Plasma-assisted synthesis of three-dimensional hierarchical NiFeOx/NiFeP electrocatalyst for highly enhanced water oxidation in alkaline media

Electrocatalytic water splitting into hydrogen and oxygen holds great promise for renewable energy conversion and usage, but its application is limited by the sluggish kinetics of the water oxidation (oxygen evolution reaction). Forming active species (metal oxide/hydroxide) on the surface of metal non-oxide compounds is proven to be an effective strategy for improving activity. In this work, O₂ plasma-assisted strategy is employed to controllably construct 4–6 nm NiFeOx amorphous layer on the surface of NiFeP. Benefiting from the synergistic effect between NiFeP and NiFeOx, large electrochemical surface area, and favorable three-dimensional hierarchical nanostructure, the resulting NiFeOx/NiFeP/NF catalyst exhibits extraordinary water oxidation performance in alkaline media. A current density of 10 mA cm⁻² is achieved at a low overpotential of 165 mV with good stability, which outperforms most of the transition-metal-based electrocatalyst reported. Density functional theory (DFT) calculations suggest that the improved electrochemical performance of NiFeOx/NiFeP can be understood as a result of the optimized electronic structure and increased active catalytic sites. This work might shed light on the design and fabrication of efficient and durable electrode materials for a variety of electrochemical applications.     

Vanadium-induced fragmentation of crystalline CoFe hydr(oxy)oxide electrocatalysts for enhanced oxygen evolution reaction

Transition metal hydr(oxy)oxides are one of the most efficient electrocatalysts towards oxygen evolution reaction (OER). However, structural-dependant and low stability severely hinder their overall performance and catalytic efficiency. Herein, we report the enhancement of OER activity via vanadium doping into the CoFe hydr(oxy)oxide electrocatalyst (denoted as CoFeV). Our results show that the incorporation of vanadium induces fragmentation of the polycrystalline structure of the CoFe hydr(oxy)oxide (CoFe) and transformation to a more active amorphous structure, greatly exposing the CoO and FeOOH active sites on the surface for OER. The optimised electrocatalyst exhibits an overpotential of 376 mV to reach a current density of 10 mA cm⁻² and requires a small Tafel slope of 26 mV dec⁻¹ for OER in alkaline media. These results illustrate that vanadium doping can efficiently improve the catalytic activity of CoFeV which can be extended to other vanadium-doped transition metal hydr(oxy)oxides as promising electrocatalysts for OER applications.     

Structural and electronic engineering of zirconium-induced bimetallic phosphides supported by nitrogen-doped carbon fibers for highly efficient oxygen evolution reaction

The exploitation of non noble metal oxygen evolution reaction (OER) electrocatalysts with tuneable electronic structures and chemical compositions is an effective way to improve water splitting, which is of great significance to the development of clean energy. Here, we report a zirconium-induced transition metal phosphide supported by nitrogen-doped carbon fiber (P–CoFeZr/NCBC) as an efficient electrocatalyst for the OER. The electrocatalyst is produced in situ on the surface of nitrogen-doped carbon fibers to form an open structure, achieving a high concentration of active sites. Additionally, the electronic structure is modified through the introduction of zirconium to increase the intrinsic activity of the electrocatalyst. These phenomena greatly enhance the oxygen evolution activity of the electrocatalyst. In alkaline media, P–CoFeZr/NCBC exhibits excellent catalytic activity and fast kinetics, with overpotentials and Tafel slopes of 300 mV and 51 mV/dec at 10 mA/cm², and displays excellent long-term stability for more than 24 h at 50 mA/cm².     

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 construction of porous and heterostructured molybdenum nitride/carbide nanobelt arrays for large current hydrogen evolution reaction

