Improved Oxygen Evolution Reaction Research Articles

Ce-4f as an electron-modulation reservoir weakening Fe-O bond to induce iron vacancies in CeFevNi hydroxide for enhancing oxygen evolution reaction

Designing novel rare-earth-transition metal composites is at the forefront of electrocatalyst research. However, the modulation of transition metal electronic structures by rare earths to induce vacancy defects and enhance electrochemical performance has rarely been reported. In this study, we systematically investigate the mechanism by which Ce-4f electron modulation weakens the Fe-O bond, thereby altering the electronic structure in CeFevNi hydroxide to improve oxygen evolution reaction (OER) performance. Theoretical calculations and experimental characterizations reveal that Ce-4f orbitals function as electron-modulation reservoirs, capable not only of retaining or donating electrons but also of influencing the material's electronic structure. Moreover, Ce-4f bands optimize the Fe lower Hubbard bands (LHB) and O-2p bands, leading to weakened Fe-O bonds and the formation of cationic vacancies. This change results in the upshift of the d-band center at the active sites, favoring the reaction energy barrier for oxygen intermediates in the OER process. The synthesized catalyst demonstrated an overpotential of 201 mV at 10 mA cm−2 and a lifetime exceeding 200 h at 100 mA cm−2 under alkaline conditions. This work offers a proof-of-concept for the application of the mechanism of rare earth-induced transition metal vacancy defects, providing a general guideline for the design and development of novel highly efficient catalysts.

Hierarchically Structured Graphene Aerogel Supported Nickel-Cobalt Oxide Nanowires as an Efficient Electrocatalyst for Oxygen Evolution Reaction.

The rational design of a heterostructure electrocatalyst is an attractive strategy to produce hydrogen energy by electrochemical water splitting. Herein, we have constructed hierarchically structured architectures by immobilizing nickel-cobalt oxide nanowires on/beneath the surface of reduced graphene aerogels (NiCoO2/rGAs) through solvent-thermal and activation treatments. The morphological structure of NiCoO2/rGAs was characterized by microscopic analysis, and the porous structure not only accelerates the electrolyte ion diffusion but also prevents the agglomeration of NiCoO2 nanowires, which is favorable to expose the large surface area and active sites. As further confirmed by the spectroscopic analysis, the tuned surface chemical state can boost the catalytic active sites to show the improved oxygen evolution reaction performance in alkaline electrolytes. Due to the synergistic effect of morphology and composition effect, NiCoO2/rGAs show the overpotential of 258 mV at the current density of 10 mA cm-2. Meanwhile, the small values of the Tafel slope and charge transfer resistance imply that NiCoO2/rGAs own fast kinetic behavior during the OER test. The overlap of CV curves at the initial and 1001st cycles and almost no change in current density after the chronoamperometric (CA) test for 10 h confirm that NiCoO2/rGAs own exceptional catalytic stability in a 1 M KOH electrolyte. This work provides a promising way to fabricate the hierarchically structured nanomaterials as efficient electrocatalysts for hydrogen production.

Cr dopant regulating d-orbital electronic configuration of NiFe spinel oxide to improve oxygen evolution reaction in Zn-air battery

Significant advancements towards enhancing the catalytic activity of spinel ferrites in oxygen evolution reaction (OER) have been achieved, however, further studies are required on this regard. In this study, Cr-doped spinel ferrite (NiFe2O4) embedded in N-doped C (Cr-NiFe2O4@NC) was synthesized via a high-temperature treatment using an NiFe Prussian blue analog as a precursor and K2Cr2O7 as a Cr source. Owing to the strong octahedral site preference energy, Cr3+ ions were successfully doped into octahedral sites and tailored the electronic configuration of NiFe2O4, as confirmed by XPS. The Cr-substituted Cr-NiFe2O4@NC exhibited the overpotential of 241 mV at 10 mA cm−2, which was smaller than that of NiFe2O4@NC (266 mV) at the same current density. Meanwhile, the Tafel slope of Cr-NiFe2O4@NC (65.32 mV dec−1) was also lower than that of NiFe2O4@NC (86.96 mV dec−1). Therefore, the Cr substitution can improve the electrocatalytic OER performance of NiFe2O4@NC. In particular, an assembled rechargeable Zn-air battery using Cr-NiFe2O4@NC as the cathode catalyst afforded a high power density (61.1 mA cm−2) together with a high specific capacity (740 mA g−1). Amazingly, it could power light-emitting diodes, demonstrating its real-world application. Density functional theory calculations revealed that Cr ion doping led to electron transfer in NiFe2O4, resulting in a density of electronic states close to the Fermi level, which probably regulated the adsorption/desorption behaviors of oxygenate intermediates (*O and *OOH), thus promoting the reaction kinetics of the OER. This study provides a helpful basis for the development of spinel oxides with high electrocatalytic OER activity and durability via cation doping.

