Nanostructured FeNi3 Incorporated with Carbon Doped with Multiple Nonmetal Elements for the Oxygen Evolution Reaction

N-Doping Fe-C@Nb2CTx MXenes with High Stability and Strong Activity for Sodium-Ion Storage and Overall Water Splitting

The development of highly stable and strongly active electrode materials for sodium-ion batteries (SIBs) and overall water splitting (OWS) is critical in sustainable energy storage and conversion systems. Here, a new electrode material N-Fe-C@Nb2CTx is introduced, with a layered sandwich structure consisting of N-doping Fe-MOF derived-nanorods (Fe-C) and Nb2CTx MXenes. Specifically, Nb2CTx obtained by etching Nb2AlC with HF acid is used as the main body to construct the layered sandwich structure with Fe-C as the filler. Benefiting from this structure, Fe-MOF grows in situ within Nb2CTx, which restrains MXenes aggregation and stacking and also alleviates the bulk effect of sodium-ion embedding/de-embedding, thus improving its stability. Again, the more exposed active sites from the layered sandwich structure and N-doping introduction ensure high reactivity as electrode materials. In addition, Fe-C nanorods strengthen the linkage between the Nb2CTx layers and N-doping enhances the ion/electron transport rate, thereby boosting the effective mass transfer and electrical conductivity. Density functional theory (DFT) calculations show that Fe-C and N-doping help increase the density of states (DOS) and material electrical conductivity. Meanwhile, the generated oxygen species (*OH and *O) in OER are captured by in situ FT-IR test. As a result, the N-Fe-C@Nb2CTx electrochemical test displays good electrochemical performance in SIBs and OWS.

Hf-Doped CoP Hollow Nanocubes as High-Performance Electrocatalyst for Oxygen Evolution Reaction

Designing and synthesizing hollow frame structures with unique three-dimensional open structures in electrocatalysis remain a challenge. Etching is an effective method to synthesize metal-organic frameworks (MOFs) with a hollow structure and rich function. Herein, we report the design and synthesis of Hf-doped CoP hollow nanocubes by selective etching and ion exchange. Different from the traditional etching method, we used acid xylenol orange solution to etch typically the (211) crystal face of ZIF-67, obtaining the unique bell-like structure, named XO-ZIF-67. Subsequently, Hf-doped CoP hollow nanocubes were formed by Hf4+ doping and simple phosphating treatment. Electrochemical tests showed that the overpotential of the obtained catalyst is only 291 mV at the current density of 10 mA cm-2 when applied in catalyzing the oxygen evolution reaction (OER). Furthermore, the catalyst shows excellent stability when running in 1 M KOH solution for 25 h.

Alloying-Triggered Phase Engineering of NiFe System via Laser-Assisted Al Incorporation for Full Water Splitting

It is challenging to design one non-noble material with balanced bifunctional performance for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for commercial sustainability at a low cost since the different electrocatalytic mechanisms are not easily matchable for each other. Herein, a self-standing hybrid system Ni₁₈ Fe₁₂ Al₇₀ , consisting of Ni₂ Al₃ and Ni₃ Fe phases, was constructed by laser-assisted aluminum (Al) incorporation towards full water splitting. It was found that the incorporation of Al could effectively tune the morphologies, compositions and phases. The results indicate that Ni₁₈ Fe₁₂ Al₇₀ delivers an extremely low overpotential to trigger both HER (η₁₀₀ =188 mV) and OER (η₁₀₀ =345 mV) processes and maintains a stable overpotential for 100 h, comparable to state-of-the-art electrocatalysts. The synergistic effect of Ni₂ Al₃ and Ni₃ Fe alloys on the HER process is confirmed based on theoretical calculation.

Boosting the Oxygen Evolution Reaction by Controllably Constructing FeNi3/C Nanorods

Transition bimetallic alloy-based catalysts are regarded as attractive alternatives for the oxygen evolution reaction (OER), attributed to their competitive economics, high conductivity and intrinsic properties. Herein, we prepared FeNi3/C nanorods with largely improved catalytic OER activity by combining hydrothermal reaction and thermal annealing treatment. The temperature effect on the crystal structure and chemical composition of the FeNi3/C nanorods was revealed, and the enhanced catalytic performance of FeNi3/C with an annealing temperature of 400 °C was confirmed by several electrochemical tests. The outstanding catalytic performance was assigned to the formation of bimetallic alloys/carbon composites. The FeNi3/C nanorods showed an overpotential of 250 mV to afford a current density of 10 mA cm−2 and a Tafel slope of 84.9 mV dec−1, which were both smaller than the other control samples and commercial IrO2 catalysts. The fast kinetics and high catalytic stability were also verified by electrochemical impendence spectroscopy and chronoamperometry for 15 h. This study is favorable for the design and construction of bimetallic alloy-based materials as efficient catalysts for the OER.

