FeN3S1─OH Single-Atom Sites Anchored on Hollow Porous Carbon for Highly Efficient pH-Universal Oxygen Reduction Reaction.

Regulating the asymmetric active center of a single-atom catalyst to optimize the binding energy is critical but challenging to improve the overall efficiency of the electrocatalysts. Herein, an effective strategy is developed by introducing an axial hydroxyl (OH) group to the Fe─N4 center, simultaneously assisting with the further construction of asymmetric configurations by replacing one N atom with one S atom, forming FeN3S1─OH configuration. This novel structure can optimize the electronic structure and d-band center shift to reduce the reaction energy barrier, thereby promoting oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activities. The optimal catalyst, FeSA-S/N-C (FeN3S1─OH anchored on hollow porous carbon) displays remarkable ORR performance with a half-wave potential of 0.92, 0.78, and 0.64 V versus RHE in 0.1 m KOH, 0.5 m H2SO4, and 0.1 m PBS, respectively. The rechargeable liquid Zn-air batteries (LZABs) equipped with FeSA-S/N-C display a higher power density of 128.35 mW cm-2, long-term operational stability of over 500 h, and outstanding reversibility. More importantly, the corresponding flexible solid-state ZABs (FSZABs@FeSA-S/N-C) display negligible voltage changes at different bending angles during the charging and discharging processes. This work provides a new perspective for the design and optimization of asymmetric configuration for single-atom catalysts applied to the area of energy conversion and storage.

"Fence" Effect Enabling a Metal-Organic Framework-Derived Single-Atom Co-N-C Catalyst for High-Performance Zn-Air Batteries.

It offers bright prospects to develop non-Pt group metal (non-PGM) electrocatalysts in the area of energy storage and conversion. Herein, we reported a simple spatial isolation strategy to synthesize Co-based electrocatalysts, using partially substituted Zn atoms in a ZnCo-ZIF precursor. The "fence" effect that originated from the partially substituted Zn atoms can yield a better isolation of Co atoms, achieving selective loading of Co species on nitrogen-doped porous carbon varying from nanoparticles to single atoms. The low boiling point of Zn enables abundant porous structures to the N-doped carbon substrate after pyrolysis. The best performing single-atom Co catalyst (Co-SAs/N-C) exhibits excellent oxygen reduction reaction activity in alkaline media. As an illustration, the rechargeable liquid Zn-air battery incorporating the Co-SAs/N-C catalyst demonstrates a substantial open circuit voltage of 1.49 V, a high specific capacity of 689.3 mAh g-1, and remarkable cycling stability over 200 h. This study paves the way for the strategic development of non-PGM electrocatalysts in battery applications.

Constructing a Self-Supported Bifunctional Multiphase Heterostructure for Electrocatalytic Overall Water Splitting.

Developing well-performing and stable bifunctional electrocatalysts is of great importance for efficient green hydrogen production through water electrolysis. Herein, a three-dimensional self-supported CoMoS3.13/FeS2/Co3S4 on carbon paper (FeCoMoS/CP) heterostructure with interconnected nanosheets for overall water splitting was fabricated by a facile hydrothermal method followed by vulcanization treatment. The FeCoMoS/CP heterostructure with high structural integrity and more accessible active sites can effectively optimize the electronic structure through component regulation to achieve enhanced catalytic activity. Significantly, the FeCoMoS/CP required overpotentials of 257 mV at 50 mA cm-2 for OER and 280 mV at 20 mA cm-2 for HER. Importantly, the assembled FeCoMoS/CP||FeCoMoS/CP alkaline electrolyzer achieved a superior cell voltage of 1.48 V at 10 mA cm-2 with superb long-term stability, which implies a remarkable electrocatalytic performance of the FeCoMoS/CP heterostructure for overall water splitting. This work provides an applicable route for synthesizing high-performance bifunctional catalysts toward water electrolysis.

3D Hollow Hierarchical Porous Carbon with Fe-N4 -OH Single-Atom Sites for High-Performance Zn-Air Batteries.

The development of innovative and efficient Fe-N-C catalysts is crucial for the widespread application of zinc-air batteries (ZABs), where the inherent oxygen reduction reaction (ORR) activity of Fe single-atom sites needs to be optimized to meet the practical application. Herein, a three-dimensional (3D) hollow hierarchical porous electrocatalyst (ZIF8@FePMPDA-920) rich in asymmetric Fe-N4 -OH moieties as the single atomic sites is reported. The Fe center is in a penta-coordinated geometry with four N atoms and one O atom to form Fe-N4 -OH configuration. Compared to conventional Fe-N4 configuration, this unique structure can weaken the adsorption of intermediates by reducing the electron density of the Fe center for oxygen binding, which decreases the energy barrier of the rate-determining steps (RDS) to accelerate the ORR and oxygen evolution reaction (OER) processes for ZABs. The rechargeable liquid ZABs (LZABs) equipped with ZIF8@FePMPDA-920 display a high power density of 123.11 mW cm-2 and a long cycle life (300 h). The relevant flexible all-solid-state ZABs (FASSZABs) also display outstanding foldability and cyclical stability. This work provides a new perspective for the structural design of single-atom catalysts in the energy conversion and storage areas.