Elucidating the green energy production potential of biomimetically fashioned [OFR-MnO/ZrO2]-MMO electrocatalyst

A new hybrid metal oxide (MnO/ZrO2) has been synthesized by an environmentally benign material from the Olea ferruginea royle (OFR)plant leaf extract as a green precursor. The hybrid material has improved oxygen evolution reaction (OER) electrochemical performance. The existence of [OFR-MnO/ZrO2] particles is confirmed by spectroscopic techniques, X-ray photoelectron spectroscopy (XPS), and elemental diffraction spectroscopic (EDS) investigations, emphasizing the role of organic species (N, O and C) during synthesis. To assess the electrode’s electrochemical performance, linear sweep voltametric and impedance measurements were performed. The electrode showed a lower linear sweep measurement of 382 mV and a Tafel value of 79 mV dec−1, suggesting MnO/ZrO2 as an efficient electrocatalytic material. These results demonstrate the potential of MnO/ZrO2 as an excellent, long-term and economical solution for a variety of applications, particularly in the area of water splitting research.

Improved Oxygen Evolution Reaction Kinetics with Titanium Incorporated Nickel Ferrite for Efficient Anion Exchange Membrane Electrolysis

Improved Oxygen Evolution Reaction Kinetics with Titanium Incorporated Nickel Ferrite for Efficient Anion Exchange Membrane Electrolysis

Cobalt‐based Co3Mo3N/Co4N/Co Metallic Heterostructure as a Highly Active Electrocatalyst for Alkaline Overall Water Splitting

AbstractAlkaline water electrolysis holds promise for large‐scale hydrogen production, yet it encounters challenges like high voltage and limited stability at higher current densities, primarily due to inefficient electron transport kinetics. Herein, a novel cobalt‐based metallic heterostructure (Co3Mo3N/Co4N/Co) is designed for excellent water electrolysis. In operando Raman experiments reveal that the formation of the Co3Mo3N/Co4N heterointerface boosts the free water adsorption and dissociation, increasing the available protons for subsequent hydrogen production. Furthermore, the altered electronic structure of the Co3Mo3N/Co4N heterointerface optimizes ΔGH of the nitrogen atoms at the interface. This synergistic effect between interfacial nitrogen atoms and metal phase cobalt creates highly efficient active sites for the hydrogen evolution reaction (HER), thereby enhancing the overall HER performance. Additionally, the heterostructure exhibits a rapid OH− adsorption rate, coupled with great adsorption strength, leading to improved oxygen evolution reaction (OER) performance. Crucially, the metallic heterojunction accelerates electron transport, expediting the afore‐mentioned reaction steps and enhancing water splitting efficiency. The Co3Mo3N/Co4N/Co electrocatalyst in the water electrolyzer delivers excellent performance, with a low 1.58 V cell voltage at 10 mA cm−2, and maintains 100 % retention over 100 hours at 200 mA cm−2, surpassing the Pt/C||RuO2 electrolyzer.

Cobalt-based Co3Mo3N/Co4N/Co Metallic Heterostructure as a Highly Active Electrocatalyst for Alkaline Overall Water Splitting.

