Engineering FeNi-based electrocatalysts conjoined with Mo2C grown on carbon spheres toward efficient water oxidation via structure and electronic modulation

The development of active non-noble electrocatalysts for oxygen evolution reaction (OER) is urgently desired to accomplish high-performance electrocatalytic water splitting. Here, we report a unique structure electrocatalyst composed of FeNi and Mo₂C heterojunction as spherical shell supported on carbon sphere as core for efficient OER. With the aid of Mo₂C incorporation, FeNi–Mo₂C@C shows increased specific surface area, more electrocatalytic active sites, improved surface water adsorption, and reduced energy barrier. Benefiting from the synergy of shell-core and heterojunction sturcture, the optimized FeNi–Mo₂C@C exhibits superior activity for OER with an overpotential of 283 mV at 10 mA cm⁻² as well as Tafel slope of 29.2 mV dec⁻¹, which is comparable to that of the benchmark ruthenium oxide. The feasible bond between structural design and electronic alteration enhances the charge transfer efficiency, conductivity, and catalytic kinetics, thus intrinsically boost the electrocatalytic performance. This study hence supports a viable strategy to develop highly-efficient non-precious OER electrocatalysts through structural and electronic modification.     

In situ construction of heterostructure NiSe–NiO nanoarrays with rich oxygen vacancy on MXene for efficient oxygen evolution

Developing high-efficiency, non-noble, earth-available electrocatalysts for the oxygen evolution reaction (OER) is vital for electrochemical energy conversion, but it is still challenging. Herein, we ingeniously designed a partial selenization method to construct NiSe–NiO heterostructure grown in situ on Ta₄C₃T_x MXene (denoted as NiSe–NiO/Ta₄C₃T_x MXene). NiSe–NiO/Ta₄C₃T_x MXene's plethora of heterointerfaces provides a wealth of active sites, fast charge and mass transfer, and favorable adsorption energies for OER intermediates, all of which contribute synergistically to the oxidation of alkaline water. As expected, taking advantage of the strong chemical and electron synergistic effects of NiSe and NiO, the synthesized NiSe–NiO/Ta₄C₃T_x MXene exhibits excellent activity for OER with a low overpotential of 255 mV at 10 mA cm⁻², a small Tafel slope of 47.4 mV dec⁻¹, as well as excellent long-term stability, exceeding that of its competitors. This study offers a novel synthetic route toward developing high-performance OER electrocatalysts for renewable energy conversion/storage systems and beyond by optimizing the catalysts' composition and architecture.     

Amorphous FeOOH decorated hierarchy capillary-liked CoAl LDH catalysts for efficient oxygen evolution reaction

Electrocatalytic water splitting is a promising route for the generation of clean hydrogen. However, the anodic oxygen evolution reaction (OER) suffers greatly from low reaction kinetics and thereby hampers the energy efficiency of alkaline water electrolysers. In recent years, tremendous efforts have been dedicated to the pursuit of highly efficient, low cost and stable electrocatalysts for oxygen evolution reaction. Herein, an amorphous FeOOH roughened capillary-liked CoAl layered double hydroxide (LDH) catalyst grown on nickel foam (denoted as FeOOH–CoAl LDH/NF) was reported for OER electrolysis. The developed FeOOH–CoAl LDH/NF electrode shows excellent OER activity with overpotentials of 228 mV and 250 mV to deliver a current density of 50 mA cm^?2 and 100 mA cm^?2 in 1.0 M KOH solution, respectively, ranking it one of the most promising OER catalysts based on transition-metal-based LDH. This is owed to the formed capillary-liked hierarchy structure with high-porosity as well as the strong electronic interaction between FeOOH and CoAl LDH. The developed morphological engineering approach to build hierarchal porous structures together with facile amorphous FeOOH modification may be extended to other layered double hydroxide catalyst for enhanced OER activities.     

