Water purification is the recent emergence in handling sewage and contaminated solution from different sources, such as industrial and domestic disposal. Such practice is being comprehensively explored to foresee whether it could be applicable in our lives since a couple of unpredicted toxic chemicals in wastewater may pose a significant risk to people. Environmentally-friendly research on water purification stands out as a promising solution, therefore, water purification by utilizing natural sunlight and nanomaterials was proposed by scientists. This thought incorporates raw environmental resource into chemical reaction of chosen materials. Literally, photocatalytic water purification is the decomposition of organic solution into less harmful products under the irradiation of sunlight. To quickly realize how photocatalytic reaction is proceeding in a compact and observable scale, a small-size of solar reactor could also be fostered because of its high surface to volume ratio. Consequently, Wang et al.¹ claimed that the technology is undergoing rapid development due to its tremendous potential in the fast testing field in environmental science. In the essay, the mechanism and its diverse application of light-induced water purification by using catalysts in micro-reactors will be discussed in an ordered manner.

Solar-driven photoelectrochemical (PEC) water splitting for hydrogen production promises to solve the impending energy and environmental crisis. The key to increase the efficiency of PEC hydrogen generation is developing high-performance catalysts and photocathodes. 3D p-type silicon (p-Si) arrays are promising architectures due to the high light harvesting and the large interfacial areas. We demonstrate its enhanced PEC performance with a photocurrent density of -37.5 mA/cm² at 0 V (vs. RHE) under simulated 100 mW/cm² (1 Sun) with an AM 1.5 G filter, which is the highest value reported for p-type Si photocathodes. The synergic effects of the excellent light harvesting of Si nanopillar (NP) array core and the good optical transparency , as well as excellent electrocatalytic activity of NiCoSe_x shell boost the production and utilization of photogenerated electrons. The Faradaic efficiency of H₂ and O₂ on p-Si/NiCoSe_x was approximately 100%, which confirmed that the photocurrent during PEC reaction was attributed to hydrogen generation. The completely enclosed core-shell structure isolated the Si NP from air and aqueous electrolyte and minimized the oxidation of silicon, leading to good stability. The design of p-Si/NiCoSe_x core/shell NP arrays offers a new strategy for preparing highly efficient photoelectrochemical solar energy conversion devices.

Setting a target on implantable medical devices such as respiration-supporting pacemaker for amyotrophic lateral sclerosis (ALS), we develop an energy harvester that could earn electrical power from the mechanical motion of liquid droplets on an electrical charged plate called “electret.” A PDMS sheet with micro fluidic channels is laminated onto a silicon substrate with built-in permanent electrical charges. A pair of capacitive electrodes is formed in the sealed fluidic channels, in which water droplets are displaced back and forth due to the applied pressure that simulates the motion of heartbeat. Typical output power of 0.17 µW/cm² is obtained at 0.47 Hz. Analytical model suggests that the extension of the electret plate to ~ 6 cm² delivers 1 mW power, which is sufficient to drive the implantable medical devices.

The use of sunlight to drive organic reactions constitutes a green and sustainable strategy for organic synthesis. Herein, we discovered that the earth-abundant aluminum oxide (Al₂O₃) though paradigmatically known to be an insulator could induce an immense increase in the selective photo-oxidation of different benzyl alcohols in the presence of a large variety of dyes and O₂. This unique phenomenon is based on the surface complexation of benzyl alcohol (BnOH) with the Brønsted base sites on Al₂O₃, which reduces its oxidation potential and causes an upshift in its HOMO for electron abstraction by the dye. The surface complexation of O₂ with Al₂O₃ also activates the adsorbed O₂ for receiving electrons from the photoexcited dyes. This discovery brings forth a new understanding on utilizing surface complexation mechanisms between the reactants and earth abundant materials to effectively achieve a wider range of photoredox reactions.

This talk will discuss some of the limitations and optimizations relating to solar fuels device. From an experimental standpoint, our work on ultra-low Pt loadings for photocatalytic hydrogen production on silicon photocathodes shows that platinum is potentially even more scalable than non-noble metal catalysts. From a modeling standpoint, our web based model at solarfuelsmodeling.com¹ allows users to model and analyze efficiencies of device given various input parameters.  By comparing experimental data to theoretically optimal data, we have quantitatively analyzed how close best in class photoabsorber materials are to reaching their thermodynamic limits.  This approach also shows what the fundamental issues (photocurrent vs. photovoltage) are holding back a particular material.  We have also branched out using this modeling for non-water splitting reactions, such as CO₂ reduction and halide production and the potential for these reactions will also be discussed.

