Enhanced Oxygen Evolution Reaction Activity Research Articles

Exclusive Strain Effect Boosts Overall Water Splitting in PdCu/Ir Core/Shell Nanocrystals

AbstractCore/shell nanocatalysts are a class of promising materials, which achieve the enhanced catalytic activities through the synergy between ligand effect and strain effect. However, it has been challenging to disentangle the contributions from the two effects, which hinders the rational design of superior core/shell nanocatalysts. Herein, we report precise synthesis of PdCu/Ir core/shell nanocrystals, which can significantly boost oxygen evolution reaction (OER) via the exclusive strain effect. The heteroepitaxial coating of four Ir atomic layers onto PdCu nanoparticle gives a relatively thick Ir shell eliminating the ligand effect, but creates a compressive strain of ca. 3.60%. The strained PdCu/Ir catalysts can deliver a low OER overpotential and a high mass activity. Density functional theory (DFT) calculations reveal that the compressive strain in Ir shell downshifts the d‐band center and weakens the binding of the intermediates, causing the enhanced OER activity. The compressive strain also boosts hydrogen evolution reaction (HER) activity and the strained nanocrystals can be served as excellent catalysts for both anode and cathode in overall water‐splitting electrocatalysis.

Exclusive Strain Effect Boosts Overall Water Splitting in PdCu/Ir Core/Shell Nanocrystals.

Core/shell nanocatalysts are a class of promising materials, which achieve the enhanced catalytic activities through the synergy between ligand effect and strain effect. However, it has been challenging to disentangle the contributions from the two effects, which hinders the rational design of superior core/shell nanocatalysts. Herein, we report precise synthesis of PdCu/Ir core/shell nanocrystals, which can significantly boost oxygen evolution reaction (OER) via the exclusive strain effect. The heteroepitaxial coating of four Ir atomic layers onto PdCu nanoparticle gives a relatively thick Ir shell eliminating the ligand effect, but creates a compressive strain of ca. 3.60%. The strained PdCu/Ir catalysts can deliver a low OER overpotential and a high mass activity. Density functional theory (DFT) calculations reveal that the compressive strain in Ir shell downshifts the d-band center and weakens the binding of the intermediates, causing the enhanced OER activity. The compressive strain also boosts hydrogen evolution reaction (HER) activity and the strained nanocrystals can be served as excellent catalysts for both anode and cathode in overall water-splitting electrocatalysis.

Spatial Confinement of a Carbon Nanocone for an Efficient Oxygen Evolution Reaction.

A major bottleneck of large-scale water splitting for hydrogen production is the lack of catalysts for the oxygen evolution reaction (OER) with low cost and high efficiency. In this work, we proposed an electrocatalyst of a curved carbon nanocone embedded with two TMN4 active sites (TM = transition metal) and used first-principles calculations to investigate their OER mechanisms and catalytic activities. In the particular spatial confinement of a curved nanocone, we found that the distance between intermediates adsorbed on two active sites is shorter than the distance between these two active sites. This finding can be used to enhance OER activity by distance-dependent interaction between intermediates through two different mechanisms. The first mechanism in which an O2 molecule is generated from two neighboring *O intermediates exhibits a linear activity trend, and the lowest overpotential is 0.27 V for the FeN4 system. In the second mechanism, selective stabilization of the *OOH intermediate is realized, leading to a new scaling relationship (ΔG*OOH = ΔG*OH + 3.04 eV) associated with a modified OER activity volcano (theoretical volcano apex at 0.29 V). The studied mechanisms of the spatial confinement of a carbon nanocone provide a new perspective for designing efficient OER catalysts.