Self-Assembly of Ir-Based Nanosheets with Ordered Interlayer Space for Enhanced Electrocatalytic Water Oxidation.

Iridium (Ir)-based electrocatalysts are widely explored as benchmarks for acidic oxygen evolution reactions (OERs). However, further enhancing their catalytic activity remains challenging due to the difficulty in identifying active species and unfavorable architectures. In this work, we synthesized ultrathin Ir-IrOx/C nanosheets with ordered interlayer space for enhanced OER by a nanoconfined self-assembly strategy, employing block copolymer formed stable end-merged lamellar micelles. The interlayer distance of the prepared Ir-IrOx/C nanosheets was well controlled at ∼20 nm and Ir-IrOx nanoparticles (∼2 nm) were uniformly distributed within the nanosheets. Importantly, the fabricated Ir-IrOx/C electrocatalysts display one of the lowest overpotential (η) of 198 mV at 10 mA cm-2geo during OER in an acid medium, benefiting from their features of mixed-valence states, rich electrophilic oxygen species (O(II-δ)-), and favorable mesostructured architectures. Both experimental and computational results reveal that the mixed valence and O(II-δ)- moieties of the 2D mesoporous Ir-IrOx/C catalysts with a shortened Ir-O(II-δ)- bond (1.91 Å) is the key active species for the enhancement of OER by balancing the adsorption free energy of oxygen-containing intermediates. This strategy thus opens an avenue for designing high performance 2D ordered mesoporous electrocatalysts through a nanoconfined self-assembly strategy for water oxidation and beyond.

Successive Anion/Cation Exchange Enables the Fabrication of Hollow CuCo2S4 Nanorods for Advanced Oxygen Evolution Reaction Electrocatalysis.

Hollow CuCo2S4 nanorods (H-CCS-Ns) have been successfully developed via a facile successive anion/cation-exchange method. The outstanding electrocatalytic performance of H-CCS-Ns is mainly attributed to its distinctive hollow structure, which accelerates the electron transfer rate and provides abundant active sites. Moreover, a mechanism study indicates that H-CCS-Ns has highly active octahedral Co3+, and the existence of Co3+ cations optimizes the adsorption of oxygen-involved intermediates, making H-CCS-Ns a promising OER electrocatalyst. Optimized H-CCS-Ns only need an ultralow overpotential of 220 mV to drive a current density of 10 mA·cm-2 and exhibit distinguished cycling stability with a negligible fluctuation for 30 h. More impressively, when H-CCS-Ns are assembled with Pt/C for overall water splitting, a voltage as low as 1.545 V is required at a current density of 10 mA·cm-2, and the catalyst shows outstanding stability for as long as 38 h. This study offers a feasible strategy to design hollow spinel catalysts for efficient OER catalysis.

Se-doping-induced sulfur vacancy engineering of CuCo2S4 nanosheets for enhanced electrocatalytic overall water splitting.

The coordination of the electronic structure and charge transfer through heteroatomic doping and sulfur vacancies is one of the most vital strategies for enhancing the electrocatalytic performance of the oxygen and hydrogen evolution reactions (OER, HER) through water splitting. Se-doped CuCo2S4 nanosheets (CuCo2S3.68Se0.32) with abundant sulfur vacancies were synthesized via a simple hydrothermal method to achieve remarkably efficient electrocatalytic water splitting. Importantly, incorporating Se in three-dimensional nanosheet structures effectively fine-tunes the electronic structure, ensuring ample accessibility of active sites for swift charge carrier transfer and improved reaction kinetics. The optimized CuCo2S3.68Se0.32 offers substantially high electrocatalytic activity with overpotentials of 65 and 230 mV at the current density of 10 mA cm-2 for HER and OER, respectively, which is comparable to commercial catalysts. Combining Se-doping and rich sulfur vacancies facilitates fast charge transport, thus significantly boosting the electrocatalytic activity. Furthermore, utilizing CuCo2S3.68Se0.32 as both the cathode and anode, a two-electrode electrolyser exhibits remarkable performance. It achieves a low voltage of 1.52 V at 10 mA cm-2 and demonstrates exceptional durability over time. This study investigates the significance of doping and vacancies in enhancing electrocatalytic activity for water splitting.

Engineering NiMoO4/NiFe LDH/rGO multicomponent nanosheets toward enhanced electrocatalytic oxygen evolution reaction.

