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The sustainable and scalable production of hydrogen through hydrogen evolution reaction (HER) and oxygen through oxygen evolution reaction (OER) in water splitting demands efficient and robust electrocatalysts. Currently, state‐of‐the‐art electrocatalysts of Pt and IrO2/RuO2 exhibit the benchmark catalytic activity toward HER and OER, respectively. However, expanding their practical application is hindered by their exorbitant price and scarcity. Therefore, the development of alternative effective electrocatalysts for water splitting is crucial. In the last few decades, substantial effort has been devoted to the development of alternative HER/OER and water splitting catalysts based on various transition metals (including Fe, Co, Ni, Mo, and atomic Pt) which show promising catalytic activities and durability. In this review, after a brief introduction and basic mechanism of HER/OER, the authors systematically discuss the recent progress in design, synthesis, and application of single atom and cluster‐based HER/OER and water splitting catalysts. Moreover, the crucial factors that can tune the activity of catalysts toward HER/OER and water splitting such as morphology, crystal defects, hybridization of metals with nonmetals, heteroatom doping, alloying, and formation of metals inside graphitic layered materials are discussed. Finally, the existing challenges and future perspectives for improving the performance of electrocatalysts for water splitting are addressed.

Noble nanoparticle(NP)-sized electrocatalysts have been exploited for diverse electrochemical reactions, in particular, for eco-friendly hydrogen economy such as water splitting. Recently, minimal amounts of single atoms (SAs) are exploited to...

The most efficient electrocatalyst for the hydrogen evolution reaction (HER) is a Pt‐based catalyst, but its high cost and nonperfect efficiency hinder wide‐ranging industrial/technological applications. Here, an electrocatalyst of both ruthenium (Ru) single atoms (SAs) and N‐doped‐graphitic(GN)‐shell‐covered nitrided‐Ru nanoparticles (NPs) (having a Ru‐Nx shell) embedded on melamine‐derived GN matrix {1: [Ru(SA)+Ru(NP)@RuNx@GN]/GN}, which exhibits superior HER activity in both acidic and basic media, is presented. In 0.5 m H2SO4/1 m KOH solutions, 1 shows diminutive “negative overpotentials” (−η = |η| = 10/7 mV at 10 mA cm−2, lowest ever) and high exchange current densities (4.70/1.96 mA cm−2). The remarkable HER performance is attributed to the near‐zero free energies for hydrogen adsorption/desorption on Ru(SAs) and the increased conductivity of melamine‐derived GN sheets by the presence of nitrided‐Ru(NPs). The nitridation process forming nitrided‐Ru(NPs), which are imperfectly covered by a GN shell, allows superb long‐term operation durability. The catalyst splits water into molecular oxygen and hydrogen at 1.50/1.40 V (in 0.1 m HClO4/1 m KOH), demonstrating its potential as a ready‐to‐use, highly effective energy device for industrial applications.

For efficient water splitting, it is essential to develop inexpensive and super-efficient electrocatalysts for the oxygen evolution reaction (OER). Herein, we report a phosphate-based electrocatalyst [Fe3Co(PO4)4@reduced-graphene-oxide(rGO)] showing outstanding OER performance (much higher than state-of-the-art Ir/C catalysts), the design of which was aided by first-principles calculations. This electrocatalyst displays low overpotential (237 mV at high current density 100 mA cm−2 in 1 M KOH), high turnover frequency (TOF: 0.54 s−1), high Faradaic efficiency (98%), and long-term durability. Its remarkable performance is ascribed to the optimal free energy for OER at Fe sites and efficient mass/charge transfer. When a Fe3Co(PO4)4@rGO anodic electrode is integrated with a Pt/C cathodic electrode, the electrolyzer requires only 1.45 V to achieve 10 mA cm−2 for whole water splitting in 1 M KOH (1.39 V in 6 M KOH), which is much smaller than commercial Ir-C//Pt-C electrocatalysts. This cost-effective powerful oxygen production material with carbon-supporting substrates offers great promise for water splitting.

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The band gap is an important parameter that determines light-harvesting capability of perovskite materials. It governs the performance of various optoelectronic devices such as solar cells, light-emitting diodes, and photodetectors. For perovskites of a formula ABX3 having a non-zero band gap, we study nonlinear mappings between the band gap and properties of constituent elements (e.g., electronegativities, electron affinities, etc) using alternating conditional expectations (ACE)a machine learning technique suitable for small data sets. We also compare ACE with other machine learning methods: decision trees, kernel ridge regression, extremely randomized trees, AdaBoost, and gradient boosting. The best performance is achieved by kernel ridge regression and extremely randomized trees. However, ACE has an advantage that it presents its results in a graphic form, helping in interpretation. The models are trained with the data obtained from density functional theory calculations. Different statistical approaches for feature selection are applied and compared: Pearson correlation, Spearman’s rank correlation, maximal information coefficient, distance correlation, and ACE. A classification task of separating metallic perovskites from nonmetallic ones is solved using support-vector machines with the radial basis function kernel.

As the race towards higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport towards higher power efficiency has been urgently demanded. Here, we unravel a hidden role of A-site cation of PSCs in carrier transport which has been largely neglected, i.e., tuning the Frhlich electron-phonon (e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Frhlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ion. The coordination to I − alleviates electron-phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses lower electron-phonon coupling by several promising organic cations including hydroxyl-ammonium cation (NH3OH + ) and possibly Li + solvating methylamine (Li + ···NH2CH3) than methyl-ammonium cation. A new perspective on the role of A-site cation could help in improving power efficiency and accelerating the application of PSCs.Solar energy is a highly efficient and eco-friendly source for future energy harvesting. In particular, inorganic/organic PSCs of ABX3 (A = Cs+, CH3NH3+, etc.; B = Pb2+; X = Cl-, Br-or I-) show extraordinary solar cell efficiencies exceeding 22 % [1] with unusual characteristics, [2][3][4] which attracts tremendous attention as most promising large-scale solar energy conversion materials.[5] One key origin of high efficiency arises from high carrier mobility (µ) 10 -8] even in the presence of defects.[9] While the carrier transport exhibits remarkable features in experiments, there is still a gap of understanding. One aspect of this difficulty stems from significant electron-phonon (e-ph) interactions, [2,8,[10][11][12][13] which complicates band pictures. Another aspect is due to the A-site cation that rotates [14] or even diffuses across the material, causing I-V hysteresis.[15] Although recent discovery of various types of cations has greatly advanced the efficiency of PSCs, [16] it also has brought about more ambiguity on the role of cations.In general, electrons in polar crystal experience the deformation potential in addition to Bloch potential due to large polarity of ionic bonding. The charge carriers are then described by polaron quasiparticles originating from coupling between electrons and phonons. For PSCs, there has been a critical debate on the source of e-ph coupling, acoustic vs. longitudinal. From the temperature dependence of µ ∼ T −3/2 around room temperature, acoustic phonons have been considered as the main source of e-ph coupling. [11,17] In this study, we unveil a hidden role of A-site cation in polaron picture of PSCs by accounting for the e-ph interactions regarding both A-site cation and LO phonon scattering at the first principles level. [18,19] We used the Frohlich polaron model [23] which is suitable for large bandwidth (W ) limit W ω ph by considering Frohlich vertex. [24] We cast light on the role o...