Chun-Jern Pan

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.

Bimetallic PtM (M = Pd, Ir) nanoparticle decorated multi-walled carbon nanotube enzyme-free, mediator-less amperometric sensor for H2O2

A new highly catalytic and intensely sensitive amperometric sensor based on PtM (where M=Pd, Ir) bimetallic nanoparticles (NPs) for the rapid and accurate estimation of hydrogen peroxide (H(2)O(2)) by electrooxidation in physiological conditions is reported. PtPd and PtIr NPs-decorated multiwalled carbon nanotube nanocatalysts (PtM/MWCNTs) were prepared by a modified Watanabe method, and were characterized by XRD, TEM, ICP, and XAS. The sensors were constructed by immobilizing PtM/MWCNTs nanocatalysts in a Nafion film on a glassy carbon electrode. Both PtPd/MWCNTs and PtIr/MWCNTs assemblies catalyzed the electrochemical oxidation of H(2)O(2). Cyclic voltammetry characterization measurements revealed that both the PtM (M=Pd, Ir)/MWCNTs/GCE possessed similar electrochemical surface areas (∼0.55 cm(2)), and electron transfer rate constants (∼1.23 × 10(-3)cms(-1)); however, the PtPd sensor showed a better performance in H(2)O(2) sensing than did the PtIr counterpart. Explanations were sought from XAS measurements to explain the reasons for differences in sensor activity. When applied to the electrochemical detection of H(2)O(2), the PtPd/MWCNTs/GC electrode exhibited a low detection limit of 1.2 μM with a wide linear range of 2.5-125 μM (R(2)=0.9996). A low working potential (0V (SCE)), fast amperometric response (<5s), and high sensitivity (414.8 μA mM(-1)cm(-2)) were achieved at the PtPd/MWCNTs/GC electrode. In addition, the PtPd/MWCNTs nanocatalyst sensor electrode also exhibited excellent reproducibility and stability. Along with these attractive features, the sensor electrode also displayed very high specificity to H(2)O(2) with complete elimination of interference from UA, AA, AAP and glucose.

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.

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.

Ultrathin TiO2-coated MWCNTs with excellent conductivity and SMSI nature as Pt catalyst support for oxygen reduction reaction in PEMFCs

The sluggish kinetics of the oxygen reduction reaction (ORR), the instability of platinum on the carbon support, and carbon corrosion are still critical issues affecting the activity and long-term durability of polymer electrolyte membrane fuel cells. An ideal solution would be to modify the catalytic supports to enhance the durability and performance of supported catalysts. Here we have synthesized multiwalled carbon nanotube (MWCNT) supported ultrathin TiO2 films (MWCNT@UT-TiO2) using a simple modified sol–gel method. Our approach takes advantage of the strong metal support interactions (SMSIs) between the MWCNT@UT-TiO2 support and platinum nanoparticles, which results in a decrease of the d-band vacancy of platinum due to electron transfer from the support, thereby enhancing the performance of the supported catalysts. Our results revealed that Pt–MWCNT@UT-TiO2 has better catalytic activity and durability compared to Pt–MWCNT and Pt–C with equivalent Pt loadings.

Robust non-carbon Ti0.7Ru0.3O2 support with co-catalytic functionality for Pt: enhances catalytic activity and durability for fuel cells

Multifunctional binary metal oxide (Ti0.7Ru0.3O2), a novel functionalised co-catalytic support for Pt, is synthesized in a simple one-step hydrothermal process at low temperature. In practical applications Ti0.7Ru0.3O2 offers both excellent improvements in electrocatalytic activity and durability over commercial carbon supported Pt and PtRu catalysts for direct methanol fuel cell (DMFC), while at the molecular level it provides advantages in terms of its high surface area, and the strong interactions between Pt and the co-catalytic support. The Ti0.7Ru0.3O2 support acts as a co-catalyst supporting Pt activity, due to the high proton conductivity of hydrated Ti0.7Ru0.3O2 which underlies a ‘bifunctional mechanism’ and the synergistic effect between Pt and Ti0.7Ru0.3O2, modifying the electronic nature of the metal particles as well, which additionally enhances CO-tolerance, the catalytic activity and durability for methanol and hydrogen oxidation. Additionally, Ti0.7Ru0.3O2 can be fabricated as a much thinner catalyst layer resulting in improving mass transport kinetics, giving a broad scope for its wider application in other fuel cells, as demonstrated here by its application in a direct methanol fuel cell (DMFC) and polymer electrolyte membrane fuel cell (PEMFC) and can also be extended to other areas such as catalytic biosensor technology.

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.

