eScience

Recent progress in cathodic reduction-enabled organic electrosynthesis: Trends, challenges, and opportunities

Huang Binbin, Sun Zemin, Sun Genban

2022, 2(3): 243-277. doi: 10.1016/j.esci.2022.04.006

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Abstract:
Compared with general redox chemistry, electrochemistry using the electron as a potent, controllable, yet traceless alternative to chemical oxidants/reductants usually offers more sustainable options for achieving selective organic synthesis. With its environmentally benign features gradually being uncovered and studied, organic electrosynthesis is currently undergoing a revival and becoming a rapidly growing area within the synthetic community. Among the electrochemical transformations, the anodically enabled ones have been far more extensively exploited than those driven by cathodic reduction, although both approaches are conceptually attractive. To stimulate the development of cathodically enabled organic reactions, this review summarizes the recently developed reductive electrosynthetic protocols, discussing and highlighting reaction features, substrate scopes, applications, and plausible mechanisms to reveal the recent trends in this area. Herein, cathodic reduction-enabled preparative organic transformations are categorized into four types: reduction of (1) unsaturated hydrocarbons, (2) heteroatom-containing carbon-based unsaturated systems, (3) saturated C-hetero or C–C polar/strained bonds, and (4) hetero-hetero linkages. Apart from net electroreductive reactions, a few examples of reductive photo-electrosynthesis as well as paired electrolysis are also introduced, which offer opportunities to overcome certain limitations and improve synthetic versatility. The electrochemically driven, transition metal-catalyzed reductive cross-couplings that have been comprehensively discussed in several other recent reviews are not included here.

Defect engineering in molybdenum-based electrode materials for energy storage

Wang Weixiao, Xiong Fangyu, Zhu Shaohua, Chen Jinghui, Xie Jun, An Qinyou

2022, 2(3): 278-294. doi: 10.1016/j.esci.2022.04.005

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Abstract:
Molybdenum-based materials have stepped into the spotlight as promising electrodes for energy storage systems due to their abundant valence states, low cost, and high theoretical capacity. However, the performance of conventional molybdenum-based electrode materials has been limited by slow diffusion dynamics and deficient thermodynamics. Applying defect engineering to molybdenum-based electrode materials is a viable method for overcoming these intrinsic limitations to realize superior electrochemical performance for energy storage. Herein, we systematically review recent progress in defect engineering for molybdenum-based electrode materials, including vacancy modulation, doping engineering, topochemical substitution, and amorphization. In particular, the essential optimization mechanisms of defect engineering in molybdenum-based electrode materials are presented: accelerating ion diffusion, enhancing electron transfer, adjusting potential, and maintaining structural stability. We also discuss the existing challenges and future objectives for defect engineering in molybdenum-based electrode materials to realize high-energy and high-power energy storage devices.

Ni single-atom sites supported on carbon aerogel for highly efficient electroreduction of carbon dioxide with industrial current densities

Guo Hui, Si Duan-Hui, Zhu Hong-Jing, Li Qiu-Xia, Huang Yuan-Biao, Cao Rong

2022, 2(3): 295-303. doi: 10.1016/j.esci.2022.03.007

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Abstract:
Finding highly efficient electrocatalysts for the CO₂ electroreduction reactions (CO₂RR) that have high selectivity and appreciable current density to meet commercial application standards remains a challenge. Because their reduction potentials are similar to that of the associated competitive hydrogen evolution reaction and the CO₂ activation kinetics are sluggish. Although single-atom catalysts (SACs) with high atom efficiency are one class of promising candidates for the CO₂RR to produce CO, single-atom active sites supported on microporous carbons are not fully exposed to substrates and thus lead to low current density. Carbon aerogels with interconnected channels and macropores can facilitate mass transport. But few reports describe utilizing them as supports to anchor SACs for efficient electrocatalysis. Herein, N-doped carbon aerogels supporting Ni single atomic catalyst sites (denoted as Ni-NCA-X, X = 10, 20) were fabricated by pyrolyzing Ni/Zn bimetallic zeolitic imidazolate framework (Ni/Zn-ZIF-8)/carboxymethylcellulose composite gels. Owing to abundant hierarchical micro-, meso-, and macropores and high CO₂ adsorption, the Ni single active sites in the optimal Ni-NCA-10 were readily accessible for the electrolyte and CO₂ molecules and thus achieved an industrial-level CO partial current density of 226 mA cm⁻², a high CO Faradaic efficiency of 95.6% at −1.0 V vs. the reversible hydrogen electrode, and a large turnover frequency of 271810 h⁻¹ in a flow-cell reactor at −1.0 V. Such excellent CO₂RR performance makes Ni-NCA-10 a rare state-of-the-art electrocatalyst for CO₂-to-CO conversion. This work provides an effective strategy for designing highly efficient electrocatalysts toward the CO₂RR to achieve industrial current density via anchoring single-atom sites on carbon aerogels.

