High-energy-density lithium-sulfur(Li-S) batteries are drawing dramatic research interests to fulfill the ever-increasing demands of electrical vehicles. However, challenges with the insulating property of sulfur and its lithiation products and its large volume expansion, and the shuttle effect of lithium polysulfides, hinder the commercial application of Li-S batteries. Lots of material design concepts have been developed to address the failure modes. Among them, hollow micro-/nanostructures with abundant compositional and geometrical feasibility have been proved fruitful in addressing the current obstacles of Li-S batteries. Here, typical examples of designing hollow micro-/nanostructures to address the problems of Li-S batteries and simultaneously improve the practical capacity and lifespan are highlighted. In particular, the great effect of structural engineering on minimizing volume change, inhibiting the shuttle effect and catalyzing polysulfide conversion are discussed systematically. Finally, future directions of hollow nanostructure design to enhance the progress of Li-S batteries are also provided.

Searching for high-activity, stability and highly cost-effective electrocatalysts for acid oxygen reaction reduction(ORR) has always been an urgent problem in polymer electrolyte membrane fuel cells(PEMFCs). Nonetheless, the electrochemical properties of various systems have their intrinsic limits and tremendous efforts have been paid out to search for highly efficient electrocatalysts by more rational control over the size, morphology, composition, and structure. In particular, single-atom catalysts(SACs) have attracted extensive interest due to theirs excellent activity, stability, selectivity and the highest metal utilization. In recent years, the number of papers in the field of SACs has increased rapidly, indicating that SACs have made great progress. This review focuses on SACs electrochemical applications in the acid ORR and introduces innovative syntheses, fuel cell performance and long-time durability.

The continuous development of solid-state electrolytes(SSEs) has stimulated immense progress in the development of all-solid-state batteries(ASSBs). Particularly, garnet-typed SSEs in formula of Li₇La₃Zr₂O₁₂(LLZO) are fctivity(<1 mS/cm), wide electrochemical window(<5 V), and good chemical electrochemical stability for lithium, which are critical factors to ensure a stable, and high performance ASSBs. This review will focus on the challenges related to LLZOs-based electrolyte, and update the recent developments in structural design of LLZOs, which are discussed in three major sections: (i) crystal structure and the lithium-ion transport mechanism of LLZO; (ii) single-site and multi-site doping of Li sites, La sites and Zr sites to enhance Li ions conductivity(LIC) and stability of LLZO; (iii) interface strategies between electrodes and LLZO to decrease interface area-specific resistance(ASR).

Abstract The semitransparent flexible organic solar cell takes advantages of flexibility, transparency, color adjustment property, which is more conducive to integrate on buildings and mobile terminals. Ascribing to the developments of narrow band gap donors and the new non-fullerene acceptors, the power conversion efficiency of semitransparent flexible organic solar cells has been achieved 10% to 12% with average visible transmittance of 17% to 21%. This review summarizes the molecular design of the most representative active layer materials, and discusses the characterization of semitransparent parameters paradigms, then we discuss how to optimize the device in combination with optical simulation, and finally list the recent development of semitransparent flexible electrodes of ITO-free organic solar cells, and give our perspectives on the next step direction.

Li-ion solid electrolytes, which are compatible with metallic lithium anodes, are the key component of all solid-state batteries. Recently, the garnet Li₇La₃Zr₂O₁₂ solid electrolyte has experienced booming development and shown great potential for its excellent overall performance. However, further understanding of its stability with lithium is required for a longer battery lifetime. In this review, latest research work on the interface between garnet-type solid electrolytes and lithium is presented, including both mechanisms governing interface stability and interface engineering methods. The development prospects and potential directions for following research are also discussed in the last section.

Rigorous assessment of heterogeneous electrocatalysts for electrochemical water splitting has been a critical issue mainly due to insufficient standard protocols to measure and report experimental data. In this perspective, we highlight some common pitfalls when measuring and reporting electrocatalytic data, which should be avoided to ensure the accuracy and reproducibility and to advance the water splitting field. We advocate to prevent the introduction of artefacts from the counter and reference electrodes, as well as the impurities in the electrolyte when conducting electrocatalyst activity measurements. In addition, we encourage the use of the electrochemically active surface area(ECSA)-normalized current densities to represent the intrinsic activity of the reported catalysts for a better comparison with previously known materials. Suitable ECSA measurement methods should be employed based on the nature of catalysts. Recommendations made in this perspective will hopefully assist in identifying advanced catalysts for water splitting research.

