Enhanced electrocatalytic performance of FeNiCoP amorphous alloys as oxygen-evolving catalysts for electrolytic water splitting application

Crystal Facets-Activity Correlation for Oxygen Evolution Reaction in Compositional Complex Alloys

Compositional complex alloys, including high and medium-entropy alloys (HEAs/MEAs) have displayed significant potential as efficient electrocatalysts for the oxygen evolution reaction (OER), but their structure-activity relationship remains unclear. In particular, the basic question of which crystal facets are more active, especially considering the surface reconstructions, has yet to be answered. This study demonstrates that the lowest index {100} facets of FeCoNiCr MEAs exhibit the highest activity. The underlying mechanism associated with the {100} facet's low in-plane density, making it easier to surface reconstruction and form amorphous structures containing the true active species is uncovered. These results are validated by experiments on single crystals and polycrystal MEAs, as well as DFT calculations. The discoveries contribute to a fundamental comprehension of MEAs in electrocatalysis and offer physics-based strategies for developing electrocatalysts.

Facile and scalable synthesis of Ni3S2/Fe3O4 nanoblocks as an efficient and stable electrocatalyst for oxygen evolution reaction

This study employed a one-step hydrothermal process to synthesize Ni3S2/Fe3O4 nanoblocks in situ on nickel foam (NF). The resulting Ni3S2/Fe3O4/NF catalyst demonstrates exceptional electrocatalytic activity for the oxygen evolution reaction (OER) and robust long-term stability. It achieves a low overpotential of only 220 mV for a current density of 10 mA cm-2 with a Tafel slope of 54.1 mV dec-1 and remains stable in 1.0 M KOH for 66 h. The binder-free self-supported three-dimensional nanoblocks enhance the reaction region and long-term stability. Electronic interactions between Fe3O4 and Ni3S2, coupled with heterogeneous interfaces, optimize the electronic structure, fostering the formation of highly reactive species. Density-functional theory (DFT) calculations confirm that Ni3S2/Fe3O4, with a heterogeneous interfacial structure, modulates the chemisorption of reaction intermediates on the catalyst surface, optimizing the Gibbs free energies (ΔG) of oxygen-containing intermediates. The synergistic effect between the two active materials within the heterogeneous structure enhances OER catalytic performance. This finding offers a valuable approach to designing efficient and stable OER electrocatalysts.

Three-dimensional ordered macroporous design of heterogeneous cobalt-iron phosphides as oxygen evolution electrocatalyst

Oxygen evolution reaction (OER) plays a key role in electrochemical conversion, which needs efficient and economical electrocatalyst to boost its kinetics for large-scale application. Herein, a bimetallic CoP/FeP2heterostructure with a three-dimensional ordered macroporous structure (3DOM-CoP/FeP2) was synthesized as an OER catalyst to demonstrate a heterogeneous engineering induction strategy. By adjusting the electron distribution and producing a lot of active sites, the heterogeneous interface enhances catalytic performance. High specific surface area is provided by the 3DOM structure. Additionally, at the solid-gas-electrolyte threephase interface, the electrocatalytic reaction exhibits good mass transfer.In situRaman spectroscopy characterization revealed that FeOOH and CoOOH reconstructed from CoP/FeP2were the true OER active sites. Consequently, the 3DOM-CoP/FeP2demonstrates superior OER activity with a low overpotentials of 300/420 mV at 10/100 mA cm-2and meritorious OER durability. It also reveals promising performance as the overall water splitting anode.

Modulating nickel-iron active species via dealloying to boost the oxygen evolution reaction

The surface structure and composition of pre-catalysts play a critical role in the surface reconstruction process toward active species during the anodic oxygen evolution reaction (OER). Surface modified methods can accelerate the OER process of alloy ribbons, but the understanding of pre-catalysts and the structure/reactivity of the reconstruction (active) species is still insufficient. Herein, we report a two-step dealloyed Ni-Fe-P alloy ribbon as a highly efficient OER electrocatalyst. By adjusting the surface-derived component, we could regulate Ni/Fe hydroxide active species on the Ni-Fe-P alloy ribbon, enhancing the OER performance. The oxidation and release of P driven by dealloying plays a key role in constructing optimal β-NiOOH/FeOOH catalytic species on Ni-Fe-P. The optimal β-NiOOH/FeOOH active species enables Ni-Fe-P alloy to obtain a 104 mV of reduction in overpotential (at 10 mA cm-2) and a 78-fold increase in current density (at overpotential: 300 mV) compared to undealloyed Ni-Fe-P. Our work provides valuable insights into the relationship between the surface structure/composition of alloy bulk electrocatalysts and surface-reconstructed species and a rational design of a surface treatment process.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Introducing High-Valence Iridium Single Atoms into Bimetal Phosphides toward High-Efficiency Oxygen Evolution and Overall Water Splitting

