Wei-Nien Su

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.

Photoelectrochemical water splitting at low applied potential using a NiOOH coated codoped (Sn, Zr) α-Fe2O3 photoanode

One of the major challenges in photoelectrochemical water splitting is to develop an efficient photoanode that can oxidize water at low applied potential. Herein, a codoped (Sn, Zr) α-Fe2O3 photoanode modified with a stable and earth abundant nickel oxyhydroxide (NiOOH) co-catalyst that can split water at low applied potential is reported. First, an unintentional gradient monodoped (Sn) α-Fe2O3 photoanode was synthesized at controlled annealing temperature that achieved a photocurrent density of 0.86 mA cm−2 at 1.23 V vs. RHE. Further doping with an optimized amount of Zr outperformed the monodoped (Sn) α-Fe2O3 photoanode providing significantly much higher photocurrent density (1.34 mA cm−2). The remarkably improved electrical conductivity and more than three times higher charge carrier density (as evidenced from electrochemical impedance spectroscopy measurements and Mott–Schottky analysis) of the codoped (Sn, Zr) α-Fe2O3 photoanode highlight the importance of codoping. The synergetic effect of codoping (Sn, Zr) led to 1.6-fold enhancement in charge separation efficiency at 1.23 V compared to the monodoped (Sn) α-Fe2O3 photoanode. The NiOOH modified codoped (Sn, Zr) α-Fe2O3 photoanode exhibited drastically lower onset potential (0.58 V) and a photocurrent density of 1.64 mA cm−2 at 1.23 V. Interestingly a 160 mV cathodic shift in photocurrent onset potential was also observed. Concomitant with this, the NiOOH modified codoped (Sn, Zr) α-Fe2O3 photoanode exhibited 1.6 to 9.5-fold enhancement in charge injection efficiency (ηinj) at the kinetic control region of 0.7 to 0.9 V compared to the unmodified codoped photoanode. Gas evolution measurements also showed that the NiOOH modified codoped α-Fe2O3 photoanode achieved an average Faradaic efficiency of 93%.

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.

Photocatalytic hydrogen production on nickel-loaded LaxNa1−xTaO3 prepared by hydrogen peroxide-water based process

A green production process for producing nickel-loaded LaxNa1−xTaO3, using a hydrogen peroxide-water based solvent system (HW-derived), is reported. The H2 evolution of the HW-derived sample is about 1.8 times higher than samples made using ethanol as a solvent. The activity of the sample can be further increased 9.3 times by depositing nanosized nickel as a co-catalyst on the surface of the La0.02Na0.98TaO3. Possible mechanisms of H2 evolution from pure water and from aqueous methanol solutions using nickel in three states (i.e.Ni metal, NiO oxide, and Ni/NiO core/shell)-La0.02Na00.98TaO3, are discussed systematically for the first time. It is clearly shown that the activity of hydrogen generation from pure water is in sequence: Ni/NiO > NiO > Ni, whereas the activity sequence with respect to aqueous methanol is: Ni > Ni/NiO > NiO. Metallic Ni presents the most active sites and favors the formation of hydrogen from aqueous methanol. The Ni in Ni/NiO core/shell induce the migration of photogenerated electrons from the bulk to catalyst surface, while NiO act as H2 evolution site and prevent water formation from H2 and O2. The recombination is interrupted by the effective capture of the holes by methanol acting as a sacrificial reagent, thereby leading to higher hydrogen evolution. In contrast, the competition between the recombination and the charge-transfer reaction occurs in pure water leading to a possible back reaction between H2 and O2 on the photocatalysts surface. The photocatalyst synthesis avoids the use of organic solvents and thereby contributes to the environmentally friendly production of hydrogen.