Ever-increasing emissions of anthropogenic carbon dioxide (CO2) cause global environmental and climate challenges. Inspired by biological photosynthesis, developing effective strategies NeuNlto up-cycle CO2 into high-value organics is crucial. Electrochemical CO2 reduction reaction (CO2RR) is highly promising to convert CO2 into economically viable carbon-based chemicals or fuels under mild process conditions. Herein, mesoporous indium supported on multi-walled carbon nanotubes (mp-In@MWCNTs) is synthesized via a facile wet chemical method. The mp-In@MWCNTs electrocatalysts exhibit high CO2RR performance in reducing CO2 into formate. An outstanding activity (current density −78.5 mA cm−2), high conversion efficiency (Faradaic efficiency of formate over 90%), and persistent stability (~30 h) for selective CO2-to-formate conversion are observed. The outstanding CO2RR process performance is attributed to the unique structures with mesoporous surfaces and a conductive network, which promote the adsorption and desorption of reactants and intermediates while improving electron transfer. These findings provide guiding principles for synthesizing conductive metal-based electrocatalysts for high-performance CO2 conversion.

The ever-increasing complexity of environmental pollutants urgently warrants the development of new detection technologies. Sensors based on the optical properties of hydrogels enabling fast and easy in situ detection are attracting increasing attention. In this paper, the data from 138 papers about different optical hydrogels (OHs) are extracted for statistical analysis. The detection performance and potential of various types of OHs in different environmental pollutant detection scenarios were evaluated and compared to those obtained using the standard detection method. Based on this analysis, the target recognition and sensing mechanisms of two main types of OHs are reviewed and discussed: photonic crystal hydrogels (PCHs) and fluorescent hydrogels (FHs). For PCHs, the environmental stimulus response, target receptors, inverse opal structures, and molecular imprinting techniques related to PCHs are reviewed and summarized. Furthermore, the different types of fluorophores (i.e., compound probes, biomacromolecules, quantum dots, and luminescent microbes) of FHs are discussed. Finally, the potential academic research directions to address the challenges of applying and developing OHs in environmental sensing are proposed, including the fusion of various OHs, introduction of the latest technologies in various fields to the construction of OHs, and development of multifunctional sensor arrays.

Supercapacitors based on two-dimensional MXene (Ti3C2Tz) have shown extraordinary performance in ultrathin electrodes with low mass loading, but usually there is a significant reduction in high-rate performance as the thickness increases, caused by increasing ion diffusion limitation. Further limitations include restacking of the nanosheets, which makes it challenging to realize the full potential of these electrode materials. Herein, we demonstrate the design of a vertically aligned MXene hydrogel composite, achieved by thermal-assisted self-assembled gelation, for high-rate energy storage. The highly interconnected MXene network in the hydrogel architecture provides very good electron transport properties, and its vertical ion channel structure facilitates rapid ion transport. The resulting hydrogel electrode show excellent performance in both aqueous and organic electrolytes with respect to high capacitance, stability, and high-rate capability for up to 300 μm thick electrodes, which represents a significant step toward practical applications.

Exploring high efficiency S-scheme heterojunction photocatalysts with strong redox ability for removing volatile organic compounds from the air is of great interest and importance. However, how to predict and regulate the transport of photogenerated carriers in heterojunctions is a great challenge. Here, density functional theory calculations were first used to successfully predict the formation of a CdS quantum dots/InVO4 atomic-layer (110)/(110) facet S-scheme heterojunction. Subsequently, a CdS quantum dots/InVO4 atomic-layer was synthesized by in-situ loading of CdS quantum dots with (110) facets onto the (110) facets of InVO4 atomic-layer. As a result of the deliberately constructed built-in electric field between the adjoining facets, we obtain a remarkably enhanced photocatalytic degradation rate for ethylene. This rate is 13.8 times that of pure CdS and 13.2 times that of pure InVO4. In-situ irradiated X-ray photoelectron spectroscopy, photoluminescence and time-resolved photoluminescence measurements were carried out. These experiments validate that the built-in electric field enhanced the dissociation of photoexcited excitons and the separation of free charge carriers, and results in the formation of S-scheme charge transfer pathways. The reaction mechanism of the photocatalytic C2H4 oxidation is investigated by in-situ electron paramagnetic resonance. This work provides a mechanistic insight into the construction and optimization of semiconductor heterojunction photocatalysts for application to environmental remediation.

