Thanks to its high gravimetric and volumetric capacities, silicon (Si) is one of the most promising alternatives to graphite for negative electrodes for lithium-ion batteries. Its practical use is nevertheless hampered by its low capacity retention, resulting from its high volume variation upon cycling driving the formation of an unstable solid electrolyte interphase (SEI). Coatings of Si particles with metal–organic frameworks (MOFs) acting as artificial SEIs were recently reported and found to lead to improved electrochemical performances in a few cases. We here developed a room temperature route to coat Si particles with the aluminum fumarate MOF (Al-fum), in conditions compatible with the aqueous formulation of state-of-the-art Si electrodes. Thanks to a variety of characterization techniques, including IR and solid-state NMR spectroscopies, powder X-ray diffraction, and scanning transmission electron microscopy coupled with energy-dispersive X-ray analysis (STEM-EDX), we show that a layer of ca. 20 nm of MOF is grown at the surface of the Si particles. Nevertheless, such a coating does no translate into any major modification of the electrochemical performance when the Si particles are integrated in electrodes with a loading of practical interest (∼2 mgSi cm–2). Postmortem characterizations revealed that Al-fum, although being highly stable toward water, evolves in the standard LP30 electrolyte through a reaction with the PF6– anions. The MOF further reacts during the first electrochemical reduction, ultimately leading to lithium aluminate phases, still located at the surface of the Si particles. Considering the growing interest of MOFs in the field of electrochemical energy storage, this let us conclude that there is probably a general need to more deeply and systematically evaluate the stability of MOFs toward battery electrolytes and electrochemical processes.

This study synthesizes the nickel–cobalt supported by the metal–organic framework CPO-27 (coordination polymer of Oslo, Ni–Co-CPO-27) and g-C3N4, showing outstanding catalytic activity and stability for oxygen reduction reaction in alkaline media, notated by NiCo2-CPO-27/PCN-HT (PCN-HT: heat-treated polymeric carbon nitrite). The half-wave potential of NiCo2-CPO-27/PCN-HT is 0.82 V, and the electron transfer number is around 3.99, which is close to the activity of Pt/C. The stability test of NiCo2-CPO-27/PCN-HT demonstrates only 0.01 V decade of the half-wave potential after 30,000 cycles. The synergistic effects of Ni–Co metals, pyridinic-N species, and graphitic-N species contribute to the outstanding performance of NiCo2-CPO-27/PCN-HT. The anion exchange membrane fuel cell (AEMFC) using NiCo2-CPO-27/PCN-HT in the cathode shows excellent performance with a maximum power density of 224.4 mW cm–2, 20% higher than AEMFC using the Pt/C under the same condition.

Solid-state batteries can be built based on thiophosphate electrolytes such as β-Li3PS4. For the preparation of these solid electrolytes, various solvent-based routes have been reported. For recycling of end-of-life solid-state batteries based on such thiophosphates, we consider the development of dissolution and recrystallization strategies for the recovery of the model compound β-Li3PS4. We show that recrystallization can only be performed in polar, slightly protic solvents such as N-methylformamide (NMF). The recrystallization is comprehensively studied, showing that it proceeds via an intermediate phase with composition Li3PS4·2NMF, which is structurally characterized. This phase has a high resistivity for the transport of lithium ions and must be removed in order to obtain a recrystallized product with a conductivity similar to the pristine material. Moreover, the recrystallization from solution results in an increase of the amorphous phase fraction next to crystalline β-Li3PS4.

