Single‐atom catalysts (SACs) with highly active sites atomically dispersed on substrates exhibit unique advantages regarding maximum atomic efficiency, abundant chemical structures, and extraordinary ...catalytic performances for multiple important reactions. In particular, M–N–C SACs (M=transition metal atom) demonstrate optimal electrocatalytic activity for the oxygen reduction reaction (ORR) and have attracted extensive attention recently. Despite substantial efforts in fabricating various M–N–C SACs, the principles for regulating the intrinsic electrocatalytic activity of their active sites have not been sufficiently studied. In this Review, we summarize the regulation strategies for promoting the intrinsic electrocatalytic ORR activity of M–N–C SACs by modulation of the center metal atoms, the coordinated atoms, the environmental atoms, and the guest groups. Theoretical calculations and experimental investigations are both included to afford a comprehensive understanding of the structure–performance relationship. Finally, future directions of developing advanced M–N–C SACs for electrocatalytic ORR and other analogous reactions are proposed.
Regulation strategies for enhancing the intrinsic electrocatalytic oxygen reduction reaction activity of M–N–C single‐atom catalysts are summarized in this review. Four components are considered in the optimization of the catalyst: the center metal atoms, the coordinated atoms, the environmental atoms, and the guest groups.
Single‐atom catalysts (SACs) have great potential in electrocatalysis. Their performance can be rationally optimized by tailoring the metal atoms, adjacent coordinative dopants, and metal loading. ...However, doing so is still a great challenge because of the limited synthesis approach and insufficient understanding of the structure–property relationships. Herein, we report a new kind of Mo SAC with a unique O,S coordination and a high metal loading over 10 wt %. The isolation and local environment was identified by high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure. The SACs catalyze the oxygen reduction reaction (ORR) via a 2 e− pathway with a high H2O2 selectivity of over 95 % in 0.10 m KOH. The critical role of the Mo single atoms and the coordination structure was revealed by both electrochemical tests and theoretical calculations.
The cat in the SAC: Mo single‐atom catalysts (SACs) with a unique O,S coordination exhibit outstanding H2O2 selectivity above 95 % in the oxygen reduction reaction (ORR). Electrochemical tests and theoretical calculations revealed the critical role of the coordination structure in SACs, highlighting new opportunities to tune the activity and selectivity in multi‐electron electrocatalysis.
Oxygen reduction and evolution reactions constitute the core process of many vital energy storage or conversion techniques. However, the kinetic sluggishness of the oxygen redox reactions and heavy ...reliance on noble-metal-based electrocatalysts strongly limit the energy efficiency of the related devices. Developing high-performance noble-metal-free bifunctional ORR and OER electrocatalysts has gained worldwide attention, where much important progress has been made during the last decade. This review systematically addresses the design principles to obtain high-performance noble-metal-free bifunctional oxygen electrocatalysts by emphasizing strategies of both intrinsic activity regulation and active site integration. A statistical analysis of the reported bifunctional electrocatalysts is further carried out to reveal the composition-performance relationship and guide further exploration of emerging candidates. Finally, perspectives for developing advanced bifunctional oxygen electrocatalysts and aqueous rechargeable metal-air batteries are proposed.
Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.
•A consistent relation between forest cover loss and water is valid across spatial scales.•Scale effect on forest-water relation is significant in large watersheds.•Water-limited watersheds are more ...sensitive to forest change.•Mixed forest dominated small watersheds are more hydrologically resilient to forest change.•Snow dominated large watersheds are more hydrologically resilient to forest change.
