In-situ self-reconstruction of Ni–Fe–Al hybrid phosphides nanosheet arrays enables efficient oxygen evolution in alkaline

Developing efficient oxygen evolution reaction (OER) electrocatalysts with earth-abundant elements is very important for sustainable H₂ generation via electrochemical water splitting. Here we design a crystalline-amorphous Ni–Fe–Al hybrid phosphides nanosheet arrays grown on NiFe foam for efficient OER application. Dynamic surface reorganization of phosphides at anodic/cathodic polarizations is probed by in situ Raman spectroscopy. The reconstructed amorphous Ni(Fe)OOH species are determined as the active phases that facilitate the OER process. This unique electrode shows highly catalytic activity toward water oxidation, achieving the current densities of 10 and 100 mA cm^?2 at 181 and 214 mV in 1 M KOH, respectively. Meanwhile, it also exhibits excellent stability at a large current density of 100 mA cm^?2 for over 60 h. This work reveals the dynamic structural transformation of pre-catalyst in realistic conditions and highlights the important role of oxyhydroxides as real reactive species in OER process with high activity.     

Fe doped metal organic framework (Ni)/carbon black nanosheet as highly active electrocatalyst for oxygen evolution reaction

To alleviate the sluggish oxygen evolution reaction (OER) kinetics, it's urgent to develop electrocatalysts with high activity and low cost. In this work, Fe doped metal organic frameworks (Ni)/carbon black composites were synthesized via a facile hydrothermal method. Benefiting from the direct use of metal organic frameworks (MOFs) for OER, numerous and highly dispersed active sites are exposed to the electrolyte and reactants. By regulating Ni/Fe ratios, a high electrochemical active surface area (ECSA) and high relative surface content of active Ni³⁺ species are obtained, which mainly contribute to the high OER activity. Besides, the introduced carbon black (CB) was found to enhance the charge-transfer efficiency of the electrocatalysts, which is also favorable for OER. The optimal Ni9Fe1-BDC-0.15CB electrocatalyst shows excellent OER activity with the low overpotential of ~290 mV at 10 mA cm⁻² and the Tafel slope of ~76.1 mV dec⁻¹, which is comparable to RuO₂ and other MOFs-based OER electrocatalysts reported in recent years.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

Electrodeposition of Ni–Fe micro/nano urchin-like structure as an efficient electrocatalyst for overall water splitting

The usage of active electrocatalysts is a useful approach to accelerate the kinetics of electrochemical reactions and to enhance the efficiency of water splitting. To fabricate active electrocatalysts, the creation of new structures that can be easily constructed has always been a research interest. Ni–Fe based alloys are generally known as active OER catalyst. However, in this study, a novel Ni–Fe micro/nano urchin-like structure is reported to be active for both HER and OER. This is the first report of the fabrication of this morphology by a fast, one-step, and affordable electrodeposition method as an efficient HER/OER electrocatalyst. The optimized Ni–Fe coating on Cu substrate demonstrated promising HER activity with low overpotentials of ?124 and ?243 mV at the current densities of ?10 and ?100 mA cm^?2, respectively. Moreover, the fabricated Ni–Fe urchin-like catalyst is highly active toward OER, requiring overpotentials of only 292 and 374 mV to deliver 10 and 100 mA cm^?2. The unique structure of the synthesized coating with an abundant number of micro/nano-scale cones is suggested to play a vital role in the superior HER/OER activity of the catalyst. This article introduces a cost-effective method for the fabrication of a novel urchin-like Ni–Fe alloy as a highly active bifunctional water splitting electrocatalyst.     

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.     

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.     

Growth of ultrathin nanosheets of nickel iron layered double hydroxide for the oxygen evolution reaction

Because of low cost and abundance, nickel-iron double layered hydroxide (NiFe LDH) is seen as a viable substitute for noble-metal-based electrodes for the oxygen evolution reaction (OER). Herein, we report the growth of NiFe LDH in the form of fine nanosheets in a single step using benzyl alcohol-mediated chemistry. The electrochemical studies clearly suggest that benzyl alcohol is capable of inducing effective chemical interaction between Ni and Fe in the NiFe LDH. The overpotential to produce benchmark 10 mA cm^?2 (η₁₀) for the NiFe LDH electrode is only ～270 mV_RHE, which is much smaller than those of benchmark IrO₂ (η₁₀ = 318 mV_RHE), nickel hydroxide (η₁₀ = 370 mV_RHE) and iron hydroxide (η₁₀ = 410 mV_RHE) for the OER. The difference of the overpotential requirement increases further with increasing current density, indicating faster kinetics of the OER at the catalytic interface of the NiFe LDH. Estimation of Tafel values verifies this notion – the Tafel slopes of NiFe LDH, Ni(OH)₂, and FeOOH are calculated to be 48.6, 55.8, and 59.3 mV dec^?1, respectively. At η = 270 mV, the turnover frequency (TOF) of the NiFe LDH is 0.48 s^?1, which is ～8 and ～11 folds higher than those of Ni(OH)₂ (0.059 s^?1) and FeOOH (0.042 s^?1). In addition to Tafel and TOF, the NiFe LDH electrode has favorable electrochemically active surface area and electrochemical impedance. The electrochemical stability of the NiFe LDH electrode is assessed by conducting potentiostatic measurements at η = 270 mV_RHE (～10 mA cm^?2) and at η = 355 mV_RHE (～30 mA cm^?2) for 24 h of continuous oxygen production.     