Because poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is water processable, thermally stable, and highly conductive, PEDOT:PSS and its composites have been considered to be one of the most promising flexible thermoelectric materials. However, the PEDOT:PSS film prepared from its commercial aqueous dispersion usually has very low conductivity, thus cannot be directly utilized for TE applications. Here, a simple environmental friendly strategy via femtosecond laser irradiation without any chemical dopants and treatments was demonstrated. Under optimal conditions, the electrical conductivity of the treated film is increased to 803.1 S cm−1 from 1.2 S cm−1 around three order of magnitude higher, and the power factor is improved to 19.0 μW m−1 K−2, which is enhanced more than 200 times. The mechanism for such remarkable enhancement was attributed to the transition of the PEDOT chains from a coil to a linear or expanded coil conformation, reduction of the interplanar stacking distance, and the removal of insulating PSS with increasing the oxidation level of PEDOT, facilitating the charge transportation. This work presents an effective route for fabricating high-performance flexible conductive polymer films and wearable thermoelectric devices.

Safe operation of electrochemical capacitors (supercapacitors) is hindered by the flammability of commercial organic electrolytes. Non-flammable Water-in-Salt (WIS) electrolytes are promising alternatives; however, they are plagued by the limited operation voltage window (typically ≤2.3 V) and inherent corrosion of current collectors. Herein, a novel deep eutectic solvent (DES)-based electrolyte which uses formamide (FMD) as hydrogen-bond donor and sodium nitrate (NaNO3) as hydrogen-bond acceptor is demonstrated. The electrolyte exhibits the wide electrochemical stability window (3.14 V), high electrical conductivity (14.01 mS cm−1), good flame-retardance, anticorrosive property, and ultralow cost (7% of the commercial electrolyte and 2% of WIS). Raman spectroscopy and Density Functional Theory calculations reveal that the hydrogen bonds between the FMD molecules and NO 3 − $$ {{\mathrm{NO}}_3}^{-} $$ ions are primarily responsible for the superior stability and conductivity. The developed NaNO3/FMD-based coin cell supercapacitor is among the best-performing state-of-art DES and WIS devices, evidenced by the high voltage window (2.6 V), outstanding energy and power densities (22.77 Wh kg−1 at 630 W kg−1 and 17.37 kW kg−1 at 12.55 Wh kg−1), ultralong cyclic stability (86% after 30 000 cycles), and negligible current collector corrosion. The NaNO3/FMD industry adoption potential is demonstrated by fabricating 100 F pouch cell supercapacitors using commercial aluminum current collectors.

Anode-free Li-metal batteries are of significant interest to energy storage industries due to their intrinsically high energy. However, the accumulative Li dendrites and dead Li continuously consume active Li during cycling. That results in a short lifetime and low Coulombic efficiency of anode-free Li-metal batteries. Introducing effective electrolyte additives can improve the Li deposition homogeneity and solid electrolyte interphase (SEI) stability for anode-free Li-metal batteries. Herein, we reveal that introducing dual additives, composed of LiAsF6 and fluoroethylene carbonate, into a low-cost commercial carbonate electrolyte will boost the cycle life and average Coulombic efficiency of NMC||Cu anode-free Li-metal batteries. The NMC||Cu anode-free Li-metal batteries with the dual additives exhibit a capacity retention of about 75% after 50 cycles, much higher than those with bare electrolytes (35%). The average Coulombic efficiency of the NMC||Cu anode-free Li-metal batteries with additives can maintain 98.3% over 100 cycles. In contrast, the average Coulombic efficiency without additives rapidly decline to 97% after only 50 cycles. In situ Raman measurements reveal that the prepared dual additives facilitate denser and smoother Li morphology during Li deposition. The dual additives significantly suppress the Li dendrite growth, enabling stable SEI formation on anode and cathode surfaces. Our results provide a broad view of developing low-cost and high-effective functional electrolytes for high-energy and long-life anode-free Li-metal batteries.

Room temperature sodium–sulfur (Na–S) batteries are severely hampered by dissolution of polysulfides into electrolytes. Herein, a facile approach is used to tune a biomass-derived carbon down to an ultrasmall 0.37 nm microporous structure for the first time as a cathode in sodium–sulfur batteries. This produced an intact uniform Na2S membrane to greatly confine the dissolution of polysulfides while realizing a direct solid phase conversion for complete reduction of sulfur to Na2S, which delivers a sulfur loading of 1 mg cm−2 (50 wt.%), an excellent rate capacity (933 mAh g−1 @ 0.1 A g−1 and 410 mAh g−1 @ 2 A g−1), long cycle performance (0.036% per cycle decay at 1 A g−1 after 1500 cycles), and a high energy density for 373 Wh kg−1 (0.1 A g−1) based on whole electrode weight (active sulfur loading + carbon), ranking the best among all reported plain carbon cathode-based room temperature sodium–sulfur batteries in terms of the cycle life and rate capacity. It is proposed that the solid Na2S produced in the ultrasmall pores (0.37 nm) can be squeezed out to grow an intact membrane on the electrode surface covering the outlet of the pores and greatly depressing the dissolution effect of polysulfides for the long cycle life. This work provides a green chemistry to recycle wastes for sustainable energies and sheds light on design of a unique pore structure to effectively block the dissolution of polysulfides for high-performance sodium–sulfur batteries.