The development of cost-effective, highly efficient and stable electrocatalysts for alkaline water electrolysis at a large current density has attracted considerable attention. Herein, we reported a one-dimensional (1D) porous Mo₂C/Mo₂N heterostructured electrocatalyst on carbon cloth as robust electrode for large current hydrogen evolution reaction (HER). The MoO₃ nanobelt arrays and urea were used as the metal and non-metal sources to fabricate the electrocatalyst by one-step thermal reaction. Due to the in-situ formed abundant high active interfaces and porous structure, the Mo₂C/Mo₂N electrocatalyst shows enhanced HER activity and kinetics, as exemplified by low overpotentials of 54, 73, and 96 mV at a current density of 10 mA cm^?2 and small Tafel slopes of 48, 59 and 60 mV dec^?1 in alkaline, neutral and acid media, respectively. Furthermore, the optimal Mo₂C/Mo₂N catalyst only requires a low overpotential of 290 mV to reach a large current density of 500 mA cm^?2 in alkaline media, which is superior to commercial Pt/C catalyst (368 mV) and better than those of recently reported Mo-based electrocatalysts. This work paves a facile strategy to construct highly efficient and low-cost electrocatalyst for water splitting, which could be extended to fabricate other heterostructured electrocatalyst for electrocatalysis and energy conversion.     

FeOOH/Ni heterojunction nanoarrays on carbon cloth as a robust catalyst for efficient oxygen evolution reaction

Developing catalysts based on transition metal-based materials for oxygen evolution reaction (OER), which are cheap and efficient, is one of the keys to increase the rate of electrolysis of water to produce hydrogen. Herein, we successfully synthesize iron hydr(oxy)oxide nano-arrays on carbon cloth (FeOOH@CC), and then metallic nickel is electrodeposited on its surface to fabricate FeOOH/Ni heterojunction nanoarrays. Notably, the optimal FeOOH/Ni heterojunction nanoarrays catalyst shows high electrocatalytic performance toward OER with a small overpotential of 257.8 mV at 50 mA cm⁻², a Tafel slope of 30.8 mV dec⁻¹ and outstanding long-term stability in alkaline media. The superior OER performance could be ascribed to the introducing of metallic nickel. The nickel in-situ grows on the surface of FeOOH, which not only can improve the conductivity of FeOOH, but also cooperate with FeOOH to form the FeOOH/Ni heterogeneous interfaces for further enhancing OER electrocatalytic activities. This work provides a simple and efficient strategy of interface engineering to fabricate oxyhydroxide/metal heterojunction nanoarrays as high-efficiency OER catalysts.     

Adjusting electronic structure coupling of Fe–Ni2P (NiFeP-MOF) multilevel structure for ultra-activity and durable catalytic water oxidation

Efficient electrocatalyst for alkaline oxygen evolution reaction is the critical core to the wide application of metal-air energy storage and water electrolysis hydrogen energy. Therefore, appropriate design of highly active and stable non-noble metal oxygen evolution electrocatalyst with good electronic structure and multilevel structure is both a goal and a challenge. Here, we report a Fe–Ni₂P electrocatalyst (NiFeP-MOF) with multilevel structure, which was obtained by anion exchange on the basis of Fe–Ni(OH)₂ (NiFe-MOF) grown on nickel foam in situ by solvothermal method. As expected, Fe substitution regulates the Ni oxidation state in the NiFeP-MOF and realizes electronic structure coupling, showing a highly active and stable oxygen evolution reaction (OER) in alkaline electrolyte solution. Specifically, the NiFeP-MOF demonstrates an ultralow overpotentials (232 mV, 10 mA cm^?2; 267 mV 100 mA cm^?2), respectively, an extremely small Tafel slope (34 mV dec^?1). Separately, the electrocatalyst shows an excellent cycle stability at 10 mA cm^?2 for 12 h (43,200 s). More importantly, this work come up with an available policy for the preparation of excellent alkaline hydrolysis electrolysis catalysts and air cathodes with excellent performance.     

Pulse-activation engineering induced lattice transformation of layered double hydroxides for efficient alkaline hydrogen evolution

In order to accelerate water dissociation kinetics and improve hydrogen evolution reaction (HER) in alkaline electrolyzer, pulse-activation engineering was proposed to induce lattice transformation of layered double hydroxides (LDHs) electrocatalysts. Physicochemical characterizations and density functional theory (DFT) calculations confirmed that FeOOH crystals diminished with a pulse potential of 1000 cycles (cls) in non-Faradaic region (?0.8～0 V) and transformed into more stable CoFeLDH nanosheet arrays in Fe-rich system. The above transformation effectively reduced H¹ adsorption energy to accelerate water decomposition for efficient hydrogen evolution reaction. This was because pulse-activation accelerated electron transfer from Co²⁺ to Fe^(3+δ)+ through intermittent input negative potential, reducing the high oxidation state of Fe^(3+δ)+ in FeOOH to generate more stable CoFeLDH. The optimized pulse-activation with CoFeLDH electrocatalyst after 1000 cls decreased the overpotential of alkaline hydrogen evolution by 36%, from 225 mV to 144 mV at ?10 mA cm^?2.     