Highly Durable and Efficient Ni-FeOx/FeNi3 Electrocatalysts Synthesized by a Facile In Situ Combustion-Based Method for Overall Water Splitting with Large Current Densities

Ni-/Fe-based materials are promising electrocatalysts for the oxygen evolution reaction (OER) but usually are not suitable for the hydrogen evolution reaction (HER). Herein, a durable and bifunctional catalyst consisting of Ni-FeO_x and FeNi₃ is prepared on nickel foam (Ni-FeO_x/FeNi₃/NF) by in situ solution combustion and subsequent calcination to accomplish efficient alkaline water splitting. Density functional theory (DFT) calculation shows that the high HER activity is attributed to the strong electronic coupling effects between FeO_x and FeNi₃ in the Janus nanoparticles by modulating ΔG_H* and electronic states. Consequently, small overpotentials (η) of 71 and 272 mV in HER and 269 and 405 mV in OER yield current densities (j) of 50 and 1000 mA cm^-2, respectively. The catalyst shows outstanding stability for 280 and 200 h in HER and OER at a j of ∼50 mA cm^-2. Also, the robustness and mechanical stability of the electrode at an elevated j of ∼500 mA cm^-2 are excellent. Moreover, Ni-FeO_x/FeNi₃/NF shows excellent water splitting activities as a bifunctional catalyst as exemplified by j of 50 and 500 mA cm^-2 at cell voltages of 1.58 and 1.80 V, respectively. The Ni-FeO_x/FeNi₃/NF structure synthesized by the novel, simple, and scalable strategy has large potential in commercial water electrolysis, and the in situ combustion method holds great promise in the fabrication of thin-film electrodes for different applications.

Bicontinuous Nanoporous Nitrogen/Carbon-Codoped FeCoNiMg Alloy as a High-Performance Electrode for the Oxygen Evolution Reaction

The kinetics of the oxygen evolution reaction (OER) in aqueous electrolytes is relatively slow, which seriously limits the energy efficiency of electricity-to-hydrogen conversion. Herein, a bicontinuous nanoporous FeCoNiMg alloy is prepared by high heat sintering method based on the nanoscale Kirkendall effect and the surface is codoped with nitrogen and carbon elements by the nitrocarburizing method (denoted NC-FeCoNiMg). The three-dimensional (3D) nanoporous NC-FeCoNiMg alloy electrode achieves superior electrocatalytic performance for the OER in alkaline media, delivering a low Tafel slope (34.6 mV dec^-1) and small overpotentials (235 and 290 mV at 10 and 100 mA cm^-2, respectively). Under consecutive high current densities, the NC-FeCoNiMg electrode still exhibits excellent long-term stability, and the OER activity even increases after testing for 100 h at a high current density of 1000 mA cm^-2. Comprehensive studies reveal that the N/C codoping of the inner and outer surfaces dramatically improves the electrocatalytic activity of the NC-FeCoNiMg electrode. This work demonstrates an efficient nanoarchitectural construction and a surface modulation strategy to increase the electrocatalytic activity and stability of transition-metal-based electrodes for the OER, holding great promise for fulfilling the requirements for the large-scale production of clean hydrogen energy.