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.     

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.     

Bimetallic-MOF derived nickel-iron phosphide nanosheets on carbon cloth for efficacious oxygen evolution reaction

It is of momentously realistic significance to exploit highly efficacious and cost-effective non-noble metal electrocatalysts for oxygen evolution reaction (OER), considering its promising renewable energy application. Herein, a self-supporting electrocatalyst composed of nickel-iron phosphide nanosheets on carbon cloth (NiFeP@CC) is proposed for OER, which are derived from the phosphating treatment of two-dimensional NiFe-MOF nanosheets. The NiFeP@CC composite possesses the synergistic effect of bimetallic NiFe phosphides in promoting the OER, the fully exposed active sites of the nano-sheet structure and the fast charge/mass transfer from the hierarchical porous structure. Owing to the above structural features, the optimized NiFeP@CC presents an impressive OER performance in alkaline solution. The overpotential and Tafel slope are as low as 229 mV and 36.4 mV dec^?1 under a current density of 10 mA cm^?2, respectively, much superior to those for the commercial IrO₂ catalyst. More excitingly, this self-supporting electrocatalyst also possesses an exceptionally high durability, showing no activity degradation for 25 h. This work offers a simple and feasible strategy for developing practically available OER catalysts with a high activity and stability.     

Facile synthesis of bimetallic Ni–Fe phosphide as robust electrocatalyst for oxygen evolution reaction in alkaline media

Bimetallic Ni–Fe phosphide electrocatalysts were in-situ synthesized through direct phosphorization of metal salts on carbon cloth (CC). The Fe dopant remarkably enhances the OER performance of Ni₂P in alkaline medium through the electronic structure modulation of Ni. The (Fe_0.5Ni_0.5)₂P/CC electrode, composed of uniform films coated on carbon fibers, delivers a low overpotential of 260 mV with a small Tafel slope of 45 mV·dec⁻¹ at the current density of 100 mA cm⁻², outperforming most reported non-noble electrocatalysts and commercial RuO₂ electrocatalyst. The (Fe_0.5Ni_0.5)₂P/CC also displays superior electrochemical stability at high current density. An appropriate Fe dopant level facilitates the in-situ transformation of Ni–Fe phosphides into active NiFeOOH during alkaline OER. This work simplifies the synthesis procedure of metal phosphides.     

Superhydrophilic 3D peony flower-like Mo-doped Ni2S3@NiFe LDH heterostructure electrocatalyst for accelerating water splitting

Constructing highly efficient nonprecious electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential to improve the efficiency of overall water splitting, but still remains lots of obstacles. Herein, a novel 3D peony flower-like electrocatalyst was synthesized by employing Mo–Ni₂S₃/NF nanorod arrays as scaffolds to in situ growth ultrathin NiFe LDH nanosheets (Mo-Ni₂S₃@NiFe LDH). As expected, the novel peony flower-like Mo–Ni₂S₃@NiFe LDH displays superior electrocatalytic activity and stability for both OER and HER in alkaline media. Low overpotentials of only 228 mV and 109 mV are required to achieve the current densities of 50 mA cm^?2 and 10 mA cm^?2 for OER and HER, respectively. Additionally, the material remarkably accelerates water splitting with a low voltage of 1.54 V at 10 mA cm^?2, which outperforms most transition metal electrodes. The outstanding electrocatalytic activity benefits from the following these features: 3D peony flower-like structure with rough surface provides more accessible active sites; superhydrophilic surfaces lead to the tight affinity between electrode with electrolyte; metallic Ni substrate and highly conductive Mo–Ni₂S₃ nanorods scaffold together with offer fast electron transfer; the nanorod arrays and porous Ni foam accelerate gas bubble release and ions transmission; the strong interfacial effect between Mo-doped Ni₃S₂ and NiFe LDH shortens transport pathway, which are benefit for electrocatalytic performance enhancement. This work paves a new avenue for construction and fabrication the 3D porous structure to boost the intrinsic catalytic activities for energy conversion and storage applications.     

