Reduced graphene oxide composite Ni3S2 microspheres grown directly on nickel foam as an efficient electrocatalyst for OER

The development of cheap, high-efficiency, and stable oxygen evolution reaction (OER) electrocatalysts is a current research hotspot. In this work, reduced graphene oxide (rGO) composite Ni₃S₂ microspheres grown directly on nickel foam (Ni₃S₂-rGO/NF) were prepared by tube furnace calcination and hydrothermal method. The Ni₃S₂-rGO/NF had excellent OER catalytic activity and stability with an overpotential of 303 mV at the current density of 100 mA cm⁻², which was 100 mV lower than that of Ni₃S₂/NF, and its Tafel slope was as low as 23 mV·dec⁻¹. The main reason for enhancing OER activity of the Ni₃S₂-rGO/NF is due to synergistic effect of Ni₃S₂ microspheres and rGO, which inhibited the production of NiS and refined the micron size of Ni₃S₂. This work offers a new method for developing stable and efficient OER catalysts.     

One-step preparation of Fe-doped Ni3S2/rGO@NF electrode and its superior OER performances

In order to improve the OER performance, Ni₃S₂-based catalysts were directly grown on Ni substrate by simultaneously doping of Fe and compositing with reduced graphene oxide (rGO). Synthesis and loading of Ni₃S₂/rGO were completed during a one-step hydrothermal process, in which Ni foam acted as support and Ni source of Ni₃S₂, as well as the subsequent current collector. It is found that either GO or Fe salt tuned Ni₃S₂ nanosheets into thinner and smaller interconnected nanosheets anchored on rGO, which enhanced the charge transfer resistance and improved the active sites. Hence, as-synthesized Fe-doped Ni₃S₂/rGO composite at 120 °C (Fe-2-Ni₃S₂/rGO@NF-120) exhibited an enhancement on OER performances: An overpotential of 247 mV at 20 mA cm⁻², and a small Tafel slope of 63 mV dec⁻¹, as well as an excellent stability of: 20 h maintaining at 20 mA cm⁻² or 50 mA cm⁻².     

3D V–Ni3S2@CoFe-LDH core-shell electrocatalysts for efficient water oxidation

Studying cheap and efficient electrocatalysts is of great significance to promote the sluggish kinetics of oxygen evolution reaction (OER). Here, we adopted a simple two-step method to successfully prepare the 3D V–Ni₃S₂@CoFe-LDH core-shell electrocatalyst. The V–Ni₃S₂@CoFe-LDH/NF shows excellent OER performance with low overpotential (190 mV at 10 mA/cm² and 240 mV at 50 mA/cm²), small Tafel slope (26.8 mV/dec) and good long-term durability. Excitingly, to reach the same current density, V–Ni₃S₂@CoFe-LDH/NF electrode even needs much smaller overpotential than RuO₂. Furthermore, the outstanding OER activity of V–Ni₃S₂@CoFe-LDH/NF is ascribed to the following reasons: (1) V–Ni₃S₂ nanorod cores improve the conductivity and ensure the fast charge transfer; (2) CoFe-LDH nanosheets interconnected with each other provide more exposed active sites; (3) the unique 3D core-shell structures are favorable for electrolyte diffusion and gas releasing. Our work indicates that building 3D core-shell heterostructure will be a useful way to design good electrocatalysts.     

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.     

A nano-spherical structure Ni3S2/Ni(OH)2 electrocatalyst prepared by one-step fast electrodeposition for efficient and durable water splitting

Developing non-noble metal catalysts with excellent electrocatalytic performance and stability is of great significance to hydrogen production by water electrolysis, but there are still problems of low activity, complex preparation and high cost. Herein, we fabricated a novel Ni₃S₂/Ni(OH)₂ dual-functional electrocatalyst by a one-step fast electrodeposition on nickel foam (NF). While maintaining the electrocatalytic performance of Ni₃S₂, the existence of heterostructure and Ni(OH)₂ co-catalyst function greatly improves the overall water splitting performance of Ni₃S₂/Ni(OH)₂–NF. Hence, It shows a low overpotential of 66 mV at 10 mA cm^?2 for HER and 249 mV at 20 mA cm^?2 for OER. The dual-functional electrocatalyst needs only 1.58 V at 20 mA cm^?2 when assembled two-electrode electrolytic cell. Impressively, the electrocatalyst also shows outstanding catalytic stability for about 800 h when 20 and 50 mA cm^?2 constant current was applied, respectively which demonstrates a potential electrocatalyst for overall water splitting.     

