NiFe-based Oxygen Evolution Reaction Electrocatalysts Research Articles

Investigation on the Distinct Catalytic Origin of Nickel-Iron Sulfides/Selenides Oxygen Evolution Electrocatalyst

Nickel-iron based electrocatalysts are efficient oxygen evolution reaction (OER) catalysts and their oxidative transition is widely reported in alkaline water splitting. However, the distinct catalytic origin governing their different catalytic performance is still unclear. Herein, NiFe chalcogenides, NixFe1−xSe and NixFe1−xS, are prepared, and their distinct OER performance and activity origin are comparatively investigated. NixFe1−xSe is found exhibits excellent OER electrocatalytic activity in 1 M KOH with an overpotential of 165 mV at 10 mA cm−2, outperforms that of NixFe1−xS (197 mV). Meanwhile, excellent stability performance is also achieved by NixFe1−xSe in anion exchange membrane water electrolyzer under high current density condition. Experimental analysis reveals that the high intrinsic activity of NixFe1−xSe is originated from the in-situ generated Se-doped γ-NiOOH species, whereas NixFe1−xS is converted into NiO/α-FeOOH during OER. Theoretical calculations show that, compared with NiO/α-FeOOH, Se/γ-NiOOH exhibits a higher degree of electron localization conducive to the stability of the Se/γ-NiOOH electronic structure, faster electron transfer, and favorable adsorption of reaction intermediates. The latter can effectively promote the transformation of *O into *OOH rate-controlling step, thereby exhibiting improved OER performance. These results provide new insights on the distinct catalytic origin of high efficiency NiFe-based OER electrocatalysts.

Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation.

In-depth insights into the structure-activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived Od-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni4+ and Fe4+ species, which generate oxygen-bridged NiIV-O-FeIV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free Od-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-NiIV-O-FeIV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

Regulating multiscale structures of nickel-iron-based electrocatalysts for efficient water oxidation

The oxygen evolution reaction (OER) has been widely studied as an efficient process in energy conversion, yet the majority of studies focus on the strategies to boost the intrinsic activity of the catalyst, rather than considering the enhancement of its mass transfer ability. Herein, we produce efficient NiFe-based OER electrocatalysts with rich oxygen vacancies and abundant transport channels by an argon plasma-engraving strategy. Compared with pristine NiFe, the obtained best NiFe-plasma catalyst yields a 12.5-fold increased current density (from 80 to ∼1,000 mA cm −2 ) at an overpotential of 300 mV, surpassing most OER catalysts in alkaline solution. Density functional theory calculations disclose that the introduction of oxygen vacancies improve the intrinsic activity and electronic conductivity, while finite element method calculations demonstrate that the construction of catalytic interface and the creation of nanoscale transport channels facilitates mass transfer. • Plasma simultaneously produces oxygen vacancies and slit pores on NiFe • FEM calculation illustrates that the slit pores can facilitate mass transport • The obtained NiFe-plasma catalyst yields a 12.5-fold increased current density Wang et al. prepare efficient NiFe-based OER electrocatalysts with abundant oxygen vacancies and transport channels by plasma engraving. Owing to the synergistic effect between high intrinsic catalytic activity and preferable mass transfer ability, the NiFe-P-200-15 exhibits a small overpotential of 302 mV to reach 1.0 A cm −2 in alkaline electrolyte.

Stable and active NiFeW layered double hydroxide for enhanced electrocatalytic oxygen evolution reaction

The electrocatalytic oxygen evolution reaction (OER) requires stable, highly active, and robust earth-abundant electrocatalysts. In the present study, we combine theoretical simulations with experimental approaches to design and synthesize NiFeW layered double hydroxide (LDH) for efficient OER in alkaline electrolytes. Density functional theory with the Hubbard U (DFT + U) calculations suggests that W, a high-valence metal, can improve the catalytic activity of Fe sites and optimize adsorption energies for OER intermediates at the surface of NiFeW LDH. Electrochemical measurements of the as-synthesized NiFeW LDH with a constant mass loading reveals that the LDH with the NiFeW molar ratio of 4:1:1 presented the highest intrinsic OER activity among all bi- and trimetallic LDH catalysts in this study. Furthermore, the nanostructures with the optimal active metal molar ratio are in situ grown on hydrophilic-treated carbon paper to fabricate an integrated 3D electrode with an overpotential of 248 mV at the catalytic current density of 20 mA cm−2 and a low Tafel slope of 68 mV dec−1 in 1.0 M KOH solution. This indicates that NiFeW LDH is among the most active NiFe-based OER electrocatalysts reported to date. X-ray photoelectron spectroscopy and elemental analysis confirm that the optimized Ni Fe W LDH presents a stable chemical composition and Ni, Fe, and W are still present in the catalyst after alkaline OER measurements. These findings offer new insights and avenues for the future design and study of stable and active multi-metal-based OER electrocatalysts.

Fluorination-enabled Reconstruction of NiFe Electrocatalysts for Efficient Water Oxidation.