Microwave-assisted growth of spherical core-shell NiFe LDH@CuxO nanostructures for electrocatalytic water oxidation reaction

The high energy demand for electrochemical water splitting arises from sluggish oxygen evolution reaction (OER) kinetics. In this regard, Layered double hydroxide (LDH) has been introduced as an outstanding catalyst for the OER due to its exceptional physiochemical and 2D infrastructure properties. Herein, we report the design and synthesiss of core-shell nanostructured electrocatalyst by rationally decorating vertically oriented NiFe LDH ultrathin nanosheets on Cu_xO support (NiFe LDH@Cu_xO) via microwave-assisted hydrothermal reaction. For OER, the NiFe LDH@Cu_xO core-shell nanostructured catalyst demonstrated promising electrocatalytic performance, requiring only 1.43 V onset potential and 270 mV overpotential at 10 mA cm^?2. The NiFe LDH@Cu_xO also outperformed pristine NiFe LDH and iridium oxide (IrO₂) in terms of electrocatalytic activity, durability, and Faradaic efficiency. The fabricated NiFe-LDH@Cu_xO electrocatalyst with outer shell NiFe-LDH ultrathin nanosheets provides numerous exposed active sites, benefits electrolyte diffusion and oxygen gas releasing and also reduces the interfacial charge transfer resistance to enhance OER activity. Furthermore, exclusive core-shell 3D infra-structure effectively prevents NiFe-LDH nanosheets agglomeration and restacking, enhancing electrochemical stability.     

Electrodeposited amorphous nickel–iron phosphide and sulfide derived films for electrocatalytic oxygen evolution

The preparation of inexpensive and efficient electrocatalysts for oxygen evolution reaction (OER) is crucial in the widespread application of water electrolyzers. A simple one-step aqueous electrodeposition method is utilized to prepare amorphous nickel-iron sulfide (Ni–Fe–S) and phosphide (Ni–Fe–P) films on Ni foam. The deposited films are highly porous, and can convert to active electrocatalysts for OER. In 1 M KOH, the Ni–Fe–S shows the highest OER activity, and requires only 230 mV overpotential to reach 0.05 A cm^?2 OER current densities. The Fe–Ni–S also sustains the 30 h 0.05 A cm^?2 galvanostatic OER test. Ex-situ characterizations show that sulfur in the Fe–Ni–S is oxidized and leached into the solution during OER, and that (oxy)hydroxide layer is formed at the surface. The adsorption energy of the hydroxyl group, an OER intermediate, is tuned by the electron interaction between the Ni and Fe, and the Ni–Fe–S exhibits the optimum hydroxyl group adsorption energy and the most facile OER kinetics. Also, higher intrinsic OER activity is observed for the electrodeposited amorphous nickel phosphide-derived film than the amorphous nickel sulfide-derived film.     

In situ growth of NiCoFe-layered double hydroxide through etching Ni foam matrix for highly enhanced oxygen evolution reaction

It is of great significance to develop the nonprecious metal oxide electrocatalysts toward oxygen evolution reaction (OER) for water splitting. Herein we report an in-situ growth of the ternary NiCoFe-layered double hydroxide nanosheets on surface etching nickel foam (NiCoFe LDHs/NF) without adding any nickel precursor. In this method, etching Ni matrix by Fe³⁺ not only provides the slowly released nickel ions, but also intensifies the Fe–Ni interaction between the directly grown active species and Ni foam. Therefore the composition, electronic structure, and morphology of the electrocatalysts can be easily regulated only by adjusting Co²⁺:Fe³⁺ ratio in the precursor solution. The obtained NiCo₁Fe₁ LDH/NF, which is formed in 1:1 Co²⁺:Fe³⁺ solution, has highest content of Ni³⁺ and Co³⁺ active sites and the largest electrochemical active area. It exhibits an outstanding OER performance with a small overpotential of 231 mV at 10 mA cm^?2 and excellent durability at 50 mA cm^?2 in 1.0 M KOH solution.     

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.     

Pulse-electrodeposited Ni–Fe–Sn films supported on Ni foam as an excellent bifunctional electrocatalyst for overall water splitting

The development of extremely active bifunctional non-noble electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is pivotal for water splitting but remains challenging. Herein, self-supported Ni–Fe–Sn electrocatalysts were fabricated on nickel foam (NF) through a simple and facile pulse electrodeposition process. Under optimal conditions, the prepared Ni–Fe–Sn electrocatalysts exhibited excellent bifunctional properties in alkaline medium and required ultralow overpotentials of only 27 and 201 mV for HER and OER, respectively, to reach the current density of 10 mA cm^?2. Importantly, the same Ni–Fe–Sn electrocatalyst can be assembled as the anode and the cathode in a two-electrode system. It demanded a fairly low applied voltage of 1.55, 1.72, and 1.87 V to produce 10, 50, and 100 mA cm^?2, respectively, and exhibited excellent long-term stability. The excellent electrocatalytic water splitting performance of the Ni–Fe–Sn film was mainly associated with its intrinsic catalytic activity derived from the modulation of the electronic structures among Ni, Fe, and Sn by using the appropriate atomic ratio of Ni: Fe: Sn.     

