found to be Mn³⁺ along with the Mn²⁺ and Co²⁺ ions on the octahedral and tetrahedral sites, respectively, where Co²⁺ ions tend to suppress the distortion, which is well supported by density functional theory analysis, molecular orbital study, and magnetic studies.

Recent Advances in Metal-Gas Batteries with Carbon-Based Nonprecious Metal Catalysts

Metal-gas batteries draw a lot of attention due to their superiorities in high energy density and stable performance. However, the sluggish electrochemical reactions and associated side reactions in metal-gas batteries require suitable catalysts, which possess high catalytic activity and selectivity. Although precious metal catalysts show a higher catalytic activity, high cost of the precious metal catalysts hinders their commercial applications. In contrast, nonprecious metal catalysts complement the weakness of cost, and the gap in activity can be made up by increasing the amount of the nonprecious metal active centers. Herein, recent work on carbon-based nonprecious metal catalysts for metal-gas batteries is summarized. This review starts with introducing the advantages of carbon-based nonprecious metal catalysts, followed by a discussion of the synthetic strategy of carbon-based nonprecious metal catalysts and classification of active sites, and finally a summary of present metal-gas batteries with the carbon-based nonprecious metal catalysts is presented. The challenges and opportunities for carbon-based nonprecious metal catalysts in metal-gas batteries are also explored.

Facile fabrication of hollow tubular covalent organic frameworks using decomposable monomer as building block

In this study, a commercial and low-toxicity hydrazide-containing building block has been used to construct azine-linked covalent organic frameworks (COFs). New style COFs were constructed between flexible formic hydrazide (FH) and 1,3,5-triformylphloroglucinal (Tp) or 1,3,5-triformylbenzene (TFB). The two resulting COFs (TpFH and TFBFH) exhibited uniform hollow tubular morphology (20–50 nm for TpFH, 50–100 nm for TFBFH). Compared to hydrazine, FH has low-toxicity and is a flexible monomer, consisting of amine and aldehyde groups. The decomposition of FH slows down the reaction rate and the as-synthesized FH-series COFs (708 m² g⁻¹ for TpFH and 888 m² g⁻¹ for TFBFH) had higher specific surface area than hydrazine-series COFs (617 m² g⁻¹ for TpAzine and 472 m² g⁻¹ for TFBAzine). A detailed time-dependent investigation was carried out to interpret the mechanism of hollow structure formation, and Ostwald ripening possibly happens during the formation of hollow COF microstructures. Considering the porous and high density N, O elements of these materials, preliminary applications of the metal ions removal from aqueous solution and gas storage were implemented.

In this study, an efficient and green strategy has been used to synthesize chemically stable COFs with a hollow microtubular structure using decomposition aldehyde-containing monomer, and high affinities toward metal ions or gas molecules.

Hollow C@SnS2/SnS nanocomposites: High efficient oxygen evolution reaction catalysts

Using structural phase transitions to enhance electrochemical properties without has received wide attention due to its large active area and excellent electron transport capacity. In this work, hollow C@SnS₂/SnS nanocomposites were successfully synthesized from hollow C@SnS₂ by controlling the temperature and time of the phase transitions. It is found that this hollow C@SnS₂/SnS nanocomposite can serve as electrocatalyst, showing excellent oxygen evolution reaction performance. The Sn (IV) heterostructure easily accepts electrons in water and plays a crucial role in the oxygen evolution reaction. Meanwhile, the low-valence Sn (II) can maintain a stable structure in the electrochemical reaction and thus exhibits good electrochemical performance with an overpotential of 380 mV, at the current density of 10 mA cm^-2, and a low Tafel slope of 63 mV dec^-1, which is far lower than that of pure SnS or SnS₂.