Self-supported hierarchical porous FeNiCo-based amorphous alloys as high-efficiency bifunctional electrocatalysts toward overall water splitting

Rationally designing an efficient and cost-effective bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a primary matter in applying electrocatalytic water splitting. Herein, a self-supported FeNiCo-based amorphous catalyst with a hierarchical micro/nanoporous structure is fabricated by dealloying an amorphous/nanocrystalline precursor. The amorphous nanoporous framework enables the prepared electrocatalyst to afford fast reaction kinetics, abundant active sites, and enhanced electrochemical active surface areas (ECSAs). Such structural advantages and the synergistic effects of the ternary transition metals contribute to a dramatic catalytic activity of this electrocatalyst under alkaline conditions, which delivers the current density of 10 mA cm⁻² at a low overpotential of 134 mV for HER and 206 mV for OER, respectively. Furthermore, a full electrolysis apparatus constructed by the self-supported hierarchical micro/nanoporous FeNiCo-based amorphous electrocatalyst as both cathode and anode acquires a dramatically low voltage of 1.58 V operating at 10 mA cm⁻² along with stability for more than 24 h for overall water splitting.     

Nanosheet self-assembled NiCoP microflowers as efficient bifunctional catalysts (HER and OER) in alkaline medium

Developing greatly efficient and steady non-noble metal bifunctional electrocatalyst is of great significance for reducing the energy consumption. In this work, we found that the construction of hierarchical nanostructures was an effective strategy to improve the catalytic performance of bimetallic transition-metal phosphide (NiCoP). Herein, we successfully synthesized the Ni_1.5Co_1.5P catalyst with porous nanosheet self-assembled microflowers (MFs) structure by sequential solvothermal, annealing and phosphorization treatment, and then adjusted the morphology of the MFs by changing the Ni/Co molar ratio to optimize its electronic structure and increase the exposed active sites, thereby improving catalytic activity of the catalyst. Specifically, the Ni_1.5Co_1.5P/MFs only required overpotentials of 141 mV and 314 mV to reach a current density of 10 mA cm⁻² toward HER and OER, respectively. Impressively, during the continuous 12 h chronoamperometry measurement, the Ni_1.5Co_1.5P/MFs displayed good durability. In conclusion, this study provided a feasible strategy to explore and prepare low-cost non-noble metal bifunctional electrocatalysts.     

Construction of P,N-codoped carbon shell coated CoP nanoneedle array with enhanced OER performance for overall water splitting

Water electrolysis to generate hydrogen (H₂) and oxygen (O₂) was a sustainable alternative for clean energy in the future but remained challenging. Herein, we fabricated a nanoneedle-like CoP core coated by a P,N-codoped carbon shell (CoP@PNC@NF). The hierarchical structure, unique nanoneedle-like morphology, CoP core, and P,N-codoped carbon shell were responsible for the high electrocatalytic activity. Electrocatalytic tests demonstrated that CoP@PNC@NF displayed low overpotentials of 137.6 and 148.4 mV, as well as Tafel slopes of 59.89 and 56.40 mV dec⁻¹ for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, at 10 mA cm⁻² in 1.0 M KOH. The bifunctional electrocatalyst CoP@PNC@NF also exhibited a low cell voltage of 1.458 V to yield 10 mA cm⁻² in the two-electrode system and could maintain the activity for 50 h. The Faradaic efficiencies of CoP@PNC@NF for both HER and OER were nearly 100%. The result outperformed the precious-metal-based electrocatalyst apparatus (RuO₂||Pt/C) and other carbon-coated transition-metal phosphides (TMPs). This work paved the way for the rational design of carbon shell-coated TMPs with low energy barriers for converting and storing electrochemical energy.     