Efficient charge extraction in lead halide perovskite nanocrystals is frequently sought-after and probed using various probe molecules. Often ignored, the chemical bonding of the molecules to the perovskite’s surface, as dictated by the terminal anchoring functional group, can have implications on the excited-state interactions between perovskite nanocrystals and the charge-shuttling molecules. Considering the remarkability of the recent work on ferrocene-based molecules in allowing charge transfer in perovskite nanocrystals, we have employed ferrocene molecule functionalized with various functional groups to understand the binding and charge-transfer process at the interface of the perovskite nanocrystal and the redox relay molecule. We evidenced that the charge transfer enhanced with enhancement in binding, as validated by the association constant evaluated as high as 1.71 × 107 M–1. In particular, the −COOH and −NMe2 functional groups led to the efficient quenching of photoluminescence (PL) emission and a decrease in photoluminescence lifetime than the other functional group analogues, showing their feasibility in charge transfer studies. More importantly, the −NMe2 functional group indicated passivation of the defects on the perovskite surface, attributed to the interaction between the lone pair of nitrogen and the undercoordinated surface Pb2+ cations. This was also evident in the transient absorption spectra, where the excited-state interaction could be analyzed better. This work opens avenues for exploring anchoring moieties in facilitating charge transfer across the perovskite interface, thus impacting its photocatalytic applications.

Developing high power density sodium-ion batteries by exploiting the high power nature of capacitive behavior has been a hot topic in recent years. However, the improvement in power density of sodium-ion batteries usually comes at the cost of a loss in energy density, so a trade-off between power and energy densities is required. Herein, we innovatively establish a connection between the capacitive contribution in the electrode material and the energy and power densities of sodium-ion batteries. The energy and power densities of sodium-ion batteries at high current densities are equilibrated by tuning the capacitive contribution in the hard carbon materials. First, it is proved that the power and energy densities are a joint function of the current density and the capacitive contribution by theoretical analysis. Then, hard carbon materials are designed and fabricated by tuning the capacitive contribution guided by the theoretical analysis to equilibrate energy and power densities of sodium-ion batteries at high current densities. Finally, the variation of the power and energy densities of the sodium-ion batteries with the current density and capacitive contribution is obtained. The results indicate that at low current densities (<1 A/g), the sodium-ion batteries employed with hard carbon anode with low capacitive contribution have a similar power density but higher energy density compared with those with high capacitive contribution. When the current density reaches or exceeds 1 A/g, the sodium-ion batteries employed with hard carbon anode with high capacitive contribution reveal both higher power and energy densities (power and energy densities are 8,316.66 Wh/kg and 251.81 W/kg at 3 A/g, respectively). These results are attributed to the various capacity decay rates of the battery and capacitive parts at different current densities, and the energy density provided by the capacitive part is limited. This work provides guidance on the introduction of capacitive contribution in electrode materials for ion batteries from a full-battery perspective.

Photoelectrochemical solar water splitting has become a potential approach for producing clean hydrogen fuels by utilizing semiconductor photoelectrodes and solar energy. Among emerging metal oxide photoelectrodes, iron vanadate (FeVO4) with its unique electronic band structure and suitable bandgap energies for absorbing visible light from the solar spectrum has become a promising photoanode. However, the reported photocurrent density of this material is still low because of the poor water oxidation kinetics and the slow separation of carriers, leading to recombination at the surface. In this study, we attempted to solve these limitations by nanostructuring the FeVO4 photoanode and modifying its surface with cocatalysts (CoOx, CoPi, and CoOx–CoPi). Both photocurrent and onset potential are significantly improved, resulting from the enhancement of charge injection and separation efficiencies. For the first time, the dual layer of oxygen evolution CoOx–CoPi catalysts is found more effective than single-layer CoOx or CoPi catalysts for the nanoporous FeVO4 photoanode with the increased photocurrent density at 1.23 V vs RHE of a 5-fold improvement compared to the pristine FeVO4. This result offers a strategy to further improve FeVO4 photoanode performance for efficient solar water splitting toward practical applications.