Phosphated IrMo bimetallic cluster for efficient hydrogen evolution reaction

Guo Xu, Wan Xin, Liu Qingtao, Li Yongcheng, Li Wenwen, Shui Jianglan

2022, 2(3): 304-310. doi: 10.1016/j.esci.2022.04.002

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Abstract:
Developing low-cost, high-performance electrocatalysts for the hydrogen evolution reaction (HER) is essential for producing hydrogen from renewable energy sources. Herein, we report phosphated IrMo bimetallic clusters supported by macroporous nitrogen-doped carbon (IrMoP/MNC) as a highly efficient alkaline HER catalyst. The experimental and theoretical results demonstrate that P and Mo synergistically tune the electronic structure of atomically dispersed Ir to improve adsorption of the reactant H₂O and desorption of the product OH⁻. P itself serves as an active site and cooperates with the nearby Ir atom to significantly enhance the HER kinetics. Even with only 2.6 wt% Ir in the catalyst, IrMoP/MNC exhibits an ultralow overpotential of 14 mV at 10 mA cm⁻², as well as an unprecedented high mass activity of 18.58 A mg_Ir⁻¹ at an overpotential of 100 mV, superior to commercial Pt/C and overwhelmingly better than other Ir-based electrocatalysts. This study demonstrates a multi-level design strategy to effectively improve the atom efficiency of a noble metal, involving spatial geometry, local electronic structure, and dual-atom synergy.

Thermally rearranged covalent organic framework with flame-retardancy as a high safety Li-ion solid electrolyte

Wang Zhifang, Zhang Yushu, Zhang Penghui, Yan Dong, Liu Jinjin, Chen Yao, Liu Qi, Cheng Peng, Zaworotko Michael J., Zhang Zhenjie

2022, 2(3): 311-318. doi: 10.1016/j.esci.2022.03.004

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Abstract:
Solid polymer electrolytes have demonstrated high promise to solve the safety problems caused by conventional liquid electrolytes in lithium ion batteries. However, the inherent flammability of most polymer electrolyte materials remains unresolved, hence hindering their further industrial application. Addressing this challenge, we designed and constructed a thermal-responsive imide-linked covalent organic framework (COF) bearing ortho-positioned hydroxy groups as precursors, which can conduct a thermal rearrangement to transform into a highly crystalline and robust benzoxazole-linked COF upon heating. Benefiting from the release of carbon dioxide through thermal rearrangement reaction, this COF platform exhibited excellent flame retardant properties. By contrast, classic COFs (e.g., boronate ester, imine, olefin, imide linked) were all flammable. Moreover, incorporating polyethylene glycol and Li salt into the COF channels can produce solid polymer electrolytes with outstanding flame retardancy, high ionic conductivity (6.42 × 10⁻⁴ S cm⁻¹) and a high lithium-ion transference number of 0.95. This thermal rearrangement strategy not only opens a new route for the fabrication of ultrastable COFs, but also provides promising perspectives to designing flame-retardant materials for energy-related applications.

