The rise of China could be the most important political development of the twenty-first century. What will China look like in the future? What should it look like? And what will China's rise mean for ...the rest of world? This book, written by China's most influential foreign policy thinker, sets out a vision for the coming decades from China's point of view.
Alkaline anion‐exchange membrane fuel cells (AEMFCs) are attracting much attention because of their potential use of nonprecious electrocatalysts. The anion‐exchange membrane (AEM) is one of the key ...components of AEMFCs. An ideal AEM should possess high hydroxide conductivity and sufficient long‐term durability at elevated temperatures in high‐pH solutions. Herein, recent progress in research into the alkaline stability behavior of cations (including quaternary ammonium, imidazolium, guanidinium, pyridinium, tertiary sulfonium, phosphonium, benzimidazolium, and pyrrolidinium) and their analogous AEMs, which have been investigated by both experimental studies and theoretical calculations, is reviewed. Effects, including conjugation, steric hindrance e, σ–π hyperconjugation, and electrons, on the alkaline stability of cations and their analogous AEMs have been discussed. The aim of this article is to provide an overview of some key factors for the future design of novel cations and their analogous AEMs with high alkaline stability.
Long‐lasting separation: The alkaline stability of an anion‐exchange membrane (AEM) is fundamentally affected by the chemical structure of the cationic groups. The alkaline stability behavior of organic cations has been recently investigated by both experimental studies and theoretical calculations. This Minireview provides an up‐to‐date summary and comparison among various organic cations in terms of the alkaline stability and analogous cation‐based AEMs
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•KMnO4 is used as activating agent and template precursor during pyrolysis process.•Hierarchical porous carbon shows high gravimetric capacitances.•The symmetric device shows high ...energy density and good electrochemical stability.
Hierarchical porous carbon materials exhibit huge potential in high performance supercapacitors. Nevertheless, developing a convenient and sustainable approach remains a big challenge. Herein, we report a novel approach for synthesis of heteroatoms-doped hierarchical porous carbon derived from chitin using KMnO4 as activating agent and template precursor with amidogen and hydroxyl assist. The formation and evolution process of the hierarchical porous carbon are researched in detail. The optimized hierarchical porous carbon materials (HPC-700) have large specific surface area, hierarchical porous framework and numerous nitrogen and oxygen functional groups. As a result, the HPC-700 electrode displays an ultrahigh specific capacitance of 412.5 F g−1 at 0.5 A g−1, good rate characteristic and remarkable electrochemical stability (only 0.4% loss after 10,000 cycles). Significantly, the constructed flexible all-solid-state symmetric supercapacitor shows a high energy density of 9.67 Wh Kg−1 and superior electrochemical stabilization in PVA/KOH sol electrolyte. The research highlights a new, convenient approach for construction of hierarchical porous carbon materials for supercapacitors.
Valleytronics is an appealing alternative to conventional charge-based electronics that aims at encoding data in the valley degree of freedom, that is, the information as to which extreme of the ...conduction or valence band carriers are occupying. The ability to create and control valley currents in solid-state devices could therefore enable new paradigms for information processing. Transition metal dichalcogenides (TMDCs) are a promising platform for valleytronics due to the presence of two inequivalent valleys with spin-valley locking
and a direct bandgap
, which allows optical initialization and readout of the valley state
. Recent progress on the control of interlayer excitons in these materials
could offer an effective way to realize optoelectronic devices based on the valley degree of freedom. Here, we show the generation and transport over mesoscopic distances of valley-polarized excitons in a device based on a type-II TMDC heterostructure. Engineering of the interlayer coupling results in enhanced diffusion of valley-polarized excitons, which can be controlled and switched electrically. Furthermore, using electrostatic traps, we can increase the exciton concentration by an order of magnitude, reaching densities in the order of 10
cm
, opening the route to achieving a coherent quantum state of valley-polarized excitons via Bose-Einstein condensation.
Purpose
CEST provides a MR contrast mechanism sensitizing to the exchange between dilute labile and bulk water protons. However, the CEST effect depends on the RF saturation duration and relaxation ...delay, which need to be long to reach its steady state. Our study aims to estimate the QUAsi‐Steady State (QUASS) CEST signal from experiments with shorter saturation and relaxation delay times.