Organic electrodes have been identified as promising energy-storage materials for aqueous zinc-ion batteries (AZIBs). Small molecular materials have ideal redox properties, high specific capacity, and structural diversity, making them a category of cathode candidates for AZIBs. However, the instability and dissolution during the extraction and insertion of H+/Zn2+ limit their application of the long-cycle stability for AZIBs. Herein, a small-molecule nanosheet (NI-DAQ, ∼14 nm in thickness) with imide linkage is designed and synthesized by the condensation of anthraquinones and anhydrides. It not only inhibits the dissolution of monomer electrodes but also boosts the reactivity and conductivity of the whole molecule by the introduction of π-conjugated imide groups and extended aromatic planes. Therefore, the NI-DAQ electrode obtains a large initial capacity of 191.9 mA h g–1 at 50 mA g–1 and superior cyclability after 3000 cycles at 500 mA g–1 with a minor average capacity fading rate of 0.01% per cycle. Moreover, in situ Fourier transform infrared (FT-IR) and ex situ X-ray photoelectron spectroscopy (XPS) characterization techniques have been implemented to investigate the redox mechanism of C═O units in AZIBs for the NI-DAQ electrode. Thus, a promising conductive molecule is developed and explored in this paper, which can provide insights into the application of organic materials in AZIBs.

Zintl-phase semiconductor materials with low intrinsic lattice thermal conductivity have been the target of study for thermoelectric (TE) applications. Herein, we report Zintl-phase TiNiSi-type SrMgSi and CaMgGe with calculated low intrinsic lattice thermal conductivity (κL) values of 2.52 and 1.90 W/m·K at room temperature, respectively. The low κL is mainly due to the strong lattice anharmonicity, which originates from the weak bonding of cations in the anionic network, and strong optical-acoustic phonon coupling. Additionally, the high band degeneracy results in good electrical properties, and excellent ZT values of ∼2.83 (n-type, 500 K) and 3.09 (n-type, 500 K) are predicted for SrMgSi and CaMgGe, respectively. The theoretical study provides a valuable direction for exploring Zintl-phase materials for efficient TE power conversion.

Lithium-ion batteries (LIBs) have been extensively employed in a range of electrical vehicles and portable devices in virtue of their high energy density and stable cycle life. However, poor performance under low temperatures hinders their application in cold climates and regions. Herein, single-shell (carbon) multiple-core (ultra-small MnO@C nanoparticles) hollow carbon nanospheres (MnO@C@HCS) were prepared by a sacrificial template method, and MnO@C@HCS showed excellent low-temperature electrochemical performance. These MnO@C cores with large surface areas can shorten diffusion lengths of lithium ions and enhance diffusion rates along their rich grain boundaries, enabling rapid charging/discharging. The hollow carbon nanosphere with a porous shell can block serious agglomeration of nanoparticles and regulate the amount of electrolyte filled in the hollow nanosphere to reduce side reactions between highly active electrode materials and electrolytes. The hollow structure formed between the core and the shell mitigates the volume expansion and contraction during cycling. The MnO@C@HCS anode exhibits high specific capacities (1027 mAh g–1 at 0.20 A g–1) and high rate performance (353 mAh g–1 at 10.00 A g–1) under room temperature. Furthermore, the MnO@C@HCS anode maintains a satisfactory discharge capacity under low temperatures (461 mAh g–1 at 0.05 A g–1 under −10 °C, 220 mAh g–1 at 0.10 A g–1 under −20 °C, respectively). The contribution of pseudocapacitance to the capacity decreases as the test temperature drops. Our strategy provides a design concept for the high-performance anode for low-temperature lithium storage.

To solve the toxicity issues related to lead-based halide perovskite solar cells, the lead-free double halide perovskite Cs2AgBiBr6 is proposed. However, reduced rate of charge transfer in double perovskites affects optoelectronic performance. We designed a series of pyridine-based small molecules with four different arms attached to the pyridine core as hole-selective materials by using interface engineering. We quantified how arm modulation affects the structure–property–device performance relationship. Electrical, structural, and spectroscopic investigations show that the N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine arm’s robust association with the pyridine core results in an efficient hole extraction for PyDAnCBZ due to higher spin density close to the pyridine core. The solar cells fabricated using Cs2AgBiBr6 as a light harvester and PyDAnCBZ as the hole selective layer measured an unprecedented 2.9% power conversion efficiency. Our computed road map suggests achieving ∼5% efficiency through fine-tuning of Cs2AgBiBr6. Our findings reveal the principles for designing small molecules for electro-optical applications as well as a synergistic route to develop inorganic lead-free perovskite materials for solar applications.

