Ni and Co Active Site Transition and Competition in Fluorine-Doped NiCo(OH)2 LDH Electrocatalysts for Oxygen Evolution Reaction

The oxygen evolution reaction (OER) performance of NiCo LDH electrocatalysts can be improved through fluorine doping. The roles of Ni and Co active sites in such catalysts remain ambiguous and controversial. In addressing the issue, this study draws upon the molecular orbital theory and proposes the active center competitive mechanism between Ni and Co. The doped F-atoms can directly impact the valence state of metal atoms or exert an indirect influence through the dehydrogenation, thereby modulating the active center. As the F-atoms are progressively aggregate, the eg orbitals of Ni and Co transition from e2 g to e1 g, and subsequently to e0 g. The corresponding valence state elevates from +2 to +3, and then to +4, signifying an initial increase followed by a subsequent decrease in the electrocatalytic performance. Furthermore, a series of F-NiCo LDH catalysts are synthesized to verify the eg orbital occupancy analysis, and the catalytic OER overpotentials are 303, 243, 240, and 246 mV at the current density of 10 mA cm-2, respectively, which coincides well with the theoretical prediction. This investigation not only provides novel mechanistic insights into the transition and competition of Ni and Co in F-NiCo LDH catalysts but also establishes a foundation for the design of high-performance catalysts.

Hollow-structured cobalt sulfide electrocatalyst for alkaline oxygen evolution reaction: Rational tuning of electronic structure using iron and fluorine dual-doping strategy

Utilizing renewable electricity for water electrolysis offers a promising way for generating high-purity hydrogen gases while mitigating the emission of environmental pollutants. To realize the water electrolysis, it is necessary to develop highly active and precious metal-free electrocatalyst for oxygen evolution reaction (OER) which incurs significant overpotential due to its complicated four-electron transfer mechanism. Hence, we propose a facile preparation method for hollow-structured Fe and F dual-doped CoS2 nanosphere (Fe-CoS2-F) as an efficient OER electrocatalyst. The uniform hollow and porous structure of Fe-CoS2-F enlarge the specific surface area and increase the number of exposed active sites. Furthermore, the Fe and F dual-dopants synergistically contributed to the adjustment of electronic structure, thereby promoting the adsorption/desorption of oxygen-containing reaction intermediates on active sites during the alkaline OER procedure. As a result, the prepared Fe-CoS2-F exhibits outstanding OER activity, characterized by a low overpotential of 298 mV to achieve a current density of 10 mA cm-2 and a Tafel slope as small as 46.0 mV dec-1. Based on computational theoretical calculations, the introduction of the dual-dopants into CoS2 structure reduce the excessively strong adsorption energy of reaction intermediate in the rate determining step, leading to effectively promoted electrocatalytic cycle for OER in alkaline environment. This study presents an effective strategy for preparing noble metal-free OER electrocatalysts with promising potential for large-scale industrial water electrolysis.

Progress in Manipulating Dynamic Surface Reconstruction via Anion Modulation for Electrocatalytic Water Oxidation

The development of efficient and economical electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the sustainable production of renewable fuels and energy storage systems; however, the sluggish OER kinetics involving multistep four proton-coupled electron transfer hampers progress in these systems. Fortunately, surface reconstruction offers promising potential to improve OER catalyst design. Anion modulation plays a crucial role in controlling the extent of surface reconstruction and positively persuading the reconstructed species' performances. This review starts by providing a general explanation of how various types of anions can trigger dynamic surface reconstruction and create different combinations with pre-catalysts. Next, the influences of anion modulation on manipulating the surface dynamic reconstruction process are discussed based on the in situ advanced characterization techniques. Furthermore, various effects of survived anionic groups in reconstructed species on water oxidation activity are further discussed. Finally, the challenges and prospects for the future development directions of anion modulation for redirecting dynamic surface reconstruction to construct highly efficient and practical catalysts for water oxidation are proposed.

Facile route of fluorine incorporation into nickel (oxy)hydroxide for improving the oxygen evolution reaction

Despite extensive development, oxygen evolution reaction (OER) catalysts still require significant overpotentials to function. In this study, we show that the overpotential of a nickel (Ni) electrode for the OER can decrease by about 100 mV with fluorine (F) incorporation, particularly by a facile electrochemical approach at room temperature.

