Iron-Doped Nickel Molybdate with Enhanced Oxygen Evolution Kinetics

Electrochemical water splitting is one of the potential approaches for making renewable energy production and storage viable. The oxygen evolution reaction (OER), as a sluggish four-electron electrochemical reaction, has to overcome high overpotential to accomplish overall water splitting. Therefore, developing low-cost and highly active OER catalysts is the key for achieving efficient and economical water electrolysis. In this work, Fe-doped NiMoO₄ was synthesized and evaluated as the OER catalyst in alkaline medium. Fe³⁺ doping helps to regulate the electronic structure of Ni centers in NiMoO₄, which consequently promotes the catalytic activity of NiMoO₄. The overpotential to reach a current density of 10 mA cm⁻² is 299 mV in 1 m KOH for the optimal Ni_0.9Fe_0.1MoO₄, which is 65 mV lower than that for NiMoO₄. Further, the catalyst also shows exceptional performance stability during a 2 h chronopotentiometry testing. Moreover, the real catalytically active center of Ni_0.9Fe_0.1MoO₄ is also unraveled based on the ex situ characterizations. These results provide new alternatives for precious-metal-free catalysts for alkaline OER and also expand the Fe-doping-induced synergistic effect towards performance enhancement to new catalyst systems.     

Crystallinity-Modulated Electrocatalytic Activity of a Nickel(II) Borate Thin Layer on Ni3B for Efficient Water Oxidation

The exploration of new efficient OER electrocatalysts based on nonprecious metals and the understanding of the relationship between activity and structure of electrocatalysts are important to advance electrochemical water oxidation. Herein, we developed an efficient OER electrocatalyst with nickel boride (Ni₃B) nanoparticles as cores and nickel(II) borate (Ni-B_i) as shells (Ni-B_i@NB) via a very simple and facile aqueous reaction. This electrocatalyst exhibited a small overpotential of 302 mV at 10 mA cm⁻² and Tafel slope of 52 mV dec⁻¹. More interestingly, it was found that the OER activity of Ni-B_i@NB was closely dependent on the crystallinity of the Ni-B_i shells. The partially crystalline Ni-B_i catalyst exhibited much higher activity than the amorphous or crystalline analogues; this higher activity originated from the enhanced intrinsic activity of the catalytic sites. These findings open up opportunities to explore nickel(II) borates as a new class of efficient nonprecious metal OER electrocatalysts, and to improve the electrocatalyst performance by modulating their crystallinity.     

Robust Polyoxometalate/Nickel Foam Composite Electrodes for Sustained Electrochemical Oxygen Evolution at High pH

The development of technologically viable electrodes for the electrochemical oxygen evolution reaction (OER) is a major bottleneck in chemical energy conversion. This article describes a facile one‐step hydrothermal route to deposit microcrystals of a robust Dexter–Silverton polyoxometalate oxygen evolution catalyst, [Co_6.8Ni_1.2W₁₂O₄₂(OH)₄(H₂O)₈], on a commercial nickel foam electrode. The electrode shows efficient and sustained electrochemical oxygen evolution at low overpotentials (360 mV at 10 mA cm⁻² against RHE, Tafel slope 126 mV dec⁻¹, faradaic efficiency (96±5) %) in alkaline aqueous solution (pH 13). Post‐catalytic analyses show no mechanical or chemical degradation and no physical detachment of the microcrystals. The results provide a blueprint for the stable “wiring” of POM catalysts to commercial metal foam substrates, thus giving access to technologically relevant composite OER electrodes.     

