Mo-incorporated three-dimensional hierarchical ternary nickel-cobalt-molybdenum layer double hydroxide for high-efficiency water splitting

Flowers-like 3D hierarchical ternary NiCoMo-layered double hydroxide (NiCoMo-LDH) spheres have been fabricated in substrate-free route via a one-pot hydrothermal method and utilized as efficient electrocatalysts for the OER and HER. The well-structured 3D hierarchical flowers were composed of numerous two-dimensional nanosheets, which inherently possess considerable electrochemical active sites, thereby enhancing catalytic activity. NiMo and CoMo binary LDHs, with similar morphology, were also prepared to illustrate the efficiency of the ternary LDH. The results indicate higher electrocatalytic activity for the ternary LDH as compared to binary LDHs under alkaline conditions. The NiCoMo-LDH required an overpotential as low as 202 and 93 mV to deliver a constant anodic and cathodic current density of 10 mA cm^?2 for the OER and HER, respectively. Furthermore, the NiCoMo-LDH exhibited remarkable HER activity, affording a low overpotential of 198 mV at a current density of ?100 mA cm^?2. Moreover, it could offer a stable current density of 10 mA cm^?2 for overall water splitting at 1.62 V in 1 M KOH with long-term stability for 20 h. The double-layer capacitance (C_dl) value indicated that the NiCoMo-LDH significantly influenced interface conductivity and the electrochemical active surface area. The ternary NiCoMo-LDH electrode yielded low Tafel slope values of 54 and 51 mVdec^?1 for the OER and HER. Owing to the efficient incorporation of Ni, Co, and Mo in a layered structure, synergetic effect, and high electrochemical surface area, the NiCoMo-LDH exhibited remarkable electrocatalytic activity. Such eco-friendly ternary LDHs can be used in rechargeable metal–air batteries for industrial applications.     

Promotional effects of trace Ni on its dual-functional electrocatalysis of Co/N-doped carbon nanotube catalysts for ORR and OER

It is still a great challenge for developing efficient dual-functional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalysts are critical to enhance the efficiency of metal-air cells and fuel cells. In this study, a one-pot vapor deposition method was used to realize the synchronously dope of N and Ni (trace) into Co/C to form Co–Ni (trace)/N-doped carbon nanotubes (Co–Ni (trace)/NCNTs). An interesting result is that injecting dicyandiamide (DCD) into Ni foam as a precursor led to the in situ formation of NCNTs, with synchronous doping of trace Ni into Co species. The cooperative effects of the Co–Ni (trace) and N-doped carbon nanotubes resulted in superior dual-functional electrocatalytic performance of Co–Ni (trace)/NCNTs for the ORR (half-wave potential E_1/2 vs. RHE: 0.83 V, electron transfer number n: 3.97) and OER (overpotential vs. RHE: 337 mV at 10 mA cm^?2, Tafel slope: 94.0 mV dec^?1). Moreover, the Co–Ni (trace)/NCNTs catalyst showed excellent stability during 20,000 s of durability testing for both ORR and OER. This study provides a feasible strategy for designing efficient nonnoble metal-catalysts for renewable energy conversion devices.     

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.     

Bifunctional nanoporous ruthenium-nickel alloy nanowire electrocatalysts towards oxygen/hydrogen evolution reaction

A class of ruthenium-nickel alloy catalysts featured with nanoporous nanowires (NPNWs) were synthesized by a strategy combining rapid solidification with two-step dealloying. RuNi NPNWs exhibit excellent electrocatalytic activity and stability for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in which the RuNi-2500 NPNWs catalyst shows an OER overpotential of 327 mV to deliver a current density of 10 mA cm^?2 and the RuNi-0 NPNWs catalyst requires the overpotential of 69 mV at 10 mA cm^?2 showing the best HER activity in alkaline media. Moreover, the RuNi-1500 NPNWs catalyst was used as the bifunctional electrocatalyst in a two-electrode alkaline electrolyzer for water splitting, which exhibits a low cell voltage of 1.553 V and a long-term stability of 24 h at 10 mA cm^?2, demonstrating that the RuNi NPNWs catalysts can be considered as promising bifunctional alkaline electrocatalysts.     