Tailoring Spin State of Perovskite Oxides by Fluorine Atom Doping for Efficient Oxygen Electrocatalysis

Promoting the initially deficient but economical catalysts to high-performing competitors is important for developing superior catalysts. Unlike traditional nano-morphology construction methods, this work focuses on intrinsic catalytic activity enhancement via heteroatom doping strategies to induce lattice distortion and optimize spin-dependent orbital interaction to alter charge transfer between catalysts and reactants. Experimentally, a series of different concentrations of fluorine-doped lanthanum cobaltate (F_x -LaCoO₃ ) exhibiting excellent electrocatalytic activity is synthesized, including a low overpotential of 390 mV at j = 10 mA cm^-2 for OER and a large half-wave potential of 0.68 V for ORR. Meanwhile, the assembled rechargeable Zn-air batteries deliver an excellent performance with a large specific capacity of 811 mAh/g_Zn under 10 mA cm^-2 and stability of charge/recharge (120 h). Theoretically, taking advantage of density functional theory calculations, it is found that the prominent OER/ORR performance arises from the spin state transition of Co³⁺ (Low spin state (LS, t_2g ⁶ e_g ⁰ ) → Intermediate spin state (IS, t_2g ⁵ e_g ¹ ) and the mediated d-band center upshift by F atom incorporation. This work establishes a novel avenue for designing superior electrocatalysts in perovskite-based oxides by regulating spin states.

Self-supported molybdenum nickel oxide catalytic electrode designed via molecular cluster-mediated electroplating and electrochemical activation for an efficient and durable oxygen evolution reaction

Efficient and durable nonprecious catalysts for the oxygen evolution reaction (OER) are crucial for practical water electrolysis for hydrogen production. A self-supported OER catalytic electrode with sufficient exposure of the catalyst and tight anchoring onto the current collector is vital for the catalytic activity and stability, and is therefore deemed to be a preferable tactic to enhance water electrolysis performance. Herein, a polyoxometalate (POM) molecular cluster-mediated electroplating and activation tactics are proposed to design a self-supported molybdenum nickel oxide (MoNiO_x) catalytic electrode for the OER. The MoNiO_x active layer can anchor tightly onto the Ni foam current collector with sufficient surface exposure and high structural stability, therefore enabling high alkali OER catalytic efficiency (222 mV at 10 mA cm^-2) and robust durability (only slight decay in catalytic efficiency upon 12 days of chronopotentiometry (V-t) test). Moreover, the easily processable electroplating and active protocol can serve as a general approach to prepare other OER catalytic electrodes by altering the reactants and current collectors. The current work paves a facile and universal way to design a highly active and durable molybdenum (Mo) based hybrid catalytic electrode for OER via molecular cluster-assisted electroplating and activation treatment.

Construction of NiCoZnS materials with controllable morphology for high-performance supercapacitors

A facile two-step hydrothermal approach with post-sulfurization treatment was put forward to construct the mixed transition metal sulfide (NiCoZnS) with a high electrochemical performance. The different morphologies of NiCoZnS materials were successfully fabricated by adjusted the Ni/Co molar ratio of the NiCoZn(OH)F precursor. Moreover, thein situphase transformation from the NiCoZn(OH)F phase to Zn_0.76Co_0.24S and NiCo₂S₄phases and lattice defects via the S^2-ion-exchange were determined by x-ray diffractometer, transmission electron microscopy and x-ray photoelectron spectroscopy techniques, which improved electric conductivity and interfacial active sites of the NiCoZnS, and so promoted the reaction kinetics. Significantly, the urchin-like NiCoZnS_1/1prepared at the Ni/Co molar ratio of 1.0 exhibited promising electrochemical performances with high capacitance and excellent cycling stability. Furthermore, the asymmetric device (NiCoZnS//AC) using NiCoZnS_1/1as the positive electrode had excellent supercapacitor performances with an energy density of 57.8 Wh·kg^-1at a power density of 750 W·kg^-1as well as a long cycle life (79.2% capacity retention after 10 000 cycles), indicating the potential application in high-performance supercapacitors.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

An Extreme Energy-Saving Carbohydrazide Oxidization Reaction Directly Driven by Commercial Graphite Paper in Alkali and Near-Neutral Seawater Electrolytes

The energy-saving anode with low oxidization potential has been an intriguing pursue for earth-abundant seawater electrolysis. In this paper, we first introduced a superior energy-saving carbohydrazide oxidization reaction catalysis system in the anode section, which can be driven by commercial graphite paper with good durability. Combining this catalysis reaction and common graphite paper, the lowest anodic potentials 0.63 V (vs RHE) and 1.09 V (vs RHE) were obtained for driving a 10 mA/cm² current density in alkali and near-neutral seawater electrolytes, respectively, outperforming all the as-reported alkali or near-neutral seawater catalysts accordingly to the best of our knowledge.

