Unveiling the Coercivity-Induced Electrocatalytic Oxygen Evolution Activity of Single-Domain CoFe2O4 Nanocrystals under a Magnetic Field

Recent Advancements on Spin Engineering Strategies for Highly Efficient Electrocatalytic Oxygen Evolution Reactions

Oxygen evolution reaction (OER) is a widely employed half-electrode reaction in oxygen electrochemistry, in applications such as hydrogen evolution, carbon dioxide reduction, ammonia synthesis, and electrocatalytic hydrogenation. Unfortunately, its slow kinetics limits the commercialization of such applications. It is therefore highly imperative to develop highly robust electrocatalysts with high activity, long-term durability, and low noble-metal contents. Previously intensive efforts have been made to introduce the advancements on developing non-precious transition metal electrocatalysts and their OER mechanisms. Electronic structure tuning is one of the most effective and interesting ways to boost OER activity and spin angular momentum is an intrinsic property of the electron. Therefore, modulation on the spin states and the magnetic properties of the electrocatalyst enables the changes on energy associated with interacting electron clouds with radical absorbance, affecting the OER activity and stability. Given that few review efforts have been made on this topic, in this review, the-state-of-the-art research progress on spin-dependent effects in OER will be briefed. Spin engineering strategies, such as strain, crystal surface engineering, crystal doping, etc., will be introduced. The related mechanism for spin manipulation to boost OER activity will also be discussed. Finally, the challenges and prospects for the development of spin catalysis are presented. This review aims to highlight the significance of spin engineering in breaking the bottleneck of electrocatalysis and promoting the practical application of high-efficiency electrocatalysts.

Spin Crossover and Exchange Effects on Oxygen Evolution Reaction Catalyzed by Bimetallic Metal Organic Frameworks

Bimetallic metal-organic frameworks (BMOFs) have shown a superior oxygen evolution reaction (OER) performance, attributed to the synergistic effects of dual metal sites. However, the significant role of these dual-metal synergies in the OER is not yet fully understood. In this study, we employed density functional theory to systematically investigate the OER performance of NiAl- and NiFe-based BMOFs by examining all possible spin states of each intermediate across diverse external potentials and pH environments. We found that the spin state featuring a shallow hole trap state and Ni ions with a higher oxidation state serve as strong oxidizing agents, promoting the OER. An external potential-induced spin crossover was observed in each intermediate, resulting in significant changes in the overall reaction and activation energies due to altered energy levels. Combining the constant potential method and the electrochemical nudged elastic band method, we mapped the minimum free energy barriers of the OER under varied external potential and pH by considering the spin crossover effect for both NiAl and NiFe BMOFs. The results showed that NiFe exhibits better OER thermodynamics and kinetics, which is in good agreement with experimentally measured OER polarization curves and Tafel plots. Moreover, we found that the improved OER kinetics of NiFe not only is attributed to lower barriers but also is a result of improved electrical conductivity arising from the synergistic effects of Ni-Fe dual-metal sites. Specifically, replacing the second metal Al with Fe leads to two significant outcomes: a reduction in both the band gap and the effective hole mass compared to NiAl, and the initiation of super- and double-exchange interactions within the Ni-F-Fe chain, thereby enhancing electron transfer and hopping and leading to the improved OER kinetics.

From Charge to Spin: An In-Depth Exploration of Electron Transfer in Energy Electrocatalysis

Catalytic materials play crucial roles in various energy-related processes, ranging from large-scale chemical production to advancements in renewable energy technologies. Despite a century of dedicated research, major enduring challenges associated with enhancing catalyst efficiency and durability, particularly in green energy-related electrochemical reactions, remain. Focusing only on either the crystal structure or electronic structure of a catalyst is deemed insufficient to break the linear scaling relationship (LSR), which is the golden rule for the design of advanced catalysts. The discourse in this review intricately outlines the essence of heterogeneous catalysis reactions by highlighting the vital roles played by electron properties. The physical and electrochemical properties of electron charge and spin that govern catalysis efficiencies are analyzed. Emphasis is placed on the pronounced influence of external fields in perturbing the LSR, underscoring the vital role that electron spin plays in advancing high-performance catalyst design. The review culminates by proffering insights into the potential applications of spin catalysis, concluding with a discussion of extant challenges and inherent limitations.