In situ growth of Ni0·85Se on graphene as a robust electrocatalyst for hydrogen evolution reaction

Seeking the efficient and robust electrocatalysts necessarily enhances performance of hydrogen evolution reaction (HER). Increasing the surface active sites is a means to improve the performance. Herein, we use the Ni_0·85Se anchored on reduction of graphene oxide (Ni_0·85Se/rGO) hybrid material skillfully established by one-step facile hydrothermal method as a robust and stable electrocatalyst applying to hydrogen evolution reaction (HER). In terms of morphology, Ni_0·85Se nanospheres composed of many nanosheets are uniformly distributed on the graphene sheet layer. We also detailedly analyze its properties. Based on the interaction between Ni_0·85Se and rGO, and the roles of graphene are as a substrate to heighten conductivity, possesses more active surface area by limiting growth of Ni_0·85Se, and increases dispersion for exposing more active surface area and enlarge ion/electron transfer rate. In HER, the Ni_0·85Se/rGO catalyst displays the overpotential of 128 mV with a common current density of 10 mA cm⁻², a small Tafel slope of 91 mV dec⁻¹, an extremely low onset potential of 37 mV, outstanding stability that a high current retention of 97.7% after 1000 cycles and well long-term stability for 18 h, outperforming the capability of Ni_0·85Se nanospheres in alkaline solution for HER. The above results indicate that the Ni_0·85Se/rGO hybrid material is a good HER ability and non-noble metal electrocatalyst has potential value in HER.     

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.     

Ni3S2@Co(OH)2 heterostructures grown on Ni foam as an efficient electrocatalyst for water oxidation

It is of high significance to design robust, low-cost and stable electrocatalysts for the oxygen evolution reaction (OER) under alkaline medium. In this communication, we present the exploitation of Ni₃S₂@Co(OH)₂ which directly grown on nickel foam (Ni₃S₂@Co(OH)₂/NF) as a robust and stable electrocatalyst for OER. Such Ni₃S₂@Co(OH)₂/NF-5h demanding overpotential of only 290 mV is less than that of Ni₃S₂@Co(OH)₂/NF-10h (310 mV), Ni₃S₂@Co(OH)₂/NF-2h (320 mV) and Ni₃S₂/NF(350 mV), respectively, to drive a geometrical catalytic current density of 35 mA cm⁻², which is also better than that of noble metal catalyst IrO₂/NF (320 mV). In addition, the Ni₃S₂@Co(OH)₂/NF-5h presents a superior long-term electrocatalytic stability, keeping its activity at 26 mA cm⁻² for 40 h.     

Effect of cation substitution in MnCo2O4 spinel anchored over rGO for enhancing the electrocatalytic activity towards oxygen evolution reaction (OER)

Developing an efficient and durable electrocatalyst for catalyzing oxygen evolution reaction in electrochemical water splitting application is greatly desired and challenging. Herein, a simple and facile strategy was followed to prepare Ni-substituted MnCo₂O₄/rGO nanocomposite as an electrocatalyst for oxygen evolution reaction (OER). The structural and morphological analyses show the successful bonding between spinel-carbon hybrid. Nickel substitution and addition of rGO disclose a drastic change in the catalytic activity of the same towards OER. Among all the electrocatalysts, Mn_1-xNi_xCo₂O₄/rGO with x = 0.6 exhibits low overpotential of 250 and 290 mV for attaining the current density of 10 mA/cm² before and after potential cycling respectively. It also exhibits low onset potential of 1.48 V with a Tafel slope of value 78 mV/dec. The electrocatalyst also shows excellent stability, high ECSA and roughness factor (R_f) which are responsible for enhanced OER performance. These properties confirm that the present hybrid material would be an excellent electrocatalyst for catalyzing OER for hydrogen energy production.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

Electrodeposition of NiFe layered double hydroxide on Ni3S2 nanosheets for efficient electrocatalytic water oxidation

