Fluoridation-induced high-performance catalysts for the oxygen evolution reaction: A mini review

Unlocking the Potential: Atomically Thin 2D Fluoritene from Exfoliated Fluorite Ore and Its Electrochemical Activity

Fluorite mineral holds significant importance because of its optoelectronic properties and wide range of applications. Here, we report the successful exfoliation of bulk fluorite ore (calcium fluoride, CaF2) crystals into atomically thin two-dimensional fluoritene (2D CaF2) using a highly scalable liquid-phase exfoliation method. The microscopic and spectroscopy characterizations show the formation of (111) plane-oriented 2D CaF2 sheets with exfoliation-induced material strain due to bond breaking, leading to the changes in lattice parameter. Its potential role in electrocatalysis is further explored for deeper insight, and a probable mechanism is also discussed. The 2D CaF2 with long-term stability shows overpotential values of 670 and 770 mV vs RHE for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, at 10 mA cm-2. Computational simulations demonstrate the unique "direct-indirect" band gap switching with odd and even numbers of layers. Current work offers new avenues for exploring the structural and electrochemical properties of 2D CaF2 and its potential applicability.

Graphene Architecture-Supported Porous Cobalt-Iron Fluoride Nanosheets for Promoting the Oxygen Evolution Reaction

The design of efficient oxygen evolution reaction (OER) electrocatalysts is of great significance for improving the energy efficiency of water electrolysis for hydrogen production. In this work, low-temperature fluorination and the introduction of a conductive substrate strategy greatly improve the OER performance in alkaline solutions. Cobalt-iron fluoride nanosheets supported on reduced graphene architectures are constructed by a one-step solvothermal method and further low-temperature fluorination treatment. The conductive graphene architectures can increase the conductivity of catalysts, and the transition metal ions act as electron acceptors to reduce the Fermi level of graphene, resulting in a low OER overpotential. The surface of the catalyst becomes porous and rough after fluorination, which can expose more active sites and improve the OER performance. Finally, the catalyst exhibits excellent catalytic performance in 1 M KOH, and the overpotential is 245 mV with a Tafel slope of 90 mV dec-1, which is better than the commercially available IrO2 catalyst. The good stability of the catalyst is confirmed with a chronoamperometry (CA) test and the change in surface chemistry is elucidated by comparing the XPS before and after the CA test. This work provides a new strategy to construct transition metal fluoride-based materials for boosted OER catalysts.

Morphology Modulation and Phase Transformation of Manganese-Cobalt Carbonate Hydroxide Caused by Fluoride Doping and Its Effect on Boosting the Overall Water Electrolysis

Increasing demands for pollution-free energy resources have stimulated intense research on the design and fabrication of highly efficient, inexpensive, and stable non-noble earth-abundant metal catalysts with remarkable catalytic activity for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Morphology control of the catalysts is widely implemented as an effective strategy to change the surface atomic coordination and increase the catalytic behavior of the catalysts. In this study, we have designed a series of Mn-Co catalyts with different morphologies on the graphite paper substrate to enhance OER and HER activities in alkaline media. The prepared catalysts with different morphologies were successfully obtained by adjusting the amount of ammonium fluoride (NH₄F) in the hydrothermal process. The electrochemical tests display that the cubic-like Mn-Co catalyst with pyramids on the faces at a concentration of 0.21 M NH₄F exhibits the best activity toward both OER and HER. The cubic-like Mn-Co catalyst with pyramids on the faces showed overpotentials of 240 and 82 mV at a current density of 10 mA cm^-2 for OER and HER, respectively. Also, the cubic-like Mn-Co catalyst with pyramids on the faces required overpotentials of 319 and 216 mV to reach the current density of 100 mA cm^-2 for OER and HER, respectively. The current density of this catalyst at η = 0.32 V was 701.05 mA cm^-2 for OER, and for HER, the current density of the catalyst was 422.89 mA cm^-2 at η = 0.23 V. The Tafel slopes of the Mn-Co catalyst with cubic-like structures with pyramids on the faces were 78 and 121 mV dec^-1 for OER and HER, respectively. A two-electrode overall water electrolysis system using this bifunctional Mn-Co catalyst exhibited low cell voltages of 1.60 in the alkaline electrolyte at the standard current density of 10 mA cm^-2 with appropriate stability. These electrochemical merits exhibit the considerable potential of the cubic-like Mn-Co catalyst with pyramids on the faces for bifunctional OER and HER applications.

