Flexible perovskite solar cells (f‐PSCs) have attracted great attention because of their unique advantages in lightweight and portable electronics applications. However, their efficiencies are far inferior to those of their rigid counterparts. Herein, a novel histamine diiodate (HADI) is designed based on theoretical study to modify the SnO2/perovskite interface. Systematic experimental results reveal that the HADI serves effectively as a multifunctional agent mainly in three aspects: 1) surface modification to realign the SnO2 conduction band upward to improve interfacial charge extraction; 2) passivating the buried perovskite surface, and 3) bridging between the SnO2 and perovskite layers for effective charge transfer. Consequently, the rigid MA‐free PSCs based on the HADI‐SnO2 electron transport layer (ETL) display not only a high champion power conversion efficiency (PCE) of 24.79% and open‐circuit voltage (VOC) of 1.20 V but also outstanding stability as demonstrated by the PSCs preserving 91% of their initial efficiencies after being exposed to ambient atmosphere for 1200 h without any encapsulation. Furthermore, the solution‐processed HADI‐SnO2 ETL formed at low temperature (100 °C) is utilized in f‐PSCs that achieve a PCE as high as 22.44%, the highest reported PCE for f‐PSCs to date.

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Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/extraction of charge carriers (for example, lithium ions). Common synthesis methods can introduce large cations such as potassium, barium and ammonium ions into the tunnels, but how these cations affect charge storage performance is not fully understood. Here, we report the role of tunnel cations in governing the electrochemical properties of electrode materials by focusing on potassium ions in α-MnO2. We show that the presence of cations inside 2 × 2 tunnels of manganese dioxide increases the electronic conductivity, and improves lithium ion diffusivity. In addition, transmission electron microscopy analysis indicates that the tunnels remain intact whether cations are present in the tunnels or not. Our systematic study shows that cation addition to α-MnO2 has a strong beneficial effect on the electrochemical performance of this material.

α-MnO2 is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li(+). The overall electrochemical performance of α-MnO2 is determined by the tunneled structure stability during its interaction with Li(+), the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal-orthorhombic-tetragonal symmetric transition during lithiation of K(+)-stabilized α-MnO2 is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 × 1 and 2 × 2 tunnels exist along c ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the ⟨100⟩ projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of α-MnO2's tetragonal unit cell along a and b lattice directions, which results in a tetragonal-orthorhombic-tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 × 2 tunnels are the preferred sites for Li(+) occupancy. The sequential Li(+) filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/α-MnO2 batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO2 as an intercalated electrode.

Amid the growing interest in rechargeable aqueous zinc-based batteries, tunnel-structured α-MnO2 has emerged as a promising cathode material owing to its low cost, high capacity and high safety.However, the precise charge storage mechanism, possibly involving proton and/or Zn ion insertion, has not been fully characterized especially at the atomistic level. Here, we report new insights through a combined investigation of atomic-scale electron microscopy, electrochemical analysis and ab initio simulations. We find that reversible Zn 2+ insertion into α-MnO2 framework is unlikely in the aqueous system, and that the charge storage process is dominated by H + insertion into the tunnel structures which are maintained upon discharging to HMnO2. Furthermore, we identify the local lattice positions for the hydroxyl (OH) groups in HxMnO2 as a function of H content. We reveal the consequent anisotropic structural change proceeding from the particle surface into the bulk, and thus account for the structural failure and capacity decay of the electrode upon cycling.Future work should consider optimizing proton insertion kinetics with enhanced host stability.

Passivating undercoordinated ions is an effective way to reduce the defect densities at the surface and grain boundaries (GBs) of perovskite materials for enhanced photovoltaic performance and stability of perovskite solar cells (PSCs). Here, (BBF) complex is chosen as a multifunctional additive, which contains both C7H9N and BF3 groups working as Lewis base and Lewis acid, respectively, can bond with Pb2+/I− and FA+ on the surface and in the GBs in the perovskite film, affording passivation of both cation and anion defects. The synergistic effect of the C7H9N and BF3 complex slows the crystallization during the perovskite film deposition to improve the crystalline quality, which reduces the trap density and the recombination in the perovskite film to suppress nonradiative recombination loss and minimizes moisture permeation to improve the stability of the perovskite material. Meanwhile, such an additive improves the energy-level alignment between the valence band of the perovskite and the highest occupied molecular orbital of the hole-transporting material, Spiro-OMeTAD. Consequently, our work achieves power conversion efficiency of 23.24%, accompanied by enhanced stability under ambient conditions and light illumination and opens a new avenue for improving the performance of PSCs through the use of a multifunctional complex.

Pumping macrocycles onto surfaces Numerous chemical processes, ranging from water purification to catalysis, involve sorption of small molecules onto surfaces. Typically, spontaneous attractive interactions favor the binding event. Feng et al . report a mechanisorption process that requires redox manipulations to pump macrocycles from bulk solution onto axles immobilized on a metal-organic framework. The resulting rotaxanes store energy through nonequilibrium charge concentration in their mechanical bonds. Ultimately, the technique could also prove useful for actively partitioning compounds with particular functionality between surface and bulk environments. —JSY

To treat impairments in hard tissues or overcome pathological calcification in soft tissues, a detailed understanding of mineralization pathways of calcium phosphate materials is needed. Here, we report a detailed mechanistic study of hydroxyapatite (HA) mineralization pathways in an artificial saliva solution via in situ liquid cell transmission electron microscopy (TEM). It is found that the mineralization of HA starts by forming ion-rich and ion-poor solutions in the saliva solution, followed by coexistence of the classical and nonclassical nucleation processes. For the nonclassical path, amorphous calcium phosphate (ACP) functions as the substrate for HA nucleation on the ACP surface, while the classical path features direct HA nucleation from the solution. The growth of HA crystals on the surface of ACP is accompanied by the ACP dissolution process. The discoveries reported in this work are important to understand the physiological and pathological formation of HA minerals, as well as to engineer the biomineralization process for bone healing and hard tissue repairs.