NiFe Oxy Hydroxide Research Articles

Modifying d–p orbital hybridization of Ni/Fe[sbnd]O species by high-valence ruthenium doping to enhance oxygen evolution performance

The electron distribution of catalysts can be modulated by high-valence metal doping, thus enhancing the intrinsic activity. Herein, we adopt Ru modification to adjust the d–p orbital hybridization of Ni-Fe oxyhydroxides, significantly increasing the oxygen evolution reaction (OER) activity. The amorphous NiFe0.5Ru0.1OxHy catalyst synthesized by sol–gel method exhibits excellent OER activity, far superior to commercial RuO2. In situ Raman and XPS results confirm that the NiOOH/FeOOH active species gradually form with increasing applied voltage. Density functional theory (DFT) calculations reveal that high-valence Ru dopants can effectively regulate the hybridization of d–p orbital of Ni/FeO, significantly increase the electron density around Fermi level, promote charge transfer, make them have more anti-bonding orbitals, enhance the binding ability of active sites to intermediates, reduce the reaction energy barrier of the rate-determining step (H2O → *OH), and thus improve the OER activity. In addition, NiFeRuOxHy as a cathode catalyst in a rechargeable zinc-air battery shows an outstanding cycle life of 600 h. This study provides a promising hybrid orbital method for designing high-performance OER catalysts for water splitting and rechargeable zinc-air batteries.

Ultrafast synthesis of biphase Ni-doped FeOOH for efficient and stable oxygen evolution at high current density

NiFe oxyhydroxides, generally reconstructed on surface during oxygen evolution reaction (OER), are real active species for water oxidation; however, their direct and convenient preparation remains challenging. Here, we develop a one-step approach to prepare biphase (α/δ) Ni-doped FeOOH catalyst in 3 min under room temperature. The core of this ultrafast method is that Fe2+ derived from the redox reaction of Fe3+ and Ni2+ accelerate Fenton-like reaction, while simultaneously producing mixed-valence Ni ions(Ni2+, Ni3+) results in not only homovalent and heterovalent doping, but also biphase Ni-doped FeOOH heterojunction with high and low crystallinity. Specifically, Ni2+ doping leads to a preferred formation of low-crystalline δ-oriented Ni-doped FeOOH with abundant oxygen vacancies, which is in favor of triggering the lattice oxygen mechanism (LOM) during OER. Benefitting from high/low crystalline biphase heterojunction and LOM, the optimized Ni-FeOOH merely needs low overpotential of 300 mV to reach 1000 mA cm−2 for OER in alkaline electrolyte and also shows excellent durability even at a high current density of 500 mA cm−2. This work provides a cost-effective strategy to fabricate highly active and robust non-noble electrocatalysts that can potentially be applied for industrial-scale OER electrolysis.

Fe-S dually modulated adsorbate evolution and lattice oxygen compatible mechanism for water oxidation.

Simultaneously activating metal and lattice oxygen sites to construct a compatible multi-mechanism catalysis is expected for the oxygen evolution reaction (OER) by providing highly available active sites and mediate catalytic activity/stability, but significant challenges remain. Herein, Fe and S dually modulated NiFe oxyhydroxide (R-NiFeOOH@SO4) is conceived by complete reconstruction of NiMoO4·xH2O@Fe,S during OER, and achieves compatible adsorbate evolution mechanism and lattice oxygen oxidation mechanism with simultaneously optimized metal/oxygen sites, as substantiated by in situ spectroscopy/mass spectrometry and chemical probe. Further theoretical analyses reveal that Fe promotes the OER kinetics under adsorbate evolution mechanism, while S excites the lattice oxygen activity under lattice oxygen oxidation mechanism, featuring upshifted O 2p band centers, enlarged d-d Coulomb interaction, weakened metal-oxygen bond and optimized intermediate adsorption free energy. Benefiting from the compatible multi-mechanism, R-NiFeOOH@SO4 only requires overpotentials of 251 ± 5/291 ± 1 mV to drive current densities of 100/500 mA cm-2 in alkaline media, with robust stability for over 300 h. This work provides insights in understanding the OER mechanism to better design high-performance OER catalysts.

