Facile construction of heterostructural Ni3(NO3)2(OH)4/CeO2 bifunctional catalysts for boosted overall water splitting

Interface engineering has aroused vitally widespread concern since it could be an effective strategy for exploring high-performance and low-cost water oxidation electrocatalysts. Herein, we report a hetero-structured Ni₃(NO₃)₂(OH)₄/CeO₂/NF (NNO/CeO₂/NF) electrode, exhibiting superior performance owning to the NO₃^? anion substitution for the OH^? in nickel hydroxide to form Ni₃(NO₃)₂(OH)₄, together with its interface synergy with ceria. In alkaline solution, the NNO/CeO₂/NF electrocatalyst could catalyze the OER with an overpotential of 330 mV to approach 50 mA cm^?2. Also, it needs only an overpotential of 120 mV to reach 10 mA cm^?2 for HER. Additionally, when a standard two-electrode water electrolyzer is fabricated by employing NNO/CeO₂/NF as both the cathode and anode, it can generate 10 mA cm^?2 at 1.64 V and operate steadily without performance degradation after 25 h. This research provides a novel perspective for reasonable design of advanced catalytic materials with improvements in the field of electrocatalysis.     

In-situ generate robust Fe–Ni derived nano-catalyst featuring surface reconstruction for enhanced oxygen evolution reaction

Oxygen evolution reaction (OER) catalysts with highly efficient and cost-effective are cardinal for hydrogen production through water electrolysis. Herein, a novel strategy based on the theory of molecular crystallization and atomic diffusion is described to construct the FeOOH@Ni₃(NO₃)₂(OH)₄/NF. It requires an overpotential of 248 mV at the current density of 100 mA cm^?2 for OER. The in-situ Raman spectroscopy test exploring the catalytic actives unravels that NiOOH is one of the real active species and a small amount of NiFe₂O₄ is generated during OER process. The analysis of the mechanism shows that NiOOH converted from the intermediate product of Ni(OH)₂ derived from Ni₃(NO₃)₂(OH)₄ in the process of OER. NiOOH and FeOOH mainly work together contributing to boosting intrinsic catalytic activity. This work may provide a new insight into fabricating strategy for other nano-catalysts. The in-situ Raman measurement provides a valid and reliable means to probe into the catalytic active site and catalytic mechanism in the catalytic process.     

Highly-stable,bifunctional, binder-free & stand-alone photoelectrode (FexNi1-xO@a-CC) for natural waters splitting into hydrogen

Photoelectrochemical (PEC) water splitting is a promising approach to boost green hydrogen production. Herein, we prepared novel binder-free photoelectrode by direct growth of iron doped nickel oxide catalyst over activated carbon cloth (Fe_xNi_1-xO@a-CC) having band gap energy of 2.2 eV for overall water splitting. Fe_xNi_1-xO@a-CC photoelectrode had shown remarkable lower potential of only 1.36 V for oxygen evolution reaction (OER) to reach 10 mA cm^?2 current density using very low photonic intensity of 8.36 × 10^?4 E/L.s. For the first time, we also reported electrical efficiency required for PEC water splitting for 1 m³ of water that is equal to 0.09 kWh/m³. Fe_xNi_1-xO@a-CC photoelectrode also exhibits low potentials of 1.44 V (OER) and ?0.210 V (HER) at 10 mA cm^?2 to split sea water. Our results confirmed that designing Fe_xNi_1-xO@a-CC photoelectrode would be an innovative step to widen green energy conversion applications using natural waters (both sea and fresh water).     

Efficient overall water splitting using nickel boride-based electrocatalysts

Currently there is tremendous interest in the discovery of low cost and efficient electrocatalysts for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). In this work, iron-doped nickel boride (Fe_xNi_1-xB) and nickel boride (NiB) were successfully grown on 3D self-supporting graphene (SSG) electrodes via a one-step reduction approach. The Fe_0.2Ni_0.8B/SSG electrode required a very low overpotential of only 263 mV for OER (the best OER activity achieved to date for a metal boride). NiB/SSG showed modest OER performance but excellent HER activity. A water electrolyzer comprising Fe_0.2Ni_0.8B/SSG and NiB/SSG delivered a current density of 10 mA cm⁻² at a voltage of only 1.62 V. Further, the Fe_0.2Ni_0.8B/SSG and NiB/SSG catalysts showed excellent stability with no deactivation observed over 14 h of testing. Results demonstrate that nickel boride-based electrocatalysts are promising lost cost alternatives to precious metal-based electrocatalysts for OER, HER and overall water splitting.     

