Enhanced Water Oxidation Research Articles

Mechanistic insights into multimetal synergistic and electronic effects in a hexanuclear iron catalyst with a [Fe3(μ3-O)(μ2-OH)]2 core for enhanced water oxidation.

Multinuclear molecular catalysts mimicking natural photosynthesis have been shown to facilitate water oxidation; however, such catalysts typically operate in organic solutions, require high overpotentials and have unclear catalytic mechanisms. Herein, a bio-inspired hexanuclear iron(III) complex I, Fe6(μ3-O)2(μ2-OH)2(bipyalk)2(OAc)8 (H2bipyalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol); OAc = acetate) with desirable water solubility and stability was designed and used for water oxidation. Our results showed that I has high efficiency for water oxidation via the water nucleophilic attack (WNA) pathway with an overpotential of only ca. 290 mV in a phosphate buffer of pH 2. Importantly, key high-oxidation-state metal-oxo intermediates formed during water oxidation were identified by in situ spectroelectrochemistry and oxygen atom transfer reactions. Theoretical calculations further supported the above identification. Reversible proton transfer and charge redistribution during water oxidation enhanced the electron and proton transfer ability and improved the reactivity of I. Here, we have shown the multimetal synergistic and electronic effects of catalysts in water oxidation reactions, which may contribute to the understanding and design of more advanced molecular catalysts.

Swelling the d/p‐Band Center Difference Induced by Heterostructure Self‐Optimization Engineering for Enhanced Water Oxidation

Swelling the <i>d</i>/<i>p</i>‐Band Center Difference Induced by Heterostructure Self‐Optimization Engineering for Enhanced Water Oxidation

Crystallinity control on Bi2MoO6 microspheres for improved photocatalytic oxygen evolution

Crystallinity of the photocatalyst has a profound impact on its reactivity. Here, flower-like Bi2MoO6 microspheres were synthesized via a solvothermal method and subsequently calcined at varying temperatures to regulate the crystallinity and particle size. The thermally treated samples exhibited enhanced photocatalytic oxygen evolution compared to the pristine Bi2MoO6. Higher calcination temperatures led to the formation of larger crystallites, significantly boosting the activity. We demonstrate that crystallinity, rather than surface area, plays a more vital role in governing the photocatalytic performance of Bi2MoO6. Improved crystallinity can thicken the space charge layer (SCL), resulting in a greater band bending that facilitates the separation of electron-hole pairs. Conversely, poor crystallinity leads to an abundance of surface and bulk defects, promoting electron-hole recombination. Overall, efficient charge separation and suppressed recombination endow the calcined Bi2MoO6 with enhanced water oxidation efficiency.

Reconstruction of Ferromagnetic/Paramagnetic Cobalt-Based Electrocatalysts under Gradient Magnetic Fields for Enhanced Oxygen Evolution.

The rational manipulation of the surface reconstruction of catalysts is a key factor in achieving highly efficient water oxidation, but it is a challenge due to the complex reaction conditions. Herein, we introduce a novel in situ reconstruction strategy under a gradient magnetic field to form highly catalytically active species on the surface of ferromagnetic/paramagnetic CoFe2O4@CoBDC core-shell structure for electrochemical oxygen evolution reaction (OER). We demonstrate that the Kelvin force from the cores' local gradient magnetic field modulates the shells' surface reconstruction, leading to a higher proportion of Co2+ as active sites. These Co sites with optimized electronic configuration exhibit more favorable adsorption energy for oxygen-containing intermediates and lower the activation energy of the overall catalytic reaction. As a result, a significant enhancement in OER performance is achieved with a large current density increment about 128 % at 1.63 V and an overpotential reduction by 28 mV at 10 mA cm-2 after reconstruction. Interestingly, after removing the external magnetic field, the activity could persist for over 100 h. This work showcases the directional surface reconstruction of catalysts under a gradient magnetic field for enhanced water oxidation.

