Electrocatalysts with single metal atoms as active sites have received increasing attention owing to their high atomic utilization efficiency and exotic catalytic activity and selectivity. This review aims to provide a comprehensive summary on the recent development of such single-atom electrocatalysts (SAECs) for various energy-conversion reactions. The discussion starts with an introduction of the different types of SAECs, followed by an overview of the synthetic methodologies to control the atomic dispersion of metal sites and atomically resolved characterization using state-of-the-art microscopic and spectroscopic techniques. In recognition of the extensive applications of SAECs, the electrocatalytic studies are dissected in terms of various important electrochemical reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Examples of SAECs are deliberated in each case in terms of their catalytic performance, structure−property relationships, and catalytic enhancement mechanisms. A perspective is provided at the end of each section about remaining challenges and opportunities for the development of SAECs for the targeted reaction.

oxygen evolution reaction or OER) processes. [1][2][3][4][5] To avoid the use of costly noble metal catalysts, nitrogen-doped porous carbon materials are proposed as the electrode materials in these batteries since they can be derived from naturally abundant biomass. The performance of these porous carbon materials as electrodes depends on the chemistry that results in the generation of OER-active pyridinic N and ORR-active quaternary N groups in a high density as well as the porosity of the materials. [6] This is because these factors determine the extent of exposure of the active sites to the relevant chemical species such as O 2 , OH − , and H 2 O and help prevent the rapid clogging of the planar electrode surface. [7][8][9][10] SiO 2 [8,11,12] and Al 2 O 3[13] microbeads have been employed widely as templates for generating nanopores in carbon-based catalysts. However, this requires multiple steps such as the bottom-up synthesis of the catalysts from carbon precursors as well as etching and purification processes to remove the templates, which increases the cost for mass production. [14][15][16][17][18] In addition, porous carbon materials synthesized by bottom-up methods are generally in the powder form and are thus not self-supporting. Thus, the fabrication of air electrodes requires an additional process, wherein the powder carbon materials are electrosprayed onto carbon cloth/paper, which then lead to the inevitable decreases Porous carbon electrodes have emerged as important cathode materials for metal-air battery systems. However, most approaches for fabricating porous carbon electrodes from biomass are highly energy inefficient as they require the breaking down of the biomass and its subsequent reconstitution into powder-like carbon. Here, enzymes are explored to effectively hydrolyze the partial cellulose in bulk raw wood to form a large number of nanopores, which helps to maximally expose the inner parts of the raw wood to sufficiently dope nitrogen onto the carbon skeletons during the subsequent pyrolysis process. The resulting carbons exhibit excellent catalytic activity with respect to the oxygen reduction and oxygen evolution reactions. As-fabricated cellulosedigested, carbonized wood plates are mechanically strong, have high conductivity, and contain a crosslinked network and natural ion-transport channels and can be employed directly as metal-free electrodes without carbon paper, polymer binders, or carbon black. When used as metal-free cathodes in zincair batteries, they result in a specific capacity of 801 mA h g −1 and an energy density of 955 W h kg −1 with the long-term stability of the batteries being as high as 110 h. This work paves the way for the ready conversion of abundant biomass into high-value engineering products for energy-related applications.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201900341.Rechargeable Zn-air batteries have emerged as a promising technology for coping with future energy demands owin...

In order to investigate the contamination levels of trace metals, surface water samples were collected from six regions along Yangtze River in Nanjing Section. The concentrations of trace metals (As, B, Ba, Be, Cd, Cr, Cu, Fe, Pb, Li, Mn, Mo, Ni, Sb, Se, Sn, Sr, V and Zn) were determined using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Total concentrations of the metals in the water samples ranged from 825.1 to 950.4 microg/L. The result was compared with international water quality guidelines. Seven metals levels were above the permissible limit as prescribed by guidelines. A preliminary risk assessment was then carried out to determine the human health risk via calculating Hazard Quotient and carcinogenic risk of the metals. Hazard Quotients of all metals were lower than unity, except As. The carcinogenic risk of As and Cd was higher than 10(-6), suggesting that those two metals have potential adverse effects on local residents.