Tingting Yao

Integrating a dual-silicon photoelectrochemical cell into a redox flow battery for unassisted photocharging

Solar rechargeable flow cells (SRFCs) provide an attractive approach for in situ capture and storage of intermittent solar energy via photoelectrochemical regeneration of discharged redox species for electricity generation. However, overall SFRC performance is restricted by inefficient photoelectrochemical reactions. Here we report an efficient SRFC based on a dual-silicon photoelectrochemical cell and a quinone/bromine redox flow battery for in situ solar energy conversion and storage. Using narrow bandgap silicon for efficient photon collection and fast redox couples for rapid interface charge injection, our device shows an optimal solar-to-chemical conversion efficiency of ∼5.9% and an overall photon–chemical–electricity energy conversion efficiency of ∼3.2%, which, to our knowledge, outperforms previously reported SRFCs. The proposed SRFC can be self-photocharged to 0.8 V and delivers a discharge capacity of 730 mAh l−1. Our work may guide future designs for highly efficient solar rechargeable devices.

Chemical Synthesis, Structural Characterization, Optical Properties, and Photocatalytic Activity of Ultrathin ZnSe Nanorods

Ultrathin ZnSe nanorods in the cubic phase have been synthesized by the reaction of selenium and zinc oleate for 30 min at 240 °C. These nanorods showed an average diameter of 2.4 nm, which is much smaller than the Bohr size of bulk ZnSe. Thus, they exhibited a remarkable quantum size effect in terms of their optical properties. The formation of the ultrathin nanorods could be attributed to the oriented attachment mechanism, which was supported by the structure of the nanorods and the control experiments. The ultrathin nanorods were transferred into an aqueous solution by ligand exchange. The performance of these nanorods as a catalyst was examined, using the photodegradation of methyl orange as a model reaction. It was found that the ultrathin nanorods possessed better photocatalytic activities than conventional ones.

Manipulating the Interfacial Energetics of n-type Silicon Photoanode for Efficient Water Oxidation

The photoanodes with heterojunction behavior could enable the development of solar energy conversion, but their performance largely suffers from the poor charge separation and transport process through the multiple interfacial energy levels involved. The question is how to efficiently manipulate these energy levels. Taking the n-Si Schottky photoanode as a prototype, the undesired donor-like interfacial defects and its adverse effects on charge transfer in n-Si/ITO photoanode are well recognized and diminished through the treatment on electronic energy level. The obtained n-Si/TiOx/ITO Schottky junction exhibits a highly efficient charge transport and a barrier height of 0.95 eV, which is close to the theoretical optimum for n-Si/ITO Schottky contact. Then, the holes extraction can be further facilitated through the variation of surface energy level, with the NiOOH coated ITO layer. This is confirmed by a 115% increase in surface photovoltage of the photoanodes. Eventually, an unprecedentedly low onset potential of 0.9 V (vs RHE) is realized for water oxidation among n-Si photoanodes. For the water oxidation reaction, the n-Si/TiOx/ITO/NiOOH photoanode presents a charge separation efficiency up to 100% and an injection efficiency greater than 90% at a wide voltage range. This work identifies the important role of interfacial energetics played in photoelectrochemical conversion.

Substrate–Electrode Interface Engineering by an Electron-Transport Layer in Hematite Photoanode

The photoelectrochemical water oxidation efficiency of photoanodes is largely limited by interfacial charge-transfer processes. Herein, a metal oxide electron-transport layer (ETL) was introduced at the substrate-electrode interface. Hematite photoanodes prepared on Li(+)- or WO3-modified substrates deliver higher photocurrent. It is inferred that a Li-doped Fe2O3 (Li:Fe2O3) layer with lower flat band potential than the bulk is formed. Li:Fe2O3 and WO3 are proved to function as an expressway for electron extraction. Via introducing ETL, both the charge separation and injection efficiencies are improved. The lifetime of photogenerated electrons is prolonged by 3 times, and the ratio of surface charge transfer and recombination rate is enhanced by 5 times with Li:Fe2O3 and 125 times with WO3 ETL at 1.23 V versus reversible hydrogen electrode. This result indicates the expedited electron extraction from photoanode to the substrate can suppress not only the recombination at the back contact interface but also those at the surface, which results in higher water oxidation efficiency.