Fabrication of hierarchically flower-like trimetallic coordination polymers via ion-exchange strategy for efficient electrocatalytic oxygen evolution

Designing a highly efficient electrocatalyst with more active sites and faster charge transfer is crucial to improve oxygen evolution reaction (OER) performance. Herein, hierarchically trimetallic Co0.7Fe0.24Ni0.06-BDC (1,4-benzenedicarboxylate, denoted as BDC) coordination polymers have been prepared via ion-exchange by immersing Co-BDC micro-flowers into bimetallic solution under mild conditions. This strategy introduces the other transition metal ions into Co-BDC micro-flower structure without destroying Co-BDC superior flower-like morphology. As electrocatalyst in OER, Co0.7Fe0.24Ni0.06-BDC exhibits an outstanding performance with a small overpotential of 291 mV at 50 mA·cm−2 and a Tafel slope of 60.8 mV·dec−1. Durability of Co0.7Fe0.24Ni0.06-BDC is demonstrated by long-term electrochemical stability for 12 h at current density of 10 mA·cm−2. This enhanced OER activity of Co0.7Fe0.24Ni0.06-BDC micro-flowers attribute to the superior morphology of micro-flowers and the synergistic effect of tri-metal, inducing the presence of abundant active sites and fast charge transfer. This work suggests that Co0.7Fe0.24Ni0.06-BDC micro-flowers have great practical potential in the field of OER.

Gold nanocrystal decorated trimetallic metal organic frameworks as high performance electrocatalysts for oxygen evolution reaction

Electronic states of active sites of catalysts play a key role in determining electrocatalytic performances. Engineering electronic states through decoration of functional entities is an effective way to advance catalyst design. In this work, Au nanocrystal decorated Fe, Co, and Ni based trimetallic metal organic frameworks (MOFs) (termed Au/(FCN)MOFs), grown in-situ on nickel plates (NP), are developed as high performance oxygen evolution reaction (OER) electrocatalysts. At optimal Au nanocrystal decoration, the Au/(FCN)MOF/NP electrode achieves significantly improved OER performances over the (FCN)MOF/NP electrode, lowering η10 and η500 from 242 and 340 mV to 216 and 290 mV, respectively, Tafel slope from 42.1 to 31.7 mV dec−1, and current density decay from 6 to 5% after 50 h operations at an initial current density of 160 mA cm-2. The enhancement is attributed to the significantly elevated oxidation states of Fe, Co, and Ni, induced by decoration of the Au nanocrystals of high electronegativity, as evidenced with high-resolution X-ray photoelectron spectroscopy and in-situ Raman spectroscopy studies. Furthermore, the main active metal centers are identified with in-situ X-ray absorption spectroscopy study to be Ni and Co, with Fe playing a supporting role to help Ni acquire high valence states to realize enhanced OER activities.

Multi‐chambered, carbon‐coated Ni0.4Fe2.6O4 nanoparticle superlattice microspheres for boosting water oxidation reaction

AbstractDeveloping active and robust non‐noble‐metal‐based electrocatalysts for the oxygen evolution reaction (OER) is of vital practical significance for accelerating the kinetics of water splitting. Here, a novel double emulsion template method is proposed to design and prepare hierarchically multichambered, carbon‐coated Ni0.4Fe2.6O4 nanoparticle superlattice microspheres (M‐NFO@C‐NSMs) for the highly efficient oxygen evolution. The high‐temperature calcination under inert gas enables an improved electrochemical property by rationally transforming the long‐chain organic capping ligands into partially graphitized uniform carbon coatings. More importantly, benefiting from the unique hierarchical superstructure with macro‐/meso‐/microporosities and three‐dimensional continuous conductive carbon frameworks, M‐NFO@C‐NSMs exhibit comprehensively enhanced OER activity in a dilute alkaline electrolyte as compared to their solid counterparts and most spinel‐based electrocatalysts reported to date. Notably, the collective property of supraparticles endowed M‐NFO@C‐NSMs with superior long‐term cyclic stability. This work sheds light on the sophisticated design of functionalized supraparticles for efficient water splitting.