The electricity consumption of buildings is tremendous; moreover, a huge amount of electricity is lost during distribution. Taking away this consumption can significantly reduce energy demand and greenhouse effect gas emission. One of the low-cost and renewable solutions to this issue is to install photovoltaic panels on the buildings themselves, namely, building-integrated photovoltaics(BIPVs). Using this technology, power generation roofs, windows, and facades can harvest solar radiation and convert to electricity for building power consumption. Semi-transparent perovskite solar cells(ST-PSCs) have attracted tremendous attention for the power generation windows, due to the excellent photoelectric properties, versatile fabrication methods, bandgap tunability, and flexibility. Here, an overview is provided on the recent progress of ST-PSCs for BIPV, which mainly focuses on the control of perovskite morphology, optical engineering for an efficient and semi-transparent ST-PSC. We also summarize recent development on various transparent electrodes and present prospects and challenges for the commercialization of ST-PSCs.

Sulfide-based solid-state electrolytes with ultrahigh lithium ion conductivities have been considered as the most promising electrolyte system to enable practical all-solid-state batteries. However, the practical applications of the sulfide-based all-solid-state batteries are hindered by severe interfacial issues as well as large-scale material preparation and battery fabrication problems. Liquid-involved interfacial treatments and preparation processes compatible with current battery manufacturing capable of improving electrode/electrolyte interface contacts and realizing the mass production of sulfide electrolytes and the scalable fabrication of sulfide-based battery component have attracted considerable attention. In this perspective, the current advances in liquid-involved treatments and processes in sulfide-based all-solid-state batteries are summarized. Then relative chemical mechanisms and existing challenges are included. Finally, future guidance is also proposed for sulfide-based batteries. Focusing on the sulfide-based all-solid-state batteries, we aim at providing a fresh insight on understandings towards liquid-involved processes and promoting the development of all-solid-state batteries with higher energy density and better safety.

High-energy-density batteries are in urgent need to solve the ever-increasing energy storage demand for portable electronic devices, electric vehicles, and renewable solar and wind energy systems. Alkali metals, typically lithium(Li), sodium(Na) and potassium(K), are considered as the promising anode materials owing to their low electrochemical potential, low density, and high theoretical gravimetric capacities. However, the problem of dendrite growth of alkali metals during their plating/stripping process will lead to low Coulombic efficiencies, a short lifespan and huge volume expansion, eventually hindering their practical commercialization. To resolve this issue, a very effective approach is engineering the anodes on structured current collectors. This review summarizes the development of the alkali metal batteries and discusses the recent advances in rational design of anode current collectors. First, the challenges and strategies of suppressing alkali-metal dendrite growth are presented. Then the special attention is paid to the novel current collector design for dendrite-free alkali metal anodes. Finally, we give conclusions and perspective on the current challenges and future research directions toward advanced anode current collectors for alkali metal batteries.

The electrochemical performances of lithium-ion batteries(LIBs) are closely related to the interphase between the electrode materials and electrolytes. However, the development of lithium-ion batteries is hampered by the formation of uncontrollable solid electrolyte interphase(SEI) and subsequent potential safety issues associated with dendritic formation and cell short-circuits during cycling. Fabricating artificial SEI layer can be one promising approach to solve the above issues. This review summarizes the principles and methods of fabricating artificial SEI for three types of main anodes: deposition-type(e.g., Li), intercalation-type(e.g., graphite) and alloy-type(e.g., Si, Al). The review elucidates recent progress and discusses possible methods for constructing stable artificial SEIs composed of salts, polymers, oxides, and nanomaterials that simultaneously passivate anode against side reactions with electrolytes and regulate Li⁺ ions transport at interfaces. Moreover, the reaction mechanism of artificial SEIs was briefly analyzed, and the research prospect was also discussed.