Despite extensive studies on hydrological responses to forest cover change in small watersheds, the hydrological responses to forest change and associated mechanisms across multiple spatial scales have not been fully understood. This review thus examined about 312 watersheds worldwide to provide a generalized framework to evaluate hydrological responses to forest cover change and to identify the contribution of spatial scale, climate, forest type and hydrological regime in determining the intensity of forest change related hydrological responses in small (<1000km2) and large watersheds (⩾1000km2). Key findings include: (1) the increase in annual runoff associated with forest cover loss is statistically significant at multiple spatial scales whereas the effect of forest cover gain is statistically inconsistent; (2) the sensitivity of annual runoff to forest cover change tends to attenuate as watershed size increases only in large watersheds; (3) annual runoff is more sensitive to forest cover change in water-limited watersheds than in energy-limited watersheds across all spatial scales; and (4) small mixed forest-dominated watersheds or large snow-dominated watersheds are more hydrologically resilient to forest cover change. These findings improve the understanding of hydrological response to forest cover change at different spatial scales and provide a scientific underpinning to future watershed management in the context of climate change and increasing anthropogenic disturbances.
Hydrogen peroxide (H2O2) is a green oxidizer widely involved in a vast number of chemical reactions. Electrochemical reduction of oxygen to H2O2 constitutes an environmentally friendly synthetic ...route. However, the oxygen reduction reaction (ORR) is kinetically sluggish and undesired water serves as the main product on most electrocatalysts. Therefore, electrocatalysts with high reactivity and selectivity are highly required for H2O2 electrosynthesis. In this work, a synergistic strategy is proposed for the preparation of H2O2 electrocatalysts with high ORR reactivity and high H2O2 selectivity. A Co−Nx−C site and oxygen functional group comodified carbon‐based electrocatalyst (named as Co–POC–O) is synthesized. The Co–POC–O electrocatalyst exhibits excellent catalytic performance for H2O2 electrosynthesis in O2‐saturated 0.10 m KOH with a high selectivity over 80% as well as very high reactivity with an ORR potential at 1 mA cm−2 of 0.79 V versus the reversible hydrogen electrode (RHE). Further mechanism study identifies that the Co−Nx−C sites and oxygen functional groups contribute to the reactivity and selectivity for H2O2 electrogeneration, respectively. This work affords not only an emerging strategy to design H2O2 electrosynthesis catalysts with remarkable performance, but also the principles of rational combination of multiple active sites for green and sustainable synthesis of chemicals through electrochemical processes.
A synergistic strategy of rational combination of multiple active sites is proposed for high‐performance H2O2 electrosynthesis. Comodification of atomic Co–Nx–C sites and oxygen functional groups on noble‐metal‐free nanocarbon electrocatalysts synergistically renders high reactivity for oxygen reduction and high selectivity for the two‐electron pathway. Consequently, high H2O2 productivity is achieved through a green and sustainable electrochemical approach.
Humans can naturally and effectively find salient regions in complex scenes. Motivated by this observation, attention mechanisms were introduced into computer vision with the aim of imitating this ...aspect of the human visual system. Such an attention mechanism can be regarded as a dynamic weight adjustment process based on features of the input image. Attention mechanisms have achieved great success in many visual tasks, including image classification, object detection, semantic segmentation, video understanding, image generation, 3D vision, multimodal tasks, and self-supervised learning. In this survey, we provide a comprehensive review of various attention mechanisms in computer vision and categorize them according to approach, such as channel attention, spatial attention, temporal attention, and branch attention; a related repository
https://github.com/MenghaoGuo/Awesome-Vision-Attentions
is dedicated to collecting related work. We also suggest future directions for attention mechanism research.
Associations between gut microbiota and colorectal cancer (CRC) have been widely investigated. However, the replicable markers for early-stage adenoma diagnosis across multiple populations remain ...elusive. Here, we perform an integrated analysis on 1056 public fecal samples, to identify adenoma-associated microbial markers for early detection of CRC. After adjusting for potential confounders, Random Forest classifiers are constructed with 11 markers to discriminate adenoma from control (area under the ROC curve (AUC) = 0.80), and 26 markers to discriminate adenoma from CRC (AUC = 0.89), respectively. Moreover, we validate the classifiers in two independent cohorts achieving AUCs of 0.78 and 0.84, respectively. Functional analysis reveals that the altered microbiome is characterized with increased ADP-L-glycero-beta-D-manno-heptose biosynthesis in adenoma and elevated menaquinone-10 biosynthesis in CRC. These findings are validated in a newly-collected cohort of 43 samples using quantitative real-time PCR. This work proves the validity of adenoma-specific markers across multi-populations, which would contribute to the early diagnosis and treatment of CRC.