Supercapacitors formed from porous carbon and graphene-oxide (GO) materials are usually dominated by either electric double-layer capacitance, pseudo-capacitance, or both. Due to these combined features, reduced GO materials have been shown to offer superior capacitance over typical nanoporous carbon materials; however, there is a significant variation in reported values, ranging between 25 and 350 F g−1. This undermines the structure (e.g., oxygen functionality and/or surface area)-performance relationships for optimization of cost and scalable factors. This work demonstrates important structure-controlled charge storage relationships. For this, a series of exfoliated graphene (EG) derivatives are produced via thermal-shock exfoliation of GO precursors and following controlled graphitization of EG (GEG) generates materials with varied amounts of porosity, redox-active oxygen groups and graphitic components. Experimental results show significantly varied capacitance values between 30 and 250 F g−1 at 1.0 A g−1 in GEG structures; this suggests that for a given specific surface area the redox-active and hydrophilic oxygen content can boost the capacitance to 250–300% higher compared to typical mesoporous carbon materials. GEGs with identical oxygen functionality show a surface area governed capacitance. This allows to establish direct structure-performance relationships between 1) redox-active oxygen functional concentration and capacitance and 2) surface area and capacitance.

Aqueous zinc-ion batteries (ZIBs) have shown great potential in the fields of wearable devices, consumer electronics, and electric vehicles due to their high level of safety, low cost, and multiple electron transfer. The layered cathode materials of ZIBs hold a stable structure during charge and discharge reactions owing to the ultrafast and straightforward (de)intercalation-type storage mechanism of Zn2+ ions in their tunable interlayer spacing and their abilities to accommodate other guest ions or molecules. Nevertheless, the challenges of inadequate energy density, dissolution of active materials, uncontrollable byproducts, increased internal pressure, and a large de-solvation penalty have been deemed an obstacle to the development of ZIBs. In this review, recent strategies on the structure regulation of layered materials for aqueous zinc-ion energy storage devices are systematically summarized. Finally, critical science challenges and future outlooks are proposed to guide and promote the development of advanced cathode materials for ZIBs.

Exploration of alternative energy storage systems has been more than necessary in view of the supply risks haunting lithium-ion batteries. Among various alternative electrochemical energy storage devices, sodium-ion battery outstands with advantages of cost-effectiveness and comparable energy density with lithium-ion batteries. Thanks to the similar electrochemical mechanism, the research and development of lithium-ion batteries have forged a solid foundation for sodium-ion battery explorations. Advancements in sodium-ion batteries have been witnessed in terms of superior electrochemical performance and broader application scenarios. Here, the strategies adopted to optimize the battery components (cathode, anode, electrolyte, separator, binder, current collector, etc.) and the cost, safety, and commercialization issues in sodium-ion batteries are summarized and discussed. Based on these optimization strategies, assembly of functional (flexible, stretchable, self-healable, and self-chargeable) and integrated sodium-ion batteries (−actuators, −sensors, electrochromic, etc.) have been realized. Despite these achievements, challenges including energy density, scalability, trade-off between energy density and functionality, cost, etc. are to be addressed for sodium-ion battery commercialization. This review aims at providing an overview of the up-to-date achievements in sodium-ion batteries and serves to inspire more efforts in designing upgraded sodium-ion batteries.

Sodium-carbon dioxide (Na-CO2) batteries are regarded as promising energy storage technologies because of their impressive theoretical energy density and CO2 reutilization, but their practical applications are restricted by uncontrollable sodium dendrite growth and poor electrochemical kinetics of CO2 cathode. Constructing suitable multifunctional electrodes for dendrite-free anodes and kinetics-enhanced CO2 cathodes is considered one of the most important ways to advance the practical application of Na-CO2 batteries. Herein, RuO2 nanoparticles encapsulated in carbon paper (RuCP) are rationally designed and employed as both Na anode host and CO2 cathode in Na-CO2 batteries. The outstanding sodiophilicity and high catalytic activity of RuCP electrodes can simultaneously contribute to homogenous Na+ distribution and dendrite-free sodium structure at the anode, as well as strengthen discharge and charge kinetics at the cathode. The morphological evolution confirmed the uniform deposition of Na on RuCP anode with dense and flat interfaces, delivering enhanced Coulombic efficiency of 99.5% and cycling stability near 1500 cycles. Meanwhile, Na-CO2 batteries with RuCP cathode demonstrated excellent cycling stability (>350 cycles). Significantly, implementation of a dendrite-free RuCP@Na anode and catalytic-site-rich RuCP cathode allowed for the construction of a symmetric Na-CO2 battery with long-duration cyclability, offering inspiration for extensive practical uses of Na-CO2 batteries.