One-step hydrothermal synthesized 3D P–MoO3/FeCo LDH heterostructure electrocatalysts on Ni foam for high-efficiency oxygen evolution electrocatalysis

Low-cost yet high-efficiency oxygen evolution reaction (OER) catalysts have attracted ardent attention to speed up the development of water electrolysis. Recent researches have shown that layered double hydroxides (LDH) are promising candidates towards OER, but further improvement is still highly demanded for its large-scale practical application in water splitting. Herein, we report a 3D P-doped MoO₃/FeCo LDH/NF (P–MoO₃/FeCo LDH/NF) ultrathin nanosheet heterostructure electrocatalyst with an extremely low overpotentials of 225 mV for delivering a current density of 10 mA cm^?2 for OER and a great durability for at least 80 h by a simple one-step hydrothermal method. Extraordinarily, the P–MoO₃/FeCo LDH catalyst achieves a high current density of 300 mA cm^?2 and even 350 mA cm^?2 at an extremely low overpotential of 297 mV and 302 mV, respectively, which is crucial for the water electrolysis industry. The remarkable performance may be attributed to that the heterostructure between P–MoO₃ and FeCo LDH not only optimizes electronic structure, thus inducing electron transfer from P–MoO₃ to FeCo LDH and then realizing fast electron transfer rates, but also produces more catalytic active sites. Moreover, the synergetic effect between MoO₃ and FeCo LDH also plays an essential role for enhancing the catalytic performances. This work explores the effect of phosphomolybdic acid on the structure, composition and performances of FeCo LDH catalysts, and also provides a simple and cost-effective way to prepare high-efficiency and low-cost layered double hydroxide electrocatalysts for OER.     

Role of sputtered WO3 underlayer and NiFeCr-LDH co-catalyst in WO3–BiVO4 heterojunction for enhanced photoelectrochemical water oxidation

WO₃–BiVO₄ (WO-60s/BVO) heterojunction was synthesized by radio-frequency sputtered WO₃ onto FTO substrate, followed by spin-coating of BiVO₄ layer. Furthermore, Cr incorporated NiFe-LDH (NiFeCr-LDH) oxygen evolution reaction (OER) co-catalyst was electrodeposited onto WO-60s/BVO. The sputtered WO₃ underlayer in the WO-60s/BVO facilitated enhanced electron-hole separation and less transient time for the electron to arrive at back contact than conventional spin-coated WO₃ layers. Incorporating Cr into NiFe-LDH increased the electrical conductivity of LDH, which resulted in an enhanced transfer of photogenerated charge-carrier and significant promotion of the OER kinetics. The heterojunction with sputtered WO₃ underlayer and NiFeCr-LDH co-catalyst attained photocurrent density of 4.9 mA cm⁻² at 1.23 V vs. RHE with an IPCE value greater than 56% in the 350–470 nm wavelength range. Moreover, the WO-60s/BVO-NiFeCr photoanode showed only 7% decay in photocurrent after 6 h with H_2, and O₂ evolution of 98 and 47 μmol cm⁻² h, respectively, suggesting high stability for OER.     

Promotion of MXene (Ti3C2Tx) as a robust electrocatalyst for oxygen evolution reaction via active sites of ZIF-67 - In situ mechanism investigations

Developing a robust and highly efficient electrocatalyst for the oxygen evolution reaction (OER) is required, due to its sluggish mechanism and high overpotential. Here, we present the Ti₃C₂T_x and ZIF-67 composites (MXene/ZIF-67) for an efficient OER. The MXene/ZIF-67_1:10 exhibits an overpotential of 366 mV, which is better than MXene (548 mV) and RuO₂ (395 mV). Moreover, it shows improved electrochemical stability at 20 mA·cm^{− 2} with only a 0.2% increase in potential after 5 h, while RuO₂ with an overpotential loss of 3.5%. After an increase in the current density of 10–20 mA cm⁻² the potential increase by 7.4%, while in RuO₂ by 25.3%. To reveal the observed performance, the composite's robustness and explain each component's role. Moreover, a detailed OER mechanism is discussed. It was realized via in situ Raman microscopy and XRD which allowed the revealing of the active species responsible for boosting OER.     

