Dr Yiyang Li

Abstract:

The development of new recyclable and inexpensive electrochemically active species for water oxidation catalysis is the most crucial step for future utilization of renewables. Particularly, transition metal complexes containing internal multiple, cooperative metal centers to couple with redox catalysts in the inorganic Keggin-type polyoxometalate (POM) framework at high potential or under extreme pH conditions would be promising candidates. However, most reported Ni-containing POMs have been highly unstable towards hydrolytic decomposition, which precludes them from application as water oxidation catalysts (WOCs). Here, we have prepared new tri-Ni-containing POMs with variable oxidation states by charge tailored synthetic strategies for the first time and developed them as recyclable POMs for water oxidation catalysts. In addition, by implanting corresponding POM anions into the positively charged MIL-101(Cr) metal–organic framework (MOF), the entrapped Ni2+/Ni3+ species can show complete recyclability for water oxidation catalysis without encountering uncontrolled hydrolysis of the POM framework. As a result, a low onset potential of approximately 1.46 V vs. NHE for water oxidation with stable WOC performance is recorded. Based on this study, rational design and stabilization of other POM-electrocatalysts containing different multiple transition metal centres could be made possible.

Abstract:

Photocatalytic splitting of seawater for hydrogen evolution has attracted a great deal of attention in recent years. However, the poor energy conversion efficiency and stability of photocatalysts in a salty environment have greatly hindered further applications of this technology. Moreover, the effects of electrolytes in seawater remain controversial. Here we present electrolyte-assisted charge polarization over an N-doped TiO2 photocatalyst, which demonstrates the stoichiometric evolution of H2 and O2 from the thermo-assisted photocatalytic splitting of seawater. Our extensive characterizations and computational studies show that ionic species in seawater can selectively adsorb on photo-polarized facets of the opposite charge, which can prolong the charge-carrier lifetime by a factor of five, leading to an overall energy conversion efficiency of 15.9 ± 0.4% at 270 °C. Using a light-concentrated furnace, a steady hydrogen evolution rate of 40 mmol g−1 h−1 is demonstrated, which is of the same order of magnitude as laboratory-scale electrolysers.

Abstract:

2D-layered transition metal chalcogenides are useful semiconductors for a wide range of opto-electronic applications. Their similarity as layered structures offers exciting possibility to modify their electronic properties by creating new heterojunction assemblies from layer-by-layer restacking of individual monolayer sheets, however, the lack of specific interaction between these layers could induce phase segregation. Here, we employed a chemical method using n-BuLi to exfoliate MoS₂ and In₂Se₃ into their monolayer-containing colloids in solution. The bulky Se atoms can be selectively leached from In₂Se₃ during Li treatment which gives positively charged surface monolayers in neutral pH whereas the strong polarization of Mo–S with moderate S leaching gives a negatively charged surface. Specific interlayer electrostatic attraction during their selective assembly gives a controllable atomic AB-type of layer stacking as supported by EXAFS, STEM with super-EDX mapping, TAS/TRPL and DFT calculations. Using this simple but inexpensive bottom-up solution method, a new photocatalyst assembled from layers for photo water splitting can be tailor-made with high activity.

Abstract:

The photocatalytic overall water splitting (POWS) reaction using particulate catalysts is considered as an ideal approach for capturing solar energy and storing it in the form of hydrogen, however, current POWS systems are hindered by the slow separation but fast recombination of the photo-generated charge carriers, hence giving unsatisfactory performances. Here we report a dramatically improved POWS system for a Au-supported Fe₃O₄/N-TiO₂ superparamagnetic photocatalyst promoted by local magnetic field effects. Strong local magnetic flux was induced by a weak external magnetic field of 180 mT, which then resulted in a quantum efficiency of 88.7% at 437 nm at 270 °C without any sacrificial reagent. The mechanism of the magnetic field effects was explored systematically and quantitatively by time-resolved spectroscopic technique and first-principles calculations, which suggested such enhancement was due to the greatly prolonged excitonic lifetime, originating from both the Lorentz force and spin-polarisation effects. By controllable manipulation of both features using local magnetic field, an unprecedented solar-to-hydrogen conversion efficiency of 11.9 ± 0.5% and an overall energy efficiency of 1.16 ± 0.05% were achieved in a particulate POWS system under AM 1.5G simulated solar illumination, which exceeds the STH goal of 10% for practical applications of POWS systems imposed by the United States Department of Energy.

Abstract:

It has come to our attention that the unit in the intrinsic activity (TOF) was not correct in y axis of Figure 4b and the TOC graphic. In addition, the label of the XPS spectra of 3%Fe-sMoS2 and 5%Fe-sMoS2 had been swooped by accident in the original Figure S17. All the descriptions and conclusion remain the same, but the labels need to be corrected. The corrected figures are shown below; the SI graphics are provided in the corrected SI file. (Figure Presented).