Due to the burning of fossil fuels, the level of carbon dioxide(CO₂) in the atmosphere gradually rises, leading to serious greenhouse effect and environmental problems. Electrocatalytic reduction of CO₂ is currently an efficient way to convert CO₂ to value-added products. Bismuth(Bi)-based nanomaterials have raised great interests due to their excellent activity and high selectivity to electrocatalytic CO₂ reduction. In this review, the fundamental principles of electrochemical CO₂ reduction reaction(CO₂RR) are introduced at first. Moreover, the recent development of Bi-based electrocatalytic materials including Bi with various nanostructures(nanoparticle, nanosheet, etc.), Bi-based compounds(Bi oxide, bimetal chalcogenide, etc.), and Bi/C nanocomposites are summarized. In the end, the future prospects and challenges of electrocatalysts for CO₂ reduction are discussed.

Thermogalvanic cells(also known as thermo-electrochemical cells) that convert waste heat energy to electricity are a new type of energy conversion device. However, the electron transfer kinetics and mass transfer of redox couples have not been thoroughly studied. Here, the ion reaction and charge transport in thermogalvanic cells are investigated by electrochemical impedance analysis. We first propose the detailed impedance model followed experimental verification on three types of electrode materials. Parameters including kinetic rate constants and ion diffusion coefficients for the electrodes are obtained by fitting the impedance data. Our study shows explicitly that impedance analysis can provide useful information on selecting suitable electrode materials for thermogalvanic cells.

A challenging but urgent task is to construct efficient and robust hydrogen evolution reaction(HER) electrocatalysts for practically feasible and sustainable hydrogen production through alkaline water electrolysis. Herein we report a simple and mild pyrolysis method to synthesize the efficient Ru nanoparticles(NPs) supported on Co-embedded N-doped carbon nanotubes(Ru/Co-NCNTs) catalyst for HER in basic media. The Ru/Co-NCNTs display remarkable performance with a low overpotential of only 35 mV at 10 mA/cm², a small Tafel slope(36 mV/dec), and a high mass activity in 1 mol/L KOH, which is superior to commercial 20% Pt/C catalyst. This excellent performance is benefited from the enhanced conductivity of N-doped carbon nanotubes(NCNTs) and high intrinsic activity triggered by synergistic coupling between Ru NPs and Co-embedded N-doped carbon nanotubes(Co-NCNTs).

The application of transition metal dichalcogenides(TMDs) as anode materials in sodium-ion batteries (SIBs) has been hindered by low conductivity and poor cyclability. Herein, we report the synthesis of Co _xFe_1−xS₂ bimetallic sulfide/sulfur-doped Ti₃C₂ MXene nanocomposites(Co _xFe_1−xS₂@S-Ti₃C₂) by a facile co-precipitation process and thermal-sulfurization reaction. The interconnected 3D frameworks consisting of MXene nanosheets can effectively buffer the volume change and enhance the charge transfer. In particular, sulfur-doped MXene nanosheets provide rich active sites for sodium storage and restrain sulfur loss during charging/discharging processes, leading the increase of specific capacity and cycling the stability of anode materials. As a result, Co _xFe_1−xS₂@S-Ti₃C₂ anodes exhibited high capacity, high rate capability and long cycle life(399 m·Ah/g at 5 A/g with an 94% capacity retention after 600 cycles).

The sphene-type solid electrolyte with high ionic conductivity has been designed for solid-state lithium metal battery. However, the practical applications of solid electrolytes are still suffered by the low relative density and long sintering time of tens of hours with large energy consumption. Here, we introduced the spark plasma sintering technology for fabricating the sphene-type Li_1.125Ta_0.875Zr_0.125SiO₅ solid electrolyte. The dense electrolyte pellet with high relative density of ca. 97.4% and ionic conductivity of ca. 1.44× 10⁻⁵ S/cm at 30 °C can be obtained by spark plasma sintering process within the extremely short time of only ca. 0.1 h. Also the solid electrolyte provides stable electrochemical window of ca. 6.0 V(vs. Li⁺/Li) and high electrochemical interface stability toward Li metal anode. With the enhanced interfacial contacts between electrodes and electrolyte pellet by the in-situ formed polymer electrolyte, the solid-state lithium metal battery with LiFePO₄ cathode can deliver the initial discharge capacity of ca. 154 mAh/g at 0.1 C and the reversible capacity of ca. 132 mAh/g after 70 cycles with high Coulombic efficiency of 99.5% at 55 °C. Therefore, this study demonstrates a rapid and energy efficient sintering strategy for fabricating the solid electrolyte with dense structure and high ionic conductivity that can be practically applied in solid-state lithium metal batteries with high energy densities and safeties.

