Yun Jeong Hwang

Facile growth of aligned WO3 nanorods on FTO substrate for enhanced photoanodic water oxidation activity

We demonstrate a facile hydrothermal method to grow oriented WO3 nanorods on a transparent conductive substrate without the assistance of any seed layer or structure directing agent for photocatalytic applications. The effects of hydrothermal growth conditions such as reaction time, precursor solution, and post annealing temperature on the crystalline phase and morphology of the WO3 on the FTO substrate are discussed. XRD studies reveal that the as-prepared orthorhombic WO3·0.33H2O nanorods are transformed to the hexagonal phase by post annealing at 400 °C. Moreover, post annealing above 500 °C converts them to monoclinic WO3 nanorods on the FTO substrate, which is the photoactive crystal phase of WO3 for water oxidation. The synthesized WO3 nanorods were revealed to have a single crystalline structure by HRTEM analysis. The photoelectrochemical water splitting properties of the annealed WO3 nanorod arrays were investigated in 0.5 M Na2SO4 under AM 1.5G illumination. The optimized WO3 nanorod arrays exhibit a photocurrent of 2.26 mA cm−2 at 1.23 V (versus RHE), and an incident photon-to-current conversion efficiency (IPCE) of as high as 35% at 400 nm for the photoelectrochemical oxidation of water. This simple hydrothermal method can allow the use of WO3 for photoanodic applications with high efficiency.

Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles

Selective electrochemical reduction of CO2 is one of the most sought-after processes because of the potential to convert a harmful greenhouse gas to a useful chemical. We have discovered that immobilized Ag nanoparticles supported on carbon exhibit enhanced Faradaic efficiency and a lower overpotential for selective reduction of CO2 to CO. These electrocatalysts were synthesized directly on the carbon support by a facile one-pot method using a cysteamine anchoring agent resulting in controlled monodispersed particle sizes. These synthesized Ag/C electrodes showed improved activities, specifically decrease of the overpotential by 300 mV at 1 mA/cm(2), and 4-fold enhanced CO Faradaic efficiency at -0.75 V vs RHE with the optimal particle size of 5 nm compared to polycrystalline Ag foil. DFT calculations enlightened that the specific interaction between Ag nanoparticle and the anchoring agents modified the catalyst surface to have a selectively higher affinity to the intermediate COOH over CO, which effectively lowers the overpotential.

Embedding covalency into metal catalysts for efficient electrochemical conversion of CO2.

CO2 conversion is an essential technology to develop a sustainable carbon economy for the present and the future. Many studies have focused extensively on the electrochemical conversion of CO2 into various useful chemicals. However, there is not yet a solution of sufficiently high enough efficiency and stability to demonstrate practical applicability. In this work, we use first-principles-based high-throughput screening to propose silver-based catalysts for efficient electrochemical reduction of CO2 to CO while decreasing the overpotential by 0.4-0.5 V. We discovered the covalency-aided electrochemical reaction (CAER) mechanism in which p-block dopants have a major effect on the modulating reaction energetics by imposing partial covalency into the metal catalysts, thereby enhancing their catalytic activity well beyond modulations arising from d-block dopants. In particular, sulfur or arsenic doping can effectively minimize the overpotential with good structural and electrochemical stability. We expect this work to provide useful insights to guide the development of a feasible strategy to overcome the limitations of current technology for electrochemical CO2 conversion.

Simple Chemical Solution Deposition of Co3O4 Thin Film Electrocatalyst for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is the key reaction in electrochemical processes, such as water splitting, metal-air batteries, and solar fuel production. Herein, we developed a facile chemical solution deposition method to prepare a highly active Co3O4 thin film electrode for OER, showing a low overpotential of 377 mV at 10 mA/cm(2) with good stability. An optimal loading of ethyl cellulose additive in a precursor solution was found to be essential for the morphology control and thus its electrocatalytic activity. Our results also show that the distribution of Co3O4 nanoparticle catalysts on the substrate is crucial in enhancing the inherent OER catalytic performance.

A monolithic and standalone solar-fuel device having comparable efficiency to photosynthesis in nature

The need for developing sustainable energy sources has generated academic and industrial attention in artificial photosynthesis, inspired by the natural process. In this study, we demonstrate a highly efficient solar energy to fuel conversion device using CO2 and water as feedstock. We developed a thin film photovoltaic technology for the light absorbing component using a low cost, solution based Cu(InxGa1−x)(SySe1−y)2 (CIGS) module fabrication method to provide sufficient potential for the conversion reactions. Our solar-fuel device uses cobalt oxide (Co3O4) nanoparticle thin film deposited with a low temperature coating method as the water oxidation catalyst and nanostructured gold film as the CO2 reduction to CO generation catalyst. We demonstrated that the integrated monolithic device operated by energy only from sunlight, in an absence of any external energy input. The individual components showed the following abilities: solar-to-power conversion efficiency of 8.58% for the CIGS photovoltaic module photoelectrode, overpotential reduction of water oxidation with the Co3O4 catalyst film by ∼360 mV at 5 mA cm−2, and Faradaic efficiency of over 90% by the nanostructured Au catalyst for CO2 reduction to CO. Remarkably, this is the first demonstration of a monolithic and standalone solar-fuel device whose solar-to-fuel conversion efficiency from CO2 and H2O is 4.23%, which is comparable with that of photosynthesis in nature.

