Oxygen Evolution Reaction Catalytic Activity Research Articles

One-Step Scalable Synthesis of 3D Self-Supported Superaerophobic Ce-Coupled Ni3S2/NiS@NF Nanobud Catalyst for Efficient Oxygen Evolution Reaction

The elaborate design of inexpensive, high-performance electrocatalysts from earth-abundant elements toward oxygen evolution reaction (OER) is critical in various (electro)chemical processes. Herein, a novel binder-free catalyst of Ce-coupled Ni3S2/NiS supported on Ni foam (Ce-Ni3S2/NiS@NF) is successfully synthesized via a facile one-step hydrothermal method that enables practical feasibility with a significant enhancement of OER activity through anchoring Ce dopants on an Ni3S2/NiS nanobud host. Ce species coupling can modulate electronic structure, which reduces the reaction energy barrier and optimizes OER catalytic activity. More profoundly, the superhydrophilic and superaerophobic properties of the Ce-Ni3S2/NiS@NF electrode further promote mass transfer. As a result, the Ce-Ni3S2/NiS@NF electrode exhibits excellent OER activity with a low overpotential of 236 and 350 mV to achieve current densities of 10 and 100 mA cm−2, respectively, and long-term durability for 24 h in alkaline medium. These results could supply valuable guidelines for the design of other OER catalysts and beyond.

Strategic Defect Engineering Enabled Efficient Oxygen Evolution Reaction in Reconstructed Metal‐Organic Frameworks

AbstractMetal‐organic frameworks (MOFs) have emerged as promising pre‐catalysts for oxygen evolution reaction (OER) due to their marvelous structural reconstruction process in strongly alkaline media. However, targeting design MOF structures to achieve excellent OER performance of reconstructed products is a challenge. Here, a strategic defect engineering is used to promote the OER performance of reconstructed products. Briefly, modified linkers with monocarboxylic acids (ferrocene carboxylic acid, FcCA) are incorporated into MOF (NiBDC‐FcCA), leading to its stepwise reconstruction into Fe‐doped Ni(OH)2 and NiOOH during the OER process, with the oxygen vacancy and strategic doping of metal Fe persisting throughout the multi‐step reconstruction. Benefiting from the synergistic interaction of oxygen vacancies and Fe doping, NiBDC‐FcCA delivers the extremely enhanced current density at 1.6 V versus reversible hydrogen electrode by ≈9 times compared with that of NiBDC. Moreover, the optimized NiBDC‐FcCA/Fe foam exhibits excellent OER catalytic activity and stability with a low overpotential of 250 mV at 200 mA cm−2 and negligible activity decay after 1200 h at 1 A cm−2. Density function theory calculations reveal that Fe doping weakens the interaction of oxygen intermediate with Ni sites, favoring the formation of OOH* to accelerate the OER process.

CoFe2O4 with the In-Situ Formed Oxygen Vacancies and Co Particles as an Efficient Bifunctional Catalyst for Rechargeable Zinc-Air Batteries.

Rechargeable zinc-air batteries (RZABs) are considered as one of the most promising clean energy device due to their abundant resources, low cost and environmental friendliness. However, their energy efficiency and cycle life are far from satisfactory due to the poor activity and stability of bi-functional electrocatalyst in air cathode. In this work, an efficient bi-functional catalyst (rGO-CoFe2O4/Co) was derived from its precursor (rGO-CoFe2O4) through a simple annealing process. Electrochemical measurements prove that rGO-CoFe2O4/Co with the in-situ formed Co nano particles and rich oxygen vacancies appears excellent oxygen reduction reaction and oxygen evolution reaction catalytic activity compared to its counterpart. Its half-wave potential is 0.81 V (vs RHE) and the OER overpotential is only 310 mV (vs RHE). In addition, rechargeable zinc-air batteries assembled with rGO-CoFe2O4/Co show the highest peak power density (128.9 mW cm-2) and cycling stability compared to rGO-CoFe2O4 and commercial Pt/C-RuO2 catalysts. This work provides a simple strategy for the design of advanced bifunctional catalysts.