Sodium-ion batteries (SIBs) with high energy density, improved safety, and low cost are exciting candidates for next-generation energy storage and electrical vehicles. Cathode materials are the core component for SIBs. Recently, an experimental study reported a promising Na2FeS2 cathode with a specific structure consisting of edge-shared and chained FeS4 tetrahedra as the host structure and a high capacity of 320 mA h g–1 for sodium storage. However, the underlying reaction mechanisms and Na migration pathways have not been fully understood. In this study, density functional theory (DFT) and DFT + U calculations are performed to study the structural stability, phase stability, electronic properties (spin polarization density of states), average voltage using total energy based on fully charged and discharged states, and Na-ion transport and diffusion channel using ab initio molecular dynamic simulations of the NaXFeS2 (X = 2, 1.5, and 1) cathode materials. It is revealed that Na2FeS2 is unstable at 0 K and possesses a theoretical capacity of 323 mA h g–1 with a low diffusion barrier of 0.40 eV in NaxFeS2 series. Moreover, some transition metals are substituted at Fe sites to evaluate the structural effect of Na2FeS2, in which Na2MnS2 exhibits excellent structural stability, low hull energy, and high theoretical capacity of 325 mA h g–1, which could be appealing for researchers in the future.

Transition-metal phosphides (TMPs) are regarded as ideal HER electrocatalysts owing to their rich reserve, flexible composition, competitive activity, and excellent stability. Yet, TMPs still suffer from poor electrical conductivity and agglomeration under a high synthetic temperature of the solid-state reaction. Benefiting from the monodisperse metal sites with organic matrix surrounded, metal–organic frameworks (MOFs) have great potential as a feedstock to produce MOF-derived TMPs with tailored microenvironments of the active sites. Herein, a series of nanostructured Ni2P@SNC were facilely fabricated by a one-step phosphorization of a well-designed Ni-MOF with red phosphorus as a “P” source. The optimal Ni2P@SNC-800 displayed an overpotential of 80 mV in 1.0 M KOH and 97 mV in 0.5 M H2SO4 at 10 mA cm–2, demonstrating excellent HER performance as well as long durability in both electrolytes. The comprehensive performance of Ni2P@SNC-800 surpasses that of other heteroatom-free MOF-derived Ni2P significantly. This work provides a facile approach for the design and synthesis of MOF-derived TMPs coated with a heteroatom-doped carbon matrix as an electrocatalyst with high activity.

High interfacial compatibility between electrolytes and electrodes is of great importance for the stable operation of all-solid-state batteries (ASSBs). Here, we report that the introduction of LiBH4 into Li2B12H12-5Li2B10H10 improves the electrochemical window to ∼3.0 V and Li-ion conductivity to 1.0 × 10–4 S cm–1 at room temperature (RT). Moreover, the Li2B12H12-5Li2B10H10-6LiBH4 electrolyte exhibits good compatibility with a metallic Li anode and TiS2 cathode, allowing the stable operation of the all-solid-state In1.3Li0.3||TiS2 cell for 120 cycles at 0.1 C and RT, with 117.8 mAh g–1 of capacity and ∼100% of the coulombic efficiency. This work illustrates that a hydroborate electrolyte with high ionic conductivity and large electrochemical stability can enable the development of an ASSB with a voltage up to 2.7 V.

Harvesting the vibration energy commonly found in engines, air compressors, and other machines is of great significance for energy recovery and reutilization. However, due to the small vibration amplitude and non-single vibration directions, the conventional vibration energy harvesters based on triboelectric nanogenerators (TENGs) have a low efficiency. In this work, we proposed a TENG with a rotational freestanding mode (RFM-TENG), which can effectively harvest the mechanical vibration energy with a small amplitude, high frequency, and multiple directions. The working principle and performance characteristics of each TENG unit were demonstrated through theoretical analysis and electrical simulations. To further improve the harvest efficiency, we prepared a room-temperature vulcanized silicone rubber (RTV) film doped with high dielectric constant halloysite nanotubes powder as the triboelectric layer, which increased the open-circuit voltage by 100% and the short-circuit current by 85% at an optimal doping ratio of 7 wt %. When the RFM-TENG was installed on an air compressor, it generated an open-circuit voltage of about 60 V and a maximum output power of 45 μW and allowed 30 commercial LEDs to light up simultaneously. RFM-TENG has the advantages of strong nonlinearity, high sensitivity, and multi-directional response and has potential applications in the field of smart factory and digital twin.