A Surface-Oxide-Rich Activation Layer (SOAL) on Ni2Mo3N for a Rapid and Durable Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is key to renewable energy technologies such as water electrolysis and metal–air batteries. However, the multiple steps associated with proton-coupled electron transfer result in sluggish OER kinetics and catalysts are required. Here we demonstrate that a novel nitride, Ni₂Mo₃N, is a highly active OER catalyst that outperforms the benchmark material RuO₂. Ni₂Mo₃N exhibits a current density of 10 mA cm⁻² at a nominal overpotential of 270 mV in 0.1 m KOH with outstanding catalytic cyclability and durability. Structural characterization and computational studies reveal that the excellent activity stems from the formation of a surface-oxide-rich activation layer (SOAL). Secondary Mo atoms on the surface act as electron pumps that stabilize oxygen-containing species and facilitate the continuity of the reactions. This discovery will stimulate the further development of ternary nitrides with oxide surface layers as efficient OER catalysts for electrochemical energy devices.     

High Valence State Sites as Favorable Reductive Centers for High-Current-Density Water Splitting

Electrochemical water splitting is a promising approach for producing sustainable and clean hydrogen. Typically, high valence state sites are favorable for oxidation evolution reaction (OER), while low valence states can facilitate hydrogen evolution reaction (HER). However, here we proposed a high valence state of Co³⁺ in Ni_9.5Co_0.5−S−FeO_x hybrid as the favorable center for efficient and stable HER, while structural analogues with low chemical states showed much worse performance. As a result, the Ni_9.5Co_0.5−S−FeO_x catalyst could drive alkaline HER with an ultra-low overpotential of 22 mV for 10 mA cm⁻², and 175 mV for 1000 mA cm⁻² at the industrial temperature of 60 °C, with an excellent stability over 300 h. Moreover, this material could work for both OER and HER, with a low cell voltage being 1.730 V to achieve 1000 mA cm⁻² for overall water splitting at 60 °C. X-ray absorption spectroscopy (XAS) clearly identified the high valence Co³⁺ sites, while in situ XAS during HER and theoretical calculations revealed the favorable electron capture at Co³⁺ and suitable H adsorption/desorption energy around Co³⁺, which could accelerate the HER. The understanding of high valence states to drive reductive reactions may pave the way for the rational design of energy-related catalysts.     

Controlled Synthesis of 3D Flower‐like Ni2P Composed of Mesoporous Nanoplates for Overall Water Splitting

Developing efficient non‐noble metal and earth‐abundant electrocatalysts with tunable microstructures for overall water splitting is critical to promote clean energy technologies for a hydrogen economy. Herein, novel three‐dimensional (3D) flower‐like Ni₂P composed of mesoporous nanoplates with controllable morphology and high surface area was prepared by a hydrothermal method and low‐temperature phosphidation as efficient electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Compared with the urchin‐like Ni_x P_y , the 3D flower‐like Ni₂P with a diameter of 5 μm presented an efficient and stable catalytic performance in 0.5 m H₂SO₄, with a small Tafel slope of 79 mV dec⁻¹ and an overpotential of about 240 mV at a current density of 10 mA cm⁻² with a mass loading density of 0.283 mg cm⁻². In addition, the catalyst also exhibited a remarkable performance for the OER in 1.0 m KOH electrolyte, with an overpotential of 320 mV to reach a current density of 10 mA cm⁻² and a small Tafel slope of 72 mV dec⁻¹. The excellent catalytic performance of the as‐prepared Ni₂P may be ascribed to its novel 3D morphology with unique mesoporous structure.     

Heterogeneous Cu1.92S@Cu3P/Ni2P Nanospheres on Nickel Foam for Effective Electrocatalytic Oxygen Evolution Reaction**

The design and exploration of highly efficient and economical electrocatalysts for large-scale hydrogen generation from electrochemical water splitting is of great importance towards hydrogen economy. Herein, we have synthesized heterogeneous Cu_1.92S@Cu₃P/Ni₂P nanospheres on nickel foam, which exhibits excellent OER activity in alkaline solution. When used as an anode, the overpotential of Cu_1.92S@Cu₃P/Ni₂P at the current density of 10 mA cm⁻² is only 211 mV, with a good stability (24 h). The heterogeneous Cu_1.92S@Cu₃P/Ni₂P nanospheres has synergistic modulation of electronic structure, abundant active sites and fast electron transfer rate, which contributes to its high OER activity.     