Fluorine‐Lodged High‐Valent High‐Entropy Layered Double Hydroxide for Efficient, Long‐Lasting Zinc‐Air Batteries

AbstractEfficient and stable bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts are urgently needed to unlock the full potential of zinc‐air batteries (ZABs). High‐valence oxides (HVOs) and high entropy oxides (HEOs) are suitable candidates for their optimal electronic structures and stability but suffer from demanding synthesis. Here, a low‐cost fluorine‐lodged high‐valent high‐entropy layered double hydroxide (HV‐HE‐LDH) (FeCoNi2F4(OH)4) is conveniently prepared through multi‐ions co‐precipitation, where F− are firmly embedded into the individual hydroxide layers. Spectroscopic detections and theoretical simulations reveal high valent metal cations are obtained in FeCoNi2F4(OH)4, which enlarge the energy band overlap between metal 3d and O 2p, enhancing the electronic conductivity and charge transfer, thus affording high intrinsic OER catalytic activity. More importantly, the strengthened metal‐oxygen (M−O) bonds and stable octahedral geometry (M−O(F)6) in FeCoNi2F4(OH)4 prevent structural reorganization, rendering long‐term catalytic stability. Furthermore, an efficient three‐phase reaction interface with fast oxygen transportation was constructed, significantly improving the ORR activity. ZABs assembled with FeCoNi2F4(OH)4@HCC (hydrophobic carbon cloth) cathodes deliver a top performance with high round‐trip energy efficiency (61.3 % at 10 mA cm−2) and long‐term stability (efficiency remains at 58.8 % after 1050 charge–discharge cycles).

Fluorine-lodged high-valent high-entropy layered double hydroxide for efficient, long-lasting zinc-air batteries.

Efficient and stable bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts are urgently needed to unlock the full potential of zinc-air batteries (ZABs). High-valence oxides (HVOs) and high entropy oxides (HEOs) are suitable candidates for their optimal electronic structures and stability but suffer from demanding synthesis. Here, a low-cost fluorine-lodged high-valent high-entropy layered double hydroxide (HV-HE-LDH) (FeCoNi2F4(OH)4) is conveniently prepared through multi-ions co-precipitation, where F- are firmly embedded into the individual hydroxide layers. Spectroscopic detections and theoretical simulations reveal high valent metal cations are obtained in FeCoNi2F4(OH)4, which enlarge the energy band overlap between metal 3d and O 2p, enhancing the electronic conductivity and charge transfer, thus affording high intrinsic OER catalytic activity. More importantly, the strengthened metal-oxygen (M-O) bonds and stable octahedral geometry (M-O(F)6) in FeCoNi2F4(OH)4 prevent structural reorganization, rendering long-term catalytic stability. Furthermore, an efficient three-phase reaction interface with fast oxygen transportation was constructed, significantly improving the ORR activity. ZABs assembled with FeCoNi2F4(OH)4@HCC (hydrophobic carbon cloth) cathodes deliver a top performance with high round-trip energy efficiency (60.6% at 10 mA cm-2) and long-term stability (efficiency remains at 58.8% after 1050 charge-discharge cycles).

Fluorine-doped carbon dots decorated on iron-cobalt selenide nanosheets as superior electrocatalysts for oxygen evolution in seawater