Core-shell Ni3Fe-based nanocomposites for the oxygen evolution reaction

To deal with energy and environmental issues, it is necessary to exploit efficient and stable electrocatalysts for the generation of clean hydrogen. Herein, we describe the synthesis of bimetallic Fe/Ni alloy encapsulated by amorphous carbon shells via a facile annealing strategy for electrocatalytic oxygen evolution reaction (OER). The ferric nickel tartrate annealed at 800 °C (Ni₃Fe₁O_x@C-800) exhibits a low OER overpotential of 264 mV at 10 mA cm^?2 and good stability in alkaline media. Compared with monometallic counterpart, bimetallic Ni₃Fe-based nanocomposites show lower OER barrier (ca. 324 kJ mol^?1) due to a cooperation mechanism between Ni and Fe sites in promoting electrocatalytic water oxidation. Compared with those annealed at other temperatures, the enhanced OER performance of Ni₃Fe₁O_x@C-800 can be ascribed to the large electrochemical surface area for exposing more active sites, smaller charge transfer, and better intrinsic activity of Ni₃Fe-based sites.     

Fe2O3/spinel NiFe2O4 heterojunctions in-situ wrapped by one-dimensional porous carbon nanofibers for boosting oxygen evolution/reduction reactions

One-dimensional (1D) nanofiber structure of electrocatalyst has attracted increasing attention in oxygen evolution/reduction reactions (OER/ORR) owing to its unique structural properties. Here, MIL-53(Fe) and Ni(NO₃)₂·6H₂O are incorporated into the electrospun carbon nanofibers (CNFs) to prepare the nickel-iron spinel-based catalysts (Fe₂O₃/NiFe₂O₄@CNFs) with 1D and porous structure. The marked Fe₂O₃/NiFe₂O₄@CNFs-2 catalyst has a tube diameter of approximately 300 nm, a high surface area of 282.4 m² g^?1 and a hydrophilic surface (contact angle of 16.5°), which obtains a promising bifunctional activity with ΔE = 0.74 V (E_1/2 = 0.84 V (ORR) and E_j10 = 1.58 V (OER)) in alkaline media. Fe₂O₃/NiFe₂O₄@CNFs-2 has a higher catalytic stability (93.35%) than Pt/C (89.36%) for 30,000 s tests via an efficient 4e^? ORR pathway. For OER, Fe₂O₃/NiFe₂O₄@CNFs-2 obtains a low overpotential of 350 mV and a high Faraday efficiency of 92.7%. NiFe₂O₄ (Ni²⁺ in tetrahedral position) relies on its variable valence states (NiOOH and/or FeOOH) to obtain good catalytic activity and stability for OER, while CNFs wrap/protect the active components (Fe–N and graphic N) in the carbon skeleton to effectively improve the charge transfer (conductivity), activity and stability for ORR. Porous 1D nanofiber structure provides abundant smooth pathways for mass transfer. It indicates that the bimetallic active substances can promote bifunctional activity by synergistically changing the oxide/spinel interface structure.     

Tailoring the activity of NiFe layered double hydroxide with CeCO3OH as highly efficient water oxidation electrocatalyst

Owing to the efficient modulation of the electronic structure of nanomaterials, rare earth elements introduction as promoters into nanomaterials has attracted great attention in oxygen evolution reaction (OER). This work demonstrates the cerium carbonate hydroxide (CeCO₃OH) in situ grown on nickel foam (NF) supported NiFe layered double hydroxide (LDH) as a novel promoter in OER process. The hybrid material (Ni_0.75Fe_0.15Ce_0.10/NF) possesses excellent performance for OER where the overpotentials at the current densities of 10 mA cm^?2 and 100 mA cm^?2 are 228 mV and 270 mV, respectively, along with the Tafel slope of 38.3 mV dec^?1. Such performance is comparable in activity to many state-of-the-art electrocatalysts. The enhanced performance in the NiFe LDH can be ascribed to the synergetic interaction between CeCO₃OH and NiFe LDH by utilizing the advantages of cerium and carbonate in OER. The novelty of our work is the exploration of CeCO₃OH as a promoter to enhance the OER performance, which expands the application of cerium-based compounds in energy storage and conversion.     

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.     

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.     

Hydrogen production via highly efficient electrocatalyst based on 3D NixCo1-x(OH)2/NiFe-AM induce overall water splitting

The main factors limiting water splitting producing hydrogen production are overpotential, activity and persistence of electrocatalysts. Herein, a novel Ni_xCo_1-x(OH)₂ coupled with NiFe amorphous compound array growing on nickel foam substrate (expressed as Ni_xCo_1-x(OH)₂/NiFe-AM) was developed by facile hydrothermal and electrodeposition methods. Significantly, Ni_xCo_1-x(OH)₂/NiFe-AM with this unique structural exhibits superior activity and stability in the two half reactions of water electrolysis. In addition, when tested in an alkaline electrolyte with a current density of 10 mA cm⁻², the overpotentials of HER and OER was 157 mV and 196 mV (60 mA cm⁻²), respectively. The stability can up to 60 h. These test results show through constructing hierarchical nano-thron architecture enhanced electrocatalytic activity to produce hydrogen and oxygen.     