Strong interaction heterointerface of NiFe oxyhydroxide/cerium oxide for efficient and stable water oxidation

NiFe oxyhydroxides (NiFeOxHy) are the benchmark catalysts for alkaline oxygen evolution reaction (OER), however, it is still challenging for these electrocatalysts to achieve both high activity and long-term stability. Herein, we demonstrate that strong interaction heterointerface of NiFeOxHy/cerium oxide (CeO2) can control the reconstruction process of NiFeOxHy and the reaction pathway of OER, which simultaneously improves the activity and stability of the catalyst. Interestingly, after electrochemical activation, with and without CeO2, NiFeOxHy oxyhydroxides exhibit completely different structures, with the former being uniformly distributed ultrafine nanosheets and the latter being spherical agglomerates of larger sized nanosheets. The strong heterointerface interaction induces the electrons of Ni and Fe to shift to Ce. The optimized catalyst exhibits a low overpotential of 240 mV at 100 mA cm−2 with the durability over 2000 h at 200 mA cm−2. The OER mechanism studies indicate that NiFeOxHy/CeO2 heterointerface follows lattice oxygen oxidation mechanism with accelerated kinetics.

Light-Induced Quantum Reconfiguration of Oxyhydroxides for Photoanodes with 4.24% Efficiency and Stability Beyond 250 Hours.

Photoelectrochemical (PEC) water splitting is attracting significant research interest in addressing sustainable development goals in renewable energy. Current state-of-the-art, however, cannot provide photoanodes with simultaneously high efficiency and long-lasting lifetime. Here, large-scale NiFe oxyhydroxides-alloy hybridized co-catalyst layer that exhibits an applied bias photon-to-current efficiency (ABPE) of 4.24% in buried homojunction-free photoanodes and stability over 250 h is reported. These performances represent an increase over the present highest-performing technology by 408% in stability and the most stable competitor by over 330% in efficiency. These results originate from a previously unexplored mechanism of light-induced atomic reconfiguration, which rapidly self-generates a catalytic-protective amorphous/crystalline heterostructure at low biases. This mechanism provides active sites for reaction and insulates the photoanode from performance degradation. Photon-generated NiFe oxyhydroxides are more than 200% higher than the quantity that pure electrocatalysis would otherwise induce, overcoming the threshold for an efficient water oxidation reaction in the device. While of immediate interest in the industry of water splitting, the light-induced NiFe oxyhydroxides-alloy co-catalyst developed in this work provides a general strategy to enhance further the performances and stability of PEC devices for a vast panorama of chemical reactions, ranging from biomass valorization to organic waste degradation, and CO2-to-fuel conversion.

Efficient Alkaline Water Oxidation Electrocatalysis Enabled via the Modulation of Sn‐Containing Anions towards NiFe Oxyhydroxide

AbstractDespite being considered as efficient non‐noble metal‐based electrocatalysts for alkaline oxygen evolution reaction (OER), NiFe (oxy)hydroxides (NiFeOxHy) typically fall short of meeting practical requirements. To address this challenge, herein, we introduce a facile in situ electrochemical incorporation technique to form Sn‐doped NiFe‐layered double hydroxide (NiFeSn‐LDH) precatalysts. Subsequently, under the alkaline OER process, the precatalyst evolves into stannate ion‐adsorbed NiFe oxyhydroxides (NiFeOOH). The presence of these stable stannate oxyanions plays a key role in mitigating Fe leaching, optimizing the electronic structure of NiFeOOH, and improving its reaction kinetics, thereby significantly enhancing alkaline OER performance. Notably, the nickel foam‐supported NiFeSn‐LDH demonstrates impressive results delivering a large current density of 100 mA cm−2 at an overpotential as low as ~260 mV and maintaining the industrial‐relevant ~500 mA cm−2 current density over 5 days with negligible activity decay, surpassing the performances of most of the transition metal‐based electrocatalysts. Comprehensive advanced characterizations of the precatalysts, before and after the OER reactions, have been performed to uncover stable residual stannate ions at the surface of NiFeOxHy and to determine their pivotal role in promoting alkaline OER.