Facile synthesis of molybdenum-based layered double hydroxide nanorods for boosting water oxidation reaction

Exploiting environmentally friendly and robust oxygen evolution reaction (OER) electrodes is still a great difficulty to promote the water oxidation for electrocatalytic water splitting. In the work, molybdenum-doped nickel copper hydrotalcite (NiCuMo_x LDH) nanoarrays have been firstly in situ grown on nickel foam (NF) via a typical hydrothermal process. When NiCuMo_0.2 LDH/NF was used as a OER electrocatalyst, it displays superior electrocatalytic performance with the need of small overpotentials of only 290 mV to drive 40 mA cm⁻² and a low Tafel slope of 39.8 mV dec⁻¹, which are almost one of the best water oxidation activities reported so far. The enhanced electrocatalytic performance is attributed to unique urchin-like structure, more exposure to active sites and enhanced charge transfer rate owing to the synergistic effect of Mo doping and NiCu LDH. The work put forward a new method for the development of efficient water oxidation electrocatalysts, which will fill the gap for the exploitation of trimetal LDH-based electrodes in large-scale water splitting applications in the future.     

Vertical Nickel–Iron layered double hydroxide nanosheets grown on hills-like nickel framework for efficient water oxidation and splitting

Herein, the vertical thin nickel–iron layered double hydroxide nanosheets grown on the hills-like nickel framework (NiFe LDH/Ni@NF) are employed for the oxygen evolution reaction (OER), securing at the low overpotentials of 197 and 270 mV to obtain the current densities of 20 and 100 mA cm⁻², respectively, with a Tafel slope of 73.34 mV dec⁻¹. The electrodeposited nickel film induces the NiFe LDH nanosheets grow vertically and thinly. As well, the nickel abundant interfaces and inner space makes this catalyst effective for OER. It was further served as the OER electrode in a water splitting system coupled the Pt/C cathode, and a cell voltage was at 1.52 and 1.67 V to achieve the current density of 10 mA cm⁻² and 50 mA cm⁻². In addition, the water electrolyzer can suffer a long time of 24 h at 50 mA cm⁻², showing the feasibility in a practical unbiased alkaline water splitting system.     

NiFe layered double hydroxide nanosheet arrays for efficient oxygen evolution reaction in alkaline media

Exploring efficient oxygen evolution reaction (OER) catalysts synthesized from low-cost and earth-abundant elements are crucial to the progression of water splitting. In this paper, NiFe layered double hydroxide (LDH) nanosheets were grown on Ni foam (NF) through a straightforward hydrothermal method. The Fe doping effects were systematically investigated by controlling Ni/Fe ratios and Fe valence states, and the in-depth influence mechanisms were discussed. The results indicate that, through controlling structure morphology and enhancing Ni²⁺ oxidation, NiFe_III(1:1)-LDH displays the best and outstanding OER performance, with a low over potential of 382 mV at 50 mA cm^?2, a low Tafel slope of 31.1 mVdec^?1 and only 20 mV increase after 10 h continuous test at 50 mA cm^?2. To our knowledge, this is one of the best OER electrocatalysts in alkaline media to date. This work provides a facile and novel strategy for the fabrication of bimetallic LDH catalysts with desired structures and compositions.     

Core-shell nanoarray structured La doped Cu(OH)2@NiCo layered double hydroxide for oxygen evolution reaction

Efficient hydrogen production via water splitting is significant because of the zero-carbon emission property. Developing low-cost and highly efficient electrocatalysts for the oxygen evolution reaction (OER), a key half-reaction of water splitting, is critical. Herein, we designed Cu(OH)₂@NiCo layered double hydroxide core-shell nanoarray supported on copper foam (CF) with different La doping amount (abbreviated as Cu(OH)₂@NiCoLa LDH/CF) via the facile electrodeposition method. Owing to the synergistic effect between La and NiCo LDH by electronic structure tuning, Cu(OH)₂@NiCoLa LDH/CF shows excellent OER performance with the lowest overpotential of 254 mV to drive the current density of 10 mA cm^?2 and outstanding long-term durability for 24 h. The idea of doping rare-earth metal into non-noble NiCo-based LDHs core-shell nanoarray structure in this work can inspire the design of other efficient electrocatalysts.     

