Spin states of metal centers in electrocatalysis.

Understanding the electronic structure of active sites is crucial in efficient catalyst design. The spin state, spin configurations of d-electrons, has been frequently discussed recently. However, its systematic depiction in electrocatalysis is lacking. In this tutorial review, a comprehensive interpretation of the spin state of metal centers in electrocatalysts and its role in electrocatalysis is provided. This review starts with the basics of spin states, including molecular field theory, crystal field theory, and ligand field theory. It further introduces the differences in low spin, intermediate spin, and high spin, and intrinsic factors affecting the spin state. Popular characterization techniques and modeling approaches that can reveal the spin state, such as X-ray absorption microscopy, electron spin resonance spectroscopy, Mössbauer spectroscopy, and density functional theory (DFT) calculations, are introduced as well with examples from the literature. The examples include the most recent progress in tuning the spin state of metal centers for various reactions, e.g., the oxygen evolution reaction, oxygen reduction reaction, hydrogen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, nitrate reduction reaction, and urea oxidation reaction. Challenges and potential implications for future research related to the spin state are discussed at the end.

A Flexible and Lightweight Biomass-Reinforced Microwave Absorber.

Developing a flexible, lightweight and effective electromagnetic (EM) absorber remains challenging despite being on increasing demand as more wearable devices and portable electronics are commercialized. Herein, we report a flexible and lightweight hybrid paper by a facile vacuum-filtration-induced self-assembly process, in which cotton-derived carbon fibers serve as flexible skeletons, compactly surrounded by other microwave-attenuating components (reduced graphene oxide and Fe3O4@C nanowires). Owing to its unique architecture and synergy of the three components, the as-prepared hybrid paper exhibits flexible and lightweight features as well as superb microwave absorption performance. Maximum absorption intensity with reflection loss as low as - 63 dB can be achieved, and its broadest frequency absorption bandwidth of 5.8 GHz almost covers the entire Ku band. Such a hybrid paper is promising to cope with ever-increasing EM interference. The work also paves the way to develop low-cost and flexible EM wave absorber from biomass through a facile method.