Defective layered double hydroxide formed by H2O2 treatment act as highly efficient electrocatalytic for oxygen evolution reaction

Electrocatalytic reaction is the important electrode reaction for many new generation electrochemistry energy and storage devices. However, the poor reaction kinetics of those electrode reaction severely restricts its application. Highly efficient electrocatalyst is essential to resolve the problem of commercial application of those electrochemistry energy and storage devices. Herein, by simple H₂O₂ treatment, the highly efficient CoFe-Layered Double Hydroxides (LDHs) electrocatalysts with multiple defects have been synthesized (noted as D-LDHs). The D-LDHs show a low overpotential of 283 mV at 10 mA cm⁻² and small Tafel slope of 39 mV dec⁻¹ for the oxygen evolution reaction (OER). The work offers a new strategy to create defects in LDHs as highly efficient electrocatalysts for OER.     

Electrochemically engineering defect-rich nickel-iron layered double hydroxides as a whole water splitting electrocatalyst

It is well proved that fabricating more defects on basal plane of layered double hydroxides (LDHs) is one of effective ways to boost the electrocatalytic performance for oxygen evolution reaction (OER). For the first time, the nickel iron LDHs (NiFe LDHs) with hierarchical morphology and abundant defects are simultaneously constructed by one-step electrodeposition (ED) strategy with easy operation, time-saving and green chemistry. Remarkably, the morphology is elaborately tailored by changing the species of doped anions which is unique. Also, the X-ray photoelectron spectroscopy (XPS) results elucidate that the Fe sites are in electron-rich state in LDHs which is revealed to enhance the catalytic activity strongly arising from the generation of oxygen vacancy. To deliver the current density of 10 mA cm⁻², the optimal NiFe LDHs require the overpotential of 128, 106 mV for OER and hydrogen evolution reaction (HER), and achieve 100 mA cm⁻² at the overpotential of 237, 242 mV, respectively. As a bifunctional electrocatalyst, the NiFe LDHs exhibit the current density of 10 mA cm⁻² at a cell voltage of 1.55 V and 100 mA cm⁻² at 1.76 V, which are lower than that of most of benchmarking materials reported previously.     

An efficient Fe2O3/FeS heterostructures water oxidation catalyst

Slow kinetics and insufficient understanding of the perceptual design of oxygen evolution reaction (OER) electrocatalysts are major obstacles. Overcoming these challenges, heterostructures have recently attracted attention because they encourage alternative OER electrocatalysts with active structural features. In this study, synthesis, characterization and electrochemical evaluation of the heterostructure of iron oxide/iron sulfide (Fe₂O₃/FeS) and its counterparts, iron oxide (Fe₂O₃) and iron sulfide (FeS) are reported. The structural features of as-synthesized electrocatalysts have been evaluated by infrared spectroscopy, powder X-ray diffraction study and scanning electron microscopy. Fe₂O₃/FeS was found to be a stable electrocatalyst for efficient water splitting, which initiates OER at a surprisingly low potential of 1.49 V (vs RHE) in 1 M potassium hydroxide. The Fe₂O₃/FeS electrocatalyst drives OER with a current density of 40 mA cm^?2 at overpotential of 370 mV and a Tafel slope of 90 mV dec^?1. Its performance is better than its counterparts (Fe₂O₃ and FeS) under similar electrochemical conditions. At an applied potential of 1.65 V (vs RHE), continuous oxygen production for several hours revealed the long-term stability and effective activity of the Fe₂O₃/FeS electrocatalyst for OER. The as-developed Fe₂O₃/FeS heterostructure provides an effective alternative low-cost metal-based electrocatalysts for OER.     