Reconstruction of Ferromagnetic/non‐magnetic Cobalt‐based Electrocatalysts under Gradient Magnetic Fields for Enhanced Oxygen Evolution

The rational manipulation of the surface reconstruction of catalysts is a key factor in achieving highly efficient water oxidation, but it is a challenge due to the complex reaction conditions. Herein, we introduce a novel in situ reconstruction strategy under a gradient magnetic field to form highly catalytically active species on the surface of ferromagnetic/non‐magnetic CoFe2O4@CoBDC core‐shell structure for electrochemical oxygen evolution reaction (OER). We demonstrate that the Kelvin force from the cores’ local gradient magnetic field modulates the shells’ surface reconstruction, leading to a higher proportion of Co2+ as active sites. These Co sites with optimized electronic configuration exhibit more favorable adsorption energy for oxygen‐containing intermediates and lower the activation energy of the overall catalytic reaction. As a result, a significant enhancement in OER performance is achieved with a large current density increment about 128% at 1.63 V and an overpotential reduction by 28 mV at 10 mA cm‐2 after reconstruction. Interestingly, after removing the external magnetic field, the activity could persist for over 100 h. This work showcases the directional surface reconstruction of catalysts under a gradient magnetic field for enhanced water oxidation.

Enhanced water oxidation stability and activity in MnO2 nanosheet arrays through Ti doping

Efficient and sustainable hydrogen production via water electrolysis relies on the utilization of highly active electrocatalysts for the oxygen evolution reaction (OER). Birnessite (δ-MnO2), a type of manganese oxide with a local atomic structure similar to the oxygen-evolving complex in photosystem II, has emerged as a highly promising OER catalyst. Despite considerable efforts have been dedicated to enhancing its activity, the crucial aspect of stability has been often overlooked. Herein, Ti ions have been selectively incorporated into the in-layered lattice of δ-MnO2 nanosheet arrays using a hydrothermal method, aiming to enhance the stability without reducing its activity. The resulting optimal Ti-MnO2 catalyst demonstrates enhanced OER activity in alkaline electrolyte, exhibiting an impressive 85 mV reduction in overpotential at 10 mA cm−2, surpassing the initial MnO2 (495 mV reduced to 410 mV). Remarkably, it shows exceptional durability, with negligible performance decrease observed over the entire 50-hour testing period. It has been demonstrated that the incorporation of Ti serves a dual purpose: it adjusts the electronic structure around Mn, enhancing activity, and simultaneously stabilizes the Mn3+ active sites, resulting in exceptional durability. Furthermore, density functional theory (DFT) calculations provide additional confirmation of the optimized electronic structure of MnO2 achieved through Ti doping, leading to a reduction in the free energy of adsorbed intermediates and expediting the kinetics of the OER process. Our findings open a pathway for enhancing OER stability and activity by doping electrocatalytic inert ions into the lattice of MnO2, applicable not only to Mn-based oxides but also other non-precious metal-based electrocatalysts.

Electronic Structure Modification of MnO2 Nanosheet Arrays with Enhanced Water Oxidation Activity and Stability by Nitrogen Plasma.

The strategic design of catalysts for the oxygen evolution reaction (OER) is crucial in tackling the substantial energy demands associated with hydrogen production in electrolytic water splitting. Despite extensive research on birnessite (δ-MnO2) manganese oxides to enhance catalytic activity by modulating Mn3+ species, the ongoing challenge is to simultaneously stabilize Mn3+ while improving overall activity. Herein, oxygen (O) vacancies and nitrogen (N) doping have been simultaneously introduced into the MnO2 through a simple nitrogen plasma approach, resulting in efficient OER performance. The optimized N-MnO2v electrocatalyst exhibits outstanding OER activity in alkaline electrolyte, reducing the overpotential by nearly 160 mV compared to pure pristine MnO2 (from 476 to 312 mV) at 10 mA cm-2, and a small Tafel slope of 89 mV dec-1. Moreover, it demonstrates excellent durability over a 122 h stability test. The introduction of O vacancies and incorporation of N not only fine-tune the electronic structure of MnO2, increasing the Mn3+ content to enhance overall activity, but also play a crucial role in stabilizing Mn3+, thereby leading to exceptional stability over time. Subsequently, density functional theory calculations validate the optimized electronic structure of MnO2 achieved through the two engineering methods, effectively lowering the intermediate adsorption free energy barrier. Our synergistic approach, utilizing nitrogen plasma treatment, opens a pathway to concurrently enhance the activity and stability of OER electrocatalysts, applicable not only to Mn-based but also to other transition metal oxides.