Electrochemically Activated NiFeOxHy for Enhanced Oxygen Evolution

To minimize the utilization of fossil fuels, the demand for H2 production through water electrolysis has considerably increased. NiFeOxHy-based electrocatalysts have attracted extensive attention owing to their excellent electrocatalytic activity in the oxygen evolution reaction (OER). To realize their commercialization, NiFeOxHy-based catalysts are being ameliorated for better performance in the OER. Here, we present a simple and scalable approach for fabricating NiFeOxHy by preparing a Ni–Fe master alloy plate followed by electrochemical activation including cyclized oxidation and reduction of metal ions. During the activation, the amorphous layer evolved only on the catalyst surface by circulating the oxidation states of Ni and Fe. The activated NiFeOxHy required only 250 mV of overpotential for 10 mA cm–2 and 30 mV dec–1 of Tafel slope, which is much better than the electrodeposited NiFeOxHy. The enhanced OER activity originates from the amorphous surface layer which contains partially oxidized Ni ions. This study suggests valuable possibilities for the design of better OER electrocatalysts using simple electrochemical methods.

Interfacial La Diffusion in the CeO2/LaFeO3 Hybrid for Enhanced Oxygen Evolution Activity.

The electrochemical oxygen evolution reaction (OER) is of great significance for energy conversion and storage. The hybrid strategy is attracting increasing interest for the development of highly active OER electrocatalysts. Regarding the activity enhancement mechanism, electron coupling between two phases in hybrids has been widely reported, but the interfacial elemental redistribution is rarely investigated. Herein, we developed a CeO2/LaFeO3 hybrid electrocatalyst for enhanced OER activity. Interestingly, a selective interfacial La diffusion from LaFeO3 to CeO2 was demonstrated by the electron energy loss spectra and elemental mapping. This redistribution of cations triggers the change of the chemical environment of interface elements for charge compensation because of the electroneutrality principle, which results in increased oxygen vacancies and high-valent Fe species that promote the OER electrocatalysis. This mechanism might be extended to other hybrid systems and inspire the design of more efficient electrocatalysts.

Surface-assembled Fe-Oxide colloidal nanoparticles for high performance electrocatalytic water oxidation

Nanoscale electrocatalytic materials having enhanced electroactive sites has been considered trendier and can drive kinetically uphill OER at much lower energy cost with high efficiency. However, very complex synthetic strategies, extensive functionalization processes, and less stability have stimulated quest for economically viable, straightforward and facile preparative methods for designing stable, robust and active nanoscale electrocatalysts engaging geologically abundant materials to ensure their industrial implications. Here we present surface-assembled Fe(OH)x/FeOx type colloidal catalytic thin-films, with or without post annealing, derived from Fe-colloidal NPs in simple carbonate system for efficient water oxidation. Comprehensive electrochemical studies including cyclic voltammetry, chronoamperometry, chronopotentiometry, impedance spectroscopy, Tafel slope analysis, mass activity, electrochemically active surface area measurements are conducted to comparatively evaluate the performance of simple (FeOx/HCO3−@FTO and annealed (FeOx/HCO3−@FTO250, FeOx/HCO3−@FTO500) catalysts for oxygen evolution reaction (OER) under employed conditions. The FeOx/HCO3−@FTO250 annealed at 250 °C initiates water oxidation at much lower overpotential of 1.52 V vs. RHE with remarkable stability during long-term electrochemical experimentations. In addition to enhanced OER activity as evidence by better onset potential (<1.55 V vs. RHE), lower Tafel slope value (36 mV dec1−) and negligible charge transfer resistance, the Fe(OH)x/HCO3−@FTO type catalyst presented excellent electroactive nature during long term controlled potential electrolysis experiments where more and more electroactive sites were getting exposed during continuous hours of electrolysis. The catalysts behave as a potential enduring, inexpensive and competent candidate for catalyzing water oxidation reaction when tested under begin conditions.

High‐Entropy Metal Sulfide Nanoparticles Promise High‐Performance Oxygen Evolution Reaction

AbstractTransition metal sulfides with a multi‐elemental nature represent a class of promising catalysts for oxygen evolution reaction (OER) owing to their good catalytic activity. However, their synthesis remains a challenge due to the thermodynamic immiscibility of the constituent multimetallic elements in a sulfide structure. Herein, for the first time the synthesis of high‐entropy metal sulfide (HEMS, i.e., (CrMnFeCoNi)Sx) solid solution nanoparticles is reported. Computational and X‐ray photoelectron spectroscopy analysis suggest that the (CrMnFeCoNi)Sx exhibits a synergistic effect among metal atoms that leads to desired electronic states to enhance OER activity. The (CrMnFeCoNi)Sx nanoparticles show one of the best activities (low overpotential 295 mV at 100 mA cm−2 in 1 m KOH solution) and good durability (only slight polarization after 10 h by chronopotentiometry) compared with their unary, binary, ternary, and quaternary sulfide counterparts. This work opens up a new synthesis paradigm for high‐entropy compound nanoparticles for highly efficient electrocatalysis applications.