Spin-Selective Oxygen Evolution Reaction in Chiral Iron Oxide Nanoparticles: Synergistic Impact of Inherent Magnetic Moment and Chirality

Electron spin polarization is identified as a promising avenue for enhancing the oxygen evolution reaction (OER), which is the bottleneck that limits the energy efficiency of water-splitting. Here, we report that both ferrimagnetic (f-Fe3O4) and superparamagnetic iron oxide (s-Fe3O4) catalysts can exhibit external magnetic field (Hext)-induced OER enhancement, and the activity is proportional to their intrinsic magnetic moment. Additionally, the chirality-induced spin selectivity (CISS) effect was utilized in synergy with Hext to get a maximum enhancement of up to 89% improvement in current density (at 1.8 V vs RHE) with a low onset potential of 270 mV in s-Fe3O4 catalysts. Spin polarization and the resultant spin selectivity suppress the production of H2O2 and promote the formation of ground state triplet O2 during the OER. Furthermore, the design of chiral s-Fe3O4 with synergistic spin potential effect demonstrates a high spin polarization of ∼42%, as measured using conductive atomic force microscopy (c-AFM).

Sodium Tartrate-Assisted Synthesis of High-Purity NiFe2O4 Nano-Microrods Supported by Porous Ketjenblack Carbon for Efficient Alkaline Oxygen Evolution

Herein, a simple two-step synthetic method was developed for the synthesis of NiFe₂O₄ nano-microrods supported on Ketjenblack carbon (NiFe₂O₄/KB). A sodium tartrate-assisted hydrothermal method was employed for the synthesis of a NiFe-MOF/KB precursor, which was then pyrolyzed under N₂ at 500 °C to yield NiFe₂O₄/KB. Benefiting from the presence of high-valence Ni³⁺ and Fe³⁺, high conductivity, and a large electrochemically active surface area, NiFe₂O₄/KB delivered outstanding OER electrocatalytic performance under alkaline conditions, including a very low overpotential of 258 mV (vs RHE) at 10 mA cm^-2, a small Tafel slope of 43.01 mV dec^-1, and excellent durability in 1.0 M KOH. Density functional theory calculations verified the superior alkaline OER electrocatalytic activity of NiFe₂O₄ to IrO₂. While both catalysts possessed a similar metallic ground state, NiFe₂O₄ offered a lower energy barrier in the rate-determining OER step (*OOH → O₂) compared to IrO₂, resulting in faster OER kinetics.

Effects of Alternating Magnetic Fields on the OER of Heterogeneous Core-Shell Structured NiFe2O4@(Ni, Fe)S/P

Composition optimization, structural design, and introduction of external magnetic fields into the catalytic process can remarkably improve the oxygen evolution reaction (OER) performance of a catalyst. NiFe₂O₄@(Ni, Fe)S/P materials with a heterogeneous core-shell structure were prepared by the sulfide/phosphorus method based on spinel-structured NiFe₂O₄ nanomicrospheres. After the sulfide/phosphorus treatment, not only the intrinsic activity of the material and the active surface area were increased but also the charge transfer resistance was reduced due to the internal electric field. The overpotential of NiFe₂O₄@(Ni, Fe)P at 10 mA cm^-2 (iR correction), Tafel slope, and charge transfer resistance were 261 mV, 42 mV dec^-1, and 3.163 Ω, respectively. With an alternating magnetic field, the overpotential of NiFe₂O₄@(Ni, Fe)P at 10 mA cm^-2 (without iR correction) declined by 45.5% from 323 mV (0 mT) to 176 mV (4.320 mT). Such enhancement of performance is primarily accounted for the enrichment of the reactive ion OH^- on the electrode surface induced by the inductive electric potential derived from the Faraday induction effect of the AMF. This condition increased the electrode potential and thus the charge transfer rate on the one hand and weakened the diffusion of the active substance from the electrolyte to the electrode surface on the other hand. The OER process was dominantly controlled by the charge transfer process under low current conditions. A fast charge transfer rate boosted the OER performance of the catalyst. At high currents, diffusion exerted a significant effect on the OER process and low OH^- diffusion rates would lead to a decrease in the OER performance of the catalyst.