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Supercapacitors (SCs) are highly promising electrochemical energy conversion and storage devices. SCs display an outstanding power performance, excellent reversibility, long-term stability, simple operation, and high feasibility for integration into electronic devices, including consumer electronics, memory backup systems, and industrial power and energy management systems. The electrode materials determine the cell capacitance, operating voltage, power density, energy density, and time constant of SCs. Transition metal-based electrode materials (TMEMs) are among the most promising electrodes for SCs, due to their outstanding energy density, specific capacitance, and quick charging/discharging rates, in addition to their ease of preparation in a high yield from low-cost and earth-abundant resources. Binary transition metal sulfides (BTMSs) possess various advantages relative to other TMEMs, including higher storage capacity, higher electrical conductivity, excellent redox properties, better specific capacitance, quicker electron/ion diffusion, and superior reversibility with long cycle life. Herein, the inventory and the recent progress in the rational design of BTMS electrodes for SCs are deliberated, spaning from the preparation methods to the operative conditions, performance, and mechanism. To help assist in the further development of BTMS electrodes for efficient and durable SCs, current underlying challenges and possible solutions are identified and addressed, with emphasis on device performance vs BTMS type and relative merits.

Hybrid organic−inorganic perovskite solar cells (HOIPs), especially CH 3 NH 3 PbI 3 (MAPbI 3 ), have received tremendous attention due to their excellent power conversion efficiency (25.2%). However, two fundamental hurdles, long-term stability and lead (Pb) toxicity, prevent HOIPs from practical applications in the solar industry. To overcome these issues, compositional engineering has been used to modify cations at A-and B-sites and anions at the X-site in the general form ABX 3 . In this work, we used the density functional theory (DFT) to incorporate Rb, Cs, and FA at the A-site to minimize the volatile nature of MA, while the highly stable Ca 2+ and Sr 2+ were mixed with the less stable Ge 2+ and Sn 2+ at the B-site to obtain a Pb-free perovskite. To further enhance the stability, we mixed the X-site anions (I/Br). Through this approach, we introduced 20 new perovskite species to the lead-free perovskite family and 7 to the leadcontaining perovskite family. The molecular dynamic (MD) simulations, enthalpy formation, and tolerance and octahedral factor study confirm that all of the perovskite alloys we introduced here are as stable as pristine MAPbI 3 . All Pb-free perovskites have suitable and direct band gaps (1.42−1.77 eV) at the Γ-point, which are highly desirable for solar cell applications. Most of our Pb-free perovskites have smaller effective masses and exciton binding energies. Finally, we show that the introduced perovskites have high absorption coefficients (10 5 cm −1 ) and strong absorption efficiencies (above 90%) in a wide spectral range (300−1200 nm), reinforcing their significant potential applications. This study provides a new way of searching for stable lead-free perovskites for sustainable and green energy applications.

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