A highly efficient A-site deficient perovskite interlaced within two dimensional MXene nanosheets as an active electrocatalyst for hydrogen production

We report a unique composite of La_0.4Sr_0.4Ti_0.9Ni_0.1O_3-δ (LSTN) nanoparticles interlaced with two dimensional Ti₃C₂T_x (MXene) nanosheets, providing high conductivity. LSTN heterostructure synthesized by the sol-gel method produces a large oxygen vacancy and creates a variable valence state while, MXene synthesized from Hydrofluoric acid (HF) treatment resulted in a highly hydrophilic and conductive surface, thereby enhancing the charge transferability. For OER, the LSTN/MXene 66.67% electrode exhibits a benchmark of 10 mA cm^?2 at a potential of 1.56 (V vs RHE) in 1 M KOH. It has exhibited the lowest Tafel slope of 44 mV dec^?1 and highest mass activity (60 mA g^?1 @ 1.59 V) due to quicker ions diffusion and increased available exposed area. Moreover, the efficient LSTN/MXene 66.67% electrode showed good long-term durability during a 24 h stability test at a current density of 100 mA cm^?2. The strong interfacial interaction and high charge transfer among LSTN nanoparticles and 2D MXene nanosheets not only provide good structural strength to the composite but also improves the redox activity of LSTN/MXene 66.67% catalyst towards OER. This work provides improved conductive properties of perovskite by developing a composite of perovskite and MXene, that has significantly enhanced electrochemical properties of the catalyst by undergoing fast kinetics.     

Metal-organic framework derived iron-nickel sulfide nanorods for oxygen evolution reaction

Highly efficient oxygen evolution reaction (OER) on noble metal-free catalysts is a major challenge for green hydrogen production. We report herein a rational preparation strategy for MOF-derived chalcogenide electrocatalysts. The optimal sulfuration time is 12 h under the conditions of the theoretical Fe/Ni ratio of 1:1 and treatment temperature at 120 °C. In this case, the pyrite Fe_0.75Ni_0.25S₂ nanorods combining with amorphous FeNiOOH formed in situ exhibit a low overpotential of 247 mV with a small Tafel slope of 47.6 mV dec^?1 at a current density of 10 mA cm^?2 in alkaline media along with high electrochemical stability for OER. The enhanced performance is derived from the synergistic effect between FeNi sulfide with favorable electrical conductivity and generated (oxy)hydroxides with high intrinsic activity. More importantly, the more active sites and appropriate mesoporous structure further facilitate electrocatalytic activity due to improved mass transfer. This facile synthesis method is a potential pathway for MOF derived highly efficient electrocatalysis for sustainable hydrogen product.     

Top-down preparation of Ni–Pd–P@graphitic carbon core-shell nanostructure as a non-Pt catalyst for enhanced electrocatalytic reactions

Electrochemical reactions such as the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and methanol oxidation reaction (MOR) are essential for energy conversion applications such as water electrolysis and fuel cells. Furthermore, Pt or Ir-related materials have been extensively utilized as electrocatalysts for the OER, ORR, and MOR. To reduce the utilization of precious metals, innovative catalyst structures should be proposed. Herein, we report a bi-metallic phosphide (Ni₂P and PdP₂) structure surrounded by graphitic carbon (Ni–Pd–P/C) with an enhanced electrochemical activity as compared to conventional electrocatalysts. Despite the low Pd content of 3 at%, Ni–Pd–P/C exhibits a low overpotential of 330 mV at 10 mA cm^?2 in the OER, high specific activity (2.82 mA cm^?2 at 0.8 V) for the ORR, and a high current density of 1.101 A mg^?1 for the MOR. The superior electrochemical performance of Ni–Pd–P/C may be attributed to the synergistic effect of the bi-metallic phosphide structure and core-shell structure formed by graphitic carbon.     