Developing oxygen evolution reaction (OER) catalysts that can efficiently and stably operate at high current densities in seawater electrolysis is urgently demanded but challenging. In this study, selenium-vacancies-rich iron-cobalt selenide nanosheets (FeCoSe-VSe) decorated with fluorine-doped carbon dots (FCDs) are rationally designed and prepared on nickel foams (NF) through the successive processes of hydrothermal treatment and selenization. Benefiting from the large accessible surface area and abundant active sites provided by FCDs and selenium vacancy defects, as well as the coupled interface between FCDs and FeCoSe-VSe that optimizes the oxygen adsorption energy and accelerates the kinetic process, the FCDs/FeCoSe-VSe/NF composite exhibits excellent OER catalytic activities with a low overpotential of 258 mV at a current density of 200 mA cm−2 in 1 M KOH and a small Tafel slope of 34.7 mV dec–1. In the electrolytes of simulated and natural seawater, the overpotentials of the FCDs/FeCoSe-VSe/NF electrode are slightly higher, measured to be 264 and 297 mV, respectively. More profoundly, the introduction of FCDs enhances the corrosion resistance of the catalyst in alkaline electrolytes, ensuring the long-term stability. This work offers a guidance to improve the electrochemical performances of catalysts for high-efficiency seawater oxidation.

Ce-doped NiCo-layered double hydroxide based self-supporting tremella-shaped nanoflower heterostructure as efficient electrocatalyst for the oxygen evolution reaction

NiCo-layered double hydroxide (NiCo-LDH) catalysts, which indicate a profound prospect of oxygen evolution reaction (OER) under alkaline conditions, are facing great challenges in terms of low electrical conductivity, restricted by poor conductivity, limited electrochemical reaction sites, and low-density of edge sites. The authors developed a simple and convenient in-situ one-pot self-assembly method. The Ce-doped defective NiCo-LDH hexagonal nanosheets and nitrogen-doped graphene fabricated the NiCoCe-LDH/RNF electrocatalyst on the nickel foam (NF) substrate, which demonstrated excellent OER activity under alkaline conditions, only needed 299 mV overvoltage to achieve 50 mA cm−2 current intensity, could last for 6 hours at 20 mA cm−2. The highly active OER benefited from the structural advantages of the novelty three-dimensional array structure. The novel geometric structure of tremella-shaped nanoflower integrated with curved nanosheets and curled edges loaded on NF provides more electrochemical reaction sites for intermediate coordination. The nitrogen-doped reduced graphene oxide accommodated better interaction between active materials and charge carriers, enhancing the conductivity and electron transfer performance of nanostructures. The effective synergistic between the heterostructures promoted the excellent OER electrocatalytic activity of NiFeCe-LDH/RNF nanocomposites. DFT calculations at the atomic level of pristine and Ce-doped NiCo-LDH exhibited that Ce-doping can reduce the Gibbs free energy of the potential-determining step and improve the catalytic activity of OER. This work has broadened the horizons for developing simple and highly available OER electrocatalysts.

P-functionalization of Ni Fe − Electrocatalysts from Prussian blue analogue for enhanced anode in anion exchange membrane water electrolysers

Efficient hydrogen generation from water-splitting is widely acknowledged as a priority route to promote the hydrogen economy. Anion exchange membrane water electrolyzers (AEMWE) offer multiple advantages in improving performance and minimizing the cost limitations of current electrolysis technologies. However, the persistence of issues related to the limited electrocatalytic activity of such materials and their poor stability under operating conditions makes developing highly active, stable, platinum-group-metal-free electrocatalysts for oxygen evolution reaction (OER) necessary.We report the development of Prussian blue analogues (PBA)-derived NiFe-based electrocatalysts through a mild aqueous phase precipitation method, followed by thermal stabilization and phosphorus doping. The formation of the NiFe-PBA-precursor with a framework nanocubic Ni(II)[Fe(III)(CN)6]2/3 structure was confirmed by X-ray diffraction, scanning electron microscopy, and inductively coupled plasma analysis. The NiFe-PBA-precursor was subjected to thermal stabilization and phosphorus doping to provide the material with enhanced OER catalytic activity and stability. The existence of OER active sites based on NiFe and NiFeP has been revealed by transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical characterization in a three-electrode cell configuration in a 1 M KOH electrolyte. NiFe-PBA and NiFeP-PBA were assembled at the anode side of an AEMWE, resulting in an excellent electrochemical performance both in terms of current density at 2.0 V using 1 M KOH (1.21 A cm−2) and durability, outperforming the benchmark catalyst.

