A multi-interfacial FeOOH@NiCo2O4 heterojunction as a highly efficient bifunctional electrocatalyst for overall water splitting

Structural engineering evoked multifunctionality in molybdate nanosheets for industrial oxygen evolution and dual energy storage devices inspired by multi-method calculations

Structural engineering, including electronic and geometric modulations, is a good approach to improve the activity of electrocatalysts. Herein, we employed FeOOH and the second metal center Ni to modulate the electronic structure of CoMoO4 and used a low temperature solvothermal route and a chemical etching method to prepare the special hollow hierarchical structure. Based on the prediction of multi-method calculations by density functional theory (DFT) and ab initial molecular dynamics (AIMD), a series of materials were fabricated. Among them, the optimal hollow FeOOH/(Ni1Co1)MoO4 by coating (NiCo)MoO4 nanosheets on FeOOH nanotubes showed excellent performances toward high current density oxygen evolution reaction (OER) in alkaline and simulated seawater solutions, hybrid supercapacitor (HSC), and aqueous battery due to the well-controlled electronic and geometric structures. The optimal FeOOH/(Ni1Co1)MoO4 required overpotentials of 225 and 546 mV to deliver 10 and 1000 mA cm-2 current densities toward alkaline OER, and maintained a good stability for 100 h at 200 mA cm-2 with negligible attenuation. The FeOOH/(Ni1Co1)MoO4//Pt/C electrolyzer exhibited a low cell voltage of 1.52 and 1.79 V to drive 10 and 200 mA cm-2 and retained a long-term durability nearly 100 h at 1.79 V. As the electrode of energy storage devices, it possessed a specific capacity of 342 mA h g-1 at 1 A g-1. HSC and SC-type battery devices were fabricated. The assembled HSC kept a capacitance retention of 94 % after 10,000 cycles. This work provided a way to fabricate effective and stable multifunctional materials for energy storage and conversion with the aid of multi-method calculations.

Al-Ce co-doped BaTiO3 nanofibers as a high-performance bifunctional electrochemical supercapacitor and water-splitting electrocatalyst

Supercapacitors and water splitting cells have recently played a key role in offering green energy through converting renewable sources into electricity. Perovskite-type electrocatalysts such as BaTiO3, have been well-known for their ability to efficiently split water and serve as supercapacitors due to their high electrocatalytic activity. In this study, BaTiO3, Al-doped BaTiO3, Ce-doped BaTiO3, and Al-Ce co-doped BaTiO3 nanofibers were fabricated via a two-step hydrothermal method, which were then characterized and compared for their electrocatalytic performance. Based on the obtained results, Al-Ce co-doped BaTiO3 electrode exhibited a high capacitance of 224.18 Fg-1 at a scan rate of 10 mVs-1, high durability during over the 1000 CV cycles and 2000 charge-discharge cycles, proving effective energy storage properties. Additionally, the onset potentials for OER and HER processes were 11 and - 174 mV vs. RHE, respectively, demonstrating the high activity of the Al-Ce co-doped BaTiO3 electrode. Moreover, in overall water splitting, the amount of the overpotential was 0.820 mV at 10 mAcm-2, which confirmed the excellent efficiency of the electrode. Hence, the remarkable electrocatalytic performance of the Al-Ce co-doped BaTiO3 electrode make it a promising candidate for renewable energy technologies owing to its high conductivity and fast charge transfer.

Fe-Incorporated Nickel-Based Bimetallic Metal-Organic Frameworks for Enhanced Electrochemical Oxygen Evolution

Developing cost-effective and high-efficiency catalysts for electrocatalytic oxygen evolution reaction (OER) is crucial for energy conversions. Herein, a series of bimetallic NiFe metal-organic frameworks (NiFe-BDC) were prepared by a simple solvothermal method for alkaline OER. The synergistic effect between Ni and Fe as well as the large specific surface area lead to a high exposure of Ni active sites during the OER. The optimized NiFe-BDC-0.5 exhibits superior OER performances with a small overpotential of 256 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 45.4 mV dec^-1, which outperforms commercial RuO₂ and most of the reported MOF-based catalysts reported in the literature. This work provides a new insight into the design of bimetallic MOFs in the applications of electrolysis.