Development of NiO/Co3O4 nanohybrids catalyst with oxygen vacancy for oxygen evolution reaction enhancement in alkaline solution

The oxygen evolution reaction (OER) is a significant reaction in water splitting and energy conversion. However, high price and sluggish kinetics catalysts prevent commercial applications. Generally, noble metals (e.g., iridium and ruthenium), which are expensive and unstable, have been used as catalysts for OER because of their high electrocatalytic activity. In this study, we report a high-performance OER catalyst with oxygen vacancies comprising NiO/Co₃O₄ nanohybrids. For OER, the NiO/Co₃O₄ heterostructure show good electrocatalytic performance with a low overpotential of 330 mV. This is higher than those of NiO, Co₃O₄, and benchmark IrO₂ candidates at current density of 10 mA cm^?2. Furthermore, the NiO/Co₃O₄ nanohybrids show long-term electrochemical stability for 10 h. The present research results show that NiO/Co₃O₄ heterostructure is an excellent electrocatalyst for OER.     

High throughput preparation of Ni–Mo alloy thin films as efficient bifunctional electrocatalysts for water splitting

The synthesis of cost-effective and high-performance electrocatalysts for water splitting is the main challenge in electrochemical hydrogen production. In this study, we adopted a high throughput method to prepare bi-metallic catalysts for oxygen/hydrogen evolution reactions (OER/HER). A series of Ni–Mo alloy electrocatalysts with tunable compositions were prepared by a simple co-sputtering method. Due to the synergistic effect between Ni and Mo, the intrinsic electrocatalytic activity of the Ni–Mo alloy electrocatalysts is improved, resulting in excellent HER and OER performances. The Ni₉₀Mo₁₀ electrocatalyst shows the best HER performance, with an extremely low overpotential of 58 mV at 10 mA cm^?2, while the Ni₄₀Mo₆₀ electrocatalyst shows an overpotential of 258 mV at 10 mA cm^?2 in OER. More significantly, the assembled Ni₄₀Mo₆₀//Ni₉₀Mo₁₀ electrolyzer only needs a cell voltage of 1.57 V to reach 10 mA cm^?2 for overall water splitting.     

Spatially confined growth of ultrathin NiFe layered double hydroxide nanosheets within carbon nanofibers network for highly efficient water oxidation

The rational design of catalysts with low cost, high efficient and robust stability toward oxygen evolution reaction (OER) is greatly desired but remains a formidable challenge. In this work, a one-pot, spatially confined strategy was reported to fabricate ultrathin NiFe layered double hydroxide (NiFe-LDH) nanosheets interconnected by ultrafine, strong carbon nanofibers (CNFs) network. The as-fabricated NiFe-LDH/CNFs catalyst exhibits enhanced OER catalytic activity in terms of low overpotential of 230 mV to obtain an OER current density of 10 mA cm^?2 and very small Tafel slope of 34 mV dec^?1, outperforming pure NiFe-LDH nanosheets assembly, commercial RuO₂, and most non-noble metal catalysts ever reported. It also delivers an excellent structural and electrocatalytic stability upon the long-term OER operation at a large current of 30 mA cm^?2 for 40 h. Furthermore, the cell assembled by using NiFe-LDH/CNFs and commercial Pt/C as anode (+) and cathode (?) ((+)NiFe-LDH/CNFs||Pt/C(?)) only requires a potential of 1.50 V to deliver the water splitting current of 10 mA cm^?2, 130 mV lower than that of (+)RuO₂||Pt/C(?) couple, demonstrating great potential for applications in cost-efficient water splitting devices.     

Co–NiFe layered double hydroxide nanosheets as an efficient electrocatalyst for the electrochemical evolution of oxygen

The development of non-precious metal catalysts for the electrochemical oxygen evolution reaction (OER) is especially important for the water electrolysis process. Herein, a two-dimensional (2D) ultrathin hybrid Co–NiFe layered double hydroxide (LDH) is synthesized via a facile hydrothermal method. In 1.0 M KOH electrolyte, Co–NiFe LDH exhibits remarkable activities for OER. At the current density of 10 mA cm⁻², it only needs an overpotential of 278 mV, which is ca. 50 mV and 20 mV lower than those for NiFe LDH (328 mV) and RuO₂ catalysts (298 mV), respectively. In addition, Co–NiFe LDH also shows impressive long-term stability for OER. Besides the stable morphology and crystal structure, the potential is always kept at 1.50 V and shows almost no attenuation during the 20 h of durability test. Changes in the electronic structure of LDH due to introduction of Co ions, as well as the large specific surface area facilitate the mass/electron transfer and the oxygen bubbles release, and thus lead to the enhanced catalytic properties for OER. This work can be informative not only for understanding the role of physical and electronic structures on OER but also for designing high-performance non-precious metal OER electrocatalysts.     