Intercalated and Surface-Adsorbed Phosphate Anions in NiFe Layered Double-Hydroxide Catalysts Synergistically Enhancing Oxygen Evolution Reaction Activity.

The oxygen evolution reaction (OER), a crucial semireaction in water electrolysis and rechargeable metal-air batteries, is vital for carbon neutrality. Hindered by a slow proton-coupled electron transfer, an efficient catalyst activating the formation of an O-H bond is essential. Here, we proposed a straightforward one-step hydrothermal procedure for fabricating PO43--modified NiFe layered double-hydroxide (NiFe LDH) catalysts and investigated the role of PO43- anions in enhancing OER. Phosphate amounts can efficiently regulate LDH morphology, crystallinity, composition, and electronic configuration. The optimized sample showed a low overpotential of 267 mV at 10 mA cm-2. Density functional theory calculations revealed that intercalated and surface-adsorbed PO43- anions in NiFe LDH reduced the Gibbs free energy in the rate-determining step of *OOH formation, balancing oxygen-containing intermediate adsorption/dissociation and promoting the OER. Intercalated phosphate ions accelerated precatalyst dehydrogenation kinetics, leading to a rapid reconstruction into active NiFe oxyhydroxide species. Surface-adsorbed PO43- interacted favorably with adsorbed *OOH on the active Ni sites, stabilizing *OOH. Overall, the synergistic effects of intercalated and surface-adsorbed PO43- anions significantly contributed to enhanced OER activity. Achieving optimal catalytic activity requires a delicate equilibrium between thermodynamic and kinetic factors by meticulously regulating the quantity of introduced PO43- ions. This endeavor will facilitate a deeper comprehension of the influence of anions in electrocatalysis for OER.

Boosting cascade electron transfer in NiFe oxyhydroxide for overall water splitting

Nickel–iron oxyhydroxides are among the most active electrocatalysts, but their sluggish kinetic of oxygen evolution reaction (OER) limits the energy efficiency toward overall water splitting. Here, we present a “cascade electron transfer” strategy through spurring unidirectional electron transfer among different metal sites in Mn-doped FeNiOOH@FeNiP to boost OER and overall water splitting. The Mn doping induces a cascade electron transfer from Ni to Fe and then to Mn via metal-O-metal bridge, thus promoting the oxidation Ni and Fe centers, which in turn help charge transfer by increasing the covalency between metal-O bonds to optimize the bonding strength between metal and adsorbed oxygen species. Consequently, the optimal Mn–FeNiOOH@FeNiP delivers a fast OER kinetics (32.1 mV dec−1) along with a low overpotential of 215 mV@10 mA cm−2. Benefiting from the synergistic effect of high conductivity, large specific surface area, and favorable OER kinetics, the catalyst only requires a low cell voltage of 1.456 V to achieve 20 mA cm−2 for overall water splitting, superior to that of a commercial RuO2ǁPt/C catalyst.