Facile Synthesis of Carbon Cloth Supported Cobalt Carbonate Hydroxide Hydrate Nanoarrays for Highly Efficient Oxygen Evolution Reaction

Developing efficient and low-cost replacements for noble metals as electrocatalysts for the oxygen evolution reaction (OER) remain a great challenge. Herein, we report a needle-like cobalt carbonate hydroxide hydrate (Co(CO₃)_0.5OH·0.11H₂O) nanoarrays, which in situ grown on the surface of carbon cloth through a facile one-step hydrothermal method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations demonstrate that the Co(CO₃)_0.5OH nanoarrays with high porosity is composed of numerous one-dimensional (1D) nanoneedles. Owing to unique needle-like array structure and abundant exposed active sites, the Co(CO₃)_0.5OH@CC only requires 317 mV of overpotential to reach a current density of 10 mA cm⁻², which is much lower than those of Co(OH)₂@CC (378 mV), CoCO₃@CC (465 mV) and RuO₂@CC (380 mV). For the stability, there is no significant attenuation of current density after continuous operation 27 h. This work paves a facile way to the design and construction of electrocatalysts for the OER.

Molybdenum oxynitride nanoparticles on nitrogen-doped CNT architectures for the oxygen evolution reaction

Transition metal-based electrocatalysts are considered the potential alternative to noble metal-based ones owing to their comparable electrocatalytic properties, durability, and low cost for the oxygen evolution reaction (OER). Herein, we report the partial nitridation of molybdenum oxide nanoparticles anchored on nitrogen-doped carbon nanotube (Mo-N-CNT) architectures for a highly active OER electrocatalyst. The molybdenum oxynitride nanoparticles are uniformly distributed on the surface of hierarchical N-CNT architectures, where nitrogen heteroatoms are incorporated through the thermal decomposition of carbon nitride. The modified surface chemistry can boost the electrocatalytic activity of Mo-N-CNT to show improved electrochemical behaviours for OER operation. The Mo-N-CNT achieves a current density of 10 mA cm^-2 with an overpotential of 344 mV, Tafel slope of 64 mV dec^-1, and current density retention of 79% during the oxidation in an alkaline electrolyte for 80 h. The enhanced electrocatalytic performance of Mo-N-CNT is attributed to the hierarchical N-CNT structure and nitridation of Mo oxides.

An Efficient RuTe2 /Graphene Catalyst for Electrochemical Hydrogen Evolution Reaction in Acid Electrolyte

Developing efficient powder catalysts for hydrogen evolution reaction (HER) in the acidic electrolyte is significant for hydrogen generation in the proton exchange membrane (PEM) water electrolysis technique. Herein, we demonstrated an efficient catalyst for HER in the acid media based on the graphene supported ruthenium telluride nanoparticles (RuTe₂ /Gr). The catalysts were easily fabricated by a facile microwave irradiation/thermal annealing approach, and orthorhombic RuTe₂ crystals were found anchored over the graphene surface. The defective structure was demonstrated in the aberration-corrected transmission electron microscopy images for RuTe₂ crystals and graphene support. This catalyst required an overpotential of 72 mV to drive 10 mA cm^-2 for HER when loading on the inert glass carbon electrode; Excellent catalytic stability in acidic media was also observed to offer 10 mA cm^-2 for 10 hours. The Volmer-Tafel mechanism was indicated on RuTe₂ /Gr catalyst by Tafel slope of 33 mV dec^-1 , similar to that of Pt/C catalysts. The high catalytic performance of RuTe₂ /Gr could be attributed to its high dispersion on the graphene surface, high electrical conductivity and low charge transfer resistance. This powder catalyst has potential application in the PEM water electrolysis technique because of its low cost and high stability.

Efficient nanointerface hybridization in a nickel/cobalt oxide nanorod bundle structure for urea electrolysis

Urea electrolysis has received great attention for the energy-relevant applications, and efficient nanostructured catalysts are required to overcome the sluggish urea oxidation kinetics. Herein, we noticed that the valence state of Ni in the hybrid Ni/Co oxide nanorods can be correlated to the catalytic capability for urea oxidation. Crystal lattice hybridization was found in the interface of Ni/Co oxide nanoparticles that assembled as a nanorod bundle structure. The more or the less of Ni2+/Ni3+ generated lower catalytic ability, and Ni/Co oxide with the optimum content of Ni2+/Ni3+ exhibited the highest catalytic ability for urea oxidation because of the efficient synergism, resulting from the formation of high valence state of Ni species and improved kinetics. A low onset potential of 1.29 V was required for the urea oxidation compared with the high onset potential of 1.52 V for water oxidation; high selectivity for urea oxidation was found in the potential below 1.50 V, and as a promising application for urea-assisted water electrolysis about 190 mV less was required to provide 10 mA cm-2 in the two-electrode system, indicating the energy-efficient nature for hydrogen evolution. The study provides some novel insights into the Ni/Co catalyst design and fabrication with efficient catalytic synergism for electrocatalysis.