Facile synthesis, optical properties and growth mechanism of elongated Mn-doped ZnSe1−xSx nanocrystals

Transition-metal doping in semiconductors is an important way to modulate their intrinsic properties or bring out new functionalities for applications. Here, elongated Mn-doped ZnSe1−xSx nanocrystals are synthesized through a facile one-pot reaction under mild conditions. These ZnSe1−xSx nanocrystals with a diameter of 2–4 nm have a cubic phase structure. EPR and PL spectra confirm the successful doping of Mn2+ into the ZnSe1−xSx nanocrystals, based on the corresponding hyperfine splitting constants and emission bands. The temporal evolution of UV-vis absorption and PL spectra indicates the growth-doping mechanism for the formation of the doped nanocrystals. Surface coating of these nanocrystals with a shell increases the quantum yield up to 35%. And the dual-function role of 1-dodecanethiol is confirmed in the formation of the elongated Mn-doped ZnSe1−xSx nanocrystals.

Design and Fabrication of a Dual-Photoelectrode Fuel Cell towards Cost-Effective Electricity Production from Biomass

A photo fuel cell (PFC) offers an attractive way to simultaneously convert solar and biomass energy into electricity. Photocatalytic biomass oxidation on a semiconductor photoanode combined with dark electrochemical reduction of oxygen molecules on a metal cathode (usually Pt) in separated compartments is the common configuration for a PFC. Herein, we report a membrane-free PFC based on a dual electrode, including a W-doped BiVO4 photoanode and polyterthiophene photocathode for solar-stimulated biomass-to-electricity conversion. Air- and water-soluble biomass derivatives can be directly used as reagents. The optimal device yields an open-circuit voltage (VOC ) of 0.62 V, a short-circuit current density (JSC ) of 775 μA cm-2 , and a maximum power density (Pmax ) of 82 μW cm-2 with glucose as the feedstock under tandem illumination, which outperforms dual-photoelectrode PFCs previously reported. Neither costly separating membranes nor Pt-based catalysts are required in the proposed PFC architecture. Our work may inspire rational device designs for cost-effective electricity generation from renewable resources.

Enhancing the Performance of Amorphous‐Silicon Photoanodes for Photoelectrocatalytic Water Oxidation

Herein, hydrogenated amorphous Si (a-Si:H) covered with a thin layer of CoOx is applied as photoanode for PEC water splitting. The thin layer of CoOx effectively protects a-Si:H from the corrosive electrolyte and quantitative oxidation of water to oxygen was observed. A high applied bias photon-to-current efficiency of 2.34 % was achieved using an intrinsic absorber and an additional p-type layer. This work shows that a-Si:H with a sandwich-like structure, in which each layer has its own functionality, can be applied as an efficient and stable photoanode for PEC water oxidation.

Facile synthesis and optical properties of ultrathin Cu-doped ZnSe nanorods

Successful doping of anisotropic semiconductor nanocrystals with impurities offers an effective pathway to manipulate their physical properties and enhance the application performances. However, such doping into anisotropic nanocrystals is seldom reported because it needs simultaneous controls in the crystal growth for a specific shape and composition engineering for transition-metal doping. Here, ultrathin Cu-doped ZnSe nanorods are synthesized by a growth–doping process. The doped nanorods are characterized by XRD and TEM techniques to reveal their crystal structure. Their optical properties are described in terms of UV-Vis absorption spectra and PL spectra. The tunable emission from 480 to 520 nm evidences the successful doping of Cu into ZnSe nanorods. The effects of the reaction time, the reaction temperature and the surface ligand on the optical properties are discussed in detail. After purification, the doped nanorods with a Cu/Zn ratio of 1% give a quantum yield of 7%. This emission could be retained for weeks in air, which is important for its future applications in many fields.

Earth-Abundant Transition-Metal Based Electrocatalysts for Water Electrolysis to Produce Renewable Hydrogen

Fundamentals of water electrolysis, and recent research progress and trends in the development of earth-abundant first-row transition-metal (Mn, Fe, Co, Ni, Cu)-based oxygen evolution reaction (OER) and hydrogen evolution (HER) electrocatalysts working in acidic, alkaline, or neutral conditions are reviewed. The HER catalysts include mainly metal chalcogenides, metal phosphides, metal nitrides, and metal carbides. As for the OER catalysts, the basic principles of the OER catalysts in alkaline, acidic, and neutral media are introduced, followed by the review and discussion of the Ni, Co, Fe, Mn, and perovskite-type OER catalysts developed so far. The different design principles of the OER catalysts in photoelectrocatalysis and photocatalysis systems are also presented. Finally, the future research directions of electrocatalysts for water splitting, and coupling of photovoltaic (PV) panel with a water electrolyzer, so called PV-E, are given as perspectives.