Alkaline water electrolysis holds promise for large-scale hydrogen production, yet it encounters challenges like high voltage and limited stability at higher current densities, primarily due to inefficient electron transport kinetics. Herein, a novel cobalt-based metallic heterostructure (Co3Mo3N/Co4N/Co) is designed for excellent water electrolysis. In operando Raman experiments reveal that the formation of the Co3Mo3N/Co4N heterointerface boosts the free water adsorption and dissociation, increasing the available protons for subsequent hydrogen production. Furthermore, the altered electronic structure of the Co3Mo3N/Co4N heterointerface optimizes ΔGH of the nitrogen atoms at the interface. This synergistic effect between interfacial nitrogen atoms and metal phase cobalt creates highly efficient active sites for the hydrogen evolution reaction (HER), thereby enhancing the overall HER performance. Additionally, the heterostructure exhibits a rapid OH- adsorption rate, coupled with great adsorption strength, leading to improved oxygen evolution reaction (OER) performance. Crucially, the metallic heterojunction accelerates electron transport, expediting the afore-mentioned reaction steps and enhancing water splitting efficiency. The Co3Mo3N/Co4N/Co electrocatalyst in the water electrolyzer delivers excellent performance, with a low 1.58 V cell voltage at 10 mA cm-2, and maintains 100 % retention over 100 hours at 200 mA cm-2, surpassing the Pt/C||RuO2 electrolyzer.

Ligand-regulated Ni-based coordination compounds to promote self-reconstruction for improved oxygen evolution reaction

The Ni-based coordination compounds, during oxygen evolution reaction (OER), self-reconstruct to produce NiOOH, which are the real active sites. Thus, encouraging the self-reconstruction of pre-catalysts to generate more NiOOH species...

Ni-Rh-based bimetallic conductive MOF as a high-performance electrocatalyst for the oxygen evolution reaction.

Metal-organic frameworks (MOFs) have recently been considered the promising catalysts due to their merits of abundant metal sites, versatile coordination groups, and tunable porous structure. However, low electronic conductivity of most MOFs obstructs their direct application in electrocatalysis. In this work, we fabricate an Ni-Rh bimetallic conductive MOF ([Ni2.85Rh0.15(HHTP)2]n/CC) grown in situ on carbon cloth. Abundant nanopores in the conductive MOFs expose additional catalytic active sites, and the advantageous 2D π-conjugated structure helps accelerate charge transfer. Owing to the introduction of Rh, [Ni2.85Rh0.15(HHTP)2]n/CC exhibited substantially improved oxygen evolution reaction (OER) activity and exhibited only an overpotential of 320mV to achieve the current density of 20mAcm-2. The remarkable OER performance confirmed by the electrochemical tests could be ascribed to the synergistic effect caused by the doped Rh together with Ni in [Ni2.85Rh0.15(HHTP)2]n/CC, thereby exhibiting outstanding electrocatalytic performance.

The role of nonmetallic ion substitution in perovskite LaCoO3 for improved oxygen evolution reaction activity

Transition metal perovskite (ABO3) is an emerging type of oxygen evolution reaction (OER) electrocatalyst that shows reasonably good activity and moderate stability. Although efforts have been made to improve perovskite’ OER performance by various element substitution at A/B-site, the influence of ion, particularly non-metallic ion, substitutions on the OER mechanism are rarely studied. More and more evidence has shown that the metal-center theory has failed to explain lots of OER-related phenomena. Therefore, it is urgent to understand how the cation and anion sites in perovskite determine OER performance. Here, we used a Fe and P co-doped LaCoO3 as a model system to explore the influence of substitution in perovskite by combinng operando/ex-situ X-ray characterization and density functional theroy (DFT). We observed enhanced OER catalytic activities in co-doped materials, which are attributed to the stronger transition-metal-oxygen-bonding-covalency (TMOBC). The detailed analyses by O K-edge XAS, electrochemical performance, and DFT suggest that the hybridization between O 2p and transition metal 3d eg orbitals could be a more credible descriptor of perovskite for OER, which is the combination of eg orbital theory and TMOBC theory. The finding in our work provides insights into the OER catalysis mechanism on metal oxides, which could guide new design of cost-effective oxide electrocatalysts.

Coordination Tuning of Metal Porphyrins for Improved Oxygen Evolution Reaction.

