The spontaneous α-to-δ phase transition of the formamidinium-based (FA) lead halide perovskite hinders its large scale application in solar cells. Though this phase transition can be inhibited by alloying with methylammonium-based (MA) perovskite, the underlying mechanism is largely unexplored. In this Communication, we grow high-quality mixed cations and halides perovskite single crystals (FAPbI)(MAPbBr) to understand the principles for maintaining pure perovskite phase, which is essential to device optimization. We demonstrate that the best composition for a perfect α-phase perovskite without segregation is x = 0.1-0.15, and such a mixed perovskite exhibits carrier lifetime as long as 11.0 μs, which is over 20 times of that of FAPbI single crystal. Powder XRD, single crystal XRD and FT-IR results reveal that the incorporation of MA is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation disorder. Moreover, we find that Br incorporation can effectively control the perovskite crystallization kinetics and reduce defect density to acquire high-quality single crystals with significant inhibition of δ-phase. These findings benefit the understanding of α-phase stabilization behavior, and have led to fabrication of perovskite solar cells with highest efficiency of 19.9% via solvent management.

Improving electrochemical activity of graphene is crucial for its various applications, which requires delicate control over its geometric and electronic structures. We demonstrate that precise control of the density of vacancy defects, introduced by Ar(+) irradiation, can improve and finely tune the heterogeneous electron transfer (HET) rate of graphene. For reliable comparisons, we made patterns with different defect densities on a same single layer graphene sheet, which allows us to correlate defect density (via Raman spectroscopy) with HET rate (via scanning electrochemical microscopy) of graphene quantitatively, under exactly the same experimental conditions. By balancing the defect induced increase of density of states (DOS) and decrease of conductivity, the optimal HET rate is attained at a moderate defect density, which is in a critical state; that is, the whole graphene sheet becomes electronically activated and, meanwhile, maintains structural integrity. The improved electrochemical activity can be understood by a high DOS near the Fermi level of defective graphene, as revealed by ab initio simulation, which enlarges the overlap between the electronic states of graphene and the redox couple. The results are valuable to promote the performance of graphene-based electrochemical devices. Furthermore, our findings may serve as a guide to tailor the structure and properties of graphene and other ultrathin two-dimensional materials through defect density engineering.

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Nanoelectrochemical approaches were used to investigate adsorption/desorption of hydrogen on Pt electrodes. These processes, which have been extensively studied over the last century, remain of current interest because of their applications in energy storage systems. The effective surface area of a nanoelectrode was found to be much larger than its geometric surface area due to surface diffusion of adsorbed redox species at the Pt/glass interface. An additional peak of hydrogen desorption was observed and attributed to the spillover of hydrogen from the Pt surface into glass. The results were compared to those obtained for underpotential deposition of copper on Pt nanoelectrodes.

Herein, the traditional views that contrast the important areas of electrocatalysis and molecular electrochemistry are challenged. By extending Laviron′s seminal concept, we show that these two domains only represent idealized limits of a much broader continuum. More importantly, we show that electrochemical systems that apparently behave experimentally as if under diffusion control (i.e. systems that obey the founding molecular electrochemistry paradigm) may be controlled by electrocatalytic steps, that is, in which the activation of electroactive substrates exclusively occurs through adsorbed intermediates. This analysis is supported through quantitative experimental and theoretical investigations on the reduction of benzyl chloride at silver electrodes. At silver cathodes, the reduction wave of benzyl chloride as monitored at the usual scan rates is dramatically shifted to more positive potentials by about 0.5 V versus that at inert (e.g. glassy carbon) electrodes. This approach, which is based on the use of fast‐scan cyclic voltammetry and simulations (KISSA‐1D), combined with our previous results from surface‐enhanced Raman spectroscopy (SERS) and density functional theory (DFT) analysis, allow us to fully unravel the mechanistic origin of this dramatic effect and quantitatively validate this mechanism, which has eluded many research groups until now. In practice, this example provides a missing link between the traditional areas of electrocatalysis and molecular electrochemistry. Furthermore, it bridges the chemical areas of organometallic/inorganic catalysis and electrochemical activation by showing that the inner‐sphere concept, as developed by Taube and Myers for inorganic reactions, applies perfectly to electrochemical reactions of molecular substrates.

Single walled carbon nanotubes (SWCNT) and room temperature ionic liquid (RTIL) were used to make a gel microelectrode for studies of the oxidation of nitric oxide (NO). The Faraday response of the gel microelectrode was contributed from two components: an outside-surface microdisk and a thin-layer cell formed by inner porous electrode materials, and enhanced by the thin-layer effect. An EC mechanism, electrochemical NO oxidation followed by a chemical oxidation, was proposed. The gel microelectrode with a Nafion coating eliminated interferences from nitrite and some biomolecules, improved stability, and had a linear response range from 100 nM to 100 mM.

Dopamine is an important neurotransmitter in mammalian central and peripheral nervous systems and is also a medicament to cure some neuropsychosis. In this work, ion transfer (IT), facilitated ion transfer (FIT) of protonated dopamine, and electron transfer (ET) between dopamine and ferrocene are investigated at the water/1,2-dichloroethane (W/DCE) interface. The IT and FIT reactions of protonated dopamine can be observed simultaneously within the same potential window. The experimental results demonstrate that dibenzo-18-crown-6, dibenzo-24-crown-8, and benzo-15-crown-5 work well with the protonated dopamine. The amperometric detection of dopamine based on either the IT or the FIT of protonated dopamine can get rid of the interference of ascorbic acid, and the lowest concentration that can be determined is approximately 0.05 microM by differential pulse voltammetry. For the ET reaction, its kinetics can be evaluated by scanning electrochemical microscopy, and the results show that the relationship between rate constants and driving force at the unmodified W/DCE interface obeys the Butler-Volmer equation in a rather wide potential region. When the W/DCE interface is modified by egg lecithin, the ET rate constants decrease with increasing concentration of egg lecithin, which indicates that egg lecithin hinders the ET reaction. When the driving force is increased to a certain degree, the linear relationship between ET rate constants and the driving force is distorted. These results will be helpful to understand both the pharmacodynamics and the neural signal transmission mechanism of dopamine at biological membranes and also provide a novel way to detect dopamine.

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