Nanostructured N-doped TiO₂ photocatalyst has been prepared via a new approach from Ti-based MOF[NH₂-MIL-125(Ti)] precursor. The success of N doping enhances light absorption and narrows the bandgap. Moreover, the as-prepared nanostructure is constructed with tiny nanoparticles and resembles a pie-like morphology inherited from the MOF, which accelerates electron transfer. Hence, as a photocatalyst for the degradation of methylene blue(MB) under visible light irradiation, the N-doped TiO₂(N-TiO₂) nanostructure shows higher photocatalytic activity with a reaction rate constant of 0.018 min⁻¹ than that of the TiO₂-P25 and TiO₂ under the visible light.

Exploring cost-effective and high-performance oxygen reduction reaction(ORR) electrocatalysts to replace precious platinum-based materials is crucial for developing electrochemical energy conversion devices but remains a great challenge. Herein, Fe single atoms anchored on nanosheet-linked, defect-rich, highly N-doped 3D porous carbon(Fe-SAs/NLPC) electrocatalysts were obtained by pyrolyzing salt-sealed Fe-doped zeolitic imidazolate frameworks(ZIFs). NaCl functions both as pore-forming agent and closed nanoreactor, which can not only lead to the formation of defects-rich three-dimensional interconnected structures with high N-doping content to expose abundant active sites, promote mass transfer and electron transfer, but also facilitate the effective incorporation of Fe to form Fe-N _x active sites without aggregation. These unique characteristics render Fe-SAs/NLPC outstanding electrocatalytic activity for ORR, with one-set potential of 0.96 V and high kinetic current density(j _K) of 33.32 mA/cm² in alkaline medium, which surpass the values of most nonprecious-metal catalysts and even commercial Pt/C.

Cathodes with high cycling stability and rate capability are required for ambient temperature sodium ion batteries in renewable energy storage application. Na₃V₂(PO₄)₃ is an attractive cathode material with excellent electrochemical stability and fast ion diffusion coefficient within the 3D NASICON structure. Nevertheless, the practical application of Na₃V₂(PO₄)₃ is seriously hindered by its intrinsically poor electronic conductivity. Herein, solvent evaporation method is presented to obtain the nitrogen-doped carbon coated Na₃V₂(PO₄)₃ cathode material, delivering enhanced electrochemical performances. N-Doped carbon layer coating serves as a highly conducting pathway, and creates numerous extrinsic defects and active sites, which can facilitate the storage and diffusion of Na⁺. Moreover, the N-doped carbon layer can provide a stable framework to accommodate the agglomeration of the electrode upon electrode cycling. N-Doped carbon coated Na₃V₂(PO₄)₃(NC-NVP) exhibits excellent long cycling life and superior rate performances than bare Na₃V₂(PO₄)₃ without carbon coating. NC-NVP delivers a stable capacity of 95.9 mA·h/g after 500 cycles at 1 C rate, which corresponds to high capacity retention(94.6%) with respect to the initial capacity(101.4 mA·h/g). Over 91.3% of the initial capacity is retained after 500 cycles at 5 C, and the capacity can reach 85 mA·h/g at 30 C rate.

The development of methanol-tolerate oxygen reduction reaction(ORR) electrocatalysts is of special significance to direct methanol fuel cells system. Iridium is known for its better methanol tolerance than platinum and able to survive in harsh acidic environment. However, its activity is relatively low and thus the approach to improve Ir’s ORR is desired. Herein, bimetallic Ir-Cu nanoparticles(NPs) with controllable Ir/Cu compositions(ca. 1:2 to 4:1, atomic ratio) are synthesized via a galvanic replacement-based chemical method. The as-synthesized Ir-Cu NPs are investigated as ORR catalysts after electrochemically leaching out the surface Cu and forming Ir-skinned structures. Around 2- to 3-fold enhancement in the intrinsic activity has been observed in these Ir-skinned Ir-Cu catalysts compared to Ir counterpart. The approach is demonstrated to be a promising way to prepare efficient Ir ORR catalysts and lower catalyst cost.