Unlocking OER potential: Tailoring layered double perovskites through self-reconstruction

The sluggish rate of the anode oxygen evolution reaction (OER) is a major bottleneck for green hydrogen production, making a room-temperature alkaline water electrolyser critical to unlocking the full potential of this technology. Using a perovskite oxide as an electrochemical matrix and forming a self-assembled oxyhydroxide (OOH) layer on the surface via a self-reconstruction activation process (SRAP) is a promising strategy to enhance OER catalytic activity. However, the mechanistic details and long-term impact of SRAP, especially its relationship to catalytic activity, oxygen collection efficiency, and catalyst dissolution, are not fully understood. To address this knowledge gap, we used layered double perovskite (LDP) PrBa0·75Ca0·25Co1·5Fe0·5O5+δ(PBCCF) and PrBa0·75Ca0·25Co1·5Fe0·4Ni0·1O5+δ (PBCCFN) as model electrochemical catalysts and then triggered SRAP. Subsequently, chronoamperometry (CA), rotating ring disk electrode (RRDE) and in-situ electrochemical quartz crystal microbalance (EQCM) were used to study the effect of the Ni-doping on PBCCF SRAP in terms of catalytic activity, oxygen collection efficiency and catalyst stability. The results showed that the OER catalytic current of the LDP PBCCF and PBCCFN was enhanced via SRAP and the oxygen collection efficiency can be maintained, suggesting that the increase in catalytic current is attributed to OER catalytic activity increase rather than electrochemical catalyst dissolution. Additionally, the effect of the catalytic activity enhancement was more significant in PBCCFN. More importantly, the stability of the LDP PBCCF and PBCCFN after SRAP are higher than simple perovskite Ba0·5Sr0·5Co0·8Fe0·2O3-δ (BSCF) because of the increase in the redeposition rate. The observations suggest that using LDP PBCCF and PBCCFN as matrices followed by triggering SRAP can provide valuable insights for designing LDP based OER electrocatalysts.

Enhanced oxygen evolution reaction activity of oxygen-deficient perovskite catalysts with A-site defects boosted by self-assembled silver nanoparticles

To enhance the competitiveness of perovskite electrocatalysts in oxygen evolution reaction (OER), it is vital to improve its catalytic activity and durability while controlling costs. Herein, we report a composite electrocatalyst of Ag/Sr0·8Ag0·2Co0·75Fe0·25O3-δ, which was designed by integrating internal structure-engineering, A-site defect, and in-situ Ag segregation. This electrocatalyst demonstrates an excellent OER performance in an alkaline electrolyte, surpassing the catalytic activity and durability of commercial RuO2 electrocatalysts. Various characterization techniques as well as density functional theory (DFT) calculations were performed to elucidate the synergistic coupling effects of Sr0·8Ag0·2Co0·75Fe0·25O3-δ with Ag nanoparticles (NPs) in the OER process. The robust interface anchoring Ag NPs on Sr0·8Ag0·2Co0·75Fe0·25O3-δ perovskite can induce ligand effects and interparticle cooperation, thereby promoting electron transfer and ion migration during OER. Meanwhile, partial Ag+ forms Ag NPs contributing to the formation of A-site defects, which results in an increase of oxygen vacancies and induces lattice expansion. A-site defects can optimize the occupancy of eg orbitals close to unity for B-site transition metals, thereby greatly enhancing OER catalytic activity. This work highlights an effective strategy to enhance OER activity of perovskite electrocatalysts.