Microwave rapid hydrolysis induced two-dimensional NiFeSe nanosheets for efficient oxygen evolution reaction

The key to the development of renewable energy is to search a cheap and efficient non-precious metal based oxygen evolution reaction (OER) electrocatalyst. The NiFe-based catalyst showed excellent catalytic properties. Herein, nickel-iron selenide (m-NiFeSe) with a two-dimensional nanosheet structure was prepared by using the nickel foam as the substrate and the electrodeposition-microwave rapid hydrolysis method. The presence of microwave not only changes the microscopic morphology, forms a new catalytic interface, increases the active area, but also appears mixed valence states of nickel (Ni²⁺/Ni³⁺) and high valence states of selenium, which makes the catalyst possess excellent basic OER electrocatalytic performance. In addition, the synergistic effect between nickel metal and iron metal is also one of the reasons for the improved property. Finally, the microwave-assisted catalysts reached electric current densities of 10 and 100 mA·cm⁻² at overpotentials of 128 and 291 mV, separately, and had good durability in the chronopotentiometry test at a current density of 100 mA·cm⁻² for 18 h. The purpose of this study is to provide a simple and feasible method for the preparation of inexpensive and high-efficiency OER electrocatalysts.     

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.     

Enhanced OER performance of NiFeB amorphous alloys by surface self-reconstruction

The design and development of cost-effective and highly efficient oxygen evolution reaction (OER) electrocatalysts are urgently desirable during the water-splitting process. Here, Ni_xFe_80-xB₂₀ (x = 70, 60, 50, 40, 30, hereafter referred to as NFB) amorphous alloys, with high mechanical strength, excellent corrosion resistance, and unique atomic structure, are fabricated as efficient water oxidation electrocatalysts in alkaline solutions. Ni₄₀Fe₄₀B₂₀ amorphous ribbons achieve only 319 mV of overpotential at 10 mA·cm^?2 with a Tafel slope of 56 mV dec^?1 and exhibit excellent long-term stability for 24 h at 10 mA·cm^?2 and 100 mA·cm^?2 in 1 M KOH solution, which outperform the commercial RuO₂ electrocatalyst. It is worth noting that the OER performance of Ni_xFe_80-xB₂₀ amorphous electrocatalysts after long-term chronopotentiometry test displays more effectively, which can be ascribed to the surface construction. Meanwhile, the analysis of the morphology and structure of the electrocatalysts reveal that continuous oxidation during the OER process induces the structural reorganization on the surface of the electrocatalysts, which can enhance the electron transfer process and adsorption of the reaction intermediates to optimize the OER performance. This study provides a shred of evidence for surface self-reconstruction of NiFeB amorphous alloys electrocatalysts during the OER process and promotes the application of amorphous alloys as functional materials in the water-splitting field.     

Bimetal metal-organic framework hollow nanoprisms for enhanced electrochemical oxygen evolution

Carboxylate-based metal-organic frameworks (MOFs) have emerged as promising electrocatalyst candidates for the water splitting and metal-air batteries. Hierarchical porous structure and redox-active metal centers with unsaturated coordination sites in MOFs facilitate the enhanced catalytic activity of oxygen evolution reaction (OER). Herein, uniform hollow structured Fe-free bi-metal (Co, Ni) MOF-74 nanoprisms are successfully synthesized using a solvothermal method and (Co₁Ni₁)₃(OH)(CH₃COO)₅ as the sacrificial templates, where Co and Ni are the metal nodes and 2,5-dihydroxyterephthalic acid (H₄DOBDC) serves as the organic ligand. At an overpotential of 300 mV, CoNi MOF-74 shows a high electrocatalytic activity towards OER in 0.1 M KOH, where the current density is 10 mA cm^?2 and the Tafel slope is 65.6 mV dec^?1. Meanwhile, CoNi MOF-74 is durable that sustains in alkaline for 100 h with 83.25% retention of current density. The improved catalytic activity can be associated with the in-situ generated amorphous Ni–Co (oxy)hydroxide, as well as the electron transfer from Ni²⁺ to Co²⁺. This work elucidates the potential application of MOF materials as efficient electrocatalysts for OER.     