Improving the Electrocatalytic Activity of a Nickel-Organic Framework toward the Oxygen Evolution Reaction through Vanadium Doping

Metal-organic frameworks (MOFs) have been considered as potential oxygen evolution reaction (OER) electrocatalysts owning to their ultra-thin structure, adjustable composition, high surface area, and high porosity. Here, we designed and fabricated a vanadium-doped nickel organic framework (V_1−x−Ni_xMOF) system by using a facile two-step solvothermal method on nickel foam (NF). The doping of vanadium remarkably elevates the OER activity of V_1−x−Ni_xMOF, thus demonstrating better performance than the corresponding single metallic Ni-MOF, NiV-MOF and RuO₂ catalysts at high current density (>400 mA cm⁻²). V_0.09−Ni_0.91MOF/NF provides a low overpotential of 235 mV and a small Tafel slope of 30.3 mV dec⁻¹ at a current density of 10 mA cm⁻². More importantly, a water-splitting device assembled with Pt/C/NF and V_0.09−Ni_0.91MOF/NF as cathode and anode yielded a cell voltage of 1.96 V@1000 mA cm⁻², thereby outperforming the-state-of-the-art RuO₂⁽⁺⁾||Pt/C⁽⁻⁾. Our work sheds new insight on preparing stable, efficient OER electrocatalysts and a promising method for designing various MOF-based materials.     

In-Situ Generated Trimetallic Molybdate Nanoflowers on Ni Foam Assisted with Microwave for Highly Enhanced Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is considered as a critical half-cell reaction of water splitting, the kinetics of which is sluggish even not favored, thus requiring highly active electrocatalysts to shrink the reaction energy barrier and improve the energy conversion efficiency. In this study, In-situ generated trimetallic molybdate nanoflowers on Ni foam by a straightforward and time-saving solvothermal method assisted with microwave, not only bring synergistic effect into full play between multiple metals, but also construct a well-defined nanoflower-like structure accompanied by larger specific area (273.3 m² g⁻¹) and smaller size than the pristine NiMoO₄. The resulting Ni_0.9Al_0.1MoO₄-NF requires a relatively low overpotential of 266 mV for OER at 10 mA cm⁻², which outperforms commercial RuO₂ catalysts (274 mV). Such excellent performance compares favorably to most previously reported NiMoO₄-based electrocatalysts for OER. This work not only supplies a facile method to construct a well-defined nanoflower-like structure on foam, but also broadens our horizons into the mechanism of OER in alkaline conditions.     

Flexible Molecular Precursors for Selective Decomposition to Nickel Sulfide or Nickel Phosphide for Water Splitting and Supercapacitance

Herein, the synthesis of three nickel(II) dithiophosphonate complexes of the type [Ni{S₂P(OR)(4-C₆H₄OMe)}₂] [R=H ( 1 ), C₃H₇ ( 2 )] and [Ni{S₂P(OR)(4-C₆H₄OEt}₂] [R=(C₆H₅)₂CH ( 3 )] is described; their structures were confirmed by single-crystal X-ray studies. These complexes were subjected to surfactant/solvent reactions at 300 °C for one hour as flexible molecular precursors to prepare either nickel sulfide or nickel phosphide particles. The decomposition of complex 2 in tri-octylphosphine oxide/1-octadecene (TOPO/ODE), TOPO/tri-n-octylphosphine (TOP), hexadecylamine (HDA)/TOP, and HDA/ODE yielded hexagonal NiS, Ni₂P, Ni₅P₄, and rhombohedral NiS, respectively. Similarly, the decomposition of complex 1 in TOPO/TOP and HDA/TOP yielded hexagonal Ni₂P and Ni₅P₄, respectively, and that of complex 3 in similar solvents led to hexagonal Ni₅P₄, with TOP as the likely phosphorus provider. Hexagonal NiS was prepared from the solvent-less decomposition of complexes 1 and 2 at 400 °C. NiS (rhom) had the best specific supercapacitance of 2304 F g⁻¹ at a scan rate of 2 mV s⁻¹ followed by 1672 F g⁻¹ of Ni₂P (hex). Similarly, NiS (rhom) and Ni₂P (hex) showed the highest power and energy densities of 7.4 kW kg⁻¹ and 54.16 W kg⁻¹ as well as 6.3 kW kg⁻¹ and 44.7 W kg⁻¹, respectively. Ni₅P₄ (hex) had the lowest recorded overpotential of 350 mV at a current density of 50 mA cm⁻² among the samples tested for the oxygen evolution reaction (OER). NiS (hex) and Ni₅P₄ (hex) had the lowest overpotentials of 231 and 235 mV to achieve a current density of 50 mA cm⁻², respectively, in hydrogen evolution reaction (HER) examinations.     