Enhancing the oxygen evolution reaction activity and stability of high-valent CoOOH by switching the catalytic pathway through doping low-valent Cu

The oxygen evolution reaction (OER) is a critical process in electrochemical energy storage and conversion systems. The adsorbate evolution mechanism (AEM) pathway possesses the characteristics of high stability but slow catalytic kinetics. We propose that combining AEM with the lattice oxidation mechanism (LOM) pathway can potentially enhance the OER catalytic activity and stability. However, the triggering of LOM is an important challenge due to the high thermodynamic activation barrier of lattice oxygen. To solve this problem, we performed theoretical calculations and experiments which suggest that the introduction of low-valent Cu in CoOOH (CuxCo1−xOOH) could directionally modulate the local coordination environment of CoO bonds. This approach can activate lattice oxygen and generate oxygen vacancies to enhance the nucleophilic attack of *OH and directly establish OO coupling, thereby facilitating the smoothly switching from AEM to LOM pathway by increasing voltage and thus activating lattice oxygen in CuxCo1−xOOH. The switching of AEM and LOM enables CuxCo1−xOOH showing an outstanding overpotential of only 252 mV (10 mA cm−2) and durability of only 2.80 % degradation after 280h. This work provides a new way for designing efficient and stable electrocatalysts with AEM and LOM pathway switching.

Magnetically Enhanced Oxygen Evolution Reaction in Mild Alkaline Electrolytes by Building Catalysts on Magnetic Frame.

Under large current densities, the excessive hydroxide ion (OH) consumption hampers alkaline water splitting involving the oxygen evolution reaction (OER). High OH concentration (≈30wt.%) is often used to enhance the catalytic activity of OER, but it also leads to higher corrosion in practical systems. To achieve higher catalytic activity in low OH concentration, catalysts on magnetic frame (CMF) are built to utilize the local magnetic convection induced from the host frame's magnetic field distributions. This way, a higher reaction rate can be achieved in relatively lower OH concentrations. A CMF model system with catalytically active CoFeOx nanograins grown on the magnetic Ni foam is demonstrated. The OER current of CoFeOx@NF receives ≈90% enhancement under 400mT (900mAcm-2 at 1.65V) compared to that in zero field, and exhibits remarkable durability over 120h. As a demonstration, the water-splitting performance sees a maximum 45% magnetic enhancement under 400mT in 1m KOH (700mAcm-2 at 2.4V), equivalent to the concentration enhancement of the same electrode in a more corrosive 2m KOH electrolyte. Therefore, the catalyst-on-magnetic-frame strategy can make efficient use of the catalysts and achieve higher catalytic activity in low OH concentration by harvesting local magnetic convection.

An Investigation of the Interface between Transition Metal Oxides (MnOx, FeOx, CoOx and NiOx)/MoO3 Composite Electrocatalysts for Oxygen Evolution Reactions

This study presents the synthesis of a multicomponent metal oxide electrocatalyst that increases the activity of the oxygen evolution reaction (OER). We synthesized transition metal oxides (MnOx, FeOx, CoOx, and NiOx) with MoO3 heterostructures through a solid-state reaction approach at low cost. In comparison to the other compositions, CoOx garnered higher attention and demonstrated superior performance on account of its large surface area and varied crystal facets. The MnOx-MoO3, FeOx-MoO3, CoOx-MoO3, and NiOx-MoO3 compositions attained an overpotential of 390 mV, 350 mV, 310 mV, and 340 mV, respectively, at a current density of 10 mA cm−2 in alkaline solution. The performance of OER was enhanced in CoOx-MoO3 at 10 mA cm−2, while FeOx-MoO3 exhibited a lower current density at 100 mA cm−2 than other metal oxides. The CoOx-MoO3 material exhibited a favorable crystal interface transition due to the presence of MoO3 oxide. For the first time, we report on the MoO3-to-(MnOx, FeOx, CoOx, and NiOx) interface crystal transition and the active surface area for OER catalytic activity in water-splitting processes. This investigation intends to develop an electrocatalyst that is capable of producing hydrogen with the use of heterostructure metal oxides.