Tapping the Potential of High-Valent Mo and W Metal Centers for Dynamic Electronic Structures in Multimetallic FeVO(OH)/Ni(OH)2 for Ultrastable and Efficient Overall Water Splitting

Rationally designing a noble metal-free electrocatalyst for OER and HER is pivotal for large-scale energy generation via water splitting. A multimetallic electrocatalyst FeVO(OH)/Ni_0.86Mo_0.07W_0.07(OH)₂, aimed at tuning the electronic structure, is fabricated and shows considerable improvement in the water-splitting reaction kinetics, aided by low Tafel slope values of 24 mV/dec for OER and 67 mV/dec for HER, respectively. By taking advantage of (e̅-e̅) repulsions at the t_2g level, we introduced high-valency Mo and W to provide a viable path for π-electron donation from oxygen 2p orbitals to vacant Mo and W orbitals for a dynamic electronic structure and an interfacial synergistic effect, which optimized the bond lengths for reaction intermediates to facilitate water splitting. The hybrid catalyst FeVO(OH)/NiMoW(OH)₂ shows intrinsic activity and durability toward OER and HER tested for 48 h at a current density of 20 mA/cm² and a cell voltage of 1.65 V @ 20 mA/cm².

NiCo2O4 nanoparticles rich in oxygen vacancies: Salt-Assisted preparation and boosted water splitting

NiCo₂O₄ is a promising catalyst toward water splitting to hydrogen. However, low conductivity and limited active sites on the surfaces hinder the practical applications of NiCo₂O₄ in water splitting. Herein, small sized NiCo₂O₄ nanoparticles rich in oxygen vacancies were prepared by a simple salt-assisted method. Under the assistance of KCl, the formed NiCo₂O₄ nanoparticles have abundant oxygen vacancies, which can increase surface active sites and improve charge transfer efficiency. In addition, KCl can effectively limit the growth of NiCo₂O₄, and thus reduces its size. In comparison with NiCo₂O₄ without the assistance of KCl, both the richer oxygen vacancies and the reduced nanoparticle sizes are favorable for the optimal NiCo₂O₄-2KCl to expose more active sites and increase electrochemical active surface area. As a result, it needs only the overpotentials of 129 and 304 mV to drive hydrogen and oxygen evolution at 10 mA cm⁻² in 1 M KOH, respectively. When NiCo₂O₄-2KCl is applied in a symmetrical water splitting cell, a voltage of ∼1.66 V is only required to achieve the current density of 10 mA cm⁻². This work shows that the salt-assisted method is an efficient method of developing highly active catalysts toward water splitting to hydrogen.

Carbon Nanotube-Coupled Seaweed-like Cobalt Sulfide as a Dual-Functional Catalyst for Overall Water Splitting

Preparation of high-efficiency dual-functional catalysts remains the bottleneck for electrochemical water splitting. To prepare a non-precious metal catalyst with high activity and stability, here, we present a seaweed-like structure consisting of transition-metal sulfide nanoplates self-assembled on carbon nanotube sponge networks (SW-CoS@CNT). By adjusting the key parameters during synthesis (e.g., the loading amount and ratio of Co and S precursors), the microstructure can be tailored in a wide range, and sulfur defects can be introduced into the nanoplates by thermal annealing. The resulting SW-CoS@CNT serves as a freestanding dual-functional catalytic electrode, showing low overpotentials of 105 and 218 mV for the hydrogen evolution reaction and the oxygen evolution reaction, respectively, which are superior to most reported transition-metal-sulfide-based catalysts in alkaline solution. Rational design of this hierarchical biomimetic structure may be useful in developing high-performance electrochemical catalysts in renewable energy and environmental fields.