Dynamic in situ reconstruction of NiSe2 promoted by interfacial Ce2(CO3)2O for enhanced water oxidation

Understanding and manipulating the structural evolution of water oxidation electrocatalysts lays the foundation to finetune their catalytic activity. Herein, we present a synthesis of NiSe2-Ce2(CO3)2O heterostructure and demonstrate the efficacy of interfacial Ce2(CO3)2O in promoting the formation of catalytically active centers to improve oxygen evolution activity. In-situ Raman spectroscopy shows that incorporation of Ce2(CO3)2O into NiSe2 causes a cathodic shift of the Ni2+→Ni3+ transition potential. Operando electrochemical impedance spectroscopy reveals that strong electronic coupling at heterogeneous interface accelerates charge transfer process. Furthermore, density functional theory calculations suggest that actual catalytic active species of NiOOH transformed from NiSe2, which is coupled with Ce2(CO3)2O, can optimize electronic structure and decrease the free energy barriers toward fast oxygen evolution reaction (OER) kinetics. Consequently, the resultant NiSe2-Ce2(CO3)2O electrode exhibits remarkable electrocatalytic performance with low overpotentials (268/304 mV @ 50/100 mA cm−2) and excellent stability (50 mA cm−2 for 120 h) in the alkaline electrolyte. This work emphasizes the significance of modulating the dynamic changes in developing efficient electrocatalyst.

Graphene Quantum Dots as Hole Extraction and Transfer Layer Empowering Solar Water Splitting of Catalyst-Coupled Zinc Ferrite Nanorods.

Despite the narrow band gap energy, the performance of zinc ferrite (ZnFe2O4) as a photoharvester for solar-driven water splitting is significantly hindered due to its sluggish charge transfer and severe charge recombination. This work reports the fabrication of a hybrid nanostructured hydrogenated ZnFe2O4 (ZFO) photoanode with enhanced photoelectrochemical water-oxidation activity through coupling N-doped graphene quantum dots (GQDs) as a hole transfer layer and Co-Pi as a catalyst. The GQDs not only reduce the surface-mediated nonradiative electron-hole pair recombination but also induce a built-in interfacial electric field leading to a favorable band alignment at the ZFO/GQDs interface, helping rapid photogenerated hole separation and serving as a conducting hole transfer highway, improve the hole transportation into the Co-Pi catalyst for enhanced water oxidation reaction kinetics. The optimized ZFO/GQD/Co-Pi hybrid photoanode delivers a 23-fold photocurrent enhancement at 1.23 V versus the reversible hydrogen electrode (RHE) and a significant 360 mV reduction in the onset potential, reaching 0.65 VRHE compared with the ZFO photoanode under 1 sun illumination in a neutral electrolytic environment. This investigation underscores the mechanism of synergistic interplay between the hole transport layer and cocatalyst in boosting the solar-illuminated water-splitting activity of the ZFO photoanode.

Ultrathin layer TAFC on BiVO4 with ligand-to-metal charge transfer enhances built-in electric field for boosting photoelectrochemical water oxidation

To reveal the mechanism of charge transfer between interfaces of BiVO4-based heterogeneous materials in photoelectrochemical water splitting system, the cocatalyst was grown in situ using tannic acid (TA) as a ligand and Fe and Co ions as metal centers (TAFC), and then uniformly and ultra-thinly coated on BiVO4 to form photoanodes. The results show that the BiVO4/TAFC achieves a superior photocurrent density (4.97 mA cm−2 at 1.23 VRHE). The charge separation and charge injection efficiencies were also significantly higher, 82.0 % and 78.9 %, respectively. From XPS, UPS, KPFM, and density functional theory calculations, Ligand-to-metal charge transfer (LMCT) acts as an electron transport highway in TAFC ultrathin layer to promote the concentration of electrons towards metal center, leading to an increase in the work function, which enhances the built-in electric field and further improves the charge transport. This study demonstrated that the LMCT pathway on TA-metal complexes enhances the built-in electric field in BiVO4/TAFC to promote charge transport and thus enhance water oxidation, providing a new understanding of the performance improvement mechanism for the surface-modified composite photoanodes.

Constructing FeNiPt@C Trifunctional Catalyst by High Spin-Induced Water Oxidation Activity for Zn-Air Battery and Anion Exchange Membrane Water Electrolyzer.