Single atoms are superior electrocatalysts having high atomic utilization and amazing activity for water oxidation and splitting. Herein, this work reports a thermal reduction method to introduce high-valence iridium (Ir) single atoms into bimetal phosphide (FeNiP) nanoparticles toward high-efficiency oxygen evolution reaction (OER) and overall water splitting. The presence of high-valence single Ir atoms (Ir⁴⁺ ) and their synergistic interaction with Ni³⁺ species as well as the disproportionation of Ni³⁺ assisted by Fe collectively contribute to the exceptional OER performance. In specific, at appropriate Ir/Ni and Fe/Ni ratios, the as-prepared Ir-doped FeNiP (Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ ) nanoparticles at a mass loading of only 35 µg cm^-2 show the overpotential as low as 232 mV at 10 mA cm^-2 and activity as high as 1.86 A mg^-1 at 1.5 V versus RHE for OER in 1.0 m KOH. Computational simulations confirm the vital role of high-valence Ir to weaken the adsorption of OER intermediates, favorable for accelerating OER kinetics. Impressively, a Pt/C||Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ two-electrode alkaline electrolyzer affords a current density of 10 mA cm^-2 at a low cell voltage of 1.42 V, along with satisfied stability. An AA battery with a nominal voltage of 1.5 V can drive overall water splitting with obvious bubbles released.

Annealing and electrochemically activated amorphous ribbons: Surface nanocrystallization and oxidation effects enhanced for oxygen evolution performance

Annealing and cyclic voltammetry (CV) are essential for the activation of amorphous alloy ribbons. Various amorphous alloy ribbons have been activated in the fields of environmental catalysts using either annealing or CV. However, the combination of the two methods for improving the oxygen evolution reaction (OER) performance has rarely been reported. This combination is expected to significantly improve the OER performance of amorphous ribbons. Here, we developed an "annealing +CV-activation" integrated strategy to treat a free-standing NiFeBSiP ribbon, which as an efficient and stable oxygen-evolving electrode. The "annealing +CV-activation" strategy induces the nanocrystallization and oxidation effects on the surface of the NiFeBSiP ribbon. The effects significantly increase the electron transfer ability, the Ni/Fe/P oxidation state and the surface area of the NiFeBSiP ribbon, which consequently leads to enhancing the OER performance. As a result, the treated ribbon exhibits a low overpotential of 269 mV at 10 mA cm^-2 and a small Tafel slope of 40.5 mV dec^-1, which are much better than the OER performance of the as-spun ribbon. The enhanced OER performance of the NiFeBSiP ribbon demonstrates the significant and promising effect of the "annealing +CV-activation" integrated strategy for designing high-efficiency amorphous alloy ribbons electrocatalysts.

Attractive Electron Delocalization Behavior of FeCoMoPB Amorphous Nanoplates for Highly Efficient Alkaline Water Oxidation

The rational design of high-performance and cost-effective electrocatalysts to overcome the kinetically sluggish water oxidation reaction is a grand challenge in water electrolysis. Transitional metals with incompletely filled d orbitals are expected to have intrinsic electronic interaction to promote the reaction kinetics, however, the construction of multiple active sites is still a bottleneck problem. Here, inspired by an amorphous alloy design strategy with chemical tunability, a noble-metal-free FeCoMoPB amorphous nanoplate for superior alkaline water oxidation is developed. The achieved overpotentials at current densities of 10, 100, and 500 mA cm^-2 are 239, 281, and 331 mV, respectively, while retaining a reliable stability of 48 h, outperforming most currently available electrocatalysts. Experimental and theoretical results reveal that the chemical complexity of the amorphous nanoplate leads to the formation of multiple active sites that is able to greatly lower the free energy of the rate-determining step during the water oxidation reaction. Moreover, the Mo element would result in an electron delocalization behavior to promote electron redistribution at its surrounding regions for readily donating and taking electrons. This amorphous alloy design strategy is expected to stimulate the development of more efficient electrocatalysts that is applicable in energy devices, such as metal-air batteries, fuel cells, and water electrolysis.