Fe‐Doped Ni3C Nanodots in N‐Doped Carbon Nanosheets for Efficient Hydrogen‐Evolution and Oxygen‐Evolution Electrocatalysis

Uniform Ni₃C nanodots dispersed in ultrathin N‐doped carbon nanosheets were successfully prepared by carburization of the two dimensional (2D) nickel cyanide coordination polymer precursors. The Ni₃C based nanosheets have lateral length of about 200 nm and thickness of 10 nm. When doped with Fe, the Ni₃C based nanosheets exhibited outstanding electrocatalytic properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For example, 2 at % Fe (atomic percent) doped Ni₃C nanosheets depict a low overpotential (292 mV) and a small Tafel slope (41.3 mV dec⁻¹) for HER in KOH solution. An outstanding OER catalytic property is also achieved with a low overpotential of 275 mV and a small Tafel slope of 62 mV dec⁻¹ in KOH solution. Such nanodot‐incorporated 2D hybrid structures can serve as an efficient bifunctional electrocatalyst for overall water splitting.     

A Dendritic Nanostructured Copper Oxide Electrocatalyst for the Oxygen Evolution Reaction

To use water as the source of electrons for proton or CO₂ reduction within electrocatalytic devices, catalysts are required for facilitating the proton‐coupled multi‐electron oxygen evolution reaction (OER, 2 H₂O→O₂+4 H⁺+4 e⁻). These catalysts, ideally based on cheap and earth abundant metals, have to display high activity at low overpotential and good stability and selectivity. While numerous examples of Co, Mn, and Ni catalysts were recently reported for water oxidation, only few examples were reported using copper, despite promising efficiencies. A rationally designed nanostructured copper/copper oxide electrocatalyst for OER is presented. This material derives from conductive copper foam passivated by a copper oxide layer and further nanostructured by electrodeposition of CuO nanoparticles. The generated electrodes are highly efficient for catalyzing selective water oxidation to dioxygen with an overpotential of 290 mV at 10 mA cm⁻² in 1 m NaOH solution.     

Interface‐Engineered Ni(OH)2/β‐like FeOOH Electrocatalysts for Highly Efficient and Stable Oxygen Evolution Reaction

Iron‐based (oxy)hydroxides are especially attractive electrocatalysts for the oxygen evolution reaction (OER) owing to their earth abundance, low cost, and nontoxicity. However, poor OER kinetics on the surface restricts the performance of the FeOOH electrocatalyst. Herein, a highly efficient and stable Ni(OH)₂/β‐like FeOOH electrocatalyst is obtained by facile electroactivation treatment. The activated Ni(OH)₂/β‐like FeOOH sample indicates an overpotential of 300 mV at 10 mA cm⁻² for the OER, and no clear current decay after 50 h of testing; this is comparable to the most efficient nickel‐ and cobalt‐based electrocatalysts on planar substrates. Furthermore, studies suggest that β‐like FeOOH plays a key role in remarkably enhancing the performance during the electroactivation process owing to its metastable tunnel structure with a lower barrier for interface diffusion of Ni²⁺ ions between the bilayer electrocatalyst. This study develops a new strategy to explore efficient and low‐cost electrocatalysts and deepens understanding of bilayer electrocatalysts for the OER.     