A Cluster-Type Self-Healing Catalyst for Stable Saline–Alkali Water Splitting

In electrocatalytic processes, traditional powder/film electrodes inevitably suffer from damage or deactivation, reducing their catalytic performance and stability. In contrast, self-healing electrocatalysts, through special structural design or composition methods, can automatically repair at the damaged sites, restoring their electrocatalytic activity. Here, guided by Pourbaix diagrams, foam metal was activated by a simple cyclic voltammetry method to synthesize metal clusters dispersion solution (MC/KOH). The metal clusters-modified hydroxylated Ni-Fe oxyhydroxide electrode (MC/NixFeyOOH) by a facile Ni-Fe metal–organic framework-reconstructed strategy, exhibiting superior performance toward the oxygen evolution reaction (OER) in the mixture of MC/KOH and saline–alkali water (MC/KOH+SAW). Specifically, using a nickel clusters-modified hydroxylated Ni-Fe oxyhydroxide electrode (NC/NixFeyOOH) for OER, the NC/NixFeyOOH catalyst has an ultra-low overpotential of 149 mV@10 mA cm−2, and durable stability of 100 h at 500 mA cm−2. By coupling this OER catalyst with an efficient hydrogen evolution reaction catalyst, high activity and durability in overall SAW splitting is exhibited. What is more, benefiting from the excellent fluidity, flexibility, and enhanced catalytic activity effect of the liquid NC, we demonstrate a self-healing electrocatalysis system for OER operated in the flowing NC/(KOH+SAW). This strategy provides innovative solutions for the fields of sustainable energy and environmental protection.

Polyoxometalate-incorporated NiFe-based oxyhydroxides for enhanced oxygen evolution reaction in alkaline media.

NiFe-based oxyhydroxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline media, but further enhancing their OER performance remains a significant challenge. Herein, we in situ incorporated polyoxometalates into NiFe oxyhydroxides to form a homogeneous/heterogeneous hybrid material, which induces the electronic interaction between Ni, Fe and Mo sites, as revealed by a variety of characterization experiments and theoretical calculations. The resulting hybrid electrocatalyst delivers a low overpotential of 203 mV at 10 mA cm-2 and a TOF of 2.34 s-1 at 1.53 V in alkaline media. This work presents a critical step towards developing high-performance OER catalysts by constructing metal-POM hybrids.

Scalable room-temperature synthesis of NiFe oxyhydroxide tailored with S atoms for enhanced oxygen evolution in alkaline seawater and domestic sewage

Developing facile and scalable method to prepare oxygen evolution reaction (OER) catalysts which is suitable for industrial water electrolysis at large operating current densities is vital for green hydrogen production, but still remains a great challenge. Herein, we developed a one-step method to in situ fabricate S-doped NiFe oxyhydroxide (S–Ni(Fe)OOH) ultrathin nanosheet on Ni foam at room temperature, which is suitable for scaled up production, by which a electrode with 500 cm2 can be facilely prepared. The prepared S–Ni(Fe)OOH shows excellent OER stability and activity in 5 M KOH, alkaline sewage and seawater, which only need 198, 275 and 284 mV to reach 500 mA cm−2, respectively. Theoretical calculations indicated that the incorporation of S modulates the chemisorption of oxygen-containing intermediates and facilitates the evolution from ∗OH to ∗O, thus enhance the intrinsic OER activity of S–Ni(Fe)OOH. The electrolyzer based on S–Ni(Fe)OOH can reach a current density of 100 mA cm−2 at a small cell voltage of 1.56 V in 1 M KOH, much smaller than the benchmark catalyst Pt/C||IrO2(1.87 V). This work provides a facile method to prepare highly efficient catalyst for alkaline sewage and seawater oxidation, which could provide references to the development of catalyst for practical hydrogen production.

Ru-promoted NiFe oxyhydroxide anchored on the hierarchical porous N-doped carbon aerogel: Electronic structures modulation for much enhanced OER/HER dual-functional characteristics