Insight into the Catalytic Activity of Amorphous Multimetallic Catalysts under a Magnetic Field toward the Oxygen Evolution Reaction

Slow kinetics in the oxygen evolution reaction (OER) remains a Gordian knot to develop an efficient and cost-effective electrocatalyst in electrochemical water splitting. In recent studies, either a synergistic effect on multimetallic catalysts or spin polarization in ferromagnetic materials is considered as a desirable way to improve water electrolysis. Herein, the OER performance of amorphous FeNiCo-based multimetallic catalysts with adjustable composition was investigated from the perspective of atomic structure. Mössbauer spectra results demonstrate that the OER activities exhibit a significant dependence on the local structure of catalysts in which a catalyst with a high content of Fe clusters of low coordination numbers tends to obtain higher activity. Furthermore, benefiting from the spin polarization of these ferromagnetic catalysts, the OER activity is notably enhanced in the presence of a magnetic field. In particular, overpotential reduction exceeding 20 mV (above 100 mA cm^-2) in alkaline OER performance is observed for strong ferromagnetic catalysts in comparison with the weak ferromagnetic ones. An increment of 65.2% in turnover frequency is achieved for the catalyst with the strongest ferromagnetism. This magnetic enhancement strategy affords an effective way of improving the water oxidation performance on amorphous ferromagnetic catalysts.

Amorphous NiCuFeP@Cu3P nanoarray for an efficient hydrogen evolution reaction

Transition metal phosphides are considered as ideal alternatives for noble metal catalysts for hydrogen evolution reactions. In this study, amorphous NiCuFeP nanosheets are uniformly coated on self-supporting Cu3P nanowire array...

Amorphous materials for elementary-gas-involved electrocatalysis: an overview

Given the critical demands on energy conversion, storage, and transportation, tremendous interest has been devoted to the field of material development related to energy harvesting, recently. As the only route towards energy utilization, the carriers with the characteristics of low carbon are regarded as the future choice, e.g., hydrogen and ammonia. To this end, electrocatalysis provides a green way to access these substances. However, the unfulfilled conversion efficiency is the bottleneck for practical application. In this review, the promising characteristics of amorphous materials and the amorphous-induced electrocatalytic enhancement (AIEE) were emphasized. In the beginning, the characteristics of amorphous materials are briefly summarized. The basic mechanism of heterogeneous electrocatalytic reactions is illustrated, including the hydrogen/oxygen evolution and oxygen/nitrogen reduction. In the third part, the electrocatalytic performance of amorphous materials is discussed in detail, and the mechanism of AIEE is highlighted. In the last section of this review, the challenges and outlook for the development of amorphous enhanced electrocatalysis are presented.

Phytic Acid-Based FeCo Bimetallic Metal-Organic Gels for Electrocatalytic Oxygen Evolution Reaction

Electrocatalysts have been developed to improve the efficiency of gas release for oxygen evolution reaction (OER), and finding a simple and efficient method for efficient electrocatalysts has inspired research enthusiasm. Herein, we report bimetallic metal-organic gels derived from phytic acid (PA) and mixed transition metal ions to explore their performance in electrocatalytic oxygen evolution reaction. PA is a natural phosphorus-rich organic compound, which can be obtained from plant seeds and grains. PA reacts with bimetallic ions (Fe³⁺ and Co²⁺ ) in a facile one-pot synthesis under mild conditions to form PA-FeCo bimetallic gels, and the corresponding aerogels are further partially reduced with NaBH₄ to improve the electrocatalytic activity. Mixed valence states of Fe(II)/Fe(III) and Co(III)/Co(II) are present in the materials. Excellent OER performance in terms of overpotential (257 mV at 20 mA cm^-2 ) and Tafel slope (36 mV dec^-1 ) is achieved in an alkaline electrolyte. This reduction method is superior to the pyrolysis method by well maintaining the gel morphology structure. This strategy is conducive to the further improvement of the performance of metal-organic electrocatalysts, and provides guidance for the subsequent application of metal-organic gel electrocatalysts.