Engineering Lithium Ions Embedded in NiFe Layered Double Hydroxide Lattices To Activate Laminated Ni2+ Sites as High-Efficiency Oxygen Evolution Reaction Catalysts

NiFe layered double hydroxides (LDHs) have been denoted as benchmark non-noble-metal electrocatalysts for the oxygen evolution reaction (OER). However, for laminates of NiFe LDHs, the edge sites are active, but the basal plane is inert, leading to underutilization as catalysts for the OER. Herein, for the first time, light and electron-deficient Li ions are intercalated into the basal plane of NiFe LDHs. The results of theoretical calculations and experiments both showed that electrons would be transferred from near Ni²⁺ to the surroundings of Li⁺, resulting in electron-deficient properties of the Ni sites, which would function as “electron-hungry” sites, to enhance surface adsorption of electron-rich oxygen-containing groups, which would enhance the effective activity for the OER. As demonstrated by the catalytic performance, the Li−NiFe LDH electrodes showed an ultralow overpotential of only 298 mV at 50 mA cm⁻², which was lower than that of 347 mV for initial NiFe LDHs and lower than that of 373 mV for RuO₂. Reasonable intercalation adjustment effectively activates laminated Ni²⁺ sites and constructs the electron-deficient structure to enhance its electrocatalytic activity, which sheds light on the functional treatment of catalytic materials.     

Atmospheric-Temperature Chain Reaction towards Ultrathin Non-Crystal-Phase Construction for Highly Efficient Water Splitting

Combining the self-sacrifice of a highly crystalline substance to design a multistep chain reaction towards ultrathin active-layer construction for high-performance water splitting with atmospheric-temperature conditions and an environmentally benign aqueous environment is extremely intriguing and full of challenges. Here, taking cobalt carbonate hydroxides (CCHs) as the initial crystalline material, we choose the Lewis acid metal salt of Fe(NO₃)₃ to induce an aqueous-phase chain reaction generating free CO₃²⁻ ions with subsequent instant FeCO₃ hydrolysis. The resultant ultrathin (∼5 nm) amorphous Fe-based hydroxide layer on CCH results in considerable activity in catalyzing the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), yielding 10/50 mA ⋅ cm⁻² at overpotentials of 230/266.5 mV for OER and 72.5/197.5 mV for HER. The catalysts can operate constantly in 1.0 M KOH over 48 and 45 h for the OER and HER, respectively. For bifunctional catalysis for alkaline electrolyzer assembly, a cell voltage as low as 1.53 V was necessary to yield 10 mA cm⁻² (1.7 V at 50 mA cm⁻²). This work rationally builds high-efficiency electrochemical bifunctional water-splitting catalysts and offers a trial in establishing a controllable nanolevel ultrathin lattice disorder layer through an atmospheric-temperature chemical route.     

A Nickel Dithiolate Water Reduction Catalyst Providing Ligand‐Based Proton‐Coupled Electron‐Transfer Pathways

A nickel pyrazinedithiolate ([Ni(dcpdt)₂]²⁻; dcpdt=5,6‐dicyanopyrazine‐2,3‐dithiolate), bearing a NiS₄ core similar to the active center of [NiFe] hydrogenase, is shown to serve as an efficient molecular catalyst for the hydrogen evolution reaction (HER). This catalyst shows effectively low overpotentials for HER (330–400 mV at pH 4–6). Moreover, the turnover number of catalysis reaches 20 000 over the 24 h electrolysis with a high Faradaic efficiency, 92–100 %. The electrochemical and DFT studies reveal that diprotonated one‐electron‐reduced species (i.e., [Ni^II(dcpdt)(dcpdtH₂)]⁻ or [Ni^II(dcpdtH)₂]⁻) forms at pH<6.4 via ligand‐based proton‐coupled electron‐transfer (PCET) pathways, leading to electrocatalytic HER without applying the highly negative potential required to generate low‐valent nickel intermediates. This is the first example of catalysts exhibiting such behavior.     