Electrochemical oxygen reduction reaction(ORR) is crucial for fuel cells and metal-air batteries, while the oxygen consumption dynamics study during ORR, which affects the ORR efficiency, is not as concerned as catalysts design does. Herein the consumption behavior of an individual oxygen bubble on Pt foils with different wettabilities during ORR was tracked by a real-time approach to reveal whether the surface wettability of electrode can influence the consumption dynamics and determine the reaction reactive zones of oxygen bubble consumption. The oxygen bubble underwent a “constant contact angle” dominant consumption model on aerophobic Pt foil, while an initial “constant radius” and the subsequent “constant contact angle” oxygen consumption models were observed on aero-philic Pt foil. Results here demonstrated that the current was proportional to the bottom contact area, rather than the three-phase contact line of the bubbles according to the fitting curves between individual bubble current and the con-sumption behavior parameters. This study highlights the important role of the gas-solid interface in influencing the efficiency of gas consumption electrochemical reactions, which shall benefit the understanding of reaction kinetics and the rational design of electrocatalysts.

Developing efficient catalysts toward both oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) is the core task for rechargeable metal-air batteries. Although integration of two active components should be an effective method to produce the bifunctional catalysts in principle, traditional techniques still can not attain fine tunable surface structure during material-hybridization process. Herein, we present a facile short-time in-situ argon(Ar) plasma strategy to fabricate a high-performance bifunctional hybrid catalyst of vacancy-rich CoFe₂O₄ synergized with defective graphene(r-CoFe₂O₄@DG). Reflected by the low voltage gap of 0.79 V in two half-reaction measurements, the striking capability to catalyze ORR/OER endows it excellent and durable performance in rechargeable Zn-air batteries, with a maximum power density of 155.2 mW/cm² and robust stability(up to 60 h). Further experimental and theoretical studies validate its remarkable bifunctional energetics root from plasma-induced surface vacancy defects and interfacial charge polarization between DG and CoFe₂O₄. This work offers more opportunities for reliable clean energy systems by rational interfacial and defect engineering on catalyst design.

To achieve structural order and strength-toughness balance similar to natural composites like nacre has been highly challenging, especially in a practically viable manner. Liu et al. developed a continuous preparation method to construct such high performance composites through a superspreading strategy at the oil/water/hydrogel interface. Exploiting the strong shear force at the interface, nanoparticles, such as nanosheets and nanotubes oriented and stacked together rapidly(as short as 358 ms) and the composites revealed the tensile strength and fracture toughness far higher those of natural nacre(9.0 and 20.4 times higher). This work has been published in Nature, 2020.

Synthesis of colloidal heterostructures with rational design and controllable precision represents a promising strategy for achieving novel properties and applications. Most recently, Zhuang et al. reported a “double-buffer-layer engineering” concept that was capable of regioselectively growing magnetic Fe₃O₄ nanodomains only at single ends of semiconductor Zn _xCd_1−xS(0⩽x⩽1) nanorods. The resulting composite nanostructures exhibited chiroptical activity due to the local magnetic fields introduced by regiospecific magnetic nanodomains, highlighting the promise of controlled colloidal chemistry in synthesizing chiroptical nanostructures in the absence of chiral molecules and helical geometries. The work has been published online in Nature Nanotechnology on January 20, 2020.

The original version of this article unfortunately con-tained a mistake. The presentation of corresponding authors was incorrect. The corrected corresponding authors labeled with asterisk are given below.

WEN Xianfang^1,3, YE Lin², CHEN Likun¹, KONG Lingce¹, YUAN Ling^1,3, XI Hailing^1,3* and ZHONG Jinyi^1,3*

Hailing Xi, hailing_xi@163.com

Jinyi Zhong, linfzjy@163.com