Controlling Li ion transport in glasses at atomic and molecular levels is key to realizing all-solid-state batteries, a promising technology for electric vehicles. In this context, Li3PS4 glass, a promising solid electrolyte candidate, exhibits dynamic coupling between the Li+ cation mobility and the PS43− anion libration, which is commonly referred to as the paddlewheel effect. In addition, it exhibits a concerted cation diffusion effect (i.e., a cation–cation interaction), which is regarded as the essence of high Li ion transport. However, the correlation between the Li+ ions within the glass structure can only be vaguely determined, due to the limited experimental information that can be obtained. Here, this study reports that the Li ions present in glasses can be classified by evaluating their valence oscillations via Bader analysis to topologically analyze the chemical bonds. It is found that three types of Li ions are present in Li3PS4 glass, and that the more mobile Li ions (i.e., the Li3-type ions) exhibit a characteristic correlation at relatively long distances of 4.0–5.0 Å. Furthermore, reverse Monte Carlo simulations combined with deep learning potentials that reproduce X-ray, neutron, and electron diffraction pair distribution functions showed an increase in the number of Li3-type ions for partially crystallized glass structures with improved Li ion transport properties. Our results show order within the disorder of the Li ion distribution in the glass by a topological analysis of their valences. Thus, considering the molecular vibrations in the glass during the evaluation of the Li ion valences is expected to lead to the development of new solid electrolytes.

Tunable bandgaps make halide perovskites promising candidates for developing tandem solar cells (TSCs), a strategy to break the radiative limit of 33.7% for single-junction solar cells. Combining perovskites with market-dominant crystalline silicon (c-Si) is particularly attractive; simple estimates based on the bandgap matching indicate that the efficiency limit in such tandem device is as high as 46%. However, state-of-the-art perovskite/c-Si TSCs only achieve an efficiency of ~32.5%, implying significant challenges and also rich opportunities. In this review, we start with the operating mechanism and efficiency limit of TSCs, followed by systematical discussions on wide-bandgap perovskite front cells, interface selective contacts, and electrical interconnection layer, as well as photon management for highly efficient perovskite/c-Si TSCs. We highlight the challenges in this field and provide our understanding of future research directions toward highly efficient and stable large-scale wide-bandgap perovskite front cells for the commercialization of perovskite/c-Si TSCs.

CuS is an encouraging photoelectrode candidate that meets the essential requirements for efficient solar-to-hydrogen production, but it has not been thoroughly studied. A CuS light absorber layer is grown by the self-assembly of copper and sulfur precursors on a carbon paper (CP) electrode. Simultaneously, rGO is introduced as a buffer layer to control the optical and electrical properties of the absorber. The well-ordered microstructural arrangement suppresses the recombination loss of electrons and holes owing to enhanced charge-carrier generation, separation, and transport. The potential reaching 10 mA cm−2 in 1.0 m KOH solution is significantly lowered to 0.87 V, and the photocurrent density at 1.23 V is 94.7 mA cm−2. The computational result reveals that the potential-determining step is sensitive to O* stability; the lower stability of O* in the thin layer of CuS/rGO decreases the free-energy gap between the initial and final states of the potential-determining step, resulting in a lowering of the onset potential. The faradaic efficiency for the photoelectrochemical oxygen evolution reaction in the optimized 2CuS/1rGO/CP photoanode is 98.60%, and the applied bias photon-to-current and the solar-to-hydrogen efficiencies are 11.2% and 15.7%, respectively, and its ultra-high performance is maintained for 250 h. These record-breaking achievement indices may be a trigger for establishing a green hydrogen economy.