Anodized Nickel Foam for Oxygen Evolution Reaction in Fe-Free and Unpurified Alkaline Electrolytes at High Current Densities

To achieve practically high electrocatalytic performance for the oxygen evolution reaction (OER), the active surface area should be maximized without severely compromising electron and mass transport throughout the catalyst electrode. Though the importance of electron and mass transport has been studied using low surface area catalysts under low current densities (∼tens of mA/cm²), the transport properties of large surface area catalysts under high operating current densities (∼500 mA/cm²) for practical OER catalysis have rarely been explored. Herein, three-dimensional (3D) hierarchically porous anodized nickel foams (ANFs) with large and variable surface areas were synthesized via electrochemical anodization of 3D nickel foam and applied as OER electrocatalysts in Fe-free and unpurified KOH electrolytes. Using Fe-free and in situ Fe-doped ANF that were prepared in Fe-free and unpurified electrolytes, respectively, we investigated the interdependent effects of active surface area and transport properties on OER activity under practically high current densities. While activity increased linearly with active surface area for Fe-free ANF, the activity of Fe-doped ANF showed a nonlinear increase with active surface area due to lower electrocatalytic activity enhancement. Detailed investigations on the possible factors (Fe incorporation, mass transport, and electron transport) identified that electron transport limitations played the major role in restricting the activity enhancement with increasing active surface area for Fe-doped ANF, although Fe-doped ANF has electron transport properties better than those of Fe-free ANF. This study exemplifies the growing significance of electron transport properties in large surface area catalysts, especially those with superb intrinsic catalytic activity and high operating current density.

Bismuth iron molybdenum oxide solid solution: a novel and durable electrocatalyst for overall water splitting

The drive for finding active bifunctional electrocatalysts for efficient overall water splitting continues in order to extract energy in the form of hydrogen as a clean fuel. Bismuth iron molybdenum oxide solid solution, composed of orthorhombic Bi2MoO6 as the major component and monoclinic Bi3(FeO4)(MoO4)2 as the minor component, has been identified as a potential electrocatalyst for the first time.

The effect of Fe(III) cations in electrolyte on oxygen evolution catalytic activity of Ni(OH)2 electrode

A facile and efficient oxygen evolution reaction (OER) catalytic system was constructed based on interconnected Ni(OH)₂ nanosheets arrays electrode and Fe(III) containing alkali electrolyte. The partial deposition of Fe(III) onto Ni(OH)₂ with heterostructure obviously enhanced the OER current density and reduced the overpotential, and the Fe(III) cations in electrolyte also furnished positive contribution to the catalytic reaction, the synergy between Ni(OH)₂ electrode and Fe(III) cations in electrolyte therefore substantially improved the OER catalytic efficiency with low overpotential (285 mV at current density of 50 mA cm^-2) and obviously enhanced current density (391 mA cm^-2 at 1.8 V). The current work expands our understanding on the effect of Fe(III) cations in electrolyte to OER efficiency of Ni based catalyst, and opens up a cost-effective and practical avenue to enhance the OER catalytic efficiency by introducing metal cations in alkali electrolyte.

Confinement of fluorine anions in nickel-based catalysts for greatly enhancing oxygen evolution activity

A simple fluorine treatment was developed to confine abundant fluorine anions into Ni-based catalysts to greatly enhance OER catalytic activity.

Modulation of crystal water in cobalt phosphate for promoted water oxidation

Cobalt phosphate tetrahydrate was identified as the optimal Co-Pi phase for electrocatalytic oxygen evolution reaction.

Quinary Defect-Rich Ultrathin Bimetal Hydroxide Nanosheets for Water Oxidation

The electronic structure of layered double hydroxides (LDHs) can be modulated by heteroatom doping and creating vacancies. The number of exposed active sites can be enriched by exfoliating the bulk structure into fewer layers. Herein, we successfully achieved multielement doping and exfoliation for Co₃Fe LDHs by one SF₆-plasma etching step at room temperature (named as Co₃Fe LDHs-SF₆). The obtained Co₃Fe LDHs-SF₆ ultrathin nanosheets display outstanding oxygen evolution reaction (OER) activity, which only needs 268 mV overpotential to reach 10 mA cm^-2. Tafel slope and charge transfer resistance are dramatically decreased indicating a faster reaction kinetic rate. The excellent OER activity can be attributed to an increased number of active sites and an optimized electronic structure modulated by the incorporation of electron-withdrawing F, electron-donating S, and abundant vacancies resulting in proper adsorption energy to oxygen species.

Room temperature thiosulfate ion redox reaction-driven synthesis of a robust porous copper–cobalt–sulfur–oxygen nanowire coating on copper foam for highly-efficient and low-cost oxygen evolution reaction

Room temperature thiosulfate ion redox reaction-driven synthesis of robust porous Cu–Co–S–O NWC on copper foam for highly-efficient and low-cost OER.

Modified bluing treatment to produce nickel–cobalt–iron spinel oxide with promoted oxygen-evolving performance

A facile modified bluing treatment approach was proposed for low-cost, time-saving and batch synthesis of a ternary NiCoFe spinel oxide catalyst with high OER performance.