The oxygen evolution reaction (OER) plays a vital role in various energy conversion applications. Up to now, the highly efficient OER catalysts are mostly based on noble metals, such as Ir- and Ru-based catalysts. Thus, it is extremely urgent to explore the non-precious electrocatalysts with great OER performance. Herein, a simple electrodeposition combined with hydrothermal method is applied to synthesize a non-precious OER catalyst with a three-dimensional (3D) core-shell like structure and excellent OER performance. In our work, NiFe layered double hydroxide (LDH) was electrodeposited on Ni₃S₂ nanosheets on nickel foam (NF), which exhibits a better performance compared with RuO₂, and a low overpotential of 200 mV is needed to reach the current density of 10 mA/cm² in 1 M KOH. Notably, the Ni₃S₂/NiFe LDH only need an overpotential of 273 mV to reach the current density of 200 mA/cm².     

In situ growth Fe and V co-doped Ni3S2 for efficient oxygen evolution reaction at large current densities

Industrial electrolysis of water is one of the effective strategies for green hydrogen production in the future. Nevertheless, the large-scale applications of water electrolysis are still intractable issues hindered by the high overpotentials and inferior reaction kinetics on the anode. Herein, a facile one-step hydrothermal method was applied to in situ growth the Fe and V co-doped Ni₃S₂ electrocatalyst on nickel foam substrate (Fe, V–Ni₃S₂/NF). In 1 M KOH electrolyte, the as-prepared Fe, V–Ni₃S₂/NF electrode exhibited an improved water oxidation activity with ultralow overpotentials of 253 and 370 mV to realize large current densities of 100 and 1000 mA/cm², respectively. More importantly, the Fe, V–Ni₃S₂/NF electrode existed an activation process during 100 h chronopotentiometry testing period. Detailed characterizations revealed that elements of V and S in the electrocatalyst were oxidized and dissolved into the electrolyte, making the electrocatalyst undergo surface reconstruction and resulting in a faster kinetic reaction rate, thus leading to enhanced oxygen evolution reaction activities. Collectively, the resultant Fe, V–Ni₃S₂/NF in this work provides new cogitation towards design and synthesis of low-cost electrocatalyst with large current densities for water oxidation.     

Hollow hexagonal NiSe–Ni3Se2 anchored onto reduced graphene oxide as efficient electrocatalysts for hydrogen evolution in wide-pH range

Integrating transition metal complexes with carbon-based materials, especially graphene, is a useful strategy for synthesizing effective hydrogen evolution catalysts. Herein, we report a design of hollow hexagonal NiSe–Ni₃Se₂ nanosheets grown on reduced graphene oxide (NiSe–Ni₃Se₂/rGO) by a simple hydrothermal method as an effective catalyst for hydrogen evolution reaction (HER) in the full pH range. In 0.5 M H₂SO₄, the NiSe–Ni₃Se₂/rGO possesses 112 mV to achieve 10 mA cm^?2 and a small Tafel slope (61 mV dec^?1). In 1.0 M PBS and 1.0 M KOH, the overpotentials are 261 and 188 mV at 10 mA cm^?2, and Tafel slopes are 103 and 92 mV dec^?1, respectively. Meanwhile, it owns good cycle stability and durability over 20 h in the whole pH range (0-14). In all solutions, the HER performance of NiSe–Ni₃Se₂/rGO is better than that of NiSe–Ni₃Se₂. This is because the rGO substrate accelerates the electron transfer and improves the electrical conductivity, increasing HER activity of catalyst.     

Interface engineering of NixSy@MoS2 heterostructured nanorods as high-efficient electrocatalysts for water splitting

Robust and low-cost oxygen evolution reaction (OER) electrocatalysts at low overpotentials play an increasingly pivotal role in clean energy storage and conversion systems. The emerging catalyst with core-shell heterostructure has excited the potentiality of non-noble-metal candidates. Herein, in order to enhance the sufficient exposure and utilization of actives sites and accelerate the electron transfer rate of catalyst, Ni_xS_y@MoS₂ core-shell nanorods decorated NiSe₂ framework (Ni_xS_y@MoS₂/NiSe₂) electrocatalyst has been successfully prepared via interface engineering. As expected, the as-prepared catalyst shows an outstanding OER activity with a small overpotential of 360 mV to drive 100 mA cm⁻² and a Tafel slope of 64 mV dec⁻¹. More importantly, a low cell voltage of 1.40 V is achieved for the Ni_xS_y@MoS₂/NiSe₂ based water splitting electrolyzer at 10 mA cm⁻², and it shows negligible decrement after continuous operation for 100 h. Furthermore, density functional theory (DFT) calculations further uncovered the synergetic catalytic effect between the Ni_xS_y@MoS₂ core-shell nanorods and the NiSe₂ framework played a key role in generating more charge carriers and declining the energy barriers in the process of forming intermediates.     

