Bing-Joe Hwang

Organometal halide perovskite solar cells: degradation and stability

Organometal halide perovskite solar cells have evolved in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) reached 20.1% late last year. The key disquiet was stability, which has been limiting practical application, but now the state of the art is promising, being measured in thousands of hours. These improvements have been achieved through the application of different materials, interfaces and device architecture optimizations, especially after the investigation of hole conductor free mesoporous devices incorporating carbon electrodes, which promise stable, low cost and easy device fabrication methods. However, this work is still far from complete. There are various issues associated with the degradation of Omh-perovskite, and the interface and device instability which must be addressed to achieve good reproducibility and long lifetimes for Omh-PSCs with high conversion efficiencies. A comprehensive understanding of these issues is required to achieve breakthroughs in stability and practical outdoor applications of Omh-PSCs. For successful small and large scale applications, besides the improvement of the PCE, the stability of Omh-PSCs has to be improved. The causes of failure and associated mechanisms of device degradation, followed by the origins of degradation, approaches to improve stability, and methods and protocols are discussed in detail and form the main focus of this review article.

Direct In situ Observation of Li2O Evolution on Li-Rich High-Capacity Cathode Material, Li[NixLi(1–2x)/3Mn(2–x)/3]O2 (0 ≤ x ≤0.5)

High-capacity layered, lithium-rich oxide cathodes show great promise for use as positive electrode materials for rechargeable lithium ion batteries. Understanding the effects of oxygen activating reactions on the cathodes surface during electrochemical cycling can lead to improvements in stability and performance. We used in situ surfaced-enhanced Raman spectroscopy (SERS) to observe the oxygen-related surface reactions that occur during electrochemical cycling on lithium-rich cathodes. Here, we demonstrate the direct observation of Li2O formation during the extended plateau and discuss the consequences of its formation on the cathode and anode. The formation of Li2O on the cathode leads to the formation of species related to the generation of H2O together with LiOH and to changes within the electrolyte, which eventually result in diminished performance. Protection from, or mitigation of, such devastating surface reactions on both electrodes will be necessary to help realize the potential of high-capacity cathode materials (270 mAhg(-1) versus 140 mAhg(-1) for LiCoO2) for practical applications.

Using hematite for photoelectrochemical water splitting: a review of current progress and challenges

Photoelectrochemical (PEC) water splitting is a promising technology for solar hydrogen production to build a sustainable, renewable and clean energy economy. Hematite (α-Fe2O3) based photoanodes offer promise for such applications, due to their high chemical stability, great abundance and low cost. Despite these promising properties, progress towards the manufacture of practical water splitting devices has been limited. This review is intended to highlight recent advancements and the limitations that still hamper the full utilization of hematite electrodes in PEC water splitting systems. We review recent progress in manipulating hematite for PEC water splitting through various approaches, focused on e.g. enhancing light absorption, water oxidation kinetics, and charge carrier collection efficiency. As the morphology affects various properties, progress in morphological characterization from thicker planar films to recent ultrathin nanophotonic morphologies is also examined. Special emphasis has been given to various ultrathin films and nanophotonic structures which have not been given much attention in previous review articles.

The synergetic effect of graphene on Cu2O nanowire arrays as a highly efficient hydrogen evolution photocathode in water splitting