A highly active oxygen evolution electrocatalyst: Ni-Fe-layered double hydroxide intercalated with the Molybdate and Vanadate anions

Efficient low-cost electrocatalysts in the oxygen evolution reaction (OER) are important components of renewable energy technologies, e.g. solar fuel synthesis and water splitting for powering fuel cells. A great deal of attention has been attracted toward LDHs due to their electrical power and they are potentially applied in the field of electrocatalysts. The present paper reports synthesis of the Ni-Fe-Molybdate and Ni-Fe-Vanadate layered double hydroxides (LDHs) using a simple co-precipitation method. Powder X-ray diffraction, Fourier transform infrared spectra, Transmission electron microscopy, and X-ray energy dispersive spectroscopy spectrum provide successful intercalation of the Vanadate and Molybdate anions. Compared to the bare glassy carbon electrode, in alkaline media, the as-obtained Ni-Fe-MoO₄-LDH represents superior electrocatalytic activity toward water oxidation with the overpotential of 491 mV at10 mA/cm² and a low Tafel slope of 23 mV/dec. Ni–Fe-MoO₄-LDH exhibits good OER activity, which is stated as low onset overpotential, small Tafel slope, and large exchange current density. The current density of the Ni–Fe-MoO₄-LDH nanosheets is about 10 mA cm⁻² at the overpotential of 0.491 (V vs SCE). This value is much larger than that of the Ni–Fe-NO₃-LDH and Ni–Fe-VO₃-LDH nanoparticles.     

Partially crystallized Ni–Fe oxyhydroxides promotes oxygen evolution

The slow oxygen evolution reaction (OER) kinetics influences hydrogen production efficiency from water splitting. To break through the bottleneck of water splitting, it is urgent to develop efficient and economic electrocatalysts. Although NiFe-based catalysts exhibit outstanding OER activity, the complicated preparation process limits their large-scale synthesis and applications. Here, partially crystallized nickel-iron oxyhydroxides are synthesized by a facile sol-gel method. When the Fe/Ni mole ratio is 0.5:1, the NiFe_0.5(OH)_x catalyst shows superior OER performance with a low OER overpotential of 265 mV and good durability. Kinetic studies show that the energy barrier of NiFe_0.5(OH)_x is only 31.5 kJ mol^?1, much smaller than those of Ni(OH)_x (41.0 kJ mol^?1) and Fe(OH)_x (44.8 kJ mol^?1). The synergistic action between Ni and Fe sites not only facilitates mass and charge transfer, but also promotes the formation of 1OOH intermediate for the OER.     

One-pot preparation of Ni3S2@3-D graphene free-standing electrode by simple Q-CVD method for efficient oxygen evolution reaction

Efficient non-noble metal catalysts for the oxygen evolution reaction (OER) are particularly important in the practical applications of electrocatalytic water splitting (ECWS). Herein, based on a simple quasi chemical vapor deposition (Q-CVD) method, we fabricate a newly Ni₃S₂@3-D graphene free-standing electrode for efficient OER applications. The Ni₃S₂@3-D graphene integrates the advantageous features of 3-D graphene and Ni₃S₂ towards OER, such as more interfacial catalytic sites, pore-rich structure, N-doped structure and good electrical conductivity. Benefiting from the favorable features, the Ni₃S₂@3-D graphene (especially 900 °C sample) exhibits excellent OER performances in alkaline medium, which includes a low on-set potential (1.53 V), low overpotential of 305 mV at a current density of 10 mA cm⁻², and a smaller Tafel slope (50 mV dec⁻¹). This catalyst also shows ultrahigh stability after chronoamperometry response at 10 mA cm⁻² for 48 h with 30% increase in the current density. The present work opens a new approach for the one-pot construction of hybrid materials between metal sulfide and graphene to increase the electrocatalytic activity of non-noble metal OER catalysts.