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.     

Crystalline nickel sulfide integrated with amorphous cobalt sulfide as an efficient bifunctional electrocatalyst for water splitting

The development of highly efficient and low-cost electrocatalysts is critical to the mass production of hydrogen from water splitting. Herein, a facile yet effective method was developed to synthesize bimetallic sulfides Ni₃S₂/CoS_x, which were aimed for use as the electrocatalysts in both HER and OER. Encouragingly, the Ni₃S₂/CoS_x demonstrated a low overpotential of 110 mV for HER at a current density of 10 mA·cm^?2. It was discovered that the surface of Ni₃S₂/CoS_x during OER process would undergo an in-situ oxidation to form MOOH (M = Co, Ni), that is, MOOH/Ni₃S₂/CoS_x were the real functioning species in catalysis, which had an excellent OER activity and a low overpotential of 226 mV. Additionally, the assembled electrolyzer required only a low cell voltage of 1.53 V to achieve a current density of 10 mA·cm^?2 in a 1 M KOH solution, and its performance was stable. Overall, this work provided a promising strategy for the facile fabrication of low-cost amorphous electrocatalysts, which is expected to promote the progress of overall water splitting.     

Tungsten promoted nickel phosphide nanosheets supported on carbon cloth: An efficient and stable bifunctional electrocatalyst for overall water splitting

It is of great significance to develop a highly active, durable and inexpensive bifunctional electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report a tungsten-doped nickel phosphide nanosheets based on carbon cloth (W–Ni₂P NS/CC) as an efficient bifunctional catalyst through simple hydrothermal and phosphorization for overall water splitting in 1 M KOH. The W–Ni₂P NS/CC exhibits excellent electrochemical performance with low overpotentials for HER (η₁₀ = 71 mV, η₅₀ = 160 mV) and OER (η₂₀ = 307 mV, η₅₀ = 382 mV) in 1 M KOH, as well as superior long-term stability. Moreover, W–Ni₂P NS/CC as a bifunctional catalyst reveals remarkable activity with a low voltage of 1.55 V to reach a current density of 20 mA cm⁻². This work provides a viable bifunctional catalyst for the overall water splitting.     

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.     

Zn-substituted MnCo2O4 nanostructure anchored over rGO for boosting the electrocatalytic performance towards methanol oxidation and oxygen evolution reaction (OER)

Mixed valence spinel oxides have emerged as an attractive and inexpensive anode electrocatalyst for water oxidation to replace noble metals based electrocatalysts. The present work demonstrates the facile synthesis of Zn substituted MnCo₂O₄ supported on 3D graphene prepared by simple hydrothermal technique and its application as an electrocatalyst for water oxidation and methanol oxidation. The physico-chemical properties of the nanocatalyst were studied using various microscopic, spectroscopic and diffraction analyses confirming the formation of the composite. The electrocatalytic performance of the prepared electrocatalyst was evaluated using potentiodynamic, potentiostatic and impedance techniques. The synthesized Zn_1-xMn_xCo₂O₄/rGO electrocatalyst with x = 0.2 and 0.4 offered the same onset potential and overpotential at 10 mA/cm². However, catalyst x = 0.4 delivered a higher current density indicating the superiority of the same over other compositions which is attributed to better kinetics that it possessed for OER as revealed by the smallest Tafel slope (80.6 mV dec⁻¹). The prepared electrocatalysts were tested for methanol oxidation in which electrocatalyst Zn_1-xMn_xCo₂O₄/rGO with x = 0.4 shows a better electrochemical performance in oxidizing methanol with the higher current density of 142.3 mA/cm². The above catalyst also revealed excellent stability and durability during both MOR and OER, suggesting that it can be utilized in practical applications.