A one dimensional (1D) Cu2O straddled with graphene is proposed as a highly promising and stable photocathode for solar hydrogen production. The Cu2O nanowire arrays modified with an optimized concentration of graphene provide much higher improved photocurrent density −4.8 mA cm−2, (which is two times that of bare 1D Cu2O, −2.3 mA cm−2), at 0 V vs. RHE under AM 1.5 illumination (100 mW cm−2) and solar conversion efficiency reaching 3.3% at an applied potential of −0.55 V vs. the Pt counter electrode. Surprisingly, 1D Cu2O with an optimum graphene concentration exhibits an inspiring photocurrent density from 2.1 to 1.1 mA cm−2 at a higher positive potential range of 0.2–0.4 V vs. RHE, which is 300–550% higher compared with that of bare 1D Cu2O. This is the highest value ever reported for a Cu2O-based photocathode at such a positive potential. After 20 minutes of standard solar irradiation, 83% of the initial photocurrent density was retained for the nanocomposite which is more than five times compared to the bare Cu2O (14.5%). A Faradic efficiency of 74% was obtained for the evolved H2 gas measurement. To get evidence for the photostability of the graphene modified photocathode, detailed characterization was carried out. The high PEC performance of the graphene/Cu2O nanocomposite is attributed to the improved crystallinity and the synergetic effect of graphene in absorbing visible light, suppressing the charge recombination and photocorrosion of the photoelectrode by preventing direct contact with the electrolyte. This inexpensive photocathode prepared free of noble metals, showed enhanced high photocurrent density with good stability and is a highly promising photocathode for solar hydrogen production.

Performance and design considerations for lithium excess layered oxide positive electrode materials for lithium ion batteries

The Li-excess oxide compound is one of the most promising positive electrode materials for next generation batteries exhibiting high capacities of >300 mA h g−1 due to the unconventional participation of the oxygen anion redox in the charge compensation mechanism. However, its synthesis has been proven to be highly sensitive to varying conditions and parameters where nanoscale phase separation may occur that affects the overall battery performance and life. In addition, several thermodynamic and kinetic drawbacks including large first cycle irreversible capacity, poor rate capability, voltage fading, and surface structural transformation need to be addressed in order to reach commercialization. This review will focus on the recent progress and performance trends over the years and provide several guidelines and design considerations based on the library of work done on this particular class of materials.

Heterostructured Cu2O/CuO decorated with nickel as a highly efficient photocathode for photoelectrochemical water reduction

Here we report the design, synthesis and characterization of a novel Cu2O/CuO heterojunction decorated with a nickel cocatalyst as a highly efficient photocathode for solar hydrogen production. The heterojunction structure was shown and examined by X-ray absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Tip-enhanced Raman spectroscopy (TERS). Due to the synergistic effect, the Cu2O/CuO heterojunction gave a remarkably improved photocurrent density (−2.1 mA cm−2), i.e. 3.1 times higher than a Cu2O photoelectrode. Additionally, the Cu2O/CuO heterojunction, when decorated with the nickel cocatalyst, showed six-fold and two-fold increases in photocurrent density (−4.3 mA cm−2) respectively when compared to Cu2O and the bare Cu2O/CuO at 0 V vs. RHE under AM 1.5 illumination (100 mW cm−2). Interestingly, the Ni decorated Cu2O/CuO photocathode showed an impressive solar conversion efficiency of 2.71% at −0.4 V vs. Pt, i.e. 467% higher compared to the bare Cu2O/CuO. After 20 minutes of standard solar illumination, 87.7% of the initial photocurrent density was retained for the nickel decorated Cu2O/CuO heterojunction. This is more than 1.5 times that of the bare Cu2O/CuO (53.6%), suggesting that surface modification with Ni not only effectively promotes water splitting but also stabilizes the photoelectrode. The enhanced photoelectrochemical performance is attributable to the efficient charge transfer and protective role of Ni, the improved crystallinity and the synergistic effect of the heterojunction in light absorption and charge separation. This inexpensive photocathode with increased photocurrent density and photostability offers a higher promise for solar hydrogen production.

Blending Cr2O3 into a NiO–Ni Electrocatalyst for Sustained Water Splitting

The rising H2 economy demands active and durable electrocatalysts based on low-cost, earth-abundant materials for water electrolysis/photolysis. Here we report nanoscale Ni metal cores over-coated by a Cr2 O3 -blended NiO layer synthesized on metallic foam substrates. The Ni@NiO/Cr2 O3 triphase material exhibits superior activity and stability similar to Pt for the hydrogen-evolution reaction in basic solutions. The chemically stable Cr2 O3 is crucial for preventing oxidation of the Ni core, maintaining abundant NiO/Ni interfaces as catalytically active sites in the heterostructure and thus imparting high stability to the hydrogen-evolution catalyst. The highly active and stable electrocatalyst enables an alkaline electrolyzer operating at 20 mA cm(-2) at a voltage lower than 1.5 V, lasting longer than 3 weeks without decay. The non-precious metal catalysts afford a high efficiency of about 15 % for light-driven water splitting using GaAs solar cells.

