The preparation of ionic liquid based iron phosphate/CNTs composite via microwave radiation for hydrogen evolution reaction and oxygen evolution reaction

Water electrolysis is a promising method for hydrogen production, so the preparation of low-cost and efficient electrocatalysts with a quick and simple procedure is crucial. Herein, iron phosphate (Fe₇(PO₄)₆) was prepared via microwave radiation using ionic liquid (IL) as iron and phosphorus dual-source. This method is simple and rapid, and the product can be directly used as electrocatalysts without further treatment. The experimental results show that the IL can influence the morphology and electrocatalytic performance. Moreover, the addition of carbon nanotubes (CNTs) is favorable for formation of iron phosphate nanoparticles to improve the catalytic activities. As hydrogen evolution reaction (HER) catalyst, this iron phosphate/CNTs exhibits an onset overpotential of 120 mV, Tafel slope of 32.9 mV dec^-1, and current densities of 10 mA cm⁻² at overpotential of 185 mV. Then, it obtains a good activity for oxygen evolution reaction (OER) with a low onset potential of 1.48 V, Tafel slope of 73.3 mV dec^-1, and it only needs an overpotential of 300 mV to drive the 10 mA cm⁻². This bifunctional catalyst also shows good durability for HER and OER. This microwave-assisted method provides an outstanding strategy to prepare iron phosphate in a simple and fast process with good catalytic performance for water splitting.     

Iron–Cobalt Phosphomolybdate with High Electrocatalytic Activity for Oxygen Evolution Reaction

Iron–cobalt phosphomolybdate (FeCoPM₁₂) nanoparticles, which are highly efficient catalytic materials for the oxygen evolution reaction (OER), were fabricated through a coprecipitation route. Compared with iron–cobalt hydroxide and state‐of‐the‐art RuO₂ electrocatalysts, the as‐prepared FeCoPM₁₂ sample exhibited robust OER catalytic activity with a low overpotential of 258 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 33 mV dec⁻¹. Moreover, the as‐synthesized sample presented preferable stability and after 10 h at 1.52 V the current density degraded by merely 8.3 %. This is ascribed to the high electrochemical stability and small porous structure of FeCoPM₁₂, which provide effective electron transmission and improve the catalytic performance for OER in alkaline media.     

CoS2 Nanoparticles Embedded in Covalent Organic Polymers as Efficient Electrocatalyst for Oxygen Evolution Reaction with Ultralow Overpotential

Cobalt disulfide (CoS₂) has been explored as attractive electrocatalyst for oxygen evolution reaction (OER). However, bulk CoS₂ sheets have limited catalytic activity due to low exposure of active sites. Herein, through an in-situ vulcanization approach, CoS₂ nanoparticles are embedded into bipyridine-containing covalent organic polymer (BP-COP). The as-prepared nanocomposite CoS₂@BP-COP exhibits high catalytic activity toward OER with an ultra-low overpotential of 270 mV (vs. RHE) at a current density of 10 mA cm⁻², a small Tafel slope of 36 mV dec⁻¹, and an excellent durability for 24 h without decay. The surface of CoS₂ is partially converted into CoOOH to form CoS₂/CoOOH as active sites under OER conditions. CoS₂@BP-COP displays superior OER catalytic activity to CoS₂ nanosheets and commercially available RuO₂ under the same conditions. The outstanding OER performance activity of CoS₂@BP-COP could be attributed to the uniform and small particle sizes of CoS₂/CoOOH distributed in BP-COP.     

Laser-Ablation-Produced Cobalt Nickel Phosphate with High-Valence Nickel Ions as an Active Catalyst for the Oxygen Evolution Reaction

Cost-effective, highly efficient and stable non-noble metal-based catalysts for the oxygen evolution reaction (OER) are very crucial for energy storage and conversion. Here, an amorphous cobalt nickel phosphate (CoNiPO₄), containing a considerable amount of high-valence Ni³⁺ species as an efficient electrocatalyst for OER in alkaline solution, is reported. The catalyst was converted from Co-doped Ni₂P through pulsed laser ablation in liquid (PLAL) and exhibits a large specific surface area of 162.5 m² g⁻¹ and a low overpotential of 238 mV at 10 mA cm⁻² with a Tafel slope of 46 mV dec⁻¹, which is much lower than those of commercial RuO₂ and IrO₂. This work demonstrates that PLAL is a powerful technology for generating amorphous CoNiPO₄ with high-valence Ni³⁺, thus paving a new way towards highly effective OER 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.     

Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction

Designing highly efficient electrocatalysts for oxygen evolution reaction (OER) plays a key role in the development of various renewable energy storage and conversion devices. In this work, we developed metallic Co₄N porous nanowire arrays directly grown on flexible substrates as highly active OER electrocatalysts for the first time. Benefiting from the collaborative advantages of metallic character, 1D porous nanowire arrays, and unique 3D electrode configuration, surface oxidation activated Co₄N porous nanowire arrays/carbon cloth achieved an extremely small overpotential of 257 mV at a current density of 10 mA cm⁻², and a low Tafel slope of 44 mV dec⁻¹ in an alkaline medium, which is the best OER performance among reported Co‐based electrocatalysts to date. Moreover, in‐depth mechanistic investigations demonstrate the active phases are the metallic Co₄N core inside with a thin cobalt oxides/hydroxides shell during the OER process. Our finding introduces a new concept to explore the design of high‐efficiency OER electrocatalysts.     

