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Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given. [ABSTRACT FROM AUTHOR]

Thermoelectrics are a class of materials that provide interconversion between heat and electricity, with desirable traits such as low thermal conductivity and low electrical resistivity. AgSbTe 2 has emerged as one of the leading materials in recent years due to its ultra-low thermal conductivity. However, one major hindrance in undoped AgSbTe 2 is its high electrical resistivity and low Seebeck coefficient due to the presence of Ag 2 Te nanoprecipitates. In this work, we leverage on the combination of an off-stoichiometric composition and a non-equilibrium process to simultaneously enhance the properties of AgSbTe 2 and its thermoelectric device performance. Microscopically, the Ag 2 Te-deficient starting composition combined with a non-equilibrium thermal process suppresses the Ag 2 Te nanoprecipitates in the material. In addition, it is evident from the density functional theory (DFT) electronic structure that Ag 2 Te deficiency results in a smaller lattice and higher density-of-states near the Fermi level, which simultaneously lower the electrical resistivity and increase the Seebeck coefficient. As a result, zT as high as 1.7 was achieved at 573 K. Additionally, when combined with a high room temperature zT of 0.75, a power conversion efficiency of 7.3% was achieved at a Δ T of 290 K. Crucially, the strategy in this work can inspire application in other ABX 2 material systems to achieve improved thermoelectric performances.

Integrated n-Si/BiVO 4 is one of the most promising candidates for unbiased photoelectrochemical water splitting. However, a direct connection between n-Si and BiVO 4 will not attain overall water splitting due to the small band offset as well as the interfacial defects at the n-Si/BiVO 4 interface that severely impede carrier separation and transport, limiting the photovoltage generation. This paper describes the design and fabrication of an integrated n-Si/BiVO 4 device with enhanced photovoltage extracted from the interfacial bi-layer for unassisted water splitting. An Al 2 O 3 /indium tin oxide (ITO) interfacial bi-layer was inserted at the n-Si/BiVO 4 interface, which promotes the interfacial carrier transport by enlarging the band offset while healing interfacial defects. When coupled to a separate cathode for hydrogen evolution, spontaneous water splitting could be realized with this n-Si/Al 2 O 3 /ITO/BiVO 4 tandem anode, with an average solar-to-hydrogen (STH) efficiency of 0.62% for over 1000 hours., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)