Probing Dynamic Self-Reconstruction on Perovskite Fluorides toward Ultrafast Oxygen Evolution

Exploring low cost, highly active, and durable electrocatalysts for oxygen evolution reaction (OER) is of prime importance to boost energy conversion efficiency. Perovskite fluorides are emerging as alternative electrocatalysts for OER, however, their intrinsically active sites during real operation are still elusive. Herein, the self-reconstruction on newly designed NiFe coupled perovskite fluorides during OER process is demonstrated. In situ Raman spectroscopy, ex situ X-ray absorption spectroscopy, and theoretical calculation reveal that Fe incorporation can significantly activate the self-reconstruction of perovskite fluorides and efficiently lower the energy barrier of OER. Benefiting from self-reconstruction and low energy barrier, the KNi_0.8 Fe_0.2 F₃ @nickel foam (KNFF2@NF) electrocatalyst delivers an ultralow overpotential of 258 mV to afford 100 mA cm^-2 and an excellent durability for 100 h, favorably rivaling most the state-of-the-art OER electrocatalysts. This protocol provides the fundamental understanding on OER mechanism associated with surface reconstruction for perovskite fluorides.

Antioxidant, Antimicrobial, and Photocatalytic Potential of Cobalt Fluoride (CoF2) Nanoparticles

Herein, CoF2 nanoparticles (NPs) are prepared by simple coprecipitation method and are characterized by various techniques, i.e., XRD, SEM/EDX, FTIR, and UV/Vis, for their structure identification. As-prepared nanostructures were used as photocatalyst, as antioxidant, and as antimicrobial agent. The degradation studies of the prepared samples were carried out for specific time for the degradation of methylene blue (MLB) dye under a UV/visible spectrophotometer to determine decolorization and change in concentration of MLB with respect to time. The antibacterial activity against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) was measured by well diffusion and serial dilution method to determine their efficiency against these two bacteria, through a dose-dependent method. The antibacterial activity was further confirmed against the experimental bacteria through calculation of minimum inhibition concentration (MIC). The antioxidant activity (radical scavenging activity) of the prepared CoF2 NPs was also assessed.

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.

Bifunctional citrate-Ni0.9Co0.1(OH)x layer coated fluorine-doped hematite for simultaneous hole extraction and injection towards efficient photoelectrochemical water oxidation

Surface modification by loading a water oxidation co-catalyst (WOC) is generally considered an efficient means to optimize the sluggish surface oxygen evolution reaction (OER) of a hematite photoanode for photoelectrochemical (PEC) water oxidation. However, the surface WOC usually exerts little impact on the bulk charge separation of hematite. Herein, an ultrathin citrate-Ni_0.9Co_0.1(OH)_x [Cit-Ni_0.9Co_0.1(OH)_x] is conformally coated on the fluorine-doped hematite (F-Fe₂O₃) photoanode for PEC water oxidation to simultaneously promote the internal hole extraction and surface hole injection of the target photoanode. Besides, the conformally coated Cit-Ni_0.9Co_0.1(OH)_x overlayer passivates the redundant surface trap states of F-Fe₂O₃. These factors result in a superior photocurrent density of 2.52 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode (V vs. RHE) for the target photoanode. Detailed investigation manifests that the hole extraction property in Cit-Ni_0.9Co_0.1(OH)_x is mainly derived from the Ni sites, while Co incorporation endows the overlayer with more catalytic active sites. This synergistic effect between Ni and Co contributes to a rapid and continuous hole migration pathway from the bulk to the interface of the target photoanode, and then to the electrolyte for water oxidation.

In-situ reconstruction of non-noble multi-metal core-shell oxyfluorides for water oxidation

The electrochemical anodic behavior of transition metal compounds plays an undeniably non-negligible role across many electrooxidation reactions. In this work, a chronopotentiometric technique was employed to activate the multicomponent non-noble metal oxyfluorides in-situ for oxygen evolution reaction (OER). It is interesting to unravel that the increasing applied current density helps to reconstruct the catalyst into nanoporous core-shell structure and introduce metal oxyhydroxide on the surface, which guarantees more channels for efficient ion/mass transportation and thus contributes to exposing more active sites for catalytic reaction. The activated five-membered oxyfluoride shows the best catalytic activity with overpotential of 348 ± 2 mV to achieve the current density of 10 mA/cm² and a Tafel slope of 110.3 ± 0.1 mV/dec, in contrast with the pristine one (532 ± 2 mV & 240.2 ± 0.1 mV/dec). It still maintains high stability after long time OER measurement, making it a promising succedaneum for noble metal catalysts. The high-entropy effect, amorphous state and high active sites density jointly contribute to its enhanced OER performance. This work provides new ideas for realizing the potential of inactive elements via entropy engineering and using electrochemical self-reconstruction to modify semiconductors for advanced water oxidation.