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.     

Trimetallic Oxyhydroxide Coralloids for Efficient Oxygen Evolution Electrocatalysis

Trimetallic oxyhydroxides are one of the most effective materials for oxygen evolution reaction (OER) catalysis, a key process for water splitting. Herein, we describe a facile wet‐chemical method to directly grow a series of coralloid trimetallic oxyhydroxides on arbitrary substrates such as nickel foam (NF) and carbon nanotubes (CNTs). The amount of iron in these oxyhydroxide sponges on NF and CNTs was precisely controlled, revealing that the electrocatalytic activity of the WCoFe trimetallic oxyhydroxides depends on the Fe amount in a volcano‐like fashion. The optimized W_0.5Co_0.4Fe_0.1/NF catalyst exhibited an overpotential of only 310 mV to deliver a large current density of 100 mA cm⁻² and a very low Tafel slope of 32 mV dec⁻¹. It also showed superior stability with negligible activity decay after use in the OER for 21 days (>500 h). X‐ray photoelectron spectroscopy revealed that the addition of Fe leads to an on average lower Co oxidation state, which contributes to the enhanced OER performance.     

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.     

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.     

Cooperative Ni(Co)-Ru-P Sites Activate Dehydrogenation for Hydrazine Oxidation Assisting Self-powered H2 Production

Water electrolysis for H₂ production is restricted by the sluggish oxygen evolution reaction (OER). Using the thermodynamically more favorable hydrazine oxidation reaction (HzOR) to replace OER has attracted ever-growing attention. Herein, we report a twisted NiCoP nanowire array immobilized with Ru single atoms (Ru₁−NiCoP) as superior bifunctional electrocatalyst toward both HzOR and hydrogen evolution reaction (HER), realizing an ultralow working potential of −60 mV and overpotential of 32 mV for a current density of 10 mA cm⁻², respectively. Inspiringly, two-electrode electrolyzer based on overall hydrazine splitting (OHzS) demonstrates outstanding activity with a record-high current density of 522 mA cm⁻² at cell voltage of 0.3 V. DFT calculations elucidate the cooperative Ni(Co)−Ru−P sites in Ru₁−NiCoP optimize H* adsorption, and enhance adsorption of *N₂H₂ to significantly lower the energy barrier for hydrazine dehydrogenation. Moreover, a self-powered H₂ production system utilizing OHzS device driven by direct hydrazine fuel cell (DHzFC) achieve a satisfactory rate of 24.0 mol h⁻¹ m⁻².     

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 Bifunctional Electrocatalyst for OER and ORR based on a Cobalt(II) Triazole Pyridine Bis-[Cobalt(III) Corrole] Complex

As alternative energy sources are essential to reach a climate-neutral economy, hydrogen peroxide (H₂O₂) as futuristic energy carrier gains enormous awareness. However, seeking for stable and electrochemically selective H₂O₂ ORR electrocatalyst is yet a challenge, making the design of—ideally—bifunctional catalysts extremely important and outmost of interest. In this study, we explore the application of a trimetallic cobalt(II) triazole pyridine bis-[cobalt(III) corrole] complex Co^IITP[Co^IIIC]₂ 3 in OER and ORR catalysis due to its remarkable physicochemical properties, fast charge transfer kinetics, electrochemical reversibility, and durability. With nearly 100 % selective catalytic activity towards the two-electron transfer generated H₂O₂, an ORR onset potential of 0.8 V vs RHE and a cycling stability of 50 000 cycles are detected. Similarly, promising results are obtained when applied in OER catalysis. A relatively low overpotential at 10 mA cm⁻² of 412 mV, Faraday efficiency 98 % for oxygen, an outstanding Tafel slope of 64 mV dec⁻¹ combined with superior stability.     

Heterostructured Inter-Doped Ruthenium–Cobalt Oxide Hollow Nanosheet Arrays for Highly Efficient Overall Water Splitting

The development of transition-metal-oxides (TMOs)-based bifunctional catalysts toward efficient overall water splitting through delicate control of composition and structure is a challenging task. Herein, the rational design and controllable fabrication of unique heterostructured inter-doped ruthenium–cobalt oxide [(Ru–Co)O_x] hollow nanosheet arrays on carbon cloth is reported. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru–Co)O_x nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, the catalyst showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm⁻² and a small Tafel slope of 23.5 mV dec⁻¹, as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm⁻². As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm⁻² for alkaline overall water splitting.     