Developing cost-efficient trifunctional catalysts capable of facilitating hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) activity is essential for the progression of energy devices. Engineering these catalysts to optimize their active sites and integrate them into a cohesive system presents a significant challenge. This study introduces a nanoflower (NFs)-like carbon-encapsulated FeNiPt nanoalloy catalyst (FeNiPt@C NFs), synthesized by substituting Co2+ ions with high-spin Fe2+ ions in Hofmann-type metal-organic framework, followed by carbonization and pickling processes. The FeNiPt@C NFs catalyst, characterized by its nitrogen-doped carbon-encapsulated metal alloy structure and phase-segregated FeNiPt alloy with slight surface oxidization, exhibits excellent trifunctional catalytic performance. This is evidenced by its activities in HER (-25mV at 10mA cm-2), ORR (half-wave potential of 0.93V), and OER (294mV at 10mA cm-2), with the enhanced water oxidation activity attributed to the high-spin state of the Fe element. Consequently, the Zn-air battery and anion exchange membrane water electrolyzer assembled by FeNiPt@C NFs catalyst demonstrate remarkable power density (168mW cm-2) and industrial-scale current density (698mA cm-2 at 1.85V), respectively. This innovative integration of multifunctional catalytic sites paves the way for the advancement of sustainable energy systems.

Ca-doped birnessite for enhanced photocatalysis of water oxidation

Birnessite is an affordable and high-efficiency catalyst for water oxidation. In this study, we attempted to doping Ca into birnessite to investigate the influence on its photocatalytic oxidation activity. The study was carried out through a combination of experimental investigations and DFT calculations of the relevant crystal and electronic structures. The analysis of Raman spectra using 2T2D correlation and DFT indicates that Ca contributes more to the high frequency regions (400–600 cm−1) than K. DFT further reveals that the introduction of Ca leads to an increase in the number of localized electrons at the Fermi energy level and in the conduction band. Photo-electrochemical experiments demonstrate a significant reduction of 100 mV/dec in the Tafel slope, along with a 1.48-fold increase in photo-current density for Ca-birnessite when compared to K-birnessite. In general, the results demonstrate that Ca-birnessite can effectively heighten visible light absorption, reduce the band gap, and enhance water oxidation activity.

Hierarchical nanoassembly of Ni3S2-MoS2 interconnected with CeO2 as a highly remarkable hybrid electrocatalyst for enhancing water oxidation and energy storage

Emerging earth-abundant multifunctional electrocatalysts with highly efficient performance are important substitutes for replacing the expensive noble metal catalysts to attain sustainable electrochemical water splitting and supercapacitor applications. Therefore, we developed a well-interconnected Ni3S2-MoS2-CeO2 (NMC) nanostructure as a multifunctional electrocatalyst for the utilization in dual purposes, mainly energy conversion and storage applications. In alkaline conditions, the NMC-1 exhibits remarkable oxygen evolution reaction (OER) performances of lower over potential (206.03 mV at 10 mA/cm2) coupled with the lowest Tafel slope (40.19 mV·dec−1). The NMC-1 further shows excellent stability, where the durability of the electrocatalyst was verified after a considerable number of cyclic voltammetry (5000 cycles) and chronopotentiometry. The coexistence of the Ce3+ and Ce4+ oxidation states generates a favorable environment for surface oxygen ion exchange. The inherent stability of CeO2 facilitates a strong and intimate electrochemical coupling with NiOOH. This synergistic interaction between CeO2, MoS2, and NiOOH further enhances the performance of the catalytic system. Notably, the hybrid structure of NMC-1 exhibited superior supercapacitor performance (417 F/g at 1 A/g). The fabricated asymmetric supercapacitor device showed efficient performance by attaining a high energy density of 4.84 Wh kg−1 at a power density of 72 W kg−1. According to the intrinsic properties of each material in NMC and the existence of synergy owing to the outstanding interfacial contact between the compounds in NMC, reasonably abundant surface-active sites are available for attaining rapid charge transfer, enhancing the inherent activity and durability of the catalyst, which is the key reason for better OER and supercapacitor performance. This study provides useful understandings into the strategic design of advanced multifunctional electrode materials for next-generation applications.

Modulation of the Phase Transformation of Fe2O3 for Enhanced Water Oxidation under a Magnetic Field

Modulation of the Phase Transformation of Fe₂O₃ for Enhanced Water Oxidation under a Magnetic Field

Activating nickel foam with trace titanium oxide for enhanced water oxidation.