Methods
The evolution of the CEST signal was modeled as a function of the bulk water longitudinal relaxation rate during the relaxation delay (Td) and spin‐lock relaxation rate during the RF saturation (Ts), from which the QUASS CEST effect is solved. Numeric simulations were programmed to compare the apparent CEST and QUASS CEST effects as a function of Ts and Td times. We also performed CEST MRI experiments from a creatine–gel pH phantom under serially varied Ts and Td times.
Results
The numeric simulation showed that although the apparent CEST effect depends on Td and Ts, the QUASS CEST solution has little dependence. Phantom results showed that the routine CEST pH contrast could be described by a nonlinear regression model (ie, ΔCESTR=ΔCESTReqapp1‐e‐R1ρapp·t). We had ΔCESTReqapp = 3.90±0.03% (P < 5e−8) and R1ρapp=0.62±0.02s‐1 (P < 5e−6). For the QUASS CEST analysis, we modeled the pH contrast as ΔCESTR=ΔCESTReqQUASS+s·t, using a linear regression model. We had ΔCESTReqQUASS=3.63±0.01% (P < 5e−9) and s=‐0.02±0.00%/s (P < 0.01), the slope of which is minimal.
Conclusions
The QUASS CEST algorithm provides a post‐processing solution that facilitates robust CEST measurement.
Purpose
CEST MRI omega plot quantifies the labile proton fraction ratio (fr) and exchange rate (ksw), yet it assumes long RF saturation time (Ts) and relaxation delay (Td). Our study aimed to test if ...a quasi–steady‐state (QUASS) CEST analysis that accounts for the effect of finite Ts and Td could improve the accuracy of CEST MRI quantification.
Methods
We modeled the MRI signal evolution using a typical CEST EPI sequence. The signal relaxes toward its thermal equilibrium following the bulk water relaxation rate during Td, and then toward its CEST steady state following the spin‐lock relaxation rate during Ts from which the QUASS CEST effect is derived. Both fr and ksw were solved from simulated conventional apparent CEST and QUASS CEST MRI. We also performed MRI experiments from a Cr‐gel phantom under serially varied Ts and Td times from 1.5 to 7.5 s.
Results
Simulation showed that, although ksw could be slightly overestimated (3%‐15%) for the range of Ts and Td, fr could be substantially underestimated by as much as 67%. In contrast, the QUASS solution provided accurate ksw and fr determination within 2%. The CEST MRI experiments confirmed that the QUASS solution enabled robust quantification of ksw and fr, superior over the omega plot analysis based on the conventional apparent CEST MRI measurements.
Conclusions
The QUASS CEST MRI algorithm corrects the effect of finite Ts and Td times on CEST measurements, thereby allowing robust and accurate CEST quantification.
Highly phosphorescent (Ph4P)2MnBr4 as a low‐cost and environmentally benign emitting material achieves peak current efficiency of 25.4 cd A−1 and external quantum efficiency (EQE) of 7.2% for ...nondoped organic light‐emitting diodes, and peak current efficiency of 32.0 cd A−1 and EQE of 9.6% for doped devices with 20% (Ph4P)2MnBr4:27% TCTA:53% 6DCZPPY as a doping emitting layer.
In this paper, a HADE (hybrid adaptive differential evolution) algorithm is proposed for the identification problem of PEMFC (proton exchange membrane fuel cell). Inspired by biological genetic ...strategy, a novel adaptive scaling factor and a dynamic crossover probability are presented to improve the adaptive and dynamic performance of differential evolution algorithm. Moreover, two kinds of neighborhood search operations based on the bee colony foraging mechanism are introduced for enhancing local search efficiency. Through testing the benchmark functions, the proposed algorithm exhibits better performance in convergent accuracy and speed. Finally, the HADE algorithm is applied to identify the nonlinear parameters of PEMFC stack model. Through experimental comparison with other identified methods, the PEMFC model based on the HADE algorithm shows better performance.
•We propose a hybrid adaptive differential evolution algorithm (HADE).•The search efficiency is enhanced in low and high dimension search space.•The effectiveness is confirmed by testing benchmark functions.•The identification of the PEMFC model is conducted by adopting HADE.
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