Pseudocapacitive materials that store charges via reversible surface or near-surface faradaic reactions are capable of overcoming the capacity limitations of electrical double-layer capacitors. Revealing the structure–activity relationship between the microstructural features of pseudocapacitive materials and their electrochemical performance on the atomic scale is the key to build high-performance capacitor-type devices containing ideal pseudocapacitance effect. Currently, the high brightness (flux), and spectral and coherent nature of synchrotron X-ray analytical techniques make it a powerful tool for probing the structure–property relationship of pseudocapacitive materials. Herein, we report a comprehensive and systematic review of four typical characterization techniques (synchrotron X-ray diffraction, pair distribution function [PDF] analysis, soft X-ray absorption spectroscopy, and hard X-ray absorption spectroscopy) for the study of pseudocapacitance mechanisms. In addition, we offered significant insights for understanding and identifying pseudocapacitance mechanisms (surface redox pseudocapacitance, intercalation pseudocapacitance, and the extrinsic pseudocapacitance phenomenon in battery materials) by combining in situ hard XAS and electrochemical analyses. Finally, a perspective for further depth of understanding into the pseudocapacitance mechanism using synchrotron X-ray analytical techniques is proposed.

It is well accepted that a lithiophilic interface can effectively regulate Li deposition behaviors, but the influence of the lithiophilic interface is gradually diminished upon continuous Li deposition that completely isolates Li from the lithiophilic metals. Herein, we perform in-depth studies on the creation of dynamic alloy interfaces upon Li deposition, arising from the exceptionally high diffusion coefficient of Hg in the amalgam solid solution. As a comparison, other metals such as Au, Ag, and Zn have typical diffusion coefficients of 10–20 orders of magnitude lower than that of Hg in the similar solid solution phases. This difference induces compact Li deposition pattern with an amalgam substrate even with a high areal capacity of 55 mAh cm−2. This finding provides new insight into the rational design of Li anode substrate for the stable cycling of Li metal batteries.

Small coin cell batteries are predominantly used for testing lithium-ion batteries (LIBs) in academia because they require small amounts of material and are easy to assemble. However, insufficient attention is given to difference in cell performance that arises from the differences in format between coin cells used by academic researchers and pouch or cylindrical cells which are used in industry. In this article, we compare coin cells and pouch cells of different size with exactly the same electrode materials, electrolyte, and electrochemical conditions. We show the battery impedance changes substantially depending on the cell format using techniques including Electrochemical Impedance Spectroscopy (EIS) and Galvanostatic Intermittent Titration Technique (GITT). Using full cell NCA-graphite LIBs, we demonstrate that this difference in impedance has important knock-on effects on the battery rate performance due to ohmic polarization and the battery life time due to Li metal plating on the anode. We hope this work will help researchers getting a better idea of how small coin cell formats impact the cell performance and help predicting improvements that can be achieved by implementing larger cell formats.

Lithium metal batteries are emerging as a strong candidate in the future energy storage market due to its extremely high energy density. However, the uncontrollable lithium dendrites and volume change of lithium metal anodes severely hinder its application. In this work, the porous Cu skeleton modified with Cu6Sn5 layer is prepared via dealloying brass foil following a facile electroless process. The porous Cu skeleton with large specific surface area and high electronic conductivity effectively reduces the local current density. The Cu6Sn5 can react with lithium during the discharge process to form lithiophilic Li7Sn2 in situ to promote Li-ions transport and reduce the nucleation energy barrier of lithium to guide the uniform lithium deposition. Therefore, more than 300 cycles at 1 mA cm−2 are achieved in the half-cell with an average Coulombic efficiency of 97.5%. The symmetric cell shows a superior cycle life of more than 1000 h at 1 mA cm−2 with a small average hysteresis voltage of 16 mV. When coupled with LiFePO4 cathode, the full cell also maintains excellent cycling and rate performance.

Iron-nitrogen-carbon (Fe-N-C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have seriously been hindered by their poor ORR performance of Fe-N-C due to the low active site density (SD) and site utilization. Herein, we reported a melamine-assisted vapor deposition approach to overcome these hindrances. The melamine not only compensates for the loss of nitrogen caused by high-temperature pyrolysis but also effectively etches the carbon substrate, increasing the external surface area and mesoporous porosity of the carbon substrate. These can provide more useful area for subsequent vapor deposition on active sites. The prepared 0.20Mela-FeNC catalyst shows a fourfold higher SD value and site utilization than the FeNC without the treatment of melamine. As a result, 0.20Mela-FeNC catalyst exhibits a high ORR activity with a half-wave potential (E1/2) of 0.861 V and 12-fold higher ORR mass activity than the FeNC in acidic media. As the cathode in a H2-O2 PEMFCs, 0.20Mela-FeNC catalyst demonstrates a high peak power density of 1.30 W cm−2, outstripping most of the reported Fe-N-C catalysts. The developed melamine-assisted vapor deposition approach for boosting the SD and utilization of Fe-N-C catalysts offers a new insight into high-performance ORR electrocatalysts.