20 Seconds to fabricate high-performance NiFe-based anode for seawater electrolysis via bidirectional pulse current method

Direct electrolysis of seawater to produce green hydrogen has attracted great attention. While the development of efficient and rapid manufacturing processes for high-performance electrodes, particularly for the oxygen evolution reaction (OER), remains trail. Here, we introduce an innovative approach to fabricate high-performance NiFe-based OER electrodes using an ultrafast and eco-friendly bidirectional pulse current (BPC) method, which remarkably shortens the production time to only 20 s. Our BPC method alternates oxidation and reduction currents within a single period, promoting the in-situ formation of porous NiFe(oxy)hydroxide nanosheets through dissolution/deposition processes. Moreover, this methodology creatively utilizes the “Cl− induced effect”, typically a hindrance in other electrochemical systems involving seawater or brine, to enhance the synthesis of electrodes from NaCl-based electrolyte devoid of additional Ni/Fe cations. The prepared NiFe-based electrode (NFF O-R 20 s) demonstrates remarkable OER catalytic activity, with optimal overpotentials (ηj) of 272, 308 and 367 mV at current densities (j) of 10, 100 and 1000 mA cm−2, respectively. Impressively, it maintains a low cell voltage of 1.59 V after enduring 1000 h of operation under the industrially current density of 300 mA cm−2, with a direct current energy consumption of 3.8 kWh Nm−3 H2 for overall alkaline seawater electrolysis. Our BPC preparation approach provides a promising path to construction high-catalytic activity, good-stability electrode, and low-energy consumption direct seawater electrolyzer.

Plasma-Induced Oxygen Defect Engineering in Perovskite Oxide for Boosting Oxygen Evolution Reaction.

Perovskite oxides are considered highly promising candidates for oxygen evolution reaction (OER) catalysts due to their low cost and adaptable electronic structure. However, modulating the electronic structure of catalysts without altering their nanomorphology is crucial for understanding the structure-property relationship. In this study, a simple plasma bombardment strategy is developed to optimize the catalytic activity of perovskite oxides. Experimental characterization of plasma-treated LaCo0.9Fe0.1O3 (P-LCFO) reveals abundant oxygen vacancies, which expose numerous active sites. Additionally, X-ray photoelectron spectroscopy and X-ray absorption fine structure analyses indicate a low Co valence state in P-LCFO, likely due to the presence of these oxygen vacancies, which contributes to an optimized electronic structure that enhances OER performance. Consequently, P-LCFO exhibits significantly improved OER catalytic activity, with a low overpotential of 294mV at a current density of 10mAcm-2, outperforming commercial RuO2. This work underscores the benefits of plasma engineering for studying structure-property relationships and developing highly active perovskite oxide catalysts for water splitting.

Modulating the valence states of transition metals in FeCoNiMnW high-entropy alloys for enhanced oxygen evolution reaction activity

Amorphous high-entropy alloys (HEA) are promising electrocatalysts with multi-element active sites and dense defect distribution, high valences of 3d transition metals (3d TMs) are beneficial for the oxygen evolution reaction (OER). However, it is still unknown how to enhance the 3d TMs valence state in amorphous HEA to improve OER catalytic activity. Herein, we successfully boosted the valence state of 3d TMs in HEA through electronegativity. The prepared FeCoNiMnW HEA exhibited significant amorphous characteristics. X-ray photoelectron spectroscopy analysis confirmed the formation of Ni3+ sites after adding electronegative W and the widespread occurrence of oxygen vacancies through amorphization. The enhanced oxidation state of Ni reduced the overpotential, while the abundant oxygen vacancies significantly facilitated electron transfer and thus boosting the rate of catalytic reactions. These characteristics lead to the amorphous FeCoNiMnW HEA exhibiting excellent OER performance. Amorphous FeCoNiMnW HEA films deposited on carbon cloth loaded with carbon nanotubes (FeCoNiMnW@CCC) only require an overpotential of 253 mV to generate a current density of 10 mA cm−2, which is much lower than RuO2 films (399 mV) and commercial RuO2 (309 mV), and shows excellent durability for 700 h at 20 mA cm−2 and 100 h at 200mA cm−2. This work provides a novel strategy for designing high-performance OER catalysts through structural and electronic modulation.