Ni/NiO@rGO as an efficient bifunctional electrocatalyst for enhanced overall water splitting reactions

Herein, we fabricated bifunctional, noble metal-free, highly efficient nickel/nickel oxide on reduced graphene oxide (Ni/NiO@rGO) by chemical synthesis approach for electrochemical water splitting reaction. Its structural and morphological characterization using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), energy dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD) represents, Ni/NiO@rGO is having Ni/NiO NPs ∼10 nm (±2 nm) on graphene oxide with face-centered cubic (FCC) crystal structure. Moreover, the presence of Ni/NiO (2.26%), O (6.56%), N (0.74%) and C (90.44%) from EDAX analysis further confirms the formation of Ni/NiO@rGO and it also supported by FTIR studies. This nanocatalyst is examined further for electrocatalytic water splitting reactions (HER and OER). It demonstrated low overpotential 582 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 63 mV dec⁻¹ obtained in 0.5 M H₂SO₄ towards HER. Also, at the other end at onset potential of 1.6 V vs. RHE towards OER. It demonstrated low overpotential 480 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 41 mV dec⁻¹ in 0.5 M KOH towards OER observed. Hydrogen fuel is eco-friendly to the environment and noteworthy performance of earth-saving reactions.     

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.     

Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition

NiFe layered double hydroxides (LDHs) is considered to be one of the LDHs electrocatalyst materials with the best electrocatalytic oxygen evolution properties. However, its poor conductivity and inherently poor electrocatalytic activity are considered to be the limiting factors inhibiting the electrocatalytic properties for oxygen evolution reaction (OER). The amorphous NiFeAl-LDHs electrocatalysts were prepared by electrodeposition with nickel foam as the support, and the D-NiFeAl-LDHs electrocatalyst with defect sites was then obtained by alkali etching. The mechanism of catalysts with defect sites in OER was analyzed. The ingenious defects can selectively accelerate the adsorption of OH⁻, thus enhancing the electrochemical activity. The D-NiFeAl-LDHs electrocatalyst had higher OER electrocatalytic activity than NiFe-LDHs electrocatalyst: its accelerated OER kinetics were mainly due to the introduction of iron and nickel defects in NiFeAl-LDHs nanosheets, which effectively adjusted the surface electronic structure and improved OER electrocatalytic performance. There was only a low overpotential of 262 mV with the current density of 10 mA cm⁻², and the Tafel slope was as low as 41.67 mV dec⁻¹. The OER electrocatalytic performance of D-NiFeAl-LDHs was even better than those of most of the reported NiFe-LDHs electrocatalysts.     

S/Se dual-doping promotes the formation of active Ni/Fe oxyhydroxide for oxygen evolution reaction of (sea)water splitting

Surface reconstruction produces metal oxyhydroxide (1OOH) active sites, and promoting surface reconstruction is essential for the design of OER electrocatalysts. In this paper, we reported that a large amount of active NiFeOOH was generated in-situ on the surface of nickel-iron sulfide selenide, thus exposing more active sites and efficiently catalyzing OER. In 1 M KOH solution, NiFeOOH(S,Se) achieves an ultra-low overpotential of 195 mV at the current density of 10 mA cm^?2, and the Tafel slope is only 31.99 mV dec^?1, showing excellent catalytic performance. When the current density is 100  mA cm^?2, the over-potential of NiFeOOH(S,Se) in KOH + seawater solution is 239 mV, which is almost equivalent to 231 mV in KOH solution. The excellent OER stability of the NiFeOOH(S,Se) catalyst in alkaline electrolytes was confirmed, and the overpotential did not change significantly after 4 days of testing in KOH + seawater solution.     

Green synthesis of CuCo2O4–CuO composite nanoparticles grown on nickel foam for high-performance oxygen evolution reaction

Among the most critical components for water-derived hydrogen fuel, the study of electrocatalysts as anode for the oxygen evolution reaction (OER) is an important topic in the water splitting process. In this work, the CuCo₂O₄–CuO composite electrode is synthesized directly on nickel foam to evaluate its potential as OER catalyst in an alkaline medium. An interesting strategy is to use flaxseed oil as a polymerizing agent in a sol-gel process combined with hydrothermal. A low overpotential value of 289 mV vs RHE at 10 mA cm⁻² current density and a Tafel slope of 71 mV dec⁻¹ were observed, which represents an electrocatalytic performance superior to other Cu and Co based oxides in the literature. Short-term stability testing of 15 h demonstrates stable overpotential with no observable secondary phase after measurements. The successful outcome of this work is a result of the promising green chemical approach herein used to obtain electrocatalysts grown directly on conductive substrates for water oxidation.     