The lower polarization intensity and charge separation efficiency are the main factors hindering the wide application of piezoelectric photoelectrocatalytic (piezo-PEC) water splitting. Here, we first introduced lattice fluorine into the bulk Bi2WO6 by fluorination treatment to improve the piezoelectric polarization intensity of Bi2WO6. In addition, the adsorbed fluorine produced by the fluorination treatment can promote the effective separation of the surface photogenerated charges, thus achieving the effective transfer and separation of the bulk and surface photogenerated charges, further realizing the efficient piezo-PEC water splitting performance. The photocurrent of Bi2WO6 with lattice fluorine and adsorbed fluorine reaches 0.298 mA/cm2 under ultrasonication by proper fluorination, which is almost 3.3 times that of pure Bi2WO6 under the same conditions. Detailed experimental data shows that the presence of lattice fluorine and adsorbed fluorine can not only effectively improve the bulk piezoelectric polarization of the photoelectric electrode but also effectively optimize the surface photoinduced charge separation. This study provides a promising approach to improve the performance of piezo-PEC by tuning the polarization intensity and surface state of the piezoelectric material.

The electrochemical reduction of carbon dioxide to ethanol is a promising way to make CO2 electrolysis economically feasible. In this work, a bimetallic catalyst of silver and copper is synthesized, and the effect of its copper content on the formation of ethanol is analyzed. By decreasing the near-surface copper content from 99 to 45% (measured by XPS) at a current density of −20 mA cm–2, the Faradaic efficiency of ethanol could be enhanced from 5 to 23%. Moreover, we show that with excess of CO, due to a lower copper and a higher silver near-surface content, the formation of ethanol is favored over ethylene.

The development of a solid electrolyte that can impede dendrite growth while still maintaining an appropriate level of conductivity is essential for improving performance of solid-state Li-ion battery. In this paper, we report the synthesis of single Li-ion conducting hairy nanoparticle (NP) materials that improved the cycling stability of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-doped poly(ethylene oxide) (PEO) solid electrolyte without significant reduction in conductivity. To unveil mechanisms leading to improved cycling stability, several characterization techniques including broadband dielectric spectroscopy, differential scanning calorimetry, small angle X-ray scattering, transmission electron microscopy, and shear rheology were used to study properties of polymer composites (PC) with added hairy NPs. It was found that hairy NPs influenced the Li/electrolyte interface and improved mechanical properties of bulk composites, all of which contributed to homogenous Li plating and stripping. The improved performance has been found in composites with concentrations of 4.8 and 9.1 weight % of added hairy NPs, which enabled Li cycling stability at 0.2 mA cm–2 critical current density (>300 h) that was otherwise not possible in either PEO-LiTFSI alone or PEO-LiTFSI composites containing a polymer identical to that attached to hairy NPs. Based on the discovered ability of hairy NP to influence bulk and interfacial properties of solid electrolyte, their use as additives is expected to be equally effective in reducing dendrite formation in other electrolytes relevant for the design of solid-state battery.