Novel Ni-doped dual MOF-derived urchin-like Co-Fe layered double hydroxides for oxygen evolution reaction

The oxygen evolution reaction (OER) plays a vital role in water splitting for renewable energy generation, making the development of cost-effective electrocatalysts essential in tackling global energy challenges. In this study, we synthesized ZIF-67@MIL-88 using a self-assembly method, with ZIF-67 serving as the precursor, followed by an aging process. The incorporation of iron-based MIL-88 significantly enhanced the surface morphology and porous structure of the catalytic material. Additionally, we prepared urchin-like CoNi@FeNi layered double hydroxides (LDHs) by etching ZIF-67@MIL-88. LDHs are known for their unique two-dimensional sheet structure and corrosion resistance. We achieved efficient OER catalytic activity through interlayer anion exchange by substituting metal cations at high temperatures. This study highlights recent advancements in the rational design of structurally unique and highly effective CoNi@FeNi-LDH composite materials using a synergistic strategy.

Surface reconstruction of defect-engineered MIL-88@Fe2O3 p-n heterojunction for enhanced electrocatalytic water and urea oxidation

Defective p-n heterojunction engineering has been under the spotlighted as a promising strategy for electrochemical oxygen evolution reaction (OER) and urea oxidation reaction (UOR). Still, it remains a great challenge how to use the space-charge region of p-n heterojunction to study the regulation mechanism of the p-n heterogeneous interface and vacancy defects on the incomplete self-reconstruction from pristine materials to (oxy)hydroxides. Herein, NH2-MIL-88b(Fe)/α-Fe2O3 p-n heterojunction with rich oxygen vacancies (denoted as p-n MIL-88@Fe2O3-OV) has been prepared using a simple one-step solvothermal method. The prepared catalyst exhibits promising electrocatalytic performance for OER and UOR, requiring low potentials of 1.50 and 1.35 V to reach the current density of 10 mA cm−2, respectively. The combined in-situ/ex-situ experimental and theoretical investigations unveil that p-n heterojunction engineering can not only facilitate charge transfer from n-type α-Fe2O3 to p-type NH2-MIL-88b(Fe) (denoted as MIL-88) and generate abundant oxygen vacancies, but also appreciably trigger phase transformation of MIL-88 into active FeOOH. The reconstructed FeOOH/α-Fe2O3 heterojunction optimizes the bonding strength towards key oxygen-containing intermediates and boosts the intrinsic activity. This work provides new insight into the self-reconstruction effect in the p-n heterojunction for enhancing the OER and UOR catalytic activity.

Dynamic Activity and Stability of Transition Metal (oxy)Hydroxide Oxygen Evolution Electrocatalysts Under Steady and Intermittent Operation

The oxygen evolution reaction (OER) plays a critical role in several energy conversion technologies, requiring catalysts that combine high activity with stability for successful commercialization. Catalysts from first-row transition metal oxides have garnered considerable attention in this context. Their noteworthy OER activity and stability in alkaline environments position them as a more economical option than their counterparts in acidic media. However, recent studies have shown that these materials tend to experience significant dissolution issues and undergo changes in their chemical composition, a phenomenon observed even during steady-state operation.1–3 Addressing these challenges is essential for advancing commercial energy conversion technologies, which often face intermittent loads and reverse currents. A deeper understanding of the dynamic reconstruction under fluctuating conditions is crucial for developing more durable and efficient electrocatalytic materials.4 In this work, we investigate the dynamic performance of nickel cobalt (oxy)hydroxide electrocatalytic films containing iron active sites in alkaline media under steady and intermittent operation. First, we systematically examine the changes in OER catalytic activity in a typical three-electrode configuration, focusing on the incorporation and redissolution of iron and cerium during different electrochemical conditioning methods. As shown in Figure 1a, conditioning using chronopotentiometry decreases the OER overpotential without significantly shifting the redox peak. In contrast, conditioning using cyclic voltammetry significantly shifts the redox peak position (Figure 1b). These differences are attributed to the (oxy)hydroxide film thickness, which forms in situ during conditioning and affects the Fe dissolution and incorporation rates based on the conditioning method.5 We use different analytical techniques to probe chemical transformations during conditioning. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) are utilized to analyze compositional changes in the electrocatalytic films. Moreover, inductively coupled plasma mass spectrometry (ICP-MS) tracks metal dissolution.Next, we utilize an electrochemical flow electrolyzer with a zero-gap configuration, integrated with online ICP-MS and post-experimental TOF-SIMS and XPS, to study the reconstruction of these electrocatalysts under intermittent conditions. A NiFe anode operating in 6 M KOH with 1 ppm of Fe3+ shows a 40-mV change after 120 cycles of alternating low and high currents, including a simulated shutdown step (Figure 1c). This cell potential change, becoming more negative with continued cycling (Figure 1d), is attributed to the shutdown step. Our findings underscore how operating conditions affect the dynamic reconstruction of electrocatalytic materials, offering critical insights for developing more robust and resilient materials suitable for commercial applications.