Sulfur-Modified Oxygen Vacancies in Iron–Cobalt Oxide Nanosheets: Enabling Extremely High Activity of the Oxygen Evolution Reaction to Achieve the Industrial Water Splitting Benchmark

The oxygen vacancies of defective iron–cobalt oxide (FeCoO_x-Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co−S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoO_x-Vo-S to exhibit much superior OER activity. FeCoO_x-Vo-S exhibits a mass activity of 2440.0 A g⁻¹ at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO₂. The Tafel slope is as low as 21.0 mV dec⁻¹, indicative of its excellent charge transfer rate. When FeCoO_x-Vo-S (anode catalyst) is paired with the defective CoP₃/Ni₂P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm⁻² and 406.0 mA cm⁻² at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved.     

Fast Modulation of d-Band Holes Quantity in the Early Reaction Stages for Boosting Acidic Oxygen Evolution

Synthesis of highly active and durable oxygen evolution reaction (OER) catalysts applied in acidic water electrolysis remains a grand challenge. Here, we construct a type of high-loading iridium single atom catalysts with tunable d-band holes character (h-HL−Ir SACs, ∼17.2 wt % Ir) realized in the early OER operation stages. The in situ X-ray absorption spectroscopy reveals that the quantity of the d-band holes of Ir active sites can be fast increased by 0.56 unit from the open circuit to a low working potential of 1.35 V. More remarkably, in situ synchrotron infrared and Raman spectroscopies demonstrate the quick accumulation of *OOH and *OH intermediates over holes-modulated Ir sites in the early reaction voltages, achieving a rapid OER kinetics. As a result, this well-designed h-HL−Ir SACs exhibits superior performance for acidic OER with overpotentials of 216 mV @10 mA cm⁻² and 259 mV @100 mA cm⁻², corresponding to a small Tafel slope of 43 mV dec⁻¹. The activity of catalyst shows no evident attenuation after 60 h operation in acidic environment. This work provides some useful hints for the design of superior acidic OER catalysts.     

From a Molecular 2Fe‐2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting

A highly active FeSe₂ electrocatalyst for durable overall water splitting was prepared from a molecular 2Fe‐2Se precursor. The as‐synthesized FeSe₂ was electrophoretically deposited on nickel foam and applied to the oxygen and hydrogen evolution reactions (OER and HER, respectively) in alkaline media. When used as an oxygen‐evolution electrode, a low 245 mV overpotential was achieved at a current density of 10 mA cm⁻², representing outstanding catalytic activity and stability because of Fe(OH)₂/FeOOH active sites formed at the surface of FeSe₂. Remarkably, the system is also favorable for the HER. Moreover, an overall water‐splitting setup was fabricated using a two‐electrode cell, which displayed a low cell voltage and high stability. In summary, the first iron selenide material is reported that can be used as a bifunctional electrocatalyst for the OER and HER, as well as overall water splitting.     

Balancing Activity and Stability in Spinel Cobalt Oxides through Geometrical Sites Occupation towards Efficient Electrocatalytic Oxygen Evolution

Designing active and stable oxygen evolution reaction (OER) catalysts are vitally important to various energy conversion devices. Herein, we introduce elements Ni and Mn into (Co)_tet(Co₂)_octO₄ nanosheets (NSs) at fixed geometrical sites, including Mn_oct, Ni_oct, and Ni_tet, to optimize the initial geometrical structure and modulate the CoCo₂O₄ surface from oxygen-excess to oxygen-deficiency. The pristine (Ni,Mn)-(Co)_tet(Co₂)_octO₄ NSs shows excellent OER activity with an overpotential of 281.6 mV at a current density of 10 mA cm⁻². Moreover, without damaging their initial activity, the activated (Act)-(Ni,Mn)-(Co)_tet(Co₂)_octO₄ NSs after surface reconstruction exhibit long-term stability of 100 h under 10 mA cm⁻², 50 mA cm⁻², or even 100 mA cm⁻². The optimal balance between electroactivity and stability leads to remarkable OER performances, providing a pivotal guideline for designing ideal electrocatalysts and inspiring more works to focus on the dynamic change of each occupation site component.     

Controlled Synthesis of Ni-Doped MoS2 Hybrid Electrode for Synergistically Enhanced Water-Splitting Process

The development of high-efficiency, low-cost, and earth-abundant electrocatalysts for overall water splitting remains a challenge. In this work, Ni-modified MoS₂ hybrid catalysts are grown on carbon cloth (Ni-Mo-S@CC) through a one-step hydrothermal treatment. The optimized Ni-Mo-S@CC catalyst shows excellent hydrogen evolution reaction (HER) activity with a low overpotential of 168 mV at a current density of 10 mA cm⁻² in 1.0 m KOH, which is lower than those of Ni-Mo-S@CC (1:1), Ni-Mo-S@CC (3:1), and pure MoS₂. Significantly, the Ni-Mo-S@CC hybrid catalyst also displays outstanding oxygen evolution reaction (OER) activity with a low overpotential of 320 mV at a current density of 10 mA cm⁻², and remarkable long-term stability for 30 h at a constant current density of 10 mA cm⁻². Experimental results and theoretical analysis based on density functional theory demonstrate that the excellent electrocatalytic performance can be attributed mainly to the remarkable conductivity, abundant active sites, and synergistic effect of the Ni-doped MoS₂. This work sheds light on a unique strategy for the design of high-performance and stable electrocatalysts for water-splitting electrolyzers.