Nitrogen and molybdenum co-doped CoP nanohoneycombs on 3D nitrogen-doped porous graphene as enhanced electrocatalyst for oxygen evolution reaction

The four-electron transfer process involved in anodic oxygen evolution reaction (OER) of electrocatalytic water splitting causes the sluggish kinetics and significantly limiting the efficiency of energy conversion. It's urgent to explore low-cost, efficient and stable electrocatalysts for OER. In the work, we design the nitrogen and molybdenum co-doped CoP with nanohoneycombs structure on three-dimensional (3D) nitrogen-doped porous graphene (N/Mo–CoP@NPG) as an efficient OER electrocatalyst. The N/Mo–CoP@NPG delivers the current density of 10 mA cm⁻² at a low overpotential value of 201 mV in 1.0 M KOH, meanwhile the electrocatalytic activity shows no obvious degradation after 50 h. The NPG substrate provides plentiful ligaments for growth of N/Mo–CoP nanohoneycombs and 3D network for rapid electronic transfer. Additionally, doping N and Mo atoms into CoP synergistically modifies the micromorphology and electronic structure, benefiting the electrocatalytic ability. This work offers a promising strategy to improve the electrocatalytic activity of transition metal phosphides.     

Facile fabrication of a binary NiCo phosphide with hierarchical architecture for efficient hydrogen evolution reactions

Exploring and designing efficient non-noble catalysts formed by element doping and nanostructure modification for the hydrogen evolution reaction (HER) is of critical importance with respect to sustainable resources. Herein, we have prepared a three-dimensional binary NiCo phosphide with hierarchical architecture (HA) composed of NiCoP nanosheets and nanowires grown on carbon cloth (CC) via a facile hydrothermal method followed by oxidation and phosphorization. Due to its unique hierarchical nanostructure, the NiCoP HA/CC electrocatalyst exhibits excellent performance and good working stability for the HER in both acidic and alkaline conditions. The obtained NiCoP HA/CC shows excellent HER activity with a low potential of 74 and 89 mV at 10 mA cm⁻², a small Tafel slope of 77.2 and 99.8 mV dec⁻¹ and long-term stability up to 24 h in acidic and alkaline electrolyte, respectively. NiCoP HA/CC, a non-noble metal material, is a promising electrocatalyst to replace noble metal-based electrocatalysts for the HER.     

Flower-like HEA/MoS2/MoP heterostructure based on interface engineering for efficient overall water splitting

The development of cheap, efficient, and active non-noble metal electrocatalysts for total hydrolysis of water (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) is of great significance to promote the application of water splitting. Herein, a heterogeneous structured electrode based on FeAlCrMoV high-entropy alloy (HEA) was synthesized as a cost-effective electrocatalyst for hydrogen and oxygen evolution reactions in alkaline media. In combination of the interfacial synergistic effect and the high-entropy coordination environment, flower-like HEA/MoS₂/MoP exhibited the excellent HER and OER electrocatalytic performance. It showed a low overpotential of 230 mV at the current density of 10 mA cm⁻² for OER and 148 mV for HER in alkaline electrolyte, respectively. Furthermore, HEA/MoS₂/MoP as both anode and cathode also exhibited an overpotential of 1.60 V for overall water splitting. This work provides a new strategy for heterogeneous structure construction and overall water splitting based on high-entropy alloys.     

Nitrogen doped carbon fibers derived from carbonization of electrospun polyacrylonitrile as efficient metal-free HER electrocatalyst

Development of durable and efficient electrocatalyst for hydrogen evolution reaction (HER) is significantly important for forwarding the commercialization of water splitting technology. In this work, we report a facile synthesis of nitrogen doped carbon fibers derived from the carbonization of the electron-spun polyacrylonitrile (PAN) membrane at 800 °C (NCFs-800) as efficient and stable metal-free electrocatalyst for HER catalysis in both acidic and alkaline mediums. Ascribing to the homogenous nitrogen dopants in electrocatalyst, NCFs-800 requires only 114.3 mV and 198.6 mV vs. RHE to achieve current density of 10 mA cm⁻² in 0.5 M H₂SO₄ and 1 M KOH electrolytes, respectively. Moreover, the HER activity is well maintained after 2000 potential cycles indicating that NCFs-800 possesses high durability in both acidic and alkaline conditions due to the fibrous structure with high corrosion resistance. Our study offers new strategy to synthesize stable and efficient metal-free electrocatalyst, which could be extended to other heteroatom doped carbon electrocatalyst.     