The nucleophilic attack of water or hydroxide on metal-oxo units forms an O-O bond in the oxygen evolution reaction (OER). Coordination tuning to improve this attack is intriguing but has been rarely realized. We herein report on improved OER catalysis by metal porphyrin 1-M (M=Co, Fe) with a coordinatively unsaturated metal ion. We designed and synthesized 1-M by sterically blocking one porphyrin side with a tethered tetraazacyclododecane unit. With this protection, the metal-oxo species generated in OER can maintain an unoccupied trans axial site. Importantly, 1-M displays a higher OER activity in alkaline solutions than analogues lacking such an axial protection by decreasing up to 150-mV overpotential to achieve 10 mA/cm2 current density. Theoretical studies suggest that with an unoccupied trans axial site, the metal-oxo unit becomes more positively charged and thus is more favoured for the hydroxide nucleophilic attack as compared to metal-oxo units bearing trans axial ligands.

One-step preparation of anti-ferromagnetic single-atom nickel to improve oxygen evolution reaction of photoelectrochemical water splitting

Noble metal single-atom catalysts (SACs) have been extensively investigated due to their unique electronic structure and high-efficiency catalytic performance. However, there are relatively few studies on the preparation of non-noble metal single atoms and their application in oxygen evolution reaction (OER). In this paper, nickel single-atom catalyst was prepared using a facile one pot method. Ni-SAC-2.5% showed uniform SACs distribution and anti-ferromagnetic property as evidenced by the aberration-corrected high resolution electron microscopy and the first-principle calculations, achieving the lowest overpotential of 220 mV (10 mA/cm2) and stable performance in 1 M KOH solution. Meanwhile, it exhibited the largest specific capacitance value of 1.77 mF/cm2 and the lowest charge transfer resistance value of 40.75 Ω due to the accelerated kinetic process. First-principle calculations further confirmed the lowest computed overpotential of 0.54 eV for Ni-SAC-2.5%. However, Ni small clusters and nanoclusters with superparamagnetic performance were observed when the Ni content was further increased to 3.5% and 4.5%, resulting in higher overpotentials of 0.85 eV and 2.01 eV for OER process as indicated by the theoretical simulation. This paper provides a facile method to prepare non-noble metal SACs and useful guidance for designing high performance OER catalysts.

Hexagonal Boron Nitride/Reduced Graphene Oxide Heterostructures as Promising Metal‐Free Electrocatalysts for Oxygen Evolution Reaction Driven by Boron Radicals

Developing highly efficient earth‐abundant alternatives to traditional noble metal catalysts is essential for clean and sustainable energy‐conversion and energy‐storage technologies, yet still challenging in limited active sites and weak resistance to electrochemical corrosion. Herein, density‐functional theory calculations demonstrate that hexagonal boron nitride (h‐BN), albeit often being considered inert, can generate boron‐active radicals at defective sites by forming heterogeneous structures with graphene‐containing point vacancies, leading to a substantial electron delocalization and charge transfer, indicating a superior catalytic activity. Experimentally, the van der Waals heterostructure is rationally designed with h‐BN nanosheets (BNNs) anchored on reduced graphene oxide (rGO) as strongly coupled composite catalysts. Despite the poor conductivity in BN and lower catalytic activity in rGO, the created heterostructures demonstrate unexpected, improved oxygen evolution reaction (OER) activity with excellent stability in alkaline electrolyte. Qualitative analysis of the valence band offset and theoretical calculation reveal that the formation of heterostructures can significantly drive the electron transfer between C and B atoms near the vacancies across the interface and cause a half‐metallic property of BN, decreasing the free energy barrier of four‐electron OER kinetics. Herein, the synthetic schemes of h‐BNNs are guided as highly active metal‐free OER electrocatalysts.