Nickel-based electrocatalysts for water oxidation suffer from low activity and poor stability. In this work, 0.015 mg cm-2 TiO2 nanosheets anchored on Ni foam addressed these problems after electrochemical activation. In situ investigations, including Raman spectra, corroborated the enhanced generation of highly active Ni(III)-O-O species on Ni foam in the presence of trace TiO2.

A facile N-doped NiFe(B) (Oxy)hydroxide monolithic electrode for enhanced water oxidation.

N-doped NiFe(B) (oxy)hydroxide can promote the catalytic activity for an alkaline oxygen evolution reaction (OER) significantly, but fabrication is difficult. Herein, we introduced a B-induction route to the N-NiFe(B) (oxy)hydroxide monolithic electrode under a relatively low temperature. We observed an excellent catalytic performance benefiting from an optimal electronic structure, enlarged surface area and improved hydrophilicity. Moreover, this mild protocol could be extended to fabricate an S-doped NiFe-based catalyst. This research could aid large-scale manufacture.

Fe-induced crystalline-amorphous interface engineering of a NiMo-based heterostructure for enhanced water oxidation.

Engineering heterostructures with a unique surface/interface structure is one of the effective strategies to develop highly active noble-metal-free catalysts for the oxygen evolution reaction (OER), because the surface/interface of catalysts is the main site for the OER. Herein, we design a coralloid NiMo(Fe)-20 catalyst with a crystalline-amorphous interface through combining a hydrothermal method and an Fe-induced surface reconfiguration strategy. That is, after Fe3+ impregnation treatment, the Ni(OH)2-NiMoO4 pre-catalyst with a complete crystalline surface is restructured into a trimetallic heterostructure with a crystalline-amorphous interface, which facilitates mass diffusion and charge transfer during the OER. As expected, self-supported NiMo(Fe)-20 exhibits excellent electrocatalytic water oxidation performance (overpotential: η-10 = 220 mV, η-100 = 239 mV) in the alkaline electrolyte, and its electrocatalytic performance hardly changes after maintaining the current density of 50 mA cm-2 for 10 hours. Furthermore, nickel foam (NF) supported commercial Pt/C and self-supported NiMo(Fe)-20 served as the cathode and anode of the Pt/C‖NiMo(Fe)-20 electrolyzer, respectively, which exhibits a lower cell voltage (E-100 = 1.53 V) than that of the Pt/C‖RuO2 electrolyzer (E-100 = 1.58 V) assembled with noble metal-based catalysts. The enhanced electrocatalytic performance of the NiMo(Fe)-20 catalyst is mainly attributed to the synergistic effect between the crystalline-amorphous interface and the coralloid trimetallic heterostructure.

Enhancing Water Oxidation Catalysis by Controlling Metal Cation Distribution in Layered Double Hydroxides (Adv. Funct. Mater. 2/2024)

Enhancing Water Oxidation Catalysis by Controlling Metal Cation Distribution in Layered Double Hydroxides (Adv. Funct. Mater. 2/2024)

Competing double-exchange/super-exchange ordering for enhanced water oxidation kinetics.

Competing double-exchange and super-exchange interactions yield rich chemistry for understanding robust oxygen evolution activity in Ru-doped lanthanum strontium manganite, with an overpotential of 300 mV at 10 mA cm-2 and a Tafel slope of 110 mV dec-1. Favourable redox potentials of Mn3+/Mn4+ and Ru4+/Ru5+ lead to improved charge transfer kinetics for OER activity.

Oxygen vacancies boosted photoelectrochemical performance of α-Fe2O3 photoanode via butane flame annealing

In this study, oxygen vacancies were introduced into the hematite (α-Fe2O3) photoanode via a simple and rapid butane flame annealing method. The presence of oxygen vacancies were demonstrated by high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The photocurrent density of optimum α-Fe2O3 electrodes was 0.67 mA cm-2 at 1.23 VRHE, about 2.7 times higher than that of N2 annealed electrode. Our study showed that butane flame annealing process can introduce oxygen vacancies defects in Fe2O3 electrodes, which would improve carrier transfer efficiency, increase electron density, enhance water oxidation activity, and narrow the light optical bandgap. The use of butane flame annealing is a simple and effective strategy to enhance the PEC performance of hematite thin films.