Rational design of Ni-induced NC @Mo2C@MoS2 sphere electrocatalyst for efficient hydrogen evolution reaction in acidic and alkaline media

Exploiting efficient and low-cost electrocatalyst for Hydrogen Evolution Reaction (HER) of water electrolysis remains a challenge. Herein, we designed an efficient electrocatalyst of Ni-induced nitrogen-doped carbon @ molybdenum carbide @ molybdenum disulfide sphere (NC@Mo₂C@MoS₂-(Ni)) by two simple coating steps following pyrolysis process. Benefiting from the regular spherical morphology, unique structure, synergistic effect between Mo₂C and MoS₂, inducement effect of elemental Ni that initial added and removed in final synthesis procedure, heteroatom N and P doping. The catalyst NC@Mo₂C@MoS₂-(Ni) exhibits relatively good catalytic performance of overpotentials of 205 and 216 mV at 10 mA cm^?2 and Tafel slopes of 61.4 and 42.7 mV dec^?1 in acidic and basic media, respectively. This work not only fabricate the electrocatalyst of NC@Mo₂C@MoS₂-(Ni) towards HER, but also provides a way to rationally design more efficient other functional electrocatalysts.     

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.     

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

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

Reduced graphene oxide-supported ruthenium nanocatalysts for highly efficient electrocatalytic hydrogen evolution reaction

Hydrogen energy has received great attention because of its advantages such as large energy density and not producing carbon dioxide, and it is currently considered to be one of the most valuable green energy sources. Therefore, the development of efficiently hydrogen production is of great importance. Hydrogen production from water electrolysis has large application prospects due to its cleanliness and no pollution. However, how to prepare an efficient, stable and low-cost electrocatalyst for this process is still challenging. Here, we develop a reduced graphene oxide-supported ruthenium (Ru) nanoparticle electrocatalyst synthesized by a simple method. The ruthenium precursors are encapsulated and isolated with N,N-dimethylformamide (DMF) (Ru³⁺-DMF), which effectively inhibits the further agglomeration growth of ruthenium. After Ru³⁺-DMF being loaded on graphene oxide, Ru is supported on reduced graphene oxide (Ru/rGO) by the liquid phase chemical reduction method and the remaining organic solvent could be removed by calcination to form a well-dispersed Ru-based electrocatalyst. Ru/rGO shows excellent electrocatalytic activity and long-term stability for hydrogen evolution reaction (HER). In a solution of 1.0 M KOH, the overpotential of 3.0 wt%Ru/rGO for the HER at 100 mA cm^?2 is only 111.7 mV, and the Tafel slope is 31.5 mV dec^?1. It exhibits better HER performance compared to commercial Pt/C and other Ru/rGO catalysts with different Ru loadings. The work could give a new strategy for the synthesis of efficient electrocatalysts.     

Surface self-reconstruction of nickel foam triggered by hydrothermal corrosion for boosted water oxidation

Electrochemical water splitting, as a promising approach to convert renewable electricity sources into chemical energy, is limited by bottleneck reaction of oxygen evolution (OER), and requires efficient/low-cost catalysts to accelerate OER dynamics. Metallic Ni, generally as the cathode of industrial alkali electrolyzer toward H₂ production, is affordable yet inactive for anodic OER process. Enabling Ni metal with high OER activity directly serving as the anode will be an exciting progress, and undoubtedly full of challenges. Here, unexpectedly, metallic Ni demonstrates OER vitality through a superficial morphological reconstruction of Ni foam (NF) via hydrothermal-etching. The surface morphological change achieves Ni hierarchical nanosheet@nanoparticle array structure (r-Ni-1), ensuring the realization of optimal electrolyte contact and more surface exposure. More importantly, such configuration benefits for further achievement of fast transformation (in several seconds) into metal oxides and/or (oxy)hydroxides during the catalysis which are believed as the real active species. Above-mentioned in-situ transformation for conventional NF generally requires high-temperature treatment or long-term electrochemical activation for several hours. The catalytic performance of r-Ni-1 indeed outperforms most bimetallic catalysts with overpotential of 330 mV to yield 60 mA cm^?2 in 1.0 M KOH, and it shows no obvious decay after a 60 h test. Our findings not only present a high-performance OER electrocatalyst, but also offer a possibility toward the simple preparation of anode materials for scale-up alkali electrolyzers.