Morphologically controlled cobalt oxide nanoparticles for efficient oxygen evolution reaction

Electrochemical water oxidation is one of the thrust areas of research today in solving energy and environmental issues. The morphological control in the synthesis of nanomaterials plays a crucial role in designing efficient electrocatalyst. In general, various synthetic parameters can direct the morphology of nanomaterials and often this is the main driving force for the electrocatalyst in tuning the rate of the oxygen evolution reaction (OER) for the electrochemical water-splitting. Here, a facile and cost-effective synthesis of spinel cobalt oxides (Co₃O₄) via a one-pot hydrothermal pathway with tunable morphology has been demonstrated. Different kinds of morphologies have been obtained by systematically varying the reaction time i.e. nanospheres, hexagon and nanocubes. Their catalytic activity has been explored towards OER in 1.0 M alkaline KOH solution. The catalyst Co₃O₄-24 h nanoparticles synthesized in 24 h reaction time shows the lowest overpotential (η) value of 296 mV at 10 mA cm^-2 current density, in comparison to that of other as-prepared catalysts i.e. Co₃O₄-pH9 (311 mV), Co₃O₄-12 h (337 mV), and Co₃O₄-6 h (342 mV) with reference to commercially available IrO₂ (415 mV). Moreover, Co₃O₄-24 h sample shows the outstanding electrochemical stability up to 25 h time.

Atomic-Level Manipulations in Oxides and Alloys for Electrocatalysis of Oxygen Evolution and Reduction

As chemical reactions and charge-transfer simultaneously occur on the catalyst surface during electrocatalysis, numerous studies have been carried out to attain an in-depth understanding on the correlation among the surface structure and composition, the electrical transport, and the overall catalytic activity. Compared with other catalysis reactions, a relatively larger activation barrier for oxygen evolution/reduction reactions (OER/ORR), where multiple electron transfers are involved, is noted. Many works over the past decade thus have been focused on the atomic-scale control of the surface structure and the precise identification of surface composition change in catalyst materials to achieve better conversion efficiency. In particular, recent advances in various analytical tools have enabled noteworthy findings of unexpected catalytic features at atomic resolution, providing significant insights toward reducing the activation barriers and subsequently improving the catalytic performance. In addition to summarizing important surface issues, including lattice defects, related to the OER and ORR in this Review, we present the current status and discuss future perspectives of oxide- and alloy-based catalysts in terms of atomic-scale observation and manipulation.

Prudent electrochemical pretreatment to promote the OER by catalytically inert "Iron incorporated metallic Ni nanowires" synthesized via the "non-classical" growth mechanism

This study provides new insight towards the non-classical "amorphous to crystalline" growth mechanism for metal nanowire synthesis and reports an electrochemical strategy to activate inactive materials into efficient electrocatalysts for the OER. Despite considerable research on transition metal oxides/hydroxides, especially NiFe based hydroxides as OER electrocatalysts, poor conductivity of these materials plagues their catalytic efficiency. In contrast, lack of catalytic centers hinders the OER performance of conductive metals. Herein, we devised a suitable precondition strategy to transform only the surface of conductive metallic Ni nanowires into active catalytic centers. The resulting material with intimate contact between the electrically conductive core and electrocatalytically active surface showed promising "specific" and "geometric" electrocatalytic activity towards the alkaline OER at different pH. Upon iron incorporation, the Fe centers incorporated at the surface as well as in the bulk of the nanowires were found to further boost the OER activity of these materials. A one-pot strategy was adopted to produce iron free/incorporated Ni nanowires covered with nano-spikes. Growth analysis revealed a unique "non-classical amorphous-to-crystalline transformation" to be responsible for the formation of metallic nanowires.

Fe3+–Co2+ species loaded on carbon as an effective pre-catalyst for oxygen evolution

An advanced electrocatalyst was synthesized by a one-step synthesis method.

2D Fe-doped NiO nanosheets with grain boundary defects for the advanced oxygen evolution reaction

NiFe_0.1O with grain boundary defects possesses a smaller ECSA (C_dl = 3.23 mF cm⁻²) than other samples. However, NiFe_0.1O shows the highest electrocatalytic OER performance.

Bimetallic Iron–Cobalt Catalysts and Their Applications in Energy-Related Electrochemical Reactions

Since the persistently increasing trend of energy consumption, technologies for renewable energy production and conversion have drawn great attention worldwide. The performance and the cost of electrocatalysts play two crucial roles in the globalization of advanced energy conversion devices. Among the developed technics involving metal catalysts, transition-metal catalysts (TMC) are recognized as the most promising materials due to the excellent properties and stability. Particularly, the iron–cobalt bimetal catalysts exhibit exciting electrochemical properties because of the interior cooperative effects. Herein, we summarize recent advances in iron–cobalt bimetal catalysts for electrochemical applications, especially hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Moreover, the components and synergetic effects of the composites and catalytic mechanism during reaction processes are highlighted. On the basis of extant catalysts and mechanism, the current issues and prospective outlook of the field are also discussed.