1. A Flexible Web-Based Approach to Modeling Tandem Photocatalytic Devices. Seger B.; Hansen, O.; Vesborg  P.C.K. Solar RRL 2017, 1, 1600013

In response to fossil-fuel shortages and ecological deterioration, human being has been diligently seeking for clean and renewable energy sources, which has placed energy storage at the forefront of technological innovation.  Such endeavors will not only provide critical technologies for grid energy storage, in particular, grid-connected intermittent energy sources such as photovoltaics and wind turbines, but are also essential to the development of electric vehicles, plug-in hybrid vehicles and other applications.  Current electrical energy storage is primarily based on batteries and supercapacitors.  Developing novel materials and material architectures that can provide high energy and power densities is particularly essential.  In this talk, the design and synthesis of novel nanocomposites for advanced energy storage applications will be presented.  Towards high-performance electrodes for energy storage, the essential prerequisites are making electrodes with faster ion transport, excellent electron conductivity, and robust electrode structure.  This provides a general design platform towards high-performance electrodes for energy storage applications.

A programmable fluid-fluid interface is a powerful platform for realizing many potential applications in the field of microfluidics, drug delivery, tunable optical components and display devices. The most fundamental challenge is to realize a reliable method capable of on-demand delivery and organization of functional components (solid objects or liquid drops) at the desired position. An ideal strategy should be able to perform transport/assembly in a reconfigurable manner under isothermal conditions, also should be free from the addition of chemicals. In this talk, I will present an overview of stimuli-controlled particle manipulation with an emphasis on our recent results on reconfigurable particle assembly/transport using bubble mediated capillary interactions. The method is capable of long-range transport, assembly and on-demand disassembly of floating objects at the water-air interface, by simply controlling the dynamics of the air bubble. Since capillarity driven assembly is applicable for microscale objects as well, the method presented could prove effective for the tunable patterning/assembly of smaller objects, in a reconfigurable manner.

Photothermal, also known as light-to-heat conversion, a seemingly primitive and ancient means of utilizing solar energy involves harvesting and converting solar irradiation by photothermal materials into heat as terminal energy for beneficial usage. Due to its high energy conversion efficiency, photothermal conversion has gained renewed research interest in the past decade and found itself certain niche applications such as water evaporation and water desalination. In this talk, we will cover (1) droplet light heating system for measuring internal light-to-heat conversion efficiency of a photomaterials; (2) interfacial heating scheme to improve light-to-water-evaporation efficiency; surface wettability is a generally accepted approach to induce self-floating photothermal materials for automated interfacial heating. (3) Rational design of heat barrier to minimize the heat loss to bulk water. Coupled and decoupled water transport channel from heat barrier will be discussed and compared. (4) A polymeric polypyrrole based self-floating and self-healing photothermal material design will be presented in the end.           

Direct ethanol fuel cells (DEFC), which convert the chemical energy stored in ethanol directly into electricity, are one of the most promising energy-conversion devices for portable, mobile and stationary power applications, primarily because ethanol is a carbon-neutral, sustainable fuel and possesses many unique physicochemical properties including high energy density and ease of transportation, storage as well as handling. However, conventional DEFCs that use acid proton exchange membranes and precious metal catalysts result in rather low performance. In our research, we use alkaline anion exchange membranes as the ion conductor in DEFCs. The change from the acid membrane to an alkaline one leads to a significant performance boost. In addition, we proposed a novel hybrid DEFC system, which consists of an alkaline anode and an acid cathode. The power density of DEFCs now is as high as 450 mW cm^-2, which represents the highest power density for this type of fuel cell.

Nowadays, while the inorganic based solar cells have already commercialized and matured for several years, the polymer based counterpart are still on their way to hit the 10% or even higher power conversion efficiency. Generally speaking, the power conversion efficiency is the multiply of open voltage, current density and fill factor. As is known to all that the absolute energy gap between the LUMO Levels of donor and acceptor should be no less than 0.3 eV. However, this thumb rule seemed having been challenged from recent reports. It should be an encouraging result if we can minimize the gap without deteriorates the charge dissociation, given the fact while the necessary gap provide the required driving force but at the same time it is one of the source of energy loss for larger open circuit voltage. In order to fulfill this aim, side chain decorating has been widely studied, so did the introducing of heteroatoms, such as fluorine. However, it is less studied the introducing of chlorine into the side chain. In this study, chlorination as an effective method to enlarge the open voltage was realized by chlorine substitution of either the electron rich or electron deficient part of PTB7-Th precursor. By combining this newly designed polymer with the widely used molecular ITIC, an evident enhancement of power conversion efficiency (PCE) up to 8.21% (no additive) with an open circuit voltage up to 1.03 V was realized, which is much higher than that of PTB7-Th counterpart (6.80%) with 0.25% DIO additive.