Enhanced Metal-Support Interactions Boost the Electrocatalytic Water Splitting of Supported Ruthenium Nanoparticles on a Ni3N/NiO Heterojunction at Industrial Current Density

Developing highly efficient and stable hydrogen production catalysts for electrochemical water splitting (EWS) at industrial current densities remains a great challenge. Herein, we proposed a heterostructure-induced-strategy to optimize the metal-support interaction (MSI) and the EWS activity of Ru-Ni₃N/NiO. Density functional theory (DFT) calculations firstly predicted that the Ni₃N/NiO-heterostructures can improve the structural stability, electronic distributions, and orbital coupling of Ru-Ni₃N/NiO compared to Ru-Ni₃N and Ru-NiO, which accordingly decreases energy barriers and increases the electroactivity for EWS. As a proof-of-concept, the Ru-Ni₃N/NiO catalyst with a 2D Ni₃N/NiO-heterostructures nanosheet array, uniformly dispersed Ru nanoparticles, and strong MSI, was successfully constructed in the experiment, which exhibited excellent HER and OER activity with overpotentials of 190 mV and 385 mV at 1000 mA cm⁻², respectively. Furthermore, the Ru-Ni₃N/NiO-based EWS device can realize an industrial current density (1000 mA cm⁻²) at 1.74 V and 1.80 V under alkaline pure water and seawater conditions, respectively. Additionally, it also achieves a high durability of 1000 h (@ 500 mA cm⁻²) in alkaline pure water.     

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO₃ perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO₃ induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO₃; the former exhibit a low onset overpotential of 380 mV at 10 mA cm⁻² and a small Tafel slope of 70.8 mV dec⁻¹. Oxygen vacancy formation is accompanied by mixed Ni²⁺/Ni³⁺ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO₃ in accordance with the enhanced OER activity that is observed.     

A Surface‐Oxide‐Rich Activation Layer (SOAL) on Ni2Mo3N for a Rapid and Durable Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is key to renewable energy technologies such as water electrolysis and metal–air batteries. However, the multiple steps associated with proton‐coupled electron transfer result in sluggish OER kinetics and catalysts are required. Here we demonstrate that a novel nitride, Ni₂Mo₃N, is a highly active OER catalyst that outperforms the benchmark material RuO₂. Ni₂Mo₃N exhibits a current density of 10 mA cm^?2 at a nominal overpotential of 270 mV in 0.1 m KOH with outstanding catalytic cyclability and durability. Structural characterization and computational studies reveal that the excellent activity stems from the formation of a surface‐oxide‐rich activation layer (SOAL). Secondary Mo atoms on the surface act as electron pumps that stabilize oxygen‐containing species and facilitate the continuity of the reactions. This discovery will stimulate the further development of ternary nitrides with oxide surface layers as efficient OER catalysts for electrochemical energy devices.     

Red Bean Pod Derived Heterostructure Carbon Decorated with Hollow Mixed Transition Metals as a Bifunctional Catalyst in Zn-Air Batteries

Design and synthesis of low-cost and efficient bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Zn-air batteries are essential and challenging. We report a facile method to synthesize heterostructure carbon consisting of graphitic and amorphous carbon derived from the agricultural waste of red bean pods. The heterostructure carbon possesses a large surface area of 625.5 m² g⁻¹, showing ORR onset potential of 0.89 V vs. RHE and OER overpotential of 470 mV at 5 mA cm⁻². Introducing hollow FeCo nanoparticles and nitrogen dopant improves the bifunctional catalytic activity of the carbon, delivering ORR onset potential of 0.93 V vs. RHE and OER overpotential of 360 mV. Electron energy-loss spectroscopy (EELS) O K-edge map suggests the presence of localized oxygen on the FeCo nanoparticles, suggesting the oxidation of the nanoparticles. Zn-air battery with these carbon-based catalysts exhibits a peak power density as high as 116.2 mW cm⁻² and stable cycling performance over 210 discharge/charge cycles. This work contributes to the advancement of bifunctional oxygen electrocatalysts while converting agricultural waste into value-added material.