The reaction of 3,5-diamino-4,4′-bis(1H-pyrazole) (3,5-H2L) with copper(II) and nickel(II) acetates under solvothermal conditions led to the four mixed-metal metal–organic frameworks (MIXMOFs) [CuxNi1–x(3,5-L)] (CuxNi1–x, x = 0.05, 0.1, 0.2, 0.5), which were thoroughly characterized in the solid state. The textural analysis unveiled their macroporous nature, with BET specific surface areas falling in the 140–240 m2/g range. Despite the low specific surface areas, their CO2 adsorption capacity at ambient temperature and pressure (highest: Cu0.05Ni0.95 and Cu0.2Ni0.8; 5.6 wt % CO2) and isosteric heat of adsorption (highest: Cu0.2Ni0.8; Qst = 26.2 kJ/mol) are reasonably high. All of the MIXMOFs were tested as heterogeneous catalysts in carbon dioxide electrochemical reduction (CO2RR) in acetonitrile solution at variable potential. The best results were obtained at E = −1.5 V vs Ag/AgCl/KClsat: besides H2 from the hydrogen evolution (HER) side reaction, CO and CH4 were the main reduction products observed under the applied conditions. Cu0.05Ni0.95 showed the best performance with an overall [CO + CH4] conversion of ∼200 ppm and a Faradaic efficiency of ∼52%. CO2RR product selectivity seems to be correlated to the most abundant metal ion in the catalyst: while the Ni-richest phase Cu0.05Ni0.95 mainly produces CO, Cu0.5Ni0.5 mostly generates CH4. The preferential CO2 adsorption sites determined through GCMC simulations are close to the metal centers. For low copper loading, a prevalent end-on interaction of the type O═C═O···NiII is observed, but the progressive increase of the copper content in the MIXMOF equals the metal–gas distances with simultaneous MII···O═C═O···MII activation by two nearby metal ions and a bridging CO2 coordination mode. The analysis of the spent catalyst revealed partial formation of metal nanoparticles under the applied strongly reducing conditions.

In this study, we present a pyrolytically derived iron-based nonprecious metal catalyst (NPMC), Fe3C embedded in heteroatom (S,N)-codoped carbon matrix, and explored it as a potential NPMC for oxygen reduction in alkaline media. The as-prepared catalysts are well characterized for their structure, crystallite size, morphology, different bonding states of the dopants, and defect levels in the carbon matrix. The optimization is performed for ideal reaction temperature and dopant amounts in Fe3C@C nanostructures. From the electrochemical study, it is found that among the different variants, the sample prepared at a temperature of 800 °C with 20 wt % dopant, i.e., Fe3C@C-SN/25-800, shows a more positive onset potential (Eonset) of 0.844 V (vs reversible hydrogen electrode (RHE)) and a low half-wave potential (E1/2) value of 0.670 V. It also shows good long-term oxygen reduction reaction (ORR) stability and methanol tolerance in a 0.1 M KOH aqueous electrolyte. The measurement of intrinsic parameters, double-layer capacitance (Cdl), and charge transfer resistance (RCT) values validate the current–voltage profile of the samples. The major active sites are identified as Fe–Nx and Nx–C in the nanostructures. Fe3C@C-SN/25-800 also exhibits considerable oxygen evolution reaction (OER) activity among its variants and requires a potential difference (ΔE = E1/2(ORR) – EJ=10 mA cm–2 (OER)) of 0.980 V for overall oxygen electrochemistry. The best electrocatalytic activity can be attributed to the combination of several factors, namely, chosen reaction temperature, dopant concentration, better graphitization, and the presence of a high amount of heteroatoms suitably aligned in the carbon matrix (pyridinic-N, thiophenic-S, etc.) that synergistically enhance the overall performance.

As photocatalysts, holey carbon nitride nanosheets (HCNS) have attracted considerable attention. Although many advances have been made in the preparation of HCNS, challenges remain in the low-cost preparation of HCNS. In this work, a one-step strategy for preparing HCNS was explored. First, melamine was placed in a small ceramic boat, which was in a large ceramic boat. Next, the gap between two ceramic boats was filled with copper oxide. Finally, the two boats were wrapped in tin foil and heat-treated. During the thermal polymerization of melamine, released ammonia is decomposed by copper oxide, and the equilibrium shifts to the right, further accelerating the release of ammonia and achieving the preparation of HCNS. Meanwhile, partial copper oxide is reduced to copper by ammonia. After a simple heat treatment, the copper in the mixture can be converted back to copper oxide and realize recycling. The obtained HCNS displays high looseness and enhanced photogenerated charge carrier separation efficiency. Our results indicate that the filled copper oxide can effectively decompose ammonia, affect the thermal condensation atmosphere, and significantly improve the photocatalytic activity of carbon nitride. This work provides a new idea for further fundamental and applied research of HCNS.