Effect of the Si/TiO2/BiVO4 Heterojunction on the Onset Potential of Photocurrents for Solar Water Oxidation

BiVO4 has been formed into heterojunctions with other metal oxide semiconductors to increase the efficiency for solar water oxidation. Here, we suggest that heterojunction photoanodes of Si and BiVO4 can also increase the efficiency of charge separation and reduce the onset potential of the photocurrent by utilizing the high conduction band edge potential of Si in a dual-absorber system. We found that a thin TiO2 interlayer is required in this structure to realize the suggested photocurrent density enhancement and shifts in onset potential. Si/TiO2/BiVO4 photoanodes showed 1.0 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode (RHE) with 0.11 V (vs RHE) of onset potential, which were a 3.3-fold photocurrent density enhancement and a negative shift in onset potential of 300 mV compared to the performance of FTO/BiVO4 photoanodes.

Printable, wide band-gap chalcopyrite thin films for power generating window applications

Printable, wide band-gap chalcopyrite compound films (CuInGaS2, CIGS) were synthesized on transparent conducting oxide substrates. The wide band-gap and defective nature of the films reveal semi-transparent and bifacial properties that are beneficial for power generating window applications. Importantly, solar cell devices with these films demonstrate a synergistic effect for bifacial illumination resulting in a 5.4–16.3% increase of the apparent power conversion efficiency compared to the simple sum of the efficiencies of the front and rear side illumination only. We also confirmed that this extra output power acquisition due to bifacial irradiation is apparently not influenced by the light intensity of the rear side illumination, which implies that weak light (e.g., indoor light) can be efficiently utilized to improve the overall solar cell efficiency of bifacial devices.

Construction of efficient CdS–TiO2 heterojunction for enhanced photocurrent, photostability, and photoelectron lifetimes

The photoefficiency of CdS/TiO2 electrodes can be enhanced by employing efficient method of CdS sensitization from which, the contact area, thickness of CdS layer, and the recombination of photoelectrons with electrolyte can be controlled. Here, we demonstrate a simple solvothermal approach of CdS quantum dots (QDs) sensitization on TiO2 nanoparticle (NP) film coated on FTO. Our new approach prevents the clogging of CdS QDs and promotes uniform deposition of QDs throughout the mesoporous TiO2 NP film. The sensitization of CdS can be controlled by the reaction time and the concentration of the precursors. The solvothermally sensitized photoanodes exhibit enhanced photocurrents and fill factors and improved photostability in aqueous solution compared to the one prepared by a conventional SILAR method. Open-circuit potential decay measurement under shutting off illumination shows that the lifetime of photoelectron is extended with solvothermally prepared CdS layer, indicating efficient suppression of recombination of the accumulated electron in TiO2 to the electrolyte. This methodology can be applied in making more efficient heterojunctions consisting of CdS and other wide band gap oxide semiconductors which could improve charge separation and mitigate charge recombination for photoelectrochemical applications.

Insight into Charge Separation in WO3/BiVO4 Heterojunction for Solar Water Splitting

Recently, the WO3/BiVO4 heterojunction has shown promising photoelectrochemical (PEC) water splitting activity based on its charge transfer and light absorption capability, and notable enhancement of the photocurrent has been achieved via morphological modification of WO3. We developed a graft copolymer-assisted protocol for the synthesis of WO3 mesoporous thin films on a transparent conducting electrode, wherein the particle size, particle shape, and thickness of the WO3 layer were controlled by tuning the interactions in the polymer/sol-gel hybrid. The PEC performance of the WO3 mesoporous photoanodes with various morphologies and the individual heterojunctions with BiVO4 (WO3/BiVO4) were characterized by measuring the photocurrents in the absence/presence of hole scavengers using light absorption spectroscopy and intensity-modulated photocurrent spectroscopy. The morphology of the WO3 photoanode directly influenced the charge separation efficiency within the WO3 layer and concomitant charge collection efficiency in the WO3/BiVO4 heterojunction, showing the smaller sized nanosphere WO3 layer showed higher values than did the plate-like or rod-like one. Notably, we observed that photocurrent density of WO3/BiVO4 was not dependent on the thickness of WO3 film or its charge collection time, implying slow charge flow from BiVO4 to WO3 can be a crucial issue in determining the photocurrent, rather than the charge separation within the nanosphere WO3 layer.

Chalcogenization-Derived Band Gap Grading in Solution-Processed CuInxGa1–x(Se,S)2 Thin-Film Solar Cells

Significant enhancement of solution-processed CuIn(x)Ga(1-x)(Se,S)2 (CIGSSe) thin-film solar cell performance was achieved by inducing a band gap gradient in the film thickness, which was triggered by the chalcogenization process. Specifically, after the preparation of an amorphous mixed oxide film of Cu, In, and Ga by a simple paste coating method chalcogenization under Se vapor, along with the flow of dilute H2S gas, resulted in the formation of CIGSSe films with graded composition distribution: S-rich top, In- and Se-rich middle, and Ga- and S-rich bottom. This uneven compositional distribution was confirmed to lead to a band gap gradient in the film, which may also be responsible for enhancement in the open circuit voltage and reduction in photocurrent loss, thus increasing the overall efficiency. The highest power conversion efficiency of 11.7% was achieved with J(sc) of 28.3 mA/cm(2), V(oc) of 601 mV, and FF of 68.6%.