Therefore, an ever-growing demand for sustainable and environmentally-friendly energy technologies are prompting scientists to explore alternative approaches to reduce the carbon footprint. [2] Among the different sustainable energy sources, solar energy is an inexhaustible source of energy (173000 Terawatt (TW), almost 10 000 times of 17.7 TW, global energy consumption in 2020) and the largest currently available on earth with a ubiquitous distribution worldwide. [3] However, its decentralized and intermittent nature poses a great challenge to our energy needs. [4] Artificial photosynthesis, imitating a natural photosynthesis, where the harvesting of sunlight directly into chemical bonds (hydrogen, (H 2 ) fuel) is a highly promising approach to address the serious issue like air pollution, greenhouse gas emission, etc. Artificial photosynthesis by means of photoelectrochemical (PEC) water splitting has been studied extensively to split water using sunlight and semiconductor into oxygen (O 2 ) and H 2 , where the generated H 2 can be stored and transported to other energy conversion systems. [5] In recent years, PEC water splitting turned out to be an elegant and ecological way to generate clean H 2 fuel. Despite having mature and commercialized technologies, photovoltaic-electrochemical (PV-EC) water splitting systems have the most significant complications in PV designs. [6] In a photocatalytic (PC) water splitting system, on the other hand, both H 2 and O 2 are generated on the same surface of the PC particles, and hence in most cases, backward complex reactions like hydrogen oxidation reaction and oxygen reduction reaction occur quickly, lowering the solar to H 2 (STH) conversion efficiency. [7] In comparison to the PC system, the physical separation of oxidation and reduction species in PEC cells makes them more practical, effective, and safer. Moreover, the PEC cells have an electrode/electrolyte interface that performs simultaneous roles of light-harvesting and electrolysis in a single reactor. Consequently, the large-scale application of PEC cells is the most achievable, even at lower operating temperature as it opens up the opportunity to improve efficiency and reduce costs through the nature of its device structure. In particular, a recent assessment of its practicability has shown that the lifespan (i.e., stability), efficiency, and capital cost of The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO 4 ), having an earthabundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO 4 is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO 4 suffers from rapid recombination of photogenerated charge carriers that have impeded further imp...

Here, a hierarchical nanostructure composed of Ni-doped α-FeOOH (Ni:FeOOH) nanosheets coupled with N-doped graphite foam (NGF) is demonstrated as a three-dimensional (3D) self-supported electrocatalyst for highly efficient and durable water oxidation. A facile, one-step directional growth of catalytically active Ni:FeOOH nanosheets on highly conducting 3D NGF results in a fully integrated, hierarchical, nanostructured electrocatalyst with (i) the high intrinsic activity of Ni:FeOOH, (ii) the outstanding electrical conductivity of NGF, and (iii) a well-defined porous structure with an enhanced active surface area. As a result, the self-supported 3D Ni:FeOOH/NGF electrocatalyst exhibits remarkable electrocatalytic activity for the oxygen evolution reaction (OER) in an alkaline solution with an overpotential of 214 mV at 10 mA/cm2, a high stability for over 60 h, a low Tafel slope of 36.2 mV dec–1, and a capability of delivering a high current density of 300 mA/cm2 at an overpotential of 368 mV. In contrast to photodeposition, electrodeposition, and hydrothermal methods for the formation/integration of (oxy)­hydroxides, this facile solution strategy for designing an attractive and efficient structure with a highly active metal (oxy)­hydroxide and highly conducting NGF provides a pathway to develop other earth-abundant electrocatalysts for a multitude of energy-conversion-device applications.

Transition-metal phosphide (TMP) nanostructures have been extensively studied for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, phase-controlled synthesis of colloidal Ni2P nanocrystals (NCs) or related heterostructures remains challenging and their use as bifunctional electrocatalysts in overall water splitting (OWS) is not systematically studied. Herein, zero-dimensional (0D) colloidal Ni2P NCs are synthesized using a robust solution-phase method and encapsulated in two-dimensional (2D) N- and S-doped graphene (NSG) nanosheets via facile ex situ sonication to form a 0D@2D Ni2P@NSG heterostructure. The interaction between surface functionalities of Ni2P NCs and defective NSG via strong van der Waals force provides a robust sheath to Ni2P NCs when encapsulated in NSG nanosheets, further enhancing the specific surface area and active site exposure. Density functional theory calculations indicate that the dual interaction of N and S dopants with Ni2P benefits the synergistic effect of optimized water and hydrogen free energy adsorption. As a result, Ni2P@NSG electrocatalysts manifest high catalytic activity toward HER and OER, and a two-electrode alkaline electrolyzer assembled by Ni2P@NSG as both an anode and a cathode requires only 1.572 V to reach a current density of 10 mA/cm2.

The performance of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell is known to be severely limited by the nonradiative recombination near the heterojunction interface and within the bulk of the CZTSSe absorber resulting from abundant recombination centers and limited carrier collection efficiency. Herein, nonradiative recombination is simultaneously reduced by incorporating small amounts of Ge and Cd into the CZTSSe absorber. Incorporation of Ge effectively increases the p‐type doping, thus successfully improving the bulk conductance and reducing the recombination in the CZTSSe bulk via enhanced quasi‐Fermi level splitting, while the incorporation of Cd greatly reduces defects near the junction region, enabling larger depletion region width and better carrier collection efficiency. The combined effects of Cd and Ge incorporation give rise to systematic improvement in open‐circuit voltage (VOC), short‐circuit current density (JSC), and fill factor (FF), enabling a high conversion efficiency of 11.6%. This study highlights the multiple cation incorporation strategy for systematically manipulating the opto‐electronic properties of kesterite materials, which may also be applicable to other semiconductors.

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