A highly stable CuS and CuS–Pt modified Cu2O/CuO heterostructure as an efficient photocathode for the hydrogen evolution reaction

A Cu2O/CuO heterostructure modified with CuS is proposed as a highly promising and stable photocathode for solar hydrogen production. The Cu2O/CuO/CuS heterostructure was synthesized by in situ growth of Cu2O/CuO via simple electrodeposition of Cu film followed by annealing in air, and then the surfaces of the heterostructure were sequentially modified by loading CuS via a successive ion layer adsorption and reaction (SILAR) approach. Experimental evidence, including Raman, XANES/EXAFS and XPS spectra, is presented for the interfacial reaction between CuS and Cu2O/CuO. The optimized Cu2O/CuO/CuS photocathode provides a remarkably enhanced photocurrent density of −5.4 mA cm−2 (i.e. >2.5 times than that of bare Cu2O/CuO) at 0 V vs. RHE under standard AM 1.5 light illumination. Due to the bicatalytic effects in suppressing electron–hole recombination, a further increase in photocurrent density to −5.7 mA cm−2 was noticed after decorating the Cu2O/CuO surface with both CuS and Pt. To the best of our knowledge, this is the highest performance yet reported for a cocatalyst modified Cu2O/CuO photoelectrode for solar water splitting. More importantly, the Cu2O/CuO heterostructure modified with optimum CuS afforded an impressive solar conversion efficiency of ABPE% = 3.6%, which is a greater than fourfold increase compared to the bare Cu2O/CuO. The stability of the bare Cu2O/CuO photocathode showed about a 44% decrease in initial photocurrent density within 1 h, whereas 85% and 92% of the initial photocurrent was maintained after 1 h when the photocathode was modified with CuS and with both CuS and Pt, respectively. This highly enhanced photoelectrochemical property is attributed to the fast transfer of photogenerated electrons resulting in suppressed electron–hole recombination and the synergistic effect of a heterojunction in light absorption and charge separation. This work demonstrates a facile strategy and potential use of low cost CuS as an efficient cocatalyst for solar hydrogen production that can be applicable in the general field of energy conversion.

Binder-free rice husk-based silicon–graphene composite as energy efficient Li-ion battery anodes

Rice husks, often neglected and considered as waste, contain constituents that could be of a potential use in advanced material applications. In this study, rice husks were used as a source of silicon dioxide for the synthesis of silicon nanoparticles (Si NPs) through magnesiothermic reduction process. The Si NPs were further used to prepare a binder-free composite system comprising Si NPs and graphene as an anode material for lithium ion battery system (LiBs). The composite system fabricated from rice husk-based Si NPs (RH-Si NPs) yielded an initial capacity of 1000 mA h g−1 at high applied current density of 1000 mA g−1. This study opens up the use of waste materials such as rice husk as a sustainable source of key components in advanced technology applications.

Nucleation and growth mechanism of electroformation of polypyrrole on a heat-treated gold/highly oriented pyrolytic graphite

Abstract Nucleation and growth mechanism of polypyrrole (PPy) films was investigated on vapor deposited gold/highly oriented pyrolytic graphite (Au/HOPG) after heat treatment by means of current–time transient ( i – t ) measurements and tapping mode atomic force microscopy. It was found that the nucleation and growth was a progressive 3-D before and after nuclei overlapping. However, the mechanism was 2-D instantaneous before nuclei overlapping and 3-D progressive after nuclei overlapping on a Au/HOPG substrate without heat treatment. The change in mechanism was due to lower surface defects on a heat-treated Au/HOPG substrate than on a bare Au/HOPG substrate. Images of PPy on a heat-treated Au/HOPG contained zig-zag and nodular structures.