Kinetically Controlled Autocatalytic Chemical Process for Bulk Production of Bimetallic Core–Shell Structured Nanoparticles

Although bimetallic core@shell structured nanoparticles (NPs) are achieving prominence due to their multifunctionalities and exceptional catalytic, magnetic, thermal, and optical properties, the rationale underlying their design remains unclear. Here we report a kinetically controlled autocatalytic chemical process, adaptable for use as a general protocol for the fabrication of bimetallic core@shell structured NPs, in which a sacrificial Cu ultrathin layer is autocatalytically deposited on a dimensionally stable noble-metal core under kinetically controlled conditions, which is then displaced to form an active ultrathin metal-layered shell by redox-transmetalation. Unlike thermodynamically controlled under-potential deposition processes, this general strategy allows for the scaling-up of production of high-quality core-shell structured NPs, without the need for any additional reducing agents and/or electrochemical treatments, some examples being Pd@Pt, Pt@Pd, Ir@Pt, and Ir@Pd. Having immediate and obvious commercial potential, Pd@Pt NPs have been systematically characterized by in situ X-ray absorption, electrochemical-FTIR, transmission electron microscopy, and electrochemical techniques, both during synthesis and subsequently during testing in one particularly important catalytic reaction, namely, the oxygen reduction reaction, which is pivotal in fuel cell operation. It was found that the bimetallic Pd@Pt NPs exhibited a significantly enhanced electrocatalytic activity, with respect to this reaction, in comparison with their monometallic counterparts.

High Coulombic efficiency aluminum-ion battery using an AlCl3-urea ionic liquid analog electrolyte

Significance To relieve humanity’s dependence on fossil fuels, grid-scale storage of renewable energy must be implemented. This requires cheap, high-rate, and long cycle life energy storage mechanisms. This work presents the development of an Al-ion battery using earth-abundant aluminum and graphite as anode and cathode, respectively, and an ionic liquid analog electrolyte composed of AlCl3 and urea that is very low-cost and eco-friendly. The battery exhibits ∼99.7% Coulombic efficiency and a substantial rate capability, with a cathode capacity of 73 mA g-1 at 100 mA g-1 (1.4 C). In recent years, impressive advances in harvesting renewable energy have led to a pressing demand for the complimentary energy storage technology. Here, a high Coulombic efficiency (∼99.7%) Al battery is developed using earth-abundant aluminum as the anode, graphite as the cathode, and a cheap ionic liquid analog electrolyte made from a mixture of AlCl3 and urea in a 1.3:1 molar ratio. The battery displays discharge voltage plateaus around 1.9 and 1.5 V (average discharge = 1.73 V) and yielded a specific cathode capacity of ∼73 mAh g−1 at a current density of 100 mA g−1 (∼1.4 C). High Coulombic efficiency over a range of charge–discharge rates and stability over ∼150–200 cycles was easily demonstrated. In situ Raman spectroscopy clearly showed chloroaluminate anion intercalation/deintercalation of graphite (positive electrode) during charge–discharge and suggested the formation of a stage 2 graphite intercalation compound when fully charged. Raman spectroscopy and NMR suggested the existence of AlCl4−, Al2Cl7− anions and [AlCl2·(urea)n]+ cations in the AlCl3/urea electrolyte when an excess of AlCl3 was present. Aluminum deposition therefore proceeded through two pathways, one involving Al2Cl7− anions and the other involving [AlCl2·(urea)n]+ cations. This battery is a promising prospect for a future high-performance, low-cost energy storage device.

O3–NaxMn1/3Fe2/3O2 as a positive electrode material for Na-ion batteries: structural evolutions and redox mechanisms upon Na+ (de)intercalation

The electrochemical properties of the O3-type NaxMn1/3Fe2/3O2 (x = 0.77) phase used as positive electrode material in Na batteries were investigated in the 1.5–3.8 V, 1.5–4.0 V and 1.5–4.3 V ranges. We show that cycling the Na cells in a wider voltage range do not induce a significant gain on long term cycling as the discharge capacities reached for the three experiments are identical after the 14th cycle. The structural changes the material undergoes from 1.5 V (fully intercalated state) to 4.3 V were investigated by operando in situ X-ray powder diffraction (XRPD) and were further characterized by ex situ synchrotron XRPD. We show that the low amount of Mn3+ ions (≈33% of total Mn+ ions) is enough to induce a cooperative Jahn–Teller effect for all MO6 octahedra in the fully intercalated state. Upon deintercalation the material exhibits several structural transitions: O′3 → O3 → P3. Furthermore, several residual phases are observed during the experiment. In particular, a small part of the O3 type is not transformed to P3 but is always involved in the electrochemical process. To explain this behaviour the hypothesis of an inhomogeneity, which is not detected by XRD, is suggested. All phases converge into a poorly crystallized phase for x ≈ 0.15. The short interslab distance of the resulting phase strongly suggests an octahedral environment for the Na+ ions. X-ray absorption spectroscopy and 57Fe Mossbauer spectroscopy were used to confirm the activity of the Mn4+/Mn3+ and Fe4+/Fe3+ redox couples in the low and high voltage regions, respectively. 57Fe Mossbauer spectroscopy also showed an increase of the disorder into the material upon deintercalation.