Facile electrodeposited amorphous Co–Mo–Fe electrocatalysts for oxygen evolution reaction

A new type of superior activity and highly cost-effective amorphous electrocatalyst Co–Mo–Fe on nickel foam (NF) supports is prepared by facile one-step rapid electrodeposition. The amorphous electrocatalyst Co–Mo–Fe/NF shows excellent oxygen evolution reaction (OER) performance, with a small overpotential of 218 mV at 10 mA cm^?2 current density in 1 M KOH. It only needs overpotential of 252 mV at 50 mA cm^?2 current density in 1 M KOH, and the Tafel slope is 45 mV dec^?1. The results show that the doping of Fe significantly improves the oxygen evolution capacity of the Co–Mo–Fe system. The synergistic effect of the three metals and the doping of the third metal iron make the oxygen evolution active sites of the whole system increase significantly. This provides a feasible direction for the oxygen evolution reaction of cobalt transition metal.     

Highly efficient catalytic CoS1.097 embedded in biomass nanosheets for oxygen evolution reaction

At present, the preparation of highly active, long-term durable, and low cost electrocatalysts toward oxygen evolution reaction is still a challenge for energy conversion and storage. This work reports CoS_1.097 embedded in flower-like biomass derived from red blood cells (RBCs) nanocatalyst, was synthesized via a one-step, hydrothermal method. Compared to pure CoS_1.097 microspheres, the generated flower-like CoS_1.097-biomass (CoS_1.097-B) nanosheets exhibits enhanced OER electrocatalytic activity with a onset potential of 1.43 V (vs. RHE), an overpotential of 260 mV at the current density of 10 mA cm⁻², and a low Tafel slope of 83 mV dec⁻¹. Moreover, the CoS_1.097-B displays good OER stability with little decrease after 400 cycles. Our work provides a reference for designing catalysts with high activity and good stability using biomass under mild conditions.     

Cu2S/Ni3S2 ultrathin nanosheets on Ni foam as a highly efficient electrocatalyst for oxygen evolution reaction

It is an inevitable choice to find efficient and economically-friendly electrocatalysts to reduce the high overpotential of oxygen evolution reaction (OER), which is the key to improve the energy conversion efficiency of water splitting. Herein, we synthesized Cu₂S/Ni₃S₂ catalysts on nickel foam (NF) with different molar ratios of Ni/Cu by a simple two-step hydrothermal method. Cu₂S/Ni₃S₂-0.5@NF (CS/NS-0.5@NF) effectively reduces the overpotential of OER, displaying small overpotentials (237 mV@100 mA cm^?2 and 280 mV@500 mA cm^?2) in an alkaline solution, along with a low Tafel slope of 44 mV dec^?1. CS/NS-0.5@NF also presents an excellent durability at a relatively high current density of 100 mA cm^?2 for 100 h. The excellent performance is benefited by the prominent structural advantages and desirable compositions. The nanosheet has a high electrochemical active surface area and the porous structure is conducive to electrolyte penetration and product release. This work provides an economically-friendly Cu-based sulfide catalyst for effective electrosynthesis of OER.     

Electroless deposition synthesis of composite catalysts Ni-Fe-P-WO3/NF with superior oxygen evolution performance

The proper construction of high efficiency, low-cost, earth-abundant oxygen evolution reaction (OER) catalyst is essential for hydrogen formation by water splitting. A novel electrocatalyst with highly active OER performance was manufactured by a simple electroless deposition method of Ni-Fe-P-WO₃ on nickel foam (NF). Benefiting from outstanding mass transfer capability of Ni-Fe-P-WO₃/NF heterogeneous structure, abundance of active sites in the amorphous architecture and etc., the Ni-Fe-P-WO₃/NF shows extremely superb electrocatalytic properties compare to noble metal catalyst IrO₂/NF for OER, which needs an overpotential of only 218 mV in 1.0 M KOH solution to achieve the current density of 10 mA cm^?2. It also has remarkable OER activity at high current density that only needs 298 mV to attain 100 mA cm^?2 current density. Moreover, the Ni-Fe-P-WO₃/NF has low Tafel slope of 42 mV dec^?1. This study offers a novel approach to the production of OER multiphase electrocatalysts from oxides and alloys.     