Electrospun cobalt-ZIF micro-fibers for efficient water oxidation under unique pH conditions

A unique electrospinning methodology to fabricate Co based ZIF microfibers has been developed which showed superior electrocatalytic water oxidation both in acidic and alkaline electrolytes.

Tailoring Surface Opening of Hollow Nanocubes and Their Application as Nanocargo Carriers

Hollow nanoparticles (NPs) are of broad interest for biomedical, optical, and catalytic applications due to their unique geometry-related physicochemical properties. The ability to engineer hollow structures with surface openings is particularly attractive since emergent properties are promised by the design of shell porosity and encapsulation of guest materials. However, it still remains challenging to precisely control the opening of the hollow structure, in terms of shape, size, and location. Here, we report a facile one-step strategy to synthesize a hollow nanostructure with well-defined cubic-shape openings at the corners, by regulating nanoscale galvanic replacement processes with specific surface-capping agents. The final product is a single-crystalline AuAg alloy which morphologically features three "belts" orthogonally wrapping around a virtual cube, denoted by nanowrapper. We demonstrate a structural tunability of our synthetic method for tailoring nanowrapper and the corresponding tuning of its plasmonic band from the visible to near-infrared (NIR) range. Advanced electron tomography techniques provide unambiguous three-dimensional (3D) visualizations to reveal an unconventional transformation pathway of sharp-cornered Ag nanocube to nanowrapper and correlate its structure with measured and computed spectroscopic properties. Importantly, we find that the surfactant, i.e., cetylpyridinium chloride (CPC), is crucial for the openings to be localized at the corners of the hollow cube and be tailored to a cubic shape in our one-step process. Furthermore, such a well-defined hollow architecture also allows a guest nano-object to be contained within, while the large openings at corners enable controlled loading/release of nanoscale cargo, a DNA-coated particle, using change of ionic conditions. This work expands our understanding of surface engineering in nanoscale galvanic replacement reactions and opens new ways toward the shape control of hollow NPs.

Ostwald Ripening Driven Exfoliation to Ultrathin Layered Double Hydroxides Nanosheets for Enhanced Oxygen Evolution Reaction

As a key half-reaction in water splitting, the oxygen evolution reaction (OER) process is kinetically sluggish. Layered double hydroxides (LDHs) are regarded as the highly promising electrocatalysts to promote the OER kinetics. However, the closely stacking layered structure of pristine bulk LDHs restricts the exposure of electrocatalytically active sites, and it remains a great challenge to find an efficient strategy to exfoliate the bulk LDHs into ultrathin and stable nanosheets with increased surface area and exposed active sites. Herein, a novel Ostwald ripening driven exfoliation (ORDE) of NiFe LDHs has been achieved in situ on the electrodes by spontaneously self-etching and redepositing via a simple hydrothermal treatment without the assistance of any exfoliating reagent or surfactant. The thermodynamically driven Ostwald ripening has been expanded to the exfoliation of two-dimensional layered materials for the first time. Compared with conventional exfoliation methods, this ORDE is a time-saving and green strategy that avoids the serious adsorption of surfactant molecules. The ORDE of NiFe LDHs is accomplished in situ on a Cu mesh electrode, which not only exhibits excellent electrical contact between LDHs catalyst and electrodes but also prevents the restacking of the exfoliated LDHs. As a result, the exfoliated ultrathin, clean, and vertically aligned NiFe nanosheets with much higher surface area and numerous exposed active edges and sites demonstrated significantly enhanced OER performances with low overpotential of 292 mV at 10 mA cm^-2 and long-term stability for more than 60 h, as well as remarkable flexibility. Additionally, bulk Ni(OH)₂ nanosheets on Ni foams have also been exfoliated by a similar mechanism, indicating this ORDE strategy can be widely extended to other 2D layered materials for novel applications.