Rationalizing Effect of Metal Substitution in Ruthenium Pyrochlores on the Lattice Oxygen Binding and Oxygen Evolution Activity

Ruthenium pyrochlores, i.e. oxides of composition A2Ru2O7 − δ, are known as advanced catalysts for the oxygen evolution reaction (OER) in acidic conditions.1 In this work, we discuss a direct correlation between the OER activity of Y1.8M0.2Ru2O7 − δ pyrochlores (M = Y, Fe, Co, Ni, Cu), the lattice oxygen binding strength/lability and the formation enthalpy of the binary MOx oxide, i.e. metals M with highter enthalpies of formation for the respective oxide weaken the oxygen binding on the surface of Y1.8M0.2Ru2O7 − δ pyrochlores that results in enhanced OER activity. DFT studies captured changes in the electronic structure of the Y1.8M0.2Ru2O7 − δ oxides induced by M substitution and revealed trends in the electronic band structure that govern the OER performance of the Y1.8M0.2Ru2O7 − δ catalysts. Specifically, an increase in the density of oxygen vacancy (VO) sites and the OER activity in Y1.8M0.2Ru2O7 − δ is attributed to the shift of the O 2p band center closer to the Fermi level. Our work introduces Y1.8Cu0.2Ru2O7 − δ as a novel OER catalyst with enhanced OER activity and, more fundamentally, emphasizes the underexplored role that substituents can play in regulating the OER activity of complex oxides, i.e. by influencing the surface oxygen binding strength. Our results can be utilized for the rational design of new compositions for oxygen electrocatalysis and other applications that involve lattice oxygen sites.(1) Kim, J.; Shih, P.-C.; Tsao, K.-C.; Pan, Y.-T.; Yin, X.; Sun, C.-J.; Yang, H., J. Am. Chem. Soc. 2017, 139, 12076-12083.

Effect of Global Fuel Starvation on Reversal Tolerant Anode Materials – Pulsed Versus Continuous Cell Reversal Events

A broad scientific consensus exists that anthropogenic emissions are major factors fueling global warming and its potential impacts on the earth’s ecosystem. On the way to a sustainable and decarbonized energy supply, "green" hydrogen is an emission-free alternative to conventional energy carriers. In this context, proton exchange membrane fuel cells (PEMFCs) offer promising properties for the electromobility by a simple scalability of the performance through the modular design of fuel cell stacks, a refueling time and range comparable with conventional combustion engines and zero emissions if "green" hydrogen is used. Despite all scientific work on this field – performance, cost and durability issues still hamper the wide commercialization of PEMFCs.Concerning the durability, global fuel starvation causes destructive and irreversible degradation at the anode catalyst. A lack of fuel supply leads to the anode potential being raised to levels where the oxygen evolution reaction (OER) and carbon oxidation reaction (COR) take place instead of the hydrogen oxidation reaction (HOR). Thus, the overall cell voltage reverses compared to normal operation. In consequence, the widely employed carbon-supported anode catalyst corrodes and the overall cell performance is reduced if appropriate mitigation strategies are absent [1]. Among other remedies, incorporating a second catalyst component with an enhanced OER activity, such as IrO2 [2], is a promising material-specific strategy. Within this study we present a technical approach on single cell level (50 cm2 geo) to investigate and evaluate the impact of short-pulsed versus long-lasting fuel starvation events on the degradation of a reversal-tolerant anode catalyst layer comprising Pt/C and IrO2 [3]. By utilizing an on-line mass spectrometer, the anode exhaust gas stream is examined in terms of O2- and CO2-amounts to reveal the origin of the degrading effect.