Developing low-cost and efficient electrocatalysts to accelerate oxygen evolution reaction (OER) kinetics is vital for water and carbon-dioxide electrolyzers. The fastest-known water oxidation catalyst, Ni(Fe)OxHy, usually produced through an electrochemical reconstruction of precatalysts under alkaline condition, has received substantial attention. However, the reconstruction in the reported catalysts usually leads to a limited active layer and poorly controlled Fe-activated sites. Here, we demonstrate a new electrochemistry-driven F-enabled surface-reconstruction strategy for converting the ultrathin NiFeOxFy nanosheets into an Fe-enriched Ni(Fe)OxHy phase. The activated electrocatalyst shows a low OER overpotential of 218 ± 5 mV at 10 mA cm-2 and a low Tafel slope of 31 ± 4 mV dec-1, which is among the best for NiFe-based OER electrocatalysts. Such superior performance is caused by the effective formation of the Fe-enriched Ni(Fe)OxHy active-phase that is identified by operando Raman spectroscopy and the substantially improved surface wettability and gas-bubble-releasing behavior.

Recent Progress on NiFe-Based Electrocatalysts for the Oxygen Evolution Reaction.

The seriousness of the energy crisis and the environmental impact of global anthropogenic activities have led to an urgent need to develop efficient and green fuels. Hydrogen, as a promising alternative resource that is produced in an environmentally friendly and sustainable manner by a water splitting reaction, has attracted extensive attention in recent years. However, the large-scale application of water splitting devices is hindered predominantly by the sluggish oxygen evolution reaction (OER) at the anode. Therefore, the design and exploration of high-performing OER electrocatalysts is a critical objective. Considering their low prices, abundant reserves, and intrinsic activities, NiFe-based bimetal compounds are widely studied as excellent OER electrocatalysts. Moreover, recent progress on NiFe-based OER electrocatalysts in alkaline environments is comprehensively and systematically introduced through various catalyst families including NiFe-layered hydroxides, metal-organic frameworks, NiFe-based (oxy)hydroxides, NiFe-based oxides, NiFe alloys, and NiFe-based nonoxides. This review briefly introduces the advanced NiFe-based OER materials and their corresponding reaction mechanisms. Finally, the challenges inherent to and possible strategies for producing extraordinary NiFe-based electrocatalysts are discussed.

Pulsed laser-assisted synthesis of defect-rich NiFe-based oxides for efficient oxygen evolution reaction

To develop highly efficient and low-cost electrocatalysts based on earth-abundant elements for oxygen evolution reaction (OER) is a great challenge for electrocatalytic water splitting. Herein, a unique type of NiFe-based oxide catalyst with abundant defects is successfully synthesized via pulsed laser ablation in a specific atmosphere containing reductive NH3. The catalyst is self-supported and assembled from ultrafine nanoparticles with size of less than 5 nm. X-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy analyses suggest that the reductive NH3 results in a large number of oxygen and nickel vacancies and lower crystallinity. Consequently, the catalyst exhibits a high-performance OER activity with a low overpotential of 223 mV at 10 mA cm−2 and a small Tafel slope of 35.0 mV dec−1, surpassing most NiFe-based OER electrocatalysts.

NiFe Oxalate Nanomesh Array with Homogenous Doping of Fe for Electrocatalytic Water Oxidation.

NiFe-based materials have shown impressive electrocatalytic activity for the oxygen evolution reaction (OER). The mutual effect between proximate Ni and Fe atoms is essential in regulating the electronic structure of the active site to boost the OER kinetics. Detailed studies confirm that the separated monometal phases in NiFe-based materials are detrimental to OER. Thus, the high-level blending of Ni and Fe in NiFe-based OER electrocatalysts is critical. Herein, an NiFe oxalate nanomesh array based on solid solutions between nickel (II) oxalate and iron (II) oxalate is prepared through a facile surfactant-free approach in the presence of the reductive oxalate anions. The integrated electrode can efficiently catalyze water oxidation to reach a current density of 50 mA cm-2 with a small overpotential of 203 mV in a 1.0 m KOH aqueous solution. The high efficiency can be attributed to the atomic level mix of Ni and Fe in the solid solutions and the hierarchical porous structure of the nanomesh array. These two aspects bring about fast kinetics, efficient mass diffusion, and quick charge transfer, which are the three major positive factors for a high-performance heterogenous electrocatalyst.

A mini review of NiFe-based materials as highly active oxygen evolution reaction electrocatalysts

Oxygen evolution reaction (OER) electrolysis, as an important reaction involved in water splitting and rechargeable metal-air batteries, has attracted increasing attention for clean energy generation and efficient energy storage. Nickel/iron (NiFe)-based compounds have been known as active OER catalysts since the last century, and renewed interest has been witnessed in recent years on developing advanced NiFe-based materials for better activity and stability. In this review, we present the early discovery and recent progress on NiFe-based OER electrocatalysts in terms of chemical properties, synthetic methodologies and catalytic performances. The advantages and disadvantages of each class of NiFe-based compounds are summarized, including NiFe alloys, electrodeposited films and layered double hydroxide nanoplates. Some mechanistic studies of the active phase of NiFe-based compounds are introduced and discussed to give insight into the nature of active catalytic sites, which could facilitate further improving NiFe based OER electrocatalysts. Finally, some applications of NiFe-based compounds for OER are described, including the development of an electrolyzer operating with a single AAA battery with voltage below 1.5 V and high performance rechargeable Zn-air batteries.