Nanorod-like NiFe metal-organic frameworks for oxygen evolution in alkaline seawater media

Searching for highly active and durable oxygen evolution reaction (OER) electrocatalysts is the key to break through the bottleneck of overall water splitting. Here, we prepare Ni_xFe-BDC (H₂BDC = terephthalic acid) nanorods with different Ni/Fe ratios by a facile solvothermal method for the OER. The optimal Ni₃Fe-BDC exhibits a low overpotential (η₁₀) of 265 mV and a Tafel slope of 90 mV·dec⁻¹ in 1 M KOH. Moreover, it shows a low η₁₀ of 280 mV and excellent stability in the mixture of 1 M NaCl and 1 M KOH, which has strong corrosion resistance to Cl⁻ anions. The role of Fe³⁺ not only increases the charge transfer rate of Ni₃Fe-BDC, but also affects the specific surface area of the catalyst with high electrochemical activity. Kinetic studies show that both Fe and Ni sites act as active centers, which catalyze synergistically to reduce the reaction kinetic energy barrier. Characterization results of the used Ni₃Fe-BDC reveal that the in situ formed rod-like Ni₃FeOOH is the active site for the OER.     

NiFe2O4–Ni3S2 nanorod array/Ni foam composite catalyst indirectly controlled by Fe3+ immersion for an efficient oxygen evolution reaction

A NiFe alloy was designed on nickel foam (NF) as a precursor using cathodic electrodeposition. NiFe₂O₄–Ni₃S₂ nanorods (NRs) composite catalysts were prepared by Fe³⁺ impregnation and further hydrothermal sulfuration methods. NiFe₂O₄–Ni₃S₂ nanosheets (NSs) were also prepared by direct hydrothermal sulfuration of the NiFe alloy for comparison. Compared to the dense NS structure of the NiFe₂O₄–Ni₃S₂ NSs/NF, the NiFe₂O₄–Ni₃S₂ NRs/NF showed better oxygen evolution performance due to its unique weed-like NR array structure composed of additional oxygen evolution reaction (OER) active sites, with a strong electron interaction for Ni and Fe and the active sulfide synergistic effect with oxides. Therefore, Driving a current density of 10 mA cm^?2 only requires an overpotential of 189 mV and the catalyst could provide 100 mA cm^?2 continuously and be constant for more than 80 h in 1.0 M KOH. This experiment indicated that Fe³⁺ immersion had an indirect regulating effect on the morphological growth of the catalyst, which provided a novel concept for designing better OER catalysts.     

SrTi0.1CoxFe0.9-xO3-δ Perovskites for enhanced oxygen evolution reaction activity

We developed a series of Fe doping in Co-based perovskites SrTi_0.1Co_xFe_0.9-xO_3-δ (x = 0.5, 0.6, 0.7, 0.9) to investigate their OER activity and stability in alkaline media. Among all the samples, SrTi_0·1Co_0·5Fe_0·4O_3-δ (donated as STCF-154) shows wonderful OER activity with an overpotential of 0.37 V, a current density of 33.65 mA cm⁻² at 1.71 V, and a Tafel slope of 94.82 mV dec⁻¹. Besides, the potential of STCF-154 remained nearly unchanged for at least 8 h at a fixed current density of 10 mA cm⁻²_disk on GC electrode. The improved activity and stability are likely originating from the highly oxidative oxygen species O₂²⁻/O⁻ formed in STCf-154, which can easily migrate from bulk STCF-154 and “spillover” to the surface of the catalyst during OER process. The Fe doping in Co-based perovskites had synergetically enhanced activity and can be considered as a good candidate for the OER in alkaline solution.     

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.     

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.     

Ternary NiCoFe nanosheets for oxygen evolution in anion exchange membrane water electrolysis

Tuning nickel-based catalyst activity and understanding electrolyte and ionomer interaction for oxygen evolution reaction (OER) is crucial to improve anion exchange membrane (AEM) water electrolyzers. Herein, an investigation of multimetallic Ni_0.6Co_0.2Fe_0.2 OER activity, coupled with in-situ Raman spectroscopy to track dynamic structure changes, was carried out and compared to other Ni catalysts. The effect of KOH concentration, KOH purity, ionomer type, and electrolyte with organic cations was evaluated. The Ni_0.6Co_0.2Fe_0.2 catalyst achieved 10 mA/cm² at 260 mV overpotential with stability over 50 h and 5000 cycles in 1 M KOH. In-situ Raman spectroscopy showed that Ni_0.6Co_0.2Fe_0.2 activity originates from promoting Ni(OH)₂/NiOOH transformation at low potentials compared to bi- and mono-metallic nickel-based catalysts. Fumion anion ionomer in the catalyst inks led to a lower OER activity than catalysts with inks containing Nafion ionomer. The OER activity of Ni_0.6Co_0.2Fe_0.2 is adversely influenced in the presence of fumion anion ionomer and benzyltrimethylammonium hydroxide (BTMAOH) with possible phenyl oxidation under applied high anodic potentials. The alkaline AEM water electrolyzer circulating 1 M KOH electrolyte, with a Pt/C cathode and a Ni_0.6Co_0.2Fe_0.2 anode, achieved 1.5 A/cm² at 2 V.