High Surface Area IrOx Synthesis By Glycine-Assisted Adams Fusion Method for Improved Electrocatalysis of Oxygen Evolution Reaction

As renewable energy generation increases, the storage of intermittently excess electricity is becoming an important challenge due to the instability of photovoltaic and wind generation. Among many approaches, hydrogen production through water electrolysis features the advantages of long-term large-scale storability [1]. Especially, proton exchange membrane water electrolysis (PEMWE) has benefits at high current operation and gas purity compared to the alkaline water electrolysis. Iridium oxide (IrO2), which is the most widely using catalysts for PEMWE at present, were developed intensely to overcome the sluggish oxygen evolution reaction (OER) kinetics and unsatisfied stability under acidic conditions [2-3]. Recent studies demonstrated amorphous IrOx have higher OER activity than rutile IrO2 owing to IrIIIOOH species. Despite the high OER activity, amorphous IrOx has issues in high Ir dissolution rate during OER which is critical for OER stability. In this study, high surface area IrOx was synthesized by Adams fusion method with glycine as an additive (denoted by IrOx (G)) to enhance OER activity and stability. From the XRD analysis, IrOx was observed with crystallite of 2.4 nm calculated from (101) by Scherrer equation. XPS results show positive peak shifts at Ir 4f and higher Oads/Olatt ratio of O 1s peak compared to rutile IrO2 which is well-known features of hydrated amorphous IrOx [4]. The specific surface areas of IrOx (G) obtained by BET equation were 385 m2 g-1 which is 170 % larger than IrO2 prepared by the common Adams fusion method (225 m2 g-1, denoted by Adams-IrO2). The result shows that glycine acts as a pore-forming agent and also prevent the development of rutile IrO2 during thermal treatment. The amorphous structure and high specific surface area of IrOx (G) are the main reasons which increase OER activity and stability. IrOx (G) exhibited 305 mV overpotential at 10 mA cm-2 which is a significantly lower overpotential than Adams-IrO2 (340 mV). Also, no significant change occurred during 6 hours of chronopotentiometry at 10 mA cm-2 which implies excellent stability at OER. References E. Hosseini, M.A. Wahid, Renew. Sust. Energy Rev., 57, 850 (2016).Faustini, M. Giraud, D. Jones, J. Rozière, M. Dupont, T.R. Porter, S. Nowak, M. Bahri, O. Ersen, C. Sanchez., Adv. Energy Mater., 9, 1802136, (2019).Li, S. Li, M. Xiao, J. Ge, C. Liu, W. Xing, Nanoscale, 9, 9291, (2017).F., Abbott, D., Lebedev, K., Waltar, M., Povia, M., Nachtegaal, E., Fabbri, C., Copéret, T. J., Schmidt, Chem. Mater., 28, 6591, (2016).

Development and performance of A-site rich perovskite-type material for enhanced oxygen evolution reaction in alkaline electrolyte

A-site rich perovskites Ba2xCoMoO6 (x = 1, 1.1, 1.2, and 1.3) were synthesised by a solid-state reaction method. The as-prepared Ba2.4CoMoO6 catalyst showed good catalytic activity towards oxygen evolution reaction (OER) activity with an overpotential of 420 mV at 10 mA cm−2, which was enhanced by 100 mV compared with that of Ba2CoMoO6, with a low Tafel slope of 79 mV dec−1, an obviously enhanced electrochemical active surface area value, and sharply improved mass and specific activities. The enhanced OER activity even surpassed the precious metal oxide IrO2 and perovskite benchmark Ba0.5Sr0.5Co0.8Fe0.2O3-δ activities.

Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer

• A small densely packed Ni 0.75 Fe 2.25 O 4 nanoparticles were grown on the Ni foam. • The binary Ni 0.75 Fe 2.25 O 4 nanoparticles exhibited excellent OER activity. • The improved activation sites due to their nanostructures enhanced OER activity. • The electrode applied to high-efficiency AEM water electrolyzer as an anode. Developing high-performance and low-cost oxygen-evolving electrodes is one of major challenges in electrochemical water splitting technology. We demonstrated that the aqueous-phase corrosion of a conventional Ni foam in the presence of exotic Fe 3+ cations can directly transform it into an electrode with high catalytic activity and stability for oxygen evolution reaction (OER). The surface of the corroded electrode consisted of densely packed, small Ni 0.75 Fe 2.25 O 4 nanoparticles with sizes less than 5 nm. This electrode required an overpotential of only 192 mV to reach an OER current density of 10 mA/cm 2 in 1 M KOH, outperforming the state-of-the-art IrO 2 catalyst by 73 mV. Density functional theory calculations revealed that the unique surface structure and iron composition of Ni 0.75 Fe 2.25 O 4 nanoparticles play a key role in achieving an improved OER activity. When coupled with a Pt/C hydrogen-evolving catalyst, the resulting anion-exchange membrane (AEM) water electrolyzer achieved an overall water-splitting current density as high as 2.0 A/cm 2 at a cell voltage of 1.9 V in 1 M KOH, which was 1.7 times that obtained from the IrO 2 and Pt/C catalyst pair and also much greater than reported values from other AEM water electrolyzers. By revisiting and exploiting a traditional corrosion process, our work opens a simple, cost-effective, and scalable route to a high-performance OER electrode for efficient AEM water electrolysis.

Enhanced oxygen evolution reaction activity of flower-like FeOOH via the synergistic effect of sulfur

S-modified FeOOH electrocatalysts were prepared via a facile hydrothermal method at 120 °C. The electrocatalyst exhibited an outstanding OER performance with an overpotential of 254 mV at 50 mA·cm −2 . The effect of S on the catalytic process was attributed to the formation of (Fe-S)-O*, which reduces the adsorption energy of OH* and increase that of O*, resulting in a smaller ΔG of the rate-determining step. • S-modified FeOOH was facilely prepared with outstanding OER activity. • Fe 4+ improves OER performance. • The (Fe-S)-O* bond was first reported. Surface modification of electrocatalysts is a convenient strategy for addressing the drawbacks arising from the sluggish kinetics of the oxygen evolution reaction (OER). However, the preparation of electrocatalysts usually involves complex procedures, rendering investigation of the catalytic mechanism difficult. In this study, S-modified FeOOH electrocatalysts were prepared via a facile hydrothermal method at 120 °C. The electrocatalyst exhibited an outstanding OER performance with an overpotential of 254 mV at 50 mA·cm −2 . Morphological, structural, and electrochemical characterizations revealed that Fe ion is the most important active center. The effect of S on the catalytic process was investigated based on the steric hindrance for OH* adsorption, changes in ΔG of the rate-determining step, and coordination mode of the active species. This is the first report on the formation of an (Fe-S)-O* bond that plays a key role in enhancing the electrochemical performance.

Effect of Reduced Graphene Oxide on the Ta2O5-IrO2 Electrocatalyst for Water Splitting

There is a great interest in the development of advanced electrocatalysts for efficient water splitting. A tantalum iridium oxide (Ta2O5-IrO2) coating is considered to be one of the best electrocatalysts for the oxygen evolution reaction (OER) in acidic media. In the present study, novel Ta2O5-IrO2-rGO coatings with varying loads of reduced graphene oxide (rGO) were designed to investigate the effects of rGO on the catalytic activity and stability of the Ta2O5-IrO2 coating for the OER. Five different electrodes comprised of Ta2O5-IrO2-rGO on a titanium substrate were fabricated with incremental weight percentages of rGO (0.0 wt.%, 1.0 wt.%, 2.0 wt.%, 5.0 wt.% and 7.5 wt.%) using a facile thermal decomposition method. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS) were employed to characterize the morphology and composition of the prepared Ta2O5-IrO2-rGO coatings. Longevity tests revealed that the incorporation of rGO into the oxide layer strongly affected the stability of the Ta2O5-IrO2-rGO electrodes. The electrochemical activities of the prepared Ta2O5-IrO2-rGO electrodes were characterized by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS). The Ta2O5-IrO2-rGO coating containing 1.0 wt.% rGO exhibited the greatest stability, along with enhanced OER activity.

