In halide perovskite solar cells the formation of secondary-phase excess lead iodide (PbI 2 ) has some positive effects on power conversion efficiency (PCE) but can be detrimental to device stability and lead to large hysteresis effects in voltage sweeps. We converted PbI 2 into an inactive (PbI 2 ) 2 RbCl compound by RbCl doping, which effectively stabilizes the perovskite phase. We obtained a certified PCE of 25.6% for FAPbI 3 (FA, formamidinium) perovskite solar cells on the basis of this strategy. Devices retained 96% of their original PCE values after 1000 hours of shelf storage and 80% after 500 hours of thermal stability testing at 85°C.

The precise control of edge geometry and crystal shape of monolayer MoS 2 is particular of importance for their applications in nanoelectronics and photo-electro catalysts. Here we reveal a crucial role of chemical potential in the determination of equilibrium shape (ES) and edge structure of monolayer MoS 2 by using density-functional theory calculations. Applying Wulff construction rule, our results demonstrate the shape evolution of monolayer MoS 2 flake from the dodecagonal shape, then to the hexagonal shape, to the triangular shape with the variation of chemical potential from the Mo-rich to the S-rich condition, and the edge structure of ES changes correspondingly from mixed zigzag/armchair edges to pure zigzag edges. This finding can be applied to explain extensive experimental observations about the morphology of MoS 2 domains. Meanwhile, the edge magnetism and electronic structures of monolayer MoS 2 domains are found to be dependent on their edge structure, which provides specific guidance for the magnetic modulation of monolayer MoS 2 and designing more effective MoS 2 -based catalysts.

The aim of this study is to provide a better understanding of performance degrading mechanisms occurring when a proton exchange membrane water electrolyzer (PEM-WE) is coupled with renewable energies, where times of operation and idle periods alternate. An accelerated stress test (AST) is proposed, mimicking a fluctuating power supply by operating the electrolyzer cell between high (3 A cm −2 geo) and low current densities (0.1 A cm −2 geo), alternating with idle periods during which no current is supplied and the cell rests at open circuit voltage (OCV). Polarization curves, periodically recorded during the OCV-AST, reveal an initial increase in activity (≈50 mV after 10 cycles) followed by a significant decrease in performance during prolonged OCV cycling due to an increasing high frequency resistance (HFR) (≈1.6-fold after 718 cycles). These performance changes can clearly be related to the OCV periods, since they are not observed in a reference experiment where the OCV period is replaced by a potential hold at 1.3 V. The origin of the phenomena, which are responsible for the initial performance gain as well as the subsequent decay are analyzed via detailed electrochemical and physical characterization of the MEAs, and an operating strategy to prevent performance degradation is proposed.