Catalytic Activities of Ir-Based Pyrochlore-Type Oxides Doped with Divalent Cations for Oxygen Evolution Reaction

Alternative energy sources to fossil fuels are essential for a sustainable society, and hydrogen energy is expected to be a solution to environmental and energy problems. Currently, proton exchange membrane water electrolyzers are becoming widely used for the electrochemical production of hydrogen due to their advantages such as high efficiency. However, the highly acidic conditions and the large overpotentials of oxygen evolution reaction (OER) at the anode are problems, and the development of catalysts that are stable and highly active in acidic aqueous solutions is important for further popularization.Ir-based oxides with pyrochlore structure (A2B2O7) are known to exhibit high catalytic activities in acidic aqueous solutions. Recently, it has been reported that the improved catalytic activities of Pr2Ir2O7 by Zn doping, which controlled the composition of A-site elements in the pyrochlore structure [1]. In this study, we synthesized Pr2-x MgxIr2O7 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5), in which a part of trivalent cation Pr3+ was replaced by divalent cation Mg2+ and improved the OER catalytic activity in acidic aqueous solutions.Pr2-x MgxIr2O7 was synthesized by the hydrothermal method. The catalyst was mixed with a conductivity aid carbon (Vulcan XC-72) and a cationic conductive ionomer (Nafion) at a weight ratio of 5:1:1 and dispersed in 2-propanol to form a catalyst ink. The catalytic activity was evaluated by cyclic voltammetry (CV) using a glassy carbon rotating disk electrode (RDE) as a working electrode, a reversible hydrogen electrode (RHE) as a reference electrode, and a platinum wire as a counter electrode. The electrolyte was a 0.1 mol dm-3 HClO4 solution saturated with oxygen, and the CV scanning range was 1.1 to 1.7 V with respect to the RHE, scanning speed was 20 mV s-1, and the RDE rotation speed was 1200 rpm. The stability of the catalyst in acidic aqueous solutions was evaluated by chronopotentiometry (CP) using the same cell.X-ray diffraction and energy-dispersive X-ray analysis measurements of the synthesized materials confirmed that the desired compounds were obtained, and CV measurements showed that the OER activity of Pr2-x MgxIr2O7 increased in the first 20 cycles for all compositions, and then stabilized over several tens of cycles. The cycle change was similar to that of Pr2Ir2O7, suggesting that the surface rearrangement proceeds in acidic aqueous solutions [2]. Linear sweep voltammogram (LSV) and OER activity normalized by Brunauer-Emmett-Teller (BET) specific surface area were measured at the 20th cycle, and the catalytic activity stabilized at a high value. The OER activity was enhanced by Mg substitution in the catalysts of Pr2-x Mg x Ir2O7.References L. Zhu, C. Ma, Z. Wang, X. Gong, L. Cao, and J. Yang, Appl. Surf. Sci., 31, 151840 (2022).C. Shang, C. Cao, D. Yu, Y. Yan, Y. Lin, H. Li, T. Zheng, X. Yan, W. Yu, S. Zhou, and J. Zeng, Adv. Mater., 31, 1805104 (2018).