Herein, we have demonstrated a novel Ru-promoted NiFe oxyhydroxide anchored on N-doped carbon aerogel (RuNi7FeOx(OH)y @NCA) aerogel electrocatalyst via a facile sol-gel process, combined with a supercritical drying technique. The optimal RuNi7FeOx(OH)y @NCA shows a typical "pearl-like" aerogel nanostructure with a three-dimensional network, showing a large specific surface area of 227.7 m2·g−1. The achieved RuNi7FeOx(OH)y @NCA exhibits high OER activity with a low overpotential of 278.0 mV at 10 mA·cm−2 and a high TOF of 0.107 s−1. Moreover, the RuNi7FeOx(OH)y @NCA aerogel displays excellent hydrogen evolution reaction (HER) activity under alkaline conditions, with a small overpotential of 99.0 mV at a current density of 10 mA·cm−2, and a small Tafel slope of 61.1 mV·dec−1. DFT calculations show that the enhanced OER activity is attributed to the improved binding strength of *OOH intermediate on the catalyst surface, and the introduced Ru with larger electron density and weaker spin density leads to the improved HER activity.

Operando Reconstruction toward Dual‐Cation‐Defects Co‐Containing NiFe Oxyhydroxide for Ultralow Energy Consumption Industrial Water Splitting Electrolyzer (Adv. Energy Mater. 10/2023)

Water Splitting In article number 2203595, Youwen Liu, Yan Liu, Jiakun Fang, Lin Yu and co-workers fabricate a Co dopant and cation vacancy coexistent NiFe oxyhydroxide by in situ Mo leaching from high-entropy Co, Mo co-doped NiFe hydroxide precursors. Dual-regulated NiFe oxyhydroxide electrodes operate stably at 8 A in practical industrial electrolyzers with ultralow energy consumption of 4.6 kWh m−3 H2, verifying the feasibility of lab-constructed novel catalysts for industrialization.

Operando Reconstruction toward Dual‐Cation‐Defects Co‐Containing NiFe Oxyhydroxide for Ultralow Energy Consumption Industrial Water Splitting Electrolyzer

AbstractNickel–iron oxygen evolution catalysts have been under the spotlight as substitutes for precious metals, however, they rarely operate efficiently in practical industrial electrolyzers due to their moderate activity. Guided by density functional theory, the interaction of cation vacancies and dopants can manipulate d band centers, thus gaining near‐optimal binding energies of the oxygenated intermediates and ultralow potentials. This principle is implemented experimentally by catalysis operando variations synthesis, more specifically, in situ Mo leaching from high‐entropy Co, Mo co‐doped NiFe hydroxide precursors form Co dopant and cation vacancy coexistent NiFe oxyhydroxide. Operando electrochemical spectroscopy uncovers that dual‐cation‐defects promote the readier oxidation transition of metal sites, thus contributing to a low overpotential of 255 mV at 100 mA cm−2. Furthermore, dual‐regulated NiFe oxyhydroxide electrodes operate stably at 8 A in practical industrial electrolyzers with ultralow energy consumption of ≈4.6 kWh m−3H2, verifying the feasibility of lab‐constructed novel catalysts towards industrialization.

Facile synthesis approach of bifunctional Co-Ni-Fe oxyhydroxide and spinel oxide composite electrocatalysts from hydroxide and layered double hydroxide composite precursors.

Zinc-air batteries (ZABs) are promising candidates for the next-generation energy storage systems, however, their further development is severely hindered by kinetically sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Facile synthesis approaches of highly active bifunctional electrocatalysts for OER and ORR are required for their practical applications. Herein, we develop a facile synthesis procedure for composite electrocatalysts composed of OER-active metal oxyhydroxide and ORR-active spinel oxide containing Co, Ni and Fe from composite precursors consisting of metal hydroxide and layered double hydroxide (LDH). Both hydroxide and LDH are simultaneously produced by a precipitation method with a controlled molar ratio of Co2+, Ni2+ and Fe3+ in the reaction solution, and calcination of the precursor at a moderate temperature provides composite catalysts of metal oxyhydroxides and spinel oxides. The composite catalyst shows superb bifunctional performances with a small potential difference of 0.64 V between a potential of 1.51 V vs. RHE at 10 mA cm-2 for OER and a half-wave potential of 0.87 V vs. RHE for ORR. The rechargeable ZAB assembled with the composite catalyst as an air-electrode exhibits a power density of 195 mA cm-2 and excellent durability of 430 hours (1270 cycles) of a charge-discharge cycle test.