Insight into the underlying competitive mechanism for the shift of the charge neutrality point in a trilayer-graphene field-effect transistor

Huang Tao, Ding Jiafen, Liu Zirui, Zhang Rui, Zhang BoLei, Xiong Kai, Zhang Longzhou, Wang Chong, Shen Shili, Li Cuiyu, Yang Peng, Qiu Feng

2022, 2(3): 319-328. doi: 10.1016/j.esci.2022.03.005

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Abstract:
Layer-number modulation in graphene has become a recent focus of research due to the superior degree of freedom that can be achieved in terms of magic-angle, wettability, superconductivity, and superlattices. However, the intrinsic transport of multilayer graphene is indistinguishable in atmospheric adsorbates and supporting environment, and its underlying charge transfer mechanism has not yet been thoroughly determined. In this study, a shift in the charge neutrality point of trilayer graphene (TLG) is demonstrated to be regulated by three governing factors: oxygen gas (O₂), water molecules (H₂O), and thermally activated electrons. Absorbed O₂ induces a high work function in semimetallic TLG, while H₂O is not an evident dopant but can strengthen binding against O₂ desorption. A simplified model is developed to elucidate the competitive mechanism and charge transfer among these two dopants (O₂, H₂O) and thermal electrons, and the model is demonstrated by work function regulation and Bader charge transfer based on density functional theory calculations. This study provides a strategy to explore transport modulation of multilayer graphene in the fields of ballistic transport and low power consumption of graphene field-effect transistors.

Sputtered MoN nanolayer as a multifunctional polysulfide catalyst for high-performance lithium–sulfur batteries

Yue Xin-Yang, Zhang Jing, Bao Jian, Bai Yi-Fan, Li Xun-Lu, Yang Si-Yu, Fu Zheng-Wen, Wang Zhen-Hua, Zhou Yong-Ning

2022, 2(3): 329-338. doi: 10.1016/j.esci.2022.03.003

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Abstract:
Two major obstacles for the practical application of lithium–sulfur batteries are sluggish redox kinetics and the shuttle effect of lithium polysulfides (LiPSs). Herein, MoN nanolayer-decorated multilayer graphene is fabricated via magnetron sputtering then serves as a multifunctional interlayer in Li–S batteries to suppress the shuttle effect and enhance redox kinetics. It is revealed that after the initial discharge process, the MoN layers break up into independent microreaction units consisting of MoN bodies and MoS₂ edges, forming a heterogeneous composite catalyst in situ. The MoN bodies not only have high sulfur affinity to trap LiPSs but also enhance their redox kinetics. At the same time, the MoS₂ edge weakens the mobility of LiPSs via the anchoring effect. As a result, Li–S cells using the interlayer present superior cycling stability under a high sulfur loading of 4.8 mg cm⁻². This work may open a new avenue for developing high-performance Li–S batteries.

Suppressed recombination for monolithic inorganic perovskite/silicon tandem solar cells with an approximate efficiency of 23%

Wang Sanlong, Wang Pengyang, Chen Bingbing, Li Renjie, Ren Ningyu, Li Yucheng, Shi Biao, Huang Qian, Zhao Ying, Grätzel Michael, Zhang Xiaodan

2022, 2(3): 339-346. doi: 10.1016/j.esci.2022.04.001

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Abstract:
Potentially temperature-resistant inorganic perovskite/silicon tandem solar cells (TSCs) are promising devices for boosting efficiency past the single-junction silicon limit. However, undesirable non-radiative recombination generally leads to a significant voltage deficit. Here, we introduce an effective strategy using nickel iodide, an inorganic halide salt, to passivate iodine vacancies and suppress non-radiative recombination. NiI₂-treated CsPbI_3-xBr_x inorganic perovskite solar cells with a 1.80 eV bandgap exhibited an efficiency of 19.53% and a voltage of 1.36 V, corresponding to a voltage deficit of 0.44 V. Importantly, the treated device demonstrated excellent operational stability, maintaining 95.7% of its initial efficiency after maximum power point tracking for 300 h under continuous illumination in a N₂ atmosphere. By combining this inorganic perovskite top cell with a narrower bandgap silicon bottom cell, we for the first time achieved monolithic inorganic perovskite/silicon TSCs, which exhibited an efficiency of 22.95% with an open-circuit voltage of 2.04 V. This work provides a promising strategy for using inorganic passivation materials to achieve efficient and stable solar cells.

2022 Vol. 2, No. 3