Lattice tensile strain cobalt phosphate with modulated hydroxide adsorption and structure transformation towards improved oxygen evolution reaction

The adsorption energy of oxygen-containing intermediates for the oxygen evolution reaction (OER) electrocatalysts plays a key role on their electrocatalytic performances. Rational optimization and regulation of the binding energy of intermediates can effectively improve the catalytic activities. Herein, the binding strength of Co phosphate to *OH was weakened by generating lattice tensile strain via Mn replacement, which modulated the electronic structure and optimized the reactive intermediates adsorption with active sites. The tensile-strained lattice structure and stretched interatomic distance were confirmed by X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectra measurements. The as-obtained Mn-doped Co phosphate exhibits excellent OER activity with an overpotential of 335 mV at 10 mA cm−2, which is much higher than pristine Co phosphate. In-situ Raman spectra and methanol oxidation reaction experiments demonstrated that Mn-doped Co phosphate with lattice tensile strain shows optimized *OH adsorption strength, and is favorable to structure reconstruction and form highly active Co oxyhydroxide intermediate during OER process. Our work provides insight into the effects of the lattice strain on the OER activity from the standpoint of intermediate adsorption and structure transformation.

Ni(OH)2 nanosheets and highly stable Ni(OH)2/GO nanocomposite for its improved OER performance

In this work, Ni(OH)2/GO nanocomposite is synthesized using an improved hummers method followed by a hydrothermal method. The oxygen evolution reaction of the Ni(OH)2/GO nanocomposite is studied. The prepared nanocomposite is characterized by XRD, SEM, TEM, XPS, and Raman techniques. The Ni(OH)2 nanosheets are well dispersed on the surface of the graphene oxide layers, which improves the surface area. The nanocomposite is able to withstand high overpotential with a negligible current loss for 12 h. The Ni(OH)2/GO nanocomposite shows an improved oxygen evolution reaction (OER) performance with a low overpotential of 1.61 V (vs. RHE) at 10 mA cm−2. The nanocomposite shows a low Tafel slope of 89.1 mV dec−1, nearly one by third of bare Ni(OH)2 nanosheets 307.1 (mV dec−1). Also, the Ni(OH)2/GO nanocomposite shows improved electrochemical stability than the bare Ni(OH)2.

In Situ Charge Modification within Prussian Blue Analogue Nanocubes by Plasma for Oxygen Evolution Catalysis.

A targeted defect-induced strategy of metal sites in a porous framework is an efficient avenue to improve the performance of a catalyst. However, achieving such an activation without destroying the ordered framework is a major challenge. Herein, a dielectric barrier discharge plasma can etch the Fe(CN)6 group of the NiFe Prussian blue analogue framework in situ through reactive oxygen species generated in the air. Density functional theory calculations prove that the changed local electronic structure and coordination environment of Fe sites can significantly improve oxygen evolution reaction catalytic properties. The modified NiFe Prussian blue analogue is featured for only 316 mV at a high current density (100 mA cm-2), which is comparable to that of commercial alkaline catalysts. In a solar cell-driven alkaline electrolyzer, the overall electrolysis efficiency is up to 64% under real operation conditions. Over 80 h long-time continuous test under 100 mA cm-2 highlights superior durability. The density functional theory calculations confirm that the formation of OOH* is the rate-determining step over Fe sites, and Fe(CN)6 vacancy and extra oxygen atoms can introduce charge redistribution to the catalyst surface, which finally enhances the oxygen evolution reaction catalytic properties by reducing the overpotential by 0.10 V. Both experimental and theoretical results suggest that plasma treatment strategy is useful for modifying the skeletal material nondestructively at room temperature, which opens up a broad prospect in the field of catalyst production.

Phosphate-modified cobalt silicate hydroxide with improved oxygen evolution reaction

Oxygen Evolution Reaction (OER) has gained significant attention due to its crucial role in renewable energy systems. The quest for efficient and low-cost OER catalysts remains a challenge of significant interest and importance. In this work, phosphate-incorporated cobalt silicate hydroxide (denoted as CoSi-P) is reported as a potential electrocatalyst for OER. The researchers first synthesized hollow spheres of cobalt silicate hydroxide Co3(Si2O5)2(OH)2 (denoted as CoSi) using SiO2 spheres as a template through a facile hydrothermal method. Phosphate (PO43−) was then introduced to layered CoSi, leading to the reconstruction of the hollow spheres into sheet-like architectures. As expected, the resulting CoSi-P electrocatalyst demonstrated low overpotential (309 mV at 10 mA·cm−2), large electrochemical active surface area (ECSA), and low Tafel slope. These parameters outperform CoSi hollow spheres and cobaltous phosphate (denoted as CoPO). Moreover, the catalytic performance achieved at 10 mA cm−2 is comparable or even better than that of most transition metal silicates/oxides/hydroxides. The findings indicate that the incorporation of phosphate into the structure of CoSi can enhance its OER performance. This study not only provides a non-noble metal catalyst CoSi-P but also demonstrates that the incorporation of phosphates into transition metal silicates (TMSs) offers a promising strategy for the design of robust, high-efficiency, and low-cost OER catalysts.

