Rechargeable aqueous Zn-ion batteries (ZIBs) are very promising for large-scale grid energy storage applications owing to their low cost, environmentally benign constituents, excellent safety, and relatively high energy density. 1, 2 Their usage, however, is largely hampered by the fast capacity fade. The cycle stability seems to be highly rate-dependent, 3 which poses an additional challenge, but can also play a pivotal role in uncovering the reaction mechanisms. The complexity of the reactions has resulted in long-standing ambiguities of the chemical pathways of Zn/MnO2 system, and has led to many controversies with regard to their nature. In this report, we present a combined experimental and theoretical study of Zn/ MnO2 cells. We found that both H + /Zn 2+ intercalation and conversion reactions occur at different voltages, and that the rapid capacity fading can clearly be ascribed to the rate-limiting and irreversible conversion reactions at a lower voltage. By avoiding the irreversible conversion reactions at ~ 1.26 V, we successfully demonstrate ultra-high power and long-life Zn/MnO2 cells which, after 1000 cycles, maintain an energy density of ~ 231 Wh kg-1 and ~ 105 Wh kg-1 at a power density of ~ 4 kW kg-1 (9C, ~ 3.1 A g-1) and ~ 15 kW kg-1 (30C, ~ 10.3 A g-1), respectively. The excellent cycle stability and power capability are superior to most reported ZIBs or even some lithium-ion batteries. The results establish accurate electrochemical reaction mechanisms and kinetics for Zn/MnO2, and identify the interplay of the voltage window and rate as the determining factors for achieving excellent cycle life. Broader Context The increasing interest and importance in large-scale grid storage technology are attributed to multiple factors, including managing peak demands, improving the grid reliability, integrating most sustainable energy sources such as solar radiation, wind, wave power, geothermal energy, etc., and further powering the energy infrastructures. Rechargeable aqueous Zn-ion batteries (ZIBs) with mild electrolytes have the advantages of low cost materials (Zn/ MnO2), manufacturing (air-and water-inert Zn anode), and recycling (mild electrolytes); relatively high energy density; and excellent safety, making them prospective candidates for large-scale grid storage. Their low cyclability, however, has remained a grand challenge, hindering the widespread applications of these attractive ZIBs. A prerequisite for improving the cycle life and electrochemical performance of Zn/MnO2 batteries is to accurately determine the reaction mechanisms, especially under different rates, which poses a considerable challenge. In our combined experimental and computational study, a concomitant intercalation and conversion reactions of H + /Zn 2+ occurring at different voltages in the Zn/MnO2 system is established. The rapid capacity fading is unambiguously ascribed to the rate-limiting and irreversible conversion reactions at a lower voltage. By mitigating or avoiding the irreversible conversion reactions at the lower ...

Two-dimensional (2D) materials and ultrathin nanosheets are advantageous for elevating the catalysis performance and elucidating the catalysis mechanism of heterogeneous catalysts, but they are mostly restricted to inorganic or organic materials based on covalent bonds. We report an electrochemical/chemical exfoliation strategy for synthesizing metal-organic 2D materials based on coordination bonds. A catechol functionalized ligand is used as the redox active pillar to construct a pillared-layer framework. When the 3D pillared-layer MOF serves as an electrocatalyst for water oxidation (pH 13), the pillar ligands can be oxidized in situ and removed. The remaining ultrathin (2 nm) nanosheets of the metal-organic layers are an efficient catalyst with overpotentials as low as 211 mV at 10 mA cm and a turnover frequency as high as 30 s at an overpotential of 300 mV.

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The electrochemical detection of hydrogen peroxide and nitrite, as important representatives of cytotoxic reactive oxygen/nitrogen (ROS, RNS) species released during cellular oxidative bursts, is reported in microfluidic devices. Platinum‐black coated platinum (Pt/Pt‐black) electrodes were microfabricated and optimized to achieve optimal detection performances. In parallel, oxidation mechanisms of hydrogen peroxide and nitrite at these microelectrodes were investigated and electrode responses were compared to theoretical predictions based on convective mass transport at microchannel electrodes. In both cases, the active surface area of Pt/Pt‐black electrodes allowed to avoid inhibition effect leading to long term stability in contrast to bare Pt electrodes. Such highly sensitive Pt/Pt‐black electrodes allowed almost five decades of concentration range and detection limits down to 10 nM, a range suitable for detection of ROS and RNS released by a few cells.