Urchin-like hierarchical ruthenium cobalt oxide nanosheets on Ti3C2Tx MXene as a binder-free bifunctional electrode for overall water splitting and supercapacitors

Synthesizing efficient electrode materials for water splitting and supercapacitors is essential for developing clean electrochemical energy conversion/storage devices. In the present work, we report the construction of a ruthenium cobalt oxide (RuCo₂O₄)/Ti₃C₂T_x MXene hybrid by electrophoretic deposition of Ti₃C₂T_x MXene on nickel foam (NF) followed by RuCo₂O₄ nanostructure growth through an electrodeposition process. Owing to the strong interactions between RuCo₂O₄ and Ti₃C₂T_x sheets, which are verified by density functional theory (DFT)-based simulations, RuCo₂O₄/Ti₃C₂T_x MXene@NF can serve as a bifunctional electrode for both water splitting and supercapacitor applications. This electrode exhibits outstanding electrocatalytic activity with low overpotentials of 170 and 68 mV at 100 A m^-2 toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The RuCo₂O₄/Ti₃C₂T_x MXene@NF-based alkaline water-splitting cell only requires 1.62 V to achieve a current density of 100 A m^-2, which is much better than that of RuO₂@NF and Pt/C@NF-assembled cells (1.75 V@100 A m^-2). The symmetric supercapacitor (SSC)-assembled electrode displays a high specific capacitance of 229 F g^-1 at 3 A g^-1. The experimental results, complemented with theoretical insights, provide an effective strategy to prepare multifunctional materials for electrocatalysis and energy storage applications.

Fe and Cu dual-doped Ni3S4 nanoarrays with less low-valence Ni species for boosting water oxidation reaction

Transition metal materials with high efficiency and durable electrocatalytic water splitting activity have attracted widespread attention among scientists. In this work, two cation co-doped Ni₃S₄ nanoarrays grown on a Ni foam support were firstly synthesized through a typical two step hydrothermal process. Cu and Fe co-doping can regulate the internal electron configuration of the material, thus reducing the activation energy of the active species. Moreover, density functional theory calculations demonstrate that a low Ni²⁺ amount improves the adsorption energy of H₂O, which facilitates the formation and reaction of intermediate species in the water splitting process. The experimental results indicate that the Cu and Fe co-doped Ni₃S₄ material has superior electrochemical activity for water oxidation reaction to pure Ni₃S₄, Fe doped Ni₃S₄ and Cu doped Ni₃S₄. The Fe-Cu-Ni₃S₄ material displays a significantly enhanced electrocatalytic performance with low overpotentials of 230 mV at 50 mA cm^-2 and 260 mV at 100 mA cm^-2 for the oxygen evolution reaction under alkaline conditions. It's worth noting that when Fe-Cu-Ni₃S₄ was used as the anode and cathode, a small cell voltage of 1.59 V at 10 mA cm^-2 was obtained to achieve stable overall water splitting. Our work will afford a novel view and guidance for the preparation and application of efficient and environmentally friendly water splitting catalysts.

Constructing oxygen vacancy-enriched Fe2O3@NiO heterojunctions for highly efficient electrocatalytic alkaline water splitting

The oxygen vacancy-enriched Fe2O3@NiO heterojunctions assembled by nanoparticles and nanosheets can be used as a highly efficient and stable dual-function electrocatalyst to achieve efficient all-water splitting.

A doping element improving the properties of catalysis: in situ Raman spectroscopy insights into Mn-doped NiMn layered double hydroxide for the urea oxidation reaction

Potential dependent in situ Raman spectra confirm that Mn doping enables a negative shift in Ni(ii) oxidation onset potential.