Rechargeable zinc–air batteries constitute promising next‐generation energy storage devices due to their intrinsic safety, low cost, and feasibility to realize high cycling current density and long ...cycling lifespan. Nevertheless, their cathodic reactions involving oxygen reduction and oxygen evolution are highly sluggish in kinetics, requiring high‐performance noble‐metal‐free bifunctional electrocatalysts that exceed the current noble‐metal‐based benchmarks. Herein, a noble‐metal‐free bifunctional electrocatalyst is fabricated, which demonstrates ultrahigh bifunctional activity and renders excellent performance in rechargeable zinc–air batteries. Concretely, atomic Co–N–C and NiFe layered double hydroxides (LDHs) are respectively selected as oxygen reduction and evolution active sites and are further rationally integrated to afford the resultant CoNC@LDH composite electrocatalyst. The CoNC@LDH electrocatalyst exhibits remarkable bifunctional activity delivering an indicator ΔE of 0.63 V, far exceeding the noble‐metal‐based Pt/C+Ir/C benchmark (ΔE = 0.77 V) and most reported electrocatalysts. Correspondingly, ultralong lifespan (over 3600 cycles at 10 mA cm−2) and excellent rate performances (cycling current density at 100 mA cm−2) are achieved in rechargeable zinc–air batteries. This work highlights the current advances of bifunctional oxygen electrocatalysis and endows high‐rate and long‐cycling rechargeable zinc–air batteries for efficient sustainable energy storage.
Ultrahigh bifunctional electrocatalytic activity for oxygen reduction and evolution is achieved with the indicator ΔE = 0.63 V, far exceeding the noble‐metal‐based benchmark and most reported electrocatalysts. Corresponding rechargeable zinc–air batteries afford ultralong lifespan over 3600 cycles at 10 mA cm−2 and ultrahigh cycling current density of 100 mA cm−2.
High‐performance bifunctional oxygen electrocatalysis constitutes the key technique for the widespread application of clean and sustainable energy through electrochemical devices such as rechargeable ...Zn–air batteries. Single‐atom electrocatalysts with maximum atom efficiency are highly considered as an alternative of the present noble‐metal‐based electrocatalysts. However, the fabrication of transition metal single‐atoms is very challenging, requiring extensive attempts of precursors with novel design principles. Herein, an all‐covalently constructed cobalt‐coordinated framework porphyrin with graphene hybridization is innovatively designed and prepared as the pyrolysis precursor to fabricate single‐atom Co–Nx–C electrocatalysts. Excellent electrochemical performances are realized for both bifunctional oxygen electrocatalysis and rechargeable Zn–air batteries with regard to reduced overpotentials, improved kinetics, and prolonged cycling stability comparable with noble‐metal‐based electrocatalysts. Design principles from multiple scales are proposed and rationalized with detailed mechanism investigation. This work not only provides a novel precursor for the fabrication of high‐performance single‐atom electrocatalysts, but also inspires further attempts to develop advanced materials and emerging applications.
Cobalt‐coordinated framework porphyrin hybridized with graphene is employed as the pyrolysis precursor to fabricate single‐atom Co–Nx–C electrocatalysts. Excellent ORR/OER bifunctional electrocatalytic performances are achieved with a small overpotential gap of 0.87 V, and corresponding Zn–air batteries afford higher power density, improved rate performance, and cycling stability for over 200 cycles beyond the noble‐metal‐based electrocatalysts.
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