Simple two-step development of TiO2/Fe2O3 nanocomposite for oxygen evolution reaction (OER) and photo-bio active applications

The simultaneous incorporation of iron (III) oxide (Fe2O3) and titanium oxide (TiO2) forms a noteworthy composite for oxygen evolution reactions, photocatalytic, and antimicrobial applications. TiO2/Fe2O3 nanocomposite was deposited utilizing a patented, single-step altered pseudo-successive ionic layer adsorption and reaction (p-SILAR) process for the deposition of oxides, which was employed in this study to form Fe2O3 for the first time. The developed nanocomposite exhibits improved electrocatalytic water splitting, photocatalytic activity, and antimicrobial performance. Controlled deposition and heat treatment enable the formation of Fe2O3 and TiO2, as verified by SEM, EAM, and XPS analysis. Incorporation of Fe2O3 significantly enhances the photocatalytic activity of P25 TiO2 nanoparticles, demonstrated by the visible light dye degradation test. The nanocomposite degrades 58% of the Rhodamine B dye in 100 min, while pure TiO2 degrades only 26%. The NC also shows higher antibacterial activity against E. coli and S. aureus compared to pure TiO2. Water splitting performance is evaluated using Electro Impedance Spectroscopy (EIS), Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV), and Tafel Slopes. The results show that TiO2/Fe2O3 NCs exhibit improved oxygen evolution reaction compared to pure TiO2.

Enhanced cycle stability of aprotic Li-O2 batteries based on a self-defensed redox mediator

LiBr as a promising redox mediator (RM) has been applied in Li-O2 batteries to improve oxygen evolution reaction kinetics and reduce overpotentials. However, the redox shuttle of Br3− can induce the unexpected reactions and thus cause the degradation of LiBr and the corrosion of Li anode, resulting in the poor cyclability and the low round-trip efficiency. Herein, MgBr2 is firstly employed with dual functions for Li-O2 batteries, which can serve as a RM and a SEI film-forming agent. The Br– is beneficial to facilitating the decomposition of Li2O2 and thus decreasing the overpotential. Additionally, a uniform SEI film containing Mg and MgO generates on Li anode surface by the in-situ spontaneous reactions of Mg2+ and Li anode in an O2 environment, which can suppress the redox shuttle of Br3− and improve the interface stability of Li anode and electrolyte. Benefiting from these advantages, the cycle life of Li-O2 battery with MgBr2 electrolyte is significantly extended.

Vacancy Promotion in Layered Double Hydroxide Electrocatalysts for Improved Oxygen Evolution Reaction Performance

Layered double hydroxides (LDHs) are promising catalysts for the oxygen evolution reaction (OER) given their modular chemistry and ease of synthesis. Herein, we report a facile strategy for inclusion of oxygen vacancies (VO) using Ce as a promoter in Co–Ni LDHs that significantly enhances the activity for OER. In situ X-ray absorption spectroscopy (XAS) uncovers an increase in octahedral Co sites and VO upon addition of Ce that promotes the transformation of the LDH into an oxyhydroxide-reactive phase more readily. The presence of an OER-active oxyhydroxide phase along with the generation of VO facilitated by the partial reduction of Ce4+ to Ce3+ under oxidizing conditions results in a better electrochemical activity of Ce-doped electrocatalysts. Density functional theory calculations further corroborate the in situ XAS experimental findings by showcasing that the presence of both Ce and VO reduces the free-energy barrier of the rate-limiting OH* deprotonation step during OER. This work showcases how an enhanced understanding of the role of VO promoters in LDH electrocatalysts can provide insights for future catalyst design in anodic reactions.