Rational hybridization of two-dimensional (2D) nanomaterials with extrinsic species has shown great promise for boosting the electrocatalytic oxygen evolution reaction (OER). To date, the rational design and engineering of heterojunctions based on three or more components has been rather limited. Herein, by taking advantage of the high intrinsic activity of NiFe layered double hydroxide (LDH), strong synergistic effects between different components, and good electronic conductivity of reduced graphene oxide (rGO), we demonstrate the successful synthesis of NiMoO4/NiFe LDH/rGO nanosheets. As a proof-of-concept demonstration, the multicomponent nanosheet catalyst with a well-modified electronic structure is applied to boost the electrochemical OER and achieve decent electrocatalytic activity in 1 M KOH electrolyte, which can deliver a current density of 10 mA cm-2 with an overpotential of merely 270 mV and a small Tafel slope of 76.2 mV dec-1, which are markedly superior to those of the commercial RuO2 catalyst (303 mV, 131.9 mV dec-1). This work is expected to provide new insights into furnishing multi-component heterostructures with extended functionalities and more advantageous merits.

Beneficial restacking of 2D nanomaterials for electrocatalysis: a case of MoS2 membranes.

Restacking of 2D nanomaterials is often deemed to be detrimental to their applications. In contrast to this common notion, here we demonstrate that tightly packed stacked MoS2 exhibits a higher electrocatalytic activity for hydrogen evolution than the more loosely stacked ones.

Salt-Assisted Synthesis of 3D Porous g-C3N4 as a Bifunctional Photo- and Electrocatalyst.

Graphitic carbon nitride (g-C3N4), characterized with a suitable bandgap, has aroused great interest as a robust and efficient catalyst for solar energy utilization. Herein, we introduce a new strategy to fabricate a three-dimensional (3D) porous g-C3N4 by a facile NaCl-assisted ball-milling strategy. The porous structure-induced advantages, such as a higher specific surface area, more efficient charge separation, and faster electron-transfer efficiency, enable the 3D porous g-C3N4 to achieve impressive properties as a bifunctional catalyst for both photocatalytic hydrogen evolution and electrocatalytic oxygen evolution reaction (OER). As a result, the 3D porous g-C3N4 exhibits a hydrogen evolution rate of 598 µmol h-1 g-1 with an apparent quantum yield of 3.31% at 420 nm for photocatalytic H2 generation, which is much higher than that of the bulk g-C3N4. Simultaneously, the porous g-C3N4 also presents an attractive OER performance with a low onset potential of 1.47 V (vs reversible hydrogen electrode) in an alkaline electrolyte after rational cobalt-doping. Accordingly, the NaCl-assisted ball-milling strategy paves the way to the rational design of a controllable porous structure.

Ultrathin molybdenum disulfide/carbon nitride nanosheets with abundant active sites for enhanced hydrogen evolution.

The exploration of highly active catalysts for hydrogen evolution reaction (HER) is beneficial to realize high catalytic activity and enhance kinetics for water splitting. Herein, flower-like molybdenum disulfide/carbon nitride (MoS2/C3N4) nanosheets with thickness of 4.6 nm and enlarged interlayer spacing of 0.64 nm were synthesized via a facile hydrothermal method. As expected, the ultrathin thickness endowed MoS2/C3N4 with abundant active sites, ensuring outstanding catalytic activity and excellent stability for HER in alkaline electrolyte. MoS2/C3N4 nanocomposites can offer an onset overpotential of 153 mV versus reversible hydrogen electrode (RHE). Notably, the Tafel slope value is only 43 mV dec-1, which is significantly better than those of reported MoS2-based hydrogen evolution catalysts, revealing superior HER performance of MoS2/C3N4, particularly in catalytic kinetics. More significantly, density functional theory (DFT) calculations further verify that rich active sites confined in ultrathin nanostructure of g-C3N4 nanolayers could increase the activity of MoS2/C3N4 and result in enhanced HER efficiency. This study indicates that rational interaction between two different 2D materials can significantly facilitate H2 generation, which endows extraordinary HER activity.

Carbon-Induced Generation of Hierarchical Structured Ni0.75Co0.25(CO3)0.125(OH)2 for Enhanced Supercapacitor Performance.

Hierarchical nanostructures with heteroatom doping have been considered as an important component in electrode materials for advanced supercapacitors. Herein, with the aid of C, N, and S codoped Ni0.75Co0.25(CO3)0.125(OH)2/C (NSH) with a hierarchical structure was synthesized through a facile one-step hydrothermal method. Notably, it is the first report on a carbon precursor as a structure inducer for designing a three-dimensional (3D) carnation-like hierarchical structure. Thanks to the carbon induction effect and the introduction of N/S dopants, the obtained NSH with a 3D architecture exhibits superior performances as electrode materials for supercapacitors. For example, NSH offers a high specific capacity of 277.3 mAh/g at 0.5 A/g. Moreover, the assembled NSH//reduced graphene oxide hydrogel-based hybrid supercapacitor exhibits high energy densities of 44.4 and 11.7 Wh/kg at power densities of 460 W/kg and 9.8 kW/kg, respectively. This result opens up opportunities for carbon-induced methods to control the morphology and structure of other similar materials.