A-site deficient perovskite nanofibers boost oxygen evolution reaction for zinc-air batteries

Among the various perovskites, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) has been theoretically proved to have the prominent intrinsic oxygen evolution reaction (OER) catalytic activity in the alkaline solutions. Whereas, most of the measured bulk BSCF samples suffer the unsatisfactory OER activities which are much more inferior than that of the state-of-the-art catalyst of IrO2. In this work, we synthesize the A-site deficient BSCF nanofibers with the diameter of 300 nm by electrospinning for the first time. Thanks to the one dimensional (1D) nanofiber structure and A-site deficiency, the optimized BSCF nanofibers (BSCF-80-ES) demonstrate the remarkably enhanced OER activity with a low potential of 1.54 V at 10 mA cm−2 in 1 M KOH and an improved oxygen reduction reaction (ORR) activity. It is noteworthy that the OER potential of BSCF-80-ES keeps almost unchanged during the long-term test, indicating an excellent OER stability. Moreover, BSCF-80-ES with the high bifunctional catalytic activity and superb stability can be used as a promising oxygen catalyst for aqueous and flexible solid-state zinc-air batteries.

Heterometallic Feed Ratio‐Dominated Oxygen Evolution Activity in Self‐Supported Metal‐Organic Framework Nanosheet Arrays Electrocatalyst

Developing earth‐abundant, highly active and long‐term durable electrocatalysts for oxygen evolution reaction (OER) is highly desirable and great challenging for large‐scale industrial application of electrochemical water splitting. Herein, in‐situ growth of uniform nanosheet arrays on nickel foam (NF) is hydrothermally achieved by varying feed ratios of FeIII and NiII salts. The feed ratio of the two active metals has significantly dominated both the morphological and electronic structures of the resultant electrocatalysts, leading to feed ratio‐dependent volcano‐type OER activity. The optimized Fe0.89Ni0.11‐BDC/NF exhibits the best OER performance, affording a low overpotential of 220 mV to drive a current density of 50 mA·cm–2 with small Tafel slope of 44.8 mV·dec–1 and long‐lasting stability over 20 hours. The synergistic effect from the FeIII and NiII species on both the morphological and electronic structure modulations have dramatically accelerated the reaction kinetics, responsible eventually for the enhanced OER activity. This work provides valuable information for nanostructured MOFs as efficient electrocatalysts.

Dynamic structure evolution of free-standing S-doped porous Co-Fe microspheres with enhanced oxygen evolution electrocatalysis in alkaline media

Herein, S-doped Co-Fe hybrid films with unique 3D porous microsphere arrays stacked by ultrathin nanoflakes directly grown on nickel foam (Co-Fe-S NFs@MS/NF) are synthesized via a single-step co-electrodeposition strategy. The synergistic effects among Fe, S and Co enable superior oxygen evolution reaction (OER) catalytic activity in basic environments. Impressively, this hybrid material can achieve further enhanced OER catalytic performance with surface self-assembly through in-situ electrochemical oxidation activation in both gentle alkaline (GA: 1.0 M KOH at 298 K) and industrial alkaline (IA: 30 wt.% KOH at 353 K) water electrolysis conditions. The transition processes are revealed in detail regarding the chemical, structural, and catalytic properties of the catalysts served in the GA and IA conditions. The pristine quasi-amorphous Co-Fe-S is found to partially transform into crystallized Co(Fe)OOH in the GA condition, while it remains amorphous nature in the IA condition. The enhanced OER activity is attributed to the formation of active Co(Fe)OOH species with massively proliferated specific surface area, optimized electronic structure, and accelerated kinetics. The activated Co-Fe-S NFs@MS/NF exhibits excellent OER activity, achieving 10 and 500 mA cm−2 at ultralow potentials of 1.453 (iR corrected) and 1.517 V with outstanding durability in the GA and IA conditions, respectively. For full water splitting, the assembled two-electrode system based on the Co-Fe-S NFs@MS/NF anode and a Co-S based cathode affords voltages of only 1.518 and 1.685 V to robustly deliver 10 and 500 mA cm−2 in GA and IA conditions over 200 h, respectively.