Boosting high-current water electrolysis: Superhydrophilic/superaerophobic nanosheet arrays of NiFe LDH with oxygen vacancies in situ grown on iron foam

Developing cost-effective and superior bifunctional electrocatalysts for alkaline water splitting is crucial to realizing hydrogen economy. However, in industrial applications, especially at high current densities, the sluggish kinetic process and the dissatisfactory prolonged stability of electrocatalysts astrict their applications. Herein, the superhydrophilic/superhydrophobic NiFe layered double hydroxide (LDH) with oxygen vacancies was designed in situ grown on iron foam (NiFe/IF) as a high-performance bifunctional electrocatalyst. The unique feature of the superhydrophilic/superaerophobic surface makes for electrolyte penetration and bubbles release, and the existence of oxygen vacancies grants the catalyst with enhanced inherent catalytic activity. Moreover, in situ Raman analysis reveals NiFe LDH could undergo surface reconstruction into active NiOOH species in the electrooxidation environment. Profiting from the above superiorities, the prepared NiFe/IF displays superior OER activity in 1 M KOH with low overpotentials of 245.2, and 480.2 mV to supply 100 and 1000 mA cm⁻², respectively. And the NiFe/IF exhibits prominent stability at 1000 mA cm⁻² under a simulated industrial condition (6 M KOH and 85 °C). Moreover, the water electrolysis device based on NiFe/IF as anode and cathode was assembled with a commercial solar cell to simulate a photovoltaic-driven water splitting system, which revealed a superior efficiency of 15.13% for solar hydrogen production.     

MOF-derived multi-metal embedded N-doped carbon sheets rich in CNTs as efficient bifunctional oxygen electrocatalysts for rechargeable ZABs

The rational design and preparation of bifunctional electrocatalysts with pleasant oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance is crucial for extensive commercial applications of rechargeable Zn–air batteries (ZABs). Herein, we report a simple method to obtain multi-metal (Fe, Ni, Zn) embedded in N-doped carbon sheets entangled with carbon nanotubes (CNTs) as superior oxygen electrocatalysts (FeNi-NCS-2). The resultant FeNi-NCS-2 exhibits an impressive electrochemical performance, providing a reversible oxygen overpotential as low as 0.758 V. The ZAB with FeNi-NCS-2 as the air cathode shows a promising capacity of 639.71 mAh g^?1 at 20 mA cm^?2, a power density of 109.8 mW cm^?2 and cycling stability of over 130 cycles at 10 mA cm^?2 with an energy efficiency of about 55%, superior to the ZAB based on Pt/C–IrO₂. The satisfactory electrocatalytic performance is mainly due to the Fe, Ni-based nanoparticles protected by graphitic carbon layers, hierarchical porous lamellar structures that promote the accessibility between the active centers and the electrolyte as well as self-growing tangled carbon nanotubes that provide fast transmission channels. This study presents a facile way for the synthesis of highly efficient ORR/OER bifunctional electrocatalysts for high-performance rechargeable ZABs.     

Boosting the kinetics of oxygen and hydrogen evolution in alkaline water splitting using nickel ferrite /N-graphene nanocomposite as a bifunctional electrocatalyst

Design, synthesize and application of metal-oxide based bifunctional electrocatalysts with sustainability and efficient activity in water splitting is significant among the wide spread researches in energy applications. Herein, bifunctional electrocatalysts composed of NiFe₂O₄ dispersed on N-doped graphene has been prepared by in-situ polymerization and characterized for further bifunctional catalytic performances. The electrocatalyst exhibited bespoken performances as cathode in HER as well as anode in OER at alkaline electrolyte. The nanocomposite N-doped graphene/NiFe₂O₄ (NGNF) exhibited low overpotential of 184 mV in HER and 340 mV in OER for attaining the current density of 10 mA/cm² which is far better than their pristine counterparts. Similarly its Tafel slopes were found to be 82.9 mV/dec and 93.2 mV/dec for HER and OER. As an electrocatalyst NGNF outperformed pure nickel ferrite and graphene/NiFe₂O₄ (GNF) as bifunctional electrocatalyst with low overpotential and Tafel slopes. This indicates the impact of graphene and N-doping on graphene in the activity of pure NF. The graphene in the composite and the N-dopants provoked the catalytic activity and tuned the electron transfer and interaction with the electrolyte. Thus, herein we endow with strategies of preparing highly efficient bifunctional electrocatalysts by coupling spinel oxides and N-doped graphene for HER and OER.