Ni–Zn nanosheet anchored on rGO as bifunctional electrocatalyst for efficient alkaline water-to-hydrogen conversion via hydrazine electrolysis

Bifunctional Ni-15 at.% Zn/rGO catalyst was fabricated by a two-step electrodeposition to be used for efficient alkaline water-to-hydrogen conversion via hydrazine electrolysis. Experiments show that the as-deposited Ni-15 at.% Zn/rGO nanosheet arrays with porous structure possesses excellent catalytic activity and stability towards both hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR). A small overpotential of 49 mV at 10 mA cm⁻² with a low Tafel slope of 26.3 mV dec⁻¹, and a retention rate of 91.4% after 12 h at 10 mA cm⁻² are observed for Ni-15 at.% Zn/rGO towards HER. Moreover, Ni-15 at.% Zn/rGO also shows an extra-high current density of 1097 mA cm⁻² at 0.6 V vs RHE with a low Tafel slope of 33.5 mV dec⁻¹, and a high durability of 90.5% after 5000 s towards HzOR. Moreover, two-electrode cell was constructed using Ni-15 at.% Zn/rGO as both cathode and anode for HER and HzOR, achieving 100 mA cm⁻² at an ultralow cell voltage of 0.418 V. The above outstanding bifunctional catalytic performance should be attributed to its large ECSA, high electrical conductivity and most importantly, its superaerophobic surface induced by the porous structure with nanosheet arrays.     

N-doped graphene quantum dots incorporated cobalt ferrite/graphitic carbon nitride ternary composite for electrochemical overall water splitting

Multicomponent electrocatalysts containing carbon supports play a crucial role in influencing the hydrogen and oxygen evolution reactions which enhance the total water splitting. Herein, we report a ternary composite with cobalt ferrite, graphitic carbon nitride, and N-doped graphene quantum dots prepared via hydrothermal technique. The purity of the samples is established by carrying out various characterization methods. The intrinsic characteristics of the obtained materials are investigated by employing electrocatalytic processes in an alkaline media toward hydrogen and oxygen evolution reactions. Cobalt ferrite/graphitic carbon nitride/N doped graphene quantum dots electrocatalyst demonstrates a very low overpotential towards hydrogen evolution reaction of 287 mV at a constant 10 mA cm^?2 current density in 1.0 M KOH. Tafel slope and R_ct values generated are 94 mV dec^?1 and 0.86 cm², respectively. Oxygen evolution reaction studies reveal an overpotential of 445 mV at 10 mA cm^?2 with a Tafel slope of 69 mV dec^?1. Finally, the cell potential needed for the cobalt ferrite/graphitic carbon nitride/N doped graphene quantum dots electrode to achieve 10 mA cm^?2 in total water splitting is only 2.0 V while displaying long-term stability.     

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni₇Fe₃ attains 100 mA cm⁻² at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec⁻¹ for OER, and porous Ni₉Fe₁ exhibits a low overpotential of 347 mV at 100 mA cm⁻² with a Tafel slope of 121.0 mV dec⁻¹ for HER. The Ni₉Fe₁//Ni₉Fe₁ requires a low cell voltage of 1.69 V to deliver 10 mA cm⁻² current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.     

Electrodeposited Ni-Co-Sn alloy as a highly efficient electrocatalyst for water splitting

Water splitting to produce hydrogen and oxygen is considered as a feasible solution to solve the current energy crisis. It is highly desirable to develop inexpensive and efficient electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this paper, nanostructured Ni-Co-Sn alloys were electrodeposited on copper foil and the excellent electrocatalytic performances for both HER and OER in alkaline media were achieved. The optimized Ni-Co-Sn electrode shows a low onset overpotential of −18 mV and a small Tafel slope of 63 mV/dec for the HER, comparable to many state-of-the-art non-precious metal HER catalysts. For the OER, it produces an overpotential of 270 mV (1.50 V vs. RHE) at current density of 10 mA/cm², which is better than that of the commercial Ir/C catalyst. In addition to high electrocatalytic activities, it exhibits good stability for both HER and OER. This is the first report that Ni-Co-Sn is served as a cost-effective and highly efficient bifunctional catalyst for water splitting and it will be of great practical value.