Efficient Polysulfide Redox Enabled by Lattice-Distorted Ni3Fe Intermetallic Electrocatalyst-Modified Separator for Lithium-Sulfur Batteries

Exploring efficient electrocatalysts for lithium-sulfur (Li-S) batteries is of great significance for the sulfur/polysulfide/sulfide multiphase conversion. Herein, we report nickel-iron intermetallic (Ni₃Fe) as a novel electrocatalyst to trigger the highly efficient polysulfide-involving surface reactions. The incorporation of iron into the cubic nickel phase can induce strong electronic interaction and lattice distortion, thereby activating the inferior Ni phase to catalytically active Ni₃Fe phase. Kinetics investigations reveal that the Ni₃Fe phase promotes the redox kinetics of the multiphase conversion of Li-S electrochemistry. As a result, the Li-S cells assembled with a 70 wt % sulfur cathode and a Ni₃Fe-modified separator deliver initial capacities of 1310.3 mA h g^-1 at 0.1 C and 598 mA h g^-1 at 4 C with excellent rate capability and a long cycle life of 1000 cycles at 1 C with a low capacity fading rate of ∼0.034 per cycle. More impressively, the Ni₃Fe-catalyzed cells exhibit outstanding performance even at harsh working conditions, such as high sulfur loading (7.7 mg cm^-2) or lean electrolyte/sulfur ratio (∼6 μL mg^-1). This work provides a new concept on exploring advanced intermetallic catalysts for high-rate and long-life Li-S batteries.

Strong electronic coupled FeNi3/Fe2(MoO4)3 nanohybrids for enhancing the electrocatalytic activity for the oxygen evolution reaction

Strong electronically coupled FeNi₃/Fe₂(MoO₄)₃ hybrid nanomaterials were successfully fabricated to enhance the electrocatalytic activity for the oxygen evolution reaction.

Chemical Structure of Fe-Ni Nanoparticles for Efficient Oxygen Evolution Reaction Electrocatalysis

Bimetallic iron-nickel-based nanocatalysts are perhaps the most active for the oxygen evolution reaction (OER) in alkaline electrolytes. Recent developments in literature have suggested that the ratio of iron and nickel in Fe-Ni thin films plays an essential role in the performance and stability of the catalysts. In this work, the metallic ratio of iron to nickel was tested in alloy bimetallic nanoparticles. Similar to thin films, nanoparticles with iron-nickel atomic compositions where the atomic iron percentage is ≤50% outperformed nanoparticles with iron-nickel ratios of >50%. Nanoparticles of Fe₂₀Ni₈₀, Fe₅₀Ni₅₀, and Fe₈₀Ni₂₀ compositions were evaluated and demonstrated to have overpotentials of 313, 327,, and 364 mV, respectively, at a current density of 10 mA/cm². While the Fe₂₀Ni₈₀ composition might be considered to have the best OER performance at low current densities, Fe₅₀Ni₅₀ was found to have the best current density performance at higher current densities, making this composition particularly relevant for electrolysis conditions. However, when stability was evaluated through chronoamperometry and chronopotentiometry, the Fe₈₀Ni₂₀ composition resulted in the lowest degradation rates of 2.9 μA/h and 17.2 μV/h, respectively. These results suggest that nanoparticles with higher iron and lower nickel content, such as the Fe₈₀Ni₂₀ composition, should be still taken into consideration while optimizing these bimetallic OER catalysts for overall electrocatalytic performance. Characterization by electron microscopy, diffraction, and X-ray spectroscopy provides detailed chemical and structural information on as-synthesized nanoparticle materials.