Anchoring Co3S4 nanowires on NiCo2O4 nanosheet arrays as high-performance electrocatalyst for hydrogen and oxygen evolution

The development of catalysts which can substitute expensive metals to efficiently split water is currently a hot research topic. Here, multi-layered NF/NiCo2O4/Co3S4 nanocomposite was prepared on 3D porous nickel foam...

γ-FeO(OH) with Multi-surface Terminations Intrinsically Active for Electrocatalytic Oxygen Evolution Reaction

Due to poor conductivity, electrocatalytic performance of independently prepared iron oxy-hydroxide (FeO(OH)) is inferior whereas in-situ derived FeO(OH) from the iron based electro(pre)catalyst shows superior oxygen evolution reaction (OER). Use...

Controllable synthesis of Cu-Ni-M (M = S, P and Se) hybrid nanoarrays for efficient water splitting reaction

Electrochemical water splitting has become one of the state of the art approaches to generate hydrogen. It is important to exploit relatively low toxicity, low cost and environmentally friendly water splitting electrocatalysts. A series of Cu-Ni-M (M = S, P and Se) materials were firstly in situ grown on Ni foam and these materials showed excellent water splitting activity. The Cu-Ni-S material shows excellent oxygen evolution reaction performance (200 mV@20 mA cm-2) and the Cu-Ni-P sample shows an effective hydrogen evolution reaction performance (52 mV@10 mA cm-2). When the Cu-Ni-S and Cu-Ni-P materials were assembled into a two-electrode system, the Cu-Ni-S/NF//Cu-Ni-P/NF electrode pairs display superior water splitting activity (1.50 V@20 mA cm-2), which is one of the best electrocatalytic activities reported so far. The experimental analysis demonstrates that the excellent performance of the Cu-Ni-S/NF and Cu-Ni-P/NF materials is attributed to the rapid electron transfer rate, increased electrocatalytically active area, more exposure to active sites and the superior synergistic catalytic factor of Ni2+ and Cu2+. It was found that amorphous oxides were in situ generated on the outside surface of the catalyst through the analysis of the catalyst after the reaction, and they were the real electrocatalytically active centers. Density functional theory demonstrates that the in situ generated Cu-doped NiOOH shows the optimal water adsorption energy compared with NiOOH. This work offers novel views for the design of relatively low toxicity, stable and inexpensive water splitting electrocatalysts.

Formation of graphene encapsulated cobalt–iron phosphide nanoneedles as an attractive electrocatalyst for overall water splitting

Hierarchical CoFe–P nanoneedles encapsulated in the graphene texture are fabricated for water splitting applications.

Coupling Co2P/CoSe2 heterostructure nanoarrays for boosting overall water splitting

Exploiting environmentally friendly and robust electrocatalysts for overall water splitting is of utmost importance in order to alleviate the excessive global energy consumption and climate change. Herein, a simple phosphoselenization method was used to prepare Co2P and CoSe2 coupled nanosheet and nanoneedle composite materials on nickel foam (Co2P/CoSe2/NF). Density functional theory calculations showed that Co2P had a higher water adsorption energy compared with CoSe2, indicating that H2O molecules are strongly adsorbed on the active sites of Co2P, which speeds up the kinetic process of water splitting. The Co2P/CoSe2-300 material displayed superior electrocatalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline medium. It's worth noting that the Co2P/CoSe2-300 composite material nanoarrays merely needed an ultralow overpotential of 280 mV to drive a current intensity of 100 mA cm-2 for OER. In addition, when a two-electrode system was constructed for overall water splitting, the current intensity of 20 mA cm-2 could be reached while requiring an ultrasmall cell voltage of 1.52 V, which is one of the best catalytic activities reported up to now. Experimental and density functional theory calculations showed that the superior electrocatalytic performance of Co2P/CoSe2-300 could be attributed to its higher electron-transfer rate, higher water adsorption energy, and the synergistic effect of Co2P and CoSe2. Our work provides a novel approach for the one-step construction of composite materials as environmentally friendly and inexpensive water splitting catalysts.