Partially crystallized Ni–Fe oxyhydroxides promotes oxygen evolution

The slow oxygen evolution reaction (OER) kinetics influences hydrogen production efficiency from water splitting. To break through the bottleneck of water splitting, it is urgent to develop efficient and economic electrocatalysts. Although NiFe-based catalysts exhibit outstanding OER activity, the complicated preparation process limits their large-scale synthesis and applications. Here, partially crystallized nickel-iron oxyhydroxides are synthesized by a facile sol-gel method. When the Fe/Ni mole ratio is 0.5:1, the NiFe0.5(OH)x catalyst shows superior OER performance with a low OER overpotential of 265 mV and good durability. Kinetic studies show that the energy barrier of NiFe0.5(OH)x is only 31.5 kJ mol−1, much smaller than those of Ni(OH)x (41.0 kJ mol−1) and Fe(OH)x (44.8 kJ mol−1). The synergistic action between Ni and Fe sites not only facilitates mass and charge transfer, but also promotes the formation of ∗OOH intermediate for the OER.

Electrochemically Activated Ni-Fe Oxyhydroxide for Mimic Saline Water Oxidation

Electrochemically Activated Ni-Fe Oxyhydroxide for Mimic Saline Water Oxidation

Triggering Lattice Oxygen Activation of Single-Atomic Mo Sites Anchored on Ni-Fe Oxyhydroxides Nanoarrays for Electrochemical Water Oxidation.

Tuning the reactivity of lattice oxygen is of significance for lowering the energy barriers and accelerating the oxygen evolution reaction (OER). Herein, single-atomic Mo sites are anchored on Ni-Fe oxyhydroxide nanoarrays by a facile metal-organic-framework-derived strategy, exhibiting superior performance toward the OER in alkaline media. Insitu electrochemical spectroscopy and isotope-labeling experiments reveal the involvement of lattice oxygen during OER cycles. Combining theoretical and experimental investigations of the electronic configuration, it is comprehensively confirmed that the incorporation of single-atomic Mo sites enables higher oxidation state of the metal and strengthened metal-oxygen hybridization, as well as the formation of oxidized ligand holes above the Fermi level. In a word, the considerable acceleration of water oxidation is achieved via enhancing the reactivity of lattice oxygen and triggering the lattice oxygen activation. This work may provide new insights for designing ideal electrocatalysts via tuning the chemical state and activating the anions ligands.

NiFe oxyhydroxide decorated TiO2 nanotube photoanodes for improved photoelectrochemical oxidation performance

NiFe oxyhydroxide decorated TiO2 nanotube photoanodes for improved photoelectrochemical oxidation performance

Dynamic active sites in NiFe oxyhydroxide upon Au nanoparticles decoration for highly efficient electrochemical water oxidation

Noble metal decoration is one of the most efficient strategies to boost the performance for water oxidation. However, identifying the role of noble metals and especially the active sites remain a persistent challenge. Here, we report a direct synthesis of Au nanoparticles (NPs) decorated NiFe oxyhydroxides via in-situ electrochemical conversion and a comprehensive study of the activity boost via a combination of systematic experimental and theoretical studies. We found that Fe doping greatly improved the electrical conductivity and the Fe sites were the dominant active centers, collectively responsible for the remarkably enhanced performance in NiFe oxyhydroxide. Upon Au NPs decoration, the content of high-valence Fe/Ni ions was substantially reduced and the formation of surface oxygen vacancy was greatly suppressed. As a result, the dominant active centers transfer to the interfacial Au sites that directly participate in catalyzing water oxidation. Our study presents a fundamental insight into the origin of activity enhancement and especially the active centers in noble metal decorated NiFe oxyhydroxide catalysts.