In Situ Transformation of Prussian-Blue Analogue-Derived Bimetallic Carbide Nanocubes by Water Oxidation: Applications for Energy Storage and Conversion

Using bimetallic Prussian blue analogue (PBA) as a precursor is effective for preparing electrocatalysts for the oxygen evolution reaction (OER); however, the role of these PBA-derived catalysts in the OER is still ambiguous. Herein, by simply controlling synthesis temperature, a bimetallic PBA-derived O,N-codoped Ni-Fe carbide, can be well tuned to optimize structure and OER performance. Importantly, by a series of ex situ and in situ investigations, real active species of NiFeO_x H_y are in situ formed on the surface during the OER, which reveals a "pre-catalyst" role of O,N-codoped Ni-Fe carbides. Furthermore, it has been successfully applied to highly efficient Zn-air batteries and outplays its RuO₂ counterpart. When applied to photoelectrocatalytic water oxidation as the co-catalyst, it improves the performance of the BiVO₄ photoanode by enhancing hole collecting and transporting ability. We believe this research not only provides a highly efficient and low-cost electrocatalyst for the OER, but also unveils the "pre-catalyst" role of PBA-derived materials in energy-storage and conversion devices.

Fluoridated Iron-Nickel Layered Double Hydroxide for Enhanced Performance in the Oxygen Evolution Reaction

Layered double hydroxides (LDHs) are very promising but still far from satisfactory for catalyzing the electrochemical oxygen evolution reaction (OER) in water electrolysis. Herein, it was found that the catalytic performance of iron-nickel LDHs for OER can be largely boosted by a facile and controllable fluoridation approach at low temperatures. Temperature dependence of the crystal structure and surface chemical state was observed for the simple fluoridation of the iron-nickel LDH. However, no significant surface roughness and electrochemical active surface area increases were found, which was probably owing to the structure change from nanosheets to nanorods. Significant improvements in the performance, including the catalytic activity, stability, efficiency, and kinetics, were found compared with the pristine iron-nickel LDH. Specifically, iron-nickel fluoride obtained at 250 °C afforded the lowest overpotential of 225 mV (no iR correction) to drive 10 mA cm^-2 loaded on an inert glassy carbon electrode with a small Tafel slope of 79 mV dec^-1 , outperforming the noble-metal IrO₂ catalyst and most of the similar Fe-Ni based catalysts. The performance improvement could be mainly attributed to the phase-structure transfer from metal-O bonding in the FeNi-LDHs to metal-F bonding after fluoridation, which means it is easier to form the real active sites of Fe-doped high-valence Ni-(oxy)hydroxide over the iron-nickel fluoride surface.

Surface chemical state evaluation of CoSe2 catalysts for the oxygen evolution reaction

The critical condition for efficient metallic Co–Se bonding construction and its significance for the oxygen evolution reaction were demonstrated.

An Fe-doped NiTe bulk crystal as a robust catalyst for the electrochemical oxygen evolution reaction

An Fe doped NiTe bulk crystal was demonstrated to exhibit an extremely active and stable performance for the electrochemical oxygen evolution reaction.

Electrochemical hydrogen evolution reaction boosted by constructing Ru nanoparticles assembled as a shell over semimetal Te nanorod surfaces in acid electrolyte

Due to its interactions with semi-metallic Te nanorods, Ru nanoparticles assembled as a shell over Te nanorod surfaces (Te@Ru) formed an excellent catalyst for the hydrogen evolution reaction in an acid electrolyte solution.

Electrochemical oxygen evolution reaction efficiently boosted by thermal-driving core-shell structure formation in nanostructured FeNi/S, N-doped carbon hybrid catalyst

Water electrolysis has not yet been implemented on a large scale due to the sluggish oxygen evolution reaction (OER). Herein, we for the first time discover an interesting core-shell structure formation driven by the Kirkendall effect in a nanostructured FeNi alloy incorporating S, N-doped carbon (FeNi/SN-C) and this structural transformation can greatly boost the alloy's catalytic ability for OER. Thermal annealing of FeNi/SN-C in air induces the formation of an Fe-rich Fe-Ni oxide shell over the Fe-Ni alloy core due to the different metal diffusion rates and oxygen coupling abilities. As a powder catalyst, an overpotential as low as 230 mV can drive 10 mA cm-2, about 30 mV less than the original catalyst; it outperforms most nonprecious metal catalysts and noble commercial IrO2 catalysts. The catalytic performances are probably derived from the oxidized Fe-rich oxidation shell in contact with the conductive FeNi/SN-C host, which chemically stabilizes and further activates the active sites formed during the reaction. It is also concluded that exposure of the metal oxide shell contributes more to the activity than the large surface area contributed by the porous carbon matrix. This work puts forward a novel and efficient strategy to optimize Fe-Ni-based catalysts for OER by in situ structure and morphology tuning.